Nb、Ti对耐热钢中δ铁素体固溶行为的影响

Effect of Nb and Ti on dissolution behavior of delta ferrite in heat-resistant steel

δ铁素体是马氏体型耐热钢中常见的有害析出相,它的存在会对材料的力学性能及耐蚀性能产生不利影响,并且难以通过常规热处理去除,因此如何去除δ铁素体是该领域的研究重点。以C-Cr-Ni-Mo马氏体型耐热钢为主要研究对象,设计了C-Cr-Ni-Mo、C-Cr-Ni-Mo-Nb、C-Cr-Ni-Mo-Ti 3种不同成分的试验钢。采用OM、SEM、EDS等手段,通过对比3种不同成分试验钢铸态组织中δ铁素体体积分数、δ铁素体固溶速率、奥氏体晶粒尺寸,研究了Nb、Ti微合金化对马氏体型耐热钢中δ铁素体固溶行为的影响并揭示其机理,为马氏体型耐热钢中δ铁素体快速固溶提供新方法。试验结果表明,Nb、Ti微合金元素作为铁素体形成元素,会加重合金成分的偏析,冷却过程中未能充分转变,导致室温下铸态组织中δ铁素体的体积分数增加,但生成的δ铁素体尺寸相对较小。δ铁素体的固溶速率与其形态及位置有关,Nb、Ti微合金化能提高δ铁素体的固溶速率,一方面是Nb、Ti微合金化会细化铸态组织中的δ铁素体,提高δ铁素体的比表面积;另一方面是通过形成含Nb、Ti的析出相,钉扎晶界,抑制奥氏体晶粒长大,延缓固溶过程中δ铁素体由不规则状向球状转变的过程,在加热过程中使更多的δ铁素体更长时间地维持在奥氏体晶界处呈不规则状,维持较大的比表面积,进而提高δ铁素体的固溶速率。

Delta ferrite is a common harmful precipitated phase in martensitic heat-resistant steel.Its existence will adversely affect the mechanical properties and corrosion resistance of the material,and it is difficult to be removed by conventional heat treatment.Therefore,how to remove delta ferrite is the focus of research in this field.C-Cr-Ni-Mo martensitic heat-resistant steel is taken as the main research object,and three kinds of experimental steels with different compositions of C-Cr-Ni-Mo,C-Cr-Ni-Mo-Nb and C-Cr-Ni-Mo-Ti are designed.By means of OM,SEM and EDS,the effects of Nb and Ti microalloying on the dissolution behavior of delta ferrite in martensitic heat-resistant steel are studied and the mechanism is revealed by comparing the volume percent of delta ferrite,the dissolution rate of delta ferrite and the austenite grain size in the as-cast microstructure of three different composition experimental steels,which provides a new method for the rapid dissolution of delta ferrite in martensitic heat-resistant steel.The experimental results show that Nb and Ti microalloying elements,as ferrite forming elements,will aggravate the segregation of alloy composition and fail to fully transform during the cooling process,resulting in an increase in the volume percent of delta ferrite in the as-cast structure at room temperature,but the size of the generated delta ferrite is relatively small.The dissolution rate of delta ferrite is related to its morphology and position.Nb and Ti microalloying can increase the dissolution rate of delta ferrite.On the one hand,Nb and Ti microalloying will refine the delta ferrite in the as-cast structure and increase the specific surface area of delta ferrite.On the other hand,by forming precipitates containing Nb and Ti,pinning grain boundaries,inhibiting austenite grain growth,delaying the transformation of delta ferrite from irregular shape to spherical shape during dissolution,more delta ferrite is maintained at austenite grain boundaries for a longer time during heating and maintain a large specific surface area,thereby increasing the dissolution rate of delta ferrite.