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
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • 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
• • 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
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • 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
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
• • 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
  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
• • 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
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • 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
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • 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
  • C22C38/18—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
  • 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
• • 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
  • C21D2211/004—Dispersions; Precipitations
• • 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
  • C21D2211/005—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

• the present invention relates to steel plates, and more specifically to steel plates and exterior panel members with excellent appearance, primarily used for example as exterior panel members for automobiles.
• Patent Document 1 describes a steel sheet for hot-dip galvanizing, which contains, by mass%, C: 0.02-0.3%, Si: 0.1-2.0%, Mn: less than 1.0%, Cr: more than 1.0-3.0%, P: 0.02% or less, S: 0.02% or less, Al: 0.014% or less, and N: 0.001-0.008%, and satisfies 2.5 ⁇ 1.5Mn%+Cr%, 4.1-2.3Mn%-1.2Cr% ⁇ Si%, with the balance being Fe and unavoidable impurities.
• Patent Document 1 also teaches that by optimizing the amounts of Mn, Cr, and Si added, it is possible to achieve both the workability of a steel sheet for hot-dip galvanizing with a tensile strength of 390 MPa or more and an appearance after processing that allows it to be used as an automotive exterior panel. Furthermore, Patent Document 1 teaches that by setting the area ratio of the main phase, ferrite, to 70% or more and the area ratio of the hard second phase, including martensite, to 30% or less, it is possible to keep the strength, yield strength, yield ratio, and strength-ductility balance all within a good range.
• the present invention describes a cold-rolled steel sheet having a composition that contains Ti* in a range that satisfies 0 ⁇ Ti* ⁇ 0.02 and further satisfies (Sb%) ⁇ (Cu%)/5, with the balance being Fe and unavoidable impurities, and that the content (mass%) of Ti element contained in precipitates less than 20 nm in size in the plate thickness surface layer portion up to 10 ⁇ m from each surface on both sides of the steel sheet is 9% or less of the total Ti content (mass%) in the steel sheet.
• Patent Document 2 also teaches that by setting the content (mass%) of Ti element contained in precipitates less than 20 nm in size in the plate thickness surface layer portion up to 10 ⁇ m from each surface on both sides of the steel sheet to 9% or less of the total Ti content (mass%) in the steel sheet, it is possible to avoid the occurrence of appearance unevenness caused by such fine Ti-based precipitates, and to obtain a cold-rolled steel sheet with excellent surface properties, and further that the cold-rolled steel sheet can be suitably used for parts that require excellent surface quality after forming, mainly for the outer panels of automobiles.
• thin and wide steel materials are relatively often required for automotive exterior plate applications such as those described in Patent Documents 1 and 2, but such thin and wide steel materials have a problem in that they are prone to bending, called heat buckling, during the manufacturing process, for example, when passing through a continuous annealing processing line (CAPL). If operation is continued with heat buckling occurring, it may lead to plate breakage, and if plate breakage occurs, it becomes necessary to stop the manufacturing line and perform recovery work, which can cause significant damage.
• CTL continuous annealing processing line
• the present invention aims to provide a steel sheet with a novel structure that can suppress the occurrence of heat buckling during the manufacturing process and achieve both strength and good appearance after forming.
• the inventors conducted research focusing on both the chemical composition and metal structure of the steel sheet.
• the inventors discovered that by optimizing the chemical composition of the steel sheet and appropriately controlling the form and amount of Nb carbonitride, the high-temperature strength of the steel sheet can be improved, thereby suppressing the occurrence of heat buckling during the manufacturing process, and further discovered that by uniformly dispersing martensite contained in a predetermined ratio in the metal structure in both the micro- and macro-regions in the metal structure, the desired high strength can be achieved based on such a hard structure, and even when strain is applied by press forming or the like, the generation of minute irregularities on the steel sheet surface can be significantly suppressed, thus completing the present invention.
• the present invention which has achieved the above object is as follows. (1) In mass%, C: 0.030-0.100%, Mn: 0.70-3.00%, Si: 0.005-1.500%, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0010-0.0150%, O: 0.0100% or less, Nb: 0.005-0.200%, Cr: 0-1.00%, Mo: 0 to 0.80%, B: 0 to 0.0100%, Ti: 0-0.200%, V: 0 to 0.500%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, W: 0 to 1.00%, Ta: 0 to 0.10%, Co: 0-3.00%, Sn: 0-1.00%, Sb: 0 to 0.200%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, REM: 0-0.0100%, Bi: 0 to 0.0500%, As
• the Nb content in all Nb carbonitrides is 0.004% or more, and the Nb content in Nb carbonitrides having a particle size of 20 nm or more is 60% or more of the Nb content in all Nb carbonitrides,
• the average grain spacing of the martensite is 2.5 ⁇ m or less,
• the chemical composition is, in mass%, Cr: 0.001-1.00%, Mo: 0.001-0.80%, B: 0.0001 to 0.0100%, Ti: 0.001 to 0.200%, V: 0.001-0.500%, Ni: 0.001 to 1.00%, Cu: 0.001 to 1.00%, W: 0.001-1.00%, Ta: 0.001 to 0.10%, Co: 0.001 to 3.00%, Sn: 0.001 to 1.00%, Sb: 0.001-0.200%, Ca: 0.0001-0.0100%, Mg: 0.0001-0.0100%, Zr: 0.0001 to 0.0100%, REM: 0.0001-0.0100%, Bi: 0.0001 to 0.0500%, and As: 0.001 to 0.10%
• the steel sheet according to the above (1) characterized in that it contains at least one of the following: (3)
• the present invention provides a steel sheet that can suppress the occurrence of heat buckling during the manufacturing process and can achieve both strength and good appearance after forming.
• the steel plate according to the embodiment of the present invention has, in mass%, C: 0.030-0.100%, Mn: 0.70-3.00%, Si: 0.005-1.500%, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0010-0.0150%, O: 0.0100% or less, Nb: 0.005-0.200%, Cr: 0-1.00%, Mo: 0 to 0.80%, B: 0 to 0.0100%, Ti: 0-0.200%, V: 0 to 0.500%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, W: 0 to 1.00%, Ta: 0 to 0.10%, Co: 0-3.00%, Sn: 0 to 1.00%, Sb: 0 to 0.200%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, REM: 0-0.0100%, Bi: 0
• the Nb content in all Nb carbonitrides is 0.004% or more, and the Nb content in Nb carbonitrides having a particle size of 20 nm or more is 60% or more of the Nb content in all Nb carbonitrides,
• the average grain spacing of the martensite is 2.5 ⁇ m or less,
• the steel sheet is characterized by having a metal structure in which the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is 1.5% or less.
• DP steel which is a mixture of soft structure made of ferrite and hard structure made of martensite
• uneven deformation is likely to occur during processing such as press forming, in which the soft structure and its surroundings are preferentially deformed, and fine irregularities are generated on the surface of the steel sheet after forming, which can cause appearance defects called ghost lines.
• the soft structure made of ferrite deforms greatly and is recessed on the surface of the steel sheet.
• the hard structure made of martensite is small in deformation. Therefore, compared to the soft structure, the hard structure does not recess on the surface of the steel sheet, but rises to be convex.
• the deformation amount varies especially in the width direction of the steel plate (the direction perpendicular to the rolling direction and the plate thickness direction), and ghost lines are generated in a band shape (striped shape).
• elements such as Mn may be added in relatively large amounts to improve the hardenability of the steel plate.
• Mn is an element that is likely to segregate in a streaky manner in the steel plate. More specifically, Mn-enriched regions such as central segregation and microsegregation are formed during casting, and the enriched regions are elongated in the rolling direction by hot rolling or cold rolling, so that Mn segregates in a streaky manner.
• a CAPL generally has a hearth roll with a crown that has a convex shape in the center. Therefore, when a steel sheet passes through a CAPL, compressive stress is applied to the steel sheet in the width direction center due to the convex crown of the hearth roll. On the other hand, since continuous annealing is performed at a relatively high temperature, the yield stress of the steel sheet decreases as the sheet temperature increases.
• the steel sheet may not be able to fully resist the compressive stress due to the decrease in yield stress at a relatively high temperature, and in such a case, a phenomenon called heat buckling occurs in which the sheet breaks and wrinkles occur.
• heat buckling occurs in which the sheet breaks and wrinkles occur.
