Advances in heating technology for low-temperature cast billets of grain-oriented silicon steel

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

　　In recent years, major grain-oriented silicon steel manufacturers worldwide have attached great importance to improvements in billet heating processes. The traditional high-temperature furnace heating method has been replaced by a combination of ordinary walking beam furnace heating and high-frequency induction furnace high-temperature short-time heating. In 1996, Nippon Steel's Yamapan plant adopted a low-temperature billet heating process of 1150 ~ 1250℃ for the production of Hi-B steel; Russia has been using a 1250 ~ 1280℃ slab heating process for the production of CGO steel. In the modern steel industry, which increasingly emphasizes energy saving, environmental protection, and cost reduction, low-temperature billet heating technology will inevitably be widely used in the production of grain-oriented silicon steel.

 

　　High-temperature billet heating technology for grain-oriented silicon steel

　　In the production of grain-oriented silicon steel, in order to obtain a single Goss texture through secondary recrystallization, fine and dispersed precipitated phase particles or grain boundary segregated elements that can effectively inhibit the normal growth of primary grains are called inhibitors and play a key role. To ensure stable magnetic properties, the coarse MnS particles precipitated during casting and solidification must be completely dissolved. Therefore, the billet heating temperature for CGO steel using MnS as an inhibitor is specified as 1350-1370℃, while for Hi-B steel using MnS+AlN as an inhibitor, due to higher manganese and carbon content than CGO steel, the heating temperature is specified as 1380-1400℃. After high-temperature heating above 1350℃, the coarse MnS particles are completely dissolved and precipitate again in a fine and dispersed state during hot rolling. Fine and dispersed AlN particles are mainly precipitated during the normalization of hot-rolled plates. The appropriate primary grain size after decarburization annealing of CGO steel is 15 ~ 25 μm, and for Hi-B steel it is 10 ~ 15 μm. This ensures complete secondary recrystallization and thus high magnetic properties. However, high-temperature billet heating has the following disadvantages:

　　Reduced yield: Due to billet oxidation, the amount of burn loss increases (3.5%-6%), which is about 4 times higher than that of ordinary carbon steel heating;

　　(1) Severe bottom slag accumulation, 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. On average, the furnace slag needs to be cleaned after heating 4,000 billets, and maintenance is required after heating about 8,000 billets. The working conditions for furnace repair are extremely poor;

　　(2) Energy waste: Mainly due to excessively high temperatures, resulting in increased fuel consumption;

　　(3) Shortened furnace life: The refractory materials in the high-temperature zone of the heating furnace, which is subjected to high-temperature thermal load for a long time, suffer severe spalling, shortening its lifespan. This not only increases maintenance costs but also reduces furnace operating rate;

　　(4) High manufacturing cost: Due to billet grain coarsening and edge grain boundary oxidation, the hot-rolled strip is prone to edge cracks, reducing the yield and increasing manufacturing costs;

　　(5) Many surface defects in the product: Poor removal of iron oxide scale on the surface of the hot-rolled strip affects the product quality;

　　(6) Unstable magnetic properties: Aluminum, silicon, and carbon in the billet surface layer combine with oxygen, reducing their content and causing uneven magnetic properties in the product and deterioration of the insulation film characteristics;

　　(7) In addition, due to billet grain coarsening, the product is prone to linear fine crystal defects, affecting magnetic stability.

　　Currently, the common process using high-temperature billet heating is: the billet is first preheated in an ordinary heating furnace at 1200℃, and then enters a high-frequency induction furnace for high-temperature short-time heating. This process consumes less energy than the traditional high-temperature heating furnace heating method, has a longer furnace life, reduces bottom slag accumulation and hot rolling edge cracks, and lowers manufacturing costs.

 

　　Low-temperature billet heating technology for grain-oriented silicon steel

　　Due to the above-mentioned disadvantages of high-temperature billet heating technology and its incompatibility with the sharing of hot rolling production lines with other steel grades, reducing the billet heating temperature is imperative. To achieve low-temperature billet heating, MnS must be excluded from the inhibitor or its effect weakened, and replaced with AlN, Cu2S, etc. This is mainly because the solid solution temperature of AlN and Cu2S is lower than that of MnS, making it more suitable for low-temperature heating. Currently, there are two main low-temperature billet heating processes used in industry: one is to form the inhibitor necessary for secondary recrystallization before cold rolling (called innate inhibitor), and the other is to perform nitriding treatment after decarburization annealing, allowing nitrogen to combine with the aluminum originally present in the steel to form fine and dispersed (Al, Si) N particles, obtaining the inhibitor necessary for secondary recrystallization (called acquired inhibitor). During nitriding treatment, the nitriding amount is controlled at (150-300) × 10-6, and the average grain size of the primary grains after decarburization annealing is controlled at 18 ~ 30 μm, so as to obtain a perfect secondary recrystallization structure and achieve 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+NH3 (mixed gas, controlling the oxidation rate PH2O/PH2 ≤ 0.04. In addition, the method of adding nitrides when applying MgO isolating agent to the steel plate surface can also be used to achieve nitriding. Using the nitriding process, the billet heating temperature can be reduced to 1150-1200℃.

　　Using innate inhibitors to produce CGO steel and using both innate and acquired inhibitors to produce Hi-B steel is another effective way to reduce the billet heating temperature, which can control the billet heating temperature at 1250-1300℃.

　　In summary, there are currently two main low-temperature billet heating production processes for grain-oriented silicon steel:

　　(1) Post-nitriding process: Only a small amount of aluminum element is added during steelmaking, mainly used for the production of Hi-B grain-oriented silicon steel. Its composition requires S mass fraction < 0.007%, and nitriding treatment is carried out after decarburization annealing. The main characteristics of this process are that the steel strip needs to undergo 750℃ × 30s nitriding treatment after decarburization annealing. (Al, Si) N particles are formed during the high-temperature annealing heating process, hindering the growth of primary grains before secondary recrystallization occurs. The appropriate size of primary grains after decarburization annealing is 18 ~ 30 μm (larger than the primary grain size of the high-temperature billet heating process). This process can reduce the billet heating temperature to 1150-1200℃, which is currently the lowest temperature used for billet heating in the industrial production of grain-oriented silicon steel;

　　(2) Cu 2 Inherent inhibitor process: When producing CGO steel, Cu2S is used as the main inhibitor, and Cu2S is completely dissolved by heating at 1250-1300℃. The fine and dispersed Cu2S particles precipitated during hot rolling play an inhibitory role, while the remaining coarse MnS particles in the hot-rolled sheet do not play an inhibitory role. The initial grain size is between the high-temperature billet heating process and the low-temperature billet heating process (15-25μm). MnS+AlN is used as an inhibitor for Hi-B steel production. Fine AlN particles are precipitated during the normalization treatment of hot-rolled sheets, and nitriding treatment is often used after decarburization annealing to further enhance the inhibition ability. This technology can reduce the billet heating temperature to 1250-1300℃.

 

　　Conclusion

　　Undeniably, high-temperature billet heating technology is a significant milestone in the history of grain-oriented silicon steel development. It is a mature process formulated after people fully understood the role of inhibitors, enabling the stable acquisition of high magnetic properties. However, in recent years, with the increasingly tense energy supply and the increasing demands for environmental protection and cost reduction, the disadvantages of high-temperature heating have become increasingly prominent. Reducing the billet heating temperature has become a hot spot in technological development that major grain-oriented silicon steel manufacturers worldwide are focusing on. With continuous research, low-temperature billet heating technology will be more widely promoted and applied, playing a positive role in promoting the production and development of grain-oriented silicon steel.