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Improve laminar cooling, improve the quality of the plate

Optimizing laminar cooling technology and improving water supply operation are usually adopted to control and improve the mechanical properties and shape of the final product.Laminar flow rapid cooling technology is becoming more and more popular, and has become a necessary cooling control link in various steel rolling production lines.

The advantage of laminar cooling technology is that during the cooling process, the cooling water flowing out of the upper and lower nozzles is relatively slow, which can form a smooth and continuous flow of water, directly falling on the fast-moving steel surface, forming a cooling uniform water surface.However, different process parameters and equipment layout parameters of the production line will affect the cooling capacity, and different cooling water temperature control and water quality will also affect the quality of the plate.Therefore, it is of great significance to explore the influence of various factors on laminar cooling technology.

Reasons why laminar cooling affects the properties of steel plates

In the laminar cooling process after rolling, the heat transfer of steel is mainly air cooling and water cooling.Air-cooled steel radiates heat to the surrounding environment, mainly by radiative heat transfer.Water cooling means that when the cooling water at medium or low temperature comes into contact with the steel at high temperature, the direct collision causes rapid heat transfer, and then a vapor layer is formed on the steel surface. The vapor layer impedes heat conduction and belongs to membrane boiling heat transfer.When the steam layer is no longer attached to the steel surface stably, the steel surface recontacts with the cooling water and again generates strong heat transfer, which belongs to nuclear boiling heat transfer.During the cooling process, austenite decomposes and latent heat is released.

The shape control problem of high strength steel plate is determined by the horizontal temperature uniformity and thickness uniformity of steel plate during the cooling process.The uneven temperature leads to the inconsistent phase transition process. The residual stress in the cooling process is too large, the straightening temperature is low, and the strength of the steel plate is increased, which makes the thermal straightener unable to straighten, or the steel plate temperature is still inconsistent after straightening, resulting in the residual stress when the cooling temperature is uniform on the cold bed.

It can be said that the plate shape problem caused by cooling is mainly caused by the following three kinds of non-uniform cooling:

One is the non-uniformity of transverse cooling.In order to ensure the uniform cooling of steel plate, non-uniform flow design or regulation, side shielding, side spraying and other measures are usually adopted to ensure the uniform cooling of steel plate.Although such a design can improve the horizontal uniformity of the temperature, some edge shielding still cannot be put into operation normally, and the problem of temperature uniformity still exists.

Second, the thickness of the direction of cooling asymmetry.In order to ensure the thickness of the direction of cooling symmetry, the cooling process of the upper and lower surfaces to get the same cooling effect, should be in the process of cold shrinkage and cooling phase transition process, to ensure the deformation of the steel plate symmetry, so that the shape of the steel plate after cooling will not change.

In order to ensure the symmetry of steel plate thickness, the ratio of cooling water on the upper and lower surface of steel plate of different thickness is different, which has been proved in many medium and thick plate rolling mills and furnace rolling production lines.For the steel plate of a certain thickness, the fluctuation range of cooling water on the upper and lower surface of the steel plate is relatively small, such as 20mm thick steel plate, and the ratio of upper and lower water is 1:1.9 ~ 1:2.1.

In the past, the flow of the upper and lower tube of the cooling control device for sheet steel of some furnace coils and rolling lines has no regulating means, and can only be manually configured, which is difficult to produce. The symmetrical cooling in the thickness direction of steel plates of different thickness cannot be realized, resulting in the shape of steel plates easy to bend after the cooling control.

Third, transverse and thickness direction of the cooling uneven mixing.Therefore, controlling the horizontal uniformity of cooling and the symmetry of thickness direction is the main way to control the flatness of the plate.

The influence of equipment problem on cooling is mainly manifested as the serious blockage of part header and nozzle.The laminar cooling water system of the furnace rolling mill is designed as a semi-filter type. Due to frequent changes in the dry and wet conditions of the collecting pipe during long-term use, the joint action of oil in the cooling water and suspended matter results in the rust and scaling of part of the lower collecting pipe, and the water column height of the lower collecting pipe varies greatly.Although during equipment maintenance, the replacement of the lower manifold adopted pickling and cleaning methods, but due to equipment design problems, these problems still can not be completely solved.

Laminar cooling water temperature will also affect the plate.Due to the problems of the northern season and the design process requirements of the initial water system, the high water temperature in summer is prone to the poor cooling effect, plate shape and bending, etc., while the low water temperature in winter and the rapid cooling also affect the shape and performance of the plate.

In order to realize the efficient laminar cooling process, the existing equipment upper and lower collecting pipes, water supply pipelines, valves can be modified, as well as the increase of automatic regulation control system, to reduce the original design is not conducive to the cooling of various factors, improve the laminar cooling quality.

Technical modification for uneven cooling

Based on the summary of production practice, the author believes that a series of technical improvement measures should be implemented to solve the above three kinds of non-uniform cooling problems.

Generally, the upper collector is made up of external water pipe, inlet water pipe and 4 rows of u-pipe. The damping mechanism in the water tank can make the water pressure on the whole length of the collector even, so as to ensure the water flow of the u-pipe is even. The damping pipe is made of stainless steel.

