Hot-rolled steel sheet

Abstract

This hot-rolled steel sheet has a predetermined chemical composition, in a microstructure at a depth position of 1 / 4 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 E value is 10.7 or more, an I value is 1.020 or more, a CS value is −8.0×105 to 8.0×105, a standard deviation of Mn concentrations is 0.60 mass % or less, a solute Cr concentration in an outermost layer region is 0.10 mass % or more, a number density of Cr oxides having a sphere equivalent radius of 0.1 μm or more at the surface is 1.0×104 pieces / cm2 or less.

Description

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to a hot-rolled steel sheet. Specifically, the present invention relates to a hot-rolled steel sheet that is formed into various shapes by press working or the like to be used, and particularly relates to a hot-rolled steel sheet that has high strength and sheet thickness reduction at critical fracture, excellent ductility and shearing property, and further has excellent fatigue property after press forming.

[0002] Priority is claimed on Japanese Patent Application No. 2022-142994, filed on Sep. 8, 2022, the content of which is incorporated herein by reference.BACKGROUND ART

[0003] 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.

[0004] 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.

[0005] 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.

[0006] 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.

[0007] In addition, steel sheets applied for vehicle members are required to have excellent fatigue property after press forming.

[0008] 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.

[0009] However, in Patent Document 1, a sheet thickness reduction at critical fracture, shearing property, and fatigue property after press forming of the hot-rolled steel sheet are not considered.PRIOR ART DOCUMENTPatent Document

[0010] Patent Document 1: WO2020 / 044445Non-Patent Document

[0011] Non-Patent Document 1: J. Webel, J. Gola, D. Britz, F. Mucklich, Materials Characterization 144 (2018) 584-596

[0012] Non-Patent Document 2: D. L. Naik, H. U. Sajid, R. Kiran, Metals 2019, 9, 546

[0013] 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. 474-485DISCLOSURE OF THE INVENTIONProblems to be Solved by the Invention

[0014] 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 and shearing property, and further having excellent fatigue property after press forming.Means for Solving the Problem

[0015] The gist of the present invention is as follows.

[0016] (1) A hot-rolled steel sheet according to one aspect of the present invention comprising, in terms of mass %, as a chemical composition.

[0017] C: 0.050% to 0.250%,

[0018] Si: 0.05% to 3.00%,

[0019] Mn: 1.00% to 4.00%,

[0020] sol. Al: 0.001% to 0.500%,

[0021] Cr: 0.060% to 2.000%;

[0022] P: 0.100% or less,

[0023] S: 0.0300% or less,

[0024] N: 0.1000% or less,

[0025] Q: 0.0100% or less,

[0026] Ti: 0% to 0.500%,

[0027] Nb: 0% to 0.500%,

[0028] V: 0% to 0.500%,

[0029] Cu: 0% to 2.00%,

[0030] Mo: 0% to 1.00%,

[0031] Ni: 0% to 2.00%,

[0032] B: 0% to 0.0100%,

[0033] Ca: 0% to 0.0200%,

[0034] Mg: 0% to 0.0200%,

[0035] REM: 0% to 0.1000%;

[0036] Bi: 0% to 0.0200%,

[0037] As: 0% to 0.100%,

[0038] Zr: 0% to 1.00%,

[0039] Co: 0% to 1.00%,

[0040] Zn: 0% to 1.00%,

[0041] W: 0% to 1.00%,

[0042] Sn: 0% to 0.05%,

[0043] a remainder comprising Fe and impurities, and

[0044] the following formulas (A) and (B) are satisfied,

[0045] in which, in a microstructure at a depth position of 1 / 4 from a surface in a sheet thickness direction,

[0046] in terms of area %,

[0047] residual austenite is less than 3.0%,

[0048] ferrite is 15.0% or more and less than 60.0%, and

[0049] pearlite is less than 5.0%,

[0050] 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,

[0051] a standard deviation of Mn concentrations is 0.60 mass % or less,

[0052] a solute Cr concentration in an outermost layer region whose starting point is the surface and ending point is a position of 5 μm depth in the sheet thickness direction is 0.10 mass % or more,

[0053] a number density of Cr oxides having a sphere equivalent radius of 0.1 μm or more at the surface is 1.0×104 pieces / cm2 or less,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,

[0055] P(i,j) in the following formulas (1) to (5) is a gray level co-occurrence matrix, L in the following formula (2) is standardized constant of possible brightness value of the SEM images, i and j in the following formulas (2) and (3) are natural numbers from 1 to the L, μx and μy 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+<semantics definitionURL="">❘<annotation encoding="Mathematica">"\[LeftBracketingBar]"< / annotation>< / semantics>i-j<semantics definitionURL="">❘<annotation encoding="Mathematica">"\[RightBracketingBar]"< / annotation>< / semantics>L(2)[Formula⁢ 3]Cluster⁢ Shade=∑ i⁢∑ j⁢(i+j-μx-μy)3⁢P⁡(i,j)(3)[Formula⁢ 4]μx=∑ i⁢∑ j⁢i⁡(P⁡(i,j))(4)[Formula⁢ 5]μy=∑ i⁢∑ j⁢j⁡(P⁡(i,j))(5)

[0056] (2) The hot-rolled steel sheet according to (1), in which an average grain size in a surface layer region whose starting point is the surface and ending point is a position of 20 μm depth in the sheet thickness direction may be less than 3.0 μm.

[0057] (3) The hot-rolled steel sheet according to (1) or (2), in which the chemical composition may comprise, in terms of mass %, one or two or more selected from the group consisting of

[0058] Ti: 0.001% to 0.500%,

[0059] Nb: 0.001% to 0.500%,

[0060] V: 0.001% to 0.500%,

[0061] Cu: 0.01% to 2.00%,

[0062] Mo: 0.01% to 1.00%,

[0063] Ni: 0.02% to 2.00%,

[0064] B: 0.0001% to 0.0100%,

[0065] Ca: 0.0005% to 0.0200%,

[0066] Mg: 0.0005% to 0.0200%,

[0067] REM: 0,0005% to 0.1000%,

[0068] Bi: 0.0005% to 0.0200%,

[0069] As: 0.001% to 0.100%,

[0070] Zr: 0.01% to 1.00%,

[0071] Co: 0.01% to 1.00%,

[0072] Zn: 0.01% to 1.00%,

[0073] W: 0.01% to 1.00%, and

[0074] Sn: 0.01% to 0.05%.Effects of the Invention

[0075] 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 and shearing property, and further having excellent fatigue property after press forming.

[0076] In addition, according to the preferable aspect according to the present invention, it is possible to obtain a hot-rolled steel sheet which has the above various properties, and further suppresses an occurrence of inner crack at bending, that is, has excellent inner crack resistance at bending.

[0077] 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 THE DRAWINGS

[0078] FIG. 1 An example of a sheared end surface of a hot-rolled steel sheet according to a present invention example.

[0079] FIG. 2 An example of a sheared end surface of a hot-rolled steel sheet according to a comparative example.

[0080] FIG. 3 A figure for explaining a press forming conducted in Example.

[0081] FIG. 4 A figure for explaining a shape of a punch used in the press forming.EMBODIMENTS OF THE INVENTION

[0082] 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.

[0083] 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

[0084] 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%, sol. Al: 0.001% to 0.500%, Cr: 0.060% 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%).

[0085] Each element will be described in detail below.

[0086] C: 0.050% to 0.250%

[0087] 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 0.055% or more, more preferably 0.060% or more, still more preferably 0.065% or more.

[0088] 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 0.150% or less.

[0089] Si: 0.05% to 3.00%

[0090] 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 0.40% or more and more preferably 0.60% or more.

[0091] 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, 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 2.50% or less, and more preferably 2.00% or less.

[0092] Mn: 1.00% to 4.00%

[0093] 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 1.10% or more and more preferably 1.20% or more.

[0094] 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 3.50% or less, more preferably 3.00% or less, and still more preferably 2.50% or less.

[0095] Ti: 0% to 0.500%

[0096] Nb: 0% to 0.500%

[0097] V: 0% to 0.500%0.06%≤Ti+Nb+V≤0.5%(A)

[0098] 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.

[0099] 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. The total content of Ti, Nb, and Vis preferably 0.080% or more, and more preferably 0.100% or more. The respective contents of Ti. Nb. and V is preferably 0.001% or more.

[0100] On the other hand, when the total content of Ti, Nb, and V exceeds 0.500%, the workability of the hot-rolled steel sheet deteriorates. Therefore, 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 total content of Ti, Nb, and Vis preferably 0.300% or less, more preferably 0.250% or less, and still more preferably 0.200% or less.

[0101] sol. Al: 0.001% to 0.500%

[0102] 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.

[0103] On the other hand, when the sol. Al content is more than 0.500%, the above effects are saturated, which is not economically preferable, and thus the sol. Al content is set to 0.500% or less. The sol. Al content is preferably 0.450% or less, more preferably 0.400% or less, and still more preferably 0.350% or less.

[0104] Note that the sol. Al means acid-soluble Al and refers to solid solution Al present in steel in a solid solution state.

[0105] Cr: 0.060% to 2.000%

[0106] Cr has an action of enhancing the hardenability of the hot-rolled steel sheet. In addition, in combination with a desired manufacturing method, Cr has an action of suppressing a growth of scales by concentrating in the outermost layer region of the hot-rolled steel sheet, and decreasing an arithmetic average roughness Ra after press forming. When the Cr content is less than 0.060%, effects by the actions cannot be obtained. Therefore, the Cr content is set to 0.060% or more. The Cr content is preferably 0.200% or more, more preferably 0.400% or more, and still more preferably 0.600% or more.

[0107] On the other hand, when the Cr content is more than 2.000%, the chemical convertibility of the hot-rolled steel sheet significantly deteriorates. Therefore, the Cr content is set to 2.000% or less. The Cr content is preferably 1.800% or less, and more preferably 1.600% or less.

[0108] P: 0.100% or less

[0109] P has an action of increasing the strength of the hot-rolled steel sheet by solid solution strengthening. Therefore, 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 0.030% or less. The lower limit of the P content does not need to be particularly specified and may be 0%, but the P content is preferably set to 0.001% from the viewpoint of the refining cost.

[0110] S: 0.0300% or less

[0111] 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 0.0050% or less. The lower limit of the S content does not need to be particularly specified and may be 0%, but the S content is preferably set to 0.0001% from the viewpoint of the refining cost.

[0112] N: 0.1000% or less

[0113] 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, and still more preferably 0.0100% or less. Although the lower limit of the N content does not need to be particularly specified and may be 0%, but the N content is preferably set to 0.0010% or more and more preferably set to 0.0020% or more to promote the precipitation of a carbonitride in a case where one or two or more of Ti, Nb. and V are contained to further refine the microstructure.

[0114] O: 0.0100% or less

[0115] 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. Although the O content may be 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.

[0116] 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.

[0117] 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.

[0118] Cu: 0.01% to 2.00%

[0119] Mo: 0.01% to 1.00%

[0120] Ni: 0.02% to 2.00%

[0121] B: 0.0001% to 0.0100%

[0122] All of Cu, Mo, Ni, 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.

[0123] 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.

[0124] 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.

[0125] As described above, Ni has an action of enhancing the hardenability of the hot-rolled steel sheet. In addition, in a case where Cu is contained, Ni has an action of effectively suppressing the grain boundary cracking of the slab caused by Cu. In order to more reliably obtain the effect by the action, the Ni content is preferably set to 0.02% or more. Since Ni is an expensive element, it is not economically preferable to contain a large content of Ni. Therefore, the Ni content is set to 2.00% or less.

[0126] 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 0.0050% or less.

[0127] Ca: 0.0005% to 0.0200%

[0128] Mg: 0.0005% to 0.0200%

[0129] REM: 0.0005% to 0.1000%

[0130] Bi: 0.0005% to 0.0200%

[0131] 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 Bi content is preferably 0.0100% or less.

[0132] 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.

[0133] As: 0.001% to 0.100%

[0134] 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.

[0135] 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.

[0136] Zr: 0.01% to 1.00%

[0137] Co: 0.01% to 1.00%

[0138] Zn: 0.01% to 1.00%

[0139] W: 0.01% to 1.00%Zr+Co+Zn+W≤1.%(B)

[0140] Sn: 0.01% to 0.05%

[0141] Here, 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.

[0142] 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. 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.

[0143] 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.

[0144] 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.

[0145] 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

[0146] Next, the microstructure of the hot-rolled steel sheet according to the present embodiment will be described.

[0147] In the microstructure at a depth position of 1 / 4 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, a solute Cr concentration in an outermost layer region whose starting point is the surface and ending point is a position of 5 μm depth in the sheet thickness direction is 0.10 mass % or more, a number density of Cr oxides having a sphere equivalent radius of 0.1 μm or more at the surface is 1.0×104 pieces / cm2 or less.

[0148] Therefore, the hot-rolled steel sheet according to the present embodiment can obtain high strength and sheet thickness reduction at critical fracture, excellent ductility and shearing property, and further obtain excellent fatigue property after press forming.

[0149] 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 depth position of 1 / 4 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.

[0150] When the hot-rolled steel sheet has the plating layer, the “surface” refers to the interface of the plating layer and the steel sheet, “depth position of 1 / 4 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.

[0151] Area Ratio of Residual Austenite: Less than 3.0%

[0152] 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%.

[0153] Since residual austenite is preferably as little as possible, the area ratio of the residual austenite may be 0%.

[0154] 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.

[0155] 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 depth position of 1 / 4 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. 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 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.

[0156] Area Ratio of Ferrite: 15.0% or more and less than 60.0%

[0157] Ferrite is a structure formed when fec 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.

[0158] 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.

[0159] Area Ratio of Pearlite: Less than 5.0%

[0160] 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.

[0161] 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%.

[0162] 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.

