BAR STEEL

Description

TECHNICAL FIELD

The present invention relates to a bar steel.

Priority is claimed on Japanese Patent Application No. 2023-001176, filed Jan. 6, 2023, the content of which is incorporated herein by reference.

BACKGROUND ART

In the manufacturing of a steel material such as a bar steel, scale is generated on a surface thereof. Scale deteriorates the surface properties of the steel material or causes surface defects during rolling of the steel material. Scale also protects the steel material from oxygen in the atmosphere and improves corrosion resistance of the steel material.

The scale is removed from the steel material when the steel material is subjected to various processings to form a mechanical structural component. However, it may take up to several months from the completion of manufacturing of the steel material to the start of processing of the steel material. During this period, the steel material is often stored in a high-moisture environment outdoors. In this situation, the scale exerts an effect of improving corrosion resistance of the steel material.

Various studies have been conducted on the scale adhering to the surface of the steel material.

Patent Document 1 discloses a method for manufacturing a Cr-containing bar steel material containing 0.10 to 2.0% of Cr (% means mass %, the same applies hereinafter for the chemical components of steel), the manufacturing method including: a step a of descaling a steel piece after taking out the steel piece from a heating furnace, a step b of hot-rolling the steel piece under an air atmosphere after step a, and a step c of descaling the steel piece after step b, in which the hot-rolling in step b is performed under predetermined conditions.

Patent Document 2 discloses a manufacturing method for a Cr-containing bar steel material having excellent descalability, the manufacturing method including: taking out a steel piece containing 0.10 to 2.00% of Cr (% means mass %, the same applies hereinafter for the chemical component of steel) from a heating furnace, descaling the steel piece, and then hot-rolling the steel piece, in which the steel piece is heated in the heating furnace for 15 minutes or longer in a temperature range of the surface temperature of the steel piece of 1000° C. or higher and 1150° C. or lower, the steel piece at a surface temperature (extraction temperature) in this temperature range is removed from the heating furnace, the steel piece is then rapidly heated at a temperature rising rate of 20° C./min or faster until the surface temperature of the steel piece reaches a temperature range of 1200° C. or higher and 1350° C. or lower from the extraction temperature in an atmosphere of an O₂ concentration of 10 vol % or more and an H₂O concentration of 5 vol % or more and 35 vol % or less, and then descaling is performed.

Patent Document 3 discloses a manufacturing method for a Cr-containing steel having excellent descalability, in which a steel piece including a Cr-containing steel containing Si: 0.05 to 0.4 mass % and Cr: 0.1 to 2.0 mass % with the balance including Fe and inevitable impurities is heat-treated immediately before descaling treatment, the heat treatment temperature is set to 1000 to 1150° C., the holding time at the heat treatment temperature is set to 1 to 30 minutes, and the heat treatment atmosphere contains 25 to 30 vol % of H₂O [water vapor] and 1 to 10 vol % of O₂ with the balance including N₂ and inevitable gas components.

Patent Document 4 discloses a method for manufacturing a high Cr-containing steel material for cold pressing having favorable descaling properties in which in a method of subjecting a steel billet containing Cr: 0.2 to 4.0 mass % to heating and soaking in a heating furnace in a combustion atmosphere, then taking out the heated steel billet from the heating furnace, and hot-rolling the same, the soaking is performed at a temperature of lower than 1100° C. for 10 minutes or longer in an atmosphere in which when the Cr concentration in the steel is designated as X mass %, the water vapor concentration in the heating furnace is (4.47X+17.1) vol. % or more, and a ratio of the water vapor concentration and an oxygen concentration (vol. %) in the heating furnace is adjusted to 10 or more.

Patent Document 5 discloses a steel wire having excellent coiling properties, the steel wire containing C: 0.4 to 0.8 mass %, Si: 1.0 to 2.5 mass %, Mn: 0.2 to 1 mass %, P: more than 0 mass % and 0.05 mass % or less, S: more than 0 mass % and 0.05 mass % or less, and Cr: 0.6 to 2 mass % with the balance being iron and inevitable impurities, in which a proportion of the surface of the steel wire in an iron oxide scale satisfies FeO: 10 to 60 vol %, Fe₂O₃: more than 0 vol % and 15 vol % or less, and balance: Fe₃O₄ and Fe₂SiO₄, an average thickness of the iron oxide scale is 0.3 to 2.0 μm, and an average grain size of the iron oxide scale is 0.2 μm or shorter.

Patent Document 6 discloses a steel wire rod in which an FeO layer including fine crystal grains having random orientation is formed as inner layer scale on a surface of steel containing C: 0.05 to 1.2 mass %, Si: 0.01 to 0.50 mass %, Mn: 0.1 to 1.5 mass %, P: 0.02 mass % or less (inclusive of 0%), S: 0.02 mass % or less (inclusive of 0%), N: 0.005 mass % or less (inclusive of 0%) with the balance including iron and inevitable impurities, an Fe₂SiO₄ layer having a thickness of 0.01 to 1.0 μm is formed in an interface between the FeO layer of the inner layer scale and the steel, and a thickness of the inner layer scale is 1 to 40% of the total scale thickness.

CITATION LIST

Patent Documents

• Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2010-264469
• Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2010-23087
• Patent Document 3: Japanese Unexamined Patent Application, First Publication No. 2010-524
• Patent Document 4: Japanese Unexamined Patent Application, First Publication No. 2009-275285
• Patent Document 5: Japanese Unexamined Patent Application, First Publication No. 2017-115228
• Patent Document 6: Japanese Unexamined Patent Application, First Publication No. 2009-263750

SUMMARY OF INVENTION

Technical Problem

Red rust may occur on the surface of a bar steel during the period from the completion of manufacturing of the bar steel to the start of processing of the bar steel. Red rust reduction is required in bar steels. The present inventors found that scale peeling occurs where red rust occurs.

A method for suppressing scale peeling has not been sufficiently studied in the prior art. For example, in the techniques of Patent Documents 1 to 5, objects thereof were to improve descalability. A method of suppressing scale peeling and a method of improving corrosion resistance of the scale peeling portion are not disclosed at all in these patent documents.

An object of the technique of Patent Document 6 is to provide a steel wire rod in which scale is hardly peeled off during cooling after hot-rolling or during storage and conveyance, and descalability is favorable and mechanical descaling properties are excellent during mechanical descaling. However, Patent Document 6 relates to a steel wire rod. The manufacturing conditions described in Patent Document 6 for suppressing scale peeling of the steel wire rod cannot be applied to the bar steel. In Examples of the technique of Patent Document 6, only the scale peeling rate at the time when the manufacturing of the steel wire rod is completed is evaluated. Whether or not the technique of Patent Document 6 can suppress scale peeling during storage and conveyance of a steel wire rod cannot be determined from Examples of Patent Document 6.

