Steel plate and its manufacturing method

Abstract

The present invention provides a steel sheet having improved bendability after plastic working and excellent tensile strength, and also provides a manufacturing method therefor.　A steel sheet according to the present invention is characterized by exhibiting a tensile strength of 1180 MPa or more, and having a prescribed chemical composition and steel structure, wherein the surface layer portion of the steel sheet has a deboronized layer where the luminescence intensities B30, B140, and B150 of B, as measured in the depth direction from the steel sheet surface by high-frequency glow discharge optical emission spectrometry, at the depth positions of 30 μm, 140 μm, and 150 μm from the steel sheet surface satisfy B30 / B150<0.90 and 0.90≤B140 / B150≤1.10.

Description

[Technical Field]

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

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

[0003] Hot-dip galvanized steel sheets used for automotive parts require not only strength but also various workability properties necessary for part formation, such as press formability and weldability. Specifically, from the standpoint of press formability, the steel sheets require excellent bendability.

[0004] Patent Document 1 discloses a steel sheet in which B is mainly in a precipitated state on the surface layer of the steel sheet and mainly in a solid solution state inside the steel sheet, thereby improving its bendability.

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

[0006] As a technique for improving the bendability of high-strength steel sheets, for example, Patent Document 3 describes a high-strength cold-rolled steel sheet manufactured by decarburizing the steel sheet, in which the surface layer is mainly composed of ferrite. Also, Patent Document 4 describes an ultra-high-strength cold-rolled steel sheet manufactured by decarburizing and annealing the steel sheet, in which the surface layer has a soft layer. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] International Publication No. 2017 / 002883 [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2015 - 155572 [Patent Document 3] Japanese Unexamined Patent Application Publication No. 10 - 130782 [Patent Document 4] Japanese Unexamined Patent Application Publication No. 5 - 195149 [Summary of the Invention] [[ID=I8]] [Problems to be Solved by the Invention]

[0008] In addition, for high - strength steel sheets used for automotive components, after being formed into parts, it is required not to break due to collision deformation. In particular, for steel sheets used for automotive components, it is necessary to have excellent bendability after plastic strain is introduced by press forming, rather than bendability before press forming. However, there has not always been sufficient consideration regarding the improvement of bendability after plastic strain is introduced.

[0009] Therefore, an object of the present invention is to provide a steel sheet having excellent tensile strength and improved bendability after plastic processing, and a method for manufacturing the same. [Means for Solving the Problems]

[0010] As a result of intensive studies to solve the above problems, the inventors of the present invention have found that by forming an appropriate boron - depleted layer on the surface layer, the bendability after plastic processing can be improved. The present invention has been completed based on such findings. The present invention includes the following aspects.

[0011] (Aspect 1) A steel sheet, where the chemical composition of the steel sheet is, in mass%, C: 0.06 - 0.30%, Si: 0.01 - 2.50%, Mn: 1.00 - 3.50%, Ti: 0.001 - 0.100%, B: 0.0005 to 0.0050%, P: 0.050% or less, S: 0.0100% or less, Al: 1.500% or less, N: 0.010% or less, O: 0.0100% or less, Cr: 0 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%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Ce: 0 to 0.0150%, Zr: 0 to 0.0100%, La: 0 to 0.0150%, Hf: 0 to 0.0100%, Bi: 0 to 0.0100%, REM other than Ce and La: 0 to 0.0100%, and The balance: Fe and impurities, In the range from 1 / 8 depth position to 3 / 8 depth position of the plate thickness of the above steel plate, the steel structure is, in area%, Ferrite: 30% or less, Tempered martensite: 40% or more, Total of retained austenite and fresh martensite: 15% or less, Total of pearlite and cementite: 5% or less, and The balance: Bainite, The surface layer of the above steel plate has a deboronized layer in which the emission intensity of B measured in the depth direction from the steel plate surface by high-frequency glow discharge optical emission spectrometry satisfies the following formulas (1) and (2), A steel plate characterized by having a tensile strength of 1180 MPa or more. B30 / B150 < 0.90 ···(1) 0.90 ≤ B140 / B150 ≤ 1.10 ···(2) B30: The above luminescence intensity of B at a depth of 30 μm from the surface of the steel plate. B140: The luminescence intensity of B at a depth of 140 μm from the surface of the steel plate. B150: The luminescence intensity of B at a depth of 150 μm from the surface of the steel plate.

[0012] (Aspect 2) The steel sheet according to embodiment 1, characterized in that the surface layer of the steel sheet satisfies the following equations (3) and (4), measured in the depth direction from the surface of the steel sheet by the above-mentioned high-frequency glow discharge emission analysis. C40 / C150 > 0.50 ···(3) 0.90 ≤ C140 / C150 ≤ 1.10 ···(4) C40: The above-mentioned emission intensity of C at a depth of 40 μm from the surface of the steel plate. C140: The above-mentioned luminescence intensity of C at a depth of 140 μm from the surface of the steel plate. C150: The above-mentioned emission intensity of C at a depth of 150 μm from the surface of the steel plate.

[0013] (Aspect 3) The steel sheet according to embodiment 1 or 2, characterized in that the tensile residual stress acting in the direction perpendicular to the rolling direction on the surface of the steel sheet is 200 MPa or less.

[0014] (Aspect 4) The steel sheet according to any one of embodiments 1 to 3, characterized in that it has a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface of the steel sheet.

[0015] (Aspect 5) A method for manufacturing steel plates, The chemical composition is expressed in mass percent. C: 0.06~0.30%, Si: 0.01~2.50%, Mn: 1.00~3.50%, Ti: 0.001~0.100%, B: 0.0005~0.0050%, P: 0.050% or less, S: 0.0100% or less, Al: 1.500% or less, N: 0.010% or less, O: 0.0100% or less, Cr: 0~1.00%, Mo: 0~1.00%, Cu: 0~1.00%, Ni: 0~1.00%, Co: 0~1.00%, W: 0~1.00%, Sn: 0~1.00%, Sb: 0~0.50%, Nb: 0~0.200%, V: 0~1.00%, As: 0~0.10%, Zn: 0~1.00%, Ca: 0~0.0100%, Mg: 0~0.0100%, Ce: 0~0.0150%, Zr: 0~0.0100%, La: 0~0.0150%, Hf: 0~0.0100%, Bi: 0~0.0100%, REM other than Ce and La: 0~0.0100%, and The remaining portion consists of a slab of Fe and impurities, which is hot-rolled at a finish rolling completion temperature of 850-950°C to obtain a hot-rolled steel sheet. The hot-rolled steel sheet is then cooled to 450-650°C and wound up in a hot-rolling process (a). The pickling process (b) involves pickling the steel sheet obtained in the hot rolling process (a) described above, The steel sheet obtained in the above pickling process (b) is cold-rolled at a reduction ratio of 30-75% to obtain a cold-rolled steel sheet in a cold-rolling process (c), A heat treatment step (d) is performed on the steel sheet obtained in the cold rolling step (c) described above, The process includes a grinding step (e) performed before or after the above pickling step (b), in which the front and back surfaces of the steel sheet obtained in the hot rolling step (a) or the steel sheet obtained in the above pickling step (b) are ground using a rotary grinding brush containing abrasive particles. The above hot rolling process (a) consists of three or more finish rolling passes, with a reduction ratio of 20% or more for each of the last three passes of the finish rolling, a time between passes of 1 second or less, an entry steel sheet temperature of 1000°C or less before the last three passes, and a time of 3 seconds or less from the completion of the final pass to the start of cooling. The above grinding step (e) is performed when the rotational speed R (revolutions / min) of the grinding brush, the diameter D (m) of the grinding brush, and the speed V (m / min) of the steel plate satisfy the following equation (5): The above heat treatment step (d) is: The steel sheet obtained in the cold rolling process (c) described above is heated from 650°C to a maximum heating temperature of Ac1+50°C or higher and 950°C or lower, at an average heating rate of 0.5 to 500°C / second (d-1), The process (d-2) involves holding the steel sheet obtained in the cold rolling process (c) above at the maximum heating temperature above for 1 to 300 seconds, A step of cooling the steel sheet obtained in the cold rolling step (c) above to a temperature below the Ms point -100°C, comprising a step (d-3) of cooling from 700°C to 500°C at an average cooling rate of 10°C / second or more, The process further comprises a step (d-4) of holding the steel sheet obtained in the cold rolling step (c) above at 200-350°C for 50-600 seconds, A method for manufacturing a steel sheet, characterized in that, in step (d-1) above, the atmosphere surrounding the steel sheet obtained in step (c) above satisfies the following equation (6) for the partial pressure of water vapor pH2O and the partial pressure of hydrogen pH2.

number

[0016] (Aspect 6) The above hot rolling process (a) further comprises a step of keeping the hot-rolled steel sheet warm in an insulated container with an inner wall covered with insulating material within 30 minutes after it has been wound up. The method for manufacturing a steel plate according to embodiment 5, characterized in that the maximum ambient temperature reached inside the above-mentioned insulated container is 500 to 650°C, and the time it takes for the ambient temperature to reach the above-mentioned maximum temperature is 1 to 8 hours.

