a process for developing continuous casting mould...
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Journal of ScientifIC & Industrial Research Vol.SS, October 1999, pp 773-7S0
A Process for Developing Continuous Casting Mould-Powder for Steel Industries
B Mazumder Regional Research Laboratory, Bhubaneswar 75 1 0 13 , India
Revised received: 2 1 January 1 999; accepted: 2 July 1 999
For selecting the right material for continuous casting mould-powder for steel industries, the importance of property-composition relationship of a mould-powder is outlined. The chemical composition of three varieties of slags (TISeO, RSP and Phosphatic) are described. Typical compositions of some commercialIy successful mould powders are discussed. The casting parameters, such as viscosity and m.elting rate of the powders are described. The inclusion removal property of the powders is outlined. It is mentioned that these inclusions are predominantly oxides of alumina and can be removed by controlIing basic/acid ratio of the powder itself. The importance of surface spreadability of the powders is indicated.
Introduction
Continuous casting mould-powder (also known as 'mould-flux') is used primarily to facilitate passage of cast through water-cooled mould. On spraying at the top of hot metal surface in the mould region, it melts at a defmite rate and forms a molten glassy slag, which passes through the interface of mould and steel strand as a thin film, thus acting as a lubricant between these two faces. Besides this, the .powder also serves as a thermal insulator and removes simultaneously the inorganic inclusions (mostly alumina).
Mould powders are manufactured using a number of inorganic raw materials and it has been found that no universal formula can be worked out which performs equally well for all steel plants under different casting conditions and grades of steel. Hence, it is important to know the property-composition relationship of a mouldpowder so as to select the right material suitable for a particular set of casting conditions and which, in tum, will help the shop-floor personnel in altering the powder composition according to their plant running parameters.
This article presents the recent information available in this area in a systematic manner with a view to select the right powder and to reduce trouble shooting at the casting bay.
Thermoprofile of the Mould Region and Selecting
Base Materials for Powders
A mould-powder since has to work as a viscous liquid and remains in direct contact with the hot metal, it needs to be non-contaminating to the hot metal and should simultaneously possess the characteristic of a glass forming slag. For these properties as well as for the economic considerations, selection of basic components for a mould-powder is restricted to S i02, AIP3 and CaO, along with few modifiers in minor quantities.
Hears et all . used a mould thermal monitoring device (basically thermocouples) in a continuous casting �lab caster at the Sidmer plant in Belgium, and noted a drop in the metal temperature of about 1 600- 1 200 nc from liquid bulk to the metal/mould-powder interface. Drop in temperature within the mould powder, from crystalline to glassy phase at the mould interface, is from 1 200 to 600 nc. A similar thermoprofile has also been supported hy Susa l:t aU (Figure I ). These confirmed temperature ranges indicate that the powder composition that is aimed to develop, should have sufficiently low liquidus temperature so that it does not solidify or become too viscous when it penetrates all the way down the mould, which, in tum, calls for a liquidus range for the mouldpowder from 1 1 50 to 1 250 ''C. This rather narrow range implies that the powder composition should have the characteristics of rapid viscosity change within this narrow limit. Experiments have established3 the desired range as 0.5 - 5 poise at 1 300 nc. On the other hand,
774 J SCI IND RES VOL.58 OCTOBER 1 999
100 \
Figure I -Various temperature regimes between strand and mould
viscosity of powder composition in molten state, can be derived theoretically using Eq.( I ) :
I n 11 = I n A + BIT
where. In A = �.242 Alp ) - 0.061 CaO �. 1 2 1 MgO
+ 0.063 CaF2-O. 1 9 Nap - 4 .8 1 6
and B =-92.59 Si02 + 283. 1 86 Alp) - 1 65.635 CaO
+ 4 1 3.646 CaF2 - 455 Lip + 290 1 2
. . . ( I )
Here, T is the temperature in "K, and Tl is the viscosity in poise.
