In recent years, through optimizing the steel rolling process and improving the performance of the rolling die equipment, the hardness, heat resistance, and impact resistance of the rolling dies have been effectively improved and significantly enhanced. This has not only reduced the die wear but also prolonged the lifespan of the rolling dies. This article focuses on analyzing aspects such as irregular detachment in the rolling groove, cracking of the die ring, and the appearance of pockmarks in the die groove of hot-rolled narrow strip steel. An improved die cooling system is implemented to adapt to the current conditions of the production line. By using this device and water supply system, the cooling effect of the die track can be effectively improved, and the width of the die ring can be appropriately increased to avoid cracks, detachment, and the selection of suitable die materials.
This system not only corrects the uneven detachment in the middle of the die track but also solves problems such as die ring cracking, pockmarks, and other issues, and can improve the efficient operation efficiency of the rolling dies.
The die is a very important piece of equipment. It directly contacts the steel billet in the rolling machine and deforms the steel billet through certain pressure during the rolling process, thereby achieving different steel quality standards. Therefore, the fatigue strength and wear resistance of the die are required to be high. Especially in recent years, as the annual increase in the demand for steel products in the market has led to excessive die wear in the steel rolling production process, thereby increasing the production input cost. Under such circumstances, how to increase the steel rolling production rate, reduce the frequency of die replacement, and extend the lifespan of the die has become a focus issue of concern in the metallurgical industry. The die operates under high temperature, high pressure, alternating cold and hot conditions, and iron oxide scale. Under the mechanical stress, friction, thermal stress, and impact load of each base plate, it is prone to wear, cracking, and peeling, which greatly shortens the lifespan of the die. The die is an important production link of the rolling machine, and its procurement cost is also relatively high. Die consumption is an important economic indicator, and it is an important indicator of production costs.
1. The Importance of Extending the Lifespan of Rolling Dies
The die is an important component. It can undergo continuous plastic deformation of metal in the rolling machine and has high consumption. It accounts for a large part of the production cost. Through a complete die management system, managers can clearly know the condition and trajectory of each die, enabling the correct matching and use of dies, maximizing the utilization rate of the dies, and formulating appropriate grinding quantities. This can not only reduce die fatigue, allowing the die to meet the standards for normal production of the machine, increase the lifespan of the die, reduce abnormal die replacement, and improve the production efficiency of the rolling line, and increase production benefits. Whether the die management system is complete, the management level is high, etc., are all key factors in reducing the production cost of the rolling line and extending the lifespan of the die.
A dedicated person is needed to manage the dies, establish a die management system, and keep records of the entry into the warehouse acceptance, matching use, die cracks, abnormal unloading, etc. Through such operations, the usage trajectory and status of each die can be managed and controlled, which is crucial for reducing die consumption and increasing the lifespan of the die.
2. Problems of Dies in Production
2.1 Irregular Detachment in the Center of the Rolling Groove
In the center of the rolling groove, all parts containing copper have an excessive chemical composition of copper, which is very likely to cause irregular detachment problems. Especially when heating the center of the entire rolling groove, a large amount of copper will continuously flow back and forth on the inner wall surface of the entire rolling groove. Once the copper forms a low-temperature melting point, it will directly significantly reduce its thermoplastic performance, and then also cause many cracks on the inner surface. As the copper flows back and forth, the cracks will continue to expand and extend, to a certain extent, causing irregular detachment problems. Take hot-rolled narrow strip steel as an example. Due to inadequate daily supervision, the rolling volume of each rolling groove is controlled by the finished product adjustment worker independently. This results in inconsistent wear of each rolling groove, and during the repair of the rolling rolls, some rolling grooves have not had all the cracks removed. Due to the existence of micro-cracks, when the rolls are re-uploaded for use, the micro-cracks rapidly grow and connect, ultimately causing macro-cracks to form, resulting in the loss of material from the rolling grooves.
2.2 Roller Ring Breakage
The roller ring rolls are subjected to 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 cross-section. During the rolling process, the roller ring experiences a large tangential tensile stress on its inner diameter, and using a single material roller ring is not conducive to increasing the service life of the rolls. Additionally, due to the increase in local stress in the roller ring and the combined effect of other stresses, it is prone to cracking and causing the roller ring to break. Understanding roller ring breakage causes is essential for improving rolling die lifespan.
