Cold rolled and annealed 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:
• 0.03-0.25%
• Mn:
• 3.5-8%
• Si:
• 0.5-2.0%
• Al:
• 0.03-2.0%
• Ti≤0.080%
• Nb≤0.080%
• V≤0.2%
• V+Ti+Nb>0.01%
• S≤0.010%
• P≤0.020%
• N≤0.008%
• and optionally including at least one of the following elements, in weight percentage:
• Mo:
• 0.1-0.5%
• Cr:
• 0.01-1%
• B:
• 0.0005-0.004%
• a remainder of the composition being iron and unavoidable impurities resulting from processing,
• the cold-rolled and heat-treated steel sheet having a microstructure consisting of, in surface fraction:
• between 10% and 30% of retained austenite, the retained austenite being present as austenite films having an aspect ratio of at least 3 and as Martensite Austenite islands, less than 8% of such Martensite Austenite islands having a size above 0.5 μm,
• at most 1% of fresh martensite,
• at most 50% of tempered martensite, and
• recovered martensite containing precipitates of at least one element chosen from the group consisting of:
• niobium, titanium and vanadium.

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

• casting a steel so as to obtain a slab, the steel having a composition comprising by weight percent:
• C:
• 0.03-0.25%
• Mn:
• 3.5-8%
• Si:
• 0.5-2.0%
• Al:
• 0.03-2.0%
• Ti≤0.080%
• Nb≤0.080%
• V≤0.2%
• V+Ti+Nb>0.01%
• S≤0.010%
• P≤0.020%
• N≤0.008%
• and optionally including at least one of the following elements, in weight percentage:
• Mo:
• 0.1-0.5%
• Cr:
• 0.01-1%
• B:
• 0.0005-0.004%
• a remainder of the composition being iron and unavoidable impurities resulting from processing;
• reheating the slab at a temperature Treheat 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 20° C. and 600° C.;
• annealing the hot rolled steel sheet, to obtain a hot-rolled and annealed steel sheet;
• cold rolling the hot-rolled and annealed steel sheet so as to obtain a cold rolled steel sheet;
• reheating the cold-rolled steel sheet to a first annealing temperature between Ae3 and Ae3+100° C. and maintaining the cold-rolled steel sheet at the first annealing temperature for a holding time between 30 s and 600 s, so as to obtain, upon annealing, a fully austenitic structure;
• quenching the cold-rolled steel sheet at a cooling rate between 0.5° C./s and 200° C./s, to a quenching temperature between 20° C. and Ms-50° C. and maintaining the cold-rolled steel sheet at the quenching temperature for a holding time between 1 and 200 s;
• reheating the cold-rolled steel sheet to a second annealing temperature comprised between TΘ and Ae3-30° C., TΘ designating the temperature above which the cementite is dissolved upon heating, and maintaining the cold-rolled steel sheet at the second annealing temperature for a time between 100 s and 2000 s;
• cooling the cold-rolled steel sheet to the room temperature;
• tempering the cold-rolled steel sheet, at a temperature between 170° C. and 500° C., for a holding time between 3 and 1200 s to obtain a cold-rolled and heat treated steel sheet.