[0017] (Aspect 7) A method for manufacturing a steel sheet according to embodiment 5 or 6, characterized in that, in the above step (d-1), the atmosphere surrounding the steel sheet obtained in the above cold rolling step (c) satisfies the following formula (7) for the partial pressure of water vapor pH2O and the partial pressure of hydrogen pH2. -4.0≦log(pH2O / pH2)≦-1.0 (7)

[0018] (Pattern 8) The above step (d-3) is a method for manufacturing a steel sheet according to any one of embodiments 5 to 7, characterized in that when the temperature of the steel sheet obtained in the above cold rolling step (c) is between the Ms point and 650°C, cooling is stopped and the sheet is allowed to cool for 0.1 to 3.0 seconds. [Effects of the Invention]

[0019] The present invention makes it possible to obtain a steel sheet that has excellent tensile strength and excellent bendability after plastic deformation. [Brief explanation of the drawing]

[0020] [Figure 1] Figure 1 is a schematic diagram showing a cross-section of a plated steel sheet 1, including a base steel sheet 2 according to one embodiment of the present invention, cut in the thickness direction. [Modes for carrying out the invention]

[0021] Hereinafter, a plated steel sheet containing a steel sheet as a base material, which is one embodiment of the present invention, will be described in detail with reference to Figure 1. Figure 1 is a schematic diagram showing a cross-section of a plated steel sheet 1, which contains a base material steel sheet 2 according to one embodiment of the present invention, cut in the thickness direction.

[0022] In the present invention, the features regarding specific positions in the thickness direction of the steel sheet are defined. In the following description, these features may be described using the positions in the thickness direction of the steel sheet with respect to the steel sheet surface as a reference.

[0023] Since the "thickness direction" and the "depth direction" of the steel sheet are synonymous, in this specification, the position in the thickness direction of the steel sheet with respect to the above-mentioned steel sheet surface may be referred to as the "depth position". In connection with this, in this specification, the "x / y depth position of the thickness (where x and y are natural numbers satisfying x < y)" means the surface in the thickness direction of the steel sheet, that is, from the steel sheet surface, in the thickness direction, the position moved toward the center of the steel sheet by a distance (depth) of x / y of the thickness. For example, when the thickness of the steel sheet is t mm, the "1 / 8 depth position of the thickness" means the position at a depth of 1t / 8 mm in the thickness direction from the steel sheet surface.

[0024] Here, regarding the "steel sheet surface" which is the reference for the position in the thickness direction of the steel sheet, that is, the depth position of the steel sheet, in this specification, in the high-frequency glow discharge optical emission spectrometry (hereinafter, may be referred to as "high-frequency GDS analysis") described later, the depth position at which the emission intensity of Fe reaches 0.7 times the internal Fe emission intensity is defined as the 0 μm position, and this 0 μm position is taken as the steel sheet surface. The internal Fe emission intensity is the Fe emission intensity in a sufficient depth region of the base steel sheet. This region is a region where there is almost no change in the Fe concentration in the depth direction and is a region judged as "steel" as common technical knowledge. The internal Fe emission intensity may be, for example, the Fe emission intensity at a sputtering time of 1000 seconds.

[0025] Note that the "steel sheet" targeted by the present invention may be the "base steel sheet" of a steel sheet having some coating on the surface, such as the plated steel sheet 1 shown in FIG. 1. In such a case, the "steel sheet surface" which is the reference for the depth position of the steel sheet is the steel sheet surface of the base steel sheet, but similar to the above, it is the depth position at which the emission intensity of Fe in the high-frequency GDS analysis reaches 0.7 times the internal Fe emission intensity, that is, the 0 μm position. For example, in the plated steel sheet 1 shown in Figure 1, the surface of the steel sheet is indicated by the dashed line "S" near the interface between the base steel sheet 2 and the plating layer 3. d This is the position where, as mentioned above, in high-frequency GDS analysis, the Fe emission intensity reaches 0.7 times the internal Fe emission intensity, i.e., the 0 μm position.

[0026] Similarly, expressions such as "a depth of 30 μm from the steel plate surface" also mean a position 30 μm away from the steel plate surface, in the thickness direction, toward the center of the steel plate. For example, in the plated steel plate 1 shown in Figure 1, the steel plate surface S d P at a depth of 30 μm 30 is the surface of the steel plate S d This position is located 30 μm away from the center of the steel plate in the thickness direction.

[0027] <Plated steel sheet> As shown in Figure 1, the plated steel sheet 1 is a plated steel sheet having a base steel sheet 2 of this embodiment and a plating layer 3 provided on both sides of the base steel sheet 2. Note that the plating layer 3 may be provided on only one side of the base steel sheet 2.

[0028] Furthermore, as shown in Figure 1, the plated steel sheet 1 has a steel sheet surface S d P at a depth of 150 μm 150 The surface layer P is defined as a region in the thickness direction of the plate. S It has.

[0029] <Base material steel plate> In this embodiment, the base steel plate 2 has the following characteristics. First, the chemical composition of the base steel sheet 2 is, in mass%, C: 0.06~0.30%, Si: 0.01~2.50%, Mn: 1.00~3.50%, Ti: 0.001~0.100%, B: 0.0005~0.0050%, P: 0.050% or less, S: 0.0100% or less, Al: 1.500% or less, N: 0.010% or less, O: 0.0100% or less, Cr: 0 - 1.00%, Mo: 0 - 1.00%, Cu: 0 - 1.00%, Ni: 0 - 1.00%, Co: 0 - 1.00%, W: 0 - 1.00%, Sn: 0 - 1.00%, Sb: 0 - 0.50%, Nb: 0 - 0.200%, V: 0 - 1.00%, As: 0 - 0.10%, Zn: 0 - 1.00%, Ca: 0 - 0.0100%, Mg: 0 - 0.0100%, Ce: 0 - 0.0150%, Zr: 0 - 0.0100%, La: 0 - 0.0150%, Hf: 0 - 0.0100%, Bi: 0 - 0.0100%, REM other than Ce and La: 0 - 0.0100%, and the balance: Fe and impurities.

[0030] Furthermore, in the range from 1 / 8 depth position to 3 / 8 depth position of the thickness of the base steel plate 2, the steel structure is, in area%, ferrite: 30% or less, tempered martensite: 40% or more, total of retained austenite and fresh martensite: 15% or less, total of pearlite and cementite: 5% or less, and the balance: bainite.

[0031] Also, the surface layer part P of the base steel plate 2 S has a boron - depleted layer P in which the emission intensity of B measured in the depth direction from the surface S of the steel plate by high - frequency glow discharge optical emission spectrometry satisfies the following formulas (1) and (2). d B30 / B150 < 0.90 ···(1) B has it. B30 / B150 < 0.90 ···(1) 0.90 ≤ B140 / B150 ≤ 1.10 ···(2) B30: Steel plate surface S d Emission intensity of B at a depth of 30 μm B140: Steel plate surface S d Emission intensity of B at a depth of 140 μm B150: Steel plate surface S d Emission intensity of B at a depth of 150 μm

[0032] Furthermore, the tensile strength of the base steel plate 2 is 1180 MPa or higher.

[0033] The following describes in detail these characteristics of the base steel sheet 2.

[0034] (chemical composition) First, the reason for limiting the chemical composition of the base steel sheet (hereinafter sometimes simply referred to as "steel sheet") according to this embodiment as described above will be explained. In this specification, all "%" used to define chemical composition refer to "mass%" unless otherwise specified. Also, in this specification, "~" indicating a numerical range is used to mean that the values ​​written before and after it are included as the lower and upper limits, respectively, unless otherwise specified.

[0035] (C: 0.06~0.30%) Carbon (C) is an essential element for ensuring the strength of steel plates. From the viewpoint of obtaining the required high strength, the C content should be 0.06% or more. The C content may be 0.07% or more, 0.08% or more, or 0.10% or more. Furthermore, from the viewpoint of workability and weldability, the C content should be 0.30% or less. The C content may be 0.29% or less, 0.28% or less, or 0.25% or less.

[0036] (Si: 0.01~2.50%) Silicon (Si) is an element that suppresses the formation of iron carbides and contributes to improved strength and formability. From the viewpoint of strength, formability, and weldability, the Si content should be 0.01 to 2.50%. The Si content may be 0.05% or more, 0.10% or more, 0.15% or more, or 0.20% or more. Alternatively, the Si content may be 2.20% or less, 2.00% or less, or 1.90% or less.

[0037] (Mn: 1.00~3.50%) Manganese (Mn) is a powerful austenite-stabilizing element and is effective in increasing the strength of steel sheets. From the viewpoint of strength, weldability, and low-temperature toughness, the Mn content should be 1.00 to 3.50%. The Mn content may be 1.10% or more, 1.30% or more, or 1.50% or more. Alternatively, the Mn content may be 3.30% or less, 3.10% or less, or 3.00% or less.

[0038] (Ti: 0.001~0.100%) Titanium (Ti) is an effective element for increasing the strength of steel sheets. From the viewpoint of strength and cost, the Ti content should be 0.001 to 0.100%. The Ti content may be 0.005% or more, 0.010% or more, 0.015% or more, or 0.020% or more. Alternatively, the Ti content may be 0.080% or less, 0.070% or less, or 0.050% or less.

[0039] (B: 0.0005~0.0050%) Boron (B) is an element that is effective in increasing the hardenability and strength of steel sheets, and is an essential element in this invention. In this invention, the bendability of the steel sheet after plastic deformation can be improved by forming a deboronized layer, as described later, on the surface layer of the steel sheet. From the viewpoint of forming an appropriate deboronized layer, the B content is set to 0.0005 to 0.0050%. The B content may be 0.0007% or more, 0.0010% or more, or 0.0015% or more. In addition, the B content may be 0.0040% or less, 0.0035% or less, or 0.0030% or less.

[0040] (P:0.050% or less) Phosphorus (P) is an element found as an impurity in steel. While it contributes to increasing the strength of steel sheets through solid solution strengthening, the P content should be 0.050% or less from the viewpoint of weldability and toughness. Preferably, the P content is 0.045% or less, 0.035% or less, or 0.020% or less. P is not an essential element, and the lower limit of the P content is 0%. However, since extremely low P content increases the cost of removing P, from an economic standpoint, the lower limit of the P content may be 0.0001%, 0.0005%, or 0.001%.

