Effects of using high-speed steel rolls and issues to pay attention to

Abstract: High-speed steel rolls have the characteristics of high hardness, good red hardness, and good wear resistance, and have been widely used in the steel rolling industry. This article reviews the composition, structure, performance, manufacturing methods and application effects of high-speed steel rolls, as well as issues that should be paid attention to in the use of high-speed steel rolls.

 

1. Basic characteristics and performance of high-speed steel rolls

 

(1) High-speed steel rolls usually use high-carbon high-speed steel, the carbon content is generally above 1.5%, and the carbide content is usually 10-20%.

(2) The working layer material of high-speed steel rolls contains high alloying elements such as vanadium, chromium, molybdenum, tungsten, and niobium. The total amount of alloying elements reaches more than 16%. Therefore, the carbides in the roll structure are mainly MC type and M2C type. , carbide has a good shape, high hardness, dispersed distribution and good wear resistance.

(3) High-speed steel rolls have good thermal stability and red hardness and have high hardness and good wear resistance under conventional rolling temperatures.

(4) High-speed steel rolls have good hardenability and hardenability, and the hardness almost does not drop from the surface of the roll body to the inside of the working layer, thus ensuring that the roll has equally good wear resistance from the outside to the inside.

(5) The surface hardness of the high-speed steel roll body is 80-90HSD, and the hardness unevenness is ≤2HSD.

(6) The core material of the high-speed steel roll is made of high-strength alloy ductile iron. The roll neck has high strength and is not easy to break.

 

performance of high-speed steel rolls

 

2. Usage of high-speed steel rolls

 

1) Baosteel’s hot continuous rolling high-speed steel rolls have good results

 

Since the use of high-speed steel, Baosteel’s hot rolling unit has achieved a historic breakthrough in roll consumption, from the original 0.7kg through hard work and adjustment of rolling mill process parameters, accumulation of roll surface determination technology, and detection and elimination of roll surface defects. /t has dropped below the current 0.4kg/t. The average millimetre rolling volume of high-speed steel rollers is 2.2-3.5 times that of high-chromium rollers. Even taking into account price factors, the performance-price ratio of high-speed steel rollers is better than that of high-chromium rollers.

 

2) Application of Meishan Iron and Steel Co., Ltd.’s hot-rolled high-speed steel rolls

 

In order to improve the surface quality of hot-rolled strip steel and improve the wear resistance and surface roughness resistance of the rolls, high-speed steel rolls are used in the work rolls of the Meishan Iron and Steel hot rolling finishing unit frame. The usage results show that the average rolling weight of high-speed steel rolls is 5500t, while the average steel passing capacity of high-chromium cast iron rolls is only 2380t. Calculated based on the steel passing capacity of 500t per hour, high-speed steel rolls can be used continuously for up to 10 hours, while high-chromium cast iron rolls can only After 4 hours of continuous use, it can be seen that the number of roll changes is reduced after using high-speed steel rolls, which improves the efficiency of the rolling mill; the rolling weight per millimetre is 1.6 times that of high-chromium cast iron rolls, which reduces the application cost of the rolls and greatly improves the cost performance.

 

3) Application of high-speed steel rolls of Zhongtian Iron and Steel Group

 

Save the cost of using rollers, improve the output and surface quality of hot-rolled rebar, and improve the wear resistance and surface roughness resistance of the rollers. Illustrated through on-site actual data,

For example: In a steel rolling line of the Second Steel Rolling Plant of Zhongtian Iron and Steel Group, the work rolls of the hot rolling finishing rolling unit frame use high-speed steel rolls.

Through stage data statistics, the data of high-speed steel rolls used by finishing rolling unit K1 shows that the rolling efficiency of high-speed steel rollers is higher than that of ordinary rolls. When finishing rolling unit K1 uses ordinary rollers, the grooves need to be replaced every 4 hours. That is to say, it needs to be replaced 3 to 4 times in one shift; after using high-speed steel rolls, small sizes are usually replaced once per shift, and large sizes are usually replaced once a day.

Regardless of large- or small-size rolling, there is a significant improvement in the steel throughput data of a single groove. At the same time, the use time of the rolls is significantly extended, which can effectively utilize the rolling time, increase the service life of the rolls, reduce the cost of roll consumption, and reduce production costs. Manpower and material cost. In addition, because the high-speed steel roll has good wear resistance and hardness, the roll wear is even and the amount of wear is small during production and use. When the roll is milled again based on the original rib, the amount of turning and milling can be reduced, and the roll can be effectively reused to cut costs.

