The 620mm strip production line of Laigang Yinshan Section Steel Co., Ltd.’s plate and strip factory has 12 flat roller mills and 4 vertical roller mills, adopting a fully continuous layout. The vertical roller RE0 and the flat roller R0 form a billet rolling unit; the vertical roller RE1 and RE2 and the four two-high rolling mills R1~R4 form the rough rolling unit; the vertical roller FE1 and the seven four-high rolling mills F1~F7 form the finishing rolling unit. The production line can produce carbon structural steel, high-quality carbon structural steel and low alloy structural steel with a width of 183~450mm and a thickness of 1.7~7.8mm, with more than 300 varieties and specifications of products.
The vertical roller mill is mainly used for descaling and edge trimming during the rolling process, and uses integral cast steel rolls; the roughing mill unit is subject to large deformation due to the high temperature of the billet, but the surface quality requirements are not high, so it uses cost-effective rollers. Cast steel and medium nickel-chromium cast iron rolls;
Because its support rollers bear large rolling forces, they require better strength, toughness, and fatigue resistance, so composite semi-steel rollers and high-alloy composite cast steel rollers are used;
The finishing rolling mill determines product quality and the stability of the rolling process. The use conditions are harsh. It not only requires good surface quality of the rolls but also high strength and wear resistance. It also requires good accident resistance. The finish rolling work rolls originally used high-nickel-chromium-molybdenum infinitely chilled cast iron rolls. Since 2003, the F2~F4 stands have adopted centrifugal composite high-speed steel rolls.
2. Usage and problems
According to the performance characteristics of high-speed steel rolls, high-speed steel rolls were applied to the Fz~F4 stands in the middle section of the finishing rolling unit. Its use process was tracked, the usage data was recorded, and compared with high-nickel-chromium-molybdenum infinitely chilled cast iron rolls. Compared, it was found that high-speed steel rolls have the following advantages:
(1) Good wear resistance
The oxide film on the surface of the roll forms quickly and is not easy to fall off, so it has good wear resistance. Under the same rolling amount, the wear amount of high-speed steel rolls can be reduced by more than half compared to high-nickel-chromium-molybdenum infinitely chilled cast iron rolls, see Table 1.
Table 1: Comparison of average wear amount of rolls in different stands
|Average wear amount of high-speed steel roll per time/mm
|Average wear amount of high nickel chromium molybdenum roll per time/mm
(2) Good thermal crack resistance
During normal use, the proportion of hot cracks in high-speed steel rolls is much smaller than that of high-nickel-chromium-molybdenum infinitely chilled cast iron rolls. Taking the roll usage statistics in 2008 as an example, high-nickel-chromium-molybdenum infinitely chilled cast iron rolls produce thermal cracks during use. The proportion of high-speed steel rolls is about 15%, while that of high-speed steel rolls is only 4%.
(3) The roller has a long service life and low roller consumption
Due to the small single grinding amount and small hardness drop of high-speed steel rollers, the number of times of use of high-speed steel rollers is about three times that of high-nickel-chromium infinitely chilled cast iron rollers, and the total rolling volume is about that of high-nickel-chromium-molybdenum infinitely chilled cast iron rollers. 6~8 times, which greatly reduces the roll consumption, reduces the roll replacement downtime, improves the rolling mill operation rate, and is of great significance to the completion of the goal of cost reduction and efficiency improvement.
(1) Large thermal deformation
High-speed steel rolls are easily deformed when exposed to heat. The middle part of the rolling deformation zone will expand due to heat in a short period of time after rolling, causing the roll gap to change. As a result, the deformation of the middle part of the strip is greater than that of the edges during the rolling process, and the finished product is easy to The occurrence of defects such as scooping or wavy shapes brings certain difficulties to plate shape control, and in severe cases can lead to steel stacking accidents.
(2) Cracked rollers are difficult to repair
Due to the high hardness of high-speed steel rolls, once cracks appear, it is extremely difficult to turn, and the cracks will quickly extend radially toward the core of the roll. If the turning is not clean and continues to be used, the cracks will expand rapidly due to heat, causing the roll to peel off on the machine, and the roll will be scrapped. At the same time, it will also lead to steel pile accidents. Therefore, the requirements for crack detection and repair of high-speed steel rolls are very high.
3. Main measures taken to address the problem
3.1 Strengthen cooling water control and improve the cooling effect of the middle part of the roll
Increase the cooling water flow of the finishing rolling unit, and at the same time improve the roll cooling water pipe, increase the diameter of several water pipes in the middle of the water pipe, increase the cooling effect of the middle part of the roll, and reduce the convexity caused by thermal expansion in this part; Re-adjust the angle of the water pipe to ensure the effective cooling of the rolls by the cooling water; at the same time, according to seasonal changes, water temperature changes and rolling specifications, the cooling water flow rate of each frame is determined, and the water volume is adjusted at any time.
