Steel plates and press-formed products

Abstract

Adopted are: a steel sheet having a chemical composition that comprises, by mass%, 0.040-0.105% of C, 1.00-2.30% of Mn, 0.005-1.500% of Si, 0.005-0.700% of Al, not more than 0.100% of P, not more than 0.0200% of S, not more than 0.0150% of N, and not more than 0.0100% of O, the remainder being Fe and impurities, and having a value of ΔC calculated from formula (1) below, where C20 is the content of C at a depth of 20μm from a surface and C60 is the content of C at a depth of 60 μm from said surface, that is 0.20-0.90 mass% / mm; and a press-molded article obtained by press-molding the steel sheet.　(1) ΔC = (C60-C20 / (0.04)

Description

Technical Field

[0001] The present invention relates to steel sheets and press-formed products.

Background Art

[0002] From the perspective of global environmental protection, there is a demand for weight reduction and improved collision safety in automobile bodies. To meet these requirements, high-strength and thin-wall designs are being considered for panel components such as door outsides. These panel components, unlike skeletal components, are visible to the human eye and thus require high appearance quality. Therefore, even high-strength steel sheets that have been conventionally applied to skeletal components need to have excellent appearance quality after forming when applied to panel components.

[0003] One issue in improving appearance quality is to suppress the occurrence of ghost lines. Ghost lines are minute irregularities on the order of several millimeters on the surface that occur when a steel sheet having a hard phase and a soft phase is press-formed, with the periphery of the soft phase deforming preferentially. Since these irregularities form streak patterns on the surface, press-formed products with ghost lines have inferior 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 contains, by mass%, C: 0.02 to 0.20%, Si: 0.7% or less, Mn: 1.5 to 3.5%, P: 0.10% or less, S: 0.01% or less, Al: 0.1 to 1.0%, N: 0.010% or less, Cr: 0.03 to 0.5%, and the annealing surface oxidation index A defined by the mathematical formula: A = 400Al / (4Cr + 3Si + 6Mn) with the contents of Al, Cr, Si, and Mn in the same terms is 2.3 or more, the balance being Fe and inevitable impurities, and further, the structure of the substrate consists of ferrite and a second phase, and the second phase is mainly martensite. A high-strength hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface of the substrate is disclosed.

[0005] Patent Document 2 discloses a high-strength cold-rolled steel sheet, a high-strength plated steel sheet, and a manufacturing method thereof, which have a tensile strength of 780 MPa or more in the surface layer and have good formability.

[0006] Patent Document 3 discloses a method for forming a high-strength member for an automobile by hot pressing, and discloses a high-strength member for an automobile and a hot pressing method thereof that can ensure hydrogen embrittlement susceptibility caused by post-processing after hot pressing without dehydrogenation treatment.

[0007] Patent Document 4 discloses a hot-dip galvanized steel sheet having a tensile strength (TS) of 980 MPa or more and excellent plating adhesion and stress corrosion cracking resistance, and a manufacturing method thereof.

[0008] Patent Document 5 discloses a hot-pressed steel sheet member, a manufacturing method thereof, and a hot-pressed steel sheet that can obtain excellent collision characteristics while having high strength.

[0009] Patent Document 6 discloses a hot-dip galvanized steel sheet, an alloyed hot-dip galvanized steel sheet, and a manufacturing method thereof, which have good elongation characteristics and bendability.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Summary of the Invention

Problems to be Solved by the Invention

[0011] The present invention has been made in view of the above circumstances. An object of the present invention is to provide a press-formed product having high strength (specifically, tensile strength: 500 MPa or more) and excellent appearance quality, and a steel sheet capable of manufacturing this press-formed product.

Means for Solving the Problems

[0012] The gist of the present invention is as follows. (1) The steel sheet according to one aspect of the present invention has a chemical composition in mass %, C: 0.040 to 0.105%, Mn: 1.00 to 2.30%, Si: 0.005 to 1.500%, Al: 0.005 to 0.700%, P: 0.100% or less, S: 0.0200% or less, N: 0.0150% or less, O: 0.0100% or less, Cr: 0 to 0.80%, Mo: 0 to 0.16%, Ti: 0 to 0.100%, B: 0 to 0.0100%, Nb: 0 to 0.060%, V: 0 to 0.50%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, W: 0 to 1.00%, Sn: 0 to 1.00%, Sb: 0 to 0.200%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, REM: 0 to 0.0100%, and the balance: Fe and impurities, Of the steel plate C content at a depth of 20 μm from the surface, C 20and C, which is the C content at a depth of 60 μm from the surface 60 and ΔC calculated from the following formula (1) is 0.20 to 0.90 mass% / mm , the tensile strength is 500 to 750 MPa is satisfied. ΔC = (C 60 - C 20 ) / (0.04) …(1) (2) The steel sheet according to (1) above, wherein the chemical composition is, in mass%, Cr: 0.01 to 0.80%, Mo: 0.01 to 0.16%, Ti: 0.001 to 0.100%, B: 0.0001 to 0.0100%, Nb: 0.001 to 0.060%, V: 0.01 to 0.50%, Ni: 0.01 to 1.00%, Cu: 0.01 to 1.00%, W: 0.01 to 1.00%, Sn: 0.01 to 1.00%, Sb: 0.001 to 0.200%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Zr: 0.0001 to 0.0100%, and REM: 0.0001 to 0.0100% may contain one or more selected from the group consisting of. (3) The steel sheet according to (1) or (2) above, wherein the chemical composition may be C: 0.040 to 0.080% in mass%. (4) The steel sheet according to any one of (1) to (3) above, wherein ΔC may be 0.30 to 0.80 mass% / mm. (5) The steel sheet according to any one of (1) to (4) above may have a plating layer on at least one surface of the steel sheet. ( 6 ) A press-formed product according to another aspect of the present invention is a press-formed product obtained by press-forming the steel sheet according to any one of (1) to ( 5 ), and C, which is the C content at a depth of 20 μm from the surface 20 and C, which is the C content at a depth of 60 μm from the surface 60 satisfy that ΔC calculated from the following formula (1) is 0.20 to 0.90 mass% / mm. ΔC=(C 60 -C 20 ) / (0.04) …(1)

