HOT-ROLLED STEEL SHEET

Description

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a hot-rolled steel sheet.

Priority is claimed on Japanese Patent Application No. 2022-163955, filed on Oct. 12, 2022, the content of which is incorporated herein by reference.

BACKGROUND ART

In recent years, from the viewpoint of protecting the global environment, efforts have been made to reduce the amount of carbon dioxide gas emitted in many fields. Vehicle manufacturers are also actively developing techniques for reducing the weight of vehicle bodies for the purpose of reducing fuel consumption. However, it is not easy to reduce the weight of vehicle bodies since the emphasis is placed on improvement in collision resistance to secure the safety of the occupants.

In order to achieve both vehicle body weight reduction and collision resistance, an investigation has been conducted to make a member thin by using a high-strength steel sheet. Therefore, there is a strong demand for a steel sheet having both high strength and excellent formability. In order to meet this demand, several techniques have been conventionally proposed.

Since there are various working methods for vehicle members, the required formability differs depending on members to which the working methods are applied. Among these, sheet thickness reduction at critical fracture and ductility is placed as important indices for formability. The sheet thickness reduction at critical fracture is a value calculated from a sheet thickness of a tensile test piece before fracture and a minimum value of a sheet thickness of the tensile test piece after fracture. When the sheet thickness reduction at critical fracture is low, it is not preferable since early fracture become to be easily occurred when tensile strain is applied during press forming.

Vehicle members are formed by press forming, and the press-formed blank sheet is often manufactured by highly productive shearing working. A blank sheet manufactured by shearing working needs to be excellent in terms of the end surface accuracy after shearing working.

For example, when a secondary sheared surface consisting of a sheared surface, a fractured surface, and a sheared surface is occurred in the appearance of the end surface after shearing working (sheared end surface), the accuracy of the sheared end surface significantly deteriorates.

The higher the strength of hot-rolled steel sheet, the more likely crack occurs from the bent inner during bending (hereinafter referred to as bent inner cracking).

The mechanism of bent inner cracking is presumed to be as follows. A compressive stress is generated in the bent inner during bending. At first, the entire bent inner is uniformly deformed as the bending proceeds, but as the amount of deformation increases, the deformation cannot be borne by the uniform deformation alone, and strain is concentrated locally, causing the deformation to proceed (occurrence of a shear deformation band). By further growing the shear deformation band, cracking is occurred along the shear band from the surface of the bent inner and grows. The reason why the bent inner cracking are likely to occur as the strength is increased is presumably because the decrease of work hardenability associated with the increase in strength makes it difficult for uniform deformation to proceed, making it easier for deformation to become uneven, leading to the formation of the shear deformation band early in processing (or under loose processing conditions). There are various working method for vehicle members as described above, since a steel sheet is often subjected to bending, it is not preferable to occur bent inner cracking during bending.

In addition, there is a case where a chemical conversion film is formed on a surface of a steel sheet for the purpose of such as improving corrosion resistance. When a Si content in a steel sheet is increased in order to increase the strength, oxides containing Si are likely to be generated and remain on a surface layer of the steel sheet, which may deteriorate a chemical convertibility of the steel sheet and prevent a sufficient formation of the chemical conversion film.

For example, Patent Document 1 discloses a hot-rolled steel sheet that can be used as a raw material for cold-rolled steel sheet with excellent surface properties after press forming, in which the degrees of Mn segregation and P segregation in the center part of sheet thickness are controlled.

PRIOR ART DOCUMENT

Patent Document

• Patent Document 1: WO2020/044445

Non-Patent Document

• Non-Patent Document 1: J. Webel, J. Gola, D. Britz, F. Mucklich, Materials Characterization 144 (2018) 584-596
• Non-Patent Document 2: D. L. Naik, H. U. Sajid, R. Kiran, Metals 2019, 9, 546
• Non-Patent Document 3: K. Zuiderveld, Contrast Limited Adaptive Histogram Equalization, Chapter VIII. 5, Graphics Gems IV. P. S. Heckbert (Eds.), Cambridge, MA, Academic Press, 1994, pp. 474-485

SUMMARY OF INVENTION

Technical Problem

However, in Patent Document 1, a sheet thickness reduction at critical fracture, bent inner cracking, and chemical convertibility of the hot-rolled steel sheet are not considered.

The present invention has been made in view of the circumstances described above, and an object of the present invention is to provide a hot-rolled steel sheet having high strength and sheet thickness reduction at critical fracture, excellent ductility, shearing property and chemical convertibility, and further suppressing an occurrence of bent inner cracking, that is, having excellent resistance to inner crack at bending.

Solution to Problem

The gist of the present invention is as follows.

(1) A hot-rolled steel sheet according to one aspect of the present invention comprising, in terms of mass %, as a chemical composition,

• • C: 0.050% to 0.250%,
  • Si: 0.05% to 3.00%,
  • Mn: 1.00% to 4.00%,
  • Ni: 0.02% to 2.00%,
  • sol. Al: 0.001% to 2.000%,
  • P: 0.100% or less,
  • S: 0.0300% or less,
  • N: 0.1000% or less,
  • O: 0.0100% or less,
  • Ti: 0% to 0.500%,
  • Nb: 0% to 0.500%,
  • V: 0% to 0.500%,
  • Cu: 0% to 2.00%,
  • Cr: 0% to 2.00%,
  • Mo: 0% to 1.00%,
  • B: 0% to 0.0100%,
  • Ca: 0% to 0.0200%,
  • Mg: 0% to 0.0200%,
  • REM: 0% to 0.1000%,
  • Bi: 0% to 0.0200%,
  • As: 0% to 0.100%,
  • Zr: 0% to 1.00%,
  • Co: 0% to 1.00%,
  • Zn: 0% to 1.00%,
  • W: 0% to 1.00%,
  • Sn: 0% to 0.05%,
  • a remainder comprising Fe and impurities, and
  • the following formulas (A) and (B) are satisfied,
  • in which, in a microstructure at a position of ¼ from a surface in a sheet thickness direction,
  • in terms of area %,
  • residual austenite is less than 3.0%,
  • ferrite is 15.0% or more and less than 60.0%, and
  • pearlite is less than 5.0%,
  • an Entropy value indicated by the following formula (1) is 10.7 or more, an Inverse difference normalized value indicated by the following formula (2) is 1.020 or more, and a Cluster Shade value indicated by the following formula (3) is −8.0×10⁵ to 8.0×10⁵, which are obtained by analyzing SEM images of the microstructure with a gray level co-occurrence matrices method,
  • a standard deviation of Mn concentrations is 0.60 mass % or less,
  • at the surface,
  • an area ratio of a region where a Ni concentration is 0.2 mass % or more is 10.0% or more,
  • an area ratio of a region where an O concentration is 3.0 mass % or more is 3.0% to 50.0%, and
  • a maximum value of sphere equivalent diameters of oxides is 5.00 μm or less,
  • a tensile strength of the hot-rolled steel sheet is 980 MPa or more,

0.06 % ≤ Ti + Nb + V ≤ 0.5 % , ( A ) Zr + Co + Zn + W ≤ 1. % , ( B )

• • here, each element symbol in the formulas (A) and (B) indicates the content of the element in terms of mass %, and 0% is substituted when the element is not contained,
  • P(i,j) in the following formulas (1) to (5) is a gray level co-occurrence matrix, L in the following formula (2) is possible Quantization levels of grayscale of the SEM images, i and j in the following formulas (2) and (3) are natural numbers from 1 to the L, μ_x and μ_y in the following formula (3) are indicated by the in the following formulas (4) and (5).

[ Formula ⁢ 1 ] Entropy = - ∑ i ∑ j P ⁡ ( i , j ) ⁢ log ⁡ ( P ⁡ ( i , j ) ) ( 1 ) [ Formula ⁢ 2 ] Inverse ⁢ difference ⁢ normalized = ∑ i ∑ j P ⁡ ( i , j ) 1 + ❘ "\[LeftBracketingBar]" i - j ❘ "\[RightBracketingBar]" L ( 2 ) [ Formula ⁢ 3 ] Cluster ⁢ Shade = ∑ i ∑ j ( i + j - μ x - μ y ) 3 ⁢ P ⁡ ( i , j ) ( 3 ) [ Formula ⁢ 4 ] μ x = ∑ i ∑ j i ⁡ ( P ⁡ ( i , j ) ) ( 4 ) [ Formula ⁢ 5 ] μ y = ∑ i ∑ j j ⁡ ( P ⁢ ( i , j ) ) ( 5 )

(2) The hot-rolled steel sheet according to (1), in which the chemical composition may comprise, in terms of mass %, one or two or more selected from the group consisting of

• • Ti: 0.001% to 0.500%,
  • Nb: 0.001% to 0.500%,
  • V: 0.001% to 0.500%.
  • Cu: 0.01% to 2.00%,
  • Cr: 0.01% to 2.00%,
  • Mo: 0.01% to 1.00%,
  • B: 0.0001% to 0.0100%,
  • Ca: 0.0005% to 0.0200%,
  • Mg: 0.0005% to 0.0200%,
  • REM: 0.0005% to 0.1000%,
  • Bi: 0.0005% to 0.0200%,
  • As: 0.001% to 0.100%,
  • Zr: 0.01% to 1.00%,
  • Co: 0.01% to 1.00%,
  • Zn: 0.01% to 1.00%,
  • W: 0.01% to 1.00%, and
  • Sn: 0.01% to 0.05%.

Advantageous Effects of Invention

According to the above aspect according to the present invention, it is possible to obtain a hot-rolled steel sheet having high strength and sheet thickness reduction at critical fracture, excellent ductility, shearing property, chemical convertibility, and resistance to inner crack at bending.

The hot-rolled steel sheet according to the above aspect of the present invention is suitable as an industrial material used for vehicle members, mechanical structural members, and building members.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 An example of a sheared end surface of a hot-rolled steel sheet according to a present invention example.

FIG. 2 An example of a sheared end surface of a hot-rolled steel sheet according to a comparative example.

DESCRIPTION OF EMBODIMENTS

The chemical composition and microstructure of a hot-rolled steel sheet according to the present embodiment will be more specifically described below.

However, the present invention is not limited only to a configuration disclosed in the present embodiment, and various modifications can be made without departing from the scope of the gist of the present invention.

The numerical limit range described below with “to” in between includes the lower limit and the upper limit. Regarding the numerical value indicated by “less than” or “more than”, the value does not fall within the numerical range. In the following description, % regarding the chemical composition is mass % unless particularly otherwise specified.

Chemical Composition

The chemical composition of the hot-rolled steel sheet according to the present embodiment includes, in terms of mass %, C: 0.050% to 0.250%, Si: 0.05% to 3.00%, Mn: 1.00% to 4.00%, Ni: 0.02% to 2.00%, sol. Al: 0.001% to 2.000%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, and a remainder of Fe and impurities, and satisfies the formula (A) (0.060%≤Ti+Nb+V≤0.500%).

Each element will be described in detail below.

C: 0.050% to 0.250%

C increases the area ratio of a hard phase and increases the strength of ferrite by bonding to a precipitation hardening element such as Ti, Nb, or V. When the C content is less than 0.050%, a desired strength cannot be obtained. Therefore, the C content is set to 0.050% or more. The C content is preferably 0.060% or more, more preferably 0.070% or more, still more preferably 0.080% or more or 0.090% or more.

On the other hand, when the C content is more than 0.250%, the ductility of the hot-rolled steel sheet deteriorates due to a decrease in the area ratio of ferrite. Therefore, the C content is set to 0.250% or less. The C content is preferably 0.200% or less, 0.150% or less or 0.120% or less.

Si: 0.05% to 3.00%

Si has an action of improving the ductility of the hot-rolled steel sheet by promoting the formation of ferrite and has an action of increasing the strength of the hot-rolled steel sheet by the solid solution strengthening of ferrite. In addition, Si has an action of making steel sound by deoxidation (suppressing the occurrence of a defect such as a blowhole in steel). When the Si content is less than 0.05%, an effect by the action cannot be obtained. Therefore, the Si content is set to 0.05% or more. The Si content is preferably 0.50% or more and more preferably 0.80% or more, 1.00% or more, 1.20% or more or 1.40% or more.

On the other hand, when the Si content is more than 3.00%, the surface properties, chemical convertibility, furthermore, ductility and weldability of the hot-rolled steel sheet significantly deteriorate, and the A₃ transformation point significantly increases. Therefore, it becomes difficult to perform hot rolling in a stable manner. In addition, ferrite is likely to generate excessively, resulting in decrease of the strength of the hot-rolled steel sheet, and austenite is likely to remain after cooling, resulting in decrease of the sheet thickness reduction at critical fracture. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.70% or less, and more preferably 2.50% or less, 2.20% or less, 2.00% or less or 1.80% or less.

Mn: 1.00% to 4.00%

Mn has an action of suppressing ferritic transformation to enhance strength of the hot-rolled steel sheet. When the Mn content is less than 1.00%, a desired strength cannot be obtained. Therefore, the Mn content is set to 1.00% or more. The Mn content is preferably 1.30% or more and more preferably 1.50% or more or 1.80% or more.

On the other hand, when the Mn content is more than 4.00%, due to the segregation of Mn, the form of the hard phase becomes a periodic band shape, and it becomes difficult to obtain a desired shearing property. Therefore, the Mn content is set to 4.00% or less. The Mn content is preferably 3.70% or less or 3.50% v or less, more preferably 3.20% or less, 3.00% or less or 2.60% or less.

