Steel sheet and press-formed product

Abstract

The present invention employs a steel sheet and a press-formed product obtained by press-forming the steel sheet, the steel sheet having a chemical composition containing, 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 balance: Fe and impurities, a C content at a position 20 μm in depth from the surface, C 20 , a C content at a position 60 μm in depth from the above surface, C 60 , and ΔC calculated by the following formula (1) is 0.20 to 0.90 mass% / mm. ΔC = (C 60 -C 20 ) / (0.04) (1).

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 lightweighting and improved crash safety in automobiles. To address these requirements, research has been conducted on increasing the strength and thinning the walls of panel components such as door panels. Unlike frame components, these panel components are visible to the public and therefore require high aesthetic quality. Thus, even high-strength steel sheets previously used in frame components must achieve excellent appearance quality after forming when applied to panel components.

[0003] To improve appearance quality, suppressing 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 a steel sheet containing both hard and soft phases during pressing. Because these irregularities form a rib-like pattern on the surface, pressed products with ghost lines exhibit poor 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 a steel sheet (substrate) and a hot-dip galvanized layer 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 surface oxidation index A during annealing, defined by the mathematical formula A = 400Al / (4Cr+3Si+6Mn), with the contents of Al, Cr, Si, and Mn as terms of the same sign, is 2.3 or more. 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 high-strength cold-rolled steel sheet, a high-strength galvanized steel sheet, and a method for manufacturing the same, which have a tensile strength of 780 MPa or more on the surface and good formability.

[0006] Patent document 3 discloses a high-strength component for automobiles and a hot-pressing method thereof, wherein, in the method of forming a high-strength component for automobiles by hot pressing, it is possible to ensure that the sensitivity to hydrogen embrittlement caused by post-processing after hot pressing is not affected without dehydrogenation treatment.

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

[0008] Patent document 5 discloses a hot-pressed steel plate component with high strength and excellent impact characteristics, its manufacturing method, and the hot-pressed steel plate.

[0009] Patent document 6 discloses a hot-dip galvanized steel sheet with good tensile and bending properties, an alloyed hot-dip galvanized steel sheet, and a method for manufacturing the same.

[0010] Existing technical documents

[0011] Patent documents

[0012] Patent Document 1: Japanese Patent Application Publication No. 2005-220430

[0013] Patent Document 2: International Publication No. 2016-121388

[0014] Patent Document 3: Japanese Patent Application Publication No. 2006-104546

[0015] Patent Document 4: International Publication No. 2013-047820

[0016] Patent Document 5: International Publication No. 2015-097882

[0017] Patent Document 6: Japanese Patent Application Publication No. 2017-48412 Summary of the Invention

[0018] The problem that the invention aims to solve

[0019] The present invention was made in view of the above-mentioned facts. The object of the present invention is to provide a pressed-formed article with high strength (specifically, tensile strength: 500 MPa or more) and excellent appearance quality, and a steel sheet capable of manufacturing the pressed-formed article.

[0020] Methods for solving problems

[0021] The main points of this invention are as follows.

[0022] (1) The chemical composition of the steel plate of one embodiment of the present invention, in mass % is:

[0023] C: 0.040~0.105%

[0024] Mn: 1.00~2.30%

[0025] Si: 0.005~1.500%

[0026] Al: 0.005~0.700%

[0027] P: below 0.100%

[0028] S: Below 0.0200%

[0029] N: below 0.0150%

[0030] O: Below 0.0100%

[0031] Cr: 0–0.80%

[0032] Mo: 0–0.16%

[0033] Ti: 0~0.100%

[0034] B: 0~0.0100%

[0035] Nb: 0~0.060%

[0036] V: 0~0.50%

[0037] Ni: 0~1.00%

[0038] Cu: 0–1.00%

[0039] W: 0~1.00%

[0040] Sn: 0~1.00%

[0041] Sb: 0~0.200%

[0042] Ca: 0~0.0100%

[0043] Mg: 0~0.0100%

[0044] Zr: 0~0.0100%

[0045] REM: 0–0.0100%, and

[0046] Remaining components: Fe and impurities.

[0047] The C content at a depth of 20 μm from the surface, i.e., C 20 The C content at a depth of 60 μm from the above surface, i.e., C 60 The ΔC calculated by the following formula (1) is 0.20 to 0.90 mass% / mm.

