Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/12—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
  • H01F1/01—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials
  • H01F1/03—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity
  • H01F1/12—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials
  • H01F1/14—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials metals or alloys
  • H01F1/147—Alloys characterised by their composition

Definitions

• the present disclosure relates to a grain-oriented electrical steel sheet and a method for forming an insulating coating.
• Grain-oriented electrical steel sheets are primarily used in transformers. Transformers are continuously excited over long periods of time, from installation to disposal, resulting in continuous energy loss. Therefore, the energy loss when magnetized by alternating current, i.e., iron loss, is a key indicator that determines the performance of a transformer.
• tension to steel sheet is effective in reducing iron loss.
• Forming a coating made of a material with a smaller thermal expansion coefficient than steel sheet at high temperatures on the steel sheet surface is an effective way to reduce iron loss.
• Forsterite-based coatings (inorganic coatings) with excellent coating adhesion are produced during the finish annealing process of electrical steel sheet when oxides on the steel sheet surface react with annealing separators, and are a coating that can apply tension to steel sheet.
• Patent Document 2 discloses a technique in which, after normal finish annealing, the steel sheet is pickled to remove surface deposits, and then chemically or electrolytically polished to a mirror finish. It has been found that even better iron loss improvement effects can be achieved by forming a tensioned insulating coating on the surface of grain-oriented electrical steel sheet that does not have a forsterite-based coating, which has been obtained using such a known method. Furthermore, in addition to improving iron loss, tensioned insulating coatings can also impart various other properties, such as corrosion resistance, heat resistance, and slip resistance.
• forsterite-based coatings also function as intermediate layers that ensure coating adhesion when forming a tension coating (tension-applying insulating coating). That is, because a forsterite-based coating is formed in a state where it penetrates deep into the steel sheet, it has excellent coating adhesion to the metal steel sheet. Therefore, when a tension-applying coating (tension coating) containing colloidal silica, phosphate, or the like as a main component is formed on the surface of a forsterite-based coating, excellent coating adhesion is achieved.
• Patent Document 3 discloses a technique for ensuring the coating adhesion of a tension-applying insulating coating by applying a coating to serve as an intermediate layer beforehand when forming the tension-applying coating.
• the technique disclosed in Patent Document 3 has a problem in that it is not possible to maintain good coating adhesion of a tensioned insulating coating having a large tension.
• grain-oriented electrical steel sheets may be subjected to stress relief annealing after being processed into a specified shape.
• grain-oriented electrical steel sheets to which the technology of Patent Document 4 is applied are not intended to be subjected to stress relief annealing, and that if stress relief annealing is performed, coating adhesion may decrease.
• the crystalline iron phosphate in the intermediate layer becomes coarse due to chemical conversion treatment, which may result in a decrease in the space factor when used as a core.
• Patent Documents 1 to 4 do not disclose grain-oriented electrical steel sheets that do not have a forsterite-based coating and that exhibit excellent coating adhesion even when subjected to stress relief annealing.
• the present disclosure aims to provide a grain-oriented electrical steel sheet that does not have a forsterite-based coating, while maintaining coating tension, corrosion resistance, phosphorus elution from the coating, and space factor and core loss when used as a core at least as good as conventional steel sheets, and that also provides excellent coating adhesion even after stress relief annealing.
• the present disclosure also aims to provide a method for forming an insulating coating that can produce the above-mentioned grain-oriented electrical steel sheet.
• a base steel plate An insulating coating formed on the surface of the base steel sheet, a grain-oriented electrical steel sheet characterized in that the insulating coating contains iron phosphate, the iron phosphate has an average particle size of 5 to 500 nm, and the amount of phosphoric acid eluted is calculated by boiling the steel sheet in boiling pure water for 10 minutes, measuring the amount of phosphoric acid eluted into the pure water, and dividing the measured amount of phosphoric acid by the area of the boiled insulating coating, thereby obtaining an amount of phosphoric acid eluted of less than 40 mg/ m2 .
