STEEL MATERIAL

Description

TECHNICAL FIELD

The present disclosure relates to a steel material, and more particularly to a steel material that will serve as a starting material for a component for machine structural use.

BACKGROUND ART

Components for machine structural use are used as automobile parts, such as crankshafts for automobiles and construction vehicles. Machine structural parts are required to have high fatigue strength.

A technique for improving the fatigue strength of components for machine structural use is disclosed in, for example, Japanese Patent Application Publication No. 2008-169411 (Patent Literature 1).

The steel material disclosed in Patent Literature 1, used as a material for machine structural parts, contains, in mass %, C: 0.15 to 0.55%, Si: 0.01 to 2.0%, Mn: 0.01 to 2.5%, Cu: 0.01 to 2.0%, Ni: 0.01 to 2.0%, Cr: 0.01 to 2.5%, Mo: 0.01 to 3.0%, and at least one kind of element selected from a group consisting of V and W in a total amount of 0.01 to 1.0%, with the balance being Fe and unavoidable impurities. For this steel material, the Larson-Miller Parameter (LMP) that gives the maximum HRC hardness at room temperature is 17.66 or more after the steel material is held at 1010 to 1050° C., cooled to 500 to 550° C. at a cooling rate of 200° C./min or more, subsequently cooled to 150° C. or less at a cooling rate of 100° C./min or more, and then heated in a temperature range of 550 to 700° C. According to this literature, the LMP that gives the maximum HRC hardness at room temperature after heat treatment performed under the above conditions is 17.66 or more to improve softening resistance and thereby improve fatigue properties.

CITATION LIST

Patent Literature

Patent Literature 1: Japanese Patent Application Publication No. 2008-169411

SUMMARY OF INVENTION

Technical Problem

In order to improve the fatigue strength of components for machine structural use, surface hardening treatment may be performed on the components for machine structural use.

One of the various surface hardening treatments is induction hardening. With induction hardening, only the areas that need to be hardened can be hardened. Induction hardening involves heating the steel material at a high temperature and thereafter cooling the steel material to form a hardened layer on the surface thereof. Induction hardening provides a greater depth of a hardened layer and a higher fatigue strength than other surface hardening treatments such as soft nitriding.

The induction hardening treatment applied to a component for machine structural use is described below based on an example in which the component for machine structural use is a crankshaft. When a crankshaft, which is a component for machine structural use, has the shape shown in FIG. 1, induction hardening is applied, for example, to a filleted round portion 1 of the crankshaft to improve the fatigue strength of the crankshaft. In this case, a hardened layer is formed at the surface layer of the filleted round portion 1.

In order to increase the depth of the hardened layer, the output of high frequency power may be increased to raise the heating temperature of induction hardening. However, when induction hardening is performed at a high temperature, the heating temperature tends to become excessively high at edge portions of the component for machine structural use. For example, when the component for machine structural use is the crankshaft shown in FIG. 1, the heating temperature becomes excessively high at edge portions 2. In particular, when the heating rate during induction hardening is high, the heating temperature is likely to become excessively high.

For example, if the heating temperature during induction hardening becomes excessively high and reaches 1350° C. or higher, a portion of the steel material may melt and crack. Such cracks are hereinafter referred to as “melting cracks” in the present description. It is preferable to suppress the occurrence of such melting cracks. That is to say, the steel material is required to have excellent melting cracks resistance.

In addition, hot working is applied to the steel material that will serve as the starting material for the component for machine structural use, during the process for producing the component for machine structural use or during the process for producing the steel material. Therefore, for the steel material, it is desired to suppress the occurrence of cracking caused by hot working (hot working cracks). That is to say, the steel material is required to have excellent hot working cracks resistance.

Furthermore, the steel material that will serve as the starting material for the component for machine structural use is subjected to machining during the process for producing the component for machine structural use. There are cases where excellent machinability is required, especially inside the steel material. For example, when the component for machine structural use is a crankshaft, machining such as drilling is performed on the central portions of both end faces thereof. Therefore, the steel material is required to have excellent machinability inside the steel material.

In the above Patent Literature 1, melting cracks resistance, hot working cracks resistance, and machinability of the steel material are not considered.

An objective of the present disclosure is to provide a steel material that has excellent melting cracks resistance, excellent hot working cracks resistance, and excellent machinability, and that, when served as a starting material for a component for machine structural use, enables the component for machine structural use to have high fatigue strength.

Solution to Problem

A steel material according to the present disclosure is a steel material whose cross section perpendicular to an axial direction thereof is circular, consisting of, by mass %,

• • C: more than 0.30 to 0.60%,
  • Si: 0.01 to 0.90%,
  • Mn: 0.50 to 1.70%,
  • P: 0.030% or less,
  • S: 0.200% or less,
  • Bi: 0.0051 to 0.2500%,
  • Al: 0.001 to 0.100%,
  • N: 0.0250% or less,
  • O: 0.0050% or less,
  • Cr: 0 to 1.30%,
  • V: 0 to 0.200%,
  • Sn: 0 to 0.1000%,
  • Sb: 0 to 0.0500%,
  • As: 0 to 0.0500%,
  • Pb: 0 to 0.09%,
  • Mg: 0 to 0.0100%,
  • Ti: 0 to 0.0400%,
  • Nb: 0 to 0.0500%,
  • W: 0 to 0.4000%,
  • Zr: 0 to 0.2000%,
  • Ca: 0 to 0.0100%,
  • Te: 0 to 0.0100%,
  • B: 0 to 0.0050%,
  • rare earth metal: 0 to 0.0100%,
  • Co: 0 to 0.0100%,
  • Se: 0 to 0.0100%,
  • In: 0 to 0.0100%,
  • Mo: 0 to 0.30%,
  • Cu: 0 to 0.50%,
  • Ni: 0 to 0.50%, and
  • with the balance being Fe and impurities,
  • and Fn defined by formula (1) is 0.45 to 1.05,
  • wherein:
  • at a depth of 0.08R from a surface of the steel material, where R defines a radius of the steel material,
    • a number density of fine Bi particles having an equivalent circular diameter of 0.1 to 1.0 μm, is 15.00 pieces/mm² or more, and
    • a number density of coarse Bi particles having an equivalent circular diameter of 10.0 μm or more, is 0.25 pieces/mm² or less,
  • at a depth of 0.65R from the surface of the steel material,
    • the number density of the fine Bi particles is less than 15.00 pieces/mm², and
    • the number density of the coarse Bi particles is more than 0.25 pieces/mm²;

Fn = C + ( Si / 10 ) + ( Mn / 5 ) - ( 5 ⁢ S / 7 ) + ( 5 ⁢ Cr / 22 ) + 1.65 V , ( 1 )

• • where, a content in percent by mass of a corresponding element is substituted for each symbol of an element in the formula (1), and if an element is not contained, “0” is substituted for the corresponding symbol of an element.

Advantageous Effects of Invention

A steel material of the present disclosure has excellent melting cracks resistance, excellent hot working cracks resistance, and excellent machinability, and when served as a starting material for a component for machine structural use, enables the component for machine structural use to have high fatigue strength.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a front view of a portion of a crankshaft, which is an example of a component for machine structural use.

FIG. 2 is a cross-sectional view taken in perpendicular to an axial direction of a steel material according to an embodiment.

FIG. 3 is a schematic diagram showing a microstructure observed in a melting cracks evaluation test in Examples.

FIG. 4 is a schematic diagram showing a microstructure observed in a melting cracks evaluation test in Examples, different from that shown in FIG. 3.

FIG. 5 is a side view of a fatigue test specimen used in a fatigue strength test in Examples.

DESCRIPTION OF EMBODIMENTS

The present inventors first conducted studies regarding a chemical composition of a steel material that, when served as a starting material for a component for machine structural use, would improve the fatigue strength of the component for machine structural use. As a result, the present inventors concluded that the component for machine structural use may have high fatigue strength when made of a steel material having a chemical composition consisting of, in percent by mass, C: more than 0.30 to 0.60%, Si: 0.01 to 0.90%, Mn: 0.50 to 1.70%, P: 0.030% or less, S: 0.200% or less, Al: 0.001 to 0.100%, N: 0.0250% or less, O: 0.0050% or less, Cr: 0 to 1.30%, V: 0 to 0.200%, Sn: 0 to 0.1000%, Sb: 0 to 0.0500%, As: 0 to 0.0500%, Pb: 0 to 0.09%, Mg: 0 to 0.0100%, Ti: 0 to 0.0400%, Nb: 0 to 0.0500%, W: 0 to 0.4000%, Zr: 0 to 0.2000%, Ca: 0 to 0.0100%, Te: 0 to 0.0100%, B: 0 to 0.0050%, rare earth metal: 0 to 0.0100%, Co: 0 to 0.0100%, Se: 0 to 0.0100%, In: 0 to 0.0100%, Mo: 0 to 0.30%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, and with the balance being Fe and impurities.

Next, the present inventors conducted studies regarding means to improve the machinability of the steel material in which the content of each element in the chemical composition is within the range described above. As a result, the present inventors discovered that excellent fatigue strength of the component for machine structural use can be obtained and the machinability of the steel material, which is a starting material for the component for machine structural use, can be improved by setting Fn defined in formula (1) to 0.45 to 1.05:

Fn = C + ( Si / 10 ) + ( Mn / 5 ) - ( 5 ⁢ S / 7 ) + ( 5 ⁢ Cr / 22 ) + 1.65 V . ( 1 )

Furthermore, the present inventors conducted studies regarding means to improve the melting cracks resistance of the steel material during induction hardening.

The C content affects the occurrence of melting cracks in the steel material during induction hardening. Specifically, the melting point at the grain boundaries is lowered by the C that segregates at the grain boundaries. As a result, melting cracks are more likely to occur. Therefore, the above chemical composition also contains Bi: 0.0051 to 0.2500%. If the Bi content is within the above range, Bi particles (inclusions) are generated in the steel material. The fine Bi particles have a pinning effect, which suppress the coarsening of crystal grains (austenite grains) in the steel material during induction hardening. If the crystal grains can be kept fine during induction hardening, the reduction of the grain boundary area can be suppressed. If the reduction of the grain boundary area can be suppressed and a certain level of grain boundary area can be ensured, the concentration per unit area of the C that segregates at the grain boundaries decreases. As a result, the occurrence of melting cracks is suppressed.

However, when the steel material contains Bi in the above range, not only fine Bi particles but also coarse Bi particles may be generated. Coarse Bi particles serve as the starting points of hot working cracks. Therefore, excessive coarse Bi particles reduce the hot working cracks resistance of the steel material.

On the other hand, the coarse Bi particles improve the machinability of the steel material. Considering the above, it seems difficult to simultaneously obtain excellent melting cracks resistance, excellent hot working cracks resistance, and excellent machinability by Bi in the above range contained in a steel material having the above chemical composition.

However, the present inventors thought that it would be possible to simultaneously obtain excellent melting cracks resistance, excellent hot working cracks resistance, and excellent machinability by varying the number density of fine Bi particles and the number density of coarse Bi particles depending on the regions in the steel material. Specifically, in the steel material, the region where melting cracks and hot working cracks are likely to occur is the surface layer region of the steel material. Therefore, the surface layer region of the steel material needs to have excellent melting cracks resistance and excellent hot working cracks resistance. On the other hand, in the internal region of the steel material, melting cracks and hot working cracks are less likely to occur. Therefore, the internal region of the steel material only needs to have high machinability.

Based on the above technical concept, the present inventors investigated and studied the relationship between the number density of fine Bi particles and the number density of coarse Bi particles in the surface layer regions and internal regions of the steel material; and the melting cracks resistance, hot working cracks resistance, and machinability of the steel material. As a result, the present inventors have discovered that in a steel material satisfying the above chemical composition and having an Fn of 0.45 to 1.05, excellent melting cracks resistance, excellent hot working cracks resistance, and excellent machinability can be simultaneously satisfied when the number density of fine Bi particles is 15.00 pieces/mm² or more and the number density of coarse Bi particles is 0.25 pieces/mm² or less in the surface layer region, and the number density of fine Bi particles is less than 15.00 pieces/mm² and the number density of coarse Bi particles is more than 0.25 pieces/mm² in the internal region, and further, when served as a starting material for a component for machine structural use, such a steel material enables the component for machine structural use to have excellent fatigue strength.

