steel sheet

Abstract

The present invention provides a steel sheet in which excellent appearance quality can be achieved in a molded product. In the steel sheet, the chemical composition is C: 0.030 to 0.145%, Si: 0 to 0.500% or less, Mn: 0.50 to 2.50%, P: 0 to 0.100%, S: 0 to 0.020%, Al: 0 to 1.000%, N: 0 to 0.0100%, etc., in mass%, the microstructure is composed of ferrite at a volume fraction of 70 to 95% and hard phase at a volume fraction of 5 to 30%, the standard deviation of Vickers hardness H 1 / 4 at a position 1 / 4 of the sheet thickness direction divided by the average value of the Vickers hardness H 1 / 4 is 0.025 or less, and the standard deviation of Vickers hardness H 1 / 2 at a position 1 / 2 of the sheet thickness direction divided by the average value of the Vickers hardness H 1 / 2 is 0.030 or less.

Description

TECHNICAL FIELD

[0001] The present application relates to a steel sheet. BACKGROUND

[0002] From the viewpoint of protecting the global environment, in order to improve the fuel consumption of automobiles, the demand for lightweighting is increasing not only for structural members such as a vehicle beam but also for panel members such as a roof panel and a door outer panel. These panel members are different from skeletal members in that they are visible to people, and thus are required to have high appearance quality. As the appearance quality, designability and surface quality can be cited.

[0003] Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet having excellent surface quality. Specifically, Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet having a steel sheet (base sheet) containing, in mass%, C: 0.02 to 0.20%, Si: 0.7% or less, Mn: 1.5 to 3.5%, P: 0.10% or less, S: 0.01% or less, Al: 0.1 to 1.0%, N: 0.010% or less, Cr: 0.03 to 0.5%, and a hot-dip galvanized layer on the surface of the base sheet, and further, an annealing surface oxidation index A defined by a mathematical expression A = 400Al / (4Cr + 3Si + 6Mn) using the contents of Al, Cr, Si, and Mn as the same category is 2.3 or more, and the balance consists of Fe and unavoidable impurities, and further, the structure of the base sheet is composed of ferrite and a second phase which is a martensite matrix.

[0004] PRIOR ART DOCUMENTS

[0005] PATENT DOCUMENT

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 2005-220430 SUMMARY

[0007] PROBLEMS TO BE SOLVED BY THE INVENTION

[0008] In order to improve the appearance quality, the occurrence of ghost lines can be cited as a technical problem. The ghost line refers to a minute unevenness on the order of 1 mm occurring on the surface when a steel sheet having a hard phase and a soft phase such as a DP (Dual Phase) steel is subjected to press forming, and the soft phase around the hard phase deforms preferentially, resulting in the minute unevenness. The unevenness forms a striped pattern on the surface, and thus the appearance quality of the press-formed product in which the ghost lines occur is poor.

[0009] With the high strength and thin walling of panel members for the lightweighting of automobiles, and further the complication of the shape, there is a tendency that the surface of the steel sheet after forming is likely to have unevenness and is likely to have ghost lines.

[0010] The present application has been made in view of the above-described circumstances. It is an object of the present application to provide a steel sheet in which excellent appearance quality can be achieved in a molded product.

[0011] Solution to the problem

[0012] The present application is characterized by the following steel sheet.

[0013] (1) A steel sheet whose chemical composition is, in mass%,

[0014] C: 0.030% to 0.145%,

[0015] Si: 0% to 0.500%,

[0016] Mn: 0.50% to 2.50%,

[0017] P: 0% to 0.100%,

[0018] S: 0% to 0.020%,

[0019] Al: 0% to 1.000%,

[0020] N: 0% to 0.0100%,

[0021] B: 0% to 0.0050%,

[0022] Mo: 0% to 0.80%,

[0023] Ti: 0% to 0.200%,

[0024] Nb: 0% to 0.10%,

[0025] V: 0% to 0.20%,

[0026] Cr: 0% to 0.80%,

[0027] Ni: 0% to 0.25%

[0028] O: 0% to 0.0100%,

[0029] Cu: 0% to 1.00%,

[0030] W: 0% to 1.00%,

[0031] Sn: 0% to 1.00%,

[0032] Sb: 0% to 0.20%,

[0033] Ca: 0% to 0.0100%,

[0034] Mg: 0% to 0.0100%,

[0035] Zr: 0%~0.0100%

[0036] REM: 0%~0.0100%,

[0037] The balance consists of iron and impurities.

[0038] The metallographic structure consists of 70–95% ferrite by volume and 5–30% hard phase by volume.

[0039] Vickers hardness H at 1 / 4 position in the thickness direction 1 / 4 The standard deviation divided by the Vickers hardness H 1 / 4 The average value X1 obtained is below 0.025.

[0040] Vickers hardness H at 1 / 2 position in the thickness direction 1 / 2 The standard deviation divided by the Vickers hardness H 1 / 2 The average value obtained by X2 is below 0.030.

[0041] (2) The steel plate according to (1) above, characterized in that the average grain diameter of the ferrite is 5.0 to 30.0 μm and the average grain diameter of the hard phase is 1.0 to 5.0 μm.

[0042] (3) The steel plate according to (1) or (2) above is characterized in that, in the region of 1 / 4 to 1 / 2 of the plate thickness direction, the area of ​​the hard phase with a continuous thickness of 100 μm or more in the rolling direction is less than 30% of the total area of ​​the hard phase.

[0043] (4) The steel plate according to any one of (1) to (3) above, characterized in that the aspect ratio Str (ISO25178) of the surface properties of the test piece after being subjected to a 5% strain by a tensile test is 0.28 or more.

[0044] (5) The steel plate according to any one of (1) to (4) above, characterized in that the Vickers hardness H at the 1 / 4 position in the thickness direction is 1 / 4 The average value is 150-300.

[0045] Vickers hardness H at 1 / 2 position in the thickness direction 1 / 2 The average value is 155-305.

[0046] (6) The steel plate according to any one of (1) to (5) above, characterized in that the hard phase is composed of any one or more of martensite, bainite, tempered martensite and pearlite.

[0047] (7) The steel sheet according to any one of (1) to (6) above, characterized in that the thickness of the steel sheet is 0.20 mm to 1.00 mm.

[0048] (8) The steel sheet according to any one of (1) to (7) above, characterized in that the steel sheet is an outer panel of an automobile.

[0049] Effects of the Invention

[0050] According to the above-described aspect of the present application, a steel sheet that can achieve excellent appearance quality in a molded product can be provided. DETAILED DESCRIPTION

[0051] <Thoughts on the process of coming up with the present invention>

[0052] The present inventors have studied a method for suppressing the occurrence of ghost lines after press forming of a high-strength steel sheet. As described above, in a steel sheet in which hard phases and soft phases are mixed, such as a DP (dual phase) steel, deformation mainly occurs around the soft phases during forming, and minute concave-convex portions are generated on the surface of the steel sheet, and thus appearance defects called ghost lines are sometimes generated. During press forming of a steel sheet, the soft phases are depressed, and the hard phases are not depressed or are deformed in a manner that becomes convex, and thus ghost lines are generated in a band shape (striped shape). The band-shaped structure is formed of hard phases such as martensite.

[0053] The present inventors have conducted intensive studies, and as a result, have found that by controlling the hot-rolled structure at the time of manufacturing a steel sheet and suppressing the band-shaped structure, the band-shaped hard phases in the final product can be suppressed.

[0054] The present application is based on the above-described insight, and the steel sheet of the present embodiment will be described in detail below. The present application is not limited to the technical solutions disclosed in the present embodiment, and various modifications can be made within the scope of the gist of the present application.

[0055] First, the chemical composition of the steel sheet of the present embodiment will be described. For the numerical value range indicated by "to" below, the lower limit value and the upper limit value are included in the range. For the numerical value with "less than" or "greater than", the numerical value is not included in the numerical value range. In the following description, the % relating to the chemical composition is mass % unless otherwise specified.

[0056] The chemical composition of the steel sheet of the present embodiment is, in mass %,

[0057] C: 0.030% to 0.145%,

[0058] Si: 0% to 0.500%,

[0059] Mn: 0.50% to 2.50%,

[0060] P: 0%~0.100%

[0061] S: 0%~0.020%

[0062] Al: 0%~1.000%

[0063] N: 0%~0.0100%

[0064] B: 0%~0.0050%

[0065] Mo: 0%–0.80%

[0066] Ti: 0%~0.200%

[0067] Nb: 0%–0.10%

[0068] V: 0%~0.20%

[0069] Cr: 0%–0.80%

[0070] Ni: 0%–0.25%

[0071] O: 0%~0.0100%

[0072] Cu: 0%~1.00%

[0073] W: 0%~1.00%

[0074] Sn: 0%~1.00%

[0075] Sb: 0%~0.20%

[0076] Ca: 0%~0.0100%

[0077] Mg: 0%~0.0100%

[0078] Zr: 0%~0.0100%

[0079] REM: 0%~0.0100%,

[0080] The balance consists of iron and impurities. The following is a description of each element.

