Steel sheet and press-formed product

Abstract

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

Description

Technical Field

[0001] This invention relates to steel plates and pressed products. Background Technology

[0002] From an environmental protection perspective, there are increasing demands for lightweight vehicle bodies and improved crash safety. To address these requirements, high-strength and thin-walled designs have been researched for panel components such as door panels. Unlike frame components, these panel components are directly visible to the human eye and therefore require high aesthetic quality. Thus, even high-strength steel sheets previously used for frame components must achieve excellent appearance quality after forming when applied to panel components.

[0003] To improve appearance quality, suppressing the formation of ghost lines can be considered as a research topic. Ghost lines are tiny irregularities on the surface, occurring on the order of millimeters, caused by the preferential deformation of the soft phase at the periphery of the steel sheet containing both hard and soft phases during the pressing process. These irregularities appear as rib-like patterns on the surface, resulting in poor appearance quality in pressed-formed products.

[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 a steel sheet (substrate) and a hot-dip galvanized layer formed on the surface of the substrate. The steel sheet (substrate) contains, by mass%, C: 0.02-0.20%, Si: 0.7% or less, Mn: 1.5-3.5%, P: 0.10% or less, S: 0.01% or less, Al: 0.1-1.0%, N: 0.010% or less, and Cr: 0.03-0.5%. Furthermore, the contents of Al, Cr, Si, and Mn are set such that the surface oxidation index A during annealing is 2.3 or more as defined by the mathematical formula with the same sign: A = 400Al / (4Cr+3Si+6Mn). The remaining portion contains Fe and unavoidable impurities. Moreover, the microstructure of the substrate includes ferrite and a second phase, which is mainly martensite.

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

[0006] Patent document 3 discloses a high-strength thin steel sheet, characterized in that the Vickers hardness at a depth of 0.05 mm from the surface of the steel sheet is 100 to 250 Hv and (Vickers hardness at a depth of 0.2 mm from the surface of ...

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

[0008] Patent document 5 discloses a high-strength steel plate with minimal degradation of properties after cutting. The steel plate is characterized by having a microstructure mainly consisting of ferrite and bainite, a Mn segregation degree (= Mn peak concentration in the center / average Mn concentration) in the thickness direction of the plate being less than 1.20, and a maximum tensile strength of 540 MPa or more.

[0009] Existing technical documents

[0010] Patent documents

[0011] Patent Document 1: Japanese Patent Application Publication No. 2005-220430

[0012] Patent Document 2: International Publication No. 2019 / 026113

[0013] Patent Document 3: Japanese Patent Application Publication No. 2006-70328

[0014] Patent Document 4: Japanese Patent Application Publication No. 2006-97102

[0015] Patent Document 5: Japanese Patent Application Publication No. 2009-263685 Summary of the Invention

[0016] The problem that the invention aims to solve

[0017] The present invention was made in view of the above-mentioned actual situation. The object of the present invention is to provide a pressed molded article with high strength and excellent appearance quality, and a steel sheet for manufacturing the pressed molded article.

[0018] Methods for solving problems

[0019] The main points of this invention are as follows.

[0020] (1) The chemical composition of the steel plate of one embodiment of the present invention, in mass % is:

[0021] C: 0.040~0.100%

[0022] Mn: 1.00~2.00%

[0023] Si: 0.005~1.500%

[0024] P: below 0.100%

[0025] S: Below 0.0200%

[0026] Al: 0.005~0.700%

[0027] N: below 0.0150%

[0028] O: Below 0.0100%

[0029] Cr: 0–0.80%

[0030] Mo: 0–0.16%

[0031] B: 0~0.0100%

[0032] Ti: 0~0.100%

[0033] Nb: 0~0.060%

[0034] V: 0~0.50%

[0035] Ni: 0~1.00%

[0036] Cu: 0–1.00%

[0037] W: 0~1.00%

[0038] Sn: 0~1.00%

[0039] Sb: 0~0.200%

[0040] Ca: 0~0.0100%

[0041] Mg: 0~0.0100%

[0042] Zr: 0~0.0100%

[0043] REM: 0~0.0100%, and

[0044] Remaining components: Fe and impurities.

[0045] The arithmetic mean waviness Wa of the steel plate is 0.10 to 0.30 μm.

[0046] (2) The steel plate according to (1) above may also contain, in terms of mass percent, one or more elements selected from the group consisting of:

[0047] Cr: 0.01~0.80%

[0048] Mo: 0.01–0.16%

[0049] B: 0.0001~0.0100%

[0050] Ti: 0.001~0.100%

[0051] Nb: 0.001~0.060%

[0052] V: 0.01~0.50%

[0053] Ni: 0.01~1.00%

[0054] Cu: 0.01~1.00%

[0055] W: 0.01~1.00%

[0056] Sn: 0.01~1.00%

[0057] Sb: 0.001~0.200%

[0058] Ca: 0.0001~0.0100%

[0059] Mg: 0.0001~0.0100%

[0060] Zr: 0.0001~0.0100%, and

[0061] REM: 0.0001~0.0100%.

[0062] (3) According to the steel plate described in (1) or (2) above, it is also possible to set the average value of Mn concentration in the region from a position 1 / 8 of the thickness of the steel plate in the thickness direction to a position 3 / 8 of the thickness of the steel plate in the thickness direction from the surface of the steel plate to the position of the steel plate. When the standard deviation of the Mn concentration is set to σ, (3σ / μ)×100≤7.0.

[0063] (4) The steel plate according to any one of (1) to (3) above may also have a decarburized layer with a thickness of 20 μm or more on the surface of the steel plate.

[0064] (5) The steel plate according to any one of (1) to (4) above may also have a coating on at least one surface of the steel plate.

[0065] (6) Another aspect of the present invention is a pressed-molded article obtained by pressing the steel plate described in any one of (1) to (5) above.

