Oscillation Mark Formation in Continuous Casting Processes

Microsoft Word - introsidor.docby
framlägges till offentlig granskning för avläggande av
Teknologie Licentiatexamen, fredag 31 oktober 2003, kl. 15.00,
B2, Brinellvägen 23, Stockholm.
Några lånade ord om min syn på forskning, undervisning och – tja, livet... Till eftertanke...
Om jag vill lyckas med att föra en människa mot ett bestämt mål måste jag finna henne där hon är, och börja just där. Den som inte kan detta lurar sig själv när hon tror att hon kan hjälpa andra. För att hjälpa någon måste jag visserligen förstå mer än vad han gör men först och främst förstå det han förstår. Om jag inte kan det så hjälper det inte att jag kan och vet mer. Vill jag ändå visa hur mycket jag kan så beror det på att jag är fåfäng och högmodig och egentligen vill bli beundrad av den andre istället för att hjälpa honom. All äkta hjälpsamhet börjar med ödmjukhet inför den jag vill hjälpa och därmed måste jag förstå: Att detta med att hjälpa inte är att vilja härska utan att vilja tjäna. Kan jag inte förstå detta kan jag inte heller hjälpa någon. Sören Kierkegaard
Oscillation Mark Formation in
Royal Institute of Technology S-100 44 Stockholm, Sweden
Abstract
Oscillation marks are ripples formed on the surface of continuously cast material. They may cause cracking and decrease the yield of the process since some material must be grinded away to avoid crack growth. A study of break-out shells, a full- scale water model study and full-scale experiments in four different plants have been performed to analyse the formation of oscillation marks. The hypothesis initiating the studies was that there is an optimal oscillation frequency. Material cast at the optimal frequency will have smaller oscillation marks and fewer cracks and, maybe most important, all marks are of the same character and depth. The optimal oscillation frequency is determined by its relation to casting velocity and interfacial tension between metal and protective medium, e.g. slag:
a2 vf ⋅
= where ( )gsurroundinmetal
σ⋅ =
The results from the experiments indicate that there is an optimal frequency at which the surface quality gets better. A theoretical analysis has been worked out. The suggestion is that the marks form as the surface tension balance controlling the meniscus shape collapses. The collapses occur when the meniscus grows too high and bulges out towards the mould wall. Calculations were performed to analyse the influence of interfacial tension on the oscillation marks. The results show that the higher the interfacial tension gets the deeper and wider will the marks get. Instead of analysing the friction forces acting in the meniscus region, it was assumed that the oscillation cause a variation of the interfacial tension. In some of the calculations, the interfacial tension was changed from one value to another at some point. The mark shape then becomes a combination of the different cases.
Oscillationsmärkesbildning vid
kontinuerliga gjutprocesser
Sammanfattning
Oscillationsmärken är tvärgående ”räfflor” som bildas på ytan av kontinuerligt gjutet material. Märkena kan orsaka sprickbildning samt minska utbytet av processen eftersom man måste slipa ämnena för att undvika spricktillväxt. En undersökning av genombrottsskal, en fullskalig vattenmodellstudie och fullskaliga experiments vid fyra olika gjutningsanläggningar har genomförts för att analysera bildningen av oscillationsmärken. Hypotesen som initierade arbetet var att det finns en optimal oscillationsfrekvens. Material som gjuts vid den optimala frekvensen får mindre oscillationsmärken och färre sprickor och, kanske viktigast, alla märken kommer att vara av samma typ och vara lika djupa. Den optimala oscillationsfrekvensen bestäms av dess relation till gjuthastighet och gränsytspänningen mellan smälta och omgivande medium, till exempel slagg:
a2 vf ⋅
= där ( )gsurroundinmetal
σ⋅ =
Resultaten från experimenten indikerar att det finns en optimal frekvens vid vilken ytkvaliteten blir bättre. En teoretisk analys har utarbetats och föreslår att oscillationsmärkena bildas när ytspänningsbalansen som bestämmer meniskens form kollapsar. Kollapsen inträffar när menisken blir för hög och bular ut mot kokillväggen. Inverkan av gränsytspänningen på märkenas profil har analyserats med hjälp av beräkningar. Resultaten visar att högre gränsytspänning ger djupare, och bredare märken. Istället för att analysera de friktionskrafter som uppkommer mellan smälta och slagg i meniskområdet, antogs det att oscillationen orsakar en variation av gränsytspänningen. I några beräkningar ändrades gränsytspänningen från ett värde till ett annat vid en viss tidpunkt. Märkets form förändrades till en kombination av formerna för de olika gränsytspänningarna.
