| Ultra-high strength stainless steel is widely used in aerospace,military ships and other important fields because of its ultra-high strength,good toughness-plastic matching and excellent corrosion resistance.In order to obtain excellent strength and toughness,high strength stainless steel is usually prepared by vacuum induction and vacuum consumable remelting.However,due to the high alloy content of ultra-high strength steel,in the process of large ingot smelting,the cooling rate difference between different positions of ingot is large,which is easy to cause composition segregation and thus affect the comprehensive properties of materials.Therefore,in this thesis,the characteristics of composition segregation and high temperature homogenization process of large-size ingot of ultra-high strength stainless steel（USS250）in C-Cr-Ni-Mo-Al-Ti alloy system are studied.By means of thermodynamic calculation,high-temperature laser confocal and directio nal solidification,the solidification structure characteristics and segregation law of USS250 ultra-high strength stainless steel ingots at different cooling rates were studied,and the vacuum consumable remelting smelting parameters were optimized by means of Meltflo w-VAR simulation calculation.According to the simulation optimization results,the vacu um consumable remelting ingot smelting of USS250 steel was carried out,and the as-cas t structure was anatomically analyzed.According to the theory of residual segregation co efficient and Fick’s second law,the homogenization system of sample-grade ingot was de signed and optimized,and the influence of homogenization system on ingot segregation was analyzed by EPMA and other means.The specific results mainly include:Through thermodynamic calculation and experimental results,it is found that the pha se transformation sequence of USS250 ultra-high strength stainless steel during cooling a nd solidification is as follows:L→L+δ→L+δ+γ→δ+γ→γ+MC→γ+MC+Laves→γ+MC+Lav es+α→γ+MC+Laves+α+Ni3（Al,Ti）.Through directional solidification experiments,it is foun d that during the solidification process of USS250 ultra-high strength stainless steel,the i ncrease of cooling rate will refine the dendrite structure of ingot and reduce the segregat ion degree of elements.The segregation of Ni,Mo and Ti elements in ingot is serious,while the segregation degree of Cr and Al elements is light.Among them,Ni,Mo,Ti a nd Al are enriched in dendrites,while Cr and C are enriched in dendrites.The functiona l relationship?2=86.7（GL R）-1/3 between secondary dendrite spacing and cooling rate was fi tted.The simulation results of Meltflow-VAR software show that with the increase of melting rate,the steady-state morphology of metal pool develops from"shallow flat"to"arc"and then to"deep V".With the increase of melting speed,the concentration gradient of elements in ingot increases and the segregation situation intensifies.At 3.2～4.2 kg/min low melting rate,the depth of molten pool is stable and the degree of element segregation is light,which is beneficial to improve the uniformity of structure.However,at the melting speed of 3.2 kg/min,the melting time is longer and the secondary dendrite spacing increases.Based on comprehensive consideration,it is determined that the melting rate of USS250 ultra-high strength stainless steel should be maintained at 4.2 kg/min,and the anatomical analysis of the ingot melted at this melting rate shows that the ingot has good composition uniformity.The homogenization kinetics curve of USS250 ultra-high strength stainless steel was drawn by Fick’s second law and residual segregation index,and the influence of different homogenization systems on microstructure microsegregation of ingot was studied.The results show that the dendrite structure in the ingot basically disappears and the element distribution is more uniform under the high temperature homogenization system of 1220℃*4 h. |
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