Steel plates and press-formed products

Abstract

This steel sheet has a chemical composition in mass% of C: 0.040 to 0.100%, Mn: 1.00 to 2.00%, Si: 0.005 to 1.500%, P: 0.100% or less, S: 0.0200% or less, Al: 0.005 to 0.700%, N: 0.0150% or less, O: 0.0100% or less, and the remainder: Fe and impurities, and has an arithmetic mean waviness Wa of 0.10 to 0.30 μm.

Description

Technology Field

[0001] The present invention relates to steel plates and press-formed articles. Background Technology

[0002] From the perspective of protecting the global environment, there is a demand for lighter vehicle bodies and improved crash safety. To meet these demands, high strength and thinning are being considered for panel-based components, such as door outers. Unlike skeletal components, these panel-based parts require high appearance quality to stand out. Therefore, even high-strength steel sheets traditionally used for skeletal components must possess excellent appearance quality after forming when applied to panel-based parts.

[0003] To improve appearance quality, suppressing the occurrence of ghost lines can be cited as a challenge. Ghost lines refer to minute irregularities on the order of millimeters that appear on the surface of steel sheets containing hard and soft phases when they are press-formed, as the area surrounding the soft phase deforms preferentially. Since these irregularities appear as streaks on the surface, press-formed products exhibiting ghost lines have reduced appearance quality.

[0004] Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet with excellent surface quality. Specifically, Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet having, in mass%, C: 0.02 to 0.20%, Si: 0.7% or less, Mn: 1.5 to 3.5%, P: 0.10% or less, S: 0.01% or less, Al: 0.1 to 1.0%, N: 0.010% or less, Cr: 0.03 to 0.5%, and a surface oxidation index A at annealing defined by the formula A=400Al / (4Cr+3Si+6Mn) with the content of Al, Cr, Si, and Mn as the same item, which is 2.3 or more, and the remainder being Fe and unavoidable impurities, and the structure of the substrate is composed of ferrite and a second phase, wherein the second phase is mainly martensite, and a hot-dip galvanized layer on the surface of the substrate.

[0005] Patent document 2 discloses a hot-dip galvanized steel sheet having an Fe-Al alloy layer at the interface between the hot-dip galvanized layer and the base steel sheet, the Fe-Al alloy layer having an average thickness of 0.1 μm to 2.0 μm and a difference between the maximum thickness and the minimum thickness in the steel sheet width direction of 0.5 μm or less, and a refinement layer in direct contact with the Fe-Al alloy layer having a difference between the maximum thickness and the minimum thickness in the steel sheet width direction of 2.0 μm or less.

[0006] Patent document 3 discloses a high-strength thin steel plate characterized in that the Vickers hardness at a depth of 0.05 mm from the surface of the steel plate is 100 to 250 Hv, and (Vickers hardness at a depth of 0.2 mm from the surface) × 0.8 or less, and the variation in Vickers hardness in the inner layer at the center of the plate thickness from a depth of 0.2 mm from the surface is 100 Hv or less, the inner layer contains bainite and martensite in a total area ratio of 80% or more, the surface roughness of the steel plate is Ra of 0.4 to 1.2 μm, and the tensile strength of the steel plate is 780 MPa or more.

[0007] Patent document 4 discloses a high-strength alloyed hot-dip galvanized steel sheet characterized by having a chemical composition in which the alloyed hot-dip galvanized layer contains Fe: 10 to 15% and Al: 0.20 to 0.45% in mass%, with the remainder being Zn and impurities, and the interfacial adhesion strength between the steel sheet and the alloyed hot-dip galvanized layer being 20 MPa or more.

[0008] Patent document 5 discloses a high-strength steel plate with low property deterioration after cutting, characterized in that the steel plate structure is mainly composed of ferrite and bainite, the degree of Mn segregation in the plate thickness direction (= central Mn peak concentration / average Mn concentration) is 1.20 or less, and the maximum tensile strength is 540 MPa or more. Prior art literature

[0009] Japanese Patent Publication No. 2005-220430, International Publication No. 2019 / 026113, Japanese Patent Publication No. 2006-70328, Japanese Patent Publication No. 2006-97102, Japanese Patent Publication No. 2009-263685 The problem to be solved

[0010] The present invention has been made in consideration of the above circumstances. The present invention aims to provide a press-formed article having high strength and excellent appearance quality, and a steel plate capable of manufacturing the press-formed article. means of solving the problem

[0011] The gist of the present invention is as follows.

[0012] (1) A steel plate according to one embodiment of the present invention has a chemical composition in mass%,

[0013] C: 0.040 to 0.100%,

[0014] Mn: 1.00 to 2.00%,

[0015] Si: 0.005 to 1.500%,

[0016] P: 0.100% or less,

[0017] S: 0.0200% or less,

[0018] Al: 0.005 to 0.700%,

[0019] N: 0.0150% or less,

[0020] O: 0.0100% or less,

[0021] Cr: 0 to 0.80%,

[0022] Mo: 0 to 0.16%,

[0023] B: 0 to 0.0100%,

[0024] Ti: 0 to 0.100%,

[0025] Nb: 0 to 0.060%,

[0026] V: 0 to 0.50%,

[0027] Ni: 0 to 1.00%,

[0028] Cu: 0 to 1.00%,

[0029] W: 0 to 1.00%,

[0030] Sn: 0 to 1.00%,

[0031] Sb: 0 to 0.200%,

[0032] Ca: 0 to 0.0100%,

[0033] Mg: 0 to 0.0100%,

[0034] Zr: 0 to 0.0100%,

[0035] REM: 0 to 0.0100%, and

[0036] Remainder: Fe and impurities,

[0037] The arithmetic mean waviness Wa is 0.10 to 0.30 μm.