[0048] ΔC=(C 60 -C 20 ) / (0.04) (1)

[0049] (2) The steel plate according to (1) above, wherein the above chemical composition may also contain one or more elements selected from the following elements in mass percent:

[0050] Cr: 0.01~0.80%

[0051] Mo: 0.01–0.16%

[0052] Ti: 0.001~0.100%

[0053] B: 0.0001~0.0100%

[0054] Nb: 0.001~0.060%

[0055] V: 0.01~0.50%

[0056] Ni: 0.01~1.00%

[0057] Cu: 0.01~1.00%

[0058] W: 0.01~1.00%

[0059] Sn: 0.01~1.00%

[0060] Sb: 0.001~0.200%

[0061] Ca: 0.0001~0.0100%

[0062] Mg: 0.0001~0.0100%

[0063] Zr: 0.0001~0.0100%, and

[0064] REM: 0.0001~0.0100%.

[0065] (3) The steel plate according to (1) or (2) above, wherein the above chemical composition may also be C: 0.040 to 0.080% by mass%.

[0066] (4) The steel plate according to any one of (1) to (3) above, wherein the above ΔC may also be 0.30 to 0.80 mass% / mm.

[0067] (5) The steel plate according to any one of (1) to (4) above may also have a coating on at least one side surface of the steel plate.

[0068] (6) The steel plate according to any one of (1) to (5) above may also have a tensile strength of 500 to 750 MPa.

[0069] (7) Another aspect of the present invention is a pressed-molded article obtained by pressing the steel plate described in any one of (1) to (6) above.

[0070] The C content at a depth of 20 μm from the surface, i.e., C 20 The C content at a depth of 60 μm from the above surface, i.e., C 60 The ΔC calculated by the following formula (1) is 0.20 to 0.90 mass% / mm.

[0071] ΔC=(C 60 -C 20 ) / (0.04) (1)

[0072] Invention Effects

[0073] According to the above-described solution of the present invention, it is possible to provide a pressed molded article with high strength and excellent appearance quality, and a steel plate capable of manufacturing the pressed molded article.

[0074] It should be noted that having excellent appearance quality means suppressing the formation of ghost lines. Detailed Implementation

[0075] The inventors of this invention have studied a method for suppressing the formation of ghost lines during the pressing and forming of high-strength steel sheets. As a result, the inventors recognized that reducing the hardness difference in steel is effective. The inventors further recognized that by decarburizing the surface layer of the steel sheet to form a homogeneous decarburized layer with a small hardness difference, the hardness difference in the steel can be reduced.

[0076] If decarburizing annealing is performed on a steel plate, the carbon content decreases from near the surface, forming a decarburized layer. The more intense the decarburizing conditions, the thicker the decarburized layer. The carbon concentration in the decarburized layer increases from near the steel plate surface towards the base material (the interior of the steel plate), but its upper limit becomes the carbon content of the base material. That is, the carbon concentration gradient from the surface to the interior of the steel plate depends on the decarburizing conditions and the carbon content of the steel plate.

[0077] Regions with low carbon concentration tend to become ferrite single-phase, thus softening the surface of the steel plate relative to its interior. It is believed that if the carbon concentration increases sharply towards the interior of the steel plate in the decarburized layer, the hardness difference increases, resulting in ghost lines after pressing. The inventors of this invention recognized that by setting the carbon concentration gradient in the decarburized layer to a desired range, the hardness difference within the decarburized layer can be reduced, thus suppressing the formation of ghost lines after pressing.

[0078] This invention is based on the above understanding, 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.

[0079] First, the chemical composition of the steel plate of this embodiment will be described. The values ​​listed below with “~” are defined within a range, including both the lower and upper limits. Values ​​expressed as “less than” or “more than” are not included in the numerical range. In the following description, “%” for chemical composition refers to “mass %” unless otherwise specified.

[0080] The chemical composition of the steel plate of this embodiment, by mass%, contains: C: 0.040–0.105%, Mn: 1.00–2.30%, Si: 0.005–1.500%, Al: 0.005–0.700%, 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. The details of each element are described below.

[0081] C: 0.040~0.105%

[0082] 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 increase the strength of the steel sheet, the C content is preferably 0.050% or more, more preferably 0.060% or more, or 0.070% or more.

[0083] Furthermore, by setting the C content to 0.105% or less, excessive hardness differences in the decarburized layer can be suppressed. As a result, the formation of ghost lines after pressing 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.