[0041] (S:0.0100% or less) S (sulfur) is an element present as an impurity in steel, and it forms MnS in steel sheets, degrading toughness and hole-expanding properties. Therefore, from the viewpoint of suppressing the degradation of toughness and hole-expanding properties, the S content should be 0.0100% or less. Preferably, the S content is 0.0050% or less, 0.0040% or less, or 0.0030% or less. S is not an essential element, and the lower limit of the S content is 0%. However, since extremely low S content increases desulfurization costs, from an economic standpoint, the lower limit of the S content may be 0.00001%, 0.00005%, or 0.0001%.

[0042] (Al: 1.500% or less) Aluminum (Al) is an element included for deoxidation of steel and does not need to be included in the final product, the steel sheet. Therefore, the lower limit of the Al content is 0%. However, in order to obtain a sufficient deoxidation effect, Al may be added during deoxidation so that the final product, the steel sheet, contains 0.0001% or more, 0.0005% or more, or 0.001% or more of Al. From the viewpoint of the load during hot rolling due to the increase in the transformation temperature of the steel, the Al content should be capped at 1.500%. Preferably, the Al content is 1.200% or less, 1.000% or less, or 0.800% or less.

[0043] (N:0.010% or less) Nitrogen (N) is an element found as an impurity in steel. If its content exceeds 0.010%, it forms coarse nitrides in the steel, degrading its bendability and hole-expanding properties. Therefore, the N content should be 0.010% or less. Preferably, the N content is 0.008% or less, 0.006% or less, or 0.005% or less. N is not an essential element, and the lower limit of the N content is 0%. However, since extremely low N content incurs high costs for nitrogen removal, from an economic standpoint, the lower limit of the N content may be 0.0001%, 0.0005%, or 0.001%.

[0044] (O:0.0100% or less) Oxygen (O) is an element present as an impurity in steel. If its content exceeds 0.0100%, it forms coarse oxides in the steel, causing it to become less flexible and its holes to widen. Therefore, the O content should be 0.0100% or less. Preferably, the O content is 0.0080% or less, 0.0060% or less, or 0.0050% or less. O is not an essential element, and the lower limit of the O content is 0%. However, from the viewpoint of manufacturing costs, the lower limit of the O content may be 0.00001%, 0.00005%, or 0.0001%.

[0045] In this embodiment, the basic chemical composition of the base steel sheet 2 is as described above. Furthermore, the base steel sheet 2 may contain the following optional elements as needed.

[0046] (Cr:0~1.00%, Mo:0~1.00%, Cu:0~1.00%, Ni:0~1.00%, Co:0~1.00%, W:0~1.00%, Sn:0~1.00%, Sb:0~0.50%, Nb:0~0.200%, V:0~1.00%, As:0~0.10%, Zn:0~1.00%) Cr (chromium), Mo (molybdenum), Cu (copper), Ni (nickel), Co (cobalt), W (tungsten), Sn (tin), Sb (antimony), Nb (niobium), V (vanadium), As (arsenic), and Zn (zinc) are all elements that are effective in increasing the strength of steel sheets. For this reason, one or more of these elements may be added as needed. From the standpoint of the benefits and costs of including these elements, the content of these elements shall be as follows: Cr: 0-1.00%, Mo: 0-1.00%, Cu: 0-1.00%, Ni: 0-1.00%, Co: 0-1.00%, W: 0-1.00%, Sn: 0-1.00%, Sb: 0-0.50%, Nb: 0-0.200%, V: 0-1.00%, As: 0-0.10%, and Zn: 0-1.00%. The content of these elements may be 0.005% or more, or 0.010% or more, respectively.

[0047] (Ca: 0-0.0100%, Mg: 0-0.0100%, Ce: 0-0.0150%, Zr: 0-0.0100%, La: 0-0.0150%, Hf: 0-0.0100%, Bi: 0-0.0100%, and REM other than Ce and La: 0-0.0100%) Ca (calcium), Mg (magnesium), Ce (cerium), Zr (zirconium), La (lanthanum), Hf (hafnium), and REM (rare earth elements) other than Ce and La are all elements that contribute to the fine dispersion of inclusions in steel. Bi (bismuth) is an element that reduces the microsegregation of substitutional alloy elements such as Mn and Si in steel. Since each of these elements contributes to improving the workability of steel sheets, one or more of these elements may be added as needed. From the viewpoint of workability and ductility, the upper limit of the content of Ca, Mg, Zr, Hf, Bi, and REM other than Ce and La is 0.0100% each, and the upper limit of the content of Ce and La is 0.0150% each. The content of these elements may be 0.0005% or more, or 0.0010% or more, respectively.

[0048] In this embodiment, the remainder of the base steel sheet 2, other than the above-mentioned components, consists of Fe and impurities. Here, the impurities contained in the remainder other than the above-mentioned components are components that are mixed in during the industrial production of steel sheets due to various factors in the manufacturing process, including raw materials such as ore and scrap. Such impurities include components that were not intentionally added to the base steel sheet 2. Furthermore, the impurities contained in this remainder include elements other than the above-described components, which are present in the steel sheet to the extent that the specific effects of the impurity elements do not affect the properties of the base steel sheet 2.

[0049] Furthermore, if the steel sheet is a surface-treated steel sheet, the above chemical composition refers to the content of the base steel sheet after the surface coating has been removed. Also, if the steel sheet is a steel sheet without a coating such as a plating layer or surface treatment layer, the above chemical composition refers to the content of the steel sheet itself.

[0050] The chemical composition of steel sheets can be measured using general analytical methods. For example, the chemical composition of steel sheets can be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). Specifically, the front and back surfaces of the steel sheet are ground to a depth of 200 μm from each surface to take test pieces, and the chemical composition of the steel sheet can be determined by measuring it using a measuring device such as the Shimadzu ICPS-8100 under conditions based on a pre-established calibration curve. C and S, which cannot be measured by ICP-AES, can be measured using combustion-infrared absorption spectroscopy, N can be measured using inert gas fusion-thermal conductivity spectroscopy, and O can be measured using inert gas fusion-nondispersive infrared absorption spectroscopy.

[0051] [Steel structure inside steel plate] Next, the reasons for limiting the internal structure of the base steel sheet 2 according to this embodiment will be explained. In this specification, all "%" used to define the steel structure refer to "area %" unless otherwise specified.

[0052] (Ferrite: 30% or less) Ferrite is a soft structure with excellent ductility. It may be included in steel sheets to improve their elongation, depending on the required strength and ductility. From the viewpoint of balancing strength and ductility, the ferrite content should be limited to 30%. The ferrite content may be 25% or less, or 20% or less. The ferrite content may be 0%, or 3% or more, 5% or more, or 10% or more.

[0053] (Tempered martensite: 40% or more) Tempered martensite is a high-strength and tough structure that also increases the tensile strength and bending load of steel sheets. To obtain the desired tensile strength and bendability, the tempered martensite content should be at least 40%. Preferably, the tempered martensite content is 50% or more, 60% or more, 70% or more, or 80% or more.

[0054] (Total of residual austenite and fresh martensite: 15% or less) Retained austenite is a microstructure that contributes to improving the ductility of steel sheets through the work-induced transformation effect. On the other hand, retained austenite transforms into as-quenched martensite through work-induced transformation, which can degrade the bendability of the steel sheet. Furthermore, fresh martensite is a brittle microstructure and can become the starting point for fracture during plastic deformation, potentially degrading the local ductility of the steel sheet. Therefore, the total content of retained austenite and fresh martensite should be 15% or less. Preferably, the total content of retained austenite and fresh martensite is 12% or less, 10% or less, or 8% or less. The total content of retained austenite and fresh martensite may be 0% or more, 1% or more, 3% or more, or 5% or more.

[0055] (Total of perlite and cementite: 5% or less) Pearlite contains hard, coarse cementite and acts as a fracture initiation point during plastic deformation. Therefore, if the combined content of pearlite and cementite exceeds 5%, it can degrade the local ductility of the steel sheet. For this reason, the pearlite content, including cementite, should be 5% or less. The combined content of pearlite and cementite may be 3% or less, or even 2% or less. Here, cementite refers to coarse particles with a circular diameter exceeding 1 μm. Minute cementite precipitated within bainite or martensite is not included.

[0056] The remaining tissue other than the above-mentioned tissue may be 0%, but if such remaining tissue exists, it is bainite. Furthermore, the bainite in the remaining tissue may be either upper bainite or lower bainite, or a mixture of the two.

[0057] The structural fraction of steel sheets is evaluated by SEM-EBSD (scanning electron beam backscatter diffraction) and SEM secondary electron imaging.

[0058] First, a sample is taken from the cross-section of the steel plate parallel to the rolling direction, which is used as the observation surface. The observation surface is then mechanically polished to a mirror finish, followed by electropolishing. Next, in one or more observation fields ranging from 1 / 8 to 3 / 8 of the steel plate thickness on the observation surface, a total of 2.0 × 10⁻⁶ -9 m 2 The above-mentioned area will be subjected to crystal structure and orientation analysis using the SEM-EBSD method. TSL's "OIM Analysys® 6.0" will be used to analyze the data obtained by the EBSD method. The step size between grading points will be set to 0.10 μm. Regions identified as FCC iron based on observations will be considered retained austenite. Furthermore, a grain boundary map will be obtained by defining boundaries where the crystal orientation difference is 15 degrees or more as grain boundaries.

[0059] Next, nital etching is performed on the same sample that underwent EBSD observation, and secondary electron imaging is performed on the same field of view as in the EBSD observation. To observe the same field of view as during EBSD measurement, it is advisable to mark the area beforehand with Vickers indentations or other indicators. From the obtained secondary electron images, the area fractions of ferrite, retained austenite, bainite, tempered martensite, fresh martensite, and pearlite are measured, respectively.

