Steel sheet and method for manufacturing same

Abstract

Provided are: a steel sheet having 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 980 MPa or more, has a prescribed chemical composition and steel structure, and in the 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; the steel sheet is further characterized in that the luminous intensities C30, C140, and C150 of C at the depth positions of 30 [mu] m, 140 [mu] m, and 150 [mu] m from the surface of the steel sheet when the surface layer part of the steel sheet is measured in the depth direction from the surface of the steel sheet by high-frequency glow discharge luminescence analysis satisfy C30 / C150 < = 0.5 and 0.90 < = C140 / C150 < = 1.10, and that the luminous intensities C30, C140, and C150 of C at the depth positions of 30 [mu] m, 140 [mu] m, and 150 [mu] m from the surface of the steel sheet satisfy C30 / C150 < = 0.5 and 0.90 < = C140 / C150 < = 1.10.

Description

TECHNICAL FIELD

[0001] The present application relates to a steel sheet and a method for manufacturing the same. BACKGROUND

[0002] In recent years, from the viewpoint of restriction of greenhouse gas emission amount accompanying countermeasures against global warming, improvement of fuel efficiency of automobiles is required, and for light weight of a vehicle body and securing of crash safety, application of high-strength steel sheets is gradually expanding. In particular, recently, demand for ultrahigh-strength steel sheets having a tensile strength of 980 MPa or more is increasing.

[0003] For a steel sheet for an automobile member, not only strength but also various workabilities required for press forming, weldability and the like are required. Specifically, from the viewpoint of press formability, the steel sheet is required to have excellent elongation (total elongation in a tensile test: El).

[0004] Generally, with high-strength of a steel sheet, press formability is deteriorated. As a means for balancing high-strength of a steel and press formability, a TRIP steel sheet (TRansformation Induced Plasticity) utilizing transformation induced plasticity of retained austenite is known.

[0005] Patent Documents 1 to 3 disclose high-strength TRIP steel sheets in which a microstructure composition ratio is controlled in a prescribed range, and elongation and hole expandability are improved. In addition, Patent Document 4 describes a high-strength steel sheet having a prescribed chemical composition, 15% or less of ferrite having an average crystal grain diameter of 2 μm or less in terms of a volume fraction, 2 to 15% of retained austenite having an average crystal grain diameter of 2 μm or less in terms of a volume fraction, 10% or less of martensite having an average crystal grain diameter of 3 μm or less in terms of a volume fraction, and the remaining portion being bainite and tempered martensite having an average crystal grain diameter of 6 μm or less, and further containing 10 or more cementite particles having a particle diameter of 0.04 μm or more in the bainite and tempered martensite grains, and describes that the high-strength steel sheet has a tensile strength of 1180 MPa or more, and has high elongation, hole expandability, and excellent bend workability accompanying the same.

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

[0007] Patent Document 6 describes a high-strength TRIP steel sheet which has excellent shape freezing property and workability by having a large processing solidification amount at the start of forming, by controlling a solid solution Si amount and a solid solution Mn amount contained in retained austenite to be a prescribed value or more.

[0008] Further, a steel sheet for automobiles is required to have excellent weldability in addition to press formability. In particular, in welding of hot-dip galvanized steel sheets to each other or welding of a hot-dip galvanized steel sheet to a non-plated steel sheet, it is necessary to suppress a liquid metal embrittlement (LME) crack. This phenomenon is a crack that occurs due to a tensile stress generated by welding when zinc that has been liquidized by welding heat input infiltrates / brittlezes into the inside of a steel sheet along a grain boundary.

[0009] Patent Document 7 discloses that the more Si contained in a steel, the more likely it is to generate such an LME crack. Therefore, in this document, a TRIP steel sheet in which Al having the same effect is added instead of a part of Si added in order to obtain retained austenite in a TRIP steel is disclosed. Further, in Patent Documents 8 and 9, a TRIP steel sheet in which Al is added instead of a part of Si is also disclosed.

[0010] Further, Patent Document 10 discloses a method for manufacturing a hot-dip galvanized steel sheet excellent in LME crack resistance, characterized by controlling an atmosphere at the time of heating annealing in a hot-dip galvanizing line.

[0011] Further, for a high-strength steel sheet for an automobile member, it is required not to be broken by deformation due to a collision after being formed into a member. In particular, with respect to a steel sheet used for an automobile member, it is necessary to be excellent in bendability after plastic strain is introduced by press forming, rather than bendability before press forming. As an invention for improving the bendability of a steel sheet for automobiles, Patent Documents 11, 12, 13, and 14 shown below are disclosed.

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

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

[0014] As a technique for improving the bendability of a high-strength steel sheet, for example, Patent Document 13 discloses a high-strength cold-rolled steel sheet in which a surface layer portion is composed of ferrite by decarburization treatment of a steel sheet. Further, Patent Document 14 discloses an ultra-high-strength cold-rolled steel sheet having a soft layer in a surface layer portion by decarburization annealing of a steel sheet.

[0015] Prior Art Documents

[0016] Patent Literature

[0017] Patent Literature 1: International Publication No. 2013 / 051238

[0018] Patent Literature 2: Japanese Patent Application Laid-Open No. 2006-104532

[0019] Patent Literature 3: Japanese Patent Application Laid-Open No. 2011-184757

[0020] Patent Literature 4: International Publication No. 2017 / 179372

[0021] Patent Literature 5: International Publication No. 2018 / 190416

[0022] Patent Literature 6: International Publication No. 2013 / 018741

[0023] Patent Literature 7: International Publication No. 2018 / 202916

[0024] Patent Literature 8: Japanese Patent Application Laid-Open No. 2011-17046

[0025] Patent Literature 9: International Publication No. 2013 / 144377

[0026] Patent Literature 10: International Publication No. 2018 / 234938

[0027] Patent Literature 11: International Publication No. 2017 / 002883

[0028] Patent Literature 12: Japanese Patent Application Laid-Open No. 2015-155572

[0029] Patent Literature 13: Japanese Patent Application Laid-Open No. Hei 10-130782

[0030] Patent Literature 14: Japanese Patent Application Laid-Open No. Hei 5-195149 SUMMARY

[0031] PROBLEMS TO BE SOLVED BY THE INVENTION

[0032] However, the improvement in the bendability after the introduction of plastic strain has not necessarily been sufficiently studied so far.

[0033] Therefore, an object of the present application is to provide a steel sheet excellent in tensile strength and improved in elongation (EL), liquid metal embrittlement (LME) crack resistance, and bendability after plastic working, and a method for producing the same.

[0034] MEANS FOR SOLVING THE PROBLEMS

[0035] The present inventors have conducted intensive studies in order to solve the above-described problems, and as a result, have found that, for a steel sheet including residual austenite, by forming an appropriate decarburized layer and a decarburized layer in a surface layer portion, that is, forming an appropriate decarburized layer after decarburization, it is possible to improve the elongation (EL), liquid metal embrittlement (LME) cracking resistance, and bendability after plastic working. The present invention is based on such insight. The present invention includes each mode described below.

[0036] (Mode 1)

[0037] A steel sheet characterized in that the chemical composition of the steel sheet is, in mass%,

[0038] C: 0.15 to 0.35%,

[0039] Si: 0.01 to 1.20%,

[0040] Mn: 1.00 to 3.50%,

[0041] Al: 0.300 to 1.500%,

[0042] Ti: 0.001 to 0.100%,

[0043] B: 0.0005 to 0.0050%,

[0044] P: 0.050% or less,

[0045] S: 0.0100% or less,

[0046] N: 0.010% or less,

[0047] O: 0.0100% or less,

[0048] Cr: 0 to 1.00%,

[0049] Mo: 0 to 1.00%,

[0050] Cu: 0 to 1.00%,

[0051] Ni: 0 to 1.00%,

[0052] Co: 0 to 1.00%,

[0053] W: 0 to 1.00%,

[0054] Sn: 0 to 1.00%,

[0055] Sb: 0 to 0.50%,

[0056] Nb: 0 to 0.200%,

[0057] V: 0 to 1.00%,

[0058] As: 0~0.10%,

[0059] Zn: 0-1.00%,

[0060] Ca: 0~0.0100%,

[0061] Mg: 0~0.0100%,

[0062] Zr: 0~0.0100%,

[0063] Hf: 0~0.0100%,

[0064] Bi: 0~0.0100%,

[0065] REM: 0 to 0.015%, and

[0066] The rest: Fe and impurities,

[0067] The steel structure within the range of 1 / 8 to 3 / 8 of the thickness of the steel plate is expressed in area %.

[0068] Ferrite: 0-50%,

[0069] Retained austenite: 6-30%,

[0070] Total of primary martensite and cementite: 0-10%,

[0071] Pearlite: less than 5%,

[0072] Tempered martensite: more than 5% and

[0073] The rest: bainite,

[0074] The surface layer of the steel sheet has a deboronized layer in which the luminescence intensity of B measured from the surface of the steel sheet in the depth direction by high-frequency glow discharge luminescence analysis satisfies the following formulas (1) and (2):

[0075] Furthermore, 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):

[0076] The tensile strength of the steel plate is 980 MPa or higher.

[0077] B30 / B150<0.90 (1)0.90≤B140 / B150≤1.10 (2)

[0078] C30 / C150≤0.50 (3)0.90≤C140 / C150≤1.10 (4)

[0079] B30: the above-described emission intensity of B at a depth position of 30 μm from the surface of the steel sheet

[0080] B140: the above-described emission intensity of B at a depth position of 140 μm from the surface of the steel sheet

[0081] B150: the above-described emission intensity of B at a depth position of 150 μm from the surface of the steel sheet

[0082] C30: the above-described emission intensity of C at a depth position of 30 μm from the surface of the steel sheet

[0083] C140: the above-described emission intensity of C at a depth position of 140 μm from the surface of the steel sheet

[0084] C150: the above-described emission intensity of C at a depth position of 150 μm from the surface of the steel sheet

[0085] (way 2)

[0086] The steel sheet according to the above-described way 1, wherein the steel sheet has a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface thereof.

