What refractories are suitable for ladle slag line&low corrosion area?

The refining ladle brick lining is mainly made of neutral bricks (high alumina bricks, Al2O3-MgO-C bricks, etc.) and MgO-C bricks. The desulfurization treatment (using saturated CaO slag and CaF2 additive) requires the slag line to be lined with MgO-C bricks. For the sake of environmental protection, it is required to treat MgO-Cr2O3 bricks needed to be treated with chromium after used. Therefore, it is limited to application under special conditions in the world, such as VOD ladle, etc., because the range of alkalinity of VOD ladle slag fluctuates greatly (0.6~ Above 4.0), the MgO-Cr2O3 bricks showed the best results.

I. Refractories for Ladle Slag Line

In the past, the refined ladle mainly used MgO-Cr2O3 brick and high aluminum brick for the slag line and the low corrosion area lining. Due to the requirements of environmental protection, nowadays, in addition to the VOD ladle, chromium-free has been achieved, and instead of MgO-C brick or MgO-CaO-C brick as the refractory material lining for the slag line, low-carbon MgO-C bricks are used for vacuum refining (including refining low-carbon and ultra-carbon steel) ladle slag line areas.

The refined ladle slag line is made of MgO-C brick or MgO-CaO-C brick, which is mainly determined by the refined steel grade, slag characteristics and operating conditions.

The results show that in the CaO-SiO2 slag (low alkalinity slag), the MgO-CaO-C brick has a relatively high resistance to erosion. The reason is that when the MgO-CaO particles are in contact with the CaO-SiO2 slag, the dissolution of fCaO from the MgO-CaO sand particles immediately reacts with the slag component to form a high melting point and 2CaO?SiO2 and 3CaO?SiO2 are solidified on the surface. The slag is highly viscous and forms a strong protective layer on the surface of the brick, thereby improving the durability of the MgO-CaO-C brick lining.

At the same time, in the CaO-Al2O3 system slag, the loss of MgO-CaO-C brick is serious and increases with the increase of CaO in the brick. The reason is that the CaO dissolved from the brick is immediately dissolved in the CaO-Al2O3 slag to form a low-melting substance such as 12CaO?7Al2O3, which is melted from the surface of the brick and discharged into the slag to accelerate the brick into the slag. On the contrary, the MgO-C brick is easily eroded by the CaO-SiO2 system slag and has high resistance to the Ca0-Al2O3 system slag.

From this point of view, it is necessary to select a refractory material for the refining ladle slag line portion according to specific refining conditions and slag characteristics.

The main damage mechanism of the refined ladle slag line MgO-C lining is the oxidation of C. In addition to the direct oxidation of C, it is mainly caused by the operating temperature (greater than 1650 ° C) and high temperature decompression operation. This is especially true under high temperature and reduced pressure conditions.

II. Refractory for Ladle Low Corrosion Area

When the refractory castable is used in the low-corrosion zone such as the side wall of the refined ladle and the bottom of the ladle, the synthetic raw materials such as fused corundum, plate-like Al2O3 and Sp (76A or 90A) rich in Al2O3 are used as the main raw materials of the refractory castable. The LCC and ULCC schemes are designed. In this way, a refractory material having high refractoriness, slag resistance, and thermodynamic stability can be produced. The Al2O3-rich Sp releases excessive amounts of Al2O3 at low temperatures, which react with CaO in the slag to form CaO?6Al2O3, accompanied by an increase in volume, such as:

And MnO and FeO in the solid solution slag lead to an increase in viscosity, thereby improving slag resistance. The appropriate amount of Sp added is 15% to 30% (mass fraction), and the corresponding MgO content is 4% to 10% (mass fraction).

In order to further improve the slag permeability of the refractory castable, the MgO refractory castable scheme is adopted, and the Mg is generated in situ by the MgO+Al2O3→Sp reaction at a high temperature, and the volume is increased (about 15%) to compensate for the sintering. The volumetric shrinkage also increases the structural compactness of the material. The addition of a small amount of SiO2 promotes the formation of Sp while controlling the amount of expansion of the refractory castable to meet the requirements of use. For the thermal expansion of Al2O3-MgO refractory castables, it can be controlled by CA.