[0066] Invention Effects

[0067] According to the above-described solution of the present invention, it is possible to provide a high-strength, high-appearance-quality pressed molded article and a steel plate for manufacturing the pressed molded article. Detailed Implementation

[0068] The inventors have investigated a method for suppressing the formation of ghost lines after pressing high-strength steel sheets. As a result, they have found it effective to reduce the hardness difference in steel and control the surface roughness of the steel sheet within a desired range. One contributing factor to the formation of hardness differences in steel is banded Mn segregation that occurs during the solidification process. If Mn segregation occurs in bands, the periphery of areas with high Mn concentration is prone to austenite transformation during annealing, resulting in the formation of hard martensite in bands after annealing following cold rolling. Consequently, the hardness difference in the steel increases, leading to the formation of ghost lines during pressing.

[0069] Generally speaking, the lower the surface roughness of the steel sheet used as raw material, the better. This is because excessive surface roughness of the steel sheet results in poor appearance quality. However, the inventors have realized that in order to suppress the formation of ghost lines in pressed products, it is important to moderately roughen the surface of the steel sheet used as raw material without reducing appearance quality.

[0070] This invention is based on the above-mentioned knowledge, and the steel plate and pressed product of this embodiment will be described in detail below. However, this invention is not limited to the configuration disclosed in this embodiment, and various modifications can be made without departing from the spirit of this invention.

[0071] First, the chemical composition of the steel plate of this embodiment will be described. For the numerical ranges specified below, enclosed in “~”, the lower and upper limits are included within these ranges. Values ​​expressed as “less than” or “more than” are not included within these ranges. In the following description, unless otherwise specified, the percentage of chemical composition is expressed as a percentage by mass.

[0072] The chemical composition of the steel plate of this embodiment, by mass%, contains: C: 0.040–0.100%, Mn: 1.00–2.00%, Si: 0.005–1.500%, P: 0.100% or less, S: 0.0200% or less, Al: 0.005–0.700%, N: 0.0150% or less, O: 0.0100% or less, and the remainder: Fe and impurities. The elements will be described below.

[0073] C: 0.040~0.100%

[0074] Carbon (C) is an element that increases the strength of steel sheets and pressed products. To obtain the desired strength, the C content is set to 0.040% or more. To further improve the strength, the C content is preferably 0.050% or more, more preferably 0.060%, 0.070%, or 0.075% or more.

[0075] Furthermore, by setting the C content to 0.100% or less, the diffusion of Mn during solidification can be promoted, thereby suppressing the formation of banded Mn segregation. As a result, the formation of ghost lines after pressing can be suppressed. Therefore, the C content is set to 0.100% or less. The C content is preferably 0.095% or less, more preferably 0.090% or less, or 0.085% or less.

[0076] It should be noted that when the Mn content is below 1.40%, the C content is preferably above 0.075%. In this way, by strictly controlling the Mn and C contents, Mn diffusion in the steel can be promoted at high temperatures, and Mn segregation can be reduced.

[0077] Mn: 1.00~2.00%

[0078] Mn is an element that improves the hardenability of steel, thus contributing to increased strength. To obtain the desired strength, the Mn content is set to 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.

[0079] Furthermore, if the Mn content is 2.00% or less, banded Mn segregation during steel solidification can be suppressed. Therefore, 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.

[0080] Si: 0.005~1.500%

[0081] Si is an element that improves the strength-formability balance of steel sheets. To achieve this effect, the Si content is set to 0.005% or more, preferably 0.010% or more.

[0082] Furthermore, Si is also an element that forms coarse Si oxides, which act as the starting point for damage. By setting the Si content to 1.500% or less, the formation of Si oxides can be suppressed, making cracking less likely. 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, and more preferably 1.000% or less.

[0083] P: below 0.100%

[0084] P is an impurity element that embrittles steel. If the P content is 0.100% or less, it can suppress the embrittlement of the steel sheet and its tendency to crack during the production process. Therefore, the P content is set to 0.100% or less. From a productivity point of view, the P content is preferably 0.050% or less, more preferably 0.030% or less, or 0.020% or less.

[0085] The lower limit for phosphorus (P) content includes 0%, but by setting the P content to 0.001% or higher, manufacturing costs can be further reduced. Therefore, the P content can also be set to 0.001% or higher.

[0086] S: below 0.0200%

[0087] S is an impurity element that forms Mn sulfides and deteriorates the formability of steel sheets, including ductility, porosity, tensile flange properties, and bending properties. If the S content is 0.0200% or less, a significant decrease in the formability of the steel sheet can be prevented. 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.

[0088] The lower limit for sulfur content includes 0%, but by setting the sulfur content to 0.0001% or higher, manufacturing costs can be further reduced. Therefore, the sulfur content can also be set to 0.0001% or higher.

[0089] Al: 0.005~0.700%

[0090] Al is the element that functions as a deoxidizer. To fully obtain the deoxidizing effect brought by Al, the Al content is set to 0.005% or more. The preferred Al content is 0.010% or more or 0.025% or more.

[0091] Furthermore, Al is also an element that forms coarse oxides, which become the starting point for damage and embrittle the steel. By setting the Al content to 0.700% or less, the formation of coarse oxides, which act as the starting point for damage, can be suppressed, and the slab can be prevented from becoming prone to cracking. Therefore, the Al content is set to 0.700% or less. The upper limit of the Al content is preferably 0.600%, 0.400%, 0.200%, or 0.100%, more preferably 0.085%, 0.070%, 0.065%, or 0.060%.

[0092] N: below 0.0150%

[0093] Nitrogen (N) is an impurity element that forms nitrides and deteriorates the formability of steel sheets, including ductility, porosity, tensile flange properties, and bending properties. If the N content is 0.0150% or less, the reduction in the formability of the steel sheet can be suppressed. Therefore, the N content is set to 0.0150% or less. Furthermore, N is also an element that causes welding defects during welding, hindering productivity. Therefore, the N content is preferably 0.0120% or less, and more preferably 0.0100% or less.

