Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/12—Accessories for subsequent treating or working cast stock in situ
  • B22D11/128—Accessories for subsequent treating or working cast stock in situ for removing
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals

Definitions

• the present invention relates to steel plates.
• DP steel (Dual Phase steel) consisting of a soft ferrite phase (first phase) and a hard second phase mainly composed of martensite as described in Patent Document 1 has a low yield strength (Yield Strength; hereinafter sometimes referred to as "YS”) and is excellent in workability during forming such as press forming, but is prone to non-uniform deformation in which the soft phase consisting of ferrite and its surroundings deform preferentially during processing. For this reason, when such DP steel is used for exterior panel parts, minute irregularities are generated on the steel plate surface after forming, which can cause appearance defects called ghost lines and stretcher strain.
• Yield Strength Yield Strength
• the present invention aims to provide a high-strength steel sheet with a new structure that has excellent appearance after forming and workability during forming.
• the present invention includes the following aspects:
• a steel plate The chemical composition of the steel plate is, in mass%, C: 0.030-0.100%, Mn: 1.00-2.50%, Si: 0.005-1.500%, Al: 0.005-0.700%, P: 0.100% or less, S: 0.0200% or less, N: 0.0150% or less, O: 0.0100% or less, Cr: 0-0.80%, Mo: 0 to 0.50%, B: 0 to 0.0100%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, V: 0 to 0.50%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, W: 0-1.00%, Sn: 0-1.00%, Sb: 0 to 0.200%, As: 0 to 0.200%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, REM: 0 to 0.0100%, and the balance: Fe and impurities;
• the area ratio of the steel plate
• the above chemical composition is, in mass %, Cr: 0.01-0.80%, Mo: 0.01-0.50%, B: 0.0001 to 0.0100%, Ti: 0.001 to 0.100%, Nb: 0.001 to 0.100%, V: 0.01-0.50%, Ni: 0.01-1.00%, Cu: 0.01 to 1.00%, W: 0.01-1.00%, Sn: 0.001 to 1.00%, Sb: 0.001-0.200%, As: 0.001-0.200%, Ca: 0.0001-0.0100%, Mg: 0.0001-0.0100%, Zr: 0.0001 to 0.0100%, and REM: 0.0001 to 0.0100%
• the average grain size of the ferrite in a region having a depth from the 3/8 depth position to the 5/8 depth position of the plate thickness is 5.0 to 30.0 ⁇ m
• the steel sheet according to any one of the above aspects 1 to 4, characterized in that the average crystal grain size of the hard phase in a region having a depth from a 3/8 depth position to a 5/8 depth position of the sheet thickness is 1.0 to 5.0 ⁇ m.
• the present invention can provide high-strength steel sheets that have excellent appearance after forming and workability during forming.
• DP steel which has a relatively low yield strength (YS)
• YS yield strength
• DP steel is prone to non-uniform deformation in which the soft phase and its surroundings are preferentially deformed during processing such as press forming. For this reason, when such DP steel is used for exterior panel parts, minute irregularities may occur on the steel sheet surface after forming, resulting in appearance defects called ghost lines.
• the soft phase parts made of ferrite are deformed so as to be concave, while the hard phase parts mainly made of martensite are deformed so as to not be concave or to be raised in a convex shape.
• minute irregularities are formed on the steel sheet surface after forming. These minute irregularities are formed so that convex parts extending roughly along the rolling direction and concave parts extending roughly along the rolling direction are aligned in the sheet width direction perpendicular to the rolling direction.
• the rolling direction can be easily identified based on the extension direction of the crystal grains of the steel sheet.
• the direction perpendicular to the rolling direction and the thickness direction means the direction perpendicular to each of the rolling direction and the thickness direction.
• the inventors In order to improve such ghost lines, the inventors first conducted detailed studies focusing on the morphology of the hard phase in the metal structure of the steel sheet. As a result, the inventors discovered that in steel sheets such as DP steel, which contain a mixture of soft and hard phases, the presence of hard phases connected in stripes (striped hard phases) in the metal structure makes the degree of ghost lines more pronounced. Furthermore, the inventors discovered that by using a method to suppress the formation of such striped hard phases and disperse the hard phases more uniformly in the metal structure, it is possible to suppress the formation of minute irregularities on the steel sheet surface after forming while fully maintaining the high strength provided by the hard phase, and as a result, to suppress the occurrence of ghost lines.
• the inventors have discovered that reducing Mn segregation during solidification in the slab casting process, in which molten steel is solidified to cast slabs, is an effective way to suppress the formation of banded hard phases. Therefore, the inventors have conducted further detailed studies on methods for reducing Mn segregation from two perspectives: center segregation and microsegregation.
• the inventors first considered that suppressing the flow of molten steel during slab casting would be effective in reducing central segregation of Mn, and conducted various studies. To explain in more detail, when molten steel solidifies, it solidifies from the surface, and the center solidifies last. At this stage, the solid phase is discharged from the liquid phase of the molten steel, and Mn in the liquid phase becomes concentrated at this stage. If the molten steel flows during solidification, this concentrated part of Mn tends to eventually gather in the center during the solidification process, resulting in noticeable central segregation of Mn. Therefore, the inventors discovered that when manufacturing steel plate, the central segregation of Mn can be significantly suppressed by appropriately controlling the solidification conditions and suppressing the flow of the molten steel.
• the inventors found that when the C and Mn contents are high, the solidification does not become ⁇ solidification during solidification, but the diffusion rate of Mn decreases and microsegregation increases, while when the Si, Al, Cr, and Mo contents are high, the diffusion of Mn during solidification is promoted, and microsegregation can be reduced.
• the present inventors have investigated further methods for improving ghost lines in addition to the above-mentioned method for reducing Mn segregation. As a result, the present inventors have found that ghost lines can be improved by reducing the hard phase fraction of a steel sheet, even if center segregation of Mn remains to a certain extent.
• the present inventors also found that the occurrence of ghost lines is also greatly influenced by the solidification structure, and that even if the center segregation of Mn is small, if coarse equiaxed crystals are generated in the solidification structure, negative segregation of Mn occurs, and the variation in the hard phase fraction in the direction perpendicular to the rolling direction and the plate thickness direction increases, making ghost lines more likely to occur. Furthermore, the present inventors found that the negative segregation of Mn can be suppressed and ghost lines can be improved by a method different from the conventional center segregation countermeasures, that is, a method of reducing the equiaxed crystal fraction and controlling the solidification structure to a columnar crystal structure.
• the inventors therefore conducted extensive research into a method for achieving both suppression of poor appearance after forming, including not only ghost lines but also stretcher strain, and workability during forming.
• a high-strength steel sheet that is excellent in both appearance after forming and workability during forming can be obtained by controlling, within specific ranges, the area ratio of the hard phase in the region having a depth from the surface of the steel sheet to 1/8 of the sheet thickness, i.e., the hard phase fraction in the surface layer of the steel sheet, and the area and variation of the hard phase in the region having a depth from 3/8 to 5/8 of the sheet thickness, i.e., the hard phase fraction in the center of the sheet thickness and its variation.
