Study Of Characteristics Of Inclusions And Property Of High Heat Input Welding For Oxide Metallurgy Steel Developed By Ca Deoxidation

Abstract:Welding structures of thick plates are widely used in fields such as ship building, sea platform, oil lines, super high artitechtures and so on. To improve the welding efficiency and quality, and lower constructing cost, gain more benefit, it is necessary to popularize high heat input welding technology. However, after high heat input welding, traditional steel form terrible microstructures which result in seriously decrease in toughness and threatens safety. So, it is one of the hot pot in steel industry of our country to develop new type of steel for high heat input welding. Oxide metallurgy is the most effective method to meet this need so far. This article has developed a new type of EH36ship plate steel which can sustain high heat input welding of400kJ/cm according to oxide metallurgy method, and manifest the relationship between the inclusion characters and the improvement of toughness of heat affected zone(HAZ).Experiment results show that, HAZ of conventional steel has terrible toughness, and Widmannstatten structure is dominant in microstructure; typical micron-level inclusions are cluster Al2O3and stringy MnS, which cannot induce formation of acicular ferrite(AF), while number of submicron-level inclusions is too small to restrict growth of austenite grains.HAZ of developed steel has excellent toughness, and AF is dominant in microstructure; typical micron-level inclusions are dispersed with alumate core and composite sulfide of Ca and Mn in the outer layer, which tend to induce formation of AF; as Ca content increases from2ppm to25ppm, mass ratio of Al/Ca decreases from19.85to0.8, and the amount of sulfide tends to increase but majority is MnS; the number of submicron-level inclusions is6times as much as that in conventional steel, which restrict growth of austenite grains more obviously.Thermodynamic calculation shows that deoxidation ability is in such order:Ca>Al>Ti>Mn>Si; adding more Ca can translate Al2O3to CaO2Al2O3and even CaO·Al2O3effectively, and initial forming temperature of sulfide, majority is MnS with little CaS, increses as Ca content increases. The calculation results accord with the chemical composition change tendency of inclusions.