The effects of the initial martensite microstructure on the microstructure and tensile properties of intercritically annealed Fe-9Mn-0.05C steel

The effects of the initial microstructure of α′ martensite on the microstructural evolution during intercritical annealing and the tensile properties of annealed specimens were investigated for Fe-9Mn-0.05C (wt.%) steel. The hot-rolled specimen with fully α′ martensitic microstructure showed a mixed microstructure of lath-shaped ferrite (α_L) and austenite (γ_L) after intercritical annealing. The α_L grains had a high density of dislocations due to inactive recovery, and also had a low Mn concentration. The γ_L grains had a low density of dislocations and high Mn and C concentrations. The α_L and γ_L grains were deformed simultaneously during the tensile test because the α_L grains were as hard as the γ_L grains due to their high dislocation density, resulting in continuous yielding. The cold-rolled specimen with a deformed α′ martensite microstructure exhibited a mixed microstructure of globular-shaped ferrite (α_G) and austenite (γ_G) after intercritical annealing. The α_G grains had a low dislocation density due to active recovery, and also had a low Mn concentration. The γ_G grains had a low dislocation density and high Mn and C concentrations. The soft α_G grains with a low dislocation density were easily deformed at the early stage of the tensile test, resulting in discontinuous yielding and a large yield point elongation.