Classifications

• • 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
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • 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
• • 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

Definitions

• the present invention relates to a hot-rolled and annealed steel sheet for use in a grain-oriented electrical steel sheet.
• Grain-oriented electrical steel sheet contains 7% or less by mass of Si and has a secondary recrystallization texture concentrated in the ⁇ 110 ⁇ 001> orientation (Goss orientation).
• the ⁇ 110 ⁇ 001> orientation means that the ⁇ 110 ⁇ plane of the crystal is aligned parallel to the rolling surface, and the ⁇ 001> axis of the crystal is aligned parallel to the rolling direction.
• the magnetic properties of grain-oriented electrical steel sheets are greatly affected by the degree of concentration in the ⁇ 110 ⁇ 001> orientation.
• the relationship between the rolling direction of the steel sheet, which is the main magnetization direction when the steel sheet is in use, and the crystal's ⁇ 001> direction, which is the direction of easy magnetization, is considered important.
• the angle between the crystal's ⁇ 001> direction and the rolling direction in practical grain-oriented electrical steel sheets is controlled to be within a range of around 5°.
• Such precise crystal orientation control is achieved by dispersing fine precipitates called inhibitors in the steel appropriately before final annealing, and by holding the steel sheet at high temperatures during final annealing.
• inhibitors increase the selective growth of Goss-oriented grains, which results in secondary recrystallization progressing during final annealing so that Goss-oriented grains grow preferentially.
• attempts have been made to highly control inhibitors in order to precisely control crystal orientation.
• Patent Document 1 discloses using MnS as an inhibitor and performing two cold rolling steps.
• Patent Documents 2 and 3 disclose the use of inhibitors to control MnS+AlN and MnS (and/or MnSe)+Sb, respectively.
• Patent Document 4 discloses a technique for preferably controlling inhibitors in order to lower the slab heating temperature for the purpose of reducing production costs.
• Patent Document 5 discloses controlling the primary recrystallized grain size and its dispersion related to an inhibitor.
• Patent Documents 6 to 8 disclose the addition of Nb, V, etc. to grain-oriented electrical steel sheets.
• Patent Documents 9 to 11 disclose techniques for improving magnetostriction by precisely controlling the atmosphere and residence time during finish annealing to form subgrain boundaries within secondary recrystallized grains. These techniques demonstrate the technical concept of expanding the temperature range over which secondary recrystallization progresses in order to form subgrain boundaries, and at the same time, they also show that improvements in magnetic flux density can be expected.
• One aspect of the present invention was made in consideration of the above-mentioned problems. Given the current demand for increasing the magnetic flux density of grain-oriented electrical steel sheets, one aspect of the present invention aims to provide a hot-rolled annealed steel sheet for grain-oriented electrical steel sheets that can increase the magnetic flux density.
• the gist of the present invention is as follows:
• a hot-rolled annealed steel sheet for grain-oriented electrical steel sheet is In mass%, C: 0.0010 to 0.10%, Si: 2.0 to 7.0%, Mn: 0.050 to 1.0%, S: 0 to 0.0350%, Se: 0 to 0.0350%, S+Se total content: 0.0030-0.0350%, Al: 0.010-0.0650%, N: 0.0040-0.0120%, Nb: 0 to 0.030%, V: 0 to 0.030%, Mo: 0 to 0.030%, Ta: 0 to 0.030%, W: 0-0.030%, Nb + V + Mo + Ta + W total content: 0.0030 to 0.030%, Cu: 0 to 0.40%, Bi: 0 to 0.010%, B: 0 to 0.080%, P: 0-0.50%, Ti: 0 to 0.0150%, Sn: 0 to 0.10%, Sb: 0 to 0.10%, Cr: 0 to 0.30%, Ni:
• the above aspect of the present invention provides a hot-rolled annealed steel sheet for grain-oriented electrical steel sheet that can increase magnetic flux density.
• FIG. 1 is a schematic diagram of particle size-number density distribution of precipitates having a circle-equivalent diameter D of 50 to 1000 nm. 1 is a flowchart of a method for manufacturing a hot-rolled annealed steel sheet for a grain-oriented electrical steel sheet according to an embodiment of the present invention.
• the term “inhibitor” is primarily used in explanations relating to the secondary recrystallization mechanism for the precipitates in the steel that characterize this embodiment, and the term “precipitates” is primarily used in explanations relating to the compound phases observed in the structure.
• the terms “inhibitor” and “precipitates” are not used with the intention of strictly distinguishing them.
• the researchers investigated how to more effectively expand the temperature range over which secondary recrystallization progresses by appropriately controlling the inhibitor morphology in the steel, and how to preferentially grow grains with a preferred crystal orientation during the secondary recrystallization process over this expanded temperature range.
• the morphology of precipitates contained in hot-rolled annealed steel sheets is optimally controlled during the manufacturing process of grain-oriented electrical steel sheets, the temperature range over which secondary recrystallization progresses can be expanded during finish annealing, allowing Goss-oriented grains to grow preferentially, and ultimately increasing the magnetic flux density of the grain-oriented electrical steel sheets obtained compared to conventional technology.
• inhibitors are tiny precipitates in steel with a diameter of approximately 1000 nm or less. These inhibitors have a pinning effect on grain boundaries, suppressing grain growth. When the temperature reaches approximately 1000°C or higher during final annealing, these inhibitors dissolve into the ⁇ -Fe phase, which is the parent phase, and the pinning effect on the grain boundaries weakens. As a result, abnormal grain growth occurs, a phenomenon known as secondary recrystallization.
• Mn-based inhibitors and Al-based inhibitors are primarily used in manufacturing methods in which the slab heating temperature before hot rolling is 1300°C or higher (hereinafter referred to as the "high-temperature slab heating process").
• Al-based inhibitors Al-based inhibitors controlled after cold rolling
• carbides and nitrides of Nb, V, Mo, Ta, W, etc. are sometimes used as secondary inhibitors.
• the steel composition and manufacturing conditions have been controlled in order to form inhibitors with the appropriate functions within the steel.
• the steel composition, hot rolling conditions, and decarburization annealing conditions are recognized as manufacturing conditions that have a significant impact on the morphology of the inhibitor, and these conditions have been precisely controlled.
• the temperature range in which secondary recrystallization progresses is expanded during the subsequent finish annealing process, and the selectivity of crystal orientation associated with the progress of secondary recrystallization is increased.
• the above effects are achieved by allowing relatively fine inhibitors and relatively coarse inhibitors to coexist in the hot-rolled and annealed steel sheet with appropriate sizes and distributions.
• secondary recrystallization begins at a relatively low temperature during the temperature rise process of finish annealing and continues up to a relatively high temperature, which is thought to expand the temperature range over which secondary recrystallization progresses.
• the driving force for the preferential growth of Goss-oriented grains is not very strong. Therefore, when grain boundary migration occurs relatively easily in grains other than Goss-oriented grains during secondary recrystallization, for example, when the decomposition of the inhibitor is rapid and the pinning effect on grain growth is weakened, resulting in a relatively high grain growth rate (when the driving force for grain growth is relatively high), crystal grains other than Goss-oriented grains also grow easily. In this case, the preferential growth of Goss-oriented grains is inhibited.
