Improving strength and ductility via ausforming combined with austempering through deformation-induced bainite refinement

The refinement of bainitic ferrite laths is one of the most critical ways to improve the combination of strength and ductility. However, low-temperature bainitic transformations in the microstructural evolution of carbide-free bainite in high-carbon steels cause challenges for both the manufacture of coils and their application in car bodies. In this study, deformation-induced bainite refinement was proposed for refining bainite and retained austenite laths using a combination of ausforming and austempering in low-carbon low-alloy steel. The deformed austenite supplied more nucleation sites for bainitic transformation and suppressed the thickening of the bainitic ferrite laths by strengthening the surrounding untransformed austenite. A tensile strength of 1283 MPa and total elongation of 23.6% were obtained in the steel with an ausforming strain of 40% before austempering. The significant increase in strength was mainly attributed to the refinement of the bainitic ferrite laths and the high accumulation of dislocations within both the bainitic ferrite and retained austenite. The more stable retained austenite improved the sustainable work-hardening capability via the continuous transformation-induced plasticity effect.