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Innovation in the world's electrical steel manufacturing process

Category: Shanli Blog Publish Time：2018-01-20

The 100+ year history of electrical steel production is also a history of continuous process innovation. The transition from hot-rolled silicon steel sheets to cold-rolled electrical steel strips represents the first major process revolution. Following the adoption of cold rolling technology, with the development of grain-oriented electrical steel, the emergence of different inhibitors, and advancements in pre-process metallurgy and casting, there have been continuous process innovations in the pre-processes, as well as simplification and continuous improvement in the post-processes. Pre-processes refer to the steps from smelting to hot rolling; the entire process from the transportation of hot-rolled coils to the electrical steel plant through various processes to final shearing is referred to as the post-process. The pre-processes have undergone the most significant changes, from steelmaking and refining to the production of hot-rolled coils, with its changes...

　　Over 100 years of electrical steel production history is also a history of continuous process innovation. The transition from hot-rolled silicon steel sheets to cold-rolled electrical steel strips represents the first major technological revolution. Following the adoption of cold rolling technology, with the development of grain-oriented electrical steel, the emergence of different inhibitors, and advancements in pre-process metallurgy and casting, continuous process innovations have emerged in the pre-processes, along with simplification and continuous improvement in the post-processes.

　　The pre-process refers to the process from smelting to hot-rolled coils; the process from the transportation of hot-rolled coils to the electrical steel plant through various processes until the finished product is cut is called the post-process. The pre-process has undergone the most changes, from steelmaking and refining to the production of hot-rolled coils. The characteristics of these changes are the reduction of slab thickness and the shortening of processes. In the post-process, regardless of whether it is grain-oriented or non-grain-oriented steel, efforts are made to achieve the final thickness in a single rolling process, eliminating intermediate annealing. In addition, some non-grain-oriented steel varieties adopt continuous acid rolling (CDCM).

　　Traditional semi-finished products are still popular in the West, particularly in the bulk varieties of non-grain-oriented steel. They originate from production in automotive sheet plants and are a type of low-carbon, low-cost steel. Final annealing and surface bluing are completed at the iron core manufacturing plant. This product was successfully developed by the original Wuhan Iron and Steel Corporation in the 1980s and once became a major material for

　　(Continued on page B09) joint-venture refrigerator compressors and motors. However, due to the rapid development of cold-rolled full-process electrical steel (non-grain-oriented steel) and the gradual reduction in costs, semi-finished products gradually withdrew from the market. There are thousands of small and medium-sized motor manufacturers in China. Due to their small scale, dispersed nature, and diverse iron core sizes, they are unable to organize integrated production. This is also an important reason why semi-finished products have not been widely promoted in China. In terms of semi-finished products themselves, iron cores made according to process requirements make the most of the material's magnetic properties and are energy-saving products.

　　Evolution of Strip Production Processes

　　1.1 Process 1: Mold Casting - Roughing - Hot Rolling

　　This process is still used by a few plants. The Allegheny plant in Pittsburgh, USA, generally uses this process for grain-oriented steel production, where slabs are obtained from mold casting, roughing, and hot rolling into coils. In the CJ7411 contract signed by the original Wuhan Iron and Steel Corporation in 1974, two major varieties, namely high-grade non-grain-oriented steel and HiB steel, had slabs obtained from mold casting and roughing. In the 1980s, Wuhan Iron and Steel Corporation carried out a large amount of equipment matching and process innovation work to change mold casting billets to continuous casting billets.

　　1.2 Process 2: Continuous Casting and Rolling of Slabs - Conventional Production Process

　　This process, which involves continuous casting of slabs (slab thickness 180-250 mm), is the main production process for all varieties, both domestically and internationally.

　　1.3 Process 3: Thin Slab Continuous Casting and Rolling Production Process

　　This process involves thin slab continuous casting (CSP, slab thickness 70-90 mm). Domestically and internationally, it has been developed for the production of electrical steel and has achieved success. The production of medium and low-grade non-grain-oriented steel has become commonplace. It has been proven that all specifications and grades of grain-oriented electrical steel can be produced in terms of magnetic properties. However, surface defects such as iron skin pressing, protective slag rolling, and roller scratches have become the main problems hindering the mass production of grain-oriented electrical steel.