[0094] The lower limit for nitrogen content includes 0%, but by setting the nitrogen content to 0.0005% or higher, manufacturing costs can be further reduced. Therefore, the nitrogen content can also be set to 0.0005% or higher.

[0095] O: Below 0.0100%

[0096] O is an impurity element that forms oxides and hinders the formability of steel sheets, including ductility, porosity, tensile flange properties, and bending. If the O content is 0.0100% or less, a significant decrease in the formability of the steel sheet can be prevented. 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.

[0097] The lower limit for O content includes 0%, but by setting the O content to 0.0001% or higher, manufacturing costs can be further reduced. Therefore, the O content can also be set to 0.0001% or higher.

[0098] The steel plate of this embodiment may also contain the following elements as optional elements to replace a portion of Fe. The content of any of the following optional elements is 0%.

[0099] Cr: 0–0.80%

[0100] Cr is an element that improves the hardenability of steel and contributes to the increase in the strength of steel plates. Since Cr is not necessarily present, the lower limit of Cr content includes 0%. In order to fully obtain the strength improvement effect brought by Cr, the Cr content is preferably 0.01% or more, more preferably 0.20% or more, and even more preferably 0.30% or more.

[0101] Furthermore, if the Cr content is below 0.80%, the formation of coarse Cr carbides, which can become the starting point for destruction, can be suppressed. Therefore, the Cr content is set to below 0.80%. To reduce alloy costs, the upper limit of the Cr content can also be set to 0.60%, 0.40%, 0.20%, 0.10%, or 0.05%, as needed.

[0102] Mo: 0–0.16%

[0103] Mo is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of steel sheets. Since Mo is not necessarily present, the lower limit of Mo content includes 0%. In order to fully obtain the strength improvement effect brought by Mo, the Mo content is preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.10% or more.

[0104] Furthermore, a Mo content of 0.16% or less can suppress the reduction in hot workability and productivity. Therefore, the Mo content is set to 0.16% or less. To reduce alloy costs, the upper limit of the Mo content can also be set to 0.12%, 0.10%, 0.08%, or 0.04% as needed.

[0105] It should be noted that by including Cr: 0.01-0.80% and Mo: 0.01-0.16%, the strength of the steel plate can be improved more reliably, and therefore it is preferred.

[0106] B: 0~0.0100%

[0107] Boron (B) is an element that inhibits phase transformation at high temperatures and contributes to improving the strength of steel plates. Since B is not necessarily present, the lower limit for B content includes 0%. To fully obtain the strength-enhancing effect brought about by B, the B content is preferably 0.0001% or more, more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0108] Furthermore, if the boron content is below 0.0100%, the formation of boron precipitates and the resulting decrease in steel plate strength can be suppressed. Therefore, the boron content is set to below 0.0100%. To reduce alloy costs, the upper limit of the boron content can also be set to 0.0050%, 0.0030%, 0.0020%, 0.0010%, or 0.0005%, as needed.

[0109] Ti: 0~0.100%

[0110] Ti is an element that reduces the amount of S, N, and O that contribute to the formation of large inclusions that act as the starting point for damage. Furthermore, Ti has the effect of refining the microstructure and improving the strength-formability balance of the steel sheet. Since Ti is not necessarily present, the lower limit for Ti content includes 0%. To fully obtain the above effects, the Ti content is preferably set to 0.001% or more, and more preferably 0.001% or more.

[0111] Furthermore, if the Ti content is 0.100% or less, the formation of coarse Ti sulfides, Ti nitrides, and Ti oxides can be suppressed, ensuring the formability of the steel sheet. Therefore, the Ti content is set to 0.100% or less. The Ti content is preferably set to 0.080% or less, and more preferably to 0.060% or less. To reduce alloy costs, the upper limit of the Ti content can also be set to 0.040%, 0.020%, 0.010%, or 0.005%, as needed.

[0112] Nb: 0~0.060%

[0113] Nitrogen (Nb) is an element that contributes to the improvement of steel sheet strength through strengthening by precipitates, grain refinement strengthening due to inhibition of ferrite grain growth, and dislocation strengthening due to inhibition of recrystallization. Since Nb is not necessarily present, the lower limit of Nb content includes 0%. To fully 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.

[0114] Furthermore, if the Nb content is 0.060% or less, recrystallization can be promoted while suppressing the retention of unrecrystallized ferrite, thus ensuring the formability of the steel sheet. Therefore, the Nb content is set to 0.060% or less. The Nb content is preferably 0.050% or less, more preferably 0.040% or less. To reduce alloy costs, the upper limit of the Nb content can also be set to 0.030%, 0.020%, 0.010%, or 0.005% as needed.

[0115] V: 0~0.50%

[0116] V is an element that contributes to the improvement of steel sheet strength through strengthening by precipitates, grain refinement strengthening due to inhibition of ferrite grain growth, and dislocation strengthening due to inhibition of recrystallization. Since V is not necessarily present, the lower limit of V content includes 0%. In order to fully obtain the strength improvement effect brought by V, the V content is preferably 0.01% or more, and more preferably 0.03% or more.

[0117] Furthermore, if the V content is below 0.50%, the excessive precipitation of carbonitrides can be suppressed, thus preventing a decrease in the formability of the steel sheet. Therefore, the V content is set to below 0.50%. To reduce alloy costs, the upper limit of the V content can also be set to 0.30%, 0.20%, 0.10%, 0.05%, or 0.02%, depending on the requirements.

[0118] Ni: 0~1.00%

[0119] Ni is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of steel plates. Since Ni is not necessarily present, the lower limit of Ni content includes 0%. In order to fully obtain the strength improvement effect brought by Ni, the Ni content is preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.20% or more.

