The mill roll is a very important piece of equipment. The rolls are in direct contact with the billet, and the billet is deformed by a certain pressure during the rolling process, so as to achieve different steel quality standards. Therefore, the requirements for the fatigue resistance and wear resistance of the roll are high, especially in recent years, with the increasing demand for rolled steel products in the market, the consumption of rolls in the steel rolling production process is too large, which in turn increases the production input cost. In this situation, how to increase the productivity of steel rolling, reduce the frequency of roll replacement, and prolong the service life of rolls has become the focus of general attention in the metallurgical industry. The rolls work under the conditions of high temperature, high pressure, alternating cold and hot, and oxidized iron scale. Under the action of mechanical stress, friction, thermal stress and impact load, the rollers of each stand are prone to wear, cracking, peeling, etc., thereby greatly shortening the service life of the rollers. The role is an important production link of the rolling mill, and its purchase cost is relatively high. Roll consumption is an important economic indicator, it is an important indicator of production costs.
1 The importance of extending the service life of the roll
The roll is an important workpiece that undergoes continuous plastic deformation of the metal in a rolling mill and is highly consumable. It accounts for a large portion of production expenses. Through the perfect roll management system, the manager can clearly know the condition and track of each roll, make the correct pairing and use of the rolls, maximize the utilization of the rolls, and formulate the appropriate grinding amount, which can not only reduce the fatigue of the rolls, but also Make the rolls meet the standard for normal production of the machine, increase the life of the rolls, reduce abnormal roll changes, improve the production efficiency of the rolling line, and increase production benefits. Whether the roll management system is sound or not, and the degree of management, etc., are the key factors to reduce the production cost of the rolling line and prolong the service life of the roll.
Special personnel is required to manage the rolls, establish a roll management system, and make records of rolls entering the warehouse for acceptance, matching use, roll cracks, and abnormal dismounting. Through a series of such operations, the used trajectory of each roll and the state of the rolls will be managed and controlled, which is crucial to reducing roll consumption and increasing roll service life.
2 Problems exist in rolls during production
2.1 Irregular falling blocks in the center of the rolling groove
There are too many chemical components containing copper in the central parts of the rolling groove, which will easily lead to irregular block falling. Especially whenever the central part of the entire rolling groove is heated, a large amount of copper may flow back and forth continuously on the inner wall surface of the central part of the entire rolling groove. Once the copper forms a low-temperature melting point, it will directly make its thermoplastic performance greatly improved. Reduced, which in turn will lead to many cracks on the surface of the inner layer. Moreover, as the copper flows back and forth, the cracks will continue to expand and extend, and to a certain extent, the problem of irregular falling blocks will appear. Taking hot-rolled narrow strip steel as an example, due to the lack of daily supervision, the rolling volume of each groove is controlled by the finished product adjuster, so the wear of each groove is inconsistent. When the roll is repaired, the cracks in individual grooves are not completely turned. Due to the existence of microscopic cracks, when it is used again on the line, the microcracks grow rapidly and connect, finally leading to the generation of macroscopic cracks, resulting in the loss of meat in the rolling groove.
2.2 Roll ring cracking
Roll rings and rolls are affected by assembly stress, thermal stress, and rolling stress during the rolling process. The stress on the roller ring is the algebraic sum of the stress values along the section. In the rolling process, the tangential tensile stress of the inner diameter of the roll ring is large, and the use of a single material roll ring is not conducive to improving the service life of the roll. At the same time, due to the increase of the local stress of the roller ring, coupled with the effects of other stresses, the roller ring is also easily damaged, causing it to crack.
2.3 Pitting in the rolling groove
Pitting on the rolling groove is a common surface defect, and its appearance is rough and uneven, also known as “pitting”. Most are continuous, but a few are partial or sporadic. Pitting This defect is allowed in the groove, but its depth cannot exceed the thickness deviation of the product. Causes of pimples are:
(1) The finished hole or front hole is worn, or there is damaged iron oxide.
(2) After being extruded to the surface of the rolled piece, the broken iron oxide is peeled off.
(3) The roll is corroded.
(4) During heating, the surface of the billet is severely oxidized.
2.4 Roll break
During the rolling process, cracks and soft spots are formed on the surface and inside of the roll due to impact, tail flicking, steel clamping, and other reasons. The cracks and soft spots of the roll will seriously affect the normal production of the rolling line and the service life of the roll. Severe cracks can cause premature spalling of the roll, ending the roll’s life prematurely.
3 Reason analysis
The reasons for these problems are improper human operation, poor cooling effect, key points of rolling material production technology, quality problems of the roll itself, etc. Improper or inappropriate cooling will generate a large temperature difference on the surface of the roller, thereby accelerating the thermal stress of the roller falling off. At the same time, if the temperature of the roller is too high, it will also affect the strength and wear resistance of the roller, resulting in burst grooves, Problems such as debonding, burning, and even breaking. In production, if the characteristics of the rolled material and the production process are not considered, the selected roll material is not suitable, or the roll and piled steel caused by human factors will also cause problems such as cracking, meat loss, and roll breakage.
In actual production operations, since metallurgical steel materials often contain brittle inclusions such as SiO2, Al2O3, or silicate, the existence of these inclusions can easily have a serious impact on the service life of the roll. Different types have different effects on the life of the roll. Normally, the more inclusions and the larger their size, the greater the hazard, especially the inclusions with sharp edges are the most harmful.