[0060] Regions containing a substructure within the grain, where cementite precipitates with multiple variants, more specifically two or more variants, are identified as tempered martensite. Regions where cementite precipitates in a lamellar pattern are identified as pearlite. Regions with relatively low brightness within the field of view, including all structures, and where no substructure is observed, are identified as ferrite. Regions with high brightness and where the substructure has not been revealed by etching are identified as fresh martensite and retained austenite. The area percentage of each structure is calculated using the point counting method to obtain the area percentage of each structure. The finer the lattice spacing used in point counting, the more accurate the value. For example, a lattice spacing of 2 μm is suitable.

[0061] If the total area ratio of each tissue obtained using the above evaluation method is less than 100%, the remaining region is judged to be bainite. If the total area ratio of each tissue obtained using the above evaluation method exceeds 100%, the area ratio of each tissue is determined by multiplying the area ratio of each tissue by 100 / (total area ratio of each tissue).

[0062] [Boron Removal] In this embodiment, the base steel sheet 2 has a surface layer P as described above. S Deboronized layer P B It has the following characteristics. In this specification, a "boron-free layer" is defined as a layer in which the emission intensity of B, measured in the depth direction from the surface of the steel plate by high-frequency glow discharge emission analysis (high-frequency GDS analysis), satisfies the following equations (1) and (2).

[0063] B30 / B150 < 0.90 ···(1) 0.90 ≤ B140 / B150 ≤ 1.10 ···(2)

[0064] Here, B30, B140, and B150 are the luminescence intensities of B at a depth of 30 μm from the steel plate surface, 140 μm from the steel plate surface, and 150 μm from the steel plate surface, respectively, when measured by high-frequency GDS analysis from the steel plate surface in the thickness direction.

[0065] High-frequency GDS analysis measurements are performed at five arbitrary locations. B30, B140, and B150 are determined by taking the average emission intensity of B at depths of 30 μm, 140 μm, and 150 μm from the steel plate surface at five arbitrary locations, respectively. The measurement conditions are as follows:

[0066] B30, B140, and B150 are measured using a high-frequency glow discharge emission spectrometer. Specifically, the surface of the steel plate to be measured is placed in an Ar atmosphere, and a voltage is applied to generate a glow plasma. The surface of the steel plate is then sputtered and analyzed in the depth direction. The elements contained in the steel plate are identified from the element-specific emission spectral wavelengths emitted when atoms are excited in the glow plasma, and the emission intensity of the identified elements is estimated.

[0067] The depth data can be estimated from the sputtering time. Specifically, by determining the relationship between sputtering time and sputtering depth in advance using a standard sample, the sputtering time can be converted to sputtering depth. Therefore, the sputtering depth converted from the sputtering time can be defined as the depth from the steel plate surface. The sputtering time should be set so that the sputtering depth is at least 150 μm or greater.

[0068] In high-frequency GDS analysis, commercially available analytical instruments can be used. In this embodiment, a high-frequency glow discharge emission spectrometer GD-Profiler2 (registered trademark) manufactured by Horiba, Ltd. is used. The detection pitch is set to 0.1 seconds. The obtained data is filtered after background removal. Filtering is performed using the moving average method. Specifically, a moving average of 51 points, consisting of the center point and 25 points before and after it, is calculated. The time values ​​corresponding to depths of 30 μm, 140 μm, and 150 μm are B30, B140, and B150, respectively. Other measurement conditions are as follows.

[0069] Ar gas pressure: 600 Pa Anode diameter: 4mmφ RF output: 35W

[0070] In this specification, as described above, the 0 μm position is defined as the depth position where the Fe emission intensity determined by high-frequency GDS analysis reaches 0.7 times the internal Fe emission intensity. In this definition, the internal Fe emission intensity may be, for example, the Fe emission intensity at a sputtering time of 1000 seconds.

[0071] The above equation (1) means that the boron concentration at a depth of 30 μm from the surface of the steel plate is less than 0.90 times the boron concentration at a depth of 150 μm. By satisfying this equation (1), damage to the metal structure near the surface of the steel plate is less likely to occur when the steel plate undergoes plastic deformation.

[0072] In formula (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, or less than 0.60. Also, B30 / B150 may be 0, but may be 0.10 or more, 0.20 or more, or 0.30 or more.

[0073] The above equation (2) means that the luminescence intensity of B at a depth of 140 μm from the surface of the steel plate is approximately equal to the luminescence intensity of B at a depth of 150 μm from the surface of the steel plate. In other words, the boron-free layer P in the present invention BThis means that the region where it can be formed extends to a depth of 150 μm from the surface of the steel plate. By satisfying equation (2), it is possible to prevent excessive softening at depths of 150 μm or more from the surface of the steel plate, thereby ensuring the strength of the steel plate.

[0074] The deboronized layer P described above B By forming this layer, the bendability after plastic deformation can be improved. The reason why this effect is obtained is not clear, but the deboronized layer P B The soft surface layer formed by this method may experience less damage to the metal structure (e.g., the formation of microvoids) when subjected to plastic deformation compared to the soft surface layer formed by the decarburization layer.

[0075] Furthermore, the deboronized layer P B Preferably, the emission intensity of B measured by the above-mentioned high-frequency GDS analysis satisfies the following equations (1') and (2).

[0076] B40 / B150 < 0.90 ···(1') 0.90 ≤ B140 / B150 ≤ 1.10 ···(2)

[0077] Here, B40 is the emission intensity of B at a depth of 40 μm from the surface of the steel plate, as measured by high-frequency GDS analysis in the thickness direction from the surface of the steel plate. Deboronized layer P that satisfies equations (1') and (2) as described above. B By forming this structure, the bendability after plastic deformation can be more reliably improved.

[0078] In the present invention, the boron-free layer is formed by means of a steel sheet, and the boron content in the steel sheet is 0.0005 to 0.0050%, and the emission intensity of bo measured by high-frequency GDS analysis satisfies at least the above formulas (1) and (2). In other words, if a region exists on the surface of the steel sheet where the boron content in the steel sheet is 0.0005 to 0.0050%, and the emission intensity of bo measured by high-frequency GDS analysis satisfies at least the above formulas (1) and (2), then that region becomes the "boron-free layer" in the present invention, regardless of the formation means.

[0079] Furthermore, the surface layer P of the base steel plate 2. S It is preferable that the degree of decarbonization (hereinafter sometimes simply referred to as "decarburization") is not large. Specifically, the surface layer P of the base steel sheet 2 S Preferably, the emission intensity of C measured in the depth direction from the surface of the steel plate by high-frequency glow discharge emission analysis (high-frequency GDS analysis) satisfies the following equations (3) and (4).

[0080] C40 / C150 > 0.50 ···(3) 0.90 ≤ C140 / C150 ≤ 1.10 ···(4)

[0081] Here, C40, C140, and C150 are the emission intensities of carbon at a depth of 40 μm from the steel plate surface, 140 μm from the steel plate surface, and 150 μm from the steel plate surface, respectively, when measured by high-frequency GDS analysis in the thickness direction from the steel plate surface.

[0082] High-frequency GDS analysis measurements are performed at five arbitrary locations. For C40, C140, and C150, the average emission intensity of C at depths of 40 μm, 140 μm, and 150 μm from the steel plate surface at five arbitrary locations is adopted. The measurement conditions are the same as those for B30, B140, and B150 described above.

[0083] Equation (3) above means that the carbon concentration at a depth of 40 μm from the surface of the steel plate is greater than 0.50 times the carbon concentration at a depth of 150 μm. By satisfying this equation (3), it is possible to prevent excessive softening near the surface of the steel plate and to ensure a certain level of fatigue strength.

[0084] In formula (3) above, C40 / C150 may be 0.70 or greater, 0.80 or greater, or 0.90 or greater. Also, C40 / C150 may be 1.10 or less, 1.05 or less, or 1.00 or less.

[0085] The degree of decarburization can be controlled by adjusting the atmosphere during the heat treatment process of the steel sheet manufacturing method described later, until the maximum heating temperature is reached. By limiting the degree of decarburization, it is possible to improve both bendability and planar bending fatigue characteristics.

[0086] Equation (4) above means that the emission intensity of carbon at a depth of 140 μm from the surface of the steel plate is approximately equal to the emission intensity of carbon at a depth of 150 μm from the surface of the steel plate. Note that the carbon concentration at a depth of 150 μm from the surface of the steel plate is approximately equal to the carbon concentration at the center of the thickness of the steel plate. By satisfying equation (4) in conjunction with equation (3) above, it is possible to prevent the metal structure from becoming excessively softened up to a depth of 150 μm from the surface of the steel plate, and to ensure the strength of the steel plate.

[0087] Previously, forming a boron-free layer on a steel sheet would lead to decarbonization, potentially impairing the fatigue properties of the steel sheet. However, as mentioned above, the boron-free layer P B By suppressing decarburization, the bendability of the steel sheet after plastic deformation can be improved without impairing its fatigue properties. Although the reason for this effect is not entirely clear, it is thought that if both a boron-free layer and a decarburized layer are present, the surface layer becomes excessively softened, significantly impairing the fatigue strength. Therefore, by suppressing decarburization, it is possible to prevent the surface layer from becoming excessively softened and to ensure a certain level of fatigue strength.

[0088] [Tensile strength: 1180 MPa or higher] In this embodiment, the tensile strength of the base steel sheet 2 is 1180 MPa or higher. Even though the base steel sheet 2 in this embodiment has such high tensile strength, the above-mentioned boron-free layer P B This provides excellent bendability after plastic deformation. The tensile strength of the base steel sheet 2 may be 1200 MPa or more, 1300 MPa or more, 1400 MPa or more, or 1500 MPa or more. The upper limit of the tensile strength of the base steel sheet 2 is not particularly limited, but from the viewpoint of toughness and formability, it may be, for example, 4000 MPa or less, 3000 MPa or less, or 2000 MPa or less.

