Cooling rate-dependent microstructural evolution: Segregation and precipitated phase in S30432 steel for ultra-supercritical power plants

The solidification characteristics, elemental segregation behavior, and the formation and growth of precipitated phases of S30432 steel at various cooling rates were investigated. The relationship between the solidus and liquidus of steel and the cooling rate is denoted as T S o l = 253.00 e − C R 244.68 + 1183.58 and T L i q = 201.92 e − C R 166.77 + 1284.17 , respectively. The solidification microstructure of steel becomes finer with the increase in cooling rate. The relationship between the secondary dendrite arm spacing and the cooling rate is as follows: λ 2 = 178.50 · C R − 1 / 3 . The segregation index of Nb increases from 2.326 to 2.836 as the cooling rate increases from 5 °C/min to 1800 °C/min. Additionally, the proportion of precipitated phases smaller than 2 μm increases from 20.00 % to 53.54 %. The calculation results show that the solid fraction of NbN beginning to precipitate decreases as the cooling rate increases. However, the acceleration of the cooling rate significantly shortens the growth time of the NbN, leading to an increase in the area fraction of the NbN, but a decreasing trend of their size.