Ultra-high strength martensitic steel with different Mn additions(1.01％,1.41％and 1.74％,mass fraction)was prepared by ultra-fast cooling process,and the differences of microstructure,such as the prior austenite grain size,high-angle grain boundary ratio,thickness of martensite lath and nanoprecipitates were compared by XRD,SEM,EBSD and TEM.The hydrogen embrittlement property of the steel was analysed by TDA.The first-principles were used to study the hydrogen trap at the interface of the Cu precipitates and steel matrix.The results show that with the increase of Mn addition,the prior austenitic grain size of 1500 MPa grade martensitic steel is about 7 μm,but the martensitic lath in higher Mn specimen is thinner and denser.First-principles calculations show that harmless hydrogen traps can be formed at the interface between Cu and martensitic steel matrix,and a large number of various interfaces in the martensitic structure help to form more dispersed hydrogen traps in the steel,so that hydrogen is not easy to accumulate locally in the material,thereby reducing the risk of hydrogen embrittlement of the material.Based on the ultra-fast cooling process,the increase of Mn addition can achieve finer microstructure of 1500 MPa grade martensitic steel,and the Cu precipitation can also improve the hydrogen embrittlement resistance of the steel.
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