Machining of High Speed Steel Roll Pass

1. Selection of cutting tools and processing technology for high-speed steel rolls

High-speed steel rolls have been recognized by iron and steel companies for their high wear resistance, high rolling capacity, and high operating rate. However, the problem of pass processing of high-speed steel rolls has always plagued users and affected their further application. Because the high-speed steel roll has good thermal stability, red hardness and good wear resistance, its hardness is close to that of ordinary carbide knives. Only by selecting a suitable high-hardness tool can the normal processing of the roll be guaranteed.

At present, high-hardness cutting tools include artificial diamond (PCD), cubic boron nitride (CBN), ceramic inserts (Si3N4-based, Al2O3-TiC-based), ultra-fine grain alloys (H1OF, H06F, ZK10), etc., which are suitable for processing The range is different. High-speed steel rolls exhibit different mechanical properties and surface hardness during use, so only by using a variety of cutting tools in a reasonable manner can the problem of roll processing be solved.

Because superhard cutting tools (PCD, PCBN) are limited by technology, cutting tool material and price, it is impossible to make cutting and forming knives. The material for cutting and forming knives currently used is mainly ultrafine grain alloy. At present, the commonly used welded turning tools usually need to be kept warm for 48~72 hours during production, and the hardness of the blade generally decreases by about HRA10, and there is welding stress. The alloy cracking phenomenon is serious during use or grinding, accounting for above 50% of scrapped tools. The forming knife made by bonding process (bonding temperature 80~120°C) retains the original hardness of the alloy, has a short production time (it can be cured for 4 hours), and has high impact resistance (the adhesive plays a significant role. Vibration damping effect), high yield, etc., which greatly improves the turning efficiency, and has been widely used in the processing of high-speed steel roll slitting passes.

1) Turning of the high-speed steel roll body

The hardness of the high-speed steel roll body is generally HSD75~90. MBN3500 polycrystalline boron nitride tool square blade (such as SNUN12T612) can be used when processing the new roll or repaired roll body, and it is equipped with a special machine clip-type tool holder for turning (CSXNR3535R12), the main deflection angle is 25°, which is easy to break chips and dissipate heat.

Due to the high hardness and high brittleness of the high-speed steel roll, before turning, the two ends of the roll body should be reversed to the process angle (recommended ≥ 5*45°), otherwise, the roll body will have serious chipping when turning to the other end. The processing is generally divided into two steps of rough turning and fine turning. During the rough turning, the margin is 0.4~0.6mm. It is better to reverse the cutting tool during the finishing turning, so as to eliminate the gap caused by the top of the tailstock. At about 0.06mm, it meets the requirements of split rolling and achieves the purpose of replacing “grinding” with “car”.

When processing new rolls, a higher cutting amount can be used (recommended value V=13~26m/min, f=0.36~0.7 m/r, a=1~1.5mm), high speed and low feed should be used for finishing, To ensure the surface roughness requirements.

When processing the repair roller, when the feed rate is too large, chipping will occur at the notch of the pass. Use a 45° machine clamp to chamfer both sides of the notch of the pass, and then turn the roll body as a whole. Processing parameters can refer to the new roll to choose a lower cutting amount. It can be known from processing experience that when the depth of cut doubles, the main cutting force also doubles; when the feed rate doubles, the main cutting force increases by 0.75 to 0.9 times. In order to improve processing efficiency, a larger feed rate is used within the range allowed by the rigidity of the machine tool during rough machining.

2) Turning of high-speed steel roll pass

(1) Processing of new roll pass of high-speed steel

The K1 roll pass of the new high-speed steel roll can be directly processed by a polycrystalline cubic boron nitride pass cutter (MBN3500). This processing method has a short service life and low efficiency due to the large machining volume and large contact area of the hole-shaped cutter. When machining on the C8450, in order to improve the machining efficiency, it is generally used for rough machining with a pass knife one size smaller than the machining pass, and then for finishing with a forming knife. For example, when machining the ψ14 round steel pass, first use the ψ12 pass cutter for rough machining, and then use the ψ14 pass cutter for finishing to size.

Because it is difficult to sharpen polycrystalline cubic boron nitride hole-shaped blades, the cost of using disposable blades is high, and machine-clamped hole-shaped blades can be used to reduce costs. After the blades are blunted, they can be sent to the manufacturer for sharpening (special CNC tool grinder), or self-made special fixtures and sharpening samples for self-grinding, self-grinding requires a higher technical level of workers.

