Steel sheet, automobile component including same, and method for producing steel sheet

Abstract

The present invention provides a steel sheet and a method for producing the steel sheet, said steel sheet being characterized by having a specific chemical composition. The steel sheet is also characterized in that: the steel structure at the position of 1 / 4 thickness from the surface contains, by area%, 85% or more of martensite, a total of 0-5% of ferrite and bainite, and 1-8% of retained austenite; and when a total of 12 dislocation densities that are obtained by measuring dislocation densities at four positions corresponding to respective apexes of a square having one side of 1 mm in a plane that is parallel to the surface at each of the position of 1 / 8 thickness, the position of 2 / 8 thickness, and the position of 3 / 8 thickness from the surface are defined as a population, the value of (standard deviation of dislocation densities) / (average of dislocation densities) is 0.05-0.50.

Description

Steel sheet, automotive parts containing the same, and method for producing the steel sheet

[0001] The present invention relates to a steel sheet, parts containing the same, and a method for producing the steel sheet.

[0002] In recent years, in view of regulations on greenhouse gas emissions accompanying measures against global warming, improvement of fuel efficiency of automobiles has been demanded, and the application of high-strength steel sheets has been increasingly expanded for weight reduction of vehicle bodies and improvement of collision safety. However, when the strength of a steel sheet is increased, properties such as elongation generally decrease, and various measures have been taken to achieve both strength and formability in high-strength steel sheets.

[0003] Steel contains a large number of linear defects called dislocations inside. It is known that deformation progresses by the movement of dislocations in the crystal, and on the other hand, suppressing the movement of dislocations is effective for increasing the strength of steel. In the prior art as well, it has been proposed to improve other properties in addition to strength by controlling the dislocation density in steel.

[0004] For example, in Patent Document 1, it has a predetermined chemical composition, and the values of the amount of solid solution B in steel solB [mass%] and the prior austenite grain size Dγ [μm] satisfy the relationship of solB·Dγ≥0.0010. Further, in terms of area ratio, polygonal ferrite is 10% or less, bainite is 30% or less, retained austenite is 6% or less, tempered martensite is 60% or more, and the number density of Fe carbides in tempered martensite is 1×10 6 / mm 2 or more, and the average dislocation density of the entire steel is 1.0×10 15 / m 2 or more and 2.0×10 16 / m 2 or less, and it is characterized by having a steel structure with an effective crystal grain size of 7.0 μm or less. Further, in Patent Document 1, by setting the average dislocation density of the entire steel to 1.0×10 15 / m 2 or more and 2.0×10 16 / m 2 or less, it is taught that it is possible to achieve both a tensile strength of 1300 MPa or more and excellent hydrogen embrittlement resistance characteristics.

[0005] Patent Document 2 describes a cold-rolled steel sheet having a predetermined chemical composition, wherein the t / 4 portion is defined as a range of 1 / 8 to 3 / 8 of the sheet thickness in the thickness direction from the surface, and the surface layer is defined as a range of 20 μm from the surface in the thickness direction, and the microstructure of the t / 4 portion contains, by volume fraction, 0% to 10.0% of retained austenite and 90.0% to 100% of one or two types of martensite and tempered martensite, the ratio of the dislocation density of the surface layer to the dislocation density of the t / 4 portion is 0.80 or more, and the ratio of the hardness of the surface layer to the hardness of the t / 4 portion is 0.90 or more. Furthermore, Patent Document 2 teaches that if the ratio of the dislocation density of the surface layer to the dislocation density of the t / 4 portion is 0.80 or more, and the ratio of the hardness of the surface layer to the hardness of the t / 4 portion is 0.90 or more, then dislocations are immobilized and a decrease in flow stress can be prevented.

[0006] Japanese Patent Publication No. 2016-050343, International Publication No. 2023 / 002910

[0007] As mentioned earlier, increasing the strength of steel sheets generally reduces properties such as elongation. Therefore, there is a high demand for steel sheets that can achieve both high strength and high elongation. In addition, in automotive parts, there is a need for high-strength parts that can suppress the localization of deformation that occurs during collisions. In connection with this, there is a need for steel sheets that can uniformly strengthen (bake harden) the entire part after forming and paint baking. Here, bake hardening is a phenomenon in which the movement of interstitial elements (mainly carbon) is inhibited by the movement and fixation of dislocations introduced into the steel sheet by press forming, etc., through a paint baking treatment at 100 to 220°C, thereby increasing the strength. This is also called strain aging. Generally, bending is the main processing method for automotive parts to which high-strength steel sheets are applied. On the other hand, in such parts, strain from forming, such as press forming, cannot be introduced to parts other than the bent parts, and therefore dislocations cannot be sufficiently introduced to parts other than the bent parts. In such cases, it becomes impossible to sufficiently increase the amount of curing harden in areas where sufficient dislocations have not been introduced through molding of the part (hereinafter sometimes referred to as strain-free areas).

[0008] Therefore, the present invention aims to provide a steel sheet and a method for manufacturing the same, which, through a novel configuration, can have improved elongation despite being high in strength, and can increase the amount of bake-hardened portion in the strain-free area.

[0009] To achieve the above objective, the inventors focused particularly on the steel structure of the steel sheet. Specifically, they first found that by optimizing the chemical composition of the steel sheet and constructing the steel structure of the steel sheet with a structure mainly composed of martensite and containing a predetermined amount of retained austenite, it is possible to significantly improve the elongation of the steel sheet while achieving high strength, for example, a tensile strength of 1470 MPa or higher. In addition, the inventors found that by controlling the variation in dislocation density in the thickness direction and in-plane direction of the steel sheet within a predetermined range, it is possible to significantly improve the amount of bake-hardened paint even in areas where sufficient strain is not introduced during press forming, etc., thus completing the present invention.

[0010] The present invention, which has achieved the above objectives, is as follows. (1) In mass percent, C: 0.16-0.40%, Si: 0.001-2.00%, Mn: 0.50-2.00%, P: 0.050% or less, S: 0.0100% or less, Al: 0.001-1.000%, Ti: 0.001-0.100%, B: 0.0005-0.0050%, N: 0.0150% or less, O: 0.0100% or less, Cr: 0-1.00%, Mo: 0-1.00%, Cu: 0-1.00%, Ni: 0-1.00%, Co: 0-1.00%, W: 0-1.00%, Ta: 0-1.00%, Sn: 0-1.00% The chemical composition consists of Sb: 0-0.50%, Nb: 0-0.200%, V: 0-1.00%, As: 0-0.100%, Zn: 0-1.000%, Ca: 0-0.0100%, Mg: 0-0.0100%, Zr: 0-0.0100%, Hf: 0-0.0100%, Bi: 0-0.0100%, REM: 0-0.0100%, and the remainder being Fe and impurities. The microstructure at a thickness of 1 / 4 of the surface contains, by area percentage, martensite: 85% or more, total of ferrite and bainite: 0-5%, and retained austenite: 1-8%. A steel sheet characterized in that, when the dislocation density of a total of 12 points obtained by measuring the dislocation density at four points corresponding to each vertex of a square with sides of 1 mm in a plane parallel to the surface at positions 1 / 8 thickness, 2 / 8 thickness, and 3 / 8 thickness from the surface is used as the population, the average value of the standard deviation of the dislocation density / dislocation density is 0.05 to 0.50. (2) The steel sheet according to (1) above, characterized in that when the concentrations of Mn, Si, Cr, Mo, and Ni at positions 1 / 8 thickness, 2 / 8 thickness, and 3 / 8 thickness from the surface are measured by EPMA, the area ratio at which ΔMs represented by the following formula 1 is 30 or more is 10% or less at all of the positions 1 / 8 thickness, 2 / 8 thickness, and 3 / 8 thickness.ΔMs = |33 × ( - [Mn]) + 17 × (<Cr> - [Cr]) + 17 × (<Ni> - [Ni]) + 7.5 × (<Si> - [Si]) + 21 × ( - [Mo])| ...Equation 1 Here, , <Si>, <Cr>, , and <Ni> are the concentrations (mass%) of Mn, Si, Cr, Mo, and Ni measured by EPMA, and [Mn], [Si], [Cr], [Mo], and [Ni] are the Mn, Si, Cr, Mo, and Ni content (mass%) in the steel sheet. (3) The steel sheet according to (1) or (2) above, characterized by having a tensile strength of 1470 MPa or more. (4) The steel sheet according to any one of (1) to (3) above, characterized by having a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on at least one surface. (5) An automobile part characterized by including a steel plate as described in any one of the above items (1) to (4).(6) (A) A hot rolling process that includes hot rolling a slab having the chemical composition described in (1) above, then winding the obtained hot-rolled steel sheet and cooling, and satisfying the following conditions (A1) and (A2): (A1) Three or more rolling passes with a reduction ratio of 20% or more in the temperature range of 950 to 1050°C, and a cumulative reduction ratio of 60% or more in the temperature range of 950 to 1050°C, and (A2) a cumulative reduction ratio of 10 to 50% in the temperature range of 850 to 950°C. (B) A cold rolling process in which the hot-rolled steel sheet is pickled and then cold-rolled with a reduction ratio of 30 to 75%. (C) A heat treatment process in which the obtained cold-rolled steel sheet is heated to a maximum heating temperature of Ac3 to 950°C and then cooled, and satisfying the following conditions (C1) to (C3). A method for manufacturing a steel sheet according to any one of (1) to (5) above, characterized in that (C1) the average cooling rate between Ms and 700°C is 40°C / s or more, the average cooling rate between Ms-200 and Ms°C is 20°C / s or less, and the cooling stop temperature is 100 to 220°C, (C2) at least one bending process is performed between Ms-200 and Ms°C using a roll with a diameter of 1700 mm or less, and (C3) after cooling stops, the cold-rolled steel sheet is held in a temperature range of 150 to 300°C for 200 to 1000 seconds, and (D) the cold-rolled steel sheet is skin-pass rolled under conditions of elongation: 0.5 to 3.0%, back tension / YP: 0.05 to 0.20, and work roll diameter: 400 to 550 mm, wherein YP is the yield point (MPa) of the cold-rolled steel sheet at the skin-pass rolling entry side. (7) The method for manufacturing a steel sheet as described in (6) above, characterized in that the hot rolling process further satisfies the following condition (A3): (A3) The winding temperature is 450 to 580°C, and the maximum temperature reached after winding is 600°C or less.

[0011] According to the present invention, it is possible to provide a steel sheet and a method for manufacturing the same that have high strength, yet possess improved elongation and can increase the amount of bake-hardened areas in the unstrained portion.

