(1) Prevent secondary oxidation of molten steel
The tundish flows into the copper mould tube, and the metal surface in the copper mould tube is continuously renewed due to the impact of the injection. When the mold flux is added to the molten steel surface in the copper mould tube, the liquid slag layer, the sintered layer and the slag layer are rapidly formed, and uniformly spread on the molten steel surface to be insulated from the air, thereby effectively preventing air from entering the molten steel. To prevent secondary oxidation of molten steel.
(2) Adiabatic insulation reduces the heat loss of molten steel
Solidification of the molten steel surface and early solidification of the meniscus shell of the meniscus will have an adverse effect on the surface of the cast slab. Because the floating inclusions in the molten steel are likely to be trapped by the solidified iron crystals, forming a hard shell structure composed of metal and oxide, which can cause serious defects after being wound into the shell. The heat preservation effect of the slag is achieved by a slag layer having a low temperature and a small bulk density covering the molten steel surface. Therefore, appropriately increasing the thickness of the slag layer can improve the insulation performance of the slag and increase the temperature of the slag layer. However, an excessively thick slag layer will prolong the sintering time of the slag at high temperatures, causing the agglomeration and slag to seriously jeopardize the risk of continuous casting production.
(3) Absorbing and dissolving non-metallic inclusions
The molten steel entering the copper mould tube is inevitably brought into the non-metallic inclusions, and the inclusions floating in the molten liquid meniscus in the liquid crystal cavity of the mold can be caught in the shell forming surface and the subcutaneous inclusion defects. From a thermodynamic point of view, silicate slag can absorb and dissolve such non-metallic inclusions, but its dissolution rate is affected by many factors. In the vigorous implementation of the economic clean steel production mode, although the inclusions in the molten steel are reduced, the performance of the slag is affected after entering the protective slag. It is hoped that the performance change of the protective slag should be controlled within the scope of the continuous casting process.
(4) Lubricating between the mold wall and the shell
A layer of liquid slag of suitable shape and uniform thickness is required between the solidified shell and the copper wall of the mold to reduce the solid-solid friction. The liquid slag layer on the molten steel surface continuously supplies lubricant between the shell and the mold wall. In order to ensure the continuous supply of liquid slag, the meniscus must be kept open, and in order to make full play of the lubricating effect, the liquid slag should have glassy properties, and there should be no precipitation of high melting point crystals in the liquid residue. The downward movement of the slab during casting and the vibration of the copper mould tube cause the slag to form a slag film between the mold wall and the shell, and in order to reduce the friction, it is necessary to maintain a certain slag consumption.
(5) Improve and control the heat transfer between the slab and the mold
The data shows that the thermal conductivity of air is about 0.09 W/(m·K), while the slag film is about 1.2 W/(m·K). During the solidification process, the shell has an air gap between the copper mould tube and the copper mould tube, so that the thermal resistance increases, so that the heat transfer in the lower part of the copper mould tube is slowed down, which is not conducive to the rapid increase of the thickness of the shell and the increase of the pulling speed. productivity. On the other hand, the air gap in the meniscus is very small, the heat transfer capacity of the slag film is too strong, the uneven cooling effect of the primary shell is large and the thickness becomes uneven, plus the flow field and temperature field are not Uniformity results in a greater thickness non-uniformity of the green body shell, and tissue stress caused by a peritectic reaction may cause cracks on the surface of the slab and under the skin. Especially for the slab of rolled plate, even the sub-crack of the unexposed slab is likely to remain on the steel plate when rolling the thick gauge steel plate, resulting in a large amount of cleaning or even high scrap rate; and thick slab production Due to the large meniscus on the section, the low temperature zone is prone to occur, and adhesion is likely to occur. Therefore, how to effectively coordinate and control the heat transfer in the meniscus area and ensure the lubrication of the slab is a very urgent problem. However, the control technology in this area at home and abroad has been difficult to meet the needs of stable production.
In order to ensure the continuous casting process and the production of defect-free slabs, in addition to the coordination of the function of the slag itself, it is equally important to control the continuous casting process conditions.
