Performance requirements and material development trends of cold rolling work rolls

Cold-rolled steel sheets (including galvanized, tin-plated, coated and enameled steel sheets) are widely used in chemical industry, light industry, automobiles, machinery, construction, water and electricity, petroleum, coal, railways and other sectors, and are indispensable and important in the national economy. Steel varieties. Rolls are essential and very important equipment components for cold-rolled plate production. The increase in demand for cold-rolled plates has also led to a substantial increase in the number of roll production companies. However, the production capacity of high-end cold-rolled forged steel rolls is still very scarce. Due to the use of imported equipment, only a few roll companies can provide some high-end rolls, and the gap can still only be filled through imports. With the gradual improvement of numerical control, modernization, automation and continuity of rolling mills, the requirements for roll quality are becoming higher and higher.

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Main performance requirements for cold rolled work rolls products

①Roll 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 rolling work roll not only has to withstand a pressure exceeding 1×104MPa, but also withstands shear stress caused by the friction between the roll and the plate interface. In order to meet the harsh requirements of use, the tensile strength σb of the cold rolling work roll is approximately 2×103MPa, and the internal structure must be uniform. The emergence of vacuum smelting, vacuum degassing, vacuum pouring and other technologies not only prevents the generation of oxides, but also improves the segregation of alloy elements in the steel ingot by controlling the injection temperature and injection speed, and improves the compactness and composition uniformity of the structure. Electroslag remelting technology brings the purity of steel into a new era. The addition of molybdenum, vanadium and other alloying elements enhances the strength of cold rolling roll steel and improves various properties of cold rolling work roll steel materials.

②Wear resistance of rollers

There are two main forms of wear of cold rolls during the cold rolling process: 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 changes from an ingot to a slab, especially when rolling various types of thin plates at high speed, the service life of the cold rolling work rolls will be greatly shortened due to severe wear and tear during the cold rolling process. This has become a big problem that needs to be solved urgently. Many literatures have extensively and profoundly discussed the wear problem of cold rolling work rolls: for example, the wear caused by aluminum oxides in cold rolling roll steel. Under the action of large cyclic rolling stress, the roll surface of the cold rolling roll may be deformed, resulting in the production of fine wear particles, which are oxidized to form abrasive particles and directly cause wear of the cold rolling work rolls. During the cold rolling process of the cold rolling work roll in the hot zone, the roll surface will immediately generate oxides due to oxidation, and then the oxides will wear the roll. When the cold roll is rolled at high speed, the surface roughness will gradually increase as the billet bites in. Therefore, whether the roll surface can be regenerated determines the subsequent rolling life of the cold rolling work roll.

To sum up, 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) Use surface treatment technology to improve rolling life;

(3) High speed steel cold roll;

(4) Adding Ti improves wear resistance;

(5) Use metal spray molding technology to improve wear resistance.

③Anti-accident performance of rolls

Accident resistance refers to the resistance to thermal cracking, spalling and thermal shock of cold rolling rolls during use. As modern rolling mills develop toward high speed and high pressure, accidents such as steel jamming, overlapping rolling, and slipping during the rolling process of cold rolling mills will cause increased friction on the surface of the cold rolls, resulting in an instantaneous temperature rise. Steel sticking occurs when the temperature reaches 1000°C. In general, the increase in local surface temperature of the cold roll causes local overheating and changes in organizational properties. Cracks on the surface of cold rolling work rolls are mainly caused by the combination of thermal stress and structural stress caused by local overheating during the rolling process. The cracks generated in the cold rolling work roll during the previous rolling process must be completely removed by grinding in time, otherwise large and deep spalling will form in the subsequent rolling process, causing early failure of the cold rolling work roll. This spalling in the form of microcrack expansion is the main cause of early failure of cold rolling work rolls during use.

