The development of a high strength Nb-bearing ship plate steel for high heat input welding

Large thickness, high strength EH40 steel plates with excellent low temperature toughness are required for the construction of 8000 TEU container ships. In order to improve the construction efficiency of the huge ship, increasing heat input was applied by the one-pass welding process. Two challenges are encountered during the development of the EH40 plate for high heat input welding. The first is the softening of the joint plate and the other is the deterioration of the HAZ toughness. In order to cope with the softening that accompanies the high heat welding input process, a promising strengthening strategy by increasing the Nb content and adding Cu and Ni is proposed. Also, in order to improve the toughness of the HAZ that is subjected to high heat input welding, Ti-B treatment is found to be a quite effective countermeasure. The BN particles that form on prior austenite grain boundaries can prevent the formation of bainite and Widmanstatten ferrite, while those forming inside the austenite grains can contribute to the formation of intragranular acicular ferrite. The predominantly intragranular ferrite microstructure thus improves the low temperature toughness of the HAZ. (AU)