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Thermodynamic Aspects on Inclusion Composition and Oxygen Activity during Ladle Treatment
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.
2008 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Two industrial studies and one set of lab scale trials have been done. In addition, a theoretical study has been done. The main focus has been on non metallic inclusion composition during the ladle refining operation in industrial steel production. Sampling has been done together with careful inclusion determination. The inclusion composition is related to different variables. In the industrial trials samples have been taken at different steps during the ladle refining period. Steel and slag composition as well as temperature and oxygen activity have been determined.

The thesis is based on five supplements with different major objectives, all related to the inclusion composition. The equilibrium top slag-steel bulk and inclusions-steel bulk were investigated by comparison between calculated and measured oxygen activity values. The oxygen activity and relation to temperature has also been discussed as well as oxygen activity and temperature gradients. The effect of vacuum pressure on inclusion composition has been evaluated in a theoretical study as well as lab scale trials. The inclusion composition has been studied during the industrial ladle treatment process. The inclusion composition was related to top slag composition and other parameters during ladle treatment.

The major findings in the thesis are the lack of equilibrium conditions with respect to top-slag and steel bulk before vacuum treatment. The inclusions have been found to be closer to equilibrium with the steel bulk. Al/Al2O3 equilibrium has been found to control the oxygen activity after Al-deoxidation. Evaluation of inclusion composition during the ladle refining has revealed that the majority of the inclusions showed a continuous composition change throughout the ladle refining process, from high Al2O3, via MgO-spinel to finally complex types rich in CaO and Al2O3. The final inclusion composition after vacuum treatment was found to be close to the top slag composition. Vacuum pressure has been found to have a theoretical effect on inclusion composition at very low pressures.

Place, publisher, year, edition, pages
Stockholm: KTH , 2008. , xii, 68 p.
Keyword [en]
Ladle treatment, thermodynamics, inclusions, steel, slag, equilibrium, ladle treatment, vacuum, oxide activity, oxygen activity.
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-4673ISBN: 978-91-7178-904-4 (print)OAI: oai:DiVA.org:kth-4673DiVA: diva2:13362
Public defence
2008-04-08, B3, Materialvetenskap, Brinellvägen 23, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20100712Available from: 2008-03-19 Created: 2008-03-19 Last updated: 2010-07-12Bibliographically approved
List of papers
1. Equilibrium between slag, steel and inclusions during ladle treatment: comparison with production data
Open this publication in new window or tab >>Equilibrium between slag, steel and inclusions during ladle treatment: comparison with production data
2007 (English)In: Ironmaking & steelmaking, ISSN 0301-9233, E-ISSN 1743-2812, Vol. 34, no 4, 312-324 p.Article in journal (Refereed) Published
Abstract [en]

The primary purpose of the investigation was to determine whether equilibrium conditions exist between slag and steel and inclusions and steel after silicon deoxidation and aluminium deoxidation. In doing so, the performance of three different models for predicting oxide component activities was also evaluated. The results from the oxide activity models were used to calculate oxygen activities in the steel melt. Industrial data such as steel, slag and inclusion component concentrations, as well as temperature, were used as input to the calculations. One general conclusion drawn was that although the results from the different models showed similar trends, the models did not predict the same actual values for specific calculations. Predicted theoretical equilibrium oxygen activity values were compared with plant measurements of oxygen activity. Here, it was found that the oxygen activity values pertaining to the equilibrium between steel and inclusions were higher than corresponding measured values. The calculations for the equilibrium between the top slag and steel melt, however, resulted in theoretical oxygen activity values considerably lower than corresponding measurements. It was thus concluded that equilibrium conditions did not exist between the top slag and the steel bulk with respect to oxygen. Overall, the models proved useful in predicting oxide activities, but the lack of equilibrium between top slag and molten steel made the calculation of oxygen activity difficult.

Keyword
models; activity; slag; steel; oxygen; oxide; SLAGS
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-8120 (URN)10.1179/174328107X168039 (DOI)000248541700006 ()2-s2.0-35348944450 (Scopus ID)
Note
QC 20100707Available from: 2008-03-19 Created: 2008-03-19 Last updated: 2017-12-14Bibliographically approved
2. The Effect of Ladle Treatment on Inclusion Composition in Tool Steel Production
Open this publication in new window or tab >>The Effect of Ladle Treatment on Inclusion Composition in Tool Steel Production
2008 (English)In: Steel Research International, ISSN 1611-3683, Vol. 79, no 4, 261-270 p.Article in journal (Refereed) Published
Abstract [en]

An in-depth SEM-investigation of the inclusion composition change in the ladle refining process during tool steel production was done. Plant trials were carried out at Uddeholm Tooling AB in Hagfors, Sweden. A series of samples was taken during ladle refining, from deslagging through vacuum treatment. The goal was evaluating the effects of the top slag and other process parameters on the inclusion composition during ladle refining. The main conclusion was that normal variations in the process have a large influence on inclusion composition. Furthermore, it was concluded that the MgO content in the top slag had a large influence on the inclusion, composition throughout the whole ladle refining process. Also, the SiO2 content in the ladle slag originating from the EAF-slag, had a noticeable effect on the inclusions. The inclusions belonging to the system Al2O3-CaO-MgO-SiO2 showed a continuous composition change throughout the ladle refining process, from high Al2O3, via MgO-spinel to finally complex types rich in CaO and Al2O3.

