Metal treatment – Compositions – Heat treating
Patent
1981-11-27
1984-09-25
Skiff, Peter K.
Metal treatment
Compositions
Heat treating
148 12C, C21D 804
Patent
active
044734140
DESCRIPTION:
BRIEF SUMMARY
FIELD OF INVENTION
The present invention relates to high tensile strength cold rolled steel sheets having excellent formability and a method for producing said steel sheets, and particularly to non-ageing high tensile strength cold rolled steel sheets having excellent formability and a tensile strength of about 35-45 kg/mm.sup.2 and a method for producing said steel sheets. Furthermore, the present invention relates to non-ageing high tensile strength hot-dip galvanized steel sheets having excellent formability and a method for producing said steel sheets.
BACKGROUND OF INVENTION
Demand of high tensile strength steel sheets has been increased in order to decrease the body weight for improving the fuel cost of automobiles. These steel sheets must satisfy the following various requirements.
As the steels satisfying these requirements, dual phase steel composed of martensite-ferrite two phase alloy structure and rephos-steel in which P, Mn, Si and the like are added to aluminium killed steel have been developed but these steels are poor in the formability and therefore cannot be used in the portions where the deep drawing is carried out, such as automobile fenders. Furthermore, these steels contain a number of elements, such as Si, Cr, Mn and the like which retard the plating ability, so that the hot-dip galvanization cannot be easily effected.
Furthermore, it has been proposed that Ti or Nb having a high affinity to C and N is incorporated in an amount of more than stoichiometric equivalent based on an amount of C or C+N to fix C and N and Mn or Si is incorporated as a solid solution reinforcing element to obtain a steel sheet having high r value and tensile strength. For example, it has been reported in "Iron and Steel", 1979, No. 11, p. 838 that an extremely low carbon steel having less than 0.01% of C which contains 0.25% of Ti, 1.5% of Mn and 0.22% of Si is continuously annealed to obtain a steel having high formability which has a tensile strength (abbreviated as TS hereinafter) of 43 kg/mm.sup.2, an yield point (abbreviated as YP hereinafter) of 22-25 kg/mm.sup.2, r value of 1.8 and an elongation of 39%. However, this steel has a demerit that Ti combines with not only C and N in the steel but also S and O, so that a large amount of inclusions are formed and the surface defects are liable to be formed. Moreover, amounts of Mn, Si and Ti added are large and it is necessary to add a large amount of alloy to effect decarburization refining to decrease c in the molten steel to less than 0.01%, so that this steel has a demerit that the cost of alloy and decarburization is higher.
Furthermore, it has been proposed in Japanese Patent Laid-Open Application No. 100,920/79 that high tensile strength steels having TS of 42-46 kg/mm.sup.2, YP of 28-30 kg/mm.sup.2, r value of 1.6-1.8 and an elongation of 32-35% are obtained by subjecting a steel containing 0.004% of C, 1.01% of Si, 0.22% of Mn, 0.025% of Al and 0.049% of Nb to a continuous annealing and then to an over ageing treatment at 400.degree. C. for 3 minutes, but r value and the elongation are low, so that the formability is not satisfactory.
SUMMARY OF INVENTION
An object of the present invention is to provide high tensile strength cold rolled steel sheets having excellent formability in which the above described various defects in the prior high tensile strength steel sheets are obviated and a method for producing said steel sheets. A further object of the present invention is to provide hot-dip galvanized high tensile strength cold rolled steel sheets having excellent formability and a method for producing said steel sheets.
The present invention consists in high tensile strength cold rolled steel sheets having excellent formability, which consist of 0.002-0.015% of C, not more than 1.2% of Si, 0.04-0.8% of Mn, 0.03-0.10% of P, 0.02-0.10% and not less than N%.times.4 of Al, C%.times.3 to {C%.times.8+0.020%} of Nb and the remainder being substantially Fe, and in a method for producing the high tensile strength cold rolled steel sheets having excell
REFERENCES:
patent: 3368886 (1968-02-01), Muta
patent: 3988173 (1976-10-01), Kawano
patent: 4339284 (1982-07-01), Hashimoto et al.
patent: 4368084 (1983-01-01), Irie et al.
Hashimoto Osamu
Irie Toshio
Sato Susumu
Kawasaki Steel Corporation
Skiff Peter K.
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