WO2012014926A1 - 熱延鋼板、冷延鋼板、亜鉛めっき鋼板およびこれらの製造方法 - Google Patents
熱延鋼板、冷延鋼板、亜鉛めっき鋼板およびこれらの製造方法 Download PDFInfo
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- WO2012014926A1 WO2012014926A1 PCT/JP2011/067070 JP2011067070W WO2012014926A1 WO 2012014926 A1 WO2012014926 A1 WO 2012014926A1 JP 2011067070 W JP2011067070 W JP 2011067070W WO 2012014926 A1 WO2012014926 A1 WO 2012014926A1
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- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21B—ROLLING OF METAL
- B21B35/00—Drives for metal-rolling mills, e.g. hydraulic drives
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- B32—LAYERED PRODUCTS
- B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
- B32B15/00—Layered products comprising a layer of metal
- B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
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- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
- C23C2/024—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by cleaning or etching
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- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
- C23C2/06—Zinc or cadmium or alloys based thereon
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- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
- C23C2/36—Elongated material
- C23C2/40—Plates; Strips
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- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
- C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
- C25D5/34—Pretreatment of metallic surfaces to be electroplated
- C25D5/36—Pretreatment of metallic surfaces to be electroplated of iron or steel
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- 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
- C21D2201/00—Treatment for obtaining particular effects
- C21D2201/05—Grain orientation
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/001—Austenite
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/002—Bainite
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/009—Pearlite
Definitions
- the present invention is excellent in local deformability such as bending, stretch flange, burring, etc., and has a low orientation dependency of formability, and is mainly used for hot-rolled steel sheets, cold-rolled steel sheets, galvanized steel sheets, and the like. It relates to the manufacturing method.
- the hot-rolled steel sheet includes a hot-rolled steel strip that serves as an original sheet such as a cold-rolled steel sheet and a galvanized steel sheet.
- This application includes Japanese Patent Application No. 2010-169670 filed in Japan on July 28, 2010, Japanese Patent Application No. 2010-169627 filed in Japan on July 28, 2010, and March 4, 2011. Japanese Patent Application No. 2011-048236 filed in Japan, Japanese Patent Application No. 2010-169230 filed in Japan on July 28, 2010, and Japanese Patent Application No.
- Non-Patent Document 1 discloses that uniform elongation, which is important for drawing or stretch forming, is reduced by increasing the strength. Therefore, in order to use high-strength steel sheets for undercarriage parts of automobile bodies, parts that contribute to collision energy absorption, etc., local deformability such as burring workability and local ductility that contributes to formability such as bending workability. It is important to improve.
- Non-Patent Document 2 discloses a method for improving uniform elongation even with the same strength by compounding the metal structure of a steel plate.
- Non-Patent Document 3 local deformability represented by bendability, hole expansion workability and burring workability is improved by inclusion control, single structure formation, and reduction in hardness difference between structures.
- a metallographic control method is disclosed. This is to improve the hole expansibility by making a single structure by controlling the structure, but in order to make a single structure, as described in Non-Patent Document 4, heat treatment from an austenite single phase. Is the basis of the manufacturing method.
- Non-Patent Document 4 the microstructure is controlled by cooling control after hot rolling, and appropriate fractions of ferrite and bainite are obtained by controlling the precipitates and the transformation structure to increase the strength. And a technique for ensuring ductility.
- any of the above techniques is a method for improving local deformability that relies on tissue control, and is greatly influenced by the formation of the base tissue.
- Non-Patent Document 5 the main phase of a product is obtained by performing large pressure reduction in a low temperature region as much as possible in an austenite region and transforming ferrite from unrecrystallized austenite.
- a technique for increasing the strength and toughness by reducing the crystal grain size of a ferrite and making it finer is disclosed.
- no consideration is given to the improvement of the local deformability that the present invention intends to solve.
- the main means is to perform structure control including inclusion control.
- structure control since it depends on the structure control, it is necessary to control the fraction and form of precipitates, structures such as ferrite and bainite, and the base metal structure is limited.
- the local deformation capability of a high-strength steel sheet is controlled by controlling the size and form of crystal grains and controlling the texture without being limited to the type of phase by controlling the texture, not the control of the base structure.
- the present invention provides a hot-rolled steel sheet, a cold-rolled steel sheet, a galvanized steel sheet, and a method for producing them.
- the present inventors newly focused on the influence of the texture of the steel sheet, and investigated and studied its effects in detail.
- the X-ray random intensity ratio of each orientation of the specific crystal orientation group is controlled from the hot rolling process, and the r value in the rolling direction, the r value in the direction perpendicular to the rolling direction, and 30 ° with respect to the rolling direction. It was also clarified that the local deformability is dramatically improved by controlling the r value in the direction of 60 °.
- the hot-rolled steel sheet according to one embodiment of the present invention is in mass%, C: 0.0001% to 0.40%, Si: 0.001% to 2.5%, Mn : 0.001% or more, 4.0% or less, P: 0.001% or more, 0.15% or less, S: 0.0005% or more, 0.03% or less, Al: 0.001% or more, 2 0.0% or less, N: 0.0005% or more, 0.01% or less, O: 0.0005% or more, 0.01% or less, and Ti: 0.001% or more, 0.20 % Or less, Nb: 0.001% or more, 0.20% or less, V: 0.001% or more, 1.0% or less, W: 0.001% or more, 1.0% or less, B: 0.0001 %: 0.0050% or less, Mo: 0.001% or more, 1.0% or less, Cr: 0.001%
- X-ray random intensity ratio of 0 or less and ⁇ 332 ⁇ ⁇ 113> crystal orientation is 1 It is 0 or more and 5.0 or less, and rC which is an r value in a direction perpendicular to the rolling direction is 0.70 or more and 1.10 or less, and is an r value in a direction forming 30 ° with respect to the rolling direction. r30 is 0.70 or more and 1.10 or less.
- rL which is r value of the said rolling direction is 0.70 or more and 1.10 or less, and the direction which makes 60 degrees with respect to the said rolling direction.
- the r value r60 may be 0.70 or more and 1.10 or less.
- the ratio of grains having a ratio dL / dt which is a ratio of the length dL in the rolling direction to the length dt in the plate thickness direction is 3.0 or less may be 50% or more and 100% or less.
- the area ratio of crystal grains having a grain size exceeding 20 ⁇ m is 0% or more and 10% or less in the total area of the metal structure of the hot-rolled steel sheet. May be.
- a cold-rolled steel sheet according to an aspect of the present invention is a cold-rolled steel sheet obtained by cold rolling the hot-rolled steel sheet described in (1) above, and at least ⁇ 100 ⁇ ⁇ 011> in the central portion of the plate thickness.
- the average value of the X-ray random intensity ratio of the ⁇ 223 ⁇ ⁇ 110> orientation group is 1.0 or more and less than 4.0, and the X-ray random intensity ratio of the crystal orientation of ⁇ 332 ⁇ ⁇ 113> is 1.0 or more R30 which is r or less in a direction perpendicular to the rolling direction and rC which is 0.70 or more and 1.10 or less and which forms 30 ° with respect to the rolling direction. Is 0.70 or more and 1.10 or less.
- rL which is an r value in the rolling direction is 0.70 or more and 1.10 or less, and is an r value in a direction forming 60 ° with respect to the rolling direction.
- r60 may be 0.70 or more and 1.10 or less.
- one or more of bainite, martensite, pearlite, and austenite are present in the cold-rolled steel sheet, and among the crystal grains of these structures,
- the ratio of grains in which dL / dt, which is the ratio of the length dL in the rolling direction and the length dt in the thickness direction, is 3.0 or less may be 50% or more and 100% or less.
- the area ratio of crystal grains having a grain size exceeding 20 ⁇ m is 0% or more and 10% or less. May be.
- a galvanized steel sheet according to one aspect of the present invention is a galvanized steel sheet further provided with a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface of the cold-rolled steel sheet according to (5).
- the average value of the X-ray random intensity ratio of the ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation group at least in the central portion of the plate thickness is 1.0 or more and less than 4.0, and ⁇ 332 ⁇ ⁇ 113>
- the X-ray random intensity ratio of the crystal orientation is 1.0 or more and 5.0 or less
- rC which is an r value in a direction perpendicular to the rolling direction is 0.7 or more and 1.10 or less
- R30 which is an r value in a direction forming 30 ° with respect to the rolling direction is 0.70 or more and 1.10 or less.
- rL that is an r value in the rolling direction is 0.70 or more and 1.10 or less, and is an r value in a direction that forms 60 ° with respect to the rolling direction.
- r60 may be 0.70 or more and 1.10 or less.
- the method for producing a hot-rolled steel sheet according to one embodiment of the present invention is, in mass%, C: 0.0001% to 0.40%, Si: 0.001% to 2.5%, Mn: 0.001% or more, 4.0% or less, P: 0.001% or more, 0.15% or less, S: 0.0005% or more, 0.03% or less, Al: 0.001% or more, 2.0% or less, N: 0.0005% or more, 0.01% or less, O: 0.0005% or more, 0.01% or less, and Ti: 0.001% or more; 20% or less, Nb: 0.001% or more, 0.20% or less, V: 0.001% or more, 1.0% or less, W: 0.001% or more, 1.0% or less, B: 0.00.
- T1 is a temperature determined by a steel plate component, and is expressed by the following formula 1.
- T1 (° C.) 850 + 10 ⁇ (C + N) ⁇ Mn + 350 ⁇ Nb + 250 ⁇ Ti + 40 ⁇ B + 10 ⁇ Cr + 100 ⁇ Mo + 100 ⁇ V (Formula 1)
- the rolling is performed at least once at a rolling rate of 30% or more in one pass. May be.
- the reduction at a reduction rate of 20% or more is at least twice or more.
- the austenite grain size may be 100 ⁇ m or less.
- Tf the temperature after the final pass
- P1 the rolling reduction in the final pass
- the temperature increase of the steel plate between each pass of the second hot rolling in the temperature range of T1 + 30 ° C. or more and T1 + 200 ° C. or less may be 18 ° C. or less.
- the method for producing a cold-rolled steel sheet according to one aspect of the present invention is hot at an Ar3 transformation temperature or higher with respect to the hot-rolled steel sheet obtained by the method for producing a hot-rolled steel sheet according to (11) above.
- pickling is performed, rolling is performed at a temperature of 20% to 90% in the cold, and annealing is performed at a temperature range of 720 ° C to 900 ° C with a holding time of 1 second to 300 seconds, 650 ° C.
- the cooling rate is 10 ° C./s to 200 ° C./s, and the temperature is maintained at 200 ° C. to 500 ° C.
- the rolling is performed at least once or more at a rolling reduction of 30% or more in one pass. May be.
- the first hot rolling in the temperature range of 1000 ° C. or more and 1200 ° C. or less is performed at least twice or more at a reduction rate of 20% or more.
- the austenite particle size may be 100 ⁇ m or less.
- Tf the temperature after the final pass
- P1 the rolling reduction in the final pass
- the temperature increase of the steel plate between each pass of the second hot rolling in the temperature range of T1 + 30 ° C. or more and T1 + 200 ° C. or less may be 18 ° C. or less.
- the method for producing a galvanized steel sheet according to one aspect of the present invention is hot at an Ar3 transformation temperature or higher with respect to the hot-rolled steel sheet obtained by the method for producing a hot-rolled steel sheet described in (11) above.
- the sheet is wound up in a temperature range of 680 ° C. or lower and room temperature or higher, pickled, rolled 20% to 90% in the cold, and heated to a temperature range of 650 ° C. or higher and 900 ° C. or lower.
- Annealing is performed at a holding time of 1 second to 300 seconds
- cooling is performed to a temperature range of 720 ° C. to 580 ° C. at a cooling rate of 0.1 ° C./s to 100 ° C./s, and galvanization is performed.
- the reduction at a reduction rate of 20% or more is at least twice or more.
- the austenite grain size may be 100 ⁇ m or less.