Ni: 0.02% to 2.00%

Ni has an action of enhancing the hardenability of the hot-rolled steel sheet. In addition, Ni is concentrated in the surface layer of the steel sheet together with a growth of scale during rough rolling, and become a precipitation nucleus for the chemical conversion film, thereby promoting a formation of the chemical conversion film that is free of lack of hiding and has good adhesion. As a result, Ni has an action of improving the chemical convertibility of the hot-rolled steel sheet. When the Ni content is less than 0.02%, the chemical convertibility of the hot-rolled steel sheet deteriorates. Therefore, the Ni content is set to 0.02% or more. The Ni content is preferably 0.04% or more, more preferably 0.06% or more, and still more preferably 0.08% or more.

On the other hand, even when the Ni content is set to more than 2.00%, the above effects are saturated and the alloy cost increases, which is not preferable. Therefore, the Ni content is set to 2,00% or less. The Ni content is preferably 1.80% or less, and more preferably 1.60% or less.

• • Ti: 0% to 0.500%
  • Nb: 0% to 0.500%
  • V: 0% to 0.500%

0.06 % ≤ Ti + Nb + V ≤ 0.5 % ( A )

Here, each element symbol in the formula (A) indicates the content of the element in terms of mass %, and 0% is substituted when the element is not contained.

Ti, Nb, and V are elements that are finely precipitated in steel as a carbide and a nitride and improve the strength of steel by precipitation hardening. When the total content of Ti, Nb, and V is less than 0.060%, these effects cannot be obtained. Therefore, the total content of Ti, Nb, and V is set to 0.060% or more. That is, the value of the middle part of the formula (A) is set to 0.060% or more. Not all of Ti, Nb, and V need to be contained, and any one thereof may be contained, and the total content thereof may be 0.060% or more. Therefore, the lower limit of each content of Ti, Nb and V is 0%. The lower limit of each content of the Ti, Nb and V may be set to 0.001%, 0.010%, 0.030% or 0.050%. The total content of Ti, Nb, and V is preferably 0.080% or more, and more preferably 0.100% or more.

On the other hand, when content of any one of Ti, Nb, and V exceeds 0.500%, or the total content of Ti, Nb, and V exceeds 0.500%, the workability of the hot-rolled steel sheet deteriorates. Therefore, the each content of Ti, Nb, and V is set to 0.500% or less, and the total content of Ti, Nb, and V is set to 0.500% or less. That is, the value of the middle part of the formula (A) is set to 0.500% or less. The each content of Ti, Nb, and V is preferably 0.400% or less or 0.300% or less, more preferably 0.250% or less, and still more preferably 0.200% or less or 0.100% or less.

sol. Al: 0.001% to 2.000%

Similar to Si, Al has an action of making steel sound by deoxidizing and has an action of enhancing the ductility of the hot-rolled steel sheet by promoting the formation of ferrite. When the sol. Al content is less than 0.001%, an effect by the action cannot be obtained. Therefore, the sol. Al content is set to 0.001% or more. The sol. Al content is preferably 0.010% or more, 0.030% or more or 0.050% or more, more preferably 0.080% or more, 0.100% or more or 0.150% or more.

On the other hand, when the sol. Al content is more than 2.000%, the above effects are saturated, which is not economically preferable. Therefore, the sol. Al content is set to 2.000% or less. The sol. Al content is preferably 1.700% or less or 1.500% or less, more preferably 1.300% or less, and still more preferably 1.000% or less.

Note that the soL. Al means acid-soluble Al and refers to solid solution Al present in steel in a solid solution state.

P: 0.100% or less

P has an action of increasing the strength of the hot-rolled steel sheet by solid solution strengthening. The P content may be set to 0%, or P may be positively contained. However, P is an element that is easily segregated, and when the P content exceeds 0.100%, the deterioration of ductility and the sheet thickness reduction at critical fracture attributed to boundary segregation becomes significant. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.050% or less, 0.030% or less, 0.020% or less or 0.015% or less.

The P content may be set to 0.001% or more, 0.003% or more or 0.005% or more from the viewpoint of the refining cost.

S: 0.0300% or less

S forms a sulfide-based inclusion in steel to decrease the ductility and the sheet thickness reduction at critical fracture of the hot-rolled steel sheet. When the S content is more than 0.0300%, the ductility and the sheet thickness reduction at critical fracture of the hot-rolled steel sheet significantly deteriorates. Therefore, the S content is set to 0.0300% or less. The S content is preferably 0.0100% or less, 0.0070% or less or 0.0050% or less.

The S content may be set to 0%, or may be set to 0.0001% or more, 0.0005% or more, 0.0010% or more or 0.0020% or more from the viewpoint of the refining cost.

N: 0.1000% or less

N has an action of decreasing the ductility and the sheet thickness reduction at critical fracture of the hot-rolled steel sheet. When the N content is more than 0.1000%, the ductility and the sheet thickness reduction at critical fracture of the hot-rolled steel sheet significantly deteriorates. Therefore, the N content is set to 0.1000% or less. The N content is preferably 0.0800% or less, more preferably 0.0700% or less or 0.0300% or less, and still more preferably 0.0150% or less or 0.0100% or less.

Although the N content may be set to 0%, but in a case where one or two or more of Ti, Nb, and V are contained to further refine the microstructure, the N content is preferably set to 0.0010% or more, and more preferably set to 0.0015% or more or 0.0020% or more to promote the precipitation of a carbonitride.

O: 0.0100% or less

When a large content of O is contained in steel, O forms a coarse oxide that becomes the starting point of fracture and causes brittle fracture and hydrogen-induced cracks. Therefore, the O content is set to 0.0100% or less. The O content is preferably 0.0080% or less and more preferably 0.0050% or less or 0.0030% or less.

The O content may be set to 0%, but the O content may be set to 0.0005% or more or 0.0010% or more to disperse a large number of fine oxides when molten steel is deoxidized.

The remainder of the chemical composition of the hot-rolled steel sheet according to the present embodiment may be Fe and an impurity. In the present embodiment, the impurities mean substances that are incorporated from ore as a raw material, a scrap, manufacturing environment, or the like and/or substances that are permitted to an extent that the hot-rolled steel sheet according to the present embodiment is not adversely affected.

Instead of a part of Fe, the hot-rolled steel sheet according to the present embodiment may contain the following elements as optional elements. In a case where these optional elements are not contained, the lower limit of the content thereof is 0%. Hereinafter, the optional elements will be described in detail.

• • Cu: 0.01% to 2.00%
  • Cr: 0.01% to 2.00%
  • Mo: 0.01% to 1.00%
  • B: 0.0001% to 0.0100%

All of Cu, Cr, Mo, and B have an action of enhancing the hardenability of the hot-rolled steel sheet. In addition, Cu and Mo have an action of being precipitated as a carbide in steel to increase the strength of the hot-rolled steel sheet. Furthermore, in a case where Cu is contained, Ni has an action of effectively suppressing the grain boundary cracking of a slab caused by Cu. Therefore, one or two or more of these elements may be contained.

As described above, Cu has an action of enhancing the hardenability of the hot-rolled steel sheet and an action of being precipitated as a carbide in steel at a low temperature to increase the strength of the hot-rolled steel sheet. In order to more reliably obtain the effect by the action, the Cu content is preferably set to 0.01% or more and more preferably set to 0.05% or more. However, when the Cu content is more than 2.00%, grain boundary cracking may occur in the slab in some cases. Therefore, the Cu content is set to 2.00% or less. The Cu content is preferably 1.50% or less and more preferably 1.00% or less, 0.70% or less or 0.50% or less.

As described above, Cr has an action of enhancing the hardenability of the hot-rolled steel sheet. In order to more reliably obtain the effect by the action, the Cr content is preferably set to 0.01% or more and more preferably set to 0.05% or more. However, when the Cr content is more than 2.00%, the chemical convertibility of the hot-rolled steel sheet significantly deteriorates. Therefore, the Cr content is set to 2.00% or less. The Cr content is preferably 1.50% or less, and more preferably 1.00% or less, 0.70% or less or 0.50% or less.

As described above, Mo has an action of enhancing the hardenability of the hot-rolled steel sheet and an action of being precipitated as a carbide in steel to increase the strength of the hot-rolled steel sheet. In order to more reliably obtain the effect by the action, the Mo content is preferably set to 0.01% or more and more preferably set to 0.02% or more. However, even when the Mo content is set to more than 1.00%, the effect by the action is saturated, which is not economically preferable. Therefore, the Mo content is set to 1.00% or less. The Mo content is preferably 0.50% or less and more preferably 0.20% or less or 0.10% or less.

As described above, B has an action of enhancing the hardenability of the hot-rolled steel sheet. In order to more reliably obtain the effect by this action, the B content is preferably set to 0.0001% or more and more preferably set to 0.0002% or more. However, when the B content is more than 0.0100%, the formability of the hot-rolled steel sheet significantly deteriorates, and thus the B content is set to 0.0100% or less. The B content is preferably set to 0.0050% or less or 0.0025% or less.

• • Ca: 0.0005% to 0.0200%
  • Mg: 0.0005% to 0.0200%
  • REM: 0.0005% to 0.1000%
  • Bi: 0.0005% to 0.0200%

All of Ca, Mg, and REM have an action of enhancing the ductility of the hot-rolled steel sheet by adjusting the shape of inclusions in steel to a preferable shape. In addition, Bi has an action of enhancing the ductility of the hot-rolled steel sheet by refining the solidification structure. Therefore, one or two or more of these elements may be contained. In order to more reliably obtain the effect by the action, it is preferable that the content of any one or more of Ca, Mg, REM, and Bi are set to 0.0005% or more. However, when the Ca content or Mg content is more than 0.0200% or when the REM content is more than 0.1000%, an inclusion is excessively formed in steel, and thus the ductility of the hot-rolled steel sheet may be conversely decreased in some cases. In addition, even when the Bi content is set to more than 0.0200%, the above effect by the action is saturated, which is not economically preferable. Therefore, the Ca content and the Mg content are set to 0.0200% or less, the REM content is set to 0.1000% or less, and the Bi content is set to 0.0200% or less. The Ca content, the Mg content and the Bi content are preferably 0.0100% or less, more preferably 0.0070% or less or 0.0040% or less. The REM content is preferably 0.0070% or less or 0.0040% or less.

Here, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanoids, and the REM content refers to the total content of these elements. In the case of the lanthanoids, the lanthanoids are industrially added in the form of misch metal.

As: 0.001% to 0.100%

As lowers an austenitizing temperature and thus refines the prior austenite grains, thereby contributing for an improvement of ductility of the hot-rolled steel sheet. In order to reliably obtain the effects, the As content is preferably set to 0.001% or more. On the other hand, since the above effects are saturated even in a case where a large content of As is contained, the As content is set to 0.100% or less.

• • Zr: 0.01% to 1.00%
  • Co: 0.01% to 1.00%
  • Zn: 0.01% to 1.00%
  • W: 0.01% to 1.00%

Zr + Co + Zn + W ≤ 1. % ( B )

• • Sn: 0.01% to 0.05%

Each element symbol in the formula (B) indicates the content of the element in terms of mass %, and 0% is substituted when the element is not contained.

Regarding Zr, Co, Zn, and W, the present inventors have confirmed that, even when a total of 1.00% or less of these elements are contained, the effect of the hot-rolled steel sheet according to the present embodiment is not impaired. Therefore, one or two or more of Zr, Co, Zn, or W may be contained in a total of 1.00% or less. That is, the value of the left side of the formula (B) may be set to 1.00% or less, 0.50% or less, 0.10% or less or 0.05% or less. The each content of Zr, Co, Zn, and W may be set to 0.50% or less, 010% or less or 0.05% or less. Since Zr, Co, Zn, and W may not be contained, the respective content may be 0%. In order to improve the strength by solid solution strengthening of the steel sheet, the respective content of Zr, Co, Zn, and W may be 0.01% or more.

In addition, the present inventors have confirmed that, even when a small content of Sn is contained, the effect of the hot-rolled steel sheet according to the present embodiment is not impaired. However, when a large content of Sn is contained, a defect may be generated during hot rolling, and thus the Sn content is set to 0.05% or less. Since Sn may not be contained, the Sn content may be 0%. In order to improve the corrosion resistance of the hot-rolled steel sheet, the Sn content may be set to 0.01% or more.

The chemical composition of the above hot-rolled steel sheet may be measured by a general analytical method. For example, inductively coupled plasma-atomic emission spectrometry (ICP-AES) may be used for measurement. sol. Al may be measured by the ICP-AES using a filtrate after a sample is decomposed with an acid by heating. C and S may be measured by using a combustion-infrared absorption method, N may be measured by using the inert gas melting-thermal conductivity method, and O may be measured using an inert gas melting-non-dispersive infrared absorption method.

When a plating layer is provided on the surface of the hot-rolled steel sheet, the chemical composition may be analyzed after the plating layer is removed by mechanical grinding or the like as necessary.

Microstructure of Hot-Rolled Steel Sheet

Next, the microstructure of the hot-rolled steel sheet according to the present embodiment will be described.

In the microstructure at a position of ¼ from a surface in a sheet thickness direction of the hot-rolled steel sheet according to the present embodiment, in terms of area %, residual austenite is less than 3.0%, ferrite is 15.0% or more and less than 60.0%, and pearlite is less than 5.0%, an Entropy value indicated by the following formula (1) is 10.7 or more, an Inverse difference normalized value indicated by the following formula (2) is 1.020 or more, and a Cluster Shade value indicated by the following formula (3) is −8.0×10⁵ to 8.0×10⁵, which are obtained by analyzing SEM images of the microstructure with a gray level co-occurrence matrices method, a standard deviation of Mn concentrations is 0.60 mass % or less, at the surface, an area ratio of a region where a Ni concentration is 0.2 mass % or more is 10.0% or more, an area ratio of a region where an O concentration is 3.0 mass % or more is 3.0% to 50.0%, and a maximum value of sphere equivalent diameters of oxides is 5.00 Um or less.

