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  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
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  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
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  • C21D8/0236—Cold rolling
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
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  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/009—Pearlite

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
• the chemical composition in mass% is C: 0.020% or more and 0.090% or less, Si: 0.200% or less, Mn: 0.45% or more and 2.10% or less, P : 0.030% or less, S: 0.020% or less, sol.
• the metal structure consists of ferrite and a second phase with a volume fraction of 0.01 to 5.0%, and the position of more than 20 ⁇ m in the plate thickness direction from the surface to the plate in the plate thickness direction from the surface
• the metal structure of the inner region which is in the range up to 1/4 of the thickness, is composed of ferrite and a second phase with a volume fraction of 2.0 to 10.0%, and the second phase in the surface layer region.
• the metal structure of the surface layer region is composed of ferrite and a second phase with a volume fraction of 0.01 to 5.0%, and the volume fraction of the second phase in the surface layer region is By making the volume fraction smaller than the volume fraction of the second phase in the inner region and further increasing the volume fraction of the second phase in the inner region, the occurrence of surface unevenness during molding is suppressed and the material strength with a tensile strength of 400 MPa or more It is described that it can be compatible with On the other hand, in the automobile industry and the like, there is a demand for further weight reduction of steel sheets, and in order to achieve such weight reduction, it becomes necessary to increase the strength of steel sheets more than ever before. Therefore, there is still a great need for a steel sheet that can solve the problem of fine unevenness that can occur on the surface of the steel sheet after forming, even when the steel sheet is strengthened to the same or higher strength than before.
• 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 focused on the morphology of the hard phase in the metallographic structure. As a result, the present inventors have found that by reducing the formation of the striped hard phase and dispersing the hard phase more uniformly in the metal structure, while maintaining the high strength based on such a hard phase, molding, etc. The inventors have found that the formation of fine irregularities on the surface of the steel sheet is remarkably suppressed even when strain is applied by, and completed the present invention.
• the present invention that has achieved the above object is as follows.
• A 10 [C] + 0.3 [Mn] - 0.2 [Si] - 0.6 [Al] - 0.05 [Cr] - 0.2 [Mo]
• [C], [Mn], [Si], [Al], [Cr] and [Mo] are the content [% by mass] of each element, and 0% when no element is contained .
• a steel plate according to an embodiment of the present invention is The chemical composition, in mass %, C: 0.040 to 0.100%, Mn: 1.00-2.50%, Si: 0.005 to 1.500%, P: 0.100% or less, S: 0.0200% or less, Al: 0.005 to 0.700%, N: 0.0150% or less, O: 0.0100% or less, Cr: 0 to 0.80%, Mo: 0-0.50%, 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, and the balance: Fe and impur
• A 10 [C] + 0.3 [Mn] - 0.2 [Si] - 0.6 [Al] - 0.05 [Cr] - 0.2 [Mo]
• [C], [Mn], [Si], [Al], [Cr] and [Mo] are the content [% by mass] of each element, and 0% when no element is contained .
• 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 focused attention on the morphology of the hard phase in the metal structure and conducted studies in order to improve such poor appearance after molding. As a result, the present inventors found that in a steel sheet such as DP steel in which a soft phase and a hard phase are mixed, the degree of ghost lines becomes remarkable due to the presence of a hard phase connected in stripes in the metal structure. I found out.
• the present inventors have found that by reducing the formation of such striped hard phases and dispersing the hard phases more uniformly in the metal structure, while sufficiently maintaining the high strength based on the hard phases, It has been found that even when strain is imparted by forming or the like, the formation of fine irregularities on the surface of the steel sheet can be remarkably suppressed, whereby the generation of ghost lines can be remarkably suppressed.
• the present inventors found that in the slab casting process in which molten steel is solidified to cast a slab, Mn segregation during solidification should be reduced. In connection with this, detailed studies were conducted on techniques for reducing Mn segregation from the two viewpoints of center segregation and micro segregation.
• the present inventors have found that the center segregation of Mn is conspicuously suppressed by appropriately controlling the conditions during solidification to suppress the flow of molten steel.
• the maximum connection length of the hard phase in the rolling direction at the half thickness position of the finally obtained steel sheet can be controlled to 80 ⁇ m or less.
