Advanced cold continuous rolling process equipment and high-precision automatic control technology
Advanced cold continuous rolling process equipment and high-precision automatic control technology
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The integrated technology of the pickling cold continuous rolling combined unit (PL-TCM unit) is an internationally recognized comprehensive technology that is complex in system, technology-intensive and highly precise. It requires profound professional and comprehensive capabilities in aspects such as unit assembly, process, machinery and the three electric systems. Only a few top companies in the world, such as SMS of Germany and Mitsubishi of Japan, have the ability to integrate. After decades of development, especially the extensive introduction of technology in the past two decades, and through digestion, absorption, improvement and innovation, China has reached a new level in the accumulation and innovation of cold continuous rolling technology, as well as in engineering design, equipment manufacturing and the independent integration of production units.
The development of cold continuous rolling production technology
To meet the requirements of variety, specification, quality and different production scales of cold-rolled strip steel, the production of cold-rolled strip steel has undergone a transformation from single-sheet rolling to coil production, as well as the development from reversible rolling to fully continuous rolling and the combination of pickling and rolling mills. At present, large-scale and high-efficiency production of high-quality cold-rolled thin strip steel is mainly carried out on continuous rolling mills.
China's production of cold-rolled strip steel started relatively late. In 1960, the first 1700mm single-stand reversible cold rolling mill was established. Subsequently, 1200mm single-stand reversible cold rolling mills, MKW1400mm off-center eight-high rolling mills, 1150mm 20-high cold rolling mills and 1250mm HC single-stand reversible cold rolling mills were successively put into operation. Over the past few decades, the technological level of cold-rolling production equipment in our country has evolved from being able to produce only low-carbon thin plates to being capable of manufacturing high-carbon steel, high-alloy steel, stainless steel cold-rolled thin plates, galvanized plates, coated steel plates, plastic composite plates and silicon steel sheets, etc. In recent years, the production of cold-rolled strip steel in our country has made considerable technological progress. The adoption of new equipment and new processes has further met the technical requirements of cold rolling production. After more than 20 years of continuous development, the technology of the pickling and cold rolling combined unit (PL-TCM unit) has become increasingly mature.
2. Development Trends and new Technologies of PL-TCM units
2.1 Development Trends of PL-TCM Units
Continuous production by connecting pickling and cold continuous rolling can more economically achieve the required thickness, straightness and surface quality of the steel strip. It can significantly enhance the production capacity and output of the rolling mill. Due to the elimination of strip threading and tailing operations, production costs are saved, and the reduction of head and tail cutting losses of the strip can also increase the total output.
The necessary condition for combining different units to form a continuous production line is that the average metal flow rate per second of the steel strip in each process production line is equal. The pickling and cold rolling combined unit can meet this condition. Moreover, due to the adoption of turbulent pickling technology and mechanical pre-descaling equipment in the pickling unit, the pickling quality and production efficiency have been significantly improved. Through the application of technologies such as six-roll UCM or UCMW rolling mills, hydraulic roll clamping and positive and negative bending rolls, CVC technology, support roll protrusion control technology, hydraulic pressing down, roll segmented cooling, and high-precision strip thickness and straightness detection systems, the plate shape, plate thickness and surface quality of cold-rolled products are reliably guaranteed.
Compared with fully continuous cold rolling mills and conventional cold rolling mills, the PL-TCM unit has a higher production capacity, can produce high-quality cold-rolled steel strips, and requires less investment, has a high yield rate, good benefits, increasingly mature process technology, and reliable equipment performance. In summary, it has the following characteristics:
1) Productivity has been significantly enhanced: As it achieves headless rolling, there is no threading or tailing process, thus greatly increasing the actual production time of the rolling mill. Compared with conventional continuous rolling mills, the productivity has increased by approximately 10% to 20%.
2) Save equipment investment: The two almost identical sets of inlet and outlet section equipment originally used in pickling and continuous rolling can be replaced by one set after being combined, such as uncoiler, coiler and welder, etc. It also eliminates the intermediate warehouse, reduces the factory building area and intermediate hoisting equipment, thus lowering the total investment by more than 15%.