2.3 Rolling Groove Blisters
Blisters on the rolling groove are a common surface defect, appearing rough and uneven, also known as “blotches”. Most are continuous, but some are local or scattered. This defect is allowed on the rolling groove, but its depth must not exceed the thickness deviation of the product. The causes of rolling groove blisters include:
(1) Wear of the finished product holes or previous holes, or presence of damaged iron oxide.
(2) After being pressed onto the surface of the rolled piece, the broken iron oxide is stripped off.
(3) The rolls are eroded.
(4) The surface of the steel billet is severely oxidized during heating.
2.4 Roll Breakage
During the rolling process, due to impacts, tailing, and steel clamping, cracks and soft spots form on the surface and inside of the rolls. These cracks and soft spots seriously affect the normal production of the rolling line and the service life of the rolls. Severe cracks can cause the rolls to fall off prematurely and end their service life prematurely. Roll breakage prevention is therefore a key part of extending rolling die lifespan.
3. Cause Analysis
The causes of these problems include improper human operation, poor cooling effect, technical key points of rolled material production, and quality issues of the rolls themselves. Improper or inappropriate cooling can generate large temperature differences on the roller surface, accelerating the thermal stress that causes the rolls to fall off. Additionally, excessively high temperatures of the rolls can also affect their strength and wear resistance, leading to problems such as cracking, cracking off, burning, and even breaking. In production, if we ignore the characteristics of the rolled material and the production process, we may choose an inappropriate roll material. Human factors such as roll curling or piled-up steel can also cause cracking off, loss of material, and roll breakage.
In actual production operations, due to the presence of SiO2, Al2O3, or silicate and other brittle inclusions in the metallurgical steel material, the presence of these inclusions has a significant impact on the service life of the rolls, and the degree of impact varies depending on the quantity, size, and type of inclusions. Generally, the more inclusions and the larger the size, the greater the hazard. Among them, the inclusions with sharp edges have the greatest hazard.
4. Process Measures
4.1 Improvement of Roll Cooling Device
We can improve the roll cooling device to use cooling water more efficiently. This enhances cooling and extends roll service life. The rolling mill generates a large amount of heat during rolling. Therefore, we must cool the rolls to control the replacement cycle and service life. We also must ensure the temperature after processing. The rolls have two cooling water tanks: primary and secondary. The primary cooling water is sprayed axially through the slit into the rolling zone. It lubricates and cools the copper tubes. The secondary cooling quickly reduces the rolled tube temperature. It prevents air from entering the rolling sleeve and prevents copper tube oxidation.The cooling device of the roll sprays water to cool the roll.
During rolling, the main drive speed is above 1300 rpm, and the secondary drive (roll) is above 700 rpm. The rolls operate at such high speeds that strict control of the flow and pressure of the cooling water is necessary. The cooling water must be continuously and fully cooled. If the roll temperature is too high, it must be replaced immediately to prevent thermal fatigue cracking. In actual production, to ensure that the roll temperature does not exceed a certain limit and to maintain stability of the temperature, the cooling speed of the rolls must be above 3500 L/h, and the pressure of the water pump should be below 0.8 MPa.
From the original square box-shaped perforated structure to the solid cylindrical nozzle, two rows of elliptical water channels are added at the cutting wedge area. The width of the water channels covering the cutting wedge is 5-8 mm. To ensure that the water at the cutting wedge area is three times that of other areas, the cooling degree of the cutting wedge area is strengthened, and the service life of the cutting wedge-shaped area of the rolling groove is improved.
4.2 Optimization of Cooling Water Parameters and Modification of the Water Supply System
Select an appropriate cooling water temperature. If it is too high, it will accelerate the breakage of the rolls and reduce the re-crystallization effect of the rolled tube, affecting the quality of the product. If it is extremely low or extremely high, it will accelerate fatigue. Therefore, in production, the cooling water temperature of the rolls should be 40-60℃.