By changing the transverse spacing or diameter of the u-shaped nozzle, the transverse water distribution can be obtained.The horizontal cooling uniformity of the steel plate can be improved by the cross configuration of four u-tubes in the transverse position.

Redesign and manufacture lower manifold 1 ~6 groups (a total of 36 sets), increase the maximum water supply of lower manifold, improve the maximum water ratio of laminar cooling system, improve the cooling capacity of the system, and ensure the cooling uniformity of the width of steel plate, ensure the stable production of high strength steel.Further improvement is made on the structure of the new lower manifold: the inner manifold is the jet hole of stainless steel, and the internal damping tube of the manifold is made of stainless steel to prevent scaling from affecting the uniformity of water flow.

Laminar flow systems are now controlled with a fixed ratio of cooling water to cooling water (1:1) and control valves are set manually.This configuration is suitable for narrow plates (width ≤2000mm) and for thin plates (thickness ≤12mm).But for the wide plate, thick plate, this control method has been unable to ensure the shape of the plate after cooling, or even after the hot straightener leveling.And because of the uneven cooling, the steel plate to the cold bed will still occur warpage.

In order to ensure that steel plates of different thickness and width are cooled evenly under different water temperature conditions, the cooling water on the upper and lower surfaces of steel plates must be automatically adjusted to make the upper and lower surfaces cool symmetrically, and the influence of temperature drop on longitudinal temperature accuracy can be compensated during the cooling process of long steel plates.

Renovate the water supply pipeline of the lower collector, remove the original flow orifice, and renovate the pipeline to meet the needs of increasing the regulating valve and improving the measurement accuracy and stability of the flowmeter.

The splash plate opposite each side spray device shall be modified accordingly to increase the drainage function.According to the field situation, it can be considered to add drainage holes and other measures on the splash-proof plate to prevent the side jet device from blowing the water on the upper surface of the steel plate during the production of high-strength structural steel, resulting in uneven cooling of the steel plate resulting in plate ladle.The newly added purging device is arranged at the outlet of the cooling control area to meet the cooling requirements of steel plates with high cooling rate and low final cooling temperature.

Optimize operations to reduce adverse effects

Reducing the impact of equipment problems on laminar flow cooling deserves great attention, which can be achieved by optimizing operations.

First of all, the high water tank is two tanks with a length of 27700mm and a diameter of 3000mm. The effective water level is 200cm (the interface shows that the actual maximum water level is 250cm and the minimum water level is 50cm). The average water level area of the high water tank is calculated according to 135m2.According to the test, when the effective water level is not less than 50cm, the set value of the flow rate of the header is maintained.When it is less than 50cm, the flow attenuation of the header cannot reach the set value, that is, the maximum decrease of water level shall not exceed 1.5m.

Secondly, the overflow of the lower manifold of 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 replenishment capacity of the floor cooling water tank pump is: one pump 400m3/h, two pumps 1200m3/h, three pumps 1800m3/h, and four pumps 2500m3/h.

Secondly, to meet the minimum overflow conditions, make the maximum amount of layer cold water through the self-cleaning filter, improve the water quality, reduce the influence of oil and suspended matter in the water on the nozzle, and reduce the overflow of hot water to the cold pool, reduce the water temperature.

It is also important to optimize operations to reduce the impact of seasons on water temperature.

Summer high temperature cooling ability is low, through the open water pump and filter pump, ensure cold pool with a low water level operation, in the production of AH70DB, X70 pipeline steel and low carbon bainite steel, according to the need of low water temperature control plate shape and performance, in a timely manner through added new water reduce water temperature of the existing system, to ensure the quality of the production sheet.

In winter, by reducing the flow rate of the upper tower and controlling the filter pump regulation, the cold pool can run at a higher water level and overflow from the cold pool to the hot pool.This not only ensures the water quality, but also meets the water temperature requirements.

Ensure water quality to improve the quality of the plate surface can not be ignored.

Laminar water quality is up to standard, directly affects the quality of the plate surface.The standards of conductivity ≤1000 S/cm~1600 S/cm, Cl-≤200mg/L~400mg/L, and Ca2+≤400mg/L~600mg/L are another factor influencing the final product of the plate.The electrical conductivity and Cl- index exceed the standard easily cause the plate surface pitting phenomenon;Excessive Ca2+ index is not only easy to cause laminar nozzle scaling and blockage, affecting the uniform cooling of the plate, but also easy to cause white spots on the surface of the plate, affecting the appearance quality.Through the daily laminar flow system water quality and new water test, to the laminar flow system water in time for new water replacement, to ensure the final formation of the plate, wood.For example, when the new water index is not good, the laminar flow system water quality is always in the standard state by adding soft water.

To sum up, based on the existing equipment layout adjustment, for the spray up and down in the tube and the water supply pipe is optimized, and increase the splash board and purging device, technical measures, such as optimizing the operation of layer of the cold water supply system, etc., can reduce the influence of three kinds of non-uniform cooling of plate, solve the plate flatness problem such as warp, improve the production capacity of furnace volume GaXian.Strictly controlling laminar water quality is also an important measure to ensure the surface quality and stable plate production.



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