• the steel plate according to one embodiment of the present invention has a chemical composition, in mass%, C: 0.030-0.100%, Mn: 1.00-2.50%, Si: 0.005-1.500%, Al: 0.005-0.700%, P: 0.100% or less, S: 0.0200% or less, N: 0.0150% or less, O: 0.0100% or less, Cr: 0-0.80%, Mo: 0 to 0.50%, B: 0 to 0.0100%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, V: 0 to 0.50%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, W: 0-1.00%, Sn: 0-1.00%, Sb: 0 to 0.200%, As: 0 to 0.200%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, REM: 0 to 0.0100%, and the balance: Fe and impur
• the area ratio of the hard phase in a region having a depth from the surface of the steel plate to a depth position of 1/8 of the plate thickness is 0.20Vm to 0.80Vm, where Vm is the area ratio of the hard phase in a region having a depth from a depth position of 3/8 to a depth position of 5/8 of the plate thickness.
• the metal structure in a region having a depth from the 3/8 depth position to the 5/8 depth position of the plate thickness contains, in area percentage, ferrite: 75 to 97% and hard phase: 3 to 25%, and the standard deviation of the area ratio of the hard phase in the direction perpendicular to the rolling direction and the plate thickness direction in the region having a depth from the 3/8 depth position to the 5/8 depth position of the plate thickness is 0.85% or less.
• the steel plate of this embodiment has a specific chemical composition
• the metal structure in the region having a depth from 3/8 to 5/8 of the plate thickness i.e., the center of the plate thickness
• This type of unique metal structure can be obtained by adopting a specific chemical composition and casting conditions, as described below.
• the steel plate of this embodiment has such a unique metal structure, that is, a metal structure in which the central segregation of Mn during solidification is small, and the hard phase fraction and its variation are small, so that the steel plate can suppress the generation of minute irregularities on the steel plate surface after forming while adequately maintaining high strength.
• the steel plate of this embodiment can significantly suppress the occurrence of ghost lines and stretcher strain while adequately maintaining high strength.
• the area ratio of the hard phase in the region having a depth from the surface of the steel plate to a depth position of 1/8 of the plate thickness i.e., the hard phase fraction in the surface layer
• the hard phase fraction in the surface layer is 0.20Vm to 0.80Vm relative to the area ratio Vm of the hard phase in the center of the plate thickness.
• YS low yield strength
• TS high tensile strength
• this makes it easier to further suppress stretcher strain that occurs during forming of the steel plate.
• the hard phase fraction in the surface layer of the steel plate is 0.80Vm or less, good bendability can be ensured, and a steel plate with excellent workability during forming can be obtained.
• a hard phase fraction of the surface layer of a steel plate of "0.20 Vm" means that the hard phase fraction Vm in the center of the plate thickness is 0.20 times (0.20 x Vm). The same is true for 0.80 Vm, etc. Note that in this specification, the area fraction (%) of the hard phase and the hard phase fraction (%) are synonymous.
• the steel plate of this embodiment is a high-strength steel plate that combines suppression of appearance defects after forming, including not only ghost lines but also stretcher strain, with excellent workability during forming, i.e., a high-strength steel plate that is excellent in both appearance after forming and workability during forming.
• a depth position of x/y in the sheet thickness means a position moved from the surface in the sheet thickness direction of the steel sheet by a distance (depth) of x/y of the sheet thickness toward the center of the steel sheet in the sheet thickness direction.
• a depth position of 1/8 of the sheet thickness means a position that is 0.25 mm deep in the sheet thickness direction from the surface of the steel sheet.
• the surface of the steel sheet in the definition of "a depth position of x/8 of the sheet thickness from the surface of the steel sheet” above means the interface between the steel sheet and the plating layer, and "sheet thickness” means the sheet thickness of the steel sheet (base material) excluding the plating layer.
• the steel plate of this embodiment has C: 0.030-0.100%, Mn: 1.00-2.50%, Si: 0.005-1.500%, Al: 0.005-0.700%, P: 0.100% or less, S: 0.0200% or less, N: 0.0150% or less, O: 0.0100% or less, Cr: 0-0.80%, Mo: 0 to 0.50%, B: 0 to 0.0100%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, V: 0 to 0.50%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, W: 0-1.00%, Sn: 0-1.00%, Sb: 0 to 0.200%, As: 0 to 0.200%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, REM: 0 to 0.0100%, and the balance: Fe and impurities.
• C is an element that increases the strength of the steel sheet.
• the C content is set to 0.030% or more.
• the C content may be 0.032% or more, 0.034% or more, or 0.035% or more.
• the C content is set to 0.100% or less.
• the C content may be 0.095% or less, 0.090% or less, or 0.080% or less.
• Mn is an element that improves the hardenability of steel and contributes to improving strength. In order to fully obtain such effects, the Mn content is set to 1.00% or more. The Mn content may be 1.02% or more, 1.04% or more, or 1.05% or more. On the other hand, if Mn is contained excessively, the diffusion of Mn during solidification is inhibited, and the microsegregation of Mn may not be sufficiently suppressed. Therefore, the Mn content is set to 2.50% or less. The Mn content may be 2.40% or less, 2.30% or less, or 2.20% or less.
• Si is a deoxidizing element for steel and is a solid solution strengthening element effective for increasing the strength of steel plate without impairing the ductility.
• Si is also an element effective for promoting the diffusion of Mn during solidification and reducing the microsegregation of Mn.
• the Si content is set to 0.005% or more.
• the Si content may be 0.008% or more, 0.010% or more, or 0.012% or more.
• the peelability of the scale may decrease and surface defects may occur. Therefore, the Si content is set to 1.500% or less.
• the Si content may be 1.200% or less, 1.000% or less, or 0.800% or less.
• Al is an element that functions as a deoxidizer and is an effective solid solution strengthening element for increasing the strength of steel.
• Al is also an effective element for promoting the diffusion of Mn during solidification and reducing the microsegregation of Mn.
• the Al content is set to 0.005% or more.
• the Al content may be 0.010% or more, 0.015% or more, or 0.020% or more.
• the Al content is set to 0.700% or less.
• the Al content may be 0.600% or less, 0.500% or less, 0.400% or less, or 0.300% or less.
• P is an element that is mixed in during the manufacturing process.
• P is also a solid solution strengthening element.
• the P content may be 0%.
• the P content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.
• the P content is set to 0.100% or less.
• the P content may be 0.080% or less, 0.075% or less, or 0.070% or less.
• S is an element that is mixed in during the manufacturing process.
• the S content may be 0%.
• the S content may be 0.0001% or more, 0.0005% or more, or 0.0008% or more.
• excessive S content may form Mn sulfides, which may reduce the formability of the steel sheet, such as ductility, hole expandability, stretch flangeability, and/or bendability. Therefore, the S content is set to 0.0200% or less.
• the S content may be 0.0150% or less, 0.0100% or less, or 0.0080% or less.
