In-situ observation of peritectic solidification of Fe-Mn-Al-C steel with medium manganese

Abstract

The design of the continuous casting process for the third generation of Fe-Mn-Al-C steel with medium manganese should be based on the understanding of the peritectic solidification mechanism. So, in the present study, Fe-5.56Mn-0.078Al-0.157C (wt%) steel was taken as the research object. By means of high-temperature confocal scanning laser microscopy (HT-CSLM), Thermo-Calc thermodynamic software and optical microscopy, the peritectic solidification process and microstructure characteristics of the medium manganese steel at different cooling rates were studied and revealed. And thus, the influence of the cooling rate on the peritectic solidification mechanism was clarified. Meanwhile, the kinetics of the γ-Fe grain wrapping the δ-Fe grain and the subsequent growth were determined. The results show that at low cooling rate of 10 °C/min, the initial δ-Fe phase is cellular. The γ-Fe phase firstly nucleates at the L/δ interface, and then grows along the δ/L interface until the L and δ-Fe phases are separated. Subsequently, the peritectic transformation occurs, that is the L and δ-Fe phases are directly transformed into the γ-Fe phase. Under the conditions with higher cooling rates of 20 and 30 °C/min, when the γ-Fe phase separates the L and δ-Fe phases completely, multiple γ-Fe grains form in a single δ-Fe grain at the same time, and the whole δ-Fe grain is covered within a very short time, which is the typical massive-like transformation. In the initial stage of the peritectic reaction, the linear velocity of the γ-Fe phase wrapping the δ-Fe phase is inversely proportional to the cooling rate, which is closely related to the size and morphology of δ-Fe grains. At cooling rates of 10, 20 and 30 °C/min, the initial migration rates of the δ/γ interface are 45.9, 8.9 and 12.5 µm/s, respectively. At higher cooling rates, the occurrence of the massive-like transformation greatly promotes the peritectic transformation, and the migration rate of the δ/γ interface increases to 16.7 and 17.8 µm/s. The initial migration rates of the γ/L interface are 6.0, 1.8 and 6.5 µm/s, respectively. It is found that the migration rate of the δ/γ interface is much larger than that of the γ/L interface. Dendrite etching experiment in the top, bottom and longitudinal sections of the medium manganese steel samples shows that the larger the cooling rate, the finer the solidification structure. • The δ/L interface of Fe-5.56Mn-0.078Al-0.157C (wt%) steel becomes more unstable with the increase of the cooling rate. • The mechanisms of the peritectic solidification of the medium Mn steel at different cooling rates are revealed. • Kinetics of the γ-Fe phase wrapping δ-Fe phase and then growing into δ-Fe and L phases are measured for the medium Mn steel. • The relationships between the dendrites arm spacings of the medium manganese steel and cooling rates are presented.