Steel sheet and method for manufacturing same

Abstract

Provided are: a steel sheet having excellent tensile strength and improved bendability after plastic working; and a method for producing the steel sheet. A steel sheet according to the present invention has a tensile strength of 1180 MPa or more, has a prescribed chemical composition and steel structure, and has, in a surface layer part of the steel sheet, the luminous intensities B30, B140, and B150 of B at depth positions of 30 [mu] m, 140 [mu] m, and 150 [mu] m from the surface of the steel sheet when measured in the depth direction from the surface of the steel sheet by high-frequency glow discharge luminescence analysis satisfy B30 / B150lt, and the luminous intensities B30, B140, and B150 of B at depth positions of 30 [mu] m, 140 [mu] m, and 150 [mu] m from the surface of the steel sheet when measured in the depth direction from the surface of the steel sheet. B140 / B150 is greater than or equal to 0.90 and less than or equal to 1.10.

Description

Technical Field

[0001] The present invention relates to a steel plate and a method for manufacturing the same. Background Art

[0002] In recent years, with the need to improve automobile fuel efficiency in response to greenhouse gas emissions regulations as part of global warming countermeasures, the use of high-strength steel sheets has been increasing to reduce vehicle weight and ensure collision safety. In particular, demand for ultra-high-strength steel sheets with a tensile strength of 980 MPa or higher has been increasing.

[0003] Hot-dip galvanized steel sheets for automotive parts are required not only to have strength but also to have various workability properties required for part forming, such as press formability and weldability. Specifically, from the perspective of press formability, the steel sheets are required to have excellent bendability.

[0004] Patent Document 1 discloses a steel sheet in which B is mainly precipitated in the surface layer of the steel sheet and mainly dissolved in the interior of the steel sheet, thereby improving bendability.

[0005] Patent document 2 discloses a high-strength steel plate having excellent delayed fracture resistance in the cut end surface and the steel plate base material, wherein the plate has a single-phase martensite structure, the region having a KAM value (Kernel Average Misorientation value) of 1° or more accounts for more than 50%, and the maximum tensile residual stress in the surface region from the surface to a depth of 1 / 4 of the plate thickness is 80 MPa or less.

[0006] As a technology for improving the bending workability of high-strength steel sheets, for example, Patent Document 3 describes a high-strength cold-rolled steel sheet produced by decarburization, with the surface layer mainly composed of ferrite. Furthermore, Patent Document 4 describes an ultra-high-strength cold-rolled steel sheet produced by decarburization annealing, with a soft layer on the surface.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: International Publication No. 2017 / 002883

[0010] Patent Document 2: Japanese Patent Application Laid-Open No. 2015-155572

[0011] Patent Document 3: Japanese Patent Application Laid-Open No. 10-130782

[0012] Patent Document 4: Japanese Patent Application Laid-Open No. 5-195149 Summary of the Invention

[0013] Problems to be solved by the invention

[0014] Furthermore, high-strength steel sheets used in automotive components are required to resist breakage due to impact deformation after being formed into components. In particular, steel sheets used in automotive components require excellent bendability not just before press-forming but after plastic strain is introduced during press-forming. However, improving bendability after plastic strain has not necessarily been fully studied to date.

[0015] Therefore, an object of the present invention is to provide a steel sheet having excellent tensile strength and improved bendability after plastic working, and a method for producing the same.

[0016] Means for solving problems

[0017] The present inventors conducted extensive research to address the above-mentioned issues and discovered that forming a suitable deboronized layer in the surface layer can improve bendability after plastic working. The present invention is based on this understanding and includes the following aspects.

[0018] (Scheme 1)

[0019] A steel plate, characterized in that it is a steel plate,

[0020] The chemical composition of the steel plate is as follows in mass %:

[0021] C: 0.06~0.30%,

[0022] Si: 0.01-2.50%,

[0023] Mn: 1.00~3.50%,

[0024] Ti: 0.001~0.100%,

[0025] B: 0.0005~0.0050%,

[0026] P: 0.050% or less,

[0027] S: 0.0100% or less,

[0028] Al: 1.500% or less,

[0029] N: 0.010% or less,

[0030] O: 0.0100% or less,

[0031] Cr: 0-1.00%,

[0032] Mo: 0-1.00%,

[0033] Cu: 0-1.00%,

[0034] Ni: 0-1.00%,

[0035] Co: 0-1.00%,

[0036] W: 0~1.00%,

[0037] Sn: 0-1.00%,

[0038] Sb: 0-0.50%,

[0039] Nb: 0~0.200%,

[0040] V: 0~1.00%

[0041] As: 0~0.10%,

[0042] Zn: 0-1.00%,

[0043] Ca: 0~0.0100%,

[0044] Mg: 0~0.0100%,

[0045] Ce: 0~0.0150%,

[0046] Zr: 0~0.0100%,

[0047] La: 0~0.0150%,

[0048] Hf: 0~0.0100%,

[0049] Bi: 0~0.0100%,

[0050] REM other than Ce and La: 0 to 0.0100%, and

[0051] The rest: Fe and impurities,

[0052] The steel structure within the range of 1 / 8 to 3 / 8 of the thickness of the steel plate is expressed in area % as follows:

[0053] Ferrite: less than 30%,

[0054] Tempered martensite: more than 40%,

[0055] Total of retained austenite and fresh martensite: less than 15%,

[0056] Total of pearlite and cementite: less than 5%, and

[0057] The rest: bainite,

[0058] The surface layer of the steel sheet has a deboronized layer in which the luminescence intensity of B measured in the depth direction from the surface of the steel sheet by high-frequency glow discharge luminescence analysis satisfies the following formulas (1) and (2):

[0059] The tensile strength of the steel plate is 1180 MPa or higher.

[0060] B30 / B150<0.90 (1) 0.90≤B140 / B150≤1.10 (2)

[0061] B30: The luminous intensity of B at a depth of 30 μm from the surface of the steel sheet

[0062] B140: The luminous intensity of B at a depth of 140 μm from the surface of the steel sheet. B150: The luminous intensity of B at a depth of 150 μm from the surface of the steel sheet (Scheme 2).

[0063] The steel plate according to the above-mentioned solution 1 is characterized in that the luminescence intensity of C in the surface layer portion of the steel plate measured from the surface of the steel plate in the depth direction by the above-mentioned high-frequency glow discharge luminescence analysis satisfies the following equations (3) and (4).

[0064] C40 / C150>0.50 (3) 0.90≤C140 / C150≤1.10 (4)

[0065] C40: The luminous intensity of C at a depth of 40 μm from the surface of the steel sheet

[0066] C140: The luminous intensity of C at a depth of 140 μm from the surface of the steel sheet C150: The luminous intensity of C at a depth of 150 μm from the surface of the steel sheet (Scheme 3)

[0067] The steel sheet according to claim 1 or 2, wherein the tensile residual stress acting in a direction perpendicular to rolling on the surface of the steel sheet is 200 MPa or less.

[0068] (Scheme 4)

[0069] The steel sheet according to any one of the above-mentioned aspects 1 to 3 is characterized in that a hot-dip galvanized layer or an alloyed hot-dip galvanized layer is provided on the surface of the steel sheet.

[0070] (Scheme 5)

[0071] A method for manufacturing a steel plate, characterized in that it is a method for manufacturing a steel plate,

[0072] With the following processes:

[0073] Hot rolling step (a) of hot rolling a slab at a finishing temperature of 850-950°C to obtain a hot-rolled steel sheet, and then cooling the hot-rolled steel sheet to 450-650°C and coiling the hot-rolled steel sheet, wherein the chemical composition of the slab is as follows in mass %:

[0074] C: 0.06~0.30%,

[0075] Si: 0.01-2.50%,

[0076] Mn: 1.00~3.50%,

[0077] Ti: 0.001~0.100%,

[0078] B: 0.0005~0.0050%,

[0079] P: 0.050% or less,

[0080] S: 0.0100% or less,

[0081] Al: 1.500% or less,

[0082] N: 0.010% or less,

[0083] O: 0.0100% or less,

[0084] Cr: 0-1.00%,

[0085] Mo: 0-1.00%,

[0086] Cu: 0-1.00%,

[0087] Ni: 0-1.00%,

[0088] Co: 0-1.00%,

[0089] W: 0~1.00%,

[0090] Sn: 0-1.00%,

[0091] Sb: 0-0.50%,

[0092] Nb: 0~0.200%,

[0093] V: 0~1.00%

[0094] As: 0~0.10%,

[0095] Zn: 0-1.00%,

[0096] Ca: 0~0.0100%,

[0097] Mg: 0~0.0100%,

[0098] Ce: 0~0.0150%,

[0099] Zr: 0~0.0100%,

[0100] La: 0~0.0150%,

[0101] Hf: 0~0.0100%,

[0102] Bi: 0~0.0100%,

[0103] REM other than Ce and La: 0 to 0.0100%, and

[0104] The rest: Fe and impurities;

[0105] a pickling step (b) of pickling the steel sheet obtained in the hot rolling step (a);

[0106] a cold rolling step (c) of cold-rolling the steel sheet obtained in the pickling step (b) at a reduction ratio of 30 to 75% to obtain a cold-rolled steel sheet;

[0107] a heat treatment step (d) of subjecting the steel sheet obtained in the cold rolling step (c) to heat treatment; and

[0108] a grinding step (e) of grinding the front and back surfaces of the steel sheet obtained in the hot rolling step (a) or the steel sheet obtained in the pickling step (b) using a rotary grinding brush containing abrasive grains before or after the pickling step (b);

[0109] In the hot rolling step (a), the finishing rolling is performed in three or more passes, the reduction ratio of each of the final three passes of the finishing rolling is 20% or more, the time between passes is within 1 second, the temperature of the steel sheet at the entry side before the final three passes is 1000° C. or less, and the time from the completion of the final pass to the start of cooling is within 3 seconds.