• N is an element that is mixed in during the manufacturing process.
• the N content may be 0%.
• the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.
• the N content is set to 0.0150% or less.
• the N content may be 0.0100% or less, 0.0080% or less, or 0.0050% or less.
• O is an element that is mixed in during the manufacturing process.
• the O content may be 0%.
• the O content may be 0.0001% or more, 0.0005% or more, or 0.0008% or more.
• the O content is set to 0.0100% or less.
• the O content may be 0.0070% or less, 0.0050% or less, or 0.0030% or less.
• the steel plate of this embodiment is as described above. Furthermore, in this embodiment, the steel plate may contain one or more of the following optional elements in place of a portion of the remaining Fe, as necessary. These optional elements are described in detail below.
• Cr is an element that enhances the hardenability of steel and contributes to improving the strength of steel plate.
• Cr is also an element that is effective in promoting the diffusion of Mn during solidification and reducing the microsegregation of Mn.
• the Cr content may be 0%, but in order to obtain these effects, the Cr content is preferably 0.01% or more.
• the Cr content may be 0.05% or more, 0.10% or more, or 0.15% or more.
• the Cr content is preferably 0.80% or less.
• the Cr content may be 0.75% or less, 0.70% or less, or 0.65% or less.
• Mo is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel sheet. Mo is also an element that is effective in promoting the diffusion of Mn during solidification and reducing microsegregation of Mn.
• the Mo content may be 0%, but in order to obtain these effects, the Mo content is preferably 0.01% or more.
• the Mo content may be 0.05% or more or 0.07% or more.
• the Mo content is preferably 0.50% or less.
• the Mo content may be 0.45% or less, 0.40% or less, or 0.35% or less.
• B is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel sheet.
• the B content may be 0%, but in order to obtain such an effect, the B content is preferably 0.0001% or more.
• the B content may be 0.0005% or more, 0.0008% or more, or 0.0010% or more.
• the B content is preferably 0.0100% or less.
• the B content may be 0.0080% or less, 0.0060% or less, or 0.0040% or less.
• Ti is an element that has the effect of reducing the amount of S, N, and O that generate coarse inclusions that act as the starting point of fracture. Ti is also a precipitation strengthening element that has the effect of refining the structure and improving the strength-formability balance of the steel sheet.
• the Ti content may be 0%, but in order to obtain these effects, the Ti content is preferably 0.001% or more.
• the Ti content may be 0.005% or more, 0.007% or more, or 0.010% or more.
• coarse Ti sulfides, Ti nitrides, and/or Ti oxides may be formed, which may reduce the formability of the steel sheet. Therefore, the Ti content is preferably 0.100% or less.
• the Ti content may be 0.080% or less, 0.075% or less, or 0.070% or less.
• Nb is a precipitation strengthening element that contributes to improving the strength of the steel sheet due to strengthening by precipitation, grain refinement strengthening by suppressing the growth of ferrite crystal grains, and/or dislocation strengthening by suppressing recrystallization.
• the Nb content may be 0%, but in order to obtain these effects, the Nb content is preferably 0.001% or more.
• the Nb content may be 0.003% or more, 0.004% or more, or 0.005% or more.
• the Nb content is preferably 0.100% or less.
• the Nb content may be 0.080% or less, 0.070% or less, or 0.060% or less.
• V is an element that contributes to improving the strength of the steel sheet due to strengthening by precipitation, grain refinement strengthening by suppressing the growth of ferrite crystal grains, and/or dislocation strengthening by suppressing recrystallization.
• the V content may be 0%, but in order to obtain these effects, the V content is preferably 0.01% or more.
• the V content may be 0.02% or more.
• the V content is preferably 0.50% or less.
• the V content may be 0.40% or less, 0.30% or less, or 0.20% or less.
• Ni is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel sheet.
• the Ni content may be 0%, but in order to obtain such an effect, the Ni content is preferably 0.01% or more.
• the Ni content may be 0.03% or more or 0.05% or more.
• the Ni content is preferably 1.00% or less.
• the Ni content may be 0.60% or less, 0.50% or less, or 0.40% or less.
• Cu is an element that exists in the steel in the form of fine particles and contributes to improving the strength of the steel sheet.
• the Cu content may be 0%, but in order to obtain such an effect, the Cu content is preferably 0.01% or more.
• the Cu content may be 0.03% or more or 0.05% or more.
• the Cu content is preferably 1.00% or less.
• the Cu content may be 0.80% or less, 0.60% or less, or 0.40% or less.
• 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 in order to obtain such an effect, the W content is preferably 0.01% or more.
• the W content may be 0.02% or more.
• the W content is preferably 1.00% or less.
• the W content may be 0.80% or less, 0.60% or less, or 0.40% or less.
• Sn is an element that suppresses the coarsening of crystal grains and contributes to improving the strength of the steel sheet.
• the Sn content may be 0%, but in order to obtain such an effect, the Sn content is preferably 0.001% or more.
• the Sn content may be 0.004% or more.
• the Sn content is preferably 1.00% or less.
• the Sn content may be 0.80% or less, 0.60% or less, or 0.40% or less.
• Sb is an element that suppresses the coarsening of crystal grains and contributes to improving the strength of the steel sheet.
• the Sb content may be 0%, but in order to obtain such an effect, the Sb content is preferably 0.001% or more.
• the Sb content may be 0.005% or more, 0.010% or more, or 0.015% or more.
• excessive Sb content may cause embrittlement of the steel sheet. Therefore, the Sb content is preferably 0.200% or less.
• the Sb content may be 0.180% or less, 0.170% or less, or 0.160% or less.
• the As content may be 0%, but in order to obtain such an effect, the As content is preferably 0.001% or more.
• the As content may be 0.002% or more, 0.004% or more, or 0.006% or more.
• the As content is preferably 0.200% or less.
• the As content may be 0.180% or less, 0.170% or less, or 0.160% or less.
• Ca, Mg, Zr and REM are elements that contribute to improving the formability of the steel sheet.
• the Ca, Mg, Zr and REM contents may be 0%, but in order to obtain such effects, the Ca, Mg, Zr and REM contents are preferably 0.0001% or more.
• the Ca, Mg, Zr and REM contents may be 0.0005% or more, 0.0010% or more, or 0.0015% or more.
• the Ca, Mg, Zr and REM contents are preferably 0.0100% or less.
• the Ca, Mg, Zr and REM contents may be 0.0080% or less, 0.0070% or less, or 0.0060% or less, respectively.
• REM is a collective term for 17 elements: scandium (Sc), atomic number 21; yttrium (Y), atomic number 39; and the lanthanides lanthanum (La), atomic number 57, through lutetium (Lu), atomic number 71.
• the REM content is the total content of these elements.