The steel material according to the present embodiment, which has been completed based on the above technical concept, has the following configuration:

[1]

A steel material whose cross section perpendicular to an axial direction thereof is circular, consisting of, by mass %,

• • C: more than 0.30 to 0.60%,
  • Si: 0.01 to 0.90%,
  • Mn: 0.50 to 1.70%,
  • P: 0.030% or less,
  • S: 0.200% or less,
  • Bi: 0.0051 to 0.2500%,
  • Al: 0.001 to 0.100%,
  • N: 0.0250% or less,
  • O: 0.0050% or less,
  • Cr: 0 to 1.30%,
  • V: 0 to 0.200%,
  • Sn: 0 to 0.1000%,
  • Sb: 0 to 0.0500%,
  • As: 0 to 0.0500%,
  • Pb: 0 to 0.09%,
  • Mg: 0 to 0.0100%,
  • Ti: 0 to 0.0400%,
  • Nb: 0 to 0.0500%,
  • W: 0 to 0.4000%,
  • Zr: 0 to 0.2000%,
  • Ca: 0 to 0.0100%,
  • Te: 0 to 0.0100%,
  • B: 0 to 0.0050%,
  • rare earth metal: 0 to 0.0100%,
  • Co: 0 to 0.0100%,
  • Se: 0 to 0.0100%,
  • In: 0 to 0.0100%,
  • Mo: 0 to 0.30%,
  • Cu: 0 to 0.50%,
  • Ni:0 to 0.50%, and
  • with the balance being Fe and impurities,
  • and Fn defined by formula (1) is 0.45 to 1.05,
  • wherein:
  • at a depth of 0.08R from a surface of the steel material, where R defines a radius of the steel material,
    • a number density of fine Bi particles having an equivalent circular diameter of 0.1 to 1.0 μm, is 15.00 pieces/mm² or more, and
    • a number density of coarse Bi particles having an equivalent circular diameter of 10.0 μm or more, is 0.25 pieces/mm² or less,
  • at a depth of 0.65R from the surface of the steel material,
    • the number density of the fine Bi particles is less than 15.00 pieces/mm², and
    • the number density of the coarse Bi particles is more than 0.25 pieces/mm²;

Fn = C + ( Si / 10 ) + ( Mn / 5 ) - ( 5 ⁢ S / 7 ) + ( 5 ⁢ Cr / 22 ) + 1.65 V , ( 1 )

• • where, a content in percent by mass of a corresponding element is substituted for each symbol of an element in the formula (1), and if an element is not contained, “0” is substituted for the corresponding symbol of an element.
    [2]

The steel material according to [1], containing one or more elements selected from a group consisting of:

• • Cr: 0.01 to 1.30%,
  • V: 0.001 to 0.200%,
  • Sn: 0.0001 to 0.1000%,
  • Sb: 0.0001 to 0.0500%,
  • As: 0.0001 to 0.0500%,
  • Pb: 0.01 to 0.09%,
  • Mg: 0.0001 to 0.0100%,
  • Ti: 0.0001 to 0.0400%,
  • Nb: 0.0001 to 0.0500%,
  • W: 0.0001 to 0.4000%,
  • Zr: 0.0001 to 0.2000%,
  • Ca: 0.0001 to 0.0100%,
  • Te: 0.0001 to 0.0100%,
  • B: 0.0001 to 0.0050%,
  • rare earth metal: 0.0001 to 0.0100%,
  • Co: 0.0001 to 0.0100%,
  • Se: 0.0001 to 0.0100%,
  • In: 0.0001 to 0.0100%,
  • Mo: 0.01 to 0.30%,
  • Cu: 0.01 to 0.50%, and
  • Ni: 0.01 to 0.50%.

Hereunder, the steel material of the present embodiment is described in detail. The symbol “%” in relation to an element means “mass percent” unless specifically stated otherwise.

Features of Steel Material of Present Embodiment

The steel material of the present embodiment satisfies the following Feature 1 to Feature 4.

(Feature 1) The chemical composition consists of, in percent by mass, C: more than 0.30 to 0.60%, Si: 0.01 to 0.90%, Mn: 0.50 to 1.70%, P: 0.030% or less, S: 0.200% or less, Bi: 0.0051 to 0.2500%, Al: 0.001 to 0.100%, N: 0.0250% or less, O: 0.0050% or less, Cr: 0 to 1.30%, V: 0 to 0.200%, Sn: 0 to 0.1000%, Sb: 0 to 0.0500%, As: 0 to 0.0500%, Pb: 0 to 0.09%, Mg: 0 to 0.0100%, Ti: 0 to 0.0400%, Nb: 0 to 0.0500%, W: 0 to 0.4000%, Zr: 0 to 0.2000%, Ca: 0 to 0.0100%, Te: 0 to 0.0100%, B: 0 to 0.0050%, rare earth metal: 0 to 0.0100%, Co: 0 to 0.0100%, Se: 0 to 0.0100%, In: 0 to 0.0100%, Mo: 0 to 0.30%, Cu: 0 to 0.50%, Ni:0 to 0.50%, and with the balance being Fe and impurities.

(Feature 2)

Fn defined by formula (1) is 0.45 to 1.05.

(Feature 3)

At the depth of 0.08R from the surface of the steel material, where R defines the radius of the steel material, the number density of fine Bi particles having an equivalent circular diameter of 0.1 to 1.0 μm, is 15.00 pieces/mm² or more, and the number density of coarse Bi particles having an equivalent circular diameter of 10.0 or more, is 0.25 pieces/mm² or less.

(Feature 4)

At the depth of 0.65R from the surface of the steel material, the number density of fine Bi particles is less than 15.00 pieces/mm², and the number density of coarse Bi particles is more than 0.25 pieces/mm².

Hereunder, Feature 1 to Feature 4 are described.

[(Feature 1) Regarding Chemical Composition]

The chemical composition of the steel material of the present embodiment contains the following elements.

C: more than 0.30 to 0.60%

Carbon (C) improves the hardness of the component for machine structural use produced using the steel material, and improves the fatigue strength of the component for machine structural use. If the content of C is 0.30% or less, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.

On the other hand, C lowers the melting point of the steel material. Therefore, if the content of C is more than 0.60%, melting cracks will be likely to occur in the steel material when induction hardening is performed on the steel material during the process for producing the component for machine structural use made of the steel material, even if the contents of other elements are within the range of the present embodiment.

Therefore, the content of C is to be more than 0.30 to 0.60%.

A preferable lower limit of the content of C is 0.31%, more preferably is 0.35%, further preferably is 0.37%, and further preferably is 0.38%.

A preferable upper limit of the content of C is 0.55%, more preferably is 0.50%, and further preferably is 0.45%.

Si: 0.01 to 0.90%

Silicon (Si) deoxidizes the steel in the steelmaking process. Si also improves the hardness of the component for machine structural use produced using the steel material, and improves the fatigue strength of the component for machine structural use. If the content of Si is less than 0.01%, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.

On the other hand, Si has a weak affinity with C. Therefore, if the content of Si is more than 0.90%, C will become more likely to segregate at the grain boundaries during heating than within the grains where Si is dissolved even if the contents of other elements are within the range of the present embodiment. As a result, melting cracks is likely to occur in the steel material when induction hardening is performed on the steel material during the process for producing the component for machine structural use made of the steel material.

Therefore, the content of Si is to be 0.01 to 0.90%.

A preferable lower limit of the content of Si is 0.02%, more preferably is 0.05%, further preferably is 0.08%, and further preferably is 0.10%.

A preferable upper limit of the content of Si is 0.70%, more preferably is 0.65%, further preferably is 0.55%, and further preferably is 0.50%.

Mn: 0.50 to 1.70%

Manganese (Mn) deoxidizes the steel in the steelmaking process. Mn also improves the hardenability of the steel material. Therefore, the hardness of the component for machine structural use produced using the steel material is improved, and the fatigue strength of the component for machine structural use is improved. Furthermore, Mn has a strong affinity with C. Therefore, during heating, C remains within the grains where Mn is dissolved. Therefore, the segregation of C at the grain boundaries is suppressed. As a result, the occurrence of melting cracks is suppressed when induction hardening is performed on the steel material during the process for producing the component for machine structural use made of the steel material. In addition, Mn combines with S to form sulfide of Mn. Therefore, Mn can suppress the formation of coarse FeS. As a result, the hot workability of the steel material during hot working is improved, and hot working cracks resistance is improved. If the content of Mn is less than 0.50%, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.

On the other hand, Mn lowers the melting point of the steel material. Therefore, if the content of Mn is more than 1.70%, melting cracks resistance will decrease when induction hardening is performed on the steel material during the process for producing the component for machine structural use made of the steel material, even if the contents of other elements are within the range of the present embodiment. In addition, if the content of Mn is more than 1.70%, the hardness of the steel material will be excessively increased even if the contents of the other elements are within the ranges of the present embodiment. In this case, the machinability of the steel material cannot be sufficiently achieved.

Therefore, the content of Mn is to be 0.50 to 1.70%.

A preferable lower limit of the content of Mn is 0.70%, more preferably is 0.80%, further preferably is 0.85%, and further preferably is 0.90%.

A preferable upper limit of the content of Mn is 1.65%, more preferably is 1.60%, further preferably is 1.55%, further preferably is 1.50%, further preferably is 1.48%, further preferably is 1.45%, further preferably is 1.43%, and further preferably is 1.40%.

P: 0.030% or less

Phosphorus (P) is an impurity. P segregates at the grain boundaries and lowers the melting point of the steel material. Therefore, melting cracks is likely to occur in the steel material when induction hardening is performed on the steel material during the process for producing the component for machine structural use made of the steel material.

Therefore, the content of P is to be 0.030% or less.

The content of P is preferably as low as possible. However, excessive reduction of the content of P increases production costs. Therefore, taking into account ordinary industrial production, a preferable lower limit of the content of P is more than 0%, more preferably is 0.001%, and further preferably is 0.002%. A preferable upper limit of the content of P is 0.028%, more preferably is 0.026%, further preferably is 0.023%, and further preferably is 0.020%.

S: 0.200% or less

Sulfur (S) forms sulfide and improves the machinability of the steel material. Even if a small amount of S is contained, the aforementioned advantageous effect will be obtained to a certain extent even if the contents of the other elements are within the ranges of the present embodiment.

However, S lowers the melting point of the steel material. Therefore, if the content of S is more than 0.200%, melting cracks will become more likely to occur in the steel material when induction hardening is performed on the steel material during the process for producing the component for machine structural use made of the steel material, even if the contents of the other elements are within the ranges of the present embodiment.

Therefore, the content of S is to be 0.200% or less.

A preferable lower limit of the content of S is more than 0%, more preferably is 0.001%, further preferably is 0.005%, further preferably is 0.010%, further preferably is 0.015%, and further preferably is 0.020%.

A preferable upper limit of the content of S is 0.150%, more preferably is 0.120%, further preferably is 0.095%, further preferably is 0.080%, further preferably is 0.075%, further preferably is 0.055%, and further preferably is 0.035%.

Bi: 0.0051 to 0.2500%

Bismuth (Bi) forms particles in the steel material and has a pinning effect, which suppresses the coarsening of crystal grains (austenite grains) in the steel material during heating in induction hardening. If the crystal grains can be kept fine, the reduction of the grain boundary area can be suppressed. Therefore, the C concentration per unit grain boundary area is reduced, and melting cracks during induction hardening is suppressed. Bi also improves the machinability of the steel material. If the content of Bi is less than 0.0051%, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.

On the other hand, if the content of Bi is more than 0.2500%, coarse Bi particles will be generated excessively in the surface layer region of the steel material even if the contents of the other elements are within the ranges of the present embodiment. The coarse Bi particles in the surface layer region are likely to serve as the starting points of cracks during hot working in the process for producing the steel material or during hot working in the process for producing the component for machine structural use. Therefore, the hot working cracks resistance of the steel material is reduced.

Therefore, the content of Bi is to be 0.0051 to 0.2500%.

A preferable lower limit of the content of Bi is 0.0060%, more preferably is 0.0070%, further preferably is 0.0100%, further preferably is 0.0150%, and further preferably is 0.0200%.

A preferable upper limit of the content of Bi is 0.2000%, more preferably is 0.1500%, further preferably is 0.1250%, further preferably is 0.1000%, further preferably is 0.0900%, further preferably is 0.0750%, and further preferably is 0.0600%.

Al: 0.001 to 0.100%

Aluminum (Al) deoxidizes the steel. If the content of Al is less than 0.001%, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.

On the other hand, if the content of Al is more than 0.100%, Al will form coarse oxide even if the contents of the other elements are within the ranges of the present embodiment. Coarse oxide reduces the fatigue strength of the component for machine structural use made of the steel material.

Therefore, the content of Al is to be 0.001 to 0.100%.

A preferable lower limit of the content of Al is 0.002%, more preferably is 0.003%, further preferably is 0.005%, and further preferably is 0.010%.

A preferable upper limit of the content of Al is 0.060%, more preferably is 0.050%, further preferably is 0.040%, further preferably is 0.030%, and further preferably is 0.025%.

N: 0.0250% or less

Nitrogen (N) forms nitride and/or carbonitride during the process for producing the component for machine structural use made of the steel material, thereby precipitation strengthening the steel material. As a result, the fatigue strength of the component for machine structural use made of the steel material is improved. Even if a small amount of N is contained, the aforementioned advantageous effect will be obtained to a certain extent even if the contents of the other elements are within the ranges of the present embodiment.

On the other hand, if the content of N is more than 0.0250%, the hot workability of the steel material will decrease even if the contents of the other elements are within the ranges of the present embodiment.

Therefore, the content of N is to be 0.0250% or less.

A preferable lower limit of the content of N is more than 0%, more preferably is 0.0001%, further preferably is 0.0005%, further preferably is 0.0010%, further preferably is 0.0020%, further preferably is 0.0030%, and further preferably is 0.0040%.

A preferable upper limit of the content of N is 0.0200%, more preferably is 0.0190%, further preferably is 0.0170%, further preferably is 0.0150%, further preferably is 0.0130%, and further preferably is 0.0100%.

O: 0.0050% or less

Oxygen (O) is an impurity. 0 forms oxide in the steel, and reduces the fatigue strength of the component for machine structural use made of the steel material.

Therefore, the content of O is to be 0.0050% or less.

The content of O is preferably as low as possible. However, excessive reduction of the content of O increases production costs. Therefore, taking into account ordinary industrial production, a preferable lower limit of the content of O is more than 0%, more preferably 0.0001%, and further preferably 0.0002%. A preferable upper limit of the content of O is 0.0030%, more preferably is 0.0025%, further preferably is 0.0020%, further preferably is 0.0015%, and further preferably is 0.0012%.

The balance of the chemical composition of the steel material of the present embodiment is Fe and impurities. Here, the term “impurities” refers to elements which are mixed in from ore and scrap as the raw material or from the production environment or the like when industrially producing the steel material, and which are not intentionally contained but are permitted within a range not adversely affecting the steel material of the present embodiment.

[Optional Elements]

The chemical composition of the steel material of the present embodiment may further contain, in lieu of a part of Fe, one or more kinds of element selected from a group consisting of:

• • Cr: 0 to 1.30%,
  • V: 0 to 0.200%,
  • Sn: 0 to 0.1000%,
  • Sb: 0 to 0.0500%,
  • As: 0 to 0.0500%,
  • Pb: 0 to 0.09%,
  • Mg: 0 to 0.0100%,
  • Ti: 0 to 0.0400%,
  • Nb: 0 to 0.0500%,
  • W: 0 to 0.4000%,
  • Zr: 0 to 0.2000%,
  • Ca: 0 to 0.0100%,
  • Te: 0 to 0.0100%,
  • B: 0 to 0.0050%,
  • rare earth metal: 0 to 0.0100%,
  • Co: 0 to 0.0100%,
  • Se: 0 to 0.0100%,
  • In: 0 to 0.0100%,
  • Mo: 0 to 0.30%,
  • Cu: 0 to 0.50%, and
  • Ni:0 to 0.50%.