[0012] <Steel Sheet> The steel sheet according to the embodiment of the present invention has the following composition in mass%, C: 0.16-0.40%, Si: 0.001-2.00%, Mn: 0.50-2.00%, P: 0.050% or less, S: 0.0100% or less, Al: 0.001-1.000%, Ti: 0.001-0.100%, B: 0.0005-0.0050%, N: 0.0150% or less, O: 0.0100% or less, Cr: 0-1.00%, Mo: 0-1.00%, Cu: 0-1.00%, Ni: 0-1.00%, Co: 0-1.00%, W: 0-1.00%, Ta: 0-1.00% The chemical composition consists of Sn: 0-1.00%, Sb: 0-0.50%, Nb: 0-0.200%, V: 0-1.00%, As: 0-0.100%, Zn: 0-1.000%, Ca: 0-0.0100%, Mg: 0-0.0100%, Zr: 0-0.0100%, Hf: 0-0.0100%, Bi: 0-0.0100%, REM: 0-0.0100%, and the remainder being Fe and impurities. The microstructure at a thickness of 1 / 4 of the surface contains, by area percentage, martensite: 85% or more, total of ferrite and bainite: 0-5%, and retained austenite: 1-8%. The method is characterized by the fact that, when the dislocation density is measured at four points corresponding to each vertex of a 1 mm square in a plane parallel to the surface at positions 1 / 8 thickness, 2 / 8 thickness, and 3 / 8 thickness from the surface, and the total of 12 dislocation densities obtained are used as the population, the ratio of the standard deviation of the dislocation density to the mean of the dislocation density is between 0.05 and 0.50.

[0013] As mentioned earlier, increasing the strength of a steel sheet generally reduces properties such as elongation. In particular, for steel sheets with extremely high strength, such as those with a tensile strength of 1470 MPa or more, the steel structure of the steel sheet generally contains martensite as the main component in order to ensure high strength. On the other hand, when a high-strength steel sheet is constructed with such a martensite-based structure, the reduction in properties such as elongation naturally becomes significant.

[0014] Therefore, the present inventors first found that by optimizing the chemical composition of the steel sheet, particularly the content of C, Si, Mn, and B, and by configuring the steel structure of the steel sheet to be mainly martensite but also containing a predetermined amount of retained austenite, more specifically, by configuring the steel structure of the steel sheet to contain 85% or more martensite and 1-8% retained austenite by area percentage, it is possible to significantly improve the elongation of the steel sheet while achieving high strength, for example, a tensile strength of 1470 MPa or more.

[0015] As mentioned earlier, bending is a relatively common method of processing automotive parts that use steel plates with the extremely high strength described above. However, in such parts, it is not possible to sufficiently introduce strain through forming, such as press forming, to areas other than the bent parts, and therefore, it is not possible to sufficiently introduce dislocations to areas other than the bent parts. In such cases, it becomes impossible to sufficiently increase the amount of bake hardening in areas where strain has not been sufficiently introduced (i.e., unstrained areas).

[0016] Therefore, the inventors conducted further studies to achieve the desired amount of bake hardening in strain-free areas, even when high-strength steel sheets having a tensile strength of, for example, 1470 MPa or more are applied to automobile parts. As a result, the inventors found that in a steel structure containing 85% or more martensite by area, by controlling the variation in dislocation density in both the thickness direction and the in-plane direction of the steel sheet to within a predetermined range, and more specifically by measuring the dislocation density at four points corresponding to each vertex of a 1 mm square in a plane parallel to the surface of the steel sheet at 1 / 8 thickness, 2 / 8 thickness, and 3 / 8 thickness positions from the surface of the steel sheet in the thickness direction, and using a total of 12 dislocation densities obtained as the population, the amount of bake hardening during paint baking can be significantly improved even in areas where sufficient strain is not introduced by press forming, etc.

[0017] To explain in more detail, martensite is generally known to have a high dislocation density and a hard structure. Therefore, in steel structures containing 85% or more martensite by area percentage, the overall dislocation density of the steel structure is naturally higher compared to steel structures with a relatively small martensite area percentage, and thus the variation in the overall dislocation density of the steel structure becomes larger. Consequently, when steel sheets with such a structure are processed by press forming or the like, the variation in dislocation density between the strained areas and the unstrained areas becomes even larger. As a result, the difference in the amount of bake-hardened paint during curing becomes particularly pronounced between the strained areas and the unstrained areas, and it becomes difficult to sufficiently increase the amount of bake-hardened paint, especially in the unstrained areas.

[0018] Therefore, the inventors have found that, as will be explained in detail later in relation to the manufacturing method, by appropriately controlling the hot rolling process and the heat treatment process after the cold rolling process, in particular, the variation in dislocation density after martensitic transformation caused by variations in austenite grain size can be significantly suppressed throughout the steel sheet. In addition, the inventors have found that, as will also be explained in detail later in relation to the manufacturing method, by appropriately controlling the skin pass rolling process, strain can be introduced uniformly throughout the steel sheet, and that by a specific combination of such controlled hot rolling, heat treatment, and skin pass rolling processes, the dislocation density can be made uniform not only in the thickness direction but also in the in-plane direction. More specifically, the inventors have found that by a specific combination of the controlled hot rolling process, heat treatment process, and skin pass rolling process described above, it is possible to control the standard deviation of the dislocation density / the average value of the dislocation density to be within the range of 0.05 to 0.50, when the dislocation density of a total of 12 points obtained at 1 / 8 thickness, 2 / 8 thickness, and 3 / 8 thickness positions in the thickness direction from the surface of the steel sheet is used as the population.

[0019] Furthermore, in the steel sheet according to the embodiment of the present invention, it is not enough to simply control the standard deviation of the dislocation density; it is extremely important to control the value obtained by normalizing the standard deviation of the dislocation density by the average value of the dislocation density, i.e., the standard deviation of the dislocation density / average value of the dislocation density, to within the range of 0.05 to 0.50. To explain in more detail, in a steel structure with a relatively small area ratio of martensite, the value of the dislocation density itself becomes small, and naturally, the standard deviation of the dislocation density also becomes small. However, in this case, the number of dislocations to which interstitial elements (mainly carbon) adhere during the paint baking process is small, so a sufficient amount of bake hardening cannot be obtained in the unstrained area. On the other hand, in a steel structure mainly composed of martensite, such as the steel sheet according to the embodiment of the present invention, i.e., a steel structure containing 85% or more martensite by area %, the value of the dislocation density itself becomes very large, so the variation in dislocation density also becomes large, and as a result the standard deviation of the dislocation density becomes large. Therefore, the inventors considered it appropriate to use a value normalized by dividing the standard deviation of the dislocation density by the average value of the dislocation density, i.e., the standard deviation of the dislocation density / average value of the dislocation density, as an indicator of the uniformity of the dislocation density, and repeated experiments. As a result, the inventors found that in a martensite-dominant structure with a high dislocation density, it is effective to control the standard deviation of the dislocation density / average value of the dislocation density within the range of 0.05 to 0.50, based on the dislocation density obtained from predetermined positions in the plate thickness direction and in-plane direction as described above. In other words, the inventors found that by controlling the standard deviation of the dislocation density / average value of the dislocation density within the range of 0.05 to 0.50, the dislocation density can be made uniform at a high level, and therefore, even in strain-free areas where sufficient strain has not been introduced by press forming, etc., a relatively high dislocation density is maintained, which significantly improves the amount of baked hardening during paint baking. Therefore, the steel sheet according to the embodiment of the present invention, despite having high strength due to its martensitic-dominant structure, exhibits improved elongation and can increase the amount of bake-hardened areas in the unstrained portion, making it particularly useful in the automotive sector where a high level of both high strength and formability is required.

[0020] The steel sheets according to embodiments of the present invention will be described in more detail below. In the following description, "%", which is the unit for the content of each element, means "mass%" unless otherwise specified. In this specification, "~", which indicates a numerical range, is used to mean that the numbers written before and after it are included as the lower limit and upper limit, respectively, unless otherwise specified.

[0021] [C: 0.16-0.40%] Carbon (C) is an essential element for ensuring the strength of steel plates. To obtain this effect fully, the C content should be 0.16% or more. The C content may be 0.18% or more, 0.20% or more, 0.22% or more, 0.24% or more, or 0.26% or more. On the other hand, if the C content is excessive, the toughness may decrease due to an excessive increase in strength, which may lead to premature fracture or a decrease in uniform elongation. For this reason, the C content should be 0.40% or less. The C content may be 0.38% or less, 0.35% or less, 0.32% or less, 0.30% or less, or 0.28% or less.

[0022] [Si: 0.001-2.00%] Silicon (Si) is an element that suppresses the formation of iron carbides and contributes to improving strength and formability. To fully obtain these effects, the Si content should be 0.001% or more. The Si content may be 0.01% or more, 0.05% or more, 0.10% or more, 0.20% or more, 0.30% or more, 0.50% or more, 0.80% or more, or 1.00% or more. On the other hand, excessive Si content may reduce the toughness of the steel, leading to premature fracture. Therefore, the Si content should be 2.00% or less. The Si content may be 1.80% or less, 1.60% or less, 1.40% or less, or 1.20% or less.

[0023] [Mn: 0.50-2.00%] Manganese (Mn) is an element that enhances hardenability and contributes to improved strength. To fully obtain this effect, the Mn content should be 0.50% or more. The Mn content may be 0.60% or more, 0.80% or more, or 1.00% or more. On the other hand, if Mn is included in excess, Mn segregation during casting becomes significant, and the martensitic transformation onset temperature changes between the segregated and non-segregated areas, resulting in a mixture of martensite formed at relatively high temperatures and martensite formed at low temperatures. As a result, the in-plane and thickness-direction variations in dislocation density become large, and it may not be possible to achieve the desired amount of bake-hardened material in the strain-free area. Therefore, the Mn content should be 2.00% or less. The Mn content may be 1.80% or less, 1.60% or less, 1.40% or less, or 1.20% or less.

[0024] [P: 0.050% or less] P (phosphorus) is a solid solution strengthening element and is effective in increasing the strength of steel plates, but excessive addition may degrade weldability and toughness. Therefore, the P content should be 0.050% or less. Preferably, the P content is 0.045% or less, 0.035% or less, or 0.020% or less. The P content may be 0%, but reducing the P content to an extreme degree will increase the cost of removing P. For this reason, from an economic standpoint, the P content may be 0.0001% or more, 0.0005% or more, or 0.001% or more.

[0025] [S: 0.0100% or less] S (sulfur) is an element contained as an impurity and can form MnS in steel, degrading toughness and hole-expanding properties. Therefore, the S content should be 0.0100% or less. Preferably, the S content is 0.0050% or less, 0.0040% or less, 0.0030% or less, or 0.0020% or less. The S content may be 0%, but extremely low S content increases desulfurization costs. For this reason, from an economic standpoint, the S content may be 0.0001% or more, 0.0003% or more, or 0.0005% or more.

