Hot-rolled steel sheet

Abstract

This hot-rolled steel plate has a prescribed chemical composition and metal structure, wherein: the pole density of the ｛001｝＜１１０＞, ｛１１１｝＜１１０＞, and ｛１１２｝＜１１０＞ orientation group is 2.0 to 8.0 in the texture of a region from a surface to a depth of 1 / 8 of the thickness from the surface; the pole density of the ｛１１０｝＜１１２＞ orientation is 2.0 to 4.0 in the texture of a region from the depth of 1 / 8 of the thickness from the surface to a depth of 1 / 2 of the thickness from the surface; and the tensile strength is 980 MPa.

Description

Technical Field

[0001] The present invention relates to a hot-rolled steel sheet. This application claims priority based on Japanese Patent Application No. 2021-168627 filed in Japan on October 14, 2021, and incorporates its content herein by reference.

Background Art

[0002] From the perspective of global environmental protection, in order to improve the fuel efficiency of automobiles, the weight reduction of automobile bodies is being promoted. In order to further reduce the weight of automobile bodies, it is necessary to increase the strength of the steel sheets applied to automobile bodies. However, generally, when the strength of a steel sheet is increased, its formability decreases.

[0003] As a method for improving the formability of steel sheets, there is a method of incorporating retained austenite into the metal structure of steel sheets. However, when retained austenite is incorporated into the metal structure of a steel sheet, although the ductility is improved, the ductility isotropy may deteriorate and the hole expansion property may deteriorate in some cases. When performing bending forming, hole expansion processing, and flanging processing, it is required that the anisotropy of ductility be reduced, that is, the ductility isotropy be excellent. Furthermore, when performing the above-mentioned processing, it is also required to have excellent hole expansion property.

[0004] Patent Document 1 discloses a hot-rolled steel sheet in which the microstructure is mainly bainite, the hard phase composed of martensite and / or austenite has an area fraction of 3% or more and less than 20%, among the hard phases present in the center of the sheet thickness, those with an aspect ratio of 3 or more account for 60% or more, the length in the rolling direction of the hard phase present in the center of the sheet thickness is less than 20 μm, the sum of the X-ray random intensity ratios in the <011> orientation and the <111> orientation as viewed from the rolling direction is 3.5 or more, and the X-ray random intensity ratio in the <001> orientation as viewed from the rolling direction is 1.0 or less.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0006] However, in Patent Document 1, it is necessary to further improve the strength in order to further reduce the weight of the automobile body. Further, in Patent Document 1, the ductile isotropy is not considered.

[0007] An object of the present invention is to provide a hot-rolled steel sheet having high strength, excellent ductile isotropy, and hole expansion property.

MEANS FOR SOLVING THE PROBLEMS

[0008] In view of the above problems, the present inventors have conducted intensive studies on the relationship between the chemical composition, the metallographic structure, and the mechanical properties of the hot-rolled steel sheet, and as a result, have obtained the following findings and completed the present invention.

[0009] The present inventors have found that in order to enhance the ductile isotropy and the hole expansion property of the hot-rolled steel sheet, it is important to control the aggregate structure in the surface layer region and the internal region of the hot-rolled steel sheet. Further, the present inventors have found that in order to control the aggregate structure in the surface layer region and the internal region of the hot-rolled steel sheet, it is particularly effective to control the finish rolling conditions.

[0010] The gist of the present invention made based on the above findings is as follows. (1) The hot-rolled steel sheet according to one aspect of the present invention has a chemical composition in mass %, C: 0.100 to 0.350%, Si: 0.010 to 3.00%, Mn: 1.00 to 4.00%, sol.Al: 0.001 to 2.000%, Si + sol.Al: 1.00% or more, Ti: 0.010 to 0.380%, P: 0.100% or less, S: Below 0.0300%, N: Below 0.1000%, O: Below 0.0100%, Nb: 0 to 0.100%, V: 0 to 0.500%, Cu: 0 to 2.00%, Cr: 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 0.020%, One or more of Zr, Co, Zn, and W: In total 0 to 1.00%, and Sn: 0 to 0.050% is contained, The balance consists of Fe and impurities, In the region from 1 / 8 of the plate thickness from the surface to 3 / 8 of the plate thickness from the surface, the metallographic structure is, in area%, Retained austenite: 10 to 20%, Fresh martensite: 10% or less, and Bainite: Consists of 70 to 90%, In the aggregate structure of the region from the surface to 1 / 8 of the plate thickness from the surface, The pole densities of the {001}<110>, {111}<110>, and {112}<110> orientation groups are 2.0 to 8.0, In the aggregate structure of the region from 1 / 8 of the plate thickness from the surface to 1 / 2 of the plate thickness from the surface, The pole density of the {110}<112> orientation is 2.0 to 4.0, The tensile strength is 980 MPa or more. (2) The hot-rolled steel sheet according to (1) above, wherein the chemical composition is, in mass%, Nb: 0.005 to 0.100%, V: 0.005 to 0.500%, Cu: 0.01 to 2.00%, Cr: 0.01 to 2.00%, Mo: 0.01 - 1.00%, Ni: 0.02 - 2.00%, B: 0.0001 - 0.0100%, Ca: 0.0005 - 0.0200%, Mg: 0.0005 - 0.0200%, REM: 0.0005 - 0.1000%, and Bi: 0.0005 - 0.020% It may contain one or more selected from the group consisting of.

Advantages of the Invention

[0011] According to the above aspect of the present invention, it is possible to provide a hot-rolled steel sheet having high strength and excellent ductility isotropy and hole expansion property.

Modes for Carrying Out the Invention

[0012] Regarding the chemical composition and metallographic structure of the hot-rolled steel sheet according to the present embodiment, it will be specifically described below. However, the present invention is not limited only to the configuration disclosed in the present embodiment, and various modifications are possible without departing from the spirit of the present invention.