[0084] Mn: 1.00~2.30%

[0085] Mn is an element that improves the hardenability of steel and contributes 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 or 1.10% or more, and more preferably 1.20% or more, 1.30% or more or 1.40% or more.

[0086] Furthermore, by setting the Mn content to 2.30% or less, it is possible to suppress the formation of hardness differences in steel. Therefore, 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.

[0087] Si: 0.005~1.500%

[0088] Si is an element that forms coarse Si oxides, which act as the initiation point for fracture. 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 or 1.000% or less, more preferably 0.800% or less, 0.600% or less or 0.500% or less.

[0089] To improve the strength-formability balance of the steel sheet, the Si content is set to 0.005% or more. Preferably, the Si content is 0.010% or more or 0.020% or more.

[0090] Al: 0.005~0.700%

[0091] Al is an element that functions as a deoxidizer. Furthermore, Al also forms coarse oxides that act as initiators of fracture, embrittled the steel. By setting the Al content to 0.700% or less, the formation of coarse oxides that act as initiators of fracture can be suppressed, thus preventing the billet from becoming prone to cracking. Therefore, the Al content is set to 0.700% or less. The Al content is preferably 0.650% or less, 0.400% or less, or 0.200% or less, more preferably 0.100% or less, 0.080% or less, or 0.060% or less.

[0092] To fully realize the deoxidation effect brought about by Al, the Al content is set to 0.005% or more. The preferred Al content is 0.010% or more, 0.020% or more, 0.030% or more, or 0.040% or more.

[0093] P: below 0.100%

[0094] Phosphorus (P) is an element that is introduced as an impurity and can cause steel to become embrittled. If the P content is 0.100% or less, it can 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 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.

[0095] 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.

[0096] S: below 0.0200%

[0097] Sulfur (S) is an element that is introduced as an impurity and is also involved in the formation of Mn sulfides, which 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 or 0.0080% or less, more preferably 0.0060% or less or 0.0040% or less.

[0098] 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.

[0099] N: below 0.0150%

[0100] Nitrogen (N) is an element that is introduced as an impurity and forms nitrides, deteriorating 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 or 0.0100% or less, more preferably 0.0080% or less or 0.0060% or less.

[0101] 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.

[0102] O: Below 0.0100%

[0103] O is an element that is introduced as an impurity and also forms oxides, hindering the formability of steel sheets, such as ductility, porosity, tensile flange properties, and bending properties. If the O content is 0.0100% or less, it can prevent a significant decrease in the formability of the steel sheet. Therefore, the O content is set to 0.0100% or less. Preferably, it is 0.0080% or less or 0.0050% or less, more preferably 0.0030% or less or 0.0020% or less.

[0104] 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.

[0105] The steel plate of this embodiment may also contain the following elements as optional elements to replace a portion of Fe. The content of the following optional elements is 0% when they are not present.

[0106] Cr: 0–0.80%

[0107] Cr is an element that improves the hardenability of steel and contributes to the strength of steel plates. Cr may or may not be present, therefore the lower limit for Cr content includes 0%. To fully obtain the strength-enhancing effect of Cr, the Cr content is preferably 0.01% or more, or 0.20% or more, and more preferably 0.30% or more.

[0108] Furthermore, if the Cr content is 0.80% or less, the formation of coarse Cr carbides that can become the starting point for fracture can be suppressed. Therefore, the Cr content is set to 0.80% or less. In order to reduce the cost of the alloy, the Cr content is preferably set to 0.60% or less or 0.40% or less, and more preferably 0.20% or less, 0.10% or less or 0.06% or less.

[0109] Mo: 0–0.16%

[0110] Mo is an element that inhibits phase transformation at high temperatures and contributes to improving the strength of steel sheets. Mo may not necessarily be present, therefore the lower limit for Mo content includes 0%. To fully obtain the strength-enhancing effect brought by Mo, the Mo content is preferably 0.01% or more, or 0.05% or more, and more preferably 0.10% or more.

[0111] Furthermore, if the Mo content is 0.16% or less, it can suppress the reduction in hot workability and thus productivity. Therefore, the Mo content is set to 0.16% or less. In order to reduce alloy costs, the Mo content is preferably set to 0.12% or less or 0.08% or less, more preferably 0.06% or less, 0.04% or less or 0.02% or less.