• a grain-oriented electrical steel sheet that does not have a forsterite-based coating, and that maintains coating tension, corrosion resistance, phosphorus elution from the coating, and space factor and core loss when used as a core at least as good as conventional ones, while also achieving excellent coating adhesion even after stress relief annealing. Furthermore, according to another aspect of the present disclosure, it is possible to provide a method for forming an insulating coating that can produce the above grain-oriented electrical steel sheet.
• This section describes a grain-oriented electrical steel sheet according to one embodiment of the present disclosure (grain-oriented electrical steel sheet according to this embodiment) and a method for forming an insulating coating that can be used to manufacture this grain-oriented electrical steel sheet.
• this disclosure is not limited to the configuration disclosed in this embodiment, and various modifications are possible within the scope of the present disclosure.
• the grain-oriented electrical steel sheet according to this embodiment has a base steel sheet and an insulating coating formed on the surface of the base steel sheet.
• the grain-oriented electrical steel sheet according to this embodiment may be made of only a base steel sheet and an insulating coating, i.e., the grain-oriented electrical steel sheet according to this embodiment may have a two-layer structure made of only a base steel sheet and an insulating coating.
• the base steel plate is made of a steel plate having the following chemical composition:
• the chemical composition of the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment may be within a range known for grain-oriented electrical steel sheets in order to obtain the properties generally required of grain-oriented electrical steel sheets.
• % relating to the chemical composition is % by mass unless otherwise specified.
• C 0.010% or less
• C (carbon) is an element effective for controlling the structure of steel sheets in the manufacturing process up to the completion of the decarburization annealing process.
• the C content is preferably 0.010% or less.
• the C content is more preferably 0.005% or less.
• a lower C content is preferable, reducing the C content to less than 0.0001% saturates the effect of structural control and simply increases manufacturing costs. Therefore, the C content may be 0.0001% or more.
• Si 2.50-4.00%
• Silicon (Si) is an element that increases the electrical resistance of grain-oriented electrical steel sheets and improves their iron loss characteristics. If the Si content is less than 2.50%, a sufficient eddy current loss reduction effect cannot be obtained. Therefore, the Si content is preferably 2.50% or more. The Si content is more preferably 2.70% or more, and even more preferably 3.00% or more. On the other hand, if the Si content exceeds 4.00%, the grain-oriented electrical steel sheet becomes embrittled and the threading property deteriorates significantly. Furthermore, the workability of the grain-oriented electrical steel sheet deteriorates, and the steel sheet may break during rolling. Therefore, the Si content is preferably 4.00% or less. The Si content is more preferably 3.80% or less, and even more preferably 3.70% or less.
• Mn 0.01-0.50%
• Mn manganese
• Mn is an element that combines with S to form MnS during the manufacturing process. This precipitate functions as an inhibitor (a suppressor of normal grain growth) and induces secondary recrystallization in the steel. Mn also improves the hot workability of the steel. If the Mn content is less than 0.01%, the above-mentioned effects cannot be fully obtained. Therefore, the Mn content is preferably 0.01% or more. The Mn content is more preferably 0.02% or more. On the other hand, if the Mn content exceeds 0.50%, secondary recrystallization does not occur, and the magnetic properties of the steel deteriorate. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment, the Mn content is preferably 0.50% or less. The Mn content is more preferably 0.20% or less, and even more preferably 0.10% or less.
• N 0.010% or less
• N nitrogen
• the N content is preferably 0.010% or less.
• the N content is more preferably 0.008% or less.
• the lower limit of the N content is not particularly specified, but reducing it to less than 0.001% would only increase the manufacturing cost, so the N content may be 0.001% or more.
• Sol. Al 0.020% or less
• Sol. Al (acid-soluble aluminum) is an element that bonds with N to form AlN, which functions as an inhibitor, during the manufacturing process of grain-oriented electrical steel sheets.
• the sol. Al content is preferably 0.020% or less.
• the sol. Al content is more preferably 0.010% or less, and even more preferably less than 0.001%.
• the sol. Al content may be 0.0001% or more.
