Steel plate and its manufacturing method

Abstract

The present invention provides a steel sheet having an improved bendability post-plastic working, and also provides a method for producing this steel sheet.　The steel sheet according to the present invention has a tensile strength of at least 980 MPa; has a prescribed chemical composition and a prescribed steel structure; and has, in the surface layer region of the steel sheet, a deboronized layer in which the B emission intensities B30, B140, and B150 —according to measurement in the depth direction from the steel sheet surface by radio frequency glow discharge optical emission spectroscopy at depth positions of 30 µm, 140 µm, and 150 µm from the steel sheet surface— satisfy B30 / B150 < 0.90 and 0.90 ≤ B140 / B150 ≤ 1.10. In addition, the C emission intensities C30, C140, and C150 for the surface layer region of the steel sheet —according to measurement in the depth direction from the steel sheet surface by radio frequency glow discharge optical emission spectroscopy at depth positions of 30 µm, 140 µm, and 150 µm from the steel sheet surface— satisfy C30 / C150 ≤ 0.5 and 0.90 ≤ C140 / C150 ≤ 1.10.

Description

[Technical Field]

[0001] This invention relates to steel plates and methods for manufacturing the same. [Background technology]

[0002] In recent years, with the need to improve fuel efficiency in automobiles due to greenhouse gas emission regulations as a measure against global warming, the application of high-strength steel sheets is expanding more and more to reduce vehicle weight and ensure collision safety. In particular, there has been a growing demand for ultra-high-strength steel sheets with a tensile strength of 980 MPa or higher.

[0003] Steel sheets used for automotive parts require not only strength but also various workability properties necessary for part formation, such as press formability and weldability. Specifically, from the standpoint of press formability, steel sheets are required to have excellent elongation (total elongation in tensile tests: El).

[0004] Generally, as the strength of steel sheets increases, their press formability deteriorates. As a means of achieving both high strength and press formability in steel, TRIP steel sheets (Transformation Induced Plasticity), which utilize the transformation-induced plasticity of retained austenite, are known.

[0005] Patent documents 1 to 3 disclose high-strength TRIP steel sheets in which the structural composition fraction is controlled within a predetermined range to improve elongation and hole expansion rate. Patent document 4 also describes a high-strength steel sheet having a predetermined chemical composition, in which 15% or less by volume fraction of ferrite with an average grain size of 2 μm or less, 2 to 15% by volume fraction of retained austenite with an average grain size of 2 μm or less, 10% or less by volume fraction of martensite with an average grain size of 3 μm or less, and the remainder being bainite and tempered martensite with an average grain size of 6 μm or less, and in which an average of 10 or more cementite particles with a particle size of 0.04 μm or more are contained within the bainite and tempered martensite grains, and it is stated that this high-strength steel sheet has a tensile strength of 1180 MPa or more, as well as high elongation, hole expansion rate, and consequently excellent bendability.

[0006] Patent Document 5 discloses a TRIP steel sheet in which the stretch flange formability is improved by limiting the area ratio of lumpy (low aspect ratio) retained austenite.

[0007] Patent Document 6 discloses a high-strength TRIP steel sheet that has a large amount of work hardening in the initial stages of forming, and excellent shape-freezing properties and workability, by controlling the amount of solid-solution Si and solid-solution Mn contained in retained austenite to be above a predetermined value.

[0008] Furthermore, in addition to press formability, automotive steel sheets require excellent weldability. In particular, when welding hot-dip galvanized steel sheets together, or when welding hot-dip galvanized steel sheets to ungalvanized steel sheets, it is necessary to suppress liquid metal embrittlement (LME) cracking. This phenomenon is a crack that occurs when zinc, which has liquefied due to the heat input during welding, infiltrates and becomes brittle inside the steel sheet along the grain boundaries, and then tensile stress generated by welding acts on it.

[0009] Patent Document 7 discloses that LME cracking is more likely to occur in steel with a higher Si content. Therefore, the same document discloses a TRIP steel sheet in which Al, which has a similar effect, is added in place of some of the Si added to obtain retained austenite in TRIP steel. Furthermore, TRIP steel sheets in which Al is added in place of some Si are also disclosed in Patent Documents 8 and 9.

[0010] Furthermore, Patent Document 10 discloses a method for manufacturing a hot-dip galvanized steel sheet with excellent LME crack resistance, characterized by controlling the atmosphere during heating and annealing in a hot-dip galvanizing line.

[0011] In addition, for high-strength steel sheets used for automotive components, it is required that they do not break due to collision deformation after being formed into parts. In particular, for steel sheets used for automotive components, it is necessary to have excellent bendability after introducing plastic strain by press forming, rather than bendability before press forming. As inventions for improving the bendability of automotive steel sheets, there are Patent Documents 11, 12, 13, and 14 shown below.

[0012] Patent Document 11 discloses a steel sheet in which B is mainly in a precipitated state in the surface layer part of the steel sheet and mainly in a solid solution state inside the steel sheet, and the bendability is improved.

[0013] Patent Document 12 discloses a high-strength steel sheet having a single-phase martensite structure, in which regions having a Kernel Average Misorientation (KAM) value of 1° or more account for 50% or more, and the maximum tensile residual stress in the surface layer region from the surface to a depth of 1 / 4 of the plate thickness is 80 MPa or less, and having excellent stress corrosion cracking resistance of the cut end face and the steel sheet base material.

[0014] As a technique for improving the bending workability of high-strength steel sheets, for example, Patent Document 13 describes a high-strength cold-rolled steel sheet in which the surface layer part is mainly composed of ferrite and is manufactured by decarburizing the steel sheet. In addition, Patent Document 14 describes an ultra-high-strength cold-rolled steel sheet having a soft layer in the surface layer part and manufactured by decarburizing annealing the steel sheet.

Prior Art Documents

Patent Documents

[0015]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

[0016] However, the improvement of bending properties after the introduction of plastic strain has not necessarily been sufficiently studied to date.

[0017] Therefore, the present invention aims to provide a steel sheet that has excellent tensile strength, and improved elongation (EL), resistance to liquid metal embrittlement (LME) cracking, and bendability after plastic deformation, as well as a method for manufacturing the same. [Means for solving the problem]

[0018] The present inventors, through diligent research to solve the above problems, have found that by forming an appropriate deboronized layer and a decarburized layer on the surface of a steel sheet containing retained austenite, that is, by forming an appropriate decarburized deboronized layer, elongation (EL), liquid metal embrittlement (LME) cracking resistance, and bendability after plastic working can be improved. The present invention was completed based on this finding. The present invention includes the following embodiments.

[0019] (Aspect 1) It is a steel plate, The chemical composition of the above steel plate is, in mass%, C: 0.15~0.35%, Si: 0.01~1.20%, Mn: 1.00~3.50%, Al: 0.300~1.500%, Ti: 0.001~0.100%, B: 0.0005~0.0050%, P: 0.050% or less, S: 0.0100% 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%, Zr: 0~0.0100%, Hf: 0~0.0100%, Bi: 0~0.0100%, REM: 0~0.015%, and The remainder consists of Fe and impurities. The steel structure in the range from 1 / 8 to 3 / 8 of the plate thickness of the above steel plate is, in area %, Ferrite: 0-50%, Residual austenite: 6-30%, Total fresh martensite and cementite: 0-10% Perlite: 5% or less Tempered martensite: 5% or more, Remaining parts: Bay Knight, The surface layer of the above steel plate has a boron-free layer in which the emission intensity of B, measured in the depth direction from the surface of the steel plate by high-frequency glow discharge emission analysis, satisfies the following equations (1) and (2). Furthermore, the surface layer of the steel plate satisfies the following equations (3) and (4) when the emission intensity of C measured in the depth direction from the surface of the steel plate by the above high-frequency glow discharge emission analysis. A steel plate characterized by having a tensile strength of 980 MPa or more. B30 / B150 < 0.90 ···(1) 0.90 ≤ B140 / B150 ≤ 1.10 ···(2) C30 / C150 ≤ 0.50 ···(3) 0.90 ≤ C140 / C150 ≤ 1.10 ···(4) B30: The above luminescence intensity of B at a depth of 30 μm from the surface of the steel plate. B140: The luminescence 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. C30: The above-mentioned emission intensity of C at a depth of 30 μm from the surface of the steel plate. C140: The above-mentioned luminescence intensity of C at a depth of 140 μm from the surface of the steel plate. C150: The above-mentioned emission intensity of C at a depth of 150 μm from the surface of the steel plate.

[0020] (Aspect 2) The steel sheet according to embodiment 1, wherein the surface of the steel sheet has a hot-dip galvanized layer or an alloyed hot-dip galvanized layer.

