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Application practice of high-speed steel rolls in CSP rolling lines

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Due to good wear resistance and hot crack resistance, the rational use of high-speed steel rolls on CSP rolling lines can not only extend the service life of the rolls, but also improve the shape and surface quality of hot-rolled strips. The use of high-speed steel can also solve problems such as plate surface, oxide film and cracks that are often encountered in roll production practice, continuously reduce production costs, and obtain better technical and economic benefits.

Keywords: high-speed steel rolls, high-speed steel roll application

rolling mill rolls

Characteristics of high-speed steel rolls

High-speed steel rolls have good wear resistance and hot crack resistance, which have obvious advantages over rolls made of ordinary materials.

Hardness

High-speed steel rolls are generally made of high-C and high-V high-speed steel. Contains more elements such as Cr, Mo, W, Nb and Co. During solidification and heat treatment, these alloying elements form primary and secondary carbides, and these carbides have higher hardness. At the same time, the high-speed steel roll has good hardenability. Within the working layer of 50 mm at room temperature, the Shore hardness decrease from the roll surface to the core is less than 3 HSD, which can ensure that the roll has good wear resistance from the outside to the inside. . The Shore hardness of the high-speed steel rolls currently used in the 1810 line is 72~78 HSD.

Thermal crack resistance

The material composition of the working layer of high-speed steel rolls contains 1.5% to 3.5% carbon elements and a large amount of strong carbide metal elements such as Cr, Mo, V, W, and Co. The matrix structure is tempered martensite and bainite, with a large number of MC type and M6C type high hardness carbides distributed. The continuous increase in the total content of alloy elements is the development trend of high-speed steel roll materials. It has currently reached more than 5% to obtain a large number of finely dispersed high-hardness carbides and a highly stable matrix structure. The carbide hardness, morphology, quantity, distribution and matrix structure and performance of high-speed steel rolls determine its excellent wear resistance, hot cracking resistance and roughness resistance.

Optimization of roll rolling conditions

In order to adapt to the use of high-speed steel, the 1810 line adjusted the roll cooling water and hot rolling oil, and optimized the rolling model.

Cooling water

The cooling water nozzles of the 1810 line finishing rolling work rolls are respectively arranged at the entrance and exit sides of the rolling mill. A water beam (one row of cooling water nozzles) is arranged at the entrance of the work roll, and two cooling water beams (three rows of cooling water nozzles) are arranged on the outlet side. There are three types of single drain nozzles, namely A, B, and C. According to the flow rate of a single water nozzle, A>B>C. Each row of work rolls on the inlet and outlet sides of the F1~F3 frame has 25 water nozzles (13 for type A, 6 for type B, and 6 for type C). The water volume is 600~700m³/h, and the high-pressure water pressure is 8.5~9 MPa.

The cooling water volume of the front frame of the 1810 line finishing rolling is relatively small, which has certain restrictions on the use of high-speed steel. In order to adapt to the use of high-speed steel rolls, the cooling water nozzle in the middle has been changed to a large-eye water nozzle, and the cooling capacity of the rolling area has been increased accordingly. Because the friction factor of high-speed steel rollers is higher than that of ordinary iron rollers, a frame using high-speed steel rollers needs to switch to a rolling model of high-speed steel rollers.

hot rolling oil

The hot rolling oil of the 1810 line’s stand using high-speed steel rollers is put into normal use. The purpose of using lubricating oil rolling in the finishing rolling mill is to reduce the rolling force and rolling energy consumption; reduce roll wear and roll consumption; improve the surface condition of the rolls and improve the surface quality of the strip. Hot rolling oil forms a high-strength lubricating film on the surface of the work roll, reducing roll surface wear, reducing friction in the roll gap area, and appropriately reducing rolling force. The hot rolling oil volume fraction of the 1810 line has been tested from 0.2% to 0.6%. According to the roll surface condition and rolling state, it was finally determined to use a hot rolling oil concentration of 0.5%, which has better results. It is lubricated with rolling oil and there is no slipping problem during use.

Grinding and flaw detection management of high-speed steel rolls

Grinding management

1) The high-speed steel roll is cooled off the machine.

The high-speed steel roll is removed from the machine for 15~20 minutes and the temperature is checked. The temperatures of F1 and F2 high-speed steel rolls were measured when the 1810-line double-line rolling thicker cold-rolled steel grade and the rolling tonnage was 2300 t. The temperature curve of F1 rack high-speed steel is gentle and normal, and the temperature is slightly higher. The temperature in the contact area between the roll and the strip is generally above 60°C, with the highest temperature being 64.4°C. The temperature curve of the high-speed steel of the F2 frame is gentle and normal, and the temperature is basically normal. The temperature in the contact area between the roll and the strip is generally around 55°C, with the highest temperature being 56.0°C. The temperatures of the upper and lower rollers are basically the same, with the lower roller slightly higher. The surface temperature of 1810-line high-speed steel rolls after coming off the machine generally does not exceed 65°C. After the high-speed steel roll is removed from the machine, the forced cooling method of water cooling is not used, but the slow cooling method of air cooling is generally required. Air cooling is generally required for 12 to 24 hours. When the surface temperature of the roll reaches room temperature, it can be ground or put on the machine for reuse.

