Additive for annealing separator

Abstract

In order to remove Nb included as an in-steel impurity in steel of a grain-oriented electrical steel sheet to be produced, provided is an additive for an annealing separator. The additive is to be added to an annealing separator for use in the production of a grain-oriented electrical steel sheet, and has a titanium oxide content of 95.00 mass% or more and a Nb content of 0.001-0.150 mass%. The additive for an annealing separator preferably has an anatase ratio of 20.0% or more. The additive for an annealing separator preferably has a specific surface area of 3.0-100.0 m2 / g.<sp / >

Description

Additives for annealing separation agents

[0001] This invention relates to an additive for annealing separating agents.

[0002] Grain-oriented electrical steel sheets are generally manufactured by hot-rolling and cold-rolling a steel slab having a predetermined composition, then subjecting the resulting steel sheet (cold-rolled sheet) to primary recrystallization annealing, and subsequently performing finish annealing including secondary recrystallization annealing (Patent Document 1).

[0003] Finish annealing is often performed at high temperatures on rolled steel sheets (i.e., coils), which can cause overlapping steel sheets to stick together. Therefore, conventionally, to prevent the steel sheets from sticking together, an annealing release agent mainly composed of magnesium oxide (MgO) is applied to the surface of the steel sheets in a slurry form dispersed in water before finish annealing.

[0004] On the surface of the steel plate, silicon dioxide (SiO₂) is formed by primary recrystallization annealing. ２ An oxide layer is formed that mainly contains ). In addition to the above-mentioned role of preventing adhesion, the annealing separating agent also has the role of preventing this oxide layer (SiO ２ By reacting with ) a forsterite coating called forsterite (Mg ２ SiO ４ It also plays a role in forming a protective coating. The forsterite coating acts as a binder, firmly adhering the insulating coating formed in a later process to the steel sheet. The insulating coating improves the magnetic properties of the grain-oriented electrical steel sheet by applying tension to the steel sheet.

[0005] International Publication No. 2016 / 158325

[0006] In recent years, there has been a growing demand for reducing carbon dioxide emissions, and the steel industry is discussing a shift from the blast furnace method to the electric arc furnace method. However, the electric arc furnace method results in a higher amount of impurities in the steel produced during the steelmaking process compared to the blast furnace method. Further investigation by the inventors of this invention into the electric arc furnace method revealed that niobium (Nb) in particular is mixed into the steel as an impurity (impurity element) during the steelmaking process, and that this Nb remains in the steel even after finish annealing, leading to a deterioration of the magnetic properties of grain-oriented electrical steel sheets.

[0007] Furthermore, based on the above findings, the inventors also re-examined grain-oriented electrical steel sheets manufactured by the blast furnace method and found that, even in the blast furnace method, Nb can be mixed into the steel during the steelmaking process, albeit in relatively small amounts.

[0008] This invention has been made in view of the above points, and aims to remove Nb, an impurity in the steel of the manufactured grain-oriented electrical steel sheet.

[0009] The inventors diligently studied to achieve the above objective. As a result, they found that a certain amount of titanium oxide (TiO2) can be added to the annealing separating agent used in the manufacture of grain-oriented electrical steel sheets. ２ We discovered that by adding an additive containing (an annealing separation agent additive), Nb can be removed from steel during finish annealing, and that the effect of removing Nb from steel differs depending on the Nb content of the additive, thus completing the present invention.

[0010] In other words, the present invention provides the following [1] to [3]: [1] An additive for annealing separation agents used in the manufacture of grain-oriented electrical steel sheets, wherein the titanium oxide content is 95.00% by mass or more and the Nb content is 0.001% by mass or more and 0.150% by mass or less. [2] The additive for annealing separation agents according to [1] above, wherein the anatase rate is 20.0% or more. [3] A specific surface area of ​​3.0 m² ２ / g or more 100.0m ２ An additive for annealing separating agents as described in [1] or [2] above, wherein the amount is less than or equal to / g.

[0011] According to the present invention, Nb can be removed from the steel of the manufactured grain-oriented electrical steel sheet.