[0026] [Al: 0.001-1.000%] Aluminum (Al) is an element that acts as a deoxidizing agent. To obtain this effect sufficiently, the Al content should be 0.001% or more. The Al content may be 0.010% or more, 0.020% or more, 0.030% or more, 0.040% or more, or 0.050% or more. On the other hand, if Al is included in excess, the effect will saturate, and including more Al in the steel plate than necessary will lead to an increase in manufacturing costs. Therefore, the Al content should be 1.000% or less. The Al content may be 0.800% or less, 0.600% or less, 0.300% or less, or 0.100% or less.

[0027] [Ti: 0.001 to 0.100%] Titanium (Ti) is an effective element for fixing nitrogen (N) present as an impurity in steel as TiN and for suppressing the precipitation of boron (B) as nitride (BN). To obtain these effects sufficiently, the Ti content should be 0.001% or more. The Ti content may be 0.005% or more, 0.010% or more, 0.015% or more, or 0.020% or more. On the other hand, if Ti is included in excess, the effect will saturate, and including more Ti in the steel sheet than necessary will lead to an increase in manufacturing costs. Therefore, the Ti content should be 0.100% or less. The Ti content may be 0.090% or less, 0.080% or less, 0.060% or less, or 0.040% or less.

[0028] [B: 0.0005 to 0.0050%] Boron (B) is an element that enhances hardenability and contributes to improved strength. To obtain this effect sufficiently, the B content should be 0.0005% or more. The B content may also be 0.0008% or more, 0.0010% or more, 0.0012% or more, 0.0015% or more, 0.0018% or more, or 0.0020% or more. On the other hand, if B is included in excess, excessive borides may be formed in the steel, which may reduce the hardenability of the steel sheet. Therefore, the B content should be 0.0050% or less. The B content may also be 0.0045% or less, 0.0040% or less, 0.0035% or less, 0.0030% or less, 0.0028% or less, or 0.0025% or less.

[0029] [N: 0.0150% or less] Nitrogen (N) is an element contained as an impurity, and if the N content is high, coarse nitrides may form in the steel, reducing its bendability and hole-expanding properties. Therefore, the N content should be 0.0150% or less. Preferably, the N content is 0.0120% or less, 0.0100% or less, 0.0080% or less, 0.0060% or less, or 0.0050% or less. The N content may be 0%, but reducing the N content to an extreme degree will increase the cost of nitrogen removal. For this reason, from an economic standpoint, the N content may be 0.0001% or more, 0.0003% or more, or 0.0005% or more.

[0030] [O: 0.0100% or less] O (oxygen) is an element contained as an impurity, and if the O content is high, coarse oxides may form in the steel, reducing its bendability and hole-expanding properties. Therefore, the O content should be 0.0100% or less. Preferably, the O content is 0.0080% or less, 0.0060% or less, or 0.0050% or less. The O content may be 0%, but extremely low O content increases manufacturing costs. For this reason, from the viewpoint of manufacturing costs, the O content may be 0.0001% or more, 0.0003% or more, or 0.0005% or more.

[0031] The basic chemical composition of the steel sheet according to the embodiment of the present invention is as described above. Furthermore, the steel sheet may optionally contain at least one of the following elements in place of a portion of the remaining Fe.

[0032] [Cr: 0-1.00%, Mo: 0-1.00%, Cu: 0-1.00%, Ni: 0-1.00%, Co: 0-1.00%, W: 0-1.00%, Ta: 0-1.000%, Sn: 0-1.00%, Sb: 0-0.50%, Nb: 0-0.200%, and V: 0-1.00%] Cr (chromium), Mo (molybdenum), Cu (copper), Ni (nickel), Co (cobalt), W (tungsten), Ta (tantalum), Sn (tin), Sb (antimony), Nb (niobium), and V (vanadium) are all elements that are effective in increasing the strength of steel sheets. The content of these elements may be 0%, but in order to obtain such an effect, at least one of these elements may be included in the steel sheet as needed. However, excessive amounts of these elements may lead to saturation of the effect and an increase in manufacturing costs. Therefore, the content of Cr, Mo, Cu, Ni, Co, W, Sn, and V should be 1.00% or less, and may be 0.60%, 0.50%, 0.30%, or 0.20%, respectively. Similarly, the Ta content should be 1.000% or less, and may be 0.600%, 0.500%, 0.30%, or 0.20%, respectively. Similarly, the Sb content should be 0.50% or less, and may be 0.30%, 0.10%, or 0.05%, respectively. Similarly, the Nb content should be 0.200% or less, and may be 0.100%, 0.060%, or 0.040%, respectively. For the lower limits of these elements, for example, the content of Cr, Mo, Cu, Ni, Co, W, Sn, Sb, and V may be 0.001% or more, or 0.01% or more. Similarly, the content of Ta and Nb may be 0.001% or more, or 0.005% or more.

[0033] [As: 0-0.100%] Arsenic (As) is an effective element for improving corrosion resistance. The As content may be 0%, but to obtain such an effect, the As content is preferably 0.001% or more, and may be 0.005% or more, or 0.010% or more. On the other hand, if the As content is excessive, the effect will saturate, and including more As than necessary in the steel sheet will lead to an increase in manufacturing costs. Therefore, the As content should be 0.100% or less, and may be 0.050% or less, 0.030% or less, or 0.020% or less.

[0034] [Zn: 0-1.000%, Ca: 0-0.0100%, Mg: 0-0.0100%, Zr: 0-0.0100%, Hf: 0-0.0100%, Bi: 0-0.0100%, and REM: 0-0.0100%] Zinc (Zn) is an effective element for controlling the shape of inclusions in steel, calcium (Ca), magnesium (Mg), zirconium (Zr), hafnium (Hf), and rare earth metals (REM) are elements that contribute to the fine dispersion of inclusions in steel, and bismuth (Bi) is an element that reduces the microsegregation of substitutional alloy elements such as Mn and Si in steel. The content of these elements may be 0%, but since each contributes to improving the workability of the steel sheet, at least one of these elements may be included in the steel sheet as needed. However, excessive amounts of these elements may lead to saturation of the effect and an increase in manufacturing costs. Therefore, the Zn content should be 1.000% or less, and may also be 0.500% or less, 0.200% or less, 0.100% or less, 0.050% or less, or 0.020% or less. Similarly, the Ca, Mg, Zr, Hf, Bi, and REM content should be 0.0100% or less, and may also be 0.0080% or less, 0.0060% or less, or 0.0030% or less. For the lower limits of these elements, for example, the Zn content may be 0.001% or more, or 0.005% or more. Similarly, the Ca, Mg, Zr, Hf, Bi, and REM content may be 0.0001% or more, or 0.0005% or more, respectively. In this specification, REM refers to the collective term for 17 elements, including scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content is the total content of these elements.

[0035] In the steel sheet according to the embodiment of the present invention, the remainder other than the above-mentioned elements consists of Fe and impurities. Impurities are components that are mixed in during the industrial manufacture of steel sheets due to various factors in the manufacturing process, including raw materials such as ore and scrap, and components that are included in a range that does not affect the effects of the present invention.

[0036] The chemical composition of the steel sheet according to the embodiment of the present invention may be measured by a general analysis method. For example, the chemical composition of the steel sheet may be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES). C and S may be measured using combustion-infrared absorption method, N may be measured using inert gas fusion-thermal conductivity method, and O may be measured using inert gas fusion-nondispersive infrared absorption method.

[0037] [Steel structure] [Martensite: 85% or more, total of ferrite and bainite: 0 - 5%, and retained austenite: 1 - 8%] In the steel sheet according to the embodiment of the present invention, the steel structure in the cross-section at the 1 / 4 thickness position from the surface of the steel sheet contains, in area%, martensite: 85% or more, total of ferrite and bainite: 0 - 5%, and retained austenite: 1 - 8%.

[0038] In the embodiment of the present invention, martensite includes as-quenched martensite (fresh martensite) and tempered martensite. Martensite is a structure with a high dislocation density and is hard, thus contributing to the improvement of strength. When the amount of martensite is small, not only does the strength such as the tensile strength simply decrease, but the amount of bake hardening also decreases. This is because when the amount of martensite is small, the number of dislocations where invasive elements (mainly carbon) adhere during the painting baking process decreases, and thus a sufficient amount of bake hardening cannot be obtained. Therefore, in order to obtain the desired tensile strength and amount of bake hardening, particularly the desired amount of bake hardening in the strain-free part, the area ratio of martensite is set to 85% or more. The area ratio of martensite may be 88% or more, 90% or more, 92% or more, or 95% or more. The upper limit is not particularly limited. For example, the area ratio of martensite may be 99% or less. From the perspective of improving uniform elongation, the lower the area ratio of martensite, the more preferable it is. For example, it may be 98% or less, 97% or less, or 96% or less.

[0039] Of ferrite and bainite, ferrite, in particular, has excellent ductility and contributes to improved elongation. However, if the total area ratio of ferrite and bainite becomes too high, the area ratio of martensite decreases, and therefore the desired strength cannot be achieved. For this reason, the total area ratio of ferrite and bainite should be 5% or less. The total area ratio of ferrite and bainite may be 4% or less, 3% or less, or 2% or less. The total area ratio of ferrite and bainite may be 0% or 1% or more.

[0040] Retained austenite improves the uniform elongation of the steel sheet through the TRIP effect, where it transforms into martensite through work-induced transformation during deformation of the steel sheet. To fully obtain this effect, the area ratio of retained austenite should be 1% or more. From the viewpoint of further improving uniform elongation, a higher area ratio of retained austenite is preferable, for example, it may be 2% or more, 3% or more, or 4% or more. However, if the area ratio of retained austenite becomes too high, the area ratio of martensite decreases, and therefore the desired strength may not be achieved. For this reason, the area ratio of retained austenite should be 8% or less. The area ratio of retained austenite may be 7% or less, 6% or less, or 5% or less.

[0041] [Residual structure: 0-10% in total] Residual structures other than martensite, ferrite, bainite, and retained austenite may account for 0% of the area. If residual structures are present, they are pearlite, etc. From the viewpoint of ensuring the above-mentioned effects based on martensite, ferrite, bainite, and retained austenite, the area ratio of the residual structures is preferably 10% or less in total, and may be, for example, 8% or less, 6% or less, 4% or less, 3% or less, or 2% or less. On the other hand, the area ratio of the residual structures may be 0.5% or more, or 1% or more.