• S 0.010% or less
• S sulfur
• MnS manganese
• the S content in the grain-oriented electrical steel sheet is preferably as low as possible. For example, less than 0.001%. However, reducing the S content in the grain-oriented electrical steel sheet to less than 0.0001% will only increase the manufacturing cost. Therefore, the S content in the grain-oriented electrical steel sheet may be 0.0001% or more.
• the chemical composition of the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment contains the above-mentioned elements, with the balance consisting of Fe and impurities.
• Sn, Cu, Se, and Sb may be contained in place of a portion of Fe in the ranges shown below.
• other elements such as W, Nb, Ti, Ni, Co, V, Cr, and Mo are contained in a total of 1.0% or less, this does not impair the effects of the grain-oriented electrical steel sheet according to this embodiment.
• impurities refer to elements that are mixed in from raw materials such as ore or scrap, or the manufacturing environment, during the industrial production of the base steel sheet, and are permissible to be contained in amounts that do not adversely affect the function of the grain-oriented electrical steel sheet according to this embodiment.
• Sn 0-0.50% Sn (tin) is an element that contributes to improving magnetic properties through controlling the primary recrystallization structure.
• the Sn content is preferably 0.01% or more.
• the Sn content is more preferably 0.02% or more, and even more preferably 0.03% or more.
• the Sn content is preferably 0.50% or less.
• the Sn content is more preferably 0.30% or less, and even more preferably 0.10% or less.
• Cu is an element that contributes to increasing the Goss orientation occupancy rate in the secondary recrystallized structure.
• the Cu content is preferably 0.01% or more.
• the Cu content is more preferably 0.02% or more, and even more preferably 0.03% or more.
• the Cu content is preferably 0.50% or less.
• the Cu content is more preferably 0.30% or less, and even more preferably 0.10% or less.
• Se is an element that has a magnetic property improving effect.
• the Se content is preferably 0.001% or more in order to effectively exhibit the magnetic property improving effect.
• the Se content is more preferably 0.003% or more, and even more preferably 0.006% or more.
• the Se content is preferably 0.020% or less, more preferably 0.015% or less, and even more preferably 0.010% or less.
• Sb 0-0.50% Sb (antimony) is an element that has a magnetic property improving effect.
• the Sb content is preferably 0.005% or more in order to effectively exhibit the magnetic property improving effect.
• the Sb content is more preferably 0.01% or more, and even more preferably 0.02% or more.
• the Sb content is preferably 0.50% or less, more preferably 0.30% or less, and even more preferably 0.10% or less.
• the chemical composition of the base steel sheet of the grain-oriented electrical steel sheet is, for example, one that contains the elements described above, with the remainder consisting of Fe and impurities.
• the chemical composition of the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment can be measured using the known ICP atomic emission spectroscopy method. However, if an insulating coating is formed on the surface, it must be removed before measurement.
• the removal method involves immersing the sheet in a highly concentrated alkaline solution (for example, a 30% sodium hydroxide solution heated to 85°C) for 20 minutes or more. Peeling can be determined visually. For small samples, removal can also be achieved by surface grinding.
• the thickness of the base steel sheet is not limited, but is preferably 0.15 to 0.35 mm from the viewpoint of reducing iron loss.
• an insulating coating is formed on a base steel sheet. That is, since a forsterite-based coating is not formed, the insulating coating is formed in direct contact with the base steel sheet. As will be described later, the insulating coating is a single layer formed by fusing a first insulating coating and a second insulating coating by heat treatment.
• the insulating coating contains iron phosphate.
• iron phosphate By including iron phosphate in the insulating coating, it is possible to suppress a decrease in coating adhesion after stress relief annealing.
• the average particle size of the iron phosphate is 5 to 500 nm. By setting the average particle size of the iron phosphate to 5 to 500 nm, it is possible to further improve coating adhesion after stress relief annealing.
• the average particle size of the iron phosphate is preferably 20 nm or more, or 50 nm or more. Furthermore, it is preferable that the average particle size of the iron phosphate is 450 nm or less, or 350 nm or less.
• the iron phosphate is preferably an Fe-P-O or Fe-P-O-Si system iron phosphate.
