Manufacturing method of grain-oriented electrical steel sheet

Abstract

We propose a method for manufacturing grain-oriented electrical steel sheets that can further improve magnetic properties and suppress fluctuations in magnetic properties and coating properties within coils without using inhibitors. This method for producing grain-oriented electrical steel sheet includes a hot rolling step in which a steel material having a composition containing C, Si, Mn, acid-soluble Al, N, and Sb, and containing S and Se so as to satisfy a predetermined relationship, an optional hot-rolled sheet annealing step, a cold rolling step, a pickling step, a primary recrystallization annealing step, and a finish annealing step, in which, in the hot-rolled sheet annealing step or intermediate annealing in the cold rolling step before producing a final cold-rolled steel sheet, the γ phase fraction of the steel structure at the annealing temperature is set to 1.0 to 25.0 volume % and the average cooling rate after annealing is set to 20 to 50°C / s, the pickling step involves pickling with a solution containing 2 to 15 mass % of Fe ions and having an acid concentration of 1 to 20 mass %, and the finish annealing step involves an annealing separator containing a Ti compound, calculated as Ti, in 0.3 to 8 parts by mass per 100 parts by mass of magnesia.

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing grain-oriented electrical steel sheets used as iron core materials for transformers. In this specification, the term "x to y" representing a numerical range means not less than x and not more than y, and includes the boundary value. [Background technology]

[0002] Grain-oriented electrical steel is a soft magnetic material used as the iron core material for transformers and large generators. Grain-oriented electrical steel has a crystal orientation that is the easy axis of magnetization of iron. <001> The steel sheet has a crystallographic texture with its axis highly aligned in the rolling direction. This texture is formed by secondary recrystallization and has a {110} Goss orientation. <001> This is achieved by preferentially growing grains with the Goss orientation. Therefore, the manufacturing method of grain-oriented electrical steel generally uses a precipitate called an inhibitor to induce secondary recrystallization of grains with the Goss orientation during secondary recrystallization annealing.

[0003] Patent Document 1 discloses a method using AlN or MnS as an inhibitor, and Patent Document 2 discloses a method using MnS or MnSe as an inhibitor, both of which have been put into industrial use. These methods using inhibitors require slab heating at high temperatures of 1300°C or higher to completely dissolve the inhibitor components. However, these methods are extremely useful for stably developing secondary recrystallized grains.

[0004] Furthermore, in order to enhance the action of these inhibitors, Patent Document 3 discloses a method of using Pb, Sb, Nb, and Te, and Patent Document 4 discloses a method of using Zr, Ti, B, Nb, Ta, V, Cr, and Mo.

[0005] Furthermore, Patent Document 5 proposes a method in which acid-soluble Al (sol. Al) is added at 0.010 to 0.060 mass%, slab heating is kept at a low temperature, and nitriding is performed in an appropriate nitriding atmosphere in the decarburization annealing process. As a result, (Al, Si)N precipitates during secondary recrystallization annealing and is used as an inhibitor. Many methods have been proposed, called nitriding methods, in which such nitriding treatment is performed during the intermediate process of manufacturing grain-oriented electrical steel sheet and (Al, Si)N or AlN is used as an inhibitor.

[0006] On the other hand, Patent Document 6 and other publications disclose a technique for developing Goss-oriented grains through secondary recrystallization in a material that does not contain inhibitor components. This technique minimizes impurities such as inhibitor components, thereby revealing the grain boundary misorientation angle dependence of the grain boundary energy of the grain boundaries during primary recrystallization and allowing Goss-oriented grains to undergo secondary recrystallization without the use of inhibitors. This effect is called the texture inhibition effect. This method does not require the fine dispersion of inhibitors in the steel, and therefore does not require high-temperature slab heating, which is previously essential. This method has significant advantages in terms of cost and maintenance.

[0007] Furthermore, Patent Document 7 discloses a technique for achieving both magnetic properties and film properties using a material that does not contain an inhibitor component. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Special Publication No. 40-15644 [Patent Document 2] Special Publication No. 51-13469 [Patent Document 3] Special Publication No. 38-08214 [Patent Document 4] Japanese Patent Application Publication No. 52-24116 [Patent Document 5] Japanese Patent Application Publication No. 03-02324 [Patent Document 6] Japanese Patent Application Laid-Open No. 2000-129356 [Patent Document 7] Japanese Patent Application Laid-Open No. 2007-138199 Summary of the Invention [Problem to be solved by the invention]

[0009] However, the prior art disclosed in the above patent document has the following problems. Grain-oriented electrical steel sheets manufactured from materials containing no inhibitors still have inferior stability in magnetic properties and coating properties compared to grain-oriented electrical steel sheets manufactured using conventional inhibitors. In particular, variations in magnetic properties and coating properties can occur in the width or length direction of the steel strip due to the coiling and finish annealing of the steel sheet. To combat global warming, various countries and regions are tightening regulations to improve energy efficiency, creating a high demand for grain-oriented electrical steel sheets with high magnetic properties. However, attempts to improve magnetic properties to meet these demands have revealed a new issue: variations in magnetic properties and coating properties within the coil. Therefore, when a technology for achieving both magnetic properties and coating properties was applied to steel types using Al-based inhibitors, the technology described in Patent Document 7 did not result in any improvement in magnetic properties.

[0010] The present invention has been made in view of the above problems, and aims to solve the above problems by proposing a method for manufacturing a grain-oriented electrical steel sheet that can further improve magnetic properties and suppress fluctuations in magnetic properties and coating properties within a coil without using an inhibitor. [Means for solving the problem]

[0011] To solve the above-mentioned problems, the inventors conducted extensive research focusing on the annealing conditions before final cold rolling and the primary recrystallization annealing conditions for materials that do not contain inhibitor components. As a result, they found that grain-oriented electrical steel sheets that combine excellent magnetic properties and coating properties can be consistently obtained by specifying the γ phase fraction of the material during annealing before final cold rolling and the cooling conditions after annealing. In addition, they found that grain-oriented electrical steel sheets that combine excellent magnetic properties and coating properties can be consistently obtained by specifying the pickling conditions before primary recrystallization annealing and the auxiliary agent conditions for magnesia in the annealing separator. Combining these findings led to the development of the present invention.