[0038] (2) The steel plate described in (1) above has the chemical composition in mass%,

[0039] Cr: 0.01 to 0.80%,

[0040] Mo: 0.01 to 0.16%,

[0041] B: 0.0001 to 0.0100%,

[0042] Ti: 0.001 to 0.100%,

[0043] Nb: 0.001 to 0.060%,

[0044] V: 0.01 to 0.50%,

[0045] Ni: 0.01 to 1.00%,

[0046] Cu: 0.01 to 1.00%,

[0047] W: 0.01 to 1.00%,

[0048] Sn: 0.01 to 1.00%,

[0049] Sb: 0.001 to 0.200%,

[0050] Ca: 0.0001 to 0.0100%,

[0051] Mg: 0.0001 to 0.0100%,

[0052] Zr: 0.0001 to 0.0100%, and

[0053] REM: 0.0001 to 0.0100%

[0054] It may contain one or more types selected from the group consisting of

[0055] (3) The steel plate described in (1) or (2) above may have an average value of Mn concentration in the region from a position spaced 1 / 8 of the plate thickness in the plate thickness direction from the surface of the steel plate to a position spaced 3 / 8 of the plate thickness in the plate thickness direction from the surface, with μ being the average value of the Mn concentration and σ being the standard deviation of the Mn concentration, so that (3σ / μ)×100≤7.0.

[0056] (4) The steel plate described in any one of (1) to (3) above may have a decarburized layer with a thickness of 20 μm or more on the surface of the steel plate.

[0057] (5) The steel plate described in any one of claims (1) to (4) above may have a plating layer on at least one surface of the steel plate.

[0058] (6) A press-formed article according to another form of the present invention is obtained by press-forming a steel plate described in any one of claims (1) to (5). Effects of the invention

[0059] According to the above aspects of the present invention, a press-formed article having high strength and excellent appearance quality, and a steel plate capable of manufacturing the press-formed article can be provided. Specific details for implementing the invention

[0060] The inventors investigated a method to suppress the occurrence of ghost lines after press forming a high-strength steel sheet. As a result, the inventors found that it is effective to reduce the hardness difference within the steel and to control the surface roughness of the steel sheet to a desired range. One of the factors causing a hardness difference within the steel is band-shaped Mn segregation that occurs during the solidification process of the steel. When band-shaped Mn segregation occurs, the area surrounding the high Mn concentration is prone to transforming into austenite during annealing; therefore, after performing annealing following cold rolling, hard martensite is formed in a band shape. As a result, it is believed that the hardness difference within the steel increases, causing ghost lines to occur during press forming.

[0061] Generally, it is considered desirable for the surface roughness of the steel sheet used as the material to be as low as possible. This is because if the surface roughness of the steel sheet is excessively high, the appearance quality deteriorates. However, the inventors have discovered that in order to suppress the occurrence of ghost lines in press-formed products, it is important to appropriately roughen the surface of the steel sheet used as the material to a degree that does not degrade the appearance quality.

[0062] The present invention is based on the above findings, and the steel plate and press-formed article according to the present embodiment will be described in detail below. However, the present invention is not limited to the configuration disclosed in the present embodiment, and various modifications are possible within the scope that does not deviate from the spirit of the present invention.

[0063] First, the chemical composition of the steel sheet according to the present embodiment will be described. In the numerical limit ranges described below separated by "to," the lower limit and the upper limit are included within the range. In numerical values ​​indicated as "less than" or "greater than," the value is not included within the numerical range. In the following description, % regarding the chemical composition is mass % unless specifically designated.

[0064] The steel sheet according to the present embodiment has a chemical composition in mass% of C: 0.040 to 0.100%, Mn: 1.00 to 2.00%, Si: 0.005 to 1.500%, P: 0.100% or less, S: 0.0200% or less, Al: 0.005 to 0.700%, N: 0.0150% or less, O: 0.0100% or less, and the remainder: Fe and impurities. Each element is described below.

[0065] C: 0.040 to 0.100%

[0066] C is an element that increases the strength of steel sheets and press-formed products. To obtain the desired strength, the C content is 0.040% or more. To further increase the strength, the C content is preferably 0.050% or more, and more preferably 0.060% or more, 0.070% or more, or 0.075% or more.

[0067] In addition, by keeping the C content at 0.100% or less, the diffusion of Mn during solidification is promoted, thereby suppressing the tendency for band-shaped Mn segregation to occur. As a result, the occurrence of ghost lines after press forming can be suppressed. Therefore, the C content is kept at 0.100% or less. The C content is preferably 0.095% or less, and more preferably 0.090% or less or 0.085% or less.

[0068] In addition, when the Mn content is 1.40% or less, it is preferable that the C content be greater than 0.075%. In this way, by strictly controlling the Mn content and C content, the diffusion of Mn in the steel at high temperatures is promoted, and Mn segregation can be reduced.

[0069] Mn: 1.00 to 2.00%

[0070] Mn is an element that increases the quenchability of steel and contributes to the improvement of strength. To obtain the desired strength, the Mn content is 1.00% or more. The Mn content is preferably 1.05% or more, 1.10% or more, or 1.20% or more, and more preferably 1.30% or more, 1.40% or more, or 1.50% or more.

[0071] In addition, if the Mn content is 2.00% or less, the occurrence of band-shaped Mn segregation during the solidification of steel can be suppressed. For this reason, the Mn content is set to 2.00% or less. The Mn content is preferably 1.85% or less, more preferably 1.80% or less, and even more preferably 1.75% or less.

[0072] Si: 0.005 to 1.500%

[0073] Si is an element that improves the strength-formability balance of steel sheets. To achieve this effect, the Si content is 0.005% or more. Preferably, it is 0.010% or more.

[0074] In addition, Si is an element that forms coarse Si oxides, which act as fracture initiators. By keeping the Si content at 1,500% or less, the formation of Si oxides can be suppressed, making it difficult for cracks to occur. As a result, the embrittlement of the steel can be suppressed. Therefore, the Si content is set to 1,500% or less. The Si content is preferably 1,300% or less, and more preferably 1,000% or less.

[0075] P: 0.100% or less

[0076] P is an impurity element and an element that embrittles steel. If the P content is 0.100% or less, it is possible to suppress the steel sheet from becoming embrittled and prone to cracking during the production process. Therefore, the P content is set to 0.100% or less. From the perspective of productivity, a P content of 0.050% or less is preferable, and 0.030% or less or 0.020% or less is more preferable.

[0077] The lower limit of the P content includes 0%, but manufacturing costs can be further reduced by making the P content 0.001% or higher. Therefore, the P content may be 0.001% or higher.

[0078] S: 0.0200% or less

[0079] S is an impurity element that forms Mn sulfides and degrades the formability of steel sheets, such as ductility, hole expansion, elongation flangeability, and bendability. If the S content is 0.0200% or less, the significant deterioration of the steel sheet's formability can be suppressed. Therefore, the S content is set to 0.0200% or less. The S content is preferably 0.0100% or less, and more preferably 0.0080% or less.

