Because it is necessary to complete the control of the molten steel temperature, the precise adjustment of trace alloy elements and the calcium treatment of the improvement of inclusions in the tundish, at the same time, because the capacity of the tundish and the temperature of the molten steel are greatly different from those of the ladle, the refractory used The materials are also different, but no matter which tundish type, the refractory materials used must meet the following requirements:
(1) It is required that the lining material is resistant to corrosion by molten steel and molten slag and has a long service life.
(2) It is required that the lining material has good thermal shock resistance and does not burst when it comes into contact with molten steel.
(3) The lining material is required to have a low thermal conductivity and small thermal expansion, so that the tundish lining has a certain degree of heat preservation and good integrity.
(4) It is required that the lining material has little pollution to the molten steel during the pouring process to ensure the quality of the molten steel.
(5) The shape and structure of the lining material are required to be easy to build and disassemble.
The main types of intermediate packages are:
(1) High temperature tundish
The tundish is set up for a specific metallurgical process, the lining is made of magnesia brick, and it is preheated to about 1500℃.
(2) Hot tundish
That is, a kind of tundish commonly used in steel plants at present, which uses fired bricks or unfired bricks or castables as the lining of the tundish. Before pouring, preheat to 800～1100℃.
(3) Cold tundish
The insulation board is used as the lining, which can be used without preheating before pouring.
As for the choice of which tundish type and which material is better for the lining, it depends on the cast steel, smelting method, baking conditions and the ability of the steel plant itself.
The tundish lining is mainly composed of the following parts:
(1) Insulation layer (10~30mm)
This layer is next to the steel shell of the tundish, and its material is usually asbestos board, insulation brick or light castable.
(2) Permanent layer (100~200mm)
This layer is in contact with the insulation layer, and its material is generally clay brick.
(3) Working layer (20~50mm)
The layer in contact with molten steel is the working layer. The lining materials of this layer mainly include: high alumina bricks, alkaline bricks (such as magnesia bricks, etc.), siliceous thermal insulation panels, magnesia thermal insulation panels, forsterite thermal insulation panels and coatings, such as magnesia, magnesia chrome coatings, etc. .
At present, castables are also used as tundish lining.
It is inlaid at the bottom of the tundish and used to install the tundish nozzle. The material is usually high aluminum.
(5) Bag bottom
The material of the bottom of the tundish is basically equivalent to that of the working layer.
The tundish cover, covering the tundish, can play the role of heat preservation and prevent molten steel from splashing. The material is usually made of clay bricks or castables.
(7) Slag retaining wall
The wall is built in the tundish, and can be a single wall or a double wall. Single-wall or double-wall slag retaining wall, as the name suggests, is used to block slag to improve the cleanliness of molten steel. A molten steel filter can also be installed on it to further remove inclusions in the molten steel. The material of the slag retaining wall is usually high-aluminum, which can be built with bricks in the tundish, or it can be made into prefabricated blocks and placed in the tundish.
There are often some problems in the use of refractory linings in the tundish. Here are a few problems for explanation.
Some of the problems that occur during the use of tundish refractory materials are the quality of the material itself, and some are related to the on-site construction and require careful observation and analysis.
Dry material has no strength
Before the tundish is put into use, the dry material of the working layer needs to have a certain low temperature strength, which is mainly obtained by vibration and baking. After the dry material is demoulded by vibration and baking, there will often be local lack of strength or low strength, which can easily cause collapse and affect the production of continuous casting cases. After long-term observation and analysis, it is concluded that the main reasons for the lack of strength or low strength of dry materials are:
①Baking problem: The tundish baking device used in the steel plant is a gas baking device. After long-term use, it will cause a large amount of tar in the pipeline or damage the burner, resulting in poor local baking effect and resulting in weak or low strength.
②The dry material is damp: The dry material is composed of 70% particles and 30% fine powder. The fine powder contains magnesia and binder. Because of the high specific surface area, the fine powder is easy to absorb water and moisture. The magnesia and the binder absorb water after water Chemical deterioration causes the bonding agent to fail and cannot produce bonding strength during baking;
③Uneven mixing of binder: When producing dry materials, various particles, fine powders and binders need to be put together and mixed evenly. Sometimes the mixing is uneven due to human factors or mixing equipment. Some dry materials have no binder or The content of the binder is seldom resulting in partial loss of strength after baking and demolding.
Solution: First of all, it is necessary to ensure the baking effect of the roaster, to regularly purge the gas pipe to remove tar and dust, and to replace the damaged burner in time. Second, it is necessary to ensure that the dry materials are dried and mixed evenly.
The turbulator floats up
Turbulators mainly play the role of stabilizing steel flow and protecting the impact area during the continuous casting production process. It belongs to the refractory material of the slag retaining wall series and is generally produced by magnesia materials. In the multi-furnace continuous pouring process, sometimes the turbulator will surging on the surface of the molten steel, which cannot stabilize the steel flow and protect the impact area, which is detrimental to the quality of the molten steel and safe production. The turbulator floats up because the density of refractory material is less than the density of molten steel, so it will float when immersed in molten steel. The turbulator floats include the floating of the turbulent frame and the overall float. The frame of the turbulator floats up because the magnesia material is immersed in high-temperature molten steel and the expansion coefficient is too large, which can easily cause cracking, and the contact part of the frame and the impact plate is a weak link, which easily causes the frame and the impact plate to separate. The overall floating is due to the installation turbulence The working layer at the bottom of the turbulator is not flat. After pouring, the molten steel penetrates into the gap between the turbulator and the working layer, causing the turbulator to float as a whole.
Solution: One is to adjust the formula of the turbulator to control the expansion under high temperature; the other is to spread a layer of refractory powder on the surface of the working layer when installing the turbulator to ensure that there is no gap between the turbulator and the working layer.
Nozzle cracking and seeping steel
The tundish nozzle with zirconium core is easy to crack during the pouring process, resulting in steel infiltration, which often forces continuous casting to block production or abnormal shutdown. Analysis believes that the nozzle cracking is mainly due to the poor thermal shock resistance of the zirconium core.
Solution: The bulk density of the zirconium core should not be too high. The higher the bulk density, the worse the thermal shock resistance. Appropriately thickening the nozzle body can prevent molten steel from seeping out.
Large package casing break
The large ladle casing is located between the ladle nozzle and the tundish, and its metallurgical function is to prevent the molten steel from splashing and being oxidized during the process of the molten steel flowing from the ladle into the tundish. The most common problem in the use of large-clad casing is fracture. There are two reasons for the fracture. One is the poor thermal shock resistance of the large-clad casing. The nozzle is separated. When the casing and the ladle nozzle are tightly adhered, the caster will apply external force to the ladle casing and cause the casing to break.
Solution: First, use materials with small thermal expansion coefficient and elastic mold to produce the casing to improve thermal shock resistance; second, when the casing and the water outlet cannot be shared, external force cannot be applied to the lower part of the casing, and the upper part of the casing can be steel External force is applied to the shell to separate it.
In addition to the above problems, in the process of communicating with some practitioners and producers in the steelmaking industry, we found that there were other problems such as collapse of the dry material, cracks in the working lining, damage to the nozzle, peeling of the slag retaining wall, steel infiltration, etc. And other issues.