• the presence of an Fe-P-O or Fe-P-O-Si system iron phosphate can improve the density of the insulating coating formed from the phosphate and amorphous silica.
• Fe-P-O system iron phosphates include Fe 2 P 2 O 7 , Fe 3 (PO 4 ) 2 , and FePO 4.
• Fe-P-O-Si system iron phosphates include FeOP 2 O 5 SiO 2 .
• the insulating coating contains Si derived from colloidal silica.
• the iron phosphate content is preferably 50 to 90 mass % and the Si content is preferably 10 to 40 mass %.
• the thickness of the insulating coating is preferably 1 to 10 ⁇ m from the viewpoint of improving the space factor.
• the thickness of the insulating coating is determined by the following method. In the flat portion, the cross section of the sample is observed with a scanning electron microscope, and the average thickness can be measured by measuring the thickness at five or more points. In this case, the position where the distribution of P element rapidly decreases from the insulating coating side is taken as the interface between the insulating coating and the base steel sheet.
• the circle-equivalent diameter of the identified iron phosphate is determined and then averaged to obtain the average particle size of the iron phosphate. Measurements are performed for at least three fields of view, and the average is calculated. The magnification is 50,000 times.
• the Si content in the insulating coating can also be measured using a transmission electron microscope and an energy dispersive elemental analyzer.
• the amount of phosphorus leached from the coating is suppressed, and therefore the amount of phosphorus leached is less than 40 mg/ m2 .
• the amount of phosphorus leached is determined by boiling the steel sheet in boiling pure water for 10 minutes, measuring the amount of phosphorus leached into the pure water, and dividing the measured amount of phosphorus by the area of the boiled insulating coating.
• the grain-oriented electrical steel sheet according to this embodiment can achieve the above-described effects regardless of the manufacturing method, but can be preferably manufactured by a manufacturing method including the following steps, for example.
• a hot rolling step in which the steel slab is heated and hot-rolled into a hot-rolled sheet;
• a cold rolling step in which the hot-rolled sheet after the pickling step is cold-rolled to obtain a steel sheet (cold-rolled sheet);
• a decarburization annealing step of subjecting the steel sheet to decarburization annealing;
• a finish annealing step of applying an annealing separator containing 10 to 100 mass% of Al 2 O 3 to the steel sheet after the decarburization annealing, drying the steel sheet,
• the method for producing a grain-oriented electrical steel sheet according to this embodiment is characterized by the (vi) finish annealing step to the (xii) heat treatment step, and the (i) hot rolling step to the (v) decarburization annealing step are not particularly limited, and known conditions can be applied.
• the hot-rolled sheet annealing process In the hot-rolled sheet annealing process, the steel sheet (hot-rolled sheet) after the hot rolling process is annealed. By performing such annealing treatment, recrystallization occurs in the steel sheet structure, making it possible to achieve good magnetic properties.
• the hot-rolled sheet annealing process of this embodiment the hot-rolled sheet manufactured through the hot rolling process may be annealed according to a known method.
• the means for heating the hot-rolled sheet during annealing is not particularly limited, and known heating methods can be adopted. For example, so-called continuous annealing may be used, or the hot-rolled sheet may be coiled and subjected to batch annealing.
• the annealing conditions are also not particularly limited, but for example, the hot-rolled sheet may be annealed in a temperature range of 900 to 1200°C for 10 seconds to 5 minutes.
• the atmosphere is not particularly limited, but it is preferable to suppress oxidation of the steel sheet, and it is preferable to perform the annealing in a non-oxidizing atmosphere such as nitrogen, argon, or hydrogen.
• [Acid washing process] In the pickling process, scale (oxides) formed on the surface of the steel sheet during the hot rolling and hot-rolled sheet annealing is removed.
• a known method is used in the pickling process of this embodiment.
• Known acids such as hydrochloric acid, sulfuric acid, and nitric acid are used as the pickling solution.
• Known pickling inhibitors, pickling accelerators, and the like may be added to the pickling solution as needed.