Advantages of the Invention

[0013] According to the above aspect of the present invention, it is possible to provide a press-formed product having high strength and excellent appearance quality, and a steel sheet capable of manufacturing this press-formed product. Note that having excellent appearance quality means that the generation of ghost lines is suppressed.

Embodiments for Carrying Out the Invention

[0014] The inventor has studied a method for suppressing the generation of ghost lines when press-forming a high-strength steel sheet. As a result, the inventor has found that it is effective to reduce the hardness difference in the steel. The inventor has found that the hardness difference in the steel can be reduced by decarburizing the surface layer of the steel sheet to form a homogeneous decarburized layer with a small hardness difference.

[0015] When the steel sheet is subjected to decarburizing annealing, the C content decreases in the region near the surface, and a decarburized layer is formed. The stronger the decarburizing conditions, the greater the thickness of the decarburized layer. The C concentration in the decarburized layer increases from the region near the surface of the steel sheet toward the base metal side (inside the steel sheet), but its upper limit is the C content of the base metal. That is, the C concentration gradient from the surface to the inside of the steel sheet depends on the decarburizing conditions and the C content of the steel sheet.

[0016] Regions with low C concentration tend to become a ferrite single phase, so the surface of the steel sheet softens compared to the interior of the steel sheet. In the decarburized layer, as the C concentration rapidly increases towards the interior of the steel sheet, the hardness difference increases, so it is considered that ghost lines occur after press forming. The inventor has found that by setting the C concentration gradient in the decarburized layer within a desired range, the hardness difference within the decarburized layer can be reduced and the occurrence of ghost lines after press forming can be suppressed.

[0017] The present invention has been made based on the above findings, and the steel sheet and press-formed product according to the present embodiment will be described in detail below. However, the present invention is not limited only to the configurations disclosed in the present embodiment, and various modifications are possible without departing from the spirit of the present invention.

[0018] First, the chemical composition of the steel sheet according to the present embodiment will be described. In the numerical limit ranges described with "~" in between below, the lower limit value and the upper limit value are included in the range. The numerical values indicated as "less than" or "exceeding" are not included in the numerical range. In the following description, % regarding the chemical composition is mass % unless otherwise specified.

[0019] The steel sheet according to the present embodiment has a chemical composition, by mass %, of C: 0.040 to 0.105%, Mn: 1.00 to 2.30%, Si: 0.005 to 1.500%, Al: 0.005 to 0.700%, P: 0.100% or less, S: 0.0200% or less, N: 0.0150% or less, O: 0.0100% or less, and the balance: Fe and impurities. Each element will be described below.

[0020] C: 0.040 to 0.105% C is an element that increases the strength of the steel sheet and the press-formed product. In order to obtain a desired strength, the C content is 0.040% or more. In order 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. Moreover, by setting the C content to 0.105% or less, the generation of excessive hardness differences in the decarburized layer can be suppressed. As a result, the generation of ghost lines after press forming can be suppressed. Therefore, the C content is set to 0.105% or less. The C content is preferably 0.090% or less, more preferably 0.080% or less.

[0021] Mn: 1.00 - 2.30% Mn is an element that enhances the hardenability of steel and contributes to the improvement of 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, more preferably 1.20% or more, 1.30% or more, or 1.40% or more. Moreover, by setting the Mn content to 2.30% or less, it is possible to suppress the occurrence of hardness differences in the 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, more preferably 1.90% or less, 1.80% or less, or 1.70% or less.

[0022] Si: 0.005 - 1.500% Si is an element that forms coarse SiO₂ that acts as a crack initiation point. By setting the Si content to 1.500% or less, the formation of SiO₂ can be suppressed, making cracking less likely to occur. As a result, embrittlement of the steel can be suppressed. Therefore, the Si content is set to 1.500% or less. The Si content is preferably 1.300% or less or 1.000% or less, more preferably 0.800% or less, 0.600% or less, or 0.500% or less. The Si content is set to 0.005% or more to improve the strength - formability balance of the steel sheet. The Si content is preferably 0.010% or more or 0.020% or more.

[0023] Al: 0.005 - 0.700% Al is an element that functions as a deoxidizer. Also, Al is an element that forms coarse oxides which serve as the starting points of fracture and embrittle the steel. By setting the Al content to 0.700% or less, the formation of coarse oxides that act as the starting points of fracture can be suppressed, and the tendency for the slab to crack can be inhibited. 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, and more preferably 0.100% or less, 0.080% or less, or 0.060% or less. The Al content is set to 0.005% or more in order to sufficiently obtain the deoxidizing effect of Al. The Al content is preferably 0.010% or more, 0.020% or more, 0.030% or more, or 0.040% or more.