Keyword
inclusions; composition; ladle; refining; tool steel; vacuum
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-8121 (URN)10.1002/srin.200806350 (DOI)000256177900003 ()2-s2.0-43749110004 (Scopus ID)
Note

QC 20100712. Uppdaterad från in press till published 20100712.

Available from: 2008-03-19 Created: 2008-03-19 Last updated: 2017-06-14Bibliographically approved
3. Effect of Temperature on Oxygen Activity during Ladle Treatment
Open this publication in new window or tab >>Effect of Temperature on Oxygen Activity during Ladle Treatment
2008 (English)In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 48, no 4, 438-445 p.Article in journal (Refereed) Published
Abstract [en]

The effect of temperature on oxygen activity in steel was studied in plant trials where temperature and oxygen activity were measured at two depths and at the same time during different parts of ladle refining. The results show that large temperature gradients exist towards the surface of the steel melt for the sampling occasion when no slag is present on the steel surface, as can be expected. It was also observed that the oxygen activity is higher in the lower measurement position than in the upper during the majority of the ladle refining. This is explained by the oxygen activity's temperature dependence. By using the data from this study and previously reported plant trials it was shown that Si-SiO2 or Al-Al2O3 equilibrium controls the oxygen activity.

Keyword
oxygen activity; temperature; steel; ladle; deoxidation; equilibrium
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-8122 (URN)10.2355/isijinternational.48.438 (DOI)000255321600008 ()2-s2.0-46749107601 (Scopus ID)
Note

QC 20100708. Uppdaterad från accepted till published (20100708).

Available from: 2008-03-19 Created: 2008-03-19 Last updated: 2017-12-14Bibliographically approved
4. The Influence of Vacuum Treatment on Inclusion Composition: Theoretical Study
Open this publication in new window or tab >>The Influence of Vacuum Treatment on Inclusion Composition: Theoretical Study
2010 (English)In: High Temperature Materials and Processes, ISSN 0334-6455, E-ISSN 2191-0324, Vol. 29, no 3, 179-187 p.Article in journal (Refereed) Published
Abstract [en]

A theoretical study has been conducted into how inclusion composition is influenced by vacuum treatment. Software for computational thermodynamics has been used to simulate how the conditions in a steel melt changed as a result of a decreased pressure. A case study was made on a low sulphur bearing steel (1.4% Cr, 1.0%C). The calculations show the effect of sulphur, oxygen, carbon and temperature on precipitated phases in the steel melt. The result shows that at low pressure, formation of carbon monoxide takes place. For 5 ppm O and 10 ppm S the CO-gas becomes thermodynamically stable below 40 torr. Oxide phases will be reduced and CaS will be the stable phase.

Keyword
vacuum, inclusions, calcium aluminates
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-8123 (URN)000283965900007 ()2-s2.0-78149246585 (Scopus ID)
Note
QC 20100709. Uppdaterad från submitted till published (20101210). Tidigare submitted till Steel Research International, med titeln "The Influence of Vacuum Treatment on Inclusion Composition: Part 1: Theoretical Study".Available from: 2008-03-19 Created: 2008-03-19 Last updated: 2017-12-14Bibliographically approved
5. The Influence of Vacuum Treatment on Inclusion Composition: Laboratory Study
Open this publication in new window or tab >>The Influence of Vacuum Treatment on Inclusion Composition: Laboratory Study
2010 (English)In: High Temperature Materials and Processes, ISSN 0334-6455, E-ISSN 2191-0324, Vol. 29, no 3, 189-202 p.Article in journal (Refereed) Published
Abstract [en]

A series of lab scale trials have been carried out with the aim of investigating the effect of vacuum treatment on inclusion chemistry. A ball bearing steel grade (1.47 wt% Cr, 0.96 wt% C) has been melted in a controlled atmosphere in magnesia and alumina crucibles. A laboratory vacuum furnace, which makes it possible to carry out sampling during vacuum treatment, was used. Samples were taken before and during vacuum treatment. Thereafter, the inclusion compositions in the samples were determined using SEM in combination with EDS. The inclusions found were mainly sulphides containing both manganese and chromium. In addition to these, MgO-Al2O3 oxides were found with varying contents of calcium. Furthermore, a few, calcium sulphides were found. Additionally, it was possible to find a small difference in the average inclusion composition in the samples taken before and during vacuum. More specifically, it was seen that the composition of the inclusions becomes more sulphide-rich during vacuum treatment. The change of average inclusion composition was not as large as suggested from theoretical computations and oxides were found at the same extent during vacuum as before. Another finding was that the inclusions from the trial where an alumina crucible was used contained no magnesia, but that the inclusions contained magnesia when a magnesia crucible was used. Even though the effect of vacuum treatment on the inclusion compositions was found to be small, it was concluded that vacuum treatment could not to be neglected when studying inclusions influenced by top slag/metal reactions.

Keyword
Experiment, vacuum, laboratory, inclusions, calcium aluminates
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-8124 (URN)000283965900008 ()2-s2.0-78149273281 (Scopus ID)
Note
QC 20100709. Uppdaterad från submitted till published (20101210). Tidigare submitted till Steel Research International, med titeln "The Influence of Vacuum Treatment on Inclusion Composition: Part 2: Laboratory Study".Available from: 2008-03-19 Created: 2008-03-19 Last updated: 2017-12-14Bibliographically approved

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