[0087] (way 3)

[0088] A method for manufacturing a steel sheet, characterized by comprising:

[0089] a hot-rolling step (a) in which a chemical composition is

[0090] C: 0.15 to 0.35%,

[0091] Si: 0.01 to 1.20%,

[0092] Mn: 1.00 to 3.50%,

[0093] Al: 0.300 to 1.500%,

[0094] Ti: 0.001 to 0.100%,

[0095] B: 0.0005 to 0.0050%,

[0096] P: 0.050% or less,

[0097] S: 0.0100% or less,

[0098] N: 0.010% or less,

[0099] O: 0.0100% or less,

[0100] Cr: 0 to 1.00%,

[0101] Mo: 0 to 1.00%,

[0102] Cu: 0 to 1.00%,

[0103] Ni: 0 to 1.00%,

[0104] Co: 0 to 1.00%,

[0105] W: 0 to 1.00%,

[0106] Sn: 0 to 1.00%,

[0107] Sb: 0 to 0.50%,

[0108] Nb: 0 to 0.200%,

[0109] V: 0 to 1.00%,

[0110] As: 0 to 0.10%,

[0111] Zn: 0 to 1.00%,

[0112] Ca: 0 to 0.0100%,

[0113] Mg: 0 to 0.0100%,

[0114] Zr: 0 to 0.0100%,

[0115] Hf: 0 to 0.0100%,

[0116] Bi: 0 to 0.0100%,

[0117] REM: 0 to 0.015%, and

[0118] the remainder: Fe and impurities, after hot-rolling the slab of the remainder: Fe and impurities at a finish rolling temperature of 850 to 950°C to obtain a hot-rolled steel sheet, the hot-rolled steel sheet is cooled to 450 to 680°C and coiled;

[0119] (b) pickling the steel sheet obtained in the hot-rolling step (a);

[0120] (c) 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;

[0121] (d) applying a heat treatment to the steel sheet obtained in the cold-rolling step (c); and

[0122] (e) before or after the pickling step (b), using a rotary abrasive brush including abrasive grains, 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),

[0123] In the hot rolling process (a), the finish rolling is performed in 3 or more passes, the reduction ratio of each of the last 3 passes of the finish rolling is 20% or more, the interval time between the passes is 1 second or less, the entry side steel sheet temperature before the last 3 passes is 1000°C or less, and the time from the end of the last pass to the start of the cooling is 3 seconds or less,

[0124] In the grinding process (e), the rotation speed R (rotation / minute) of the grinding brush, the diameter D (m) of the grinding brush, and the sheet passing speed V (m / minute) of the steel sheet satisfy the following equation (5),

[0125] The heat treatment process (d) further includes:

[0126] Process (d-1) of heating the steel sheet obtained in the cold rolling process (c) at an average heating rate of 0.5 to 500°C / second from 650°C to a maximum heating temperature of Ac1+50°C or higher and 950°C or lower,

[0127] Process (d-2) of holding the steel sheet obtained in the cold rolling process (c) at the maximum heating temperature for 1 to 300 seconds,

[0128] Process (d-3) of cooling the steel sheet obtained in the cold rolling process (c) to a temperature of Ms point-30°C or lower at an average cooling rate of 10°C / second or higher from 700°C to 500°C, and

[0129] Process (d-4) of holding the steel sheet obtained in the cold rolling process (c) at 300 to 450°C for 100 to 600 seconds,

[0130] In the process (d-1), in the atmosphere surrounding the steel sheet obtained in the cold rolling process (c), the water vapor partial pressure pH2O and the hydrogen partial pressure pH2 satisfy the following equation (6),

[0131] [Mathematical formula 1]

[0132]

[0133] -1.0≤log(pH2O / pH2)≤-0.1 (6).

[0134] (Manner 4)

[0135] The steel sheet manufacturing method according to the above-described manner 3, characterized in that,

[0136] The hot rolling process (a) includes a process of, after coiling the hot rolled steel sheet, covering the inner wall with a heat insulating material in a heat insulating container within 30 minutes,

[0137] The maximum reached temperature of the atmosphere temperature inside the above-mentioned heat-insulating container is 500 to 650°C, and the time until the above-mentioned atmosphere temperature reaches the above-mentioned maximum reached temperature is 1 to 8 hours.

[0138] Effects of the Invention

[0139] According to the present application, a steel sheet having excellent tensile strength and excellent elongation (EL), LME crack resistance, and bending property after plastic working can be obtained. BRIEF DESCRIPTION OF DRAWINGS

[0140] Figure 1 is a view schematically showing a cross section cut in the sheet thickness direction of a plated steel sheet 1 including a base steel sheet 2 of one embodiment of the present application. DETAILED DESCRIPTION

[0141] Hereinafter, the present application will be described in detail with reference to Figure 1 A plated steel sheet including a steel sheet of one embodiment of the present application as a base steel sheet will be described in detail. Note that, Figure 1 is a view schematically showing a cross section cut in the sheet thickness direction of a plated steel sheet 1 including a base steel sheet 2 of one embodiment of the present application.

[0142] In the present application, characteristics of a specific position in the sheet thickness direction of a steel sheet are specified. In the following description, the characteristics are sometimes described using a position in the sheet thickness direction of a steel sheet with reference to the surface of the steel sheet.

[0143] Note that the "sheet thickness direction" of a steel sheet is the same as the "depth direction"; therefore, in this specification, a position in the sheet thickness direction of a steel sheet with reference to the surface of the steel sheet is sometimes referred to as a "depth position".

[0144] In this connection, in this specification, an "x / y depth position of the sheet thickness (here, x and y are natural numbers satisfying x < y)" means a position at a distance (depth) of x / y of the sheet thickness from the surface in the sheet thickness direction of a steel sheet, i.e., the surface of the steel sheet in the sheet thickness direction toward the center portion of the steel sheet. For example, in the case where the sheet thickness of a steel sheet is t mm, a "1 / 8 depth position of the sheet thickness" means a position at a depth of 1 t / 8 mm in the sheet thickness direction from the surface of the steel sheet.

[0145] Here, regarding the "steel plate surface," which serves as a reference for the position in the thickness direction of the steel plate, that is, the depth position of the steel plate, 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 Fe luminescence intensity reaches 0.7 times the internal Fe luminescence intensity is defined as the 0 μm position, and this 0 μm position is regarded as the steel plate surface. The internal Fe luminescence intensity is the Fe luminescence intensity in a sufficient depth region of the parent steel plate. This region is an area where the Fe concentration in the depth direction hardly changes, and is determined to be "steel" based on technical common sense. The internal Fe luminescence intensity can be, for example, the Fe luminescence intensity at a sputtering time of 1000 seconds.

[0146] It should be noted that the "steel plate" that is the subject of the present invention may be Figure 1 The "base steel plate" is a steel plate having some coating on its surface, such as the plated steel plate 1 shown. In this case, the "steel plate surface" serving as the reference for the depth position of the steel plate is the steel plate surface of the base steel plate, but as described above, it is the depth position at which the Fe luminescence intensity in high-frequency GDS analysis reaches 0.7 times the Fe luminescence intensity inside, that is, the 0 μm position.

[0147] For example, in Figure 1 In the plated steel sheet 1 shown, the surface of the steel sheet is near the interface between the base steel sheet 2 and the plated layer 3, which is indicated by the dashed line “S 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.

[0148] 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, 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 was moved by a distance of 30 μm toward the center of the steel plate in the plate thickness direction.

[0149] <Plated Steel Sheet>

[0150] like Figure 1 As shown, 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. Alternatively, the plating layer 3 may be provided on only one surface of the base steel sheet 2.

[0151] In addition, if Figure 1 As shown, the plated steel sheet 1 has a surface layer portion P S , the surface part PS It is divided into the steel plate surface S d To a depth of 150 μm 150 The area in the thickness direction of the plate.

[0152] <Base Steel Plate>

[0153] Furthermore, in the present embodiment, the base material steel plate 2 has the following characteristics.

[0154] First, the chemical composition of the base steel plate 2 is expressed in mass %.

[0155] C: 0.15-0.35%,

[0156] Si: 0.01-1.20%,

[0157] Mn: 1.00~3.50%,

[0158] Al: 0.300~1.500%,

[0159] Ti: 0.001~0.100%,

[0160] B: 0.0005~0.0050%,

[0161] P: 0.050% or less,

[0162] S: 0.0100% or less,

[0163] N: 0.010% or less,

[0164] O: 0.0100% or less,

[0165] Cr: 0-1.00%,

[0166] Mo: 0-1.00%,

[0167] Cu: 0-1.00%,

[0168] Ni: 0-1.00%,

[0169] Co: 0-1.00%,

[0170] W: 0~1.00%,

[0171] Sn: 0-1.00%,

[0172] Sb: 0-0.50%,

[0173] Nb: 0~0.200%,

[0174] V: 0~1.00%

[0175] As: 0~0.10%,

[0176] Zn: 0 to 1.00%,

[0177] Ca: 0 to 0.0100%,

[0178] Mg: 0 to 0.0100%,

[0179] Zr: 0 to 0.0100%,

[0180] Hf: 0 to 0.0100%,

[0181] Bi: 0 to 0.0100%,

[0182] REM: 0 to 0.015%, and

[0183] the remainder: Fe and impurities.

[0184] The steel structure in the range of the 1 / 8 depth position to the 3 / 8 depth position of the plate thickness of the base material steel plate 2, in terms of area %, is ferrite: 0 to 50%, retained austenite: 6 to 30%, the total of primary martensite and cementite: 0 to 10%, pearlite: 5% or less, tempered martensite: 5% or more, and the remainder: bainite.

[0185] Further, the surface layer portion P S has a luminescence intensity of B measured in the depth direction from the steel plate surface S d satisfying the following formula (1) and formula (2). B .

