Metal treatment – Process of modifying or maintaining internal physical... – Utilizing disclosed mathematical formula or relationship
Reexamination Certificate
2000-04-10
2001-08-14
Yee, Deborah (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Utilizing disclosed mathematical formula or relationship
C148S541000, C148S547000, C148S652000, C148S661000
Reexamination Certificate
active
06273971
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method of manufacturing a cold rolled steel-sheet adapted for use in the preparation of vehicle outer panels such as doors, hoods, and fenders, particularly, to a method of manufacturing a cold rolled steel sheet excellent in its resistance to natural aging to suppress the yield elongation in tension, in its panel formability, in its panel shapeability, and in its dent resistance, i.e., resistance to a local concavity.
BACKGROUND ART
The steel used for preparation of vehicle outer panels such as doors, hoods, and fenders is required on one hand to be soft and exhibits an excellent formability before formation of the outer panel and is also required on the other hand to exhibit a high mechanical strength after the panel forming and painting so as to improve the dent resistance.
In recent years, a bake-hardenable (BH) cold rolled steel sheet has been developed. as a steel sheet meeting these requirements and is widely used nowadays. Particularly, vigorous studies are being made on a method of manufacturing a BH steel sheet having an excellent deep-drawability by annealing a cold rolled steel sheet prepared by adding carbonitriding elements such as Nb and Ti at an atomic ratio of 1 or less in terms of the carbon equivalent to an extra low carbon steel having the carbon content lowered to about 50 ppm.
For example, Japanese Patent Publication (Kokoku) No. 60-46166 discloses a technique of annealing an extra low carbon steel having Nb or Ti added thereto at a high temperature, i.e., about 900° C. The technique disclosed in this prior art is desirable in that, since the steel is annealed at a high temperature, it is possible to improve the bake-hardenability (BH), i.e., the degree of hardening of the steel that can be achieved when a strained steel is baked, and to improve an r-value providing an index of deep-drawability. However, this technique gives rise to a difficulty that the ferrite grains may be coarsened so as to deteriorate the surface state of the steel sheet. Also, since the steel sheet itself is softened, the mechanical strength of the steel sheet after the forming and painting is not satisfactory, even if high BH property is imparted to the steel sheet.
Japanese Patent Disclosure (Kokai) No. 61-276928 discloses a technique of manufacturing a BH steel sheet by annealing an extra low carbon steel having Nb added thereto at a temperature range of about 700 to 850° C. The annealing temperature disclosed in this prior art is relatively low, compared with that disclosed in JP '166 noted above. The low annealing temperature is desirable in respect of the surface and the yield strength of the steel sheet, but is not satisfactory for improving the r-value and BH property.
Further, although each of the prior arts exemplified above is intended to improve the BH property of a steel sheet, the improvement in the BH property promotes the natural aging. It follows that a yield point elongation is likely to be generated during the storage at room temperature, with the result that a stretcher strain may be generated in the panel forming. In order to prevent these difficulties, an improvement in the BH property is limited.
DISCLOSURE OF INVENTION
An object of the present invention is to provide a method of manufacturing a cold rolled steel sheet excellent in panel formability and dent resistance required for a steel sheet used as a vehicle outer panel while maintaining a resistance to natural aging.
Another object of the present invention is to provide a method of manufacturing a hot-dip zinc coated steel sheet excellent in panel formability and dent resistance required for a steel sheet used as a vehicle outer panel while maintaining a resistance to natural aging.