[0163] 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 depth position of 1 / 4 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 depth position of 1 / 4 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 (JSM-7001F manufactured by JEOL) and an EBSD detector (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 fields is set to 5.

[0164] Furthermore, a backscattered electron images are photographed at the respective same observed visual fields where the above-mentioned crystal orientation information were obtained. Note that when photographing the images, the acceleration voltage is set to 15 kV, 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. The area ratios of 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.

[0165] 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.

[0166] The area ratio of a remainder of the microstructure is obtained by subtracting the area ratios of residual austenite, ferrite, and pearlite from 100%.

[0167] In the present embodiment, the rolling direction of the hot-rolled steel sheet is determined by the following method.

[0168] 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 the surface of a sheet at an arbitrary depth in a range of 1 / 4 to 1 / 2 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

[0169] 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.

[0170] 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.

[0171] 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.

[0172] Cluster Shade Value: −8.0×105 to 8.0×105 Cluster Shade value (CS value) indicates skewness 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.

[0173] 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.

[0174] 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.

[0175] 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.

[0176] The E value, the I value and the CS value can be obtained by the following method.

[0177] 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 depth position of 1 / 4 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.

[0178] 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.

[0179] 179 matrices of the frequency values sampled by the above method are expressed as pk (k=0 . . . 89, 91, . . . 179) where k is a rotation angle from the original image. pk'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.

[0180] P(i,j) in the following formulas (1) to (5) is the gray level co-occurrence matrix, the value at the ith row and jth 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, μx and μy in the following formula (3) are indicated by the following formulas (4) and (5)

[0181] In the following formulas (1′) to (5′), the value at the ith row and the jth column of the matrix P is expressed as Pij.[Formula⁢ 6]Entropy=-∑ i⁢∑ j⁢P⁡(i,j)⁢log⁡(P⁡(i,j))(1)[Formula⁢ 7]Inverse⁢ difference⁢ normalized=∑ i⁢∑ j⁢P⁡(i,j)1+<semantics definitionURL="">❘<annotation encoding="Mathematica">"\[LeftBracketingBar]"< / annotation>< / semantics>i-j<semantics definitionURL="">❘<annotation encoding="Mathematica">"\[RightBracketingBar]"< / annotation>< / semantics>L(2)[Formula⁢ 8]Cluster⁢ Shade=∑ i⁢∑ j⁢(i+j-μx-μy)3⁢P⁡(i,j)(3)[Formula⁢ 9]μx=∑ i⁢∑ j⁢i⁡(P⁡(i,j))(4)[Formula⁢ 10]μy=∑ i⁢∑ j⁢j⁡(P⁡(i,j))(5)[Formula⁢ 11]Entropy=-∑ i=1,j=1i=256,j=256⁢Pij⁢log⁢Pij(1′)[Formula⁢ 12]Inverse⁢ difference⁢ normalized=∑ i=1,j=1i=256,j=256⁢Pij / (1+<semantics definitionURL="">❘<annotation encoding="Mathematica">"\[LeftBracketingBar]"< / annotation>< / semantics>i-j<semantics definitionURL="">❘<annotation encoding="Mathematica">"\[RightBracketingBar]"< / annotation>< / semantics> / 256)(2′)[Formula⁢ 13]Cluster⁢ Shade=∑ i=1,j=1i=256,j=256⁢(i+j-μx-μy)3⁢Pij(3′)[Formula⁢ 14]μx=∑ i=1,j=1i=256,j=256⁢i⁡(Pij)(4′)[Formula⁢ 15]μy=∑ i=1,j=1i=256,j=256⁢j⁡(Pij)(5′)

[0182] Standard Deviation of Mn Concentrations: 0.60 mass % or Less

[0183] 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.

[0184] 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 depth position of 1 / 4 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 depth position of 1 / 4 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.

[0185] Solute Cr Concentration in Outermost Layer Region: 0.10 mass % or More

[0186] The present inventors has found that in a case where a solute Cr concentration in the outermost layer region (a region of the 5 μm depth from the surface in the sheet thickness direction) is high, by decreasing a number density of Cr oxides having a sphere equivalent radius of 0.1 μm or more at the surface, a deterioration of the fatigue property after press forming of the hot-rolled steel sheet can be suppressed. When the solute Cr concentration in the outermost layer region is less than 0.10 mass %, the deterioration of the fatigue property after press forming of the hot-rolled steel sheet cannot be suppressed. Therefore, the solute Cr concentration in the outermost layer region is set to 0.10 mass % or more. The solute Cr concentration in the outermost layer region is preferably 0.20 mass % or more, and more preferably 0.40 mass % or more.

[0187] The solute Cr concentration in the outermost layer region may be set to 5.00 mass % or less.

[0188] Note that the region of the 5 μm depth from the surface in the sheet thickness direction in the present embodiment refers to a layered region having a depth in the sheet thickness direction whose starting point is the surface of the hot-rolled steel sheet and ending point is a position of the depth of 5 μm in the sheet thickness direction. 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 as described above.

[0189] The solute Cr concentration in the outermost layer region can be analyzed by GD-MS (Glow Discharge-Mass Spectrometry) analysis. The GD-MS analysis is an analytical method for tracing changes in composition from the surface of the hot-rolled steel sheet to the depth direction over a course of discharge time.

[0190] In the present embodiment, a sample is collected from an arbitrary position of the hot-rolled steel sheet, and an average value of Cr concentrations in mass % in the region of the 5 μm depth from the surface in the sheet thickness direction (the region whose starting point is the surface and ending point is the position of the 5 μm depth in the sheet thickness direction) by analyzing the sample with GD-MS analysis in the sheet thickness direction. This operation is performed at arbitrary 3 or more positions (preferably 5 or more positions), and the solute Cr concentration in the outermost layer region is obtained by calculating the average value of the obtained values.

[0191] When the hot-rolled steel sheet has the plating layer, a depth position with an Fe concentration of 90 mass % or more when GD-MS analysis is performed is regarded as the interface of the plating layer and the hot-rolled steel sheet, that is, the surface of the hot-rolled steel sheet.

[0192] Number Density of Cr Oxides Having Sphere Equivalent Radius of 0.1 μm or More at Surface: 1.0×104 pieces / cm2 or Less

[0193] When the number density of Cr oxides having the sphere equivalent radius of 0.1 μm or more at the surface is more than 1.0×104 pieces / cm2, a surface roughness increases when the hot-rolled steel sheet is press formed. Since this surface roughness deteriorates the fatigue property of pressed parts, it is preferable that the surface roughness is small. The Cr oxides defined here have the sphere equivalent radius of 0.1 μm or more and are relatively coarse. It is considered that the coarse Cr oxides impede sliding between the hot-rolled steel sheet and a die, which causes the increase of surface roughness. In general, since Cr oxides are present at a bottom of the scale and have high adhesion to a base steel, it is difficult to reduce the number density of Cr oxides at the surface of Cr-containing steel. In the present embodiment, this problem is solved by controlling the temperature during descaling, and the temperature and rolling reduction of rough rolling. Therefore, the number density of Cr oxides having the sphere equivalent radius of 0.1 μm or more at the surface is set to 1.0×104 pieces / cm2 or less. The number density of Cr oxides having the sphere equivalent radius of 0.1 μm or more at the surface is preferably 0.8×104 pieces / cm2 or less, and more preferably 0.6×104 pieces / cm2 or less.

[0194] The number density of Cr oxides having the sphere equivalent radius of 0.1 μm or more at the surface may be set to 0.1×104 pieces / cm2 or more.

[0195] The number density of Cr oxides at the surface is measured by the following method.

[0196] A sample is cut out from the hot-rolled steel sheet so that the surface in the sheet thickness direction is the observation plane. The observation plane is degreased at 60° C. for 60 seconds using FC-E6403 manufactured by Nihon Parkerizing Co., Ltd . . . and then immersed in acetone and subjected ultrasonic cleaning for 90 seconds. Then, 10 or more visual fields are observed at a magnification of 3000 times. The composition of precipitates can be measured by EDS (energy dispersive X-ray spectrometer). The number of regions containing Cr and O with the sphere equivalent radius of 0.1 μm or more is counted in each visual field, and the number density of Cr oxides is obtained by dividing the number by the measured area. Note that Cr and O are each detected to be 20 atom % or more when the precipitate is analyzed by EDS, the precipitate is regarded as a Cr oxide.

[0197] When the hot-rolled steel sheet has the plating layer, the above measurements are performed after removing the plating layer by pickling with fuming nitric acid.

[0198] Average Grain Size in Surface Layer Region: Less Than 3.0 μm

[0199] By refining grain sizes in the surface layer region (the region of 20 μm depth from the surface in the sheet thickness direction), inner crack at bending of the hot-rolled steel sheet can be suppressed. The higher the strength of hot-rolled steel sheet, the more likely crack occurs from the bent inner during bending (hereinafter referred to as inner crack at bending). The mechanism of inner crack at bending 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 occurs along the shear band from the surface of the bent inner and grows. The reason why the inner crack at bending 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).

[0200] Note that the region of the 20 μm depth from the surface in the sheet thickness direction in the present embodiment refers to a layered region having a depth in the sheet thickness direction whose starting point is the surface of the hot-rolled steel sheet and ending point is the position of the 20 μm depth in the sheet thickness direction.

[0201] By the present inventors' research, it has revealed that inner crack at bending becomes prominent in steel sheets with grade of tensile strength of 980 MPa or more. The inventors have also found that the finer the grain size in the surface layer region of the hot-rolled steel sheet, the more local strain concentration is suppressed, and inner crack at bending is less likely to occur. In order to obtain the above effect, it is preferable that the average grain size in the surface layer region of the hot-rolled steel sheet is set to less than 3.0 μm. Therefore, in the present embodiment, the average grain size in the surface layer region may be set to less than 3.0 μm. The average grain size in the surface layer region is more preferably 2.5 μm or less. The lower limit of the average grain size in the surface layer region is not particularly specified, but may be set to 0.5 μm.

[0202] The grain sizes in the surface layer region are measured by EBSP-OIM (Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy) method. EBSP-OIM method is performed using a device that combines a scanning electron microscope and an EBSP analyzer, and OIM Analysis (registered trademark) manufactured by AMETEK. The analyzable area of the EBSP-OIM method is the area that can be observed with the SEM. Although it depends on the resolution of the SEM, the EBSP-OIM method can analyze with a minimum resolution of 20 nm.

[0203] In a cross section of the hot-rolled steel sheet in a direction parallel to the rolling direction and in the sheet thickness direction, measurements are performed in at least 5 visual fields at a magnification of 1200 times in the surface layer region of the 20 μm depth from the surface in the sheet thickness direction. A location where an angle difference between adjacent measurement points is 5° or more is defined as a grain boundary, and the grain size of area average is calculated.

[0204] Note that since the residual austenite is not a structure generated by a phase transformation at 600° C. or lower and does not have an effect of dislocation accumulation, the residual austenite is not the subject of analysis in this measurement method. In the EBSP-OIM method, the residual austenite having a crystal structure of fcc can be excluded from the subject of analysis.Tensile Strength Properties

[0205] 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 1 / 4 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.

[0206] 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.

[0207] 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

[0208] 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

[0209] 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 conversion treatment liquid) after plating.Manufacturing Conditions

[0210] 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.

[0211] In the suitable method for manufacturing the hot-rolled steel sheet according to the present embodiment, the following steps (1) to (11) 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.

[0212] (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.

[0213] (2) Descaling is performed one or more times in a temperature range of 1150° C. or higher before rough rolling, descaling is performed two or more times in a temperature range of 1130° C. or higher during rough rolling, and a maximum value of a total rolling reduction between each descaling in the temperature range of 1130° C. or higher is set to less than 40%.

[0214] (3) 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.

[0215] (4) Stress of 170 kPa or more is loaded to the steel sheet from rolling at one stand before a final stand of hot rolling to start of rolling of the final stand.

[0216] (5) 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.

[0217] (6) Stress of less than 200 kPa is loaded to the steel sheet from rolling at the final stand of hot rolling to the steel sheet is cooled to 800° C.

[0218] (7) The steel sheet is cooled to a temperature range of the finishing temperature Tf−50° C. or lower within 1 second after finishing of hot rolling, and then cooling is performed to a temperature range of 600° C. to 780° C. at an average cooling rate of 50° C. / s or faster. Here, cooling to the temperature range of the finishing temperature Tf−50° C. or lower within 1 second after finishing of hot rolling is a more preferable cooling condition.

[0219] (8) 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.

[0220] (9) After the slow cooling, the steel sheet is cooled 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.

[0221] (10) The steel sheet is cooled so that an average cooling rate at a temperature range of coiling temperature to 450° C. is 50° C. / s or faster.

[0222] (11) The steel sheet is coiled in a temperature range of 350° C. or lower.

[0223] By adopting the above manufacturing method, a hot-rolled steel sheet having high strength and sheet thickness reduction at critical fracture, excellent ductility and shearing property, and further having excellent fatigue property after press forming can be stably manufactured.

[0224] (1) Slab, Slab Temperature and Retention Time on Hot Rolling

[0225] 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.

[0226] 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.

[0227] 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.

[0228] 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 includes 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.

[0229] (2) Descaling Before Rough Rolling: One or More Times in Temperature Range of 1150° C. or Higher, Descaling During Rough Rolling: Two or More Times in Temperature Range of 1130° C. or Higher, and Maximum Value of Total Rolling Reduction Between Each Descaling in Temperature Range of 1130° C. or Higher: Less Than 40%

[0230] By controlling descaling condition before rough rolling, descaling condition during rough rolling, and rolling condition between each descaling, the number density of Cr oxides at the surface of the hot-rolled steel sheet can be preferably controlled. Descaling can be performed by water splaying.

