Development of Thick YP460 MPa Class Steel Plates for Large Heat-input Welding for Very Large Containerships

摘要

In connection with a recent trend toward large scaled containerships, itrnbecomes more important than ever to ensure hull girder longitudinalrnstrength around the deck openings in order to assure the safety andrnreliability of the hull construction. For this reason, YP460 MPa classrnsteel plates have been applied, which exceed YP390 MPa class, thernhighest strength standard of the conventional TMCP (ThermornMechanical Control Process) steel plate.rnFor practical application of YP460 MPa class steel plate, it is necessaryrnto ensure further safety of the ships, as well as to apply high efficiencyrnlarge heat-input welding.rnThis report describes the development concept of YP460 MPa classrnsteel plates, and the material characteristics of the base metal andrnwelded joints. The concept is as follows.rnFirst, to reduce MA that may cause the HAZ (Heat Affected Zone)rntoughness deterioration, C addition and Ceq were reduced as much asrnpossible. In addition, TiN was added for the purpose of refinement ofrnthe HAZ structure, and to prevent enlargement of γ grains from largernheat-input welding.rnSecond, to achieve the YP460 MPa class high strength, transformationrntoughening using a uniform intense cooling process was applied.rnThird, improvements of the base metal fracture toughness werernachieved by refinement of crystal grains using strictly temperaturerncontrolled TMCP.rnThe results were as follows.rn(1)As for the base metal toughness, vE-40 was more than 340 J, whichrncleared the specification value of 53 J. It showed a favorable fracturerntransition temperature of -80℃ or below.rn(2)As for the brittle crack arresting characteristics, the temperaturerngradient brittle fracture test (ESSO) at the minimum design temperaturern(-10℃) showed a favorable stress intensity factor Kca of 7000rnN/mm1.5 or more.rn(3)Even by 42 kJ/mm large heat-input welding using single electro gasrnarc welding, the joint strength cleared the specification value, and thernjoint toughness posed a favorable Charpy absorbed energy of 53 J orrnmore, at all notch positions in the V-notch Charpy impact test at a testrntemperature of -20°C.