Level Of T91 Heat-resistant Steel Continuous Casting Solidification Process Simulation And Microstructure Research

T91(9Cr-1Mo-V-Nb-N) is the representative of9-12%Cr ferritic/martensitic heat-resistant steels, which has been widely used in high temperature structural components such as main steam pipe, superheater tube and resuperheater tube in supercritical power plants due to excellent high temperature properties. At present, many tube mill plants use rolled round billets and continuous casting round billets to manufacture T91seamless tube. Compared to the rolled round billet, although the continuous casting one is simple to produce, its quality is poor, leading to low yield. It is well known that there is close relationship between the quality of billet and solidification process, so it meaningful to investigate to solidification process and microstructure of billet to improve its quality and yield. In this work, heat transfer behavior and equilibrium phase constitutes as function of temperature during horizontal continuous casting of T91steel are studied by combing numerical calculation and experiments, getting the following results:(1) It is found by numerical simulation of COMSOL Multiphysics software that the heat flux density along copper sleeve is greater than that along graphite one, surface reheating occurs and the temperature of slab surface is increased by100℃at intersection of copper sleeve and graphite one due to their different conductivity and intensity of cooling. Moreover, the temperature rise is about100℃when casting speed increases from2.1m/min to3.3m/min at constant superheat, while surface reheating decreases by20℃, which reduce propensity of internal cracking. In the meantime, at the constant casing speed, the temperature rise is only about20℃when the degree of superheat increases from30℃to45℃, which indicates casting speed has more obvious effect on surface temperature than superheat.(2) The precipitates of T91steel as function of temperature during equilibrium solidification is studied using Thermo-Calc calculation software, the result indicates MX carbonitrides, M23C6carbides and Z-phase are major equilibrium precipitates at400℃～1600℃, the initial precipitation temperature of M23C6and MX is866℃and1264℃, respectively. With decreasing temperature, the Z-phase progressively precipitates at expense of MX at about770℃, which imply that a-Fe, M23C6, Z-phase are major equilibrium phases in T91steel during service temperature.(3) The α-Feâ†'γ-Fe transformation start temperature Ac1and magnetic transition are determined to be856℃and749℃, respectively at heating during differential thermal analysis for a normalised and tempered sample, thereinto Ac1agree well with calculated equilibrium phase temperature Aei (=820℃); the martensite transformation is clearly observed at around400℃during cooling. Microstructure characterization indicates that the matrix microstructure of T91steel is tempered martensite that has bcc structure, M23C6carbides mainly distribute along prior austenite grain boundary and lath boundary, while MX carbonitrides precipitate within grain. Moreover, no Z-phase is observed.