This article introduces the use effects of high-speed steel rolls and the main causes of wear during use. According to the causes, the roll cooling water system, cooling water pipes and nozzle layout are reasonably designed to increase the rolling capacity of high-speed steel. Reduce the number of roll and slot changes in the production line.
The bar production line of Shandong Shiheng Special Steel Group Co., Ltd. is a domestic full-continuous bar rolling production line. It was completed and put into operation in April 2005. The designed production capacity is 600,000 t/a. The main products are φ16~φ40mm hot-rolled ribbed steel bars and round steel. . Due to the severe market situation, fierce competition and small profit margins for hot-rolled ribbed steel bars of φ16mm and above, the company has proposed the strategic development goal of “small specification”, that is, focusing on the development and production of φ10~φ14mm hot-rolled ribbed steel bars. In order to solve the problems of many small-sized rolling passes and low hourly output, the bar production line developed a three-slit rolling process for φ12 and φ14mm hot-rolled ribbed steel bars in November 2005. On the basis of rolling, the four-slit rolling process of φ12 and φ10mm hot-rolled ribbed steel bars has been developed, which has greatly improved the daily production level of small specifications and achieved significant economic benefits. In addition, high-speed steel rolls have been used in the selection of K1 roll materials for small-sized hot-rolled ribbed steel bars. Its single-groove rolling volume far exceeds that of chromium-molybdenum infinitely chilled ductile iron rolls, which has a great impact on improving production line operation rate and output. significant effect.
2. Effects of using high-speed steel rolls
High-speed steel rollers have been used in the bar production line since 2006, and the effect is very stable. The comparison of the rolling volume with the chromium-molybdenum infinitely chilled ductile iron rollers is shown in Table 1.
Table 1: Comparison of finished hole rolling amounts between high-speed steel rolls and CrMoQT cast iron rolls
|Finished hole rolling amount/t
|CrMoQT cast iron roll
When the high-speed steel roller reaches a certain rolling volume, the changes in surface conditions of the hot-rolled ribbed steel bars are shown in Table 2.
Table 2: Effect of high-speed steel rollers on the surface quality of finished bars at different rolling amounts
|Bar surface quality
|A small amount of pitting appears on the surface
|Slight indentation on the surface
|Surface indentation height 0.2mm
|Surface indentation height 0.3mm
3. Main reasons for wear of high-speed steel rolls
High-speed steel rolls are characterized by good wear resistance, high roll body stiffness, and high hardness. However, groove wear will also occur during use, thus affecting the rolling amount of the groove. The main reason for the wear of the rolling groove is that during the use of the roll, thermal fatigue occurs on the surface of the rolling groove, forming a network of fatigue cracks. As the rolling time continues to increase, the cracks continue to expand, eventually forming small pieces of peeling, causing the steel bars to The surface is pitted, and band-like rolling marks will gradually appear in the parts with greater stress (such as the rolling groove connected to the slit belt), which will affect the surface quality of the steel bar and force the production line to replace the rolling groove.
Thermal fatigue is defined as the destruction of materials caused by alternating thermal stress under the effects of heating and cooling cycles.
In the formula, σ is the thermal stress, α is the linear expansion coefficient of the material, E is the elastic modulus of the material, and T is the roll temperature.
It can be seen that the main factor affecting the thermal fatigue of high-speed steel rolls is temperature changes. Temperature change amplitude: △T=(Tmax-Tmin)/2. As the temperature amplitude increases, the thermal fatigue strength of the material decreases and the number of damage cycles decreases. In addition, if the temperature change amplitude remains unchanged, as the average temperature increases, the thermal fatigue strength of the material also decreases.
The main reason for the thermal fatigue of the roll is that when the roll comes into contact with the high-temperature steel billet, the roll surface temperature can rise to 500~600°C; when it comes into contact with the cooling water, the temperature of the work roll quickly drops below 70~120°C. This periodic heating and cooling causes a changing temperature field on the roll surface, thus producing obvious periodic stress. When the thermal stress exceeds the fatigue limit of the material, small network-like thermal cracks, known as turtles, will appear on the roll surface. cracks, resulting in a decrease in the surface quality of the rolling groove.
During the use of the roll, not only hot cracks may appear, but also new sources of cracks may be formed due to scratches on the rolling groove when the slit belt passes through the rolling groove. Cracks and surface cracks on the surface of the roll are oxidized due to working stress, residual stress and cooling, causing the stress at the crack tip to increase sharply exceed the allowable stress of the material and expand into the inside of the roll. When the crack develops at a certain angle with the roll surface or even expands in a direction parallel to the roll surface, it will cause the roll surface to peel off and form rolling marks on the surface of the steel bar, affecting the surface quality of the steel bar.
4. Main measures to reduce wear of high-speed steel rolls
The main measure to reduce the wear of high-speed steel rolls is to enhance the cooling effect of the rolls and inhibit the spread of hot cracks on the roll surface. The main measures taken are:
(1) A new medium-pressure turbid water system has a rated flow of 230m3/h and a head of 110m. Through experiments, it was found that the cooling water pressure requirement for the roll should be controlled at 0.6~0.7MPa. The greater the water pressure, the easier it is for water to splash out from the roll surface after it comes into contact with the roll surface. The cooling effect is insufficient and waste is caused. If the water pressure is too low, the temperature of the roll surface will only decrease to a limited extent, which may easily cause cracks on the roll surface.
(2) The cooling water row should be aligned with the hole shape; when installing the cooling water row, try to be as close to the deformation area of the rolling groove as possible and install it at the outlet. The nozzle of the cooling water row should be flat.
(3) The curvature of the cooling water row should match the curvature of the roll. The distance from the roll should be about 30mm. The length of the water row should be able to cover 2/5 of the roll circumference.
(4) The cooling water pressure is controlled at 0.6~0.7MPa.
(5) Each cooling water pipe on the cooling water row should be twice the width of the rolling groove to ensure that the rolling groove can be completely cooled.
(6) There are 10 nozzles arranged in the cooling water pipe, of which 4 are arranged longitudinally and 6 are arranged transversely. There are two consecutive transverse nozzles near the roll deformation area, and the rest are arranged vertically and horizontally in a staggered manner.
(7) The spray angle of the cooling water pipe nozzle is 60°~70° with the radial direction of the roll. The nozzle and the cooling water pipe are an integrated structure, and the spray direction is opposite to the rotation direction of the roll.
(8) An additional thin water pipe with a diameter of φ8mm is introduced into the deformation zone at the outlet of each rolling groove to enhance cooling of the rolling groove.
According to the above requirements, the cooling water for the finished rolls of φ12mm hot-rolled ribbed steel bars divided into four parts is specially designed (as shown in the picture) to ensure the cooling effect of the rolls. A rubber plate with a thickness of 3mm is fixed around each cooling water pipe as a water retaining plate. A rolling mill waterproof device is designed outside the entire rolling mill to prevent rolling tank cooling water from splashing outside the rolling mill during the production process, which not only increases the roll cooling effect but also reduces water waste.
Cooling water layout diagram of φ12mm hot-rolled ribbed steel bar quarter-cut finished product rolls
Since the bar production line began to use high-speed steel rolls in 2006, through focused research on the wear mechanism of high-speed steel rolls, combined with actual production conditions, the finishing rolling cooling water system and roll cooling water drainage have been optimized many times to improve rolling groove cooling. The effect is to maximize the advantages of high-speed steel rolls, reduce the number of roll changes and groove changes in the production line, reduce the labour intensity of employees, and improve the production line operation rate and economic benefits.