[0042] [Identification of Steel Structure and Calculation of Area Ratio] The identification of the steel structure and the calculation of the area ratio are performed by using a secondary electron image taken with a FE-SEM (field emission scanning electron microscope, for example, JSM-7200F manufactured by JEOL Ltd., measured at an acceleration voltage of 15 kV) and X-ray diffraction method. First, a sample is taken with a plate thickness cross-section in a direction perpendicular to the plate surface of the steel plate as the observation surface. After mechanically polishing the observation surface to a mirror finish, etching is performed using nital solution. Next, in one or more observation fields in the range of 1 / 8 thickness to 3 / 8 thickness centered on the 1 / 4 thickness position from the surface of the steel plate on the observation surface, a secondary electron image is taken at a magnification of 3000 times for a total area of 2.0×10 -9 m 2 or more. From the obtained secondary electron image, the area ratios of the total of martensite and retained austenite, pearlite (if present), and the total of ferrite and bainite are measured respectively. First, a region with high brightness and no underlying structure revealed by etching is judged as fresh martensite and retained austenite. Next, a region with an underlying structure and multiple cementites with different elongation directions precipitated is judged as tempered martensite. Next, a region where cementite is precipitated in a lamellar shape is judged as pearlite (the total of pearlite and cementite). The remainder other than the above structures is judged as ferrite and bainite. For reference, a region with low brightness and no underlying structure can be judged as ferrite, and a region not corresponding to any of the above can be judged as bainite. The area ratio of each structure identified in this way is calculated by the point counting method. For the area ratio of martensite, it can be obtained by subtracting the area ratio of retained austenite determined by the X-ray diffraction method described below from the total area ratio of tempered martensite, fresh martensite, and retained austenite.

[0043] The area fraction of retained austenite is measured by X-ray diffraction. Specifically, the steel plate is polished mechanically and chemically from the surface to a depth of 1 / 4 in the thickness direction. Then, using MoKα1 rays as characteristic X-rays on the polished sample, the integral intensity ratio of the diffraction peaks of the bcc phase (200) and (211) and the fcc phase (200), (220), and (311) is used to calculate the structural fraction of retained austenite, which is then defined as the area fraction of retained austenite.

[0044] [Standard deviation of dislocation density / Average value of dislocation density: 0.05 to 0.50] In the steel sheet according to the embodiment of the present invention, when the dislocation density of a total of 12 points obtained by measuring the dislocation density at four points corresponding to each vertex of a square with sides of 1 mm in a plane parallel to the surface at positions 1 / 8 thickness, 2 / 8 thickness, and 3 / 8 thickness from the surface is used as the population, the standard deviation of dislocation density / average value of dislocation density is controlled to 0.05 to 0.50. In a steel structure mainly composed of martensite, that is, a steel structure containing 85% or more martensite by area %, by controlling the standard deviation of dislocation density / average value of dislocation density within this range, as explained above, the dislocation density can be made uniform at a high level, and as a result, even in strain-free areas where sufficient strain has not been introduced by press forming, etc., a relatively high dislocation density is present, making it possible to significantly improve the amount of bake hardening during paint baking. From the viewpoint of further increasing the amount of curing by baking, a lower standard deviation of dislocation density / average value of dislocation density is preferable, for example, it may be 0.45 or less, 0.40 or less, 0.35 or less, 0.30 or less, 0.25 or less, or 0.20 or less. Although a lower limit is preferable, it is difficult to reduce the standard deviation of dislocation density to 0 (zero). Therefore, the standard deviation of dislocation density / average value of dislocation density should be 0.05 or more, for example, 0.06 or more, 0.08 or more, 0.10 or more, or 0.12 or more.

[0045] [Method for Determining the Standard Deviation / Average Value of Dislocation Density] The standard deviation / average value of dislocation density is determined by the following method. Samples are prepared by mechanically grinding and chemically polishing steel plates to 1 / 8 thickness, 2 / 8 thickness, and 3 / 8 thickness, and X-ray diffraction is performed. Here, four points on a grid with 1.0 mm spacing in the vertical and horizontal directions are measured at each thickness position. The X-ray irradiation diameter is focused using a 1.0 mmφ collimator. The dislocation density at each measurement point is determined from the obtained X-ray diffraction profile using the modified Williamson-Hall method and the modified Warren-Averbach method. Specifically, the dislocation density is determined according to the method described in ISIJ Int. vol. 50 (2010) pp. 875-882. The average value and standard deviation are determined from the dislocation densities of the 12 points obtained. Finally, the ratio of the standard deviation of the dislocation density to the average of the dislocation density is determined from the average and standard deviation of the obtained dislocation density.

[0046] [Area ratio at which ΔMs is 30 or more: 10% or less at all positions at the 1 / 8 thickness, 2 / 8 thickness, and 3 / 8 thickness positions] In a preferred embodiment of the present invention, the steel sheet is controlled such that when the concentrations of Mn, Si, Cr, Mo, and Ni at each of the 1 / 8 thickness, 2 / 8 thickness, and 3 / 8 thickness positions from the surface are measured by EPMA (electron probe microanalyzer), the area ratio at which ΔMs represented by the following formula 1 is 30 or more is 10% or less at all positions at the 1 / 8 thickness, 2 / 8 thickness, and 3 / 8 thickness positions. ΔMs = |33 × ( - [Mn]) + 17 × (<Cr> - [Cr]) + 17 × (<Ni> - [Ni]) + 7.5 × (<Si> - [Si]) + 21 × ( - [Mo])| ...Equation 1 Here, , <Si>, <Cr>, , and <Ni> are the concentrations (mass%) of Mn, Si, Cr, Mo, and Ni measured by EPMA, and [Mn], [Si], [Cr], [Mo], and [Ni] are the Mn, Si, Cr, Mo, and Ni content (mass%) in the steel sheet.

[0047] The ΔMs expressed in Equation 1 above represents the absolute value of the difference between the martensitic transformation initiation temperature at the 1 / 8 thickness, 2 / 8 thickness, and 3 / 8 thickness positions from the surface and the martensitic transformation initiation temperature calculated from the (average) chemical composition of the steel sheet. Therefore, the fact that the area ratio in which ΔMs is 30 or more is 10% or less at all of the 1 / 8 thickness, 2 / 8 thickness, and 3 / 8 thickness positions means that local variations in the martensitic transformation initiation temperature within the steel structure are sufficiently suppressed. As a result, local variations in dislocation density within the steel structure can be suppressed, making it possible to further significantly improve the amount of baked hardening during paint curing.

[0048] More specifically, if the initiation temperature of martensitic transformation is high, auto-tempering will occur between the completion of the transformation and the cooling to room temperature. As a result, some of the dislocations introduced during the transformation to martensite will annihilate each other through thermal vibrations as they go through this temperature history. Therefore, it is thought that the dislocation density in steel structures that undergo martensitic transformation at higher temperatures will be relatively low, and the dislocation density in steel structures that undergo martensitic transformation at lower temperatures will be relatively high. Consequently, even within the same martensitic structure, regions with high and low dislocation densities will be formed due to variations in the martensitic transformation initiation temperature. Therefore, in order to reduce such local variations in dislocation density, it is important to reduce the local variation in the martensitic transformation initiation temperature. In other words, by reducing the local variation in the martensitic transformation initiation temperature, it is possible to reduce the local variation in dislocation density within the transformed martensitic structure, and as a result, it is thought that the amount of bake-hardened paint during baking can be significantly improved.

[0049] As a result of further investigation from this perspective, the inventors have found that by appropriately controlling the coiling temperature in the hot rolling process and the maximum temperature reached after coiling, as will be explained in detail later in relation to the manufacturing method, when the concentrations of Mn, Si, Cr, Mo, and Ni at the 1 / 8 thickness, 2 / 8 thickness, and 3 / 8 thickness positions from the surface of the steel sheet are measured by EPMA, the area ratio at which ΔMs represented by the above formula 1 is 30 or more can be controlled to be 10% or less at all of the 1 / 8 thickness, 2 / 8 thickness, and 3 / 8 thickness positions, thereby reducing localized dislocation density variations within the steel structure, and as a result, the amount of baked hardening during paint baking can be significantly improved. From the viewpoint of further improving the amount of baked hardening, it is preferable that the area ratio at which ΔMs is 30 or more is small, for example, it may be 8% or less, 6% or less, 5% or less, 4% or less, or 3% or less at all of the 1 / 8 thickness, 2 / 8 thickness, and 3 / 8 thickness positions. The lower limit is not particularly limited, but for example, the area ratio in which ΔMs is 30 or more may be 0.5% or more or 1% or more at all of the 1 / 8 thickness positions, 2 / 8 thickness positions, and 3 / 8 thickness positions.

[0050] [Method for Determining the Area Ratio for which ΔMs is 30 or More] The area ratio for which ΔMs, expressed in Equation 1 above, is 30 or more is determined using an FE-EPMA (electron emission electron beam microanalyzer) as follows. A sample is taken from the cross-section of the steel plate perpendicular to the plate surface, and the observation surface is mechanically polished to a mirror finish. A total of 500,000 points are measured in rectangular areas of 50 μm in the thickness direction and 100 μm in the width direction, centered at the 1 / 8 thickness, 2 / 8 thickness, and 3 / 8 thickness positions of the sample, with a pitch of 0.10 μm. ΔMs at each measurement point is calculated using Equation 1 from the Mn, Si, Cr, Mo, and Ni concentrations at each measurement point, and the number of measurement points for which ΔMs is 30 or more is determined. This number is then divided by the total number of measurement points to obtain the area ratio. For measurement, for example, a JEOL JXA-8530F is used, with an acceleration voltage of 15 kV. The characteristic X-ray spectroscopy method is wavelength-dispersive. The spectroscopic crystal should be selected appropriately depending on the element being analyzed; for example, LiF can be used for Mn and TAP for Si. The output Si and Mn concentrations are values ​​converted from the detection intensity of characteristic X-rays to mass percent using the program included with the JXA-8530, assuming that calibration with standard materials has been performed.

[0051] [Plate Thickness] The steel plate according to the embodiment of the present invention is not particularly limited, but generally has a plate thickness of 0.6 to 6.0 mm. For example, the plate thickness may be 1.0 mm or more, 1.2 mm or more, or 1.4 mm or more, and / or 5.0 mm or less, 4.0 mm or less, 3.0 mm or less, or 2.5 mm or less.

[0052] [Plating Layer] The steel sheet according to the embodiment of the present invention may be a plated steel sheet having a plating layer on at least one surface, preferably both surfaces. The plating layer is not particularly limited, but may be, for example, a hot-dip galvanized layer (GI), an alloyed hot-dip galvanized layer (GA), or an electroplated galvanized layer (EG). These zinc plating layers may have any composition known to those skilled in the art, and may contain additive elements such as Al and Mg in addition to Zn. Furthermore, the amount of the plating layer is not particularly limited and may be a general amount.