• the inhibitor decomposition rate should be kept as slow as possible, the grain growth rate during secondary recrystallization should be made relatively fast compared to the inhibitor decomposition rate, and secondary recrystallization should be sustained for a long period of time.
• the rate of temperature rise in the temperature range where the inhibitor strength weakens should be slowed down, the inhibitor dissolution rate should be slowed, and the resulting growth rate of secondary recrystallized grains should be made relatively fast compared to the inhibitor decomposition rate.
• this method inevitably results in a longer total finish annealing time, resulting in a decrease in productivity.
• the steel composition, casting conditions, hot rolling conditions, and hot-rolled sheet annealing conditions in a composite and inseparable manner, relatively fine precipitates and relatively coarse precipitates of appropriate size and distribution are allowed to coexist in the hot-rolled and annealed steel sheet after the hot-rolled sheet annealing process.
• the morphology of the precipitates is preferably controlled by adding auxiliary inhibitor-forming elements.
• the morphology of the above precipitates is defined based on hot-rolled annealed steel sheet (steel sheet immediately before cold rolling).
• the hot-rolled annealed steel sheet according to this embodiment has, in mass%, C: 0.0010 to 0.10%, Si: 2.0 to 7.0%, Mn: 0.050 to 1.0%, S: 0 to 0.0350%, Se: 0 to 0.0350%, S+Se total content: 0.0030-0.0350%, Al: 0.010-0.0650%, N: 0.0040-0.0120%, Nb: 0 to 0.030%, V: 0 to 0.030%, Mo: 0 to 0.030%, Ta: 0 to 0.030%, W: 0-0.030%, Nb + V + Mo + Ta + W total content: 0.0030 to 0.030%, Cu: 0 to 0.40%, Bi: 0 to 0.010%, B: 0 to 0.080%, P: 0-0.50%, Ti: 0 to 0.0150%, Sn: 0 to 0.10%, Sb: 0 to 0.10%, Cr: 0 to 0.30%, Ni: 0-1.0%, and the balance being Fe and
• the hot-rolled and annealed steel sheet has an average grain size of 20.0 to 21.5 ⁇ m.
• the hot-rolled annealed steel sheet according to this embodiment has a chemical composition that includes basic elements, optional elements as needed, and the balance being Fe and impurities.
• the hot-rolled annealed steel sheet according to this embodiment contains, as base elements (major alloying elements), the following mass fractions: C: 0.0010-0.10%, Si: 2.0-7.0%, Mn: 0.050-1.0%, S+Se total content: 0.0030-0.0350%, Al: 0.010-0.0650%, N: 0.0040-0.0120%, and Nb+V+Mo+Ta+W total content: 0.0030-0.030%.
• Carbon (C) is an element effective in controlling the primary recrystallization structure during the manufacturing process.
• C content of the hot-rolled annealed steel sheet may be 0.0010 to 0.10%.
• the preferred upper limit of the C content is 0.0850%, or 0.0750%.
• C is purified in the decarburization annealing process and the finish annealing process described below, and after the finish annealing process, the C content is 0.0050% or less.
• the C content may be more than 0% or may be 0.0010% or more, taking into account productivity in industrial production.
• Si 2.0-7.0%
• Silicon (Si) increases the electrical resistance of grain-oriented electrical steel sheets and reduces iron loss. If the Si content is less than 2.0%, austenite transformation occurs during finish annealing, damaging the crystal orientation of the grain-oriented electrical steel sheet. On the other hand, if the Si content exceeds 7.0%, cold workability decreases, making cracks more likely to occur during cold rolling. Therefore, the Si content of hot-rolled annealed steel sheets should be 2.0 to 7.0%.
• the preferred lower limit of the Si content is 2.50%, more preferably 3.0%.
• the preferred upper limit of the Si content is 4.50%, more preferably 4.0%.
• part of the inhibitor function may be performed by carbides, nitrides, carbonitrides, or the like of Nb group elements.
• the amount of precipitated MnS and MnSe, which act as inhibitors may be controlled to be small. Therefore, the upper limit of the Mn content is preferably 0.50%, and more preferably 0.20%.
• S 0-0.0350%
• S+Se total content 0.0030-0.0350%
• Sulfur (S) and selenium (Se) combine with Mn to precipitate as MnS or MnSe, functioning as inhibitors.
• S content of the hot-rolled annealed steel sheet should be 0 to 0.0350%
• Se content should be 0 to 0.0350%
• the total S + Se content should be 0.0030 to 0.0350%.
• a total S and Se content of 0.0030 to 0.0350% is preferable because it stabilizes secondary recrystallization.
• the total content of S and Se is 0.0030 to 0.0350%
• the hot-rolled annealed steel sheet may contain only one of S or Se in its chemical composition, with the content being 0.0030 to 0.0350%.
• the hot-rolled annealed steel sheet may contain both S and Se, with the total content being 0.0030 to 0.0350%.
• Al 0.010-0.0650%
• Aluminum (Al) combines with N to precipitate as AlN or (Al,Si)N, functioning as an inhibitor.
• the Al content of the hot-rolled annealed steel sheet should be 0.010 to 0.0650%.
• AlN or (Al,Si)N precipitates in a favorable form through nitriding in the low-temperature slab heating process, stabilizing secondary recrystallization, particularly in the high-temperature range.
• the Al content When the Al content is below 0.010%, the amount of AlN or (Al,Si)N precipitated, which functions as an inhibitor, is insufficient, hindering the proper progress of secondary recrystallization. Furthermore, when the Al content exceeds 0.0650%, the amount of AlN or (Al,Si)N precipitated, which functions as an inhibitor, becomes excessive, hindering the proper progress of secondary recrystallization.
• the lower limit of the Al content is preferably 0.020%, more preferably 0.0250%. From the viewpoint of stability of secondary recrystallization, the upper limit of the Al content is preferably 0.040%, and more preferably 0.030%.
• N 0.0040 ⁇ 0.0120%
• Nitrogen (N) combines with Al and precipitates as AlN or (Al, Si)N, functioning as an inhibitor.
• the N content of hot-rolled annealed steel sheet may be 0.0040 to 0.0120%. Note that in the low-temperature slab heating process, N may be added to the steel by nitriding during the manufacturing process. If the N content exceeds 0.0120%, blisters, a type of defect, are more likely to occur in the steel sheet.
• the upper limit of the N content is preferably 0.010%, more preferably 0.0090%.
• N is purified in the finish annealing process, and after the finish annealing process, the N content is 0.0050% or less.
• Nb+V+Mo+Ta+W total content 0.0030-0.030%
• Nb 0-0.030%
• V 0-0.030%
• Mo 0-0.030%
• Ta 0-0.030%
• W 0-0.030%
• the Nb content is set to 0 to 0.030%
• the V content is set to 0 to 0.030%
• the Mo content is set to 0 to 0.030%
• the Ta content is set to 0 to 0.030%
• the W content is set to 0 to 0.030%
• the total content of Nb + V + Mo + Ta + W is set to 0.0030 to 0.030%.
• the lower limit of the content of Nb, V, Mo, Ta, and/or W is preferably 0.0040%, and more preferably 0.0050%.
• the upper limit of the content of Nb, V, Mo, Ta, and/or W is preferably 0.020%, more preferably 0.010%.