[0112] Ti: 0~0.100%

[0113] Ti is an element that reduces the amount of S, N, and O that contribute to the formation of large inclusions that act as initiation points for fracture. Furthermore, Ti has the effect of refining the microstructure and improving the strength-formability balance of the steel sheet. Ti may not necessarily be present; therefore, 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 0.010% or more.

[0114] 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 0.075% or less or 0.060% or less, and more preferably to 0.040% or less or 0.020% or less.

[0115] B: 0~0.0100%

[0116] Boron (B) is an element that inhibits phase transformation at high temperatures and contributes to improving the strength of steel plates. Boron may not necessarily be present, therefore the lower limit for B content includes 0%. To fully obtain the strength-enhancing effect brought about by Bo, the B content is preferably 0.0001% or more, or 0.0005% or more, and more preferably 0.0010% or more.

[0117] Furthermore, if the boron content is 0.0100% or less, the formation of boron precipitates can be suppressed, thereby reducing the strength of the steel sheet. Therefore, the boron content is set to 0.0100% or less. In order to reduce alloy costs, the boron content is preferably set to 0.0080% or less or 0.0060% or less, and more preferably to 0.0040% or less, 0.0030% or less, or 0.0015% or less.

[0118] Nb: 0~0.060%

[0119] Nb is an element that contributes to improving the strength of steel sheets through strengthening by precipitates, grain refinement strengthening by inhibiting ferrite grain growth, and dislocation strengthening by inhibiting recrystallization. Nb may not necessarily be present; therefore, the lower limit for Nb content includes 0%. To fully obtain the above effects, the Nb content is preferably set to 0.001% or more, or 0.005% or more, and more preferably 0.010% or more.

[0120] 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, and more preferably 0.040%, 0.030%, or 0.015% or less.

[0121] V: 0~0.50%

[0122] V is an element that contributes to the strength improvement of steel sheets through strengthening by precipitates, grain refinement strengthening by inhibiting ferrite grain growth, and dislocation strengthening by inhibiting recrystallization. V may not necessarily be present, therefore the lower limit of V content includes 0%. To fully obtain the strength improvement effect brought about by V, the V content is preferably 0.01% or more, more preferably 0.03% or more.

[0123] Furthermore, if the V content is 0.50% or less, the excessive precipitation of carbonitrides can be suppressed, thereby reducing the formability of the steel sheet. Therefore, the V content is set to 0.50% or less. In order to reduce alloy costs, the V content is preferably set to 0.30% or less or 0.10% or less, and more preferably to 0.08%, 0.06%, or 0.03% or less.

[0124] Ni: 0~1.00%

[0125] Ni is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of steel sheets. Ni may not necessarily be present, therefore the lower limit for Ni content includes 0%. To fully obtain the strength-enhancing effect brought by Ni, the Ni content is preferably 0.01% or more, or 0.05% or more, and more preferably 0.20% or more.

[0126] Furthermore, if the Ni content is 1.00% or less, the reduction in weldability of the steel sheet can be suppressed. Therefore, the Ni content is set to 1.00% or less. In order to reduce alloy costs, the Ni content is preferably set to 0.70% or less or 0.50% or less, and more preferably to 0.30% or less, 0.15% or less or 0.08% or less.

[0127] Cu: 0~1.00%

[0128] Cu is an element that exists in steel in the form of fine particles and contributes to the strength of steel sheets. Cu may not necessarily be present, therefore 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 or 0.05% or more, and more preferably 0.15% or more.

[0129] Furthermore, if the Cu content is 1.00% or less, the reduction in weldability of the steel sheet can be suppressed. Therefore, the Cu content is set to 1.00% or less. In order to reduce alloy costs, the Cu content is preferably set to 0.70% or less or 0.50% or less, and more preferably to 0.30% or less, 0.15% or less or 0.08% or less.

[0130] W: 0~1.00%

[0131] W is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of steel plates. W may not necessarily be present, therefore the lower limit for W content includes 0%. To fully obtain the strength-enhancing effect brought about by W, the W content is preferably 0.01% or more, or 0.03% or more, and more preferably 0.10% or more.

[0132] Furthermore, if the W content is 1.00% or less, it can suppress the reduction in hot workability and thus productivity. Therefore, the W content is set to 1.00% or less. In order to reduce alloy costs, the W content is preferably set to 0.70% or less or 0.50% or less, and more preferably to 0.30% or less, 0.15% or less or 0.08% or less.

