Low Carbon MgO-C Refractories for Clean Steel Making in Steel Ladles
In recent years, with the demand for high-quality steelmaking, constant efforts have been made to diminish the impurities during steelmaking to produce high-grade and clean steel products to meet the stringent requirement of end-users; particularly for some critical grades and applications. For this purpose, many researchers have studied on diminishing or controlling the impurities in the steel by using advanced refining technology. A higher amount of flake graphite addition in MgO-C refractory in steel ladle gives better non-wettability to the molten metal and slag to provide better corrosion resistance. However, the presence of a higher amount of carbon increases the thermal conductivity of the refractory leading to an increase in heat loss, steel ladle shell deformation, and high carbon pickup to the molten steel. As a result, the refractory manufacturers have faced serious challenges of stringent quality requirements from the market and to develop smart and innovative refractory materials. The present paper discusses the use of nano-carbon in MgO-C refractories for steel ladle application to reduce the carbon pickup from the refractories in the ladle to produce clean steel. In this work, varying amounts of flake graphite with magnesia aggregate in MgO-C refractories were studied along with two types of nano-carbon introduced in the matrix part of the refractories. These two types of nano-carbon have the same DBP absorption number but differ in iodine absorption number and surface area. Refractory samples were fabricated using industrial friction screw press and the samples were coked in 1000°C/4 hrs. in reducing the atmosphere. The effect of types and amount of nano-carbon on the properties of MgO-C refractories have been examined. Apparent porosity, bulk density, cold crushing strength, spalling resistance, and oxidation resistance have been evaluated. The corrosion resistance test was conducted at at1650°C/4 hrs. to measure the extent of slag penetration and corrosion. The hot modulus of rupture (HMOR) was measured and the effect of nano-carbon on HMOR values has been evaluated. The effect of different amounts of nano-carbon varying from 0.5 to 1.5% on the physical, chemical, mechanical, thermo-mechanical and corrosion resistance properties of MgO C refractory has also been studied in detail。
Carbon bonded MgO-C refractories present unique Thermo-mechanical and chemical properties. Therefore, they have established high duty refractory products in steel making applications, particularly for converters, EAF and steel treatment ladles. However, this kind of refractory suffers two main drawbacks of poor oxidation resistance and low mechanical strength due to the presence of carbon(graphite, carbon black, and pyrolytic carbon from the resins or pitches). An approach to overcoming these problems has been usually made by incorporating the additives (antioxidants) [1-3] and surface modification or improving the graphite properties [4-6].In the last decade, the composition of MgO-Crefractories has been improved under ecological and economical aspects, especially in terms of binders and additives used for reinforced oxidation resistance and better thermo-mechanical properties. The presence of a higher amount of graphite in the brick gives better-wettability to the molten metal and slag to give rise to better corrosion resistance. But high carbon content increases the thermal conductivity of the brick leading to an increase in heat loss of the process. At the same time, high carbon content increases carbon pickup to the molten steel, which deteriorates the property of the steel. The release of carbon dioxide during tempering and preheating during lining causes environmental pollution and an increase of Carbon FootPrint.The reduction of carbon content in MgO-C brick decreases the thermal conductivity and affects the thermal spalling resistance as well as corrosion resistance. So the reduction of fixed carbon content in MgO-C brick without hampering the property itself is a great challenge. This can be achieved by using high surface area carbon source to the matrix part of the brick since a small amount of carbon addition inMgO-C refractory cover the whole matrix of the brick.
Nanoparticles in refractories
The refractory industry is highly matured and in order to counteract stiff competition from the foreign markets, the only way is to develop new technologies. Thus, the use of nanoparticles has brought about a revolution in the refractories field by exhibiting remarkable performance [8-10] by altering structural, microstructural, chemical, and mechanical properties at high temperatures due to its high surface to volume ratio  and higher reactivity with the matrix. Refractory manufacturers are actively engaged and lots of work have undertaken in the research front application of different types of the lower amount of nano-particle addition in various refractories without sacrificing much of the key performance determining properties [11-16]. Researchers are actively engaged to look into the whole aspects including the use of different nanoparticles to lower down the amount as well as to ensure uniform distribution of minor additives throughout the matrix.
Use of carbon
Traditionally, MgO-C bricks contain 10-20% carbon depending upon their application requirement in various steelmaking furnaces and vessels because of their excellent corrosion and thermal shock resistance. MgO-C brick faces various problems long term applications due to its graphite content. Higher graphite content in the brick increases the carbon pickup by steel, which is not desirable for good quality steel because it requires carbon content as low as possible. Another problem is heat loss, because of its high thermal conductivity, which leads to a reduction in molten metal temperature. If the carbon content is reduced in the bricks the thermal and thermo-mechanical properties such as thermal spalling resistance, corrosion resistance at high temperatures are decreased which are related to the properties (aspect ratio and surface area) of carbon. Hence, a high surface area containing MgO-carbon MgO-C bricks with desired properties is required. The particle size of the carbon material added to the brick has a great impact on the properties and performance of the brick. A small number of nanomaterials can be used in place of micron-sized materials due to their high surface area and mono-modal particle size distribution. The oxidation behavior of nano-carbon is very similar to that of fine graphite with average particle size of 5
μm. Nano-carbons with higher graphitic ability showed better oxidation resistance than those with low graphitic ability. The service life of MgO-C brick depends upon the carbon oxidation. The oxidation resistance could be enhanced by optimizing the typesof carbon source and antioxidants.