[0089] The tensile strength (TS) of a steel plate can be measured as follows: First, a No. 5 test specimen according to JIS Z 2241:2011 is taken from the center of the width of the steel plate to be measured, with the longitudinal direction perpendicular to the rolling direction. Next, the tensile strength TS (MPa) can be measured by performing a tensile test in accordance with JIS Z 2241:2011 using this test specimen.

[0090] Furthermore, if it is difficult to take a test specimen from the steel plate to be measured, the Vickers hardness of the steel plate can be measured, and the tensile strength value can be derived from the following correlation formula ("Correlation between static strength parameters," Norihiko Hasegawa, Junichi Arai, and Michishichi Tanaka, "Materials," Vol. 39, No. 442, pp. 859-863) using the measured Vickers hardness. Hv = 0.301 × TS + 5.701 However, in the above formula, Hv represents Vickers hardness and TS represents tensile strength (MPa).

[0091] The Vickers hardness of steel plates can be measured according to JIS Z 2244:2009. Specifically, the Vickers hardness of a steel plate is obtained by taking 10 measurements at a depth of 1 / 4 of the plate thickness with a load of 1 kgf (approximately 9.80 N), and taking the average of these 10 measurements. At this time, the distance between measurement positions should be at least three times the distance between indentations.

[0092] [Tensile residual stress acting perpendicular to the rolling direction: 200 MPa or less] In this embodiment, it is preferable that the base steel sheet 2 has a tensile residual stress of 200 MPa or less acting in the direction perpendicular to the rolling direction on the surface of the steel sheet. This tensile residual stress can be measured by X-ray diffraction, and specifically, it should be measured in accordance with the standard conditions described in "Standard for X-ray Stress Measurement Method (1997 Edition)" (1997), published by the X-ray Materials Strength Division Committee of the Japan Society for Materials Science. For the measuring device, for example, the AutoMate II manufactured by Rigaku Corporation may be used.

[0093] The tensile residual stress acting perpendicular to the rolling direction on the surface of the steel plate is preferably 100 MPa or less, more preferably 60 MPa or less, and even more preferably 40 MPa or less. Note that the direction perpendicular to the rolling direction refers to the direction perpendicular to both the rolling direction and the thickness direction.

[0094] Reducing the tensile residual stress acting perpendicular to the rolling direction on the steel plate surface in this manner is particularly effective in improving fatigue strength. As in the base steel plate 2 of this embodiment, the surface layer P S When a soft layer is present, the effect of tensile residual stress becomes particularly pronounced.

[0095] [Plating layer] As described above, in this embodiment, the base steel sheet 2 has plating layers 3 on both sides. The plating layer 3 may be a hot-dip galvanized layer having any known composition, or an alloyed hot-dip galvanized layer. The plating layer 3 may also contain additive elements such as Al in addition to Zn. Furthermore, the amount of plating layer 3 is not particularly limited and can be a general amount. The plating layer 3 may be provided on only one side of the base steel sheet 2, or it may not be provided on any surface of the base steel sheet 2. In the steel sheet of the present invention, it is not essential to have a plating layer on the surface of the steel sheet.

[0096] (Thickness of steel plate) The thickness of the steel sheet of the present invention is not particularly limited and can be the same as that of steel sheets used for automobile parts, for example. Examples of such steel sheet thicknesses include 0.5 to 3.0 mm. The thickness of the steel sheet may be 0.7 mm or more, 0.8 mm or more, or 1.0 mm or more. Alternatively, the thickness of the steel sheet may be 2.8 mm or less, 2.5 mm or less, or 2.0 mm or less.

[0097] <Method of manufacturing steel plates> Next, a method for manufacturing a steel sheet according to one embodiment of the present invention will be described. The following description is intended to illustrate a characteristic method for manufacturing a steel sheet according to one embodiment of the present invention, and is not intended to limit the steel sheet to one manufactured by the following manufacturing method.

[0098] The method for manufacturing steel sheets includes a hot rolling step (a) (hereinafter sometimes simply referred to as "step (a)") in which a slab having a specific chemical composition is hot-rolled to obtain a hot-rolled steel sheet; a grinding step (e) (hereinafter sometimes simply referred to as "step (e)") in which the hot-rolled steel sheet is ground with a rotary grinding brush; a pickling step (b) (hereinafter sometimes simply referred to as "step (b)") in which the sheet is pickled after grinding; a cold rolling step (c) (hereinafter sometimes simply referred to as "step (c)") in which the hot-rolled steel sheet is cold-rolled after pickling to obtain a cold-rolled steel sheet; and a heat treatment step (d) (hereinafter sometimes simply referred to as "step (d)") in which the cold-rolled steel sheet is heat-treated.

[0099] The following describes in detail the preferred conditions for these processes.

[0100] [Hot rolling process (a)] First, a hot-rolling process (a) is performed in which a slab having the following specific chemical composition is hot-rolled under predetermined conditions to obtain a hot-rolled steel sheet, and then this hot-rolled steel sheet is cooled to a predetermined temperature and wound up. In the hot-rolling process, the slab having the following specific chemical composition is heated before hot-rolling.

[0101] Here, regarding the chemical composition of the slab, when the chemical composition of the final steel sheet is analyzed according to the analytical method described above, it can be confirmed that there is virtually no difference between it and the chemical composition of the slab. Therefore, the chemical composition of the slab is basically the same as that of the steel plate described above. That is, the chemical composition of the slab is, in mass%, C: 0.06~0.30%, Si: 0.01~2.50%, Mn: 1.00~3.50%, Ti: 0.001~0.100%, B: 0.0005~0.0050%, P: 0.050% or less, S: 0.0100% or less, Al: 1.500% or less, N: 0.010% or less, O: 0.0100% or less, Cr: 0~1.00%, Mo: 0~1.00%, Cu: 0~1.00%, Ni: 0~1.00%, Co: 0~1.00%, W: 0~1.00%, Sn: 0~1.00%, Sb: 0~0.50%, Nb: 0~0.200%, V: 0~1.00%, As: 0~0.10%, Zn: 0~1.00%, Ca: 0~0.0100%, Mg: 0~0.0100%, Ce: 0~0.0150%, Zr: 0~0.0100%, La: 0~0.0150%, Hf: 0~0.0100%, Bi: 0~0.0100%, REM other than Ce and La: 0~0.0100%, and The remainder consists of Fe and impurities. Furthermore, the preferred content of each component in the chemical composition of the slab is basically the same as that of the steel sheet described above.

[0102] In the hot rolling process, the heating temperature of the slab is not particularly limited, but it is generally preferable to set it to 1150°C or higher in order to sufficiently dissolve borides, carbides, etc. While it is preferable to cast the steel slab using a continuous casting method from the viewpoint of manufacturability, it may also be manufactured by ingot casting or thin slab casting.

[0103] (rough rolling) In this manufacturing method, the heated slab may be subjected to rough rolling before finish rolling for purposes such as adjusting the plate thickness. The conditions for such rough rolling are not particularly limited, but from the viewpoint of recrystallization during hot rolling, it is preferable to perform the rough rolling so that the total reduction ratio at 1050°C or higher is 60% or more. The total reduction ratio may be, for example, 90% or less.

[0104] (Finishing rolling) Next, the aforementioned slab is hot-rolled in a finish rolling process to obtain a hot-rolled steel sheet. The entry temperature for the finish rolling process is not particularly limited, but it is preferably set to 900-1050°C in order to obtain a suitable microstructure for the hot-rolled steel sheet. Furthermore, the total reduction ratio in the finish rolling process is preferably set to 70-95%.

[0105] In this manufacturing method, in order to form the boron-free layer described above, finish rolling is performed in three or more passes. Furthermore, the reduction ratio of each of the last three passes in the finish rolling is set to 20% or more, the interval between passes is set to within 1 second, the temperature of the entry steel sheet before the last three passes is set to 1000°C or less, and the temperature at the end of the finish rolling is set to 850-950°C. In addition, the time from the end of the final pass to the start of cooling is set to within 3 seconds. Performing finish rolling under these conditions promotes ferrite transformation by accumulating strain in austenite and softens the surface of the hot-rolled steel sheet, thereby promoting the introduction of strain into the surface by grinding in the next process. As a result, the boron-free layer described above can be formed on the final steel sheet product. The number of finish rolling passes is not particularly limited, as long as the last three passes satisfy the above conditions.

[0106] In this specification, "final three passes" refers to the three passes in the finishing rolling process that are counted from the final pass: the third pass, the second pass, and the third pass.

[0107] (Winding temperature: 450~650℃) The hot-rolled steel sheet, after the finish rolling described above, is cooled to a predetermined winding temperature and then wound. At this time, the winding temperature is set to 450-650°C from the viewpoint of the strength and workability of the hot-rolled sheet. The winding temperature may be 500°C or higher. Alternatively, the winding temperature may be 620°C or lower.

[0108] After winding is complete, a process of keeping the wound hot-rolled steel sheet warm in an insulated container may be carried out to promote the formation of a boron-free layer in the heat treatment process described later. As an example of a warming process, within 30 minutes after winding is complete, the hot-rolled steel sheet may be placed in an insulated container with an inner wall covered with insulating material for warming. In this case, the warming conditions can be set so that the maximum temperature of the atmosphere inside the container is 500 to 650°C, and the time it takes for the atmosphere temperature to reach the above maximum temperature can be 1 to 8 hours. When warming is carried out under such conditions, the surface layer of the hot-rolled steel sheet softens further, promoting the introduction of strain in the subsequent grinding process, and further promoting the formation of a boron-free layer in the heat treatment process described later, thereby further improving the bendability of the steel sheet after plastic deformation.