[0021] (Aspect 3) A method for manufacturing steel plates, The chemical composition is expressed in mass percent. C: 0.15~0.35%, Si: 0.01~1.20%, Mn: 1.00~3.50%, Al: 0.300~1.500%, Ti: 0.001~0.100%, B: 0.0005~0.0050%, P: 0.050% or less, S: 0.0100% 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%, Zr: 0~0.0100%, Hf: 0~0.0100%, Bi: 0~0.0100%, REM: 0~0.015%, and The remainder: A hot rolling process (a) in which a slab containing Fe and impurities is hot-rolled at a finish rolling completion temperature of 850-950°C to obtain a hot-rolled steel sheet, and then the hot-rolled steel sheet is cooled to 450-680°C and wound up, The pickling process (b) involves pickling the steel sheet obtained in the hot rolling process (a) described above, The steel sheet obtained in the above pickling process (b) is cold-rolled at a reduction ratio of 30-75% to obtain a cold-rolled steel sheet in a cold-rolling process (c), A heat treatment step (d) is performed on the steel sheet obtained in the cold rolling step (c) described above, The process includes a grinding step (e) performed before or after the above pickling step (b), in which the front and back surfaces of the steel sheet obtained in the hot rolling step (a) or the steel sheet obtained in the above pickling step (b) are ground using a rotary grinding brush containing abrasive particles. The above hot rolling process (a) consists of three or more finish rolling passes, with a reduction ratio of 20% or more for each of the last three passes of the finish rolling, a time between passes of 1 second or less, an entry steel sheet temperature of 1000°C or less before the last three passes, and a time of 3 seconds or less from the completion of the final pass to the start of cooling. The above grinding step (e) is performed when the rotational speed R (revolutions / min) of the grinding brush, the diameter D (m) of the grinding brush, and the speed V (m / min) of the steel plate satisfy the following equation (5): The above heat treatment step (d) is: The steel sheet obtained in the cold rolling process (c) described above is heated 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 (d-1), The process (d-2) involves holding the steel sheet obtained in the cold rolling process (c) above at the maximum heating temperature for 1 to 300 seconds, A step of cooling the steel sheet obtained in the cold rolling step (c) above to a temperature below the Ms point -30°C, comprising a step (d-3) of cooling from 700°C to 500°C at an average cooling rate of 10°C / second or more, The process further comprises a step (d-4) of holding the steel sheet obtained in the cold rolling step (c) above at 300-450°C for 100-600 seconds, A method for manufacturing a steel sheet, characterized in that, in the above step (d-1), the atmosphere surrounding the steel sheet obtained in the above cold rolling step (c) satisfies the following equation (6) for the partial pressure of water vapor pH2O and the partial pressure of hydrogen pH2.

number

[0022] (Aspect 4) The above hot rolling process (a) includes a step of keeping the hot-rolled steel sheet warm in an insulated container with an inner wall covered with insulating material within 30 minutes after it has been wound up. A method for manufacturing a steel plate according to embodiment 3, characterized in that the maximum ambient temperature reached inside the above-mentioned insulated container is 500 to 650°C, and the time it takes for the ambient temperature to reach the above-mentioned maximum temperature is 1 to 8 hours. [Effects of the Invention]

[0023] The present invention makes it possible to obtain a steel sheet that has excellent tensile strength, elongation (EL), resistance to LME cracking, and bendability after plastic deformation. [Brief explanation of the drawing]

[0024] [Figure 1] Figure 1 is a schematic diagram showing a cross-section of a plated steel sheet 1, including a base steel sheet 2 according to one embodiment of the present invention, cut in the thickness direction. [Modes for carrying out the invention]

[0025] Hereinafter, a plated steel sheet containing a steel sheet as a base material, which is one embodiment of the present invention, will be described in detail with reference to Figure 1. Figure 1 is a schematic diagram showing a cross-section of a plated steel sheet 1, which contains a base material steel sheet 2 according to one embodiment of the present invention, cut in the thickness direction.

[0026] This invention defines characteristics of specific positions in the thickness direction of a steel plate. In the following description, these characteristics may be explained using the position in the thickness direction of the steel plate with respect to the surface of the steel plate.

[0027] Since the "thickness direction" and "depth direction" of a steel plate are synonymous, in this specification, the position in the thickness direction of the steel plate relative to the surface of the steel plate may be referred to as the "depth position." 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)" means a position that moves from the surface in the plate thickness direction of the steel plate, that is, the steel plate surface, toward the center of the steel plate by a distance (depth) of x / y of the plate thickness in the plate thickness direction. For example, when the plate thickness of the steel plate is t mm, the "1 / 8 depth position of the plate thickness" means a position with a depth of 1t / 8 mm from the steel plate surface in the plate thickness direction.

[0028] Here, regarding the "steel plate surface" which is the reference for the position in the plate thickness direction of the steel plate, that is, the depth position of the steel plate, in this specification, in the high-frequency glow discharge emission analysis (hereinafter sometimes referred to as "high-frequency GDS analysis") described later, the depth position at which the emission intensity of Fe reaches 0.7 times the internal Fe emission intensity is defined as the 0 μm position, and this 0 μm position is taken as the steel plate surface. The internal Fe emission intensity is the Fe emission intensity in a sufficient depth region of the base metal steel plate. This region is a region where there is almost no change in the Fe concentration in the depth direction and is a region judged as "steel" as a matter of common technical knowledge. The internal Fe emission intensity may be, for example, the Fe emission intensity at a sputtering time of 1000 seconds.

[0029] Note that the "steel plate" targeted by the present invention may be the "base metal steel plate" of a steel plate having some coating on the surface, such as the plated steel plate 1 shown in FIG. 1. In such a case, the "steel plate surface" which is the reference for the depth position of the steel plate is the steel plate surface of the base metal steel plate, but similar to the above, it is the depth position at which the emission intensity of Fe in the high-frequency GDS analysis reaches 0.7 times the internal Fe emission intensity, that is, the 0 μm position. For example, in the plated steel plate 1 shown in FIG. 1, the steel plate surface is the position indicated by the symbol "S" with a broken line near the interface between the base metal steel plate 2 and the plating layer 3. d This position is, as described above, the depth position at which the emission intensity of Fe reaches 0.7 times the internal Fe emission intensity in the high-frequency GDS analysis, that is, the 0 μm position.

[0030] Similarly, expressions such as "a depth of 30 μm from the steel plate surface" also mean a position 30 μm away from the steel plate surface, in the thickness direction, toward the center of the steel plate. For example, in the plated steel plate 1 shown in Figure 1, the steel plate surface S d P at a depth of 30 μm 30 is the surface of the steel plate S d This position is located 30 μm away from the center of the steel plate in the thickness direction.

[0031] <Plated steel sheet> As shown in Figure 1, 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 sides of the base steel sheet 2. Note that the plating layer 3 may be provided on only one side of the base steel sheet 2.

[0032] Furthermore, as shown in Figure 1, the plated steel sheet 1 has a steel sheet surface S d P at a depth of 150 μm 150 The surface layer P is defined as a region in the thickness direction of the plate. S It has.

[0033] <Base material steel plate> In this embodiment, the base steel plate 2 has the following characteristics. First, the chemical composition of the base steel sheet 2 is, in mass%, C: 0.15~0.35%, Si: 0.01~1.20%, Mn: 1.00~3.50%, Al: 0.300~1.500%, Ti: 0.001~0.100%, B: 0.0005~0.0050%, P: 0.050% or less, S: 0.0100% 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 to 1.00%, W: 0 to 1.00%, Sn: 0 to 1.00%, Sb: 0 to 0.50%, Nb: 0 to 0.200%, V: 0 to 1.00%, As: 0 to 0.10%, Zn: 0 to 1.00%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Hf: 0 to 0.0100%, Bi: 0 to 0.0100%, REM: 0 to 0.015%, and the balance: Fe and impurities.

[0034] In the range of 1 / 8 depth position to 3 / 8 depth position of the base steel plate 2, the steel structure is, in area%, ferrite: 0 to 50%, retained austenite: 6 to 30%, total of fresh martensite and cementite: 0 to 10%, pearlite: 5% or less, tempered martensite: 5% or more, and the balance: bainite.

[0035] Also, the surface layer P of the base steel plate 2 S is a deboronized layer P having a B emission intensity measured in the depth direction from the steel plate surface S d that satisfies the following formulas (1) and (2) by high-frequency glow discharge optical emission spectrometry. B It has. B30 / B150 < 0.90 ··· (1) 0.90 ≤ B140 / B150 ≤ 1.10 ··· (2) B30: B emission intensity at a depth position of 30 μm from the steel plate surface S d B140: B emission intensity at a depth position of 140 μm from the steel plate surface S d B150: B emission intensity at a depth position of 150 μm from the steel plate surface S d

[0036] ​​​Furthermore, the surface layer P of the base steel plate 2 S The surface of the steel plate S was determined by high-frequency glow discharge emission analysis. d The luminescence intensity of C measured in the depth direction satisfies the following equations (3) and (4). C30 / C150 ≤ 0.50 ···(3) 0.90 ≤ C140 / C150 ≤ 1.10 ···(4) C30: Steel plate surface S d Emission intensity of C at a depth of 30 μm C140: Steel plate surface S d Emission intensity of C at a depth of 140 μm C150: Steel plate surface S d Emission intensity of C at a depth of 150 μm

[0037] Furthermore, the tensile strength of the base steel plate 2 is 980 MPa or higher.

[0038] The following describes in detail these characteristics of the base steel sheet 2.

[0039] (chemical composition) First, the reason for limiting the chemical composition of the base steel sheet (hereinafter sometimes simply referred to as "steel sheet") according to the embodiment of the present invention as described above will be explained. In this specification, all "%" used to define chemical composition refer to "mass%" unless otherwise specified. Also, in this specification, "~" indicating a numerical range is used to mean that the values ​​written before and after it are included as the lower and upper limits, respectively, unless otherwise specified.

