Article ID: ISIJINT-2018-026
To understand the γα transformation and control the carbon contents among the phases involved is most important in the design of high-strength and high-ductility low carbon steels. Using a previously-developed field emission-electron probe microanalyzer (FE-EPMA) which is able to suppress hydrocarbon contamination to a very low level, the carbon concentrations at γ/α interfaces and within the austenite were analyzed in Fe-xC-2.0Si-yMn (x=0.15 or 0.20, y=1.5 or 2.0 in mass%) steels isothermally transformed at 750°C and 800°C. The paraequilibrium (PE) model gives a good accounting of carbon enrichment in the 1.5 mass% Mn steel held for 15 s, whereas the NPLE/PLE transition model of local equilibrium gives a much better accounting in the 2.0 mass% Mn steel than PE. However, the interfacial carbon concentration agrees with the composition shown by the NPLE/PLE transition line in both alloys when annealed for 1800 s. In the 1.5 mass% Mn steel, the carbon concentration at the interface in austenite seems to have shifted from PE to NPLE during annealing.