WO2023149002A1 - Steel sheet - Google Patents

Steel sheet Download PDF

Info

Publication number
WO2023149002A1
WO2023149002A1 PCT/JP2022/031748 JP2022031748W WO2023149002A1 WO 2023149002 A1 WO2023149002 A1 WO 2023149002A1 JP 2022031748 W JP2022031748 W JP 2022031748W WO 2023149002 A1 WO2023149002 A1 WO 2023149002A1
Authority
WO
WIPO (PCT)
Prior art keywords
less
steel sheet
str
content
rolling
Prior art date
Application number
PCT/JP2022/031748
Other languages
French (fr)
Japanese (ja)
Inventor
諭 弘中
泰弘 伊藤
真衣 永野
克哉 中野
Original Assignee
日本製鉄株式会社
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by 日本製鉄株式会社 filed Critical 日本製鉄株式会社
Publication of WO2023149002A1 publication Critical patent/WO2023149002A1/en

Links

Classifications

    • 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
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

  • the present invention relates to steel sheets.
  • the surface oxidation index A during annealing is 2.3 or more, the balance is composed of Fe and unavoidable impurities, and the structure of the substrate is composed of ferrite and a second phase, and the second phase is mainly martensite.
  • Patent Document 1 describes that the high-strength hot-dip galvanized steel sheet has excellent surface quality and a tensile strength of 590 MPa or more, which are suitable for use mainly as automotive structural parts such as members and lockers. It is
  • an object of the present invention is to provide a high-strength steel sheet having an improved post-forming appearance through a novel configuration.
  • the present inventors conducted studies with a particular focus on the surface properties of steel sheets.
  • the present inventors found that in a steel plate made of DP steel composed of a composite structure of a soft phase made of ferrite and a hard phase mainly composed of martensite, etc., the aspect ratio (Str) of the surface texture is a ghost line.
  • the present inventors while controlling the initial Str within a specific range, by suppressing the variation of Str when applying tensile strain within a predetermined range, martensite The present inventors have found that the generation of ghost lines on the steel sheet surface can be remarkably suppressed even during processing such as press molding while maintaining high strength based on the hard phase mainly composed of, etc., and completed the present invention.
  • the present invention that has achieved the above object is as follows.
  • a steel plate according to an embodiment of the present invention is The chemical composition, in mass %, C: 0.020 to 0.100%, Mn: 1.00-2.50%, 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, Si: 0 to 1.500%, Cr: 0 to 0.80%, Mo: 0-0.50%, B: 0 to 0.0100%, Ti: 0 to 0.100%, Nb: 0 to 0.060%, 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: 0-0.0100%, Zr: 0 to 0.0100%, REM: 0 to 0.0100%, and the balance: Fe and impurities,
  • the metal structure in mass %
  • DP steel which has a relatively low yield strength, is often used from the viewpoint of avoiding surface defects called surface distortion that occur during press forming.
  • the soft phase and its surroundings are preferentially deformed during processing such as press forming. Deformation is likely to occur, and minute unevenness may occur on the surface of the steel sheet after forming, resulting in appearance defects called ghost lines.
  • the soft phase composed of ferrite is dented, while the hard phase mainly composed of martensite, etc.
  • the present inventors found that in a steel plate made of DP steel composed of a composite structure of a soft phase made of ferrite and a hard phase mainly composed of martensite, etc., the aspect ratio (Str) of the surface texture is a ghost line. found to have a significant effect on development.
  • Str which is the texture aspect ratio of the surface texture
  • JIS B0681-2:2018 indicates the strength of the anisotropy of the surface. and is known to take values in the range of 0 to 1.
  • the anisotropy becomes strong and lines appear on the surface. Become.
  • the present inventors found that the initial (that is, before forming by press forming or the like) surface of the steel sheet (if a coating layer exists on the surface of the steel sheet, the surface of the coating layer) Str of 0.35 ⁇ It has been found that control within the range of 0.75 is very effective in suppressing the generation of ghost lines on the surface of the steel sheet when strain is imparted by forming such as press forming. Since ghost lines are associated with streak patterns on the surface of the steel sheet, from the viewpoint of suppressing the occurrence of the ghost lines, it is preferable that the value of Str is more isotropic, and therefore closer to 1 is preferable. is generally expected.
  • the value of Str is not too low, and that it is set to a value equal to or higher than a specific value. From this point of view, the present inventors found that the initial Str of the surface of the steel sheet should be 0.35 or more. On the other hand, according to the experimental results of the present inventors, it was found that when Str is closer to 1, ghost lines are more pronounced after press molding, and therefore the appearance after molding may be deteriorated. Therefore, from the viewpoint of suppressing the generation of ghost lines, the initial Str must be controlled within an appropriate range, specifically within the range of 0.35 to 0.75. By controlling the initial Str within such an appropriate range, it is possible to maintain good appearance both before and after molding.
  • Str is closer to 1, the steel sheet after forming, relative to the initial more isotropic surface texture. It is thought that the presence of fine unevenness on the surface becomes too conspicuous, degrading the appearance.
  • Str is a moderate value, that is, if there are streaks that are not noticeable to the naked eye, fine unevenness that occurs on the surface of the steel sheet after forming will not be noticeable due to the effects of the streaks that are present from the beginning. It is thought that the streaks present in the hollow and the fine irregularities formed on the surface of the steel sheet after forming cancel each other out, and as a result, the occurrence of ghost lines is suppressed or reduced.
  • the present inventors considered that the decarburization treatment of the surface layer of the steel sheet in the annealing process was appropriate in order to control the initial Str within a desired range.
  • internal oxides such as Si and Mn formed on the surface layer of the steel sheet during the hot rolling process should be reduced as much as possible. I have found that it is preferable.
  • the present inventors suppressed the upper limits of the Si and Mn contents in the steel sheet to 1.500% and 2.50% or less, respectively, while maintaining the coiling temperature and
  • the atmosphere, temperature and time of the annealing process within predetermined ranges, the formation of internal oxides during the hot rolling process can be sufficiently suppressed or reduced, and related to this, appropriate decarburization during the annealing process. It has been found that the treatment can be achieved and thus the desired initial Str at the surface of the steel sheet.
  • ⁇ Str is considered to be closely related to the Mn segregation in the steel in addition to the initial Str value, and the initial Str is 0 It is considered that the value of ⁇ Str can be made smaller by controlling the Mn segregation in the steel in addition to controlling it within the range of 0.35 to 0.75. Ghost lines are considered to be conspicuous due to the presence of hard phases connected in a striped pattern in the metal structure. It is considered effective to reduce the Mn segregation of Mn segregation is affected by various alloying elements contained in the steel sheet, and its degree becomes particularly pronounced when the C and Mn contents in the steel sheet become high.
  • the initial Str is controlled within the range of 0.35 to 0.75 while sufficiently maintaining high strength based on the hard phase contained in the steel sheet.
  • ⁇ Str to 0.15 or less when a 5% tensile strain is applied, not only can the appearance before molding be maintained well, but strain is applied by molding such as press molding. Even in this case, it is possible to remarkably suppress the occurrence of appearance defects such as ghost lines on the surface of the steel sheet. Therefore, according to embodiments of the present invention, it is possible to provide a high-strength steel sheet having improved post-forming appearance in addition to good pre-forming appearance.
  • C is an element that increases the strength of the steel sheet.
  • the C content is made 0.020% or more.
  • the C content may be 0.025% or more, 0.030% or more, 0.035% or more, 0.040% or more, or 0.050% or more.
  • the C content should be 0.100% or less.
  • the C content may be 0.095% or less, 0.090% or less, 0.085% or less, 0.080% or less, or 0.070% or less.
  • Mn is an element that enhances the hardenability of steel and contributes to the improvement of strength.
  • the Mn content is set to 1.00% or more.
  • the Mn content may be 1.20% or more, 1.30% or more, 1.40% or more, or 1.50% or more.
  • Mn is excessively contained, the diffusion of Mn during solidification is inhibited, Mn segregation cannot be sufficiently suppressed, and ⁇ Str may not be controlled within the desired range.
  • Mn is excessively contained, internal oxides are excessively formed during hot rolling, and the decarburization treatment of the steel sheet surface layer during the subsequent annealing process cannot be appropriately controlled. It may not be possible to control Str within a desired range. Therefore, the Mn content should be 2.50% or less.
  • the Mn content may be 2.25% or less, 2.10% or less, 2.00% or less, 1.85% or less, or 1.75% or less.
  • P is an element mixed in during the manufacturing process.
  • the P content may be 0%.
  • the P content may be 0.0001% or more, 0.0005% or more, 0.001% or more, or 0.005% or more.
  • an excessive P content may lower the toughness of the steel sheet. Therefore, the P content should be 0.100% or less.
  • the P content may be 0.070% or less, 0.060% or less, 0.040% or less, or 0.020% or less.
  • S is an element 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.
  • the S content should be 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 deoxidizing agent and is an effective element for increasing the strength of steel.
  • Al is also an effective element for promoting the diffusion of Mn during solidification and reducing Mn segregation.
  • the Al content should be 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 should be 0.700% or less.
  • the Al content may be 0.600% or less, 0.400% or less, 0.300% or less, 0.150% or less, 0.100% or less, or 0.070% or less.
  • N is an element 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 should be 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 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 should be 0.0100% or less.
  • the O content may be 0.0070% or less, 0.0040% or less, 0.0040% or less, or 0.0020% or less.
  • the steel sheet may contain one or more of the following optional elements in place of part of the remaining Fe, if necessary. These optional elements are described in detail below. The lower limits of the contents of these optional elements are all 0%.
  • Si is a deoxidizing element for steel, and is an effective element for increasing the strength without impairing the ductility of the steel sheet.
  • Si is also an effective element for promoting diffusion of Mn during solidification and reducing Mn segregation.
  • the Si content may be 0%, but in order to obtain these effects, the Si content is preferably 0.001% or more or 0.005% or more.
  • the Si content may be 0.010% or more, 0.050% or more, 0.100% or more, or 0.150% or more.
  • Si is excessively contained, internal oxides are excessively formed during hot rolling, and the decarburization treatment of the steel sheet surface layer during the subsequent annealing process cannot be appropriately controlled.
  • the Si content should be 1.500% or less.
  • the Si content may be 1.400% or less, 1.200% or less, 1.000% or less, 0.800% or less, 0.600% or less, 0.500% or less, or 0.300% or less .
  • Cr 0 to 0.80%
  • Cr is an element that increases the hardenability of steel and contributes to the improvement of the strength of the steel sheet. Moreover, Cr is also an element effective in promoting diffusion of Mn during solidification and reducing Mn segregation.
  • the Cr content may be 0%, the Cr content is preferably 0.001% or more in order to obtain these effects.
  • the Cr content may be 0.01% or more, 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 improvement in strength of the steel sheet. Mo is also an effective element for promoting the diffusion of Mn during solidification and reducing Mn segregation.
  • the Mo content may be 0%, but in order to obtain these effects, the Mo content is preferably 0.001% or more.
  • the Mo content may be 0.01% or more, 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 improvement in strength of the steel sheet.
  • the B content may be 0%, the B content is preferably 0.0001% or more in order to obtain such effects.
  • 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 amounts of S, N, and O that generate coarse inclusions that act as starting points for fracture. In addition, Ti has the effect of refining the structure and improving the strength-formability balance of the steel sheet.
  • the Ti content may be 0%, the Ti content is preferably 0.001% or more in order to obtain these effects. The Ti content may be 0.005% or more, 0.007% or more, or 0.010% or more.
  • the Ti content is preferably 0.100% or less.
  • the Ti content may be 0.080% or less, 0.070% or less, 0.060% or less, or 0.030% or less.
  • Nb is an element that contributes to the improvement of the strength of the steel sheet due to strengthening by precipitates, grain refinement strengthening by suppressing the growth of ferrite grains, and/or dislocation strengthening by suppressing recrystallization.
  • the Nb content may be 0%, the Nb content is preferably 0.001% or more in order to obtain these effects.
  • the Nb content may be 0.005% or more, 0.007% or more, or 0.010% or more.
  • the Nb content is preferably 0.060% or less.
  • the Nb content may be 0.050% or less, 0.040% or less, or 0.030% or less.
  • V is an element that contributes to the improvement of the strength of the steel sheet due to strengthening by precipitates, grain refinement strengthening by suppressing the growth of ferrite grains, and/or dislocation strengthening by suppressing recrystallization.
  • the V content may be 0%, the V content is preferably 0.001% or more in order to obtain these effects.
  • the V content may be 0.005% or more, 0.01% or more, or 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 improvement in strength of the steel sheet.
  • the Ni content may be 0%, the Ni content is preferably 0.001% or more in order to obtain such effects.
  • the Ni content may be 0.01% or more, 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 steel in the form of fine particles and contributes to improving the strength of the steel sheet.
  • the Cu content may be 0%, the Cu content is preferably 0.001% or more in order to obtain such effects.
  • the Cu content may be 0.01% or more, 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 improvement in strength of the steel sheet.
  • the W content may be 0%, the W content is preferably 0.001% or more in order to obtain such effects.
  • the W content may be 0.01% or more, 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, 0.20% or less, or 0.15% or less.
  • Sn is an element that suppresses the coarsening of crystal grains and contributes to the improvement of the strength of the steel sheet.
  • the Sn content may be 0%, the Sn content is preferably 0.001% or more in order to obtain such effects.
  • the Sn content may be 0.01% or more, 0.05% or more, or 0.08% or more.
  • an excessive Sn content may cause embrittlement of the steel sheet. Therefore, the Sn content is preferably 1.00% or less.
  • the Sn content may be 0.80% or less, 0.50% or less, 0.20% or less, or 0.15% or less.
  • Sb is an element that suppresses the coarsening of crystal grains and contributes to the improvement of the strength of the steel sheet.
  • the Sb content may be 0%, the Sb content is preferably 0.001% or more in order to obtain such effects.
  • the Sb content may be 0.003% or more, 0.005% or more, or 0.010% or more.
  • an excessive Sb content may cause embrittlement of the steel sheet. Therefore, the Sb content is preferably 0.200% or less.
  • the Sb content may be 0.150% or less, 0.100% or less, 0.050% or less, or 0.020% or less.
  • Ca, Mg, Zr, and REM are elements that contribute to improving the formability of steel sheets.
  • 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 each preferably 0.0001% or more. , 0.0005% or more, 0.0010% or more, or 0.0015% or more.
  • an excessive content of these elements may reduce the ductility of the steel sheet.
  • the Ca, Mg, Zr and REM contents are each preferably 0.0100% or less, and 0.0080% or less, 0.0060% or less, 0.0030% or less or 0.0020% or less. good too.
  • REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanide (La) with atomic number 57 to lutetium (Lu) with atomic number 71, which are lanthanoids.
  • the REM content is the total content of these elements.
  • the balance other than the above elements consists of Fe and impurities.
  • Impurities are components and the like that are mixed due to various factors in the manufacturing process, including raw materials such as ores and scraps, when steel sheets are manufactured industrially.
  • Impurities such as 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.
  • Impurities may contain 0.100% or less in total.
  • the chemical composition of the steel sheet can 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).
  • ICP-AES Inductively Coupled Plasma-Atomic Emission Spectrometry
  • C and S can be measured using a combustion-infrared absorption method
  • N can be measured using an inert gas fusion-thermal conductivity method
  • O can be measured using an inert gas fusion-nondispersive infrared absorption method.
  • the metal structure of the steel sheet consists of ferrite: 70 to 97% and hard phase: 3 to 30%, more specifically ferrite: 70 to 95% and hard phase: 5 to 30% in terms of area %. Configured.
  • the strength of the steel sheet is maintained within an appropriate range, more specifically, a tensile strength of 500 MPa or more is achieved, and the appearance after forming is improved.
  • the area fraction of the hard phase may be 5% or more, 7% or more, 10% or more, or 12% or more.
  • the area fraction of ferrite may be 95% or less, 93% or less, 90% or less, or 88% or less.
  • the area fraction of the hard phase is 28% or less, 26% or less, 23% or less, 20% or less, 18% or less, 16% or less, or 14% or less. There may be.
  • the area fraction of ferrite may be 72% or more, 74% or more, 77% or more, 80% or more, 82% or more, 84% or more, or 86% or more.
  • the hard phase refers to a structure harder than ferrite, and includes at least one of martensite, bainite, tempered martensite and pearlite, or at least one of them. and in particular at least one of martensite, bainite, tempered martensite and perlite. From the viewpoint of improving the strength of the steel sheet, the hard phase preferably consists of at least one of martensite, bainite and tempered martensite, or at least one of them. It is more preferable to have In the embodiment of the present invention, the metal structure of the steel sheet preferably has little retained austenite. Specifically, the retained austenite is preferably less than 1% or less than 0.5% in terms of area %. , 0%.
  • Identification of the metal structure and calculation of the area fraction are performed as follows. First, the metal structure (microstructure) is observed from the W/4 position or 3W/4 position of the width W of the obtained steel plate (that is, the W/4 position in the width direction from either end of the steel plate in the width direction). A sample (size is roughly 20 mm in the rolling direction, 20 mm in the width direction, and the thickness of the steel plate) is taken. Then, using an optical microscope, the metal structure (microstructure) at 1/2 thickness from the surface is observed, and the surface of the steel sheet (when plating is present, the surface excluding the plating layer) to 1/2 thickness Calculate the area fraction of the hard phase up to the thickness.
  • the plate thickness cross-section in the direction perpendicular to the rolling direction is polished as an observation surface and etched with a repeller reagent.
  • classify the "microstructure" from optical micrographs at 500x or 1000x magnification.
  • each structure is color-coded, for example, black for bainite and pearlite, white for martensite (including tempered martensite), and gray for ferrite. can be easily distinguished from hard tissue.
  • the non-gray areas showing ferrite are the hard phases.
  • the maximum brightness value L max and the minimum brightness value L min of the image are obtained from the image, and pixels with brightness from L max ⁇ 0.3 (L max ⁇ L min ) to L max
  • the part is defined as a white area
  • the part having pixels from L min to L min + 0.3 (L max - L min ) is defined as a black area
  • the other part is defined as a gray area.
  • Calculate the area fraction of Image analysis is performed in the same manner as above for a total of 10 observation fields of view to measure the area fraction of the hard phase, and these area fractions are averaged to calculate an average value.
  • the average value is defined as the area fraction of the hard phase, and the remainder is defined as the area fraction of ferrite.
  • the area fraction of retained austenite can be measured by X-ray diffraction of the observation surface.
  • the Str of the surface of the steel sheet (the surface of the plating layer when the steel sheet has a plating layer) is 0.35 to 0.75.
  • the Str of the surface of the steel sheet must be controlled within a suitable range as described above. and/or may be 0.70 or less, 0.65 or less, or 0.60 or less.
  • the difference ⁇ Str between the initial Str and the Str after applying 5% tensile strain is 0.15 or less.
  • ⁇ Str is preferably as low as possible, and may be, for example, 0.12 or less, 0.10 or less, 0.08 or less, or 0.05 or less.
  • ⁇ Str When tensile strain is applied, the value of Str is generally smaller than before applying tensile strain (sometimes it does not change), so even if measurement errors etc. are considered, ⁇ Str will be on the negative side. cannot be of great value. Therefore, although the lower limit is not particularly limited, ⁇ Str may be -0.03 or more, 0.00 or more, or 0.01 or more, for example.
  • Str and ⁇ Str are determined as follows. First, a No. 5 tensile test piece of JIS Z2241: 2011 with a direction (C direction) perpendicular to the rolling direction (L direction) as the test direction is taken from a position at least 100 mm away from the end surface of the steel plate, and then the sampled steel plate. Three-dimensional image analysis is performed using a VK-X250/150 shape analysis laser microscope manufactured by Keyence Corporation on the surface of the sample (the surface of the coating layer if there is a coating layer on the surface of the steel plate sample), and JIS The initial Str is determined according to the provisions of B0681-2:2018.
  • the target area of the three-dimensional image analysis shall be 5 mm (C direction) ⁇ 2 mm (L direction) or more.
  • Str after applying a tensile strain of 5% is determined by performing the same measurement as above, and finally the initial Str ⁇ Str is determined by subtracting Str after applying 5% tensile strain from .
  • the steel plate according to the embodiment of the present invention is not particularly limited, but has a thickness of 0.1 to 2.0 mm, for example.
  • a steel plate having such a thickness is suitable for use as a material for lid members such as doors and hoods.
  • 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 is measured with a micrometer.
  • the steel sheet according to the embodiment of the present invention is a cold-rolled steel sheet, and may further include a plating layer on the surface for the purpose of improving corrosion resistance.
  • 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 coating layer or an electroplating layer on its surface.
  • the hot-dip plating layer is, for example, a hot-dip galvanizing layer (GI), an alloyed hot-dip galvanizing 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, hot-dip Zn - Including Al-Mg-Si alloy plating layer, etc.
  • the electroplated layer includes, for example, an electrogalvanized layer (EG), an electroplated Zn—Ni alloy layer, and the like.
  • the plating layer is a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electro-galvanized layer.
  • the coating amount of the plating layer is not particularly limited, and a general coating amount may be used.
  • a steel sheet having the above chemical composition and metallographic structure can achieve a high tensile strength, specifically a tensile strength of 500 MPa or more.
  • the tensile strength is preferably 540 MPa or higher, more preferably 570 MPa or higher or 600 MPa or higher.
  • the upper limit is not particularly limited, for example, the tensile strength may be 980 MPa or less, 850 MPa or less, 750 MPa or less, 700 MPa or less, or 650 MPa or less.
  • Tensile strength is measured by taking a JIS Z2241:2011 No. 5 tensile test piece from a steel plate with the test direction perpendicular to the rolling direction and performing a tensile test in accordance with JIS Z2241:2011.
  • the steel sheet according to the embodiment of the present invention has a high strength, specifically a tensile strength of 500 MPa or more, it can maintain an excellent appearance even after forming such as press working. For this reason, the steel plate according to the embodiment of the present invention is very useful for use as outer panel parts such as roofs, hoods, fenders, doors, etc., which require high designability in automobiles.
  • a steel sheet manufacturing method includes: hot rolling a slab having the chemical composition described above in relation to steel plate and then coiling at a temperature below 550° C. (hot rolling process); A step of cold rolling the obtained hot-rolled steel plate so that the cumulative reduction ratio is 50 to 90% (cold rolling step); A step of holding the obtained cold-rolled steel sheet in an atmosphere with a dew point of ⁇ 20 to 5° C. in a temperature range of 750° C. or higher for 30 to 200 seconds (annealing step); A step of skin-pass rolling at a rolling reduction of 0.6% or less using rolls having an average roughness Ra (pass-rolling step) is characterized by including Each step will be described in detail below.
  • a slab having the chemical composition described above in relation to steel 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 produced 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 heating temperature is preferably less than 1300° C. from an economical point of view.
  • the heated slab may optionally be subjected to rough rolling before finish rolling for thickness adjustment and the like.
  • Hot rolling is not particularly limited, but is generally carried out under such conditions that the completion temperature of finish rolling is 650° C. or higher. This is because if the finish rolling completion temperature is too low, the rolling reaction force increases, making it difficult to stably obtain a desired plate thickness.
  • the upper limit is not particularly limited, but generally the finish rolling completion temperature is 950° C. or less.
  • the finish rolled steel sheet is then coiled at a temperature below 550°C.
  • a high coiling temperature may promote the formation of internal oxides such as Si and Mn in the surface layer of the hot-rolled steel sheet. Since the formed internal oxide cannot be sufficiently removed even by the subsequent pickling, the subsequent cold rolling and annealing steps, especially the annealing step, are performed in a state containing a relatively large amount of internal oxide. Become. This time, the present inventors have found that it is necessary to moderately and uniformly decarburize the surface layer of the steel sheet in the annealing process in order to control the initial Str on the steel sheet surface within the range of 0.35 to 0.75. Found.
  • the decarburization treatment in the annealing process cannot be made appropriate. That is, the decarburization of the surface layer of the steel sheet in the annealing process is inhibited by the internal oxides, and the variation becomes large. As a result, Str on the surface of the finally obtained steel sheet cannot be controlled within the desired range.
  • the coiling temperature to less than 550 ° C. and reliably suppressing or reducing the formation of internal oxides such as Si and Mn in the surface layer of the hot-rolled steel sheet, appropriate decarburization treatment is performed in the subsequent annealing process.
  • the coiling temperature in the hot rolling process is set to less than 550°C, preferably 500°C, while suppressing the Si and Mn contents in the steel sheet to 1.500% or less and 2.50% or less, respectively.
  • the formation of internal oxides in the hot rolling process can be sufficiently suppressed or reduced to achieve appropriate decarburization treatment in the subsequent annealing process and eventually the desired Str on the final steel sheet surface.
  • the lower limit of the coiling temperature is not particularly limited, but if the coiling temperature is too low, the strength of the hot-rolled steel sheet becomes excessively high, which may impair the cold-rollability. Therefore, the winding temperature is preferably 450° C. or higher.
  • Cold rolling process The obtained hot-rolled steel sheet is appropriately pickled to remove scales, and then subjected to a cold-rolling process.
  • the cumulative rolling reduction is, for example, 50 to 90%.
  • the annealing temperature is very important to appropriately decarburize the surface layer of the steel sheet at the above dew point, annealing temperature and holding time in order to achieve the desired initial Str.
  • the dew point is -15 to 0°C and the holding time is 50 to 150 seconds.
  • the upper limit of the annealing temperature is not particularly limited, for example, the annealing temperature is preferably 900° C. or lower from the viewpoint of suppressing coarsening of crystal grains and ensuring sufficient strength.
  • the cold-rolled steel sheet after the annealing process is cooled in the next cooling process.
  • the cooling process is not particularly limited, and any appropriate conditions may be appropriately selected so as to obtain the metal structure containing the ferrite and hard phase described above in relation to the steel sheet and the hard phase in a predetermined area fraction. good.
  • the average cooling rate from the annealing temperature is 5 to 50° C./sec.
  • the average cooling rate is set to 5° C./second or more, excessive transformation to ferrite can be suppressed and the amount of hard phases such as martensite produced can be increased to obtain the desired strength.
  • the average cooling rate to 50° C./sec or less, the steel sheet can be cooled more uniformly in the width direction.
  • the surface of the obtained cold-rolled steel sheet may be subjected to a plating treatment, if necessary.
  • the plating process may be a process such as hot-dip plating, hot-dip alloying, electroplating, or the like.
  • the steel sheet may be subjected to hot-dip galvanizing treatment as the plating treatment, or alloying treatment may be performed after the hot-dip galvanizing treatment.
  • Specific conditions for the plating treatment and alloying treatment are not particularly limited, and may be any appropriate conditions known to those skilled in the art.
  • the alloying temperature may be 450-600°C.
  • the cold-rolled steel sheet or plated steel sheet is pass-rolled at a rolling reduction of 0.6% or less using rolls having an arithmetic mean roughness Ra of 1.3 ⁇ m or less.
  • Arithmetic mean roughness Ra is measured in accordance with JIS B0601:2013.
  • temper rolling is performed on a steel sheet after annealing or plating for the purpose of correcting the shape of the steel sheet, adjusting the surface roughness, or the like.
  • the surface texture of the steel sheet finally obtained is extremely important, and such surface texture is created especially by the preceding hot rolling and annealing steps.
  • the temper rolling process it is necessary to roll under relatively mild conditions in order to maintain the surface properties of the cold-rolled steel sheet or plated steel sheet thus produced.
  • the arithmetic mean roughness Ra of the rolls used in temper rolling is more than 1.3 ⁇ m and / or the rolling reduction of temper rolling is more than 0.6%
  • the roughness of the rolls is on the steel sheet surface. It may be strongly transferred, and the surface texture created especially in the hot rolling process and the annealing process may be partially or wholly destroyed, and as a result, the initial Str may be outside the desired range.
  • the surface properties created in the previous process can be sufficiently maintained. It is possible to appropriately correct the shape of the steel sheet while achieving the initial Str within the desired range by using the Preferably, the arithmetic mean roughness Ra of the rolls is 1.2 ⁇ m or less, and the rolling reduction of temper rolling is 0.5% or less.
  • steel sheets according to embodiments of the present invention were produced under various conditions, and the properties of tensile strength and post-forming appearance of the obtained steel sheets were investigated.
  • a slab having a chemical composition shown in Table 1 and a thickness of 200 to 300 mm was cast by a continuous casting method.
  • the balance other than the components shown in Table 1 is Fe and impurities.
  • the obtained slab is subjected to a hot rolling process (heating temperature of 1200 ° C. and finish rolling end temperature of 800 ° C.), a cold rolling process (cumulative reduction rate of 80%), an annealing process, 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 coiling temperature condition I (less than 550 ° C.) in the hot rolling process and the annealing process condition II (dew point: -20 to 5 ° C., annealing temperature: 750 ° C. or higher, and holding time 30 to 200 seconds)
  • Table 2 shows the cases where the conditions are satisfied ( ⁇ ) and the cases where the conditions are not satisfied ( ⁇ , A or B). Specifically, the winding was performed at a temperature of 500.degree. C. in the example satisfying the condition I, and the winding was performed at a temperature of 650.degree.
  • the annealing process was performed under the conditions of a dew point of -5°C, an annealing temperature of 800°C, and a holding time of 150 seconds.
  • condition A dew point -40 ° C., annealing temperature 800 ° C. and holding time 60 seconds
  • condition B dew point 8 ° C., annealing temperature 800 ° C. and holding time 280 seconds
  • An annealing step was performed.
  • the surface of the obtained cold-rolled steel sheet was appropriately plated to form a hot-dip galvanized layer (GI), an alloyed hot-dip galvanized layer (GA), or an electro-galvanized layer (EG). Further, when the chemical composition of the sample taken from the produced cold-rolled steel plate was analyzed, there was no change from the chemical composition of the slab shown in Table 1. Finally, the obtained cold-rolled steel sheet or plated steel sheet was subjected to temper rolling. In each example, when the condition III of the skin pass rolling process (roll arithmetic mean roughness Ra: -1.3 ⁇ m or less and rolling reduction: 0.6% or less) is satisfied ( ⁇ ) and not satisfied ( ⁇ ) Each is shown in Table 2.
  • skin pass rolling is performed at a rolling reduction of 0.5% using rolls having an arithmetic mean roughness Ra of 1.2 ⁇ m, while in an example that does not satisfy Condition III, , and rolls having an arithmetic mean roughness Ra of 1.8 ⁇ m were used to perform skin pass rolling at a rolling reduction of 1.0%.
  • the properties of the obtained steel sheets were measured and evaluated by the following methods.
  • [Str and ⁇ Str] Str and ⁇ Str were determined as follows. First, a No. 5 tensile test piece of JIS Z2241: 2011 with a direction (C direction) perpendicular to the rolling direction (L direction) as the test direction is taken from a position at least 100 mm away from the end surface of the steel plate, and then the sampled steel plate. Three-dimensional image analysis is performed using a VK-X250/150 shape analysis laser microscope manufactured by Keyence Corporation on the surface of the sample (the surface of the coating layer if there is a coating layer on the surface of the steel plate sample), and JIS Initial Str was determined according to the provisions of B0681-2:2018. The target area for three-dimensional image analysis was 10 mm (C direction) ⁇ 2 mm (L direction).
  • Str after applying 5% tensile strain is determined by performing the same measurement as above, and finally the initial Str ⁇ Str was determined by subtracting Str after 5% tensile strain was applied from .
  • Tensile strength was measured by taking a No. 5 tensile test piece of JIS Z2241:2011 from a steel plate with the longitudinal direction perpendicular to the rolling direction and performing a tensile test in accordance with JIS Z2241:2011.
  • the appearance after molding was evaluated by the degree of ghost lines generated on the surface of the door outer after molding.
  • the surface after press molding was ground with a grindstone, and the striped patterns at intervals of several millimeters on the surface were judged to be ghost lines, and were rated on a scale of 1 to 5 depending on the degree of occurrence of the striped pattern.
  • An arbitrary area of 100 mm x 100 mm was visually checked, and the case where no streak pattern was confirmed was rated as "1", and the case where the maximum length of the streak pattern was 20 mm or less was rated as "2", and the maximum length of the streak pattern.
  • a steel sheet with a tensile strength of 500 MPa or more and a post-forming appearance evaluation of 3 or less was evaluated as a high-strength steel sheet with an improved post-forming appearance.
  • Table 2 shows the results.
  • the hard phase included or was at least one of martensite, bainite, tempered martensite and pearlite.
  • the area ratio of retained austenite was less than 1% in all the examples.
  • Comparative Example 4 the roll roughness in the temper rolling process was rough and the rolling reduction was high, so the roll roughness was strongly transferred to the steel plate surface, and the initial Str was 0.75. became super. As a result, the appearance after molding deteriorated. In Comparative Example 5, none of the manufacturing conditions I to III were satisfied, and the initial Str exceeded 0.75. As a result, the appearance after molding deteriorated. In Comparative Example 11, the coiling temperature in the hot rolling process was high, which probably promoted the formation of internal oxides such as Mn in the surface layer of the hot rolled steel sheet. As a result, the decarburization treatment in the annealing process could not be made appropriate, the initial Str was out of the desired range, and the appearance after molding deteriorated.
  • Comparative Examples 21 and 22 the C or Mn content was high, and diffusion of Mn during solidification during slab casting was inhibited, and Mn segregation could not be sufficiently suppressed. As a result, ⁇ Str could not be suppressed to 0.15 or less, and the appearance after molding deteriorated.
  • Comparative Example 23 since the Si content was high, it is considered that the formation of internal oxides was promoted in the surface layer of the hot-rolled steel sheet during the hot-rolling process. As a result, the decarburization treatment in the annealing process could not be made appropriate, the initial Str was out of the desired range, and the appearance after molding deteriorated. In Comparative Examples 24 and 25, sufficient strength was not obtained due to the low C or Mn content.
  • Inventive Examples 1-3, 6-10, 14-17, 19, 20 and 26-29 have a predetermined chemical composition and metallographic structure, especially the initial Str and tensile strength of the steel plate surface.

