Hot-rolled steel sheet

Abstract

The hot-rolled steel sheet has a prescribed chemical composition; with respect to the metal structure, the residual austenite is less than 3.00% in area%, the Rcf value, which represents the ratio of the average cross-sectional unit before and after plastic deformation, is 2.00 or more, and the tensile strength of the hot-rolled steel sheet is 980 MPa or more.

Description

TECHNICAL FIELD

[0001] The present application relates to a hot-rolled steel sheet. Specifically, it relates to a hot-rolled steel sheet utilized by being formed into various shapes by press working or the like, particularly a hot-rolled steel sheet which is high in strength and small in deterioration of crack propagation stop property after plastic deformation.

[0002] This application claims priority based on Japanese Patent Application No. 2021-113549 filed on July 8, 2021, and the contents thereof are hereby incorporated by reference. BACKGROUND

[0003] In recent years, reduction of carbon dioxide emissions has been pursued in many fields from the viewpoint of protecting the global environment. In automobile manufacturers as well, technical development of vehicle body weight reduction for the purpose of low fuel consumption has been actively conducted. However, in order to ensure the safety of passengers, emphasis has been placed on the improvement of collision resistance, and thus vehicle body weight reduction is not easy.

[0004] In order to reduce the emission of greenhouse gases by vehicle body weight reduction, the use of high-strength steel sheets to thin the members has been studied. Therefore, there is a strong demand for steel sheets that have both high strength and excellent formability, and in order to meet these requirements, some technologies have been proposed in the past. On the other hand, it is described in Non-Patent Literature 1 that plastic deformation becomes difficult with the high-strength of steel sheets, and in general, the crack propagation stop property is reduced.

[0005] In addition, in the case of a strong working portion such as a bent portion, the strength increases due to work hardening because it is subjected to a large plastic deformation at press forming, and thus the crack propagation stop property is further reduced, and sometimes press cracking occurs in the portion subjected to a large plastic deformation. The deterioration of the crack propagation stop property after plastic deformation has been a problem with thick plate materials used in marine or structural steels in the past, but with the high-strength in recent years, it has become necessary to study at the time of forming of hot-rolled steel sheets as automobile materials.

[0006] As for a technology for improving the toughness after plastic deformation, for example, a large structure steel sheet is disclosed in Patent Literature 1, which is excellent in crack propagation stop property after plastic deformation by setting the ferrite grain size of the surface layer to 3 μm or less in addition to strict control of impurity elements.

[0007] A large structure steel sheet is disclosed in Patent Literature 2, which is excellent in crack propagation stop property after plastic deformation by making the ferrite crystal grains having a flatness ratio of 2 or more and a short axis diameter of 5 μm or less and containing subgrains having an equivalent circle diameter of 3 μm or less in the ferrite crystal.

[0008] PRIOR ART DOCUMENTS

[0009] PATENT LITERATURE

[0010] Patent Literature 1: Japanese Patent No. 3499085

[0011] Patent Literature 2: Japanese Patent No. 3467767

[0012] Non-Patent Literature

[0013] Non-Patent Literature 1: Takashi TAKAHASHI, Osamu KAWANO, Hiroshi SHIODA, Susumu SUZUKI: Iron and Steel, 99, (2013), 4, 312-321 SUMMARY

[0014] PROBLEMS TO BE SOLVED BY THE INVENTION

[0015] The technologies disclosed in Patent Literatures 1 and 2 are all technologies related to steel sheets for large structures, and do not target hot-rolled steel sheets. Furthermore, both are designed as microstructures with ferrite as the main body, and since the strength of the steel sheets is 450 to 700 MPa, it is sometimes difficult to apply the technologies disclosed in Patent Literatures 1 and 2 to high-strength hot-rolled steel sheets of 980 MPa or more with bainite or martensite as the main body.

[0016] The present application was made in view of the above problems of the conventional technologies, and aims to provide a hot-rolled steel sheet having high strength and small deterioration of crack propagation stop property after plastic deformation.

[0017] MEANS FOR SOLVING THE PROBLEMS

[0018] The present inventors repeatedly conducted in-depth studies on the relationship between the chemical composition of hot-rolled steel sheets and the mechanical properties of the metal microstructure, and as a result, obtained the following insights (a) to (e), and completed the present application.

[0019] (a) In order to obtain excellent tensile (maximum) strength, it is preferable to effectively utilize a hard microstructure. That is, it is preferable to include martensite or bainite in the metal microstructure.

[0020] (b) However, since a hard microstructure is a microstructure with insufficient crack propagation stop property, it is not possible to ensure excellent crack propagation stop property when only a metal microstructure with the same as the main body is produced.

[0021] (c) Furthermore, since work hardening occurs by being subjected to plastic deformation, the crack propagation stop property after plastic deformation is further reduced.

[0022] (d) In order to make a high-strength hot-rolled steel sheet have excellent crack propagation stop property, and to suppress the reduction of the crack propagation stop property after plastic deformation, it is effective to obtain a microstructure that is fine, and has a bent crack propagation path after plastic deformation.

[0023] Specifically, making the cross-sectional unit after plastic deformation fine is effective for suppressing deterioration of the crack propagation stop property after plastic deformation.

[0024] (e) To make the cross-sectional unit after plastic deformation fine, controlling the heating conditions, hot rolling conditions, and cooling conditions after hot rolling of the slab is effective. Thus, it is possible to make the austenite grains fine and the orientation difference between the generated structures at the bcc phase transition large, and it is possible to make the cross-sectional unit after plastic deformation fine.

[0025] The gist of the present application based on the above knowledge is as follows.

[0026] (1) The chemical composition of the hot-rolled steel sheet according to one aspect of the present application contains, in mass%:

[0027] C: 0.040 to 0.400%,

[0028] Si: 0.05 to 3.00%,

[0029] Mn: 1.00 to 4.00%,

[0030] sol. Al: 0.001 to 0.500%,

[0031] P: 0.100% or less,

[0032] S: 0.0300% or less,

[0033] N: 0.1000% or less,

[0034] O: 0.0100% or less,

[0035] Ti: 0 to 1.000%,

[0036] V: 0 to 1.000%,

[0037] Nb: 0 to 1.000%,

[0038] Cu: 0 to 2.00%,

[0039] Cr: 0 to 2.00%,

[0040] Mo: 0 to 1.00%,

[0041] Ni: 0 to 2.00%,

[0042] B: 0 to 0.0100%,

[0043] Ca: 0 to 0.0200%,

[0044] Mg: 0 to 0.0200%,

[0045] REM: 0 to 0.1000%,

[0046] Bi: 0 to 0.020%,

[0047] one or two or more of Zr, Co, Zn and W: total 0 to 1.00% and

[0048] Sn: 0 to 0.05%,

[0049] the remainder containing Fe and impurities;

[0050] for the metal structure,

[0051] in area %: retained austenite is less than 3.00%,

[0052] Rcf value, which indicates the ratio of the average cross-sectional unit before and after plastic deformation, is 2.00 or more;

[0053] the tensile strength of the hot-rolled steel sheet is 980 MPa or more.

[0054] (2) The hot-rolled steel sheet according to the above (1), wherein the above chemical composition can also contain, in mass %, one or two or more selected from the group consisting of:

[0055] Ti: 0.010 to 1.000%,

[0056] V: 0.010 to 1.000%,

[0057] Nb: 0.010 to 1.000%,

[0058] Cu: 0.01 to 2.00%,

[0059] Cr: 0.01 to 2.00%,

[0060] Mo: 0.01 to 1.00%,

[0061] Ni: 0.02 to 2.00%,

[0062] B: 0.0001 to 0.0100%,

[0063] Ca: 0.0005 to 0.0200%,

[0064] Mg: 0.0005 to 0.0200%,

[0065] REM: 0.0005 to 0.1000% and

[0066] Bi: 0.0005 to 0.020%.

[0067] (3) The hot-rolled steel sheet according to the above (1) or (2), wherein the metal structure can also be 15.00 to 60.00% ferrite and 40.00 to 85.00% martensite in area%.

[0068] (4) The hot-rolled steel sheet according to the above (1) or (2), wherein the metal structure can also be 50.00% or more bainite in area%.