[0013] In the numerical limit range described with "~" sandwiched below, the lower limit value and the upper limit value are included in the range. The numerical values indicated by "<" or ">" are not included in the numerical range. In the following description, % regarding the chemical composition is mass % unless otherwise specified.

[0014] Chemical Composition The hot-rolled steel sheet according to the present embodiment has a chemical composition, in mass %, of C: 0.100 - 0.350%, Si: 0.010 - 3.00%, Mn: 1.00 - 4.00%, sol.Al: 0.001 - 2.000%, Si + sol.Al: 1.00% or more, Ti: 0.010 - 0.380%, 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. Hereinafter, each element will be described in detail.

[0015] C: 0.100 to 0.350% C is an element necessary to obtain the desired strength. If the C content is less than 0.100%, it becomes difficult to obtain the desired strength. Therefore, the C content shall be 0.100% or more. The C content is preferably 0.120% or more, 0.150% or more. On the other hand, if the C content exceeds 0.350%, the transformation rate becomes slow, making it easy to generate MA (mixed phase of fresh martensite and retained austenite), and it becomes difficult to obtain excellent ductility isotropy and hole expansion property. Therefore, the C content shall be 0.350% or less. The C content is preferably 0.330% or less, 0.310% or less.

[0016] Si: 0.010 to 3.00% Si has the effect of delaying the precipitation of cementite. By this effect, the amount of austenite remaining untransformed, that is, the area ratio of retained austenite can be increased. Also, the amount of solid-solved C in the hard phase can be kept large, and the strength can be increased by preventing the coarsening of cementite. Also, Si itself has the effect of increasing the strength of the hot-rolled steel sheet by solid solution strengthening. Also, Si has the effect of purifying the steel by deoxidation (suppressing the occurrence of defects such as blowholes in the steel). If the Si content is less than 0.010%, the effects by the above actions cannot be obtained. Therefore, the Si content shall be 0.010% or more. The Si content is preferably 0.50% or more, 1.00% or more, 1.20% or more, 1.50% or more. On the other hand, if the Si content exceeds 3.00%, the precipitation of cementite is significantly delayed, and the amount of retained austenite becomes excessive, which is not preferable. Also, the surface properties and chemical conversion treatability of the hot-rolled steel sheet, further, the ductility and weldability are significantly deteriorated, and the A3 transformation point is significantly increased. Thereby, it becomes difficult to stably perform hot rolling. Therefore, the Si content shall be 3.00% or less. The Si content is preferably 2.70% or less, 2.50% or less.

[0017] Mn: 1.00 to 4.00% Mn has the effect of suppressing ferrite transformation and strengthening the hot-rolled steel sheet. When the Mn content is less than 1.00%, the desired strength cannot be obtained. Therefore, the Mn content should be 1.00% or more. The Mn content is preferably 1.50% or more, 1.80% or more. On the other hand, when the Mn content exceeds 4.00%, the ductility isotropy and hole expansion property of the hot-rolled steel sheet deteriorate. Therefore, the Mn content should be 4.00% or less. The Mn content is preferably 3.70% or less, 3.50% or less.

[0018] sol.Al: 0.001 - 2.000% Similar to Si, sol.Al has the effect of purifying the steel by deoxidation and promoting the formation of retained austenite by suppressing the precipitation of cementite from austenite. When the sol.Al content is less than 0.001%, the effect of the above action cannot be obtained. Therefore, the sol.Al content should be 0.001% or more. The sol.Al content is preferably 0.010% or more. On the other hand, when the sol.Al content exceeds 2.000%, the above effect saturates and it is not economically preferable. Furthermore, the A3 transformation point rises significantly, making it difficult to stably perform hot rolling. Therefore, the sol.Al content should be 2.000% or less. The sol.Al content is preferably 1.500% or less, 1.300% or less. In this embodiment, sol.Al means acid-soluble Al, indicating the solid-solution Al present in the steel in a solid-solution state.

[0019] Si + sol.Al: 1.00% or more Both Si and sol.Al have the effect of delaying the precipitation of cementite, and by this effect, the amount of austenite remaining untransformed, that is, the area ratio of retained austenite, can be increased. When the total content of Si and sol.Al is less than 1.00%, the effect of the above action cannot be obtained. Therefore, the total content of Si and sol.Al should be 1.00% or more. Preferably 1.20% or more, 1.50% or more. Note that Si in "Si + sol.Al" indicates the content in mass % of Si, and sol.Al indicates the content in mass % of sol.Al.

[0020] Ti: 0.010 - 0.380% Ti is an element effective for suppressing the recrystallization and grain growth of austenite between stands in hot rolling. By suppressing the recrystallization of austenite between stands, strain can be accumulated more. As a result, the microstructure of the hot-rolled steel sheet can be preferably controlled. When the Ti content is less than 0.010%, the above effects cannot be obtained. Therefore, the Ti content is set to 0.010% or more. Preferably, it is 0.050% or more, 0.070% or more, or 0.080% or more. On the other hand, when the Ti content exceeds 0.380%, inclusions caused by TiN are generated, and the toughness of the hot-rolled steel sheet deteriorates. Therefore, the Ti content is set to 0.380% or less. Preferably, it is 0.320% or less or 0.300% or less.

[0021] P: 0.100% or less P is an element generally contained in steel as an impurity, but it has the effect of increasing the strength of the hot-rolled steel sheet by solid solution strengthening. Therefore, P may be actively contained. However, P is an element that easily segregates, and when the P content exceeds 0.100%, the deterioration of the hole expansion property and the isotropy of ductility due to grain boundary segregation becomes remarkable. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.030% or less. Although there is no particular need to specify the lower limit of the P content, from the viewpoint of refining cost, it is preferably 0.001%.

[0022] S: 0.0300% or less S is an element contained in steel as an impurity, which forms sulfide-based inclusions in the steel and deteriorates the hole expansion property and the ductility isotropy of the hot-rolled steel sheet. When the S content exceeds 0.0300%, the hole expansion property and the ductility isotropy of the hot-rolled steel sheet deteriorate significantly. Therefore, the S content shall be 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, but from the viewpoint of refining cost, it is preferably 0.0001%.