[0040] (C: 0.15~0.35%) Carbon (C) is an essential element for achieving the desired steel sheet strength and ensuring EL (electrolytic strength) through the formation of retained austenite, and the C content should be 0.15% or more. The C content may be 0.16% or more, 0.17% or more, or 0.18% or more. Furthermore, from the viewpoint of ensuring resistance to LME cracking and flexibility after pre-strain application, the C content should be 0.35% or less. The C content may be 0.30% or less, 0.28% or less, or 0.25% or less.

[0041] (Si: 0.01~1.20%) Silicon (Si) is an element that suppresses the formation of iron carbides and contributes to strength and securing electroluminescence (EL) through the formation of retained austenite. However, excessive Si content degrades LME crack resistance. Therefore, the Si content should be between 0.01% and 1.20%. The Si content may also be 0.05% or more, 0.10% or more, 0.15% or more, or 0.20% or more. Alternatively, the Si content may be 1.10% or less, 1.00% or less, or 0.90% or less.

[0042] (Mn: 1.00~3.50%) Manganese (Mn) is a powerful austenite-stabilizing element and is effective in increasing the strength of steel sheets. From the viewpoint of strength, weldability, and low-temperature toughness, the Mn content should be 1.00 to 3.50%. The Mn content may be 1.10% or more, 1.30% or more, or 1.50% or more. Alternatively, the Mn content may be 3.30% or less, 3.10% or less, or 3.00% or less.

[0043] (Al: 0.300~1.500%) Aluminum (Al) is an element included in steel for deoxidation. Furthermore, Al suppresses the formation of iron carbides and contributes to improving EL (electrical strength) by promoting the formation of retained austenite. To fully obtain these effects, the Al content should be 0.300% or higher. The Al content may also be 0.400% or higher, or 0.500% or higher. On the other hand, excessive Al content can cause embrittlement of the steel and degrade its bendability after pre-straining; therefore, the Al content should be capped at 1.500%. Preferably, the Al content is 1.200% or less, 1.000% or less, or 0.800% or less.

[0044] (Ti: 0.001~0.100%) Titanium (Ti) is an effective element for increasing the strength of steel sheets. From the viewpoint of strength and cost, the Ti content should be 0.001 to 0.100%. The Ti content may be 0.005% or more, 0.010% or more, 0.015% or more, or 0.020% or more. Alternatively, the Ti content may be 0.080% or less, 0.070% or less, or 0.050% or less.

[0045] (B: 0.0005~0.0050%) Boron (B) is an element that is effective in increasing the hardenability and strength of steel sheets, and is an essential element in this invention. In addition, B segregates at the austenite grain boundaries during spot welding, reinforcing the austenite grain boundaries and improving LME crack resistance. In this invention, the bendability after plastic deformation can be improved by forming a deboronized layer, described later, on the surface layer of the steel sheet. From the viewpoint of forming an appropriate deboronized layer, the B content is set to 0.0005 to 0.0050%. The B content may be 0.0007% or more, 0.0010% or more, or 0.0015% or more. Alternatively, the B content may be 0.0040% or less, 0.0035% or less, or 0.0030% or less.

[0046] (P:0.050% or less) Phosphorus (P) is an element found as an impurity in steel. While it contributes to increasing the strength of steel sheets through solid solution strengthening, the P content should be 0.050% or less from the viewpoint of weldability and toughness. Preferably, the P content is 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, since extremely low P content increases the cost of removing P, from an economic standpoint, the lower limit of the P content may be 0.0001%, 0.0005%, or 0.001%.

[0047] (S:0.0100% or less) S (sulfur) is an element present as an impurity in steel, and it forms MnS in steel sheets, degrading toughness and hole-expanding properties. Therefore, from the viewpoint of suppressing the degradation of toughness and hole-expanding properties, the S content should be 0.0100% or less. Preferably, the S content is 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, since extremely low S content increases desulfurization costs, from an economic standpoint, the lower limit of the S content may be 0.00001%, 0.00005%, or 0.0001%.

[0048] (N:0.010% or less) Nitrogen (N) is an element found as an impurity in steel. If its content exceeds 0.0100%, it forms coarse nitrides in the steel, degrading its bendability and hole-expanding properties. Therefore, the N content should be 0.0100% or less. Preferably, the N content is 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, since extremely low N content incurs high costs for nitrogen removal, from an economic standpoint, the lower limit of the N content may be 0.0001%, 0.0005%, or 0.001%.

[0049] (O:0.0100% or less) Oxygen (O) is an element present as an impurity in steel. If its content exceeds 0.0100%, it forms coarse oxides in the steel, causing it to become less flexible and its holes to widen. 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. O is not an essential element, and the lower limit of the O content is 0%. However, from the viewpoint of manufacturing costs, the lower limit of the O content may be 0.00001%, 0.00005%, or 0.0001%.

[0050] In this embodiment, the basic chemical composition of the base steel sheet 2 is as described above. Furthermore, the base steel sheet 2 may contain the following optional elements as needed.

[0051] (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%) 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 that are effective in increasing the strength of steel sheets. Therefore, one or more of these elements may be added as needed. From the standpoint of the benefits and costs of including these elements, the content of these elements shall be as follows: 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%, and Zn: 0-1.00%. The content of these elements may be 0.005% or more, or 0.010% or more, respectively.

[0052] (Ca:0~0.0100%, Mg:0~0.0100%, Zr:0~0.0100%, Hf:0~0.0100%, Bi:0~0.0100%, REM:0~0.015%) Ca (calcium), Mg (magnesium), Zr (zirconium), Hf (hafnium), and REM (rare earth elements) are all elements that contribute to the fine dispersion of inclusions in steel. Bi (bismuth) is an element that reduces the microsegregation of substitutional alloy elements such as Mn and Si in steel. Since each of these elements contributes to improving the workability of steel sheets, one or more of these elements may be added as needed. From the viewpoint of workability and ductility, the upper limit of the content of Ca, Mg, Zr, Hf, and Bi is 0.0100% each. Similarly, from the same viewpoint, the upper limit of the REM content is 0.015%. The REM content may be 0.010% or less. The content of Ca, Mg, Zr, Hf, Bi, and REM may be 0.0005% or more, or 0.0010% or more, respectively.

[0053] In this embodiment, the remainder of the base steel sheet 2, other than the above-mentioned components, consists of Fe and impurities. Here, the impurities contained in the remainder other than the above-mentioned components are components that are mixed in during the industrial production of steel sheets due to various factors in the manufacturing process, including raw materials such as ore and scrap. Such impurities include components that were not intentionally added to the base steel sheet 2. Furthermore, the impurities contained in this remainder include elements other than the above-described components, which are present in the steel sheet to the extent that the specific effects of the impurity elements do not affect the properties of the base steel sheet 2.

[0054] Furthermore, if the steel sheet is a surface-treated steel sheet, the above chemical composition refers to the content of the base steel sheet after the surface coating has been removed. Also, if the steel sheet is a steel sheet without a coating such as a plating layer or surface treatment layer, the above chemical composition refers to the content of the steel sheet itself.

[0055] The chemical composition of steel sheets can be measured using general analytical methods. For example, the chemical composition of steel sheets can be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). Specifically, the front and back surfaces of the steel sheet are ground to a depth of 200 μm from each surface to take test pieces, and the chemical composition of the steel sheet can be determined by measuring it using a measuring device such as the Shimadzu ICPS-8100 under conditions based on a pre-established calibration curve. C and S, which cannot be measured by ICP-AES, can be measured using combustion-infrared absorption spectroscopy, N can be measured using inert gas fusion-thermal conductivity spectroscopy, and O can be measured using inert gas fusion-nondispersive infrared absorption spectroscopy.

[0056] [Steel structure inside steel plate] Next, the reasons for limiting the internal structure of the base steel sheet 2 according to this embodiment will be explained. In this specification, unless otherwise specified, all "%" used to define the steel structure refer to "area %".

[0057] (Ferrite: 0-50%) Ferrite is a soft structure with excellent ductility. It may be included in steel sheets to improve their elongation, depending on the required strength and ductility. From the viewpoint of ensuring strength, the ferrite content should be limited to 50%. The ferrite content may be 45% or less, 40% or less, 35% or less, or 30% or less. The ferrite content may be 0%, 3% or more, 5% or more, or 10% or more.

[0058] (Residual austenite: 6-30%) Retained austenite is a microstructure that contributes to improving the ductility of steel sheets through the work-induced transformation effect. Therefore, the lower limit of the retained austenite content is set at 6%. Preferably, the retained austenite content is 7% or more, 8% or more, 9% or more, or 10% or more. On the other hand, retained austenite transforms into as-quenched martensite through work-induced transformation, which may degrade the bendability of the steel sheet. Also, because retained austenite is a brittle microstructure, it can become the starting point for fracture during plastic deformation, which may degrade the local ductility of the steel sheet. Therefore, the total retained austenite content is set at an upper limit of 30%. Preferably, the retained austenite content is 25% or less, 20% or less, or 18% or less.

[0059] (Total of fresh martensite and cementite: 0-10%) Fresh martensite and cementite are brittle structures and can act as fracture initiation points during plastic deformation, potentially degrading the local ductility of the steel sheet. For similar reasons, fresh martensite and cementite can degrade the bendability after pre-straining. Therefore, the total content of fresh martensite and cementite should be limited to 10%. Preferably, the total content of fresh martensite and cementite is 8% or less, 7% or less, or 6% or less. Here, cementite refers to coarse particles with a circular diameter exceeding 1 μm. This does not include minute cementite precipitated in bainite or martensite. The lower limit for the total content of fresh martensite and cementite is 0%. The total content of fresh martensite and cementite may be 1% or more, or 2% or more.

