In recent years, through technical measures such as optimizing the steel rolling process and improving the performance of the roll assembly, the hardness, thermal crack resistance and impact resistance of the rolls have been effectively improved and significantly enhanced. As a result, while reducing roll consumption, the service life of the rolls has been prolonged. The article focuses on analyzing the irregular block detachment in the middle of the rolling groove, the cracking of the roll ring, and the appearance of pitting in the roll groove of hot-rolled narrow strip steel. Implement improved roll cooling to adapt to the current conditions of the production line. By adopting this device and the water supply device, the cooling effect of the roller track can be effectively improved, and the width of the roller ring can be appropriately increased to avoid cracking and falling off, and suitable roller materials can be selected.

This system can not only correct the uneven lumps in the middle of the raceway, but also solve problems such as cracks and pockmarks in the roller ring, and improve the efficient operation efficiency of the rolls.

Rolls are a very important piece of equipment. The rolls come into direct contact with the steel billet. During the rolling process, the steel billet is deformed under certain pressure to meet different steel quality standards. Therefore, high requirements are placed on the fatigue strength and wear resistance of the rolls. Especially in recent years, with the annual increase in the market demand for rolled steel products, the roll consumption in the steel rolling production process has become excessive, thereby increasing the production input cost. Under such circumstances, how to enhance the productivity of steel rolling, reduce the frequency of roll replacement and extend the service life of rolls has become a focus of widespread concern in the metallurgical industry. The rolls operate under conditions of high temperature, high pressure, alternating cold and heat, and iron oxide scale. Under the influence of mechanical stress, friction, thermal stress and impact load, the rollers of each base are prone to wear, cracking and spalling, which greatly shortens the service life of the rolls. Rolls are an important production link in rolling mills, and their procurement costs are also relatively high. Roll consumption is an important economic indicator and a significant indicator of production costs.

1 The importance of extending the service life of rolls

Rolls are important workpieces that can continuously perform plastic deformation on metals in rolling mills and have high consumability. It accounts for a large part of the production cost. Through a well-developed roll management system, managers can clearly understand the condition and trajectory of each roll, enabling the correct pairing and use of rolls, maximizing the utilization rate of rolls, and setting appropriate grinding allowances. This not only reduces roll fatigue, ensuring that the rolls meet the standards for normal machine production, but also extends the lifespan of the rolls and reduces abnormal roll replacement. It can enhance the production efficiency of the rolling line and increase production benefits. Whether the management system for rolls is sound and to what extent it is managed are all key factors in reducing the production cost of the rolling line and extending the service life of the rolls.

It is necessary to have a dedicated person in charge of managing the rolls, establish a roll management system, and keep records of the rolls' entry into the warehouse for acceptance, pairing for use, cracking of the rolls, and abnormal removal from the machine. Through a series of such operations, the usage trajectory and status of each roll will be managed and controlled, which is crucial for reducing roll consumption and increasing the service life of rolls.

2 Problems existing in the rolls during production

2.1 Irregular lumps fall off at the center of the rolling groove

If there is an excessive amount of copper-containing chemical components in all the central parts of the rolling groove, it is very likely to cause irregular chipping problems. Especially when the central part of the entire rolling bath is heated, a large amount of copper may constantly flow back and forth on the inner wall surface layer of the central part of the entire rolling bath. Once the copper reaches a low melting point, its thermoplastic performance will be greatly reduced, and then many cracks will easily appear on the inner surface layer. Moreover, as the copper flows back and forth, the cracks will continue to expand and extend. To a certain extent, irregular chipping problems will occur. Take hot-rolled narrow strip steel as an example. Due to inadequate daily supervision, the rolling amount of each rolling groove is controlled by the finished product adjuer themselves, which leads to inconsistent wear of each rolling groove. During the repair of the rolls, the cracks in some rolling grooves are not all turned off. Due to the existence of micro-cracks, when it is put back into use, the micro-cracks grow and connect rapidly, eventually leading to the formation of macroscopic cracks and causing the meat to fall off during the grooving process.

2.2 The roller ring has cracked

Roller ring rolls are subject to assembly stress, thermal stress and rolling stress during the rolling process. The stress acting on the roller ring is the algebraic sum of the stress values along the cross-section. During the rolling process, the inner diameter tangential tensile stress of the roll ring is large. Using a single material for the roll ring is not conducive to increasing the service life of the rolls. At the same time, due to the increase in local stress on the roller ring, coupled with the effect of other stresses, the roller ring is also prone to damage, causing it to crack.

