Improved Mechanical Properties of a Quenched and Partitioned Medium-Carbon Bainitic Steel by Control of Bainitic Isothermal Transformation

A triplex microstructure of bainite, martensite and retained austenite in the 0.3C-1.4Si-1.8Mn (wt.%) steel was obtained by the bainitic isothermal transformation combined with Q&P process (B-QP process). The effects of bainite morphology and content on the mechanical properties have been studied by controlling the isothermal holding temperature and time. Results show that with isothermal holding temperature increasing from 320 to 400 °C, the bainite changes from lower bainite to upper bainite and the ultimate tensile strength and toughness decrease. Compared to the Q&T, 360 °C B-QP and 400 °C B-QP processes, the microstructure of the 320 °C B-QP-treated sample consists of lower bainite, martensite and retained austenite, and it exhibits the best combination of strength (1617 MPa), elongation (18.6%), the product of strength and elongation (PSE, 30.1 GPa%) and maximum impact toughness (103 J) when the lower bainite content is about 50%. In addition, the work hardening behaviors indicate that both the 400 °C B-QP sample with upper bainite morphology and the 320 °C B-QP sample containing 75% lower bainite exhibit the higher work hardening rate at the high strain level and larger uniform elongation.