[0081] (C: 0.030%~0.145%)

[0082] Carbon (C) is an element that increases the strength of steel plates. To obtain the desired strength, the C content is set to 0.030% or more. To further improve the strength, the C content is preferably 0.035% or more, more preferably 0.040% or more, even more preferably 0.050% or more, and even more preferably 0.060% or more.

[0083] In addition, by making the C content 0.145% or less, the diffusion of Mn at the time of solidification is promoted, whereby the problem of the easy occurrence of band-shaped Mn segregation can be suppressed. As a result, the occurrence of ghost lines after press forming of the steel sheet can be suppressed. Therefore, the C content is set to 0.145% or less. The C content is preferably 0.110% or less, more preferably 0.090% or less.

[0084] (Si: 0% to 0.500%)

[0085] Si is a deoxidizing element of steel, and is an element effective for improving strength without impairing the ductility of the steel sheet. By making the Si content 0.500% or less, the occurrence of surface defects due to the decrease in scale spalling property can be suppressed. Therefore, the Si content is set to 0.500% or less. The Si content is preferably 0.450% or less, more preferably 0.250% or less, further preferably 0.100% or less.

[0086] The lower limit of the Si content includes 0%, but in order to improve the strength-formability balance of the steel sheet, the Si content can also be set to 0.0005% or more or 0.0010% or more, more preferably greater than 0.090%, further preferably 0.100% or more.

[0087] (Mn: 0.50% to 2.50%)

[0088] Mn is an element that improves the hardenability of steel and contributes to the improvement of strength. In order to obtain the desired strength, the Mn content is set to 0.50% or more. The Mn content is preferably 1.20% or more, more preferably 1.40% or more, further preferably greater than 1.60%, more further preferably 1.65% or more.

[0089] In addition, when the Mn content is 2.50% or less, the occurrence of striped Mn segregation at the time of solidification of the steel can be suppressed. Therefore, the Mn content is 2.50% or less. The Mn content is preferably 2.25% or less, more preferably 2.00% or less, further preferably 1.80% or less.

[0090] (P: 0% to 0.100%)

[0091] P is an element that embrittles steel. When the P content is 0.100% or less, the problem of the embrittlement of the steel sheet and the easy occurrence of cracking in the production process can be suppressed. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.080% or less, more preferably 0.050% or less.

[0092] The lower limit of the P content includes 0%, but by making the P content 0.001% or more, the production cost can be further reduced. Therefore, the P content can also be 0.001% or more.

[0093] (S: 0% to 0.020%)

[0094] S is an element that forms a Mn sulfide, deteriorating formability such as ductility, hole expandability, stretch flangeability, and bendability of the steel sheet. When the S content is 0.020% or less, a significant decrease in formability of the steel sheet can be suppressed. Therefore, the S content is set to 0.020% or less. The S content is preferably 0.010% or less, more preferably 0.008% or less.

[0095] The lower limit of the S content includes 0%, but by making the S content 0.0001% or more, production costs can be further reduced. Therefore, the S content can also be 0.0001% or more.

[0096] (Al: 0% to 1.000%)

[0097] Al is an element that functions as a deoxidizing material, and is an element effective for improving the strength of steel. By making the Al content 1.000% or less, castability can be improved, and therefore productivity can be improved. Therefore, the Al content is set to 1.000% or less. The Al content is preferably 0.650% or less, more preferably 0.600% or less, and further preferably 0.500% or less.

[0098] The lower limit of the Al content includes 0%, but in order to sufficiently obtain the deoxidizing effect based on Al, the Al content can also be set to 0.005% or more.

[0099] (N: 0% to 0.0100%)

[0100] N is an element that forms a nitride, deteriorating formability such as ductility, hole expandability, stretch flangeability, and bendability of the steel sheet. When the N content is 0.0100% or less, a decrease in formability of the steel sheet can be suppressed. Therefore, the N content is set to 0.0100% or less. In addition, N is also an element that causes a welding defect at the time of welding, hindering productivity. Therefore, the N content is preferably 0.0080% or less, more preferably 0.0070% or less, and further preferably 0.0040% or less.

[0101] The lower limit of the N content includes 0%, but by making the N content 0.0005% or more, production costs can be further reduced. Therefore, the N content can also be set to 0.0005% or more.

[0102] The steel sheet of the present embodiment can contain the following elements as optional elements. When the following optional elements are not contained, the content thereof is 0%.

[0103] (B: 0% to 0.0050%)

[0104] B is an element that suppresses phase transformation at high temperatures and is useful for increasing the strength of the steel sheet. B is not necessarily contained, and thus the lower limit of the B content includes 0%. In order to sufficiently obtain the B-based strength increasing effect, the B content is preferably 0.0001% or more, more preferably 0.0005% or more, and further preferably 0.0010% or more.

[0105] In addition, when the B content is 0.0050% or less, the generation of B precipitates can be suppressed, and thus the strength of the steel sheet can be prevented from decreasing. Therefore, the B content is set to 0.0050% or less, and preferably 0.0030% or less. The B content can also be 0.0001% to 0.0050%.

[0106] (Mo: 0% to 0.80%)

[0107] Mo is an element that suppresses phase transformation at high temperatures and is useful for increasing the strength of the steel sheet. Mo is not necessarily contained, and thus the lower limit of the Mo content includes 0%. In order to sufficiently obtain the Mo-based strength increasing effect, the Mo content is preferably 0.001% or more, more preferably 0.05% or more, and further preferably 0.10% or more.

[0108] In addition, when the Mo content is 0.80% or less, the hot workability can be suppressed from decreasing, and thus the productivity can be prevented from decreasing. Therefore, the Mo content is set to 0.80% or less, and preferably 0.40% or less, and more preferably 0.20% or less. The Mo content can be 0.001% to 0.80%, or 0% to 0.40%.

[0109] Note that, by simultaneously containing both Cr and Mo, and setting the contents to Cr: 0.20% to 0.80% and Mo: 0.05% to 0.80%, the strength of the steel sheet can be more reliably increased, and thus this is preferable.

[0110] (Ti: 0% to 0.200%)

[0111] Ti is an element that has an effect of reducing the amounts of S, N, and O, which are factors in the generation of coarse inclusions that act as fracture initiation points. In addition, Ti has an effect of refining the structure and improving the strength-formability balance of the steel sheet. Ti is not necessarily contained, and thus the lower limit of the Ti content includes 0%. In order to sufficiently obtain the above effects, the Ti content is preferably set to 0.001% or more, and more preferably 0.010% or more.

[0112] In addition, when the Ti content is 0.200% or less, formation of coarse Ti sulfides, Ti nitrides, and Ti oxides can be suppressed, and formability of the steel sheet can be ensured. Therefore, the Ti content is set to 0.200% or less. The Ti content is preferably 0.080% or less, and more preferably 0.060% or less. The Ti content can also be 0% to 0.100%, and can also be 0.001% to 0.200%.

[0113] (Nb: 0% to 0.10%)

[0114] Nb is an element that contributes to an increase in strength of the steel sheet through precipitation strengthening, fine-grain strengthening based on suppression of ferrite grain growth, and dislocation strengthening based on suppression of recrystallization. Nb need not necessarily be contained, and therefore 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, more preferably 0.005% or more, and further preferably 0.01% or more.

[0115] In addition, when the Nb content is 0.10% or less, recrystallization can be promoted and the remaining of unrecrystallized ferrite can be suppressed, and formability of the steel sheet can be ensured. Therefore, the Nb content is set to 0.10% or less. The Nb content is preferably 0.05% or less, and more preferably 0.04% or less. The Nb content can also be 0.001% to 0.10%.

[0116] (V: 0% to 0.20%)

[0117] V is an element that contributes to an increase in strength of the steel sheet through precipitation strengthening, fine-grain strengthening based on suppression of ferrite grain growth, and dislocation strengthening based on suppression of recrystallization. V need not necessarily be contained, and therefore the lower limit of the V content includes 0%. In order to sufficiently obtain the strength-increasing effect based on V, the V content is preferably 0.001% or more, more preferably 0.01% or more, and further preferably 0.03% or more.

[0118] In addition, when the V content is 0.20% or less, precipitation of a large amount of carbonitrides can be suppressed and formability of the steel sheet can be decreased. Therefore, the V content is set to 0.20% or less. The V content is preferably 0.10% or less. The V content can be 0% to 0.10%, or 0.001% to 0.20%.

[0119] (Cr: 0% to 0.80%)

[0120] Cr is an element that improves the quenchability of steel and contributes to an increase in strength of the steel sheet. Cr need not necessarily be contained, and therefore the lower limit of the Cr content includes 0%. In order to sufficiently obtain the strength-increasing effect based on Cr, the Cr content is preferably 0.001% or more, further preferably 0.20% or more, and particularly preferably 0.30% or more.

[0121] In addition, when the Cr content is 0.80% or less, formation of coarse Cr carbides that can become a fracture initiation point can be suppressed. Therefore, the Cr content is set to 0.80% or less. The Cr content is preferably 0.70% or less, and more preferably 0.50% or less. The Cr content can be 0% to 0.70%, or 0.001% to 0.80%.

[0122] (Ni: 0% to 0.25%)

[0123] Ni is an element that suppresses phase transformation at high temperatures and contributes to an increase in the strength of the steel sheet. Ni is not necessarily contained, and therefore the lower limit of the Ni content includes 0%. In order to sufficiently obtain the Ni-based strength increase effect, the Ni content is preferably 0.001% or more, and more preferably 0.05% or more.

