MICROSTRUCTURE EVOLUTION IN THE HEAT-AFFECTED ZONE OF Zr -Ti MICROALLOYED HIGH-STRENGTH HIGH-TOUGHNESS OFFSHORE STRUCTURAL STEELS

摘要

The microstructural evolution and mechanical properties of heat-affected zone of high-strength offshore structural steels was investigated. Micron and sub-micron complex Zr-Ti oxides were observed and evenly dispersed in the heat-affected zone. MnS was spheroidized and refined. The acicular ferrite grains formed on complex oxides in the heat-affected zone effectively partitioned austenite grains and thus refined microstructures. In the meantime, the low carbon content and suitable amount of Ni, Cr, Cu and Mo additions were adopted in the design of high-strength steels. Therefore, bainitic microstructure was obtained in the heat-affected zone within a large heat input welding range (25-100 kJ/cm). The V-notch impact absorbed energy (180 J) of the coarse-grained heat-affected zone for the steel plates welded by submerged arc welding at a heat input of 50 kJ/cm indicated that high-strength offshore structural steels had excellent impact toughness at -60 °C.