Steel sheet

Abstract

To provide a steel sheet that can achieve an excellent quality of appearance in a molded article.SOLUTION: In a steel sheet, a 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% or the like in mass%, a metallographic structure consists of ferrite having the volume fraction of 70 to 95% and a hard phase having the volume fraction of 5 to 30%, a value X1 obtained by dividing the standard deviation of Vickers hardness H1 / 4 at a sheet thickness direction 1 / 4 position by the average value of the Vickers hardness H1 / 4 is 0.025 or less, and a value X2 obtained by dividing the standard deviation of Vickers hardness H1 / 2 at a sheet thickness direction 1 / 2 position by the average value of Vickers hardness H1 / 2 is 0.030 or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a steel sheet. [Background technology]

[0002] From the perspective of protecting the global environment, there is a growing need to reduce the weight of not only structural components such as members but also panel components such as roofs and door outers in order to improve fuel efficiency. Unlike skeletal components, these panel components are visible to the public, so they also require high appearance quality. Appearance quality includes design and surface quality.

[0003] Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet with excellent surface quality. Specifically, Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet containing, by mass%, 0.02 to 0.20% C, 0.7% or less Si, 1.5 to 3.5% Mn, 0.10% or less P, 0.01% or less S, 0.1 to 1.0% Al, 0.010% or less N, and 0.03 to 0.5% Cr, wherein the steel sheet has a surface oxidation index A during annealing of 2.3 or more, defined by the formula A = 400Al / (4Cr + 3Si + 6Mn) where the contents of Al, Cr, Si, and Mn are the same as those of the other elements, with the balance being Fe and unavoidable impurities, and further wherein the steel sheet (substrate) has a structure consisting of ferrite and a second phase, the second phase being mainly martensite, and wherein the steel sheet has a hot-dip galvanized layer on the surface of the substrate. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-220430 Summary of the Invention [Problem to be solved by the invention]

[0005] One challenge to improving appearance quality is to suppress the occurrence of ghost lines. Ghost lines are minute irregularities on the order of 1 mm that appear on the surface when steel sheets with hard and soft phases, such as dual phase (DP) steel, are press-formed. This occurs when the area around the soft phase deforms preferentially. These irregularities appear as streaks on the surface, so press-formed products with ghost lines have poor appearance quality.

[0006] As panel parts become stronger and thinner to reduce the weight of automobiles, and as their shapes become more complex, the surface of steel sheets after forming tends to become uneven, making ghost lines more likely to occur.

[0007] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a steel sheet that can realize excellent appearance quality in formed products. [Means for solving the problem]

[0008] The present invention relates to the following steel sheet.

[0009] (1) Chemical composition in mass % C: 0.030%~0.145%, Si: 0% to 0.500%, Mn: 0.50%~2.50%, P: 0%~0.100%, S: 0%~0.020%, Al: 0% to 1.000%, N: 0% to 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% to 0.0100%, Cu: 0% to 1.00%, W: 0% to 1.00%, Sn: 0% to 1.00%, Sb: 0% to 0.20%, Ca: 0% to 0.0100%, Mg: 0% to 0.0100%, Zr: 0% to 0.0100%, REM: 0%~0.0100%, the balance being iron and impurities; The metal structure is composed of ferrite with a volume fraction of 70 to 95% and a hard phase with a volume fraction of 5 to 30%, Vickers hardness H at 1 / 4 position in the thickness direction 1 / 4 The standard deviation of the Vickers hardness H 1 / 4 The value X1 divided by the average value of is 0.025 or less, Vickers hardness H at 1 / 2 position in the thickness direction 1 / 2 The standard deviation of the Vickers hardness H 1 / 2 The value X2 divided by the average value is 0.030 or less, Steel plate.

[0010] (2) The steel sheet according to (1) above, wherein the ferrite has an average grain size of 5.0 to 30.0 μm, and the hard phase has an average grain size of 1.0 to 5.0 μm.

[0011] (3) The steel sheet according to (1) or (2), characterized in that in the region of 1 / 4 to 1 / 2 in the sheet thickness direction, the area of ​​hard phases connected in the rolling direction by 100 μm or more is 30% or less of the area of ​​the entire hard phases.

[0012] (4) The steel sheet according to any one of (1) to (3), characterized in that the aspect ratio Str (ISO25178) of the surface texture of a test piece after applying 5% strain in a tensile test is 0.28 or more.

[0013] (5) Vickers hardness H at 1 / 4 position in the thickness direction 1 / 4 The average value is 150 to 300. Vickers hardness H at 1 / 2 position in the thickness direction 1 / 2The steel sheet according to any one of (1) to (4) above, wherein the average value of is 155 to 305.

[0014] (6) The steel sheet according to any one of (1) to (5), wherein the hard phase comprises at least one of martensite, bainite, tempered martensite, and pearlite.

[0015] (7) The steel sheet according to any one of (1) to (6), characterized in that the thickness of the steel sheet is 0.20 mm to 1.00 mm.

[0016] (8) The steel sheet according to any one of (1) to (7), characterized in that the steel sheet is an outer panel of an automobile. [Effects of the Invention]

[0017] According to the above aspect of the present invention, it is possible to provide a steel sheet that can realize excellent appearance quality in a formed product. DETAILED DESCRIPTION OF THE INVENTION

[0018] <How the present invention was conceived> The present inventors have investigated a method for suppressing the occurrence of ghost lines after press-forming of high-strength steel sheets. As described above, in steel sheets that contain a mixture of hard and soft phases, such as DP (Dual Phase) steel, deformation occurs mainly around the soft phase during forming, resulting in minute irregularities on the steel sheet surface, which can cause appearance defects known as ghost lines. Ghost lines occur in a band-like (striped) shape when the soft phase is depressed during press-forming of the steel sheet, while the hard phase is not depressed or is instead deformed by rising up to become convex. The band-like structure is formed in a hard phase such as martensite.

[0019] As a result of extensive research, the present inventors have found that it is possible to suppress the band-shaped hard phase in the final product by controlling the hot-rolled structure during the production of steel sheet and suppressing the band-shaped structure.

[0020] The present invention has been made based on the above findings, and the steel sheet according to this embodiment will be described in detail below. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications are possible within the scope of the present invention.

[0021] First, the chemical composition of the steel sheet according to this embodiment will be described. Below, numerical ranges indicated with "to" include the lower and upper limits. Numerical values ​​indicated as "less than" or "greater than" do not include the numerical range. In the following description, percentages relating to chemical composition are mass percent unless otherwise specified.

[0022] The steel sheet according to this embodiment has a chemical composition, in mass%, C: 0.030%~0.145%, Si: 0% to 0.500%, Mn: 0.50%~2.50%, P: 0%~0.100%, S: 0%~0.020%, Al: 0% to 1.000%, N: 0% to 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% to 0.0100%, Cu: 0% to 1.00%, W: 0% to 1.00%, Sn: 0% to 1.00%, Sb: 0% to 0.20%, Ca: 0% to 0.0100%, Mg: 0% to 0.0100%, Zr: 0% to 0.0100%, REM: 0%~0.0100%, The balance is iron and impurities. Each element will be explained below.

[0023] (C: 0.030% to 0.145%) C is an element that increases the strength of a steel sheet. To obtain a desired strength, the C content is set to 0.030% or more. To further increase 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 still more preferably 0.060% or more. Furthermore, by setting the C content to 0.145% or less, the diffusion of Mn during solidification is promoted, which makes it possible to suppress the likelihood of band-shaped Mn segregation. 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, and more preferably 0.090% or less.

[0024] (Si: 0% to 0.500%) Silicon is a deoxidizing element for steel and is effective in increasing the strength of steel sheets without impairing their ductility. By setting the Si content to 0.500% or less, it is possible to suppress the occurrence of surface defects due to a decrease in scale peelability. 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, and even more preferably 0.100% or less. The lower limit of the Si content includes 0%, but in order to improve the balance between strength and formability of the steel sheet, the Si content may be 0.0005% or more or 0.0010% or more, more preferably more than 0.090%, and even more preferably 0.100% or more.