2.3 Pitting appears in the rolling groove

Pockmarks on the grooves are a common surface defect, with a rough and uneven appearance, also known as "pitting spots". Most of them are continuous, but there are also a few that are local or sporadic. The pitting defect is allowed in the rolling groove, but its depth must not exceed the thickness deviation of the product. The causes of pitting are:

(1) The holes in the finished product or the front holes are worn, or there is damaged iron oxide.

(2) After being extruded onto the surface of the rolled piece, the broken iron oxide is peeled off.

(3) The rolls are eroded.

(4) During the heating period, the surface of the steel billet was severely oxidized.

2.4 Roll fracture

During the rolling process, due to reasons such as impact, tailing, and steel inclusion, cracks and soft spots form on the surface and inside the rolls. Cracks and soft spots on the rolls can seriously affect the normal production of the rolling line and the service life of the rolls. Severe cracks can cause the rolls to peel off prematurely and end their service life prematurely.

3 Cause analysis

The causes of these problems include: improper human operation, poor cooling effect, key points of rolling production technology, and quality issues of the rolls themselves, etc. Improper or inappropriate cooling can create a significant temperature difference on the surface of the rollers, thereby accelerating the thermal stress that causes the rollers to fall off. At the same time, if the temperature of the rollers is too high, it will also affect the strength and wear resistance of the rollers, leading to problems such as slot explosion, meat loss, burning and cracking, and even breakage. In production, if the characteristics of the rolled material and the production process are not taken into account, the selected roller material is inappropriate, or problems such as roller rolling and steel stacking are caused by human factors, problems such as trough burst, meat drop and roller breakage may also occur.

In actual production operations, due to the fact that metallurgical steel materials often contain brittle inclusions such as SiO2, Al2O3 or silicate, the presence of these inclusions can easily have a significant impact on the service life of the rolls. Moreover, the degree of influence on the service life of the rolls varies depending on the quantity, size and type of inclusions. Generally speaking, the more inclusions there are and the larger their size, the greater the hazard. Among them, inclusions with sharp edges are the most harmful.

4 Process measures

4.1 Improvement of the roll cooling device

To enhance the utilization efficiency of cooling water and improve its cooling effect, methods such as improving the performance of the roll cooling device can be adopted to extend the service life of the rolls. During the rolling process, the rolling mill generates a large amount of heat. Therefore, to ensure the replacement cycle of the rolls, their service life, and the temperature after processing, cooling is necessary. There are two cooling water tanks for the rolls, namely the primary cooling water tank and the secondary cooling water tank. The primary cooling water is axially sprayed into the rolling area through a slit to lubricate and cool the copper tubes. Secondary cooling is to rapidly lower the temperature of the rolled tubes, prevent the gas flow from entering the rolling sleeve, and avoid the oxidation of the copper tubes. The water spray ring of the cooling device for the rolls is used to cool the rolls.

During rolling, the main drive speed is above 1300rpm, and the auxiliary drive (rolls) is above 700rpm. When the rolls operate at such a high speed, strict control over the flow and pressure of the cooling water is required. Continuous and sufficient cooling water must be provided. If the temperature of the rolls is too high, they must be replaced in time to prevent thermal fatigue cracking. In actual production, to ensure that the temperature of the rolls is not too high and to maintain temperature stability, it is necessary to guarantee that the cooling rate of the rolls is above 3500L/h and the pressure of the water pump is below 0.8MPa.

From the original square box-type structure with holes to the solid cylindrical nozzle, two rows of elliptical water slits are added at the cutting wedge position. The width of the cutting wedge covered by the water slits is 5 to 8mm. Ensure that the water in the cutting wedge area is three times that in other areas to enhance the cooling of the cutting wedge area and improve the rolling groove life of the cutting wedge area.

4.2 Optimization of cooling water parameters and renovation of the water supply system

Select an appropriate cooling water temperature. If it is too high, it will accelerate the breakage of the rolls. If it is too low, it will affect the recrystallization of the rolled tubes and the quality of the products. If it is extremely low or high, it will accelerate fatigue. Therefore, in production, the cooling water temperature of the rolls should be 40 to 60 degrees Celsius.

