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Inhibitor in oriented silicon steel

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  1 Introduction
  Oriented silicon steel refers to Si-Fe soft magnetic material with a mass fraction of 2.9% to 3.5%. It is mainly used to make various distribution transformers and large generator stators. Its performance seriously affects the efficiency of national power transmission. Compared with other kinds of soft magnetic materials, oriented silicon steel has more stable performance and can be used in certain environments such as high temperature and impact; and its price is relatively cheap, the process is more mature, and it is suitable for mass production in industry. The process and equipment used in the production of oriented silicon steel are very complex, the chemical composition is extremely strict, the specified composition range is narrow, the content of impurities is extremely low, and the performance factors are many. The manufacturing capacity and quality measure the steel of a country. Metallurgical level.
  In terms of magnetic properties, the magnetic properties of oriented silicon steel along the rolling direction are closely related to the strength of the secondary recrystallization {110}<110> texture (Goss texture); the enhancement of Goss orientation grain composition in the finished oriented silicon steel sheet can be The magnetic performance is greatly improved. In the production of oriented silicon steel, in order to control the recrystallization process and obtain a single sharp Goss texture, the grain inhibitor (the metastable second phase precipitates the particle) is indispensable. From the perspective of the current industrialized stable production of oriented silicon steel products, reasonable control of inhibitor precipitation is the key to ensure the texture and properties of oriented silicon steel [1]. Therefore, how to utilize and control the precipitation of inhibitors and play their role has always been a hot issue in the research of oriented silicon steel.
 
  2. Inhibitors promote the formation of Goss texture
  Secondary recrystallization is an abnormal growth behavior of crystal grains, that is, only a small part of crystal grains in the primary recrystallized matrix suddenly and rapidly grow into larger-sized crystal grains. The calculation of the grain growth rate in the secondary recrystallization process is shown in Equations 1, 2 [2]: dR/dt = MP (1) P = AE{(1/RC) - (K/R) - (KZ / A) } (2) where M is the grain boundary mobility; P is the total driving force of grain growth; A is the geometric coefficient; E is the average grain boundary energy of the primary recrystallized grain; Rc is the critical dimension of the primary recrystallized grain; K is the ratio of the secondary recrystallized grains to the grain boundary energy of the primary recrystallized grains; R is the secondary recrystallized grain size; Z is the Zener factor, that is, the driving force of the secondary recrystallization driving force from the primary matrix storage ( P1), the resistance of the secondary recrystallized grain growth (P2) and the pinning force of the inhibitor (P3) (see Figure 1).
  According to the Zener formula [3], the inhibition force Z of the second phase particles on grain boundary migration can be expressed as follows:
  Z = 3σf4r (3) where σ is the grain boundary energy; f is the second phase particle volume fraction; r is the second phase particle average radius. The larger the number of precipitates, the smaller the size, the stronger the pinning ability for the first recrystallized grain growth, and the higher the driving force for the primary matrix to be stored in the secondary recrystallization.
  For the mechanism that the inhibitor promotes the preferential growth of Goss oriented grains in the high temperature annealing stage, there are mainly two theories of coincident position lattice model (CSL) and high energy grain boundary model (HE). The coincident position lattice model considers that the CSL grain boundaries have lower energy than other common high angle grain boundaries. In the temperature rising phase of secondary recrystallization annealing, before the inhibitor particles are not coarsened or decomposed, the Goss grains with high coincident lattice grain boundaries have weaker grain boundary energy, and the pinning force of the particles is weaker than that of ordinary crystals. In the boundary, when the particles are coarsened or decomposed, this part of the grain boundary can preferentially get rid of the secondary recrystallization of the pinning of the particles, which leads to the abnormal growth of the Goss grains. The high-energy grain boundary model considers that the growth of Goss grains is based on the physical properties of high-energy grain boundaries with orientation differences between 20° and 45°。 High-energy grain boundaries have more point defects than other grain boundaries, which leads to higher grain boundary migration rate and grain boundary diffusion rate, and the particle coarsening speed on high-energy grain boundaries is faster than other grain boundaries. Therefore, in the secondary recrystallization process, the Goss grains can be quickly separated from the particles and migrated.
