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A Study of EAF Austenitic and Duplex Stainless Steelmaking Slags Characteristics
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. (Applied Process Metallurgy)
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 [en]
EAF, Slag, Duplex steel, 304L stainless steel, Characterization, Microstructural evolution, Spinels, Basicity, Computational thermodynamics, Slag foaming
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-12064ISBN: 978-91-7415-571-6 (print)OAI: oai:DiVA.org:kth-12064DiVA: diva2:300888
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
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, E-ISSN 1743-2812, 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: 2017-12-11Bibliographically 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, 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.

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: 2017-12-11Bibliographically approved

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