In view of the above circumstances, an object of the present invention is to provide a bar steel capable of suppressing the generation of red rust in a period from the completion of manufacturing to the start of processing.

Solution to Problem

The gist of the present invention is as follows.

• • (1) A bar steel according to an aspect of the present invention includes a matrix and a layer covering a surface of the matrix and containing Si, Cr, Fe, and O, in which the matrix contains, by unit mass %, C: 0.10 to 0.30%, Si: 0.51 to 2.40%, Mn: 0.50 to 1.50%, P: 0.050% or less, S: 0.050% or less, Cr; 0.05 to 2.00%, Al: 0.010 to 0.100%, N: 0.003 to 0.030%, and O: 0.0050% or less, the balance includes iron and impurities, and a Cr/Si mass concentration ratio of the layer is 0.10 or more.
  • (2) In the bar steel according to (1), preferably, the matrix further contains, by unit mass %, one or more of Mo: 0.50% or less, Cu: 0.50% or less, Ni: 0.50% or less, W: 0.500% or less, V: 0.50% or less, Bi: 0.10% or less, Co: 0.500% or less, Nb: 0.10% or less, Ti: 0.20% or less, Ca: 0.0015% or less, Pb: 0.09% or less, Sn: 0.10% or less, and B: 0.007% or less.
  • (3) In the bar steel according to (2), preferably, the matrix contains, by unit mass %, one or more of Bi: 0.10% or less, Pb: 0.09% or less, Sn: 0.10% or less, and Ca: 0.0015% or less.
  • (4) In the bar steel according to (2), preferably, the matrix contains, by unit mass %, one or more of Mo: 0.50% or less, W: 0.500% or less, B: 0.007% or less, Cu: 0.50% or less, Ni: 0.50% or less, and Co: 0.500% or less.
  • (5) In the bar steel according to (2), preferably, the matrix contains, by unit mass %, one or more of Ti: 0.20% or less, Nb: 0.10% or less, and V: 0.50% or less.
  • (6) The bar steel according to any one of (1) to (5), preferably, further includes a scale layer covering the layer.

Advantageous Effects of Invention

According to the present invention, it is possible to provide a bar steel capable of suppressing the generation of red rust in a period from the completion of manufacturing to the start of processing.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 A schematic cross-sectional view of a surface of a bar steel according to the present embodiment.

FIG. 2 A flowchart of a manufacturing method for a bar steel according to the present embodiment.

FIG. 3 A perspective view explaining an analysis position of an iron-based oxide layer of the bar steel.

FIG. 4 A schematic cross-sectional view showing a place where a line analysis region for measuring a Cr/Si mass concentration ratio is arranged.

FIG. 5A A schematic cross-sectional view showing an interface between a matrix and an iron-based carbide layer of the bar steel, and a line analysis region for measuring a Cr/Si mass concentration ratio.

FIG. 5B A schematic cross-sectional view showing the interface between the matrix and the iron-based carbide layer of the bar steel, and the line analysis region for measuring a Cr/Si mass concentration ratio.

DESCRIPTION OF EMBODIMENTS

A bar steel 1 according to an aspect of the present invention includes a matrix 11 and a layer (iron-based oxide layer 12) covering the matrix 11 and containing Si, Cr, Fe, and O, in which the matrix 11 has a predetermined chemical component, and a Cr/Si mass concentration ratio of the iron-based oxide layer 12 is 0.10 or more. First, a technique knowledge for attaining the bar steel according to the present embodiment will be described.

The present inventors have performed micro-observation and composition analysis of an interface between a scale layer 13 and the matrix 11. As a result, it has been found that the layer containing Si, Cr, Fe, and O (iron-based oxide layer 12) is formed at the interface between the scale layer 13 and matrix 11. The present inventors have found that Cr may be enriched in the iron-based oxide layer 12. The present inventors have found that the bar steel 1 in which the iron-based oxide layer 12 with enriched Cris formed has excellent corrosion resistance.

The scale layer 13 is easily peeled off from the bar steel 1, while the iron-based oxide layer 12 is hardly peeled off from the bar steel 1. The present inventors have confirmed that the iron-based oxide layer 12 remains at a portion where the scale layer 13 is peeled off. The present inventors estimate that the iron-based oxide layer 12 in which Cr is enriched enhances corrosion resistance at a portion where the scale layer 13 is peeled off.

The inventors have further conducted studies on a method of enriching Cr in the iron-based oxide layer 12. As a result, the inventors have found that Cr can be enriched in the iron-based oxide layer 12 by setting the Cr amount of a steel piece before hot-rolling within a predetermined range and optimizing the rolling conditions in a rough rolling stage of the hot-rolling and the cooling conditions after the end of the hot-rolling.

Next, a specific composition of the bar steel 1 according to the present embodiment will be described. Hereinafter, the unit “mass %” of the content of the element is simply described as “%”.

The bar steel 1 according to an aspect of the present invention includes at least the matrix 11 and the iron-based oxide layer 12 covering the matrix 11. The matrix 11 is also referred to as a base metal. The matrix 11 is a steel having a chemical component described below. Si and Cr contained in the matrix 11 greatly affect the composition of the iron-based oxide layer 12. Other elements do not directly affect the composition of the iron-based oxide layer 12 and the corrosion resistance of the bar steel, but are used to enhance the mechanical properties and the like of bar steel 1.

(C: 0.10 to 0.30%)

C increases the strength of steel. When C is insufficient, the hardness, tensile strength, and the like of the bar steel are impaired. The C content is set to 0.10% or more. The C content may be 0.12% or more, 0.14% or more, or 0.16% or more.

When the C content is excessive, the hardness of the bar steel becomes excessive, and the machinability of the bar steel is impaired. Therefore, the C content is set to 0.30% or less. The C content may be 0.28% or less, 0.26% or less, or 0.25% or less.

(Si: 0.51 to 2.40%)

Si forms the iron-based oxide layer 12 covering the matrix 11 and enhances the corrosion resistance of the bar steel 1. Si also increases the strength of steel. Therefore, a Si content is set to 0.51% or more. The Si content may be 0.60% or more, 0.80% or more, or 1.00% or more.

When the Si content is excessive, the hardness of the bar steel becomes excessive, and the cutting property, machinability, and manufacturability of the bar steel are impaired. Therefore, the Si content is set to 2.40% or less. The Si content may be set to 2.00% or less, 1.80% or less, or 1.60% or less.

(Mn: 0.50 to 1.50%)

Mn increases the strength of steel. When Mn is insufficient, the hardness, tensile strength, and the like of the bar steel are impaired. Therefore, a Mn content is set to 0.50% or more. The Mn content may be 0.55% or more, 0.60% or more, or 0.70% or more.