When processing K1 holes with ultra-fine grain alloy inserts (H03F, H06F), the processing time is about twice that of polycrystalline cubic boron nitride hole-type cutters, and the processing efficiency is relatively low, but the inserts have good impact resistance and are easy to grind. Convenient, and suitable for machining machine tools with less rigidity.

The K2~K4 pre-slitting pass of the new high-speed steel roll can be processed by the slitting forming knife on the C8450. Due to the large size of the hole pattern, the resistance to the knife is large, and it is difficult to feed manually. The motorized feed can be used if the rigidity of the machine tool allows. The following two points should be guaranteed: one is that the cutting speed and feed rate are about 1/5~1/4 of the K1 roller, and the other is that the overall rigidity of the machine tool is good, and the lower part of the tool rod can be rigidly supported when it extends out for a long time.

When processing rolls on CNC roll lathes, try to use larger size V-shaped positioning groove machine clamping blades (RCGX090700) under the condition of ensuring non-interference. Due to the low repeat positioning accuracy of the tapered positioning groove (difficult to process the inner and outer tapered surfaces), it is difficult to ensure that the upper plane of the blade is parallel to the centre plane of the rolled workpiece during clamping, resulting in deviations in processing dimensions. When the blade is clamped, it is only necessary to make an angle of 45° between the centre of the V-shaped positioning groove of the blade and the axis of the workpiece (to ensure certain friction between the blade and the tool holder, so as to prevent the blade from being pulled out when the blade cuts out the hole pattern, resulting in processing errors). Just lightly tighten the blade pressure plate. In finishing machining, MBN3500 inserts are generally used to ensure that the pass is turned to size at one time, otherwise, repeated positioning errors will occur during tool change machining, which will affect the machining accuracy.

(2) Processing of high-speed steel repair roller pass

Due to the work-hardening phenomenon of high-speed steel rolls, especially the processing of finished ribbed steel passes is more difficult. For the finished roll of round steel K1, it can be directly processed with a polycrystalline cubic boron nitride pass cutter (MBN3500), or roughed with an MBN2500 blade and then finished with an H03F pass cutter to size, or divided into rough and fine with H03F pass cutter Car to size. For the finished roll with ribbed steel bar K1, due to the existence of crescent groove transverse ribs, it belongs to intermittent cutting. H06F is used for rough machining, and the tool must be manually fed, and the feed rate per revolution is about 0.1mm. The blade, if it idles for many turns without feeding the knife, the blade will become blunt quickly. Before processing the rolls, prepare 2~3 forming knives for rough machining, which is beneficial to save sharpening time, and finally use MBN3500 or H03F knives for finishing to size.

When processing the repair roll pass on the CNC roll lathe, the MBN3500 blade can be used for rough and fine turning. The general recommended cutting amount is V=8~15m/min, f=0.3~0.5m/r, a=0.5~1.5 mm. In rough machining, lower cutting speed and higher feed rate should be selected. When finishing, choose a higher cutting speed and maximum feed rate, and the cutting depth should be smaller. In order to improve processing efficiency and reduce blade consumption, the ribbed steel bar K1 finished roll is first rough-machined on a common roll lathe to remove the transverse ribs and hard spots, and then the pass is finished on a CNC roll lathe. Rough and fine turning directly on the CNC roll lathe can save about 1/3 of the processing time.

3) Processing of transverse rib groove

When processing high-speed steel rolls, it is best to use a rotary engraving and milling machine (such as XK9350FA), milling transverse ribs at the flying knife station, and engraving at the swing head station. When clamping the cutter bar, use a special centring template to adjust, adjust the cutter bar, template and pass to a straight line, then apply chalk chips in the pass of the roll, use a single-stage way to feed the knife, and observe the centring of the transverse rib Finally, fine-tune the A-axis to ensure that the centre of the tool holder is in the centre of the hole.

When the β value of the angle between the transverse rib and the axis is processed according to the national standard, β should be 60°~65° as much as possible. Because the transverse rib and the axis are close to perpendicular, it is conducive to the de-grooving of the transverse rib of the steel bar in the roll and reduces the wear of the transverse rib groove. The inclination angle of the cutter head in the cutter holder is small, which is conducive to the adoption of a larger feed rate. At the same time, it can be programmed in a double-head mode. Manual or automatic programming is simple, and fast, and processing efficiency is high. The angle (90°-β) of the cutter head press-fitting ejector rod should be accurate, generally controlled at about ±2°.

H06F (good impact resistance) is often used for the milling cutter head, and the feed rate is 0.04~0.06mm/r. If this value is too low, it will increase the wear of the tool. 8 grooves, each groove takes 1.5~3h when processing threaded 12K1 roll.