[0080] The lower limit of the S content includes 0%, but manufacturing costs can be further reduced by making the S content 0.0001% or higher. Therefore, the S content may be 0.0001% or higher.

[0081] Al: 0.005 to 0.700%

[0082] Al is an element that functions as a deoxidizing agent. In order to sufficiently obtain the deoxidizing effect by Al, the Al content is 0.005% or more. The Al content is preferably 0.010% or more or 0.025% or more.

[0083] In addition, Al is an element that forms coarse oxides that serve as fracture initiation points and embrittles steel. By keeping the Al content at 0.700% or less, the formation of coarse oxides acting as fracture initiation points can be suppressed, thereby preventing the cast billet from becoming prone to cracking. For this reason, the Al content is kept at 0.700% or less. The upper limit of the Al content is preferably 0.600%, 0.400%, 0.200%, or 0.100%, and more preferably 0.085%, 0.070%, 0.065%, or 0.060%.

[0084] N: 0.0150% or less

[0085] N is an impurity element that forms nitrides and degrades the formability of steel sheets, such as ductility, hole expansion, elongation flangeability, and bendability. If the N content is 0.0150% or less, the deterioration of the steel sheet's formability can be suppressed. Therefore, the N content is set to 0.0150% or less. In addition, N is an element that impairs productivity by causing welding defects during welding. Therefore, the N content is preferably 0.0120% or less, and more preferably 0.0100% or less.

[0086] The lower limit of the N content includes 0%, but manufacturing costs can be further reduced by making the N content 0.0005% or higher. Therefore, the N content may be 0.0005% or higher.

[0087] O: 0.0100% or less

[0088] O is an impurity element that forms oxides and impairs formability, such as ductility, hole expansion, elongation flangeability, and bendability of the steel sheet. If the O content is 0.0100% or less, the significant decrease in the formability of the steel sheet can be suppressed. Therefore, the O content is set to 0.0100% or less. Preferably, it is 0.0080% or less, and more preferably 0.0050% or less.

[0089] The lower limit of the O content includes 0%, but manufacturing costs can be further reduced by making the O content 0.0001% or more. Therefore, the O content may be 0.0001% or more.

[0090] The steel sheet according to the present embodiment may contain the following elements as optional elements instead of some of Fe. The content of the following optional elements is 0%.

[0091] Cr: 0 to 0.80%

[0092] Cr is an element that increases the quenchability of steel and contributes to the improvement of the strength of steel sheets. Since Cr is not necessarily required to be included, the lower limit of the Cr content includes 0%. In order to sufficiently obtain the strength improvement effect due to Cr, the Cr content is preferably 0.01% or more, more preferably 0.20% or more, and even more preferably 0.30% or more.

[0093] In addition, if the Cr content is 0.80% or less, the formation of coarse Cr carbides, which can serve as fracture initiation points, can be suppressed. Therefore, the Cr content is set to 0.80% or less. To reduce alloy costs, the upper limit of the Cr content may be set to 0.60%, 0.40%, 0.20%, 0.10%, or 0.05% as needed.

[0094] Mo: 0 to 0.16%

[0095] Mo is an element that suppresses phase transformation at high temperatures and contributes to the improvement of the strength of steel sheets. Since Mo is not strictly necessary to be included, the lower limit of the Mo content includes 0%. To sufficiently obtain the strength improvement effect due to Mo, the Mo content is preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.10% or more.

[0096] In addition, if the Mo content is 0.16% or less, the decrease in hot workability can be suppressed, thereby preventing a decrease in productivity. For this reason, the Mo content is set to 0.16% or less. To reduce alloy costs, the upper limit of the Mo content may be set to 0.12%, 0.10%, 0.08%, or 0.04% as needed.

[0097] In addition, it is desirable to include both Cr: 0.01 to 0.80% and Mo: 0.01 to 0.16% so that the strength of the steel plate can be improved more reliably.

[0098] B: 0 to 0.0100%

[0099] B is an element that suppresses phase transformation at high temperatures and contributes to the improvement of the strength of the steel sheet. Since B is not necessarily required to be included, the lower limit of the B content includes 0%. In order to sufficiently obtain the strength improvement effect due to B, the B content is preferably 0.0001% or more, more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0100] In addition, if the B content is 0.0100% or less, the formation of B precipitates can be suppressed, thereby preventing a decrease in the strength of the steel sheet. For this reason, the B content is set to 0.0100% or less. In order to reduce alloy costs, the upper limit of the B content may be set to 0.0050%, 0.0030%, 0.0020%, 0.0010%, or 0.0005% as needed.

[0101] Ti: 0 to 0.100%

[0102] Ti is an element that has the effect of reducing the amounts of S, N, and O, which generate coarse inclusions acting as fracture initiation points. In addition, Ti has the effect of refining the microstructure and improving the strength-formability balance of the steel sheet. Since Ti is not strictly required to be included, the lower limit of the Ti content includes 0%. To sufficiently obtain the above effects, it is preferable to have a Ti content of 0.001% or more, and it is more preferable to have a Ti content of 0.001% or more.

[0103] In addition, if the Ti content is 0.100% or less, the formation of coarse Ti sulfides, Ti nitrides, and Ti oxides can be suppressed, and the formability of the steel sheet can be secured. For this reason, the Ti content is set to 0.100% or less. It is preferable to set the Ti content to 0.080% or less, and more preferable to set it to 0.060% or less. To reduce alloy costs, the upper limit of the Ti content may be set to 0.040%, 0.020%, 0.010%, or 0.005% as needed.

[0104] Nb: 0 to 0.060%

[0105] Nb is an element that contributes to the improvement of the strength of steel sheets through strengthening by precipitation, finer strengthening by inhibiting the growth of ferrite grains, and dislocation strengthening by inhibiting recrystallization. Since Nb is not necessarily required to be included, the lower limit of the Nb content includes 0%. To sufficiently obtain the above effects, the Nb content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.

[0106] In addition, if the Nb content is 0.060% or less, recrystallization can be promoted to suppress the remaining unrecrystallized ferrite, and the formability of the steel sheet can be secured. For this reason, the Nb content is set to 0.060% or less. The Nb content is preferably 0.050% or less, and more preferably 0.040% or less. To reduce alloy costs, the upper limit of the Nb content may be set to 0.030%, 0.020%, 0.010%, or 0.005% as needed.