[0133] Sn: 0~1.00%

[0134] Sn is an element that inhibits grain coarsening and contributes to improving the strength of steel sheets. Sn may not necessarily be present, therefore the lower limit for Sn content includes 0%. To fully obtain the effects of Sn, the Sn content is more preferably 0.01% or higher.

[0135] Furthermore, if the Sn content is 1.00% or less, it can suppress the steel sheet from becoming embrittled and breaking during rolling. Therefore, the Sn content is set to 1.00% or less. In order to reduce alloy costs, the Sn content is preferably set to 0.70% or less or 0.50% or less, and more preferably to 0.30% or less, 0.15% or less or 0.08% or less.

[0136] Sb: 0~0.200%

[0137] Sb is an element that inhibits grain coarsening and contributes to improving the strength of steel plates. Sb may not necessarily be present, therefore the lower limit for Sb content includes 0%. To fully achieve the above effects, the Sb content is preferably 0.001% or more, or 0.005% or more.

[0138] Furthermore, if the Sb content is 0.200% or less, it can suppress the steel sheet from becoming embrittled and breaking during rolling. Therefore, the Sb content is set to 0.200% or less. To reduce alloy costs, the Sb content is preferably set to 0.100% or less or 0.050% or less, more preferably 0.030% or less, 0.010% or less, or 0.005% or less.

[0139] Ca: 0~0.0100%

[0140] Mg: 0~0.0100%

[0141] Zr: 0~0.0100%

[0142] REM: 0~0.0100%

[0143] Ca, Mg, Zr, and REM are elements that contribute to improving the formability of steel sheets. The presence of Ca, Mg, Zr, and REM is not mandatory; therefore, the lower limit for the content of these elements includes 0%. To fully achieve the effect of improving formability, the content of each of these elements is preferably 0.0001% or more, more preferably 0.0010% or more.

[0144] Furthermore, if the contents of Ca, Mg, Zr, and REM are all 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, they are 0.0050% or less or 0.0030% or less.

[0145] REM (Rare Earth Metals) refers to the group of elements belonging to the lanthanide series.

[0146] 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 and / or during the steelmaking process, or elements that are permissible within a range that does 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.

[0147] The chemical composition of the aforementioned steel plate can be determined using general analytical methods. For example, it can be determined using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Furthermore, C and S can be determined using combustion-infrared absorption spectroscopy, N can be determined using inert gas melting-thermal conductivity spectroscopy, and O can be determined using inert gas melting-non-dispersive infrared absorption spectroscopy.

[0148] When a steel plate has a coating on its surface, the chemical composition can be analyzed after removing the coating by mechanical grinding.

[0149] The C content at a depth of 20 μm from the surface, i.e., C 20 The C content at a depth of 60 μm from the above surface, i.e., C 60 ΔC calculated using the following formula (1): 0.20~0.90 mass% / mm

[0150] ΔC=(C 60 -C 20 ) / (0.04) (1)

[0151] ΔC represents the carbon concentration gradient in the decarburized layer formed on the surface, specifically in the region between a depth of 20 μm and a depth of 60 μm from the surface. By setting ΔC to 0.20–0.90 wt% / mm, a sharp increase in the carbon concentration gradient in the decarburized layer can be suppressed. As a result, ghost lines can be suppressed after pressing.

[0152] In the steel sheet having the chemical composition of this embodiment, a ΔC value below 0.20 wt% / mm indicates insufficient decarburization or excessive decarburization from the steel sheet surface to very deep depths. Insufficient decarburization leads to significant uneven hardness in the base material, making it difficult to suppress ghost lines. Conversely, excessive decarburization can result in softening, failing to achieve the desired steel sheet strength. Therefore, ΔC is set to 0.20 wt% / mm or higher. Furthermore, if ΔC exceeds 0.90 wt% / mm, the hardness difference within the decarburized layer becomes significant, making it difficult to suppress ghost lines. ΔC is preferably set to 0.30 wt% / mm or higher, 0.35 wt% / mm or higher, 0.40 wt% / mm or higher, or 0.45 wt% / mm or higher. Furthermore, ΔC is preferably set to 0.80 wt% / mm or lower, or 0.75 wt% / mm or lower.

