1
Mould powders in continuous casting
Hamouda Assia
1, Balaska Adel
21Centre National de Recherche Scientifique et Technique en Soudage et Contrôle, Unité de Recherche Appliqué en Sidérurgie Métallurgie, URASM/CSC; B.P 196, 23000 Annaba, ALGERIE.
2Centre de Recherche Scientifique et Technique en Soudage et Contrôle, Unité de Recherche Appliqué en Sidérurgie Métallurgie, URASM/CSC; B.P 196, 23000 Annaba, ALGERIE.
Email: [email protected]
Abstract– Continuous casting of steel is a process in which liquid steel is continuously solidified into a strand of metal. The components of the continuous casting steel system are the ladle, tundish and mold. Mould powders are spread on the surface of the bath of the molten metal of the ladle and tundish. The quality of steel produced by continuous casting is strongly dependant on the performance of mould powders. Mould powders govern the steel production in terms of production rate, cleanliness, surface condition, defects and environment of casting operation. These powders are SiO2 oxides based materials.
Diatomaceous earth, or simply diatomite, formerly called Kieselguhr, is a sedimentary rock of biological origin formed by the accumulation at the bottom of the ocean of siliceous skeletons of diatoms, or unicellular algae. Diatomite is a highly porous silica (SiO2) rich material, major applications are thermal insulation. Melting behaviour of mould powders is one of the most important performance parameters for mould powders and is mainly dependant on carbon which is an essential constituent of mould powders. Carbon blacks have lately attracted considerable attention for use in mould powders due to fine size. A study was conducted to see the effect of addition of carbon black and diatomite to natural silica (sand) for a mould powder used in Arcelor Mittal Steel Annaba.
Keywords– continuous casting, mould powders, oxides, diatomite
1. Introduction
Continuous casting is the manufacturing process as used for the production of steel. It consists in forming a crust of liquid steel into a water-cooled mould made of copper.
During casting, defects can be formed on the surface of the solidified products, such as cracks, defects lubrication of oscillation marks. In some serious cases, there is a breakthrough of steel. The steel industry, becoming more demanding, the mastery of these surface defects became necessary. The origin of these defects is related not only to the parameters of continuous casting, but also to the use of casting powders or mould powders[1].
The use of mould powders is a common practice by steelworkers. In the usual way, it covers the free surface of the molten metal of a powder. This powder has several functions. It prevents oxidation of the metal by isolating it from the air, reduces waste thermal properties, and trap inclusions that float to the surface of the steel. It is composed of oxides such as silica[2]. It is deposited on the surface of the liquid steel to form a layer of a few cm thick. In the vicinity of the interface metal-powder, the powder becomes liquid, allowing it to seep between the mold wall and the skin during solidification of the cast product, and play its role as lubrification.
Mould powders were introduced into the continuous casting process about 40 years ago to replace oils used to provide lubrication to the steel strand [3]. They were found to be beneficial since they provided lower heat transfer between the steel and the mould and hence could be operated with much lower levels of superheat. Over the years it was found that the choice of mould powders could have a distinct effect on the casting performance and the surface quality of the cast-product. Mould powders are classified into synthetic or
raw mixed powders. It was regarded by many to be a “ kitchen recipe”. However, there has been a determined effort, to gain an understanding of how mould powders affect casting performance. Consequently, the expertise of local raw materials is very interessant, in order to improve sustainable development. Nevertheless, mould powder design still remains random.
Silica, with the chemical formula SiO2 and relative molar mass of 60.084, exhibits a complex polymorphism characterized by a large number of reversible and irreversible phase transformations usually associated with important relative volume changes. Industrially, silica products are classified into two main groups: (i) natural silica products, quartzite, silica sand, and diatomite and (ii) specialty silicas including fumed silica, silica gel, microsilica, precipitated silica, fused silica, and vitreous silica. Diatomaceous earth, or simply diatomite, formerly called Kieselguhr, is a sedimentary rock of biological origin formed by the accumulation at the bottom of the ocean of siliceous skeletons of diatoms, or unicellular algae. Diatomite is a highly porous material, major applications are thermal insulation [4].
