Description：The work roll is a part of the roll. The work roll in the roll of the rolling mill is mainly subject to high temperature, so it is required to have good heat resistance, wear resistance and high hardness.
Keyword：work rolls，chilled rolls，rolling mill rolls
What is a work rolls？
The work roll is a part of the roll. The work roll in the roll of the rolling mill is mainly subject to high temperature, so it is required to have good heat resistance, wear resistance and high hardness.
What are the requirements for the material of cold-rolled work rolls? How is its heat treatment carried out?
For the material of cold-rolled work rolls, it is mainly required to have high and uniform hardness, and to have good wear resistance and crack resistance. Therefore, its surface hardness must meet the standard.
The heat treatment of work rolls is mainly quenching, generally there are two types of induction surface hardening and overall quenching.
The development trend of cold rolled work rolls:
Cold rolled sheet (including galvanized, tinned, coated and enamelled steel sheets) is widely used in chemical industry, Light industry, automobile, machinery, construction, hydropower, petroleum, coal, railway and other departments. Rolls are necessary and very important equipment components for cold-rolled sheet production. The increase in demand for cold-rolled sheets has also led to a significant increase in roll manufacturers. With the gradual improvement of numerical control, modernization, automation and continuity of rolling mills, higher and higher requirements are put forward for the quality of rolls.
The main performance requirements of cold rolled work roll products:
Work rolls strength
The deformation resistance during cold rolling is much higher than that during hot rolling. In the initial stage of biting, the surface of the cold-rolled work roll not only bears a pressure of more than 1×104MPa, but also bears the shear stress caused by the friction between the roll and the plate interface. In order to meet the harsh requirements during use, the tensile strength σb of the cold-rolled work roll is about 2×103MPa, and the internal structure must be uniform. Vacuum smelting, vacuum degassing, vacuum pouring and other technologies not only prevent the generation of oxides, but also improve the segregation of alloy elements in the ingot by controlling the injection temperature and injection speed, and improve the compactness of the structure and the uniformity of the composition. Electroslag remelting technology has brought the purity of steel into a new era. The addition of alloying elements such as molybdenum and vanadium enhances the strength of the cold-rolled roll steel, and at the same time improves the properties of the steel material for the cold-rolled work roll.
Work roll wear resistance
There are two main forms of wear of cold rolls during cold rolling: macroscopic wear in the radial direction and decrease in surface roughness. The amount of macroscopic wear determines the life of the cold roll. When the billet is changed from a steel ingot to a slab, especially when rolling various types of thin plates at high speed, the service life of the cold working roll will be greatly shortened due to the severe wear and tear in the cold rolling process, which has become an urgent problem. big problem. The wear problem of cold rolled work rolls has been discussed extensively and deeply: for example, the wear caused by aluminum oxides in cold rolled steel; Fine wear particles are produced, and the particles are oxidized to form abrasive particles, which directly lead to the wear of cold-rolled work rolls; during the cold-rolling process of cold-rolled work rolls in the hot zone, the roll surface will immediately generate oxides due to oxidation, and then by Oxides cause wear to the roll; when the cold roll is rolled at high speed, the surface roughness will gradually increase with the biting of the billet. Therefore, whether the roll surface can be regenerated determines the subsequent rolling service life of the cold-rolled work roll.
In summary, the measures to improve the wear resistance of cold rolls are as follows:
(1) Increase the content of chromium to improve the service life of cold rolls;
(2) Using surface treatment technology to improve rolling service life;
(3) High speed steel cold roll;
(4) Add Ti to improve wear resistance;
(5) Metal spray molding technology is adopted to improve wear resistance.
Anti-accident performance of work rolls
Anti-accident performance refers to the thermal cracking, anti-stripping and thermal shock resistance of cold rolls during use. With the development of modern rolling mills in the direction of high speed and high pressure, accidents such as steel jamming, overlapping rolling, and slipping during the rolling process of cold rolling mills will cause intensified friction on the surface of cold rolling rolls, resulting in instantaneous temperature rise. When the temperature is as high as 1000°C, it will cause sticking to steel. In general, the local surface temperature rise of the cold rolling roll causes local overheating and changes in the structure and properties. The cracks on the surface of the cold-rolled work roll are mainly caused by the joint action of thermal stress and structural stress caused by local overheating during the rolling process. The cracks generated by the cold-rolled work rolls in the previous rolling process must be thoroughly removed by grinding in time, otherwise large and deep spalling will be formed in the subsequent rolling process, causing early failure of the cold-rolled work rolls. This spalling in the form of microcrack propagation is the most important cause of early failure of cold-rolled work rolls during service.
