Hot-rolled steel sheet

Abstract

This hot-rolled steel sheet has a desired chemical composition, and the metallographic structure at a position 1 / 4 from the surface in the sheet thickness direction has, in area%, less than 3.0% of retained austenite, 15.0% or more but less than 60.0% of ferrite, less than 5.0% of pearlite, an E value of 10.7 or more, an I value of 1.020 or more, a CS value of-8.0 * 105 to 8.0 * 105, and a standard deviation of Mn concentration of 0.60 mass% or less. The area ratio of a region in which the Ni concentration is 0.2 mass% or more is 10.0% or more, the area ratio of a region in which the O concentration is 3.0 mass% or more is 3.0-50.0%, the maximum value of the equivalent sphere diameter of the oxide is 5.00 [mu] m or less, and the tensile strength of the hot-rolled steel sheet is 980 MPa or more.

Description

Technical Field

[0001] The present invention relates to hot rolled steel sheets.

[0002] This application claims priority based on Japanese Patent Application No. 2022-163955 filed in Japan on October 12, 2022, the contents of which are incorporated herein by reference. Background Art

[0003] In recent years, from the perspective of protecting the global environment, many fields are working to reduce carbon dioxide emissions. Automobile manufacturers are also actively developing technologies to reduce vehicle weight for the purpose of reducing fuel consumption. However, in order to ensure the safety of passengers, the focus is also placed on improving collision resistance, so reducing vehicle weight is not easy.

[0004] In order to achieve both lightweight and crash resistance, thin-walled parts using high-strength steel sheets are being studied. Therefore, there is a strong demand for steel sheets that have both high strength and excellent formability, and several technologies have been proposed to meet these requirements.

[0005] Since automobile parts have various processing styles, the required formability varies depending on the parts used. Among them, the limit fracture thickness reduction rate and ductility are positioned as important indicators of formability. The limit fracture thickness reduction rate is a value obtained based on the minimum value of the plate thickness of the tensile test piece before fracture and the plate thickness of the tensile test piece after fracture. When the limit fracture thickness reduction rate is low, it is easy to break early when the tensile strain in press forming is applied, so it is not preferred.

[0006] Automobile parts are formed by press forming, but the blank sheets for press forming are often manufactured by shearing with high productivity. In the blank sheets manufactured by shearing, it is necessary to have excellent end surface accuracy after shearing.

[0007] For example, if the shape of the end surface after shearing (sheared end surface) generates a secondary sheared surface of sheared surface-fracture surface-sheared surface, the accuracy of the sheared end surface will be significantly deteriorated.

[0008] In addition, it is known that the higher the strength of the steel plate, the easier it is to generate cracks from the inside of the bend during the bending process (hereinafter, the cracks generated on the inside of the bend during the bending process are referred to as in-bend cracks). The mechanism of in-bend cracks is inferred as follows. Compressive stress is generated on the inside of the bend during the bending process. Initially, the whole inside of the bend is processed while being uniformly deformed, but if the processing amount increases, the deformation cannot be borne by only uniform deformation, and the deformation continues to progress due to the local concentration of strain (a shear deformation band occurs). The shear deformation band further grows and cracks are generated and grow along the shear band from the inside surface of the bend. The reason why in-bend cracks are easily generated as the strength of the steel plate increases is presumed to be that the work hardening ability decreases as the strength increases, making it difficult to perform uniform deformation, and deformation deviation is easily generated, thereby generating a shear deformation band in the early stage of processing (or under mild processing conditions). As described above, automobile parts have various processing styles, and there are many cases where steel plates are subjected to bending processing, so it is not preferred to generate in-bend cracks.

[0009] In addition, a chemical conversion film is sometimes formed on the surface of a steel sheet for the purpose of improving corrosion resistance, etc. If the Si content in the steel sheet is increased for high strength, oxides containing Si are easily generated and remain on the surface of the steel sheet, thereby deteriorating the chemical conversion treatability of the steel sheet and sometimes failing to fully form a chemical conversion film.

[0010] For example, Patent Document 1 discloses a hot-rolled steel sheet which is a raw material for a cold-rolled steel sheet and has excellent surface properties after press working, wherein the Mn segregation and the P segregation in the center of the sheet thickness are controlled.

[0011] Prior art literature

[0012] Patent Literature

[0013] Patent Document 1: International Publication No. 2020 / 044445

[0014] Non-patent literature

[0015] Non-patent document 1: J. Webel, J. Gola, D. Britz, F. Mucklich, Materials Characterization 144 (2018) 584-596

[0016] Non-patent document 2: DL Naik, HU Sajid, R. Kiran, Metals 2019, 9, 546

[0017] Non-patent document 3: K. Zuiderveld, Contrast Limited Adaptive Histogram Equalization, Chapter VIII.5, Graphics Gems IV. PS Heckbert (Eds.), Cambridge, MA, Academic Press, 1994, pp. 474-485 Summary of the invention

[0018] Problems to be solved by the invention

[0019] However, Patent Document 1 does not consider the critical fracture thickness reduction rate, bending cracking, and chemical conversion treatability of the hot-rolled steel sheet.

[0020] The present invention has been completed in view of the above-mentioned actual situation, and its purpose is to provide a hot-rolled steel plate, which has high strength and ultimate fracture thickness reduction rate, and has excellent ductility, shear workability and chemical conversion treatment property, and suppresses the occurrence of bending cracking, that is, it has excellent resistance to bending cracking.

[0021] Means for solving problems

[0022] The gist of the present invention is as follows.

[0023] (1) The chemical composition of the hot-rolled steel sheet according to one embodiment of the present invention contains, in terms of mass %, the following:

[0024] C: 0.050~0.250%,

[0025] Si: 0.05-3.00%,

[0026] Mn: 1.00~4.00%,

[0027] Ni: 0.02-2.00%,

[0028] Sol.Al: 0.001~2.000%,

[0029] P: 0.100% or less,

[0030] S: 0.0300% or less,

[0031] N: 0.1000% or less,

[0032] O: 0.0100% or less,

[0033] Ti: 0~0.500%,

[0034] Nb: 0~0.500%,

[0035] V: 0~0.500%、

[0036] Cu: 0-2.00%,

[0037] Cr: 0~2.00%,

[0038] Mo: 0-1.00%,

[0039] B: 0~0.0100%,

[0040] Ca: 0~0.0200%,

[0041] Mg: 0~0.0200%,

[0042] REM: 0~0.1000%、

[0043] Bi: 0~0.0200%,

[0044] As: 0~0.100%,

[0045] Zr: 0-1.00%,

[0046] Co: 0-1.00%,

[0047] Zn: 0-1.00%,

[0048] W: 0-1.00%, and

[0049] Sn: 0~0.05%,

[0050] The remaining part contains Fe and impurities.

[0051] And satisfy the following formulas (A) and (B),

[0052] The metal structure at a position 1 / 4 from the surface in the plate thickness direction is calculated in area %:

[0053] Retained austenite is less than 3.0%,

[0054] Ferrite is 15.0% or more and less than 60.0%,

[0055] Pearlite is less than 5.0%,

[0056] The entropy value (entropy value) expressed by the following formula (1) obtained by analyzing the SEM image of the metal structure using the gray level co-occurrence matrix method is 10.7 or more,

[0057] The inverse difference normalized value represented by the following formula (2) is 1.020 or more,

[0058] The Cluster Shade value expressed by the following formula (3) is -8.0×10 5 ~8.0×10 5 , the standard deviation of the Mn concentration is 0.60 mass % or less,

[0059] In the surface,

[0060] The area ratio of the region where the Ni concentration is 0.2 mass % or more is 10.0% or more,

[0061] The area ratio of the region where the O concentration is 3.0 mass % or more is 3.0 to 50.0%,

[0062] The maximum value of the equivalent spherical diameter of the oxide is less than 5.00 μm.

[0063] The hot-rolled steel sheet has a tensile strength of 980 MPa or more.

[0064] 0.060%≤Ti+Nb+V≤0.500%(A)

[0065] Zr+Co+Zn+W≤1.00%(B)

[0066] In the above formulae (A) and (B), each element symbol represents the content of the element in mass %, and 0% is substituted when the element is not contained.

[0067] Wherein, P(i, j) in the following equations (1) to (5) is a gray level co-occurrence matrix, L in the following equation (2) is the number of levels of the gray scale that the SEM image can take, i and j in the following equations (2) and (3) are natural numbers from 1 to L, and μ in the following equation (3) is x and μ y They are represented by the following formulae (4) and (5), respectively.

[0068] [Mathematical formula 1]

[0069] Entropy=-Σ i Σ j P(i,j)log(P(i,j))…(1)

[0070] [Mathematical formula 2]

[0071]

[0072] [Mathematical formula 3]

[0073] Cluster Shade=Σ i Σ j (i+j-μ x -μ y ) 3P(i, j)…(3)

[0074] [Formula 4]

[0075] μ x =Σ i Σ j i(P(i,j))…(4)

[0076] [Formula 5]

[0077] μ y =∑ i Σ j (P(i, j))…(5)

[0078] (2) The hot-rolled steel sheet described in (1) above, wherein the chemical composition may contain, in terms of mass %, one or more selected from the group consisting of the following elements:

[0079] Ti: 0.001~0.500%,

[0080] Nb: 0.001~0.500%,

[0081] V: 0.001~0.500%,

[0082] Cu: 0.01-2.00%,

[0083] Cr: 0.01~2.00%,

[0084] Mo: 0.01~1.00%,

[0085] B: 0.0001~0.0100%,

[0086] Ca: 0.0005~0.0200%,

[0087] Mg: 0.0005~0.0200%,

[0088] REM: 0.0005~0.1000%、

[0089] Bi: 0.0005~0.0200%,

[0090] As: 0.001~0.100%,

[0091] Zr: 0.01~1.00%,

[0092] Co: 0.01~1.00%,

[0093] Zn: 0.01~1.00%,

[0094] W: 0.01 to 1.00%, and

[0095] Sn: 0.01~0.05%.

[0096] Effects of the Invention

[0097] According to the above aspects of the present invention, a hot-rolled steel sheet having high strength and ultimate fracture thickness reduction rate and excellent ductility, shear workability, chemical conversion treatability and bending internal cracking resistance can be obtained.

[0098] The hot-rolled steel sheet according to the above aspect of the present invention is suitable as an industrial raw material used in automobile parts, mechanical structural parts, and building parts. BRIEF DESCRIPTION OF THE DRAWINGS

[0099] Figure 1 This is an example of a sheared end surface of a hot-rolled steel sheet according to an example of the present invention.