[0024] P: 0.100% or less P is an element that is mixed in as an impurity and is also an element that embrittles the steel. When the P content is 0.100% or less, the embrittlement of the steel plate and the tendency to crack in the production process can be suppressed. Therefore, the P content is set to 0.100% or less. From the perspective of productivity, the P content is preferably 0.050% or less, and more preferably 0.030% or less or 0.020% or less. The lower limit of the P content includes 0%, but by setting the P content to 0.001% or more, the manufacturing cost can be further reduced. Therefore, the P content may also be set to 0.001% or more.

[0025] S: 0.0200% or less S is an element that is mixed in as an impurity and is also an element that forms Mn sulfide and degrades the formability of the steel plate, such as ductility, hole expansion property, elongation flange property, and bendability. When the S content is 0.0200% or less, the significant degradation of the formability of the steel plate can be suppressed. Therefore, the S content is set to 0.0200% or less. The S content is preferably 0.0100% or less or 0.0080% or less, and more preferably 0.0060% or less or 0.0040% or less. The lower limit of the S content includes 0%, but by setting the S content to 0.0001% or more, the manufacturing cost can be further reduced. Therefore, the S content may also be set to 0.0001% or more.

[0026] N: Below 0.0150% N is an element that mixes in as an impurity, forms nitrides, and is also an element that degrades the formability such as the ductility, hole expandability, stretch flange formability, and bendability of the steel sheet. When the N content is 0.0150% or less, it is possible to suppress a decrease in the formability of the steel sheet. Therefore, the N content is set to 0.0150% or less. Further, N is also an element that generates welding defects during welding and inhibits productivity. Therefore, the N content is preferably 0.0120% or less or 0.0100% or less, and more preferably 0.0080% or less or 0.0060% or less. The lower limit of the N content includes 0%, but by setting the N content to 0.0005% or more, the manufacturing cost can be further reduced. Therefore, the N content may be 0.0005% or more.

[0027] O: 0.0100% or less O is an element that mixes in as an impurity, forms oxides, and is also an element that inhibits the formability such as the ductility, hole expandability, stretch flange formability, and bendability of the steel sheet. When the O content is 0.0100% or less, it is possible to suppress a significant decrease in the formability of the steel sheet. Therefore, the O content is set to 0.0100% or less. Preferably 0.0080% or less or 0.0050% or less, more preferably 0.0030% or less or 0.0020% or less. The lower limit of the O content includes 0%, but by setting the O content to 0.0001% or more, the manufacturing cost can be further reduced. Therefore, the O content may be 0.0001% or more.

[0028] The steel sheet according to the present embodiment may contain the following elements as optional elements instead of a part of Fe. The content when not containing the following optional elements is 0%.

[0029] Cr: 0 to 0.80% Cr is an element that enhances the hardenability of steel and contributes to the improvement of the strength of the steel plate. Since it is not necessarily required to contain Cr, the lower limit of the Cr content includes 0%. In order to sufficiently obtain the strength improvement effect by Cr, the Cr content is preferably 0.01% or more or 0.20% or more, and more preferably 0.30% or more. Also, when the Cr content is 0.80% or less, the formation of coarse Cr carbides that can become the starting point of fracture can be suppressed. Therefore, the Cr content is set to 0.80% or less. For the reduction of alloy cost, the Cr content is preferably 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.

[0030] Mo: 0 - 0.16% Mo is an element that suppresses the phase transformation at high temperatures and contributes to the improvement of the strength of the steel plate. Since it is not necessarily required to contain Mo, the lower limit of the Mo content includes 0%. In order to sufficiently obtain the strength improvement effect by Mo, the Mo content is preferably 0.01% or more or 0.05% or more, and more preferably 0.10% or more. Also, when the Mo content is 0.16% or less, it is possible to suppress the decrease in hot workability and the resulting decrease in productivity. Therefore, the Mo content is set to 0.16% or less. For the reduction of alloy cost, the Mo content is preferably 0.12% or less or 0.08% or less, and more preferably 0.06% or less, 0.04% or less or 0.02% or less.

[0031] Ti: 0 - 0.100% Ti is an element that has the effect of reducing the amounts of S, N, and O that generate coarse inclusions acting as the starting point of fracture. Also, Ti has the effect of refining the structure and enhancing the strength-formability balance of the steel plate. Since it is not necessarily required to contain Ti, the lower limit of the Ti content includes 0%. In order to sufficiently obtain the above effects, the Ti content is preferably 0.001% or more, and more preferably 0.010% or more. In addition, when the Ti content is 0.100% or less, the formation of coarse Ti sulfide, Ti nitride, and Ti oxide can be suppressed, and the formability of the steel sheet can be ensured. Therefore, the Ti content is set to 0.100% or less. The Ti content is preferably 0.075% or less or 0.060% or less, and more preferably 0.040% or less or 0.020% or less.

