The effects of heat-treatment parameters on the mechanical properties and microstructures of a low-carbon dual-phase steel

Abstract

The austenite decomposition phase transformation in a low-carbon dual-phase (DP) steel is studied as a function of inter-critical annealing parameters: annealing time, annealing temperature, and cooling rate. The austenization kinetics are obtained by dilatometric analyses, water-quenched metallography, and thermodynamic calculations, and the austenite formed at different inter-critical annealing temperatures influences the subsequent formation of secondary phases during continuous cooling. Results indicate that the austenite volume fraction increases with both annealing temperature and time; although their effect on the final microstructure and mechanical properties lessens when the cooling rate decreases to the slow cooling regime below air cooling. Interestingly, the yield strength (YS) values of samples cooled at ∼1 °C/s with sand cooling, are 75∼100 MPa greater than the corresponding values at a rate of ∼6 °C/s with air-cooling. The results indicate that the yield strength is positively correlated with yield-point elongation but negatively correlated with the ultimate tensile strength. The effects of the cooling rate on the mechanical properties can be explained by the formation of Cottrell atmospheres of carbon together with the increase of the pearlite volume fraction. Based on the secondary phase components and strength evolutions, there are four possible outcomes for the different cooling rates: (I) martensite (M); (II) ferrite-martensite (FM); (III) coexistence of martensite and pearlite (MP); and (IV) ferrite-pearlite (FP). Our results reveal that the microstructure-property correlation can be illustrated schematically as a function of cooling rates and can thereby direct the heat-treatment parameters required to obtain desirable mechanical properties.