High-performance high-speed steel has high wear resistance and hardenability, especially the red hardness at high temperature, which makes high-performance high-speed steel more suitable for manufacturing rolls and roll rings.
Therefore, high-speed steel rolls are not only suitable for plate and strip mills, but also for wire and bar mills, and have achieved remarkable results.
Chemical properties
Physical properties
Material characteristics
The high-speed steel roll ring is a steel-based roll ring, which implements the national cast steel roll standard GB/T1503-2008. Its main alloy components are vanadium, nickel, tungsten, molybdenum, niobium, cobalt, and other elements, the carbon content is less than 1.6%, and the phosphorus content must be controlled below 0.03%, the chromium content must be less than 5%, and the sulfur content must be controlled below 0.03%.
This fully guarantees the material’s high wear resistance and impact toughness, and at the same time ensures that the roll ring will not be used during use. The phenomenon of micro cracks, falling blocks, and ring explosions appear. In particular, the content of tungsten is strictly controlled, and segregation will not occur during centrifugal casting, so the hardness drop after each re-grinding is small, and the amount of steel passing will not be significantly reduced.
Manufacturing process
Process design review→tooling design→modeling→smelting→centrifugal casting→heat preservation→unpacking→flaw detection→heat treatment after rough machining→flaw detection→hardness testing→semi-finishing and corresponding testing→finishing and flaw detection→metallographic testing→factory inspection→wrap up and store.
All roll rings are inspected 3 times one by one to ensure the inner quality of the mill rolls, and are tempered three times to obtain a good metallographic structure to ensure good product performance.
The outer layer of the roll contains relatively high Cr, Mo, W, V, Ni, and Nb elements, and the matrix structure is martensite + MC type carbide. The organization has extremely high abrasion resistance, good thermal crack resistance, and bite resistance. The hardness of the working layer can reach 80-90HSD, and the hardness drop is less than 1HSD. The use of HSS rolls can greatly reduce the number of grinding, reduce roll consumption, and improve the surface quality of the rolled material. have a broad vision of application.
- High-speed steel rolls have high carbon content and high alloying elements such as tungsten, molybdenum, vanadium, chromium, niobium, etc. Therefore, the types of carbides in the roll structure are mainly MC type and M2C type, and the carbide hardness is high and has good wear resistance.
- The HSS roller has good thermal stability and the roll surface has high hardness and good wear resistance at the rolling temperature.
- The high-speed steel rolls and rings have good hardenability, and the hardness from the surface of the roll body to the inside of the working layer is almost not reduced, so as to ensure that the roll has the same good wear resistance from the inside to the outside.
- 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 mold exists for a long time without falling off, making the high-speed steel roll wear-resistant Significantly increased.
- The formation of an oxidation mold on the surface of the HSSmill rolls during rolling reduces the friction between the rolled material and the roll surface. Therefore, it is easy to cause slippage during the rolling process, which makes the adjustment of the rolling mill more difficult.
- The high-speed steel material has a large expansion coefficient and good thermal conductivity. It is easy to cause roll shape deformation during rolling and affect the accuracy of the rolled material. Therefore, when the high-speed steel roll is used on the plate and strip rolling mill, not only the cooling system design must be changed, but also Also change the design of the roll shape.
- Since the core of the HSSrolling rings is made of alloy ductile iron, graphite steel, or forged steel, the roll diameter is high in strength.
- Because the high-speed steel material has good wear resistance, but the ability to resist accidents is poor, the effect of the use of high-speed steel rolls depends not only on the quality of the high-speed steel mill roll itself but also more importantly on the use conditions and maintenance of the roll.
Benefit
- Increased Productivity: HSS mill rolls are designed to have a longer service life than traditional casting mill rolls, reducing downtime for maintenance and replacement, resulting in higher output and reduced production costs.
- Improved Quality: HSS mill rolls have superior wear resistance, thermal conductivity, can reduce defects and improve surface finish for steel wire, resulting in higher customer satisfaction.
- Reduced Total Cost of Ownership: HSS mill rolls have ability to maintain shape and properties for extended periods of use enable them to handle higher loads and rolling speeds, which ultimately translates to improved profits for the manufacturer.
Improper roll alignment during the rolling process can be corrected by adjusting the roll gap, roll bending, or roll shifting. These adjustments can be made manually or through computer control systems. The exact method used will depend on the specific type of rolling mill and the severity of the alignment issue. It’s important to maintain proper roll alignment to prevent issues such as steel sticking or stacking and to ensure consistent product quality.