Landscapes

  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Sheet Steel (AREA)

Abstract

Provided is a steel sheet that has a chemical composition including 0.020-0.100% of C, 1.00-2.50% of Mn, not more than 0.100% of P, not more than 0.0200% of S, 0.005-0.700% Al, not more than 0.0150% of N, not more than 0.0100% of O, etc., the remaining portion being Fe and impurities, that has a metal structure including, in area%, 70-97% of ferrite and 3-30% of a hard phase, and in which the Str of the surface is 0.35-0.75 and a difference ΔStr between the Str and an Str obtained after the steel sheet is imparted with a 5%-tensile strain is not more than 0.15.

Description

鋼板steel plate
　本発明は、鋼板に関する。 The present invention relates to steel sheets.
　自動車業界では、燃費向上の観点から車体の軽量化が求められている。車体の軽量化と衝突安全性を両立するためには、使用する鋼板の高強度化が有効な方法の一つであり、このような背景から高強度鋼板の開発が進められている。 In the automotive industry, there is a demand for lighter vehicle bodies from the perspective of improving fuel efficiency. Increasing the strength of the steel plate used is one of the effective ways to achieve both weight reduction of the vehicle body and collision safety.
　これに関連して、特許文献１では、基板とした鋼板の表面に溶融亜鉛めっき層を有する溶融亜鉛めっき鋼板であって、前記基板が、ｍａｓｓ％で、Ｃ：０．０２～０．２０％、Ｓｉ：０．７％以下、Ｍｎ：１．５～３．５％、Ｐ：０．１０％以下、Ｓ：０．０１％以下、Ａｌ：０．１～１．０％、Ｎ：０．０１０％以下、Ｃｒ：０．０３～０．５％を含有し、かつ、Ａｌ、Ｃｒ、Ｓｉ、Ｍｎの含有量を同号項とした数式：Ａ＝４００Ａｌ／（４Ｃｒ＋３Ｓｉ＋６Ｍｎ）で定義された焼鈍時表面酸化指数Ａが２．３以上であり、残部がＦｅおよび不可避的不純物からなり、さらに、前記基板の組織が、フェライトおよび第２相からなり、該第２相がマルテンサイト主体のものであることを特徴とする高強度溶融亜鉛めっき鋼板が記載されている。また、特許文献１では、当該高強度溶融亜鉛めっき鋼板は、主にメンバー、ロッカー等の自動車の構造部品としての使途に好適な、優れた表面品質と５９０ＭＰａ以上の引張強度とを有することが記載されている。 In relation to this, Patent Document 1 discloses a hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface of a steel sheet used as a substrate, wherein the substrate has a mass% of C: 0.02 to 0.20%. , Si: 0.7% or less, Mn: 1.5 to 3.5%, P: 0.10% or less, S: 0.01% or less, Al: 0.1 to 1.0%, N: 0 .010% or less, Cr: 0.03 to 0.5%, and the contents of Al, Cr, Si, and Mn are the same items: A = 400Al / (4Cr + 3Si + 6Mn) The surface oxidation index A during annealing is 2.3 or more, the balance is composed of Fe and unavoidable impurities, and the structure of the substrate is composed of ferrite and a second phase, and the second phase is mainly martensite. A high-strength hot-dip galvanized steel sheet characterized by is described. In addition, Patent Document 1 describes that the high-strength hot-dip galvanized steel sheet has excellent surface quality and a tensile strength of 590 MPa or more, which are suitable for use mainly as automotive structural parts such as members and lockers. It is
特開２００５－２２０４３０号公報Japanese Patent Application Laid-Open No. 2005-220430
　近年、さらなる燃費向上の要求に関連して、特許文献１において記載されるメンバー等の構造部品だけでなく、ルーフ、フード、フェンダー及びドア等の外板部品についても軽量化のニーズが高まっている。これらの外板部品は、上記のような構造部品とは異なり、人目に触れるため、強度等の特性だけでなく、意匠性や面品質も重要であり、したがって成形後の外観に優れることが求められる。一方で、このような軽量化の要求に関連して、これらの外板部品に用いられる鋼板においてもさらなる高強度化や薄肉化が求められている。加えて、これらの外板部品における形状の複雑化に伴い、成形後の鋼板表面は凹凸が生じやすくなる傾向にあり、このような凹凸が生じた場合には外観が低下するという問題がある。 In recent years, in connection with the demand for further improvement in fuel efficiency, there is an increasing need for weight reduction not only for structural parts such as the members described in Patent Document 1, but also for outer panel parts such as roofs, hoods, fenders, and doors. . Unlike the structural parts described above, these outer panel parts are visible to the public, so not only characteristics such as strength, but also design and surface quality are important. Therefore, excellent appearance after molding is required. be done. On the other hand, in connection with such a demand for weight reduction, steel sheets used for these outer panel parts are also required to have higher strength and thinner thickness. In addition, as the shape of these outer panel parts becomes more complicated, the surface of the steel sheet after forming tends to become uneven, and when such unevenness occurs, there is a problem that the appearance deteriorates.
　より具体的には、例えば、特許文献１に記載されるような軟質のフェライトとマルテンサイトを主体とする硬質の第２相とからなるＤＰ鋼（複合組織鋼）の場合には、プレス成形などの加工時にフェライトからなる軟質相及びその周辺が優先的に変形する不均一変形が起こりやすい。このため、このような軟質相と硬質相から構成される複合組織鋼を利用した場合には、成形後の鋼板表面に微小な凹凸が生じることで、ゴーストラインと呼ばれる外観不良が発生することがある。 More specifically, for example, in the case of DP steel (composite structure steel) composed of soft ferrite and a hard second phase mainly composed of martensite as described in Patent Document 1, press forming etc. Non-uniform deformation tends to occur in which the soft phase composed of ferrite and its surroundings are preferentially deformed during processing. For this reason, when using a composite structure steel composed of such a soft phase and a hard phase, fine unevenness occurs on the surface of the steel sheet after forming, which may cause appearance defects called ghost lines. be.
　そこで、本発明は、新規な構成により、改善された成形後外観を有する高強度鋼板を提供することを目的とする。 Therefore, an object of the present invention is to provide a high-strength steel sheet having an improved post-forming appearance through a novel configuration.
　本発明者らは、上記目的を達成するために、特に鋼板の表面性状に着目して検討を行った。その結果、本発明者らは、フェライトからなる軟質相とマルテンサイト等を主体とする硬質相の複合組織で構成されるＤＰ鋼からなる鋼板において、表面性状のアスペクト比（Ｓｔｒ）がゴーストラインの発生に大きく影響することを見出し、これに関連して、初期のＳｔｒを特定の範囲内に制御しつつ、引張歪を付与したときのＳｔｒの変動を所定の範囲内に抑えることで、マルテンサイト等を主体とする硬質相に基づく高強度を維持しつつ、プレス成形等の加工時においても、鋼板表面におけるゴーストラインの発生を顕著に抑制することができることを見出し、本発明を完成させた。 In order to achieve the above objectives, the present inventors conducted studies with a particular focus on the surface properties of steel sheets. As a result, the present inventors found that in a steel plate made of DP steel composed of a composite structure of a soft phase made of ferrite and a hard phase mainly composed of martensite, etc., the aspect ratio (Str) of the surface texture is a ghost line. In relation to this, while controlling the initial Str within a specific range, by suppressing the variation of Str when applying tensile strain within a predetermined range, martensite The present inventors have found that the generation of ghost lines on the steel sheet surface can be remarkably suppressed even during processing such as press molding while maintaining high strength based on the hard phase mainly composed of, etc., and completed the present invention.
　上記目的を達成し得た本発明は下記のとおりである。
　（１）化学組成が、質量％で、
　Ｃ　：０．０２０～０．１００％、
　Ｍｎ：１．００～２．５０％、
　Ｐ　：０．１００％以下、
　Ｓ　：０．０２００％以下、
　Ａｌ：０．００５～０．７００％、
　Ｎ　：０．０１５０％以下、
　Ｏ　：０．０１００％以下、
　Ｓｉ：０～１．５００％、
　Ｃｒ：０～０．８０％、
　Ｍｏ：０～０．５０％、
　Ｂ　：０～０．０１００％、
　Ｔｉ：０～０．１００％、
　Ｎｂ：０～０．０６０％、
　Ｖ　：０～０．５０％、
　Ｎｉ：０～１．００％、
　Ｃｕ：０～１．００％、
　Ｗ　：０～１．００％、
　Ｓｎ：０～１．００％、
　Ｓｂ：０～０．２００％、
　Ｃａ：０～０．０１００％、
　Ｍｇ：０～０．０１００％、
　Ｚｒ：０～０．０１００％、
　ＲＥＭ：０～０．０１００％、並びに
　残部：Ｆｅ及び不純物であり、
　金属組織が、面積％で、
　フェライト：７０～９７％、及び
　硬質相：３～３０％であり、
　表面のＳｔｒが０．３５～０．７５であり、
　前記Ｓｔｒと５％引張歪付与後のＳｔｒとの差ΔＳｔｒが０．１５以下である、鋼板。
　（２）前記化学組成が、質量％で、
　Ｓｉ：０．００５～１．５００％、
　Ｃｒ：０．００１～０．８０％、
　Ｍｏ：０．００１～０．５０％、
　Ｂ　：０．０００１～０．０１００％、
　Ｔｉ：０．００１～０．１００％、
　Ｎｂ：０．００１～０．０６０％、
　Ｖ　：０．００１～０．５０％、
　Ｎｉ：０．００１～１．００％、
　Ｃｕ：０．００１～１．００％、
　Ｗ　：０．００１～１．００％、
　Ｓｎ：０．００１～１．００％、
　Ｓｂ：０．００１～０．２００％、
　Ｃａ：０．０００１～０．０１００％、
　Ｍｇ：０．０００１～０．０１００％、
　Ｚｒ：０．０００１～０．０１００％、及び
　ＲＥＭ：０．０００１～０．０１００％
からなる群から選択される１種又は２種以上を含む、上記（１）に記載の鋼板。
　（３）前記硬質相が、マルテンサイト、ベイナイト、焼き戻しマルテンサイト及びパーライトの少なくとも１種からなる、上記（１）又は（２）に記載の鋼板。 The present invention that has achieved the above object is as follows.
(1) chemical composition, in mass %,
C: 0.020 to 0.100%,
Mn: 1.00-2.50%,
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,
Si: 0 to 1.500%,
Cr: 0 to 0.80%,
Mo: 0-0.50%,
B: 0 to 0.0100%,
Ti: 0 to 0.100%,
Nb: 0 to 0.060%,
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: 0-0.0100%,
Zr: 0 to 0.0100%,
REM: 0 to 0.0100%, and the balance: Fe and impurities,
The metal structure, in area %,
ferrite: 70 to 97%, and hard phase: 3 to 30%,
Str of the surface is 0.35 to 0.75,
A steel sheet, wherein the difference ΔStr between the Str and the Str after application of 5% tensile strain is 0.15 or less.
(2) the chemical composition, in mass %,
Si: 0.005 to 1.500%,
Cr: 0.001 to 0.80%,
Mo: 0.001 to 0.50%,
B: 0.0001 to 0.0100%,
Ti: 0.001 to 0.100%,
Nb: 0.001 to 0.060%,
V: 0.001 to 0.50%,
Ni: 0.001 to 1.00%,
Cu: 0.001 to 1.00%,
W: 0.001 to 1.00%,
Sn: 0.001 to 1.00%,
Sb: 0.001 to 0.200%,
Ca: 0.0001 to 0.0100%,
Mg: 0.0001-0.0100%,
Zr: 0.0001-0.0100%, and REM: 0.0001-0.0100%
The steel sheet according to (1) above, comprising one or more selected from the group consisting of:
(3) The steel sheet according to (1) or (2) above, wherein the hard phase comprises at least one of martensite, bainite, tempered martensite and pearlite.
　本発明によれば、良好な成形前外観に加えて、改善された成形後外観を有する高強度鋼板を提供することができる。 According to the present invention, it is possible to provide a high-strength steel sheet having an improved appearance after forming in addition to a good appearance before forming.
＜鋼板＞
　本発明の実施形態に係る鋼板は、
　化学組成が、質量％で、
　Ｃ　：０．０２０～０．１００％、
　Ｍｎ：１．００～２．５０％、
　Ｐ　：０．１００％以下、
　Ｓ　：０．０２００％以下、
　Ａｌ：０．００５～０．７００％、
　Ｎ　：０．０１５０％以下、
　Ｏ　：０．０１００％以下、
　Ｓｉ：０～１．５００％、
　Ｃｒ：０～０．８０％、
　Ｍｏ：０～０．５０％、
　Ｂ　：０～０．０１００％、
　Ｔｉ：０～０．１００％、
　Ｎｂ：０～０．０６０％、
　Ｖ　：０～０．５０％、
　Ｎｉ：０～１．００％、
　Ｃｕ：０～１．００％、
　Ｗ　：０～１．００％、
　Ｓｎ：０～１．００％、
　Ｓｂ：０～０．２００％、
　Ｃａ：０～０．０１００％、
　Ｍｇ：０～０．０１００％、
　Ｚｒ：０～０．０１００％、
　ＲＥＭ：０～０．０１００％、並びに
　残部：Ｆｅ及び不純物であり、
　金属組織が、面積％で、
　フェライト：７０～９７％、及び
　硬質相：３～３０％であり、
　表面のＳｔｒが０．３５～０．７５であり、
　前記Ｓｔｒと５％引張歪付与後のＳｔｒとの差ΔＳｔｒが０．１５以下であることを特徴としている。 <Steel plate>
A steel plate according to an embodiment of the present invention is
The chemical composition, in mass %,
C: 0.020 to 0.100%,
Mn: 1.00-2.50%,
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,
Si: 0 to 1.500%,
Cr: 0 to 0.80%,
Mo: 0-0.50%,
B: 0 to 0.0100%,
Ti: 0 to 0.100%,
Nb: 0 to 0.060%,
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: 0-0.0100%,
Zr: 0 to 0.0100%,
REM: 0 to 0.0100%, and the balance: Fe and impurities,
The metal structure, in area %,
ferrite: 70 to 97%, and hard phase: 3 to 30%,
Str of the surface is 0.35 to 0.75,
The difference ΔStr between the Str and the Str after applying 5% tensile strain is 0.15 or less.
　ルーフやドア等の外板部品においては、プレス成形等の際に生じる面歪と呼ばれる面欠陥を回避する観点から、降伏強度が比較的低いＤＰ鋼が用いられる場合が多い。しかしながら、先に述べたとおり、フェライトからなる軟質相とマルテンサイト等を主体とする硬質相が混在するＤＰ鋼の場合、プレス成形などの加工時に軟質相及びその周辺が優先的に変形する不均一変形が起こりやすく、成形後の鋼板表面に微小な凹凸が生じることで、ゴーストラインと呼ばれる外観不良が発生することがある。より詳しく説明すると、プレス成形などの加工時には、フェライトからなる軟質相が凹む一方で、マルテンサイト等を主体とする硬質相は凹まないかむしろ凸となるように盛り上がって変形することで、ゴーストラインがバンド状（縞状）に生じることとなる。これに対し、従来技術においては、例えば、軟質相と硬質相を含む鋼板の内部組織をより適切なものとする観点から、このような成形後の外観不良を改善しようとする試みがなされている。一方で、ゴーストラインは外観不良であることから、鋼板の表面性状もゴーストラインの発生に大きく寄与することが考えられる。そこで、本発明者らは、今回、鋼板の内部組織よりはむしろ、鋼板の表面性状に特に着目して検討を行った。その結果、本発明者らは、フェライトからなる軟質相とマルテンサイト等を主体とする硬質相の複合組織で構成されるＤＰ鋼からなる鋼板において、表面性状のアスペクト比（Ｓｔｒ）がゴーストラインの発生に大きく影響することを見出した。 　For outer panel parts such as roofs and doors, DP steel, which has a relatively low yield strength, is often used from the viewpoint of avoiding surface defects called surface distortion that occur during press forming. However, as mentioned earlier, in the case of DP steel, which has a mixture of a soft phase composed of ferrite and a hard phase mainly composed of martensite, etc., the soft phase and its surroundings are preferentially deformed during processing such as press forming. Deformation is likely to occur, and minute unevenness may occur on the surface of the steel sheet after forming, resulting in appearance defects called ghost lines. In more detail, during processing such as press molding, the soft phase composed of ferrite is dented, while the hard phase mainly composed of martensite, etc. will occur in a band shape (stripe shape). On the other hand, in the prior art, for example, from the viewpoint of making the internal structure of a steel sheet containing a soft phase and a hard phase more appropriate, attempts have been made to improve such poor appearance after forming. . On the other hand, since ghost lines are a poor appearance, it is conceivable that the surface texture of the steel sheet also greatly contributes to the generation of ghost lines. Therefore, the inventors of the present invention have focused on the surface properties of the steel sheet rather than the internal structure of the steel sheet. As a result, the present inventors found that in a steel plate made of DP steel composed of a composite structure of a soft phase made of ferrite and a hard phase mainly composed of martensite, etc., the aspect ratio (Str) of the surface texture is a ghost line. found to have a significant effect on development.
　表面性状のテクスチャーのアスペクト比（Ｔｅｘｔｕｒｅ　ａｓｐｅｃｔ　ｒａｔｉｏ）であるＳｔｒは、ＪＩＳ　Ｂ０６８１－２：２０１８にて規定されている表面性状の空間パラメータ（Ｓｐａｔｉａｌ　ｐａｒａｍｅｔｒｅｓ）のひとつであり、表面の異方性の強さを示し、０～１の範囲の値をとることが知られている。一般的に、Ｓｔｒの値が０に近くなると、異方性が強くなり表面に筋目などが生じ、一方でＳｔｒの値が１に近くなると、表面が方向に依存せず等方的なものとなる。本発明者らは、検討の結果、初期（すなわちプレス成形等による成形前）の鋼板の表面（鋼板の表面にめっき層が存在する場合には、めっき層の表面）のＳｔｒを０．３５～０．７５の範囲内に制御することが、プレス成形等の成形によって歪が付与された場合に、鋼板表面におけるゴーストラインの発生を抑制する上で非常に有効であることを見出した。ゴーストラインは鋼板表面の筋模様と関連していることから、当該ゴーストラインの発生を抑制する観点からは、Ｓｔｒの値はより等方的であることが好ましく、それゆえ１により近い方が好ましいと一般的には予想される。例えば、Ｓｔｒがより低い値の場合、すなわちＳｔｒが０（ゼロ）により近く、それゆえ表面の異方性がより強く鋼板表面に筋目が目立つ場合には、当然ながらプレス成形等による成形後の外観は劣化したものとなる。したがって、成形後外観を改善するためには、Ｓｔｒの値は低すぎないことが好ましく、特定の値以上の値とすることが好ましい。このような観点から、本発明者らは、初期の鋼板表面のＳｔｒを０．３５以上とする必要があることを見出した。一方で、本発明者らの実験結果によれば、Ｓｔｒが１により近くなると、プレス成形後にゴーストラインの発生が顕著となり、それゆえ成形後外観が劣化する場合があることがわかった。したがって、ゴーストラインの発生を抑制する観点からは、初期のＳｔｒは適度な範囲に制御する必要があり、具体的には０．３５～０．７５の範囲内に制御する必要がある。初期のＳｔｒをこのような適度な範囲内に制御することで、成形前と成形後の両方の外観を良好な状態に維持することが可能となる。 Str, which is the texture aspect ratio of the surface texture, is one of the spatial parameters of the surface texture specified in JIS B0681-2:2018, and indicates the strength of the anisotropy of the surface. and is known to take values in the range of 0 to 1. In general, when the value of Str is close to 0, the anisotropy becomes strong and lines appear on the surface. Become. As a result of investigation, the present inventors found that the initial (that is, before forming by press forming or the like) surface of the steel sheet (if a coating layer exists on the surface of the steel sheet, the surface of the coating layer) Str of 0.35 ~ It has been found that control within the range of 0.75 is very effective in suppressing the generation of ghost lines on the surface of the steel sheet when strain is imparted by forming such as press forming. Since ghost lines are associated with streak patterns on the surface of the steel sheet, from the viewpoint of suppressing the occurrence of the ghost lines, it is preferable that the value of Str is more isotropic, and therefore closer to 1 is preferable. is generally expected. For example, when Str is a lower value, that is, when Str is closer to 0 (zero), and therefore the surface anisotropy is stronger and streaks are more noticeable on the surface of the steel sheet, the appearance after forming by press forming etc. becomes deteriorated. Therefore, in order to improve the appearance after molding, it is preferable that the value of Str is not too low, and that it is set to a value equal to or higher than a specific value. From this point of view, the present inventors found that the initial Str of the surface of the steel sheet should be 0.35 or more. On the other hand, according to the experimental results of the present inventors, it was found that when Str is closer to 1, ghost lines are more pronounced after press molding, and therefore the appearance after molding may be deteriorated. Therefore, from the viewpoint of suppressing the generation of ghost lines, the initial Str must be controlled within an appropriate range, specifically within the range of 0.35 to 0.75. By controlling the initial Str within such an appropriate range, it is possible to maintain good appearance both before and after molding.
　何ら特定の理論に束縛されることを意図するものではないが、例えば、Ｓｔｒの値が１により近い場合には、当初のより等方的な表面性状との相対的な関係で、成形後に鋼板表面に生じる微小な凹凸の存在が目立ちすぎてしまい、外観が劣化するものと考えられる。一方で、Ｓｔｒが適度な値であり、すなわち肉眼では目立たない程度の筋目などが存在する場合には、当初から存在する筋目の影響で成形後に鋼板表面に生じる微小な凹凸が目立たないか又は当初から存在する筋目と成形後に鋼板表面に生じる微小な凹凸とが打ち消しあうなどして、結果的にゴーストラインの発生が抑制又は低減されるものと考えられる。いずれにしても、ゴーストラインの発生を抑制するためには、Ｓｔｒの値は１により近い方が好ましいと一般的に予想されることから、このような一般的な予想に反して、初期のＳｔｒを０．３５～０．７５の範囲内に制御することで、成形後のゴーストラインの発生が抑制又は低減されるという事実は極めて意外であり、また驚くべきことである。 While not intending to be bound by any particular theory, for example, if the value of Str is closer to 1, the steel sheet after forming, relative to the initial more isotropic surface texture. It is thought that the presence of fine unevenness on the surface becomes too conspicuous, degrading the appearance. On the other hand, if Str is a moderate value, that is, if there are streaks that are not noticeable to the naked eye, fine unevenness that occurs on the surface of the steel sheet after forming will not be noticeable due to the effects of the streaks that are present from the beginning. It is thought that the streaks present in the hollow and the fine irregularities formed on the surface of the steel sheet after forming cancel each other out, and as a result, the occurrence of ghost lines is suppressed or reduced. In any case, it is generally expected that the value of Str should be closer to 1 in order to suppress the occurrence of ghost lines. The fact that the occurrence of ghost lines after molding is suppressed or reduced by controlling the is within the range of 0.35 to 0.75 is extremely unexpected and surprising.
　鋼板の製造方法に関連して後で詳しく説明するが、本発明者らは、初期のＳｔｒを所望の範囲内に制御するためには、焼鈍工程における鋼板表層の脱炭処理を適切なものとすることが有効であり、またこのような適切な脱炭処理を実現するためには、熱間圧延工程の際に鋼板の表層に形成されるＳｉ及びＭｎ等の内部酸化物をできる限り低減することが好ましいことを見出した。より具体的には、本発明者らは、鋼板中のＳｉ及びＭｎ含有量の上限値をそれぞれ１．５００％及び２．５０％以下に抑制しつつ、とりわけ熱間圧延工程の巻き取り温度並びに焼鈍工程の雰囲気、温度及び時間を所定の範囲内に制御することで、熱間圧延工程における内部酸化物の形成を十分に抑制又は低減するとともに、これに関連して焼鈍工程において適切な脱炭処理を実現し、ひいては鋼板表面において所望の初期Ｓｔｒを実現することができることを見出した。 As will be described in detail later in relation to the steel sheet manufacturing method, the present inventors considered that the decarburization treatment of the surface layer of the steel sheet in the annealing process was appropriate in order to control the initial Str within a desired range. In order to realize such an appropriate decarburization treatment, internal oxides such as Si and Mn formed on the surface layer of the steel sheet during the hot rolling process should be reduced as much as possible. I have found that it is preferable. More specifically, the present inventors suppressed the upper limits of the Si and Mn contents in the steel sheet to 1.500% and 2.50% or less, respectively, while maintaining the coiling temperature and By controlling the atmosphere, temperature and time of the annealing process within predetermined ranges, the formation of internal oxides during the hot rolling process can be sufficiently suppressed or reduced, and related to this, appropriate decarburization during the annealing process. It has been found that the treatment can be achieved and thus the desired initial Str at the surface of the steel sheet.