[0124] In addition, when the Ni content is 0.25% or less, a decrease in the weldability of the steel sheet can be suppressed. Therefore, the Ni content is set to 0.25% or less. The Ni content is preferably 0.20% or less, and more preferably 0.15% or less. The Ni content can also be 0.001% to 0.20%.

[0125] Hereinafter, the preferable content of each of O, Cu, W, Sn, Sb, Ca, Mg, Zr, and REM, which are arbitrary additive elements, is described. However, these O, Cu, W, Sn, Sb, Ca, Mg, Zr, and REM do not contribute to the reduction of the ghost line within the content range of the following examples. In other words, in the present embodiment, O, Cu, W, Sn, Sb, Ca, Mg, Zr, and REM have no influence on the effect of reducing the continuous hard phase by applying a high reduction ratio to the latter half of the finishing in the hot rolling process described later, and thereby enabling the reduction of surface unevenness after forming.

[0126] (O: 0% to 0.0100%)

[0127] O is an element that is mixed in during the manufacturing process. The O content can be 0%. Note that by setting the O content to 0.0001% or more, the refining time can be shortened and productivity can be improved. Therefore, the O content can also be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, when the O content is 0.0100% or less, formation of coarse oxides can be suppressed, and formability such as ductility, hole expandability, stretch flangeability, and / or bendability of the steel sheet can be improved. Therefore, the O content is set to 0.0100% or less. The O content can also be 0.0070% or less, 0.0040% or less, or 0.0020% or less.

[0128] (Cu: 0% to 1.00%)

[0129] Cu is an element which exists in the steel in the form of fine particles and which contributes to the improvement of the strength of the steel sheet. The Cu content can be 0%, but in order to obtain this effect, the Cu content is preferably 0.001% or more. The Cu content can also be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, by making the Cu content 1.00% or less, the weldability of the steel sheet can be made good. Therefore, the Cu content is 1.00% or less. The Cu content can also be 0.60% or less, 0.40% or less, or 0.20% or less.

[0130] (W: 0% ~ 1.00%)

[0131] W is an element which suppresses phase transformation at high temperatures and which contributes to the improvement of the strength of the steel sheet. The W content can be 0%, but in order to obtain this effect, the W content is preferably 0.001% or more. The W content can also be 0.01% or more, 0.02% or more, or 0.10% or more. On the other hand, by making the content of W 1.00% or less, the hot workability can be improved and the productivity can be improved. Therefore, the W content is 1.00% or less. The W content can also be 0.80% or less, 0.50% or less, or 0.20% or less.

[0132] (Sn: 0% ~ 1.00%)

[0133] Sn is an element which suppresses grain coarsening and which contributes to the improvement of the strength of the steel sheet. The Sn content can be 0%, but in order to obtain this effect, the Sn content is preferably 0.001% or more. The Sn content can also be 0.01% or more, 0.05% or more, or 0.08% or more. On the other hand, by making the Sn content 1.00% or less, the embrittlement of the steel sheet can be suppressed. Therefore, the Sn content is 1.00% or less. The Sn content can also be 0.80% or less, 0.50% or less, or 0.20% or less.

[0134] (Sb: 0% ~ 0.20%)

[0135] Sb is an element which suppresses grain coarsening and which contributes to the improvement of the strength of the steel sheet. The Sb content can be 0%, but in order to obtain this effect, the Sb content is preferably 0.001% or more. The Sb content can also be 0.01% or more, 0.05% or more, or 0.08% or more. On the other hand, by making the Sn content 0.20% or less, the embrittlement of the steel sheet can be suppressed. Therefore, the Sb content is 0.20% or less. The Sb content can also be 0.18% or less, 0.15% or less, or 0.12% or less.

[0136] (Ca: 0% ~ 0.0100%)

[0137] (Mg: 0% ~ 0.0100%)

[0138] (Zr: 0% to 0.0100%)

[0139] (REM: 0% to 0.0100%)

[0140] Ca, Mg, Zr and REM are elements that contribute to improvement of formability of the steel sheet. The contents of Ca, Mg, Zr and REM can be 0%, but in order to obtain the effect, the contents of Ca, Mg, Zr and REM are each preferably 0.0001% or more, and can be 0.0005% or more, 0.0010% or more or 0.0015% or more. On the other hand, by making the contents of Ca, Mg, Zr and REM each 0.0100% or less, the ductility of the steel sheet can be ensured. Therefore, the contents of Ca, Mg, Zr and REM can each be set to 0.0100% or less, and can be 0.0080% or less, 0.0060% or less or 0.0030% or less. REM in the present specification means a total of 17 kinds of elements of scandium (Sc) having an atomic number of 21, yttrium (Y) having an atomic number of 39 and lanthanoids having an atomic number of 57 of lanthanum (La) to lutetium (Lu) having an atomic number of 71, and the content of REM is a total content of these elements.

[0141] The balance of the chemical composition of the steel sheet of the present embodiment can be Fe and impurities. As the impurities, elements that are mixed from a steel raw material or a scrap and / or in a steel production process, or that are allowed within a range not impairing the characteristics of the steel sheet of the present embodiment can be exemplified. As the impurities, H, Na, Cl, Co, Zn, Ga, Ge, As, Se, Tc, Ru, Rh, Pd, Ag, Cd, In, Te, Cs, Ta, Re, Os, Ir, Pt, Au, Pb, Bi, Po can be exemplified. The impurities can contain 0.200% or less in total.

[0142] The chemical composition of the above steel sheet can be measured by a conventional analysis method. For example, measurement by ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) can be performed. Note that C and S can be measured by combustion-infrared absorption method, and N can be measured by non-active gas fusion-thermal conductivity method. In the case where the steel sheet has a plating layer on the surface, the plating layer on the surface can be removed by mechanical grinding, and then the analysis of the chemical composition can be performed.

[0143] (microstructure composed of 70 to 95% in volume fraction of ferrite and 5 to 30% in volume fraction of hard phase)

[0144] By making the volume fraction of the hard phase in the metallographic structure 5% or more, the strength of the steel sheet can be sufficiently increased. Therefore, the volume fraction of the hard phase is set to 5% or more. On the other hand, by making the volume fraction of the hard phase 30% or less, the hard phase can be more uniformly dispersed, and thus surface unevenness at the time of forming can be reduced, and the appearance after forming can be improved.

[0145] In addition, the balance other than the hard phase in the metallographic structure is ferrite, and the volume fraction of the ferrite is 70 to 95%. Further, the volume fraction of the ferrite is preferably 72% or more, and more preferably 75% or more. In addition, the volume fraction of the hard phase is preferably 28% or less, and more preferably 25% or less. The sum of the volume fractions of the ferrite and the hard phase in the metallographic structure is 100%.

[0146] In the steel sheet of the present embodiment, the hard phase is a hard structure harder than the ferrite, and is composed of, for example, any one or more of martensite, bainite, tempered martensite, and pearlite. In view of improving the strength, the hard phase is preferably composed of one or more of martensite, bainite, and tempered martensite, and more preferably composed of martensite.

[0147] The volume fraction of the hard phase in the metallographic structure can be obtained by the following method.

[0148] A sample (the size is approximately 20 mm in the rolling direction x 20 mm in the width direction x the thickness of the steel sheet) for metallographic structure (microstructure) observation is collected from the 1 / 4W position or the 3 / 4W position of the plate width W of the obtained steel sheet (i.e., the position of 1 / 4W in the width direction from either of the widthwise end portions of the steel sheet), and the metallographic structure (microstructure) at the position of 1 / 2 thickness from the surface is observed with an optical microscope, and the area fraction of the hard phase from the surface (the surface from which the plating layer is removed in the case where plating is present) to the position of 1 / 2 thickness is calculated. For the preparation of the sample, the plate thickness cross section in the rolling direction is polished as the observation surface, and etched with LePera reagent.

[0149] The "microstructure" is classified from the optical microscope photograph at a magnification of 500x or 1000x. When the optical microscope observation is performed after LePera etching, since, for example, bainite and pearlite are black, martensite (including tempered martensite) is white, and ferrite is gray, each structure can be observed by color discrimination, and thus the discrimination of the ferrite and the hard structure other than the ferrite can be easily performed. In the optical microscope photograph, the area other than the gray color indicating the ferrite is the hard phase.

[0150] Ten fields of view were observed at 500x or 1000x magnification in a region extending from the surface of a steel plate etched with LePera reagent to half the plate thickness in the thickness direction. Image analysis was performed using Adobe Photoshop CS5 software to determine the area fraction of the hard phase. As an image analysis technique, for example, the maximum brightness value L of the image was obtained. max and minimum brightness value L min The lightness value is L. max -0.3(L max -L min ) to L max A portion of the pixels is defined as the white area, and the pixels with L... min To L min +0.3(L max -L min The pixels of a region are defined as black areas, and the remaining areas are defined as gray areas. The area outside the gray areas, i.e., the hard phase area fraction, is calculated. Image analysis is performed on a total of 10 observation sites in the same manner as above to determine the hard phase area fraction. These area fractions are averaged to calculate the average value, which is then used as the volume fraction.