[0186] B30 / B150 < 0.90 (1) 0.90 ≤ B140 / B150 ≤ 1.10 (2)

[0187] B30: the luminescence intensity of B at a depth position of 30 μm from the steel plate surface S d .

[0188] B140: the luminescence intensity of B at a depth position of 140 μm from the steel plate surface S d .

[0189] B150: the luminescence intensity of B at a depth position of 150 μm from the steel plate surface S d .

[0190] Further, the surface layer portion P S of the base material steel plate 2 has a luminescence intensity of C measured in the depth direction from the steel plate surface S d satisfying the following formula (3) and formula (4).

[0191] C30 / C150 < 0.50 (3) 0.90 < C140 / C150 < 1.10 (4)

[0192] C30: Emission intensity of C at a depth position of 30 μm from the surface S of the steel sheet d C30: Emission intensity of C at a depth position of 30 μm from the surface S of the steel sheet

[0193] C140: Emission intensity of C at a depth position of 140 μm from the surface S of the steel sheet d C140: Emission intensity of C at a depth position of 140 μm from the surface S of the steel sheet

[0194] C150: Emission intensity of C at a depth position of 150 μm from the surface S of the steel sheet d C150: Emission intensity of C at a depth position of 150 μm from the surface S of the steel sheet

[0195] Further, the tensile strength of the base steel sheet 2 is 980 MPa or more.

[0196] Hereinafter, these features in the base steel sheet 2 will be described in detail.

[0197] (Chemical composition)

[0198] First, the reason for limiting the chemical composition of the base steel sheet (hereinafter, sometimes simply referred to as "steel sheet") of the embodiment of the present application as described above will be described. Note that the "%" prescribed in the present specification is "mass %" unless otherwise specified. Also, in the present specification, "~" indicating a numerical range is used with the meaning that the numerals recited before and after it are included as lower limit and upper limit values, unless otherwise specified.

[0199] (C: 0.15 to 0.35%)

[0200] C (carbon) is an element necessary for ensuring EL due to the desired steel sheet strength and the generation of retained austenite, and the C content is 0.15% or more. The C content can be 0.16% or more, 0.17% or more, or 0.18% or more. Also, from the viewpoint of ensuring LME crack resistance and imparting bendability after pre-straining, the C content is 0.35% or less. The C content can be 0.30% or less, 0.28% or less, or 0.25% or less.

[0201] (Si: 0.01 to 1.20%)

[0202] Si (silicon) is an element that suppresses the generation of iron carbide and contributes to the ensuring of EL due to the strength and the generation of retained austenite. However, if the Si content is excessive, the LME crack resistance deteriorates. Therefore, the Si content is 0.01 to 1.20%. The Si content can be 0.05% or more, 0.10% or more, 0.15% or more, or 0.20% or more. Also, the Si content can be 1.10% or less, 1.00% or less, or 0.90% or less.

[0203] (Mn: 1.00 to 3.50%)

[0204] Mn (manganese) is a strong austenite stabilizing element and is an element effective for high strength of the steel sheet. From the viewpoints of strength, weldability, and low-temperature toughness, the Mn content is 1.00 to 3.50%. The Mn content can be 1.10% or more, 1.30% or more, or 1.50% or more. In addition, the Mn content can be 3.30% or less, 3.10% or less, or 3.00% or less.

[0205] (Al: 0.300 to 1.500%)

[0206] Al (aluminum) is an element contained for deoxidization of the steel. In addition, Al is an element that suppresses generation of iron carbide and contributes to improvement of EL due to generation of retained austenite. From the viewpoint of sufficiently obtaining such effects, the Al content is 0.300% or more. The Al content can be 0.400% or more or 0.500% or more. On the other hand, if Al is excessively contained, the steel becomes brittle and the bendability after imparting a pre-strain deteriorates, and thus the Al content is capped at 1.500%. The Al content is preferably 1.200% or less, 1.000% or less, or 0.800% or less.

[0207] (Ti: 0.001 to 0.100%)

[0208] Ti (titanium) is an element effective for high strength of the steel sheet. From the viewpoints of high strength and cost, the Ti content is 0.001 to 0.100%. The Ti content can be 0.005% or more, 0.010% or more, 0.015% or more, or 0.020% or more. In addition, the Ti content can be 0.080% or less, 0.070% or less, or 0.050% or less.

[0209] (B: 0.0005 to 0.0050%)

[0210] B (boron) is an element that improves hardenability of the steel sheet and is effective for high strength, and is a necessary element in the present application. In addition, B segregates at the austenite grain boundaries at the time of spot welding, and improves LME crack resistance by strengthening the austenite grain boundaries. In the present application, by forming a boron-depleted layer described later in the surface layer portion of the steel sheet, the bendability after plastic working can be improved. From the viewpoint of forming an appropriate boron-depleted layer, the B content is set to 0.0005 to 0.0050%. The B content can be 0.0007% or more, 0.0010% or more, or 0.0015% or more. In addition, the B content can be 0.0040% or less, 0.0035% or less, or 0.0030% or less.

[0211] (P: 0.050% or less)

[0212] P (phosphorus) is an element contained in the steel as an impurity. It also contributes to the high strength of the steel sheet by solid solution strengthening, but from the viewpoints of weldability and toughness, the P content is 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 P removal cost becomes high, and thus from the viewpoint of economy, the lower limit of the P content can be 0.0001%, 0.0005%, or 0.001%.

[0213] (S: 0.0100% or less)

[0214] S (sulfur) is an element contained in the steel as an impurity, and is an element that deteriorates toughness and hole expandability by forming MnS in the steel sheet. Therefore, from the viewpoint of suppressing deterioration of toughness and hole expandability, the S content is 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, in order to extremely reduce the S content, the desulfurization cost becomes high, and thus from the viewpoint of economy, the lower limit of the S content can be 0.00001%, 0.00005%, or 0.0001%.

[0215] (N: 0.010% or less)

[0216] N (nitrogen) is an element contained in the steel as an impurity, and is an element that deteriorates bendability and hole expandability by forming coarse nitrides in the steel when the content exceeds 0.0100%. Therefore, the N content is 0.0100% 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, in order to extremely reduce the N content, the N removal cost becomes high, and thus from the viewpoint of economy, the lower limit of the N content can be 0.0001%, 0.0005%, or 0.001%.

[0217] (O: 0.0100% or less)

[0218] O (oxygen) is an element contained in the steel as an impurity, and is an element that deteriorates bendability and hole expandability by forming coarse oxides in the steel when the content exceeds 0.0100%. Therefore, the O content is 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 viewpoint of manufacturing cost, the lower limit of the O content can be 0.00001%, 0.00005%, or 0.0001%.

[0219] In the present embodiment, the base steel sheet 2 has the above-described basic chemical composition. Furthermore, the base steel sheet 2 can also contain, as necessary, any of the following optional elements.

[0220] (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 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%)

[0221] 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 for high-strength steel sheets. Therefore, one or two or more of these elements can be added as necessary. From the viewpoint of the effects and costs associated with the inclusion of these elements, the contents of these elements are respectively 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 to 1.00%, Sb: 0 to 0.50%, Nb: 0 to 0.200%, V: 0 to 1.00%, As: 0 to 0.10%, and Zn: 0 to 1.00%. The contents of these elements can each be 0.005% or more or 0.010% or more.

[0222] (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%)

[0223] Ca (calcium), Mg (magnesium), Zr (zirconium), Hf (hafnium), and REM (rare earth element) are all elements that contribute to the fine dispersion of inclusions in steel. Bi (bismuth) is an element that mitigates microscopic segregation of substitution-type alloying elements such as Mn and Si in steel. These elements each contribute to the improvement of the workability of the steel sheet, and therefore one or two or more of these elements can also be added as necessary. From the viewpoints of workability and ductility, the upper limits of the contents of Ca, Mg, Zr, Hf, and Bi are respectively 0.0100%. Also, from the same viewpoints, the upper limit of the REM content is 0.015%. Also, the REM content can be 0.010 or less. The contents of Ca, Mg, Zr, Hf, Bi, and REM can each be 0.0005% or more or 0.0010% or more.

[0224] In the present embodiment, the remaining portion other than the above-described respective components of the base material steel sheet 2 is composed of Fe and impurities. Here, the impurities contained in the remaining portion other than the above-described respective components are components that are mixed due to various reasons in the manufacturing process, represented by raw materials such as ores, scrap, and the like, when a steel sheet is manufactured industrially. The impurities include substances that are not intentionally added to the base material steel sheet 2. In addition, the impurities contained in the remaining portion are elements other than the above-described respective components described above, and the effects unique to the elements of the impurities are also included in the elements contained in the steel sheet, within a range that does not affect the characteristics of the base material steel sheet 2.

[0225] Note that, in the case where the steel sheet is a surface-treated steel sheet, the above-described chemical composition is the content with respect to the base material steel sheet from which the surface-coating is peeled off. In addition, in the case where the steel sheet is a steel sheet that is not coated with a plated layer, a surface treatment layer, or the like, the above-described chemical composition is the content with respect to the steel sheet itself.

[0226] The chemical composition of the steel sheet can be measured by a general analysis method. For example, the chemical composition of the steel sheet can be measured using Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES). Specifically, test pieces are collected by grinding the front and back surfaces of the steel sheet to a depth of 200 μm from the respective steel sheet surfaces, and the chemical composition of the steel sheet can be specified by measuring using a measuring device such as an ICP S-8100 manufactured by Shimadzu Corporation under conditions based on a standard curve prepared in advance. C and S, which cannot be measured by ICP-AES, can be measured using a combustion-infrared absorption method, N can be measured using an inert gas fusion-thermal conductivity method, and O can be measured using an inert gas fusion-non-dispersive infrared absorption method.