• the present inventors considered that it is effective to promote the diffusion of Mn during solidification in order to reduce the microsegregation, and conducted various studies. In order to accelerate the diffusion of Mn, it is effective to create a structure in which Mn easily diffuses. Therefore, the present inventors focused on the ⁇ phase, in which Mn diffuses at a high rate, and experimentally examined the degree of influence of each element in steel on the microsegregation of Mn in order to set the solidification mode to be ⁇ solidification.
• the present inventors have found that when the C and Mn contents are high, ⁇ solidification does not occur during solidification, and the diffusion rate of Mn decreases and microsegregation tends to increase, but Si, Al, Cr As for and Mo, it was found that when the contents thereof are high, the diffusion of Mn is promoted during solidification, and the microsegregation can be reduced. More specifically, the present inventors set the index A defined by the content of these elements together with the coefficient considering the degree of influence on microsegregation, that is, the index A represented by the following formula 1 to 1.10% By controlling the following, the micro-segregation of Mn can be remarkably suppressed. It has been found that the following can be controlled.
• A 10 [C] + 0.3 [Mn] - 0.2 [Si] - 0.6 [Al] - 0.05 [Cr] - 0.2 [Mo]
• [C], [Mn], [Si], [Al], [Cr] and [Mo] are the content [% by mass] of each element, and 0% when no element is contained .
• the hard phase at the thickness 1/2 position and the thickness 1/4 position of the steel plate It is possible to control the maximum length of connection in the rolling direction within a predetermined range, that is, to remarkably suppress the formation of striped hard phases in the metal structure of the steel sheet finally obtained, and to increase the hard phases throughout the metal structure. Uniform dispersion becomes possible.
• the steel sheet according to the embodiment of the present invention even when strain is applied by forming such as press forming while sufficiently maintaining high strength based on the hard phase, generation of fine unevenness on the steel sheet surface can be remarkably suppressed, thereby making it possible to remarkably suppress the occurrence of appearance defects such as ghost lines. Therefore, according to embodiments of the present invention, it is possible to provide a high strength steel sheet with improved post forming appearance.
• C is an element that increases the strength of the steel sheet.
• the C content is made 0.040% or more.
• the C content may be 0.045% or more, 0.050% or more, 0.055% or more, or 0.060% or more.
• the C content should be 0.100% or less.
• the C content may be 0.095% or less, 0.090% or less, 0.080% 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.
• 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.
• 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 the diffusion of Mn during solidification and reducing microsegregation of Mn.
• the Si content should be 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.
• the Si content should be 1.500% or less.
• Si content is 1.400% or less, 1.200% or less, 1.000% or less, 0.850% or less, less than 0.600%, 0.550% or less, 0.500% or less, or 0.300% It may be below.
• 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 microsegregation of Mn.
• 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, 0.0030% 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%.
• Cr is an element that increases the hardenability of steel and contributes to the improvement of the strength of the steel sheet. Cr is also an effective element for promoting the diffusion of Mn during solidification and reducing microsegregation of Mn.
• 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 element effective in promoting the diffusion of Mn during solidification and reducing microsegregation of Mn.
• the Mo content may be 0%, but in order to obtain these effects, the Mo content is preferably 0.001% 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, 0.060% or less, 0.050% 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.
• index A 1.10% or less
• the chemical composition of the steel sheet according to the embodiment of the present invention requires that the index A represented by the following formula 1 is 1.10% or less.
• A 10 [C] + 0.3 [Mn] - 0.2 [Si] - 0.6 [Al] - 0.05 [Cr] - 0.2 [Mo]
• [C], [Mn], [Si], [Al], [Cr] and [Mo] are the content [% by mass] of each element, and 0% when no element is contained .
• Index A may be 1.08% or less, 1.05% or less, 1.03% or less, 1.00% or less, 0.98% or less, or 0.95% or less.
• the lower limit of the index A is not particularly limited, for example, the index A is 0.65% or more, 0.70% or more, 0.75% or more, 0.80% or more, 0.85% or more, 0.88% or more, or 0.90% or more.
• 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 95% and hard phase: 5 to 30% in terms of area %, more specifically ferrite: 70 to 95% and hard phase: 5 to 30% only. 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 7% or more, 10% or more, or 12% or more.
• the area fraction of ferrite may be 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 maximum connecting length in the rolling direction of the hard phase at the half thickness position of the steel sheet is 80 ⁇ m or less.
