Influence of Relative Amount of Ni and Mn on the Cast Microstructure of δ-Ferrite Containing Medium-Mn Steels

The nominal composition of Fe–xMn–yNi–3Al–1Si–0.2C with x and y having the values of (6, 5, 4) and (0, 1, 2), respectively, has been ingot cast in an open-air induction furnace. Room temperature microstructures comprise δ-ferrite, martensite, and austenite. Martensite/austenite (M/A) matrix is found between δ-ferrite dendrites and inside a single δ-ferrite grain. Further, δ-ferrite is of vermicular and lacy morphologies. The lacy morphology represents the allotropic transformation of δ-ferrite into austenite during solidification. The austenite fraction in 6Mn0Ni, 5Mn1Ni, and 4Mn2Ni is 0.12, 0.07, and 0.04, respectively. The microhardness of δ-ferrite is similar in each steel. The M/A matrix in 6Mn0Ni is softer (471 ± 13.10 HV_0.05) than 5Mn1Ni (557 ± 18.60 HV_0.05) and 4Mn2Ni (534 ± 6.99 HV_0.05). Some local regions of M/A in 6Mn0Ni are even softer, with a value of (382 ± 10.39 HV_0.05) due to a very high amount of austenite. The kinetics show that the severity of segregation is higher in 6Mn0Ni, followed by 5Mn1Ni and 4Mn2Ni, resulting in higher enrichment of austenite stabilizers in the last solidifying liquid. Subsequently, localized soft M/A regions form. Hence, a more heterogeneous microstructure forms in 6Mn0Ni, followed by 5Mn1Ni and 4Mn2Ni.