Abstract: Coarse austenite grain formed in the initial solidification process is an important factor to affect the transverse cracking of medium carbon microalloyed steels.The initial solidification process of P510L steels were observed by the confocal laser scanning microscope in order to inspect the formation of δ and γ phases and the peritectic reaction，and explore the temperature for the completion of γ transformation. The results showed that:1) With the cooling rate of 25℃/s， the cooling mode of P510L steel was that δferrite precipitated from the liquid phase first， then the peritectic reaction (L+δ→γ)occured between the δferrite phase and the liquid phase， then they entered to the threephase coexistence region. With the liquid phase disappeared， the remain δ phase transformed into γ phase by the solid state diffusion;2) In the initial solidification process， parts of the austenite grains annexed and grew up first， then the remain δferrite transformed into austenite with the allotropic heterogeneous， and the fully austenite region achieved at last; 3) A more accurate method about determining the temperature for the completion of γ transformation was sought by insitu dynamic observation， and the accuracy of a model for predicting the austenite grain size was thus improved.

Key words: coarse austenite, insitu observation, initial solidification, temperature for completion of austenite transformation, a model for predicting the austenite grain size