[0025] (Mn: 0.50% to 2.50%) Mn is an element that improves the hardenability of steel and contributes to improving its strength. 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, even more preferably more than 1.60%, and even more preferably 1.65% or more. Furthermore, if the Mn content is 2.50% or less, the occurrence of striped Mn segregation during solidification of the steel can be suppressed. Therefore, the Mn content is set to 2.50% or less. The Mn content is preferably 2.25% or less, more preferably 2.00% or less, and even more preferably 1.80% or less.

[0026] (P:0%~0.100%) P is an element that embrittles steel. If the P content is 0.100% or less, it is possible to prevent the steel sheet from becoming embrittled and prone to cracking during the production process. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.080% or less, and more preferably 0.050% or less. Although the lower limit of the P content includes 0%, the P content may be set to 0.001% or more, since the production costs can be further reduced by setting the P content to 0.001% or more.

[0027] (S:0%~0.020%) S is an element that forms manganese sulfides and deteriorates the formability of steel sheets, such as ductility, hole expandability, stretch flangeability, and bendability. If the S content is 0.020% or less, a significant decrease in the formability of the steel sheets can be suppressed. Therefore, the S content is set to 0.020% or less. The S content is preferably 0.010% or less, and more preferably 0.008% or less. Although the lower limit of the S content includes 0%, by setting the S content to 0.0001% or more, the manufacturing cost can be further reduced, and therefore the S content may be set to 0.0001% or more.

[0028] (Al: 0% to 1.000%) Al is an element that functions as a deoxidizer and is effective in increasing the strength of steel. By setting the Al content to 1.000% or less, castability can be improved, thereby increasing productivity. 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 even more preferably 0.500% or less. The lower limit of the Al content includes 0%, but in order to obtain a sufficient deoxidizing effect of Al, the Al content may be set to 0.005% or more.

[0029] (N:0%~0.0100%) N is an element that forms nitrides and deteriorates the formability of steel sheets, such as ductility, hole expandability, stretch flangeability, and bendability. When the N content is 0.0100% or less, the deterioration of formability of the steel sheets can be suppressed. Therefore, the N content is set to 0.0100% or less. N is also an element that causes welding defects during welding and hinders productivity. Therefore, the N content is preferably 0.0080% or less, more preferably 0.0070% or less, and even more preferably 0.0040% or less. Although the lower limit of the N content includes 0%, the production cost can be further reduced by setting the N content to 0.0005% or more, and therefore the N content may be set to 0.0005% or more.

[0030] The steel sheet according to this embodiment may contain the following elements as optional elements. When the following optional elements are not contained, the content is 0%.

[0031] (B: 0% to 0.0050%) B is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel sheet. Since B is not necessarily contained, the lower limit of the B content includes 0%. In order to fully obtain the strength-improving effect of B, the B content is preferably 0.0001% or more, more preferably 0.0005% or more, and even more preferably 0.0010% or more. Furthermore, if the B content is 0.0050% or less, it is possible to prevent the formation of B precipitates and the resulting decrease in strength of the steel sheet. Therefore, the B content is set to 0.0050% or less, and preferably 0.0030% or less. The B content may be 0.0001% to 0.0050%.

[0032] (Mo: 0% to 0.80%) Mo is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of steel sheets. Mo is not necessarily contained, so the lower limit of the Mo content includes 0%. To fully obtain the strength-improving effect of Mo, the Mo content is preferably 0.001% or more, more preferably 0.05% or more, and even more preferably 0.10% or more. Furthermore, if the Mo content is 0.80% or less, it is possible to suppress a decrease in hot workability and a decrease in productivity. Therefore, the Mo content is set to 0.80% or less, preferably 0.40% or less, and more preferably 0.20% or less. The Mo content may be 0.001% to 0.80%, or may be 0% to 0.40%. It is preferable to include both Cr and Mo in such a manner that the Cr content is 0.20% to 0.80% and the Mo content is 0.05% to 0.80%, since this can more reliably improve the strength of the steel sheet.

[0033] (Ti: 0% to 0.200%) Ti is an element that has the effect of reducing the amounts of S, N, and O, which generate coarse inclusions that act as fracture initiation sites. Ti also has the effect of refining the structure and improving the strength-formability balance of the steel sheet. Since Ti is not necessarily contained, the lower limit of the Ti content includes 0%. To fully obtain the above effects, the Ti content is preferably 0.001% or more, and more preferably 0.010% or more. Furthermore, when the Ti content is 0.200% or less, the formation of coarse Ti sulfides, Ti nitrides, and Ti oxides can be suppressed, and the formability of the steel sheet can be ensured. Therefore, the Ti content is set to 0.200% or less. The Ti content is preferably set to 0.080% or less, and more preferably set to 0.060% or less. The Ti content may be 0% to 0.100%, or may be 0.001% to 0.200%.

[0034] (Nb: 0% to 0.10%) Nb is an element that contributes to improving the strength of steel sheets by strengthening through precipitates, strengthening through grain refinement by inhibiting the growth of ferrite crystal grains, and strengthening through dislocations by inhibiting recrystallization. Since Nb is not necessarily contained, the lower limit of the Nb content includes 0%. To fully obtain the above effects, the Nb content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.01% or more. Furthermore, when the Nb content is 0.10% or less, recrystallization is promoted and the remaining unrecrystallized ferrite can be suppressed, thereby ensuring the formability of the steel sheet. 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 may be 0.001% to 0.10%.

[0035] (V:0%~0.20%) V is an element that contributes to improving the strength of steel sheets by strengthening through precipitates, strengthening through grain refinement by inhibiting ferrite grain growth, and strengthening through dislocations by inhibiting recrystallization. V is not necessarily contained, so the lower limit of the V content includes 0%. To fully obtain the strength-improving effect of V, the V content is preferably 0.001% or more, more preferably 0.01% or more, and even more preferably 0.03% or more. Furthermore, if the V content is 0.20% or less, it is possible to prevent a large amount of carbonitrides from precipitating, which would otherwise reduce the formability of the steel sheet. Therefore, the V content is set to 0.20% or less. The V content is preferably 0.10% or less. The V content may be 0% to 0.10%, or may be 0.001% to 0.20%.

[0036] (Cr: 0% to 0.80%) Cr is an element that improves the hardenability of steel and contributes to improving the strength of steel sheet. Since Cr is not necessarily contained, the lower limit of the Cr content includes 0%. To fully obtain the strength-improving effect of Cr, the Cr content is preferably 0.001% or more, more preferably 0.20% or more, and particularly preferably 0.30% or more. Furthermore, if 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. The Cr content is preferably 0.70% or less, and more preferably 0.50% or less. The Cr content may be 0% to 0.70%, or may be 0.001% to 0.80%.

[0037] (Ni: 0% to 0.25%) Ni is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of steel sheets. Ni is not necessarily contained, so the lower limit of the Ni content includes 0%. In order to fully obtain the strength-improving effect of Ni, the Ni content is preferably 0.001% or more, and more preferably 0.05% or more. Furthermore, when the Ni content is 0.25% or less, it is possible to prevent the weldability of the steel sheet from decreasing. 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 may be 0.001% to 0.20%.

[0038] Below, preferred contents of O, Cu, W, Sn, Sb, Ca, Mg, Zr, and REM are described as optional elements. However, none of O, Cu, W, Sn, Sb, Ca, Mg, Zr, and REM contribute to reducing ghost lines within the content ranges exemplified below. In other words, in this embodiment, O, Cu, W, Sn, Sb, Ca, Mg, Zr, and REM do not affect the effect of reducing the anisotropy of the surface irregularities after forming by applying a later-stage large reduction in which the reduction rate is increased in the latter half of the finish rolling in the hot rolling process described below, thereby reducing the amount of connected hard phases.

[0039] (O:0%~0.0100%) O is an element that is mixed in during the manufacturing process. The O content may be 0%. Note that by setting the O content to 0.0001% or more, the refining time can be shortened and productivity can be increased. Therefore, the O content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if the O content is 0.0100% or less, the 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 may be 0.0070% or less, 0.0040% or less, or 0.0020% or less.

