回火温度对高强弹簧钢微观组织和冲击性能的影响

Effect of Tempering Temperature on Microstructure and Impact Properties of Two High-strength Leaf Spring Steels

用光学显微镜、扫描电镜和透射电镜观察了两种高强弹簧钢50CrMnMoVNb和50CrMnSiVNb钢的微观组织,并对其冲击韧性进行了对比分析。结果表明:50CrMnMoVNb钢的带状偏析程度比50CrMnSiVNb钢更显著,大角度晶界占总晶界比例更高,回火脆性程度更轻。两种弹簧钢冲击韧性的对比结果表明,淬火和在150~400℃回火的50CrMnSiVNb钢其冲击韧性更优。此温度范围内回火的冲击韧性主要受带状偏析程度的影响,偏析带更容易发生解理断裂,进而使冲击裂纹扩展路径更平直;而在400~500℃回火后50CrMnMoVNb钢的冲击韧性更优,主要受回火脆性和大角度晶界比例的影响。在回火过程中板条界面处的薄膜状碳化物使回火脆性大幅度恶化冲击韧性,而大角度晶界对裂纹扩展更强的阻碍作用消耗了更多的能量,使冲击韧性提高。

The effect of tempering temperature on the microstructure and impact toughness of two high-strength leaf spring steels 50CrMnSiVNb and 50CrMnMoVNb for automobile was comparatively studied by means of optical microscope,scanning electron microscope,transmission electron microscope and impact tester.The results show that compared with those of 50CrMnSiVNb steel,there are more segregation bands along with a larger proportion of large-angle grain boundaries in the microstructure of 50CrMnMoVNb steel,while the later steel shows less temper brittleness.When comparing the impact toughness of the two leaf spring steels,it is found that being quenched and then tempered in the range of 150~400℃for the two steels,the 50CrMnSiVNb steel presents better impact toughness.The impact toughness of the steel tempered in this range is mainly affected by the degree of banded segregation,which is more prone to cleavage fracture and leads to a straighter impact crack propagation path;In the contrast,after the two steels were tempered in the range of 400~500℃,the 50CrMnMoVNb steel shows better impact toughness,and the impact toughness in this region is mainly affected by the tempering brittleness and the proportion of large-angle grain boundary.The tempering brittleness caused by the thin-film like carbides at the interface of the laths during tempering greatly worsens the impact toughness,while the large angle grain boundary has a stronger barrier effect to crack propagation and consumes more energy,leading to the improvement of the impact toughness.