[0023] N: 0.1000% or less N is an element contained in steel as an impurity and has an effect of deteriorating the hole expansion property and the ductility isotropy of the hot-rolled steel sheet. When the N content exceeds 0.1000%, the hole expansion property and the ductility isotropy of the hot-rolled steel sheet deteriorate significantly. Therefore, the N content shall be 0.1000% or less. The N content is preferably 0.0800% or less, 0.0700% or less. The lower limit of the N content does not need to be particularly specified, but in order to promote the precipitation of carbonitrides, the N content is preferably 0.0010% or more, and more preferably 0.0020% or more.

[0024] O: 0.0100% or less When a large amount of O is contained in the steel, it forms coarse oxides that become the starting points of fracture and cause brittle fracture and hydrogen-induced cracking. Therefore, the O content shall be 0.0100% or less. The O content is preferably 0.0080% or less, 0.0050% or less. In order to disperse a large number of fine oxides during the deoxidation of the molten steel, the O content may be 0.0005% or more, 0.0010% or more.

[0025] The remainder of the chemical composition of the hot-rolled steel sheet according to this embodiment consists of Fe and impurities. In this embodiment, the impurities refer to elements mixed from ores, scraps as raw materials, or the manufacturing environment, etc., or elements intentionally added in trace amounts, which are allowed within a range that does not adversely affect the hot-rolled steel sheet according to this embodiment.

[0026] In addition to the above elements, the chemical composition of the hot-rolled steel sheet according to this embodiment may contain the following elements as optional elements. The lower limit of the content when the above optional elements are not contained is 0%. Hereinafter, each optional element will be described in detail.

[0027] Nb: 0.005 to 0.100% and V: 0.005 to 0.500% Both Nb and V, like Ti, are elements that suppress the recrystallization and grain growth of austenite between stands during hot rolling. Therefore, one or more of these elements may be contained. In order to more surely obtain the effect by the above action, it is preferable that the Nb content is 0.005% or more or the V content is 0.005% or more. However, even if these elements are contained in excess, the effect by the above action is saturated and it is not economically preferable. Therefore, the Nb content is 0.100% or less and the V content is 0.500% or less.

[0028] 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% Cu, Cr, Mo, Ni and B all have the effect of enhancing the hardenability of the hot-rolled steel sheet. Further, Cr and Ni have the effect of stabilizing retained austenite, and Cu and Mo have the effect of precipitating carbides in the steel to increase the strength of the hot-rolled steel sheet. Furthermore, Ni has the effect of effectively suppressing the intergranular cracking of the slab caused by Cu when Cu is contained. Therefore, one or more of these elements may be contained.

[0029] As described above, Cu has the effect of enhancing the hardenability of the steel sheet and the effect of precipitating as carbides in the steel at low temperature to increase the strength of the hot-rolled steel sheet. In order to more surely obtain the effect by the above action, it is preferable that the Cu content is 0.01% or more. However, if the Cu content exceeds 2.00%, intergranular cracking of the slab may occur. Therefore, the Cu content is 2.00% or less.

[0030] As described above, Cr has the effects of enhancing the hardenability of the steel sheet and stabilizing retained austenite. In order to more surely obtain the effects by the above actions, it is preferable that the Cr content is 0.01% or more. However, when the Cr content exceeds 2.00%, the chemical conversion treatment property of the hot-rolled steel sheet significantly deteriorates. Therefore, the Cr content is set to 2.00% or less.

[0031] As described above, Mo has the effects of enhancing the hardenability of the steel sheet and increasing the strength by precipitating carbides in the steel. In order to more surely obtain the effects by the above actions, it is preferable that the Mo content is 0.01% or more. However, even if the Mo content exceeds 1.00%, the effects by the above actions are saturated and it is not economically preferable. Therefore, the Mo content is set to 1.00% or less.

[0032] As described above, Ni has the effect of enhancing the hardenability of the steel sheet. Further, when Cu is contained, Ni has the effect of effectively suppressing the intergranular cracking of the slab caused by Cu. In order to more surely obtain the effects by the above actions, it is preferable that the Ni content is 0.02% or more. Since Ni is an expensive element, it is not economically preferable to contain a large amount. Therefore, the Ni content is set to 2.00% or less.

[0033] As described above, B has the effect of enhancing the hardenability of the steel sheet. In order to more surely obtain the effects by this action, it is preferable that the B content is 0.0001% or more. However, when the B content exceeds 0.0100%, the hole expansion property and the ductility isotropy of the hot-rolled steel sheet significantly deteriorate. Therefore, the B content is set to 0.0100% or less.

[0034] 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% Ca, Mg, and REM all have the effect of enhancing the formability of hot-rolled steel sheets by controlling the shape of inclusions to a favorable shape. Also, Bi has the effect of enhancing the formability of hot-rolled steel sheets by refining the solidification structure. Therefore, one or more of these elements may be contained. In order to more surely obtain the effect by the above actions, it is preferable that any one or more of Ca, Mg, REM, and Bi be 0.0005% or more. However, when the Ca content or Mg content exceeds 0.0200%, or when the REM content exceeds 0.1000%, inclusions may be excessively generated in the steel, and conversely, the hole expansion property and ductility isotropy of the hot-rolled steel sheet may be deteriorated. Also, even when the Bi content exceeds 0.020%, the effect by the above actions saturates, which is not economically preferable. Therefore, the Ca content and Mg content are 0.0200% or less, the REM content is 0.1000% or less, and the Bi content is 0.020% or less. The Bi content is preferably 0.010% or less.

[0035] Here, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanoids, and the content of the above REM refers to the total content of these elements. In the case of lanthanoids, industrially, they are added in the form of mischmetal.