• Nb, V, Mo, Ta, and W may be collectively referred to as "Nb group elements.”
• the hot-rolled annealed steel sheet according to this embodiment contains one or more Nb group elements selected from the Nb group consisting of Nb, V, Mo, Ta, and W in a total amount of 0.0030 to 0.030 mass%.
• Nb group element precipitates when used as inhibitors, when the total content of Nb group elements in the hot-rolled annealed steel sheet is 0.030% or less (preferably 0.0030% or more and 0.030% or less), the morphology of the Nb group element precipitates is favorably controlled and the secondary recrystallization progression temperature range is favorably expanded. As a result, Goss-oriented grains grow favorably, and the magnetic flux density of the final grain-oriented electrical steel sheet is favorably increased.
• Nb group elements function favorably as inhibitors.
• Carbides, nitrides, or carbonitrides of Nb group elements precipitate non-equilibrium during cooling from high temperatures and are thought to act as precipitation nuclei for the subsequent precipitation of MnS and AlN. Therefore, compared to when Nb group elements are not contained, when Nb group elements are contained, there are more precipitation sites for MnS and AlN, and as a result, MnS and AlN are thought to be more likely to form as fine precipitates.
• the coexistence of fine inhibitors and coarse inhibitors expands the secondary recrystallization progression temperature range, and precipitates of Nb group elements are thought to be particularly effective in expanding the secondary recrystallization progression temperature range toward the lower temperature side.
• the total content of Nb group elements is preferably 0.0040% or more, and more preferably 0.0050% or more. Furthermore, the total content of Nb group elements is preferably 0.020% or less, and more preferably 0.010% or less. If the total content of Nb group elements is below 0.0030%, there will be a shortage of Nb group element precipitates that act as precipitation nuclei, making it difficult to refine MnS and AlN. On the other hand, if the total content of Nb group elements exceeds 0.030%, the precipitation temperature range of Nb group element precipitates will be too high, making the Nb group element precipitates likely to become coarse and low-density.
• the difference between the precipitation temperature range of Nb group element precipitates and the precipitation temperature range of MnS and AlN will be large, making it difficult for the Nb group element precipitates to effectively function as precipitation nuclei for refining MnS and AlN.
• the total content of Nb group elements is 0.0030 to 0.030%
• the hot-rolled annealed steel sheet may contain at least one element selected from the group consisting of Nb, V, Mo, Ta, and W in its chemical composition, with the content being 0.0030 to 0.030%.
• the hot-rolled annealed steel sheet may contain at least two elements selected from the group consisting of Nb, V, Mo, Ta, and W, with the total content being 0.0030 to 0.030%.
• Cu 0-0.40% Bi: 0 ⁇ 0.010% B: 0-0.080% P: 0 to 0.50% Ti: 0 ⁇ 0.0150% Sn: 0-0.10% Sb: 0-0.10% Cr: 0-0.30% Ni: 0 to 1.0% Copper (Cu), bismuth (Bi), boron (B), phosphorus (P), titanium (Ti), tin (Sn), antimony (Sb), chromium (Cr), and nickel (Ni) may be contained according to known purposes. There is no need to set a lower limit for the content of these optional elements, and the lower limit may be 0%.
• the chemical composition of the hot-rolled annealed steel sheet according to this embodiment may be measured using a general steel analysis method.
• the chemical composition of the hot-rolled annealed steel sheet may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry).
• ICP-AES Inductively Coupled Plasma-Atomic Emission Spectrometry
• the chemical composition is determined by measuring a 35 mm square test piece taken from the hot-rolled annealed steel sheet using ICP-AES under conditions based on a pre-prepared calibration curve.
• C and S may be measured using the combustion-infrared absorption method
• N may be measured using the inert gas fusion-thermal conductivity method.
• the precipitates (inhibitors) contained in the hot-rolled annealed steel sheet according to this embodiment may be of any type, as long as the precipitation form of the precipitates is controlled.
• the precipitates may be formed from elements contained in the hot-rolled annealed steel sheet.
• Mn-based precipitates Mn-containing precipitates
• Al-based precipitates Al-containing precipitates
• Nb-group element-containing precipitates may be carbides, nitrides, or carbonitrides.
• compounds of optional elements such as Bi and B, and complex compounds with the above elements may also be included.
• the inventors' investigations have revealed that the above-described effects obtained in this embodiment are primarily due to the size and distribution of the precipitates contained in the hot-rolled annealed steel sheet. Therefore, the size and distribution of the precipitates are specified in the hot-rolled annealed steel sheet according to this embodiment.
• the precipitates that should be controlled in the hot-rolled annealed steel sheet according to this embodiment are precipitates with an equivalent circle diameter D of 50 to 1000 nm.
• equivalent circle diameter refers to the diameter of a circle when the area of a precipitate is converted into a circle with the same area. This equivalent circle diameter is the same as the equivalent sphere diameter.
• precipitates with an excessively large equivalent circle diameter D may have a negative effect on the growth of secondary recrystallized grains in the final stage of secondary recrystallization. Furthermore, the formation of precipitates with an excessively large equivalent circle diameter D may reduce the number of precipitates (number density) contained in the hot-rolled annealed steel sheet. Furthermore, precipitates with an excessively large equivalent circle diameter D are less likely to function as inhibitors. Therefore, it is preferable that the average equivalent circle diameter D of the precipitates is 1000 nm or less. In the hot-rolled annealed steel sheet according to this embodiment, the size and distribution of precipitates with an equivalent circle diameter D of 50 to 1000 nm are controlled as precipitates that have the effect of expanding the secondary recrystallization progression temperature range.
• Figure 1 shows a schematic diagram of the particle size-number density distribution of precipitates with a circular equivalent diameter D of 50 to 1000 nm.
• Figure 1 shows examples of Dp, f(Dp), and Wp.
• Dp exceeding 300 nm is inappropriate because it results in fewer fine precipitates for expanding the secondary recrystallization temperature range.
• the upper limit of Dp is preferably 275 nm, and more preferably 250 nm.
• the secondary recrystallization proceeding temperature range is preferably expanded.
• the lower limit of Dp is preferably 125 nm, and more preferably 150 nm.
• f(Dp) is 1,000,000 particles/g or more
• the precipitates necessary for secondary recrystallization to occur are sufficiently precipitated, and the pinning effect is preferably achieved.
• the upper limit of f(Dp) is not particularly limited, but it may be set to, for example, 50,000,000 particles/g.
• the ratio of Wp to Dp becomes a favorable value, and the secondary recrystallization temperature range is preferably expanded.
• the lower limit of Wp/Dp is preferably 1.2.
• Wp/Dp is 2.0 or less, the ratio of Wp to Dp becomes a favorable value, and primary recrystallized grains grow uniformly during normal grain growth, resulting in a favorable grain structure before secondary recrystallization.
• the upper limit of Wp/Dp is preferably 1.75.
• the particle size-number density distribution of precipitates with a circular equivalent diameter D of 50 to 1000 nm can be determined as follows:
• the extraction residue (precipitate) can be recovered from the electrolytic extraction solution.
• the size and distribution of the recovered precipitates can be measured using the FFF (Field Flow Fractionation) method.
