HIGH STRENGTH COLD ROLLED STEEL SHEET HAVING SUPERIOR LOCAL DUCTILITY AND STABLE BAKING HARDENABILITY OF COATING, PRODUCED BY CONTINUOUS ANNEALING

摘要

PURPOSE: To provide superior local ductility and strain ageing characteristic and stable baking hardenability of coating by continuously annealing an Al killed steel sheet of specific composition and specifying solid-solution carbon content and transgranular cementite density, respectively. ;CONSTITUTION: This steel has a chemical composition consisting of, by weight, 0.01-0.04% C, 0.005-0.4% Si, 0.05-0.30% Mn, 0.08-0.13% P, 0.003-0.02% S, 0.015-0.15% Al, ≤0.0040% N, and the balance Fe with inevitable impurities. This low carbon Al killed steel is hot-rolled, cold-rolled, continuously annealed, and temper-rolled. By this treatment, solid-solution carbon content is regulated to 3-12ppm. Transgranular cementite density (ρ, piece/mm²) satisfies an inequality ρ≤(0.034×Al+3.0)×10⁵ in the relation with aging index (Al, MPa). By this method, the cold rolled steel sheet in which LDH₀ as (100°C×1h) local ductility index after artificial aging treatment is regulated to 22mm and which has 30-80MPa baking hardenability of coating and 380-450MPa class strength level, can be obtained.;COPYRIGHT: (C)1995,JPO