[0107] V: 0 to 0.50%

[0108] V is an element that contributes to the improvement of the strength of steel sheets through strengthening by precipitation, finer strengthening by inhibiting the growth of ferrite grains, and dislocation strengthening by inhibiting recrystallization. Since V is not necessarily required to be included, the lower limit of the V content includes 0%. To sufficiently obtain the strength improvement effect due to V, the V content is preferably 0.01% or more, and more preferably 0.03% or more.

[0109] In addition, if the V content is 0.50% or less, it is possible to suppress the deterioration of the formability of the steel sheet caused by the precipitation of a large amount of carbonitrides. For this reason, the V content is set to 0.50% or less. In order to reduce alloy costs, the upper limit of the V content may be set to 0.30%, 0.20%, 0.10%, 0.05%, or 0.02% as needed.

[0110] Ni: 0 to 1.00%

[0111] Ni is an element that suppresses phase transformation at high temperatures and contributes to the improvement of the strength of steel sheets. Since Ni is not necessarily required to be included, the lower limit of the Ni content includes 0%. In order to sufficiently obtain the strength improvement effect due to Ni, the Ni content is preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.20% or more.

[0112] In addition, if the Ni content is 1.00% or less, the deterioration of the weldability of the steel plate can be suppressed. For this reason, the Ni content is set to 1.00% or less. In order to reduce alloy costs, the upper limit of the Ni content may be set to 0.60%, 0.40%, 0.20%, 0.10%, or 0.03% as needed.

[0113] Cu: 0 to 1.00%

[0114] Cu is an element that exists in steel in the form of fine particles and contributes to the improvement of the strength of the steel sheet. Since Cu is not necessarily required to be included, the lower limit of the Cu content includes 0%. In order to sufficiently obtain the strength improvement effect due to Cu, the Cu content is preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.15% or more.

[0115] In addition, if the Cu content is 1.00% or less, the deterioration of the weldability of the steel sheet can be suppressed. For this reason, the Cu content is set to 1.00% or less. In order to reduce alloy costs, the upper limit of the Cu content may be set to 0.60%, 0.40%, 0.20%, 0.10%, or 0.03% as needed.

[0116] W: 0 to 1.00%

[0117] W is an element that suppresses phase transformation at high temperatures and contributes to the improvement of the strength of the steel sheet. Since W is not necessarily required to be included, the lower limit of the W content includes 0%. In order to sufficiently obtain the strength improvement effect due to W, the W content is preferably 0.01% or more, more preferably 0.03% or more, and even more preferably 0.10% or more.

[0118] In addition, if the W content is 1.00% or less, the decrease in hot workability can be suppressed, thereby preventing a decrease in productivity. For this reason, the W content is set to 1.00% or less. In order to reduce alloy costs, the upper limit of the W content may be set to 0.50%, 0.20%, 0.10%, 0.05%, or 0.02% as needed.

[0119] Sn: 0 to 1.00%

[0120] Sn is an element that suppresses grain coarsening and contributes to the improvement of steel sheet strength. Since Sn is not strictly required to be included, the lower limit of the Sn content includes 0%. To obtain a sufficient effect from Sn, a Sn content of 0.01% or more is more preferable.

[0121] In addition, if the Sn content is 1.00% or less, the steel sheet can be suppressed from becoming embrittled and breaking during rolling. For this reason, the Sn content is set to 1.00% or less. In order to reduce alloy costs, the upper limit of the Sn content may be set to 0.50%, 0.20%, 0.10%, 0.05%, or 0.02% as needed.

[0122] Sb: 0 to 0.200%

[0123] Sb is an element that suppresses grain coarsening and contributes to the improvement of the strength of the steel sheet. Since Sb is not necessarily required to be included, the lower limit of the Sb content includes 0%. To sufficiently obtain the above effect, the Sb content is preferably 0.001% or more, and more preferably 0.005% or more.

[0124] In addition, if the Sb content is 0.200% or less, the steel sheet can be suppressed from becoming embrittled and breaking during rolling. For this reason, the Sb content is set to 0.200% or less. In order to reduce alloy costs, the upper limit of the Sb content may be set to 0.100%, 0.070%, 0.040%, 0.010%, or 0.005% as needed.

[0125] Ca: 0 to 0.0100%

[0126] Mg: 0 to 0.0100%

[0127] Zr: 0 to 0.0100%

[0128] REM: 0 to 0.0100%

[0129] Ca, Mg, Zr, and REM are elements that contribute to the improvement of the formability of steel sheets. Since Ca, Mg, Zr, and REM do not necessarily have to be included, the lower limit of the total content of these elements is 0%. In order to sufficiently obtain the effect of improving formability, the content of each of these elements is preferably 0.0001% or more, and more preferably 0.0010% or more. In order to sufficiently obtain the above effect, it is not necessary to include all of the above elements, and it is sufficient if the content of any one of them is 0.0001% or more.

[0130] In addition, if the content of each of Ca, Mg, Zr, and REM is 0.0100% or less, the decrease in ductility of the steel sheet can be suppressed. Therefore, the content of each of these elements is set to 0.0100% or less. Preferably, it is 0.0050% or less. To reduce alloy costs, if necessary, the upper limit of the content of each of Ca, Mg, Zr, and REM may be set to 0.0030%, 0.0020%, 0.0010%, or 0.0003%, respectively.

[0131] REM (Rare Earth Metal) refers to a total of 17 elements consisting of Sc, Y, and lanthanides, and the content of REM refers to the total content of these elements.

[0132] The remainder of the chemical composition of the steel sheet according to the present embodiment may be Fe and impurities. Examples of impurities include those inevitably incorporated from steel raw materials or scrap and / or during the steelmaking process, or elements permitted within a range that does not impair the properties of the steel sheet according to the present embodiment. Examples of impurities include H, Na, Cl, Co, Zn, Ga, Ge, As, Se, Y, Tc, Ru, Rh, Pd, Ag, Cd, In, Te, Cs, Ta, Re, Os, Ir, Pt, Au, Pb, Bi, and Po. The impurities may be contained in a total amount of 0.100% or less.