[0100] Figure 2 This is an example of a sheared end surface of a hot-rolled steel sheet of a comparative example. DETAILED DESCRIPTION

[0101] The chemical composition and metallographic structure of the hot-rolled steel sheet according to the present embodiment will be described in more detail below. However, the present invention is not limited to the configuration disclosed in the present embodiment, and various modifications can be made within the scope of the present invention.

[0102] For the numerical ranges described below with "-", the lower limit and upper limit are included in the range. For the numerical values ​​expressed as "lower than" or "exceeding", the value is not included in the numerical range. In the following description, % of the chemical composition is mass % unless otherwise specified.

[0103] Chemical composition

[0104] The chemical composition of the hot-rolled steel sheet according to the present embodiment includes, by mass%, C: 0.050-0.250%, Si: 0.05-3.00%, Mn: 1.00-4.00%, Ni: 0.02-2.00%, sol.Al: 0.001-2.000%, P: less than 0.100%, S: less than 0.0300%, N: less than 0.1000%, O: less than 0.0100%, and the remainder: Fe and impurities, and satisfies formula (A) (0.060%≤Ti+Nb+V≤0.500%).

[0105] Each element is described in detail below.

[0106] C: 0.050~0.250%

[0107] C increases the area ratio of the hard phase and combines with precipitation strengthening elements such as Ti, Nb, and V to increase the strength of ferrite. When the C content is less than 0.050%, the desired strength cannot be obtained. Therefore, the C content is set to 0.050% or more. The C content is preferably 0.060% or more, more preferably 0.070% or more, and further preferably 0.080% or more or 0.090% or more.

[0108] On the other hand, when the C content exceeds 0.250%, the area ratio of ferrite decreases, thereby reducing the ductility of the hot-rolled steel sheet. Therefore, the C content is set to 0.250% or less. The C content is preferably 0.200% or less, 0.150% or less, or 0.120% or less.

[0109] Si: 0.05~3.00%

[0110] Si has the effect of promoting the formation of ferrite to improve the ductility of the hot-rolled steel sheet and the effect of solid solution strengthening the ferrite to improve the strength of the hot-rolled steel sheet. In addition, Si has the effect of making the steel sound by deoxidation (suppressing the generation of defects such as pores in the steel). If the Si content is less than 0.05%, the effect brought by the above-mentioned effect cannot be obtained. Therefore, the Si content is set to 0.05% or more. The Si content is preferably 0.50% or more, more preferably 0.80% or more, 1.00% or more, 1.20% or more or 1.40% or more.

[0111] However, when the Si content exceeds 3.00%, the surface properties and chemical conversion treatability of the hot-rolled steel sheet, as well as the ductility and weldability, are significantly deteriorated, and A 3 The transformation point rises significantly. As a result, it is difficult to perform hot rolling stably. In addition, ferrite is easily generated excessively, the strength of the hot-rolled steel sheet is reduced, and austenite is easily retained after cooling, and the limit fracture thickness reduction rate is reduced. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.70% or less, and more preferably 2.50% or less, 2.20% or less, 2.00% or less, or 1.80% or less.

[0112] Mn: 1.00~4.00%

[0113] Mn has the effect of suppressing ferrite transformation and improving the strength of hot-rolled steel sheets. When the Mn content is less than 1.00%, the desired strength cannot be obtained. Therefore, the Mn content is set to 1.00% or more. The Mn content is preferably 1.30% or more, and more preferably 1.50% or more or 1.80% or more.

[0114] On the other hand, when the Mn content exceeds 4.00%, the morphology of the hard phase becomes periodic bands due to the segregation of Mn, and it is difficult to obtain the desired shear workability. Therefore, the Mn content is set to 4.00% or less. The Mn content is preferably 3.70% or less or 3.50% or less, and more preferably 3.20% or less, 3.00% or less, or 2.60% or less.

[0115] Ni: 0.02~2.00%

[0116] Ni has the effect of improving the hardenability of hot-rolled steel sheets. In addition, Ni concentrates on the surface of the steel sheet as the oxide scale grows during rough rolling, thereby becoming the precipitation nucleus of the chemical conversion treatment film, promoting the formation of a chemical conversion treatment film with no exposed bottom and good adhesion. Thus, it has the effect of improving the chemical conversion treatment property of the hot-rolled steel sheet. When the Ni content is less than 0.02%, the chemical conversion treatment property of the hot-rolled steel sheet deteriorates. Therefore, the Ni content is set to be greater than 0.02%. The Ni content is preferably greater than 0.04%, more preferably greater than 0.06%, and further preferably greater than 0.08%.

[0117] On the other hand, even if the Ni content exceeds 2.00%, the above effect is saturated and the alloy cost increases, which is not preferred. Therefore, the Ni content is set to 2.00% or less. The Ni content is preferably 1.80% or less, and more preferably 1.60% or less.

[0118] Ti: 0~0.500%

[0119] Nb: 0~0.500%

[0120] V: 0~0.500%

[0121] 0.060%≤Ti+Nb+V≤0.500%(A)

[0122] In the above formula (A), each element symbol represents the content of the element in mass %, and when the element is not contained, 0% is substituted.

[0123] Ti, Nb and V are elements that precipitate finely in steel as carbides and nitrides and improve the strength of steel through precipitation strengthening. If the total content of Ti, Nb and V is less than 0.060%, these effects cannot be obtained. Therefore, the total content of Ti, Nb and V is made 0.060% or more. That is, the value of the middle side of the above formula (A) is made 0.060% or more. It should be noted that it is not necessary to contain all of Ti, Nb and V, as long as any one of them is contained and the total content is 0.060% or more. Therefore, the lower limits of the contents of Ti, Nb and V are 0% respectively. The lower limits of the contents of Ti, Nb and V can be 0.001%, 0.010%, 0.030% or 0.050% respectively. The total content of Ti, Nb and V is preferably 0.080% or more, and more preferably 0.100% or more.

[0124] On the other hand, when the content of any one of Ti, Nb and V exceeds 0.500%, or when the total content of Ti, Nb and V exceeds 0.500%, the workability of the hot-rolled steel sheet deteriorates. Therefore, the content of each of Ti, Nb and V is set to 0.500% or less, and the total content of Ti, Nb and V is set to 0.500% or less. That is, the value of the middle side of the above formula (A) is set to 0.500% or less. The content of each of Ti, Nb and V is preferably 0.400% or less or 0.300% or less, more preferably 0.250% or less, and further preferably 0.200% or less or 0.100% or less.

[0125] Sol.Al: 0.001~2.000%

[0126] Al, like Si, has the function of deoxidizing steel to make it sound, and has the function of promoting the formation of ferrite and improving the ductility of hot-rolled steel sheets. If the sol.Al content is less than 0.001%, the effects of the above-mentioned functions cannot be obtained. Therefore, the sol.Al content is set to 0.001% or more. The sol.Al content is preferably 0.010% or more, 0.030% or more, or 0.050% or more, and more preferably 0.080% or more, 0.100% or more, or 0.150% or more.

[0127] On the other hand, when the sol.Al content exceeds 2.000%, the above effect is saturated and it is not economically preferred. Therefore, the sol.Al content is set to 2.000% or less. The sol.Al content is preferably 1.700% or less or 1.500% or less, more preferably 1.300% or less, and even more preferably 1.000% or less.

[0128] In addition, sol.Al refers to acid-soluble Al, and means solid-solution Al present in the steel in a solid-solution state.

[0129] P: 0.100% or less

[0130] P is also an element that has the effect of improving the strength of the hot-rolled steel sheet by solid solution strengthening. The P content can be 0%, or P can be actively contained. However, P is an element that is easily segregated. When the P content exceeds 0.100%, the ductility of the hot-rolled steel sheet and the reduction rate of the ultimate fracture thickness due to grain boundary segregation become significantly reduced. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.050% or less, 0.030% or less, 0.020% or less, or 0.015% or less.

[0131] From the viewpoint of refining cost, the P content may be 0.001% or more, 0.003% or more, or 0.005% or more.

[0132] S: 0.0300% or less

[0133] S forms sulfide inclusions in steel and reduces the ductility and ultimate fracture thickness reduction rate of the hot rolled steel sheet. When the S content exceeds 0.0300%, the ductility and ultimate fracture thickness reduction rate of the hot rolled steel sheet are significantly reduced. Therefore, the S content is set to 0.0300% or less. The S content is preferably 0.0100% or less, 0.0070% or less, or 0.0050% or less.

[0134] The S content may be 0%, but from the viewpoint of refining cost, it may be 0.0001% or more, 0.0005% or more, 0.0010% or more, or 0.0020% or more.

[0135] N: 0.1000% or less

[0136] N has the effect of reducing the ductility and the ultimate fracture thickness reduction rate of the hot-rolled steel sheet. When the N content exceeds 0.1000%, the ductility and the ultimate fracture thickness reduction rate of the hot-rolled steel sheet are 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 or 0.0300% or less, and further preferably 0.0150% or less or 0.0100% or less.

[0137] The N content may be 0%, but when one or more of Ti, Nb and V is contained to further refine the metal structure, in order to promote the precipitation of carbonitrides, the N content is preferably 0.0010% or more, more preferably 0.0015% or more or 0.0020% or more.

[0138] O: 0.0100% or less

[0139] If O is contained in a large amount in steel, it forms coarse oxides that become fracture starting points, causing brittle fracture and hydrogen cracking. Therefore, the O content is set to 0.0100% or less. The O content is preferably 0.0080% or less, and more preferably 0.0050% or less or 0.0030% or less.

[0140] The O content may be 0%, but in order to disperse a large amount of fine oxides during deoxidation of molten steel, it may be 0.0005% or more or 0.0010% or more.

[0141] The remainder of the chemical composition of the hot-rolled steel sheet of the present embodiment may be Fe and impurities. In the present embodiment, impurities refer to substances mixed from ore, waste materials or manufacturing environment as raw materials, and / or substances allowed within the range that do not adversely affect the hot-rolled steel sheet of the present embodiment.

[0142] The hot-rolled steel sheet of the present embodiment may contain the following elements as optional elements in place of a portion of Fe. When these optional elements are not contained, the lower limit of the content is 0%. The optional elements will be described in detail below.

[0143] Cu: 0.01~2.00%

[0144] Cr: 0.01~2.00%

[0145] Mo: 0.01~1.00%

[0146] B: 0.0001~0.0100%

[0147] Cu, Cr, Mo and B all have the effect of improving the hardenability of the hot-rolled steel sheet. In addition, Cu and Mo have the effect of precipitating as carbides in steel to improve the strength of the hot-rolled steel sheet. In addition, Ni has the effect of effectively suppressing the grain boundary cracking of the slab caused by Cu when Cu is contained. Therefore, one or more of these elements may also be contained.

