This article briefly introduces the development of magnesia-calcium carbon bricks and its application in ladles, and analyzes and studies its effects on inclusions and oxygen content in steel during use.
Keywords: ladle; magnesia-calcium carbon brick; application
At present, refractory materials used in ladles are developing in a direction that less pollutes molten steel and even plays a certain purification role while ensuring their use. The corrosion resistance of the aluminum-magnesium unburned bricks used in the non-slag line parts of the 225 t ladle wall of Shougang Second Steelmaking Plant and the reduction of molten steel pollution are not ideal. Especially after the adoption of the new LF process and changes in product structure, it is more difficult to meet demand. To this end, research and development work has been carried out on magnesia-calcium carbon bricks for the non-slag line parts of the ladle wall.
Selection of raw materials and production technology
Selection of raw materials
The main raw material of magnesia-calcium carbon bricks is magnesia-calcium sand, which combines magnesium oxide and calcium oxide to complement each other’s advantages. The reasons for choosing magnesia calcium sand raw materials are:（1）Calcium oxide has a melting point as high as 2570°C and is strongly alkaline. From a metallurgical perspective, magnesia-calcium sand has good thermodynamic stability, does not pollute molten steel, can capture non-metallic inclusions in molten steel, keep molten steel clean, and is beneficial to controlling the form of sulfides in steel. (2) Compared with magnesium oxide, calcium oxide is more stable in contact with carbon, and its thermodynamic stability is better than magnesium-carbon combination. (3) Magnesia-calcium sand has obvious creep properties at 1260°C, small linear expansion, and good high-temperature toughness, which is beneficial to improving the spalling resistance of refractory materials during use. (4) When in contact with steel slag, calcium oxide reacts preferentially with the silica in the steel slag to form dicalcium silicate. Its high viscosity can adhere to the brick working surface to form a protective layer. At the same time, it can increase the viscosity of the slag, increase the wetting angle, and inhibit further penetration of the slag. This makes the deteriorated layer thinner, preventing decarburization of the bricks, and the surface of the bricks is prone to slag, which is beneficial to the gunning repair of the ladle during use. (5) The large amount of free calcium oxide in the magnesia-calcium sand has good plasticity under high temperature conditions and can buffer the thermal stress caused by temperature fluctuations; it will not produce a thick layer of deterioration during use, which can improve the thermal shock resistance. The physical and chemical indicators of synthetic magnesium calcium sand are shown in Table 1.
Table 1 Physical and chemical properties of synthetic magnesia-calcium sand
|High calcium sand
|Low calcium sand
Although magnesia calcium sand has many advantages, free calcium oxide is easily hydrated. The reaction formula is as follows:
Only when T>547℃, calcium hydroxide can be completely decomposed; and when T<547℃, calcium oxide has a tendency to hydrate. Hydration of calcium oxide not only releases heat, but also increases the volume by 96.5%, causing the calcium oxide refractory material to be completely pulverized and unusable. Therefore, when preparing magnesium calcium sand, titanium oxide and the like are added to the ingredients to promote sintering, so as to increase the density of the material, make the calcium oxide grains grow and form relatively stable large grains. At the same time, magnesia-calcium sand fine powder is more easily hydrated than coarse particles due to specific surface area and other reasons, so it must first pass through a 0.5 mm screen when making bricks. Discard the <0.5 mm part, and soak the magnesia-calcium sand particles in a certain concentration of silica sol solution for a certain period of time to form a protective film on the surface of the particles to prevent hydration of calcium oxide. When making bricks, the matrix part uses fused magnesia, and the carbon source uses L-198 flake graphite. In addition, in order to prevent the influence of moisture contained in ordinary phenolic resin binders on calcium oxide, anhydrous resin with moisture ≠0.3% is specially selected as the binder.
Magnesia-calcium carbon bricks use synthetic high-calcium magnesia sand and flake graphite as the main raw materials, anhydrous resin as the binding agent, and appropriate amounts of metal and non-oxide antioxidants and multi-level ingredients. When batching, follow the particle → anhydrous resin → graphite → premix → discharging process, and mold it with a 1000t press. In order to prevent hydration reaction, magnesia-calcium carbon bricks are specially kiln-specific when baking. The time from entering the kiln to leaving the kiln is >20 h, and the maximum temperature is 220°C. The processing time for T>200°C is ≠6h. The bricks baked at low temperature are dipped in wax at 70~100℃. After inspection, the finished bricks are plastic-sealed and vacuum-packed. Their physical and chemical properties are shown in Table 2.
Table 2 Physical and chemical performance indicators of magnesia-calcium carbon bricks
|Volume density g.cm-3
Note: The compressive strength is the measured value under the condition of 110℃×16h; the flexural strength is the measured value under the high temperature condition of 1400℃×0.5h.