[0053] As described above, the steel sheet according to the embodiment of the present invention achieves high strength, more specifically 1470 MPa or higher, through a steel structure mainly composed of martensite, while also improving elongation and significantly improving the amount of bake-hardened material during paint curing, even in strain-free areas where sufficient strain has not been introduced by press forming or the like. Therefore, the steel sheet according to the embodiment of the present invention is particularly useful for use in parts in technical fields that require a high level of both high strength and formability, and is especially useful for use in automotive parts. In a preferred embodiment, an automotive part (including not only passenger cars but also parts used in large vehicles such as trucks) containing the steel sheet according to the embodiment of the present invention is provided. Examples of automotive parts include structural parts such as front pillars, center pillars, side sills, and cross members, as well as bumpers, and other structural and reinforcing parts that require strength. These parts only need to contain the steel sheet according to the embodiment of the present invention in at least a portion of them, and therefore at least a portion of these parts will satisfy the characteristics of the steel sheet described above. In forming processes such as press forming, the characteristics of the steel sheet do not change significantly before and after forming in areas of the steel sheet that do not come into direct contact with the mold, or in areas of the steel sheet that come into direct contact with the mold but undergo relatively little processing.

[0054] [Mechanical Properties] [Tensile Strength (TS) and Uniform Elongation (uEL)] According to the steel sheet having the above chemical composition and steel structure, high tensile strength, specifically a tensile strength (TS) of 1470 MPa or more, can be achieved. The tensile strength is preferably 1500 MPa or more, 1600 MPa or more, 1700 MPa or more, or 1800 MPa or more. There is no particular upper limit, but for example, the tensile strength may be 2300 MPa or less, 2200 MPa or less, or 2000 MPa or less. Furthermore, according to the steel sheet according to the embodiment of the present invention, despite having such a very high tensile strength, improved elongation, specifically a uniform elongation (uEL) of 4.0% or more, can be achieved. For example, the uniform elongation may be 4.5% or more, 5.0% or more, 5.5% or more, or 6.0% or more. There is no particular upper limit, but for example, the uniform elongation may be 10.0% or less, 8.0% or less, or 7.0% or less. Tensile strength and uniform elongation are determined by taking a JIS No. 5 test specimen from a direction (direction C) where the longitudinal direction of the test specimen is preferably parallel to the direction perpendicular to the rolling direction of the steel sheet, and performing a tensile test in accordance with JIS Z 2241:2022. If the rolling direction of the steel sheet cannot be determined, a JIS No. 5 test specimen may be taken from any direction within the surface of the steel sheet. If it is difficult to take a JIS No. 5 test specimen, a JIS No. 13B test specimen may be used, or a small test specimen with a similar shape to a JIS No. 13B test specimen may be used.

[0055] [Baking Hardness (0% BH)] According to the steel sheet having the above chemical composition and steel structure, despite having the above-mentioned very high tensile strength, improved elongation can be achieved, and the amount of baking hardness during paint baking can be significantly improved even in strain-free areas where sufficient strain has not been introduced by press forming, etc. For example, when heat treatment is performed at 170°C for 20 minutes without adding pre-strain, a baking hardness (0% BH) of 70 MPa or more can be achieved. For example, 0% BH may be 80 MPa or more, 90 MPa or more, 100 MPa or more, or 110 MPa or more. There is no particular upper limit, but for example, 0% BH may be 200 MPa or less, 180 MPa or less, or 170 MPa or less. The amount of bake-hardened steel (0% BH) is determined in the same way as in TS measurement. A JIS No. 5 test specimen is taken from a direction (C direction) where the longitudinal direction of the test specimen is preferably parallel to the direction perpendicular to the rolling direction of the steel sheet. Then, the test specimen is heat-treated at 170°C for 20 minutes without applying pre-strain, and the increase in yield point before and after the heat treatment is measured. If the rolling direction of the steel sheet cannot be determined, a JIS No. 5 test specimen may be taken from any direction within the surface of the steel sheet. If it is difficult to take a JIS No. 5 test specimen, a JIS No. 13B test specimen may be used, or a small test specimen with a similar shape to a JIS No. 13B test specimen may be used.

[0056] <Method for Manufacturing Steel Sheets> Next, preferred methods for manufacturing steel sheets according to embodiments of the present invention will be described. The following description is intended to illustrate characteristic methods for manufacturing steel sheets according to embodiments of the present invention, and is not intended to limit the steel sheets to those manufactured by the manufacturing methods described below.

[0057] A method for manufacturing a steel sheet according to an embodiment of the present invention includes: (A) a hot rolling step that includes hot rolling a slab having the chemical composition described in (1) above, then winding the obtained hot-rolled steel sheet and cooling, satisfying the following conditions (A1) and (A2): (A1) performing three or more rolling passes with a reduction ratio of 20% or more in the temperature range of 950 to 1050°C, and having a cumulative reduction ratio of 60% or more in the temperature range of 950 to 1050°C, and (A2) having a cumulative reduction ratio of 10 to 50% in the temperature range of 850 to 950°C; (B) a cold rolling step of pickling the hot-rolled steel sheet and then cold rolling it with a reduction ratio of 30 to 75%; (C) a heat treatment step of heating the obtained cold-rolled steel sheet to a maximum heating temperature of Ac3 to 950°C and then cooling, satisfying the following conditions (C1) to (C3). (C1) The average cooling rate between Ms and 700°C is 40°C / s or more, the average cooling rate between Ms-200 and Ms°C is 20°C / s or less, and the cooling stop temperature is 100 to 220°C. (C2) At least one bending process is performed between Ms-200 and Ms°C using a roll with a diameter of 1700 mm or less. (C3) After cooling stops, the cold-rolled steel sheet is held in a temperature range of 150 to 300°C for 200 to 1000 seconds. (D) A skin pass rolling process is performed on the cold-rolled steel sheet under the conditions of elongation: 0.5 to 3.0%, back tension / YP: 0.05 to 0.20, and work roll diameter: 400 to 550 mm, wherein YP is the yield point (MPa) of the cold-rolled steel sheet at the skin pass rolling entry side.

[0058] [(A) Hot Rolling Process] First, a slab having the chemical composition described above in relation to a steel sheet is hot-rolled in a hot rolling process, and the resulting hot-rolled steel sheet is then wound up and cooled. The hot rolling process must satisfy the following conditions (A1) and (A2): (A1) Three or more rolling passes are performed in the temperature range of 950 to 1050°C with a reduction ratio of 20% or more, and the cumulative reduction ratio in the temperature range of 950 to 1050°C is 60% or more, and (A2) The cumulative reduction ratio in the temperature range of 850 to 950°C is 10 to 50%.

[0059] The slabs used contain a relatively large amount of alloying elements, and in particular, slabs after continuous casting contain coarse Ti carbides. Therefore, it is necessary to solid-solve the alloying elements in the slab, and in particular, it is necessary to sufficiently dissolve the Ti. Accordingly, the slab is heated before hot rolling, and it is preferable that the heating temperature be 1200°C or higher. There is no particular upper limit, but if the heating temperature of the slab is too high, the yield will decrease due to scale-off. For this reason, it is preferable that the heating temperature of the slab be 1300°C or lower. From the viewpoint of manufacturability, it is preferable to cast the slab by the continuous casting method, but it may also be manufactured by the ingot-making method or the thin-slab casting method.

[0060] [(A1) Rolling passes of 20% or more reduction at 950-1050°C: 3 or more times, and cumulative reduction at 950-1050°C: 60% or more] In this manufacturing method, the heated slab is subjected to 3 or more rolling passes of 20% or more reduction at a temperature of 950-1050°C, and the cumulative reduction at a temperature of 950-1050°C is controlled to be 60% or more. By performing such rolling, the austenite grains can be made uniform through repeated recrystallization, and in the final steel structure, it is possible to control the standard deviation of dislocation density / average value of dislocation density to be within the range of 0.05 to 0.50.

[0061] While not intended to be bound by any particular theory, it is believed that the onset temperature of martensitic transformation during the subsequent heat treatment process varies depending on the grain size of the austenite grains. More specifically, austenite grains with larger grain sizes have a higher onset temperature of martensitic transformation compared to austenite grains with smaller grain sizes, and auto-tempering proceeds between the completion of the transformation and cooling to room temperature. As a result, some of the dislocations introduced during the transformation to martensite annihilate each other through thermal vibrations as they go through this temperature history. On the other hand, austenite grains with smaller grain sizes undergo martensitic transformation at a lower temperature, so the dislocation density in the resulting martensitic structure is thought to be higher than that in the martensitic structure transformed from austenite grains with larger grain sizes. In this case, the variation in dislocation density in the final steel structure becomes significant, and as a result, it becomes impossible to control the standard deviation of dislocation density / average value of dislocation density within the range of 0.05 to 0.50. Therefore, in order to suppress or reduce such variations in dislocation density in the steel microstructure and control the standard deviation of dislocation density / average value of dislocation density to be within the range of 0.05 to 0.50, it is extremely important to reduce the variations in austenite grain size before martensitic transformation. Accordingly, in this manufacturing method, sufficient reduction is applied in the temperature range of 950 to 1050°C, which corresponds to the recrystallization temperature range. More specifically, three or more rolling passes with a reduction ratio of 20% or more are performed in the temperature range of 950 to 1050°C, and the cumulative reduction ratio in the temperature range of 950 to 1050°C is controlled to be 60% or more. This allows for repeated recrystallization to standardize the austenite grains and enables the realization of more uniform austenite grain size.

[0062] In this specification, the cumulative reduction ratio in the temperature range of 950 to 1050°C refers to the reduction ratio determined by the thickness of the plate at 950°C relative to the thickness of the plate at 1050°C. If there are fewer than three rolling passes with a reduction ratio of 20% or more in the temperature range of 950 to 1050°C, and / or if the cumulative reduction ratio in the temperature range of 950 to 1050°C is less than 60%, recrystallization cannot proceed sufficiently. As a result, the austenite grains cannot be sufficiently uniformized, and it may become impossible to control the standard deviation of the dislocation density / average value of the dislocation density in the final steel structure to be within the desired range. There is no particular upper limit, but for example, the reduction ratio of each rolling pass in the temperature range of 950 to 1050°C may be 60% or less or 50% or less. Similarly, there is no particular upper limit, but for example, the number of rolling passes with a reduction ratio of 20% or more in the temperature range of 950 to 1050°C may be five or less or four or less. Furthermore, there is no particular upper limit to the cumulative reduction rate in the temperature range of 950 to 1050°C; for example, the cumulative reduction rate in that temperature range may be 90% or less, or 80% or less.

