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

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.

Automated summary

The bake-hardening behavior of a medium Mn steel was investigated after intercritical annealing. The steel annealed at 760°C exhibited a higher bake-hardening response compared to the steel annealed at 680°C. This difference was attributed to the presence of martensite in the 760°C annealed steel, which generated dislocations during quenching and prestraining. Baking further increased the strength by pinning more dislocations and precipitating fine carbides, while also stabilizing the austenite phase and enhancing the ductility.