Masonry and test results
The magnesia-calcium-carbon test brick is a wedge-shaped, layer-laminated door with a height of 230 mm and a working thickness of 180 mm (the working thickness of the currently used aluminum-magnesia unburned bricks is 200 mm). Dry masonry is used, and the gaps between the bricks and the wall are filled with dry magnesia powder combined with solid resin and high-temperature asphalt. For comprehensive masonry ladles, magnesia-calcium carbon bricks are only used in the non-slag line parts of the ladle wall. The slag lines are magnesia-carbon bricks, and the bottom of the ladle is made of alumina-magnesium spinel carbon bricks.
Taking into account the different service life of argon-blown breathable bricks, seat bricks, nozzles and slag line magnesia carbon bricks. The magnesia-calcium carbon test brick is divided into three stages for industrial testing: first, a bottom-blown brick is built, which is decommissioned after 45 times of use, and the bottom-blown brick and the through-steel brick are replaced. The second is to build a bottom-blown brick and stop using it after 45 times. The bottom-blown brick and the steel seat brick are replaced and the slag line is repaired. The third is to lay a bottom-blown brick and plan to stop using it and dump it after using it 25 to 45 times.
A total of 10 complete ladle industrial tests were conducted since 2001. The service life of the ladle was 100 to 128 times, with an average of 116.8 times. The thickness of the remaining bricks was 55 to 85 mm, and the average erosion rate was 0.90 to 1.15mm/time. Compared with the existing aluminum-magnesium unburned bricks for the ladle wall, when the wall thickness is reduced by 20 mm, the service life is increased by 20 to 43 times and the erosion rate is reduced by 50%, achieving good test results.
Analysis of erosion mechanism
In order to understand the changes and erosion of magnesia-calcium carbon bricks during use, metallographic samples were made from original bricks and residual bricks after use, and thin sections were ground for transmitted light observation and observed under an optical microscope. At the same time, a carbon film was sprayed on the surface of the reflective sample, and the phase composition was analyzed by an electron probe.
The aggregate and fine powder in the original magnesia-calcium carbon brick are evenly distributed, and weak hydration causes cracks to appear at the boundaries of the aggregate particles. There are also metal antioxidants such as metallic aluminum. It can be seen from Figure 1 that the internal structure of magnesia-calcium sand particles is uniform and dense. However, since there is a certain time interval between taking the bricks and preparing the samples, it can be seen that the calcium oxide has been partially hydrated during observation. At the same time, silicates with high melting points in the image phase are not recognized due to hydration.
Fig.1 Microstructure of raw MgO-CaO-C brick
There is no obvious slag layer on the working surface of the remaining bricks, but it is not smooth. The aggregate particles are protruding and corroded, and the cross section is gray. The remaining bricks are 2~3 mm away from the working surface and are a metamorphic layer. There are strong magnesium oxide crystal rings around the magnesia-calcium sand particles in the layer. The size of the magnesium oxide crystal in the ring is 5 to 10 times that of the magnesium oxide crystal in the center of the particle. The closer to the non-working surface, the thinner the magnesium oxide crystal ring and the smaller the crystal. The magnesium calcium carbon residual bricks are shown in Figure 2, Figure 3 (magnesium oxide crystal ring close to the working surface) and Figure 4 (magnesium oxide crystal ring close to the original brick layer).
Fig.2 Microstructure of residual MgO-CaO-C brick
Fig．3 MicrostructureofresidualMgO—CaO-C brick
Fig.4 Microstructure of residual MgO-CaO-C brick
Formation of magnesium oxide crystal ring
From the magnesium oxide crystal ring, it can be seen that the spaces between the magnesium oxide crystals are filled with an amorphous gel structure. Its main components are calcium oxide and aluminum oxide (see samples 1 to 3 in Table 3), of which the aluminum oxide content is 32% to 42%. According to the CaO-Al₂O₃ system phase diagram analysis, this compound should be in the 3CaO-Al₂O₃ region. The formation of CaO-Al₂O₃ compounds will have an impact on the large amounts of calcium oxide and metallic aluminum in magnesia-calcium carbon bricks.
At high temperatures, the metallic aluminum of magnesia-calcium carbon bricks will volatilize and migrate to the hot surface. Aluminum trioxide is generated by the reaction with oxygen and aluminum trioxide is generated according to the reaction formula AlC₃(s)+6C0(g)→2Al₂O₃(s)+9C(s), which can be attached to the edges of magnesia-calcium sand particles and in the matrix. Magnesia-calcium sand particles contain a large amount of free calcium oxide, and the calcium oxide at the edges will react with the attached aluminum oxide to form CaO-Al₂O₃ compounds. It can be seen from the CaO-Al₂O₃ phase diagram that when the absorption of aluminum oxide in calcium oxide is <38%, the liquid phase will decompose at 1539°C. If the aluminum oxide content is 38% to 42.8%, a large amount of liquid phase can be melted and decomposed at 1360°C; the liquid phase will be completely formed at 1539°C. Therefore, when used at around 1600°C, this filling phase will undoubtedly exist in the form of a complete liquid phase. At the edge of the particle, the MgO-CaO crystal has good original crystallization. At this time, the calcium oxide has transferred to the liquid phase, and the magnesium oxide crystal is promoted by the liquid phase to grow, thereby forming a magnesium oxide crystal ring with a certain width. The liquid phase is filled between the magnesium oxide crystals.