[0133] The chemical composition of the steel plate described above can be measured by general analytical methods. For example, it can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). In addition, C and S can be measured using the combustion-infrared absorption method, N using the inert gas melt-thermal conductivity method, and O using the inert gas melt-non-dispersive infrared absorption method.

[0134] If the steel plate has a plating layer on its surface, the plating layer on the surface should be removed by mechanical grinding, and then the chemical composition should be analyzed.

[0135] Arithmetic mean waviness Wa: 0.10 to 0.30㎛

[0136] Generally, it is considered desirable from the perspective of appearance quality that the smaller the arithmetic mean waviness Wa of the steel sheet used as the material, the better. However, the inventors have discovered that in order to suppress the occurrence of ghost lines in a press-formed product, the surface of the steel sheet used as the material is appropriately roughened, specifically by making the arithmetic mean waviness Wa 0.10 μm or more, thereby suppressing the occurrence of ghost lines in the press-formed product. Therefore, in the steel sheet according to the present embodiment, the arithmetic mean waviness Wa is 0.10 μm or more. Preferably, it is 0.13 μm or more.

[0137] In addition, if the arithmetic mean waviness Wa is excessively large, the appearance quality of the steel sheet itself deteriorates, and low appearance quality is maintained even after press forming. Therefore, the arithmetic mean waviness Wa is set to 0.30㎛ or less. Preferably, it is 0.25㎛ or less.

[0138] In addition, the arithmetic mean waviness Wa refers to the arithmetic mean waviness of the steel sheet when the steel sheet does not have a plating layer, and the arithmetic mean waviness of the plating layer when the steel sheet has a plating layer on its surface.

[0139] In this embodiment, the arithmetic mean waviness Wa is obtained by the following method.

[0140] A 50 mm × 50 mm test specimen is cut from a position spaced at least 10 mm away from the end face of the steel plate. Next, using a laser displacement measuring device (Keyence VK-X1000), three lines of the profile are measured along a direction perpendicular to the rolling direction. From the obtained results, a wave curve is obtained by sequentially applying contour curve filters with cutoff values ​​λc and λf to the cross-sectional curve in accordance with JIS B 0601:2013. Specifically, from the obtained measurement results, components with a wavelength λc of 0.8 mm or less and components with a wavelength λf of 2.5 mm or more are removed to obtain the wave curve. Based on the obtained wave curve, the arithmetic mean wave degree is calculated in accordance with JIS B 0601:2013, and the average value of the three lines is calculated. The arithmetic mean of the calculated average values ​​of the three lines is set as the arithmetic mean wave degree Wa of the steel plate.

[0141] If the steel plate has a plating layer on its surface, the line analysis described above should be performed on the surface of the plating layer.

[0142] (3σ / μ)×100≤7.0

[0143] In the steel plate according to the present embodiment, when the average value of Mn concentration in the region from a position spaced 1 / 8 of the plate thickness in the plate thickness direction from the surface of the steel plate to a position spaced 3 / 8 of the plate thickness in the plate thickness direction from the surface (a region from a depth of 1 / 8 to a depth of 3 / 8 from the surface of the steel plate) is denoted as μ in unit mass% and the standard deviation of said Mn concentration is denoted as σ in unit mass%, it is preferable that (3σ / μ)×100 ≤ 7.0. By making (3σ / μ)×100 7.0 or less, the occurrence of Mn segregation in the steel plate can be further reduced and the occurrence of ghost lines can be further suppressed, thereby obtaining a press-formed product with superior appearance quality. It is more preferable that (3σ / μ)×100 be 6.5 or less. The lower limit of (3σ / μ)×100 is not specifically limited, but may be 0. Since lowering (3σ / μ)×100 increases manufacturing costs, the lower limit may be set to 2.0, 4.0, or 5.0. If necessary, the upper limit of (3σ / μ)×100 may be set to 11.0, 10.0, 9.0, or 8.0.

[0144] In this embodiment, the average value μ of the Mn concentration and the standard deviation σ of the Mn concentration are obtained by the following method.

[0145] After mirror-polishing the cross-section of the steel plate thickness, the Mn concentration is measured at 600 points at a predetermined depth position in the rolling direction of the steel plate at a measurement interval of 1 μm. By calculating the average value of the obtained Mn concentrations, the Mn concentration (mass%) at the predetermined depth position is obtained. This operation is performed at intervals of 1 μm in the plate thickness direction, from a position spaced 1 / 8 of the plate thickness from the surface of the steel plate in the plate thickness direction to a position spaced 3 / 8 of the plate thickness from the surface in the plate thickness direction. By calculating the average value (arithmetic mean) of all obtained Mn concentrations, the average value of the Mn concentration μ is obtained. In addition, by calculating the standard deviation from all obtained Mn concentrations, the standard deviation σ of the Mn concentration is obtained.

[0146] The device used is an electronic probe micro analyzer (EPMA), and the measurement conditions are an acceleration voltage of 15 kV.

[0147] The steel sheet according to the present embodiment may have a plating layer on at least one surface of the steel sheet. Examples of the plating layer include a zinc plating layer and a zinc alloy plating layer, and an alloyed zinc plating layer and an alloyed zinc alloy plating layer obtained by performing an alloying treatment thereon.

[0148] The zinc plating layer and the zinc alloy plating layer are formed by hot-dip plating, electroplating, or deposition plating. Since sufficient adhesion between the surface of the steel sheet and the zinc plating layer can be ensured when the Al content of the zinc plating layer is 0.5 mass% or less, it is preferable that the Al content of the zinc plating layer be 0.5 mass% or less.

[0149] In the case where the zinc plating layer is a hot-dip galvanized layer, in order to increase the adhesion between the steel plate surface and the zinc plating layer, the Fe content of the hot-dip galvanized layer is preferably 3.0 mass% or less.

[0150] When the zinc plating layer is an electro-galvanized layer, the Fe content of the electro-galvanized layer is preferably 0.5 mass% or less in terms of improving corrosion resistance.

[0151] The zinc plating layer and the zinc alloy plating layer may contain one or more of Al, Ag, B, Be, Bi, Ca, Cd, Co, Cr, Cs, Cu, Ge, Hf, Zr, I, K, La, Li, Mg, Mn, Mo, Na, Nb, Ni, Pb, Rb, Sb, Si, Sn, Sr, Ta, Ti, V, W, Zr, and REM within a range that does not impair the corrosion resistance and formability of the steel sheet. In particular, Ni, Al, and Mg are effective in improving the corrosion resistance of the steel sheet.