3) Enhance the service life of the rolls: Due to the realization of headless rolling, there is no threading or tailing, which reduces the impact of the steel strip on the rolls and lowers the wear of the rolls, thereby increasing their service life. According to statistics, for every 1,000 tons of cold-rolled strip steel produced, changing the work roll set can reduce the production by more than 60%.
4) Reduced scrap rate: When steel strips are stored in intermediate inventory, they often suffer from problems such as coil collapse, coil damage, and rust due to improper storage or prolonged storage, which reduces the yield of finished products. However, such problems clearly do not exist in the acid rolling combined unit. In addition, since there is no threading, the length of the super-thick steel strips at the head and tail is reduced, so the yield rate is relatively improved.
5) Improvement in product quality: Due to the absence of strip penetration, the scratches on the work rollers have been significantly reduced, greatly enhancing the surface quality of the finished steel strip.
6) Low cost: It is generally believed that the production cost of acid rolling units is reduced by about 20% to 25%, of which 10% to 12% comes from the improvement of product quality and yield rate, 3% comes from the binding tape and anti-rust oil of pickled steel coils, and the remaining comes from the improvement of roll life and labor savings, etc.
7) The shortcomings lie in the large size of the unit, the large number of equipment and the complex types. In addition, due to the limitation of the pickling process speed, the production capacity of the rolling mill cannot be fully utilized.
The acid rolling combined unit has excellent technical and economic benefits. Many continuous rolling equipment manufacturers around the world are investing a large amount of funds and human resources to develop and improve the PL-TCM technology. From the perspective of users, most metallurgical product producers are also competing to invest in building new PL-TCM units or converting old cold continuous rolling units into PL-TCM units, making PL-TCM units the current development trend in cold rolling production.
2.2 New Technology of PL-TCM Unit
The new technologies adopted by the PL-TCM unit (as shown in Figure 1) mainly include:
1) Turbulent pickling technology: Shallow tank turbulent pickling is a highly efficient pickling form carried out by the relatively rapid movement between the steel strip and the pickling medium during operation. It has the advantages of high pickling efficiency, low acid consumption and less discharge. The most crucial part of turbulent acid washing is the acid washing tank. The acid washing effect is achieved through the optimized design of the acid washing flow channel and high-pressure injection, and the acid washing process is completed through rinsing and drying.
2) Adoption of laser welding machines: The use of laser welding machines can meet the continuous production requirements of multiple steel grades. The weld seams are relatively smooth, with high strength, reducing the rate of broken belts and improving the quality of weld seams.
3) Adoption of high-precision quality control technology: including control technologies for plate thickness, plate shape and surface quality.
4) Application of intelligent control technologies such as neural networks: The introduction of a neural network system into the rolling control model has enhanced the setting accuracy of rolling parameters and improved the system's self-adaptive and self-learning capabilities to a certain extent.
5) Application of UC rolling mill: UC rolling mill, also known as universal convexity control rolling mill, is an extension of the basic concept of HC rolling mill. UC rolling mill is also equipped with an intermediate bending roller device to control special strip plate shapes. Its working roller diameter is smaller than that of HC rolling mill, and its compound control ability of plate shapes is stronger.
6) Combined application of the intermediate roller CVC system and the working roller EDC system: The EDC system consists of two major parts: the edge cooling system of the rolls and the EDC roll type. The edge cooling system of the rolls is an additional temperature control system with a more precise control range set at the edge of the rolls in addition to the original segmented cooling system, to achieve the control of the edge convexity of the rolls.
7) The adoption of Carrosel coiler: The application of this coiler makes the equipment layout at the outlet of the rolling mill more compact, and the switching of the rolls faster and more convenient. As a result, the length of the steel strip that has thickness deviations due to the loss of post-tension after leaving the last rolling mill at the tail end is reduced. Meanwhile, when the thickness of the cold-rolled strip is relatively thin, the head of the strip is prone to jumping up. It is required that the coiler be as close as possible to the exit of the last rolling mill. The Carrosel coiler precisely solves this problem.