[0109] [Grinding process (e)] Next, a grinding process (e) is performed in which the front and back surfaces of the wound steel sheet are ground using a rotary grinding brush. Examples of brushes that can be used in the grinding process include the Hotani D-100-33. The grinding conditions are such that the rotation speed R (revolutions / min) of the grinding brush, the diameter D (m) of the grinding brush, and the speed V (m / min) of the steel sheet pass through satisfy the following equation (5). When grinding is performed under conditions that satisfy equation (5), strain is introduced into the surface layer of the steel sheet, which promotes the diffusion of boron in the heat treatment process described later, and the deboronized layer formed in the heat treatment process described later expands.

[0110]

number

[0111] In formula (5), (R·D) / V may be 11 or greater, 13 or greater, or 15 or greater. Also, there is no particular upper limit to (R·D) / V, but (R·D) / V may be 60 or less, 55 or less, or 50 or less.

[0112] Such a process (e) needs to be carried out between the completion of hot rolling and before cold rolling, and may be carried out either before or after the pickling process described later.

[0113] [Acid washing process (b)] Next, a pickling process (b) is carried out, in which the steel sheet after the hot rolling process (a) or grinding process (e) is pickled. The pickling method in the pickling process should follow conventional methods. In addition, skin pass rolling may be performed in the pickling process to correct the shape of the hot-rolled coil and improve pickling properties.

[0114] [Cold rolling process (c)] Next, a cold rolling process (c) is carried out, in which the steel sheet after the pickling process (b) or grinding process (e) is subjected to cold rolling. In the cold rolling process, the reduction ratio of the cold rolling is set to 30-75%, taking into consideration the accumulation of strain and the load on the cold rolling mill due to the rolling load. For example, the reduction ratio may be 40% or more. Alternatively, the reduction ratio may be 70% or less, or 60% or less.

[0115] [Heat treatment process (d)] Next, a heat treatment process (d) is carried out to heat the steel sheet obtained in the cold rolling process (c). The heat treatment process is carried out sequentially as follows: a process (d-1) in which the steel sheet obtained in process (c) is heated from 650°C to a maximum heating temperature of Ac1+50°C or higher and 950°C or lower at an average heating rate of 0.5 to 500°C / second; a process (d-2) in which the steel sheet is held at the above maximum heating temperature for 1 to 300 seconds; a process (d-3) in which the steel sheet is cooled to a temperature of Ms point (martensitic transformation point) - 100°C or lower, with the cooling from 700°C to 500°C at an average cooling rate of 10°C / second or higher; and a process (d-4) in which the steel sheet is held at 200 to 350°C for 50 to 600 seconds.

[0116] Boron removal can be sufficiently carried out by first softening the surface layer of the steel sheet by controlling the hot rolling conditions as described above, then introducing a large amount of strain into the surface layer of the steel sheet in the grinding process, and further, in the above steps (d-1) to (d-4) of this heat treatment process, i.e., the heating and soaking process, the reaction between H2O in the atmosphere and B on the surface of the steel sheet to form an oxide.

[0117] In the above-described process (d-1), the average heating rate to the maximum heating temperature is set to 0.5 to 500°C / second, from the viewpoint of promoting ferrite recrystallization and suppressing austenite coarsening. The average heating rate may be 1.0°C / second or higher, or 2.0°C / second or higher. Alternatively, the average heating rate may be 400°C / second or lower, or 300°C / second or lower. Here, "average heating rate" refers to the value obtained by dividing the difference between 650°C and the maximum heating temperature by the time required to reach the maximum heating temperature from 650°C.

[0118] In the above-described process (d-1), the maximum heating temperature is set to Ac1 + 50°C or higher and 950°C or lower, from the viewpoint of promoting austenitization and suppressing the coarsening of the austenite diameter. In the above-described process (d-2), the holding time at the maximum heating temperature is set to 1 to 300 seconds, from the viewpoint of the progression of austenitization and productivity. During holding at the maximum heating temperature, it is not necessary to keep the steel sheet at a constant temperature, and it may fluctuate within the above-described maximum temperature range. Here, "holding" means maintaining the temperature within a range of ±20°C, preferably ±10°C, of ​​a predetermined temperature, without exceeding the defined upper and lower limits.

[0119] After holding at the maximum heating temperature, the steel plate is cooled to a temperature below the Ms point -100°C in the above-described step (d-3). At this time, it is cooled from 700°C to 500°C at an average cooling rate of 10°C / second or more. The average cooling rate from 700°C to 500°C may be 20°C / second or more, 30°C / second or more, or 50°C / second or more.

[0120] To obtain the desired structure, after cooling to a temperature below the Ms point -100°C, the material is held at 200-350°C for 50-600 seconds in step (d-4) described above. "Holding" in step (d-4) does not require maintaining a constant temperature, as described above; it means maintaining the temperature within a range of ±20°C, preferably ±10°C. Note that if the holding time in step (d-4) is shorter than 50 seconds, the area ratio of fresh martensite to retained austenite increases, and the desired bendability after applying a 2% pre-strain cannot be obtained.

[0121] In the above-described step (d-4), holding in the temperature range of 200 to 350°C may be performed by reheating after cooling to below 200°C, or if the final temperature of the cooling to below 200°C is 200°C or higher, it may be performed in the middle of the subsequent cooling process.

[0122] Furthermore, in the above-mentioned process (d-1), the atmosphere surrounding the steel sheet when heating from 650°C to the maximum heating temperature is controlled so that the water vapor partial pressure pH2O and hydrogen partial pressure pH2 satisfy the following equation (6). If the right-hand side of equation (6) is less than -4.0, the boron removal reaction will not proceed sufficiently, and the desired bendability after applying 2% pre-strain cannot be obtained.

[0123] -4.0≦log(pH2O / pH2) ···(6) pH2O: partial pressure of water vapor pH2: Hydrogen partial pressure

[0124] Note that log(pH2O / pH2) on the right-hand side of equation (6) may be -3.8 or greater, or -3.6 or greater. Also, log(pH2O / pH2) on the right-hand side of equation (6) may be -0.1 or less, or -0.5 or less.

[0125] Furthermore, in step (d-1) described above, the atmosphere surrounding the steel sheet when heating from 650°C to the maximum heating temperature may be controlled so that the water vapor partial pressure pH2O and hydrogen partial pressure pH2 satisfy the following equation (7). By controlling the atmosphere during heating to satisfy this equation (7), decarburization of the steel sheet surface is suppressed, and the planar bending fatigue strength after plastic deformation can be improved. Furthermore, by optimizing the atmosphere during heating as described above, it is preferable to make the planar bending fatigue strength after 2% pre-strain application and paint baking 0.35 times or more the tensile strength, and more preferably 0.40 times or more or 0.45 times or more the tensile strength.

[0126] -4.0≦log(pH2O / pH2)≦-1.0 (7) pH2O: partial pressure of water vapor pH2: Hydrogen partial pressure

[0127] Furthermore, in step (d-3) described above, when the steel plate is cooled to a temperature of Ms point - 100°C or lower, cooling may be stopped or the cooling rate may be sufficiently reduced to provide a cooling period of 0.1 to 3.0 seconds in the section where the temperature of the steel plate is between Ms point and 650°C. By providing such a cooling period during the cooling process, the temperature difference between the surface and the interior of the steel plate can be made uniform, and the tensile residual stress on the surface of the steel plate that occurs after cooling can be reduced. Furthermore, by optimizing these cooling conditions, it is preferable to make the tensile residual stress acting perpendicular to the rolling direction on the surface of the steel plate 200 MPa or less, and more preferably 150 MPa or less or 100 MPa or less. Here, sufficiently reducing the cooling rate means, for example, making the average cooling rate 5.0°C / second or less. This average cooling rate may also be 0°C / second. Furthermore, within the above temperature range, the temperature may rise due to reheating, etc.

[0128] Furthermore, in the above-described step (d-3), the cooling interval may be 0.2 seconds or longer, or 0.3 seconds or longer. Alternatively, the cooling interval may be 2.5 seconds or less, 2.0 seconds or less, or 1.5 seconds or less.

[0129] As described above, a plating layer may be formed on the surface of the steel sheet of the present invention. The plating layer can be, for example, a hot-dip galvanized layer. Alternatively, if necessary, an alloying treatment may be performed after the formation of the hot-dip galvanized layer to form an alloyed hot-dip galvanized layer. The formation of the plating layer and the alloying treatment can be carried out according to conventional methods and are not particularly limited. The plating treatment can be performed during the cooling process from the maximum heating temperature to below the Ms point - 100°C. In this case, cooling should be stopped once at the plating treatment temperature, and after the plating treatment is completed, the sheet should be cooled to below the Ms point - 100°C at an average cooling rate of 10°C / second or more.

[0130] By the above manufacturing method, a steel sheet of the present invention, which has excellent tensile strength and bendability, can be obtained. Bendability is evaluated by applying a 2% pre-strain to a test piece taken from the steel sheet to be evaluated, and then performing a bending test according to the method specified in German Association of the Automotive Industry (VDA) standard 238-100, and determining the maximum bending angle obtained from the test. Furthermore, depending on the above manufacturing conditions, improvements in planar bending fatigue characteristics can also be achieved. Planar bending fatigue characteristics are evaluated by applying a 2% pre-strain to a test piece taken from the steel sheet to be evaluated, and then performing a planar bending fatigue test in accordance with JIS Z 2275:1978, and determining the fatigue limit ratio (= fatigue strength / tensile strength) obtained from the test. The reason for applying a 2% pre-strain during evaluation is that the steel sheet of the present invention is intended to be used as a component. [Examples]

[0131] Next, embodiments of the present invention will be described. The conditions in these embodiments are just one example of conditions adopted to confirm the feasibility and effectiveness of the present invention. The present invention is not limited to this one example of conditions. The present invention can adopt various conditions as long as they do not depart from the spirit of the invention and achieve the objectives of the present invention.