[0148] As mentioned above, Cu has the effect of improving the hardenability of hot-rolled steel sheets and the effect of precipitating as carbides in steel at low temperatures to improve the strength of hot-rolled steel sheets. In order to more reliably obtain the effects brought about by the above-mentioned effects, it is preferred that the Cu content be set to 0.01% or more, and more preferably to 0.05% or more. However, if the Cu content exceeds 2.00%, 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, 0.70% or less, or 0.50% or less.

[0149] As mentioned above, Cr has the effect of improving the hardenability of hot-rolled steel sheets. In order to more reliably obtain the effect brought about by the above-mentioned effect, it is preferred 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 treatability of the hot-rolled steel sheet is significantly reduced. Therefore, the Cr content is set to 2.00% or less. The Cr content is preferably 1.50% or less, and more preferably 1.00% or less, 0.70% or less, or 0.50% or less.

[0150] As mentioned above, Mo has the effect of improving the hardenability of hot-rolled steel sheets and the effect of improving the strength of hot-rolled steel sheets by precipitating as carbides in steel. In order to more reliably obtain the effects brought about by the above effects, it is preferred that the Mo content be set to 0.01% or more, and more preferably to 0.02% or more. However, even if the Mo content is set to more than 1.00%, the effects brought about by the above effects are saturated, which is not economically preferred. Therefore, the Mo content is set to 1.00% or less. The Mo content is preferably 0.50% or less, and more preferably 0.20% or less or 0.10% or less.

[0151] As mentioned above, B has the effect of improving the hardenability of hot-rolled steel sheets. In order to more reliably obtain the effect brought about by this effect, it is preferred that the B content is 0.0001% or more, and more preferably 0.0002% or more. However, when the B content exceeds 0.0100%, the formability of the hot-rolled steel sheet is significantly reduced, so the B content is set to 0.0100% or less. The B content is preferably 0.0050% or less or 0.0025% or less.

[0152] Ca: 0.0005~0.0200%

[0153] Mg: 0.0005~0.0200%

[0154] REM: 0.0005~0.1000%

[0155] Bi: 0.0005~0.0200%

[0156] Ca, Mg and REM all have the effect of improving the ductility of the hot-rolled steel sheet by adjusting the shape of the inclusions in the steel to a preferred shape. In addition, Bi has the effect of improving the ductility of the hot-rolled steel sheet by refining the solidification structure. Therefore, one or more of these elements may also be contained. In order to more reliably obtain the effects brought about by the above-mentioned effects, it is preferred to set the content of any one or more of Ca, Mg, REM and Bi to 0.0005% or more. However, when the Ca content or the Mg content exceeds 0.0200%, or when the REM content exceeds 0.1000%, sometimes excessive inclusions are generated in the steel, which reduces the ductility of the hot-rolled steel sheet. In addition, even if the Bi content is set to more than 0.0200%, the effect brought about by the above-mentioned effects is saturated, which is not economically preferred. Therefore, the Ca content and the Mg content are set to less than 0.0200%, the REM content is set to less than 0.1000%, and the Bi content is set to less than 0.0200%. The Ca content, the Mg content, and the Bi content are preferably 0.0100% or less, more preferably 0.0070% or less or 0.0040% or less. The REM content is preferably 0.0070% or less or 0.0040% or less.

[0157] Here, REM refers to a total of 17 elements including Sc, Y and lanthanoid elements, and the above REM content refers to the total content of these elements. In the case of lanthanoid elements, they are added in the form of mixed rare earth metals in industry.

[0158] As: 0.001~0.100%

[0159] As helps to improve the ductility of hot-rolled steel sheets by reducing the austenite single-phase transformation temperature and refining the original austenite grains. In order to reliably obtain this effect, it is preferred that the As content be set to 0.001% or more. On the other hand, even if a large amount of As is contained, the above effect is saturated, so the As content is set to 0.100% or less.

[0160] Zr: 0.01~1.00%

[0161] Co: 0.01~1.00%

[0162] Zn: 0.01~1.00%

[0163] W: 0.01~1.00%

[0164] Zr+Co+Zn+W≤1.00%(B)

[0165] Sn: 0.01~0.05%

[0166] The symbol of each element in the above formula (B) represents the content of the element in mass %, and 0% is used when the element is not contained.

[0167] Regarding Zr, Co, Zn and W, the present inventors have confirmed that even if these elements are contained in a total of less than 1.00%, the effect of the hot-rolled steel sheet involved in this embodiment is not impaired. Therefore, one or more of Zr, Co, Zn and W may be contained in a total of less than 1.00%. That is, the value on the left side of the above formula (B) may be set to less than 1.00%, or may be set to less than 0.50%, less than 0.10% or less than 0.05%. The contents of Zr, Co, Zn, W and Sn may be less than 0.50%, less than 0.10% or less than 0.05%, respectively. Since Zr, Co, Zn and W may not be contained, the contents of each may be 0%. In order to improve the strength by solid solution strengthening of the steel sheet, the contents of Zr, Co, Zn and W may be more than 0.01%, respectively.

[0168] In addition, the present inventors have also confirmed that even if a small amount of Sn is contained, the effect of the hot-rolled steel sheet involved in the present embodiment is not impaired. However, if a large amount of Sn is contained, defects sometimes occur during hot rolling, so the Sn content is 0.05% or less. Since Sn may not be contained, the Sn content may be 0%. In order to improve the corrosion resistance of the hot-rolled steel sheet, the Sn content may be set to 0.01% or more.

[0169] The chemical composition of the hot-rolled steel sheet can be measured by general analytical methods. For example, ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) can be used for measurement. Furthermore, sol.Al can be measured by ICP-AES using the filtrate after the sample is heated and decomposed with acid. C and S can be measured using the combustion-infrared absorption method, N can be measured using the inert gas melting-thermal conductivity method, and O can be measured using the inert gas melting-non-dispersive infrared absorption method.

[0170] When the hot-rolled steel sheet has a coating on its surface, the chemical composition can be analyzed after the coating is removed by mechanical grinding or the like as needed.

[0171] Metal structure of hot rolled steel plate

[0172] Next, the metal structure of the hot-rolled steel sheet according to the present embodiment will be described.

[0173] In the hot-rolled steel sheet of the present embodiment, the metal structure at a position 1 / 4 from the surface in the plate thickness direction is, in terms of area %, retained austenite less than 3.0%, ferrite 15.0% or more and less than 60.0%, and pearlite less than 5.0%, and the entropy value represented by the following formula (1) obtained by analyzing the SEM image of the above metal structure using the gray level co-occurrence matrix method is 10.7 or more, the inverse difference normalized value represented by the following formula (2) is 1.020 or more, and the cluster shade value represented by the following formula (3) is -8.0×10 5 ~8.0×10 5 The standard deviation of Mn concentration is less than 0.60 mass %, the area ratio of the region with Ni concentration of more than 0.2 mass % on the surface is more than 10.0%, the area ratio of the region with O concentration of more than 3.0 mass % is 3.0-50.0%, and the maximum value of the equivalent spherical diameter of the oxide is less than 5.00 μm.

[0174] In addition, in this embodiment, the structure fraction, entropy value, inverse difference normalized value, cluster shade value and standard deviation of Mn concentration of the metal structure in the region at the 1 / 4 position from the surface in the plate thickness direction are specified. The reason for this is that the metal structure at this position represents the representative metal structure of the steel plate.

[0175] In addition, the "surface" mentioned here refers to the interface between the coating and the steel plate when the hot-rolled steel plate has a coating, and the "position at a depth of 1 / 4 from the surface" refers to a position at a depth of 1 / 4 of the plate thickness in the plate thickness direction from the surface of the hot-rolled steel plate.

[0176] Area ratio of retained austenite: less than 3.0%

[0177] Retained austenite is a metal structure that exists in a face-centered cubic lattice even at room temperature. Retained austenite has the effect of improving the ductility of hot-rolled steel sheets through transformation induced plasticity (TRIP). On the other hand, retained austenite transforms into high-carbon martensite during shearing, and thus becomes the starting point for crack generation during deformation, and becomes the reason for the reduction in the limit fracture thickness reduction rate. When the area ratio of retained austenite is 3.0% or more, the above-mentioned effect becomes apparent, and the limit fracture thickness reduction rate of the hot-rolled steel sheet decreases. Therefore, the area ratio of retained austenite is set to less than 3.0%. The area ratio of retained austenite is preferably less than 1.5%, and more preferably less than 1.0%.

[0178] The less the retained austenite, the better. Therefore, the area ratio of retained austenite may be 0%.

[0179] The area ratio of retained austenite can be measured by methods such as X-ray diffraction, EBSP (Electron Back Scattering Diffraction Pattern) analysis, and magnetic measurement. In the present embodiment, the area ratio of retained austenite is measured by X-ray diffraction.

[0180] In the determination of the area ratio of retained austenite based on X-ray diffraction of the present embodiment, first, in the cross section at a position 1 / 4 from the surface in the thickness direction of the hot-rolled steel sheet, a sample is collected in a manner that a metal structure in an area of ​​1 mm or more at any position in the rolling direction and 1 mm or more centered in the direction orthogonal to the rolling direction and the thickness direction can be observed, and the sample is thickened by mechanical grinding and chemical grinding in a manner that a position 1 / 4 from the surface in the thickness direction becomes the surface. For the above-mentioned sample, an X-ray diffraction device (e.g., Rigaku RINT-2500, Co-Kα ray) is used to obtain the integrated intensity of 6 peaks, α(110), α(200), α(211), γ(111), γ(200), and γ(220). Next, the volume ratio of retained austenite is calculated from the above-mentioned integrated intensity using the intensity average method. The volume ratio of the retained austenite obtained is regarded as the area ratio of the retained austenite.

[0181] Area ratio of ferrite: 15.0% or more and less than 60.0%

[0182] Ferrite is a structure formed when fcc transforms to bcc at a relatively high temperature. Ferrite has a high work hardening rate, so it has the effect of improving the strength-ductility balance of hot-rolled steel sheets. In order to obtain the above effect, the area ratio of ferrite is set to 15.0% or more. It is preferably 20.0% or more, more preferably 25.0% or more, and further preferably 30.0% or more.