[0152] The zinc plating layer or zinc alloy plating layer may be an alloyed zinc plating layer or an alloyed zinc alloy plating layer that has undergone alloying treatment. When alloying treatment is performed on a hot-dip zinc plating layer or a hot-dip zinc alloy plating layer, it is preferable to set the Fe content of the hot-dip zinc plating layer (alloyed zinc plating layer) or the hot-dip zinc alloy plating layer (alloyed zinc alloy plating layer) after alloying treatment to 7.0 to 13.0 mass% in order to improve the adhesion between the steel sheet surface and the alloyed plating layer. By performing alloying treatment on a steel sheet having a hot-dip zinc plating layer or a hot-dip zinc alloy plating layer, Fe is introduced into the plating layer, and the Fe content is increased. By doing so, the Fe content can be made 7.0 mass% or more. That is, a zinc plating layer having an Fe content of 7.0 mass% or more is an alloyed zinc plating layer or an alloyed zinc alloy plating layer.

[0153] The Fe content in the plating layer can be obtained by the following method. Only the plating layer is dissolved and removed using a 5 volume% aqueous HCl solution to which an inhibitor has been added. The Fe content in the plating layer (mass%) is obtained by measuring the Fe content in the obtained solution using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry).

[0154] The steel sheet according to the present embodiment may have a decarburization layer with a thickness of 20 μm or more on the surface of the steel sheet, regardless of whether or not there is a plating layer. By making the thickness of the decarburization layer 20 μm or more, band-shaped Mn segregation that causes stripes is reduced, and the appearance quality after press forming is further improved.

[0155] In this embodiment, the thickness of the decarburization layer is measured by the following method.

[0156] For any three locations on the steel plate, the C concentration in the region extending from the surface of the steel plate to a position spaced halfway along the depth direction (plate thickness direction) is measured at 1 µm depth intervals. The region with a C concentration less than or equal to half the C concentration at the position spaced halfway along the plate thickness from the surface is considered as a decarburized layer, and the thickness of the decarburized layer is obtained by determining its thickness.

[0157] For the measurement, a marker-type high-frequency glow discharge luminescence surface analyzer (GD-Profiler) manufactured by Horiba Seisakusho Co., Ltd. is used.

[0158] The plate thickness of the steel plate according to the present embodiment is not limited to a specific range, but considering generality and manufacturability, it is preferably 0.2 to 2.0 mm. By making the plate thickness 0.2 mm or more, it becomes easier to maintain the shape of the steel plate flat, thereby improving dimensional accuracy and shape accuracy. Therefore, the plate thickness is preferably 0.2 mm or more. More preferably, it is 0.4 mm or more.

[0159] Meanwhile, if the plate thickness is 2.0 mm or less, it becomes easier to apply appropriate deformation and control the temperature during the manufacturing process, thereby enabling the acquisition of a homogeneous structure. For this reason, the plate thickness is preferably 2.0 mm or less. More preferably, it is 1.5 mm or less.

[0160] The steel sheet according to the present embodiment preferably has a tensile strength of 500 to 750 MPa. By making the tensile strength 500 MPa or higher, it can be suitably applied to panel-type parts. By making the tensile strength 750 MPa or lower, press formability can be improved, and the deterioration of appearance quality due to the occurrence of ghost lines can also be suppressed. The lower limit of the tensile strength may be 540 MPa, 580 MPa, or 600 MPa, and the upper limit may be 680 MPa or 660 MPa.

[0161] In addition, tensile strength is evaluated in accordance with JIS Z 2241:2011. The test specimen shall be a No. 5 specimen of JIS Z 2241:2011. The sampling location for the tensile test specimen shall be the 1 / 4 portion from the end in the plate width direction, and the direction perpendicular to the rolling direction shall be the length direction.

[0162] Next, a press-formed article according to the present embodiment, which can be manufactured by press-forming the steel plate described above, will be described. The press-formed article according to the present embodiment has the same chemical composition as the steel plate described above. In addition, the press-formed article according to the present embodiment may have the plating layer described above on at least one surface.

[0163] Since the press-formed product according to the present embodiment is obtained by press-forming the steel plate described above, the occurrence of ghost lines is suppressed, and the appearance quality is excellent. Specific examples of the press-formed product include, for instance, panel-type parts such as door outers of an automobile body.

[0164] In the press-formed product according to the present embodiment, excellent appearance quality means that no streaks (i.e., ghost lines) with intervals of several millimeters occurring on the surface are observed. In other words, the maximum length of streaks with intervals of several millimeters observed when visually inspecting any area of ​​100 mm × 100 mm is 50 mm or less. It is preferable that the maximum length of the streaks is 20 mm or less. Furthermore, it is more preferable that no streaks are observed at all.

[0165] In the press-formed product according to the present embodiment, the occurrence of ghost lines is suppressed, so Wz, which is the sum of the maximum peak height Zp and the maximum trough height Zv of the wave curve, is 0.60 μm or less.

[0166] In addition, by manufacturing a press-formed product using a steel plate in which 3σ / μ is preferably controlled, a press-formed product with better appearance quality can be obtained. That is, a press-formed product can be obtained in which Wz, the sum of the maximum peak height Zp and the maximum trough height Zv of the wave curve, is 0.40 μm or less.

[0167] Wz is obtained by obtaining the wave curve of the surface of the press-formed product in accordance with JIS B 0601:2013, finding the maximum peak height Zp and the maximum valley height Zv, and calculating the sum of these.

[0168] Next, a method for manufacturing a steel plate according to the present embodiment will be described.

[0169] The steel sheet according to the present embodiment can achieve the effect if it possesses the above characteristics, regardless of the manufacturing method. Furthermore, it may be a steel sheet instead of a steel sheet. However, by using steel having the chemical composition described above and controlling, for example, the following conditions (I) to (IV) in combination and inseparably, a steel sheet with a preferably controlled arithmetic mean waviness Wa can be stably manufactured. In addition, to preferably control 3σ / μ, it is preferable to control condition (V) in addition to the following conditions (I) to (IV). In addition, to preferably control the thickness of the decarburization layer, it is preferable to control condition (VI) in addition to the following conditions (I) to (IV). Furthermore, conditions (V) and (VI) are optional conditions.

[0170] Below, each condition is explained.