[0231] Before rough rolling, descaling is preferably performed one or more times in the temperature range of 1150° C. or higher. By performing descaling one or more times in the temperature range of 1150° C. or higher before rough rolling, primary scales formed in a heating furnace can be removed and the occurrence of subsequent descaling failures can be suppressed. As a result, the number density of Cr oxides at the surface of the hot-rolled steel sheet can be preferably controlled. The upper limit of number of descaling in the temperature range of 1150° C. or higher is not particularly limited, but may be set to 5 times or less.

[0232] Rolling and descaling are performed multiple times in rough rolling.

[0233] Descaling is performed between rolling and rolling, or after multiple rolling. In the present embodiment, it is preferable that descaling is performed two or more times in the temperature range of 1130° C. or higher during rough rolling, and the maximum value of the total rolling reduction between each descaling in the temperature range of 1130° C. or higher is set to less than 40%. By performing descaling two or more times in the temperature range of 1130° C. or higher, a scale thickness formed in a previous stage of rough rolling can be reduced or the scales can be removed, thereby preferably controlling the number density of the Cr oxides at the surface of the hot-rolled steel sheet.

[0234] By setting the maximum value of the total rolling reduction between each descaling in the temperature range of 1130° C. or higher to less than 40%, the scales do not get caught between the base steel during rolling between descaling, and the number density of the Cr oxides at the surface of the hot-rolled steel sheet can be preferably controlled.

[0235] The total rolling reduction between each descaling in the temperature range of 1130° C. or higher can be expressed as {(t0−t1) / t0}×100(%), where t0 is a sheet thickness before nth descaling in the temperature range of 1130° C. or higher, and t1 is an outlet sheet thickness after the n+1th descaling in the temperature range of 1130° C. or higher. Only one rolling or multiple rolling may be performed between the nth descaling and the n+1th descaling.

[0236] (3) Rolling Reduction of Hot Rolling: 90% or More in Total in Temperature Range of 850° C. to 1100° C.

[0237] 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.

[0238] Note that the hot rolling referred to herein includes rough rolling and finish rolling.

[0239] The total 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 to and an outlet sheet thickness after rolling of the final stand in the temperature range is t1.

[0240] (4) 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

[0241] 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.

[0242] 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 6th stand (F6).

[0243] 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.

[0244] 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.

[0245] (5) Rolling Reduction at Final Stand of Hot Rolling: 8% or More, Finishing Temperature Tf: 900° C. or Higher and Lower Than 1010° C.

[0246] 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.

[0247] (6) Stress Loaded to Steel Sheet From Rolling at Final Stand of Hot Rolling To Steel Sheet Being Cooled to 800° C.: Less than 200 kPa

[0248] It is preferable that stress of less than 200 kPa is loaded to the steel sheet from 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.

[0249] (7) Steel Sheet Being Cooled to Temperature Range of Finishing Temperature Tf−50° C. or Lower Within 1 Second After Finishing of Hot Rolling, and Then Accelerated Cooling Being Performed to Temperature Range of 600° C. to 780° C. at Average Cooling Rate of 50° C. / s or Faster

[0250] In order to suppress the growth of austenite grain refined by hot rolling, it is more preferable that the steel sheet is cooled by 50° C. or more within 1 second after the finishing of hot rolling, that is, the cooling amount for 1 second after the finishing of hot rolling is 50° C. or more. In order to cool the steel sheet to the temperature range of the finishing temperature Tf−50° C. or lower within 1 second after the finishing of hot rolling, cooling with a fast average cooling rate may be performed immediately after the finishing of hot rolling, for example, spraying of cooling water to the surface of the steel sheet may be performed. By cooling the steel sheet to the temperature range of Tf-50° C. or lower within 1 second after the finishing of hot rolling, it is possible to refine the grain sizes in the surface layer and to increase the inner crack resistance at bending.

[0251] In addition, when accelerated cooling is performed to the temperature range of 780° C. or lower at an average cooling rate of 50° C. / s or faster after the above cooling, it is possible to suppress the formation of ferrite and pearlite with a small amount of precipitation hardening. Accordingly, the strength of the hot-rolled steel sheet improves. 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.

[0252] 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.

[0253] (8) 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

[0254] 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.

[0255] 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.

[0256] 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.

[0257] (9) 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

[0258] 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.

[0259] 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.

[0260] (10) Average Cooling Rate at Temperature Range of Coiling Temperature to 450° C.: 50° C. / s or Faster

[0261] 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.

[0262] 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.

[0263] (11) Coiling Temperature: 350° C. or Lower

[0264] 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

[0265] 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.

[0266] Steels having a chemical composition shown in Tables 1 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.

[0267] Note that the average cooling rate of slow cooling was set to slower than 5° C. / s. 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.

[0268] The area ratio of the microstructure, the E value, the I value, the CS value, the standard deviation of the Mn concentrations, the solute Cr concentration in the outermost layer region, the number density of Cr oxides having the sphere equivalent radius of 0.1 μm or more at the surface, the average grain size in the surface layer region, 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 Table5A to Table 6B.

[0269] Note that the remainder of the microstructure was one or two or more of bainite, martensite and tempered martensite.

[0270] Evaluation Method of Properties of Hot-Rolled Steel Sheets Tensile Properties

[0271] 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.

[0272] Sheet thickness reduction at critical fracture

[0273] The sheet thickness reduction at critical fracture of the hot-rolled steel sheet was evaluated by tensile test.

[0274] 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.

[0275] 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.

[0276] Shearing property (Evaluation of Secondary Sheared Surface)

[0277] The shearing property of the hot-rolled steel sheet was evaluated by a punching test.

[0278] 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.

[0279] 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.Fatigue Property After Press Forming

[0280] The fatigue property after press forming was evaluated by an arithmetic average roughness Ra of the surface of the hot-rolled steel sheet after press forming.

[0281] The surface A in the sheet thickness direction of the hot-rolled steel sheet was ground to reduce a sheet thickness of 1.6 mm or less of a test piece 10, and press forming was performed by pressing a punch 1 at a load of 2.5 t against the other surface B of the test piece 10 as shown in FIG. 3, while being pulled in one direction D. The shape of the punch is shown in FIG. 4 (FIG. 9 of Japanese Patent No. 5655394). The hot-rolled steel sheet after the press forming was straightened with a leveler, and the arithmetic average roughness Ra of the surface was measured by the following method.

[0282] For the sample surface of 1000 mm×1000 mm, the measurement points were set at 200 mm intervals in a direction perpendicular to the rolling direction and the sheet thickness direction, and the surface roughness was measured at each measurement point. However, the measurement length at each measurement point was set to 5 mm. A roughness curve was obtained by sequentially applying contour curve filters with cutoff values Δc and λs to the measured cross-sectional curve obtained by the measurement. Specifically, from the obtained measurement results, components with a wavelength λc of 0.8 mm or less and components with a wavelength λs of 2.5 μm or more were removed to obtain the roughness curve. Based on the obtained roughness curve, the arithmetic average roughness Ra of each measurement point was calculated in accordance with JIS B 0601:2013. By calculating the average value of obtained value, the arithmetic average roughness Ra of the surface of the hot-rolled steel sheet after press forming was obtained.

[0283] For the hot-rolled steel sheet after press forming, in a case where the arithmetic average roughness Ra of the surface was 3.0 μm or less, the hot-rolled steel sheet was determined as having excellent fatigue property after press forming and being successful. On the other hand, in a case where the arithmetic average roughness Ra of the surface was more than 3.0 μm, the hot-rolled steel sheet was determined as not having excellent fatigue property after press forming and not being successful.Inner Crack Resistance at Bending

[0284] The inner crack resistance at bending was evaluated by the following bending test.

[0285] A strip-shaped test piece of 100 mm×30 mm was cut out from a position of 1 / 2 from an end surface in a direction perpendicular to the rolling direction and the sheet thickness direction of the hot-rolled steel sheet 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 parallel to a direction perpendicular to the rolling direction and sheet thickness 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. Thus, the minimum bend radii at which cracks did not occur were obtained to investigate the inner crack resistance at bending. A value obtained by dividing the average value (R) of the minimum bend radii in the L axis and in the C axis by the sheet thickness (t) was regarded as the critical bend R / t and used as an index value of the inner crack resistance at bending. In a case where R / t was 2.5 or less, the hot-rolled steel sheet was determined to be excellent in the inner crack resistance at bending.