[0063] [(A2) Cumulative reduction ratio in the temperature range of 850-950°C: 10-50%] In this manufacturing method, sufficient rolling is performed in the recrystallization temperature range of 950-1050°C as described above, and the steel sheet, which has therefore undergone sufficient recrystallization, is then rolled in the temperature range of 850-950°C to a cumulative reduction ratio of 10-50%. In this specification, the cumulative reduction ratio in the temperature range of 850-950°C refers to the reduction ratio determined by the thickness of the sheet at 850°C relative to the thickness of the sheet at 950°C. Here, since 850-950°C corresponds to the non-recrystallization temperature range, if excessive rolling is performed in this temperature range to a reduction ratio exceeding 50%, recrystallization will not occur, resulting in an uneven austenite grain size. As a result, it may become impossible to control the standard deviation of dislocation density / average value of dislocation density in the final steel structure to be within the desired range. Therefore, in this manufacturing method, the cumulative reduction ratio in the temperature range of 850 to 950°C is controlled to be within the range of 10 to 50%, thereby suppressing excessive rolling and enabling the realization of a more uniform austenite grain size.

[0064] [(A3) Winding temperature: 450 to 580°C, and maximum temperature reached after winding: 600°C or less] In a preferred embodiment of the hot rolling process in this manufacturing method, in addition to the conditions (A1) and (A2) above, the winding after hot rolling is controlled to satisfy the conditions that the winding temperature is 450 to 580°C and the maximum temperature reached after winding is 600°C or less. Phase transformation continues even after the hot-rolled steel sheet obtained by hot rolling is wound. As a result, heat is generated due to such phase transformation, and the temperature of the hot-rolled coil rises. Here, if the maximum temperature reached after winding exceeds 600°C, distribution of elements such as Mn, Si, Cr, Mo, and Ni occurs in the steel. In this case, localized variations in the martensitic transformation onset temperature occur in the steel structure. As a result, in the final steel structure, when the concentrations of Mn, Si, Cr, Mo, and Ni at the 1 / 8 thickness, 2 / 8 thickness, and 3 / 8 thickness positions from the surface of the steel sheet are measured by EPMA, it may become impossible to control the area ratio at which ΔMs, represented by the following formula 1, is 30 or more to be 10% or less at all of the aforementioned 1 / 8 thickness, 2 / 8 thickness, and 3 / 8 thickness positions. ΔMs = |33 × ( - [Mn]) + 17 × (<Cr> - [Cr]) + 17 × (<Ni> - [Ni]) + 7.5 × (<Si> - [Si]) + 21 × ( - [Mo])| ...Equation 1 Here, , <Si>, <Cr>, , and <Ni> are the concentrations (mass%) of Mn, Si, Cr, Mo, and Ni measured by EPMA, and [Mn], [Si], [Cr], [Mo], and [Ni] are the Mn, Si, Cr, Mo, and Ni content (mass%) in the steel sheet.

[0065] If the above equation 1 is not satisfied, the amount of bake-hardened paint during baking will be somewhat lower compared to when it is satisfied. Therefore, in a preferred embodiment of the hot rolling process in this manufacturing method, for example, by appropriately adjusting the amount of cooling water during winding, the winding temperature is controlled to be within the range of 450 to 580°C, while the maximum temperature reached after winding is controlled to be 600°C or lower. As a result, in the final steel structure, by combining the control of the standard deviation / average value of the dislocation density and the reduction of local martensitic transformation onset temperature based on equation 1, it is possible to further significantly improve the amount of bake-hardened paint during baking. The lower limit of the maximum temperature reached after winding is not particularly limited, but for example, the maximum temperature reached after winding may be 470°C or higher or 500°C or higher.

[0066] [(B) Cold Rolling Process] Next, the obtained hot-rolled steel sheet is pickled in the cold rolling process and then cold-rolled at a reduction ratio of 30-75%. Pickling can be carried out under conditions suitable for removing oxide scale, and may be done once or in multiple steps to ensure complete removal of oxide scale. The reduction ratio for cold rolling should be 30% or more, preferably 40% or more, taking into consideration the desired steel structure and sheet thickness. On the other hand, excessive reduction will result in excessive rolling load and increase the load on the cold rolling mill. For this reason, the reduction ratio should be 75% or less, preferably 70% or less or 60% or less.

[0067] [(C) Heat Treatment Process] Next, the obtained cold-rolled steel sheet is heated to a maximum heating temperature of Ac3 to 950°C in the heat treatment process, and then cooled. In addition, the heat treatment process must satisfy the following conditions (C1) to (C3): (C1) The average cooling rate between Ms and 700°C is 40°C / s or more, the average cooling rate between Ms-200 and Ms°C is 20°C / s or less, and the cooling stop temperature is 100 to 220°C; (C2) At least one bending process is performed between Ms-200 and Ms°C using a roll with a diameter of 1700 mm or less; and (C3) After cooling stops, the cold-rolled steel sheet is held in a temperature range of 150 to 300°C for 200 to 1000 seconds.

[0068] To ensure sufficient austenitization and obtain the desired microstructure through subsequent cooling, the cold-rolled steel sheet must be heated to a maximum heating temperature of Ac3 to 950°C. If austenitization is insufficient, a large amount of ferrite may be formed in the final microstructure, making it impossible to achieve the desired martensite area ratio. On the other hand, if the maximum heating temperature exceeds 950°C, the austenite will grow excessively, preventing the desired grain uniformity from being achieved. As a result, it may become impossible to control the standard deviation / average value of the dislocation density within the desired range in the final microstructure. In this manufacturing method, Ac3 (°C) is calculated based on the following formula 2. In the formula below, the mass percentage of the element is substituted for the element symbol. For elements that are not present, 0 mass% is substituted. Ac3(°C)=912-230.5×C+31.6×Si-20.4×Mn-39.8×Cu-18.1×Ni-14.8×Cr+16.8×Mo+100×Al...Formula 2

[0069] [(C1) Average cooling rate between Ms and 700°C: 40°C / s or more, average cooling rate between Ms-200 and Ms°C: 20°C or less, and cooling stop temperature: 100 to 220°C] After reaching the maximum heating temperature of Ac3 to 950°C, in order to obtain the desired steel structure, the average cooling rate between Ms and 700°C is controlled to be 40°C / s or more, the average cooling rate between Ms-200 and Ms°C is controlled to be 20°C or less, and the cooling stop temperature is controlled to be 100 to 220°C. If the average cooling rate between Ms and 700°C is less than 40°C / s, a relatively large amount of ferrite and / or bainite may be formed, and sufficient strength may not be achieved. Therefore, the average cooling rate between Ms and 700°C is set to be 40°C / s or more, preferably 44°C / s or more. There is no particular upper limit, but for example, the average cooling rate between Ms and 700°C may be 150°C / s or less. In this manufacturing method, Ms (°C) is calculated based on the following formula 3. Substitute the mass percentage of the element for the element symbol in the formula below. For elements not present, substitute 0 mass%. Ms (°C) = 561 - 474 × C - 33 × Mn - 17 × Cr - 17 × Ni - 21 × Mo - 7.5 × Si + 10 × Co ... Formula 3

[0070] If the average cooling rate between Ms-200 and Ms°C exceeds 20°C / s, the cooling rate will vary between the surface and interior of the steel sheet due to the relatively fast average cooling rate. As a result, a difference in dislocation density after martensitic transformation occurs in the thickness direction of the sheet, and it may become impossible to control the standard deviation of dislocation density / average dislocation density within the desired range in the final steel structure. Therefore, the average cooling rate between Ms-200 and Ms°C should be 20°C / s or less, preferably 18°C / s or less. The lower limit is not particularly limited, but for example, the average cooling rate between Ms-200 and Ms°C may be 10°C / s or more. Also, if the cooling stop temperature is below 100°C, the austenite cannot be sufficiently stabilized, and it becomes impossible to achieve the desired retained austenite area ratio in the final steel structure. Therefore, the cooling stop temperature should be 100°C or higher, preferably 140°C or higher. On the other hand, if the cooling stop temperature exceeds 220°C, insufficient cooling can lead to differences in dislocation density in the thickness direction after martensitic transformation, making it impossible to control the standard deviation of dislocation density / average dislocation density within the desired range in the final resulting steel microstructure. Therefore, the cooling stop temperature should be 220°C or lower, preferably 215°C or lower.

[0071] [(C2) Perform at least one bending process using a roll with a diameter of 1700 mm or less between Ms-200 and Ms°C] In addition to keeping the average cooling rate between Ms-200 and Ms°C below 20°C / s, it is necessary to perform at least one bending process using a roll with a diameter of 1700 mm or less within this temperature range. As described above, controlling the average cooling rate in this temperature range to below 20°C / s can reduce the difference in dislocation density after martensitic transformation in the thickness direction of the sheet. However, controlling the average cooling rate alone may not always be sufficient to sufficiently reduce or suppress the difference in dislocation density in the thickness direction of the sheet. Therefore, in this manufacturing method, by intentionally performing at least one bending process using a roll with a diameter of 1700 mm or less within the same temperature range, the dislocation density of the martensite transformed at a relatively high temperature is increased, and the difference in dislocation density between it and the martensite transformed at a lower temperature is further reduced or suppressed. As a result, the standard deviation of the dislocation density / average value of the dislocation density in the final steel structure can be controlled to be within the desired range. If the diameter of the roll exceeds 1700 mm, it becomes impossible to sufficiently introduce dislocations into the steel plate. There is no particular lower limit, but for example, the diameter of the roll may be 800 mm or more. Also, there is no particular upper limit, but for example, the number of bending operations using a roll with a diameter of 1700 mm or less between Ms-200 and Ms°C may be 3 or less, or 2 or less. Such bending operations can be achieved, for example, using hearth rolls in the downstream stage of a continuous annealing furnace.

[0072] [(C3) After cooling stops, the cold-rolled steel sheet is held at 150-300°C for 200-1000 seconds] After cooling stops, the cold-rolled steel sheet is held at a temperature of 150-300°C for 200-1000 seconds. This operation stabilizes the austenite, making it possible to achieve the desired retained austenite area ratio in the final resulting steel structure. If the holding temperature is less than 150°C and / or the holding time is less than 200 seconds, the austenite cannot be sufficiently stabilized, and the desired retained austenite area ratio cannot be achieved in the final resulting steel structure. Preferably, the holding temperature is 200°C or higher and the holding time is 250 seconds or higher.

[0073] On the other hand, if the holding temperature exceeds 300°C and / or the holding time exceeds 1000 seconds, the austenite will decompose, and similarly, the desired retained austenite area ratio will not be achieved in the final steel structure. Preferably, the holding temperature is 280°C or lower and the holding time is 500 seconds or lower.

