The mill rolls used in the hot-rolled bar finishing rolling mill frame and wire rod intermediate rolling mill frame are mostly made of bainitic steel, high NiCrMo infinite cold-hardened cast iron, and high-speed steel. These materials have good wear resistance, high strength, and resistance to hot cracking. However, due to various reasons, phenomena such as groove bottom cracks, dislodged grooves, and poor wear resistance may occur during the use of rolls. To prevent and reduce such phenomena, it is equally important for steel rolling manufacturers to establish a scientific maintenance system for their use while strictly adhering to manufacturing processes and improving quality. The following requirements should be met during use:
- Ensure cooling conditions.
Effective cooling of the rolls is the key to preventing groove bottom cracks, dislodged grooves, and poor wear resistance. Low cooling water flow rate, low pressure, high temperature (≥40℃), and cooling system problems can lead to inadequate cooling effects. When the roll is undercooled or cooled unevenly, surface cracks caused by thermal fatigue can occur. Cooling conditions require a water pressure of 0.3-0.6 Mpa and a water flow rate of 15-25m3/h. For single grooves, especially for bar precision rolling mill frame rolls, a dedicated arc-shaped cooling water pipe should be used (see the attached schematic diagram of a certain steel mill’s cooling water pipe). The cooling range is about half the circumference, and the cooling water pipe is installed at the steel outlet groove to strengthen the cooling of the steel outlet groove. Among these nozzles, the nozzle near the exit guide should be the main nozzle (the nozzle closest to the roll), and the water ejected from this nozzle should account for 30% of the total water in the entire groove. The ejection angle is 20-30°, which is the angle between the ejected water column and the line connecting the nozzle to the center of the roll. The nozzle mouth is 50-80mm away from the roll surface. For rough and medium rolling passes with wider roll holes, flat-shaped nozzles should be used, or wide notches with a spacing of 50-80mm and a width of 3-5mm should be cut directly on the arc-shaped pipe that is perpendicular to the arc-shaped pipe, so as to increase the cooling area and effectively cool the roll.
- Reasonably determine the width of the collar.
The basic parameters of the collar are the collar height and collar width. The collar height is the depth of the groove. When designing the hole type, not only should the length of the roll surface be fully utilized, the number of grooves should be increased, and the utilization rate of the roll should be improved, but also the width and strength of the collar should be considered to ensure the safety and reliability of the groove. The collar width should be able to withstand a certain amount of lateral pressure generated by the expansion of the billet and facilitate the installation and adjustment of the guide device. Practice has proved that a collar width-to-height ratio of 1.2~2.0 for cast iron rolls is conducive to normal rolling. Especially when rolling high-alloy, hard materials or low-temperature rolling, the rolling force increases, and attention should be paid to appropriately increasing the collar width. If the collar width is too small, the strength will be reduced, the resistance to the expansion of the billet will be weakened, and the collar may crack, so the size of the collar width is closely related to dislodged grooves.
- Scientifically design the hole type.
Improper hole type design or irregular incoming material shape may cause the hole type to be excessively filled or under-filled, increase the lateral pressure or uneven pressure, cause improper guiding, affect the quality of the finished product, and also cause rolling accidents. Sharp corners in the hole type will cause stress concentration. When the rolling temperature is low, the rolling force exceeds the allowable stress of the roll material, and the processing accuracy of the crescent groove for ribbed steel bars and marks is not enough, groove bottom loss, roll shoulder peeling, and dislodged grooves may occur.
- Install the guide properly and check the guide position in a timely manner.
As the name implies, the guide plays a guiding and positioning role during the rolling process. Therefore, it also plays a crucial role in the smoothness of the rolling process. Therefore, during installation, it should be carefully checked to ensure that the inlet guide, hole type, and outlet guide are on the same straight line and are firmly installed. Regular checks should be performed to prevent deviation during rolling due to vibration, which may cause incoming material to be fed into the wrong hole type or to enter the small collar, resulting in steel pile-up, steel jamming, small collar cracking, and uneven groove bottom wear.
- Establish a scientific roll-changing cycle system.
The roll-changing cycle is determined by the quality of the roll material, the rolling process, and the degree of wear of the roll. The roll-changing cycle should be set according to the actual situation of the production line and the characteristics of the rolling process. Generally, the roll-changing cycle for precision rolling mill frame rolls is about 1-2 days, and for wire rod intermediate rolling mill frame rolls, it is about 2-3 days. When changing the rolls, it is necessary to carefully check the condition of the new rolls, clean the groove, and apply lubricating oil to the roll surface before installation. After installation, the roll gap should be adjusted to ensure the normal rolling gap. During the rolling process, attention should be paid to the wear of the rolls, and timely measures should be taken to prevent excessive wear, which may lead to groove deformation, dislodged grooves, groove bottom cracks, and other problems.
In summary, the use of mill rolls in hot-rolled bar precision rolling mill frame and wire rod intermediate rolling mill frame requires strict adherence to the manufacturing process, rigorous quality control, and scientific maintenance systems to ensure the normal operation of the rolling process and the production of high-quality products.