[0132] Steel with various chemical compositions was cast to produce slabs. These slabs were then hot-rolled to produce hot-rolled steel sheets. Furthermore, these hot-rolled steel sheets were sequentially subjected to grinding, cold-rolling, and heat treatment to produce cold-rolled steel sheets.

[0133] A portion of the obtained cold-rolled steel sheets was subjected to plating. Chemical composition analysis of samples taken from the obtained steel sheets confirmed that there was no change from the chemical composition of the slab. The chemical compositions of these steel sheets are shown in Table 1. The remainder, other than the components shown in Table 1, consists of Fe and impurities. For plated steel sheets, the chemical composition is that of the base steel sheet from which the surface plating layer was removed under the conditions described above. Underlined entries in Table 1 indicate that the chemical composition is outside the scope of the present invention.

[0134] [Table 1]

[0135] [Table 2]

[0136] The hot rolling conditions were as described in Table 2. In Table 2, R1 entry temperature refers to the entry temperature of the steel sheet at the third pass counting from the final pass of the finish rolling process. R1 represents the reduction ratio at the third pass counting from the final pass. R2 represents the reduction ratio at the second pass counting from the final pass. R3 represents the reduction ratio at the final pass. Also, t1 represents the time from the end of the third pass counting from the final pass to the start of the second pass counting from the final pass. t2 represents the time from the end of the second pass counting from the final pass to the start of the final pass. t3 represents the time from the end of the final pass to the start of cooling. Finally, R3 exit temperature refers to the temperature of the steel sheet at the end of the final pass, i.e., the temperature at which the finish rolling is complete.

[0137] Subsequently, both sides of the hot-rolled steel sheet were ground using a rotary grinding brush containing abrasive grains. The grinding conditions were set so that the value of (R·D) / V, calculated from the rotational speed R (revolutions / min) of the grinding brush, the diameter D (m) of the grinding brush, and the sheet passing speed V (m / min) of the hot-rolled steel sheet, was as shown in Table 3. Next, the ground steel sheet was pickled. Furthermore, the pickled steel sheet was cold-rolled at the reduction ratio shown in Table 3.

[0138] Subsequently, the cold-rolled steel sheets were subjected to heat treatment. The heat treatment involved heating to the maximum heating temperature, holding, and then cooling. For some steel sheets, a cooling interval was provided in the Ms~650°C range during the cooling process, during which the cooling was stopped. In Table 3, entries with a cooling time of 0 seconds indicate examples where no cooling interval was provided. Furthermore, after cooling to a temperature below Ms point -100°C, the sheets were held at 200~350°C. Steel sheet No. 15, whose cooling completion temperature to below Ms point -100°C was higher than the holding temperature at 200~350°C, was held during the subsequent cooling process after the completion of cooling to a temperature below Ms point -100°C. Steel sheets other than No. 15 were reheated to a predetermined temperature after the completion of cooling to below Ms point -100°C and then held. These conditions, as well as the log(pH2O / pH2) values ​​from 650°C to the maximum heating temperature, are shown in Table 3. Here, pH2O is the partial pressure of water vapor, and pH2 is the partial pressure of hydrogen. In addition, in Table 3, Ms represents the martensitic transformation point (°C) of the steel used.

[0139] In Table 3, the Ac1 point (°C), which serves as the reference range for setting the maximum heating temperature for heat treatment, was determined according to the following formula. The Ac1 points for each steel sheet are shown in Table 1. Ac1=723-10.7[Mn]-16.9[Ni]+29.1[Si]+16.9[Cr] In the above formula, [Mn], [Ni], [Si], and [Cr] represent the content (mass %) of each element.

[0140] Furthermore, in Table 3, the Ms point (°C) was calculated according to the following formula. Ms=561-474[C]-33[Mn]-7.5[Si]-17[Cr]-17[Ni]-21[Mo]+10[Co] In the above formula, [C], [Mn], [Si], [Cr], [Ni], [Mo], and [Co] represent the content (mass %) of each element.

[0141] Subsequently, some of the steel sheets underwent continuous hot-dip galvanizing, and some of those underwent alloying treatment. The plating conditions were not special, but rather known and general conditions. In Table 3, GA refers to alloyed hot-dip galvanized steel sheets. GI refers to hot-dip galvanized steel sheets that have not undergone alloying treatment. CR refers to cold-rolled steel sheets that have not been plated.

[0142] In Tables 2 and 3, underlines next to various numerical values ​​indicate that they are outside the scope of the present invention, that the manufacturing conditions are such that the steel sheet of the present invention cannot be obtained, or that the various properties of the steel sheet are undesirable.

[0143] [Table 3]

[0144] For the obtained steel plates, the emission intensities of B (B30, B40, B140, and B150) were measured at depths of 30 μm, 40 μm, 140 μm, and 150 μm from the steel plate surface, respectively, using the high-frequency glow discharge emission spectrometry (high-frequency GDS analysis) method described above. Similarly, the emission intensities of C (C40, C140, and C150) were measured at depths of 40 μm, 140 μm, and 150 μm from the steel plate surface. These measurement results are shown in Table 4 below.

[0145] Furthermore, a No. 5 tensile test specimen, conforming to JIS Z 2241:2011, was taken from the center of the width of the obtained steel sheet, with the longitudinal direction perpendicular to the rolling direction. A tensile test was then performed using this specimen in accordance with JIS Z2241:2011, and the tensile strength (TS) was measured. For steel sheets No. 30 and No. 31, the tensile strength was measured with the plating intact, without removing the plating from the plated steel sheet. In this example, the standard for tensile strength (1180 MPa or higher) was the same as for unplated steel sheets. The measurement results of the tensile strength of each steel sheet are shown in Table 4 below.

[0146] Furthermore, a tensile test specimen with a parallel section width of 30 mm was taken from the center of the obtained steel sheet, with the longitudinal direction perpendicular to the rolling direction. After applying a 2% pre-strain, a rectangular sample measuring 30 mm wide x 60 mm long was taken from the parallel section. Subsequently, to simulate the paint baking process of automobiles, a heat treatment was performed at 170°C for 20 minutes. The heat-treated specimens were subjected to a bending test according to the method specified in German Association of the Automotive Industry (VDA) standard 238-100, and the maximum bending angle was measured. A maximum bending angle of 60 degrees or more was judged to indicate good "bendability". The bending direction was carried out so that the rolling direction was parallel to the bending ridge. For steel sheets No. 30 and No. 31, the maximum bending angle was measured with the plating intact without removing the plating from the plated steel sheet. In this embodiment, the standard for the maximum bending angle (60 degrees or more) was the same as for unplated steel sheets. The measurement results for the maximum bending angle of each steel plate are shown in Table 4 below.

[0147] Furthermore, a No. 1 test piece (25 mm wide) as described in JIS Z 2275:1978 was taken from the center of the width of the obtained steel sheet, with the longitudinal direction perpendicular to the rolling direction. A 2% pre-strain was applied to the center of the test piece using a tensile testing machine. The amount of strain was controlled by attaching a GL5 mm strain gauge to the center of the test piece. Subsequently, a heat treatment was performed at 170°C for 20 minutes to simulate the paint baking process of an automobile. After the heat treatment, a plane bending fatigue test was performed on the test piece in accordance with JIS Z 2275:1978, and the fatigue limit ratio (= fatigue strength / tensile strength) was measured. For the measurement results, a fatigue limit ratio of 0.35 or higher was judged to indicate good "plane fatigue bending characteristics". For No. 30 and No. 31, the fatigue limit ratio was measured with the plating intact without removing the plating from the plated steel sheet. In this embodiment, the standard for the fatigue limit ratio (0.35 or higher) was the same as for unplated steel sheets. The measurement results for the fatigue limit ratio of each steel plate are shown in Table 4 below.

[0148] Here, we will explain the points to be aware of when evaluating the present invention. The features of the present invention, such as the chemical composition, structural integrity, B concentration distribution, and C concentration distribution of the steel sheet, are defined in areas unrelated to the surface coating. On the other hand, the mechanical properties of the steel sheet (i.e., tensile strength, bendability, and plane fatigue bending properties) can generally be considered to change slightly depending on whether or not the surface coating is present. Even in such a situation, in the present invention, we use a steel sheet with the same surface condition as when it is used to determine whether the mechanical properties of that steel sheet fall within the scope of the present invention. This is because, for those who use steel sheets with a surface coating, the mechanical properties in the as-coated state are important, rather than the mechanical properties in the state after the coating has been removed. Therefore, in the examples of the present invention, the mechanical properties of tensile strength, bendability, and plane fatigue bending properties are evaluated in the as-coated state for plated steel sheets (steel sheets No. 30 and No. 31), and in the unplated state for unplated steel sheets (steel sheets other than No. 30 and No. 31).

[0149] In Table 4, "α" in the microstructure represents ferrite. "TM" represents tempered martensite. "FM+γ" represents the sum of fresh martensite and retained austenite. "P+θ" represents the sum of pearlite and cementite. And "B" represents bainite. In Table 4, the underlines next to various numerical values ​​indicate that they are outside the scope of the present invention, that the manufacturing conditions are such that the steel sheet of the present invention cannot be obtained, or that the various properties of the steel sheet are undesirable.