We improved the roll cooling water pipeline. The water supply changed from low pressure to medium pressure. The water pump motor now uses a variable frequency motor. It achieves a water pressure of 0.8 MPa. The water pressure adjusts automatically. Cooling water parameters optimization is therefore a key measure for improving rolling die lifespan.
4.3 Optimization of Hole Pattern Configuration
Due to the unsatisfactory cooling effect of the cooling water, for the hot rolled narrow strip φ550, on the basis of the original hole pattern, the hole pattern of the I-frame rolling mill adopts the principle of forced widening. K1 steel has a relatively high temperature. We therefore set the forced widening passes on K3 and K4. The bottom of the forced widening groove is designed as a straight line. We also need to avoid wrinkles on the rolled piece surface. Wrinkles can cause unqualified finished product quality. We set the slope of the forced widening hole pattern at 17.7°. We adjust the width of the K3 and K4 rolls to 10.8 mm after forced widening on K6. This ensures no tailing occurs.
4.4 Optimization of Roll Material
Taking the commonly used Q215 type hot rolled narrow strip as an example, its initial rolling groove center spacing is 19.0 mm, and the width of the middle roll ring is 7.8 mm, which meets the theoretical design indicators. However, from the perspective of the overall fracture of the roll ring, the width is still very narrow, which is prone to cracks under fatigue conditions, eventually causing the roll ring to crack. Therefore, from the perspective of roll length, a 22.0 mm roll gap was designed, and the width of the middle roll ring was adjusted to 10.8 mm. Roll material optimization is therefore essential for improving rolling die lifespan.
4.5 Modification of Automatic Thickness Configuration
Due to the influence of factors such as entry thickness, tension, friction coefficient, and deformation resistance, the tensioning force of the rolling mill will also change, and the related deformation curve will have a certain impact on the exit thickness. The automatic thickness control (Automatic Gauge Control, AGC) mainly uses model algorithms to automatically control the thickness of the strip, the condition of the rolling equipment, and external disturbances, so as to automatically set the extrusion pressure, speed, and the size of the roll gap of the rolling mill, and ensure reasonable thickness deviation. Usually, AGC adopts a hydraulic press system, which has high precision and is relatively fast. The quality control of the rolling mill is related to the precise control of the slab thickness, and the number of hydraulic AGC equipment will have an impact on the precision of the rolling mill. For the 6th rolling mill of the steel wire finishing, the control thickness of the rough rolling is (27-2.7) mm, and the deviation rate is 10%.
4.6 Avoiding Improper Operation
In the production process, roll damage caused by improper operation is also very common. Therefore, first, it is necessary to prevent direct contact between the guide and the roll. Second, the roll cooling water switch must be strictly executed to prevent the head after replacement from being dry. Third, strengthen inspection to prevent problems with the roll drums. Once a roll drum is found to be faulty, the machine must be stopped immediately. The processing can only be stopped after the rolled material has cooled to room temperature. Fourth, strictly control the shape of the rolled material in accordance with the process regulations to avoid excessive material shapes in a single pass. Fifth, cut off the black heads at the head of the rolled piece cleanly and prohibit over-black steel.
4.7 The Repair Amount Should Be Adequate
To keep the output of each rolling groove relatively stable and prevent excess cracks, the rolling process must strictly follow the regulations. Excess cracks will greatly reduce the rolling volume of the rolling machine and significantly affect product quality.For example, the turning amount of the Q215 hot rolled narrow strip finished rolling roller must reach 600 mm. If some rolling grooves still cannot be completely eliminated, they should no longer be processed and used.
5. Conclusion
Taking the above measures greatly improves the service life of the roll drums, reduces the number of roll drum replacements, and decreases the labor intensity of the workers. Correcting the uneven blockage in the middle of the roll solves problems such as roll ring cracks and spots. The grooves caused by the cooling effect are almost zero, thereby improving the quality of the products. The average effective working efficiency has been improved. In short, how to improve rolling die lifespan depends on a combination of roll cooling device improvement, cooling water parameters optimization, hole pattern optimization, and roll material optimization.
E-mail: cykc@cykcgroup.com.cn