[0153] It should be noted that when the steel plate has a coating on its surface, the "surface" in "the area at a depth of 20 μm from the surface" and "the area at a depth of 60 μm from the surface" refers to the interface between the coating and the base material. It should also be noted that when performing GDS analysis using the method described later to determine the Fe content from the surface, the depth where the Fe content is 95% by mass or higher is considered the interface between the coating and the base material.

[0154] Furthermore, the ΔC is specified at depths greater than 20 μm from the surface because C concentrations below 20 μm from the surface do not affect ghost lines.

[0155] ΔC is obtained through the following method.

[0156] For any three locations on the steel plate, the carbon content (mass%) was determined by glow discharge optical emission spectrometry (GDS analysis) from the surface of the steel plate along the depth direction (thickness direction) to 100 μm. The carbon content (C) at a depth of 20 μm from the surface was also measured. 20 C content at a depth of 60 μm from the surface (C 60 ΔC (mass% / mm) is calculated using equation (1) above. ΔC is obtained by calculating the average value of ΔC at the three locations.

[0157] For the determination, a Marcus-type high-frequency glow discharge luminescent surface analyzer (GD-Profiler) manufactured by Horiba Corporation was used.

[0158] The steel plate of this embodiment may also have a coating on at least one side surface. Examples of coatings include zinc coatings, zinc alloy coatings, alloyed zinc coatings, and alloyed zinc alloy coatings obtained by alloying these coatings.

[0159] 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.

[0160] 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.

[0161] 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.

[0162] Zinc coatings and zinc alloy coatings may also 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 the steel sheet.

[0163] 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 incorporated 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.

[0164] 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 inhibitors. The Fe content (mass%) in the resulting solution is determined by ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry).

[0165] The tensile strength (TS) of the steel plate in this embodiment is 500 MPa or higher. Alternatively, the tensile strength can be 500 to 750 MPa. By setting the tensile strength to 500 MPa or higher, the steel plate of this embodiment can be appropriately applied to panel system components such as door exterior panels. The tensile strength is preferably 550 MPa or higher, or 600 MPa or higher.

[0166] Furthermore, by setting the tensile strength to 750 MPa or less, it is possible to suppress the deterioration of the appearance after compression molding. The tensile strength is preferably 700 MPa or less.

[0167] Tensile strength was evaluated according to JIS Z 2241:2011. The test piece was set as test piece No. 5 of JIS Z 2241:2011. 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.

[0168] 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 flatness of the steel plate shape, thereby improving dimensional and shape accuracy. Therefore, a plate thickness of 0.2 mm or more is preferred. More preferably, it is 0.4 mm or more.

[0169] 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.

[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. Even after pressing, the C concentration gradient in the decarburized layer does not change; therefore, the C content of the pressed-molded article of this embodiment at a depth of 20 μm from the surface, i.e., C2, is... 20 The C content at a depth of 60 μm from the above surface, i.e., C60 The ΔC calculated by the following formula (1) is 0.20 to 0.90 mass% / mm.

[0171] ΔC=(C 60 -C 20 ) / (0.04) (1)

[0172] The aforementioned C concentration gradient is preferably set to 0.30 wt% / mm or more, 0.35 wt% / mm or more, 0.40 wt% / mm or more, or 0.45 wt% / mm or more, and preferably to 0.80 wt% / mm or less or 0.75 wt% / mm or less. It should be noted that the ΔC of the pressed molded article is obtained using the same method as for the steel sheet.

[0173] The pressed-molded article of this embodiment is obtained by pressing the aforementioned steel sheet, thus suppressing the formation of ghost lines and resulting in excellent appearance quality. Excellent appearance quality means that no striped 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.

[0174] Specific examples of pressed molded products include panel components such as door panels of automobile bodies.

[0175] Next, the manufacturing method of the steel plate according to this embodiment will be described.

[0176] The steel sheet of this embodiment is independent of the manufacturing method; its effects can be obtained as long as it has the above-described characteristics. However, by using steel with the above-described chemical composition and annealing it under the following conditions after hot rolling and cold rolling, it is possible to stably manufacture a steel sheet with preferably controlled ΔC (C concentration gradient).