[0040] (Cu: 0% to 1.00%) Cu is an element present in steel in the form of fine particles and contributes to improving the strength of steel sheet. The Cu content may be 0%, but to obtain this effect, the Cu content is preferably 0.001% or more. The Cu content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, by setting the Cu content to 1.00% or less, the weldability of the steel sheet can be improved. Therefore, the Cu content is set to 1.00% or less. The Cu content may be 0.60% or less, 0.40% or less, or 0.20% or less.

[0041] (W:0%~1.00%) W is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel sheet. The W content may be 0%, but to obtain this effect, the W content is preferably 0.001% or more. The W content may be 0.01% or more, 0.02% or more, or 0.10% or more. On the other hand, by setting the W content to 1.00% or less, hot workability can be improved and productivity can be increased. Therefore, the W content is set to 1.00% or less. The W content may be 0.80% or less, 0.50% or less, or 0.20% or less.

[0042] (Sn: 0% to 1.00%) Sn is an element that suppresses coarsening of crystal grains and contributes to improving the strength of steel sheet. The Sn content may be 0%, but to obtain this effect, the Sn content is preferably 0.001% or more. The Sn content may be 0.01% or more, 0.05% or more, or 0.08% or more. On the other hand, by setting the Sn content to 1.00% or less, embrittlement of the steel sheet can be suppressed. Therefore, the Sn content is set to 1.00% or less. The Sn content may be 0.80% or less, 0.50% or less, or 0.20% or less.

[0043] (Sb: 0% to 0.20%) Sb is an element that suppresses coarsening of crystal grains and contributes to improving the strength of steel sheet. The Sb content may be 0%, but to obtain this effect, the Sb content is preferably 0.001% or more. The Sb content may be 0.01% or more, 0.05% or more, or 0.08% or more. On the other hand, by setting the Sn content to 0.20% or less, embrittlement of the steel sheet can be suppressed. Therefore, the Sb content is set to 0.20% or less. The Sb content may be 0.18% or less, 0.15% or less, or 0.12% or less.

[0044] (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 improving the formability of steel sheet. The Ca, Mg, Zr, and REM contents may be 0%, but to achieve these effects, the Ca, Mg, Zr, and REM contents are preferably 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, by limiting the Ca, Mg, Zr, and REM contents to 0.0100% or less, the ductility of the steel sheet can be ensured. Therefore, the Ca, Mg, Zr, and REM contents may be 0.0100% or less, 0.0080% or less, 0.0060% or less, or 0.0030% or less. In this specification, REM is a collective term for 17 elements: scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content is the total content of these elements.

[0045] The balance of the chemical composition of the steel sheet according to this embodiment may be Fe and impurities. Examples of impurities include elements that are mixed in from steel raw materials or scrap and / or during the steelmaking process, or elements that are acceptable to the extent that they do not impair the properties of the steel sheet according to this embodiment. Examples of impurities include 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, and Po. The total content of impurities may be 0.200% or less.

[0046] The chemical composition of the steel sheet described above may be measured by a common analytical method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S may be measured using a combustion-infrared absorption method, and N may be measured using an inert gas fusion-thermal conductivity method. If the steel sheet has a coating layer on its surface, the coating layer may be removed by mechanical grinding before analyzing the chemical composition.

[0047] (The metal structure consists of 70 to 95% ferrite by volume fraction and 5 to 30% hard phase by volume fraction) By setting the volume fraction of the hard phase in the metal structure to 5% or more, the strength of the steel sheet can be sufficiently improved. Therefore, the volume fraction of the hard phase is set to 5% or more. On the other hand, by setting the volume fraction of the hard phase to 30% or less, the hard phase can be dispersed more uniformly, which reduces surface irregularities during forming and improves the appearance after forming. The remainder of the metal structure other than the hard phase is ferrite, and the volume fraction of the ferrite is 70 to 95%. The volume fraction of ferrite is preferably 72% or more, more preferably 75% or more. The volume fraction of the hard phase is preferably 28% or less, more preferably 25% or less. The total volume fraction of the ferrite and hard phase in the metal structure is 100%.

[0048] In the steel sheet according to this embodiment, the hard phase is a hard structure harder than ferrite, and is composed of, for example, one or more of martensite, bainite, tempered martensite, and pearlite. From the viewpoint of improving strength, the hard phase is preferably composed of one or more of martensite, bainite, and tempered martensite, and more preferably composed of martensite.

[0049] The volume fraction of the hard phase in the metal structure can be determined by the following method. A sample for metallographic (microstructure) observation (size: approximately 20 mm in the rolling direction × 20 mm in the width direction × steel plate thickness) is taken from the W / 4 or 3W / 4 position of the sheet width W of the obtained steel sheet (i.e., W / 4 position in the width direction from either end of the steel sheet), and the metallographic (microstructure) is observed from the surface to 1 / 2 the sheet thickness using an optical microscope, and the area fraction of the hard phase from the surface of the steel sheet (the surface excluding the plating layer if plating is present) to 1 / 2 the sheet thickness is calculated. To prepare the sample, the sheet thickness cross section perpendicular to the rolling direction is polished as the observation surface and etched with LePeller's reagent.

[0050] The "microstructure" is classified from optical microscope photographs at 500 or 1000x magnification. When observing with an optical microscope after Lepera corrosion, each structure is observed in a different color; for example, bainite and pearlite are black, martensite (including tempered martensite) is white, and ferrite is gray, making it easy to distinguish between ferrite and other hard structures. In optical microscope photographs, the areas other than the gray color that indicates ferrite are the hard phase.

[0051] Ten fields of view were observed at 500x or 1000x magnification in the area from the surface of the steel plate etched with LePeller's reagent to a position halfway through the thickness in the thickness direction, and image analysis was performed using Adobe Photoshop CS5 image analysis software to determine the area fraction of the hard phase. max and the minimum brightness value L min and are obtained from the image, and the brightness is L max -0.3(L max -L min ) to L max The area with pixels up to L is the white area. min From L min +0.3(L max -L min ) are defined as black regions, and the other regions as gray regions, and the area fraction of the hard phase, which is the region other than the gray regions, is calculated. Image analysis is performed in the same manner as above to measure the area fraction of the hard phase for a total of 10 observation fields, and these area fractions are averaged to calculate an average value, which is used as the volume fraction.

[0052] (Vickers hardness H at 1 / 4 position in the thickness direction 1 / 4 Standard deviation σ 1 / 4 Vickers hardness H 1 / 4 The average value of H AVE1 / 4 (The value X1 divided by is 0.025 or less) The inventors have found that when there is a large deviation in the Vickers hardness distribution of a steel sheet, the hard phases tend to connect in a band-like shape, and as a result, ghost lines tend to occur easily in products formed by press-forming the steel sheet. In particular, they have focused on the deviation in the Vickers hardness distribution in regions relatively close to the surface of the steel sheet. They have then discovered that in areas where there is little deviation in the Vickers hardness distribution in the rolling direction of the steel sheet, ghost lines are formed as if they are interrupted midway, and that this makes it possible to suppress poor appearance due to long ghost lines. As a result, the Vickers hardness H at the 1 / 4 position in the sheet thickness direction was 1 / 4 Standard deviation σ 1 / 4 Vickers hardness H 1 / 4 The average value of H AVE1 / 4 It has been discovered that setting the value X1 divided by 0.025 or less is effective in improving the surface quality of the steel sheet and the surface of the pressed product of this steel sheet.

[0053] In this embodiment, the Vickers hardness refers to the hardness according to the Vickers hardness test of JIS Z 2244: 2009. The Vickers hardness here is HV0.2, which is the Vickers hardness at a test force of 1.9614 N (0.2 kgf).

[0054] In this embodiment, the object of observation for Vickers hardness is a cross section parallel to the thickness direction and rolling direction of the steel plate (a cross section perpendicular to the width direction), that is, a cross section at the center in the width direction of the steel plate.