• the chemical composition of the steel sheet is: Cr: 0.01-0.80%, Mo: 0.01-0.50%, B: 0.0001 to 0.0100%, Ti: 0.001 to 0.100%, Nb: 0.001 to 0.100%, V: 0.01-0.50%, Ni: 0.01-1.00%, Cu: 0.01 to 1.00%, W: 0.01-1.00%, Sn: 0.001 to 1.00%, Sb: 0.001-0.200%, As: 0.001-0.200%, Ca: 0.0001-0.0100%, Mg: 0.0001-0.0100%, Zr: 0.0001 to 0.0100%, and REM: 0.0001 to 0.0100%
• the steel sheet contains such optional elements, it is possible to more reliably maintain high strength while significantly suppressing the occurrence of defective appearance after forming, such as ghost lines, and it is also possible to provide a steel sheet with excellent workability during forming.
• the remainder other than the above elements consists of Fe and impurities.
• impurities refer to components that are mixed in due to various factors in the manufacturing process, including raw materials such as ores and scraps, when the steel plate is industrially manufactured.
• impurities include H, Na, Cl, Co, Zn, Ga, Ge, As, Se, Y, Tc, Ru, Rh, Pd, Ag, Cd, In, Te, Cs, Ta, Re, Os, Ir, Pt, Au, Pb, Bi, and Po.
• the total amount of impurities may be 0.100% or less.
• index A 0.45% or more
• the chemical composition of the steel sheet is preferably such that index A, expressed by the following formula (1), is 0.45% or more.
• A [Si]+10[P]+0.6[Al]+8[Ti]+9[Nb]...(1)
• [Si], [P], [Al], [Ti] and [Nb] are the contents of each element in mass%, and when the corresponding element is not contained, it is 0%.
• Index A is determined by the content of the solid solution strengthening elements Si, P, and Al, and the precipitation strengthening elements Ti and Nb, and the larger this value is, the higher the strength can be obtained with a smaller hard phase fraction.
• index A By setting index A to 0.45% or more, it is possible to obtain high strength while controlling the hard phase fraction of the steel plate below a certain level.
• Index A may be 0.48% or more, 0.50% or more, or 0.52% or more.
• the upper limit of index A is not particularly limited, but for example, index A may be 1.50% or less, 1.20% or less, or 1.00% or less.
• the chemical composition of steel plate can be measured by a general analytical method.
• the chemical composition of steel plate can be measured by inductively coupled plasma atomic emission spectrometry (ICP-AES). Specifically, a 35 mm square test piece is taken from the steel plate at a depth of approximately 1/2 the plate thickness, and the chemical composition of the steel plate can be identified by measuring the specimen under conditions based on a calibration curve created in advance using a measuring device such as Shimadzu Corporation's ICPS-8100.
• ICP-AES inductively coupled plasma atomic emission spectrometry
• C and S which cannot be measured by ICP-AES, can be measured using the combustion-infrared absorption method, N using the inert gas fusion-thermal conductivity method, and O using the inert gas fusion-non-dispersive infrared absorption method.
• the metal structure of the steel sheet has an area ratio of the hard phase in a region having a depth from the surface of the steel sheet to a 1/8 depth position of the sheet thickness (i.e., the hard phase fraction of the surface layer) of 0.20Vm to 0.80Vm, where Vm is the area ratio of the hard phase in a region having a depth from a 3/8 depth position to a 5/8 depth position of the sheet thickness (i.e., the hard phase fraction in the center of the sheet thickness).
• the hard phase fraction of the surface layer of the steel sheet is 0.20Vm or more, a low yield strength (YS) and a high tensile strength (TS) can be ensured, so that a high-strength steel sheet having excellent workability during forming can be obtained. Furthermore, this also makes it easier to suppress stretcher strain that occurs during forming of the steel sheet. In addition, when the hard phase fraction of the surface layer of the steel sheet is 0.8Vm or less, good bendability can be ensured, so that a steel sheet having excellent workability during forming can be obtained.
• the area ratio of the hard phase in the surface layer may be 0.30 Vm or more, 0.35 Vm or more, 0.40 Vm or more, or 0.45 Vm or more.
• the area ratio of the hard phase in the surface layer may be 0.79 Vm or less, 0.78 Vm or less, 0.77 Vm or less, or 0.76 Vm or less.
• the metal structure in the region having a depth from 3/8 to 5/8 of the plate thickness: ferrite: 75-97% and hard phase: 3-25% contains, in area %, 75 to 97% ferrite and 3 to 25% hard phase.
• the area fraction of the hard phase in the plate thickness center portion may be 4% or more, 5% or more, 6% or more, 7% or more, or 8% or more. Similarly, the area fraction of ferrite in the plate thickness center portion may be 96% or less, 95% or less, or 94% or less. On the other hand, from the viewpoint of further improving the appearance after forming, the area fraction of the hard phase in the plate thickness center portion may be 22% or less, 20% or less, or 18% or less. Similarly, the area fraction of ferrite in the center portion of the sheet thickness may be 78% or more, 80% or more, 82% or more, 85% or more, 90% or more, or 92% or more.
• the hard phase means a structure harder than ferrite, and is composed of at least one of martensite, bainite, tempered martensite, and pearlite, for example.
• the hard phase in the region having a depth from the 3/8 depth position to the 5/8 depth position of the plate thickness is preferably composed of at least one of martensite, bainite, and tempered martensite, and more preferably composed of martensite.
• the metal structure of the steel plate contains little retained austenite, and specifically, the retained austenite is preferably less than 1% or less than 0.5% by area, and more preferably 0%.
• the identification of the metal structure and the calculation of the area fraction are performed as follows.
• the rolling direction of the obtained steel sheet is specified.
• the rolling direction of the steel sheet can be determined as follows. A test piece is taken so that a cross section parallel to the surface of the steel sheet can be observed. The cross section of the test piece taken in the sheet thickness direction is mirror-polished and then observed using an optical microscope.
• the observation surface is a surface parallel to the surface of the steel sheet at an arbitrary depth position in the range of 1/4 to 1/2 of the sheet thickness in the sheet thickness direction, and the direction parallel to the extension direction of the crystal grains in the observation surface is determined to be the rolling direction.
• the direction perpendicular to the rolling direction and thickness direction of the steel plate is defined as the plate width direction, and the length of the steel plate in the plate width direction is defined as the plate width W.
• a sample for observing the metal structure (microstructure) is taken from the W/4 position or 3W/4 position of the plate width W (i.e., a position W/4 in the plate width direction from any end of the steel plate in the plate width direction).
• the size of the sample is, for example, 20 mm in the rolling direction ⁇ 20 mm in the plate width direction ⁇ thickness of the steel plate.
• the metal structure is observed using an FE-SEM (field emission scanning electron microscope, for example, JEOL's "JSM-7200F", acceleration voltage: 15 kV, light source: FE, device resolution: 1.2 nm).
• the observation field is a 100 ⁇ m x 100 ⁇ m area corresponding to the observation target position in the thickness direction of the steel plate.
• a 50 ⁇ m x 100 ⁇ m area (50 ⁇ m in the plate thickness direction, and 100 ⁇ m in the observation field in the direction perpendicular to the plate thickness direction) is observed in the thickness direction of the steel plate from the surface of the steel plate to a depth position of 1/8 of the plate thickness.