• the present inventors first conducted research into improving the high-temperature strength of a steel sheet from the viewpoints of both the chemical composition and metal structure of the steel sheet in order to suppress or reduce the occurrence of such heat buckling.
• the present inventors found that, from the viewpoint of the chemical composition of the steel sheet, the high-temperature strength of the steel sheet can be improved by controlling the index A represented by the following formula 1 to 0.50% or more, and thus the occurrence of heat buckling can be suppressed or reduced.
• the index A it is believed that by controlling the index A to 0.50% or more, not only can the high-temperature strength of the steel sheet be improved, but also the Ac3 point of the steel sheet can be lowered.
• the steel sheet in the first heat treatment process corresponding to the CAPL after cold rolling, the steel sheet needs to be heated to a temperature higher than the Ac3 point at which the steel sheet becomes austenite single phase, more specifically, to Ac3+10°C or higher.
• the heating temperature in the first heat treatment process can be lowered, and in connection with this, it is possible to suppress the decrease in the yield stress of the steel sheet due to the increase in the sheet temperature during heating. Therefore, by controlling the index A to 0.50% or more, it is possible to significantly improve the resistance of the steel sheet to the above-mentioned compressive stress that causes heat buckling, based on the active improvement of the high-temperature strength of the steel sheet itself and the suppression of the decrease in yield stress due to the decrease in the heating temperature in the first heat treatment process.
• the inventors have found that, from the viewpoint of the metal structure of the steel sheet, the high-temperature strength of the steel sheet can be improved by controlling the form and amount of Nb carbonitrides so that a relatively large amount of Nb carbonitrides having an appropriate size are present in the steel sheet, more specifically, by controlling the form and amount of Nb carbonitrides so that the Nb content in all Nb carbonitrides is 0.004% or more and the Nb content in Nb carbonitrides having a particle size of 20 nm or more is 60% or more of the Nb content in all Nb carbonitrides.
• Nb carbonitrides By controlling the form and amount of Nb carbonitrides within such a range, it is possible to cause a sufficient amount of Nb carbonitrides having a particle size of 20 nm or more, which is effective in improving high-temperature strength, to be present in the steel. Therefore, in combination with the control of the chemical composition by the index A described above, the high-temperature strength of the steel sheet can be significantly improved, and as a result, it becomes possible to significantly suppress or reduce the occurrence of heat buckling during the manufacturing process of the steel sheet.
• the inventors have investigated means for achieving the desired high strength by optimizing the ratio of ferrite, which is a soft structure, and martensite, which is a hard structure, in the metal structure, while further improving the appearance after forming.
• the inventors have focused on the distribution state of martensite, which is a hard structure in the metal structure, and more specifically, have investigated controlling the distribution of martensite from a viewpoint different from that of reducing Mn segregation.
• the inventors have found that by forming the metal structure in the steel sheet before final annealing with a structure mainly composed of bainite and/or martensite, and then final annealing the steel sheet having such a metal structure under specified conditions, it is possible to uniformly disperse martensite in both the micro-region and the macro-region in the finally obtained metal structure, without necessarily depending on the presence or absence or the degree of Mn segregation.
• the inventors have found that by subjecting a steel sheet having a metal structure consisting of bainite and/or martensite to final annealing under predetermined conditions, the average grain spacing of martensite can be controlled to 2.5 ⁇ m or less in the micro region, and the standard deviation of the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction can be controlled to 1.5% or less in the macro region.
• the average grain spacing of martensite can be densely and uniformly dispersed in the micro region.
• the standard deviation of the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is 1.5% or less.
• the variation of the hard structure in the macro region can be significantly reduced.
• a metal structure in which martensite, which is a hard structure, is finely and uniformly dispersed throughout the steel sheet can be formed.
• the deformation amount of the steel sheet can be made more uniform, especially in the width direction, even during forming such as press forming, and it is possible to achieve an excellent post-forming appearance in which appearance defects such as ghost lines are significantly suppressed.
• the average particle spacing of martensite is controlled to 2.5 ⁇ m or less
• the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is controlled to 1.5% or less.
• the martensite structure has substructures such as packets, blocks, and laths in the prior austenite grains, and therefore has many different interfaces inside compared to structures such as ferrite. Bainite is also a structure that has many different interfaces inside, similar to the case of martensite.
• the steel plate according to the embodiment of the present invention ensures good formability by controlling the area ratio of ferrite, a soft structure, to 75-95%, while controlling the area ratio of martensite, a hard structure, to 5-25% and further controlling the chemical composition of the steel plate within a specified range to ensure high strength with a tensile strength of 540 MPa or more.
• C is an element that secures a certain amount of martensite and improves the strength of the steel sheet.
• C is also an austenite stabilizing element and is effective in lowering the Ac3 point.
• the C content is set to 0.030% or more.
• the C content may be 0.040% or more or 0.050% or more.
• the C content is set to 0.100% or less.
• the C content may be 0.090% or less, 0.080% or less, 0.070% or less, or 0.060% or less.
• Mn is an element that improves hardenability and contributes to improving the strength of the steel sheet. Mn is also an austenite stabilizing element and is effective in lowering the Ac3 point. In order to fully obtain these effects, In addition, the Mn content is 0.70% or more. The Mn content may be 0.80% or more, 1.00% or more, 1.20% or more, or 1.50% or more. In a preferred method for producing the steel sheet, the metal structure of the steel sheet before final annealing is changed to bainite and/or annealed steel in order to uniformly disperse martensite in both the micro- and macro-regions in the final metal structure.
• Mn hardenability by adding Mn
• Mn is contained in an excessive amount, Mn
• the effect of segregation cannot be sufficiently counteracted, and the standard deviation of the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction may not be controlled within a desired range. Not more than 3.00%.
• the Mn content may be 2.80% or less, 2.50% or less, 2.20% or less, or 2.00% or less.
• Silicon is an element that improves the strength of steel sheet by solid solution strengthening. In order to fully obtain such an effect, the silicon content is set to 0.005% or more. The silicon content is set to 0.010% or more. The Si content may be 0.100% or more, 0.200% or more, 0.300% or more, or 0.400% or more. On the other hand, if the Si content is excessive, it becomes difficult to remove scale formed during hot rolling, This may cause deterioration of the appearance. Therefore, the Si content is set to 1.500% or less. In addition, since Si is a ferrite stabilizing element, reducing the Si content can lower the Ac3 point. Therefore, the Si content may be 1.200% or less, 1.000% or less, 0.900% or less, 0.800% or less, 0.700% or less, or 0.600% or less. .
• P is an impurity element that embrittles welds and deteriorates plating properties. For this reason, the P content is set to 0.100% or less.
• S is an impurity element that impairs weldability and also impairs manufacturability during casting and hot rolling. For this reason, the S content is set to 0.0200% or less.
• the lower the S content the more preferable it is.
• the lower limit is not particularly limited, and the upper limit is 0.0150% or less, 0.0120% or less, 0.0100% or less, 0.0060% or less, or 0.0030% or less.
• the S content is set to 0.0001%. or more, or 0.0002% or more, or 0.0005% or more.
• Al is an element that functions as a deoxidizer and is effective in increasing the strength of steel.
• the Al content may be 0%, but in order to fully obtain these effects, The Al content is preferably 0.001% or more.
• the Al content may be 0.005% or more, 0.010% or more, 0.025% or more, or 0.050% or more. If Al is contained in excess, coarse oxides may form, which may reduce toughness. Therefore, the Al content is set to 1.000% or less.
• Al is a ferrite stabilizing element, Al By reducing the Al content, the Ac3 point can be lowered. Therefore, the Al content may be 0.800% or less, 0.600% or less, or 0.300% or less.
• N is an element effective in improving the high-temperature strength of a steel sheet by forming carbonitrides with Nb.
• the N content is set to 0.0010% or more.
• the N content may be 0.0015% or more, 0.0020% or more, 0.0025% or more, or 0.0030% or more.
• the N content is set to 0.0150% or less.
• the N content is set to 0.0120% or less, 0.0100% or less, 0.0080% or less, or 0.0060% or less. This is also fine.
• O is an element that causes blowholes during welding. Therefore, the O content is set to 0.0100% or less.
• the O content is set to 0.0080% or less, 0.0050% or less, 0.