The thesis includes the following supplements: Supplement 1 Thoughts about the Initial Solidification Process during Continuous Casting of Steel Fredriksson, H. and Elfsberg, J. Scandinavian Journal of Metallurgy 2002, Vol. 31, pp. 292-297 I prepared samples and performed microscopy studies. I also took part in writing the report. Supplement 2 Experimental Study of the Formation of Oscillation Marks in Continuous Casting of Steel Billets Elfsberg, J., Widell, B., Fredriksson, H. 4th European Continuous Casting Conference, Oct 14-15 2002, Birmingham, England. I performed the experiments, most of the evaluations and report writing. Supplement 3 Oscillation Mark Formation on Continuously Cast Copper Elfsberg, J., Fredriksson, H. ISRN:KTH:MG-INR-03:02 SE TRITA-MG-2003:02 I performed the experiments, most of the evaluations and report writing. Supplement 4 Oscillation Mark Formation on Continuously Cast Stainless Steel and Carbon Steel Slabs Elfsberg J., Fredriksson H. ISRN:KTH:MG-INR-03:03 SE TRITA-MG-2003:03 I performed the experiments, most of the evaluations and report writing. Supplement 5 Theoretical Study of Oscillation Mark Formation in Continuous Casting Processes Elfsberg J., Fredriksson H. ISRN:KTH:MG-INR-03:04 SE TRITA-MG-2003:04 I did the calculations and the report writing.
Contents 1. Introduction…………………………………………………………………...... 1 2. History and Principles of Continuous Casting…………………………….. 1 History 1 The Principles of Conventional Continuous Casting 3 Casting Powder 4 Oscillation Parameters 5 3. Review on the Formation of Oscillation marks……………………………. 6 4. Experimental work………………………………………………………….. 10 Study of break-out shells 10 DDS- Steel Slabs 11 Fundia Special Bar AB – Steel Billets 11 Fundia Armeringsstål A/S – Steel Billets 11 Outokumpu Copper AB – Copper Strips 12 Outokumpu Copper AB – Water Model 12 Why modelling liquid metal flow using water 12 Avesta Polarit AB – Stainless Steel Slabs 13 5. Experimental Results……………………………………………………….. 14 Study of break-out shells 14 DDS- Steel Slabs 16 Fundia Special Bar AB – Steel Billets 17 Fundia Armeringsstål A/S – Steel Billets 19 Outokumpu Copper AB – Copper Strips 19 Outokumpu Copper AB – Water Model 20 Avesta Polarit AB – Stainless Steel Slabs 21 6. Theoretical background……………………………………………………... 24 6.1 Heat Transfer……………………………………………………………….. 24 Conduction 24 Radiation 25 Convection 25 Heat transport in Continuous Casting 25 Heat transport in the Model for Oscillation Mark Formation 26 The Heat Transfer from the Melt to the Shell 27 The Heat Transfer from the Shell to the Mould 28 Determination of Solid Shell Growth Rate and Shell Tip Radius 28 Solidification in the z-direction 29 6.2 Surface Tension Balance and Angles between Phases…………………… 30 6.3 Pressure balance and Meniscus Shape……………………………………. 32
Derivation of the Laplace Capillary Constant 33 Pressure Balance and Angles between the Phases 35 7. Theoretical analysis………………………………………………………….. 36 7.1 The Model for the Oscillation Mark Formation…………………………... 36 The First Approach 37 The Second Approach 37 Optimal Oscillation Frequency 38 7.2 Calculation of Oscillation Mark Profile…………………………………… 39 The First Approach 39 The Second Approach 40 8. Results of Calculations………………………………………………………. 40 The First Approach 40 The Second Approach 41 9. Discussion……………………………………………………………………. 44 10. Future Works………………………………………………………………... 46 11. Acknowledgements…………………………………………………………. 47 12. References…………………………………………………………………... 48 Supplements 1-5
1
1. Introduction All metal manufacturing includes a solidification process. The metal is either atomised to a powder, or it is cast. There is several casting methods used today. Many of the methods use the mould only once, for example sand mould casting, the lost-wax method and precision casting. For larger quantities of liquid metal, there are basically two methods: ingot casting and continuous casting. Ingot casting normally uses a permanent mould made by cast iron isolated by ceramic material. There are limitations with the ingot casting methods. The structure in the centre of the ingot will get coarse which decreases the strength of the metal and when casting larger quantities of metal the handling of the ingots gets very complex. To be able to increase the manufacturing capacity without decreasing the quality, the development of a continuous casting process started in 1856. Since then continuous casting has grown to be the major technique for casting steel. Copper is today continuously cast using a similar design as for steel casting. The heart of a continuous casting machine is the mould. The mould is an open- ended tube in which the metal is poured. The liquid metal is protected by either a gas, inert or reductive, or some melted compound. For steel the most common protection media are casting powder, i.e. an oxide mixture, or rape-seed oil. In the mould the liquid metal starts to solidify and during the solidification, surface defects called oscillation marks are formed. The oscillation marks appears as grooves perpendicular to the casting direction. They are typically 0.1-1 mm deep. The oscillation marks may work as initiation point for cracks and beneath them there is a zone with higher risk for inclusions, pores and segregation. There may also be cracks on the cast surface formed in the mould. It is clear that the surface quality of the cast material is mainly determined by the process in the mould. The formation of oscillation marks has been extensively examined by many authors and there are several models describing the formation of oscillation marks in continuous casting. To be able to control the formation of marks, the mechanism of the formation must be fully known. This work suggests that there is an optimal oscillation frequency. At the optimal oscillation frequency, it is suggested that no, or very small, oscillation marks form. 2. History and Principles of continuous casting History To be able to increase the casting capacity without decreasing the quality, the development of a continuous casting process started in 1856. Henri Bessemer suggested a model in which liquid metal was poured between to water-cooled rolls.
