| Interstitial free(IF)steel is mainly used to manufacture automobile panel.For the cold-rolled sheet,not only the properties such as deep-drawing,non-aging,but also perfect surface quality is required.In the IF steel procuction process of one of steel plant in China,there exist serious problem of high frequance of surface defects and lower productivity.Quality tracking results show that,the major factors are the high total oxygen content and the high content of large scale non-metallic inclusions in steel.The technology of liquid steel magnesium treatment has the outstanding function of inclusion controlling.It is extremely significant in decreasing defects probability and increasing productivity that the magnesium treatment technology can be used in IF steel production.For this purpose,in this study,the thermodynamic calculation,the high temperature simulation experiments and the industry experiments were conducted in turn focusing on the IF liquid steel system.Based on the fundamental results,the key technology of the magnesium treatment was developed and was applied in industrialized mass production of IF steel,and the controlling was significant effects were obtained on the steel cleaness and the surface quality of cold-rolled plate,which lays a good foundation for the expanding of the magnesium treatment technology in the field of IF steel.Thermodynamic analysis results show that,under the temperature of 1873K,Ti3O5?Al2O3?MgAl2O4?MgO and liquid slag phase exist stably in the liquid steel system of Fe-Mg-Al-Ti-O.With the increasing of magnesium content in the liquid steel,the stable region of Ti3O5 decreases,the region of liquid slag phase expanses gradually with the system transformation from Al-Ti-O to the Al-Ti-Mg-O and Mg-Ti-O,and the coexistence phase transform from Al2O3 phase to MgAl2O4 and MgO phase correspondingly.The results of deoxidation experiments showed that,under the condition of alumanium deoxidation,irregular or cluster shap pure Al2O3 inclusions existed in the steel with the average diameter of exceeding 2?m.Undter the condition of magnesium treatment after alumanium deoxidation,the pure Al2O3 type inclusion transformed to the Mg-Al-O type inclusion,and the inclusion diameter decreased.Under condition of magnesium treatment after alumanium deoxidation and titanium alloying,the inclusion transformed from Al-Ti-O type to the Mg-Al-Ti-O type with the increasing of number density and decreasing of diameter.In industrial magnesium treatment experiments,MgAl alloy with 5%mass percent of magnesium and 20-30mm in diameter was added into the liquid steel after decarbeonization as deoxization agent during RH refining process,results showed that,the steel compositions can be controlled stably,which met the requirement of RH refining function.Comparing with aluminum dexization,Mg-Al-Ti-O type inclusions existed in steel mainly substituting for Al-Ti-O type inclusions,and the number density of inclusion increased with the decreasing of diameter.In the continuous casting slab,the total oxygen content can be controlling lower than 20ppm,and the inclusion aggregation degree near the surface decreaed,the defect rate induced by Al-base inclusion decreased markedly,the properties such as yield strength,tensile strength,elongation,plastic strain ratio and strain hardening index changed not obviously.The results of industrial application of magnesium treatment technology on IF steel showed that,the liquid steel content C,Mn,P,N,Ti etc.can be cotrolled stably;the of total oxygen content(exceeding 85%of all the tracking heats)less than or equal to 20 ppm;the mechanical properties such as the yield strength,the tensile strength,the elongation rate and the impact energy are are all meet the requirements of national standard,especially,the defect rate induced by Al-base inclusion decreased markedly,and the surface quality was enhance significantly. |
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