The single hot thermocouple technique(SHTT) and high temperature equilibrium technique were combined to investigate the phase diagram of the CaO-SiO_2-5%MgO-20%Al_2O_3-TiO_2 system.The 1300 ℃ to 1500 ℃ liquidus lines are calculated according to the thermodynamic equations based on the pseudo-melting temperatures measured by the single hot thermocouple technique.The phase equilibria relationships are experimentally determined at 1400 ℃ using the high temperature equilibria technique followed by X-ray fluorescence(XRF),X-ray diffraction(XRD),scanning electron microscopy(SEM) and energy dispersive X-ray spectroscopy(EDX) analysis.The liquid phase(L),melilite solid solution phase((C_2MS_2,C_2AS)_(ss)),diopside phase(CMS_2) and perovskite phase(CaOTiO_2) are found.Coupled with the liquidus lines and equilibria results,the phase diagram is constructed for the specified region of the CaO-SiO_2-5%MgO-20%Al_2O_3-TiO_2 system.
Spinel phase is considered to be the optimal phase for stabilization of chromium in stainless steel slag.In order to restrain chromium leaching from slag for the effective environmental protection,Al_2O_3 was utilized for the modification treatment,and the effects on the enrichment and stabilization of chromium were investigated. The mineral phases and the existence state of chromium in slag with various Al_2O_3 contents at different basicities( w_(CaO)/w_(SiO_2) were analyzed by scanning electron microscopy-energy dispersive spectroscopy( SEM-EDS) and X-ray diffraction( XRD). The results showed that chromium mainly existed in the glass and spinel phases at basicity of 1. 0 and 1. 5. As the slag basicity increased to 2. 0,chromium was also found in periclase phase. Al_2O_3 in the stainless steel slag reacted with MgO and Cr_2O_3,which could generate the Al-rich Mg(Cr_xAl_(1- x))_2O_4 solid solution. Moreover,the addition of Al_2O_3 was favorable to reduce the solubility of chromium oxide in liquid phase and suppress the precipitation of periclase phase. The experimental results demonstrated that Al_2O_3 modification has a positive influence on the enrichment and the stabilization of chromium in the stainless steel slag.
The conventional mould fluxes can not be applied to the continuous casting of RE alloyed heat resistant steel, because severe slag-metal interface reactions occur generally in the mold. To restrain the interface reaction and improve conditions for continuous casting, a new mould flux based on aluminate system was devised. The viscous properties were investigated. Scanning electron microscopy and X-ray diffraction were applied to detect and characterize the crystalline phases in the continuous cooling process. The results showed that appropriate addition of CeO_2 could avoid the precipitation of CaO and decrease the viscosity of the mould flux. Increasing the mass ratio of CaO /Al_2O_3, especially to a value exceeding 1, could worsen the stability of the mould flux. With a content of less than 14 wt.%, Li_2O could reduce the viscosity and breaking temperature, but its effect could be weakened for the promoted precipitation of LiAlO_2. To obtain a mould flux with stable viscous properties, such as viscosity and breaking temperature, appropriate contents of CeO_2 and Li_2O should be controlled to around 10 wt.% and 14 wt.%, while the mass ratio of CaO /Al_2O_3 should not be more than 1.
Descaling of hot-rolled stainless steel is generally implemented through pickling process in HNO3-HF mixed acids,which induces severe environmental concerns of nitrogen oxide(NOx)gases and nitrites.According to the electrochemical measurement,the mass loss test and the appearance analysis,a new pickling process which employed HCl-based solution was proposed and evaluated to provide theoretical basis for the development of environment-friendly and highly effective pickling process.Under the experimental condition,the HCl-based solution can compete with ordinary HNO3-HF mixed acids in terms of pickling efficiency,surface integrity and the removal of Crdepleted layer.The descaling process of 430hot-rolled stainless steel in the HCl-based solution consisted of three steps.The descaling solution penetrated the oxide scales into the underlying metal,dissolving the Cr-depleted layer and part of substrates.The oxide scales peeled off from the stainless steel and a polished and smooth surface was exposed.The descaled stainless steel was dissolved uniformly in the HCl-based solution;therefore,the pickling duration was vital for desired surface integrity.Under the static condition,the appropriate descaling time of 430 hotrolled stainless steel in HCl-based solution was 400 s.
Ying-ying YUECheng-jun LIUPei-yang SHIMao-fa JIANGLi-yan QINGuang-wei FAN
