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Changes in inclusion characteristics during sampling of liquid steel
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.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.
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2012 (English)In: Ironmaking & Steelmaking, ISSN 0301-9233, Vol. 39, no 1, 67-75 p.Article in journal (Refereed) Published
Abstract [en]

In order to improve the process control during the steelmaking process, it is essential to obtain knowledge, as well as interpret information, on the characteristics of non-metallic inclusions in liquid steel samples. The purpose of the present study is to evaluate the effect of different sampling conditions on the inclusion characteristics. The changes in number, size and composition of primary inclusions due to the precipitation of secondary inclusions are studied. This is investigated in laboratory scale samples from an Fe-10 mass-%Ni alloy as well as industrial steel samples with varying contents of oxygen and sulphur. The results show that in most cases, the inclusion population can be separated, using a size condition, into primary (>= 0.6 mu m) and secondary (< 0.6 mu m) inclusions. Overall, the Lollipop sample with a 6 mm thickness can be recommended, because most of the secondary inclusions in low sulphur steels can be removed from the total particle size distribution. However, during sampling of liquid steel containing high levels of sulphur, a significant amount of sulphides precipitate heterogeneously onto primary inclusions, making it impossible to use size or morphology to separate the population. Finally, the numbers of secondary inclusions in the steel samples were found to increase significantly with an increased sample cooling rate. However, the number of primary inclusions was found to be almost constant and independent of the sample cooling rate.

Place, publisher, year, edition, pages
2012. Vol. 39, no 1, 67-75 p.
Keyword [en]
Inclusions, Characteristics, Sampling, Precipitation
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-26096DOI: 10.1179/1743281211Y.0000000044ISI: 000298488800010Scopus ID: 2-s2.0-84255167102OAI: oai:DiVA.org:kth-26096DiVA: diva2:369919
Note
QC 20101112. Updated from submitted to published. 20120127 Available from: 2010-11-12 Created: 2010-11-12 Last updated: 2012-01-27Bibliographically approved
In thesis
1. An Experimental Study of a Liquid Steel Sampling Process
Open this publication in new window or tab >>An Experimental Study of a Liquid Steel Sampling Process
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

During the steelmaking process samples are taken from the liquid steel, mainly to assess the chemical composition of the steel. Recently, methods for rapid determination of inclusion characteristics (size and composition) have progressed to the level where they can be implemented in process control. Inclusions in steel can have either good or detrimental effects depending on their characteristics (size, number, composition and morphology). Thereby, by determination of the inclusion characteristics during the steelmaking process it is possible to steer the inclusion characteristics in order to increase the quality of the steel. However, in order to successfully implement these methods it is critical that the samples taken from the liquid steel represent the inclusion characteristics in the liquid steel at the sampling moment.

 

The purpose of this study is to investigate the changes in inclusion characteristics during the liquid steel sampling process. Experimental studies were carried out at steel plants to measure filling velocity and solidification rate in real industrial samples. The sampling conditions for three sample geometries and two slag protection types were determined. Furthermore, the dispersion of the total oxygen content in the samples was evaluated as a function of sample geometry and type of slag protection. In addition, the effects of cooling rate as well as oxygen and sulfur content on the inclusion characteristics were investigated in laboratory and industrial samples. Possibilities to separate primary (existing in the liquid steel at sampling moment) and secondary (formed during cooling and solidification) inclusions depending on size and composition were investigated. Finally, in order to evaluate the homogeneity and representative of the industrial samples the dispersion of inclusion characteristics in different zones and layers of the samples were investigated.

 

It was concluded that the type of slag protection has a significant effect on the filling velocity and the sampling repeatability. Furthermore, that the thickness of the samples is the main controlling factor for the solidification rate. It was shown that top slag can contaminate the samples. Therefore, the choice of slag protection type is critical to obtain representative samples. It was shown that the cooling rate has a significant effect on the number of secondary precipitated inclusions. However, the number of primary inclusions was almost constant and independent on the cooling rate. In most cases it is possible to roughly separate the secondary and primary oxide inclusions based on the particle size distributions. However, in high-sulfur steels a significant amount of sulfides precipitate heterogeneously during cooling and solidification. This makes separation of secondary and primary inclusions very difficult. Moreover, the secondary sulfides which precipitate heterogeneously significantly change the characteristics (size, composition and morphology) of primary inclusions. The study revealed that both secondary and primary inclusions are heterogeneously dispersed in the industrial samples. In general, the middle zone of the surface layer is recommended for investigation of primary inclusions.

 

Place, publisher, year, edition, pages
Stockholm: KTH, 2010. xiii, 52 p.
Keyword
liquid steel sampling, inclusion characteristics, sampling conditions, sample homogeneity.
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-26048 (URN)978-91-7415-792-5 (ISBN)
Public defence
2010-11-26, E3, Lindstedsvägen 3, KTH, Stockholm, 09:00 (English)
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Supervisors
Note
QC 20101112Available from: 2010-11-12 Created: 2010-11-10 Last updated: 2011-04-20Bibliographically approved

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