[0012] [Additive for Annealing Separating Agent] The additive for annealing separating agent of this embodiment is an additive added to an annealing separating agent used in the manufacture of grain-oriented electrical steel sheets, and has a titanium oxide content of 95.00% by mass or more and an Nb content of 0.001% by mass or more and 0.150% by mass or less. Hereinafter, "additive for annealing separating agent" will also be simply referred to as "additive".

[0013] When manufacturing grain-oriented electrical steel sheets, finish annealing is performed using an annealing separator containing the additive of this embodiment. This removes Nb from the steel in the manufactured grain-oriented electrical steel sheets. The reason for this is presumed to be as follows.

[0014] First, a forsterite coating is formed by performing finish annealing using an annealing separation agent. At this time, by adding the additive of this embodiment to the annealing separation agent, (Ti,Nb)N, a composite nitride of Ti and Nb, is generated in the forsterite coating. The (Ti,Nb)N in the forsterite coating reduces the Nb activity in the forsterite coating during finish annealing. As a result, the Nb activity difference between the forsterite coating and the steel sheet becomes large, and therefore, Nb present inside the steel sheet (Nb in the steel) can easily diffuse into the forsterite coating, and as a result, Nb in the steel is removed.

[0015] The additives of this embodiment will be described in more detail below.

[0016] <Titanium dioxide content> The additive is of the composition formula TiO ２ It contains titanium(IV) oxide represented by . More specifically, the titanium oxide content of the additive is 95.00% by mass or more. If the titanium oxide content is too low, the effect of removing Nb from steel will not be fully exhibited. This is presumed to be because (Ti,Nb)N will not grow easily in the forsterite coating, and the Nb activity will increase, thereby suppressing the diffusion and transfer of Nb from steel into the forsterite coating. For the reason that the effect of removing Nb from steel is superior, the titanium oxide content of the additive is preferably 95.50% by mass or more, more preferably 96.00% by mass or more, even more preferably 96.50% by mass or more, particularly preferably 97.00% by mass or more, and most preferably 97.50% by mass or more.

[0017] On the other hand, even if the titanium oxide content is too high, the effect of removing Nb from steel may not be sufficiently exhibited in some cases. It is presumed that this is because when the titanium oxide content is too high, the reactivity of titanium oxide decreases, which delays the nucleation and growth of (Ti, Nb)N in the forsterite film, thereby increasing the Nb activity and suppressing the diffusion and migration of Nb from steel into the forsterite film. For this reason, for the reason of obtaining a more excellent effect of removing Nb from steel, the titanium oxide content of the additive is preferably less than 100.00 mass%, more preferably 99.99 mass% or less.

[0018] The titanium oxide content is determined in accordance with the method for determining "purity (TiO ２ )" described in JIS K 8703:2011 "Titanium (IV) Oxide (Reagent)".

[0019] 〈Nb Content〉 The Nb content of the additive is 0.150 mass% or less. If the Nb content is too high, the effect of removing Nb from steel cannot be sufficiently exhibited. It is presumed that this is because in the formed forsterite film, the Nb activity increases, and as a result, the diffusion and migration of Nb from steel into the forsterite film is suppressed.

[0020] For the reason of obtaining a more excellent effect of removing Nb from steel, the Nb content of the additive is preferably 0.110 mass% or less, more preferably 0.070 mass% or less, still more preferably 0.030 mass% or less, and particularly preferably 0.010 mass% or less.

[0021] On the other hand, even if the Nb content is too low, the effect of removing Nb from steel may not be sufficiently exhibited. It is presumed that this is because when the Nb content is too low, the nucleation and growth of (Ti, Nb)N in the forsterite film is delayed, which increases the Nb activity and suppresses the diffusion and migration of Nb from steel into the forsterite film. For this reason, for the reason of obtaining an excellent effect of removing Nb from steel, the Nb content of the additive is 0.001 mass% or more. The Nb content of the additive is preferably 0.002 mass% or more, more preferably 0.003 mass% or more.

