Martensitic steel with delayed fracture resistance and manufacturing method

A cold rolled and annealed martensitic steel sheet is provided. The steel sheet includes by weight percent, 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 remainder of the composition includes iron and unavoidable impurities resulting from melting. The microstructure is 100% martensitic and a prior austenite grain size is lower than 20 μm. The steel sheet has a delayed fracture resistance of at least 24 hours during an acid immersion U-bend test. A method, part, structural member and vehicle are also provided.