• the metal structure of a region having a depth from 3/8 to 5/8 of the thickness of the steel plate i.e., the center of the plate thickness
• a 100 ⁇ m x 100 ⁇ m region between 3/8 and 5/8 of the thickness of the steel plate in the plate thickness direction is observed.
• the resolution for observing either region is 1280 x 960 pixels.
• the plate thickness cross section in the plate width direction is polished as the observation surface and etched by nital etching.
• the "microstructure" is classified from SEM photographs magnified at 500 or 1000 times. Ferrite and hard phases can be distinguished from each other by the difference in brightness.
• the above 100 ⁇ m x 100 ⁇ m area is observed at a magnification of 500x or 1000x, and image analysis is performed using image analysis software Image J (Ver. 1.54f) to determine the area fraction of the hard phase.
• Image J Ver. 1.54f
• the secondary electron image of the metal structure is binarized based on differences in brightness, and the black parts of the image data are considered to be ferrite and the uncorroded white parts are considered to be the hard phase, and the area fraction of the hard phase is calculated.
• three measurement points are measured, and image analysis is performed at these measurement points in the same manner as above to measure the area fraction of the hard phase, and these area fractions are arithmetically averaged to calculate the average value.
• This average value is considered to be the area fraction of the hard phase, and the remainder is considered to be the area fraction of ferrite.
• the only difference is the observation field in the plate thickness direction of the steel plate, and the area fraction of the hard phase can be measured in the same way.
• the area fraction of retained austenite can be measured by X-ray diffraction on the above observation surface. Specifically, using Co-K ⁇ radiation, the integrated intensity of a total of six peaks, ⁇ (110), ⁇ (200), ⁇ (211), ⁇ (111), ⁇ (200), and ⁇ (220), at a depth position of 1/2 the plate thickness is obtained, and the volume fraction of retained austenite is calculated using the intensity averaging method, and the obtained volume fraction of retained austenite is regarded as the area fraction of retained austenite.
• the metal structure of the steel sheet has a standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction and the sheet thickness direction in a region having a depth from the 3/8 depth position to the 5/8 depth position of the sheet thickness (i.e., the sheet thickness center portion) of 0.85% or less.
• the standard deviation of the hard phase fraction means the standard deviation of the area fraction of the hard phase itself.
• the standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction and the plate thickness direction in the region having a depth from the 3/8 depth position to the 5/8 depth position of the plate thickness is 0.85% or less, i.e., the variation in the hard phase fraction in the direction perpendicular to the rolling direction and the plate thickness direction is below a certain level, so that poor appearance after forming can be significantly suppressed.
• the standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction and the plate thickness direction in the region having a depth from the 3/8 depth position to the 5/8 depth position of the plate thickness may be 0.80% or less, 0.75% or less, 0.70% or less, or 0.65% or less.
• the lower limit of the standard deviation of the hard phase fraction is not particularly limited, but for example, the standard deviation of the hard phase fraction may be 0.01% or more, 0.05% or more, 0.10% or more, 0.15% or more, or 0.20% or more.
• the ratio of the average value of the area fraction of the hard phase to its standard deviation is 0.10 or less (i.e., standard deviation of hard phase fraction/average value of area fraction of hard phase ⁇ 0.10).
• the above ratio is preferably 0.09 or less, 0.08 or less, or 0.07 or less.
• the lower limit of the above ratio is 0, but the lower limit may be set to 0.01 if necessary.
• the standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction and thickness direction of the metal structure at the center of the plate thickness can be determined as follows. First, a 100 ⁇ m x 100 ⁇ m region between 3/8 and 5/8 depth positions of the plate thickness in a cross section parallel to the direction perpendicular to the rolling direction and thickness direction of the steel plate and perpendicular to the surface of the steel plate is observed at a magnification of 500x or 1000x using an FE-SEM (field emission scanning electron microscope, for example JEOL JSM-7200F, accelerating voltage: 15 kV, light source: FE, instrument resolution: 1.2 nm) to obtain a secondary electron image. The resolution is 1280 x 960 pixels.
• FE-SEM field emission scanning electron microscope, for example JEOL JSM-7200F, accelerating voltage: 15 kV, light source: FE, instrument resolution: 1.2 nm
• image analysis is performed on this secondary electron image using image analysis software Image J (Ver. 1.54f), and the area fraction of the hard phase is measured every 100 ⁇ m within an 8 mm range in the direction perpendicular to the rolling direction and thickness direction of the steel plate (plate width direction) from the W/4 position to the 3W/4 position of the plate width W, and the standard deviation is calculated.
• the observation range in the direction perpendicular to the rolling direction and thickness direction (plate width direction) may be less than 8 mm or more than 8 mm.
• the lower limit of the observation range of the standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction and thickness direction (plate width direction) is 4 mm, and the upper limit is 12 mm.
• the tensile strength (TS) of a steel plate can be measured by taking a No. 5 tensile test piece of JIS Z2241:2011, whose longitudinal direction is perpendicular to the rolling direction and plate thickness direction, from the steel plate and conducting a tensile test in accordance with JIS Z2241:2011. Note that if the No. 5 tensile test piece cannot be taken from the steel plate of the measurement sample (for example, if the size of the measurement sample is small), a tensile test piece of any size, whose longitudinal direction is perpendicular to the rolling direction and plate thickness direction, can be used instead of the No. 5 tensile test piece.
• the average grain size of ferrite in the metal structure in the region having a depth from the 3/8 depth position to the 5/8 depth position of the steel plate thickness is preferably 5.0 to 30.0 ⁇ m.
• the average grain size of ferrite may be 6.0 ⁇ m or more or 7.0 ⁇ m or more.
• the average grain size of ferrite may be 27.0 ⁇ m or less, 21.0 ⁇ m or less, 15.0 ⁇ m or less, or 11.0 ⁇ m or less.
• the average grain size of ferrite in the center portion of the sheet thickness is determined as follows. First, the KAM value of the ferrite is calculated by KAM (Kernel Average Misorientation) analysis and GAIQ (Grain Average Image Quality) analysis in an electron probe micro analyzer (EPMA) measurement, which is a crystal analysis method using a SEM.
• KAM Kernel Average Misorientation
• GAIQ Gram Average Image Quality
• a field emission scanning electron microscope e.g., JEOL's "JSM-7001F"
• EBSD analysis can be performed using, for example, TSL's "OIM Analysis 7".
• EBSD analysis a 50 ⁇ m x 50 ⁇ m area between 3/8 and 5/8 depth positions of the plate thickness of a steel plate whose plate thickness cross section has been mirror-polished is analyzed at intervals (pitch) of 0.05 ⁇ m.
• KAM analysis is an analysis in which the average value of the orientation difference (°) between a "certain pixel” that is the measurement point and all adjacent pixels is taken as the KAM value of the "certain pixel", and a KAM map based on the local crystal orientation difference can be created.
• the KAM value in ferrite is analyzed.
• the area where ferrite exists in the EBSD measurement results is determined by the GAIQ analysis described below.