- Tf the temperature after the final pass
- P1 the rolling reduction in the final pass
- the temperature increase of the steel sheet between each pass of the second hot rolling in the temperature range of T1 + 30 ° C. or more and T1 + 200 ° C. or less may be 18 ° C. or less.
- the main structure is not limited, and even when elements such as Nb and Ti are added, the influence on anisotropy is small, the local deformability is excellent, and the orientation dependency of the formability is small. Hot rolled steel sheets, cold rolled steel sheets, and galvanized steel sheets can be obtained.
- FIG. 6 is a diagram showing a relationship between an average value of X-ray random intensity ratios of ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation groups and a sheet thickness / minimum bending radius in a hot-rolled steel sheet. It is a figure which shows the relationship between the X-ray random intensity ratio of ⁇ 332 ⁇ ⁇ 113> orientation group in a hot-rolled steel plate, and board thickness / minimum bending radius. The relationship between rC which is r value of the orthogonal
- FIG. 1 It is a figure which shows the relationship between r30 which is r value of the direction which makes 30 degrees with respect to the rolling direction in a hot-rolled steel plate, and plate
- FIG. 6 is a diagram showing a relationship between an average value of X-ray random intensity ratios of ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation groups and a sheet thickness / minimum bending radius in a cold rolled steel sheet. It is a figure which shows the relationship between the X-ray random intensity ratio of ⁇ 332 ⁇ ⁇ 113> orientation group, and sheet thickness / minimum bending radius in a cold-rolled steel sheet. It is a figure which shows the relationship between rC which is r value of the orthogonal
- FIG. 6 is a diagram showing a relationship between an average value of X-ray random intensity ratios of ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation groups and a plate thickness / minimum bending radius in a galvanized steel sheet. It is a figure which shows the relationship between the X-ray random intensity ratio of ⁇ 332 ⁇ ⁇ 113> azimuth
- FIG. 3 is a graph showing a relationship between a total rolling reduction in a temperature range of T1 ° C. or more and less than T1 + 30 ° C. and an average value of X-ray random intensity ratios of ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation groups in a hot-rolled steel sheet. is there.
- the pass having a reduction rate of 30% or more in the temperature range of T1 + 30 ° C. to T1 + 200 ° C. is defined as the large reduction pass. It is a figure which shows the relationship between the waiting time after starting the last path
- the maximum temperature increase amount of the steel plate between each pass during the reduction in the temperature range of T1 + 30 ° C. to T1 + 200 ° C., and the pass having a reduction rate of 30% or more in the temperature range of T1 + 30 ° C. to T1 + 200 ° C. is defined as the large reduction pass.
- FIG. It is a figure which shows the relationship between the austenite grain size after rough rolling, and rC which is r value of a perpendicular direction with the rolling direction in a cold-rolled steel plate. It is a figure which shows the relationship between the austenite grain size after rough rolling, and r30 which is r value of the direction which makes 30 degrees with respect to the rolling direction in a cold-rolled steel plate.
- Hot-rolled steel sheet (1) X-ray random intensity ratio of ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation group in the central part of the thickness, which is a thickness range of 5/8 to 3/8 from the surface of the steel sheet X-ray random intensity ratio of ⁇ 332 ⁇ ⁇ 113> crystal orientation: The average value of the X-ray random intensity ratios of ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation groups in the central part of the thickness that is 5/8 to 3/8 from the surface of the steel sheet This is a particularly important characteristic value in the form.
- the thickness / minimum bending radius required for processing the undercarriage part and the skeleton part is d / Rm satisfies 1.5 or more.
- a hole expansibility and a small limit bending characteristic it is preferably 4.0 or less, and more preferably less than 3.0.
- the anisotropy of the mechanical properties of the steel sheet becomes extremely strong. As a result, even if the local deformability in one direction is improved, the material in a direction different from that direction is significantly deteriorated.
- the aforementioned plate thickness / minimum bending radius ⁇ 1.5 cannot be satisfied.
- the X-ray random strength ratio is preferably less than 4.0.
- the X-ray random intensity ratio of ⁇ 332 ⁇ ⁇ 113> crystal orientation in the central portion of the plate thickness which is a plate thickness range of 5/8 to 3/8 from the surface of the steel plate, is as shown in FIG. If it is 5.0 or less, the thickness / minimum bending radius ⁇ 1.5 required for processing the undercarriage parts is satisfied. More desirably, it is 3.0 or less. If it exceeds 5.0, the anisotropy of the mechanical properties of the steel sheet becomes extremely strong. As a result, even if the local deformability in only one direction is improved, the material in a direction different from that direction is significantly deteriorated. Therefore, the thickness / minimum bending radius ⁇ 1.5 cannot be satisfied with certainty. On the other hand, the current general continuous hot rolling process is difficult to realize, but when the X-ray random intensity ratio is less than 1.0, there is a concern about deterioration of local deformability.
- r30 which is an r value in a direction forming 30 ° with respect to the rolling direction: This r30 is important in this embodiment. That is, as a result of intensive studies by the present inventors, it has been found that good local deformability cannot always be obtained even when the X-ray intensities of the various crystal orientations described above are appropriate. As shown in FIG. 4, it is essential that r30 is 1.10 or less simultaneously with the X-ray intensity.
- dL / dt ratio of bainite, martensite, pearlite and austenite grains As a result of further pursuing local deformability, the present inventors have almost no dependency on the direction of bending when the texture and r value are satisfied and the crystal grains are equiaxed. I found. As an index representing this equiaxed property, dL / dt which is a ratio of dL which is the length in the hot rolling direction of the crystal grains in these structures to dt which is the length in the plate thickness direction is 3.0 or less. The proportion of grains having excellent equiaxedness is 50% or more and 100% or less of these crystal grains.
- each tissue can be determined as follows. Perlite is identified by observation of the structure with an optical microscope. Next, the crystal structure is determined using EBSD (Electron Back Scattering Diffraction), and the crystal having the fcc structure is defined as austenite. Bcc-structured ferrite, bainite and martensite can be identified by the Kernel Average Misorientation, that is, the KAM method, provided in the EBSP-OIM TM .
- the KAM method is a first approximation that is six adjacent hexagonal pixels of measurement data, or a second approximation that is 12 outside the pixel, or a third approximation that is 18 outside the pixel. It is a value calculated by averaging each azimuth difference and calculating each pixel for the value of the center pixel. By performing this calculation so as not to cross the grain boundary, a map expressing the orientation change in the grain can be created. This map represents the strain distribution based on local orientation changes in the grains.
- the condition for calculating the azimuth difference between adjacent pixels in the EBSP-OIM TM is set as a third approximation, and this azimuth difference is set to 5 ° or less.
- Bainite or martensite which is a low-temperature transformation product, is defined as ferrite at 1 ° or less. This is because the polygonal pro-eutectoid ferrite transformed at high temperature is formed by diffusion transformation, so the dislocation density is small and the intra-granular distortion is small, so the intra-granular difference in crystal orientation is small. This is because, based on various investigation results, the ferrite volume fraction obtained by optical microscope observation and the area fraction of the area obtained by the third approximation of the orientation difference measured by the KAM method are almost in good agreement.
- the smaller area ratio of crystal grains with a grain size of more than 20 ⁇ m out of the total area It is necessary to be 0% or more and 10% or less. If it exceeds 10%, the bendability deteriorates.
- the crystal grains mentioned here refer to ferrite, pearlite, bainite, martensite, and austenite crystal grains.
- the present invention can be applied to all types of hot-rolled steel sheets, and if the above limitations are satisfied, the formability of the hot-rolled steel sheets, such as bending workability and hole-expandability, will not be limited to a combination of structures. Improve.
- the X-ray random intensity ratio of ⁇ 332 ⁇ ⁇ 113> crystal orientation in the central portion of the plate thickness which is the thickness range of 5/8 to 3/8 from the surface of the steel plate, is as shown in FIG. If it is 5.0 or less, the thickness / minimum bending radius ⁇ 1.5 required for processing of the skeleton component is satisfied. More desirably, it is 3.0 or less. If this exceeds 5.0, the anisotropy of the mechanical properties of the steel sheet becomes extremely strong. As a result, the local deformability only in a certain direction is improved, but the material in a direction different from that direction is significantly deteriorated. Therefore, the thickness / minimum bending radius ⁇ 1.5 cannot be satisfied with certainty. On the other hand, the current general continuous hot rolling process is difficult to realize, but when the X-ray random intensity ratio is less than 1.0, there is a concern about deterioration of local deformability.
- r30 which is an r value in a direction forming 30 ° with respect to the rolling direction: This r30 is important in this embodiment. That is, as a result of intensive studies by the present inventors, it has been found that good local deformability cannot always be obtained even if the X-ray random intensity ratios of various crystal orientations described above are appropriate. As shown in FIG. 10, it is essential that r30 is 1.10 or less simultaneously with the above X-ray random intensity ratio.
- Exceptional local deformability can be obtained by setting the lower limit of r30 to 0.70.
- rL and r60 can obtain better local deformability when rL is 1.10 or less and r60 is 0.70 or more.
- dL / dt ratio of bainite, martensite, pearlite and austenite grains As a result of further pursuing local deformability, the present inventors have almost no dependency on the direction of bending when the texture and r value are satisfied and the crystal grains are equiaxed. I found. As an index representing this equiaxed property, dL / dt which is a ratio of dL which is the length in the cold rolling direction of the crystal grains in these structures to dt which is the length in the plate thickness direction is 3.0 or less. The proportion of grains having excellent equiaxedness is 50% or more and 100% or less of these crystal grains.
- each tissue can be determined as follows. Perlite is identified by observation of the structure with an optical microscope. Next, the crystal structure is determined using EBSD, and the crystal having the fcc structure is set to austenite. Bcc-structured ferrite, bainite and martensite can be identified by the Kernel Average Misorientation, that is, the KAM method, provided in the EBSP-OIM TM .
- the KAM method is a first approximation that is six adjacent hexagonal pixels of measurement data, or a second approximation that is 12 outside the pixel, or a third approximation that is 18 outside the pixel. It is a value calculated by averaging each azimuth difference and calculating each pixel for the value of the center pixel. By performing this calculation so as not to cross the grain boundary, a map expressing the orientation change in the grain can be created. This map represents the strain distribution based on local orientation changes in the grains.
- the condition for calculating the azimuth difference between adjacent pixels in the EBSP-OIM TM is set as a third approximation, and this azimuth difference is set to 5 ° or less.
- Bainite or martensite which is a low-temperature transformation product, is defined as ferrite at 1 ° or less. This is because the polygonal pro-eutectoid ferrite transformed at high temperature is formed by diffusion transformation, so the dislocation density is small and the intra-granular distortion is small, so the intra-granular difference in crystal orientation is small. This is because, based on various investigation results, the ferrite volume fraction obtained by optical microscope observation and the area fraction of the area obtained by the third approximation of the orientation difference measured by the KAM method are almost in good agreement.
- the smaller area ratio of crystal grains with a grain size of more than 20 ⁇ m out of the total area It is necessary to be 0% or more and 10% or less. If it exceeds 10%, the bendability deteriorates.
- the crystal grains mentioned here refer to ferrite, pearlite, bainite, martensite, and austenite crystal grains.
- the present invention can be applied to all types of cold-rolled steel sheets, and if the above limitation is satisfied, the present invention is not limited to a combination of structures, and local deformability such as bending workability and hole-expandability of cold-rolled steel sheets is achieved. Improve dramatically.
- X-ray random strength of ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation group in the central portion of the thickness that is 5/8 to 3/8 from the surface of the steel plate Average value of ratio, X-ray random intensity ratio of ⁇ 332 ⁇ ⁇ 113> crystal orientation: The average value of the X-ray random intensity ratios of ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation groups in the central portion of the plate thickness that is 5/8 to 3/8 from the surface of the steel plate is the present embodiment. Are particularly important characteristic values. As shown in FIG.
