Improvement in Toughness of Large Heat-Input Weld Bond of 60kg/mm² Grade High Tensile Strength Steel Plates due to Boron Addition and Low Silicon Content

Abstract

In order to obtain superior toughness of weld bond of large heat-input welded steel plate, the roles of boron addition and low silicon content have been investigated. BN particles precipitate on the austenite grain boundaries in weld bond during cooling after welding. Boron addition less than B/N=0.5 promotes the formation of polygonal ferrites, suppressing degenarate pearlite and martensite islands, and decreases the free nitrogen of weld bond, and consequently notch toughness is largely improved. However, excess boron addition (B/N>0.5) produces degenerate pearlite and or martensite islands and deteriorates the toughness. The role of low silicon content is to reduce the formation of martensite islands by replacing pearlitic reaction and this also improve the toughness of weld bond. The newly developed low Si-Cu-Ni-Mo-V-B steel which has 60kg/mm² grade tensile strength shows very excellent notch toughness at the weld bond after large heat input welding.