[0032] B: 0 to 0.0100% B is an element that suppresses phase transformation at high temperatures and contributes to the improvement of the strength of the steel sheet. Since B does not necessarily have to be contained, the lower limit of the B content includes 0%. In order to sufficiently obtain the strength improvement effect by B, the B content is preferably 0.0001% or more or 0.0005% or more, and more preferably 0.0010% or more. In addition, when the B content is 0.0100% or less, the generation of B precipitates and the decrease in the strength of the steel sheet can be suppressed. Therefore, the B content is set to 0.0100% or less. For reducing the alloy cost, the B content is preferably 0.0080% or less or 0.0060% or less, and more preferably 0.0040% or less, 0.0030% or less, or 0.0015% or less.

[0033] Nb: 0 to 0.060% Nb is an element that contributes to the improvement of the strength of the steel sheet by precipitation strengthening, grain refinement strengthening by suppressing the growth of ferrite crystal grains, and dislocation strengthening by suppressing recrystallization. Since Nb does not necessarily have to be contained, the lower limit of the Nb content includes 0%. In order to sufficiently obtain the above effects, the Nb content is preferably 0.001% or more or 0.005% or more, and more preferably 0.010% or more. In addition, when the Nb content is 0.060% or less, recrystallization can be promoted and the remaining of unrecrystallized ferrite can be suppressed, and the formability of the steel sheet can be ensured. 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% or less, 0.030% or less, 0.015% or less.

[0034] V: 0 to 0.50% V contributes to the improvement of the strength of the steel sheet by precipitation strengthening, grain refinement strengthening by suppressing the growth of ferrite grains, and dislocation strengthening by suppressing recrystallization. Since V does not necessarily have to be contained, the lower limit of the V content includes 0%. In order to sufficiently obtain the strength improvement effect by V, the V content is preferably 0.01% or more, and more preferably 0.03% or more. Also, when the V content is 0.50% or less, it is possible to suppress the decrease in the formability of the steel sheet due to the precipitation of a large amount of carbonitrides. Therefore, the V content is set to 0.50% or less. In order to reduce the alloy cost, the V content is preferably 0.30% or less or 0.10% or less, and more preferably 0.08% or less, 0.06% or less, or 0.03% or less.

[0035] Ni: 0 to 1.00% Ni suppresses the phase transformation at high temperatures and contributes to the improvement of the strength of the steel sheet. Since Ni does not necessarily have to be contained, the lower limit of the Ni content includes 0%. In order to sufficiently obtain the strength improvement effect by Ni, the Ni content is preferably 0.01% or more or 0.05% or more, and more preferably 0.20% or more. Also, when the Ni content is 1.00% or less, it is possible to suppress the decrease in the weldability of the steel sheet. Therefore, the Ni content is set to 1.00% or less. In order to reduce the alloy cost, the Ni content is preferably 0.70% or less or 0.50% or less, and more preferably 0.30% or less, 0.15% or less, or 0.08% or less.

[0036] Cu: 0 to 1.00% Cu exists in the steel in the form of fine particles and contributes to the improvement of the strength of the steel sheet. Since Cu does not necessarily have to be contained, the lower limit of the Cu content includes 0%. In order to sufficiently obtain the strength improvement effect by Cu, the Cu content is preferably 0.01% or more or 0.05% or more, and more preferably 0.15% or more. Also, when the Cu content is 1.00% or less, it is possible to suppress a decrease in the weldability of the steel sheet. Therefore, the Cu content is set to 1.00% or less. In order to reduce the alloy cost, the Cu content is preferably 0.70% or less or 0.50% or less, and more preferably 0.30% or less, 0.15% or less, or 0.08% or less.

[0037] W: 0 to 1.00% W is an element that suppresses the phase transformation at high temperatures and contributes to the improvement of the strength of the steel sheet. Since W does not necessarily have to be contained, the lower limit of the W content includes 0%. In order to sufficiently obtain the strength improvement effect by W, the W content is preferably 0.01% or more or 0.03% or more, and more preferably 0.10% or more. Also, when the W content is 1.00% or less, it is possible to suppress a decrease in hot workability and a decrease in productivity. Therefore, the W content is set to 1.00% or less. In order to reduce the alloy cost, the W content is preferably 0.70% or less or 0.50% or less, and more preferably 0.30% or less, 0.15% or less, or 0.08% or less.

[0038] Sn: 0 to 1.00% Sn is an element that suppresses the coarsening of crystal grains and contributes to the improvement of the strength of the steel sheet. Since Sn does not necessarily have to be contained, the lower limit of the Sn content includes 0%. In order to sufficiently obtain the effect by Sn, the Sn content is more preferably 0.01% or more. Also, when the Sn content is 1.00% or less, it is possible to suppress embrittlement of the steel sheet and breakage during rolling. Therefore, the Sn content is set to 1.00% or less. In order to reduce the alloy cost, the Sn content is preferably 0.70% or less or 0.50% or less, and more preferably 0.30% or less, 0.15% or less, or 0.08% or less.

[0039] Sb: 0 to 0.200% Sb is an element that suppresses the coarsening of crystal grains and contributes to the improvement of the strength of the steel sheet. Since it is not necessarily required to contain Sb, the lower limit of the Sb content includes 0%. In order to sufficiently obtain the above effects, the Sb content is preferably 0.001% or more or 0.005% or more. Also, when the Sb content is 0.200% or less, it is possible to suppress embrittlement of the steel sheet and breakage during rolling. Therefore, the Sb content is set to 0.200% or less. For the reduction of alloy cost, the Sb content is preferably 0.100% or less or 0.050% or less, and more preferably 0.030% or less, 0.010% or less or 0.005% or less.