[0036] One or more of Zr, Co, Zn, and W: in total 0 to 1.00%, and Sn: 0 to 0.050% Regarding Zr, Co, Zn, and W, the inventors have confirmed that even when these elements are contained in total of 1.00% or less, the effect of the hot-rolled steel sheet according to this embodiment is not impaired. Therefore, one or more of Zr, Co, Zn, and W may be contained in total of 1.00% or less. Also, the inventors have confirmed that even when a small amount of Sn is contained, the effect of the hot-rolled steel sheet according to this embodiment is not impaired. However, since flaws may occur during hot rolling, the Sn content is 0.050% or less.

[0037] The chemical composition of the hot-rolled steel sheet described above may be measured by a general analysis method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Note that sol.Al may be measured by ICP-AES using the filtrate after the sample is decomposed by heating with an acid. C and S may be measured using the combustion-infrared absorption method, N may be measured using the inert gas fusion-thermal conductivity method, and O may be measured using the inert gas fusion-non-dispersive infrared absorption method.

[0038] Metallographic structure of the hot-rolled steel sheet Next, the metallographic structure of the hot-rolled steel sheet according to the present embodiment will be described. In the hot-rolled steel sheet according to the present embodiment, the metallographic structure in the region from 1 / 8 of the plate thickness from the surface to 3 / 8 of the plate thickness from the surface is, in area %, retained austenite: 10 to 20%, fresh martensite: 10% or less, and bainite: 70 to 90%. In the aggregate structure in the region from the surface to 1 / 8 of the plate thickness from the surface, the pole densities of the {001}<110>, {111}<110>, and {112}<110> orientation groups are 2.0 to 8.0. In the aggregate structure in the region from 1 / 8 of the plate thickness from the surface to 1 / 2 of the plate thickness from the surface, the pole density of the {110}<112> orientation is 2.0 to 4.0.

[0039] In the present embodiment, the area ratios of retained austenite, fresh martensite, and bainite at the position 1 / 4 of the plate thickness from the surface (the region from 1 / 8 of the plate thickness from the surface to 3 / 8 of the plate thickness from the surface) in the plate thickness cross-section parallel to the rolling direction are defined. The reason is that the metallographic structure at this position shows a typical metallographic structure of the hot-rolled steel sheet.

[0040] Retained austenite: 10 - 20% Retained austenite is a structure that improves the hole expansion property and ductility isotropy of hot-rolled steel sheets. If the area ratio of retained austenite is less than 10%, the desired hole expansion property and ductility isotropy cannot be obtained. Therefore, the area ratio of retained austenite should be 10% or more. Preferably, it is 12% or more or 13% or more. On the other hand, if the area ratio of retained austenite exceeds 20%, the desired strength cannot be obtained. Therefore, the area ratio of retained austenite should be 20% or less. Preferably, it is 18% or less, 17% or less.

[0041] Fresh martensite: 10% or less Since fresh martensite is a hard structure, it contributes to the improvement of the strength of hot-rolled steel sheets. However, fresh martensite also has poor hole expansion property and ductility isotropy. If the area ratio of fresh martensite exceeds 10%, the desired hole expansion property and ductility isotropy cannot be obtained. Therefore, the area ratio of fresh martensite should be 10% or less. Preferably, it is 8% or less, 6% or less, 4% or less or 2% or less. The area ratio of fresh martensite may be 0%.

[0042] Bainite: 70 - 90% Bainite is a structure that improves the strength and ductility isotropy of hot-rolled steel sheets. If the area ratio of bainite is less than 70%, the desired strength cannot be obtained. Therefore, the area ratio of bainite should be 70% or more. Preferably, it is 73% or more, 75% or more or 77% or more. On the other hand, if the area ratio of bainite exceeds 90%, the strength becomes too high and the desired hole expansion property cannot be obtained. Therefore, the area ratio of bainite should be 90% or less. Preferably, it is less than 90%, 88% or less or 85% or less.

[0043] Among the above-mentioned structures, the area ratio of the structures other than retained austenite is measured by the following method. A test piece is sampled from the hot-rolled steel sheet so that the metallographic structure in the region from the surface to 1 / 4 of the sheet thickness (the region from 1 / 8 of the sheet thickness from the surface to 3 / 8 of the sheet thickness from the surface) in the plate thickness cross-section parallel to the rolling direction can be observed. Next, after polishing the plate thickness cross-section, the polished surface is nitrided, and a 30 μm × 30 μm region is observed for the microstructure using an optical microscope and a scanning electron microscope (SEM). The observation regions shall be at least 3 regions. Image analysis is performed on the microstructure photographs obtained by this microstructure observation to obtain the area ratio of bainite. Then, for the same observation positions, after Lepera etching, microstructure observation is performed using an optical microscope and a scanning electron microscope, and image analysis is performed on the obtained microstructure photographs to obtain the area ratio of fresh martensite.

[0044] In the above microstructure observation, each microstructure is identified by the following method. Since fresh martensite is a microstructure with a high dislocation density and having substructures such as blocks and packets inside the grains, it can be distinguished from other metallographic structures according to the electron channeling contrast image using a scanning electron microscope.

[0045] A microstructure that is an aggregate of lath-shaped crystal grains and does not contain Fe-based carbides with a major axis of 20 nm or more inside the microstructure, or a microstructure that contains Fe-based carbides with a major axis of 20 nm or more inside the microstructure and the Fe-based carbides have a single variant, that is, Fe-based carbides extending in the same direction, is regarded as bainite. Here, the Fe-based carbides extending in the same direction refer to those with a difference in the extension direction of the Fe-based carbides within 5°.

[0046] The area ratio of retained austenite is measured by the following method. In this embodiment, the area ratio of retained austenite is measured by X-ray diffraction. First, in the plate thickness cross-section parallel to the rolling direction of the hot-rolled steel sheet, at a depth of 1 / 4 of the plate thickness from the surface (the region from 1 / 8 of the plate thickness from the surface to 3 / 8 of the plate thickness from the surface), using Co-Kα rays, 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, and the volume ratio of retained austenite is calculated using the intensity averaging method. This volume ratio of retained austenite is regarded as the area ratio of retained austenite.

