Evolution of microstructure and its influence on tensile properties in thermo-mechanically controlled processed (TMCP) quench and partition (Q&P) steel

Controlled hot rolling is performed on low carbon Q&P steel and is subsequently followed by a direct Q&P (DQP) treatment and a separate Q&P treatment (SQP). Two different levels of strain viz. 0.51 and 1.1 are accomplished in the thermomechanical controlled process (TMCP) prior to the Q&P treatment. The microstructures of the steels processed through both the DQP and SQP methods with different thickness reductions, contain primarily small lath packets with fine martensite laths and thin films of inter-lath austenite. Comparatively higher martensite volume fractions and fine lath packets is noticed in the DQP method. Tendency of higher retained austenite fraction is observed in the SQP method. Presence of a high fractions of high angle grain boundaries (HAGB) in the martensite laths indicates a fully recrystallized prior austenite grains in both steels. In general, prior thermo-mechanical treatment improves the partition kinetics which is supported by the high value of calculated carbon content of the austenite, Cϒ, in the DQP steels. Combination of TMCP and Q&P process has resulted in remarkable increase in strength with adequate ductility compared to a simple Q&P treatment alone. Maximum strength of about 1398 MPa with a total elongation of 14% is achieved in the 1.1DQP steel. The study suggests that performing a TMCP prior to Q&P promotes grain refinement and formation of high fractions of HAGBs that are beneficial to improve the tensile properties.