　本発明者らは、さらなる検討により、単に初期の鋼板表面のＳｔｒを０．３５～０．７５の範囲内に制御しただけでは、ゴーストラインの発生を抑制又は低減する観点からは必ずしも十分ではなく、初期のＳｔｒの制御に加えて、引張歪を付与したときのＳｔｒの変動、すなわち初期のＳｔｒと５％引張歪付与後のＳｔｒとの差ΔＳｔｒを所定の範囲内に抑えることが重要であることを見出した。より具体的には、本発明者らは、初期のＳｔｒを０．３５～０．７５の範囲内に制御することに加えて、当該初期のＳｔｒから５％引張歪を付与した後のＳｔｒを引き算した値ΔＳｔｒ（つまり、ΔＳｔｒ＝初期Ｓｔｒ－５％引張歪付与後Ｓｔｒ）を０．１５以下に制御することが重要であることを見出した。 Through further studies, the present inventors have found that simply controlling the initial Str of the steel plate surface within the range of 0.35 to 0.75 is not necessarily sufficient from the viewpoint of suppressing or reducing the occurrence of ghost lines. , In addition to controlling the initial Str, it is important to suppress the variation of Str when applying tensile strain, that is, the difference ΔStr between the initial Str and Str after applying 5% tensile strain, within a predetermined range. I found out. More specifically, the present inventors, in addition to controlling the initial Str within the range of 0.35 to 0.75, the Str after applying 5% tensile strain from the initial Str It was found that it is important to control the subtracted value ΔStr (that is, ΔStr = initial Str - Str after applying 5% tensile strain) to 0.15 or less.
　何ら特定の理論に束縛されることを意図するものではないが、ΔＳｔｒは、初期のＳｔｒの値に加えて、鋼中のＭｎ偏析とも密接に関連していると考えられ、初期のＳｔｒを０．３５～０．７５の範囲内に制御することに加えて鋼中のＭｎ偏析を低減することで、ΔＳｔｒの値をより小さくすることが可能になるものと考えられる。ゴーストラインは、金属組織中に縞状に連結した硬質相が存在することでその程度が顕著になると考えられ、一方で、このような縞状硬質相の生成を抑制するためには、鋼中のＭｎ偏析を低減することが有効であると考えられる。Ｍｎ偏析は、鋼板中に含まれる種々の合金元素によって影響を受け、とりわけ鋼板中のＣ及びＭｎ含有量が高くなると、その度合いが特に顕著なものとなる。これは、Ｃ及びＭｎ含有量が高くなると、スラブ鋳造の際の凝固時におけるＭｎの拡散速度が低下することに起因している。したがって、初期の鋼板表面のＳｔｒを０．３５～０．７５の範囲内に制御しつつ、Ｍｎ偏析に関連する鋼板の化学組成を適切なものとすること、とりわけ鋼板中のＣ及びＭｎ含有量を適切なものとすることで、ΔＳｔｒを所望の範囲内すなわち０．１５以下に抑えることが可能となる。逆に言うと、ΔＳｔｒが０．１５以下に抑制されている場合には、鋼中のＭｎ偏析が十分に抑制又は低減されていると考えられ、これに関連して鋼板の金属組織において縞状硬質相の生成が十分に抑制されているために、プレス成形等の加工時においても、鋼板表面におけるゴーストラインの発生を顕著に抑制又は低減することが可能となる。 Although not intending to be bound by any particular theory, ΔStr is considered to be closely related to the Mn segregation in the steel in addition to the initial Str value, and the initial Str is 0 It is considered that the value of ΔStr can be made smaller by controlling the Mn segregation in the steel in addition to controlling it within the range of 0.35 to 0.75. Ghost lines are considered to be conspicuous due to the presence of hard phases connected in a striped pattern in the metal structure. It is considered effective to reduce the Mn segregation of Mn segregation is affected by various alloying elements contained in the steel sheet, and its degree becomes particularly pronounced when the C and Mn contents in the steel sheet become high. This is due to the fact that the higher the C and Mn contents, the lower the diffusion rate of Mn during solidification during slab casting. Therefore, while controlling the initial Str of the steel sheet surface within the range of 0.35 to 0.75, the chemical composition of the steel sheet related to Mn segregation is appropriate, especially the C and Mn contents in the steel sheet is appropriate, ΔStr can be suppressed within a desired range, that is, 0.15 or less. Conversely, when ΔStr is suppressed to 0.15 or less, Mn segregation in the steel is considered to be sufficiently suppressed or reduced. Since the formation of the hard phase is sufficiently suppressed, it is possible to remarkably suppress or reduce the generation of ghost lines on the steel sheet surface even during processing such as press forming.
　したがって、本発明の実施形態に係る鋼板によれば、鋼板中に含まれる硬質相に基づいて高強度を十分に維持しつつ、初期のＳｔｒを０．３５～０．７５の範囲内に制御することに加えて、５％引張歪を付与したときのΔＳｔｒを０．１５以下に抑えることにより、成形前の外観を良好に維持できることはもちろんのこと、プレス成形等の成形によって歪が付与された場合においても、鋼板表面におけるゴーストライン等の外観不良の発生を顕著に抑制することが可能となる。それゆえ、本発明の実施形態によれば、良好な成形前外観に加えて、改善された成形後外観を有する高強度鋼板を提供することが可能となる。 Therefore, according to the steel sheet according to the embodiment of the present invention, the initial Str is controlled within the range of 0.35 to 0.75 while sufficiently maintaining high strength based on the hard phase contained in the steel sheet. In addition, by suppressing ΔStr to 0.15 or less when a 5% tensile strain is applied, not only can the appearance before molding be maintained well, but strain is applied by molding such as press molding. Even in this case, it is possible to remarkably suppress the occurrence of appearance defects such as ghost lines on the surface of the steel sheet. Therefore, according to embodiments of the present invention, it is possible to provide a high-strength steel sheet having improved post-forming appearance in addition to good pre-forming appearance.
　以下、本発明の実施形態に係る鋼板についてより詳しく説明する。以下の説明において、各元素の含有量の単位である「％」は、特に断りがない限り「質量％」を意味するものである。また、本明細書において、数値範囲を示す「～」とは、特に断りがない場合、その前後に記載される数値を下限値および上限値として含む意味で使用される。 The steel sheets according to the embodiments of the present invention will be described in more detail below. In the following description, the unit of content of each element, "%", means "% by mass" unless otherwise specified. In addition, in this specification, the term "to" indicating a numerical range is used to include the numerical values before and after it as lower and upper limits, unless otherwise specified.
［Ｃ：０．０２０～０．１００％］
　Ｃは、鋼板の強度を高める元素である。このような効果を十分に得るために、Ｃ含有量は０．０２０％以上とする。Ｃ含有量は０．０２５％以上、０．０３０％以上、０．０３５％以上、０．０４０％以上又は０．０５０％以上であってもよい。一方で、Ｃを過度に含有すると、凝固時のＭｎの拡散が阻害され、Ｍｎ偏析を十分に抑制することができず、ΔＳｔｒを所望の範囲内に制御することができない場合がある。したがって、Ｃ含有量は０．１００％以下とする。Ｃ含有量は０．０９５％以下、０．０９０％以下、０．０８５％以下、０．０８０％以下又は０．０７０％以下であってもよい。 [C: 0.020 to 0.100%]
C is an element that increases the strength of the steel sheet. In order to sufficiently obtain such effects, the C content is made 0.020% or more. The C content may be 0.025% or more, 0.030% or more, 0.035% or more, 0.040% or more, or 0.050% or more. On the other hand, if C is contained excessively, diffusion of Mn during solidification is inhibited, Mn segregation cannot be sufficiently suppressed, and ΔStr may not be controlled within a desired range. Therefore, the C content should be 0.100% or less. The C content may be 0.095% or less, 0.090% or less, 0.085% or less, 0.080% or less, or 0.070% or less.
［Ｍｎ：１．００～２．５０％］
　Ｍｎは、鋼の焼入れ性を高めて、強度の向上に寄与する元素である。このような効果を十分に得るために、Ｍｎ含有量は１．００％以上とする。Ｍｎ含有量は１．２０％以上、１．３０％以上、１．４０％以上又は１．５０％以上であってもよい。一方で、Ｍｎを過度に含有すると、凝固時のＭｎの拡散が阻害され、Ｍｎ偏析を十分に抑制することができず、ΔＳｔｒを所望の範囲内に制御することができない場合がある。加えて、Ｍｎを過度に含有すると、熱間圧延の際に内部酸化物が過度に形成して、その後の焼鈍工程の際の鋼板表層の脱炭処理を適切に制御することができず、初期のＳｔｒを所望の範囲内に制御することができない場合がある。したがって、Ｍｎ含有量は２．５０％以下とする。Ｍｎ含有量は２．２５％以下、２．１０％以下、２．００％以下、１．８５％以下又は１．７５％以下であってもよい。 [Mn: 1.00 to 2.50%]
Mn is an element that enhances the hardenability of steel and contributes to the improvement of strength. In order to sufficiently obtain such effects, the Mn content is set to 1.00% or more. The Mn content may be 1.20% or more, 1.30% or more, 1.40% or more, or 1.50% or more. On the other hand, if Mn is excessively contained, the diffusion of Mn during solidification is inhibited, Mn segregation cannot be sufficiently suppressed, and ΔStr may not be controlled within the desired range. In addition, if Mn is excessively contained, internal oxides are excessively formed during hot rolling, and the decarburization treatment of the steel sheet surface layer during the subsequent annealing process cannot be appropriately controlled. It may not be possible to control Str within a desired range. Therefore, the Mn content should be 2.50% or less. The Mn content may be 2.25% or less, 2.10% or less, 2.00% or less, 1.85% or less, or 1.75% or less.
［Ｐ：０．１００％以下］
　Ｐは、製造工程で混入する元素である。Ｐ含有量は０％であってもよい。しかしながら、Ｐ含有量を０．０００１％未満に低減するためには精錬に時間を要し、生産性の低下を招く。したがって、Ｐ含有量は０．０００１％以上、０．０００５％以上、０．００１％以上又は０．００５％以上であってもよい。一方で、Ｐを過度に含有すると、鋼板の靭性が低下する場合がある。したがって、Ｐ含有量は０．１００％以下とする。Ｐ含有量は０．０７０％以下、０．０６０％以下、０．０４０％以下又は０．０２０％以下であってもよい。 [P: 0.100% or less]
P is an element mixed in during the manufacturing process. The P content may be 0%. However, in order to reduce the P content to less than 0.0001%, refining takes time, resulting in a decrease in productivity. Therefore, the P content may be 0.0001% or more, 0.0005% or more, 0.001% or more, or 0.005% or more. On the other hand, an excessive P content may lower the toughness of the steel sheet. Therefore, the P content should be 0.100% or less. The P content may be 0.070% or less, 0.060% or less, 0.040% or less, or 0.020% or less.
［Ｓ：０．０２００％以下］
　Ｓは、製造工程で混入する元素である。Ｓ含有量は０％であってもよい。しかしながら、Ｓ含有量を０．０００１％未満に低減するためには精錬に時間を要し、生産性の低下を招く。したがって、Ｓ含有量は０．０００１％以上、０．０００５％以上又は０．００１０％以上であってもよい。一方で、Ｓを過度に含有すると、Ｍｎ硫化物を形成し、鋼板の延性、穴広げ性、伸びフランジ性及び／又は曲げ性などの成形性を低下させる場合がある。したがって、Ｓ含有量は０．０２００％以下とする。Ｓ含有量は０．０１００％以下、０．００６０％以下又は０．００４０％以下であってもよい。 [S: 0.0200% or less]
S is an element mixed in during the manufacturing process. The S content may be 0%. However, in order to reduce the S content to less than 0.0001%, refining takes time, resulting in a decrease in productivity. Therefore, the S content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, when S is contained excessively, Mn sulfide is formed, and formability such as ductility, hole expansibility, stretch flangeability and/or bendability of the steel sheet may be deteriorated. Therefore, the S content should be 0.0200% or less. The S content may be 0.0100% or less, 0.0060% or less, or 0.0040% or less.
［Ａｌ：０．００５～０．７００％］
　Ａｌは、脱酸剤として機能する元素であり、鋼の強度を高めるのに有効な元素である。また、Ａｌは、凝固時のＭｎの拡散を促進させてＭｎ偏析を低減するのに有効な元素でもある。これらの効果を十分に得るために、Ａｌ含有量は０．００５％以上とする。Ａｌ含有量は０．０１０％以上、０．０２０％以上又は０．０２５％以上であってもよい。一方で、Ａｌを過度に含有すると、鋳造性が悪化して生産性が低下する場合がある。したがって、Ａｌ含有量は０．７００％以下とする。Ａｌ含有量は０．６００％以下、０．４００％以下、０．３００％以下、０．１５０％以下、０．１００％以下又は０．０７０％以下であってもよい。 [Al: 0.005 to 0.700%]
Al is an element that functions as a deoxidizing agent and is an effective element for increasing the strength of steel. Al is also an effective element for promoting the diffusion of Mn during solidification and reducing Mn segregation. In order to sufficiently obtain these effects, the Al content should be 0.005% or more. The Al content may be 0.010% or more, 0.020% or more, or 0.025% or more. On the other hand, if Al is contained excessively, the castability may deteriorate and the productivity may decrease. Therefore, the Al content should be 0.700% or less. The Al content may be 0.600% or less, 0.400% or less, 0.300% or less, 0.150% or less, 0.100% or less, or 0.070% or less.
［Ｎ：０．０１５０％以下］
　Ｎは、製造工程で混入する元素である。Ｎ含有量は０％であってもよい。しかしながら、Ｎ含有量を０．０００１％未満に低減するためには精錬に時間を要し、生産性の低下を招く。したがって、Ｎ含有量は０．０００１％以上、０．０００５％以上又は０．００１０％以上であってもよい。一方で、Ｎを過度に含有すると、窒化物が形成し、鋼板の延性、穴広げ性、伸びフランジ性及び／又は曲げ性などの成形性が低下する場合がある。したがって、Ｎ含有量は０．０１５０％以下とする。Ｎ含有量は０．０１００％以下、０．００８０％以下又は０．００５０％以下であってもよい。 [N: 0.0150% or less]
N is an element mixed in during the manufacturing process. The N content may be 0%. However, in order to reduce the N content to less than 0.0001%, refining takes time, resulting in a decrease in productivity. Therefore, the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, when N is excessively contained, nitrides are formed, and formability such as ductility, hole expansibility, stretch flangeability and/or bendability of the steel sheet may be deteriorated. Therefore, the N content should be 0.0150% or less. The N content may be 0.0100% or less, 0.0080% or less, or 0.0050% or less.
［Ｏ：０．０１００％以下］
　Ｏは、製造工程で混入する元素である。Ｏ含有量は０％であってもよい。しかしながら、Ｏ含有量を０．０００１％未満に低減するためには精錬に時間を要し、生産性の低下を招く。したがって、Ｏ含有量は０．０００１％以上、０．０００５％以上又は０．００１０％以上であってもよい。一方で、Ｏを過度に含有すると、粗大な酸化物が形成し、鋼板の延性、穴広げ性、伸びフランジ性及び／又は曲げ性などの成形性が低下する場合がある。したがって、Ｏ含有量は０．０１００％以下とする。Ｏ含有量は０．００７０％以下、０．００４０％以下、０．００４０％以下又は０．００２０％以下であってもよい。 [O: 0.0100% or less]
O is an element mixed in during the manufacturing process. The O content may be 0%. However, in order to reduce the O content to less than 0.0001%, refining takes time, resulting in a decrease in productivity. Therefore, the O content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, when O is excessively contained, coarse oxides are formed, and formability such as ductility, hole expansibility, stretch flangeability and/or bendability of the steel sheet may be deteriorated. Therefore, the O content should be 0.0100% or less. The O content may be 0.0070% or less, 0.0040% or less, 0.0040% or less, or 0.0020% or less.
　本発明の実施形態に係る鋼板の基本化学組成は上記のとおりである。さらに、当該鋼板は、必要に応じて、残部のＦｅの一部に替えて以下の任意選択元素のうち１種又は２種以上を含有してもよい。以下、これらの任意選択元素について詳しく説明する。これらの任意選択元素の含有量の下限は、すべて０％である。 The basic chemical composition of the steel sheet according to the embodiment of the present invention is as described above. Furthermore, the steel sheet may contain one or more of the following optional elements in place of part of the remaining Fe, if necessary. These optional elements are described in detail below. The lower limits of the contents of these optional elements are all 0%.
［Ｓｉ：０～１．５００％］
　Ｓｉは、鋼の脱酸元素であり、鋼板の延性を損なわずに強度を高めるのに有効な元素である。また、Ｓｉは、凝固時のＭｎの拡散を促進させてＭｎ偏析を低減するのに有効な元素でもある。Ｓｉ含有量は０％であってもよいが、これらの効果を得るためには、Ｓｉ含有量は０．００１％以上又は０．００５％以上であることが好ましい。Ｓｉ含有量は０．０１０％以上、０．０５０％以上、０．１００％以上又は０．１５０％以上であってもよい。一方で、Ｓｉを過度に含有すると、熱間圧延の際に内部酸化物が過度に形成して、その後の焼鈍工程の際の鋼板表層の脱炭処理を適切に制御することができず、初期のＳｔｒを所望の範囲内に制御することができない場合がある。したがって、Ｓｉ含有量は１．５００％以下とする。Ｓｉ含有量は１．４００％以下、１．２００％以下、１．０００％以下、０．８００％以下、０．６００％以下、０．５００％以下又は０．３００％以下であってもよい。 [Si: 0 to 1.500%]
Si is a deoxidizing element for steel, and is an effective element for increasing the strength without impairing the ductility of the steel sheet. In addition, Si is also an effective element for promoting diffusion of Mn during solidification and reducing Mn segregation. The Si content may be 0%, but in order to obtain these effects, the Si content is preferably 0.001% or more or 0.005% or more. The Si content may be 0.010% or more, 0.050% or more, 0.100% or more, or 0.150% or more. On the other hand, if Si is excessively contained, internal oxides are excessively formed during hot rolling, and the decarburization treatment of the steel sheet surface layer during the subsequent annealing process cannot be appropriately controlled. It may not be possible to control Str within a desired range. Therefore, the Si content should be 1.500% or less. The Si content may be 1.400% or less, 1.200% or less, 1.000% or less, 0.800% or less, 0.600% or less, 0.500% or less, or 0.300% or less .
［Ｃｒ：０～０．８０％］
　Ｃｒは、鋼の焼入れ性を高め、鋼板の強度の向上に寄与する元素である。また、Ｃｒは、凝固時のＭｎの拡散を促進させてＭｎ偏析を低減するのに有効な元素でもある。Ｃｒ含有量は０％であってもよいが、これらの効果を得るためには、Ｃｒ含有量は０．００１％以上であることが好ましい。Ｃｒ含有量は０．０１％以上、０．１０％以上、０．２０％以上又は０．３０％以上であってもよい。一方で、Ｃｒを過度に含有すると、破壊の起点となる粗大なＣｒ炭化物が形成する場合がある。したがって、Ｃｒ含有量は０．８０％以下であることが好ましい。Ｃｒ含有量は０．７０％以下、０．６０％以下又は０．５０％以下であってもよい。 [Cr: 0 to 0.80%]
Cr is an element that increases the hardenability of steel and contributes to the improvement of the strength of the steel sheet. Moreover, Cr is also an element effective in promoting diffusion of Mn during solidification and reducing Mn segregation. Although the Cr content may be 0%, the Cr content is preferably 0.001% or more in order to obtain these effects. The Cr content may be 0.01% or more, 0.10% or more, 0.20% or more, or 0.30% or more. On the other hand, if Cr is contained excessively, coarse Cr carbides may be formed, which serve as starting points for fracture. Therefore, 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：０～０．５０％］
　Ｍｏは、高温での相変態を抑制し、鋼板の強度の向上に寄与する元素である。また、Ｍｏは、凝固時のＭｎの拡散を促進させてＭｎ偏析を低減するのに有効な元素でもある。Ｍｏ含有量は０％であってもよいが、これらの効果を得るためには、Ｍｏ含有量は０．００１％以上であることが好ましい。Ｍｏ含有量は０．０１％以上、０．０５％以上又は０．０７％以上であってもよい。一方で、Ｍｏを過度に含有すると、熱間加工性が低下して生産性が低下する場合がある。したがって、Ｍｏ含有量は０．５０％以下であることが好ましい。Ｍｏ含有量は０．４０％以下、０．３０％以下又は０．２０％以下であってもよい。 [Mo: 0 to 0.50%]
Mo is an element that suppresses phase transformation at high temperatures and contributes to improvement in strength of the steel sheet. Mo is also an effective element for promoting the diffusion of Mn during solidification and reducing Mn segregation. The Mo content may be 0%, but in order to obtain these effects, the Mo content is preferably 0.001% or more. The Mo content may be 0.01% or more, 0.05% or more, or 0.07% or more. On the other hand, when Mo is contained excessively, the hot workability may deteriorate and the productivity may decrease. Therefore, 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：０～０．０１００％］
　Ｂは、高温での相変態を抑制し、鋼板の強度の向上に寄与する元素である。Ｂ含有量は０％であってもよいが、このような効果を得るためには、Ｂ含有量は０．０００１％以上であることが好ましい。Ｂ含有量は０．０００５％以上、０．００１０％以上又は０．００１５％以上であってもよい。一方で、Ｂを過度に含有すると、Ｂ析出物が生成して鋼板の強度が低下する場合がある。したがって、Ｂ含有量は０．０１００％以下であることが好ましい。Ｂ含有量は０．００８０％以下、０．００６０％以下又は０．００３０％以下であってもよい。 [B: 0 to 0.0100%]
B is an element that suppresses phase transformation at high temperatures and contributes to improvement in strength of the steel sheet. Although the B content may be 0%, the B content is preferably 0.0001% or more in order to obtain such effects. The B content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, if B is contained excessively, B precipitates may be formed and the strength of the steel sheet may be lowered. Therefore, 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：０～０．１００％］
　Ｔｉは、破壊の起点として作用する粗大な介在物を発生させるＳ、Ｎ及びＯ量を低減する効果を有する元素である。また、Ｔｉは組織を微細化し、鋼板の強度－成形性バランスを高める効果がある。Ｔｉ含有量は０％であってもよいが、これらの効果を得るためには、Ｔｉ含有量は０．００１％以上であることが好ましい。Ｔｉ含有量は０．００５％以上、０．００７％以上又は０．０１０％以上であってもよい。一方で、Ｔｉを過度に含有すると、粗大なＴｉ硫化物、Ｔｉ窒化物及び／又はＴｉ酸化物が形成して鋼板の成形性が低下する場合がある。したがって、Ｔｉ含有量は０．１００％以下であることが好ましい。Ｔｉ含有量は０．０８０％以下、０．０７０％以下、０．０６０％以下又は０．０３０％以下であってもよい。 [Ti: 0 to 0.100%]
