| 摘要 |
PROBLEM TO BE SOLVED: To provide a steel sheet having a tensile strength of 1180 MPa or more and which is superior in delayed fracture resistance.SOLUTION: A cold rolled steel sheet having a tensile strength of 1180 MPa or more includes a coating on a surface thereof, the coating having vanadium compounds and Si compounds as principal components. The coating amount of the vanadium compound is 1 mg/mor more in terms of V, and the coating amount of the Si compound is 0.5 mg/mor more in terms of Si. The coating thickness is less than 5 μm. In the coating, pentavalent vanadium compounds themselves are reduced during a corrosion process to form a mixture of bivalent to tetravalent compounds. As described above, the vanadium compounds undergo a cathode reaction being a hydrogen generation reaction in the corrosion process, thereby reducing an amount of hydrogen generation on the surface of the steel sheet. Also, the coating in which bivalent, trivalent and tetravalent vanadium compounds are mixed contributes as a barrier coating that suppresses corrosion of the steel plate. In order for the vanadium compounds to exert a high corrosion suppressing function, it is advantageous that the environment is in high pH conditions, and this is attained by the Si compounds having a function to keep the environment in the high pH conditions. Therefore, composite addition of the Si compounds with the vanadium compounds improves delayed fracture resistance.SELECTED DRAWING: None |