• FFF Field Flow Fractionation
• the FFF device used may be a Wyatt Eclipse AF4 device (Wyatt Technology Europe, Germany).
• the measurement sample dispersion solution may be an aqueous solution of sodium dodecyl sulfate at a concentration of 300 mg/mL.
• the cell may have a channel length of 275 mm and an asymmetric diamond-shaped channel spacer with a thickness of 350 ⁇ m.
• the separation membrane may be a regenerated cellulose ultrafiltration membrane with a molecular weight of 30 kDa.
• the type and number of standard samples can be selected according to the particle size distribution of the extraction residue to be measured; for example, polystyrene latex standard particles with a particle size of 29 to 500 nm can be selected.
• the size of the standard particles must be directly confirmed in advance using a TEM (Transmission Electron Microscope) or similar. It is sufficient to measure 500 or more particles. The long side of each standard particle is measured and the average value calculated. Furthermore, six different particle diameters, for example, 29 nm, 48 nm, 100 nm, 200 nm, 300 nm, and 500 nm, can be used for the standard particles.
• the FFF device eluent (hereinafter referred to as channel flow) should be set to 1.0 mL/min, the cross flow to 0.5 mL/min, and the time should be 1 minute. Then, for focusing before sample injection, the focus flow should be set to 3.0 mL/min and the time should be 1 minute. Next, for focusing, the sample should be injected at 0.2 mL/min for 2 minutes. After sample injection, the focusing time should be 1 minute.
• a calibration curve can be created by using the time when the flow started as the reference point and correlating the time until particles are detected with the average particle size of the standard particles measured in advance. The maximum time for particle detection is 35 minutes, and the injection volume of the liquid in which the sample is dispersed is 0.1 to 0.4 mL.
• the particle size of the nanoparticles contained in the nanoparticle dispersion sample to be measured can be measured.
• the effluent from the FFF device (a solution containing precipitates separated by size) can be analyzed for components using a conventional ICP (Inductively Coupled Plasma) mass spectrometer.
• ICP Inductively Coupled Plasma
• the "mode diameter” corresponds to the particle diameter (particle diameter category) at which the number density value is greatest in the above-mentioned histogram of particle diameter and number density (particle diameter-number density distribution of precipitates).
• f(Dp), Wp, etc. After smoothing the measurement data obtained using the FFF method.
• the simple moving average method can be used to smooth the measurement data obtained using the FFF method.
• the value of f(Dp) can be calculated by considering the top three digits as significant.
• the average grain size of the hot-rolled annealed steel sheet according to this embodiment is 20.0 to 21.5 ⁇ m.
• relatively fine precipitates and relatively coarse precipitates coexist with appropriate sizes and distribution
• the average grain size of the hot-rolled annealed steel sheet (steel sheet after hot-rolled sheet annealing) is an appropriate grain size. This is achieved by lowering the first-stage annealing temperature and also lowering the second-stage annealing temperature during hot-rolled sheet annealing.
• the average grain size of hot-rolled annealed steel sheet is preferably 21.5 ⁇ m or less, and more preferably 21.0 ⁇ m.
• the lower limit of the average grain size is not particularly limited in terms of the effect of broadening the precipitate size distribution. However, in order to reduce the average grain size, it is better to use a higher second-stage annealing temperature, but in that case, fine precipitates will be scarce and the precipitate size distribution will not be sufficiently broadened. For this reason, for example, the average grain size of hot-rolled annealed steel sheet may be 20.0 ⁇ m or more.
• the average grain size of the hot-rolled annealed steel sheet according to this embodiment can be determined based on the intersecting method of JIS G0551:2013. For example, an L-section (a cross section normal to the direction perpendicular to the rolling direction) of the hot-rolled annealed steel sheet can be photographed using an optical microscope at a magnification of 200x, and the grain size of the cross-sectional structure can be measured along the thickness direction of the sheet based on the intersecting method described above. This measurement can be carried out at least five times at different measurement locations to determine the average grain size.
• the processes subsequent to the cold rolling process shown in Figure 2 i.e., the cold rolling process, decarburization annealing process, annealing separator application process, and finish annealing process, are manufacturing processes for grain-oriented electrical steel sheet (finish-annealed steel sheet).
• finish-annealed steel sheet The effects of the hot-rolled annealed steel sheet according to this embodiment can be confirmed in the final product, which is grain-oriented electrical steel sheet, so the conditions for controlling these processes will also be described later.
• the method for producing a hot-rolled annealed steel sheet includes a casting process, a hot-rolling process, and a hot-rolled sheet annealing process, In the casting process, In mass%, C: 0.0010 to 0.10%, Si: 2.0 to 7.0%, Mn: 0.050 to 1.0%, S: 0 to 0.0350%, Se: 0 to 0.0350%, S+Se total content: 0.0030-0.0350%, Al: 0.010-0.0650%, N: 0.0040-0.0120%, Nb: 0 to 0.030%, V: 0 to 0.030%, Mo: 0 to 0.030%, Ta: 0 to 0.030%, W: 0-0.030%, Nb + V + Mo + Ta + W total content: 0.0030 to 0.030%, Cu: 0 to 0.40%, Bi: 0 to 0.010%, B: 0 to 0.080%, P: 0-0.50%, Ti: 0 to 0.0150%, Sn: 0 to 0.10%, Si
• the hot-rolled steel sheet after the hot rolling process is heated and subjected to first-stage annealing in a temperature range of 1040 to 1080°C, and second-stage annealing in a lower temperature range of 810 to 880°C, and then cooled at an average cooling rate of 5 to 80°C/second to obtain a hot-rolled annealed steel sheet.
• the soaking temperature of the slab is set to more than 1030 ° C. and less than 1180 ° C., so that a portion of the precipitates contained in the slab is preferably solutionized (for example, 12 to 85 volume % of the precipitates are solutionized based on the precipitates contained in the slab after the casting process), and in order to make this solution state uniform within the slab, the soaking time of the slab is set to more than 70 minutes; and During rough rolling, the rolling temperature is set to 940 to 1070° C., and the rolling reduction is set to 82 to 95%.
• the above slab heating can be performed by soaking at a specified temperature for a specified time without temporarily increasing the heating temperature during the slab heating process.
• the above slab soaking temperature refers to the surface temperature of the slab
• the slab soaking time refers to the holding time after the slab surface temperature reaches the above soaking temperature.
• the solution state of the precipitates on the slab surface will be preferably controlled.
• the solution state of the precipitates will be preferably controlled all the way to the center of the slab.
• the chemical composition of the slab affects the "solution state of precipitates before rough rolling" mentioned above.
• the chemical composition of the slab must not only satisfy the chemical composition requirements for hot-rolled annealed steel sheet mentioned above, but must also be controlled in conjunction with other manufacturing conditions that affect the "solution state of precipitates before rough rolling.”
• An example of a method for manufacturing slabs is as follows: Molten steel is produced (smelted). Slabs are manufactured using this molten steel. For example, slabs may be manufactured using continuous casting. Alternatively, ingots may be manufactured using the molten steel, and the ingots may be bloomed to manufacture slabs.
• the thickness of the slab is, for example, 150 to 350 mm. The thickness of the slab is preferably 220 to 280 mm. So-called thin slabs with a thickness of 10 to 70 mm may also be used as slabs.
