This article analyzes the causes of heat treatment deformation in large alloy semi-steel working rolls, including conveyor deformation, linear expansion differences, improper support, and lifting methods. It also provides practical preventive measures to control roll neck bending and ensure machining quality.
The alloy semi-steel rolling rods typically contain a carbon content ranging from 1.4% to 2.3%, and their mechanical properties fall between those of steel and iron. These rods have the characteristics of small hardness drop and high wear resistance, and are widely used in the rough and intermediate rolling machines of profile steel, rail and beam, and bar rolling machines, as well as the rough and finishing rolls and vertical rolls of the hot-rolled strip continuous rolling machine, and the roll rings and roll sleeves of universal rolling machines and cantilever rolling machines.
The manufacturing process flow of semi-steel working rolls used in large steel rolling machines is: pouring – rough processing – heat treatment – fine processing. During the heat treatment process, the main heat treatment process is high-temperature diffusion + normalizing and tempering. The transmission side roll necks of such rolls often have a large length-to-diameter ratio. These roll necks bend and deform easily during heat treatment. Figure illustrates this problem. This bending causes the center hole to shift off position. As a result, the machining allowance becomes insufficient. Operators can only correct the issue through reheat treatment. This correction delays the construction schedule. It also increases the overall product cost.
For example, the semi-steel 180 rolling rods with a specification of 1000×1100mm used in the profile steel rolling machine of a certain steel mill, the main dimensions are shown in Figure.
The long diameter-to-diameter ratio of the lower roll neck of this rolling rod is 5.2. We encountered a recurring problem during manufacturing. The transmission side roll necks bent and deformed after heat treatment. The center hole also shifted off position. These issues caused quality accidents during processing. Figure shows the main heat treatment process diagram for this rolling rod.
Basic Information of Problem Rolling Rods
The company supplies this type of rolling rods to a certain steel mill, and 18 of them are produced. Among them, 17 of them have problems. Some rolling rods have small deformation amounts, and after reworking the center hole, they can meet the processing requirements. Other rolling rods have large deformation amounts and can only be corrected by reheat treatment. The center hole deviation of 6 severely deformed rolling rods is shown in Table and the non-conformity rate is 33%.
Rolling Rod Process Parameters
This type of rolling rod is used in the rolling machine frame of the profile steel rolling machine, and the material is ZUB180CrNiMo. The quality accident is the bending deformation of the transmission side roll neck after heat treatment. The chemical composition parameters required by the process are shown in Table.
Causes and Analysis of Rolling Rod Deformation
We pad each part during thermal treatment in the kiln. This prevents deformation, as shown in the figure. However, several factors affect the rolling rod during heat treatment. These factors include its own expansion. They also include conveyor deformation. Unreasonable support also affects the rod. Lifting methods also play a role. We will now analyze each influencing factor one by one.
Linear expansion of the metal material itself
The thermal expansion of metal follows the formula δ = a·d·(T – T0). Here, “a” represents the linear expansion coefficient. “d” stands for the roll diameter. “T” is the heating temperature, and “T0” is room temperature.
The roll body and roll neck have different diameters. This difference causes them to expand at different rates during heat treatment. The roll body expands more. The roll neck expands less.
We calculated the expansion amounts. The roll body expands by 11.73 mm. The roll neck expands by only 4.5 mm. The difference between them reaches 7 mm.
We heat the roll for a long time at 940°C. This temperature far exceeds the material’s plastic deformation threshold. Gravity then pulls the roll neck end downward. It moves 7 mm relative to the roll body. Figure 5 illustrates this downward movement.
Conveyor deformation
Conveyor deformation is a common problem in large-scale heat treatment. The company processed this rolling rod using a RT3-1700-12 resistance furnace made by an industrial furnace manufacturer. The structure of the conveyor is shown in Figure.
Its driving mechanism is 3 sets of motors + reducer + gear drive, without resistance belts. The main structure from bottom to top is wheel frame, I-beam frame, base plate, heat-resistant concrete, and heat-resistant bricks. We collaborated with the equipment department. We measured the deformation amount. Table 3 records the measured data. We also plotted the data as a line graph in Figure 7. The data and the line graph clearly show the following. The middle part of the conveyor protrudes significantly under high-temperature conditions.The main reason is that the temperature difference between the inside and outside of the conveyor frame is large, and the expansion amount of the upper part of the frame is large, while the expansion amount of the lower part of the frame is small. Therefore, the end of the roll neck near the conveyor edge is 36 mm to 40 mm lower than the roll body near the conveyor middle.
Unreasonable support
Due to the relatively large length-to-diameter ratio of the driven side roller neck, proper support must be provided to prevent deformation. Firstly, if a roller car with a resistance band is used, the support points should be selected on the resistance band cover plate. During high-temperature operation, the cover plate will deform, affecting the support effect. Secondly, if the support point spacing is too large, the roller neck will also deform. Thirdly, after supporting, if the firmness of the support is not checked, problems such as loose insulation bricks and gaps between the insulation bricks and the roller neck will occur, thereby affecting the support effect.
Lifting and hoisting method
We lift the high-temperature rolls from the roller car to the air-cooling machine during normalizing and air-cooling. The rolls remain in the plastic deformation temperature range at this stage. Figure 8 shows the lifting point at the end of the roll neck. This lifting position subjects the roll body to greater deformation. Gravity causes this deformation.
Factors affecting roll deformation
Among them, the deformation of the roller car is the main cause of roll deformation, followed by the linear expansion of the metal itself. Inappropriate support and the lifting method can be reduced the influence of these factors on roll deformation by formulating corresponding systems.
Measures to prevent roll deformation
Measures for dealing with the deformation of the roller car
1)We lay a certain thickness of heat-insulating asbestos felt between the bottom plate of the roller car and the heat-resistant concrete. This felt reduces the temperature difference between the inner and outer parts of the roller car’s metal structure.
2) After calculation, under the condition of ensuring the structural strength of the roller car, 100mm deep and 3mm wide expansion joints should be gas-cut on the longitudinal frame beams of the roller car’s I-beam every 3m from the top downward to alleviate the difference in expansion between the inside and outside of the roller car. After these two measures, the measured relative deformation of the roller car was reduced to 5mm to 10mm.
For the linear expansion of the metal material itself, the reverse deformation amount should be calculated in advance.We place a 5mm to 7mm thick wooden board under the roll body when loading the rolls into the furnace. The wooden board ignites during heating. The burning board creates a gap between the roll body and the metal pad block. This gap compensates for the linear expansion difference between the roll body and the roll neck.
Measures for dealing with unreasonable support
1) When formulating the heating furnace loading plan, the resistance furnace with a resistance band should not be arranged with similar rolls. Secondly, formulate an inspection record for entering the furnace, conduct knocking experiments at each support point, and keep records.
Select an appropriate lifting method
When the rolling mill rolls are heated out of the furnace, lift the roll body and neck junction to avoid bending the roller neck due to gravity. See the figure.
We implemented the above improvement measures. We then produced 20 rolls of this type. Table 4 shows their deformation amounts. The data demonstrates that we controlled the roll neck bending within 8mm. This result meets the processing allowance requirements.
Two main reasons cause the deformation of large alloy semi-steel rolls during high-temperature heat treatment. First, the roller car deforms during the process. Second, the roll body and roll neck have a large difference in linear expansion. These two factors together lead to the bending problem.Choosing an appropriate roller car structure and using wooden boards for support can reduce the deformation of the roller car.
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