[0151] Vickers hardness H at 1 / 4 position along the thickness of the plate 1 / 4 Standard deviation σ 1 / 4 Divide by Vickers hardness H 1 / 4 The average value H AVE1 / 4 The obtained value X1 is below 0.025.

[0152] The inventors have discovered that if the Vickers hardness distribution of a steel sheet deviates significantly, the hard phase tends to band together, resulting in a higher likelihood of ghost lines appearing in the molded product obtained by pressing the steel sheet. In particular, the inventors have noted deviations in the Vickers hardness distribution closer to the surface of the steel sheet. Furthermore, the inventors have found that in the rolling direction of the steel sheet, in areas with smaller deviations in the Vickers hardness distribution, ghost lines form with interruptions midway, which can suppress appearance defects caused by longitudinally elongated ghost lines. As a result, it was found that the Vickers hardness H at the 1 / 4 position in the thickness direction... 1 / 4 Standard deviation σ 1 / 4 Divide by Vickers hardness H 1 / 4 The average value H AVE1 / 4 The obtained value X1 is below 0.025, which is effective in improving the surface quality of the steel plate and the molded product obtained by pressing the steel plate.

[0153] Further, in the present embodiment, the Vickers hardness refers to the hardness obtained based on the Vickers hardness test of JIS Z 2244:2009. The Vickers hardness here is the Vickers hardness at a test force of 1.9614 N (0.2 kgf), that is, HV0.2.

[0154] In the present embodiment, the observation object of the Vickers hardness is a cross section parallel to the sheet thickness direction and the rolling direction of the steel sheet (a cross section orthogonal to the width direction), and is a cross section in the center of the width direction of the steel sheet.

[0155] Further, the observation at the "1 / 4 position in the sheet thickness direction" refers to the observation of 50 points as measurement points at an interval of 150 μm in the rolling direction at a position reaching 1 / 4 in the sheet thickness direction from the surface of the steel sheet, and 50 points as measurement points at an interval of 150 μm in the rolling direction at a position reaching 1 / 4 in the sheet thickness direction from the back of the steel sheet. Thus, by taking 150 μm x 50 = 7.5 mm as the observation object in the rolling direction, it is possible to measure the Vickers hardness while including both the position where the ghost line is generated and the position where the ghost line is not generated. That is, by taking the observation object to be a sufficient length in the rolling direction, it is possible to suppress the defect that only the position where there is no ghost line can be measured, and it is possible to suppress only the ghost line can be measured. Thus, it is possible to make a more accurate surface quality judgment taking into account the presence or absence of the ghost line.

[0156] Further, the observation object at the 1 / 4 position in the sheet thickness direction can also not be the above-described manner. The interval in the rolling direction of the observation object can be less than 150 μm, or more than 150 μm, but the upper limit of the interval in the rolling direction is 400 μm, and the lower limit is 50 μm. In addition, the measurement points in the rolling direction can be less than 50 points, or more than 50 points, but the lower limit of the measurement points in the rolling direction is 30 points. In order to make a more accurate surface quality judgment taking into account the position where the ghost line is present and the position where the ghost line is not present, the length of the observation object in the rolling direction is preferably 5 mm or more. In addition, in the present embodiment, the constitution of the cross section in the center of the width direction of the steel sheet is described, but it can also not be the above-described manner. As long as the same constitution as described in the constitution of the cross section is present in at least one of the cross sections in the middle of the width direction of the steel sheet.

[0157] The present inventors have found that, in order to suppress the generation of the ghost line in the press-formed product, by reducing the deviation of the Vickers hardness distribution in the rolling direction in the vicinity of the surface of the steel sheet, specifically, by taking the value X1 to be 0.025 or less, it is possible to suppress the generation of the ghost line. Therefore, in the present embodiment, the value X1 is taken to be 0.025 or less. The value X1 is preferably 0.020 or less. Note that the lower limit of the value X1 is zero.

[0158] (Vickers hardness H 1 / 2standard deviation σ of the Vickers hardness H 1 / 2 divided by the average value H of the Vickers hardness H 1 / 2 AVE1 / 2 The value X2 obtained is 0.030 or less

[0159] As described above, by making the value XI 0.025 or less, generation of a ghost line in a molded product obtained by press molding the steel sheet can be suppressed. The present inventors also focused on the deviation of the Vickers hardness distribution in a region deeper from the surface of the steel sheet. As a result, it was found that by making the standard deviation σ of the Vickers hardness H 1 / 2 1 / 2 divided by the average value H of the Vickers hardness H 1 / 2 AVE1 / 2 The value X2 obtained is 0.030 or less, which is effective for further improving the surface quality of the steel sheet and the surface of a molded product obtained by press molding the steel sheet.

[0160] In the present embodiment, the observation at the "1 / 2 position in the thickness direction" means that 50 points are observed as measurement points at an interval of 150 μm in the rolling direction at a position reaching 1 / 2 in the thickness direction from the surface of the steel sheet. The observation at the "1 / 2 position in the thickness direction" and the observation at the "1 / 4 position in the thickness direction" mean that the positions in the thickness direction are different for the observed sites, and the observation contents are the same except for this.

[0161] The present inventors and others found that in order to further reliably suppress generation of a ghost line in a press molded product, by reducing the deviation of the Vickers hardness distribution in the rolling direction at the center of the steel sheet, specifically, by making the value X2 0.030 or less, generation of a ghost line can be suppressed. Therefore, in the present embodiment, the value X2 is set to 0.030 or less. The value X2 is preferably 0.025 or less. Note that the lower limit of the value X2 is zero.

[0162] (the average grain diameter of ferrite is 5.0 to 30.0 μm)

[0163] By making the average grain diameter of ferrite 30.0 μm or less, a decrease in appearance after molding can be suppressed. Therefore, the average grain diameter of ferrite is preferably set to 30.0 μm or less. More preferably, it is set to 15.0 μm or less.

[0164] ​​​On the other hand, by making the average grain diameter of ferrite 5.0 μm or more, the occurrence of aggregation of ferrite grains having {001} orientation can be suppressed. Although each of the grains of ferrite having {001} orientation is small, if these grains aggregate, the deformation concentrates in the aggregated portion, and thus by suppressing the aggregation of these grains, the appearance after forming can be suppressed from deteriorating. Therefore, the average grain diameter of ferrite is preferably set to 5.0 μm or more. More preferably, 8.0 μm or more, further preferably 10.0 μm or more, and still more preferably 15.0 μm or more.

[0165] The average grain diameter of ferrite in the steel sheet can be found by the following method. Specifically, in a region of the steel sheet etched with LePera reagent up to a position of 1 / 2 of the sheet thickness from the surface, 10 fields of view are observed at a magnification of 500x, and the area fraction occupied by ferrite and the number of ferrite grains are calculated respectively by image analysis using the image analysis software of "Photoshop CS5" of Adobe, Inc. as described above. These are added up, and the average area fraction per ferrite grain is calculated by dividing the area fraction occupied by ferrite by the number of ferrite grains. From the average area fraction and the number of grains, the equivalent circle diameter is calculated, and the obtained equivalent circle diameter is taken as the average grain diameter of ferrite.

[0166] (Average grain diameter of hard phase is 1.0 to 5.0 μm)

[0167] By making the average grain diameter of hard phase 5.0 μm or less, the appearance after forming can be suppressed from deteriorating. Therefore, the average grain diameter of hard phase in the steel sheet is preferably set to 5.0 μm or less. More preferably, 4.5 μm or less, and further preferably 4.0 μm or less.

[0168] On the other hand, by making the average grain diameter of hard phase 1.0 μm or more, the occurrence of aggregation of hard phase grains can be suppressed. By making each of the grains of hard phase small and suppressing the aggregation of these grains, the appearance after forming can be suppressed from deteriorating. Therefore, the average grain diameter of hard phase in the steel sheet is preferably set to 1.0 μm or more. More preferably, 1.5 μm or more, and further preferably 2.0 μm or more.

[0169] The average grain diameter of the hard phase can be determined as follows. Specifically, in a region of the steel sheet from the surface to a position of 1 / 2 of the sheet thickness in the sheet thickness direction after etching with LePera reagent, 10 fields of view are observed at a magnification of 500x, and the area fraction occupied by the hard phase and the number of hard phase particles are calculated using the image analysis software "Photoshop CS5" of Adobe, Inc. as described above. These are summed, and the average area fraction per hard phase particle is calculated by dividing the area fraction occupied by the hard phase by the number of hard phase particles. From the average area fraction and the number of particles, the equivalent circle diameter is calculated, and the equivalent circle diameter obtained is taken as the average grain diameter of the hard phase.

[0170] (the area of the hard phase continuous in the rolling direction for 100 μm or more in the region of 1 / 4 to 1 / 2 in the sheet thickness direction is 30% or less relative to the total area of the hard phase)

[0171] By making the area of the hard phase continuous in the rolling direction for 100 μm or more 30% or less relative to the total area of the hard phase, elongation and continuous deformation of the hard phase in the rolling direction at the time of press forming of the steel sheet and the concave deformation of the soft phase around the hard phase can be suppressed, and the generation of easily visible ghost lines can be suppressed. Therefore, in the present embodiment, the area of the hard phase continuous in the rolling direction for 100 μm or more in the region of 1 / 4 to 1 / 2 in the sheet thickness direction is preferably 30% or less relative to the total area of the hard phase. This ratio is more preferably 20% or less. The lower limit of this ratio is 0%.