[0047] Pole density of {001}<110>, {111}<110>, and {112}<110> orientation groups in the microstructure of the region from the surface to 1 / 8 of the plate thickness from the surface: 2.0 to 8.0 If the pole density of the {001}<110>, {111}<110>, and {112}<110> orientation groups in the microstructure of the region from the surface to 1 / 8 of the plate thickness from the surface (hereinafter sometimes referred to as the surface layer region) is less than 2.0, the isotropy of ductility and hole expansion property of the hot-rolled steel sheet deteriorate. Therefore, the pole density of the {001}<110>, {111}<110>, and {112}<110> orientation groups in the microstructure of the surface layer region shall be 2.0 or more. Preferably, it is 2.2 or more, 2.5 or more, or 2.7 or more. If the pole density of the {001}<110>, {111}<110>, and {112}<110> orientation groups in the microstructure of the surface layer region exceeds 8.0, the isotropy of ductility and hole expansion property of the hot-rolled steel sheet deteriorate. Therefore, the pole density of the {001}<110>, {111}<110>, and {112}<110> orientation groups in the microstructure of the surface layer region shall be 8.0 or less. Preferably, it is 7.5 or less or 7.0 or less.

[0048] Pole density of the {110}<112> orientation in the microstructure of the region from 1 / 8 of the plate thickness from the surface to 1 / 2 of the plate thickness from the surface: 2.0 to 4.0 When the pole density in the {110}<112> orientation in the texture of the region from 1 / 8 of the plate thickness from the surface to 1 / 2 of the plate thickness from the surface (hereinafter sometimes referred to as the internal region) exceeds 4.0, the ductility isotropy and hole expansion property of the hot-rolled steel sheet deteriorate. Therefore, the pole density in the {110}<112> orientation in the texture of the internal region shall be 4.0 or less. Preferably it is 3.6 or less, 3.2 or less, or 3.0 or less. From the viewpoint of suppressing strength deterioration, the pole density in the {110}<112> orientation in the texture of the internal region shall be 2.0 or more. Preferably it is 2.3 or more or 2.5 or more.

[0049] The pole density is measured using a device combining a scanning electron microscope and an EBSD analyzer and OIM Analysis (registered trademark) manufactured by AMETEK. The pole densities of the {001}<110>, {111}<110>, and {112}<110> orientation groups in the texture of the surface layer region, and the pole density of {110}<112> in the texture of the internal region are obtained from the crystal orientation distribution function (ODF: Orientation Distribution Function) that displays the three-dimensional texture, which is calculated using the orientation data measured by the EBSD (Electron Back Scattering Diffraction) method and the spherical harmonic function.

[0050] Note that the measurement range for the surface layer region is the region from the surface to 1 / 8 of the plate thickness from the surface, and for the internal region is the region from 1 / 8 of the plate thickness from the surface to 1 / 2 of the plate thickness from the surface. The measurement pitch shall be 5 μm / step.

[0051] {hkl} is a crystal plane parallel to the rolling plane. <uvw>represents a crystal direction parallel to the rolling direction. That is, {hkl} <uvw>In the direction of the plate surface normal, {hkl}, and in the rolling direction <uvw>shows the crystal facing the direction.

[0052] In addition, the rolling direction of the hot-rolled steel sheet can be determined by the following method. First, a test piece is taken so that the plate thickness cross-section of the hot-rolled steel sheet can be observed. After finishing the plate thickness cross-section of the taken test piece by mirror polishing, it is observed using an optical microscope. The observation range is the entire thickness of the plate thickness, and a region with a dark brightness is determined as an inclusion. Among the inclusions, in the inclusions whose major axis length is 40 μm or more, the direction parallel to the direction in which the inclusion extends is determined as the rolling direction.

[0053] Mechanical properties The hot-rolled steel sheet according to this embodiment has a tensile (maximum) strength of 980 MPa or more. By setting the tensile strength to 980 MPa or more, it is possible to contribute to the weight reduction of the vehicle body. More preferably, the tensile strength is 1180 MPa or more. The upper limit does not particularly need to be limited, but it may be 1470 MPa, 1300 MPa or less, or 1200 MPa or less. The difference between the total elongation in the C direction and the total elongation in the L direction ((total elongation in the L direction - total elongation in the C direction) / total elongation in the C direction), which is an index of the isotropy of ductility, is preferably ±3.0% or less. The hole expansion rate, which is an index of hole expandability, is preferably 40% or more.

[0054] The tensile strength TS and the total elongation EL are measured in accordance with JIS Z 2241:2011 using a No. 5 test piece of JIS Z 2241:2011. The sampling position of the tensile test piece may be the 1 / 4 part from the end in the plate width direction, and the direction perpendicular to the rolling direction (C direction) may be set as the longitudinal direction. For the total elongation EL, the total elongation in the L direction is measured by also performing a tensile test on a tensile test piece with the direction parallel to the rolling direction (L direction) as the longitudinal direction.

[0055] The hole expansion rate λ is measured in accordance with JIS Z 2256:2010 using a No. 5 test piece of JIS Z 2241:2011. The sampling position of the hole expansion test piece may be the 1 / 4 part from the end in the plate width direction of the hot-rolled steel sheet.

[0056] Plate thickness The plate thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited, but may be 1.2 to 8.0 mm. By setting the plate thickness of the hot-rolled steel sheet to 1.2 mm or more, it becomes easy to secure the rolling completion temperature and reduce the rolling load, and hot rolling can be easily performed. Therefore, the plate thickness of the hot-rolled steel sheet according to the present embodiment may be 1.2 mm or more. Preferably it is 1.4 mm or more. Further, when the plate thickness is set to 8.0 mm or less, it becomes difficult to control the aggregate structure, and it may be difficult to obtain the aggregate structure described above. Therefore, the plate thickness may be 8.0 mm or less. Preferably it is 6.0 mm or less.