Hereunder, these optional elements are described.

[First Group: Cr and V]

The chemical composition of the steel material of the present embodiment may further contain the following elements of the first group in lieu of a part of Fe. These elements are optional elements and each of them improve the fatigue strength of the component for machine structural use.

[First Group]

One or more kinds of elements selected from a group consisting of:

• • Cr: 0 to 1.30%, and
  • V: 0 to 0.200%.
  • Cr: 0 to 1.30%

Chromium (Cr) is an optional element, and does not have to be contained. That is, the content of Cr may be 0%.

When contained, that is, when the content of Cr is more than 0%, Cr improves the hardenability of the steel material. Therefore, the hardness of the component for machine structural use made of the steel material is improved, and the fatigue strength of the component for machine structural use is improved. If even a small amount of Cr is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of Cr is more than 1.30%, sufficient machinability of the steel material will not be achieved even if the contents of the other elements are within the ranges of the present embodiment.

Therefore, the content of Cr is to be 0 to 1.30%, and if contained, the content of Cr is to be 1.30% or less.

A preferable lower limit of the content of Cr is 0.01%, more preferably is 0.05%, further preferably is 0.10%, further preferably is 0.12%, further preferably is 0.14%, more preferably is 0.16%, and more preferably is 0.18%.

A preferable upper limit of the content of Cr is 1.25%, more preferably is 1.20%, further preferably is 1.10%, further preferably is 1.00%, and further preferably is 0.90%.

V: 0 to 0.200%

Vanadium (V) is an optional element, and does not have to be contained. That is, the content of V may be 0%.

When contained, that is, when the content of V is more than 0%, V is precipitated in the ferrite of the steel material as a V precipitate during the process for producing the component for machine structural use made of the steel material. This improves the hardness of the ferrite in the steel material. As a result, the fatigue strength of the component for machine structural use is improved. If even a small amount of V is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of V is more than 0.200%, the above effects will be saturated and the production costs will be even higher.

Therefore, the content of V is to be 0 to 0.200%, and if contained, the content of V is to be 0.200% or less.

A preferable lower limit of the content of V is 0.0010%, more preferably is 0.003%, further preferably is 0.005%, further preferably is 0.007%, further preferably is 0.010%, further preferably is 0.015%, and further preferably is 0.020%.

A preferable upper limit of the content of V is 0.180%, more preferably is 0.160%, further preferably is 0.140%, further preferably is 0.120%, further preferably is 0.100%, further preferably is 0.080%, further preferably is 0.050%, and further preferably is 0.049%.

[Second Group: Sn, Sb, as, and Pb]

The chemical composition of the steel material of the present embodiment may further contain the following elements of the second group in lieu of a part of Fe. These elements are optional elements and each of them improve the machinability of the steel material.

[Second Group]

One or more kinds of elements selected from a group consisting of:

• • Sn: 0 to 0.1000%,
  • Sb: 0 to 0.0500%,
  • As: 0 to 0.0500%, and
  • Pb: 0 to 0.09%.

Sn: 0 to 0.1000%

Tin (Sn) is an optional element, and does not have to be contained. That is, the content of Sn may be 0%.

When contained, that is, when the content of Sn is more than 0%, Sn segregates at the interface between the matrix and the inclusions and embrittles the steel material. This improves the machinability of the steel material. If even a small amount of Sn is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of Sn is more than 0.1000%, Sn will excessively segregate even if the contents of the other elements are within the ranges of the present embodiment. In this case, the hot workability of the steel material is reduced.

Therefore, the content of Sn is to be 0 to 0.1000%, and if contained, the content of Sn is to be 0.1000% or less.

A preferable lower limit of the content of Sn is 0.0001%, more preferably is 0.0010%, and further preferably is 0.0020%.

A preferable upper limit of the content of Sn is 0.0500%, more preferably is 0.0100%, further preferably is 0.0090%, further preferably is 0.0080%, further preferably is 0.0070%, further preferably is 0.0060%, further preferably is 0.0050%, and further preferably is 0.0040%.

Sb: 0 to 0.0500%

Antimony (Sb) is an optional element, and does not have to be contained. That is, the content of Sb may be 0%.

When contained, that is, when the content of Sb is more than 0%, Sb segregates at the interface between the matrix and the inclusions and embrittles the steel material. This improves the machinability of the steel material. If even a small amount of Sb is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of Sb is more than 0.0500%, Sb will excessively segregate even if the contents of the other elements are within the ranges of the present embodiment. In this case, the hot workability of the steel material is reduced.

Therefore, the content of Sb is to be 0 to 0.0500%, and if contained, the content of Sb is to be 0.0500% or less.

A preferable lower limit of the content of Sb is 0.0001%, more preferably is 0.0010%, and further preferably is 0.0020%.

A preferable upper limit of the content of Sb is 0.0400%, more preferably is 0.0300%, further preferably is 0.0200%, further preferably is 0.0100%, further preferably is 0.0080%, and further preferably is 0.0060%.

As: 0 to 0.0500%

Arsenic (As) is an optional element, and does not have to be contained. That is, the content of As may be 0%.

When contained, that is, when the content of As is more than 0%, As segregates at the interface between the matrix and the inclusions and embrittles the steel material. This improves the machinability of the steel material. If even a small amount of As is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of As is more than 0.0500%, As will excessively segregate even if the contents of the other elements are within the ranges of the present embodiment. In this case, the hot workability of the steel material is reduced.

Therefore, the content of As is to be 0 to 0.0500%, and if contained, the content of As is to be 0.0500% or less.

A preferable lower limit of the content of As is 0.0001%, more preferably is 0.0010%, and further preferably is 0.0020%.

A preferable upper limit of the content of As is 0.0100%, more preferably is 0.0070%, further preferably is 0.0060%, further preferably is 0.0050%, and further preferably is 0.0040%.

Pb: 0 to 0.09%

Lead (Pb) is an optional element, and does not have to be contained. That is, the content of Pb may be 0%.

When contained, that is, when the content of Pb is more than 0%, Pb generates Pb particles in the matrix and embrittles the steel material. This improves the machinability of the steel material. If even a small amount of Pb is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of Pb is more than 0.09%, Pb particles will be excessively generated even if the contents of the other elements are within the ranges of the present embodiment. In this case, the hot workability of the steel material is reduced.

Therefore, the content of Pb is to be 0 to 0.09%, and if contained, the content of Pb is to be 0.09% or less.

A preferable lower limit of the content of Pb is 0.01%, more preferably is 0.02%, and further preferably is 0.03%.

A preferable upper limit of the content of Pb is 0.08%, more preferably is 0.07%, further preferably is 0.06%, and further preferably is 0.05%.

[Third Group: Mg]

The chemical composition of the steel material of the present embodiment may further contain the following elements of the third group in lieu of a part of Fe.

[Third Group]

Mg: 0 to 0.0100%

Magnesium (Mg) is an optional element, and does not have to be contained. That is, the content of Mg may be 0%.

When contained, that is, when the content of Mg is more than 0%, Mg deoxidizes the steel. If even a small amount of Mg is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of Mg is more than 0.0100%, Mg will form coarse oxide even if the contents of the other elements are within the ranges of the present embodiment. Coarse oxide reduces the fatigue strength of the component for machine structural use made of the steel material.

Therefore, the content of Mg is to be 0 to 0.0100%, and if contained, the content of Mg is to be 0.0100% or less.

A preferable lower limit of the content of Mg is 0.0001%, more preferably is 0.0003%, and further preferably is 0.0005%.

A preferable upper limit of the content of Mg is 0.0050%, more preferably is 0.0045%, and further preferably is 0.0040%.

[Fourth Group: Ti, Nb, W, and Zr]

The chemical composition of the steel material of the present embodiment may further contain the following elements of the fourth group in lieu of a part of Fe. These elements are optional elements and each of them form precipitates and have a pinning effect, which refines the crystal grains in the steel material, thereby improving the toughness of the component for machine structural use made of the steel material.

[Fourth Group]

One or more kinds of elements selected from a group consisting of

• • Ti: 0 to 0.0400%,
  • Nb: 0 to 0.0500%,
  • W: 0 to 0.4000%, and
  • Zr: 0 to 0.2000%.

Ti: 0 to 0.0400%

Titanium (Ti) is an optional element, and does not have to be contained. That is, the content of Ti may be 0%.

When contained, that is, when the content of Ti is more than 0%, Ti forms precipitates (carbide and/or carbonitride). These precipitates have a pinning effect, which refines the crystal grains in the steel material. This improves the toughness of the component for machine structural use. If even a small amount of Ti is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of Ti is more than 0.0400%, the above effects will be saturated and the production costs will be high.

Therefore, the content of Ti is to be 0 to 0.0400%, and if contained, the content of Ti is to be 0.0400% or less.

A preferable lower limit of the content of Ti is 0.0001%, more preferably is 0.0010%, further preferably is 0.0050%, and further preferably is 0.0080%.

A preferable upper limit of the content of Ti is 0.0300%, more preferably is 0.0200%, further preferably is 0.0175%, and further preferably is 0.0150%.

Nb: 0 to 0.0500%

Niobium (Nb) is an optional element, and does not have to be contained. That is, the content of Nb may be 0%.

When contained, that is, when the content of Nb is more than 0%, Nb, like Ti, forms a precipitate and refines the crystal grains in the steel material, thereby improving the toughness of the component for machine structural use. If even a small amount of Nb is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of Nb is more than 0.0500%, the above effects will be saturated and the production costs will be high.

Therefore, the content of Nb is to be 0 to 0.0500%, and if contained, the content of Nb is to be 0.0500% or less.

A preferable lower limit of the content of Nb is 0.00010%, more preferably is 0.0010%, further preferably is 0.0050%, and further preferably is 0.0080%.

A preferable upper limit of the content of Nb is 0.0200%, more preferably is 0.0175%, and further preferably is 0.0150%.

W: 0 to 0.4000%

Tungsten (W) is an optional element, and does not have to be contained. That is, the content of W may be 0%.

When contained, that is, when the content of W is more than 0%, W, like Ti, forms a precipitate and refines the crystal grains in the steel material, thereby improving the toughness of the component for machine structural use. If even a small amount of W is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of W is more than 0.4000%, the above effects will be saturated and the production costs will be high.

Therefore, the content of W is to be 0 to 0.4000%, and if contained, the content of W is to be 0.4000% or less.

A preferable lower limit of the content of W is 0.0001%, more preferably is 0.0050%, and further preferably is 0.0500%.

A preferable upper limit of the content of W is 0.3500%, more preferably is 0.3000%, and further preferably is 0.2000%.

Zr: 0 to 0.2000%

Zirconium (Zr) is an optional element, and does not have to be contained. That is, the content of Zr may be 0%.

When contained, that is, when the content of Zr is more than 0%, Zr, like Ti, forms a precipitate and refines the crystal grains in the steel material, thereby improving the toughness of the component for machine structural use. If even a small amount of Zr is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of Zr is more than 0.2000%, the above effects will be saturated and the production costs will be high.

Therefore, the content of Zr is to be 0 to 0.2000%, and if contained, the content of Zr is to be 0.2000% or less.

A preferable lower limit of the content of Zr is 0.00010%, more preferably is 0.0010%, further preferably is 0.0020%, and further preferably is 0.0050%.

A preferable upper limit of the content of Zr is 0.1500%, more preferably is 0.1000%, further preferably is 0.0500%, and further preferably is 0.0100%.

[Fifth Group: Ca, Te, B, and Rare Earth Metal (REM)]

The chemical composition of the steel material of the present embodiment may further contain the following elements of the fifth group in lieu of a part of Fe. These elements are optional elements and each of them improve the machinability of the steel material.

[Fifth Group]

One or more kinds of elements selected from a group consisting of:

• • Ca: 0 to 0.0100%,
  • Te: 0 to 0.0100%,
  • B: 0% to 0.0050%, and
  • rare earth metal: 0% to 0.0100%.

Ca: 0 to 0.0100%

Calcium (Ca) is an optional element, and does not have to be contained. That is, the content of Ca may be 0%.

When contained, that is, when the content of Ca is more than 0%, Ca improves the machinability of the steel material. If even a small amount of Ca is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of Ca is more than 0.0100%, Ca will form coarse oxide even if the contents of the other elements are within the ranges of the present embodiment. Coarse oxide reduces the fatigue strength of the component for machine structural use made of the steel material.

Therefore, the content of Ca is to be 0 to 0.0100%, and if contained, the content of Ca is to be 0.0100% or less.

A preferable lower limit of the content of Ca is 0.0001%, more preferably is 0.0005%, further preferably is 0.0010%, and further preferably is 0.0015%.

A preferable upper limit of the content of Ca is 0.0085%, more preferably is 0.0070%, further preferably is 0.0050%, and further preferably is 0.0030%.

Te: 0 to 0.0100%

Tellurium (Te) is an optional element, and does not have to be contained. That is, the content of Te may be 0%.

When contained, that is, when the content of Te is more than 0%, Te improves the machinability of the steel material. If even a small amount of Te is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of Te is more than 0.0100%, the hot workability of the steel material will decrease even if the contents of the other elements are within the ranges of the present embodiment.

Therefore, the content of Te is to be 0 to 0.0100%, and if contained, the content of Te is to be 0.0100% or less.

A preferable lower limit of the content of Te is 0.0001%, more preferably is 0.0003%, and further preferably is 0.0010%.

A preferable upper limit of the content of Te is 0.0090%, more preferably is 0.0085%, further preferably is 0.0080%, and further preferably is 0.0040%.