• the hot rolling step is a step in which a slab is heated to a predetermined temperature and hot rolled (rough rolling and finish rolling) to obtain a hot-rolled steel sheet.
• the slab after the casting process is heated, rough rolled, and then finish rolled to produce a hot-rolled steel sheet with a specified thickness of 1.8 to 3.5 mm. After finish rolling is complete, the hot-rolled steel sheet is coiled at the specified temperature.
• the soaking temperature of the slab is set to more than 1030°C and less than 1180°C, which preferably brings some of the precipitates contained in the slab into solution (for example, 12 to 85 volume % of the precipitates are brought into solution, based on the precipitates contained in the slab at room temperature after the casting process), and in order to make this solution state uniform within the slab, the slab should be heated for a soaking time of more than 70 minutes.
• Preferably bringing some of the precipitates contained in the slab into solution before rough rolling is necessary to achieve a favorable final balance between the amounts of relatively coarse precipitates that remain precipitated during the slab heating stage (undissolved precipitates) and relatively fine precipitates that do not precipitate during the slab heating stage but precipitate after hot rolling (re-precipitated precipitates).
• the Wp value which is the main technical feature of this embodiment, can be increased by controlling the size difference between the relatively coarse precipitates that remain precipitated during the slab heating stage (residual precipitates) and the relatively fine precipitates that do not precipitate during the slab heating stage but precipitate after hot rolling (re-precipitated precipitates).
• solution state of precipitates before rough rolling refers to the “solution state of precipitates before rough rolling" in an equilibrium state, not a non-equilibrium state.
• a non-equilibrium state for example, the solution state of precipitates will be uneven near the surface and near the center in the thickness direction. If a slab in this non-equilibrium state is subjected to rough rolling, it will ultimately become difficult to control the size and distribution of precipitates contained in the steel sheet after the hot-rolled sheet annealing process.
• the value obtained by subtracting the temperature at the center of the slab from the temperature at the surface of the slab within the range of more than -10°C and less than 50°C during slab heating and extraction.
• the temperature difference is less than -10°C, the steel plate surface will be less likely to stretch, resulting in significant defects.
• the temperature difference is more than 50°C, the solution state of the precipitates will be uneven in the thickness direction, making it difficult to control the size of the precipitates.
• the heating temperature may be temporarily increased during slab heating in order to shorten the soaking time. In this case, it is effective to keep the difference between the surface temperature at the maximum temperature reached and the surface temperature at the time of heating and extraction of the slab to 80°C or less. In this case, after the temperature is reduced from the maximum temperature reached, it is preferable to hold the slab in a low-temperature region of the slab heating furnace for at least 20 minutes or more, so that the difference between the surface temperature and the central temperature at the time of extraction from the slab heating furnace is less than 50°C. It is even more preferable to keep the difference between the surface temperature and the central temperature of the slab to be 0 to 30°C.
• the low-temperature slab heating process which involves heating a slab at temperatures below 1280°C
• solution state of the precipitates is suitably controlled, and a slab in which the solution state of the precipitates is in equilibrium is subjected to rough rolling.
• the above-mentioned “solution state of precipitates before rough rolling” is a characteristic that is affected by both the steel composition and the hot rolling conditions (slab heating conditions).
• those skilled in the art can control material properties, including precipitation behavior, and so as long as they understand the effect that each of the above conditions has on the "solution state,” they can combine the above conditions to control the "solution state.”
• the "solution state of the precipitates before rough rolling” may be controlled by temporarily increasing the heating temperature during the slab heating process and then holding the temperature for a certain period of time after cooling.
• a method is shown in which the "solution state of the precipitates before rough rolling" is controlled by soaking at a predetermined temperature for a predetermined period of time without temporarily increasing the heating temperature during the slab heating process.
• the soaking temperature during slab heating should be greater than 1030°C and less than 1180°C, and the soaking time should be greater than 70 minutes. In this case, it is easy to favorably bring some of the precipitates contained in the slab into solution (for example, it is easy to bring 12 to 85 volume % of the precipitates into solution, based on the precipitates contained in the slab at room temperature after the casting process).
• Nb-group elements When the content of Nb-group elements is within the above range, it is ultimately possible to have both fine and coarse inhibitors coexist, even if the slab heating temperature is 1100°C or higher. For example, if the slab heating temperature is high and solution formation of AlN, MnS, etc. is promoted during the slab heating stage, these AlN and MnS are likely to re-precipitate as coarse particles in subsequent processes. However, when the content of Nb-group elements is within the above range, the precipitates of Nb-group elements act as precipitation nuclei for MnS and AlN, reducing the size of the re-precipitated AlN and MnS.
• Nb-group element precipitates carbonitrides
• the precipitates of Nb-group elements themselves are more likely to precipitate as finer precipitates than AlN, etc.
• the lower limit temperature during slab soaking may be above 1030°C. Note that as the soaking temperature decreases, the solution of precipitates is also suppressed, but when the content of Nb group elements is within the above range, it is easier to preferably bring some of the precipitates contained in the slab into solution (for example, the lower limit of the solution rate of precipitates may be 12% by volume). When these conditions are met, it is ultimately possible to achieve the coexistence of fine inhibitors and coarse inhibitors.
• the mechanism by which the above effects are achieved is thought to be related to the fact that precipitates (carbonitrides) of Nb group elements precipitate more easily than MnS or AlN (MnS in particular does not precipitate easily without support such as dislocation multiplication caused by rolling, and when it does precipitate, its size increases), and that precipitates of Nb group elements function as precipitation nuclei for the precipitation of MnS and AlN, suppressing the coarsening of the re-precipitated AlN and MnS.
• a soaking time of 70 minutes or less is too short, making it difficult to control the solution state of the precipitates to an equilibrium state.
• the soaking time may be set to 2 hours.
• the soaking time during slab heating may be relaxed by changing the hot-rolled sheet annealing conditions described below.
• appropriate precipitate control may be possible even with a soaking time of 70 minutes or less.
• desired properties may be obtained (e.g., magnetic flux density B8 of 1.935 T or more) by controlling other conditions and hot-rolled sheet annealing conditions within appropriate ranges.
• Controlling the "solution state of precipitates before rough rolling" to the above conditions is necessary to achieve a favorable final balance between the amount of relatively coarse precipitates (residual precipitates) that remain precipitated during the slab heating stage and the amount of relatively fine precipitates (re-precipitated precipitates) that precipitate after hot rolling.
• the above-mentioned slab soaking temperature refers to the surface temperature of the slab
• the slab soaking time refers to the time the slab's surface temperature is maintained after it reaches the soaking temperature. For example, this is affected by the steel composition and heating rate, but if the slab's surface temperature reaches the soaking temperature during slab heating, the solution state of the precipitates on the slab surface will be suitably controlled. Furthermore, if the slab's surface temperature is maintained for the soaking time after reaching the soaking temperature, the solution state of the precipitates will be suitably controlled all the way to the center of the slab.
• the specific value of the solution ratio there are no particular limitations on the specific value of the solution ratio.
• the "solution state of precipitates before rough rolling" can be favorably controlled by controlling the steel composition and slab heating conditions.
• the specific value of the solution ratio can be determined using integrated thermodynamic calculation software.