2
Figure 1. The twin-roll caster suggested by Bessemer [1] The process was by then hard to control and the cast material was of very poor quality. In 1887 R M Daelen suggested a process using a vertical water-cooled mould open in its top and bottom. In this process problems with sticking occurred when casting steel and not until 1933, when Siegfried Junghans introduced the concept of mould oscillation, a functional continuous casting process for steel could be developed. In Junghans development the mould was moved downward at a velocity equal to the casting speed for approximately three- quarters of each cycle followed by a rapid return to the starting position. There were therefore no relative motion between the mould and the strand shell during the down-stroke. In 1954 there was a major break-through in continuous casting steel processes. A new oscillation profile, suggested by Concast/Halliday, produced a condition called negative strip. Negative strip is when the down-stroke velocity in each cycle exceeds the casting speed. Sinusoidal oscillation was first used on two Russian slab casters installed in 1959. Presently, sinusoidal is essentially the standard mode of oscillation worldwide. It is relatively simple to design and has the advantage of lower moments of inertia and smaller jerk (the rate of change of acceleration with respect to time). If the jerk is too high, problems related to vibration, noise and wear of moving parts and bearings can occur. However, non-sinusoidal oscillation has received renewed attention and with sophisticated hydraulic oscillators, a wide variety of profiles may come in use. The benefits of continuous casting compared to ingot casting are: Higher yield
Smaller number of necessary manufacturing steps
More mechanised casting process
3
A lot of development of the continuous casting processes has taken place the last decades. For example different electromagnetic devices have been introduced. Electromagnetic fields can be used for braking and for stirring the liquid metal in the mould. It is also possible to press the melt away from the mould walls with a strong electromagnetic field. Today conventional continuous casting, described in figure 2, is the dominant casting process for steel. In the future, electromagnetic casting may get in common use as well as the twin-roll caster suggested by Bessemer in 1856. Many of the problems can today be handled, but the process is still very expensive [1]. The Principles of Conventional Continuous Casting Figure 2 shows the principal design of a conventional continuous casting machine:
Figure 2. A continuous casting machine [1]. Melt flows from a ladle to a tundish and further to a mould. Between the ladle and the tundish, and from tundish to mould, ceramic tubes protect the melt. Beneath the mould there are a secondary cooling zone and a straightening zone. From a ladle, the liquid metal flows down into a tundish. The tundish acts as distributor if there is more than one mould, it evens the temperature and the composition of the melt and it makes it possible for inclusions to leave the melt. From the tundish the melt flows to the mould/moulds. A mould for steel casting is normally made by copper which is water-cooled; the surface towards the liquid metal is often covered with a protective layer by for example chromium. To avoid sticking in the mould it is oscillated. The casting process is started with the help of a dummy bar on which the metal freezes, so when it is moved downwards a strand can be drawn out from the mould. The velocity of the strand and the cooling must be controlled so that the solid shell is strong enough when the
4
strand leaves the mould. Beneath the mould there is a secondary cooling zone in which water is sprayed on the surface of the strand. Further down in the machine, the strand is bent so it gets horizontal and then straightened. When the strand is solid throughout the whole cross section it can be cut to proper lengths, often with an oxy-gas torch. The casting can be protected or not protected. In protected casting, the melt is not in contact with air at any time before it leaves the mould. In the ladle, a slag layer on the top of the melt protects it. The melt flows through a ceramic tube from the ladle to the tundish, the outlet of the ceramic tube is beneath the surface in the tundish, i.e. the tube is submerged. The melt surface is covered with some oxide mixture in the ladle and flows through one or more nozzles from the tundish to the mould/moulds. These nozzles are submerged in the melt in the mould. In the mould, casting powder, i.e. an oxide mixture, covers the melt surface. The casting powder protects the metal from contact with the air and may act as a lubricant in the meniscus region. In unprotected casting of steel rape-seed oil is often used as protective media. The