B: 0 to 0.0050%

Boron (B) is an optional element, and does not have to be contained. That is, the content of B may be 0%.

When contained, that is, when the content of B is more than 0%, B combines with N to form BN, which improves the machinability of the steel material. B also segregates at the grain boundaries and contributes to grain boundary strengthening, thereby improving the fatigue strength of the component for machine structural use made of the steel material. If even a small amount of B is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of B is more than 0.0050%, the hot workability of the steel material will decrease even if the contents of the other elements are within the ranges of the present embodiment.

Therefore, the content of B is to be 0 to 0.0050%, and if contained, the content of B is to be 0.0050% or less.

A preferable lower limit of the content of B is 0.00010%, more preferably is 0.0005%, and further preferably is 0.0010%.

A preferable upper limit of the content of B is 0.0040%, more preferably is 0.0035%, and further preferably is 0.0030%.

Rare Earth Metal (REM): 0 to 0.0100%

Rare earth metal (REM) is an optional element, and does not have to be contained. That is, the content of REM may be 0%.

When contained, that is, when the content of REM is more than 0%, REM improves the machinability of the steel material. If even a small amount of REM is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of REM is more than 0.0100%, the hot workability of the steel material will decrease even if the contents of the other elements are within the ranges of the present embodiment.

Therefore, the content of REM is to be 0 to 0.0100%, and if contained, the content of REM is to be 0.0100% or less.

A preferable lower limit of the content of REM is 0.00010%, more preferably is 0.0005%, and further preferably is 0.0010%.

A preferable upper limit of the content of REM is 0.0090%, more preferably is 0.0080%, further preferably is 0.0070%, and further preferably is 0.0040%.

The term “REM” as used in the present description means one or more kinds of element selected from the group consisting of scandium (Sc) which is the element with atomic number 21, yttrium (Y) which is the element with atomic number 39, and the elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71 that are lanthanoids. Further, in the present description the term “content of REM” refers to the total content of these elements.

[Sixth Group: Co, Se, and In]

The chemical composition of the steel material of the present embodiment may further contain the following elements of the sixth group in lieu of a part of Fe. These elements are optional elements and each of them suppress decarburization of the steel material.

[Sixth Group]

One or more kinds of elements selected from a group consisting of:

• • Co: 0 to 0.0100%,
  • Se: 0 to 0.0100%, and
  • In: 0 to 0.0100%.

Co: 0 to 0.0100%

Cobalt (Co) is an optional element, and does not have to be contained. That is, the content of Co may be 0%.

When contained, that is, when the content of Co is more than 0%, Co suppresses decarburization of the steel material during the process for producing. If even a small amount of Co is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of Co is more than 0.0100%, the hot workability of the steel material will decrease even if the contents of the other elements are within the ranges of the present embodiment.

Therefore, the content of Co is to be 0 to 0.0100%, and if contained, the content of Co is to be 0.0100% or less.

A preferable lower limit of the content of Co is 0.0001%, more preferably is 0.0005%, further preferably is 0.0010%, and further preferably is 0.0030%. A preferable upper limit of the content of Co is 0.0090%, more preferably is 0.0080%, and further preferably is 0.0070%.

Se: 0 to 0.0100%

Selenium (Se) is an optional element, and does not have to be contained. That is, the content of Se may be 0%.

When contained, that is, when the content of Se is more than 0%, Se suppresses decarburization of the steel material during the process for producing. If even a small amount of Se is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of Se is more than 0.0100%, Se will cause hot working cracks even if the contents of the other elements are within the ranges of the present embodiment.

Therefore, the content of Se is to be 0 to 0.0100%, and if contained, the content of Se is to be 0.0100% or less.

A preferable lower limit of the content of Se is 0.0001%, more preferably is 0.0010%, and further preferably is 0.0020%.

A preferable upper limit of the content of Se is 0.0090%, more preferably is 0.0080%, and further preferably is 0.0070%.

In: 0 to 0.0100%

Indium (In) is an optional element, and does not have to be contained. That is, the content of In may be 0%.

When contained, that is, when the content of In is more than 0%, In suppresses decarburization of the steel material during the process for producing. If even a small amount of In is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of In is more than 0.0100%, the hot workability of the steel material will decrease even if the contents of the other elements are within the ranges of the present embodiment.

Therefore, the content of In is to be 0 to 0.0100%, and if contained, the content of In is to be 0.0100% or less.

A preferable lower limit of the content of In is 0.0001%, more preferably is 0.0005%, and further preferably is 0.0010%.

A preferable upper limit of the content of In is 0.0090%, more preferably is 0.0080%, and further preferably is 0.0070%.

[Seventh Group: Mo, Cu, and Ni]

The chemical composition of the steel material of the present embodiment may further contain the following elements of the seventh group in lieu of a part of Fe. These elements are optional elements and each of them improve the fatigue strength of the component for machine structural use made of the steel material.

[Seventh Group]

One or more kinds of elements selected from a group consisting of:

• • Mo: 0 to 0.30%,
  • Cu: 0 to 0.50%, and
  • Ni:0 to 0.50%.

Mo: 0 to 0.30%

Molybdenum (Mo) is an optional element, and does not have to be contained. That is, the content of Mo may be 0%.

When contained, that is, when the content of Mo is more than 0%, Mo improves the fatigue strength of the component for machine structural use made of the steel material. If even a small amount of Mo is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of Mo is more than 0.30%, the hardness of the steel material will be excessively high and the hot workability of the steel material will decrease even if the contents of the other elements are within the ranges of the present embodiment.

Therefore, the content of Mo is to be 0 to 0.30%, and if contained, the content of Mo is to be 0.30% or less.

A preferable lower limit of the content of Mo is 0.01%, more preferably is 0.05%, and further preferably is 0.10%.

A preferable upper limit of the content of Mo is 0.20%, more preferably is 0.17%, and further preferably is 0.15%.

Cu: 0 to 0.50%

Copper (Cu) is an optional element, and does not have to be contained. That is, the content of Cu may be 0%.

When contained, that is, when the content of Cu is more than 0%, Cu improves the fatigue strength of the component for machine structural use made of the steel material. If even a small amount of Cu is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of Cu is more than 0.50%, melting cracks will be likely to occur in the steel material during induction hardening even if the contents of the other elements are within the ranges of the present embodiment.

Therefore, the content of Cu is to be 0 to 0.50%, and if contained, the content of Cu is to be 0.50% or less.

A preferable lower limit of the content of Cu is 0.01%, and more preferably is 0.02%.

A preferable upper limit of the content of Cu is 0.20%, more preferably is 0.10%, and further preferably is 0.05%.

Ni: 0 to 0.50%

Nickel (Ni) is an optional element, and does not have to be contained. That is, the content of Ni may be 0%.

When contained, that is, when the content of Ni is more than 0%, Ni improves the fatigue strength of the component for machine structural use made of the steel material. If even a small amount of Ni is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of Ni is more than 0.50%, melting cracks will be likely to occur in the steel material during induction hardening even if the contents of the other elements are within the ranges of the present embodiment.

Therefore, the content of Ni is to be 0 to 0.50%, and if contained, the content of Ni is to be 0.50% or less.

A preferable lower limit of the content of Ni is 0.01%, and more preferably is 0.02%.

A preferable upper limit of the content of Ni is 0.20%, more preferably is 0.10%, and further preferably is 0.05%.

[(Feature 2) Regarding Fn]

In the steel material of the present embodiment, Fn defined by formula (1) is 0.45 to 1.05:

Fn = C + ( Si / 10 ) + ( Mn / 5 ) - ( 5 ⁢ S / 7 ) + ( 5 ⁢ Cr / 22 ) + 1.65 V , ( 1 )

• • where, a content in percent by mass of a corresponding element is substituted for each symbol of an element in the formula (1), and if an element is not contained, “0” is substituted for the corresponding symbol of an element.

Fn is an index of the fatigue strength of the component for machine structural use made of the steel material and the machinability of the steel material. If Fn is less than 0.45, even if the steel material satisfies Features 1, 3, and 4, the component for machine structural use made of the steel material will not have sufficient fatigue strength. On the other hand, if Fn is more than 1.05, even if the steel material satisfies Features 1, 3, and 4, the machinability of the steel material will decrease.

Therefore, Fn is to be 0.45 to 1.05.

A preferable lower limit of Fn is 0.50, more preferably is 0.55, and further preferably is 0.60.

A preferable upper limit Fn is preferably 1.00, more preferably is 0.95, and further preferably is 0.90.

[(Feature 3) Number Density of Bi Particles at 0.08R Depth Position D_0.08R]

FIG. 2 is a cross-sectional view taken in a direction perpendicular to the axial direction of the steel material of the present embodiment. As shown in FIG. 2, a cross section 10 taken in perpendicular to the axial direction of the steel material is circular. The radius of the cross section 10 is defined as R. The position at a depth of 0.08R measured from a surface D_0.00R of the steel material is defined as a “0.08R depth position D_0.08R”.

In the steel material of the present embodiment, at 0.08R depth position D_0.08R, the number density of fine Bi particles, which are Bi particles having an equivalent circular diameter of 0.1 to 1.0 μm, is 15.00 pieces/mm² or more, and the number density of coarse Bi particles, which are Bi particles having an equivalent circular diameter of 10.0 μm or more, is 0.25 pieces/mm² or less.

As shown in FIG. 2, the region ranging from the surface D_0.00R of the steel material to the 0.08R depth position D_0.08R is defined as a surface layer region SA. The surface layer region SA of the steel material is the region near the surface of the steel material that comes into contact with processing tools such as dies or rolls during the hot working process in the process for producing the steel material, or during the hot working process in the process for producing the component for machine structural use made of the steel material. Therefore, the surface layer region SA is required to have sufficient hot working cracks resistance. In addition, the surface layer region SA is a region whose temperature becomes high due to high-frequency induction heating when induction hardening is performed in the process for producing the component for machine structural use made of the steel material. Therefore, the surface layer region SA is required to have sufficient melting cracks resistance. Therefore, the surface layer region SA of the steel material is required to have sufficient hot working cracks resistance and sufficient melting cracks resistance.

As described above, during heating in induction hardening, fine Bi particles, which are Bi particles having an equivalent circular diameter of 0.1 to 1.0 μm, have a pinning effect, which suppresses the coarsening of crystal grains in the surface layer region SA. By increasing the number density of fine Bi particles in the surface layer region SA, the crystal grains can be kept in a fine state by the pinning effect. Therefore, the reduction of the grain boundary area is suppressed, and the occurrence of melting cracks is suppressed. If the number density of fine Bi particles is 15.00 pieces/mm² or more at the 0.08R depth position D_0.08R in the surface layer region SA, the crystal grains can be kept fine during the heating in induction hardening. As a result, the steel material can have sufficient melting cracks resistance.

In addition, the surface layer region SA is subjected to a large external force from hot working processing tools during hot working. During hot working, in the surface layer region SA where the steel material receives a large external force, coarse Bi particles, which are Bi particles having an equivalent circular diameter of 10.0 μm or more serve as the starting points of hot working cracks. Therefore, in the surface layer region SA of the steel material, it is preferable that the number density of coarse Bi particles is as low as possible. If the number density of coarse Bi particles is 0.25 pieces/mm² or less at the 0.08R depth position D_0.08R in the surface layer region SA, the number density of coarse Bi particles in the surface layer region SA is sufficiently low. Therefore, the steel material can have sufficient hot working cracks resistance.

Therefore, 0.08R depth position D_0.08R, the number density of fine Bi particles is 15.00 pieces/mm² or more and the number density of coarse Bi particles is 0.25 pieces/mm² or less.

A preferable lower limit of the number density of fine Bi particles at the 0.08R depth position D_0.08R is 20.00 pieces/mm², more preferably is 25.00 pieces/mm², and further preferably is 30.00 pieces/mm².

An upper limit of the number density of fine Bi particles at the 0.08R depth position D_0.08R is not particularly limited. When the steel material satisfies the Features 1 and 2, an upper limit of the number density of fine Bi particles at the 0.08R depth position D_0.08R is, for example, 1000.00 pieces/mm².

A preferable upper limit of the number density of coarse Bi particles at the 0.08R depth position D_0.08R is 0.20 pieces/mm², more preferably is 0.16 pieces/mm², further preferably is 0.12 pieces/mm², and further preferably is 0.08 pieces/mm².

It is preferable that the number density of coarse Bi particles at the 0.08R depth position D_0.08R is as low as possible. A preferable lower limit of the number density of coarse Bi particles at the 0.08R depth position D_0.08R is 0.03 pieces/mm², and more preferably is 0.00 pieces/mm².

In addition to the above-mentioned fine Bi particles and coarse Bi particles, the surface layer region SA also contains Bi particles having an equivalent circular diameter of more than 1.0 and less than 10.0 μm. However, in the surface layer region SA, the number density of fine Bi particles and coarse Bi particles is more strongly correlated with hot working cracks and melting cracks during induction hardening than the number density of Bi particles with an equivalent circular diameter of more than 1.0 and less than 10.0 μm. Therefore, in the present embodiment, the number densities of fine Bi particles and coarse Bi particles at the 0.08R depth position D_0.08R in the surface layer region SA are used as indices of hot working cracks and melting cracks during induction hardening of the steel material.

[(Feature 4) Number Density of Bi Particles at 0.65R Depth Position D_0.65R]

As shown in FIG. 2, the position at a depth of 0.65R measured from the surface D_0.00R of the steel material is defined as a “0.65R depth position D_0.65R”. The position at a depth of 1.00R measured from the surface D_0.00R of the steel material is defined as a “1.00R depth position D_1.00R”.

In the steel material of the present embodiment, at the 0.65R depth position D_0.65R, the number density of fine Bi particles, which are Bi particles having an equivalent circular diameter of 0.1 to 1.0 μm, is less than 15.00 pieces/mm², and the number density of coarse Bi particles, which are Bi particles having an equivalent circular diameter of 10.0 μm or more, is more than 0.25 pieces/mm².