[0110] In the grinding step (e), the rotation speed R (rpm) of the grinding brush, the diameter D (m) of the grinding brush, and the speed V (m / min) of the steel plate passing through the grinding brush satisfy the following formula (5):

[0111] The heat treatment step (d) further comprises:

[0112] a step (d-1) of heating the steel sheet obtained in the cold rolling step (c) from 650°C to a maximum heating temperature of Ac1+50°C or higher and 950°C or lower at an average heating rate of 0.5 to 500°C / s;

[0113] a step (d-2) of holding the steel sheet obtained in the cold rolling step (c) at the maximum heating temperature for 1 to 300 seconds;

[0114] a step (d-3) of cooling the steel sheet obtained in the cold rolling step (c) to a temperature of Ms point - 100°C or lower, and cooling the steel sheet from 700°C to 500°C at an average cooling rate of 10°C / s or higher;

[0115] a step (d-4) of holding the steel sheet obtained in the cold rolling step (c) at 200 to 350° C. for 50 to 600 seconds,

[0116] In the step (d-1), in the atmosphere surrounding the steel sheet obtained in the step (c), the water vapor partial pressure pH2O and the hydrogen partial pressure pH2 satisfy the following formula (6).

[0117] [Mathematical formula 1]

[0118]

[0119] -4.0≤log(pH2O / pH2) (6)

[0120] (Scheme 6)

[0121] The method for manufacturing a steel plate according to the fifth embodiment is characterized in that the hot rolling step (a) further comprises the steps of: after the hot rolled steel plate is coiled, keeping the hot rolled steel plate warm in a heat-insulating container having an inner wall covered with a heat insulating material for 30 minutes or less;

[0122] The maximum temperature of the atmosphere inside the thermally insulating container is 500 to 650° C., and the time until the atmosphere temperature reaches the maximum temperature is 1 to 8 hours.

[0123] (Scheme 7)

[0124] The method for manufacturing a steel plate according to the above-mentioned scheme 5 or 6 is characterized in that, in the above-mentioned step (d-1), in the atmosphere surrounding the steel plate obtained in the above-mentioned cold rolling step (c), the water vapor partial pressure pH2O and the hydrogen partial pressure pH2 satisfy the following formula (7).

[0125] -4.0≤log(pH2O / pH2)≤-1.0 (7)

[0126] (Scheme 8)

[0127] The method for manufacturing a steel plate according to any one of the above-mentioned solutions 5 to 7 is characterized in that in the above-mentioned step (d-3), the temperature of the steel plate obtained in the above-mentioned cold rolling step (c) is between the Ms point and 650°C, cooling is stopped, and cooling is carried out for 0.1 to 3.0 seconds.

[0128] Effects of the Invention

[0129] According to the present invention, a steel sheet excellent in tensile strength and excellent in bendability after plastic working can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0130] Figure 1 FIG. is a schematic cross-sectional view obtained by cutting a plated steel sheet 1 including a base steel sheet 2 according to an embodiment of the present invention in the thickness direction of the plate. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0131] Hereinafter, a plated steel sheet having a steel sheet according to an embodiment of the present invention as a base steel sheet will be described in detail with reference to Figure 1 It should be noted that Figure 1 FIG. is a schematic cross-sectional view obtained by cutting a plated steel sheet 1 including a base steel sheet 2 according to an embodiment of the present invention in the thickness direction of the plate.

[0132] In the present invention, characteristics of specific positions in the thickness direction of the steel sheet are defined. In the following description, these characteristics are sometimes described using positions in the thickness direction of the steel sheet with respect to the steel sheet surface as a reference.

[0133] It should be noted that since the "thickness direction" and the "depth direction" of the steel sheet have the same meaning, in this specification, the position in the thickness direction of the steel sheet with respect to the above-mentioned steel sheet surface is sometimes referred to as the "depth position".

[0134] In connection with this, in this specification, the "x / y depth position of the plate thickness (where x and y are natural numbers satisfying x < y)" refers to a position that is only moved a distance (depth) of x / y of the plate thickness in the thickness direction from the surface of the steel sheet in the thickness direction of the steel sheet, that is, the steel sheet surface, toward the center of the steel sheet. For example, when the plate thickness of the steel sheet is t mm, the "1 / 8 depth position of the plate thickness" refers to a position that is 1t / 8 mm deep in the thickness direction from the steel sheet surface.

[0135] Among them, regarding the "steel sheet surface" that is the reference for the position in the thickness direction of the steel sheet, that is, the depth position of the steel sheet, in this specification, in the high-frequency glow discharge luminescence analysis (hereinafter, sometimes referred to as "high-frequency GDS analysis") described later, the depth position where the luminescence intensity of Fe reaches 0.7 times the luminescence intensity of Fe inside is defined as the 0 μm position, and this 0 μm position is set as the steel sheet surface. The luminescence intensity of Fe inside is the luminescence intensity of Fe in a sufficient depth region of the base steel sheet. This region is a region where there is basically no change in the concentration of Fe in the depth direction and is a region judged as "steel" as a technical common sense. The luminescence intensity of Fe inside can be set as the luminescence intensity of Fe at a sputtering time of 1000 seconds, for example.

[0136] It should be noted that the "steel plate" that is the subject of the present invention may be Figure 1 As shown in the plated steel sheet 1, the "base steel sheet" is a steel sheet having some kind of coating on the surface. In this case, the "steel sheet surface" that serves as the reference for the depth position of the steel sheet is the steel sheet surface of the base steel sheet. However, as described above, this is the depth position where the Fe luminescence intensity in high-frequency GDS analysis reaches 0.7 times the Fe luminescence intensity inside, that is, the 0 μm position.

[0137] For example, in Figure 1 In the plated steel sheet 1 shown in FIG, the surface of the steel sheet is represented by the symbol “S” indicated by the dotted line near the interface between the base steel sheet 2 and the plated layer 3. d " As described above, this position is the depth position where the luminescence intensity of Fe reaches 0.7 times the luminescence intensity of Fe inside in high-frequency GDS analysis, that is, the 0 μm position.

[0138] In addition, the expression "a depth position of 30 μm from the surface of the steel plate" also refers to a position that is 30 μm away from the surface of the steel plate toward the center of the steel plate in the thickness direction. Figure 1 In the plated steel sheet 1 shown in FIG, the distance S from the steel sheet surface is d The depth position P is 30 μm 30 From the steel plate surface S d The position is moved by a distance of 30 μm toward the center of the steel plate in the plate thickness direction.

[0139] <Plated Steel Sheet>

[0140] like Figure 1 As shown in , the plated steel sheet 1 is a plated steel sheet having a base steel sheet 2 of this embodiment and a plating layer 3 provided on both surfaces of the base steel sheet 2. In addition, the plating layer 3 may be provided on only one surface of the base steel sheet 2.

[0141] In addition, the plated steel sheet 1 is Figure 1 As shown in FIG, the steel plate surface S is divided into d To a depth position P of 150 μm 150 The surface layer P of the region in the plate thickness direction S .

[0142] <Base Steel Plate>

[0143] Furthermore, in the present embodiment, the base material steel plate 2 has the following characteristics.

[0144] First, the chemical composition of the base steel plate 2 is expressed in mass % as follows:

[0145] C: 0.06~0.30%,

[0146] Si: 0.01-2.50%,

[0147] Mn: 1.00~3.50%,

[0148] Ti: 0.001~0.100%,

[0149] B: 0.0005~0.0050%,

[0150] P: 0.050% or less,

[0151] S: 0.0100% or less,

[0152] Al: 1.500% or less,

[0153] N: 0.010% or less,

[0154] O: 0.0100% or less,

[0155] Cr: 0-1.00%,

[0156] Mo: 0-1.00%,

[0157] Cu: 0-1.00%,

[0158] Ni: 0-1.00%,

[0159] Co: 0-1.00%,

[0160] W: 0~1.00%,

[0161] Sn: 0-1.00%,

[0162] Sb: 0-0.50%,

[0163] Nb: 0~0.200%,

[0164] V: 0~1.00%

[0165] As: 0~0.10%,

[0166] Zn: 0-1.00%,

[0167] Ca: 0~0.0100%,

[0168] Mg: 0~0.0100%,

[0169] Ce: 0~0.0150%,

[0170] Zr: 0~0.0100%,

[0171] La: 0~0.0150%,

[0172] Hf: 0~0.0100%,

[0173] Bi: 0~0.0100%,

[0174] REM other than Ce and La: 0 to 0.0100%, and

[0175] The remainder: Fe and impurities.

[0176] Furthermore, the steel structure within the range of the 1 / 8 depth position to the 3 / 8 depth position of the plate thickness of the base steel plate 2 is, in terms of area %, ferrite: 30% or less, tempered martensite: 40% or more, the total of retained austenite and primary martensite: 15% or less, the total of pearlite and cementite: 5% or less, and the balance: bainite.

[0177] In addition, the surface layer P of the base steel plate 2 S The high frequency glow discharge luminescence analysis is performed on the steel plate surface. d The luminous intensity of B measured along the depth direction satisfies the following formula (1) and formula (2): B .

[0178] B30 / B150<0.90 (1) 0.90≤B140 / B150≤1.10 (2)

[0179] B30: Distance from steel plate surface S d The luminous intensity of B at a depth of 30 μm

[0180] B140: Distance from steel plate surface S d The luminous intensity of B at a depth of 140 μm

[0181] B150: Distance from steel plate surface S d The luminescence intensity of B at a depth of 150 μm is 1180 MPa or more.

[0182] These features of the base material steel plate 2 will be described in detail below.

[0183] (Chemical Composition)

[0184] First, the reasons for limiting the chemical composition of the base steel plate (hereinafter sometimes referred to as the "steel plate") according to this embodiment as described above will be explained. It should be noted that, unless otherwise specified, all "%" used in this specification to specify chemical composition refers to "mass %." Furthermore, in this specification, "to" indicating a numerical range is used to include the preceding and following numerical values ​​as the lower and upper limits unless otherwise specified.

[0185] (C: 0.06-0.30%)

[0186] Carbon (C) is an essential element for ensuring the strength of steel sheets. To achieve the desired high strength, the C content is set to 0.06% or higher. Alternatively, the C content may be 0.07%, 0.08%, or 0.10%. Furthermore, from the perspective of workability and weldability, the C content is set to 0.30% or lower. Alternatively, the C content may be 0.29%, 0.28%, or 0.25% or lower.

[0187] (Si: 0.01-2.50%)

[0188] Si (silicon) is an element that suppresses the formation of iron carbides and contributes to improved strength and formability. From the perspectives of strength, formability, and weldability, the Si content is set to 0.01 to 2.50%. The Si content may be 0.05% or higher, 0.10% or higher, 0.15% or higher, or 0.20% or higher. Alternatively, the Si content may be 2.20% or lower, 2.00% or lower, or 1.90% or lower.

