Application of high-speed steel rolls in bar hot continuous rolling finishing mill

High-speed steel rolls have good wear resistance, water permeability and high-temperature red hardness, but turning and milling are difficult. Carbide tools should be used and reasonable cutting parameters should be selected. During use, the cooling water volume should be sufficient and maintenance should be strengthened to ensure stable operation of the rolling mill and avoid the expansion of rolling groove cracks. Practice has proved that after using high-speed steel rolls, the number of roll changes is reduced, the roll consumption is reduced, and the annual economic benefit reaches 620,000 yuan. However, there are surface crack problems in the use of high-speed steel rolls. Abnormal scrapping of rolls should be reduced from the aspects of roll manufacturing, rolling mill maintenance and roll maintenance.

Keyword: High-speed steel rolls

The small and medium-sized production line of Laigang Bar Factory is a fully continuous rolling equipment imported from DANIELI Company. The entire production line has a total of 16 rolling mills, which are divided into three parts: rough, medium and finish rolling. The main products are hot-rolled ribbed steel bars. The application of high-speed steel rolls to replace high-nickel-chromium cast iron rolls in finishing rolling units is an important project to increase production capacity. Since 2004, the application of hot-rolled ribbed steel bars with specifications of φ18, φ20, and φ25mm in K1 and K2 passes has shown that high-speed steel rolls have good wear resistance and hardenability, which is helpful for improving product negative rate, improving rolling mill operation rate, It has significant effects in reducing roll consumption and reducing operator labor intensity.

Characteristics of high-speed steel rolls (h2)

High-speed steel rolls contain a large amount of alloying elements such as W, Mo, Cr, V, and Cn. Higher C content and V content result in high hardness MC carbide, which can improve the wear resistance of the roll. The higher Cr content causes the roll to contain a certain amount of MC₃ carbide, which can improve the roughness resistance of the roll surface and reduce the rolling force. Co can improve the red hardness of high-speed steel rolls, allowing high-speed steel rolls to be used in hot rolling mills. High-speed steel rolls have high hardness and are difficult to turn and mill. The key is to use appropriate processing tools and turning parameters. If it is a flat roller outer circle, imported cubic boron nitride (CBN) or cemented carbide K01 series can be used for processing. Domestic tools can use carbide tools such as YD500, YD05 or YG6A. The turning speed should be about 10m/min. The feed depth should be 1~3mm, and the feed amount 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.

For the finished roll, if the ribs need to be milled on the rolling groove, a flying cutter milling machine should be used. The milling cutter can be YD500, YD05 and other carbide milling cutters. When the rib spacing is small, in order to avoid cutting back, the milling method is adopted every other milling. The milling speed and feed amount are also lower than those for processing high-nickel-chromium cast iron rollers. The milling depth of the groove can be 0.2~0.5mm shallower than that of high-nickel-chromium cast iron rollers. . This can not only ensure the rolling amount, but also reduce the processing difficulty and shorten the processing time. For the repair of finished rollers, it is not necessary to clean the ribs.

The depth of 0.5~0.8mm can be retained. On this basis, the ribs can be milled to ensure the repair quality of the groove. It can also reduce the weight of the roll and increase the number of times the roll is used.

The hardness, thermal cracking resistance and thermal shock resistance are better than those of high-nickel-chromium cast iron rolls, and their wear resistance is 4 to 5 times that of high-nickel-chromium cast iron rolls. Therefore, high-speed steel rolls are quickly promoted in hot rolling production. , but how to make good use of high-speed steel rolls is still a major issue in the current steel industry. A comparison of the characteristics of high-speed steel rolls and high-nickel-chromium cast iron rolls is shown in Table 1.

In short, it is very difficult for high-speed steel rolls to process the outer circle, pass turning, or rib milling. The processing time is 3 to 4 times that of high-nickel-chromium cast iron rolls.

Use of high-speed steel rolls

Cooling of high-speed steel rolls

The cooling conditions of high-speed steel rolls need to be stronger than those of high-nickel-chromium cast iron rolls (>600m/h). The amount of cooling water should be as sufficient as possible, and flat nozzles should be used instead of conical nozzles. And 3/4 of the cooling water is used for cooling the tappet side of the roll, and the surface temperature of the roll is controlled below 50°C2. The cooling system is an annular series of nozzles that ensure a continuous and sufficient flow of water to the rolling groove. The main nozzle should be located near the exit guide. The amount of water sprayed from the nozzle is 30% of the total water amount in the entire rolling groove, and the spray angle is 20° to 30°. The function of this nozzle is to ensure that the cooling water is quickly sprayed to the part of the rolling groove that has just separated from the rolled piece. The water pressure is controlled at (4~6)×10⁵Pa, and the water volume in each tank is at least 500Lmin. The water flow cannot be scattered, and the cooling water needs to be precipitated to reduce the content of solid particles as much as possible to avoid damage to the rolls.