[0227] [Steel structure inside the steel sheet]

[0228] Next, reasons for limiting the internal structure of the base material steel sheet 2 of the present embodiment will be described. Note that, in the present specification, "%" for specifying a steel structure is "area %" unless otherwise specified.

[0229] (Ferrite: 0 to 50%)

[0230] Ferrite is a structure that is excellent in ductility but is soft. In order to improve the elongation of the steel sheet, it can be contained depending on the strength and ductility required. From the viewpoint of securing strength, the ferrite content is capped at 50%. The ferrite content can be 45% or less, 40% or less, 35% or less, or 30% or less. The ferrite content can be 0%, or 3% or more, 5% or more, or 10% or more.

[0231] (Retained Austenite: 6 to 30%)

[0232] The retained austenite is a structure that contributes to the improvement of the ductility of the steel sheet by the effect of the processing-induced phase transformation. Therefore, the lower limit of the retained austenite content is 6%. The retained austenite content is preferably 7% or more, 8% or more, 9% or more, or 10% or more. On the other hand, the retained austenite is transformed into martensite in the quenched state due to the processing-induced phase transformation, and thus sometimes deteriorates the bendability of the steel sheet. In addition, the retained austenite is a brittle structure, and thus becomes a starting point of damage at the time of plastic deformation, and sometimes deteriorates the local ductility of the steel sheet. Therefore, the total of the retained austenite content is capped at 30%. The retained austenite content is preferably 25% or less, 20% or less, or 18% or less.

[0233] (Combined amount of proeutectoid ferrite and cementite: 0 to 10%)

[0234] The proeutectoid ferrite and the cementite are brittle structures, and thus become a starting point of damage at the time of plastic deformation, and sometimes deteriorate the local ductility of the steel sheet. For the same reason, the proeutectoid ferrite and the cementite sometimes deteriorate the bendability after the pre-strain is imparted. Therefore, the combined amount of the proeutectoid ferrite and the cementite is capped at 10%. The combined amount of the proeutectoid ferrite and the cementite is preferably 8% or less, 7% or less, or 6% or less. Here, the cementite is targeted for coarse particles having a circular equivalent diameter of more than 1 μm. The fine cementite precipitated in the bainite or the martensite is not included. In addition, the lower limit of the combined amount of the proeutectoid ferrite and the cementite is 0%. The combined amount of the proeutectoid ferrite and the cementite can be 1% or more or 2% or more.

[0235] (pearlite: 5% or less)

[0236] The pearlite contains hard and coarse cementite, and thus becomes a starting point of damage at the time of plastic deformation, and thus sometimes deteriorates the local ductility of the steel sheet, particularly when the pearlite content exceeds 5%. For the same reason, the pearlite sometimes deteriorates the bendability after the pre-strain is imparted. Therefore, the pearlite content is made to be 5% or less. The pearlite content can be 3% or less or 2% or less. In addition, the lower limit of the pearlite content is 0%. The pearlite content can be 1% or more or 2% or more.

[0237] (tempers martensite: 5% or more)

[0238] The tempered martensite is a structure that is high in strength and strong in toughness, and is also a structure that improves the tensile strength and the bending load of the steel sheet. In order to obtain the desired tensile strength and the bendability, the lower limit of the tempered martensite content is 5% or more. The tempered martensite content is preferably 10% or more, 15% or more, 20% or more, 25% or more, or 30% or more. The upper limit of the tempered martensite content is not particularly limited, and is, for example, 94%. The tempered martensite content can be 92% or less, 90% or less, or 88% or less.

[0239] (Remainder: Bainite)

[0240] 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.

[0241] The fraction of the steel structure of the steel sheet was evaluated by a secondary electron image taken using a field emission scanning electron microscope (FE-SEM) and an X-ray diffraction method as described below.

[0242] First, a sample was collected from a thickness section parallel to the rolling direction of the steel plate, i.e., a thickness section at the center of the width of the steel plate, as the observation surface. The observation surface of the sample was mechanically polished to a mirror finish and then etched with Nital.

[0243] Next, in one or more observation fields ranging from the 1 / 8 depth position to the 3 / 8 depth position centered at the 1 / 4 depth position of the steel plate thickness on the observation surface, a total of 2.0×10 -9 m 2 A secondary electron image is taken of an area with an area above the above range.

[0244] The area fraction of ferrite, retained austenite, bainite, tempered martensite, fresh martensite, cementite and pearlite is measured respectively from the secondary electron image obtained. The region with lower organization and cementite having multiple variants and precipitated in the grain is judged as tempered martensite. The region where cementite is precipitated in a lamellar shape is judged as pearlite (or the total of pearlite and cementite). The region where brightness is relatively small and the lower organization is not confirmed in the field of view comprising various organizations is judged as ferrite. The region where brightness is relatively large and the lower organization is not revealed by etching is judged as fresh martensite, retained austenite and cementite. The region not belonging to any of the above-mentioned regions is judged as bainite.

[0245] The area ratio of each structure is calculated by the point counting method to obtain the area ratio of each structure. The total area ratio of fresh martensite and cementite can be obtained by subtracting the area ratio of retained austenite obtained by the X-ray diffraction method described below.

[0246] The abundance ratio of retained austenite is measured by X-ray diffraction.

[0247] First, by mechanical polishing and chemical polishing, the surface of the steel sheet to a depth of 1 / 4 of the thickness of the steel sheet was removed. Then, the existence ratio of retained austenite was calculated from the integrated intensity ratio of the diffraction peaks of (200) and (211) of the bcc phase and (200), (220), and (311) of the fcc phase obtained from the polished sample using MoKα1 line as characteristic X-rays. The obtained existence ratio was regarded as the area ratio.

[0248] In the case where the total area ratio of each structure obtained by the above evaluation method is less than 100%, the remaining area was determined as bainite. In the case where the total area ratio of each structure obtained by the above evaluation method exceeds 100%, the value obtained by multiplying the area ratio of each structure by 100 / (the total area ratio of each structure) was regarded as the area ratio of each structure.

[0249] [De-boron layer]

[0250] In the present embodiment, the base steel sheet 2 is as described above in the surface layer portion P S has a de-boron layer P B In the present specification, a layer in which the luminescence intensity of B measured from the surface of the steel sheet in the depth direction by high-frequency glow discharge spectroscopy (high-frequency GDS analysis) satisfies the following equations (1) and (2) is defined as a "de-boron layer".

[0251] B30 / B150 < 0.90 (1) 0.90 ≤ B140 / B150 ≤ 1.10 (2)

[0252] Here, B30, B140, and B150 are the luminescence intensity of B at a depth position of 30 μm from the surface of the steel sheet, the luminescence intensity of B at a depth position of 140 μm from the surface of the steel sheet, and the luminescence intensity of B at a depth position of 150 μm from the surface of the steel sheet, respectively, when measured in the thickness direction of the steel sheet based on high-frequency GDS analysis.

[0253] The measurement by high-frequency GDS analysis was performed at any of 5 positions. B30, B140, and B150 used the average value of the luminescence intensity of B at a depth position of 30 μm, 140 μm, and 150 μm from the surface of the steel sheet at any of the 5 positions, respectively. The measurement conditions were as follows.

[0254] B30, B140 and B150 are measured using a high frequency glow discharge optical emission spectrometer, respectively. Specifically, the following method is used: the surface of the steel sheet to be measured is made to be an Ar atmosphere, and in a state where a voltage is applied to generate glow plasma, the surface of the steel sheet is sputtered while being analyzed in the depth direction. Then, based on the element-specific emission spectrum wavelength emitted by atoms excited in the glow plasma, the elements contained in the steel sheet are identified, and the emission intensity of the identified elements is estimated.

[0255] The data in the depth direction can be estimated from the sputtering time. Specifically, by previously finding the relationship between the sputtering time and the sputtering depth using a standard sample, the sputtering time can be converted into the sputtering depth. Therefore, the sputtering depth converted from the sputtering time can be defined as the depth from the surface of the steel sheet. The sputtering time is set to at least a sputtering depth of 150 μm.

[0256] In the high frequency GDS analysis, a commercially available analysis device can be used. In the present embodiment, a high frequency glow discharge optical emission spectrometer GD-Profiler2 (registered trademark) manufactured by HORIBA, Ltd. is used. The detection interval is set to 0.1 seconds. The obtained data is subjected to filtering processing after removing the background. The filtering is performed by the moving average method. Specifically, the moving average of a total of 51 points including the center point and the 25 points before and after the center point is found. The values corresponding to the times of 30 μm depth, 140 μm depth and 150 μm depth are B30, B140 and B150, respectively. As for other measurement conditions, they are as described below.

[0257] Ar gas pressure: 600 Pa

[0258] Anode diameter: 4 mmφ

[0259] RF output: 35 W

[0260] Further, in the present specification, as described above, the depth position at which the emission intensity of Fe based on the high frequency GDS analysis reaches 0.7 times the emission intensity of Fe inside is defined as the 0 μm position, but the emission intensity of Fe inside in this definition can be, for example, the emission intensity of Fe at a sputtering time of 1000 seconds.

[0261] The above formula (1) means that the boron concentration at a depth position of 30 μm from the surface of the steel sheet is less than 0.90 times the boron concentration at a depth position of 150 μm. By satisfying this formula (1), when the steel sheet is subjected to plastic working, it is difficult to cause damage to the metal structure near the surface of the steel sheet.

[0262] 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. Furthermore, B30 / B150 may be 0, or 0.10 or more, 0.20 or more, or 0.30 or more.

[0263] The above formula (2) means that the luminous intensity of B at a depth of 140 μm from the steel sheet surface is substantially equal to the luminous intensity of B at a depth of 150 μm from the steel sheet surface. B The region is from the steel sheet surface to a depth of 150 μm. By satisfying this formula (2), excessive softening of the steel sheet at a depth of 150 μm or more from the steel sheet surface can be prevented, and the strength of the steel sheet can be ensured.

