WO2023149002A1 - Tôle d'acier - Google Patents

Tôle d'acier 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
English (en)
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/fr

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

L'invention concerne une tôle d'acier qui a une composition chimique comprenant 0,020 à 0,100 % de C, 1,00 à 2,50 % de Mn, pas plus de 0,100 % de P, pas plus de 0,0200 % de S, 0,005 à 0,700 % d'Al, pas plus de 0,0150 % de N, pas plus de 0,0100 % de O, etc., la partie restante étant du Fe et des impuretés, qui a une structure métallique comprenant, en % surfacique, 70 à 97 % de ferrite et 3 à 30 % d'une phase dure, et dans laquelle la Str de la surface est de 0,35 à 0,75 et une différence ΔStr entre la Str et une Str obtenue après que la tôle d'acier a subi une déformation en traction de 5 % n'est pas supérieure à 0,15.
PCT/JP2022/031748 2022-02-01 2022-08-23 Tôle d'acier WO2023149002A1 (fr)

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 (fr) 2023-08-10

Family

ID=87552033

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/JP2022/031748 WO2023149002A1 (fr) 2022-02-01 2022-08-23 Tôle d'acier

Country Status (1)

Country Link
WO (1) WO2023149002A1 (fr)

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2005220430A (ja) 2004-02-09 2005-08-18 Jfe Steel Kk 表面品質に優れる高強度溶融亜鉛めっき鋼板
WO2016121388A1 (fr) * 2015-01-28 2016-08-04 Ｊｆｅスチール株式会社 Tôle d'acier laminée à froid à haute résistance, tôle d'acier plaquée à haute résistance, et leur procédé de fabrication
WO2020145256A1 (fr) * 2019-01-07 2020-07-16 日本製鉄株式会社 Tôle d'acier et procédé pour la fabriquer
WO2020145259A1 (fr) * 2019-01-07 2020-07-16 日本製鉄株式会社 Plaque d'acier et son procédé de fabrication

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2005220430A (ja) 2004-02-09 2005-08-18 Jfe Steel Kk 表面品質に優れる高強度溶融亜鉛めっき鋼板
WO2016121388A1 (fr) * 2015-01-28 2016-08-04 Ｊｆｅスチール株式会社 Tôle d'acier laminée à froid à haute résistance, tôle d'acier plaquée à haute résistance, et leur procédé de fabrication
WO2020145256A1 (fr) * 2019-01-07 2020-07-16 日本製鉄株式会社 Tôle d'acier et procédé pour la fabriquer
WO2020145259A1 (fr) * 2019-01-07 2020-07-16 日本製鉄株式会社 Plaque d'acier et son procédé de fabrication

Similar Documents

Publication Publication Date Title
CN109715838B (zh) 用于生产扁钢产品的方法和扁钢产品
EP3128027B1 (fr) Tôle en acier laminée à froid à grande résistance mécanique, ayant un rapport élevé de limite d'élasticité, et son procédé de production
JP4635525B2 (ja) 深絞り性に優れた高強度鋼板およびその製造方法
EP2811047B1 (fr) Tôle en acier galvanisée par immersion à chaud, et procédé de fabrication de celle-ci
KR101379973B1 (ko) 고강도 용융 아연 도금 강판 및 그 제조 방법
WO2011004779A1 (fr) Tôle d’acier à haute résistance et procédé de fabrication associé
EP2500445A1 (fr) Feuille d'acier de haute résistance ayant une excellente aptitude au traitement et une excellente aptitude au durcissement de cuisson d'une peinture et son procédé de fabrication
EP3257962A1 (fr) Tôle d'acier galvanisée par immersion à chaud à haute résistance et procédé de fabrication s'y rapportant
KR20020021646A (ko) 변형 시효 경화 특성이 우수한 열연 강판, 냉연 강판 및용융 아연 도금 강판, 그리고 이들의 제조 방법
EP2749665B1 (fr) Tôle d'acier galvanisée par immersion à chaud à haute résistance qui présente une excellente aptitude à l'emboutissage, et procédé de production de cette dernière
JP4507851B2 (ja) 高強度冷延鋼板およびその製造方法
US11332804B2 (en) High-strength cold-rolled steel sheet, high-strength coated steel sheet, and method for producing the same
CN113302322B (zh) 钢板及其制造方法
WO2008082146A9 (fr) Feuille d'acier à revêtement zn possédant d'excellentes propriétés mécaniques et qualité de surface et son procédé de fabrication
CN113348259A (zh) 高强度热浸镀锌钢板和其制造方法
CN112154222A (zh) 高强度钢板及其制造方法
CN113260721B (zh) 钢板及其制造方法
WO2022181761A1 (fr) Tôle d'acier
JP4367205B2 (ja) 鋼板の歪時効処理方法および高強度構造部材の製造方法
JP7111252B2 (ja) 被覆鋼部材、被覆鋼板およびそれらの製造方法
CN116490630A (zh) 成型性和表面质量优异的高强度镀覆钢板及其制造方法
WO2023149002A1 (fr) Tôle d'acier
EP4249619A1 (fr) Feuille d'acier plaquée présentant d'excellentes résistance, formabilité et qualité de surface, et son procédé de fabrication
EP4089188B1 (fr) Tôle d'acier, et procédé de fabrication de celle-ci
JP2004218018A (ja) 加工性と歪時効硬化特性に優れる高強度冷延鋼板および高強度めっき鋼板ならびにそれらの製造方法

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