[0286] 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 a crack being present.TABLE 1Chemical composition (Mass %) remainder is Fe and impuritiesSteelTi + Nb +No.CSiMnTiNbVVsol. AlCrPSNONoteA0.1830.611.150.0820.0820.2820.7260.0180.00330.00260.0027Present Invention ExampleB0.0720.711.450.1270.1270.2750.9980.0110.00210.00320.0027Present Invention ExampleC0.0540.753.020.1130.0200.1240.2570.2680.6010.0190.00380.00300.0028Present Invention ExampleD0.0722.751.460.1280.1280.0860.3670.0100.00080.00220.0030Present Invention ExampleE0.0680.061.410.1220.1220.4321.6640.0160.00150.00440.0019Present Invention ExampleF0.0580.883.670.1210.1210.2551.2730.0210.00120.00250.0028Present Invention ExampleG0.0990.751.020.1240.1240.2161.4860.0170.00250.00420.0041Present Invention ExampleH0.0880.571.160.0820.0820.2771.8540.0140.00070.00320.0033Present Invention ExampleI0.0812.012.550.1170.1170.2820.0620.0160.00120.00330.0040Present Invention ExampleJ0.0860.411.330.1690.1690.4630.9910.0100.00300.00420.0039Present Invention ExampleK0.0861.251.680.0630.0630.2720.9690.0180.00390.00330.0035Present Invention ExampleL0.0640.611.420.1280.1280.2191.1290.0410.00320.00300.0027Present Invention ExampleM0.0820.761.220.1350.1350.3740.9880.0140.00760.00480.0031Present Invention ExampleN0.0740.631.350.1200.1200.2691.2370.0180.00240.02080.0023Present Invention ExampleO0.0680.521.260.1250.1250.0021.1660.0100.00200.00240.0080Present Invention ExampleP0.0840.811.230.1330.1330.2120.0300.0180.00240.00270.0035Comparative ExampleQ0.0270.601.410.1320.1320.2721.2570.0200.00260.00280.0037Comparative ExampleR0.2750.901.370.1330.1330.2521.1750.0160.00380.00430.0030Comparative ExampleS0.0833.181.350.1300.1300.2421.0230.0130.00160.00420.0031Comparative ExampleT0.0800.990.770.1320.1320.2550.3670.0180.00130.00410.0032Comparative ExampleU0.0710.781.270.0430.0430.2191.1610.0090.00150.00320.0029Comparative ExampleUnderlines indicate that values are outside the range of the present invention.TABLE 2Chemical composition (Mass %) remainder is Fe and impuritiesSteelZr + Co +No.CuMoNiBCaMgREMBiAsZrCoZnWZn + WSnNoteA0.00700.061PresentInventionExampleBPresentInventionExampleC0.190.130.13PresentInventionExampleD0.300.30PresentInventionExampleE0.270.27PresentInventionExampleF0.100.170.27PresentInventionExampleG0.370.0026PresentInventionExampleHPresentInventionExampleI0.01PresentInventionExampleJ0.37PresentInventionExampleKPresentInventionExampleLPresentInventionExampleM0.00240.0027PresentInventionExampleN0.0104PresentInventionExampleOPresentInventionExamplePComparativeExampleQComparativeExampleRComparativeExampleSComparativeExampleTComparativeExampleUComparativeExampleUnderlines indicate that values are outside the range of the present invention.TABLE 3AMaximumvalue ofNumber ofNumbertotal rollingtimes ofof timesreductionRetentionRetentiondescalingof descalingbetween eachtime intime inat 1150° C.at 1130° C.descaling attemperaturetemperatureor higheror higher1130° C. or higherrange ofrange ofbeforeduringduring700° C. toHeating1100° C.roughroughroughManufacturingSteel850° C.temperatureor higherrollingrollingrollingNo.No.s° C.sTimesTimes% 1A1390127796302625 2B1189126495872622 3B 754127690572629 4B1399126550432627 5B1499126989870622 6B14221253896821— 7B1336126089792648 8B1375127394342624 9B114012358896262310B114512539447263411B133612509125262112B132312569471262113B152112488917283314B144412639036262415B158112399285263116B157312659467263117B129912519512263318B121412379483262119C1382133589032632LoadedLoadedstress fromstress toTotalrolling atsteel sheetrollingone standfrom rollingreduction inbefore finalat finaltemperaturestand toRollingstand torange ofstart ofFinishingreductionsteel sheet850° C. torolling attemperatureat finalis cooledManufacturing1100° C.final standTfstandto 800° C.No.%kPa° C.%kPaNote 19320093316194Present Invention Example 29621895916183Present Invention Example 39621097214190Comparative Example 49622497016188Comparative Example 59623893316178Comparative Example 69622092714194Comparative Example 79616094818186Comparative Example 88521294613188Comparative Example 99415796113191Comparative Example10942191035 18180Comparative Example1194230958 5192Comparative Example129521693915219Comparative Example139421793415192Present Invention Example149421993718190Comparative Example159621692814170Comparative Example169521097516187Comparative Example179523194417177Comparative Example189621297713185Comparative Example199420691214175Present Invention ExampleUnderlines indicate that values are outside the range of the present invention or are not preferable manufacturing conditions.TABLE 3BMaximumvalue ofNumber ofNumbertotal rollingtimes ofof timesreductionRetentionRetentiondescalingof descalingbetween eachtime intime inat 1150° C.at 1130° C.descaling attemperaturetemperatureor higheror higher1130° C. or higherrange ofrange ofbeforeduringduring700° C. toHeating1100° C.roughroughroughManufacturingSteel850° C.temperatureor higherrollingrollingrollingNo.No.s° C.sTimesTimes%20D149612699580262221D128612429317263422E116912418841263223F119312319003262624G123112479203263325H130412579317263126I150612578889143827J133012359024262628K98411988966262829L152812519028262530M150612709334262631N139612679493262132O154012529172262733P123812438973262534Q148212709363262635R158512709215262736S155512728917262137T130612688988262638U157212309346263139B13781247900200—40G123412559051263441A1384127098222625LoadedLoadedstress fromstress toTotalrolling atsteel sheetrollingone standfrom rollingreduction inbefore finalat finaltemperaturestand toRollingstand torange ofstart ofFinishingreductionsteel sheet850° C. torolling attemperatureat finalis cooledManufacturing1100° C.final standTfstandto 800° C.No.%kPa° C.%kPaNote20932151005 18191Present Invention Example219523793317188Comparative Example229322395116180Present Invention Example239322590741178Present Invention Example249420292610170Present Invention Example259520993416188Present Invention Example269522993918186Present Invention Example279621992116181Present Invention Example289621293618181Present Invention Example299323696217186Present Invention Example309423596315192Present Invention Example319420994714180Present Invention Example329322294516174Present Invention Example339621197418179Comparative Example349519992417191Comparative Example359619897818194Comparative Example369323895117176Comparative Example379620093016190Comparative Example389520097713182Comparative Example399421491715201Comparative Example40932038968168Comparative Example419219892616198Comparative ExampleUnderlines indicate that values are outside the range of the present invention or are not preferable manufacturing conditions.TABLE 4ASlowAverageCoolingcoolingAveragecoolingamounttime incoolingrate infor 1 secondAverageCooling stoptemperaturerate intemperatureaftercoolingtemperaturerange oftemperaturerange offinishingrate ofof600° C.range ofcoilingof hotacceleratedacceleratedto450° C.temperatureCoilingManufacturingSteelrollingcoolingcooling780° C.to 600° C.to 450° C.temperatureNo.No.° C.° C. / s° C.s° C. / s° C. / s° C.Note 1A791177596.845111320Present InventionExample 2B781177573.34313350Present InventionExample 3B77 917534.43810150Comparative Example 4B80 887183.2398050Comparative Example 5B88 846874.5389150Comparative Example 6B93 886944.53610350Comparative Example 7B87 837184.1428050Comparative Example 8B881277623.43515150Comparative Example 9B841097024.0399250Comparative Example10B841167313.03913350Comparative Example11B911127193.14410650Comparative Example12B80 867283.44013050Comparative Example13B381117634.03915750Present InventionExample14B931237361.24214050Comparative Example15B94 406303.4398250Comparative Example16B931107953.34113550Comparative Example17B871107154.37515650Comparative Example18B891167413.12110650Comparative Example19C94 907005.93810650Present InventionExampleUnderlines indicate that values are outside the range of the present invention or are not preferable manufacturing conditions.TABLE 4BAverageCoolingAveragecoolingamountSlow coolingcoolingrate infor 1 secondAverageCooling stoptime inrate intemperatureaftercoolingtemperaturetemperaturetemperaturerange offinishingrate ofofrange ofrange ofcoilingof hotacceleratedaccelerated600° C. to450° C.temperatureCoilingManufacturingSteelrollingcoolingcooling780° C.to 600° C.to 450° C.temperatureNo.No.° C.° C. / s° C.s° C. / s° C. / s° C.Note20D851086132.14314850Present InventionExample21D801237363.738 2650Comparative Example22E91917124.541 8650Present InventionExample23F761107398.038 6750Present InventionExample24G931127193.73912350Present InventionExample25H931207013.23214350Present InventionExample26I551047703.23911850Present InventionExample27J89907743.34412650Present InventionExample28K75986288.04511350Present InventionExample29L96916883.84514350Present InventionExample30M78827103.13915550Present InventionExample31N821157614.441131170 Present InventionExample32O861147574.44513850Present InventionExample33P771106993.945 8650Comparative Example34Q861077423.53611950Comparative Example35R86836973.440 9550Comparative Example36S89927564.04313350Comparative Example37T921207243.63710850Comparative Example38U801047384.63615050Comparative Example39B89846833.94110350Comparative Example40G931147203.14314350Comparative Example41A821207446.249107360 Comparative ExampleUnderlines indicate that values are outside the range of the present invention or are not preferable manufacturing conditions.TABLE 5ASheetResidualRemainder inManufacturingthicknessFerriteaustenitePearlitemicrostructureE valueI valueNo.Steel No.mmArea %Area %Area %Area %—— 1A2.316.80.00.083.211.81.046 2B3.232.70.00.067.311.91.047 3B3.243.10.00.056.911.01.037 4B3.237.10.00.062.911.81.038 5B3.233.60.00.066.411.11.066 6B3.236.00.00.064.011.21.062 7B3.239.10.00.060.910.51.068 8B3.240.10.00.059.911.41.048 9B3.236.50.00.063.510.51.03510B3.221.40.00.078.610.31.04811B3.234.40.00.065.610.31.07612B3.233.30.00.066.710.41.05313B2.944.30.00.055.711.41.03514B2.910.10.00.089.911.41.04615B2.925.80.010.0 64.211.01.05616B2.9 9.80.00.090.211.51.05817B2.943.50.00.056.511.71.03418B2.931.10.00.068.911.11.06619C2.923.70.00.076.310.81.076NumberAverageSolute Crdensitygrainconcentrationof Crsize inMnin outermostoxides atsurfacestandardlayersurfacelayerManufacturingCS valuedeviationregion×104regionNo.×105Mass %Mass %pieces / cm2μmNote 1−5.90.471.090.32.4Present Invention Example 2 2.80.441.400.42.0Present Invention Example 3−1.10.631.800.42.2Comparative Example 4 3.20.611.400.42.2Comparative Example 5−6.30.472.001.52.4Comparative Example 6−3.40.451.801.22.3Comparative Example 7 3.40.441.402.72.2Comparative Example 8−3.60.611.300.52.1Comparative Example 9−0.60.391.600.42.0Comparative Example10 2.50.441.300.52.6Comparative Example11 2.60.361.200.52.3Comparative Example12−5.20.441.300.52.2Comparative Example13−2.10.462.000.53.2Present Invention Example14−6.30.421.400.52.1Comparative Example15−2.00.421.900.52.2Comparative Example16−4.80.371.400.42.2Comparative Example17−8.50.441.300.52.2Comparative Example18 8.30.421.200.42.4Comparative Example19 1.70.480.840.32.4Present Invention ExampleUnderlines indicate that values are outside the range of the present invention or properties are not preferable.TABLE 5BSheetResidualRemainder inManufacturingthicknessFerriteaustenitePearlitemicrostructureE valueI valueNo.Steel No.mmArea %Area %Area %Area %——20D2.936.01.60.062.411.51.07521D2.956.08.80.035.211.71.06922E2.933.90.00.066.111.41.03323F3.618.20.00.081.810.71.04224G2.931.70.00.068.311.71.03225H2.937.50.00.062.511.01.03226I2.943.50.00.056.511.61.05127J2.941.70.00.058.311.91.07228K1.630.90.00.069.111.21.07329L2.944.00.00.056.011.21.03630M2.943.70.00.056.311.81.06631N2.940.50.00.059.511.41.04732O2.938.40.00.061.611.41.06433P2.942.70.00.057.311.61.04134Q2.980.40.00.019.611.91.06635R2.9 5.00.00.095.011.81.07136S2.967.55.40.027.111.31.03937T2.972.50.00.027.511.21.07838U2.955.70.00.044.311.91.07939B3.238.00.00.062.011.51.07340G2.960.10.00.139.811.91.03841A2.317.70.00.082.311.81.010NumberAverageSolute Crdensitygrainconcentrationof Crsize inMnin outermostoxides atsurfacestandardlayersurfacelayerManufacturingCS valuedeviationregion×104regionNo.×105Mass %Mass %pieces / cm2μmNote20−4.70.400.440.22.5Present Invention Example212.80.470.550.22.2Comparative Example222.60.462.830.92.3Present Invention Example23−4.50.571.910.52.0Present Invention Example24−1.50.352.530.42.3Present Invention Example25−3.90.433.340.92.0Present Invention Example26−5.00.430.120.12.2Present Invention Example27−4.30.411.190.52.1Present Invention Example28−4.40.421.450.42.5Present Invention Example290.10.451.810.42.2Present Invention Example30−2.50.391.190.52.5Present Invention Example313.30.361.860.32.2Present Invention Example321.90.442.330.42.4Present Invention Example330.80.390.060.12.5Comparative Example34−2.60.421.760.52.3Comparative Example35−4.70.421.880.62.4Comparative Example362.10.421.840.52.5Comparative Example37−2.10.351.250.52.1Comparative Example38−2.10.432.320.62.0Comparative Example39−3.20.421.683.12.2Comparative Example40−1.60.332.660.31.8Comparative Example41−5.70.461.100.32.3Comparative ExampleUnderlines indicate that values are outside the range of the present invention or properties are not preferable.TABLE 6APresence orSheetArithmeticabsence ofthicknessaverageTensileTotalsecondaryreductionroughness RaCriticalstrengthelongationTS ×shearedat criticalafter pressbendManufacturingSteelTSElElsurfacefractureformingR / tNo.No.MPa%MPa · %—%μm—Note 1A139410.314325Absence71.92.42.3Present Invention Example 2B103715.616179Absence73.72.22.1Present Invention Example 3B101515.715919Presence71.11.92.3Comparative Example 4B102615.315663Presence70.82.12.3Comparative Example 5B103615.315890Absence72.43.42.3Comparative Example 6B103915.215780Absence72.43.32.1Comparative Example 7B103515.516037Presence73.43.72.3Comparative Example 8B101415.315506Presence73.22.42.4Comparative Example 9B100915.615708Presence73.32.62.2Comparative Example10B106512.713550Presence73.91.92.5Comparative Example11B101315.916089Presence71.22.42.3Comparative Example12B101515.615836Presence74.22.32.3Comparative Example13B100715.815916Absence73.72.22.8Present Invention Example14B105011.512053Absence70.72.02.1Comparative Example15B 97312.912539Absence74.41.92.4Comparative Example16B108211.111977Absence73.02.12.3Comparative Example17B103515.115605Absence58.12.42.4Comparative Example18B103915.015571Absence55.42.42.3Comparative Example19C125411.614532Absence73.22.42.3Present Invention ExampleUnderlines indicate that corresponding values are outside the range of the present invention or not preferable properties.TABLE 6BPresence orSheetArithmeticabsence ofthicknessaverageTensileTotalsecondaryreductionroughness RaCriticalstrengthelongationTS ×shearedat criticalafter pressbendManufacturingSteelTSElElsurfacefractureformingR / tNo.No.MPa%MPa · %—%μm—Note20D102617.517951Absence62.31.92.4Present Invention Example21D 98618.318005Absence54.92.12.5Comparative Example22E103015.916335Absence72.12.82.4Present Invention Example23F119611.213451Absence71.72.22.2Present Invention Example24G101015.315503Absence70.82.12.3Present Invention Example25H102415.716117Absence70.52.92.3Present Invention Example26I100815.715824Absence71.32.82.2Present Invention Example27J 99816.216202Absence72.02.12.4Present Invention Example28K 98315.715453Absence70.62.12.3Present Invention Example29L100514.314322Absence65.82.12.2Present Invention Example30M 99814.914866Absence69.02.42.3Present Invention Example31N103014.114507Absence64.82.62.1Present Invention Example32O102715.115528Absence70.82.22.3Present Invention Example33P101715.615877Absence74.93.52.4Comparative Example34Q 79421.216825Absence72.12.12.1Comparative Example35R1438 8.311953Absence71.72.22.2Comparative Example36S 94217.216249Absence54.52.02.7Comparative Example37T 89817.515750Absence72.02.32.3Comparative Example38U 95616.816046Absence74.42.42.3Comparative Example39B101915.215513Absence70.23.92.3Comparative Example40G 97617.316885Absence70.12.02.5Comparative Example41A138211.115304Presence71.92.42.3Comparative ExampleUnderlines 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 and shearing property, and further has excellent fatigue property after press forming. In addition, among the present invention examples, the hot-rolled steel sheets whose average grain size in the surface layer region is less than 3.0 μm has the above various properties, and further has excellent inner crack resistance 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 APPLICABILITYAccording 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 and shearing property, and further having excellent fatigue property after press forming. In addition, according to the preferable aspect according to the present invention, it is possible to obtain a hot-rolled steel sheet which has the above various properties, and further suppresses the occurrence of inner crack at bending, that is, has excellent inner crack resistance 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.

Examples

examples

[0265]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.

[0266]Steels having a chemical composition shown in Tables 1 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.

[0267]Note that the average cooling rate of slow cooling was set to slower than 5° C. / s. In addition, since the measurement lower limit of the coiling temperature shown in Table 4A and Table 4B...

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to a hot-rolled steel sheet. Specifically, the present invention relates to a hot-rolled steel sheet that is formed into various shapes by press working or the like to be used, and particularly relates to a hot-rolled steel sheet that has high strength and sheet thickness reduction at critical fracture, excellent ductility and shearing property, and further has excellent fatigue property after press forming.

[0002] Priority is claimed on Japanese Patent Application No. 2022-142994, filed on Sep. 8, 2022, the content of which is incorporated herein by reference.BACKGROUND ART

[0003] 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.

[0004] 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.

[0005] 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.

[0006] 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.

[0007] In addition, steel sheets applied for vehicle members are required to have excellent fatigue property after press forming.

[0008] 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.