2) High speed steel roll grinding.

The hardness of high-speed steel rolls is generally higher than that of ordinary material rolls. In the grinding process of high-speed steel rolls, great attention must be paid to the grinding quality of the rolls. You can reselect the grinding wheel material and adjust the grinding wheel hardness, particle size and other parameters to greatly improve the grinding efficiency. In addition, in terms of controlling grinding quality, grinding parameters can be optimized. For example, controlling the feed amount and grinding current during fine grinding, using periodic variable speed control technology, and controlling the number of polishing passes and polishing current, etc., make the surface quality of the high-speed steel roll fully meet production requirements when it is put on the machine.

Flaw detection management

According to different usage conditions, different flaw detection management is carried out for high-speed steel rolls that are normally removed from the machine and abnormally removed from the machine.

1) Normally dismount the high-speed steel roll.

In addition to measuring the temperature, the high-speed steel rolls that normally come off the machine need to be completely cooled, and then the roll shape is measured on the grinder. After the machine is removed, the roll shape of the old roll is compared with the roll shape of the new roll. Observe the performance of the high-speed steel roll in use on the machine. The amount of wear and the degree of maintenance of the roll shape. If the wear of the roll type is small and the wear amount of the roll is also relatively small, the high-speed steel roll does not need to be ground, and the bearing seat can continue to be used on the machine after being refueled.

At present, after a 1810-line high-speed steel roll is used more than three times, the surface of the roll will be damaged to some extent, and the roll shape retention ability will also decrease accordingly. Grinding needs to be arranged according to the conditions of the roll surface and roll shape. The grinding amount of high-speed steel rolls that normally come off the machine is generally 0.1~0.2 mm. After passing the eddy current inspection, they will continue to be put on the machine.

2) High-speed steel rolls that are abnormally off the machine.

Due to the influence of thinning and variety steel production, accidents in line 1810 tend to increase. After the accident roll was removed from the machine, focus was placed on flaw detection and monitoring. Once cracks were found, they were marked and manually confirmed by ultrasonic waves. Comprehensive inspection of the surface and subsurface of high-speed steel rolls can prevent the expansion of cracks due to mechanical damage to the rolls and avoid the expansion of accidents. After using high-speed steel work rolls, the roller grinding workshop strengthened the manual flaw detection of the rolls. After each high-speed steel roll is used abnormally, a comprehensive manual ultrasonic inspection is performed on it to ensure the internal quality of the high-speed steel work roll is qualified.

Effect of using high-speed steel rollers

Cost of use of high speed steel rolls

Line 1810 uses a total of 6 high-speed steel rolls from two manufacturers. It was put into use on November 10, 2014, and the data for one year until the end of November 2015 are shown in Table 1.

Roll numberunit priceRoll neck/mmOn-machine wear amount/mmmm rolling amount/(t ·mm⁻)Cost per ton of steel
Y348054.5780.8060.0312753.80.475
Y348154.5780.8040.03112753.80.475
Y348254.5789.6750.03413206.90.459
Y348354.5789.6090.04213206.90.459
Y347139785.5460.01910639.20.407
Y347339785.4650.03510639.20.407

At present, the work roll consumption of F1~F3 single frame of 1810 line is about 0.81~0.9 yuan/t. The current use cost of high-speed steel rollers is about half of that of ordinary iron rollers. We will continue to work on the use of high-speed steel rolls in line 1810, increase the number of uses, and expand to F1 and F3 stands. The cost of rolls in line 1810 will be significantly lower.

The high-speed steel roll oxide film is well formed

A better oxide film helps improve the surface quality of the strip. The high-speed steel rolls of the 1810 line F2 stand are generally used 3 to 4 times on the machine, and the oxide film and roughness will change accordingly after each time they are removed from the machine. The oxide film will be uniform and complete after one time off the machine. The oxidation film after the second disassembly showed slight annular uneven wear and annular color difference. After being removed from the machine three times, the oxide film was unevenly worn. The upper roller showed uneven annular wear. The oxide film on the lower roller, especially in the middle rolling area, fell off, and a thinner oxide film was regenerated. Compile a map for the use of high-speed steel rolls. Different roll surface conditions match different rolling material arrangements, see Table 2. Surface classification of high-speed steel rolls, see Figure 1.