(1) Prevent secondary oxidation of molten steel
The tundish flows into the copper mould tube, and the metal surface in the copepr mould tube is continuously renewed due to the impact of the injection. When the mold flux is added to the molten steel surface in the copper mould tube, the liquid slag layer, the sintered layer and the slag layer are rapidly formed, and uniformly spread on the molten steel surface to be insulated from the air, thereby effectively preventing air from entering the molten steel. To prevent secondary oxidation of molten steel.
(2) Adiabatic insulation reduces the heat loss of molten steel
Solidification of the molten steel surface and early solidification of the meniscus shell of the meniscus will have an adverse effect on the surface of the cast slab. Because the floating inclusions in the molten steel are likely to be trapped by the solidified iron crystals, forming a hard shell structure composed of metal and oxide, which can cause serious defects after being wound into the shell. The heat preservation effect of the slag is achieved by a slag layer having a low temperature and a small bulk density covering the molten steel surface. Therefore, appropriately increasing the thickness of the slag layer can improve the insulation performance of the slag and increase the temperature of the slag layer. However, an excessively thick slag layer will prolong the sintering time of the slag at high temperatures, causing the agglomeration and slag to seriously jeopardize the risk of continuous casting production.
(3) Absorbing and dissolving non-metallic inclusions
The molten steel entering the copper mould tube is inevitably brought into the non-metallic inclusions, and the inclusions floating in the molten liquid meniscus in the liquid crystal cavity of the mold can be caught in the shell forming surface and the subcutaneous inclusion defects. From a thermodynamic point of view, silicate slag can absorb and dissolve such non-metallic inclusions, but its dissolution rate is affected by many factors. In the vigorous implementation of the economic clean steel production mode, although the inclusions in the molten steel are reduced, the performance of the slag is affected after entering the protective slag. It is hoped that the performance change of the protective slag should be controlled within the scope of the continuous casting process.
(4) Lubricating between the mold wall and the shell
A layer of liquid slag of suitable shape and uniform thickness is required between the solidified shell and the copper wall of the mold to reduce the solid-solid friction. The liquid slag layer on the molten steel surface continuously supplies lubricant between the shell and the mold wall. In order to ensure the continuous supply of liquid slag, the meniscus must be kept open, and in order to make full play of the lubricating effect, the liquid slag should have glassy properties, and there should be no precipitation of high melting point crystals in the liquid residue. The downward movement of the slab during casting and the vibration of the copper mould tube cause the slag to form a slag film between the mold wall and the shell, and in order to reduce the friction, it is necessary to maintain a certain slag consumption.
(5) Improve and control the heat transfer between the slab and the mold
The data shows that the thermal conductivity of air is about 0.09 W/(m·K), while the slag film is about 1.2 W/(m·K). During the solidification process, the shell has an air gap between the copper mould tube and the copper mould tube, so that the thermal resistance increases, so that the heat transfer in the lower part of the copper mould tube is slowed down, which is not conducive to the rapid increase of the thickness of the shell and the increase of the pulling speed. productivity. On the other hand, the air gap in the meniscus is very small, the heat transfer capacity of the slag film is too strong, the uneven cooling effect of the primary shell is large and the thickness becomes uneven, plus the flow field and temperature field are not Uniformity results in a greater thickness non-uniformity of the green body shell, and tissue stress caused by a peritectic reaction may cause cracks on the surface of the slab and under the skin. Especially for the slab of rolled plate, even the sub-crack of the unexposed slab is likely to remain on the steel plate when rolling the thick gauge steel plate, resulting in a large amount of cleaning or even high scrap rate; and thick slab production Due to the large meniscus on the section, the low temperature zone is prone to occur, and adhesion is likely to occur. Therefore, how to effectively coordinate and control the heat transfer in the meniscus area and ensure the lubrication of the slab is a very urgent problem. However, the control technology in this area at home and abroad has been difficult to meet the needs of stable production.
In order to ensure the continuous casting process and the production of defect-free slabs, in addition to the coordination of the function of the slag itself, it is equally important to control the continuous casting process conditions.
