| T347H austenitic heat-resistant steel is mainly used for high-temperature components of superheaters and reheaters of ultra-supercritical boilers.Based on18Cr-8Ni type austenitic steel,a certain amount of Nb is added to improve the thermal strength of steel by dispersing Nb(C,N)phase.Compared with the traditional ferrite heat resistant steel,it has good creep resistance,steam oxidation resistance and corrosion resistance.TP347H steel has a high Nb content and is prone to liquid chromatography or eutectic precipitation during solidification,leading to coarse Nb-containing particles and cluster or strip distribution,resulting in poor processing performance and low yield of steel.At the same time,it will lead to the decline of the steel’s permanent creep property and affect the service life of the steel tube.In this paper,Nb phase dissolved in TP347H of continuous casting billet and domestic TP347H finished tube,imported TP347H tube,were studied to find out the optimal process to solve segregation,and finally to improve the market.The Nb segregation of TP347H austenitic heat resistant steel has been studied in detail by means of Thermo-Calc,solid solution treatment and XRD,OM,SEM and TEM.ProCAST simulation software was used to calculate and analyze the effects of casting speed and superheat on equiaxial crystal and central segregation of continuous casting billet.The main results are as follows:The effects of different alloy elements content on the structure and properties of TP347H and steel were calculated by Thermo-Calc.The results showed that>0.06%C,>0.6%Nb,>0.4%Si content had obvious influence on the solidification structure of austenitic heat-resistant steel.Ni had significant influence on precipitation temperature and transition end temperature of high-temperature ferrite.Ti and Mn have no obvious effect on the solidification of Nb-containing austenitic steel.The optimal alloying elements were designed as 0.04%C,0.2%Si,2.0%Mn,12.5%Ni,0.4%Nb.Microscopic characterization of 180mm×180mm continuous casting billets showed that Nb distribution in the central area of TP347H steel was extremely uneven,mainly in the form of clusters and chains.Nb content in the core reached about 2times of the average content,and eutectic reactants were layered distribution.[Nb][C]solid solubility product formula of domestic TP347H steel tube and imported TP347H tube at 1200～1350℃was obtained by solid solution treatment,chemical extraction of precipitated powder and XRD analysis of the powder.ProCAST simulation analysis shows that when the superheat is 35℃,the casting speed is 1.15m/min,the cooling water of the mold is 110m~3/h,and the specific water of the secondary cooling zone is 0.25L/kg,increasing the proportion of central fine isoaxial crystal region is the best. |
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