Abstract: This paper analyzes the causes of carbonized chromium steel roll rings fragmentation within the high-speed wire rod rolling unit of our factory. The analysis also references the performance characteristics of tungsten carbide rolls used in similar high-speed applications. Based on the specific circumstances, improvement measures are proposed.
1. Introduction
The second high-speed line of a certain steel rolling plant adopts a fully automatic continuous non-twisting high-speed rolling process. The designed production capacity is 450,000 tons per year, and the designed product specifications are Φ5.5 – 16mm hot-rolled wire rods. The steel types mainly include ordinary carbon steel, high-quality carbon steel, low-alloy steel, etc. The finishing rolling unit of the production line adopts the domestic Morgan fifth-generation model, and all 10 rolling mills use tungsten carbide rolls (tungsten carbide hard alloy roller rings). During actual production, due to various reasons, the roller ring breakage phenomenon occurred multiple times, seriously affecting the economic benefits of the enterprise. Compared with conventional roll materials, tungsten carbide rolls offer higher wear resistance and thermal cracking resistance, yet they remain susceptible to fragmentation under improper operating conditions.
2. Analysis of the Causes of Roll Ring Breakage
2.1 Steel Piling within the Rolling Unit
During steel piling, the heat of the rolled piece is transferred to the adjacent roll rings, causing the temperature of the roll rings to rise and concentrating thermal stress. However, during the accident handling process, the surface temperature of the roll rings rapidly drops, making it easy for roll ring breakage to occur. Even tungsten carbide rolls, despite their superior thermal stability, can fail under such extreme thermal cycling. The main reasons for steel piling within the rolling unit are as follows:
(1) Improper roll matching. The Morgan high-speed wire rod finishing rolling unit adopts collective transmission, and the transmission speed ratios of each machine frame are different. Therefore, when matching the rolls for the finishing rolling unit, in addition to meeting the gear transmission ratios between each machine frame, it is also required that the outer diameter error of the roll rings of the same frame be controlled within ±0.05mm. Otherwise, the rolled piece will shake between the two machine frames, ultimately causing steel piling.
(2) Poor alignment of the rolling line. In the finishing rolling unit, the guides between each machine frame and the section from the exit of the unit to the carding machine have various guide channels such as gas seals, water tanks, recovery sections, and pinch rolls. During high-speed operation of the rolled piece, if it is obstructed, it will lead to the occurrence of steel piling.
(3) Faults in the guides of the finishing rolling unit. If the oil and gas lubrication of the guide bearings in the finishing rolling unit is insufficient or the cooling is inadequate, the bearings will be damaged, the rollers will not rotate flexibly or get stuck, and may even burst, resulting in steel piling.
(4) Defects in the raw material. If the raw material billet has defects, during the rolling process of the finishing rolling unit, due to local defects in the cross-section of the rolled piece that cannot withstand normal pressure deformation, it will fracture, thereby causing steel piling.
2.2 Inappropriate Roll Gap Setting
The roll gaps of each rolling machine frame should be adjusted according to the rolling process program. The program specifies a standard roll gap range.
If the roll gap of a certain frame is adjusted too small (even close to 0mm):
Friction will occur between the roll rings
Friction may also occur between the roll ring and the inlet/outlet guide
This friction causes:
The stress on the roll ring to rise sharply
The surface temperature of the roll ring to rise sharply
Consequently, a breakage accident occurs.
Note on tungsten carbide rolls:
They are particularly sensitive to localized overloading
This sensitivity is most severe under zero-gap conditions
Localized overloading can initiate catastrophic fracture
2.3 Abnormal Impact Force on Roll Rings
(1) Excessive incoming material size of the finishing machine. The finishing machine requires strict control of incoming material size.
The standard range depends on product specifications.
If the incoming material is too large, the first roll ring bears excessive impact force.
This increases both deformation and rolling force.
Consequently, the finishing rolling unit suffers a breakage accident. Even high-performance tungsten carbide rolls cannot withstand such repeated overloading without risk of fragmentation.
(2) Incorrect alignment of the guide. The installation requirements for the guides of each rolling machine are that the rolled piece can pass smoothly through the guides and not cause impact on the roll rings. Otherwise, during each pass of the rolled piece, the rolled piece will cause abnormal impact force on the roll rings, ultimately leading to the occurrence of a breakage accident.
2.4 Poor Cooling Effect of Roll Rings
In actual production, the service life of the tungsten carbide rolls is closely related to the surface cooling effect. When the surface cooling of the roll rings is insufficient, the surface temperature will rise abnormally, the wear and crack rate will increase, greatly reducing the service life of the tungsten carbide rolls, and in severe cases, a breakage accident will occur. The main reasons for poor cooling effect are as follows:
(1) Abnormal cooling water pressure. The cooling water pressure for the roll rings is generally required to be controlled within 0.4-0.6MPa. If the water pressure is too high, the cooling water is prone to splash and cannot promptly remove the heat on the surface of the roll rings, affecting the cooling effect. If the water pressure is too low, the cooling water cannot break through the vapor layer formed on the surface of the roll rings and affect the cooling or the water flow is relatively insufficient, unable to promptly remove the heat on the surface of the roll rings, affecting the cooling.
