Effect of isothermal bainite treatment on microstructure and mechanical properties of 0.22C-1.63Mn-1.5Al cold-rolled TRIP steel

Microstructure and mechanical properties of the 1.5Al%-alloyed cold rolled TRIP steel annealed at three isothermal bainite temperature(IBT,370～450℃) were researched by means of SEM,X-ray,tensile test and prediction calculation on fraction of phases.The evolution of austenite during IBT and the effect of microstructure on mechanical properties were investigated.The results show that the substitution of Si by Al to produce a high performance TRIP steel is feasible by optimizing the process of heat treatment.Addition of aluminium in the alloys inhibits cementite formation during bainite transformation region.So a significant amount austenite is retained in steel at room temperature.The stability of retained austenite obtained by isothermal bainite treatment at 400℃ is higher than that at other temperatures,and instantaneous strain hardening exponent (n_(incr)) maintains to a higher value during further straining,which results in a larger elongation.The tensile strength of the steel treated at 400℃ for 180s is 756MPa,elongation is 34.2%.Some martensite is probably formed after treating at 450℃ for 180s,which is detrimental to properties.