 

3. Problems that should be paid attention to when using high-speed steel rollers

 

The main problems of high-speed steel rolls:

 

(1) Due to poor heat dissipation, a temperature difference is easily formed on the roll, which reduces the service life of the roll and may cause bursting and cracking. The cooling capacity of the roll must be enhanced to keep the roll surface temperature below 65°C.

(2) During use, there may occasionally be segregation and lack of alloy components during the roll production process, resulting in roll interruption during use; at the same time, there may also be cases where the material width of the previous K2 rolling mill is too large and the K1 reduction amount is large. The deformation resistance and front-to-back slipping increase sharply, causing the metal to flow quickly. Under the repeated action of the rolled piece, filamentous meat loss occurs in the rolling groove, or periodic block loss occurs, resulting in the “notch effect” of the transverse ribs.

 

3.1 Control of roller surface temperature

 

Roller surface temperature control is mainly to control the thermal crown of the roll, limit the thickness of the oxide film on the roll surface, reduce cracks and prevent peeling. The thermal conductivity of high-speed steel roll material is relatively low and requires good cooling conditions. The amount of cooling water should be as sufficient as possible. The scientific approach should be to obtain the highest intensity of cooling at the position where the rolled piece has just separated from the roll, and then gradually reduce the amount of water along the arc of the roll. At the same time, the water pressure, water temperature, and water quality of the cooling water will all affect the use of the roll.

In order to achieve a good cooling effect, flat nozzles are used instead of conical nozzles, and the cooling water on the tapping side of the roll should account for 3/4. The cooling water volume in the middle should be twice as high as the cooling water volume at the end of the roll body. By changing the original Cooling water pipeline, control water pressure (0.5-0.7MPa), water volume and nozzle angle. The water flow ejected from the nozzle must hit the roll at a certain angle and cannot be perpendicular to the roll surface. It usually forms an angle of 20-30° with the roll surface. And it is opposite to the direction of rotation of the roller. The final cooling effect obtained should be based on the roller surface temperature. Usually, the temperature of the roll surface should be lower than 60°C 10-20 minutes after the roll is removed from the machine.

The thickness of the oxide film on the roll surface is an important indicator. If the cooling water is too small, the oxide film generated by the high temperature of the roll surface will be too thick, brittle and easy to peel off; if the cooling water of the roll is too large, the oxide film on the roll surface will be too thin and the roll will be damaged. The roll surface is prone to roughness, both of which will lead to oxide scale defects on the surface of hot rolled strip steel.

High-stress cracks caused by mechanical overload, slippage, overheating and impact must be removed. Thermal cracks caused by rolling mill shutdown are less dangerous than stress cracks and can be controlled in a better critical state.

 

3.2 Control of load size

 

Reasonable load is crucial to the oxide film protection of high-speed steel rolls. The direct cause of oxide film damage is excessive rolling load. During the production process, the load of key stands can be appropriately distributed to other stands, or the rolling load (rolling load) can be appropriately adjusted to the roughing stand to reduce the number of plates. The thickness of the billet before entering the finishing rolling unit is to protect the oxide film on the surface of the high-speed steel roll with a large rolling load.

 

3.3 Roller lubrication effect

 

Due to the large friction coefficient of high-speed steel rolls, under the same conditions, the rolling load must be increased by 10%-20%. To ensure a reasonable rolling load, lubrication is the most effective method.

The use of rolling lubrication can reduce the friction coefficient of the roll and make the friction force become constant. At the same time, it can reduce the temperature of the roll surface, reduce the thermal cracks and surface shear stress of the roll, and improve the surface condition of the roll. The original rolls were lubricated with 3# grease when they went online. Now the lubrication system has been improved and centralized lubrication is used to ensure the lubrication effect of the rolls.

 

3.4 Maintenance of high-speed steel rolls in slit rolling

 

(1) Before the roll comes into contact with the red steel, the cooling water needs to be turned on in advance to cool the rolling groove. In addition to the circular cooling cluster pipes installed at the exit of the rolling mill, aiming at the rolling groove to continuously spray and cool the roll surface, additional water distribution bags need to be installed on the guides at the entrance and exit of the rolling mill to further forcefully cool the rolling groove and prevent the high-speed steel rolls from rolling. An effective way to create thermal cracks.

(2) High-speed steel rolls have poor accident resistance, and the failure of stacking ribbed steel bars in split rolling occurs more frequently. After stacking, a large amount of red steel gets stuck in the rolling groove, which will cause the temperature of the roller surface to rise rapidly. When the accident happens, the cooling water cannot be turned off immediately. Instead, the cooling water needs to be used to cool the stacked rolled parts until the colour turns black, and then the cooling water is turned off to deal with the accident.

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