As shown in FIG. 2, the area ranging from the 0.65R depth position D_0.65R to the 1.00R depth position D_1.00R is defined as an internal region CA. The internal region CA of the steel material is less exposed to external forces during hot working and to the heat during induction hardening than the surface layer region SA. Therefore, the internal region CA is not required to have higher hot working cracks resistance or melting cracks resistance than the surface layer region SA. On the other hand, the internal region CA may be subjected to machining during the process for producing the component for machine structural use made of the steel material. In this case, the internal region CA is required to have sufficient machinability.

During machining, coarse Bi particles serve as a starting point for peeling chips from the body of the steel material. Therefore, coarse Bi particles improve the machinability of the steel. Therefore, it is preferable that the number density of coarse Bi particles in the internal region CA is as high as possible. If the number density of coarse Bi particles is more than 0.25 pieces/mm² at the 0.65R depth position D_0.65R in the internal region CA, sufficient machinability can be achieved in the steel material.

On the other hand, fine Bi particles have little effect on the machinability of the steel. Furthermore, when the number density of fine Bi particles is high, the number density of coarse Bi particles is correspondingly low. Therefore, it is preferable that the number density of fine Bi particles in the internal region CA is as low as possible. If the number density of fine Bi particles is less than 15.00 pieces/mm² at the 0.65R depth position D_0.65R in the internal region CA, coarse Bi particles are likely to be generated in the internal region CA. As a result, the steel material can have sufficient machinability.

Therefore, at the 0.65R depth position D_0.65R, the number density of fine Bi particles is less than 15.00 pieces/mm² and the number density of coarse Bi particles is more than 0.25 pieces/mm².

A preferable upper limit of the number density of fine Bi particles at the 0.65R depth position D_0.65R is 14.00 pieces/mm², more preferably is 10.00 pieces/mm², and further preferably is 7.50 pieces/mm².

It is preferable that the number density of fine Bi particles at the 0.65R depth position D_0.65R is as low as possible. A preferable lower limit of the number density of fine Bi particles at the 0.65R depth position D_0.65R is 5.00 pieces/mm², more preferably is 3.00 pieces/mm², further preferably is 1.50 pieces/mm², and further preferably is 0.00 pieces/mm².

A preferable lower limit of the number density of coarse Bi particles at the 0.65R depth position D_0.65R is 0.26 pieces/mm², more preferably is 0.30 pieces/mm², further preferably is 0.40 pieces/mm², and further preferably is 0.50 pieces/mm².

An upper limit of the number density of coarse Bi particles at the 0.65R depth position D_0.65R is not particularly limited. However, when the steel material satisfies the Features 1 and 2, an upper limit of the number density of coarse Bi particles at the 0.65R depth position D_0.65R is, for example, 10.00 pieces/mm², and more preferably is 7.00 pieces/mm².

In addition to the above-mentioned fine Bi particles and coarse Bi particles, the internal region CA also contains Bi particles having an equivalent circular diameter of more than 1.0 and less than 10.0 μm. However, in the internal region CA, the number density of coarse Bi particles is more strongly correlated with machinability than the number density of Bi particles with an equivalent circular diameter of more than 1.0 and less than 10.0 μm. Therefore, in the present embodiment, the number density of coarse Bi particles at the 0.65R depth position D_0.65R in the internal region CA is used as an index of the machinability of the steel material.

[Method for Measuring Number Densities of Fine Bi Particles and Coarse Bi Particles at the 0.08R Depth Position D_0.08R]

The number density of fine Bi particles and the number density of coarse Bi particles at the 0.08R depth position D_0.08R can be measured by the following method.

A test specimen that includes the 0.08R depth position D_0.08R is taken from a cross section that is parallel to the axial direction of the steel material and includes the central axis of the steel material. Of the test specimen taken, the cross section parallel to the axial direction of the steel material and including the central axis of the steel material is used as the observation surface.

The observation surface is mirror polished. Using a scanning electron microscope (SEM) at a magnification of 1000×, a rectangular observation region including the 0.08R depth position D_0.08R is selected from the observation surface after mirror polishing. The total area of the observation region is 25.6 mm². The observation region is selected so that the 0.08R depth position D_0.08R is located at the center position of the observation region. Furthermore, the observation region is divided into 624 (26×24) rectangular fields of view of 202.5 μm×202.5 μm.

Based on the backscattered electron images in the fields of view obtained by SEM observation, the number densities of coarse Bi particles and fine Bi particles are measured using a well-known particle analysis method of image analysis. Specifically, particles in the steel material are identified based on the interface between the matrix of the steel material and particles. The particles referred to here are inclusions or precipitates. Image analysis is performed to determine the equivalent circular diameter of the identified particles. Specifically, the area of each identified particle is calculated. The diameter of a circle having the same area as the calculated area is regarded as the equivalent circular diameter (μm) of the particle.

Among the particles observed in the backscattered electron image obtained by the above SEM observation, particles having an equivalent circular diameter of 0.1 to 1.0 μm and having a Bi content of 50 mass % or more as a result of point analysis of the particle composition using an energy dispersive X-ray spectroscopy (EDX) equipped in the SEM are specified as fine Bi particles. Among the particles observed in the backscattered electron image obtained by the SEM observation, particles having an equivalent circular diameter of 10.0 μm or more and having a Bi content of 50 mass % or more as a result of point analysis of the particle composition using the EDX are specified as coarse Bi particles. The acceleration voltage for EDX analysis is 20 kV. Since Bi is a heavy element, it is observed as high brightness in the backscattered electron image. Therefore, Bi particles may be identified based on their brightness.

Based on the total number of fine Bi particles identified in each field of view and the total area (25.6 mm²) of the plurality of fields of view constituting the above observation region, the number density (pieces/mm²) of fine Bi particles at the 0.08R depth position D_0.08R is calculated. Similarly, based on the total number of coarse Bi particles identified in each field of view and the total area (25.6 mm²) of the plurality of fields of view, the number density (pieces/mm²) of coarse Bi particles at the 0.08R depth position D_0.08R is calculated.

[Method for Measuring Number Densities of Fine Bi Particles and Coarse Bi Particles at the 0.65R Depth Position D_0.65R]

The number density of fine Bi particles and the number density of coarse Bi particles at the 0.65R depth position D_0.65R can be measured by the following method.

A test specimen that includes the 0.65R depth position D_0.65R is taken from a cross section that is parallel to the axial direction of the steel material and includes the central axis of the steel material. Of the test specimen taken, the cross section parallel to the axial direction of the steel material and including the central axis of the steel material is used as the observation surface.

The observation surface is mirror polished. Using an SEM at a magnification of 1000×, a rectangular observation region including the 0.65R depth position D_0.65R is selected from the observation surface after mirror polishing. The total area of the observation region is 25.6 mm². The observation region is selected so that the 0.65R depth position D_0.65R is located at the center position of the observation region. Furthermore, the observation region is divided into 624 (26×24) rectangular fields of view of 202.5 μm×202.5 μm.

Based on the backscattered electron images in the fields of view obtained by SEM observation, the number densities of coarse Bi particles and fine Bi particles are measured using a well-known particle analysis method of image analysis. Specifically, particles in the steel material are identified based on the interface between the matrix of the steel material and particles. The particles referred to here are inclusions or precipitates. Image analysis is performed to determine the equivalent circular diameter of the identified particles. Specifically, the area of each identified particle is calculated. The diameter of a circle having the same area as the calculated area is regarded as the equivalent circular diameter (μm) of the particle.

Among the particles observed in the backscattered electron image obtained by the above SEM observation, particles having an equivalent circular diameter of 0.1 to 1.0 μm and having a Bi content of 50 mass % or more as a result of point analysis of the particle composition using the EDX are specified as fine Bi particles. Among the particles observed in the backscattered electron image obtained by the SEM observation, particles having an equivalent circular diameter of 10.0 μm or more and having a Bi content of 50 mass % or more as a result of point analysis of the particle composition using the EDX are specified as coarse Bi particles. The acceleration voltage for EDX analysis is 20 kV. Since Bi is a heavy element, it is observed as high brightness in the backscattered electron image. Therefore, Bi particles may be identified based on their brightness.

Based on the total number of fine Bi particles identified in each field of view and the total area (25.6 mm²) of the plurality of fields of view constituting the above observation region, the number density (pieces/mm²) of fine Bi particles at the 0.65R depth position D_0.65R is calculated. Similarly, based on the total number of coarse Bi particles identified in each field of view and the total area (25.6 mm²) of the plurality of fields of view, the number density (pieces/mm²) of coarse Bi particles at the 0.65R depth position D_0.65R is calculated.

Advantageous Effects of Steel Material of Present Embodiment

As described above, the steel material of the present embodiment satisfies Feature 1 to Feature 4. Therefore, the steel material can simultaneously achieve excellent melting cracks resistance, excellent hot working cracks resistance, and excellent machinability. Furthermore, the component for machine structural use made of the steel material can achieve high fatigue strength.

Preferred Use and Shape of Steel Material of Present Embodiment

The steel material of the present embodiment can be widely used, for example, as starting material for a component for machine structural use. The steel material of the present embodiment is particularly suitable for the case where hot working such as hot forging, induction hardening, and machining are performed in the process for producing the component for machine structural use. However, even if one or more of the processes of hot working, induction hardening, and machining is not performed, the steel material of the present embodiment can be used as a material for a component for machine structural use.

Note that a cross section of the steel material perpendicular to the axial direction of the steel material is circular. The diameter of the steel material in a cross section perpendicular to the axial direction is not particularly limited, but is, for example, 10 to 200 mm.

[Production Method]

An example of a method for producing the steel material of the present embodiment will now be described. A steel material satisfying Feature 1 to Feature 4 may be produced by a production method other than the production method described hereinafter. However, the production method described hereinafter is a preferred example of a method for producing a steel material of the present embodiment.

An example of a method for producing a steel material of the present embodiment includes the following processes. Note that Process 3 is an optional process and may or may not be performed.

• • (Process 1) Refining process
  • (Process 2) Casting process
  • (Process 3) Hot working process

Hereunder, each process is described.

[(Process 1) Refining Process]

In the refining process, molten steel having a chemical composition that satisfies the aforementioned Feature 1 and Feature 2 is produced. The refining process includes a primary refining process and a secondary refining process.

In the primary refining process, molten iron produced by a well-known method is refined in a converter to produce molten steel. In the secondary refining process, alloying elements are added to the molten steel to adjust the chemical composition of the molten steel to satisfy Feature 1 and Feature 2. Specifically, in the secondary refining process, the components of the molten steel other than Bi are adjusted while the molten steel is stirred using a well-known refining method. Thereafter, Bi is added to the molten steel using a wire, and the molten steel is stirred to adjust the Bi component. The secondary refining process satisfies the following conditions.

(Condition 1)

The time t0 from the addition of Bi to the molten steel to the end of stirring in the secondary refining process is more than 15 minutes and less than 60 minutes.

(Condition 2)

The stirring power density c for the molten steel after the addition of Bi to the molten steel is 10 to 100 W/t. Here, the stirring power density c (W/t) is defined by the following formula (A):

ε = 0.0285 × Q × T / W × log ⁡ ( 1 + 513.5 × Z / V ⁢ 1 ) , ( A )

• • where the flow rate (NL/min) of the gas injected into the ladle containing the molten steel is substituted to Q in formula (A). The molten steel temperature (K) is substituted to T. The mass (t) of the molten steel is substituted to W. The depth (m) of the molten steel in the ladle is substituted to Z. The degree of vacuum (torr) in the atmosphere containing the molten steel during stirring is substituted to V1.

Hereunder, Condition 1 and Condition 2 are described.

[Condition 1: Time t0]

In the secondary refining process, the time t0 from the addition of Bi to the molten steel to the end of stirring in the secondary refining process is more than 15 minutes and less than 60 minutes.

If the time t0 from the addition of Bi to the end of stirring in the secondary refining process is 15 minutes or less, Bi does not diffuse sufficiently in the molten steel. In this case, excessive coarse Bi particles are generated in the produced steel material. Therefore, the number density of coarse Bi particles at the 0.08R depth position D_0.08R becomes excessively high.

On the other hand, if the time t0 from the addition of Bi to the end of stirring in the secondary refining process is 60 minutes or more, Bi is likely to aggregate in the molten steel. In this case, the number density of fine Bi particles in the produced steel material decreases. Therefore, the number density of fine Bi particles at the 0.08R depth position D_0.08R becomes excessively low.

In the secondary refining process, if the time t0 from the addition of Bi to the end of stirring in the secondary refining process is more than 15 minutes and less than 60 minutes, Bi will diffuse sufficiently in the molten steel. Therefore, assuming that Condition 2, Condition 3 and Condition 4 described below are satisfied, the number density of fine Bi particles and the number density of coarse Bi particles at the 0.08R depth position D_0.08R and the 0.65R depth position D_0.65R in the steel material are within appropriate ranges.

Note that the temperature of the molten steel after the addition of Bi to the end of stirring in the secondary refining process is 1510 to 1630° C.

[Condition 2: Stirring Power Density ε]

The stirring power density c for the molten steel after the addition of Bi to the molten steel is 10 to 100 W/t.

If the stirring power density c for the molten steel after the addition of Bi to the molten steel is less than 10 W/t, Bi does not diffuse sufficiently in the molten steel. In this case, excessive coarse Bi particles are generated in the produced steel material. Therefore, the number density of coarse Bi particles at the 0.08R depth position D_0.08R becomes excessively high.

On the other hand, if the stirring power density c for the molten steel after the addition of Bi to the molten steel is more than 100 W/t, Bi is likely to aggregate in the molten steel. In this case, the number density of fine Bi particles in the produced steel material decreases. Therefore, the number density of fine Bi particles at the 0.08R depth position D_0.08R becomes excessively low.

