JPH0587600B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for electrolytic treatment of grain-oriented silicon steel sheets, and in particular, the present invention relates to an electrolytic treatment method for grain-oriented silicon steel sheets, and in particular, to effectively finish the surface to a smooth state using an industrially low-cost method, thereby improving iron loss characteristics. The present invention relates to an electrolytic treatment method suitable for achieving significant improvements. Unidirectional silicon steel sheets are products in which secondary recrystallized grains are accumulated in the (110) [001], that is, Goss orientation, and are mainly used as iron cores for transformers and other electrical equipment. Therefore, unidirectional silicon steel sheets are required to have high magnetic flux density (represented by the B ₁₀ value) and low iron loss (represented by the W _17/50 value). Particularly recently, from the standpoint of energy conservation, it has been desired to further reduce iron loss in order to reduce power loss in transformers and the like. The conventional method for manufacturing unidirectional silicon steel sheets involves hot rolling a material containing 2.0 to 4.0% by weight of Si (hereinafter simply expressed as %), followed by one or two cooling steps including intermediate annealing. The final plate thickness is obtained by inter-rolling, and after decarburization annealing, an annealing separator containing MgO as the main component is applied, and then it is wound into a coil, followed by secondary recrystallization annealing and purification annealing, and then as necessary. A commonly used method is to apply a phosphate-based insulating coating. In addition, during the purification annealing described above, the oxidized layer mainly composed of SiO ₂ generated on the surface of the steel sheet after decarburization annealing reacts with MgO in the annealing separator to form a forsterite (Mg ₂ SiO ₄ ) coating. is formed. (Prior art) By the way, in order to improve the iron loss characteristics of unidirectional silicon steel sheets, the glassy film formed on the surface of the steel sheet during purification annealing is removed, and then the nitrides near the interface between the base steel and the glassy film are removed. It has been reported that it is possible to significantly reduce iron loss by removing layers containing impurities such as iron and sulfides and creating a smooth surface (for example, Japanese Patent Publications Nos. 52-24499 and 56-4150). each publication). A common method for mirror-finishing a steel plate surface is as follows:
There are mechanical polishing using buffs, brushes, etc., chemical polishing that dissolves the surface chemically, and electrolytic polishing that dissolves the surface electrochemically. Among these methods, when using mechanical polishing, it is difficult to polish the steel plate without causing distortion.
Further, since this processing strain cannot be completely removed even by strain relief annealing, iron loss increases. Therefore, in order to stably reduce iron loss, it is necessary to achieve a mirror finish by chemical polishing or electrolytic polishing, but in the case of chemical polishing, the amount of polishing and the polishing surface deteriorate due to deterioration of the polishing bath. In contrast, in the case of electrolytic polishing, the amount of polishing and the polishing surface can be controlled much more easily than in chemical polishing because it is an electrochemical process. Therefore, from an industrial point of view, it can be said that electrolytic polishing is more advantageous as a mirror polishing treatment. (Problems to be Solved by the Invention) However, although all of these techniques have a very clear effect of reducing iron loss, they have not yet been industrially implemented today. The reason is that HF, which is used as a chemical polishing liquid,
This is because +H ₂ O ₂ and H ₃ PO ₄ +H ₂ O ₂ are expensive, resulting in high costs. Similarly, phosphoric acid baths, sulfuric acid baths, phosphoric acid-sulfuric acid baths, and perchloric acid baths, which are commonly used as electrolytic polishing solutions, all have high concentration of acid as their main component, and also contain chromate as an additive. The cost is high due to the use of fluoric acid, organic compounds, etc., and there are many unresolved problems in processing steel sheets in large quantities, such as homogeneity, production, and early deterioration of the liquid, so it is not implemented as an industrial project. has not yet been reached. Another important drawback that hinders industrialization is that it is difficult for insulating coats to adhere to mirror-polished surfaces. That is, the conventionally known phosphate-based coats and lamic coats have poor adhesion due to their mirror surfaces and cannot withstand actual use. The present invention advantageously solves the above-mentioned problems, and aims to propose an electrolytic treatment that is advantageously suitable for surface treatment that can be easily industrialized, instead of mirror polishing treatment by electrolytic polishing or chemical polishing. (Means for Solving the Problems) The inventors reexamined the influence of surface conditions on iron loss, and as a result, they obtained the knowledge described