Description：rolling mill Rolls is a very important equipment. The roll has direct contact with the billet and deform the billet through a certain pressure during the rolling process, so as to achieve different steel standards. Therefore, the fatigue resistance strength and wear resistance of the rollers are relatively high.
Keyword：Rolling mill rolls life, steel rolling process
In recent years, by optimizing the steel rolling process, improving the performance of the rolling mill rolls device and other technical measures, the hardness, thermal crack resistance and impact resistance of the roll have been effectively improved and greatly improved, thus reducing the roll consumption and extending the service life of the roll. This paper analyzes the analysis of irregular stripping, roll ring cracking and cracking of roll groove. Implement improved roll cooling to accommodate the current condition of the production line.
Roller is a very important equipment. The roll has direct contact with the billet and deform the billet through a certain pressure in the rolling process, so as to achieve different steel standards. Therefore, the fatigue strength and wear resistance of rollers are high, especially in recent years, as the market demand for steel rolling products increases year by year, the roller consumption in the rolling production process is too large, and then increases the production input cost. In this situation, how to improve the productivity of rolling steel, reduce the frequency of roll replacement and prolong the service life of roll has become the focus of general attention in the metallurgical industry. The roll work at high temperature, high pressure, alternating hot and cold, and oxide skins. Under the action of mechanical stress, friction, thermal stress and impact load, the roller of each seat is easy to produce wear, cracking and peeling, thus greatly shortening the service life of the roller. Roll is an important production link of rolling mill, and its procurement cost is also relatively high. Roll consumption is an important economic index, and it is an important index of production cost.
Importance of extending the service life of the rolling mill rolls
Roll is an important workpiece, which can carry out continuous plastic deformation of the metal on the rolling mill, with high consumption. It accounts for a large portion of the production costs. Through perfect roll management system, management personnel can clearly know the condition and trajectory of each roll, make the roll correctly matching and use, maximize the utilization of the roll, make the appropriate grinding amount, not only can reduce the roll fatigue, make the roll meet machine normal production standards, improve the life of the roll, reduce the abnormal roll, can improve the production efficiency of rolling line, increase the production efficiency. Whether the roll management system is sound and how the management degree is are the key factors to reduce the production cost of the rolling line and extend the service life of the roll.
Special personnel are required to manage the roll, establish the roll management system, and record the roll’s entering into the warehouse for acceptance, matching and use, roll crack, and abnormal machine unloading. Through a series of such operations, the use trajectory of each roll and the state of the roll will be managed and controlled, which is crucial to reducing the roll consumption and increasing the roll service life.
Problems in rolling mill rolls in production
Irregular drop at the center of rolling groove
There are too many chemical components of all the copper in the center of the rolling groove, which can easily lead to irregular drop problems. Especially whenever the central part of the whole rolling tank is heated, a large amount of copper may flow back and forth in the inner wall surface of the central part of the whole rolling tank. Once the copper forms a low temperature melting point, it will directly make its thermoplastic properties greatly reduced, and then it will lead to the occurrence of many cracks on the inner surface. And as the copper flows back and forth, the cracks will continue to expand and extend, and to a certain extent, there will be some irregular falling blocks. Taking hot-rolled narrow strip steel as an example, due to the inadequate daily supervision, the rolling amount of each rolling groove is controlled by the finished product adjustment worker himself, so the wear of each rolling groove is inconsistent. When the roller is repaired, the cracks of some rolling grooves are not completely cut off. Due to the existence of micro cracks, when the micro cracks grow up and connect quickly, it eventually leads to the macro cracks, resulting in the rolling groove.
Roller ring collapse
The roll ring roll is affected by the assembly stress, thermal stress and rolling stress in the rolling process. The stress of the roller ring is the algebra and sum of the stress values along the section. In the rolling process, the tensile stress of the inner diameter of the roller ring is large, and the use of a single material roller ring is not conducive to improve the service life of the roll. At the same time, due to the local stress increase of the roller ring, coupled with other stresses, it is also easy to damage the roller ring, resulting in cracking.
Hemp point appears in the rolling groove
The hemp spot on the rolling groove is a common surface defect, its appearance is rough, uneven, also known as “hemp spot”. Most are continuous, but a few are local or sporadic. The defect of hemp point is allowed in the rolling tank, but its depth should not exceed the thickness deviation of the product. The causes of hemp spots are:
(1) Wear of finished holes or front holes, or damaged iron oxide.
(2) Peel off the broken iron oxide after extruding it to the surface of the rolling piece.
(3) The roll is eroded.
(4) During heating, the surface of the billet is severely oxidized.
In the rolling process, cracks and soft points are formed on the surface and inside of the roll due to impact, tail swing, steel clamp and other reasons. The cracks and soft points of roll will seriously affect the normal production of rolling line and the length of roll use time. Severe cracks can cause the rolls to peel off prematurely, ending the roll use early.
The reasons for these problems are: improper human operation, poor cooling effect, key points of rolling production technology, roller quality problems. Improper or improper cooling will produce a large temperature difference on the surface of the roller, thus accelerating the production of the roller off the thermal stress, at the same time, the temperature of the roller is too high, will also affect the strength of the roller and wear resistance of the performance, resulting in the burst, meat, crack and even fracture and other problems. In production, if the characteristics of rolling material and production process are not considered, the roller material is not suitable, or due to human reasons caused by roll roll, steel pile and other reasons, will also produce burst trough, meat, broken roller and other problems.