If the stirring power density c for the molten steel after the addition of Bi to the molten steel is 10 to 100 W/t, Bi will diffuse sufficiently in the molten steel. Therefore, assuming that Condition 1, Condition 3 and Condition 4 described below are satisfied, the number density of fine Bi particles and the number density of coarse Bi particles at the 0.08R depth position D_0.08R and the 0.65R depth position D_0.65R in the steel material are within appropriate ranges.

[(Process 2) Casting Process]

In the casting process, using molten steel, a cast bloom is produced by a well-known continuous casting method. The casting process is performed under the following conditions.

(Condition 3)

When the solidification cooling rate at a depth of 15 mm from the surface in the cross section perpendicular to the longitudinal direction of the cast bloom is defined as the surface solidification cooling rate, the surface solidification cooling rate is 550° C./min or more.

(Condition 4)

When the shape of the cross section perpendicular to the longitudinal direction of the cast bloom is rectangular, the internal solidification cooling rate is defined as the solidification cooling rate at the midpoint between the surface of the cast bloom and the center point of the cast bloom on the line passing through the centers of the long sides of the rectangular cross section in its width direction, and when the shape of the cross section is circular, the internal solidification cooling rate is defined as the solidification cooling rate at the midpoint of the radius R (i.e., a depth of R/2), and the internal solidification cooling rate is 100° C./min or less.

Hereunder, Condition 3 and Condition 4 are described.

[Condition 3: Surface Solidification Cooling Rate]

Bi particles are generated during solidification. That is to say, Bi particles are formed by crystallization. Therefore, in order for the steel material to satisfy Feature 3, it is preferable to increase the solidification cooling rate at the surface layer of the cast bloom and solidify the steel before the Bi particles become coarse. Therefore, in the casting process, the cooling rate of the cast bloom in the mold of the continuous casting machine is adjusted so that the solidification cooling rate at a depth of 15 mm from the surface of the produced cast bloom is 550° C./min or more.

Here, the cooling rate from the liquidus temperature to the solidus temperature in the temperature range of steel in the casting process is defined as the solidification cooling rate (° C./min). As described above, the solidification cooling rate at a depth of 15 mm from the surface of the cast bloom is defined as the “surface solidification cooling rate”. The surface solidification cooling rate is determined by the following method.

A test specimen that includes a depth of 15 mm from the surface of the cast bloom is taken from a cross section that is perpendicular to the longitudinal direction of the cast bloom produced using the continuous casting method. For example, if the cross section perpendicular to the longitudinal direction of the cast bloom is rectangular, a test specimen is taken that includes a depth of 15 mm from the center position of the surface of the cast bloom in the width direction thereof (if the cross section is rectangular, the center position of the surface in the width direction corresponding to the long sides). The surface of the test specimen corresponding to the cross section perpendicular to the longitudinal direction of the cast bloom is defined as an observation surface. Of the observation surface, a region of 5 mm×5 mm centered around a depth of 15 mm from the surface of the cast bloom is defined as an observation region. In the observation region, the secondary dendrite arm spacing is measured at 10 locations, and the arithmetic mean value is defined as λ2 (μm). The surface solidification cooling rate V (° C./min) is measured using the measured dendrite secondary arm spacing λ2 (μm).

V = ( λ2 / 770 ) - 1 / 0.41

The region located at a depth of 15 mm from the surface of the cast bloom solidifies while passing through the mold of the continuous casting machine. Therefore, the surface solidification cooling rate is adjusted by the cooling mechanism in the mold. An upper limit of the surface solidification cooling rate is not particularly limited. A preferable lower limit of the surface solidification cooling rate is 600° C./min or more, and more preferably is 700° C./min or more. Note that a preferable casting speed for achieving a surface solidification cooling rate of 550° C./min or more is, for example, 0.6 m/min or less.

[Condition 4: Internal Solidification Cooling Rate]

As described above, Bi particles are generated during solidification. Therefore, in order for the steel material to satisfy Feature 4, it is preferable to slow down the solidification cooling rate inside the cast bloom and solidify the steel after coarsening the Bi particles. Therefore, in the casting process, the cooling rate of the cast bloom is adjusted so that the internal solidification cooling rate is 100° C./min or less.

Here, the internal solidification cooling rate based on the above definition is determined by the following method.

When the shape of the cross section perpendicular to the longitudinal direction of the cast bloom produced by the continuous casting method is rectangular, a test specimen is taken that includes the midpoint between the surface of the cast bloom and the center point of the cast bloom on the line passing through the centers of the long sides of the rectangular cross section in its width direction, and when the shape of the cross section is circular, a test specimen is taken that includes the midpoint of the radius R (a depth of R/2). The surface of the test specimen corresponding to the cross section perpendicular to the longitudinal direction of the cast bloom is defined as an observation surface. Of the observation surface, a region of 5 mm×5 mm centered around the above midpoint is defined as an observation region. In the observation region, the secondary dendrite arm spacing is measured at 10 locations, and the arithmetic mean value is defined as λ2 (μm). The internal solidification cooling rate V (° C./min) is measured using the measured dendrite secondary arm spacing λ2 (μm).

V = ( λ2 / 770 ) - 1 / 0.41

The internal solidification cooling rate can be adjusted, for example, by changing the size of the mold. Specifically, the internal solidification cooling rate can be adjusted by adjusting the area of a transverse cross section of the mold. Furthermore, in the group of rolls arranged downstream of the mold of the continuous casting machine, a plurality of fluid nozzles for cooling the cast bloom are disposed between the rolls. Therefore, the internal solidification cooling rate can be adjusted by adjusting the flow rate of the fluid (coolant such as water, air, or a mixture of coolant and air) injected from the plurality of fluid nozzles. A lower limit of the internal solidification cooling rate is not particularly limited, but a preferable lower limit is 10° C./min or more. A preferable upper limit is 50° C./min or less, and more preferably is 25° C./min or less.

[(Process 3) Hot Working Process]

The hot working process is an optional process. That is to say, the hot working process may or may not be performed. When the hot working process is to be performed, the cast bloom produced in the casting process is subjected to hot working in the hot working process to produce a steel material.

The hot working process may be, for example, only a well-known rough rolling process, or may include a well-known rough rolling process and a well-known finish rolling process performed after the rough rolling process. In the rough rolling process, for example, a billet is produced from the heated cast bloom or steel ingot by hot rolling performed using a blooming mill, or a blooming mill and a continuous billet mill. In the finish rolling process, for example, the heated billet is subjected to finish rolling using a well-known continuous rolling mill to produce a steel material (steel bar). The heating temperature in the rough rolling process is, for example, 1000 to 1300° C. The heating temperature in the finish rolling process is, for example, 1000 to 1300° C.

In the above hot working process, a steel material is produced by hot rolling. However, the steel material may be produced by hot working other than hot rolling. For example, the steel material may be produced by hot forging instead of hot rolling. The steel material may be produced by performing hot rolling and hot forging. Even when hot forging is performed in the hot working process, the heating temperature is, for example, 1000 to 1300° C.

The steel material of the present embodiment is produced by the above production process. As described above, the hot working process may be omitted. That is to say, the steel material of the present embodiment may be a cast product (a cast bloom).

[Method for Producing a Component for Machine Structural Use]

As described above, the steel material of the present embodiment is used as a starting material for the component for machine structural use. The method for producing the component for machine structural use is well known, and is, for example as follows.

The steel material of the present embodiment is subjected to hot working to produce an intermediate product having a rough shape for a component for machine structural use (e.g. a crankshaft). The hot working is, for example, hot forging. The produced intermediate product is left to cool in the air.

The intermediate product after cooling is subjected to machining to cut the intermediate product into the specified shape. The intermediate product after machining is subjected to the well-known induction hardening (tempering is omitted), or the well-known induction hardening and the well-known tempering. The component for machine structural use is produced by the above process.

Examples

The effects of the steel material of the present embodiment will be described more specifically with reference to Examples. The conditions in the following Examples are one example of conditions adopted to confirm the feasibility and advantageous effects of the steel material of the present embodiment. Therefore, the steel material of the present embodiment is not limited to the examples of conditions.

Steel materials having the chemical compositions shown in Table 1-1 to Table 1-3 were produced.

TABLE 1-1
Test	Chemical Composition (unit: mass %, balance: Fe and impurities)

No.	C	Si	Mn	P	S	Bi	Al	N	O

1	0.60	0.05	1.17	0.010	0.015	0.0215	0.035	0.0080	0.0018
2	0.37	0.08	1.66	0.002	0.145	0.0953	0.028	0.0094	0.0023
3	0.50	0.35	1.60	0.019	0.050	0.1080	0.002	0.0128	0.0017
4	0.42	0.44	1.48	0.030	0.077	0.0196	0.005	0.0220	0.0003
5	0.45	0.80	1.30	0.003	0.019	0.0052	0.030	0.0073	0.0012
6	0.38	0.11	0.93	0.008	0.095	0.0076	0.097	0.0175	0.0020
7	0.46	0.26	0.81	0.014	0.023	0.0394	0.025	0.0052	0.0013
8	0.36	0.86	0.90	0.015	0.059	0.2450	0.039	0.0040	0.0019
9	0.52	0.74	0.80	0.001	0.017	0.0531	0.016	0.0123	0.0009
10	0.54	0.39	0.90	0.005	0.020	0.0480	0.022	0.0030	0.0012
11	0.43	0.68	0.65	0.013	0.028	0.0213	0.049	0.0052	0.0009
12	0.33	0.50	0.82	0.018	0.014	0.2373	0.021	0.0042	0.0008
13	0.55	0.32	0.77	0.007	0.172	0.0104	0.033	0.0082	0.0030
14	0.47	0.30	0.84	0.020	0.060	0.0331	0.020	0.0050	0.0040
15	0.45	0.41	0.58	0.016	0.070	0.0411	0.075	0.0057	0.0016
16	0.58	0.77	1.63	0.014	0.031	0.0650	0.021	0.0058	0.0007
17	0.44	0.89	1.00	0.014	0.197	0.1824	0.023	0.0151	0.0014
18	0.57	0.02	1.42	0.006	0.035	0.0403	0.025	0.0036	0.0005
19	0.39	0.53	0.64	0.012	0.047	0.0409	0.062	0.0103	0.0013
20	0.56	0.25	1.24	0.019	0.139	0.0410	0.004	0.0092	0.0049
21	0.34	0.56	0.88	0.029	0.043	0.0423	0.010	0.0039	0.0016
22	0.46	0.20	0.87	0.013	0.123	0.0341	0.018	0.0102	0.0010
23	0.59	0.59	0.52	0.017	0.085	0.0745	0.029	0.0072	0.0016
24	0.32	0.19	0.66	0.009	0.025	0.0092	0.012	0.0033	0.0018
25	0.53	0.83	1.68	0.017	0.065	0.0152	0.053	0.0063	0.0010
26	0.48	0.27	1.54	0.015	0.052	0.0602	0.026	0.0040	0.0020
27	0.48	0.89	0.95	0.016	0.030	0.0415	0.026	0.0045	0.0034
28	0.40	0.23	0.79	0.007	0.026	0.2179	0.037	0.0198	0.0012
29	0.43	0.14	0.71	0.012	0.012	0.1998	0.014	0.0051	0.0011
30	0.31	0.47	0.75	0.025	0.040	0.0520	0.045	0.0065	0.0010
31	0.41	0.61	0.80	0.015	0.010	0.0599	0.040	0.0055	0.0015
32	0.45	0.21	1.39	0.026	0.027	0.0793	0.024	0.0060	0.0012
33	0.35	0.17	0.82	0.015	0.037	0.1276	0.009	0.0047	0.0015
34	0.47	0.29	1.12	0.023	0.055	0.0830	0.007	0.0246	0.0008
35	0.49	0.65	0.76	0.011	0.033	0.1488	0.027	0.0115	0.0014
36	0.53	0.71	0.86	0.016	0.189	0.0721	0.024	0.0055	0.0028
37	0.50	0.43	0.94	0.015	0.001	0.0411	0.022	0.0040	0.0012
38	0.45	0.26	0.90	0.014	0.023	0.0825	0.021	0.0007	0.0009
39	0.32	0.02	0.53	0.009	0.025	0.0532	0.025	0.0044	0.0008
40	0.34	0.17	0.66	0.009	0.072	0.0384	0.030	0.0053	0.0018
41	0.58	0.87	1.68	0.003	0.011	0.0430	0.021	0.0050	0.0013
42	0.59	0.88	1.63	0.002	0.015	0.0632	0.022	0.0060	0.0011
43	0.50	0.05	1.12	0.020	0.015	0.0431	0.032	0.0045	0.0022
44	0.45	0.22	0.93	0.012	0.020	0.2439	0.025	0.0050	0.0013
45	0.47	0.19	1.31	0.022	0.055	0.1781	0.062	0.0154	0.0014
46	0.42	0.35	0.85	0.013	0.050	0.0774	0.027	0.0084	0.0010
47	0.49	0.61	0.74	0.016	0.024	0.1487	0.014	0.0111	0.0013
48	0.46	0.20	0.85	0.015	0.027	0.0352	0.004	0.0041	0.0007
49	0.32	0.45	1.26	0.011	0.033	0.0212	0.020	0.0096	0.0011
50	0.45	0.25	0.61	0.018	0.070	0.0504	0.044	0.0103	0.0028
51	0.53	0.36	1.05	0.016	0.045	0.1246	0.018	0.0077	0.0012
52	0.55	0.40	0.77	0.009	0.012	0.0617	0.025	0.0062	0.0017
53	0.48	0.32	0.92	0.014	0.033	0.1040	0.020	0.0065	0.0013
54	0.51	0.39	0.85	0.010	0.023	0.0599	0.028	0.0070	0.0015
55	0.40	0.30	0.95	0.007	0.094	0.0045	0.035	0.0053	0.0015
56	0.41	0.41	0.80	0.005	0.028	0.2590	0.017	0.0125	0.0005

TABLE 1-2
Test	Chemical Composition (unit: mass %, balance: Fe and impurities)