3.2 Random tracking of roll temperature
After the high-speed steel roll comes off the machine, a dedicated person is responsible for measuring and recording the roll temperature. The measurement is carried out in several different periods to ensure the accuracy of the temperature measurement. By clearing water pipes, adjusting water volume promptly, and controlling the rolling rhythm and roll changing time, we ensure that the roll temperature meets the usage requirements, reduce the adverse impact on the plate shape, and thereby reduce the production of various types of waste products.
3.3 Strengthen crack detection and turning efforts
Given the characteristics of cracks in high-speed steel rolls, various detection methods such as colouring and ultrasonic detection are used for cross-detection to avoid cracks in high-speed steel rolls. To reduce the occurrence of cracks, arrange for dedicated personnel to guard the F₂~F4 frames. If accidents such as steel stacking occur, the operator should try to remove the rolled pieces from the frame. If the rolled pieces stop in the frame, the roll gap should be lifted immediately. And prioritize the scrap steel between high-speed steel rolls. Before starting rolling again, it should be cooled with cooling water and the rolling mill should be allowed to idle for more than 3 minutes to ensure that the rolls are cooled evenly before rolling.
To solve the problem of the high hardness of high-speed steel rolls and difficulty in turning, we adopt the method of multiple passes with less feed, rough turning with blunt tools and fine turning with sharp tools, to turn the cracks cleanly without cutting, and at the same time, use a special high-hardness alloy blade are used, and round knives are used to remove defects and square knives are used to remove fatigue to ensure both turning quality and production progress and the use effect is good.
4. Exploration of the application of high-speed steel rolls under different process conditions
4.1 Mature use of the middle section of finishing rolling
According to the performance characteristics of high-speed steel rolls, they are used in the F2~F4 stands in the front section of finishing rolling. It is found that the frequency of cracks in the F2 stand rolls is greater, resulting in a decrease in the cost-effectiveness of the use of high-speed steel rolls. Considering factors such as rolling mill efficiency, assembly speed and roll cost-effectiveness, the high-speed steel rolls are concentrated on the F3 and F4 stands. At the same time, according to the product specifications produced, the roll replacement cycle of the F3 and F4 stands is extended to the original 2~3 times, and the effect is good after operation.
4.2 Application exploration on finished racks
Due to the thermal expansion of high-speed steel rolls, the convexity in the middle of the roll shape increases, making the plate shape difficult to control, and various literature indicates that high-speed steel rolls are not suitable for use in finished machine frames. However, in the past two years, when the Laigang strip steel production line was developing high-strength and high-quality alloy steel, it boldly used high-speed steel rolls in the finished machine frame, making full use of its high hardness and strong impact resistance to successfully roll a series of high-quality alloy steels. New value-added narrow-band varieties, such as 65Mn steel, etc.
For rolling high-strength high-quality alloy steel, during the rolling process, due to the temperature drop at the strip head and the high hardness of the steel itself, the impact on the high-nickel-chromium-molybdenum infinitely chilled cast iron rolls of the finished rolling frame is very serious, and the roll surface Damage will cause chromium marks to appear on the surface of the strip, affecting the surface quality of the product and even interrupting production. This not only reduces the service life of the roll but is also not conducive to improving the operating rate. Therefore, when rolling this type of steel, the finished machine frame uses high-speed steel rolls with high roll surface hardness and good impact resistance. The surface quality of the strip is guaranteed and chrome stamping waste caused by damage to the roll surface is eliminated. The operating rate of the rolling mill is improved, and the economic benefits are considerable.
The use of high-speed steel rolls on some frames of narrow-strip steel finishing mills has achieved certain results. It can not only extend the overall service life of the rolls and reduce roll consumption, but also reduce roll replacement downtime and improve the effective operation rate of the rolling mill.
When developing new varieties, Laiwu Iron and Steel Co., Ltd. made full use of the high hardness characteristics of high-speed steel rolls, successfully used high-speed steel rolls in finished machine frames, and achieved good results. However, it is still necessary to further improve the thermal crack resistance of high-speed steel rolls, reduce the production cost of high-speed steel rolls, and realize that all finishing rolling units use high-speed steel rolls. This will reduce roll consumption in the steel rolling mill, reduce rolling costs, and improve labour intensity. It is of great significance to improve various indicators such as the effective operating rate of the rolling mill.