Microstructure characteristics of spot segregation of 25CrMo steel ingot
ZOU Chang-fei1,2,YANG Jie-ming3,WEI Xian-yi1,2,LIANG Yu1,2,YANG Ming1,2
(1. School of Materials and Metallurgy, Guizhou University, Guiyang 550025, Guizhou, China 2. The Key Laboratory for Mechanical Behavior and Microstructure of Materials of Guizhou Province, Guiyang 550025, Guizhou, China 3. Shougang Guiyang Special Steel Co., Ltd., Guiyang 550000, Guizhou, China)
Abstract:The freckle type segregation area of CrMo steel ingot was analyzed in the current study, which show that the elements of segregation area were stretching along the direction of forging with smaller size, the segregation in the primary ingot casting would be called the similar size of the spot segregation but in three dimensions. In this segregation area, with the relatively high element concentration of Cr and Mo, the bainite structures were formed after cooling and distributed in cross with the structures of the ferrite and pearlite in the matrix structure. Besides, the enrichment of inclusions in segregation area was obvious and partial areas existed air hole existed in some areas. The emergence of spot segregation in steel mainly was due to reducing the degree of under cooling at the front of solidified shell. Under constitutional super cooling condition, highly melting point constituent element Cr and Mo will partially enrich on the surface of dendritic with the temperature decreasing, the MnS inclusion in liquid steel was gathered by physical absorption of the surface of dendritic. At the same time,the viscosity of liquid phase in dendritic area increased, leading to the difficult release of gas, which will partially form air hole after solidification. The formation of spot segregation in ingots will be effectively reduced by improving the solidification rate of liquid steel and reducing its retention time in solid-liquid phase region.
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