No.	Cr	V	Sn	Sb	As	Pb	Mg	Ti	Nb	W	Zr

1	—	—	—	—	—	—	—	—	—	—	—
2	—	—	—	—	—	—	—	—	—	—	—
3	—	—	—	—	—	—	—	—	—	—	—
4	—	—	—	—	—	—	—	—	—	—	—
5	—	—	—	—	—	—	—	—	—	—	—
6	—	—	—	—	—	—	—	—	—	—	—
7	—	—	—	—	—	—	—	—	—	—	—
8	—	—	—	—	—	—	—	—	—	—	—
9	—	—	—	—	—	—	—	—	—	—	—
10	0.12	—	—	—	—	—	—	—	—	—	—
11	—	—	—	—	—	—	—	—	—	—	—
12	—	—	0.0078	—	—	—	—	—	—	—	—
13	—	—	—	0.0034	—	—	—	—	—	—	—
14	—	—	—	—	0.0025	—	—	—	—	—	—
15	—	—	—	—	—	0.03	—	—	—	—	—
16	—	—	—	—	—	—	—	—	—	—	—
17	—	—	—	—	—	—	0.0020	—	—	—	—
18	—	—	—	—	—	—	—	0.0120	—	—	—
19	—	—	—	—	—	—	—	—	0.0490	—	—
20	—	—	—	—	—	—	—	—	—	0.0900	—
21	—	—	—	—	—	—	—	—	—	—	0.1500
22	—	—	—	—	—	—	—	—	—	—	—
23	—	—	—	—	—	—	—	—	—	—	—
24	—	—	—	—	—	—	—	—	—	—	—
25	—	—	—	—	—	—	—	—	—	—	—
26	—	—	—	—	—	—	—	—	—	—	—
27	—	—	—	—	—	—	—	—	—	—	—
28	—	—	—	—	—	—	—	—	—	—	—
29	—	—	—	—	—	—	—	—	—	—	—
30	—	—	—	—	—	—	—	—	—	—	—
31	—	—	—	—	—	—	—	—	—	—	—
32	1.02	0.048	0.0971	—	—	—	—	—	—	—	—
33	1.22	—	—	0.0485	—	—	0.0005	—	—
34	0.02	—	—	—	—	0.09	0.0010	—	—	0.0050	—
35	0.29	—	—	—	—	—	—	0.0400	0.0050	—	—
36	0.57	—	—	—	—	—	—	—	—	—	0.0014
37	—	—	—	—	—	—	—	—	—	—	—
38	—	—	—	—	—	—	—	—	—	—	—
39	0.05	—	—	—	—	—	—	—	—	—	—
40	—	—	—	—	—	—	—	—	—	—	—
41	0.29	—	—	—	—	—	—	—	—	—	—
42	—	0.047	—	—	—	—	—	—	—	—	—
43	—	—	—	—	—	—	—	—	—	—	—
44	—	—	—	—	—	—	—	—	—	—	—
45	—	—	—	—	—	—	—	—	—	—	—
46	—	—	—	—	—	—	—	—	—	—	—
47	—	—	—	—	—	—	—	—	—	—	—
48	—	—	—	—	—	—	—	—	—	—	—
49	—	—	—	—	—	—	—	—	—	—	—
50	—	—	—	—	—	—	—	—	—	—	—
51	—	—	—	—	—	—	—	—	—	—	—
52	—	—	—	—	—	—	—	—	—	—	—
53	—	—	—	—	—	—	—	—	—	—	—
54	—	—	—	—	—	—	—	—	—	—	—
55	—	—	—	—	—	—	—	—	—	—	—
56	—	—	—	—	—	—	—	—	—	—	—

TABLE 1-3
Test	Chemical Composition (unit: mass %, balance: Fe and impurities)

No.	Ca	Te	B	REM	Co	Se	In	Mo	Cu	Ni	Fn

1	—	—	—	—	—	—	—	—	—	—	0.83
2	—	—	—	—	—	—	—	—	—	—	0.61
3	—	—	—	—	—	—	—	—	—	—	0.82
4	—	—	—	—	—	—	—	—	—	—	0.71
5	—	—	—	—	—	—	—	—	—	—	0.78
6	—	—	—	—	—	—	—	—	—	—	0.51
7	—	—	—	—	—	—	—	—	—	—	0.63
8	—	—	—	—	—	—	—	—	—	—	0.58
9	—	—	—	—	—	—	—	—	—	—	0.74
10	—	—	—	—	—	—	—	—	—	—	0.77
11	—	—	—	—	—	—	—	—	—	—	0.61
12	—	—	—	—	—	—	—	—	—	—	0.53
13	—	—	—	—	—	—	—	—	—	—	0.61
14	—	—	—	—	—	—	—	—	—	—	0.63
15	—	—	—	—	—	—	—	—	—	—	0.56
16	—	—	—	—	—	—	—	—	—	—	0.96
17	—	—	—	—	—	—	—	—	—	—	0.59
18	—	—	—	—	—	—	—	—	—	—	0.83
19	—	—	—	—	—	—	—	—	—	—	0.54
20	—	—	—	—	—	—	—	—	—	—	0.73
21	—	—	—	—	—	—	—	—	—	—	0.54
22	0.0006	—	—	—	—	—	—	—	—	—	0.57
23	—	0.0077	—	—	—	—	—	—	—	—	0.69
24	—	—	0.0025	—	—	—	—	—	—	—	0.45
25	—	—	—	0.0020	—	—	—	—	—	—	0.90
26	—	—	—	—	0.0013	—	—	—	—	—	0.78
27	—	—	—	—	—	0.0090	—	—	—	—	0.74
28	—	—	—	—	—	—	0.0055	—	—	—	0.56
29	—	—	—	—	—	—	—	0.11	—	—	0.58
30	—	—	—	—	—	—	—	—	0.10	—	0.48
31	—	—	—	—	—	—	—	—	—	0.10	0.62
32	—	—	—	—	—	—	—	—	—	—	1.04
33	—	—	—	—	0.0044	—	—	—	—	—	0.78
34	—	0.0013	—	—	—	0.0041	—	0.29	—	—	0.69
35	—	—	0.0018	—	—	—	—	—	—	—	0.75
36	0.0024	—	—	—	—	—	—	—	0.24	0.14	0.77
37	—	—	—	—	—	—	—	—	—	—	0.73
38	—	—	—	—	—	—	—	—	—	—	0.64
39	—	—	—	—	—	—	—	—	—	—	0.42
40	—	—	—	—	—	—	—	—	—	—	0.44
41	—	—	—	—	—	—	—	—	—	—	1.06
42	—	—	—	—	—	—	—	—	—	—	1.07
43	—	—	—	—	—	—	—	—	—	—	0.72
44	—	—	—	—	—	—	—	—	—	—	0.64
45	—	—	—	—	—	—	—	—	—	—	0.71
46	—	—	—	—	—	—	—	—	—	—	0.59
47	—	—	—	—	—	—	—	—	—	—	0.68
48	—	—	—	—	—	—	—	—	—	—	0.63
49	—	—	—	—	—	—	—	—	—	—	0.59
50	—	—	—	—	—	—	—	—	—	—	0.55
51	—	—	—	—	—	—	—	—	—	—	0.74
52	—	—	—	—	—	—	—	—	—	—	0.74
53	—	—	—	—	—	—	—	—	—	—	0.67
54	—	—	—	—	—	—	—	—	—	—	0.70
55	—	—	—	—	—	—	—	—	—	—	0.55
56	—	—	—	—	—	—	—	—	—	—	0.59

Specifically, the refining process (the primary refining process and the secondary refining process) was performed using a 70-ton converter. In the primary refining process, molten iron produced by a well-known method was refined in a converter under the same conditions.

After the primary refining process, the secondary refining process was performed. Specifically, refining treatment was performed using a ladle furnace (LF), and then RH vacuum degassing treatment was performed. The contents of the elements other than Bi were adjusted by these processes. Thereafter, Bi was further added to the molten steel using a wire and the Bi content was adjusted by stirring the molten steel.

The time t0 (minutes) from the addition of Bi to the molten steel to the end of stirring in the secondary refining process was as shown in Table 2. Furthermore, the stirring power density c (W/t) during stirring was as shown in Table 2. Note that after Bi was added to the molten steel, the molten steel temperature was 1510 to 1630° C. until the end of stirring in the secondary refining process. The molten steel was produced by the above processes.

	TABLE 2
			Condition	Condition
	Condition		3	4	Surface Layer	Internal
	1		Surface	Internal	Region	Region

Time t0

Condition

Solidi-

Solidi-

Number

Number

Number

Number

from

2

fication

fication

Density

Density of

Density of

Density of

addition of

Stirring

Cooling

Cooling

of Fine

Coarse Bi

Fine Bi

Coarse Bi

Bi to end of

Power

Rate

Rate

Bi Particles

Particles

Particles

Particles

Hot

Test

stirring

Density ε

(° C./

(° C./

(pieces/

(pieces/

(pieces/

(pieces/

Working

Melting

Machin-

Fatigue

No.

(minutes)

(W/t)

min)

min)

mm2)

mm2)

mm2)

mm2)

Cracks

Cracks

ability

Strength

1	16	37	612	48	25.40	0.00	6.64	0.51	PASS	PASS	PASS	PASS
2	30	51	655	73	35.09	0.04	8.13	1.76	PASS	PASS	PASS	PASS
3	19	66	802	48	35.21	0.08	8.68	2.34	PASS	PASS	PASS	PASS
4	17	55	603	5	23.06	0.00	6.37	0.5	PASS	PASS	PASS	PASS
5	21	25	643	23	16.26	0.00	5.55	0.31	PASS	PASS	PASS	PASS
6	59	51	900	39	16.92	0.04	5.35	0.43	PASS	PASS	PASS	PASS
7	21	34	560	20	31.66	0.00	7.43	0.47	PASS	PASS	PASS	PASS
8	57	56	663	64	38.69	0.23	9.93	5.12	PASS	PASS	PASS	PASS
9	20	50	637	51	33.10	0.04	7.86	0.63	PASS	PASS	PASS	PASS
10	20	12	567	57	31.81	0.00	7.11	0.43	PASS	PASS	PASS	PASS
11	18	49	620	43	22.63	0.00	6.45	0.39	PASS	PASS	PASS	PASS
12	23	52	589	90	38.18	0.12	14.30	4.89	PASS	PASS	PASS	PASS
13	18	31	580	30	18.95	0.00	5.67	0.35	PASS	PASS	PASS	PASS
14	45	53	643	38	27.51	0.00	6.92	0.55	PASS	PASS	PASS	PASS
15	52	78	592	23	29.70	0.00	7.66	0.55	PASS	PASS	PASS	PASS
16	17	46	675	49	32.40	0.00	7.54	1.13	PASS	PASS	PASS	PASS
17	19	52	570	22	36.31	0.12	9.26	4.06	PASS	PASS	PASS	PASS
18	19	92	620	70	33.22	0.00	7.50	0.51	PASS	PASS	PASS	PASS
19	43	48	611	41	31.38	0.00	7.31	0.31	PASS	PASS	PASS	PASS
20	38	33	622	20	32.20	0.00	7.35	0.35	PASS	PASS	PASS	PASS
21	25	40	590	3	32.79	0.04	7.66	0.31	PASS	PASS	PASS	PASS
22	18	15	653	33	29.82	0.00	6.80	0.27	PASS	PASS	PASS	PASS
23	20	50	750	82	33.10	0.04	8.09	1.09	PASS	PASS	PASS	PASS
24	50	46	841	50	18.60	0.04	5.82	0.31	PASS	PASS	PASS	PASS
25	19	46	561	18	20.52	0.00	5.90	0.39	PASS	PASS	PASS	PASS
26	55	98	632	10	32.59	0.00	7.74	1.06	PASS	PASS	PASS	PASS
27	35	47	600	36	32.59	0.04	7.66	0.51	PASS	PASS	PASS	PASS
28	17	80	583	40	37.32	0.16	9.34	4.85	PASS	PASS	PASS	PASS
29	16	49	700	7	37.20	0.20	9.50	4.22	PASS	PASS	PASS	PASS
30	23	76	980	96	32.71	0.00	7.70	0.63	PASS	PASS	PASS	PASS
31	33	49	602	15	32.32	0.00	7.46	0.74	PASS	PASS	PASS	PASS
32	17	47	599	24	33.49	0.04	7.93	1.17	PASS	PASS	PASS	PASS
33	18	58	613	25	35.88	0.04	9.07	2.38	PASS	PASS	PASS	PASS
34	40	43	633	97	33.61	0.04	7.93	1.29	PASS	PASS	PASS	PASS
35	27	61	686	40	35.99	0.08	9.07	3.05	PASS	PASS	PASS	PASS
36	22	55	694	24	33.88	0.00	8.13	1.29	PASS	PASS	PASS	PASS
37	17	52	581	47	29.90	0.00	7.58	0.39	PASS	PASS	PASS	PASS
38	23	46	621	46	33.53	0.04	7.82	1.21	PASS	PASS	PASS	PASS
39	20	33	582	44	31.30	0.00	7.00	0.35	PASS	PASS	PASS	FAIL
40	19	35	600	18	31.30	0.00	7.00	0.35	PASS	PASS	PASS	FAIL
41	21	45	641	43	32.79	0.04	7.93	0.86	PASS	PASS	FAIL	PASS
42	22	50	651	23	32.79	0.04	7.93	0.86	PASS	PASS	FAIL	PASS
43	9	42	600	72	28.92	0.35	4.46	0.39	FAIL	PASS	PASS	PASS
44	14	54	636	47	34.04	0.98	14.11	4.42	FAIL	PASS	PASS	PASS
45	61	39	610	95	13.95	0.16	10.86	2.19	PASS	FAIL	PASS	PASS
46	68	50	573	32	9.14	0.08	6.33	0.39	PASS	FAIL	PASS	PASS
47	21	8	588	35	33.10	0.43	9.14	1.60	FAIL	PASS	PASS	PASS
48	22	5	59	6	25.75	0.27	3.52	0.39	FAIL	PASS	PASS	PASS
49	18	102	577	61	7.82	0.00	3.24	0.31	PASS	FAIL	PASS	PASS
50	30	108	619	50	9.50	0.04	4.06	0.59	PASS	FAIL	PASS	PASS
51	16	52	530	38	11.69	0.47	8.52	1.09	FAIL	FAIL	PASS	PASS
52	19	45	370	22	8.13	0.31	5.47	0.47	FAIL	FAIL	PASS	PASS
53	23	40	583	103	32.83	0.12	20.79	0.23	PASS	PASS	FAIL	PASS
54	25	50	601	114	26.50	0.08	15.67	0.16	PASS	PASS	FAIL	PASS
55	19	48	644	11	13.01	0.00	4.92	0.12	PASS	FAIL	FAIL	PASS
56	17	33	591	24	35.21	0.35	13.40	5.08	FAIL	PASS	PASS	PASS

Using the molten steel, a cast bloom was produced by the continuous casting method. The solidification cooling rate (° C./min) was adjusted during casting. The surface solidification cooling rate and the internal solidification cooling rate were as shown in Table 2. Note that the surface solidification cooling rate and the internal solidification cooling rate were determined by the methods described above in [Condition 3: Surface solidification cooling rate] and [Condition 4: Internal solidification cooling rate].