[0040] Ca: 0 to 0.0100% Mg: 0 to 0.0100% Zr: 0 to 0.0100% REM: 0 to 0.0100% Ca, Mg, Zr, and REM are elements that contribute to the improvement of the formability of the steel sheet. Since it is not necessarily required to contain Ca, Mg, Zr, and REM, the lower limit of the content of these elements includes 0%. In order to sufficiently obtain the formability improvement effect, the content of each of these elements is preferably 0.0001% or more, and more preferably 0.0010% or more. Also, when the content of Ca, Mg, Zr, and REM is 0.0100% or less respectively, it is possible to suppress a decrease in the ductility of the steel sheet. Therefore, the content of each of these elements is set to 0.0100% or less. Preferably, it is 0.0050% or less or 0.0030% or less. REM (Rare Earth Metal) means a group of elements belonging to the lanthanoid series.

[0041] The balance of the chemical composition of the steel sheet according to this embodiment may be Fe and impurities. Examples of the impurities include those inevitably mixed from steel raw materials or scraps and / or in the steelmaking process, or elements that are allowed within a range that does not inhibit the properties of the steel sheet according to this embodiment. Examples of the impurities include H, Na, Cl, Co, Zn, Ga, Ge, As, Se, Y, Tc, Ru, Rh, Pd, Ag, Cd, In, Te, Cs, Ta, Re, Os, Ir, Pt, Au, Pb, Bi, and Po. The impurities may be contained in a total amount of 0.100% or less.

[0042] The chemical composition of the steel sheet described above may be measured by a general analysis method. For example, it may be measured using ICP - AES (Inductively Coupled Plasma - Atomic Emission Spectrometry). Note that C and S may be measured using the combustion - infrared absorption method, N may be measured using the inert gas fusion - thermal conductivity method, and O may be measured using the inert gas fusion - non - dispersive infrared absorption method. When the steel sheet has a plating layer on its surface, the plating layer on the surface may be removed by mechanical grinding, and then the chemical composition may be analyzed.

[0043] C, which is the C content at a depth of 20 μm from the surface 20 and C, which is the C content at a depth of 60 μm from the surface 60 and ΔC calculated from the following formula (1): 0.20 - 0.90 mass% / mm ΔC=(C 60 -C 20 ) / (0.04) …(1) ΔC indicates the C - concentration gradient in the region from a depth of 20 μm from the surface to a depth of 60 μm from the surface in the decarburized layer formed on the surface layer. By setting ΔC to 0.20 - 0.90 mass% / mm, a rapid increase in the C - concentration gradient in the decarburized layer can be suppressed. As a result, the generation of ghost lines after press forming can be suppressed.

[0044] In the steel sheet having the chemical composition of this embodiment, when ΔC is less than 0.20% by mass / mm, it means that decarburization has not occurred sufficiently or decarburization has progressed excessively from the steel sheet surface to a very deep position. When decarburization has not occurred sufficiently, the influence of the hardness variation of the base material becomes significant and it becomes difficult to suppress the generation of ghost lines. On the other hand, when excessive decarburization occurs, softening progresses and the desired steel sheet strength may not be obtained. Therefore, ΔC should be 0.20% by mass / mm or more. Also, when ΔC exceeds 0.90% by mass / mm, the hardness difference in the decarburized layer becomes significant and it becomes difficult to suppress the generation of ghost lines. It is preferable that ΔC is 0.30% by mass / mm or more, 0.35% by mass / mm or more, 0.40% by mass / mm or more, or 0.45% by mass / mm or more. Also, it is preferable that ΔC is 0.80% by mass / mm or less or 0.75% by mass / mm or less.

[0045] When the steel sheet has a plating layer on its surface, the "surface" in the "depth position 20 μm from the surface" and the "region at a depth position 60 μm from the surface" refers to the interface between the plating layer and the base material. When GDS analysis is performed by the method described later and the Fe content is measured from the surface, the depth position at which the Fe content becomes 95% by mass or more is regarded as the interface between the plating layer and the base material. Also, the reason for defining ΔC at a depth position of 20 μm or more from the surface is that the C concentration less than 20 μm from the surface does not affect the ghost lines.

[0046] ΔC is obtained by the following method. For any three locations on the steel sheet, the C content (% by mass) is measured from the surface of the steel sheet in the depth direction (plate thickness direction) up to 100 μm by glow discharge optical emission spectrometry (GDS analysis). The C content (C 20 ) at a depth position 20 μm from the surface and the C content (C 60 ) at a depth position 60 μm from the surface, and from the above formula (1), ΔC (% by mass / mm) is calculated. ΔC is obtained by calculating the average value of ΔC at the three locations. For the measurement, a Marcus type high-frequency glow discharge optical emission surface analyzer (GD-Profiler) manufactured by Horiba, Ltd. is used.

[0047] The steel sheet according to this embodiment may have a plating layer on at least one surface of the steel sheet. Examples of the plating layer include a zinc plating layer, a zinc alloy plating layer, an alloyed zinc plating layer obtained by subjecting these to an alloying treatment, and an alloyed zinc alloy plating layer.