[0172] The measurement method of the above ratio in the present embodiment is as follows. First, for a cross section parallel to the sheet thickness direction and the rolling direction of the steel sheet and a cross section in the center in the width direction of the steel sheet, an observation range (continuous hard phase observation range) of 400 μm in the rolling direction in the region of 1 / 4 to 1 / 2 in the sheet thickness direction from the surface of the steel sheet is defined. Note that the length of the continuous hard phase observation range in the rolling direction can be less than 400 μm (for example, 300 μm) or can be a value greater than 400 μm (for example, 500 μm). Among these, the lower limit of the length of the continuous hard phase observation range in the rolling direction is 250 μm.

[0173] Next, in the continuous hard phase observation range, the area AR1 of the hard phase continuous in the rolling direction for 100 μm or more is measured. Specifically, in the continuous hard phase observation range, the hard phase continuous in the rolling direction for 100 μm or more is extracted by image processing using the measurement method of the hard phase described above. In this case, "continuous" means that the grain boundaries of the hard phase are in contact. Next, in the continuous hard phase observation range, the total area of the hard phase AR2 is measured using the measurement method of the hard phase described above. Then, AR1 / AR2 is calculated.

[0174] (the aspect ratio Str of the surface properties of the test piece after 5% strain applied by the tensile test is 0.28 or more)

[0175] The aspect ratio Str of the surface properties in the test piece after 5% strain applied by the tensile test (hereinafter, referred to as "test piece after stretching") is an index indicating anisotropy of the unevenness of the surface of a molded product obtained by molding (e.g., press molding) the steel sheet. Further, the aspect ratio Str is defined by ISO (International Organization for Standardization) 25178, and is a value between 0 and 1. The closer the aspect ratio Str is to 0, the greater the anisotropy, and there are striations on the surface of the observation range. On the other hand, the closer the aspect ratio Str is to 1, the less the surface shape of the observation range depends on a particular direction.

[0176] For example, in a case where there are micro-height convex shapes extending in a prescribed first direction on the surface of the observation range, and a plurality of the convex shapes are arranged in a second direction orthogonal to the above-described first direction, the surface shape observed from the first direction and the surface shape observed from the second direction are greatly different in regularity. In such a case, the surface shape observed from the first direction and the surface shape observed from the second direction are greatly different, the anisotropy is large, and the aspect ratio Str becomes a value close to 0. On the other hand, in a case where the unevenness on the surface of the test piece after stretching has no directionality, and there are no convex shapes or concave shapes extending long in one direction, the aspect ratio Str becomes a value close to 1. In order to improve the surface quality of the surface of the molded product, it is preferable that the aspect ratio Str of the surface of the test piece after stretching be large, and the anisotropy in the surface shape be small. Therefore, the aspect ratio Str of the surface properties of the test piece after stretching is preferably 0.28 or more. By making the aspect ratio Str of the test piece after stretching 0.28 or more, the ghost lines on the surface of the molded product are not too long, and it is possible to reduce the degree of reduction in the surface quality due to the ghost lines. It is preferable that the aspect ratio Str of the test piece after stretching be 0.30 or more, and more preferably 0.35 or more.

[0177] The measurement method of the aspect ratio Str of the test piece after stretching in the present embodiment is as follows. Specifically, a JIS No. 5 test piece is cut from a position 1 / 4 apart from the end of the steel sheet in the sheet width direction in a direction (width direction) at right angles to the rolling direction of the steel sheet, and the surface of the test piece is polished with polishing paper, so that the surface becomes a mirror surface. Next, 5% strain is applied to the test piece by a tensile test. The unevenness of the surface of the test piece after 5% strain is applied is measured with a laser microscope. The aspect ratio Str is calculated from the measurement result. Further, the aspect ratio Str can be calculated by processing the coordinate data of the surface shape obtained by the laser microscope with an analysis software in accordance with ISO 25178. In the analysis, an S filter is not used, and the L filter is set to 0.8 mm.

[0178] the average value H of the Vickers hardness H at the position of 1 / 4 in the thickness direction 1 / 4 AVE1 / 4 is 150 to 300

[0179] By making the average value H of the Vickers hardness H at the position of 1 / 4 in the thickness direction 1 / 4 AVE1 / 4 150 or more, it is possible to ensure that the tensile strength of the steel sheet is 540 MPa or more. In addition, by making the average value H of the Vickers hardness H at the position of 1 / 4 in the thickness direction 1 / 4 AVE1 / 4 300 or less, at the position of 1 / 4 in the thickness direction of the steel sheet, the steel sheet does not excessively harden, and it is possible to sufficiently exert the effect of flattening the unevenness on the surface at the time of rolling of the steel sheet.

[0180] The Vickers hardness in the present embodiment refers to the hardness obtained according to the Vickers hardness test of JIS Z 2244:2009. The average value H of the Vickers hardness H at the position of 1 / 4 in the thickness direction 1 / 4 AVE1 / 4 is determined by the following method. At the position of 1 / 4 in the thickness direction from the surface and the back surface of the steel sheet, 50 points are each determined at an interval of 150 μm in the rolling direction, and 100 points are collectively determined, and the average value thereof is taken as H AVE1 / 4 .

[0181] The average value H of the Vickers hardness H at the position of 1 / 2 in the thickness direction 1 / 2 AVE1 / 2 is 155 to 305

[0182] By making the average value H of the Vickers hardness H at the position of 1 / 2 in the thickness direction 1 / 2 AVE1 / 2 155 or more, it is possible to ensure that the tensile strength of the steel sheet is 540 MPa or more. In addition, by making the average value H of the Vickers hardness H at the position of 1 / 2 in the thickness direction 1 / 2 AVE1 / 2 305 or less, at the position of 1 / 2 in the thickness direction of the steel sheet, the steel sheet does not excessively harden, and it is possible to sufficiently exert the effect of flattening the unevenness on the surface at the time of rolling of the steel sheet.

[0183] The average value H of the Vickers hardness H at the position of 1 / 2 in the thickness direction 1 / 2 AVE1 / 2 is determined by the same method as the average value H of the Vickers hardness H at the position of 1 / 4 in the thickness direction 1 / 4 AVE1 / 4 except that the measurement position in the thickness direction is different.

[0184] The width of the steel sheet is 1000 mm or more​​​​​​​​​

[0185] The formed steel sheet of this embodiment is suitable for use as an automotive panel. Examples of automotive panels include panel-type components such as door panels. Examples of panel-type components include engine hood panels, side panels such as protective panels, door panels, and roof panels.

[0186] In automotive panels, the strength of hot-rolled steel sheets used in manufacturing, similar to that of automotive structural components, is continuously increased. Furthermore, as automotive panels become thinner, the reduction rate in the cold rolling process during steel sheet manufacturing also increases. Moreover, the width of automotive panel steel sheets, especially those for door panels, sometimes exceeds 1000 mm, and the width of steel sheets for engine hoods sometimes exceeds 1500 mm. The reduction load (mill load) during the cold rolling process tends to be greater for such wide steel sheets. For example, in steel sheets with a tensile strength of 540 MPa, the reduction load during cold rolling is particularly large when the width is around 1500 mm or more; similarly, in steel sheets with a tensile strength of 780 MPa, the reduction load during cold rolling is particularly large when the width is around 1200 mm or more.

[0187] If the increased reduction load during cold rolling is not addressed, the accuracy of the steel sheet shape will deteriorate. Furthermore, previous methods to address this increased reduction load during cold rolling, such as softening annealing before cold rolling and performing the cold rolling process in two stages, have resulted in low productivity and increased manufacturing costs.

[0188] On the other hand, in this embodiment, the steel plate is as follows: (i) it has the chemical composition and metallographic structure of this embodiment, and (ii) the Vickers hardness H at 1 / 4 position in the thickness direction is... 1 / 4 Standard deviation σ 1 / 4 Divide by Vickers hardness H 1 / 4 The average value H AVE1 / 4 The obtained value X1 is below 0.025, and the Vickers hardness H at the 1 / 2 position in the thickness direction of (iii) is... 1 / 2 Standard deviation σ 1 / 2 Divide by Vickers hardness H 1 / 2 The average value H AVE1 / 2 The obtained value X2 is less than 0.030. Thus, even for wide panels like those described above, (a) it is possible to reduce the rolling load during cold rolling by making the hot-rolled sheet structure softer, and (b) it is possible to reduce the ghost lines of the formed product.

[0189] (The thickness of the steel plate is 0.20~1.00mm)

[0190] The sheet thickness of the steel sheet of the present embodiment is not limited to a particular range, but is preferably 0.20 to 1.00 mm in view of general use and manufacturability. By setting the sheet thickness to 0.20 mm or more, it becomes easy to maintain the shape of the molded product to be flat, and it is possible to improve the dimensional accuracy and shape accuracy. Therefore, the sheet thickness is preferably 0.20 mm or more, preferably 0.35 mm or more, and more preferably 0.40 mm or more.

