High strength steel and production method

Abstract

The present invention provides a cold-rolled and annealed steel sheet of thickness between 0.7 mm and 2 mm, mechanical strength ranging from 1180 MPa to 1320 MPa, wherein the hole expansion ratio Ac % is greater than 20% and the bending angle is greater than or equal to 40°. The chemical composition includes, the contents being expressed as weight percent: 0.09%≤C≤0.11%, 2.6%≤Mn≤2.8%, 0.20%≤Si≤0.55%, 0.25%≤Cr<0.5%, 0.025%≤Ti≤0.040%, 0.0015%≤B≤0.0025%, 0.005%≤Al≤0.18%, 0.08%≤Mo≤0.15%, 0.020%≤Nb≤0.040%, 0.002%≤N≤0.007%, 0.0005%≤S≤0.005%, 0.001%≤P≤0.020%, Ca≤0.003%, the remainder being iron and inevitable impurities resulting from processing. The sheet has a microstructure including martensite or lower bainite. A sum of the surface fractions of martensite and lower is between 40% and 70%, from 15% to 45% of the surface fraction is low-carbide bainite, from 5% to less than 20% of the surface fraction is ferrite, the non-recrystallized ferrite fraction relative to the total ferrite fraction is less than 15%, and less than 5% of the surface fraction of residual austenite is in the form of islands. The fraction of former austenitic grains in which the size is less than at least one micrometer represents 40% to 60% of the total population of the former austenitic grains.