[0012] <Experiment 1> Steel slabs containing, by mass, 0.015–0.070% C, 2.98–3.16% Si, 0.10–0.12% Mn, 0.0035–0.0052% N, 0.0040–0.065% sol. Al, 0.0015–0.0025% S, and 0.04–0.08% Sb, with the remainder consisting of Fe and unavoidable impurities, were produced by continuous casting. The slabs were heated to 1200°C and then hot-rolled to a 2.6 mm thick hot-rolled steel sheet. The objective of this experiment was to significantly change the C content to alter the γ phase fraction of the steel microstructure at the annealing temperature. The hot-rolled steel sheets were then annealed at 1050°C for 30 s to obtain hot-rolled annealed sheets. The hot-rolled annealed steel sheet was then pickled to remove surface scale and cold-rolled to a thickness of 0.27 mm. The final cold-rolled steel sheet was then immersed in a 7% by mass hydrochloric acid solution containing 5% by mass Fe ions at 50°C for 10 seconds to pickle the surface, yielding a pickled cold-rolled steel sheet. The pickled cold-rolled steel sheet was then subjected to primary recrystallization annealing with decarburization at 840°C for 150 seconds in a humid atmosphere of 55% by volume H2-45% by volume N2 with a dew point of 60°C, yielding a primarily recrystallized annealed steel sheet. A magnesia-based annealing separator containing 1.5 parts by mass of TiO2 (Ti equivalent) per 100 parts by mass of magnesia was then applied to the primarily recrystallized annealed steel sheet, followed by secondary recrystallization annealing at 1200°C for 15 hours to obtain a finish-annealed steel sheet. Secondary recrystallization annealing was performed in a nitrogen atmosphere below 1000°C and in an Ar atmosphere above 1000°C. During the temperature rise process of the secondary recrystallization annealing, the residence time in the temperature range of 800 to 900°C was set to 75 hours. After that, the finish-annealed sheet was washed with water to remove unreacted annealing separator, and a coating containing magnesium phosphate, colloidal silica, and chromic acid as its main components was applied.

[0013] The magnetic flux density B8 of the obtained samples was measured using the method specified in JIS C2550. The magnetic flux density B8 refers to the magnetic flux density when excited at 800 A / m. In this experiment, the conditions were changed for each coil. The final coil was divided into four sections, and the properties were evaluated at each end. The worst value was used as the representative value for the original coil. Since large variations in properties result in poorer values, good properties can be determined by small variations within the coil. The following experiments were also evaluated using representative values. The γ-phase fraction at 1050°C, the hot-rolled sheet annealing temperature, was calculated using the composition of each steel slab. The results were plotted against the magnetic flux density B8 obtained above. The results in Figure 1 indicate that good magnetic properties are obtained when the γ-phase fraction is in the range of 1.0 to 25.0 volume percent. The γ-phase fraction can be calculated using equilibrium calculation software such as Thermo-Calc (Thermo-Calc software AB). In this experiment, Thermo-Calc ver. 2020b was used, and the database used was TCFE10:Steels / Fe Alloys v10.1. Only elements available in this database were used in the calculation.

[0014] <Experiment 2> Steel slab A was produced by continuous casting. The mass ratio of steel slab A was 0.042% C, 3.35% Si, 0.07% Mn, 0.0041% N, 0.0070% sol.Al, 0.0032% S, and 0.04% Sb, with the remainder consisting of Fe and unavoidable impurities. Steel slab A had a γ-phase fraction of 9.9% by volume at 1070°C. Steel slab B was also produced by continuous casting. The mass ratio of steel slab B was 26.8% by volume. Steel slab A and steel slab B were subjected to slab heating at 1220°C, followed by hot rolling to obtain hot-rolled steel sheets with a thickness of 2.4 mm. The hot-rolled steel sheets were then annealed at 1070°C for 20 seconds to obtain hot-rolled annealed steel sheets. The cooling rate between 800 and 400°C during the cooling process after annealing was varied. The hot-rolled annealed steel sheets were then pickled to remove surface scale, and then cold-rolled to obtain final cold-rolled steel sheets with a thickness of 0.27 mm. The final cold-rolled steel sheets were then immersed in a 5% by mass hydrochloric acid solution containing 7% by mass of Fe ions at 65°C for 5 seconds to obtain pickled surfaces, resulting in pickled cold-rolled steel sheets. The pickled cold-rolled steel sheets were then subjected to primary recrystallization annealing with decarburization at 850°C for 120 seconds in a humid atmosphere of 55% by volume H2-45% by volume N2 with a dew point of 55°C to obtain primary recrystallization annealed steel sheets. Next, a magnesia-based annealing separator was added, with 1.0 part by mass of TiO2 (calculated as Ti) added to 100 parts by mass of magnesia, and applied to the primarily recrystallized annealed sheet. Secondary recrystallization annealing was performed at 1200°C for 5 hours in a hydrogen atmosphere to obtain a finish-annealed sheet. An Ar atmosphere was generally used during the temperature increase and decrease of the secondary recrystallization annealing. During this temperature increase process, the residence time in the temperature range of 800-900°C was 100 hours, during which time a nitrogen atmosphere was used. The finish-annealed sheet was then washed with water to remove any unreacted annealing separator, and a coating primarily composed of magnesium phosphate, colloidal silica, and chromic acid was applied.

[0015] The magnetic flux density B8 of the obtained samples was measured in the same manner as in Experiment 1. The results of the obtained magnetic flux density B8, organized by the cooling rate of the hot-rolled sheet annealing, are shown in Figure 2. From these results, it can be seen that for steel slab A, good magnetic properties are obtained, with a magnetic flux density B8 of 1.92 T or more when the cooling rate of the hot-rolled sheet annealing is in the range of 20 to 50 °C / s. It can also be seen that for steel slab B, the magnetic flux density B8 remains below 1.92 T even when the cooling rate is changed.