At 1 600 "C, viscosity of steel ranges between 0.002 -0.006 poise, while that of the slag remains above 0.02 poise. At 1450 "C, this value drops abruptly to 0.8-0.2 poise. Thus, our search for a cheap raw material (e.g. slag) starts with selecting source which possesses the major chemical components such as SiO" AI ° , CaO
_ 2 3 and alkali while characteristically show a sharp viscosity change in above temperature range. Figure 2 shows the general viscosity-temperature relation for both basic and acid slags. The pattern of curves clearly suggests that a basic slag will be more appropriate for the powder composition than the acid slag.
The three possible varieties of slags analysed in this laboratory, hav� chemical compositions as indicated in Table 1 . For both slow and high speed casting. mould powders generated from these slags, performed satisfactorily under TISCO casting conditions. TISCO slags, however, contain excess FeO, which needs to be removed by acid leaching before use.
Iron contamination of the slag may alter surface tension of the slag which is undesirable for the powder. AI-
Temperature Figure 2 - A typical viscosity-temperature curve for
acidic and basic slags
Table 1 - Chemical analysis of three basic slags
Chemical TISCO slag RSP slag Phosphatic slag component
Si02·% 1 5 34 37.9
CaO.% 33 34 47.3
AIP3'% 6 22.5 0.64
MgO.% 6.5 6.0 4.0
FeO.% 30 2.7
MnO.% 3.5 3
PP5'% 7.2 0.23 6.0
Alkali Nil Nil Nil
though interfacial-tension (contact angle) can be lowered (y decreases exponentially by certain agents like 0, S, B, Co, Ca, etc. and is raised by AI, Mn, Ti, etc.), contamination of the liquid metal strand by these elements needs to be avoided as much as possible. Control of interfacial tension within certain limits, reduces the severity of oscillation marks on the cast billet. y for iron melt is around 1 .8 Newton/metre (at 1 600 "C) or 1 83-1 87 J/m2), while that for basic slag is 0.4-0.5 Newtonl metre and decreases with increase in temperature. y also varies with the variation in steel grade (e.g. Ti-stabilised stainless steel and high Mn-steel).
Typical compositions of some commercially successful mould powders are shown in Table 24. As can be seen in this table, all these powders have a close CaO:Si02 : AIP3 ratio, and these basic constituents comprise more than 60% of the total composition. Alkali fluoride (e.g. NaP) being a strong viscosity modifier, Mills diagram5 in fact has expressed diagramatically these c1os� conditions within a narrow viscosity range
t
••
MAJUMDER : CONTINUOUS CASTING MOULD-POWDER FOR STEEL INDUSTRY 775
Table 2 - Typical composition of mould-powdersw and casting parameters
Chemical composition,% Flux A Flux B Flux C
CaO 24 - 26 31 - 34 28 - 32
Si02 26 - 28 36 - 38 31 - 34
AIP3 8 - 1 0 7 - 9 5 - 8
(Nap + Kp) 5 - 6 5 - 8 6 - 8
Fluoride 4 - 6 5 - 6 4 - 5
Carbon 1 0 . 1 4 4 - 6 1 4 - 1 6
Viscosity at 1 300 "C (poise) 3- 4 4 - 5 4 - 5
Melting range (± 20 nC)
Softening point (nC) 1 040 1 080 1 050
Melting point (nC) 1 1 00 I I S0 1 1 40
Flow point (nC) 1 1 40 1 1 80 1 1 60
CASTING PARAMETERS
Format Slab Bloom Bloom
Casting speed (metre/.min) 0.8- 1 .00 1 .2- 1 .4 0.8- 1 .2
Section (mm) 1 300 x 170 1 60 x 1 60 200 x 200
Steel grade Low carbon (AI-killed) Stainless Low carbon
I )fa.F
O.S 0.4
- P - ._ . ;;.- .--",. ..... ,. - -Q "1:--- _ - - _ ........ - ---
..
0.& '- x Ceo
0.8 0.& 0 .• '- x Caci
0.8
Figure 3 - Mills ternary diagram depicting yiscosity-temp-composition relations of mould powders
with respect to characteristic temperature interval and is shown in Figure 3 .
