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 (αL) and austenite (γL) after intercritical annealing. The αL grains had a high density of dislocations due to inactive recovery, and also had a low Mn concentration. The γL grains had a low density of dislocations and high Mn and C concentrations. The αL and γL grains were deformed simultaneously during the tensile test because the αL grains were as hard as the γL 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 (αG) and austenite (γG) after intercritical annealing. The αG grains had a low dislocation density due to active recovery, and also had a low Mn concentration. The γG grains had a low dislocation density and high Mn and C concentrations. The soft αG 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.