• a GAIQ (Grain Average Image Quality) analysis is performed on the EBSD measurement results obtained under the same measurement conditions as the KAM value to obtain the GAIQ value. Based on this GAIQ value, the region is divided into two regions: those with a GAIQ value of 50,000 or more, and those with a GAIQ value of less than 50,000.
• the region (grain) with a GAIQ value of 50,000 or more is determined to be a region where ferrite exists.
• the region (grain) with a GAIQ value of less than 50,000 is determined to be a region where a hard phase exists.
• the GAIQ analysis is an analysis in which the average value within one grain of the IQ value, which indicates the clarity of the Kikuchi pattern at "one pixel," the measurement point, is taken as the GAIQ value of that grain.
• the circle equivalent diameter is calculated for all crystal grains (ferrite grains) located in the region corresponding to the ferrite determined by the GAIQ analysis.
• the crystal grains are defined as the region surrounded by the grain boundary, which is the boundary between regions whose crystal orientations differ by 15° or more.
• the value obtained by arithmetically averaging these is then defined as the average crystal grain size of the ferrite.
• the above-mentioned average grain size of ferrite is measured at five locations, and the arithmetic average value thereof is determined as the average grain size of ferrite.
• the average grain size of the hard phase in the metal structure in the region having a depth from the 3/8 depth position to the 5/8 depth position of the steel plate thickness is preferably 1.0 to 5.0 ⁇ m.
• the average grain size of the hard phase may be 1.2 ⁇ m or more, 1.5 ⁇ m or more, 1.7 ⁇ m or more, or 2.0 ⁇ m or more.
• the average grain size of the hard phase may be 4.7 ⁇ m or less, 4.5 ⁇ m or less, 4.2 ⁇ m or less, 4.0 ⁇ m or less, 3.5 ⁇ m or less, or 3.0 ⁇ m or less.
• the average crystal grain size of the hard phase in the center portion of the sheet thickness is determined as follows. First, a sample having a steel plate cross section perpendicular to the plate surface is taken, and the cross section obtained by etching the steel plate cross section with a Nital reagent is used as the observation surface. A region of 100 ⁇ m ⁇ 100 ⁇ m within the range of 3/8 to 5/8 plate thickness positions of the observation surface is used as the observation region, and the hard phase is identified using an FE-SEM (for example, JSM-7200F manufactured by JEOL, measured at an acceleration voltage of 15 kV). Specifically, the metal structure is binarized based on the difference in brightness using image analysis software Image J (Ver. 1.54f), and the hard phase is identified.
• FE-SEM for example, JSM-7200F manufactured by JEOL, measured at an acceleration voltage of 15 kV
• the black part of the image data is ferrite, and the white part that is not corroded is the hard phase.
• the circle equivalent diameter of all the identified hard phases is calculated. This operation is performed in three observation regions, and the circle equivalent diameters of all the obtained hard phases are arithmetically averaged, and the obtained value is determined as the average crystal grain size of the hard phase.
• the thickness of the steel plate is not particularly limited and can be appropriately determined depending on the type of the final product.
• the steel plate may have a thickness of 0.1 to 2.0 mm.
• a steel plate having such a thickness is suitable for use as a material for a cover member such as a door or a hood.
• the thickness of the steel plate may be 0.2 mm or more, 0.3 mm or more, or 0.4 mm or more.
• the thickness of the steel plate may be 1.8 mm or less, 1.5 mm or less, 1.2 mm or less, or 1.0 mm or less.
• the thickness of the steel plate 0.2 mm or more, it is easy to maintain the shape of the molded product flat, and an additional effect of improving the dimensional accuracy and shape accuracy can be obtained.
• the thickness 1.0 mm or less the weight reduction effect of the member becomes remarkable.
• the thickness of the steel plate is measured by a micrometer.
• the steel sheet of the present embodiment may further have a plating layer on the surface for the purpose of improving corrosion resistance or the like.
• the plating layer may be provided on one side or both sides of the steel sheet.
• the plating layer may be either a hot-dip plating layer or an electroplating layer.
• the hot-dip plating layer include a hot-dip galvanized layer (GI), a hot-dip galvannealed layer (GA), a hot-dip aluminum plating layer, a hot-dip Zn-Al alloy plating layer, a hot-dip Zn-Al-Mg alloy plating layer, and a hot-dip Zn-Al-Mg-Si alloy plating layer.
• the electroplating layer examples include an electrogalvanized layer (EG) and an electrogalvanized Zn-Ni alloy plating layer.
• the plating layer is preferably a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electrogalvanized layer.
• the coating weight of the plating layer is not particularly limited, and may be a general coating weight.
• the steel sheet of this embodiment having the above-mentioned specific chemical composition and metal structure, high tensile strength, specifically tensile strength of 500 MPa or more can be achieved.
• the tensile strength of the steel sheet is preferably 540 MPa or more, more preferably 600 MPa or more.
• the upper limit of the tensile strength is not particularly limited, but may be, for example, 980 MPa or less or 850 MPa or less. By setting the tensile strength to 850 MPa or less, it becomes easier to ensure the workability when the steel sheet is press-formed.
• an exterior plate member particularly an automobile exterior plate member, is provided that includes the steel plate according to the embodiment of the present invention.
• automobile exterior plate members include roofs, hoods, fenders, doors, and the like, which require high designability.
• These exterior plate members, particularly automobile exterior plate members only need to include the steel plate according to the embodiment of the present invention in at least a portion of these exterior plate members, and therefore at least a portion of these exterior plate members satisfy the chemical composition and metal structure characteristics described above. In areas of the steel plate that are relatively less processed in forming such as press forming, the characteristics of the metal structure do not change particularly before and after forming.
• the method for manufacturing steel sheet of this embodiment includes at least a casting process for casting a slab having the above-mentioned specific chemical composition, a hot rolling process for hot rolling the cast slab, a pickling process for removing the oxide film (scale) formed during hot rolling, a cold rolling process for cold rolling the hot-rolled steel sheet after pickling, and an annealing process for holding the cold-rolled steel sheet in a predetermined atmosphere and at a predetermined temperature range.
• the manufacturing method of the steel sheet of this embodiment may optionally include other steps such as a cooling step for cooling the cold-rolled steel sheet after annealing, a plating step for forming a plating layer on the surface of the cold-rolled steel sheet after cooling, and a skin-pass rolling step for subjecting the steel sheet after the plating step to skin-pass rolling.
• the casting step is a step of casting a slab having the above-mentioned specific chemical composition.
• the casting step includes performing soft reduction using a continuous casting machine having a plurality of reduction rolls adjacent to each other in the conveying direction of the slab, the roll pitch of the adjacent reduction rolls being 290 mm or less.
• soft reduction refers to a reduction having a reduction gradient of 0.6 mm or more per meter in the casting direction.
• the steel plate of this embodiment is essential to have a unique metal structure with a lower hard phase fraction than conventional DP steel and small variation in the hard phase fraction in the rolling direction and the direction perpendicular to the plate thickness direction at the center of the plate thickness.
