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The Effect of Ladle Treatment on Steel Cleanness in Tool Steels
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Uddeholms AB.
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The aim of the present work was to get an overview of the steel cleanness in tool steel. Plant trial studies of three steel grades were done with different focuses.

Firstly the change of the inclusion characteristics during the vacuum degassing in the ladle was looked upon. The top slag composition was altered and sampling was made before and after vacuum degassing and the results showed that the top slag composition has an effect on the inclusion composition. Thermodynamic calculations indicated that the oxygen activity for the steel/inclusion was twice as large as that of the steel/top slag before vacuum degassing. However, after vacuum degassing the oxygen activity values were close.

In order to study the inclusion characteristics during vacuum degassing the process was interrupted at five pre-determined time points for sampling. During vacuum the number of the smaller inclusions was decreased. However, for the larger inclusions (>11.2µm) the number of inclusions had increased. Throughout the degassing process the inclusion composition was found to approach the top slag composition and at the end of the process only one type of inclusion composition was found.

The removal of hydrogen, nitrogen and sulphur was studied. Samples were taken before and after vacuum degassing. It was found that the removal kinetics of hydrogen and nitrogen can be described with first-order reactions. When 10 minutes of vacuum degassing have passed, the removal of hydrogen and nitrogen is more or less finished for the studied steel grades.

Two steel grades with similar process route and with only a few differences in steel composition were studied. The sum of FeO and MnO was found to be a clear indicator for when reoxidation had taken place. The results from oxygen activity calculations indicate that calculations of oxygen activities with multivalence slag species such as Fe and Cr requires measurements for validations to ensure that reliable results are obtained.

Abstract [sv]

Syftet med denna studie var att få en överblick av verktygsståls renhet. Studier på tre olika stålsorter utfördes.

Inneslutningsbilden under vakuumbehandling undersöktes. Resultaten är baserade på industriförsök likväl som termodynamiska beräkningar. Försöken gjordes på två olika stålsorter där toppslaggens sammansättning varierades och stål- och slaggprover tog ut före och efter vakuumbehandling. Proverna analyserades för att bestämma den kemiska sammansättningen på stål och slagg, men inneslutningsbilden undersöktes också. Med hjälp av termodynamiska beräkningar beräknades syreaktiviteten för jämvikterna stål/inneslutning och stål/toppslag. Beräkningarna jämfördes sinsemellan samt med uppmätt syreaktivitet från stålsmältan. Resultaten visar att toppslaggens sammansättning har en påverkan på inneslutningarnas sammansättning. Specifikt så fanns att när CaO-halten i toppslaggen ökar syns en ökning av CaO i inneslutningssammansättningen. De termodynamiska beräkningarna visar att syreaktiviten för jämvikten stål/inneslutning är dubbelt så stor som den för jämvikten stål/topplagg före vakuumbehandling. Dock är syreaktiviteterna för de olika jämvikterna lika efter vakuumbehandlingen.

För att studera hur inneslutningsbilden utvecklas under vakuumbehandling avbröts vakuumbehandlingen vid fem förutbestämda tidpunkter. Vid varje avbrott av vakuumbehandlingen togs stål- och slaggprover som analyserades för att bestämma totalsyre, inneslutningsmängd och inneslutningssammansättning. Före vakuumbehandlingen var antalet inneslutningar stort, medan antalet små inneslutningar minskade under vakuumbehandlingen. Däremot konstaterades att antalet stora inneslutningarna (>11.2μm) ökade under avgasningen. Vad beträffar inneslutningssammansättningen syntes en variation i analys före vakuumbehandlingen som sedan förändrades till att likna toppslaggens sammansättning under vakuumbehandlingstiden. I slutet av processen märktes endast en typ av inneslutningssammansättning i stålsmältan. Vad som också noterades var att efter ungefär 10 minuter av vakuumbehandling så hade inneslutningsantalet nått sitt minimumvärde.

För att erhålla rent stål är väte-, kväve- och svavelreningen också av stor vikt under vakuumbehandlingen. Dessa tre element studerades genom att prover togs vid olika tillfällen i processen; före, under (vid fem olika förutbestämda tidpunkter) och efter vakuumbehandling. De kemiska sammansättningarna för stål- och slaggproverna användes för att beräkna väte-, kväve- och svavelreningarna. Beräkningarna visade att för väte och kväve så kan kinetiken beskrivas med en förstagradsekvation. Specifikt så gäller detta för stålsorten i studien, samt under förutsättningen att svavelhalten är lägre än 0,003 vikts-%. Dessutom så visade resultaten att väte- och kvävereningen är i stort sett klar för den studerade stålsorten efter 10 minuters vakuumbehandling. När det gäller svavel så kan en förstagradsekvation inte användas vid beräkningarna Istället uppvisar svavel en tendens att följa jämviktshalten av svavel genom hela avgasningsprocessen.

