Abstract: Aiming at the problems of high processing difficulty and low processing efficiency of high-speed steel rolls, this paper expounds from the aspects of the characteristics of high-speed steel rolls, tool materials, machine tool performance, processing technology and application, and clarifies the influence of elements such as tool selection, processing technology, machine tool requirements and roll application on the application of high-speed steel rolls.
Key words: High-speed steel rolls, cutting tools, processing technology, machine tool cooling
The 260 mill is an efficient bar rolling production line, which mainly produces threaded steel and mining anchor steel bars with diameters of Ф12 mm to Ф50 mm, while taking into account functional steel bars for nuclear power and high-speed rail. The mill layout adopts a 6-4-7 process form, with 1-10 frames arranged horizontally and vertically alternately to achieve full continuous tension-free rolling.
Manufacturers traditionally use ductile iron, infinite chilled cast iron, and high nickel-chromium infinite chilled composite cast iron as roller materials. In recent years, with the promotion of high-speed steel rollers in bar rolling lines, the finishing mill train of the 260 mill has fully adopted high-speed steel rollers. This change has significantly improved the mill’s operating rate, negative difference control capability, and steel surface quality.
The extremely high hardness of the high-speed steel roller increases processing difficulty and limits processing efficiency, which restricts its wider promotion and application.
| Roll Material | Roll Surface Hardness / HSD | Carbides Proportion / % | Carbides Size | Tensile Strength / MPa | Compressive Strength / MPa | Fracture Toughness / (MPa m^1/2) | Wear Resistance / Level |
| High-Speed Steel | 78 ~ 90 | 10 ~ 20 | < 100 | 700 ~ 900 | 2700 ~ 3000 | 25 ~ 28 | 4 ~ 5 |
| High Chromium Cast Iron | 70 ~ 80 | 20 ~ 30 | 100 ~ 150 | 600 ~ 800 | 1800 ~ 2200 | 21 ~ 24 | 3 |
| High Nickel Chromium Cast Iron | 75 ~ 85 | 30 ~ 40 | 200 ~ 500 | 400 ~ 600 | 2000 ~ 2500 | 18 ~ 25 | 1 |
What is a high-speed steel roller?
High-speed steel roll, full name high-carbon high-speed steel composite roll, is one of the fastest-growing new materials for hot rolling rolls. Manufacturers use high-carbon high-speed steel for the roll’s working layer and usually choose ductile iron, graphite steel, or forged steel for the core. They produce the roll by centrifugal casting or the CPC composite process.
Special heat treatment hardens this roll even when cooled in air. Its microstructure usually contains tempered martensite, bainite, and evenly distributed fine carbides, providing the following significant advantages:
Extremely high wear resistance and toughness.
Good thermal stability and hardenability.
Excellent thermal crack resistance.
The rolling amount of a single groove is 4 to 5 times that of a cast iron roll.
Lower dependence on cooling water than carbide rolls.
High-speed steel rolls are particularly suitable for finished bar rolling, pre-cutting, cutting hole rolling, and other processes. Its hardness can reach HSD78 to 90, while ensuring wear resistance, it also has good impact resistance and thermal fatigue resistance.
Processing Challenges of High-Speed Steel Rollers
Although high-speed steel rolls have excellent performance, they are difficult to process. The main reasons are:
- High hardness and fast tool wear.
- Carbide distribution is dispersed, and cutting resistance is large.
- Different manufacturers add different alloy elements, resulting in different cutting performance.
Therefore, special optimization is required in selecting tool materials, formulating processing technology and configuring machine tool performance.
Common added elements include tungsten, molybdenum, chromium, vanadium, niobium, etc. These carbide-forming elements improve the performance of the rolls, but also increase the difficulty of processing.
Tool material selection
High temperature, high pressure, and severe friction act on the tool when machining high-speed steel rolls, creating extremely harsh working conditions. Therefore, the selection of tool material is crucial. The current mainstream materials are superhard materials such as diamond and cubic boron nitride (CBN). Compared with traditional carbide tools, these tools have a cutting speed that is 10 to 20 times higher and a smoother machining surface.
(1) High hardness
CBN is second only to diamond in hardness and has extremely strong wear resistance. Manufacturers prefer this material to machine high-speed steel rolls. They commonly produce two types of CBN tools: polycrystalline sheets made of CBN and binder, and composite sheets that bond CBN onto a carbide sheet using a binder.The most widely used tool models are MBN3500 and MBN5000. They have performed well in practice and are ideal for machining high-speed steel rolls.