• O is set to less than 0.0001%. If the O content is reduced, the production cost increases significantly, which is economically disadvantageous. Therefore, the O content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.
• Nb is an element effective in increasing the index A and improving the high-temperature strength of the steel sheet.
• Nb content is set to 0.005% or more.
• the Nb content is set to 0.010% or more, and 0.015% or more.
• the Nb content is set to 0.200% or less.
• the Nb content is set to 0.150% or less, 0.100% or less, 0 It may be 0.080% or less or 0.060% or less.
• the steel plate may contain at least one of the following optional elements in place of a portion of the remaining Fe, if necessary, for the purpose of improving the properties.
• the steel sheet may contain at least one of Cr: 0-1.00%, Mo: 0-0.80%, B: 0-0.0100%, Ti: 0-0.200%, V: 0-0.500%, Ni: 0-1.00%, Cu: 0-1.00%, W: 0-1.00%, Ta: 0-0.10%, Co: 0-3.00%, Sn: 0-1.00%, Sb: 0-0.200%, Ca: 0-0.0100%, Mg: 0-0.0100%, Zr: 0-0.0100%, REM: 0-0.0100%, Bi: 0-0.0500%, and As: 0-0.10%.
• These optional elements will be described in detail below.
• Cr 0-1.00%
• Cr is an element that improves hardenability and contributes to improving the strength of the steel sheet, similar to Mn.
• the Cr content may be 0%, but in order to obtain the above effect, the Cr content should be 0.001% or more.
• the Cr content may be 0.01% or more, 0.10% or more, or 0.20% or more.
• the Cr content is preferably 1.00% or less, and may be 0.80% or less, 0.60% or less, or 0.40% or less.
• Mo is an element that contributes to improving the high-temperature strength of steel sheets. This effect can be obtained even with a small amount of Mo.
• the Mo content may be 0%, but in order to obtain the above effect, Preferably, the Mo content is 0.001% or more.
• the Mo content may be 0.01% or more, 0.02% or more, 0.05% or more, or 0.10% or more. However, if Mo is contained excessively, hot workability may deteriorate, and productivity may decrease. Therefore, the Mo content is preferably 0.80% or less.
• the Mo content is preferably 0.60% or less. % or less, 0.50% or less, 0.40% or less, or 0.20% or less.
• B is an element that suppresses the formation of ferrite and pearlite during the cooling process from austenite and promotes the formation of martensite. B is also an element that is beneficial for increasing the strength of steel. These effects are only seen in small amounts.
• the B content may be 0%, but in order to obtain the above effects, the B content is preferably 0.0001% or more. The B content is preferably 0.0005% or more.
• the B content is preferably 0.0100% or less.
• the B content may be 0.0080% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.
• Ti is an element effective in controlling the morphology of carbides. Ti can promote an increase in the strength of ferrite.
• the Ti content may be 0%, but in order to obtain these effects, the Ti content must be less than 0.
• the Ti content is preferably 0.001% or more.
• the Ti content may be 0.002% or more, 0.010% or more, 0.020% or more, or 0.040% or more.
• an excessive Ti content However, the effect of Ti is saturated and there is a risk of an increase in manufacturing costs. Therefore, the Ti content is preferably 0.200% or less, more preferably 0.100% or less, 0.080% or less, or 0.050% or less. It may be the following.
• V is an element effective in controlling the morphology of carbides, and is also an element effective in refining the structure and improving the toughness of the steel plate.
• the V content may be 0%, but if the above effects are not to be obtained, To obtain this, the V content is preferably 0.001% or more.
• the V content may be 0.005% or more, 0.010% or more, or 0.050% or more. If V is contained in an excessive amount, a large amount of precipitates may be formed, which may reduce the toughness. Therefore, the V content is preferably 0.500% or less.
• the V content is preferably 0.400% or less. It may be 0.200% or less or 0.100% or less.
• Ni is an element effective in improving the strength of a steel sheet.
• the Ni content may be 0%, but in order to obtain the above effect, the Ni content is preferably 0.001% or more.
• the Ni content may be 0.01% or more, or 0.05% or more.
• the Ni content is set to 1.00 % or less.
• the Ni content may be 0.80% or less, 0.40% or less, or 0.20% or less.
• Cu is an element that contributes to improving the strength of the steel sheet. This effect can be obtained even with a small amount of Cu.
• the Cu content may be 0%, but in order to obtain the above effect, the Cu content must be 0%.
• the Cu content is preferably 0.001% or more.
• the Cu content may be 0.01% or more or 0.05% or more.
• excessive Cu content may cause red shortness and deteriorate the hardness during hot rolling. Therefore, the Cu content is preferably 1.00% or less.
• the Cu content is preferably 0.80% or less, 0.60% or less, 0.30% or less, or It may be 0.20% or less.
• W is an element effective in controlling the morphology of carbides and improving the strength of steel sheets.
• the W content may be 0%, but in order to obtain these effects, the W content must be 0.001% or more.
• the W content may be 0.01% or more, or 0.05% or more.
• the W content is set to 1.
• the W content may be 0.80% or less, 0.40% or less, or 0.20% or less.
• Ta is an element that is effective in controlling the morphology of carbides and improving the strength of steel sheets.
• the Ta content may be 0%, but in order to obtain these effects, the Ta content should be 0.001%.
• the Ta content may be 0.01% or more, or 0.03% or more.
• the Ta content is preferably 0.10% or less.
• the Ta content is preferably 0.08% or less, 0.06% or less, or 0.04% or less. It's fine if there is.
• Co is an element that is effective in improving the strength of steel sheets.
• the Co content may be 0%, but in order to obtain the above effect, the Co content must be 0.001% or more.
• the Co content may be 0.01% or more, 0.05% or more, or 0.10% or more.
• the Co content is preferably 3.00% or less.
• the Co content is preferably 2.00% or less, 1.00% or less, 0.50% or less, or 0.20% or less. % or less.
• Sn is an element that may be contained in a steel sheet when scrap is used as the raw material for the steel sheet. In addition, Sn may cause embrittlement of ferrite. Therefore, the smaller the Sn content, the better.
• the Sn content may be 0.10% or less, 0.040% or less, or 0.02% or less.
• the Sn content may be 0%, but Sn Reducing the Sn content to less than 0.001% leads to an excessive increase in refining costs. Therefore, the Sn content is set to 0.001% or more, 0.005% or more, or 0.01% or more. good.
• Sb is an element that can be contained in a steel sheet when scrap is used as the raw material for the steel sheet.
• Sb may strongly segregate at grain boundaries and cause embrittlement of the grain boundaries. Therefore, the smaller the Sb content, the better, and it is preferably 0.200% or less.
• the Sb content may be 0.100% or less, 0.040% or less, or 0.020% or less.
• the Sb content may be 0%, but reducing the Sb content to less than 0.001% will lead to an excessive increase in refining costs. % or more, or 0.010% or more.
• 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 contents of Ca, Mg, Zr and REM are each preferably 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. If these elements are contained in excess, the ductility of the steel sheet may decrease.
• the Ca, Mg, Zr and REM contents are each preferably 0.0100% or less, 0.0080% or less, and 0. It may be 0.0060% or less, 0.0040% or less, or 0.0020% or less.
• REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71.
• REM is a general term for the 17 elements, and the REM content is the total content of these elements.
• Bi is an element that has the effect of improving formability by refining the solidification structure.
• the Bi content may be 0%, but in order to obtain such an effect, the Bi content should be 0.0001%.
• the Bi content is preferably 0.0500% or less, more preferably 0.0400% or less, 0.0200% or less, 0.0100% or less, or 0.0050% or less. % or less.
• As is an element that can be contained in a steel sheet when scrap is used as the raw material for the steel sheet.
• As is an element that strongly segregates at grain boundaries, and the lower the As content, the better.
• the As content is preferably 0.10% or less, and may be 0.04% or less or 0.02% or less.
• the As content may be 0%, but the As content Reducing As to less than 0.001% leads to an excessive increase in refining costs, so the As content may be 0.001% or more, 0.005% or more, or 0.01% or more.
• the remainder excluding the above elements consists of Fe and impurities.
• Impurities are elements that are mixed in from the steel raw materials and/or during the steelmaking process, and whose presence is permitted to the extent that they do not impair the properties of the steel plate according to the embodiment of the present invention.