[0191] On the other hand, by making the sheet thickness 1.00 mm or less, the effect of weight reduction of the member becomes large. Therefore, the sheet thickness is preferably 1.00 mm or less, preferably 0.70 mm or less, and more preferably 0.60 mm or less. The sheet thickness of the steel sheet can be measured with a micrometer.

[0192] (Tensile strength of the steel sheet is 540 to 980 MPa)

[0193] The tensile strength of the steel sheet of the present embodiment is not limited to a particular range, but is preferably 540 to 980 MPa. By making the tensile strength of the steel sheet 540 MPa or more, it is possible to realize a thin-walled and high-strength steel sheet. In addition, by making the tensile strength of the steel sheet 980 MPa or less, it is easy to ensure the formability when the steel sheet is subjected to press working.

[0194] The tensile strength is measured by collecting a JIS No. 5 tensile test piece having a direction at right angles to the rolling direction as a length direction from the steel sheet and performing a test according to JIS (Japanese Industrial Standards) Z2241:2011 Metal Materials Tensile Test Method.

[0195] The steel sheet of the present embodiment can have a plated layer on at least one surface of the steel sheet. As the plated layer, a zinc plated layer and a zinc alloy plated layer, and an alloyed zinc plated layer and an alloyed zinc alloy plated layer obtained by applying an alloying treatment to these can be listed.

[0196] The zinc plated layer and the zinc alloy plated layer are formed by a hot-dip plating method, an electroplating method, or an evaporation plating method. When the Al content of the zinc plated layer is 0.5 mass% or less, it is possible to sufficiently ensure the adhesion between the surface of the steel sheet and the zinc plated layer, and therefore the Al content of the zinc plated layer is preferably 0.5 mass% or less.

[0197] In the case where the zinc plated layer is a hot-dip galvanized layer, in order to improve the adhesion between the surface of the steel sheet and the zinc plated layer, the Fe content of the hot-dip galvanized layer is preferably 3.0 mass% or less.

[0198] In the case where the zinc plated layer is an electroplated zinc plated layer, the Fe content of the electroplated zinc plated layer is preferably 0.5 mass% or less in consideration of improving corrosion resistance.

[0199] In a range where the corrosion resistance and formability of the steel sheet are not impaired, the zinc plating layer and the zinc alloy plating layer can contain one or two 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. In particular, Ni, Al, and Mg are effective in improving the corrosion resistance of the steel sheet.

[0200] The zinc plating layer or the zinc alloy plating layer can be an alloyed zinc plating layer or an alloyed zinc alloy plating layer to which an alloying treatment has been applied. In the case where the hot-dip zinc plating layer or the hot-dip zinc alloy plating layer is subjected to the alloying treatment, the Fe content of the hot-dip zinc plating layer (alloyed zinc plating layer) or the hot-dip zinc alloy plating layer (alloyed zinc alloy plating layer) after the alloying treatment is preferably 7.0 to 13.0 mass% from the viewpoint of improving the adhesion between the surface of the steel sheet and the plated layer. By applying the alloying treatment to the steel sheet having the hot-dip zinc plating layer or the hot-dip zinc alloy plating layer, Fe is introduced into the plated layer, and the Fe content increases. Thus, the Fe content can be made to be 7.0 mass% or more. That is, the 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.

[0201] The Fe content in the plated layer can be obtained by the following method. The plated layer is dissolved and removed only with a 5% by volume aqueous HCl solution to which a suppressor has been added. The Fe content in the plated layer (mass%) is obtained by measuring the Fe content in the obtained solution with ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry).

[0202] (Steel sheet is an outer panel of an automobile)

[0203] Next, a press-formed product that can be manufactured by press-forming the above-described steel sheet will be described. The press-formed product has the same chemical composition as the above-described steel sheet. In addition, the above-described press-formed product can have the above-described plated layer on at least one surface. The above-described press-formed product is a product obtained by press-forming the above-described steel sheet, and thus the occurrence of ghost lines is suppressed, and the appearance quality is excellent. As a result, an automobile having a high commodity value can be realized by making the appearance excellent, which is directly perceived by a consumer. As a specific example of the press-formed product, for example, a panel-like member such as an outer panel of a door of a vehicle body (an outer panel of an automobile) can be cited as described above. As the panel-like member, an outer panel of a hood, a side panel such as a fender, a door outer panel, a roof panel, and the like can be cited.

[0204] <Regarding the manufacturing method>

[0205] Next, a preferred manufacturing method of the steel sheet of the present embodiment will be described. The steel sheet of the present embodiment can obtain its effects as long as it has the above-described characteristics regardless of the manufacturing method. However, it is preferable if it is manufactured stably by the following method.

[0206] Specifically, the steel sheet of the present embodiment can be manufactured by a manufacturing method including the following steps (i) to (iv).

[0207] (i) a slab forming step of solidifying molten steel having the above-described chemical composition to form a slab;

[0208] (ii) a hot rolling step of heating the slab to perform hot rolling at a finish rolling temperature of 950°C or lower, and then coiling at 450 to 650°C after obtaining a hot-rolled steel sheet;

[0209] (iii) a cold rolling step of uncoiling the coiled hot-rolled steel sheet and performing cold rolling at a cumulative reduction ratio RCR of 50 to 90% to obtain a cold-rolled steel sheet;

[0210] (iv) a step of annealing the cold-rolled steel sheet and then forming the above-described plated layer as necessary.

[0211] Each of the steps will be described below.

[0212] [Slab forming step]

[0213] In the slab forming step, molten steel having a prescribed chemical composition is formed into a slab. The method of the slab forming step is not limited. For example, molten steel having the above-described chemical composition can be smelted by a converter or an electric furnace, and a slab can be manufactured by a continuous casting method. Instead of the continuous casting method, an ingot casting method, a thin slab casting method, or the like can be used.

[0214] [Hot rolling step]

[0215] Before hot rolling, the slab is heated to 1100°C or higher. By heating the temperature to 1100°C or higher, the rolling reaction force does not become excessively large in continuous hot rolling, and the target product thickness can be easily obtained. In addition, the accuracy of the slab shape can be improved, and coiling can be smoothly performed.

[0216] The upper limit of the heating temperature is not limited, but from an economic point of view, the slab heating temperature is preferably less than 1300°C.

[0217] In the hot rolling step, the slab heated to the above-described heating temperature is hot-rolled. In the hot rolling, finish rolling is performed after rough rolling. In the finish rolling, multiple reductions are performed.

[0218] The finish rolling is performed by a plurality of continuous rolling mills, and the reduction rate of the rolling mills in the latter half is made larger than that of the rolling mills in the former half. The reduction rate of the finish rolling in the former half is made smaller than 35%, and the reduction rate of the finish rolling in the latter half is made 35% or more. Thereby, the reduction rate of the finish rolling in the latter half can be increased, and as a result, the hot-rolled sheet as a hot-rolled product can be moderately softened. Therefore, the load of the rolling mill at the cold rolling step can be reduced. Also, the generation of hard phases such as pearlite and martensite in the structure of the hot-rolled sheet in a belt shape, and the generation of hard phases such as martensite in the structure of the molded product as a final product in a belt shape can be suppressed.

[0219] The ratio P2 / P1 of the reduction rate P1 of the rolling mills in the former half to the reduction rate P2 of the rolling mills in the latter half is preferably more than 1.0 and 1.6 or less. By making P2 / P1 more than 1.0, the hot-rolled sheet can be sufficiently softened, and the generation of hard phases in the structure of the molded product as a final product in a belt shape can be suppressed. Also, by making P2 / P1 1.6 or less, the load on the rolling mills in the latter half can be reduced.

[0220] The reduction rate of the final rolling mill is preferably 40% or more. Thereby, the generation of hard phases such as pearlite and martensite in the structure of the hot-rolled sheet in a belt shape, and the generation of hard phases such as martensite in the structure of the molded product as a final product in a belt shape can be more easily suppressed.

[0221] The rolling mills of the finish rolling are provided, for example, in a continuous manner of 7 rolling mills. In the present embodiment, the 1st rolling mill to the 3rd rolling mill are the rolling mills in the former half, and the 5th rolling mill to the 7th rolling mill are the rolling mills in the latter half. The number of the rolling mills is not limited, as long as the reduction rate of the rolling mills in the latter half is made larger than that of the rolling mills in the former half.

[0222] The finish rolling temperature is 950°C or less. By making the finish rolling temperature 950°C or less, the average grain diameter of the hot-rolled sheet can not become too large. In this case, the average grain diameter of the final product sheet can be also reduced, and sufficient yield strength and high surface quality after molding can be ensured.

[0223] The coiling temperature in the hot rolling step is preferably set to 450 to 650°C. By making the coiling temperature 650°C or less, the grain diameter can be made small, and sufficient strength of the steel sheet can be ensured. Further, the scale thickness can be suppressed, and thereby the pickling property can be sufficiently ensured. Also, by making the coiling temperature 450°C or more, the strength of the hot-rolled sheet can not be excessively increased, and the load on the equipment for the cold rolling step can be suppressed, and the productivity can be further improved.