[0016] <Experiment 3> The final cold-rolled steel sheet obtained from steel slab A in Experiment 2 was immersed in a 5% by mass hydrochloric acid solution containing 0–20% by mass Fe ions at 50°C for 5 s to obtain pickled cold-rolled steel sheets. The pickled cold-rolled steel sheets were then subjected to primary recrystallization annealing with decarburization in a humid atmosphere of 50% by volume H2-50% by volume N2 with a dew point of 52°C at 850°C for 180 s to obtain primary recrystallization annealed steel sheets. Subsequently, a magnesia-based annealing separator was applied, with TiO2 added at 1.5 parts by mass (Ti equivalent) per 100 parts by mass of magnesia. The final annealed steel sheets were then subjected to secondary recrystallization annealing at 1200°C for 5 hours in a hydrogen atmosphere to obtain final annealed steel sheets. During the secondary recrystallization annealing process, the residence time in the temperature range of 800–900°C was 100 hours. The finish-annealed steel sheets were then rinsed with water to remove any unreacted annealing separator, and a coating consisting primarily of magnesium phosphate, colloidal silica, and chromic acid was applied. The resulting samples were evaluated for coating adhesion. Coating adhesion was evaluated by wrapping the steel sheets around cylinders of various diameters and determining the smallest diameter at which the coating did not peel off. A smaller minimum diameter indicates better coating adhesion. Figure 3 shows the results of the coating adhesion evaluation, organized by the amount of Fe ions in the hydrochloric acid solution. These results indicate that good coating properties can be obtained when the Fe ions in the hydrochloric acid solution are in the range of 2 to 15 mass%.

[0017] <Experiment 4> The final cold-rolled steel sheet obtained from steel slab A in Experiment 2 was pickled under different pickling conditions to obtain pickled cold-rolled steel sheets. Pickling condition A involved immersion in a 10% by mass hydrochloric acid solution containing 7% by mass of Fe ions at 75°C for 3 s. Pickling condition B involved immersion in a 10% by mass hydrochloric acid solution containing 20% ​​by mass of Fe ions at 75°C for 3 s. The resulting pickled cold-rolled steel sheet was subjected to primary recrystallization annealing with decarburization at 835°C for 180 s in a humid atmosphere of 55% by volume H2-45% by volume N2 with a dew point of 56°C to obtain a primarily recrystallized annealed steel sheet. Subsequently, a magnesia-based annealing separator containing 0 to 12 parts by mass of TiO2 (in terms of Ti) per 100 parts by mass of magnesia was applied to the primarily recrystallized annealed steel sheet, and secondary recrystallization annealing was performed at 1200°C for 5 hours to obtain a final annealed steel sheet. The secondary recrystallization annealing was performed in a nitrogen atmosphere below 1000°C, and in an Ar atmosphere above 1000°C. During the temperature increase process of the secondary recrystallization annealing, the residence time in the temperature range of 800 to 900°C was set to 60 hours. The finish-annealed sheet was then washed with water to remove unreacted annealing separator, and a coating consisting primarily of magnesium phosphate, colloidal silica, and chromic acid was applied.

[0018] The coating adhesion of the obtained samples was evaluated in the same manner as in Experiment 3. The evaluation results of coating adhesion were organized by the amount of TiO2 added (equivalent to Ti) per 100 parts by mass of magnesia, and are shown in Figure 4. The results in Figure 4 show that under pickling condition A, good coating adhesion can be obtained when the amount of TiO2 added (equivalent to Ti) is in the range of 0.3 to 8.0 parts by mass per 100 parts by mass of magnesia. On the other hand, under pickling condition B, good coating adhesion cannot be obtained even if the amount of TiO2 added is changed.

[0019] Experiments 1 and 2 identified conditions for obtaining good magnetic properties. While the reasons for this are not entirely clear, the inventors believe the following: In Experiment 1, good magnetic properties were obtained by controlling the γ-phase fraction during annealing before cold rolling to obtain the final cold-rolled steel sheet. Even in a composition system that does not use inhibitors, small amounts of Al and N, or Mn and S and Se, form precipitates and are presumed to affect grain growth. The γ-phase formed by heating a steel strip at high temperatures has a higher amount of dissolved elements than the α-phase. That is, the amount of dissolved inhibitor-forming elements is high in the γ-phase and low in the α-phase, resulting in a heterogeneous state. Because the amount of inhibitor-forming elements is originally low, a high γ-phase fraction promotes this heterogeneous state. Therefore, it is possible that inhibitors are formed only in the γ-phase region, resulting in non-uniform grain growth and degraded magnetic properties. Therefore, it is presumed that the γ-phase fraction during the annealing process should be kept low, at 25% by volume or less. Furthermore, if the γ phase fraction is extremely low, the grain size becomes coarse during annealing before cold rolling to obtain the final cold-rolled steel sheet, which is thought to significantly change the texture before secondary recrystallization and affect the magnetic properties.

[0020] The influence of the average cooling rate between 800 and 400°C in the cooling process after annealing before cold rolling to obtain the final cold-rolled steel sheet is believed to be as described in Patent Document 7. Here, in this experiment, the influence of the γ phase fraction was also newly confirmed, and it was newly discovered that, unless it is within the above-mentioned appropriate range, good magnetic properties cannot be obtained even if the average cooling rate between 800 and 400°C is within the appropriate range described in Patent Document 7. In other words, it was newly discovered that good magnetic properties cannot be obtained unless these two conditions are specified.

[0021] In Experiments 3 and 4, conditions for obtaining good coating adhesion were identified. Experiment 3 showed that the amount of Fe ions in the pickling performed before the primary recrystallization annealing accompanied by decarburization significantly affected coating adhesion. While the reason for this is not entirely clear, the inventors speculate as follows: It is believed that Fe ions promote the cleaning of the steel sheet surface with acid through their catalytic effect. That is, when there are few Fe ions, the reactivity in the acid solution is poor, and the cleaning of the steel sheet surface is insufficient. Furthermore, when there are too many Fe ions, the pickling effect is reduced, and this also results in insufficient cleaning of the steel sheet surface. It is believed that this surface cleaning affects the coating properties. The effects obtained in Experiment 4 are believed to be as described in Patent Document 7. It was newly discovered in this experiment that, as a prerequisite, pickling must be performed with an acid solution containing a certain range of Fe ions.

[0022] The present invention has been made based on this finding, and the method for producing a grain-oriented electrical steel sheet according to the present invention, which advantageously solves the above-mentioned problems, is configured as follows.

