Effect of isothermal martensite and bainite transformation on microstructure and properties in a medium-carbon high-strength Q P steel

The effect of the amount of isothermal martensite and bainite on the microstructure and properties in a medium-carbon quenching and partitioning (Q P) steel was investigated by designing the different Q P treatment parameters. The results show that the amount of isothermal martensite increased gradually with the increase in quenching time. The increase in isothermal martensite amount improved the product of strength and elongation (PSE) of Q P steels. In addition, the increase in carbides amount and the recovery in prior martensite with longer partitioning time led to an increase in PSE first and then, a decrease. It implies that a higher PSE could be obtained by the selection of a suitable partitioning time. Furthermore, the effect of bainite transformation during partitioning on PSE was investigated by designing the different partitioning temperatures, including 300, 400 (below bainite starting temperature, Bs) and 480 °C (above Bs). The results show that compared with the samples partitioned at temperature above Bs, the bainite transformation was only detected when the samples were partitioned at temperature below Bs. The bainite transformation amount increased with the decreasing partitioning temperature, leading to the inhibition of carbides precipitation and more stable RA and thus, resulting in the highest PSE.