[0189] (Mn: 1.00-3.50%)

[0190] Mn (manganese) is a powerful austenite-stabilizing element and is effective in increasing the strength of steel plates. From the perspectives of strength, weldability, and low-temperature toughness, the Mn content is set to 1.00-3.50%. The Mn content can also be 1.10% or higher, 1.30% or higher, or 1.50% or higher. Furthermore, the Mn content can be 3.30% or lower, 3.10% or lower, or 3.00% or lower.

[0191] (Ti: 0.001~0.100%)

[0192] Ti (titanium) is an element effective in increasing the strength of steel sheets. From the perspectives of high strength and cost, the Ti content is set to 0.001% to 0.100%. The Ti content can also be 0.005% or higher, 0.010% or higher, 0.015% or higher, or 0.020% or higher. Furthermore, the Ti content can be 0.080% or lower, 0.070% or lower, or 0.050% or lower.

[0193] (B: 0.0005~0.0050%)

[0194] Boron (B) is an element that improves the hardenability of the steel plate and is effective for high strength, and is an essential element in the present invention. In the present invention, by forming a deboronized layer described later on the surface of the steel plate, the bendability of the steel plate after plastic working can be improved. From the perspective of forming a suitable deboronized layer, the B content is set to 0.0005% to 0.0050%. The B content may also be 0.0007% or more, 0.0010% or more, or 0.0015% or more. In addition, the B content may be 0.0040% or less, 0.0035% or less, or 0.0030% or less.

[0195] (P: 0.050% or less)

[0196] Phosphorus (P) is an element contained in steel as an impurity. It contributes to the high strength of steel plates through solid solution strengthening, but from the perspective of weldability and toughness, the P content is set to 0.050% or less. The P content is preferably 0.045% or less, 0.035% or less, or 0.020% or less. P is not an essential element, and the lower limit of the P content is 0%. However, in order to extremely reduce the P content, the cost of removing P increases, so from an economical perspective, the lower limit of the P content may also be 0.0001%, 0.0005%, or 0.001%.

[0197] (S: 0.0100% or less)

[0198] S (sulfur) is an element contained in steel as an impurity. It forms MnS in steel plates, which degrades toughness and hole expandability. Therefore, to suppress deterioration of toughness and hole expandability, the S content is set to 0.0100% or less. The S content is preferably 0.0050% or less, 0.0040% or less, or 0.0030% or less. S is not an essential element, and the lower limit of the S content is 0%. However, extreme reductions in the S content increase desulfurization costs, so from an economical perspective, the lower limit of the S content may be 0.00001%, 0.00005%, or 0.0001%.

[0199] (Al: 1.500% or less)

[0200] Al (aluminum) is an element contained for deoxidation of steel and is not necessarily contained in the final product, i.e., the steel plate. Therefore, the lower limit of the Al content is 0%. However, to achieve a sufficient deoxidation effect, Al can be added during deoxidation so that the final product, i.e., the steel plate, contains 0.0001% or more, 0.0005% or more, or 0.001% or more of Al. From the perspective of the load during hot rolling caused by increasing the transformation temperature of the steel, the upper limit of the Al content is set to 1.500%. The Al content is preferably 1.200% or less, 1.000% or less, or 0.800% or less.

[0201] (N: 0.010% or less)

[0202] Nitrogen (N) is an element contained in steel as an impurity. If its content exceeds 0.010%, it forms coarse nitrides in the steel, degrading bendability and hole expandability. Therefore, the N content is set to 0.010% or less. The N content is preferably 0.008% or less, 0.006% or less, or 0.005% or less. N is not an essential element, and the lower limit of the N content is 0%. However, extreme reductions in the N content increase the cost of denitrification, so from an economical perspective, the lower limit of the N content may be 0.0001%, 0.0005%, or 0.001%.

[0203] (O: 0.0100% or less)

[0204] O (oxygen) is an element contained in steel as an impurity. If its content exceeds 0.0100%, it forms coarse oxides in the steel, degrading bendability and hole expandability. Therefore, the O content is set to 0.0100% or less. The O content is preferably 0.0080% or less, 0.0060% or less, or 0.0050% or less. O is not an essential element, and the lower limit of the O content is 0%. However, from the perspective of manufacturing costs, the lower limit of the O content may be 0.00001%, 0.00005%, or 0.0001%.

[0205] In the present embodiment, the basic chemical composition of the base steel plate 2 is as described above. Furthermore, the base steel plate 2 may contain the following optional elements as needed.

[0206] (Cr: 0 to 1.00%, Mo: 0 to 1.00%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Co: 0 to 1.00%, W: 0 to 1.00%, Sn: 0~1.00%, Sb: 0~0.50%, Nb: 0~0.200%, V: 0~1.00%, As: 0~0.10%, Zn: 0~1.00%)

[0207] Cr (chromium), Mo (molybdenum), Cu (copper), Ni (nickel), Co (cobalt), W (tungsten), Sn (tin), Sb (antimony), Nb (niobium), V (vanadium), As (arsenic), and Zn (zinc) are all elements effective in increasing the strength of steel sheets. Therefore, one or more of these elements may be added as needed. From the perspective of the effects and costs associated with their inclusion, the contents of these elements are set to 0-1.00% Cr, 0-1.00% Mo, 0-1.00% Cu, 0-1.00% Ni, 0-1.00% Co, 0-1.00% W, 0-1.00% Sn, 0-1.00% Sb, 0-0.50% Nb, 0-0.200% V, 0-1.00% As, and 0-1.00% Zn, respectively. The contents of these elements may be 0.005% or more, or 0.010% or more, respectively.

[0208] (Ca: 0-0.0100%, Mg: 0-0.0100%, Ce: 0-0.0150%, Zr: 0-0.0100%, La: 0-0.0150%, Hf: 0-0.0100%, Bi: 0-0.0100%, and REM other than Ce and La: 0-0.0100%)

[0209] Ca (calcium), Mg (magnesium), Ce (cerium), Zr (zirconium), La (lanthanum), Hf (hafnium), and REMs (rare earth elements) other than Ce and La are all elements that contribute to the fine dispersion of inclusions in steel. Bi (bismuth) is an element that reduces the microsegregation of substitutional alloying elements such as Mn and Si in steel. These elements each contribute to improving the workability of the steel sheet, so one or more of these elements may be added as needed. From the perspective of workability and ductility, the upper limit of the content of Ca, Mg, Zr, Hf, Bi, and REMs other than Ce and La is set to 0.0100%, and the upper limit of the content of Ce and La is set to 0.0150%. The content of these elements may also be set to 0.0005% or more, or 0.0010% or more, respectively.

[0210] In this embodiment, the remainder of the base steel plate 2 other than the aforementioned components includes Fe and impurities. The impurities contained in the remainder other than the aforementioned components are components that are introduced during industrial steel plate production through various manufacturing processes, typically raw materials such as ore and scrap. These impurities include components that are not intentionally added to the base steel plate 2. Furthermore, the impurities contained in the remainder include elements other than the aforementioned components, as well as elements present in the steel plate within a range where the unique effects of the impurities do not affect the properties of the base steel plate 2.

[0211] It should be noted that, in the case of surface-treated steel sheets, the chemical composition described above refers to the content of the base steel sheet after the surface coating has been removed. Furthermore, in the case of steel sheets without coatings such as plating or surface-treated layers, the chemical composition described above refers to the content of the steel sheet itself.

[0212] The chemical composition of the steel plate can be measured by general analytical methods. For example, the chemical composition of the steel plate can be measured using inductively coupled plasma emission spectrometry (ICP-AES). Specifically, by grinding the front and back of the steel plate to a depth of 200 μm from the surface of each steel plate to collect a test piece, the chemical composition of the steel plate can be determined using a measuring device such as ICPS-8100 manufactured by Shimadzu Corporation and measuring under conditions based on a pre-made standard curve. C and S, which cannot be measured by ICP-AES, can be measured using the combustion-infrared absorption method, N using the inert gas fusion-thermal conductivity method, and O using the inert gas fusion-non-dispersive infrared absorption method.

[0213] [Steel structure inside the steel plate]

[0214] Next, the reasons for limiting the internal structure of the base steel plate 2 according to the present embodiment will be described. Note that, in this specification, "%" indicating the steel structure refers to "area %" unless otherwise specified.

[0215] (Ferrite: less than 30%)

[0216] Ferrite is a soft structure with excellent ductility. To improve the elongation of the steel sheet, it may be contained depending on the required strength and ductility. From the perspective of balancing strength and ductility, the ferrite content is set at an upper limit of 30%. The ferrite content may be 25% or less, or 20% or less. The ferrite content may be 0%, 3% or more, 5% or more, or 10% or more.

[0217] (Tempered martensite: more than 40%)

[0218] Tempered martensite is a high-strength and tough structure that improves the tensile strength and bending load of steel sheets. To achieve the desired tensile strength and bendability, the tempered martensite content is set at a lower limit of 40%. The tempered martensite content is preferably 50%, 60%, 70%, or 80%.

[0219] (Total of retained austenite and fresh martensite: less than 15%)

[0220] Retained austenite is a structure that contributes to the improvement of the ductility of the steel sheet through the effect of work-induced phase transformation. On the other hand, because retained austenite transforms into quenched martensite through work-induced phase transformation, it sometimes degrades the bendability of the steel sheet. In addition, because primary martensite is a brittle structure, it becomes the starting point of failure during plastic deformation, sometimes degrading the local ductility of the steel sheet. Therefore, the combined content of retained austenite and primary martensite is set to 15% or less. The combined content of retained austenite and primary martensite is preferably 12% or less, 10% or less, or 8% or less. The combined content of retained austenite and primary martensite can also be 0% or more, 1% or more, 3% or more, or 5% or more.

[0221] (Total of pearlite and cementite: 5% or less)

[0222] Pearlite contains hard and coarse cementite, which becomes the starting point of destruction during plastic deformation. Therefore, if the total content of pearlite and cementite exceeds 5%, the local ductility of the steel plate may be degraded. Therefore, the total content of pearlite and cementite is set to 5% or less. The total content of pearlite and cementite can also be 3% or less, or 2% or less. Among them, cementite is set to coarse structures with a circle-converted diameter exceeding 1μm. It does not include fine cementite precipitated in bainite or martensite.

