The main failure modes of Liugang Zhongjin's 950 mm six-high six-stand stainless steel narrow strip cold tandem roll are burst rolls (flaking), fractures, cracks, etc. This paper analyzes the causes of roll failure, and the results show that internal quality defects of the roll, unreasonable rolling process, and improper use of roll management are the main causes of roll failure. Through the implementation of measures such as strengthening the non-destructive inspection during the use of the rolls, optimizing the rolling process, strengthening the quality monitoring of raw materials, and standardizing the management of the use of rolls, the monthly strip breakage rate of stainless steel narrow strip cold tandem rolling has been reduced from 1.59% to 0.53%, and the number of roll failures has been reduced from 19 sticks/month is reduced to a stable control of 2~5 sticks/month, which effectively improves the production efficiency of the tandem mill and reduces production costs.
As a large-scale tool in steel rolling production, the performance and quality of the roll directly determine the output of the rolling mill and the quality of the product. Therefore, the use and management of rolls is very important in cold tandem rolling production. Liugang Zhongjin's 950mm six-high six-stand cold tandem cold rolling mill is the fourth domestic stainless steel narrow strip cold tandem rolling production line, and it is currently one of the high-level stainless steel narrow strip cold rolling mills in China. Since the start of production in July 2018, roll bursts (flaking), broken rolls, cracks and other accidents have frequently occurred during use. Therefore, analyzing the causes of roll failure and taking corresponding specific measures to improve the use and management of rolls and reduce the accident rate of rolls is of great significance for stabilizing production and reducing production costs.
The main forms of cold roll failure
The roll diameter of Liugang Zhongjin's 950mm six-high six-stand stainless steel narrow strip cold tandem rolling mill is small. The main technical parameters of the rolls are shown in Table 1. Since the start of production, roll scrap accidents have occurred frequently due to roll material, use management, rolling technology and other factors. Among them, the number of work rolls due to roll burst (flaking), roll breakage, and crack failure has been 54, 16, and 8, respectively. F1~F4 work rolls; 5 middle rolls are scrapped due to burst rolls; 7 support rolls are scrapped due to cracks and broken rolls. It can be seen that the main forms of cold roll failure are bursting (flaking), broken rolls, and cracks. The typical failure macroscopic morphology is shown in Figure 1.
Analysis on the Causes of Roll Failure
Roll material and shape design
Sampling and analysis of rolls due to spalling accidents, the source of fatigue, fatigue fracture zone, instantaneous fracture zone, and metallographic sample preparation position of the roll spalling are shown in Figure 2. After machining, the sample was corroded by a 4% nitric acid alcohol solution. The inclusions in the sample were observed and analyzed with a scanning electron microscope energy spectrometer. Granular Al2O3 inclusions and TiN precipitates were found inside the sample and near the cracks. The maximum size of the precipitates is 74.5 μm, and the maximum size of TiN precipitates is 18.3 μm. The micro morphology of sample inclusions, precipitates and internal cracks is shown in Figure 3.
It can be seen from Figure 3 that there are Al2O3 inclusions in the internal cracks of the roll, which may be caused by the roll manufacturer not using the electroslag remelting process during the smelting of the roll blank. When the roll is constantly subjected to alternating tensile and compressive stress during use on the machine, the separation of the inclusions and the matrix of the roll causes the discontinuity of the metal structure, resulting in internal cracks. As the crack extends, it will eventually cause the roll to burst (peel off) or even break the roll. In addition, the roll shape design and processing are improper, and the transition fillet design at the part where the cross-sectional size changes is unreasonable, resulting in stress concentration and roll breaking accidents. In the actual production process, the support rolls were broken due to the unreasonable design of the transition fillet when they were subjected to high rolling pressure.
Rolling process and raw material quality
The cold roll is subjected to strong rolling pressure, bending moment, shear stress and other internal and external forces during rolling, resulting in cracks in the roll and peeling of the working surface, and the welds, inclusions, edge cracks, etc. of the rolled pieces can easily lead to instantaneous high temperatures, resulting in Accelerated wear of rolls, fatigue damage, etc. [3]. Therefore, the rolling process and the quality of raw materials have a significant impact on the service life of the roll.
(1) The degree of cold working hardening of stainless steel is relatively high. With the increase of deformation, the strength of the rolled piece increases rapidly, and the rolling force increases. When the total reduction is 67.5%, the tensile strength of the rolled piece is as high as 2000MPa. When the local stress of the roll exceeds the strength limit of the roll material, it will cause plastic deformation of the roll, and then cracks. With the action of the alternating tension and compression stress, the cracks propagate and cause the roll to peel off or break.
(2) The rolling tension control fluctuates or slips and causes the belt to break, so that the roll surface contacts the broken belt, and is subjected to the combined action of local overload thermal shock, temperature rise, and sliding friction, which causes the roll surface to produce different degrees of burns, resulting in the roll surface Flaking.
(3) Severe wedge shape or uneven hardness in the width direction of the raw material leads to the difference in the rolling force between the operating side of the rolling mill and the transmission side or the local overload thermal shock during the continuous rolling process, causing the pressure on the side with greater rolling force to exceed the strength limit of the roll material. The stress is concentrated, the edge of the roll is crushed and peeled, or the surface of the roll is peeled off due to the thermal shock of overload at the position with higher hardness.
