25CrMo钢锭点状偏析区微观组织特点

 为了研究钢锭点状偏析区微观组织特点，对CrMo钢锻坯中的斑点偏析区域进行了分析研究。研究表明，钢坯中的元素偏析区沿锻造方向拉伸且尺寸较小，可判断原铸锭中的偏析为3个维度方向尺寸相近的点状偏析。偏析区中铬、钼元素质量分数相对较高，在冷却后点状偏聚区形成贝氏体组织，与基体的铁素体、珠光体组织呈交错分布的特征，且夹杂物在偏析区富集比较明显，局部偏析区还存在气孔。钢中的点状偏析的出现主要原因是钢液凝固前沿过冷度降低，在成分过冷条件下，高熔点铬、钼组元在枝晶表面局部富集，随温度的下降，钢液中MnS夹杂物同时受枝晶表面的物理吸附作用聚集，同时增加了枝晶区域液相的黏度，导致气体溢出困难，在凝固后局部形成气孔。通过提高钢液凝固速度，降低钢液在固液两相区的停留时间，将有效缓解钢锭中斑点偏析的形成。

Microstructure characteristics of spot segregation of 25CrMo steel ingot

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 pri-mary ingot casting would be called the similar size of the spot segregation but in three dimensions. In this segregation ar-ea,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 inclu-sions in segregation area was obvious and partial areas existed air hole existed in some areas. The emergence of spot seg-regation in steel mainly was due to reducing the degree of under cooling at the front of solidified shell. Under constitu-tional 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 sur-face 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.