A study on tundish powders and their influence on tundish slag chemistry and steel cleanness for carbon and stainless steels

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ISSN 1018-5593
European Commission
technical steel research
Steelmaking
A study of tundish powders and
their influence on tundish slag chemistry
and steel cleanness for carbon
and stainless steels
STEEL RESEARCH European Commission
technical steel research
Steelmaking
A study of tundish powders and
their influence on tundish slag chemistry
and steel cleanness for carbon
and stainless steels
H. Watkinson, K. Bain, V. Ludlow
British Steel, Teesside Technology Centre
Eston Road - PO Box 11
Grangetown
Middlesbrough TS6 6UB
United Kingdom
Contract No 7210-CC/803
1 August 1990 to 31 January 1994
Final report
Directorate-General
Science, Research and Development
1997 EUR 17845 EN LEGAL NOTICE
Neither the European Commission nor any person acting on behalf of the Commission
is responsible for the use which might be made of the following information.
A great deal of additional information on the European Union is available on the Internet.
It can be accessed through the Europa server (http://europa.eu.int).
Cataloguing data can be found at the end of this publication.
Luxembourg: Office for Official Publications of the European Communities, 1997
ISBN 92-828-1776-8
© European Communities, 1997
Reproduction is authorised provided the source is acknowledged.
Printed in Luxembourg
PRINTED ON WHITE CHLORINE-FREE PAPER CONTENTS
Page
1. OBJECTIVES 7
2. INTRODUCTION
3. SURVEY OF COMMERCIAL TUNDISH POWDERS IN USE WITHIN
BRITISH STEEL AT THE START OF THE PROJECT 8
4. ASSESSMENT OF COMMERCIAL TUNDISH POWDERS DURING CASTING 10
4.1 Sampling Methods 10
4.2 Assessment of Inclusion Pick-Up1
4.3 Results of Tundish Powder and Slag Sampling 1
5. POWDER DESIGN PHILOSOPHY7
5.1 Chemical Composition
5.2 Thermal Insulation8
5.3 Good Inclusion Assimilation Capabilities9
5.4 Formation of Non-Reactive Slag
6. LABORATORY SCALE ASSESSMENT OF EXPERIMENTAL POWDERS 20
6.1 Thermal Insulation Properties 20
6.2 Spreadability of Powders2
6.3 Small Scale Melting Experiments
6.4 Thermodynamic Modelling of Experimental Tundish Powder Behaviour 23
6.5 Large Scale Pilot Plants
7. PRODUCTION SCALE DEVELOPMENT OF BASIC TUNDISH
COVER POWDER4
7.1 Assessment of Inclusion Flotation within the Tundish 2
7.2 Experimental Arrangement 2
7.3 Evaluation of Experimental Blend 4
7.4n ofl Blend 10 Series6
8. SOURCE OF MANGANESE OXIDE IN TUNDISH SLAG 32
9. EVALUATION OF INCLUSION ASSIMILATION BY TUNDISH SLAG 33
10. CONCLUSIONS 3
11. FURTHER WORK4
12. REFERENCES5
TABLES9
FIGURES 70
APPENDIX 1: TECHNICAL ANNEX 101 LIST OF TABLES
1. Tundish Cover Powders in General Use in British Steel at the Start of the Project (1990)
2. Details of Commercial Tundish Powder Sampling Trials
3. Average Chemical Analyses of As-Received Commercial Tundish Powders Used During
Sampling Trials
4. Variation in the Principal Components of the Liquid Slag During Casting with Powder
F
5.n in the Principals of the Liquid Slag During Casting with Powder
K
6(a). Normalised Slag Analysis Scunthorpe Works Slab Tundish, Trial 1, Powder Β
6(b).d Slagses Slab, Trial 2,r Β
6(c).d Slag Analysise Works Slab Tundish, Trial 3, Powder Β
6(d).d Slags Teesside Works Bloom Tundish , Trial 1,r A
6(e). Normalised Slag Analysises Bloomh , Trial 2, Powder A
6(f).d Slags Teesside Worksm Tundish , Trial 3,r A
6(g).d Slag Analysis Teesside Works Bloom Tundish , Trial 4, Powder A
7. Variation in the Principal Components of the Liquid Slag During Casting with Powder
Al
8.n in the Principals of the Liquid Slag During Casting with Powder
O
9. Variations in the Principle Components of Liquid Slag During Casting with Powder I
10. Comparison of Thermal Insulation Properties and Bulk Densities for Selected
Commercial and Experimental Tundish Powders
11. Composition and Overall Chemistry of Experimental Tundish Powders
12. Particle Size Distribution of Commercial Tundish Powders
