New ultrahigh-strength Mn-alloyed TRIP steels with improved formability manufactured by intercritical annealing

We have developed novel ultrahigh-strength cold-rolled transformation-induced plasticity (TRIP) steel sheets manufactured by a simple intercritical annealing process, whose total and uniform elongation of are up to 24.8% and 9.3% respectively at the ultimate tensile strength of 1 GPa; and 20.0% and 9.0% at 1.2 GPa. They exhibit a better combination of the tensile properties than the present commercial cold-rolled steel sheets for automotive. Such an improvement is achieved by the new designs of chemistries and intercritical annealing (IA) process, i.e. alloying with the 5 wt% Mn content and the IA of martensite rather than the conventional cold rolled microstructures. Both lead to ultrafine austenite grains with enhanced stability formed during IA and retained in the matrix of ferrite and martensite afterwards. Finally, the improvement of elongation is attributed to austenite-to-martensite TRIP effect during the tensile deformation.