Use of antioxidants
Nano-carbon addition requires some special kind of antioxidant in order to improve the oxidation resistance but also reinforce the strength-based ceramic bonding since the rate of oxidation for nano-carbon is higher than that of graphite. Metal carbide along with metal powder addition can be a suitable antioxidant for nano-carbon added MgO-C bricks. In the present investigation, two types of nanocarbon were used along with flake graphite, in order to develop a new generation low carbon MgO-C brick with superior properties.
Raw materials and refractories fabrication
Commercially available high purity fused magnesia with a large crystal size of 500–1500 μm(bulk density of 3.54 g/cm³, 97.5% MgO) having different sizes: coarser, medium, and fines were taken. Different size fraction has been taken in order to maintain the granulometry of the mixture. Natural flake graphite containing 94% fixed carbon, antioxidants, two types of nano-carbons (NC-1 and NC-2), and liquid resin with a viscosity of 9000 cps as a binder and other additives were taken as base raw materials for fabrication of MgO-C refractories. All the raw materials were mixed by using a high intensive mixer machine at room temperature by following the standard commercial mixing practice. After mixing, bricks were pressed with a specific pressure of 2 T/cm2 with the help of the hydraulic press. The pressed bricks were tempered at 200-220°C. After tempering, the bricks were subjected to furtherproperty characterization.
Particle size analysis and surface area
The particle size and its distribution of flake graphite and nano-carbons was measured by laser scattering technique in a computer-controlled particle size analyzer (Malvern, Mastersizer 2000, UK).In order to determine the surface area of the same, Brunauer-Emmett-Teller (BET) analysis has been done on a Quantachrome (USA) machine. For BETanalysis, liquid nitrogen has been used to cover the surface area of the particles.
AP, BD and CCS
After pressing, the pressed brick samples were tempered by following the standard temperature-time curve up to 220°C in a tempering kiln. Coking was carried out at 1000°C for 4 hrs to study the materials refractory properties under a reducing atmosphere(carbon bed). Since the cured samples pass through heat treatment, which results in a thermally stable refractory and crystallization. The test samples were cut from the tempered brick as per the standard. Apparent porosity (AP), Bulk density (BD), and cold crushing strength (CCS) were measured as per the standard of IS: 1528, Part-12 (1974) for AP and BD, and IS: 1528, Part-4 (1974) for CCS tested for both tempered and coked samples. Each value of AP, BDand CCS is of an average of five parallel samples.
Hot modulus of rupture (HMOR) is determined by theconventional three-point bending test conforming to ASTM C133-97, using HMOR testing apparatus(Netzsch 422, Germany). All the specimens forHMOR testing are dried at 110°C after wet cutting,without pre-firing in air atmosphere. The heating ratefor HMOR testing is 5°C/min till the final firingtemperature of 1400°C in air atmosphere with asoaking time of 30 min.
For the oxidation resistance test, cylindrical samples(diameter 50 mm, height 50 mm) were cut from the tempered bricks and placed in an electrically heated furnace under a normal atmosphere at 1400°C for 3 with a heating rate of 5°C/min. After natural cooling, the samples were horizontally cut into two pieces. After the oxidation test, the black surface remaining was measured at eight different locations and the average value was noted down.
Slag corrosion test
Mercury porosimetry test for PSD analysis
Microstructure analysis was done for the slag corrosion tested samples by using optical microscopy analysis.
Several formulations have been made by using various types (flake graphite and nano-carbon ofNC-1 and NC-2) and amounts of carbon, and additives (metal powders) in MgO-C refractories. The variation of apparent porosity with varying amounts of graphite and nano-carbons percentage has been studied in the MgO-C refractory. The optimum apparent porosity has been achieved in refractory containing a particular amount of flake graphite and two types of nano-carbon were reported here (Table 2). The apparent porosity of NC-2 showed a slightly low value in comparison to NC-1 and flake graphite added MgO-C refractory for every composition. The specific surface area of NC-2 was higher than-1, therefore NC-2 nano carbon caused more efficient filling of the pores between the aggregates of the magnesia. Overall, the effect of nanocarbon addition was successful in reducing the apparent porosity while there was a decrease in graphite content. However, a few test results which have given desired properties are shown in Table 2. For comparison purposes, the conventional graphite added MgO-C bricks used in steel ladle are also reported.