• it is important to control the solidification structure during casting to be columnar.
• the superheat ⁇ T (the difference between the molten steel temperature and the solidification temperature of the molten steel) of the molten steel having the above-mentioned specific chemical composition is set to 25°C or more, and the segment pressing force is set to 450 tons or more, thereby controlling the solidification structure to a columnar crystal structure with an equiaxed crystal ratio of 15% or less, and center segregation can be suppressed while using a method different from the conventional center segregation countermeasure.
• the superheat ⁇ T is preferably 30°C or more.
• the superheat ⁇ T is preferably 40°C or less.
• the molten steel temperature is the molten steel temperature in the tundish and can be obtained by actual measurement.
• the solidification temperature can be obtained from the chemical composition of the molten steel using a known solidification temperature estimation formula.
• the equiaxed crystal ratio (%) can be calculated by taking an etched print of the thickness cross section of the slab in the width direction (the direction perpendicular to the conveying direction and the thickness direction), visually determining the boundary between the columnar crystal structure and the equiaxed crystal structure, measuring the thickness (mm) of the equiaxed crystal structure in the center of the slab thickness and the thickness (mm) of the slab, and dividing the thickness of the equiaxed crystal structure by the thickness of the slab and multiplying the result by 100.
• soft reduction is performed using a continuous casting machine in which the roll pitch of adjacent reduction rolls is 290 mm or less, which suppresses the flow of molten steel during solidification and reduces the concentration of Mn in the center. This makes it possible to suppress central segregation of Mn. It is preferable that the roll pitch of adjacent reduction rolls is 280 mm or less.
• the hot rolling step is a step of hot rolling the cast slab.
• the heating temperature is not particularly limited, from an economical point of view, it is preferable that the heating temperature is less than 1300 ° C.
• the heated slab is subjected to rough rolling and finish rolling.
• the finish rolling end temperature is preferably 950°C or less. By setting the finish rolling end temperature at 950°C or less, the average crystal grain size of the hot-rolled steel sheet and the final product can be reduced, ensuring sufficient yield strength and high surface quality after forming.
• the air-cooling time t (seconds) after finish rolling is set to a time that satisfies the following formula (3). 0.067t ⁇ ln(1300/(FT+273))...(3)
• t in the formula (3) represents the air cooling time (seconds) after finish rolling
• FT represents the surface temperature (° C.) of the steel sheet after finish rolling.
• the internal oxide layer will be easily removed in the subsequent pickling process, and as a result, the internal oxide layer can be minimized.
• the hot-rolled steel sheet obtained in this manner is coiled at a predetermined coiling temperature (CT).
• CT coiling temperature
• the coiling temperature is set to a temperature of 580°C or less.
• the coiling temperature may be, for example, a temperature of 450°C or more.
• the pickling process is a process for removing scale formed during hot rolling.
• the hot-rolled steel sheet that is continuously transported is immersed in a pickling tank in which an acidic cleaning solution is stored, thereby removing scale formed on the surface of the hot-rolled steel sheet.
• an acidic cleaning solution for example, hydrochloric acid, sulfuric acid, or the like can be used.
• the pickling time and the pickling temperature are set according to the chemical components (Si and Mn) of the steel sheet and the coiling temperature (CT).
• the pickling time and the pickling temperature are set so as to satisfy the following formula (4).
• T represents the pickling temperature (°C)
• tA represents the pickling time (seconds)
• [Si] represents the Si content (mass%)
• [Mn] represents the Mn content (mass%)
• CT represents the coiling temperature after hot rolling (°C).
• the internal oxide layer is more easily removed, and as a result, the internal oxide layer can be reduced as much as possible.
• the pickling time tA can be changed arbitrarily by adjusting the conveying speed of the hot-rolled steel sheet.
• the cold rolling step is a step of cold rolling the hot-rolled steel sheet after pickling.
• the cold rolling step it is preferable to cold roll the hot-rolled steel sheet so that the cumulative reduction is, for example, 50 to 90%.
• the annealing step is a step of holding the cold-rolled steel sheet in a predetermined atmosphere and at a predetermined temperature range.
• conditions such as oxygen potential are controlled by humid annealing, i.e., high dew point annealing, so as to satisfy the following three conditions (i) to (iii): (i) Heating zone oxygen potential: -0.9 ⁇ log(pH 2 O/pH 2 ) ⁇ -0.3 (ii) isotropic oxygen potential: -2.4 ⁇ log(pH 2 O/pH 2 ) ⁇ -0.8 (iii) Heating zone outlet/soaking zone temperature: Ac1+20 ⁇ T ⁇ Ac3-30
• pH2O is the water vapor partial pressure (Pa) of the heating zone atmosphere and the soaking zone atmosphere
• pH2 is the hydrogen partial pressure (Pa) of the heating zone atmosphere and the soaking zone atmosphere
• T is the temperature (°C) of the outlet side of the heating zone and the soaking zone atmosphere.
• the Ac1 point (°C) and Ac3 point (°C) are calculated according to the chemical composition of the steel sheet by the following formula.
• Ac1 751-27C+18Si-12Mn-23Cu-23Ni+24Cr+23Mo-40V-6Ti+230Nb-169Al
• Ac3 911-436C+30Si-25Mn-5Cr+15Mo+136Ti-19Nb+101Al
• the cooling step is a step of cooling the cold-rolled steel sheet after annealing.
• the cooling step it is preferable to cool the cold-rolled steel sheet so that the average cooling rate from the soaking temperature is 5 to 50 ° C. / sec.
• the average cooling rate By setting the average cooling rate to 5 ° C. / sec or more, excessive transformation into ferrite can be suppressed and the amount of hard phase such as martensite produced can be increased to obtain the desired strength.
• the average cooling rate to 50 ° C. / sec or less, the steel sheet can be cooled more uniformly in the sheet width direction (the direction perpendicular to the rolling direction and the sheet thickness direction).
• the plating step is a step of forming a plating layer on the surface of the steel sheet by performing a plating treatment on the surface of the cold-rolled steel sheet after cooling.
• the plating treatment include hot-dip plating, alloying hot-dip plating, and electroplating.
• the plating treatment may involve hot-dip galvanizing on the surface of the steel sheet, or the hot-dip galvanizing treatment may be followed by an alloying treatment.
• the specific conditions of the plating treatment and the alloying treatment are not particularly limited, and any appropriate conditions known to those skilled in the art may be adopted.
• the alloying temperature may be 450 to 600°C.
• the plating process is a process carried out for the purpose of improving corrosion resistance, etc., and is not an essential process. Therefore, if the steel sheet does not include a plating layer, there is no need to carry out such a plating process.
• a steel plate according to one embodiment of the present invention was manufactured under various conditions, and the tensile strength, workability, and post-forming appearance characteristics of the resulting steel plate were examined.
• a slab having the chemical composition shown in Table 1 was cast by continuous casting using a continuous casting machine equipped with multiple reduction rolls arranged with a roll pitch of 290 mm or less, with a light reduction with a reduction gradient of 0.6 mm or more per 1 m in the casting direction.