　　1.4 Process 4: Thin Strip Continuous Casting - Near-Net-Shape Production Process (Electrical Steel, Duplex Stainless Steel, Cold-Rolled AHSS, Aluminum Alloy)

　　Thin strip continuous casting - near-net-shape production process is the most advanced process, originating from Nucor Corporation in the United States. It has been industrialized. Baosteel in China has developed it for a long time, and now it is used in the Ningbo steel plant, but mainly for the production of ordinary carbon steel, and the production is not stable. Shagang has decided to introduce Nucor's technology and equipment in a complete set, with a capacity of 500,000 tons per line, but mainly for the production of ordinary carbon steel. Although the production of electrical steel using thin strip casting and rolling has been under development for many years, a breakthrough was made after 2008 when the research team of the State Key Laboratory of Rolling Stock in Northeastern University made significant progress in the production process and variety development of electrical steel, which may lead to a revolutionary change in the century-old production process of electrical steel.

　　1.4.1 Comparison of Basic Parameters of Castrip with Thin Slab Continuous Casting and Thick Slab Continuous Casting

　　In the forming process of different slab thicknesses, due to different cooling rates and solidification times, the internal structure of the cast billet is affected, ultimately resulting in significant differences in the properties of the finished product. A comparison of the basic casting parameters of Castrip, thin slab continuous casting, and thick slab continuous casting is shown in Table 1.

　　1.4.2 Comparison of Investment, Cost, and Energy Consumption and Emissions of Thin Slab Continuous Casting and Thin Strip Casting and Rolling

　　Advantages of the thin strip continuous casting process: green, environmentally friendly, and low cost. The production line length is: thick slab continuous casting is greater than 600m, thin slab continuous casting is about 400m, and thin strip casting and rolling is less than 60m. A comparison of the investment, cost, energy consumption, and emissions of thin slab continuous casting and thin strip casting and rolling is shown in Figure 1.

　　2 Evolution and Mechanism Analysis of Grain-Oriented Electrical Steel Process

　　The history of the development of electrical steel is essentially a history of the development of steelmaking technology, metallography, physical metallurgy, metal heat treatment, and analysis and testing technology. The improvement of steelmaking equipment technology

　　has made it possible to improve the smelting of ultra-clean steel and the control accuracy of trace elements. The improvement of detection methods has provided a scientific basis for a deep understanding of the influence of various factors on the final magnetic properties and the innovation of reasonable processes. Under the above conditions, it has become possible to produce high-grade non-grain-oriented steel. A deep understanding of the requirements for composition, inhibitors, and texture evolution in the entire process of grain-oriented steel production has pointed out the direction for improving the magnetic properties of grain-oriented steel. Since the discovery of Goss texture, scientists worldwide engaged in electrical steel research have focused on the relationship between inhibitor selection, evolution, and the final Goss texture.

　　2.1 The Role of Each Process in the Thick Slab Continuous Casting Process with AlN as the Main Inhibitor (Slab High-Temperature Heating + Single Cold Rolling Method)

　　In the early days, silicon steel was produced using a thick slab continuous casting process (slab high-temperature heating + double cold rolling method) with MnS as the inhibitor. Later, the thick slab continuous casting process with AlN as the main inhibitor and the role of each process are shown in Figure 2.

　　Under controlled conditions of various factors, this process can achieve excellent magnetic properties. Nippon Steel uses this process to produce world-class high-magnetic-induction oriented electrical steel, named HiB, and takes pride in it. To reduce the drawbacks of heating slabs to 1400℃, Nippon Steel has done a lot of work, such as heating slabs in an ordinary furnace to 1200℃, and then transferring them to a vertical slab induction heating furnace to reach 1400℃. Because the slabs enter the furnace at a high temperature of 1050℃, it only takes 40 minutes to reach 1400℃, with a holding time of 15 minutes. The total cycle time from entry to exit is 55 minutes. Therefore, the time the slab stays at high temperatures is greatly reduced. Coupled with nitrogen protection, slag is reduced, furnace life is extended, slab surface defects are improved, and the yield rate is increased. Nippon Steel's Hirohata plant has been using this process to this day. When Wuhan Iron and Steel Corporation (WISCO) introduced this technology, it also used this process to produce general oriented steel and high-magnetic-induction oriented electrical steel. The hot rolling No. 3 plant still uses this process. A vertical slab induction heating furnace was added in 2008, but it was not used ideally. This process has a total of 14 steps, with 6 steps in the front and 8 steps in the back.