(2) Abnormal cooling water nozzles or pipelines. In actual production, due to abnormal cooling water nozzles or pipelines, the roll ring cooling fails, the surface temperature rises abnormally, and ultimately leads to a breakage accident. The main reasons are as follows:
① The cooling water pipeline is damaged and there is leakage.
The cooling water is largely diverted before reaching the roll rings, causing the surface of the roll rings to fail to achieve the expected cooling effect and ultimately resulting in an accident.
② The cooling nozzle is clogged.
Due to poor water quality, some impurities accumulate at the cooling nozzle, causing the cooling water not to be able to spray onto the surface of the roll rings normally, affecting the cooling effect.
③ The cooling nozzle is offset.
Due to low installation technical level or during the accident handling process, the position of the cooling nozzle was shifted, preventing the cooling water from cooling the high-temperature area of the roller ring, causing the temperature of the roller ring to rise and eventually leading to the accident.
2.5 The installation and disassembly temperature of the roller ring does not meet the requirements
During the installation process, the temperature difference between the installation shaft, the conical sleeve, and the roller ring should be controlled within 15 degrees.
If the temperature difference is too large, thermal expansion will vary significantly.
This will first cause abnormal interference between the roller ring and the conical sleeve.
As a result, the conical sleeve may automatically fall off.
Additionally, if the roller ring temperature is too low, internal stress will concentrate during pressurization.
When the temperature rises during production, the stress increases.
This can lead to a broken roller accident.
Such problems are particularly common during winter production.
Therefore, insulation measures must be taken for effective control.
Tungsten carbide rolls have a relatively low thermal expansion coefficient.
As a result, they require even more precise temperature control during installation compared to conventional roll materials.
2.6 The working pressure of the roller ring installation is not standard
During the installation of the roller ring, the working pressure is lower than the standard requirement, resulting in insufficient interference between the roller ring and the conical sleeve, and the conical sleeve may fall off. Eventually, a broken roller accident occurs. On the contrary, if operators set the installation pressure higher than the standard, they create two problems.
First, the abnormally large interference between the roller ring, conical sleeve, and installation shaft directly causes a broken roller.
Second, this same issue prevents them from removing the broken roller during disassembly. The brittle nature of tungsten carbide rolls makes them especially vulnerable to fracture under excessive installation pressure.
2.7 There are foreign objects on the surface of the roller ring, conical sleeve, or installation shaft
During installation, operators must thoroughly clean the roller ring, conical sleeve, and installation shaft.
If they fail to remove oxide scale, rust, or other impurities from these surfaces, the friction force between the three components will decrease.
This reduction in friction can cause the roller ring to slip or fall off.
Ultimately, such slippage leads to a broken roller accident.
For tungsten carbide rolls, even minor surface contamination can create stress concentration points that initiate cracks.
2.8 The surface of the conical sleeve and installation shaft is severely worn
After long-term use, the surface of the conical sleeve and installation shaft will show certain wear. The conical sleeve and the installation shaft cannot achieve the expected interference fit.
This makes the assembly prone to exit and slip.
These problems ultimately cause a broken roller accident. Proper mating surface condition is critical for the reliable performance of tungsten carbide rolls.
3. Relevant improvement measures
(1) During daily production, follow all standard requirements.
Strengthen on-site process supervision.
This will reduce finishing rolling unit accidents.
Pay special attention to operating conditions.
(2) Regularly check the control of material size to ensure that the size of the head, middle and tail of each rolling machine is within the standard range.
(3) Utilize each downtime to inspect the cooling water system of each rolling pass, ensuring that the cooling effect on the surface of the roll ring meets the standard requirements. Adequate cooling is particularly essential for tungsten carbide rolls to prevent thermal fatigue and fragmentation.
(4) Before installation, monitor the temperature of the roll ring and the installation shaft to ensure that the temperature difference is within a reasonable range. Especially during the winter production process, apply insulation treatment to the roll ring before it is put into operation. This precaution is even more important for tungsten carbide rolls due to their sensitivity to thermal shock.
4. Conclusion
By analyzing the causes of roll ring breakage and taking corresponding control measures, good results have been achieved in the actual production process. This has effectively reduced the occurrence rate of breakage accidents, shortened unplanned downtime, and improved production efficiency. The findings are particularly relevant for mills using tungsten carbide rolls, as proper installation, cooling, and process control are essential.