[0009] However, in Patent Document 1, a sheet thickness reduction at critical fracture, shearing property, and fatigue property after press forming of the hot-rolled steel sheet are not considered.PRIOR ART DOCUMENTPatent Document

[0010] Patent Document 1: WO2020 / 044445Non-Patent Document

[0011] Non-Patent Document 1: J. Webel, J. Gola, D. Britz, F. Mucklich, Materials Characterization 144 (2018) 584-596

[0012] Non-Patent Document 2: D. L. Naik, H. U. Sajid, R. Kiran, Metals 2019, 9, 546

[0013] 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. 474-485DISCLOSURE OF THE INVENTIONProblems to be Solved by the Invention

[0014] 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 and shearing property, and further having excellent fatigue property after press forming.Means for Solving the Problem

[0015] The gist of the present invention is as follows.

[0016] (1) A hot-rolled steel sheet according to one aspect of the present invention comprising, in terms of mass %, as a chemical composition.

[0017] C: 0.050% to 0.250%,

[0018] Si: 0.05% to 3.00%,

[0019] Mn: 1.00% to 4.00%,

[0020] sol. Al: 0.001% to 0.500%,

[0021] Cr: 0.060% to 2.000%;

[0022] P: 0.100% or less,

[0023] S: 0.0300% or less,

[0024] N: 0.1000% or less,

[0025] Q: 0.0100% or less,

[0026] Ti: 0% to 0.500%,

[0027] Nb: 0% to 0.500%,

[0028] V: 0% to 0.500%,

[0029] Cu: 0% to 2.00%,

[0030] Mo: 0% to 1.00%,

[0031] Ni: 0% to 2.00%,

[0032] B: 0% to 0.0100%,

[0033] Ca: 0% to 0.0200%,

[0034] Mg: 0% to 0.0200%,

[0035] REM: 0% to 0.1000%;

[0036] Bi: 0% to 0.0200%,

[0037] As: 0% to 0.100%,

[0038] Zr: 0% to 1.00%,

[0039] Co: 0% to 1.00%,

[0040] Zn: 0% to 1.00%,

[0041] W: 0% to 1.00%,

[0042] Sn: 0% to 0.05%,

[0043] a remainder comprising Fe and impurities, and

[0044] the following formulas (A) and (B) are satisfied,

[0045] in which, in a microstructure at a depth position of 1 / 4 from a surface in a sheet thickness direction,

[0046] in terms of area %,

[0047] residual austenite is less than 3.0%,

[0048] ferrite is 15.0% or more and less than 60.0%, and

[0049] pearlite is less than 5.0%,

[0050] 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,

[0051] a standard deviation of Mn concentrations is 0.60 mass % or less,

[0052] a solute Cr concentration in an outermost layer region whose starting point is the surface and ending point is a position of 5 μm depth in the sheet thickness direction is 0.10 mass % or more,

[0053] a number density of Cr oxides having a sphere equivalent radius of 0.1 μm or more at the surface is 1.0×104 pieces / cm2 or less,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,

[0055] P(i,j) in the following formulas (1) to (5) is a gray level co-occurrence matrix, L in the following formula (2) is standardized constant of possible brightness value of the SEM images, i and j in the following formulas (2) and (3) are natural numbers from 1 to the L, μx and μy 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+<semantics definitionURL="">❘<annotation encoding="Mathematica">"\[LeftBracketingBar]"< / annotation>< / semantics>i-j<semantics definitionURL="">❘<annotation encoding="Mathematica">"\[RightBracketingBar]"< / annotation>< / semantics>L(2)[Formula⁢ 3]Cluster⁢ Shade=∑ i⁢∑ j⁢(i+j-μx-μy)3⁢P⁡(i,j)(3)[Formula⁢ 4]μx=∑ i⁢∑ j⁢i⁡(P⁡(i,j))(4)[Formula⁢ 5]μy=∑ i⁢∑ j⁢j⁡(P⁡(i,j))(5)

[0056] (2) The hot-rolled steel sheet according to (1), in which an average grain size in a surface layer region whose starting point is the surface and ending point is a position of 20 μm depth in the sheet thickness direction may be less than 3.0 μm.

[0057] (3) The hot-rolled steel sheet according to (1) or (2), in which the chemical composition may comprise, in terms of mass %, one or two or more selected from the group consisting of

[0058] Ti: 0.001% to 0.500%,

[0059] Nb: 0.001% to 0.500%,

[0060] V: 0.001% to 0.500%,

[0061] Cu: 0.01% to 2.00%,

[0062] Mo: 0.01% to 1.00%,

[0063] Ni: 0.02% to 2.00%,

[0064] B: 0.0001% to 0.0100%,

[0065] Ca: 0.0005% to 0.0200%,

[0066] Mg: 0.0005% to 0.0200%,

[0067] REM: 0,0005% to 0.1000%,

[0068] Bi: 0.0005% to 0.0200%,

[0069] As: 0.001% to 0.100%,

[0070] Zr: 0.01% to 1.00%,

[0071] Co: 0.01% to 1.00%,

[0072] Zn: 0.01% to 1.00%,

[0073] W: 0.01% to 1.00%, and

[0074] Sn: 0.01% to 0.05%.Effects of the Invention

[0075] 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 and shearing property, and further having excellent fatigue property after press forming.

[0076] In addition, according to the preferable aspect according to the present invention, it is possible to obtain a hot-rolled steel sheet which has the above various properties, and further suppresses an occurrence of inner crack at bending, that is, has excellent inner crack resistance at bending.

[0077] 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 THE DRAWINGS

[0078] FIG. 1 An example of a sheared end surface of a hot-rolled steel sheet according to a present invention example.

[0079] FIG. 2 An example of a sheared end surface of a hot-rolled steel sheet according to a comparative example.

[0080] FIG. 3 A figure for explaining a press forming conducted in Example.

[0081] FIG. 4 A figure for explaining a shape of a punch used in the press forming.EMBODIMENTS OF THE INVENTION

[0082] 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.

[0083] 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

[0084] 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%, sol. Al: 0.001% to 0.500%, Cr: 0.060% 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%).

[0085] Each element will be described in detail below.

[0086] C: 0.050% to 0.250%

[0087] 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 0.055% or more, more preferably 0.060% or more, still more preferably 0.065% or more.

[0088] 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 0.150% or less.

[0089] Si: 0.05% to 3.00%

[0090] 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 0.40% or more and more preferably 0.60% or more.

[0091] 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, 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 2.50% or less, and more preferably 2.00% or less.

[0092] Mn: 1.00% to 4.00%

[0093] 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 1.10% or more and more preferably 1.20% or more.

[0094] 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 3.50% or less, more preferably 3.00% or less, and still more preferably 2.50% or less.

[0095] Ti: 0% to 0.500%

[0096] Nb: 0% to 0.500%

[0097] V: 0% to 0.500%0.06%≤Ti+Nb+V≤0.5%(A)

[0098] 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.

[0099] 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. The total content of Ti, Nb, and Vis preferably 0.080% or more, and more preferably 0.100% or more. The respective contents of Ti. Nb. and V is preferably 0.001% or more.

[0100] On the other hand, when the total content of Ti, Nb, and V exceeds 0.500%, the workability of the hot-rolled steel sheet deteriorates. Therefore, 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 total content of Ti, Nb, and Vis preferably 0.300% or less, more preferably 0.250% or less, and still more preferably 0.200% or less.

[0101] sol. Al: 0.001% to 0.500%

[0102] 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.

[0103] On the other hand, when the sol. Al content is more than 0.500%, the above effects are saturated, which is not economically preferable, and thus the sol. Al content is set to 0.500% or less. The sol. Al content is preferably 0.450% or less, more preferably 0.400% or less, and still more preferably 0.350% or less.

[0104] Note that the sol. Al means acid-soluble Al and refers to solid solution Al present in steel in a solid solution state.

[0105] Cr: 0.060% to 2.000%

[0106] Cr has an action of enhancing the hardenability of the hot-rolled steel sheet. In addition, in combination with a desired manufacturing method, Cr has an action of suppressing a growth of scales by concentrating in the outermost layer region of the hot-rolled steel sheet, and decreasing an arithmetic average roughness Ra after press forming. When the Cr content is less than 0.060%, effects by the actions cannot be obtained. Therefore, the Cr content is set to 0.060% or more. The Cr content is preferably 0.200% or more, more preferably 0.400% or more, and still more preferably 0.600% or more.

[0107] On the other hand, when the Cr content is more than 2.000%, the chemical convertibility of the hot-rolled steel sheet significantly deteriorates. Therefore, the Cr content is set to 2.000% or less. The Cr content is preferably 1.800% or less, and more preferably 1.600% or less.

[0108] P: 0.100% or less

[0109] P has an action of increasing the strength of the hot-rolled steel sheet by solid solution strengthening. Therefore, 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 0.030% or less. The lower limit of the P content does not need to be particularly specified and may be 0%, but the P content is preferably set to 0.001% from the viewpoint of the refining cost.

[0110] S: 0.0300% or less

[0111] 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 0.0050% or less. The lower limit of the S content does not need to be particularly specified and may be 0%, but the S content is preferably set to 0.0001% from the viewpoint of the refining cost.

[0112] N: 0.1000% or less

[0113] 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, and still more preferably 0.0100% or less. Although the lower limit of the N content does not need to be particularly specified and may be 0%, but the N content is preferably set to 0.0010% or more and more preferably set to 0.0020% or more to promote the precipitation of a carbonitride in a case where one or two or more of Ti, Nb. and V are contained to further refine the microstructure.

[0114] O: 0.0100% or less

[0115] 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. Although the O content may be 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.

[0116] 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.

[0117] 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.

[0118] Cu: 0.01% to 2.00%

[0119] Mo: 0.01% to 1.00%

[0120] Ni: 0.02% to 2.00%

[0121] B: 0.0001% to 0.0100%

[0122] All of Cu, Mo, Ni, 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.

[0123] 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.

[0124] 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.

[0125] As described above, Ni has an action of enhancing the hardenability of the hot-rolled steel sheet. In addition, in a case where Cu is contained, Ni has an action of effectively suppressing the grain boundary cracking of the slab caused by Cu. In order to more reliably obtain the effect by the action, the Ni content is preferably set to 0.02% or more. Since Ni is an expensive element, it is not economically preferable to contain a large content of Ni. Therefore, the Ni content is set to 2.00% or less.

[0126] 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 0.0050% or less.

[0127] Ca: 0.0005% to 0.0200%

[0128] Mg: 0.0005% to 0.0200%

[0129] REM: 0.0005% to 0.1000%

[0130] Bi: 0.0005% to 0.0200%

[0131] 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 Bi content is preferably 0.0100% or less.

[0132] 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.

[0133] As: 0.001% to 0.100%

[0134] 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.

[0135] 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.

[0136] Zr: 0.01% to 1.00%

[0137] Co: 0.01% to 1.00%

[0138] Zn: 0.01% to 1.00%

[0139] W: 0.01% to 1.00%Zr+Co+Zn+W≤1.%(B)

[0140] Sn: 0.01% to 0.05%

[0141] Here, 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.

[0142] 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. 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.

[0143] 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.

[0144] 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.

[0145] 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

[0146] Next, the microstructure of the hot-rolled steel sheet according to the present embodiment will be described.

[0147] In the microstructure at a depth position of 1 / 4 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, a solute Cr concentration in an outermost layer region whose starting point is the surface and ending point is a position of 5 μm depth in the sheet thickness direction is 0.10 mass % or more, a number density of Cr oxides having a sphere equivalent radius of 0.1 μm or more at the surface is 1.0×104 pieces / cm2 or less.

[0148] Therefore, the hot-rolled steel sheet according to the present embodiment can obtain high strength and sheet thickness reduction at critical fracture, excellent ductility and shearing property, and further obtain excellent fatigue property after press forming.

[0149] 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 depth position of 1 / 4 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.

[0150] When the hot-rolled steel sheet has the plating layer, the “surface” refers to the interface of the plating layer and the steel sheet, “depth position of 1 / 4 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.

[0151] Area Ratio of Residual Austenite: Less than 3.0%

[0152] 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%.

[0153] Since residual austenite is preferably as little as possible, the area ratio of the residual austenite may be 0%.

[0154] 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.

[0155] 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 depth position of 1 / 4 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. 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 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.

[0156] Area Ratio of Ferrite: 15.0% or more and less than 60.0%

[0157] Ferrite is a structure formed when fec 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.

[0158] 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.

[0159] Area Ratio of Pearlite: Less than 5.0%

[0160] 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.

[0161] 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%.

[0162] 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.

[0163] 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 depth position of 1 / 4 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 depth position of 1 / 4 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 (JSM-7001F manufactured by JEOL) and an EBSD detector (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 fields is set to 5.

[0164] Furthermore, a backscattered electron images are photographed at the respective same observed visual fields where the above-mentioned crystal orientation information were obtained. Note that when photographing the images, the acceleration voltage is set to 15 kV, 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. The area ratios of 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.

[0165] 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.

[0166] The area ratio of a remainder of the microstructure is obtained by subtracting the area ratios of residual austenite, ferrite, and pearlite from 100%.

[0167] In the present embodiment, the rolling direction of the hot-rolled steel sheet is determined by the following method.

[0168] 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 the surface of a sheet at an arbitrary depth in a range of 1 / 4 to 1 / 2 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

[0169] 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.

[0170] 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.

[0171] 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.

[0172] Cluster Shade Value: −8.0×105 to 8.0×105 Cluster Shade value (CS value) indicates skewness 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.

[0173] 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.

[0174] 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.

[0175] 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.

[0176] The E value, the I value and the CS value can be obtained by the following method.

[0177] 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 depth position of 1 / 4 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.

[0178] 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.

[0179] 179 matrices of the frequency values sampled by the above method are expressed as pk (k=0 . . . 89, 91, . . . 179) where k is a rotation angle from the original image. pk'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.