below. The first point is that it is mainly the surface oxide that has a large effect on the hysteresis loss, and that the surface irregularities do not necessarily have to be mirror-like. Here, the specular state is an optical concept, and although it is not quantitatively defined, it refers to a surface roughness of 0.4 μm or less, preferably 0.1 μm or less in center line average roughness. Figure 2 shows each iron loss of a grain-oriented silicon steel sheet with oxides on its surface, a grain-oriented silicon steel sheet that has been subjected to mirror polishing treatment, and a grain-oriented silicon steel sheet that has been further pickled and has a rough surface. As is clear from the figure, even if the mirror surface is lost due to pickling, the iron loss has not deteriorated much. In order to obtain a silicon steel sheet with low hysteresis loss in this way, it is not necessarily necessary to make the surface of the steel sheet a mirror surface.In other words, it is necessary to make the surface of the steel sheet a magnetically smooth surface, that is, to prevent the movement of the domain walls that cause hysteresis loss. What is necessary is to provide a surface that is free from oxidation and has excellent film adhesion. Therefore, electrolytic polishing and chemical polishing are not indispensable conditions, and surface treatment means can be freely selected. However, it goes without saying that strain on the surface of the steel sheet during the magnetic smoothing process of silicon steel sheets should be avoided as much as possible since it will degrade core loss. Strain-free polishing methods are suitable. Here we will touch on the mirror polishing phenomenon that characterizes the electrolytic polishing method. In electrolytic polishing, when a current is passed through a strong acid or weak alkaline electrolyte using the surface to be polished as an anode, the metal flows out from the surface as ions due to an electrolytic reaction, but the metal flows between the electrolyte on the metal surface. A viscous film forms. Since this viscous film is thinner at the convex portions of the surface, more current flows through the convex portions than at the concave portions, causing more melting in the convex portions than in the concave portions, resulting in a mirror-like finish with no irregularities. Therefore, chemical polishing and electrolytic polishing can be said to be methods for smoothing metal surfaces, completely independent of crystal grain size and orientation. The second finding is that when a silicon steel sheet is subjected to anodic electrolysis treatment with an aqueous chloride solution, the surface properties of the steel sheet vary greatly depending on the crystal grain orientation on the surface of the steel sheet. In particular, {110} planes, which are strongly accumulated on the surface of grain-oriented silicon steel sheets that have been finish annealed, form a unique network structure microscopically, although they appear as surfaces with grain boundaries in their macroscopic appearance. Moreover, we newly discovered that this surface is magnetically smooth. And NaCl, KCl and
Chlorides such as NH ₄ Cl are extremely low in cost compared to chemicals used in conventional chemical polishing and electrolytic polishing, have high current efficiency and conductivity of the electrolyte, are less corrosive, and have a lower bath composition. Since it is extremely easy to maintain and control, the equipment load and operating costs are also extremely low. It was also found that the surface of a steel sheet anodically electrolyzed with this chloride aqueous solution had better adhesion to the insulating coat than a mirror surface obtained by chemical polishing or electrolytic polishing. However, when a strip of grain-oriented silicon steel sheet is subjected to continuous anodic electrolysis treatment in an aqueous chloride solution, arc spots, speckled stains, or minute scratches are likely to occur on the surface of the treated steel sheet. There were variations in magnetic properties and adhesion strength of the coating film formed after electrolysis. When the cause of this was investigated, it was confirmed that iron hydroxide precipitates adhering to the steel plate surface adhered to the steel plate surface when passing through the conductor roll, causing deterioration of the surface quality. Incidentally, a large amount of iron hydroxide precipitate inevitably accumulates in the bath as a steel plate undergoes anodic electrolysis. Therefore, we investigated various methods for improving the surface properties of steel sheets after electrolytic treatment, and found that before the steel sheet passes through the conductor roll, a clean liquid is applied to the steel sheet to cool it and to wash away the precipitates from the surface. was found to be effective. That is, this invention introduces a finish-annealed grain-oriented silicon steel sheet that is positively charged by a conductor roll into an electrolytic cell filled with an electrolytic solution made of an aqueous solution containing one or more water-soluble chlorides. When continuously performing magnetic smoothing treatment by electrolysis, a portion of the electrolyte is continuously extracted from the electrolytic cell and solid-liquid separation is performed to determine the concentration of precipitates in the extracted electrolyte.
