Effect of bainitic isothermal transformation plus Q&P process on the microstructure and mechanical properties of 0.2C bainitic steel

A novel bainitic isothermal transformation plus quenching and partitioning (B-QP) process was developed for a 0.2C-1.5Si-1.8Mn bainitic steel. The microstructure and mechanical properties of the steel treated by different heat treatment process were investigated. The results show that the B3-QP (the subscript '3' indicates the isothermal holding of 3 min for the bainitic transformation) samples exhibit a multiphase microstructure containing carbide-free bainite, martensite, film-like retained austenite. The microstructure is much finer than that of the Q&T samples with full martensite and the B30-QP samples with a bainite microstructure. Although the Q&T sample has the highest tensile strength (1455 MPa), the ductility and the product of strength and elongation (PSE) are lower. In contrast, the B3-QP and B30-QP samples exhibit better ductility and higher PSE values of 26.3 GPa% and 28.2 GPa%, respectively. Furthermore, the B3-QP sample exhibits maximum impact toughness value of 121 Jcm−2, which is about 20% and 55% higher than those of Q&T sample (101 J cm−2) and B30-QP sample (78 J cm−2) respectively.