[0023] [1] In mass%, the steel contains C: 0.01 to 0.10%, Si: 2.5 to 4.0%, Mn: 0.04 to 0.50%, acid-soluble Al: 0.0030 to 0.0100%, N: 0.0020 to 0.0060%, and Sb: 0.03 to 0.30%, and the S and Se contents satisfy S + 0.405Se ≦ 0.0050%, Optionally, Group A: at least one selected from Sn: 0.005 to 0.500%, Cr: 0.005 to 0.500%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, Bi: 0.005 to 0.500%, P: 0.005 to 0.200%, Mo: 0.005 to 0.500%, and Co: 0.001 to 0.500%; Group B: at least one selected from B: 0.0001 to 0.0025, Nb: 0.001 to 0.020%, Ti: 0.0005 to 0.0400%, V: 0.001 to 0.020%, and W: 0.001 to 0.020%; Group C: at least one selected from Zn: 0.0005 to 0.020%, Zr: 0.001 to 0.020%, Pb: 0.0001 to 0.0100%, As: 0.001 to 0.020%, Ag: 0.001 to 0.050%, Au: 0.001 to 0.050%, Ga: 0.0001 to 0.0050%, Ge: 0.0001 to 0.0050%, Ca: 0.0005 to 0.020%, Mg: 0.0005 to 0.020%, REM: 0.0005 to 0.0200%, and Hf: 0.001 to 0.020%; Contains at least one component of The method includes a hot rolling step of heating a steel material having a composition with the balance consisting of Fe and unavoidable impurities to a temperature of 1250°C or less and hot rolling it to obtain a hot-rolled steel sheet; a hot-rolled sheet annealing step of optionally subjecting the hot-rolled steel sheet to hot-rolled sheet annealing to obtain a hot-rolled annealed sheet; a cold rolling step of cold-rolling the hot-rolled steel sheet or the hot-rolled annealed sheet once or at least twice with intermediate annealing in between to obtain a final cold-rolled steel sheet; a pickling step of pickling the final cold-rolled steel sheet to obtain a pickled cold-rolled steel sheet; a primary recrystallization annealing step of subjecting the pickled cold-rolled steel sheet to primary recrystallization annealing with decarburization to obtain a primary recrystallization annealed sheet; and a finish annealing step of applying an annealing separator mainly composed of magnesia to the surface of the primary recrystallization annealed sheet and subjecting it to secondary recrystallization annealing, and the final cold-rolled steel sheet is subjected to hot-rolled sheet annealing in the hot-rolled sheet annealing step or intermediate annealing in the cold-rolling step, and during annealing before being made into a final cold-rolled steel sheet, the γ phase fraction of the steel structure at the annealing temperature is set to 1.0 to 25.0 volume % and the average cooling rate between 800 and 400°C in the cooling process after annealing is set to 20 to 50°C / s, the final cold-rolled steel sheet is pickled in the pickling step with a solution containing 2 to 15 mass % of Fe ions and having an acid concentration of 1 to 20 mass %, and the finish annealing step uses an annealing separator containing at least one Ti compound selected from TiO2, TiO3·H2O, TiO·(OH)2 and Ti(OH)4 in an amount of 0.3 to 8 mass parts, calculated as Ti, per 100 mass parts of magnesia. [2] In the method for producing a grain-oriented electrical steel sheet according to the above [1], in the temperature-raising process of the finish annealing step, the residence time in the temperature range of 800 to 900°C is 40 to 150 hours. [3] In the method for producing a grain-oriented electrical steel sheet according to the above [1] or [2], in the final annealing step, the annealing separator further contains at least one Sr compound selected from SrSO4, Sr(OH)2·8H2O, SrCO3, and Sr(NO)3 in an amount of 0.2 to 8 parts by mass, calculated as Sr, per 100 parts by mass of magnesia. [Effects of the Invention]

[0024] According to the present invention, by using a material that does not contain inhibitor components, the γ phase fraction of the material during annealing before final cold rolling, the cooling conditions after annealing, the pickling conditions before primary recrystallization annealing, and the auxiliary agent conditions for magnesia in the annealing separator during finish annealing are specified, and the resulting grain-oriented electrical steel sheet can achieve good magnetic properties and coating properties. [Brief explanation of the drawings]

[0025] [Figure 1] 10 is a graph showing the effect of the γ phase fraction of a material on magnetic flux density B8. [Figure 2] 1 is a graph showing the effect of the cooling rate in the annealing of a hot-rolled sheet on magnetic flux density B8. [Figure 3] 1 is a graph showing the effect of Fe ion concentration in a hydrochloric acid solution on coating adhesion. [Figure 4] 1 is a graph showing the effect of the TiO2 content in the annealing separator on the coating adhesion. DETAILED DESCRIPTION OF THE INVENTION

[0026] Hereinafter, a method for manufacturing a grain-oriented electrical steel sheet according to an embodiment of the present invention will be described.

[0027] <Steel material for manufacturing grain-oriented electrical steel sheets> First, the chemical composition of the steel material used in manufacturing the grain-oriented electrical steel sheet according to this embodiment and the reasons for limiting this composition will be described. In the following description, unless otherwise specified, the notation "%" in the chemical composition of the steel material means "% by mass."

[0028] C: 0.01 to 0.10% If the C content is less than 0.01%, the steel structure of the material will be a single α phase, with no γ phase appearing, resulting in poor magnetic properties. In addition, the steel will become embrittled during casting or hot rolling, resulting in defects that hinder manufacturing. On the other hand, if the C content exceeds 0.10%, it will be difficult to reduce it to 0.005% or less, at which point magnetic aging does not occur, by annealing accompanied by decarburization. Therefore, the C content should be in the range of 0.01 to 0.10%. Preferably, the C content is in the range of 0.025 to 0.06%.

[0029] Si: 2.5 to 4.0% Si is an element necessary for increasing the resistivity of steel and reducing iron loss. The above effects are not sufficient if the Si content is less than 2.5%. On the other hand, if the Si content exceeds 4.0%, workability decreases, making it difficult to manufacture by rolling. Therefore, the Si content is set to the range of 2.5 to 4.0%. Preferably, the Si content is in the range of 2.8 to 3.6%.

[0030] Mn: 0.04 to 0.50% Mn is an element necessary for improving the hot workability of steel. The above effect is not sufficient if the Mn content is less than 0.04%. On the other hand, if the Mn content exceeds 0.50%, the magnetic flux density of the finished sheet decreases. Therefore, the Mn content is set to the range of 0.03 to 0.50%. Preferably, the Mn content is in the range of 0.05 to 0.30%.