• physical treatments such as shot blasting of the steel sheet may be performed before pickling in order to penetrate the pickling solution into the interface between the scale and the steel sheet and improve the pickling efficiency.
• Cold rolling process In the cold rolling process, the steel sheet after the pickling process is cold rolled to obtain a cold-rolled sheet.
• the cold rolling may be a single cold rolling (a series of cold rolling without intermediate annealing) or may be multiple cold rollings with intermediate annealing between them, with the cold rolling being interrupted and at least one or two or more intermediate annealings being performed before the final pass of the cold rolling process.
• the cold rolling conditions may follow known methods.
• the cold rolling reduction of grain-oriented electrical steel sheet has a significant effect on its magnetic properties.
• the final reduction has a particularly large effect, and the final reduction can be set to 80-95%.
• the final reduction is the cumulative reduction of cold rolling, and if intermediate annealing is performed, it is the cumulative reduction of cold rolling after final intermediate annealing.
• the steel sheet is held at a temperature of 800 to 1200° C. for 5 to 180 seconds, for example.
• the annealing atmosphere is not particularly limited, but it is preferable to perform the annealing in a non-oxidizing atmosphere such as nitrogen, argon, or hydrogen to prevent oxidation of the steel sheet.
• the annealing method may be either so-called continuous annealing or batch annealing in a coil shape, or other methods.
• the number of intermediate annealings is preferably three or less, taking into consideration the manufacturing cost.
• decarburization annealing process the cold-rolled steel sheet after the grinding step is subjected to decarburization annealing, which removes (decarburizes) carbon from the steel sheet, which adversely affects magnetic properties, and also causes primary recrystallization of the cold-rolled steel sheet.
• the decarburization annealing conditions are not limited, but the annealing is performed in a nitrogen-hydrogen mixed atmosphere for decarburization, with the oxygen potential increased by humidification.
• the humidification temperature (dew point) is determined from the viewpoint of the annealing temperature required for recrystallization and the oxygen potential that allows decarburization at the annealing temperature.
• the annealing temperature is, for example, about 700 to 900° C., and since annealing is generally performed in a continuous annealing process, soaking is performed for about 60 seconds.
• Nitriding treatment may be carried out between the decarburization annealing step and the finish annealing step described below.
• the decarburization-annealed steel sheet is subjected to nitriding treatment by maintaining it at approximately 700 to 850°C in a nitriding atmosphere (an atmosphere containing hydrogen, nitrogen, and a nitriding gas such as ammonia).
• a nitriding atmosphere an atmosphere containing hydrogen, nitrogen, and a nitriding gas such as ammonia.
• AlN is used as an inhibitor, it is preferable to set the nitrogen concentration of the steel sheet to 40 ppm or more by the nitriding process.
• the nitrogen concentration of the steel sheet exceeds 1000 ppm, excess AlN remains in the steel sheet even after the completion of secondary recrystallization in the finish annealing. Such AlN causes iron loss degradation. For this reason, it is preferable to set the nitrogen concentration of the steel sheet after the nitriding process to 1000 ppm or less.
• an annealing separator containing 10 to 100 mass% of Al 2 O 3 is applied to the steel sheet after the decarburization annealing step (or nitriding treatment step), the steel sheet is dried, and then final annealing is performed.
• a forsterite-based coating is formed on the surface of the steel sheet (cold-rolled sheet) by applying an annealing separator mainly composed of MgO and performing final annealing.
• an annealing separator containing Al 2 O 3 is used so as not to form a forsterite-based coating.
• the proportion of Al2O3 may be 100% by mass, but from the viewpoint of preventing Al2O3 from seizing onto the steel sheet surface, in the method for producing a grain-oriented electrical steel sheet according to this embodiment, it is preferable that the annealing separator contains MgO. While the MgO content may be 0%, to obtain the above effect, the proportion of MgO is preferably 5% by mass or more. When MgO is contained, the proportion of MgO is 90% by mass or less to ensure 10% by mass or more of Al2O3 . The proportion of MgO is preferably 50% by mass or less. It is sufficient that the total of Al2O3 and MgO exceeds 50% by mass in terms of solid content relative to the annealing separator.