[0055] Observation at the "1 / 4 position in the sheet thickness direction" refers to observations in which 50 measurement points are measured at a 150 μm pitch in the rolling direction at a position 1 / 4 of the way from the front surface of the steel sheet in the sheet thickness direction, and 50 measurement points are measured at a 150 μm pitch in the rolling direction at a position 1 / 4 of the way from the back surface of the steel sheet in the sheet thickness direction. By observing a length of 150 μm × 50 = 7.5 mm in the rolling direction, Vickers hardness can be measured in both areas where ghost lines occur and areas where ghost lines do not occur. In other words, by ensuring that the length of the observation target is sufficient in the rolling direction, the defect of measuring only areas without ghost lines can be prevented and the measurement of only ghost lines can be prevented. This allows for more accurate surface quality assessment that takes into account the presence or absence of ghost lines.

[0056] The observation target at the 1 / 4 position in the sheet thickness direction does not have to be as described above. The pitch in the rolling direction of the observation target may be less than 150 μm or more than 150 μm, but the upper limit of the pitch in the rolling direction is 400 μm, and the lower limit is 50 μm. The number of measurement points in the rolling direction may be less than 50 or more than 50, but the lower limit of the measurement points in the rolling direction is 30. It is preferable that the length of the observation target in the rolling direction be 5 mm or more to perform a more accurate surface quality assessment taking into account the positions where ghost lines are present and where they are not. In addition, in this embodiment, the configuration of the cross section at the center in the width direction of the steel sheet is described, but this does not have to be the case. It is sufficient that at least one of the cross sections at the middle in the width direction of the steel sheet has the same configuration as described in the cross section configuration.

[0057] The present inventors have found that the occurrence of ghost lines in press-formed products can be suppressed by reducing the bias in the Vickers hardness distribution in the rolling direction near the steel sheet surface, specifically by setting the value X1 to 0.025 or less. Therefore, in this embodiment, the value X1 is set to 0.025 or less. Preferably, the value X1 is 0.020 or less. The lower limit of the value X1 is zero.

[0058] (Vickers hardness H at 1 / 2 position in the thickness direction 1 / 2 Standard deviation σ 1 / 2 Vickers hardness H 1 / 2 The average value of H AVE1 / 2 (The value X2 divided by is 0.030 or less) As mentioned above, by setting the value X1 to 0.025 or less, it is possible to suppress the occurrence of ghost lines in a product formed by press-forming a steel sheet. The inventors also focused on the deviation of the Vickers hardness distribution in a deep region from the surface of the steel sheet. As a result, the Vickers hardness H 1 / 2 Standard deviation σ 1 / 2 Vickers hardness H 1 / 2 The average value of H AVE1 / 2 It has been discovered that setting the value X2 obtained by dividing by 0.030 or less is effective in further improving the surface quality of the steel sheet and the surface of the molded product obtained by press-forming this steel sheet.

[0059] In this embodiment, observation at the "half position in the sheet thickness direction" refers to observation at 50 measurement points at a pitch of 150 μm in the rolling direction at a position halfway from the surface of the steel sheet in the sheet thickness direction. The observation at the "half position in the sheet thickness direction" and the observation at the "quarter position in the sheet thickness direction" are the same except that the positions of the observation points in the sheet thickness direction are different.

[0060] The present inventors have found that the occurrence of ghost lines in press-formed products can be more reliably suppressed by reducing the bias in the Vickers hardness distribution in the rolling direction at the center of the steel sheet, specifically by setting the value X2 to 0.030 or less. Therefore, in this embodiment, the value X2 is set to 0.030 or less. Preferably, the value X2 is 0.025 or less. The lower limit of the value X2 is zero.

[0061] (Average grain size of ferrite is 5.0 to 30.0 μm) By setting the average grain size of ferrite to 30.0 μm or less, deterioration of the appearance after molding can be suppressed. Therefore, the average grain size of ferrite is preferably set to 30.0 μm or less, and more preferably set to 15.0 μm or less. On the other hand, by setting the average grain size of ferrite to 5.0 μm or more, it is possible to prevent particles with the {001} orientation of ferrite from agglomerating. Even if the individual particles with the {001} orientation of ferrite are small, if these particles aggregate, deformation will be concentrated in the aggregated areas. Therefore, by preventing the aggregation of these particles, it is possible to prevent deterioration of the appearance after molding. Therefore, it is preferable to set the average grain size of ferrite to 5.0 μm or more. It is more preferable to set it to 8.0 μm or more, even more preferable to set it to 10.0 μm or more, and even more preferable to set it to 15.0 μm or more.

[0062] The average grain size of ferrite in a steel sheet can be determined by the following method. Specifically, a region from the surface of a steel sheet etched with LePeller's reagent to a position halfway through the sheet thickness in the sheet thickness direction is observed in 10 fields at 500x magnification, and image analysis is performed in the same manner as above using Adobe Photoshop CS5 image analysis software to calculate the area fraction occupied by ferrite and the number of ferrite particles. These are then added together, and the area fraction occupied by ferrite is divided by the number of ferrite particles to calculate the average area fraction per ferrite particle. The circle equivalent diameter is calculated from this average area fraction and the number of particles, and the resulting circle equivalent diameter is used as the average grain size of ferrite.

[0063] (The average grain size of the hard phase is 1.0 to 5.0 μm) By making the average crystal grain size of the hard phase 5.0 μm or less, deterioration of the appearance after forming can be suppressed. Therefore, the average crystal grain size of the hard phase in the steel sheet is preferably 5.0 μm or less, more preferably 4.5 μm or less, and even more preferably 4.0 μm or less. On the other hand, by making the average crystal grain size of the hard phase 1.0 μm or more, it is possible to prevent the particles of the hard phase from agglomerating. By making the individual particles of the hard phase small and preventing the aggregation of these particles, it is possible to prevent deterioration of the appearance after forming. Therefore, it is preferable that the average crystal grain size of the hard phase in the steel sheet is 1.0 μm or more. It is more preferably 1.5 μm or more, and even more preferably 2.0 μm or more.

[0064] The average crystal grain size of the hard phase can be determined by the following method. Specifically, 10 fields of view are observed at 500x magnification in the region from the surface of a steel sheet etched with LePeller's reagent to a position halfway through the sheet thickness in the sheet thickness direction. Image analysis is performed in the same manner as above using Adobe Photoshop CS5 image analysis software to calculate the area fraction occupied by the hard phase and the number of hard phase particles. These are then added together and the area fraction occupied by the hard phase is divided by the number of hard phase particles to calculate the average area fraction per hard phase particle. The circle equivalent diameter is calculated from this average area fraction and the number of particles, and the resulting circle equivalent diameter is used as the average crystal grain size of the hard phase.

[0065] (In the region of 1 / 4 to 1 / 2 in the thickness direction, the area of ​​hard phases connected in the rolling direction by 100 μm or more is 30% or less of the area of ​​the total hard phases) By making the area of ​​hard phases connected by 100 μm or more in the rolling direction 30% or less of the area of ​​the total hard phases, when the steel sheet is press-formed, the protruding deformation of the hard phases and the recessed deformation of the soft phases around the hard phases are prevented from continuing long in the rolling direction, thereby suppressing the occurrence of easily visible ghost lines. Therefore, in this embodiment, it is preferable that the area of ​​hard phases connected by 100 μm or more in the rolling direction in the region 1 / 4 to 1 / 2 in the sheet thickness direction is 30% or less of the area of ​​the total hard phases. It is more preferable that this ratio is 20% or less. The lower limit of this ratio is 0%.

[0066] The method for measuring the above ratio in this embodiment is as follows. First, an observation range (consolidated hard phase observation range) is defined in a cross section of the steel sheet parallel to the sheet thickness direction and rolling direction, at the center of the steel sheet in the width direction, which is a region of ¼ to ½ in the sheet thickness direction from the surface of the steel sheet and which is 400 μm in the rolling direction. Note that the length of the consolidated hard phase observation range in the rolling direction may be less than 400 μm (e.g., 300 μm) or may be a value exceeding 400 μm (e.g., 500 μm). However, the lower limit of the length of the consolidated hard phase observation range in the rolling direction is 250 μm.

[0067] Next, in the connected hard phase observation range, the area AR1 of the hard phases connected by 100 μm or more in the rolling direction is measured. Specifically, in the connected hard phase observation range, the hard phases connected by 100 μm or more in the rolling direction are extracted by image processing using the above-mentioned hard phase measurement method. In this case, "connected" means that the crystal grain boundaries of the hard phases are in contact. Next, in the connected hard phase observation range, the area AR2 of all hard phases is measured using the above-mentioned hard phase measurement method. Then, AR1 / AR2 is calculated.

