Development of YP420 N/mm~2 Class Steel Plate with Excellent Low Temperature HAZ Toughness for Offshore Structures

摘要

Recently, the installation areas of offshore structures has expanded intoarctic areas, and as a result, steel plates with excellent low temperatureHAZ toughness are now required. A new high strength steel plate forultra-low temperature service in the Arctic Ocean has been developedto respond to this demand. The steel was designed to improve the lowtemperature HAZ toughness of multi-pass weld joints. The coarse grainHAZ (CGHAZ) and inter-critically reheated CGHAZ (ICCGHAZ)formed during multi-pass welding cause reduced low temperature HAZtoughness of weld joints because of their coarse grain size andformation of the brittle martensite-austenite constituent (M-A).In the developed steel, the chemical composition was optimized toreduce the amount of M-A in the CGHAZ by lowering the C contentand to enhance the decomposition of M-A into ferrite and cementite inthe ICCGHAZ by decreasing the Si content. Fine Ca inclusions werealso controlled to improve low temperature HAZ toughness bysuppressing austenite grain growth during welding and enhancing finebainite formation during the cooling process after welding. YP420N/mm~2 class steel with a -40°C CTOD temperature was successfullydeveloped based on this alloy design concept.This steel achieves excellent performance of the base metal and weldjoints by applying advanced HAZ microstructure control technologyusing Ca inclusions. The basic concept of the developed steel isdescribed in this paper, and the properties of the steel are alsointroduced.