两相区退火处理冷轧0.1C-5Mn中锰钢的氢脆敏感性

Hydrogen Embrittlement of Intercritically Annealed Cold-Rolled 0.1C-5Mn Steel

对0.1C-5Mn中锰钢冷轧后在650℃进行不同保温时间的两相区逆相变退火处理,利用电化学充氢和慢应变速率拉伸(SSRT)实验研究了其氢脆敏感性。结果表明,冷轧后中锰钢在退火过程中发生奥氏体逆转变,在退火10 min时可获得优异的强度和塑性配合。随着退火时间延长,可扩散H含量及氢脆敏感性增加,特别是氢脆敏感性的增加幅度十分显著。充氢断口起裂区呈现典型的空心韧窝及包含奥氏体(变形后转变为马氏体)晶粒的实心韧窝的混合断裂模式,这种实心韧窝本质上是在应力作用下氢致裂纹沿奥氏体与铁素体的界面萌生与扩展而形成的一种脆性沿晶断裂。氢脆断裂行为主要与退火过程中逆转变奥氏体的含量及其机械稳定性等因素有关。

Medium-Mn steel typically alloyed with（3%-10%）Mn（mass fraction） has recently regained significant interest as one of the most promising candidates for the third-generation automobile steel due to its excellent combination of ultra-high strength and ductility as well as relatively low material cost and industrial feasibility. Considering the ever increasing strength level as well as the comparatively high amount of reverted austenite（RA） of medium-Mn steel, special attention began to be given to its hydrogen embrittlement（HE） behavior for ensuring the safety service of components made of this kind of steel. However, the effect of RA on HE of medium-Mn steel has not been fully understood. For this purpose, the susceptibility to HE of a cold-rolled medium-Mn steel 0.1 C-5 Mn intercritically annealed at650 ℃ for different time to obtain different amounts of RA was investigated by using electrochemical hydrogen charging, thermal desorption spectrometry（TDS）, slow strain rate test（SSRT） and SEM. The results show that the annealed samples exhibit a dual-phase microstructure of reverted globular shaped RA and ferrite. The ultimate tensile strength（sb） increases while the yield strength decreases with increasing annealing time, and both the total elongation（d） and the product of sbto d（sb×d） initially increase and then decrease with increasing annealing time. That is to say, an excellent combination of strength and ductility could be obtained when the tested steel was annealed at 650 ℃ for 10 min. However, the results of TDS and SSRT show that both the absorbed diffusible hydrogen concentration and the susceptibility to HE increase with increasing annealing time, and the latter is more significant. SEM analysis of the fracture surfaces of fractured samples revealed that the hydrogen-charged annealed sample was fractured to leave both dimples filled with grains and empty dimples while the uncharged annealed specimen was ductile fractured to leave only empty dimples. The dimples filled with grains were basically a brittle intergranular cracking occurring along the boundaries of RA and/or martensite（formerly RA） grains by the hydrogen-assisted cracking mechanism. It is thus concluded that the HE behavior of intercritically annealed cold-rolled medium-Mn steel is primarily controlled by both the amount and mechanical stability of RA.