[0048] The zinc plating layer and the zinc alloy plating layer are formed by a hot-dip plating method, an electroplating method, or a vapor deposition plating method. When the Al content of the zinc plating layer is 0.5 mass% or less, sufficient adhesion between the surface of the steel sheet and the zinc plating layer can be ensured. Therefore, the Al content of the zinc plating layer is preferably 0.5 mass% or less.

[0049] When the zinc plating layer is a hot-dip zinc plating layer, the Fe content of the hot-dip zinc plating layer is preferably 3.0 mass% or less in order to enhance the adhesion between the steel sheet surface and the zinc plating layer. When the zinc plating layer is an electroplated zinc plating layer, the Fe content of the electroplated zinc plating layer is preferably 0.5 mass% or less from the viewpoint of improving corrosion resistance.

[0050] The zinc plating layer and the zinc alloy plating layer may contain one or more of Al, Ag, B, Be, Bi, Ca, Cd, Co, Cr, Cs, Cu, Ge, Hf, Zr, I, K, La, Li, Mg, Mn, Mo, Na, Nb, Ni, Pb, Rb, Sb, Si, Sn, Sr, Ta, Ti, V, W, Zr, REM as long as they do not impair the corrosion resistance and formability of the steel sheet. In particular, Ni, Al, and Mg are effective in improving the corrosion resistance of the steel sheet.

[0051] The zinc plating layer or zinc alloy plating layer may be an alloyed zinc plating layer or an alloyed zinc alloy plating layer subjected to an alloying treatment. When an alloying treatment is applied to the hot-dip zinc plating layer or hot-dip zinc alloy plating layer, from the viewpoint of improving the adhesion between the steel sheet surface and the alloyed plating layer, the Fe content of the hot-dip zinc plating layer (alloyed zinc plating layer) or hot-dip zinc alloy plating layer (alloyed zinc alloy plating layer) after the alloying treatment is preferably 7.0 to 13.0 mass%. By applying an alloying treatment to a steel sheet having a hot-dip zinc plating layer or a hot-dip zinc alloy plating layer, Fe is incorporated into the plating layer, and the Fe content increases. Thereby, the Fe content can be made 7.0 mass% or more. That is, a zinc plating layer having an Fe content of 7.0 mass% or more is an alloyed zinc plating layer or an alloyed zinc alloy plating layer.

[0052] The Fe content in the plating layer can be obtained by the following method. Only the plating layer is dissolved and removed using a 5 vol% HCl aqueous solution added with an inhibitor. The Fe content in the plating layer (mass%) is obtained by measuring the Fe content in the obtained dissolution solution using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry).

[0053] The tensile strength (TS) of the steel sheet according to this embodiment is 500 MPa or more. Also, the tensile strength may be 500 to 750 MPa. By setting the tensile strength to 500 MPa or more, the steel sheet according to this embodiment can be suitably applied to panel parts such as door outsides. The tensile strength is preferably 550 MPa or more or 600 MPa or more. Also, by setting the tensile strength to 750 MPa or less, it is possible to suppress deterioration of the appearance after press forming. The tensile strength is preferably 700 MPa or less.

[0054] The tensile strength is evaluated in accordance with JIS Z 2241:2011. The test piece shall be a No. 5 test piece of JIS Z 2241:2011. The sampling position of the tensile test piece shall be the 1 / 4 part from the end in the sheet width direction, and the direction perpendicular to the rolling direction shall be the longitudinal direction.

[0055] The plate thickness of the steel sheet according to this embodiment is not limited to a specific range, but considering versatility and manufacturability, 0.2 to 2.0 mm is preferable. By setting the plate thickness to 0.2 mm or more, it becomes easy to maintain the flatness of the steel sheet shape, and the dimensional accuracy 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. On the other hand, when the plate thickness is 2.0 mm or less, it becomes easy to apply appropriate strain and control the temperature during the manufacturing process, and a homogeneous structure can be obtained. Therefore, the plate thickness is preferably 2.0 mm or less. More preferably, it is 1.5 mm or less.

[0056] Next, a press-formed product according to this embodiment, which can be manufactured by press-forming the above-described steel sheet, will be described. The press-formed product according to this embodiment has the same chemical composition as the above-described steel sheet. Further, the press-formed product according to this embodiment may be provided with the above-described plating layer on at least one surface. Since the C concentration gradient in the decarburized layer does not change even after press-forming, the press-formed product according to this embodiment has a C content at a depth of 20 μm from the surface, which is C 20 and a C content at a depth of 60 μm from the surface, which is C 60 and ΔC calculated from the following formula (1) is 0.20 to 0.90 mass% / mm. ΔC=(C 60 -C 20 ) / (0.04) …(1)

[0057] The above C concentration gradient is preferably 0.30 mass% / mm or more, 0.35 mass% / mm or more, 0.40 mass% / mm or more, or 0.45 mass% / mm or more, and preferably 0.80 mass% / mm or less or 0.75 mass% / mm or less. Note that ΔC of the press-formed product is obtained by the same method as that of the steel sheet.

