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Analysis of Largest Sulfide Inclusions in Low Carbon Steel by Using Statistics of Extreme Values
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.
2011 (English)In: Steel Research International, ISSN 1611-3683, Vol. 82, no 4, 313-322 p.Article in journal (Refereed) Published
Abstract [en]

The statistics of extreme values was applied for the determination of the largest sulfide inclusions with different morphology in low carbon steel samples by using both two-dimensional (2D) observations on the polished cross section and three-dimensional (3D) observations on a surface of a film filter after electrolytic extraction of the samples. It was found that the globular, rod-like and dendritic sulfides in the molten steel sample as well as the elongated sulfides in the rolled steel sample can be successfully extracted from the both samples, and analyzed precisely by using extreme value analysis in 3D. Based on the geometrical considerations of the probability for measurement of the true length of rod-like and elongated inclusions on a cross section, it was found that this probability for inclusions decreases dramatically with an increasing real aspect ratio value of them. Particularly for the determination of the true length for elongated inclusions in the rolled steel sample by 2D investigations on a metal cross section, it is required to be cut investigating section of steel sample within +/- 1 degree against rolling direction. Therefore, a 3D observation is considered to be more preferable and accurate than the conventional cross sectional observation in 2D, due to the possibility for the measurements of the real size of them.

Place, publisher, year, edition, pages
2011. Vol. 82, no 4, 313-322 p.
Keyword [en]
aspect ratio, cross section, electrolytic extraction, extreme value analysis, maximum size, sulfide inclusion
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-33942DOI: 10.1002/srin.201000141ISI: 000290050900002Scopus ID: 2-s2.0-79953685471OAI: oai:DiVA.org:kth-33942DiVA: diva2:421783
Note
QC 20110609Available from: 2011-06-09 Created: 2011-05-23 Last updated: 2012-03-28Bibliographically approved
In thesis
1. Control of Alloy Composition and Evaluation of Macro Inclusions during Alloy Making
Open this publication in new window or tab >>Control of Alloy Composition and Evaluation of Macro Inclusions during Alloy Making
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In order to obtain a good performance and predict the properties of alloys, it is necessary to control the contents of alloying elements and to evaluate a largest inclusion in the product. Thus, improved techniques for both control of alloy elements and evaluation of the large inclusion in products will enable us to provide better qualities of the final products.

In the case of one Ni alloy, (NW2201, >99 mass%Ni), the precise control technique of Mg content is important to obtain a good hot-workability. Hereby, the slag/metal reaction experiments in a laboratory have been carried out at 1873 K, so that the equilibrium Mg content and kinetic behavior can be understood. More addition of Al in the melt as well as higher CaO/Al2O3 value of slag resulted in higher amount of Mg content in Ni. For the same conditions of Al content and slag composition, the mass transfer coefficient of Mg in molten Ni was determined as 0.0175 cm/s. By applying several countermeasures regarding the equilibrium and kinetic process to the plant trials, the value of the standard deviation for the Mg content in an alloy was decreased till 0.003 from 0.007 mass%.

The size measurements of largest inclusions in the various alloys (an Fe-10mass%Ni alloy, 17CrMo4 of low-C steel and 304 stainless steel) were carried out by using statistics of extreme values (SEV). In order to improve the prediction accuracy of this method, three dimensional (3D) observations were applied after electrolytic extraction. In addition, the relationship of extreme value distribution (EVD) in the different stages of the production processes was studied. This was done to predict the largest inclusion in the products at an early stage of the process.

A comparison of EVDs for single Al2O3 inclusion particles obtained by 2D and 3D observations has clarified that 3D observations result in more accurate EVD because of the absence of pores. Also, it was found that EVD of clusters were larger than that of single particles. In addition, when applying SEV to sulfide inclusions with various morphologies, especially for elongated sulfides, the real maximum sizes of them were able to be measured by 3D observations. Geometrical considerations of these particles clarified the possibility of an appearance of the real maximum inclusion sizes on a cross section to be low. The EVDs of deoxidation products in 304 stainless steel showed good agreement between the molten steel and slab samples of the same heat. Furthermore, the EVD of fractured inclusion lengths in the rolled steel were estimated from the initial sizes of undeformed inclusions which were equivalent with fragmented inclusions. On the other hand, from the viewpoint of inclusion width, EVD obtained from perpendicular cross section of strips was found to be useful to predict the largest inclusion in the final product with less time consumption compared to a slab sample.

In summary, it can be concluded that the improvement of the techniques by this study has enabled to precisely control of alloy compositions as well as to evaluate the largest inclusion size in them more accurately and at an earlier stage of the production process.

Place, publisher, year, edition, pages
Stockholm: KTH, 2010. 53 p.
Keyword
slag/metal reaction, equilibrium, kinetics, largest inclusion, statistics of extreme values, electrolytic extraction, cross section, inclusion morphology
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-27773 (URN)978-91-7415-846-5 (ISBN)
Public defence
2011-01-14, B1、, Brinellvägen 23, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20101222Available from: 2010-12-22 Created: 2010-12-22 Last updated: 2012-03-28Bibliographically approved

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