Maintenance and maintenance of high-speed steel rolls

Since high-speed steel materials are highly brittle and have poor accident resistance, management must be strengthened to ensure stable operation of the rolling mill and reduce steel accumulation and bearing burning. When piles of steel occur, the upper roller should be quickly lifted and the roller should be continued to cool. The water can be stopped only after the rolling groove and the rolled piece have cooled (below 50°C). An oxide film will form on the surface of the high-speed steel roll slots during the rolling process. The oxide film starts out orange or light blue, and will turn into dark blue or purple-black with continued use. When the oxide film thickens to a certain extent or is scratched from the outside, the oxide film easily falls off and forms meteor spots on the surface of the slots. In severe cases, the roller surface becomes textured or even pitted. During the use of the rolling groove, it is difficult to avoid thermal cracks on the surface of the roll due to thermal fatigue. However, if you continue to use the roll after the thermal cracks appear, or if the cracks are not completely repaired during grinding, the cracks will continue to expand during further use. Meat loss occurs. Therefore, the key to the performance of high-speed steel rolls lies in the use conditions of the rolls and the maintenance of the rolls.

Effect of using high-speed steel rollers

The use of high-speed steel rolls makes up for the shortcomings of high-nickel-chromium cast iron rolls such as low wear resistance, frequent roll and groove changes, easy meat loss, rough surface quality of rolled materials, and poor dimensional accuracy, which is conducive to giving full play to the production capacity of the equipment. It is beneficial to the smooth flow of production, reduces the labor intensity of rolling line operators, and has significant economic benefits.

Reduce the time of changing rollers and slots and improve production stability

Reduced roll consumption

Applications show that the number of times high-speed steel rolls are rebuilt is three times that of alloy cast iron rolls, the dimensions of the finished product are stable, and the surface quality is improved. The comparison of the effects of using high-speed steel and high-nickel-chromium cast iron rolls on the K1 frame is shown in Table 2.

The high-speed steel roller is used for the K2 frame, and the grinding amount is only 2~3mm, while the grinding amount of the high-nickel-chromium cast iron roller is 6~7 mm. Using high-speed steel rolls, the steel yield rate and cut-to-length rate are improved.

Economic benefit analysis

The rolling capacity of one set of high-nickel-chromium cast iron rolls is 45,000 tons, while the rolling capacity of one set of high-speed steel rolls is 480,000 tons. After using high-speed steel rollers, the downtime for slot change and frame change was reduced by 52.25 hours per year, and 12 sets of cast iron rollers were saved, generating economic benefits of 620,000 yuan.

Questions and Suggestions

At present, the main problem in the use of high-speed steel rolls is surface cracks, which are divided into mechanical cracks and thermal cracks. Mechanical cracks are mainly caused by the strong impact load generated at the moment when the rolled piece bites into the rolling mill; thermal cracks are generally caused by steel accumulation, and severe ones can lead to the scrapping of the roll.

To reduce the surface cracks of high-speed steel rolls, it is necessary to carry out the aspects of roll manufacturing, rolling mill maintenance and roll maintenance. Communicate with suppliers to improve the roll composition and manufacturing process, try to eliminate defects in the manufacturing process, and improve its thermal crack resistance. Improve the condition of rolling mill equipment.

When using high-nickel-chromium cast iron rolls, the K2 pass changes the groove once per shift, and the K1 pass changes the groove 3 or 4 times per shift. The wear of the groove is large, which to a certain extent restricts the further improvement of production capacity. The single-groove rolling volume using high-speed steel rollers is 3-4 times that of high-nickel-chromium cast iron rollers. The rolling groove is replaced once every 3 or 4 shifts of K2 pass. The K1 pass changes the groove once per shift, and the wear of the groove is very small, which reduces the time occupied by changing grooves and rollers, and improves the operating rate of the rolling mill and the stability of production.

Ensure its stable operation and reduce steel accumulation and bearing burning; when repairing the roll, if there are cracks in the rolling groove, the rolling groove can be scrapped first. After repeated repairs, it is determined that the cracks have been turned clean before continued use.