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Analysis and control of the causes of cracking of high-speed steel rolls

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The working conditions of high-speed steel rolls were systematically analyzed, and it was clarified that the main cause of roll cracking was the combined effect of thermal fatigue, mechanical, friction and other stresses. By implementing measures such as improved cooling water application and optimization of process parameters, the occurrence of tank explosion accidents has been reduced and production efficiency has been improved.

keyword: high-speed steel rolls

The Bar Factory of Laiwu Company 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, and prestressed precision-rolled threads. In order to improve the surface quality and production efficiency of steel bars, the use of high-carbon and high-vanadium series high-speed steel rolls has been promoted in finishing rolling units since 2010, 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 rollers are significantly better than ordinary rollers. The amount of steel passed in 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 rollers have a high cost performance. .

However, under the working conditions of the original rolls, it was found that the finished groove K1 and the slit and pre-slit grooves K3 and K4 frequently experienced groove explosions. The main manifestations are that the threaded cross ribs at the bottom of the K1 rolling groove of the finished product fell off and the roll rings in the middle of the two adjacent rolling grooves cracked, and the cutting wedges of the K3 and K4 rolling grooves cracked, causing production accidents and a decrease in the steel qualification rate, which brought harm to production and product quality. to have a greater impact. 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.

Analysis of the causes of high-speed steel rolls bursting

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 kind of cooling and heating conditions causes a continuously circulating temperature gradient in the radial direction of the roll, and fatigue thermal stress is generated around the rolling groove. As the rolling time increases, the thermal stress continues to increase, especially when the rolling is abnormal. For example, accidental steel jamming, circulating water failure, etc. will cause local overheating of the roller surface, a sudden increase in thermal stress, and then the formation of vertical cracks along the crystalline distribution. Once the thermal fatigue cracks are generated, they will rapidly expand along the interface between the carbide and the matrix. . 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 limit of the strength of the roll, stress concentration points such as the transverse ribs at the bottom of the rolling groove or the groove spacer ring will burst (groove explosion), causing an accident.

Cracks caused by mechanical and frictional stress

During the rolling process, the mechanical stress acting on the roll mainly includes: direct shear stress generated by the rolling load, bending stress and torsional shear stress generated by the torque necessary to drive the roll to overcome the rolling friction. The friction force generated by the relative sliding between the roll and the rolled piece. 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 improper use of cooling water or “blackhead steel” caused by flying shear errors. Low-temperature edge rolling, low-temperature wire drawing or other hard impurities caused by improper adjustment will produce severe impact loads after entering the rolling groove, which is 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 explosion 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.

Measures to control high-speed steel rolls bursting

Improving 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 roll. In severe cases, scale will form and block the pipes or nozzles. To this end, the rod factory added equipment such as a high-efficiency inclined plate chemical oil remover and a sludge filter press on the basis of the natural sedimentation of chemicals in the original advection tank. The cooling return water from the rolling mill flows into the iron sheet ditch and flows into the swirl sedimentation tank. After preliminary sedimentation, it is upgraded to the sewage chemical degreaser. After dosing, precipitation, oil removal and pH adjustment, it flows into the hot water well of the pumping station. It is lifted to the cooling tower by a cooling pump to cool down, and then pumped to the cold water well, and then sent to the turbid ring water users by the circulating water pump. . 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 on the next page.

parametersunitcontrol valueparametersunitcontrol value
PH 7~9total ironMg/L≤1.0
TurbidityNTU<30Oil contentMg/L≤5
total hardnessMg/L(CaCO3)<500ConductivityUs/cm<1500
ChlorideMg/L(Cr)≤400bacteria个/mL<1.0*105

Water volume and water pressure requirements

During production, the roll cooling water pressure should be maintained between 0.4 and 0.6 MPa. Cooling nozzles with different flow rates are used according to different rolling specifications and speeds. When rolling Φ10~Φ50 mm steel bars, the cooling water volume in a single slot is generally controlled at 300~500 L/min. Adjust the water volume and pressure according to changes in ambient temperature. Especially in northern winter, when the ambient temperature drops below -10°C, operational monitoring should be strengthened. When the roll starts rolling in the cold state, the frequency of steel passing should not be too fast, so that the roll has a slow temperature rise process. After testing, after stopping the steel passing, the difference between the surface temperature of the rolling groove and the ambient temperature is preferably controlled within 15°C.

Change the traditional rolling groove cooling method

Traditional rolling groove cooling generally has a cooling nozzle on the upper and lower rollers at the entrance and exit of the rolled piece. The size of the nozzle and the spray angle are random and cannot be standardized, resulting in uneven cooling of the rolling groove, excessive roll temperature, and often bursting of the rolling groove due to cooling problems. In order to ensure the cooling requirements of high-speed steel rolls and avoid a sharp increase in thermal stress, an annularly distributed combined spray device is specially designed according to the different requirements for cooling water volume according to rolling specifications (see Figure 1). Made of stainless steel, the width of the jet water curtain exceeds the width of the rolling groove slot by 2 to 4 cm. The spray water curtain forms an angle of 30° along the tangent direction of the roll. The water volume in 1/2 of the rolling groove at the outlet end accounts for 70% of the total water volume. Try not to spray the imported guide cooling water directly onto the rolled piece to prevent blackhead steel. The main nozzle is located near the exit guide, and the water volume is 30% of the total water volume in the entire rolling tank. Ensure that the cooling water is sprayed into 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 the generation of thermal 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.

Optimize groove process parameters

1) Increase the center distance of the rolling groove.

In order to avoid bursting between grooves, the center distance of rolling grooves should be appropriately increased while meeting the requirements of imported guides. Generally, when rolling Φ10~Φ14 mm ribbed steel bars, the center distance of the rolling groove is increased from the original 10 mm to 15 mm. The center distance of the rolling groove is increased from the original 15 mm to 20 mm for Φ16~Φ25 mm. The center distance of the rolling groove is increased from Φ25 mm. The distance is increased to 25 mm. At the same time, the notch fillet is enlarged from the original 0.8 mm to 1.2 mm to reduce stress concentration.

2) Optimize K3 and K4 cutting wedges.

The wedge tip arc radius at the pre-cutting point is generally designed to be 1.4~2.0 mm, and the wedge tip radius at the cutting pass is generally designed to be 0.6~0.9 mm. From a design perspective, these two passes of rolling grooves are required to have high wear resistance. It 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.

Schematic diagram of rebar K1 roller three-section cooling spray device

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 meat loss or periodic block loss in the rolling groove.

Strict process discipline

Strictly control the steel passing amount to prevent excessive use of the rolls. The recommended steel passing amount for the single groove of the ribbed steel K1 roll is 400 to 450 t, and the pre-slit roll K4 roll is 2 000 to 2 500 t. When repairing the roll, ensure that the edge of the rolling groove, All cracks at the bottom should be turned away, especially at the notch. There should be no black skin or small cracks. 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 of 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.

backup rolls

When hot rolling work rolls are actually working, they often bear the combined effects of thermal fatigue, mechanical, friction and other stresses at the same time. In order 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 cracks in high-speed steel rolls 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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Product advantage

The world No. 1 in crude steel production steel plant, over 70% of the HSS for bar and wire are from us.

LMM high-speed steel rolls have higher steel passing per groove (times) due to their good wear resistance, which saves roll changing time, improves rolling mill operation rate, reduces roll consumption, and improve the overall efficiency of the factory.
Generally, the amount of steel passing in a single groove (times) is 3 to 5 times that of cast iron rolls.

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