When engraving in the hole pattern, first adjust the machine tool to the swing head mode, and use the H03F rod milling cutter to process about 36 characters in 6 slots, and the engraving time is about 10 minutes (5 characters). After the milling cutter head and rod milling cutter are blunt, they should be ground in time, and use a special sharpening sample to compare the size of the cutter head until they are ground to a qualified size.

2. Selection of machine tools and adjustment of processing system rigidity

When processing high-speed steel rolls, special roll machine tools with better rigidity should be selected, such as C8450, CK8463, XK9350FA and other machine tools, and equipped with four-jaw heavy-duty chucks and rotary sleeve tailstocks. When the roll is processed, it is clamped by one clamp and one top (the front end of the main shaft is preferably installed with an axially positioned top, and the jaws are clamped at the transmission end), and a fixed top is installed in the tailstock so that the roll diameter can be controlled by a single claw for the entire roll The end jumps, and the fixed top rotates with the sleeve, the vibration is small during processing, and the processing efficiency is high. If the jaws are short, a flat-head sleeve can be used for clamping, and set screws should be installed around to reduce the gap between the flathead and the sleeve.

When clamping a large roll, a special arc jaw support block is used to ensure surface contact between the jaw and the roll to reduce vibration. If the tailstock is of non-rotary type when the rotary centre is used for clamping, the runout usually reaches 0.1mm, and the rotary centre has a low bearing capacity and will be damaged soon.

When the ordinary fixed tip is used, it is easy to burn out due to the high heat. It is best to make a special inlaid carbide tip and use an infusion device to make a drip lubrication device, which is used to cool the tip and the taper hole of the roll to support the tapered surface. At the same time, if the space permits, try to use the centre bracket. The lathe tool rod should have sufficient strength and be directly pressed on the tool holder, with the extension length as short as possible. If necessary, a support rod should be installed to improve its rigidity.

When machining transverse ribs on XK9350FA, ensure that the milling cutter shank has sufficient rigidity, and use the largest diameter cutter shank within the allowable range of the hole size. At the same time, the cutter bar should be clamped in the way of one clamp and on top, and the top adopts a rotary tip (the fixed tip rubs against the toolbar, causing the clamping cone surface to be elliptical, and the toolbar radially jumps, resulting in inconsistent size and depth of the transverse rib).

The support block adopts the adjustable gap type, and the gap between the support block and the cutter bar must be adjusted in time every time 2~3 slotted transverse ribs are processed, so as to prevent the size of the transverse rib from being out of tolerance or the toolbar from being broken due to excessive clearance. Should have graphite lubricant to ensure adequate wear resistance.

3. The influence of other factors on roll processing

1) Rolling process

Roll cooling water should be clean and have sufficient water pressure and water volume. When the water pressure is low and the water volume is insufficient, the roll-hardening phenomenon will be serious. Generally, the hardness of the upper roll is about HSD5 higher than that of the lower roll, which is prone to bursting. The pass edge collapse is serious, and the roll body is heavy and the vehicle volume is large. Affected by the amount of rolling and the material of the roll, the hardness of the roll body increases slightly after each rolling, generally increasing HSD2~5 each time. Therefore, in order to ensure normal turning and reasonable rolling capacity, the hardness of new rolls is ideal at HSD78~83.

When the rolling volume of the rolling groove exceeds 1/3 of the specified value (recommended 300-500t for a single groove of the ribbed steel bar K1 roll, 1500-2500t for the pre-splitting roll K4 roll, and a higher single-groove rolling volume of the smooth round steel bar), The hardness of the roll body increases significantly, and the cracks in the transverse ribs spread deeply. The weight of the roll body is 2 to 3 times that of the normal load, resulting in a decrease in the number of roll reloads, a significant decrease in the overall rolling capacity, and low-cost performance. Therefore, a reasonable rolling process can ensure the normal processing of high-speed steel rolls.

2) Reasonable processing time

The hardness of high-speed steel rolls is higher than that of cast iron rolls by HSD20 or more, and its hardness is close to that of ordinary cemented carbide. During the rolling process, the surface is hardened by cold work, and the roll body is hard and brittle. The normal processing time is 2~3 times that of cast iron rolls. times. The high-hardness cutting tools (PCBN, H03F, H06F) currently used have poor impact resistance, and the high-speed steel rolls must not be processed with the cutting amount used when processing cast iron, otherwise, the cutting head will break when touched, even the best cutting tools cannot be used normally. (Generally, it takes 2~3h/slot to process roll ψ370*650 ribbed steel pass, and 1.5~3h/slot to mill transverse ribs)

Article source: “Machining of High-Speed Steel Roll Pass” Wu Debao, Liu Penggang