The work presented here will help to minimize defaults related to the continuous casting powders. In this perspective, it was first necessary to understand the behavior of the mould powders in factory, and then be able to determine the physicochemical characteristics that are likely to account for their performance in the laboratory.
The behavior of the powders used to Arcelor Mittal Steel Annaba has been studied. The powders of the continuous casting of steel are used for the tundish and the ladle. The ladle powders are made of sand. The addition of diatomite increases the thermal insulation. The addition of carbon to the powders can help to reduce the consumption rate. There are several kinds of carbon: amorphous graphite, coke and
A. Hamouda et al. / IJRRXXX, Vol. 2, No. 1, pp. xx-xx, March 2012 2 carbon black. We chose carbon black for his availability. In
this work, mixtures of sand, diatomite and carbon black were studied.
2. Experimental
Samples of diatomite were scanned using a Philips XL 30 electron microscope. The characterization of the powders as well as the phase identification have been followed by XRD measurements using a Siemens D500 diffractometer with Cu Kα radiation, allowing us to obtain mineralogic analysis, lattice parameters and phase proportions. The thermal analysis were carried out on a STA 409 PC Lux Mettler Toledo 822 apparatus at a continuous heating rate of 5
°C/min within the temperature range 23-1500 °C.
The XRD patterns were analyzed using MAUD program [5] which is based on the Rietveld and Warren–Averbach methods in combination with Fourier analysis [6, 7]. The procedure consists in modeling the diffraction profiles by analytical functions in order to characterize the structure of the powders. During fitting procedure, the background and peak profiles for all phases are evaluated. A specially prepared corundum Al2O3 standard, having no size and strain broadening, was used to estimate the instrumental contribution of broadening correction, peak asymmetry, peak gaussianity and peak broadening parameters. Then all these parameters are incorporated in the MAUD program for an accurate determination of the sample structural parameters.
After the refinement of the instrumental parameters, the positions of the peaks are corrected by successive refinements for systematic errors taking into account the zero shift error and the sample displacement error. Next, the background is refined as a three-degree polynomial followed by the refinement of the crystal structure parameters. The accuracy of the results is related to the value of the quality factor of fitting, GoF, which is close to unity.
3. Results
This study was undertaken to Arcelor Mittal Steel Annaba. The mould powders are cover powders produced by CODESID [8]. CODESID powders have a high consumption rates relatively to other products such as fly ash rice powder.
This presents an ideal behavior (low consumption rate) as covering powder because it’s carbon rich (30 %). The consumption rate of the powders is related to the melting rate of the powders. The addition of carbon is necessary to regulate the rate of melting. It also provides thermal insulation of the liquid metal. The melting rate depends on the type of carbon, its concentration and the particle size of carbon.
Accordingly, these fine particles have a large surface in contact with the other mineral particles. The carbon particles are formed in aggregates. The size of the aggregates is a feature of the carbon black which influences the quality of powders
The powder mixture of the mould powder of our study consists of sand, diatomite and carbon black. Diatomite is a siliceous amorphous structure. Diatomite used in this study is an Algerian diatomite deposit from the Sig-Mascara.
Diatomite frustules are mainly divided into categories, centric (discoid) and pennate (elongated to filiform). Fig. 1 shows an electron micrograph of diatomite with magnification in order of 300 and 1000. The plates indicate
only centric types of diatom. These latter have a radius of approximately 40 μm. It can be inferred from the scanning micrograph that diatomite has a large void volume. In addition to its highly porous structure. The high porosity of this material was the main reasons for choosing it as refractory powder.
Figure 1. Scan electron micrography of diatomite
XRD pattern of the diatomite powder (Fig. 2) shows in addition to the crystalline Bragg peaks, an overlap of the peak around 20° corresponding to the amorphous silica. The best fit with Rietveld method has been obtained using at least a combination of four cristallines phases, calcite, α-quartz, ankérie and tridimite.
In order to calculate the amorphous/crystalline fraction of the composites, X-ray data refinement was made using the program MAUD. The modelling of the silica glass has been introduced as a part of the general Rietveld refinement [9].
