EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

The desired microstructure of advanced high strength steels is often multiphase in nature, combining softer (ferrite, retained austenite) and harder (martensite, bainite) phase constituents in order to achieve high strength-ductility combinations. The thermomechanical processing of these steels may involve isothermal heat treatments around the Ms temperature. The presence of pre-existing martensite provides additional nucleation sites available for subsequent isothermal transformation, in turn accelerating the transformation kinetics. In the present work, a 0.2C-3Mn-2Si (wt.%) steel (Ms temperature: 350 °C) was investigated using Q&P annealing treatments. A quench-stop temperature of 340 °C was applied, resulting into approximately 17% prior martensite. The partitioning temperature was varied in the range of 380 – 450 °C. The bainite formation kinetics measured at these temperatures were compared to the results for bainitic holding treatments at the same temperatures without a pre-quench. The initial transformation kinetics were at least two times higher due to presence of prior martensite. The bainite fraction varied in a comparatively narrow range (40-60%) in the Q&P treatments compared to the treatments without a pre-quench where it varied between 6-75%, with a general decreasing trend with increasing temperature. At temperatures of 420-450 °C the bainite fraction was significantly higher in cases with prior martensite, which also resulted in less fresh martensite and more retained austenite. These observations indicate that pre-existing martensite can enhance the bainite formation and thereby increase the stabilization of austenite.