An excessive amount of Mn may interfere with the carburizing treatment of the bar steel. When the bar steel is used as a material for a carburized part, an excessive amount of Mn may increase the amount of residual austenite on the surface of the part after carburizing treatment to reduce the surface hardness of the part, thereby reducing the fatigue strength of the part. Therefore, the Mn content is set to 1.50% or less. The Mn content may be 1.45% or less, 1.40% or less, or 1.35% or less.

(P: 0.050% or Less)

P is a component mixed into the matrix as an impurity. When a P content is excessive, the mechanical properties of the steel material are impaired. Therefore, the P content is set to 0.050% or less. The P content may be 0.040% or less, 0.030% or less, or 0.020% or less.

P is not required to improve the corrosion resistance of the bar steel. Therefore, the lower the P content, the more preferable, and the P content may be 0%. However, when the P content is lower, the refining cost increases. In order to reduce the refining cost, the P content may be set to more than 0%, 0.001% or more, or 0.005% or more.

(S: 0.050% or Less)

S is a component mixed into the matrix as an impurity. When a S content is excessive, coarse inclusions are generated in the matrix, and the mechanical properties of the steel material are impaired. Therefore, the S content is set to 0.050% or less. The S content may be 0.040% or less, 0.030% or less, or 0.020% or less.

S is not required to improve the corrosion resistance of the bar steel. Therefore, the lower the S content, the more preferable, and the S content may be 0%. However, when the S content is lower, the refining cost increases. In order to reduce the refining cost, the S content may be set to more than 0%, 0.001% or more, or 0.005% or more.

(Cr: 0.05 to 2.00%)

Cr is enriched in the iron-based oxide layer 12 covering the matrix 11 to enhance the corrosion resistance of the iron-based oxide layer 12. Thereby, Cr dramatically increases the corrosion resistance of the bar steel 1. Cr increases the tempering softening resistance of the steel and improves the surface fatigue strength of the steel. Therefore, a Cr content is set to 0.05% or more. The Cr content may be set to 0.10% or more, 0.20% or more, 0.50% or more, or 1.00% or more.

When the Cr content is excessive, while the above-described effect is saturated, the hardness of the steel material is increased, and the manufacturing cost is increased. Therefore, the Cr content is set to 2.00% or less. The Cr content may be set to 1.80% or less, 1.50% or less, 1.20% or less, 0.50% or less, or 0.30% or less.

(Al: 0.010 to 0.100%)

Al forms fine inclusions and refines γ grains. Therefore, an Al content is set to 0.010% or more. The Al content may be set to 0.015% or more or 0.020% or more.

When the Al content is excessive, coarse inclusions are generated in the matrix, and the mechanical properties of the bar steel are impaired. Therefore, the Al content is set to 0.100% or less. The Al content may be set to 0.090% or less, 0.080% or less, 0.070% or less, 0.050% or less, 0.040% or less, or 0.030% or less.

(N: 0.003 to 0.030%)

N refines the grains of the matrix. Therefore, a N content is set to 0.003% or more. The N content may be set to 0.005% or more, 0.010% or more, or 0.015% or more.

When the N content is excessive, the high-temperature ductility of the matrix is impaired, and the manufacture yield of the bar steel is reduced. Therefore, the N content is set to 0.030% or less. The N content may be set to 0.028% or less, 0.025% or less, or 0.020% or less.

(O: 0.0050% or Less)

O is a component mixed into the matrix as an impurity. When an O content is excessive, coarse oxides are generated in the matrix, and the mechanical properties of the steel material are impaired. Therefore, the O content is set to 0.0050% or less. The O content may be 0.0040% or less, 0.0030% or less, or 0.0020% or less.

O is not required to improve the corrosion resistance of the bar steel. Therefore, the lower the O content is, the more preferable it is, and the O content may be 0%. However, when the O content is lower, the refining cost increases. In order to reduce the refining cost, the O content may be set to more than 0%, 0.0001% or more, or 0.0005% or more.

(Balance: Iron and Impurities)

The balance of the chemical component of the matrix is iron and impurities. The impurity means, for example, a raw material such as ore or scrap, or a component mixed due to various factors of a manufacture step when a steel material is industrially manufactured, and is acceptable within a range not adversely affecting the bar steel according to the present embodiment.

The matrix may further contain one or more of the following optional elements. Thereby, the characteristics of the bar steel are further improved. However, the following optional elements are not essential for solving the problem of the bar steel according to the present embodiment. Therefore, the lower limit of the following optional elements may be 0%.

(Mo: Preferably More than 0% and 0.50% or Less)

Mo enhances the hardenability of the steel. Therefore, a Mo content may be set to more than 0%, 0.10% or more, or 0.15% or more.

When the Mo content is 0.50% or less, excessive hardening of the steel can be avoided, and the workability of the bar steel can be improved. Therefore, the Mo content may be set to 0.50% or less, 0.40% or less, or 0.30% or less.

(Cu: Preferably More than 0% and 0.50% or Less)

Cu enhances the hardenability of the steel. Therefore, a Cu content may be set to more than 0%, 0.10% or more, or 0.15% or more.

When the Cu content is 0.50% or less, excessive hardening of the steel can be avoided, and the workability of the bar steel can be improved. Therefore, the Cu content may be set to 0.50% or less, 0.40% or less, or 0.30% or less.

(Ni: Preferably More than 0% and 0.50% or Less)

Ni enhances the hardenability of the steel. Therefore, a Ni content may be set to more than 0%, 0.10% or more, or 0.15% or more.

When the Ni content is 0.50% or less, excessive hardening of the steel can be avoided, and the workability of the bar steel can be improved. Therefore, the Ni content may be set to 0.50% or less, 0.40% or less, or 0.30% or less.

(W: Preferably More than 0% and 0.500% or Less)

W enhances the hardenability of the steel. Therefore, a W content may be set to more than 0%, 0.100% or more, or 0.150% or more.

When the W content is 0.500% or less, excessive hardening of the steel can be avoided, and the workability of the bar steel can be improved. Therefore, the W content may be set to 0.500% or less, 0.400% or less, or 0.300% or less.

(V: Preferably More than 0% and 0.50% or Less)

V forms fine carbides, nitrides, and/or carbonitrides. Thereby, V suppresses grain growth, contributes to fine homogenization of the microstructure, and improves the fatigue properties of the bar steel. Therefore, a V content may be set to more than 0%, 0.10% or more, or 0.15% or more.

When the V content is 0.50% or less, the generation of a hard coarse carbide can be suppressed, and the machinability of the bar steel can be secured. Therefore, the V content may be set to 0.50% or less, 0.40% or less, or 0.30% or less.