This is an electrolytic treatment method for a grain-oriented silicon steel sheet, which is characterized in that the electrolytic solution is adjusted to 100 ppm or less, and then the electrolytic solution is supplied to the surface of the steel sheet at the inlet side of a conductor roll, and then returned to the electrolytic cell. Further, in the embodiment, it is advantageous to use a precipitating flocculant for solid-liquid separation of the electrolyte. This invention will be specifically explained below. In this invention, a slab for a silicon steel plate is hot rolled according to a conventional method, then cold rolled with intermediate annealing to achieve the final thickness, decarburized annealed, and then final finish annealed. give As an annealing separator during this final annealing, an annealing separator containing MgO as a main component has conventionally been mainly used in order to simultaneously form a forsterite film. For example,
The main component is Al ₂ O ₃ , etc., and inert MgO, Ca, etc.
A separation agent containing an Sr compound may also be used. Next, the surface oxidation layer of the final finish annealing is removed. Removal methods include chemical methods such as pickling and mechanical methods such as emery polishing. Although there are no particular limitations, if the surface oxide layer is removed by mechanical methods, distortion may easily occur inside the board. Since such strain cannot be relieved even by a subsequent electrolytic treatment, it is preferable to remove surface oxides by a pickling treatment. Then, the surface from which the surface oxide layer has been removed is magnetically smoothed by anodic electrolytic treatment. The electrolytic bath is an aqueous solution containing one or more water-soluble chlorides, and the water-soluble chlorides here mean HCl, NH ₄ Cl, and chlorides of various metals. All of these have a magnetic smoothing effect on grain-oriented silicon steel sheets after finish annealing, but in actual operation, alkali metals and alkaline earth metals are used to prevent metal precipitation on the cathode. It is preferable to select from chloride, NH ₄ Cl, HCl, AlCl ₃ , etc. Further, the concentration is desirably 20 g/or more in order to ensure the electrical conductivity of the bath. Note that seawater can also be used in this invention due to its composition and concentration. The bath temperature can be arbitrarily selected from room temperature or above, but if the temperature is too high, water will evaporate significantly, so room temperature to about 90°C is suitable. The current density can be set in a range from about 5 A/dm ² to several hundred A/dm ² . However, when the bath temperature is low, using a high current density exceeding 100 A/ ^dm2 tends to cause uneven surface treatment, so if you try to widen the current density range, you can increase the bath temperature by 40
It is better to keep it above ℃. In addition, from the standpoint of reducing iron loss, the amount of electricity in electrolysis and the amount of electrolytic removal in this discovery are respectively
It is preferable that the thickness be 300 C/dm ² or more and 1 μm or more per side. Further, the changes in the bath caused by the electrolytic reaction are shown below using an aqueous NaCl solution as an example. Note Anode: Fe+2Cl ^- →FeCl ₂ +2e ^- …(1) Cathode: 2Na ⁺ +2H ₂ O+2e ^- →2NaOH+H ₂ ↑
…(2) Bulk: FeCl ₂ +2NaOH→2NaCl+Fe(OH) ₂
↓ ...(3) That is, FeCl ₂ produced by equation (1) and NaOH produced by equation (2) automatically regenerate NaCl through the reaction shown in equation (3). Therefore, the basic method for controlling the bath is to generate it using equation (3).
All that is required is the removal of the Fe(OH) ₂ precipitate, the replenishment of water, and the replenishment of NaCl carried out of the system by the plate.
Compared to conventional chemical polishing or electrolytic polishing, it is much easier and cheaper. This point is also one of the reasons why the method of the present invention is industrially superior. By the way, the amount of steel plate melted to reduce iron loss is 1 μm or more per side, and when treated continuously, all of the dissolved iron becomes iron hydroxide, so the amount that forms and precipitates in the bath is quite large. many. For this reason, the precipitates tend to adhere to the steel plate, and if the precipitates pass through the conductor roll while remaining adhered, they will stick to the steel plate and cause defects such as stains, scratches, and arc spots. These defects naturally have an adverse effect on the magnetic properties and adhesion of the coating. Grain-oriented silicon steel sheets are thin at 0.2 to 0.35 mm, and contain a large amount of Si, so their resistivity is several times higher than that of ordinary steel.In addition, they are used to increase line speed and improve productivity. The input current increases, and as a result, the heat generated by the steel plate increases significantly.
For this reason, iron hydroxide precipitates have a strong tendency to stick to steel sheets. Therefore, the present invention eliminates the adverse effects of the precipitate concentration by the following method, which will be explained in detail with reference to FIG. 1. FIG. 1 schematically shows an example of an electrolytic treatment apparatus having three vertical electrolytic cells, which is directly used in the present invention.
