Application of Microstructural Engineering to Produce Third Generation Dual-Phase Steel Having Lean Alloy Composition

The present work proposes methods to enhance the mechanical properties of dual phase steels through microstructural engineering, via optimizing the composition and thermo-mechanical treatment (TMT). Research on the development of steels with good combination of strength and ductility having alloying addition less than 5% has gained importance for automotive application. The current research is focused on developing dual phase steels which could meet the demand of 3^rd generation AHSS (UTS x %TE > 22000 MPa%). Four different compositions of dual phase (DP) steel were optimized with varying carbon equivalent value and melted in a vacuum arc melting furnace. The solidified alloys were subjected to three different intercritical annealing treatments followed by water quenching. Also, the as-cast alloy were subjected to eight different types of TMT cycles, such as sequential quenching, combination of deformation and intercritical annealing, and intermediate quenching. Amongst different compositions, the sample having highest carbon equivalent qualified for 3^rd generation AHSS for three different TMT cycles. The TMT having 60% intermediate cold rolling step showed UTS x %TE value 25323 MPa%, TMT having 40% intermediate hot rolling followed by sequential quenching showed 22529 MPa%, and intermediate quenching treatment showed 25804 MPa%. Results showed that intermediate deformation promotes the formation of fine bainite. Microstructures consisted of 26-40% martensitic phase with ~ 10% bainite and ~ 10 µm ferritic grain size have shown exceptional combination of strength and ductility. The key behind such exceptional combinations of strength and ductility was ascribed to the presence of fine soft ferrite and bainite phase, evenly distributed at the interfaces of hard martensitic phase. The bainite acts either as crack arrester or crack deflection point resulting in an increase in the resistance to crack propagation. Additionally, presence of γ-fiber texture also contributed to the ductility and toughness in material.