A method for heat treating a roll for a roughing stand for a wire rod mill

Abstract

The application discloses a kind of heat treatment methods of roll for wire bar roughing stand, the diameter of the roll is 300-1000mm, heat treatment method includes the following steps: S1, normalizing: the roll is heated to 820-860 ℃, and the temperature of the whole roll section is uniform, then the roll is cooled to room temperature in air;S2, tempering: the roll after normalizing treatment is heated to 680-720 ℃ again, and then kept for 10-15 hours;S3, cooling: the roll after keeping is cooled using cooling device, first spray+water spray cooling t1 time, then spray cooling t2 time again, improve the quenching cooling intensity of roll, obtain tempering sorbite structure;The application improves the quenching cooling intensity of roll, improves the matrix strength of roll.

Description

Technical Field

[0001] This invention relates to a heat treatment method for rolls used in roughing mills for wire rods, belonging to the field of roll manufacturing technology. Background Technology

[0002] In recent years, with the development of high-speed and continuous rolling of wire rod mills at home and abroad, and the continuous improvement of the performance of rolled materials, especially the long length of rolled materials and the long heating time of rolls in recent years, steel mills have increasingly higher requirements for the applicability of rolls. They not only require rolls to have good wear resistance and high strength, but also better resistance to thermal cracking and toughness.

[0003] Currently, wire rod roughing mill stands generally use pearlitic ductile iron rolls, with roll diameters typically ranging from 300 to 1000 mm. The heat treatment process during roll fabrication generally involves normalizing followed by high-temperature tempering, and cooling is usually achieved using methods such as... Figure 3 The cooling device shown includes four rollers 15 driven by a power mechanism to support and rotate the roll. The roll 17 is placed on the rollers 15, which support the roll body and are driven by the power mechanism to rotate the roll 17. Cooling is achieved using a fan 16 for air cooling and water pipes for water cooling. Under the force of the fan 16, the water flowing from the pipes is sprayed in a mist, achieving both air cooling and atomized water cooling, ultimately resulting in a pearlitic microstructure for the roll. Rolls processed according to this heat treatment process have the following problems during use:

[0004] 1. The rolls have poor wear resistance, severe hot cracking at the bottom of the grooves, and low single-pass rolling yield. Due to the high rolling temperature and slow rolling speed of the roughing mill stand, the surface temperature of the roll body is relatively high. Products with simple spray cooling have a pearlitic structure as the matrix and low strength. Under the continuous impact of the steel billet, hot cracks are prone to appear at the bottom of the roll grooves.

[0005] 2. During the rolling process, hot cracks are prone to appear at the bottom of the groove, and the wear resistance is poor, which makes it impossible to maintain a good roll pass. This results in the inability to guarantee a good material shape after rough rolling, affecting the quality of subsequent rolling.

[0006] 3. Significant differences in performance between the inner and outer working layers of the rolls. For the heat-treated rolls described above, the hardness difference between the inner and outer working layers within a 100mm diameter layer can generally reach over 5 HSD. This leads to a significant decrease in wear resistance during later use, resulting in shorter on-machine rolling time and impacting rolling efficiency. Summary of the Invention

[0007] To solve the above-mentioned technical problems, this invention provides a heat treatment method for rolls used in roughing mills for wire rods, which improves the quenching and cooling intensity of the rolls, changes the microstructure, obtains tempered sorbite microstructure, improves the strength of the roll matrix, and improves the overall performance of the rolls by more than 1.5 times.

[0008] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0009] A heat treatment method for rolls used in a roughing mill for wire rod, wherein the diameter of the rolls is 300-1000 mm, and the heat treatment method includes the following steps:

[0010] S1. Normalizing: Heat the roll to 820-860℃ and hold for 5-10 hours to make the temperature of the entire roll cross section uniform. Then cool the roll to room temperature in the air.

[0011] S2. Tempering: Reheat the normalized rolls to 680-720℃ and then hold for 10-15 hours.

[0012] S3, Cooling: The heat-insulated rolls are cooled using a cooling device. First, spray cooling + water cooling is performed for time t1, and then spray cooling is performed for time t2 to improve the quenching cooling intensity of the rolls and obtain tempered sorbite structure. Among them, when the roll diameter is 300-500mm in S3, t1 is 5-10min and t2 is 10-15min.

[0013] When the roll diameter is 500-800mm, t1 is 10-20min, and t2 is 15-25min;

[0014] When the roll diameter is 800-1000mm, t1 is 20-30min, and t2 is 25-30min.

