The cooling of molten steel in the copper mould tube is called primary cooling, and its cooling effect can be measured by the size of the heat flow transmitted through the mould wall.
1) Primary cooling effect:
Primary cooling is water cooling of the copper mould tube. Its function is to ensure that the cast billet forms a certain primary billet shell in the tube.
2) Primary cooling principle:
Primary cooling water is based on experience, and it is determined that under certain process conditions, the molten steel can form a sufficient billet shell thickness in the crystallizer and ensure the safe operation of the crystallizer. Usually, the water supply around the crystallizer is 2L/min.mm. The temperature difference between the inlet and outlet water does not exceed 8C, the outlet water temperature is preferably controlled at 45-50C, and the water pressure is controlled at 0.4-0.6Mpa.
Copper mould tube water quality
Generally, the following technical conditions are met to prevent scaling on the copper plate surface in the crystallizer water tank, which affects the heat transfer of the crystallizer.
- Solids are not more than 10mg/L.
- The total suspended matter is not more than 400m/L.
- Sulfate is not more than 150m/L.
- Chloride is not more than 100m/L.
- Total hardness (calculated as CaCO3) is not more than 10 mg/L.
- PH value is 7.5—9.5.
- Industrial clean water is used for small square billets, and soft water is often used for slabs.
Role of copper mould tube
â—†At the highest possible pulling speed, ensure that the billet leaves the copper mould tube with a shell of sufficient thickness, so that the continuous casting process can proceed safely, and at the same time determine the production capacity of the continuous casting machine;
â—†The molten steel in the mould tube transfers heat to the copper plate steadily, so that the thickness of the surrounding shell can grow evenly, ensuring the surface quality of the billet.
Behavioral characteristics of shell growth in the copper mould tube
(1) When the molten steel enters the tube and contacts the copper plate, a meniscus with a smaller radius will be formed on the upper part of the bar due to the surface tension and density of the molten steel. At the root of the meniscus, due to the fast cooling rate (up to 100C/S), the primary shell is formed quickly, and the molten steel continuously flows into the mould tube. New primary shells are continuously generated, and the thickness of the generated shells continues to increase.
(2) The solidified shell shrinks inward due to phase change and detaches from the copper plate of the copper tube until it is balanced with the static pressure of the molten steel.
(3) Due to the reason in item (2), an air gap is generated between the primary shell and the copper plate. In this way, the shell begins to recover due to insufficient cooling, and its strength decreases. The static pressure of the molten steel pushes the shell to the copper plate again.
(4) The above process is repeated until the shell leaves the copper mould tube. The unevenness of the shell always exists, and most surface defects originate from this process.
(5) The heat transfer at the corner is two-dimensional, and the solidification starts fastest, shrinks earliest, and forms air gaps earliest. The shell in the corner area is the thinnest, which is also the weak link for the generation of corner cracks and steel leakage. In addition to forced cooling by the mould tube wall, the molten steel injected into the tube also transfers heat through radiation from the steel surface and conduction in the direction of billet drawing. The ratio of heat transfer is about 30:0.15:0.03. The thermal resistance of each section is about: 26% for the billet shell; 71% for the air gap; 1% for the copper wall of the tube; 2% for the interface between the copper wall and the cooling water.
Factors affecting heat transfer in the copper mould tube
- Coppper tube taper
- Product protection slag
- Cooling water quality
- Copper tube material
- Steel composition
Studies have shown that the growth of the billet shell thickness obeys the root mean square law:
K is affected by various factors and varies within a certain range. The K value of the slab copper tube is generally 18~22mm. mm-0.5.
Secondary cooling in continuous casting
1.Function of secondary cooling in continuous casting:
Secondary cooling refers to the cooling process of the billet in the copper mould tube in the secondary cooling section of the continuous casting machine. Its purpose is to spray water to cool the billet with liquid core to make it completely solidify so as to achieve uniform cooling during the billet drawing process.
2. Principles for determining the strength of secondary cooling in continuous casting:
Secondary cooling is usually considered in combination with the heat transfer of the billet and the metallurgical quality of the billet. As the billet leaves the copper mould tube, a large amount of water cooling is used to quickly increase the thickness of the billet shell. As the billet moves in the secondary cooling zone, the billet shell thickness increases and the amount of water spraying gradually decreases. Therefore, the secondary cooling zone can be divided into several cooling zones. Each cooling section controls the water volume separately. At the same time, the secondary cooling water volume is adjusted in a targeted manner considering the steel type’s sensitivity to cracks.
3. Secondary cooling water volume and water pressure:
For ordinary carbon steel and low alloy steel, the cooling intensity is: 1.0-1.2L/Kg steel.
For low carbon steel and high carbon steel, the cooling intensity is: 0.6-0.8L/Kg steel.
For steel types with strong thermal crack sensitivity, the cooling intensity is:
0.4-0.6L/Kg steel.
The water pressure is 0.1-0.5MPa
4. Heat transfer and solidification in the secondary cooling zone
1) Characteristics of heat transfer in secondary cooling
The process of complete solidification of the billet from the moment it leaves the crystallizer is called secondary cooling. The main methods and proportions of heat transfer in secondary cooling are:
- Heat transfer methods account for about %
- Cooling water heating and evaporation 55%
- Bill radiation 25%
- Roller conduction 17%
- Air convection 3%
When the equipment and process conditions are constant, the heat transfer of the slab radiation and roller conduction does not change much, and the heat transfer of the spray water is dominant. The heat in the center of the billet is conducted to the surface of the billet through the billet shell. When the spray water droplets hit the surface of the billet, they will take away a certain amount of heat, and the surface temperature of the billet will suddenly drop, forming a large temperature gradient between the center and the surface, which becomes the driving force for cooling the billet.
From the heat transfer method of secondary cooling, it can be explained that to improve the cooling efficiency of the secondary cooling zone, it is necessary to study the heat exchange between the spray water droplets and the high-temperature billet. It can be expressed by the convection heat transfer equation:
To improve the cooling efficiency of the secondary cooling zone and ensure the quality of the slab, it is necessary to increase the h value and the reasonable distribution of the values
2) Factors affecting heat transfer in the secondary cooling zone
- Surface temperature of the ingot
- Water flow density
- Water droplet velocity
- Water droplet diameter
- Surface state of the ingot
- Nozzle usage status