[0120] Furthermore, a Ni content of 1.00% or less can suppress the reduction in weldability of the steel sheet. Therefore, the Ni content is set to 1.00% or less. To reduce alloy costs, the upper limit of the Ni content can also be set to 0.60%, 0.40%, 0.20%, 0.10%, or 0.03% as needed.

[0121] Cu: 0~1.00%

[0122] Cu is an element that exists in steel in the form of fine particles and contributes to the strength of steel sheets. Since Cu is not necessarily present, the lower limit of Cu content includes 0%. In order to fully obtain the strength improvement effect brought by Cu, the Cu content is preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.15% or more.

[0123] Furthermore, a Cu content of 1.00% or less can suppress the reduction in weldability of the steel sheet. Therefore, the Cu content is set to 1.00% or less. To reduce alloy costs, the upper limit of the Cu content can also be set to 0.60%, 0.40%, 0.20%, 0.10%, or 0.03% as needed.

[0124] W: 0~1.00%

[0125] W is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of steel plates. Since W is not necessarily present, the lower limit of W content includes 0%. In order to fully obtain the strength improvement effect brought by W, the W content is preferably 0.01% or more, more preferably 0.03% or more, and even more preferably 0.10% or more.

[0126] Furthermore, if the W content is below 1.00%, it can suppress the decrease in hot workability and productivity. Therefore, the W content is set to below 1.00%. To reduce alloy costs, the upper limit of the W content can also be set to 0.50%, 0.20%, 0.10%, 0.05%, or 0.02% as needed.

[0127] Sn: 0~1.00%

[0128] Sn is an element that inhibits grain coarsening and contributes to improving the strength of steel sheets. Since Sn is not necessarily present, the lower limit of Sn content includes 0%. To fully obtain the effects brought by Sn, the Sn content is more preferably 0.01% or more.

[0129] Furthermore, if the Sn content is below 1.00%, it can suppress the embrittlement of the steel sheet and prevent breakage during rolling. Therefore, the Sn content is set to below 1.00%. To reduce alloy costs, the upper limit of the Sn content can also be set to 0.50%, 0.20%, 0.10%, 0.05%, or 0.02% as needed.

[0130] Sb: 0~0.200%

[0131] Sb is an element that inhibits grain coarsening and contributes to improving the strength of steel sheets. Since Sb is not necessarily present, the lower limit of Sb content includes 0%. To fully obtain the above-mentioned effects, the Sb content is preferably 0.001% or more, and more preferably 0.005% or more.

[0132] Furthermore, if the Sb content is below 0.200%, it can suppress the embrittlement of the steel sheet and prevent breakage during rolling. Therefore, the Sb content is set to below 0.200%. To reduce alloy costs, the upper limit of the Sb content can also be set to 0.100%, 0.070%, 0.040%, 0.010%, or 0.005% as needed.

[0133] Ca: 0~0.0100%

[0134] Mg: 0~0.0100%

[0135] Zr: 0~0.0100%

[0136] REM: 0~0.0100%

[0137] Ca, Mg, Zr, and REM are elements that contribute to improving the formability of steel sheets. Since Ca, Mg, Zr, and REM are not necessarily present, the lower limit of the total content of these elements includes 0%. To fully obtain the effect of improving formability, the content of each of these elements is preferably 0.0001% or more, more preferably 0.0010% or more. To fully obtain the above effect, it is not necessary to contain all of the above elements; the content of any one of them only needs to be 0.0001% or more.

[0138] Furthermore, if the contents of Ca, Mg, Zr, and REM are each below 0.0100%, the reduction in the ductility of the steel sheet can be suppressed. Therefore, the contents of these elements are each set to 0.0100% or less, preferably 0.0050% or less. To reduce alloy costs, the upper limits of the contents of Ca, Mg, Zr, and REM can also be set to 0.0030%, 0.0020%, 0.0010%, or 0.0003%, respectively, as needed.

[0139] REM (Rare Earth Metal) refers to a total of 17 elements including Sc, Y, and the lanthanides. The content of REM refers to the total content of these elements.

[0140] The remaining portion of the chemical composition of the steel plate in this embodiment may also be Fe and impurities. Examples of impurities include elements that inevitably mix in from steel raw materials or scrap iron and / or during the steelmaking process, or elements that are permissible within a range that do not impair the properties of the steel plate of this 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 total amount of impurities may also be 0.100% or less.

[0141] The chemical composition of the aforementioned steel plates can be determined using common analytical methods. For example, ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) can be used. It should be noted that C and S can be determined using the combustion-infrared absorption method, N can be determined using the inert gas melting-thermal conductivity method, and O can be determined using the inert gas melting-non-dispersive infrared absorption method.

[0142] When a steel plate has a coating on its surface, the chemical composition can be analyzed after removing the coating by mechanical grinding.

[0143] Arithmetic mean waviness Wa: 0.10–0.30 μm

[0144] Generally, the smaller the arithmetic mean waviness Wa of the steel sheet used as raw material, the better from the viewpoint of appearance quality. However, the inventors have realized that in order to suppress the generation of ghost lines in the pressed and molded article, the generation of ghost lines in the pressed and molded article can be suppressed by moderately roughening the surface of the steel sheet used as raw material, specifically by setting the arithmetic mean waviness Wa to 0.10 μm or more. Therefore, in the steel sheet of this embodiment, the arithmetic mean waviness Wa is set to 0.10 μm or more. Preferably, it is 0.13 μm or more.

[0145] Furthermore, if the arithmetic mean waviness Wa is too large, the appearance quality of the steel sheet itself decreases, and the low appearance quality is maintained after pressing. Therefore, the arithmetic mean waviness Wa is set to 0.30 μm or less. Preferably, it is 0.25 μm or less.