- the X-ray random intensity ratio of the ⁇ 332 ⁇ ⁇ 113> crystal orientation in the central portion of the plate thickness that is 5/8 to 3/8 from the surface of the steel plate is as shown in FIG.
- the thickness / bending radius ⁇ 1.5 required for the processing of the undercarriage part that is required most recently is satisfied. Desirably, it is 3.0 or less. If this exceeds 5.0, the anisotropy of the mechanical properties of the steel sheet becomes extremely strong. As a result, the local deformability only in a certain direction is improved, but the material in a direction different from that direction is significantly deteriorated. Therefore, the thickness / bending radius ⁇ 1.5 cannot be satisfied with certainty.
- the current general continuous hot rolling process is difficult to realize, but when the X-ray random intensity ratio is less than 1.0, there is a concern about deterioration of local deformability.
- RC which is the r value in the direction perpendicular to the rolling direction:
- This rC is important in this embodiment. That is, as a result of intensive studies by the present inventors, it has been found that even if only the above-mentioned X-ray random intensity ratios of various crystal orientations are appropriate, good hole expandability and bendability cannot always be obtained. As shown in FIG. 15, it is essential that rC is 0.70 or more simultaneously with the above X-ray random intensity ratio. By setting the upper limit of rC described above to 1.10, more excellent local deformability can be obtained.
- R30 which is an r value in a direction forming 30 ° with respect to the rolling direction:
- This r30 is important in this embodiment. That is, as a result of intensive studies by the present inventors, it has been found that good local deformability cannot always be obtained even if the X-ray random intensity ratios of various crystal orientations described above are appropriate. As shown in FIG. 16, it is essential that r30 is 1.10 or less simultaneously with the X-ray random intensity ratio. By setting the lower limit of r30 described above to 0.70, better local deformability can be obtained.
- RL which is the r value in the rolling direction
- r60 which is the r value in the direction forming 60 ° with respect to the rolling direction
- the above-described limitation on the X-ray intensity ratio of the crystal orientation and the limitation on the r value are as follows. It is not synonymous with each other, and good local deformability cannot be obtained unless both limitations are satisfied at the same time.
- the present invention can be applied to galvanized steel sheets in general, and if the above-mentioned limitations are satisfied, the present invention is not limited to the combination of structures, and local deformability such as bending workability and hole expansibility of galvanized steel sheets is achieved. Improve dramatically.
- the main orientations included in the ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation groups described above are ⁇ 100 ⁇ ⁇ 011>, ⁇ 116 ⁇ ⁇ 110>, ⁇ 114 ⁇ ⁇ 110>, ⁇ 113 ⁇ ⁇ 110>, ⁇ 112 ⁇ ⁇ 110>, ⁇ 335 ⁇ ⁇ 110> and ⁇ 223 ⁇ ⁇ 110>.
- the X-ray random intensity ratio in each direction can be measured by using a method such as X-ray diffraction or EBSD (Electron Back Scattering Diffraction).
- a method such as X-ray diffraction or EBSD (Electron Back Scattering Diffraction).
- EBSD Electro Back Scattering Diffraction
- a plurality of pole figures preferably among the three-dimensional texture calculated by the vector method and ⁇ 110 ⁇ , ⁇ 100 ⁇ , ⁇ 211 ⁇ , ⁇ 310 ⁇ pole figures May be obtained from a three-dimensional texture calculated by the series expansion method using three or more).
- the intensities of [1-10], (113) [1-10], (112) [1-10], (335) [1-10], (223) [1-10] may be used as they are. 1 with an upper line representing minus 1 is represented by -1.
- the average value of the ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation groups is an arithmetic average of the above-mentioned orientations.
- the thickness of the steel sheet is reduced from the surface to a predetermined thickness by mechanical polishing or the like, and then the strain is removed by chemical polishing or electrolytic polishing. What is necessary is just to adjust and measure a sample according to the above-mentioned method so that a suitable surface may become a measurement surface in the range of 3/8. In the plate width direction, it is desirable to collect at a position of 1/4 or 3/4 from the end.
- the above-mentioned limitation of the X-ray intensity is satisfied not only in the vicinity of the plate thickness 1 ⁇ 2 but also as much as possible, so that the local deformability is further improved.
- the material characteristics of the entire steel plate can be representatively represented.
- the X-ray random intensity ratio of the azimuth is defined.
- the crystal orientation represented by ⁇ hkl ⁇ ⁇ uvw> indicates that the normal direction of the plate surface is parallel to ⁇ hkl ⁇ and the rolling direction is parallel to ⁇ uvw>.
- each r value described above is evaluated by a tensile test using a JIS No. 5 tensile test piece.
- Tensile strain is usually in the range of 5 to 15% in the case of a high-strength steel sheet, and may be evaluated in the range of uniform elongation.
- the direction in which the bending process is performed differs depending on the processed part, and is not particularly limited. According to the present invention, similar characteristics can be obtained in any bending direction.
- the dL / dt and particle size of pearlite can be determined by binarization processing and the point count method in the structure observation with an optical microscope.
- the grain sizes of ferrite, bainite, martensite, and austenite are measured by measuring the orientation in a measurement step of 0.5 ⁇ m or less at a magnification of 1500 times in the analysis of the orientation of the steel sheet by the EBSD method described above. It is obtained by determining a position where the orientation difference of the matching measurement points exceeds 15 ° as a grain boundary and obtaining the equivalent circle diameter.
- dL / dt can be obtained by simultaneously obtaining the lengths of the grains in the rolling direction and the plate thickness direction.
- % Of content is mass%. Since the cold-rolled steel sheet and the galvanized steel sheet of the present invention use the hot-rolled steel sheet in the present invention as the original sheet, the components of the steel sheet are as follows for any of the hot-rolled steel sheet, the cold-rolled steel sheet, and the galvanized steel sheet. .
- C is an element that is basically contained, and the lower limit is set to 0.0001% because the lower limit value obtained from practical steel is used. If the upper limit exceeds 0.40%, workability and weldability deteriorate, so the upper limit is set to 0.40%. In addition, since excessive C addition deteriorates spot weldability remarkably, 0.30% or less is more desirable.
- Si is an effective element for increasing the mechanical strength of the steel sheet, but if it exceeds 2.5%, workability deteriorates or surface flaws occur, so 2.5% is the upper limit. On the other hand, since it is difficult to make Si less than 0.001% in practical steel, 0.001% is made the lower limit.
- Mn is an element effective for increasing the mechanical strength of the steel sheet, but if it exceeds 4.0%, the workability deteriorates, so 4.0% is the upper limit. On the other hand, since it is difficult to make Mn less than 0.001% in practical steel, 0.001% is made the lower limit. However, in order to avoid an extreme increase in steelmaking cost, it is desirable to set it to 0.01% or more. Since Mn suppresses the formation of ferrite, if it is desired to ensure elongation by including a ferrite phase in the structure, it is desirable to make it 3.0% or less. In addition to Mn, when an element such as Ti that suppresses the occurrence of hot cracking due to S is not sufficiently added, it is desirable to add an amount of Mn that satisfies Mn / S ⁇ 20 by weight%.
- the upper limits of P and S are 0.15% or less for P and 0.03% or less for S in order to prevent deterioration of workability and cracking during hot rolling or cold rolling.
- Each lower limit is set to 0.001% for P and 0.0005% for S as possible values in the current general refining (including secondary refining).
- S since extreme desulfurization becomes cost too high, 0.001% or more is more desirable.
- Al is added 0.001% or more for deoxidation. However, when deoxidation is sufficiently necessary, addition of 0.01% or more is more desirable. Moreover, since Al significantly raises the ⁇ ⁇ ⁇ transformation point, it is an effective element particularly when directing hot rolling below the Ar3 point. However, if the amount is too large, the weldability becomes poor, so the upper limit is made 2.0%.
- N and O are impurities, and both are set to 0.01% or less so as not to deteriorate the workability.
- the lower limit is set to 0.0005%, which is possible for both elements by current general refining (including secondary refining). However, 0.001% or more is desirable in order to suppress an extreme increase in steelmaking costs.
- Ti, Nb, B, Mg, REM are conventionally used as elements used to increase mechanical strength by precipitation strengthening, or to control inclusions and refine precipitates in order to improve local deformability.
- Ca, Mo, Cr, V, W, Cu, Ni, Co, Sn, Zr, As may be included.
- Ti, Nb, V, and W are solid solution elements and have an effect of contributing to refinement of crystal grains.
- Ti is 0.001% or more
- Nb is 0.001% or more
- V is 0.001% or more
- W is 0.001.
- % Or more must be added.
- Ti and Nb have the effect of improving the material through mechanisms such as carbon and nitrogen fixation, structure control, and fine grain strengthening in addition to precipitation strengthening.
- V is effective for precipitation strengthening, and is less effective than Mo or Cr when the deterioration of local deformability due to strengthening by addition is small, and high strength and better hole expansibility and bendability are required.
- B has the effect of improving the material through mechanisms such as carbon and nitrogen fixation, precipitation strengthening, and fine grain strengthening.
- Mo and Cr have the effect of improving the material in addition to the effect of increasing the mechanical strength. In order to obtain these effects, it is necessary to add 0.0001% or more of B, 0.001% or more of Mo, Cr, Ni, and Cu, and 0.0001% or more of Co, Sn, Zr, and As.
- the upper limit of B is 0.0050%
- the upper limit of Mo is 1.0%
- the upper limit of Cr, Ni, Cu is 2.0%
- the upper limit of Co is 1.0%
- the upper limit of Sn and Zr is 0.2%
- the upper limit of As is 0.50%.
- the upper limit of B is 0.005% and the upper limit of Mo is 0.50%. From the viewpoint of cost, it is more desirable to select B, Mo, Cr, As among the above-described additive elements.
- Mg, REM, and Ca are important additive elements for detoxifying inclusions and further improving local deformability.
- the lower limit of the addition amount for obtaining this effect is 0.0001%, respectively.
- the upper limit was made 0.010% for Mg, 0.1% for REM, and 0.010% for Ca.
- the galvanized steel sheet of the present invention has a galvanized layer formed by galvanization on the surface of the cold-rolled steel sheet of the present invention, but galvanization is effective in both hot dip galvanizing and electrogalvanizing. It is done. Moreover, it is good also as an alloying galvanized steel plate represented by the automotive use by performing an alloying process after galvanization.
- the effect of the present invention is not lost, and any of organic film formation, film lamination, organic salt / inorganic salt treatment, non-chromic treatment, etc. The effect is obtained.
- the production method prior to hot rolling is not particularly limited. That is, various secondary smelting may be performed following melting by a blast furnace, an electric furnace, etc., and then casting may be performed by a method such as normal continuous casting, casting by an ingot method, or thin slab casting. In the case of continuous casting, it may be cooled to a low temperature and then heated again and then hot rolled, or the cast slab may be hot rolled after casting without being cooled to a low temperature. Scrap may be used as a raw material.
- the hot-rolled steel sheet according to this embodiment is obtained when the following requirements are satisfied.
- the austenite grain size after rough rolling that is, before finish rolling is important. As shown in FIGS. 19 and 20, the austenite grain size before finish rolling may be 200 ⁇ m or less.
- rough rolling is performed by rolling in a temperature range of 1000 ° C. or more and 1200 ° C. or less, and at least 20% in this temperature range. What is necessary is just to reduce once or more by the above reduction ratio. However, in order to further improve the homogeneity, increase the elongation, and improve the local deformability, it is desirable to perform rolling at least once at a rolling reduction of at least 40% in a temperature range of 1000 ° C. or more and 1200 ° C. or less.
- the austenite particle size is more desirably 100 ⁇ m or less, and for that purpose, it is desirable to perform the reduction two or more times at a reduction rate of 20% or more. Desirably, it is twice or more at a rolling reduction of 40% or more. Finer grains can be obtained as the rolling reduction ratio and the number of times of rolling are reduced. However, rolling exceeding 70% or rough rolling exceeding 10 times may cause a decrease in temperature or excessive production of scale. Thus, reducing the austenite grain size before finish rolling is effective for improving local deformability through control of rL and r30 through promoting recrystallization of austenite in subsequent finish rolling.
