Accelerated Ferrite-to-Austenite Transformation During Intercritical Annealing of Medium-Manganese Steels Due to Cold-Rolling

• Main Authors: Josh J. Mueller, David K. Matlock, John G. Speer, Emmanuel De Moor
• Format: Article
• Language: English
• Published: MDPI AG 2019-08-01
• Series: Metals
• Subjects: intercritical annealing; medium manganese steel; phase field simulation
• Online Access: https://www.mdpi.com/2075-4701/9/9/926

Prior cold deformation is known to influence the ferrite-to-austenite (<i>&#945;</i> &#8594; &#947;) transformation in medium-manganese (Mn) steels that occurs during intercritical annealing. In the present study, a 7Mn steel with ultra-low residual carbon content and varying amounts of prior cold deformation was intercritically annealed using various heating rates in a dilatometer. The study was conducted using an ultra-low carbon steel so that assessments of austenite formation during intercritical annealing would reflect the effects of cold deformation on the &#945; &#8594; &#947; transformation and Mn partitioning and not effect cementite formation and dissolution or paraequilibrium partitioning induced austenite growth from carbon. Increasing prior cold deformation was found to decrease the <i>A</i>_c1 temperature, increase austenite volume fraction during intercritical annealing, and increase the amount of austenite nucleation sites. Phase field simulations were also conducted in an attempt to simulate the apparent accelerated &#945; &#8594; &#947; transformation with increasing prior cold deformation. Mechanisms for accelerated &#945; &#955; &#8594; &#947; transformation explored with phase field simulations included an increase in the amount of austenite nucleation sites and an increased Mn diffusivity in ferrite. Simulations with different amounts of austenite nucleation sites and Mn diffusivity in ferrite predicted significant changes in the austenite volume fraction during intercritical annealing.