High strength cold rolled and galvannealed steel sheet and manufacturing process thereof

A cold rolled and galvannealed steel sheet having a chemical composition comprising, in weight %:

• C:
• 0.15-0.25%
• Mn:
• 2.4-3.5%
• Si:
• 0.30-0.90%
• Cr:
• 0.30-0.70%
• Mo:
• 0.05-0.35%
• Al:
• 0.001-0.09%
• Ti:
• 0.01-0.06%
• B:
• 0.0010-0.0040%
• Nb 0.01-0.05%
• P≤0.020%
• S≤0.010% and
• N≤0.008%;
• a remainder of the composition being iron and unavoidable impurities resulting from processing, the steel sheet having a microstructure consisting of, in surface fraction:
• martensite from 80% to 90%, ferrite and bainite.

A process for manufacturing a cold rolled and galvannealed steel sheet comprising the following and successive steps:

• casting a steel to obtain a semi-product, the semi product having a composition comprising, in weight %:
• C:
• 0.15-0.25%
• Mn:
• 2.4-3.5%
• Si:
• 0.30-0.90%
• Cr:
• 0.30-0.70%
• Mo:
• 0.05-0.35%
• Al:
• 0.001-0.09%
• Ti:
• 0.01-0.06%
• B:
• 0.0010-0.0040%
• Nb 0.01-0.05%
• P≤0.020%
• S≤0.010% and
• N≤0.008%;
• a remainder of the composition being iron and unavoidable impurities resulting from processing;
• reheating the slab to a temperature Treheat comprised from 1150° C. to 1300° C.;
• hot rolling the reheated slab with a final rolling temperature comprised from 850° C. to 950° C., so to obtain a hot rolled steel sheet;
• then
• cooling said steel sheet to a coiling temperature Tcoil comprised from 250° C. to 650° C.;
• then
• coiling the steel sheet at said temperature Tcoil so to obtain a coiled steel sheet;
• then
• pickling the steel sheet;
• annealing the steel sheet to an annealing temperature TA comprised from 500° C. to 650° C. and maintaining the steel sheet at said temperature TA for a holding time tA;
• optionally further pickling the steel sheet;
• cold rolling the hot-rolled steel sheet with a reduction rate between 20% and 80%, to obtain a cold rolled steel sheet;
• heating the cold rolled steel sheet to a soaking temperature Tsoak comprised from Ac1 to Ac3 and maintaining the steel sheet at said temperature Tsoak for a holding time tsoak comprised from 30 s to 200 s, in order to obtain between 85% and 95% of austenite and between 5% and 15% of ferrite;
• cooling the steel sheet to a temperature comprised from 440° C. to 480° C.;
• coating the steel sheet by continuous dipping in a zinc bath at a temperature TZn comprised from 450° C. to 480° C.;
• reheating the steel sheet to a galvannealed temperature TGA comprised from 510° C. and 550° C., and maintaining the steel sheet at the temperature TGA for a holding time tGA comprised from 10 s and 30 s;
• and
• cooling the reheated steel sheet to room temperature to obtain a cold rolled and galvannealed steel sheet.