Steel plates and press-formed products

Abstract

A steel plate having a chemical composition in mass%, C: 0.040 to 0.105%, Mn: 1.00 to 2.30%, Si: 0.005 to 1.500%, Al: 0.005 to 0.700%, P: 0.100% or less, S: 0.0200% or less, N: 0.0150% or less, O: 0.0100% or less, and the remainder: Fe and impurities, and containing C20, which is the C content at a depth of 20 μm from the surface, and C60, which is the C content at a depth of 60 μm from the surface, and a ΔC calculated from the following formula (1) having a mass% to 0.20 to 0.90 mass% / mm, and a press-formed article obtained by press-forming the steel plate is employed. ΔC=(C60-C20) / (0.04) … (1)

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-type components, such as door outers. Unlike skeletal components, these panel-type components are visible, so high appearance quality is required. Therefore, even high-strength steel sheets traditionally used for skeletal components must possess excellent appearance quality after forming when applied to panel-type components.

[0003] To improve appearance quality, suppressing the occurrence of ghost lines can be cited as a challenge. Ghost lines are minute irregularities on the surface, measuring on the order of millimeters, that occur when a steel sheet having hard and soft phases is 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 suffer from reduced appearance quality.

[0004] For example, 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 also having a surface oxidation index A of 2.3 or more when annealed, defined by the formula A=400Al / (4Cr+3Si+6Mn) with the content of Al, Cr, Si, and Mn as the same item, and the remainder being Fe and unavoidable impurities, and also having a structure of the substrate consisting 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 high-strength cold-rolled steel sheet, a high-strength plated steel sheet having a tensile strength of 780 MPa or more of the surface layer and good formability, and a method for manufacturing the same.

[0006] Patent document 3 discloses a high-strength member for automobiles and a hot pressing method thereof, wherein, in a method for forming a high-strength member for automobiles by hot pressing, the susceptibility to hydrogen embrittlement caused by post-processing after hot pressing is secured without dehydrogenation treatment.

[0007] Patent document 4 discloses a hot-dip galvanized steel sheet having a tensile strength (TS) of 980 MPa or more and excellent plating adhesion and delayed fracture resistance, and a method for manufacturing the same.

[0008] Patent document 5 discloses a hot-pressed steel sheet member capable of obtaining excellent impact characteristics while having high strength, a method for manufacturing the same, and a steel sheet for hot pressing.

[0009] Patent document 6 discloses hot-dip galvanized steel sheets having good elongation characteristics and bendability, alloyed hot-dip galvanized steel sheets, and a method for manufacturing the same. Prior art literature

[0010] Japanese Patent Publication No. 2005-220430, International Publication No. 2016-121388, Japanese Patent Publication No. 2006-104546, International Publication No. 2013-047820, International Publication No. 2015-097882, Japanese Patent Publication No. 2017-48412 The problem to be solved

[0011] 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 (specifically, tensile strength: 500 MPa or more) and excellent appearance quality, and a steel plate capable of manufacturing the press-formed article. means of solving the problem

[0012] The gist of the present invention is as follows.

[0013] (1) A steel plate according to one embodiment of the present invention has a chemical composition in mass%,

[0014] C: 0.040 to 0.105%,

[0015] Mn: 1.00 to 2.30%,

[0016] Si: 0.005 to 1.500%,

[0017] Al: 0.005 to 0.700%,

[0018] P: 0.100% or less,

[0019] S: 0.0200% or less,

[0020] N: 0.0150% or less,

[0021] O: 0.0100% or less,

[0022] Cr: 0 to 0.80%,

[0023] Mo: 0 to 0.16%,

[0024] Ti: 0 to 0.100%,

[0025] B: 0 to 0.0100%,

[0026] Nb: 0 to 0.060%,

[0027] V: 0 to 0.50%,

[0028] Ni: 0 to 1.00%,

[0029] Cu: 0 to 1.00%,

[0030] W: 0 to 1.00%,

[0031] Sn: 0 to 1.00%,

[0032] Sb: 0 to 0.200%,

[0033] Ca: 0 to 0.0100%,

[0034] Mg: 0 to 0.0100%,

[0035] Zr: 0 to 0.0100%,

[0036] REM: 0 to 0.0100%, and

[0037] Remainder: Fe and impurities,

[0038] C content at a depth of 20㎛ from the surface 20 and, C content at a depth of 60㎛ from the above surface 60 And, the ΔC calculated from the following formula (1) is 0.20 to 0.90 mass% / mm.