[0057] Plating layer The hot-rolled steel sheet according to the present embodiment having the above-described chemical composition and metal structure may be provided with a plating layer on the surface for the purpose of improving corrosion resistance or the like to form a surface-treated steel sheet. The plating layer may be an electroplating layer or a hot-dip plating layer. Examples of the electroplating layer include electro-galvanizing and electro Zn-Ni alloy plating. Examples of the hot-dip plating layer include hot-dip galvanizing, alloyed hot-dip galvanizing, hot-dip aluminum plating, hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating, and hot-dip Zn-Al-Mg-Si alloy plating. The plating adhesion amount is not particularly limited and may be the same as in the prior art. Further, it is also possible to further enhance the corrosion resistance by performing an appropriate chemical conversion treatment (for example, application and drying of a silicate-based chromium-free chemical conversion treatment solution) after plating.

[0058] Next, a suitable manufacturing method of the hot-rolled steel sheet according to the present embodiment will be described. A suitable manufacturing method of the hot-rolled steel sheet according to the present embodiment includes the following steps (a) to (d). In the following description, the temperature refers to the surface temperature of the steel sheet unless otherwise specified.

[0059] (a) A heating step of heating a slab having the above-described chemical composition in a temperature range of 1100°C or higher and lower than 1350°C. (b) A finish rolling process of finish rolling the slab after heating using a rolling mill having a plurality of stands, satisfying the following conditions (I) to (V). (I) The finish rolling start temperature is set to 850°C or higher. (II) In each of the last four stands among the plurality of stands, rolling is performed so that σ represented by the following formula (1) becomes 40 to 80. σ = exp(0.753 + 3000 / T) × ε 0.21 × ε’ 0.13 …(1) Here, T is the temperature (°C) immediately before entering each stand, ε is the equivalent plastic strain, and ε’ is the strain rate. (III) The inter-pass time between each of the last four stands is set to 0.1 to 10.0 seconds. (IV) The cumulative reduction ratio of the last four stands is set to 60% or higher. (V) The finish rolling completion temperature is set to 850 to 1000°C. (c) An air cooling step of air cooling for 2.0 to 4.0 seconds after the finish rolling is completed, and then cooling so that the average cooling rate up to the temperature range of 450 to 550°C is 100°C / s or higher. (d) A winding step of performing winding after cooling. Hereinafter, each step will be described.

[0060] (a) Heating step In the heating step, it is preferable to heat the slab having the above-described chemical composition in a temperature range of 1100°C or higher and less than 1350°C. The method for manufacturing the slab does not particularly need to be limited, and a common method of melting molten steel having the above-described chemical composition in a converter or the like and making it into a slab by a casting method such as continuous casting can be applied. Note that an ingot-making - slab-splitting method may be used.

[0061] In the slab, most of the carbonitride-forming elements such as Ti exist in the slab in a non-uniform distribution as coarse carbonitrides. Coarse precipitates (carbonitrides) existing in a non-uniform distribution deteriorate various properties of the hot-rolled steel sheet (such as tensile strength, ductility, hole expansion property, etc.). Therefore, the slab before hot rolling is heated in a desired temperature range to dissolve the coarse precipitates. In order to sufficiently dissolve these coarse precipitates before hot rolling, it is preferable that the heating temperature of the slab is 1100 °C or higher. However, if the heating temperature of the slab becomes too high, it will cause the generation of surface defects and a decrease in yield due to scale-off. Therefore, it is preferable that the heating temperature of the steel material is less than 1350 °C.

[0062] The slab is heated in a temperature range of 1100 °C or higher and less than 1350 °C and held for a predetermined time. However, if the holding time exceeds 4800 seconds, the amount of scale generation increases. As a result, scale biting and the like are likely to occur in the subsequent finishing rolling process, and the surface quality of the hot-rolled steel sheet may deteriorate. Therefore, the holding time in the temperature range of 1100 °C or higher and less than 1350 °C is preferably 4800 seconds or less.

[0063] Rough rolling process Between the heating process and the finishing rolling process, rough rolling may be performed on the slab. Rough rolling only needs to be able to obtain a desired sheet bar size, and its conditions are not particularly limited.

[0064] (b) Finishing rolling process In the finishing rolling process, the heated slab is finish-rolled using a rolling mill having a plurality of stands. At this time, it is preferable to satisfy the following conditions (I) to (V). Note that it is preferable to perform descaling before finishing rolling or during rolling between the rolling stands of finishing rolling.

[0065] (I) Finishing rolling start temperature: 850 °C or higher The finish rolling start temperature (the inlet temperature of the first pass of finish rolling) is preferably 850°C or higher. If the finish rolling start temperature is less than 850°C, rolling in some of the plurality of rolling stands (especially the first half stands) will be carried out at the two-phase region temperature of ferrite + austenite. As a result, a worked structure may remain after finish rolling, and the strength and ductility of the hot rolled steel sheet may deteriorate. Therefore, the finish rolling start temperature is preferably 850°C or higher. Note that the finish rolling start temperature may be 1100°C or lower in order to suppress the coarsening of austenite.

[0066] (II) In each of the last four stands, σ represented by the following formula (1): 40 to 80 σ = exp(0.753 + 3000 / T)·ε 0.21 ·ε’ 0.13 …(1) Here, T is the temperature (°C) immediately before entering each stand (i.e., the inlet temperature), ε is the equivalent plastic strain, and ε’ is the strain rate. The fact that σ is 40 to 80 in each of the last four stands can be paraphrased as that σ of the fourth stand from the last, σ of the third stand from the last, σ of the second stand from the last, and σ of the final stand are all 40 to 80.