These objects can be achieved as follows:
1. A method of manufacturing a cold rolled steel sheet excellent in its resistance to natural aging and in panel properties, comprising steps of:
preparing steel consisting of 0.005 to 0.012% by weight of C, 0.01 to 0.4% by weight of Si, 0.15 to 1.0% by weight of Mn, 0.01 to 0.08% by weight of P, at most 0.02% by weight of S, 0.01 to 0.1% by weight of sol. Al, at most 0.004% by weight of N, and at least one element selected from the group consisting of 0.01 to 0.2% by weight of Nb and 0.04 to 0.1% by weight of Ti, X in formula (1) defined by C, Nb, and Ti contents falling within a range of between 1.2 and 2.5, i.e., 1.2≦X≦2.5, and the balance of essentially Fe and unavoidable impurities:
X=(12/93) (Nb%/C%)+(12/48) (Ti*%/C%) (1)
where Ti*%=Ti%−(48/14)N%−(48/32)S%
when Ti* in the above equation is not larger than 0, Ti* is regarded as 0;
melting the steel;
applying a hot rolling and a cold rolling to the molten steel;
soaking the cold rolled steel sheet at T(° C.) meeting a formula Ac3≧T(° C.)≧157log(X)+737; and
cooling the steel sheet after the soaking step at a cooling rate meeting a formula R(° C./sec)≦−35+162/X.
2. The method defined above, wherein the steel further contains 0.0002 to 0.002% by weight of B.
3. The method defined above, wherein the cold rolled steel has a yield point of 212 to 233 MPa, a 2% BH, i.e., degree of hardening achieved by baking after impartation of 2% strain, of 0 to 11 MPa, a residual depression &dgr;30 after application and, then, removal of 30 kgf of load of 0.18 to 0.25 mm, a spring back amount &rgr; of 1 to 4%, and a restoration amount of yield point elongation &Dgr;YPel after storage for 6 months at 25° C. of 0%.
4. The method defined above, wherein the cold rolled steel has a yield point of 220 to 231 MPa, a 2% BH, i.e., degree of hardening achieved by baking after impartation of 2% strain, of 5 to 14 MPa, a residual depression &dgr;30 after application and, then, removal of 30 kgf of load of 0.2 to 0.23 mm, a spring back amount &rgr; of 4%, and a restoration amount of yield point elongation &Dgr;YPel after storage for 6 months at 25° C. of 0%.
5. The method defined above, wherein the cold rolled steel has a yield point of 220 to 233 MPa, a 2% BH, i.e., degree of hardening achieved by baking after impartation of 2% strain, of 0 to 3 MPa, a residual depression &dgr;30 after application and, then, removal of 30 kgf of load of 0.21 to 0.25 mm, a spring back amount &rgr; of 2 to 4%, and a restoration amount of yield point elongation &Dgr;YPel after storage for 6 months at 25° C. of 0%.
6. The method of manufacturing a hot-dip zinc coated steel sheet being excellent in resistance to natural aging and in panel properties, further comprising the step of galvanizing the cold rolled steel sheet.
According to the present invention, it is possible to manufacture stably a cold rolled steel sheet and a molten zinc-coated steel sheet excellent in resistance to natural aging, in dent resistance after the panel forming, and in shapeability required for a steel sheet used as a vehicle outer panel by controlling appropriately the steel chemical composition, and annealing conditions such as the annealing temperature and cooling rate. Therefore, the present invention is highly valuable in the steel industry and the vehicle industry.
REFERENCES:
patent: 4589931 (1986-05-01), Yasuda
patent: 510718 (1992-10-01), None
patent: 0 691 415 (1996-01-01), None
patent: 0 816 524 (1998-01-01), None
Japanese Patent Application Kokoku Publication No. 60-46166 published Oct. 15, 1985.
Japanese Patent Application Kokai Publication No. 61-276928 published Dec. 6, 1986.
Patent Abstracts of Japan, vol. 011, No. 139, May 7, 1987 & JP 61 276928, Dec. 6, 1986.
Patent Abstracts of Japan, vol. 098, No. 006, Apr. 30, 1998 & JP 10 036916, Feb. 10, 1998.
Patent Abstracts of Japan, vol. 095, No. 005, Jun. 30, 1995 & JP 07 034138, Feb. 3, 1995.
Patent Abstracts of Japan, vol. 013, No. 042, Jan. 30, 1989 & JP 63 241122, Oct. 6, 1998.
Fujita Takeshi
Hiasa Michihiro
Inoue Tadashi
Ishii Takeo
Kitano Fusato
Nixon & Vanderhye
NKK Corporation
Yee Deborah
LandOfFree
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