Evading the strength-ductility compromise in medium manganese steel by a novel low temperature warm rolling treatment

Abstract

Strength-ductility trade-off is an enduring problem in a wide range of metals and alloys including the advanced high-strength steels. In this work, we propose a novel non-isothermal low-temperature warm rolling (WR) treatment which was applied after the conventional intercritical annealing (IA) in order to overcome the strength-ductility compromise in a medium manganese steel, Fe-5.1Mn-0.22C-0.85Si-0.42Al (composition in wt%). IA + WR treatment yielded a simultaneous improvement in tensile properties, wherein yield strength (YS), ultimate tensile strength (UTS), and ductility improved by 32%, 12%, and 6% respectively, compared to the conventional IA treatment. Despite an increase in the yield point elongation (2%) and Lüders strain (3%) in the IA + WR specimen, the ductility of the same improved as compared to the IA specimen. Microstructure of the IA + WR specimen was tailored by introducing a significantly higher dislocation density in the constituent ferrite and retained austenite (RA) phases along with a relatively higher density of planar defects (stacking faults and nano-twins) in the RA phase. X-ray line profile analysis quantified the increased dislocation density (order ∼1015 vs 1014 m−2) and twin fault probability (∼1.35% vs ∼0.12%) in the RA phase of the IA + WR compared to the IA specimen. The improvement in YS was primarily due to the increase in the dislocation density of the constituent phases in the IA + WR specimen. Introduction of a high density of lattice defects in the RA phase during the WR treatment above the Md30 temperature enhanced the mechanical stability of the RA phase, leading to a decreased rate of TRIP effect and formation of hierarchical twins during tensile deformation. The near-ubiquitous strength-ductility trade-off was overcome via a tailored microstructure consisting of regulated austenite stability leading to discontinuous TRIP effect, hierarchical twinning in RA phase, and favorable texture in the constituent phases. The present study provides a pathway to tailor the microstructure for escaping the strength-ductility paradox in different metastable FCC alloys such as stainless steel, HEAs, TWIP steels etc.