By using two research methods, dynamic slag resistance and static slag resistance, combined with research methods such as X-ray diffraction analysis, electron microscopy, energy spectrum analysis and chemical analysis, the cause of corrosion of magnesia carbon bricks with ladle slag wire for a special steel was studied Analyzed. The results show that under static slag-resistant conditions, a certain special steel slag has little corrosion on magnesia-carbon bricks, and the molten slag penetrates into the material along the low-melting phase of the material matrix and magnesia grain boundaries. Under dynamic slag resistance conditions, the corrosion of magnesia carbon bricks by molten slag is more serious. In order to improve the corrosion resistance of magnesia carbon bricks to a special steel slag, it is necessary to reduce the amount of impurities in the magnesia carbon bricks and optimize the matrix composition and microstructure of the magnesia carbon bricks.
Keywords: a special steel, slag, magnesia carbon brick, etching
Carbon refractory materials are widely used in the steel and metallurgical industry due to their good thermal shock stability and excellent resistance to slag erosion. Due to the non-wetting property of slag to graphite, magnesium carbon refractory materials have a long service life, so they are often used in parts with harsh operating conditions such as ladle slag lines to extend the overall service life of the ladle lining.
Currently, Wuhan Iron and Steel General Plant uses MgO-C bricks as the working layer material of the ladle slag line when smelting a certain special steel. During use, it was found that the service life of the slag wire bricks used for smelting a certain special steel is lower than that of ordinary ladle slag wire bricks. This article studies the corrosion of magnesium carbon refractory materials by a certain special steel slag and analyzes and discusses it.
Fused magnesia particles (68wt%), fused magnesia fine powder (20wt%) and flake graphite (12wt%) are used as the main raw materials. The chemical composition of the raw materials is shown in Table 1. Mix the raw materials and add 4wt% phenolic resin as a binding agent. The chemical composition of a certain special steel slag is shown in Table 2.
Table 1 Chemical composition of raw materials (wt.%)
Table 2 Chemical composition of slag of a certain special steel (wt.%)
Dynamic slag resistance experiment
The refractory material mixture is pressed into a crucible on an isostatic press at a pressure of 200 MPa. The inner diameter of the crucible is φ 70mm-80mm and the height is 230mm. The crucible was dried at 230°C for 24 hours. Put a crucible containing a certain special steel into an industrial induction furnace, fill it with magnesium material around it, and then the industrial induction furnace starts to energize the special steel to melt it. When the steel sample is nearly completely melted, a special steel slag is quickly added. The experimental diagram is shown in Figure 1. During the experiment, the surface of the slag was kept in a slightly agitated state. The experimental time was 150 minutes and the experimental temperature was 1600°C.
Figure 1 Schematic diagram of anti-slag experiment
Static slag resistance experiment
Drill the magnesia carbon bricks, the inner diameter of the hole is φ43mm, and the height is 65mm. The magnesium carbon crucible was dried at 230°C for 24 hours. Then put 130 g of a certain special steel slag into the crucible, then put the crucible together with the steel slag into a sagger, cover it with coke, send it to a high-temperature furnace, and keep it at 1600°C for 3 hours.
After the experiment, the crucible was cut along the center, and electron microscopy, energy spectrum analysis (JEOL, JXA-8800R) and X-ray analysis (XPERT PRO PW3040 Philips) were performed on the sample after the experiment. The residue in the crucible after the static experiment was analyzed by X-ray diffraction.
Results and discussion
Static slag resistance experiment
The results of the static slag resistance experiment are shown in Figure 2. It can be seen from Figure 2 that under the conditions of static slag resistance experiment, the corrosion of magnesium carbon refractory material by molten slag is very small, and the penetration layer is almost unobservable. In fact, this is because under static conditions, after the reaction between the slag and the refractory material reaches equilibrium, the equilibrium product isolates the slag and the refractory material layer, and the related reactions are then balanced.
Figure 3 shows the XRD analysis of steel slag before and after the test. The results show that the main components of the slag have changed. The main mineral phase of the steel slag before the test is Akermanite, and its chemical formula is:
[Ca1.53Na0.51][Mg0.39Al0.41Fe0.16][Si2O7]. After the experiment, the chemical composition of the steel slag changed. The main mineral phase is Gehlenite (the chemical formula is:
[Ca1.96Na0.05][Mg0.24Al0.64Fe0.12][Si1.39Al0.61O7]), forsterite (Forsterite, chemical formula: MgSiO4) and pyrope (Pyrope, chemical formula: [Mg0. 151Ca0.849]3Al2 [SiO4]3) composition. The main reason is that the MgO in the magnesium carbon material dissolves into the slag and reacts with other oxides in the slag. Table 3 shows the chemical composition analysis of steel slag after the test.