[0264] By forming the above-mentioned deboronized layer P B , can improve the bendability after plastic working. The reason for this effect is not clear, but it is believed that the deboronized 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 the surface soft layer formed by the decarburized layer.

[0265] It should be noted that, as mentioned above, B (boron) segregates at the austenite grain boundaries during spot welding and improves the LME crack resistance by strengthening the austenite grain boundaries. According to this mechanism, the deboronized layer P B The formation of the deboronized layer P reduces the amount of B in the austenite grain boundaries, so it is initially thought that the LME crack resistance is deteriorated. However, it is actually confirmed that even if the deboronized layer P is formed, the LME crack resistance is deteriorated. B The LME crack resistance is not deteriorated. This is because the temperature near the welding point rises due to the heat input during spot welding, and the B inside the steel plate diffuses to the surface layer P of the steel plate. S , so even if a deboronized layer P is formed B , the surface layer P of the steel plate S The amount of segregated B in the austenite grain boundaries in the steel plate will also be sufficiently increased. Therefore, it is considered that even in the surface layer portion P of the steel plate before welding, S A deboronized layer P is formed B , the effect of improving the LME crack resistance brought about by B can also be obtained.

[0266] Furthermore, the base material steel plate 2 of this embodiment is formed by S The surface portion P of the base steel plate 2 has a decarburized layer. S Decarburization (hereinafter sometimes referred to as "decarburization") can further improve the LME crack resistance. Specifically, the surface layer P of the base steel plate 2 SThe luminescence intensity of C measured from the surface of the steel sheet in the depth direction by high frequency glow discharge spectrometry (high frequency GDS analysis) satisfies the following formula (3) and formula (4).

[0267] C30 / C150≤0.50 (3) 0.90≤C140 / C150≤1.10 (4)

[0268] Here, C30, C140, and C150 are the luminescence intensity of C at a depth position of 30 μm from the surface of the steel sheet, the luminescence intensity of C at a depth position of 140 μm from the surface of the steel sheet, and the luminescence intensity of C at a depth position of 150 μm from the surface of the steel sheet, respectively, when measured in the thickness direction of the steel sheet based on high frequency GDS analysis.

[0269] The measurement of high frequency GDS analysis is performed at any 5 positions. C30, C140, and C150 use the average value of the luminescence intensity of C at a depth position of 30 μm, 140 μm, and 150 μm from the surface of the steel sheet at any 5 positions. The measurement conditions are the same as those of B30, B140, and B150 described above.

[0270] The above formula (3) means that decarburization proceeds at least to a depth position of 30 μm from the surface of the steel sheet. By performing decarburization in such a manner as to satisfy this formula (3), the LME crack resistance can be further improved.

[0271] In the above formula (3), C30 / C150 can be 0.45 or less, 0.40 or less, or 0.35 or less. In addition, C30 / C150 can be 0, or 0.10 or more, 0.15 or more, or 0.20 or more.

[0272] The degree of decarburization can be controlled by adjusting the atmosphere up to the maximum heating temperature in the heat treatment of the steel sheet manufacturing method described later.

[0273] The above formula (4) means that the luminescence intensity of C at a depth position of 140 μm from the surface of the steel sheet is approximately equal to the luminescence intensity of C at a depth position of 150 μm from the surface of the steel sheet. That is, it means that the decarburization depth is 140 μm or less. In addition, the C concentration at a depth position of 150 μm from the surface of the steel sheet is approximately equal to the C concentration at the center of the thickness of the steel sheet. In the case where formula (4) is not satisfied, that is, in the case where decarburization proceeds excessively, the tensile strength is excessively reduced, and sometimes the desired tensile strength cannot be obtained.

[0274] [Tensile strength: 980 MPa or more]

[0275] In the present embodiment, the tensile strength of the base steel sheet 2 is 980 MPa or more. The base steel sheet 2 of the present embodiment, even if the tensile strength is such a high strength, has the decarburized boron-depleted layer P having the above decarburizationB The LME crack resistance and the bending property after plastic working are also excellent. The tensile strength of the base steel sheet 2 can be 1180 MPa or higher, 1200 MPa or higher, 1300 MPa or higher, 1400 MPa or higher, or 1500 MPa or higher. Note that the upper limit of the tensile strength of the base steel sheet 2 is not particularly limited, and can be, for example, 4000 MPa or lower, 3000 MPa or lower, or 2000 MPa or lower from the viewpoint of toughness and formability.

[0276] Note that the tensile strength (TS) of the steel sheet can be determined as follows. First, a No. 5 test piece of JIS Z 2241:2011 having a length direction at right angles to the rolling direction is taken from the center of the width of the steel sheet to be measured. Next, a tensile test according to JIS Z 2241:2011 is performed using the test piece, whereby the tensile strength TS (MPa) can be determined.

[0277] In addition, in a case where it is difficult to take a test piece from the steel sheet to be measured, the Vickers hardness of the steel sheet is determined, and the value of the tensile strength can be derived from the following correlation formula ("Correlation between static strength parameters", Norihiko Hasegawa, Junichi Arai, Michihiro Tanaka, "Materials", Vol. 39, No. 442, P859-863) using the determined value of the Vickers hardness.

[0278] Hv = 0.301 x TS + 5.701

[0279] In the above formula, Hv represents the Vickers hardness, and TS represents the tensile strength (MPa).

[0280] The Vickers hardness of the steel sheet can be determined according to JIS Z 2244:2009. Specifically, the Vickers hardness of the steel sheet can be determined at a position of 1 / 4 of the thickness of the steel sheet with a load of 1 Kgf (about 9.80 N) for 10 times, and the average of the 10 determined values is obtained. At this time, the interval of the determined positions is ensured to be a distance of 3 times or more of the indentation.

[0281] [Plating layer]

[0282] As described above, in the present embodiment, the base steel sheet 2 has a plating layer 3 on both surfaces. The plating layer 3 can be a hot-dip galvanized layer or an alloyed hot-dip galvanized layer having an arbitrary composition known in the art. The plating layer 3 can contain an additive element such as Al in addition to Zn. In addition, the attachment amount of the plating layer 3 is not particularly limited, and can be a general attachment amount.

[0283] Note that the plating layer 3 can be provided only on one surface of the base steel sheet 2, or can not be provided on any surface of the base steel sheet 2. It is not necessary for the steel sheet of the present application to have a plating layer on the surface of the steel sheet.

[0284] (thickness of steel sheet)

[0285] The thickness of the steel sheet of the present application is not particularly limited, and can be set to the same thickness as a steel sheet used as an automobile component, for example. As the thickness of such a steel sheet, for example, a thickness of 0.5 to 3.0 mm can be mentioned. The thickness of the steel sheet can be 0.7 mm or more, 0.8 mm or more, or 1.0 mm or more. In addition, the thickness of the steel sheet can be 2.8 mm or less, 2.5 mm or less, or 2.0 mm or less.

[0286] <manufacturing method of steel sheet>

[0287] Next, a manufacturing method of a steel sheet of one embodiment of the present application will be described. The following description is an example of a description intended to illustrate a characteristic method of manufacturing a steel sheet of one embodiment of the present application, and is not intended to limit the steel sheet to a steel sheet manufactured by the following manufacturing method.

[0288] The manufacturing method of the steel sheet includes a hot rolling step (a) (hereinafter, sometimes referred to simply as "step (a)") of hot rolling a slab having a specific chemical composition to obtain a hot-rolled steel sheet, a grinding step (e) (hereinafter, sometimes referred to simply as "step (e)") of grinding the hot-rolled steel sheet by a rotary grinding brush, a pickling step (b) (hereinafter, sometimes referred to simply as "step (b)") of pickling after grinding, a cold rolling step (c) (hereinafter, sometimes referred to simply as "step (c)") of cold rolling the hot-rolled steel sheet after pickling to obtain a cold-rolled steel sheet, and a heat treatment step (d) (hereinafter, sometimes referred to simply as "step (d)") of subjecting the cold-rolled steel sheet to heat treatment.

[0289] Next, preferred conditions and the like of these steps will be described in detail.

[0290] [hot rolling step (a)]

[0291] First, the hot rolling step (a) is performed, and after the hot rolling of a slab having the following specific chemical composition is performed under prescribed conditions to obtain a hot-rolled steel sheet, the hot-rolled steel sheet is cooled to a prescribed temperature and coiled. In the hot rolling step, the slab having the following specific chemical composition is heated before hot rolling.

[0292] Here, regarding the chemical composition of the slab, if the chemical composition of the finally obtained steel sheet is analyzed according to the above analysis method, it can be confirmed that there is substantially no difference from the chemical composition of the slab.

[0293] Therefore, the chemical composition of the slab is substantially the same as the chemical composition of the above steel sheet. That is, the chemical composition of the slab is, in mass %,

[0294] C: 0.15 to 0.35%,

[0295] Si: 0.01 to 1.20%,

[0296] Mn: 1.00 to 3.50%,

[0297] Al: 0.300 to 1.500%,

[0298] Ti: 0.001 to 0.100%,

[0299] B: 0.0005 to 0.0050%,

[0300] P: 0.050% or less,

[0301] S: 0.0100% or less,

[0302] N: 0.010% or less,

[0303] O: 0.0100% or less,

[0304] Cr: 0 to 1.00%,

[0305] Mo: 0 to 1.00%,

[0306] Cu: 0 to 1.00%,

[0307] Ni: 0 to 1.00%,

[0308] Co: 0 to 1.00%,

[0309] W: 0 to 1.00%,

[0310] Sn: 0 to 1.00%,

[0311] Sb: 0 to 0.50%,

[0312] Nb: 0 to 0.200%,

[0313] V: 0 to 1.00%,

[0314] As: 0 to 0.10%,

[0315] Zn: 0 to 1.00%,

[0316] Ca: 0 to 0.0100%,

[0317] Mg: 0 to 0.0100%,

[0318] Zr: 0 to 0.0100%,

[0319] Hf: 0 to 0.0100%,

[0320] Bi: 0 to 0.0100%,

[0321] REM: 0-0.015%, and

[0322] The remainder: Fe and impurities.

