Fundamentals of Steel Complex Deoxidation with Multiple Deoxidizers

摘要

As demand for high quality steel increases every year, deoxidation is always a key to minimize the free oxygen of the molten steel and improve the cleanliness of the steel. The word "deoxidation" appeared for the first time in 1916 in reported literatures~1 as reviewed by Turkdogan~2. Non-metallic inclusions are detrimental to both the castabiliry of the steel and the quality of products.~3One of the main challenges in the production of high quality steels is the control of inclusions in the steel. Most of indigenous inclusions are produced during the deoxidation of the molten steel.~4 The deoxidation equilibrium relationship by single deoxidizer has been maturely predicted by thermodynamic calculation, such as Al-O~(5,6), Mg-O~7, Ca-O~(8,9), Si-O~(10), Ti-O~(11,12), Zr-O~(13,14), Cr-O~(15), V-O~(16), La-O~(17), Ce-O~9, etc. The equilibrium constant of the deoxidation reactions, log K, and interaction coefficients of alloy elements were measured and summarized by many researchers, as examples published by JSPS.~(18) Jung et al~(19,20) calculated the deoxidation curves of extensive alloy elements and introduced the application of thermodynamic databases to steelmaking process. Whereas, thermodynamic calculation of complex deoxidation - with multiple deoxidizers - to predict formation of inclusions was rarely reported before 2000. In recent years, Suzuki et al~(21) investigated deoxidation equilibrium of chromium stainless steel with Al and Si. Ohta et af~(14) calculated the stability phase diagram of Al-Mg-O system and Al-Ca-O system at 1873 K. Kang and Lee~(22) calculated the stability diagram of Al-Si-Mn-O in liquid iron at 1873 K using FactSage. Seo et at~(23)studied deoxidation equilibrium among Mg, Al and O in liquid iron in the presence of MgO-Al_2O_3 spinel. Jung et al~(24) reported that a liquid Al_2O_3-Ti_2O_3-TiO_2, inclusion phase could exist in Al-killed Ti-bearing steels at 1873 K and calculated the stability diagram for Al-Ti deoxidation at 1873 K. Suzuki et at~(21) calculated the stability diagram of Al-Si-0 in stainless steel at 1873 K. Ono et al~(25-27) calculated the stability regions of MgTi_2O_4, MgO, and Ti_2O_3 at 1873 K and the phase diagram of TiO_2-MgO-Al_2O_3 system at 1873 K. Itoh et al~(28,29) calculated the stability diagram of Al-Mg-Ca-0 in liquid iron at 1873 K. In the current study, the thermodynamic calculation of steel complex deoxidation with multiple deoxidizers were extensively performed, including deoxidation by single-deoxidizer, dual-deoxidizer and tri-deoxidizer, using the in-house thermodynamic computational code of the authors. The curves for these deoxidation process were obtained with validated measurement data from literatures. The typical morphology of inclusions generated during these deoxidation process was also given.