• the remainder other than the components shown in Table 1 is Fe and impurities.
• the various conditions in this casting process are shown in Table 2.
• the casting condition " ⁇ T” is the condition that superheat ⁇ T ⁇ 25°C
• the casting condition "pressing force” is the condition that segment pressing force ⁇ 450 tons.
• cases where these conditions are met are indicated as "OK” and cases where they are not met are indicated as "NG”, and are shown in Table 2.
• the obtained slab was subjected to a hot rolling process.
• the various conditions in the hot rolling process are shown in Table 2.
• the "air-cooling time" of the hot rolling conditions for each steel sheet is a condition that the air-cooling time after finish rolling satisfies the following formula (3)
• the "coiler temperature” of the hot rolling conditions is a condition that the coiler temperature (CT) ⁇ 580°C. 0.067t ⁇ ln(1300/(FT+273))...(3)
• t represents the air cooling time (seconds) after finish rolling
• FT represents the surface temperature (° C.) of the steel sheet after finish rolling.
• the hot-rolled steel sheet after the hot rolling process was subjected to a pickling process.
• Various conditions in the pickling process are shown in Table 2.
• the "time and temperature" of the pickling conditions are conditions in which the pickling time and the pickling temperature satisfy the following formula (4).
• T represents the pickling temperature (°C)
• tA represents the pickling time (seconds)
• [Si] represents the Si content (mass%)
• [Mn] represents the Mn content (mass%)
• CT represents the coiling temperature after hot rolling (°C).
• the pickled hot-rolled steel sheets were then subjected to a cold rolling process (cumulative reduction rate 80%), an annealing process (soaking temperature 800°C) and a cooling process (average cooling rate 10°C/sec) to produce steel sheets.
• the surfaces of the obtained steel sheets were appropriately plated to form a hot-dip galvanized layer (GI), a galvannealed layer (GA) or an electrolytic galvanized layer (EG) consisting of the plating types shown in Table 2.
• GI hot-dip galvanized layer
• GA galvannealed layer
• EG electrolytic galvanized layer
• the steel sheet was heated under the conditions of a heating zone temperature of 780° C., a dew point of ⁇ 2° C., and a hydrogen concentration of 2.6 vol.%, a soaking zone temperature of 780° C., a dew point of ⁇ 40° C., and a hydrogen concentration of 2.7 vol.%, and then the steel sheet was subjected to a hot-dip galvanizing treatment and then to an alloying treatment at 560° C.
• steel sheets No. 8 and No. 33 are examples in which the surface of the steel sheet was not plated.
• Heating zone oxygen potential -0.9 ⁇ log(pH 2 O/pH 2 ) ⁇ -0.3
• isotropic oxygen potential -2.4 ⁇ log(pH 2 O/pH 2 ) ⁇ -0.8
• Heating zone outlet/soaking zone temperature Ac1+20 ⁇ T ⁇ Ac3-30
• Ratio of hard phase fraction at 3/8 to 5/8 depth positions of plate thickness; Vs/Vm in Table 2 means the ratio Vs/Vm of the hard phase fraction Vs in the region having a depth from the surface of the steel plate to 1/8 depth position of the plate thickness to the hard phase fraction Vm in the region having a depth from 3/8 to 5/8 depth position of the plate thickness. Therefore, this ratio Vs/Vm being 0.20 to 0.80 is synonymous with "the area ratio of the hard phase in the region having a depth from the surface of the steel plate to a depth position of 1/8 of the plate thickness being 0.20Vm to 0.80Vm.”
• the metal structure and various properties of the surface and center of the plate were measured and evaluated using the methods described above or below.
• a close-contact bending test was performed by the push-bending method of the metal material bending test method of JIS Z2248:2022 to evaluate the workability.
• a rectangular test piece having a width of 30 mm and a length of 100 mm was taken from the steel plate so that the direction parallel to the rolling direction of the steel plate was the axial direction of the bending test.
• the test pieces after the close-contact bending test were visually evaluated for the presence or absence of cracks at the ridge of the bend apex, and the absence of cracks was deemed to have passed, and was shown in Table 2 as "OK”. If cracks were found in the above visual evaluation, the test piece was deemed to have failed, and was shown in Table 2 as "NG".
• the ratio Vs/Vm of the hard phase fraction Vs of the metal structure of the surface layer to the hard phase fraction Vm of the metal structure of the sheet thickness center portion was 0.17 (the hard phase fraction Vs of the metal structure of the surface layer was 0.17Vm), and the yield ratio was high and stretcher strain was generated, resulting in a poor appearance after forming.
• the standard deviation of the hard phase fraction in the metal structure at the center of the plate thickness was 0.88%, and ghost lines were generated, resulting in poor appearance after forming.
• the ratio Vs/Vm of the hard phase fraction Vs of the metal structure of the surface layer to the hard phase fraction Vm of the metal structure of the sheet thickness center portion was 0.17 (the hard phase fraction Vs of the metal structure of the surface layer was 0.17Vm), and the yield ratio was high and stretcher strain was generated, resulting in poor appearance after forming.
• the hot rolling step was performed under the condition of a coiling temperature of 690° C.
• the pickling step was performed under the condition of a pickling temperature of 65° C. and a pickling time of 80 seconds, i.e., under the condition not satisfying the above formula (4)
• the annealing step was performed under the condition of a soaking zone oxygen potential value of ⁇ 0.5
• the ratio Vs/Vm of the hard phase fraction Vs of the metal structure of the surface layer to the hard phase fraction Vm of the metal structure of the sheet thickness center portion was 0.00 (the hard phase fraction Vs of the metal structure of the surface layer was 0.00Vm)
• the yield ratio was high
• stretcher strain was generated, resulting in a poor appearance after forming.
• the hot rolling step was performed under the conditions of a finish rolling temperature of 920°C and an air cooling time of 1.5 seconds, i.e., conditions not satisfying the above formula (3)
• the pickling step was performed under the conditions of a coiling temperature of 570°C, a pickling temperature of 55°C, and a pickling time of 90 seconds, i.e., conditions not satisfying the above formula (4)
• the ratio Vs/Vm of the hard phase fraction Vs of the metal structure of the surface layer to the hard phase fraction Vm of the metal structure of the sheet thickness center portion was 0.13 (the hard phase fraction Vs of the metal structure of the surface layer was 0.13Vm), the yield ratio was high, and stretcher strain occurred, resulting in a poor appearance after forming.
• the steel sheets Nos. 1, 2, 7-13, 18-20, and 23-28 which are examples of the present invention, maintained a high tensile strength of 500 MPa or more while exhibiting excellent workability in press forming and the like, and even when strain was imparted by press forming, the occurrence of ghost lines and stretcher strain on the steel sheet surface was significantly suppressed.

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• 2024
  • 2024-09-05 JP JP2025535112A patent/JP7820682B2/ja active Active
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  • 2024-09-05 KR KR1020267006058A patent/KR20260041896A/ko active Pending
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