• the annealing separator may further contain chloride.
• the inclusion of chloride in the annealing separator provides the effect of making it more difficult for a forsterite-based coating to form.
• the chloride content is not particularly limited and may be 0%, but to obtain the above effect, a content of 0.5 to 10.0 mass% is preferred.
• Examples of effective chlorides include bismuth chloride, calcium chloride, cobalt chloride, iron chloride, and nickel chloride.
• the conditions for the finish annealing are not limited, but for example, conditions in which the steel is held at a temperature of 1150 to 1250° C. for 10 to 60 hours can be adopted.
• annealing separator removal process In the annealing separator removing step, excess annealing separator is removed from the steel sheet after the finish annealing step. For example, excess annealing separator can be removed by washing with water.
• the steel sheet after the annealing separator removal step is pickled with 0.1 to 5.0 mass % of an inorganic acid for 10 to 60 seconds. If the conditions for light pickling are not favorable, excess annealing separator may remain on the surface of the steel sheet, increasing the surface roughness and reducing the space factor, while if the acid is too strong, the surface of the steel sheet may be etched, resulting in reduced magnetic properties.
• First insulation film forming step In the first insulating coating formation process, the steel sheet after the water-rinsing process is immersed for 5 to 150 seconds in a treatment solution having a liquid temperature of 30 to 85°C and a metal phosphate concentration of 1.00 to 10.00 mass %, and after the treatment solution is washed away with water, the steel sheet is dried, thereby forming a first insulating coating on the surface of the steel sheet.
• the concentration of the metal phosphate is less than 1.00% by mass, the formation of the first insulating coating may take too long, which may be disadvantageous in terms of cost, so the concentration of the metal phosphate may be 1.00% by mass or more.
• the first insulating coating may be formed with a partial thickness, ultimately resulting in a non-uniform insulating coating. If the immersion time is less than 5 seconds, it is not possible to precipitate iron phosphate in the insulating coating of the grain-oriented electrical steel sheet, whereas if it exceeds 150 seconds, it takes too long and is disadvantageous in terms of cost, so the holding time may be 150 seconds or less.
• the sheet temperature is below 700°C, the tension will be low and the magnetic properties will be inferior. Therefore, it is preferable that the sheet temperature be 700°C or higher. On the other hand, if the sheet temperature is above 950°C, the rigidity of the steel sheet will decrease and it will be prone to deformation. In this case, strain may be introduced into the steel sheet due to transportation, etc., resulting in inferior magnetic properties. Therefore, it is preferable that the sheet temperature be 950°C or lower. Furthermore, if the retention time is less than 10 seconds, the elution property will be poor. Therefore, the retention time is preferably 10 seconds or more. On the other hand, if the retention time is more than 90 seconds, the coating adhesion will be reduced, and if an attempt is made to avoid the reduction in coating adhesion, productivity will be poor. Therefore, the retention time is preferably 90 seconds or less.
• the coating liquid contains a metal phosphate and colloidal silica in an amount of 30 to 150 parts by mass of colloidal silica per 100 parts by mass of the metal phosphate.
• the total amount of the metal phosphate and colloidal silica in the coating liquid, calculated as solid content, is sufficient as long as it exceeds 50% by mass. If the amount of colloidal silica is less than 30 parts by mass, the space factor and iron loss may deteriorate. Therefore, the amount of colloidal silica is preferably 30 parts by mass or more. If the amount of colloidal silica exceeds 150 parts by mass, the adhesion, film tension, corrosion resistance, elution, space factor, and iron loss may deteriorate. Therefore, the amount of colloidal silica is preferably 150 parts by mass or less.
• metal phosphate for example, one or a mixture of two or more selected from aluminum phosphate, zinc phosphate, magnesium phosphate, nickel phosphate, copper phosphate, lithium phosphate, cobalt phosphate, etc. can be used.