[0150] [Table 4]

[0151] Steel plate No. 6 exhibited poor bendability because the grinding process conditions were not appropriate, resulting in the formation of a proper boron-free layer.

[0152] Steel plate No. 7 exhibited poor bendability because the reduction ratio during the finish rolling process in the hot rolling stage was inappropriate, resulting in the formation of a proper boron-free layer.

[0153] Steel plate No. 8 failed to achieve the desired tensile strength because the maximum heating temperature during the heat treatment process was low, resulting in a high ferrite content.

[0154] Steel plate No. 9 exhibited poor bendability because the finishing rolling temperature during the hot rolling process was too high, resulting in the formation of an inadequate boron-free layer.

[0155] Steel sheet No. 15 exhibited inferior bendability because its high cooling stop temperature during the heat treatment process resulted in a higher proportion of fresh martensite and retained austenite.

[0156] Steel plate No. 16 was not ground with a brush, and therefore a proper boron-removed layer was not formed, resulting in poor bendability.

[0157] Steel plate No. 17 exhibited poor bendability because the time between finish rolling passes in the hot rolling process was too long, resulting in insufficient formation of a proper boron-free layer.

[0158] Steel sheet No. 18 had a low holding temperature after cooling during the heat treatment process, resulting in a high proportion of fresh martensite and retained austenite, which led to poor bendability.

[0159] Steel sheet No. 19 had a short holding time after cooling during the heat treatment process, resulting in a higher proportion of fresh martensite and retained austenite, which led to poorer bendability.

[0160] Steel plate No. 20 had a log(pH2O / pH2) value less than -4.0 during the heat treatment process, resulting in an insufficient boron-free layer formation and thus poor bendability.

[0161] Steel plate No. 32 failed to achieve the desired tensile strength due to its low carbon content in its chemical composition.

[0162] Steel plate No. 33 had a high carbon content in its chemical composition, resulting in poor bendability.

[0163] Steel plate No. 34 had a low B content in its chemical composition, resulting in the formation of an inadequate boron-free layer, which led to poor bendability.

[0164] Steel sheet No. 35 had a high Mn content in its chemical composition, resulting in a high proportion of fresh martensite and retained austenite combined, which led to poor bendability.

[0165] Steel sheet No. 36 had a low Mn content in its chemical composition, a high ferrite fraction, and a low tempered martensite fraction, resulting in poor bendability.

[0166] Steel sheet No. 37 had a high Si content in its chemical composition, resulting in a high total fraction of fresh martensite and retained austenite, which led to poor bendability.

[0167] Steel plate No. 2 is an example of an inventive steel plate with excellent bendability. Because log(pH2O / pH2) in the heat treatment process was greater than -1.0, and the cooling time was shorter than 0.1 seconds, the fatigue limit ratio was slightly lower compared to other examples of the present invention.

[0168] Steel plate No. 3 is an example of an inventive steel plate with excellent bendability. Because log(pH2O / pH2) in the heat treatment process was greater than -1.0, the fatigue limit ratio was slightly lower compared to other examples of the invention.

[0169] Steel plate No. 13 is an example of an inventive steel plate with excellent bendability. As a result of the cooling time during the heat treatment process being less than 0.1 seconds, the fatigue limit ratio was slightly lower compared to other examples of the present invention. [Explanation of Symbols]

[0170] 1. Plated steel sheet 2 Base steel plate 3 Plating layer Sd steel plate surface P S surface layer P B Deboron-free layer P 30 A depth of 30 μm from the surface of the steel plate. P 150 A depth of 150 μm from the surface of the steel plate.

Claims

1. It is a steel plate, The chemical composition of the steel plate is, in mass%, C: 0.06-0.30%, Si: 0.01-2.50%, Mn: 1.00-3.50%, Ti: 0.001 to 0.100%, B: 0.0005 to 0.0050%, P: 0.050% or less, S: 0.0100% or less, Al: 1.500% or less, N: 0.010% or less, O: 0.0100% or less, Cr: 0-1.00%, Mo: 0-1.00%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Co: 0-1.00%, W: 0-1.00%, Sn: 0-1.00%, Sb: 0 to 0.50%, Nb: 0 to 0.200%, V: 0 to 1.00%, As: 0 to 0.10%, Zn: 0 to 1.00%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Ce: 0 to 0.0150%, Zr: 0 to 0.0100%, La: 0 to 0.0150%, Hf: 0-0.0100%, Bi: 0 to 0.0100%, REM other than Ce and La: 0 to 0.0100%, and The remainder consists of Fe and impurities. The steel structure in the range from 1 / 8 depth to 3 / 8 depth of the steel plate thickness is, in area %, Ferrite: 30% or less Tempered martensite: 40% or more, Total of residual austenite and fresh martensite: 15% or less. Total amount of perlite and cementite: 5% or less, Remaining parts: Bay Knight, The surface layer of the steel plate has a boron-free layer in which the emission intensity of B, measured in the depth direction from the surface of the steel plate by high-frequency glow discharge emission analysis, satisfies the following equations (1) and (2). A steel plate characterized by having a tensile strength of 1180 MPa or more. B30 / B150<0.90...(1) 0.90 ≤ B140 / B150 ≤ 1.10 ... (2) B30: The luminescence intensity of B at a depth of 30 μm from the surface of the steel plate. B140: The luminescence intensity of B at a depth of 140 μm from the surface of the steel plate. B150: The luminescence intensity of B at a depth of 150 μm from the surface of the steel plate.

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

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

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

5. A method for manufacturing a steel plate according to Claim 1, The chemical composition is expressed in mass percent. C: 0.06-0.30%, Si: 0.01-2.50%, Mn: 1.00-3.50%, Ti: 0.001 to 0.100%, B: 0.0005 to 0.0050%, P: 0.050% or less, S: 0.0100% or less, Al: 1.500% or less, N: 0.010% or less, O: 0.0100% or less, Cr: 0-1.00%, Mo: 0-1.00%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Co: 0-1.00%, W: 0-1.00%, Sn: 0-1.00%, Sb: 0 to 0.50%, Nb: 0 to 0.200%, V: 0 to 1.00%, As: 0 to 0.10%, Zn: 0 to 1.00%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Ce: 0 to 0.0150%, Zr: 0 to 0.0100%, La: 0 to 0.0150%, Hf: 0-0.0100%, Bi: 0 to 0.0100%, REM other than Ce and La: 0 to 0.0100%, and The remainder: A hot rolling process (a) in which a slab consisting of Fe and impurities is hot-rolled at a finish rolling completion temperature of 850 to 950°C to obtain a hot-rolled steel sheet, and then the hot-rolled steel sheet is cooled to 450 to 650°C and wound up, The pickling step (b) involves pickling the steel sheet obtained in the hot rolling step (a), The steel sheet obtained in the pickling step (b) is cold-rolled at a reduction ratio of 30 to 75% to obtain a cold-rolled steel sheet in a cold-rolling step (c), A heat treatment step (d) is performed on the steel sheet obtained in the cold rolling step (c), The process includes a grinding step (e) performed before or after the pickling step (b), in which the front and back surfaces of the steel sheet obtained in the hot rolling step (a) or the steel sheet obtained in the pickling step (b) are ground using a rotary grinding brush containing abrasive particles. The hot rolling process (a) consists of three or more finish rolling passes, with a reduction ratio of 20% or more for each of the last three passes of the finish rolling, a time between passes of 1 second or less, an entry steel sheet temperature of 1000°C or less before the last three passes, and a time of 3 seconds or less from the completion of the final pass to the start of cooling. The grinding step (e) is performed when the rotational speed R of the grinding brush (revolutions / min), the diameter D of the grinding brush (m), and the speed V of the steel plate passing through (m / min) satisfy the following equation (5): The heat treatment step (d) is The steel sheet obtained in the cold rolling process (c) is heated from 650°C to a maximum heating temperature of Ac1 + 50°C or more and 950°C or less, at an average heating rate of 0.5 to 500°C / second (d-1), The process (d-2) involves holding the steel sheet obtained in the cold rolling process (c) at the maximum heating temperature for 1 to 300 seconds, A step of cooling the steel sheet obtained in the cold rolling step (c) to a temperature below the Ms point -100°C, comprising a step (d-3) of cooling from 700°C to 500°C at an average cooling rate of 10°C / second or more, The process further comprises a step (d-4) of holding the steel sheet obtained in the cold rolling step (c) at 200 to 350°C for 50 to 600 seconds, In the above step (d-1), the atmosphere surrounding the steel sheet obtained in the cold rolling step (c) is the water vapor partial pressure pH ２ O and hydrogen partial pressure pH ２ A method for manufacturing steel plates, characterized in that the following equation (6) is satisfied. [Math 1]

6. The hot rolling process (a) further comprises a step of keeping the hot-rolled steel sheet warm in an insulated container with an inner wall covered with insulating material within 30 minutes after it has been wound up. The method for manufacturing a steel plate according to claim 5, characterized in that the maximum temperature reached by the ambient temperature inside the insulated container is 500 to 650°C, and the time it takes for the ambient temperature to reach the maximum temperature is 1 to 8 hours.

7. In the above step (d-1), the atmosphere surrounding the steel sheet obtained in the cold rolling step (c) is the water vapor partial pressure pH ２ O and hydrogen partial pressure pH ２ A method for manufacturing a steel sheet according to claim 5 or 6, characterized in that the following formula (7) is satisfied. -4.0≦log(pH ２ O / ph ２ )≦-1.0 ・・・(7)

8. The method for manufacturing a steel sheet according to any one of claims 5 to 7, characterized in that step (d-3) involves stopping the cooling when the temperature of the steel sheet obtained in the cold rolling step (c) is between the Ms point and 650°C, and allowing it to cool for 0.1 to 3.0 seconds.

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