Effects of Intercritical Annealing on Microstructure Evolution and Tensile Properties of Direct-Quenched Low-Carbon Steels Containing Cr and Mo

This study investigates the effects of intercritical annealing (IA) on the microstructural evolution and tensile properties of direct-quenched low-carbon steels containing Cr and Mo. The steels were subjected to IA at different temperatures (740, 780, and 820 °C) to examine variations in ferrite-martensite morphology and phase distribution. The microstructural analysis revealed that increasing IA temperature led to an increase in the martensite volume fraction and a transformation from a lath to a fibrous microstructure in all steels, with the CrMo-added steel showing the most significant change, from 17.3% at 740 °C to 54.1% at 820 °C. The addition of Cr and Mo influenced phase transformation kinetics by reducing the mobility of the ferrite/austenite interface at lower IA temperatures due to the solute drag effect. Tensile testing showed an unusual trend where higher martensite fractions resulted in a decrease in strength and an increase in uniform elongation across all compositions, deviating from conventional dual-phase steel behavior. Notably, in the CrMo-added steel, the yield strength decreased from 685 MPa at 740 °C to 572 MPa at 820 °C, while uniform elongation increased from 6.1 to 7.4%. This phenomenon was attributed to changes in mobile dislocation density, hetero-deformation-induced strengthening, and enhanced martensite plasticity at higher IA temperatures. These findings offer valuable insights for optimizing IA conditions and alloy design to develop cost-effective, high-performance low-carbon dual-phase steels with enhanced mechanical properties.