Understanding of γ→α transformation during intercritical annealing is important to achieve precise control over the mechanical properties of low-carbon steels. And control of the carbon contents in the phases is the most important factor in alloy design for achieving high strength and high ductility. However, it is unusually difficult to determine the carbon contents in multiphase structures with high accuracy. So, we have developed new methods for suppressing hydrocarbon contamination during field emission (FE) EPMA measurements.

Carbon enrichment at γ/α interface and carbon concentration of γ phase in Fe-0.15%C-2%Si-(1.5, 2.0)%Mn steels isothermally transformed at 750 and 800°C was measured using developed FE-EPMA. The paraequilibrium (PE) model gives much better predictions for carbon enrichment in 1.5%Mn steel for 15 s. The NPLE/PLE transition model of local equilibrium gives much better predictions in 2.0%Mn steel. But the interfacial carbon concentration agrees with the composition of PLE/NPLE transition line in all alloys annealing for 1800 s. Furthermore, carbon enrichment shifted from the PE to NPLE model during annealing.