In the actual production operation, because metallurgical steel materials often contain SiO2, Al2O3 or silicate and other brittle inclusions, the existence of these inclusions is easy to cause serious impact on the service life of the roll, and according to the number, size, type of inclusions, the impact on the roll life of the degree is not the same. Usually, the more inclusions, the larger the size, the greater the harm, especially with the sharp edges of the most harmful.
Steel rolling process measures
Improvement of the roll cooling unit
In order to improve the utilization efficiency of the cooling water and enhance the cooling effect of the cooling water, the method of improving the performance of the roller cooling device can be adopted to extend the service life of the roller. The mill produces a lot of heat during the rolling process, so the roll needs to cool the replacement cycle, service life and ensure the temperature after processing. Two cooling water tanks of rollers, primary cooling water and secondary cooling water are sprayed into the rolling area through the slit axis to lubricate and cool the copper pipe; the secondary cooling is to quickly reduce the temperature of the rolling pipe to avoid air flow into the rolling sleeve and prevent the copper pipe oxidation. The cooling device of the roll is used to cool the roll.
In rolling, the main transmission speed is above 1300 rpm, and the secondary transmission (roll) is above 700 rpm. The roll working at such high speeds requires strict control of the flow and pressure of the cooling water. The cooling water must be cooled continuously and adequately. If the roll temperature is too high, it must be replaced in time to prevent thermal fatigue cracking. In actual production, in order to ensure that the temperature of the roll should not be too high and maintain the temperature stability, the cooling speed of the roll must be above 3500L / h and the pressure of the pump should be below 0.8MPa.
From the original square box hole structure to the solid cylinder nozzle, two columns of elliptical water joints are added to the cutting wedge, and the width of the cutting wedge is 5-8 mm; ensure that the water in the cutting wedge is 3 times higher to strengthen the cooling of the cutting wedge and improve the rolling life of the cutting wedge.
Optimization of cooling water parameters and transformation of water supply system
Choosing the appropriate cooling water temperature, if too high, will accelerate the fracture of the roller, reduce it will affect the recrystallization of the rolling pipe, affect the quality of the product. If it is extreme low temperature and extreme high temperature, it will accelerate fatigue. Therefore, in production, the cooling water temperature of the roller should be 40~60℃.
The roller cooling water pipeline is improved from turbid low water supply to turbid water supply. The frequency conversion motor makes the water pressure reach 0.8 MPa, and the water pressure can be adjusted automatically.
Because the cooling water effect is not ideal, hot rolled narrow strip steel φ 550 on the basis of the original hole type, for I mill hole forced width principle with K1 steel temperature is higher, so the forced width channel in the K3, K4, forced the bottom of the groove design for straight, at the same time to avoid rolling surface wrinkles cause finished surface quality is unqualified, the forced width hole slope as 17.7, K3, K4 horizontal used after forced width and K6 rolling groove size made the corresponding changes, to ensure that no tail phenomenon.
Roller material optimization
Taking Q215 hot rolled narrow strip steel as an example, the initial center spacing of the rolling groove is 19.0 mm, while the width of the middle roller ring is 7.8 mm, which reaches the theoretical design index. However, from the point of view of the overall fracture of the roller ring, the width of the roller ring is still very narrow, which is easy to crack under fatigue conditions, and finally cause the cracking of the roller ring. For this purpose, a 22.0 mm roller seam is designed, and the width of the middle roller ring is adjusted to 10.8 mm.
Automatic control thickness configuration modification
Due to the influence of the inlet thickness, tension, friction coefficient, deformation resistance and other factors, the binding force of the rolling mill will also change, and the related deformation curve will have a certain impact on the outlet of the thickness. Automatic thickness control mainly uses the model algorithm to automatically control the strip thickness, rolling equipment condition, external disturbance, so as to automatically set the juice pressure, speed, the size of the mill roll joint, and ensure a reasonable thickness deviation. Usually AGC uses hydraulic lowering system, this mechanism has high accuracy and is relatively fast. The quality control of the mill is related to the precise control of the slab thickness, and the quantity of hydraulic QGC equipment will have a certain influence on the accuracy of the mill. No.6 rolling mill, the rough rolling control thickness is (27~2.7) mm, but the deviation rate is 10%.
Avoid improper operation
In the production process, the roll damage caused by improper use is also very common, so, one is to prevent the direct contact between the guide and the roll. Second, the roll cooling water switch should be strictly implemented to prevent the replacement of the head without moisture. Third, we should strengthen the spot inspection to prevent problems with the roll. Once the roll appears, it must be stopped immediately, and the winding roll material is cold to room temperature. Fourth, we should control the shape of the rolling pieces strictly according to the process system, and put an end to the phenomenon of excessive single movement shape. Fifth, the black head of the head of the rolled piece should be cut clean and prohibit passing over the black steel.
Repair amount should be in place
In order to ensure that the yield of each rolling tank can maintain a relatively stable state, there can be no excess cracks, otherwise, the rolling amount of the mill will be greatly reduced, and if there are excess cracks, it will have a great impact on the quality of the product. Therefore, in the rolling process, must be strictly in accordance with the provisions, such as Q215 hot rolled narrow strip steel finished rolling roll must reach 600 mm, if the individual rolling tank still can not be completely eliminated, it can be no longer processing, use.
By taking the above measures, the service life of rollers can be greatly improved, the number of changing rollers can be reduced, and the labor intensity of workers can be reduced. By correcting the uneven falling block in the middle of the roll, the problems such as the roll ring crack and the hemp point are solved, and the grooves produced by the cooling effect are almost zero, so that the quality of the products is improved. Improvement in average effective work efficiency.