[0171] (I) The winding temperature must be 550℃ or higher.

[0172] (II) The pickling time should be 50 seconds or more.

[0173] (III) The arithmetic mean roughness Ra of the rolling roll surface of the final pass of cold rolling is set to 0.2 to 0.7 μm.

[0174] (IV) The reduction rate of temper rolling is set to 0.3 to 0.7%, and the arithmetic mean roughness Ra of the rolling roll is set to 1.5 to 3.5 μm.

[0175] (V) Heat the slab to a temperature range of 1200°C or higher and maintain it at that temperature range for at least 5 hours.

[0176] (VI) Perform annealing with a dew point (average dew point in the annealing furnace) of -20°C or higher and a residence time of the steel sheet in a temperature range of 700°C or higher of 50 to 400 seconds.

[0177] (I) Winding temperature: 550℃ or higher

[0178] By setting the coiling temperature after hot rolling to a high temperature range of 550°C or higher, scale is more likely to form on the surface of the steel sheet. As a result, irregularities are more likely to form on the surface of the steel sheet after pickling. The coiling temperature is more preferably 600°C or higher, and even more preferably 650°C or higher.

[0179] (II) Pickling time: 50 seconds or more

[0180] In the pickling process after coiling and before cold rolling, if the pickling time is 50 seconds or more, irregularities are likely to occur on the surface of the steel sheet. It is more preferable to make the pickling time 70 seconds or more.

[0181] (III) Arithmetic mean roughness Ra of the rolling rolls in the final pass of cold rolling: 0.2 to 0.7 μm

[0182] By making the arithmetic mean roughness Ra of the rolling roll surface of the final pass in cold rolling after pickling 0.2 to 0.7 μm, suitable irregularities can be formed on the surface of the steel sheet during cold rolling. It is more preferable to make the arithmetic mean roughness Ra of the rolling roll 0.3 μm or more.

[0183] Since conventional rolling rolls do not possess the aforementioned arithmetic mean roughness Ra, the steel sheet according to the present embodiment cannot be manufactured. To manufacture the steel sheet according to the present embodiment, it is preferable to use a special rolling roll in the final pass of cold rolling.

[0184] (IV) Reduction rate of temper rolling: 0.3 to 0.7%, arithmetic mean roughness Ra of rolling rolls: 1.5 to 3.5㎛

[0185] In temper rolling after annealing (or after plating if the material is plated), irregularities can be formed on the surface of the steel sheet by setting the reduction rate to 0.3 to 0.7% and the arithmetic mean roughness Ra of the rolling roll surface to 1.5 to 3.5 μm. It is more preferable to set the reduction rate during temper rolling to 0.5% or more, and it is more preferable to set the arithmetic mean roughness Ra of the rolling roll surface to 2.3 μm or more.

[0186] (V) Slab heating temperature and holding time: 5 hours or more in a temperature range of 1200℃ or higher

[0187] Condition (V) is an arbitrary condition. By heating the slab in a temperature range of 1200°C or higher for 5 hours or more, the 3σ / μ in the region from a position spaced 1 / 8 of the plate thickness in the plate thickness direction from the surface of the steel plate to a position spaced 3 / 8 of the plate thickness in the plate thickness direction from the surface (a region from 1 / 8 depth from the surface of the steel plate to 3 / 8 depth from the surface of the steel plate) can be preferably controlled. As a result, the occurrence of Mn segregation in the steel plate can be further reduced, and a press-formed product with better appearance quality can be obtained.

[0188] (VI) Dew point: Residence time of steel sheet in the temperature range of -20℃ or higher to 700℃ or higher: 50 to 400 seconds

[0189] Condition (VI) is an optional condition. In this embodiment, annealing may be performed on the steel sheet after cold rolling obtained by the method described above. By setting the dew point during annealing (average dew point in the annealing furnace) to -20°C or higher, and setting the residence time of the steel sheet in the temperature range of 700°C or higher to 50 to 400 seconds, the surface of the steel sheet can be stably decarburized. By doing so, a decarburized layer with a thickness of 30 μm or more can be formed on the surface of the steel sheet. In addition, although there is no need to specifically set an upper limit for the dew point, it may be around 10°C.

[0190] In addition to the conditions described above, although not specifically limited, it is desirable to satisfy, for example, the following conditions.

[0191] After heating the steel slab to a temperature range of 1100°C or higher, hot rolling is performed. After hot rolling, coiling is performed, followed by pickling. After pickling, cold rolling is performed. It is preferable that the cumulative reduction rate during cold rolling be 30 to 90%. After cold rolling, annealing is performed. Subsequently, if necessary, the plating layer described above is formed. In addition, it is preferable to perform temper rolling thereafter.

[0192] Next, a method for manufacturing a press-formed product according to the present embodiment will be described. The method of press forming is not particularly limited. For example, for automotive panel parts such as door outers, it is possible to form them by applying pressure to a steel sheet with a blank holder and a die, then applying deformation to the steel sheet by pressing and contacting a punch, and then stretching it. Such forming is referred to as drawing forming or extrusion forming.

[0193] Examples

[0194] Next, embodiments of the present invention will be described. However, the conditions in the embodiments are examples of conditions adopted to verify the feasibility and effects of the present invention. The present invention is not limited to these examples of conditions. The present invention may adopt various conditions as long as the objectives of the present invention are achieved without departing from the gist of the present invention.

[0195] Steel having the chemical composition shown in Table 1 was melted, and slabs with a thickness of 240 to 300 mm were manufactured by continuous casting. Using the obtained slabs, cold-rolled steel sheets and galvanized steel sheets were manufactured according to conditions (I) to (V) described below. In addition, in Table 2, "OK" was written in the condition column if the condition was satisfied, and "NG" was written in the condition column if the condition was not satisfied. In addition, the thickness of the obtained steel sheets and galvanized steel sheets was 0.2 to 2.0 mm.

[0196] In addition, annealing was performed after cold rolling.

[0197] The manufacturing conditions other than conditions (I) to (VI) are as follows. The slab was heated to a temperature range of 1100°C or higher and then hot-rolled. After hot-rolling, coiling was performed, followed by pickling. After pickling, cold rolling was performed with a cumulative reduction rate of 30 to 90%. After cold rolling, annealing was performed, and if necessary, an alloyed hot-dip galvanized layer (GA), a hot-dip galvanized layer (GI), and an electroplated layer (EG) were formed. After that, temper rolling was performed.

