Measures and Research on Steel Rolling Process To Improve Roll Life

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, increasing the productivity of steel rolling, reducing the frequency of roll replacement, and prolonging the service life of rolls have 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 roll 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

Rolls undergo continuous plastic deformation of metal in rolling mills and are highly consumable, forming a large part of production expenses. An efficient roll management system allows managers to monitor roll conditions, pair and use rolls correctly, and set appropriate grinding amounts. This reduces roll fatigue, ensures rolls meet production standards, extends roll life, minimizes abnormal roll changes, improves production efficiency, and increases benefits. Proper roll management is crucial to reducing production costs and extending roll service life.

Special personnel are 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 with Rolls During Production

2.1 Irregular falling blocks in the centre of the rolling groove

There are too many chemical components containing copper in the central parts of the rolling groove. This easily leads to irregular block falling. When the central part of the rolling groove is heated, a large amount of copper may flow continuously on its inner wall. If copper forms a low-temperature melting point, it greatly improves its thermoplastic performance, leading to many cracks on the inner layer’s surface. As copper flows, the cracks expand and extend, causing irregular falling blocks.

For hot-rolled narrow strip steel, the lack of daily supervision means the finished product adjuster controls the rolling volume of each groove, resulting in inconsistent wear. During roll repair, cracks in individual grooves are not completely turned. Microscopic cracks grow rapidly and connect when used again, leading to macroscopic cracks and the loss of material 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 operation, poor cooling effect, technical points of rolling material production, and quality problems of the roll itself. 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, and 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.

Improving cooling water pipelines and using frequency conversion motors for pumps ensure stable pressure and temperature control.

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

The rolling force of the rolling mill changes due to factors like entrance thickness, tension, friction coefficient, and deformation resistance. This affects the deformation curve and exit thickness. The Automatic Gauge Control (AGC) uses model algorithms to control strip thickness, rolling equipment conditions, and external disturbances. It sets the rolling mill’s pressure, speed, and roll gap to ensure thickness deviation is reasonable. Typically, AGC uses a hydraulic press-down system for high precision and speed. Accurate slab thickness control impacts rolling mill quality. The number of hydraulic QGC equipment affects precision. For steel wire at the No. 6 rolling mill, rough rolling thickness is 27 to 2.7 mm with a 10% deviation rate.

4.6 Avoid improper operation

In the production process, roll damage caused by improper use is also very common. Preventing direct guide and roll contact, ensuring proper cooling water use, and maintaining material shape and blackhead removal prevent roll damage and ensure smooth production.

4.7 The amount of restoration should be in place

Maintaining stable rolling groove output without redundant cracks prevents production issues and maintains product quality. Strictly following rolling regulations, such as ensuring Q215 hot-rolled narrow strip steel roll turning reaches 600mm, is essential.

 

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