- the austenite grain boundary after rough rolling that is, before finish rolling, functions as one of the recrystallization nuclei during finish rolling.
- the in order to confirm the austenite grain size after rough rolling it is desirable to cool the plate piece before finishing rolling as much as possible, and the plate piece is cooled at a cooling rate of 10 ° C./s or more.
- the structure of the cross section is etched to raise the austenite grain boundary and measured with an optical microscope. At this time, 20 fields of view or more are measured by image analysis or a point count method at a magnification of 50 times or more.
- the X-ray random intensity ratio of the crystal orientation is in the range of the above-mentioned value, based on the T1 temperature described in Formula 1 determined by the steel plate component in the finish rolling after the rough rolling, as T1 + 30 Processing is performed with a large rolling reduction in a temperature range of not less than T.degree. C. and not more than T1 + 200.degree. C., preferably in a temperature range of not less than T.sub.1 + 50.degree. C.
- the T1 temperature itself is obtained empirically, and the inventors have found through experiments that recrystallization in the austenite region of each steel is promoted based on the T1 temperature.
- the total rolling reduction is 50% or more, preferably 70% or more, and the temperature rise of the steel sheet between passes is preferably 18 ° C. or less. On the other hand, it is not desirable that the total rolling reduction exceeds 90% from the viewpoints of securing temperature and excessive rolling load.
- at least one pass in the rolling in the temperature range of T1 + 30 ° C. to T1 + 200 ° C. is preferably 30% or more, preferably 40%. Reduction is performed at the above reduction ratio. On the other hand, if it exceeds 70% in one pass, there is a concern that the shape may be hindered. When higher workability is required, it is more desirable to make the final two passes 30% or more.
- the total rolling reduction at T1 ° C. or more and less than T1 + 30 ° C. is less than 30%.
- a rolling reduction of 10% or more is desirable, but when the local deformability is more important, the rolling reduction is preferably 0%.
- the austenite texture develops, and in the finally obtained hot-rolled steel sheet, at least
- the average value of the X-ray random intensity ratio of the ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation groups in the central portion of the thickness that is 5/8 to 3/8 from the surface of the steel plate is 6.0.
- the X-ray random intensity ratio in each crystal orientation in which the X-ray random intensity ratio of the ⁇ 332 ⁇ ⁇ 113> crystal orientation is 5.0 or less cannot be obtained.
- rolling is performed at a temperature higher than the specified temperature range, or if a reduction ratio smaller than the specified reduction ratio is adopted, it may cause coarsening or mixed grains, and the grain size exceeding 20 ⁇ m.
- the area ratio increases. Whether or not the above-described rolling is performed can be determined by actual results or calculation based on the rolling load, sheet thickness measurement, and the like. Also, the temperature can be measured with an inter-stand thermometer, or can be obtained by either or both of them because calculation simulation considering processing heat generation or the like can be performed from line speed, rolling reduction, etc. .
- the hot rolling performed as described above ends at a temperature of Ar3 or higher.
- the end temperature of hot rolling is less than Ar3, since it includes two-phase rolling of an austenite region and a ferrite region, accumulation in ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation groups becomes strong, and the result As a result, local deformability is significantly deteriorated.
- rL and r60 are rL of 0.70 or more and r60 of 1.10 or less, respectively, further satisfactory plate thickness / minimum bending radius ⁇ 2.0 is satisfied.
- a pass having a reduction rate of 30% or more in a temperature range of T1 + 30 ° C. or more and T1 + 200 ° C. or less is set as a large reduction pass, cooling is started after the final pass of the large reduction passes is completed. It is desirable that the waiting time t (second) satisfies the above-mentioned formula 2 and the temperature rise of the steel plate between the passes is 18 ° C. or less.
- FIG. 26 and FIG. 27 show the relationship between the temperature rise amount of the steel sheet between passes during the rolling at T1 + 30 ° C.
- a sheet bar may be joined after rough rolling, and finish rolling may be performed continuously.
- the coarse bar may be wound once in a coil shape, stored in a cover having a heat retaining function as necessary, and rewound again before joining. Moreover, you may wind up after hot rolling.
- the hot-rolled steel sheet may be subjected to skin pass rolling as necessary.
- Skin pass rolling has the effect of preventing stretcher strain generated during processing and shape correction.
- the structure of the hot-rolled steel sheet obtained in this embodiment is mainly composed of ferrite, but it may contain compounds such as pearlite, bainite, martensite, austenite, and carbonitride as metal structures other than ferrite. Since the crystal structure of martensite and bainite is the same as or similar to that of ferrite, these structures may be mainly used instead of ferrite.
- the steel sheet according to the present invention can be applied not only to bending, but also to composite forming mainly composed of bending, overhanging, drawing, and bending.
- the production method prior to hot rolling is not particularly limited. That is, various secondary smelting may be performed following melting by a blast furnace, an electric furnace, etc., and then casting may be performed by a method such as normal continuous casting, casting by an ingot method, or thin slab casting. In the case of continuous casting, it may be cooled to a low temperature and then heated again and then hot rolled, or the cast slab may be hot rolled after casting without being cooled to a low temperature. Scrap may be used as a raw material.
- the cold-rolled steel sheet having excellent local deformability according to this embodiment is obtained when the following requirements are satisfied.
- the austenite grain size after rough rolling that is, before finish rolling is important. As shown in FIGS. 28 and 29, it is desirable that the austenite grain size before the finish rolling is small, and if it is 200 ⁇ m or less, the above value is satisfied.
- rough rolling is performed in a temperature range of 1000 ° C. or more and 1200 ° C. or less, and once at a rolling reduction of at least 20% or more. What is necessary is just to reduce above. Finer grains can be obtained as the reduction ratio and the number of reductions are larger.
- the austenite particle size is more desirably 100 ⁇ m or less, and for that purpose, it is desirable to perform the reduction two or more times at a reduction rate of 20% or more. Desirably, it is twice or more at a rolling reduction of 40% or more. Finer grains can be obtained as the rolling reduction ratio and the number of times of rolling are reduced. However, rolling exceeding 70% or rough rolling exceeding 10 times may cause a decrease in temperature or excessive production of scale. Thus, reducing the austenite grain size before finish rolling is effective for improving local deformability through control of rL and r30 through promoting recrystallization of austenite in subsequent finish rolling.
- the influence of the refinement of the austenite grain size on the local deformability is presumed to be because the austenite grain boundary after rough rolling, that is, before finish rolling, functions as one of the recrystallization nuclei during finish rolling.
- the in order to confirm the austenite grain size after rough rolling it is desirable to cool the plate piece before finishing rolling as much as possible, and the plate piece is cooled at a cooling rate of 10 ° C./s or more.
- the cross-sectional structure is etched, and the austenite grain boundary is lifted up and measured with an optical microscope. At this time, 20 fields of view or more are measured by image analysis or a point count method at a magnification of 50 times or more.
- the average value of the X-ray random intensity ratios of ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation groups in the central portion of the plate thickness which is the thickness range of 5/8 to 3/8 from the surface of the steel plate, and ⁇ 332 ⁇
- processing is performed with a large rolling reduction in a temperature range of T1 + 50 ° C. or more and T1 + 100 ° C.
- T1 + 30 ° C. or more and T1 + 200 ° C. or less large pressure reduction is performed in a temperature range of T1 + 30 ° C. or more and T1 + 200 ° C. or less, and then light pressure reduction is performed at T1 ° C. or more and less than T1 + 30 ° C. 8
- the X-ray random intensity ratio of the ⁇ 332 ⁇ ⁇ 113> crystal orientation are controlled to drastically improve the local deformability of the final hot rolled product.
- the T1 temperature itself is determined empirically, and the inventors have found through experiments that recrystallization in the austenite region of each steel is promoted based on the T1 temperature.
- the total rolling reduction is 50% or more, more desirably 60% or more, and further desirably 70% or more.
- the total rolling reduction exceeds 90% from the viewpoint of securing temperature and excessive rolling load.
- at least one pass in the rolling in the temperature range of T1 + 30 ° C. to T1 + 200 ° C. is preferably 30% or more, preferably 40%. Rolling is performed at the above rolling reduction.
- it exceeds 70% in one pass there is a concern that the shape may be hindered.
- the rolling reduction at T1 ° C. or more and less than T1 + 30 ° C. is less than 30%. From the viewpoint of plate shape, a rolling reduction of 10% or more is desirable, but when the local deformability is more important, the rolling reduction is preferably 0%. If the rolling reduction at T1 ° C. or more and less than T1 + 30 ° C.
- the recrystallized austenite grains expand, and if the retention time is short, recrystallization does not proceed sufficiently and local deformability deteriorates. That is, in the manufacturing conditions according to the present embodiment, the austenite is uniformly and finely recrystallized in finish rolling to control the texture of the hot rolled product and improve local deformability such as hole expandability and bendability. be able to.
- the austenite texture develops, and in the cold-rolled steel sheet finally obtained, at least
- the average value of the X-ray random intensity ratio of the ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation groups in the central portion of the thickness that is 5/8 to 3/8 thickness from the surface of the steel plate is 4.0.
- the X-ray random intensity ratio in each crystal orientation is less than 5.0 and the X-ray random intensity ratio of the crystal orientation of ⁇ 332 ⁇ ⁇ 113> is 5.0 or less.
- rolling is performed at a temperature higher than the specified temperature range, or if a reduction ratio smaller than the specified reduction ratio is adopted, it may cause coarsening or mixed grains, resulting in a crystal having a grain size exceeding 20 ⁇ m.
- the area ratio of grains increases.
- the temperature can be measured with an inter-stand thermometer, or can be obtained by either or both of them because calculation simulation considering processing heat generation or the like can be performed from line speed, rolling reduction, etc. .
- the hot rolling performed as described above ends at a temperature of Ar3 or higher.
- hot rolling is completed at less than Ar3, since it includes two-phase rolling of an austenite region and a ferrite region, accumulation in ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation groups becomes stronger, resulting in local Deformability is significantly deteriorated.
- rL and r60 are rL of 0.70 or more and r60 is 1.10 or less, further satisfactory plate thickness / minimum bending radius ⁇ 2.0 is satisfied.
- the cooling after rolling at the final rolling stand of the rolling in the temperature range of T1 + 30 ° C. or more and T1 + 200 ° C. or less greatly affects the austenite grain size, which is the equiaxed grain fraction of the structure after cold rolling annealing, It strongly affects the coarse grain fraction. Therefore, when a pass having a reduction rate of 30% or more in a temperature range of T1 + 30 ° C. or more and T1 + 200 ° C. or less is set as a large reduction pass, the process waits until cooling is started after the final pass of the large reduction passes is completed.
- the time t needs to satisfy Equation 4 above. On the longer side than this, coarsening progresses and the elongation decreases significantly. On the short time side, sufficient recrystallization cannot be obtained and the anisotropy increases. For this reason, the thickness / minimum bending radius ⁇ 2.0 cannot be satisfied.
- the cooling pattern after hot rolling is not particularly defined, and the effect of the present invention can be obtained even if a cooling pattern for controlling the structure suitable for each purpose is adopted.
- a sheet bar may be joined after rough rolling, and finish rolling may be performed continuously.
- the coarse bar may be wound once in a coil shape, stored in a cover having a heat retaining function as necessary, and rewound again before joining.
- the steel sheet that has been subjected to the above hot rolling is cold-rolled at a reduction rate of 20% to 90%. If it is less than 20%, it is difficult to cause recrystallization in the subsequent annealing step, the equiaxed grain fraction is lowered, and crystal grains after annealing are coarsened. When the rolling reduction exceeds 90%, the texture develops during annealing, and the anisotropy becomes strong. For this reason, the cold reduction ratio is set to 20% or more and 90% or less.
