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  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
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  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
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  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0273—Final recrystallisation annealing
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  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite
• • C—CHEMISTRY; METALLURGY
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  • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite
• • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• This disclosure relates to steel sheets.
• Patent Document 3 discloses a steel sheet having a specific chemical composition and metal structure, in which the value X1 obtained by dividing the standard deviation in the thickness direction of the average Mn concentration in the rolling direction at the 1/4 position in the thickness direction by the average Mn concentration is 0.025 or less.
• Patent Document 4 also discloses a steel sheet having a specific chemical composition, a metal structure in a surface region consisting of ferrite and a second phase with a volume fraction of 0.01 to 5.0%, a metal structure in an inner region consisting of ferrite and a second phase with a volume fraction of 2.0 to 10.0%, the volume fraction of the second phase in the surface region being smaller than the volume fraction of the second phase in the inner region, the average grain size of the second phase in the surface region being 0.01 to 4.0 ⁇ m, and an intensity ratio X ODF ⁇ 001 ⁇ / ⁇ 111 ⁇ between the ⁇ 001 ⁇ orientation and the ⁇ 111 ⁇ orientation of ferrite being 0.60 or more and less than 2.00.
• the purpose of this disclosure is to provide a high-strength steel sheet with an improved post-forming appearance through a novel configuration.
• the present inventors conducted detailed studies focusing on the morphology of the hard phase in the metal structure. As a result, the present inventors discovered that by suppressing the generation of hard phases connected in stripes (striped hard phases) and dispersing the hard phases in the metal structure more uniformly, the high strength based on such hard phases is maintained while improving poor appearance after forming. Specifically, the present inventors discovered that by reducing the central segregation of Mn during solidification, which is a factor in the generation of the striped hard phases, and by reducing the hard phase fraction and minimizing its variation, it is possible to significantly improve poor appearance after forming while adequately maintaining high strength.
• the chemical composition, in mass%, is C: 0.030 to 0.100%, Mn: 1.00 to 2.50%, Si: 0.005 to 1.500%, P: 0.100% or less, S: 0.0200% or less, Al: 0.005 to 0.700%, N: 0.0150% or less, O: 0.0100% or less, Cr: 0 to 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 to 1.00%, Sn: 0 to 1.00%, Sb: 0 to 0.200%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, REM: 0 to 0.0100%, and the balance: Fe and impurities, and index A represented by the following formula (1) is 0.45% or
• A [Si] + 10 [P] + 0.6 [Al] + 8 [Ti] + 9 [Nb] ...
• [Si], [P], [Al], [Ti] and [Nb] are the contents of each element in mass%, and when no element is contained, it is 0%.
• the above chemical composition is, in mass %, Cr: 0.01 to 0.80%, Mo: 0.01 to 0.50%, B: 0.0001 to 0.0100%, Ti: 0.001 to 0.100%, Nb: 0.001 to 0.100%, V: 0.01 to 0.50%, Ni: 0.01 to 1.00%, Cu: 0.01 to 1.00%, W: 0.01 to 1.00%, Sn: 0.01 to 1.00%, Sb: 0.001 to 0.200%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Zr: 0.0001 to 0.0100%, and REM: 0.0001 to 0.0100%
• Aspect 4 The steel sheet according to any one of Aspects 1 to 3, wherein the ferrite has an average crystal grain size of 5.0 to 30.0 ⁇ m, and the hard phase has an average crystal grain size of 1.0 to 5.0 ⁇ m.
• the steel sheet according to one embodiment of the present disclosure has a chemical composition, in mass%, C: 0.030 to 0.100%, Mn: 1.00 to 2.50%, Si: 0.005 to 1.500%, P: 0.100% or less, S: 0.0200% or less, Al: 0.005 to 0.700%, N: 0.0150% or less, O: 0.0100% or less, Cr: 0 to 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 to 1.00%, Sn: 0 to 1.00%, Sb: 0 to 0.200%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, REM: 0 to 0.0100%, and the balance: Fe and impur
• A [Si] + 10 [P] + 0.6 [Al] + 8 [Ti] + 9 [Nb] ...
• [Si], [P], [Al], [Ti] and [Nb] are the contents of each element in mass%, and when no element is contained, it is 0%.
• DP steel which has a relatively low yield strength, is often used in order to avoid surface defects called surface distortions that occur during press forming and the like.
• DP steel which is a mixture of a soft phase made of ferrite and a hard phase mainly made of martensite, etc.
