Description：The high-speed steel rolls that people call at present are all high-carbon high-speed steel composite rolls, that is, the working layer material of the rolls is made of high-carbon high-speed steel. The core material of the roll is made of ductile iron, graphite steel or forged steel, etc., and the two different materials are made by centrifugal casting or composited by CPC process. It is essentially different from standard types of tungsten-molybdenum high-speed steel such as traditional M2 and M4. Under normal rolling conditions, the service life of high-speed steel rolls is more than three times that of alloy cast iron rolls.
Keyword：High speed steel rolls
Characteristics of High Speed Steel Rolls
1. High-speed steel rolls have high carbon content and high alloying elements such as vanadium, chromium, tungsten, molybdenum, and niobium. Therefore, the types of carbides in the roll structure are mainly MC type and M2C type, and the carbides have high hardness and good wear resistance.
2. The high-speed steel roll has good thermal stability. At the rolling temperature, the roll surface has high hardness and good wear resistance.
3. The high-speed steel roll has good hardenability, and the hardness hardly decreases from the surface of the roll body to the inside of the working layer, thus ensuring that the roll has equally good wear resistance from the outside to the inside.
4. 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. This uniform, thin and dense oxide film exists for a long time without falling off, which significantly improves the wear resistance of high-speed steel rolls.
5. The formation of oxide film on the roll surface of high-speed steel rolls during rolling reduces the friction between the rolled material and the roll surface, so it is easy to cause slippage during the rolling process, making the adjustment of the rolling mill more difficult.
6. The high-speed steel material has a large expansion coefficient and good thermal conductivity. It is easy to cause changes in the roll shape during rolling and affect the precision of the rolled material. Therefore, when high-speed steel rolls are used in plate and strip mills, not only the design of the cooling system must be changed, but also the design of the roll shape must be re-changed.
7. Since the core of the high-speed steel roll is made of alloy ductile iron, graphite steel or forged steel, the roll neck has high strength. the
8. Due to the good wear resistance and poor accident resistance of high-speed steel materials, the use effect of high-speed steel rolls depends not only on the quality of high-speed steel rolls themselves, but more importantly on the use conditions of the rolls and the maintenance of the rolls.
Processing of High Speed Steel Rolls
After the hardness of the roll increases, the processing difficulty of the roll increases accordingly. For the pass processing of high-speed steel rolls used in wire and bar mills, especially the cutting process of rolling threaded steel rolls during repeated use, due to intermittent cutting , its processing is more difficult, so choosing a reasonable processing tool and cutting amount is a prerequisite for determining whether the high-speed steel roll can be used normally on an online bar mill.
In the selection of processing tools, imported cubic boron nitride (CBN) or imported cemented carbide K01 series, such as Swedish Sandvik HIP and other brands, can be used if conditions permit. If it is a domestic tool, you can choose cemented carbide tools such as YD500 or YD05 or YG6A. For the turning of rolled rebar rolls, carbide tools must be used.
For the cutting amount of high-speed steel rolls, if the outer circle of flat rolls is processed, the turning speed should be about 10m/min, the cutting depth should be 1-3mm, and the feed rate should be 0.2-0.5mm/r. If it is hole processing or repeated turning of threaded holes, the turning speed should be about 5m/min, and the cutting depth should be 0.1-0.2mm, which is ideal. the
For the rolling of rebar, ribs need to be milled on the rolling groove. The milling machine can be a flying cutter milling machine, and the milling cutter can be a hard alloy milling cutter such as YD500 or YD05 or YTI. the
In short, for high-speed steel rolls, whether it is turning of the pass or milling of thread grooves, it is very difficult, and the processing time is 3-4 times that of alloy cast iron rolls.
Cooling conditions and requirements for high-speed steel rolls in wire and bar mills
During the rolling process, the cooling of the rolls serves three purposes:
One is to prevent the roll from overheating and eventually peeling off;
The second is to prevent the temperature of the roll surface from being too high, resulting in changes in the structure of the roll surface, affecting the wear resistance of the roll and the surface quality of the rolled material;
The third is to prolong the formation time of thermal fatigue cracks and prevent the expansion of thermal fatigue cracks.
The heat of the roll mainly comes from the contact with the rolled piece, but the deformation of the rolled piece also has a certain influence on the heat of the roll. It is generally considered that the temperature in the contact area between the rolled piece and the roll is the average value of the rolled piece temperature and the roll temperature. Therefore, the instantaneous temperature of the roll surface where the roll is in contact with the rolled piece can reach about 500°C. In particular, the introduction of endless rolling technology has more stringent requirements on the bearing conditions of the rolls, and how to quickly cool the roll surface temperature is extremely important. The ideal cooling system should be a series of nozzles distributed in a circular shape, which can ensure a continuous and sufficient water flow to the rolling groove. Among these nozzles, the nozzle near the exit guide should be the main nozzle. The amount of water sprayed from this nozzle It is 30% of the total water in the entire rolling groove, and the spraying angle is 20-30 degrees. The function of this nozzle is to ensure that the cooling water is sprayed quickly to the part of the rolling groove that just leaves the rolling piece, which is particularly important. The water pressure is controlled above 0.5Mpa, the water volume of each groove of a single roll is not less than 300L/min, and 70%-80% of the water volume should be guaranteed at the outlet side. The cooling water must be completely flushed in the rolling groove. Since the cooling condition of the lower roll is worse than that of the upper roll, the water volume of the lower roll should be more than that of the upper roll. The cooling water should be precipitated and the content of solid particles should be reduced as much as possible. the
Although the roll water cooling device, cooling method, cooling water volume, water pressure and nozzle angle of each rolling mill are quite different. However, reducing the roll surface temperature, keeping the roll cooling with uniform temperature distribution, and ensuring the cooling effect of the roll surface are all crucial to improving the service life of the roll and improving the surface quality of the rolled material. When the cooling method and water pressure are similar, increasing the cooling water volume of the roll has a significant effect on reducing the temperature of the roll and reducing thermal cracks on the roll surface.
Handling of High Speed Steel Roll Rolling Accidents
According to statistics, the consumption of abnormal use of metallurgical rolls will account for about 30% of the total roll consumption. Therefore, it is extremely important to reduce the consumption of rolls by strengthening the on-site management of the enterprise, strict rolling process, ensuring the normal operation of the rolling mill and reducing rolling accidents. When the high-speed steel rolls used in the wire and bar rolling mill have steel jamming or stacking, the water should be stopped immediately, and the upper roll should be lifted quickly to make the rolled material leave the roll surface and at the same time remove the rolled material accumulated on the lower roll immediately, so that The heated rolling groove is slowly cooled in the air. When the accident is handled and restarted, the groove must be changed, and the groove must be separated by 2-3 grooves. It is absolutely not allowed to let the cooling water rush or flow to the heated rolling groove. If it is avoided but cannot change the groove, the roll must be changed, and then it will be used again after cooling down naturally, otherwise, the cooling water will be poured directly on the overheated rolling groove, and cracks will definitely appear in the rolling groove.