• “Thermo-Calc” is known as a commonly available integrated thermodynamic calculation software.
• the solution ratio was calculated from the chemical composition and temperature of the slab using "Thermo-Calc" (2019a ver.) and used as a reference.
• Hot rolling is carried out following the slab heating described above.
• Hot rolling is generally divided into rough rolling and finish rolling.
• it is important to control the rolling temperature and reduction rate of the rough rolling after controlling the "solution state of the precipitates before rough rolling" described above.
• the rolling temperature should be controlled to 940-1070°C and the reduction ratio to 82-95%.
• the reduction rate By setting the reduction rate within the above range, stress-induced precipitation occurs, making it possible to precipitate fine, large amounts of precipitates. If the rough rolling reduction rate is smaller than the above lower limit, fewer dislocations are introduced by the rolling process, and there are fewer precipitation sites available for stress-induced precipitation, resulting in larger precipitate particle sizes and a smaller Wp value.
• the upper limit of the rough rolling reduction rate is not particularly limited, but it may be set to 95%, taking into account the performance of the rolling mill, etc.
• the rolling temperature for rough rolling is higher than the above upper limit, deformation-induced precipitation of precipitates of MnS, AlN, Nb-group elements, etc. will occur on the higher temperature side of the precipitation nose or near the nose, resulting in a larger critical precipitation radius for the precipitates that re-precipitate during hot rolling. This reduces the size difference with the relatively coarse precipitates (residual precipitates) that have been precipitated since the slab heating stage, resulting in a smaller Wp value.
• there is no particular lower limit for the rolling temperature for rough rolling but since a lower temperature will harden the slab and reduce rollability, it is sufficient to roll at, for example, 940°C or higher.
• the rough rolling temperature is defined as the average of the start and end temperatures of rough rolling.
• Nb group elements when Nb group elements are suitably contained in the chemical composition, in addition to MnS and AlN, precipitates of Nb group elements (particularly carbides and nitrides) will precipitate during rough rolling. These Nb group element precipitates act as precipitation nuclei for the subsequent precipitation of MnS and AlN, resulting in finer re-precipitation of MnS and AlN. Therefore, when Nb group elements are suitably contained in the chemical composition, it is sufficient to control the various control conditions, such as the solution state of the precipitates (for example, the solution rate of the precipitates before rough rolling), the rough rolling temperature, and the rough rolling reduction, as described above.
• the solution state of the precipitates for example, the solution rate of the precipitates before rough rolling
• the rough rolling temperature for example, the rough rolling temperature
• rough rolling reduction as described above.
• the solution state of the precipitates is not controlled appropriately when an Nb group element is contained (for example, if the "solution rate of precipitates before rough rolling" is lower than 12% by volume), the precipitates will not be fully dissolved when the slab is heated, just as in the case where an Nb group element is not contained, and therefore fewer fine precipitates will re-precipitate during hot rolling. This reduces Wp and makes it impossible to sufficiently expand the secondary recrystallization progression temperature range during finish annealing.
• the solution state of the precipitates is not controlled appropriately when an Nb group element is contained (for example, if the "solution rate of precipitates before rough rolling" is higher than 85% by volume), then most of the precipitates will be dissolved when the slab is heated, just as in the case where an Nb group element is not contained, and therefore fewer relatively coarse precipitates (undissolved precipitates) will remain in the slab. This makes it impossible to sufficiently expand the secondary recrystallization progression temperature range during finish annealing.
• the reason why the rough rolling reduction rate should be controlled as above when Nb group elements are suitably contained, compared to when Nb group elements are not suitably contained, is thought to be as follows: When Nb group elements are contained, precipitates of Nb group elements are more likely to precipitate finely in the steel, so the number of fine precipitates contained in the steel even before rough rolling is greater, compared to when Nb group elements are not contained. Therefore, when Nb group elements are contained, there are more precipitation sites for precipitates, and deformation-induced precipitation is more likely to occur even when the reduction rate is lowered. For this reason, it is thought that the rough rolling reduction rate should be controlled as above.
• the rough rolling reduction is less than 82%, as in the case where Nb group elements are not contained, fewer dislocations are introduced by the rolling process, and there are fewer precipitation sites available for processing-induced precipitation, resulting in larger precipitate particle sizes and a smaller Wp value.
• the upper limit of the rough rolling reduction is preferably 93%.
• the reason why the rough rolling temperature should be controlled as described above when Nb group elements are suitably contained, compared to when Nb group elements are not suitably contained, is thought to be as follows.
• Nb group elements are contained, as mentioned above, the number of fine precipitates contained in the steel even before rough rolling is greater, compared to when Nb group elements are not contained. Therefore, when Nb group elements are contained, there are more precipitation sites for precipitates, and the precipitates that re-precipitate during hot rolling are more likely to be fine. For this reason, it is thought that the rough rolling temperature should be controlled as described above.
• the rolling temperature for rough rolling is higher than 1070°C, all precipitates of MnS, AlN, and Nb group elements will undergo processing-induced precipitation at temperatures higher than the precipitation nose of the precipitate, and the critical precipitation radius of the precipitates that re-precipitate during hot rolling will become larger. As a result, the size difference between these precipitates and the relatively coarse precipitates (residual precipitates) that have been precipitated since the slab heating stage will become smaller, resulting in a smaller Wp value.
• the upper limit of the rolling temperature for rough rolling is preferably 1065°C, and more preferably 1040°C.
• the size and distribution of precipitates are favorably controlled.
• the particle size-number density distribution of precipitates is controlled within the above range.
• the slab soaking temperature during slab heating before rough rolling and the rolling temperature during rough rolling are controlled with a purpose. These temperatures are not caused by a natural temperature drop that occurs when the slab is removed from the slab heating furnace and subjected to rough rolling.
• the slab soaking temperature and rough rolling temperature are not controlled with a purpose.
• the slab soaking temperature and rough rolling temperature are controlled with a purpose. For example, even if the slab soaking temperature is high within the above range, the rough rolling temperature is controlled to be within the above range. Similarly, even if the slab soaking temperature is low within the above range, the rough rolling temperature is controlled to be within the above range.
• the conditions for finish rolling in the hot rolling process are not particularly limited, and normal hot rolling conditions may be used.
• the hot-rolled sheet annealing process is a process in which the hot-rolled steel sheet after the hot-rolling process is annealed to obtain a hot-rolled annealed steel sheet.
• the hot-rolled sheet annealing process is generally performed to control the steel sheet structure, such as the recrystallization rate, residual strain, and crystal grain size, and to preferably adjust the morphology of precipitates in the steel by annealing the hot-rolled steel sheet after the hot-rolling process.
• the annealing conditions for the hot-rolled sheet annealing process may be the hot-rolled sheet annealing conditions described below.
• the precipitates contained in the hot-rolled annealed steel sheet after the hot-rolled sheet annealing process are controlled to have a particle size-number density distribution within the ranges described above.
• the hot-rolled steel sheet after the hot rolling process is heated and subjected to first-stage annealing for recrystallization in a temperature range of 1040 to 1080°C.
• Second-stage annealing is then performed at a lower temperature range of 810 to 880°C.
• the steel sheet is then cooled at an average cooling rate of 5 to 80°C/s.
• the first-stage annealing temperature is preferably 1040 to 1060°C.
