Development of Copper Precipitation-hardened 780 N/mm~2 High-Strength Steel with Lower preheating Temperature Characteristics

摘要

To improve the working environment associated with the preheating of conventional HT780 high-strength steel, a new HT780 steel with a preheating temperature that can be lowered by applying new hardening techniques has been developed. The essential points of the development are: (1) the hardness of the heat-affected zone (HAZ) is reduced by lowering the carbon content and without adding boron; (2) the base metal is hardened through the precipitation of copper, niobium and vanadium; and (3) effective precipitation-hardening and grain refinement are accomplished by the direct quenching and tempering (DQT) process. The new HT780 steel has a maximum HAZ hardness far lower than that of conventional HT780 steel. It does not crack when welded if preheated to 50 deg C, and excels in cold cracking resistance. The new HT780 steel also has superior linear heating properties and unlike conventional HT780 steel it can be water cooled. Futhermore, the new HT780 steel is comparable to conventional HT780 steel in weld joint toughness and fatigue properties.