[0068] (The aspect ratio Str (ISO25178) of the surface texture of the test piece after applying 5% strain in a tensile test is 0.28 or more.) The aspect ratio Str of the surface texture of a test piece after applying 5% strain in a tensile test (hereinafter referred to as the "post-tensile test piece") is an index that indicates the anisotropy of the surface irregularities of a formed product obtained by forming (e.g., press-forming) a steel sheet. The aspect ratio Str is specified in ISO (International Organization for Standardization) 25178 and is a value between zero and one. The closer the aspect ratio Str is to zero, the greater the anisotropy, and streaks will be present on the surface of the observed area. On the other hand, the closer the aspect ratio Str is to one, the less dependent the surface shape of the observed area is on a specific direction.

[0069] For example, if the surface of the observation area has convex shapes of minute height extending in a predetermined first direction, and multiple convex shapes are arranged along a second direction perpendicular to the first direction, the surface shape viewed from the first direction will have significantly different regularity from the surface shape viewed from the second direction. In such cases, the surface shape viewed from the first direction differs significantly from the surface shape viewed from the second direction, resulting in high anisotropy and an aspect ratio Str close to zero. On the other hand, if the uneven shape on the surface of the tensile test specimen is non-directional and there are no convex or concave shapes extending long in one direction, the aspect ratio Str will be close to 1. To improve the surface quality of the molded product, it is preferable that the aspect ratio Str of the surface of the tensile test specimen be large and the anisotropy of the surface shape be small. Therefore, it is preferable that the aspect ratio Str of the surface texture of the tensile test specimen be 0.28 or more. By having an aspect ratio Str of 0.28 or more, the ghost lines on the surface of the molded product are not excessively long, and the degree of degradation of surface quality caused by the ghost lines can be reduced. Preferably, the aspect ratio Str of the test piece after tension is 0.30 or more, more preferably 0.35 or more.

[0070] In this embodiment, the aspect ratio Str of a test specimen after tensioning is measured as follows. Specifically, a JIS No. 5 test specimen is cut out from a position 1 / 4 of the way from the edge of the steel sheet in the width direction, perpendicular to the rolling direction of the steel sheet (width direction), and the surface of this test specimen is polished with abrasive paper to a mirror finish. Next, a tensile test is performed on the test specimen to impart 5% strain. The surface irregularities of the test specimen to which 5% strain has been imparted are measured using a laser microscope. The aspect ratio Str is calculated from the measurement results. Note that the aspect ratio Str can be calculated by processing the coordinate data of the surface shape obtained with a laser microscope using analysis software in accordance with ISO 25178. In the analysis, an S filter was not used, and an L filter of 0.8 mm was used.

[0071] (Vickers hardness H at 1 / 4 position in the thickness direction 1 / 4 The average value of H AVE1 / 4 is 150-300) Vickers hardness H at 1 / 4 position in the thickness direction 1 / 4 The average value of H AVE1 / 4 By ensuring that the Vickers hardness H at the 1 / 4 position in the thickness direction is 150 or more, the tensile strength of the steel sheet can be ensured to be 540 MPa or more. 1 / 4 The average value of H AVE1 / 4 By making the hardness 300 or less, the steel sheet does not become excessively hard at the 1 / 4 position in the sheet thickness direction, and the effect of leveling out surface irregularities during rolling of the steel sheet is fully exhibited.

[0072] The Vickers hardness in this embodiment refers to the hardness according to the JIS Z 2244:2009 Vickers hardness test. 1 / 4 The average value of H AVE1 / 4 is measured by the following method. Measurements are taken at 50 points at 150 μm intervals in the rolling direction at positions 1 / 4 of the way from the front and back of the steel sheet in the sheet thickness direction, for a total of 100 points, and the average value is taken as H AVE1 / 4 It was decided.

[0073] (Vickers hardness H at 1 / 2 position in the thickness direction 1 / 2 The average value of H AVE1 / 2 is 155-305) Vickers hardness H at 1 / 2 position in the thickness direction 1 / 2 The average value of H AVE1 / 2 By ensuring that the Vickers hardness H at the 1 / 2 position in the thickness direction is 155 or more, the tensile strength of the steel sheet can be ensured to be 540 MPa or more. 1 / 2 The average value of H AVE1 / 2 By making the hardness 305 or less, the steel plate does not become excessively hard at the half position in the plate thickness direction, and the effect of leveling out the surface irregularities during rolling of the steel plate is fully exhibited.

[0074] Vickers hardness H at 1 / 2 position in the thickness direction 1 / 2 The average value of H AVE1 / 2 The measurement method is the Vickers hardness H at the 1 / 4 position in the thickness direction, except that the measurement position in the thickness direction is different. 1 / 4 The average value of H AVE1 / 4 The measurement method is the same as that of

[0075] (The width of the steel plate is 1000mm or more) The molded product of the steel sheet of this embodiment is suitable for use as an automobile panel. Examples of the automobile panel include panel parts such as door outers. Examples of the panel parts include hood outer panels, quarter panels such as fender panels, door outer panels, and roof panels. As with automotive structural components, efforts are being made to increase the strength of such automotive panels, and the strength of hot-rolled steel sheets during the manufacturing process of steel sheets for automotive panels is also increasing. Furthermore, as automotive panels become thinner, the reduction ratio in the cold rolling process during steel sheet manufacturing is also increasing. Some automotive panel steel sheets, particularly those for door panels, have widths exceeding 1000 mm, and some steel sheets for hood panels have widths exceeding 1500 mm. Such wide steel sheets tend to have a large reduction load (rolling mill load) during the cold rolling process. For example, for steel sheets with a tensile strength of 540 MPa, the reduction load during cold rolling becomes particularly large when the width is approximately 1500 mm or greater, while for steel sheets with a tensile strength of 780 MPa, the reduction load during cold rolling becomes particularly large when the width is approximately 1200 mm or greater. Unless such an increase in rolling load during cold rolling is addressed, the accuracy of the steel sheet shape will deteriorate. Furthermore, conventional methods for addressing such an increase in rolling load during cold rolling have involved performing softening annealing before cold rolling or performing the cold rolling process in two steps, but these have resulted in low productivity and increased manufacturing costs. On the other hand, in this embodiment, (i) the chemical composition and metal structure of this embodiment are present, and (ii) the Vickers hardness H 1 / 4 Standard deviation σ 1 / 4 Vickers hardness H 1 / 4 The average value of H AVE1 / 4 The value X1 divided by is 0.025 or less, and (iii) the Vickers hardness H 1 / 2 Standard deviation σ 1 / 2 Vickers hardness H 1 / 2 The average value of H AVE1 / 2The steel sheet has a value X2 of 0.030 or less, which is obtained by dividing X by 0.030. This makes it possible to (a) reduce the rolling load during cold rolling by making the hot-rolled sheet structure softer, and (b) reduce ghost lines in the formed product, even in the case of wide panels such as those described above.

[0076] (The steel plate thickness is 0.20 to 1.00 mm) The thickness of the steel sheet according to this embodiment is not limited to a specific range, but is preferably 0.20 to 1.00 mm in consideration of versatility and manufacturability. By making the thickness 0.20 mm or more, it becomes easier to maintain the shape of the formed product flat, and dimensional accuracy and shape accuracy can be improved. Therefore, the thickness is preferably 0.20 mm or more, more preferably 0.35 mm or more, and more preferably 0.40 mm or more. On the other hand, by making the plate thickness 1.00 mm or less, the effect of reducing the weight of the member is greater. Therefore, the plate thickness is preferably 1.00 mm or less, more preferably 0.70 mm or less, and more preferably 0.60 mm or less. The plate thickness of the steel plate can be measured with a micrometer.

