Enhancing the Robustness and Efficiency in the Production of Medium Mn Steels by Al Addition

Abstract

The narrow process window during intercritical annealing and discontinuous yielding have limited the commercialization of medium Mn steels. In this study, a double-annealing process based on the commercial continuous annealing line is proposed. The cold-rolled medium Mn steels were first fully austenitized and quenched during the first annealing, followed by intercritical annealing for reverted austenite transformation. The microstructure of duplex lath-shaped austenite and ferrite is produced and steel exhibits a desirable continuous yielding during tensile deformation. Al is added into the medium Mn steel to enlarge the process window and to improve the partitioning efficiency of Mn. The produced steel is more robust with temperature fluctuation during the industrial process due to the enlarged intercritical region. Mn partitioning is more efficient owing to the elevated annealing temperature, which results in the improvement of ductility in the Al-added steel with increased austenite stability.

Temperature-Dependent Microstructural Evolution of Al-Rich Medium-Mn Steel During Intercritical Annealing

“…[19] Moreover, aluminum increases the A c1 and A c3 temperatures, which accelerates diffusion and enables faster achievement of the equilibrium state. [13] In addition, extending the intercritical range increases the temperature range of the mentioned technological window, [20] which results in a lower sensitivity of the process to possible deviations of process parameters.…”

Austenite/ferrite interface movement during intercritical annealing of a medium Mn steel

“…Then it is feasible and convenient to verify the growth of austenite, namely the austenite/ferrite interface movement, through the change in the volume fraction of γ in the annealed microstructure of medium Mn steel, which is batch annealed. Besides, the addition of Al into medium Mn steels is beneficial for improving the stability of γ because of more Mn partitioning in γ at elevated annealing temperature [14]. Therefore, the growth of lath-type γ , that is the γ / α interface movement, in medium Mn steel with relatively high Mn and Al addition during isothermal annealing was theoretically studied.…”