[0177] (Annealing after hot rolling)

[0178] First, a hot-rolled steel sheet is obtained by hot rolling a slab having the above-mentioned chemical composition under general conditions. The obtained hot-rolled steel sheet is then subjected to a single annealing process in a high-temperature zone under atmospheric conditions. This single annealing is performed at an annealing temperature of 550–700°C for at least 2 hours. By annealing in a high-temperature zone after hot rolling, internal oxides of Si and Mn are formed in the surface layer of the steel sheet. As a result, surface concentration of Si and Mn can be suppressed during annealing after cold rolling, promoting decarburization. Therefore, ΔC can be preferably controlled.

[0179] If the annealing temperature is below 550°C or the annealing time is less than 2 hours, it is not possible to optimally control the ΔC of the steel plate.

[0180] After the aforementioned annealing, a steel sheet or strip of the desired thickness is manufactured by pickling followed by cold rolling with a cumulative reduction of 70% or more. By setting the cumulative reduction of cold rolling to 70% or more, austenite recrystallization is promoted during annealing after cold rolling, and the increase in the austenite fraction is suppressed. As a result, the fraction of ferrite with a high carbon diffusion coefficient increases during annealing after cold rolling, which promotes decarburization.

[0181] It should be noted that the cumulative reduction rate here is expressed as {1-(thickness of the plate after cold rolling / thickness of the plate before cold rolling)}×100(%).

[0182] By further performing a secondary annealing after cold rolling, a steel sheet with the desired mechanical properties is obtained. At this time, for example, by setting the dew point (average dew point in the annealing furnace) during secondary annealing to above -10°C and setting the residence time of the steel sheet in a temperature range above 700°C to 50–400 seconds, the surface of the steel sheet can be stably decarburized. The upper limit of the dew point does not need to be specifically set, but it can be set to around 10°C. If the dew point is too low or the residence time is too short, decarburization will not be sufficient, and ΔC cannot be optimally controlled. Furthermore, if the residence time is too long, sufficient tensile strength may not be obtained. It should be noted that the annealing temperature is, for example, around 750–850°C.

[0183] There are no special limitations beyond the conditions mentioned above, but the following conditions are preferred, for example.

[0184] The slab is heated to a temperature range above 1100°C and then hot-rolled. After hot rolling, it is coiled, annealed once, and then pickled. The finishing temperature of hot rolling is preferably above 900°C, and the coiling temperature is preferably below 650°C. After pickling, it is cold-rolled. Alternatively, a second annealing can be performed after cold rolling, followed by the formation of the aforementioned coating as needed.

[0185] Next, the manufacturing method of the pressed molded article according to this embodiment will be described.

[0186] To maintain the resulting microstructure and thus suppress ghost lines, the pressing forming method is preferably cold working. There are no particular limitations on the cold working method, but any method that forms the steel sheet by moving the die and punch relative to each other is acceptable.

[0187] Example

[0188] The following describes embodiments of the present invention. However, the conditions described in these embodiments are merely examples used to confirm the feasibility and effectiveness of the present invention. The present invention is not limited to these single examples. Various conditions may be used in the present invention as long as they do not depart from the spirit and purpose of the invention.

[0189] Steel with the chemical composition shown in Table 1 is smelted and continuously cast to produce slabs with a thickness of 240–300 mm. The resulting slabs are heated to a temperature range above 1100°C and then hot-rolled. After hot rolling, the slabs are coiled, annealed once under the conditions shown in Table 2, and then pickled. The finishing temperature of the hot rolling is set above 900°C, and the coiling temperature is set below 650°C. After pickling, cold rolling is performed with a cumulative reduction of 70–90%. After cold rolling, a second annealing is performed under the conditions shown in Table 2 to form an alloyed hot-dip galvanized layer (GA), a hot-dip galvanized layer (GI), or an electro-galvanized layer (EG), as needed. Through the above method, the steel sheets and coated steel sheets shown in Table 2 are obtained. It should be noted that the thickness of the obtained steel sheets and coated steel sheets is 0.2–2.0 mm.

[0190] After cold rolling and annealing, a roughly semi-cylindrical simulated component (press-formed product) simulating the outer panel of a door is manufactured by pressing using steel sheet and galvanized steel sheet. During the pressing of this simulated component, the material (steel sheet or galvanized steel sheet) is actively fed into the mold, and at any location on the surface of the simulated component, the ratio of strain in the direction perpendicular to any direction along the surface of the simulated component to strain in that direction (or any direction thereof) is set to approximately 1. That is, pressing is performed in an anisotropic manner, where no strain is generated at any location on the surface of the simulated component.

