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Practice and discussion of domestic PC steel rod production line

in recent years, with the rapid promotion and application of pre gravity concrete pipe piles, the production level of domestic PC steel rod production line has also been greatly improved after continuous exploration and mutual reference. Its important signs: the production line speed, diameter 9.0 from 30 ~ 40 m/min to 100 m/min, diameter 10.7 from 20 ~ 30 m/min to 90 m/min; The elongation after fracture is stably controlled between 8.5% and 9.0%; The author has the honor to organize and participate in the independent research and development of the production line of a PC steel bar production enterprise, which lasted for 4 years. I am willing to share and discuss the insights with you, and hope to help the development of domestic PC steel bar production line

I. composition and characteristics of PC steel bar production line

1. Composition of production line: raw material setting out, descaling, wire drawing (after wire drawing alone, it is necessary to set out again), straightening, traction, quenching heating, quenching water cooling, tempering heating, tempering water cooling, traction, shearing, branching fork and finished product winding

2. the production line can be divided into two types: the continuous drawing production line and the separate drawing production line according to whether the drawing is connected with the subsequent process

the main advantage of wire drawing linkage is that it can make full use of the heat energy generated by wire drawing and save 10 ~ 40kW power consumption according to different production specifications and speeds; Compared with separate wire drawing, it can reduce the setting out process after wire drawing, so it not only saves the stacking yard of semi-finished products after wire drawing, but also saves 1 ~ 1.5 personnel per shift for each line; The second is to reduce the material loss of each joint by about 2 meters. Of course, the premise of continuous wire drawing is that the consumption of wire drawing die is relatively low, otherwise the operation of the whole production line will be affected due to frequent die change, and the length of waste wire caused by each die change shutdown is 1 ~ 1.5 times of the linear speed, which is equivalent to the waste wire of 30 ~ 50 joints drawn separately. Therefore, the necessary premise of continuous wire drawing is that the service life of each die is not less than 40 ~ 50 tons, Which way to adopt the production line should be determined by the enterprise according to the specific situation

3. setting out can be divided into two types: twisting and untwisting

4. Due to environmental constraints, mechanical descaling is generally used, which is not as effective as chemical descaling, and is affected by the aging of raw materials

5. Wire drawing can be divided into separate wire drawing and continuous wire drawing according to the linkage relationship with subsequent processes. Continuous wire drawing adopts gear speed change, which has high efficiency and low wear, and the heat on the steel bar can be used. Drawing die can be divided into passive rotation and active rotation. Since the active rotary die can accurately control the wire drawing lead, it is conducive to extending the service life of the die and improving productivity

6. The "rule of man" characteristics of private enterprises leading in the high-end manufacturing of power transmission and distribution are more obvious in the horizontal and vertical directions, breaking the foreign monopoly. If the continuous drawing process is adopted, the horizontal alignment can be simplified or even cancelled

7. traction is divided into two ways: full active drive of upper and lower wheels and passive drive of upper and lower wheels: full active drive can increase the traction force, but there are five main aspects in the uneven traction wheel: after uniform wear, the traction line speed at each point of each traction wheel will be inconsistent, thus increasing the loss of the traction mechanism. Power transmission is divided into hard connection and soft connection (mainly belt). For heating traction, the former has little effect on the tension of the steel bar in the heating zone during shearing, and the latter can reduce the anti rotation torque of the drive during shearing

8. quenching heating can be divided into two stages (table) heating and three stages (table) heating. The high-speed line should adopt three-stage heating, which is beneficial to reduce the power of a single heating power supply and facilitate manufacturing, and to improve the heating efficiency

9. The arrangement and design of quenching water cooling system have a great impact on product quality and production, and are highly dependent on experience

10. The reasonable arrangement of tempering heating and quenching water cooling has a great impact on product quality and energy consumption, which needs to be reasonably balanced. At the same time, the tempering heating frequency affects the distribution gradient of product hardness, which directly affects the roll welding of pipe piles

11. shearing can be divided into synchronous shearing and asynchronous shearing. It is better to adopt synchronous cutting for high-speed lines

12. The finished product winding mechanism is divided into horizontal and vertical. Horizontal type is not easy to arrange wires and has poor molding, but it is conducive to high-speed and safe production; The vertical line is neat, but it is difficult for the finished product to enter the plate and accelerate the synchronization, and the requirements for operators are also high

II. Current situation of domestic PC steel bar production line and bottle diameter to improve production speed

due to various reasons, PC steel bar production line development units do not take PC steel bar as an industry to operate for a long time and devote themselves to research, so they lack effective human and financial investment and close cooperation with PC steel bar production enterprises. At present, they mainly stay in the imitation stage, and it is difficult to effectively break through the production capacity of the production line in a short time. Next, the author will share with colleagues his experience in developing PC steel bar production line and the bottle diameter encountered in recent years