• the second-stage annealing temperature is preferably 830 to 870°C.
• the average heating rate to the first-stage annealing temperature is preferably 5°C/s or more.
• the steel sheet is preferably held for 20 seconds or more during second-stage annealing.
• the average cooling rate after second-stage annealing is preferably 10°C/s or more, and more preferably 20°C/s or more. While there is no particular upper limit to the average cooling rate, to prevent breakage during cold rolling, the average cooling rate is preferably 50°C/s or less, and more preferably less than 40°C/s.
• the average cooling rate mentioned above refers to the temperature range from the second annealing temperature to 500°C divided by the time required for cooling.
• the first-stage annealing temperature 1040-1080°C
• annealing at a temperature higher than 1080°C is likely to increase the amount of AlN that goes into solution in the first-stage annealing temperature range, and the number of fine AlN particles that precipitate during second-stage annealing is likely to increase.
• Nb-group elements are likely to attach to the fine AlN and precipitate during cooling after second-stage annealing, which may result in a small Wp.
• annealing at a temperature lower than 1040°C makes it difficult for precipitates due to Nb-group elements to go into solution sufficiently, and the number of precipitates due to Nb-group elements that precipitate during cooling after second-stage annealing is likely to decrease, which may result in a small Wp.
• the precipitates of Nb-group elements that precipitate during subsequent cooling can be preferably finely precipitated, thereby preferably increasing Wp.
• the subsequent cooling begins in the high-temperature range, making it easier for large Nb-group elements to precipitate during cooling, which can result in a small Wp.
• the average grain size of the hot-rolled annealed steel sheet (steel sheet after hot-rolled sheet annealing) can be controlled to preferably 20.0 to 21.5 ⁇ m.
• the method for manufacturing hot-rolled annealed steel sheet comprises a casting process, a hot-rolling process, and a hot-rolled sheet annealing process.
• Hot-rolled annealed steel sheet manufactured by comprehensively controlling the above conditions in each process has favorably controlled precipitate size and distribution, and the particle size-number density distribution of the precipitates is controlled within the above range.
• the secondary recrystallization progression temperature range is expanded during finish annealing, the selective growth of Goss-oriented grains is enhanced, and the magnetic flux density of the grain-oriented electrical steel sheet is improved.
• the amount of relatively coarse precipitates (residual precipitates) that remain precipitated during the slab heating stage is controlled primarily by the slab soaking temperature and time during slab heating before rough rolling, and the amount of relatively fine precipitates (re-precipitated precipitates) is controlled by the subsequent manufacturing conditions, thereby controlling the characteristics of the hot-rolled annealed steel sheet within the above-mentioned ranges.
• the secondary recrystallization progression temperature range is expanded during finish annealing, the selective growth of Goss-oriented grains is enhanced, and the magnetic flux density of the grain-oriented electrical steel sheet is improved.
• the manufacturing method for grain-oriented electrical steel sheet includes a cold rolling process, a decarburization annealing process, an annealing separator application process, and a finish annealing process. If necessary, it may also include an insulating coating formation process and a magnetic domain control process. These processes may employ well-known, general process conditions. Below, we will explain an example of a manufacturing method that applies nitriding treatment as a low-temperature slab heating process.
• the cold rolling step is a step of cold rolling the hot-rolled annealed sheet obtained in the hot-rolled sheet annealing step once, or cold rolling the hot-rolled annealed sheet multiple times (two or more times) via annealing (intermediate annealing) (for example, a total cold rolling rate of 80 to 95%) to obtain a cold-rolled steel sheet having a thickness of, for example, 0.10 to 0.50 mm.
• the decarburization annealing step is a step in which the cold-rolled steel sheet obtained in the cold rolling step is subjected to decarburization annealing (for example, at 700 to 900°C for 1 to 3 minutes) to obtain a decarburization annealed steel sheet in which primary recrystallization has occurred.
• decarburization annealing By subjecting the cold-rolled steel sheet to decarburization annealing, C contained in the cold-rolled steel sheet is removed.
• the decarburization annealing is preferably performed in a humid atmosphere in order to remove "C" contained in the cold-rolled steel sheet.
• the nitriding treatment is carried out to adjust the strength of the inhibitor in secondary recrystallization.
• the nitrogen content of the steel sheet may be increased to about 40 to 300 ppm at any timing between the start of the above-mentioned decarburization annealing and the start of secondary recrystallization in the finish annealing described below.
• nitriding treatment examples include a treatment in which a steel sheet is annealed in an atmosphere containing a gas with nitriding ability such as ammonia, and a treatment in which a decarburization-annealed steel sheet coated with an annealing separator containing a powder with nitriding ability such as MnN is finish-annealed.
• the annealing separator application step is a step of applying an annealing separator to the decarburized annealed steel sheet.
• an annealing separator containing MgO as a main component or an annealing separator containing alumina as a main component can be used.
• the decarburized annealed steel sheet is wound into a coil and then finish-annealed in the next finish-annealing process.
• the final annealing step is a step in which the decarburized annealed steel sheet coated with the annealing separator is subjected to final annealing to cause secondary recrystallization.
• the growth of primary recrystallized grains is suppressed by an inhibitor, and secondary recrystallization is allowed to proceed, thereby preferentially growing ⁇ 110 ⁇ 001> oriented grains and improving magnetic flux density.
• the temperature range in which secondary recrystallization progresses expands during finish annealing, causing preferential growth of ⁇ 100 ⁇ 011> oriented grains to an extent not previously seen, resulting in a dramatic improvement in magnetic flux density.
• abnormal grain growth of secondary recrystallized grains occurs during finish annealing, and after finish annealing, the secondary recrystallized grains occupy the entire sheet surface. The few secondary recrystallized grains cover the entire steel sheet surface, and the grain size of each secondary recrystallized grain increases.
• the finish annealing conditions for "expanding the secondary recrystallization progression temperature range" disclosed in the above-mentioned Patent Documents 9 to 11 may be applied as needed.
• the secondary recrystallization progression temperature range can be expanded even more favorably.
• the following insulating coating formation process and magnetic domain control process are not necessary from the viewpoint of concentrating the crystal orientation in ⁇ 110 ⁇ 001>, but are steps that are adopted for general grain-oriented electrical steel sheets to improve practical magnetic properties.
• the insulating coating formation step is a step of forming an insulating coating on the grain-oriented electrical steel sheet (finish-annealed steel sheet) after the finish-annealing step.
• An insulating coating mainly composed of phosphate and colloidal silica or an insulating coating mainly composed of alumina sol and boric acid may be formed on the steel sheet after the finish-annealing step.
• the magnetic domain control step is a step of subdividing the magnetic domains of the grain-oriented electrical steel sheet. This step is carried out at an appropriate timing after cold rolling. For example, localized micro-strains or localized grooves may be formed in the grain-oriented electrical steel sheet by a known method such as laser, plasma, mechanical method, or etching.
• Grain-oriented electrical steel sheet obtained using the hot-rolled annealed steel sheet according to this embodiment A brief description will be given of the grain-oriented electrical steel sheet produced using the hot-rolled annealed steel sheet according to this embodiment.