[0060] (Perlite: 5% or less) Pearlite contains hard, coarse cementite and acts as a fracture initiation point during plastic deformation. Therefore, a pearlite content exceeding 5% can degrade the local ductility of the steel sheet. For similar reasons, pearlite can degrade the bendability after pre-straining. For this reason, the pearlite content should be 5% or less. The pearlite content may also be 3% or less, or 2% or less. Furthermore, the lower limit for the pearlite content is 0%. The pearlite content may also be 1% or more, or 2% or more.

[0061] (Tempered martensite: 5% or more) Tempered martensite is a high-strength and tough structure that also increases the tensile strength and bending load of steel sheets. To obtain the desired tensile strength and bendability, the tempered martensite content should be at least 5%. Preferably, the tempered martensite content is 10% or more, 15% or more, 20% or more, 25% or more, or 30% or more. There is no particular upper limit to the tempered martensite content, but for example, it is 94%. The tempered martensite content may be 92% or less, 90% or less, or 88% or less.

[0062] (Remaining copies: Baynight) The remaining tissue other than the above-mentioned tissue may be 0%, but if such remaining tissue exists, it is bainite. Furthermore, the bainite in the remaining tissue may be either upper bainite or lower bainite, or a mixture of the two.

[0063] The fraction of the steel structure of a steel sheet is evaluated using secondary electron images captured with a field emission scanning electron microscope (FE-SEM) and X-ray diffraction, as follows.

[0064] First, a sample is taken from the thickness cross-section of the steel plate, parallel to the rolling direction, at the center of the width direction of the steel plate, and the observation surface is used. The observation surface of the sample is mechanically polished to a mirror finish, and then etched using Nital solution. Next, in one or more observation fields centered on the 1 / 4 depth position of the steel plate thickness on the observation surface, within the range of 1 / 8 to 3 / 8 depth positions, a total of 2.0 × 10 -9 m 2 A secondary electron image will be taken for the area specified above. From the obtained secondary electron images, the area fractions of ferrite, retained austenite, bainite, tempered martensite, fresh martensite, cementite, and pearlite are measured. Regions with a substructure within the grain and cementite precipitated with multiple variants are identified as tempered martensite. Regions where cementite precipitates in a lamellar pattern are identified as pearlite (or the sum of pearlite and cementite). Regions with relatively low brightness within the field of view, including all structures, and where no substructure is observed, are identified as ferrite. Regions with relatively high brightness and where the substructure is not revealed by etching are identified as fresh martensite, retained austenite, and cementite. Regions that do not fall into any of the above categories are identified as bainite. The area percentage of each tissue is calculated using the point counting method. The total area percentage of fresh martensite and cementite can be determined by subtracting the area percentage of retained austenite, which is obtained by the X-ray diffraction method described later.

[0065] The relative abundance of retained austenite is measured by X-ray diffraction. First, the steel plate is polished mechanically and chemically from the surface to a depth of 1 / 4 of its thickness in the thickness direction. Then, using MoKα1 radiation as the characteristic X-ray 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 taken as the relative abundance of retained austenite. The obtained relative abundance is considered as the area fraction.

[0066] If the total area ratio of each tissue obtained using the above evaluation method is less than 100%, the remaining region is judged to be bainite. If the total area ratio of each tissue obtained using the above evaluation method exceeds 100%, the area ratio of each tissue is determined by multiplying the area ratio of each tissue by 100 / (total area ratio of each tissue).

[0067] [Boron Removal] In this embodiment, the base steel sheet 2 has a surface layer P as described above. S Deboronized layer P B It has the following characteristics. In this specification, a "boron-free layer" is defined as a layer in which the emission intensity of B, measured in the depth direction from the surface of the steel plate by high-frequency glow discharge emission analysis (high-frequency GDS analysis), satisfies the following equations (1) and (2).

[0068] B30 / B150 < 0.90 ···(1) 0.90 ≤ B140 / B150 ≤ 1.10 ···(2)

[0069] Here, B30, B140, and B150 are the luminescence intensities of B at a depth of 30 μm from the steel plate surface, 140 μm from the steel plate surface, and 150 μm from the steel plate surface, respectively, when measured by high-frequency GDS analysis in the thickness direction from the steel plate surface.

[0070] High-frequency GDS analysis measurements are performed at five arbitrary locations. B30, B140, and B150 are determined by taking the average emission intensity of B at depths of 30 μm, 140 μm, and 150 μm from the steel plate surface at five arbitrary locations, respectively. The measurement conditions are as follows:

[0071] B30, B140, and B150 are measured using a high-frequency glow discharge emission spectrometer. Specifically, the surface of the steel plate to be measured is placed in an Ar atmosphere, and a voltage is applied to generate a glow plasma. The surface of the steel plate is then sputtered and analyzed in the depth direction. The elements contained in the steel plate are identified from the element-specific emission spectral wavelengths emitted when atoms are excited in the glow plasma, and the emission intensity of the identified elements is estimated.

[0072] The depth data can be estimated from the sputtering time. Specifically, by determining the relationship between sputtering time and sputtering depth in advance using a standard sample, the sputtering time can be converted to sputtering depth. Therefore, the sputtering depth converted from the sputtering time can be defined as the depth from the steel plate surface. The sputtering time should be set so that the sputtering depth is at least 150 μm or greater.

[0073] In high-frequency GDS analysis, commercially available analytical instruments can be used. In this embodiment, a high-frequency glow discharge emission spectrometer GD-Profiler2 (registered trademark) manufactured by Horiba, Ltd. is used. The detection pitch is set to 0.1 seconds. The obtained data is filtered after background removal. Filtering is performed using the moving average method. Specifically, a moving average of 51 points, consisting of the center point and 25 points before and after it, is calculated. The time values ​​corresponding to depths of 30 μm, 140 μm, and 150 μm are B30, B140, and B150, respectively. Other measurement conditions are as follows.

[0074] Ar gas pressure: 600 Pa Anode diameter: 4mmφ RF output: 35W

[0075] In this specification, as described above, the 0 μm position is defined as the depth position where the Fe emission intensity determined by high-frequency GDS analysis reaches 0.7 times the internal Fe emission intensity. In this definition, the internal Fe emission intensity may be, for example, the Fe emission intensity at a sputtering time of 1000 seconds.

[0076] The above equation (1) means that the boron concentration at a depth of 30 μm from the surface of the steel plate is less than 0.90 times the boron concentration at a depth of 150 μm. By satisfying this equation (1), damage to the metal structure near the surface of the steel plate is less likely to occur when the steel plate undergoes plastic deformation.

[0077] In (1) above, B30 / B150 may be 0.80 or less, less than 0.80, 0.70 or less, less than 0.70, 0.60 or less, less than 0.60, 0.50 or less, or less than 0.50. Also, B30 / B150 may be 0, but may be 0.10 or more, 0.20 or more, or 0.30 or more.

[0078] Equation (2) above means that the luminescence intensity of B at a depth of 140 μm from the surface of the steel plate is approximately equal to the luminescence intensity of B at a depth of 150 μm from the surface of the steel plate. In other words, the boron-free layer P in this embodiment B This means that the region where this can form extends to a depth of 150 μm from the surface of the steel plate. By satisfying equation (2), it is possible to prevent excessive softening at depths greater than 150 μm from the surface of the steel plate and to ensure the strength of the steel plate.

[0079] The deboronized layer P described above B By forming this layer, the bendability after plastic deformation can be improved. The reason why this effect is obtained is not clear, but the deboronized layer P B The soft surface layer formed by this method may experience less damage to the metal structure (e.g., the formation of microvoids) during plastic deformation compared to the soft surface layer formed by the decarburization layer.

[0080] As mentioned above, boron (B) segregates at the austenite grain boundaries during spot welding, strengthening the austenite grain boundaries and improving LME crack resistance. According to this mechanism, the boron-free layer P B The formation of the boron-free layer P reduces the boron content at the austenite grain boundaries, which might seem to degrade LME crack resistance. However, in reality, the deboron-free layer PB It was confirmed that even when this is formed, the LME crack resistance does not deteriorate. This is because the temperature rises near the welding point due to the heat input during spot welding, causing B inside the steel plate to rise to the surface layer P of the steel plate. S Because it diffuses to the surface layer P of the steel plate, S The amount of segregation B at the austenite grain boundary in the deboronized layer P B This is thought to be because even if a layer is formed, it will become sufficiently large. Therefore, the surface layer P of the steel plate before welding S Deboronized layer P B Even if a layer was formed, it is considered that the improvement in LME crack resistance due to B was obtained.

[0081] Furthermore, the base steel sheet 2 of this embodiment has a surface layer P S Having a decarburized layer, that is, the surface layer P of the base steel sheet 2 S By decarbonizing (hereinafter sometimes simply referred to as "decarburization") the LME crack resistance can be further improved. Specifically, the surface layer P of the base steel sheet 2 S This means that the emission intensity of C, measured in the depth direction from the surface of the steel plate by high-frequency glow discharge emission analysis (high-frequency GDS analysis), satisfies the following equations (3) and (4).

