Research and Improvement of Cooling Water for Finishing Work Rolls in Hot Continuous Rolling Mill

The improvement of the rolling rhythm of hot rolling directly affects the temperature, and structure, and changes the law of the material in the production process. Due to the high-tempo high-speed rolling, the heat transfer between the material and the roll is increased, which directly affects the temperature of the roll and shape control changes.

The temperature increase of the work rolls of the finishing mill brought about by the increase of the rolling rhythm will increase the peeling of the roll surface and the scale defects of the product roll system. The thermal crown of the roll is an important factor affecting the loading gap of the strip mill.

In the hot-strip continuous rolling mill, the work rolls are in direct contact with the high-temperature rolling stock. When the cooling is insufficient, the thermal crown of the rolls can reach hundreds or even thousands of microns. Significant changes have occurred in the degree, resulting in a decline in the quality indicators of the product shape process.

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1 Problems in the use of work roll cooling water and its analysis and improvement

With the improvement of the rolling rhythm, the rolling gap of the finishing mill is reduced, and the roll temperature is gradually increased. The temperature of the F1-F4 rolls is increased from 70°C to 85°C. The cooling capacity of the work rolls can no longer meet the production needs. F1-F4 roll oxide film peeling often occurs, especially in the production of pickled plates with a thickness of less than 3.0mm and tinplate steel specifications with a thickness of less than 2.0mm.

Since the strip steel is thin, the pure rolling time of the finish rolling is 120~130s, which is about 30s longer than the conventional pure rolling time. The rolling time is longer, and the high temperature of the roll may easily cause the oxide film of the work roll to peel off, and the roll system of the strip steel will appear. Scale defects, resulting in unplanned roll changes. It will affect production organization, product quality, and cost control. When the work roll is off the machine, the oxide film often peels off.

The control of the work roll temperature is determined by the pressure of the work roll cooling water, the water temperature, the distribution of the water inlet and outlet water, the spray angle, the arrangement of the nozzles, and the flow rate of the nozzles and other factors. Due to the improvement of the pressure of the work roll cooling water system, the pumping station and the main water supply pipeline need to be modified, and the changes are relatively large. Improvements were therefore made by analyzing work roll cooling headers and nozzles.

Roll cooling optimizes the nozzle type and nozzle arrangement for roll cooling to improve the distribution of roll-cooling water and avoid interference between cooling watermarks, so as to improve the heat exchange coefficient to improve cooling efficiency, slow down the thermal expansion of the roll, and stabilize the roll Thermal crowning can achieve the purpose of improving the shape control ability and rolling stability.

Without changing the water supply system and the main water supply pipeline, in order to give full play to the role of cooling water for the work rolls and meet the temperature requirements of the rolls under fast-paced rolling. By optimizing the nozzle arrangement and water volume ratio of the inlet and outlet, the water volume ratio of the original design work roll inlet and outlet is 30% and 70% and adjusted to 15% and 85%. At the same time, by adjusting the nozzle spacing and nozzle deflection angle, the cooling water interference between the nozzles is avoided, the cooling water coverage is increased, and the cooling capacity is improved.

Improved work roll cooling technology.

(1) Design of the original cooling headers for F1-F4 work rolls of line 2250:

F1-F4 mill outlet upper and lower headers, existing nozzle models, and layout parameters. The total water volume of the F1-F2 outlet headers is 536.51m/h (8941.85L/ min), the total water volume of the F3-F4 outlet header is
447.15m/h (7451.9L/min).

According to the above data, the spray watermarks of the upper, middle, and lower rows of nozzles in the cooling water of F1-F4 rolls do not interfere, but the spray watermarks of the same row of nozzles on the rolls all interfere; in addition, the water volume of each nozzle along the length of the roll body is equal, that is The water volume in the middle of the roll is consistent with the water volume on both sides, which will easily cause the temperature in the middle of the roll to be too high. The direct impact is that the crown in the middle of the roll is too large, and the strip is prone to center waves during rolling.

(2) Improvement of F1-F4 work roll cooling technology on line 2250:

F1-F4 rolling mill exit upper header, after modification, by increasing the flow rate of cooling water nozzles in the middle row of work rolls, no nozzles are arranged on both sides of the middle row to improve the cooling in the middle of the roll body Capacity, so that the water volume of the upper header at the outlet of F1-F2 is increased to 544.16m/h (9069.28L/min), and the water volume of the upper header at the outlet of F3-F4 is increased to 599.25m/h (9987.44L/min).

The corresponding F1-F4 rolling mill outlet lower header, nozzle type, and arrangement are designed symmetrically with the upper header, and the modified lower header flow rate is equal to the upper header flow rate.

(3) The original cooling header design for F1-F3 work rolls of Line 1580:

F1-F3 rolling mill outlet headers, the total water volume of existing F1-F3 outlet headers is 476.5m/h (7941.01l/min): 1580 Line F1- F4 The spray watermarks of different rows of nozzles do not interfere, and the spray watermarks of the same row of nozzles on the roll do not interfere; the water volume of the same row of nozzles along the length of the roll body is high in the middle and low on both sides, that is, the water volume in the middle of the roll is large and the water volume on both sides Small, the water distribution in the middle part is very even.