[0039] ΔC=(C 60 -C 20 ) / (0.04) … (1)

[0040] (2) The steel plate described in (1) above has the chemical composition in mass%,

[0041] Cr: 0.01 to 0.80%,

[0042] Mo: 0.01 to 0.16%,

[0043] Ti: 0.001 to 0.100%,

[0044] B: 0.0001 to 0.0100%,

[0045] Nb: 0.001 to 0.060%,

[0046] V: 0.01 to 0.50%,

[0047] Ni: 0.01 to 1.00%,

[0048] Cu: 0.01 to 1.00%,

[0049] W: 0.01 to 1.00%,

[0050] Sn: 0.01 to 1.00%,

[0051] Sb: 0.001 to 0.200%,

[0052] Ca: 0.0001 to 0.0100%,

[0053] Mg: 0.0001 to 0.0100%,

[0054] Zr: 0.0001 to 0.0100%, and

[0055] REM: 0.0001 to 0.0100%

[0056] It may contain one or more types selected from the group consisting of

[0057] (3) The steel plate described in (1) or (2) above may have a chemical composition of C: 0.040 to 0.080% in mass%.

[0058] (4) The steel plate described in any one of (1) to (3) above may have a ΔC of 0.30 to 0.80 mass% / mm.

[0059] (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.

[0060] (6) The steel plate described in any one of (1) to (5) above may have a tensile strength of 500 to 750 MPa.

[0061] (7) A press-formed article according to another aspect of the present invention is a press-formed article obtained by press-forming a steel plate described in any one of claims (1) to (6), and

[0062] C content at a depth of 20㎛ from the surface 20 and, C content at a depth of 60㎛ from the above surface 60 And, the ΔC calculated from the following formula (1) is 0.20 to 0.90 mass% / mm.

[0063] ΔC=(C 60 -C 20 ) / (0.04) … (1) Effects of the invention

[0064] 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.

[0065] In addition, having excellent appearance quality means that the occurrence of ghost lines is suppressed. Specific details for implementing the invention

[0066] The inventors examined a method to suppress the occurrence of ghost lines when press-forming high-strength steel sheets. As a result, the inventors found that reducing the hardness difference within the steel is effective. The inventors found that the hardness difference within the steel can be reduced by decarburizing the surface layer of the steel sheet to form a homogeneous decarburized layer with a small hardness difference.

[0067] When decarburization annealing is performed on a steel sheet, the carbon content is reduced starting from the region near the surface, forming a decarburized layer. The more intense the decarburization conditions, the greater the thickness of the decarburized layer. The carbon concentration in the decarburized layer increases from the region near the surface of the steel sheet toward the base material side (inside the steel sheet), but its upper limit is the carbon content of the base material. In other words, the carbon concentration gradient from the surface to the interior of the steel sheet depends on the decarburization conditions and the carbon content of the steel sheet.

[0068] Since regions with low C concentration tend to become a single ferrite phase, the surface of the steel sheet softens relative to the interior of the steel sheet. It is believed that ghost lines occur after press forming because the difference in hardness increases when the C concentration in the decarburized layer increases rapidly toward the interior of the steel sheet. The inventors have discovered that by setting the C concentration gradient in the decarburized layer to a desired range, the difference in hardness within the decarburized layer can be reduced, thereby suppressing the occurrence of ghost lines after press forming.

[0069] The present invention is based on the above findings, and the steel plate and press-formed article according to the present embodiment are 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.

[0070] 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 otherwise.