oil burns and cracks, forming a reducing atmosphere in the mould. In continuous casting of thin copper strips either a salt mixture or a controlled, inert or reducing, atmosphere is used as protection from oxygen. As the melt fills the mould, the following occurs: in the upper part of the mould, a meniscus forms. The meniscus is the melt forming a convex upper surface just at the mould wall. The meniscus is curved because there is an interfacial tension between liquid metal and protective media [2]. Beneath this meniscus, a solid shell forms as the metal gets close to the wall. The solidified shell is continuously pulled downwards, at the same time new melt is poured down into the mould. The moving shell grows and shrinks when it is strong enough to withstand the metallostatic pressure. Then an air gap forms between the shell and the mould. Casting Powder The casting powder can either be a mechanical mixture of fine grain oxides or a pre-melted and granuled mixture. Granuled powders are preferable since they do not get packed and may thus be fed by automatic feeders. The properties considered important are viscosity, basicity, melt temperature, melting rate and degree of crystallisation. These properties can be connected to the composition of the powder. Another important factor is the particle size distribution. Particles of the same size will always have empty space in between. If there are particles of a wide range of sizes, the particles will pack tighter and air passage gets more difficult. The main ingredients of casting powders are CaO, SiO2, Al2O3, MgO, Na2O and CaF2. The acidic oxide SiO2 forms SiO42- which forms a silicate network. This network will bind up and contain the other oxides. Al2O3, which is an amphoteric
5
oxide, can replace the SiO42- in the network. The Al2O3 molecule can form two negative ions, AlO4-, and will thus increase the viscosity strongly if there is excess oxygen present. The following oxides are basic and decrease the viscosity since they break the silicate network: CaO, MgO, BaO, SrO, Na2O, Li2O, K2O. Also fluoride ions lower the viscosity [4], [5]. Graphite is also included (3-6 %) to control the melt rate. The casting powder has several functions [6], [7], [8], [9], [10], [11], [12]: Protect the metal from oxidation
Thermally isolate the upper surface to prevent meniscus solidification
Absorb inclusions from the melt and dissolve them
Lubricate the surface between mould and shell
Create a homogeneous heat transport
Determine the meniscus shape by determining the metal/slag interfacial
tension.
The composition of the casting powder may change throughout the casting due to reactions between steel and slag. As the composition changes, the essential properties of the slag will also change. Reactions between the metal and the slag will result in major mass transport between them. Mass transport across the interface will decrease the surface tension of the liquid metal. Oscillation Parameters The formation of the oscillation marks is regarded to be influenced by the mould oscillation. There are a number of parameters describing the oscillation, for example: Instantaneous mould velocity
v s f f t m = ⋅ ⋅ ⋅ ⋅ ⋅

π πcos [m/min] Equation 1
where s is the stroke length [mm], f is the oscillation frequency [1/min] and t is the cycle time [s]. Average mould velocity
fs2vaverage ⋅⋅= [m/min] Equation 2
t fc = 60 [s] Equation 3

N [s] Equation 4
where VC is the casting velocity [m/min]. The oscillation parameters should be chosen so that the negative strip time is large enough for avoiding sticking. Typical values are 0.2-0.3 seconds [1]. The distance between oscillation marks has been assumed to follow the expression:
p V f G= [mm] Equation 5
[3], [6], [13], [14]. 3. Review on the Formation of Oscillation Marks There is a lot of work done in the field of formation of oscillation marks. Some of the suggestions on formation mechanisms are here reviewed. Sato presented in 1979 [15] the idea of the formation on a “secondary” meniscus formed due to pressure variations caused by the oscillation. He suggests that the marks are formed in two steps, first the solid meniscus shell is lifted by the upward moving mould. This lift causes the formation of two convex surfaces, ab and bc in figure 3b. Then, as the mould turns, the two convex surfaces are forged together and the mark is formed. Figure 3 shows the formation of a mark during casting operation without the use of casting powder. Figure 4 shows the conditions for casting with the use of casting powder. The mechanisms for the two cases are the same, but the presence of the slag will cause a different pressure term [15].
7
Figure 3. The oscillation mark formation during continuous casting without the use of slag. The mark forms during the upstroke. The solid meniscus us lifted by the mould movement [15].…