Martensitic steel with delayed fracture resistance and manufacturing method

摘要

A cold-rolled annealed martensitic steel sheet is provided. The steel sheet is in% by weight, 0.30 ≦ C ≦ 0.5%, 0.2 ≦ Mn ≦ 1.5%, 0.5 ≦ Si ≦ 3.0%, 0.02 ≦ Ti ≦ 0.05%, 0.001 ≦ N ≦ 0.008%, 0.0010 ≦ B ≦ 0.0030%, 0.01 ≦ Nb ≦ 0.1%, 0.2 ≦ Cr ≦ 2.0%, P ≦ 0.02% , S ≦ 0.005%, Al ≦ 1%, Mo ≦ 1% and Ni ≦ 0.5%. The balance of the composition contains iron and inevitable impurities resulting from melting. The microstructure is 100% martensite and the grain size of the prior austenite is less than 20 μm. The steel sheet has delayed fracture resistance of at least 24 hours between acid immersion U-bend tests. Methods, parts, structural members and vehicles are also provided.