• the hot-rolled annealed steel sheet according to this embodiment has both relatively fine precipitates and relatively coarse precipitates of a preferred size and distribution. Therefore, in the grain-oriented electrical steel sheet obtained using the hot-rolled annealed steel sheet according to this embodiment, Goss-oriented grains grow preferentially, resulting in a preferred increase in magnetic flux density. Furthermore, the grain-oriented electrical steel sheet manufactured using the hot-rolled annealed steel sheet according to this embodiment does not suffer from a deterioration in other properties due to the increased magnetic flux density, and can therefore be used in the same applications as conventional ones.
• the grain-oriented electrical steel sheet manufactured using the hot-rolled annealed steel sheet according to this embodiment contains, as a base element (major alloying element), 2.0 to 7.0% Si (silicon) by mass fraction.
• Impurities refer to elements that are mixed in from raw materials such as ore or scrap during industrial steel production, or from the manufacturing environment, etc.
• the upper limit for the total impurity content may be, for example, 5%.
• optional elements may be contained.
• optional elements such as Nb, V, Mo, Ta, W, C, Mn, S, Se, Al, N, Cu, Bi, B, P, Ti, Sn, Sb, Cr, and Ni may be contained.
• These optional elements may be contained according to the purpose. Therefore, there is no need to set a lower limit for these optional elements, and the lower limit may be 0%.
• These optional elements may also be contained as impurities.
• grain-oriented electrical steel sheet undergoes relatively large changes in chemical composition (reduction in content) through decarburization annealing and purification annealing during secondary recrystallization.
• Purification annealing can reduce the content of some elements to a level that cannot be detected by standard analytical methods (1 ppm or less).
• the chemical composition of the final product differs from that of the starting material, the slab, but the optional elements listed above are elements contained in the slab that remain in the final product, and the content of each element will not exceed the content range stated above for the slab, but will be within a range that depends on the content in the slab and the subsequent manufacturing process.
• the above chemical composition is that of grain-oriented electrical steel sheet. If the grain-oriented electrical steel sheet to be measured has an insulating coating on its surface, remove the coating using a known method before measuring the chemical composition.
• the grain-oriented electrical steel sheet manufactured using the hot-rolled annealed steel sheet according to this embodiment may have an intermediate layer disposed in contact with the grain-oriented electrical steel sheet (silicon steel sheet), and an insulating coating disposed in contact with the intermediate layer.
• the intermediate layer may be a layer mainly made of oxide, a layer mainly made of carbide, a layer mainly made of nitride, a layer mainly made of boride, a layer mainly made of silicide, a layer mainly made of phosphide, a layer mainly made of sulfide, or a layer mainly made of an intermetallic compound.
• These intermediate layers are formed primarily to ensure adhesion between the silicon steel sheet and the insulating coating, and may be any known layer formed by heat treatment in an atmosphere with controlled oxidation-reduction, chemical vapor deposition (CVD), physical vapor deposition (PVD), or the like.
• Typical examples of the insulating coating include an insulating coating that is primarily composed of phosphate and colloidal silica and has an average thickness of 0.1 to 10 ⁇ m, and an insulating coating that is primarily composed of alumina sol and boric acid and has an average thickness of 0.5 to 8 ⁇ m.
• Hot-rolled and annealed steel sheets were manufactured using slabs with the chemical compositions shown in Tables 1 and 2.
• the chemical compositions of the manufactured hot-rolled and annealed steel sheets were equivalent to the chemical compositions of the slabs shown in Tables 1 and 2. These chemical compositions were measured based on the method described above. In Tables 1 and 2, "-" indicates that no control or manufacturing was carried out with the content in mind, and that the content was not measured.
• the above hot-rolled and annealed steel sheets were manufactured based on the manufacturing conditions shown in Tables 3 to 9.
• Slab heating was performed by soaking at a specified temperature for a specified time without temporarily increasing the heating temperature during the slab heating process.
• the soaking temperature shown in the table indicates the surface temperature of the slab after heating, and the soaking time shown in the table indicates the slab heating time from when the slab surface temperature reached the soaking temperature.
• the hot-rolled steel sheet after the hot rolling process was annealed.
• the hot-rolled steel sheet after the hot rolling process was annealed under the annealing conditions shown in Tables 3 to 9.
• the heating rate to the first-stage annealing temperature was an average of 5°C/second or more
• the holding time in second-stage annealing was 20 seconds or more.
• second-stage annealing was not performed.
• the average grain size and precipitation morphology of the produced hot-rolled annealed steel sheets were investigated using the methods described above.
• the precipitation morphology of precipitates with a circular equivalent diameter D of 50 to 1000 nm is shown in Tables 10 to 16.
• Dp represents the most frequent diameter
• f(Dp) represents the number density of the most frequent diameter
• Wp represents the half-width of the most frequent diameter.
• the hot-rolled and annealed steel sheets were then cold-rolled and decarburized under known conditions.
• the cold-rolling reduction was 90.7% and the sheet thickness was 0.26 mm.
• the decarburization annealing temperature was between 830 and 860°C, and the steel sheets were annealed for 90 seconds.
• the steel sheets were then nitrided in a mixed atmosphere of hydrogen, nitrogen, and ammonia, resulting in a nitrogen content of 0.020 to 0.023 mass% (200 to 230 ppm).
• a coating solution for forming an insulating coating primarily composed of phosphate and colloidal silica with chromium, was applied to the primary coating (intermediate layer) formed on the surface of the manufactured grain-oriented electrical steel sheet (finish-annealed steel sheet). The sheet was then heated and held in an atmosphere of 75% by volume hydrogen:25% by volume nitrogen, and then cooled to form an insulating coating.
• the manufactured grain-oriented electrical steel sheet When viewed on a cross section with the cutting direction parallel to the thickness direction, the manufactured grain-oriented electrical steel sheet had an intermediate layer placed in contact with the grain-oriented electrical steel sheet (silicon steel sheet) and an insulating coating placed in contact with this intermediate layer.
• the intermediate layer was a forsterite coating with an average thickness of 2 ⁇ m
• the insulating coating was an insulating coating primarily composed of phosphate and colloidal silica with an average thickness of 1 ⁇ m.
• the obtained grain-oriented electrical steel sheets were evaluated for various properties. The evaluation results are shown in Tables 10 to 16.
• the magnetic flux density B8 (T) in the rolling direction of the steel sheet when excited at 800 A/m was measured.
• a magnetic flux density B8 of 1.945 T or more was judged to be acceptable.
• iron loss W17 /50 (W/kg) defined as the power loss per unit weight (1 kg) of the steel sheet, was also measured under conditions of an AC frequency of 50 Hz and an excitation magnetic flux density of 1.7 T.
• the particle size-number density distribution of the precipitates contained in the hot-rolled annealed steel sheet was well controlled in the inventive examples, and all of them exhibited excellent magnetic flux density as grain-oriented electrical steel sheets.
• the comparative examples did not have well controlled particle size-number density distribution of the precipitates contained in the hot-rolled annealed steel sheet, and did not achieve the magnetic flux density desirable for grain-oriented electrical steel sheets.
• the above aspects of the present invention make it possible to provide a hot-rolled annealed steel sheet for grain-oriented electrical steel sheet that can increase magnetic flux density, as well as a manufacturing method thereof, and therefore have high industrial applicability.

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