[0180] P(i,j) in the following formulas (1) to (5) is the gray level co-occurrence matrix, the value at the ith row and jth 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, μx and μy in the following formula (3) are indicated by the following formulas (4) and (5)

[0181] In the following formulas (1′) to (5′), the value at the ith row and the jth column of the matrix P is expressed as Pij.[Formula⁢ 6]Entropy=-∑ i⁢∑ j⁢P⁡(i,j)⁢log⁡(P⁡(i,j))(1)[Formula⁢ 7]Inverse⁢ difference⁢ normalized=∑ i⁢∑ j⁢P⁡(i,j)1+<semantics definitionURL="">❘<annotation encoding="Mathematica">"\[LeftBracketingBar]"< / annotation>< / semantics>i-j<semantics definitionURL="">❘<annotation encoding="Mathematica">"\[RightBracketingBar]"< / annotation>< / semantics>L(2)[Formula⁢ 8]Cluster⁢ Shade=∑ i⁢∑ j⁢(i+j-μx-μy)3⁢P⁡(i,j)(3)[Formula⁢ 9]μx=∑ i⁢∑ j⁢i⁡(P⁡(i,j))(4)[Formula⁢ 10]μy=∑ i⁢∑ j⁢j⁡(P⁡(i,j))(5)[Formula⁢ 11]Entropy=-∑ i=1,j=1i=256,j=256⁢Pij⁢log⁢Pij(1′)[Formula⁢ 12]Inverse⁢ difference⁢ normalized=∑ i=1,j=1i=256,j=256⁢Pij / (1+<semantics definitionURL="">❘<annotation encoding="Mathematica">"\[LeftBracketingBar]"< / annotation>< / semantics>i-j<semantics definitionURL="">❘<annotation encoding="Mathematica">"\[RightBracketingBar]"< / annotation>< / semantics> / 256)(2′)[Formula⁢ 13]Cluster⁢ Shade=∑ i=1,j=1i=256,j=256⁢(i+j-μx-μy)3⁢Pij(3′)[Formula⁢ 14]μx=∑ i=1,j=1i=256,j=256⁢i⁡(Pij)(4′)[Formula⁢ 15]μy=∑ i=1,j=1i=256,j=256⁢j⁡(Pij)(5′)

[0182] Standard Deviation of Mn Concentrations: 0.60 mass % or Less

[0183] 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.

[0184] 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 depth position of 1 / 4 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 depth position of 1 / 4 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.

[0185] Solute Cr Concentration in Outermost Layer Region: 0.10 mass % or More

[0186] The present inventors has found that in a case where a solute Cr concentration in the outermost layer region (a region of the 5 μm depth from the surface in the sheet thickness direction) is high, by decreasing a number density of Cr oxides having a sphere equivalent radius of 0.1 μm or more at the surface, a deterioration of the fatigue property after press forming of the hot-rolled steel sheet can be suppressed. When the solute Cr concentration in the outermost layer region is less than 0.10 mass %, the deterioration of the fatigue property after press forming of the hot-rolled steel sheet cannot be suppressed. Therefore, the solute Cr concentration in the outermost layer region is set to 0.10 mass % or more. The solute Cr concentration in the outermost layer region is preferably 0.20 mass % or more, and more preferably 0.40 mass % or more.

[0187] The solute Cr concentration in the outermost layer region may be set to 5.00 mass % or less.

[0188] Note that the region of the 5 μm depth from the surface in the sheet thickness direction in the present embodiment refers to a layered region having a depth in the sheet thickness direction whose starting point is the surface of the hot-rolled steel sheet and ending point is a position of the depth of 5 μm in the sheet thickness direction. 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 as described above.

[0189] The solute Cr concentration in the outermost layer region can be analyzed by GD-MS (Glow Discharge-Mass Spectrometry) analysis. The GD-MS analysis is an analytical method for tracing changes in composition from the surface of the hot-rolled steel sheet to the depth direction over a course of discharge time.

[0190] In the present embodiment, a sample is collected from an arbitrary position of the hot-rolled steel sheet, and an average value of Cr concentrations in mass % in the region of the 5 μm depth from the surface in the sheet thickness direction (the region whose starting point is the surface and ending point is the position of the 5 μm depth in the sheet thickness direction) by analyzing the sample with GD-MS analysis in the sheet thickness direction. This operation is performed at arbitrary 3 or more positions (preferably 5 or more positions), and the solute Cr concentration in the outermost layer region is obtained by calculating the average value of the obtained values.

[0191] When the hot-rolled steel sheet has the plating layer, a depth position with an Fe concentration of 90 mass % or more when GD-MS analysis is performed is regarded as the interface of the plating layer and the hot-rolled steel sheet, that is, the surface of the hot-rolled steel sheet.

[0192] Number Density of Cr Oxides Having Sphere Equivalent Radius of 0.1 μm or More at Surface: 1.0×104 pieces / cm2 or Less

[0193] When the number density of Cr oxides having the sphere equivalent radius of 0.1 μm or more at the surface is more than 1.0×104 pieces / cm2, a surface roughness increases when the hot-rolled steel sheet is press formed. Since this surface roughness deteriorates the fatigue property of pressed parts, it is preferable that the surface roughness is small. The Cr oxides defined here have the sphere equivalent radius of 0.1 μm or more and are relatively coarse. It is considered that the coarse Cr oxides impede sliding between the hot-rolled steel sheet and a die, which causes the increase of surface roughness. In general, since Cr oxides are present at a bottom of the scale and have high adhesion to a base steel, it is difficult to reduce the number density of Cr oxides at the surface of Cr-containing steel. In the present embodiment, this problem is solved by controlling the temperature during descaling, and the temperature and rolling reduction of rough rolling. Therefore, the number density of Cr oxides having the sphere equivalent radius of 0.1 μm or more at the surface is set to 1.0×104 pieces / cm2 or less. The number density of Cr oxides having the sphere equivalent radius of 0.1 μm or more at the surface is preferably 0.8×104 pieces / cm2 or less, and more preferably 0.6×104 pieces / cm2 or less.

[0194] The number density of Cr oxides having the sphere equivalent radius of 0.1 μm or more at the surface may be set to 0.1×104 pieces / cm2 or more.

[0195] The number density of Cr oxides at the surface is measured by the following method.

[0196] A sample is cut out from the hot-rolled steel sheet so that the surface in the sheet thickness direction is the observation plane. The observation plane is degreased at 60° C. for 60 seconds using FC-E6403 manufactured by Nihon Parkerizing Co., Ltd . . . and then immersed in acetone and subjected ultrasonic cleaning for 90 seconds. Then, 10 or more visual fields are observed at a magnification of 3000 times. The composition of precipitates can be measured by EDS (energy dispersive X-ray spectrometer). The number of regions containing Cr and O with the sphere equivalent radius of 0.1 μm or more is counted in each visual field, and the number density of Cr oxides is obtained by dividing the number by the measured area. Note that Cr and O are each detected to be 20 atom % or more when the precipitate is analyzed by EDS, the precipitate is regarded as a Cr oxide.

[0197] When the hot-rolled steel sheet has the plating layer, the above measurements are performed after removing the plating layer by pickling with fuming nitric acid.

[0198] Average Grain Size in Surface Layer Region: Less Than 3.0 μm

[0199] By refining grain sizes in the surface layer region (the region of 20 μm depth from the surface in the sheet thickness direction), inner crack at bending of the hot-rolled steel sheet can be suppressed. The higher the strength of hot-rolled steel sheet, the more likely crack occurs from the bent inner during bending (hereinafter referred to as inner crack at bending). The mechanism of inner crack at bending 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 occurs along the shear band from the surface of the bent inner and grows. The reason why the inner crack at bending 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).

[0200] Note that the region of the 20 μm depth from the surface in the sheet thickness direction in the present embodiment refers to a layered region having a depth in the sheet thickness direction whose starting point is the surface of the hot-rolled steel sheet and ending point is the position of the 20 μm depth in the sheet thickness direction.

[0201] By the present inventors' research, it has revealed that inner crack at bending becomes prominent in steel sheets with grade of tensile strength of 980 MPa or more. The inventors have also found that the finer the grain size in the surface layer region of the hot-rolled steel sheet, the more local strain concentration is suppressed, and inner crack at bending is less likely to occur. In order to obtain the above effect, it is preferable that the average grain size in the surface layer region of the hot-rolled steel sheet is set to less than 3.0 μm. Therefore, in the present embodiment, the average grain size in the surface layer region may be set to less than 3.0 μm. The average grain size in the surface layer region is more preferably 2.5 μm or less. The lower limit of the average grain size in the surface layer region is not particularly specified, but may be set to 0.5 μm.

[0202] The grain sizes in the surface layer region are measured by EBSP-OIM (Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy) method. EBSP-OIM method is performed using a device that combines a scanning electron microscope and an EBSP analyzer, and OIM Analysis (registered trademark) manufactured by AMETEK. The analyzable area of the EBSP-OIM method is the area that can be observed with the SEM. Although it depends on the resolution of the SEM, the EBSP-OIM method can analyze with a minimum resolution of 20 nm.

[0203] In a cross section of the hot-rolled steel sheet in a direction parallel to the rolling direction and in the sheet thickness direction, measurements are performed in at least 5 visual fields at a magnification of 1200 times in the surface layer region of the 20 μm depth from the surface in the sheet thickness direction. A location where an angle difference between adjacent measurement points is 5° or more is defined as a grain boundary, and the grain size of area average is calculated.

[0204] Note that since the residual austenite is not a structure generated by a phase transformation at 600° C. or lower and does not have an effect of dislocation accumulation, the residual austenite is not the subject of analysis in this measurement method. In the EBSP-OIM method, the residual austenite having a crystal structure of fcc can be excluded from the subject of analysis.Tensile Strength Properties

[0205] 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 1 / 4 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.

[0206] 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.

[0207] 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

[0208] 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

[0209] 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 conversion treatment liquid) after plating.Manufacturing Conditions

[0210] 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.

[0211] In the suitable method for manufacturing the hot-rolled steel sheet according to the present embodiment, the following steps (1) to (11) 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.

[0212] (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.

[0213] (2) Descaling is performed one or more times in a temperature range of 1150° C. or higher before rough rolling, descaling is performed two or more times in a temperature range of 1130° C. or higher during rough rolling, and a maximum value of a total rolling reduction between each descaling in the temperature range of 1130° C. or higher is set to less than 40%.

[0214] (3) 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.

[0215] (4) Stress of 170 kPa or more is loaded to the steel sheet from rolling at one stand before a final stand of hot rolling to start of rolling of the final stand.

[0216] (5) 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.

[0217] (6) Stress of less than 200 kPa is loaded to the steel sheet from rolling at the final stand of hot rolling to the steel sheet is cooled to 800° C.

[0218] (7) The steel sheet is cooled to a temperature range of the finishing temperature Tf−50° C. or lower within 1 second after finishing of hot rolling, and then cooling is performed to a temperature range of 600° C. to 780° C. at an average cooling rate of 50° C. / s or faster. Here, cooling to the temperature range of the finishing temperature Tf−50° C. or lower within 1 second after finishing of hot rolling is a more preferable cooling condition.

[0219] (8) 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.

[0220] (9) After the slow cooling, the steel sheet is cooled 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.

[0221] (10) The steel sheet is cooled so that an average cooling rate at a temperature range of coiling temperature to 450° C. is 50° C. / s or faster.

[0222] (11) The steel sheet is coiled in a temperature range of 350° C. or lower.

[0223] By adopting the above manufacturing method, a hot-rolled steel sheet having high strength and sheet thickness reduction at critical fracture, excellent ductility and shearing property, and further having excellent fatigue property after press forming can be stably manufactured.

[0224] (1) Slab, Slab Temperature and Retention Time on Hot Rolling

[0225] 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.

[0226] 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.

[0227] 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.

[0228] 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 includes 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.

[0229] (2) Descaling Before Rough Rolling: One or More Times in Temperature Range of 1150° C. or Higher, Descaling During Rough Rolling: Two or More Times in Temperature Range of 1130° C. or Higher, and Maximum Value of Total Rolling Reduction Between Each Descaling in Temperature Range of 1130° C. or Higher: Less Than 40%

[0230] By controlling descaling condition before rough rolling, descaling condition during rough rolling, and rolling condition between each descaling, the number density of Cr oxides at the surface of the hot-rolled steel sheet can be preferably controlled. Descaling can be performed by water splaying.

[0231] Before rough rolling, descaling is preferably performed one or more times in the temperature range of 1150° C. or higher. By performing descaling one or more times in the temperature range of 1150° C. or higher before rough rolling, primary scales formed in a heating furnace can be removed and the occurrence of subsequent descaling failures can be suppressed. As a result, the number density of Cr oxides at the surface of the hot-rolled steel sheet can be preferably controlled. The upper limit of number of descaling in the temperature range of 1150° C. or higher is not particularly limited, but may be set to 5 times or less.

[0232] Rolling and descaling are performed multiple times in rough rolling.

[0233] Descaling is performed between rolling and rolling, or after multiple rolling. In the present embodiment, it is preferable that descaling is performed two or more times in the temperature range of 1130° C. or higher during rough rolling, and the maximum value of the total rolling reduction between each descaling in the temperature range of 1130° C. or higher is set to less than 40%. By performing descaling two or more times in the temperature range of 1130° C. or higher, a scale thickness formed in a previous stage of rough rolling can be reduced or the scales can be removed, thereby preferably controlling the number density of the Cr oxides at the surface of the hot-rolled steel sheet.

[0234] By setting the maximum value of the total rolling reduction between each descaling in the temperature range of 1130° C. or higher to less than 40%, the scales do not get caught between the base steel during rolling between descaling, and the number density of the Cr oxides at the surface of the hot-rolled steel sheet can be preferably controlled.

