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Development of Heating Technology for Low-temperature Cast Billet of Oriented Silicon Steel

  • Time of issue:2018-10-24 18:38

(Summary description)With the continuous deepening of research, the low-temperature casting billet heating technology will be more widely promoted and applied, which will play a positive role in promoting the production and development of oriented silicon steel.    In recent years, major oriented silicon steel production plants in the world have attached great importance to the improvement of the casting billet heating process. The traditional high-temperature furnace heating method has been replaced by ordinary walking furnace heating + high-frequency induction furnace high-temperature short-time heating.

Development of Heating Technology for Low-temperature Cast Billet of Oriented Silicon Steel

(Summary description)With the continuous deepening of research, the low-temperature casting billet heating technology will be more widely promoted and applied, which will play a positive role in promoting the production and development of oriented silicon steel.    In recent years, major oriented silicon steel production plants in the world have attached great importance to the improvement of the casting billet heating process. The traditional high-temperature furnace heating method has been replaced by ordinary walking furnace heating + high-frequency induction furnace high-temperature short-time heating.

  • Categories:Industry News
  • Author:Li Jun
  • Origin:"Steel"
  • Time of issue:2018-10-24 18:38
  • Views:

  With the continuous deepening of research, the low-temperature casting billet heating technology will be more widely promoted and applied, which will play a positive role in promoting the production and development of oriented silicon steel.

  In recent years, major oriented silicon steel production plants in the world have attached great importance to the improvement of the casting billet heating process. The traditional high-temperature furnace heating method has been replaced by ordinary walking furnace heating + high-frequency induction furnace high-temperature short-time heating. In 1996, Nippon Steel's Bapan Plant has used 1150 ~ 1250 ℃ low temperature casting billet heating process to produce Hi-B steel; Russia has been using 1250 ~ 1280 ℃ slab heating process to produce CGO steel. In the modern iron and steel industry, which is increasingly pursuing energy saving, environmental protection and cost reduction, the low-temperature casting billet heating process will surely be widely used in the production of oriented silicon steel.

  Oriented silicon steel high temperature casting billet heating technology

  In the production process of oriented silicon steel, in order to obtain a single Goss texture through secondary recrystallization, the fine and dispersed precipitated phase particles or grain boundary segregation elements that can effectively inhibit the normal growth of the primary grains are called inhibitors. Sexual effect. In order to ensure stable magnetic properties, the coarse MnS particles precipitated during the casting and condensation process must be completely dissolved. Therefore, the heating temperature of CGO steel casting billet with MnS as inhibitor is specified as 1350-1370℃, and the heating temperature of Hi-B steel with MnS+AlN as inhibitor is higher than CGO steel because of higher manganese and carbon content than CGO steel. The heating temperature is specified as 1380-1400 ℃. When the cast slab is heated at a high temperature higher than 1350℃, the coarse MnS particles are completely dissolved and then precipitated in a finely dispersed state during the hot rolling process. The finely dispersed AlN particles are mainly precipitated during the normalization process of the hot-rolled sheet. The suitable initial grain size after decarburization annealing for CGO steel is 15-25 μm, and that for Hi-B steel is 10-15 μm. This can ensure that the secondary recrystallization is complete and high magnetic properties can be obtained. However, the heating of high-temperature cast slab has the following disadvantages:

  The yield rate is reduced: the burning loss increased (3.5%-6%) due to the over-oxidation of the cast slab, which is about 4 times higher than the heating burning loss of ordinary carbon steel;

  (1) Slag accumulation at the bottom of the furnace and low output: the melting point of the formed SiO2 oxide layer is only 1205℃, so the oxide layer melts in the high-temperature heating furnace and flows to the bottom of the furnace. The average heating of 4 000 billets requires cleaning of the slag and heating. 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) Shortened furnace life: The refractory lining in the high temperature zone of the heating furnace that has been subjected to high temperature and heat load for a long time will be severely peeled off and the life will be shortened, which not only increases maintenance costs, but also reduces the furnace operation rate;

  (4) High manufacturing cost: due to the coarsening of the slab grain and the oxidation of the edge grain boundary, the hot-rolled strip is prone to edge cracks, the yield rate is reduced, and the manufacturing cost is just as strong;

  (5) Many product surface defects: poorly removed oxide scale on the surface of hot-rolled strip steel, which affects the physical quality of the product;

  (6) The magnetic properties are unstable: the aluminum, silicon and carbon in the surface of the cast slab are combined with oxidation, reducing the content, resulting in uneven magnetic properties of the product and deterioration of the insulating film characteristics;

  (7) In addition, due to the coarsening of the slab grains, the product is prone to linear fine crystal defects, which affects the magnetic stability.