[0031] Acid soluble Al: 0.0030~0.0100% When producing grain-oriented electrical steel sheets by inducing secondary recrystallization in a composition system without using an inhibitor, Al is an impurity element. Therefore, the acid-soluble Al content must be 0.0100% or less. However, if the acid-soluble Al content is 0.0030% or more, the oxide film formed on the steel sheet surface during primary recrystallization annealing, which involves decarburization, becomes dense. This suppresses nitrogen absorption during secondary recrystallization annealing, improves the concentration of secondary recrystallized grains in the Goss orientation, and improves magnetic properties. Therefore, in this embodiment, the acid-soluble Al content is set to a range of 0.0030 to 0.0100%. Preferably, the acid-soluble Al content is set to a range of 0.0040 to 0.0080%.

[0032] N: 0.0020~0.0060% When producing grain-oriented electrical steel sheet by inducing secondary recrystallization in a component system without using an inhibitor, N (nitrogen) is an impurity element. Therefore, the N content must be 0.0060% or less. However, to suppress nitrogen absorption during secondary recrystallization annealing, the N content must be 0.0020% or more. In this embodiment, the N content is in the range of 0.0020 to 0.0060%. Preferably, the N content is in the range of 0.0030 to 0.0050%.

[0033] Sb: 0.03 to 0.30% Sb very effectively suppresses the increase in nitrogen content in steel sheet during secondary recrystallization annealing. Sb is an essential element for obtaining excellent magnetic properties and stabilizing the magnetic properties. To fully exert this effect, an Sb content of 0.03% or more is required. However, if the Sb content exceeds 0.30%, the decarburization property during primary recrystallization annealing becomes very poor, making the steel unsuitable for industrial mass production. Therefore, the Sb content is limited to the range of 0.03 to 0.30%. Preferably, the Sb content is in the range of 0.04 to 0.15%.

[0034] The S and Se contents must satisfy the condition S + 0.405Se ≤ 0.0050% When producing grain-oriented electrical steel sheets by inducing secondary recrystallization in a component system without using an inhibitor, S and Se are impurity elements. Here, the S equivalent is expressed as S + 0.405Se. The element symbols in the formula represent the content. In this embodiment, the S equivalent must be 0.0050% or less. If the S equivalent exceeds 0.0050%, secondary recrystallization becomes difficult, leading to deterioration of magnetic properties. Preferably, the S equivalent is 0.0040 mass% or less.

[0035] The above is the basic composition of the steel material for manufacturing the grain-oriented electrical steel sheet according to this embodiment. Optionally, it may further contain at least one element from groups A to C below.

[0036] Group A: at least one selected from Sn: 0.005 to 0.500%, Cr: 0.005 to 0.500%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, Bi: 0.005 to 0.500%, P: 0.005 to 0.200%, Mo: 0.005 to 0.500%, and Co: 0.001 to 0.500% The inclusion of at least one of Sn, Cr, Cu, Ni, Bi, P, Mo, and Co improves the recrystallization texture, thereby improving the final magnetic properties. Addition of less than the lower limit of each element produces little effect. Addition exceeding the upper limit, on the other hand, saturates the effect and results in the disadvantage of increased costs. Therefore, when adding at least one of Sn, Cr, Cu, Ni, Bi, P, Mo, and Co, the content must be within the above range.

[0037] Group B: at least one selected from B: 0.0001 to 0.0025, Nb: 0.001 to 0.020%, Ti: 0.0005 to 0.0400%, V: 0.001 to 0.020%, and W: 0.001 to 0.020%. By including at least one of B, Nb, Ti, V, and W, the carbides and nitrides of these elements are formed finely, which allows for finer grain sizes after annealing. As a result, bending properties are improved and threading problems can be suppressed. Addition of less than the lower limit of each element has little effect. On the other hand, addition of more than the upper limit saturates the effect, resulting in disadvantages that increase costs. Therefore, when at least one of B, Nb, Ti, V, and W is added, the content should be within the above range.

[0038] Group C: at least one selected from Zn: 0.0005 to 0.020%, Zr: 0.001 to 0.020%, Pb: 0.0001 to 0.0100%, As: 0.001 to 0.020%, Ag: 0.001 to 0.050%, Au: 0.001 to 0.050%, Ga: 0.0001 to 0.0050%, Ge: 0.0001 to 0.0050%, Ca: 0.0005 to 0.020%, Mg: 0.0005 to 0.020%, REM: 0.0005 to 0.0200%, and Hf: 0.001 to 0.020%. By including at least one of Zn, Zr, Pb, As, Ag, Au, Ga, Gr, Ca, Mg, REM, and Hf, these elements concentrate at grain boundaries or form compounds at the grain boundaries, strengthening the grain boundaries. As a result, defects caused by grain boundary fracture can be suppressed. Addition below the lower limit of each element's content has little effect. On the other hand, addition above the upper limit saturates the effect, resulting in disadvantages that increase costs. Therefore, when at least one of Zn, Zr, Pb, As, Ag, Au, Ga, Gr, Ca, Mg, REM, and Hf is added, the content should be within the above range.

[0039] The chemical composition of the steel material used to manufacture the grain-oriented electrical steel sheet according to this embodiment is such that the remainder other than the above-mentioned component composition is substantially Fe and unavoidable impurities.

[0040] <Method of manufacturing grain-oriented electrical steel sheets> Next, a method for manufacturing the grain-oriented electrical steel sheet according to this embodiment will be described. The method for producing a grain-oriented electrical steel sheet according to this embodiment includes a hot rolling step, and optionally a hot-rolled sheet annealing step, a cold rolling step, a pickling step, a primary recrystallization annealing step, and a finish annealing step. The steel material for the grain-oriented electrical steel sheet according to this embodiment can be produced by a method for producing a general electrical steel sheet. That is, molten steel with a predetermined composition adjustment may be used as a steel material by a normal ingot-making method or a continuous casting method to produce, for example, a steel slab. Alternatively, a thin cast piece with a thickness of 100 mm or less may be produced by a direct casting method. Of the above-mentioned additive elements, those that are difficult to add in intermediate steps after casting are preferably added at the molten steel stage.

[0041] Hot rolling process A steel material, such as a steel slab, is heated by a conventional method and hot-rolled to form a hot-rolled steel sheet. The composition of this embodiment does not require high-temperature heating to dissolve the inhibitor. Therefore, it is essential to keep the slab heating temperature low, at 1250°C or lower, in order to reduce energy costs.