[0077] (The tensile strength of the steel plate is 540 to 980 MPa) The tensile strength of the steel sheet according to this embodiment is not limited to a specific range, but is preferably 540 to 980 MPa. When the tensile strength of the steel sheet is 540 MPa or more, a thin and high-strength steel sheet can be realized. Furthermore, when the tensile strength of the steel sheet is 980 MPa or less, formability when the steel sheet is press-formed can be easily ensured. Tensile strength is measured by taking a JIS No. 5 tensile test piece from the steel plate, with the longitudinal direction perpendicular to the rolling direction, and conducting a test in accordance with JIS (Japanese Industrial Standards) Z2241:2011 Metallic Material Tensile Test Method.

[0078] 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, and a zinc alloy plating layer obtained by alloying these layers.

[0079] The zinc plating layer and zinc alloy plating layer are formed by hot-dip plating, electroplating, or vapor deposition plating. 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, so the Al content of the zinc plating layer is preferably 0.5 mass% or less. When the zinc plating layer is a hot-dip galvanized layer, the Fe content of the hot-dip galvanized layer is preferably 3.0 mass % or less in order to improve the adhesion between the steel sheet surface and the zinc plating layer. When the zinc plating layer is an electrogalvanized layer, the Fe content of the electrogalvanized layer is preferably 0.5 mass % or less from the viewpoint of improving corrosion resistance.

[0080] The zinc plating layer and zinc alloy plating layer may contain one or more of Al, Ag, B, Be, Bi, Ca, Cd, Co, Cr, Cs, Cu, Ge, Hf, Zr, I, K, La, Li, Mg, Mn, Mo, Na, Nb, Ni, Pb, Rb, Sb, Si, Sn, Sr, Ta, Ti, V, W, Zr, and REM, to the extent that the corrosion resistance and formability of the steel sheet are not impaired. Ni, Al, and Mg are particularly effective in improving the corrosion resistance of the steel sheet.

[0081] The zinc plating layer or zinc alloy plating layer may be a zinc alloy plating layer or zinc alloy plating layer that has been subjected to an alloying treatment. When the hot-dip galvanized layer or hot-dip zinc alloy plating layer is subjected to an alloying treatment, the Fe content of the hot-dip galvanized layer (galvannealed layer) or hot-dip zinc alloy plating layer (galvannealed zinc alloy plating layer) after the alloying treatment is preferably 7.0 mass % to 13.0 mass % from the viewpoint of improving adhesion between the steel sheet surface and the alloying plating layer. By subjecting a steel sheet having a hot-dip galvanized layer or hot-dip zinc alloy plating layer to an alloying treatment, Fe is incorporated into the plating layer, increasing the Fe content. This makes it possible to set the Fe content to 7.0 mass % or more. In other words, a zinc plating layer with an Fe content of 7.0 mass % or more is a zinc alloy plating layer or zinc alloy plating layer.

[0082] The Fe content in the plating layer can be determined by the following method: The plating layer alone is dissolved and removed using a 5% by volume HCl aqueous solution containing an inhibitor. The Fe content in the resulting solution is measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) to obtain the Fe content (mass%) in the plating layer.

[0083] (The steel plate is the outer panel of the car) Next, a press-formed product that can be produced by press-forming the above-described steel sheet will be described. This press-formed product has the same chemical composition as the above-described steel sheet. Furthermore, the press-formed product may have the above-described plating layer on at least one surface. Since the press-formed product is obtained by press-forming the above-described steel sheet, the occurrence of ghost lines is suppressed, resulting in excellent appearance quality. As a result, an automobile with high marketability can be realized due to its excellent appearance that directly catches the consumer's eye. Specific examples of press-formed products include panel parts (automotive outer panel) such as outer doors of an automobile body, as described above. Examples of panel parts include outer hood panels, quarter panels such as fender panels, outer door panels, roof panels, etc.

[0084] <Manufacturing method> Next, a preferred method for manufacturing the steel sheet according to this embodiment will be described. The steel sheet according to this embodiment can achieve the effects described above regardless of the manufacturing method. However, the following method is preferred because it can be manufactured stably.

[0085] Specifically, the steel sheet according to this embodiment can be produced by a production method including the following steps (i) to (iv). (i) a slab forming step of solidifying molten steel having the above chemical composition to form a slab; (ii) a hot rolling process in which the slab is heated and hot rolled so that the rolling end temperature is 950°C or less to obtain a hot-rolled steel sheet, and then the hot-rolled steel sheet is coiled at 450 to 650°C; (iii) a cold rolling step in which the coiled hot-rolled steel sheet is uncoiled and subjected to cold rolling at a cumulative rolling reduction (RCR) of 50 to 90% to obtain a cold-rolled steel sheet; (iv) annealing the cold-rolled steel sheet and then forming the above-mentioned coating layer as necessary; Each step will be described below.

[0086] [Slab forming process] In the slab forming process, molten steel having a predetermined chemical composition is formed into a slab. The manufacturing method for the slab forming process is not limited. For example, molten steel having the above-mentioned chemical composition can be produced using a converter or an electric furnace, and a slab produced by continuous casting can be used. Instead of continuous casting, an ingot casting method, thin slab casting method, etc. may also be used.

[0087] [Hot rolling process] Prior to hot rolling, the slab is heated to 1100°C or higher. By setting the heating temperature at 1100°C or higher, the rolling reaction force during the subsequent hot rolling is not excessively large, making it easier to obtain the desired product thickness. This also increases the precision of the plate shape, allowing for smooth coiling. There is no need to limit the upper limit of the heating temperature, but from an economical point of view, it is preferable that the slab heating temperature be less than 1300°C.

[0088] In the hot rolling process, the steel billet heated to the above heating temperature is hot rolled. During the hot rolling, rough rolling is followed by finish rolling. In the finish rolling, multiple reductions are performed. Finish rolling is performed using multiple consecutive rolling stands, with the reduction rate of the latter rolling stands being greater than that of the former rolling stands. The reduction rate of the former finish rolling is set to less than 35%, and the reduction rate of the latter finish rolling is set to 35% or more. This allows the reduction rate of the latter finish rolling to be increased, resulting in the hot-rolled sheet being appropriately softened after hot rolling. This reduces the load on the rolling mill during the cold rolling process. Furthermore, the formation of band-like hard phases such as pearlite and martensite in the structure of the hot-rolled sheet can be suppressed, and the formation of band-like hard phases such as martensite can also be suppressed in the structure of the formed product, which is the final product. The ratio P2 / P1, which is the ratio of the reduction P1 in the first rolling stand to the reduction P2 in the second rolling stand, is preferably greater than 1.0 and less than 1.6. By making P2 / P1 greater than 1.0, the hot-rolled sheet can be sufficiently softened and the formation of band-like hard phases in the structure of the final product, the formed article, can be suppressed. Furthermore, by making P2 / P1 less than 1.6, the load on the second rolling stand can be reduced. The reduction ratio in the final rolling stand is preferably 40% or more, which makes it possible to more easily prevent band-like formation of hard phases such as pearlite and martensite in the structure of the hot-rolled sheet, and also more easily prevent band-like formation of hard phases such as martensite in the structure of the formed product, which is the final product.

[0089] For example, seven rolling stands are provided in succession in the finish rolling. In this embodiment, the first to third stands are the first half stands, and the fifth to seventh stands are the second half stands. The number of rolling stands is not limited, as long as the rolling ratio of the second half stands among the multiple rolling stands is made greater than the rolling ratio of the first half stands.

[0090] The rolling end temperature is set to 950°C or lower. By setting the rolling end temperature to 950°C or lower, the average crystal grain size of the hot-rolled steel sheet does not become excessively large. In this case, the average crystal grain size of the final product sheet can also be made small, ensuring sufficient yield strength and high surface quality after forming.

[0091] The coiling temperature in the hot rolling process is preferably 450 to 650°C. By setting the coiling temperature to 650°C or less, the crystal grain size can be made small, and sufficient steel sheet strength can be ensured. Furthermore, by suppressing the scale thickness, sufficient pickling properties can be ensured. Furthermore, by setting the coiling temperature to 450°C or more, the strength of the hot-rolled steel sheet does not increase excessively, and the load on the equipment performing the cold rolling process can be suppressed, thereby further increasing productivity.

[0092] [Cold rolling process] In the cold rolling process, cold rolling is performed at a cumulative reduction ratio (RCR) of 50 to 90% to obtain a cold-rolled steel sheet. By cold-rolling a hot-rolled steel sheet to which a predetermined residual stress has been imparted at the above cumulative reduction ratio, ferrite having a desired texture can be obtained after annealing and cooling.

