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Characteristics, manufacturing, application and development trend of high speed steel rolls

Author: Publications:2018-3-13 14:45:43 Click:

    The characteristics of high-speed steel rolls, the characteristics of high-speed steel rolls are mainly reflected in the following seven aspects:


    1) Carbide hardness is high. In the conventionally used rolls, the distribution on the substrate is mostly Fe3C type or M7C3 type eutectic carbide, and the structure is coarse and the hardness is low. High-speed steel rolls generally use high-C, high-V high-speed steel, and also contain more alloying elements such as Cr, Mo, W, Nb, and Co. These alloying elements form primary and secondary carbonization during solidification and heat treatment. Object. The high chromium cast iron roll carbides are M7C3 (2500 Hv) and M23C6 (1600 Hv). In the infinite chilled cast iron roll, the carbide is Fe3C (l300 Hv). The high-speed steel rolls mainly contain MC (3000 Hv), M7C3 (2500 Hv) and M6C (2000 Hv), which can replace the ordinary rolls, thereby improving the wear resistance. In addition, as the V content increases, the bone-like M6C-type carbide in the high-speed steel structure is converted to the granular VC type.


    2) Good thermal stability. High-speed steel rolls contain more elements such as W, Cr, Mo, V and Nb, which have better thermal stability. It is found that the hardness of ordinary roll materials decreases significantly with increasing temperature, while high-speed steel rolls are at 600. °C still maintains 500 Hv, which will greatly improve the wear resistance of the roll. This tendency is more pronounced especially when the Co element distributed in the matrix structure is added.


    3) An oxide film is easily formed during use. During the rolling process, a continuous, dense, uniform, and good adhesion oxide film is formed on the surface of the roll, which not only reduces the friction coefficient between the roll and the rolled material, but also improves the dimensional accuracy and surface quality of the rolled material. Ordinary rolls are used in the initial stage due to the absence of oxide film protection and wear. In the use of high-speed steel rolls, the oxide film can be formed quickly, and the appearance of the oxide film can reduce the friction between the strip and the surface of the roll, prevent the roll from sticking to the steel, and improve the life of the roll. Compared with ordinary rolls, high-speed steel rolls have lower initial wear, which is very advantageous for improving the wear resistance of high-speed steel rolls.


    4) Good hardenability. The high-speed steel roll has good hardenability. At room temperature, within 50mm of the working layer, the hardness drop from the surface of the roll to the core is less than 3HS, which ensures that the roll has good wear resistance from the outside to the inside.


    5) Excellent strength and toughness. High-speed steel rolls are superior to high-chromium cast iron rolls and infinite chilled cast iron rolls, and semi-steel rolls, compared to general infinite chilled cast iron, high-chromium cast iron and semi-steel rolls. Close.


    6) Good resistance to thermal cracking. High-speed steel can effectively inhibit the formation and expansion of cracks due to its unique structural characteristics. In actual use, it has been found that high speed steel rolls do have better thermal crack resistance than high chromium cast iron rolls. Therefore, in the event of a general production accident, the impact of the accident can be eliminated by normal grinding or by adding a small amount of grinding, which simplifies the process and saves costs.


    7) Good wear resistance. Compared with semi-steel, high-chromium cast iron and high-nickel-chromium infinite chilled cast iron rolls, high-speed steel roll rolling tonnage has been greatly improved, F1-F4 frame is about 10000t, F5 frame is about 6000t, showing high-speed steel roll has Good wear resistance. The increase in tonnage of high-speed steel roll rolling results in a significant drop in roll consumption. Under the same rolling conditions, the roll consumption of high-speed steel rolls is 1/5 of that of high-chromium cast iron rolls in F1 frame, and F2 frame is high-chromium cast iron roll. The 1/7, F4 rack is 1/13 of the high nickel chrome infinite chilled cast iron roll, and the F5 rack is 1/8 of the high nickel chrome infinite chilled cast iron roll.


    High-speed steel roll manufacturing technology, manufacturing high-speed steel rolls mainly include forging, casting, spray forming, hot isostatic pressing and other technologies. The popularization and use of forged high-speed steel rolls is progressing slowly; the most common methods for manufacturing high-speed steel rolls in industrial production include centrifugal casting, continuous casting outer layer forming, electroslag remelting and liquid metal electroslag welding.


    2.1 Cast high speed steel rolls


    The improvement of the casting high-speed steel roll manufacturing technology mainly focuses on improving the purity and uniformity of the steel, improving the roll strength and the metallurgical combination of the outer layer of the roll and the roll core. Different mills, different stands of the same mill, have different requirements on the performance of the rolls, and the appropriate manufacturing method can be selected according to the required roll performance, roll specifications and production cost.


    1) Centrifugal casting method. The main feature of the centrifugal casting rolls is that the liquid outer layer material and the core material are poured into the mold at regular intervals. Centrifugal rotation time, roll casting time, casting temperature and prevention of segregation of outer layer elements and interface oxidation of inner and outer materials are the key to the success of this method. High-speed steel contains more elements such as W, Cr, Mo and V, and these elements and the carbides formed by them have a large difference in density. Under ordinary centrifugal casting conditions, the elemental segregation of high-speed steel rolls is severe. Kawasaki Steel Corporation of Japan analyzed the segregation of centrifugal casting high speed steel rolls mainly due to the segregation of MC type carbides, which seriously affected the wear resistance of the rolls. It was found that MC type carbides are mainly segregation of primary crystal VC, due to the density of VC and molten steel. The difference is large. The method of preventing VC segregation is to increase the density of MC type carbide by adding Nb element, and to limit the addition of segregation elements W and Mo, and select the basic component of 2.0% C-6.0%V-7.0%Cr-2.5%Mo, and add 1.0%~1.5%. Nb was tested. As a result, due to the formation of MC-type composite carbides (V, Mo and Nb-based carbides) with higher density, the density is close to that of molten steel, which reduces VC and effectively controls the centrifugal casting of high-speed steel rolls. Carb segregation. However, the red hardness of the W-free low-speed high-speed steel roll is remarkably lowered, and the wear resistance is lowered. In addition, Nb increases the quenching temperature of the steel and lowers the temperature at which the secondary hardening peak occurs. In the high-speed steel casting process, the Nb-based MC-type carbide is coarser than the V-based MC-type carbide, which causes the roll to be easily peeled off during use. Therefore, how to determine the optimal ratio of V and Nb compound addition under given process conditions, control the production of coarse NbC, increase the W and Mo content in the rolls, ensure the high hardness steel and wear resistance, and eliminate the high speed. Segregation of steel rolls is an urgent problem to be solved in the production of centrifugally cast high-speed steel rolls.



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