[0042] Hot-rolled sheet annealing process Next, the hot-rolled steel sheet is annealed as needed to obtain a hot-rolled annealed sheet. When the hot-rolled steel sheet annealing is performed before the final cold rolling, for the reasons mentioned above, the calculated γ phase fraction at the annealing temperature of the steel sheet as a raw material is set to a range of 1.0 to 25.0 volume % in order to suppress property variations within the steel sheet coil. The average cooling rate between 800 and 400°C during the cooling process after annealing is set to a range of 20 to 50°C / s. The annealing temperature is preferably set to a range of 900 to 1200°C. If the annealing temperature is less than 900°C, the recrystallized grains become finer, the Goss nuclei in the primary recrystallized structure decrease, and the magnetic properties deteriorate. On the other hand, if the annealing temperature exceeds 1200°C, the crystal grains become coarse, which makes cracks more likely to occur during the subsequent rolling process, which is disadvantageous from the viewpoint of manufacturability.

[0043] Cold rolling process Hot-rolled steel sheets or hot-rolled annealed sheets are cold-rolled once or twice or more times with intermediate annealing in between to produce final cold-rolled steel sheets. Cold rolling with intermediate annealing in between is performed as necessary. The intermediate annealing temperature is also preferably in the range of 900 to 1200°C for the same reasons as for annealing hot-rolled sheets. In the final cold-rolling to produce the final cold-rolled steel sheets, it is preferable to increase the cold-rolling temperature to 100 to 300°C and to perform aging treatment in the range of 100 to 300°C once or multiple times during the cold rolling, as this changes the primary recrystallization texture and improves the magnetic properties.

[0044] Pickling process The final cold-rolled steel sheet is pickled in a solution containing 2 to 15% by mass of Fe ions and having an acid concentration of 1 to 20% by mass to obtain a pickled cold-rolled steel sheet. It is essential to pickle the steel sheet with an acid solution containing Fe ions before primary recrystallization annealing. It is believed that Fe ions promote cleaning of the steel sheet surface with acid through their catalytic effect. The Fe ion concentration can be calculated, for example, by measuring the electrical conductivity, liquid temperature, and ultrasonic propagation velocity of the acid solution using an ultrasonic concentration meter. This method was also adopted in this embodiment. It can also be measured by an extraction method using a chelating agent. There are no limitations on the type of acid, and hydrochloric acid, phosphoric acid, nitric acid, sulfuric acid, etc. can be used depending on the purpose.

[0045] Primary recrystallization annealing process The pickled cold-rolled steel sheet is subjected to primary recrystallization annealing accompanied by decarburization to obtain a primary recrystallization annealed sheet. In the primary recrystallization annealing accompanied by decarburization, the annealing temperature is preferably in the range of 800 to 900°C from the viewpoint of decarburization. Furthermore, the atmosphere is preferably a humid atmosphere. From the viewpoint of texture, the heating rate to the holding temperature is preferably 100°C / s or more.

[0046] Finishing annealing process In the final annealing, an annealing separator containing magnesia as its main component is applied to the surface of the primary recrystallization annealed steel sheet. "Magnetic oxide" means that the magnesia (MgO) content in the annealing separator is 60% by mass or more in terms of solid content. The magnesia content in the annealing separator is preferably 80% by mass or more in terms of solid content. Magnesia is applied to the steel sheet as a slurry solution in which magnesia is suspended in water. To prevent viscosity increase, the slurry solution is preferably maintained at a constant temperature within a range of 5 to 30°C. It is essential that the annealing separator contain at least one Ti compound selected from TiO2, TiO3·H2O, TiO·(OH)2, and Ti(OH)4 in an amount of 0.3 to 8 parts by mass, calculated as Ti, per 100 parts by mass of magnesia. This is to obtain good coating adhesion.

[0047] It is also preferable to combine 0.2 to 8 parts by mass of a Sr compound (calculated as Sr) with the Ti compound per 100 parts by mass of magnesia. This is effective in further improving and stabilizing the magnetic properties and coating properties. The Sr compound can be at least one selected from SrSO4, Sr(OH)2·8H2O, SrCO3, and Sr(NO)3.

[0048] In order to further improve the uniformity of the coating, it is more preferable to add to the annealing separator at least one selected from the group consisting of oxides such as CaO, sulfides such as MgSO4·7H2O and SnSO4, B-based compounds such as Na2B4O7, and Sb-based compounds such as Sb2O3 and Sb2(SO4)3, either alone or in combination.

[0049] Furthermore, the magnesia (MgO) used in the annealing separator preferably has a hydration amount, i.e., a weight loss upon ignition at 1000°C for 1 hour, of 1 to 5 mass%. If the hydration amount of magnesia is less than 1 mass%, the formation of a forsterite coating film will be insufficient. On the other hand, if the hydration amount of magnesia exceeds 5 mass%, too much moisture will be carried between the coil layers, resulting in a large amount of additional oxidation of the steel sheet. As a result, there is a risk that a good forsterite coating film will not be obtained.

[0050] Subsequently, secondary recrystallization annealing is performed to achieve secondary recrystallization and purify impurities in the steel. Here, the residence time in the temperature range of 800 to 900°C during the temperature rise process is preferably 40 to 150 hours. In a composition system that does not use an inhibitor, secondary recrystallization occurs in this temperature range, so a longer residence time tends to improve magnetic properties. By performing final annealing, it is possible to develop the secondary recrystallization structure and form a forsterite film. Final annealing is preferably performed at 800°C or higher to induce secondary recrystallization.

[0051] From the viewpoint of purification, it is preferable to hold the temperature at 1100°C or higher for at least one hour. More preferably, it is preferable to hold the temperature at 1150°C or higher for at least three hours. Since the introduction of an H2 atmosphere promotes purification, it is preferable to use an atmosphere containing 50% or more of H2 in the high temperature range. However, this may cause deterioration in the bendability of the product, in which case it is preferable to use an Ar atmosphere in the high temperature range.

[0052] After the secondary recrystallization annealing, it is useful to perform water washing, brushing, or pickling to remove any adhering annealing separator.

[0053] After that, further flattening annealing and shape correction are effective in reducing iron loss.