13.e Sizen of Experimentalhs
14. Spreading Test Results of Raw Materials and Powder Blends
15. Composition and Overall Chemistry of Experimental Tundish Powders
16. Chemical Analysis of the Trial Samples Taken Using Blend 4
17. Analysis of the Principal Oxides in Various Size Fractions of Blend 4 Powder and 10 min
Slag Sample from Blend 4 Trial
18(a). Normalised Slag Analyses for Blend 10 Plant Experiments
18(b).d Slags for Plant Standard Powder
19(a).d Slag Analyses for Blend 10A Plants
19(b).d Slags for Plant Standard Powder
20(a). Normalised Slag Analyses for Blend 10B Plant Experiments
20(b).d Slags for Plant Powder
21.d Slag Analyses for Blend Bl Plants
22. Normalised Slags ford 10A-LC Plant Experiments
23. Mould Metal Carbon Analysis Using Blend 10A-LC
24.d Tundish Slag Analyses for Blend 10A + Tracers Plant Experiment
25.d Mould Slag? for Blend 10A +s Plantt
LIST OF FIGURES
1 Schematic of Tundish Cover Core Sampling Device
2c ofh Powder Layer Structure
3. Micrograph of Tundish Cover Core Sample. Powder Β Scunthorpe Slabcaster
4 Photomicrograph of Sinter Dissolving into Liquid Slag Layer, Powder Β - Scunthorpe
Slabcaster
5. Oxide Pick-Up in the Fused Slag, Trial 5 - Powder A
6. Dark Field Illuminated Micrograph of Powder F "As-Received"
7 Metallic Droplets Held in a Sintered Alumina Matrix from the Metal Interface
Powder F
8. Dark Field Illuminated Micrograph of Powder Κ "As-Received" 9. Mullite Lathes Crystallising from MnO-Al203-Si02 Glass in the Fused Slag Layer of
Powder Β
10. CaO-MgO-Al203-Si02 Pseudo Ternary Diagrams
11. Variation of Thermal Insulation with Particle Size for Bubble Alumina
12.n ofln withe Size for Magnesite
13. Effect of Particle Size Distribution on Experimental Blend 4 Thermal Insulation
14. Apparatus for Testing the Flowability of Powders
15. Comparison of Fusion Times with Thermal Insulation Values for Experimental Powders
16. Percent Liquid Slag Formed at Equilibrium by the Bulk Composition of the
Experimental Powder
17. Factors Influencing Interaction of Minerals in Tundish Cover Powders
18. Effect of a Second Mineral on the Liquidus Temperature of Calcined Dolomite
19.t of Alumina Pick-Upon Liquid Slag Layer Chemistry of the Experimental Powder
at1530°C
20. Schematic Diagram of 'Pod' Type Metal Samplers
21. Position Metal Samples Were Obtained in Tundish
22. Variation of Steel Total Oxygen - With Dam/Weir Tundish
23.n of Steelln - Nonrh
24. Schematic Layout of Tundish Used for Powder Evaluation Experiments
25. First Plant Evaluation Experiment Using Blend 10 Mould Metal Oxygen Contents
26.ttn Using Blend 10 Tundish Metal Oxygen Contents
27. Second Plant Evaluation Trial Using Blend 10 Mould Metal Oxygen Contents
28. Third Plantn Experiment Using Blend 10 Mould Metal Oxygen Contents
29. Firstt Evaluationtgd 10Adlns
30. Kapoor-Frohberg Tundish Slag MnO Activity Prediction
31. Variation of Tundish Slag MnO with Steel Mnfor High Silica Tundish Powders
32. Sensitivity of Slag Analysis to Changes in Slag Mass A STUDY OF TUNDISH POWDERS AND THEIR INFLUENCE ON TUNDISH SLAG
CHEMISTRY AND STEEL CLEANNESS FOR CARBON AND STAINLESS STEELS.
British Steel pic
ECSC Agreement No. 7210.CC/803
Final Technical Report
1. OBJECTIVES
The aim of the research was to develop the composition of tundish cover powders to optimise
inclusion entrapment, whilst maintaining insulating properties, and thereby improve the
cleanness of both carbon and stainless steel. A secondary aim was to evaluate the potential of
suitable powders to promote refining in the tundish.
The programme of work was planned in a series of well defined stages:-
(i) A survey of tundish powders in current use within British Steel at the time of project
initiation (1990).
(ii) Assessment of available powder types.
(iii)t of commercial powders identified in (i) and (ii) during use on specified
British Steel casters whilst casting low carbon aluminium killed and medium carbon
killed steels as well as stainless steels.
