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Intercritical annealing (IA) at various temperatures followed by quenching and partitioning (IAQP) treatments was conducted on a cold-rolled Fe-0.2C-1.42Si-1.87Mn (wt%) sheet steel. Optimized microstructure and enhanced mechanical properties were achieved through appropriate adjustment of IA temperatures. The steel which was annealed at 1,033 K for 600 s, then quenched to 573 K and partitioned at 693 K for 20 min, designated as 1033QP steel, exhibits maximum 16.3 vol% retained austenite (RA) with good mechanical properties (ultimate tensile strength 886 MPa and total elongation 27%). It was found that the thermal and mechanical stabilities of RA are mainly influenced by the combined effect of its average carbon content and amount of adjacent martensite. Furthermore, the transformation-induced plasticity effect increased the peak n-values observed at the second stage of the work hardening of IAQP steels.

In order to avoid the appearance of soft particles composed of ferrite or pearlite in the actual production of new bainitic steel, the phase transformation behaviour and bainite formation kinetics were investigated by DIL805A dilatometer, optical microscopy, scanning electron microscopy and Vickers-durometer. The results show that the soft particles cannot appear when the cooling rate exceeds 0.025 K s −1 , and this condition can be ensured by direct spray cooling in production. The local activation energy decreases with increasing transformed bainite volume fraction ( f b ), and the average energy is about 136.7 kJ mol −1 . The local Avrami exponent mainly lies between 0.5 and 3 in a wide f b range, indicating that the dominating mechanism of bainite formation is two-dimensional and one-dimensional growth.