Development of New Third-Generation Medium Manganese Advanced High-Strength Steels Elaborating Hot-Rolling and Intercritical Annealing

The effect of hot rolling and austenite revert transformation (ART) on the microstructure development and mechanical properties for a series of newly developed manganese steels with small carbon and/or boron content was investigated in this study. The cast steels were hot rolled, ART annealed and subjected to tensile testings after each step. The carbon-bearing alloy revealed a significant combination of ultimate tensile strength (UTS) and elongation (El. pct) with 1.1 GPa and 42 pct values, respectively. The boron-bearing alloy exhibited a UTS of 760 MPa having 30 pct elongation. Moreover, the carbon and boron alloyed steel had a fully martensitic structure with a UTS of 1.4 GPa and a negligible elongation. X-ray diffraction, SEM and electron backscatter diffraction analyses were used to rationalize the mechanical behaviour of the developed alloys. The higher ductility values for the boron alloyed and carbon alloyed steels were attributed to the presence of the austenite phase in conjunction with α′- and ε-martensite and the transformation-induced martensite during the tensile loadings. The ART annealing at 290 °C, 450 °C and 510 °C increased the austenite fraction in the final microstructure of the boron alloyed and carbon alloyed steels but had an insignificant influence on the boron and carbon alloyed steels. It was concluded that the α′ → γ transformation occurring at higher ART annealing temperature is the dominant factor for the improved mechanical properties in the developed alloys.