[0015] A further improvement of the technical solution of the present invention is that: the cooling device in S3 includes a trolley driven by a power mechanism to support and drive the rotation of the roll, and four trolleys are provided and distributed below the roll body; it also includes a spray nozzle and a water spray nozzle provided on a fixed crossbeam, which are supplied with water by a water supply device and moved by a moving device, and the spray nozzle and water spray nozzle are evenly spaced in a row, with the spray nozzle in the upper row and the water spray nozzle in the lower row; two sets of the spray nozzle and water spray nozzle, as well as their water supply device and moving device, are provided, respectively located on both sides of the roll;

[0016] The water supply device includes a constant temperature water tank with two outlets, and the two outlets are respectively connected to the first branch pipe and the second branch pipe through the first main pipe and the second main pipe. The tail ends of the spray nozzle and the spray nozzle are respectively connected to the first branch pipe and the second branch pipe through water pipes. A high-pressure water pump is installed on both the first main pipe and the second main pipe.

[0017] The moving device includes a lead screw that passes through and engages with threaded holes at both ends of a fixed crossbeam. The top of the lead screw is connected to the drive shaft of a motor, and the bottom is mounted on a moving frame. The bottom of the moving frame is provided with a pulley, and a matching slide rail is provided below the pulley. The top of the moving frame is provided with a hinge hole in which the lead screw can rotate. A push-pull cylinder that can push the moving frame is provided on one side.

[0018] The motor, push-pull cylinder, power mechanism, and high-pressure water pump are all electrically connected to the PLC controller.

[0019] A further improvement of the technical solution of the present invention is that: during cooling in S3, the distance between the spray nozzle and the water spray nozzle in the cooling device and the roll is controlled to be 300-500mm.

[0020] A further improvement of the technical solution of the present invention is that: valves are provided on the first main pipe, the second main pipe, the first branch pipe and the second branch pipe in the water supply device, and a switch is provided on each spray nozzle and water spray nozzle, and the switches are electrically connected to the PLC controller.

[0021] A further improvement of the technical solution of the present invention is that: the inlet of the constant temperature water tank is connected to the water pipe network of the factory, and a valve is installed on the inlet pipe; a heating rod and a thermometer are installed inside the constant temperature water tank; and the heating rod, the thermometer, and the valve on the inlet pipe are all electrically connected to the PLC controller, and the water temperature in the constant temperature water tank is set to 20°C by the PLC controller.

[0022] A further improvement of the technical solution of the present invention is that: each of the movable frames is provided with two push-pull cylinders located at both ends of the movable frame, and the push-pull cylinders are fixed by a fixed column.

[0023] A further improvement of the technical solution of the present invention is that: the movable frame is configured as a trapezoidal structure consisting of a top plate, a bottom plate and two side plates, and the movable frame is provided with an upright plate for fixing the motor.

[0024] A further improvement of the technical solution of the present invention is that: two baffles are provided on the upright plate, and the lower end of the baffles is provided with a semi-circular hole that can accommodate the neck of the rolling mill.

[0025] A further improvement of the technical solution of the present invention is that: four pulleys are provided and distributed at the four corners of the movable frame.

[0026] A further improvement of the technical solution of the present invention is that: a bearing is provided in the hinge hole, and the inner ring of the bearing is fixed to the lead screw, and the outer ring is fixed to the inside of the hinge hole.

[0027] The technological advancements achieved by this invention due to the adoption of the above technical solutions are as follows:

[0028] This invention, by limiting the water cooling + spray cooling method and time, can significantly improve the quenching cooling intensity and uniformity, improve the microstructure of pearlitic roll matrix, obtain tempered sorbite microstructure, improve the roll matrix strength, increase the roll body hardness uniformity to 1-2 HSD, increase the single rolling amount of roll by 1.5 times, and greatly improve the surface cracks on the bottom of the groove after exiting the mill.