• non-uniform deformation is likely to occur during processing such as press forming, in which the soft phase and its surroundings are preferentially deformed. This non-uniform deformation can cause minute irregularities on the surface of the steel sheet after forming, which can result in appearance defects called ghost lines.
• the soft phase made of ferrite deforms so as to be concave while the hard phase mainly made of martensite, etc. does not concave or rather deforms so as to rise in a convex shape.
• minute irregularities are formed on the surface of the steel sheet 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 width direction perpendicular to the rolling direction.
• the rolling direction can be easily determined based on the extension direction of the crystal grains of the steel sheet.
• the direction perpendicular to the rolling direction is the direction perpendicular to the rolling direction and the thickness direction.
• the present inventors therefore conducted detailed studies focusing on the morphology of the hard phase in the metal structure in order to improve such poor appearance after forming.
• the present inventors first discovered that in steel sheets in which a soft layer and a hard phase are mixed, such as DP steel, the degree of ghost lines becomes significant due to the presence of hard phases connected in stripes in the metal structure.
• the present inventors then discovered that by suppressing the formation of such striped hard phases and dispersing the hard phases in the metal structure more uniformly, it is possible to suppress the formation of minute irregularities on the steel sheet surface after forming while adequately maintaining high strength, and as a result, to suppress the occurrence of ghost lines.
• the present inventors have found that reducing Mn segregation during solidification in the slab casting process, in which molten steel is solidified to cast slabs, is effective in suppressing the formation of banded structures associated with hard phases.
• the present inventors have conducted detailed studies on methods for reducing Mn segregation from two perspectives: center segregation and microsegregation.
• the present inventors believed that suppressing the flow of molten steel during slab casting was 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 finally the center solidifies. At this time, the solid phase is discharged from the liquid phase of the molten steel, so that Mn in the liquid phase becomes concentrated at this stage. If the molten steel flows during solidification, such concentrated Mn areas tend to eventually gather in the center during the solidification process, resulting in noticeable central segregation of Mn. Therefore, the present inventors discovered that when manufacturing steel plate, the central segregation of Mn can be significantly suppressed by appropriately controlling the conditions during solidification to suppress the flow of such molten steel.
• the present inventors have conducted various studies, believing that promoting the diffusion of Mn during solidification is effective in reducing the microsegregation of Mn. In order to promote the diffusion of Mn, it is effective to create a structure in which Mn can easily diffuse. Therefore, the present inventors have focused on the ⁇ phase, in which Mn has a fast diffusion rate, and have experimentally investigated the influence of each element in the steel on the microsegregation of Mn in order to set the solidification mode to ⁇ solidification.
• the present inventors have found that when the C and Mn contents are high, ⁇ solidification does not occur during solidification, the diffusion rate of Mn decreases and microsegregation increases, but when the Si, Al, Cr and Mo contents are high, the diffusion of Mn during solidification is promoted and microsegregation can be reduced.
• ghost lines can be improved by the above-mentioned methods for reducing Mn segregation, in order to obtain a sufficient improvement effect over the entire length and width of the coil, the present inventors have investigated further methods for improving ghost lines in addition to the above-mentioned methods for reducing Mn segregation. As a result, the present inventors have found that ghost lines can be improved even if a certain amount of central segregation of Mn remains by first reducing the hard phase fraction of the steel sheet.
• the present inventors have found that the occurrence of ghost lines is also greatly influenced by the solidification structure, and even if the central segregation of Mn is small, if coarse equiaxed crystals are generated in the solidification structure, negative segregation of Mn occurs, which increases the variation in the hard phase fraction in the direction perpendicular to the rolling direction, and worsens the poor appearance after forming. Therefore, unlike conventional center segregation countermeasures (Note: It is common knowledge among those skilled in the art that an increase in the equiaxed crystal fraction is necessary to improve center segregation. For example, Takaho Kawawa et al.: “Tetsu to Hagane", vol. 60 (1974) No. 5, pp.
• the steel sheet has a specific chemical composition as described above, and has a unique metal structure that is made up of a lower hard phase fraction than conventional DP steel and has a small variation in the hard phase fraction in the direction perpendicular to the rolling direction.
• a metal structure can be obtained by adopting a specific chemical composition and casting conditions and controlling the solidification structure during casting to form columnar crystals.
• 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 it is possible to 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 appearance defects after forming, such as ghost lines, while adequately maintaining high strength.
