Microstructure–properties relationship in a one-step quenched and partitioned steel

To comprehensively analyze the relationship between the microstructure and mechanical properties in a one-step quenched and partitioned (Q&P) steel, Q&P processes with different partitioning time were applied to a low-carbon steel. Microstructures were characterized by means of EPMA, XRD, EBSD and TEM. The dislocation density of martensite was calculated using the Williamson–Hall method. Mechanical properties were measured by uniaxial tensile tests. Results show that the microstructures consist of lath martensite accompanying with both film-like inter-lath retained austenite and blocky retained austenite. Martensite laths broaden with prolonged partitioning time. The amount of retained austenite increases first and decreases with the critical partitioning time of 100 s. The relation between the microstructure and properties was clarified by analyzing the stress–strain curves stage by stage combining with the substructure of martensite and the condition of retained austenite. The presence of retained austenite decreases the elastic limit and influences on the yield strength for its early plastic deformation. Two kinds of nano-scaled carbide appearing in the specimens partitioned longer than 100 s promote the austenite decomposition and play the main role in increasing the yield strength. The interaction of the dislocations in martensite and the transformation induced plasticity (TRIP) effect of retained austenite increase the work hardening rate and improve both the ultimate tensile strength (UTS) and the uniform elongation.