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Chul-Kim, Jong (Hyundai Steel Company) | Park, Byung-Jun (Hyundai Steel Company) | Park, Ki-Jung (Hyundai Steel Company) | Hwang, Sung-Doo (Hyundai Steel Company) | Suh, Yong-Chan (Hyundai Steel Company)
Despite its short business history in heavy plate production which began its first operation 2010, Hyundai Steel has been developing high quality steel plates for offshore applications. Since 2013, the state-ofthe art technology of rolling, HART (Hyundai’s Advanced Rolling Technology), has been applied to produce high strength and extra heavy gauge steel plates for applications such as ship-building, offshore structure and construction. Owing to its capability to exquisitely controlling of hot rolling passes schedule, HART allows the production of high strength, heavy gauge steel plates with refined microstructure compared with conventional TMCP (Thermo-Mechanically Controlled Processed) steel plates. The present study aims at investigating the effects of rolling condition on Charpy impact toughness of SMYS 460- MPa-class offshore structural steel plates in strain-aged condition. The microstructure of the steel fabricated from HART process is compared with the steel manufactured by conventional TMCP process.
In response to the worldwide increase in demand for energy resources, the areas of crude oil drilling and exploration are being extended to cold regions such as Sakhalin and the North Sea (Tomita et al, 1994). In order to secure the lifetime of structures in extreme weather conditions, it is required to use steels with higher strength and better toughness than conventional steels. Meanwhile, pipe products usually used for structural member to support the loads of facilities. Press bending or roll bending are commonly applied to produce pipe products, and these bending process deteriorates the mechanical properties of steel. In particular, the deterioration of toughness has a significant influence on the integrity of structures, and in order to overcome this problem, demand for strain aging properties of steel is increasing (Torizuka el al, 2006). Grain refinement is the most effective way to improve toughness of steels in harsh environments (Tanaka, 1981). Thermo-mechanically control process is generally used for heavy plate rolling where high strength and toughness are required. In 2013, HYUNDAI STEEL developed a state-of-the-art heavy plate rolling process, HART (Hyundai’s Advanced Rolling Technology), that can make finer and more uniform microstructure than conventional TMCP process by delicately controlling rolling pass schedule, rolling temperature, etc. Through this process, it is possible to manufacture high-strength heavy plates with outstanding strain aging impact toughness and applying them to many offshore projects. In this study, the effect of the rolling conditions on the microstructural change and strain aging impact toughness of the SMYS 460-MPa-class offshore structural steel plates manufactured by conventional and HART process.
Hase, Kazukuni (JFE Steel Corporation) | Ichimiya, Katsuyuki (JFE Steel Corporation) | Terazawa, Yusuke (JFE Steel Corporation) | Fujiwara, Takaki (JFE Steel Corporation) | Hayashi, Kenji (JFE Steel Corporation) | Aoki, Masahiro (JFE Steel Corporation)
Recently, the installation areas of offshore structures has expanded into arctic areas, and as a result, steel plates with excellent low temperature HAZ toughness are now required. A new high strength steel plate for ultra-low temperature service in the Arctic Ocean has been developed to respond to this demand. The steel was designed to improve the low temperature HAZ toughness of multi-pass weld joints. The coarse grain HAZ (CGHAZ) and inter-critically reheated CGHAZ (ICCGHAZ) formed during multi-pass welding cause reduced low temperature HAZ toughness of weld joints because of their coarse grain size and formation of the brittle martensite-austenite constituent (M-A).
In the developed steel, the chemical composition was optimized to reduce the amount of M-A in the CGHAZ by lowering the C content and to enhance the decomposition of M-A into ferrite and cementite in the ICCGHAZ by decreasing the Si content. Fine Ca inclusions were also controlled to improve low temperature HAZ toughness by suppressing austenite grain growth during welding and enhancing fine bainite formation during the cooling process after welding. YP420 N/mm2 class steel with a -40°C CTOD temperature was successfully developed based on this alloy design concept.
This steel achieves excellent performance of the base metal and weld joints by applying advanced HAZ microstructure control technology using Ca inclusions. The basic concept of the developed steel is described in this paper, and the properties of the steel are also introduced.