[0069] (5) The hot-rolled steel sheet according to the above (1) or (2), wherein the metal structure can also be more than 85.00% martensite in area%.

[0070] Effects of Invention

[0071] According to the above aspect of the present application, a hot-rolled steel sheet having high strength and a small deterioration in crack propagation stop property after plastic deformation can be obtained.

[0072] The hot-rolled steel sheet of the above aspect of the present application is suitable as an industrial raw material used in automobile members, mechanical structural members, and further building members. DETAILED DESCRIPTION

[0073] The chemical composition and the metal structure of the hot-rolled steel sheet of the present embodiment are described in more detail below. However, the present application is not limited only to the composition disclosed in the present embodiment, and various modifications can be made within the scope of the gist of the present application.

[0074] With respect to the numerical limit range written with "to" below, the lower limit value and the upper limit value are included in the range. With respect to the numerical value expressed as "less than" or "more than", the value is not included in the numerical range. In the following description, the % of the chemical composition of the steel sheet is mass % unless otherwise specified.

[0075] 1. Chemical Composition

[0076] The hot-rolled steel sheet of the present embodiment contains, in mass %, C: 0.040 to 0.400%, Si: 0.05 to 3.00%, Mn: 1.00 to 4.00%, sol. Al: 0.001 to 0.500%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, and the balance: Fe and impurities. Each element is described in detail below.

[0077] (1-1) C: 0.040 to 0.400%

[0078] C The strength of the hot-rolled steel sheet is increased by increasing the fraction of the hard phase and lowering the phase transition point of the hard phase. When the C content is less than 0.040%, it becomes difficult to obtain the desired strength. Therefore, the C content is set to 0.040% or more. The C content is preferably 0.060% or more, more preferably 0.070% or more, and still more preferably 0.080% or more.

[0079] On the other hand, when the C content exceeds 0.400%, a large amount of carbide is generated in the structure, thereby promoting the generation of internal defects at the time of plastic deformation, and the deterioration of the crack propagation stop property after plastic deformation becomes large. As a result, the desired Rcf value cannot be obtained. Therefore, the C content is set to 0.400% or less. The C content is preferably 0.300% or less, more preferably 0.250% or less, and still more preferably 0.150% or less.

[0080] (1-2) Si: 0.05 to 3.00%

[0081] Si has an effect of increasing the strength of the hot-rolled steel sheet by solid solution strengthening at normal temperature and an effect of improving the toughness of the hot-rolled steel sheet by solid solution softening at low temperature. In addition, Si has an effect of soundness of steel by deoxidation (inhibition of generation of defects such as pores in steel). When the Si content is less than 0.05%, the effect by the above effects cannot be obtained. Therefore, the Si content is set to 0.05% or more. The Si content is preferably 0.50% or more, and more preferably 0.80% or more.

[0082] However, when the Si content exceeds 3.00%, the surface properties and chemical conversion treatment properties of the steel sheet, and further the ductility and weldability are significantly deteriorated, and the surface energy at the time of fracture is reduced. Thereby, the generation and propagation of cracks at the time of plastic deformation become easy, and the deterioration of the crack propagation stop property after plastic deformation becomes large. As a result, the desired Rcf value cannot be obtained. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.70% or less, and more preferably 2.50% or less.

[0083] (1-3) Mn: 1.00 to 4.00%

[0084] Mn has an effect of increasing the strength of the hot-rolled steel sheet by inhibiting the ferrite phase transition and an effect of improving the toughness of the hot-rolled steel sheet by solid solution softening at low temperature. When the Mn content is less than 1.00%, the desired tensile strength cannot be obtained. Therefore, the Mn content is set to 1.00% or more. The Mn content is preferably 1.30% or more, and more preferably 1.50% or more.

[0085] On the other hand, when the Mn content exceeds 4.00%, the effect of the reduction in the surface energy at the time of fracture becomes large, and thus the generation and propagation of cracks at the time of plastic deformation become easy, and the deterioration in the crack propagation stopping property after plastic deformation becomes large. As a result, the desired Rcf value cannot be obtained. Therefore, the Mn content is set to 4.00% or less. The Mn content is preferably 3.70% or less, and more preferably 3.50% or less.

[0086] (1-4) sol. Al: 0.001 to 0.500%

[0087] Al has the same effect as Si to deoxidize steel and to make steel sound, and has the effect to show solid solution softening at low temperatures and to improve the toughness of the hot-rolled steel sheet. When the sol. Al content is less than 0.001%, the effect by the above effect cannot be obtained. Further, when the sol. Al content is less than 0.001%, the desired Rcf value 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.

[0088] On the other hand, when the sol. Al content exceeds 0.500%, the above effect is saturated, and is not preferable in terms of economy, and thus the sol. Al content is set to 0.500% or less. The sol. Al content is preferably 0.300% or less, and more preferably 0.100% or less.

[0089] Note that sol. Al means acid-soluble Al, and indicates solid solution Al present in steel in a solid solution state.

[0090] (1-5) P: 0.100% or less

[0091] P is an element generally contained as an impurity, but is also an element having the effect to improve the strength of the hot-rolled steel sheet by solid solution strengthening. Therefore, P can also be actively contained, but P is an element that easily segregates, and if the P content exceeds 0.100%, the reduction in the grain boundary strength due to grain boundary segregation becomes significant, and grain boundary cracking becomes easy to occur. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.030% or less.

[0092] The lower limit of the P content is not particularly specified, but from the viewpoint of the refining cost, it is preferable to set it to 0.001% or more.

[0093] (1-6) S: 0.0300% or less

[0094] S is an element contained as an impurity, which forms sulfide inclusions in the steel and promotes the generation of cracks. If the S content exceeds 0.0300%, the generation of cracks during plastic deformation becomes significant, and the crack propagation stop property after plastic deformation is significantly reduced. Therefore, the S content is set to 0.0300% or less. The S content is preferably 0.0050% or less.

[0095] The lower limit of the S content is not particularly specified, but from the viewpoint of refining cost, it is preferably set to 0.0001% or more.

[0096] (1-7) N: 0.1000% or less

[0097] N is an element contained in the steel as an impurity, which has an effect of promoting the generation of cracks with the impurity as a starting point. If the N content exceeds 0.1000%, the generation of cracks during plastic deformation becomes significant, and the crack propagation stop property after plastic deformation is significantly reduced. 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.

[0098] The lower limit of the N content is not particularly specified, but it can also be set to 0.0001% or more. Furthermore, in the case where one or two or more of Ti, Nb, and V are contained to further refine the metal structure, in order to promote the precipitation of carbonitride, the N content is preferably set to 0.0010% or more, and more preferably set to 0.0020% or more.

[0099] (1-8) O: 0.0100% or less

[0100] If O is contained in a large amount in the steel, coarse oxides that become the starting points of fractures are formed, and brittle fracture, hydrogen-induced cracking occurs. 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.

[0101] In order to disperse fine oxides in a large amount at the time of deoxidization of the molten steel, the O content can also be set to 0.0005% or more, or 0.0010% or more.

[0102] The remaining portion of the chemical composition of the hot-rolled steel sheet of the present embodiment can also be Fe and impurities. In the present embodiment, the so-called impurities refer to components mixed from ores, scrap iron, or manufacturing environments, and / or components that are allowed within a range that does not adversely affect the hot-rolled steel sheet of the present embodiment.

[0103] The hot-rolled steel sheet of the present embodiment can also contain the following elements as optional elements in place of a part of Fe. The lower limit of the content in the case where the optional elements are not contained is 0%. Hereinafter, the optional elements are described in detail.

[0104] (1-9) Ti: 0.010 to 1.000%, Nb: 0.010 to 1.000%, and V: 0.010 to 1.000%

[0105] Ti, Nb, and V are elements that are finely precipitated in the steel as carbides and nitrides, and increase the strength of the steel by precipitation strengthening. Therefore, one or two or more of these elements can be contained. In order to more reliably obtain this effect, the contents of Ti, Nb, and V are preferably each set to 0.010% or more. Note that it is not necessary to contain all of Ti, Nb, and V, and it is sufficient that the content of any one of them is 0.010% or more. The contents of Ti, Nb, and V are each preferably 0.060% or more, and more preferably 0.080% or more.