[0058] Since the press-formed product according to this embodiment is obtained by press-forming the above-described steel sheet, the generation of ghost lines is suppressed, and the appearance quality is excellent. Excellent appearance quality means that stripe patterns (i.e., ghost lines) with an interval of several millimeters generated on the surface are not observed. More specifically, when any area of 100 mm × 100 mm is visually confirmed, the maximum length of the streak pattern with an interval of several millimeters is 50 mm or less. The maximum length of the streak pattern is preferably 20 mm or less. Further, it is more preferable that no streak pattern is observed at all.

[0059] As a specific example of the press-formed product, for example, panel parts such as the outer door of an automobile body can be mentioned.

[0060] Next, the manufacturing method of the steel sheet according to this embodiment will be described. The steel sheet according to this embodiment can obtain the above effects as long as it has the above characteristics regardless of the manufacturing method. However, by using the steel having the above-described chemical composition and performing annealing under the following conditions after hot rolling and cold rolling, a steel sheet with preferably controlled ΔC (C concentration gradient) can be stably manufactured.

[0061] (Annealing after hot rolling) First, a hot-rolled steel sheet is obtained by performing hot rolling on the slab having the above-described chemical composition under general conditions. The obtained hot-rolled steel sheet is subjected to primary annealing in an atmospheric atmosphere at a high temperature range. This primary annealing is performed under the conditions of an annealing temperature of 550 to 700 °C and an annealing time of 2 hours or more. By performing annealing at a high temperature range after hot rolling, internal oxides of Si and Mn are formed on the surface layer of the steel sheet. As a result, the surface enrichment of Si and Mn is suppressed and decarburization is promoted in the annealing after cold rolling. Thereby, ΔC can be preferably controlled. When the annealing temperature is less than 550 °C or the annealing time is less than 2 hours, ΔC of the steel sheet cannot be preferably controlled.

[0062] After performing the above annealing, pickling treatment is carried out, and cold rolling with a cumulative reduction ratio of 70% or more is performed to produce a steel sheet or steel strip having a desired thickness. By setting the cumulative reduction ratio of cold rolling to 70% or more, austenite recrystallization is promoted during annealing after cold rolling, and an increase in the austenite fraction can be suppressed. As a result, during annealing after cold rolling, the ferrite fraction with a large diffusion coefficient of C increases, and decarburization is promoted.

[0063] Note that the cumulative reduction ratio mentioned here is expressed as {1 - (thickness of the steel sheet after cold rolling / thickness of the steel sheet before cold rolling)} × 100 (%).

[0064] After cold rolling, further secondary annealing is carried out to obtain a steel sheet having desired mechanical properties. At that time, for example, by setting the dew point (average dew point in the annealing furnace) during secondary annealing to -10°C or more and the residence time of the steel sheet in the temperature range of 700°C or more to 50 to 400 seconds, the surface of the steel sheet can be stably decarburized. The upper limit of the dew point does not particularly need to be specified, but it may be about 10°C. If the dew point is too low or the above residence time is too short, decarburization does not proceed sufficiently, and ΔC cannot be preferably controlled. Also, if the above residence time is too long, sufficient tensile strength may not be obtained. Note that the temperature during annealing is, for example, about 750 to 850°C.

[0065] Except for the above-described conditions, there are no particular limitations, but for example, it is preferable to satisfy the following conditions. The slab is heated to a temperature range of 1100°C or more and then hot-rolled. After hot rolling, coiling is performed, primary annealing is performed, and then pickling is performed. The finish rolling temperature of hot rolling is preferably 900°C or more, and the coiling temperature is preferably 650°C or less. After pickling, cold rolling is performed. After cold rolling, secondary annealing is carried out, and thereafter, if necessary, the above-described plating layer may be formed.

[0066] Next, a method for manufacturing a press-formed product according to the present embodiment will be described. For the press forming method, cold working is preferred to maintain the obtained texture and suppress the generation of ghost lines. The cold working method is not particularly limited as long as the steel sheet can be formed by relatively moving the die and the punch.

Examples

[0067] Next, examples of the present invention will be described. The conditions in the examples are one example of the conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to this one example of conditions. The present invention can adopt various conditions as long as it does not deviate from the gist of the present invention and achieves the object of the present invention.

[0068] Steel having the chemical composition shown in Table 1 was melted and a slab with a thickness of 240 to 300 mm was produced by continuous casting. After heating the obtained slab in a temperature range of 1100 °C or higher, hot rolling was performed. After hot rolling, coiling was performed, and after primary annealing under the conditions shown in Table 2, pickling was performed. The finish rolling temperature of the hot rolling was 900 °C or higher, and the coiling temperature was 650 °C or lower. After pickling, cold rolling was performed with a cumulative reduction ratio of 70 to 90%. After cold rolling, secondary annealing was carried out under the conditions shown in Table 2, and an alloying hot-dip galvanized layer (GA), a hot-dip galvanized layer (GI), and an electrolytic galvanized layer (EG) were formed as necessary. By the above method, the steel sheets and plated steel sheets shown in Table 2 were obtained. The thickness of the obtained steel sheets and plated steel sheets was 0.2 to 2.0 mm.

[0069] After annealing after cold rolling, using the steel sheets and plated steel sheets, a substantially semi-cylindrical simulated part (press-formed part) simulating a door outer was manufactured by press forming. When press-forming this simulated part, the material (steel sheet or plated steel sheet) was actively made to flow into the mold, and at any position on the surface of the simulated part, the ratio of the strain in the direction perpendicular to an arbitrary direction (that arbitrary direction) to the strain in an arbitrary direction along the surface of the simulated part was made to be about 1. That is, press forming was performed so that strain anisotropy did not occur at any position on the surface of the simulated part.