[0223] The remaining structure other than the above structure may be 0%, but if such a remaining structure exists, it is bainite. Furthermore, the bainite in the remaining structure may be either upper bainite or lower bainite, or a mixture thereof.

[0224] The steel structure fraction of the steel plate was evaluated by the SEM-EBSD method (electron beam backscatter diffraction method) and SEM secondary electron image observation.

[0225] First, a sample was collected from a cross section of the steel plate parallel to the rolling direction as the observation surface. The observation surface was mechanically polished to a mirror finish and then electrolytically polished. Next, a total of 2.0×10 -9 m 2The crystal structure and orientation analysis of the area above the area was performed by SEM-EBSD method. The analysis of the data obtained by EBSD method was performed using "OIM Analysys (registered trademark) 6.0" manufactured by TSL. In addition, the interval between scores (step) was set to 0.10 μm. The area judged to be FCC iron by the observation results was set as retained austenite. Furthermore, the crystal orientation difference was 15 degrees or more as the boundary to obtain a crystal grain boundary map.

[0226] Next, the same sample used for EBSD observation is etched with Nital, and a secondary electron image is obtained for the same field of view as the EBSD observation. To observe the same field of view as the EBSD measurement, a mark such as a Vickers indentation can be made in advance. The resulting secondary electron image is used to determine the area fractions of ferrite, retained austenite, bainite, tempered martensite, fresh martensite, and pearlite.

[0227] The area in which the lower structure is present in the crystal and cementite is precipitated in multiple variants, more specifically, in more than two variants, is judged to be tempered martensite. The area in which cementite is precipitated in a lamellar form is judged to be pearlite. The area in which the brightness is relatively low in the field of view containing various structures and the lower structure is not confirmed is judged to be ferrite. The area in which the brightness is large and the lower structure is not present by etching is judged to be primary martensite and retained austenite. The area ratio of each structure is calculated by using the point counting method as the area ratio of each structure. The finer the grid spacing when performing point counting, the more accurate the value can be obtained. The grid spacing is set to, for example, 2 μm intervals.

[0228] If the total area ratio of each structure obtained by the above-mentioned evaluation method is less than 100%, the remaining area is determined to be bainite. If the total area ratio of each structure obtained by the above-mentioned evaluation method exceeds 100%, the area ratio of each structure is determined by multiplying the area ratio of each structure by 100 / (total area ratio of each structure).

[0229] [Deboronized layer]

[0230] In this embodiment, the base steel plate 2 has a surface layer portion P as described above. S There is a deboronized layer P B In this specification, a portion where the luminescence intensity of B measured in the depth direction from the surface of the steel sheet by high-frequency glow discharge luminescence analysis (high-frequency GDS analysis) satisfies the following equations (1) and (2) is defined as a "deboronized layer."

[0231] B30 / B150<0.90 (1) 0.90≤B140 / B150≤1.10 (2)

[0232] Among them, B30, B140 and B150 are the luminescence intensity of B at a depth of 30 μm from the surface of the steel plate, the luminescence intensity of B at a depth of 140 μm from the surface of the steel plate, and the luminescence intensity of B at a depth of 150 μm from the surface of the steel plate, respectively, when measured from the surface of the steel plate along the thickness direction using high-frequency GDS analysis.

[0233] High-frequency GDS analysis was performed at five random locations. B30, B140, and B150 were the average values ​​of the B emission intensities at depths of 30 μm, 140 μm, and 150 μm from the steel sheet surface, respectively. The measurement conditions were as follows.

[0234] B30, B140, and B150 were measured using a high-frequency glow discharge luminescence analyzer. Specifically, the following method was used: the surface of the steel plate to be measured was placed in an Ar atmosphere, and while applying voltage to generate glow plasma, the surface of the steel plate was sputtered while analysis was performed along the depth direction. The elements contained in the steel plate were then identified based on the wavelengths of the element-specific luminescence spectra produced by excited atoms in the glow plasma, and the luminescence intensity of the identified elements was estimated.

[0235] Depth data can be estimated from the sputtering time. Specifically, by pre-determining the relationship between sputtering time and sputtering depth using standard samples, the sputtering time can be converted into sputtering depth. Therefore, the sputtering depth converted from the sputtering time can be defined as the depth from the steel sheet surface. The sputtering time is set so that the sputtering depth exceeds 150 μm.

[0236] In high-frequency GDS analysis, a commercially available analysis device can be used. In this embodiment, a high-frequency glow discharge luminescence analysis device GD-Profiler2 (registered trademark) manufactured by Horiba, Ltd. is used. The detection interval is set to 0.1 seconds. The obtained data is filtered after removing the background. The filtering is performed by the moving average method. Specifically, the moving average of a total of 51 points of the center point + 25 points before and after is calculated. The values ​​of the time equivalent to 30μm depth, 140μm depth and 150μm depth are B30, B140 and B150, respectively. Regarding other measurement conditions, they are as follows.

[0237] Ar gas pressure: 600Pa

[0238] Anode diameter:

[0239] RF output power: 35W

[0240] It should be noted that in this specification, as described above, the depth position where the luminescence intensity of Fe analyzed by high-frequency GDS reaches 0.7 times the internal Fe luminescence intensity is defined as the 0 μm position, but the internal Fe luminescence intensity in this definition can be set to, for example, the luminescence intensity of Fe when the sputtering time is 1000 seconds.

[0241] The above formula (1) means that the boron concentration at a depth of 30 μm from the steel sheet surface is less than 0.90 times the boron concentration at a depth of 150 μm. By satisfying this formula (1), damage to the metal structure near the steel sheet surface is less likely to occur when the steel sheet is subjected to plastic working.

[0242] In the above formula (1), B30 / B150 may be 0.80 or less, less than 0.80, 0.70 or less, less than 0.70, 0.60 or less, or less than 0.60. In addition, B30 / B150 may be 0, but may be 0.10 or more, 0.20 or more, or 0.30 or more.

[0243] The above formula (2) means that the luminous intensity of B at a depth of 140 μm from the steel plate surface is approximately equal to the luminous intensity of B at a depth of 150 μm from the steel plate surface. B The area is from the surface of the steel plate to a depth of 150 μm.

[0244] By satisfying the formula (2), it is possible to prevent excessive softening at a depth of 150 μm or more from the steel sheet surface, thereby ensuring the strength of the steel sheet.

[0245] By forming the above-mentioned deboronized layer P B , can improve the bendability after plastic working. The reason for such effect is not clear, but it is believed that it may be due to the removal of the boron layer P B The formed surface soft layer is less susceptible to damage to the metal structure (eg, generation of micropores) during plastic working than a surface soft layer formed by a decarburized layer.

[0246] It should be noted that the deboronized layer P B It is preferable that the emission intensity of B measured by the above-mentioned high-frequency GDS analysis satisfies the following formulas (1′) and (2).

[0247] B40 / B150<0.90 (1')

[0248] 0.90≤B140 / B150≤1.10 (2)

[0249] Here, B40 is the luminescence intensity of B at a depth of 40 μm from the surface of the steel sheet when measured from the surface of the steel sheet along the sheet thickness direction by high-frequency GDS analysis.

[0250] By forming a deboronized layer P satisfying the equations (1') and (2) B , which can more reliably improve the bendability after plastic working.

[0251] In the present invention, the formation method of the deboronized layer is not particularly limited as long as the boron content in the steel sheet is 0.0005% to 0.0050% and the luminescence intensity of boron measured by high-frequency GDS analysis satisfies at least the above-mentioned formulas (1) and (2). In other words, as long as there is a region in the surface layer of the steel sheet where the boron content in the steel sheet is 0.0005% to 0.0050% and the luminescence intensity of boron measured by high-frequency GDS analysis satisfies at least the above-mentioned formulas (1) and (2), such region constitutes the "deboronized layer" in the present invention, regardless of the formation method.

[0252] In addition, the surface layer P of the base steel plate 2 S It is preferable that the degree of decarburization (hereinafter sometimes referred to as "decarburization") is not large. Specifically, the surface layer portion P of the base steel plate 2 S The luminescence intensity of C measured in the depth direction from the surface of the steel sheet by high-frequency glow discharge luminescence analysis (high-frequency GDS analysis) preferably satisfies the following formulas (3) and (4).

[0253] C40 / C150>0.50 (3) 0.90≤C140 / C150≤1.10 (4)

[0254] Among them, C40, C140 and C150 are the luminescence intensity of C at a depth of 40 μm from the surface of the steel plate, the luminescence intensity of C at a depth of 140 μm from the surface of the steel plate, and the luminescence intensity of C at a depth of 150 μm from the surface of the steel plate, respectively, when measured from the surface of the steel plate along the thickness direction using high-frequency GDS analysis.

[0255] High-frequency GDS analysis was performed at five random locations. C40, C140, and C150 were the average values ​​of the C emission intensities at five random locations at depths of 40 μm, 140 μm, and 150 μm from the steel sheet surface, respectively. The measurement conditions were the same as those for B30, B140, and B150 described above.

[0256] The above formula (3) means that the carbon concentration at a depth of 40 μm from the steel plate surface is greater than 0.50 times the carbon concentration at a depth of 150 μm. By satisfying this formula (3), excessive softening near the steel plate surface is prevented, making it easier to ensure a certain fatigue strength.

[0257] In the above formula (3), C40 / C150 may be 0.70 or more, 0.80 or more, or 0.90 or more. In addition, C40 / C150 may be 1.10 or less, 1.05 or less, or 1.00 or less.

[0258] The degree of decarburization can be controlled by adjusting the atmosphere until the steel sheet reaches the maximum heating temperature during the heat treatment in the steel sheet manufacturing method described below. By not increasing the degree of decarburization, bendability can be improved, and plane bending fatigue properties can be improved.

[0259] The above formula (4) means that the luminescence intensity of C at a depth of 140 μm from the steel plate surface is approximately equal to the luminescence intensity of C at a depth of 150 μm from the steel plate surface. It should be noted that the C concentration at a depth of 150 μm from the steel plate surface is approximately equal to the C concentration at the center of the steel plate thickness. By satisfying this formula (4) together with the above formula (3), excessive softening of the metal structure up to a depth of 150 μm from the steel plate surface is prevented, making it easier to maintain the strength of the steel plate.

