Abstract
High strength/high elongation continues to be the primary challenge and focus for medium-Mn steels. It is elucidated herein via critical experimental analysis that the cumulative contribution of transformation-induced plasticity (TRIP) and microstructural constituents governs high strength/high elongation in 0.2C–3Al–(6–8.5)Mn–Fe steels. This was enabled by an effective heat treatment involving a combination of intercritical hardening and tempering to obtain high strength/high ductility. An excellent combination of high ultimate tensile strength of 935–1112 MPa and total elongation of 35–40% was obtained when the steels were subjected to intercritical hardening in the temperature range of 700–750 °C and low tempering at 200 °C. The intercritical hardening impacted the coexistence of austenite, ferrite, and martensite, such that the deformation behavior varied with the Mn content. The excellent obtained properties of the steels are attributed to the cumulative contribution of the enhanced TRIP effect of austenite and the microstructural constituents, ferrite and martensite. The discontinuous TRIP effect during deformation involved stress relaxation, which was responsible for the high ductility. Lamellar austenite, unlike the equiaxed microstructure, is envisaged to induce stress relaxation during martensitic transformation, resulting in the discontinuous TRIP effect.
Highlights
Transformation-induced plasticity (TRIP) steels have good crashworthiness, superior ductility, and high strength
As regards medium-Mn steels, the steel is first heated in the austenitic region and quenched to obtain a complete martensite microstructure
Effective and simple two-stage heat treatment process was adopted: (1) Intercritical hardening—the as-hot-rolled steels were heated in the intercritical is weakened
Summary
Transformation-induced plasticity (TRIP) steels have good crashworthiness, superior ductility, and high strength. The high-manganese twinning-induced plasticity (TWIP) steels with PSE of ~60 ±. In conventional TRIP steel, a two-stage heat treatment process is adopted to stabilize austenite during austempering [17]. As regards medium-Mn steels, the steel is first heated in the austenitic region and quenched to obtain a complete martensite microstructure. Annealing followed by air cooling stabilizes a high retained austenite fraction. Ms and Mf. Annealing at the quenching temperature or above is subsequently conducted to facilitate the partitioning of carbon from martensite to austenite [18,19,20]. For medium-manganese steel, two different traditional heat treatments (Q&P and ART processes) are described. A novel heat treatment is proposed for the experimental medium-Mn steels to obtain high strength/high elongation, constituting the objective of the study
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