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Progress in heating technology of oriented silicon steel low temperature slab

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  With the deepening of research, the low-temperature slab heating technology will be more widely promoted and applied, and will play an active role in promoting the development of oriented silicon steel.
  In recent years, the world's major oriented silicon steel production plants have attached great importance to the improvement of the slab heating process. The traditional high-temperature furnace heating method has been replaced by the ordinary stepping furnace heating + high-frequency induction furnace high-temperature short-time heating method. In 1996, Nippon Steel Bagua Plant has used Hi-B steel for heating at a low temperature of 1150 ~ 1250 °C; Russia has been producing CGO steel with a slab heating process of 1250 ~ 1280 °C. In the modern steel industry, which is increasingly pursuing energy conservation, environmental protection and cost reduction, the low temperature slab heating process will be widely used in the production of oriented silicon steel.
 
  Oriented silicon steel high temperature slab heating technology
  In the production process of oriented silicon steel, in order to obtain a single Goss texture by secondary recrystallization, small, dispersed precipitated phase or grain boundary segregation elements capable of effectively suppressing the normal growth of primary grains are called inhibitors. Sexual effect. In order to ensure stable magnetic properties, the coarse MnS particles precipitated during the casting condensation must be completely dissolved. Therefore, the heating temperature of CGO steel slab with MnS as inhibitor is specified as 1350-1370 °C, and Hi-B steel with MnS+AlN as inhibitor is higher than CGO steel due to MnS+AlN. The heating temperature is specified as 1380-1400. °C. The slab is heated at a high temperature higher than 1350 ° C, and the coarse MnS particles are completely dissolved, and are precipitated in a fine dispersion state during the hot rolling. The finely dispersed AlN particles are mainly precipitated during the normalization of the hot rolled sheet. The suitable primary grain size of CGO steel after decarburization annealing is 15 ~ 25 μm, and Hi-B steel is 10 ~ 15μm. This ensures that the secondary recrystallization is complete to obtain high magnetic properties. However, high temperature slab heating has the following disadvantages:
 
  The finished product rate is reduced: the burning loss is increased by 3.5%-6% due to the oxidation of the slab; the heating loss of the carbon steel is about 4 times higher;
  (1) Slag accumulation at the bottom of the furnace is serious and the yield is low: the melting point of the formed SiO2 oxide layer is only 1205 °C. Therefore, in the high-temperature heating furnace, the oxide layer is melted and flows to the bottom of the furnace, and the average heating of 4 000 slabs is to clean the slag and heat it. About 8,000 will be overhauled, and the working conditions for repairing the furnace are extremely poor;
  (2) Energy waste: mainly due to excessive temperature, fuel consumption increases;
  (3) The life of the furnace is shortened: the refractory material in the high temperature zone of the furnace with high temperature and long-term heat load is severely peeled off and the service life is shortened, which not only increases the maintenance cost but also reduces the furnace operation rate;
  (4) High manufacturing cost: Due to grain coarsening of the slab and oxidation of the edge grain boundary, the hot rolled strip is prone to edge cracking, the finished product rate is lowered, and the manufacturing cost is reduced;
  (5) Many surface defects of the product: the surface of the hot-rolled strip is poorly removed, which affects the physical quality of the product;
  (6) Unstable magnetic properties: aluminum, silicon and carbon in the surface layer of the slab are oxidized to reduce the content, resulting in uneven magnetic properties of the product and poor properties of the insulating film;
  (7) In addition, due to the coarse grain of the slab, the product is prone to linear fine crystal defects, which affects the magnetic stability.
  At present, the general process of heating with high-temperature slab is: the slab is preheated by a common heating furnace at 1200 ° C, and then enters a high-frequency induction furnace for high-temperature short-time heating. This process consumes less energy than the conventional high-temperature heating furnace, and the furnace life is longer, which reduces the slag and hot-rolled edge cracks at the bottom of the furnace, and the manufacturing cost is reduced.
 