[0071] The steel sheet according to the present embodiment has a chemical composition in mass% of C: 0.040 to 0.105%, Mn: 1.00 to 2.30%, Si: 0.005 to 1.500%, Al: 0.005 to 0.700%, P: 0.100% or less, S: 0.0200% or less, N: 0.0150% or less, O: 0.0100% or less, and the remainder: Fe and impurities. Each element is described below.

[0072] C: 0.040 to 0.105%

[0073] 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 of the steel sheet, the C content is preferably 0.050% or more, and more preferably 0.060% or more or 0.070% or more.

[0074] In addition, by keeping the C content at 0.105% or less, the occurrence of excessive hardness differences in the decarburized layer can be suppressed. As a result, the occurrence of ghost lines after press forming can be suppressed. Therefore, the C content is set to 0.105% or less. The C content is preferably 0.090% or less, and more preferably 0.080% or less.

[0075] Mn: 1.00 to 2.30%

[0076] 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 or 1.10% or more, more preferably 1.20% or more, 1.30% or more or 1.40% or more.

[0077] In addition, by keeping the Mn content at 2.30% or less, it is possible to suppress the occurrence of hardness differences within the steel. For this reason, the Mn content is set to 2.30% or less. The Mn content is preferably 2.10% or less or 2.00% or less, and more preferably 1.90% or less, 1.80% or less, or 1.70% or less.

[0078] Si: 0.005 to 1.500%

[0079] Si is an element that forms coarse Si oxides, which act as fracture initiation points. 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 steel can be suppressed. Therefore, the Si content is set to 1,500% or less. The Si content is preferably 1,300% or less or 1,000% or less, and more preferably 0,800% or less, 0,600% or less, or 0,500% or less.

[0080] The Si content is set to 0.005% or more to improve the strength-formability balance of the steel sheet. The Si content is preferably 0.010% or more or 0.020% or more.

[0081] Al: 0.005 to 0.700%

[0082] Al is an element that functions as a deoxidizer. In addition, Al is an element that embrittles steel by forming coarse oxides that serve as fracture initiation points. 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 Al content is preferably 0.650% or less, 0.400% or less, or 0.200% or less, and more preferably 0.100% or less, 0.080% or less, or 0.060% or less.

[0083] The Al content is set to 0.005% or more to sufficiently obtain the deoxidation effect by Al. The Al content is preferably 0.010% or more, 0.020% or more, 0.030% or more, or 0.040% or more.

[0084] P: 0.100% or less

[0085] P is an element incorporated as an impurity and is also 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.

[0086] 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.

[0087] S: 0.0200% or less

[0088] S is an element incorporated as an impurity and forms Mn sulfides, which 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 decrease in the formability of the steel sheet can be suppressed. Therefore, the S content is set to 0.0200% or less. The S content is preferably 0.0100% or less or 0.0080% or less, and more preferably 0.0060% or less or 0.0040% or less.

[0089] 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.

[0090] N: 0.0150% or less

[0091] N is an element incorporated as an impurity and forms nitrides, which 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 or 0.0100% or less, and more preferably 0.0080% or less or 0.0060% or less.

[0092] 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.

[0093] O: 0.0100% or less

[0094] O is an element incorporated as an impurity and forms oxides, which 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 or 0.0050% or less, and more preferably 0.0030% or less or 0.0020% or less.

[0095] 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.

[0096] 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%.

[0097] Cr: 0 to 0.80%

[0098] 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 or 0.20% or more, and more preferably 0.30% or more.

[0099] In addition, if the Cr content is 0.80% or less, the formation of coarse Cr carbides, which can serve as the starting point for fracture, can be suppressed. For this reason, the Cr content is set to 0.80% or less. To reduce alloy costs, it is preferable to set the Cr content to 0.60% or less or 0.40% or less, and it is more preferable to set it to 0.20% or less, 0.10% or less, or 0.06% or less.

