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

The size effect of α' martensite constituents before annealing on the microstructure and tensile properties of intercritically annealed medium Mn steel was systematically investigated. The cold-rolled Fe-9Mn-0.05C (wt%) steel was austenized at three different temperatures, water-quenched to room temperature to vary the sizes of the martensite constituents, and then annealed at 640°C for 10min. When the austenizing temperature increased, the sizes of the prior austenite (γ) grains and of the packets and blocks of α' martensite increased; however, the width of the laths changed only insignificantly. The annealed specimens had a dual-phase microstructure with lath-shaped ferrite (α) and retained γ (γ_R) phases. The volume fraction of γ_R decreased with increasing austenizing temperature because the specimen austenized at the higher temperature underwent a slower reverse transformation in the early stage of intercritical annealing. The slowed kinetics of the reverse transformation with increasing austenizing temperature was attributed to the reduction in area of block boundaries which provide nucleation sites for the reverted γ. The widths of the α and γ_R laths were almost independent of the austenizing temperature. The partitioning of Mn and C atoms from α into γ_R laths became active with increasing austenizing temperature, resulting in a more stable γ_R. The specimen austenized at higher temperature exhibited a lower strain hardening rate (SHR) due to the less active transformation-induced plasticity (TRIP) in γ_R with the higher phase stability. The ultimate tensile strength decreased with increasing austenizing temperature because the SHR was lowered by the less active TRIP with increasing austenizing temperature. The uniform elongation increased with increasing austenizing temperature due to delayed necking caused primarily by the flow stress, which dropped with increasing austenizing temperature.