[0224] [Cold rolling step]

[0225] In the cold rolling step, cold rolling is performed with a cumulative reduction ratio RCR of 50 to 90% to obtain a cold-rolled steel sheet. By performing cold rolling on a hot-rolled steel sheet to which a prescribed residual stress is applied with the above cumulative reduction ratio, a ferrite having a desired texture can be obtained after annealing and cooling.

[0226] By making the cumulative reduction ratio RCR 50% or more, the thickness of the billet in the hot rolling step can be sufficiently ensured by back-calculation from the thickness of the steel sheet, and the hot rolling step can be performed. In addition, by making the cumulative reduction ratio RCR 90% or less, the rolling load does not become excessively large, and the uniformity of the material in the width direction of the steel sheet can be sufficiently ensured. Furthermore, the stability of production can also be sufficiently ensured. Therefore, the cumulative reduction ratio RCR in the cold rolling is set to 50 to 90%.

[0227] [Annealing Step]

[0228] In the annealing step, the cold-rolled steel sheet is heated to a soaking temperature of 750 to 900°C and held. By making the soaking temperature 750°C or more, recrystallization of ferrite and reverse transformation from ferrite to austenite are sufficiently performed, and a desired texture can be obtained. On the other hand, by making the soaking temperature 900°C or less, the grains become densified, and sufficient strength can be obtained. Furthermore, the heating temperature does not become excessively high, and productivity can be improved.

[0229] [Cooling Step]

[0230] In the cooling step, the cold-rolled steel sheet after soaking in the annealing step is cooled. The cooling is performed at an average cooling rate from the soaking temperature of 5.0 to 50°C / sec. By making the above average cooling rate 5.0°C / sec or more, the ferrite phase transformation is not excessively promoted, the amount of generation of hard phases such as martensite can be increased, and a desired strength can be obtained. In addition, by making the average cooling rate 50°C / sec or less, the steel sheet can be more uniformly cooled in the width direction of the steel sheet.

[0231] [Plating Step]

[0232] The cold-rolled steel sheet obtained by the above method can also be subjected to a plating step in which a plated layer is formed on the surface.

[0233] [Alloying Step]

[0234] The plated layer formed in the plating step can be subjected to alloying. In the alloying step, the alloying temperature is, for example, 450 to 600°C.

[0235] According to the manufacturing method described above, by applying the high reduction ratio in the latter half of the finishing in the hot rolling step, a continuous steel sheet with a small amount of hard phase can be manufactured. Thereby, in the molded product after molding, the anisotropy of the surface unevenness shape is small, the generation of ghost lines can be suppressed, and excellent appearance quality can be obtained. Also, in terms of the manufacturability of the steel sheet, the hot rolled sheet can also be moderately softened, and the cold rolling workability can also be improved, rather than having to perform soft annealing and 2 times cold rolling.

[0236] Further, in the present embodiment, for the steel sheet after the hot rolling process, shape correction is not performed using a flattener as a shape correction device. In order to ensure high appearance quality, a high surface property is required for the steel sheet of the present embodiment. Therefore, in the present embodiment, a steel sheet that requires shape correction using a flattener cannot be used. In other words, the manufacturing method of the present embodiment is not envisaged using a special hot rolling process that includes arranging a flattener on the exit side of the finishing mill. Thus, the flattener is not combined with the manufacturing method of the steel sheet in the present embodiment.

[0237] Example

[0238] Next, an example of the present application will be described. Note that the conditions in the example are one example of conditions adopted in order to confirm the feasibility and effects of the present application, and the present application is not limited to this example of conditions. Various conditions can be adopted as long as the gist of the present application is not exceeded and the object of the present application can be achieved.

[0239] Steels having the chemical compositions shown in Table 1 for Steel Billet Nos. A to K were melted and cast to manufacture slabs having a thickness of 200 to 300 mm. For a part of the obtained slabs, hot rolling was performed under the conditions shown in Table 2, and coiling was performed. Further, at the time of finishing in the hot rolling, seven mills were arranged in series, and the first three mills (1st mill to 3rd mill) were arranged as the front half mills, and the last three mills (5th mill to 7th mill) were arranged as the rear half mills.

[0240] Then, the coiled material was uncoiled, and for the obtained hot rolled sheet, test pieces were cut out and the tensile strength was measured. The tensile strength was evaluated based on JIS Z 2241:2011. The test pieces were No. 5 test pieces of JIS Z 2241:2011. The collection position of the test pieces was the 1 / 4 portion from the end portion in the sheet width direction, and the direction perpendicular to the rolling direction was the length direction.

[0241] After pickling, cold rolling was performed at the cumulative reduction ratio RCR shown in Table 2 to obtain steel sheets A1 to K1.

[0242] After that, annealing and cooling were performed under conditions of the soaking temperature and the cooling rate (average cooling rate) shown in Table 3. In addition, a part of the steel sheets was subjected to various plating to form a plated layer on the surface, and was subjected to alloying treatment at the alloying temperature shown in Table 3. In Table 4, CR indicates no plating, GI indicates hot-dip galvanizing, GA indicates alloyed hot-dip galvanizing, and EG indicates electro-galvanizing.

[0243] For the obtained product sheets No. Al a to Kl a (that is, product sheets No. Al a to A2a, Bl a to B2a, Cl a to C2a, Dl a to D5a, El a, Fl a, Gl a, Hl a, Il a, Jl a, and Kl a), the sheet width and the sheet thickness were measured.

[0244] In addition, the tensile strength of the product sheets No. Al a to Kl a was measured. The tensile strength was evaluated based on JIS Z 2241:2011. The test piece was No. 5 test piece of JIS Z 2241:2011. The collection position of the tensile test piece was the 1 / 4 portion from the end portion in the sheet width direction, with the direction perpendicular to the rolling direction as the length direction. In the case where the obtained tensile strength was 540 MPa or more, it was regarded as high strength, and was determined to be acceptable. On the other hand, in the case where the obtained tensile strength was less than 540 MPa, it was regarded as poor strength, and was determined to be unacceptable.

[0245] In addition, the volume fraction of ferrite and hard phase in the microstructure of the obtained product sheets No. Al a to Kl a was measured by the above-described method. The sum of the volume fractions of the hard phase and the ferrite in the microstructure of the product sheets No. Al a to Kl a was 100%.

[0246] In addition, the average grain diameter of ferrite and the average grain diameter of hard phase in the microstructure of the obtained product sheets No. Al a to Kl a were measured by the above-described method.

[0247] The results are shown in Table 4.

[0248] [Table 1]

[0249] [Table 1]

[0250]

[0251] The underlined portions indicate ranges outside the scope of the present application.

[0252] [Table 2]

[0253] [Table 2]

[0254]

[0255] The underlined portions indicate ranges outside the preferred scope of the present application.

[0256] [Table 3]

[0257] [Table 3]

[0258]

[0259] [Table 4]

[0260] [Table 4]

[0261]

[0262] The underlined portions are outside the scope of the present application or outside the preferred range.

[0263] Further, for the obtained product sheets No. Al a to Kl a, for the position 1 / 4 from the surface in the sheet thickness direction, the Vickers hardness H of 50 points was measured at a measurement interval of 150 μm in the rolling direction by the above method. 1 / 4 Then, for the position 1 / 4 from the back surface in the sheet thickness direction, the Vickers hardness H of 50 points was measured at a measurement interval of 150 μm in the rolling direction by the above method. 1 / 4 Then, the standard deviation σ of the Vickers hardness H of these 50 points was calculated. 1 / 4 1 / 4 The value Xl obtained by dividing the standard deviation σ of the Vickers hardness H of these 50 points by the average value H of the Vickers hardness H of 50 points was calculated. 1 / 4 AVE1 / 4

[0264] Further, for the obtained product sheets No. Al a to Kl a, for the position 1 / 2 from the surface in the sheet thickness direction, the Vickers hardness H of 50 points was measured at a measurement interval of 150 μm in the rolling direction by the above method. 1 / 2 Then, the standard deviation σ of the Vickers hardness H of these 50 points was calculated. 1 / 2 1 / 2 The value X2 obtained by dividing the standard deviation σ of the Vickers hardness H of these 50 points by the average value H of the Vickers hardness H of 50 points was calculated. 1 / 2 AVE1 / 2

[0265] Further, for the obtained product sheets No. Al a to Kl a, in the region of 1 / 4 to 1 / 2 in the sheet thickness direction, the area ratio of the hard phase of 100 μm or more in the rolling direction was measured by the above method.

[0266] Further, for each product sheet No. Al a to Kl a, the surface was made into a mirror surface state using an abrasive paper or the like to obtain a tensile test piece, and the aspect ratio Str of the surface properties after applying a strain of 5% to the tensile test piece by a tensile test was measured by the above method.

[0267] ​​​​​​In addition, for each of the product plates No. Al a to Kl a, a surface thereof was made into a mirror surface state with abrasive paper or the like to obtain a tensile test piece, and the surface roughness Wa (arithmetic mean waviness) after applying a 5% strain to the tensile test piece by a tensile test was measured by the following method. 50 profile lines were measured in a direction at right angles to the rolling direction with a laser displacement measuring device (Keyence VK-X1000). At this time, components having a wavelength of 0.8 mm or less and 2.5 mm or more were removed. From the obtained results, the arithmetic mean waviness was calculated in accordance with JIS B0601:2013, and the average of the total of 50 profile lines was calculated. Thus, the surface roughness Wa of the product plate was obtained.

