Cold rolled and heat-treated steel sheet and method of manufacturing the same

A cold-rolled and heat-treated steel sheet, made of a steel having a composition comprising, by weight percent:

• C:
• 0.3-0.4%
• Mn:
• 2.0-2.6%
• Si:
• 0.8-1.6%
• Al:
• 0.01-0.6%
• Mo:
• 0.15-0.5%
• Cr:
• 0.3-1.0%
• Nb≤0.06%
• Ti≤0.06%
• Ni≤0.8%
• S≤0.010%
• P≤0.020%
• N≤0.008%
• Cu≤0.03%
• and optionally one or more of the following elements, in weight percentage:
• B:
• 0.0003-0.005%
• V≤0.2%
• a remainder of the composition being iron and unavoidable impurities resulting from processing,
• the steel sheet having a microstructure consisting of, in surface fraction:
• between 15% and 30% of retained austenite, said retained austenite having a carbon content of at least 0.7%;
• between 70% and 85% of tempered martensite;
• at most 5% of fresh martensite;
• and
• at most 5% of bainite.

A method for manufacturing a cold-rolled and heat-treated steel sheet, comprising the following successive steps:

• casting a steel to obtain a slab, said steel having a composition comprising, by weight percent:
• C:
• 0.3-0.4%
• Mn:
• 2.0-2.6%
• Si:
• 0.8-1.6%
• Al :
• 0.01-0.6%
• Mo:
• 0.15-0.5%
• Cr:
• 0.3-1.0%
• Nb≤0.06%
• Ti≤0.06%
• Ni≤0.8%
• S≤0.010%
• P≤0.020%
• N≤0.008%
• Cu≤0.03%
• and optionally one or more of the following elements, in weight percentage:
• B:
• 0.0003-0.005%
• V≤0.2%
• a remainder of the composition being iron and unavoidable impurities resulting from processing;
• reheating the slab at a temperature Treheat comprised between 1150° C. and 1300° C.,
• hot rolling the reheated slab at a temperature higher than Ar3 to obtain a hot rolled steel sheet;
• coiling the hot rolled steel sheet at a coiling temperature Tcoil between 200° C. and 700° C.;
• optionally pickling said hot rolled steel sheet;
• optionally annealing the hot rolled steel sheet, to obtain a hot-rolled and annealed steel sheet;
• optionally pickling said hot-rolled and annealed steel sheet;
• cold rolling the hot-rolled and annealed steel sheet to obtain a cold rolled steel sheet;
• reheating the cold-rolled steel sheet to an annealing temperature between Ac3 and Ac3+100° C. and maintaining the cold-rolled steel sheet at the annealing temperature for a holding time between 30 s and 600 s, to obtain, upon annealing, a fully austenitic structure;
• quenching the cold-rolled steel sheet at a cooling rate comprised between 0.1° C./s and 200° C./s, to a quenching temperature Tq between (Ms-140° C.) and (Ms-75° C.) and optionally maintaining it at Tq for a holding time between 1 and 200 s;
• reheating the cold-rolled steel sheet to a partitioning temperature between 350° C. and 500° C., and maintaining the cold-rolled steel sheet at the partitioning temperature for a partitioning time between 30 s and 2000 s;
• and
• cooling the cold-rolled steel and heat-treated sheet to room temperature.