Describe the working principle of magnetic particle inspection.
Answer: Use a magnetic field to locally magnetize the roll, and magnetic flux leakage will occur at the position where the roll has cracks. The magnetic powder coated on the roll surface will gather at the crack, so that the crack on the roll surface can be detected.
What is the working principle of eddy current testing?
Answer: The alternating current is used to generate an alternating magnetic field on the surface of the roll. The alternating magnetic field will generate a special reflection signal at the defect of the roll. The signal is decomposed and processed to determine the type and depth of the defect on the roll surface. This method is suitable for surface flaw detection.
What are the performance characteristics of ultrasound?
① When the ultrasonic wave propagates in the medium, it has the characteristic of reflection on the interface of different mediums. When the platform encounters a defect, and the size of the defect is equal to or greater than the wavelength of the ultrasonic wave, the ultrasonic wave will be reflected back on the defect surface, and the flaw detector can display the reflected wave. If the size of the defect is smaller than the wavelength, the sound wave will be diffracted and cannot be reflected.
② The directionality of ultrasonic waves is good. The higher the frequency, the better the directivity. It radiates into the medium with a very narrow beam, and it is easy to determine the location of the defect.
③ Ultrasonic transmission energy is large. For example, the energy transmitted by an ultrasonic wave with a frequency of 1 million Hz is equivalent to 1 million times that of a sound wave with the same amplitude and a frequency of 1000 Hz.
What is the function of ultrasonic testing of mill rolls?
Ultrasonic flaw detection is divided into surface glass flaw detection and internal flaw detection. The main purpose of surface wave flaw detection is to find surface defects and prevent early peeling of rolls, while internal flaw detection can comprehensively judge the metallurgical quality of rolls, detect internal shrinkage cavities, porosity, cracks, Defects such as slag inclusions can prevent accidents such as roll bursting. First of all, this is very important in the manufacturing process of rolls. Whether the roll is manufactured or used, it is a very complicated process of stress. When manufacturing rolls, quenching, tempering, heating and cooling , High-temperature heating and rapid cooling during quenching will generate huge stress inside the roll, and the roll after quenching and low-temperature tempering still retains high residual stress inside. Some defects inside the roll, triggered by these internal stresses, are prone to expansion and cracking. Severe cracking (burst) poses a great threat to the safety of personnel and equipment in the roll manufacturing process. Therefore, in the roll manufacturing process, ultrasonic flaw detection must be relied on to find and control defects in steel. In the cold rolling field, in addition to using ultrasonic waves to detect internal defects during roll acceptance, more surface waves are used to detect the surface defects of the rolls to prevent the rolls from being damaged on the machine and causing peeling.
How to prevent mill roll marks?
Preventing or delaying the occurrence of roll marks is a very important topic at the cold rolling site, and involves a wide range of content. Such as the cleanliness of the rolling site, the cleanliness of the rolling oil, the surface quality of the incoming material, the roll change system and so on. Generally speaking, for cold rolling mills, it is necessary to achieve clean rolling and reduce the introduction of foreign matter. By improving the performance of the roll, it is also possible to better prevent or reduce the probability of roll printing. In recent years, major roll manufacturers at home and abroad have introduced roll-resistant rolls. The main technical improvement is to improve the roll material and manufacturing process, so that the hardness value of the roll matrix can be improved.
What are the factors affecting the wear resistance of the mill roll, and how to improve the wear resistance of the roll?
We know that the matrix metallographic structure of conventional cold roll materials is composed of carbide, tempered martensite and a small amount of retained austenite. The highest hardness of tempered martensite is about 750-850HV, while the hardness of carbide can be changed according to its alloying elements, and the hardness distribution is 900-2500HV, or even higher. With the increase of carbide hardness and the optimization of the number and distribution, the wear resistance of the roll can be improved. Therefore, the first way to effectively improve the wear resistance of the roll is to select the appropriate chemical composition of the material. The chemical composition design must ensure that the steel can form high-hardness carbides; the second is to choose a reasonable heat treatment process so that the carbides in the steel can be distributed finely and evenly; the third is to ensure a certain matrix strength. Improve the wear resistance of the substrate.
What are the commonly used materials for backup rollers?
The traditional support roll material is mostly 9CR2Mo, with a carbon content of about 0.9%, a Cr content of 2%, and a small amount of Mo. This material is a typical cold-rolled work roll material, which can obtain high surface hardness and certain wear resistance after heating and severe cooling. In the past, there was no special back-up roll material for the production of forged steel rolls, so this material was often used as a back-up roll after quenching and tempering to medium-low hardness. However, with the improvement of rolling mill design requirements and changes in rolling conditions, the shortcomings of this material began to appear. These changes include:
1. Deeper hardened layer depth requirements. Modern rolling mills often require that the unilateral depth of the work roll is more than 25mm, at most 50mm, and the unilateral depth of the backup roll is more than 35mm, at most 75mm. 9Cr2Mo is affected by the content of its own alloy elements, and its hardenability cannot meet such requirements.
2. Changes in working conditions require special materials to adapt. Modern rolling mills have higher and higher requirements for rolls. Work rolls require wear resistance, accident resistance, roll mark resistance, and high uniformity, while the surface of backup rolls requires fatigue resistance and wear resistance become more important. Due to the high carbon content of 9Cr2Mo, there are more carbides in the matrix. If these carbides are not handled properly and the distribution state is not ideal, the fatigue strength of the matrix will be reduced. In addition, the Cr/C ratio in the material determines that the carbide of the material is M3C type, the hardness is low (about 900Hv), and the contribution to the wear resistance of the roll is not enough.
Based on the above situation, the roll manufacturer will work with the design and user to improve the material. The MC3 and MC5 materials of the work rolls have become the mainstream materials of the cold-rolled work rolls; the 70Cr3Mo series have also become the preferred materials of the back-up rolls. In some high-speed and heavy-duty rolling mills, the backup roll material even uses 50Cr4MoV and 45Cr5MoV series materials. The overall development trend is a higher amount of alloying elements (mainly Cr, but also Ni and Mo adjustments), and a lower carbon content. Of course, there are also corresponding process improvements from the manufacturer, so as to achieve deeper hardened layer depth, better fatigue resistance and wear resistance, and uniformity of surface hardness.
What is the development direction of cold rolling?
The development of cold rolling rolls is driven by the pull of the cold rolling industry and the promotion of roll manufacturing related technologies. From the perspective of the demand of the cold rolling industry, the requirements for increasing wear resistance, roughness retention performance, accident resistance performance, uniformity and other indicators are getting higher and higher. The purpose is to achieve free rolling, rolling high-grade plates, reduce roll consumption, etc. Driven by these requirements, roll manufacturing enterprises are also making continuous efforts to improve and innovate roll manufacturing technology. The specific performance is
(1) The alloy composition of forged steel rolls is increasing continuously. The application of 5%Cr, 8%Cr, semi-high-speed steel, high-speed steel and other high-alloy steels on cold rolls is a trend. At the same time, the material of the roll will maintain a variety to meet the needs of different rolling conditions.
(2) The application of new roll blank composite manufacturing technology. In order to meet the requirements of high wear resistance on the surface of the roll and high comprehensive mechanical properties of the core. Some manufacturers have been exploring the trial of electroslag casting, CPC, OSPRAY and other new surface composite technologies in the preparation of roll blanks.
(3) The application of new roll heat treatment and processing technology. The application of large-scale CNC machine tools, the trial of computer-automatically controlled heating furnaces and quenching equipment can increase the manufacturing accuracy and surface uniformity of rolls, and meet the needs of high-end rolling mills.