- the cold-rolled steel sheet is then held in a temperature range of 720 ° C. or higher and 900 ° C. or lower for 1 to 300 seconds. Accordingly, the reverse transformation from the ferrite does not proceed sufficiently at a low temperature or for a short time, and the second phase cannot be obtained in the subsequent cooling step, so that sufficient strength cannot be obtained. On the other hand, if the temperature exceeds 900 ° C. or the holding is continued for 300 seconds or more, the crystal grains are coarsened, so that the area ratio of crystal grains having a grain size of 20 ⁇ m or less increases.
- the cooling rate between 650 degreeC and 500 degrees C at the cooling rate of 10 degrees C / s or more and 200 degrees C / s or less. If the cooling rate is less than 10 ° C./s, or if the end point temperature is more than 500 ° C., pearlite is generated, so that the local deformability is lowered. On the other hand, even if the cooling rate exceeds 200 ° C./s, the pearlite suppression effect is saturated, and conversely, the controllability of the cooling end point temperature is significantly deteriorated.
- the structure of the cold-rolled steel sheet obtained in the present embodiment contains ferrite, but it may contain compounds such as pearlite, bainite, martensite, austenite, and carbonitride as metal structures other than ferrite.
- pearlite is desirably 5% or less in order to deteriorate local ductility. Since the crystal structure of martensite or bainite is the same as or similar to that of ferrite, it may be a structure mainly composed of ferrite, bainite, or martensite.
- cold-rolled steel sheet according to the present invention can be applied not only to bending, but also to composite forming mainly composed of bending, overhanging, drawing, and bending.
- the production method preceding hot rolling is not particularly limited. That is, various secondary smelting may be performed following melting by a blast furnace, an electric furnace, etc., and then casting may be performed by a method such as normal continuous casting, casting by an ingot method, or thin slab casting. In the case of continuous casting, it may be cooled once to a low temperature and then heated again and then hot rolled, or the cast slab may be hot-rolled after casting without being cooled to a low temperature. Scrap may be used as a raw material.
- the galvanized steel sheet having excellent local deformability according to the present embodiment can be obtained when the following requirements are satisfied.
- the austenite grain size after rough rolling that is, before finish rolling is important. As shown in FIGS. 32 and 33, it is desirable that the austenite grain size before finish rolling is small, and the above-described value is satisfied if it is 200 ⁇ m or less.
- rough rolling is performed in a temperature range of 1000 ° C. or more and 1200 ° C. or less as shown in FIG. 21, and the rolling is performed once or more at a rolling reduction of at least 20% or more. do it.
- austenite grain size before finish rolling is effective in improving local deformability through control of rL and r30 through promoting recrystallization of austenite in subsequent finish rolling.
- the reason why the refinement of the austenite grain size affects the local deformability is presumed to be because the austenite grain boundary after rough rolling, that is, before finish rolling, functions as one of the recrystallization nuclei during finish rolling. .
- the structure of the cross section is etched to raise the austenite grain boundary and measured with an optical microscope. At this time, 20 fields of view or more are measured by image analysis or a point count method at a magnification of 50 times or more. Further, in order to enhance the local deformability, 100 ⁇ m or less is desirable.
- the T1 temperature determined by the steel plate component defined by the formula 1 is set in the finish rolling after the rough rolling.
- the rolling is performed with a large rolling reduction in the temperature range of T1 + 30 ° C. or more and T1 + 200 ° C., preferably in the temperature range of T1 + 50 ° C.
- the local deformability of the final hot rolled product is dramatically improved.
- the T1 temperature itself is determined empirically, and the inventors have found through experiments that recrystallization in the austenite region of each steel is promoted based on the T1 temperature.
- At least one pass in the rolling in the temperature range of T1 + 30 ° C. to T1 + 200 ° C. is preferably 30% or more, preferably 40%. It is desirable to perform rolling at the above rolling reduction. On the other hand, if it exceeds 70% in one pass, there is a concern that the shape may be hindered. When higher workability is required, it is more desirable to make the final two passes 30% or more.
- the total rolling reduction between T1 ° C. and T1 + 30 ° C. is less than 30%.
- a rolling reduction of 10% or more is desirable from the plate shape, when the local deformability is more important, the rolling reduction is preferably 0%.
- the austenite is uniformly and finely recrystallized in finish rolling to control the texture of hot rolled products and improve local deformability such as hole expandability and bendability. it can.
- the average value of the X-ray random intensity ratios of ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation groups in the central portion of the plate thickness that is at least 5/8 to 3/8 from the surface of the steel plate is 4.
- An X-ray random intensity ratio of each crystal orientation of less than 0 and an X-ray random intensity ratio of ⁇ 332 ⁇ ⁇ 113> crystal orientation of 5.0 or less cannot be obtained.
- the hot rolling performed as described above ends at a temperature of Ar3 or higher.
- the hot rolling is finished at less than Ar3, it includes a two-phase rolling of an austenite region and a ferrite region, so that the accumulation in ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation groups becomes strong, resulting in local deformation. The performance is significantly degraded.
- rL and r60 are respectively set to rL of 0.70 or more and r60 of 1.10 or less, further satisfactory plate thickness / minimum bending radius ⁇ 2.0 is satisfied.
- a pass having a reduction rate of 30% or more in a temperature range of T1 + 30 ° C. or more and T1 + 200 ° C. or less is set as a large reduction pass, cooling is started after the final pass of the large reduction passes is completed. It is important that the waiting time t (seconds) satisfies the condition defined in Equation 6 above.
- FIGS. 38 and 39 show the temperature rise of the steel sheet during the rolling at T1 + 30 ° C. or higher and T1 + 200 ° C. or lower, and the relationship between the waiting time t and rL and r60. It is effective for obtaining uniform recrystallized austenite that the waiting time t satisfies Equation 6 and further suppresses the temperature rise of the steel sheet between T1 + 30 ° C. and T1 + 200 ° C. to 18 ° C. or less between passes.
- the cooling pattern after hot rolling is not particularly defined, and the effect of the present invention can be obtained by taking a cooling pattern for controlling the structure suitable for each purpose.
- the coiling temperature exceeds 680 ° C., there is a concern that surface oxidation may progress or the bendability after cold rolling or annealing may be adversely affected. Therefore, the coiling temperature is set to 680 ° C. or lower and room temperature or higher.
- sheet bars may be joined after rough rolling, and finish rolling may be performed continuously.
- the coarse bar may be wound once in a coil shape, stored in a cover having a heat retaining function as necessary, and rewound again to perform bonding.
- the hot-rolled steel sheet may be subjected to skin pass rolling as necessary. Skin pass rolling has the effect of preventing stretcher strain generated during processing and shape correction.
- the hot-rolled steel sheet After the hot-rolled steel sheet has been pickled, it is cold-rolled so that the cold reduction ratio is 20% or more and 90% or less. If the rolling reduction is less than 20%, there is a concern that a sufficient cold-rolled recrystallized structure is not formed and mixed grains are formed. Moreover, when it exceeds 90%, there exists a concern of the fracture
- the effect of the present invention can be obtained even if the annealing heat treatment pattern is a heat treatment pattern for controlling the structure for each purpose.
- the temperature is raised to a temperature range of at least 650 ° C. to 900 ° C., and a holding time of 1 second to 300 seconds is obtained.
- a holding time 1 second to 300 seconds.
- the holding temperature range is less than 650 ° C. or the holding time is less than 1 second, a sufficient recovery recrystallization structure cannot be obtained. Further, if the holding temperature range exceeds 900 ° C.
- the structure of the galvanized steel sheet obtained in the present embodiment is mainly composed of ferrite, but it may contain compounds such as pearlite, bainite, martensite, austenite, and carbonitride as a metal structure other than ferrite. Since the crystal structure of martensite and bainite is the same as or similar to that of ferrite, these structures may be mainly used instead of ferrite.
- the galvanized steel sheet according to the present invention can be applied not only to bending, but also to composite molding mainly composed of bending such as bending, overhanging and drawing.
- these steels are reheated as they are or once cooled to room temperature, heated to a temperature range of 900 ° C. to 1300 ° C., and then hot rolled under the conditions of Table 2 or Table 3, Finally, a hot-rolled steel sheet having a thickness of 2.3 mm or 3.2 mm was obtained.
- Table 1 shows the chemical composition of each steel
- Tables 2 and 3 show the production conditions
- Tables 4 and 5 show the structure and mechanical properties.
- As an index of local deformability the hole expansion rate and the critical bending radius by 90 ° V-bending were used.
- C direction bending and 45 ° direction bending were performed, and the ratio was used as an index of orientation dependency of formability.
- the tensile test and the bending test were compliant with JIS Z2241 and Z2248 V-block 90 ° bending tests, and the hole expansion test was compliant with the Iron Federation standard JFS T1001.
- the X-ray random intensity ratio is determined by using the above-mentioned EBSD at the central part of the plate thickness in the region of 5/8 to 3/8 of the cross section parallel to the rolling direction, with the width direction being 1/4 of the position from the end. And measured at a pitch of 0.5 ⁇ m. The r value in each direction was measured by the method described above.
- These steels are cast, or after being cooled to room temperature after being cast, are reheated and heated to a temperature range of 900 ° C. to 1300 ° C., and then hot-rolled under the conditions shown in Table 7 to 2 to 5 mm. After forming a thick hot-rolled steel sheet, it was pickled, cold-rolled to a thickness of 1.2 to 2.3 mm, and annealed under the annealing conditions shown in Table 7. Then, 0.5% skin pass rolling was performed and used for material evaluation.
- Table 6 shows the chemical composition of each steel, and Table 7 shows the production conditions.
- Table 8 shows the structure and mechanical characteristics of each.
- indices of local deformability the hole expansion rate and the critical bending radius by 90 ° V-bending were used.
- C direction bending and 45 ° direction bending were performed, and the ratio was used as an index of orientation dependency of formability.
- the tensile test and the bending test were compliant with JIS Z2241 and Z2248 V-block 90 ° bending test, and the hole expansion test was compliant with the iron standard JFS T1001.
- the X-ray random intensity ratio is the center of the plate thickness in the region of 3/8 to 5/8 of the cross section parallel to the rolling direction using the above-mentioned EBSD, and the width direction is 1/4 of the position from the end. , Measured at a pitch of 0.5 ⁇ m. The r value in each direction was measured by the method described above.
- these steels are reheated as they are or once cooled to room temperature, heated to a temperature range of 900 ° C. to 1300 ° C., and then hot-rolled under the conditions shown in Table 10 to a thickness of 2 to 5 mm.
- hot-rolling the steel sheet pickling and cold-rolling, cold-rolling to a thickness of 1.2 to 2.3 mm, annealing under the annealing conditions shown in Table 10, and hot-dip galvanizing bath It was used for continuous annealing and hot dip galvanizing or hot galvannealing. Then, 0.5% skin pass rolling was performed and used for material evaluation.
- Table 9 shows the chemical composition of each steel
- Table 10 shows the production conditions
- Table 11 shows the structure and mechanical properties under each production condition.
- the tensile test and the bending test were compliant with JIS Z 2241 and Z 2248 V-block 90 ° bending tests, and the hole expansion test was compliant with the ironwork standard JFS T1001.
- the X-ray random intensity ratio is 0 with respect to the position where the width direction is 1/4 from the end portion at the central portion of the region of 3/8 to 5/8 of the cross section parallel to the rolling direction using the above-mentioned EBSD. Measured at a pitch of 5 ⁇ m. The r value in each direction was measured by the method described above.
- the main structure is not limited, and in addition to the control of the crystal grain size and form, the texture is controlled so that local deformation can be achieved even if Nb, Ti, or the like is added.
- a hot-rolled steel sheet, a cold-rolled steel sheet, and a galvanized steel sheet with excellent performance and less orientation dependency of formability can be obtained.
- the present invention has high applicability in the steel industry. Further, in the present invention, the strength of the steel sheet is not specified, but as described above, the formability decreases as the strength is increased. Therefore, the strength is particularly effective when the tensile strength is 440 MPa or more. large.
