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

Abstract

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.

Keywords

• Medium-Mn steel (MMnS)
• Austenite reversion
• Nano-precipitation
• Synchrotron X-ray diffraction
• Three-dimensional atom probe tomography