As users' demands for cold-rolled sheet and strip products become increasingly strict, it has driven the continuous progress and development of cold-rolling related technologies, mainly including the following two aspects: First, how to improve and stably maintain quality; The second is how to produce at low cost and high efficiency. The most concerned quality indicators for cold-rolled products include mechanical properties, surface quality and dimensional accuracy. Among them, dimensional accuracy includes thickness accuracy, plate shape accuracy and edge drop accuracy. How to continuously improve quality based on user demands and maintain it stably is the goal and pursuit of every cold-rolling enterprise. The costs of the cold rolling process mainly include electricity consumption, emulsion consumption, and roll consumption, etc. How to reduce these consumables while ensuring stable production and high quality is directly related to the competitiveness of the products.
Centering on these two issues, three major aspects of cold rolling core technologies have gradually taken shape, including stable plate passing technology, surface control technology and precision rolling technology.
The stable plate passing technologies include: continuous rolling technology, process setting technology (rolling strategy and rolling regulations, roll gap setting, dynamic specification change, etc.), steel coil weight increase, vibration suppression technology, etc. Surface control technologies include: rolling lubricant technology, comprehensive prevention and control measures for slippage and thermal slip damage and corresponding process lubrication technology, improvement of roll material, and roll roughening technology, etc. Precision rolling technology includes: thickness control technology, plate shape control technology and edge drop control technology. Thickness control technologies include: hydraulic pressing technology, automatic thickness control technology, etc.
At present, the thickness control accuracy of advanced units internationally reaches ±0.5% to ±1.2% (varying according to different thicknesses). Plate shape control technology includes: hydraulic bending roll technology, UC series rolling mills and CVC series rolling mills, automatic plate shape control technology, etc. At present, the plate shape control accuracy of advanced units internationally has reached 5-12IU (varying according to different thicknesses). Edge drop control technologies include K-WRS rolling mills, EDC technology, etc. At present, the advanced edge drop control index internationally has reached 5μ (generally referring to the difference between the thickness at a position 100mm from the edge and that at a position 15mm from the edge).
3. Computer control system for cold continuous rolling mill
So far, the cold continuous rolling production line equipment and computer control systems for strip steel introduced by our country have encompassed all the core technologies in the world that master the production of cold-rolled strip steel. Although China has already acquired the design and manufacturing capabilities in mechanical equipment, the core control technology of cold continuous rolling, which serves as the "brain", has long been in the hands of a few companies such as SMS of Germany and Mitsubishi Hitachi of Japan. Due to the confidentiality of their core technologies, some key models and control functions introduced into the system are usually presented in a "black box" form, with only a few parameter interfaces visible. This greatly restricts the optimization of new functions, the development of new products, and future system upgrades and renovations, especially in today's era of continuous hardware and system software upgrades. More enterprises encounter situations where models and other software cannot be upgraded or even applied when replacing hardware.
To break through the "black box" limitations of key cold rolling technologies and establish a control system with independent intellectual property rights, the National Engineering Research Center for High Efficiency Rolling of University of Science and Technology Beijing has successively participated in the digestion, development and on-site commissioning of process control mathematical model systems for multiple production lines such as the pickling and cold continuous rolling combined units of Baosteel, Ansteel, Shougang and Wuhan Iron and Steel. After more than ten years of research and development, application and technological accumulation, the National Engineering Research Center for High Efficiency Rolling of University of Science and Technology Beijing has gone through a path from introduction, digestion and absorption to continuous optimization and improvement, and has now developed to independent innovation. It has the ability to independently design, integrate and develop the automatic control system of cold continuous rolling mills. In terms of service methods, it has the ability to provide cold continuous rolling process technology consultation, factory design, sub-item supply, and cooperate with heavy machinery factories for turnkey projects. It has promoted the independent innovation and localization process of large-scale high-end pickling cold continuous rolling mills, and enhanced China's core competitiveness in cold rolling control systems.