[0106] On the other hand, if the content of any one of Ti, Nb, and V exceeds 1.000%, the workability of the hot-rolled steel sheet deteriorates. Therefore, the contents of Ti, Nb, and V are each set to 1.000% or less. It is preferable that they be 0.800% or less, and more preferably 0.500% or less.

[0107] (1-10) Cu: 0.01 to 2.00%, Cr: 0.01 to 2.00%, Mo: 0.01 to 1.00%, Ni: 0.02 to 2.00%, and B: 0.0001 to 0.0100%

[0108] Cu, Cr, Mo, Ni, and B each have an effect of increasing the hardenability of the hot-rolled steel sheet. Furthermore, Ni has an effect of effectively suppressing the grain boundary cracking of the slab due to Cu when Cu is contained. Therefore, one or two or more of these elements can be contained.

[0109] Cu has an effect of increasing the hardenability of the hot-rolled steel sheet as described above. In order to more reliably obtain the effect resulting from the above effect, 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 exceeds 2.00%, the grain boundary cracking of the slab sometimes occurs. 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.

[0110] As described above, Cr has an effect of improving the hardenability of the hot-rolled steel sheet. In order to more reliably obtain the effect resulting from the above effect, it is preferable to set the Cr content to 0.01% or more, and more preferably to 0.05% or more. However, when the Cr content exceeds 2.00%, the chemical conversion treatment property of the hot-rolled steel sheet is significantly reduced. Therefore, the Cr content is set to 2.00% or less.

[0111] As described above, Mo has an effect of improving the hardenability of the hot-rolled steel sheet and an effect of improving the strength of the hot-rolled steel sheet by precipitating as carbides in the steel. In order to more reliably obtain the effect resulting from the above effects, it is preferable to set the Mo content to 0.01% or more, and more preferably to 0.02% or more. However, even when the Mo content is set to exceed 1.00%, the effect resulting from the above effects is saturated, and is not preferable in terms of economy. 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.

[0112] As described above, Ni has an effect of improving the hardenability of the hot-rolled steel sheet. In addition, Ni has an effect of effectively suppressing the grain boundary cracking of the slab due to Cu when Cu is contained. In order to more reliably obtain the effect resulting from the above effect, the Ni content is preferably set to 0.02% or more. Ni is an element that is expensive, and therefore it is not preferable to contain a large amount in terms of economy. Therefore, the Ni content is set to 2.00% or less.

[0113] As described above, B has an effect of improving the hardenability of the hot-rolled steel sheet. In order to more reliably obtain the effect resulting from the above effect, it is preferable to set the B content to 0.0001% or more, and more preferably to 0.0002% or more. However, when the B content exceeds 0.0100%, the formability of the hot-rolled steel sheet is significantly reduced, and therefore the B content is set to 0.0100% or less. The B content is preferably set to 0.0050% or less.

[0114] (1-11) Ca: 0.0005 to 0.0200%, Mg: 0.0005 to 0.0200%, REM: 0.0005 to 0.1000%, and Bi: 0.0005 to 0.020%

[0115] Ca, Mg and REM all have the effect of improving the crack propagation stop property of the hot-rolled steel sheet by adjusting the shape of inclusions in the steel to a preferred shape. In addition, Bi has the effect of improving the crack propagation stop property of the hot-rolled steel sheet by refining the solidification structure. Therefore, one or two or more of these elements can be contained. In order to more reliably obtain the effects resulting from the above effects, it is preferable to set one or more of Ca, Mg, REM and Bi to 0.0005% or more. However, if the Ca content or the Mg content exceeds 0.0200% or the REM content exceeds 0.1000%, inclusions are excessively generated in the steel, and sometimes the crack propagation stop property of the hot-rolled steel sheet is reduced. In addition, even if the Bi content is set to exceed 0.020%, the effects resulting from the above effects are saturated, and are 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 0.020% or less. The Bi content is preferably 0.010% or less.

[0116] Here, REM refers to a total of 17 elements including Sc, Y and lanthanoid elements, and the content of the above REM refers to the total content of these elements. In the case of lanthanoid elements, they are added in the form of a mixed rare earth metal in industry.

[0117] (1-12) One or two or more of Zr, Co, Zn and W: 0 to 1.00% in total, and Sn: 0 to 0.05%

[0118] With regard to Zr, Co, Zn and W, the present inventors have confirmed that even if these elements are contained in a total of 1.00% or less, the effects of the hot-rolled steel sheet of the present embodiment are not impaired. Therefore, one or two or more of Zr, Co, Zn and W in a total of 1.00% or less can be contained.

[0119] In addition, the present inventors have confirmed that even if Sn is contained in a small amount, the effects of the hot-rolled steel sheet of the present embodiment are not impaired. However, if Sn is contained in a large amount, defects are sometimes generated at the time of hot-rolling, and therefore the Sn content is set to 0.05% or less.

[0120] The chemical composition of the hot-rolled steel sheet described above can be measured by a general analysis method. For example, it can be measured by using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Note that sol. Al can be measured by using a filtrate obtained by heating and decomposing a sample with an acid, by ICP-AES. C and S can be measured by using a combustion-infrared absorption method, N by using an inert gas fusion-thermal conductivity method, and O by using an inert gas fusion-non-dispersive infrared absorption method.

[0121] 2. Metal structure of hot-rolled steel sheet

[0122] Next, the metal structure of the hot-rolled steel sheet of the present embodiment will be described.

[0123] The metal structure of the hot-rolled steel sheet of the present embodiment is such that the area % of residual austenite is less than 3.00%, and the Rcf value, which represents the ratio of the average cross-sectional unit before and after plastic deformation, is 2.0 or more.

[0124] Therefore, the hot-rolled steel sheet of the present embodiment can achieve high strength, and also excellent crack propagation stop property after plastic deformation.

[0125] Note that in the present embodiment, the fraction of structure and the Rcf value in the metal structure at the position of 1 / 4 depth from the surface (the region of 1 / 8 depth from the surface to 3 / 8 depth from the surface) in the cross section parallel to the rolling direction and at the center position in the sheet width direction are defined. The reason for this is that the metal structure at this position represents the representative metal structure of the steel sheet.

[0126] (2-1) Area fraction of residual austenite: less than 3.00%

[0127] Residual austenite is a metal structure that exists as fcc even at room temperature. The carbon concentration of the structure around residual austenite can become the starting point of crack generation by phase transition to hard martensite at the time of plastic deformation. When the area fraction of residual austenite is 3.00% or more, the above effect is significant, and the crack propagation stop property after plastic deformation is significantly reduced. Therefore, the area fraction of residual austenite is set to be less than 3.00%. The area fraction of residual austenite is preferably 2.00% or less, less than 1.50% or 1.00% or less, more preferably less than 1.00% or less than 0.50%. The less the residual austenite, the more preferable, and therefore the area fraction of residual austenite can also be 0.00%.

[0128] As a method for measuring the area ratio of residual austenite, there are a method using X-ray diffraction, a method using EBSP (Electron Back Scattering Diffraction Pattern) analysis, a method using magnetic measurement, and the like. In the present embodiment, the area ratio of residual austenite is measured by X-ray diffraction.

[0129] In the measurement of the area ratio of residual austenite by X-ray diffraction in the present embodiment, first, in a cross section parallel to the rolling direction at a position 1 / 4 deep from the surface (a region from 1 / 8 deep from the surface to 3 / 8 deep from the surface) of the hot-rolled steel sheet and at a central position in the sheet width direction, the integrated intensity of the total of 7 peaks of a (110), a (200), a (211), g (111), g (200), and g (220) is calculated by using Co-Ka rays, and the area ratio of residual austenite is calculated using the intensity average method.

[0130] The metal structure of the hot-rolled steel sheet of the present embodiment can include ferrite, martensite, bainite, and pearlite in addition to residual austenite.

[0131] The area ratio of ferrite can also be set to 60.00% or less, 50.00% or less, or 45.00% or less. In addition, the area ratio of ferrite can also be set to 0.00% or more or 0.05% or more.

[0132] The area ratio of martensite can also be set to 100.00% or less or 99.00% or less. In addition, the area ratio of martensite can also be set to 0.00% or more, 1.00% or more, or 1.50% or more.

