Analysis and Control of Burst Grooving in High-Speed Steel Rolls

This article systematically analyzes the operating conditions of high-speed steel rolls and clarifies that the main cause of roll cracking is the combined effect of thermal fatigue, mechanical, friction and other stresses. By implementing measures such as improving cooling water application and optimizing process parameters, cracking has been reduced. The occurrence of slot accidents has improved production efficiency.

The Bar Factory of the Laiwu Branch of Shandong Iron and Steel Co., Ltd. currently has 4 steel bar production lines, with an annual output of 2.7 million tons of construction rebar, mining anchor steel, prestressed precision-rolled thread, etc. In order to improve the surface quality and production efficiency of steel bars, 2010 Starting from 2009, the company began to promote the use of high-carbon and high-vanadium series high-speed steel rolls in the finishing rolling mill, replacing the original high-chromium cast iron and infinitely chilled ductile iron rolls.

After comparison in use, the wear resistance and surface roughness resistance of high-speed steel rolls are significantly better than ordinary rolls. The steel passing capacity of a single groove can be increased by 5 to 6 times. The surface quality and dimensional accuracy of the rolled material are significantly improved, and the cost-effectiveness of the rolls is high.

However, under the working conditions of the original rolls, it was found that the finished groove K1 and the slit, pre-slit grooves K3 and K4 frequently experienced groove explosions, mainly manifested as the thread transverse ribs at the bottom of the finished K1 rolling groove falling off and two-phase The middle roll ring of adjacent rolling grooves cracked, and the cutting wedges of K3 and K4 rolling grooves cracked, causing production accidents and a decrease in the steel qualification rate, which had a great impact on production and product quality. To this end, the bar factory has comprehensively optimized the working conditions and process parameters of high-speed steel rolls to adapt to its own characteristics and reduce accidents.

 

HSS roll, cracking

 

1. Analysis of causes of roll cracking

 

1) Cracks caused by fatigue thermal stress

 

When the roll comes into contact with the high-temperature rolling stock, the surface temperature rises rapidly and then the water-cooling temperature drops sharply. This cooling and heating condition causes a continuous cyclic temperature gradient in the radial direction of the roll, and fatigue thermal stress is generated around the rolling groove. With the rolling, As time goes by, thermal stress continues to increase. Especially when rolling is under abnormal conditions, such as accidental steel jamming, circulating water failure, etc., it will cause local overheating of the roll surface, a sudden increase in thermal stress, and subsequently produce cracks along the grain distribution. Vertical cracks and thermal fatigue cracks will rapidly propagate along the interface between carbide and matrix once they are generated. Due to the high carbon content of high-speed steel rolls (1.5%~2.0%), the total alloy content exceeds 15%, the thermal expansion coefficient is large, and it has high crack sensitivity. When the stress reaches the strength limit of the roll, the transverse ribs at the bottom of the rolling groove will Stress concentration points such as groove spacer rings may burst (groove bursting) occasionally, causing accidents.

 

2) Cracks caused by mechanical and frictional stress

 

During the rolling process, the mechanical stress acting on the roll mainly includes direct shear stress produced by the rolling load, bending stress and torsional shear stress produced by the torque necessary to drive the roll to overcome the rolling friction. The roll and Friction are caused by relative sliding between rolling stock. The mechanical impact load generated at the moment when the rolled piece bites in and is thrown out will also intensify the effect of mechanical stress. Abnormalities in the production process, such as “black steel” caused by improper use of cooling water or flying shearing errors, low-temperature rolling, low-temperature wire drawing or other hard debris caused by improper adjustment, will cause severe impact after entering the rolling groove. load, extremely destructive. Although high-speed steel rolls have high hardness and wear resistance, their toughness and impact resistance are low. When mechanical stress cracks reach a certain limit, it is easy to cause rolling groove burst accidents. It was found on-site that mechanical stress cracks have no obvious distribution characteristics and are oblique cracks. They generally expand in the opposite direction of rolling and the expansion speed is fast. Once they occur, it is difficult to control.

 

2. Control measures

 

1) Improve the quality of turbid ring water in rolling mills

 

The cooling water of the rolling mill system is in direct contact with the cooling equipment, and the water quality after use is poor. The main pollution factors are suspended solids, PH, waste oil, etc. If the water quality does not meet the requirements, it will affect the cooling effect of the rolls. In severe cases, scale will form in the pipes or nozzles and blockage. Therefore, the bar factory added a high-efficiency inclined plate chemical oil remover based on the natural precipitation of chemicals in the original advection tank. Mud filter press and other equipment. The cooling water of the rolling mill flows into the iron sheet ditch and flows into the cyclonic sedimentation tank. After preliminary sedimentation, it is raised to the sewage chemical degreaser. After chemical addition, sedimentation, oil removal, and PH value adjustment, it flows into the hot water well of the pumping station and is used for cooling. After the pump is lifted to the cooling tower to cool down, it is pumped to the cold water well, and then the circulating water is pumped to the turbid ring water users. According to the water quality requirements for roll cooling, the water quality of the turbid ring water of the rolling mill should meet the requirements of Table 1.