4 process measures
4.1 Improvement of roll cooling device
In order to improve the utilization efficiency of cooling water and enhance the cooling effect of cooling water, the method of improving the performance of the roll cooling device can be adapted to prolong the service life of the roll. The rolling mill will generate a lot of heat during the rolling process. Therefore, cooling is required to ensure the replacement cycle, service life, and temperature after processing the rolls. Two cooling water tanks for the roll, primary cooling water, and secondary cooling water, the primary cooling water is sprayed axially to the rolling area through the slit to lubricate and cool the copper tube; the secondary cooling is to quickly reduce the temperature of the rolled tube, to avoid airflow into the rolling envelope, to prevent oxidation of copper tubes. The water spray ring of the cooling device of the roll is used to cool the roll.
During rolling, the main transmission speed is above 1300rpm, and the auxiliary transmission (roller) is above 700rpm. The rolls work at such a high speed that the flow and pressure of the cooling water need to be strictly controlled. The cooling water must be continuously and sufficiently cooled. 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 is not too high and maintain temperature stability, it is necessary to ensure that the cooling rate of the roll is above 3500L/h, and the pressure of the water pump should be below 0.8MPa.
From the original square box-type structure with holes to the solid cylindrical nozzle, two rows of elliptical water seams are added to the split wedge, and the width of the split wedge covered by the water seams is 5~8mm; to ensure the safety of the split wedge The water is 3 times that of other parts to enhance the cooling degree of the cutting edge and improve the groove life of the cutting wedge.
4.2 Optimization of cooling water parameters and transformation of the water supply system
Choose the appropriate cooling water temperature, if it is too high, it will accelerate the fracture of the roll, if it is lowered, it will affect the recrystallization of the rolled tube and the quality of the product, if it is extremely low temperature and extremely high temperature, it will accelerate fatigue, so, in During production, the cooling water temperature of the roll should be 40~60°C.
The roll cooling water pipeline has been improved, from a low turbid water supply to a medium turbid water supply, and the water pump motor adopts a frequency conversion motor to make the water pressure reach 0.8MPa, and the water pressure can be automatically adjusted.
4.3 Optimization of pass configuration
Due to the unsatisfactory effect of cooling water, on the basis of the original pass of the hot-rolled narrow strip steel φ550, the pass of the I-stand rolling mill adopts the principle of forced widening. In view of the high temperature of K1 steel, the pass of forced widening is set at For K3 and K4, the bottom of the forced widening groove is designed as a straight line. At the same time, in order to avoid wrinkles on the surface of the rolled piece and cause the unqualified surface quality of the finished product, the slope of the forced widening pass is set at 17.7°, and the K3 1. After K4 adopts forced widening, corresponding changes are made to K6’s rolling groove size to ensure that there will be no tailing phenomenon.
4.4 Roll material optimization
Taking Q215 hot-rolled narrow-strip commonly used varieties as an example, its initial rolling groove center distance is 19.0mm, and the middle roll ring width is 7.8mm, which has reached the theoretical design index. However, from the point of view of the overall fracture of the roll ring, the width of the roll ring is still very narrow, which is prone to cracks under fatigue conditions, and finally causes the roll ring to crack. For this reason, from the perspective of roll length, a roll gap of 22.0 mm is designed, and the width of the middle roll ring is adjusted to 10.8 mm.
4.5 Automatic control thickness configuration modification
Due to the influence of the entrance thickness, tension, friction coefficient, deformation resistance and other factors, the rolling force of the rolling mill will also change accordingly, and the related deformation curve will have a certain impact on the thickness of the exit. Automatic thickness control (Automatic Controller Controller, AGC) mainly uses model algorithms to automatically control strip thickness, rolling equipment conditions, external disturbances, etc., so as to automatically set the juicing pressure, speed, and the size of the roll gap of the rolling mill, and ensure reasonable thickness deviation. Usually, AGC adopts a hydraulic press-down system, which has high precision and correspondingly is relatively fast. The quality control of the rolling mill has a certain relationship with the precise control of the thickness of the slab, and the number of hydraulic QGC equipment will have a certain impact on the precision of the rolling mill. Steel wire finishes rolling No. 6 rolling mill, the rough rolling control thickness is (27~2.7) mm, and the deviation rate is 10%.
4.6 Avoid improper operation
In the production process, roll damage caused by improper use is also very common. Therefore, it is necessary to prevent direct contact between the guide and the roll. Second, the cooling water switch of the roll must be strictly implemented to prevent the replaced head from being dehydrated. Third, spot checks must be strengthened to prevent problems with the rolls. Once rolls appear, the machine must be stopped immediately, and the processing can only be stopped after the winding roll material cools down to room temperature. Fourth, the material shape of the rolled piece must be strictly controlled in accordance with the processing system to prevent the phenomenon that the material shape of a single rack is too large. Fifth, the blackheads on the head of the rolled piece must be cut clean, and black steel is prohibited.
4.7 The amount of restoration should be in place
In order to ensure that the output of each rolling groove can maintain a relatively stable state, no redundant cracks can appear, otherwise, the rolling capacity of the rolling mill will be greatly reduced, and if there are redundant cracks, it will affect the quality of the product. have a great impact. Therefore, during the rolling process, the rolling must be carried out strictly according to the regulations. For example, the turning amount of the finished roll of Q215 hot-rolled narrow strip steel must reach 600mm.
In summary, the steel rolling process measures to improve the life of mill rolls are:
1 Improvement of roll cooling device
2 Optimization of cooling water parameters and transformation of the water supply system
3 Optimization of pass configuration
4 Roll material optimization
5 Automatic control thickness configuration modification
6 Avoid improper operation
7 The amount of restoration should be in place