为了应对汽车发动机排气温度将大幅升高至1050 ℃的要求, 本工作以新设计的3种不同N含量(0~0.55%, 质量分数)的Nb稳定化奥氏体耐热铸钢为研究对象, 通过在1000 ℃, 50 MPa条件下的蠕变性能测试和蠕变前后的组织分析, 研究了N对奥氏体耐热铸钢1000 ℃蠕变性能的影响规律. 结果表明: 根据N添加量的不同, 合金之间的最小蠕变速率相差接近一个数量级. 合金的主要析出相为NbC, Nb(C, N)和富Cr相. 随着N含量的增加, 合金中NbC转变为Nb(C, N), 其形貌逐渐从草书体状转变为不规则的混合片块状, 并最终转变为多面体块状. 其中, 草书体状NbC能有效强化铸钢的晶界和枝晶间区域, 因而有利于提高高温蠕变性能. 富Cr相在晶界的粗化连接是合金蠕变断裂的主要裂纹源, 不利于蠕变性能. 富Cr相的二次析出还会降低奥氏体基体固溶的C含量, 从而降低其固溶强化能力. 草书体状NbC的析出及富Cr相含量的降低是新型奥氏体耐热铸钢蠕变性能提高的主要原因.

To comply with more stringent environmental and fuel consumption regulations in recent years, automotive gasoline engines equipped with turbochargers are increasingly used to improve fuel efficiency. As a result, exhaust gas temperatures are now reaching 1050 ℃, about 200 ℃ higher than the conventional temperature. Hence, there is an urgent demand in automobile industries to develop novel and economic austenitic heat-resistant steels that are durable against these increased temperatures. In this study, the effects of N addition on creep behavior at 1000 ℃ and 50 MPa are investigated in a series of Nb-bearing austenitic heat-resistant cast steels, which are developed for exhaust component applications. Microstructures before and after creep rupture tests are carefully characterized to illustrate the microstructural evolution during creep deformation. The results of creep tests show approximately an order of magnitude increase in the minimum creep rate among the experimental alloys with variations of N addition. Microstructural analyses indicate that the morphology of NbC and Nb(C, N) is changed from “Chinese-script” to mixed flake-blocks, and then to faceted blocks as N additions increase. The best creep property occurs in an alloy with “Chinese-script” NbC, which could effectively strengthen the grain boundaries and interdendritic regions. The Cr-rich phases are adverse to creep properties, in particular those coarsened and coalesced phases along grain boundaries. They act as crack sources and accelerate the propagation of creep cracks. Moreover, the secondary precipitation of Cr-rich phase results in a significant decrease of C concentration in the matrix and thus reduces the solution strengthening ability during creep deformation. This study suggests that the strengthening of these austenitic cast steels can be achieved through the exploit of primary NbC and Nb(C, N) and the elimination of Cr-rich phases, and therefore, N additions should be strictly controlled.