(iv) Assessment of commercial powders in laboratory scale experiments to measure
insulation and fusion rates.
(v) With the knowledge gained from (i) to (iv), design a series of powders with good alumina
assimilation whilst maintaining good insulation properties.
(vi) Test the new powder blends on pilot scale experiments, followed by plant scale tests.
2. INTRODUCTION
The continuous casting tundish serves two major roles, firstly as a reservoir and distributor of steel
to the mould(s) and secondly, and increasingly more important, as an extension to secondary
steelmaking operations* 1·2). This latter aspect has resulted in most major steel producers
investing heavily in the development of various types of tundish technology<3>4>5>6>7>8,9,10>. These
technologies all seek to improve and exploit the opportunity which the tundish presents for
flotation of non-metallic inclusions with the objective of improving steel cleanness. At the
inception of this project there was a great wealth of published information concerning the
optimisation of tundish design for a flotation of inclusions, but there was little reported data on the
performance and behaviour of tundish cover powders*'-.11»12.13.14', the design of which are critical
for optimum assimilation of inclusions floated to the metal surface.
A tundish cover powder should satisfy three basic requirements'1'; thermal insulation to minimise
heat loss from the steel surface, prevention of reoxidation of the steel by the atmosphere and
assimilation of non-metallic inclusions. In addition to these three basic criteria the slag derived
from the powder should cause no intrinsic reoxidation of the liquid steel and should have a low oxygen transport potential. Low powder consumption and minimum production costs are also
requirements.
At the start of the project in 1990 the majority of tundish cover powders in general use within
British Steel for the production of carbon steels were either acidic (strip grade casters) or neutral
(long product and plate grade casters). Rice husk ash (RHA) based powders (acidic) were known to
provide good insulation, hence maximum sequence length, at low cost, but were unsuitable for
inclusion assimilation. Acidic powders were also know to produce exchange reactions between the
molten slag and the steel'5' and were therefore unsuitable for certain aluminium killed steels.
Alumino-silicate powders (neutral) provided some insulation and, as was thought at the time,
some of the benefits of a basic powder at a low cost, but without the problems of crusting of basic
powders experienced within British Steel at the time.
No basic powders were in general use within British Steel at the start of the project, but in the
stainless casting area, when casting certain steel grades critical to carbon pick-up and during the
casting of specific clean steels, trials had been and were being undertaken using basic tundish
powders based on lime, lime-alumina or magnesite, with or without carbon. British Steel's
experience with these basic powders was that they did absorb alumina but they had poor
insulation characteristics unless a very deep cover was maintained, and they often formed and
impenetrable crust, sometimes after less than 30 mins casting, which prevented movement of ladle
shrouds and tundish stopper mechanisms.
The knowledge of tundish cover powders at the time of thus suggested that their performance was
a compromise between insulation and inclusion assimilation, with a powder efficient at both
requiring development.
A duplex powder practice had been developed at the now closed British Steel plant at Ravenscraig
for use on critical clean steels but the disadvantage was that the lower active powder layer could
not be replenished without affecting significantly the powder/slag cover.
The development of tundish cover powder compositions to optimise inclusion assimilation whilst
maintaining sufficient insulating properties was the main objective of the research. This was
achieved by a detailed survey of tundish powders then in use in British Steel and a critical
assessment of their behaviour and performance during use by physical and chemical examination
of powder and slag samples taken during casting. Based on the data gathered during the
assessment, a chemical composition and a range of mineral formulations were devised for a powder
best suited to the absorption of alumina; alumina being considered the most problematic steel
cleanness issue within British Steel. Evaluation of the new powder formulations was first by
laboratory scale studies and then by full scale production plant experiments.
3. SURVEY OF COMMERCIAL TUNDISH POWDERS IN USE WITHIN BRITISH
STEEL AT THE START OF THE PROJECT
At the start of the project the general tundish powder philosophy within British Steel was to use
powders which provided what was perceived as maximum thermal insulation at minimum cost. A
survey of the tundish powders in use at that time, Table 1, indicated that nevertheless there was a
wide range ofhs in use, with some plants using more than one powder of a similar
type. The reason being was that the plants wanted more than a single source of supply. The
survey showed there to be no overall preference for any given powder blend.
Powder samples obtained for chemical analysis showed that the compositions were within the
manufacturers specified ranges, but there were variations of composition from batch to batch.
This was expected as the specified composition ranges were wide.
Particle size distributions by sieve analysis were determined for selected samples of powder, and
pétrographie examination of both the sieved fractions and raw powder was undertaken. These
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