[0235] The total rolling reduction between each descaling in the temperature range of 1130° C. or higher can be expressed as {(t0−t1) / t0}×100(%), where t0 is a sheet thickness before nth descaling in the temperature range of 1130° C. or higher, and t1 is an outlet sheet thickness after the n+1th descaling in the temperature range of 1130° C. or higher. Only one rolling or multiple rolling may be performed between the nth descaling and the n+1th descaling.

[0236] (3) Rolling Reduction of Hot Rolling: 90% or More in Total in Temperature Range of 850° C. to 1100° C.

[0237] 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.

[0238] Note that the hot rolling referred to herein includes rough rolling and finish rolling.

[0239] The total 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 to and an outlet sheet thickness after rolling of the final stand in the temperature range is t1.

[0240] (4) 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

[0241] 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.

[0242] 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 6th stand (F6).

[0243] 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.

[0244] 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.

[0245] (5) Rolling Reduction at Final Stand of Hot Rolling: 8% or More, Finishing Temperature Tf: 900° C. or Higher and Lower Than 1010° C.

[0246] 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.

[0247] (6) Stress Loaded to Steel Sheet From Rolling at Final Stand of Hot Rolling To Steel Sheet Being Cooled to 800° C.: Less than 200 kPa

[0248] It is preferable that stress of less than 200 kPa is loaded to the steel sheet from 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.

[0249] (7) Steel Sheet Being Cooled to Temperature Range of Finishing Temperature Tf−50° C. or Lower Within 1 Second After Finishing of Hot Rolling, and Then Accelerated Cooling Being Performed to Temperature Range of 600° C. to 780° C. at Average Cooling Rate of 50° C. / s or Faster

[0250] In order to suppress the growth of austenite grain refined by hot rolling, it is more preferable that the steel sheet is cooled by 50° C. or more within 1 second after the finishing of hot rolling, that is, the cooling amount for 1 second after the finishing of hot rolling is 50° C. or more. In order to cool the steel sheet to the temperature range of the finishing temperature Tf−50° C. or lower within 1 second after the finishing of hot rolling, cooling with a fast average cooling rate may be performed immediately after the finishing of hot rolling, for example, spraying of cooling water to the surface of the steel sheet may be performed. By cooling the steel sheet to the temperature range of Tf-50° C. or lower within 1 second after the finishing of hot rolling, it is possible to refine the grain sizes in the surface layer and to increase the inner crack resistance at bending.

[0251] In addition, when accelerated cooling is performed to the temperature range of 780° C. or lower at an average cooling rate of 50° C. / s or faster after the above cooling, it is possible to suppress the formation of ferrite and pearlite with a small amount of precipitation hardening. Accordingly, the strength of the hot-rolled steel sheet improves. 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.

[0252] 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.

[0253] (8) 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

[0254] 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.

[0255] 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.

[0256] 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.

[0257] (9) 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

[0258] 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.

[0259] 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.

[0260] (10) Average Cooling Rate at Temperature Range of Coiling Temperature to 450° C.: 50° C. / s or Faster

[0261] 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.

[0262] 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.

[0263] (11) Coiling Temperature: 350° C. or Lower

[0264] 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

[0265] 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.

[0266] Steels having a chemical composition shown in Tables 1 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.

[0267] Note that the average cooling rate of slow cooling was set to slower than 5° C. / s. 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.

[0268] The area ratio of the microstructure, the E value, the I value, the CS value, the standard deviation of the Mn concentrations, the solute Cr concentration in the outermost layer region, the number density of Cr oxides having the sphere equivalent radius of 0.1 μm or more at the surface, the average grain size in the surface layer region, 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 Table5A to Table 6B.

[0269] Note that the remainder of the microstructure was one or two or more of bainite, martensite and tempered martensite.

[0270] Evaluation Method of Properties of Hot-Rolled Steel Sheets Tensile Properties

[0271] 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.

[0272] Sheet thickness reduction at critical fracture

[0273] The sheet thickness reduction at critical fracture of the hot-rolled steel sheet was evaluated by tensile test.

[0274] 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.

[0275] 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.

[0276] Shearing property (Evaluation of Secondary Sheared Surface)

[0277] The shearing property of the hot-rolled steel sheet was evaluated by a punching test.

[0278] 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.

[0279] 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.Fatigue Property After Press Forming

[0280] The fatigue property after press forming was evaluated by an arithmetic average roughness Ra of the surface of the hot-rolled steel sheet after press forming.

[0281] The surface A in the sheet thickness direction of the hot-rolled steel sheet was ground to reduce a sheet thickness of 1.6 mm or less of a test piece 10, and press forming was performed by pressing a punch 1 at a load of 2.5 t against the other surface B of the test piece 10 as shown in FIG. 3, while being pulled in one direction D. The shape of the punch is shown in FIG. 4 (FIG. 9 of Japanese Patent No. 5655394). The hot-rolled steel sheet after the press forming was straightened with a leveler, and the arithmetic average roughness Ra of the surface was measured by the following method.

[0282] For the sample surface of 1000 mm×1000 mm, the measurement points were set at 200 mm intervals in a direction perpendicular to the rolling direction and the sheet thickness direction, and the surface roughness was measured at each measurement point. However, the measurement length at each measurement point was set to 5 mm. A roughness curve was obtained by sequentially applying contour curve filters with cutoff values Δc and λs to the measured cross-sectional curve obtained by the measurement. Specifically, from the obtained measurement results, components with a wavelength λc of 0.8 mm or less and components with a wavelength λs of 2.5 μm or more were removed to obtain the roughness curve. Based on the obtained roughness curve, the arithmetic average roughness Ra of each measurement point was calculated in accordance with JIS B 0601:2013. By calculating the average value of obtained value, the arithmetic average roughness Ra of the surface of the hot-rolled steel sheet after press forming was obtained.

[0283] For the hot-rolled steel sheet after press forming, in a case where the arithmetic average roughness Ra of the surface was 3.0 μm or less, the hot-rolled steel sheet was determined as having excellent fatigue property after press forming and being successful. On the other hand, in a case where the arithmetic average roughness Ra of the surface was more than 3.0 μm, the hot-rolled steel sheet was determined as not having excellent fatigue property after press forming and not being successful.Inner Crack Resistance at Bending

[0284] The inner crack resistance at bending was evaluated by the following bending test.

[0285] A strip-shaped test piece of 100 mm×30 mm was cut out from a position of 1 / 2 from an end surface in a direction perpendicular to the rolling direction and the sheet thickness direction of the hot-rolled steel sheet 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 parallel to a direction perpendicular to the rolling direction and sheet thickness 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. Thus, the minimum bend radii at which cracks did not occur were obtained to investigate the inner crack resistance at bending. A value obtained by dividing the average value (R) of the minimum bend radii in the L axis and in the C axis by the sheet thickness (t) was regarded as the critical bend R / t and used as an index value of the inner crack resistance at bending. In a case where R / t was 2.5 or less, the hot-rolled steel sheet was determined to be excellent in the inner crack resistance at bending.

