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Petrographical study of microstructural evolution of EAF duplex stainless steelmaking slags
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy. (Applied Process Metallurgy)
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.
2011 (English)In: Ironmaking & steelmaking, ISSN 0301-9233, E-ISSN 1743-2812, 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.

Place, publisher, year, edition, pages
2011. Vol. 38, no 2, 90-100 p.
Keyword [en]
Duplex stainless steel, EAF, Slag, Microstructural characterisation, Microstructural evolution, Magnesiochromite spinels
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-31873DOI: 10.1179/030192310X12731438631769ISI: 000287020400002Scopus ID: 2-s2.0-79751469756OAI: oai:DiVA.org:kth-31873DiVA: diva2:408067
Note
QC 20110403Available from: 2011-04-03 Created: 2011-03-28 Last updated: 2017-12-11Bibliographically approved
In thesis
1. A Study of EAF Austenitic and Duplex Stainless Steelmaking Slags Characteristics
Open this publication in new window or tab >>A Study of EAF Austenitic and Duplex Stainless Steelmaking Slags Characteristics
2010 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The high temperature microstructure of the solid phases within the electric arc furnace (EAF) slag has a large effect on the process features such as foamability of the slag, chromium recovery, consumption of the ferroalloys and the wear rate of the refractory. The knowledge of the microstructural and compositional evolution of the slag phases during the EAF process stages is necessary for a good slag praxis.

In supplement 1, an investigation of the typical characteristics of EAF slags in the production of the AISI 304L stainless steel was carried out. In addition, compositional and microstructural evolution of the slag during the different EAF process stages was also investigated. Computational thermodynamics was also 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. Furthermore, the influence of different parameters (MgO wt%, Cr2O3 wt%, temperature and the top slag basicity) on the amount of the spinel phase in the slag was studied. In supplement 2, a novel study to characterize the electric arc furnace (EAF) slags in the production of duplex stainless steel at the process temperatures was performed. The investigation was focused on determining the microstructural and compositional evolution of the EAF slag during and at the end of the refining period.

Slag samples were collected from 14 heats of AISI 304L steel (2 slag samples per heat) and 7 heats of duplex steel (3 slags sample per heat). Simultaneously with each slag sampling, the temperature of the slag was measured. The selected slag samples were studied both using SEM-EDS and LOM. In some cases (supplement 2), X-ray diffraction (XRD) analyses were also performed on fine-powdered samples to confirm the existence of the observed phases.

It was observed that at the process temperature and at all process stages, the stainless steel EAF slag consists mainly of liquid oxides, magnesiochromite spinel particles and metallic droplets. Under normal operation and at the final stages of the EAF, 304L steelmaking slag contains 2-6 wt% magnesiochromite spinel crystals. It was also found that, within the compositional range of the slag samples, the only critical parameter affecting the amount of solid spinel particles in the slag is the chromium oxide content. Petrographical investigation of the EAF duplex stainless steelmaking showed that, before FeSi-addition, the slag samples contain large amounts of undissolved particles and the apparent viscosity of the slag is higher, relative to the subsequent stages. In this stage, the slag also includes solid stoichiometric calcium chromite. It was also found that, after FeSi-injection into the EAF and during the refining period, the composition and the basicity of the slag in the EAF duplex steelmaking and EAF stainless steelmaking are fairly similar. This indicates that, during the refining period, the basic condition for the utilization of an EAF foaming-slag praxis, in both austenitic and duplex stainless steel cases, is the same. Depending on the slag basicity, the slag may contain perovskite and/or dicalcium silicate too. More specifically, the duplex stainless steel slag samples with a higher basicity than 1.55 found to contain perovskite crystals.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2010. xii, 46 p.
Keyword
EAF, Slag, Duplex steel, 304L stainless steel, Characterization, Microstructural evolution, Spinels, Basicity, Computational thermodynamics, Slag foaming
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-12064 (URN)978-91-7415-571-6 (ISBN)
Presentation
2010-03-26, D3, KTH, Lindstedtvägen 5, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20110413Available from: 2010-03-05 Created: 2010-03-01 Last updated: 2011-04-13Bibliographically approved
2. 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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