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AssigneeKawasaki Steel Corporation
InventorsToshio Irie, Susumu Sato, Osamu Hashimoto
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The present invention provides 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-{C%.times.8+0.020%} of Nb and the remainder being substantially Fe, as the high tensile strength steel sheets which reduce the weight of automobiles and the like. The steel sheet is produced by hot rolling a steel slab having the above described composition to obtain a hot rolled coil, in the hot rolling the total reduction rate being at least 90%, the rolling speed in the finishing rolling being at least 40 m/min and the coiling-up temperature being at least 600.degree. C.; cold rolling the above described hot rolled coil in a conventional process to obtain a cold rolled steel strip having a final gauge; subjecting the above described cold rolled steel strip to a continuous annealing at a temperature of 700.degree.-900.degree. C. for 10 sec-5 min and then cooling the annealed strip to 500.degree. C. at a rate of at least 60.degree. C./min. The steel sheets having the above described composition, except that Si is not more than 0.5%, are suitable for the continuous hot-dip galvanizing.
A method for producing high tensile strength cold rolled steel sheets having excellent formability, a tensile strength of more than 35 kg/mm.sup.2 and a r value larger than 2, and which comprises hot rolling a steel slab consisting 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-{C%.times.8+0.020%} of Nb and the remainder being substantially Fe and unavoidable impurities, to obtain a hot rolled coil, in said hot rolling the total reduction rate being at least 90%, the rolling speed in the finishing rolling being at least 40 m/min and the coiling-up temperature being at least 600.degree. C., cold rolling the above-described hot rolled coil in a conventional process to obtain a cold rolled steel strip having a final gauge, subjecting the cold rolled steel strip to a continuous annealing at a temperature of 700.degree.-900.degree. C. for 10 sec-5 min and then cooling the annealed strip to 500.degree. C. to a rate of at least 60.degree. C./min.
A method for producing non-ageing high tensile strength hot-dip galvanized cold rolled steel sheets having excellent press formability, a tensile strength of more than 35 Kg/mm.sup.2 and a r value larger than 2, and which comprises hot rolling a steel slab consisting of 0.002-0.015% of C, not more than 0.50% of Si, 0.04-1.2% of Mn, 0.03-0.10% of P, 0.02-0.10% and not less than N%.times.4 of Al, 0.01-0.08% and C%.times.3-{C%.times.8+0.020%} of Nb and the remainder being substantially Fe and unavoidable impurities, to obtain a hot rolled coil, in said hot rolling the total reduction rate being at least 90%, the rolling speed in the finishing rolling being at least 40 m/min and the coiling-up temperature being at least 600.degree. C., cold rolling the above-described hot rolled coil in a conventional process to obtain a cold rolled steel strip having a final gauge, subjecting the cold rolled steel strip to a soaking at a temperature of 700.degree.-900.degree. C. for 10 sec-5 min, then cooling the soaked strip at a rate of at least 60.degree. C./min and continuously hot-dip galvanizing the cooled strip.
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