Contrary to mould casting, continuous casting is a dynamic process, where viscosity requirements also relate to casting speed for establishing a dynamic equilibrium. Until viscosity value and casting speed are properly matched, the cast slab will acquire oscillation marks on
its surface. Optimum casting speed can be increased by lowering mould-powder viscosity at the corresponding temperatures. This inverse relation occurs because increasing casting-speed requires better lubrication and hence the lower viscosity figure. On the other side, casting speed can be raised only with decrease in carbon content of the steel. Casting condition is generally de-
776 J SCI IND RES VOL.58 OCTOBER 1999
Table 3 - Recommended mould-powder at various carbon and casting spered9
Carbon,% 0.8- 1 .35 0.08-0. 1 8 Less than 0.08
Casting speed (metre/min) 1 .27 1 .77 2.29
Si02% 36.5-38.5 35-37 33-35
CaO, % 30-32 32-34 28-30
CaO/Si02, % 0.78-0.87 0.86-0.97 0.80-0.90
AIP3'% 6.50-7.50 6.50-7.50 5.50-6.50
MgO,% 1 -2 1 -2 1 .5-2.5
Nap,% 7.5-8.5 6.5-7.5
KP,% 0. 1 -0.4 0. 1 -0.4
Lip,% 0.5- 1 .0 0.75- 1 .25
F,% 6.5-8.5 7.5-9.5 6-8
RP,% 8. 1 -9.9 7.35-9. 1 5 1 2- 1 4
Total C,% 4.5-5.5 4.5-5.5 4.5-5.6
Fusion ranges Initial deformation temp,OC 1 040- 1 060 1 060- 1 090 1 1 1 0- 1 130
Softening temp. range, °C 1 070- 1090 1 080- 1 1 00 1 1 30- 1 500
Hemispherical temp, °C 1 090- 1 1 1 0 1 090- 1 1 20 1 1 50- 1 1 70
Fluid temperature, °C 1 1 00- 1 1 30 1 1 J 5� 1 1 35 1 1 60- 1 1 80
Vitrification rate (%) 75-80 75-80 75-80
Crystallization temperature, °C 1 060- 1 090 1 060- 1 080 1 1 60- 1 1 80
Viscosity at I 300°C, Poise 2.8-3.2
Bulk density, glcc 47.5-52
Particle size, % -200 BS 75
Thermal conductivity, Btu/ft2 3350
termined from : section to be cast, grade of steel, superheat of the mould, casting speed, mould oscil lation frequency and amplitude, and negative strip condition of mould oscillation. Mould flux. viscosity, which is primarily responsible for effective mould lubrication, is required to be optimized for obtaining a uniform flux film for a given casting speed condition4• This is shown in Figure 44.
A Japanese plant study with CC-slab castor, expresses flux viscosity and casting speed (proportional to friction force) and the pin hole defect (see Figure 5); this optimum set of conditions varies with casting parameters . Several models have been proposed for the estimation of viscosities from chemical compositions of slag, but
1 .8-2.2 1 .3- 1 .7
48-52 50
75 75
2900 2900
not all of them are applicable to casting powders since no allowance is made for CaF2 content. The models due to MaCauley and Apelian6 and due to Riboud et al. 7 were designed specifically for casting powders and were found to hold good in actual tests. It may also be noted in Figure 5 that product of'Y and Vc (casting speed) is about 2.5 for optimum performance at 1 3 0 0 °C, which corresponds to a viscosity of about 1 . 1 5 poise for the mould powder at that temperature range. Based on actual casting conditions followed at Tata Steel (Jamshedpur), Chatterji9 has recommended specific powder compositions for various grade steels, provided a refractory shroud is used between tumdish and the mould. These are given in Table 3 .