Masonry of high alumina bricks in the low-corrosion zone of refined ladle is another solution. Modern refined ladle uses natural andalusite and bauxite to produce high-alumina bricks. Compared with bauxite, the former has higher purity and thermo-mechanical stability (such as hot strength). Because it contains more SiO2, the corrosion resistance is poor. Especially in the condition of alkaline slag, the high-aluminum brick masonry refining ladle lining produced with bauxite clinker as the main raw material has the following shortcomings and is being eliminated.

1. Due to volume shrinkage, molten steel and slag are severely infiltrated and eroded, especially at the brick joint to form a thick slag layer.

2. Due to the inherent brittleness of the brick and its structural structure, the ladle lining forms a thicker stripping strip and slag layer.

3. Due to the inherent wet ability of the lining to the molten steel and slag, the erosion and penetration of the slag is more serious, and flaking occurs.

In order to overcome the above shortcomings of high-aluminum bricks in refined ladle, Al2O3-MgO-C bricks and Al2O3-Sp-C bricks have been developed to replace them. The former has the following advantages compared with high-aluminum bricks:

(1) Good high temperature resistance and resistance to tissue spalling;

(2) Good resistance to corrosion of molten steel and slag;

(3) Good residual expansion, even at higher temperatures, cracks will not occur at the joints of the bricks.

The reason is that when the temperature is higher than 1650 ° C, the high alumina brick shows a significant shrinkage, which causes the molten steel and slag to penetrate into the brick joint. Unlike high alumina bricks, Al2O3-MgO-C bricks do not shrink during the continuous casting temperature range (1650~1670 °C).

The results of comparative slag resistance test on high alumina brick, Al2O3-MgO-C brick and MgO-C brick by rotary impregnation method show that the erosion and penetration of high alumina brick are serious, while the erosion and penetration of MgO-C brick is the smallest. The Al2O3-MgO-C brick is somewhere in between. Field use also shows that Al2O3-MgO-C bricks have a higher service life than high-aluminum bricks and are therefore widely used to refine refractory materials in low-corrosion areas of ladle.

The alkaline bricks lining the low-corrosion zone of the brick refining ladle are mainly MgO-CaO bricks and MgO-C bricks. In order to prevent hydration of the burnt dolomite bricks, the organic matter may be impregnated to reduce the apparent porosity to less than 10%, and the ladle may be removed when heated before use. Rich MgO dolomite bricks are used between the slag line and the steel line to reduce wear in this area. One advantage of the dolomite brick for ladle is the reaction capacity with molten steel. The dolomite brick lining serves as a reaction zone for the refined ladle, and the contact surface with the brick is large. However, the wear rate of the dolomite brick lining is generally higher than that of the MgO-C brick or the Al2O3-MgO-C brick.

When the MgO-C brick is used to refine the side wall of the ladle, the C content is generally less than 15%. For magnesia refractories, in order to improve the thermal peeling resistance of bricks, C is indispensable and affects the high thermal expansion of MgO. C added to MgO bricks can improve slag resistance, but lower oxidation resistance. In general, the problem of easy oxidation of MgO-C bricks is overcome by the addition of an antioxidant.

In MgO-C bricks, the problem with adding metal antioxidants is to increase the thermal conductivity (which can be increased to 10-15W/(m?K), which leads to excessive heat loss of the molten steel. Therefore, for the whole sidewall (slag line and wire) MgO-C lining designed refining ladle, you need to consider installing a thermal insulation layer.

In addition, the trial results show that the MgO-C brick has achieved good results in the impact of the steel. However, when carbon-containing refractories are fully used in ladle, MgO-C bricks having a carbon content of less than 10% (mass fraction) are preferred from the viewpoint of temperature and inclusions. However, at this time, it should be considered that low carbonization will cause the damage to be accelerated due to spalling.

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