Cold rolled annealed steel sheet with high hole expansion ratio and manufacturing process thereof

A cold rolled annealed steel sheet having a chemical composition comprising, 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%,
• a 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
• a microstructure including in surface fraction, 35% to 45% of islands of martensite and retained austenite (M-A), a total of the retained austenite being higher than or equal to 24%, and a remainder consisting of bainitic ferrite.

A method for producing a steel sheet comprising the following steps:

• providing a steel semi-product with composition comprising, 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%,
• a 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;
• heating the the steel semi-product to temperature between 1150° C. and 1300° C., so to obtain a reheated steel semi-product;
• hot rolling the reheated steel semi-product with a final rolling temperature higher than or equal to 800° C., so to obtain a hot rolled steel sheet;
• coiling the hot rolled steel sheet at a temperature Tcoil between 400° C. and 590° C., so to obtain a coiled steel sheet;
• optionally, heat treating said coiled steel sheet;
• cold rolling the coiled steel sheet with a reduction rate between 30% and 80%, so to obtain a cold rolled steel sheet;
• heating the cold rolled steel sheet at a heating rate VH between 2° C./s and 50° C./s to a soaking temperature Tsoak higher than Ac3+20° C. and lower than 1000° C., for a duration tsoak higher than 60 s, so to obtain an annealed steel sheet;
• cooling the annealed steel sheet with a cooling rate VC between 20° C./s and 1000° C./s to an overaging temperature TOA higher than 385° C. and lower than 450° C.;
• and
• holding the cooled annealed steel sheet at overaging temperature TOA for a duration tOA higher than or equal to 270 s.