Hss roller have excellent wear resistance, and their service life on the finishing stand of hot tandem mills is 3 to 6 times that of high-chromium ferrous rolls, so they are highly regarded .However, there are still some problems in the use of high-speed steel rolls, such as large thermal expansion, large friction coefficient, easy fall off of oxide film, etc. These problems have always plagued the use of high-speed steel rolls. so how to improve these problems is of great significance to on-site guidance.
The performance of Hss roller
Usually Hss roller are used on the machine several times between the two grindings, without grinding during the period, and air-cooled after being off the machine. Figure 1 shows the measured roll profile curve and roll profile deviation curve of the high-speed steel work rolls of the F1 stand of the hot-rolled 2300 line after 6 times of continuous use. Excellent wear resistance of the roll, but after the 5th use, the roughness increases.
Surface oxide film
The hss roller is in contact with the hot strip, and a uniformly distributed oxide film is formed on the roll surface, and the oxide film is usually silver-gray or light blue. Figure 2. The oxide film on the roll surface of the F1 stand high-speed steel rolls of Benxi Steel’s hot rolling line 2300 after the fifth time off and the oxide film morphology of the high-chromium cast iron rolls after one time off. The maximum roughness of the high-speed steel rolls was measured. The roughness Ra is 1.5 μm, and the maximum roughness Ra of the high-chromium ferrous roll is 1.34 μm. The roughness of the high-speed steel roll after five times of running is close to that of the high-chromium ferrous roll after one running, which shows that the high-speed steel roll has Good roughness resistance.
The formation and peeling of oxide film on high-speed steel rolls is a dynamic balance process composed of oxidation and friction and wear. In the initial stage of the use of the roll, the material on the surface of the roll quickly forms an oxide film at high temperature. During the continuous use, the oxide film is continuously thinned. At the same time, through the expansion of ions through the oxide film, the working layer material under the oxide film is continuously oxidized, and the oxide film is maintained in a relatively stable state. thickness value. Hot cracks in the oxide film of high-speed steel rolls are hereditary. With the increase of rolling times, the number, depth and width of hot cracks all show an increasing trend, and the cracks parallel to the axis direction are more obvious. The roughness also has a certain heritability, and the roughness value of the roll surface increases with the increase of rolling times.
The hss rolls are continuously in contact with the hot strip, and the rolls thermally expand. Figure 4 shows the thermal crown of the F1 stand of Benxi Steel’s hot rolling line 2300 for 30 minutes after the first to third time off. From the actual measurement results, it can be known that the roll surface temperature is 55 °C, and the maximum thermal crown is is 120 μm. During rolling, there are three main factors affecting the load roll gap: the original roll shape of the roll, the thermal expansion of the roll and the wear of the roll. The load gap determines the profile and shape of the strip, so it is necessary to control the wear and thermal crown of the rolls.
Countermeasures for the use and maintenance of high-speed steel rolls
The thermal expansion of high-speed steel rolls has an important relationship with the roll surface temperature. Once the roll surface temperature is unevenly distributed, it is easy to produce uneven thermal expansion, followed by the occurrence of abnormal shedding of the oxide film, which ultimately affects the surface quality of the product. Therefore, in order to ensure the continuous use of high-speed steel rolls and the surface quality of products, it is necessary to precisely control the roll surface temperature of high-speed steel rolls. Aiming at the problem of uneven surface temperature of the rolls, Benxi Steel optimized the distribution of cooling water for the 2300-line hot-rolled rolls, increased the amount of cooling water in the middle of the roll body, and promoted uniform temperature distribution, thereby ensuring the integrity of the oxide film. Large thermal expansion was previously considered the main cause of unstable rolling of high-speed steel rolls, but in fact, thermal expansion becomes relatively significant because of their small wear. Thermal expansion should be as small as possible, so the middle of the roll surface of the work roll is required to be cooled as soon as possible。
The hot rolling lubrication technology is to spray oil-water mixture on the roll surface before the rolling stock enters the roll gap. The combustion of hot rolling lubricants can form combustion residues. At the same time, the lubricants are rapidly vaporized and decomposed under high temperature and high pressure in the deformation zone, forming high temperature and high pressure air cushions. The combustion residues and air cushions can isolate the roll from the strip. , play a lubricating role and reduce the friction coefficient between the roll and the strip. Hot rolling process lubrication can form a protective film covering the roll surface of high-speed steel rolls, reduce rolling force, roll temperature and wear, improve roll surface condition, and at the same time prevent iron oxide scale from pressing in and improve strip surface quality. In order to ensure the lubricating effect, it is necessary to regularly steam clean the main pipeline of lubricating and rolling, and at the same time, measure the amount of lubricating oil sprayed during each repair, and confirm the difference between the actual value and the set value of the lubricating oil flowmeter. If there is any abnormality, maintenance processing is required. Usually, when lubricated rolling is put into use, the rolling force can be reduced by about 20%.
Use and maintain
◆ The number of times of use between two grindings is determined
The number of times of use between two grindings is mainly determined by the shape of the oxide film on the roll surface, the roughness value and the grade of the rolled material after the machine is turned off. In addition, it is also related to whether it is close to the finished rack. When the oxide film and roughness are in good condition, the upstream rack is used relatively more times, and the downstream rack is less. Usually, it can be used 3 to 6 times. And the oxide film and roughness of the roll surface are still kept well.
◆ Temperature measurement of lower rolls
In addition to the key observation of the oxide film on the roll surface, the temperature distribution of the roll surface should also be detected. Through the detection of the temperature of the roll surface, the maintenance of the cooling water nozzle of the work roll can be guided to avoid the occurrence of the surface quality problem of the strip steel caused by the peeling of the oxide film caused by the problem of the cooling water nozzle. Sometimes it is necessary to comprehensively judge the surface of the strip steel. .
◆ Flaw detection work
For high-speed steel that is repeatedly loaded between the two grindings, air cooling is required after the rolls are unloaded to eliminate the influence of thermal expansion on the roll gap. Once abnormal rolling occurs during the use of high-speed steel rolls, it is easy to cause severe mesh-like thermal cracks on the roll surface, and the high-speed steel rolls need to be replaced immediately. After the roll is cooled, an automatic or portable ultrasonic flaw detector should be used to detect cracks on the roll surface. If cracks are found, they should be removed to avoid carrying cracks on the machine for use.
(1) Thermal crown and wear are very important factors in hot rolling production. Although the wear problem has been solved by rolling with high-speed steel rolls, the thermal crown of high-speed steel rolls must be considered.
(2) A uniform and complete oxide film is the key factor to ensure the multiple use of high-speed steel rolls between two grindings, and the roughness is continuously inherited and gradually deteriorated during multiple uses.
(3) The comprehensive investment of high-speed steel roll cooling, roll shape optimization and process lubrication can give full play to the excellent performance of high-speed steel rolls.
(4) High-speed steel rolls must have strict requirements for use and management, and hot crack detection on the roll surface is an important means to avoid failure.
Founded in 2007, LMM GROUP mainly produces all kinds of metallurgical rolls for various types of steel, bar, high-speed wire and strip hot and cold plate and strip rolling mills.
The company covers an area of 100,000 square meters, including two complete production lines for cast rolls and forged steel rolls, processing workshops, product research and development centers, testing centers, training centers and office buildings and other supporting facilities. The factory environment is very beautiful.
LMM have been supplied to world’s top steel manufacturer Arcelormittal, TATA Steel, EZZ steel etc. We do OEM for Concast and Danieli for a long time