[0191] For the obtained steel sheet, coated steel sheet, and simulated part (pressed product), ΔC is calculated using the method described above. It should be noted that since the ΔC of the steel sheet and coated steel sheet is the same value as that of the simulated part, the ΔC of the simulated part is not recorded in the table.

[0192] Furthermore, the tensile strength of the steel sheet and the appearance quality of the simulated part were evaluated using the following methods. It should be noted that since there is no significant difference between the tensile strength of the steel sheet and the tensile strength of the simulated part (pressed product), the evaluation focused on whether the steel sheet possessed the expected tensile strength for a simulated part.

[0193] tensile strength

[0194] Tensile strength was evaluated according to JIS Z 2241:2011. The test piece was designated as test piece No. 5 according to JIS Z 2241:2011. The tensile test piece was collected at a distance of 1 / 4 of the distance from the end in the width direction of the plate, with the direction perpendicular to the rolling direction defined as the length direction. A tensile strength of 500 MPa or higher was considered high strength and deemed acceptable. Conversely, a tensile strength lower than 500 MPa was considered poor strength and deemed unacceptable.

[0195] Appearance quality

[0196] Appearance quality is evaluated by the degree of ghost lines appearing on the surface of the molded simulated part. The pressed surface is polished with a mold, and stripes spaced several millimeters apart are identified as ghost lines. These ghost lines are scored from 1 to 5 based on the degree of ghost lines. A 100mm x 100mm area is visually inspected. A score of "1" indicates no ghost lines are observed; "2" indicates a maximum ghost line length of 20mm or less; "3" indicates a maximum ghost line length exceeding 20mm but less than 50mm; "4" indicates a maximum ghost line length exceeding 50mm but less than 70mm; and "5" indicates a maximum ghost line length exceeding 70mm. A score of "3" or lower is considered excellent and deemed acceptable. Conversely, a score of "4" or higher is considered poor and deemed unacceptable.

[0197] [Table 1]

[0198]

[0199] [Table 2]

[0200]

[0201] The underlined part indicates that the invention is outside the scope of the invention or that the characteristics are not preferred.

[0202] As can be seen from Table 2, the pressed-molded article of this invention has high strength and excellent appearance quality. Furthermore, it can be seen that the steel plate of this invention can be manufactured into a pressed-molded article with high strength and excellent appearance quality.

[0203] On the other hand, it can be seen that the pressed-molded articles of the comparative examples have poor strength or deteriorated appearance quality. Furthermore, it can be seen that the steel plates of the comparative examples cannot be manufactured into pressed-molded articles with high strength and excellent appearance quality.

[0204] Industrial availability

[0205] According to the above-described solution of the present invention, it is possible to provide a pressed molded article with high strength and excellent appearance quality, and a steel plate capable of 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：0.040～0.105%、 Mn: 1.00~2.30%, Si: 0.005~1.500%, Al：0.005～0.700%、 P: Below 0.100% S: Below 0.0200% N: below 0.0150% O: Below 0.0100% Cr：0～0.80%、 Mo: 0–0.16% Ti: 0~0.100%, B：0～0.0100%、 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 carbon content (C) at a depth of 20 μm from the surface of the steel plate is... 20 The C content at a depth of 60 μm from the surface, i.e., C 60 The ΔC calculated by the following formula (1) is 0.20~0.90 mass% / mm. The tensile strength of the steel plate is 500–750 MPa. ΔC=(C 60 -C 20 ) / (0.04) (1)。 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 following: Cr：0.01～0.80%、 Mo: 0.01–0.16% Ti: 0.001~0.100% B：0.0001～0.0100%、 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 chemical composition, expressed as mass%, is C: 0.040–0.080%.

4. The steel plate according to claim 1 or 2, characterized in that, The ΔC is 0.30 to 0.80 mass% / mm.

5. The steel plate according to claim 1 or 2, characterized in that, The steel plate has a coating on at least one side surface.

6. A compressed molded article, characterized in that, It is a pressed-formed article obtained by pressing the steel plate according to any one of claims 1 to 5. The C content at a depth of 20 μm from the surface, i.e., C 20 The C content at a depth of 60 μm from the surface, i.e., C 60 The ΔC calculated by the following formula (1) is 0.20~0.90 mass% / mm. ΔC=(C 60 -C 20 ) / (0.04) (1)。

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