The produced cast bloom was subjected to hot working. Specifically, the cast bloom was subjected to rough rolling to produce a billet having a transverse cross section of 180 mm×180 mm. The heating temperature of the cast bloom during rough rolling was 1250° C.

Furthermore, the billet was subjected to hot forging equivalent to finishing rolling to produce a steel material (steel bar) having a diameter of 97 mm. The heating temperature for the billet during hot forging was 1250° C. The forging was performed using an air hammer (model 600HP) manufactured by OTANI MACHINERY MFG. CO., LTD. The billet was heated at 1250° C. for 1 hour and forged to a cross section of 120 mm×120 mm. Thereafter, the billet was heated again at 1250° C. for 1 hour and forged so that its transverse cross section is an octagon circumscribing a circle having a diameter of 97 mm. Thereafter, the billet was heated again at 1250° C. for 1 hour and forged into a steel bar (round steel) with a circular cross section and a diameter of 97 mm. The steel material was produced by the above production process.

[Evaluation Test]

The following evaluation tests were performed on the steel materials of each test number.

• • (Test 1) Number density measurement test of fine Bi particles and coarse Bi particles at the 0.08R depth position D_0.08R
  • (Test 2) Number density measurement test of fine Bi particles and coarse Bi particles at the 0.65R depth position D_0.65R
  • (Test 3) Hot working cracks evaluation test
  • (Test 4) Melting cracks evaluation test
  • (Test 5) Machinability evaluation test (drill life test)
  • (Test 6) Fatigue strength evaluation test (rotating bending fatigue test)

Each evaluation test is described hereunder.

[(Test 1) Number Density Measurement Test of Fine Bi Particles and Coarse Bi Particles at the 0.08R Depth Position D_0.08R]

For the steel material of each test number, the number density of fine Bi particles (pieces/mm²) at the 0.08R depth position D_0.08R and the number density of coarse Bi particles (pieces/mm²) at the 0.08R depth position D_0.08R were determined using the method described above in [Method for measuring number densities of fine Bi particles and coarse Bi particles at the 0.08R depth position D_0.08R]. The results are shown in Table 2.

[(Test 2) Number Density Measurement Test of Fine Bi Particles and Coarse Bi Particles at the 0.65R Depth Position D_0.65R]

For the steel material of each test number, the number density of fine Bi particles (pieces/mm²) at the 0.65R depth position D_0.65R and the number density of coarse Bi particles (pieces/mm²) at the 0.65R depth position D_0.65R were determined using the method described above in [Method for measuring number densities of fine Bi particles and coarse Bi particles at the 0.65R depth position D_0.65R]. The results are shown in Table 2.

[(Test 3) Hot Working Cracks Evaluation Test]

The surface of the produced steel material of each test number was observed. As a result of the observation, if two or more apparent cracks were observed on the surface of the steel material per meter in the longitudinal direction of the steel material, it was determined that hot working cracks had occurred. On the other hand, as a result of the observation, if two or more apparent cracks were not observed on the surface of the steel material per meter in the longitudinal direction of the steel material, it was determined that hot working cracks was suppressed. Here, an apparent crack refers to a crack that is 3 mm or more in length and that can be observed with the naked eye or with a simple magnifying glass.

The evaluation results of hot working cracks are shown in the “Hot Working Cracks” column of Table 2. The occurrence of hot working cracks is indicated as “FAIL”, and the suppression of hot working cracks is indicated as “PASS”.

[(Test 4 to Test 6) Regarding Melting Cracks Evaluation Test, Machinability Evaluation Test, and Fatigue Strength Evaluation Test]

[Production of Simulated Intermediate Product of Component for Machine Structural Use]

Heat treatment was performed to simulate the hot forging during the process for producing the component for machine structural use made of the steel material of each test number. Specifically, the steel material was heated and held at 1100° C. for 30 minutes. Thereafter, the steel material was left to cool in the air. Hereinafter, the steel material subjected to the above heat treatment are referred to as a “simulated intermediate product of the component for machine structural use (or simply a simulated intermediate product)”. The simulated intermediate product of the component for machine structural use was a steel bar (round steel) with a diameter of 97 mm.

[(Test 4) Melting Cracks Evaluation Test]

A test specimen of 10 mm in width, 3 mm in thickness, and 100 mm in length was machined from the region including the surface layer region SA of the simulated intermediate product of the component for machine structural use. The longitudinal direction of the test specimen was parallel to the longitudinal direction of the simulated intermediate product. In addition, the central axis parallel to the longitudinal direction of the test specimen coincided with the 0.08R depth position D_0.08R.

A hardening test simulating induction hardening was performed on the test specimen using a thermal cycle testing device manufactured by Fuji Electronic Industrial Co., Ltd. Specifically, the test specimen was heated to 1400° C. at a heating rate of 100° C./sec. The test specimen was held at 1400° C. for 15 seconds. Thereafter, the test specimen was water-cooled.

After water cooling, the test specimen was cut in a direction perpendicular to the longitudinal direction of the test specimen at the center position in the longitudinal direction of the test specimen. The cut surface was defined as the observation surface. The observation surface was subjected to mechanical polishing. The observation surface after mechanical polishing was etched with picral etchant. The center position of the etched observation surface was observed with an optical microscope at a magnification of 400×, and the presence or absence of melting cracks was visually checked. The field of view for observation was set to 250 μm×400 μm.

When an apparently etched region having a length of 5 μm or more at a grain boundary was observed on the observed surface, it was determined that melting cracks had occurred. The apparently etched region having a width of 5 μm or more at a grain boundary refers to, for example, each of the regions indicated by the reference numerals 15 in FIG. 3. On the other hand, when no etched region was observed at the grain boundaries as in FIG. 4, it was determined that melting cracks were suppressed. The evaluation results of melting cracks are shown in the “Melting Cracks” column of Table 2. The occurrence of melting cracks is indicated as “FAIL”. The suppression of melting cracks is indicated as “PASS”.

[(Test 5) Machinability Evaluation Test (Drill Life Test)]

A test specimen for the machinability evaluation test was cut out from the simulated intermediate product of the component for machine structural use. Specifically, a hole was drilled at an arbitrary position in the area corresponding to the internal region CA of the cross section perpendicular to the longitudinal direction of the simulated intermediate product with a diameter of 97 mm. The drill used was a model SD3.0 drill manufactured by NACHI-FUJIKOSHI CORP. As a drilling condition, a feed was set to 0.25 mm/rev and a drilling depth was set to 9 mm per hole. The lubricant was a water-soluble cutting oil.

A hole was drilled under the above drilling conditions to evaluate the machinability of the steel material. A maximum cutting speed VL1000 (m/min) was used as the evaluation index. The maximum cutting speed VL1000 is the maximum cutting speed of a drill capable of drilling a hole with a cumulative hole depth of 1000 mm.

Based on the maximum cutting speed VL1000, machinability was evaluated as follows:

• • VL1000 is 20 m/min or more: Excellent machinability (“PASS”)
  • VL1000 is less than 20 m/min: Poor machinability (“FAIL”)

The “Machinability” column of Table 2 shows “PASS” or “FAIL” as the evaluation results.

[(Test 6) Fatigue Strength Evaluation Test (Rotating Bending Fatigue Test)]

The fatigue strength was evaluated by the following test method, using a fatigue test specimen assumed to be the component for machine structural use made of the steel material.

The fatigue test specimen shown in FIG. 5 was prepared from the simulated intermediate product of the component for machine structural use. The fatigue test specimen was a round bar test specimen, in which the diameter D1 of its parallel portion was 8 mm and the diameter of its grip portion was 12 mm. A fatigue test specimen was prepared by machining from the R/2 position (i.e. the midpoint of the radius) of a cross section perpendicular to the longitudinal direction of the simulated intermediate product of the component for machine structural use. The longitudinal direction of the fatigue test specimen was parallel to the longitudinal direction of the simulated intermediate product. It is common technical knowledge known to those skilled in the art that if the rotating bending fatigue strength of a test specimen before induction hardening is sufficiently high, the rotating bending fatigue strength of the test specimen after induction hardening will also be sufficiently high.

The parallel portion of the fatigue test specimen was subjected to finishing polishing to adjust the surface roughness. Specifically, the center line average roughness (Ra) of the surface was set to 3.0 μm or less and the maximum height (Rmax) was set to 9.0 μm or less in accordance with JIS B 0601:2001.

Using the fatigue test specimen, an Ono-type rotating bending fatigue test was performed at room temperature (23° C.), in an air atmosphere, and under a condition of fully-reversed tension-compression at a rotational speed of 3600 rpm. The fatigue test was performed on a plurality of test specimens by applying different stresses, and the highest stress at which the test specimen did not break after 107 cycles was determined as the fatigue strength (MPa).

If the fatigue strength obtained was 300 MPa or more, it was determined that sufficient fatigue strength was achieved. The results of the fatigue strength evaluation are shown in the “Fatigue Strength” column of Table 2. A fatigue strength of 300 MPa or more was rated as “PASS”, and a fatigue strength of less than 300 MPa was rated as “FAIL”.

[Test Results]

The test results are shown in Tables 1-1 to Table 1-3 and Table 2.

In Tables 1-1 to Table 1-3 and Table 2, the steel materials of Test No, 1 to 38 had appropriate chemical compositions and satisfied formula (1). Furthermore, the production conditions were also appropriate. Therefore, the steel materials of these test numbers satisfied Feature 1 to Feature 4. As a result, no hot working cracks or melting cracks was observed, and excellent hot working cracks resistance and excellent melting cracks resistance were achieved. Furthermore, the maximum cutting speed VL1000 of each steel material was 20 m/min or more, and excellent machinability was achieved. Furthermore, the fatigue strength of each steel material was 300 MPa or more, and the fatigue strength of the component for machine structural use made of the steel material was high.

On the other hand, in Test No. 39 and 40, Fn was less than 0.45. Therefore, the fatigue strength of the component for machine structural use made of these steel materials was low.

In Test No. 41 and 42, Fn was more than 1.05. Therefore, the machinability of the steel materials was low.

In Test No. 43 and 44, the time t0 from the addition of Bi to the end of stirring was 15 minutes or less. Therefore, the number density of coarse Bi particles at the 0.08R depth position D_0.08R was more than 0.25 pieces/mm². As a result, hot working cracks occurred.

In Test No. 45 and 46, the time t0 from the addition of Bi to the end of stirring was 60 minutes or more. Therefore, the number density of fine Bi particles at the 0.08R depth position D_0.08R was less than 15.00 pieces/mm². As a result, melting cracks occurred.

In Test No. 47 and 48, the stirring power density E of the molten steel after the addition of Bi was less than 10 W/t. Therefore, the number density of coarse Bi particles at the 0.08R depth position D_0.08R was more than 0.25 pieces/mm². As a result, hot working cracks occurred.

In Test No. 49 and 50, the stirring power density E of the molten steel after the addition of Bi was more than 100 W/t. Therefore, the number density of fine Bi particles at the 0.08R depth position D_0.08R was less than 15.00 pieces/mm². As a result, melting cracks occurred.

In Test No. 51 and 52, the surface solidification cooling rate during casting was less than 550° C./min. Therefore, the number density of fine Bi particles at the 0.08R depth position D_0.08R was less than 15.00 pieces/mm², and the number density of coarse Bi particles at the 0.08R depth position D_0.08R was more than 0.25 pieces/mm². As a result, hot working cracks and melting cracks occurred.

In Test No. 53 and 54, the internal solidification cooling rate during casting was more than 100° C./min. Therefore, the number density of fine Bi particles at the 0.65R depth position D_0.65R was 15.00 pieces/mm² or more, and the number density of coarse Bi particles at the 0.65R depth position D_0.65R was 0.25 pieces/mm² or less. As a result, the machinability of the steel material was low.

In Test No. 55, the Bi content was too low. Therefore, the number density of fine Bi particles at the 0.08R depth position D_0.08R was less than 15.00 pieces/mm², and the number density of coarse Bi particles at the 0.65R depth position D_0.65R was 0.25 pieces/mm² or less. As a result, melting cracks occurred, and furthermore, the machinability of the steel material was low.

In Test No. 56, the Bi content was too high. Therefore, the number density of coarse Bi particles at the 0.08R depth position D_0.08R was more than 0.25 pieces/mm². As a result, hot working cracks occurred.

The embodiment of the present disclosure has been described above. However, the foregoing embodiment is merely an example for implementing the present disclosure. Accordingly, the present disclosure is not limited to the above embodiment, and the above embodiment can be appropriately modified and implemented within a range that does not deviate from the gist of the present disclosure.

REFERENCE SIGNS LIST

• • 1 Filleted Round Portion
  • 2 Edge of Crankshaft
  • 15 Melting Cracks