Austenite reversion and nano-precipitation during a compact two-step heat treatment of medium-Mn steel containing Cu and Ni

Breaking the strength-ductility paradox of very-low (<= 0.05 wt.%)-C medium (3-12 wt.%)-Mn steels (MMnS) has been a hard-wired topic, since in these steels multi-step heat treatments are usually required to obtain austenite for improved ductility and precipitates for higher strength. In this study, a compact two-step heat treatment comprising short (2 min) annealing and tempering was developed to investigate the synergetic effect of austenite reversion and nano-precipitation on the tensile behavior of a very-low-C MMnS containing 1.5 wt.% Cu and 1.5 wt.% Ni. The annealing step promoted considerable amount of reverted austenite (33 vol.%), and the annealing was short to prevent Cu and Ni from partitioning into austenite, since they were supposed to maintain in the ferrite phase and then promote the nano-precipitation in the subsequent tempering stage. During the subsequent tempering step, the nano-precipitates with Cu concentration of 20-50 at.% in the precipitation core and enriched with Cu, Ni, Al and Mn were observed in the ferrite phase. The volume fraction of reverted austenite reached 38.5 vol.% after tempering, which led to the ultimate tensile strength of 1222 MPa and total elongation of 29% by the transformation induced plasticity during plastic deformation. The current study demonstrates the beneficial influence of the compact two-step heat treatment on the austenite reversion and nano-precipitation behavior of very-low-C MMnS with the addition of Cu and Ni, which subsequently enables an enhanced strain hardening behavior, thereby improving the mechanical property profile of the MMnS. (C) 2022 The Author(s). Published by Elsevier B.V.

Automated summary

This study developed a compact two-step heat treatment to improve the ductility and strength of low carbon medium manganese steels (MMnS) through the synergistic effect of austenite reversion and nano-precipitation. The annealing step promoted the formation of reverted austenite, while the subsequent tempering step resulted in the formation of nano-precipitates enriched with Cu, Ni, Al, and Mn.