[0074] [(D) Skin Pass Rolling Process] Finally, the cold-rolled steel sheet is subjected to skin pass rolling under the following conditions: elongation: 0.5–3.0%, back tension / YP: 0.05–0.20, and work roll diameter: 400–550 mm. By subjecting the cold-rolled steel sheet to such skin pass rolling, the desired retained austenite area ratio can be achieved in the final resulting steel structure, and the standard deviation of dislocation density / average of dislocation density can be controlled within the desired range. Here, the elongation is defined by the following equation 4 using the entry speed V1 (m / sec) and exit speed V2 (m / sec) of the cold-rolled steel sheet. Elongation = (V2–V1) / V1 × 100 …Equation 4

[0075] If the elongation rate is less than 0.5%, uniform strain cannot be introduced into the cold-rolled steel sheet, and the ratio of the standard deviation of dislocation density to the average of dislocation density in the final steel structure may exceed 0.50. On the other hand, if the elongation rate exceeds 3.0%, retained austenite may be transformed into martensite due to excessive skin pass rolling, and the area ratio of retained austenite in the final steel structure may be less than 1.0%. Preferably, the elongation rate is between 0.6% and 2.5%.

[0076] Furthermore, YP is the yield point (MPa) of the cold-rolled steel sheet at the skin pass rolling entry side. In this process, the back tension (MPa) applied in skin pass rolling is normalized by YP, i.e., the back tension / YP is controlled to be within the range of 0.05 to 0.20. This allows for the uniform introduction of strain into the cold-rolled steel sheet, and as a result, it becomes possible to control the standard deviation of dislocation density / average value of dislocation density within the desired range. On the other hand, if the back tension / YP is less than 0.05 or greater than 0.20, it is not possible to uniformly introduce strain into the cold-rolled steel sheet, and the standard deviation of dislocation density / average value of dislocation density in the final steel structure may exceed 0.50. YP can be determined from a cold-rolled steel sheet produced without skin pass rolling by taking a JIS No. 5 test specimen from the direction in which the longitudinal direction of the test specimen is parallel to the rolling direction of the steel sheet (L direction) and performing a tensile test in accordance with JIS Z 2241:2022. If a yield point is found, the lower yield point shall be defined as YP. If no yield point is found, the yield strength at which the plastic elongation reaches 0.2%, as read using the offset method specified in JIS Z 2241:2022, shall be defined as YP.

[0077] The diameter of the work rolls (WR diameter) used in skin pass rolling is also important for introducing uniform strain. If the work roll diameter is too small or too large, it will not be possible to introduce uniform strain into the cold-rolled steel sheet, and the standard deviation of the dislocation density / average value of the dislocation density will not be controllable within the desired range. Therefore, in this process, in order to control the standard deviation of the dislocation density / average value of the dislocation density within the range of 0.05 to 0.50, the work roll diameter is set to 400 to 550 mm, preferably 450 to 500 mm.

[0078] According to the steel sheet manufactured by the above manufacturing method, by configuring the steel structure of the steel sheet to contain 85% or more martensite and 1-8% retained austenite by area percent, it is possible to significantly improve the elongation of the steel sheet while achieving high strength, for example, a tensile strength of 1470 MPa or more. Furthermore, by a specific combination of controlled hot rolling, heat treatment, and skin pass rolling processes, when the dislocation density of a total of 12 points obtained at the 1 / 8 thickness position, 2 / 8 thickness position, and 3 / 8 thickness position from the surface of the steel sheet in the thickness direction is used as the population, the standard deviation of the dislocation density / average value of the dislocation density can be controlled to be within the range of 0.05 to 0.50. As a result, it is possible to significantly improve the amount of bake hardening during paint baking even in strain-free areas where sufficient strain has not been introduced by press forming, etc. Therefore, the steel sheet manufactured by the above manufacturing method is particularly useful in the automotive field, where a high level of both high strength and formability is required.

[0079] The steel sheet according to the embodiment of the present invention can be used, for example, as various automobile parts as described above. Therefore, in addition to the case of taking a sample from a wound coil, the sampling locations when taking a sample from such automobile parts will be described below.

[0080] [Sampling Locations] When sampling from a wound coil, the outermost edge of the coil may have a changed surface condition. Therefore, samples should be taken from the outermost edge (1st turn) of the coil, starting from the 3rd turn and beyond, avoiding the area within 100 mm of the widthwise end. On the other hand, when sampling from automotive parts, the following locations (i) to (iv) should be avoided: (i) within 20 mm of the toe of a spot weld, and within 20 mm of the bead toe of an arc / laser weld (ii) processed areas with a radius of curvature less than 15 mm, and within 5 mm of such processed areas (iii) the end within 5 mm of the cut end face of the part (iv) within 5 mm of any area where red rust is visible

[0081] The present invention will be described in more detail below with reference to examples, but the present invention is not limited in any way to these examples.

[0082] In the following examples, steel sheets according to the embodiment of the present invention were manufactured under various conditions, and the tensile strength (TS), uniform elongation (uEL), and bake hardening amount (0%BH) of the strain-free portion of the obtained steel sheets were investigated.

[0083] First, molten steel was cast using a continuous casting method to form slabs with various chemical compositions as shown in Table 1. Next, in the hot rolling process, these slabs were heated under the conditions shown in Table 2, then hot-rolled, and the resulting hot-rolled steel sheets were wound and cooled under the conditions shown in Table 2. The "Number of reduction passes at 950-1050°C" shown in Table 2 refers to the number of rolling passes with a reduction ratio of 20% or more in the temperature range of 950-1050°C, and the actual reduction ratios were 20-30%. Next, the resulting hot-rolled steel sheets were pickled and cold-rolled at the reduction ratios shown in Table 2. The resulting cold-rolled steel sheets were heated to the maximum heating temperature shown in Table 2 in the heat treatment process, and then cooled under the conditions shown in Table 2. During cooling, bending was performed the number of times shown in Table 2 using a roll with a diameter of 800 mm between Ms-200 and Ms°C. Finally, the cold-rolled steel sheet was subjected to skin pass rolling under the conditions shown in Table 2 to obtain a cold-rolled steel sheet with a thickness of 1.4 mm.

[0084]

[0085]

[0086]

[0087]

[0088] The properties of the obtained steel plates were measured and evaluated by the following method.

[0089] [Tensile Strength (TS) and Uniform Elongation (uEL)] Tensile strength (TS) and uniform elongation (uEL) were determined by taking a JIS No. 5 test specimen from the direction in which the longitudinal direction of the test specimen is parallel to the direction perpendicular to the rolling direction of the steel plate (direction C), and performing a tensile test in accordance with JIS Z 2241:2022.

[0090] [Baking Hardening Amount (0% BH)] The baking hardening amount (0% BH) was determined by taking a JIS No. 5 test specimen from the direction in which the longitudinal direction of the test specimen is parallel to the direction perpendicular to the rolling direction of the steel sheet (direction C), then heat-treating the test specimen at 170°C for 20 minutes without applying pre-strain, and measuring the increase in yield point before and after the heat treatment.

[0091] Steel sheets with a tensile strength (TS) of 1470 MPa or higher, a uniform elongation (uEL) of 4.0% or higher, and a bake hardening rate (0%BH) of 70 MPa or higher were evaluated as having high strength, improved elongation, and increased bake hardening rate in the strain-free area. The results are shown in Table 3.

[0092] In Table 3, "M," "α+B," and "Retained γ" represent the area percentages of "martensite," "ferrite and bainite," and "retained austenite," respectively. Furthermore, the "Standard Deviation / Mean Value of Dislocation Density" in Table 3 represents the standard deviation / mean value of dislocation density when the population consists of 12 dislocation densities obtained by measuring the dislocation density at four points corresponding to each vertex of a 1 mm square in a plane parallel to the surface at 1 / 8 thickness, 2 / 8 thickness, and 3 / 8 thickness positions from the steel plate surface. Furthermore, the "Area ratio for ΔMs≧30 (1 / 8t)", "Area ratio for ΔMs≧30 (2 / 8t)", and "Area ratio for ΔMs≧30 (3 / 8t)" in Table 3 represent the area ratios in which ΔMs, expressed by the following formula 1, is 30 or greater when the concentrations of Mn, Si, Cr, Mo, and Ni at positions 1 / 8 thickness, 2 / 8 thickness, and 3 / 8 thickness from the surface of the steel plate are measured by EPMA. ΔMs = |33 × ( - [Mn]) + 17 × (<Cr> - [Cr]) + 17 × (<Ni> - [Ni]) + 7.5 × (<Si> - [Si]) + 21 × ( - [Mo])| ...Equation 1 Here, , <Si>, <Cr>, , and <Ni> are the concentrations (mass%) of Mn, Si, Cr, Mo, and Ni measured by EPMA, and [Mn], [Si], [Cr], [Mo], and [Ni] are the Mn, Si, Cr, Mo, and Ni content (mass%) in the steel sheet.

[0093]

[0094] Referring to Tables 1-3, Comparative Example 11 showed a decrease in TS due to its low C content. Comparative Example 12, due to its high C content, experienced a decrease in steel toughness due to an excessive increase in strength, resulting in the steel plate fracturing before reaching maximum stress during the tensile test (premature fracture). Comparative Example 13 also experienced premature fracture due to a decrease in steel toughness caused by its high Si content. Comparative Example 14 showed a decrease in TS because its low Mn content prevented sufficient martensite formation. Furthermore, the insufficient martensite formation resulted in a reduced number of dislocations, leading to a decrease in 0%BH. Comparative Example 15 showed a decrease in 0%BH because its high Mn content resulted in a mixture of martensite formed at high and low temperatures due to Mn segregation during casting, leading to increased in-plane and thickness-direction variations in dislocation density. Comparative Example 16 had a low B content, which reduced its hardenability and prevented sufficient martensite formation, resulting in decreased TS and 0% BH. In Comparative Example 17, the high B content likely led to excessive boride formation in the steel, reducing the hardenability of the steel sheet. As a result, sufficient martensite was not formed, leading to decreased TS and 0% BH.

