| Super austenitic stainless steel is a kind of austenitic stainless steel with high Cr,Ni,Mo and up to 50%of alloy elements content.It has excellent mechanical properties and corrosion resistance with pitting resistance equivalent number≥40.However,the higher alloying degree leads to the redistribution and diffusion of a large number of solute elements in the solidification process of super austenitic stainless steel,which easily produce serious segregation and a large number of hard brittle phase precipitation,and form coarse solidification microstructure and columnar crystal.It seriously deteriorates the hot workability and corrosion resistance of super austenitic stainless steel,and limits the further development of the super austenitic stainless steel preparation and application.In this paper,the solidification microstructure evolution,element segregation and second phase precipitation of 7Mo super austenitic stainless steel were systematically studied by thermodynamic calculation,mold casting experiment,directional solidification experiment,high temperature confocal microscope in-situ observation experiment and continuous casting solidification simulation experiment.The effects of rare earth Ce on microstructure and element segregation were investigated,and the influence of solidification parameters on solidification microstructure and element segregation were analyzed by continuous casting simulation experiments.The main research results are as follows:(1)The solidification microstructure and solidification path of super austenitic stainless steel were predicted by thermodynamic software.The results show that the equilibrium solidification path of super austenitic stainless steel is L→L+γ→L+y+δ→γ.The non-equilibrium solidification path is L→L+γ→L+γ+δ→L+γ+δ+σ→L+γ+σ→L+γ+σ+hcp.The main precipitated phase is σ phase,and the precipitation temperatures in non-equilibrium solidification is 1292℃.The solidification path and subsequent solid phase transformation of super austenitic stainless steel were determined by analyzing the directional solidification microstructure with solid-liquid interface:L→L+γ→L+γ+δ→γ+δ→γ+δ+(σ→γ2)→γ+(σ+γ2).The precipitation mode of σ phase is:σ phase is formed by eutectoid decomposition of δ-ferrite(δ→σ+γ2).(2)The solidification microstructure of 500 kg super austenitic stainless steel ingot was experimentally observed.The results show that the solidification microstructure of the ingot along the radial direction is significantly different.The core microstructure of the ingot is coarse,and the surface layer microstructure of the ingot is fine.The average secondary dendrite spacing of the measured core is 97.5 μm,and the surface layer is 26.4 μm.The size and morphology of the precipitated phase change significantly.The size of the precipitated phase in the core of the ingot is coarse and connected into a network.The size of the precipitated phase at the edge of the ingot is small and discontinuous.The cooling rate at different positions of the ingot and the variation of the secondary dendrite arm spacing and cooling rate at different positions of the ingot was obtained by simulating the solidification process of the ingot.The relationship between secondary dendrite arm spacing and cooling rate of super austenitic stainless steel ingot is λ2=215.88(CR)-0.29.(3)The micro-segregation of the ingot was measured.The results showed that the Ni element in the super austenitic stainless steel had negative segregation,and the Cr Mo and Mn elements had positive segregation,the segregation of Mo element was the most serious.From the edge to the center of the ingot,the segregation coefficient of Mo increases continuously,and the edge,3/4R,1/2R,1/4R and core of the ingot are 1.03,1.21,1.24,1.25 and 1.35,respectively.(4)The rare earth Ce element significantly refined the solidification microstructure of super austenitic stainless steel.It is beneficial to reduce the secondary dendrite spacing,reduce the element segregation and inhibit the second phase precipitation.The Ce promotes the solidification nucleation of super austenitic stainless steel and inhibits grain growth.The combined effect of two factors refines the solidification microstructure of super austenitic stainless steel.(5)The solidification process of super austenitic stainless steel hot simulated continuous casting slab was studied by continuous casting simulation test device,It was found that the secondary dendrite spacing at the solid-liquid interface decreases from 31.77 μm to 26.61 μm with the increase of casting speed.The secondary dendrite spacing at the solid-liquid interface decreases from 28.45 μm to 27.82 μm with the increase of superheat.The solute partition coefficient of Mo element is more sensitive to the change of cooling rate and superheat than other elements. |
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