Work rolls hardened layer depth
By increasing the depth of the hardened layer of the cold roll, the actual diameter of the roll can be increased, and at the same time, the service life of the cold roll can be effectively extended; the use cost of the cold roll can be effectively reduced due to the reduction of the number of times of re-quenching. When the chromium content in steel for cold rolling rolls increases from 2% to 3%, the depth of hardened layer of cold rolling work rolls will also increase from 10mm to 15mm. In order to improve the hardness of the cold-rolled work rolls, we can add an appropriate amount of nickel element to the traditional chromium-based forged steel cold-rolled work rolls to increase the hardness of the cold-rolled rolls and increase the depth of the hardened layer by 7~8mm. By the mid-1980s, liquid nitrogen cryogenic treatment and dual-frequency induction heating technology were widely used, and the content of alloying element chromium was increased from 3% to 5%. Not only the depth of the hardened layer will exceed 50mm, but the initial surface hardness value of the cold roll will also increase. Will be as high as 100(HSD).
Material of cold rolled work rolls
High chrome cold roll
By increasing the content of the Cr element, the hardenability of the cold-rolled work roll material can be improved. Mitsubishi Steel Corporation has developed a new type of high-chromium forged steel cold roll material. The test results show that the wear resistance of the cold roll with a Cr content of 10% is twice as high as that of a cold roll with a Cr content of 3%. When the Cr element content is further increased to 18%, the wear resistance of the cold rolling roll will also be improved accordingly. By adjusting the balance of carbon and chromium, not only can a higher hardness be obtained at a lower quenching temperature, but also the sensitivity to fracture of the roll material can be reduced, and the occurrence of roll breakage accidents can be reduced. The cold rolled roll has a strong ability to resist roll breakage and good roughness retention, and can be adapted to rolling under harsh working conditions such as high rolling force and high product precision.
High alloyed improved cold rolll
In order to improve the mechanical properties and performance of the new cold-rolled work rolls, the material of the 5% Cr cold-rolled work rolls has been improved by changing the percentage content of elements such as Mo and V or adding an appropriate amount of strong carbide-forming elements such as Ti, Nb, etc., can form a new Cr5 series roll material, and the wear resistance and thermal shock strength of the new Cr5 series material cold roll can be significantly improved.
Nitrogen can strengthen the mechanical properties of the roll. It has a strong affinity with alloying elements such as chromium, vanadium, titanium, and niobium in steel, and can form stable nitrides that exist in steel in different forms. Nitrides are more likely to form fine and dispersed second phases, inhibit the growth of austenite grains, have a significant effect of refining grains, and significantly improve the mechanical properties of the roll material such as strength and hardness. Therefore, we can improve the comprehensive performance of cold-rolled work rolls by controlling the nitrogen content in cold-rolled roll materials, which also provides an important direction for the development of roll materials.
Forged semi-high-speed steel cold rolls
During the actual use of cold-rolled work rolls, production accidents such as steel jamming, sticking to steel, and strip breakage may occur, which will cause changes in the local structure and hardness of the cold-rolled work rolls made of Cr3 and Cr5 series, resulting in changes in the roll surface. Microcracks or massive spalling will occur locally, which will reduce the anti-accident ability of cold rolling rolls. The material of the forged semi-high-speed steel cold roll is developed on the basis of the material of the 5%Cr cold-rolled work roll. Due to the addition of a large amount of strong carbide-forming elements Cr, Mo, and V, the type of carbide obtained after quenching is mainly M7C3 type. (HV2500) and MC type (HV3000) are mainly carbides, and the existence of high-hardness carbides ensures that forged semi-high-speed steel rolls have good wear resistance and high accident resistance.
High speed steel cold rolls
In 1988, high-speed steel rolls were first put into practical production and application in Japan, and then the United States also began to use high-speed steel rolls in the early 1990s. Compared with Japan and the United States, the practical application of high-speed steel rolls in Europe started late but developed rapidly. At present, my country’s large-scale high-speed steel roll manufacturing is still in its infancy, and key technologies have not yet been broken through. High-speed steel rolls are only used in a few large iron and steel enterprises headed by Baosteel. Carrying out research on the manufacturing and application technology of high-speed steel rolls can lay the foundation for the popularization and application of new high-speed steel rolls, so it has very important practical value.
At present, high-speed steel cold rolls have been used in dozens of steel companies such as Inland Steel Company of the United States, Dofasco Company of Canada, Akers Company of Sweden, and Nippon Steel Company, and the use effect is good. The typical enterprise that adopts electroslag remelting + traditional forging process to produce high-speed steel cold rolls is Akers Company, and the typical enterprise that adopts CPC method to produce high-speed steel cold rolls is Nippon Steel Corporation. The high-speed steel roll material contains a large amount of carbide-forming elements, and the carbide types are mainly MC and M2C, which will greatly improve the strength, surface roughness, and wear resistance of cold rolls. Therefore, the use of high-speed steel rolls can significantly reduce roll consumption, reduce the number of roll changes, and help reduce rolling costs and improve plate quality. Due to the high carbon and alloy content of high-speed steel cold rolling rolls, its production process is more complicated. At present, there are no reliable smelting, forging, and heat treatment methods, so its processing technology is poor, and the yield is also low. This is also a problem that needs to be solved in the future key technologies.