[0029] This invention also provides a complementary cooling system that achieves the required water-cooling + spray cooling method and time, and utilizes a PLC controller to achieve automated equipment control and one-button operation for production. Additionally, a specially designed water baffle is installed at the roller diameter position to prevent stress concentration and cracking at the roller's side and root arc during spray quenching. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the main structure of the present invention;

[0031] Figure 2 This is a top view of the structure of the present invention;

[0032] Figure 3 This is a schematic diagram of the cooling device mentioned in the background section of this invention;

[0033] Figure 4 This is a metallographic structure image at 50x magnification in an embodiment of the present invention;

[0034] Figure 5 This is a metallographic structure image at 100x magnification in an embodiment of the present invention;

[0035] Figure 6 This is a metallographic structure image at 400x magnification in an embodiment of the present invention;

[0036] Among them, 1. Spray nozzle, 2. Water spray nozzle, 3. Water pipe, 4. High-pressure water pump, 5. Constant temperature water tank, 6. Fixed crossbeam, 7. Screw, 8. Motor, 9. Moving frame, 10. Pulley, 11. Slide rail, 12. Push-pull cylinder, 13. Fixed column, 14. Water baffle, 15. Drag wheel, 16. Fan, 17. Roller. Detailed Implementation

[0037] The existing heat treatment method for rolls used in linear bar roughing mill stands is normalizing followed by high-temperature tempering, with air cooling and atomized water cooling used for cooling. However, rolls manufactured according to this heat treatment process have the following problems during use:

[0038] 1. The rolls have poor wear resistance, severe hot cracking at the bottom of the grooves, and low single-pass rolling yield. Due to the high rolling temperature and slow rolling speed of the roughing mill stand, the surface temperature of the roll body is relatively high. Products with simple spray cooling have a pearlitic structure as the matrix and low strength. Under the continuous impact of the steel billet, hot cracks are prone to appear at the bottom of the roll grooves.

[0039] 2. During the rolling process, hot cracks are prone to appear at the bottom of the groove, and the wear resistance is poor, which makes it impossible to maintain a good roll pass. This results in the inability to guarantee a good material shape after rough rolling, affecting the quality of subsequent rolling.

[0040] 3. Significant differences in performance between the inner and outer working layers of the rolls. For the heat-treated rolls described above, the hardness difference between the inner and outer working layers within a 100mm diameter layer can generally reach over 5 HSD. This leads to a significant decrease in wear resistance during later use, resulting in shorter on-machine rolling time and impacting rolling efficiency.

[0041] The present invention provides a heat treatment method for rolls used in roughing mills for wire rods, which can significantly improve the quenching and cooling strength of the rolls, change the microstructure, obtain tempered sorbite microstructure, improve the strength of the roll matrix, and improve the overall performance of the rolls by more than 1.5 times.

[0042] The present invention will be further described in detail below with reference to embodiments:

[0043] A heat treatment method for rolls used in a roughing mill for wire rods includes the following steps, wherein the diameter of the rolls is 300-1000 mm:

[0044] S1. Normalizing: Heat the roll to 820-860℃ and hold for 5-10 hours to make the temperature of the entire roll cross section uniform. Then cool the roll to room temperature in the air.

[0045] S2. Tempering: Reheat the normalized rolls to 680-720℃ and then hold for 10-15 hours.

[0046] S3, Cooling: The heat-insulated rolls are cooled using a cooling device. First, spray cooling + water cooling is performed for time t1, and then spray cooling is performed for time t2 to improve the quenching cooling intensity of the rolls and obtain tempered sorbite structure. Among them, when the roll diameter is 300-500mm in S3, t1 is 5-10min and t2 is 10-15min.

[0047] When the roll diameter is 500-800mm, t1 is 10-20min, and t2 is 15-25min;

[0048] When the roll diameter is 800-1000mm, t1 is 20-30min, and t2 is 25-30min.

[0049] A matching cooling device is also provided for the above-mentioned cooling.

[0050] like Figure 1 and 2As shown, a cooling device for the rolls of a wire rod roughing mill includes four rollers 15 driven to rotate by a power mechanism. These rollers 15 are distributed below the roll body to support and drive the rolls to rotate. The power mechanism is electrically connected to a PLC controller, which controls the running time of the rollers 15.

[0051] The cooling device also includes spray nozzles 1 and water spray nozzles 2, which are supplied with water by a water supply device and moved by a moving device. Two sets of spray nozzles 1 and water spray nozzles 2, along with their water supply and moving devices, are provided, located on both sides of the roll 17. The spray nozzles 1 and water spray nozzles 2 are mounted on a fixed crossbeam 6 and are evenly spaced in a row, with spray nozzles 1 in the upper row and water spray nozzles 2 in the lower row. The distance between any two adjacent spray nozzles 1 is 150mm, and the distance between any two adjacent water spray nozzles 2 is also 150mm. Furthermore, the distance between the upper row of spray nozzles 1 and the lower row of water spray nozzles 2 is 200mm.