[0093] By setting the cumulative reduction rate RCR to 50% or more, the thickness of the billet in the hot rolling process can be ensured by back-calculating from the thickness of the steel plate, making it practical to carry out the hot rolling process. Furthermore, by setting the cumulative reduction rate RCR to 90% or less, the rolling load does not become too large, and uniformity of the material properties in the plate width direction can be sufficiently ensured. Furthermore, production stability can be sufficiently ensured. Therefore, the cumulative reduction rate RCR in cold rolling is set to 50-90%.

[0094] [Annealing process] In the annealing process, the cold-rolled steel sheet is heated to a soaking temperature of 750 to 900°C and then held there for annealing. A soaking temperature of 750°C or higher allows for sufficient recrystallization of ferrite and reverse transformation from ferrite to austenite, resulting in the desired texture. On the other hand, a soaking temperature of 900°C or lower densifies the crystal grains, resulting in sufficient strength. Furthermore, the heating temperature is not excessively high, allowing for increased productivity.

[0095] [Cooling process] In the cooling step, the cold-rolled steel sheet after the soaking step in the annealing step is cooled. During cooling, the average cooling rate from the soaking temperature is 5.0 to 50°C / second. When the average cooling rate is 5.0°C / second or more, the ferrite transformation is not excessively promoted, and the amount of hard phases such as martensite produced is increased, thereby achieving the desired strength. Furthermore, when the average cooling rate is 50°C / second or less, the steel sheet can be cooled more uniformly in the width direction of the steel sheet.

[0096] [Plating process] The cold-rolled steel sheet obtained by the above method may further be subjected to a plating step of forming a plating layer on the surface.

[0097] [Alloying process] The plated layer formed in the plating step may be alloyed at an alloying temperature of, for example, 450 to 600°C.

[0098] According to the above manufacturing method, by applying a large reduction in the latter stage of finish rolling in the hot rolling process, in which the reduction rate is increased, a steel sheet with fewer connected hard phases can be obtained. As a result, in the formed product after forming, the anisotropy of the surface irregularities is reduced, the occurrence of ghost lines can be suppressed, and excellent appearance quality can be obtained. Moreover, in terms of the manufacturability of the steel sheet, the hot-rolled sheet can also be moderately softened, and cold-rolling workability can be improved without requiring softening annealing or double cold rolling.

[0099] In this embodiment, the steel sheet after hot rolling is not subjected to shape correction using a leveler as a shape correction device. The steel sheet of this embodiment is required to have high surface properties to ensure high appearance quality. Therefore, steel sheets that require shape correction using a leveler cannot be used in this embodiment. In other words, the steel sheet of this embodiment is not intended to be manufactured using a manufacturing method that includes a special hot rolling process in which a leveler is placed on the stand exit side of the finish rolling mill. Therefore, a leveler is not combined with the steel sheet manufacturing method of this embodiment. [Example]

[0100] Next, examples of the present invention will be described. Note that the conditions in the examples are examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0101] Steels having the chemical compositions shown in billet Nos. A to K in Table 1 were melted and continuously cast into slabs with thicknesses of 200 to 300 mm. Some of the obtained slabs were hot rolled under the conditions shown in Table 2 and then coiled. For the finish rolling in the hot rolling, seven rolling stands were set up in succession, with the first three stands (stands 1 to 3) being the first half stands and the last three stands (stands 5 to 7) being the second half stands.

[0102] The coil was then uncoiled, and test specimens were cut from the resulting hot-rolled sheet to measure tensile strength. The tensile strength was evaluated in accordance with JIS Z 2241:2011. The test specimens used were No. 5 test specimens of JIS Z 2241:2011. The tensile test specimens were taken from a quarter section from the end in the sheet width direction, with the direction perpendicular to the rolling direction as the longitudinal direction.

[0103] After pickling, the steel sheets were cold rolled at the cumulative reduction ratio RCR shown in Table 2 to obtain steel sheets A1 to K1.

[0104] Thereafter, annealing and cooling were performed under the conditions of the soaking temperature and cooling rate after heating (average cooling rate) shown in Table 3. In addition, some of the steel sheets were subjected to various plating treatments to form plating layers on the surface, and then subjected to alloying treatment at the alloying temperatures shown in Table 3. In Table 4, CR indicates no plating, GI indicates hot-dip galvanized, GA indicates galvannealed, and EG indicates electrogalvanized.

[0105] The widths and thicknesses of the obtained product plates Nos. A1a to K1a (that is, product plates Nos. A1a to A2a, B1a to B2a, C1a to C2a, D1a to D5a, E1a, F1a, G1a, H1a, I1a, J1a, and K1a) were measured.

[0106] In addition, the tensile strength of the product sheets Nos. A1a to K1a was measured. The tensile strength was evaluated in accordance with JIS Z 2241:2011. The test pieces were No. 5 test pieces of JIS Z 2241:2011. The tensile test pieces were taken from a quarter section from the end in the sheet width direction, with the direction perpendicular to the rolling direction as the longitudinal direction. When the obtained tensile strength was 540 MPa or more, it was judged to have high strength and pass. On the other hand, when the obtained tensile strength was less than 540 MPa, it was judged to have poor strength and fail.

[0107] Furthermore, the volume fractions of ferrite and hard phases in the metallographic structures of the obtained product sheets Nos. A1a to K1a were measured by the method described above. In the metallographic structures of the product sheets Nos. A1a to K1a, the total volume fraction of the hard phase and ferrite was 100%.

[0108] Furthermore, the average grain size of the ferrite and the average grain size of the hard phase in the metallographic structure of the obtained product sheets Nos. A1a to K1a were measured by the above-mentioned methods.

[0109] The results are shown in Table 4.

[0110] [Table 1]

[0111] [Table 2]

[0112] [Table 3]

[0113] [Table 4]

[0114] In addition, for the obtained product sheets No. A1a to K1a, the Vickers hardness H was measured at 50 points at 1 / 4 positions from the surface in the sheet thickness direction at measurement intervals of 150 μm in the rolling direction. 1 / 4 Furthermore, the Vickers hardness H was measured at 50 points at 1 / 4 positions in the thickness direction from the back surface at intervals of 150 μm in the rolling direction. 1 / 4 The Vickers hardness H of these 100 points was measured by the above-mentioned method. 1 / 4 Standard deviation σ 1 / 4 Vickers hardness H of 100 points 1 / 4 The average value of H AVE1 / 4 The value X1 was calculated by dividing by .

[0115] In addition, for the obtained product sheets No. A1a to K1a, the Vickers hardness H was measured at 50 points at 1 / 2 positions from the surface in the sheet thickness direction at measurement intervals of 150 μm in the rolling direction. 1 / 2 The Vickers hardness H of these 50 points was measured by the method described above. 1 / 2 Standard deviation σ 1 / 2 Vickers hardness H of 50 points 1 / 2 The average value of H AVE1 / 2 The value X2 was calculated by dividing by .

[0116] Furthermore, the area ratio of hard phases connected over 100 μm or more in the rolling direction in the region of 1 / 4 to 1 / 2 in the sheet thickness direction was measured for the obtained product sheets Nos. A1a to K1a by the method described above.

[0117] Furthermore, for each of the product sheets Nos. A1a to K1a, the surface was polished to a mirror finish using abrasive paper or the like, and a tensile test specimen was subjected to a 5% strain in a tensile test, after which the aspect ratio Str of the surface texture was measured using the method described above.

[0118] Additionally, for each of the product sheets Nos. A1a to K1a, the surface of the tensile test specimens was polished to a mirror finish using abrasive paper or similar, and the surface roughness Wa (arithmetic mean waviness) was measured after applying a 5% strain in a tensile test using the following method. Using a laser displacement measuring device (Keyence VK-X1000), 50 lines of the profile were measured along a direction perpendicular to the rolling direction. Components with wavelengths of 0.8 mm or less and 2.5 mm or more were removed. From the results obtained, the arithmetic mean waviness was calculated in accordance with JIS B 0601:2013, and the average value of all 50 lines was calculated. This gave the surface roughness Wa of the product sheet.