Genom att kontrollera processen finns stora möjligheter att erhålla en bra renhet på stålet. Två stålsorter studerades i denna specifika studie, där de hade en liknande processväg samt endast ix

några små skillnader i stålanalysen. Stål- och slaggprover samlades in och analyserades. Summan av FeO och MnO visade sig vara en klar indikation på när reoxidation har skett, men mängden "carry-over" slagg från ljusbågsugnen kunde inte predikteras med hjälp av någon specifik indikator. Dessutom så beräknades syreaktiviten och beräkningarna jämfördes sedan med uppmätt syre i stålet. Resultaten indikerar att beräkningar av syreaktivitet med toppslagger som innehåller multivalenselement, såsom Fe och Cr, kräver valideringar med mätningar för att trovärdiga predikteringar ska erhållas.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2016. , 51 p.
Keyword [en]
oxygen activity, inclusion characteristics, vacuum degassing, desulphurization, slag, ladle, hydrogen, nitrogen, deslagging
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-187368ISBN: 978-91-7595-951-1 (print)OAI: oai:DiVA.org:kth-187368DiVA: diva2:929977
Public defence
2016-06-10, B3, Brinellvägen 23, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20160523

Available from: 2016-05-23 Created: 2016-05-20 Last updated: 2016-06-09Bibliographically approved
List of papers
1. Effect of top slag composition on inclusion characteristics during vacuum degassing of tool steel
Open this publication in new window or tab >>Effect of top slag composition on inclusion characteristics during vacuum degassing of tool steel
2007 (English)In: Steel Research International, ISSN 1611-3683, Vol. 78, no 7, 522-530 p.Article in journal (Refereed) Published
Abstract [en]

The focus of the study was to investigate the effect of the chemical composition of the top slag on the inclusion chemical composition during vacuum treatment of a plastic mould tool steel. Sampling was done before and after vacuum degassing. The chemical composition of the inclusions was determined by using SEM combined with EDX. The results showed that several inclusion compositions were found before vacuum degassing, while only one main composition of inclusions was present after vacuum degassing. Furthermore, the composition of the top slag was found to have a great influence on the composition of the inclusions found in samples taken after vacuum degassing. The present study also shows that the vacuum degassing effectively reduces the number of inclusions in steel. Finally, the thermodynamic calculations of the activities using Wagner's equation were found to predict a lower oxygen activity value than the calculations made using the Thermo-Calc software.

Keyword
inclusions, vacuum degassing, ladle, steel, slag, composition
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-16849 (URN)000248554300002 ()2-s2.0-34547700322 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2016-05-23Bibliographically approved
2. Change of inclusion characteristics during vacuum degassing of tool steel
Open this publication in new window or tab >>Change of inclusion characteristics during vacuum degassing of tool steel
2006 (English)In: Steel Research International, ISSN 1611-3683, Vol. 77, no 6, 392-400 p.Article in journal (Refereed) Published
Abstract [en]

The inclusion characteristics were studied during vacuum degassing by interrupting the operation at five different times after the start of operation. Slag and steel samples were collected and thereafter assessed with respect to steel and slag composition, total oxygen content, size distribution of inclusions and chemical composition of inclusions. The main conclusion is that the number of inclusions in the different size classes as well as the total oxygen content seem to reach a minimum value after around ten minutes of vacuum degassing. Furthermore, it seems to be a consistent trend that, during vacuum degassing, the top slag influences the inclusion composition.

Keyword
inclusions, vacuum degassing, ladle, steel, ladle treatment, mass-transfer, interface
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-15832 (URN)000238898200003 ()2-s2.0-33745627756 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2016-05-23Bibliographically approved
3. Removal of hydrogen, nitrogen and sulphur from tool steel during vacuum degassing
Open this publication in new window or tab >>Removal of hydrogen, nitrogen and sulphur from tool steel during vacuum degassing
2013 (English)In: Ironmaking & steelmaking, ISSN 0301-9233, E-ISSN 1743-2812, Vol. 40, no 3, 199-205 p.Article in journal (Refereed) Published
Abstract [en]

The removal of hydrogen, nitrogen and sulphur during vacuum degassing of a tool steel grade was studied. Both slag and steel samples were collected before and after vacuum treatment for 10 heats. From these, the chemical compositions of steel and slag were determined. Thereafter, the removal rates of sulphur, hydrogen and nitrogen were calculated. For sulphur and hydrogen only first order reaction models were considered. However, for nitrogen both first order and second order kinetic models were studied. The results showed that the removal of hydrogen and nitrogen can be described with first order reaction models. However, the removal rate of sulphur follows the equilibrium sulphur content at all stages during the vacuum degassing. It is also clear that after 10 min of degassing the hydrogen and nitrogen removal is more or less finished for the studied steel grade. However, the sulphur refining can still be carried out for longer times to reach lower values.

Keyword
Ladle refining, Steel, Hydrogen, Nitrogen, Sulphur, Vacuum degassing
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-120278 (URN)10.1179/1743281212Y.0000000029 (DOI)000316015800004 ()2-s2.0-84876148635 (Scopus ID)
Note

QC 20130408

Available from: 2013-04-08 Created: 2013-04-04 Last updated: 2017-12-06Bibliographically approved
4. The role of process control on the steel cleanliness
Open this publication in new window or tab >>The role of process control on the steel cleanliness
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The possibilities of obtaining a good process control in order to reach a good cleanliness of the steel were studied based on plant trials. Steel and slag samples were taken from two steel grades with a slightly different composition, but with similar processing conditions. Thereafter, the chemical compositions of the steel and slag samples were determined. The results show that the sum of the FeO and MnO was found to be a clear indicator for when reoxidation had taken place. No conclusive indicator was found to predict the extent of slag carry-over from the electric arc furnace. However, most of the variation in deoxidation seem to originate from the accuracy of the slag raking. Steel grades with low amounts of deoxidisers such as C and Si naturally were found to result in higher oxygen contents at the start of deoxidation. Furthermore, calculations of oxygen activities in the steel melt were made and the results were compared to measurements with varying results. Overall, the results indicate that calculation of oxygen activities with multivalence slag species such as Fe and Cr requires additional measurements to obtain accurate results.

Keyword
ladle, deslagging, oxygen activity, cleanness, sulphur, stirring
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-187367 (URN)
Note

QC 20160523

Available from: 2016-05-20 Created: 2016-05-20 Last updated: 2016-05-23Bibliographically approved

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