Oxidation film and wear of hot-rolled high-speed steel rolls

High-speed steel rolls have excellent wear resistance, and their service life on the finishing rolling stand of a hot continuous rolling mill is 3 to 6 times that of high-chromium iron rolls, so they are highly regarded. However, there are still certain problems in the use of high-speed steel rolls, such as large thermal expansion, large friction coefficient, and easy shedding of oxide films. These problems have always plagued the use of high-speed steel rolls. Therefore, how to improve these problems is of great significance to on-site guidance.

 

hot-rolled, high-speed steel rolls

 

1. Performance of high-speed steel rolls

1) Wear and tear

Usually, high-speed steel rolls are put on the machine several times between two grindings. They are not ground during this period and are air-cooled after being removed from the machine. Through the measured roll profile curve and roll profile deviation curve of the high-speed steel work rolls of the F1 stand of Bengang’s hot rolling line 2300 after being used continuously for 6 times, it can be seen that the roll profile is maintained well and the wear amount is <0.15 mm, which reflects the high-speed steel The roller has excellent wear resistance, but after the fifth use, the roughness increases.

2) Surface oxide film

The high-speed steel roll is in contact with the hot strip steel, and a uniformly distributed oxide film is formed on the roll surface. The oxide film is usually silver-grey or light blue. The oxide film on the surface of the high-speed steel roll of the F1 stand of the Benxi Iron and Steel Co., Ltd. 2300 line hot rolling line after the fifth time of use and the oxide film morphology of the high-chromium cast iron roll after the first time of use. The measured maximum roughness Ra of the high-speed steel roll is The maximum roughness Ra of the 1.5 μm high-chromium iron roll is 1.34 μm. The roughness of the high-speed steel roll after five times of use is close to that of the high-chromium iron roll after one use. This shows that the high-speed steel roll has good resistance. Roughness.

Sun Dale et al. proposed that the formation and shedding of oxide film on high-speed steel rolls is a dynamic balance process composed of oxidation and friction and wear. During the initial use of the roll, the roll surface material rapidly generates an oxide film at high temperatures. During continued use, the oxide film continues to become thinner. 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 at a relatively stable level. thickness value. Hot cracks in the oxide film of high-speed steel rolls are hereditary. With the increase in rolling times, the number, depth and width of hot cracks all show an increasing trend, and cracks parallel to the axis are more obvious. Roughness also has a certain hereditary nature, and the roll surface roughness value shows an increasing trend as the number of rolling times increases.

3) Thermal expansion

The high-speed steel roll is in constant contact with the hot strip steel, causing thermal expansion of the roll. The actual measurement results show that the roll surface temperature is 55°C and the maximum thermal crown is 120 μm.

During rolling, there are three main factors that affect 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 roll gap determines the cross-sectional profile and plate shape of the strip, so the wear and thermal crown of the roll need to be controlled.

2. High-speed steel roll usage and maintenance strategies

1) Roll cooling

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, uneven thermal expansion will easily occur, followed by 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 accurately control the surface temperature of high-speed steel rolls.

In response to the problem of uneven temperature on the roll surface, Benxi Iron and Steel optimized the distribution of cooling water in the hot rolling line 2300 line and increased the amount of cooling water in the middle part of the roll body to promote uniform temperature distribution and ensure the integrity of the oxide film. Large thermal expansion was previously regarded as the main reason for the unstable rolling of high-speed steel rolls, but in fact it is because of its small wear that thermal expansion becomes relatively significant. 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 quickly as possible.

2) Rolling lubrication

Hot rolling lubrication technology is to spray an oil-water mixture onto the roll surface before the rolled piece enters the roll gap. The combustion of hot rolling lubricant can form combustion residues. At the same time, the lubricant vaporizes and decomposes rapidly under the high temperature and high pressure in the deformation zone, forming a high-temperature and high-pressure air cushion. The combustion residues and air cushion can isolate the roll from the strip. , plays a lubricating role and reduces the friction coefficient between the roller and the strip.

The lubrication of the hot rolling process can form a protective film covering the surface of the high-speed steel roll, reduce the rolling force and roll temperature wear, improve the condition of the roll surface, and at the same time prevent the intrusion of iron oxide scale and improve the surface quality of the strip. In order to ensure the lubrication effect, the main pipeline of lubricating and rolling needs to be steam cleaned regularly. At the same time, the spray volume of lubricating oil should be measured during each scheduled repair, and the difference between the actual value and the set value of the lubricating oil flow meter should be confirmed. If there are any abnormalities, maintenance needs to be carried out. Normally, when lubricated rolling is put into use, the rolling force can be reduced by about 20%.

3) Use and maintenance

(1) Determine the number of uses between two grindings

The number of uses between two grindings is mainly determined by the oxide film shape, roughness value and rolled material grade of the roll surface after discharging. 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 often, and the downstream rack is used less. Normally, it can be used 3 to 6 times. The oxide film and roughness of the roller surface are still in good condition.

(2) Measurement of the temperature of the discharging rollers

In addition to observing the oxide film on the roll surface, the temperature distribution on the roll surface must also be detected. Through the detection of the temperature of the roll surface, the inspection and maintenance of the cooling water nozzle of the work roll can be guided to avoid the strip surface quality problems caused by the peeling of the oxide film due to the problem of the cooling water nozzle. Sometimes it is necessary to make a comprehensive judgment based on the strip surface condition. .

(3) Flaw detection work

The high-speed steel rolls that are put on the machine repeatedly between two grindings need to be air-cooled after being removed from the machine to eliminate the impact of thermal expansion on the roll gap. Once abnormal rolling occurs during the use of high-speed steel rollers, it is easy to cause serious local network-like hot cracks on the roller surface, and the high-speed steel roller needs 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 any cracks are found, they should be removed to avoid carrying the cracks onto the machine.

Conclusion

(1) Thermal crowns 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 problem of high-speed steel rolls must be considered.

(2) A uniform and complete oxide film is a key factor in ensuring that high-speed steel rolls are used multiple times between grinding. The roughness is continuously inherited and gradually deteriorates during multiple uses.

(3) Comprehensive investment in high-speed steel roll cooling, roll profile 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 usage and management requirements, and thermal crack detection on the roll surface is an important means to avoid failure.

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