High strength and high formability steel sheet and manufacturing method

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

• 0.10%≤C≤0.25%
• 3.5%≤Mn≤6.0%
• 0.5%≤Si≤2.0%
• 0.3%≤Al≤1.2%
• with Si+Al≥0.8%
• 0.10%≤Mo≤0.50%
• S≤0.010%
• P≤0.020%
• N≤0.008%
• and optionally at least one of the following elements selected from a group consisting of Cr, Ti, Nb, V and B, such that by weight percent:
• 0.01%≤Cr≤1.0%
• 0.010%≤Ti≤0.080%
• 0.010%≤Nb≤0.080%
• 0.010%≤V≤0.30%
• 0.0005%≤B≤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 45% of ferrite, having an average grain size of at most 1.3 μm, a product of the surface fraction of ferrite by the average grain size of the ferrite being of at most 35 μm %,
• between 8% and 30% of retained austenite, said retained austenite having an Mn content higher than 1.1*Mn %, Mn % designating the Mn content of the steel,
• at most 8% of fresh martensite,
• at most 2.5% of cementite, and
• partitioned martensite.