Mechanical Properties of Ausformed Carbide-Free Bainite

In this study, the effect of ausforming temperature and strain on the bainite transformation in a model alloy containing 0.39 pct C, 2.0 pct Mn, 1.9 pct Si, and 0.5 pct Cr (mass percent) was investigated. Samples were deformed at 300 °C, followed by isothermal holding to monitor the phase transformation kinetics. Dilatometry results confirm the acceleration of transformation kinetics when the material is deformed to small strains (0.05 to 0.20). Tensile testing results show that, for long isothermal holding times, no improvement in strength or ductility is achieved as a result of ausforming in comparison with the non-deformed state. However, the benefit of ausforming lies in its ability to accelerate the bainite transformation kinetics and to reduce the isothermal holding time needed to avoid premature fracture due to the presence of a high volume-fracture of fresh martensite. Fracture analysis reveals ductile fracture, with cracks along martensitic regions within the microstructure. The distribution of martensite in the final microstructure was associated with bands of Mn segregation. This banding effect was less visible within the ausformed microstructure because bainite was able to nucleate within both the high- and low-Mn regions. A second advantage of ausforming is, therefore, to reduce the microstructure heterogeneity associated with compositional banding within the steel sheets and, by so doing, eliminate crack propagation within the bands. In spite of the fact that ausforming accelerates the bainite transformation kinetics, the required transformation times for the present steel are still too long for use within industrial continuous annealing lines.