(Bi: Preferably More than 0% and 0.10% or Less)

Bi improves the machinability of the steel. Therefore, a B content may be set to more than 0%, 0.01% or more, or 0.03% or more.

When the Bi content is 0.10% or less, the occurrence of cracks during rolling of the bar steel can be suppressed. Therefore, the B content may be set to 0.10% or less, 0.08% or less, or 0.05% or less.

(Co: Preferably More than 0% and 0.500% or Less)

Co enhances the hardenability of the steel. Therefore, a Co content may be set to more than 0%, 0.100% or more, or 0.150% or more.

When the Co content is 0.500% or less, excessive hardening of the steel can be avoided, and the workability of the bar steel can be improved. Therefore, the Co content may be set to 0.500% or less, 0.400% or less, or 0.300% or less.

(Nb: Preferably More than 0% and 0.10% or Less)

Nb forms fine carbides, nitrides, and/or carbonitrides. Thereby, Nb suppresses grain growth, contributes to fine homogenization of the microstructure, and improves the fatigue properties of the bar steel. Therefore, a Nb content may be set to more than 0%, 0.01% or more, or 0.03% or more.

When the Nb content is 0.10% or less, the generation of a hard coarse carbide can be suppressed, and the machinability of the bar steel can be secured. Therefore, the Nb content may be set to 0.10% or less, 0.08% or less, or 0.05% or less.

(Ti: Preferably More than 0% and 0.20% or Less)

Ti forms fine carbides, nitrides, and/or carbonitrides. Thereby, Ti suppresses grain growth, contributes to fine homogenization of the microstructure, and improves the fatigue properties of the bar steel. Therefore, a Ti content may be set to more than 0%, 0.01% or more, or 0.03% or more.

When the Ti content is 0.20% or less, the generation of a hard coarse carbide can be suppressed, and the machinability of the bar steel can be secured. Therefore, the Ti content may be set to 0.20% or less, 0.18% or less, or 0.15% or less.

(Ca: Preferably More than 0% and 0.0015% or Less)

Ca improves the machinability of the steel. Therefore, a Ca content may be set to more than 0%, 0.0002% or more, or 0.0005% or more.

When the Ca content is 0.0015% or less, the fatigue strength of the steel can be improved. Therefore, the Ca content may be set to 0.0015% or less, 0.0012% or less, or 0.0010% or less.

(Pb: Preferably More than 0% and 0.09% or Less)

Pb improves the machinability of the steel. Therefore, a Pb content may be set to more than 0%, 0.01% or more, or 0.03% or more.

When the Pb content is 0.09% or less, the forgeability of the steel can be improved. Therefore, the Pb content may be set to 0.09% or less, 0.06% or less, or 0.05% or less.

(Sn: Preferably More than 0% and 0.10% or Less)

Sn improves the machinability of the steel. Therefore, a Sn content may be set to more than 0%, 0.01% or more, or 0.03% or more.

When the Sn content is 0.10% or more, hot forging cracks occur. Therefore, the Sn content may be set to 0.10% or less, 0.08% or less, or 0.05% or less.

(B: Preferably More than 0% and 0.007% or Less)

B enhances the hardenability of the steel. Therefore, a B content may be set to more than 0%, 0.001% or more, or 0.002% or more:

When the B content is 0.007% or less, saturation of the above-described effect can be avoided, and the cost of the raw material of the steel can be reduced, Therefore, the B content may be set to 0.007% or less, 0.006% or less, or 0.005% or less.

A method of measuring the chemical component of the matrix is not particularly limited. For example, an analysis method or the like defined in JIS G 1201:2014 can be used to specify the chemical component of the matrix.

(Iron-Based Oxide Layer 12)

The layer containing Si, Cr, Fe, and O (iron-based oxide layer 12) is formed to cover the surface of the matrix 11. Fe and Si contained in the iron-based oxide layer 12 are derived from Fe and Si contained in the matrix 11. The bar steel 1 is manufactured by hot bar steel rolling of slabs. At the time of cooling after the hot-rolling, the surface of the steel is oxidized to form the iron-based oxide layer 12.

Takeda et al., “Influence of Si Concentration on the Structure and Adhesion of Primary Scales of Si Containing Steel” (Kobe Steel Engineering Reports Vol. 55 No. 1 (April 2005) p31) describes that “In high-temperature oxidation of steel, Si is known to affect the growth rate of scale and its properties. Si in the steel material forms SiO₂ on the alloy surface and then reacts with FeO to form Fe₂SiO₄ (fayalite)”. Since the iron-based oxide layer 12 of the bar steel 1 according to the present embodiment is formed on the surface of the matrix 11 containing Si, it is estimated that the iron-based oxide layer 12 is a layer containing fayalite. However, since the iron-based oxide layer 12 of the bar steel 1 according to the present embodiment contains Cr, it is not the same as the scale described in this document.

(Cr/Si Mass Concentration Ratio of Iron-Based Oxide Layer 12: 0.10 or More)

The iron-based oxide layer 12 of the bar steel 1 according to present embodiment contains Cr. Cr improves corrosion resistance of the iron-based oxide layer 12, prevents corrosion of the matrix 11 covered with the iron-based oxide layer 12, and suppresses occurrence of red rust, Like Fe and Si, Cr contained in the iron-based oxide layer 12 is also derived from Cr contained in the matrix 11.

In the iron-based oxide layer 12 of the bar steel 1 according to the present embodiment, a proportion of a Cr concentration and a Si concentration is defined. Specifically, a Cr/Si mass concentration ratio of the iron-based oxide layer 12 is 0.10 or more. The Cr/Si mass concentration ratio of the iron-based oxide layer 12 is an enrichment index of Cr in the iron-based oxide layer 12. By setting the Cr/Si mass concentration ratio of the iron-based oxide layer 12 to 0.10 or more, the corrosion resistance of the bar steel 1 can be secured. The Cr/Si mass concentration ratio of the iron-based oxide layer 12 may be set to 0.50 or more, 1.00 or more, or 5.00 or more.

The higher the Cr/Si mass concentration ratio of the iron-based oxide layer 12 is, the more preferable it is. Therefore, the upper limit of the Cr/Si mass concentration ratio of the iron-based oxide layer 12 is not particularly limited. According to the results, the Cr/Si mass concentration ratio in the iron-based oxide layer does not necessarily coincide with the Cr/Si mass concentration ratio of the matrix. Considering the chemical component of the matrix described above, the Cr/Si mass concentration ratio of the matrix is 3.92 at the maximum, while the Cr/Si mass concentration ratio of the iron-based oxide layer 12 may exceed 24. In consideration of the upper limit of Cr of the matrix, the Cr/Si mass concentration ratio of the iron-based oxide layer 12 may be set to 40 or less, 35 or less, or 30 or less.