[0183] On the other hand, ferrite has low strength, so if the area ratio is excessive, desired strength cannot be obtained. Therefore, the ferrite area ratio is set to less than 60.0%. Therefore, it is preferably 50.0% or less, and more preferably 45.0% or less.

[0184] Area ratio of pearlite: less than 5.0%

[0185] Pearlite is a lamellar metal structure in which cementite is precipitated in layers between ferrites. It is a soft metal structure compared with bainite and martensite. If the area ratio of pearlite is 5.0% or more, carbon is consumed by cementite contained in pearlite, and the strength of martensite and bainite as the remaining structure decreases, and the desired strength cannot be obtained. Therefore, the area ratio of pearlite is set to less than 5.0%. The area ratio of pearlite is preferably 3.0% or less.

[0186] In order to improve the stretch flangeability of the hot-rolled steel sheet, the area ratio of pearlite is preferably reduced as much as possible, and the area ratio of pearlite is more preferably 0%.

[0187] The hot-rolled steel sheet of the present embodiment includes, as the remaining structure other than retained austenite, ferrite and pearlite, hard structures including one or more of bainite, martensite and tempered martensite in a total area ratio exceeding 32.0% and not more than 85.0%.

[0188] The area ratio of ferrite and pearlite is determined by the following method. First, a sample is collected in the center of the direction orthogonal to the rolling direction and the plate thickness direction in such a way that the metal structure in the area 1 / 4 of the position from the surface in the plate thickness direction can be observed in the plate thickness section parallel to the rolling direction. The sample is set to a size of about 10 mm that can be observed in the rolling direction. Then, after the above-mentioned sample section is ground and finished into a mirror surface, it is ground for 8 minutes at room temperature using colloidal silica with a particle size of 0.25 μm that does not contain an alkaline solution to remove the strain introduced into the surface of the sample. For the area of ​​200 μm or more at any position in the rolling direction of the above-mentioned sample section and 200 μm or more centered on the 1 / 4 position from the surface in the plate thickness direction, the measurement interval of 0.1 μm in the rolling direction and the plate thickness direction is measured by electron backscatter diffraction to obtain crystal orientation information. In the above measurement, an EBSD analysis device consisting of a thermal field emission scanning electron microscope (e.g., JSM-7001F manufactured by JEOL) and an EBSD detector (e.g., DVC5 detector manufactured by TSL) was used. At this time, the vacuum degree in the EBSD analysis device was 9.6×10 -5 Pa or less, the acceleration voltage was 15 kV, the irradiation current level was 13, and the electron beam irradiation level was 62. In addition, the number of observation fields was 5 fields.

[0189] Furthermore, a reflected electron image is taken in each of the same viewing fields in which the above-mentioned crystal orientation information is obtained. In addition, in the shooting of the image, the acceleration voltage is set to 15 kV, the irradiation current level is set to 12-13, and the irradiation level of the electron beam is set to 62. The focal length (WD: Waorking Distance) is set to 5 mm. According to the reflected electron image and the above-mentioned crystal orientation information, ferrite and pearlite are identified. First, in the reflected electron image, the grains in which cementite precipitates in lamellar form are determined. In the reflected electron image, cementite is observed with a white contrast. The cementite in the pearlite has a lamellar morphology, and the white contrast grains with a lamellar morphology observed at intervals of less than 1.0 μm are regarded as pearlite grains. By calculating the area ratio of the grains, the area ratio of pearlite is obtained. Then, for grains other than the grains determined as pearlite, the "Grain Average Misorientation" function in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device was used to determine that the region with a Grain Average Misorientation value of 1.0° or less in the obtained crystal orientation information was ferrite. At this time, the Grain Tolerance Angle was set to 15°, and the area ratio of the region determined as ferrite was calculated, thereby obtaining the area ratio of ferrite.

[0190] The area ratio of the residual structure is obtained by subtracting the area ratios of retained austenite, ferrite, and pearlite from 100%.

[0191] In addition, in the present embodiment, the rolling direction of the hot-rolled steel sheet is determined by the following method.

[0192] First, a test piece is collected in such a way that a cross section parallel to the surface of the hot-rolled steel sheet can be observed. The cross section of the test piece collected in the plate thickness direction is finished by mirror polishing and then observed using an optical microscope. The observation surface is set to be a surface parallel to the plate surface at any depth in the range of 1 / 4 to 1 / 2 in the plate thickness direction, and in this observation surface, the direction parallel to the extension direction of the crystal grains is determined as the rolling direction.

[0193] Entropy value: 10.7 or above

[0194] Inverse difference normalized value: 1.020 or more

[0195] In order to suppress the generation of secondary shear planes, it is important to form a fracture surface after the shear plane is fully formed, and it is necessary to suppress the early generation of cracks from the tip of the tool during shearing. For this purpose, it is important that the periodicity of the metal structure is low and the uniformity of the metal structure is high. In this embodiment, the generation of secondary shear planes is suppressed by controlling the Entropy value (E value) representing the periodicity of the metal structure and the Inverse difference normalized value (I value) representing the uniformity of the metal structure.

[0196] The E value represents the periodicity of the metal structure. When the brightness is arranged periodically due to the formation of a banded structure, that is, when the periodicity of the metal structure is high, the E value decreases. In the present embodiment, since it is necessary to form a metal structure with low periodicity, it is necessary to increase the E value. If the E value is lower than 10.7, secondary shear surfaces are likely to be generated. Starting from the periodically arranged structure, cracks are generated from the tip of the shearing tool at the very early stage of the shearing process to form a fracture surface, and then a shear surface is formed again. From this, it is estimated that secondary shear surfaces are likely to be generated. Therefore, the E value is set to be greater than 10.7. Preferably, it is greater than 10.8, and more preferably, it is greater than 11.0. The higher the E value, the more preferred it is. There is no particular upper limit, and it can be less than 13.0, less than 12.5, or less than 12.0.

[0197] The I value represents the uniformity of the metal structure. The larger the area of ​​the region with a certain brightness, the higher the I value. The so-called high I value means that the uniformity of the metal structure is high. In the present embodiment, it is necessary to form a metal structure with high uniformity, so it is necessary to increase the I value. If the I value is lower than 1.020, due to the influence of the hardness distribution of the precipitates and the element concentration difference in the grains, cracks are generated from the tip of the shearing tool in the very early stage of the shearing process to form a fracture surface, and then a shear surface is formed again. Thus, it is estimated that secondary shear surfaces are easily generated. Therefore, the I value is set to be greater than 1.020. Preferably, it is greater than 1.025, and more preferably greater than 1.030. The higher the I value, the more preferred it is. The upper limit is not particularly specified, and it can be less than 1.200, less than 1.150, or less than 1.100.

[0198] Cluster Shade value: -8.0×10 5 ~8.0×10 5

[0199] The Cluster Shade value (CS value) indicates the strain rate of the metal structure. The CS value is a positive value when there are many points with brightness exceeding the average value of the brightness in the image obtained by photographing the metal structure, and a negative value when there are many points with brightness below the average value.

[0200] In the secondary electron image of the electron microscope, the brightness increases at the part with large surface unevenness of the observed object, and decreases at the part with small surface unevenness. The surface unevenness of the observed object is greatly affected by the particle size and intensity distribution in the metal structure. For the CS value in this embodiment, it increases when the deviation of the strength of the metal structure is large or the tissue unit is small, and decreases when the deviation of the strength is small or the tissue unit is large.

[0201] In this embodiment, it is important to keep the CS value within the desired range close to 0. The CS value is less than -8.0×10 5 When the limit fracture thickness reduction rate of the hot-rolled steel sheet is reduced, it is speculated that this is because there are large grains in the metal structure, and these grains are preferentially destroyed during the limit deformation. Therefore, the CS value is -8.0×10 5 Above. Preferably -7.5×10 5 More preferably -7.0×10 5 above.

[0202] On the other hand, the CS value exceeds 8.0×10 5 When the limit fracture thickness reduction rate of the hot-rolled steel sheet is 8.0×10 5 Below. Preferably 7.5×10 5 Below, more preferably 7.0×10 5 the following.

[0203] The E value, I value and CS value can be obtained by the following method.

[0204] In the present embodiment, in order to calculate the E value, I value and CS value, in the shooting area of ​​the SEM image taken, in the center in the direction orthogonal to the rolling direction and the plate thickness direction, in the cross section parallel to the rolling direction, the thickness direction is set to 160 μm × 160 μm, and the number of observation fields is set to 5 fields. In the shooting of the SEM image, a SU-6600 Schottky electron gun manufactured by Hitachi High-Technologies Co., Ltd. is used, the emitter is set to tungsten, and the acceleration voltage is set to 1.5 kV. Based on the above settings, the SEM image is output at a magnification of 1000 times and a gray scale of 256 levels.

[0205] Next, the obtained SEM image was cut out of an area of ​​880×880 pixels (the actual size of the observation area was 160μm×160μm), and the resulting image was subjected to a smoothing process described in Non-Patent Document 3, with the contrast enhancement limit ratio set to 2.0 and the tile grid size set to 8×8. The smoothed SEM image was rotated counterclockwise from 0 to 179 degrees at intervals of 1 degree, except for 90 degrees, and images were created at intervals of 1 degree, thereby obtaining a total of 179 images. Next, for each of these 179 images, the GLCM method described in Non-Patent Document 1 was used to collect the frequency values ​​of the brightness between adjacent pixels in a matrix format.

[0206] Set k to the rotation angle from the original image, and express the matrix of 179 frequency values ​​collected by the above method as p k (k=0...89, 91,...179). For each image, for all k (k=0...89, 91...179) pairs of p generated k The sum is then calculated to normalize the 256×256 matrix P so that the sum of each component is 1. Furthermore, the E value, I value, and CS value are calculated using the following equations (1) to (5) described in Non-Patent Document 2. It should be noted that the average value obtained by measuring all the fields of view is calculated.

[0207] P(i, j) in the following equations (1) to (5) is a grayscale co-occurrence matrix, and the value of the i-th row and j-th column of the matrix P is marked as P(i, j). In addition, as described above, since a 256×256 matrix P is used for calculation, if you want to emphasize this point, the following equations (1) to (5) can be corrected to the following equations (1') to (5'). Here, L in the following equation (2) is the number of grayscale levels (Quantization levels of grayscale) that the SEM image can take. In the present embodiment, as described above, the SEM image is output with a grayscale of 256 levels, so L is 256. i and j in the following equations (2) and (3) are natural numbers from 1 to the above L, and μ in the following equation (3) is x and μ y They are represented by the following formulas (4) and (5), respectively.