[0286] 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 a crack being present.TABLE 1Chemical composition (Mass %) remainder is Fe and impuritiesSteelTi + Nb +No.CSiMnTiNbVVsol. AlCrPSNONoteA0.1830.611.150.0820.0820.2820.7260.0180.00330.00260.0027Present Invention ExampleB0.0720.711.450.1270.1270.2750.9980.0110.00210.00320.0027Present Invention ExampleC0.0540.753.020.1130.0200.1240.2570.2680.6010.0190.00380.00300.0028Present Invention ExampleD0.0722.751.460.1280.1280.0860.3670.0100.00080.00220.0030Present Invention ExampleE0.0680.061.410.1220.1220.4321.6640.0160.00150.00440.0019Present Invention ExampleF0.0580.883.670.1210.1210.2551.2730.0210.00120.00250.0028Present Invention ExampleG0.0990.751.020.1240.1240.2161.4860.0170.00250.00420.0041Present Invention ExampleH0.0880.571.160.0820.0820.2771.8540.0140.00070.00320.0033Present Invention ExampleI0.0812.012.550.1170.1170.2820.0620.0160.00120.00330.0040Present Invention ExampleJ0.0860.411.330.1690.1690.4630.9910.0100.00300.00420.0039Present Invention ExampleK0.0861.251.680.0630.0630.2720.9690.0180.00390.00330.0035Present Invention ExampleL0.0640.611.420.1280.1280.2191.1290.0410.00320.00300.0027Present Invention ExampleM0.0820.761.220.1350.1350.3740.9880.0140.00760.00480.0031Present Invention ExampleN0.0740.631.350.1200.1200.2691.2370.0180.00240.02080.0023Present Invention ExampleO0.0680.521.260.1250.1250.0021.1660.0100.00200.00240.0080Present Invention ExampleP0.0840.811.230.1330.1330.2120.0300.0180.00240.00270.0035Comparative ExampleQ0.0270.601.410.1320.1320.2721.2570.0200.00260.00280.0037Comparative ExampleR0.2750.901.370.1330.1330.2521.1750.0160.00380.00430.0030Comparative ExampleS0.0833.181.350.1300.1300.2421.0230.0130.00160.00420.0031Comparative ExampleT0.0800.990.770.1320.1320.2550.3670.0180.00130.00410.0032Comparative ExampleU0.0710.781.270.0430.0430.2191.1610.0090.00150.00320.0029Comparative ExampleUnderlines indicate that values are outside the range of the present invention.TABLE 2Chemical composition (Mass %) remainder is Fe and impuritiesSteelZr + Co +No.CuMoNiBCaMgREMBiAsZrCoZnWZn + WSnNoteA0.00700.061PresentInventionExampleBPresentInventionExampleC0.190.130.13PresentInventionExampleD0.300.30PresentInventionExampleE0.270.27PresentInventionExampleF0.100.170.27PresentInventionExampleG0.370.0026PresentInventionExampleHPresentInventionExampleI0.01PresentInventionExampleJ0.37PresentInventionExampleKPresentInventionExampleLPresentInventionExampleM0.00240.0027PresentInventionExampleN0.0104PresentInventionExampleOPresentInventionExamplePComparativeExampleQComparativeExampleRComparativeExampleSComparativeExampleTComparativeExampleUComparativeExampleUnderlines indicate that values are outside the range of the present invention.TABLE 3AMaximumvalue ofNumber ofNumbertotal rollingtimes ofof timesreductionRetentionRetentiondescalingof descalingbetween eachtime intime inat 1150° C.at 1130° C.descaling attemperaturetemperatureor higheror higher1130° C. or higherrange ofrange ofbeforeduringduring700° C. toHeating1100° C.roughroughroughManufacturingSteel850° C.temperatureor higherrollingrollingrollingNo.No.s° C.sTimesTimes% 1A1390127796302625 2B1189126495872622 3B 754127690572629 4B1399126550432627 5B1499126989870622 6B14221253896821— 7B1336126089792648 8B1375127394342624 9B114012358896262310B114512539447263411B133612509125262112B132312569471262113B152112488917283314B144412639036262415B158112399285263116B157312659467263117B129912519512263318B121412379483262119C1382133589032632LoadedLoadedstress fromstress toTotalrolling atsteel sheetrollingone standfrom rollingreduction inbefore finalat finaltemperaturestand toRollingstand torange ofstart ofFinishingreductionsteel sheet850° C. torolling attemperatureat finalis cooledManufacturing1100° C.final standTfstandto 800° C.No.%kPa° C.%kPaNote 19320093316194Present Invention Example 29621895916183Present Invention Example 39621097214190Comparative Example 49622497016188Comparative Example 59623893316178Comparative Example 69622092714194Comparative Example 79616094818186Comparative Example 88521294613188Comparative Example 99415796113191Comparative Example10942191035 18180Comparative Example1194230958 5192Comparative Example129521693915219Comparative Example139421793415192Present Invention Example149421993718190Comparative Example159621692814170Comparative Example169521097516187Comparative Example179523194417177Comparative Example189621297713185Comparative Example199420691214175Present Invention ExampleUnderlines indicate that values are outside the range of the present invention or are not preferable manufacturing conditions.TABLE 3BMaximumvalue ofNumber ofNumbertotal rollingtimes ofof timesreductionRetentionRetentiondescalingof descalingbetween eachtime intime inat 1150° C.at 1130° C.descaling attemperaturetemperatureor higheror higher1130° C. or higherrange ofrange ofbeforeduringduring700° C. toHeating1100° C.roughroughroughManufacturingSteel850° C.temperatureor higherrollingrollingrollingNo.No.s° C.sTimesTimes%20D149612699580262221D128612429317263422E116912418841263223F119312319003262624G123112479203263325H130412579317263126I150612578889143827J133012359024262628K98411988966262829L152812519028262530M150612709334262631N139612679493262132O154012529172262733P123812438973262534Q148212709363262635R158512709215262736S155512728917262137T130612688988262638U157212309346263139B13781247900200—40G123412559051263441A1384127098222625LoadedLoadedstress fromstress toTotalrolling atsteel sheetrollingone standfrom rollingreduction inbefore finalat finaltemperaturestand toRollingstand torange ofstart ofFinishingreductionsteel sheet850° C. torolling attemperatureat finalis cooledManufacturing1100° C.final standTfstandto 800° C.No.%kPa° C.%kPaNote20932151005 18191Present Invention Example219523793317188Comparative Example229322395116180Present Invention Example239322590741178Present Invention Example249420292610170Present Invention Example259520993416188Present Invention Example269522993918186Present Invention Example279621992116181Present Invention Example289621293618181Present Invention Example299323696217186Present Invention Example309423596315192Present Invention Example319420994714180Present Invention Example329322294516174Present Invention Example339621197418179Comparative Example349519992417191Comparative Example359619897818194Comparative Example369323895117176Comparative Example379620093016190Comparative Example389520097713182Comparative Example399421491715201Comparative Example40932038968168Comparative Example419219892616198Comparative ExampleUnderlines indicate that values are outside the range of the present invention or are not preferable manufacturing conditions.TABLE 4ASlowAverageCoolingcoolingAveragecoolingamounttime incoolingrate infor 1 secondAverageCooling stoptemperaturerate intemperatureaftercoolingtemperaturerange oftemperaturerange offinishingrate ofof600° C.range ofcoilingof hotacceleratedacceleratedto450° C.temperatureCoilingManufacturingSteelrollingcoolingcooling780° C.to 600° C.to 450° C.temperatureNo.No.° C.° C. / s° C.s° C. / s° C. / s° C.Note 1A791177596.845111320Present InventionExample 2B781177573.34313350Present InventionExample 3B77 917534.43810150Comparative Example 4B80 887183.2398050Comparative Example 5B88 846874.5389150Comparative Example 6B93 886944.53610350Comparative Example 7B87 837184.1428050Comparative Example 8B881277623.43515150Comparative Example 9B841097024.0399250Comparative Example10B841167313.03913350Comparative Example11B911127193.14410650Comparative Example12B80 867283.44013050Comparative Example13B381117634.03915750Present InventionExample14B931237361.24214050Comparative Example15B94 406303.4398250Comparative Example16B931107953.34113550Comparative Example17B871107154.37515650Comparative Example18B891167413.12110650Comparative Example19C94 907005.93810650Present InventionExampleUnderlines indicate that values are outside the range of the present invention or are not preferable manufacturing conditions.TABLE 4BAverageCoolingAveragecoolingamountSlow coolingcoolingrate infor 1 secondAverageCooling stoptime inrate intemperatureaftercoolingtemperaturetemperaturetemperaturerange offinishingrate ofofrange ofrange ofcoilingof hotacceleratedaccelerated600° C. to450° C.temperatureCoilingManufacturingSteelrollingcoolingcooling780° C.to 600° C.to 450° C.temperatureNo.No.° C.° C. / s° C.s° C. / s° C. / s° C.Note20D851086132.14314850Present InventionExample21D801237363.738 2650Comparative Example22E91917124.541 8650Present InventionExample23F761107398.038 6750Present InventionExample24G931127193.73912350Present InventionExample25H931207013.23214350Present InventionExample26I551047703.23911850Present InventionExample27J89907743.34412650Present InventionExample28K75986288.04511350Present InventionExample29L96916883.84514350Present InventionExample30M78827103.13915550Present InventionExample31N821157614.441131170 Present InventionExample32O861147574.44513850Present InventionExample33P771106993.945 8650Comparative Example34Q861077423.53611950Comparative Example35R86836973.440 9550Comparative Example36S89927564.04313350Comparative Example37T921207243.63710850Comparative Example38U801047384.63615050Comparative Example39B89846833.94110350Comparative Example40G931147203.14314350Comparative Example41A821207446.249107360 Comparative ExampleUnderlines indicate that values are outside the range of the present invention or are not preferable manufacturing conditions.TABLE 5ASheetResidualRemainder inManufacturingthicknessFerriteaustenitePearlitemicrostructureE valueI valueNo.Steel No.mmArea %Area %Area %Area %—— 1A2.316.80.00.083.211.81.046 2B3.232.70.00.067.311.91.047 3B3.243.10.00.056.911.01.037 4B3.237.10.00.062.911.81.038 5B3.233.60.00.066.411.11.066 6B3.236.00.00.064.011.21.062 7B3.239.10.00.060.910.51.068 8B3.240.10.00.059.911.41.048 9B3.236.50.00.063.510.51.03510B3.221.40.00.078.610.31.04811B3.234.40.00.065.610.31.07612B3.233.30.00.066.710.41.05313B2.944.30.00.055.711.41.03514B2.910.10.00.089.911.41.04615B2.925.80.010.0 64.211.01.05616B2.9 9.80.00.090.211.51.05817B2.943.50.00.056.511.71.03418B2.931.10.00.068.911.11.06619C2.923.70.00.076.310.81.076NumberAverageSolute Crdensitygrainconcentrationof Crsize inMnin outermostoxides atsurfacestandardlayersurfacelayerManufacturingCS valuedeviationregion×104regionNo.×105Mass %Mass %pieces / cm2μmNote 1−5.90.471.090.32.4Present Invention Example 2 2.80.441.400.42.0Present Invention Example 3−1.10.631.800.42.2Comparative Example 4 3.20.611.400.42.2Comparative Example 5−6.30.472.001.52.4Comparative Example 6−3.40.451.801.22.3Comparative Example 7 3.40.441.402.72.2Comparative Example 8−3.60.611.300.52.1Comparative Example 9−0.60.391.600.42.0Comparative Example10 2.50.441.300.52.6Comparative Example11 2.60.361.200.52.3Comparative Example12−5.20.441.300.52.2Comparative Example13−2.10.462.000.53.2Present Invention Example14−6.30.421.400.52.1Comparative Example15−2.00.421.900.52.2Comparative Example16−4.80.371.400.42.2Comparative Example17−8.50.441.300.52.2Comparative Example18 8.30.421.200.42.4Comparative Example19 1.70.480.840.32.4Present Invention ExampleUnderlines indicate that values are outside the range of the present invention or properties are not preferable.TABLE 5BSheetResidualRemainder inManufacturingthicknessFerriteaustenitePearlitemicrostructureE valueI valueNo.Steel No.mmArea %Area %Area %Area %——20D2.936.01.60.062.411.51.07521D2.956.08.80.035.211.71.06922E2.933.90.00.066.111.41.03323F3.618.20.00.081.810.71.04224G2.931.70.00.068.311.71.03225H2.937.50.00.062.511.01.03226I2.943.50.00.056.511.61.05127J2.941.70.00.058.311.91.07228K1.630.90.00.069.111.21.07329L2.944.00.00.056.011.21.03630M2.943.70.00.056.311.81.06631N2.940.50.00.059.511.41.04732O2.938.40.00.061.611.41.06433P2.942.70.00.057.311.61.04134Q2.980.40.00.019.611.91.06635R2.9 5.00.00.095.011.81.07136S2.967.55.40.027.111.31.03937T2.972.50.00.027.511.21.07838U2.955.70.00.044.311.91.07939B3.238.00.00.062.011.51.07340G2.960.10.00.139.811.91.03841A2.317.70.00.082.311.81.010NumberAverageSolute Crdensitygrainconcentrationof Crsize inMnin outermostoxides atsurfacestandardlayersurfacelayerManufacturingCS valuedeviationregion×104regionNo.×105Mass %Mass %pieces / cm2μmNote20−4.70.400.440.22.5Present Invention Example212.80.470.550.22.2Comparative Example222.60.462.830.92.3Present Invention Example23−4.50.571.910.52.0Present Invention Example24−1.50.352.530.42.3Present Invention Example25−3.90.433.340.92.0Present Invention Example26−5.00.430.120.12.2Present Invention Example27−4.30.411.190.52.1Present Invention Example28−4.40.421.450.42.5Present Invention Example290.10.451.810.42.2Present Invention Example30−2.50.391.190.52.5Present Invention Example313.30.361.860.32.2Present Invention Example321.90.442.330.42.4Present Invention Example330.80.390.060.12.5Comparative Example34−2.60.421.760.52.3Comparative Example35−4.70.421.880.62.4Comparative Example362.10.421.840.52.5Comparative Example37−2.10.351.250.52.1Comparative Example38−2.10.432.320.62.0Comparative Example39−3.20.421.683.12.2Comparative Example40−1.60.332.660.31.8Comparative Example41−5.70.461.100.32.3Comparative ExampleUnderlines indicate that values are outside the range of the present invention or properties are not preferable.TABLE 6APresence orSheetArithmeticabsence ofthicknessaverageTensileTotalsecondaryreductionroughness RaCriticalstrengthelongationTS ×shearedat criticalafter pressbendManufacturingSteelTSElElsurfacefractureformingR / tNo.No.MPa%MPa · %—%μm—Note 1A139410.314325Absence71.92.42.3Present Invention Example 2B103715.616179Absence73.72.22.1Present Invention Example 3B101515.715919Presence71.11.92.3Comparative Example 4B102615.315663Presence70.82.12.3Comparative Example 5B103615.315890Absence72.43.42.3Comparative Example 6B103915.215780Absence72.43.32.1Comparative Example 7B103515.516037Presence73.43.72.3Comparative Example 8B101415.315506Presence73.22.42.4Comparative Example 9B100915.615708Presence73.32.62.2Comparative Example10B106512.713550Presence73.91.92.5Comparative Example11B101315.916089Presence71.22.42.3Comparative Example12B101515.615836Presence74.22.32.3Comparative Example13B100715.815916Absence73.72.22.8Present Invention Example14B105011.512053Absence70.72.02.1Comparative Example15B 97312.912539Absence74.41.92.4Comparative Example16B108211.111977Absence73.02.12.3Comparative Example17B103515.115605Absence58.12.42.4Comparative Example18B103915.015571Absence55.42.42.3Comparative Example19C125411.614532Absence73.22.42.3Present Invention ExampleUnderlines indicate that corresponding values are outside the range of the present invention or not preferable properties.TABLE 6BPresence orSheetArithmeticabsence ofthicknessaverageTensileTotalsecondaryreductionroughness RaCriticalstrengthelongationTS ×shearedat criticalafter pressbendManufacturingSteelTSElElsurfacefractureformingR / tNo.No.MPa%MPa · %—%μm—Note20D102617.517951Absence62.31.92.4Present Invention Example21D 98618.318005Absence54.92.12.5Comparative Example22E103015.916335Absence72.12.82.4Present Invention Example23F119611.213451Absence71.72.22.2Present Invention Example24G101015.315503Absence70.82.12.3Present Invention Example25H102415.716117Absence70.52.92.3Present Invention Example26I100815.715824Absence71.32.82.2Present Invention Example27J 99816.216202Absence72.02.12.4Present Invention Example28K 98315.715453Absence70.62.12.3Present Invention Example29L100514.314322Absence65.82.12.2Present Invention Example30M 99814.914866Absence69.02.42.3Present Invention Example31N103014.114507Absence64.82.62.1Present Invention Example32O102715.115528Absence70.82.22.3Present Invention Example33P101715.615877Absence74.93.52.4Comparative Example34Q 79421.216825Absence72.12.12.1Comparative Example35R1438 8.311953Absence71.72.22.2Comparative Example36S 94217.216249Absence54.52.02.7Comparative Example37T 89817.515750Absence72.02.32.3Comparative Example38U 95616.816046Absence74.42.42.3Comparative Example39B101915.215513Absence70.23.92.3Comparative Example40G 97617.316885Absence70.12.02.5Comparative Example41A138211.115304Presence71.92.42.3Comparative ExampleUnderlines 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 and shearing property, and further has excellent fatigue property after press forming. In addition, among the present invention examples, the hot-rolled steel sheets whose average grain size in the surface layer region is less than 3.0 μm has the above various properties, and further has excellent inner crack resistance 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 APPLICABILITYAccording 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 and shearing property, and further having excellent fatigue property after press forming. In addition, according to the preferable aspect according to the present invention, it is possible to obtain a hot-rolled steel sheet which has the above various properties, and further suppresses the occurrence of inner crack at bending, that is, has excellent inner crack resistance 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.

Examples

examples

[0265]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.

[0266]Steels having a chemical composition shown in Tables 1 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.

[0267]Note that the average cooling rate of slow cooling was set to slower than 5° C. / s. In addition, since the measurement lower limit of the coiling temperature shown in Table 4A and Table 4B...

Claims

1. A hot-rolled steel sheet 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%;sol. Al: 0.001% to 0.500%;Cr: 0.060% 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%;Mo: 0% to 1.00%;Ni: 0% to 2.00%;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%; anda remainder comprising Fe and impurities,the following formulas (A) and (B) are satisfied,wherein a microstructure at a depth position of 1 / 4 from a surface in a sheet thickness direction has,in terms of area %,residual austenite at less than 3.0%,ferrite at 15.0% or more and less than 60.0%, andpearlite at 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, anda 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,a solute Cr concentration in an outermost layer region whose starting point is the surface and ending point is a position of 5 μm depth in the sheet thickness direction is 0.10 mass % or more,a number density of Cr oxides having a sphere equivalent radius of 0.1 μm or more at the surface is 1.0×104 pieces / cm2 or less,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×and μy in the following formula (3) are indicated by 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+<semantics definitionURL="">❘<annotation encoding="Mathematica">"\[LeftBracketingBar]"< / annotation>< / semantics>i-j<semantics definitionURL="">❘<annotation encoding="Mathematica">"\[RightBracketingBar]"< / annotation>< / semantics>L(2)[Formula⁢ 3]Cluster⁢ Shade=∑ i⁢∑ j⁢(i+j-μx-μy)3⁢P⁡(i,j)(3)[Formula⁢ 4]μx=∑ i⁢∑ j⁢i⁡(P⁡(i,j))(4)[Formula⁢ 5]μy=∑ i⁢∑ j⁢j⁡(P⁡(i,j))._(5)2. The hot-rolled steel sheet according to claim 1, wherein an average grain size in a surface layer region whose starting point is the surface and ending point is a position of 20 μm depth in the sheet thickness direction is less than 3.0 μm.

3. The hot-rolled steel sheet according to claim 1, wherein the chemical composition comprises, in terms of mass %, one or more 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%;Mo: 0.01% to 1.00%;Ni: 0.02% to 2.00%;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%; andSn: 0.01% to 0.05%.

4. The hot-rolled steel sheet according to claim 2, wherein the chemical composition comprises, in terms of mass %, one or more 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%;Mo: 0.01% to 1.00%;Ni: 0.02% to 2.00%;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%; andSn: 0.01% to 0.05%.

Images