STEEL SHEET SUPERIOR IN DELAYED FRACTURE RESISTANCE

• Main Authors: OTSUKA SHINJI, MATSUZAKI AKIRA, TSUCHIMOTO KAZUAKI, YOSHIMI NAOTO, TAIRA SHOICHIRO
• Format: Patent
• Language: English; Japanese
• Published: 05.09.2016
• Subjects: CHEMICAL SURFACE TREATMENT; CHEMISTRY; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATIONOR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY IONIMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; COATING MATERIAL WITH METALLIC MATERIAL; COATING METALLIC MATERIAL; DIFFUSION TREATMENT OF METALLIC MATERIAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION INGENERAL; METALLURGY; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THESURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION

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 【課題】引張り強度１１８０ＭＰａ以上を有する鋼板であって、耐遅れ破壊特性に優れた鋼板を提供する。【解決手段】引張り強度が１１８０ＭＰａ以上の冷延鋼板の表面に、バナジウム化合物とＳｉ化合物を主成分とし、バナジウム化合物の付着量がＶ換算で１ｍｇ／ｍ２以上、Ｓｉ化合物の付着量がＳｉ換算で０．５ｍｇ／ｍ２以上、皮膜厚が５μｍ未満の皮膜を有する。皮膜中の５価のバナジウム化合物は、腐食過程において自身は還元されて２価〜４価の化合物が混在したものになる。このように、腐食過程における水素発生反応であるカソード反応をバナジウム化合物が担うことによって、鋼板表面の水素発生量を低下させる。また、２価、３価、４価のバナジウム化合物が混在した皮膜は鋼板の腐食を抑制するバリア皮膜として寄与する。バナジウム化合物が高い腐食抑制機能を発揮するには環境が高ｐＨである方が有利であるが、Ｓｉ化合物は高ｐＨに保持する機能を有するため、バナジウム化合物との複合添加によって、耐遅れ破壊特性が向上する。【選択図】 なし