• the steel plate of this embodiment has C: 0.030 to 0.100%, Mn: 1.00 to 2.50%, Si: 0.005 to 1.500%, P: 0.100% or less, S: 0.0200% or less, Al: 0.005 to 0.700%, N: 0.0150% or less, O: 0.0100% or less, Cr: 0 to 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 to 1.00%, Sn: 0 to 1.00%, Sb: 0 to 0.200%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, REM: 0 to 0.0100%, and the balance: Fe and impurities, and has a specific chemical composition in
• A [Si] + 10 [P] + 0.6 [Al] + 8 [Ti] + 9 [Nb] ...
• [Si], [P], [Al], [Ti] and [Nb] are the contents of each element in mass%, and when no element is contained, it is 0%.
• 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 0.035% or more, 0.040% or more, or 0.050% 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.
• the Mn content is set to 1.00% or more.
• the Mn content may be 1.20% or more, 1.30% or more, or 1.40% or more.
• the Mn content is set to 2.50% or less.
• the Mn content may be 2.25% or less, 2.00% or less, or 1.85% 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 0.010% or more, 0.050% or more, or 0.100% or more.
• the Si content is set to 1.500% or less.
• the Si content may be 1.000% or less, 0.500% 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 0.001% or more.
• the P content is set to 0.100% or less.
• the P content may be 0.060% or less, 0.040% or less, or 0.020% 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 0.0010% 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 0.0100% or less, 0.0060% or less, or 0.0040% or less.
• Al 0.005 to 0.700%
• 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, 0.020% or more, or 0.025% or more.
• the Al content is set to 0.700% or less.
• the Al content may be 0.600% or less, 0.400% or less, 0.300% or less, 0.200% or less, or 0.100% 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 0.0010% or more.
• 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.
• 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 0.001% or more, and more preferably 0.01% or more.
• the Cr content may be 0.10% or more, 0.20% or more, or 0.30% or more.
• the Cr content is preferably 0.80% or less.
• the Cr content may be 0.70% or less, 0.60% or less, or 0.50% 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 the microsegregation of Mn.
• the Mo content may be 0%, but in order to obtain these effects, the Mo content is preferably 0.001% or more, and more 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 0.40% or less, 0.30% or less, or 0.20% 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, 0.0010% or more, or 0.0015% 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 0.0030% 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, 0.060% or less, or 0.030% 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 0.005% or more, 0.007% or more, or 0.010% or more.
• the Nb content is preferably 0.100% or less.
• the Nb content may be 0.060% or less, 0.040% or less, or 0.030% 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 0.001% or more, more preferably 0.005% or more, and even more 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, 0.20% or less, or 0.10% 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 0.001% or more, and more 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, 0.40% or less, or 0.20% 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 0.001% or more, and more 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 0.60% or less, 0.40% or less, or 0.20% 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 0.001% or more, and more preferably 0.01% or more.
• the W content may be 0.02% or more or 0.10% or more.
• the W content is preferably 1.00% or less.
• the W content may be 0.80% or less, 0.50% or less, or 0.20% 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, and more preferably 0.01% or more.
• the Sn content may be 0.05% or more or 0.08% or more.
• the Sn content is preferably 1.00% or less.
• the Sn content may be 0.80% or less, 0.50% or less, or 0.20% 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 0.01% or more, 0.05% or more, or 0.08% or more.
• excessive Sn content may cause embrittlement of the steel sheet. Therefore, the Sb content is preferably 1.00% or less, and more preferably 0.20% or less.
• the Sb content may be 0.10% or less, 0.05% or less, or 0.01% 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, 0.0060% or less, or 0.0030% 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 to 0.80%, Mo: 0.01 to 0.50%, B: 0.0001 to 0.0100%, Ti: 0.001 to 0.100%, Nb: 0.001 to 0.100%, V: 0.01 to 0.50%, Ni: 0.01 to 1.00%, Cu: 0.01 to 1.00%, W: 0.01 to 1.00%, Sn: 0.01 to 1.00%, Sb: 0.001 to 0.200%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Zr: 0.0001 to 0.0100%, and REM: 0.0001 to 0.0100%
• the steel sheet contains such optional elements, it is possible to more reliably maintain high strength and significantly suppress the occurrence of appearance defects after forming, such as ghost lines.
• 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 impurities may be contained in a total amount of 0.100% or less.
• index A 0.45% or more
• the chemical composition of the steel sheet is such that index A, represented by the following formula (1), is 0.45% or more.