In the present embodiment, the microstructural ratios, the Entropy value, the Inverse difference normalized value, the Cluster Shade value and the standard deviation of the Mn concentrations in the microstructure at the position of ¼ from the surface in the sheet thickness direction of the hot-rolled steel sheet are specified. The reason therefor is that the microstructure at this position indicates a typical microstructure of the steel sheet.

When the hot-rolled steel sheet has the plating layer, the “surface” referred to herein refers to the interface of the plating layer and the steel sheet, “depth position of ¼ from surface” referred to herein refers to a position that is a depth of ¼ of the sheet thickness from the surface of the hot-rolled steel sheet in the sheet thickness direction.

Area Ratio of ResidualAustenite:Less than 3.0%

Residual austenite is a microstructure that is present as a face-centered cubic lattice even at room temperature. Residual austenite has an action of enhancing the ductility of the hot-rolled steel sheet by transformation-induced plasticity (TRIP). On the other hand, residual austenite transforms into high-carbon martensite during shearing working, becomes a starting point of cracking during deformation, and becomes the cause of decrease of the sheet thickness reduction at critical fracture. When the area ratio of the residual austenite is 3.0% or more, the action is actualized, and the sheet thickness reduction at critical fracture of the hot-rolled steel sheet decreases. Therefore, the area ratio of the residual austenite is set to less than 3.0%. The area ratio of the residual austenite is preferably less than 1.5% and more preferably less than 1.0%.

Since residual austenite is preferably as little as possible, the area ratio of the residual austenite may be 0%.

As the measurement method of the area ratio of the residual austenite, there are methods by X-ray diffraction, EBSP (electron back scattering diffraction pattern) analysis, and magnetic measurement and the like. In the present embodiment, the area ratio of the residual austenite is measured by X-ray diffraction.

In the measurement of the area ratio of the residual austenite by X-ray diffraction in the present embodiment, first, in a cross section at a position of ¼ from the surface in the sheet thickness direction of the hot-rolled steel sheet, a sample is collected so that the microstructure at a region of 1 mm or more in an arbitrary position of the rolling direction and 1 mm or more centered in a direction perpendicular to the rolling direction and the sheet thickness direction can be observed, and a sheet thickness of the sample is reduced by mechanical grinding and chemical grinding so that the position of ¼ from the surface in the sheet thickness direction is a surface. For the sample, the integrated intensities of a total of 6 peaks of α(110), α(200), α(211), γ(111), γ(200), and γ(220) are obtained using a X-ray diffraction device (for example, Rigaku RIINT-2500, Co-Kα rays). Next, the volume ratio of the residual austenite is calculated using the strength averaging method from the integrated intensities. The obtained volume ratio is regarded as an area ratio of the residual austenite.

Area Ratio of Ferrite:15.0% or more and less than 60.0%

Ferrite is a structure formed when fcc transforms into bcc at a relatively high temperature. Ferrite has a high work hardening rate and thus has an action of enhancing the strength-ductility balance of the hot-rolled steel sheet. In order to obtain the above action, the area ratio of the ferrite is set to 15.0% or more. The area ratio of the ferrite is preferably 20.0% or more, more preferably 25.0% or more, and still more preferably 30.0% or more.

On the other hand, since ferrite has a low strength, a desired tensile strength cannot be obtained when the area ratio is excessive. Therefore, the area ratio of the ferrite is set to less than 60.0%. The area ratio of the ferrite is preferably 50.0% or less and more preferably 45.0% or less.

Area Ratio of Pearlite:Less than 5.0%

Pearlite is a lamellar microstructure in which cementite is precipitated in layers between ferrite and is a soft microstructure as compared with bainite and martensite. When the area ratio of the pearlite is 5.0% or more, carbon is consumed by cementite that is contained in pearlite, and the strengths of martensite and bainite, which are the remainder in microstructure decrease, and a desired strength cannot be obtained. Therefore, the area ratio of the pearlite is set to less than 5.0%. The area ratio of the pearlite is preferably 3.0% or less.

In order to improve the stretch flangeability of the hot-rolled steel sheet, the area ratio of the pearlite is preferably reduced as much as possible, and the area ratio of the pearlite is still more preferably 0%.

The hot-rolled steel sheet according to the present embodiment contains a full hard structure consisting of one or two or more of bainite, martensite, and tempered martensite in a total area ratio of more than 32.0% and 85.0% or less as the remainder in microstructure other than residual austenite, ferrite, and pearlite.

Measurement of the area ratios of ferrite and pearlite is performed by the following method. First, in a center in a direction perpendicular to the rolling direction and the sheet thickness direction, a sample is collected so that the microstructure in a region of a position of ¼ from the surface in the sheet thickness direction in a sheet thickness cross section parallel to the rolling direction can be observed. The size of the sample is set to a size that can be observed by about 10 mm in the rolling direction. Next, after mirror finishing the cross section of the sample by polishing, the observed cross section of the sample is polished at room temperature with colloidal silica having grain size of 0.25 μm not containing an alkaline solution for 8 minutes, thereby removing strain introduced into the surface layer of the sample. For a region of 200 μm or more at an arbitrary position in the rolling direction and 200 μm or more centered in a position of ¼ from the surface in the sheet thickness direction of the cross section of the sample, crystal orientation information is obtained by a measurement using electron backscatter diffraction at a measurement interval of 0.1 μm in the rolling direction and the sheet thickness direction. For the measurement, an EBSD analyzer configured of a thermal field emission scanning electron microscope (for example, JSM-7001F manufactured by JEOL) and an EBSD detector (for example, DVC5 type detector manufactured by TSL) is used. At this time, the degree of vacuum inside the EBSD analyzer is set to 9.6×10⁻⁵ Pa or less, the acceleration voltage is set to 15 kV, the irradiation current level is set to 13, and the electron beam irradiation level is set to 62. Note that the number of observation visual fields is set to 5.

Furthermore, a backscattered electron images are photographed at the respective same visual fields where the above-mentioned crystal orientation information were obtained. Note that when photographing the images, the acceleration voltage is set to 15 k, the irradiation current level is set to 12 to 13, and the electron beam irradiation level is set to 62. The working distance (WD) is set to 5 mm. Ferrite and pearlite are identified from the backscattered electron images and the crystal orientation information. First, the grains in which cementite precipitates in a lamellar shape are identified in the backscattered electron images. In the backscattered electron images, cementite is observed as a white contrast. The cementite in the pearlite has a lamellar shape, and the grains in which the lamellar white contrast is observed at intervals of 1.0 μm or less are regarded as grains of pearlite. The area ratio of the grains is calculated, thereby obtaining the area ratio of pearlite. After that, for grains except the grains determined as pearlite, regions where the grain average misorientation value is 1.0° or less are determined as ferrite using the obtained crystal orientation information and a “Grain Average Misorientation” function installed in software “OIM Analysis (registered trademark)” included in the EBSD analyzer. At this time, the Grain Tolerance Angle is set to 15°, the area ratio of the region determined as ferrite is obtained, thereby obtaining the area ratio of ferrite.

The area ratio of a remainder of the microstructure is obtained by subtracting the area ratios of residual austenite, ferrite, and pearlite from 100%.

In the present embodiment, the rolling direction of the hot-rolled steel sheet is determined by the following method.

First, a test piece is collected so that a cross section parallel to the surface of the hot-rolled steel sheet can be observed. The cross section of the test piece collected in the sheet thickness is observed using an optical microscope after finishing by mirror polishing. The observation plane is set to a plane parallel to a plate surface at an arbitrary depth in a range of ¼ to ½ in the sheet thickness direction, the extending direction of grains in the observation plane is determined as the rolling direction.

Entropy Value: 10.7 or More

Inverse Difference Normalized Value: 1.020 or More

In order to suppress the occurrence of the secondary sheared surface, it is important to form a fractured surface after a sheared surface is sufficiently formed, and there is a need to suppress the early occurrence of cracking from the cutting edge of the tool during shearing working. In order for that, it is important that the periodicity of the microstructure is low and the uniformity of the microstructure is high. In the present embodiment, the occurrence of the secondary sheared surface is suppressed by controlling the Entropy value (E value) that indicates the periodicity of the microstructure and the Inverse difference normalized value (I value) that indicates the uniformity of the microstructure.

The E value represents the periodicity of the microstructure. In a case where the brightness is periodically arranged due to an influence of the formation of a band-like structure or the like, that is, the periodicity of the microstructure is high, the E value decreases. In the present embodiment, since there is a need to make the microstructure poorly periodic, it is necessary to increase the E value. When the E value is less than 10.7, a secondary sheared surface is likely to be occurred. From periodically arranged structures as starting points, cracking occurs from the cutting edge of a shearing tool in an extremely early stage of shearing working to form a fractured surface, and then a sheared surface is formed again. It is presumed that this makes it likely for a secondary sheared surface to be occurred. Therefore, the E value is set to 10.7 or more. The E value is preferably 10.8 or more and more preferably 11.0 or more. The E value is preferably as high as possible, and the upper limit is not particularly specified and may be set to 13.0 or less, 12.5 or less, or 12.0 or less.

The I value represents the uniformity of the microstructure and increases as the area of a region having certain brightness increases. A high I value means that the uniformity of the microstructure is high. In the present embodiment, since there is a need to make the microstructure highly uniform, it is necessary to increase the I value. When the I value is less than 1.020, due to an influence of the hardness distribution attributed to precipitates in grains and an element concentration difference, cracking occurs from the cutting edge of a shearing tool in an extremely early stage of shearing working to form a fractured surface, and then a sheared surface is formed again. It is presumed that this makes it likely for a secondary sheared surface to be occurred. Therefore, the I value is set to 1.020 or more. The I value is preferably 1.025 or more and more preferably 1.030 or more. The I value is preferably as high as possible, and the upper limit is not particularly specified and may be set to 1.200 or less, 1.150 or less, or 1.100 or less.

Cluster Shade Value: −8.0×10⁵ to 8.0×10⁵

Cluster Shade value (CS value) indicates a degree of strain of the microstructure. The CS value becomes a positive value when there are many points with a brightness higher than the average brightness in images obtained by photographing the microstructure, and become a negative value when there are many points with a brightness lower than the average brightness.

In a secondary electron image of an electron microscope, the brightness becomes high where the surface unevenness of the observed object is large, and the brightness becomes low where the unevenness is small. The unevenness of the surface of the observed object is greatly affected by the grain size and the strength distribution in the microstructure. The CS value in the present embodiment becomes high when the variation in the strength of the microstructure is large or the structure unit is small, and becomes low when the variation in the strength is small or the structure unit is large.

In the present embodiment, it is important to keep the CS value in a desired range close to 0. When the CS value is less than −8.0×10⁵, the sheet thickness reduction at critical fracture of the hot-rolled steel sheet decreases. This is presumably because large grains exist in the microstructure and these grains are preferentially fractured during extreme deformation. Therefore, the CS value is set to −8.0×10⁵ or more. The CS value is preferably −7.5×10⁵ or more, and more preferably −7.0×10⁵ or more.

On the other hand, when the CS value is more than 8.0×10⁵, the sheet thickness reduction at critical fracture of the hot-rolled steel sheet decreases. This is presumably because there is a large variation in microscopic strength in the microstructure, strain during extreme deformation is concentrated locally, and fracture is more likely to occur. Therefore, the CS value is set to 8.0×10⁵ or less. The CS value is preferably 7.5×10⁵ or less, and more preferably 7.0×10⁵ or less.

The E value, the I value and the CS value can be obtained by the following method.

In the present embodiment, the photographing region of SEM images photographed for calculating the E value, the I value and the CS value are set to, in a center in a direction perpendicular to the rolling direction and the sheet thickness direction, 160 μm×160 μm centered in a position of ¼ from the surface in the sheet thickness direction, and the number of the observation fields is set to 5. The SEM image is photographed using an SU-6600 Schottky electron gun manufactured by Hitachi High-Technologies Corporation with a tungsten emitter and an acceleration voltage of 1.5 kV. Based on the above settings, the SEM image is output at a magnification of 1000 times and a gray scale of 256 gradations.

Next, on an image obtained by cutting out the obtained SEM image into a 880×880-pixel region (an actual size of the observation region is 160 μm×160 μm), a smoothing treatment described in Non-Patent Document 3, in which the contrast-enhanced limit magnification is set to 2.0 and the tile grid size is 8×8 is performed. The smoothed SEM image is rotated counterclockwise from 0 degrees to 179 degrees in increments of 1 degree, excluding 90 degrees, and an image is created at each angle, thereby obtaining a total of 179 images. Next, from each of these 179 images, the frequency values of brightness between adjacent pixels are sampled in a matrix form using the GLCM method described in Non-Patent Document 1.

179 matrices of the frequency values sampled by the above method are expressed as p_k (k=0 . . . 89, 91, . . . 179) where k is a rotation angle from the original image. p_k's generated for individual images are summed for all k's (k=0 . . . 89, 91, . . . 179), and then 256×256 matrices P standardized so that the total of individual components becomes 1 are calculated. Furthermore, the E value, the I value and the CS value are each calculated using the following formulas (1) to (5) described in Non-Patent Document 2. Note that the average value obtained by measuring the entire visual fields is calculated.