[0146] It should be noted that the arithmetic mean waviness Wa refers to the arithmetic mean waviness of the steel plate when it has no coating, and to the arithmetic mean waviness of the coating when it has a coating on its surface.

[0147] In this embodiment, the arithmetic mean ripple degree Wa is obtained by the following method.

[0148] A 50mm × 50mm test piece was cut from a position at least 10mm away from the end face of the steel plate. Next, using a laser displacement measuring device (KEYENCE VK-X1000), the profiles of three lines were measured along a direction perpendicular to the rolling direction. Based on the results, according to JIS B0601:2013, a waviness curve was obtained by sequentially applying profile curve filters with cutoff values ​​of λc and λf to the cross-sectional curve. Specifically, components with wavelengths λc below 0.8mm and wavelengths λf above 2.5mm were removed from the measured results to obtain the waviness curve. Based on the obtained waviness curve, the arithmetic mean waviness was calculated according to JIS B0601:2013, and the average value of the three lines was calculated. The arithmetic mean of the calculated values ​​of the three lines was set as the arithmetic mean waviness Wa of the steel plate.

[0149] When the steel plate has a coating on its surface, the above-mentioned line analysis can be performed on the surface of the coating.

[0150] (3σ / μ)×100≤7.0

[0151] For the steel sheet of this embodiment, the average Mn concentration in the region from a position 1 / 8 of the thickness of the steel sheet in the thickness direction to a position 3 / 8 of the thickness of the steel sheet in the thickness direction (a region from 1 / 8 depth to 3 / 8 depth from the surface of the steel sheet) is set as μ in units of mass%, and the standard deviation of the Mn concentration is set as σ in units of mass%. Preferably, (3σ / μ)×100 ≤ 7.0. By setting (3σ / μ)×100 to 7.0 or less, the generation of Mn segregation in the steel sheet can be further reduced, the generation of ghost lines can be further suppressed, and a pressed molded product with better appearance quality can be obtained. (3σ / μ)×100 is more preferably set to 6.5 or less. The lower limit of (3σ / μ)×100 is not particularly limited and can also be set to 0. In order to reduce (3σ / μ)×100 and increase manufacturing costs, the lower limit can also be set to 2.0, 4.0 or 5.0. The upper limit of (3σ / μ)×100 can also be set to 11.0, 10.0, 9.0 or 8.0 as needed.

[0152] In this embodiment, the average value μ of Mn concentration and the standard deviation σ of Mn concentration are obtained by the following method.

[0153] After mirror grinding of the steel plate's cross-section, the Mn concentration is measured at 600 points at 1 μm intervals along the rolling direction of the steel plate at a specified depth. The average Mn concentration (mass%) at the specified depth is obtained by calculating the average value of the obtained Mn concentrations. This operation is performed every 1 μm along the thickness direction, starting from a position 1 / 8 of the thickness away from the surface of the steel plate in the thickness direction, and ending at a position 3 / 8 of the thickness away from the surface in the thickness direction. The average Mn concentration μ is obtained by calculating the average value (arithmetic mean) of all obtained Mn concentrations. Furthermore, the standard deviation σ of the Mn concentration is obtained by calculating the standard deviation from all obtained Mn concentrations.

[0154] The apparatus used was an electron probe microanalyzer (EPMA), and the measurement conditions were that the accelerating voltage was set to 15 kV.

[0155] The steel plate of this embodiment may also have a coating on at least one surface of the steel plate. Examples of coatings include zinc coatings and zinc alloy coatings, as well as alloyed zinc coatings and alloyed zinc alloy coatings obtained by alloying these coatings.

[0156] Zinc coatings and zinc alloy coatings are formed by hot-dip galvanizing, electroplating, or vapor deposition. If the Al content of the zinc coating is less than 0.5% by mass, the adhesion between the steel plate surface and the zinc coating can be sufficiently ensured. Therefore, the Al content of the zinc coating is preferably less than 0.5% by mass.

[0157] When the zinc coating is a hot-dip galvanized layer, in order to improve the adhesion between the steel plate surface and the zinc coating, the Fe content of the hot-dip galvanized layer is preferably 3.0% by mass or less.

[0158] When the zinc coating is an electroplated zinc coating, the Fe content of the electroplated zinc coating is preferably 0.5% by mass or less from the perspective of improving corrosion resistance.

[0159] Zinc coatings and zinc alloy coatings may contain one or more of the following elements, within a range that does not impair the corrosion resistance and formability of the steel sheet: 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. In particular, Ni, Al, and Mg are effective in improving the corrosion resistance of steel sheets.

[0160] The zinc coating or zinc alloy coating can also be an alloyed zinc coating or an alloyed zinc alloy coating that has undergone alloying treatment. When alloying treatment is performed on the hot-dip galvanized layer or hot-dip galvanized alloy layer, from the viewpoint of improving the adhesion between the steel sheet surface and the alloyed coating, it is preferable to set the Fe content of the alloyed hot-dip galvanized layer (alloyed zinc coating) or hot-dip galvanized alloy layer (alloyed zinc alloy coating) to 7.0 to 13.0% by mass. By performing alloying treatment on the steel sheet having the hot-dip galvanized layer or hot-dip galvanized alloy layer, Fe is introduced into the coating, increasing the Fe content. Therefore, the Fe content can be set to 7.0% by mass or more. That is, a zinc coating with an Fe content of 7.0% by mass or more is an alloyed zinc coating or an alloyed zinc alloy coating.

[0161] The Fe content in the coating can be obtained by the following method: The coating is dissolved and removed using a 5% (v / v) aqueous solution of HCl with added inhibitor. The Fe content (mass %) in the resulting solution is determined by ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry).