[0082] C30 / C150 ≤ 0.50 ···(3) 0.90 ≤ C140 / C150 ≤ 1.10 ···(4)

[0083] Here, C30, C140, and C150 are the emission intensities of carbon at a depth of 30 μm from the steel plate surface, 140 μm from the steel plate surface, and 150 μm from the steel plate surface, respectively, when measured by high-frequency GDS analysis in the thickness direction from the steel plate surface.

[0084] High-frequency GDS analysis measurements are performed at five arbitrary locations. For C30, C140, and C150, the average emission intensity of C at depths of 30 μm, 140 μm, and 150 μm from the steel plate surface at five arbitrary locations is adopted. The measurement conditions are the same as those for B30, B140, and B150 described above.

[0085] Equation (3) above means that decarburization has progressed to a depth of at least 30 μm from the surface of the steel sheet. By decarburizing to satisfy this equation (3), resistance to LME cracking can be further improved.

[0086] In the above formula (3), C30 / C150 may be 0.45 or less, 0.40 or less, or 0.35 or less. Also, C30 / C150 may be 0, but may be 0.10 or more, 0.15 or more, or 0.20 or more.

[0087] The degree of decarburization can be controlled in the heat treatment of the steel sheet manufacturing method described later by adjusting the atmosphere until the maximum heating temperature is reached.

[0088] Equation (4) above means that the emission intensity of carbon at a depth of 140 μm from the surface of the steel plate is approximately equal to the emission intensity of carbon at a depth of 150 μm from the surface of the steel plate. In other words, it means that the decarburization depth is 140 μm or less. Note that the carbon concentration at a depth of 150 μm from the surface of the steel plate is approximately equal to the carbon concentration at the center of the thickness of the steel plate. If equation (4) is not satisfied, that is, if decarburization proceeds excessively, the tensile strength may decrease excessively, and the desired tensile strength may not be obtained.

[0089] [Tensile strength: 980 MPa or higher] In this embodiment, the tensile strength of the base steel sheet 2 is 980 MPa or higher. Even though the base steel sheet 2 in this embodiment has such high tensile strength, the above-mentioned decarburized deboronized layer P BBy possessing this feature, it exhibits excellent resistance to LME cracking and bendability after plastic deformation. The tensile strength of the base steel sheet 2 may be 1180 MPa or higher, 1200 MPa or higher, 1300 MPa or higher, 1400 MPa or higher, or 1500 MPa or higher. There is no particular upper limit to the tensile strength of the base steel sheet 2, but from the viewpoint of toughness and formability, it may be, for example, 4000 MPa or lower, 3000 MPa or lower, or 2000 MPa or lower.

[0090] The tensile strength (TS) of a steel plate can be measured as follows: First, a No. 5 test specimen according to JIS Z 2241:2011 is taken from the center of the width of the steel plate to be measured, with the longitudinal direction perpendicular to the rolling direction. Next, the tensile strength TS (MPa) can be measured by performing a tensile test in accordance with JIS Z 2241:2011 using this test specimen.

[0091] Furthermore, if it is difficult to take a test specimen from the steel plate to be measured, the Vickers hardness of the steel plate can be measured, and the tensile strength value can be derived from the following correlation formula ("Correlation between static strength parameters," Norihiko Hasegawa, Junichi Arai, and Michishichi Tanaka, "Materials," Vol. 39, No. 442, pp. 859-863) using the measured Vickers hardness. Hv = 0.301 × TS + 5.701 However, in the above formula, Hv represents Vickers hardness and TS represents tensile strength (MPa).

[0092] The Vickers hardness of steel plates can be measured according to JIS Z 2244:2009. Specifically, the Vickers hardness of a steel plate is obtained by taking 10 measurements at a depth of 1 / 4 of the plate thickness with a load of 1 kgf (approximately 9.80 N), and taking the average of these 10 measurements. At this time, the distance between measurement positions should be at least three times the distance between indentations.

[0093] [Plating layer] As described above, in this embodiment, the base steel sheet 2 has plating layers 3 on both sides. The plating layer 3 may be a hot-dip galvanized layer having any known composition, or an alloyed hot-dip galvanized layer. The plating layer 3 may also contain additive elements such as Al in addition to Zn. Furthermore, the amount of plating layer 3 is not particularly limited and can be a general amount. The plating layer 3 may be provided on only one side of the base steel sheet 2, or it may not be provided on any surface of the base steel sheet 2. In the steel sheet of the present invention, it is not essential to have a plating layer on the surface of the steel sheet.

[0094] (Thickness of steel plate) The thickness of the steel sheet of the present invention is not particularly limited and can be the same as that of steel sheets used for automobile parts, for example. Examples of such steel sheet thicknesses include 0.5 to 3.0 mm. The thickness of the steel sheet may be 0.7 mm or more, 0.8 mm or more, or 1.0 mm or more. Alternatively, the thickness of the steel sheet may be 2.8 mm or less, 2.5 mm or less, or 2.0 mm or less.

[0095] <Method of manufacturing steel plates> Next, a method for manufacturing a steel sheet according to one embodiment of the present invention will be described. The following description is intended to illustrate a characteristic method for manufacturing a steel sheet according to one embodiment of the present invention, and is not intended to limit the steel sheet to one manufactured by the following manufacturing method.

[0096] The method for manufacturing steel sheets includes a hot rolling step (a) (hereinafter sometimes simply referred to as "step (a)") in which a slab having a specific chemical composition is hot-rolled to obtain a hot-rolled steel sheet; a grinding step (e) (hereinafter sometimes simply referred to as "step (e)") in which the hot-rolled steel sheet is ground with a rotary grinding brush; a pickling step (b) (hereinafter sometimes simply referred to as "step (b)") in which the sheet is pickled after grinding; a cold rolling step (c) (hereinafter sometimes simply referred to as "step (c)") in which the hot-rolled steel sheet is cold-rolled after pickling to obtain a cold-rolled steel sheet; and a heat treatment step (d) (hereinafter sometimes simply referred to as "step (d)") in which the cold-rolled steel sheet is heat-treated.

[0097] The following describes in detail the preferred conditions for these processes.

[0098] [Hot rolling process (a)] First, a hot-rolling process (a) is performed in which a slab having the following specific chemical composition is hot-rolled under predetermined conditions to obtain a hot-rolled steel sheet, and then this hot-rolled steel sheet is cooled to a predetermined temperature and wound up. In the hot-rolling process, the slab having the following specific chemical composition is heated before hot-rolling. Here, regarding the chemical composition of the slab, when the chemical composition of the final steel sheet is analyzed according to the analytical method described above, it can be confirmed that there is virtually no difference between it and the chemical composition of the slab. 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, in mass%, C: 0.15~0.35%, Si: 0.01~1.20%, Mn: 1.00~3.50%, Al: 0.300~1.500%, Ti: 0.001~0.100%, B: 0.0005~0.0050%, P: 0.050% or less, S: 0.0100% 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%, Zr: 0~0.0100%, Hf: 0~0.0100%, Bi: 0~0.0100%, REM: 0~0.015%, and The remainder consists of Fe and impurities. Furthermore, the preferred content of each component in the chemical composition of the slab is basically the same as that of the steel sheet described above.

[0099] In the hot rolling process, the heating temperature of the slab is not particularly limited, but it is generally preferable to set it to 1150°C or higher in order to sufficiently dissolve borides, carbides, etc. While it is preferable to cast the steel slab using a continuous casting method from the viewpoint of manufacturability, it may also be manufactured by ingot casting or thin slab casting.

[0100] (rough rolling) In this manufacturing method, the heated slab may be subjected to rough rolling before finish rolling for purposes such as adjusting the plate thickness. The conditions for such rough rolling are not particularly limited, but from the viewpoint of recrystallization during hot rolling, it is preferable to perform the rough rolling so that the total reduction ratio at 1050°C or higher is 60% or more. The total reduction ratio may be, for example, 90% or less.

[0101] (Finishing rolling) Next, the aforementioned slab is hot-rolled in a finish rolling process to obtain a hot-rolled steel sheet. The entry temperature for the finish rolling process is not particularly limited, but it is preferably set to 900-1050°C in order to obtain a suitable microstructure for the hot-rolled steel sheet. Furthermore, the total reduction ratio in the finish rolling process is preferably set to 70-95%.

[0102] In this manufacturing method, in order to form the boron-free layer described above, finish rolling is performed in three or more passes. Furthermore, the reduction ratio of each of the last three passes in the finish rolling is set to 20% or more, the interval between passes is set to within 1 second, the temperature of the entry steel sheet before the last three passes is set to 1000°C or less, and the temperature at the end of the finish rolling is set to 850-950°C. In addition, the time from the end of the final pass to the start of cooling is set to within 3 seconds. Performing finish rolling under these conditions promotes ferrite transformation by accumulating strain in austenite and softens the surface of the hot-rolled steel sheet, thereby promoting the introduction of strain into the surface by grinding in the next process. As a result, the boron-free layer described above can be formed on the final steel sheet product. The number of finish rolling passes is not particularly limited, as long as the last three passes satisfy the above conditions.

[0103] In this specification, "final three passes" refers to the three passes in the finishing rolling process that are counted from the final pass: the third pass, the second pass, and the third pass.

[0104] (Wind-up temperature: 450~680℃) The hot-rolled steel sheet, after the finish rolling described above, is cooled to a predetermined winding temperature and then wound. At this time, the winding temperature is set between 450 and 680°C from the viewpoint of the strength and workability of the hot-rolled sheet. The winding temperature may be 500°C or higher. Alternatively, the winding temperature may be 620°C or lower.

