Effect of cooling rate on the inclusion precipitation behavior in unidirectionally solidified Fe-Mn-C-Al TWIP alloys

During the solidification process, the precipitation of inclusions is inevitable due to micro-segregation, especially AlN and MnS in TWIP steel. In order to reasonably control the precipitation of inclusion, the cooling rate as an important factor in the casting process was studied through the unidirectional solidification experiments. The combined analyses using ASEM-EDS, EPMA and optical microscope revealed that the number density of precipitated inclusion decreased along the solidification direction with the decrease of the solid fraction. The number density of Al₂O₃, which existed in the liquid alloy was basically stable. On the contrary, the number density of MnS decreased and that of AlN increased with the increasing of cooling rate. Moreover, in the early stage of the solidification (f_s<0.1), the order of Al concentration curve slope was 0.58 K/s >0.38 K/s >0.1 K/s due to the entrapment of precipitated AlN by the solid-liquid interface moving relatively fast. Temperature was the main factor to affect the AlN precipitation and the particles were precipitated at the early stage of the solidification process. AlN inclusions can become the core to form the composite particles and have the positive effect on fining grains.