  Oriented silicon steel low temperature slab heating technology
  Due to the above shortcomings of the high-temperature slab heating technology, and it is not conducive to the common hot-rolling production line of oriented silicon steel and other steel grades, it is imperative to reduce the heating temperature of the slab. In order to achieve low temperature slab heating, it is necessary to exclude MnS or weaken MnS in the inhibitor, and replace it with AlN, Cu2S, and the like. This is mainly because the solid solution temperature of AlN and Cu2S is lower than that of MnS, and it is more suitable for low-temperature heating. At present, there are two main types of low-temperature slab heating processes used in industry: one is an inhibitor necessary for secondary recrystallization before cold rolling (called an innate inhibitor), and the other is decarburization annealing. After nitriding, the nitrogen is combined with the original aluminum in the steel to form fine dispersed (Al, Si) N sites, and the necessary inhibitors (called acquired inhibitors) for secondary recrystallization are obtained. When nitriding treatment, the nitriding amount is controlled at (150-300) X10-6, and the average grain size of the primary grains after decarburization annealing is controlled at 18 ~ 30μm, so as to obtain a perfect secondary recrystallized structure and obtain a high B800. value. The nitriding treatment and the decarburization annealing are carried out in the same continuous annealing furnace, that is, after the decarburization annealing, the steel strip passes through H2+N2+NH (mixed gas, and the oxidation rate is controlled to be PH2O/PH2 ≤0. 04. In addition, it can also be used in The method of adding nitride when the surface of the steel plate is coated with MgO is used to achieve the purpose of nitriding. The nitriding process can reduce the heating temperature of the slab to 1150 to 1200 °C.
  The use of innate inhibitors to produce CGO steel and the simultaneous use of innate inhibitors and acquired inhibitors to produce Hi-B steel is another effective way to reduce the heating temperature of the slab, and the heating temperature of the slab can be controlled at 1250-1300 °C.
 
  To sum up, oriented silicon steel currently has the following two low-temperature slab heating production processes:
  (1) Late nitriding process: only a small amount of aluminum is added during steel making, which is mainly used to produce Hi-B oriented silicon steel. The composition requires an S mass fraction of <0. 007%, and is subjected to nitriding treatment after decarburization annealing. The main feature of this process is that the steel strip needs to be nitrided at 750 °C for 30s after decarburization annealing. The (Al,Si) N mass is formed during the high temperature annealing and the primary grain growth is prevented before the secondary recrystallization occurs. The appropriate size of the primary grains after decarburization annealing is 18 ~ 30 μm (greater than the primary grain size of the high temperature slab heating process). The process can reduce the heating temperature of the slab to 1150 to 1200 ° C, which is the lowest temperature used for heating the slab in the current industrial production of oriented silicon steel;
  (2) Cu2S innate inhibitor process: Cu2S is the main inhibitor when producing CGO steel, and Cu2S is completely dissolved by heating at 1250-1300 °C. The fine dispersed Cu2S particles precipitated during hot rolling act as inhibitors, while the coarse MnS particles remaining in the hot rolled sheet do not inhibit. The initial grain size is between the high temperature slab heating process and the low temperature slab heating process (15-25 μm). The production of Hi-B steel uses MnS+AlN as an inhibitor, and the hot rolled sheet is often treated to precipitate fine AIN particles. After decarburization annealing, nitriding treatment is often used to further enhance the inhibition ability. This technology reduces the slab heating temperature to 1250 to 1300 °C.
 
  Conclusion
  It is undeniable that high-temperature slab heating technology is an important milestone in the history of oriented silicon steel. It is a mature process that can be stably obtained to obtain high magnetic properties after fully understanding the role of inhibitors. However, in recent years, with the increasing supply of energy and the increasing requirements for environmental protection and cost reduction, the shortcomings of high-temperature heating have become more and more prominent. The reduction of the heating temperature of the slab has become the focus of the world's major oriented silicon steel production plants. Technology development hotspots. With the deepening of research, the low-temperature slab heating technology will be more widely promoted and applied, and will play an active role in promoting the development of oriented silicon steel.