HIGH TENSILE COLD-ROLLED STEEL SHEET AND PROCESS FOR PRODUCTION THEREOR

摘要

A high tensile cold-rolled steel sheet which has a composition containing by mass C: 0.001 to 0.05%, Si: 0.4% or below, Mn: 0.5 to 2.0%, P: 0.08% or below, S: 0.005% or below, Al: 0.05% or below, and N: 0.0080 to 0.0250% with at least 0.0050% of solid-soluted N at an N/Al ratio of 0.30 or above and an N/C ratio of 0.40 or above with the balance being Fe and unavoidable impurities and a composite structure composed of 95.0 to 99.5% (in volume fraction) of ferrite phase and 0.5 to 5.0% (in volume fraction) of a phase formed at low temperature and which exhibits a tensile strength of 340MPa or above. The steel sheet does not contain a large amount of expensive alloying elements and makes it possible to produce 340 to 440MPa grade high tensile cold-rolled steel sheets which have yield ratios of 55% or below and are excellent in press formability or hot-dip zinc-coated steel sheets easily at low costs even when the cooling rate after annealing varies.