(4) Raw material inclusions, edge cracks and poor weld quality lead to strip breakage during the continuous rolling process, and thermal shock causes the roll surface to stick to steel, cracks or peels.
Roll usage management
In the use and maintenance of cold rolling rolls, it is necessary to focus on several aspects such as roll change cycle, grinding amount, maintenance of defective rolls, stress recovery, and evaluation mechanism. In the early stage of production, the extensive management of roll use resulted in a high roll accident rate. The main reasons were:
(1) New rolls entering the factory, production accident rolls, etc., are used on the machine without flaw detection after being ground;
(2) The roll change cycle is unscientific, and the service cycle is too long. Under the action of the cycle stress, the work hardened layer due to the fatigue mechanism deepens, which increases the brittleness of the material, thereby increasing the possibility of cracks and spalling;
(3) There is no professional roll disassembly and assembly equipment, and the disassembly and assembly of the roll are completely completed by hand, which causes wear of the roll, chock, seal ring, etc., resulting in poor assembly quality and accuracy of the roll, and even burnout and jamming of the roll bearing, which causes the roll to twist. Break
(4) The shortage of spare parts for the rollers caused the lower rollers to be ground on the upper grinder without sufficient time for stress release. When the temperature of the roller surface is high, the grinding will cause burns and deteriorate the stress state of the roller surface, which is not conducive to restoring the normal stress. , In severe cases, it will cause cracks or peeling when grinding on the grinder;
(5) Insufficient staff training has led to unclear roll grinding, recording, inspection and responsibility for accidents. It is necessary to formulate defective roller handling specifications and inspection standards to prevent defective rollers from being used on the machine.
Improvement measures and effects
Increase the intensity of roll inspection and inspection
Due to the small roll diameter, traditional large roll companies are not very enthusiastic about production. The rolls currently in use are produced by smaller roll manufacturers in China, and the quality of the rolls fluctuates greatly. During the initial stage of operation of the stainless steel narrow strip cold tandem mill, there were many accidents of spalling, roll breaking and cracking caused by roll quality problems. In order to avoid the use of unqualified rolls on the machine, it is required to conduct a flaw detection inspection on each new roll and the accident roll before it is placed on the machine. The rolls that are normally planned to be replaced are randomly checked. Using the existing detection method combining ultrasonic flaw detection and magnetic particle flaw detection, using the good penetration and directivity of ultrasonic waves, it can locate, quantify and qualitate the internal defects of the roll. At the same time, magnetic particle flaw detection assists in detecting the surface quality of the roll. The eddy current flaw detection of the grinding machine found that the rolls with defects on the surface and inside were refused to be used on the machine, and the roll non-destructive testing management system was formulated.
Strengthen the quality control of raw materials
At this stage, the quality of the upstream raw materials of stainless steel narrow strip fluctuates greatly. Quality problems such as raw material edge cracks, inclusions, and peeling cause frequent strip breakage in cold continuous rolling, resulting in roll burning, steel sticking, cracks, and spalling. Therefore, it is necessary to strengthen raw material quality monitoring. The strip steel with the risk of strip breakage must be dealt with according to corresponding measures; Optimize the rolling process, reasonably allocate the reduction of each frame of F1-F6, and control the total reduction rate between 50% and 70%; reduce the hardness of the work roll Reduce from 90~93HSD to 86~90HSD; adjust the rolling force difference between the operating side and the transmission side of the rolling mill and the tension fluctuation range between the stands to prevent unilateral waves in the shape control, prevent the strip from breaking, causing the roll to stick to the steel, Cracks, even peeling.
Standardize roll use management system
Formulate a standardized roll management system:
(1) A reasonable roll change cycle can not only give full play to the use performance of the rolls, but also ensure the rolling quality;
(2) In order to ensure the release of the residual stress of the lower rolls and restore the normal stress, a certain number of spare parts for the rolls must be guaranteed. Normally, the rolls of the off-machine cannot be arranged for grinding immediately. The work rolls, intermediate rolls, and backup rolls are required to be off-machine for 12, 24, and 48 hours before being ground;
(3) Increase the roll disassembly and assembly platform to improve the quality and accuracy of roll disassembly and assembly; (4) Establish a roll evaluation mechanism. Number the rolls, quickly identify them, and manage roll files to track the usage of each roll.
Improved effect
By strengthening the non-destructive inspection during the use of the rolls, optimizing the rolling process, strengthening the quality monitoring of the raw materials, and standardizing the use and management of the rolls, the monthly stripping rate of 950 mm cold tandem rolling has been reduced from the highest 1.59% to 0.53%, and the monthly roll failure count has been reduced from the highest 19 The support is reduced to a stable control at 2~5, which effectively improves the production efficiency of the continuous rolling mill and reduces the production cost.
Concluding remarks
Spalling, broken rolls, and cracks are the main forms of roll failure. During roll use, non-destructive flaw detection can be strengthened, rolling process and raw material quality monitoring are optimized, roll use management is standardized, and rolls with quality problems can be avoided from unintended use and production can be reduced. Accidents, reduce the number of roll failures, stabilize production, and reduce production costs.
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