(2) Good strength and toughness
The tool must withstand high cutting and impact forces, so it must have sufficient strength and toughness. Although polycrystalline diamond (PCD) has high hardness, it is brittle and has poor impact resistance, making it unsuitable for steel materials. On the contrary, CBN tools have better thermal stability, bending strength and fracture toughness than ceramics and cemented carbide, making them more suitable for processing iron materials such as high-speed steel rollers.
(3) High wear resistance and heat resistance
The higher the hardness of the tool, the stronger its wear resistance. CBN tools maintain high hardness at high temperatures, that is, they have good red hardness. In addition, they have strong resistance to high-temperature plastic deformation and can maintain stable performance even after long working hours.
(4) Good thermal conductivity
Thermal conductivity directly affects the tool’s thermal shock resistance and crack resistance. The thermal stability of CBN composite blades can reach 1000℃, and their red hardness is strong, which can effectively reduce the impact of cutting heat on tool life.
(5) Good processability and economy
Manufacturers synthesize CBN tools from CBN micropowder and a small amount of binder under high temperature and high pressure. They make blades of various shapes and angles, which perfectly meet the efficient processing needs of CNC machine tools.
Table 2 shows the comparison of mechanical properties of different tool materials (such as cemented carbide, diamond, and CBN), providing users with a technical reference to select tools.
3. High-speed steel roller processing
The working layer of the high-speed steel roller body is a fine carbide with high hardness dispersed in the martensite matrix. During the cutting process, it produces a violent high-frequency impact on the tool edge. At the same time, the temperature during roller body processing has little effect on the hardness, which accelerates the wear of the tool, making the processing of high-speed steel rollers difficult and the cutting efficiency low. Therefore, the processing of high-speed steel rollers should adopt high speed, low cutting amount, and frequent tool change methods to overcome the difficulties in the processing process.
3.1 Roller hole processing
Because the new high-speed steel roller hole undergoes a large amount of rough processing that causes fast tool wear and short tool life, operators generally use a blade slightly smaller than the base circle diameter of the hole to cut the groove shape on the roller body first. Then, they use a CBN tool to perform cyclic processing according to the CNC machining program. The cutting amount adopts different parameters according to the different parts of the processing hole. Select the general cutting depth as 0.3 mm to 0.4 mm and set the cutting speed between 60 m/min and 80 m/min. Adjust the feed rate in the processing program according to needs. Set the feed rate for the roller ring part at about 0.8 mm/r, for the hole groove arc part between 0.3 mm/r and 0.5 mm/r, and for the hole groove bottom (base circle large arc) between 0.3 mm/r and 0.4 mm/r for fine processing. Figure 1 shows the schematic diagram of different parts of the roller groove.
3.2 Common problems in hole processing
3.2.1 The hole does not meet the template or dimensional accuracy
(1) There is a gap at the bottom of the groove. The feed rate of the hole finishing can be appropriately reduced, or the tool compensation can be reduced in the CNC machining program.
(2) The groove is asymmetrical, and the gap is large on one side. A new blade needs to be replaced. If the problem still exists, consider increasing the screw compensation value in the program.
(3) The groove depth is not enough. If tool wear is ruled out, reduce the feed rate of the groove bottom arc processing or increase the tool compensation value appropriately.
3.2.2 Tool damage forms
(1) Chipping.
Several small notches or a small piece of chipping on the cutting edge is a common form of damage. If the tool has a micro-chipped edge, it can continue to cut within the allowable wear limit. If it exceeds the tolerance, it needs to be replaced and the cutting amount should be appropriately reduced.
(2) Breakage.
This is generally caused by material fatigue after tool wear and failure to replace in time or excessive impact load. The feed rate needs to be reduced.
(3) Peeling.
Shell-shaped peeling occurs on the front cutting edge of the blade. This is caused by the mismatch between the cutting speed and feed rate selection. Appropriately increase the cutting speed and reduce the feed rate.
3.3 Milling of transverse ribs (taking XK500C multifunctional CNC thread milling machine as an example)
Operators should use a flying cutter milling machine (such as the XK9350 or XK500 series) to process the transverse ribs of high-speed steel rollers at the flying cutter station and process the trademark characters at the swing head station. After clamping the tool bar, they should adjust the position of the tool head. Then, they should use a special hole-type centering template for alignment (as shown in Figure 2). Operators should align the tool head, template, and hole type on the tool bar in a straight line, confirm that the tool head aligns with the center of the base circle hole type, remove the centering template, and withdraw the tool. Start the processing program, use the manual (hand-cranked pulser) to slowly feed the tool until the tool head touches the bottom of the groove and produces milling scratches. Check the scratches to confirm the accuracy of the tool. If the tool head is still not centered, use the handwheel for fine adjustment.
