研究了在对称应变控制条件下1100 MPa级调质态高强钢的低周疲劳性能，借助OM、SEM、TEM等手段对高强钢在低周疲劳载荷下的微观组织、断口形貌、裂纹扩展特性、夹杂物形态等进行了研究。结果表明，调质态1100 MPa高强钢具有优异的低周疲劳性能，主要有2个原因：一是由于夹杂物形态为近圆形，直径为2~5 μm，低于夹杂物引起疲劳裂纹萌生的临界尺寸，裂纹萌生于试样表面，提高了疲劳裂纹萌生寿命；二是原奥氏体晶界、马氏体板条包/束界、夹杂物/孔洞都会诱导裂纹偏转，使裂纹走向曲折，降低了裂纹扩展速率，提高了疲劳裂纹扩展寿命。

The low-cycle fatigue (LCF) behavior of 1100 MPa grade tempered high strength steel under symmetrical strain control conditions was studied at the strain amplitude ranges of 0.4%~1.2% in this work. The LCF properties of quenching and tempering high strength steel were examined by means of OM, SEM and TEM. The microstructure changes, fracture morphology, crack propagation characteristics and inclusion morphology were studied in detail. The results show that the cyclic hardening and cyclic softening depend on strain amplitude. At the low strain amplitude of 0.4%, rapid cyclic hardening occurs in initial 10 cyc, and then the stress remains almost unchanged until the sample breaks. While at the strain amplitude ranges of 0.5%~1.2%, the cyclic hardening reaches a peak at the first few cycles, followed by the remarkable cyclic softening until the sample fails. The cyclic softening is mainly due to the recovery of some martensite lath under low-cycle fatigue loading and the decrease of dislocation density in the laths. 1100 MPa grade high-strength steel is found to obey LCF Manson-Coffin relationship. The high-strength steel has excellent LCF performance for two main reasons, which is related with the shape and size of inclusions. One is that the shape of the inclusion is nearly circular, and the diameter is 2~5 μm, which is lower than the critical dimension of the inclusions causing fatigue crack initiation. The crack is initiated on the surface of the sample. This increases fatigue crack initiation life. The other one is that the original austenite grain boundary, the martensite packet/block boundary and the inclusions or cavities can induce the crack deflection, reducing the crack propagation rate and increasing fatigue crack propagation life.