Bake hardening of 7Mn–3Al-0.2C steel: Influence of intercritical annealing temperature

Abstract

© 2022 Elsevier B.V.Bake-hardening behavior in a medium Mn steel was investigated after intercritical annealing at either 680 °C or 760 °C. After 2% prestraining and baking at 180 °C for 20 min, the 760 °C -annealed steel exhibited bake hardening response (BHR) equal to 156 ± 3 MPa, while the BHR of 680°C-annealed counterpart was 60 ± 1 MPa. The BHR difference was attributed to martensite in the 760°C-annealed steel, which generated dislocations near adjacent austenite and ferrite on martensite transformation during quenching and prestraining. Baking enabled pinning of more dislocations by carbon and precipitation of fine carbides by martensite tempering, which could effectively increase the strength. Moreover, baking changed the state of strain incompatibility between the phases and stabilized austenite in the 760°C-annealed steel. As a result, strain hardening rate stayed at a high level, which enhanced ductility.