• index A 0.50% or more
• the chemical composition of the steel sheet according to the embodiment of the present invention requires that the index A represented by the following formula 1 is 0.50% or more.
• A [C]-0.1[Si]+0.3([Mn]-0.5)-0.3[Al]+0.1[Cr]+0.6[Mo]-[Ti]+15[Nb ] ...
• [C], [Si], [Mn], [Al], [Cr], [Mo], [Ti] and [Nb] are the contents [mass %] of each element, When no Cr content is contained, the value is 0%.
• the primary alloying step which will be described in detail later in relation to the manufacturing method of the steel plate, is performed. It is effective to lower the heating temperature required in the heat treatment process (i.e., Ac3+10°C or higher). By lowering the heating temperature required in the first heat treatment process, the yield of the steel sheet caused by the increase in sheet temperature during heating can be reduced.
• the inventors set the index A defined by the contents of these elements together with a coefficient considering the degree of influence, that is, the index A expressed by the above formula 1, to 0.50% or more.
• the resistance of the steel sheet to the compressive stress is improved based on the positive improvement of the high-temperature strength of the steel sheet itself and the suppression of the decrease in the yield stress caused by the decrease in the heating temperature in the first heat treatment process.
• the larger the index A the more preferable it is.
• the upper limit of the index A is not particularly limited, but the index A may be, for example, 0.60% or more, 0.65% or more, 0.70% or more, 0.75% or more, or 0.80% or more. , 2.00% or less, 1.80% or less, 1.50% or less, 1.30% or less, or 1.10% or less.
• the chemical composition of the steel plate according to the embodiment of the present invention may be measured by a general analytical method.
• the chemical composition of the steel plate 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 may be measured using the inert gas fusion-thermal conductivity method
• O may be measured using the inert gas fusion-non-dispersive infrared absorption method.
• ferrite Since ferrite is a soft structure, it is easily deformed and contributes to improving elongation. When the area ratio of ferrite is 75% or more, sufficient formability can be obtained. From the viewpoint of improving formability, the higher the area ratio of ferrite, the more preferable it is, and it may be, for example, 78% or more, 80% or more, 82% or more, or 85% or more. On the other hand, if ferrite is contained excessively, the desired strength may not be achieved in the steel plate. Therefore, the area ratio of ferrite is 95% or less. The area ratio of ferrite may be 93% or less, 90% or less, or 87% or less.
• Martensite is a structure with high dislocation density and hardness, and therefore contributes to improving tensile strength.
• the area ratio of martensite By setting the area ratio of martensite to 5% or more, it is possible to ensure a tensile strength of, for example, 540 MPa or more. From the viewpoint of improving strength, the higher the area ratio of martensite, the more preferable it is, and it may be, for example, 7% or more, 10% or more, or 13% or more.
• the area ratio of martensite is 25% or less, it is possible to ensure formability and appearance.
• the area ratio of martensite may be 22% or less, 20% or less, 18% or less, or 15% or less.
• "martensite” includes not only as-quenched martensite (so-called fresh martensite) but also tempered martensite.
• the remaining structure other than ferrite and martensite may be 0% in area ratio, but when the remaining structure exists, the remaining structure is at least one of bainite, pearlite, and retained austenite.
• the area ratio of the remaining structure i.e., at least one of bainite, pearlite, and retained austenite, may be 10% or less in total, for example, 8% or less, 6% or less, 4% or less, or 2% or less.
• the area ratio of the remaining structure 0% in order to make the area ratio of the remaining structure 0%, a high level of control is required in the manufacturing process of the steel plate, which may lead to a decrease in yield. Therefore, the area ratio of the remaining structure may be 0.5% or more, or 1% or more.
• Identification of the metal structure and calculation of the area ratio are performed by FE-SEM (field emission scanning electron microscope) and optical microscope after corrosion using Nital reagent (3% nitric acid ethanol solution) and X-ray diffraction method.
• the structure observation by FE-SEM and optical microscope is performed at a magnification of 500 to 50,000 times for a 100 ⁇ m ⁇ 100 ⁇ m area in the steel plate cross section in the direction perpendicular to the plate surface.
• three measurement points are set, and the area ratio is determined by calculating the average value of the measured values.
• the length in the plate thickness direction is reduced while securing a measurement area of 10,000 ⁇ m 2.
• a measurement area of 20 ⁇ m in the plate thickness direction and 500 ⁇ m in the direction perpendicular to the plate thickness direction may be observed.
• the measurement length in the plate thickness direction is 10 ⁇ m or more, preferably 50 ⁇ m or more. The same applies to the "100 ⁇ m ⁇ 100 ⁇ m area" in the following description.
• plate thickness x/y position (where x and y are natural numbers satisfying x ⁇ y) refers to a position moved in the plate thickness direction from the surface (plate surface) of the steel plate in the plate thickness direction toward the center of the steel plate by a distance (depth) of x/y of the plate thickness t.
• depth a distance of x/y of the plate thickness t.
• plate thickness 1/8 position refers to a position that is 0.25 mm deep in the plate thickness direction from the surface of the steel plate.
• the surface of the steel plate refers to the interface between the steel plate and the coating
• plate thickness t refers to the thickness of the steel plate (base material) excluding the coating.
• the area ratios of ferrite and martensite are determined by the following procedure. First, the observation surface of the sample is etched with a Nital reagent (a 3% nitric acid in ethanol solution), and then a 100 ⁇ m x 100 ⁇ m area within the range of 1/8 to 3/8 of the plate thickness, centered at the 1/4 position, is observed with an FE-SEM (e.g., JEOL's JSM-7200F, measured at an acceleration voltage of 15 kV and a magnification of 500 to 2000 times). In Nital corrosion, martensite and retained austenite are not corroded, so the area ratio of the uncorroded area corresponds to the total area ratio of martensite and retained austenite.
• a Nital reagent a 3% nitric acid in ethanol solution
• the metal structure is binarized according to differences in brightness using the image analysis software Image J (Ver. 1.54f), and the black parts of the image data are ferrite, and the uncorroded white parts are the total structure of martensite and retained austenite. Therefore, the area ratio of ferrite is calculated from the area ratio of the black area, while the area ratio of martensite is calculated by subtracting the area ratio of retained austenite measured by the X-ray diffraction method described later from the area ratio of this uncorroded area.
• the area ratio of martensite calculated by this method also includes the area ratio of tempered martensite.
• the area fraction of retained austenite is calculated by X-ray diffraction.
• the specimen is removed from the plate surface to a depth of 1/4 in the plate thickness direction by mechanical polishing and chemical polishing. More specifically, the specimen is thinned to the vicinity of the observation position by mechanical polishing, and then thinned to the target position by chemical polishing (with hydrofluoric acid).
• the structure fraction of retained austenite is calculated from the integrated intensity ratio of the diffraction peaks of (200) and (211) of the bcc phase and (200), (220), and (311) of the fcc phase obtained at the 1/4 plate thickness position using, for example, a Rigaku X-ray diffraction device (RINT2500, X-ray output 40 kV-200 mA). The general five-peak method is used for this calculation.
• the calculated structure fraction of retained austenite is determined as the area fraction of retained austenite.
• the residual structures of bainite, pearlite, and retained austenite may be present at a total ratio of 0 to 10%. That is, in the steel plate according to this embodiment, ferrite and martensite are the main metal structures, and the residual structures of bainite, pearlite, and retained austenite are metal structures that may be unavoidably generated during manufacturing. Therefore, there is essentially no positive technical significance in identifying the residual structures of bainite, pearlite, and retained austenite or measuring their area ratios. From the chemical composition and manufacturing method of the steel plate described in this specification, it is clear that the residual structure in the steel plate according to this embodiment is bainite, pearlite, retained austenite, or a composite thereof. The following methods can be used to identify and measure the residual structures of bainite and pearlite. The method for measuring the area ratio of retained austenite is as described above.
• bainite and calculation of the area ratio are carried out as follows. First, the observation surface of the sample is corroded with Nital reagent, and then a 100 ⁇ m x 100 ⁇ m area within the range of 1/8 to 3/8 of the plate thickness, centered at 1/4 of the plate thickness, is observed with an FE-SEM (e.g., JEOL's JSM-7200F, measured at an acceleration voltage of 15 kV and a magnification of 500 to 2000 times). From the position and arrangement of cementite contained within the structure in this observation area, bainite is identified as follows.