[0133] The area ratio of bainite can also be set to 100.00% or less or 96.00% or less. In addition, the area ratio of bainite can also be set to 0.00% or more or 0.01% or more.

[0134] Note that the area ratios of residual austenite, ferrite, bainite, and martensite described above are applicable to all of the three steel types (DP steel, bainitic steel, and martensitic steel) described later.

[0135] Hereinafter, preferred area ratios of each structure in each of the three steel types (DP steel, bainitic steel, and martensitic steel) will be described.

[0136] (2-2) DP Steel

[0137] In the case where the metal structure contains less than 3.00% of residual austenite, and ferrite, martensite, and a trace of remaining structure in terms of area ratio, it is preferable that the area ratio of ferrite be 15.00 to 60.00%, the area ratio of martensite be 40.00 to 85.00%, and the area ratio of the remaining structure be less than 45.00%. By setting the area ratios of the respective structures as described above, it is possible to improve the strength and ductility in balance.

[0138] The respective structures in the present embodiment are described below.

[0139] Area ratio of ferrite: 15.00 to 60.00%

[0140] Ferrite is a structure generated when the fcc phase is transformed into the bcc phase at a relatively high temperature. Ferrite has a high work hardening rate, and thus has an effect of improving the strength-ductility balance of the hot-rolled steel sheet. By setting the area ratio of ferrite to be 15.00% or more, it is possible to sufficiently obtain the effect by the above effect. Therefore, the area ratio of ferrite is preferably set to be 15.00% or more. The area ratio of ferrite is more preferably 20.00% or more, 25.00% or more, or 30.00% or more.

[0141] On the other hand, since ferrite has low strength, if the area ratio is excessive, it is sometimes not possible to obtain the desired tensile strength. Therefore, the area ratio of ferrite is preferably set to be 60.00% or less. The area ratio of ferrite is more preferably 55.00% or less, and still more preferably 50.00% or less.

[0142] Area ratio of martensite: 40.00 to 85.00%

[0143] Martensite is a structure generated when the fcc phase is transformed into the bcc phase at a relatively low temperature. Martensite is a structure containing fine grains with high dislocation density, and has an effect of improving the strength of the hot-rolled steel sheet. By setting the area ratio of martensite to be 40.00% or more, it is possible to sufficiently obtain the effect by the above effect. Therefore, the area ratio of martensite is preferably set to be 40.00% or more. The area ratio of martensite is preferably 50.00% or more.

[0144] On the other hand, since martensite has insufficient ductility, if the area ratio is excessive, the ductility of the hot-rolled steel sheet is sometimes reduced. Therefore, the area ratio of martensite is preferably set to be 85.00% or less. The area ratio of martensite is more preferably 80.00% or less, still more preferably 75.00% or less, or 70.00% or less.

[0145] Area ratio of remaining structure: less than 45.00%

[0146] In the present embodiment, as the remaining structure, pearlite and bainite that total less than 45.00% can also be included. The area ratio of the remaining structure can also be 10.00% or less or 5.00% or less. The remaining structure can also not be included, and the area ratio can total 0.00%.

[0147] (2-3) Bainitic Steel

[0148] In the case where the metal structure includes, in terms of area ratio, less than 3.00% of residual austenite, and bainite and a trace amount of a remaining structure, it is preferable that the area ratio of the bainite be 50.00% or more and the area ratio of the remaining structure be less than 50.00%. By setting the area ratios of the respective structures as described above, it is possible to simultaneously improve strength, ductility, and expandability.

[0149] The respective structures in the present embodiment are described below.

[0150] Area ratio of bainite: 50.00% or more

[0151] Bainite is a structure that is generated when an fcc phase is transformed into a bcc phase at a low temperature. Bainite is a structure that includes fine grains and carbides, and has the effect of improving the strength, ductility, and expandability of a hot-rolled steel sheet in balance. By setting the area ratio of the bainite to be 50.00% or more, it is possible to sufficiently obtain the effects resulting from the above effect. Therefore, the area ratio of the bainite is preferably set to be 50.00% or more. The area ratio of the bainite is more preferably 80.00% or more, 85.00% or more, or 90.00% or more.

[0152] The upper limit is not particularly specified, but can be set to be 100.00% or less.

[0153] Remaining structure: less than 50.00%

[0154] In the present embodiment, as the remaining structure, pearlite, ferrite, and martensite that total less than 50.00% can also be included. The area ratio of the remaining structure is more preferably 20.00% or less, 15.00% or less, or 10.00% or less. The remaining structure can also not be included, and the area ratio can total 0.00%.

[0155] (2-4) Martensitic Steel

[0156] In the case where the metal structure includes, in terms of area ratio, less than 3.00% of residual austenite, martensite, and a trace amount of a remaining structure, it is preferable that the total of the area ratios of the martensite be 85.00% or more and the area ratio of the remaining structure be less than 15.00%. By setting the area ratios of the respective structures as described above, it is possible to simultaneously improve strength and expandability.

[0157] The respective structures in the present embodiment are described below.

[0158] Total area ratio of martensite: more than 85.00%

[0159] As described above, the martensite has an effect of increasing the strength of the hot-rolled steel sheet. In addition, the martensite has an effect of increasing the hole expandability of the hot-rolled steel sheet because the crystal orientation of the structure is random. By setting the area ratio of the martensite to be more than 85.00%, the effect brought by the above effects can be sufficiently obtained. Therefore, the area ratio of the martensite is preferably set to be more than 85.00%. The area ratio of the martensite is more preferably 90.00% or more, 93.00% or more, or 95.00% or more. The upper limit is not particularly specified, but can be set to be 100.00% or less.

[0160] Remaining structure: less than 15.00%

[0161] In the present aspect, as the remaining structure, the pearlite, the ferrite, and the bainite, which are less than 15.00% in total, can also be included. The area ratio of the remaining structure is more preferably 10.00% or less, 7.00% or less, or 5.00% or less. The remaining structure can also not be included, and the area ratio can be 0.00% in total.

[0162] (2-5) Area ratio of pearlite: less than 5.00%

[0163] The pearlite is a lamellar metal structure in which the cementite is precipitated in layers between the ferrites, and is a soft metal structure if compared with the bainite or the martensite. If the area ratio of the pearlite is 5.00% or more, the carbon is consumed by the cementite included in the pearlite, the strength of the martensite and the bainite decreases, and the tensile strength of 980 MPa or more cannot be obtained in some cases. Therefore, in any of the aspects described above, the area ratio of the pearlite can be set to be less than 5.00%. The area ratio of the pearlite is more preferably 3.00% or less. In order to increase the stretch flangeability of the hot-rolled steel sheet, the area ratio of the pearlite is preferably as low as possible, and the area ratio of the pearlite is more preferably 0.00%.

[0164] Note that the area ratio of the pearlite referred to here is an area ratio that applies to all of the three steel types (DP steel, bainitic steel, and martensitic steel) described above.

[0165] The structure other than the retained austenite is measured by the following method.

[0166] The area ratio of ferrite and pearlite was measured by the following method. In a cross section parallel to the rolling direction, a sample was collected in a manner that the observable distance surface was 1 / 4 depth of the plate thickness (a region from 1 / 8 depth of the distance surface to 3 / 8 depth of the distance surface) and the central position in the plate width direction. The cross section parallel to the rolling direction of the sample was finished to a mirror surface, and the strain introduced into the surface layer of the sample was removed by polishing for 8 minutes at room temperature using colloidal silica not containing an alkaline solution.

[0167] At an arbitrary position in the length direction of the sample cross section, a region of length 50 μm, 1 / 4 depth of the plate thickness from the distance surface (a region from 1 / 8 depth of the plate thickness to 3 / 8 depth of the plate thickness), and the central position in the plate width direction was measured at a measurement interval of 0.1 μm by electron backscatter diffraction to obtain crystal orientation information. The number of measurement points was set to at least 500 points. For the measurement, an EBSD device composed of a hot field emission type scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) was used. At this time, the degree of vacuum in the EBSD device was set to 9.6 x 10 -5 Pa or less, the acceleration voltage was set to 15 kV, the irradiation current level was set to 13, and the irradiation level of the electron rays was set to 62.