[0100] Mo: 0 to 0.16%

[0101] Mo is an element that contributes to the improvement of the strength of steel sheets by suppressing phase transformation at high temperatures. Since Mo is not necessarily required to be included, the lower limit of the Mo content includes 0%. In order to sufficiently obtain the strength improvement effect due to Mo, the Mo content is preferably 0.01% or more or 0.05% or more, and more preferably 0.10% or more.

[0102] In addition, if the Mo content is 0.16% or less, it is possible to suppress the decrease in productivity caused by reduced hot workability. For this reason, the Mo content is set to 0.16% or less. To reduce alloy costs, it is preferable to set the Mo content to 0.12% or less or 0.08% or less, and it is more preferable to set it to 0.06% or less, 0.04% or less, or 0.02% or less.

[0103] Ti: 0 to 0.100%

[0104] 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 refines the microstructure, thereby 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 that the Ti content be 0.001% or more, and more preferable that it be 0.010% or more.

[0105] 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, thereby ensuring the formability of the steel sheet. For this reason, the Ti content is set to 0.100% or less. It is preferable that the Ti content be 0.075% or less or 0.060% or less, and more preferable that it be 0.040% or less or 0.020% or less.

[0106] B: 0 to 0.0100%

[0107] B is an element that contributes to the improvement of the strength of steel sheets by suppressing phase transformation at high temperatures. 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 or 0.0005% or more, and more preferably 0.0010% or more.

[0108] In addition, if the B content is 0.0100% or less, it is possible to suppress the formation of B precipitates and the resulting decrease in the strength of the steel sheet. For this reason, the B content is set to 0.0100% or less. To reduce alloy costs, it is preferable to set the B content to 0.0080% or less or 0.0060% or less, and it is more preferable to set it to 0.0040% or less, 0.0030% or less, or 0.0015% or less.

[0109] Nb: 0 to 0.060%

[0110] 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, it is preferable that the Nb content be 0.001% or more or 0.005% or more, and more preferable that it be 0.010% or more.

[0111] In addition, if the Nb content is 0.060% or less, recrystallization can be promoted to suppress the remaining unrecrystallized ferrite, thereby ensuring the formability of the steel sheet. 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, 0.030% or less, and 0.015% or less.

[0112] V: 0 to 0.50%

[0113] 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.

[0114] 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. To reduce alloy costs, it is preferable to set the V content to 0.30% or less or 0.10% or less, and it is more preferable to set it to 0.08% or less, 0.06% or less, or 0.03% or less.

[0115] Ni: 0 to 1.00%

[0116] Ni is an element that contributes to the improvement of the strength of steel sheets by suppressing phase transformation at high temperatures. 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 or 0.05% or more, and more preferably 0.20% or more.

[0117] 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. To reduce alloy costs, it is preferable to set the Ni content to 0.70% or less or 0.50% or less, and it is more preferable to set it to 0.30% or less, 0.15% or less, or 0.08% or less.

[0118] Cu: 0 to 1.00%

[0119] 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 or 0.05% or more, and more preferably 0.15% or more.

[0120] In addition, if the Cu content is 1.00% or less, the deterioration of the weldability of the steel plate can be suppressed. For this reason, the Cu content is set to 1.00% or less. To reduce alloy costs, it is preferable to set the Cu content to 0.70% or less or 0.50% or less, and it is more preferable to set it to 0.30% or less, 0.15% or less, or 0.08% or less.

[0121] W: 0 to 1.00%

[0122] W is an element that contributes to the improvement of the strength of steel sheets by suppressing phase transformation at high temperatures. 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 or 0.03% or more, and more preferably 0.10% or more.

[0123] In addition, if the W content is 1.00% or less, the decrease in productivity due to reduced hot workability can be suppressed. For this reason, the W content is set to 1.00% or less. To reduce alloy costs, it is preferable to set the W content to 0.70% or less or 0.50% or less, and it is more preferable to set it to 0.30% or less, 0.15% or less, or 0.08% or less.

[0124] Sn: 0 to 1.00%

[0125] Sn is an element that contributes to the improvement of steel sheet strength by suppressing grain coarsening. 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.

