High-precision rolling and heat treatment technology and equipment for thin strip steel

　　The article introduces a high-precision rolling and heat treatment process and equipment technology for thin strip steel. Based on previous work in the direction of advanced cold rolling, this research continuously improves the control accuracy of the high-precision cold rolling shape control system and promotes its application, carrying out research and application of edge thinning control technology; In terms of advanced continuous annealing and coating technology, it continuously promotes the research and development of direct flame impact rapid heating technology, oxidation-free rapid cooling technology, and hot-dip galvanizing quality control technology, and has carried out research and application of warm rolling process and equipment technology for cast-rolled thin strips, etc.

 

High-precision rolling of thin strip steel

and heat treatment process and equipment technology

Preface

 

　　Advanced cold rolling, heat treatment, and coating processes and equipment technologies are key to producing high-end cold-rolled strip steel products. Since the center was approved, based on the previous work in the direction of advanced cold rolling, the control accuracy of the high-precision cold rolling shape control system has been continuously improved and its promotion and application has been promoted. Research and application of edge thinning control technology have been carried out; In terms of advanced continuous annealing and coating technology, the research and development of direct flame impact rapid heating technology, oxidation-free rapid cooling technology, and hot-dip galvanizing quality control technology have been continuously promoted, and research and application of warm rolling process and equipment technology for cast-rolled thin strips have been carried out.

 

Main Research Progress

 

　　2.1 High-precision cold rolling shape and edge thinning control technology

　　High-precision cold rolling shape and edge thinning control technology is the core technology to ensure the quality of cold-rolled strip steel. The independent research and development of high-precision cold rolling shape and edge thinning control and equipment technology is of great significance to improving China's core competitiveness in the rolling technology field.

　　Modern mainstream shape control cold rolling mills usually have multiple shape adjustment mechanisms involved in the shape closed-loop feedback control, making their control model and system more complex. Different from the pattern recognition method based on orthogonal decomposition, RAL, based on the independently researched influence efficiency function theory, constructs the influence efficiency functions of various shape control actuators such as mill tilt, work roll bending, intermediate roll bending, and intermediate roll runout through numerical analysis methods. The shape control evaluation function is constructed by the influence efficiency of the shape control actuator, the deviation between the measured shape value and the target value, the shape control influence factor, and the adjustment amount of each shape control actuator. The minimum value of the adjustment amount of each actuator under the condition that the evaluation function is minimized is calculated by the optimization method as the output value of a closed-loop control, so as to realize the multi-variable optimal shape control (Figure 1).

 

 

　　The shape closed-loop feedback control uses the control efficacy coefficient of each shape adjustment mechanism and the measured shape value of each measurement section of the shape roll, and uses the linear least squares principle to establish the shape control effect evaluation function to solve the optimal adjustment amount of each shape adjustment mechanism. In view of the importance of the shape control efficacy coefficient in the shape control system, a self-learning model of the shape control efficacy is formulated. In order to obtain an accurate shape control efficacy coefficient, by using artificial intelligence methods such as neural networks, the accuracy of these prior values is continuously improved based on the measured shape data.

　　The RAL cold rolling shape control system has a series of proprietary technologies, mainly reflected in the following aspects: a self-learning model of the shape control efficacy coefficient online is developed, which is suitable for shape control processes of any strip specifications, materials, and mill types, improving the versatility of the control system; a multi-variable optimal shape optimization method based on dynamic constraint convex quadratic programming is developed, which does not require solving the gradient and Hesse matrix of the objective function, and does not require the objective function to be continuously differentiable; a dynamic substitution control mechanism for shape adjustment mechanisms is introduced, which solves the problem of shape control when the actuator is saturated, and opens up new ideas for in-depth research on shape control technology; a segmented cooling control of work rolls based on fuzzy control is developed, which does not rely on mathematical models and solves the non-linear, time-varying large hysteresis problems that are difficult to solve by conventional control.

　　The successful development of RAL cold rolling shape control technology has formed a unique technical system in the international cold-rolled strip steel production field, forming a core technology system with completely independent intellectual property rights for control systems and process control, and has achieved huge economic benefits in practical applications. By the end of 2016, the high-precision cold rolling shape control technology has been promoted and applied in 11 cold rolling units, including Ansteel Putian 1450mm cold tandem mill, Ansteel Tiantie 1800mm cold tandem mill, Siwenkode 1450mm cold tandem mill, and Shandong Guanzhou 1450mm cold tandem mill, which greatly meets the increasingly high quality requirements of subsequent units and users.

　　In terms of edge thinning control technology, the UCM of Ansteel's 1500mm silicon steel cold rolling mill is transformed into a single-end taper work roll runout function UCMW cold tandem mill. On this basis, a multi-specification strip product edge thinning process control model and adaptive automatic control system for work roll runout, speed, and bending force are developed. The requirements for the convexity quality of hot-rolled materials are greatly reduced, the edge thinning control is further improved, the 2σ value is less than 3.15μm, the quality and yield of cold-rolled products are improved by 2.7%, and the control accuracy and product quality of edge thinning of silicon steel thin strips are greatly improved. In 2016, the "Core Technology Research and Industrial Application of Cold-Rolled Silicon Steel Edge Thinning Control" project jointly completed by Ansteel and Northeastern University won the first prize of the Metallurgical Science and Technology Award in 2016.