[0260] It should be noted that, in the past, if a deboronized layer is formed in a steel sheet, decarburization is performed, which may impair the fatigue properties of the steel sheet. B By suppressing decarburization, the bendability of the steel sheet after plastic working can be improved without compromising fatigue properties. The reason for this effect is not clear, but it is believed that if both a deboronized layer and a decarburized layer are present, the surface layer will become excessively softened, significantly impairing fatigue strength. Therefore, it is believed that suppressing decarburization can prevent excessive softening of the surface layer and ensure a certain fatigue strength.

[0261] [Tensile strength: 1180MPa or more]

[0262] In this embodiment, the tensile strength of the base steel plate 2 is 1180 MPa or more. Even if the base steel plate 2 of this embodiment is a steel plate with such a high tensile strength, the base steel plate 2 has the above-mentioned deboronized layer P. B, and the bendability after plastic working is also excellent. The tensile strength of the base steel plate 2 may be 1200 MPa or more, 1300 MPa or more, 1400 MPa or more, or 1500 MPa or more. It should be noted that the upper limit of the tensile strength of the base steel plate 2 is not particularly limited, but from the perspective of toughness and formability, it may be, for example, 4000 MPa or less, 3000 MPa or less, or 2000 MPa or less.

[0263] The tensile strength (TS) of a steel plate can be measured as follows. First, a JIS Z 2241:2011 No. 5 test piece is collected from the width center of the steel plate to be measured, with the longitudinal direction perpendicular to the rolling direction. This test piece is then used to perform a tensile test in accordance with JIS Z 2241:2011 to measure the tensile strength TS (MPa).

[0264] In addition, when it is difficult to collect a test piece from the steel plate to be measured, the Vickers hardness of the steel plate can be measured. Using the measured value of the Vickers hardness, the tensile strength value can be derived from the following correlation formula ("Correlation between Static Strength Parameters", Norihiko Hasegawa, Junichi Arai, Michishichi Tanaka, "Materials", Vol. 39, No. 442, P859-863).

[0265] Hv=0.301×TS+5.701

[0266] In the above formula, Hv represents Vickers hardness, and TS represents tensile strength (MPa).

[0267] The Vickers hardness of a steel plate can be measured according to JIS Z 2244:2009. Specifically, the Vickers hardness of a steel plate can be measured 10 times at a depth of 1 / 4 of the plate thickness, under a load of 1 kgf (approximately 9.80 N), and the average of these 10 measured values ​​can be obtained. The distance between the measurement points should be at least three times the indentation length.

[0268] [Tensile residual stress acting at right angles to rolling: 200 MPa or less]

[0269] In this embodiment, the base steel plate 2 preferably has a tensile residual stress acting in a direction perpendicular to rolling on the surface of the steel plate of 200 MPa or less. This tensile residual stress can be measured by X-ray diffraction, specifically according to the standard conditions described in the "X-ray Material Strength Division Committee of the Japan Society of Materials, 'X-ray Stress Measurement Method Standard (1997 Edition)'" (1997, Japan Society of Materials). For example, the measuring apparatus can be the AutoMate II manufactured by Rigaku Corporation.

[0270] The tensile residual stress acting in a direction perpendicular to rolling on the surface of the steel sheet is preferably 100 MPa or less, more preferably 60 MPa or less, and even more preferably 40 MPa or less.

[0271] It should be noted that the rolling perpendicular direction refers to a direction perpendicular to the rolling direction and the plate thickness direction.

[0272] Reducing the tensile residual stress acting in the direction perpendicular to the rolling process on the surface of the steel sheet is particularly effective in improving fatigue strength. S In the case of a soft layer in the steel, the influence of the tensile residual stress becomes particularly significant.

[0273] [Plating]

[0274] As described above, in this embodiment, the coating 3 is provided on both surfaces of the base steel plate 2. The coating 3 may be a hot-dip galvanized layer or an alloyed hot-dip galvanized layer having any known composition. The coating 3 may contain additive elements such as Al in addition to Zn. The coating 3 is not particularly limited in its application amount and may be a typical application amount.

[0275] It should be noted that the plating layer 3 may be provided on only one surface of the base steel plate 2, or may not be provided on any surface of the base steel plate 2. In the steel plate of the present invention, it is not essential to have a plating layer on the surface of the steel plate.

[0276] (Thickness of steel plate)

[0277] The thickness of the steel plate of the present invention is not particularly limited and can be, for example, the same thickness as steel plates used for automotive parts. Examples of such steel plate thicknesses include 0.5 to 3.0 mm. The steel plate thickness can also be 0.7 mm or greater, 0.8 mm or greater, or 1.0 mm or greater. Furthermore, the steel plate thickness can be 2.8 mm or less, 2.5 mm or less, or 2.0 mm or less.

[0278] <Method for Manufacturing Steel Sheet>

[0279] Next, a method for manufacturing a steel plate according to an embodiment of the present invention will be described. The following description is intended to illustrate a characteristic method for manufacturing a steel plate according to an embodiment of the present invention and is not intended to limit the steel plate to steel plates manufactured by the following manufacturing method.

[0280] The method for manufacturing a steel plate includes: a hot rolling step (a) of hot-rolling a slab having a specific chemical composition to obtain a hot-rolled steel plate (hereinafter sometimes referred to as “step (a)”); a grinding step (e) of grinding the hot-rolled steel plate using a rotary grinding brush (hereinafter sometimes referred to as “step (e)”); a pickling step (b) of pickling after grinding (hereinafter sometimes referred to as “step (b)”); a cold rolling step (c) of cold-rolling the pickled hot-rolled steel plate to obtain a cold-rolled steel plate (hereinafter sometimes referred to as “step (c)”); and a heat treatment step (d) of heat-treating the cold-rolled steel plate (hereinafter sometimes referred to as “step (d)”).

[0281] Hereinafter, preferred conditions and the like of these steps will be described in detail.

[0282] [Hot rolling process (a)]

[0283] First, the following hot rolling step (a) is performed: a slab having the following specific chemical composition is hot rolled under predetermined conditions to obtain a hot-rolled steel sheet, which is then cooled to a predetermined temperature and coiled. In the hot rolling step, the slab having the following specific chemical composition is heated before hot rolling.

[0284] Here, regarding the chemical composition of the slab, when the chemical composition of the finally obtained steel plate is analyzed according to the above-mentioned analysis method, it can be confirmed that there is substantially no difference from the chemical composition of the slab.

[0285] Therefore, the chemical composition of the slab is basically the same as that of the steel plate described above. That is, the chemical composition of the slab is calculated in mass % as follows:

[0286] C: 0.06~0.30%,

[0287] Si: 0.01-2.50%,

[0288] Mn: 1.00~3.50%,

[0289] Ti: 0.001~0.100%,

[0290] B: 0.0005~0.0050%,

[0291] P: 0.050% or less,

[0292] S: 0.0100% or less,

[0293] Al: 1.500% or less,

[0294] N: 0.010% or less,

[0295] O: 0.0100% or less,

[0296] Cr: 0-1.00%,

[0297] Mo: 0-1.00%,

[0298] Cu: 0-1.00%,

[0299] Ni: 0-1.00%,

[0300] Co: 0-1.00%,

[0301] W: 0~1.00%,

[0302] Sn: 0-1.00%,

[0303] Sb: 0-0.50%,

[0304] Nb: 0~0.200%,

[0305] V: 0~1.00%

[0306] As: 0~0.10%,

[0307] Zn: 0-1.00%,

[0308] Ca: 0~0.0100%,

[0309] Mg: 0~0.0100%,

[0310] Ce: 0~0.0150%,

[0311] Zr: 0~0.0100%,

[0312] La: 0~0.0150%,

[0313] Hf: 0~0.0100%,

[0314] Bi: 0~0.0100%,

[0315] REM other than Ce and La: 0 to 0.0100%, and

[0316] The remainder: Fe and impurities.

[0317] It should be noted that the preferred content of each component in the chemical composition of the slab is also basically the same as the chemical composition of the steel plate described above.

[0318] In the hot rolling process, the heating temperature of the slab is not particularly limited, but is generally preferably set to 1150°C or higher in order to fully dissolve borides, carbides, etc. It should be noted that the steel slab used is preferably cast by continuous casting from the perspective of manufacturability, but can also be produced by ingot casting or thin slab casting.

[0319] (Rough rolling)

[0320] In this manufacturing method, the heated slab may be subjected to rough rolling before finish rolling for purposes such as thickness adjustment. The conditions for such rough rolling are not particularly limited, but from the perspective of recrystallization during hot rolling, rough rolling is preferably performed so that the total reduction ratio at 1050°C or above is 60% or greater. For example, the total reduction ratio may be 90% or less.

[0321] (Finishing rolling)

[0322] Next, the slab is hot-rolled through finish rolling to produce a hot-rolled steel sheet. The entry temperature during finish rolling is not particularly limited, but is preferably set to 900-1050°C to achieve an optimal microstructure of the hot-rolled steel sheet. Furthermore, the total reduction ratio during finish rolling is preferably set to 70-95%.

[0323] In the present manufacturing method, in order to form the above-mentioned deboronized layer, finishing rolling is performed for more than three passes. Furthermore, the reduction rate of each of the final three passes in finishing rolling is set to more than 20%, the time between passes is set to less than 1 second, the temperature of the steel plate at the entry side before the final three passes is set to less than 1000°C, and the finishing temperature is set to 850-950°C. Furthermore, the time from the completion of the final pass to the start of cooling is set to less than 3 seconds. If finishing rolling is performed under such conditions, the surface of the hot-rolled steel plate is softened by promoting the ferrite transformation by utilizing the strain accumulation to austenite, which can promote the introduction of strain to the surface caused by grinding in the next process. As a result, the above-mentioned deboronized layer can be formed on the final product, i.e., the steel plate. The number of finishing rolling passes is not particularly limited as long as the final three passes meet the above-mentioned conditions.

[0324] In this specification, “final three passes” refers to three passes, namely, the first pass, the second pass, and the third pass, counted from the final pass, among three or more passes in finish rolling.

[0325] (Coiling temperature: 450~650℃)

[0326] The hot-rolled steel sheet after finish rolling is cooled to a predetermined coiling temperature and then coiled. The coiling temperature is set to 450°C to 650°C from the perspective of the strength and workability of the hot-rolled sheet. The coiling temperature may be set to 500°C or higher. Alternatively, the coiling temperature may be set to 620°C or lower.

