Structure–mechanical property relationship in a low carbon Nb–Cu microalloyed steel processed through a three-step heat treatment: The effect of tempering process

We describe here the impact of tempering temperature and tempering time on the microstructure and mechanical property in a low carbon Nb–Cu microalloyed steel via a three-step heat treatment. After tempering process, the microstructure primarily comprised of ferrite, retained austenite and tempered bainite/martensite. The ferrite matrix with ultrafine grain size was film-like and enriched with nanometer-sized niobium-containing and copper precipitates. The volume fraction of ferrite increased with the increase in tempering temperature and tempering time. Retained austenite had average grain size less than 1 μm and was enriched with copper precipitates that contributes to enhance the stability of austenite. The retained austenite revealed high thermal stability and remained stable in the range of 20–30% when tempering temperature and time changed. High strength and good ductility were obtained in the tempering temperature range of 450–550 °C, or by prolonging tempering time at 500 °C, where the yield strength was ~750 MPa and the product of tensile strength and % elongation was ~32 GPa%, which is attributed to the cooperation of multiphase microstructure, stable retained austenite and nanometer-sized precipitates.