[0323] 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 above-mentioned steel plate.

[0324] In the hot rolling process, the heating temperature of the slab is not particularly limited, but is generally preferably 1150°C or higher in order to fully dissolve borides, carbides, etc. It should be noted that from the perspective of manufacturability, the steel slab used is preferably cast by continuous casting, but can also be produced by ingot casting or thin slab casting.

[0325] (Rough rolling)

[0326] In this production method, rough rolling may be performed before finish rolling to adjust the thickness of the heated slab. The conditions for such rough rolling are not particularly limited; however, from the perspective of recrystallization during hot rolling, rough rolling is preferably performed at a total reduction ratio of 60% or greater at a temperature of 1050°C or higher. For example, the total reduction ratio may be 90% or less.

[0327] (Finishing rolling)

[0328] Next, the slab is hot-rolled by finish rolling to obtain a hot-rolled steel sheet. The finish rolling entry temperature is not particularly limited, but is preferably 900-1050°C to achieve an appropriate microstructure of the hot-rolled steel sheet. Furthermore, the total reduction ratio during finish rolling is preferably 70-95%.

[0329] In the present manufacturing method, in order to form the above-mentioned deboronized layer, more than three passes of finishing rolling are performed, and the reduction rate of each of the final three passes in the finishing rolling is set to be more than 20%, the pass interval time is within 1 second, the temperature of the steel plate on the entry side before the final three passes is below 1000°C, and the finishing temperature of finishing rolling is 850-950°C. Furthermore, the time from the end of the final pass to the start of cooling is set to within 3 seconds. If finishing rolling is carried out under such conditions, the ferrite transformation is promoted by the accumulation of strain in austenite, and the surface of the hot-rolled steel plate is softened, thereby promoting the introduction of strain to the surface based on grinding in the next process. As a result, the above-mentioned deboronized layer can be formed on the steel plate as the final product. The number of finishing rolling passes is not particularly limited as long as the final three passes meet the above-mentioned conditions.

[0330] 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.

[0331] (Coiling temperature: 450~680℃)

[0332] The hot-rolled steel sheet after the finish rolling is cooled to a predetermined coiling temperature and coiled. At this time, the coiling temperature is 450 to 680°C from the viewpoint of the strength and workability of the hot-rolled sheet. The coiling temperature can be 500°C or higher. In addition, the coiling temperature can be set to 620°C or lower.

[0333] After the coiling is completed, a process of heat-insulating the hot-rolled steel sheet after the coiling can be performed for the purpose of promoting the formation of the boron-depleted layer in the subsequent heat treatment process. As an example of the heat-insulating process, the hot-rolled steel sheet can be stored in a heat-insulating container having an inner wall covered with a heat-insulating material and heat-insulated within 30 minutes after the coiling is completed. At this time, the heat-insulating conditions can set the maximum reached temperature of the atmosphere in the container to 500 to 650°C and the time for which the atmosphere temperature reaches the maximum reached temperature to 1 to 8 hours. If the heat-insulating is performed under such conditions, the surface layer of the hot-rolled steel sheet is further softened, the strain introduction in the subsequent grinding process is promoted, the formation of the boron-depleted layer in the subsequent heat treatment process is further promoted, and thus the bendability after the plastic working of the finally obtained steel sheet can be further improved.

[0334] [Grinding process (e)]

[0335] Next, a grinding process (e) of grinding the front and back surfaces of the steel sheet after the coiling using a rotary grinding brush is performed. As a brush that can be used in the grinding process, for example, D-100-33 manufactured by Hotani Co., Ltd. can be cited. With respect to the grinding conditions, the rotation speed R (rotations / minute) of the grinding brush, the diameter D (m) of the grinding brush, and the sheet passing speed V (m / minute) of the steel sheet satisfy the following equation (5). If the grinding is performed under conditions satisfying such equation (5), the strain is introduced to the surface layer of the steel sheet, the diffusion of boron in the subsequent heat treatment process is promoted, and the boron-depleted layer formed in the subsequent heat treatment process is enlarged.

[0336] [Mathematical equation 2]

[0337]

[0338] Note that in equation (5), (R·D) / V can be 11 or higher, 13 or higher, or 15 or higher. In addition, the upper limit of (R·D) / V is not particularly limited, and (R·D) / V can be 60 or lower, 55 or lower, or 50 or lower.

[0339] Such process (e) needs to be performed during the period from the end of the hot-rolling to before the cold-rolling, and can be performed during either of the period before the pickling process described later or the period after the pickling process.

[0340] [Pickling process (b)]

[0341] Next, a pickling step (b) of pickling the steel sheet after the hot rolling step (a) or the grinding step (e) is performed. The pickling method in the pickling step can be performed according to a conventional method. In addition, in the pickling step, in order to improve the shape correction of the hot rolled coil and the pickling property, skin pass rolling can also be performed.

[0342] [cooling step (c)]

[0343] Next, a cold rolling step (c) of cold rolling the steel sheet after the pickling step (b) or the grinding step (e) is performed. In the cold rolling step, the reduction rate of cold rolling is set to 30 to 75% in consideration of the accumulation of strain and the load on the cold rolling mill due to the rolling load. For example, the reduction rate can be set to 40% or more. In addition, the reduction rate can be 70% or less or 60% or less.

[0344] [heat treatment step (d)]

[0345] Next, a heat treatment step (d) of heat treating the steel sheet obtained in the cold rolling step (c) is performed.

[0346] The heat treatment step is performed in the order of a step (d-1) of heating the steel sheet obtained in the step (c) at an average heating rate of 0.5 to 500°C / sec from 650°C to a maximum heating temperature of Ac1+50°C or higher and 950°C or lower, 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 (martensite transformation point)-30°C or lower, wherein the cooling is performed at an average cooling rate of 10°C / sec or higher from 700°C to 500°C, and a step (d-4) of holding the steel sheet at 300 to 450°C for 100 to 600 seconds.

[0347] As described above, on the basis of the softening of the surface layer portion of the steel sheet by the control of the hot rolling conditions, a large amount of strain is introduced to the surface layer portion of the steel sheet in the grinding step, and further, in the steps (d-1) to (d-4) of the heat treatment step, that is, the temperature raising / soaking step, H2O in the atmosphere reacts with B on the surface of the steel sheet to become an oxide, whereby sufficient boron removal can be performed.

[0348] In the step (d-1), the average heating rate from 650°C to the maximum heating temperature is 0.5 to 500°C / sec from the viewpoint of allowing recrystallization of ferrite to proceed while suppressing the coarsening of austenite. The average heating rate can be 1.0°C / sec or more or 2.0°C / sec or more. In addition, the average heating rate can be 400°C / sec or less or 300°C / sec or less. Here, the "average heating rate" means a 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.

[0349] In the above process (d-1), the maximum heating temperature is Ac1+ 50°C or higher and 950°C or lower from the viewpoint of allowing the austenitization to proceed while suppressing the coarsening of the austenite diameter. In addition, in the above process (d-2), the holding time at the maximum heating temperature is 1 to 300 seconds from the viewpoints of the austenitization and productivity. In the holding at the maximum heating temperature, it is not necessarily required to hold the steel sheet at a certain temperature, and it can be varied within the range of the above maximum temperature. Here, the "holding" means maintaining the temperature within the range of the prescribed temperature ± 20°C, preferably ± 10°C, within the range of not more than the prescribed upper and lower limits.

[0350] After the holding at the maximum heating temperature, in the above process (d-3), the steel sheet is cooled to a temperature of Ms point - 30°C or lower, at this time, the average cooling temperature is 10°C / sec or higher from 700°C to 500°C. The average cooling rate from 700°C to 500°C can be 20°C / sec or higher, 30°C / sec or higher, or 50°C / sec or higher.

[0351] In order to obtain the desired structure, after the cooling to a temperature of Ms point - 30°C or lower, in the above process (d-4), the holding is performed at 300 to 450°C for 100 to 600 seconds. The "holding" in this process (d-4) is the same as above, and it is not required to hold at a certain temperature, and it means maintaining the temperature within the range of the prescribed temperature ± 20°C, preferably ± 10°C.

[0352] In the above process (d-4), the atmosphere around the steel sheet when heating from 650°C to the maximum heating temperature is controlled in such a manner that the water vapor partial pressure pH2O and the hydrogen partial pressure pH2 satisfy the following formula (6). If log (pH2O / pH2) in the formula (6) is less than -1.0, the decarburization reaction cannot sufficiently proceed, and the desired LME crack resistance cannot be obtained. In addition, when log (pH2O / pH2) in the formula (6) exceeds -0.1, the improvement effect of the bendability is saturated, and the strength of the steel sheet can be decreased.

[0353] -1.0 ≤ log (pH2O / pH2) ≤ -0.1 (6)

[0354] pH2O: water vapor partial pressure

[0355] pH2: hydrogen partial pressure

[0356] Note that log (pH2O / pH2) in the formula (6) can be -0.9 or higher or -0.8 or higher. In addition, log (pH2O / pH2) in the formula (6) can be -0.2 or lower or -0.3 or lower.

[0357] As described above, a plated layer can be formed on the surface of the steel sheet of the present application. The plated layer can be provided as a hot-dip galvanized layer, for example. In addition, as needed, an alloying treatment can be performed after the formation of the hot-dip galvanized layer, as an alloyed hot-dip galvanized layer. The formation of the plated layer and the formation of the alloying treatment can be performed according to a conventional method, and are not particularly limited. The plating treatment can also be performed in the middle of cooling from the maximum heating temperature to a temperature of the Ms point - 30°C or less. In this case, the primary cooling is ended at the plating treatment temperature, and after the plating treatment is ended, the steel sheet can be cooled to a temperature of the Ms point - 30°C or less at an average cooling rate of 10°C / sec or more. In addition, the plating treatment can be performed after the above-described holding at 300 to 450°C, or can be performed after the heat treatment step is ended and the steel sheet is cooled to room temperature, and the steel sheet is heated again to the plating bath temperature.