Table 2: Properties of developed MgO-C bricks
Particle size distribution and surface area
Table 3: Chemical composition and somecharacteristics of NC-1 and NC-2 nano-carbons
Pore size distribution
AP, BD and CCS of tempered bricks
There is a marginal increase in BD and decrease in AP with respect to a certain amount of nano-carbon addition in the tempered bricks as compared to conventional MgO-C bricks as mentioned in Table 2.This is mainly due to the nano-carbon along with graphite added in MgO-C refractory had effectively filled the pores and voids and were in correct proportion to cover the pores in magnesia matrix. When the MgO-C refractory contains lower than the specified amount of graphite along with nanocarbon, high apparent porosity had been attained again. It could be due to the fact that graphite along with nanocarbon was not sufficient to cover the pores in the magnesia matrix. Therefore, nano carbon was not able to fill up the pores of the matrix, and hence apparent porosity expected to increase. Thus, the optimum apparent porosity has been achieved refractory containing a certain amount of graphite and nano carbons. Moreover, apparent porosity of NC-2showed slightly low value in comparison to NC-1 for every composition. As the particle size of NC-2 was higher than NC-1, therefore NC-2 nano carbon caused more efficient filling of the pores between the aggregates of the magnesia. Overall, the effect of a no carbon addition was successful in reducing the apparent porosity while there was decrease graphite content.No significant change in BD value for two types of nanocarbon added MgO-C refractory. The increase in BD values as compared to conventional one may be due to two reasons: First there was an increase in the percentage of magnesia in the matrix. Secondly,nano-carbon effectively fill up the pores and voids,hence increase in the BD values. Higher CCS value was observed for the samples with the addition of nano-carbon is mainly due to the low amount of graphite. The CCS values of NC-2were greater than that of samples containing graphite and NC-1. This may be due to the densification of the matrix by effective packing thus the strength is more prominent as supported by Griffith’s rule. MgO-C Refractories
AP, BD and CCS after coking
The effect of nano-carbon addition on AP, BD andCCS in MgO-C refractories after coking is given inTable 2. Coked CCS of samples containing nano-carbon is more in comparison to that of conventional graphite containing bricks. The fine particle size of nano-carbon occupies the pores and voids in the brick matrix thereby densify the matrix, whereas, flake graphite containing bricks releases the stress during coking. Due to this, the strength after coking is decreased. MgO-C Refractories
Highest HMOR value was observed in the qualityhaving nano-carbon. This is mainly due to thedensification of the matrix and less oxidation ofcarbon. In case of conventional MgO-C refractories,presence of more amount of flake graphite restrict thecontact between the aggregate and matrix part ofmagnesia grains thereby the bond strength is gettingreduced as compared to lower amount ofnano-carbon added MgO-C bricks.
Oxidation resistance and strength
Spalling resistance test
In general, it is expected to have low thermal shock resistance for the bricks containing a lower amount of carbon than the higher amount of conventional flake graphite added one. However, no remarkable change in thermal shock value was observed irrespective of varying amounts of carbon in the brick.This is mainly due to the presence of lower residual fracture energy in nano-carbon added based bricks as compared to the conventional flake graphite added one, which in turn helps to improve the spalling resistance. This can be correlated with the gap between the aggregates: Both nano carbons had a high value of Dibutyl phthalate (DBP) absorption.DBP is the ability of the material to form the gap between the aggregate. Increase the gap between the aggregate did not mean to increase in apparent porosity. The increased distance of separation between aggregates was already filled up by nano-carbon, therefore it could not affect the apparent porosity. High DBP means it has a high tendency to form a gap between the aggregates. This space between the aggregate may help to decrease the elasticity of the refractory material. It could be due to the reason that NC-2 had lesser particle diameter than NC-1 and as the particle diameter became small. The gaps between the aggregates were able to accommodate more carbon nanoparticles without increasing porosity resulting in lower modulus of elasticity. As the modulus of elasticity of the MgO-Crefractory decreased then the thermal spalling resistance has been expected to improve. Therefore the improvement in the stress relaxation of refractory can be observed. The thermal stress in the refractory was less which in turn expected to improve the thermal shock resistance of MgO-C refractories with the inclusion of nano-carbon. MgO-C Refractories
Slag corrosion test
Therefore, it prevented the attack of slag and they did not allow it to penetrate into the MgO-C refractory and prevent the corrosion of magnesia. Thus, the slag resistance properties of NC-2 are higher than the NC-1. Moreover, with a decrease in the percentage of graphite and inclusion of nano-carbon in carbon-containing refractory was better than the flake graphite added one. The nano-carbon fills up the pores and did not allow the slag to penetrate. Hence there was increasing in slag corrosion resistance.
Microstructure analysis of corroded samples
The microscopic observation of the corrosion tested sample of Figure 4(c) was given in Figure 5 (a-f). Figure 5(a) shows the microstructure away from the hot face of the brick (middle part) after the slag corrosion test. The matrix part of the brick showing graphite flakes and fused magnesia (M) grains. Figure 5 (b and c) shows the slag and brick interface near the working face. The slag has penetrated the refractory material into the open pores (P) by capillary forces and the iron present in the slag diffuses through the grain boundaries .
Probable wear mechanism:
MgO-C bricks containing graphite and nano-carbon incombination with small amount of metal additives areequivalent and in some cases better than that ofconventional graphite content (12%) bricks toproduce clean steel. Presence of sub micron pores inthe matrix part of MgO-C refractories improves theproperties like corrosion, spalling and oxidationresistance. MgO-C Refractories