  At present, the general process for heating high-temperature cast slabs is as follows: the cast slabs are first preheated in an ordinary heating furnace at 1200°C, and then entered into a high-frequency induction furnace for high-temperature and short-time heating. This process consumes less energy than traditional high-temperature heating furnace heating methods, the furnace body has a longer service life, reduces bottom slag accumulation and hot rolling edge cracks, and reduces manufacturing costs.

  oriented silicon steel low temperature casting billet heating technology

  Due to the above-mentioned shortcomings of the high-temperature casting billet heating technology, and it is not conducive to the use of oriented silicon steel and other steel grades to share the hot rolling production line, it is imperative to reduce the billet heating temperature. In order to achieve low-temperature casting billet heating, MnS must be eliminated or the effect of weakening MnS must be eliminated from the inhibitor, and AlN, Cu2S, etc. must be used instead. This is mainly because the solid solution temperature of AlN and Cu2S is lower than that of MnS, which is more suitable for low temperature heating. At present, there are mainly two kinds of low-temperature casting billet heating processes used in the industry: one is the inhibitor (called innate inhibitor) necessary for the formation of secondary recrystallization before cold rolling, and the other is the decarburization annealing After nitriding, the nitrogen is combined with the original aluminum in the steel to form fine and dispersed (Al, Si) N particles, and the inhibitor necessary for secondary recrystallization (called acquired inhibitor) is obtained. During nitriding treatment, the amount of nitriding is controlled at (150-300) X10-6, and the average grain size of the primary grains after decarburization annealing is controlled at 18 ~ 30μm, in order to obtain a perfect secondary recrystallized structure and obtain a high B800 value. Nitriding treatment and decarburization annealing are carried out in the same continuous annealing furnace, that is, after decarburization annealing, the steel strip passes through H2+N2+NH (mixed gas, controlling the oxidation rate PH2O/PH2≤0.04. In addition, it can also be used in The method of adding nitride when coating the MgO release agent on the surface of the steel plate to achieve the purpose of nitriding. The nitriding process can reduce the heating temperature of the cast slab to 1150-1200°C.

  The use of innate inhibitors to produce CGO steel and the use of both innate inhibitors and acquired inhibitors to produce Hi-B steel is another effective way to reduce the heating temperature of the cast slab. The heating temperature of the cast slab can be controlled at 1250 to 1300°C.

  In summary, oriented silicon steel currently mainly has the following two low-temperature casting billet heating production processes:

  (1) Late nitriding process: only a small amount of aluminum is added during steelmaking, which is mainly used to produce Hi-B oriented silicon steel. Its composition requires S mass fraction <0.007%, and nitriding treatment is carried out after decarburization annealing. The main feature of this process is that the steel strip needs to be nitrided at 750 ℃ ​​X 30s after decarburization annealing. (Al, Si) N particles are formed during the high temperature annealing and heating process, which hinders the growth of the primary grains before the secondary recrystallization occurs. The proper size of the primary grains after decarburization annealing is 18-30 μm (larger than the primary grain size of the high-temperature casting billet heating process). This process can reduce the slab heating temperature to 1150-1200℃, which is the lowest temperature used for slab heating in the current industrial production of oriented silicon steel;

  (2) Cu2S innate inhibitor process: Cu2S is the main inhibitor in the production of CGO steel, and Cu2S is heated at 1250 to 1300°C to achieve complete solid solution. The fine and dispersed Cu2S particles precipitated during hot rolling act as inhibitors, while the remaining coarse MnS particles in the hot-rolled sheet do not. The initial grain size is between the high-temperature slab heating process and the low-temperature slab heating process (15-25μm). In the production of Hi-B steel, MnS+AlN is used as an inhibitor. The hot-rolled sheet is often treated to precipitate fine AIN particles. After decarburization and annealing, nitriding is often used to further strengthen the suppression ability. This technology can reduce the heating temperature of the cast billet to 1250 to 1300°C.

  Conclusion

  It is undeniable that the high-temperature billet heating technology is an important milestone in the history of the development of oriented silicon steel. It is a mature process that can stably obtain high magnetic properties after people have fully realized the role of inhibitors. However, in recent years, with the increasing shortage of energy supply and the increasing requirements for environmental protection and cost reduction, the shortcomings of high-temperature heating have become more and more prominent. Lowering the heating temperature of slabs has become the concern of major oriented silicon steel manufacturers in the world. Technology development hotspots. With the continuous deepening of research, the low-temperature casting billet heating technology will be more widely promoted and applied, which will play a positive role in promoting the production and development of oriented silicon steel.

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