In the production process of the furnace coil rolling line, the laminar cooling technology and the method of improving the water supply operation are usually adopted to control and improve the comprehensive mechanical properties and plate shape of the final product. The application of laminar rapid cooling technology is becoming more and more common, and it has become a necessary cooling control link for various steel rolling production lines.
The advantage of laminar cooling technology is: during its cooling process, the cooling water flowing from the upper and lower nozzles is relatively slow, and can form a smooth and continuous water flow, which directly falls on the surface of the fast-moving steel to form a uniform cooling water surface. However, different process parameters and equipment layout parameters of the production line will affect the cooling capacity, and the cooling water temperature control and water quality will also affect the quality of the plate. Therefore, it is of great significance to discuss the influence of various factors on laminar cooling technology.
The reason why laminar cooling affects the performance of steel plate
In the laminar cooling process after rolling, the heat transfer of the steel is mainly air-cooled and water-cooled. In air cooling, steel radiates heat to the surrounding environment, mainly radiant heat transfer. Water cooling means that when low-temperature cooling water comes into contact with high-temperature steel, direct collision causes rapid heat transfer, and then a steam layer is formed on the surface of the steel. The steam layer hinders heat conduction and belongs to film-like boiling heat transfer. When the steam layer is no longer steadily attached to the surface of the steel, the surface of the steel re-contacts with the cooling water, and again generates a strong heat transfer, which belongs to nuclear boiling heat transfer. During the cooling of the steel plate, austenite decomposes and the latent heat of phase transformation is released.
The shape control problem of high-strength steel plate is determined by the temperature uniformity in the transverse direction and the temperature uniformity in the thickness direction of the steel plate during cooling. Uneven temperature leads to inconsistent phase transformation process, excessive residual stress during cooling, low straightening temperature, and increased steel plate strength, which makes the thermal straightener unable to straighten, or the steel plate temperature is still inconsistent after straightening, cooling to temperature on a cold bed Residual stress occurs when uniform.
It can be said that the shape problem caused by cooling is mainly caused by the following three types of uneven cooling:
One is lateral cooling unevenness. In view of the problem that the original upper header flow is evenly distributed in the horizontal direction, which is not conducive to uniform cooling of the steel plate. To ensure uniform cooling in the horizontal direction, the header flow rate is usually designed or adjusted with uneven flow in the lateral direction. Blocking, side spraying and other measures. Although this design will improve the lateral uniformity of the temperature, there are still some edge shields that cannot be put into operation normally, and the problem of temperature uniformity still exists.
The second is the asymmetry of the cooling in the thickness direction. In order to ensure symmetrical cooling in the thickness direction, the same cooling effect is obtained on the upper and lower surfaces during cooling. The deformation of the steel plate should be symmetrical during the shrinking process and during the cooling phase change, so that the shape of the steel plate will not change after cooling.
In order to ensure the symmetry of the thickness of the steel plate, the cooling water ratio of the upper and lower surfaces of the steel plate with different thicknesses is different, which has been proved in the production of many medium and heavy plate coil factories and furnace coil production lines. For a certain thickness of steel plate, the cooling water volume ratio of the upper and lower surfaces of the steel plate is smaller than the fluctuation range. For example, for a 20mm thick steel plate, the ratio of upper and lower water is 1: 1.9 to 1: 2.1.
In the past, the flow control of the upper and lower headers of some coil cooling line plate cooling devices did not have adjustment methods, and they could only be configured manually. The production was difficult, and symmetrical cooling in the thickness direction could not be achieved for different thickness steel plates, resulting in steel plates after controlled cooling Easy to shape dangling.
Third, the cooling is unevenly mixed in the lateral and thickness directions. Therefore, controlling the lateral uniformity of the cooling and the symmetry in the thickness direction is the main way to control the flatness of the plate. Reducing the temperature difference between the head and tail of the length direction can also partially control the flatness of the plate.
The impact of equipment problems on cooling is mainly manifested in the serious blockage of some headers and nozzles. The laminar cooling water system of the furnace coil mill is designed to be semi-filtered. Due to the frequent changes in the dry and wet conditions of the header during long-term use, the combined effect of oil and suspended solids in the cooling water causes some of the lower header to rust, and the scale is serious. , And the height of the water column of the lower header is uneven and the difference is large. Although during the equipment overhaul, methods such as pickling and cleaning were used for the replacement of the lower header, these problems could not be completely resolved due to equipment design issues.
Laminar cooling water temperature will also affect the board. Due to the seasonal problems in the north and the requirements of the initial water system design process, high water supply temperature in summer is prone to abnormalities such as poor cooling effect and plate shape scoop. Winter water temperature is too low and cooling too fast also affects the shape and performance.
To achieve an efficient laminar cooling process, it is possible to reduce the various factors that are not conducive to cooling in the original design and improve laminar cooling by modifying the existing equipment up and down headers, water supply pipes, valves, and adding an automatic adjustment control system quality.
Technical transformation for uneven cooling
Through the summary of production practice, the author believes that for the above three types of uneven cooling that cause plate shape problems, a series of technical improvement measures should be implemented.
Generally, the encrypted upper header is composed of an outer water pipe, a water inlet pipe and 4 rows of U-shaped pipes. The damping mechanism in the water tank can make the water pressure of the entire length of the header uniform, thereby ensuring uniform water flow of the U-shaped pipe. Material.