[0327] After coiling is completed, in order to promote the formation of a deboronized layer in the heat treatment process described later, a process of keeping the coiled hot-rolled steel sheet warm in an insulating container may be implemented. As an example of a heat preservation process, the hot-rolled steel sheet may be placed in an insulating container whose inner wall is covered with an insulating material for heat preservation within 30 minutes after coiling is completed. At this time, the heat preservation conditions may set the maximum temperature of the atmosphere in the container to 500-650°C, and the time for the atmosphere temperature to reach the above-mentioned maximum temperature to 1-8 hours. If heat preservation is performed under such conditions, the surface layer of the hot-rolled steel sheet will be further softened, which will promote the introduction of strain in the subsequent grinding process and further promote the formation of a deboronized layer in the heat treatment process described later, thereby further improving the bendability of the final steel sheet after plastic working.

[0328] [Grinding process (e)]

[0329] Next, a grinding process (e) is performed in which the surface and back of the coiled steel plate are ground using a rotary grinding brush. As brushes that can be used in the grinding process, for example, D-100-33 manufactured by HOTANI Corporation can be cited. Regarding the grinding conditions, the rotation speed R (rev / min) of the grinding brush, the diameter D (m) of the grinding brush, and the plate passing speed V (m / min) of the steel plate satisfy the following formula (5). If grinding is performed under the conditions satisfying such formula (5), strain is introduced into the surface layer of the steel plate, and the diffusion of boron can be promoted in the heat treatment process described later, and the deboronized layer formed in the heat treatment process described later is expanded.

[0330] [Mathematical formula 2]

[0331]

[0332] In formula (5), (R·D) / V may be 11 or greater, 13 or greater, or 15 or greater. The upper limit of (R·D) / V is not particularly limited, but (R·D) / V may be 60 or less, 55 or less, or 50 or less.

[0333] Such a step (e) needs to be performed between the completion of hot rolling and before cold rolling, and may be performed before or after the pickling step described later.

[0334] [Pickling step (b)]

[0335] Next, a pickling step (b) is performed to pickle the steel sheet after the hot rolling step (a) or the grinding step (e). The pickling method in the pickling step can be conventional. Furthermore, skin pass rolling may be performed during the pickling step to correct the shape of the hot-rolled coil and improve pickling resistance.

[0336] [Cold rolling process (c)]

[0337] Next, a cold rolling step (c) is performed on the steel sheet after the pickling step (b) or the grinding step (e). In the cold rolling step, the cold rolling reduction ratio is set to 30-75% in consideration of the accumulation of strain and the load on the cold rolling mill caused by the rolling load. For example, the reduction ratio may be set to 40% or more. Alternatively, the reduction ratio may be set to 70% or less, or 60% or less.

[0338] [Heat treatment step (d)]

[0339] Next, a heat treatment step (d) is performed on the steel sheet obtained in the cold rolling step (c). The heat treatment steps are performed in order: a step (d-1) of heating the steel sheet obtained in the step (c) from 650°C to a maximum heating temperature of Ac1+50°C or higher and 950°C or lower at an average heating rate of 0.5 to 500°C / second; a step (d-2) of holding the steel sheet at the maximum heating temperature for 1 to 300 seconds; a step (d-3) of cooling the steel sheet to a temperature of Ms point (martensitic transformation point) -100°C or lower, a step (d-4) of cooling the steel sheet from 700°C to 500°C at an average cooling rate of 10°C / second or higher; and a step (d-5) of holding the steel sheet at 200 to 350°C for 50 to 600 seconds.

[0340] Deboronization can be fully carried out by softening the surface of the steel plate by controlling the hot rolling conditions as described above, introducing a large amount of strain into the surface of the steel plate during the grinding process, and then, in the above-mentioned steps (d-1) to (d-4) of the heat treatment process, that is, the heating / soaking process, the H2O in the atmosphere reacts with the B on the surface of the steel plate to form oxides.

[0341] In the above-mentioned step (d-1), from the viewpoint of recrystallizing ferrite and suppressing the coarsening of austenite, the average heating rate to the maximum heating temperature is set to 0.5 to 500°C / second. The average heating rate may be 1.0°C / second or more or 2.0°C / second or more. In addition, the average heating rate may be 400°C / second or less or 300°C / second or less. Here, the "average heating rate" refers to the value obtained by dividing the difference between 650°C and the maximum heating temperature by the time required to reach the maximum heating temperature from 650°C.

[0342] In the above-mentioned step (d-1), from the viewpoint of advancing austenitization and suppressing the coarsening of the austenite diameter, the maximum heating temperature is set to Ac1+50°C or higher and 950°C or lower. In addition, in the above-mentioned step (d-2), from the viewpoint of advancing austenitization and productivity, the holding time at the maximum heating temperature is set to 1 to 300 seconds. During the holding at the maximum heating temperature, it is not necessary to maintain the steel plate at a constant temperature, and the temperature may also vary within the above-mentioned maximum temperature range. Here, "maintaining" means maintaining the temperature within the range of ±20°C, preferably ±10°C, of ​​the specified temperature, within the range not exceeding the specified upper and lower limits.

[0343] After holding at the maximum heating temperature, the steel plate is cooled to a temperature of Ms point minus 100°C or lower in the aforementioned step (d-3). During this process, cooling is performed from 700°C to 500°C at an average cooling rate of 10°C / second or higher. The average cooling rate from 700°C to 500°C may also be set to 20°C / second or higher, 30°C / second or higher, or 50°C / second or higher.

[0344] To obtain the desired structure, after cooling to a temperature below the Ms point -100°C, the steel is held at 200-350°C for 50-600 seconds in the aforementioned step (d-4). The "holding" in this step (d-4), as described above, does not necessarily mean holding at a constant temperature; rather, it means maintaining the steel within a range of ±20°C, preferably ±10°C, of ​​the specified temperature. It should be noted that in step (d-4), if the holding time is shorter than 50 seconds, the area ratios of fresh martensite and retained austenite increase, and the desired bendability after applying a 2% prestrain cannot be achieved.

[0345] In the above-mentioned step (d-4), the maintenance in the temperature range of 200 to 350°C can be carried out by reheating after the above-mentioned cooling to below 200°C is carried out, or if the end temperature of the above-mentioned cooling to below Ms point -100°C is above 200°C, it can also be carried out in the middle of the subsequent cooling process.

[0346] Furthermore, in the aforementioned step (d-1), the atmosphere surrounding the steel sheet during heating from 650°C to the maximum heating temperature is controlled so that the water vapor partial pressure pH2O and the hydrogen partial pressure pH2 satisfy the following equation (6). If the right side of the equation (6) is less than -4.0, the deboronization reaction does not proceed sufficiently, and the desired bendability after imparting a 2% prestrain cannot be achieved.

[0347] -4.0≤log(pH2O / pH2) (6)

[0348] pH2O: partial pressure of water vapor

[0349] pH2: Hydrogen partial pressure

[0350] It should be noted that the log(pH2O / pH2) on the right side of formula (6) may be greater than or equal to -3.8 or greater than or equal to -3.6. In addition, the log(pH2O / pH2) on the right side of formula (6) may be less than or equal to -0.1 or less than or equal to -0.5.

[0351] Furthermore, in the above-mentioned step (d-1), the atmosphere around the steel plate during heating from 650°C to the maximum heating temperature can also be controlled so that the water vapor partial pressure pH2O and the hydrogen partial pressure pH2 satisfy the following formula (7). By controlling the atmosphere during heating so as to satisfy this formula (7), decarburization on the surface of the steel plate can be suppressed, and the plane bending fatigue strength after plastic working can be improved. Furthermore, by optimizing the above-mentioned atmosphere during heating, the plane bending fatigue strength after applying 2% prestrain and after coating and baking is preferably set to 0.35 times or more of the tensile strength, and more preferably set to 0.40 times or more or 0.45 times or more of the tensile strength.

[0352] -4.0≤log(pH2O / pH2)≤-1.0 (7)

[0353] pH2O: partial pressure of water vapor

[0354] pH2: Hydrogen partial pressure

[0355] In addition, in the above-mentioned step (d-3), when the steel plate is cooled to a temperature below the Ms point - 100°C, the cooling may be stopped or the cooling rate may be sufficiently reduced in the range of the steel plate temperature to the Ms point ~ 650°C, and a cooling interval of 0.1 to 3.0 seconds may be set. By setting such a cooling interval during cooling, the temperature difference between the surface and the interior of the steel plate can be uniformed, and the tensile residual stress on the surface of the steel plate generated after cooling can be reduced. Furthermore, by optimizing such cooling conditions, the tensile residual stress acting in the direction perpendicular to the rolling on the surface of the steel plate is preferably set to 200 MPa or less, more preferably to 150 MPa or less or 100 MPa or less. Among them, the cooling rate is sufficiently reduced, for example, the average cooling rate is set to 5.0°C / second or less. The average cooling rate may also be 0°C / second. Furthermore, if it is within the above-mentioned temperature range, the temperature may be raised by reheating, etc.

[0356] It should be noted that, in the above-mentioned step (d-3), the cooling period may be 0.2 seconds or more or 0.3 seconds or more. In addition, the cooling period may be 2.5 seconds or less, 2.0 seconds or less, or 1.5 seconds or less.

[0357] As described above, a coating may also be formed on the surface of the steel sheet of the present invention. The coating may be, for example, a hot-dip galvanized layer. In addition, as needed, an alloying treatment may be performed after the formation of the hot-dip galvanized layer to form an alloyed hot-dip galvanized layer. The formation of the coating and the alloying treatment may be performed according to conventional methods and are not particularly limited. The coating treatment may be performed during the cooling process from the maximum heating temperature to a temperature below the Ms point - 100°C. In this case, cooling is completed once at the coating treatment temperature, and after the coating treatment is completed, cooling is performed to a temperature below the Ms point - 100°C at an average cooling rate of 10°C / second or more.