[0208] In the following equations (1') to (5'), the value of the i-th row and j-th column of the matrix P is denoted as P ij .

[0209] [Mathematical formula 6]

[0210] Entropy=-∑ i ∑ j P(i,j)log(P(i,j))…(1)

[0211] [Mathematical formula 7]

[0212]

[0213] [Mathematical formula 8]

[0214] Cluster Shade=∑ i ∑ j (+j-μ x -μ y ) 3 P(i, j)…(3)[Formula 9]

[0215] μ x =∑ i ∑ j i(P(i, j))…(4)

[0216] [Formula 10]

[0217] μ y =∑ i ∑j j (P(i, j))…(5)

[0218] [Mathematical formula 11]

[0219]

[0220] [Mathematical formula 12]

[0221]

[0222] [Mathematical formula 13]

[0223]

[0224] [Mathematical formula 14]

[0225]

[0226] [Mathematical formula 15]

[0227]

[0228] Standard deviation of Mn concentration: 0.60 mass % or less

[0229] The standard deviation of the Mn concentration of the hot-rolled steel sheet of the present embodiment is 0.60% by mass or less. Thus, the hard phase can be evenly dispersed, and cracks can be prevented from being generated from the tip of the shearing tool at an extremely early stage of the shearing process. As a result, the generation of secondary shear surfaces can be suppressed. The standard deviation of the Mn concentration is preferably 0.50% by mass or less, and more preferably 0.47% by mass or less. For the lower limit of the standard deviation of the Mn concentration, the smaller the value, the better, but due to the constraints of the manufacturing process, the actual lower limit is 0.10% by mass.

[0230] The standard deviation of Mn concentration can be obtained by the following method. First, a sample is collected in the center of the rolling direction and the direction orthogonal to the plate thickness direction in such a way that an area at a position 1 / 4 of the surface in the plate thickness direction can be observed in the plate thickness section parallel to the rolling direction. Although the sample also depends on the measuring device, it is set to a size of about 10 mm that can be observed in the rolling direction. Then, after the above sample is mirror-polished, an electron probe microanalyzer (EPMA) is used to measure the standard deviation of the Mn concentration. The measurement conditions are as follows: the acceleration voltage is set to 15 kV, the magnification is set to 5000 times, and the distribution image of the Mn concentration in the range of 20 μm in the rolling direction and 20 μm in the plate thickness direction of the sample with the position 1 / 4 of the surface in the plate thickness direction as the center is measured. More specifically, the measurement interval is set to 0.1 μm, and the Mn concentration of more than 40,000 places is measured. Next, the standard deviation is calculated based on the Mn concentration obtained from all the measurement points, thereby obtaining the standard deviation of the Mn concentration.

[0231] Area ratio of the region where the Ni concentration in the surface is 0.2 mass % or more: 10.0% or more

[0232] In this embodiment, in order to improve the resistance to bending cracking, as described later, in the surface of the hot-rolled steel sheet, the area with an O concentration of 3.0 mass% or more is controlled to be 3.0% or more in terms of area %. However, when the area with an O concentration of 3.0 mass% or more in the surface is 3.0% or more in terms of area ratio, the chemical conversion treatment property of the hot-rolled steel sheet deteriorates. The inventors have found that, on the surface of the hot-rolled steel sheet, even if the area with an O concentration of 3.0 mass% or more is 3.0% or more in terms of area ratio, the chemical conversion treatment property of the hot-rolled steel sheet can be improved by increasing the area ratio of the area with a Ni concentration of 0.2 mass% or more. When the area ratio of the area with a Ni concentration of 0.2 mass% or more in the surface is less than 10.0%, the chemical conversion treatment property of the hot-rolled steel sheet cannot be fully improved. Therefore, in the surface, the area ratio of the area with a Ni concentration of 0.2 mass% or more is 10.0% or more. It is preferably 15.0% or more, and more preferably 20.0% or more.

[0233] The upper limit of the area ratio of the region having a Ni concentration of 0.2 mass % or more on the surface is not particularly limited, and may be 100.0% or less, 60.0% or less, 50.0% or less, or 40.0% or less.

[0234] Even if the area ratio of the region with a Ni concentration of less than 0.2 mass % is increased on the surface, the chemical conversion treatability of the hot-rolled steel sheet cannot be sufficiently improved. Therefore, in the present embodiment, it is important to increase the area ratio of the region with a Ni concentration of 0.2 mass % or more.

[0235] Area ratio of the region where the O concentration in the surface is 3.0 mass % or more: 3.0 to 50.0%

[0236] When the area ratio of the region with an O concentration of 3.0 mass % or more on the surface is less than 3.0%, the bending internal cracking resistance of the hot-rolled steel sheet deteriorates. Therefore, the area ratio of the region with an O concentration of 3.0 mass % or more on the surface is set to 3.0% or more, preferably 5.0% or more, and more preferably 10.0% or more.

[0237] On the other hand, when the area ratio of the region with an O concentration of 3.0 mass % or more on the surface exceeds 50.0%, the chemical conversion treatability of the hot-rolled steel sheet deteriorates. Therefore, the area ratio of the region with an O concentration of 3.0 mass % or more on the surface is set to 50.0% or less. Preferably, it is 40.0% or less, and more preferably, it is 30.0% or less.

[0238] In the surface, even if the area ratio of the region where the O concentration is lower than 3.0 mass % is controlled, it will not have a significant impact on the bending internal cracking resistance and chemical conversion treatability of the hot-rolled steel sheet. Therefore, in this embodiment, it is important to control the area ratio of the region where the O concentration is greater than 3.0 mass %.

[0239] The area ratio of the region where the Ni concentration is 0.2 mass % or more and the area ratio of the region where the O concentration is 3.0 mass % or more were measured by the following method.

[0240] The sample is collected in such a way that the surface of the hot-rolled steel sheet can be observed. For the sample, the observation surface is degreased at 60°C for 60 seconds using FC-E6403 manufactured by Nihon Parkerizing, and then immersed in acetone for 90 seconds of ultrasonic cleaning, thereby performing surface treatment of the sample. For the sample after surface treatment, an electron probe microanalyzer (EPMA) is used to map and analyze Ni and O. The measurement conditions are to set the acceleration voltage to 15kV, set the magnification to 500 times, and measure the distribution image of the sample in the rolling direction of 200μm and in the width direction of the sample plate. More specifically, the measurement interval is set to 1μm, and the Ni concentration and O concentration are measured at more than 40,000 locations per field of view. At least 5 fields of view are measured. By dividing the measurement points where the Ni concentration is 0.2% by mass or more by all the measurement points, the area ratio of the region where the Ni concentration is 0.2% by mass or more is obtained. Furthermore, the area ratio of the region where the O concentration is 3.0 mass % or more is obtained by dividing the measurement points where the O concentration is 3.0 mass % or more by all the measurement points.

[0241] In addition, when the hot-rolled steel sheet has oxide scale on the surface, the sample was subjected to pickling treatment under the following conditions and then subjected to the above-mentioned surface treatment.

[0242] The pickling treatment can be carried out by conventional methods, for example, by immersing in hydrochloric acid with a hydrochloric acid concentration of 3 to 10% by volume at a temperature of 85 to 98°C for 20 to 300 seconds. In addition, pickling can be carried out once or multiple times as needed. For the above-mentioned pickling time (20 to 300 seconds), it refers to the time of the pickling when only one pickling is carried out, and refers to the total time of these picklings when multiple picklings are carried out. By setting the pickling temperature to above 85°C, the oxides on the surface can be fully removed, so it is preferred. The upper limit of the pickling temperature is not particularly limited, and is actually about 98°C. When the pickling time exceeds 300 seconds, the surface roughness becomes too coarse, the surface properties deteriorate, and further, the unevenness remaining after cold rolling produces notches, and sometimes the bendability of the hot-rolled steel sheet deteriorates. The upper limit of the pickling time is preferably 200 seconds.

[0243] In the case where the hot-rolled steel sheet has a surface treatment film such as a coating and a coating on the surface, the surface of the substrate obtained after removing the surface treatment film is subjected to the above-mentioned pickling treatment and then provided for the above-mentioned surface treatment. The method for removing the surface treatment film can be appropriately selected according to the type of the surface treatment film within the range that does not affect the surface roughness of the substrate. For example, when the surface treatment film is a zinc coating such as electrogalvanizing, electroplating Zn-Ni alloy, hot-dip galvanizing, alloyed hot-dip galvanizing, hot-dip Zn-Al alloy, hot-dip Zn-Al-Mg alloy, and hot-dip Zn-Al-Mg-Si alloy, the zinc coating can be dissolved using dilute hydrochloric acid to which an inhibitor is added. Thus, only the zinc coating can be peeled off from the steel sheet. An inhibitor refers to an additive used to inhibit the change in roughness caused by preventing excessive dissolution of the substrate. For example, a solution of a corrosion inhibitor "IBIT No.700BK" for hydrochloric acid pickling made by Asahi Chemical Industry Co., Ltd. can be added to hydrochloric acid diluted to 5% by volume in a manner to become a concentration of 0.6 g / L. In addition, when the surface treatment film is an aluminum coating such as hot-dip aluminum coating, according to the description of JIS G 3314: 2019, it is immersed in a dilute hydrochloric acid aqueous solution to which a sodium hydroxide aqueous solution and hexamethylenetetramine are added in sequence until the foaming caused by the dissolution of the coating ends, thereby dissolving the Al coating. In addition, when the surface treatment film is an electrodeposition coating, a stripping agent (Neo River SP-751: manufactured by Sancai Chemical Co., Ltd.) is used to strip the electrodeposition coating.

[0244] Maximum equivalent spherical diameter of oxide on the surface: 5.00 μm or less

[0245] When the maximum value of the equivalent spherical diameter of the oxide in the surface exceeds 5.00 μm, the bending cracking resistance in the hot-rolled steel sheet deteriorates. Therefore, the maximum value of the equivalent spherical diameter of the oxide in the surface is 5.00 μm or less. In order to further improve the bending cracking resistance of the hot-rolled steel sheet, the maximum value of the equivalent spherical diameter of the oxide is preferably 4.50 μm or less, and more preferably 4.00 μm or less.

[0246] Since it may be technically difficult to reduce the maximum value of the equivalent spherical diameter of the oxide to less than 1.00 μm, the maximum value of the equivalent spherical diameter of the oxide may be 1.00 μm or more.

[0247] The oxides referred to here refer to precipitates containing O.

[0248] The maximum value of the equivalent spherical diameter of the oxide is measured by the following method.

