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Innovation of electrical steel process in the world

  • Time of issue:2018-01-20 15:52

(Summary description)The production history of electrical steel for more than 100 years is also a history of continuous technological innovation. The change from hot-rolled silicon steel sheet to cold-rolled electrical steel strip is the first major technological change.

Innovation of electrical steel process in the world

(Summary description)The production history of electrical steel for more than 100 years is also a history of continuous technological innovation. The change from hot-rolled silicon steel sheet to cold-rolled electrical steel strip is the first major technological change.

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  • Time of issue:2018-01-20 15:52
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  The production history of electrical steel for more than 100 years is also a history of continuous technological innovation. The change from hot-rolled silicon steel sheet to cold-rolled electrical steel strip is the first major technological change. After the cold rolling process is adopted, the development of oriented electrical steel is different The emergence of inhibitors and the technological advancement of smelting and casting in the previous process, the continuous innovation of the process flow in the previous process, and the simplification and continuity of the process flow in the subsequent process.

  The pre-process is from smelting→hot-rolled coil; the hot-rolled coil is transported to the electrical steel plant through different processes until the finished product is cut. The whole process is called the post-process. The most changes in these processes are the previous processes, from steelmaking and refining to the production of hot-rolled coils. The features of the changes are thinning of slabs and shortening of processes. Regardless of the oriented and non-oriented steel in the later process, we try our best to take one rolling pass to reach the product thickness, eliminating the need for intermediate annealing, and the non-oriented steel adopts continuous pickling (CDCM) for some varieties of non-oriented steel.

  The semi-process varieties in the traditional process are still popular in the West. The non-oriented bulk varieties are derived from the production in the automobile plate production plant. It is a low-carbon steel and low-cost. Its final annealing and surface bluing are in the iron. The core manufacturing plant is completed. This product was successfully developed by the former Wuhan Iron and Steel Company in the 1980s and once became a Chinese and foreign company.

  The main materials for compressors and motors of joint venture refrigerators of version B09 were transferred. However, due to the rapid development of cold-rolled full-process electrical steel (non-oriented steel), the cost was gradually reduced, so the semi-process gradually withdrew from the market. There are thousands of small and medium-sized motor manufacturers in China. Due to their small scale, scattered production and diversified core sizes, it is impossible to organize intensive production. This is also an important reason why semi-process products cannot be promoted in China. As far as the semi-processed product itself is concerned, the iron core made according to the process requirements is the best use of the material's own magnetism and is an energy-saving product.

  1 The evolution of the strip production process

  1.1 Process 1: Die casting-blooming-hot rolling

  This process is still used in a few factories. The general grain-oriented steel production process of the Arrigeni Plant in Pittsburgh, USA is that slabs are obtained from die casting, blooming and hot rolling into coils. The original WISCO introduced and signed the CJ7411 contract in 1974. Among the two major varieties, high-grade non-oriented steel and HiB steel, the slabs are formed by die casting and blooming. In the 1980s, Wuhan Iron and Steel did a lot of equipment matching and process innovation in order to change the die casting billet to the continuous casting billet.

  1.2 Process 2: Slab continuous casting and rolling-conventional production process

  This process, namely continuous slab casting (slab thickness 180-250mm), is the main production process for all varieties regardless of domestic and foreign.

  1.3 Process 3: Thin slab continuous casting and rolling production process

  The process is thin slab continuous casting (CSP, billet thickness 70-90mm). Both domestic and foreign development and production of electrical steel have achieved results. So far, the production of non-oriented medium and low grades has become normal. It has been proven magnetically that it can produce oriented electrical steels of all specifications and grades, but so far, surface defects such as iron sheet intrusion, mold slag entrapment, and furnace roll scratches have become the main problems restricting the mass production of oriented electrical steel.

  1.4 Process 4: Thin strip continuous casting-near-net forming production process (electrical steel, duplex stainless steel, cold-rolled AHSS, aluminum alloy)

  Thin strip continuous casting-the near net shape production process is the most advanced process. It originated from the Nucor Company in the United States and has been industrialized. After a long period of development, Baosteel in China is now at Ningbo Steel Plant, but it mainly produces plain carbon steel. Stablize. Shagang decided to introduce Nucor's technology and equipment in a complete set, each with a production capacity of 500,000 tons, but mainly for the production of plain carbon steel. Although the use of thin strip casting to produce electrical steel has been researched and developed for many years, the breakthrough is that after 2008, the research team of the State Key Laboratory of Rolling Steel of Northeastern University has made significant progress in the development of electrical steel production technology and variety, which may make it possible to produce electrical steel for a century. The steel process technology has produced a disruptive revolution.

