采用超高温激光共聚焦扫描显微镜对15CrMoG钢包晶凝固过程进行了原位动态观察。发现冷却速率为5和15 ℃/min时，δ相以胞状方式析出；而当冷却速率增加至100 ℃/min时，δ相以枝晶方式析出。通过包晶相形核热力学分析表明，初始δ相凝固过程中L/δ界面处浓度梯度的存在增加包晶γ相Gibbs自由能成核势垒。随着冷却速率的增加，穿过L/δ界面浓度梯度变陡，导致包晶相γ形核所需过冷度增加，进而降低了包晶反应温度和提高了包晶反应速率。另外，冷却速率的增加导致包晶转变(δ→γ)模式发生改变，冷却速率为5 ℃/min时，δ→γ转化界面呈现溶质扩散控制的平面形态；冷却速率为15 ℃/min时，δ→γ转化界面呈现溶质扩散控制的胞状形态；而冷却速率为100 ℃/min时，出现界面过程控制的δ→γ块状转变。基于不同包晶转变δ→γ模式体积收缩的差异，讨论了亚包晶钢连铸调控机理。

Cast defects of hypo-peritectic steel such as uneven growth of strand shell, crack formation and oscillation marks formation were found to occur frequently during continuous casting of steels. In industry, measures such as high-basicity casting powder, hot-top mold and reduction of mold cooling strength were usually used in the investigations, but a reasonable explanation for these measures has been lacking. In this work, solidification of 15CrMoG steel at different cooling rates were observed with an ultra high temperature confocal scanning laser microscope. The precipitation of the δ-phase was in a cellular manner when the cooling rates were 5 and 15 ℃/min, whereas it was in a dendrite manner when the cooling rate was increased to 100 ℃/min. Thermodynamic analysis of the peritectic phase nucleation showed that a concentration gradient existed at the L/δ interface during the solidication of initial δ phase which led to an increase in the Gibbs free energy barrier for the nucleation of the peritectic γ phase. As the cooling rate increased, the concentration gradient across the L/δ interface became steeper, resulting in an increase in the nucleation undercooling of the peritectic γ phase. This, in turn, decreased the temperature and increased the peritectic reaction rate. In addition, an increase in the cooling rate led to a change in the mode of peritectic transformation (δ→γ). A diffusion-controlled δ→γ transformation occurred due to the progression of planar and cellular interfaces at cooling rates of 5 and 15 ℃/min, respectively. However, a large δ→γ transformation, which was controlled by the interface process, occurred when the cooling rate was increased to 100 ℃/min. The difference in volume shrinkage of the different modes of peritectic transformation (δ→γ) led to a discussion of the control mechanism of continuous casting of hypo-peritectic steel.