• the concentration of the coating liquid is 10.0 to 40.0 mass%. If the concentration is less than 10.0 mass%, there is a risk of coating defects such as patterns occurring. Therefore, it is preferable that the concentration be 10.0 mass% or more. On the other hand, if the concentration is more than 40.0 mass%, the space factor and iron loss may deteriorate. Therefore, it is preferable that the concentration be 40.0 mass% or less.
• the coating solution may contain additional elements such as vanadium, tungsten, molybdenum, and zirconium.
• Colloidal silica can be of type S or type C.
• Type S colloidal silica refers to colloidal silica in which the silica solution is alkaline
• type C colloidal silica refers to silica in which the silica particle surface is aluminum-treated and the silica solution is alkaline to neutral.
• Type S colloidal silica is widely used and relatively inexpensive, but caution is required as there is a risk of aggregation and precipitation when mixed with an acidic metal phosphate solution.
• Type C colloidal silica is stable even when mixed with a metal phosphate solution and there is no risk of precipitation, but it is relatively expensive due to the large number of processing steps required. It is preferable to use the appropriate type depending on the stability of the coating liquid to be prepared.
• the surface of the insulating coating (the surface of the insulating coating provided on the grain-oriented electrical steel sheet) may be irradiated with energy rays to refine the 180° magnetic domains. By refining the magnetic domains, it is possible to further reduce the iron loss of the grain-oriented electrical steel sheet.
• the magnetic domain subdivision process can be carried out by any known method.
• one method is to narrow the width of the 180° magnetic domains (subdivide the 180° magnetic domains) by forming linear or point-like grooves that extend in a direction that intersects the rolling direction at specified intervals along the rolling direction.
• a mechanical groove forming method using gears or the like, a chemical groove forming method in which grooves are formed by electrolytic etching, and a thermal groove forming method using laser irradiation can be used. If the insulating coating is damaged by the formation of stress-strained portions or grooves, and the insulating properties and other characteristics are deteriorated, the insulating coating may be formed again to repair the damage.
• the steel sheet is heated to a temperature range of 700 to 900°C in a non-oxidizing atmosphere having a nitrogen content of 50 to 100% by volume and a hydrogen content of 0 to 50% by volume, and is held at this temperature range for 10 to 180 minutes, whereby iron phosphate is generated, which grows to an appropriate size and fuses the first insulating coating and the second insulating coating to form an insulating coating.
• the atmosphere during the heat treatment is preferably a hydrogen-nitrogen mixed gas, with a hydrogen content of 50% by volume or less. If the hydrogen content exceeds 50% by volume, not only will the cost increase, but the atmosphere will be too reducing, which may cause a silica layer to form on the surface and peel off the insulating coating.
• the conditions in the example are merely an example of conditions adopted to confirm the feasibility and effects of the present disclosure, and the present disclosure is not limited to this example.
• Various conditions may be adopted in the present disclosure as long as they do not deviate from the gist of the present disclosure and the objectives of the present disclosure are achieved.
• a slab containing, by mass%, 0.08% C, 3.31% Si, 0.028% sol. Al, and 0.008% N was cast, heated, and hot-rolled to produce a 2.2 mm hot-rolled sheet. This hot-rolled sheet was then annealed by holding it at 1100°C for 10 seconds.
• the steel was pickled under known conditions and then cold-rolled to 0.22 mm without intermediate annealing to obtain a steel sheet (cold-rolled sheet). This steel sheet was then subjected to decarburization annealing at 830°C for 3 minutes.
• an annealing separator containing 48 mass% MgO, 47 mass% Al2O3 , and 5 mass% BiCl3 was applied and dried, and then finish annealing was performed by heating to 1200°C and holding for 20 hours. After finish annealing, the steel sheet was washed with water to remove excess annealing separator, and it was found that no forsterite-based coating was formed on the steel sheet surface.
• a first insulating coating was formed on this steel sheet using a treatment solution containing a mixture of metal phosphate and additives shown in Table 1. Thereafter, a coating liquid containing a metal phosphate and colloidal silica in the ratio shown in Table 2 was applied and dried to form a second insulating coating.
