The main links in the cold roll manufacturing process are smelting, forging, heat treatment, processing, and inspection.
During the working process, the cold rolls have to bear a lot of rolling stress, coupled with problems such as weld seams, inclusions, and edge cracks in the rolled pieces, it is easy to cause instantaneous high temperatures, which makes the working rolls subject to strong thermal shock, resulting in cracks, sticking, and peeling even scrapped. Therefore, cold rolls must have the ability to resist cracking and spalling caused by bending stress, torsional stress, and shear stress, as well as high wear resistance, high contact fatigue strength, high fracture toughness, and thermal shock strength. . So how to improve the service life of rolls has always been a major problem faced by the roll manufacturing industry.
The service life of cold rolls not only depends on the manufacturing quality but also is closely related to maintenance during use. If it is not used properly, the use effect will change greatly.
Fatigue spalling is the most common form of failure in roll use. It originates from the fatigue cracks on the surface of the roll. If the cracks cannot be completely removed before being put on the machine, they will fatigue and expand in the hardened layer to form a typical fatigue zone, and then fatigue spalling will occur. Prevention of such accidents should run through the entire process of roll use and operation. Users should understand the origin of these cracks and preventive measures.
1. Roll storage and preheating
The microstructure of the roll is very sensitive to sudden changes in temperature, so it is recommended to store the roll in an environment with a suitable temperature and use sleepers as a pad to keep the roll clean.
The roll cannot start suddenly under a strong load, nor can it stop running suddenly. The reasonable operation is: the roll runs under light load for a period of time, and gradually loads and starts. It is beneficial for the roll to be slightly preheated not exceeding 80 °C, but the process is slow, and it is absolutely forbidden to contact the roll with flame.
Experienced grinders know that when grinding high-hardness rolls, careless operations such as sudden feed, large feed rate, and improper selection of grinding wheels will cause grinding cracks, resulting in roll peeling.
3. Work hardening
During rolling, the contact fatigue stress on the surface of the roll body produces work hardening, which accumulates continuously, and cracks appear. Therefore, regular grinding is required to remove the fatigue layer.
4. Rolling accident
The surface cracks of the roll also originate from the surface damage caused by rolling accidents, such as: slipping, overloading, sticking steel, broken strip, etc. Therefore, for the normal operation of the rolling mill, we recommend that users establish strict rolling operating procedures and equipment regulations.
5. Roll management
The user should establish a roll use file to record the data during the use of the roll, such as accidents, grinding, use stand position, rolling volume, turnover times, roll maintenance, etc., so as to facilitate evaluation and feedback to the manufacturer for joint research and improve the roll. life.
Flaw detection standards for cold rolls
The non-destructive testing of rolls is usually carried out with ultrasonic flaw detectors. In the cold roll flaw detection standard, the roll body is usually divided into the surface area, the central area, and the middle area according to the force condition, and then the critical size of the defects allowed to exist in each area is stipulated respectively according to the principle of fracture mechanics. Flaw detection workers should have knowledge of roll manufacturing in order to determine the nature of defects, and also have knowledge of roll used in order to estimate whether these defects may cause service damage under specific use conditions.