COATED SEIZURE-HARDENING TYPE COLD-ROLLED STEEL SHEET HAVINGEXCELLENT AGING RESISTANCE AND METHOD OF PRODUCTION THEREOF

摘要

A cold-rolled steel sheet having a composition controlled in the contents of C, P, S and N, and containing 1.0 % or less of Si, 3.0 % or less of Mn, 0.010.20 % of Al and 0.001-0.2 % of Ti, wherein the ratio (A) of (AIQUENCH-AI) to AIQUENCH is 0.4 or more and the AIQUENCH value is 30 or more, or the ratio (M/G) of the mean intergranular angle M (degree) to the mean crystal grain diameter G (.mu.m) is 0.8 or more. In addition, 0.001 to 0.2 wt.% of Nb or 0.0001 to 0.0080 wt.% of B may be contained, or 0.001 to 0.2 wt.% of Nb and 0.0001 to 0.0080 wt.% of B may be contained in combination. Alternatively, a steel blank having a composition controlled in the contents of Si, Mn, S, Al and N to have suitable values, containing 0.005 to 0.02 wt.% of C, 0.05 wt.% or less of P and 0.025 to 0.19 wt.% of Nb and satisfying the relation 0.7 x (C/12) Nb/93 1.2 x (C/12) (where C is the content of C (wt.%), and Nb is the content of Nb (wt.%)) is heated, finish-rolled at a temperature within the range of 960 to 650 ~C, coiled, cold rolled at a draft of 50 to 95 %, and thereafter recrystallization-annealed at an annealing temperature of 750 to 920 ~C. The steel material may further contain B and Ti. Thus it has become possible to obtain excellent coated seizure-hardening type cold-rolled sheet which has excellent aging resistance and can be manufactured industrially stably.