• FE-SEM e.g., JEOL's JSM-7200F
• Bainite is classified into upper bainite and lower bainite, and in upper bainite, cementite or retained austenite exists at the interface of lath-shaped bainitic ferrite.
• upper bainite cementite exists inside lath-shaped bainitic ferrite, there is one type of crystal orientation relationship between bainitic ferrite and cementite, and the cementite has the same variant.
• upper bainite and lower bainite can be identified. In the present invention, these are collectively referred to as bainite, and the area ratio of the identified bainite is calculated based on image analysis.
• Pearlite is identified and its area ratio calculated using the following procedure. First, the observation surface of the sample is corroded with Nital reagent, and then the area from 1/8 to 3/8 of the plate thickness, centered at 1/4 of the plate thickness, is observed using an SEM (e.g., JEOL's JSM-7200F, measured at an acceleration voltage of 15 kV and magnification of 500 to 2000 times). Areas in which lamellar cementite is observed in the SEM observation image are identified as pearlite, and the area ratio of this area is calculated based on image analysis.
• SEM e.g., JEOL's JSM-7200F
• the amount of Nb in all Nb carbonitrides is 0.004% or more, and the amount of Nb in Nb carbonitrides having a particle size of 20 nm or more is 60% or more of the amount of Nb in all Nb carbonitrides]
• the steel plate according to the embodiment of the present invention contains Nb carbonitrides in the metal structure, the Nb content in all Nb carbonitrides is 0.004% or more, and the Nb content in Nb carbonitrides having a particle size of 20 nm or more is controlled to 60% or more of the Nb content in all Nb carbonitrides.
• Nb carbonitrides include not only NbCN but also NbC and NbN, and further include NbCN, NbC and NbN in which a part of Nb is replaced by one or more other elements such as Ti.
• Nb carbonitrides By controlling the form and amount of Nb carbonitrides within the above range, it is possible to cause a sufficient amount of Nb carbonitrides having a particle size of 20 nm or more, which is effective for improving high-temperature strength, to be present in the steel, and therefore, in combination with the control of the chemical composition by the index A described above, the high-temperature strength of the steel plate can be significantly improved. As a result, it is possible to reliably suppress or reduce the occurrence of heat buckling during the manufacturing process of the steel plate.
• the ratio of the Nb amount in all Nb carbonitrides and the Nb amount in Nb carbonitrides having a particle size of 20 nm or more to the total Nb carbonitrides is preferably as large as possible.
• the Nb amount in all Nb carbonitrides may be 0.006% or more, 0.008% or more, 0.010% or more, or 0.012% or more.
• the upper limit is not particularly limited, but for example, the Nb amount in all Nb carbonitrides may be 0.100% or less, 0.060% or less, 0.040% or less, or 0.030% or less.
• the Nb amount in Nb carbonitrides having a particle size of 20 nm or more may be 62% or more, 65% or more, 68% or more, or 70% or more of the Nb amount in all Nb carbonitrides.
• the upper limit is not particularly limited, but for example, the Nb amount in Nb carbonitrides having a particle size of 20 nm or more may be 95% or less, 90% or less, or 85% or less of the Nb amount in all Nb carbonitrides. If the particle size of the Nb carbonitride is 20 nm or more, the effect of improving high-temperature strength can be obtained, and if the particle size is too large, the effect is not significantly reduced. Therefore, the upper limit of the particle size of the Nb carbonitride is not particularly limited, but the particle size of the Nb carbonitride may be, for example, 1000 nm or less, i.e., 1 ⁇ m or less.
• a test piece is taken from the 1/2 position of the plate thickness, and the taken test piece is electrolyzed at a constant current in an electrolytic solution (10% by volume acetylacetone-1% by mass tetramethylammonium chloride-methanol), and the precipitates attached to the test piece after electrolysis are dispersed in an aqueous sodium hexametaphosphate solution, and then filtered and collected with a porous filter having a pore size of 0.02 ⁇ m ⁇ (20 nm ⁇ ).
• the amount of Nb contained in the precipitates on the filter is measured by ICP emission spectroscopy, and the content of Nb in the steel precipitated as Nb precipitates having a particle size of 20 nm or more collected on the filter is obtained.
• the amount of Nb in the filtrate having a particle size of less than 20 nm contained in the filtrate that has passed through the filter is measured by ICP emission spectroscopy.
• the total mass of Nb precipitated as Nb carbonitrides is calculated by adding together the amount of Nb in Nb precipitates having a particle size of 20 nm or more and the amount of Nb in Nb precipitates having a particle size of less than 20 nm, and the obtained value is determined as the amount of Nb in all Nb carbonitrides.
• the amount of Nb precipitated as Nb carbonitrides having a particle size of 20 nm or more is used to calculate the ratio of the amount of Nb precipitated as Nb carbonitrides to the total mass of Nb, and the calculated value is determined as the ratio of the amount of Nb in Nb carbonitrides having a particle size of 20 nm or more to the amount of Nb in all Nb carbonitrides.
• the average particle spacing of the martensite which is a hard structure
• the average particle spacing of the martensite is an index that indicates the uniformity of the hard structure distribution in the micro region. The smaller the average particle spacing of the martensite, the more densely and uniformly the hard structure is dispersed, and therefore the higher the uniformity. The more uniform the deformation amount of the steel sheet during press forming is, particularly in the width direction of the steel sheet, the better the appearance of the steel sheet after press forming.
• the deformation amount of the steel sheet is strongly affected by the distribution state of the hard structure, in order to make the deformation amount of the steel sheet uniform in the width direction of the steel sheet, it is necessary to make the distribution of the hard structure in the metal structure uniform.
• the deformation amount of the steel sheet can be made more uniform in the width direction even during forming such as press forming, and as a result, a good post-forming appearance can be achieved.
• the average grain spacing of martensite is preferably 2.4 ⁇ m or less, more preferably 2.2 ⁇ m or less, and most preferably 2.0 ⁇ m or less or 1.8 ⁇ m or less. Although there is no particular lower limit, for example, the average grain spacing of martensite may be 0.5 ⁇ m or more, 0.8 ⁇ m or more, or 1.0 ⁇ m or more.
• the average grain spacing of martensite is determined as follows. First, a sample having a steel sheet cross section perpendicular to the sheet surface is taken, and the cross section is used as the observation surface. A region of 100 ⁇ m ⁇ 100 ⁇ m within the range of 1/8 to 3/8 of the sheet thickness centered at 1/4 of the sheet thickness is used as the observation region of this observation surface, and martensite is identified using FE-SEM (for example, JEOL JSM-7200F, measured at an acceleration voltage of 15 kV and a magnification of 1000 to 5000 times). Specifically, the image analysis software Image J (Ver.
• the black part of the image data is ferrite, and the uncorroded white part is the total structure of martensite and retained austenite.
• the white structure can be regarded as martensite.
• the distance between the centers (centers of gravity) of all adjacent martensite grains among the identified martensite grains is calculated as the particle spacing based on image analysis, and the average of the calculated particle spacings is obtained. This operation is performed in the other two observation regions, and the average of the three values obtained is determined as the average particle spacing of martensite (strictly speaking, particles including martensite and/or retained austenite).
• Standard deviation in area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is 1.5% or less
• the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is controlled to 1.5% or less.
• the standard deviation is an index representing the uniformity of the hard structure in the macro region. The appearance, which is an issue during press forming, depends on the minute irregularities on the steel sheet surface caused by the difference in the amount of deformation in the width direction of the steel sheet.
• the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is preferably 1.4% or less, more preferably 1.2% or less, and most preferably 1.0% or less.
• the lower limit is not particularly limited, but the standard deviation may be, for example, 0.1% or more, 0.3% or more, or 0.5% or more.
• the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is determined as follows. First, a metal structure image of a steel plate cross section in a region of 50 mm in the direction perpendicular to the rolling direction and the plate thickness direction is obtained. In the case of an image of 10 mm or smaller, multiple images may be obtained and joined to make 50 mm.
• the cross section is observed at 0°, 45°, 90°, and 135° to an arbitrary direction, and the cross section with the highest aspect ratio of the precipitates among them is determined as the cross section parallel to the rolling direction, and the direction perpendicular to the plate thickness direction is determined as the direction perpendicular to the rolling direction and the plate thickness direction.