[0168] Further, a reflection electron image was captured in the same field of view. First, ferrite and cementite that precipitated in a layered manner were specified from the reflection electron image, and the area ratio of the grains was calculated to obtain the area ratio of the pearlite. Thereafter, for the grains other than the grains determined to be pearlite, the "Grain Average Misorientation" function mounted in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device was used for the obtained crystal orientation information, and a region in which the Grain Average Misorientation value was 1.0° or less was determined to be ferrite. The area ratio of the region determined to be ferrite was calculated to obtain the area ratio of the ferrite.

[0169] After polishing the same observation surface as in the above-described measurement, etching with nitric acid-ethanol was performed, and observation was performed on a 30 μm x 30 μm region in a position 1 / 4 of the plate thickness from the surface (a region from 1 / 8 depth in the plate thickness direction from the surface to 3 / 8 depth in the plate thickness direction from the surface) using an optical microscope and a scanning electron microscope (SEM) for at least three regions. The area ratio of bainite was obtained by image analysis of the microstructure photographs obtained by this microstructure observation. Thereafter, for the same observation position, after Lepera etching, microstructure observation was performed using an optical microscope and a scanning electron microscope, and the area ratio of martensite was calculated by image analysis of the obtained microstructure photographs.

[0170] In the above-described microstructure observation, each microstructure was identified by the following method.

[0171] Martensite is distinguished from other metal microstructures if an electron channeling contrast image using a scanning electron microscope is used because it is a microstructure having a high dislocation density and having a lower microstructure such as a block and a packet within a grain.

[0172] A microstructure that is a collection of grains in a lath shape and does not contain Fe-based carbide having a length of 20 nm or more in the inside of the microstructure and is not martensite and a microstructure that contains Fe-based carbide having a length of 20 nm or more in the inside of the microstructure and the Fe-based carbide has a single variant, that is, Fe-based carbide elongated in the same direction, is regarded as bainite. Here, the Fe-based carbide elongated in the same direction means a carbide in which the difference in the elongation direction of the Fe-based carbide is within 5°.

[0173] Note that the rolling direction of the hot-rolled steel sheet is determined by the following method.

[0174] First, a test piece is collected in a manner in which the plate thickness section of the hot-rolled steel sheet can be observed. The plate thickness section of the collected test piece is finished by mirror polishing, and observation is performed using an optical microscope. The observation range is set to the entire thickness of the plate thickness, and a direction parallel to the extension direction of the grain is determined as the rolling direction.

[0175] Rcf value indicating the ratio of the average cross-sectional unit before and after plastic deformation: 2.00 or more

[0176] Generally, the crack propagation stop property of the hot-rolled steel sheet after plastic deformation decreases due to the increase in strength by dislocation introduction at the time of plastic deformation. The crack is formed on a cleavage facet called a cleavage facet unit, and thus, to suppress crack propagation, it is important to make the cleavage facet unit fine to bend the propagation path. In the present embodiment, the decrease in the crack propagation stop property after plastic deformation is suppressed by controlling the Rcf (Ratio of cleavage facet) value indicating the ratio of the cleavage facet unit before and after plastic deformation.

[0177] The Rcf value indicates the ratio of the average cleavage facet unit before and after plastic deformation, and is expressed by the following formula using the average cleavage facet unit before plastic deformation, that is, the Cf1 value, and the average cleavage facet unit after plastic deformation, that is, the Cf2 value.

[0178] Rcf = Cf1 / Cf2

[0179] In the case of the hot-rolled steel sheet subjected to plastic deformation, the crack propagation stop property decreases due to the increase in strength by work hardening due to dislocation introduction. On the other hand, the crack propagation stop property is also affected by the cleavage facet unit, and the finer the cleavage facet unit, the more the propagation path is bent, and the more the propagation of the crack is suppressed. Therefore, in order to exhibit a high crack propagation stop property also after plastic deformation, it is necessary to increase the Rcf value.

[0180] If the Rcf value is less than 2.00, the effect of the increase in strength by dislocation introduction becomes larger than the effect of the fineness of the cleavage facet unit, and thus it is presumed that the crack propagation stop property decreases. Therefore, the Rcf value is set to 2.00 or more. It is preferably 2.20 or more, and more preferably 2.30 or more.

[0181] The higher the Rcf value, the more preferable, and thus the upper limit is not particularly specified, but it can be set to 5.00 or less, 4.00 or less, or 3.00 or less.

[0182] The Cf1 value and the Cf2 value can be obtained by the following method.

[0183] In the present embodiment, in order to calculate the Cf1 value and the Cf2 value, it is necessary to generate a brittle fracture. As a test method for generating a brittle fracture, for example, it is sufficient to produce a 2.5 mm small-size V-notch test piece in which the width direction (C direction) of the hot-rolled steel sheet becomes the direction of the test piece length, and to perform a Charpy impact test at -196°C in accordance with JIS Z 2242:2018. In the case of a test piece in which the sheet thickness of the hot-rolled steel sheet is less than 2.5 mm, it is sufficient to perform a test on the entire thickness.

[0184] Note that, by the above-described method, the direction at right angles to the rolling direction is determined as the width direction of the hot-rolled steel sheet.

[0185] The photographing region of the SEM image taken to calculate the Cf1 value and the Cf2 value is set to a position at a depth of 1 / 4 of the thickness of the steel sheet from the surface in the cross section parallel to the rolling direction (a region from a depth of 1 / 8 of the thickness from the surface to a depth of 3 / 8 of the thickness from the surface), and a central position in the sheet width direction. For the photographing of the SEM image, a SU-6600 Schottky electron gun manufactured by Hitachi High-Technologies, Inc. is used, the emitter is set to tungsten, the accelerating voltage is set to 20 kV, and the photographing magnification is set to 9.6 x 104 -5 The accelerating voltage is set to 1.5 kV in a vacuum of 10" Pa. The photographing magnification is set to 1000x, and the photographing field number is set to 3 or more.

[0186] In the photographed SEM image, a ductile fracture portion called a tear ridge is photographed as a bright contrast. A region surrounded by the tear ridge is set as one cleavage facet, the equivalent circle diameter is calculated from the area of each cleavage facet, and the cross-sectional unit of each cleavage facet is set. From the obtained cross-sectional units, the area average diameter weighted by the area of each cleavage facet is calculated, and the average cross-sectional unit is set.

[0187] By the above-described processing of the hot-rolled steel sheet before plastic deformation and the hot-rolled steel sheet after plastic deformation, the Cf1 value and the Cf2 value are calculated, respectively.

[0188] Note that, regarding the Charpy impact test after plastic deformation, JIS No. 5 tensile test pieces are produced in a direction in which the width direction (C direction) of the hot-rolled steel sheet becomes the test piece length direction, and various test pieces are collected after a compression pre-strain of 10% is given to the steel material with respect to the test piece length direction.

[0189] Standard deviation of Mn concentration: 0.60 mass% or less

[0190] In the hot-rolled steel sheet of the present embodiment, the standard deviation of the Mn concentration at a position at a depth of 1 / 4 of the thickness of the steel sheet from the surface (a region from a depth of 1 / 8 of the thickness from the surface to a depth of 3 / 8 of the thickness from the surface) and a central position in the sheet width direction can also be set to 0.60 mass% or less. Thereby, the development of a region in which the Mn locally concentrates and the fracture energy decreases can be suppressed, and the generation of local cracks at the time of plastic deformation and the decrease in the crack propagation stop property after plastic deformation can be further suppressed.

[0191] The standard deviation of the Mn concentration is preferably 0.50 mass% or less, more preferably 0.47 mass% or less. The lower limit of the standard deviation of the Mn concentration is more preferably smaller in terms of suppressing a decrease in fracture energy, but the practical lower limit is 0.10 mass% in terms of constraints on the manufacturing process.

