| Delayed fracture is one of the leading factors in affecting high strength of machinery steels. With various kinds of high strength steel sheets, especially of ultra-high strength increasingly being used, the evaluation of the delayed fracture behavior has become very urgent. Up to now, there is a scarcity of studies on delayed fracture behavior of high strength steel sheets, especially of steel sheets after hot stamping. In this paper, Hydrogen induced delayed fracture behavior of automobile ultra-high strength steel sheets after different processing was studied. The effects of hot stamping, carbon content in steel and conventional heat treatment on the delayed fracture behavior of Mn-B type ultra-high strength steel sheets and the effects of austenite content and stability on medium manganese steel with high strength and high ductility were discussed. Therefore, this study may provide theoretical basis and technical support for the application of ultra-high strength steel sheets in automobile production.Carbon content and tempering temperature was shown to have a significant effect on hydrogen embrittlement susceptibility and notched tensile strength of Mn-B type ultra-high strength steel sheet. Hydrogen embrittlement susceptibility of tested steels increases with rising carbon content, while the hydrogen embrittlement susceptibility of tested steels is no longer increasing above0.3%carbon content in tested steels. Notched tensile strength of tested steels decreases significantly after hydrogen charging, compared to that of steels before hydrogen charging. Hydrogen embrittlement susceptibility of Mn-B type ultra-high strength steel sheet reduces with the rising tempering temperature. Yet,600℃tempered tested steels are becoming less susceptible to hydrogen infection.Due to the impact of the transfer time of samples, heat conduction of moulds, circulating cooling, water temperature and the holding time of parts after forming during hot stamping process, Mn-B type steel sheet with much lower carbon content(≤0.3%) has experienced self tempering in different degrees, which can contribute to the higher delayed fracture resistance of the tested steel than as-quenched sample. As a result, the hot stamping sample gets a higher critical delayed fracture stress. The critical delayed fracture stress σc of the steel sheet could be further improved by tempering at200℃. Therefore, the steel has good delayed fracture resistance after hot stamping, and the delayed fracture resistance of the steel sheet could be further improved by tempering. Likewise, delayed fracture resistance of Mn-B type ultra-high strength steel sheet after hot stamping reduces with the rising C content. Therefore, considering the strength and the delayed fracture resistance, Mn-B type steel with0.2%C content is suitable to be used as steel for hot stamping. The strength level of hot stamping steel can be further improved by increasing the C content, but not more than0.3%. The highest delayed fracture resistance of1500MPa grade Mn-B type steel sheet could be obtained when tempered at200℃with little strength decrease. What’s more, the steel has good delayed fracture behavior after hot stamping.Hydrogen induced delayed fracture of high strength and ductility medium manganese steel is influenced by the content and stability of reverted transformation austenite. The annealed sample with ferrite+austenite structure has higher delayed fracture resistance than as-quenched samples with martensite structure. However, delayed fracture resistance of tested steels decreases with rising austenite content and declining stability of austenite, while the annealed sample with10%austenite content has higher delayed fracture resistance. This shows that tested steel with best mechanical property does not have highest delayed fracture resistance. This can be explained by the phenomenon that delayed fracture sensitivity of tested steel increases with the rising prestrain plastic deformation. Besides, hydrogen embrittlement susceptibility of tested steel with different austenite content differs according to the prestrain. |
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