[0358] By the above production method, the steel sheet of the present application having excellent tensile strength and excellent elongation (EL), LME crack resistance, and bending property after plastic working can be obtained. For the bending property, after a test piece collected from the steel sheet to be evaluated is given a pre-strain of 2%, a bending test is performed by a method prescribed as VDA (Verband der Automobilindustrie) standard 238-100, the maximum bending angle is measured, and evaluation is performed thereby.

[0359] Example

[0360] Next, an example of the present application will be described. The conditions in this example are one example of conditions adopted in order to confirm the possibility of implementation and effects of the present application. The present application is not limited to this one example of conditions. Various conditions can be adopted as long as the purpose of the present application is achieved without departing from the gist of the present application.

[0361] Steels having various chemical compositions were cast, and slabs were produced. Using these slabs, hot-rolled steel sheets were produced by hot-rolling. Further, the hot-rolled steel sheets were subjected to grinding treatment, cold-rolling, and heat treatment in this order, and cold-rolled steel sheets were produced.

[0362] A plating treatment was performed on a part of the obtained cold-rolled steel sheets. The chemical composition of the test pieces collected from the obtained steel sheets was analyzed, and it was confirmed that there was no change in the chemical composition from that of the slabs. The chemical compositions of these steel sheets are shown in Table 1. The remaining portion other than the components shown in Table 1 is Fe and impurities. Note that, regarding the plated steel sheets, the chemical composition of the base steel sheet from which the plated layer on the surface was peeled off under the above-described conditions. The underlines attached to the chemical compositions in Table 1 indicate that they are outside the scope of the present application.

[0363]

[0364] Table 2

[0365] The hot rolling was performed under the conditions described in Table 2. In Table 2, the Rl entry side temperature refers to the entry side steel sheet temperature of the third pass counted from the final pass of the finish rolling. Rl refers to the reduction ratio of the third pass counted from the final pass. R2 refers to the reduction ratio of the second pass counted from the final pass. R3 refers to the reduction ratio of the final pass. In addition, tl refers to the time from the end of the third pass counted from the final pass to the start of the second pass counted from the final pass. t2 refers to the time from the end of the second pass counted 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 the cooling. Also, the R3 exit side temperature refers to the temperature of the steel sheet at the end of the final pass, i.e., the finish rolling end temperature.

[0366] Then, the front and back surfaces 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 (rpm) of the grinding brush, the diameter D (m) of the grinding brush, and the sheet passing speed V (m / min) of the steel sheet was the value shown in Table 3. Next, the ground steel sheet was pickled. Further, the pickled steel sheet was cold-rolled at the reduction ratios described in Table 3. The thickness of the cold-rolled sheet was 1.4 mm in all.

[0367] Then, the cold-rolled steel sheet was subjected to heat treatment. The heat treatment was held after heating to the maximum heating temperature and was cooled. Further, after cooling to a temperature of Ms point - 30°C or lower, it was held at 300 to 450°C. These conditions and the value 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 addition, in Table 3, Ms is the martensite transformation point (°C) of the steel used.

[0368] Note that, in Table 3, the Ac1 point (°C) serving as a reference for the set range of the maximum heating temperature of the heat treatment was calculated according to the following formula. The Ac1 point for each steel sheet 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 addition, in Table 3, the Ms point (°C) was calculated 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] Then, a part of the steel sheet was subjected to continuous hot dip galvanizing treatment, and a part of the same was further subjected to alloying treatment. The plating conditions were not particularly specified, and were set to general conditions known per se. GA in Table 3 refers to an alloyed hot dip galvanized steel sheet. In addition, GI means a hot dip galvanized steel sheet which was not subjected to alloying treatment. CR means a cold-rolled steel sheet which was not subjected to plating.

[0375] Note that the underlines given to various numerical values and the like in Tables 2 and 3 indicate that the manufacturing conditions are outside the scope of the present application, that the steel sheet cannot be obtained by the present application, or that the various characteristics of the steel sheet are not preferable.

[0376] Table 3

[0377] For the obtained steel sheet, the luminescence intensity B of B at each depth position of 30 μm, 140 μm, and 150 μm from the surface of the steel sheet when the measurement based on high frequency GDS analysis was performed in the thickness direction of the steel sheet was measured by the above-described method based on high frequency glow discharge spectroscopy analysis (high frequency GDS analysis). Similarly, the luminescence intensity C of C at each depth position of 30 μm, 140 μm, and 150 μm from the surface of the steel sheet was measured. These measurement results are shown in Table 4 below.

[0378] In addition, from the central portion in the width direction of the obtained steel sheet, a No. 5 tensile test piece of JIS Z 2241 : 2011 was taken out in a direction at right angles to the rolling direction as the length direction, and a tensile test according to JIS Z 2241 : 2011 was performed using the test piece to measure the tensile strength (TS) and the elongation (EL). Note that for the steel sheets of No. 30 and No. 31, the plating was not peeled off from the plated steel sheet, and the tensile strength was measured in the plated state. In the present embodiment, the reference for the tensile strength (980 MPa or more) was the same as for the steel sheet which was not subjected to plating. In addition, the reference for the balance between the tensile strength and the elongation was that if TS 1.5 x EL / 1000 was 440 or more, it was judged to be good. These results are shown in Table 4 below.

[0379] In addition, from the center of the width of the obtained steel sheet, a tensile test piece having a parallel portion width of 30 mm in a direction at right angles to the rolling direction was taken, and after applying a pre-strain of 2%, a rectangular test piece having a width of 30 mm and a length of 60 mm was taken from the parallel portion. Subsequently, in order to simulate the painting and baking process of an automobile, heat treatment was performed at 170°C for 20 minutes. For the test piece after the heat treatment, a bend test was performed by the method prescribed in VDA (Verband der Automobilindustrie) Standard 238-100, and the maximum bending angle was measured. For the measurement result thereof, the maximum bending angle of 60 degrees or more was judged to be good in bendability. The bending direction was performed in such a manner that the rolling direction was parallel to the bend edge. Note that for the steel sheets of Nos. 30 and 31, the plating was not peeled off from the plated steel sheet, and the maximum bending angle in the plated state was measured. In the present example, the reference of the maximum bending angle (60 degrees or more) was the same as that of the steel sheet on which plating was not performed.

[0380] The measurement result of the maximum bending angle of each steel sheet is shown in Table 4 below.

[0381] In addition, in order to evaluate the liquid metal embrittlement (LME) crack resistance of the spot-welded portion, a test piece having a width of 150 mm and a length of 50 mm was cut from the obtained steel sheet, and a two-piece set spot-welding test was performed. The sheet set was one set of the steel sheet shown in Table 3 and a commercially available alloyed hot-dip galvanized steel sheet (SGCC: JIS G 3346, plating adhesion amount 60 g / m 2 , sheet thickness 1.4 mm), and welding was performed in a state in which the angle was 5 degrees. The test machine was a stationary spot-welding test machine driven by a servo motor. The power source was set to single-phase alternating current 50 Hz, the pressing force was set to 400 kgf, the current time was set to 20 cycles, and the holding time was set to 5 cycles. The welding current value was set to a current value in which the diameter of the nugget was 4.0 times, 4.5 times, 5.0 times, and 5.5 times the sheet thickness (t: mm). An electrode having a tip diameter of φ 6 mm and a tip curvature radius R of 40 mm was used. The nugget portion of the sample after welding was observed in cross section. In the cross-sectional observation, a case in which a crack of 0.2 mm or more was confirmed in any of the above welding current values was judged to be “POOR”, a case in which a crack of 0.1 mm or more but less than 0.2 mm was confirmed in any of the above welding current values was judged to be “GOOD”, and a case in which a crack of 0.1 mm or more was not confirmed in any of the above welding current values was judged to be “EX”. The evaluation result of the LME crack resistance of each steel sheet is shown in Table 4 below.

[0382] In this evaluation of the present application, points to be noted are described. As the features of the steel sheet of the present application, the chemical composition, the steel structure, the B concentration distribution, and the C concentration distribution, and the like, are defined for the region irrespective of the surface coating. On the other hand, the mechanical properties of the steel sheet are generally considered to change slightly depending on the presence or absence of the surface coating. Even in such a situation, in the present application, the steel sheet in the same surface state as that at the time of use is used, and it is determined whether or not the mechanical properties of the steel sheet fall within the range of the present application. This is because, for a person who uses the steel sheet with the surface coating, the mechanical properties in the state without the coating are not important, but the mechanical properties in the state with the coating are important. Therefore, in the present application example, for the plated steel sheets (steel sheets of Nos. 16, 17, 20, 21, 23, and 24), the mechanical properties such as the tensile strength, the elongation, and the bendability (maximum bending angle) are evaluated in the state with the plating. For the non-plated steel sheets (steel sheets other than Nos. 16, 17, 20, 21, 23, and 24), the mechanical properties such as the tensile strength, the elongation, and the bendability (maximum bending angle) are evaluated in the state without the plating.

[0383] In addition, in Table 4, "a" of the microstructure means ferrite. In addition, "γ" means residual austenite. "FM + θ" means the total of the primary martensite and cementite. "P" means pearlite. "TM" means tempered martensite. Furthermore, "B" means bainite.

[0384] The underlines given to various numerical values and the like in Table 4 indicate that the manufacturing conditions of the steel sheet of the present application cannot be obtained, or the various properties of the steel sheet are not preferable, outside the range of the present application.

[0385] Table 4

[0386]

[0387] The log (pH2O / pH2) of the No. 2 steel sheet is less than -1.0, and the decarburization reaction is not sufficiently performed, so that it becomes a result that the LME crack resistance is poor.