• A [Si] + 10 [P] + 0.6 [Al] + 8 [Ti] + 9 [Nb] ...
• [Si], [P], [Al], [Ti] and [Nb] are the contents of each element in mass%, and when no element is 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 the steel sheet may be measured by a general analytical method.
• the chemical composition of the steel sheet may be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES).
• C and S may 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 is composed of 75 to 97% ferrite and 3 to 25% hard phase, in terms of area percent.
• the metal structure of the steel sheet such a composite structure, it is possible to suppress poor appearance after forming while maintaining the strength of the steel sheet within an appropriate range, more specifically, achieving a tensile strength of 500 MPa or more.
• the area fraction of the hard phase may be 7% or more, 10% or more, 12% or more, or 15% or more.
• the area fraction of ferrite may be 93% or less, 90% or less, 88% or less, or 85% or less.
• the area fraction of the hard phase may be 24% or less, 22% or less, or 20% or less.
• the area fraction of ferrite may be 76% or more, 78% or more, or 80% or more.
• the hard phase refers to a structure harder than ferrite, and is composed of at least one of martensite, bainite, tempered martensite, and pearlite, for example.
• the hard phase 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. First, a sample (having a size of, for example, 20 mm in the rolling direction ⁇ 20 mm in the width direction ⁇ thickness of the steel plate) for observing the metal structure (microstructure) is taken from the W/4 position or 3W/4 position of the sheet width W of the obtained steel plate (i.e., a position W/4 in the width direction from either end of the steel plate). Next, the metal structure (microstructure) is observed using a scanning electron microscope (SEM).
• SEM scanning electron microscope
• the observation visual field is a total of 10 visual fields, including (i) one visual field centered in the thickness direction at a position 125 ⁇ m from the steel sheet surface (surface excluding the plating layer if plating is present) (hereinafter referred to as the "125 ⁇ m thickness position”); (ii) one visual field centered in the thickness direction at a position 1/2 the thickness from the steel sheet surface (hereinafter referred to as the "1/2 thickness position”); and (iii) eight visual fields divided at intervals that divide the distance between the 125 ⁇ m thickness position and the 1/2 thickness position into nine equal parts between (i) and (ii).
• the thickness cross section perpendicular to the rolling direction is polished as an observation surface and etched by nital corrosion.
• the "microstructure" is classified from SEM photographs at a magnification of 500 or 1000 times. Ferrite and hard phases can be distinguished from each other by the difference in brightness.
• the above 10 observation fields are observed at a magnification of 500x or 1000x, and image analysis is performed using image analysis software "Photoshop (registered trademark) CS5" manufactured by Adobe, to determine the area fraction of the hard phase. Ferrite and hard phase are binarized based on the difference in brightness, and the area fraction of the hard phase is calculated. For a total of 10 observation fields, image analysis is performed in the same manner as above to measure the area fraction of the hard phase, and these area fractions are averaged to calculate an average value. This average value is the area fraction of the hard phase, and the remainder is the area fraction of ferrite.
• the metal structure of the steel sheet has a standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction of 0.75% 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 is 0.75% or less, that is, the variation in the hard phase fraction in the direction perpendicular to the rolling direction is equal to or less than a certain level, so that the appearance defects after forming can be significantly suppressed.
• 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 ratio is preferably 0.09 or less, 0.08 or less, or 0.07 or less.
• the lower limit of the ratio is 0, but may be 0.01 if necessary.
• the standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction of the metal structure can be obtained as follows. First, the area between a position 50 ⁇ m from one surface of the steel plate and a position 50 ⁇ m from the other surface of the steel plate in a cross section parallel to the direction perpendicular to the rolling direction of the steel plate and perpendicular to the steel plate surface is observed with a scanning electron microscope (SEM) at a magnification of 500 times or 1000 times to obtain an SEM photograph.
• SEM scanning electron microscope
• This SEM photograph is subjected to image analysis using image analysis software in the same manner as the above-mentioned area fraction of the hard phase, and the area fraction of the hard phase is measured every 100 ⁇ m within a range of 8 mm in the direction perpendicular to the rolling direction of the steel plate, and its standard deviation is calculated.
• the observation range in the direction perpendicular to the rolling direction may be less than 8 mm or may be 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 is 4 mm
• the upper limit is 12 mm.
• the standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction may be 0.65% or less, 0.55% or less, or 0.45% or less, from the viewpoint of further improving the appearance after forming.