Figure 2 Cross-sectional view of the crucible after static slag resistance test
Figure 3 XRD analysis of steel slag before and after the experiment (left: original slag, right: residue after test)
Table 3 Chemical composition analysis of steel slag after the test (wt.%)
Figure 4 SEM image of hot surface of MgO-C brick after slag resistance test
A significant difference in the chemical composition of the steel slag before and after the experiment is that the MgO content in the steel slag increased by nearly 80% after the experiment. The contents of CaO, MnO, TiO2 and TFe decreased by a certain amount, while the SiO2 content did not change. The analysis results show that the magnesium oxide in the magnesia carbon bricks dissolved into the steel slag during the experiment and formed new mineral phases with other components, such as forsterite and pyrope garnet.
Figure 4 shows the SEM picture of the hot surface of the magnesia carbon brick after the slag resistance test. From the results, it can be seen that the slag penetrated into the magnesia grain boundaries and formed a low-melting phase of magnesia-calcium-silicon mixed oxide. The large magnesia grains are decomposed into small grains and then the small grains are dissolved into the steel slag. (The chemical composition of point 1 is: MgO 26.60%, SiO2 40.36% and CaO 33.05%)
Dynamic slag resistance experiment
Figure 5 shows the cross-sectional view of the magnesium carbon crucible after the dynamic slag resistance test.
Figure 5 Interface diagram of MgO-C crucible after dynamic slag resistance experiment
From the cross section of the crucible after dynamic slag resistance, it can be found that the magnesium carbon material is severely corroded by the molten slag. Figure 6 is a typical photo of the hot surface of the magnesium carbon crucible slag line. It can be found from the figure that the reaction layer becomes loose and a calcium magnesium silicon low-melting oxide phase is formed. The morphology of graphite in the reaction layer remains intact, and no decarburization layer was found in the microstructural analysis. Moreover, the penetration layer formed by molten slag is extremely thin, which means that the erosion of magnesium carbon materials by molten slag is mainly melting loss.
Figure 6 SEM image of the hot surface of the magnesium carbon material after dynamic slag resistance
The oxidation of graphite and the dissolution of magnesium oxide in the slag are the main causes of corrosion of magnesia carbon bricks. It can be seen from the study that the slag penetrates into the magnesia grain boundaries and reacts with the magnesia to form a low melting phase, which causes the dissolution of the magnesia in the slag, resulting in the destruction of the integrity of the material. Graphite can prevent the contact between magnesia particles and slag to a certain extent. After magnesia and slag come into contact, slag will penetrate into the magnesia grain boundaries. Magnesia grains will decompose into small magnesia grains under the penetration of slag, thus causing the destruction of magnesia grains.
Generally, the relevant reactions during the etching process of magnesia carbon bricks are as follows:
2C(s)+O2(g) → 2CO(g) ( 1)
C(s)+O2(g) → CO2(g) (2)
(FeO)slag+ C(s) →Fe(l)+CO(g) (4)
(MnO)slag+ C(s) →Mn(l)+CO(g) (5)
Equations (4) and (5) indicate that the graphite in magnesia carbon bricks will be oxidized under the action of oxides in the slag. Once the graphite is oxidized, the slag will further corrode the material, and the penetration of the slag into the bricks will be aggravated.
However, it can be seen from our experimental results that the penetration of slag into magnesium carbon materials is not obvious. The shape of graphite in the reaction layer remains intact, indicating that the main cause of erosion of magnesia carbon bricks is the destruction of magnesia particles by molten slag. The main path of destruction is the penetration of magnesia grain boundaries and the decomposition of magnesia grains. The slag reacts with periclase and forms a low melting phase in the grain boundaries, thus causing material damage. Of course, part of the graphite located on the surface of the reaction layer will also be oxidized under the action of oxides in the slag, thereby aggravating the erosion of magnesia by the slag. The corroded magnesia is lost into the slag along with the low-melting phase under the stirring action of the slag liquid, causing melting loss of the magnesia-carbon material.
MgO-C bricks have good resistance to slag corrosion under static slag resistance experimental conditions. The periclase will dissolve into the slag. As time goes by, the dissolution reaction will reach equilibrium, and the dissolution of periclase will stop. However, under the conditions of dynamic slag resistance experiments, the stirred slag seriously erodes magnesium carbon materials. The molten slag penetrates into the magnesia grain boundaries, causing the decomposition of the magnesia grains. The small grains formed during the decomposition process will be lost into the molten slag, thereby destroying the overall structure of the magnesia carbon material. At the same time, part of the graphite located in the reaction layer will be oxidized by the oxides in the slag, thereby intensifying the erosion of the magnesium carbon material by the slag.
To improve the slag resistance of magnesia carbon materials, the purity of magnesia should be improved. The magnesia used to produce slag line magnesia carbon bricks should be in good crystallization condition, and the impurity content in the grain boundaries should be as low as possible. At the same time, the graphite content in magnesia carbon bricks should be controlled within an appropriate range to achieve a balance between slag resistance and thermal shock stability. Because graphite is easily oxidized, oxygen in the air and oxides in the slag will cause it to oxidize. This will destroy the overall performance of the magnesium carbon composite material.