  1.4.1 Comparison of basic parameters between Castrip and thin slab continuous casting and thick slab continuous casting

  During the forming process of different slab thickness, the cooling rate is different, and the solidification time is different, which affects the internal structure of the cast slab, and finally makes the performance of the finished product vary widely. The comparison of basic casting parameters between 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 between thin slab continuous casting and thin strip casting and rolling

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

  2 oriented electrical steel process evolution and mechanism analysis

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

  The improvement of    technology makes it possible to improve the precision of smelting ultra-pure steel and the control of trace elements. The improvement of detection methods has a scientific basis for a deep understanding of the influence of various factors on the final magnetism and innovative and reasonable technology. Under the above premise conditions, it is possible to produce high-grade non-oriented steel. A deep understanding of the composition requirements, inhibitors and texture evolution of the whole process in the production of oriented steel has pointed out the direction for us to improve the magnetic level of oriented steel. Since the discovery of the Goss texture, scientists around the world 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 + one-time cold rolling method)

  In the early days, silicon steel was produced by the thick slab continuous casting process (slab high temperature heating + secondary 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. Shown.

  This process can obtain very good magnetic properties under various factors controlled conditions. Nippon Steel uses this process to produce the world's top high magnetic induction oriented electrical steel named HiB, and is proud of it. In order to reduce the shortcomings of slab heating at 1400℃, Nippon Steel has done a lot of work. The temperature of the slab entering the furnace is as high as 1050°C, rising to 1400°C, it only takes 40 minutes, holding 15 minutes, and the total time for a cycle to be out of the furnace is 55 minutes. Therefore, the residence time of the slab in the high temperature section is greatly shortened. The slag is reduced, the life of the furnace is prolonged, the surface defects of the slab are also improved, and the yield rate is increased. Nippon Steel's Guanghata Plant has been using this process until now. When the original WISCO was introduced, it also used this process to produce general oriented steel and high magnetic induction oriented electrical steel, and it is still used in the third branch of the hot rolling mill. In 2008, a slab induction heating vertical furnace was added, but the use was not ideal. There are 14 processes in this process, of which there are 6 pre-processes and 8 post-processes.

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

  The advantage of lowering the heating temperature of the slab is to prevent the generation of liquid slag, which can reduce the shutdown time of the heating furnace, increase the yield rate and save energy; reduce unnecessary coarse grains in the slab; compared with the traditional process, it is possible to cancel reheating and Pre-rolled.

  Due to the advantages and huge economic benefits brought by the reduction of slab heating, electrical steel workers in various countries have done a lot of work on how to reduce the heating temperature of slabs. The first result obtained was the former Soviet Union metallurgical workers, who used their original Cu2S+AlN As an inhibitor. The heating temperature of the slab is reduced to 1280-1320℃, and the secondary rolling method is still used in the subsequent process. At the same time, other processes are adjusted accordingly to industrially produce high-quality general oriented electrical steel P17/50≤1.2W/kg, B8≥1.88T.

  This process was first used in China WISCO in 2002-2004. The steel grade was named QRD. According to this process, a general oriented steel production plant with an annual output of 160,000 tons was constructed. Because the process control factors are relatively easy and stable, it is The magnetic level is also at a stable high level. 0.30mm thick P17/50 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, such as laser scoring, 0.30mm thick P17/50 reaches 1.0W /kg is entirely possible. At present, most of the grain-oriented electrical steel produced by private enterprises are based on this process.

  2.3 The process of nitriding after low-temperature heating of thick slabs with AlN as the main inhibitor and the role of each process (one-shot cold rolling method)