• the maximum connection length of the hard phase in the rolling direction at the half thickness position of the steel sheet may be 10 ⁇ m or more or 20 ⁇ m or more.
• the maximum connecting length of the hard phase in the rolling direction at the position of 1/4 of the plate thickness of the steel plate is 40 ⁇ m or less.
• the lower limit is not particularly limited, for example, the maximum connecting length of the hard phase in the rolling direction at the position of 1/4 of the thickness of the steel sheet may be 5 ⁇ m or more or 8 ⁇ m or more.
• Microsegregation of Mn is related to the entire region in the plate thickness direction of the steel plate, but in the embodiment of the present invention, typically the maximum coupling in the rolling direction of the hard phase at the 1/4 position of the plate thickness of the steel plate By observing and controlling the length, microsegregation of Mn is evaluated and suppressed.
• the length of the connected hard phase observation range in the rolling direction may be less than 800 ⁇ m or greater than 800 ⁇ m.
• the lower limit of the length of the connecting hard phase observation range in the rolling direction is 600 ⁇ m
• a hard phase having a length connected in the rolling direction is extracted by image processing.
• "connected” means that the crystal grain boundaries of the hard phase are in contact.
• the one with the longest connection length in the rolling direction is determined as the “maximum connection length in the rolling direction of the hard phase at the 1/2 plate thickness position”.
• the maximum connection length in the rolling direction of the hard phase at the 1/4 position of the plate thickness is defined as “a region of 100 ⁇ m in the plate thickness direction centered at the position of 1/2 thickness from the steel plate surface” is defined as “1/4 thickness from the steel plate surface. Measured in the same manner as in the measurement of the maximum connection length in the rolling direction of the hard phase at the 1/2 position in the plate thickness, except that it was changed to "a region of 100 ⁇ m in the plate thickness direction centered on the position of It is determined.
• the average grain size of ferrite in the metallographic structure is 5.0-30.0 ⁇ m.
• the average grain size of ferrite may be 7.0 ⁇ m or more, 8.0 ⁇ m or more, 9.0 ⁇ m or more, or 10.0 ⁇ m or more.
• the average grain size of ferrite may be 27.0 ⁇ m or less, 25.0 ⁇ m or less, 20.0 ⁇ m or less, 16.0 ⁇ m or less, 14.0 ⁇ m or less, or 12.0 ⁇ m or less.
• the average grain size of ferrite in a steel sheet is determined as follows. First, 10 fields of view were observed 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 position of 1/2 of the plate thickness in the plate thickness direction, and image analysis was performed using Adobe "Photoshop CS5". Image analysis is performed using software, and the area fraction of ferrite and the number of ferrite particles in each field of view are calculated. Next, the area fraction of ferrite and the number of ferrite particles in 10 fields of view are totaled, and the total area fraction of ferrite is divided by the total number of ferrite particles to calculate the average area fraction per ferrite particle.
• the equivalent circle diameter is calculated from the average area fraction and the number of particles, and the obtained equivalent circle diameter is determined as the average crystal grain size of ferrite.
• the average grain size of the hard phases in the metallographic structure is between 1.0 and 5.0 ⁇ m.
• controlling the average crystal grain size of the hard phase within such a fine range can further improve the appearance of the steel sheet, particularly the appearance after forming. It becomes possible.
• the average grain size of the hard phase may be 1.2 ⁇ m or more, 1.5 ⁇ m or more, 1.7 ⁇ m or more, or 2.0 ⁇ m or more.
• the average grain size of the hard phase may be 4.7 ⁇ m or less, 4.5 ⁇ m or less, 4.2 ⁇ m or less, 4.0 ⁇ m or less, 3.8 ⁇ m or less, 3.6 ⁇ m or less, or 3.4 ⁇ m or less. good.
• the average grain size of the hard phase is determined as follows. First, 10 fields of view were observed 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 position of 1/2 of the plate thickness in the plate thickness direction, and image analysis was performed using Adobe "Photoshop CS5". Image analysis is performed using software, and the area fraction of the hard phase and the number of particles of the hard phase in each field of view are calculated. Next, the area fraction of the hard phase and the number of ferrite particles in 10 fields of view are totaled, and the total area fraction of the hard phase is divided by the total number of particles of the hard phase to obtain the average area fraction per hard phase particle.
