Study On Thermodynamic Characters And Physical Parameters Of High-Al₂O₃Refining Slags Used In Steelmaking

High-Al2O3refining slags are widely used in the refining processes of special steels with high quality. High content of Al2O3normally existed in the slags during the production of the steel deoxidized by aluminum, especially the high-aluminum steels. Refining phase is a critical phase to remove the inclusions in the steel, hence this paper conducted a systematical study on the thermodynamic characters and parameters of physical properties of high-Al2O3refining slags used in special steel making processes. By means of calculation through mathematic model, measurement of molten temperature, as well as phase analysis of slag et al., the physical properties were investigated for the slags used in the secondary refining.In accordance with the co-existence theory, Matlab software was applied to calculate the activity of Al2O3in the slag system of CaO-Al2O3-MgO. Based on the previous procedure, it is determined that the area of lower Al2O3activity is the area to measure the dissolution rate. The results calculated according to the co-existence theory indicated that in the CaO-Al2O3-MgO slag system, when the MgO content is in constant, the activity of Al2O3in the slag will decrease as the content of CaO increases; while, if the CaO content is constant, the activity of Al2O3in the slag will decrease with the increase of MgO content. The lowest Al2O3activity is in the area where CaO and MgO are all saturated. Measurement of the rate of alumina rods dissolving into CaO-Al2O3-MgO slag system will be conducted under the temperature of1873K. Moreover, Scanning Electron Microscope and Energy Dispersive Spectrometer (SEM and EDS) were adopted to observe the reaction boundary layers between the alumina rods and the melted slag. In the end, the author investigated the effect of CaO-Al2O3-MgO slag system on the cleanness of molten steel and analyzed the variation of the inclusions in the steel.A calculation model of surface tension was set up for CaO-Al2O3-SiO2system, CaO-MgO-Al2O3-SiO2system and other slag systems, as well as Fe-Ni alloy. Meanwhile, a calculation model was established for interface tension between CaO-Al2O3slag and Fe-Ni alloy, also CaO-MgO-Al2O3-SiO2-CaF2slag and molten iron. The calculation results indicated that with the increase of basicity of slag and the content of MgO, the surface tension and interface tension will increase correspondingly. Under different contents of the Al2O3, the surface tension and interface tension are closely relevant to the basicity of slag. If the content of CaF2in slag is higher, the interface tension value will become smaller.Melting point method was conducted to measure the melting characteristics of the CaO-MgO-Al2O3-SiO2refining slag and the results showed that, at the constant content of SiO2in the slag, when MgO content are5wt%and10wt%, the low flowing temperature of slag will be obtained at CaO contents of51wt%and48wt%; while, when MgO=15wt%, CaO content must be kept at around41wt%in order to obtain relatively lower flowing temperature. Thus, during the process of secondary refining, there is a must to strictly control the compositions of the refining slag in order to obtain the refining slag with lower melting point.By using the method of rotating cylinder, RTW-10type instrument for physical characteristics test of melt was adopted to conduct a viscosity measurement to the quaternary refining slag system of CaO-MgO-Al2O3-SiO2. The results showed that with the increase of the temperature, the viscosity of the slags will decrease; when the content of MgO reaches to5wt%, viscosity curve of slag with relatively higher CaO/Al2O3has an obvious inflection point, and the viscosity value is relatively low at a high temperature, which is0.3PaS; when the MgO content is10wt%, lower CaO/Al2O3leads to smaller viscosity values of slags. An increase of MgO content is a significant reason to increase the viscosity of the slag.The experiments of the influences of CaO-Al2O3-MgO slag to the cleanness of the liquid steel showed that the slag composition of50%CaO-45%Al2O3-5%MgO maximizes its deoxidation rate constan. The main types of inclusions in the experimental process are aluminum nitride and magnesium aluminate spinel together with sulfide which is compounded with them.The author has also conducted an exploratory study to precipitate phases of the refining slags. X-Ray Diffraction (XRD) was used to conduct a qualitative analysis of the compositions of the slag, hence the compositions of the slag was preliminarily determined. Based on XRD results, a microcopy observation was employed in determining the phases of slags combined with SEM and EDS.