[0052] The water supply device includes a constant-temperature water tank, inside which are installed heating rods and thermometers, both electrically connected to a PLC controller. The PLC controller can be configured to maintain the water temperature in the tank at 20°C. When the water temperature is below 20°C, the heating rods will activate to heat the water until it reaches 20°C, at which point heating will cease. The constant-temperature water tank has two outlets and one inlet. The inlet is connected to the plant's water network and is equipped with a valve. This valve is also electrically connected to the PLC controller. When the water temperature in the tank exceeds 20°C, the PLC controller opens the valve on the inlet, injecting cooler groundwater. The two outlets are connected to a first main pipe and a second main pipe, respectively, and to a first branch pipe and a second branch pipe. The ends of the spray nozzles 1 and 2 are connected to the first and second branch pipes via water pipes 3, respectively. High-pressure water pumps are installed on both the first and second main pipes. Valves are installed on the first main pipe, second main pipe, first branch pipe, and second branch pipe of the water supply device. Each spray nozzle 1 and water spray nozzle 2 is equipped with a switch, and the switches and the high-pressure water pump are electrically connected to the PLC controller. The spraying time of spray nozzle 1 and water spray nozzle 2 are controlled by the PLC controller.

[0053] The moving device includes two lead screws 7, which pass through threaded holes at both ends of the fixed crossbeam 6 and engage with the threaded holes. Rotating the lead screws 7 causes the fixed crossbeam 6 to move up and down along the lead screws 7, thereby driving the spray nozzle 1 and the water spray nozzle 2 to move up and down. The power for rotating the lead screws 7 comes from a motor 8 at the top, and the drive shaft of the motor 8 is connected to the top end of the lead screw 7. A moving frame 9 is provided at the bottom of the lead screw 7, and a vertical plate for fixing the motor 8 is provided on the moving frame 9.

[0054] The movable frame 9 is configured as a trapezoidal structure consisting of a top plate, a bottom plate, and two side plates. The top plate has a hinge hole within which a lead screw 7 can rotate. A bearing is installed within the hinge hole, with the inner ring of the bearing fixed to the lead screw and the outer ring fixed to the inside of the hinge hole. Four pulleys 10 are located at the bottom of the movable frame 9, distributed at the four corners. A matching slide rail 11 is located below each pulley 10. A push-pull cylinder 12 is installed on one side of each movable frame 9, capable of moving it. Each movable frame 9 has two push-pull cylinders 12 located at both ends, and these cylinders are fixed by a fixed column 13. The motor 8 and the push-pull cylinders 12 are all electrically connected to a PLC controller. The PLC controller controls the motor 8 to rotate, which drives the spray nozzle 1 and water spray nozzle 2 to move up and down. It controls the push-pull cylinder 12 to extend and retract, which drives the spray nozzle 1 and water spray nozzle 2 to move left and right. The distance between the spray nozzle 1 and water spray nozzle 2 and the outer side of the roller is adjusted, which is generally 300-500mm.

[0055] The first stand of a wire rod roughing mill at a domestic steel plant was produced using the aforementioned cooling device and heat treatment process, followed by performance testing. The test results are as follows:

[0056] 1. Hardness difference detection within the working layer

[0057] Distance from roller surface (mm) 5 15 20 25 30 35 40 45 Hardness HSD 55 55 54 54 54 54 53 53

[0058] 2. Metallographic structure testing, such as Figure 4-5 As shown in the figure, the microstructure is tempered sorbite + carbide, with graphite area of ​​6.05%, carbide area of ​​11.23%, and matrix microhardness of 334.15 Hm.

[0059] 3. Roll diameter tensile strength test: 675 MPa

[0060] The test product was used on the first stand of the roughing mill, with a single rolling capacity of 11,500 tons, twice that of the previous products. After rolling, the product surface quality was good, with no hot cracks, and the grinding allowance was 12mm, a decrease of 45%. Currently, the equipment is in normal operation, and the products produced are being used in more than 30 steel mills at home and abroad, all achieving the above results.