Ti is an element that has the effect of reducing the amounts of S, N, and O that generate coarse inclusions that act as starting points for fracture. In addition, Ti has the effect of refining the structure and improving the strength-formability balance of the steel sheet. Although the Ti content may be 0%, the Ti content is preferably 0.001% or more in order to obtain these effects. The Ti content may be 0.005% or more, 0.007% or more, or 0.010% or more. On the other hand, when Ti is contained excessively, coarse Ti sulfides, Ti nitrides and/or Ti oxides are formed, which may deteriorate 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, 0.070% or less, 0.060% or less, or 0.030% or less.
［Ｎｂ：０～０．０６０％］
　Ｎｂは、析出物による強化、フェライト結晶粒の成長抑制による細粒化強化、及び／又は再結晶の抑制による転位強化に起因して鋼板の強度の向上に寄与する元素である。Ｎｂ含有量は０％であってもよいが、これらの効果を得るためには、Ｎｂ含有量は０．００１％以上であることが好ましい。Ｎｂ含有量は０．００５％以上、０．００７％以上又は０．０１０％以上であってもよい。一方で、Ｎｂを過度に含有すると、未再結晶フェライトが増加して鋼板の成形性が低下する場合がある。したがって、Ｎｂ含有量は０．０６０％以下であることが好ましい。Ｎｂ含有量は０．０５０％以下、０．０４０％以下又は０．０３０％以下であってもよい。 [Nb: 0 to 0.060%]
Nb is an element that contributes to the improvement of the strength of the steel sheet due to strengthening by precipitates, grain refinement strengthening by suppressing the growth of ferrite grains, and/or dislocation strengthening by suppressing recrystallization. Although the Nb content may be 0%, the Nb content is preferably 0.001% or more in order to obtain these effects. The Nb content may be 0.005% or more, 0.007% or more, or 0.010% or more. On the other hand, if Nb is contained excessively, non-recrystallized ferrite increases and the formability of the steel sheet may deteriorate. Therefore, the Nb content is preferably 0.060% or less. The Nb content may be 0.050% or less, 0.040% or less, or 0.030% or less.
［Ｖ：０～０．５０％］
　Ｖは、析出物による強化、フェライト結晶粒の成長抑制による細粒化強化、及び／又は再結晶の抑制による転位強化に起因して鋼板の強度の向上に寄与する元素である。Ｖ含有量は０％であってもよいが、これらの効果を得るためには、Ｖ含有量は０．００１％以上であることが好ましい。Ｖ含有量は０．００５％以上、０．０１％以上又は０．０２％以上であってもよい。一方で、Ｖを過度に含有すると、炭窒化物が多量に析出して鋼板の成形性が低下する場合がある。したがって、Ｖ含有量は０．５０％以下であることが好ましい。Ｖ含有量は０．４０％以下、０．２０％以下又は０．１０％以下であってもよい。 [V: 0 to 0.50%]
V is an element that contributes to the improvement of the strength of the steel sheet due to strengthening by precipitates, grain refinement strengthening by suppressing the growth of ferrite grains, and/or dislocation strengthening by suppressing recrystallization. Although the V content may be 0%, the V content is preferably 0.001% or more in order to obtain these effects. The V content may be 0.005% or more, 0.01% or more, or 0.02% or more. On the other hand, if V is contained excessively, a large amount of carbonitrides may be precipitated to deteriorate the formability of the steel sheet. Therefore, 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：０～１．００％］
　Ｎｉは、高温での相変態を抑制し、鋼板の強度の向上に寄与する元素である。Ｎｉ含有量は０％であってもよいが、このような効果を得るためには、Ｎｉ含有量は０．００１％以上であることが好ましい。Ｎｉ含有量は０．０１％以上、０．０３％以上又は０．０５％以上であってもよい。一方で、Ｎｉを過度に含有すると、鋼板の溶接性が低下する場合がある。したがって、Ｎｉ含有量は１．００％以下であることが好ましい。Ｎｉ含有量は０．６０％以下、０．４０％以下又は０．２０％以下であってもよい。 [Ni: 0 to 1.00%]
Ni is an element that suppresses phase transformation at high temperatures and contributes to improvement in strength of the steel sheet. Although the Ni content may be 0%, the Ni content is preferably 0.001% or more in order to obtain such effects. The Ni content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, when Ni is contained excessively, the weldability of the steel sheet may deteriorate. Therefore, 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：０～１．００％］
　Ｃｕは、微細な粒子の形態で鋼中に存在し、鋼板の強度の向上に寄与する元素である。Ｃｕ含有量は０％であってもよいが、このような効果を得るためには、Ｃｕ含有量は０．００１％以上であることが好ましい。Ｃｕ含有量は０．０１％以上、０．０３％以上又は０．０５％以上であってもよい。一方で、Ｃｕを過度に含有すると、鋼板の溶接性が低下する場合がある。したがって、Ｃｕ含有量は１．００％以下であることが好ましい。Ｃｕ含有量は０．６０％以下、０．４０％以下又は０．２０％以下であってもよい。 [Cu: 0 to 1.00%]
Cu is an element that exists in steel in the form of fine particles and contributes to improving the strength of the steel sheet. Although the Cu content may be 0%, the Cu content is preferably 0.001% or more in order to obtain such effects. The Cu content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, when Cu is contained excessively, the weldability of the steel sheet may deteriorate. Therefore, 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：０～１．００％］
　Ｗは、高温での相変態を抑制し、鋼板の強度の向上に寄与する元素である。Ｗ含有量は０％であってもよいが、このような効果を得るためには、Ｗ含有量は０．００１％以上であることが好ましい。Ｗ含有量は０．０１％以上、０．０２％以上又は０．１０％以上であってもよい。一方で、Ｗを過度に含有すると、熱間加工性が低下して生産性が低下する場合がある。したがって、Ｗ含有量は１．００％以下であることが好ましい。Ｗ含有量は０．８０％以下、０．５０％以下、０．２０％以下又は０．１５％以下であってもよい。 [W: 0 to 1.00%]
W is an element that suppresses phase transformation at high temperatures and contributes to improvement in strength of the steel sheet. Although the W content may be 0%, the W content is preferably 0.001% or more in order to obtain such effects. The W content may be 0.01% or more, 0.02% or more, or 0.10% or more. On the other hand, when W is excessively contained, the hot workability may deteriorate and the productivity may decrease. Therefore, the W content is preferably 1.00% or less. The W content may be 0.80% or less, 0.50% or less, 0.20% or less, or 0.15% or less.
［Ｓｎ：０～１．００％］
　Ｓｎは、結晶粒の粗大化を抑制し、鋼板の強度の向上に寄与する元素である。Ｓｎ含有量は０％であってもよいが、このような効果を得るためには、Ｓｎ含有量は０．００１％以上であることが好ましい。Ｓｎ含有量は０．０１％以上、０．０５％以上又は０．０８％以上であってもよい。一方で、Ｓｎを過度に含有すると、鋼板の脆化を引き起こす場合がある。したがって、Ｓｎ含有量は１．００％以下であることが好ましい。Ｓｎ含有量は０．８０％以下、０．５０％以下、０．２０％以下又は０．１５％以下であってもよい。 [Sn: 0 to 1.00%]
Sn is an element that suppresses the coarsening of crystal grains and contributes to the improvement of the strength of the steel sheet. Although the Sn content may be 0%, the Sn content is preferably 0.001% or more in order to obtain such effects. The Sn content may be 0.01% or more, 0.05% or more, or 0.08% or more. On the other hand, an excessive Sn content may cause embrittlement of the steel sheet. Therefore, the Sn content is preferably 1.00% or less. The Sn content may be 0.80% or less, 0.50% or less, 0.20% or less, or 0.15% or less.
［Ｓｂ：０～０．２００％］
　Ｓｂは、結晶粒の粗大化を抑制し、鋼板の強度の向上に寄与する元素である。Ｓｂ含有量は０％であってもよいが、このような効果を得るためには、Ｓｂ含有量は０．００１％以上であることが好ましい。Ｓｂ含有量は０．００３％以上、０．００５％以上又は０．０１０％以上であってもよい。一方で、Ｓｂを過度に含有すると、鋼板の脆化を引き起こす場合がある。したがって、Ｓｂ含有量は０．２００％以下であることが好ましい。Ｓｂ含有量は０．１５０％以下、０．１００％以下、０．０５０％以下又は０．０２０％以下であってもよい。 [Sb: 0 to 0.200%]
Sb is an element that suppresses the coarsening of crystal grains and contributes to the improvement of the strength of the steel sheet. Although the Sb content may be 0%, the Sb content is preferably 0.001% or more in order to obtain such effects. The Sb content may be 0.003% or more, 0.005% or more, or 0.010% or more. On the other hand, an excessive Sb content may cause embrittlement of the steel sheet. Therefore, the Sb content is preferably 0.200% or less. The Sb content may be 0.150% or less, 0.100% or less, 0.050% or less, or 0.020% or less.
［Ｃａ：０～０．０１００％］
［Ｍｇ：０～０．０１００％］
［Ｚｒ：０～０．０１００％］
［ＲＥＭ：０～０．０１００％］
　Ｃａ、Ｍｇ、Ｚｒ及びＲＥＭは、鋼板の成形性の向上に寄与する元素である。Ｃａ、Ｍｇ、Ｚｒ及びＲＥＭ含有量は０％であってもよいが、このような効果を得るためには、Ｃａ、Ｍｇ、Ｚｒ及びＲＥＭ含有量はそれぞれ０．０００１％以上であることが好ましく、０．０００５％以上、０．００１０％以上又は０．００１５％以上であってもよい。一方で、これらの元素を過度に含有すると、鋼板の延性が低下する場合がある。したがって、Ｃａ、Ｍｇ、Ｚｒ及びＲＥＭ含有量はそれぞれ０．０１００％以下であることが好ましく、０．００８０％以下、０．００６０％以下、０．００３０％以下又は０．００２０％以下であってもよい。本明細書におけるＲＥＭとは、原子番号２１番のスカンジウム（Ｓｃ）、原子番号３９番のイットリウム（Ｙ）及びランタノイドである原子番号５７番のランタン（Ｌａ）～原子番号７１番のルテチウム（Ｌｕ）の１７元素の総称であり、ＲＥＭ含有量はこれら元素の合計含有量である。 [Ca: 0 to 0.0100%]
[Mg: 0 to 0.0100%]
[Zr: 0 to 0.0100%]
[REM: 0 to 0.0100%]
Ca, Mg, Zr, and REM are elements that contribute to improving the formability of steel sheets. 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 each preferably 0.0001% or more. , 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, an excessive content of these elements may reduce the ductility of the steel sheet. Therefore, the Ca, Mg, Zr and REM contents are each preferably 0.0100% or less, and 0.0080% or less, 0.0060% or less, 0.0030% or less or 0.0020% or less. good too. In this specification, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanide (La) with atomic number 57 to lutetium (Lu) with atomic number 71, which are lanthanoids. , and the REM content is the total content of these elements.
　本発明の実施形態に係る鋼板において、上記の元素以外の残部はＦｅ及び不純物からなる。不純物とは、鋼板を工業的に製造する際に、鉱石やスクラップ等のような原料を始めとして、製造工程の種々の要因によって混入する成分等である。不純物として、例えば、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及びＰｏが挙げられる。不純物は、合計で０．１００％以下含んでもよい。 In the steel sheet according to the embodiment of the present invention, the balance other than the above elements consists of Fe and impurities. Impurities are components and the like that are mixed due to various factors in the manufacturing process, including raw materials such as ores and scraps, when steel sheets are manufactured industrially. Impurities such as 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. Impurities may contain 0.100% or less in total.
　鋼板の化学組成は、一般的な分析方法によって測定すればよい。例えば、鋼板の化学組成は、誘導結合プラズマ発光分光分析（ＩＣＰ－ＡＥＳ：Ｉｎｄｕｃｔｉｖｅｌｙ　Ｃｏｕｐｌｅｄ　Ｐｌａｓｍａ－Ａｔｏｍｉｃ　Ｅｍｉｓｓｉｏｎ　Ｓｐｅｃｔｒｏｍｅｔｒｙ）を用いて測定すればよい。Ｃ及びＳは燃焼－赤外線吸収法を用い、Ｎは不活性ガス融解－熱伝導度法を用い、Ｏは不活性ガス融解－非分散型赤外線吸収法を用いて測定すればよい。 The chemical composition of the steel sheet can be measured by a general analytical method. For example, the chemical composition of the steel sheet may be measured using Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES). C and S can be measured using a combustion-infrared absorption method, N can be measured using an inert gas fusion-thermal conductivity method, and O can be measured using an inert gas fusion-nondispersive infrared absorption method.
［フェライト：７０～９７％、及び硬質相：３～３０％］
　鋼板の金属組織は、面積％で、フェライト：７０～９７％、及び硬質相：３～３０％からなり、より具体的にはフェライト：７０～９５％、及び硬質相：５～３０％のみから構成される。鋼板の金属組織をこのような複合組織とすることで、鋼板の強度を適切な範囲内に維持しつつ、より具体的には５００ＭＰａ以上の引張強さを達成しつつ、成形後の外観を向上させることが可能となる。鋼板の強度をより高める観点から、硬質相の面積分率は、５％以上、７％以上、１０％以上又は１２％以上であってもよい。同様に、フェライトの面積分率は、９５％以下、９３％以下、９０％以下又は８８％以下であってもよい。一方で、成形後の外観をより向上させる観点から、硬質相の面積分率は、２８％以下、２６％以下、２３％以下、２０％以下、１８％以下、１６％以下又は１４％以下であってもよい。同様に、フェライトの面積分率は、７２％以上、７４％以上、７７％以上、８０％以上、８２％以上、８４％以上又は８６％以上であってもよい。 [ferrite: 70-97% and hard phase: 3-30%]
The metal structure of the steel sheet consists of ferrite: 70 to 97% and hard phase: 3 to 30%, more specifically ferrite: 70 to 95% and hard phase: 5 to 30% in terms of area %. Configured. By making the metal structure of the steel sheet into such a composite structure, the strength of the steel sheet is maintained within an appropriate range, more specifically, a tensile strength of 500 MPa or more is achieved, and the appearance after forming is improved. It is possible to From the viewpoint of increasing the strength of the steel sheet, the area fraction of the hard phase may be 5% or more, 7% or more, 10% or more, or 12% or more. Similarly, the area fraction of ferrite may be 95% or less, 93% or less, 90% or less, or 88% or less. On the other hand, from the viewpoint of further improving the appearance after molding, the area fraction of the hard phase is 28% or less, 26% or less, 23% or less, 20% or less, 18% or less, 16% or less, or 14% or less. There may be. Similarly, the area fraction of ferrite may be 72% or more, 74% or more, 77% or more, 80% or more, 82% or more, 84% or more, or 86% or more.
　本発明の実施形態に係る鋼板において、硬質相は、フェライトよりも硬い組織を言うものであり、例えばマルテンサイト、ベイナイト、焼き戻しマルテンサイト及びパーライトの少なくとも１種を含むか又はそれらの少なくとも１種からなり、特にはマルテンサイト、ベイナイト、焼き戻しマルテンサイト及びパーライトの少なくとも１種である。鋼板の強度向上の観点からは、硬質相は、マルテンサイト、ベイナイト及び焼き戻しマルテンサイトの少なくとも１種からなること又はそれらの少なくとも１種であることが好ましく、マルテンサイトからなること又はマルテンサイトであることがより好ましい。本発明の実施形態においては、鋼板の金属組織には、残留オーステナイは少ないことが好ましく、具体的には、残留オーステナイトは、面積％で、１％未満又は０．５％未満であることが好ましく、０％であることがより好ましい。 In the steel sheet according to the embodiment of the present invention, the hard phase refers to a structure harder than ferrite, and includes at least one of martensite, bainite, tempered martensite and pearlite, or at least one of them. and in particular at least one of martensite, bainite, tempered martensite and perlite. From the viewpoint of improving the strength of the steel sheet, the hard phase preferably consists of at least one of martensite, bainite and tempered martensite, or at least one of them. It is more preferable to have In the embodiment of the present invention, the metal structure of the steel sheet preferably has little retained austenite. Specifically, the retained austenite is preferably less than 1% or less than 0.5% in terms of area %. , 0%.
［金属組織の同定及び面積分率の算出］
　金属組織の同定及び面積分率の算出は以下のようにして行われる。まず、得られた鋼板の板幅ＷのＷ／４位置又は３Ｗ／４位置（すなわち、鋼板のいずれかの幅方向端部から幅方向にＷ／４の位置）から金属組織（ミクロ組織）観察用の試料（サイズは、おおむね、圧延方向に２０ｍｍ×幅方向に２０ｍｍ×鋼板の厚さ）を採取する。次いで、光学顕微鏡を用いて表面から板厚１／２厚における金属組織（ミクロ組織）の観察を行い、鋼板の表面（めっきが存在する場合はめっき層を除いた表面）から板厚１／２厚までの硬質相の面積分率を算出する。試料の調整として、圧延直角方向の板厚断面を観察面として研磨し、レペラー試薬にてエッチングする。次に、倍率５００又は１０００倍の光学顕微鏡写真から「ミクロ組織」を分類する。レペラー腐食後に光学顕微鏡観察を行なうと、例えばベイナイト及びパーライトは黒、マルテンサイト（焼き戻しマルテンサイトを含む）は白、フェライトは灰色と、各組織が色分けして観察されるので、フェライトとそれ以外の硬質組織との判別を容易に行うことができる。光学顕微鏡写真で、フェライトを示す灰色以外の領域が硬質相である。 [Identification of metal structure and calculation of area fraction]
Identification of the metal structure and calculation of the area fraction are performed as follows. First, the metal structure (microstructure) is observed from the W/4 position or 3W/4 position of the width W of the obtained steel plate (that is, the W/4 position in the width direction from either end of the steel plate in the width direction). A sample (size is roughly 20 mm in the rolling direction, 20 mm in the width direction, and the thickness of the steel plate) is taken. Then, using an optical microscope, the metal structure (microstructure) at 1/2 thickness from the surface is observed, and the surface of the steel sheet (when plating is present, the surface excluding the plating layer) to 1/2 thickness Calculate the area fraction of the hard phase up to the thickness. For preparation of the sample, the plate thickness cross-section in the direction perpendicular to the rolling direction is polished as an observation surface and etched with a repeller reagent. Next, classify the "microstructure" from optical micrographs at 500x or 1000x magnification. When observed with an optical microscope after repeller corrosion, each structure is color-coded, for example, black for bainite and pearlite, white for martensite (including tempered martensite), and gray for ferrite. can be easily distinguished from hard tissue. In the optical micrograph, the non-gray areas showing ferrite are the hard phases.
　レペラー試薬にてエッチングした鋼板の表面から板厚方向に板厚１／２位置までの領域において５００倍又は１０００倍の倍率にて１０視野観察し、Ａｄｏｂｅ社製「Ｐｈｏｔｏｓｈｏｐ　ＣＳ５」の画像解析ソフトを用いて画像解析を行い、硬質相の面積分率を求める。画像解析手法として、例えば、画像の最大明度値Ｌmaxと最小明度値Ｌminとを画像から取得し、明度がＬmax－０．３（Ｌmax－Ｌmin）からＬmaxまでの画素を持つ部分を白色領域、ＬminからＬmin＋０．３（Ｌmax－Ｌmin）の画素を持つ部分を黒色領域、それ以外の部分を灰色領域と定義して、灰色領域以外の領域である硬質相の面積分率を算出する。合計１０箇所の観察視野について、上記と同様に画像解析を行って硬質相の面積分率を測定し、これらの面積分率を平均して平均値を算出する。この平均値を硬質相の面積分率とし、残部をフェライトの面積分率とする。なお、観察面積は板厚方向１５０μｍ、圧延方向２５０μｍ（この場合の観察面積は１５０×２５０＝３７５００μｍ２）とする。
　なお、残留オーステナイトの面積分率の測定が必要な場合、前記観察面に対するＸ線回析により、残留オーステナイトの面積分率を測定することができる。具体的には、Ｃｏ－Ｋα線を用いて、板厚方向１／４位置のα（１１０）、α（２００）、α（２１１）、γ（１１１）、γ（２００）、γ（２２０）の計６ピークの積分強度を求め、強度平均法を用いて残留オーステナイトの体積分率を算出し、得られた残留オーステナイトの体積分率を、残留オーステナイトの面積分率とする。 Observe 10 fields of view at a magnification of 500 times or 1000 times in the area from the surface of the steel plate etched with the repeller reagent to the plate thickness 1/2 position in the plate thickness direction, and use the image analysis software "Photoshop CS5" manufactured by Adobe. Image analysis is performed using this to determine the area fraction of the hard phase. As an image analysis method, for example, the maximum brightness value L max and the minimum brightness value L min of the image are obtained from the image, and pixels with brightness from L max −0.3 (L max −L min ) to L max The part is defined as a white area, the part having pixels from L min to L min + 0.3 (L max - L min ) is defined as a black area, and the other part is defined as a gray area. Calculate the area fraction of Image analysis is performed in the same manner as above for a total of 10 observation fields of view to measure the area fraction of the hard phase, and these area fractions are averaged to calculate an average value. The average value is defined as the area fraction of the hard phase, and the remainder is defined as the area fraction of ferrite. The observation area is 150 μm in the plate thickness direction and 250 μm in the rolling direction (in this case, the observation area is 150×250=37500 μm 2 ).
When it is necessary to measure the area fraction of retained austenite, the area fraction of retained austenite can be measured by X-ray diffraction of the observation surface. Specifically, using Co-Kα rays, α(110), α(200), α(211), γ(111), γ(200), γ(220) at the quarter position in the thickness direction The integrated intensity of a total of six peaks is obtained, the volume fraction of retained austenite is calculated using the intensity average method, and the obtained volume fraction of retained austenite is defined as the area fraction of retained austenite.
［表面のＳｔｒ：０．３５～０．７５］
　本発明の実施形態では、鋼板の表面（鋼板の表面にめっき層が存在する場合には、めっき層の表面）のＳｔｒは０．３５～０．７５である。初期（すなわち製造されたまま）の鋼板の表面のＳｔｒをこのような範囲内に制御し、かつ後で説明するΔＳｔｒを所定の範囲内に抑えることで、プレス成形等の成形によって歪が付与された場合であっても、鋼板表面におけるゴーストラインの発生を顕著に抑制又は低減することが可能となる。先に述べたとおり、Ｓｔｒの値が０により近く、それゆえ表面の異方性がより強く鋼板表面に筋目が目立つ場合には、当然ながらプレス成形等による成形後の外観は劣化したものとなる。一方で、Ｓｔｒの値が１により近く、それゆえより等方的な表面性状の場合であっても、プレス成形等による成形後には、当初のより等方的な表面性状との相対的な関係で、ゴーストラインの程度が顕著となり、外観が劣化してしまう。したがって、Ｓｔｒの値は低すぎても高すぎても、鋼板表面におけるゴーストラインの発生を抑制又は低減するという観点からは不利に作用する可能性が高い。したがって、本発明の実施形態においては、鋼板表面のＳｔｒは、上記のとおり適度な範囲内に制御する必要があり、例えば、Ｓｔｒは０．４０以上、０．４５以上若しくは０．５０以上であってもよく、及び／又は０．７０以下、０．６５以下若しくは０．６０以下であってもよい。 [Surface Str: 0.35 to 0.75]