[0054] When steel sheets are used in a stack, it is effective to apply an insulating coating to the surface of the steel sheet before or after planarization annealing in order to improve iron loss. A coating that can impart tension to the steel sheet is preferred to reduce iron loss. The use of a tension coating method using a binder, or a coating method in which an inorganic substance is vapor-deposited on the surface of the steel sheet by physical vapor deposition or chemical vapor deposition is preferred because these methods provide excellent coating adhesion and a significant iron loss reduction effect. [Example]

[0055] The embodiments of the present invention will be further explained by way of examples. Note that the present invention is not limited to the manufacturing conditions and product performance shown in the following examples. The embodiments can achieve the desired performance within the scope of the present invention.

[0056] Example 1 A steel slab containing, by mass, 0.042% C, 3.22% Si, 0.0040% acid-soluble Al, 0.0042% N, 0.06% Sb, 0.15% Mn, and 0.0030% Se, with the remainder consisting of Fe and unavoidable impurities, was produced by continuous casting. The steel slab was subjected to slab heating by soaking at 1200°C for 40 minutes, and then hot-rolled to obtain a 2.6 mm thick hot-rolled steel sheet. The hot-rolled steel sheet was then hot-annealed at 1100°C for 45 seconds to obtain a hot-rolled annealed sheet. The hot-rolled annealed sheet was pickled to remove surface scale, and then cold-rolled to obtain a 1.8 mm thick cold-rolled steel sheet. Subsequently, intermediate annealing was performed at 1000°C for 50 seconds. The samples before the intermediate annealing were analyzed for their composition. Using the obtained values, the γ phase fraction at the intermediate annealing temperature of 1000°C was calculated using Thermo-Calc ver. 2020b, which was found to be 11.6% by volume. Furthermore, the cooling rate between 800 and 400°C after the intermediate annealing was changed as shown in Table 1. Subsequently, cold rolling was performed to obtain final cold-rolled steel sheets with a thickness of 0.27 mm. The steel sheets were then immersed in an acid containing the amount of Fe ions listed in Table 1 to obtain pickled cold-rolled steel sheets. The pickling conditions were uniform: an acid concentration of 5% by mass, a solution temperature of 60°C, and immersion for 5 seconds. The pickled cold-rolled steel sheets were subjected to primary recrystallization annealing with decarburization at 840°C for 180 seconds in a humid atmosphere of 50% by volume H2 and 50% by volume N2 with a dew point of 55°C, to obtain primary recrystallization-annealed steel sheets. Furthermore, a magnesia-based annealing separator was applied to the primary recrystallization-annealed steel sheets. The Ti compounds listed in Table 1 were added to the annealing separator. Subsequently, secondary recrystallization annealing was performed by holding at 1200°C for 5 hours. A nitrogen atmosphere was used below 1000°C, and an Ar atmosphere was used above 1000°C. During this temperature increase process, the residence time in the temperature range of 800 to 900°C was 75 hours. The unreacted annealing separator was then removed by rinsing with water, and a coating consisting primarily of magnesium phosphate, colloidal silica, and chromic acid was applied.

[0057] The magnetic properties B8 (magnetic flux density when excited at 800 A / m) of the obtained samples were measured using the method described in JIS C2550, and the results are also shown in Table 1. In this example, the conditions were changed for each coil, and the final coil was divided into four parts and the magnetic properties were evaluated at each end, with the worst value being taken as the representative value for the original coil. In addition, the coating adhesion was evaluated by wrapping the steel sheet around cylinders of various diameters and using the smallest diameter at which the coating did not peel off. The results are also shown in Table 1. Table 1 shows that good magnetic properties and coating properties can be obtained by using conditions that are compatible with the present invention.

[0058] [Table 1]

[0059] Example 2 A steel slab containing, by mass, 0.029% C, 2.98% Si, 0.0060% acid-soluble Al, 0.0035% N, 0.12% Sb, 0.08% Mn, and 0.0030% Se, with the remainder consisting of Fe and unavoidable impurities, was produced by continuous casting. The steel slab was subjected to slab heating by soaking at 1200°C for 40 minutes, and then hot-rolled to obtain a 2.6 mm thick hot-rolled steel sheet. The γ phase fraction at the hot-rolled sheet annealing temperature of 1050°C was calculated using Thermo-Calc ver. 2020b based on the steel slab's chemical composition, and was found to be 8.8% by volume. The hot-rolled steel sheet was then annealed at 1050°C for 45 seconds to obtain a hot-rolled annealed sheet. The cooling rate from 800 to 400°C after hot-rolled sheet annealing was 25°C / s. The hot-rolled and annealed steel sheets were pickled to remove surface scale and then cold-rolled to a thickness of 0.23 mm. The steel sheets were then immersed in nitric acid containing 5% Fe ions by mass to obtain pickled cold-rolled steel sheets. The pickling conditions were 5% nitric acid at a solution temperature of 60°C for 5 seconds. The pickled cold-rolled steel sheets were subjected to primary recrystallization annealing with decarburization at 840°C for 150 seconds in a humid atmosphere of 50% H2 by volume and 50% N2 by volume with a dew point of 52°C to obtain primarily recrystallized annealed steel sheets. The primarily recrystallized annealed steel sheets were then coated with a magnesia-based annealing separator, to which 3 parts TiO2 (calculated as Ti) was added per 100 parts magnesia by mass. Sr compounds listed in Table 2 were also added. The steel sheets were then subjected to secondary recrystallization annealing at 1220°C for 10 hours in a hydrogen atmosphere. During this temperature increase process, the residence time in the temperature range of 800 to 900°C was changed variously as shown in Table 2. After that, the workpiece was washed with water to remove unreacted annealing separator, and then a coating containing magnesium phosphate, colloidal silica, and chromic acid as its main components was applied.

[0060] The magnetic flux density B8 (magnetic flux density when excited at 800 A / m) of the obtained sample was measured using the method described in JIS C2550, and the results are also shown in Table 2. In this example, the conditions were changed for each coil, and the final coil was divided into four parts and the magnetic properties were evaluated at each end, with the worst value being taken as the representative value for the original coil. In addition, the coating adhesion was evaluated by wrapping the steel sheet around cylinders of various diameters and using the smallest diameter at which the coating did not peel off. The results are also shown in Table 2. Table 2 shows that good magnetic properties and coating properties can be obtained by using conditions that are compatible with the present invention.