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Abstract
Description
本願は、2010年7月28日に日本に出願された特願2010-169670号と、2010年7月28日に日本に出願された特願2010-169627号と、2011年3月4日に日本に出願された特願2011-048236号と、2010年7月28日に日本に出願された特願2010-169230号と、2011年3月4日に日本に出願された特願2011-048272号と、2010年9月13日に日本に出願された特願2010-204671号と、2011年3月4日に日本に出願された特願2011-048246号と、2011年3月4日に日本に出願された特願2011-048253号とに基づき優先権を主張し、これらの内容をここに援用する。
従って、自動車車体の足回り部品や、衝突エネルギー吸収に寄与する部品等に高強度鋼板を用いるには、バーリング加工性や、曲げ加工性等の成形性に寄与する局部延性などの局部変形能を改善することが重要となる。
しかし、上記のいずれの技術も組織制御に頼った局部変形能の改善方法であり、ベースの組織形成に大きく影響されてしまう。
(1)すなわち、本発明の一態様にかかる熱延鋼板は、質量%で、C:0.0001%以上、0.40%以下、Si:0.001%以上、2.5%以下、Mn:0.001%以上、4.0%以下、P:0.001%以上、0.15%以下、S:0.0005%以上、0.03%以下、Al:0.001%以上、2.0%以下、N:0.0005%以上、0.01%以下、O:0.0005%以上、0.01%以下、を含有し、さらに、Ti:0.001%以上、0.20%以下、Nb:0.001%以上、0.20%以下、V:0.001%以上、1.0%以下、W:0.001%以上、1.0%以下、B:0.0001%以上、0.0050%以下、Mo:0.001%以上、1.0%以下、Cr:0.001%以上、2.0%以下、Cu:0.001%以上、2.0%以下、Ni:0.001%以上、2.0%以下、Co:0.0001%以上、1.0%以下、Sn:0.0001%以上、0.2%以下、Zr:0.0001%以上、0.2%以下、As:0.0001%以上、0.50%以下、Mg:0.0001%以上、0.010%以下、Ca:0.0001%以上、0.010%以下、REM:0.0001%以上、0.1%以下、のうちの1種又は2種以上を含有し、残部が鉄および不可避的不純物からなり、少なくとも鋼板表面から5/8~3/8の板厚範囲である板厚中央部における{100}<011>~{223}<110>方位群のX線ランダム強度比の平均値が1.0以上6.0以下でかつ、{332}<113>の結晶方位のX線ランダム強度比が1.0以上5.0以下であり、圧延方向に対して直角方向のr値であるrCが0.70以上1.10以下でかつ、前記圧延方向に対して30°をなす方向のr値であるr30が0.70以上1.10以下である。
ここで、前記T1は鋼板成分により決定される温度であり、下式1で表される。
T1(℃)=850+10×(C+N)×Mn+350×Nb+250×Ti+40×B+10×Cr+100×Mo+100×V・・・(式1)
t1≦t≦t1×2.5・・・(式2)
ここで、t1は下式3で表される。
t1=0.001×((Tf-T1)×P1)2-0.109×((Tf-T1)×P1)+3.1・・・(式3)
ここで、Tfは前記最終パス後の温度であり、P1は前記最終パスにおける圧下率である。
t1≦t≦t1×2.5・・・(式4)
ここで、t1は下式5で表される。
t1=0.001×((Tf-T1)×P1)2-0.109×((Tf-T1)×P1)+3.1・・・(式5)
ここで、Tfは前記最終パス後の温度であり、P1は前記最終パスにおける圧下率である。
t1≦t≦t1×2.5・・・(式6)
ここで、t1は下式7で表される。
t1=0.001×((Tf-T1)×P1)2-0.109×((Tf-T1)×P1)+3.1・・・(式7)
ここで、Tfは前記最終パス後の温度、P1は前記最終パスにおける圧下率である。
1.熱延鋼板について
(1)鋼板の表面から5/8~3/8の板厚範囲である板厚中央部における{100}<011>~{223}<110>方位群のX線ランダム強度比の平均値、{332}<113>の結晶方位のX線ランダム強度比:
鋼板の表面から5/8~3/8の板厚範囲である板厚中央部における{100}<011>~{223}<110>方位群のX線ランダム強度比の平均値は、本実施形態において特に重要な特性値である。
(2)圧延方向と直角方向のr値であるrC:
このrCは、本実施形態において重要である。すなわち、本発明者等が鋭意検討した結果、上述した種々の結晶方位のX線ランダム強度比だけが適正であっても、必ずしも良好な穴拡げ性や曲げ性が得られないことが判明した。図3に示すように、上記のX線ランダム強度比と同時に、rCが0.70以上であることが必須である。
(3)圧延方向に対して30°をなす方向のr値であるr30:
このr30は、本実施形態において重要である。すなわち、本発明者等が鋭意検討した結果、上述した種々の結晶方位のX線強度が適正であっても、必ずしも良好な局部変形能が得られないことが判明した。図4に示すように、上記のX線強度と同時に、r30が1.10以下であることが必須である。
(4)圧延方向のr値であるrLおよび圧延方向に対して60°をなす方向のr値であるr60:
さらに、本発明者等が鋭意検討した結果、上述した種々の結晶方位のX線ランダム強度比とrCおよびr30だけでなく、図5、図6に示すように、さらに圧延方向のrLが0.70以上でかつ、圧延方向に対して60°をなす方向のr値であるr60が1.10以下であれば、板厚/最小曲げ半径≧2.0を満たすことが判明した。
本発明者らは、さらに局部変形能を追求した結果、上記の集合組織およびr値を満たした上で、結晶粒の等軸性に優れたときに、曲げ加工の方向依存性がほぼなくなることを見出した。この等軸性を表す指標としては、これら組織中の結晶粒の熱間圧延方向の長さであるdLと板厚方向の長さであるdtの比であるdL/dtが、3.0以下の等軸性に優れた粒の割合が、これら結晶粒のうち、50%以上100%以下である。50%未満では、圧延方向であるL方向または圧延方向に対して直角方向であるC方向の曲げ性Rが劣化する。
各組織の判定は、以下のように行うことができる。
光学顕微鏡による組織観察にて、パーライトを特定する。次にEBSD(Electron Back Scattering Diffraction;電子線後方散乱回折法)を用いて、結晶構造を判定し、fcc構造の結晶をオーステナイトとする。bcc構造のフェライト、ベイナイトおよびマルテンサイトは、EBSP-OIMTMに装備されているKernel Average Misorientation、すなわちKAM法にて識別することができる。KAM法は測定データのうちのある正六角形のピクセルの隣り合う6個である第一近似、もしくはさらにその外側12個である第二近似、もしくはさらにその外側の18個である第三近似のピクセル間の方位差を平均し、その値をその中心のピクセルの値とする計算を各ピクセルに行うことにより算出される値である。粒界を越えないようにこの計算を実施することで粒内の方位変化を表現するマップを作成できる。このマップは粒内の局所的な方位変化に基づくひずみの分布を表している。
本発明の実施例においては、EBSP-OIMTMにおいて隣接するピクセル間の方位差を計算する条件を第三近似として、この方位差が5°以下とし、上記の方位差第三近似において、1°超が低温変態生成物であるベイナイトもしくはマルテンサイト、1°以下がフェライトと定義した。これは、高温で変態したポリゴナルな初析フェライトは拡散変態で生成するので、転位密度が小さく、粒内の歪みが少ないため、結晶方位の粒内差が小さく、これまで発明者らが実施してきた様々な調査結果より、光学顕微鏡観察で得られるフェライト体積分率とKAM法にて測定した方位差第三近似1°で得られるエリアの面積分率がほぼよい一致をするためである。
さらに、曲げ性は、結晶粒の等軸性の影響を強く受け、その効果が大きいことを見出した。その理由は明らかではないが、曲げ変形は、局部的にひずみが集中するモードであり、全ての結晶粒が均一に、等価にひずみを受ける状態が曲げ性には有利と考えられる。粒径の大きな結晶粒が多い場合には、等方性化と等軸粒化が十分であっても、局部的な結晶粒が歪むことにより、その局部的に歪む結晶粒の方位により、曲げ性に大きなばらつきが出て、曲げ性の低下を引き起こすと考えている。そのため、等方性化と等軸粒化の効果により、ひずみの局部化を抑え、曲げ性を向上させるためには、全面積のうち、粒径20μmを超える結晶粒の占める面積割合が少ない方がよく、0%以上10%以下である必要がある。10%より多いと曲げ性が劣化する。ここで言う結晶粒とは、フェライト、パーライト、ベイナイト、マルテンサイト、およびオーステナイトの結晶粒を言う。
(1)鋼板の表面から5/8~3/8の板厚範囲である板厚中央部における{100}<011>~{223}<110>方位群のX線ランダム強度比の平均値、{332}<113>の結晶方位のX線ランダム強度比:
鋼板の表面から5/8~3/8の板厚範囲である板厚中央部における{100}<011>~{223}<110>方位群のX線ランダム強度比の平均値は本実施形態において、特に重要な特性値である。
このrC値は、本実施形態において重要である。すなわち、本発明者等が鋭意検討した結果、上述した種々の結晶方位のX線ランダム強度比だけが適正であっても、必ずしも良好な穴拡げ性や曲げ性が得られないことが判明した。図9に示すように、上記のX線ランダム強度比と同時に、rCが0.70以上であることが必須である。
このr30は、本実施形態において重要である。すなわち、本発明者等が鋭意検討した結果、上述した種々の結晶方位のX線ランダム強度比が適正であっても、必ずしも良好な局部変形能が得られないことが判明した。図10に示すように、上記のX線ランダム強度比と同時に、r30が1.10以下であることが必須である。
さらに本発明者等が鋭意検討した結果、上述した種々の結晶方位のX線ランダム強度比とrCおよびr30だけでなく、図11、図12に示すように圧延方向のrLおよび圧延方向に対して60°をなす方向のr値であるr60が、それぞれrLが0.70以上でかつ、r60が1.10以下であれば、さらに良好な板厚/最小曲げ半径≧2.0を満たすことが判明した。
本発明者らは、さらに局部変形能を追求した結果、上記の集合組織およびr値を満たした上で、結晶粒の等軸性に優れたときに、曲げ加工の方向依存性がほぼなくなることを見出した。この等軸性を表す指標としては、これら組織中の結晶粒の冷間圧延方向の長さであるdLと板厚方向の長さであるdtの比であるdL/dtが、3.0以下の等軸性に優れた粒の割合が、これら結晶粒のうち、50%以上100%以下である。50%未満では、圧延方向であるL方向または圧延方向に対して直角方向であるC方向の曲げ性Rが劣化する。
各組織の判定は、以下のように行うことができる。
光学顕微鏡による組織観察にて、パーライトを特定する。次にEBSDを用いて、結晶構造を判定し、fcc構造の結晶をオーステナイトとする。bcc構造のフェライト、ベイナイトおよびマルテンサイトは、EBSP-OIMTMに装備されているKernel Average Misorientation、すなわちKAM法にて識別することができる。KAM法は測定データのうちのある正六角形のピクセルの隣り合う6個である第一近似、もしくはさらにその外側12個である第二近似、もしくはさらにその外側の18個である第三近似のピクセル間の方位差を平均し、その値をその中心のピクセルの値とする計算を各ピクセルに行うことにより算出される値である。粒界を越えないようにこの計算を実施することで粒内の方位変化を表現するマップを作成できる。このマップは粒内の局所的な方位変化に基づくひずみの分布を表している。