The control system of modern pickling cold continuous rolling mills is generally composed of a three-level computer control system. The three-level computer serves as the production management level, responsible for the input of production plans and the management of product data. The secondary computer serves as the process control level. The first-level computer serves as the basic level of automation.
3.1 Process Automation Control System
The core function of the process control system of the pickling cold continuous rolling mill is to provide reasonable load distribution and rolling setting parameters for the basic automation system of the rolling mill, and optimize the model through self-adaptation and self-learning. The rolling mill section also includes various model calculations (such as rolling force, forward slip, deformation resistance and friction coefficient, bending roll force and roll movement amount, roll thermal expansion and wear, strip steel temperature, etc.). The functions of the process control system also include: communication with basic automation and production management levels; Steel coil tracking Measurement value collection and processing; Statistics on the quality of finished steel strips and equipment operation data; Management of production planning data, raw material data, equipment data and actual performance data of strip steel production; It provides human-machine interfaces, report output and team management, etc.
The cold continuous rolling process automation system, on the one hand, serves as a bridge between the first-level basic automation and the third-level production management system, and should meet the requirements of high-speed data processing capacity. On the other hand, since multiple control functions ultimately act on the deformation zone of the steel strip, there is a strong coupling between functions, and information needs to be exchanged with each other. Therefore, the function of high-speed communication capability should be met.
3.1.1 Material Tracking
Material tracking in the secondary process control system is different from that in the basic automation system. Its tracking function mainly focuses on the unified coordination of the flow of steel coil materials, collecting various tracking information from the basic automation, conducting corresponding calculations, and transmitting the calculation results to the basic automation. Trigger data collection and processing, and perform functions such as model adaptation and self-learning.
3.1.2 Model Setting Calculation
The core and main task of computer control in the cold continuous rolling process is to calculate the set values of each mechanism through various mathematical models. To achieve correct setting and calculation, it should also have functions such as material tracking, data collection and processing, model self-adaptation and self-learning. The setting calculation is divided into pre-setting calculation, re-calculation and post-calculation (model adaptation), as well as the dynamic variable specification calculation required for the pickling - rolling mill combined unit, etc. For each set value, three calculations are conducted. The raw data is received from L3 or PDI, and the first calculation is carried out to determine whether the steel coil can be rolled. The steel coils are loaded onto the production line and undergo a second calculation to reconfirm whether the steel coils can be rolled. The third calculation is carried out before the steel strip arrives at the rolling mill to ensure that the accurate set value is sent to the first level.
The calculation set values for cold continuous rolling are divided into two major parts. One part is the thickness setting calculation value. Another part is the calculation value for setting the plate shape. The important input data for plate shape setting is the convexity of the steel strip of the raw material, which is provided to the process automation by L3 or raw material data. When the weld seam reaches a certain position before the first frame, each steel coil undergoes a plate shape pre-setting once, and the set value is sent to the basic automation system. The plate shape setting value is adaptively adjusted online by comparing the calculated value with the measured value.
3.1.3 Load Distribution
One of the important conditions for a process automation computer to calculate the set value is to rationally allocate the reduction amount of each frame based on the incoming material data. Reasonable load distribution can achieve energy-saving rolling, fully exert the production capacity of the rolling mill and ensure product quality. When calculating the set values of each frame, the rolling specification is an important basis for the distribution of the reduction load. A well-selected rolling specification can enable the best performance of the process and equipment. Rolling code calculation is based on PDI data such as the incoming thickness, width, steel grade and finished product thickness of the strip steel, as well as parameters such as roll parameters, motor capacity limit conditions and rolling load limit conditions. Under the premise of meeting process requirements and equipment safety, it formulates load distribution for each stand, rolling speed, tension between stands, etc.
3.1.4 Defect treatment of strip steel
The incoming material defects received by the process computer are included in the PDI data. For each type of defect (including surface scratch issues, surface cleanliness issues, surface morphology issues, etc.), the following information should be sent: defect type, defect start (strip steel position), defect stop (strip steel position), degree (ignore, decelerate, stop), and all orders
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