新型热轧纳米析出强化超高强汽车板的疲劳性能研究

Study on Fatigue Property of New Type Hot-rolled Nano Precipitation Strengthening Ultra-high Strength Automobile Strip

利用MTS-810疲劳试验系统进行高周疲劳试验,研究新型热轧Nb-Ti微合金化抗拉强度700 MPa级车厢板和780 MPa级大梁的疲劳性能,探讨晶粒尺寸、第二相(析出物、夹杂物)等对疲劳性能的影响机理。结果表明,在载荷比R=–1和循环基数为107条件下,700 MPa级车厢板和780 MPa级大梁钢的条件疲劳强度分别为438 MPa和443 MPa,疲劳强度比分别为0.61和0.57,明显高于一般钢材;试验钢的疲劳裂纹源包含驻留滑移带(Persistent slip bands,PSB)和大尺寸析出物或夹杂物;析出物尺寸越细小,发生裂纹失稳扩展所需要的临界断裂应力越高,不易形成疲劳裂纹源;第二相尺寸相当的前提下,带尖角的方形第二相较圆形或椭圆形第二相更易在交变载荷过程中发生应力集中,形成疲劳裂纹源;晶粒的超细化、析出物的纳米化、颗粒状或短棒状碳化物、低夹杂物水平是新型超高强汽车板具有优良抗疲劳性能的主要原因。

By high cycle fatigue test on MTS-810 material testing system, the fatigue property of new developed hot-rolled Nb and Ti microalloying 700 MPa grade carriage strip and 780 MPa grade crossbeam strip are studied, and the effects of grain size, second phase （precipitation and inclusion）, high/low angle grain boundaries and carbide on fatigue property are investigated. The result indicate that, when the load ratio R of-1 and cycle base number of 107, the conditions fatigue strength of 700 MPa grade carriage strip and 780 MPa grade crossbeam strip are 438 MPa and 443 MPa, respectively. The fatigue strength ratio of 700 MPa grade carriage strip and 780 MPa grade crossbeam strip are 0.61 and 0.57, respectively, which higher than general steel. Fatigue crack source of experimental steel are persistent slip bands（PSB） and coarse precipitation or inclusion. When the size of precipitation is small, the critical fracture stress of crack instability propagation is higher, so nano scale precipitation very hard to become fatigue crack source. When the size of second phase is nearly, contrast to rounded closed angle coarse precipitation or inclusion, the quadrate precipitation or inclusion are easily to generate stress concentration and become to fatigue crack source. The reasons of experimental steels having excellent anti-fatigue performance are ultra fine grain size, nanometer precipitation and granular or short bar carbides.