In an embodiment of the present invention, the Str of the surface of the steel sheet (the surface of the plating layer when the steel sheet has a plating layer) is 0.35 to 0.75. By controlling Str on the surface of the initial (that is, as-manufactured) steel sheet within such a range and suppressing ΔStr, which will be described later, within a predetermined range, strain is imparted by forming such as press forming. Even in this case, it is possible to remarkably suppress or reduce the occurrence of ghost lines on the surface of the steel sheet. As mentioned above, when the value of Str is closer to 0 and therefore the surface anisotropy is stronger and the streaks are conspicuous on the surface of the steel sheet, the appearance after forming by press forming etc. naturally deteriorates. . On the other hand, even if the value of Str is closer to 1 and therefore has a more isotropic surface texture, the relative relationship with the initial more isotropic surface texture after forming, such as by press molding, is Therefore, the degree of ghost lines becomes conspicuous and the appearance deteriorates. Therefore, if the value of Str is too low or too high, there is a high possibility that it will work disadvantageously from the viewpoint of suppressing or reducing the occurrence of ghost lines on the surface of the steel sheet. Therefore, in the embodiment of the present invention, the Str of the surface of the steel sheet must be controlled within a suitable range as described above. and/or may be 0.70 or less, 0.65 or less, or 0.60 or less.
［Ｓｔｒと５％引張歪付与後のＳｔｒとの差ΔＳｔｒ：０．１５以下］
　本発明の実施形態では、初期のＳｔｒと５％引張歪付与後のＳｔｒとの差ΔＳｔｒは０．１５以下である。ここで、ΔＳｔｒとは、初期のＳｔｒから５％の引張歪を付与した後のＳｔｒを引き算した値をいうものであり、すなわちΔＳｔｒ＝初期Ｓｔｒ－５％引張歪付与後Ｓｔｒである。初期のＳｔｒを上記の範囲に制御しつつ、ΔＳｔｒを０．１５以下に抑えることで、プレス成形等の加工時におけるゴーストラインの発生を確実に抑制又は低減することが可能となる。先に述べたとおり、ΔＳｔｒが０．１５以下に抑制されている場合には、鋼中のＭｎ偏析が十分に抑制又は低減されていると考えられる。これに関連して、鋼板の金属組織において縞状硬質相の生成が十分に抑制されているために、プレス成形等の加工時においても、鋼板表面におけるゴーストラインの発生を顕著に抑制又は低減することが可能となる。ゴーストラインの発生を抑制又は低減する観点からは、ΔＳｔｒは低いほど好ましく、例えば、０．１２以下、０．１０以下、０．０８以下又は０．０５以下であってもよい。引張歪を付与した場合、Ｓｔｒの値は引張歪を付与する前と比較して一般的には小さくなることから（変化しない場合もある）、測定誤差等を考慮してもΔＳｔｒがマイナス側に大きな値となることはない。したがって、下限は特に限定されないが、例えば、ΔＳｔｒは、－０．０３以上、０．００以上又は０．０１以上であってもよい。 [Difference between Str and Str after applying 5% tensile strain ΔStr: 0.15 or less]
In an embodiment of the present invention, the difference ΔStr between the initial Str and the Str after applying 5% tensile strain is 0.15 or less. Here, ΔStr is the value obtained by subtracting the Str after applying 5% tensile strain from the initial Str, that is, ΔStr = Initial Str - Str after applying 5% tensile strain. By suppressing ΔStr to 0.15 or less while controlling the initial Str within the above range, it is possible to reliably suppress or reduce the generation of ghost lines during processing such as press molding. As described above, when ΔStr is suppressed to 0.15 or less, Mn segregation in steel is considered to be sufficiently suppressed or reduced. In relation to this, since the formation of striped hard phases is sufficiently suppressed in the metallographic structure of the steel sheet, the occurrence of ghost lines on the steel sheet surface is significantly suppressed or reduced even during processing such as press forming. becomes possible. From the viewpoint of suppressing or reducing the occurrence of ghost lines, ΔStr is preferably as low as possible, and may be, for example, 0.12 or less, 0.10 or less, 0.08 or less, or 0.05 or less. When tensile strain is applied, the value of Str is generally smaller than before applying tensile strain (sometimes it does not change), so even if measurement errors etc. are considered, ΔStr will be on the negative side. cannot be of great value. Therefore, although the lower limit is not particularly limited, ΔStr may be -0.03 or more, 0.00 or more, or 0.01 or more, for example.
［Ｓｔｒ及びΔＳｔｒの測定］
　Ｓｔｒ及びΔＳｔｒは、以下のようにして決定される。まず、鋼板の端面から１００ｍｍ以上離れた位置から、圧延方向（Ｌ方向）に直角な方向（Ｃ方向）を試験方向とするＪＩＳ　Ｚ２２４１：２０１１の５号引張試験片を採取し、次いで採取した鋼板試料の表面（鋼板試料の表面にめっき層が存在する場合には、めっき層の表面）に対してキーエンス社製のＶＫ－Ｘ２５０／１５０形状解析レーザ顕微鏡を用いて三次元画像解析を行い、ＪＩＳ　Ｂ０６８１－２：２０１８の規定に準拠して初期のＳｔｒが決定される。三次元画像解析の対象領域は５ｍｍ（Ｃ方向）×２ｍｍ（Ｌ方向）以上とする。次に、鋼板試料に対して単軸における５％の引張歪を付与した後、先と同様の測定を行うことで５％の引張歪を付与した後のＳｔｒが決定され、最後に初期のＳｔｒから５％引張歪付与後のＳｔｒを引き算することによりΔＳｔｒが決定される。 [Measurement of Str and ΔStr]
Str and ΔStr are determined as follows. First, a No. 5 tensile test piece of JIS Z2241: 2011 with a direction (C direction) perpendicular to the rolling direction (L direction) as the test direction is taken from a position at least 100 mm away from the end surface of the steel plate, and then the sampled steel plate. Three-dimensional image analysis is performed using a VK-X250/150 shape analysis laser microscope manufactured by Keyence Corporation on the surface of the sample (the surface of the coating layer if there is a coating layer on the surface of the steel plate sample), and JIS The initial Str is determined according to the provisions of B0681-2:2018. The target area of the three-dimensional image analysis shall be 5 mm (C direction)×2 mm (L direction) or more. Next, after applying a uniaxial tensile strain of 5% to the steel plate sample, Str after applying a tensile strain of 5% is determined by performing the same measurement as above, and finally the initial Str ΔStr is determined by subtracting Str after applying 5% tensile strain from .
［板厚］
　本発明の実施形態に係る鋼板は、特に限定されないが、例えば０．１～２．０ｍｍの板厚を有する。このような板厚を有する鋼板は、ドアやフード等の蓋物部材の素材として用いる場合に好適である。板厚は０．２ｍｍ以上、０．３ｍｍ以上、０．４ｍｍ以上であってもよい。同様に、板厚は１．８ｍｍ以下、１．５ｍｍ以下、１．２ｍｍ以下又は１．０ｍｍ以下であってもよい。例えば、板厚を０．２ｍｍ以上とすることで、成形品形状を平坦に維持することが容易になり、寸法精度及び形状精度が向上するという追加の効果を得ることができる。一方、板厚を１．０ｍｍ以下とすることで部材の軽量化効果が顕著となる。鋼板の板厚はマイクロメータによって測定される。 [Thickness]
The steel plate according to the embodiment of the present invention is not particularly limited, but has a thickness of 0.1 to 2.0 mm, for example. A steel plate having such a thickness is suitable for use as a material for lid members such as doors and hoods. The plate thickness may be 0.2 mm or more, 0.3 mm or more, or 0.4 mm or more. Similarly, 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. For example, by setting the plate thickness to 0.2 mm or more, it becomes easy to maintain the shape of the molded product flat, and an additional effect of improving dimensional accuracy and shape accuracy can be obtained. On the other hand, by setting the plate thickness to 1.0 mm or less, the effect of reducing the weight of the member becomes remarkable. The plate thickness of the steel plate is measured with a micrometer.
［めっき］
　本発明の実施形態に係る鋼板は、冷間圧延鋼板であるが、耐食性の向上等を目的として、表面にめっき層をさらに含んでもよい。めっき層は、溶融めっき層及び電気めっき層のいずれでもよい。つまり、本発明の実施形態に係る鋼板は、その表面に溶融めっき層又は電気めっき層を有する冷間圧延鋼板であってもよい。溶融めっき層は、例えば、溶融亜鉛めっき層（ＧＩ）、合金化溶融亜鉛めっき層（ＧＡ）、溶融アルミニウムめっき層、溶融Ｚｎ－Ａｌ合金めっき層、溶融Ｚｎ－Ａｌ－Ｍｇ合金めっき層、溶融Ｚｎ－Ａｌ－Ｍｇ－Ｓｉ合金めっき層等を含む。電気めっき層は、例えば、電気亜鉛めっき層（ＥＧ）、電気Ｚｎ－Ｎｉ合金めっき層等を含む。好ましくは、めっき層は、溶融亜鉛めっき層、合金化溶融亜鉛めっき層、又は電気亜鉛めっき層である。めっき層の付着量は、特に制限されず一般的な付着量でよい。 [Plating]
The steel sheet according to the embodiment of the present invention is a cold-rolled steel sheet, and may further include a plating layer on the surface for the purpose of improving corrosion resistance. 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 coating layer or an electroplating layer on its surface. The hot-dip plating layer is, for example, a hot-dip galvanizing layer (GI), an alloyed hot-dip galvanizing 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, hot-dip Zn - Including Al-Mg-Si alloy plating layer, etc. The electroplated layer includes, for example, an electrogalvanized layer (EG), an electroplated Zn—Ni alloy layer, and the like. Preferably, the plating layer is a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electro-galvanized layer. The coating amount of the plating layer is not particularly limited, and a general coating amount may be used.
［機械特性］
　上記の化学組成及び金属組織を有する鋼板によれば、高い引張強さ、具体的には５００ＭＰａ以上の引張強さを達成することができる。引張強さは、好ましくは５４０ＭＰａ以上、より好ましくは５７０ＭＰａ以上又は６００ＭＰａ以上である。上限は特に限定されないが、例えば、引張強さは９８０ＭＰａ以下、８５０ＭＰａ以下、７５０ＭＰａ以下、７００ＭＰａ以下又は６５０ＭＰａ以下であってもよい。引張強さを８５０ＭＰａ以下とすることで、鋼板をプレス加工する際の成形性を確保しやすいという利点がある。引張強さは、圧延方向に直角な方向を試験方向とするＪＩＳ　Ｚ２２４１：２０１１の５号引張試験片を鋼板から採取し、ＪＩＳ　Ｚ２２４１：２０１１に準拠して引張試験を行うことで測定される。 [Mechanical properties]
A steel sheet having the above chemical composition and metallographic structure can achieve a high tensile strength, specifically a tensile strength of 500 MPa or more. The tensile strength is preferably 540 MPa or higher, more preferably 570 MPa or higher or 600 MPa or higher. Although the upper limit is not particularly limited, for example, the tensile strength may be 980 MPa or less, 850 MPa or less, 750 MPa or less, 700 MPa or less, or 650 MPa or less. By setting the tensile strength to 850 MPa or less, there is an advantage that it is easy to ensure the formability when press working the steel plate. Tensile strength is measured by taking a JIS Z2241:2011 No. 5 tensile test piece from a steel plate with the test direction perpendicular to the rolling direction and performing a tensile test in accordance with JIS Z2241:2011.
　本発明の実施形態に係る鋼板は、高強度、具体的には５００ＭＰａ以上の引張強さを有するにもかかわらず、プレス加工等の成形後においても優れた外観を維持することができる。このため、本発明の実施形態に係る鋼板は、例えば、自動車において高い意匠性が求められるルーフ、フード、フェンダー及びドア等の外板部品として使用するのに非常に有用である。 Although the steel sheet according to the embodiment of the present invention has a high strength, specifically a tensile strength of 500 MPa or more, it can maintain an excellent appearance even after forming such as press working. For this reason, the steel plate according to the embodiment of the present invention is very useful for use as outer panel parts such as roofs, hoods, fenders, doors, etc., which require high designability in automobiles.
＜鋼板の製造方法＞
　次に、本発明の実施形態に係る鋼板の好ましい製造方法について説明する。以下の説明は、本発明の実施形態に係る鋼板を製造するための特徴的な方法の例示を意図するものであって、当該鋼板を以下に説明するような製造方法によって製造されるものに限定することを意図するものではない。 <Manufacturing method of steel plate>
Next, a preferred method for manufacturing the steel sheet according to the embodiment of the present invention will be described. The following description is intended to exemplify the characteristic method for manufacturing the steel sheet according to the embodiment of the present invention, and is limited to the steel sheet manufactured by the manufacturing method as described below. is not intended to
　本発明の実施形態に係る鋼板の製造方法は、
　鋼板に関連して上で説明した化学組成を有するスラブを熱間圧延し、次いで５５０℃未満の温度で巻き取る工程（熱間圧延工程）、
　得られた熱間圧延鋼板を累積圧下率が５０～９０％となるように冷間圧延する工程（冷間圧延工程）、
　得られた冷間圧延鋼板を露点が－２０～５℃の雰囲気中７５０℃以上の温度域で３０～２００秒間保持する工程（焼鈍工程）、及び
　前記冷間圧延鋼板を１．３μｍ以下の算術平均粗さＲａを有するロールを用いて０．６％以下の圧下率で調質圧延する工程（調質圧延工程）
を含むことを特徴としている。以下、各工程について詳しく説明する。 A steel sheet manufacturing method according to an embodiment of the present invention includes:
hot rolling a slab having the chemical composition described above in relation to steel plate and then coiling at a temperature below 550° C. (hot rolling process);
A step of cold rolling the obtained hot-rolled steel plate so that the cumulative reduction ratio is 50 to 90% (cold rolling step);
A step of holding the obtained cold-rolled steel sheet in an atmosphere with a dew point of −20 to 5° C. in a temperature range of 750° C. or higher for 30 to 200 seconds (annealing step); A step of skin-pass rolling at a rolling reduction of 0.6% or less using rolls having an average roughness Ra (pass-rolling step)
is characterized by including Each step will be described in detail below.
［熱間圧延工程］
　まず、鋼板に関連して上で説明した化学組成を有するスラブが熱間圧延に供される。使用するスラブは、生産性の観点から連続鋳造法によって鋳造することが好ましいが、造塊法又は薄スラブ鋳造法によって製造してもよい。スラブは、熱間圧延に先立ち１１００℃以上に加熱することが好ましい。加熱温度を１１００℃以上とすることで、熱間圧延において圧延反力が過度に大きくならず、目的とする製品厚を得やすくすることができる。加熱温度の上限は特に限定されないが、経済上の観点から、加熱温度は１３００℃未満とすることが好ましい。また、加熱されたスラブに対し、板厚調整等のために、任意選択で仕上げ圧延の前に粗圧延を施してもよい。このような粗圧延は、所望のシートバー寸法が確保できればよく、その条件は特に限定されない。熱間圧延は、特に限定されないが、一般的には仕上げ圧延の完了温度が６５０℃以上となるような条件下で行われる。仕上げ圧延の完了温度が低すぎると、圧延反力が高まり、所望の板厚を安定して得ることが困難となるからである。上限は特に限定されないが、一般的には仕上げ圧延の完了温度は９５０℃以下である。 [Hot rolling process]
First, a slab having the chemical composition described above in relation to steel 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 produced 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. By setting the heating temperature to 1100° C. or higher, the rolling reaction force in hot rolling does not become excessively large, and the target product thickness can be easily obtained. Although the upper limit of the heating temperature is not particularly limited, the heating temperature is preferably less than 1300° C. from an economical point of view. In addition, the heated slab may optionally be subjected to rough rolling before finish rolling for thickness adjustment and the like. Conditions for such rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured. Hot rolling is not particularly limited, but is generally carried out under such conditions that the completion temperature of finish rolling is 650° C. or higher. This is because if the finish rolling completion temperature is too low, the rolling reaction force increases, making it difficult to stably obtain a desired plate thickness. The upper limit is not particularly limited, but generally the finish rolling completion temperature is 950° C. or less.
［巻き取り］
　次に、仕上げ圧延された鋼板は、５５０℃未満の温度で巻き取られる。巻き取り温度が高いと、熱間圧延鋼板の表層においてＳｉ及びＭｎ等の内部酸化物の形成が促進されてしまう場合がある。形成された内部酸化物はその後の酸洗によっても十分に除去することができないため、内部酸化物を比較的多く含む状態で以降の冷間圧延工程及び焼鈍工程、とりわけ焼鈍工程が行われることになる。今回、本発明者らによって、鋼板表面における初期のＳｔｒを０．３５～０．７５の範囲内に制御するためには、焼鈍工程において鋼板表層を適度かつ均一に脱炭する必要があることが見出された。しかしながら、熱間圧延工程において内部酸化物が比較的多く生成した状態で、その後の焼鈍工程を実施した場合には、焼鈍工程における脱炭処理を適切なものとすることができない。すなわち、焼鈍工程における鋼板表層の脱炭が内部酸化物によって阻害されてそのばらつきが大きくなり、結果として最終的に得られる鋼板の表面におけるＳｔｒを所望の範囲内に制御することができなくなる。これに対し、巻き取り温度を５５０℃未満として、熱間圧延鋼板の表層におけるＳｉ及びＭｎ等の内部酸化物の形成を確実に抑制又は低減することで、以降の焼鈍工程において適切な脱炭処理を実現し、ひいては最終的に得られる鋼板の表面において所望の初期Ｓｔｒを実現することが可能となる。これに関連して、単に巻き取り温度を５５０℃未満に低減したとしても、鋼板中のＳｉ及び／又はＭｎ含有量が過度に高いと、これらの元素の内部酸化を十分に抑制することができない場合がある。このような場合には、同様に焼鈍工程における鋼板表層の脱炭のばらつきが大きくなり、最終的に得られる鋼板の表面におけるＳｔｒを所望の範囲内に制御することができなくなる。したがって、本製造方法では、鋼板中のＳｉ及びＭｎ含有量をそれぞれ１．５００％以下及び２．５０％以下に抑制しつつ、熱間圧延工程の巻き取り温度を５５０℃未満、好ましくは５００℃以下に制御することにより、当該熱間圧延工程における内部酸化物の形成を十分に抑制又は低減することが以降の焼鈍工程における適切な脱炭処理、ひいては最終的な鋼板表面における所望のＳｔｒを実現する上で極めて重要なものとなる。巻き取り温度の下限は特に限定されないが、巻き取り温度が低すぎると、熱間圧延鋼板の強度が過大となり、冷間圧延性を損なう場合がある。したがって、巻き取り温度は４５０℃以上であることが好ましい。 [Take-up]
The finish rolled steel sheet is then coiled at a temperature below 550°C. A high coiling temperature may promote the formation of internal oxides such as Si and Mn in the surface layer of the hot-rolled steel sheet. Since the formed internal oxide cannot be sufficiently removed even by the subsequent pickling, the subsequent cold rolling and annealing steps, especially the annealing step, are performed in a state containing a relatively large amount of internal oxide. Become. This time, the present inventors have found that it is necessary to moderately and uniformly decarburize the surface layer of the steel sheet in the annealing process in order to control the initial Str on the steel sheet surface within the range of 0.35 to 0.75. Found. However, when the subsequent annealing process is performed in a state in which a relatively large amount of internal oxides are generated in the hot rolling process, the decarburization treatment in the annealing process cannot be made appropriate. That is, the decarburization of the surface layer of the steel sheet in the annealing process is inhibited by the internal oxides, and the variation becomes large. As a result, Str on the surface of the finally obtained steel sheet cannot be controlled within the desired range. On the other hand, by setting the coiling temperature to less than 550 ° C. and reliably suppressing or reducing the formation of internal oxides such as Si and Mn in the surface layer of the hot-rolled steel sheet, appropriate decarburization treatment is performed in the subsequent annealing process. and thus the desired initial Str on the surface of the finally obtained steel sheet. In this connection, if the Si and/or Mn content in the steel sheet is excessively high, internal oxidation of these elements cannot be sufficiently suppressed even if the coiling temperature is simply reduced to less than 550°C. Sometimes. In such a case, decarburization of the surface layer of the steel sheet in the annealing process similarly becomes highly variable, and Str on the surface of the finally obtained steel sheet cannot be controlled within a desired range. Therefore, in the present manufacturing method, the coiling temperature in the hot rolling process is set to less than 550°C, preferably 500°C, while suppressing the Si and Mn contents in the steel sheet to 1.500% or less and 2.50% or less, respectively. By controlling the following, the formation of internal oxides in the hot rolling process can be sufficiently suppressed or reduced to achieve appropriate decarburization treatment in the subsequent annealing process and eventually the desired Str on the final steel sheet surface. will be extremely important in doing so. The lower limit of the coiling temperature is not particularly limited, but if the coiling temperature is too low, the strength of the hot-rolled steel sheet becomes excessively high, which may impair the cold-rollability. Therefore, the winding temperature is preferably 450° C. or higher.
［冷間圧延工程］
　得られた熱間圧延鋼板は、スケールを除去するために適宜酸洗処理を施され、次いで冷間圧延工程に供される。冷間圧延工程では、例えば、累積圧下率が５０～９０％となるように熱間圧延鋼板に冷間圧延を施すことが好ましい。累積圧下率をこのような範囲に制御することで、所望の板厚を確保し、さらに板幅方向の材質の均一性を十分に確保しつつ、圧延荷重が過大となって圧延が困難となることを防ぐことができる。 [Cold rolling process]
The obtained hot-rolled steel sheet is appropriately pickled to remove scales, and then subjected to a cold-rolling process. In the cold-rolling step, it is preferable to cold-roll the hot-rolled steel sheet so that the cumulative rolling reduction is, for example, 50 to 90%. By controlling the cumulative rolling reduction within such a range, it is possible to secure the desired strip thickness and sufficiently ensure the uniformity of the material in the strip width direction, but the rolling load becomes excessive and rolling becomes difficult. can be prevented.
［焼鈍工程］
　得られた冷間圧延鋼板は、焼鈍工程において、露点が－２０～５℃の雰囲気中７５０℃以上の温度域で３０～２００秒間保持される。露点が－２０℃未満であるか、焼鈍温度が７５０℃未満であるか、及び／又は保持時間が３０秒未満である場合には、鋼板表層の脱炭が不十分となり、Ｓｔｒが１により近い値となり、０．７５以下に制御することができなくなる。一方で、露点が５℃超であるか及び／又は保持時間が２００秒超である場合には、鋼板表層の脱炭が進行しすぎてしまい、Ｓｔｒが０により近い値となり、０．３５以上に制御することができなくなる。したがって、焼鈍工程においては、上記の露点、焼鈍温度及び保持時間において鋼板表層を適度に脱炭することが所望の初期Ｓｔｒを実現する上で非常に重要なものとなる。好ましくは、露点は－１５～０℃であり、保持時間は５０～１５０秒である。また、焼鈍温度の上限は特に限定されないが、例えば、結晶粒の粗大化を抑制して十分な強度を確保する観点から、焼鈍温度は、９００℃以下とすることが好ましい。 [Annealing process]
