Study Of Characteristics Of Inclusions And Property Of High Heat Input Welding For Steel Plate Developed By Mg Deoxidation

:Heavy steel plates have been widely applied to the large-scale welded structures areas of shipbuilding, offshore structures, oil pipeline fields and high-raise building construction. In order to decrease the costs of constructions and increase economic benefits by improving the welding efficiency and quality, the heat input of welding is improved continually. During the thermal cycle of high heat input welding, the microstructure of heat affected zone (HAZ) is deteriorated seriously, resulting in sharply reduced toughness of HAZ, which has a strongly adverse effect on the mechanical properties of the steel plates, so it has been the key subject in the fields of steel research to improving the toughness of HAZ. It is well known that oxide metallurgy technology utilizing the fine particles imparted during steelmaking is the most effective method to improve the toughness of HAZ. In this thesis, the characteristics of inclusions and property of high heat input welding for oxide metallurgy steel plates developed by Mg deoxidation were studied.In this study, EH36ship plate steel was selected for an urgent need of the property of high heat input welding. The ingots containing different Mg content were melted utilizing the oxide metallurgy technology developed by Mg deoxidiation and then rolled into50mm heavy steel plates by TMCP. The mechanical properties of them were examined by impact and tensile tests. The high heat welding thermal simulation experiments were used to study the effects of Mg content in steel on welding HAZ toughness. The effects of Mg content in steel on characteristics of inclusions were studied systematically by SEM observation. For understanding the evolution of inclusions in steel, thermodynamic calculations were applied.It has been showed that the HAZ toughness was improved significantly by utilizing the oxide metallurgy technology with Mg deoxidiation. The typical inclusions in steel deoxidized by Mg were complex inclusions comprising central oxides containing Mg and peripheral MnS, which were effective to induce the formation of intragranular acicular ferrites (IAF) and thus improved the HAZ toughness. With the increase of Mg content in the steel from0to27,38and99ppm, the phase of central oxide inclusions tends to change sequentially from Al2O3to (Mg-Al-Ti-O), and MgO, which was verified by thermodynamic calculations. Furthermore, the micro-size inclusions were refined by Mg dexodidation. When Mg content in steel is lower, the dominance mechanism of improving HAZ toughness is to utilize the microstructure refinement of IAF induced by (Mg-Al-Ti-O)-MnS complex inclusions. When Mg content in steel is higher, the HAZ toughness is improved by the synergistic effect of both the microstructure refinement of IAF induced by MgO-MnS complex inclusions and the grain refinement of pinning effect of TiN.This thesis contains38figures,16tables and99references.