[0358] By the above manufacturing method, the steel plate of the present invention having excellent tensile strength and excellent bendability can be obtained. With regard to bendability, a test piece collected from the steel plate as the evaluation object is given a pre-strain of 2%, and then a bending test is performed according to the method specified in the 238-100 standard of the German Association of the Automotive Industry (VDA), and the evaluation is performed by the maximum bending angle obtained from the test. In addition, according to the above manufacturing conditions, it is also possible to seek improvement in plane bending fatigue characteristics. With regard to plane bending fatigue characteristics, a test piece collected from the steel plate as the evaluation object is given a pre-strain of 2%, and then a plane bending fatigue test is performed according to JIS Z2275:1978, and the evaluation is performed by the fatigue limit ratio (= fatigue strength / tensile strength) obtained from the test. It should be noted that the reason for giving a pre-strain of 2% during the evaluation is because it is assumed that the steel plate of the present invention is used as a component.

[0359] Example

[0360] Next, an embodiment of the present invention will be described. The conditions in this embodiment are an example of conditions adopted to confirm the feasibility and effectiveness of the present invention. The present invention is not limited to this example of conditions. As long as it does not deviate from the main purpose of the present invention and achieves the purpose of the present invention, the present invention can adopt various conditions.

[0361] Steels with various chemical compositions are cast to create slabs. These slabs are then hot-rolled to produce hot-rolled steel sheets. Furthermore, these hot-rolled steel sheets are sequentially ground, cold-rolled, and heat-treated to produce cold-rolled steel sheets.

[0362] A portion of the resulting cold-rolled steel sheet was subjected to a plating treatment. The chemical composition of the samples collected from the resulting steel sheet was analyzed, confirming that the chemical composition remained unchanged from the slab. The chemical compositions of these steel sheets are shown in Table 1. The remainder other than the components shown in Table 1 is Fe and impurities. It should be noted that the chemical composition of the plated steel sheet refers to the chemical composition of the base steel sheet after the surface coating has been stripped under the above-mentioned conditions. Underlined chemical compositions in Table 1 indicate compositions outside the scope of the present invention.

[0363]

[0364] Table 2

[0365] Hot rolling conditions are as described in Table 2. In Table 2, R1 entry temperature refers to the entry temperature of the steel plate at the third pass from the final pass of finishing rolling. R1 refers to the reduction rate at the third pass from the final pass. R2 refers to the reduction rate at the second pass from the final pass. R3 refers to the reduction rate at the final pass. In addition, t1 refers to the time from the end of the third pass from the final pass to the start of the second pass from the final pass. t2 refers to the time from the end of the second pass from the final pass to the start of the final pass. t3 refers to the time from the end of the final pass to the start of cooling. Moreover, R3 exit temperature refers to the temperature of the steel plate at the end of the final pass, that is, the finishing temperature.

[0366] The hot-rolled steel sheets were then ground on both sides using a rotating grinding brush containing abrasive grains. The grinding conditions were set so that the (R·D) / V value, calculated from the grinding brush rotation speed R (r / min), the brush diameter D (m), and the hot-rolled steel sheet feed rate V (m / min), corresponded to the values ​​shown in Table 3. The ground steel sheets were then pickled. Furthermore, the pickled steel sheets were cold rolled at the reduction ratios listed in Table 3.

[0367] After that, the cold-rolled steel sheet is heat-treated. After the heat treatment is completed, the steel sheet is held and cooled. For some steel sheets, a cooling zone is set in the Ms to 650°C range during cooling to stop cooling. In Table 3, the cooling time is 0 seconds, which is an example without a cooling zone. Furthermore, after cooling to a temperature below Ms point - 100°C, the steel sheet is held at 200 to 350°C. The steel sheet No. 15, whose cooling end temperature to Ms point - 100°C is higher than the holding temperature of 200 to 350°C, is held in the next cooling process after cooling to a temperature below Ms point - 100°C. Steel sheets other than No. 15 are heated to a specified temperature after cooling to a temperature below Ms point - 100°C and held. These conditions and the values ​​of log (pH2O / pH2) from 650°C to the maximum heating temperature are shown in Table 3. Here, pH2O is the water vapor partial pressure, and pH2 is the hydrogen partial pressure. In Table 3, Ms is the martensitic transformation point (° C.) of the steel used.

[0368] In Table 3, the Ac1 point (° C.) serving as a reference for the setting range of the maximum heating temperature for heat treatment was determined according to the following formula. The Ac1 point of each steel plate is shown in Table 1.

[0369] Ac1=723-10.7[Mn]-16.9[Ni]+29.1[Si]+16.9[Cr]

[0370] In the above formula, [Mn], [Ni], [Si], and [Cr] refer to the content (mass %) of each element.

[0371] In Table 3, the Ms point (° C.) was obtained according to the following formula.

[0372] Ms=561-474[C]-33[Mn]-7.5[Si]-17[Cr]-17[Ni]-21[Mo]+10[Co]

[0373] In the above formula, [C], [Mn], [Si], [Cr], [Ni], [Mo], and [Co] refer to the content (mass %) of each element.

[0374] Subsequently, a portion of the steel sheets were subjected to continuous hot-dip galvanizing, and a portion thereof was further subjected to alloying. The plating conditions were not special and were generally known. In Table 3, GA refers to alloyed hot-dip galvanized steel sheets. Furthermore, GI refers to hot-dip galvanized steel sheets that were not alloyed. CR refers to cold-rolled steel sheets that were not plated.

[0375] In addition, underlined values ​​in Tables 2 and 3 indicate that the values ​​are outside the range of the present invention, or that the steel sheet production conditions cannot achieve the values ​​of the present invention, or that the various properties of the steel sheet are not preferred.

[0376] Table 3

[0377]

[0378] The resulting steel plates were measured using the aforementioned high-frequency glow discharge luminescence analysis (high-frequency GDS analysis) method. The luminescence intensities B30, B40, B140, and B150 of B at depths of 30 μm, 40 μm, 140 μm, and 150 μm from the steel plate surface were measured using high-frequency GDS analysis along the plate thickness. Similarly, the luminescence intensities C40, C140, and C150 of C were measured at depths of 40 μm, 140 μm, and 150 μm from the steel plate surface. These measurement results are shown in Table 4 below.

[0379] In addition, from the width center of the obtained steel plate, a No. 5 tensile test piece of JIS Z 2241:2011 was collected with the direction perpendicular to the rolling direction as the longitudinal direction. The tensile test piece was used to perform a tensile test in accordance with JIS Z 2241:2011 to measure the tensile strength (TS). It should be noted that for steel plates No. 30 and No. 31, the coating was not peeled off from the plated steel plate, and the tensile strength in the plated state was measured. In this embodiment, the tensile strength benchmark (1180 MPa or more) was set to the same as that of the steel plate without plating. The results of the tensile strength measurements of each steel plate are shown in Table 4 below.

[0380] In addition, from the width center of the obtained steel plate, a tensile test piece with a parallel portion width of 30 mm and a length direction at right angles to the rolling direction was collected. After giving a pre-strain of 2%, a rectangular specimen with a width of 30 mm and a length of 60 mm was collected from the parallel portion. Next, in order to simulate the paint baking process of an automobile, a heat treatment was performed at 170°C for 20 minutes. For the test piece after heat treatment, a bending test was performed by the method specified in 238-100 of the German Association of the Automotive Industry (VDA) standard to measure the maximum bending angle. For the measurement results, a maximum bending angle of 60 degrees or more was judged to be good "bendability". The bending direction was implemented in a manner such that the rolling direction became parallel to the bending ridgeline. It should be noted that for steel plates No. 30 and No. 31, the coating was not peeled off from the plated steel plate, and the maximum bending angle in the plated state was measured. In this embodiment, the benchmark for the maximum bending angle (60 degrees or more) was set to be the same as that of the steel plate without plating. The measurement results of the maximum bending angle of each steel plate are shown in Table 4 below.

[0381] In addition, from the width center of the obtained steel plate, a No. 1 test piece (width 25 mm) described in JIS Z 2275: 1978 was collected with the longitudinal direction being perpendicular to the rolling direction, and a 2% prestrain was applied to the central part of the test piece using a tensile testing machine. The strain amount was controlled by attaching a GL5 mm strain gauge to the central part of the test piece. Thereafter, a heat treatment was performed at 170°C for 20 minutes to simulate the paint baking process of an automobile. The test piece after heat treatment was subjected to a plane bending fatigue test in accordance with JIS Z 2275: 1978 to measure the fatigue limit ratio (= fatigue strength / tensile strength). For the measurement results, a fatigue limit ratio of 0.35 or more was judged to be good for the "plane fatigue bending characteristics". It should be noted that for No. 30 and No. 31, the coating was not peeled off from the plated steel sheet, and the fatigue limit ratio in the plated state was measured. In this example, the fatigue limit ratio criterion (0.35 or more) was set to be the same as that of the steel sheet not plated. The measurement results of the fatigue limit ratio of each steel sheet are shown in Table 4 below.

[0382] Here, we will explain the points that should be noted in the evaluation of the present invention. The characteristics of the present invention, namely, the chemical composition, steel structure, B concentration distribution, and C concentration distribution of the steel sheet, define regions unrelated to surface coating. On the other hand, it is believed that the mechanical properties of the steel sheet (i.e., tensile strength, bendability, and in-plane fatigue bending properties) generally vary slightly depending on the presence or absence of surface coating. Even in this situation, the present invention uses steel sheets with the same surface condition as when the steel sheet was used to determine whether the mechanical properties of the steel sheet fall within the scope of the present invention. This is because, for those who use steel sheets with surface coatings, the mechanical properties in the coated state are more important than the mechanical properties after the coating is removed. Therefore, in the present invention examples, the mechanical properties of tensile strength, bendability, and in-plane fatigue bending properties were evaluated for the plated steel sheets (steel sheets No. 30 and No. 31) in the plated state, and for the unplated steel sheets (steel sheets other than No. 30 and No. 31) in the unplated state.

[0383] In Table 4, "α" in the microstructure refers to ferrite. "TM" refers to tempered martensite. "FM+γ" refers to the sum of primary martensite and retained austenite. "P+θ" refers to the sum of pearlite and cementite. Furthermore, "B" refers to bainite.

[0384] Underlined values ​​in Table 4 indicate that the values ​​are outside the range of the present invention, that the steel sheet of the present invention cannot be obtained under production conditions, or that the various properties of the steel sheet are not preferred.

[0385] Table 4

[0386]

[0387] In the steel sheet No. 6, the grinding process conditions were not suitable, and a suitable deboronized layer was not formed, resulting in poor bendability.