[0198] Conditions (I) through (VI) in the table are as follows.

[0199] (I) The winding temperature must be 550℃ or higher.

[0200] (II) The pickling time should be 50 seconds or more.

[0201] (III) The arithmetic mean roughness Ra of the rolling roll surface of the final pass of cold rolling is set to 0.2 to 0.7 μm.

[0202] (IV) The reduction rate of temper rolling is set to 0.3 to 0.7%, and the arithmetic mean roughness Ra of the rolling roll is set to 1.5 to 3.5 μm.

[0203] (V) Heat the slab to a temperature range of 1200°C or higher and maintain it at that temperature range for at least 5 hours.

[0204] (VI) Perform annealing with a dew point (average dew point in the annealing furnace) of -20°C or higher and a residence time of the steel sheet in a temperature range of 700°C or higher of 50 to 400 seconds.

[0205] Next, using the manufactured steel plate and plated steel plate, a roughly semi-cylindrical mock part (press-formed product) simulating a door outer was manufactured by press forming. When press-forming this mock part, the material (steel plate or plated steel plate) was actively fed into the mold, and the ratio of deformation in the direction perpendicular to the direction (that arbitrary direction) to deformation in any direction along the surface of the mock part was made to be approximately 1 at any position on the surface of the mock part. That is, press forming was performed so that anisotropy of deformation does not occur at any position on the surface of the mock part.

[0206] For the obtained steel sheet and plated steel sheet, the arithmetic mean waviness Wa, the average value μ and standard deviation σ of Mn concentration, tensile strength, and the thickness of the decarburization layer were determined by the method described above.

[0207] If the obtained tensile strength was 500 MPa or higher, it was judged to be high strength and passed. On the other hand, if the obtained tensile strength was less than 500 MPa, it was judged to be low strength and failed.

[0208] In addition, the appearance quality of the simulated part was evaluated by the following method.

[0209] The appearance quality was evaluated based on the degree of ghost lines occurring on the surface of the simulated part after forming. The surface after press forming was ground, and streaks of several millimeters intervals occurring on the surface were judged as ghost lines, and a rating from 1 to 5 was assigned based on the degree of streak occurrence. An arbitrary area of ​​100 mm × 100 mm was visually inspected, and "1" was assigned if no streaks were observed at all, "2" if the maximum length of the streaks was 20 mm or less, "3" if the maximum length of the streaks was greater than 20 mm and less than or equal to 50 mm, "4" if the maximum length of the streaks was greater than 50 mm and less than or equal to 70 mm, and "5" if the maximum length of the streaks exceeded 70 mm. If the evaluation was "3" or lower, the appearance quality was considered excellent and the part was judged to pass. On the other hand, if the evaluation was "4" or higher, the appearance quality was considered poor and the part was judged to fail.

[0210] In addition, the appearance quality was evaluated more strictly by "Wz, which is the sum of the maximum peak height Zp and the maximum trough height Zv of the wave curve." Using the same method as when calculating the arithmetic mean waviness Wa, the surface wave curve of a press-formed product (simulated part) was obtained in accordance with JIS B 0601:2013. From this wave curve, the maximum peak height Zp and the maximum trough height Zv were calculated, and Wz was obtained by calculating their sum. If the obtained Wz was 0.40 μm or less, the appearance quality was judged to be superior.

[0211]

[0212]

[0213] Looking at Table 2, it can be seen that the press-formed article according to the example of the present invention has high strength and excellent appearance quality. In addition, it can be seen that the steel plate according to the example of the present invention was able to produce a press-formed article having high strength and excellent appearance quality. Furthermore, it can be seen that the example of the present invention, in which 3σ / μ was 7.0 or less, had better appearance quality after press forming.

[0214] Meanwhile, it can be seen that the press-formed product of the comparative example has reduced strength or deteriorated appearance quality. Furthermore, it can be seen that the steel sheet of the comparative example could not be used to manufacture a press-formed product having high strength and excellent appearance quality. Industrial applicability

[0215] According to the above aspects of the present invention, a press-formed article having high strength and excellent appearance quality, and a steel plate capable of manufacturing the press-formed article can be provided.

Claims

Claim 1 The chemical composition, in mass%, is C: 0.040 to 0.100%, Mn: 1.00 to 2.00%, Si: 0.005 to 1.500%, P: 0.100% or less, S: 0.0200% or less, Al: 0.005 to 0.700%, N: 0.0150% or less, O: 0.0100% or less, Cr: 0 to 0.80%, Mo: 0 to 0.16%, B: 0 to 0.0100%, Ti: 0 to 0.100%, Nb: 0 to 0.060%, V: 0 to 0.50%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, W: 0 to 1.00%, Sn: 0 to A steel plate characterized by having 1.00%, Sb: 0 to 0.200%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, REM: 0 to 0.0100%, and remainder: Fe and impurities, with an arithmetic mean waviness Wa of 0.10 to 0.30 μm, and when the average value of Mn concentration in the region from a position spaced 1 / 8 of the plate thickness in the plate thickness direction from the surface of the steel plate to a position spaced 3 / 8 of the plate thickness in the plate thickness direction from the surface is set to μ and the standard deviation of said Mn concentration is set to σ, (3σ / μ)×100≤7.

0. Claim 2 In claim 1, the chemical composition is, in mass%, Cr: 0.01 to 0.80%, Mo: 0.01 to 0.16%, B: 0.0001 to 0.0100%, Ti: 0.001 to 0.100%, Nb: 0.001 to 0.060%, V: 0.01 to 0.50%, Ni: 0.01 to 1.00%, Cu: 0.01 to 1.00%, W: 0.01 to 1.00%, Sn: 0.01 to 1.00%, Sb: 0.001 to 0.200%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Zr: 0.0001 to A steel plate characterized by containing one or more types selected from the group consisting of 0.0100%, and REM: 0.0001 to 0.0100%. Claim 3 delete Claim 4 A steel plate according to claim 1 or 2, characterized in that the steel plate has a decarburization layer having a thickness of 20㎛ or more on its surface. Claim 5 A steel plate according to claim 1 or 2, characterized by having a plating layer on at least one surface of the steel plate. Claim 6 A press-formed product characterized by being obtained by press-forming a steel plate as described in claim 1 or 2.

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