• the obtained image is divided into 100 ⁇ m (0.1 mm) in the direction perpendicular to the rolling direction and the plate thickness direction, and the area ratio of martensite in the entire plate thickness is calculated for each divided range. Based on the martensite area ratio calculated from each of the total 500 divided images, the standard deviation in the area ratio of martensite is calculated.
• the area ratio of martensite in each divided region is calculated according to the procedure described in the section "Identification of metal structure and calculation of area ratio".
• the measurement results of the steel sheet cross section in a region of 50 mm in the direction perpendicular to the rolling direction and the sheet thickness direction may be used instead of the measurement results of each divided region.
• the average grain size of ferrite in the metal structure is 3.0 to 25.0 ⁇ m.
• the average grain size of ferrite may be 5.0 ⁇ m or more, 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 22.0 ⁇ m or less, 20.0 ⁇ m or less, 16.0 ⁇ m or less, 14.0 ⁇ m or less, or 12.0 ⁇ m or less.
• the average grain size of ferrite in steel plate is determined as follows. First, a sample having a steel plate cross section perpendicular to the plate surface is taken, and the cross section is used as the observation surface. A 100 ⁇ m x 100 ⁇ m region within the range of 1/8 to 3/8 plate thickness positions, centered at the 1/4 plate thickness position, is used as the observation region on this observation surface, and martensite is identified using an FE-SEM (e.g., JEOL's JSM-7200F, measured at an acceleration voltage of 15 kV and a magnification of 500 to 2000 times). Specifically, the metal structure is binarized based on the difference in brightness using image analysis software Image J (Ver. 1.54f), and ferrite is identified.
• FE-SEM e.g., JEOL's JSM-7200F
• the black parts of the image data are ferrite, and the uncorroded white parts are the combined structure of martensite and retained austenite.
• the circle equivalent diameter of all identified ferrite is calculated. This operation is repeated in the other two observation areas, and the arithmetic average of the circular equivalent diameters of all the ferrite particles obtained in the three observation areas is calculated, and the obtained value is determined as the average grain size of the ferrite.
• the average grain size of martensite in the metal structure is 1.0 to 5.0 ⁇ m.
• the average grain size of martensite 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 martensite may be 4.7 ⁇ m or less, 4.5 ⁇ m or less, 4.2 ⁇ m or less, 4.0 ⁇ m or less, 3.8 ⁇ m or less, 3.6 ⁇ m or less, or 3.4 ⁇ m or less.
• the average crystal grain size of martensite is determined as follows. First, a sample having a steel plate cross section perpendicular to the plate surface is taken, and the cross section is used as the observation surface. A 100 ⁇ m x 100 ⁇ m region within the range of 1/8 to 3/8 plate thickness positions centered at the 1/4 plate thickness position is used as the observation region on this observation surface, and martensite is identified using an FE-SEM (for example, JEOL's JSM-7200F, measured at an acceleration voltage of 15 kV and a magnification of 500 to 2000 times). Specifically, the image analysis software Image J (Ver. 1.54f) is used to binarize the metal structure based on the difference in brightness, and martensite is identified.
• FE-SEM for example, JEOL's JSM-7200F, measured at an acceleration voltage of 15 kV and a magnification of 500 to 2000 times.
• the black parts of the image data are ferrite, and the uncorroded white parts are the combined structure of martensite and retained austenite.
• the white structure is regarded as martensite.
• the circle equivalent diameter of all identified martensite is calculated. This operation is repeated in the other two observation regions, and the circle equivalent diameters of all the martensite particles obtained in the three observation regions are arithmetically averaged, and the obtained value is determined as the average crystal grain size of the martensite (strictly speaking, particles containing martensite and/or retained austenite).
• the average aspect ratio of martensite in the metal structure is 2.5 or more.
• the average aspect ratio of martensite may be 2.6 or more, 2.8 or more, or 3.0 or more.
• the upper limit is not particularly limited, but for example, the average aspect ratio of martensite may be 4.0 or less, 3.8 or less, or 3.6 or less.
• the average aspect ratio of martensite is determined as follows. First, a sample having a steel plate cross section perpendicular to the plate surface is taken, and the cross section is used as the observation surface. A 100 ⁇ m x 100 ⁇ m area is taken from this observation surface within the range of 1/8 to 3/8 plate thickness positions, centered at the 1/4 plate thickness position, and martensite is identified using an FE-SEM (e.g., JEOL JSM-7200F, measured at an acceleration voltage of 15 kV and a magnification of 1000 to 5000 times). Specifically, the image analysis software Image J (Ver. 1.54f) is used to binarize the metal structure based on the difference in brightness, and martensite is identified.
• FE-SEM e.g., JEOL JSM-7200F
• the black parts of the image data are ferrite, and the uncorroded white parts are the combined structure of martensite and retained austenite.
• the white structure is considered to be martensite.
• the aspect ratios of all martensite grains are calculated from the obtained image data using the image analysis software Image J (Ver. 1.54f).
• the aspect ratios of the particles (crystal grains) on the image can be measured using a function built into the image analysis software Image J (Ver. 1.54f).
• This operation is then performed in the other two observation regions, and the aspect ratios of all martensite grains obtained in the three observation regions are arithmetically averaged, and the obtained value is determined as the average aspect ratio of martensite (strictly speaking, particles containing martensite and/or retained austenite).
• the steel plate according to the embodiment of the present invention has a plate thickness of, for example, 0.1 to 2.0 mm, but is not particularly limited thereto.
• a steel plate having such a plate thickness is suitable for use as a material for exterior plate members such as doors and hoods as automobile members.
• the plate thickness may be 0.2 mm or more, 0.3 mm or more, or 0.4 mm or more.
• the plate thickness may be 1.8 mm or less, 1.5 mm or less, 1.2 mm or less, or 1.0 mm or less.
• the plate thickness 0.2 mm or more it becomes easier to maintain the shape of the molded product flat, and an additional effect of improving dimensional accuracy and shape accuracy can be obtained.
• the plate thickness 1.0 mm or less the weight reduction effect of the member becomes remarkable.
• the plate thickness of the steel plate is measured by a micrometer.
• the steel sheet according to the embodiment of the present invention is a cold-rolled steel sheet, but may further include a plating layer on the surface for the purpose of improving corrosion resistance or the like.
• the plating layer may be either a hot-dip plating layer or an electroplating layer. That is, the steel sheet according to the embodiment of the present invention may be a cold-rolled steel sheet having a hot-dip plating layer or an electroplating layer on its surface.
• the hot-dip plating layer includes, for example, 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, a hot-dip Zn-Al-Mg-Si alloy plating layer, and the like.
• the electroplating layer includes, for example, an electrogalvanized layer (EG), an electrogalvanized Zn-Ni alloy plating layer, and the like.
• the plating layer is 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 540 MPa or more can be achieved.
• the tensile strength is preferably 570 MPa or more, more preferably 600 MPa or more.
• the upper limit is not particularly limited, but for example, the tensile strength may be 980 MPa or less, 900 MPa or less, 850 MPa or less, 830 MPa or less, or 800 MPa or less.
• the tensile strength is measured by taking a tensile test piece No. 5 of JIS Z2241:2011 from the steel plate, with the test direction being perpendicular to the rolling direction and the plate thickness direction, and performing a tensile test in accordance with JIS Z2241:2011.
• the steel plate according to the embodiment of the present invention can suppress the occurrence of heat buckling during the manufacturing process, and can achieve both high strength, for example a tensile strength of 540 MPa or more, and an excellent appearance after forming such as press working. For this reason, the steel plate according to the embodiment of the present invention is particularly useful for use in parts in technical fields where both of these properties are required.
• 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. Examples of the exterior plate members of an automobile include roofs, hoods, fenders, doors, and the like, which require high designability.
• exterior plate members particularly the exterior plate members of an automobile, 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 will satisfy the chemical composition and metal structure characteristics described above.
• the characteristics of the metal structure do not change particularly before and after forming.
• the method for producing a steel sheet according to an embodiment of the present invention includes: A hot rolling process comprising finish rolling a slab having the chemical composition described above in relation to the steel sheet, followed by coiling, the hot rolling process satisfying the following conditions (a) to (d): (a) The finish rolling entry temperature is 1000 to 1080°C; (b) The reduction ratio of the rolling pass two passes before the final pass is 30% or more; (c) a ratio of the rolling reduction of the rolling pass two passes before the final pass/the rolling reduction of the final pass is 1.5 to 2.5; and (d) a coiling temperature is 520 to 670° C.