[0126] In addition, if the Sn content is 1.00% or less, it is possible to suppress the steel sheet from becoming embrittled and breaking during rolling. For this reason, the Sn content is set to 1.00% or less. To reduce alloy costs, it is preferable to set the Sn content to 0.70% or less or 0.50% or less, and it is more preferable to set it to 0.30% or less, 0.15% or less, or 0.08% or less.

[0127] Sb: 0 to 0.200%

[0128] Sb is an element that contributes to the improvement of the strength of steel sheets by suppressing grain coarsening. Since Sb is not strictly 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 or 0.005% or more.

[0129] In addition, if the Sb content is 0.200% or less, it is possible to suppress the steel sheet from becoming embrittled and breaking during rolling. For this reason, the Sb content is set to 0.200% or less. To reduce alloy costs, it is preferable to set the Sb content to 0.100% or less or 0.050% or less, and it is more preferable to set it to 0.030% or less, 0.010% or less, or 0.005% or less.

[0130] Ca: 0 to 0.0100%

[0131] Mg: 0 to 0.0100%

[0132] Zr: 0 to 0.0100%

[0133] REM: 0 to 0.0100%

[0134] Ca, Mg, Zr, and REM are elements that contribute to the improvement of the formability of steel sheets. Since Ca, Mg, Zr, and REM are not necessarily required to be included, the lower limit of the 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.

[0135] In addition, if the content of Ca, Mg, Zr, and REM is 0.0100% or less each, the decrease in ductility of the steel sheet can be suppressed. Therefore, the content of these elements is set to 0.0100% or less each. Preferably, it is 0.0050% or less or 0.0030% or less.

[0136] REM (Rare Earth Metal) refers to a group of elements belonging to the lanthanide series.

[0137] 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.

[0138] 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.

[0139] If the steel plate has a plating layer on its surface, the plating layer on the surface can be removed by mechanical grinding, and then the chemical composition can be analyzed.

[0140] C content at a depth of 20㎛ from the surface 20 and, C content at a depth of 60㎛ from the above surface 60 and ΔC calculated from the following formula (1): 0.20 to 0.90 mass% / mm

[0141] ΔC=(C 60 -C 20 ) / (0.04) … (1)

[0142] ΔC represents the C concentration gradient in the region from a depth of 20 μm from the surface to a depth of 60 μm from the surface within the decarburized layer formed on the surface layer. By setting ΔC to 0.20 to 0.90 mass% / mm, a rapid increase in the C concentration gradient in the decarburized layer can be suppressed. As a result, the occurrence of ghost lines after press forming can be suppressed.

[0143] In a steel sheet having the chemical composition of the present embodiment, a ΔC of less than 0.20 mass% / mm indicates that decarburization has not occurred sufficiently or that decarburization has progressed excessively to a very deep location from the surface of the steel sheet. If decarburization has not occurred sufficiently, the influence of fluctuations in the hardness of the base material becomes significant, making it difficult to suppress the occurrence of ghost lines. On the other hand, if excessive decarburization occurs, softening proceeds, and the desired steel sheet strength may not be obtained. Therefore, ΔC is set to 0.20 mass% / mm or more. Furthermore, if ΔC exceeds 0.90 mass% / mm, the difference in hardness within the decarburization layer becomes significant, making it difficult to suppress the occurrence of ghost lines. It is preferable that ΔC be 0.30 mass% / mm or more, 0.35 mass% / mm or more, 0.40 mass% / mm or more, or 0.45 mass% / mm or more. In addition, it is preferable that ΔC be 0.80 mass% / mm or less or 0.75 mass% / mm or less.

[0144] In addition, when the steel plate has a plating layer on its surface, the “surface” in the “area at a depth of 20 μm from the surface” and the “region at a depth of 60 μm from the surface” refers to the interface between the plating layer and the base material. Furthermore, when GDS analysis is performed by the method described below and the Fe content is measured from the surface, the depth position where the Fe content is 95 mass% or more is considered to be the interface between the plating layer and the base material.

