Cold-rolled and heat-treated steel plates and their manufacturing methods

Abstract

This invention relates to cold-rolled and heat-treated steel sheets, said steel comprising, by weight percentage: 0.17% ≤ carbon ≤ 0.25%, 2% ≤ manganese ≤ 3%, 0.9% ≤ silicon ≤ 2%, 0% ≤ aluminum ≤ 0.09%, 0.01% ≤ molybdenum ≤ 0.2%, 0% ≤ phosphorus ≤ 0.02%, 0% ≤ sulfur ≤ 0.03%, 0% ≤ nitrogen ≤ 0.09%, and optionally one or more of the following elements: 0% ≤ chromium ≤ 0.3%, 0% ≤ niobium ≤ 0.06%, 0% ≤ titanium ≤ 0.06%, 0% ≤ vanadium ≤ 0.1%, 0% ≤ calcium ≤ 0.005%, 0% ≤ boron ≤ 0. The steel sheet contains 0.10%, 0% ≤ magnesium ≤ 0.05%, 0% ≤ zirconium ≤ 0.05%, 0% ≤ cerium ≤ 0.1%, and the balance includes iron and unavoidable impurities. The microstructure comprises the following: 50% to 80% bainite, 10% to 30% retained austenite, 15% to 50% distributed martensite, 0% to 10% ferrite, and 0% to 5% fresh martensite by area fraction, and a ferrite-rich layer extending up to 50 micrometers from both surfaces of the steel sheet, the average ferrite content of such ferrite-rich layer being 55% to 80% by area fraction.

Images