Claims

1. A heat treatment method for rolls used in a roughing mill stand for wire rod, characterized in that: The diameter of the roll is 300-1000 mm, and the heat treatment method includes the following steps: S1. Normalizing: Heat the roll to 820-860℃ and hold for 5-10 hours to make the temperature of the entire roll cross section uniform. Then cool the roll to room temperature in the air. S2. Tempering: Reheat the normalized rolls to 680-720℃ and then hold for 10-15 hours. S3. Cooling: The heat-insulated rolls are cooled using a cooling device. First, spray cooling + water cooling is performed for time t1, and then spray cooling is performed for time t2 to improve the quenching cooling intensity of the rolls and obtain tempered sorbite structure. Among them, when the roll diameter is 300-500mm in S3, t1 is 5-10min and t2 is 10-15min. When the roll diameter is 500-800mm, t1 is 10-20min and t2 is 15-25min; When the roll diameter is 800-1000mm, t1 is 20-30min and t2 is 25-30min; The cooling device in S3 includes a trolley (15) driven by a power mechanism to support and drive the rotation of the roll. There are four trolleys (15) distributed below the roll body. It also includes a spray nozzle (1) and a water spray nozzle (2) installed on a fixed crossbeam (6), which are supplied by a water supply device and moved by a moving device. The spray nozzle (1) and the water spray nozzle (2) are evenly spaced in a row, with the spray nozzle (1) in the upper row and the water spray nozzle (2) in the lower row. There are two sets of the spray nozzle (1) and the water spray nozzle (2), as well as their water supply device and moving device, which are located on both sides of the roll (17). The water supply device includes a constant temperature water tank with two outlets, and the two outlets are connected to the first branch pipe and the second branch pipe through the first main pipe and the second main pipe respectively. The tail ends of the spray nozzle (1) and the spray nozzle (2) are connected to the first branch pipe and the second branch pipe through the water pipe (3) respectively. A high pressure water pump is installed on both the first main pipe and the second main pipe. The moving device includes a lead screw (7) that passes through threaded holes at both ends of a fixed crossbeam (6) and engages with the threaded holes. The top of the lead screw (7) is connected to the drive shaft of a motor (8), and the bottom is mounted on a moving frame (9). The bottom of the moving frame (9) is provided with a pulley (10), and a matching slide rail (11) is provided below the pulley (10). The top is provided with a hinge hole in which the lead screw (7) can rotate. A push-pull cylinder (12) that can push the moving frame (9) is provided on one side. The motor (8), push-pull cylinder (12), power mechanism and high-pressure water pump are all electrically connected to the PLC controller; The inlet of the constant temperature water tank is connected to the water pipe network of the factory, and a valve is installed on the inlet pipe. The constant temperature water tank is equipped with a heating rod and a thermometer. The heating rod, thermometer and valve on the inlet pipe are all electrically connected to the PLC controller. The water temperature in the constant temperature water tank is set to 20℃ by the PLC controller. The resulting roll microstructure is tempered sorbite + carbides.

2. The heat treatment method for rolls used in a wire rod roughing mill stand according to claim 1, characterized in that: During cooling in S3, the distance between the spray nozzle (1) and the water spray nozzle (2) in the cooling device and the roll is controlled to be 300-500mm.

3. The heat treatment method for rolls used in a wire rod roughing mill stand according to claim 1, characterized in that: Valves are installed on the first main pipe, the second main pipe, the first branch pipe and the second branch pipe in the water supply device. Each spray nozzle (1) and spray nozzle (2) is equipped with a switch and the switches are electrically connected to the PLC controller.

4. The heat treatment method for rolls used in a wire rod roughing mill stand according to claim 1, characterized in that: Each of the movable frames (9) is provided with two push-pull cylinders (12) located at both ends of the movable frame (9), and the push-pull cylinders (12) are fixed by a fixed column (13).

5. The heat treatment method for rolls used in a roughing mill stand for wire rods according to claim 1, characterized in that: The mobile frame (9) is configured as a trapezoidal structure consisting of a top plate, a bottom plate and two side plates, and the mobile frame (9) is provided with a vertical plate for fixing the motor (8).

6. The heat treatment method for rolls used in a roughing mill stand for wire rods according to claim 5, characterized in that: The upright plate is provided with two baffles (14), and the lower end of the baffles (14) is provided with a semi-circular hole that can accommodate the neck of the roll.

7. The heat treatment method for rolls used in a wire rod roughing mill stand according to claim 1, characterized in that: The pulleys are provided in four positions and are located at the four corners of the movable frame (9).

8. The heat treatment method for rolls used in a wire rod roughing mill stand according to claim 1, characterized in that: A bearing is installed inside the hinge hole, with the inner ring of the bearing fixed to the lead screw and the outer ring fixed inside the hinge hole.

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