Inhibitors in grain-oriented silicon steel

　　3.1 MnS

　　MnS is the first compound used as an inhibitor in the history of grain-oriented silicon steel manufacturing, and traditional grain-oriented silicon steel mainly uses MnS as an inhibitor. MnS generally nucleates and precipitates preferentially at crystal defects such as dislocations and grain boundaries during the hot rolling stage, affected by the heating and hot rolling process of the cast billet [4]. In order to ensure that MnS can be finely and dispersedly precipitated during hot rolling, the cast billet needs to be heated to 1350 ℃ or even higher temperature to ensure that the coarse MnS particles in the cast billet can be fully dissolved, which will bring a series of problems to the production, such as more oxide slag, large burning loss, low yield, and many surface defects of the product. In the hot rolling stage, MnS begins to precipitate at about 1250 ℃, precipitates fastest at about 1150 ℃, and precipitation basically stops at about

　　950 ℃. During the precipitation of MnS, the number and average size of particles are continuously increasing, and the increasing rate depends on the content of Mn and S.

　　Generally, the mass fraction of Mn in grain-oriented silicon steel is specified as 0.05%~0.12%, and the mass fraction of sulfur is 0.015%~0.030%. The coarse MnS in the cast billet generally precipitates as spherical nanoparticles (see Figure 2) after slab reheating and hot rolling, with a size less than 100 nm and a distribution density of approximately 1012~1014 per cubic centimeter. This has a good effect of inhibiting primary grain growth. In grain-oriented silicon steel using MnS as an inhibitor, the secondary recrystallization temperature is usually between 870℃ and 925℃. During the secondary recrystallization annealing process, as the temperature increases, the MnS particles will gradually coarsen or dissolve, and their pinning effect on the grain boundaries will weaken until it disappears. After the secondary recrystallization is completed, at a temperature >1150℃ and in a pure H2 atmosphere, it is decomposed and volatilized by the reaction of S with H2 to generate H2S, completing the purification of the grain-oriented silicon steel product.

　　3.2AlN

　　In order to reduce the reheating temperature, AlN obtained from the subsequent hot rolling process is mainly used to replace MnS to produce low-temperature grain-oriented silicon steel. At present, two methods can be used to precipitate AlN particles with high inhibition strength in the process stage before the final high-temperature annealing: one is to perform a two-stage normalization annealing of (1120～1150)℃×(2～3)min→(920～950)℃×(2～3)min on the hot-rolled sheet before cold rolling to obtain a large number of nano-scale AlN, which is a primary inhibitor; the other is to obtain a sufficient number of nano-scale AlN through (750～900)℃×(5～60)s continuous nitriding treatment after decarburization annealing, which is a secondary inhibitor. During normalization annealing and nitriding heat treatment, the number and average size of particles also increase continuously, and the rate of increase depends on the content of acid-soluble aluminum (Als) and N in the steel.

　　The mass fraction of Als in grain-oriented silicon steel is generally controlled at 0.015%~0.035%, and the mass fraction of N is generally controlled at 0.006%~0.013%. AlN initially precipitates during hot rolling. Due to the large volume of the slab, the cooling is slow during the cooling stage, resulting in AlN obtained in the hot rolling stage being slightly larger than the effective inhibition size. Therefore, hot rolling usually uses a fast rolling and fast cooling process to minimize AlN precipitation. After normalization of the hot-rolled sheet, AlN precipitates as rectangular or irregular nanoparticles (see Figure 3), with a size of about 20～100 nm, and has a strong inhibitory effect. When using high-temperature normalization and single large-reduction cold rolling to produce grain-oriented silicon steel with AlN as the main inhibitor, the magnetic properties of the product are unstable. By using AlN and MnS together, a sharper Goss texture can be obtained after high-temperature annealing. In the high-temperature annealing process in a pure H2 atmosphere, when the temperature rises to 1100℃～1200℃, AlN will be absorbed by the glass film on the surface of the thin plate and rapidly decompose. The decomposed N will react with H2 to generate NH3 and volatilize, achieving the purification of the grain-oriented silicon steel product.