[0105] After winding is complete, a process of keeping the wound hot-rolled steel sheet warm in an insulated container may be carried out to promote the formation of a boron-free layer in the heat treatment process described later. As an example of a warming process, within 30 minutes after winding is complete, the hot-rolled steel sheet may be placed in an insulated container with an inner wall covered with insulating material for warming. In this case, the warming conditions can be set so that the maximum temperature of the atmosphere inside the container is 500 to 650°C, and the time it takes for the atmosphere temperature to reach the above maximum temperature can be 1 to 8 hours. When warming is carried out under such conditions, the surface layer of the hot-rolled steel sheet softens further, promoting the introduction of strain in the subsequent grinding process, and further promoting the formation of a boron-free layer in the heat treatment process described later, thereby further improving the bendability of the steel sheet after plastic deformation.

[0106] [Grinding process (e)] Next, a grinding process (e) is performed in which the front and back surfaces of the wound steel sheet are ground using a rotary grinding brush. Examples of brushes that can be used in the grinding process include the Hotani D-100-33. The grinding conditions are such that the rotation speed R (revolutions / min) of the grinding brush, the diameter D (m) of the grinding brush, and the speed V (m / min) of the steel sheet pass through satisfy the following equation (5). When grinding is performed under conditions that satisfy equation (5), strain is introduced into the surface layer of the steel sheet, which promotes the diffusion of boron in the heat treatment process described later, and the deboronized layer formed in the heat treatment process described later expands.

[0107]

number

[0108] In formula (5), (R·D) / V may be 11 or greater, 13 or greater, or 15 or greater. Also, there is no particular upper limit to (R·D) / V, but (R·D) / V may be 60 or less, 55 or less, or 50 or less.

[0109] Such a process (e) needs to be carried out between the completion of hot rolling and before cold rolling, and may be carried out either before or after the pickling process described later.

[0110] [Acid washing process (b)] Next, a pickling process (b) is carried out, in which the steel sheet after the hot rolling process (a) or grinding process (e) is pickled. The pickling method in the pickling process should follow conventional methods. In addition, skin pass rolling may be performed in the pickling process to correct the shape of the hot-rolled coil and improve pickling properties.

[0111] [Cold rolling process (c)] Next, a cold rolling process (c) is carried out, in which the steel sheet after the pickling process (b) or grinding process (e) is subjected to cold rolling. In the cold rolling process, the reduction ratio of the cold rolling is set to 30-75%, taking into consideration the accumulation of strain and the load on the cold rolling mill due to the rolling load. For example, the reduction ratio may be 40% or more. Alternatively, the reduction ratio may be 70% or less, or 60% or less.

[0112] [Heat treatment process (d)] Next, a heat treatment process (d) is carried out, in which the steel sheet obtained in the cold rolling process (c) is subjected to heat treatment. The heat treatment process is carried out sequentially as follows: (d-1) heating the steel sheet obtained in process (c) from 650°C to a maximum heating temperature of Ac1+50°C to 950°C at an average heating rate of 0.5 to 500°C / second; (d-2) holding the steel sheet at the above maximum heating temperature for 1 to 300 seconds; (d-3) cooling the steel sheet to a temperature of Ms point (martensitic transformation point) -30°C or lower, specifically cooling from 700°C to 500°C at an average cooling rate of 10°C / second or more; and (d-4) holding the steel sheet at 300 to 450°C for 100 to 600 seconds.

[0113] Boron removal can be sufficiently carried out by first softening the surface layer of the steel sheet by controlling the hot rolling conditions as described above, then introducing a large amount of strain into the surface layer of the steel sheet in the grinding process, and further, in the above steps (d-1) to (d-4) of this heat treatment process, i.e., the heating and soaking process, the reaction between H2O in the atmosphere and B on the surface of the steel sheet to form an oxide.

[0114] In the above-described process (d-1), the average heating rate to the maximum heating temperature is set to 0.5 to 500°C / second, from the viewpoint of promoting ferrite recrystallization and suppressing austenite coarsening. The average heating rate may be 1.0°C / second or higher, or 2.0°C / second or higher. Alternatively, the average heating rate may be 400°C / second or lower, or 300°C / second or lower. Here, "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.

[0115] In the above-described process (d-1), the maximum heating temperature shall be between Ac1 + 50°C and 950°C, from the viewpoint of promoting austenitization and suppressing the coarsening of the austenite diameter. Furthermore, in the above-described process (d-2), the holding time at the maximum heating temperature shall be between 1 and 300 seconds, from the viewpoint of the progression of austenitization and productivity. During holding at the maximum heating temperature, it is not necessary to keep the steel sheet at a constant temperature, and it may fluctuate within the above-described maximum temperature range. Here, "holding" means maintaining the temperature within a range of ±20°C, preferably ±10°C, of ​​the predetermined temperature, without exceeding the defined upper and lower limits.

[0116] After holding at the maximum heating temperature, in step (d-3) described above, the steel plate is cooled to a temperature of Ms point -30°C or lower, while cooling from 700°C to 500°C at an average cooling temperature of 10°C / second or more. The average cooling rate from 700°C to 500°C may be 20°C / second or more, 30°C / second or more, or 50°C / second or more.

[0117] To obtain the desired tissue, after cooling to a temperature below the Ms point -30°C, the tissue is held at 300-450°C for 100-600 seconds in step (d-4) described above. In step (d-4), "holding" does not mean holding at a constant temperature, as described above, but rather maintaining it within a range of ±20°C, preferably ±10°C, of ​​the predetermined temperature.

[0118] In the above-described process (d-4), when heating from 650°C to the maximum heating temperature, the atmosphere surrounding the steel sheet is controlled so that the water vapor partial pressure pH2O and hydrogen partial pressure pH2 satisfy the following equation (6). If log(pH2O / pH2) in equation (6) is less than -1.0, the decarburization reaction will not proceed sufficiently, and the desired LME crack resistance will not be obtained. Also, if log(pH2O / pH2) in equation (6) exceeds -0.1, the effect of improving bendability will saturate, and the strength of the steel sheet may decrease.

[0119] -1.0≦log(pH2O / pH2)≦-0.1 (6) pH2O: partial pressure of water vapor pH2: Hydrogen partial pressure

[0120] Note that log(pH2O / pH2) in formula (6) may be -0.9 or greater, or -0.8 or greater. Also, log(pH2O / pH2) in formula (6) may be -0.2 or less, or -0.3 or less.

[0121] As described above, a plating layer may be formed on the surface of the steel sheet of the present invention. The plating layer can be, for example, a hot-dip galvanized layer. Alternatively, if necessary, 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 plating layer and the alloying treatment may be carried out according to conventional methods and are not particularly limited. The plating treatment may be performed during the cooling process from the maximum heating temperature to a temperature of Ms point - 30°C or lower. In this case, the cooling should be stopped once at the plating treatment temperature, and after the plating treatment is completed, the sheet should be cooled to a temperature of Ms point - 30°C or lower at an average cooling rate of 10°C / second or more. Alternatively, the plating treatment may be performed after holding at 300-450°C as described above, or after the heat treatment process is completed and the sheet has cooled to room temperature, the steel sheet may be reheated to the plating bath temperature and the plating treatment may be performed.

[0122] By the above manufacturing method, a steel sheet of the present invention can be obtained that has excellent tensile strength, elongation (EL), resistance to LME cracking, and bendability after plastic deformation. The bendability is evaluated by applying a 2% pre-strain to a test piece taken from the steel sheet to be evaluated, and then performing a bending test according to the method specified in German Association of the Automotive Industry (VDA) standard 238-100, and measuring the maximum bending angle. [Examples]

[0123] Next, embodiments of the present invention will be described. The conditions in these embodiments are just one example of conditions adopted to confirm the feasibility and effectiveness of the present invention. The present invention is not limited to this one example of conditions. The present invention can adopt various conditions as long as they do not depart from the spirit of the invention and achieve the objectives of the present invention.

[0124] Steel with various chemical compositions was cast to produce slabs. These slabs were then hot-rolled to produce hot-rolled steel sheets. Furthermore, these hot-rolled steel sheets were sequentially subjected to grinding, cold-rolling, and heat treatment to produce cold-rolled steel sheets.

[0125] A portion of the obtained cold-rolled steel sheets was subjected to plating. Chemical composition analysis of samples taken from the obtained steel sheets confirmed that there was no change from the chemical composition of the slab. The chemical compositions of these steel sheets are shown in Table 1. The remainder, other than the components shown in Table 1, consists of Fe and impurities. For plated steel sheets, the chemical composition is that of the base steel sheet from which the surface plating layer was removed under the conditions described above. Underlined entries in Table 1 indicate that the chemical composition is outside the scope of the present invention.

[0126] [Table 1]

[0127] [Table 2]

[0128] The hot rolling conditions were as described in Table 2. In Table 2, R1 entry temperature refers to the entry temperature of the steel sheet at the third pass counting from the final pass of the finish rolling process. R1 represents the reduction ratio at the third pass counting from the final pass. R2 represents the reduction ratio at the second pass counting from the final pass. R3 represents the reduction ratio at the final pass. Also, t1 represents the time from the end of the third pass counting from the final pass to the start of the second pass counting from the final pass. t2 represents the time from the end of the second pass counting from the final pass to the start of the final pass. t3 represents the time from the end of the final pass to the start of cooling. Finally, R3 exit temperature refers to the temperature of the steel sheet at the end of the final pass, i.e., the temperature at which the finish rolling is complete.