[0070] For the obtained steel sheet, plated steel sheet, and simulated parts (press-formed parts), ΔC was determined by the above-described method. Since the ΔC of the steel sheet and the plated steel sheet was the same as that of the simulated parts, the ΔC of the simulated parts is not shown in the table. Also, the tensile strength of the steel sheet and the appearance quality of the simulated parts were evaluated by the following method. Since there was no significant difference between the tensile strength of the steel sheet and that of the simulated parts (press-formed parts), it was evaluated whether the steel sheet had the desired tensile strength as a simulated part at the steel sheet stage.

[0071] Tensile strength The tensile strength was evaluated in accordance with JIS Z 2241:2011. The test piece was the No. 5 test piece of JIS Z 2241:2011. The sampling position of the tensile test piece was 1 / 4 from the end in the sheet width direction, and the direction perpendicular to the rolling direction was defined as the longitudinal direction. When the obtained tensile strength was 500 MPa or more, it was judged as qualified as having high strength. On the other hand, when the obtained tensile strength was less than 500 MPa, it was judged as unqualified as having inferior strength.

[0072] Appearance quality The appearance quality was evaluated by the degree of ghost lines generated on the surface of the simulated parts after forming. The surface after press forming was polished with a grindstone, and the stripe patterns with an interval on the order of several millimeters generated on the surface were judged as ghost lines, and scored from 1 to 5 according to the degree of occurrence of the stripe patterns. An arbitrary area of 100 mm × 100 mm was visually confirmed. When no stripe pattern was confirmed, it was rated as "1". When the maximum length of the stripe pattern was 20 mm or less, it was rated as "2". When the maximum length of the stripe pattern was more than 20 mm and 50 mm or less, it was rated as "3". When the maximum length of the stripe pattern was more than 50 mm and 70 mm or less, it was rated as "4". When the maximum length of the stripe pattern exceeded 70 mm, it was rated as "5". When the evaluation was "3" or less, it was judged as qualified as having excellent appearance quality. On the other hand, when the evaluation was "4" or more, it was judged as unqualified as having inferior appearance quality.

[0073]

Table 1

[0074]

Table 2

[0075] From Table 2, it can be seen that the press-formed product according to the inventive example is of high strength and has excellent appearance quality. Also, it can be seen that the steel sheet according to the inventive example was able to produce a press-formed product of high strength and excellent appearance quality.

[0076] On the other hand, it can be seen that the press-formed product according to the comparative example is inferior in strength or has deteriorated appearance quality. Also, it can be seen that the steel sheet according to the comparative example was unable to produce a press-formed product of high strength and excellent appearance quality.

Industrial Applicability

[0077] According to the above aspect of the present invention, it is possible to provide a press-formed product of high strength and excellent appearance quality, and a steel sheet capable of manufacturing this press-formed product.

Claims

1. The chemical composition is by 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, Cr: 0 to 0.80%, Mo: 0 to 0.16%, Ti: 0 to 0.100%, B: 0 to 0.0100%, Nb: 0 to 0.060%, V: 0 to 0.50%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, W: 0 to 1.00%, Sn: 0 to 1.00%, Sb: 0 to 0.200%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, REM: 0 to 0.0100%, and the balance: Fe and impurities, C, which is the C content at a depth of 20 μm from the surface of the steel sheet ２０ and C, which is the C content at a depth of 60 μm from the surface ６０ where ΔC calculated from the following formula (1) is 0.20 to 0.90 mass% / mm, a steel sheet characterized in that the tensile strength is 500 to 750 MPa. ΔC = (C ６０ - C ２０ ) / (0.04) …(1)

2. The chemical composition is by mass%, Cr: 0.01 to 0.80%, Mo: 0.01 to 0.16%, Ti: 0.001 to 0.100%, B: 0.0001 to 0.0100%, Nb: 0.001 to 0.060%, V: 0.01 to 0.50%, Ni: 0.01 to 1.00%, Cu: 0.01 to 1.00%, W: 0.01 to 1.00%, Sn: 0.01 to 1.00%, Sb: 0.001 to 0.200%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Zr: 0.0001 to 0.0100%, and REM: 0.0001 to 0.0100% The steel sheet according to claim 1, characterized by containing one or more selected from the group consisting of.

3. The chemical composition is by mass%, and C is 0.040 to 0.080%. The steel sheet according to claim 1 or 2, characterized by this.

4. The steel sheet according to any one of claims 1 to 3, characterized in that the ΔC is 0.30 to 0.80 mass% / mm.

5. The steel sheet according to any one of claims 1 to 4, characterized by having a plating layer on at least one surface of the steel sheet.

6. A press-formed product obtained by press-forming the steel sheet according to any one of claims 1 to 5, C, which is the C content at a depth of 20 μm from the surface ２０ and C, which is the C content at a depth of 60 μm from the surface ６０ and a ΔC calculated from the following formula (1) is 0.20 to 0.90 mass% / mm, and the press-formed product is characterized by this. ΔC = (C ６０ - C ２０ ) / (0.04) …(1)

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