Microstructure evolution and mechanical properties of sub-zero treated medium Mn steels

Abstract

The microstructure evolution and mechanical properties such as tensile properties and formability have been investigated for sub-zero hardened Fe-(10, 8)Mn-0.2C-2Al medium Mn steels. Yield strength increased and total elongation decreased with martensitic transformation after sub-zero treatment. In addition, both of yield strength and total elongation increased simultaneously after subsequent tempering. It was revealed that the evolution of the microstructure includes twinned martensite with decrease of the amount of carbon in the austenite after sub-zero treatment, and the formation of iron carbide after subsequent tempering treatment through TEM-analysis. Alloys showed good bendability in 3-point bending test, but poor Hole Expansion Ratio (HER) value. It could be deduced that their poor HER would be related to mechanically transformed martensite during punching, compared with the high HER value of the samples made by drilling.