[0145] In addition, the reason for defining ΔC at a depth of 20㎛ or more from the surface is that C concentrations of less than 20㎛ from the surface do not affect ghost lines.

[0146] ΔC is obtained by the following method.

[0147] For any three locations on the steel plate, the C content (mass%) is measured up to 100 µm in the depth direction (plate thickness direction) from the surface of the steel plate by Glow Discharge Optical Emission Spectrometry (GDS analysis). The C content (C 20 ) and, C content (C) at a depth of 60㎛ from the surface 60 ) and from the above equation (1), ΔC (mass % / mm) is calculated. By calculating the average value of ΔC at three locations, ΔC is obtained.

[0148] For the measurement, a Marcus-type high-frequency glow discharge luminescence surface analyzer (GD-Profiler) manufactured by Horiba Seisakusho Inc. is used.

[0149] 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 on these.

[0150] 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.

[0151] 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.

[0152] 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.

[0153] 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.

[0154] 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 galvanized layer or a hot-dip galvanized layer, it is preferable to set the Fe content of the hot-dip galvanized layer (alloyed zinc plating layer) or the hot-dip galvanized 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 galvanized layer or a hot-dip galvanized 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.

[0155] 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).

[0156] The tensile strength (TS) of the steel plate according to the present embodiment is 500 MPa or higher. Additionally, the tensile strength may be 500 to 750 MPa. By making the tensile strength 500 MPa or higher, the steel plate according to the present embodiment can be suitably applied to panel-type parts such as door outers. The tensile strength is preferably 550 MPa or higher or 600 MPa or higher.

[0157] In addition, by keeping the tensile strength at 750 MPa or less, the deterioration of the appearance after press forming can be suppressed. The tensile strength is preferably 700 MPa or less.

[0158] 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.

[0159] 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.

[0160] 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.

[0161] Next, a press-formed article according to the present embodiment, which can be manufactured by press-forming the steel sheet described above, will be described. The press-formed article according to the present embodiment has the same chemical composition as the steel sheet 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. Since the C concentration gradient in the decarburization layer does not change even after press-forming, the press-formed article according to the present embodiment has a C content at a depth of 20 μm from the surface. 20 and, C content at a depth of 60㎛ from the above surface 60 And, the ΔC calculated from the following formula (1) is 0.20 to 0.90 mass% / mm.

[0162] ΔC=(C 60 -C 20 ) / (0.04) … (1)

[0163] The above C concentration gradient is preferably 0.30 mass% / mm or more, 0.35 mass% / mm or more, 0.40 mass% / mm or more, or 0.45 mass% / mm or more, and preferably 0.80 mass% / mm or less, or 0.75 mass% / mm or less. In addition, the ΔC of the press-formed product is obtained by the same method as when it is a steel sheet.

[0164] 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. Excellent appearance quality means that no stripe patterns (i.e., ghost lines) with intervals of several millimeters occurring on the surface are observed. In other words, when visually inspecting an arbitrary area of ​​100 mm × 100 mm, the maximum length of stripe patterns with intervals of several millimeters is 50 mm or less. It is preferable that the maximum length of the stripe pattern is 20 mm or less. Furthermore, it is more preferable that no stripe patterns are observed at all.

[0165] Specific examples of press-formed products include, for instance, panel-type parts such as door outers of automobile bodies.

[0166] Next, a method for manufacturing a steel plate according to the present embodiment will be described.

[0167] The steel sheet according to the present embodiment has the above characteristics, regardless of the manufacturing method, and the effect is obtained. However, by using steel having the chemical composition described above and performing annealing under the following conditions after hot rolling and after cold rolling, it is possible to stably manufacture a steel sheet in which ΔC (C concentration gradient) is preferably controlled.