  In summary, the role of the inhibitor in oriented silicon steel is mainly to suppress the growth of normal crystal grains in the primary recrystallization annealing stage as the secondary recrystallization storage driving force, and to promote the abnormal length of Goss oriented grains during the secondary recrystallization annealing. Big. Although the finely distributed fine precipitates can pin the grain boundaries and inhibit the growth of the primary grains, whether it can be used as an effective inhibitor can be aggregated and dissolved with the increase of temperature during high temperature annealing. It is finally determined by H2 removal at a temperature of 1 200 °C, so as not to inhibit the growth of the primary grain and also hinder the growth of the secondary recrystallized grains. Inhibitors can be classified into two types, one being a compound such as a sulfide or a nitride, and the other being a simple element having a tendency to segregate at a grain boundary. This article combines its own research and published literature at home and abroad to briefly introduce the types of commonly used inhibitors and their effects.
 
  3. Compound inhibitors commonly used in oriented silicon steel
  3.1MnS
  MnS is the first compound used in the history of oriented silicon steel production as an inhibitor. Traditional oriented silicon steel mainly uses MnS as an inhibitor. MnS is generally preferentially nucleated in the hot rolling stage at the crystal defects such as dislocations and grain boundaries, which are affected by the heating and hot rolling process of the slab [4]. In order to ensure that the MnS can be finely dispersed during the hot rolling process, the slab needs to be heated to a temperature of 1350 ° C or higher to ensure that the coarse MnS particles in the slab can be fully dissolved, which will bring a series of problems to the production. Such as more oxidized slag, large burning loss, low yield, and many surface defects. In the hot rolling stage, MnS starts to precipitate at about 1250 ° C, and the precipitation is fastest at about 1150 ° C.
  The precipitation basically stopped at 950 °C. During the precipitation of MnS, the number of particles and the average size are constantly increasing, and the rate of increase depends on the contents of Mn and S.
  Generally, the mass fraction of Mn in the oriented silicon steel is 0.05% to 0.12%, and the mass fraction of sulfur is 0.015% to 0.030%. The coarse MnS in the slab is generally precipitated by spherical nano-scale precipitates after slab reheating and hot rolling (see Figure 2). The size is less than 100nm and the distribution density is about 1012~1014 per cubic centimeter. The effect of suppressing the growth of the primary grain is suppressed. With MnS as the suppressed oriented silicon steel, the secondary recrystallization temperature is usually between 870 ° C and 925 ° C. During the secondary recrystallization annealing process, as the temperature increases, the MnS particles will gradually coarsen or dissolve, and the pinning effect on the grain boundaries will become weaker and weaker. After the secondary recrystallization is sufficiently completed, H2S is generated by the reaction of S with H2 at a temperature of >1150 ° C and a pure H 2 atmosphere to be decomposed and volatilized, and the purification of the oriented silicon steel product is completed.
  3.2AlN
  In order to reduce the reheating temperature, AlN, which is obtained in the subsequent process section of hot rolling, is used to replace the MnS to produce medium and low temperature oriented silicon steel. At this stage, two methods can be used to precipitate high-inhibition strength AlN particles in the process stage before the final high-temperature annealing: one is through the hot-rolled plate (1120 ~ 1150) ° C × (2 ~ 3) min → (920 ~ 950) Two-stage normalized annealing at °C × (2 ~ 3) min obtained a large amount of nano-sized AlN before cold rolling, which is a primary inhibitor; the other is through decarburization annealing after passing the sample (750 ~ 900) A continuous nitriding treatment of °C × (5 to 60) s to obtain a sufficient amount of nano-sized AlN is a secondary inhibitor. During the normalizing and nitriding heat treatment, the number and average size of the particles are also constantly increasing, and the rate of increase depends on the acid-soluble aluminum (Als) content and the N content in the steel.
