Nowadays, mill rolls are more and more widely used, especially high-speed steel rolls. At present, high-speed steel rolls have been widely used in hot-rolled strip mills to replace high-chromium cast iron rolls, which greatly prolongs the service life of rolls. There are many differences between steel rolls and traditional high-chromium iron rolls, which should be paid attention to in use. The main differences are as follows:
1. Thermal convexity
Compared with high-chromium iron rolls, the thermal expansion of high-speed steel rolls is much larger, mainly because the thermal conductivity of high-speed steel is higher than that of high-chromium cast iron (the thermal conductivity of high-speed steel is 25.5W/(m K), high-chromium The thermal conductivity of cast iron is 20W/(m·K). For this reason, heat is more easily transmitted to the core material through the working layer of the high-speed steel roll, resulting in a greater thermal crown of the high-speed steel roll. Therefore, the high-speed steel roll is used during use There should be better cooling conditions and an improved mathematical model of thermal crown when using HSS rolls.
2. Oxidation of the roll surface during rolling
The formation of oxide film starts from the combination of matrix structure and carbide. Due to the different content of carbide, the formation and growth rate of oxide film on high-speed steel is not as good as that of high-chromium cast iron; the peeling off of high-chromium cast iron roll is due to typical mechanical The crack spreads to the subcutaneous layer, causing the small particles of the roll material to peel off and the surface of the roll to be roughened. However, the surface of the high-speed steel roll is much smoother, not easy to peel off, and has a long service life.
3. Rolling force
Using high-speed steel rolls on hot strip mills, in some cases, the friction coefficient is increased by 3%-5%, while in other cases, the friction coefficient may be greatly increased according to different rolling conditions. It has been noticed that the use of high-speed steel rolls will increase the temperature of the strip surface by about 50°C. Generally speaking, the lower surface of the strip will be 30°C higher than the temperature of the upper surface.
4. Roll into the iron oxide scale
The problem of rolling in scale is related to the thin thickness, low carbon content and low silicon content of the rolled material. Rolling into the scale often occurs on the lower surface of the strip, because the lower roll is often worn more severely, making the surface of the roll worse. This shows that the rolling in of scale is related to the damage of the roll surface. The use of high-speed steel rolls will increase the surface temperature of the rolled material and promote the formation of high-hardness oxides due to the increase in the friction coefficient. Therefore, inter-stand cooling and skin cooling are used. It is better to solve the problem of rolling into the oxide scale.
5. Thermal cracks
Compared with hot-rolled strip steel rolls of other materials, high-speed steel rolls have good thermal crack resistance, especially very good resistance to thermal crack growth.
Since the residual stress of high-speed steel rolls is higher than that of high-chromium cast iron rolls, high-speed steel rolls are usually very sensitive to mechanical cracks. When subjected to strong impact loads, mechanical cracks are prone to occur due to rapid concentration of load and local overload. Under the combined action of residual stress and rolling stress, cracks rapidly expand in the depth direction, resulting in spalling or catastrophic fracture.
Based on the above, compared with traditional high-chromium iron rolls, high-speed steel rolls have many advantages, are more advanced than traditional high-chromium iron rolls, and have a longer service life. However, it is still necessary to compare what people choose. The choice is based on merit.