[0022] The Nb content is determined in accordance with the method for determining the "niobium content" described in JIS M 8321:1999 "Titanium Ore - Method for Determination of Niobium".

[0023] <Anatase Rate> The proportion (in %) of anatase-type titanium oxide among the titanium oxide contained in the additive is called the "anatase rate." The higher the anatase rate of the additive, the better the adhesion of the forsterite coating to the steel sheet (hereinafter also simply referred to as "adhesion") in the manufactured grain-oriented electrical steel sheet. This is presumed to be because, during finish annealing, the reactivity of titanium oxide increases, promoting the formation of Ti-enriched areas at the interface between the steel sheet and the forsterite coating, which improves the adhesion between the two. For this reason, the anatase rate of the additive is, for example, 10.0% or more, and for the reason of excellent adhesion, 20.0% or more is preferable, 40.0% or more is more preferable, 60.0% or more is even more preferable, and 80.0% or more is particularly preferable.

[0024] On the other hand, if the anatase content of the additive is too high, adhesion may decrease. This is presumed to be because, during the finish annealing, the reaction between titanium oxide and the forsterite coating occurs intensively in a short period of time, resulting in the uneven formation of the Ti-enriched areas mentioned above. For this reason, for the sake of superior adhesion, the anatase content of the additive is preferably less than 100.0%, and more preferably 99.0% or less.

[0025] The anatase fraction is determined by powder X-ray diffraction. More specifically, assuming that the sample consists of anatase phase (anatase-type titanium dioxide), rutile phase (rutile-type titanium dioxide), and amorphous phase (amorphous titanium dioxide), the fraction of each phase is determined using powder X-ray diffraction, and the anatase fraction is calculated by subtracting the fractions of the rutile and amorphous phases from 100%.

[0026] <Specific Surface Area> If the specific surface area of ​​the additive is too small, the aesthetic quality of the resulting grain-oriented electrical steel sheet may decrease. This is presumed to be because slight variations are more likely to occur in the reaction of the additive on the steel sheet, resulting in the formation of patterns in the forsterite coating. For this reason, the specific surface area of ​​the additive should be, for example, 1.0 m².２ The reason that it is 3.0 m² or more and the resulting grain-oriented electrical steel sheet has excellent aesthetic appeal is that it is 3.0 m². ２ Preferably 7.0 m / g or more. ２ A value of 1 / g or higher is more preferable.

[0027] On the other hand, if the specific surface area of ​​the additive is too large, the aesthetic quality of the resulting grain-oriented electrical steel sheet may decrease. This is presumed to be because the additive tends to aggregate, and during the finish annealing process, the aggregated additive causes cosmetic defects called black spots to form. For this reason, the specific surface area of ​​the additive should be, for example, 120.0 m². ２ The reason that the amount is less than / g and the resulting grain-oriented electrical steel sheet has excellent aesthetic appeal is 100.0m ２ Preferably less than / g, and 70.0m ２ More preferably less than or equal to 40.0 m ２ It is even more preferable to be less than or equal to 20.0 m ２ A value of less than or equal to / g is particularly preferred.

[0028] The specific surface area is determined using nitrogen gas as the adsorbate gas, in accordance with the "static volume method" described in JIS Z 8830:2013 "Method for measuring the specific surface area of ​​powders (solids) by gas adsorption".

[0029] <Impurities> In additives, the content of impurity elements other than Nb is not particularly limited. However, from the viewpoint of the aesthetic quality of the resulting grain-oriented electrical steel sheet, Fe: 0.005% by mass or less, Al: 0.005% by mass or less, Si: 0.010% by mass or less, Na: 0.003% by mass or less, and Cl: 0.005% by mass or less are preferred.

[0030] <Preparation Method> Additives are obtained, for example, by preparing two or more titanium dioxide powders having specific purities (titanium dioxide content), Nb content, anatase rate, and specific surface area as raw materials and mixing them. In this case, the values ​​of titanium dioxide content, Nb content, anatase rate, and specific surface area derived from the mixing ratio of the raw materials (unit: mass%) are treated as the values ​​of the additive produced. Alternatively, one type of raw material (titanium dioxide powder) may be used as an additive as is.