P(i,j) in the following formulas (1) to (5) is the gray level co-occurrence matrix, the value at the i^th row and j^th column of the matrix P is expressed as P(i,j). As described above, since the calculation is performed using the 256×256 matrices P, when this point is emphasized, the following formulas (1) to (5) can be modified to the following formulas (1′) to (5′). Here, L in the following formula (2) is possible Quantization levels of grayscale of the SEM images, since the SEM image is output in grayscales of 256 gradations as described above in the present embodiment, L is 256. i and j in the following formulas (2) and (3) are natural numbers from 1 to the L, μ_x and μ_y in the following formula (3) are indicated by the following formulas (4) and (5) In the following formulas (1′) to (5′), the value at the i^th row and the j^th column of the matrix P is expressed as P_ij.

[ Formula ⁢ 6 ] Entropy = - ∑ i ∑ j P ⁡ ( i , j ) ⁢ log ⁡ ( P ⁡ ( i , j ) ) ( 1 ) [ Formula ⁢ 7 ] Inverse ⁢ difference ⁢ normalized = ∑ i ∑ j P ⁡ ( i , j ) 1 + ❘ "\[LeftBracketingBar]" i - j ❘ "\[RightBracketingBar]" L ( 2 ) [ Formula ⁢ 8 ] Cluster ⁢ Shade = ∑ i ∑ j ( i + j - μ x - μ y ) 3 ⁢ P ⁡ ( i , j ) ( 3 ) [ Formula ⁢ 9 ] μ x = ∑ i ∑ j i ⁡ ( P ⁡ ( i , j ) ) ( 4 ) [ Formula ⁢ 10 ] μ y = ∑ i ∑ j j ⁡ ( P ⁢ ( i , j ) ) ( 5 ) [ Formula ⁢ 11 ] Entropy = - ∑ i = 1 , j = 1 i = 256 , j = 256 P ⁡ ( i , j ) ⁢ log ⁡ ( P ⁡ ( i , j ) ) ( 1 ′ ) [ Formula ⁢ 12 ] Inverse ⁢ difference ⁢ normalized = ∑ i = 1 , j = 1 i = 256 , j = 256 P ij / ( 1 + ❘ "\[LeftBracketingBar]" i - j ❘ "\[RightBracketingBar]" / 256 ) ( 2 ′ ) [ Formula ⁢ 13 ] Cluster ⁢ Shade = ∑ i = 1 , j = 1 i = 256 , j = 256 ( i + j - μ x - μ y ) 3 ⁢ P ij ( 3 ′ ) [ Formula ⁢ 14 ] μ x = ∑ i = 1 , j = 1 i = 256 , j = 256 i ⁡ ( P ij ) ( 4 ′ ) [ Formula ⁢ 15 ] μ y = ∑ i = 1 , j = 1 i = 256 , j = 256 j ⁡ ( P ij ) ( 5 ′ )

Standard Deviation of Mn Concentrations: 0.60 Mass % or Less

The standard deviation of the Mn concentrations of the hot-rolled steel sheet according to the present embodiment is 0.60 mass % or less. This makes it possible to uniformly disperse the hard phase and makes it possible to prevent the occurrence of cracking from the cutting edge of the shearing tool in an extremely early stage of shearing working. As a result, the occurrence of the secondary sheared surface can be suppressed. The standard deviation of the Mn concentrations is preferably 0.50 mass % or less and more preferably 0.47 mass % or less. The value of the lower limit of the standard deviation of the Mn concentrations is desirably as small as possible from the viewpoint of suppressing excessively large burrs, but the substantial lower limit is 0.10 mass % due to restrictions in the manufacturing process.

The standard deviation of the Mn concentrations can be obtained by the following method. First, at a center in a direction perpendicular to the rolling direction and the sheet thickness direction, a sample is collected so that a region of a position of ¼ from the surface in the sheet thickness direction in a cross section parallel to the rolling direction can be observed. The size of the sample depends on a measurement device, but is set to a size that can be observed by about 10 mm in the rolling direction. Next, after mirror polishing the sample, the standard deviation of the Mn concentrations is measured using an electron probe micro analyzer (EPMA). As the measurement conditions, the acceleration voltage is set to 15 kV and the magnification is set to 5000 times, and a distribution image of Mn concentrations in a range of 20 μm in the rolling direction of the sample and 20 μm centered in the position of ¼ from the surface in the sheet thickness direction of the sample. More specifically, the Mn concentrations of 40000 or more points are measured with the measurement interval of to 0.1 μm. Next, the standard deviation is calculated based on the Mn concentrations obtained from all of the measurement points, thereby obtaining the standard deviation of the Mn concentrations.

Area Ratio of Region Where Ni Concentration is 0.2 Mass % or More at Surface: 10.0% or More

In the present embodiment, in order to improve the resistance to inner crack at bending, the area ratio of the region where the O concentration is 3.0 mass % or more is controlled to 3.0% or more at the surface of the hot-rolled steel sheet. However, when the region where the O concentration is 3.0 mass % or more exists 3.0% or more in terms of area ratio at the surface, the chemical convertibility of the hot-rolled steel sheet deteriorates. The present inventors have found that the chemical convertibility of the hot-rolled steel sheet can be improved by increasing the area ratio of the region where the Ni concentration is 0.2 mass % or more even when the area ratio of the region where the O concentration is 3.0 mass % or more is 3.0% or more at the surface of the hot-rolled steel sheet. When the area ratio of the region where the Ni concentration is 0.2 mass % or more at the surface is less than 10.0%, the chemical convertibility of the hot-rolled steel sheet cannot be sufficiently improved. Therefore, the area ratio of the region where the Ni concentration is 0.2 mass % or more at the surface is set to 10.0% or more. The area ratio of the region is preferably 15.0% or more, and more preferably 20.0% or more.

The upper limit of the area ratio of the region where the Ni concentration is 0.2 mass % or more at the surface is particularly limited, but may be set to 100.0% or less, 60.0% or less, 50.0% or less or 40.0% or less.

The chemical convertibility of the hot-rolled steel sheet cannot be sufficiently improved by increasing the area ratio of the region where the Ni concentration is less than 0.2 mass % at the surface, it is important to increase the area ratio of the region where the Ni concentration is 0.2 mass % or more at the surface in the present embodiment.

Area Ratio of Region Where O Concentration is 3.0 mass % or More at Surface: 3.0% to 50.0%

When the area ratio of the region where the O concentration is 3.0 mass % or more at the surface is less than 3.0%, the resistance to inner crack at bending of the hot-rolled steel deteriorates. Therefore, the area ratio of the region where the O concentration is 3.0 mass % or more at the surface is set to 3.0% or more. The area ratio of the region is preferably 5.0% or more, and more preferably 10.0% or more.

On the other hand, the area ratio of the region where the O concentration is 3.0 mass % or more at the surface is more than 50.0%, the chemical convertibility of the hot-rolled steel sheet deteriorates. Therefore, the area ratio of the region where the O concentration is 3.0 mass % or more at the surface is set to 50.0% or less. The area ratio of the region is preferably 40.0% or less, and more preferably 30.0% or less.

The resistance to inner crack at bending and the chemical convertibility of the hot-rolled steel sheet are not significantly affected even when the area ratio of the region where the O concentration is less than 3.0 mass % at the surface is controlled, it is important to control the area ratio of the region where the O concentration is 3.0 mass % or more at the surface in the present embodiment.

The area ratio of the region where the Ni concentration is 0.2 mass % or more and the area ratio of the region where the O concentration is 3.0 mass % v or more are measured by the following method.

A sample is collected so that the surface of the hot-rolled steel sheet can be observed. For the sample, an observation surface is degreased at 60° C. for 60 seconds using FC-E6403 manufactured by Nihon Parkerizing Co., Ltd., and then a surface treatment is performed by immersing in acetone for 90 seconds and performing ultrasonic cleaning. For the sample after the surface treatment, mapping analysis for Ni and O is performed using an electron probe microanalyzer (EPMA). The measurement conditions are set to an acceleration voltage of 15 kV and a magnification of 500 times, and a distribution image is measured at a range of 200 μm in the rolling direction of the sample and 200 μm in the sheet width direction of the sample. More specifically, the measurement interval is set to 1 μm, and the Ni concentration and the O concentration are measured at 40000 or more points per a visual field. Measurement is performed for at least 5 visual fields. The area ratio of the area where the Ni concentration is 0.2 mass % or more is obtained by dividing the measurement points where the Ni concentration is 0.2 mass % or more by the total measurement points. In addition, the area ratio of the area where the O concentration is 3.0 mass % or more is obtained by dividing the measurement points where the O concentration is 3.0 mass % or more by the total measurement points.

Note that when the hot-rolled steel sheet has scale on the surface, the sample is subjected to the above surface treatment after a pickling treatment under the following condition.

The pickling treatment may be performed in a standard method, for example, by immersing the sample in hydrochloric acid having a hydrochloric acid concentration of 3 vol % to 10 vol % at a temperature of 85° C. to 98° C. for 20 to 300 seconds. The pickling may be performed once, or may be performed several times as necessary. The above pickling time (20 to 300 seconds) means the time of the pickling when pickling is performed only once, and means the total time of the pickling when pickling is performed several times. By setting the pickling temperature to 85° C. or higher. it is preferable since the oxides in the surface layer can be sufficiently removed. The upper limit of the pickling temperature is not particularly limited, but is practically about 98° C. When the pickling time is longer than 300 seconds, the surface roughness becomes excessively rough and the surface properties deteriorate, and furthermore, an unevenness remaining after cold rolling may cause a notch-like effect, which may deteriorate the bendability of the hot-rolled steel sheet. The upper limit of the pickling time is preferably 200 seconds.

When the hot-rolled steel sheet has a surface treatment film such as a plating layer or a coating, the above-mentioned pickling treatment is performed for the surface of the base steel obtained after removing the surface treatment film, and then the above-mentioned surface treatment is performed. The method for removing the surface treatment film can be appropriately selected according to the kind of the surface treatment film as long as it does not affect the surface roughness of the base steel. For example, when the surface treatment film is a galvanized layer such as electrogalvanizing, electro Zn—Ni alloy plating, hot-dip galvanizing, hot-dip galvannealing, hot-dip Zn—Al alloy plating, hot-dip Zn—Al—Mg alloy plating, or hot-dip Zn—Al—Mg—Si alloy plating, the galvanized layer may be dissolved using dilute hydrochloric acid with an inhibitor added. This allows only the galvanized layer to be peeled off from the steel sheet. The inhibitor is an additive used for prevent excessive dissolution of the base steel and suppress changes in roughness. For example, hydrochloric acid diluted to 5 vol % with a corrosion inhibitor for hydrochloric acid pickling “Ibit No. 700BK” manufactured by Asahi Chemical Industry Co., Ltd. added to a concentration of 0.6 g/L can be used. In addition, when the surface treatment film is an aluminum plating layer such as hot-dip aluminum plating, in accordance with the description of JIS G 3314:2019, the aluminum plating is dissolved by sequentially immersing in an aqueous sodium hydroxide solution and a dilute hydrochloric acid solution to which hexamethylenetetramine is added until foaming caused by dissolution of the plating subsides. In addition, when the surface treatment film is an electrodeposition coating, the electrodeposition coating is peeled off using a stripper (Neo River SP-751, manufactured by Sansai Kako Co., Ltd.).

Maximum Value of Sphere equivalent diameters of Oxides at Surface: 5.00 μm or Less

When the maximum value of sphere equivalent diameters of oxides at the surface is more than 5.00 μm, the resistance to inner crack at bending of the hot-rolled steel sheet deteriorates. Therefore, the maximum value of sphere equivalent diameters of oxides at the surface is set to 5.00 μm or less. In order to further improve the resistance to inner crack at bending of the hot-rolled steel sheet, the maximum value of sphere equivalent diameters of oxides is preferably set to 4.50 m or less, and more preferably 4.00 μm or less.

Since it may be technically difficult to set the maximum value of sphere equivalent diameters of oxides to less than 1.00 ™, the maximum value of sphere equivalent diameters of oxides may be set to 1.00 μm or more.

Note that the oxides referred to herein refer to precipitates containing O.

The maximum value of sphere equivalent diameters of the oxides is measured by the following method.

A sample is collected from the hot-rolled steel sheet so that the surface can be observed. The surface of the sample is degreased at 60° C. for 60 seconds using FC-E6403 manufactured by Nihon Parkerizing Co., Ltd., and then a surface treatment is performed by immersing in acetone for 90 seconds and performing ultrasonic cleaning. For the sample after the surface treatment, precipitates are identified by observing at least 10 visual fields at a magnification of 3000 times. The composition of the precipitates is measured using an EDS (energy dispersive X-ray spectroscope). Among the precipitates, sphere equivalent diameters are calculated for precipitates containing O. In the observation visual field, the equivalent sphere diameters of all precipitates containing O are calculated. Among the obtained sphere equivalent diameters, the maximum value is regarded as the maximum value of sphere equivalent diameters of the oxides.

Note that when the precipitate is subjected to EDS analysis and O is detected at 15 atom % or more, the precipitate is regarded as an oxide.

In addition, when the hot-rolled steel sheet has scale or the surface treatment film on the surface, these are removed by the above-mentioned method, and then the above-mentioned surface treatment is performed before the measurement.

Tensile Strength Properties

Among the mechanical properties of the hot-rolled steel sheets, the tensile strength properties (tensile strength and total elongation) are evaluated according to JIS Z 2241: 2011. A test piece is set to a No. 5 test piece of JIS Z 2241: 2011. The sampling position of the test piece may be set to a position of ¼ from the end surface in a direction perpendicular to the rolling direction and the sheet thickness direction, and the sheet width direction may be set to the longitudinal direction.

