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 the rolls. However, there are many differences between high-speed steel rolls and traditional high-chromium iron rolls. Pay attention to.
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 transferred 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, high-speed steel rolls should have better cooling conditions during use, and the mathematical model of the thermal crowns should be improved when using high-speed steel 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 the oxide film in high-speed steel is not as good as that of high-chromium cast iron;
The skin spalling of high chromium cast iron rolls is due to the typical mechanical crack propagation throughout the subcutaneous layer, causing small particles of the roll material to peel off and the roll surface 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 in 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 to 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.
6. Peeling off.
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.
To sum up, compared with traditional high-chromium ferro-rolls, high-speed steel rolls have many advantages, are more advanced than traditional high-chromium ferro-rolls, and have a longer service life. But what kind of specific choice, or according to the customer’s own needs to decide.