• the lower limit of the standard deviation 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 tensile strength (TS) of a steel plate can be measured by taking a JIS Z2241:2011 No. 5 tensile test piece from the steel plate, with the longitudinal direction perpendicular to the rolling direction, and conducting a tensile test in accordance with JIS Z2241:2011.
• the average grain size of ferrite in the metal structure is preferably 5.0 to 30.0 ⁇ m.
• the average grain size of ferrite may be 7.0 ⁇ m or more, 8.0 ⁇ m or more, 9.0 ⁇ m or more, or 10.0 ⁇ m or more.
• the average grain size of ferrite may be 27.0 ⁇ m or less, 25.0 ⁇ m or less, 20.0 ⁇ m or less, or 16.0 ⁇ m or less.
• the average grain size of ferrite in the steel sheet is determined as follows. First, in the region from the surface of the steel sheet etched with Nital reagent to the 1/2 position in the sheet thickness direction, 10 fields of view in the sheet thickness direction are observed at a magnification of 500 times, which is the same as the measurement of the area fraction of ferrite and hard phase described above. Image analysis is performed using image analysis software "Photoshop (registered trademark) CS5" manufactured by Adobe, and the area fraction of ferrite and the number of ferrite particles in each field are calculated.
• the area fraction of ferrite and the number of ferrite particles in the 10 fields of view are summed, and the average area fraction per ferrite particle is calculated by dividing the total area fraction of ferrite by the total number of ferrite particles. From this average area fraction and the number of particles, the circle equivalent diameter is calculated, and the obtained circle equivalent diameter is determined as the average grain size of ferrite.
• the average crystal grain size of the hard phase in the metal structure is preferably 1.0 to 5.0 ⁇ m.
• the average crystal 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 crystal grain size of the hard phase may be 4.7 ⁇ m or less, 4.5 ⁇ m or less, 4.2 ⁇ m or less, or 4.0 ⁇ m or less.
• the average grain size of the hard phase is determined as follows. First, in the region from the surface of the steel sheet etched with Nital reagent to the 1/2 position in the sheet thickness direction, 10 fields of view in the sheet thickness direction are observed at a magnification of 500 times, which is the same as the measurement of the area fraction of the ferrite and hard phase described above. Image analysis is performed using image analysis software "Photoshop (registered trademark) CS5" manufactured by Adobe, and the area fraction of the hard phase and the number of particles of the hard phase in each field are calculated.
• the area fraction of the hard phase and the number of particles of ferrite in the 10 fields of view are each summed, and the total area fraction of the hard phase is divided by the total number of particles of the hard phase to calculate the average area fraction per hard phase particle. From this average area fraction and the number of particles, the circle equivalent diameter is calculated, and the obtained circle equivalent diameter is determined as the average grain size of the hard phase.
• the thickness of the steel plate is not particularly limited, but for example, 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 becomes easier 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 include a plating layer on the surface for the purpose of improving corrosion resistance, etc.
• 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 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.
• a high tensile strength specifically a tensile strength of 500 MPa or more
• the tensile strength of the steel plate is preferably 540 MPa or more, more preferably 600 MPa or more.
• the upper limit of the tensile strength is not particularly limited, but the tensile strength may be, for example, 980 MPa or less or 850 MPa or less. By setting the tensile strength to 850 MPa or less, there is an advantage that the formability when the steel plate is press-processed can be easily ensured.
• the steel plate of this embodiment has high strength, specifically a tensile strength of 500 MPa or more, yet can maintain an excellent appearance even after forming such as press working. For this reason, the steel plate of this embodiment is very useful for use as exterior panel parts such as roofs, hoods, fenders, and doors, which require high design quality in automobiles.
• the manufacturing method of the steel plate of this embodiment includes a casting process for casting a slab having the above-mentioned specific chemical composition.
• a casting process includes performing soft reduction using a continuous casting machine equipped with multiple 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 reduction with a reduction gradient of 0.6 mm or more per meter in the casting direction.
• 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, so that the solidification structure is controlled to a columnar crystal structure with an equiaxed crystal ratio of 15% or less, and center segregation can also be suppressed while using a method different from the conventional center segregation countermeasure. It is more preferable that the superheat ⁇ T is 30°C or more. In addition, it is preferable that the superheat ⁇ T is 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 by using a known solidification temperature estimation formula.
