Influence of Silicon and Aluminum on the Microstructures of as-cast Advanced High-Strength Steels

Abstract

The third-generation advanced high-strength steels exhibit increased strength and formability due to their chemical composition and precise controlled thermomechanical processing. The chemical composition also affects the properties of the semi-finished steel. Reliable production of these steels requires that the effects of chemical composition on microstructure development be understood during all steps of processing. This study focuses on the as-cast microstructure by characterizing laboratory-cast ingots containing 0.2 mass% C, with varying Mn, Si and Al concentrations. Isothermal heat treatments were also conducted to verify Thermo-Calc predictions of austenitization behavior. The microstructure constituents were compared against Thermo-Calc diffusion simulations. Results show that increasing the Al and Si concentrations increases the A₃ transformation temperature and the amount of ferrite in the as-cast state. Microstructures at room temperature are mainly bainite and ferrite. Steels containing C, Mn, and Si have proeutectoid ferrite and those C, Mn, Si, and Al alloy have primary ferrite. A steel alloyed with only Al has a mixture of primary and proeutectoid ferrite where proeutectoid ferrite regrows from the primary dendrite remnants.