[0192] After mirror polishing a cross section (L cross section) parallel to the rolling direction of the hot-rolled steel sheet, the standard deviation of the Mn concentration is measured at a depth of 1 / 4 of the sheet thickness from the surface of the steel sheet (a region from a depth of 1 / 8 of the sheet thickness from the surface to a depth of 3 / 8 of the sheet thickness from the surface) and at the central position in the sheet width direction using an electron probe micro-analyzer (EPMA). The measurement conditions are that the acceleration voltage is set to 15 kV, the magnification is set to 5000 times, a distribution image of a range of 20 μm in the sheet thickness direction of the sample is measured (a region from a depth of 1 / 8 of the sheet thickness from the surface to a depth of 3 / 8 of the sheet thickness from the surface), and more specifically, the measurement interval is set to 0.1 μm, and the Mn concentration is measured at 40,000 points or more. Then, the standard deviation is calculated based on the Mn concentrations obtained from all the measurement points, and the standard deviation of the Mn concentration is obtained.

[0193] 3. Tensile properties

[0194] The tensile properties (tensile strength) of the hot-rolled steel sheet are evaluated in accordance with JIS Z 2241:2011. The test piece is set to No. 5 test piece of JIS Z 2241:2011. The collection position of the tensile test piece is set to 1 / 4 of the end portion in the sheet width direction, and the direction perpendicular to the rolling direction is set to the length direction.

[0195] The tensile (maximum) strength of the hot-rolled steel sheet of the present embodiment is 980 MPa or more. It is preferably 1000 MPa or more. If the tensile strength is less than 980 MPa, the applicable members are limited, and the contribution to the weight reduction of the vehicle body is small. The upper limit is not particularly limited, but it can be set to 1780 MPa or less, 1500 MPa or less, or 1300 MPa or less from the viewpoint of suppressing die wear.

[0196] 4. Sheet thickness

[0197] The sheet thickness of the hot-rolled steel sheet of the present embodiment is not particularly limited, but can be set to 1.20 to 8.00 mm. When the sheet thickness of the hot-rolled steel sheet is less than 1.20 mm, it is sometimes difficult to secure the finish rolling temperature, and the rolling load becomes too large, making it difficult to hot- roll. Therefore, the sheet thickness of the hot-rolled steel sheet of the present embodiment can also be set to 1.20 mm or more. It is preferable to be 1.40 mm or more. On the other hand, when the sheet thickness exceeds 8.00 mm, the influence of the standard deviation of the Mn concentration sometimes becomes significant, making it difficult to obtain the desired Rcf value. Therefore, the sheet thickness can also be set to 8.00 mm or less. It is preferable to be 6.00 mm or less or 3.00 mm or less.

[0198] 5. Other

[0199] (5-1) Plating layer

[0200] The hot-rolled steel sheet of the present embodiment having the above-described chemical composition and metal structure can also be made into a surface-treated steel sheet by providing a plating layer on the surface for the purpose of improving corrosion resistance and the like. The plating layer can be an electroplated layer or a hot-dipped plating layer. As the electroplated layer, an electroplated zinc layer, an electroplated Zn-Ni alloy layer, and the like can be exemplified. As the hot-dipped plating layer, a hot-dipped zinc layer, an alloyed hot-dipped zinc layer, a hot-dipped aluminum layer, a hot-dipped Zn-Al alloy layer, a hot-dipped Zn-Al-Mg alloy layer, a hot-dipped Zn-Al-Mg-Si alloy layer, and the like can be exemplified. The plating adhesion amount is not particularly limited, and can be set as in the past. Furthermore, by performing a proper chemical conversion treatment (for example, application and drying of a chromium-free chemical conversion treatment liquid of a silicate type) after plating, it is also possible to further improve the corrosion resistance.

[0201] 6. Manufacturing conditions

[0202] A preferred manufacturing method of the hot-rolled steel sheet of the present embodiment having the above-described chemical composition and metal structure is described below.

[0203] In order to obtain the hot-rolled steel sheet of the present embodiment, it is effective to perform heating of the slab under prescribed conditions, then perform hot-rolling, accelerate cooling to a prescribed temperature range, and thereafter perform slow cooling as necessary, and control the cooling history until coiling.

[0204] In the preferred manufacturing method of the hot-rolled steel sheet of the present embodiment, the following steps (1) to (7) are performed in this order. Note that 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.

[0205] (1) The slab is heated and held at a temperature region of 1100°C or higher for 6000 seconds or more. Note that, during heating, it is more preferable to further heat after holding at a temperature region of 700 to 850°C for 900 seconds or more and then hold at a temperature region of 1100°C or higher for 6000 seconds or more.

[0206] (2) Hot rolling is performed in a temperature region of 850 to 1100°C so that the total reduction in thickness becomes 90% or more.

[0207] (3) The starting temperature of hot rolling is set to 850°C or higher and lower than 930°C, the rolling temperature of the first two stages of the first stage to the final stage of hot rolling is set to 850°C or higher and lower than 950°C, and the reduction ratio of the rolling is set to lower than 30%.

[0208] (4) The rolling temperature of the final stage and the stage preceding the final stage of hot rolling is set to 930°C or higher and lower than 1010°C, the reduction ratio of the rolling is set to 50% or more, and the finishing temperature of the rolling is set to 950°C or higher and lower than 1010°C.

[0209] (5) After completion of hot rolling, cooling is performed at an average cooling rate of 50°C / sec or more within 1.0 seconds, and the starting temperature of the cooling is set to 850°C or higher and lower than 960°C.

[0210] (6) Cooling is performed at an average cooling rate of 50°C / sec or more to a temperature region of 600 to 730°C, and slow cooling is performed at an average cooling rate of lower than 5°C / s for 2.0 seconds or more in the temperature region of 600 to 730°C. Thereafter, cooling is performed at an average cooling rate of 50°C / s or more to a temperature region of 350°C or lower.

[0211] Note that, slow cooling can not be performed, and in the case where slow cooling is not performed, cooling is performed at an average cooling rate of 50°C / s or more to a temperature region of 350°C or lower.

[0212] (7) Coiling is performed in a temperature region of 350°C or lower.

[0213] (6-1) Slab, slab temperature at the time of hot rolling, and holding time

[0214] The slab for hot rolling can use a slab obtained by continuous casting, a slab obtained by casting and blooming, or the like, and can use a slab obtained by subjecting them to hot working or cold working as needed. The slab for hot rolling is preferably heated and held at a temperature region of 1100°C or higher for 6000 seconds or more. Further, at the holding at 1100°C or higher, the steel sheet temperature can be allowed to fluctuate in the temperature region of 1100°C or higher, or can be set constant. By holding at a temperature region of 1100°C or higher for 6000 seconds or more, the austenite grains at the time of heating the slab can be made uniform. By making the austenite grains uniform, recrystallization of the austenite in the pre-hot rolling section (the first two stages of the first stage to the final stage of hot rolling) described later can be suppressed, and as a result, the desired Rcf value can be obtained. When the holding temperature is lower than 1100°C or the holding time is less than 6000 seconds, it becomes difficult to make the austenite grains uniform, and recrystallization of the austenite in the pre-hot rolling section described later cannot be suppressed, and as a result, the desired Rcf value cannot be obtained at times.

[0215] Further, at the time of heating the slab, after holding at a temperature region of 700 to 850°C for 900 seconds or more, further heating can be performed and holding at a temperature region of 1100°C or higher for 6000 seconds or more. Note that at the holding at a temperature region of 700 to 850°C, the steel sheet temperature can be allowed to fluctuate in the temperature region, or can be set constant. At the austenite phase transformation in the temperature region of 700 to 850°C, Mn is distributed between ferrite and austenite, and by extending the phase transformation time, Mn can diffuse within the ferrite region. Thus, the Mn micro-segregation unevenly present in the slab can be eliminated, and the standard deviation of the Mn concentration can be significantly reduced. If the standard deviation of the Mn concentration is large, regions in which the fracture energy is reduced by local concentration of Mn develop, and cracking at the time of plastic deformation is promoted, and the desired Rcf value cannot be obtained at times.

[0216] The hot rolling is preferably performed using a reversible mill or a tandem mill as a multi-pass rolling. In particular, from the viewpoint of industrial productivity and the viewpoint of stress load on the steel sheet in rolling, it is more preferable that at least the final two stages be set to hot rolling using a tandem mill.

