Abstract: Combining actual production experience and analyzing the reasons for the wear of profiled steel rolls based on the wear mechanism, it has a certain reference effect for the selection of rolls, roll replacement and production in the profiled steel workshop.
0 Preface
The wear of the roll has an important influence on the quality of the product and the service life of the roll itself. Different types of rolling mill rolls have different wear patterns. The section steel is rolled with a pass, and the deformation between the rolls is more complicated. Aiming at the wear problem inside the roll pass of the current section steel rolling mill, combined with the wear mechanism, explore its wear law.
1. Wear mechanism
In the section steel rolling workshop, on-site technicians generally judge whether to replace the groove or roll off the line for repair based on the single groove over-steel amount of the roll (the tonnage of billets rolled by a single groove). During this period, due to the friction between the rolling piece and the roll, and there are relative sliding phenomena such as forward slip and backward slip between the two, the roll is worn out. According to the wear mechanism, wear can usually be divided into adhesive wear, abrasive wear, corrosion wear, fatigue wear, erosion wear, fretting wear and impact wear [1]. The factors affecting wear are very complex, including working conditions (load, speed, movement mode, etc.), environmental factors (humidity, temperature and ambient atmosphere, etc.), media factors, lubrication conditions, the composition of the roll material, the metallographic structure and the surface of the roll The physical, chemical, and mechanical properties of the product. A slight change in any of these factors may change the wear mechanism [2]. The wear of the roll is often the result of the combined action of several mechanisms [3]. Under a certain working condition, one mechanism plays a major role, while also causing the effects of other mechanisms. When the working conditions change, the mechanism that plays the main role may change, leading to changes in other accompanying mechanisms. In actual working conditions, one side is a high-temperature rolled product, and the other side is a forced water-cooled roll. In the process of contact between the two, due to the oxidizing atmosphere formed by the high temperature rolling piece and the cooling water, the bonding force of the metal bond is relatively weak, and it is easy to transfer the material from the rolling piece to the roll, forming loose abrasive grains. With the extension of the rolling time, the wear of the roll is intensified, and the surface roughness increases rapidly, which is a prerequisite for corrosion and wear. With long-term periodic mechanical loads and large temperature fluctuations, contact fatigue and wear are inevitable [4- 6]. The form of wear between the rolled piece and the roll is mainly adhesive wear and corrosive wear. The wear of the section steel roll is usually expressed by the maximum change in the radial dimension after rolling a certain weight of billet, that is, the maximum wear. The calculation of the maximum wear amount generally uses Archard wear model [6], see formula (1)
It can be seen from formula (1) that the wear of the roll is proportional to the contact load and inversely proportional to the hardness.
2. The law of wear
Figure 1 shows the roll matching diagram of a channel steel rolling mill in a section steel workshop. The internal wear of the channel steel roll pass of different specifications in the picture shows that the wear of the roll pass is very uneven. The decreasing direction in Fig. 1a indicates that the measuring point gradually penetrates into the inside of the pass along the side wall, and it is obvious that the wear is increasing; the increasing direction of the roll diameter in Fig. 1b indicates that the measuring point is continuously moving to the center position along the bottom of the pass. The wear is gradually reduced, but the magnitude of the change is not as large as the side wall. In general, the wear peak point is located at the corner of the pass. Due to the different specifications, pass sizes and rolling quantities of rolled pieces, the wear of each roll is not the same. Figure 1c The wear of the roll is obviously greater than that of Figure 1a. The general trend of the wear of the roll can be seen from the profile curve before and after the wear of the roll: the maximum value of the internal wear of the pass appears at the corner of the pass; the wear value at the bottom is generally greater than At the side wall, the wear uniformity of the outer circle of the bottom is also better than that of the side surface.
