Mill Roll TechnologyII Hot Rolling High Speed Steel Roll Oxide Film And Wear

High speed steel rolls have excellent wear resistance, and their service life in finishing mill stands of hot rolling mills is 3~6 times that of high chromium iron rolls, so they are highly respected.However, there are still some problems in the use of high speed steel rolls, such as large thermal expansion, large friction coefficient, easy to fall off oxide film, etc., these problems have been plaguing the use of high speed steel rolls, so, how to improve these problems is of great significance for on-site guidance.

1.?The performance of high-speed steel rolls Abrasion: Usually, high speed steel rolls are used on the machine several times between two grinding, during which no grinding, air cooling after the machine.Figure 1 shows the measured roll profile curve and roll profile deviation curve of HSS work roll of F1 frame of 2300 hot rolled line of This Steel after 6 consecutive times of use. It can be seen that the roll profile is maintained well and the wear quantity is less than 0.15mm, which reflects the excellent wear resistance of HSS rolls. However, roughness increases after the 5th time of use.

Surface oxide film HSS Rolls are in contact with tropical steel and the roll surface forms a uniformly distributed oxide film, usually silver gray or light blue.Figure 2 shows the morphology of oxide film on HSS roll surface and high chromium cast iron roll surface after the fifth discharge of HSS roll and high chromium iron roll surface after the first discharge of HSS roll. The measured maximum roughness Ra of HSS roll is 1.5 μm and Ra of high chromium iron roll is 1.34 μm.The roughness of HSS roller after 5 times of rolling down is close to that of high chromium iron roller after 1 time of rolling down, which shows that HSS roller has good roughness resistance.

The formation and shedding of oxide film on high speed steel rolls is a dynamic equilibrium process composed of oxidation and friction and wear.At the early stage of roller use, the surface material rapidly generates oxide film under high temperature, and the oxide film is continuously thinned in the process of continuous use. At the same time, the working layer material under the oxide film is continuously oxygenated and the oxide film maintains a relatively stable thickness value.The hot cracks in oxide film of high speed steel roll are hereditary. With the increase of rolling times, the number, depth and width of hot cracks increase, and the cracks parallel to the axis are more obvious.Roughness also has a certain heritability. The roughness value of roll surface increases with the increase of rolling times.

Thermal expansion The HSS roll is in continuous contact with the tropical steel and the roll develops thermal expansion.FIG. 4 shows the thermal crown of rolls of F1 stand of 2300 hot rolling line of This steel for 30 min after the first to third discharges. According to the actual measurement results, the temperature of roll surface is 55℃ and the maximum thermal crown is 120 μm.During rolling, there are three main factors affecting the load roll gap: the original roll shape, the thermal expansion of the roll and the wear of the roll.The load roll gap determines the profile and shape of strip, so it is necessary to control the wear and thermal crown of roll.

2.high speed steel roll use and maintenance countermeasures Roll cooling There is an important relationship between the thermal expansion of high speed steel roll and the temperature of the roll surface. Once the temperature of the roll surface is unevenly distributed, it is easy to produce uneven thermal expansion, followed by the abnormal fall off 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 roll and product surface quality, it is necessary to accurately control the surface temperature of high speed steel roll.In order to solve the problem of uneven roller surface temperature,the cooling water distribution of hot rolling line 2300 has been optimized to increase the cooling water quantity in the middle part of the roller body and promote the uniform temperature distribution, so as to ensure the integrity of oxide film.In the past, high thermal expansion was considered as the main reason for the unstable rolling of high speed steel rolls, but in fact, because of its small wear, thermal expansion becomes relatively significant.The thermal expansion should be as small as possible, so the middle of the work roll should be cooled as soon as possible.

The lubrication of rolling Hot rolling lubrication technology is to spray oil and water mixture onto the roll surface before rolling parts enter the roll gap.Hot rolling lubricant combustion can form burning residue, and the lubricant in the deformation zone of high temperature and high pressure under rapid vaporization and decomposition, forming high temperature and high pressure air cushion, combustion residue and air cushion can be separated from the roller and strip steel, play lubrication, reduce the friction coefficient between roller and strip.Hot rolling process lubrication can form a protective film covering the surface of HSS roll, reduce rolling force, mild wear of roll, improve the state of roll surface, at the same time can prevent oxide sheet pressing into the strip, improve the surface quality.In order to ensure the lubrication effect, it is necessary to carry out steam cleaning on the main pipe road of lubrication rolling regularly, and at the same time, measure the amount of lubricating oil gushing during each fixed repair, and confirm the difference between the actual value of lubricating oil flowmeter and the set value. If there is any abnormality, it is necessary to maintain and deal with it.Generally, when lubricating rolling is put into use, the rolling force can be reduced by about 20%.

Use the maintenance ◆ The use times of two grinding rooms are determined The number of grinding times is mainly determined by the morphology of oxide film on roll surface, roughness value and the grade of rolling material.This is also related to whether it is close to the finished frame. In the case that the oxide film and roughness are in good condition, the upstream frame is used relatively more times, and the downstream frame is less. Usually, it can be used 3-6 times, and the oxide film and roughness of the roll surface still remain good.

Roller temperature measurement 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 roller surface, it can guide the maintenance of the nozzle of the working roller cooling water, and avoid the quality problems of the strip surface caused by the spalling of the oxide film due to the nozzle of the cold water. Sometimes, it is necessary to make a comprehensive judgment based on the surface condition of the strip.

◆ Flaw detection work Air cooling is required for HSS which is put on the machine repeatedly between two grinding processes to eliminate the influence of thermal expansion on roll gap.Once abnormal rolling occurs in the process of high speed steel roll, it is easy to cause serious mesh hot crack on the roll surface, and it is necessary to replace the high speed steel roll immediately.After roller cooling, automatic or portable ultrasonic flaw detector should be used to detect the crack on the roll surface. If the crack is found, it should be removed and avoided to carry the crack on the machine.

3.concluding remarks (1) Thermal crown and wear are very important factors in hot rolling production. Although the wear problem has been solved by using high speed steel rolls, the thermal crown of high speed steel rolls must be considered.

(2) Uniform and complete oxide film is the key factor to ensure repeated use of HSS roller between two grinding, and roughness is continuously inherited and gradually deteriorated in the process of repeated use.

(3) The comprehensive input of HSS roll cooling, roll shape optimization and process lubrication can give full play to the excellent performance of HSS roll.

(4) High speed steel roll should have strict use and management requirements, and hot crack detection on roll surface is an important means to avoid failure.

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