[0217] (6-2) Reduction ratio of hot rolling: total reduction of 90% or more in the temperature region of 850 to 1100°C

[0218] By performing the hot rolling in which the total of the reductions in thickness becomes 90% or more in the temperature range of 850 to 1100°C, the refinement of the recrystallized austenite grains can be mainly sought, and the accumulation of strain energy into the non-recrystallized austenite grains can be promoted. Also, the recrystallization of the austenite can be promoted, and the atomic diffusion of Mn is promoted, and the standard deviation of the Mn concentration can be reduced. As a result, the generation of cracks at the time of plastic deformation can be promoted, and the desired Rcf value can be obtained. Therefore, it is preferable to perform the hot rolling in which the total of the reductions in thickness becomes 90% or more in the temperature range of 850 to 1100°C. If the total of the reductions in the temperature range of 850 to 1100°C is less than 90%, the standard deviation of the Mn concentration sometimes becomes high, and the development of a region in which the fracture energy is reduced due to the local concentration of Mn cannot be suppressed, and the generation of cracks at the time of plastic deformation is promoted. Thus, the desired Rcf value sometimes cannot be obtained.

[0219] Note that the reduction in thickness in the temperature range of 850 to 1100°C can be expressed as {(t0-t1) / t0} x 100 (%) when the entry thickness before the initial rolling in the rolling in the temperature range is set to t0, and the exit thickness after the final stage of the rolling in the temperature range is set to t1.

[0220] (6-3) The starting temperature of the hot rolling is 850°C or higher and lower than 930°C, the rolling temperature of the first stage to the second stage before the final stage of the hot rolling is 850°C or higher and lower than 950°C, and the reduction in the rolling is lower than 30%

[0221] It is preferable that the starting temperature of the hot rolling be set to 850°C or higher and lower than 930°C, the rolling temperature of the first stage to the second stage before the final stage of the hot rolling be set to 850°C or higher and lower than 950°C, and the reduction in the rolling of the first stage to the second stage before the final stage of the hot rolling be set to lower than 30%. By setting the starting temperature of the hot rolling to a relatively low temperature, and performing the pre-stage of the hot rolling at a low temperature and a low reduction, the recrystallization in the pre-stage of the hot rolling can be suppressed, and the strain can be accumulated in the austenite grains. As a result, the non-recrystallized austenite having a high dislocation density in the grains can be maintained to the post-stage of the rolling. Thus, the desired Rcf value can be obtained. When the starting temperature of the rolling is 930°C or higher, the rolling temperature of the first stage to the second stage before the final stage of the hot rolling is 950°C or higher, or the reduction in the rolling is 30% or higher, the recrystallization of the austenite in the first stage to the second stage before the final stage of the hot rolling cannot be suppressed, and as a result, the desired Rcf value sometimes cannot be obtained. Further, when the starting temperature of the hot rolling is lower than 850°C, or the rolling temperature of the first stage to the second stage before the final stage of the hot rolling is lower than 850°C, it becomes difficult to set the rolling temperature of the final stage and the pre-stage of the final stage of the hot rolling to 930°C or higher, and as a result, the desired Rcf value sometimes cannot be obtained.

[0222] (6-4) The rolling temperature of the final stage and the stage preceding the final stage of hot rolling: 930°C or higher and lower than 1010°C, the reduction ratio of the rolling: 50% or higher, the finish rolling temperature: 950°C or higher and lower than 1010°C

[0223] It is preferable that the rolling temperature of the final stage and the stage preceding the final stage of hot rolling be set to 930°C or higher and lower than 1010°C, the reduction ratio of the final stage and the stage preceding the final stage be set to 50% or higher, and the finish rolling temperature (the temperature after the rolling of the final stage) be set to 950°C or higher. By setting the reduction ratio of the final stage and the stage preceding the final stage of hot rolling to 50% or higher and the finish rolling temperature to 950°C or higher, recrystallization of austenite in the latter stage of rolling is promoted. By the grains having high dislocation density due to the suppression of recrystallization in the former stage of rolling, recrystallization occurs in the latter stage of rolling, and the orientation difference between the structures generated from the recrystallized austenite becomes large. Thus, the Rcf value can be increased, and the desired Rcf value can be obtained. It is presumed that, in the structures having a large orientation difference, the interface cracking propagation stopping property is improved by the generation of crystal rotation at the time of plastic deformation, and the Rcf value is increased. When the rolling temperature of the final stage and the stage preceding the final stage of hot rolling is lower than 930°C, the reduction ratio of the final stage and the stage preceding the final stage is lower than 50%, or the finish rolling temperature is lower than 950°C, the recrystallization of austenite is sometimes insufficient, and the desired Rcf value cannot be obtained.

[0224] Further, by setting the rolling temperature and the finish rolling temperature of the final stage and the stage preceding the final stage of hot rolling to be lower than 1010°C, the structure can be refined by suppressing the coarsening of the austenite grain diameter. Thus, the generation of cracks at the time of plastic deformation can be suppressed, and the Rcf value can be increased.

[0225] (6-5) The average cooling rate of cooling within 1.0 seconds after the completion of hot rolling: 50°C / sec or higher, the cooling start temperature: 850°C or higher and lower than 960°C

[0226] In order to suppress the growth of the austenite grains that are refined by hot rolling, it is preferable that cooling be performed at an average cooling rate of 50°C / sec or higher within 1.0 seconds after the completion of hot rolling, and the cooling start temperature be set to 850°C or higher and lower than 960°C. In order to perform cooling at an average cooling rate of 50°C / sec or higher within 1.0 seconds after the completion of hot rolling, it is only necessary to perform cooling at a large average cooling rate immediately after the completion of hot rolling, for example, by spraying cooling water to the surface of the steel sheet. By setting the cooling start temperature to 850°C or higher and lower than 960°C, and performing cooling at an average cooling rate of 50°C / sec or higher within 1.0 seconds after the completion of hot rolling, the austenite grains and the structures generated thereafter can also be refined. Thus, the generation of cracks at the time of plastic deformation can be suppressed, and the Rcf value can be increased.

[0227] The cooling start temperature referred to herein is the temperature immediately before cooling at an average cooling rate of 50°C / sec or more is performed, for example, the temperature immediately before cooling water is sprayed onto the surface of the steel sheet.

[0228] Further, the average cooling rate referred to herein is the value obtained by dividing the temperature drop of the steel sheet from the start of the accelerated cooling (when the steel sheet is introduced into the cooling device) to the completion of the accelerated cooling (when the steel sheet is discharged from the cooling device) by the required time from the start of the accelerated cooling to the completion of the accelerated cooling.

[0229] (6-6) Cooling at an average cooling rate of 50°C / sec or more to a temperature range of 600 to 730°C, and performing slow cooling at an average cooling rate of less than 5°C / s for 2.0 seconds or more in the temperature range of 600 to 730°C. Thereafter, cooling at an average cooling rate of 50°C / s or more to a temperature range of 350°C or less.

[0230] Note that slow cooling can not be performed, and in the case where slow cooling is not performed, cooling at an average cooling rate of 50°C / s or more to a temperature range of 350°C or less.

[0231] After the above cooling, by cooling at an average cooling rate of 50°C / sec or more to a temperature range of 600 to 730°C, the generation of ferrite and pearlite having low strength can be suppressed. Thus, the strength of the hot-rolled steel sheet is increased.

[0232] In the cooling, by performing slow cooling at an average cooling rate of less than 5°C / s for 2.0 seconds or more in the temperature range of 600 to 730°C, the bainite can be sufficiently precipitated. Thus, the structure is refined, and therefore the strength and the crack propagation stop property of the hot-rolled steel sheet can be balanced. Note that the average cooling rate referred to herein is the value obtained by dividing the temperature drop of the steel sheet from the cooling stop temperature of the accelerated cooling to the stop temperature of the slow cooling by the required time from the stop of the accelerated cooling to the stop of the slow cooling.

[0233] Note that by performing slow cooling in the high-temperature range (660 to 730°C) in the temperature range of 600 to 730°C, the above-described DP steel can be stably manufactured. Further, by performing slow cooling in the low-temperature range (600°C or more and less than 660°C) in the temperature range of 600 to 730°C, the above-described bainitic steel can be stably manufactured.

[0234] In order to suppress the area ratio of the pearlite and obtain a desired tensile strength, it is preferable to set the average cooling rate from the cooling stop temperature of the slow cooling to the coiling temperature to 50°C / sec or more. Thus, the parent phase structure can be made hard.