• the equiaxed crystal ratio (%) can be calculated by taking an etched print of the slab's cross-section in 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 at the center of the slab's 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 more preferable that the roll pitch of adjacent reduction rolls is 280 mm or less.
• the manufacturing method of the steel plate of this embodiment may include other steps such as a hot rolling step, a cold rolling step, an annealing step, and a cooling step in addition to the casting step described above. Furthermore, this manufacturing method may optionally include a plating step. These steps are not particularly limited, and may be carried out under any appropriate conditions selected appropriately so as to obtain a metal structure containing the ferrite and hard phases described above in relation to the steel plate at a predetermined area fraction. Preferred conditions for these steps will be briefly described below.
• the slab Prior to hot rolling, the slab is preferably heated to 1100°C or higher.
• the heating temperature By setting the heating temperature to 1100°C or higher, the rolling reaction force is not excessively large in hot rolling, and the target product thickness can be easily obtained.
• the upper limit of the heating temperature is not particularly limited, but from an economical point of view, the heating temperature is preferably less than 1300°C.
• the heated slab is subjected to rough rolling and finish rolling.
• the hot-rolled steel sheet obtained in this manner is wound at a winding temperature of, for example, 450 to 650°C.
• the finish rolling end temperature is preferably 950°C or less.
• the finish rolling end temperature is preferably 950°C or less.
• Cold rolling process The hot-rolled steel sheet obtained by the hot rolling process is subjected to an appropriate pickling treatment to remove scale, and then to a cold rolling process.
• the cumulative reduction is, for example, 50 to 90%.
• the annealing step it is preferable to perform an annealing treatment in which the cold-rolled steel sheet is heated to a soaking temperature of 750 to 900°C and held at that temperature.
• a soaking temperature 750 to 900°C and held at that temperature.
• the soaking temperature 750°C or higher, it is possible to sufficiently advance the recrystallization of ferrite and the reverse transformation from ferrite to austenite, thereby making it possible to obtain a desired metal structure in the final product.
• the soaking temperature at 900°C or lower, it is possible to densify the crystal grains and obtain sufficient strength.
• the cold-rolled steel sheet after the annealing step is cooled.
• the average cooling rate is set 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 is set to 50 ° C. / sec or less, the steel sheet can be cooled more uniformly in the width direction.
• a plating treatment may be applied to the surface of the obtained cold-rolled steel sheet.
• the plating treatment include hot-dip plating, alloying hot-dip plating, and electroplating.
• the steel sheet surface may be subjected to hot-dip galvanizing treatment as the plating treatment, or the alloying treatment may be performed after the hot-dip galvanizing 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.
• steel sheets according to an embodiment of the present disclosure were manufactured under various conditions, and the tensile strength and post-forming appearance characteristics of the resulting steel sheets were examined.
• casting condition (I) is a condition that superheat ⁇ T is ⁇ 25°C
• casting condition (II) is a condition that segment pressing force is ⁇ 450 tons.
• cases where these conditions are met are shown in Table 2.
• the obtained slab was subjected to a hot rolling process (heating temperature 1200°C, finish rolling end temperature 900°C, and coiling temperature 550°C), a cold rolling process (cumulative rolling reduction 80%), an annealing process (soaking temperature 800°C), and a cooling process (average cooling rate 10°C/sec) to produce a cold-rolled steel sheet with a thickness of 0.4 mm.
• the surface of the obtained cold-rolled steel sheet was appropriately plated to form a hot-dip galvanized layer (GI), a galvannealed layer (GA), or an electrolytic galvanized layer (EG).
• GI hot-dip galvanized layer
• GA galvannealed layer
• EG electrolytic galvanized layer
• the properties of the resulting steel plates were measured and evaluated using the following methods.
• the tensile strength of the steel plate was measured by taking a tensile test piece according to JIS Z2241:2011 No. 5, with the longitudinal direction perpendicular to the rolling direction, from the steel plate and conducting a tensile test in accordance with JIS Z2241:2011.
• the measured tensile strength of the steel plate was used to calculate the relationship between the tensile strength (TS) of the steel plate and the hard phase fraction (Vm): (TS-180,000/TS)/Vm.
• the appearance of the steel sheet after forming was evaluated based on the degree of ghost lines that appeared on the surface of the door outer after forming.
• the surface after press forming was ground with a grindstone, and stripes that appeared on the surface at intervals of the order of several mm were judged to be ghost lines, and were scored from 1 to 5 based on the following criteria according to the degree of occurrence of the stripes.

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