Transformation and Precipitation Behavior in the New Conceptual TMCP Process Utilizing Heat Treatment On-line Process

摘要

New conceptual TMCP process for manufacturing high strength steel plates, which is applied an on-line heat treatment immediately after accelerated cooling (ACC), was developed. Transformation and precipitation behavior in the new TMCP process was investigated and compared with those in conventional ACC process and quenching and tempering process (Q+T). In the ACC process and Q+T process, microstructures were consisted of bainitic ferrite and second phase, such as cementite or martensite-austenite constituent (MA). And fine carbides, which were formed randomly, were observed in Q+T steel. On the other hand, in the new TMCP process polygonal ferrite was observed in addition to bainitic ferrite and cementite, and two kinds of precipitation forms, random precipitation and row precipitation, were observed. It was found that ferrite transformation is promoted during heating after accelerated cooling, which brings row precipitation of fine carbides. Furthermore, Control of the formation of MA this new TMCP process. In the conventional ACC process, MA constituents are formed from carbon enriched untransformed austenite during air cooling after ACC, and formation of MA is hard to prevent for higher strength steels. On the other hand, carbon enrichment to untransformed austenite can be prevented by carbide formation during on-line heat treatment after ACC. It was demonstrated that homogeneous microstructure with very low amount of MA constituents was achieved by the new TMCP process. And, absence of brittle phase brought excellent resistance to hydrogen induced cracking in NACE sour environment. In this paper, details of the metallurgical and mechanical feature of this new TMCP steel were discussed, and application to sour resistant linepipe steel was introduced.