The obtained cold-rolled steel sheet is held in an atmosphere with a dew point of -20 to 5°C in a temperature range of 750°C or higher for 30 to 200 seconds in the annealing process. When the dew point is less than −20° C., the annealing temperature is less than 750° C., and/or the holding time is less than 30 seconds, decarburization of the surface layer of the steel sheet is insufficient, and Str is closer to 1. value and cannot be controlled below 0.75. On the other hand, when the dew point is more than 5°C and/or the holding time is more than 200 seconds, the decarburization of the surface layer of the steel sheet proceeds too much, Str becomes a value closer to 0, and is 0.35 or more. you will not be able to control it. Therefore, in the annealing process, it is very important to appropriately decarburize the surface layer of the steel sheet at the above dew point, annealing temperature and holding time in order to achieve the desired initial Str. Preferably, the dew point is -15 to 0°C and the holding time is 50 to 150 seconds. Although the upper limit of the annealing temperature is not particularly limited, for example, the annealing temperature is preferably 900° C. or lower from the viewpoint of suppressing coarsening of crystal grains and ensuring sufficient strength.
［冷却工程］
　焼鈍工程後の冷間圧延鋼板は次の冷却工程において冷却される。冷却工程は、特に限定されず、鋼板に関連して上で説明したフェライトと硬質相を所定の面積分率で含む金属組織が得られるように任意の適切な条件を適宜選択して実施すればよい。例えば、冷却工程においては、焼鈍温度からの平均冷却速度が５～５０℃／秒となるように冷却することが好ましい。平均冷却速度を５℃／秒以上とすることで、フェライトへの過剰な変態を抑制するとともに、マルテンサイト等の硬質相の生成量を多くして所望の強度を得ることができる。また、平均冷却速度を５０℃／秒以下とすることで、幅方向において鋼板をより均一に冷却することができる。 [Cooling process]
The cold-rolled steel sheet after the annealing process is cooled in the next cooling process. The cooling process is not particularly limited, and any appropriate conditions may be appropriately selected so as to obtain the metal structure containing the ferrite and hard phase described above in relation to the steel sheet and the hard phase in a predetermined area fraction. good. For example, in the cooling step, it is preferable to cool so that the average cooling rate from the annealing temperature is 5 to 50° C./sec. By setting the average cooling rate to 5° C./second or more, excessive transformation to ferrite can be suppressed and the amount of hard phases such as martensite produced can be increased to obtain the desired strength. Further, by setting the average cooling rate to 50° C./sec or less, the steel sheet can be cooled more uniformly in the width direction.
［めっき工程］
　耐食性の向上等を目的として、必要に応じて、得られた冷間圧延鋼板の表面にめっき処理を施してもよい。めっき処理は、溶融めっき、合金化溶融めっき、電気めっき等の処理であってよい。例えば、めっき処理として鋼板に溶融亜鉛めっき処理を行ってもよく、溶融亜鉛めっき処理後に合金化処理を行ってもよい。めっき処理及び合金化処理の具体的な条件は特に限定されず、当業者に公知の任意の適切な条件であってよい。例えば、合金化温度は４５０～６００℃であってもよい。 [Plating process]
For the purpose of improving corrosion resistance, etc., the surface of the obtained cold-rolled steel sheet may be subjected to a plating treatment, if necessary. The plating process may be a process such as hot-dip plating, hot-dip alloying, electroplating, or the like. For example, the steel sheet may be subjected to hot-dip galvanizing treatment as the plating treatment, or alloying treatment may be performed after the hot-dip galvanizing treatment. Specific conditions for the plating treatment and alloying treatment are not particularly limited, and may be any appropriate conditions known to those skilled in the art. For example, the alloying temperature may be 450-600°C.
［調質圧延工程］
　最後に、冷間圧延鋼板又はめっき鋼板は、１．３μｍ以下の算術平均粗さＲａを有するロールを用いて０．６％以下の圧下率で調質圧延される。算術平均粗さＲａは、ＪＩＳ　Ｂ０６０１：２０１３の規定に準拠して測定される。一般に、調質圧延は、鋼板の形状矯正や表面粗さの調整等を目的として、焼鈍やめっき処理後の鋼板に施されるものである。本製造方法では、最終的に得られる鋼板の表面性状が極めて重要であり、このような表面性状は、とりわけ先の熱間圧延工程及び焼鈍工程によって作り込まれている。このため、調質圧延工程では、このようにして作り込まれた冷間圧延鋼板又はめっき鋼板の表面性状を維持すべく、比較的穏やかな条件下で圧延を実施する必要がある。例えば、調質圧延において用いられるロールの算術平均粗さＲａが１．３μｍ超であるか及び／又は調質圧延の圧下率が０．６％超であると、ロールの粗度が鋼板表面に強く転写されてしまい、特に熱間圧延工程及び焼鈍工程において作り込んだ表面性状が部分的に又は全体的に壊され、結果として初期Ｓｔｒが所望の範囲外となる場合がある。これに対し、１．３μｍ以下の算術平均粗さＲａを有するロールを用いて０．６％以下の圧下率で調質圧延することで、先の工程で作り込んだ表面性状を十分に維持して所望の範囲内の初期Ｓｔｒを達成しつつ、鋼板の形状を適切に矯正することが可能である。好ましくは、ロールの算術平均粗さＲａは１．２μｍ以下であり、調質圧延の圧下率は０．５％以下である。 [Temperature rolling process]
Finally, the cold-rolled steel sheet or plated steel sheet is pass-rolled at a rolling reduction of 0.6% or less using rolls having an arithmetic mean roughness Ra of 1.3 μm or less. Arithmetic mean roughness Ra is measured in accordance with JIS B0601:2013. In general, temper rolling is performed on a steel sheet after annealing or plating for the purpose of correcting the shape of the steel sheet, adjusting the surface roughness, or the like. In this production method, the surface texture of the steel sheet finally obtained is extremely important, and such surface texture is created especially by the preceding hot rolling and annealing steps. Therefore, in the temper rolling process, it is necessary to roll under relatively mild conditions in order to maintain the surface properties of the cold-rolled steel sheet or plated steel sheet thus produced. For example, if the arithmetic mean roughness Ra of the rolls used in temper rolling is more than 1.3 μm and / or the rolling reduction of temper rolling is more than 0.6%, the roughness of the rolls is on the steel sheet surface. It may be strongly transferred, and the surface texture created especially in the hot rolling process and the annealing process may be partially or wholly destroyed, and as a result, the initial Str may be outside the desired range. On the other hand, by using rolls having an arithmetic mean roughness Ra of 1.3 μm or less and skin pass rolling at a reduction rate of 0.6% or less, the surface properties created in the previous process can be sufficiently maintained. It is possible to appropriately correct the shape of the steel sheet while achieving the initial Str within the desired range by using the Preferably, the arithmetic mean roughness Ra of the rolls is 1.2 μm or less, and the rolling reduction of temper rolling is 0.5% or less.
　以下、実施例によって本発明をより詳細に説明するが、本発明はこれらの実施例に何ら限定されるものではない。 The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.
　以下の実施例では、本発明の実施形態に係る鋼板を種々の条件下で製造し、得られた鋼板の引張強さ及び成形後外観の特性について調べた。 In the following examples, steel sheets according to embodiments of the present invention were produced under various conditions, and the properties of tensile strength and post-forming appearance of the obtained steel sheets were investigated.
　まず、連続鋳造法により表１に示す化学組成を有しかつ厚さが２００～３００ｍｍのスラブを鋳造した。表１に示す成分以外の残部はＦｅ及び不純物である。次に、得られたスラブに対し、熱間圧延工程（加熱温度１２００℃及び仕上げ圧延終了温度８００℃）、冷間圧延工程（累積圧下率８０％）、焼鈍工程、並びに冷却工程（平均冷却速度１０℃／秒）を実施して、板厚が０．４ｍｍの冷間圧延鋼板を製造した。各例において、熱間圧延工程における巻き取り温度の条件Ｉ（５５０℃未満）と焼鈍工程の条件ＩＩ（露点：－２０～５℃、焼鈍温度：７５０℃以上、及び保持時間３０～２００秒）を満たす場合（〇）と満たさない場合（×、Ａ又はＢ）をそれぞれ表２に示す。具体的には、条件Ｉを満たす例では、５００℃の温度で巻き取りを行い、一方で条件Ｉを満たさない例では、６５０℃の温度で巻き取りを行った。また、条件ＩＩを満たす例では、露点－５℃、焼鈍温度８００℃及び保持時間１５０秒の条件下で焼鈍工程を実施した。一方、条件ＩＩを満たさない例では、条件Ａ（露点－４０℃、焼鈍温度８００℃及び保持時間６０秒）又は条件Ｂ（露点８℃、焼鈍温度８００℃及び保持時間２８０秒）の条件下で焼鈍工程を実施した。 First, a slab having a chemical composition shown in Table 1 and a thickness of 200 to 300 mm was cast by a continuous casting method. The balance other than the components shown in Table 1 is Fe and impurities. Next, the obtained slab is subjected to a hot rolling process (heating temperature of 1200 ° C. and finish rolling end temperature of 800 ° C.), a cold rolling process (cumulative reduction rate of 80%), an annealing process, and a cooling process (average cooling rate 10° C./sec) to produce a cold-rolled steel sheet with a thickness of 0.4 mm. In each example, the coiling temperature condition I (less than 550 ° C.) in the hot rolling process and the annealing process condition II (dew point: -20 to 5 ° C., annealing temperature: 750 ° C. or higher, and holding time 30 to 200 seconds) Table 2 shows the cases where the conditions are satisfied (○) and the cases where the conditions are not satisfied (×, A or B). Specifically, the winding was performed at a temperature of 500.degree. C. in the example satisfying the condition I, and the winding was performed at a temperature of 650.degree. In addition, in the example satisfying Condition II, the annealing process was performed under the conditions of a dew point of -5°C, an annealing temperature of 800°C, and a holding time of 150 seconds. On the other hand, in an example that does not satisfy condition II, under the conditions of condition A (dew point -40 ° C., annealing temperature 800 ° C. and holding time 60 seconds) or condition B (dew point 8 ° C., annealing temperature 800 ° C. and holding time 280 seconds) An annealing step was performed.
　次に、得られた冷間圧延鋼板の表面に適宜めっき処理を施し、溶融亜鉛めっき層（ＧＩ）、合金化溶融亜鉛めっき層（ＧＡ）又は電気亜鉛めっき層（ＥＧ）を形成した。また、製造した冷間圧延鋼板から採取した試料について化学組成を分析したところ、表１に示すスラブの化学組成と変化がなかった。最後に、得られた冷間圧延鋼板又はめっき鋼板に対し、調質圧延を実施した。各例において、調質圧延工程の条件ＩＩＩ（ロールの算術平均粗さＲａ：－１．３μｍ以下、及び圧下率：０．６％以下）を満たす場合（〇）と満たさない場合（×）をそれぞれ表２に示す。具体的には、条件ＩＩＩを満たす例では、１．２μｍの算術平均粗さＲａを有するロールを用いて０．５％の圧下率で調質圧延を行い、一方で条件ＩＩＩを満たさない例では、１．８μｍの算術平均粗さＲａを有するロールを用いて１．０％の圧下率で調質圧延を行った。 Next, the surface of the obtained cold-rolled steel sheet was appropriately plated to form a hot-dip galvanized layer (GI), an alloyed hot-dip galvanized layer (GA), or an electro-galvanized layer (EG). Further, when the chemical composition of the sample taken from the produced cold-rolled steel plate was analyzed, there was no change from the chemical composition of the slab shown in Table 1. Finally, the obtained cold-rolled steel sheet or plated steel sheet was subjected to temper rolling. In each example, when the condition III of the skin pass rolling process (roll arithmetic mean roughness Ra: -1.3 μm or less and rolling reduction: 0.6% or less) is satisfied (○) and not satisfied (×) Each is shown in Table 2. Specifically, in an example that satisfies Condition III, skin pass rolling is performed at a rolling reduction of 0.5% using rolls having an arithmetic mean roughness Ra of 1.2 μm, while in an example that does not satisfy Condition III, , and rolls having an arithmetic mean roughness Ra of 1.8 μm were used to perform skin pass rolling at a rolling reduction of 1.0%.
　得られた鋼板の特性は以下の方法によって測定及び評価した。 The properties of the obtained steel sheets were measured and evaluated by the following methods.
［Ｓｔｒ及びΔＳｔｒ］
　Ｓｔｒ及びΔＳｔｒは、以下のようにして決定した。まず、鋼板の端面から１００ｍｍ以上離れた位置から、圧延方向（Ｌ方向）に直角な方向（Ｃ方向）を試験方向とするＪＩＳ　Ｚ２２４１：２０１１の５号引張試験片を採取し、次いで採取した鋼板試料の表面（鋼板試料の表面にめっき層が存在する場合には、めっき層の表面）に対してキーエンス社製のＶＫ－Ｘ２５０／１５０形状解析レーザ顕微鏡を用いて三次元画像解析を行い、ＪＩＳ　Ｂ０６８１－２：２０１８の規定に準拠して初期のＳｔｒを決定した。三次元画像解析の対象領域は１０ｍｍ（Ｃ方向）×２ｍｍ（Ｌ方向）とした。次に、鋼板試料に対して単軸における５％の引張歪を付与した後、先と同様の測定を行うことで５％の引張歪を付与した後のＳｔｒを決定し、最後に初期のＳｔｒから５％引張歪付与後のＳｔｒを引き算することによりΔＳｔｒを決定した。 [Str and ΔStr]
Str and ΔStr were determined as follows. First, a No. 5 tensile test piece of JIS Z2241: 2011 with a direction (C direction) perpendicular to the rolling direction (L direction) as the test direction is taken from a position at least 100 mm away from the end surface of the steel plate, and then the sampled steel plate. Three-dimensional image analysis is performed using a VK-X250/150 shape analysis laser microscope manufactured by Keyence Corporation on the surface of the sample (the surface of the coating layer if there is a coating layer on the surface of the steel plate sample), and JIS Initial Str was determined according to the provisions of B0681-2:2018. The target area for three-dimensional image analysis was 10 mm (C direction)×2 mm (L direction). Next, after applying 5% tensile strain in a uniaxial direction to the steel plate sample, Str after applying 5% tensile strain is determined by performing the same measurement as above, and finally the initial Str ΔStr was determined by subtracting Str after 5% tensile strain was applied from .
［引張強さ］
　引張強さは、圧延方向に直角な方向を長手方向とするＪＩＳ　Ｚ２２４１：２０１１の５号引張試験片を鋼板から採取し、ＪＩＳ　Ｚ２２４１：２０１１に準拠して引張試験を行うことで測定した。 [Tensile strength]
Tensile strength was measured by taking a No. 5 tensile test piece of JIS Z2241:2011 from a steel plate with the longitudinal direction perpendicular to the rolling direction and performing a tensile test in accordance with JIS Z2241:2011.
［成形後外観］
　成形後外観は、成形後のドアアウタの表面に発生するゴーストラインの程度により評価した。プレス成形後の表面を砥石掛けし、表面に生じた数ｍｍオーダー間隔の縞模様を、ゴーストラインと判断し、筋模様の発生程度によって１～５で評点付けした。１００ｍｍ×１００ｍｍの任意の領域を目視で確認し、筋模様が全く確認されなかった場合を「１」とし、筋模様の最大長さが２０ｍｍ以下の場合を「２」とし、筋模様の最大長さが２０ｍｍ超、５０ｍｍ以下の場合を「３」とし、筋模様の最大長さが５０ｍｍ超、７０ｍｍ以下の場合を「４」とし、筋模様の最大長さが７０ｍｍを超える場合を「５」とした。評価が「３」以下であった場合、成形後外観に優れるとして合格と判定した。一方、評価が「４」以上であった場合、成形後外観に劣るとして不合格と判定した。 [Appearance after molding]
The appearance after molding was evaluated by the degree of ghost lines generated on the surface of the door outer after molding. The surface after press molding was ground with a grindstone, and the striped patterns at intervals of several millimeters on the surface were judged to be ghost lines, and were rated on a scale of 1 to 5 depending on the degree of occurrence of the striped pattern. An arbitrary area of 100 mm x 100 mm was visually checked, and the case where no streak pattern was confirmed was rated as "1", and the case where the maximum length of the streak pattern was 20 mm or less was rated as "2", and the maximum length of the streak pattern. If the length is over 20 mm and 50 mm or less, "3"; and When the evaluation was "3" or less, it was determined to be acceptable because the appearance after molding was excellent. On the other hand, when the evaluation was "4" or more, it was judged to be unacceptable because the appearance after molding was inferior.
　引張強さが５００ＭＰａ以上及び成形後外観の評価が３以下の場合を、改善された成形後外観を有する高強度鋼板として評価した。その結果を表２に示す。表２に示す金属組織において、硬質相はマルテンサイト、ベイナイト、焼き戻しマルテンサイト及びパーライトの少なくとも１種を含むか又はそれらの少なくとも１種であった。また、Ｘ線回析による残留オーステナイト測定の結果、残留オーステナイトの面積率は全ての例で１％未満であった。 A steel sheet with a tensile strength of 500 MPa or more and a post-forming appearance evaluation of 3 or less was evaluated as a high-strength steel sheet with an improved post-forming appearance. Table 2 shows the results. In the metallographic structure shown in Table 2, the hard phase included or was at least one of martensite, bainite, tempered martensite and pearlite. As a result of measurement of retained austenite by X-ray diffraction, the area ratio of retained austenite was less than 1% in all the examples.
　表２を参照すると、比較例４では、調質圧延工程におけるロール粗さが粗く、圧下率も高かったために、ロールの粗度が鋼板表面に強く転写されてしまい、初期のＳｔｒが０．７５超となった。その結果として成形後外観が劣化した。比較例５では、製造条件における条件Ｉ～ＩＩＩを全て満足せず、初期のＳｔｒが０．７５超となった。その結果として成形後外観が劣化した。比較例１１では、熱間圧延工程において巻き取り温度が高かったために、熱間圧延鋼板の表層においてＭｎ等の内部酸化物の形成が促進されてしまったものと考えられる。その結果として、焼鈍工程における脱炭処理を適切なものとすることができず、初期のＳｔｒが所望の範囲外となり、成形後外観が劣化した。比較例１２では、焼鈍工程における露点が低かったため、鋼板表層の脱炭が不十分で、Ｓｔｒが０．７５超となり、成形後外観が悪化した。比較例１３では、焼鈍工程における露点が高く焼鈍時間も長かったために、鋼板表層の脱炭が進行しすぎてしまったものと考えられる。その結果としてＳｔｒが０．３５未満となり、成形後外観が劣化した。比較例１８では、製造条件における条件Ｉ及びＩＩを満足せず、初期のＳｔｒが０．３５未満となった。その結果として成形後外観が劣化した。比較例２１及び２２では、Ｃ又はＭｎ含有量が高く、スラブ鋳造の際の凝固時におけるＭｎの拡散が阻害され、Ｍｎ偏析を十分に抑制することができなかったと考えられる。その結果として、ΔＳｔｒを０．１５以下に抑えることができず、成形後外観が劣化した。比較例２３では、Ｓｉ含有量が高かったために、熱間圧延工程において熱間圧延鋼板の表層で内部酸化物の形成が促進されてしまったものと考えられる。その結果として、焼鈍工程における脱炭処理を適切なものとすることができず、初期のＳｔｒが所望の範囲外となり、成形後外観が劣化した。比較例２４及び２５では、Ｃ又はＭｎ含有量が低かったために、十分な強度が得られなかった。 Referring to Table 2, in Comparative Example 4, the roll roughness in the temper rolling process was rough and the rolling reduction was high, so the roll roughness was strongly transferred to the steel plate surface, and the initial Str was 0.75. became super. As a result, the appearance after molding deteriorated. In Comparative Example 5, none of the manufacturing conditions I to III were satisfied, and the initial Str exceeded 0.75. As a result, the appearance after molding deteriorated. In Comparative Example 11, the coiling temperature in the hot rolling process was high, which probably promoted the formation of internal oxides such as Mn in the surface layer of the hot rolled steel sheet. As a result, the decarburization treatment in the annealing process could not be made appropriate, the initial Str was out of the desired range, and the appearance after molding deteriorated. In Comparative Example 12, since the dew point in the annealing step was low, decarburization of the surface layer of the steel sheet was insufficient, Str exceeded 0.75, and the appearance after forming deteriorated. In Comparative Example 13, the decarburization of the surface layer of the steel sheet progressed excessively because the dew point in the annealing step was high and the annealing time was long. As a result, Str became less than 0.35, and the appearance after molding deteriorated. In Comparative Example 18, the conditions I and II in the manufacturing conditions were not satisfied, and the initial Str was less than 0.35. As a result, the appearance after molding deteriorated. In Comparative Examples 21 and 22, the C or Mn content was high, and diffusion of Mn during solidification during slab casting was inhibited, and Mn segregation could not be sufficiently suppressed. As a result, ΔStr could not be suppressed to 0.15 or less, and the appearance after molding deteriorated. In Comparative Example 23, since the Si content was high, it is considered that the formation of internal oxides was promoted in the surface layer of the hot-rolled steel sheet during the hot-rolling process. As a result, the decarburization treatment in the annealing process could not be made appropriate, the initial Str was out of the desired range, and the appearance after molding deteriorated. In Comparative Examples 24 and 25, sufficient strength was not obtained due to the low C or Mn content.
　これとは対照的に、本発明例１～３、６～１０、１４～１７、１９、２０及び２６～２９では、所定の化学組成及び金属組織を有し、とりわけ鋼板表面の初期Ｓｔｒ及び引張歪付与後のΔＳｔｒをそれぞれ０．３５～０．７５及び０．１５以下に制御することにより、引張強さ５００ＭＰａ以上の高強度を維持しつつ、プレス成形によって歪が付与された場合においても、鋼板表面におけるゴーストラインの発生を顕著に抑制して、改善された成形後外観を達成することができた。 In contrast, Inventive Examples 1-3, 6-10, 14-17, 19, 20 and 26-29 have a predetermined chemical composition and metallographic structure, especially the initial Str and tensile strength of the steel plate surface. By controlling the ΔStr after straining to 0.35 to 0.75 and 0.15 or less, respectively, while maintaining a high tensile strength of 500 MPa or more, even when strain is imparted by press molding, It was possible to remarkably suppress the generation of ghost lines on the surface of the steel sheet and achieve an improved appearance after forming.