In the hot-rolled steel sheet according to the present embodiment, the tensile strength is 980 MPa or more. The tensile strength is preferably 1000 MPa or more. When the tensile strength is less than 980 MPa, applicable components are limited and contribution for vehicle body weight reduction is small. The upper limit does not need to be particularly limited and may be set to 1780 MPa from the viewpoint of suppressing the wearing of a die.

The total elongation of the hot-rolled steel sheet according to the present embodiment is preferably set to 10.0% or more, and the product of the tensile strength and the total elongation (TS×El) is preferably set to 13000 MPa·% or more. The total elongation is more preferably set to 11.0% or more and still more preferably set to 13.0% or more. In addition, the product of the tensile strength and the total elongation is more preferably set to 14000 MPa-% or more and still more preferably 15000 MPa-% or more. By setting the total elongation to 10.0% or more and the product of the tensile strength and the total elongation to 13000 MPa-% or more, it is possible to significantly contribute to vehicle body weight reduction without limiting applicable components.

Sheet Thickness

The sheet thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited and may be set to 0.5 to 8.0 mm. When the sheet thickness of the hot-rolled steel sheet is less than 0.5 mm, it may become difficult to secure the rolling finishing temperature and the rolling force may become excessive, which makes hot rolling difficult. Therefore, the sheet thickness of the hot-rolled steel sheet according to the present embodiment may be set to 0.5 mm or more. The sheet thickness is preferably 1.2 mm or more or 1.4 mm or more. On the other hand, when the sheet thickness is more than 8.0 mm, it becomes difficult to refine the microstructure, and it may be difficult to obtain the above microstructure. Therefore, the sheet thickness may be set to 8.0 mm or less. The sheet thickness is preferably 6.0 mm or less.

Plating Layer

The hot-rolled steel sheet according to the present embodiment having the above-described chemical composition and microstructure may be provided with a plating layer on the surface for the purpose of improving corrosion resistance and the like and thereby made into a surface-treated steel sheet. The plating layer may be an electro plating layer or a hot-dip plating layer. Examples of the electro plating layer include electrogalvanizing, electro Zn—Ni alloy plating, and the like. Examples of the hot-dip plating layer include hot-dip galvanizing, hot-dip galvannealing, hot-dip aluminum plating, hot-dip Zn—Al alloy plating, hot-dip Zn—Al—Mg alloy plating, hot-dip Zn—Al—Mg—Si alloy plating, and the like. The plating adhesion amount is not particularly limited and may be the same as before. In addition, it is also possible to further enhance the corrosion resistance by performing an appropriate chemical conversion treatment (for example, the application and drying of a silicate-based chromium-free chemical treatment liquid) after plating.

Manufacturing Conditions

A suitable method for manufacturing the hot-rolled steel sheet according to the present embodiment having the above-described chemical composition and microstructure is as follows.

In the suitable method for manufacturing the hot-rolled steel sheet according to the present embodiment, the following steps (1) to (12) are sequentially performed. The temperature of the slab and the temperature of the steel sheet in the present embodiment refer to the surface temperature of the slab and the surface temperature of the steel sheet. In addition, stress refers to tension that is loaded in the rolling direction of the steel sheet.

• • (1) The slab is retained in a temperature range of 700° C. to 850° C. for 900 seconds or longer, then, further heated, and retained in a temperature range of 1100° C. or higher for 6000 seconds or longer.
  • (2) Times of rough rolling is set to 5 times or more, and at all stands of rough rolling, rolling are performed at the temperature range of lower than 1130° C., rolling are performed at rolling reductions of less than 50%, descaling are performed at inlets of rolling, and rolling are performed within 2.0 seconds after descaling.
  • (3) A maximum temperature reached from completion of rough rolling to 120 seconds pass, and to start of descaling of finish rolling is set to 1000° C. to 1170° C., and retention at this maximum temperature is performed for 1.0 second or longer.
  • (4) Hot rolling is performed in a temperature range of 850° C. to 1100° C. so that a rolling reduction is 90% or more in total.
  • (5) Stress of 170 kPa or more is loaded to the steel sheet from completion of rolling at one stand before a final stand of hot rolling to start of rolling of the final stand.
  • (6) The rolling reduction at the final stand of hot rolling is set to 8% or more, and hot rolling is finished so that a finishing temperature Tf is 900° C. or higher and lower than 1010° C.
  • (7) Stress of less than 200 kPa is loaded to the steel sheet from completion of rolling at the final stand of hot rolling to the steel sheet is cooled to 800° C.
  • (8) After completion of hot rolling, accelerated cooling is performed to a temperature range of 600° C. to 780° C. at an average cooling rate of 50° C./s or faster.
  • (9) Slow cooling at an average cooling rate of slower than 5° C./s is performed in the temperature range of 600° C. to 780° C. for 2.0 seconds or longer.
  • (10) After the slow cooling, cooling is performed so that an average cooling rate at a temperature range of 450° C. to 600° C. is 30° C./s or faster and slower than 50° C./s.
  • (11) Cooling is performed so that an average cooling rate at a temperature range of coiling temperature to 450° C. is 50° C./s or faster.
  • (12) Coiling is performed in a temperature range of 350° C. or lower.

By adopting the above manufacturing method, a hot-rolled steel sheet having high strength and sheet thickness reduction at critical fracture, excellent ductility, shearing property, chemical convertibility, and resistance to inner crack at bending can be stably manufactured.

(1) Slab, Slab Temperature and Retention Time at Hot Rolling

As the slab that is subjected to hot rolling, a slab obtained by continuous casting, a slab obtained by casting and blooming, or the like can be used. In addition, if necessary, it is possible to use the above slabs after hot working or cold working.

The slab that is subjected to hot rolling is preferably retained in a temperature range of 700° C. to 850° C. for 900 seconds or longer during slab heating, then, further heated, and retained in a temperature range of 1100° C. or higher for 6000 seconds or longer. Note that during retention in the temperature range of 700° C. to 850° C., the steel sheet temperature may be fluctuated or be maintained constant in this temperature range. In addition, during retention at 1100° C. or higher, the steel sheet temperature may be fluctuated or be maintained constant in the temperature range of 1100° C. or higher.

In austenite transformation in the temperature range of 700° C. to 850° C., Mn is distributed between ferrite and austenite, and Mn can be diffused into the ferrite region by extending the transformation time. Accordingly, the Mn microsegregation unevenly distributed in the slab can be eliminated, and the standard deviation of the Mn concentrations can be significantly reduced. In addition, by retention the slab in the temperature range of 1100° C. or higher for 6000 seconds or longer, the Mn concentrations can be significantly reduced.

In the hot rolling, it is preferable to use a reverse mill or a tandem mill for multi-pass rolling. Particularly, from the viewpoint of industrial productivity and the viewpoint of stress loading on the steel sheet during the rolling, at least the final two stands are more preferably hot rolling in which a tandem mill is used. Hot rolling is a process including rough rolling and finish rolling, and each rolling is performed multiple times (stages). Rough rolling is a process of rolling a slab to a minimal thickness of 25 mm, and finish rolling is a process of rolling the sheet after rough rolling to an objected sheet thickness.

(2) Rolling Reduction, Descaling, and Rolling Conditions in Rough Rolling

Rolling and descaling are performed multiple times in rough rolling. In the present embodiment, it is preferable that the time times of rough rolling are set to 5 times or more, and at all stands of rough rolling, rolling are performed at the temperature range of lower than 1130° C., rolling are performed at the rolling reductions of less than 50%, descaling are performed at the inlets of rolling, and rolling are performed within 2.0 seconds after descaling.

By setting the time times of rough rolling to 5 times or more, and at all stands of rough rolling, by performing rolling at the temperature range of lower than 1130° C., by performing rolling at the rolling reductions of less than 50%, and by performing descaling at the inlets of rolling, the thickness of the scale formed in the previous step can be sufficiently reduced or the scale can be sufficiently removed, and an excessive increase in the scale thickness can be suppressed. When the scale is formed on the surface of the steel sheet, Ni is concentrated at the surface of the steel sheet with a growth of the scale. Therefore, if the growth rate of the scale is slowed, Ni may be difficult to concentrate at the surface of the steel sheet. By setting the time times of rough rolling to 5 times or more, and at all stands of rough rolling, by performing rolling at the temperature range of lower than 1130° C., by performing rolling at the rolling reductions of less than 50%, and by performing descaling at the inlets of rolling, as a result, the area ratio of the region where the Ni concentration is 0.2 mass % or more can be preferably controlled.

Note that performing rolling at the temperature range of lower than 1130° C. at all stands of rough rolling means the inlet temperature of all stands of rough rolling is in the temperature range of lower than 1130° C. Descaling can be performed by water spray.

At all stands of rough rolling, rolling is preferably performed within 2.0 seconds after descaling. When longer than 2.0 seconds are passed after descaling, the oxides at the surface may become excessively coarse. Therefore, rolling are preferably performed within 2.0 seconds after descaling.

(3) Retention Condition After Completion of Rough Rolling.

It is preferable that the maximum temperature reached from completion of rough rolling to 120 seconds pass, and to start of descaling of finish rolling is set to 1000° C. to 1170° C., and retention at this maximum temperature is performed for 1.0 second or longer. By setting the maximum temperature reached from completion of rough rolling to 120 seconds pass, and to start of descaling of finish rolling to 1000° C. to 1170° C., the area ratio of the region where the O concentration is 3.0 mass % or more can be preferably controlled. When the maximum temperature is lower than 1000° C. or higher than 1170° C., the area ratio of the region where the O concentration is 3.0 mass % or more may not be preferably controlled

Note that the temperature may be increased to the temperature range of 1000° C. to 1170° C. by heating, or may be increased by processing heat generated by rolling without heating. There is a case where the exit temperature after the final stand of rough rolling is 1000° C. or higher, in this case, the temperature may be retained in the temperature range of 1000° C. to 1170° C. for 1.0 second or longer without increasing the temperature, or the temperature may be increased to the temperature of 1170° C. or lower and then retained in the temperature range of 1000° C. to 1170° C. for 1.0 second or longer.

In addition, in the retention at the temperature range of 1000° C. to 1170° C., the steel sheet temperature may be fluctuated or be maintained constant in the temperature range of 1000° C. to 1170° C.

(4) Rolling Reduction of Hot Rolling: 90% or More in Total in Temperature Range of 850° C. to 1100° C.

When the hot rolling is performed so that the total rolling reduction is 90% or more in the temperature range of 850° C. to 1100° C., mainly recrystallized austenite grains are refined, and accumulation of strain energy into the unrecrystallized austenite grains is promoted. In addition, the recrystallization of austenite is promoted, and the atomic diffusion of Mn is promoted, which makes it possible to reduce the standard deviation of the Mn concentrations. Therefore, it is preferable to perform the hot rolling so that the total rolling reduction is 90% or more in the temperature range of 850° C. to 1100° C.

Note that the hot rolling referred to herein includes rough rolling and finish rolling.

The rolling reduction in the temperature range of 850° C. to 1100° C. can be expressed as {(t₀−t₁)/t₀}×100(%), where an inlet sheet thickness before rolling of the first rolling in the temperature range is t₀ and an outlet sheet thickness after rolling of the final stand in the temperature range is t₁.

(5) Stress Loaded to Steel Sheet From Rolling at One Stand before Final Stand of Hot Rolling To Start of Rolling at Final Stand: 170 kPa or More

The stress of 170 kPa or more is preferably loaded to the steel sheet from rolling at the one stand before the final stand of hot rolling to start of rolling at the final stand. This make it possible to reduce the number of grains having a {110}<001> crystal orientation in the recrystallized austenite after the rolling at the one stand before the final stand. Since {110}<001> is a crystal orientation that is difficult to recrystallize, recrystallization by the rolling of the final stand can be effectively promoted by suppressing the formation of this crystal orientation. As a result, the band-like structure of the hot-rolled steel sheet is improved, the periodicity of the microstructure is reduced, and the E value increases.

Note that the rolling at the one stand before the final stand of hot rolling referred to herein means the rolling at one stand before the final stage of finish rolling. For example, when finish rolling is performed with 7 stands of F1, F2, . . . , F6, and F7 it means rolling at the 6^th stand (F6).

When the stress that is loaded to the steel sheet is less than 170 kPa, a desired E value may not be obtained. The stress that is loaded to the steel sheet is preferably 190 kPa or more.

Note that the stress that is loaded to the steel sheet refers to a tension in the longitudinal direction of the steel sheet, and can be controlled by adjusting the roll rotation speed during tandem rolling, can be determined by dividing the load in the rolling direction measured in the rolling stand by the cross sectional area of the steel sheet being passed.

(6) Rolling Reduction at Final Stand of Hot Rolling: 8% or More, Finishing Temperature Tf: 900° C. or Higher and Lower Than 1010° C.

It is preferable that the rolling reduction at the final stand of the hot rolling is set to 8% or more and the finishing temperature Tf is set to 900° C. or higher. When the rolling reduction at the final stand of the hot rolling is set to 8% or more, it is possible to promote recrystallization caused by the final stand rolling. As a result, the band-like structure of the hot-rolled steel sheet is improved, the periodicity of the microstructure is reduced, and the E value increases. When the finishing temperature Tf is set to 900° C. or higher, it is possible to suppress an excessive increase in the number of ferrite nucleation sites in austenite. As a result, the formation of ferrite in the final structure (the microstructure of the hot-rolled steel sheet after manufacturing) is suppressed, and the hot-rolled steel sheet having high strength can be obtained. In addition, when Tf is set to lower than 1010° C., it is possible to suppress the coarsening of the austenite grain sizes and to obtain a desired E value by reducing the periodicity of the microstructure.