[0129] Subsequently, both sides of the hot-rolled steel sheet were ground using a rotary grinding brush containing abrasive grains. The grinding conditions were set so that the value of (R·D) / V, calculated from the rotation speed R (revolutions / min) of the grinding brush, the diameter D (m) of the grinding brush, and the steel sheet passing speed V (m / min), was as shown in Table 3. Next, the ground steel sheet was pickled. Furthermore, the pickled steel sheet was cold-rolled at the reduction ratio shown in Table 3. The thickness of the cold-rolled sheet was 1.4 mm in all cases.

[0130] Subsequently, the cold-rolled steel sheets were subjected to heat treatment. The heat treatment involved heating to the maximum heating temperature, holding the temperature, and then cooling. After further cooling to a temperature below the Ms point -30°C, the temperature was held at 300-450°C. These conditions, as well as the log(pH2O / pH2) values ​​from 650°C to the maximum heating temperature, are shown in Table 3. Here, pH2O is the partial pressure of water vapor, and pH2 is the partial pressure of hydrogen. In Table 3, Ms is the martensitic transformation point (°C) of the steel used.

[0131] In Table 3, the Ac1 point (°C), which serves as the reference range for setting the maximum heating temperature for heat treatment, was determined according to the following formula. The Ac1 points for each steel sheet are shown in Table 1. Ac1=723-10.7[Mn]-16.9[Ni]+29.1[Si]+16.9[Cr] In the above formula, [Mn], [Ni], [Si], and [Cr] represent the content (mass %) of each element.

[0132] Furthermore, in Table 3, the Ms point (°C) was calculated according to the following formula. Ms=561-474[C]-33[Mn]-7.5[Si]-17[Cr]-17[Ni]-21[Mo]+10[Co] In the above formula, [C], [Mn], [Si], [Cr], [Ni], [Mo], and [Co] represent the content (mass %) of each element.

[0133] Subsequently, some of the steel sheets underwent continuous hot-dip galvanizing, and some of those underwent alloying treatment. The plating conditions were not special, but rather known and general conditions. In Table 3, GA refers to alloyed hot-dip galvanized steel sheets. GI refers to hot-dip galvanized steel sheets that have not undergone alloying treatment. CR refers to cold-rolled steel sheets that have not been plated.

[0134] In Tables 2 and 3, underlines next to various numerical values ​​indicate that they are outside the scope of the present invention, that the manufacturing conditions are such that the steel sheet of the present invention cannot be obtained, or that the various properties of the steel sheet are undesirable.

[0135] [Table 3]

[0136] For the obtained steel plates, the emission intensities of B (B30, B140, and B150) were measured at depths of 30 μm, 140 μm, and 150 μm from the steel plate surface using the high-frequency glow discharge emission spectrometry (high-frequency GDS analysis) method described above, in the thickness direction from the steel plate surface. Similarly, the emission intensities of C (C30, C140, and C150) were measured at depths of 30 μm, 140 μm, and 150 μm from the steel plate surface. These measurement results are shown in Table 4 below.

[0137] Furthermore, a No. 5 tensile test specimen, conforming to JIS Z 2241:2011, was taken from the center of the width of the obtained steel sheet, with the longitudinal direction perpendicular to the rolling direction. A tensile test was then performed using this specimen in accordance with JIS Z2241:2011, and the tensile strength (TS) and elongation (EL) were measured. For steel sheets No. 30 and No. 31, the tensile strength was measured with the plating intact, without removing the plating from the plated steel sheet. In this embodiment, the tensile strength standard (980 MPa or higher) was the same as for unplated steel sheets. The standard for the balance between tensile strength and elongation was TS. 1.5 A ×EL / 1000 value of 440 or higher was considered satisfactory. These results are shown in Table 4 below.

[0138] Furthermore, a tensile test specimen with a parallel section width of 30 mm was taken from the center of the obtained steel sheet, with the longitudinal direction perpendicular to the rolling direction. After applying a 2% pre-strain, a rectangular sample measuring 30 mm wide x 60 mm long was taken from the parallel section. Subsequently, to simulate the paint baking process of automobiles, a heat treatment was performed at 170°C for 20 minutes. The heat-treated specimens were subjected to a bending test according to the method specified in German Association of the Automotive Industry (VDA) standard 238-100, and the maximum bending angle was measured. A maximum bending angle of 60 degrees or more was judged to indicate good bendability. The bending direction was carried out so that the rolling direction was parallel to the bending ridge. For steel sheets No. 30 and No. 31, the maximum bending angle was measured with the plating intact without removing the plating from the plated steel sheet. In this embodiment, the standard for the maximum bending angle (60 degrees or more) was the same as for unplated steel sheets. The measurement results for the maximum bending angle of each steel plate are shown in Table 4 below.

[0139] Furthermore, to evaluate the resistance to liquid metal embrittlement (LME) cracking of the spot welds, test specimens measuring 150 mm wide x 50 mm long were taken from the obtained steel plates, and a two-piece spot welding test was performed. The plate combination consisted of the steel plates shown in Table 3 and commercially available alloyed hot-dip galvanized steel sheets (SGCC: JIS G 3346, plating adhesion 60 g / m²). 2Two pieces of metal (1.4 mm thick) were welded together as a pair, with a welding angle of 5 degrees. A servo motor-driven stationary spot welding test machine was used for the test. The power supply was single-phase AC 50 Hz, the applied pressure was 400 kgf, the energizing time was 20 cycles, and the hold time was 5 cycles. The welding current values ​​were set so that the diameter of the molten nugget was 4.0 times, 4.5 times, 5.0 times, and 5.5 times √t (t: plate thickness / mm), respectively. A chromium copper electrode with a tip diameter of φ6 mm and a tip radius of curvature R of 40 mm was used. Cross-sectional observation of the nugget portion was performed on the welded samples. In the cross-sectional observation, cracks of 0.2 mm or more were observed at any of the welding current values ​​mentioned above and were judged as "POOR (fail)", cracks of 0.1 to less than 0.2 mm were observed at any of the welding current values ​​mentioned above and were judged as "GOOD (pass)", and no cracks of 0.1 mm or more were observed at any of the welding current values ​​mentioned above and were judged as "EX (pass)". The evaluation results of the LME crack resistance of each steel plate are shown in Table 4 below.

[0140] Here, we will explain the points to be aware of when evaluating the present invention. The features of the present invention, such as the chemical composition, structural integrity, B concentration distribution, and C concentration distribution of the steel sheet, are defined in areas unrelated to the surface coating. On the other hand, the mechanical properties of a steel sheet can generally be slightly altered depending on whether or not it is surface coated. Even in such a situation, in the present invention, we use a steel sheet with the same surface condition as when it is used to determine whether or not its mechanical properties fall within the scope of the present invention. This is because, for those who use coated steel sheets, the mechanical properties in the coated state are important, rather than the mechanical properties in the state after the coating has been removed. Therefore, in the examples of the present invention, the mechanical properties such as tensile strength, elongation, and bendability (maximum bending angle) are evaluated in the as-plated state for plated steel sheets (steel sheets No. 16, 17, 20, 21, 23, and 24), and in the unplated state for unplated steel sheets (steel sheets other than No. 16, 17, 20, 21, 23, and 24).

[0141] In Table 4, "α" in the microstructure represents ferrite. "γ" represents retained austenite. "FM+θ" represents the sum of fresh martensite and cementite. "P" represents pearlite. "TM" represents tempered martensite. And "B" represents bainite. In Table 4, the underlines next to various numerical values ​​indicate that they are outside the scope of the present invention, that the manufacturing conditions are such that the steel sheet of the present invention cannot be obtained, or that the various properties of the steel sheet are undesirable.

[0142] [Table 4]

[0143] Steel plate No. 2 had a log(pH2O / pH2) value less than -1.0, indicating that the decarburization reaction did not proceed sufficiently, resulting in poor resistance to LME cracking.

[0144] Steel plate No. 3 was not brush-ground, and therefore a proper boron-removing layer was not formed. As a result, the maximum bending angle after pre-strain application was small, leading to poor bendability.

[0145] Steel plate No. 5 was wound at a low temperature, resulting in the formation of an inadequate boron-free layer. This led to a smaller maximum bending angle after pre-strain application, resulting in poor bendability.

[0146] In steel plate No. 6, the reduction ratio during the finish rolling process in the hot rolling stage was inappropriate, resulting in the formation of a proper boron-free layer. This led to a smaller maximum bending angle after pre-straining and consequently, poorer bendability.

[0147] In steel sheet No. 7, the inter-pass time during the finish rolling process in the hot rolling stage was long, resulting in insufficient formation of a proper boron-free layer. This led to a smaller maximum bending angle after pre-strain application and consequently, poorer bendability.

[0148] In steel plate No. 8, the high temperature at which the finish rolling was completed during the hot rolling process prevented the formation of a proper boron-free layer. As a result, the maximum bending angle after pre-strain application was small, leading to poor bendability.

[0149] In steel sheet No. 9, the entry-side steel sheet temperature was high three passes before the final pass in the hot rolling process, preventing the formation of a proper boron-free layer. As a result, the maximum bending angle after pre-strain application was small, leading to poor bendability.