[0388] In the steel sheet No. 7, the reduction ratio in the finish rolling in the hot rolling process was not appropriate, and an appropriate deboronized layer was not formed, resulting in poor bendability.

[0389] In the steel sheet No. 8, the maximum heating temperature in the heat treatment step was low, and the ferrite fraction was high, so the desired tensile strength was not obtained.

[0390] In the steel sheet No. 9, the finishing temperature in the hot rolling process was high, and thus an appropriate deboronized layer was not formed, resulting in poor bendability.

[0391] In the steel sheet No. 15, the cooling stop temperature in the heat treatment step was high, and the total fraction of fresh martensite and retained austenite increased, resulting in inferior bendability.

[0392] Since the steel sheet No. 16 was not ground with a brush, a suitable deboronized layer was not formed, resulting in poor bendability.

[0393] In the steel sheet No. 17, the time between passes of the finish rolling in the hot rolling process was long, and thus a suitable deboronized layer was not formed, resulting in inferior bendability.

[0394] In the steel sheet No. 18, the holding temperature after cooling in the heat treatment step was low, and the total fraction of fresh martensite and retained austenite was high, resulting in inferior bendability.

[0395] In the steel sheet No. 19, the holding time after cooling in the heat treatment step was short, and the total fraction of fresh martensite and retained austenite was high, resulting in inferior bendability.

[0396] In the steel sheet No. 20, log (pH2O / pH2) in the heat treatment step was less than -4.0, and an appropriate deboronized layer was not formed, resulting in poor bendability.

[0397] The steel plate No. 32 did not achieve the desired tensile strength because of the low C content in the chemical composition.

[0398] The steel plate No. 33 had a high C content in its chemical composition, resulting in poor bendability.

[0399] Steel sheet No. 34 had a low B content in its chemical composition and therefore did not form a suitable deboronized layer, resulting in poor bendability.

[0400] The steel sheet No. 35 has a high Mn content in its chemical composition, resulting in a high total fraction of fresh martensite and retained austenite, which results in poor bendability.

[0401] The steel sheet No. 36 has a low Mn content in the chemical composition, a high ferrite fraction, and a low tempered martensite fraction, resulting in inferior bendability.

[0402] The steel sheet No. 37 had a high Si content in its chemical composition, resulting in a high total fraction of fresh martensite and retained austenite, which resulted in inferior bendability.

[0403] Steel sheet No. 2 is an invention example of a steel sheet with excellent bendability. Since log(pH2O / pH2) during the heat treatment step is greater than -1.0 and the cooling time is shorter than 0.1 seconds, the fatigue limit is slightly lower than that of the invention example.

[0404] Steel sheet No. 3 is an invention example of a steel sheet having excellent bendability. Since log(pH2O / pH2) in the heat treatment step is greater than -1.0, the fatigue limit is slightly lower than that of the invention example.

[0405] Steel sheet No. 13 is an invention example of a steel sheet with excellent bendability. The cooling time during the heat treatment step was less than 0.1 seconds, resulting in a fatigue limit slightly lower than that of the invention example.

[0406] Explanation of symbols

[0407] 1 Coated steel sheet

[0408] 2. Base steel plate

[0409] 3 coating

[0410] S d Steel plate surface

[0411] P S Surface

[0412] P B Deboronized layer

[0413] P 30 At a depth of 30 μm from the steel plate surface

[0414] P 150 At a depth of 150 μm from the steel plate surface

Claims

1. A steel plate, characterized in that: It is a steel plate, the chemical composition of the steel plate is calculated as follows in mass %: C：0.06～0.30％、 Si: 0.01-2.50%, Mn: 1.00~3.50%, Ti: 0.001~0.100%, B：0.0005～0.0050％、 P: 0.050% or less, S: 0.0100% or less, Al: 1.500% or less, N: 0.010% 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％、 Sn: 0-1.00%, Sb: 0-0.50%, Nb: 0~0.200%, V：0～1.00％、 As: 0~0.10%, Zn: 0-1.00%, Ca: 0~0.0100%, Mg: 0~0.0100%, Ce: 0~0.0150%, Zr：0～0.0100％、 La: 0~0.0150%, Hf: 0~0.0100%, Bi: 0~0.0100%, REM other than Ce and La: 0 to 0.0100%, and The rest: Fe and impurities, The steel structure within the range of 1 / 8 to 3 / 8 of the thickness of the steel plate is calculated as follows in terms of area %: Ferrite: less than 30%, Tempered martensite: more than 40%, Total of retained austenite and fresh martensite: less than 15%, Total of pearlite and cementite: less than 5%, and The rest: bainite, The surface layer of the steel plate has a deboronized layer in which the luminescence intensity of B measured from the surface of the steel plate in the depth direction by high-frequency glow discharge luminescence analysis satisfies the following formulas (1) and (2): The tensile strength of the steel plate is above 1180 MPa. B30 / B150<0.90 (1) 0.90≤B140 / B150≤1.10 (2) B30: the luminous intensity of B at a depth of 30 μm from the surface of the steel plate B140: the luminous intensity of B at a depth of 140 μm from the surface of the steel plate B150: the luminescence intensity of B at a depth of 150 μm from the surface of the steel plate.

2. The steel plate according to claim 1, wherein The luminescence intensity of C in the surface layer of the steel plate measured from the surface of the steel plate in the depth direction by the high-frequency glow discharge luminescence analysis satisfies the following equations (3) and (4): C40 / C150>0.50 (3) 0.90≤C140 / C150≤1.10 (4) C40: the luminous intensity of C at a depth of 40 μm from the surface of the steel plate. C140: the luminous intensity of C at a depth of 140 μm from the surface of the steel plate. C150: the luminous intensity of C at a depth of 150 μm from the surface of the steel plate.

3. The steel plate according to claim 1 or 2, characterized in that: The tensile residual stress acting in the direction perpendicular to rolling on the surface of the steel plate is 200 MPa or less.

4. The steel plate according to any one of claims 1 to 3, characterized in that A hot-dip galvanized layer or an alloyed hot-dip galvanized layer is provided on the surface of the steel plate.

5. A method for manufacturing a steel plate, characterized in that: It is a method of manufacturing steel plates, With the following processes: Hot rolling step (a) of hot rolling a slab at a finishing temperature of 850 to 950° C. to obtain a hot-rolled steel sheet, and then cooling the hot-rolled steel sheet to 450 to 650° C. and coiling the sheet, wherein the chemical composition of the slab is as follows in mass %: C：0.06～0.30％、 Si: 0.01-2.50%, Mn: 1.00~3.50%, Ti: 0.001~0.100%, B：0.0005～0.0050％、 P: 0.050% or less, S: 0.0100% or less, Al: 1.500% or less, N: 0.010% 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％、 Sn: 0-1.00%, Sb: 0-0.50%, Nb: 0~0.200%, V：0～1.00％、 As: 0~0.10%, Zn: 0-1.00%, Ca: 0~0.0100%, Mg: 0~0.0100%, Ce: 0~0.0150%, Zr：0～0.0100％、 La: 0~0.0150%, Hf: 0~0.0100%, Bi: 0~0.0100%, REM other than Ce and La: 0 to 0.0100%, and The rest: Fe and impurities; a pickling step (b) of pickling the steel sheet obtained in the hot rolling step (a); a cold rolling step (c) of cold-rolling the steel sheet obtained in the pickling step (b) at a reduction ratio of 30 to 75% to obtain a cold-rolled steel sheet; a heat treatment step (d) of subjecting the steel sheet obtained in the cold rolling step (c) to heat treatment; and a grinding step (e) of grinding the front and back surfaces of the steel sheet obtained in the hot rolling step (a) or the steel sheet obtained in the pickling step (b) using a rotary grinding brush containing abrasive grains before or after the pickling step (b); In the hot rolling step (a), the finishing rolling is performed in three or more passes, the reduction ratio of each of the final three passes of the finishing rolling is 20% or more, the time between passes is within 1 second, the temperature of the steel sheet at the entry side before the final three passes is 1000° C. or less, and the time from the completion of the final pass to the start of cooling is within 3 seconds. In the grinding step (e), the rotation speed R (rpm) of the grinding brush, the diameter D (m) of the grinding brush, and the steel plate passing speed V (m / min) satisfy the following formula (5): The heat treatment step (d) further comprises: a step (d-1) of heating the steel sheet obtained in the cold rolling step (c) from 650°C to a maximum heating temperature of Ac1+50°C or higher and 950°C or lower at an average heating rate of 0.5 to 500°C / second; a step (d-2) of maintaining the steel sheet obtained in the cold rolling step (c) at the maximum heating temperature for 1 to 300 seconds; a step (d-3) of cooling the steel sheet obtained in the cold rolling step (c) to a temperature of Ms point - 100°C or lower, and cooling the steel sheet from 700°C to 500°C at an average cooling rate of 10°C / s or higher; and a step (d-4) of holding the steel sheet obtained in the cold rolling step (c) at 200 to 350° C. for 50 to 600 seconds, In the step (d-1), in the atmosphere surrounding the steel sheet obtained in the cold rolling step (c), the water vapor partial pressure pH2O and the hydrogen partial pressure pH2 satisfy the following formula (6): -4.0≤log(pH2O / pH2) (6).

6. The method for manufacturing a steel plate according to claim 5, wherein: The hot rolling step (a) further comprises the following steps: after the hot rolled steel sheet is coiled, the hot rolled steel sheet is kept warm in a heat-insulating container whose inner wall is covered with a heat-insulating material within 30 minutes; The maximum temperature of the atmosphere inside the thermally insulated container is 500 to 650° C., and the time until the atmosphere reaches the maximum temperature is 1 to 8 hours.

7. The method for manufacturing a steel plate according to claim 5 or 6, wherein: In the step (d-1), in the atmosphere surrounding the steel sheet obtained in the cold rolling step (c), the water vapor partial pressure pH2O and the hydrogen partial pressure pH2 satisfy the following formula (7), -4.0≤log(pH2O / pH2)≤-1.0 (7).

8. The method for manufacturing a steel plate according to any one of claims 5 to 7, wherein: In the step (d-3), when the temperature of the steel sheet obtained in the cold rolling step (c) is between the Ms point and 650° C., cooling is stopped and the steel sheet is allowed to cool for 0.1 to 3.0 seconds.

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