  Since 1989, Nippon Steel Yawata Steel has developed low-temperature slab heating (1100-1150°C), called SL steel. Its magnetic level and iron loss are equivalent to HiB steel, and B8 is slightly inferior to HiB steel. Around 1.89-1.91T. The reason for the decrease of the slab temperature is to reduce MnS as the inhibitor, and mainly AlN. The AlN part comes from steelmaking and undergoes high temperature normalization in the post-process to precipitate dispersed particles. In addition, the steel strip is subjected to 750°C (or 900°C) nitriding treatment after the initial recrystallization and decarburization annealing to form (Si, Al)N in the steel strip, which can be used in a high-temperature box furnace (bell furnace or ring furnace) In this method, it is converted into AlN to inhibit the initial recrystallization and promote the occurrence and growth of secondary recrystallization. This method is called the acquired inhibitor. In the two processes of high and low temperature slab heating, the size of the first recrystallized grains produced during decarburization annealing is obviously different. The first recrystallization of high temperature process is significantly smaller than the first recrystallized grains of low temperature slab heating, the former is 10-12μm, The latter is 20-23μm. At present, the low-temperature heating process of slabs has become the mainstream process for the production of high magnetic induction oriented steel in my country. Wuhan Iron and Steel, Baosteel and Shougang are constantly optimizing this process.

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

  After the first energy crisis in the 1970s, the world steel industry was facing a major issue of structural adjustment.

  The core of the adjustment is to require the simplification of the long-term process used for a long time in order to reduce investment, save energy, and produce thinner and wider products. After the mid-1980s, many international companies, including German SMS and Demag, Italy's Danieli, Austria's VAI, Japan's Sumitomo Metal, etc., have rushed to conduct a lot of research and development and make breakthroughs. On July 21, 1989, the American Nucor Steel Company adopted the CSP process developed by SMS Company to successfully construct a production line at the Crawfordsville plant in Indiana, USA, marking the first industrialization of thin slab continuous casting and rolling technology in the world.

  In March 1999, the CSP production line of ThyssenKrupp in Germany was called the second-generation thin slab continuous casting and rolling production line, and it produced non-oriented steel for the first time with a silicon content of less than 2.4% and 50,000 tons. (Molten steel). In August 2001, a new CSP production line was built in the Italian AST plant. This line highlights the development and progress of the CSP process in the production of high-alloyed special steels (stainless steel, electrical steel, etc.). Its design capacity is 1 million tons, and its product structure is: 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). Electrical steel has Si=1.0%, 1.8% (non-oriented) and 3.0%. (Oriented steel). Italian experts once believed that CSP can be used to produce all grades including high magnetic induction oriented steel.

  The thin slab continuous casting and rolling process has obvious advantages compared with the traditional thick slab process. The process is significantly shortened, the required equipment is greatly simplified, the occupation of the production site area is correspondingly reduced, the unit product construction investment is significantly reduced, the energy consumption is reduced, the labor quota is reduced, the yield rate is high, the operation time is short, and the production cost is compared with traditional thick slab continuous casting Decrease by 18%.

  The production of electrical steel with CSP in my country was promoted by the National Science and Technology Support Program "New Generation of Recyclable Steel Process Technology" project. At that time, Academician Qian Yong of the Chinese Academy of Engineering paid special attention and promotion to this project and passed the continuous casting of Beijing Iron and Steel Research Institute. The center and the factory are combined, and the qualified factories are selected to carry out the test and research work. Finally, in March 2005, China's first hot rolled coil equivalent to 50W540 electrical steel was produced in Maanshan Iron & Steel.

  At present, the production of medium and low grade non-oriented steels with CSP in my country has become normal (Wuyang Iron and Steel, Maanshan Iron and Steel), and high grades are 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 oriented steel has completed the national Steel grade development plan issued by the Ministry of Science and Technology. Wu Gang has made a major breakthrough in the development of high magnetic induction oriented steel, and basically established and improved the process standard. The current thickness of 0.27mm P1.7/50≤1.0W/kg, B8≥1.90T, can be mass-produced, after optimization, further There is potential to improve magnetic properties, but due to the inherent problems caused by the CSP process itself, the surface quality of the product still needs to be further improved.

  The nature of the ESP and CSP process is basically the same. The difference lies in endless rolling, and has many advantages over CSP, but it is rarely used in electrical steel. In the future, it is very important to use ESP process to develop different varieties of electrical steel. Meaningful topics.

  3 R&D of the production process of electrical steel by continuous strip casting and rolling process

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

  As early as 1958, Northeastern University in my country started the research on the process route of using a reducing twin-roll cast-rolling machine to adopt light reduction and rapid casting-rolling, and cast-rolled electrical steel plates and cast iron plates in the laboratory, and cast-rolled them in 1960. The steel plate and iron plate with a width of 600mm and a thickness of 2-2.5mm each exceeded 100 tons, achieving the international leading level at that time. Later, due to various domestic factors, the research work was interrupted. It was not until the 1980s that Northeastern University resumed research work.

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