The Cr/Si mass concentration ratio of the iron-based oxide layer is specified by EPMA component analysis according to the following procedure.

First, the bar steel is embedded in a resin for cross-section observation 5 and cut. The cut surface is preferably perpendicular to the longitudinal direction of the bar steel 1. However, when the iron-based oxide layer 12 is thin, in order to increase the apparent thickness of the iron-based oxide layer 12 at the cut surface, an angle formed by the cut surface and the longitudinal direction of the bar steel 1 may be set to less than 90 degrees. Next, the cut surface is polished. The cut surface is observed using EPMA to specify the position of the iron-based oxide layer 12.

Next, the Si concentration, the Fe concentration, the Cr concentration, and Mn concentration of the iron-based oxide layer 12 are linearly analyzed. The procedure of the line analysis will be described with reference to FIGS. 3, 4, 5A, and 5B. FIG. 3 is a perspective view of the bar steel 1. A region 1C shown in FIG. 3 includes the surface of the bar steel 1 and the vicinity thereof in the cross section of the bar steel 1. The region 1C is enlarged and observed to set a region to be subjected to line analysis. FIG. 4 is an enlarged view of the region 1C. Each of regions 2 shown in FIG. 4 is subjected to line analysis. FIGS. 5A and 5B are enlarged views of the regions 2. FIG. 5A is an enlarged view of a portion to which the scale layer 13 is not attached, and FIG. 5B is an enlarged view of a portion to which the scale layer 13 is attached. In FIGS. 5A and 5B, Reference Number 3 denotes a starting point of line analysis, and Reference Number 4 denotes a line analysis region. Reference Number 5 denotes a resin for cross-section observation.

As shown in FIGS. 5A and 5B, the extending direction of the line analysis region is perpendicular to an interface between the iron-based oxide layer 12 and the matrix 11. Fine irregularities visible by microscopic observation occur at the interface between the iron-based oxide layer 12 and the matrix 11. Therefore, the direction perpendicular to the interface between the iron-based oxide layer 12 and the matrix 11 does not necessarily coincide with the direction perpendicular to the surface of the bar steel 1, which is specified by observing the bar steel 1 with the naked eye.

The line analysis region includes at least the iron-based oxide layer 12 within a range of 10 μm from the interface between the matrix 11 and the iron-based oxide layer 12. By this line analysis, the position where the Si concentration is maximized in the iron-based oxide layer 12 within 10 μm from the interface between the matrix 11 and the iron-based oxide layer 12 is specified. The Si concentration and the Cr concentration at this position are measured, and a value obtained by dividing the Cr concentration by the Si concentration is calculated. The line analysis conditions of the iron-based oxide layer 12 are as follows.

• • Measurement magnification: 1000 times
  • Acceleration voltage: 15.0 kV
  • Irradiation current: 4.993×10⁻⁸ A
  • Measurement interval: 0.20 μm
  • Number of measurement points: 500 (line analysis length: 100 μm)

Measurement of the Cr/Si mass concentration ratio by the above-described method is performed at five locations. As shown in FIG. 4, the interval between the regions 2 where the line analysis is performed is set to 50 μm. An average value of Cr/Si mass concentration ratios specified in each of the regions 2 is regarded as the Cr/Si mass concentration ratio of the iron-based oxide layer 12 of the bar steel 1.

The composition other than the Cr/Si mass concentration ratio of the iron-based oxide layer 12, for example, the thickness is not particularly limited. The iron-based oxide layer 12 has a large number of irregularities, and has a non-uniform thickness. Although the iron-based oxide layer 12 having a uniform thickness is shown in FIG. 1, irregularities are omitted here for convenience of description. It is extremely difficult to quantitatively evaluate the thickness of the iron-based oxide layer 12. When the present inventors observed the cross sections of various iron-based oxide layers 12, it was qualitatively confirmed that the thickness of the iron-based oxide layer 12 tended to be proportional to the Si content of the matrix 11. When the iron-based oxide layer 12 is extremely thin, Cr enriched in the iron-based oxide layer 12 is not detected, and the measurement value of the Cr/Si mass concentration ratio does not become 0.10 or more. In other words, the iron-based oxide layer 12 in which the measurement value of the Cr/Si mass concentration ratio is 0.10 or more naturally has a thickness sufficient for securing corrosion resistance of the bar steel 1. As long as the iron-based oxide layer 12 is not removed and the Si content of the matrix 11 is in the above-described range, the bar steel 1 necessarily includes the iron-based oxide layer 12 having a thickness sufficient for ensuring corrosion resistance.

The component of the iron-based oxide layer 12 is also not particularly limited. The component of the iron-based oxide layer 12 usually has a value corresponding to the component of the matrix 11. This is because the iron-based oxide layer 12 is an oxide layer generated by oxidation of the surface of the matrix 11. Therefore, as long as the component of the matrix 11 is within the above range, the component of the iron-based oxide layer 12 naturally contains Fe and Si as main components.

(Scale Layer 13)

The bar steel 1 may further include the scale layer 13 covering the iron-based oxide layer 12. Since the scale layer 13 suppresses corrosion of the matrix 11, the corrosion resistance of the bar steel 1 is further improved. However, the scale layer 13 easily falls off from the bar steel 1. As shown in FIG. 1, the iron-based oxide layer 12 remains even where the scale layer 13 has fallen off. In the bar steel 1 according to the present embodiment, since the corrosion resistance of the iron-based oxide layer 12 is enhanced, the corrosion of the matrix 11 is suppressed even where the scale layer 13 has fallen off. Therefore, the bar steel 1 may not have the scale layer 13.

(Other Compositions of Bar Steel)

Other compositions of the bar steel according to the present embodiment are not particularly limited. For example, the cross-sectional shape of the bar steel is not particularly limited. Examples of the cross-sectional shape of the bar steel include a circle, a square, a rectangle, a hexagon, and an octagon. The size of the bar steel is also not particularly limited. For example, the diameter or equivalent circle diameter of the bar steel may be set to 18 mm or longer, 20 mm or longer, or 30 mm or longer. The diameter or equivalent circle diameter of the bar steel may be set to 180 mm or shorter, 150 mm or shorter, or 120 mm or shorter. The bar steel may be a bar-in-coil, that is, a steel material obtained by winding a hot-rolled bar steel in a coil shape as it is long.

(Manufacturing Method for Bar Steel)

Next, a manufacturing method for a bar steel according to another aspect of the present invention will be described. According to the manufacturing method described below, a bar steel having an iron-based oxide layer in which Cris enriched can be obtained. However, it should be noted that the bar steel satisfying the requirements described above is regarded as the bar steel according to the present embodiment regardless of the manufacturing method for the bar steel. That is, the range of the bar steel according to the present embodiment described above is not limited by the manufacturing method for a bar steel described below.