　　2.2 The role of each process in the thick slab continuous casting process using Cu2S+AlN as an inhibitor (medium-temperature slab heating + secondary cold rolling method)

　　The advantages of lowering the slab heating temperature are preventing the formation of liquid slag, reducing furnace downtime for maintenance, improving yield and energy saving; reducing unwanted coarse grains in the slab; and potentially eliminating reheating and pre-rolling compared to traditional processes.

　　Due to the advantages and significant economic benefits of lowering slab heating temperature, electrical steel workers worldwide have done a lot of work on how to lower the slab heating temperature. The first achievement was made by Soviet metallurgical workers, who used their original Cu2S+AlN as an inhibitor. This reduced the slab heating temperature to 1280-1320℃, while the subsequent processes still used the secondary rolling method. At the same time, other process steps were adjusted accordingly, and high-quality general oriented electrical steel P17/50≤1.2W/kg, B8≥1.88T was produced industrially.

　　This process was first used in Wuhan Iron and Steel Corporation (WISCO), China, from 2002 to 2004. The steel grade was named QRD, and a specialized production plant with an annual output of 160,000 tons of general oriented steel was built according to this process. Because the control factors of this process are relatively easy and stable, its magnetic level is also at a stable high level. For 0.30mm thick P17/50, the value is 1.05-1.2W/kg, B8=1.88-1.90T. If the composition and process are optimized, there is still some potential for magnetic improvement. For example, using laser scribing, it is entirely possible to achieve 1.0W/kg for 0.30mm thick P17/50. Currently, most privately-owned enterprises producing oriented electrical steel mainly use this process.

　　2.3 Process and role of each process in the nitriding process after low-temperature heating of thick slabs using AlN as the main inhibitor (single cold rolling method)

　　Since 1989, Nippon Steel Yawata Steel Works has developed low-temperature slab heating (1100-1150℃), called SL steel. Its magnetic level and iron loss are comparable to HiB steel, with B8 slightly lower than HiB steel, at around 1.89-1.91T. The reason for the lower slab temperature is the reduction of MnS as an inhibitor, with AlN as the main inhibitor. Part of the AlN comes from steelmaking and is precipitated and dispersed as particles during high-temperature normalization in subsequent processes. In addition, after the steel strip undergoes primary recrystallization decarburization annealing, it undergoes 750℃ (or 900℃) nitriding treatment, forming (Si,Al)N in the steel strip, which is converted to AlN in a high-temperature box furnace (hood furnace or ring furnace), inhibiting initial recrystallization and promoting the occurrence and growth of secondary recrystallization. This method is called an artificially obtained inhibitor. In the two processes of high and low temperature slab heating, the primary recrystallization grain size produced during decarburization annealing is significantly different. The primary recrystallization of the high-temperature process is significantly smaller than that of the low-temperature slab heating process, with the former being 10-12μm and the latter 20-23μm. Currently, low-temperature slab heating has become the mainstream process for producing high-magnetic-induction oriented steel in China. WISCO, Baosteel, and Shougang are constantly optimizing this process.

　　2.4 Application of thin slab continuous casting (CSP) process using AlN as an inhibitor in electrical steel

　　In the 1970s, after the first energy crisis, the world's steel industry faced a major challenge of structural adjustment.

　　The core of the adjustment was to simplify the long-process technology that had been used for a long time to reduce investment, save energy, and produce thinner and wider products. Since the mid-1980s, many international companies, including SMS and Demag from Germany, Danieli from Italy, voestalpine from Austria, and Sumitomo Metal from Japan, have conducted extensive research and development and achieved breakthroughs. On July 21, 1989, Nucor Steel Company in the United States successfully put into operation a production line using the CSP process developed by SMS Company at its Crawfordsville plant in Indiana, marking the first industrialization of thin slab continuous casting and rolling technology in the world.