Using the horizontal spacing or nozzle diameter of the U-shaped tube nozzle can obtain the lateral water distribution of the convex shape; through the cross configuration of the horizontal position of the 4 groups of U-shaped tubes, the uniformity of the horizontal cooling of the steel plate can be improved.
Redesign and manufacture 1 to 6 sets of lower headers (total 36 headers), increase the maximum water supply of the lower headers, increase the maximum water ratio of the laminar cooling system, improve the cooling capacity of the system, and ensure the width of the steel plate The cooling uniformity in the direction ensures the stable production of high-strength steel. The new lower header structure is further improved: the inner tube is a stainless steel nozzle, and the inner damping tube of the header is made of stainless steel to prevent scaling from affecting the uniformity of the water flow.
Laminar flow system now uses a fixed upper and lower cooling water ratio (1: 1) control mode, and the control valve is manually set. This configuration is suitable for narrow plates (width≤2000mm) and thinner plates (thickness≤12mm). However, for wide plates and medium-thick plates, this control method can no longer guarantee the shape of the plate after cooling control, even after being leveled by a thermal straightener. Moreover, due to the uneven cooling, the steel plate will still warp after reaching the cooling bed.
In order to ensure that the steel plates of different thicknesses and widths are cooled uniformly under different water temperature conditions, the cooling water on the upper and lower surfaces of the steel plate must be automatically adjusted to cool the upper and lower surfaces symmetrically, and the temperature drop can be compensated for the effect of longitudinal temperature accuracy during the cooling of the long steel plate .
Transform the lower header water supply pipeline, remove the original flow orifice plate, and transform the pipeline to meet the needs of adding regulating valves and improving the accuracy and stability of the flowmeter measurement.
Each group of side spray devices must be modified correspondingly to increase the drainage function. According to the site conditions, measures such as adding drainage holes on the splash plate can be considered to prevent the side spray device from blowing the surface water on the steel plate when producing high-strength structural steel, resulting in uneven cooling of the steel plate and causing the steel plate to scoop. The newly added purge device is arranged at the outlet of the cooling control zone to meet the needs of high cooling rate and low final cooling temperature steel plate cooling control.
Optimized operation to reduce adverse effects
Reducing the impact of equipment problems on laminar cooling deserves high attention, which can be achieved through optimized operation.
First, the high water tank is two 27700mm long and 3000mm diameter tanks, the effective water level is 200cm (the interface shows that the actual highest water level is 250cm, and the lowest water level is 50cm). The average water level area of the high water tank is calculated as 135m2. After testing, when the effective water level is not less than 50cm, the header flow rate remains at the set value; when it is less than 50cm, the header flow rate attenuation cannot reach the set value, that is, the maximum drop in water level does not exceed 1.5m.
Secondly, the overflow of the header under laminar flow is 500m3 / h. Laminar flow pump water supply capacity: one pump 900m3 / h, two pumps 1700m3 / h, three pumps 2300m3 / h, four pumps 3000m3 / h. Therefore, the effective water replenishment capacity of the cold water tank pump is: one pump 400m3 / h, two pumps 1200m3 / h, three pumps 1800m3 / h, four pumps 2500m3 / h.
Second, to meet the minimum overflow conditions, make the maximum amount of cold water through the self-cleaning filter, improve water quality, reduce the impact of grease and suspended matter on the nozzle, and reduce the overflow of hot water to the cold pool and reduce the water temperature.
In addition, it is also important to optimize the operation to reduce the influence of the season on the water temperature.
In summer, the temperature is high and the cooling tower capacity is low. By turning on the cooperation of the tank pump and the filter pump, the cold water pool is guaranteed to run at a low water level. When producing pipeline steel such as AH70DB and X70 and low-carbon bainite steel, the low water temperature control is required The shape and performance of the plate are timely reduced by adding new water to reduce the water temperature of the existing system to ensure the quality of the produced plate.
In winter, reduce the flow of the upper tower, control the filter pump adjustment, ensure that the cold pool runs at a higher water level, and realize the overflow of the cold pool to the hot pool. This not only ensures the water quality, but also meets the water temperature requirements.
Ensuring water quality and improving the surface quality of the board can not be ignored.
Whether the laminar water quality meets the standard directly affects the surface quality of the plate. The standards of conductivity ≤1000μS / cm ~ 1600μS / cm, Cl-≤200mg / L ~ 400mg / L, Ca2 + ≤400mg / L ~ 600mg / L are another factors that affect the final material of the plate. Excessive conductivity and Cl-index easily cause pitting corrosion on the surface of the plate; excessive Ca2 + index not only easily causes lamination of the nozzle of the laminar flow and affects the uniform cooling of the plate, but also causes white marks on the surface of the plate to affect the appearance quality. Through the daily test of the water quality and fresh water of the laminar flow system, the new water is replaced in time for the laminar flow system water exceeding the standard, so as to ensure that the sheet is finally formed and finished. For example, when the new water index is not good, supplement the soft water to ensure that the water quality of the laminar flow system is always in a standard state.
In summary, by adjusting the existing equipment layout, the upper and lower headers and lower water supply pipes of the spray are optimized and reformed, and technical improvements such as splash plates and purge devices are added to optimize the cooling water supply system. Operation methods, etc., can reduce the impact of three types of uneven cooling on the plate, solve the plate shape problems such as plate warpage, and improve the production capacity of the furnace coil rolling line. Strictly controlling the quality of laminar flow water is also an important measure to ensure the surface quality of the plate and stabilize the plate production.
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