The Ladle Shroud,Stopper& Submerged Nozzle of Continuous Casting

Functional refractory components such as the ladle shroud, the stopper, and the submerged nozzle are collectively referred to as the three major components of continuous casting.Currently, the more suitable molding method generally uses isostatic pressing.Molding pressure is generally divided into 50Mpa, 100Mpa, 125Mpa, etc. Different pressures are used according to different parts of the nozzle and particle gradation.

✳ Ladle Shroud

When molten steel is poured from the ladle to the tundish, in order to avoid oxidation and splashing, the shroud is installed at the lower end of the slide gate at the bottom of the ladle. One end is connected to the lower nozzle, and the other end is inserted into the molten steel in the tundish for sealing and protection pouring.

The functions of the shroud are as follows:

(1) Prevent secondary oxidation of molten steel and improve the quality of the steel;

(2) Reduce the deposition of oxidation products of easily oxidized elements in the steel on the inner wall of the nozzle, extending its service life;

(3) The shroud can be used multiple times to reduce refractory consumption.

The fused quartz shroud is formed by the mud casting method. It is characterized by good thermal shock resistance, high mechanical strength and resistance to acidic slag erosion, and good chemical stability. However, it is easy to form low melts with oxides such as iron and manganese in molten steel or slag. At high temperatures, quartz reacts with carbon to be decomposed and gasified, resulting in poor corrosion resistance, which is not conducive to smelting clean steel.

The aluminum carbon shroud uses high-purity raw materials to reduce the impurity content, improve the organizational structure according to reasonable particle gradation, and improve the corrosion resistance; adjust the content of natural graphite, and use the non-wetting property of coarse-grained flake graphite to molten steel to reduce nodules at the shroud; improve the shape to reduce nozzle cracks; apply micro-powder technology and add an appropriate amount of Al2O3 micro-powder to enhance high-temperature strength and thermal stability.

Aluminum carbon shroud has good thermal shock resistance and strong adaptability to steel types. In order to prevent the carbon on the surface of the shroud from being oxidized during baking and use, an anti-oxidation coating is applied to the surface of the shroud. The anti-oxidation coating is mainly composed of feldspar, quartz, clay and other raw materials. They are made into glaze by wet grinding, and is applied to the surface of the shroud manually or mechanically. This coating forms a glaze layer in the range of 700°C to 1000°C, thus protect graphite from oxidation or minimal oxidation.

For the aluminum carbon long shroud, on the basis of adding appropriate low expansion materials (fused silica, aluminum titanate), toughening materials (zirconia), steel fiber reinforcement, etc., in order to further improve its performance, the Al2O3 content of the shroud is increased, and the SiO2 content is reduced, so as to ensure the thermal shock performance and improve the service life. In addition, there are also chrome corundum-mullite shrouds and unshaped shrouds made of Al2O3-SiC-C castables.

✳ Monoblock Stopper

Characteristics of the monoblock stopper: They are all formed by isostatic pressing. Its shape and size depend on the capacity of the tundish, the height of the molten steel surface,the trumpet shape and hole size of the tundish nozzle. The stopper head of the monoblock stopper has a hollow one, an argon blowing hole or a breathable plug.The fixing method is the key, one is fixed with metal pins, and the other is fixed with threads.

The function of the stopper is mainly used to open and close the tundish. In addition to automatically controlling the flow of molten steel from the tundish to the mold, it can also blow argon and other inert gases into the tundish through the argon blowing hole. The rod has the function of controlling the flow of steel and purifying it.

The monoblock stopper material is generally aluminum carbon. The head is equipped with an argon blowing hole or a porous plug. During pouring, the argon gas is blown from the stopper rod hole through the blowing hole or porous plug to the submerged nozzle.

In order to adapt to different refining conditions and the continuous casting needs of steel types, the monoblock stopper head has different materials such as aluminum carbon, magnesium carbon, spinel carbon and zirconium carbon. The rod body material is mostly aluminum carbon. In some long life tundish continuous casting processes, the rod body will be based on the actual needs which will be composite design with slag line material to improve the corrosion resistance of the stopper rod body.

✳ Submerged Nozzle

In order to improve the quality of the cast slab, the submerged nozzle is provided between the tundish and the mold. Its main functions are:

(1) to prevent secondary oxidation and nitridation of molten steel and splashing of molten steel.

(2) to adjust the flow state and injection speed of molten steel.

(3) Preventing non-metallic inclusions in the mold slag from being involved in the molten steel, which plays an important role in promoting the floating of inclusions in the molten steel.

(4) It has a decisive influence on the edge casting billet yield and billet quality.

The submerged nozzle has a certain porosity and also have breathability. The outside air penetrates into the nozzle under the negative pressure generated by the flow of molten steel, and contacts the molten steel to oxidize it. Therefore, the outer surface of the shroud and submerged nozzle must be coated with an anti-oxidation glaze layer. Regardless of the material and structure of the submerged nozzle, it must meet the following conditions:

(1) Ensure the flow rate of molten steel at normal pulling speed;

(2) Make the heat flow distribution in the mold and the cross section of the slab as uniform as possible;

(3) It is conducive to the rapid melting of mold powder;

(4) It is conducive to the floating of inclusions and does not entrain slag;

(5) Avoid violent stirring of the molten steel level in the mold;

(6) Easy to install.

In the early stage of continuous casting, a fused quartz submerged nozzle with good thermal stability and high purity is used, which can meet the casting needs of ordinary carbon steel, low manganese steel, aluminum-killed steel and other steel types. However, in the case where high manganese steel and high alkalinity slag corrosion is very serious, aluminum carbon submerged nozzles are widely used.The raw materials of the aluminum carbon submerged nozzle are special-grade alumina, fused corundum or sintered corundum, etc., with graphite and antioxidant added. The product has the advantages of corrosion resistance and thermal shock resistance.

In order to solve the serious erosion of the slag line of the submerged nozzle and prevent the clogging of the nozzle caused by the adhesion of alumina, a series of new immersed nozzles were developed based on aluminum carbon: Al2O3-C/ZrO2-C composite nozzle, ( MgO-C)-(Al2O3-C)-(ZrO2-C) composite nozzle, etc.

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