As shown in FIG. 2, the manufacturing method for a bar steel according to the present embodiment includes;

• • (S1) a step of subjecting a steel piece to a first descaling;
  • (S2) a step of rough rolling the steel piece;
  • (S3) a step of subjecting the steel piece to a second descaling;
  • (S4) a step of finish rolling the steel piece to obtain a bar steel; and
  • (S5) a step of cooling the bar steel,
  • in which the steel piece contains, by unit mass %, C: 0.10 to 0.30%, Si: 0.51 to 2.40%, Mn: 0.50 to 1.50%, P: 0.050% or less, S: 0.050% or less, Cr: 0.05 to 2.00%, Al: 0.010 to 0.100%, N: 0.003 to 0.030%, and O: 0.0050% or less, the balance includes iron and impurities, the number of passes of rough rolling is set to 5 or more, a rolling reduction in all passes of the rough rolling is set to 10% or more and 30% or less, a rolling temperature in all passes of the rough rolling is set to 950° C. or higher and 1100° C. or lower, and in the cooling, an average cooling rate in a range from a finish rolling finishing temperature to 500° C. is set to 1.0° C./s or slower.

First, a technique idea of the manufacturing conditions of the bar steel according to the present embodiment will be described. In order to form an iron-based oxide layer in which Cr is sufficiently enriched, it is necessary to apply a manufacturing method that satisfies the following two requirements to a steel piece.

• • (A) The steel piece immediately before finish rolling has been sufficiently descaled.
  • (B) After completion of the finish rolling, the bar steel is slowly cooled in a temperature range in which the iron-based oxide layer is formed.
  • (A) When the steel piece immediately before finish rolling has not been sufficiently descaled, the Cr/Si mass concentration ratio of the iron-based oxide layer of the bar steel is not sufficiently increased. This is presumed to be because the oxide layer present on the surface of the steel piece before finish rolling hinders the enrichment of Cr at the time of forming the iron-based oxide layer after finish rolling.

The important condition for sufficiently descaling the steel piece immediately before finish rolling is not the descaling condition but the condition of rough rolling before finish rolling. The temperature and rolling reduction during rough rolling affect the denseness of the scale formed on the surface of the steel piece after rough rolling. When the oxide layer formed after rough rolling is dense, the oxide layer cannot be sufficiently removed even by descaling (second descaling) after rough rolling.

• • (B) When the cooling rate after completion of the finish rolling is too high, the Cr/Si mass concentration ratio of the iron-based oxide layer of the bar steel cannot be sufficiently increased. This is because the growth of the iron-based oxide layer stops before Cr is sufficiently enriched in the iron-based oxide layer.

Next, specific contents of the manufacturing conditions of the bar steel according to the present embodiment will be described in detail.

(S1 First Descaling)

In the manufacturing method for a bar steel according to the present embodiment, first, the steel piece is heated, and then the steel piece is descaled. The heating of the steel piece is carried out in order to bring the steel piece to a temperature suitable for rough rolling. Descaling is removing an oxide layer (for example, scale or the like) on the surface of the steel piece generated by heating by injection of high-pressure water. To distinguish from descaling of a steel piece after rough rolling, descaling of a steel piece before rough rolling is referred to as first descaling.

The chemical component of the steel piece is identical to the chemical component of the matrix of the bar steel. Therefore, the chemical component of the steel piece is within the range of the chemical component of the matrix described above. A manufacturing method for a steel piece is not particularly limited. A steel piece manufactured by ordinary continuous casting can be used in the manufacturing method for a bar steel according to the present embodiment.

In the first descaling, the surface pressure of water injected to the steel piece is set to be in a range of 5 to 30 MPa. When the surface pressure is too high, the temperature of the steel material is excessively lowered. In this case, a large amount of defects are generated on the surface of the steel material in the rough rolling performed subsequently to the first descaling. As a result, the quality of the bar steel is deteriorated, or the bar steel cannot be manufactured. When the surface pressure is too low, removal of coarse scale generated during heating before descaling becomes insufficient.

(S2 Rough Rolling)

The descaled steel piece is hot-rolled. The hot-rolling includes rough rolling and finish rolling. The rough rolling is rolling for bringing the shape of the steel piece close to an intended bar steel shape. The finish rolling is rolling for matching the shape of the roughly rolled steel piece with the intended bar steel shape. Normal hot-rolling equipment includes, as a separate body, rough equipment for rough rolling and rolling equipment for finish rolling.

In the rough rolling, it is necessary to set the number of passes to 5 or more. It is necessary to set the rolling reduction to 10% or more and 30% or less and the rolling temperature to 950° C. or higher and 1100° C. or lower in all passes. When these requirements are not satisfied, a dense oxide layer, which cannot be completely removed by descaling, is formed on the surface of the steel piece after rough rolling, and this oxide layer is considered to prevent the enrichment of Cr in the fayalite. In the manufacturing method according to the present embodiment, the formation of a dense oxide layer is suppressed, and the Cr concentration of the iron-based oxide layer is increased.

(S3 Second Descaling)

After the rough rolling, the steel piece is descaled again. To distinguish from descaling of a steel piece before rough rolling, descaling of a steel piece after rough rolling is referred to as second descaling.

In the second descaling, the surface pressure of water injected to the steel piece is set to be in a range of 2.0 to 5.0 MPa. When the surface pressure is too high, the temperature of the steel piece is excessively lowered. As a result, the time during which Cris enriched in the iron-based carbide is insufficient, and the Cr concentration in the iron-based carbide is insufficient. When the surface pressure is too low, an intended iron-based carbide cannot be obtained.

Even when the surface pressure and the injection time in the second descaling are appropriate, the oxide layer that adversely affects the iron-based carbide is not sufficiently removed when the condition of the rough rolling is inappropriate.

(S4 Finish Rolling)

After the second descaling, the steel piece is finish-rolled. The rolling reduction and the finish rolling temperature in the finish rolling are not particularly limited, and can be appropriately set according to the chemical component of the steel piece and the shape of the bar steel. A bar steel having a predetermined shape is obtained by finish rolling. For example, the finish rolling end temperature may be set within a range of 850 to 1000° C.

(S5 Cooling)

After the finish rolling, the bar steel is cooled. Here, the average cooling rate in a range from a finish rolling finishing temperature to 500° C. is set to 1.0° C./s or slower. The average cooling rate in a range from a finish rolling finishing temperature to 500° C. is a value obtained by dividing the difference between the finish rolling finishing temperature and 500° C. by the time required for the surface temperature of the bar steel to decrease from the finish rolling finishing temperature to 500° C.