The method approximates the amorphous phase as a nanocrystalline solid where the long-range order is lost. The model is not sufficient to describe exactly the amorphous structure, moreover, from a fitting point of view, it provides nearly the same results as those obtained by reverse Monte Carlo simulation (RMC) [10]. The Rietveld analysis of amorphous phases is based in an improved microcrystallinity model that includes microstrains. The disorder is statistically introduced by microstrain effect, leading to strong line broadening on the diffraction pattern. According to the traditional microcrystallinity model, an amorphous solids consist of small domains with a crystalline structure. This method was first applied to multicomponent flouride glasses [11]. It was also applied to thin films of amorphous SiO2 on Si-wafers in which data evaluation was made using the programs ARITVE [8] and LAXS [12]. It was also shown how this approach could be successfully used to determine the amorphous fraction in ceramic materials containing a glassy phase, and in synthetic mixtures of Al2O3 and SiO2, where it is very important to assess the product quality by the check on the amorphous silica content [13, 14]. The analysis
A. Hamouda et al. / IJRRXXX, Vol. 2, No. 1, pp. xx-xx, March 2012 3 was started assuming the structure of the cristallines and
amorphous phases, all the used structures are already in the database of the MAUD program. The obtained quantitative analysis of the sample using the MAUD program are listed in Table 1.
Figure 2. XRD pattern of the diatomite powder
Table 1. Quantitative analysis of diatomite phases
phases structure Proportion (%)
Amorphous silica SiO2 57
quartz SiO2 23
calcite CaCO3 9
ankérite Ca(Fe,Mg)(CO3)2 8
tridimite SiO2 3
The ATG/DSC profile of the diatomite powder show tow steps in the thermal curves of the diatomites, although other minor peaks may also be present.The first main step, produced at temperatures between 25 and 100° ± 10°C, is related with mechanically trapped and/or physiosorbed water, its quantity depending on the particle size and surface area of the samples, as well as on the relative humidity to which they had been exposed prior to their analysis. This peak does not depend on the chemical nature of the sample, but it is related only with morphological and textural parameters [15]. The second step, at around 750° ± 80°C, corresponds to the decomposition of alkaline earth metal carbonates, always present in diatomite, and finally, at 895°C, the crystallization behavior occurs[16]. Between these two steps, two losses are detected. A lower temperature weight loss corresponds to the evolution/decomposition of organic matter, a usual component of diatomite, and the higher one to the loss of constituonal water from clay minerals that can sometimes impurity the material.
A mixture of sand and diatomite has been studied in order to define the mixture having optimum physical properties.
Diatomite and sand mixtures with 20, 30 and 40% sand were characterized. Density, moisture and loss on ignition are summarized in the following table.
Too much sand would lead to a glassy crust above the liquid metal. This crust solidifies and prevents the measurement of the temperature of the liquid metal. The increase of diatomite above 80% leads to decrease in the
density of the latter; the density of the diatomite used in this study is about 0.3 g/cm3. This is consistent with literature which states that the density of diatomite varies from 0.19 g/cm3to 0.275g/cm [4].
Powders must be dense. The density must be > 0.69 g/cm3. Diatomite has a loss on ignition (LOI) at 1000 ° C ≤ 22% higher while the sand has a low loss on ignition of about
≤ 2.8%. The LOI must be <16%. The mixture of sand and diatomaceous earth with 20% sand is a good compromise with a density of 0,738 g/cm3 and an ignition loss of 12.2%.
Table 2. Physical properties of sand and diatomite mixtures
Carbon black is another form of carbon just like diamond and graphite. Although has a completely different atomic structure. It is similar to graphite in some respects. It is made from the incomplete combustion of liquid aromatic hydrocarbons. As the liquid aromatic hydrocarbon decomposes in a furnace it forms carbon radicals and fragments which recombine into essentially spherical particles. Depending on the process parameters, the carbon black particle size can vary from 15 to 75 nm. Such fine particle size can give them immense surface area. However, carbon black particles do not exist individually and the particles fuse into primary aggregates, which are the smallest dispersible units. Therefore aggregate size is another important characteristic of carbon black. This carbon black may have small amounts of chemisorbed oxygen on their surface in form of organic compounds such as phenols and carboxylic acids. There is another term associated with carbon black structure grade. Structure grade of a carbon black essentially reflects its extent of aggregation of primary particles. Higher aggregate size means a higher grade. Larger aggregate size leads to more voids into the structure. Higher area grades are more difficult to disperse due to greater number of contacts between aggregates.