1. Several problems in PC steel bar production line: of course, the heat treatment process is the first. In order to make it easy to explore a reasonable process and stabilize the product quality, the process conditions should be stabilized as much as possible. It is suggested that the water system should adopt constant pressure water supply, and the cooling tower should adopt constant temperature control (preferably spray quenching energy flow control), which is also conducive to energy conservation; In addition, it is the control of tempering power, because the tempering temperature is mainly affected by many factors, such as linear speed, quenching hardness and cooling conditions, tempering heating power and so on. When one of these factors is abnormal, it will inevitably cause tempering temperature fluctuations. At this time, the quality of tempering power control mode is very critical to ensure the stability and consistency of product quality and power consumption; Another problem should be included in the drawing die consumption, because die consumption involves many factors, such as die quality, descaling, drawing lubricant, die head cooling, wire drawing lead control and control dead zone, die head driving characteristics, and so on. It is not easy to solve it well. Another is that there is no reasonable method to remove the residual lubricant and oxide skin on the steel bar after wire drawing, which is easy to cause blockage by sintering in the first ring. There is also a problem of dust pollution. The invasion of lubricants and oxide scales in dust to on-site production personnel, heating power supply and control equipment cannot be ignored, but it is also difficult to solve

2. Current equipment status of PC steel bar production line. AC drive is generally used for wire drawing. The main advantage is that the motor is resistant to environmental pollution. The disadvantage is that the inverter has high power, energy consumption braking is unrealistic, and the cost of inverter braking is not easy to accept, so it is difficult to stop in an emergency; DC drive is easy to realize four quadrant operation and stop quickly. In high-power applications, the cost is low. AC drive has the main disadvantage of large maintenance of DC motor. In the transmission of heat treatment production line, the cost of using AC drive is low, but the anti electric disturbance and overload capacity of the former is lower than that of the latter, which is particularly profound for enterprises that have several AC drive and DC drive production lines at the same time. In terms of heating equipment, parallel inverter is widely used in heating power supply, so the loss of filter reactor is quite large, and the electrical corrosion is also very serious. Using series inverter can get rid of the reactor, which is difficult to reliably solve the problem of overcurrent protection, which may be an obstacle to the application of series inverter power supply; At present, the commutation angle of inverter circuit is difficult to be controlled stably and automatically, which is unfavorable for the accurate control of tempering heating power. For ultrasonic quenching heating, load change is easy to cause inverter tube damage; Because the production line works continuously all year round, the comprehensive heating efficiency must be paid enough attention. For example, the high current circuit should be designed from the economic density rather than the safety density. At the same time, the efficiency advantage should be fully reflected in the price of the equipment, which is beneficial to the healthy development of the equipment manufacturing industry, At the same time, it is also conducive to reducing the production cost of PC steel bars (generally speaking, the increased selling price by improving the power efficiency is enough to recover the electricity saved in one year). In terms of automatic control of transmission and heating, the functions of diagnosis, abnormal record filing and fault tracing should be introduced to facilitate maintenance and improve the quality assurance system; At the same time, the real-time closed-loop control of wire drawing die lead and driving torque is added to create conditions for reducing die consumption

3. The main bottle diameter of developing high-speed production line is to explore and improve the corresponding heat treatment process. First of all, we should solve the contradiction between rapid heating and low temperature gradient of the cross-section of the heated material expected by diathermy, which involves the reasonable segmentation of the heating area, the reasonable arrangement and collocation of heating frequency and power of each section, the balance between diathermy and heating efficiency, the reasonable setting of soaking area and many other aspects. Secondly, we should solve the permeability of rapid water-cooling quenching and explore a reasonable quenching curve and process

4. Another obstacle to the development of high-speed production lines is the lack of reverse zigzag testing machines. Generally, machines used for compression resistance, pressure holding, bending resistance, tearing and peeling, adhesion, shear and other performance of metal and non-metal composite materials and products, rubber and other industrial materials, lack of high reliability supporting equipment that fits the production site and is suitable for the cost. First of all, it is reflected in the heating power supply. Domestic SCR or IGBT power supply that can provide frequency of 6 ~ 10kHz and power of 500 ~ 1000kW is very few, and IGBT power supply with frequency of 40 ~ 60KHZ and power of 250 ~ 400KW is also few, and the mature and stable performance of products is less. Secondly, in mechanical equipment, in order to reduce the resistance torque of heating traction during high-speed wire cutting, there is an urgent need for a synchronous cutting device with appropriate cost and reliable work, or an asynchronous cutting device that can better control the amount of steel bar arch out during cutting; In order to cooperate with the winding of high-speed finished products, it is advisable to adopt automatic wire arrangement/winding, but there is no corresponding product supply in China; The linear speed of active rotary die bearing is limited, and high-speed applications may need to find another way; High speed production should adopt the continuous drawing process, otherwise it is difficult to increase the speed due to the large inertia of single drawing, which is bound to increase the ratio between the drawing unit and the heat treatment line. If the continuous drawing process is adopted, the problem of drawing die consumption will inevitably emerge; The surface wear resistance of the wire drawing drum needs to be improved, and so on, which shows that the maturity and improvement of the high-speed production line is a system and process, which requires the concerted efforts of colleagues in the industry to push the production level of PC steel bars to a new level in China. Limited to the level, the above must have many defects, and I hope my colleagues will not hesitate to give advice

appendix: Main Indicators of the existing domestic high-speed production line

specifications( φ Mm)

linear speed (M/min) 10090

tensile strength (MPA) ≥ 1420 ≥ 1420 ≥ 1420

elongation after fracture (%) 8 ~ 98 ~ 98 ~ 9

unit power consumption (degrees/ton) 380320310

note: the power consumption is on the low voltage side of the transformer, and the line voltage = 390v, cos φ= Measured at 0.95. Its value is closely related to process, heating coupling degree, power efficiency and other factors. (end)

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