3. Analysis of the causes of uneven wear on the surface of the hole
3.1, metal sliding
When rolling a special-shaped section rolled piece, each part of the rolled piece does not contact the roll at the same time. At the exit, not all parts of the rolled piece are positively slippery. At the bottom corner of the upper roll groove (point A in Figure 2), the roll diameter is the smallest and the circumferential speed is the smallest, so the forward slip is the largest. As the roll diameter increases (A→B, A→C), the circumferential speed increases and the forward slip value decreases. Under the condition of a certain roll diameter, the linear velocity of the roll surface is equal to the velocity of the rolled piece, and the forward slip is zero. When the diameter of the upper roll continues to increase, the peripheral speed of the roll exceeds the roll-out speed of the rolled piece, and the rolled piece appears back slip when it leaves the roll. Different forward sliding speeds result in different friction around the hole pattern, causing uneven wear of the hole pattern [7]. The maximum wear occurs at the location with the greatest friction, so the absolute wear at the bottom corner of the hole is the largest, and the wear at the bottom of the hole is much larger than the side wall.
3.2. Uneven roller surface hardness
The new roll must be heat treated during the manufacturing process to ensure that the roll surface forms a working layer with a depth of (50-60) mm. Not only the surface hardness is higher, but the hardness uniformity is better, that is, the hardness difference along the entire roll surface is very small. Because the surface of the rolls used in the section steel rolling mill has a pass of tens of centimeters deep, the pass size will not meet the production requirements after the roll is used, and a new turning roll car is needed. With the increase of the number of turnings, the chilled layer of the roll decreases. Part of the hole
The type is in the cold hard layer with high wear resistance, while the basic part of the hole type is cut into the transition layer. The pass gradually cuts deeper into the transition layer, resulting in a more uneven distribution of the surface hardness of the roll. At the same time, contact between the roll and the 1000 ℃ high temperature rolling material will change the surface structure of the roll. Therefore, there is a large hardness difference on the surface of the roll, which aggravates the unevenness of the internal wear of the pass [7].
3.3. In the rolling process with uneven rolling temperature, the rolling force required for different temperature areas is different. There is a negative exponential relationship between the temperature of the rolled piece and the deformation resistance of the rolled piece [8]. See formula (2)
K=Me-mT (2)
In the formula, K————Deformation resistance of rolling piece, N T————Rolling piece temperature, ℃
M, m—Constant depending on the characteristics of the metal itself
The difference in deformation resistance results in different rolling loads, so the friction between the rolling piece and the contact surface of the roll is naturally different, resulting in different degrees of roll surface wear. There are similar laws in the surface temperature distribution of H-shaped steel and channel steel, that is, the temperature of the flange end and the flange root, the entire flange and the web is different, usually about several tens of degrees Celsius. The temperature difference leads to the difference in deformation resistance, which affects the deformation of the rolled piece. Since the temperature of the web part of the rolled piece is much higher than that of the flange part, the deformation resistance of the former is smaller than that of the latter. Similarly, the deformation resistance of the flange end of the rolled piece is greater than that of the root. Therefore, from the influence of temperature distribution, the roll pass wear shows a trend of increasing gradually from the web to the corresponding position of the flange.
3.4. Uneven metal deformation
The rolling piece undergoes three-dimensional plastic deformation in the pass, so its deformation process should not use a single reduction rate like the strip. First of all, the reduction rate of the rolled flange and the web is different. Taking into account the overall limitation of the rolled piece, that is, the part with large extension is restricted by the part with small extension, the former is the longitudinal compressive stress, and the latter is the longitudinal tensile stress. The existence of additional tensile stress changes the original flow direction of the metal and complicates the internal deformation of the rolled piece [9]. Secondly, even in the direction of the entire web or flange of the rolled piece, the reduction rate is different, and the small difference in the reduction rate will cause a large fluctuation in the rolling force, which results in the different wear of each part of the roll pass. An important factor.
4 Conclusion
The uniqueness of roll wear in section steel mills: the wear value of the bottom of the pass is greater than that of the side wall, and there is obvious unevenness in the wear of the roll. Due to the particularity of section steel rolling, the three-dimensional deformation in the roll pass leads to differences in the sliding speed of different parts of the rolling piece, and uneven deformation of the rolling piece. Differences in roll materials and production processes make the roll surface hardness distribution uneven; differences in cooling conditions make the surface temperature distribution of the rolled pieces uneven. The combined effect of these factors leads to uneven roll wear in the section steel mill. Therefore, the surface strengthening process of the roll used in the section steel rolling mill can be formulated in accordance with the wear characteristics of the roll surface.
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