High-speed steel is an alloy steel with complex composition, containing carbide-forming elements such as tungsten, molybdenum, chromium, and vanadium. The total amount of alloying elements is 10%~25%. In 1988, high-speed steel rolls were first applied to a hot strip rolling mill in Japan, and then wire rod, bar and cold strip rolling mills were also used, and satisfactory results were obtained. Now high-speed steel rolls are developed on bar and wire rolling mills, and have a wide range of application fields and application prospects.
Advantages of high-speed steel rolls
1. At present, the high-speed steel rolls that people call are all high-carbon high-speed steel composite rolls, which have high carbon content and high alloying elements such as vanadium, chromium, molybdenum, and niobium. Therefore, the types of carbides in the roll structure are as follows: MC type is the main type, with high carbide hardness and good wear resistance.
2. High-speed steel rolls can still maintain high hardness under high heat during use, and the HRC can be above 60. This is the most important characteristic of high-speed steel – red hardness.
3. During the use of the roll, under good cooling conditions, a thin and dense oxide film is formed on the surface of the roll body, and this oxide film is very uniform and does not fall off for a long time so that the wear resistance of the high-speed steel roll is significantly improved.
4. The core of the high-speed steel roll is made of alloy ductile iron, graphite steel or forged steel, so the roll neck has high strength and is not prone to roll breakage.
5. The high-speed steel roll has good thermal stability and good hardenability. At the rolling temperature, the roll surface has high hardness and good wear resistance, and the hardness of the outer surface of the roll body is almost the same as that of the inner working layer. Consistent, thus ensuring that the roll has equally good wear resistance from the outside to the inside.
Application of High-Speed Steel Rolls in Continuous Bar Rolling Mill
Japan’s Nisshin Steel Co., Ltd. first applied high-speed steel rolls to hot rolling mills in the world. Compared with the original high-chromium cast steel rolls and hemp-mouthed fine-grained alloy cast iron rolls, the high-speed steel rolls have better wear resistance and smaller roughness after wear. The main effects of using high-speed steel rolls are: due to the less wear of the rolls, the strip crown is small, and the displacement cooperation with the intermediate rolls can disperse the wear degree, improve the surface quality of the strip steel, and facilitate disordered rolling. The number of roll changes in the hot rolling production line The workload in the grinding roll workshop is reduced, the number of roll changes is reduced by about 60% for F1~F3 stands, and the number of roll changes for F4~F7 stands is reduced by about 30%. The total workload of the rolling workshop can be reduced by 30%. Rolling line The productivity is increased, and the unit consumption of the roll is reduced.
European and American countries started late in the development of high-speed steel rolls and began to use high-speed steel rolls in the 1990s. For example, Dofasco in Canada has used more than 200 high-speed steel rolls of 22 different types manufactured by 9 factories in the six years since 1992. At present, all high-speed steel rolls have been used on F2, F3 and F4 frames. After the steel roll is rolled, the roll consumption is significantly reduced, the operating rate of the rolling mill is significantly improved, and the surface quality of the strip is also improved by 20%.
Recently, the new HSS7 high-speed steel roll was applied to the 2050mm hot rolling mill of Vanderbijlpark Steel Works in South Africa. The rolling capacity per mm was increased from 5000T of high chromium cast iron roll to 12000T, and the problem of strip quality degradation caused by the iron oxide scale was reduced by two-thirds. The surface quality of hot-rolled strip steel has been greatly improved.
Because high-speed steel rolls have more advantages than other rolls, more and more manufacturers use this roll. Generally, it is first used on finishing mills F1~F4 and then expanded to F5 and F6. Now there is a tendency to use high-speed steel rolls on all 7 stands.
High-speed steel rolls have also been well applied in cold-rolled strip mills. After replacing 5% Cr forged steel rolls with high-speed steel rolls, the steel passing capacity has increased from 600T to more than 2500T, and the appearance quality of strip steel has improved significantly. High-speed steel is used to roll large-angle steel, and its wear resistance reaches 8.4 times that of cast semi-steel rolls.
Maintenance of High-Speed Steel Rolls
Due to the high brittleness of high-speed steel materials and poor accident resistance, it is necessary to strengthen management to ensure the normal operation of the rolling mill and reduce damage to the rolls caused by rolling accidents. When steel jamming or stacking occurs, the upper roll should be lifted quickly, and the roll should continue to be cooled. When the temperature of the rolled piece is lowered, the water can be stopped for disposal. If the high-speed steel roll has been operated for a period of time without water cooling due to an accident, the roll must be unloaded at this time, allowed to cool slowly, and re-grinded before being put into use. (If the cooling water is poured on the hot roll, there will be serious consequences)
The surface of the groove hole of the high-speed steel roll will form an oxide film during the rolling process. The oxide film is orange or light blue at first, and it will become dark blue or purple-black when it continues to be used. When the oxide film thickens to a certain extent or there is a When scratched, the oxide film is easy to fall off, and meteor spots are formed on the surface of the slot. In severe cases, the surface of the roller is roughened and even pitted.
The factors that lead to the groove change of high-speed steel rolls are the roughening of the groove and the appearance of small thermal cracks on the surface of the groove. During the use of the rolling groove, it is difficult to prevent thermal cracks on the surface of the roll due to thermal fatigue. However, if the thermal cracks appear and continue to be used, or the cracks are not thoroughly ground during grinding, then the cracks will be damaged during further use. It will continue to expand, lose meat, or even break, eventually causing the roll to be scrapped ahead of time.
In short, the quality of high-speed steel rolls also depends on the use conditions of the rolls and the maintenance of the rolls.