Levelsurface conditionUse grinding arrangementsRemark
1The oxide film is uniform and complete, dark blue or light black.No grinding, can roll all varieties1(a)
2The oxide film is uniform and complete, silver-gray in colorNo grinding, can roll all varieties1(b)
3The oxide film is uniform and complete, and the surface is roughenedOrdinary varieties without grinding or rolling1(c)
4The oxide film is relatively complete and has small meteor spots.Ordinary varieties without grinding or rolling1(d)
5The oxide film is annularly worn and color difference occurs.Only low-grade varieties can be rolled1(e)
6The oxide film continuously falls off, forming spots.grinding1(f)
7The oxide film is severely scratched and peeled offgrinding1(g)
8The oxide film peels off in large areasgrinding1(h)
9Any superficial anomalygrinding1(h)

High-speed steel rolls have little on-machine wear

The average on-machine wear of high-speed steel rolls is generally less than 0.05 mm, and they have strong wear resistance and roll shape retention capabilities. The deviation of the wear curve of the high-speed steel rolls in the F1 and F2 stands is generally around 0.02 mm after a single rolling cycle of 2300t. The deviation of the wear curve after three uses on the machine is generally around 0.05 mm. Compared with ordinary high-chromium iron rolls, it has excellent roll shape retention capabilities.

Strip quality

1) Surface quality and plate quality.

Optimize the roll shape of high-speed steel work rolls and improve plate shape accuracy. The thermal expansion coefficient of high-speed steel rolls is larger than that of high-chromium iron work rolls. When producing thicker carbon steel, the temperature of the slab entering the F2 rolling mill is about 960°C. When the high-speed steel work rolls bite the steel, the thermal expansion ratio is high. The degree of ferrochrome work rolls is larger, and the negative crown of high-speed steel rolls needs to be appropriately increased. Before 2008, the high-chromium iron work rolls made of ordinary 1810 line materials used a crown of -0.15, while the high-speed steel rolls used a crown of -0.3. Because the 1810 line currently produces a lot of thin-spec products, the rolling force of the high-speed steel rollers used in the F2 stand is relatively large, and the deflection deformation of the rollers is also relatively large. Through the rolling test, -0.1, -0.12, -0.15, – were used respectively. Test rolling with a crown of 0.2 (high-speed steel rolls with large negative crown are prone to double-sided corrugation defects). At the same time, because F1, F2, and F3 of the 1810 line are PC rolling mills, the F2 stand uses a PC angle of 0.2~0.3. According to the actual production practice, the high-speed steel roll uses a -0.1 crown to cooperate with the PC, which is suitable for controlling thin gauge rolling. It has the best effect on flatness control and rolling stability. The negative crown currently used for 1810-line high-speed steel rolls is -0.1, and ordinary iron rolls use -0.25 crown. In actual production, the crown is also adjusted according to the cross section and variety.

When the rolling tonnage reaches 1900, 2100, and 2300 t, the strip surface quality and plate shape quality are checked on the line. See Table 3 and Figure 2 for details.

No.Slab numberRolling thickness/mmRolling tonnage/tSurface QualityShape quality
15105109E304.01900Slightly pockmarkedNormal, no obvious bright bands, unilateral waves, or bilateral waves
25105110E104.02100Slightly pockmarkedNormal, no obvious bright bands, unilateral waves, or bilateral waves
35205694D704.02300Slightly pockmarkedNormal, no obvious bright bands, unilateral waves, or bilateral waves

2) Convexity control.

The rolling tonnage of the rolls reaches 1900, 2100, and 2300t. The average crown of the HMI line and the measured three-point thickness are shown in Table 4 for details.

No.Slab numberNominal thickness/mmRolling tonnage/tThickness 1/mmThickness 2/mmThickness 3/mmActual crown/μmHMI average convexity/μm
15105109E304.019003.943.983.953546
25105110E104.021003.963.993.991528
35205694D704.02.3003.964.03.992535

But when the roll tonnage reaches 2100t, the multi-function instrument detects that the convexity of the second half of the strip is slightly smaller than the wedge shape. At the end of the rolling process, the strip crown appears to be less than 30 μm and slightly smaller than the wedge shape. However, the F1~F3 rolling mill PC has room for small adjustments to improve the crown at 0.1°~0.3°, and further experiments are needed to observe the crown value to make a more accurate evaluation.

Conclusion

High-speed steel rolls have excellent wear resistance, thermal crack resistance and other performance properties. They also have characteristics such as large thermal expansion, large friction factor, and sensitivity to mechanical cracks. Only the reasonable use of high-speed steel roll cooling and rolling lubrication, as well as the optimal control of its thermal expansion and wear models, can the excellent performance of high-speed steel rolls be fully utilized. Therefore, establishing a usage map of high-speed steel rolls, matching different roll surface conditions with different rolling material arrangements, and rationally arranging the eddy current and ultrasonic flaw detection of the rolls can effectively avoid abnormal failure of the rolls, extend the service life of the rolls, and improve the quality of the hot-rolled strips. Improve plate shape and surface quality, reduce production costs, and achieve higher technical and economic benefits.

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The world No. 1 in crude steel production steel plant, over 70% of the HSS for bar and wire are from us.

LMM high-speed steel rolls have higher steel passing per groove (times) due to their good wear resistance, which saves roll changing time, improves rolling mill operation rate, reduces roll consumption, and improve the overall efficiency of the factory.
Generally, the amount of steel passing in a single groove (times) is 3 to 5 times that of cast iron rolls.

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