Prediction and evaluation of optimum quenching temperature and microstructure in a 1300 MPa ultra-high-strength Q&P steel

The quenching and partitioning steel is the representative of the third generation of advanced high-strength steel. The effect of quenching temperature on the microstructure and mechanical property of ferrite-containing quenching and partitioning steel was studied by intercritical annealing quenching and partitioning processes. When preparing a test steel with a tensile strength of 1300 MPa and total elongation of 19%, it is found that the actual optimum quenching temperature was lower than that calculated according to the constrained carbon equilibrium. The results indicate that the martensite start temperature of the austenite was overestimated when considering the diffusion of carbon only. Austenite grain size which is affected by low temperature and the existence of ferrite during intercritical annealing influenced the optimum quenching temperature. A scheme considering the diffusion of various alloying elements and austenite grain size was proposed and verified. Using this scheme, the optimum quenching temperature of intercritically annealed quenching and partitioning steel with complex microstructures was well predicted.