Microstructural Evolution and Dynamic Partitioning Behavior in Quenched and Partitioned Steels

Abstract

Direct quenching and dynamic partitioning (DQ&P) process is a promising approach and there continues to be lack of understanding concerning cooling condition in the hot strip rolling line. Here, the authors have simulated DQ&P process in low carbon Si–Mn steel using a thermal simulator. The effect of quenching temperature, relaxation process, and cooling rate on microstructure and hardness is studied. The results suggest that retained austenite is less sensitive to quenching temperature and the volume fraction of retained austenite including film‐like and blocky shape is ≈7–11%. The relaxation process leads to ferrite transformation and recovery of defects, resulting in increase in retained austenite, and reduced hardness. It is interesting that the quenching temperature and hardness do not show an inverse relation. It is important that the isothermal transformation at high quenching temperature below Ms promotes retained austenite. Moreover, the low coiling cooling rate less than 0.1 °C s−1 from quenching temperature to room temperature is adequate for carbon partitioning. In the view of thermodynamics and kinetics, an appropriate processing window of coiling temperature between 200 and 320 °C plus low cooling rate less than 0.1 °C s−1 is proposed for hot rolled DQ&P steels.