A strong and ductile medium Mn steel manufactured via ultrafast heating process

Ultrafast heating (UFH) at the rates of 10-300 degrees C/s was employed as a new strategy to anneal a cold rolled 7wt% Mn steel, followed by the immediate cooling. Severely deformed strain-induced martensite and lightly-deformed thermal martensite, both had been already enriched with C and Mn before, transformed to fine and coarse austenite grains during the UFH, leading to the bimodal size distribution. Compared with the long intercritical annealing (IA) process, the UFH processes produced larger fraction of RA grains (up to 37%) with a high density of dislocation, leading to the significant increase in yield strength by 270 MPa and the product of strength and elongation up to 55 GPa% due to the enormous work hardening capacity. Such a significant strengthening is first attributed to high density dislocations preserved after UFH and then to the microstructural refinement and the precipitation strengthening; whilst the sustainable work hardening is attributed to the successive TRIP effect during deformation, resulting from the large fraction of RA instantly formed with the bimodal size distribution during UFH. Moreover, the results on the microstructural characterization, thermodynamics calculation on the reverse transformation temperature and the kinetic simulations on the reverse transformation all suggest that the austenitization during UFH is displacive and involves the diffusion and partition of C. Therefore, we propose that it is a bainite-like transformation. (c) 2022 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

Automated summary

Ultrafast heating (UFH) followed by immediate cooling was employed to anneal cold rolled 7wt% Mn steel, resulting in a significant increase in yield strength and work hardening capacity. The high density dislocations preserved after UFH, microstructural refinement, and precipitation strengthening are attributed to the substantial strengthening effect observed. Furthermore, the sustainable work hardening is believed to be influenced by the successive TRIP effect during deformation.