(7) Stress Loaded to Steel Sheet From Completion of Rolling at Final Stand of Hot Rolling to Steel Sheet Being Cooled to 800° C.: Less than 200 kPa

It is preferable that stress of less than 200 kPa is loaded to the steel sheet from the completion of the rolling of the final stand of hot rolling to the steel sheet is cooled to 800° C. By loading the stress of less than 200 kPa to the steel sheet, the recrystallization of austenite preferentially proceeds in the rolling direction, and an increase in the periodicity of the microstructure can be suppressed. As a result, the E value can be set to a desired value. The stress that is loaded to the steel sheet is more preferably 180 kPa or less.

(8) After Completion of Hot Rolling, Accelerated Cooling Being Performed to Temperature Range of 600° C. to 780° C. at Average Cooling Rate of 50° C./s or Faster

After the completion of hot rolling, it is preferable that accelerated cooling is performed to the temperature range of 780° C. or lower at the average cooling rate of 50° C./s or faster. This makes it possible to suppress the formation of ferrite and pearlite with a small amount of precipitation hardening, thereby improving the strength of the hot-rolled steel sheet.

Note that the average cooling rate referred to herein refers to a value obtained by dividing the temperature drop width of the steel sheet from the start of accelerated cooling (when introducing the steel sheet into cooling equipment) to the completion of accelerated cooling (when deriving the steel sheet from the cooling equipment) by the time required from the start of accelerated cooling to the completion of accelerated cooling.

The upper limit of the cooling rate is not particularly specified, but when the cooling rate is increased, the cooling equipment becomes large and the equipment cost increases. Therefore, considering the equipment cost, the average cooling rate is preferably 300° C./s or slower. In addition, the cooling stop temperature of the accelerated cooling is preferably set to 600° C. or higher in order to perform the slow cooling described below.

(9) Slow Cooling at Average Cooling Rate of Slower Than 5° C./s Being Performed in Temperature Range of 600° C. to 780° C. for 2.0 Seconds or Longer

When slow cooling at an average cooling rate of slower than 5° C./s is performed in a temperature range of 600° C. to 780° C. for 2.0 seconds or longer, it is possible to sufficiently precipitate the precipitation-hardened ferrite. This makes it possible to achieve both strength and ductility of the hot-rolled steel sheet.

Note that the average cooling rate referred to herein refers to a value obtained by dividing the temperature drop width of the steel sheet from the cooling stop temperature of the accelerated cooling to the stop temperature of the slow cooling by the time required from the stop of the accelerated cooling to the stop of the slow cooling.

The slow cooling time is preferably 3.0 seconds or longer. The upper limit of the slow cooling time is determined by the equipment layout and may be set to approximately shorter than 10.0 seconds. In addition, the lower limit of the average cooling rate of the slow cooling is not particularly provided and may be set to 0° C./s or faster since heating the steel sheet without cooling accompanies a huge equipment investment.

(10) After Slow Cooling, Cooling Being Performed So That Average Cooling Rate at Temperature Range of 450° C. to 600° C. Being 30° C./s or Faster and Slower Than 50° C./s

After finishing the above slow cooling, cooling is preferably performed so that the average cooling rate in the temperature range of 450° C. to 600° C. is 30° C./s or faster and slower than 50° C./s. By setting the average cooling rate in the above temperature range to 30° C./s or faster and slower than 50° C./s, the CS value can be set to a desired value. When the average cooling rate is 50° C./s or faster, a flat lath-like structure with low brightness is likely to be generated, and the CS value becomes less than −8.0×10⁵. When the average cooling rate is slower than 30° C./s, the concentration of carbon in the untransformed portion is promoted, the strength of the hard structure increases, and the strength difference with the soft structure increases, thereby becoming the CS value to more than 8.0×10⁵.

Note that the average cooling rate referred to herein refers to a value obtained by dividing the temperature drop width of the steel sheet from the cooling stop temperature of the slow cooling with the average cooling rate of slower than 5° C./s to the cooling stop temperature of the cooling with the average cooling rate of 30° C./s or faster and slower than 50° C./s by the time required from the stop of the slow cooling with the average cooling rate is slower than 5° C./s to stop of the cooling with the average cooling rate of 30° C./s or faster and slower than 50° C./s.

(11) Average Cooling Rate at Temperature Range of Coiling Temperature to 450° C.: 50° C./s or Faster

In order to suppress the area ratio of pearlite and residual austenite, and obtain a desired strength and formability, the average cooling rate at the temperature range of the coiling temperature to 450° C. is preferably set to 50° C./s or faster. This makes it possible to harden a mother-phase structure.

Note that the average cooling rate referred to herein refers to a value obtained by dividing the temperature drop width of the steel sheet from the cooling stop temperature of the cooling with the average cooling rate of 30° C./s or faster and slower than 50° C./s to the coiling temperature by the time required from the stop of the cooling with the average cooling rate of 30° C./s or faster and slower than 50° C./s to coiling.

(12) Coiling Temperature: 350° C. or Lower

The coiling temperature is set to 350° C. or lower. When the coiling temperature is set to 350° C. or lower, the amount of an iron carbide precipitated is reduced, and the variation in the hardness distribution in the hard phase can be reduced. As a result, it is possible to increase the I value, and thereby suppressing the occurrence of the secondary sheared surface.

EXAMPLES

Next, the effects of one aspect of the present invention will be described more specifically by way of examples, but the conditions in the examples are condition examples adopted for confirming the feasibility and effects of the present invention. The present invention is not limited to these condition examples. 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.

Steels having a chemical composition shown in Tables I and 2 were melted and continuously cast to manufacture slabs having a thickness of 240 to 300 mm. The obtained slabs were used to obtain hot-rolled steel sheets shown in Table 5A to Table 6B under the manufacturing conditions shown in Table 3A to Table 4B.

Note that the average cooling rate of slow cooling was set to slower than 5° C./s. In addition, the retention time at the maximum temperature shown in Table 3A and Table 3B was set to 1.0 second or longer. In addition, since the measurement lower limit of the coiling temperature shown in Table 4A and Table 4B is 50° C., the actual coiling temperatures of the examples with a value of 50° C. are 50° C. or lower.

The area ratio of the microstructure, the E value, the I value, the CS value, the standard deviation of the Mn concentrations, the area ratio of the region where the Ni concentration is 0.2 mass % or more, the area ratio of the region where the O concentration is 3.0 mass % or more, the maximum value of sphere equivalent diameters of oxides, the tensile strength TS, and the total elongation El of each the obtained hot-rolled steel sheets were obtained by the above methods. The obtained results are shown in Table 5A to Table 6B.

Evaluation Method of Properties of Hot-Rolled Steel Sheets

Tensile Properties

In a case where the tensile strength TS was 980 MPa or more, the total elongation El was 10.0% or more, and the tensile strength TS×total elongation El was 13000 MPa-% or more, the hot-rolled steel sheet was determined as having high strength and excellent ductility, and being successful. In a case where any one was not satisfied, the hot-rolled steel sheet was determined as not having high strength and excellent ductility, and not being successful.

Sheet Thickness Reduction at Critical Fracture

The sheet thickness reduction at critical fracture of the hot-rolled steel sheet was evaluated by tensile test.

The tensile test was conducted in the similar manner as when the tensile properties were evaluated. When the sheet thickness before the tensile test was set to t₁, and the minimum sheet thickness at the center part of the width direction (lateral direction) of the tensile test piece after fracture was set t₂, the sheet thickness reduction at critical fracture was obtained by calculating the value of (t₁−t₂)×100/t₁. The tensile test was conducted 5 times, and the sheet thickness reduction at critical fracture was obtained by calculating the average value of 3 times excluding the maximum and the minimum values of the sheet thickness reduction at critical fracture.

In a case where the sheet thickness reduction at critical fracture was 60.0% or more, the hot-rolled steel sheet was determined as having high sheet thickness reduction at critical fracture, and being successful. On the other hand, in a case where the sheet thickness reduction at critical fracture was less than 60.0%, the hot-rolled steel sheet was determined as not having high sheet thickness reduction at critical fracture, and not being successful.

Shearing property (Evaluation of Secondary Sheared Surface)

The shearing property of the hot-rolled steel sheet was evaluated by a punching test.

Three punched holes were produced in each example with a hole diameter of 10 mm, a clearance of 10%, and a punching speed of 3 m/s. Next, a cross section perpendicular to the rolling direction and a cross section parallel to the rolling direction of the punched hole were each embedded in a resin, and the cross-sectional profile was photographed with a scanning electron microscope. In the obtained observation photographs, the sheared end surfaces as shown in FIG. 1 or FIG. 2 can be observed. FIG. 1 is an example of a sheared end surface of a hot-rolled steel sheet according to the present invention example, and FIG. 2 is an example of a sheared end surface of a hot-rolled steel sheet according to a comparative example. In FIG. 1, the sheared end surface is a sheared end surface with a shear droop, a sheared surface, a fractured surface, and a burr. On the other hand, in FIG. 2, the sheared end surface is a sheared end surface with a shear droop, a sheared surface, a fractured surface, a sheared surface, a fractured surface, and a burr. Here, the shear droop is an R-like smooth surface region, the sheared surface is the region of a punched end surface separated by shear deformation, the fractured surface is the region of a punched end surface separated by a crack initiated from the vicinity of the cutting edge, and a burr is a surface having projections protruding from the lower surface of the hot-rolled steel sheet.

In a case where, for example, a sheared surface, a fractured surface, and a sheared surface as shown in FIG. 2 appeared on two surfaces perpendicular to the rolling direction and two surfaces parallel to the rolling direction in the obtained sheared end surface, a secondary sheared surface was determined to be formed. 4 surfaces for each punched hole, that is, a total of 12 surfaces were observed, and, in a case where there was no surface on which a secondary sheared surface appeared, the hot-rolled steel sheet was determined as having excellent shearing property and being successful, and mentioned as “Absence” in Tables. On the other hand, in a case where even a single secondary sheared surface was formed, the hot-rolled steel sheet was determined as not having excellent shearing property and not being successful, and mentioned as “Presence” in Tables.

Chemical Convertibility

A sample of 150 mm×70 mm was collected from the hot-rolled steel sheet after pickling, and after chemical conversion treatment using a chemical treatment solution (PB-SX35) manufactured by Nihon Parkerizing Co., Ltd., 3 points (center and both ends) along the length of the test piece were observed at a magnification of 1000 times using a scanning electron microscope (SEM) to observe the degree of adhesion of the grains of the zinc phosphate film. The pickling condition was the same as those for the pickling treatment described above.

In a case where the zinc phosphate crystals of the chemical conversion film were densely attached, it was evaluated as “Good”, and in a case the zinc phosphate crystals were sparse and there were small gaps between adjacent crystals (parts where the zinc phosphate film was not attached, commonly called “lack of hiding”), it was evaluated as “Fair”. In a case where there were areas where the chemical conversion film was clearly not coated, it was evaluated as “Bad”. In this evaluation, examples evaluated as Good and Fair were determined to be successful.

Resistance to Inner Crack at Bending

The resistance to inner crack at bending was evaluated by the following bending test.

From a position of ½ in the width direction of the hot-rolled steel sheet after pickling, a strip-shaped test piece of 100 mm×30 mm was cut out to obtain a bending test piece. For both a bend where the bending ridge was parallel to the rolling direction (L direction) (L-axis bending) and a bend where the bending ridge was perpendicular to the rolling direction (C direction) (C-axis bending), the test according to a V block test (the bending angle was 90°) of JIS Z 2248: 2022 was conducted. And thus, the minimum bend radii at which cracks were not occurred were obtained to investigate the resistance to inner crack at bending. A value obtained by dividing the average value of the minimum bend radii in the L axis and in the C axis by the sheet thickness was regarded as the critical bend R/t and used as an index value of the resistance to inner crack at bending.

In a case where R/t was 2.5 or lower, the hot-rolled steel sheet was determined to be excellent in the resistance to inner crack at bending, and to be successful. On the other hand, in a case where R/t was higher than 2.5, the hot-rolled steel sheet was determined not to be excellent in the resistance to inner crack at bending, and not to be successful.

Here, regarding the presence or absence of cracks, a cross section obtained by cutting the test piece after the test on a plane parallel to the bending direction and perpendicular to the sheet surface was mirror polished, then, cracks were observed with an optical microscope, and a case where the length of the crack observed in the bend inner of the test piece exceeds 30 μm was determined as crack being present.