[0067] If there is even one stand where σ is less than 40, the strain required for the development of the aggregated structure in the surface layer region may not be suitably imparted in each of the last four stands. As a result, in the aggregated structure of the region from the surface to a depth of 1 / 8 of the plate thickness from the surface, the pole densities of the {001}<110>, {111}<110>, and {112}<110> orientation groups may not be preferably controlled. Also, the aggregated structure of the internal region may not be preferably controlled. Therefore, σ in each of the last four stands is preferably 40 or higher. In addition, if there is even one stand where σ exceeds 80, the above-mentioned texture will not develop, and dynamic recrystallization may occur, resulting in randomization of the texture. As a result, the ductility isotropy and hole expansion property of the hot-rolled steel sheet may deteriorate. Therefore, it is preferable that σ in each of the last four stands is 80 or less.

[0068] Incidentally, the equivalent plastic strain ε can be obtained by ε = (2 / √3) × (h / H), where h is the thickness of the incoming side plate and H is the thickness of the outgoing side plate. Also, the strain rate ε' can be obtained by ε' = ε / t, where t is the rolling time (s). In addition, the rolling time t refers to the time during which the steel sheet is in contact with the rolling roll and strain is applied to the steel sheet.

[0069] (III) Inter-pass time between each of the last four stands: 0.1 to 10.0 seconds If there is even one inter-pass where the inter-pass time exceeds 10.0 seconds between each of the last four stands, recovery and recrystallization during the inter-pass will proceed. As a result, it becomes difficult to accumulate strain, and it may not be possible to obtain the desired texture in the surface region and the internal region. Therefore, it is preferable that the inter-pass time between each of the last four stands is 10.0 seconds or less. Although it is preferable that the inter-pass time between each of the last four stands is short, there are limitations in terms of the installation space of each stand and the rolling speed when shortening the inter-pass time. Therefore, it is preferably 0.1 seconds or more. Incidentally, the fact that the inter-pass time between each of the last four stands is 0.1 to 10.0 seconds can be paraphrased as meaning that the inter-pass time between the fourth last stand and the third last stand, the inter-pass time between the third last stand and the second last stand, and the inter-pass time between the second last stand and the final stand are all 0.1 to 10.0 seconds.

[0070] (IV) Cumulative reduction ratio of the last four stands: 60% or more If the cumulative reduction ratio of the last four stands is less than 60%, the dislocation density introduced into the unrecrystallized austenite may be small. When the dislocation density introduced into the unrecrystallized austenite becomes small, it becomes difficult to obtain a desired microstructure, and the hole expansion property and ductility isotropy of the hot-rolled steel sheet may deteriorate. Therefore, it is preferable that the cumulative reduction ratio of the last four stands be 60% or more. In addition, if the cumulative reduction ratio of the last four stands exceeds 97%, the shape of the hot-rolled steel sheet may deteriorate. Therefore, the cumulative reduction ratio of the last four stands may be 97% or less.

[0071] In addition, the cumulative reduction ratio of the last four stands can be expressed as {1 - (t1 / t0)} × 100 (%) where t0 is the inlet sheet thickness of the fourth stand from the last and t1 is the outlet sheet thickness of the final stand.

[0072] (V) Finish rolling completion temperature: 850 - 1000 °C If the finish rolling end temperature (the temperature on the outlet side of the final stand) is less than 850 °C, the rolling will be carried out in the two-phase region temperature of ferrite + austenite. As a result, the processed microstructure may remain after rolling, and the strength and ductility isotropy of the hot-rolled steel sheet may deteriorate. Therefore, it is preferable that the finish rolling completion temperature be 850 °C or more. Also, in the slab having the chemical composition according to this embodiment, the unrecrystallized austenite region is generally in the temperature range of 1000 °C or less. Therefore, if the finish rolling completion temperature exceeds 1000 °C, the austenite grains will grow, and the martensite grain length of the hot-rolled steel sheet obtained after cooling will increase. As a result, it becomes difficult to obtain a desired microstructure, and the strength and ductility isotropy of the hot-rolled steel sheet may deteriorate. Therefore, it is preferable that the finish rolling completion temperature be 1000 °C or less.

[0073] (c) Cooling process In the cooling process, it is preferable to air-cool for 2.0 - 4.0 seconds after the finish rolling is completed, and then cool so that the average cooling rate to the temperature range of 550 - 450 °C is 100 °C / s or more.

[0074] Air cooling time: 2.0 to 4.0 seconds After finish rolling, it is preferable to perform air cooling for 2.0 to 4.0 seconds. If the air cooling time is less than 2.0 seconds or more than 4.0 seconds, a desired amount of bainite may not be obtained. Therefore, it is preferable to perform air cooling for 2.0 to 4.0 seconds. In this embodiment, air cooling refers to cooling with an average cooling rate of less than 10°C / s.

[0075] In this embodiment, it is preferable to install a cooling facility at the subsequent stage of the finish rolling facility and perform cooling after the above air cooling while passing the steel plate after finish rolling through this cooling facility. Note that the cooling here does not include the above air cooling. The cooling facility is preferably a facility capable of cooling the steel plate at an average cooling rate of 100°C / s or more. As such a cooling facility, for example, a water cooling facility using water as a cooling medium can be exemplified.

[0076] The average cooling rate in the cooling process is the value obtained by dividing the temperature drop range of the steel plate from the start of cooling to the end of cooling by the required time from the start of cooling to the end of cooling. The start of cooling is defined as the introduction of the steel plate into the cooling facility, and the end of cooling is defined as the discharge of the steel plate from the cooling facility. In addition, among the cooling facilities, there are facilities without an air cooling section in the middle and facilities having one or more air cooling sections in the middle. In this embodiment, any cooling facility may be used. Even when using a cooling facility having an air cooling section, the average cooling rate from the start of cooling to the end of cooling may be 100°C / s or more.

[0077] Average cooling rate from the air cooling end temperature to the temperature range of 450 to 550°C: 100°C / s or more If the average cooling rate from the air cooling end temperature to the temperature range of 450 to 550°C is less than 100°C / s, ferrite is likely to be formed, and a desired amount of bainite may not be obtained. Therefore, the average cooling rate from the air cooling end temperature to the temperature range of 450 to 550°C is preferably 100°C / s or more.