  The mass fraction of Als in oriented silicon steel is generally controlled at 0.015% to 0.035%, and the mass fraction of N is generally controlled at 0.006% to 0.013%. AlN is initially precipitated during hot rolling. Due to the large volume of the slab, the cooling in the cooling stage is slow, and the AlN obtained in the hot rolling stage is slightly larger than the effective suppression size. Therefore, hot rolling is usually performed by rapid rolling and rapid cooling. It is desirable to minimize the precipitation of AlN. After the hot-rolled sheet is normalized, AlN precipitates in a rectangular or irregular shape of nano-scale precipitates (see Fig. 3), and has a size of about 20 to 100 nm, which has a strong inhibitory effect. AlN is the main inhibitor, and the magnetic properties of the product obtained when the oriented silicon steel is produced by high-temperature normalization and cold rolling at one time is unstable. By using AlN together with MnS, a sharper Gauss can be obtained after high-temperature annealing. Texture. In the high-temperature annealing process of pure H2 atmosphere, when the temperature is raised to 1100 ° C ~ 1200 ° C, AlN will be absorbed by the glass film on the surface of the thin plate and rapidly decomposed, and the decomposed N and H2 will react to form NH3 and volatilize. Purification of oriented silicon steel products.
  3.3 copper sulfide
  In order to reduce the slab reheating temperature and improve the magnetic properties, many manufacturers add a small amount of Cu in the production of oriented silicon steel to produce Hi-B steel heated at medium temperature (1250 ° C ~ 1300 ° C). The main types of Cu-containing inhibitors in Cu-oriented silicon steel include Cu2S, (Cu, Mn) 1.8S and Cu1.8S. The solid solution temperature of Cu2S is about 100 ° C lower than that of MnS, and the particle coarsening speed is much smaller than that of MnS. The size of the Cu2S particles has a very significant pinning effect on grain boundary migration at 30 to 100 nm. The size of Cu2S particles precipitated in oriented silicon steel is mainly within 10~50nm. The morphology and distribution of typical Cu2S particles are shown in Fig. 4. Cu2S particles can be precipitated in the stages of continuous casting, hot rolling, intermediate annealing and primary recrystallization annealing. Initially, a large amount of precipitation starts in the hot rolling stage. In the process before high temperature annealing, the number of Cu2S particles is increasing, and the size is increased. The trend of growing up [5].
  (Cu, Mn) 1.8S and Cu1.8S are generally used as auxiliary inhibitors, mainly in the hot rolling stage. Cu1.8S is mainly formed when the Cu mass fraction in steel is 0.15% to 0.20%, and its solid solution temperature is 1200 °C to 1250 °C, and the precipitation peak is about 1000 °C. A large amount of fine (Cu, Mn) 1.8S is precipitated during the γ→α phase transition at 1150 °C during finish rolling. The (Cu, Mn) 1.8S size is finer than that of (Cu, Mn)S, and the size is about 30 to 50 nm. As the Cu content increases, the ratio of Cu to Mn in the precipitate increases continuously, and the precipitate size increases. Will continue to decrease [6]. Oriented silicon steel is produced by using copper-sulfur compounds as inhibitors. The inhibitory ability of steel inhibitors is between oriented silicon steels with MnS and AlN as main inhibitors [5].
 
  4. Ordinal solute inhibitors commonly used in oriented silicon steel
  4.1Sn
  Sn is one of the more commonly used auxiliary inhibitors for high magnetic induction oriented silicon steel. The addition of proper amount of Sn can significantly improve the magnetic properties of steel. The mass fraction in steel is generally controlled at 0.05% to 0.10%. Sn is a surface active element that initially precipitates along the grain boundary during the crimping and normalization process, which contributes to the uniform distribution of the γ phase and the α phase during the normalization process, so that a larger ferrite crystal is obtained after normalization. The granules further promote the formation of more deformation zones during cold rolling and increase the number of secondary recrystallized nucleuses, thereby reducing iron loss.