• the equivalent circle diameter is calculated from the average area fraction and the number of particles, and the obtained equivalent circle diameter is determined as the average crystal grain size of the hard phase.
• 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 method for manufacturing a steel sheet according to an embodiment of the present invention is a casting process for casting a slab having the chemical composition described above in relation to the steel sheet, comprising a plurality of reduction rolls adjacent in the conveying direction of the slab, It is characterized by including a casting process including performing soft reduction using a continuous casting machine in which the roll pitch between adjacent reduction rolls is 290 mm or less.
• A 10 [C] + 0.3 [Mn] - 0.2 [Si] - 0.6 [Al] - 0.05 [Cr] - 0.2 [Mo]
• [C], [Mn], [Si], [Al], [Cr] and [Mo] are the content [% by mass] of each element, and 0% when no element is contained .
• a continuous casting machine configured with a plurality of reduction rolls having a relatively short roll pitch of 290 mm or less, preferably 280 mm or less, is used to perform soft reduction. , significantly suppresses the flow of molten steel during solidification, thereby making it possible to reduce such concentration of Mn in the center. Therefore, by performing a casting process that includes a combination of a roll pitch of 290 mm or less and a light reduction, the maximum connection length of the hard phase in the rolling direction at the position of 1/2 of the plate thickness of the finally obtained steel plate is 80 ⁇ m or less. Reliable control becomes possible.
• the term "light reduction” refers to reduction having a reduction gradient of 0.6 mm or more per 1 m in the casting direction.
• the manufacturing method may include a hot rolling process, a cold rolling process, an annealing process, and a cooling process. Further, the manufacturing method may optionally include a plating step. These steps are not particularly limited, and any appropriate conditions are appropriately selected so as to obtain the metal structure containing the ferrite and hard phase described above in relation to the steel sheet in a predetermined area fraction. do it. Preferred conditions for each step are briefly described below.
• the slab is 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 is then subjected to rough rolling and finish rolling, and the resulting hot rolled steel sheet is coiled at a coiling temperature of, for example, 450-650.degree.
• the finishing temperature of finish rolling is preferably 950° C. or lower.
• the finish rolling end temperature By setting the finish rolling end temperature to 950°C or less, the average grain size of the hot-rolled steel sheet and the final product can be reduced, and it is possible to ensure sufficient yield strength and high surface quality after forming. Become. Further, by setting the coiling temperature to 450 to 650° C., it is possible to reduce the average crystal grain size and suppress the growth of scale.
• 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 step it is preferable to perform an annealing treatment in which the cold-rolled steel sheet is heated to a soaking temperature of 750 to 900° C. and held.
• a soaking temperature 750 to 900° C.
• recrystallization of ferrite and reverse transformation from ferrite to austenite can be sufficiently advanced, and a desired metal structure can be obtained in the final product.
• the soaking temperature to 900° C. or less, the crystal grains can be densified and sufficient strength can be obtained.
• 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.
• 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.
• Table 2 shows the cases where casting condition (I): with light reduction and casting condition (II): roll pitch of 290 mm or less are satisfied (OK) and not satisfied (NG).
• the casting condition (I) is OK
• the reduction is performed with a reduction gradient of 0.7 mm or more per 1 m in the casting advancing direction, while the example where such a light reduction is not performed is NG.
• the roll pitch was set to 270 mm
• the roll pitch was set to 360 mm.
• the obtained slab is subjected to a hot rolling process (heating temperature 1200 ° C., finish rolling end temperature 900 ° C. and coiling temperature 550 ° C.), cold rolling process (cumulative rolling reduction rate 80%), annealing process (uniform A heat temperature of 800° C.) and a cooling process (average cooling rate of 10° C./sec) were carried out to produce a cold-rolled steel sheet with a thickness of 0.4 mm.
• the surfaces of the obtained cold-rolled steel sheets were 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.
• the properties of the obtained steel sheets were measured and evaluated by the following methods.
• Tensile strength was measured by taking a No. 5 tensile test piece of JIS Z2241:2011 from the steel plate with the direction perpendicular to the rolling direction as the test direction, and performing a tensile test according to 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.
• Inventive Examples 1-3, 6-10, 13-16, 18 and 25-31 have a given chemical composition and metallographic structure, especially 1/2 and 1/4 thickness.

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