[0061] [Table 2]

[0062] Example 3 A steel slab having the chemical composition shown in Table 3, with the remainder consisting of Fe and unavoidable impurities, was produced by continuous casting. The steel slab was subjected to slab heating by soaking at 1230°C for 40 minutes, and then hot-rolled to obtain a 2.2 mm-thick hot-rolled steel sheet. The γ phase fraction at the hot-rolled sheet annealing temperature of 1050°C was calculated using Thermo-Calc ver. 2020b based on the chemical composition of the steel slab, and the results are also shown in Table 3. The hot-rolled steel sheet was then annealed at 1050°C for 50 seconds to obtain a hot-rolled annealed steel sheet. The cooling rate between 800°C and 400°C after hot-rolled annealing was 50°C / s. The hot-rolled annealed steel sheet was pickled to remove surface scale, and then cold-rolled to obtain a final cold-rolled steel sheet with a thickness of 0.23 mm. The final cold-rolled steel sheet was then immersed in hydrochloric acid containing 11% by mass of Fe ions to obtain a pickled cold-rolled steel sheet. The pickling conditions were immersion in 11% hydrochloric acid at 60°C for 3 seconds. The pickled cold-rolled steel sheets were subjected to primary recrystallization annealing with decarburization at 840°C for 150 seconds in a humid atmosphere of 52% H2 by volume and 48% N2 by volume, with a dew point of 54°C, to obtain primary recrystallization-annealed steel sheets. The primary recrystallization-annealed steel sheets were then coated with a magnesia-based annealing separator, to which TiO2 was added at 0.5 parts by mass per 100 parts by mass of magnesia (equivalent to Ti). Secondary recrystallization annealing was then performed at 1220°C for 10 hours in a hydrogen atmosphere. During this heating process, the dwell time in the temperature range of 800–900°C was 100 hours. The steel sheets were then rinsed with water to remove unreacted annealing separator, and coated with a coating primarily composed of magnesium phosphate, colloidal silica, and chromic acid.

[0063] The magnetic flux density B8 (magnetic flux density when excited at 800 A / m) of the obtained samples was measured using the method described in JIS C2550, and the results are also shown in Table 3. As in the experiment, the conditions were changed for each coil, and the final coil was divided into four parts and the properties were evaluated at each end, with the worst value being taken as the representative value for the original coil. In addition, the coating adhesion was evaluated by wrapping the steel sheet around cylinders of various diameters and using the smallest diameter where the coating did not peel off. The results are also shown in Table 3. From Table 3, it can be seen that good magnetic properties and film properties can be obtained by setting conditions that are compatible with the present invention.

[0064] Table 3

Claims

1. In mass%, C: 0.01-0.10%, Si: 2.5-4.0%, Mn: 0.04-0.50%, Acid-soluble Al: 0.0030 to 0.0100%, N: 0.0020 to 0.0060%, and Sb: 0.03 to 0.30%; The contents of S and Se satisfy S + 0.405Se ≦ 0.0050%, Optionally, Group A; at least one selected from Sn: 0.005 to 0.500%, Cr: 0.005 to 0.500%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, Bi: 0.005 to 0.500%, P: 0.005 to 0.200%, Mo: 0.005 to 0.500%, and Co: 0.001 to 0.500%, Group B; At least one selected from B: 0.0001 to 0.0025%, Nb: 0.001 to 0.020%, Ti: 0.0005 to 0.0400%, V: 0.001 to 0.020%, and W: 0.001 to 0.020%, Group C; At least one selected from Zn: 0.0005 to 0.020%, Zr: 0.001 to 0.020%, Pb: 0.0001 to 0.0100%, As: 0.001 to 0.020%, Ag: 0.001 to 0.050%, Au: 0.001 to 0.050%, Ga: 0.0001 to 0.0050%, Ge: 0.0001 to 0.0050%, Ca: 0.0005 to 0.020%, Mg: 0.0005 to 0.020%, REM: 0.0005 to 0.0200%, and Hf: 0.001 to 0.020%; and containing at least one component selected from the group consisting of a hot rolling step of heating a steel material having a component composition with the balance being Fe and unavoidable impurities to a temperature of 1250°C or less and hot rolling the material to obtain a hot-rolled steel sheet; Optionally, a hot-rolled sheet annealing step of subjecting the hot-rolled steel sheet to hot-rolled sheet annealing to obtain a hot-rolled annealed sheet; a cold rolling step in which the hot-rolled steel sheet or the hot-rolled annealed sheet is cold-rolled once or twice or more times with intermediate annealing therebetween to form a final cold-rolled steel sheet; A pickling step of pickling the final cold-rolled steel sheet to obtain a pickled cold-rolled steel sheet; a primary recrystallization annealing step of subjecting the pickled cold-rolled steel sheet to primary recrystallization annealing accompanied by decarburization to obtain a primary recrystallization annealed sheet; a finish annealing process in which an annealing separator containing magnesia as a main component is applied to the surface of the primary recrystallization annealed sheet, and secondary recrystallization annealing is performed; Including, Before cold rolling to obtain the final cold-rolled steel sheet, hot-rolled sheet annealing in the hot-rolled sheet annealing step or intermediate annealing in the cold rolling step is performed, During annealing before the final cold-rolled steel sheet is obtained, the γ phase fraction of the steel structure at the annealing temperature is set to 1.0 to 25.0 volume%, and the average cooling rate between 800 and 400 ° C. during the cooling process after annealing is set to a range of 20 to 50 ° C. / s, In the pickling step, the final cold-rolled steel sheet is pickled with a solution containing 2 to 15 mass% of Fe ions and having an acid concentration of 1 to 20 mass%, In the final annealing step, TiO ２ , TiO ３ ・H ２ O, TiO・(OH) ２ and Ti(OH) ４ and a method for producing a grain-oriented electrical steel sheet, the method comprising the step of: using an annealing separator containing, in terms of Ti, at least one Ti compound selected from the group consisting of:

2. 2. The method for producing a grain-oriented electrical steel sheet according to claim 1, wherein the temperature rise process in the finish annealing step involves a residence time in the temperature range of 800 to 900°C of 40 to 150 hours.

3. In the final annealing step, the annealing separator further contains SrSO ４ , Sr(OH) ２ ・8H ２ O, SrCO ３ and Sr(NO) ３ 3. The method for producing a grain-oriented electrical steel sheet according to claim 1, wherein the magnesia contains at least one Sr compound selected from the group consisting of Sr, Sr-equivalent, and Sr-based compounds in an amount of 0.2 to 8 parts by mass per 100 parts by mass of magnesia.

Images