 

　　2.2 Advanced Continuous Annealing and Coating Technology

　　With the increasingly stringent market requirements for the performance and quality of cold-rolled products, the existing key technologies of continuous annealing and coating urgently need to be broken through, and high-efficiency and low-cost heating and cooling technologies are important breakthrough directions. The advanced cold rolling direction has made important progress in the research and development of direct flame impact (DFI) rapid heating technology, oxidation-free rapid cooling technology, and hot-dip galvanizing quality control technology, laying a solid foundation for further industrial application. The specific progress is as follows:

　　1) Aiming at the problems existing in the existing DFI technology, a narrow-gap multi-flame burner, flame pulse combustion control technology, and flame width control technology with independent intellectual property rights are developed. The pulse combustion control system is greatly simplified, the cost is reduced, and the flame width control method and device are simple and reliable; for the independently developed burner, finite element simulation research and experimental research are carried out on its combustion efficiency, flame characteristics, heating rate, and pollution emission, and the burner structure and combustion process parameters are optimized to obtain flame characteristic data (Figure 4). The thin plate heating rate reaches 180℃/(s·mm). By adjusting the air-fuel ratio, the degree of oxidation on the surface of the strip is controlled, laying the foundation for the design of the DFI heating system; on this basis, a DFI heating furnace (Figure 5) and corresponding workpiece transportation, gas supply control, etc. are designed and manufactured. The system is applied to RAL and Wuhan Iron and Steel thin strip warm rolling heating, realizing the breakthrough of industrial prototype machines.

　　2) Experimental research on oxidation-free rapid cooling technology was carried out. A cooling medium circulation supply system and an atomization injection device were designed to verify the feasibility and safety of oxidation-free cooling technology. The effects of cooling medium pressure and flow rate on the cooling rate of the sample and the limit cooling rate of samples with different thicknesses were studied. The results show that as the injection pressure of the cooling medium increases, the cooling rate of the sample increases. For a 1 mm sample, when the cooling medium injection pressure is 0.25 MPa, the cooling rate is close to 240 ℃/s (Figure 6 (a)), and the surface quality is good (Figure 6 (b)).

 

 

　　3) Based on the above technology, a feasibility study report and scheme design for a 1500 MPa ultra-high-strength steel production line were completed. The characteristics of this production line are high product strength, fast heating and cooling rates, short production line length, and the ability to co-produce wide and multi-flow narrow bands.

　　4) In terms of coating and plating technology and equipment technology, Northeastern University and Ansteel Group have cooperated to achieve a breakthrough in the core technology of quality control of hot-dip galvanized products. They successfully developed a coating thickness control system and achieved industrial application. The actual average coating thickness deviation is less than 3.2 g/m2, which is 1.13% higher than the international advanced level; the coating uniformity 2σ is 1.25 g/m2, which is 10.7% higher than the international advanced level, creating an annual economic benefit of more than 60 million yuan. In 2015, the project "Research and Industrial Application of Core Technology for Quality Control of Hot-Dip Galvanized Strip Coating" completed by Ansteel, Northeastern University, and other units won the second prize of the National Metallurgical Science and Technology Award. In 2016, breakthroughs were made in the industrialization of mist spray control zinc flower technology and galvanizing defect control technology. Aiming at the existing problems of the existing production line, application development contracts were signed with Hegang and Handan Iron and Steel to solve galvanizing defects such as zinc flow lines, edge oblique lines, and uneven zinc flower sizes, reduce the transition materials for variety changes, and improve the yield rate.

 

　　2.3 Research and development platform construction of new thin strip continuous casting + temper rolling process for preparing high-precision thin strip products

　　Thin strip continuous casting + temper rolling is a new full-process green manufacturing technology, mainly used for continuous casting, temper rolling, and subsequent cold rolling deep processing of high silicon steel, high-strength steel, and special alloy materials. It can be used for special rolling of various difficult-to-deform metal thin strip materials. This direction cooperates with the short-process direction to independently design and develop the first domestically produced thin strip continuous casting + temper rolling pilot line (Figure 7). Using direct-fired flame rapid heating, coil insulation, and online anti-oxidation technology, an innovative technological route for rapid solidification to improve material properties and warm processing of difficult-to-deform metal materials has been formed: 1) Multi-process section full-process nitrogen micro-spray technology for casting strip, hot leveling, and coil temper rolling has been developed, effectively solving the oxidation problem in the casting, temper rolling, and finished product processes of thin strips; 2) The independently developed direct-fired rapid heating and coil winding system has been successfully applied to this thin strip casting and rolling line. By applying the coupled control technology of coil box insulation and direct-fired flame rapid heating, the problem of constant temperature control of the roll and workpiece deformation zone during temper rolling has been solved, and the 3-4 mm high silicon steel thin cast strip coil material has been successfully prepared into 0.27 mm continuous coil high magnetic thin strip material through rapid variable temperature temper rolling process control. 3) A device for online automatic width adjustment of direct-fired rapid heating flame nozzles has been developed. Combined with nozzle combination distribution optimization, the technical problems of direct-fired rapid heating for different incoming material widths and uniform surface temperature of strip materials have been solved. At present, the above-mentioned advanced technologies have been successfully applied in the RAL laboratory and the thin strip casting and rolling demonstration line of Wuhan Iron and Steel National Silicon Steel Engineering Technology Center to prepare high silicon thin strip magnetic materials.

