Development Of Mould Fluxes For Bloom Continuous Casting Of High Carbon Steel

Panzhihua Iron and Steel Co (PISC) is to install bloom continuous casting machine (CCM) for the production of high carbon steels, including U71Mn and PD3. Continuous casting of high carbon steel is difficult due to its relatively low shear strength at high temperatures. To ensure a trouble-free bloom continuous casting of high carbon steels and to improve product quality, mould fluxes have been widely used worldwide. In this study, the state-of-the-art in this area has been reviewed; a systematic investigation in the physical chemical properties of mould fluxes and the performance under the conditions of the CCM at PISC in particular has been conducted. The main purpose was to identify the key parameters that control the behavior of mould fluxes particularly sintering potential and the product quality. The knowledge gained from this research will be directly used in guiding the commercial production of high carbon steel in PISC.By establishing and using a mathematical model, it was confirmed that the lubrication behavior is controlled by mainly the properties of fluxes. The formation of slag rim was analyzed. Using a viscometer and differential thermal analyzer (DTA1700), the melting temperature, viscosity, crystallizing temperature and their dependence on the components of fluxes in the CaO-SiO2-Al2O3-Na2O-Cap2-MgO system have been studied. The slag column method and the melting ratio method were used to show the relationship between the carbon materials added and the melting rate of fluxes. By utilizing an X-ray diffractometer, electronic probe and optical microscope, the mineralogical composition and microscopic characteristics of fluxes during the sintering process were investigated in detail. In light of these observations, sintering mechanisms and their link with the carbon materials and the pre-treatment procedure of raw materials of fluxes, have been discussed. According to the suggested sintering mechanism, the formation mechanism of the slag rim has also been analyzed. A new evaluative method has been proposed, which quantitatively describes the sintering process of mould fluxes.A thermal simulation testing machine (Gleeble-1500) was employed to determine the high-temperature mechanic properties of the heavy rail steel-U71Mn. The test demonstrated that the heavy rail steel possesses a low plasticity and tension strength at high temperatures. The breakout could occur under the condition of large frictional force in the CCM mould. Consequently, the research on continuous casting mouldfluxes for heavy rail steel should focus on improving lubrication and decreasing casting resistance force. Under the condition studied, the mathematical model also suggested that the elevated melting temperature, increased viscosity and the formation of slag rim could worsen the lubrication for continuous casting strands. The increase in basicity and the contents of Na2O, CaF2, MgO could lower the melting temperature and viscosity of molten fluxes. However, the elevated basicity and Na2O content result in an increase in the crystallization temperature. In assessing the effects of carbon materials on the melting rate, the melting ratio method was found to be more authentic and reliable than the slag column method. Possessing a stronger effect on the melting rate than other carbon materials studied, the intermediate super carbon black, can efficiently hinder the sintering of fluxes.The study on the sintering property revealed that the sintering process of fluxes consists of solid-solid reactions and liquid phase reactions. The solid-solid reaction often starts at about 600℃, resulting mineral phases such as Ca2SiO2F2 and Ca4F2Si2O7 along with some liquid phase which is responsible for the densification. The bulk density-temperature relationship, the densification rate and starting temperature can be used to assess quantitatively the sintering potential of fluxes. The densification rate and temperature play the key role in controlling the formation of slag rim. Addition of carbon materials, pre-melting and pre-heat...