In Situ Observation of P91 High‐Alloy Steel Solidification Characteristics at Different Cooling Rates

To clarify the effect of different radial cooling intensities on the formation of central cracks in large round bloom continuous casting, it is necessary to study the solidification characteristics and dynamics of P91 high‐alloy steel at different cooling rates (CRs) to improve the central defects. In this article, the solidification characteristics of P91 high‐alloy steel at different CRs are studied by using the Thermo‐Calc software, high‐temperature laser confocal microscopy, scanning electron microscopy, and optical microscopy. Meanwhile, the growth kinetics of δ‐Fe and γ‐Fe phases under different CRs are determined. The results show that the solidification path of P91 high‐alloy steel is L(L + δ‐Fe) → (L + γ‐Fe + δ‐Fe) → (δ‐Fe + γ‐Fe). The δ‐Fe and γ‐Fe phase precipitation process is divided into two stages. Stage I is the nucleation and rapid growth phase, in which a high undercooling is required. Stage II is the slow growth stage, where the undercooling decreases and remains constant. The initial growth linear velocities of the δ‐Fe phase are 0.51, 2.72, and 2.09 μm s −1 at CRs of 10, 50, and 100 °C min −1 , respectively, while those of the γ‐Fe phase are 0.10, 1.42, and 1.41 μm s −1 .