[0268] In addition, the product of the tensile strength and the aspect ratio Str of the surface properties of the tensile test piece after the tensile test was calculated for each of the product plates No. Al a to Kl a. The tensile strength TS x the aspect ratio Str is an index in which the higher it is, the more it indicates that the anisotropy of the concave-convex shape of the surface is small despite being high in strength and low in workability.

[0269] The results are shown in Table 5.

[0270] [Table 5]

[0271] [Table 5]

[0272]

[0273] The underlined indicates outside the range of the present application or outside the preferred range.

[0274] As shown in Tables 1 to 5, the aspect ratio Str of the surface state of the test piece after stretching in the examples has a tendency to be significantly higher than the aspect ratio Str of the surface state of the test piece after stretching in the comparative examples, whereby the anisotropy of the concave-convex shape of the surface is small, and it is excellent in strength and surface quality. In more detail, the tensile strength of the examples is all greater than 540 MPa, and it is high-strength. Also, the aspect ratio Str of the surface state of the test piece after stretching in the examples is 0.28 or more, the area of the hard phase of 100 μm or more is 30% or less with respect to the total area of the hard phase, and the ghost line is sufficiently suppressed. Also, in the examples, it is shown that the tensile strength TS x the aspect ratio Str is greater than 200 and is sufficiently high, and although it is high-strength and the workability is low, the anisotropy of the concave-convex shape of the surface is small. Also, the average value of (tensile strength of product sheet - tensile strength of hot-rolled sheet) in the 10 examples is 77, and the average value of (tensile strength of product sheet - tensile strength of hot-rolled sheet) in the 8 comparative examples is about 54. That is, in the examples, the difference between the tensile strength of the product sheet and the tensile strength of the hot-rolled sheet is sufficiently produced, and the softening of the hot-rolled sheet is achieved. Especially, for the product sheet of a large width suitable for an automobile engine hood sheet, an automobile door sheet, it is confirmed that the load of the rolling mill in the cold rolling process is reduced.

[0275] On the other hand, in the product plates No. A2a, B2a as comparative examples, the reduction ratio in the latter half of the finishing in the hot rolling is small, so that the stripe-shaped concave-convex on the surface of the steel plate cannot be sufficiently flattened, the area ratio of the hard phase of 100 μm or more in the rolling direction in the region of 1 / 4 to 1 / 2 of the rolling direction is more than 40%, further, the aspect ratio Str of the surface state of the test piece after the tensile test is less than 0.28, and the tensile strength TS x the aspect ratio Str is less than 180, so that the surface quality after the molding is low. Further, in the product plates No. C2a, D2a as comparative examples, the reduction ratio in the latter half of the finishing in the hot rolling is small, so that the stripe-shaped concave-convex on the surface of the steel plate cannot be sufficiently flattened, the area ratio of the hard phase of 100 μm or more in the rolling direction in the region of 1 / 4 to 1 / 2 of the rolling direction is more than 30%, further, the aspect ratio Str of the surface state of the test piece after the tensile test is less than 0.28, and the tensile strength TS x the aspect ratio Str is less than 170, so that the surface quality after the molding is low. Further, in the product plate No. D5a as a comparative example, the ratio P2 / P1 of the reduction ratio P1 in the former half to the reduction ratio P2 in the latter half of the finishing in the hot rolling is in the range of more than 1.0 and 1.6 or less, but the reduction ratio in the latter half is small, so that the stripe-shaped concave-convex on the surface of the steel plate cannot be sufficiently flattened, the area ratio of the hard phase of 100 μm or more in the rolling direction in the region of 1 / 4 to 1 / 2 of the rolling direction is more than 30%, further, the aspect ratio Str of the surface state of the test piece after the tensile test is less than 0.28, and the tensile strength TS x the aspect ratio Str is less than 170, so that the surface quality after the molding is low.

[0276] Further, in the product plate No. E1a as a comparative example, the content of carbon exceeds the preferable range, so that the band-shaped Mn segregation is easily generated. As a result, the area ratio of the hard phase of 100 μm or more in the rolling direction in the region of 1 / 4 to 1 / 2 of the rolling direction is more than 30%, further, the tensile strength TS x the aspect ratio Str is less than 180, so that the surface quality after the molding is low. Further, in the product plate No. F1a as a comparative example, the content of carbon does not reach the preferable range, the volume fraction of ferrite is too large and the volume fraction of the hard phase is small, so that the tensile strength of the product plate is low, and does not reach 540 MPa. Further, in the product plate No. G1a as a comparative example, the content of Mn exceeds the preferable range, so that the band-shaped Mn segregation is generated at the solidification of the steel. As a result, the area ratio of the hard phase of 100 μm or more in the rolling direction in the region of 1 / 4 to 1 / 2 of the rolling direction is more than 40%, further, the tensile strength TS x the aspect ratio Str is less than 170, so that the surface quality after the molding is low.

[0277] Here, the product sheets No. Ala and A2a, No. Bla and B2a, No. Cla and C2a, No. Dla and D2a having the same sheet thickness were compared. The surface roughness Wa of the product sheets No. Ala, Bla, Cla, Dla as examples was 0.058 μm, 0.055 μm, 0.058 μm, 0.055 μm, respectively. On the other hand, the surface roughness Wa of the product sheets No. A2a, B2a, C2a, D2a as comparative examples was 0.050 μm, 0.053 μm, 0.056 μm, 0.055 μm, respectively. Thus, the surface roughness Wa of the product sheet No. Ala as an example was higher than that of the product sheet No. A2a as a comparative example, and the surface roughness Wa of the product sheets No. Bla, Cla, Dla as examples was higher than that of the product sheets No. B2a, C2a, D2a as comparative examples, respectively. On the other hand, the aspect ratio Str of the product sheets No. Ala, Bla, Cla, Dla as examples was higher than that of the product sheets No. A2a, B2a, C2a, D2a as comparative examples. Thus, for the product sheets No. Ala, Bla, Cla, Dla as examples, although the surface roughness Wa was higher than that of the product sheets No. A2a, B2a, C2a, D2a as comparative examples, respectively, the aspect ratio Str was high, and it was confirmed that the anisotropy of the unevenness of the surface was small, and the surface quality was excellent.

[0278] Industrial Applicability

[0279] According to the above-described mode of the present application, a steel sheet in which excellent appearance quality can be achieved in a molded product can be provided.

Claims

1. A steel sheet having a chemical composition, in mass%, of C: 0.030% to 0.145%, Si: 0% to 0.500%, Mn: 0.50% to 2.50%, P：0％～0.100％、 S：0％～0.020％、 Al：0％～1.000％、 N：0％～0.0100％、 B：0％～0.0050％、 Mo: 0% to 0.80%, Ti: 0% to 0.200%, Nb: 0% to 0.10%, V：0％～0.20％、 Cr：0％～0.80％、 Ni: 0% to 0.25% O：0％～0.0100％、 Cu: 0% to 1.00%, W：0％～1.00％、 Sn: 0% to 1.00%, Sb: 0% to 0.20%, Ca: 0% to 0.0100%, Mg: 0% to 0.0100%, Zr：0％～0.0100％、 REM: 0% to 0.0100%, the balance being iron and impurities, a microstructure consisting of 70 to 95% by volume of ferrite and 5 to 30% by volume of hard phases, Vickers hardness H at a position 1 / 4 from the plate thickness direction 1 / 4 a value X1 obtained by dividing the standard deviation of the Vickers hardness H 1 / 4 by the average value of the Vickers hardness H is 0.025 or less, Vickers hardness H at a position 1 / 2 of the plate thickness direction 1 / 2 The value X2 obtained by dividing the standard deviation of the Vickers hardness H 1 / 2 by the average value of the Vickers hardness H is 0.030 or less.

2. The steel sheet according to claim 1, characterized by the average grain diameter of the ferrite being 5.0 to 30.0 μm and the average grain diameter of the hard phases being 1.0 to 5.0 μm.

3. Steel sheet according to claim 1 or 2, characterized in that, In a region of 1 / 4 to 1 / 2 of the sheet thickness direction, the area of the hard phases of 100 μm or more in the rolling direction is 30% or less relative to the total area of the hard phases.

4. The steel sheet according to any one of claims 1 to 3, characterized by, The aspect ratio Str (ISO 25178) of the surface properties of a test piece to which 5% strain is applied by a tensile test is 0.28 or more.

5. The steel sheet according to any one of claims 1 to 4, characterized by, Vickers hardness H at 1 / 4 position in plate thickness direction 1 / 4 150 to 300, Vickers hardness H at 1 / 2 position in plate thickness direction 1 / 2 The average value is 155 to 305.

6. The steel sheet according to any one of claims 1 to 5, characterized by, The hard phases consist of one or more of martensite, bainite, tempered martensite, and pearlite.

7. The steel sheet according to any one of claims 1 to 6, characterized by, The sheet thickness of the steel sheet is 0.20 mm to 1.00 mm.

8. The steel sheet according to any one of claims 1 to 7, characterized by, The steel sheet is an automobile outer panel.

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