	TABLE 1
	Chemical composition (mass %) remainder being Fe and impurities

Steel								Ti +
No.	C	Si	Mn	Ni	Ti	Nb	V	Nb + V	sol. Al	P	S	N	O
A	0.075	0.76	1.87	0.10	0.125			0.125	0.362	0.009	0.0033	0.0030	0.0026
B	0.075	1.52	1.82	0.06	0.126			0.126	0.338	0.012	0.0042	0.0024	0.0024
C	0.216	1.47	1.23	0.07	0.124			0.124	0.312	0.014	0.0011	0.0021	0.0039
D	0.138	0.25	1.97	0.08	0.182			0.182	1.440	0.010	0.0048	0.0047	0.0031
E	0.158	2.80	1.35	0.15	0.121			0.121	0.009	0.020	0.0043	0.0038	0.0044
F	0.062	1.52	2.74	0.12	0.126			0.126	0.334	0.012	0.0032	0.0037	0.0035
G	0.087	2.66	1.78	0.08	0.138			0.138	0.033	0.016	0.0016	0.0047	0.0024
H	0.057	1.42	3.75	0.07	0.124			0.124	1.920	0.011	0.0035	0.0030	0.0045
I	0.074	1.72	2.31	0.07	0.059	0.016		0.075	0.345	0.013	0.0024	0.0030	0.0033
J	0.075	1.42	1.74	0.06			0.162	0.162	0.361	0.018	0.0058	0.0046	0.0037
K	0.089	1.58	1.82	0.05		0.092		0.092	0.302	0.020	0.0033	0.0050	0.0034
L	0.085	1.48	2.01	0.08	0.101	0.013	0.042	0.156	0.342	0.013	0.0034	0.0025	0.0022
M	0.075	1.35	1.82	0.09	0.122			0.122	0.332	0.016	0.0033	0.0028	0.0034
N	0.079	1.38	1.87	1.20	0.124			0.124	0.331	0.010	0.0045	0.0036	0.0042
O	0.073	1.70	1.76	0.02	0.127			0.127	0.352	0.020	0.0035	0.0035	0.0039
P	0.045	1.52	1.83	0.08	0.132			0.132	0.346	0.015	0.0040	0.0027	0.0034
Q	0.265	1.69	1.82	0.06	0.098			0.098	0.309	0.021	0.0033	0.0050	0.0042
R	0.078	3.16	1.85	0.05	0.129			0.129	0.361	0.014	0.0035	0.0024	0.0042
S	0.090	1.49	0.84	0.08	0.138			0.138	0.351	0.017	0.0034	0.0025	0.0022
T	0.076	1.65	1.75	0.01	0.124			0.124	0.368	0.009	0.0043	0.0039	0.0027
U	0.073	1.59	1.82	0.08	0.052			0.052	0.392	0.011	0.0034	0.0040	0.0044
Underlines indicate that values are outside the range of the present invention.

	TABLE 2
	Chemical composition (mass %) remainder being Fe and impurities

Steel

Zr + Co +

No.

Cu

Cr

Mo

B

Ca

Mg

REM

Bi

As

Zr

Co

Zn

W

Zn + W

Sn

A
B
C					0.0027	0.0018
D								0.0020
E
F												0.16		0.16
G				0.0038
H															0.03
I
J							0.0024		0.042
K												0.09	0.15	0.24
L										0.24				0.24
M	0.15										0.23			0.23
N		0.24		0.0024
O			0.13												0.02
P
Q
R
S
T
U
Underlines indicate that values are outside the range of the present invention.

TABLE 3A
							Number of
							times of
		Retention		Retention		Maximum	rough	Number of
		time in		time in		temperature	rolling at	times of
		temperature		temperature	Number of	of inlet	rolling	descaling
		range of		range of	times of	temperature	reduction	performed at
		700° C. to	Heating	1100° C. or	rough	of rough	of less	inlet of
Manufacturing	Steel	850° C.	temperature	higher	rolling	rolling	than 50%	rolling
No.	No.	s	° C.	s	Times	° C.	Times	Times
1	A	1282	1210	9431	6	1076	6	6
2	B	1264	1220	9014	6	1114	6	6
3	B	824	1233	8976	6	1109	6	6
4	B	1384	1224	5794	6	1120	6	6
5	B	1345	1220	9246	4	1125	4	4
6	B	1346	1229	9119	6	1121	5	6
7	B	1248	1230	9260	6	1121	6	5
8	B	1357	1224	9307	6	1113	6	6
9	B	1365	1221	9339	6	1110	6	6
10	B	1362	1243	9346	6	1128	6	6
11	B	1340	1232	9619	6	1125	6	6
12	B	1435	1245	9533	6	1126	6	6
13	B	1134	1220	9153	6	1112	6	6
14	B	1395	1228	8986	6	1115	6	6
15	B	1295	1222	9382	6	1105	6	6
16	B	1351	1235	9414	6	1110	6	6
17	B	1234	1231	9753	6	1113	6	6
18	B	1334	1235	9511	6	1125	6	6
19	B	1365	1233	9035	6	1100	6	6
20	B	1345	1249	9632	6	1119	6	6
21	B	1195	1242	9701	6	1099	6	6

			Maximum
		Number of	temperature		Loaded stress to
		times of	reached from		steel sheet from
		rolling	completion of	Sheet	completion of
		performed	rough rolling	thickness	rolling at one
		within 2.0	to 120 seconds	reduction in	stand before final
		seconds	pass, and to	temperature	stand to start of
		after	start of finish	range of 850° C.	rolling at final
	Manufacturing	descaling	descaling	to 1100° C.	stand
	No.	Times	° C.	%	kPa	Note
	1	6	1085	96	204	Present Invention
						Example
	2	6	1125	94	226	Present Invention
						Example
	3	6	1118	96	229	Comparative Example
	4	6	1121	93	226	Comparative Example
	5	4	1130	92	225	Comparative Example
	6	6	1115	93	229	Comparative Example
	7	6	1120	95	221	Comparative Example
	8	5	1124	95	222	Comparative Example
	9	6	992	93	207	Comparative Example
	10	6	1179	94	204	Comparative Example
	11	6	1125	87	210	Comparative Example
	12	6	1135	93	165	Comparative Example
	13	6	1121	96	226	Comparative Example
	14	6	1105	93	219	Comparative Example
	15	6	1086	93	229	Comparative Example
	16	6	1121	94	220	Comparative Example
	17	6	1097	93	205	Comparative Example
	18	6	1124	95	211	Comparative Example
	19	6	1111	95	206	Comparative Example
	20	6	1128	93	226	Comparative Example
	21	6	1110	91	216	Comparative Example
	Underlines indicate that values are outside the range of the present invention or are not preferable manufacturing conditions.

TABLE 3B
		Retention		Retention		Maximum		Number of
		time in		time in		temperature	Number of	times of
		temperature		temperature	Number of	of inlet	times of rough	descaling
		range of		range of	times of	temperature	rolling at rolling	performed at
		700° C. to	Heating	1100° C. or	rough	of rough	reduction of	inlet of
Manufacturing	Steel	850° C.	temperature	higher	rolling	rolling	less than 50%	rolling
No.	No.	s	° C.	s	Times	° C.	Times	Times
22	C	954	1350	7268	6	1101	6	6
23	D	1209	1342	9352	6	1101	6	6
24	E	1251	1295	9693	6	1128	6	6
25	F	1856	1205	11500	8	1110	8	8
26	G	1549	1230	9748	8	1124	8	8
27	H	2006	1230	10825	6	1105	6	6
28	I	1502	1195	9332	8	1108	8	8
29	J	1506	1232	9030	6	1125	6	6
30	K	1260	1221	8963	6	1121	6	6
31	L	1295	1203	9191	6	1116	6	6
32	M	1496	1210	9071	6	1103	6	6
33	N	1416	1203	9132	6	1104	6	6
34	O	1449	1240	9500	6	1114	6	6
35	P	1510	1249	9110	6	1110	6	6
36	Q	1536	1240	9394	8	1126	8	8
37	R	1438	1352	9524	8	1114	8	8
38	S	1575	1210	9570	6	1113	6	6
39	T	1157	1240	9492	6	1105	6	6
40	U	1424	1200	9052	6	1102	6	6
41	B	1232	1252	7364	6	1132	6	6
42	B	1252	1230	8245	6	1115	6	6

			Maximum
			temperature
		Number of	reached from		Loaded stress to
		times of	completion		steel sheet from
		rolling	of rough	Sheet	completion of
		performed	rolling to 120	thickness	rolling at one
		within 2.0	seconds pass,	reduction in	stand before final
		seconds	and to start	temperature	stand to start of
		after	of finish	range of 850° C.	rolling at final
	Manufacturing	descaling	descaling	to 1100° C.	stand
	No.	Times	° C.	%	kPa	Note
	22	6	1112	96	208	Present Invention
						Example
	23	6	1110	96	235	Present Invention
						Example
	24	6	1152	96	192	Present Invention
						Example
	25	8	1121	93	208	Present Invention
						Example
	26	8	1148	94	201	Present Invention
						Example
	27	6	1032	96	225	Present Invention
						Example
	28	8	1056	93	175	Present Invention
						Example
	29	6	1124	95	219	Present Invention
						Example
	30	6	1130	93	200	Present Invention
						Example
	31	6	1125	93	228	Present Invention
						Example
	32	6	1112	96	224	Present Invention
						Example
	33	6	1113	95	225	Present Invention
						Example
	34	6	1125	94	206	Present Invention
						Example
	35	6	1119	94	222	Comparative Example
	36	8	1135	96	221	Comparative Example
	37	8	1125	95	209	Comparative Example
	38	6	1122	93	219	Comparative Example
	39	6	1045	94	217	Comparative Example
	40	6	1087	94	224	Comparative Example
	41	6	1142	92	215	Comparative Example
	42	6	1125	94	225	Comparative Example
	Underlines indicate that values are outside the range of the present invention or are not preferable manufacturing conditions.

TABLE 4A
					Average cooling
				Loaded stress to steel	rate to temperature
			Rolling	sheet from completion	range of 600° C. to	Starting
		Finishing	reduction	of rolling at final stand	780° C. after	temperature
		temperature	at final	to steel sheet being	completion of	of slow
Manufacturing	Steel	Tf	stand	cooled to 800° C.	hot rolling	cooling
No.	No.	° C.	%	kPa	° C./s	° C.
1	A	945	17	191	108	674
2	B	972	17	193	91	712
3	B	926	13	186	83	701
4	B	924	15	184	83	705
5	B	942	16	184	72	742
6	B	930	16	183	106	706
7	B	972	15	184	98	716
8	B	931	15	186	107	720
9	B	930	14	184	124	718
10	B	931	15	185	105	725
11	B	942	15	178	65	725
12	B	943	17	182	103	685
13	B	1029	13	188	109	642
14	B	946	7	198	91	641
15	B	950	15	212	82	712
16	B	973	13	188	41	713
17	B	941	13	185	97	801
18	B	966	15	181	82	695
19	B	942	16	186	87	675
20	B	933	16	198	109	687
21	B	935	13	183	117	701

				Average
			Average	cooling
			cooling	rate in
		Slow cooling	rate in	temperature
		time in	temperature	range of
		temperature	range	coiling
		range of 600° C.	of 450° C.	temperature	Coiling
	Manufacturing	to 780° C.	to 600° C.	to 450° C.	temperature
	No.	s	° C./s	° C./s	° C.	Note
	1	4.2	38	92	50	Present Invention
						Example
	2	4.0	45	137	50	Present Invention
						Example
	3	3.8	37	102	50	Comparative Example
	4	4.2	36	122	50	Comparative Example
	5	3.2	41	98	50	Comparative Example
	6	4.0	41	102	50	Comparative Example
	7	4.3	42	95	50	Comparative Example
	8	4.2	36	98	50	Comparative Example
	9	4.1	36	94	50	Comparative Example
	10	4.0	38	100	50	Comparative Example
	11	4.0	46	105	50	Comparative Example
	12	3.8	38	112	50	Comparative Example
	13	3.8	36	102	50	Comparative Example
	14	3.8	44	142	50	Comparative Example
	15	3.9	35	97	50	Comparative Example
	16	4.0	38	109	50	Comparative Example
	17	4.2	44	87	50	Comparative Example
	18	1.3	40	92	50	Comparative Example
	19	4.0	65	142	50	Comparative Example
	20	3.8	22	105	50	Comparative Example
	21	4.2	43	46	50	Comparative Example
	Underlines indicate that values are outside the range of the present invention or are not preferable manufacturing conditions.

TABLE 4B
					Average cooling
				Loaded stress to steel	rate to temperature
				sheet from completion	range of 600° C. to
		Finishing	Rolling	of rolling at final stand	780° C. after	Starting
		temperature	reduction at	to steel sheet being	completion of hot	temperature of
Manufacturing	Steel	Tf	final stand	cooled to 800° C.	rolling	slow cooling
No.	No.	° C.	%	kPa	° C./s	° C.
22	C	905	14	187	111	621
23	D	911	14	181	123	702
24	E	945	15	191	95	682
25	F	904	42	187	91	714
26	G	971	14	192	87	702
27	H	912	16	196	103	725
28	I	952	17	184	105	703
29	J	980	14	184	116	695
30	K	956	13	195	98	711
31	L	925	15	175	85	720
32	M	946	15	192	97	700
33	N	930	9	186	114	680
34	O	971	14	193	111	652
35	P	953	15	176	96	671
36	Q	943	17	193	97	674
37	R	920	16	182	88	674
38	S	966	16	195	95	690
39	T	955	17	186	85	678
40	U	952	16	187	97	668
41	B	956	10	186	68	698
42	B	962	9	192	57	725

	Slow cooling
	time in	Average cooling	Average cooling rate in
	temperature	rate in temperature	temperature range of
	range of 600° C.	range of 450° C. to	coiling temperature to	Coiling
Manufacturing	to 780° C.	600° C.	450° C.	temperature
No.	s	° C./s	° C./s	° C.	Note
22	6.5	41	62	50	Present
					Invention
					Example
23	3.9	39	148	50	Present
					Invention
					Example
24	4.2	41	87	190	Present
					Invention
					Example
25	5.2	44	138	216	Present
					Invention
					Example
26	4.1	44	122	50	Present
					Invention
					Example
27	6.0	35	58	315	Present
					Invention
					Example
28	3.9	41	125	50	Present
					Invention
					Example
29	4.1	40	152	50	Present
					Invention
					Example
30	2.7	35	153	50	Present
					Invention
					Example
31	3.9	37	138	50	Present
					Invention
					Example
32	4.0	40	124	50	Present
					Invention
					Example
33	3.9	36	101	50	Present
					Invention
					Example
34	3.8	42	129	50	Present
					Invention
					Example
35	3.8	44	123	50	Comparative
					Example
36	4.2	44	134	50	Comparative
					Example
37	4.2	45	115	50	Comparative
					Example
38	3.8	37	131	50	Comparative
					Example
39	4.0	41	115	50	Comparative
					Example
40	3.8	44	138	50	Comparative
					Example
41	4.0	35	72	50	Comparative
					Example
42	4.3	48	72	380	Comparative
					Example
Underlines indicate that values are outside the range of the present invention or are not preferable manufacturing conditions.