[0162] Regardless of the presence or absence of a coating, the steel sheet in this embodiment can have a decarburized layer with a thickness of 20 μm or more on its surface. By setting the thickness of the decarburized layer to 20 μm or more, the banded Mn segregation that causes the ribbed pattern is reduced, and the appearance quality after pressing is further improved.

[0163] In this embodiment, the thickness of the decarburized layer is determined by the following method.

[0164] For any three locations on the steel plate, the carbon concentration is measured at 1 μm depths, extending from the surface of the steel plate to a position half the thickness of the plate in the depth direction (thickness direction). The thickness of the decarburized layer is obtained by considering the region with a carbon concentration less than half the thickness of the plate at the position half the thickness of the plate from the surface as the decarburized layer.

[0165] For the determination, a Marcus-type high-frequency glow discharge luminescent surface analyzer (GD-Profiler) manufactured by Horiba Corporation was used.

[0166] The thickness of the steel plate in this embodiment is not limited to a specific range, but is preferably 0.2 to 2.0 mm considering versatility and manufacturability. By setting the plate thickness to 0.2 mm or more, it becomes easier to maintain the shape of the steel plate flat, and dimensional and shape accuracy can be improved. Therefore, the plate thickness is preferably 0.2 mm or more. More preferably, it is 0.4 mm or more.

[0167] On the other hand, if the plate thickness is 2.0 mm or less, it becomes easier to apply appropriate strain and control the temperature during the manufacturing process, resulting in a homogeneous microstructure. Therefore, a plate thickness of 2.0 mm or less is preferred. More preferably, it is 1.5 mm or less.

[0168] The tensile strength of the steel plate in this embodiment is preferably 500 to 750 MPa. By setting the tensile strength to 500 MPa or higher, it can be appropriately applied to panel components. By setting the tensile strength to 750 MPa or lower, the formability of the sheet can be improved, and the deterioration of appearance quality caused by ghost lines can be suppressed. The lower limit of the tensile strength can also be set to 540 MPa, 580 MPa, or 600 MPa, and the upper limit can also be set to 680 MPa or 660 MPa.

[0169] It should be noted that tensile strength was evaluated according to JIS Z 2241:2011. The test piece was set as test piece No. 5 of JIS Z2241:2011. The tensile test piece was collected at a distance of 1 / 4 from the end in the width direction of the plate, and the direction perpendicular to the rolling direction was taken as the length direction.

[0170] Next, the pressed-molded article of this embodiment, which can be manufactured by pressing the aforementioned steel sheet, will be described. The pressed-molded article of this embodiment has the same chemical composition as the aforementioned steel sheet. Furthermore, the pressed-molded article of this embodiment may also have the aforementioned coating on at least one surface.

[0171] Since the pressed-molded article of this embodiment is obtained by pressing the aforementioned steel sheet, ghost lines are suppressed, resulting in excellent appearance quality. Specific examples of pressed-molded articles include panel components such as the outer door panel of a car body.

[0172] In the pressed molded article of this embodiment, "excellent appearance quality" means that no rib-like patterns (i.e., ghost lines) with intervals of several millimeters are observed on the surface. In other words, when visually inspecting any area of ​​100mm × 100mm, the maximum length of the rib-like patterns with intervals of several millimeters is 50mm or less. The maximum length of the rib-like patterns is preferably 20mm or less. Furthermore, it is more preferable that the rib-like patterns are completely invisible.

[0173] The pressed product of this embodiment suppresses the generation of ghost lines, so the sum of the maximum mountain height Zp and the maximum valley height Zv of the waviness curve, i.e., Wz, is less than 0.60 μm.

[0174] Furthermore, by using steel plates with a preferably controlled 3σ / μ to manufacture pressed products, pressed products with superior appearance quality can be obtained. That is, pressed products with a waviness curve where the sum of the maximum mountain height Zp and the maximum valley height Zv, i.e., Wz, is less than 0.40 μm can be obtained.

[0175] Wz is obtained by the following method: according to JIS B 0601:2013, the surface waviness curve of the pressed molded product is obtained, the maximum mountain height Zp and the maximum valley height Zv are calculated, and their sum is obtained.

[0176] Next, the method for manufacturing the steel plate according to this embodiment will be described.

[0177] The steel sheet of this embodiment is independent of the manufacturing method; its effects can be obtained as long as it possesses the aforementioned characteristics. Furthermore, it may not be a steel sheet but a steel strip. However, by using steel with the aforementioned chemical composition, and by controlling the following conditions (I) to (IV) in a composite and inseparable manner, it is possible to stably manufacture a steel sheet with preferably controlled arithmetic mean waviness Wa. Furthermore, to preferably control 3σ / μ, it is preferable to further control condition (V) in addition to controlling the following conditions (I) to (IV). Furthermore, to preferably control the thickness of the decarburized layer, it is preferable to further control condition (VI) in addition to controlling the following conditions (I) to (IV). It should be noted that conditions (V) and (VI) are optional.

[0178] The following is an explanation of each condition.

[0179] (I) Set the winding temperature to 550°C or higher.

[0180] (II) Set the pickling time to 50 seconds or more.

[0181] (III) Set the arithmetic mean roughness Ra of the roll surface of the final cold rolling pass to 0.2 to 0.7 μm.

[0182] (IV) Set the reduction rate of the quenching and tempering rolling to 0.3 to 0.7% and the arithmetic mean roughness Ra of the rolls to 1.5 to 3.5 μm.

[0183] (V) Heat the slab to a temperature range of 1200°C or higher and maintain it in that temperature range for more than 5 hours.

[0184] (VI) Perform annealing with the dew point (average dew point in the annealing furnace) set above -20°C and the residence time of the steel plate in the temperature range above 700°C set to 50 to 400 seconds.

[0185] (I) Winding temperature: 550℃ or above

[0186] By setting the coiling temperature after hot rolling to a high-temperature range of 550°C or higher, oxide scale easily forms on the surface of the steel sheet. As a result, unevenness easily forms on the surface of the pickled steel sheet. A coiling temperature of 600°C or higher is more preferred, and even more preferably 650°C or higher.