　　In March 1999, ThyssenKrupp's CSP production line, known as the second-generation thin slab continuous casting and rolling production line, was put into operation and produced non-oriented steel for the first time, with a silicon content of less than 2.4% and a capacity of 50,000 tons (molten steel). In August 2001, a new CSP production line was built at the AST plant in Italy. This line highlights the CSP process's pioneering and progress in producing special steel grades with high alloy content (stainless steel, electrical steel, etc.). Its design capacity is 1 million tons, with product structures including austenitic stainless steel, ferritic stainless steel, non-oriented electrical steel (Si% <3.5%), and a small amount of C75 and C100 high-carbon steel.

　　In October 2002, a total of 115,000 tons were produced (including 88,000 tons of stainless steel, 17,000 tons of electrical steel, and 10,000 tons of ferritic stainless steel). The electrical steel included Si=1.0%, 1.8% (non-oriented) and 3.0% (oriented steel). Italian experts once believed that CSP could produce all grades of high-magnetic-induction oriented steel.

　　The thin slab continuous casting and rolling process has obvious advantages compared to the traditional thick slab process. The process is significantly shortened, the equipment required is greatly simplified, the production area is reduced accordingly, the unit product construction investment is significantly reduced, energy consumption is reduced, labor personnel are reduced, the yield rate is high, the operation time is short, and the production cost is 18% lower than the traditional thick slab continuous casting.

　　China's use of CSP to produce electrical steel was promoted by the national science and technology support plan "New Generation Recyclable Steel Process Technology." At that time, Academician Gan Yong of the Chinese Academy of Engineering paid special attention to and promoted this project. Through the cooperation between the continuous casting center of the Beijing Iron and Steel Research Institute and the factory, factories with the necessary conditions were selected to conduct experimental research. Finally, in March 2005, Ma Steel produced China's first hot-rolled coil equivalent to 50W540 electrical steel.

　　Currently, the production of medium and low-grade non-oriented steel using CSP in China has become commonplace (Wuhan Iron and Steel, Maanshan Iron and Steel), while high-grade steel is still under development. With the joint efforts of the Continuous Casting Center of Beijing Iron and Steel Research Institute and Wuhan Iron and Steel, the steel grade development plan assigned by the Ministry of Science and Technology has been completed. Wuhan Iron and Steel has made a major breakthrough in the development of high-magnetic-induction oriented steel, basically establishing and improving the process standards. Currently, 0.27mm thick P1.7/50≤1.0W/kg, B8≥1.90T, can be mass-produced. Further improvement of magnetic properties is potentially possible through optimization, but due to inherent problems of the CSP process itself, the surface quality of the product still needs further improvement.

　　The ESP and CSP processes are basically the same in nature, differing only in the absence of head rolling, and ESP has many advantages over CSP. However, its application in electrical steel is not common. Research and development of different types of electrical steel using the ESP process in the future is a very meaningful topic.

　　Research and Development of Thin Strip Continuous Casting and Rolling Process for Electrical Steel Production

　　In the 1980s, many manufacturers in the United States, Japan, and Germany announced the successful use of twin-roll or single-roll casting and rolling of stainless steel and electrical steel. In 1984, Kawasaki Steel Corporation in Japan used the twin-roll method to cast and roll high-silicon steel and carbon steel with a thickness of 0.2-0.6mm and a width of 500mm.

　　Northeastern University in China started research on the process route of using a different diameter twin-roll casting and rolling machine and adopting a light-pressure fast casting and rolling process as early as 1958. Electrical steel plates and cast iron plates were cast and rolled in the laboratory, and more than 100 tons of steel plates and iron plates with a width of 600mm and a thickness of 2-2.5mm were cast and rolled in 1960, achieving a leading international level at that time. Later, due to various domestic factors, the research work was interrupted until the 1980s, when Northeastern University resumed its research.

Keywords: Innovation in the world's electrical steel manufacturing process