TABLE 5A
		Sheet		Residual		Remainder in	E	I
Manufacturing	Steel	thickness	Ferrite	austenite	Pearlite	microstructure	value	value
No.	No.	mm	Area %	Area %	Area %	Area %	—	—
1	A	2.6	24.3	0.0	0.0	75.7	11.5	1.068
2	B	2.9	44.2	0.0	0.0	55.8	11.4	1.082
3	B	2.9	37.1	0.0	0.0	62.9	11.9	1.032
4	B	2.9	30.2	0.0	0.0	69.8	11.4	1.057
5	B	2.9	48.2	0.0	0.0	51.8	11.3	1.043
6	B	2.9	41.9	0.0	0.0	58.1	11.5	1.041
7	B	2.9	44.5	0.0	0.0	55.5	11.3	1.062
8	B	2.9	42.5	0.0	0.0	57.5	11.4	1.035
9	B	2.9	33.4	0.0	0.0	66.6	11.4	1.057
10	B	2.9	35.2	0.0	0.0	64.8	11.2	1.052
11	B	2.9	39.9	0.0	0.0	60.1	11.2	1.058
12	B	2.9	41.9	0.0	0.0	58.1	10.3	1.071
13	B	2.9	38.7	0.0	0.0	61.3	10.5	1.059
14	B	2.9	33.8	0.0	0.0	66.2	10.6	1.078
15	B	2.9	35.9	0.0	0.0	64.1	10.3	1.054
16	B	2.9	58.2	0.0	5.4	36.4	11.3	1.072
17	B	2.9	52.9	0.0	7.8	39.3	11.4	1.067
18	B	2.9	11.7	0.0	0.0	88.3	11.2	1.056
19	B	2.9	40.3	0.0	0.0	59.7	11.3	1.067
20	B	2.9	33.6	0.0	8.7	57.7	11.7	1.079
21	B	2.9	38.5	8.1	0.0	53.4	11.1	1.076

			Area	Area
			ratio of	ratio of
			region	region	Maximum
			where Ni	where O	value of
			concentration	concentration	sphere
		Mn	is 0.2	is 3.0	equivalent
	CS	standard	mass %	mass %	diameters
Manufacturing	value	deviation	or more	or more	of oxides
No.	×105	Mass %	%	%	μm	Note
1	2.5	0.39	32.2	25.2	3.52	Present
						Invention
						Example
2	0.1	0.45	24.1	11.7	1.51	Present
						Invention
						Example
3	−2.5	0.64	22.9	14.6	1.74	Comparative
						Example
4	2.5	0.61	23.4	15.9	2.01	Comparative
						Example
5	−2.4	0.42	7.9	10.6	2.63	Comparative
						Example
6	−2.6	0.46	9.0	25.7	3.54	Comparative
						Example
7	−2.3	0.45	8.5	41.4	4.79	Comparative
						Example
8	−2.5	0.42	25.1	44.6	5.40	Comparative
						Example
9	−2.6	0.43	24.4	52.0	3.52	Comparative
						Example
10	−2.4	0.43	22.3	2.6	3.19	Comparative
						Example
11	−2.6	0.62	25.1	24.5	2.63	Comparative
						Example
12	−1.9	0.40	22.7	19.5	3.25	Comparative
						Example
13	−0.5	0.38	25.4	23.7	1.62	Comparative
						Example
14	−5.4	0.43	21.3	19.7	2.47	Comparative
						Example
15	0.1	0.38	22.4	23.6	3.80	Comparative
						Example
16	2.6	0.40	20.6	14.6	3.35	Comparative
						Example
17	0.4	0.38	24.2	22.3	2.15	Comparative
						Example
18	2.5	0.45	21.0	19.5	1.84	Comparative
						Example
19	−8.4	0.43	22.9	23.9	3.66	Comparative
						Example
20	9.2	0.45	23.7	21.8	3.35	Comparative
						Example
21	−3.7	0.40	21.2	21.7	2.72	Comparative
						Example
Underlines indicate that values are outside the range of the present invention or properties are not preferable.

TABLE 5B
		Sheet		Residual		Remainder in	E	I
Manufacturing	Steel	thickness	Ferrite	austenite	Pearlite	microstructure	value	value
No.	No.	mm	Area %	Area %	Area %	Area %	—	—
22	C	2.6	16.9	0.0	0.0	83.1	11.5	1.056
23	D	2.6	18.7	0.0	0.0	81.3	11.4	1.068
24	E	2.6	52.2	2.4	0.0	45.4	11.7	1.089
25	F	1.4	21.9	0.0	0.0	78.1	11.6	1.087
26	G	2.6	48.2	1.2	0.0	50.6	11.6	1.055
27	H	6.4	15.2	0.0	0.0	84.8	11.4	1.082
28	I	2.9	35.6	0.0	0.0	64.4	10.8	1.063
29	J	2.9	40.6	0.0	0.0	59.4	11.6	1.064
30	K	2.9	26.7	0.0	0.0	73.3	11.5	1.025
31	L	2.9	39.2	0.0	0.0	60.8	11.6	1.072
32	M	2.9	37.5	0.0	0.0	62.5	11.6	1.081
33	N	2.9	34.2	0.0	0.0	65.8	11.5	1.058
34	O	2.9	44.6	0.0	0.0	55.4	11.8	1.058
35	P	2.9	82.9	0.0	0.0	17.1	11.4	1.054
36	Q	2.9	7.6	0.0	0.0	92.4	11.4	1.040
37	R	2.9	65.4	6.7	0.0	27.9	11.9	1.078
38	S	2.9	73.5	0.0	0.0	26.5	11.8	1.069
39	T	2.9	41.3	0.0	0.0	58.7	11.2	1.072
40	U	2.9	53.4	0.0	0.0	46.6	11.5	1.059
41	B	2.9	43.6	0.0	0.0	56.4	11.2	1.056
42	B	2.9	35.4	2.5	0.0	62.1	11.6	1.013

			Area	Area
			ratio of	ratio of
			region	region	Maximum
			where Ni	where O	value of
			concentration	concentration	sphere
		Mn	is 0.2	is 3.0	equivalent
	CS	standard	mass % or	mass % or	diameters
Manufacturing	value	deviation	more	more	of oxides
No.	×105	Mass %	%	%	μm	Note
22	1.6	0.40	24.2	15.9	1.52	Present
						Invention
						Example
23	−2.6	0.45	21.4	9.9	3.17	Present
						Invention
						Example
24	2.4	0.41	51.8	14.8	3.06	Present
						Invention
						Example
25	−0.1	0.48	46.7	23.9	3.39	Present
						Invention
						Example
26	0.2	0.44	20.5	28.8	1.97	Present
						Invention
						Example
27	2.4	0.55	23.7	22.6	2.51	Present
						Invention
						Example
28	−0.9	0.45	21.7	17.6	3.39	Present
						Invention
						Example
29	−2.2	0.39	23.2	16.4	1.79	Present
						Invention
						Example
30	−1.0	0.43	22.1	22.4	1.66	Present
						Invention
						Example
31	−3.8	0.44	23.4	16.2	2.45	Present
						Invention
						Example
32	−2.9	0.42	38.9	23.7	1.52	Present
						Invention
						Example
33	−5.6	0.38	100.0	15.5	3.28	Present
						Invention
						Example
34	2.4	0.45	18.2	21.4	3.70	Present
						Invention
						Example
35	−2.4	0.44	25.4	20.1	2.05	Comparative
						Example
36	0.4	0.36	24.3	14.6	2.51	Comparative
						Example
37	1.4	0.45	25.9	27.9	3.91	Comparative
						Example
38	−3.3	0.32	35.1	21.2	2.73	Comparative
						Example
39	1.6	0.37	7.6	16.4	2.37	Comparative
						Example
40	−3.6	0.43	31.8	23.2	1.97	Comparative
						Example
41	0.1	0.56	9.5	10.2	1.64	Comparative
						Example
42	−2.5	0.46	25.4	12.5	1.82	Comparative
						Example
Underlines indicate that values are outside the range of the present invention or properties are not preferable.

TABLE 6
					Presence or	Sheet
					absence of	thickness
		Tensile	Total		secondary	reduction at	Critical
		strength	elongation	TS ×	sheared	critical	bend	Chemical
Manufacturing	Steel	TS	El	El	surface	fracture	R/t	convertibility
No.	No.	MPa	%	MPa %	—	%	—	—	Note
1	A	1015	14.5	14718	Absence	71.3	2.3	Good	Present Invention
									Example
2	B	1045	15.6	16302	Absence	70.4	2.1	Good	Present Invention
									Example
3	B	1018	15.5	15779	Presence	72.4	2.1	Good	Comparative Example
4	B	1011	15.8	15974	Presence	73.5	2.1	Good	Comparative Example
5	B	986	16.2	15973	Absence	74.2	2.1	Bad	Comparative Example
6	B	1021	15.4	15723	Absence	73.4	2.1	Bad	Comparative Example
7	B	1008	15.6	15725	Absence	74.2	2.0	Bad	Comparative Example
8	B	1025	15.3	15683	Absence	72.3	2.6	Fair	Comparative Example
9	B	1024	15.2	15565	Absence	71.5	2.1	Bad	Comparative Example
10	B	1026	15.3	15698	Absence	76.2	2.6	Good	Comparative Example
11	B	1019	15.4	15693	Presence	72.2	2.1	Good	Comparative Example
12	B	1027	15.6	16021	Presence	72.2	2.1	Good	Comparative Example
13	B	1021	15.5	15826	Presence	72.7	2.1	Good	Comparative Example
14	B	1025	15.2	15580	Presence	74.3	2.1	Good	Comparative Example
15	B	1035	15.4	15939	Presence	74.3	2.1	Good	Comparative Example
16	B	945	13.7	12947	Absence	73.2	2.1	Good	Comparative Example
17	B	952	13.6	12947	Absence	74.4	2.1	Good	Comparative Example
18	B	1012	11.8	11942	Absence	74.0	2.1	Good	Comparative Example
19	B	1021	16.2	16540	Absence	58.2	2.1	Good	Comparative Example
20	B	973	13.3	12941	Absence	52.9	2.1	Good	Comparative Example
21	B	1019	15.4	15693	Absence	57.1	2.1	Good	Comparative Example
22	C	1422	10.6	15073	Absence	71.1	2.3	Good	Present Invention
									Example
23	D	1125	11.6	13050	Absence	63.2	2.3	Good	Present Invention
									Example
24	E	984	17.8	17515	Absence	60.4	2.3	Good	Present Invention
									Example
25	F	1123	13.1	14711	Absence	73.8	2.1	Good	Present Invention
									Example
26	G	984	17.9	17614	Absence	67.5	2.3	Good	Present Invention
									Example
27	H	1348	10.3	13884	Absence	73.8	2.2	Good	Present Invention
									Example
28	I	1031	15.2	15671	Absence	71.7	2.1	Good	Present Invention
									Example
29	J	988	17.2	16994	Absence	63.9	2.1	Good	Present Invention
									Example
30	K	1109	13.8	15304	Absence	71.6	2.1	Good	Present Invention
									Example
31	L	1025	15.6	15990	Absence	74.5	2.1	Good	Present Invention
									Example
32	M	1024	15.7	16077	Absence	71.3	2.1	Good	Present Invention
									Example
33	N	1009	16.1	16245	Absence	70.8	2.1	Good	Present Invention
									Example
34	O	1016	15.9	16154	Absence	73.5	2.1	Fair	Present Invention
									Example
35	P	824	22.1	18210	Absence	74.9	2.1	Good	Comparative Example
36	Q	1210	9.3	11253	Absence	73.6	2.1	Good	Comparative Example
37	R	978	17.9	17506	Absence	52.0	2.1	Good	Comparative Example
38	S	951	18.0	17118	Absence	74.6	2.1	Good	Comparative Example
39	T	1021	15.3	15621	Absence	73.8	2.1	Bad	Comparative Example
40	U	872	18.2	15870	Absence	71.1	2.1	Good	Comparative Example
41	B	1015	15.5	15733	Absence	71.2	2.1	Bad	Comparative Example
42	B	1025	12.8	13120	Presence	65.2	2.1	Bad	Comparative Example
Underlines indicate that corresponding values are outside the range of the present invention or not preferable properties.

From Table 5A to Table 6B, it is found that the hot-rolled steel sheets according to the present invention examples have high strength and sheet thickness reduction at critical fracture, excellent ductility, shearing property, chemical convertibility, and resistance to inner crack at bending.

On the other hand, it is found that the hot-rolled steel sheets according to the comparative examples were deteriorated in one or more of the above properties.

INDUSTRIAL APPLICABILITY

According to the above aspect according to the present invention, it is possible to obtain a hot-rolled steel sheet having high strength and sheet thickness reduction at critical fracture, excellent ductility, shearing property, chemical convertibility, and resistance to inner crack at bending.

The hot-rolled steel sheet according to the above aspect of the present invention is suitable as an industrial material used for vehicle members, mechanical structural members, and building members.