COLD ROLLED ANNEALED STEEL SHEET WITH HIGH HOLE EXPANSION RATIO AND MANUFACTURING PROCESS THEREOF

摘要

A cold rolled annealed steel sheet having a chemical composition including, in weight %: 0.30%≤C≤0.50%, 1.00%≤Mn≤2.50%, 1.00%≤Si≤2.00%, Al≤2.00%, Cr≤0.100%, 0.100%≤Mo≤0. 500%, 0.020%≤Nb≤0.200%, B≤0.0005%, P≤0.02%, S≤0.005%, N≤0.01%, the remainder being Fe and unavoidable impurities, with the percentages in carbon, manganese, chromium, molybdenum and boron are such that the alloy satisfies the following condition:; 250% C+120% Mn−200% Cr+200% Mo−10000% B≥320,;and wherein the microstructure comprises in surface fraction, 35% to 45% of islands of martensite and retained austenite (M-A), the total retained austenite is higher than or equal to 24%, the remainder consisting of bainitic ferrite.