[0150] Steel plate No. 10 failed to achieve the desired tensile strength because the maximum heating temperature during the heat treatment process was low, resulting in a high ferrite content.

[0151] In steel sheet No. 11, the holding temperature during the heat treatment process was lower than the required 300-450°C, resulting in a lower fraction of retained austenite and a higher total fraction of fresh martensite and cementite. As a result, TS 1.5 The ×EL / 1000 ratio was less than 440, resulting in a poor balance between tensile strength and elongation. Additionally, the maximum bending angle after pre-strain application was small, indicating poor bendability.

[0152] Steel sheet No. 12 had a shorter holding time at 300-450°C during the heat treatment process, resulting in a lower fraction of retained austenite, a higher total fraction of fresh martensite and cementite, and lower elongation. As a result, TS 1.5 The ×EL / 1000 ratio was less than 440, resulting in a poor balance between tensile strength and elongation. Additionally, the maximum bending angle after pre-strain application was small, indicating poor bendability.

[0153] Steel plate No. 13 suffered from poor bendability because the grinding process conditions were not appropriate, resulting in the formation of a proper boron-free layer. This led to a smaller maximum bending angle after pre-strain application.

[0154] Steel plate No. 14 had a low average cooling rate between 700°C and 500°C during the heat treatment process, resulting in a high ferrite fraction and failing to achieve the desired tensile strength.

[0155] In the heat treatment process for steel sheet No. 15, the holding temperature was too high during the process where it should have been held at 300-450°C. As a result, the retained austenite fraction was low, pearlite was formed, and the desired tensile strength could not be obtained. 1.5 The ×EL / 1000 value was less than 440, resulting in a poor balance between tensile strength and elongation.

[0156] In steel sheet No. 19, the high cooling end temperature during the heat treatment process resulted in a high total fraction of fresh martensite and cementite, and the absence of tempered martensite. Consequently, the maximum bending angle after pre-strain application was small, resulting in poor bendability.

[0157] Steel plate No. 33 failed to achieve the desired tensile strength due to its low carbon content in its chemical composition.

[0158] Steel plate No. 34 had a high carbon content in its chemical composition, resulting in a small maximum bending angle after pre-straining, poor bendability, and also poor resistance to LME cracking.

[0159] Steel plate No. 35 had a high Si content in its chemical composition, resulting in poor resistance to LME cracking.

[0160] Steel sheet No. 36 had a low Mn content in its chemical composition, resulting in a high ferrite fraction and a low retained austenite fraction. Furthermore, pearlite was formed, but tempered martensite was not, and consequently, the desired tensile strength could not be obtained.

[0161] Steel plate No. 37 had a high Mn content in its chemical composition, resulting in a high total fraction of fresh martensite and cementite. Consequently, the maximum bending angle after pre-strain application was small, resulting in poor bendability.

[0162] Steel plate No. 38 had a low Al content in its chemical composition, resulting in a low retained austenite fraction. As a result, TS 1.5 The ×EL / 1000 value was less than 440, resulting in a poor balance between tensile strength and elongation.

[0163] Steel plate No. 39 had a high Al content in its chemical composition, resulting in a small maximum bending angle after pre-straining and poor bendability.

[0164] Steel plate No. 40 had a low B content in its chemical composition, and a proper boron-free layer was not formed. As a result, the maximum bending angle after pre-strain application was small, resulting in poor bendability and poor resistance to LME cracking. [Explanation of Symbols]

[0165] 1. Plated steel sheet 2 Base steel plate 3 Plating layer S d steel plate surface P S surface layer P B Deboron-free layer P 30 A depth of 30 μm from the surface of the steel plate. P 150 A depth of 150 μm from the surface of the steel plate.

Claims

1. It is a steel plate, The chemical composition of the steel plate is, in mass%, C: 0.15-0.35%, Si: 0.01 to 1.20%, Mn: 1.00-3.50%, Al: 0.300-1.500%, Ti: 0.001 to 0.100%, B: 0.0005 to 0.0050%, P: 0.050% or less, S: 0.0100% or less, N: 0.010% or less, O: 0.0100% or less, Cr: 0-1.00%, Mo: 0-1.00%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Co: 0-1.00%, W: 0-1.00%, Sn: 0-1.00%, Sb: 0 to 0.50%, Nb: 0 to 0.200%, V: 0 to 1.00%, As: 0 to 0.10%, Zn: 0 to 1.00%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Hf: 0-0.0100%, Bi: 0 to 0.0100%, REM: 0-0.015%, and The remainder consists of Fe and impurities. The steel structure in the range from 1 / 8 to 3 / 8 of the thickness of the steel plate is, in area %, Ferrite: 0-50%, Residual austenite: 6-30%, Total fresh martensite and cementite: 0-10% Perlite: 5% or less Tempered martensite: 5% or more, Remaining parts: Bay Knight, The surface layer of the steel plate has a deboronized layer in which the emission intensity of B, measured in the depth direction from the surface of the steel plate by high-frequency glow discharge emission analysis, satisfies the following equations (1) and (2). Furthermore, the surface layer of the steel plate satisfies the following equations (3) and (4) when the emission intensity of C measured in the depth direction from the surface of the steel plate by high-frequency glow discharge emission analysis. A steel plate characterized by having a tensile strength of 980 MPa or more. B30 / B150<0.90...(1) 0.90 ≤ B140 / B150 ≤ 1.10 ... (2) C30 / C150≦0.50...(3) 0.90 ≤ C140 / C150 ≤ 1.10 ... (4) B30: The luminescence intensity of B at a depth of 30 μm from the surface of the steel plate. B140: The luminescence 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. C30: The luminescence intensity of C at a depth of 30 μm from the surface of the steel plate. C140: The luminescence intensity of C at a depth of 140 μm from the surface of the steel plate. C150: The luminescence intensity of C at a depth of 150 μm from the surface of the steel plate.

2. The steel sheet according to claim 1, wherein the surface of the steel sheet has a hot-dip galvanized layer or an alloyed hot-dip galvanized layer.

3. A method for manufacturing a steel sheet according to Claim 1, The chemical composition is expressed in mass percent. C: 0.15-0.35%, Si: 0.01 to 1.20%, Mn: 1.00-3.50%, Al: 0.300-1.500%, Ti: 0.001 to 0.100%, B: 0.0005 to 0.0050%, P: 0.050% or less, S: 0.0100% or less, N: 0.010% or less, O: 0.0100% or less, Cr: 0-1.00%, Mo: 0-1.00%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Co: 0-1.00%, W: 0-1.00%, Sn: 0-1.00%, Sb: 0 to 0.50%, Nb: 0 to 0.200%, V: 0 to 1.00%, As: 0 to 0.10%, Zn: 0 to 1.00%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Hf: 0-0.0100%, Bi: 0 to 0.0100%, REM: 0-0.015%, and The remainder: A hot rolling process (a) in which a slab containing Fe and impurities is hot-rolled at a finishing rolling completion temperature of 850 to 950°C to obtain a hot-rolled steel sheet, and then the hot-rolled steel sheet is cooled to 450 to 680°C and wound up, The pickling step (b) involves pickling the steel sheet obtained in the hot rolling step (a), The steel sheet obtained in the pickling step (b) is cold-rolled at a reduction ratio of 30 to 75% in a cold-rolling step (c) to obtain a cold-rolled steel sheet, A heat treatment step (d) is performed on the steel sheet obtained in the cold rolling step (c), The process includes a grinding step (e) performed before or after the pickling step (b), in which 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) are ground using a rotary grinding brush containing abrasive particles. The hot rolling process (a) consists of three or more finish rolling passes, with a reduction ratio of 20% or more for each of the last three passes of the finish rolling, a time between passes of 1 second or less, an entry steel sheet temperature of 1000°C or less before the last three passes, and a time of 3 seconds or less from the completion of the final pass to the start of cooling. The grinding step (e) is performed when the rotational speed R of the grinding brush (revolutions / min), the diameter D of the grinding brush (m), and the speed V of the steel plate passing through (m / min) satisfy the following equation (5): The heat treatment step (d) is The steel sheet obtained in the cold rolling process (c) is heated 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 (d-1), The process (d-2) involves holding the steel sheet obtained in the cold rolling process (c) at the maximum heating temperature for 1 to 300 seconds, A step of cooling the steel sheet obtained in the cold rolling step (c) to a temperature of Ms point -30°C or lower, comprising a step (d-3) of cooling from 700°C to 500°C at an average cooling rate of 10°C / second or more, The process further comprises a step (d-4) of holding the steel sheet obtained in the cold rolling step (c) at 300 to 450°C for 100 to 600 seconds, In the above step (d-1), the atmosphere surrounding the steel sheet obtained in the cold rolling step (c) is the water vapor partial pressure pH ２ O and hydrogen partial pressure pH ２ A method for manufacturing steel plates, characterized in that the following equation (6) is satisfied. [Math 1] -1.0≦log(pH ２ O / ph ２ )≦-0.1 ・・・(6)

4. The hot rolling process (a) includes a step of keeping the hot-rolled steel sheet warm in an insulated container with an inner wall covered with insulating material within 30 minutes after it has been wound up. The method for manufacturing a steel plate according to claim 3, characterized in that the maximum temperature reached by the atmosphere inside the insulated container is 500 to 650°C, and the time it takes for the atmosphere temperature to reach the maximum temperature is 1 to 8 hours.

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