Microstructure and properties of heat affected zone at high heat input for 610 MPa class high strength steel

Microstructure and properties of heat affected zone (HAZ) at high heat input for 610 MPa class high strength oil tank steel were investigated by electro gas welding and thermal simulation experiments. The results show that the main microstructure of CGHAZ for TMCP steel is coarse lath bainite after high heat input welding. The lowest value of charpy impact energy is 77 J at the site away from fusion line 0.5 mm. Impact energy of HAZ fully can meet the standard requirement (47 J). Brittle M-A component which can decrease impact energy seriously is not observed in HAZ of Nb0.026% TMCP steel, which is related with the decline of solid solution Nb content in the steel. Solid solution Nb can decrease γ-α transition temperature, and promote the formation of granular bainite. The content of Nb is reduced and a small amount of Ti (0.013%) is added in Nb0.014% RQ+T steel, which bring about the reduction of M-A component drastically. Impact energies of Nb0.014% RQ+T steel are close to the values of Nb0.026% TMCP steel. ©, 2015, Editorial Office of Transactions of Materials and Heat Treatment. All right reserved.