Effects of Alloy Elements on Carbon Partitioning in Early Stages of Proeutectoid Ferrite Transformation in Low Carbon Mn-Si Steels

Controlling the carbon concentration and distribution among constituent phases is one of the most important issues for achieving high strength and ductility in the design of steel. The carbon distribution near the alpha/gamma interface in the early stage of isothermal holding at 750 degrees C was measured and visualized in Fe-C-Mn-Si alloys containing 2 mass% Si and 1.5 or 2 mass% Mn using a high precision FE-EPMA developed recently by the authors, and the results were compared with the theory of ferrite growth in multi-component low alloy steel. The carbon concentrations at alpha/gamma interfaces in austenite were generally between the NPLE/PLE and paraequilibrium alpha/(alpha + gamma) boundary concentrations. In alloys with carbon contents smaller than the NPLE/PLE boundary, the alpha/gamma interfaces appeared to migrate under a condition close to paraequilibrium or with partially developed spikes of alloy elements in the early stages. On the other hand, in alloys with a bulk composition on the boundary and its higher carbon concentration side, Mn enrichment was observed at the interfaces, and the carbon concentrations tended to be higher than those in alloys with lower carbon contents, albeit there were variations at individual interfaces.