Coated seizure-hardening type cold-rolled steel sheet having excellent aging resistance and method of production thereof

Abstract

Disclosed are bake-hardenable sheet steel with good anti-aging property and a method for producing it. While controlling its C, P, S and N contents, the chemical composition of the sheet steel is defined to comprise not larger than 1.0% of Si, not larger than 3.0% of Mn, from 0.01 to 0.20% of Al and from 0.001 to 0.2% of Ti, in terms of % by weight. The value A (=(AI,−AI)/AI,) of the sheet steel is defined to be not smaller than 0.4 and the value AI,thereof to be not smaller than 30; or the ratio of the mean misorientation, M (degree), to the mean grain size, G (&mgr;m), M/G, of the sheet steel is defined to be not smaller than 0.8. The steel may additionally contain from 0.001 to 0.2% of Nb and/or from 0.0001 to 0.0080% of B. While controlling its Si, Mn, S, Al and N contents, the chemical composition of a steel slab is defined to comprise from 0.005 to 0.02% of C, not larger than 0.05% of P and from 0.025 to 0.19% of Nb, with satisfying the condition of 0.7×(C/12)≦Nb/93≦1.2×(C/12) (where C indicates the C content (wt. %), and Nb indicates the Nb content (wt. %)). To produce bake-hardenable sheet steel from it, the slab is heated, hot-rolled at a finishing delivery temperature of from 960 to 650° C., coiled at a temperature of from 750 to 400° C., then cold-rolled to a reduction of from 50 to 95%, and thereafter annealed for recrystallization at a temperature of from 750 to 920° C. The slab may additionally contain B and/or Ti. The invention stably produces bake-hardenable sheet steel on an industrial scale.