• a cold rolling process in which the obtained hot-rolled steel sheet is cold-rolled at a rolling reduction of 70% or more.
• the method is characterized by including a first annealing step including heating the obtained cold-rolled steel sheet to a temperature of Ac3+10°C or higher, and a second annealing step including heating the cold-rolled steel sheet and holding it at a temperature of (Ac1+20) to 820°C for 10 to 500 seconds.
• a first annealing step including heating the obtained cold-rolled steel sheet to a temperature of Ac3+10°C or higher
• a second annealing step including heating the cold-rolled steel sheet and holding it at a temperature of (Ac1+20) to 820°C for 10 to 500 seconds.
• a slab having the chemical composition described above in relation to the steel plate is subjected to hot rolling.
• the slab to be used is preferably cast by a continuous casting method from the viewpoint of productivity, but may be manufactured by an ingot casting method or a thin slab casting method.
• the slab is preferably heated to 1100 ° C or higher prior to hot rolling.
• the upper limit of the heating temperature is not particularly limited, but from an economic viewpoint, the heating temperature is preferably 1300 ° C or lower.
• the heated slab may be subjected to rough rolling before finish rolling as an option, for plate thickness adjustment, etc. Such rough rolling is not particularly limited as long as the desired sheet bar dimensions can be secured.
• Finish rolling entry temperature 1000 to 1080° C.
• the heated slab or the slab that has been rough-rolled as necessary is then subjected to finish rolling.
• the finish rolling needs to be performed under conditions where the inlet temperature of the finish rolling is 1000 to 1080 ° C.
• Nb carbonitrides can be appropriately precipitated in the hot rolling process. Therefore, due to the appropriate precipitation of such Nb carbonitrides, the high-temperature strength of the steel sheet can be sufficiently improved, and the occurrence of heat buckling can be significantly suppressed even by heat treatment at high temperatures in the subsequent primary annealing process or the like.
• the inlet temperature of the finish rolling is higher than 1080 ° C, recrystallization is likely to occur, making it difficult to accumulate strain in the later stage of the finish rolling, and it becomes impossible to promote the precipitation of Nb carbonitrides having a grain size of 20 nm or more. As a result, it becomes impossible to achieve the desired ratio of the Nb amount in Nb carbonitrides having a grain size of 20 nm or more to the Nb amount in all Nb carbonitrides.
• the finish rolling entry temperature is lower than 1000°C, although strain accumulates in the latter stages of finish rolling, precipitation of Nb carbonitrides itself becomes difficult to occur, and the grain size of the Nb carbonitrides also becomes small. As a result, it becomes impossible to achieve a desired ratio of the amount of Nb in Nb carbonitrides having a grain size of 20 nm or more to the amount of Nb in all Nb carbonitrides.
• the finish rolling is performed using a tandem rolling mill consisting of a plurality of rolling stands, for example, five or more rolling stands.
• it is important to control the reduction ratio in the latter stage of the finish rolling and more specifically, it is important to control the reduction ratio of the rolling pass two passes before the final pass to 30% or more, and to control the ratio of the reduction ratio of the rolling pass two passes before the final pass/the reduction ratio of the final pass to 1.5 to 2.5.
• the reduction rate of the rolling pass two passes before the final pass is low and the ratio of the reduction rate of the rolling pass two passes before the final pass/the reduction rate of the final pass is less than 1.5, the accumulation of strain after the reduction in the rolling pass two passes before the final pass becomes insufficient, and the precipitation of Nb carbonitrides with a particle size of 20 nm or more becomes insufficient.
• the reduction rate of the final pass is high and the ratio of the reduction rate of the rolling pass two passes before the final pass/the reduction rate of the final pass is less than 1.5, the accumulation of strain after the reduction in the final pass becomes significant, and the amount of fine Nb carbonitrides precipitated after finish rolling increases, that is, the ratio of Nb carbonitrides with a particle size of 20 nm or more decreases. Therefore, in either case, it becomes impossible to achieve the desired ratio of the amount of Nb in Nb carbonitrides having a particle size of 20 nm or more relative to the amount of Nb in all Nb carbonitrides.
• the reduction rate of the rolling pass two passes before the final pass is high or the reduction rate of the final pass is low, and in this regard the ratio of the reduction rate of the rolling pass two passes before the final pass/the reduction rate of the final pass exceeds 2.5, the accumulation of strain after the reduction in the rolling pass two passes before the final pass becomes too large, making it easier for recrystallization to occur.
• Nb carbonitrides with a grain size of 20 nm or more it becomes difficult for Nb carbonitrides with a grain size of 20 nm or more to precipitate, and it becomes impossible to achieve the desired ratio of the amount of Nb in Nb carbonitrides having a grain size of 20 nm or more to the amount of Nb in all Nb carbonitrides.
• the finish-rolled material is coiled at a coiling temperature of 520 to 670° C.
• the coiling temperature exceeds 670°C, the alloy may be added to the cementite in the metal structure.
• the metal structure in the first annealing process becomes a structure mainly composed of bainite and/or martensite. Therefore, the desired dispersion state of martensite cannot be obtained even by the subsequent secondary annealing step.
• the average grain spacing of martensite cannot be controlled to 2.5 ⁇ m or less, and/or the martensite grains in the direction perpendicular to the rolling direction and the sheet thickness direction are not uniform. In other words, it becomes impossible to obtain a metal structure in which martensite is uniformly dispersed in both the micro-region and the macro-region. In this case, the occurrence of ghost lines and the like cannot be sufficiently suppressed, and the appearance after molding is deteriorated.
• the obtained hot-rolled steel sheet is subjected to an appropriate pickling treatment to remove scale, and then to a cold rolling process.
• the hot-rolled steel sheet is cold-rolled so that the reduction is 70% or more.
• the desired plate thickness can be secured, and the recrystallization during heating in the subsequent primary annealing process can be completed early to ensure the desired precipitation state of Nb carbonitrides. If the reduction is less than 70%, recrystallization is delayed during heating in the primary annealing process, and strain remains until a high temperature is reached.
• the reduction in the cold rolling process is 90% or less. By setting the reduction rate to 90% or less, it is possible to prevent the rolling load from becoming excessively large, which makes the rolling difficult.
• the number of rolling passes and the reduction rate for each pass are not particularly limited, and may be appropriately set so that the reduction rate of the entire cold rolling is within the above range.
• the obtained cold-rolled steel sheet is heated to a temperature of Ac3+10°C or more in the next primary annealing step.
• the Ac3 point (°C) is determined by cutting a small piece from the cold-rolled steel sheet and calculating the thermal expansion of the small piece during heating from room temperature to 1000°C at 10°C/s.
• the metal structure in the steel sheet after cooling can be reliably composed of a structure mainly composed of bainite and/or martensite, for example, full bainite or full martensite.
• the structure mainly composed of bainite and/or martensite refers to a structure containing at least one of bainite and martensite in a total area ratio of 90% or more
• full bainite refers to a structure composed of 100% bainite in area ratio
• full martensite refers to a structure composed of 100% martensite in area ratio.
• the bainite and/or martensite structure has many different interfaces inside compared to structures such as ferrite.
• the metal structure of the steel sheet before the secondary annealing process i.e., the final annealing process
• a structure mainly composed of bainite and/or martensite it becomes possible to disperse and generate a very large number of carbides that can become nucleation sites of austenite on these interfaces in the stage of heating such a metal structure in the secondary annealing.
• austenite is generated finely and uniformly throughout the steel sheet from the nucleation sites dispersed in such a large number, and then martensite is generated from the austenite, so that in the metal structure obtained after the secondary annealing, the average grain spacing of martensite is controlled to 2.5 ⁇ m or less, and the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is controlled to 1.5% or less.
• the heating temperature in the first annealing step is less than Ac3+10°C, austenitization will be insufficient, and the metal structure in the steel sheet will not be composed mainly of bainite and/or martensite even after subsequent cooling, meaning that the total area ratio of bainite and martensite will not be 90% or more.
• the heating temperature in the first annealing step be 1050°C or less.
• the holding time at the above heating temperature is preferably 10 to 500 seconds.