3. Trademark processing and milling head material
Milling head material selection
Milling heads are often made of H06 or 6EF. According to the hardness of high-speed steel rollers, the recommended milling feed is 0.03 mm/r ~ 0.05 mm/r. During operation, check the wear of the cutter head frequently and replace it in time to prevent the increase of processing resistance, damage to the tool, and increase processing costs.
Trademark character processing
Operators should adjust the machine tool to the swing head processing mode before processing trademark characters. They should use the same tool setting method as for milling cross ribs and select Φ8 mm ~ Φ10 mm bar cutters. One pass should reach the standard depth, and they should set or adjust the font size and spacing in the processing program.
4. Roller clamping method and machine tool requirements
Roller clamping method
High-speed steel roller clamping usually adopts a four-claw heavy-duty chuck or transition sleeve, combined with a tailstock rotary sleeve type large center, to form a “two tops and one clamp” method.
The rod and wire mill often uses large bolts to fix the roller head, and the center hole of the round head is usually M48 mm or larger. Engineers need to customize the tailstock center as an umbrella-shaped rotary structure to enhance the clamping rigidity.
Due to the large chamfer of the transmission end, the standard clamping claw cannot clamp it. A transition sleeve should be used. The inner hole of the sleeve should be larger than the roller head size. Install 4 to 6 bolts on the outer circumference of the sleeve to fine-tune the ovality.
Machine tool requirements
CNC machine tools must have high rigidity and stability to avoid vibration of surrounding equipment affecting the processing accuracy. Ensure the tool table provides sufficient rigidity after clamping the roller.
Use a large-sized tool bar as much as possible while meeting the minimum arc requirement of the hole type. If the toolbar extends a long distance, install an auxiliary tool top to support the tool and reduce elastic deformation.”
Precautions for the use of flying cutter milling machine
When processing the transverse ribs, operators should keep the gap between the supporting copper block and the tool bar small.Too large a gap will result in insufficient rigidity of the toolbar. Operators should regularly adjust the copper block during processing to prevent the tool bar from breaking due to wear.
5. Precautions for the use of high-speed steel rolls
Cooling requirements
The cooling water of the rolls must be clean and low temperature, ensuring sufficient water pressure and water volume to prevent thermal cracks and microstructure phase changes.
The ideal cooling system uses an annular nozzle to ensure continuous water flow to the rolling groove. Maintain the water temperature below 40 ℃, control the water pressure between 0.4 MPa and 0.6 MPa, and supply 300 L/min to 500 L/min of water to each groove. Direct 70% to 80% of the water flow toward the outlet side, and align it with the rolling groove to prevent scattering.
Handling of special situations
High-speed steel rolls are highly sensitive to thermal cracks. Turn on the cooling water before the steel enters to cool the rolling groove. If a steel jam or pile-up occurs, keep the cooling water running until the temperature drops, then stop the water and promptly clear the steel pile.
If the accident occurs and the rolling groove temperature exceeds 60 ℃, replace the slot hole and resume production after skipping 2 slots.
Conclusion: High-speed steel rolls have broad application prospects
High-speed steel rolls have high hardness and excellent wear resistance. Under the same rolling conditions, compared with chilled cast iron rolls and high nickel-chromium composite cast iron rolls, they have less wear and significantly increase the rolling volume of a single groove.
This performance not only greatly improves the surface quality of steel but also effectively controls the negative difference in dimensional accuracy, and its advantages in modern bar mills are becoming increasingly obvious.
At present, the material of the machining tool is still one of the key factors restricting the development of high-speed steel rolls. However, with the continuous advancement of CBN tool technology, combined with reasonable processing technology and high-rigidity CNC machine tools, the difficulty of roll machining is gradually decreasing, and the processing efficiency is also steadily improving.
At the same time, the continuous optimization of the cooling system will further improve the service life and stability of the rolls. The promotion and application of high-speed steel rolls will comprehensively improve the production level and equipment capabilities of steel mills, and promote the development of steel rolling technology in the direction of high efficiency and precision.
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