Claims (3)

  1. 　化学組成が、質量％で、
    　Ｃ　：０．０２０～０．１００％、
    　Ｍｎ：１．００～２．５０％、
    　Ｐ　：０．１００％以下、
    　Ｓ　：０．０２００％以下、
    　Ａｌ：０．００５～０．７００％、
    　Ｎ　：０．０１５０％以下、
    　Ｏ　：０．０１００％以下、
    　Ｓｉ：０～１．５００％、
    　Ｃｒ：０～０．８０％、
    　Ｍｏ：０～０．５０％、
    　Ｂ　：０～０．０１００％、
    　Ｔｉ：０～０．１００％、
    　Ｎｂ：０～０．０６０％、
    　Ｖ　：０～０．５０％、
    　Ｎｉ：０～１．００％、
    　Ｃｕ：０～１．００％、
    　Ｗ　：０～１．００％、
    　Ｓｎ：０～１．００％、
    　Ｓｂ：０～０．２００％、
    　Ｃａ：０～０．０１００％、
    　Ｍｇ：０～０．０１００％、
    　Ｚｒ：０～０．０１００％、
    　ＲＥＭ：０～０．０１００％、並びに
    　残部：Ｆｅ及び不純物であり、
    　金属組織が、面積％で、
    　フェライト：７０～９７％、及び
    　硬質相：３～３０％であり、
    　表面のＳｔｒが０．３５～０．７５であり、
    　前記Ｓｔｒと５％引張歪付与後のＳｔｒとの差ΔＳｔｒが０．１５以下である、鋼板。     The chemical composition, in mass %,
    C: 0.020 to 0.100%,
    Mn: 1.00-2.50%,
    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,
    Si: 0 to 1.500%,
    Cr: 0 to 0.80%,
    Mo: 0-0.50%,
    B: 0 to 0.0100%,
    Ti: 0 to 0.100%,
    Nb: 0 to 0.060%,
    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: 0-0.0100%,
    Zr: 0 to 0.0100%,
    REM: 0 to 0.0100%, and the balance: Fe and impurities,
    The metal structure, in area %,
    ferrite: 70 to 97%, and hard phase: 3 to 30%,
    Str of the surface is 0.35 to 0.75,
    A steel sheet, wherein the difference ΔStr between the Str and the Str after application of 5% tensile strain is 0.15 or less.
  2. 　前記化学組成が、質量％で、
    　Ｓｉ：０．００５～１．５００％、
    　Ｃｒ：０．００１～０．８０％、
    　Ｍｏ：０．００１～０．５０％、
    　Ｂ　：０．０００１～０．０１００％、
    　Ｔｉ：０．００１～０．１００％、
    　Ｎｂ：０．００１～０．０６０％、
    　Ｖ　：０．００１～０．５０％、
    　Ｎｉ：０．００１～１．００％、
    　Ｃｕ：０．００１～１．００％、
    　Ｗ　：０．００１～１．００％、
    　Ｓｎ：０．００１～１．００％、
    　Ｓｂ：０．００１～０．２００％、
    　Ｃａ：０．０００１～０．０１００％、
    　Ｍｇ：０．０００１～０．０１００％、
    　Ｚｒ：０．０００１～０．０１００％、及び
    　ＲＥＭ：０．０００１～０．０１００％
    からなる群から選択される１種又は２種以上を含む、請求項１に記載の鋼板。     The chemical composition, in mass %,
    Si: 0.005 to 1.500%,
    Cr: 0.001 to 0.80%,
    Mo: 0.001 to 0.50%,
    B: 0.0001 to 0.0100%,
    Ti: 0.001 to 0.100%,
    Nb: 0.001 to 0.060%,
    V: 0.001 to 0.50%,
    Ni: 0.001 to 1.00%,
    Cu: 0.001 to 1.00%,
    W: 0.001 to 1.00%,
    Sn: 0.001 to 1.00%,
    Sb: 0.001 to 0.200%,
    Ca: 0.0001 to 0.0100%,
    Mg: 0.0001-0.0100%,
    Zr: 0.0001-0.0100%, and REM: 0.0001-0.0100%
    The steel sheet according to claim 1, comprising one or more selected from the group consisting of
  3. 　前記硬質相が、マルテンサイト、ベイナイト、焼き戻しマルテンサイト及びパーライトの少なくとも１種からなる、請求項１又は２に記載の鋼板。 The steel sheet according to claim 1 or 2, wherein the hard phase consists of at least one of martensite, bainite, tempered martensite and pearlite.
PCT/JP2022/031748 2022-02-01 2022-08-23 Steel sheet WO2023149002A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
JP2022014470 2022-02-01
JP2022-014470 2022-02-01

