对20Mn2SiCrNiMo贝氏体/马氏体复相高强钢进行淬火-配分-回火(Q-P-T)工艺处理, 并采用紧凑拉伸试样进行疲劳实验. 结果表明, Q-P-T工艺参数对贝氏体/马氏体复相高强钢的疲劳断裂特性具有显著影响, 经过合理的Q-P-T工艺处理的贝氏体/马氏体复相高强钢具有较高的疲劳门槛值(ΔKth=13.2 MPam1/2)及较低的裂纹扩展速率. 显微组织及裂纹扩展路径观察显示, Q-P-T处理的贝氏体/马氏体复相高强钢的显微组织包括贝氏体、马氏体和残余奥氏体薄膜, 其中“柳叶状”贝氏体及纳米级残余奥氏体薄膜可以阻碍裂纹扩展, 迫使裂纹转折、分叉或“间断”, 这是贝氏体/马氏体复相高强钢疲劳断裂性能改善的主要因素.
Recently, low-cost advanced high strength steels (AHSS) with high toughness and fatigue limit have been developed in order to ensure the safety and lightweight of the engineering components. As promising candidates for next generation of AHSS, the bainite/martensite multiphase high strength steels exhibit excellent combination of strength and toughness due to the refined multiphase microstructure and retained austenite films located between bainitic ferrite laths. The previous works showed that the mechanical properties of bainite/martensite multiphase steels can be further improved through quenching-partitioning-tempering (Q-P-T) process. In the present work, the effect of Q-P-T process on the microstructure and fatigue behaviors of steels was investigated, and the relationship between the microstructure and the fatigue crack propagation was discussed in detail. Here, a 20Mn2SiCrNiMo bainite/martensite multiphase steel was treated by Q-P-T processes: (1) quenching to 200 ℃, partitioning at 280 ℃ for 45 min and finally tempering at 250 ℃ for 2 h (QPT200 sample); (2) quenching to 320 ℃, partitioning at 360 ℃ for 45 min and finally tempering at 250 ℃ for 2 h (QPT320 sample). Microstructure observations showed that the QPT200 sample consisted of leaf-shaped bainite, martensite and filmy retained austenite (RA), while some blocky martensite/austenite (M/A) islands were observed in QPT320 sample. The volume fractions of retained austenite in QPT200 and QPT320 samples are 4.5% and 9.8%, respectively. The fatigue crack propagation rate da/dN and threshold value of fatigue cracking ΔKth were measured using compact-tensile specimens. The results showed that the Q-P-T process parameters had a significant influence on the microstructures and fatigue properties of the bainite/martensite multiphase steels. The bainite/martensite multiphase steel after appropriate Q-P-T treatment (QPT 200 sample in the present work) has higher ΔKth and lower da/dN, which originates from the resistance on fatigue crack propagation due to the presence of leaf-shaped bainite and nanometer-sized retained austenite films. Furthermore, although the volume fraction of retained austenite in QPT320 sample is higher than that in QPT200 sample, the ΔKth of QPT 320 sample is lower than that of QPT200 sample. It is suggested that the effect of retained austenite on the fatigue behaviors depends on its volume fraction, size and morphology.
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