[0119] The product of the tensile strength of each of the product sheets Nos. A1a to K1a and the aspect ratio Str of the surface texture of the test specimen after tension was calculated. The higher the tensile strength TS x aspect ratio Str, the higher the strength and the lower the processability, but the smaller the anisotropy of the surface irregularities.

[0120] The results are shown in Table 5. [Table 5]

[0121] As shown in Tables 1 to 5, the aspect ratios Str of the surface texture of the tensile test specimens in the Examples tended to be significantly higher than the aspect ratios Str of the surface texture of the tensile test specimens in the Comparative Examples, resulting in less anisotropy in the surface irregularities and excellent strength and surface quality. More specifically, all of the Examples had high strength, with tensile strengths exceeding 540 MPa. Furthermore, in the Examples, the aspect ratios Str of the surface texture of the tensile test specimens were 0.28 or higher, and the area of ​​connected hard phases 100 μm or larger was 30% or less of the area of ​​the total hard phase, thereby sufficiently suppressing ghost lines. Moreover, all of the Examples had a sufficiently high tensile strength TS × aspect ratio Str exceeding 200, indicating low anisotropy in the surface irregularities despite high strength and poor processability. Furthermore, the average value of (tensile strength of product sheet - tensile strength of hot-rolled sheet) for the 10 examples was 77, while the average value of (tensile strength of product sheet - tensile strength of hot-rolled sheet) for the 8 comparative examples was approximately 54. That is, in the examples, there was a sufficient difference between the tensile strength of the product sheet and the tensile strength of the hot-rolled sheet, and softening of the hot-rolled sheet was achieved. In particular, it was demonstrated that the load on the rolling mill in the cold rolling process was reduced for wide product sheets suitable for automobile hood panels and automobile door panels.

[0122] On the other hand, in the comparative product sheets Nos. A2a and B2a, the reduction ratio in the latter half of the finish rolling in the hot rolling was small, so the streaky irregularities on the steel sheet surface could not be sufficiently smoothed, and in the region 1 / 4 to 1 / 2 of the way along the rolling direction, the area ratio of hard phases connected by 100 μm or more in the rolling direction exceeded 40%.In addition, the aspect ratio Str of the surface properties of the test specimens after tension was less than 0.28, and further, the tensile strength TS × aspect ratio Str was less than 180, so the surface quality after forming was poor. Furthermore, in the comparative example product sheets Nos. C2a and D2a, the reduction ratio in the latter half of the finish rolling in the hot rolling was small, so the streaky irregularities on the steel sheet surface could not be sufficiently smoothed out, and in the region 1 / 4 to 1 / 2 of the way along the rolling direction, the area ratio of hard phases connected by 100 μm or more in the rolling direction exceeded 30%. In addition, the aspect ratio Str of the surface properties of the test specimens after tension was less than 0.28, and furthermore, the tensile strength TS × aspect ratio Str was less than 170, so the surface quality after forming was poor. Furthermore, in the comparative example, product sheet No. D5a, the ratio P2 / P1 of the reduction rate P1 in the first half of the finish rolling in the hot rolling to the reduction rate P2 in the second half was in the range of more than 1.0 and not more than 1.6. However, because the reduction rate in the second half was small, the streaky irregularities on the steel sheet surface could not be sufficiently smoothed, and in the region 1 / 4 to 1 / 2 of the rolling direction, the area ratio of hard phases connected by 100 μm or more in the rolling direction exceeded 30%. In addition, the aspect ratio Str of the surface properties of the test piece after tension was less than 0.28. Furthermore, the tensile strength TS × aspect ratio Str was less than 170, and the surface quality after forming was poor.

[0123] In addition, in the comparative example, product sheet No. E1a, the carbon content exceeded the preferred range, which led to the occurrence of band-like Mn segregation. As a result, in the region from 1 / 4 to 1 / 2 in the rolling direction, the area ratio of hard phases connected 100 μm or more in the rolling direction exceeded 30%, and the tensile strength TS × aspect ratio Str was below 180, resulting in poor surface quality after forming. In the comparative example, product sheet No. F1a, the carbon content did not reach the preferred range, and the volume fraction of ferrite was excessive and the volume fraction of the hard phase was low, resulting in a low tensile strength of the product sheet, below 540 MPa. In the comparative example, product sheet No. G1a, the Mn content exceeded the preferred range, which led to the occurrence of band-like Mn segregation during solidification of the steel. As a result, in the region 1 / 4 to 1 / 2 of the rolling direction, the area ratio of hard phases connected in the rolling direction by 100 μm or more exceeded 40%, and the tensile strength TS × aspect ratio Str was below 170, resulting in poor surface quality after forming.

[0124] Here, we compare product sheets Nos. A1a and A2a, Nos. B1a and B2a, Nos. C1a and C2a, and Nos. D1a and D2a, which have the same thickness. The surface roughness Wa of product sheets Nos. A1a, B1a, C1a, and D1a, which are examples, is 0.058 μm, 0.055 μm, 0.058 μm, and 0.055 μm, respectively. On the other hand, the surface roughness Wa of product sheets Nos. A2a, B2a, C2a, and D2a, which are comparative examples, is 0.050 μm, 0.053 μm, 0.056 μm, and 0.055 μm, respectively. Thus, the surface roughness Wa of the example product sheet No. A1a is greater than the surface roughness Wa of the comparative example product sheet No. A2a, and the surface roughness Wa of the example product sheets No. B1a, C1a, and D1a is also greater than the surface roughness Wa of the comparative example product sheets No. B2a, C2a, and D2a, respectively. Meanwhile, the aspect ratios Str of the example product sheets No. A1a, B1a, C1a, and D1a are all greater than the aspect ratios Str of the comparative example product sheets No. A2a, B2a, C2a, and D2a. Thus, although the surface roughness Wa of the example product sheets No. A1a, B1a, C1a, and D1a is greater than the surface roughness Wa of the comparative example product sheets No. A2a, B2a, C2a, and D2a, respectively, the high aspect ratios Str demonstrate that the anisotropy of the surface irregularities is small and the surface quality is excellent. [Industrial Applicability]

[0125] According to the above aspect of the present invention, it is possible to provide a steel sheet that can realize excellent appearance quality in a formed product.

Claims

1. The chemical composition is in mass percent, C: 0.030% to 0.145%, Si: 0% to 0.500%, 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%, B: 0% to 0.0050%, Mo: 0% to 0.80%, Ti: 0% to 0.200%, Nb: 0% to 0.10%, V: 0% to 0.20%, Cr: 0% to 0.80%, Ni: 0% to 0.25% O: 0% to 0.0100%, Cu: 0% to 1.00%, W: 0% to 1.00%, Sn: 0% to 1.00%, Sb: 0% to 0.20%, Ca: 0% to 0.0100%, Mg: 0% to 0.0100%, Zr: 0% to 0.0100%, REM: 0% to 0.0100%, the balance being iron and impurities; The metal structure is composed of ferrite with a volume fraction of 70 to 95% and a hard phase with a volume fraction of 5 to 30%, Vickers hardness H at 1 / 4 position in the plate thickness direction １／４ The standard deviation of the Vickers hardness H １／４ The value X1 divided by the average value of is 0.025 or less, Vickers hardness H at 1 / 2 position in the plate thickness direction １／２ The standard deviation of the Vickers hardness H １／２ The value X2 divided by the average value of is 0.030 or less, Steel plate.

2. 2. The steel sheet according to claim 1, wherein the ferrite has an average grain size of 5.0 to 30.0 μm, and the hard phase has an average grain size of 1.0 to 5.0 μm.

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

4. The steel plate according to any one of claims 1 to 3, characterized in that the aspect ratio Str (ISO25178) of the surface texture of a test piece after applying 5% strain in a tensile test is 0.28 or more.

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

6. The steel plate according to any one of claims 1 to 5, wherein the hard phase comprises at least one of martensite, bainite, tempered martensite, and pearlite.

7. The steel plate according to any one of claims 1 to 6, characterized in that the steel plate has a thickness of 0.20 mm to 1.00 mm.

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

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