Abstract

Medium Mn steels are a class of the new-generation ultra-high-strength materials used in automotives. However, despite excellent ductility, they may suffer from delayed cracking and thus cause serious concerns. In this study, several medium Mn steels were tested with different prestrain and hydrogen charging conditions. The interaction and synergistic effects of prestrain and hydrogen content on hydrogen-induced delayed cracking behavior are investigated. The threshold stress of hydrogen-induced cracking (HIC) decreased during dynamic hydrogen charging under a constant load. In the process of dynamic hydrogen charging, for M7B and M10B steels, the normalized stress intensity factor σ/σ_b and the corresponding threshold stress σ_HIC decreased sharply as prestrain increased. This is because the volume fraction of retained austenite decreased with an increase in prestrain. Similarly, σ_HIC was reduced and the critical hydrogen content dropped drastically with increasing prestrain. For M7C, the influence of prestrain on threshold stress and hydrogen concentration was less than that of M7B. This is because the different treatment processes leads to a different stability of the retained austenite. By observing the SEM fractographs, the fracture surface of medium Mn steels showed different fracture characteristics, such as dimple fractures and intergranular and transgranular modes.

中文翻译：

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预应变对中锰钢氢致延迟裂纹的影响

摘要

中锰钢是汽车中使用的新一代超高强度材料。然而，尽管它们具有出色的延展性，但它们可能遭受延迟开裂的困扰，从而引起严重的关注。在这项研究中，以不同的预应变和充氢条件测试了几种中锰钢。研究了预应变和氢含量对氢诱导的延迟裂化行为的相互作用和协同效应。在恒定载荷下动态充氢期间，氢致裂纹（HIC）的阈值应力降低。在动态充氢，对于M7B和M10B钢中，归一化的应力强度因子的过程σ / σ _b和相应的阈值应力σ _HIC随着预应力的增加急剧下降。这是因为残余奥氏体的体积分数随着预应变的增加而降低。类似地，σ _HIC减少，临界氢含量随预应变急剧下降。对于M7C，预应变对阈值应力和氢浓度的影响小于M7B。这是因为不同的处理过程导致残留奥氏体的稳定性不同。通过观察SEM断口图，中锰钢的断裂表面表现出不同的断裂特性，例如凹痕断裂，晶间和晶间模式。