④Depth of hardened layer

By increasing the depth of the hardened layer of the cold roll, the actual diameter of the roll can be increased, and the service life of the cold rolling work roll can be effectively extended; due to the reduction in the number of re-quenchings, the cost of using the cold roll can be effectively reduced. When the chromium content in cold rolling roll steel increases from 2% to 3%, the depth of the hardened layer of the cold rolling work roll will also increase from 10mm to 15mm. In order to improve the hardness of the cold rolling work roll, we can add an appropriate amount of nickel element to the traditional chromium-based forged steel cold rolling work roll steel. While increasing the hardness of the cold rolling roll, the depth of the hardened layer can also be increased by 7~8mm. By the mid-1980s, liquid nitrogen cryogenic treatment and dual-frequency induction heating technology were widely used. The content of the alloy element chromium is increased from 3% to 5%. Not only will the depth of the hardened layer exceed 50mm, but the initial surface hardness value of the cold rolling roll will also be as high as 100 (HSD).

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Material development trends of cold rolling work rolls

① High chromium cold roll

By increasing the content of Cr element, the hardenability of the cold rolling work roll material can be improved. Mitsubishi Steel has developed a new type of high-chromium forged steel cold roll material. Test results prove that the wear resistance of a 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. By adjusting the balance of carbon and chromium, not only can higher hardness be obtained at a lower quenching temperature, but the susceptibility to breakage of the roll material can also be reduced, reducing the occurrence of roll breakage accidents. Cold rolling rolls have strong resistance to roll breakage and good roughness retention, and can be adapted to rolling under harsh working conditions such as large rolling force and high product precision requirements.

② Highly alloyed improved cold rolling roll

In order to improve the mechanical properties and performance of the new cold-rolled work roll, improvements were made on the basis of the 5% Cr cold-rolled work rolls material, 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 new materials for Cr5 series rolls. The wear resistance and thermal shock strength of the new Cr5 series material cold rolls can be significantly improved.

Nitrogen can strengthen the mechanical properties of the roll. It has a strong affinity with alloy elements such as chromium, vanadium, titanium, and niobium in steel, and can generate stable nitrides that exist in steel in different forms. Nitride is more likely to form fine and dispersed second phases, inhibit the growth of austenite grains, and has a significant effect of refining grains, significantly improving the strength, hardness and other mechanical properties of the roll material. Therefore, by controlling the nitrogen content in the cold rolling roll material, we can improve the overall performance of the cold rolling work roll, which also provides an important direction for the development of roll materials.

③ Forged semi-high speed steel cold roll

During the actual use of cold rolling work rolls, production accidents such as steel jamming, steel sticking, and belt breakage may occur. The local structure and hardness of the cold rolling work roll body made of Cr3 and Cr5 series materials will change, causing local micro-cracks or massive peeling on the roll surface, thereby reducing the accident resistance of the cold rolling roll. The material of forged semi-high speed steel cold rolling rolls is developed based on the material of 5% Cr cold rolling work rolls. Due to the large addition 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) carbide are mainly used. The existence of high-hardness carbide ensures that the forged semi-high-speed steel roll has good wear resistance and high accident resistance.

④High speed steel cold roll

High-speed steel rolls were first put into practical production and application in Japan in 1988, and then the United States also began to adopt 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 later 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 steel companies headed by Baosteel. Carrying out research on the manufacturing and use technology of high-speed steel rolls can lay the foundation for the promotion and application of new high-speed steel rolls, and therefore 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 in the United States, Dofasco Company in Canada, Akers Company in Sweden, and Nippon Steel Company, and the use results are good. The material of high-speed steel rolls contains a large number of carbide-forming elements. The carbide types are mainly MC and M2C types, which will greatly improve the strength, surface roughness and wear resistance of cold rolling rolls. Therefore, the use of high-speed steel rolls can significantly reduce roll consumption and reduce the number of roll changes, which helps 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 complex. At present, there are no reliable smelting, forging, and heat treatment methods, so the processing technology is poor and the yield rate is also low. This is also a key technology that needs to be solved in the future.

 

MM GROUP is one of the professional roll manufacturing base in China, which supply all kinds of large-size rolls for iron and steel enterprises with production capacity of 100,000 tons of all kinds of hot strip mill rolls, section mil rolls, rod mil rolls, cold rolling m rolls, casting and forging backup rolls.

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