HT780 high strength steel plate is planned to be applied to structural members. For building use, the following properties and process should be required: maintaining toughness of the heat affected zone, restraining hydrogen crack in weld metal, reducing the pre-heat temperature, lowering the yield ratio and so on. This paper represents the results of development of new HT780 thick plate and new welding process satisfying the above requirements. Furthermore, it is also examined tensile mechanical properties of joints of this new HT780 thick plates welded by the new process.