[0249] The sample is collected from the hot-rolled steel sheet in such a way that the surface can be observed. The surface of the sample is degreased at 60°C for 60 seconds using FC-E6403 manufactured by Nihon Parkerizing, and then immersed in acetone for 90 seconds of ultrasonic cleaning, thereby performing surface treatment. For the sample after surface treatment, more than 10 fields of view are observed at a magnification of 3000 times to determine the precipitate. The composition of the precipitate is measured by EDS (energy dispersive X-ray spectrometer). Among the precipitates, the equivalent spherical diameter is calculated for the precipitate containing O. In the observation field, the equivalent spherical diameter of all precipitates containing O is calculated. The maximum value among the equivalent spherical diameters obtained is regarded as the maximum value of the equivalent spherical diameter of the oxide.

[0250] Furthermore, when EDS analysis was performed on the precipitate and 15 atomic % or more of O was detected, the precipitate was regarded as an oxide.

[0251] In addition, when the hot-rolled steel sheet has oxide scale or surface treatment film on the surface, these are removed by the above-mentioned method, and then the above-mentioned surface treatment is performed, and then the measurement is performed.

[0252] Tensile strength properties

[0253] The tensile strength characteristics (tensile strength, total elongation) of the mechanical properties of the hot-rolled steel sheet were evaluated according to JIS Z 2241: 2011. The test piece was a No. 5 test piece of JIS Z 2241: 2011. The sampling position of the test piece was set to the position 1 / 4 from the end face in the direction perpendicular to the rolling direction and the plate thickness direction, and the plate width direction was set to the length direction of the test piece.

[0254] The tensile strength of the hot-rolled steel sheet of the present embodiment is 980 MPa or more. Preferably, it is 1000 MPa or more. If the tensile strength is lower than 980 MPa, the applicable parts are limited, and the contribution to the lightweighting of the vehicle body is small. The upper limit does not need to be particularly limited, but from the viewpoint of suppressing die wear, it can also be set to 1780 MPa.

[0255] In addition, the total elongation of the hot-rolled steel sheet of the present embodiment is preferably set to 10.0% or more, and the product of the tensile strength and the total elongation (TS×El) is preferably set to 13000 MPa·% or more. The total elongation is more preferably 11.0% or more, and further preferably 13.0% or more. In addition, the product of the tensile strength and the total elongation is more preferably 14000 MPa·% or more, and further preferably 15000 MPa·%MPa or more. By making the total elongation 10.0% or more and the product of the tensile strength and the total elongation 13000 MPa·% or more, the applicable parts are not limited, which can greatly contribute to the lightweighting of the vehicle body.

[0256] Plate thickness

[0257] The plate thickness of the hot-rolled steel plate involved in the present embodiment is not particularly limited, and can also be set to 0.5 to 8.0 mm. When the plate thickness of the hot-rolled steel plate is less than 0.5 mm, it is sometimes difficult to ensure the rolling end temperature, and the rolling load is too large, making it difficult to perform hot rolling. Therefore, the plate thickness of the hot-rolled steel plate involved in the present embodiment can also be set to more than 0.5 mm. Preferably, it is more than 1.2 mm or more than 1.4 mm. On the other hand, when the plate thickness exceeds 8.0 mm, it becomes difficult to refine the metal structure, and it is sometimes difficult to obtain the above-mentioned metal structure. Therefore, the plate thickness can also be set to less than 8.0 mm. Preferably, it is less than 6.0 mm.

[0258] Plating

[0259] For the hot-rolled steel sheet with the above-mentioned chemical composition and metal structure involved in the present embodiment, for the purpose of improving corrosion resistance, etc., it is also possible to have a coating on the surface and make a surface-treated steel sheet. The coating can be an electroplated layer or a hot-dip coating. As the electroplated layer, examples include electrogalvanizing, electroplating Zn-Ni alloy, etc. As the hot-dip coating, examples 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, hot-dip Zn-Al-Mg-Si alloy plating, etc. There is no particular restriction on the coating adhesion amount, and it can also be the same as before. In addition, it is also possible to implement appropriate chemical conversion treatment (for example, coating and drying of chromium-free chemical conversion treatment liquid of silicate system) after plating to further improve corrosion resistance.

[0260] Manufacturing conditions

[0261] A suitable method for producing the hot-rolled steel sheet of the present embodiment having the above-mentioned chemical composition and metal structure is as follows.

[0262] A suitable manufacturing method for the hot-rolled steel sheet according to the present embodiment is to carry out the following steps (1) to (12) in sequence. The slab temperature and the steel sheet temperature in the present embodiment refer to the slab surface temperature and the steel sheet surface temperature. In addition, stress refers to the tension applied along the rolling direction of the steel sheet.

[0263] (1) After the slab is held in a temperature range of 700°C to 850°C for 900 seconds or more, it is further heated and held in a temperature range of 1100°C or more for 6000 seconds or more.

[0264] (2) The number of rough rolling times is set to 5 or more. During the entire rough rolling stage, rolling is performed in a temperature range lower than 1130°C and at a reduction ratio lower than 50%. Descaling is performed on the entry side of rolling, and rolling is performed within 2.0 seconds after descaling.

[0265] (3) The maximum temperature reached after 120 seconds from the end of rough rolling and before the start of descaling in finish rolling is set to 1000 to 1170° C., and the maximum temperature is maintained for 1.0 second or more.

[0266] (4) Hot rolling is performed in a temperature range of 850 to 1100°C with a total reduction ratio of 90% or more.

[0267] (5) A stress of 170 kPa or more is applied to the steel sheet from the end of the rolling before the final stage of hot rolling to the start of the final stage of rolling.

[0268] (6) The rolling reduction in the final stage of hot rolling is set to 8% or more, and the hot rolling is completed so that the rolling end temperature Tf is 900°C or more and lower than 1010°C.

[0269] (7) From the end of the final stage of hot rolling until the steel sheet is cooled to 800° C., a stress of less than 200 kPa is applied to the steel sheet.

[0270] (8) After the hot rolling is completed, the steel is accelerated cooled to a temperature range of 600 to 780° C. at an average cooling rate of 50° C. / s or more.

[0271] (9) In the temperature range of 600 to 780°C, slow cooling is performed at an average cooling rate of less than 5°C / s for 2.0 seconds or more.

[0272] (10) After the slow cooling is completed, cooling is performed so that the average cooling rate in the temperature range of 450 to 600° C. is 30° C. / s or more and less than 50° C. / s.

[0273] (11) Cooling is performed so that the average cooling rate in the temperature range from the coiling temperature to 450°C becomes 50°C / s or more.

[0274] (12) Coiling is performed in a temperature range below 350°C.

[0275] By adopting the above-mentioned production method, a hot-rolled steel sheet having high strength and ultimate fracture thickness reduction rate, as well as excellent ductility, shear workability, chemical conversion treatability and bending crack resistance can be stably produced.

[0276] (1) Slab, slab temperature and holding time during hot rolling

[0277] The slabs to be used for hot rolling may be those obtained by continuous casting, those obtained by casting / blowing, etc. In addition, slabs subjected to hot working or cold working may be used as required.

[0278] For the slab to be hot-rolled, it is preferred that: when the slab is heated, it is kept in a temperature range of 700 to 850°C for more than 900 seconds, and then further heated and kept in a temperature range of 1100°C or more for more than 6000 seconds. Furthermore, during the holding in the temperature range of 700 to 850°C, the temperature of the steel plate may be varied within the temperature range, or may be set constant. In addition, during the holding at 1100°C or more, the temperature of the steel plate may be varied within the temperature range of 1100°C or more, or may be set constant.

[0279] In the austenite transformation in the temperature range of 700-850°C, Mn is distributed between ferrite and austenite, and by extending the transformation time, Mn can diffuse in the ferrite region. As a result, the Mn microsegregation in the slab can be eliminated, and the standard deviation of the Mn concentration can be significantly reduced. In addition, by maintaining the temperature range above 1100°C for more than 6000 seconds, the standard deviation of the Mn concentration can be significantly reduced.

[0280] In hot rolling, as a multi-pass rolling, a reversible rolling mill or a tandem rolling mill is preferably used. In particular, from the viewpoint of industrial productivity and the viewpoint of stress load on the steel plate during rolling, it is more preferred that at least the final two stages are set as hot rolling using a tandem rolling mill. Hot rolling is a process including rough rolling and finish rolling, and multiple (stage) rolling is performed respectively. Rough rolling is a process of rolling the slab to a minimum of 25 mm, and finish rolling is a process of rolling the rough-rolled plate to a target plate thickness.

[0281] (2) Reduction rate, descaling, and rolling conditions in rough rolling

[0282] In the rough rolling, rolling and descaling are performed multiple times. In the present embodiment, it is preferred that the number of rough rolling is set to 5 or more times, and in the whole rough rolling section, rolling is performed in a temperature range lower than 1130°C, and rolling is performed at a reduction ratio lower than 50%, and descaling is performed on the entry side of rolling, and rolling is performed within 2.0 seconds after descaling.

[0283] By setting the number of rough rolling to 5 or more times, rolling is performed in a temperature range below 1130°C in the entire rough rolling section, and rolling is performed at a reduction rate of less than 50%, and descaling is performed on the entry side of rolling, the thickness of the scale formed in the previous section can be sufficiently reduced, or the scale can be sufficiently removed, and the excessive increase in the thickness of the scale can be suppressed. If scale is generated on the surface of the steel plate, Ni concentrates on the surface of the steel plate while it grows. Therefore, if the growth rate of the scale slows down, it is sometimes difficult for Ni to concentrate on the surface of the steel plate. By setting the number of rough rolling to 5 or more times, rolling is performed in a temperature range below 1130°C in the entire rough rolling section, and rolling is performed at a reduction rate of less than 50%, and descaling is performed on the entry side of rolling, as a result, in the hot-rolled steel plate, the area ratio of the region where the Ni concentration in the surface is 0.2 mass% or more can be preferably controlled.

[0284] It should be noted that, during the entire rough rolling process, rolling is performed in a temperature range lower than 1130° C. means that the entry temperature during the entire rough rolling process is in a temperature range lower than 1130° C. Descaling can be performed by water jetting.

[0285] In the whole rough rolling section, it is preferred to perform rolling within 2.0 seconds after descaling. If more than 2.0 seconds have passed after descaling, the oxide on the surface may become too coarse. Therefore, it is preferred to perform rolling within 2.0 seconds after descaling.

