Comparison of the Mechanical Behavior of a Low Carbon-Manganese Steel in Dual Phase and Ferritic-Bainitic Heat Treatment Conditions

Three low carbon steel grades with Mn content varying between 1.5 and 2.5% were subjected to a dual phase heat treatment involving slow cooling in the inter critical temperature range followed by water quenching to generate a hard martensite and soft ferritic phase constitution. The same three steels were subjected to two different heat treatments that could generate hard bainitic ferrite and a soft ferritic phase. The three steels were austenitized at 870 degrees C, well above Ac3 temperature, 850 degrees C very close to Ac-3 temperature and 790 degrees C in the inter critical temperature range. Post austenitization one set of samples were forced air cooled and another set was subjected to isothermal holding in a salt bath at 450 degrees C. Compared to dual phase heat treatment, a ferritic bainitic treatment gives a moderate loss in strength but better ductility was obtained in the forced air cooled condition. The strain hardening exponent was significantly lower for the dual phase steel with martensite content. The strain hardening exponent were much higher for ferritic bainitic condition with distinct slope for the initial deformation in the ferrite phase followed by bainitic phase and retained austenite phase. The ferritic-bainitic condition with isothermal bainitic holding condition gave almost double the ductility and the tensile elongation exceeded 20 GPa.%. With some modification dual phase steels can be modified into ferritic bainitic steels with attractive formability properties. The various aspects of structural evolution and properties has been brought out.