本発明の実施例においては、EBSP-OIMTMにおいて隣接するピクセル間の方位差を計算する条件を第三近似として、この方位差が5°以下とし、上記の方位差第三近似において、1°超が低温変態生成物であるベイナイトもしくはマルテンサイト、1°以下がフェライトと定義した。これは、高温で変態したポリゴナルな初析フェライトは拡散変態で生成するので、転位密度が小さく、粒内の歪みが少ないため、結晶方位の粒内差が小さく、これまで発明者らが実施してきた様々な調査結果より、光学顕微鏡観察で得られるフェライト体積分率とKAM法にて測定した方位差第三近似1°で得られるエリアの面積分率がほぼよい一致をするためである。
さらに、曲げ性は、結晶粒の等軸性の影響を強く受け、その効果が大きくなることを見出した。その理由は明らかではないが、曲げ変形は、局部的にひずみが集中するモードであり、全ての結晶粒が均一に、等価にひずみを受ける状態が曲げ性には有利と考えている。粒径の大きな結晶粒が多い場合、等方性化と等軸粒化が十分であっても、局部的な結晶粒が歪むことにより、その局部的に歪む結晶粒の方位により、曲げ性に大きなばらつきが出て、曲げ性の低下を引き起こすと考えている。そのため、等方性化と等軸粒化の効果により、ひずみの局部化を抑え、曲げ性を向上させるためには、全面積のうち、粒径20μmを超える結晶粒の占める面積割合が少ない方がよく、0%以上10%以下である必要がある。10%より多いと曲げ性が劣化する。ここで言う結晶粒とは、フェライト、パーライト、ベイナイト、マルテンサイト、およびオーステナイトの結晶粒を言う。
(1))鋼板の表面から5/8~3/8の板厚範囲である板厚中央部における{100}<011>~{223}<110>方位群のX線ランダム強度比の平均値、{332}<113>の結晶方位のX線ランダム強度比:
鋼板の表面から5/8~3/8の板厚範囲である板厚中央部における{100}<011>~{223}<110>方位群のX線ランダム強度比の平均値は本実施形態において、特に重要な特性値である。図13に示すように、鋼板の表面から5/8~3/8の板厚範囲である板厚中央部における板面のX線回折を行い、ランダム試料に対する各方位の強度比を求めたときの、{100}<011>~{223}<110>方位群の平均値が4.0未満であれば、直近要求される足回り部品の加工に必要な板厚/最小曲げ半径≧1.5を満たす。さらに穴拡げ性や小さな限界曲げ特性を必要とする場合には、3.0未満が望ましい。4.0以上では鋼板の機械的特性の異方性が極めて強くなり、その結果、ある方向のみの局部変形能は改善するものの、その方向とは異なる方向での材質が著しく劣化するため、板厚/最小曲げ半径≧1.5を満足できなくなる。
このrCは、本実施形態において重要である。すなわち、本発明者等が鋭意検討した結果、上述した種々の結晶方位のX線ランダム強度比だけが適正であっても、必ずしも良好な穴拡げ性や曲げ性が得られないことが判明した。図15に示すように、上記のX線ランダム強度比と同時に、rCが0.70以上であることが必須である。
上述のrCの上限を、1.10とすることで、より優れた局部変形能を得ることができる。
このr30は、本実施形態において重要である。すなわち、本発明者等が鋭意検討した結果、上述した種々の結晶方位のX線ランダム強度比が適正であっても、必ずしも良好な局部変形能が得られないことが判明した。図16に示すように、上記のX線ランダム強度比と同時に、r30が1.10以下であることが必須である。
上述のr30の下限を、0.70とすることで、よりすぐれた局部変形能を得ることができる。
更に本発明者等が鋭意検討した結果、上述した種々の結晶方位のX線ランダム強度比とrCおよびr30だけでなく、図17、図18のように圧延方向のrLおよび圧延方向に対して60°をなす方向のr60が、それぞれrLが0.70以上でかつ、r60が1.10以下であれば、更に良好な板厚/最小曲げ半径≧2.0を満たすことが判明した。
上述のrL値およびr60値は、rLが1.10以下、r60が0.70以上であることで、よりすぐれた局部変形能を得ることができる。
本発明は亜鉛めっき鋼板の全般に適用できるものであり、上記の限定が満たされれば、組織の組み合わせに制限されることなく、亜鉛めっき鋼板の曲げ加工性や穴広げ性などの局部変形能が飛躍的に向上する。
また、{100}<011>~{223}<110>方位群の平均値とは、上記の各方位の相加平均である。上記の全ての方位の強度を得ることができない場合には、{100}<011>、{116}<110>、{114}<110>、{112}<110>、{223}<110>の各方位の相加平均で代替しても良い。
また、フェライト、ベイナイト、マルテンサイトおよびオーステナイトの粒径は、前述のEBSD法による鋼板の方位の解析において、例えば、1500倍の倍率にて、0.5μm以下の測定ステップで方位測定を行い、隣り合う測定点の方位差が15°を超えた位置を粒境界として定め、その円相当径を求めることで得られる。その際、圧延方向および板厚方向の粒の長さについても、同時に求めることでdL/dtが得られる。
本発明の冷延鋼板および亜鉛めっき鋼板は、本発明における熱延鋼板を原板としているため、鋼板の成分については、熱延鋼板、冷延鋼板、亜鉛めっき鋼板のいずれについても以下の通りである。
これら効果を得るためには、Bは0.0001%以上、Mo、Cr、Ni、Cuは0.001%以上、Co、Sn、Zr、Asは0.0001%以上を添加する必要がある。しかし、過度の添加は逆に加工性を劣化させるので、Bの上限を0.0050%、Moの上限を1.0%、Cr、Ni、Cuの上限を2.0%、Coの上限を1.0%、Sn、Zrの上限を0.2%、Asの上限を0.50%とする。特に加工性が強く要求される場合は、Bの上限を0.005%、Moの上限を0.50%とすることが望ましい。また、コストの観点から、上記の添加元素のうち、B、Mo、Cr、Asを選択することがより望ましい。
また、本発明の亜鉛めっき鋼板は、本発明の冷延鋼板の表面に亜鉛めっき処理による亜鉛めっき層を有するものであるが、亜鉛めっきは、溶融亜鉛めっきと電気亜鉛めっきのいずれでも効果が得られる。また、亜鉛めっき後合金化処理をして、自動車用途に代表される合金化亜鉛めっき鋼板としてもよい。
加えて、本発明の高強度亜鉛めっき鋼板にさらに表面処理しても本発明の効果を失うものでなく、有機皮膜形成、フィルムラミネート、有機塩類/無機塩類処理、ノンクロ処理等の何れでも本発明の効果が得られる。
次に本実施形態に係る熱延鋼板の製造方法について述べる。
優れた局部変形能を実現するためには、所定のX線ランダム強度比をもつ集合組織を形成させること、各方向のr値の条件を満たすこと、および粒形状を制御することが重要である。これらを満たすための製造条件の詳細を以下に記す。
粗圧延後のオーステナイト粒径を確認するためには、仕上げ圧延に入る前の板片を可能な限り急冷することが望ましく、10℃/s以上の冷却速度で板片を冷却して、板片断面の組織をエッチングしてオーステナイト粒界を浮き立たせて光学顕微鏡にて測定する。この際、50倍以上の倍率にて20視野以上を、画像解析やポイントカウント法にて測定する。
このT1温度自体は経験的に求めたものであり、T1温度を基準として、各鋼のオーステナイト域での再結晶が促進されることを、発明者等は実験により知見した。
図26、図27に、T1+30℃以上T1+200℃以下での圧下時のパス間の鋼板の温度上昇量、前記待ち時間tとrLおよび、r60の関係を示す。T1+30℃以上T1+200℃以下での各パス間の鋼板の温度上昇が18℃以下で、tが前記式2を満たす場合に、rLが0.70以上、r60が1.10以下である均一な再結晶オーステナイトを得ることができる。
前記待ち時間tがt1×2.5を超えると、粗粒化が進み、伸びが著しく低下する。また、t1よりも短いと異方性が高くなり、等軸粒分率が低減する。
また、熱間圧延後には、巻取りを行ってよい。
優れた局部変形能を実現するためには、亜鉛めっき処理を行った後の鋼板において、X線ランダム強度比をもつ集合組織を形成させることおよび各方向のr値の条件を満たすことが重要である。これらを満たすための製造条件の詳細を以下に記す。
熱間圧延に先行する製造方法は特に限定するものではない。すなわち、高炉や電炉等による溶製に引き続き各種の2次製錬を行い、次いで、通常の連続鋳造、インゴット法による鋳造、または薄スラブ鋳造などの方法で鋳造すればよい。連続鋳造の場合には一度低温まで冷却したのち、再度加熱してから熱間圧延しても良いし、鋳造スラブを低温まで冷却せずに鋳造後にそのまま熱延しても良い。原料にはスクラップを使用しても構わない。
粗圧延後のオーステナイト粒径を確認するためには、仕上げ圧延に入る前の板片を可能な限り急冷することが望ましく、10℃/s以上の冷却速度で板片を冷却して、板片断面の組織をエッチングしてオーステナイト粒界を浮き立たせて光学顕微鏡にて測定する。この際、50倍以上の倍率にて20視野以上を、画像解析やポイントカウント法にて測定する。さらに局部変形能を高めるためには100μm以下が望ましい。
図34~図37に各温度域での圧下率と各方位のX線ランダム強度比の関係を示す。
前記待ち時間tが前記式6を満たし、さらにT1+30℃以上T1+200℃以下での鋼板の温度上昇を各パス間において18℃以下に抑えることが、均一な再結晶オーステナイトを得るのに有効である。
その後、常法に従い、亜鉛めっき処理を行って亜鉛めっき鋼板を得る。
本発明に係る亜鉛めっき鋼板は曲げ加工だけでなく、曲げ、張り出し、絞り等、曲げ加工を主体とする複合成形にも適用できる。
局部変形能の指標として、穴拡げ率、および90°V字曲げによる限界曲げ半径を用いた。曲げ試験はC方向曲げと45°方向曲げを行い、その比率を使って成形性の方位依存性の指標とした。引っ張り試験および曲げ試験はJIS Z2241およびZ2248のVブロック90°曲げ試験に、穴拡げ試験は鉄連規格JFS T1001に、それぞれ準拠した。X線ランダム強度比は、前述のEBSDを用いて、圧延方向に平行な断面の5/8~3/8の領域の板厚中央部で、幅方向が端部から1/4の位置に対して0.5μmピッチで測定した。また、各方向のr値については、前述した方法により測定した。
局部変形能の指標として穴拡げ率および90°V字曲げによる限界曲げ半径を用いた。なお、引っ張り試験および曲げ試験はJIS Z 2241およびZ 2248のVブロック90°曲げ試験に、穴拡げ試験は鉄連規格JFS T1001にそれぞれ準拠した。X線ランダム強度比は前述のEBSDを用いて圧延方向に平行な断面の3/8~5/8の領域の板厚中央部で、幅方向が端部から1/4の位置に対して0.5μmピッチで測定した。また、各方向のr値については、前述した方法により測定した。
また、本発明において、鋼板の強度については規定していないが、前述の通り高強度化するほど成形性が低下するため、高強度鋼板、例えば、引張強度で440MPa以上となる場合に特に効果が大きい。
Claims (25)
- 質量%で、
C :0.0001%以上、0.40%以下、
Si:0.001%以上、2.5%以下、
Mn:0.001%以上、4.0%以下、
P :0.001%以上、0.15%以下、
S :0.0005%以上、0.03%以下、
Al:0.001%以上、2.0%以下、
N :0.0005%以上、0.01%以下、
O :0.0005%以上、0.01%以下、
を含有し、さらに、
Ti:0.001%以上、0.20%以下、
Nb:0.001%以上、0.20%以下、
V:0.001%以上、1.0%以下、
W:0.001%以上、1.0%以下、
B :0.0001%以上、0.0050%以下、
Mo:0.001%以上、1.0%以下、
Cr:0.001%以上、2.0%以下、
Cu:0.001%以上、2.0%以下、
Ni:0.001%以上、2.0%以下、
Co:0.0001%以上、1.0%以下、
Sn:0.0001%以上、0.2%以下、
Zr:0.0001%以上、0.2%以下、
As:0.0001%以上、0.50%以下、
Mg:0.0001%以上、0.010%以下、
Ca:0.0001%以上、0.010%以下、
REM:0.0001%以上、0.1%以下、
のうちの1種又は2種以上を含有し、
残部が鉄および不可避的不純物からなり;