[0187] (II) Pickling time: 50 seconds or more

[0188] During pickling after coiling and before cold rolling, setting the pickling time to 50 seconds or more makes it easier to create unevenness on the surface of the steel sheet. A pickling time of 70 seconds or more is more preferable.

[0189] (III) Arithmetic mean surface roughness Ra of the rolls in the final cold rolling pass: 0.2–0.7 μm

[0190] After pickling, by setting the arithmetic mean roughness Ra of the roll surface in the final pass of cold rolling to 0.2 to 0.7 μm, a suitable unevenness can be formed on the surface of the steel sheet during cold rolling. The arithmetic mean roughness Ra of the roll is more preferably set to 0.3 μm or more.

[0191] Since conventional rolling mill rolls do not possess the aforementioned arithmetic mean roughness Ra, it is impossible to manufacture the steel sheet of this embodiment. To manufacture the steel sheet of this embodiment, it is preferable to use special rolling mill rolls in the final pass of cold rolling.

[0192] (IV) Reduction rate for quenching and tempering rolling: 0.3–0.7%; arithmetic mean surface roughness Ra of the rolls: 1.5–3.5 μm

[0193] In the quenching and tempering rolling process after annealing (or after plating if the material is coated), by setting the reduction rate to 0.3 to 0.7% and the arithmetic mean roughness Ra of the roll surface to 1.5 to 3.5 μm, it is possible to form unevenness on the surface of the steel sheet. More preferably, the reduction rate during quenching and tempering rolling is set to 0.5% or more, and the arithmetic mean roughness Ra of the roll surface is set to 2.3 μm or more.

[0194] (V) Heating temperature and holding time of slab: 5 hours or more in a temperature range above 1200°C.

[0195] Condition (V) is optional. By heating the slab in a temperature range of 1200°C or higher for 5 hours or more, it is possible to preferably control the 3σ / μ in the region from a position 1 / 8 of the slab thickness away from the surface of the steel sheet in the thickness direction to a position 3 / 8 of the slab thickness away from the surface of the steel sheet in the thickness direction (a region from 1 / 8 of the depth to 3 / 8 of the depth from the surface of the steel sheet). As a result, the generation of Mn segregation in the steel sheet can be further reduced, and pressed products with better appearance quality can be obtained.

[0196] (VI) Dew point: Residence time of steel plates in temperature ranges above -20°C and above 700°C: 50 to 400 seconds

[0197] Condition (VI) is optional. In this embodiment, the cold-rolled steel sheet obtained by the above method can also be annealed. By setting the dew point (average dew point in the annealing furnace) during annealing to -20°C or higher, and setting the residence time of the steel sheet in a temperature range of 700°C or higher to 50 to 400 seconds, the surface of the steel sheet can be stably decarburized. This allows a decarburized layer with a thickness of 30 μm or more to be formed on the surface of the steel sheet. It should be noted that the upper limit of the dew point does not need to be specifically defined, but it can be set to around 10°C.

[0198] There are no special restrictions beyond the conditions mentioned above, but the following conditions are preferred.

[0199] For the steel billet, the slab is heated to a temperature range of 1100°C or higher and then hot-rolled. After hot rolling, it is coiled and then pickled. After pickling, it is cold-rolled. The cumulative reduction rate during cold rolling is preferably set to 30-90%. After cold rolling, it is annealed. Then, the above-mentioned coating is formed as needed. Furthermore, it is preferable to perform temper rolling thereafter.

[0200] Next, the manufacturing method of the press-formed article according to this embodiment will be described. The press-forming method is not particularly limited. For example, for automotive panel components such as car door panels, the article can be formed by pressing a steel sheet with a blank holder and a die, then applying strain and stretching the steel sheet by contacting a punch. This type of forming is called deep drawing or bulging.

[0201] Example

[0202] Next, embodiments of the present invention will be described. However, the conditions in these embodiments are merely examples of conditions adopted to confirm the feasibility and effectiveness of the present invention. The present invention is not limited to these specific examples of conditions. Various conditions may be used in the present invention as long as they do not depart from the spirit and purpose of the invention.

[0203] Steel having the chemical composition shown in Table 1 is smelted and continuously cast to produce slabs with a thickness of 240–300 mm. Using the obtained slabs, cold-rolled steel sheets and clad steel sheets are manufactured under conditions (I) to (V) described later. It should be noted that in Table 2, "OK" is marked in the column for conditions that are met, and "NG" is marked in the column for conditions that are not met. Furthermore, the thickness of the obtained steel sheets and clad steel sheets is 0.2–2.0 mm.

[0204] In addition, annealing is performed after cold rolling.

[0205] Manufacturing conditions other than (I) to (VI) are set as follows: The slab is heated to a temperature range of 1100°C or higher and then hot-rolled. After hot rolling, it is coiled and then pickled. After pickling, it is cold-rolled with a cumulative reduction of 30% to 90%. After cold rolling, annealing is performed to form an alloyed hot-dip galvanized layer (GA), a hot-dip galvanized layer (GI), or an electroplated layer (EG), as needed. Then, temper rolling is performed.

[0206] Conditions (I) to (VI) in the table are as follows.

[0207] (I) Set the winding temperature to 550°C or higher.

[0208] (II) Set the pickling time to 50 seconds or more.

[0209] (III) Set the arithmetic mean roughness Ra of the roll surface of the final cold rolling pass to 0.2 to 0.7 μm.

[0210] (IV) Set the reduction rate of the quenching and tempering rolling to 0.3 to 0.7% and the arithmetic mean roughness Ra of the rolls to 1.5 to 3.5 μm.

[0211] (V) Heating the slab to a temperature range above 1200°C, and holding it in this temperature range for more than 5 hours.

