"In-situ" Observation of the .DELTA./.GAMMA. Phase Transformation on the Surface of Low Carbon Steel Containing Phosphorus at Various Cooling Rates

In this paper, the solid δ/γ phase transformation is observed “in-situ” on the surface of low carbon steels containing different phosphorus concentrations by the Confocal Scanning Laser Microscope (CSLM) at various cooling rates. The effects of phosphorus and cooling rate on the δ/γ phase transformation are discussed based on the experimental results and mathematical calculation. Phosphorus is shown to decrease the TA4 temperature and to increase the TA3 temperature at various cooling rates. This effect is enhanced by the redistribution of phosphorus from the γ phase to the δ phase during the transformation. Some retained δ phase is kept until the γ/α phase transformation in steel containing high phosphorus (0.2% P). The retained δ phase could retard the γ grain growth and promote the α phase precipitation. At the slow cooling rate, the γ cells appear first from the triple points or δ grain boundaries and then spread with finger-like patterns. While at the rapid cooling rate, the γ cells appear first from the δ grain boundaries with sword-like patterns, and spread sharply into both sides of initial δ grain boundary.