[0286] (3) Maintenance conditions after rough rolling

[0287] It is preferred that the maximum temperature reached from the end of rough rolling to 120 seconds and before the start of descaling of finish rolling is set to 1000-1170°C, and the maximum temperature is maintained for more than 1.0 second. By setting the maximum temperature reached from the end of rough rolling to 120 seconds and before the start of descaling of finish rolling to a temperature range of 1000-1170°C, the area ratio of the region with an O concentration of 3.0 mass% or more can be preferably controlled. If the maximum temperature is lower than 1000°C or exceeds 1170°C, the area ratio of the region with an O concentration of 3.0 mass% or more may not be preferably controlled.

[0288] It should be noted that the temperature may be raised to a temperature range of 1000 to 1170°C by heating, or the temperature may be raised by processing heat generated by rolling without heating. There are also cases where the outlet temperature after rolling in the final stage of rough rolling is 1000°C or higher, but in this case, the temperature may be maintained in the temperature range of 1000 to 1170°C for more than 1.0 second without raising the temperature, or the temperature may be raised to a temperature below 1170°C and then maintained in the temperature range of 1000 to 1170°C for more than 1.0 second.

[0289] In addition, during the maintenance in the temperature range of 1000 to 1170° C., the temperature of the steel sheet may be varied in the temperature range of 1000 to 1170° C. or may be kept constant.

[0290] (4) Hot rolling reduction rate: 90% or more in the temperature range of 850 to 1100°C

[0291] By performing hot rolling with a total reduction ratio of 90% or more in the temperature range of 850 to 1100°C, the recrystallized austenite grains are mainly refined, and the accumulation of strain energy in the non-recrystallized austenite grains is promoted. In addition, the recrystallization of austenite can be promoted, and the atomic diffusion of Mn can be promoted, reducing the standard deviation of the Mn concentration. Therefore, it is preferred to perform hot rolling with a total reduction ratio of 90% or more in the temperature range of 850 to 1100°C.

[0292] It should be noted that the hot rolling mentioned here includes rough rolling and finish rolling.

[0293] It should be noted that, for the rolling reduction rate in the temperature range of 850 to 1100° C., the inlet plate thickness before the initial rolling in the rolling in this temperature range is set to t 0 The exit plate thickness after rolling in the final stage of rolling in this temperature range is set as t 1 When {(t 0 -t 1 ) / t 0}×100(%).

[0294] (5) Stress applied to the steel sheet from the time of rolling before the final stage of hot rolling to the start of the final stage of rolling: 170 kPa or more

[0295] Preferably, a stress of 170 kPa or more is applied to the steel sheet from the last rolling stage before the last rolling stage of hot rolling to the start of the last rolling stage. <001> The number of grains with the crystal orientation of {110} <001> This is a crystal orientation that is difficult to recrystallize, so by suppressing the formation of this crystal orientation, the recrystallization caused by the final stage of rolling can be effectively promoted. As a result, the banded structure of the hot-rolled steel sheet can be improved, the periodicity of the metal structure can be reduced, and the E value can be increased.

[0296] It should be noted that the rolling before the final stage of hot rolling mentioned here refers to the rolling before the final stage of final rolling. For example, when the finish rolling is performed in 7 passes of F1, F2 ... F6, F7, it refers to the sixth pass (F6).

[0297] When the stress applied to the steel sheet is less than 170 kPa, the E value may not be set to a desired value. The stress applied to the steel sheet is more preferably 190 kPa or more.

[0298] The stress applied to the steel sheet refers to the tension applied along the length direction of the steel sheet, which can be controlled by adjusting the roll rotation speed during continuous rolling and can be obtained by dividing the load in the rolling direction measured on the rolling stand by the cross-sectional area of ​​the passed sheet.

[0299] (6) The final stage of hot rolling has a reduction ratio of 8% or more, and the hot rolling end temperature Tf is 900°C or more and less than 1010°C.

[0300] It is preferred that the reduction rate in the final stage of hot rolling is set to 8% or more, and the hot rolling end temperature Tf is set to 900°C or more. By setting the reduction rate in the final stage of hot rolling to 8% or more, the reduction in the final stage can be used to promote recrystallization. As a result, the banded structure of the hot-rolled steel sheet can be improved, the periodicity of the metal structure can be reduced, and the E value can be increased. By setting the hot rolling end temperature Tf to 900°C or more, the excessive increase in the number of ferrite nucleation sites in austenite can be suppressed. As a result, the formation of ferrite in the final structure (the metal structure of the hot-rolled steel sheet after manufacture) can be suppressed, and a high-strength hot-rolled steel sheet can be obtained. In addition, by setting Tf below 1010°C, the coarsening of the austenite grain size can be suppressed, the periodicity of the metal structure can be reduced, and the E value can be set to the desired value.

[0301] (7) Stress applied to the steel sheet from the end of the final stage of hot rolling to the point where the steel sheet is cooled to 800°C: less than 200 kPa

[0302] It is preferred that a stress of less than 200 kPa is applied to the steel plate from the end of the final stage of hot rolling to the cooling of the steel plate to 800°C. By applying a stress of less than 200 kPa to the steel plate, the recrystallization of austenite proceeds preferentially in the rolling direction, and the periodic increase of the metal structure can be suppressed. As a result, the E value can be set to a desired value. The stress applied to the steel plate is more preferably less than 180 kPa.

[0303] (8) After hot rolling, accelerate cooling to a temperature range of 600 to 780°C at an average cooling rate of 50°C / s or more.

[0304] After hot rolling, it is preferred that accelerated cooling be performed at an average cooling rate of 50°C / s or more to a temperature range of 780°C or less. This can suppress the formation of ferrite and pearlite with small precipitation strengthening amounts, and improve the strength of the hot-rolled steel sheet.

[0305] It should be noted that the average cooling rate referred to here refers to the value obtained by dividing the temperature drop of the steel plate from the start of accelerated cooling (when the steel plate is introduced into the cooling equipment) to the end of accelerated cooling (when the steel plate is taken out of the cooling equipment) by the time required from the start of accelerated cooling to the end of accelerated cooling.

[0306] There is no particular upper limit on the cooling rate, but if the cooling rate is increased, the cooling equipment will be large-scale, which will increase the equipment cost. Therefore, considering the equipment cost, it is preferably 300°C / s or less. In addition, in order to perform the slow cooling described later, the cooling stop temperature of the accelerated cooling can be 600°C or more.

[0307] (9) Slow cooling with an average cooling rate of less than 5°C / s for 2.0 seconds or more in a temperature range of 600 to 780°C

[0308] By performing slow cooling at an average cooling rate of less than 5°C / s for 2.0 seconds or more in a temperature range of 600 to 780°C, precipitation-strengthened ferrite can be sufficiently precipitated. Thus, both strength and ductility of the hot-rolled steel sheet can be achieved.

[0309] The average cooling rate referred to herein is a value obtained by dividing the temperature drop of the steel plate from the cooling stop temperature of the accelerated cooling to the slow cooling stop temperature by the time required from the stop of the accelerated cooling to the stop of the slow cooling.

[0310] The time for slow cooling is preferably 3.0 seconds or more. The upper limit of the time for slow cooling is determined by the equipment design, but it can be roughly less than 10.0 seconds. In addition, the lower limit of the average cooling rate of slow cooling is not specifically set, but heating without cooling will result in a large investment in equipment, so it can be set to 0°C / s or more.

[0311] (10) After the slow cooling is completed, the cooling is performed at an average cooling rate of 30°C / s or more and less than 50°C / s in the temperature range of 450°C to 600°C.

[0312] Preferably, after the above slow cooling is completed, cooling is performed in a manner that the average cooling rate in the temperature range of 450 to 600°C is 30°C / s or more and less than 50°C / s. By making the average cooling rate in the above temperature range 30°C / s or more and less than 50°C / s, the CS value can be made a desired value. When the average cooling rate is 50°C / s or more, a flat lath-shaped structure with low brightness is easily generated, and the CS value is less than -8.0×10 5 When the average cooling rate is lower than 30°C / s, carbon is concentrated in the non-transformed part, the strength of the hard structure increases, and the strength difference with the soft structure increases, so the CS value exceeds 8.0×105 .

[0313] It should be noted that the average cooling rate referred to herein refers to a value obtained by dividing the temperature drop of the steel plate from the cooling stop temperature of slow cooling with an average cooling rate of less than 5°C / s to the cooling stop temperature of cooling with an average cooling rate of 30°C / s or more and less than 50°C / s by the time required from the stop of slow cooling with an average cooling rate of less than 5°C / s to the stop of cooling with an average cooling rate of 30°C / s or more and less than 50°C / s.

[0314] (11) Average cooling rate in the temperature range from coiling temperature to 450°C: 50°C / s or more

[0315] In order to suppress the area ratio of pearlite and retained austenite and obtain desired strength and formability, the average cooling rate in the temperature range from the coiling temperature to 450° C. is preferably set to 50° C. / s or more. This can harden the parent phase structure.

[0316] It should be noted that the average cooling rate referred to herein refers to a value obtained by dividing the temperature drop of the steel plate from the cooling stop temperature to the coiling temperature when the average cooling rate is 30°C / s or more and less than 50°C / s by the time required from the stop of cooling to coiling when the average cooling rate is 30°C / s or more and less than 50°C / s.

[0317] (12) Coiling temperature: below 350°C

[0318] The coiling temperature is set to 350° C. or less. By setting the coiling temperature to 350° C. or less, the precipitation amount of iron carbides can be reduced, and the deviation of the hardness distribution in the hard phase can be reduced. As a result, the I value can be increased, and the generation of secondary shear planes can be suppressed.

[0319] Example

[0320] Next, the effect of one scheme of the present invention is described in more detail by way of an embodiment, but the conditions in the embodiment are a conditional example adopted to confirm the feasibility and effect of the present invention, and the present invention is not limited to this conditional example. The present invention may adopt various conditions as long as it does not deviate from the gist of the present invention and achieves the purpose of the present invention.

[0321] Steels having the chemical compositions shown in Tables 1 and 2 were melted and continuously cast to produce slabs having a thickness of 240 to 300 mm. The obtained slabs were used to produce hot-rolled steel sheets shown in Tables 5A to 6 under the production conditions shown in Tables 3A to 4B.

[0322] It should be noted that the average cooling rate of slow cooling is set to less than 5°C / s. In addition, the holding time at the maximum temperature described in Table 3A and Table 3B is set to more than 1.0 second. In addition, the measurement lower limit of the coiling temperature described in Table 4A and Table 4B is 50°C, so the actual coiling temperature of the example described as 50°C is below 50°C.