　　3.3 Copper Sulfide

　　In order to reduce the slab reheating temperature and improve magnetic properties, many manufacturers add a small amount of Cu to produce medium-temperature heating (1250℃～1300℃) Hi-B steel during the production of grain-oriented silicon steel. The main types of Cu inhibitors in Cu-added grain-oriented silicon steel include Cu2S, (Cu, Mn)1.8S, and Cu1.8S. The solid solution temperature of Cu2S is about 100℃ lower than that of MnS, and its particle coarsening rate is much smaller than that of MnS. Cu2S particles have a very significant pinning effect on grain boundary migration when their size is 30～100 nm. The size of Cu2S particles precipitated in grain-oriented silicon steel is mainly within 10～50 nm. The typical morphology and distribution of Cu2S particles are shown in Figure 4. Cu2S particles can precipitate in stages such as continuous casting, hot rolling, intermediate annealing, and primary recrystallization annealing. They begin to precipitate in large quantities in the hot rolling stage; in the process before high-temperature annealing, the number of Cu2S particles is continuously increasing, and the size is continuously growing [5].

　　(Cu, Mn)1.8S and Cu1.8S are usually used as auxiliary inhibitors, mainly precipitating in the hot rolling stage. Cu1.8S is mainly generated when the mass fraction of Cu in steel is 0.15%~0.20%, its solid solution temperature is 1200℃～1250℃, and the precipitation peak is around 1000℃. A large number of fine (Cu, Mn)1.8S will precipitate during the γ→α phase transformation during precision rolling at 1150℃. The size of (Cu, Mn)1.8S is smaller than that of (Cu, Mn)S, about 30～50 nm. With the increase of Cu content, the ratio of Cu to Mn in the precipitate increases continuously, and the precipitate size decreases continuously [6]. When copper sulfide is used as an inhibitor to produce grain-oriented silicon steel, the inhibition ability of the inhibitor in the steel is between that of grain-oriented silicon steel with MnS and AlN as the main inhibitors [5].

 

 

　　4. Commonly Used Single-Element Solute Inhibitors in Grain-Oriented Silicon Steel

　　4.1 Sn

　　Sn is one of the commonly used auxiliary inhibitors for high-magnetic induction grain-oriented silicon steel. Adding an appropriate amount of Sn can significantly improve the magnetic properties of the steel. Its mass fraction in the steel is generally controlled at 0.05%~0.10%. Sn is a surface-active element that initially precipitates along grain boundaries during coiling and normalization. It helps to achieve a uniform distribution of γ phase and α phase during normalization, resulting in larger ferrite grains after normalization, further promoting the formation of more deformation bands during cold rolling, increasing the number of secondary recrystallization nuclei, and thus reducing iron loss.

　　Sn can segregate at the interface between MnS, AlN, and the matrix, thereby inhibiting the coarsening of these particles, making them finer and more dispersed, enhancing the inhibition strength of the inhibitors in the steel and improving the orientation and magnetic properties of the steel. The research by Fan Lifeng et al. [7] shows that compared with steel without Sn, after adding 0.19% Sn, the precipitates in the steel become finer, and the inhibition ability of the precipitates is significantly increased. At the same time, they used scanning Auger microprobe to study the segregation of Sn along grain boundaries in the high-temperature annealing stage, and found that the Sn content segregated at the grain boundaries is about 17.41 times the average Sn content in the steel, proving that Sn will segregate at the grain boundaries during the final secondary recrystallization annealing process, hindering the normal growth of primary grains, and playing the role of an auxiliary inhibitor. This segregation of Sn mainly occurs on ordinary high-angle grain boundaries, so it will increase the difference in the migration rate between the coincident site lattice grain boundaries dominated by Σ9 grain boundaries around the Goss secondary nuclei and ordinary high-angle grain boundaries, promoting the preferential growth of Goss grains.

　　4.2 Sb

　　Sb is a grain boundary segregation solute element. Adding a small amount of Sb in combination with a compound inhibitor during the production of grain-oriented silicon steel can achieve a better inhibition effect and is more conducive to the formation of Goss texture. Fan Lifeng et al. [8] studied hot-rolled sheets of grain-oriented silicon steel containing Sb and found that with the increase of Sb content, the Goss texture content in the hot-rolled sheet increases, reaching the maximum value when the mass fraction of Sb is 0.04%.

　　During the decarburization annealing stage, Sb elements will segregate at the grain boundaries, inhibiting decarburization and primary recrystallization, and preventing the nucleation and growth of {111} plane grains, significantly increasing the {110} component and the number of secondary nuclei. The addition of Sb will enhance the inhibition ability during the high-temperature annealing stage, increase the starting temperature of secondary recrystallization, thereby improving the accuracy of Goss orientation, promoting the formation of sharp Goss texture, and ultimately improving the magnetic properties of grain-oriented silicon steel [9, 10]. Kawasaki Corporation [11] studied the addition of an appropriate amount of Sb to grain-oriented silicon steel with MnSe and AlN as the main inhibitors, and found that the segregation of Sb at the grain boundaries can reduce the primary grain size while significantly inhibiting the growth of grains in the center of the steel sheet, and its segregation on the steel sheet surface can also prevent the decomposition of surface MnSe and nitrogen increase during high-temperature annealing.