[0235] Note that the average cooling rate here refers to a value obtained by dividing the temperature drop of the steel sheet from the stop temperature of the slow cooling at an average cooling rate of less than 5°C / s to the coiling temperature by the required time from the stop of the slow cooling at an average cooling rate of less than 5°C / s to the coiling.

[0236] The upper limit of the time for slow cooling is determined depending on the equipment design, but it is sufficient to set it to be less than 10.0 seconds. Further, the lower limit of the average cooling rate of the slow cooling is not particularly set, but the temperature rise in the case of no cooling is accompanied by a large investment on the equipment, and thus it can be set to be 0°C / s or more.

[0237] Note that the slow cooling can not be performed. By performing the accelerated cooling at an average cooling rate of 50°C / s or more to a temperature region of 350°C or less without the slow cooling, the generation of ferrite and pearlite having low strength can be suppressed, and the generation of martensite can be promoted. Thus, the microstructure is refined, and the above-described martensitic steel can be stably manufactured, and the strength and the crack propagation stop property of the hot-rolled steel sheet can be balanced.

[0238] The average cooling rate here refers to a value obtained by dividing the temperature drop of the steel sheet from the start of the accelerated cooling (when the steel sheet is introduced into the cooling equipment) to the completion of the accelerated cooling (when the steel sheet is discharged from the cooling equipment) by the required time from the start of the accelerated cooling to the completion of the accelerated cooling.

[0239] The upper limit of the cooling rate is not particularly specified, but if the cooling rate is accelerated, the cooling equipment becomes large-scale, and the equipment cost becomes high. Thus, if the equipment cost is considered, it is preferable to be 300°C / s or less.

[0240] (6-7) Coiling temperature: 350°C or less

[0241] The coiling temperature is set to be 350°C or less. By setting the coiling temperature to be 350°C or less, the amount of precipitation of iron carbide can be reduced, and the unevenness of the hardness distribution in the hard phase can be reduced. As a result, the starting point and the propagation path of the crack are reduced, the generation of the crack at the time of plastic deformation can be suppressed, and the desired Rcf value can be obtained.

[0242] Examples

[0243] Next, the effects of one aspect of the present application are more specifically described by examples, but the conditions in the examples are one example of conditions adopted in order to confirm the practicability and effects of the present application, and the present application is not limited to this one example of conditions. As long as the purpose of the present application is achieved without departing from the gist of the present application, various conditions can be adopted in the present application.

[0244] Steels having the chemical compositions shown in Tables 1 and 2 were melted, and slabs having a thickness of 240 to 300 mm were manufactured by continuous casting. Using the obtained slabs, the hot-rolled steel sheets shown in Tables 5A and 5B were obtained by the manufacturing conditions shown in Tables 3A to 4.

[0245] Note that, by cooling to a temperature region of 350°C or lower at a desired average cooling rate without performing slow cooling, a martensitic steel was obtained. Further, regarding the example in which slow cooling was performed, by performing slow cooling in a high-temperature range (660 to 730°C) in the temperature region of 600 to 730°C, a DP steel was obtained, and by performing slow cooling in a low-temperature range (600°C or higher and lower than 660°C) in the temperature region of 600 to 730°C, a bainitic steel was obtained.

[0246] For the obtained hot-rolled steel sheets, the area fraction of the metal structure, the Rcf value, the standard deviation of the Mn concentration, and the tensile strength TS were obtained by the above-described methods. The obtained measurement results are shown in Tables 5A and 5B.

[0247] Evaluation method of characteristics of hot-rolled steel sheets

[0248] (1) Tensile characteristics

[0249] In the case where the tensile strength TS was 980 MPa or higher, it was determined to be a hot-rolled steel sheet having high strength and was judged to be acceptable. On the other hand, in the case where the tensile strength TS was lower than 980 MPa, it was determined to be a hot-rolled steel sheet not having high strength and was judged to be unacceptable.

[0250] (2) Crack propagation stop characteristics

[0251] The crack propagation stop characteristics were evaluated by a Charpy impact test. In accordance with JIS Z 2242:2018, 2.5 mm small-size V-notched test pieces were produced in a direction in which the width direction (C direction) of the hot-rolled steel sheet became the test piece length direction, and a Charpy impact test was performed at -196°C. For test pieces in which the plate thickness of the hot-rolled steel sheet was less than 2.5 mm, the test was performed with the full thickness.

[0252] Further, JIS No. 5 B tensile test pieces were produced in a direction in which the width direction (C direction) of the hot-rolled steel sheet became the test piece length direction, and after giving a compression pre-strain of 10% to the steel with respect to the test piece length direction, the above-described V-notched test pieces were collected. By performing a Charpy impact test on the test pieces at -196°C using the above-described method, the absorbed energy after plastic deformation was obtained.

[0253] In a case where the reduction rate of absorbed energy after plastic deformation ((absorbed energy before plastic deformation - absorbed energy after plastic deformation) / absorbed energy before plastic deformation) is 30.00% or less, it is determined to be acceptable because the deterioration of the crack propagation stop property before and after plastic deformation is small. On the other hand, in a case where the reduction rate of absorbed energy after plastic deformation exceeds 30.00%, it is determined to be unacceptable because the deterioration of the crack propagation stop property before and after plastic deformation is large.

[0254] Table 1

[0255]

[0256] The underlined is outside the scope of the present application.

[0257]

[0258]

[0259]

[0260] Table 4

[0261]

[0262] The underlined indicates that the manufacturing conditions are not preferable.

[0263]

[0264]

[0265] It is observed from Table 5A and Table 5B that the hot-rolled steel sheets of the examples of the present application have high strength and the deterioration of the crack propagation stop property after plastic deformation is small. On the other hand, it is observed that the hot-rolled steel sheets of the comparative examples do not have one or more of the above-described properties.

Claims

1. A hot-rolled steel sheet characterized by, The chemical composition contains, in mass%: C：0.040～0.400％、 Si: 0.05 to 3.00%, Mn: 1.00 to 4.00%, sol. Al: 0.001 to 0.500%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, Ti: 0 to 1.000%, V：0～1.000％、 Nb: 0 to 1.000%, Cu: 0 to 2.00%, Cr：0～2.00％、 Mo: 0 to 1.00%, Ni: 0 to 2.00%, B：0～0.0100％、 Ca: 0 to 0.0200%, Mg: 0 to 0.0200%, REM: 0 to 0.1000%, Bi: 0 to 0.020%, one or two or more of Zr, Co, Zn, and W: total 0 to 1.00%, and Sn: 0 to 0.05%, the remainder consisting of Fe and impurities; for the metal structure, in area%: retained austenite is less than 3.00%, an Rcf value, which indicates a ratio of an average cross-sectional unit before plastic deformation, Cf1, to an average cross-sectional unit after plastic deformation, Cf2, is 2.00 or more; the hot-rolled steel sheet has a tensile strength of 980 MPa or more, wherein the Rcf value is expressed as Rcf = Cf1 / Cf2 using the average cross-sectional unit before plastic deformation, Cf1, and the average cross-sectional unit after plastic deformation, Cf2.

2. Hot-rolled steel sheet according to claim 1, characterized in that, The chemical composition contains, in mass%: Ti: 0.010 to 1.000%, V：0.010～1.000％、 Nb: 0.010 to 1.000%, Cu: 0.01 to 2.00%, Cr：0.01～2.00％、 Mo: 0.01 to 1.00%, Ni: 0.02 to 2.00%, B：0.0001～0.0100％、 Ca: 0.0005 to 0.0200%, Mg: 0.0005 to 0.0200%, REM: 0.0005 to 0.1000%, and Bi: 0.0005 to 0.020%.

3. Hot-rolled steel sheet according to claim 1 or 2, wherein, The metal structure has, in area%: ferrite is 15.00 to 60.00%, and martensite is 40.00 to 85.00%.

4. The hot-rolled steel sheet according to claim 1 or 2, wherein, The metal structure has, in area%: bainite is 50.00% or more.

5. The hot-rolled steel sheet according to claim 1 or 2, wherein, The metal structure has, in area%: martensite is more than 85.00%.

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