In the temperature range from the finish rolling finishing temperature to 500° C., an iron-based oxide layer is formed on the surface of the bar steel. In this temperature range, by slowly cooling the bar steel at an average cooling rate of 1.0° C./s or slower, Cr contained in the matrix of the bar steel is transferred to the iron-based oxide layer, and the Cr/Si mass concentration ratio of the iron-based oxide layer is increased.

EXAMPLES

The effect of one aspect of the present invention is described more specifically with reference to examples. However, the conditions in the examples are merely one condition example adopted to confirm the operability and effects of the present invention. The present invention is not limited to this one condition example. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

Steel pieces having the chemical components shown in Tables 1A and 1B were prepared. A bar steel was manufactured by applying any manufacturing method of the following patterns 1 to 5 to these steel pieces.

(Pattern 1)

The steel piece was subjected to first descaling, rough rolling, second descaling, finish rolling, and cooling. The rough rolling temperature was set to 1020° C., and the finish rolling temperature was set to 980° C., In the rough rolling, rolling with a rolling reduction of 10% or more and 30% or less was performed for five passes or more. After the finish rolling, the average cooling rate in a range from a finish rolling finishing temperature to 500° C. was set to 1.0° C./s or slower.

(Pattern 2)

The steel piece was subjected to first descaling, rough rolling, second descaling, finish rolling, and cooling. The rough rolling temperature was set to 1080° C., and the finish rolling temperature was set to 870° C. In the rough rolling, the rolling reduction per pass was set to 5%. Instead, the number of passes was increased from that in pattern 1 to be set to 20%. After the finish rolling, the average cooling rate in a range from a finish rolling finishing temperature to 500° C. was set to 1.0° C./s or slower.

(Pattern 3)

The steel piece was subjected to first descaling, rough rolling, second descaling, finish rolling, and cooling. The rough rolling temperature was set to 1020° C., and the finish rolling temperature was set to 1000° C. In the rough rolling, the rolling reduction per pass was set to 30 to 40%. Instead, the number of passes was decreased from that in pattern 1 to be set to 3. After the finish rolling, the average cooling rate in a range from a finish rolling finishing temperature to 500° C. was set to 1.0° C./s or slower.

(Pattern 4)

The steel piece was subjected to first descaling, rough rolling, finish rolling, and cooling. The rough rolling temperature was set to 1020° C., and the finish rolling temperature was set to 1000° C. In the rough rolling, the rolling reduction per pass was set to 20%, and the number of passes was set to 5. On the other hand, the second descaling after the rough rolling was omitted. After the finish rolling, the average cooling rate in a range from a finish rolling finishing temperature to 500° C. was set to 1.0° C./s or slower.

(Pattern 5)

The steel piece was subjected to first descaling, rough rolling, second descaling, finish rolling, and cooling. The rough rolling temperature was set to 1020° C., and the finish rolling temperature was set to 980° C. In the rough rolling, rolling with a rolling reduction of 10% or more was performed for five passes or more. However, after the finish rolling, the average cooling rate in a range from a finish rolling finishing temperature to 500° C. was set to 10° C./s or faster.

(Pattern 6)

The steel piece was subjected to rough rolling, second descaling, finish rolling, and cooling without performing first descaling. The rough rolling temperature was set to 1020° C., and the finish rolling temperature was set to 980° C. In the rough rolling, rolling with a rolling reduction of 10% or more was performed for five passes or more. After the finish rolling, the average cooling rate in a range from a finish rolling finishing temperature to 500° C. was set to 1.0° C./s or slower.

(Pattern 7)

The steel piece was subjected to first descaling and rough rolling and was subjected to finish rolling and cooling without performing second descaling. The rough rolling temperature was set to 1020° C., and the finish rolling temperature was set to 980° C. In the rough rolling, rolling with a rolling reduction of 10% or more was performed for five passes or more. After the finish rolling, the average cooling rate in a range from a finish rolling finishing temperature to 500° C. was set to 1.0° C./s or slower.

The Cr/Si mass concentration ratio of the iron-based oxide layer of the bar steel obtained by the above-described procedure was identified by the above-described procedure and described in Table 2. When the Cr/Si mass concentration ratio of the iron-based oxide layer cannot be measured because the iron-based oxide layer was not formed or the iron-based oxide layer was very thin, the symbol “−” is described in Table 2.

For reference, the thickness of the iron-based oxide layer of the bar steel having a Cr/Si mass concentration ratio of 0.10 or more was measured. In the line analysis for measuring the Cr/Si mass concentration ratio, a region where a Si concentration of twice or more the Si concentration of the matrix was detected was estimated as an iron-based oxide layer, and the thickness of the region was measured. In the bar steel having a Cr/Si mass concentration ratio of 0.10 or more, the thickness of the region was approximately in a range of 0.4 μm to 5.0 μm.

The rust resistance of the bar steel obtained by the above-described procedure was evaluated. A test for evaluating rust resistance was a wet test described in JIS K 2246:2018. Conditions of the wet test were as follows.

• • Test temperature: 50° C.
  • Relative humidity: 95%
  • Test time: 240 hours
  • Test piece shape: cylindrical shape having a diameter of 20 to 80 mm and a length of 50 mm

The success or failure of the rust resistance was determined based on the rusting state of the surface of the test piece. A sample having a rust area fraction of 10% or less was determined to have excellent rust resistance, and was evaluated as “GOOD” in Table 2. The evaluation results of rust resistance are described in Table 2.

	TABLE 1A
	Chemical component (mass %)※1

Steel	C	Si	Mn	P	S	Cr	Al	N	O	Remark

1	0.19	1.03	0.67	0.008	0.009	0.82	0.035	0.015	0.0008	Present
										Invention
										Example
2	0.22	1.21	0.52	0.015	0.010	0.61	0.044	0.018	0.0010	Present
										Invention
										Example
3	0.28	0.88	1.01	0.011	0.005	0.25	0.021	0.005	0.0007	Present
										Invention
										Example
4	0.14	1.72	0.62	0.012	0.008	0.28	0.028	0.011	0.0011	Present
										Invention
										Example
5	0.17	2.10	0.82	0.005	0.007	1.25	0.055	0.014	0.0008	Present
										Invention
										Example
6	0.23	0.59	1.30	0.008	0.011	0.38	0.032	0.013	0.0013	Present
										Invention
										Example
7	0.16	1.56	1.41	0.007	0.015	0.44	0.041	0.010	0.0007	Present
										Invention
										Example
8	0.20	1.10	0.60	0.005	0.008	0.33	0.062	0.021	0.0006	Present
										Invention
										Example
9	0.19	1.39	0.92	0.008	0.033	0.55	0.031	0.016	0.0010	Present
										Invention
										Example
10	0.20	1.21	1.36	0.007	0.011	0.09	0.051	0.013	0.0008	Present
										Invention
										Example
11	0.20	0.60	0.55	0.009	0.018	1.54	0.070	0.021	0.0015	Present
										Invention
										Example
12	0.21	1.38	1.15	0.018	0.014	0.26	0.031	0.021	0.0009	Present
										Invention
										Example
13	0.19	0.92	0.90	0.014	0.011	0.38	0.044	0.028	0.0013	Present
										Invention
										Example
14	0.20	1.04	1.20	0.006	0.006	0.27	0.030	0.014	0.0006	Present
										Invention
										Example
15	0.19	0.70	0.82	0.013	0.008	0.03	0.055	0.022	0.0007	Comparative
										Example
16	0.21	0.10	1.05	0.006	0.012	0.51	0.042	0.017	0.0013	Comparative
										Example
※1The balance of the chemical component indicates Fe and impurities.
※3The underline indicates that a numerical value is outside the range of the present invention.