　　4.3Mo

　　Mo is a ferrite-forming element. During the hot rolling stage, it can inhibit the phase transformation process of ferrite → austenite + ferrite → ferrite, significantly inhibiting surface dynamic recrystallization, thereby promoting the formation of a larger number of stronger Goss textures during the hot rolling stage, reducing the iron loss of grain-oriented silicon steel products, and improving magnetic properties. Inokuti et al. [12] studied the effect of Mo on the microstructure of intermediate annealing sheets of grain-oriented silicon steel by adding 0.013% Mo by mass to grain-oriented silicon steel with MnSe as the main inhibitor, and found that after adding Mo, the grain size of the intermediate annealing sheet decreased from 21μm to 19μm, and the volume fraction of Goss texture increased from 8%～22% to 20.7%～40.2%, and it was analyzed that Mo mainly promotes the formation of more shear deformation in {111}<112> oriented grains during the primary cold rolling stage, thereby promoting the formation of Goss texture in the intermediate annealing sheet. The addition of Mo will significantly improve the inhibition ability of MnS and AlN, increase the secondary recrystallization temperature by 15℃～20℃, and enhance the sharpness of the Goss texture in the final high-temperature annealing stage. Adding Mo can also effectively reduce the billet heating temperature. When the Mo-containing steel is heated at 1310℃, the hot-rolled texture obtained is similar to the hot-rolled texture obtained when the Mo-free steel is heated at 1380℃, and the surface texture of the decarburized sheet can still maintain a sufficiently strong (111) orientation component. During high-temperature heating of the billet, Mo will also enrich on the surface and prevent surface and grain boundary oxidation, or form fine Mo2Si3 near the surface, preventing the formation of FeS and preventing grain boundary cracks [13].

　　5. Current Status of Inhibitor Application in Grain-Oriented Silicon Steel Production

　　According to the difference in magnetic properties, grain-oriented silicon steel can be divided into ordinary grain-oriented silicon steel (CGO) and high-permeability grain-oriented silicon steel (Hi-B). The magnetic properties of CGO steel are between 1.80～1.85T, and the magnetic properties of Hi-B steel are greater than 1.89T. At present, there are only more than ten companies in the world that can industrially produce grain-oriented silicon steel products. In order to reduce the reheating temperature of slabs and improve the magnetic properties of products as much as possible, the types of inhibitors used in production have changed from the initial single MnS to the combined use of MnS, AlN, Cu2S and various single-element solute inhibitors.

　　In the production of CGO steel, at present, companies usually use MnS or Cu2S as the main inhibitor and AlN as an auxiliary inhibitor, and the reheating temperature is 1250℃～1300℃, such as Upper Iset and Novolipetsk plants in Russia, POSCO in South Korea, and ThyssenKrupp in Germany. The production of Hi-B steel usually uses AlN as the main inhibitor, MnS, MnSe, and Cu2S as auxiliary inhibitors, and adds a small amount of Sn, Sb, etc., such as ThyssenKrupp in Germany and JFE in Japan, and the reheating temperature is below 1300℃. Nippon Steel, China Baowu Group, and POSCO in South Korea, etc., have produced products with higher inhibitor inhibition strength and better magnetic properties by using the post-cold rolling and annealing nitriding process without changing the type of Hi-B steel inhibitor, and the reheating temperature has been reduced to below 1250℃.

 

　　5. Conclusion

　　Inhibitors are indispensable for the production of grain-oriented silicon steel. The main function of inhibitors is to inhibit the growth of normal grains during the primary recrystallization annealing stage to store driving force for secondary recrystallization, and to promote the abnormal growth of Goss-oriented grains during the secondary recrystallization annealing stage. Some inhibitors can also promote the formation of Goss texture during the hot rolling stage and the formation of secondary recrystallization nuclei during the decarburization annealing stage. There are many types of inhibitors. Compound inhibitors are usually used as main inhibitors, and single-element solute inhibitors are usually used as auxiliary inhibitors. The combination of the two can achieve a better inhibition effect. Using AlN as the main inhibitor, Cu2S as the auxiliary inhibitor, adding an appropriate amount of Sn or Sb, and combining with the nitriding process is the development direction of producing high-quality grain-oriented silicon steel.