A strip 1 of silicon steel sheet passes through a conductor roll 2 and a sink roll 3 and is sequentially subjected to anodic electrolysis. Note that 4 is a cathode and 5 is a snubber roll. At this time, the iron hydroxide precipitate generated by dissolving the steel plate is dispersed and accumulated in the electrolytic solution 6. Therefore, a portion of the electrolytic solution 6 is continuously extracted from the electrolytic cell 7 and guided to the stock solution tank 9 via the pump 8. Next, the electrolytic solution 6 is sent to a coagulation tank 10, and a precipitated coagulant is fed into this tank from a precipitated coagulant tank 11. The precipitated coagulant is added to the electrolytic solution to form a floc, and this liquid is converted into a solid-liquid. It is sent to the separation device 12.
At this time, it is preferable to use a pump that creates a gentle flow that does not destroy the coagulated and precipitated flocs, or to simply use gravity to transfer without using a pump. As the solid-liquid separator 12, various types of filters, centrifuges, thickeners, magnetic separators, etc. may be used alone or in combination. The iron hydroxide precipitate separated here is either dehydrated in the sludge treatment device 13 or disposed of as is. On the other hand, the liquid after solid-liquid separation is supplied from the spray 15 on the inlet side of the conductor roll to the surface of the steel plate through the treatment liquid tank 14, and washes away the deposits of iron hydroxide deposited in the electrolytic cell 7. At this time, the concentration of precipitates in the liquid coming out of the spray 15 needs to be 100 ppm or less. This is because when the precipitate concentration exceeds 100 ppm, defects such as arc spots, dirt, and scratches are likely to occur due to the adhesion of the precipitates. Further, the pressure and flow rate of the spray are set to conditions sufficient to wash away the precipitates adhering to the surface of the steel plate, but at the same time, it is desirable to determine this while also taking into consideration the cooling of the steel plate that has generated heat due to Joule heat. In addition, in this invention, depending on the capacity of the solid-liquid separator, it is not necessary to add a settling and flocculant. Further, the processing capacity of the solid-liquid separator is preferably set so as to maintain the precipitate concentration in the electrolytic cell 7 at 2% or less. This is because if the precipitate concentration exceeds 2%, the viscosity of the liquid increases and the electrical conductivity decreases, which may seriously impede the operation. When using a precipitating flocculant in this invention,
For example, anionic, depending on the pH of the electrolyte used,
Polyacrylamide-based or polyacrylic acid ester-based polymer flocculants, which are available in various types such as nonionic and cationic types, are suitable. Although FIG. 1 shows an example of a vertical electrolytic cell, the present invention is not limited to this, and may be applied to other types of horizontal cells conventionally used for continuous electrolytic treatment of steel plate strips. suit advantageously. (Function) When continuous electrolysis is carried out according to the present invention, defects such as arc spots, dirt, and minute scratches are less likely to occur on the surface of the steel plate after treatment, because the cleaning liquid sprayed on the conductor roll entry side adheres to the surface of the steel plate. This is because the deposits of iron hydroxide coming out from the electrolytic cell are no longer present on the contact surface between the strip and the conductor roll. Furthermore, the ability to cool the steel plate that has generated heat due to Joule heat by spraying the cleaning liquid onto the surface of the steel plate is also advantageous in preventing deposits from adhering to the steel plate. In addition, the iron loss of products obtained by anodic electrolysis in chloride aqueous solutions is better than that of products with mirror surfaces obtained by conventional methods such as electrolytic polishing and chemical polishing. The reasons for this have not been completely elucidated, but firstly, geometric smoothness is not required to be very high in order to be magnetically smooth, and secondly, in the method of the present invention, the grain boundaries are not step-like or Since groove-shaped recesses are formed, the width of the magnetic domain becomes narrower, which is expected to reduce iron loss.Thirdly, electrolytic polishing is thought to cause deterioration due to an oxide film formed non-uniformly on the mirror surface. It is presumed that this is because this does not occur with the product of the present invention. (Example) Example 1 C: 0.040%, Si: 3.38%, Se: 0.016%, Mo:
A hot rolled sheet with a composition containing 0.011% and Sb: 0.027% is cold rolled twice including intermediate annealing to a thickness of 0.23mm.
It was made into a cold-rolled plate with a certain thickness. Next, this steel plate was heated to 830℃.
After decarburization and primary recrystallization annealing in wet hydrogen,
After applying an annealing separator mainly composed of MgO and Al ₂ O ₃ , it was wound into a coil and heated at 850℃ for 50 hours.