[0323] For the obtained hot-rolled steel sheet, the area ratio of the metal structure, E value, I value, CS value, standard deviation of Mn concentration, area ratio of the region with Ni concentration of 0.2 mass % or more, area ratio of the region with O concentration of 3.0 mass % or more, maximum value of equivalent spherical diameter of oxide, tensile strength TS and total elongation El were obtained by the above method. The obtained measurement results are shown in Tables 5A to 6.

[0324] Evaluation method of characteristics of hot-rolled steel sheets

[0325] Tensile properties

[0326] When the tensile strength (TS) is 980 MPa or more, the total elongation (El) is 10.0% or more, and the tensile strength (TS) × total elongation (El) is 13000 MPa·% or more, the hot-rolled steel sheet is judged as qualified as having high strength and excellent ductility. If any one of the above conditions is not met, the hot-rolled steel sheet is judged as not having high strength and excellent ductility and is judged as unqualified.

[0327] Limit fracture thickness reduction rate

[0328] The ultimate fracture thickness reduction rate of the hot-rolled steel sheet was evaluated by a tensile test.

[0329] The tensile test was performed in the same manner as in the evaluation of tensile properties. The plate thickness before the tensile test was denoted as t 1 The minimum value of the plate thickness in the width direction (short side direction) of the tensile test piece after fracture is set as t 2 When (t 1 -t 2 )×100 / t 1 The limit breaking thickness reduction rate was obtained by performing five tensile tests and calculating the average of three times excluding the maximum and minimum values ​​of the limit breaking thickness reduction rate.

[0330] When the limit breaking thickness reduction rate is 60.0% or more, the hot rolled steel sheet is judged as having a high limit breaking thickness reduction rate and is qualified. On the other hand, when the limit breaking thickness reduction rate is less than 60.0%, the hot rolled steel sheet is judged as not having a high limit breaking thickness reduction rate and is unqualified.

[0331] Shear processability (secondary shear surface evaluation)

[0332] The shear workability of the hot-rolled steel sheet was evaluated by a punching test.

[0333] For each example, three punched holes were made with a hole diameter of 10 mm, a clearance of 10%, and a punching speed of 3 m / s. Next, the cross-section of the punched hole at right angles to the rolling direction and the cross-section parallel to the rolling direction were buried in resin, and the cross-sectional shapes were photographed using a scanning electron microscope. In the obtained observation photographs, it can be observed that Figure 1 or Figure 2 The shear end face shown. It should be noted that Figure 1 This is an example of a sheared end surface of a hot-rolled steel sheet according to an example of the present invention. Figure 2 This is an example of the shear end surface of the hot-rolled steel plate of the comparative example. Figure 1 In the figure, it is the shear end surface of collapse edge-shear surface-fracture surface-flash edge. On the other hand, in Figure 2 In the figure, it is a shear end surface of collapsed edge-shear surface-fracture surface-shear surface-fracture surface-flash. Here, collapsed edge refers to the area of ​​the R-shaped smooth surface, the shear surface refers to the area of ​​the punched end surface separated by shear deformation, the fracture surface refers to the area of ​​the punched end surface separated by cracks generated near the blade tip, and the flash refers to the surface with protrusions protruding from the lower surface of the hot-rolled steel plate.

[0334] In the obtained shear end faces, two faces perpendicular to the rolling direction and two faces parallel to the rolling direction, for example, when the following Figure 2 In the case of the shear plane-fracture plane-shear plane shown in FIG. 1 , it is determined that a secondary shear plane is formed. Four planes of each punched hole, a total of 12 planes, are observed. If none of the planes have a secondary shear plane, it is determined to be acceptable as a hot-rolled steel sheet having excellent shear workability, and is recorded as "no" in the table. On the other hand, even if one secondary shear plane is formed, it is determined to be unacceptable as a hot-rolled steel sheet not having excellent shear workability, and is recorded as "yes" in the table.

[0335] Chemical conversion treatment

[0336] A 150 mm × 70 mm sample was collected from the hot-rolled steel plate after pickling, and after chemical conversion treatment using a chemical conversion treatment solution (PB-SX35) manufactured by Nihon Parkerizing Co., Ltd., a scanning electron microscope (SEM) was used to observe three locations (central part and both ends) along the length direction of the test piece at a magnification of 1000 times to observe the degree of adhesion of the grains of the zinc phosphate film. The pickling conditions were set to the same conditions as the above pickling treatment.

[0337] The case where the zinc phosphate crystals of the chemical conversion treatment film are densely attached is evaluated as "Good", and the case where the zinc phosphate crystals are sparse and a small gap is found between adjacent crystals (the part where the zinc phosphate film is not attached, generally called the "exposed bottom") is evaluated as "Fair". The case where the part not covered with the chemical conversion treatment film is clearly visible is evaluated as "Bad". In this evaluation, the examples evaluated as Good and Fair are judged to be qualified.

[0338] Resistance to internal cracking during bending

[0339] The bending crack resistance was evaluated by the following bending test.

[0340] A 100 mm × 30 mm strip-shaped test piece was cut out from the 1 / 2 position in the width direction of the hot-rolled steel plate after pickling to obtain a bending test piece. For both bending in which the bending ridge is parallel to the rolling direction (L direction) (L-axis bending) and bending in which the bending ridge is parallel to the direction perpendicular to the rolling direction (C direction) (C-axis bending), a test was performed according to the V-block method (bending angle θ is 90°) of JIS Z 2248:2022. Thus, the minimum bending radius at which no cracking occurs was determined. The resistance to internal cracking during bending was investigated. The value obtained by dividing the average value of the minimum bending radius of the L axis and the C axis by the plate thickness was taken as the limit bending R / t, which was used as an index value for resistance to internal cracking during bending.

[0341] When R / t is 2.5 or less, the hot-rolled steel sheet is judged as having excellent bending internal cracking resistance and is qualified. On the other hand, when R / t exceeds 2.5, the hot-rolled steel sheet is judged as not having excellent bending internal cracking resistance and is unqualified.

[0342] Regarding the presence or absence of cracks, the cross section of the test piece after the test was cut along a plane parallel to the bending direction and perpendicular to the plate surface, mirror-polished, and then cracks were observed under an optical microscope. When the length of the crack observed on the inner side of the bend of the test piece exceeded 30 μm, it was judged that cracks were present.

[0343]

[0344]

[0345]

[0346]

[0347]

[0348]

[0349]

[0350]

[0351] Table 6

[0352]

[0353] The underlined characters indicate that the properties are outside the scope of the present invention or are not preferred.

[0354] As can be seen from Tables 5A to 6, the hot-rolled steel sheets of the examples of the present invention have high strength and ultimate fracture thickness reduction rate, and are excellent in ductility, shear workability, chemical conversion treatability, and bending crack resistance.

[0355] On the other hand, it was found that the hot-rolled steel sheets of the comparative examples deteriorated in at least one of the above-mentioned characteristics.

[0356] Industrial Applicability

[0357] According to the above aspects of the present invention, a hot-rolled steel sheet having high strength and ultimate fracture thickness reduction rate and excellent ductility, shear workability, chemical conversion treatability and bending crack resistance can be obtained.

[0358] The hot-rolled steel sheet of the present invention is suitable as an industrial raw material used in automobile parts, machine structural parts and building parts.

Claims

1. A hot rolled steel plate, characterized in that: Its chemical composition contains by mass %: C：0.050～0.250％、 Si: 0.05-3.00%, Mn: 1.00~4.00%, Ni: 0.02-2.00%, Sol.Al: 0.001~2.000%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, Ti: 0~0.500%, Nb: 0~0.500%, V：0～0.500％、 Cu: 0-2.00%, Cr：0～2.00％、 Mo: 0-1.00%, B：0～0.0100％、 Ca: 0~0.0200%, Mg: 0~0.0200%, REM: 0~0.1000%、 Bi: 0~0.0200%, As: 0~0.100%, Zr：0～1.00％、 Co: 0-1.00%, Zn: 0-1.00%, W: 0-1.00%, and Sn: 0~0.05%, The remaining part contains Fe and impurities. And satisfy the following formulas (A) and (B), The metal structure at a position 1 / 4 from the surface in the plate thickness direction is calculated in area %: Retained austenite is less than 3.0%, Ferrite is 15.0% or more and less than 60.0%, Pearlite is less than 5.0%, The entropy value represented by the following formula (1) obtained by analyzing the SEM image of the metal structure using the gray level co-occurrence matrix method is 10.7 or more. The inverse difference normalized value represented by the following formula (2) is 1.020 or more. The Cluster Shade value represented by the following formula (3) is -8.0×10 5 ~8.0×10 5 , the standard deviation of the Mn concentration is 0.60 mass % or less, In the surface, The area ratio of the region where the Ni concentration is 0.2 mass % or more is 10.0% or more, The area ratio of the region where the O concentration is 3.0 mass % or more is 3.0 to 50.0%, The maximum value of the equivalent spherical diameter of the oxide is less than 5.00 μm. The tensile strength of the hot-rolled steel plate is above 980 MPa. 0.060%≤Ti+Nb+V≤0.500%(A) Zr+Co+Zn+W≤1.00%(B) The symbol of each element in the above formula (A) and (B) represents the content of the element in mass %, and 0% is substituted when the element is not contained. Wherein, P(i, j) in the following equations (1) to (5) is a gray level co-occurrence matrix, L in the following equation (2) is the number of levels of the gray scale that the SEM image can take, i and j in the following equations (2) and (3) are natural numbers from 1 to L, and μ in the following equation (3) is x and μ y They are represented by the following formulas (4) and (5), respectively: Entropy=-∑ i ∑ j P(i,j)log(P(i,j))…(1) Cluster Shade=Σ i S j (i+j-μ x -m y ) 3 P(i,j) …(3) m x =S i S j i(P(i,j))…(4) μ y ∑ i ∑ j j(P(i,j))…(5)。 2. The hot rolled steel sheet according to claim 1, characterized in that: The chemical composition contains, by mass%, one or more selected from the group consisting of the following elements: Ti: 0.001~0.500%, Nb: 0.001~0.500%, V：0.001～0.500％、 Cu: 0.01-2.00%, Cr：0.01～2.00％、 Mo: 0.01~1.00%, B：0.0001～0.0100％、 Ca: 0.0005~0.0200%, Mg: 0.0005~0.0200%, REM: 0.0005~0.1000%、 Bi: 0.0005~0.0200%, As: 0.001~0.100%, Zr：0.01～1.00％、 Co: 0.01~1.00%, Zn: 0.01~1.00%, W: 0.01 to 1.00%, and Sn: 0.01~0.05%.

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