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A Study of EAF High-Chromium Stainless Steelmaking Slags Characteristics and Foamability
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.
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 [en]
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: urn:nbn:se:kth:diva-33754ISBN: 978-91-7415-968-4 (print)OAI: oai:DiVA.org:kth-33754DiVA: diva2:417230
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
List of papers
1. Petrographic and thermodynamic study of slags in EAF stainless steelmaking
Open this publication in new window or tab >>Petrographic and thermodynamic study of slags in EAF stainless steelmaking
2010 (English)In: Ironmaking & steelmaking, ISSN 0301-9233, Vol. 37, no 6, 425-436 p.Article in journal (Refereed) Published
Abstract [en]

A study of the typical characteristics of electrical arc furnace (EAF) slags in the production of the stainless steel (AISI 304L) was carried out. Twenty-eight slag samples were taken from 14 heats. Simultaneously with each slag sampling, the temperature of the steel was measured, and one steel sample was taken. The selected slag samples were studied both using SEM-EDS and light optical microscopy. Computational thermodynamics was used as a tool to predict the equilibrium phases in the top slag as well as the amount of these phases at the process temperatures. It was observed that, at process temperature, the stainless EAF slag generally consists of liquid oxides, spinel particles and metallic droplets. Under normal operation, the amount of spinel particles is 26 wt-%. In addition, the influence of the slag temperature, basicity, MgO content and Cr2O3 content on the amount of spinel precipitates and thereby on the foaming index of the top slag is also illustrated and discussed. More specifically, it was found that, within the compositional range of the slag samples, the critical parameter affecting the amount of solid spinel particles in the slag is the chromium oxide content.

Keyword
Stainless steel, EAF, Slag, Microstructural characterisation, Computational thermodynamics, Slag foaming, Metallic droplets, Spinel particles
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-26812 (URN)10.1179/030192310X12646889256022 (DOI)000281466900005 ()2-s2.0-77956204070 (Scopus ID)
Note
QC 20101209Available from: 2010-12-09 Created: 2010-11-29 Last updated: 2011-05-17Bibliographically approved
2. Petrographical study of microstructural evolution of EAF duplex stainless steelmaking slags
Open this publication in new window or tab >>Petrographical study of microstructural evolution of EAF duplex stainless steelmaking slags
2011 (English)In: Ironmaking & steelmaking, ISSN 0301-9233, Vol. 38, no 2, 90-100 p.Article in journal (Refereed) Published
Abstract [en]

A novel study to characterise electric arc furnace (EAF) slags in the production of duplex stainless steel at the process temperatures was performed. The investigation is focused on determining the microstructural evolution of the EAF slag during and at the end of the refining period. In this regard, slag sampling was done at three stages from seven EAF duplex stainless steel heats (21.5-22.5 wt-%Cr, 1.6-5.7 wt-%Ni and 0.3-3.2 wt-%Mo). More specifically, the samples were collected before FeSi addition, after FeSi injection and just before tapping. Collected samples were analysed by light optical microscopy and SEM energy dispersive X-ray spectroscopy to characterise the high temperature microstructure of the slag phases. In addition, X-ray diffraction analysis was used to verify the petrographical results. It was observed that at all process stages, the duplex steel slag contains molten oxides, magnesiochromite spinels and metallic droplets. However, before the FeSi addition, the slag also contains calcium chromite crystals. In this stage, the slag has a high viscosity, which drops to lower level after FeSi injection. Furthermore, depending on the basicity, the slag may contain other solid phases such as perovskite and calcium silicate.

Keyword
Duplex stainless steel, EAF, Slag, Microstructural characterisation, Microstructural evolution, Magnesiochromite spinels
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-31873 (URN)10.1179/030192310X12731438631769 (DOI)000287020400002 ()2-s2.0-79751469756 (Scopus ID)
Note
QC 20110403Available from: 2011-04-03 Created: 2011-03-28 Last updated: 2011-05-17Bibliographically approved
3. Computational thermodynamics as a tool to study the microstructural evolution of EAF duplex stainless steelmaking slags
Open this publication in new window or tab >>Computational thermodynamics as a tool to study the microstructural evolution of EAF duplex stainless steelmaking slags
2012 (English)In: Ironmaking & steelmaking, ISSN 0301-9233, 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.

National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-33761 (URN)10.1179/1743281211Y.0000000020 (DOI)000298488800008 ()2-s2.0-84255167110 (Scopus ID)
Note
Updated from submitted to published. QC 20120127Available from: 2011-05-16 Created: 2011-05-16 Last updated: 2012-01-27Bibliographically approved
4. Influence of slag properties on the amount of solid precipitates in high-chromium EAF steelmaking
Open this publication in new window or tab >>Influence of slag properties on the amount of solid precipitates in high-chromium EAF steelmaking
2011 (English)In: Steel Grips - Journal of Steel and Related Materials, ISSN 1611-4442, Vol. 9, 355-359 p.Article in journal (Other academic) Published
Abstract [en]

The total amount of solid particles as a parameter influencing the viscosity and the foaming properties of the slag was investi-gated. In this context, the amount of these particles for different process conditions was quantified. In addition, the effect of the process conditions on the total amount of the particles was studied. More specifically, some parameter studies were carried out in order to determine the influence of chromium oxide (Cr2O3) content, calcium oxide (CaO) content, basicity and temperature on the total amount of solid particles. The interactions between the process conditions were also taken into account. The results were visualized in diagrams and thereafter discussed in detail.

Keyword
EAF, slag, amount of solid precipitates, viscosity, foamability, computational thermodynamics, high-chromium stainless steel, basicity
Identifiers
urn:nbn:se:kth:diva-33772 (URN)
Note
QC 20120328. Updated from submitted to published.Available from: 2011-05-17 Created: 2011-05-17 Last updated: 2012-03-28Bibliographically approved
5. Evaluation of the foamability of the EAF stainless steelmaking slags using computational thermodynamics and thermophysics
Open this publication in new window or tab >>Evaluation of the foamability of the EAF stainless steelmaking slags using computational thermodynamics and thermophysics
2011 (English)Report (Other academic)
Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2011. 21 p.
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-33773 (URN)
Note
QC 20110517Available from: 2011-05-17 Created: 2011-05-17 Last updated: 2011-05-17Bibliographically approved

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