The cooling water pipeline for the rolls has been improved, changing from turbid low-pressure water supply to turbid medium-pressure water supply. The water pump motor adopts a variable-frequency motor, which enables the water pressure to reach 0.8MPa and can be automatically adjusted.

4.3 Optimization of hole type configuration

Due to the unsatisfactory cooling water effect, for the hot-rolled narrow strip steel φ550, on the basis of the original hole type, the principle of forced widening was adopted for the hole type of the I-stand rolling mill. Considering that the temperature of K1 steel is relatively high, the passes for forced widening were set on K3 and K4, and the bottom of the forced widening groove was designed as a straight line. At the same time, to avoid wrinkles on the surface of the rolled piece, which may cause the surface quality of the finished product to be substandard, The slope of the hole type for forced widening was set at 17.7°. After the K3 and K4 transverse sections were subjected to forced widening, the rolling groove size of K6 was also modified accordingly to ensure that no tail-pulling phenomenon occurred.

4.4 Optimization of roll material

Taking the commonly used Q215 type hot-rolled narrow strip steel as an example, the initial center spacing of the rolling grooves is 19.0mm, while the width of the middle roll ring is 7.8mm, reaching the theoretical design index. However, from the perspective of the overall fracture of the roller ring, the width of the roller ring is still very narrow, and it is prone to cracking under fatigue conditions, eventually causing the roller ring to crack. For this purpose, from the perspective of the roller length, a 22.0mm roller gap was designed, and the width of the middle roller ring was adjusted to 10.8mm.

4.5 Automatic control thickness configuration modification

Due to the influence of factors such as the thickness of the inlet, tension, friction coefficient, and deformation resistance, the binding force of the rolling mill will also change accordingly, and the related deformation curve will have a certain impact on the thickness of the outlet. Automatic thickness control (AutomaticControllerController, AGC) model algorithm was mainly used to strip thickness, the condition of rolling equipment, external disturbances, such as automatic control, so as to achieve automatic setting juicing pressure, speed, the size of the mill roll gap, and ensure the reasonable thickness deviation. Usually, AGC adopts a hydraulic downward pressure system, which has very high precision and is correspondingly relatively fast. The quality control of the rolling mill is related to the precise control of the slab thickness to a certain extent, and the number of hydraulic QGC equipment will have a certain impact on the accuracy of the rolling mill. The No. 6 fine rolling mill for steel wire has a rough rolling control thickness of (27 to 2.7) mm, and the deviation rate is 10%.

4.6 Avoid improper operation

During the production process, damage to the rolls caused by improper use is also very common. Therefore, first of all, it is necessary to prevent the direct contact between the guide and the rolls. Second, strictly follow the cooling water switch of the rolls to prevent the replaced heads from having no moisture. Thirdly, it is necessary to strengthen inspection and spot checks to prevent problems with the rolls. Once a roll rolls occur, the machine must be stopped immediately. The processing can only be halted after the material wound around the rolls cools to room temperature. Fourth, the shape of the rolled pieces must be strictly controlled in accordance with the process system to prevent the phenomenon of overly large material shapes in a single batch. Fifth, the black ends at the head of the rolled piece must be cut clean, and over-black steel is strictly prohibited.

4.7 The amount of repairs must be in place

To ensure that the output of each rolling slot remains relatively stable, no extra cracks should occur. Otherwise, the rolling capacity of the rolling mill will be greatly reduced. Moreover, if extra cracks appear, it will have a significant impact on the quality of the product. For this reason, during the rolling process, rolling must be carried out strictly in accordance with the regulations. For instance, the turning allowance of the finished Q215 hot-rolled narrow strip steel rolls must reach 600mm. If individual rolling grooves still cannot be completely eliminated, it can be indicated that they will no longer be processed or used.

5 Conclusion

By taking the above measures, the service life of the rolls can be greatly prolonged, the frequency of roll replacement can be reduced, and the labor intensity of workers can be decreased. By correcting the uneven lumps in the middle of the rolls, problems such as cracks and pockmarks in the roll rings were solved. The grooves caused by the cooling effect were almost zero, thereby improving the quality of the products. The improvement of average effective working efficiency.