The mixture of sand and diatomite is mixed with carbon black (4 %). The behavior of this powder is studied in a steelworks (ACO2) of Arcelor Mittal Steel Annaba. A comparative study is done with the powder without carbon black and fly ash rice powder. The ratio of the consumption of fly ash rice and the old powder (without carbon black) is equal to 1/30. The ratio of the consumption of fly ash rice and the new powder is equal to 1/10.
4. Conclusion
The diatomite is a highly porous material. It’s made of amorphous silica (57 %) which is safer than crystalline silica.
mixture of sand and silica leads to an optimal mold powder with a density of 0.738 g/cm3. The main control of insulation is the density. The diatomite provides thermal insulation to prevent bridging and steel floaters. The addition of carbon black conducts to a caging effect of mould powder particles, which affects the melting rate and the then consumption rate of mould powder.
Proportion of sand
(%) 40 30 20
Density (g/cm3) 1.326 0.926 0.738
Moisture (%) 0.36 0.29 0.26
LOI (%) 19.9 12.7 12.2
A. Hamouda et al. / IJRRXXX, Vol. 2, No. 1, pp. xx-xx, March 2012 4
References
[1] S. Junghans, “Process and Device for the Casting of Metal Strands, (Patent style)” 750301, 1933.
[2] R. Carli, C. Righi, Composite Materials Formulation Chemistry, Ind.
Chem. Master Course, Faculty of Science, University of Milan, 2004.
[3] A. Cruz, F. Ch´avez, A. Romero, E. Palacios, V. Arredondo, Journal of Materials Processing Technology Vol 182, 2007, pp. 358–362.
[4] F. Cardarelli, Materials handbook (Handbook style), 2nded, 2008, Springer.
[5] L. Lutterotti, MAUD, CPD, Newsletter (IUCr), 24, 2000.
[6] L. Lutterotti, R. Capostrini, R. Di Maggio, S. Gialanella, J. Met. Nano Mater, 2000.
[7] B.E. Warren, X-ray Diffraction, first ed. Addison–Wesley, Reading, Massachussets, 1969.
[8] S. Akroun, S. Bouchama, R. Boulahdid, R. Ferhani, H. Goubi, A.
Kessal, A.Merdjani, S.Saadou, INAPI (Patent style), 1987.
[9] R. A. Young, Edit. International Union of Crystallography, Oxford University Press, New YorK, 1993.
[10] A. Le Bail, J. Non-Cryst. Solids, 1995.
[11] A. Le Bail, C. Jacobini, R. de Pape, J. Phys. (Paris) Coll.1985.
[12] P. Lecante, A. Mosset and J. Galy, J. Of Appl. Crystall, 1985.
[13] L. Lutterotti, R. Ceccato, R. Dal Maschio and E. Pagani, Qunatitative analysis of silicate glass in ceramic materials by the Rietveld method, Mater.Sci. Forum,1998.
[14] S. A. Palomares. Sanchez, S. Ponce. Castaneda, J. R. Martinez, F.
Ruiz, Determination of phases of alpha-Fe2O3-SiO2 compound by Rietveld refinement, Revista Mexicana de Fisica, 48, 2002, pp. 438- [15] R.W. Soaresa, M.V.A. Fonsecaa, R. Neumanb, V.J. Menezesa, A.O.442 Lavinasc, J. Dweck, An application of differential thermal analysis to determine the change in thermal properties of mold powders used in continuous casting of steel slabs, Thermochimica Acta 318, 1998, pp.
131-136.
[16] G. Robert Hill, N. Da Costa, V. Robert Law, Characterization of a mould flux glass, Journal of Non-Crystalline Solids 351, 2005, pp. 69–
74.