The mill rolls used in the hot-rolled bar finishing rolling mill frame and wire rod intermediate rolling mill frame are mostly made of bainitic steel, high NiCrMo infinite cold-hardened cast iron, and high-speed steel. These materials have good wear resistance, high strength, and resistance to hot cracking. However, due to various reasons, phenomena such as groove bottom cracks, dislodged grooves, and poor wear resistance may occur during the use of rolls. To prevent and reduce such phenomena, it is equally important for steel rolling manufacturers to establish a scientific maintenance system for their use while strictly adhering to manufacturing processes and improving quality. The following requirements should be met during use:
- Ensure cooling conditions.
Effective cooling of the rolls is the key to preventing groove bottom cracks, dislodged grooves, and poor wear resistance. Low cooling water flow rate, low pressure, high temperature (≥40℃), and cooling system problems can lead to inadequate cooling effects. When the roll is undercooled or cooled unevenly, surface cracks caused by thermal fatigue can occur. Cooling conditions require a water pressure of 0.3-0.6 Mpa and a water flow rate of 15-25m3/h. For single grooves, especially for bar precision rolling mill frame rolls, a dedicated arc-shaped cooling water pipe should be used (see the attached schematic diagram of a certain steel mill’s cooling water pipe). The cooling range is about half the circumference, and the cooling water pipe is installed at the steel outlet groove to strengthen the cooling of the steel outlet groove. Among these nozzles, the nozzle near the exit guide should be the main nozzle (the nozzle closest to the roll), and the water ejected from this nozzle should account for 30% of the total water in the entire groove. The ejection angle is 20-30°, which is the angle between the ejected water column and the line connecting the nozzle to the center of the roll. The nozzle mouth is 50-80mm away from the roll surface. For rough and medium rolling passes with wider roll holes, flat-shaped nozzles should be used, or wide notches with a spacing of 50-80mm and a width of 3-5mm should be cut directly on the arc-shaped pipe that is perpendicular to the arc-shaped pipe, so as to increase the cooling area and effectively cool the roll.
- Reasonably determine the width of the collar.
The basic parameters of the collar are the collar height and collar width. The collar height is the depth of the groove. When designing the hole type, not only should the length of the roll surface be fully utilized, the number of grooves should be increased, and the utilization rate of the roll should be improved, but also the width and strength of the collar should be considered to ensure the safety and reliability of the groove. The collar width should be able to withstand a certain amount of lateral pressure generated by the expansion of the billet and facilitate the installation and adjustment of the guide device. Practice has proved that a collar width-to-height ratio of 1.2~2.0 for cast iron rolls is conducive to normal rolling. Especially when rolling high-alloy, hard materials or low-temperature rolling, the rolling force increases, and attention should be paid to appropriately increasing the collar width. If the collar width is too small, the strength will be reduced, the resistance to the expansion of the billet will be weakened, and the collar may crack, so the size of the collar width is closely related to dislodged grooves.
- Scientifically design the hole type.
Improper hole type design or irregular incoming material shape may cause the hole type to be excessively filled or under-filled, increase the lateral pressure or uneven pressure, cause improper guiding, affect the quality of the finished product, and also cause rolling accidents. Sharp corners in the hole type will cause stress concentration. When the rolling temperature is low, the rolling force exceeds the allowable stress of the roll material, and the processing accuracy of the crescent groove for ribbed steel bars and marks is not enough, groove bottom loss, roll shoulder peeling, and dislodged grooves may occur.
- Install the guide properly and check the guide position in a timely manner.
As the name implies, the guide plays a guiding and positioning role during the rolling process. Therefore, it also plays a crucial role in the smoothness of the rolling process. Therefore, during installation, it should be carefully checked to ensure that the inlet guide, hole type, and outlet guide are on the same straight line and are firmly installed. Regular checks should be performed to prevent deviation during rolling due to vibration, which may cause incoming material to be fed into the wrong hole type or to enter the small collar, resulting in steel pile-up, steel jamming, small collar cracking, and uneven groove bottom wear.
- Establish a scientific roll-changing cycle system.
The roll-changing cycle is determined by the quality of the roll material, the rolling process, and the degree of wear of the roll. The roll-changing cycle should be set according to the actual situation of the production line and the characteristics of the rolling process. Generally, the roll-changing cycle for precision rolling mill frame rolls is about 1-2 days, and for wire rod intermediate rolling mill frame rolls, it is about 2-3 days. When changing the rolls, it is necessary to carefully check the condition of the new rolls, clean the groove, and apply lubricating oil to the roll surface before installation. After installation, the roll gap should be adjusted to ensure the normal rolling gap. During the rolling process, attention should be paid to the wear of the rolls, and timely measures should be taken to prevent excessive wear, which may lead to groove deformation, dislodged grooves, groove bottom cracks, and other problems.
In summary, the use of mill rolls in hot-rolled bar precision rolling mill frame and wire rod intermediate rolling mill frame requires strict adherence to the manufacturing process, rigorous quality control, and scientific maintenance systems to ensure the normal operation of the rolling process and the production of high-quality products.
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