)
1
MAJUMDER : CONTINUOUS CASTING MOULD-POWDER FOR STEEL INDUSTRY 777
... '" '" �.�'" ..... 'e./JIIIa 100 _ _ _ _ _ �
JUa *'u __ � ........ 1 m/� '" '"
1 a · 3 • JIIda crI �1
Figure 4 - Viscosity - film thickness relationship at different temperatures
Melting Rate of Casting Powders After viscosity, the next important parameter is 'melt
ing rate' of the powder. In order that the powder maintains a constant height at all times of casting operation, and that mould-flux - casting-speed maintains a real time dynamic equilibrium condition, the rate of melting of the casting powder needs to be adjusted within a certain limit. Optimum powder consumption during casting is given by Eq.(2):
. . . (2)
where, C is the consumption rate (kglcm2.sec); Vc is the casting spered (metre/min) and Tl is the viscosity in poise. The thickness of casting powder over hot metal is kept constant by balancing the melting rate and the powder consumption rate. The powder melting rate (g/cm2.sec) = af. Vr (where, 'a' is a constnat, f is a correction factor for difference in layer thickness of mould-powder between production castor and laboratory apparatus, and Vr is the vitrification rate (%). Thickness of the powder layer 'd' , is given by the expression (3)9:
d = 0.02 Vr /(A. V. W) . . . (3)
where, W is the powder consumption rate (kg/ton metal) and A is the cross-section of the mould.
For continuous lubrication of the emerging strand, an optimum depth of flux pool is to be maintained and this depth relates to melting ranges and rate of melting of the powder. Optimum depth in most cases has been found 10 to be 20±1 O mm. The degree of crystallinity controls the rate of melting for uniform supply of liquid flux throughout casting operation. In practice, the author has observed that crystalline size of about - 1 50 BS mesh of all constituents satisfies these requirements.
3
2 Friction force
1
Figure 5 - Optimization of viscosity with casting speed (proportional to friction force) and pin hole defect
Since mould flux in use is in a state of dynamic equilibrium, a homogeneous melting is essential for continuous mould lubrication. In order to achieve this, mould flux technology now has moved from multi-phase material to the use of prefused amorphous base materials. In India, still multiphase materials are in common use, and as a thumb rule, standards are followed at 80% vitrification rate corresponding to melting point of 1050 °C and a thickness of 1 0 mm under lab test.
It may be noted here that the formation of a chilled slag ring which prevents the flow of flux, can occur if the molten flux depth becomes too high or dark slag practice is not followed. In dark slag practice, the powder is opaque in the molten stage and this is achieved by adding channel grade carbon black. Because of its opacity and radiation losses, consequent chilling is reduced. Severity of longitudinal cracking in slabs has also been found to relate with magnitude of heat flux transferred from strand to mould. It is believed to be caused by thermal stresses of the shell, resulting from difference in thermal contraction of the &-ferrite and austenite phasel l . Accordingly, it is necessary to produce thin, uniform, shell in the steel miniscus region to minimize longitudinal crackingl2. l3. This can be achieved by reducing the heat flux between the shell and the mould, and by minimizing the fluctuation in heat flux in both slag infiltration amd mould level controI'4. Heat fluxes have been measured in plants and by simulation experimentsI4. 19. These results suggest that contribution from radiation conduction is significantly lower than the lattice conduction (by phonons) 19.20. However, Holzhausor et al. 17 have contradicted these results and stated that where slag films is predominantly glassy, radiation conduction is twice that of lattice conduction. Studies by Susa et al. 17 identified three distinct layers in the powder during use (Figure 6)- a slag layer containing glassy
778 J SCI IND RES VOL.58 OCTOBER 1999
r-----------'-. -
_ -_""'UIpdd 1Ieel-_-_
=- --.. ' ... : -!'f'-:-:-:-:-:. -........ '"+-+-++1 ... -:.. Of .,..l !
Figure 6 - Formation of crystalline. glassy and amorphous phases of the powder during use
and crystalline layers, and another glassy layer formed from liquid slag . . The glassy phase has a sharp increase in heat capacity at the glass transition temperature (Tg)' which in this case is around 600 °C. Crystallization of slag film (between 520-700 0c) is also associated with strong exothermicity l . Thermal diffusivity decreases at glass transition temperature (around 500 0C). The radiative heat transfer is important when the slag film is predominantly glassy and crystallization of the slag film does have significant effect on the heat transfer between strand and mould. Thus, the conclusion is that conductive heat transfer is the principal mechanism affecting the heat transfer between the strand and the mould I . Contribution of radiation heat transfer to overall heat transfer does not exceed 1 0% of that originating from the conductive heat transfer. Crystallization of the slag is the primary factor affecting heat transfer via radiation conduction; the effects of the transition metal oxies (e.g. FeO, MnO, etc.) on the absorption coefficients are much smaller than crystallization effect.