[0078] (d) Coiling process In the coiling process, it is preferable to coil the steel sheet cooled to a temperature range of 450 to 550°C. Since the steel sheet is coiled immediately after cooling, the coiling temperature is approximately equal to the cooling stop temperature. If the coiling temperature is less than 450°C, the desired amount of bainite cannot be obtained, and the hole expansion property and the isotropy of ductility may deteriorate. On the other hand, if the coiling temperature exceeds 550°C, a large amount of ferrite and pearlite may be generated, and the desired strength may not be obtained. Therefore, the coiling temperature is preferably in the temperature range of 450 to 550°C.

[0079] After coiling, it may be air-cooled. After coiling, according to a conventional method, the hot-rolled steel sheet may be subjected to temper rolling, or pickling may be performed to remove the scale formed on the surface. Alternatively, further, plating treatments such as aluminum plating, aluminum-zinc plating, aluminum-silicon plating, hot-dip galvanizing, electro-galvanizing, and alloyed hot-dip galvanizing, or chemical conversion treatment may be performed.

[0080] By the above-described preferred manufacturing method, the hot-rolled steel sheet according to the present embodiment can be stably manufactured.

Examples

[0081] Next, examples of the present invention will be described. The conditions in the examples are one set of conditions adopted for confirming the feasibility and effects of the present invention, and the present invention is not limited to this one set of conditions. The present invention can adopt various conditions as long as it does not deviate from the gist of the present invention and achieves the object of the present invention.

[0082] Molten steel having the chemical compositions shown in Table 1 was melted in a converter, and slabs were obtained by continuous casting. Next, these slabs were heated under the conditions shown in Table 2A and Table 2B, rough rolling was performed, and then finish rolling was performed under the conditions shown in Table 2A and Table 2B. After completion of the finish rolling, cooling was performed under the conditions shown in Table 3A and Table 3B, and coiling was performed to obtain hot-rolled steel sheets having the plate thicknesses shown in Table 3A and Table 3B. In addition, in the heating process, the holding time at the heating temperatures described in Table 2A and Table 2B was set to 4800 seconds or less.

[0083] In addition, the cooling after finish rolling (excluding air cooling) was performed by water cooling. The steel plate was passed through a water cooling facility having no air cooling section in the middle. The average cooling rate in Table 3A and Table 3B is a value obtained by dividing the temperature drop width of the steel plate from the time of introducing the water cooling facility to the time of discharging the water cooling facility by the required passing time of the steel plate with respect to the water cooling facility.

[0084] Test pieces were sampled from the obtained hot-rolled steel plates, and the area ratio of each structure, the pole density of the aggregate structure, the tensile strength, the total elongation in the C direction and the L direction, and the hole expansion ratio were measured by the above-described method. The obtained results are shown in Table 4A and Table 4B.

[0085] When the obtained tensile strength was 980 MPa or more, it was determined to be qualified as having high strength. On the other hand, when the obtained tensile strength was less than 980 MPa, it was determined to be unqualified as not having high strength.

[0086] When the difference between the total elongation in the C direction and the total elongation in the L direction obtained was ±3.0% or less, it was determined to be qualified as having excellent ductility isotropy. On the other hand, when the difference between the total elongation in the C direction and the total elongation in the L direction was more than ±3.0%, it was determined to be unqualified as not having excellent ductility isotropy.

[0087] When the obtained hole expansion ratio was 40% or more, it was determined to be qualified as having excellent hole expandability. On the other hand, when the hole expansion ratio was less than 40%, it was determined to be unqualified as not having excellent hole expandability.

[0088]

Table 1

[0089]

Table 2A

[0090]

Table 2B

[0091]

Table 3A

[0092]

Table 3B

[0093]

Table 4A

[0094]

Table 4B

[0095] As can be seen from Table 4A and Table 4B, in the examples of the present invention, a hot-rolled steel sheet having high strength and excellent ductility isotropy and hole expansion property was obtained. On the other hand, in the comparative examples where the chemical composition and / or the metal structure were not within the ranges defined in the present invention, one or more of the above characteristics were inferior.

Industrial Applicability

[0096] According to the above aspect of the present invention, it is possible to provide a hot-rolled steel sheet having high strength and excellent ductility isotropy and hole expansion property.< / uvw> < / uvw> < / uvw>

Claims

1. The chemical composition is in mass %, C: 0.100 to 0.350%, Si: 0.010 to 3.00%, Mn: 1.00 to 4.00%, sol. Al: 0.001 to 2.000%, Si + sol. Al: 1.00% or more, Ti: 0.010 to 0.380%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, Nb: 0 to 0.100%, V: 0 to 0.500%, Cu: 0 to 2.00%, Cr: 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 0.020%, One or more of Zr, Co, Zn, and W: in total 0 to 1.00%, and Sn: 0 to 0.050% is contained, the balance consists of Fe and impurities, in the region from 1 / 8 of the plate thickness from the surface to 3 / 8 of the plate thickness from the surface, the metallographic structure is in area %, retained austenite: 10 to 20%, fresh martensite: 10% or less, and bainite: consisting of 70 to 90%, in the aggregate structure in the region from the surface to 1 / 8 of the plate thickness from the surface, the pole densities of the {001}<110>, {111}<110>, and {112}<110> orientation groups are 2.0 to 8.0, in the aggregate structure in the region from 1 / 8 of the plate thickness from the surface to 1 / 2 of the plate thickness from the surface, the pole density of the {110}<112> orientation is 2.0 to 4.0, A hot-rolled steel sheet characterized in that the tensile strength is 980 MPa or more.

2. The chemical composition is in mass %, Nb: 0.005 to 0.100%, V: 0.005 to 0.500%, Cu: 0.01 to 2.00%, Cr: 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%, and Bi: 0.0005 to 0.020% The hot-rolled steel sheet according to claim 1, characterized by containing one or more selected from the group consisting of

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