[0212] (VI) Performing annealing in which the dew point (average dew point in the annealing furnace) is set to -20°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.

[0213] Subsequently, using the produced steel sheet and coated steel sheet, a substantially semi-cylindrical simulated component (pressed product) simulating an outer door panel of an automobile is manufactured by press forming. When press-forming the simulated component, the material (steel sheet or coated steel sheet) is actively caused to flow into the mold, such that at any position on the surface of the simulated component, the ratio of the strain in a direction perpendicular to an arbitrary direction along the surface of the simulated component to the strain in said arbitrary direction is approximately 1. That is, the press forming is performed in such a manner that no strain anisotropy occurs at any position on the surface of the simulated component.

[0214] For the obtained steel sheet and coated steel sheet, the arithmetic average waviness Wa, the average value μ and standard deviation σ of Mn concentration, the tensile strength, and the thickness of the decarburized layer are obtained by the method described above.

[0215] When the obtained tensile strength is 500 MPa or higher, it is judged as acceptable due to its high strength. On the other hand, when the obtained tensile strength is lower than 500 MPa, it is judged as unacceptable due to insufficient strength.

[0216] In addition, the appearance quality of the simulated component is evaluated by the following method.

[0217] The appearance quality is evaluated based on the degree of slip ridges generated on the surface of the formed simulated component. When the pressed surface is brought into contact with a grinding stone, the streak-like patterns with an interval of several millimeters generated on the surface are determined as slip ridges, and a score of 1 to 5 is given according to the generation degree of the streak-like patterns. An arbitrary area of 100 mm × 100 mm is checked visually: when no streak-like pattern is observed at all, the score is set to "1"; when the maximum length of the streak-like pattern is 20 mm or less, the score is set to "2"; when the maximum length of the streak-like pattern is more than 20 mm and 50 mm or less, the score is set to "3"; when the maximum length of the streak-like pattern is more than 50 mm and 70 mm or less, the score is set to "4"; when the maximum length of the streak-like pattern is more than 70 mm, the score is set to "5". When the evaluation is "3" or lower, it is judged as acceptable due to excellent appearance quality. On the other hand, when the evaluation is "4" or higher, it is judged as unacceptable due to poor appearance quality.

[0218] Furthermore, the appearance quality is evaluated more rigorously using the sum of the maximum mountain height Zp and the maximum valley height Zv of the waviness curve, i.e., Wz. Using the same method as when calculating the arithmetic mean waviness Wa, the waviness curve of the pressed molded part (simulated component) is obtained according to JIS B0601:2013. From this waviness curve, the maximum mountain height Zp and the maximum valley height Zv are calculated, and their sum is obtained to get Wz. When the obtained Wz is below 0.40 μm, the appearance quality is considered superior.

[0219]

[0220]

[0221] Observing Table 2, it is evident that the pressed-molded article of the present invention has high strength and excellent appearance quality. Furthermore, it is evident that the steel sheet of the present invention can be used to manufacture pressed-molded articles with high strength and excellent appearance quality. Moreover, it is evident that the present invention examples with 3σ / μ of 7.0 or less exhibit even better appearance quality after pressing.

[0222] On the other hand, it was found that the pressed-molded articles of the comparative examples had poor strength or deteriorated appearance quality. Furthermore, it was found that the steel plates of the comparative examples could not be used to manufacture pressed-molded articles with high strength and excellent appearance quality.

[0223] Industrial availability

[0224] According to the above-described solution of the present invention, it is possible to provide a high-strength, high-appearance-quality pressed molded article and a steel plate for manufacturing the pressed molded article.

Claims

1. A steel plate, characterized in that, The chemical composition, expressed as a percentage by mass, is as follows: C：0.040～0.100%、 Mn: 1.00~2.00% Si: 0.005~1.500%, P: Below 0.100% S: Below 0.0200% Al：0.005～0.700%、 N: below 0.0150% O: Below 0.0100% Cr：0～0.80%、 Mo: 0–0.16% B：0～0.0100%、 Ti: 0~0.100%, Nb: 0~0.060%, V：0～0.50%、 Ni: 0~1.00%, Cu: 0~1.00%, W：0～1.00%、 Sn: 0~1.00% Sb: 0~0.200%, Ca: 0~0.0100%, Mg: 0~0.0100%, Zr：0～0.0100%、 REM: 0~0.0100%, and Remaining components: Fe and impurities. The arithmetic mean waviness Wa of the steel plate is 0.10–0.30 μm. The average Mn concentration in the region from a position 1 / 8 of the thickness of the steel plate along the thickness direction to a position 3 / 8 of the thickness of the steel plate along the thickness direction is set as μ, and the standard deviation of the Mn concentration is set as σ, where (3σ / μ)×100≤7.

0.

2. The steel plate according to claim 1, characterized in that, The chemical composition, expressed as a percentage by mass, contains one or more elements selected from the group consisting of: Cr：0.01～0.80%、 Mo: 0.01–0.16% B：0.0001～0.0100%、 Ti: 0.001~0.100% Nb: 0.001~0.060%, V：0.01～0.50%、 Ni: 0.01~1.00%, Cu: 0.01~1.00%, W：0.01～1.00%、 Sn: 0.01~1.00% Sb: 0.001~0.200% Ca: 0.0001~0.0100% Mg: 0.0001~0.0100%, Zr: 0.0001~0.0100%, and REM: 0.0001~0.0100%.

3. The steel plate according to claim 1 or 2, characterized in that, The steel plate has a decarburized layer with a thickness of more than 20 μm on its surface.

4. The steel plate according to any one of claims 1 to 3, characterized in that, At least one surface of the steel plate has a coating.

5. A compressed molded article, characterized in that, It is obtained by pressing and forming the steel plate according to any one of claims 1 to 4.

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