| Along with the rapid development of chemical industry, construction, shipbuilding and other manufacturing, engineering structure is more and more development in the direction of large scale and light weight. More excellent performance such as high strength, high toughness and good weldabilty are required for steels. In large heat input welding condition, grain coarsening and induced cracks occur in welding heat affected zone (HAZ) of the plate, which degrade toughness severely and restrict the application of large heat input welding technology. Oxide metallurgy is the most effective technology to enhance the toughness in HAZ, which refine the tissue in HAZ by particles pinning effect and promoting the growth of intragranular ferrite.In this research, the nucleation and growth of many kinds of inclusions in the steel with Ti-Zr/Mg deoxidization were studied based on the metallurgical thermodynamic calculations. In order to study the impact on inclusions and weld heat affected zone by Ti-Zr/Mg deoxidation, we used metallographic microscope and Image Pro Plus 6.0 to count the number of inclusions as well as the size. SEM and EDS are used to analyse the morphology and component of inclusions. Thermal simulation experiment was carried out in the laboratory to explore the effect on the tissue by peak temperature and holding time. Gleeble 3800 thermal simulation testing machine was used for welding thermal simulation experiment to study the effect on HAZ toughness by Ti-Zr/Mg deoxidation. CLSM was used for welding simulation to observe the changement of tissue online and count the austenite grain size. The conclusions are following:(1) Thermodynamic calculations show the following results:in the steel with Ti-Zr deoxidation, Ti2O3, Ti3O5, TiO2 are easy to generate in steelmaking and solidification front, TiO can generate when the oxygen content stay at a high level; Mg treatment can help combine Al2O3 and MgO to generate MgO·Al2O3 to achieve the purpose of inclusion modification.(2) The analysis of inclusions shows the following results:Ti-Zr/Mg deoxidation can help decrease the average diameter, area percentage and total number of inclusions. The inclusions in Ti-Zr deoxidation steel are mostly wrapped by MnS, and the internal part is Al2O3-SiO2-CaO-TiOx-ZrO2. Some inclusions in Mg deoxidation steel are wrapped by MnS, and the internal part is MgO·Al2O3. The others are MgO·Al2O3 and MnS.(3) Mechanical properties testing shows the following results:Ti-Zr deoxidation can improve the impact resistance, Mg deoxidation can improve the tensile elongation. Tissue observation indicates that, Ti-Zr deoxidation can refine the tissue, Mg deoxidation can refine the tissue significantly. The results indicate that Ti-Zr/Mg deoxidation can improve the toughness of the steel.(4) Thermal simulation shows the following results:the tissues in Ti-Zr deoxidation steel after thermal simulation are ferrite and bainite, which have good mechanical properties. With the growth of peak temperature and holding time, the growth speed of ferrit grains is low. The tissue in Mg deoxidation steel is ferrite. When the peak temperature and holding time grow, the growth speed of ferrit grains is low. The results indicate that Ti-Zr/Mg deoxidation can introduce a large number of fine and dispersed inclusions. Inclusions which stay around the grain boundary can pin the grain boundaries, hinder the growth of grain boundary, refine the tissue to improve the mechanical properties of the steel.(5) Welding thermal simulation shows the following results:when the heat input is 100 kJ/cm, the impace energy of HAZ in the steel with Ti-Zr deoxidation is 82.38% higher than that of the steel without Ti-Zr deoxidation. The result indicates that Ti-Zr deoxidation can significantly improve the HAZ toughness.(6) The observation of CLSM shows the following results:the austenite grain is fine and the content of intragranular acicular ferrite is high in Ti-Zr/Mg deoxidation steel. The tissue in Ti-Zr deoxidation steel is acicular ferrite, which can increase the strength and toughness of the steel. The tissue in Mg deoxidation steel is ferrite, and the average diameter of ferrite is 6.74% smaller than that of the steel without Mg deoxidation. The results indicate that Ti-Zr/Mg deoxidation can refine the tissue and improve the toughness of the steel. |
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