• grooves 20 ⁇ m deep and 50 ⁇ m wide were formed on the surface of the steel sheet at intervals of 6 mm in a direction tilted by 80° from the rolling direction using a tooth profile. Thereafter, heat treatment was carried out under the conditions shown in Table 2 to obtain grain-oriented electrical steel sheets.
• the obtained grain-oriented electrical steel sheets were evaluated using the methods described above for the presence or absence of iron phosphate in the insulating coating, its identification, and average crystal grain size.
• the chemical composition of the base steel plate (excluding the oxide layer) was Si: 3.30 mass%, C: 0.0018 mass%, Mn: 0.06 mass%, sol. Al: 0.002 mass%, with the remainder being Fe and impurities.
• the resulting grain-oriented electrical steel sheets were also measured for coating adhesion, coating tension, corrosion resistance, elution, space factor, and iron loss using the following procedures.
• Coating Adhesion A sample of 30 mm wide and 300 mm long was taken from the grain-oriented electrical steel sheet, and Cellotape (registered trademark) was attached to the inside of the bent portion. After bending the sample using a 10 mm diameter cylinder, the Cellotape was peeled off from the rear half and attached to a piece of white drawing paper to evaluate the bending adhesion. A score of A or higher was deemed to be excellent in coating adhesion and was judged to be pass. On the other hand, a score of B or lower was deemed to be poor in coating adhesion and was judged to be fail. AA: No peeling A: Almost no peeling B: Peeling of several mm observed C: Peeling of 1/3 to 1/2 observed D: Peeling over almost the entire surface
• Coating tension was calculated by back-calculating from the state of curvature when one side of the insulating coating was peeled off. If the obtained coating tension was 4.0 MPa or more, it was judged to have high coating tension and pass. On the other hand, if the obtained coating tension was less than 4.0 MPa, it was judged to not have high coating tension and fail.
• Corrosion resistance was evaluated by a salt spray test in accordance with JIS Z 2371:2015. A 5% by volume NaCl aqueous solution was allowed to drip onto a sample taken from the grain-oriented electrical steel sheet in a 35°C atmosphere for 7 hours. The rust area of the sample was evaluated according to the following criteria. A score of 5 or higher was considered to have excellent corrosion resistance and was judged to be acceptable. On the other hand, a score of 4 or lower was considered to have poor corrosion resistance and was judged to be unacceptable.
• Leachability Leachability was evaluated based on the amount of phosphoric acid eluted from the sample.
• a sample taken from the grain-oriented electrical steel sheet was boiled in boiling pure water for 10 minutes, and the amount of phosphoric acid eluted in the pure water was measured.
• the amount of phosphoric acid eluted (mg/m 2 ) was calculated by dividing the measured amount of phosphoric acid by the area of the insulating coating of the boiled grain-oriented electrical steel sheet.
• the amount of phosphoric acid eluted in the pure water was calculated by cooling the pure water (solution) into which the phosphoric acid had eluted, diluting the cooled solution with pure water, and measuring the phosphoric acid concentration using ICP-AES.
• the space factor was measured by a method conforming to JIS C 2550-5:2020. Thirty samples were used, each 30 mm wide and 320 mm long. After measuring the total mass of the samples, the space factor was calculated by measuring the distance between the upper and lower backing plates sandwiching the laminate under a pressure of 1 MPa. When the space factor was 97.0% or more, it was determined that the core would have a high space factor and was judged to be acceptable. On the other hand, when the space factor was less than 97.0%, it was determined that the core would not have a high space factor and was judged to be unacceptable.
• Iron Loss Iron loss W17/50 (iron loss per mass at a magnetic flux density amplitude of 1.7 T and 50 Hz) was measured in accordance with the Single Sheet Magnetic Property Measurement Method (Single Sheet Tester: SST) of JIS C2556:2015.
• SST Single Sheet Magnetic Property Measurement Method
• Table 3 shows that the grain-oriented electrical steel sheet according to the present invention maintains coating tension, corrosion resistance, phosphorus elution from the coating, space factor when used as a core, and iron loss at levels equal to or greater than conventional levels, while also achieving excellent coating adhesion even after stress relief annealing.

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