Melting rate of powder is also influenced by the thermal conductivity of the powder itself. Carbon retards melt down by not allowing the particles of the flux to come in contact and get agglomerated. Since molten flux is semi-transparent, heat flow through it could be by both conduction and radiation, the lattice factor being more predominant. The overall conductivity is larger than 1 W /rnI°K. Since the variation in thermal conductivity with composition of flux is not that much so as to cause difference in heat flux, the observed influence of powder composition on heat flux may be explained by variation in slag thickness and/or thermal resistance at mould/flux interface. Typical value of thermal conductivities found with commercial powders in use are in the
range 1 -3 W/rnI°K between 200- 1 000 0c . Very fine carbon powder (channel black) is being used to achieve these insulating effects and a thumb rule of linear correlation with temperature has been used by the author to determine concentration.
Inclusion Removal Property of Powders
Since the mould-powder works on the surface of the slab being cast, it has an important role in removing inclusions from the surface, imparting it a good surface finish. These inclusions are predominantly oxides of alumina and can be removed by controlling basic/acid ratio of the powder itself. This ratio is expressed by the 'Basicity Index' and is represented by Eq.(4):
B = ( 1 .53 CaO + 1 .5 1 MgO + 1 .94 Na20
+ 3.55 Li20 + 1 .53 CaF2)/( 1 .4 Si02 + 0. 1 AlP3)
The higher the B value, more is the absorption capacity. In general, commercial mould powders have 'B' values in the range 0.70- 1 .20. High temperature reaction products formed from the raw materials and corresponding melting point of the compounds exist by reactions among Si02 ' CaO and AI203 mainly, can be predicted from the phase diagram (Figure 7)28.29: Besides CaO and Si02• alkalies also take part in dissolving the alumina, although . concentrations of the latter is low. Chemical composition of the powder also affects corrosion of the submerged entry nozzle. Errosion increases with decreasing viscosity or increasing fluoride content. It can be shown by application of thermodynamics that Nap and CaF 2 in the mixture react at high temperature to form NaF, and accordingly concentration of Na20 and CaF2 in the powder is effectively represented in the phase diagram as NaF concentration. Since slag consists of silicate chains made up of Si04 -4 tetrahedron30•31 , it is reasonable (as it can be readily replaced by AIO/ tetrahedra) to represent AIP3 concentration with Si02 (both of them are network former in glass composition). Since almost all commercial mould- powders contain less than 1 0% AIP3' their substitution remains still more valid. The silicate chain in such compositions can be broken by P, Na+, and Ca2+ ions. As represented by Mills diagram, most commercial powders have liquidus temperature in the range 1 1 50-1 200 0c . Comparatively excess Nap lowers the liquidus temperature more than CaF2. Nakamura et al. 32 have reported corrosion results of zir-
MAJUMDER : CONTINUOUS CASTING MOULD-POWDER FOR STEEL I NDUSTRY 779
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conia-graphite refractories with various mould powders. These studies indicate erosion by CaF2 as dependent upon viscosity of the melt. MgO has been found3 to be less efficient than CaO as a 'network breaker' . Viscosity of the powder, on the other hand, depends on the basicity of the powder. Hanch and Potschkoe33 reported that corrosion of submerged nozzle preceded by a mechanism which inxolved dissolution of carbon from the refractory into both steel and slag phases. In al l commercial powders, fluoride is added in the form of fluorspar (CaF,) or Na
3AIFfi'
-
The last desired property of the powder is to have sufficient surfaced spreadabi lity. Low bulk density en-
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I-
I I I I
1
- - - - S;ca
N�C5:LJ:J.. M.i l.�c.l. MC t..E (y.) -
sures this result, as low density along with turbulence and air convection results in easy displacement of the powder across the surface of hot metal. Low bulk density also imparts superior thermal insulation to the open hot surface of the metal . Bulk density of commercial powders are thus controlled within a value of 0.7 to 0.9 glee, corresponding to a consumption rate of around 0.5 to 1 .0 kg I ton steel .
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