[0388] The No. 3 steel sheet is not subjected to brush grinding, and an appropriate decarburized layer is not formed, so that the maximum bending angle after the pre-strain is made small, and it becomes a result that the bendability is poor.

[0389] The coiling temperature of the No. 5 steel sheet is low, and an appropriate decarburized layer is not formed, so that the maximum bending angle after the pre-strain is made small, and it becomes a result that the bendability is poor.

[0390] The reduction rate of the finish rolling in the hot rolling step of the No. 6 steel sheet is not appropriate, and an appropriate decarburized layer is not formed, so that the maximum bending angle after the pre-strain is made small, and it becomes a result that the bendability is poor.

[0391] No. 7 steel sheet has long interval time between passes in finish rolling in hot rolling process, and thus an appropriate deboronized layer is not formed, and as a result, the maximum bending angle after imparting pre-strain becomes small, and thus the bending property is poor.

[0392] No. 8 steel sheet has high finish rolling end temperature in hot rolling process, and thus an appropriate deboronized layer is not formed, and as a result, the maximum bending angle after imparting pre-strain becomes small, and thus the bending property is poor.

[0393] No. 9 steel sheet has high entry side steel sheet temperature before the last 3 passes in hot rolling process, and thus an appropriate deboronized layer is not formed, and as a result, the maximum bending angle after imparting pre-strain becomes small, and thus the bending property is poor.

[0394] No. 10 steel sheet has low maximum heating temperature in heat treatment process, and thus the ferrite fraction is high, and thus the desired tensile strength is not obtained.

[0395] No. 11 steel sheet has low holding temperature in the process of holding at 300 to 450°C in heat treatment process, and thus the fraction of residual austenite becomes low, and the total fraction of primary martensite and cementite becomes high. As a result, the TS 1.5 × EL / 1000 is less than 440, and thus the balance between the tensile strength and the elongation is poor, and in addition, the maximum bending angle after imparting pre-strain becomes small, and thus the bending property is poor.

[0396] No. 12 steel sheet has short holding time in the process of holding at 300 to 450°C in heat treatment process, and thus the fraction of residual austenite becomes low, and the total fraction of primary martensite and cementite becomes high, and thus the elongation becomes low. As a result, the TS 1.5 × EL / 1000 is less than 440, and thus the balance between the tensile strength and the elongation is poor, and in addition, the maximum bending angle after imparting pre-strain becomes small, and thus the bending property is poor.

[0397] No. 13 steel sheet has inappropriate conditions in grinding process, and thus an appropriate deboronized layer is not formed, and as a result, the maximum bending angle after imparting pre-strain becomes small, and thus the bending property is poor.

[0398] No. 14 steel sheet has small average cooling speed between 700 to 500°C in heat treatment process, and thus the ferrite fraction becomes high, and thus the desired tensile strength is not obtained.

[0399] No. 15 steel sheet has high holding temperature in the process of holding at 300 to 450°C in heat treatment process, and thus the fraction of residual austenite becomes low, and in addition, pearlite is formed, and thus the desired tensile strength is not obtained. As a result, the TS 1.5 × EL / 1000 is less than 440, and thus the balance between the tensile strength and the elongation is poor.

[0400] The cooling end temperature in the heat treatment step of the No. 19 steel sheet is high, so the total fraction of the primary martensite and cementite becomes high, and in addition, tempered martensite is not formed, and as a result, the maximum bending angle after imparting a pre-strain becomes small, and this is the cause of poor bendability.

[0401] The C content of the chemical composition of the No. 33 steel sheet is low, so the desired tensile strength is not obtained.

[0402] The C content of the chemical composition of the No. 34 steel sheet is high, so the maximum bending angle after imparting a pre-strain becomes small, and this is the cause of poor bendability, and in addition, this is the cause of poor LME crack resistance.

[0403] The Si content of the chemical composition of the No. 35 steel sheet is high, so this is the cause of poor LME crack resistance.

[0404] The Mn content of the chemical composition of the No. 36 steel sheet is low, so the ferrite fraction is high, the retained austenite fraction becomes low, and in addition, pearlite is formed, and tempered martensite is not formed, and as a result, the desired tensile strength is not obtained.

[0405] The Mn content of the chemical composition of the No. 37 steel sheet is high, so the total fraction of the primary martensite and cementite becomes high, and as a result, the maximum bending angle after imparting a pre-strain becomes small, and this is the cause of poor bendability.

[0406] The Al content of the chemical composition of the No. 38 steel sheet is low, so the retained austenite fraction becomes low. As a result, the TS 1.5 × EL / 1000 is less than 440, and this is the cause of poor balance between the tensile strength and the elongation.

[0407] The Al content of the chemical composition of the No. 39 steel sheet is high, so the maximum bending angle after imparting a pre-strain becomes small, and this is the cause of poor bendability.

[0408] The B content of the chemical composition of the No. 40 steel sheet is low, and an appropriate deboronized layer is not formed, so the maximum bending angle after imparting a pre-strain becomes small, and this is the cause of poor bendability, and in addition, this is the cause of poor LME crack resistance.

[0409] Explanation of Reference Signs

[0410] 1 Plated Steel Sheet

[0411] 2 Base Steel Sheet

[0412] 3 Plating Layer

[0413] S d Steel Sheet Surface

[0414] P S Surface Layer Portion

[0415] PB boron-depleted layer

[0416] P 30 depth position 30 μm from the surface of the steel sheet

[0417] P 150 depth position 150 μm from the surface of the steel sheet

Claims

1. A steel sheet characterized by, the steel sheet is 0.15 to 0.35% in mass %, Si: 0.01 to 1.20%, Mn: 1.00 to 3.50%, Al：0.300～1.500％、 Ti: 0.001 to 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 to 1.00%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Co: 0 to 1.00%, W：0～1.00％、 Sn: 0 to 1.00%, Sb: 0 to 0.50%, Nb: 0 to 0.200%, V：0～1.00％、 As: 0 to 0.10%, Zn: 0 to 1.00%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr：0～0.0100％、 Hf: 0 to 0.0100%, Bi: 0 to 0.0100%, REM: 0 to 0.015%, and the remainder: Fe and impurities, the steel structure in the range of 1 / 8 depth position to 3 / 8 depth position of the sheet thickness of the steel sheet is 0 to 50% in area %, ferrite: 0 to 50%, retained austenite: 6 to 30%, total of primary martensite and cementite: 0 to 10%, pearlite: 5% or less, tempered martensite: 5% or more, and the remainder: bainite, the surface layer portion of the steel sheet has a boron-depleted layer in which the emission intensity of B measured from the steel sheet surface in the depth direction by high-frequency glow discharge emission spectrometry satisfies the following formulas (1) and (2), further, the surface layer portion of the steel sheet has an emission intensity of C measured from the steel sheet surface in the depth direction by the high-frequency glow discharge emission spectrometry satisfying the following formulas (3) and (4), and the tensile strength of the steel sheet is 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 emission intensity of B at a depth position of 30 μm from the steel sheet surface B140: the emission intensity of B at a depth position of 140 μm from the steel sheet surface B150: the emission intensity of B at a depth position of 150 μm from the steel sheet surface C30: the emission intensity of C at a depth position of 30 μm from the steel sheet surface C140: the emission intensity of C at a depth position of 140 μm from the steel sheet surface C150: the emission intensity of C at a depth position of 150 μm from the steel sheet surface.

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

3. A method of manufacturing a steel sheet, characterized by, provided with: a hot-rolling process (a) having a chemical composition of C: 0.15 to 0.35% in mass %, Si: 0.01 to 1.20%, Mn: 1.00 to 3.50%, Al：0.300～1.500％、 Ti: 0.001 to 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 to 1.00%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Co: 0 to 1.00%, W：0～1.00％、 Sn: 0 to 1.00%, Sb: 0 to 0.50%, Nb: 0 to 0.200%, V：0～1.00％、 As: 0 to 0.10%, Zn: 0 to 1.00%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr：0～0.0100％、 Hf: 0 to 0.0100%, Bi: 0 to 0.0100%, REM: 0 to 0.015%, and the balance: Fe and impurities, and the hot-rolled steel sheet is cooled to 450 to 680°C and coiled after the finish rolling of the slab at a finish rolling end temperature of 850 to 950°C; an acid pickling step (b) of acid 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 acid pickling step (b) at a reduction rate 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 a 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 acid pickling step (b) using a rotary grinding brush containing abrasive grains before or after the acid pickling step (b), in the hot-rolling step (a), the finish rolling is performed in three or more passes, the reduction rate of each of the final three passes of the finish rolling is 20% or more, the inter-pass interval time is 1 second or less, the entry-side steel sheet temperature before the final three passes is 1000°C or less, and the time from the end of the final pass to the start of the cooling is 3 seconds or less, in the grinding step (e), the rotation speed R (rotations / minute) of the grinding brush, the diameter D (m) of the grinding brush, and the sheet passing speed V (m / minute) of the steel sheet satisfy the following formula (5), the heat treatment step (d) further includes: 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 holding 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 - 30°C or lower, wherein the cooling is performed from 700°C to 500°C at an average cooling rate of 10°C / second or more; and 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 step (d-1), the water vapor partial pressure pH2O and the hydrogen partial pressure pH2 in the atmosphere around the steel sheet obtained in the cold-rolling step (c) satisfy the following formula (6) [Mathematical formula 1] -1.0 ≤ log (pH2O / pH2) ≤ -0.1 (6).

4. The method of manufacturing a steel sheet according to claim 3, characterized in that the hot-rolling step (a) includes a step of heat-insulating the hot-rolled steel sheet by covering the inner wall of a heat-insulating container with a heat-insulating material within 30 minutes after the coiling of the hot-rolled steel sheet, The maximum reached temperature of the atmosphere temperature in the interior of the heat-insulating container is 500 to 650°C, and the time for which the atmosphere temperature reaches the maximum reached temperature is 1 to 8 hours.

Images