Publications (1)

Publication Number Publication Date
WO2023149002A1 true WO2023149002A1 (en) 2023-08-10

Family

ID=87552033

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/JP2022/031748 WO2023149002A1 (en) 2022-02-01 2022-08-23 Steel sheet

Country Status (1)

Country Link
WO (1) WO2023149002A1 (en)

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2005220430A (en) 2004-02-09 2005-08-18 Jfe Steel Kk High strength hot dip galvanized steel sheet having excellent surface quality
WO2016121388A1 (en) * 2015-01-28 2016-08-04 Ｊｆｅスチール株式会社 High-strength cold-rolled steel sheet, high-strength plated steel sheet, and method for manufacture thereof
WO2020145256A1 (en) * 2019-01-07 2020-07-16 日本製鉄株式会社 Steel sheet and method for manufacturing same
WO2020145259A1 (en) * 2019-01-07 2020-07-16 日本製鉄株式会社 Steel plate and manufacturing method thereof

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2005220430A (en) 2004-02-09 2005-08-18 Jfe Steel Kk High strength hot dip galvanized steel sheet having excellent surface quality
WO2016121388A1 (en) * 2015-01-28 2016-08-04 Ｊｆｅスチール株式会社 High-strength cold-rolled steel sheet, high-strength plated steel sheet, and method for manufacture thereof
WO2020145256A1 (en) * 2019-01-07 2020-07-16 日本製鉄株式会社 Steel sheet and method for manufacturing same
WO2020145259A1 (en) * 2019-01-07 2020-07-16 日本製鉄株式会社 Steel plate and manufacturing method thereof

Similar Documents

Publication Publication Date Title
CN109715838B (en) Method for producing a flat steel product and flat steel product
EP3128027B1 (en) High-strength cold rolled steel sheet having high yield ratio, and production method therefor
JP4635525B2 (en) High-strength steel sheet excellent in deep drawability and manufacturing method thereof
EP2811047B1 (en) Hot-dip galvanized steel sheet and production method therefor
KR101379973B1 (en) High-strength hot-dip galvanized steel sheet and manufacturing method therefor
EP2500445A1 (en) High-strength steel sheet having excellent processability and paint bake hardenability, and method for producing of high-strength steel sheet
WO2011004779A1 (en) High-strength steel sheet and manufacturing method therefor
EP3257962A1 (en) High-strength hot-dip galvanized steel sheet and manufacturing method therefor
KR20020021646A (en) Hot rolled steel plate, cold rolled steel plate and hot dip galvanized steel plate being excellent in strain aging hardening characteristics, and method for their production
EP2749665B1 (en) High strength hot dip galvanized steel sheet having excellent deep- drawability, and method for producing same
JP4507851B2 (en) High-strength cold-rolled steel sheet and manufacturing method thereof
US11332804B2 (en) High-strength cold-rolled steel sheet, high-strength coated steel sheet, and method for producing the same
WO2008082146A9 (en) High strength zn-coated steel sheet having excellent mechanical properites and surface quality and the method for manufacturing the same
CN113348259A (en) High-strength hot-dip galvanized steel sheet and method for producing same
CN113302322B (en) Steel sheet and method for producing same
JP7111252B2 (en) Coated steel member, coated steel plate and manufacturing method thereof
WO2022181761A1 (en) Steel sheet
CN112154222A (en) High-strength steel sheet and method for producing same
CN113260721B (en) Steel sheet and method for producing same
JP4367205B2 (en) Strain aging treatment method for steel sheet and method for producing high-strength structural member
CN116490630A (en) High-strength plated steel sheet excellent in formability and surface quality, and method for producing same
WO2023149002A1 (en) Steel sheet
EP4249619A1 (en) Plated steel sheet having excellent strength, formability and surface quality, and manufacturing method therefor
EP4089188B1 (en) Steel sheet and method of manufacturing the same
JP2004218018A (en) High-strength cold-rolled steel sheet which is excellent in workability and strain age-hardening property, high-strength plated steel sheet and their production methods

Legal Events

Date Code Title Description
121 Ep: the epo has been informed by wipo that ep was designated in this application

Ref document number: 22924907

Country of ref document: EP

Kind code of ref document: A1

ENP Entry into the national phase

Ref document number: 2023578364

Country of ref document: JP

Kind code of ref document: A