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Computational thermodynamics as a tool to study the microstructural evolution of EAF duplex stainless steelmaking slags
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Computational Thermodynamics.ORCID iD: 0000-0001-5031-919X
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.
2012 (English)In: Ironmaking & steelmaking, ISSN 0301-9233, E-ISSN 1743-2812, Vol. 39, no 1, 51-58 p.Article in journal (Refereed) Published
Abstract [en]

Computational thermodynamics was used as a tool to study and determine the microstructural evolution of the electrical arc furnace (EAF) slag during and at the end of the refining period in the production of duplex stainless steel. At all the process stages, the slag contains magnesiochromite spinels. Before FeSi addition to the furnace, the slag can also contain calcium chromite crystals. After FeSi addition, the amount of magnesiochromite spinels decreases considerably from similar to 15 to similar to 6 wt-%. In addition, dependent on the basicity and the temperature, the slag may contain solid dicalcium silicate. More specifically, during the refining stage of the EAF duplex steelmaking process, a slag basicity of > 1.55 leads to the precipitation of the dicalcium silicate phase. It has been shown that computational thermodynamics can be used as a powerful and relative reliable tool to study the slag (or slag/steel) equilibriums for different stainless steelmaking processes.

Place, publisher, year, edition, pages
2012. Vol. 39, no 1, 51-58 p.
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-33761DOI: 10.1179/1743281211Y.0000000020ISI: 000298488800008Scopus ID: 2-s2.0-84255167110OAI: oai:DiVA.org:kth-33761DiVA: diva2:417303
Note
Updated from submitted to published. QC 20120127Available from: 2011-05-16 Created: 2011-05-16 Last updated: 2017-12-11Bibliographically approved
In thesis
1. A Study of EAF High-Chromium Stainless Steelmaking Slags Characteristics and Foamability
Open this publication in new window or tab >>A Study of EAF High-Chromium Stainless Steelmaking Slags Characteristics and Foamability
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

A good slag practice is essential for production of a high-quality stainless steel. In addition, the electrical and material efficiency of the electric arc furnace (EAF) can considerably be improved by a good slag practice. The metallurgical properties of the slag are strongly influenced by its high-temperature microstructure. Thus, characterization of the phases within the EAF slag as well as the determination of the amount of these phases is of high importance.In addition, the knowledge about the chemical composition of the liquid slag and solid phases at the process temperatures is instrumental in developing a good slag practice.In order to study the slag in EAF high-chromium stainless steelmaking, slag samples were collected from 14 heats of AISI 304L steel (two samples per heat) and 7 heats of duplex steel (three samples per heat).The selected slag samples were petrographically studied both using scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM-EDS) and light optical microscopy (LOM). In some cases, X-ray diffraction (XRD) analyses were also performed. Moreover, computational thermodynamics was used to determine the equilibrium phases in the EAF steelmaking slags at the process temperatures. In addition, parameter studies were performed on the factors influencing the equilibria.More specifically, a petrographical and thermodynamic characterization was performed on the EAF austenitic steelmaking slags. Thereafter, the microstructural evolution of the slag during the EAF duplex steelmaking process was investigated. Moreover, an investigation with focus on the total amount of precipitates within the high-chromium stainless steelmaking slags was done. Finally, the foamability of these slags was quantified and evaluated.The petrographic investigations showed that, during the refining stage, in both austenitic and duplex cases, the main constituent of the EAF slag is a melt consisting of liquid oxides. In addition, the slag samples contain solid spinel particles. However, before ferrosilicon-addition (FeSi), the slag may also contain solid stoichiometric calcium chromite. Moreover, depending on the slag basicity, the slag may contain solid dicalcium silicate at the process temperatures.The evolution of the slag during the refining stage of the EAF was graphically illustrated in the calculated isothermal phase diagrams for the slag system Al2O3-Cr2O3-CaO-MgO-SiO2-TiO2.It was found that the only critical parameter affecting the amount of solid spinel particles in the slag is the chromium-oxide content. More specifically, it was shown that the amount of the spinel particles in the slag increases with an increased chromium-oxide content of the slag. It wasvialso shown that a higher basicity and a lower temperature of the slag contribute to the dicalcium silicate precipitation.In order to evaluate and quantify the foamability of the slags, the slag’s physical properties influencing its foaming index were determined. Computational thermodynamics was used as a tool to calculate the weight fractions of the solid phases within the slag at different EAF process stages. The computational thermophysics was used to estimate the viscosity of the liquid part of the slag samples at the process temperatures. The apparent viscosity of the samples was calculated by combining the above results. By estimating the density, surface tension and the foaming-gas bubble size, the foaming index of the slag samples were quantified. It could be shown that the foaming index of the EAF high-chromium stainless steelmaking slag may be on its minimum as the slag’s basicity takes a value in the range of 1.2 – 1.5. A basicity value of around 1.50 – 1.60 can be suitable for enhancing the foaming index of the slag, during the refining period in EAF high-chromium stainless steelmaking.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2011. xii, 66 p.
Keyword
High-chromium stainless steel, EAF, Slag, Microstructural characterization, Microstructural Evolution, Computational thermodynamics, Solid particles, Viscosity, Foamability, Basicity
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-33754 (URN)978-91-7415-968-4 (ISBN)
Public defence
2011-05-24, Sal F3, Lindstedtvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Available from: 2011-05-17 Created: 2011-05-16 Last updated: 2011-05-17Bibliographically approved

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