| Super austenitic stainless steel is a single-phase austenitic stainless steel with high alloying elements（such as Cr,Ni,Mo and N）,low carbon content and copper content.The high Mo content greatly improves the pitting corrosion resistance and crevice corrosion resistance of super austenitic stainless steel S31254.However,during the production process of the ingot,due to the different cooling rates at different positions,the element diffusion rates are inconsistent,resulting in serious precipitation at the grain boundary front.A large number of precipitation phases make it difficult to melt back,and it is easy to cause grain growth during the melt back process.Secondly,the precipitation of the precipitation phase also causes an increase in flow stress,which makes hot processing difficult,easily leads to rolling cracks,and reduces the yield.Therefore,improving the composition segregation during the solidification process of the as-cast structure,reducing the precipitation of the precipitates during the solidification and deformation process,and reducing the hot rolling cracking have become the urgent problems to be solved by S31254.This paper mainly focuses on the element segregation.Serious secondary phase precipitation and high hot deformation resistance caused by the super austenitic stainless steel S31254（6Mo）during the preparation and hot working process.Two super austenitic stainless steels,S31254 and S31254-B（added 40 ppm boron）,were prepared.The effect of boron on the precipitation and microstructure of S31254 was systematically studied.Through the homogenization meltback experiment,the optimal homogenization meltback system was explored and the effect of B element addition was compared.The temperature of 950℃,1000℃,1050℃,1100℃,1200℃and the strain rate of 0.01 s-1,0.1 s-1,1 s-1,10 s-1 are desinged.The thermal deformation experiment with the deformation amount of 70%.Through the analysis of the stress-strain curve,the effects of temperature,strain rate and deformation on the flow stress in the hot deformation of stainless steel are obtained.Due to the severe precipitation at 950°C,the role of B in the thermal deformation process of 950°C and 0.01s-1 was analyzed and compared between S31254 and S31254-B.Scanning electron microscopy（SEM）and electron backscatter diffraction（EBSD）were used to study the effect of B addition on the microstructures such as deformation degree,deformation precipitation,microstructure cracking and recovery recrystallization during thermal deformation.The main conclusions of this study are as follows:（1）Through the observation of the as-cast microstructures of S31254-B and S31254,it is found that the element segregation is the most serious at the central position of the two samples,forming well-developed dendrites or equiaxed crystals.Among them,Mo element segregation is the most serious,and Mo-richσphase is formed between dendrites.Compared with S31254,the size of dendrites in S31254-B is significantly reduced,the segregation of Mo element is reduced,the number of precipitates is less,and the size is reduced.In the subsequent homogenization and remelting process,the optimal heat treatment system of S31254 is 1250℃,4h,and the optimal heat treatment system of S31254-B is 1200℃,4h,1250℃,2h.The B element accelerates the homogenization and melting back of S31254.（2）Through the analysis of the stress-strain curve about the hot compression test,it is found that the dynamic recrystallization recovery of the S31254 and S31254-B samples is the most obvious when the strain rate is 0.01 s-1 and 0.1 s-1.The flow stress decreases as the heat distortion temperature and the strain rate increases.In the curve at 950℃,0.01 s-1,the work hardening rate and flow stress obviously decrease with the addition of B.（3）The addition of B reduces the precipitation during the deformation process,making the deformed structure more uniform,and slowing down the tendency of cracking.Especially near the edge,the microcracks of S31254-B are obviously less than those of S31254.（4）The grain analysis of the deformed structure shows that the B element significantly increases the proportion of subgrains and recrystallized grains,reducing the proportion of deformed grains,and promoting the“softening”of the structure by dynamic recrystallization.Secondly,the addition of B will also change the distribution of dislocations,slow down the accumulation of dislocations,reducing the dislocation density.Overall,the addition of element B improves the hot workability of S31254 super austenitic stainless steel. |
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