[0031] Now, in the present embodiment, the annealing separator is prepared by adding an additive (additive for annealing separator) to magnesium oxide powder as described later. That is, first, the additive is prepared, and then the prepared additive is added to the magnesium oxide powder. In this case, it is conceivable to separately add Nb-free titanium oxide powder and Ti-free niobium oxide powder to the magnesium oxide powder without preparing the additive. However, in such a case, the effect of removing Nb from steel may be insufficient. This is presumed to be because Ti atoms and Nb atoms are less likely to be in proximity to each other, making it difficult for the aforementioned (Ti,Nb)N to form. Therefore, in order to sufficiently obtain the effect of removing Nb from steel, it is preferable to first prepare the additive (additive for annealing separator), and then add the prepared additive to the magnesium oxide powder. In this case, Ti atoms and Nb atoms are likely to be in proximity to each other, making it easy for the aforementioned (Ti,Nb)N to form.

[0032] [Preparation of Annealing Separator] The annealing separator is obtained by adding the obtained additive to, for example, magnesium oxide (MgO) powder. In this case, the addition amount of the additive is preferably 5 to 20 parts by mass, more preferably 8 to 15 parts by mass, relative to 100 parts by mass of the magnesium oxide powder.

[0033] [Production of Grain-Oriented Electrical Steel Sheet] A grain-oriented electrical steel sheet is produced using the annealing separator obtained in this manner. The production method for the grain-oriented electrical steel sheet is not particularly limited, and conventionally known production methods can be employed. For example, a steel slab having a component composition for grain-oriented electrical steel sheet is subjected to hot rolling and cold rolling, the obtained steel sheet (cold-rolled sheet) is subjected to primary recrystallization annealing, then the annealing separator is applied to the steel sheet, and thereafter finish annealing including secondary recrystallization annealing is performed. Thereby, a forsterite coating is formed on the surface of the steel sheet.

[0034] The coating amount of the annealing separator (total value including both front and back surfaces), expressed as dry mass, is, for example, 5.0 to 25.0 g / m ２ , but is not limited thereto. In addition, the conditions of primary recrystallization annealing and finish annealing are also not particularly limited, and conventionally known conditions can be appropriately employed.

[0035] Thereafter, a treatment liquid for forming an insulating coating containing phosphate, colloidal silica, etc. is applied onto the surface of the forsterite coating, and baking is performed to form the insulating coating. In this way, a grain-oriented electrical steel sheet comprising a steel sheet, a forsterite coating and an insulating coating is obtained. There are no particular limitations on the component composition of the treatment liquid for forming an insulating coating, baking conditions, and the like, and conventionally known materials can be appropriately employed.

[0036] According to the present embodiment, during finish annealing, Nb present inside the steel sheet (Nb in steel) diffuses and moves into the forsterite coating and is removed, so that ultimately, a grain-oriented electrical steel sheet with a small Nb content in steel and excellent magnetic properties can be obtained.

[0037] There is no problem in using the obtained annealing separator for a steel sheet with a low Nb content (or a steel sheet containing no Nb at all).

[0038] Hereinafter, the present invention will be specifically described with reference to Examples. However, the present invention is not limited to the Examples described below.

[0039] [Test 1] <Preparation of Additive> First, titanium oxide powder having the purity, Nb content, anatase rate and specific surface area shown in Table 1 below was prepared as a raw material. When the sum of the purity and the Nb content does not equal 100% by mass, the remainder is impurities (the same applies hereinafter). Next, the prepared raw material (titanium oxide powder) was mixed for 1 hour using a ribbon mixer at the ratios shown in Table 2 below to obtain an additive for an annealing separator (additive). The titanium oxide content, Nb content, anatase rate and specific surface area of the obtained additive are shown in Table 2 below.

[0040] <Preparation of Annealing Separator> An annealing separator was obtained by adding 12 parts by mass of the additive to 100 parts by mass of magnesium oxide (MgO) powder and mixing the mixture.