In the actual use process, the following problems were found through the shooting experiment on the surface of the work roll: the nozzles in the bottom row of the upper header and the upper row of the lower header are not clear when spraying, and some nozzles cannot be sprayed. The phenomenon on the surface of the roll causes insufficient cooling of the roll.

At the same time, the flow rate of this part of the nozzles accounts for a relatively large proportion of the overall flow rate; each row of nozzles, because the deflection angle is only 15°, only the deflection angle of the uppermost row of nozzles in the upper header is 30°, resulting in a wide range of cooling coverage. If it is too small, it is easy to cause an extreme drop in the roll temperature in the cooling area and produce a very obvious temperature return in the non-cooling area.

At present, the 1580 hot continuous rolling is mainly narrow and thin. The change in the rolling rhythm has led to a high rise in the surface temperature of the roll at this stage, and the problem of the high surface temperature of the roll needs to be solved urgently.

(4) Improvement of F1-F3 work roll cooling technology on line 1580:

Adjust the nozzle deflection angle, inclination angle, and nozzle type by modifying the outlet nozzle arrangement.

The coverage width of the nozzle spray is wider, and the coverage of the spray is increased, which further improves the cooling efficiency; when two adjacent nozzles or two adjacent rows of nozzles spray, there is superposition without interference, which avoids the reduction of cooling intensity caused by interference;

When spraying with three rows of nozzles, the sprayed water is distributed in two steps. The minimum width of the uniform distribution of the first step is about 800mm, which is in line with the width of the narrowest rolled material. The width of wide-width rolled stock.

At the same time, considering the interference between the spray nozzles and the wiper board, the nozzle spray angle of the bottom row of the upper header is adjusted to increase the spray angle of this row of nozzles, so as to avoid the interference between the water spray and the wiper board.

For the uppermost row of nozzles in the lower header, since the cooling water scraper of the lower work roll is sealed by the weight of the guide plate on the roll body, considering that the angle of the nozzle is too high, the water will be concentrated in the gap between the scraper and the roll body, which will affect the lower scraping. The water sealing effect of the water plate reduces the spray angle of the upper nozzle of the lower header, avoiding the failure of the wiper to seal the water due to the rebound of the sprayed water into the gap between the work roll body and the wiper when the nozzle sprays problem, improve the water sealing effect of the wiper blade.

2 Transformation effect of work roll cooling water

2.1 Roller temperature

Through the improvement of the cooling technology of the work rolls of the finishing mill, the temperature of the rolls has dropped significantly. According to the measurement results of the roll surface temperature after the roll off the machine in the rolling rhythm and similar product roll period, the temperature of the roll surface drops by 10°C to 15°C through the above technical transformation, as shown in the table (as shown in the table) is typical.

2.2 Defects of an iron sheet of roller system

After the cooling headers of the work rolls are put on the machine, the products with defective iron sheets in the roll system show a downward trend, the occurrence rate of iron oxide scales in thin-gauge tinplate products is relatively reduced by 18%, and the total degraded product rate of iron oxide scales in hot-rolled roll systems is reduced by 2%.

2.3 Roll Gap Shape and Convexity

Good work roll cooling and thermal crown control are effective measures to reduce work roll consumption, control strip shape, and increase production yield. The cooling effect of the work roll cooling water on the work roll is not only affected by the water volume and nozzle density, but also by the distance, angle, and deflection angle between the cooling nozzle and the roll surface. The model expression of the cooling effect is as follows.

Among them: Q is the cooling capacity; A is the striking area of the cooling water on the roll; Δt is the temperature difference between the cooling water and the working roll surface; γ is the wrap angle formed by the cooling water hitting position on the roll; related constants.

The layout of the cooling nozzles of the modified work rolls, under the conditions of the largest and smallest roll diameters, the spray watermarks of different rows of nozzles do not interfere, and the spray watermarks of the same row of nozzles on the roll do not interfere; The water volume is high in the middle and low on both sides, that is, the water volume in the middle of the roll is large and the water volume on both sides is small. The water volume distribution in the middle part is very uniform, and the difference between the maximum and minimum flow is 3%.

Through the implementation of the technical plan for improving the cooling water of the work rolls of the finishing mill, the following effects have been achieved:

(1) The maximum temperature of each stand is reduced by about 10°C, and the distribution of the hot roll shape of the roll is significantly improved.

(2) The effect of the hot roll shape on the effective roll gap crown is significantly reduced.

Through the improvement of work roll cooling technology, the increase of roll temperature caused by the long-term use of high-speed steel rolls and the influence of temperature field temperature on the shape and crown of the roll gap can be effectively reduced, thereby providing better shape control for high-speed steel rolls under the conditions of use. Good process conditions.

3 Conclusion

(1) Improve the cooling water of the work rolls. The temperature of the rolls is controlled below 80°C, which is more than 10°C lower than that before the improvement. The number of roll changes. At the same time, it reduces the incidence of iron sheets in the strip steel roll system.

(2) By controlling the cooling, the phenomenon of high temperature in the middle of the work roll body direction and low temperature at both ends is solved, correspondingly reducing the different crown differences in the direction of the work roll body direction, and improving the thermal crown of the work rolls. Degree, to avoid bad plate shape.