临界区退火对热轧中锰TRIP钢P偏聚的影响

Effect of intercritical annealing on phosphorus segregation in hot-rolled medium-manganese TRIP steel

用场发射扫描电子显微(FE-SEM)和透射电镜(TEM)结合能谱仪,观察热轧中锰TRIP钢在不同退火温度、相同时效制度下的P偏聚行为。结果显示,650℃退火+时效后的试样更容易观察到P偏聚区,呈散点状,较为弥散地分布在视场内,而750℃退火+时效后的试样P偏聚更为集中,呈狭长带状分布,偏聚区集中而数量少。2种退火温度下,试样中的P偏聚区都与碳化物重叠分布在一起。650和750℃均为试样临界区(铁素体+奥氏体)温度,同一温度下,P在铁素体中的扩散系数是在奥氏体中的扩散系数的26倍,P更容易在铁素体晶界偏聚,但扩散系数不足以解释退火温度对于P偏析的影响。根据电子背散射衍射(EBSD)菊池线分析,含P析出相主要为正交系的FeP相及MnxFe1-xP的(Fe、Mn)P固溶相,与Fe-Mn-C-P系优势区图计算结果一致,属于FeP相与Mn7C3碳化物相共存区。

Field emission scanning electron microscope and transmission electron microscope combined with energy dispersive spectrometer were used to observe the phosphorus segregation behavior in hot-rolled medium-manganese TRIP steel.The samples were annealed at different temperatures,followed by the same aging process at 560℃ for 50h.The results show that particle-like phosphorus is dispersively distributed in the samples annealed at 650℃, while the sample annealed at 750℃ shows uneven narrow-band-like phosphorus distribution.The entire phosphorus segregation zone overlaps with carbides precipitation.Both 650and 750℃ are intercritical region (ferrite + austenite)temperatures.The segregation usually occurs at ferrite grain boundaries.At the same temperature,the diffusion coefficient of phosphorus in ferrite is 26 times than in austenite.However,it is insufficient to explain the effect of annealing temperature on phosphorus segregation by diffusion coefficient.According to the Kikuchi line analysis in the electron backscatter diffraction,the phosphorus-containing precipitate phase is mainly FeP in orthogonal crystal system and MnxFe1-xP solid solution,which is in accord with the calculation results in Fe-Mn-C-P system by Factsage software.