[0041] <Manufacturing of Grain-Oriented Electrical Steel Sheet> A steel slab was prepared having a composition containing C: 0.045 mass%, Si: 3.25 mass%, Mn: 0.070 mass%, Al: 80 mass ppm, N: 40 mass ppm, S: 20 mass ppm, and Nb: 40 mass ppm, with the remainder being Fe and unavoidable impurities. The prepared steel slab was heated to 1200°C, and then hot-rolled to obtain a 2.2 mm thick hot-rolled sheet. The obtained hot-rolled sheet was subjected to hot-rolled sheet annealing at 1000°C for 30 s, and then cold-rolled to obtain a steel sheet (cold-rolled sheet) with a final thickness of 0.30 mm. Next, the obtained steel sheet was subjected to primary recrystallization annealing at 850°C for 90 s, which also served as decarburization annealing. After that, 15.0 g / m² of dry mass was applied to the surface of the steel sheet. ２ An annealing separator was applied in the specified amount (total value for both front and back surfaces). The steel sheet coated with the annealing separator was wound up, and the resulting coil was heated at a rate of 25°C / h and held at 1200°C for 20 hours for finish annealing. In this way, a forsterite coating was formed on the surface of the steel sheet. Subsequently, monomagnesium phosphate (100 parts by mass in terms of solid content) and colloidal silica (SiO₂) were applied to the surface of the forsterite coating. ２ (100 parts by mass on a solid content basis) and CrO ３ An insulating coating-forming treatment solution containing (10 parts by mass in terms of metallic elements) was applied, and a baking process was carried out at 900°C for 1 min, which also served as a smoothing annealing. In this way, a grain-oriented electrical steel sheet was obtained, comprising a steel sheet, a forsterite coating, and an insulating coating.

[0042] <Nb content in steel> A 50 mm x 50 mm test piece was cut from the obtained grain-oriented electrical steel sheet, and the insulating coating and forsterite coating on both sides were removed by polishing. Then, the amount of Nb present inside the steel sheet (Nb content in steel) was measured in accordance with the method described in JIS G 1258-4:2007. Although this method is described in this JIS as being applicable to quantitative determination of 10 mass ppm or more, it can also be used to quantify amounts below this level, so the Nb content in steel was measured in units of 1 mass ppm in accordance with this method. The results are shown in Table 2 below.

[0043] <Magnetic Properties> From the obtained grain-oriented electrical steel sheet, a 280 mm x 30 mm test piece was cut out with the rolling direction as the longitudinal direction, and the iron loss W was measured in accordance with JIS C 2556:2015 "Method for measuring the magnetic properties of electrical steel strip using a single sheet tester". １７／５０ The iron loss per 1 kg of steel plate was calculated when the steel plate was magnetized to a magnetic flux density of 1.7 T in an AC magnetic field with an excitation frequency of 50 Hz. Iron loss W １７／５０ The lower the value, the better the magnetic properties can be considered. Iron loss W １７／５０ Table 2 below indicates the following: if the value is 0.75 W / kg or less, select "A"; if it is 0.88 W / kg or less but greater than 0.75 W / kg, select "B"; and if it is greater than 0.88 W / kg, select "C". From a practical standpoint, "A" or "B" is preferred, and "A" is more preferred.

[0044] <Adhesion> Five 300 mm x 30 mm test pieces were cut from the obtained grain-oriented electrical steel sheet with the rolling direction as the longitudinal direction. Next, the test pieces were wrapped around round bars of different diameters at 5 mm intervals, and the minimum diameter at which the forsterite coating did not peel off the steel sheet was confirmed at two locations (one on the front and one on the back) for each test piece. The average of the 10 minimum diameters obtained was calculated and this was determined as the minimum peel diameter (unit: mm) for that grain-oriented electrical steel sheet. The smaller the minimum peel diameter, the better the adhesion of the forsterite coating to the steel sheet can be evaluated. If the minimum peel diameter was 20 mm or less, it was marked "A", if it was between 20 mm and 40 mm, it was marked "B", and if it was greater than 40 mm, it was marked "C", as shown in Table 2 below. From a practical standpoint, "A" or "B" is preferred, and "A" is more preferred.