• austenite can be generated finely and uniformly from the carbides throughout the steel sheet while maintaining the state in which the carbides are dispersed on the interfaces.
• austenite can be generated finely and uniformly from the carbides throughout the steel sheet while maintaining the state in which the carbides are dispersed on the interfaces.
• martensite can be appropriately generated from the finely dispersed austenite, and as a result, the average particle spacing of martensite is controlled to 2.5 ⁇ m or less, and the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is controlled to 1.5% or less.
• the desired metal structure as described above cannot be obtained.
• the heating temperature in the secondary annealing process is less than Ac1+20°C or the holding time is less than 10 seconds, the desired metal structure as described above cannot be obtained.
• the heating temperature exceeds 820°C, the area ratio of austenite becomes too high, and the area ratio of ferrite cannot be increased to 75% or more.
• the holding time exceeds 500 seconds, the austenite grains become coarse, and the martensite grains obtained by subsequent cooling are also relatively coarse. In such a case, it becomes impossible to obtain a fine martensite structure in which the average grain spacing of martensite is controlled to 2.5 ⁇ m or less.
• a plating treatment may be applied to the surface of the obtained cold-rolled steel sheet as necessary.
• the plating treatment may be a treatment such as hot-dip plating, alloying hot-dip plating, or electroplating.
• the steel sheet 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 may be any appropriate conditions known to those skilled in the art.
• the alloying temperature may be 450 to 600°C.
• steel sheets according to the embodiments of the present invention were manufactured under various conditions, and the occurrence of heat buckling during the manufacturing process, as well as the tensile strength and post-forming appearance characteristics of the resulting steel sheets were investigated.
• a slab having the chemical composition shown in Table 1 and a thickness of 200 to 300 mm was cast by continuous casting.
• the remainder other than the components shown in Table 1 was Fe and impurities.
• the obtained slab was heated to a temperature of 1100 to 1300°C, and then hot rolling was performed.
• the hot rolling was performed by performing rough rolling and finish rolling. More specifically, the rough rolling conditions were the same in all examples and comparative examples, and the finish rolling was performed using a tandem rolling mill consisting of seven rolling stands.
• the reduction ratio of the rolling pass two passes before the final pass in the finish rolling (F5 rolling pass) was 30%.
• Condition I Finishing roll entry temperature 1000-1080°C
• Condition II Ratio of rolling reduction of the rolling pass two passes before the final pass/rolling reduction of the final pass (F5/F7 rolling reduction ratio) 1.5-2.5
• Condition III Coiling temperature 520-670°C.
• the finishing roll entry temperature is 1050°C
• the finishing roll entry temperature is 950°C (Comparative Example 9) or 1120°C (Comparative Example 19).
• the F5/F7 rolling reduction ratio is 2.0, while in the example where Condition II is not met, the F5/F7 rolling reduction ratio is 1.2 (Comparative Example 2) or 3.0 (Comparative Example 20).
• the winding temperature was 600°C, while in the examples that did not satisfy condition III, the winding temperature was 470°C (Comparative Example 10) or 700°C (Comparative Example 21).
• the obtained hot-rolled steel sheet was pickled, followed by cold rolling, primary annealing (holding time of 100 seconds at a specified heating temperature, and an average cooling rate of 40°C/sec to 200°C after annealing), and secondary annealing (holding time of 100 seconds at a heating temperature of 770°C, and an average cooling rate of 15°C/sec to 500°C after annealing) to produce a cold-rolled steel sheet with a thickness of 0.4 mm.
• the heating temperature of 770°C satisfies the requirement of Ac1+20°C or more for all of the invention examples and comparative examples.
• Table 2 shows the cases where cold rolling condition IV (reduction rate of 70% or more) and primary annealing condition V (heating temperature Ac3+10°C or more) are met and where they are not met.
• cold rolling was performed at a rolling reduction of 80%
• cold rolling was performed at a rolling reduction of 60%
• the primary annealing was performed by heating to Ac3+15°C or higher, i.e., 900°C.
• the primary annealing was performed by heating to a temperature lower than Ac3 (Comparative Examples 4 and 20).
• 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 electrogalvanized layer (EG).
• GI hot-dip galvanized layer
• GA galvannealed layer
• EG electrogalvanized layer
• the alloying conditions were 550°C for 20 seconds.
• the properties of the obtained steel sheets were measured and evaluated by the following methods.
• the occurrence of heat buckling during the manufacturing process the occurrence of buckling was checked after the first annealing, and if no buckling occurred, it was judged that no heat buckling had occurred and was deemed to have passed (OK), and if buckling occurred, it was judged that heat buckling had occurred and was deemed to have failed (NG).
• TS Tensile strength
• the tensile strength (TS) was measured by taking a No. 5 tensile test piece of JIS Z2241:2011 from the steel plate, the longitudinal direction of which was perpendicular to the rolling direction and the plate thickness direction, and conducting a tensile test in accordance with JIS Z2241:2011.
• the appearance after forming was evaluated based on the degree of ghost lines that appeared on the surface of the door outer after forming.
• a formed part simulating the door outer a pressed part was used in which a steel plate blanked to 600 mm square was press-formed so that the radius of curvature R at the center was 1200 mm.
• the surface after press forming was ground with a grindstone, and stripes that appeared on the surface at intervals of several mm were judged to be ghost lines, and were scored from 1 to 5 depending on the degree of occurrence of the stripes.
• the evaluation was "4" or more, the appearance after forming was judged to be poor and the specimen was judged to be unsatisfactory.
• the appearance after forming was evaluated using a pressed member simulating a door outer, but the evaluation object may be a formed member that can be estimated to have been given a strain of 2.5% by press forming, or a test piece taken from a steel plate to which a pre-strain of 2.5% has been similarly given may be evaluated, and similar evaluations can be made by such test methods.
• a test piece taken from a steel plate a JIS No. 5 test piece having a longitudinal direction perpendicular to the rolling direction and the plate thickness direction and given a pre-strain of 2.5% may be evaluated.
• the proportion of Nb carbonitrides with a particle size of 20 nm or more decreased, and heat buckling occurred in the first annealing process.
• the heating temperature in the first annealing step was low, so austenitization was insufficient, and it is believed that the metal structure in the steel sheet could not be constituted by a structure mainly composed of bainite and/or martensite even after subsequent cooling.
• the average particle spacing of martensite exceeded 2.5 ⁇ m, and the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction exceeded 1.5%, resulting in a poor appearance after forming.
• Comparative Example 20 the F5/F7 reduction ratio in the finish rolling was high, so that the accumulation of strain after reduction in the F5 rolling pass was too large, and recrystallization was promoted. As a result, the proportion of Nb carbonitrides with a particle size of 20 nm or more was reduced, and heat buckling occurred in the first annealing process.
• Comparative Example 20 as in Comparative Example 4, the heating temperature in the first annealing process was low, so that austenitization was insufficient, and it is considered that the metal structure in the steel sheet could not be composed of a structure mainly composed of bainite and/or martensite even after subsequent cooling.
• Comparative Example 28 since the value of index A was low, the high-temperature strength of the steel sheet could not be sufficiently improved, and heat buckling occurred in the primary annealing step.
• the C or Mn content was high, so the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction exceeded 1.5%, resulting in poor appearance after forming.
• the C or Mn content was low, so sufficient strength was not obtained.
• all of the steel sheets according to the examples of the invention have a prescribed chemical composition and metal structure, and by appropriately controlling the proportions of ferrite and martensite in the metal structure, a TS of 540 MPa or more is achieved, and the average particle spacing of martensite is controlled to 2.5 ⁇ m or less in the microscopic region, while in the macroscopic region the standard deviation in the area ratio of martensite in the directions perpendicular to the rolling direction and plate thickness direction is controlled to 1.5% or less.
• the Nb content in all Nb carbonitrides was controlled to 0.004% or more, and the Nb content in Nb carbonitrides with a grain size of 20 nm or more was controlled to 60% or more of the Nb content in all of the Nb carbonitrides, and by combining this with the control of the chemical composition by index A, it was possible to significantly suppress the occurrence of heat buckling during the manufacturing process of the steel sheets.
• all of the Nb carbonitrides with a grain size of 20 nm or more in the invention examples had a grain size of 1000 nm or less, i.e., 1 ⁇ m or less.

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