Observation of γ→α transformation in ultralow-carbon steel under a high temperature optical microscope

Abstract

The basic composition of IF steel is Fe-ultralow-carbon (<0.02 mass%). There is little research on the transformation behavior and microstructures of ultralow-carbon steel compared to low-carbon steels, and the details, for instance, transformation mechanism and the ratio of grain sizes of γ and α remain unclear. Therefore, in situ observation of the γ→α transformation in an Fe-0.004 % C steel was performed at a cooling rate of 0.5-18°C/s under a high-temperature optical microscope. The main microstructure of ferrite was αq, and with an increase in the cooling rate, the fraction of Widmanstätten ferrite-like structure and ferrite having a severely ragged interface increased. The growth rate of αq was 1 × 10-4-9 × 10-4 m/s which increased with the cooling rate. The growth rate of αq decreased to about half when the amount of carbon increased to 0.01%. The ratio of γ grain size to α grain size was about 1.2, and this value is considerably smaller than the values reported for the low-carbon steels. αq crossed γ grains frequently and some αq grains were larger than γ grains. Usually the curvature of the αq/γ interface did not change at the intersection of the boundaries of αq and γ. This shows that αq/γ interfaces are usually incoherent in ultralow-carbon steels. Transformation temperature was in the single-phase region of α. Therefore, the γ→αq transformation observed in the present research is thought to be massive transformation. The terminology of mechanisms of γ→α transformation in ultralow-carbon steels and the microstructures of generated α was discussed.