少なくとも鋼板表面から5/8~3/8の板厚範囲である板厚中央部における{100}<011>~{223}<110>方位群のX線ランダム強度比の平均値が1.0以上6.0以下でかつ、{332}<113>の結晶方位のX線ランダム強度比が1.0以上5.0以下であり;
圧延方向に対して直角方向のr値であるrCが0.70以上1.10以下でかつ、前記圧延方向に対して30°をなす方向のr値であるr30が0.70以上1.10以下である;
ことを特徴とする熱延鋼板。 - 前記圧延方向のr値であるrLが0.70以上1.10以下でかつ、前記圧延方向に対して60°をなす方向のr値であるr60が0.70以上1.10以下であることを特徴とする請求項1に記載の熱延鋼板。
- 前記熱延鋼板中にベイナイト、マルテンサイト、パーライトおよびオーステナイトの1種または2種以上が存在し、これら組織の結晶粒のうち、前記圧延方向の長さdLと板厚方向の長さdtの比であるdL/dtが3.0以下である粒の割合が50%以上100%以下であることを特徴とする請求項1または2に記載の熱延鋼板。
- 前記熱延鋼板の金属組織の全面積のうち、粒径が20μmを超える結晶粒の面積割合が0%以上10%以下であることを特徴とする請求項1または2に記載の熱延鋼板。
- 請求項1に記載の熱延鋼板を冷間圧延した冷延鋼板であって、
少なくとも前記板厚中央部における{100}<011>~{223}<110>方位群のX線ランダム強度比の平均値が1.0以上4.0未満でかつ、{332}<113>の結晶方位のX線ランダム強度比が1.0以上5.0以下であり;
前記圧延方向に対して直角方向のr値であるrCが0.70以上1.10以下でかつ、前記圧延方向に対して30°をなす方向のr値であるr30が0.70以上1.10以下である;
ことを特徴とする冷延鋼板。 - 前記圧延方向のr値であるrLが0.70以上1.10以下でかつ、前記圧延方向に対して60°をなす方向のr値であるr60が0.70以上1.10以下であることを特徴とする請求項5に記載の冷延鋼板。
- 前記冷延鋼板中にベイナイト、マルテンサイト、パーライトおよびオーステナイトの1種または2種以上が存在し、これら組織の結晶粒のうち、前記圧延方向の長さdLと板厚方向の長さdtの比であるdL/dtが3.0以下である粒の割合が50%以上100%以下であることを特徴とする請求項5または6に記載の冷延鋼板。
- 前記冷延鋼板の金属組織の全面積のうち、粒径が20μmを超える結晶粒の面積割合が0%以上10%以下であることを特徴とする請求項5または6に記載の冷延鋼板。
- 請求項5に記載の冷延鋼板の表面に、さらに、溶融亜鉛めっき層または、合金化溶融亜鉛めっき層を備えた亜鉛めっき鋼板であって、
少なくとも前記板厚中央部における{100}<011>~{223}<110>方位群のX線ランダム強度比の平均値が1.0以上4.0未満でかつ、{332}<113>の結晶方位のX線ランダム強度比が1.0以上5.0以下であり;
前記圧延方向に対して直角方向のr値であるrCが0.70以上1.10以下でかつ、前記圧延方向に対して30°をなす方向のr値であるr30が0.70以上1.10以下である;
ことを特徴とする亜鉛めっき鋼板。 - 前記圧延方向のr値であるrLが0.70以上1.10以下でかつ、前記圧延方向に対して60°をなす方向のr値であるr60が0.70以上1.10以下であることを特徴とする請求項9に記載の亜鉛めっき鋼板。
- 質量%で、
C :0.0001%以上、0.40%以下、
Si:0.001%以上、2.5%以下、
Mn:0.001%以上、4.0%以下、
P :0.001%以上、0.15%以下、
S :0.0005%以上、0.03%以下、
Al:0.001%以上、2.0%以下、
N :0.0005%以上、0.01%以下、
O :0.0005%以上、0.01%以下、
を含有し、さらに、
Ti:0.001%以上、0.20%以下、
Nb:0.001%以上、0.20%以下、
V:0.001%以上、1.0%以下、
W:0.001%以上、1.0%以下、
B :0.0001%以上、0.0050%以下、
Mo:0.001%以上、1.0%以下、
Cr:0.001%以上、2.0%以下、
Cu:0.001%以上、2.0%以下、
Ni:0.001%以上、2.0%以下、
Co:0.0001%以上、1.0%以下、
Sn:0.0001%以上、0.2%以下、
Zr:0.0001%以上、0.2%以下、
As:0.0001%以上、0.50%以下、
Mg:0.0001%以上、0.010%以下、
Ca:0.0001%以上、0.010%以下、
REM:0.0001%以上、0.1%以下、
のうちの1種又は2種以上を含有し、
残部が鉄および不可避的不純物からなる鋼塊またはスラブを、
1000℃以上1200℃以下の温度域で、20%以上の圧下を少なくとも1回以上行う第1の熱間圧延を行い、オーステナイト粒径を200μm以下とし;
T1+30℃以上T1+200℃以下の温度範囲で、圧下率の合計が50%以上である第2の熱間圧延を行い;
T1℃以上T1+30℃未満の温度範囲で、圧下率の合計が30%未満である第3の熱間圧延を行い;
Ar3変態温度以上で熱間圧延を終了する;
ことを特徴とする熱延鋼板の製造方法。
ここで、前記T1は鋼板成分により決定される温度であり、下式1で表される。
T1(℃)=850+10×(C+N)×Mn+350×Nb+250×Ti+40×B+10×Cr+100×Mo+100×V・・・(式1) - T1+30℃以上T1+200℃以下の温度範囲での前記第2の熱間圧延において、1パスで30%以上の圧下率の圧下を少なくとも1回以上行うことを特徴とする請求項11に記載の熱延鋼板の製造方法。
- 1000℃以上1200℃以下の温度域での前記第1の熱間圧延において、20%以上の圧下率の圧下を少なくとも2回以上行い、オーステナイト粒径を100μm以下とすることを特徴とする請求項11または12に記載の熱延鋼板の製造方法。
- T1+30℃以上T1+200℃以下の温度域における30%以上の圧下率であるパスを大圧下パスとした場合、前記大圧下パスのうちの最終パスが完了した後から冷却を開始するまでの待ち時間tが、下式2を満たすことを特徴とする請求項11または12に記載の熱延鋼板の製造方法。
t1≦t≦t1×2.5・・・(式2)
ここで、t1は下式3で表される。
t1=0.001×((Tf-T1)×P1)2-0.109×((Tf-T1)×P1)+3.1・・・(式3)
ここで、Tfは前記最終パス後の温度であり、P1は前記最終パスにおける圧下率である。 - T1+30℃以上T1+200℃以下の温度域における前記第2の熱間圧延の各パス間の鋼板の温度上昇を18℃以下とすることを特徴とする請求項14に記載の熱延鋼板の製造方法。
- 請求項11に記載の熱延鋼板の製造方法で得られた前記熱延鋼板に対して、Ar3変態温度以上で熱間圧延を終了した後、
酸洗し;
冷間にて20%以上90%以下の圧延を行い;
720℃以上900℃以下の温度域で1秒以上300秒以下の保持時間で焼鈍し;
650℃から500℃の間の冷却速度が10℃/s以上200℃/s以下である加速冷却を行い;
200℃以上500℃以下の温度にて保持する;
ことを特徴とする冷延鋼板の製造方法。 - T1+30℃以上T1+200℃以下の温度範囲での前記第2の熱間圧延において、1パスで30%以上の圧下率の圧下を少なくとも1回以上行うことを特徴とする請求項16に記載の冷延鋼板の製造方法。
- 1000℃以上1200℃以下の温度域での前記第1の熱間圧延において20%以上の圧下率の圧下を少なくとも2回以上行い、オーステナイト粒径を100μm以下とすることを特徴とする請求項16または17に記載の冷延鋼板の製造方法。
- T1+30℃以上T1+200℃以下の温度域における30%以上の圧下率であるパスを大圧下パスとした場合、前記大圧下パスのうちの最終パスが完了した後から冷却を開始するまでの待ち時間tが、下式4を満たすことを特徴とする請求項16または17に記載の冷延鋼板の製造方法。
t1≦t≦t1×2.5・・・(式4)
ここで、t1は下式5で表される。
t1=0.001×((Tf-T1)×P1)2-0.109×((Tf-T1)×P1)+3.1・・・(式5)
ここで、Tfは前記最終パス後の温度であり、P1は前記最終パスにおける圧下率である。 - T1+30℃以上T1+200℃以下の温度域における前記第2の熱間圧延の各パス間の鋼板の温度上昇を18℃以下とすることを特徴とする請求項19に記載の冷延鋼板の製造方法。
- 請求項11に記載の熱延鋼板の製造方法で得られた前記熱延鋼板に対して、Ar3変態温度以上で熱間圧延を終了した後、
680℃以下室温以上の温度域で巻き取り;
酸洗し;
冷間にて20%以上90%以下の圧延を行い;
650℃以上900℃以下の温度域まで昇温し;
1秒以上300秒以下の保持時間で焼鈍し;
0.1℃/s以上100℃/s以下の冷却速度で720℃以下580℃以上の温度域まで冷却をし;
亜鉛めっき処理を行う;
ことを特徴とする亜鉛めっき鋼板の製造方法。 - T1+30℃以上T1+200℃以下の温度範囲での前記第2の熱間圧延において、1パスで30%以上の圧下率の圧下を少なくとも1回以上行うことを特徴とする請求項21に記載の亜鉛めっき鋼板の製造方法。
- 1000℃以上1200℃以下の温度域での前記第1の熱間圧延において、20%以上の圧下率の圧下を少なくとも2回以上行い、オーステナイト粒径を100μm以下とすることを特徴とする請求項21または22に記載の亜鉛めっき鋼板の製造方法。
- T1+30℃以上T1+200℃以下の温度域における30%以上の圧下率であるパスを大圧下パスとした場合、前記大圧下パスのうちの最終パスが完了した後から冷却を開始するまでの待ち時間tが、下式6を満たすことを特徴とする請求項21または22に記載の亜鉛めっき鋼板の製造方法。
t1≦t≦t1×2.5・・・(式6)
ここで、t1は下式7で表される。
t1=0.001×((Tf-T1)×P1)2-0.109×((Tf-T1)×P1)+3.1・・・(式7)
ここで、Tfは前記最終パス後の温度、P1は前記最終パスにおける圧下率である。 - T1+30℃以上T1+200℃以下の温度域における前記第2の熱間圧延の各パス間の鋼板の温度上昇を18℃以下とすることを特徴とする請求項24に記載の亜鉛めっき鋼板の製造方法。
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TWI439554B (zh) | 2014-06-01 |
EP2599887A4 (en) | 2017-10-11 |
US9587319B2 (en) | 2017-03-07 |
CN103038383A (zh) | 2013-04-10 |
BR112013001864A2 (pt) | 2016-05-31 |
US20160130711A1 (en) | 2016-05-12 |
US9273370B2 (en) | 2016-03-01 |
MX342629B (es) | 2016-10-07 |
JP5163835B2 (ja) | 2013-03-13 |
EP2599887B1 (en) | 2021-12-01 |
EP2599887A1 (en) | 2013-06-05 |
KR101514157B1 (ko) | 2015-04-21 |
CN103038383B (zh) | 2014-12-24 |
US20130153091A1 (en) | 2013-06-20 |
KR20130021460A (ko) | 2013-03-05 |
MX2013000984A (es) | 2013-03-07 |
CA2806626C (en) | 2016-04-05 |
TW201213558A (en) | 2012-04-01 |
JPWO2012014926A1 (ja) | 2013-09-12 |
CA2806626A1 (en) | 2012-02-02 |
BR112013001864B1 (pt) | 2019-07-02 |
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