 

Table 1 Water quality control standards for rolling mill turbid ring water

ParameterUnitControl value
PH 7~9
TurbidityNTU<30
Total hardnessMg/L (calculated as CaCO3)<500
ChlorideMg/L (calculated as Cl-)≤400
Total ironMg/L≤1.0
Oil contentMg/L≤5
ConductivityUs/cm<1500
BacteriaPieces/mL<1.0*105

 

2) Water volume and water pressure requirements

 

During production, the cooling water pressure of the roll should be maintained between 0.4~0.6MPa. Cooling nozzles with different flow rates are used according to different rolling specifications and speeds. When rolling Φ10~Φ50mm steel bars, the cooling water volume of a single groove is generally controlled at 300~500L/ min. According to changes in ambient temperature, the water volume and water pressure are adjusted in time. Especially in winter, when the ambient temperature drops below -10°C, operation monitoring should be strengthened. When the roll is rolling in a cold state, the frequency of steel passing should not be too fast. Let the roll have a slow heating process. After testing, stop passing steel. Finally, it is advisable to control the difference between the rolling groove surface temperature and the ambient temperature within 15°C.

 

3) Change the traditional rolling groove cooling method

 

The traditional groove cooling is generally at the entrance of the rolled piece, and a cooling nozzle is installed on the upper and lower rollers at both ends of the outlet. The size of the nozzle and the spray angle are random and cannot be standardized, resulting in uneven cooling of the groove and excessive roll temperature, often due to Cooling problems causing the rolling grooves to burst. To ensure the cooling requirements of high-speed steel rolls and avoid a sharp increase in thermal stress, a combined spray device with annular distribution is specially designed according to the different requirements for cooling water volume according to the rolling specifications. It is made of stainless steel. The width of the spray water curtain exceeds the width of the rolling groove slot by 2~4cm. The spray water curtain forms an angle of 30° along the tangent direction of the roll. The water volume of the outlet end ring 1/2 of the rolling groove accounts for 70% of the total water volume. The imported guide cooling Try not to spray water directly onto the rolled parts to prevent blackhead steel. The main nozzle is located near the exit guide. The water volume is 30% of the total water volume in the entire rolling groove. It ensures that the cooling water is sprayed on the part of the rolling groove that has just separated from the rolled piece. The purpose is to quickly replace the heat on the rolling groove and reduce heat loss. Generation of stress. Since the cooling conditions of the lower roller are worse than those of the upper roller, when designing the cooling nozzle of the lower roller, the total water volume is calculated to increase by 10% to 15% compared with the upper roller.

 

4) Optimize groove rolling process parameters

 

(1) Increase the centre distance of rolling grooves

 

In order to avoid bursting between grooves, the centre distance of rolling grooves should be appropriately increased while meeting the requirements of imported guides. General rolling Φ10~Φ
For 14mm ribbed steel bars, the centre distance of the rolling grooves is increased from the original 10mm to 15mm, the centre distance of the rolling grooves for Φ16~Φ25 is increased from the original 15mm to 20mm, and the centre distance of the rolling grooves above Φ25mm is increased to 25mm. At the same time, the notch fillet is increased from the original 0.8mm to 1.2mm to reduce stress concentration.

 

(2) Optimize K3 and K4 cutting wedges.

 

The arc radius of the wedge tip at the pre-cutting point is generally designed to be 1.4~2.0mm, and the wedge tip radius at the cutting pass is generally designed to be 0.6~0.9mm. From a design perspective, it is required that the two passes of rolling grooves must have high Wear resistance and must also have a certain degree of toughness, otherwise, excessive wear of the wedge tip or collapse of the wedge tip will easily occur. Therefore, under the premise of meeting the cutting requirements, the arc radius should be as high as possible.

 

(3) Strictly control the ratio of K2 material width to rolling groove inner diameter to not exceed 1.7 times.

 

Prevent the width of K2 from being too large, causing the reduction of K1 to be too large, causing the metal to flow quickly, and the repeated action of the rolled piece to cause filamentous flesh to fall out of the rolling groove or periodic lumps to form.

 

5) Strict process discipline

 

Strictly control the steel passing amount to prevent overuse of the rolls. The recommended steel passing amount for the single groove of the ribbed steel K1 roll is 400~450t, and the pre-slit roll K4 roll is 2000~2500t. When repairing the roll, ensure that all the edges of the rolling groove and the bottom cracks are turned There should be no black skin or small cracks at the notches.

Strictly install the exit guide device to prevent the guide from directly contacting the rolling groove.

Strictly control the amount of cooling water at the roll inlet to reduce the chill on the head of the rolled piece.

Strengthen dynamic monitoring during the production process to reduce roll-wrapping failures. When wrapping the roll, cooling water must be used to cool all the scrap products wrapped around the roll to room temperature before water is allowed to be stopped for processing scrap steel.

 

3. Conclusion

 

When hot rolling work rolls are working, they often bear the combined effects of thermal fatigue, mechanical, friction and other stresses at the same time. To reduce the occurrence of high-speed steel roll cracking accidents, the use conditions of the rolls must be comprehensively considered. After the implementation of the above measures, the number of high-speed steel roll cracks has been reduced from 4 to 5 times per month to less than once at present, effectively reducing process failures and improving production efficiency and product quality.

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