Kinetics of Reduction of SiO 2 in SiO 2 -Al 2 O 3 -CaO Slags by Al in Fe-Al(-Si) Melts
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INTRODUCTION
A wide range of Al contents are used in various steel grades from Al-killed steels to TRIP (TRansformation Induced Plasticity) steels containing up to 2 wt pct of Al.[1–5] The reactivity of dissolved Al in the melt poses a considerable challenge during processing in terms of undesirable reactions between the melt and its environment, i.e., atmosphere, refractory vessels, and slags. As the primary network-former in the slag, SiO2 is largely responsible for the slag properties and is a major reducible oxide by the Al dissolved in the steel melt during ladle and casting processes.[6] The reaction of Al with SiO2 can cause major Si pick- up from slag in Sirestricted grade steels and lead the steel to be out of specification. The relevant reaction and Gibbs free energy of the reaction are shown in Reaction [1] and Eq. [2],[7] respectively. The underline denotes that the element is dissolved in liquid Fe, and the parenthesis denotes that the component is present in the mixture of oxide melts. Al and Si are in 1 wt pct standard state in liquid Fe. SiO2 and Al2O3 activities are in respect to pure solid state. 4Al þ 3ðSiO2 Þ ¼ 2ðAl2 O3 Þ þ 3Si
½1
DG0R ¼ 720; 680 þ 133T ðJ=molÞ
½2
JIWON PARK, Graduate Student, SEETHARAMAN SRIDHAR, Professor, and RICHARD J. FRUEHAN, U.S. Steel Professor, are with the Department of Materials Science and Engineering, Center for Iron and Steelmaking Research, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213. Contact e-mail: sridhars@ andrew.cmu.edu Manuscript submitted March 23, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
SiO2 reduction by Al is of great importance to TRIP steels production, because Al content in the melt is exceptionally high up to 2 wt pct.[1–4] Al is a major alloying element of TRIP steel, which is added to stabilize retained austenite that transforms into martensite under strain.[2] The results of this reaction, Al increment, and SiO2 loss change the viscosity of the mold flux leading to degeneration of process and product quality.[2,3,8] It is expected that a large amount of Al is consumed by slag during the casting process resulting in replacement of Al by Si in the metal which alters the metal chemistry. Unwanted Si pick-up from the slag degrades the surface quality and coatability by forming oxides on the as-cast slab.[2] This disturbs the galvanizing process and makes it difficult to use TRIP steels in commercial automobile parts. More importantly, the decrease of SiO2 in mold flux will cause problems in the continuous casting process. The mold flux is a molten oxide mixture that is used to aid the continuous casting process by providing lubrication at the mold/strand gap and controlling heat transfer. It is also used to protect the molten steel from oxidation and to absorb non-metallic inclusions. The key properties of the molten mold flux are viscosity, crystallization, glass formation, and thermal conductivity; these are highly dependent upon the silicate structure and thus SiO2 content.[6,9] A replacement of SiO2 by Al2O3 can ca
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