　　Engineering application of thin strip casting and rolling + coil temper rolling technology for preparing high magnetic induction silicon steel has achieved a breakthrough. In combination with the national "863" project for the development of high silicon steel for energy-saving motors, Northeastern University and Wuhan Iron and Steel Group Corporation signed a cooperation project. Utilizing the cutting-edge technological advantages of the Northeastern University Steel Common Technology Collaborative Innovation Center in the field of thin strip casting and rolling + temper rolling under the "2011 Plan", the first domestically produced "500 mm thin strip casting and rolling industrialization demonstration line" was built in the pilot plant of Wuhan Iron and Steel National Silicon Steel Engineering Center, and a "thin strip continuous casting + coil temper rolling prototype equipment for preparing 6.5% Si and other difficult-to-deform thin strip materials" with completely independent intellectual property rights was developed. At the same time, industrialization demonstration application was implemented in the Wuhan Iron and Steel pilot plant. At present, the project has completed the hot load test of the entire line of process equipment. On April 16, 2017, the expert group of the Ministry of Science and Technology High-Tech Center conducted acceptance of the "Development of High Silicon Steel for Energy-Saving Motors" project in Wuhan Iron and Steel and conducted on-site inspections of the thin strip continuous casting + coil temper rolling industrialization demonstration line. The evaluation opinions given were: 1) A complete set of production processes for 6.5% Si steel with vacuum smelting, twin-roll casting, multi-pass reversible hot rolling, temper rolling, and full-process atmosphere protection was proposed, and key equipment for near-net-shape manufacturing of 6.5% Si steel was developed; 2) Temper rolling of 6.5% Si thin strip steel was realized, and 6.5% Si steel cold-rolled thin strip materials were successfully rolled; 3) A hot/temper/cold three-use rolling mill (Figure 8) with temper rolling as the main function and both hot rolling and cold rolling functions was developed. Through key technologies such as rolling line atmosphere protection, hot coil box-type winding insulation, and direct-fired rapid heating, the advanced short-process preparation of 6.5% Si steel thin strip coil materials was realized. At present, the thin strip casting and rolling short-process production technology of the RAL laboratory has been applied industrially in many ways and has signed industrial production lines for preparing high value-added thin strip products with Hebei Jingye and Jiangsu Yancheng and other related enterprises and implemented them.

　　Relying on the RAL laboratory thin strip casting and rolling + temper rolling experimental platform. Based on the process equipment flow of the production line, the latest wireless communication technologies such as on-site wireless communication bus network and Wi-Fi network are established. A wireless information communication module system for sensors is developed, and the wireless network topology structure in an industrial environment is designed. On the thin strip casting and rolling experimental line, an integrated communication system for fieldbus and sensor wireless information rapid transmission has been successfully developed, realizing rapid and reliable data transmission from sensor nodes to aggregation nodes and then to PLC through Zigbee protocol and TCP/IP protocol, meeting the different process characteristics and needs in steel production. Through the collection, processing, and transmission of information status of equipment in the metallurgical production line area, the mobile terminal system application is transplanted to the industrial control system, and the equipment data status is displayed in three-dimensional images on a typical mobile client, realizing the full-line dynamic simulation of the experimental and production processes of the thin strip casting and rolling experimental research platform (such as smelting, casting and rolling, temper rolling, coil, and collection, etc.) and data monitoring and real-time dynamic monitoring of each process section. At the same time, it greatly simplifies the interconnection and information exchange between control devices in various process areas, intelligently integrates wired and wireless network data, solves the need for rapid and flexible intelligent perception of detection data in the production process, and provides technical support for comprehensive control of product quality and performance under complex process conditions.

　　An industrial wireless network architecture has been developed and designed, wirelessly connecting various process control areas and production line instruments, simplifying the information exchange between equipment in process areas, and suitable for flexible adjustment and quality control of production processes; aiming at the storage and processing of big data in the production process, a high-speed data acquisition system based on the fieldbus network has been developed, supporting wireless data high-speed data acquisition, and analyzing the needs of the distributed information processing platform for the defects of centralized storage and computation. At present, a standard analog signal wireless sensor template has been developed and high-speed signal transmission has been realized.

 

　　Conclusion

　　The research and development of high-precision rolling and heat treatment technology for thin strip steel has provided important technical support for the production of high-end cold-rolled strip steel products. In particular, the green thin-strip cast rolling technology for high-magnetic-induction electrical steel has fundamentally changed the traditional production process and component design of electrical steel. It allows for the production of high-quality electrical steel with different silicon content and thicknesses at lower costs, with simpler processes, higher quality, and less investment. This is of great significance to the sustainable development of China's national economy and national security.