HIGH-TENSILE, COLD-ROLLED STEEL PLATE WITH EXCELLENT FORMABILITY AND PROCESS FOR ITS PRODUCTION, AS WELL AS HIGH-TENSILE, GALVANIZED STEEL PLATE WITH EXCELLENT FORMABILITY, AND PROCESS FOR ITS PRODUCTION

摘要

PCT No. PCT/JP81/00068 Sec. 371 Date Nov. 27, 1981 Sec. 102(e) Date Nov. 27, 1981 PCT Filed Mar. 30, 1981 PCT Pub. No. WO81/02900 PCT Pub. Date Oct. 15, 1981.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, 0.03-10% of P, 0.02-0.10% and not less than N%x4 of Al, C%x3-{C%x8+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 DEG 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 DEG -900 DEG C. for 10 sec-5 min and then cooling the annealed strip to 500 DEG C. at a rate of at least 60 DEG 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.