Optimizing strength & ductility combination of medium-carbon TRIP steel via multi-stage work hardening

Abstract

The effect of the retained austenite (RA) stability on mechanical properties of a medium-carbon transformation-induced-plasticity (TRIP) steel at different deformation temperatures was studied. Two steels were obtained by adjusting the annealing temperature from 780 °C to 820 °C and controlling the isothermal bainitic process, and the carbon content of RA in the steels was 1.2% and 1.4%, respectively. At 25 °C, ultimate tensile strength, yield strength and total elongation of the steel after intercritical annealing at 780 °C were 1390 MPa, 870 MPa and 40%, respectively. When the deformation temperature increased to 200 °C, yield strength nearly kept constant (860 MPa) while total elongation significantly increased to 73%. The stability of RA had a significant influence on the deformation mechanisms of the steel, hence leading to the distinct variation of the mechanical properties. The RA stability increased with increasing deformation temperature to 200 °C, reducing the rate of transformation from RA to martensite and making the TRIP effect more persistently. Meanwhile, due to low solubility of Cu atom and the large number of dislocations acting as fast diffusion channels for Cu and C atoms in transformed martensite, the joint action of the increasing temperature and the continuous TRIP effect promoted the formation of numerous nanosized precipitates including η carbides and Cu particles in the newly formed martensite. As a result, the work hardening rate was significantly increased, improving the ductility without sacrificing the strength. This study provides a strategy to control the deformation mechanisms of medium-carbon TRIP steel, which can tailor the stability of RA according to the requirement of application, so as to obtain a good combination of strength and ductility.