Combining a novel cyclic pre-quenching and two-stage heat treatment in a low-alloyed TRIP-aided steel to significantly enhance mechanical properties through microstructural refinement

A novel heat treatment involving combining of cyclic pre-quenching prior to two-stage heat treatment (inter critical annealing (IA) and isothermal bainite transformation (IBT)) is proposed to refine the microstructure and significantly enhance the mechanical properties of a Fe-0.21C-1.40Si-1.47Mn-0.025Nb cold rolled transformation induced plasticity (TRIP) aided steel. The effect of cyclic numbers of pre-quenching heat treatment on the microstructure and mechanical properties of the experimental steel was studied. With the increase of pre-quenching heat treatment cycles from 0 to 3, the average ferrite grain size first decreased from 2.4 mu m to 1.3 mu m and then increased to 1.7 mu m. The volume fraction of retained austenite (RA) and average RA grain size decreased from 15.8% to 11.5% and 0.42 mu m-0.23 mu m, respectively. The cyclic pre-quenching process led to smaller RA grain size and the increase of lath-like and granular RA increased RA stability. The number of pre-quenching cycles had little effect on ultimate tensile strength (UTS), but had a significant influence on total elongation (TEL). The rate of RA transformation played an important role in governing the mechanical properties of the experimental steel during different stages of plastic deformation. Combined 2 cyclic pre-quenching and two-stage heat treatment, the experimental sample exhibited an excellent combination of mechanical properties with the product of UTS and TEL (PSE) of 30.7 GPa% which was larger than that of without pre-quenching treated sample (23.5 GPa%) because of grain refinement, microstructural homogenization and sustained TRIP effect.