[0045] <Aesthetic Appearance> The surface appearance of the obtained grain-oriented electrical steel sheet was visually inspected, and the length including defects such as patterns (distance in the rolling direction) was measured. The ratio of this length to the total coil length was determined as the appearance defect rate. The smaller the appearance defect rate, the better the aesthetic appearance. Since the insulating coating generally transmits visible light, the aesthetic appearance of the forsterite coating is the subject of evaluation. If the appearance defect rate was 1.0% or less, it was marked "A"; if it was between 1.0% and 5.0%, it was marked "B"; and if it was above 5.0%, it was marked "C," as shown in Table 2 below. From a practical standpoint, "A" or "B" is preferred, and "A" is more preferred.

[0046]

[0047]

[0048] <Summary of Evaluation Results> As shown in Table 2 above, Nos. 1-2 to 1-11, which used additives with a titanium oxide content of 95.00% by mass or more and an Nb content of 0.001% by mass or more and 0.150% by mass or less, had a lower Nb content in the resulting grain-oriented electrical steel sheets and better magnetic properties compared to No. 1-1, which used additives with an Nb content of less than 0.001% by mass; No. 1-12, which used additives with an Nb content of more than 0.150% by mass; and No. 1-13, which used additives with a titanium oxide content of less than 95.00% by mass.

[0049] Furthermore, when comparing samples No. 1-2 to No. 1-6, which differ only in the Nb content of the additive, samples No. 1-2 to No. 1-4, with an Nb content of 0.001 to 0.009 mass%, had a lower Nb content in the resulting grain-oriented electrical steel sheets and better magnetic properties compared to samples No. 1-5 to No. 1-6, which did not meet this requirement.

[0050] Furthermore, when comparing Nos. 1-7 to Nos. 1-11, which differ only in the titanium oxide content of the additive, Nos. 1-8 to Nos. 1-10, which have a titanium oxide content of 97.60 to 99.99 mass%, had a lower Nb content in the resulting grain-oriented electrical steel sheets and better magnetic properties compared to Nos. 1-7 and Nos. 1-11, which do not meet this requirement.

[0051] [Test 2] <Preparation of Additives, etc.> The raw materials (titanium oxide powder) shown in Table 3 below were mixed for 1 hour using a ribbon mixer in the proportions shown in Table 4 below to obtain an additive for annealing separation agent (additive). The titanium oxide content, Nb content, anatase rate, and specific surface area of ​​the obtained additive are shown in Table 4 below. Next, using the obtained additive, an annealing separation agent was prepared in the same manner as in Test 1 above, and grain-oriented electrical steel sheets were manufactured to measure the amount of Nb in the steel and perform various evaluations. The results are shown in Table 4 below.

[0052]

[0053]

[0054] <Summary of Evaluation Results> As shown in Table 4 above, when comparing No. 2-1 to No. 2-5, which differ only in the anatase content of the additive, No. 2-3 to No. 2-4, which have anatase content of 81.0 to 99.0%, had a smaller minimum peel diameter and better adhesion compared to No. 2-1 to No. 2-2 and No. 2-5, which do not meet this requirement.

[0055] Furthermore, comparing No. 2-6 to No. 2-11, which differ only in the specific surface area of ​​the additive, the specific surface areas range from 7.2 to 19.4 m². ２ Nos. 2-8 to 2-9, which meet the requirement of / g, had a lower rate of appearance defects and better aesthetics compared to Nos. 2-6 to 2-7 and Nos. 2-10 to 2-11, which do not meet this requirement.

Claims

1. An additive for annealing separation agents used in the manufacture of grain-oriented electrical steel sheets, wherein the titanium oxide content is 95.00% by mass or more, and the Nb content is 0.001% by mass or more and 0.150% by mass or less.

2. The additive for annealing separation agent according to claim 1, wherein the anatase content is 20.0% or more.

3. Specific surface area is 3.0 m² ２ / g or more 100.0m ２ An additive for annealing separating agents according to claim 1 or 2, wherein the amount is less than or equal to / g.

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