Table 3 Electron probe analysis results
Reasons for cracks in magnesia-calcium sand
Among the protruding MgO-CaO particles on the working surface, the strong magnesium oxide crystals are not aggregated into rings (Figure 5). However, it was cemented by high melting point substances like a hard shell, and microcracks were found 0.5~0.8 mm away from the hot surface (Figure 6). The particles in Figure 7 were continuously taken from 4 fillings at the crack on the working surface for electron probe analysis (see samples No. 4 to 7 in Table 3). The aluminum oxide content in the filling between the particles of the working surface is only 1.25%, and the content becomes higher toward the inside, up to 40%. In addition, it can be seen from the composition calculation that the cement between the magnesium oxide crystals on the hot surface corresponds to dicalcium silicate, which is proved by the test results of the electron probe (see samples No. 8~11, 13, and 14 in Table 3).
Fig.5 MgO crystal in MgO-CaO sand aggregate
Fig.6 Microcrack at 0.5~0.8 mm awayfrom heat surface
Table 4 shows the composition of steel slag after refining. It can be seen that at high temperatures, the slag should be in the miscellaneous phase zone in the ladle, that is, there is a large amount of high melting point dicalcium silicate. When the slag comes into contact with the above-mentioned magnesia-calcium sand particles that already have magnesium oxide crystal rings, no new chemical reaction will occur, only physical penetration. When the slag penetrates into the brick at high temperature, it first breaks through the liquid phase in the magnesium oxide crystal ring and migrates into the brick together with it. When reaching a certain depth, the aluminum-containing liquid phase is enriched and stops infiltrating, while the high melting point dicalcium silicate in the slag precipitates and remains in the cement phase of the working surface, tightly combining with the magnesium oxide crystals to form a hard shell.
Fig.7 Bulgy MgO-CaO sand particle
Table 4 Composition of steel slag after refining
Figure 8 shows that there is dicalcium silicate containing aluminum oxide in the matrix. The composition is shown in sample No. 12 in Table 3. It is believed that cracks are caused by differences in composition and thermal expansion coefficient between the combination of magnesium oxide crystals and dicalcium silicate structural ceramics on the working surface and the aluminum oxide enrichment zone.
Fig.8 CS and CaO-AlO₃in matrix
Impact on steel quality
It can be seen from Table 5 that when other operating conditions remain unchanged and the steel type is the same, the ladle wall is changed from aluminum-magnesia unburned bricks to magnesia-calcium carbon bricks. The [O] content has decreased, and the total amount of inclusions in the steel has decreased significantly.
Table 5 Changes in [O] content before and after hydrogen blowing
|3 min sample/ppm
The statistical analysis results are shown in Table 6. The number of inclusions and the area ratio of the 3 min samples in the test package and the comparison package are basically the same. There are big differences in the test data of the finished samples. The number of inclusions in the test package decreased by 16.34% and the area ratio decreased by 82.74%; the number of inclusions in the comparison package increased by 3.11% and the area ratio decreased by 49.42%.
Table 6 Statistical results of inclusion analysis
|Sample package sample
|Comparison package sample
|Number of inclusions
|Total area of inclusions
|Number of inclusions
|Total area of inclusions
Note: The data in the table are all average values of test statistics.
Silicate inclusions in steel float to a certain extent during the refining and argon blowing process. This effect can be seen from the change in the average area ratio of the comparison package. The test bag contains free calcium oxide in the magnesia-calcium carbon brick, which has the ability to capture silicate inclusions, causing more inclusions to float during the argon blowing process, thus purifying the molten steel. In addition, it was also found from the test data that while the number of inclusions in the test package decreased after the argon blowing treatment, the proportion of the number of inclusions of various particle sizes in the total inclusions also changed greatly. That is to say, the proportion of small particle size inclusions is greatly increased, which means that the removal rate of large particle size inclusions is higher during the argon blowing process.
(1) The average service life of magnesia-calcium carbon bricks used in non-slag line linings of ladles has increased by 37.57 times compared with 2000 under the condition that the thickness of the ladle wall is reduced by 20 mm.
(2) The petrographic analysis results show that the main cause of brick damage is spalling, that is, the cracks that appear between the ceramic bonding of magnesia-calcium sand particles in the metamorphic layer and the aluminum oxide enrichment zone, which causes spalling when the temperature changes rapidly. At the same time, when the ladle is in use, the ladle is left at low temperature for a long time due to small slag repair lines and bricks, which causes the calcium oxide in the bricks to hydrate, causing the bricks to become loose and peel off.
(3) Image analysis of inclusions in steel shows. The use of magnesia-calcium carbon bricks in the non-slag line of the ladle wall can reduce the oxygen content and the number of inclusions in the steel, reduce the inclusion particle size, and change the inclusion particle size distribution, which is beneficial to improving the quality of molten steel.
(4) Free calcium oxide in magnesia-calcium carbon bricks has no significant effect on removing sulfur from steel due to the presence of carbon in the bricks.