Next recrystallization annealing and purification annealing were performed at 1200°C for 5 hours. Thereafter, the unreacted annealing separator was removed, flattening annealing was performed to correct the winding curls, and a test material was obtained. The oxide film on the surface of the sample material was removed by pickling, and then anodic electrolytic treatment was performed using the vertical electrolytic cell shown in FIG. 1 under the following conditions. (1) Electrolyte: NaCl, 200g/, 60℃ (2) Iron hydroxide precipitate concentration in the tank: 3500ppm (3) Current density: 70A/dm ² (4) Electricity: 2400 coulombs/dm ² ( (Dissolution of 6 μm on one side) Furthermore, while performing electrolytic treatment, a portion of the electrolytic solution was extracted from the electrolytic cell and introduced into a thickener, where it was separated into solid and liquid by sedimentation. The precipitate concentration in the electrolytic solution after the sedimentation treatment was about 60 ppm. The operation was then carried out while spraying the treated electrolyte onto the strip surface at the entrance of the conductor roll. Washing with water after electrolytic treatment,
It was dried and wound into coils. Table 1 shows the results of examining the magnetic properties and surface appearance of the products. For comparison, a similar investigation was also conducted when the electrolyte was operated without solid-liquid separation and with simple circulating spraying. These results are also listed in Table 1.

[Table] The product obtained according to the present invention (conforming example) showed a defect-free surface and extremely low core loss, but the comparative example had frequent speckled stains on the arc spots, which caused magnetic properties to deteriorate. The results were also inferior to the conforming examples. Example 2 The same test material as in Example was prepared, the oxide film on the surface of the test material was removed by pickling, and then anodic electrolysis was performed using the same electrolytic bath as in Example 1 under the following conditions. Ivy. (1) Electrolyte: NaCl (100g), NH ₄ Cl (150g/),
50℃ (2) Concentration of iron hydroxide precipitate in the tank: 2800ppm (3) Current density: 90A/dm ² (4) Amount of electricity: 2000 coulombs/dm ² (dissolution of 5 μm on one side) Further electrolytic treatment is carried out in the electrolytic tank. A portion of the electrolyte is extracted from the liquid, a polyacrylamide-based polymer precipitating flocculant and Fe ₃ O ₄ powder are added to this liquid to form a floc, and then a solid-liquid is separated by a magnetic separation device using a permanent magnet. separated. The precipitate concentration in the solution after treatment was approximately 30 ppm. The operation was carried out while spraying this liquid onto the strip surface on the inlet side of the conductor roll. After electrolytic treatment, it was washed with water, dried, and wound into a coil. Table 2 shows the results of investigating the magnetic properties and surface appearance of the products. For comparison, a similar investigation was also conducted when the electrolyte was operated with simple circulation spraying without solid-liquid separation. These results are also listed in Table 2.

[Table] The product obtained according to the present invention (conforming example) showed a defect-free surface and extremely low core loss, but the comparative example had many arc spots and speckled stains, which caused the magnetic properties to deteriorate. The results were also inferior to the conforming examples. (Effects of the Invention) The method of the present invention can be used as a low-cost electrolytic treatment method for the purpose of reducing iron loss of finish-annealed grain-oriented silicon steel sheets, and as a method for obtaining products without surface defects even in continuous operation. This is extremely advantageous and can facilitate the realization of industrialization, which has been difficult in the past.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing an example of an electrolytic treatment apparatus, and FIG. 2 is a graph showing the relationship between surface condition and iron loss. 1...Strip, 2...Conductor roll, 3
... sink roll, 4... cathode, 5... snubber roll, 6... electrolyte, 7... electrolytic cell, 8... pump, 9...
Raw solution tank, 10... flocculating tank, 11... settling flocculant tank, 12... solid-liquid separator, 13... sludge treatment device, 14... treated liquid tank, 15... spray.

Claims (1)

[Claims] 1. A finish-annealed grain-oriented silicon steel sheet that is positively charged by a conductor roll is placed in an electrolytic cell filled with an electrolytic solution consisting of an aqueous solution containing one or more water-soluble chlorides. When introducing and continuously performing magnetic smoothing treatment by electrolysis, a portion of the electrolyte is continuously extracted from the electrolytic cell and solid-liquid separation is performed to keep the concentration of precipitates in the extracted electrolyte to 100 ppm or less. A method for electrolytic treatment of a grain-oriented silicon steel sheet, characterized in that the electrolytic solution is then supplied to the surface of the steel sheet at the inlet side of a conductor roll, and then returned to the electrolytic cell. 2. The method according to claim 1, in which the solid-liquid separation of the electrolytic solution uses a precipitating and coagulating agent.