[0095] In Comparative Example 18, the maximum heating temperature in the heat treatment process was too low, resulting in insufficient austenitization. This led to the formation of a large amount of ferrite in the final steel structure, and the desired martensite area ratio could not be obtained. As a result, TS and 0%BH decreased. In Comparative Example 19, the average cooling rate between Ms and 700°C in the heat treatment process was too low, resulting in the formation of a relatively large amount of ferrite and bainite, and a decrease in TS. In Comparative Example 20, the average cooling rate between Ms-200°C and Ms°C in the heat treatment process was too high, which is thought to have caused a difference in the cooling rate between the surface and interior of the steel sheet, resulting in a difference in dislocation density after martensitic transformation in the thickness direction. As a result, the standard deviation of dislocation density / average value of dislocation density exceeded 0.50, and 0%BH decreased. In Comparative Example 21, the cooling stop temperature in the heat treatment process was too low, preventing sufficient stabilization of the austenite and the desired retained austenite area ratio from being obtained. As a result, uEL decreased. In Comparative Example 22, the cooling stop temperature in the heat treatment process was too high, resulting in insufficient cooling and a difference in dislocation density after martensitic transformation in the thickness direction of the plate. As a result, the standard deviation of dislocation density / average value of dislocation density exceeded 0.50, and the 0% BH decreased. In Comparative Example 23, bending was not performed between Ms-200 and Ms°C in the heat treatment process, which is thought to have prevented sufficient reduction of the difference in dislocation density in the thickness direction of the plate. As a result, the standard deviation of dislocation density / average value of dislocation density exceeded 0.50, and the 0% BH decreased.

[0096] In Comparative Example 24, the elongation rate in the skin pass rolling process was low, preventing uniform strain from being introduced into the cold-rolled steel sheet. As a result, the standard deviation of dislocation density / average value of dislocation density in the final steel structure exceeded 0.50, leading to a decrease in 0% BH. In Comparative Example 25, the high elongation rate likely caused excessive skin pass rolling, resulting in the transformation of retained austenite into martensite. Consequently, the area ratio of retained austenite fell to less than 1.0%, resulting in a decrease in uEL. In Comparative Example 26, the hot rolling process involved fewer than three rolling passes with a reduction ratio of 20% or more in the temperature range of 950-1050°C. This prevented sufficient recrystallization, resulting in insufficient granulation of the austenite grains. Consequently, the standard deviation of dislocation density / average value of dislocation density exceeded 0.50, leading to a decrease in 0% BH. In Comparative Example 27, the hot rolling process involved fewer than three rolling passes with a reduction ratio of 20% or more in the temperature range of 950-1050°C, and the cumulative reduction ratio in the same temperature range was less than 60%. As a result, recrystallization could not proceed sufficiently, and the austenite grains could not be sufficiently uniformly formed. Consequently, the standard deviation of the dislocation density / average value of the dislocation density exceeded 0.50, and the 0% BH decreased. In Comparative Example 28, the hot rolling process involved a reduction ratio of more than 50% in the temperature range of 850-950°C, which corresponds to the non-recrystallization temperature range. As a result, the standard deviation of the dislocation density / average value of the dislocation density in the final steel structure exceeded 0.50, and the 0% BH decreased.

[0097] In Comparative Example 30, it is believed that the austenite decomposed due to the high holding temperature during the heat treatment process. As a result, the area ratio of retained austenite was less than 1.0%, and the uEL decreased. In Comparative Example 31, it is believed that the austenite could not be sufficiently stabilized because the holding time in the temperature range of 150 to 300°C during the heat treatment process was too short. As a result, the area ratio of retained austenite was less than 1.0%, and the uEL decreased. In Comparative Example 33, it is believed that the austenite grew excessively due to the high maximum heating temperature during the heat treatment process, and the austenite grains could not be sufficiently uniformized. As a result, the standard deviation of dislocation density / average value of dislocation density exceeded 0.50, and the 0% BH decreased. In Comparative Example 35, it is believed that the austenite could not be sufficiently stabilized because the holding temperature during the heat treatment process was too low. As a result, the area ratio of retained austenite was less than 1.0%, and the uEL decreased. In Comparative Examples 36 and 37, the back tension / YP in the skin pass rolling process was outside the range of 0.05 to 0.20, which prevented uniform strain from being introduced into the cold-rolled steel sheet. As a result, the standard deviation of dislocation density / average value of dislocation density exceeded 0.50, and the 0%BH decreased. Similarly, in Comparative Examples 38 and 39, the work roll diameter (WR diameter) in the skin pass rolling process was outside the range of 400 to 550 mm. As a result, uniform strain could not be introduced into the cold-rolled steel sheet, and the standard deviation of dislocation density / average value of dislocation density exceeded 0.50, and the 0%BH decreased.

[0098] In contrast, in all the examples (inventive examples) of steel sheets, by having a predetermined chemical composition and appropriately controlling each condition in the manufacturing method, the steel structure can be configured to contain martensite: 85% or more, ferrite and bainite combined: 0-5%, and retained austenite: 1-8% by area percent, thereby achieving a tensile strength of 1470 MPa or more and a uniform elongation of 4% or more. Furthermore, in all the examples (inventive examples) of steel sheets, when the dislocation density obtained by measuring the dislocation density at four points corresponding to each vertex of a 1 mm square in a plane parallel to the surface at 1 / 8 thickness, 2 / 8 thickness, and 3 / 8 thickness positions from the surface of the steel sheet, is used as the population, the dislocation density can be made uniform at a high level by controlling the standard deviation of the dislocation density / average value of the dislocation density to within the range of 0.05 to 0.50, which in turn significantly improved 0% BH.

[0099] In particular, in Examples 1 to 10, 32, and 34, where the winding after hot rolling was controlled to satisfy the conditions that the winding temperature was 450 to 580°C and the maximum temperature reached after winding was 600°C or less, when the concentrations of Mn, Si, Cr, Mo, and Ni at the 1 / 8 thickness, 2 / 8 thickness, and 3 / 8 thickness positions from the steel sheet surface were measured by EPMA, the area ratio at which ΔMs represented by the above formula 1 was 30 or more was controlled to 10% or less at all of the 1 / 8 thickness, 2 / 8 thickness, and 3 / 8 thickness positions. As a result, in these examples, the 0% BH was 90 MPa or more, and compared to Example 29 (0% BH was 85 MPa), which did not satisfy the requirement that "the area ratio at which ΔMs is 30 or more is 10% or less at all of the 1 / 8 thickness, 2 / 8 thickness, and 3 / 8 thickness positions," the amount of bake hardening in the strain-free section was further improved.

Claims

1. In mass percent, C: 0.16-0.40%, Si: 0.001-2.00%, Mn: 0.50-2.00%, P: 0.050% or less, S: 0.0100% or less, Al: 0.001-1.000%, Ti: 0.001-0.100%, B: 0.0005-0.0050%, N: 0.0150% or less, O: 0.0100% or less, Cr: 0-1.00%, Mo: 0-1.00%, Cu: 0-1.00%, Ni: 0-1.00%, Co: 0-1.00%, W: 0-1.00%, Ta: 0-1.00%, Sn: 0-1.00% The chemical composition consists of Sb: 0-0.50%, Nb: 0-0.200%, V: 0-1.00%, As: 0-0.100%, Zn: 0-1.000%, Ca: 0-0.0100%, Mg: 0-0.0100%, Zr: 0-0.0100%, Hf: 0-0.0100%, Bi: 0-0.0100%, REM: 0-0.0100%, and the remainder being Fe and impurities. The microstructure at a thickness of 1 / 4 of the surface contains, by area percentage, martensite: 85% or more, total of ferrite and bainite: 0-5%, and retained austenite: 1-8%. A steel plate characterized in that, when the dislocation density is measured at four points corresponding to each vertex of a 1 mm square in a plane parallel to the surface at positions 1 / 8 thickness, 2 / 8 thickness, and 3 / 8 thickness from the surface, a total of 12 dislocation densities are used as the population, and the average value of the standard deviation of the dislocation density / dislocation density is 0.05 to 0.

50.

2. The steel sheet according to claim 1, characterized in that when the concentrations of Mn, Si, Cr, Mo, and Ni at positions 1 / 8 thickness, 2 / 8 thickness, and 3 / 8 thickness from the surface are measured by EPMA, the area ratio at which ΔMs represented by the following formula 1 is 30 or more is 10% or less at all of the aforementioned positions 1 / 8 thickness, 2 / 8 thickness, and 3 / 8 thickness. ΔMs = |33 × ( - [Mn]) + 17 × (<Cr> - [Cr]) + 17 × (<Ni> - [Ni]) + 7.5 × (<Si> - [Si]) + 21 × ( - [Mo])| ...Equation 1 Here, , <Si>, <Cr>, , and <Ni> are the concentrations (mass%) of Mn, Si, Cr, Mo, and Ni measured by EPMA, and [Mn], [Si], [Cr], [Mo], and [Ni] are the Mn, Si, Cr, Mo, and Ni content (mass%) in the steel sheet.

3. The steel plate according to claim 1 or 2, characterized in that it has a tensile strength of 1470 MPa or more.

4. The steel sheet according to any one of claims 1 to 3, characterized in that it has a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on at least one surface.

5. An automobile part characterized by comprising a steel plate as described in any one of claims 1 to 4.

6. (A) A hot rolling step comprising hot rolling a slab having the chemical composition described in claim 1, then winding the obtained hot-rolled steel sheet and cooling, satisfying the following conditions (A1) and (A2): (A1) Three or more rolling passes with a reduction ratio of 20% or more are performed in the temperature range of 950 to 1050°C, and the cumulative reduction ratio in the temperature range of 950 to 1050°C is 60% or more, and (A2) The cumulative reduction ratio in the temperature range of 850 to 950°C is 10 to 50%. (B) A cold rolling step comprising pickling the hot-rolled steel sheet and then cold rolling it with a reduction ratio of 30 to 75%. (C) A heat treatment step comprising heating the obtained cold-rolled steel sheet to a maximum heating temperature of Ac3 to 950°C and then cooling, satisfying the following conditions (C1) to (C3). A method for manufacturing a steel sheet according to any one of claims 1 to 5, characterized in that (C1) the average cooling rate between Ms and 700°C is 40°C / s or more, the average cooling rate between Ms-200 and Ms°C is 20°C / s or less, and the cooling stop temperature is 100 to 220°C, (C2) at least one bending process is performed between Ms-200 and Ms°C using a roll with a diameter of 1700 mm or less, and (C3) after cooling stops, the cold-rolled steel sheet is held in a temperature range of 150 to 300°C for 200 to 1000 seconds, and (D) the cold-rolled steel sheet is skin-pass rolled under the conditions of elongation: 0.5 to 3.0%, back tension / YP: 0.05 to 0.20, and work roll diameter: 400 to 550 mm, wherein YP is the yield point (MPa) of the cold-rolled steel sheet at the skin-pass rolling entry side.

7. The method for manufacturing a steel sheet according to claim 6, characterized in that the hot rolling step further satisfies the following condition (A3). (A3) The winding temperature is 450 to 580°C, and the maximum temperature reached after winding is 600°C or less.

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