| During the continuous casting of high aluminum steel, Al in the molten steel reacts with SiO2in the mold slag, which can cause SiO2in the mold slag decreased dramatically while Al2O3increased greatly. As a result, the melting temperature as well as the viscosity rised dramatically and the glass property are deteriorated. Thus, the surface quality of the slab and the continuous casting process are affected.On the basis of thermodynamic and kinetic analysis for the change of Al2O3in the mold slag for20Mn23AlV high aluminum steel, it is found that:In order to prevent [Al] reacted with (SiC2), it is required to control the content of SiO2to be lower than5%and that of Al2O3to be more than30%. The change of Al2O3in the mold slag during continuous casting of high aluminum steel which is mainly affected by the initial Al2O3concentration w0of the mold slag, the reaction equilibrium concentration w*of Al2O3at the slag-steel interface, the mass transfer coefficient kF,Al2O3in the molten steel, and the absorption coefficient in the molten steel.The activity model of Al2O3-BaO-B2O3ternary slag system is established, and the influence of BaO/Al2O3, B2O3content and temperature on the activity of the system is investigated. The equal activity curves of Al2O3, BaO, B2O3are obtained. According to the activity calculation of Al2O3-BaO-B2O3system, the composition for high aluminum steel mold slag is selected as Al2O3:40～50%, BaO:30-40%, B2O3:10-20%. By using the quadratic general rotary testing, the composition range for the high aluminum steel is selected as (CaO+BaO)/Al2O3:0.7-1.1,(B2O3+SiO2):12.5-20%, CaF2:7.5-12.5%,(Na2O+Li2O):11～17%, MnO:2-6%.According to the characteristics of20Mn23AlV steel and the requirements of performance of the mold slag in the continuous casting process, a new kind of mold slag was developed for the casting of20Mn23AlV steel, which is based on the composition range of Al2O3-BaO-B2O3high aluminum mold slag. The melting temperature, the viscosity and the corresponding crystallization rate have become to be1042℃,3.11dPa.s and15%, compared to the previous980℃,2.45dPa.s, and0%before the continous casting. By the comparative study on the crystalline morphology of the mold slag of the high aluminum steel with SEM and XRD, it is found that the silicate network structure is the main formation in the CaO-Al2O3-SiO2mold slag for high aluminum steel. During continuous casting, the crystallization rate is increased with the growing concentration of A12O3and the decrease of SiO2The main crystals as Na3MgAlSi2O8, Na4Al2Si209, CaAl2Si208, Ba2Al2Si208, NaA102, Ca7MgAl10O23, Na4Ca3(A102)10are precipitated with the increase of A12O3content. At the same time, Al203-BaO-B2O3mold slag for high aluminum steel has a high B2O3content and good noncrystalline form. The content of Al2O3only has a slight increase, and a small amount of crystals NaAlSi3O8、CaAl2Si2O8were precipitated in this mold slag during continuous casting.Thermogravimetric analysis and differential thermal analysis of the new high aluminum steel mold slag were carried out from room temperature to1300℃. The adsorption water is removed from the mold slag at100℃; the B2O3which has a low melting temperature in the mold slag is melting at about440℃; and the carbonates in the mold slag are decomposing in the range of500-690℃. The glass transition temperature of the mold slag is about840℃. The heat absorption in the mold slag at about950℃is mainly caused by the melting of sodium carbonate. The melting temperature of the mold slag is about1150℃, and the NaF which is a volatile gas will also generate when the temperature is above the1150℃.The specific heat capacity of the new high aluminum steel mold slag and the previous common mold slag from room temperature to1200℃is measured with the differential scanning calorimetry method. When the temperature ranges from room temperature to1200℃, the specific heat capacity of the new high aluminum steel mold slag rises from1.046J/(g-K) to2.291J/(g-K), the total heat absorption is about1925.6J/g, and the average specific heat capacity is about1.638J/(g-K). However, the specific heat capacity of the common mold slag rises from1.046J/(g-K) to2.514J/(g-K), and the total heat absorption about1751.7J/g, and the average specific heat capacity is about1.491J/(g-K). The key innovations of this paper are as follows:(1) A kinetic model of the absorption of the Al2O3in mold slag for high aluminum steel is set up, and the change in the concentration of Al2O3in the mold slag during continuous casting is discussed;(2) The activity model for ternary slag system of Al2O3-BaO-B2O3is calculated, and the equal activity curves are also obtained;(3) A new kind of mold slag which is based on the Al2O3-BaO-B2O3ternary system is developed for the high aluminum steel20Mn23AlV. |
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