[0168] (Annealing after hot rolling)

[0169] First, a hot-rolled steel sheet is obtained by performing hot rolling under general conditions on a slab having the chemical composition described above. On the obtained hot-rolled steel sheet, a first annealing is performed in a high-temperature zone in an atmospheric environment. This first annealing is performed under conditions of an annealing temperature of 550 to 700°C and an annealing time of 2 hours or more. By performing annealing in a high-temperature zone after hot rolling, internal oxides of Si and Mn are formed on the surface layer of the steel sheet. As a result, surface enrichment of Si and Mn is suppressed during annealing after cold rolling, thereby promoting decarburization. This allows for the desirable control of ΔC.

[0170] If the annealing temperature is less than 550℃ or the annealing time is less than 2 hours, the ΔC of the steel sheet cannot be controlled in a desirable way.

[0171] After performing the above annealing, pickling treatment is performed, and cold rolling with a cumulative reduction rate of 70% or more is performed to manufacture a steel sheet or steel strip having a desired thickness. By making the cumulative reduction rate of cold rolling 70% or more, austenite recrystallization is promoted during annealing after cold rolling, and the increase in the austenite fraction can be suppressed. As a result, during annealing after cold rolling, the ferrite fraction, which has a high diffusion coefficient of C, increases, and decarburization is promoted.

[0172] In addition, the cumulative reduction rate referred to here is expressed as {1 - (plate thickness after cold rolling / plate thickness before cold rolling)} × 100 (%).

[0173] After cold rolling, a steel sheet having desired mechanical properties is obtained by further performing a second annealing. At that time, for example, the dew point during the second annealing (the average dew point inside the annealing furnace) is set to -10°C or higher, and the residence time of the steel sheet in the temperature range of 700°C or higher is set to 50 to 400 seconds, thereby allowing the surface of the steel sheet to be decarburized stably. Although there is no need to specifically set an upper limit for the dew point, it may be set to approximately 10°C. If the dew point is too low or the residence time is too short, decarburization does not proceed sufficiently, and ΔC cannot be controlled favorably. Furthermore, if the residence time is too long, sufficient tensile strength may not be obtained. In addition, the temperature during annealing is, for example, approximately 750 to 850°C.

[0174] In addition to the conditions described above, although not specifically limited, it is desirable to satisfy, for example, the following conditions.

[0175] After heating the slab to a temperature range of 1100°C or higher, hot rolling is performed. After hot rolling, coiling is performed, first annealing is performed, and then pickling is performed. The finishing rolling temperature of the hot rolling is preferably 900°C or higher, and the coiling temperature is preferably 650°C or lower. After pickling, cold rolling is performed. After cold rolling, second annealing is performed, and then, if necessary, the plating layer described above may be formed.

[0176] Next, a method for manufacturing a press-formed product according to the present embodiment will be described.

[0177] Cold working is preferred for the press forming method to maintain the obtained structure and suppress the occurrence of ghost lines. The cold working method is not particularly limited, but it is sufficient if the steel sheet can be formed by moving the die and the punch relative to each other.

[0178] Examples

[0179] 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.

[0180] 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. The obtained slabs were heated to a temperature range of 1100°C or higher, and then hot rolling was performed. After hot rolling, coiling was performed, followed by first annealing under the conditions shown in Table 2, and then pickling was performed. The finishing rolling temperature of the hot rolling was set to 900°C or higher, and the coiling temperature was set to 650°C or lower. After pickling, cold rolling was performed with a cumulative reduction rate of 70 to 90%. After cold rolling, second annealing was performed under the conditions shown in Table 2, and, if necessary, an alloyed hot-dip galvanizing layer (GA), a hot-dip galvanizing layer (GI), and an electro-galvanizing layer (EG) were formed. By the above method, the steel sheets and galvanized steel sheets shown in Table 2 were obtained. In addition, the thickness of the obtained steel plate and plated steel plate was 0.2 to 2.0 mm.