	TABLE 1B
	Chemical component (mass %)※1

Steel

Mo

Cu

Ni

W

V

Bi

Co

Nb

Ti

Ca

Pb

Sn

B

Remark

1	0.28													Present
														Invention
														Example
2	0.15							0.04						Present
														Invention
														Example
3														Present
														Invention
														Example
4		0.11	0.100											Present
														Invention
														Example
5						0.020					0.04			Present
														Invention
														Example
6												0.01		Present
														Invention
														Example
7										0.0010				Present
														Invention
														Example
8														Present
														Invention
														Example
9	0.20				0.10		0.034							Present
														Invention
														Example
10		0.08							0.10					Present
														Invention
														Example
11														Present
														Invention
														Example
12	0.15			0.100										Present
														Invention
														Example
13							0.180							Present
														Invention
														Example
14													0.002	Present
														Invention
														Example
15														Comparative
														Example
16														Comparative
														Example
※1The balance of the chemical component indicates Fe and impurities.
※2Blanks indicate that no alloying element is intentionally added.

TABLE 2
		Rolling		Rust
Test No.	Steel	pattern	Cr/Si	resistance	Remark

1	1	1	18.5	GOOD	Present Invention
					Example
2	2	1	11.2	GOOD	Present Invention
					Example
3	3	1	7.1	GOOD	Present Invention
					Example
4	4	1	6.5	GOOD	Present Invention
					Example
5	5	1	20.6	GOOD	Present Invention
					Example
6	6	1	10.5	GOOD	Present Invention
					Example
7	7	1	11.6	GOOD	Present Invention
					Example
8	8	1	8.9	GOOD	Present Invention
					Example
9	9	1	18.2	GOOD	Present Invention
					Example
10	10	1	2.1	GOOD	Present Invention
					Example
11	11	1	17.6	GOOD	Present Invention
					Example
12	12	1	3.5	GOOD	Present Invention
					Example
13	13	1	5.9	GOOD	Present Invention
					Example
14	14	1	1.6	GOOD	Present Invention
					Example
15	15	1	0.05	BAD	Comparative
					Example
16	16	1	—	BAD	Comparative
					Example
17	3	2	—	BAD	Comparative
					Example
18	5	3	—	BAD	Comparative
					Example
19	1	4	—	BAD	Comparative
					Example
20	6	5	0.04	BAD	Comparative
					Example
21	2	6	—	BAD	Comparative
					Example
22	7	7	0.05	BAD	Comparative
					Example

Invention Examples 1 to 14 in which the chemical component of the slab and the manufacturing conditions were appropriate had excellent corrosion resistance. In the iron-based oxide layers of these Invention Examples, the Cr/Si mass concentration ratio was increased.

In Comparative Example 15, Cr in the slab was insufficient. In Comparative Example 15, it was presumed that Cr was not sufficiently supplied to the iron-based oxide layer during the cooling period after the finish rolling. In Comparative Example 15, the Cr/Si mass concentration ratio of the iron-based oxide layer was insufficient. In Comparative Example 15, corrosion resistance could not be secured.

In Comparative Example 16, Si in the slab was insufficient. Therefore, in Comparative Example 16, the iron-based oxide layer was not formed. Therefore, in Comparative Example 16, corrosion resistance could not be secured.

In Comparative Example 17, the chemical component of the slab was appropriate, but the rolling reduction in the rough rolling was insufficient. As a result, in Comparative Example 17, it was presumed that the oxide phases became dense before the finish rolling, and these were not completely removed by the second descaling. In Comparative Example 17, an iron-based oxide layer was not appropriately generated on the surface of the matrix. In Comparative Example 17, corrosion resistance could not be secured.

In Comparative Example 18, the chemical component of the slab was appropriate, but the rolling reduction in the rough rolling was excessive. As a result, in Comparative Example 18, it was presumed that the oxide phases bit into the matrix before the finish rolling, and these were not completely removed by the second descaling. In Comparative Example 18, a large amount of scale was caught in the matrix, and an iron-based oxide layer was not appropriately generated on the surface of the matrix. In Comparative Example 18, corrosion resistance could not be secured.

In Comparative Example 19, the chemical component of the slab was appropriate, but the second descaling was not performed. As a result, in Comparative Example 19, a large amount of scale adhered to the surface of the slab before finish rolling. In Comparative Example 19, an iron-based oxide layer was not appropriately generated on the surface of the matrix. In Comparative Example 19, corrosion resistance could not be secured.

In Comparative Example 20, the chemical component of the slab was appropriate, but the cooling rate before finish rolling was excessive. As a result, in Comparative Example 20, it was presumed that the formation reaction (that is, oxidation of the surface of the bar steel) of the iron-based oxide layer was completed before Cr was sufficiently supplied to the iron-based oxide layer. In Comparative Example 20, the Cr/Si mass concentration ratio of the iron-based oxide layer was insufficient. In Comparative Example 20, corrosion resistance could not be secured.

In Comparative Example 21, the chemical component of the slab was appropriate, but the first descaling after heating was not performed. As a result, in Comparative Example 21, the scale generated at the time of heating could not be removed, and the target iron-based oxide layer could not be formed by cooling after rolling. In Comparative Example 21, corrosion resistance could not be secured.

In Comparative Example 22, the chemical component of the slab was appropriate, but the second descaling before finish rolling was not performed. As a result, in Comparative Example 22, the scale generated at the time of rolling could not be removed, and the target iron-based oxide layer could not be formed by cooling after rolling. In Comparative Example 22, corrosion resistance could not be secured.

REFERENCE SIGNS LIST

• • 1 Bar steel
  • 11 Matrix
  • 12 Iron-based oxide layer
  • 13 Scale layer
  • 3 Starting point of line analysis
  • 4 Line analysis region
  • 5 Resin for cross-section observation