  Sn can be segregated at the interface between MnS and AlN and the matrix, thereby suppressing the coarsening of these particles, making them finer and more dispersed, and enhancing the inhibitory strength of the inhibitor in steel and improving the orientation and magnetic properties of the steel. Fan Lifeng et al. [7] showed that after adding Sn with a mass fraction of 0.19%, the precipitates in the steel became finer and the inhibitory ability of precipitates increased significantly. At the same time, they used the scanning Auger microprobe to study the segregation of Sn along the grain boundary in the high temperature annealing stage. It was found that the Sn content segregated at the grain boundary was about 17.41 times of the average Sn content in the steel, which proved the final second re During the crystallization annealing, Sn will be segregated on the grain boundaries and hinder the normal growth of the primary grains, which will act as an auxiliary inhibitor. This segregation of Sn occurs mainly at ordinary high-angle grain boundaries, and therefore increases the moving rate of coincident lattice grain boundaries and common large-angle grain boundaries around the Σ9 grain boundary around the Goss secondary crystal nucleus. Poor, promote the preferential growth of Goss grains.
  4.2Sb
  Sb is a grain boundary segregation-type solute element. When a small amount of Sb is added in the production process of oriented silicon steel and used together with a compound inhibitor, it can better suppress the effect and contribute to the formation of Goss texture. Fan Lifeng et al. [8] studied the hot-rolled Sb-oriented silicon steel sheet and found that the content of Goss in the hot-rolled sheet increased with the increase of Sb content, and reached the maximum when the Sb mass fraction was 0.04%.
  Sb element segregates at the grain boundary during the decarburization annealing stage, inhibits decarburization and primary recrystallization, and prevents nucleation and growth of {111} surface grains, significantly increasing the number of {110} components and secondary crystal nuclei. . Adding Sb will enhance the inhibition ability of high temperature annealing stage, increase the starting temperature of secondary recrystallization, improve the accuracy of Goss orientation, promote the formation of sharp Goss texture, and finally improve the magnetic properties of oriented silicon steel [9,10] ]. Kawasaki [11] added an appropriate amount of Sb to oriented silicon steel with MnSe and AlN as main inhibitors, and found that the segregation of Sb at the grain boundary can significantly inhibit the central crystal of the steel plate while reducing the primary grain. The growth of the granules, its segregation on the surface of the steel sheet can also prevent the decomposition of surface MnSe and nitrogenation during high temperature annealing.
  4.3Mo
  Mo is a ferrite forming element, which can inhibit the phase transformation process of ferrite→austenite+ferrite→ferrite during the hot rolling stage, and significantly inhibit the dynamic recrystallization of the surface layer, thereby promoting the formation of the number in the hot rolling stage. The multi-strength Goss texture reduces iron loss and improves magnetic properties of oriented silicon steel products. Inokuti et al. [12] studied the influence of Mo on the microstructure of oriented silicon steel intermediate annealed sheets by adding Mo with a mass fraction of 0.013% in oriented silicon steel with MnSe as the main inhibitor. It was found that the grain size of the intermediate annealed sheet after adding Mo was reduced from 21 μm. As small as 19μm, the volume fraction of Goss texture increased from 8% to 22% to 20.7%~40.2%. It is considered that Mo mainly forms more in {111}<112> oriented grains by promoting one cold rolling stage. Shear deformation promotes the formation of Goss texture in the intermediate annealed sheet. The addition of Mo significantly increases the inhibition ability of MnS and AlN, and increases the secondary recrystallization temperature by 15 ° C to 20 ° C to enhance the sharpness of the Goss texture in the final high temperature annealing stage. Adding Mo can also effectively reduce the heating temperature of the slab. When the Mo-containing steel is heated at 1310 °C, the hot-rolled texture is similar to the hot-rolled texture when the 1380 °C slab without Mo steel is heated, and the obtained decarburization is obtained. The surface texture of the board still maintains a sufficiently strong (111) orientation component. When the slab is heated at a high temperature, Mo will also enrich the surface and prevent surface and grain boundary oxidation, or form a small M near the surface.