[0181] After cold rolling and annealing, a roughly semi-cylindrical mock part (press-formed product) simulating a door outer was manufactured by press forming using a steel sheet and a plated steel sheet. When press-forming this mock part, the material (steel sheet or plated steel sheet) was actively fed into the mold so that, at any position on the surface of the mock part, the ratio of deformation in the direction perpendicular to that direction (that arbitrary direction) to deformation in any direction along the surface of the mock part was approximately 1. In other words, press forming was performed so that anisotropy of deformation does not occur at any position on the surface of the mock part.

[0182] For the obtained steel sheet, plated steel sheet, and simulated part (press-formed product), ΔC was calculated by the method described above. Also, since the ΔC of the steel sheet and plated steel sheet and the ΔC of the simulated part were the same value, the ΔC of the simulated part is not listed in the table.

[0183] In addition, the tensile strength of the steel plate and the appearance quality of the simulated part were evaluated by the following method. Furthermore, since there is no significant difference between the tensile strength of the steel plate and the tensile strength of the simulated part (press-formed product), the steel plate was evaluated to determine whether it possessed the tensile strength required for a simulated part.

[0184] tensile strength

[0185] Tensile strength was evaluated in accordance with JIS Z 2241:2011. The test specimen was a No. 5 specimen of JIS Z 2241:2011. The tensile test specimen was taken at the 1 / 4 mark from the end in the plate width direction, and the direction perpendicular to the rolling direction was considered the length direction. If the obtained tensile strength was 500 MPa or higher, it was judged to be high strength and accepted. On the other hand, if the obtained tensile strength was less than 500 MPa, it was judged to be low strength and rejected.

[0186] Appearance quality

[0187] 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 polished with a grinding wheel, and the resulting striped patterns at intervals of several millimeters were judged as ghost lines. A rating of 1 to 5 was assigned based on the degree of occurrence of the striped patterns. An arbitrary area of ​​100 mm × 100 mm was visually inspected, and "1" was assigned if no striped patterns were observed at all, "2" if the maximum length of the striped pattern was 20 mm or less, "3" if the maximum length of the striped pattern was greater than 20 mm and less than or equal to 50 mm, "4" if the maximum length of the striped pattern was greater than 50 mm and less than or equal to 70 mm, and "5" if the maximum length of the striped pattern exceeded 70 mm. If the rating was "3" or lower, it was judged as passing with excellent appearance quality. On the other hand, if the rating was "4" or higher, it was judged as failing with poor appearance quality.

[0188]

[0189]

[0190] 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.

[0191] 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

[0192] 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 0.105%, Mn: 1.00 to 2.30%, Si: 0.005 to 1.500%, Al: 0.005 to 0.700%, P: 0.100% or less, S: 0.0200% or less, N: 0.0150% or less, O: 0.0100% or less, Cr: 0 to 0.80%, Mo: 0 to 0.16%, Ti: 0 to 0.100%, B: 0 to 0.0100%, 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 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 the remainder: Fe and impurities, and C content at a depth of 20 µm from the surface. 20 and, C content at a depth of 60㎛ from the above surface 60 A steel plate characterized by having a ΔC calculated from the following formula (1) of 0.20 to 0.90 mass% / mm and a tensile strength of 500 MPa or more and 750 MPa or less. ΔC=(C 60 -C 20 ) / (0.04) … (1) Claim 2 In claim 1, the chemical composition is, in mass%, Cr: 0.01 to 0.80%, Mo: 0.01 to 0.16%, Ti: 0.001 to 0.100%, B: 0.0001 to 0.0100%, 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 A steel plate according to claim 1 or 2, characterized in that the chemical composition is C: 0.040 to 0.080% in mass%. Claim 4 A steel plate according to claim 1 or 2, characterized in that the ΔC is 0.30 to 0.80 mass% / mm. 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 delete Claim 7 A press-formed product obtained by press-forming a steel plate described in claim 1 or 2, wherein the C content at a depth of 20 μm from the surface is C 20 and, C content at a depth of 60㎛ from the above surface 60 A press-molded article characterized by ΔC calculated from the following formula (1) being 0.20 to 0.90 mass% / mm. ΔC=(C 60 -C 20 ) / (0.04) … (1)

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