Manufacturing method of grain-oriented electromagnetic steel strip

Abstract

To provide a method for realizing an industrial standard technology of growing a grain group having a crystal orientation on the whole of a steel strip by joining a directional electromagnetic steel strip having a Goss crystal orientation with a steel strip after primary recrystallization which does not have a certain crystal orientation.SOLUTION: In a manufacturing method of the directional electromagnetic steel strip, the distribution of the grain in a predetermined region near the joined surface is set to an area fraction of the grain having a grain size of 1.8 times or more with respect to the average grain size of the whole steel strip excluding the welding heat affected zone of the steel strip: less than 70%, and an area fraction of the oxide: less than 5%. Thereafter, in a direction perpendicular to the rolling direction, a temperature gradient in which the directional electromagnetic steel strip side is raised to a high temperature is applied to induce grain growth.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing a grain-oriented electrical steel strip having low iron loss and suitable for use as an iron core material for a transformer or the like. [Background technology]

[0002] Grain-oriented electrical steel strips are soft magnetic materials that are mainly used as iron core materials for transformers and rotating machines, and are required to have high magnetic flux density, low iron loss, and low magnetostriction as magnetic properties. To achieve these properties, the secondary recrystallized grains in the steel strips are oriented in the {110} direction. <001> It is important to concentrate the grains in the Goss orientation to a high degree and to reduce impurities in the steel strip.

[0003] The iron loss of grain-oriented electrical steel strips is expressed as the sum of hysteresis loss, which depends on factors such as crystal orientation and purity, and eddy current loss, which depends on factors such as sheet thickness, resistivity, and magnetic domain size. Therefore, known methods for reducing iron loss include reducing hysteresis loss by increasing the degree of integration of the crystal orientation into the Goss orientation to improve magnetic flux density, and reducing eddy current loss by increasing the content of elements such as Si, which increase electrical resistance, reducing the thickness of the steel strip, or subdividing the magnetic domains.

[0004] Among these methods for reducing iron loss, a commonly used technique for improving magnetic flux density is to use a precipitate called an inhibitor when manufacturing grain-oriented electrical steel strips, to create a mobility difference at the grain boundaries during secondary recrystallization annealing, thereby causing preferential growth of only the Goss orientation.

[0005] For example, Patent Document 1 discloses a method using AlN or MnS as an inhibitor, and Patent Document 2 discloses a method using MnS or MnSe as an inhibitor, both of which are industrially put into practical use as production methods that require slab heating at high temperatures.

[0006] However, the techniques disclosed in Patent Documents 1 and 2 above have the problem that the slab needs to be heated to a high temperature of 1250° C. or higher in the heating step prior to hot rolling, which requires a great deal of energy.

[0007] To solve the above problems, Patent Document 3, for example, proposes a so-called "nitriding treatment technology" in which inhibitor-forming elements, such as Al, N, Mn, S, and Se, are not completely dissolved in the steel during the slab heating stage, and after decarburization annealing, nitriding treatment is performed while the steel strip is running, thereby forming inhibitors whose main component is (Al, Si)N, and thereby achieving magnetic properties comparable to those achieved during high-temperature slab heating even during low-temperature slab heating at 1200°C or less.

[0008] On the other hand, Patent Document 4 proposes a technique for developing Goss-oriented crystal grains without containing inhibitor-forming components. In this technique, by eliminating impurities such as inhibitor-forming components as much as possible, the grain boundary energy of the grain boundaries during primary recrystallization is made more apparent, and secondary recrystallization of crystal grains having the Goss orientation can be achieved without using an inhibitor. This effect of controlling recrystallization by texture is called the texture inhibition effect.

[0009] In this technique, since an inhibitor is not used, there is no need to perform high-temperature purification annealing after secondary recrystallization annealing. In addition, since there is no need to finely disperse the inhibitor in the steel in advance, there is no need to heat the steel slab at high temperatures. Therefore, this technique, which does not use an inhibitor, has significant advantages in terms of cost and maintenance. Furthermore, since there is no need to heat the slab at high temperatures, it can also be applied to a technique in which thin slabs are produced and directly hot-rolled.

[0010] Furthermore, although grain-oriented electrical steel strips are polycrystalline bodies mainly composed of Goss-oriented crystal grains of about 10 mm in size, they can also be considered as an aggregate of single crystals with Goss orientation. For this reason, studies have been conducted in the past to apply single crystal growth techniques, particularly those used in the semiconductor field, to directly grow Goss-oriented crystal grains without using inhibitors or texture formation techniques.

[0011] For example, Patent Documents 5 and 6 disclose a technology for increasing the degree of concentration in the Goss orientation and improving magnetic flux density by appropriately controlling the temperature gradient when performing secondary recrystallization annealing by applying a temperature gradient to the boundary between the primary recrystallization region and the secondary recrystallization region. <001> It is possible to preferentially grow grains with a crystal orientation close to the so-called just Goss orientation that matches the orientation.

[0012] Patent Document 7 discloses a technique for obtaining a Goss-oriented crystal by grain boundary migration after bonding a single crystal after secondary recrystallization as a seed crystal with a primary recrystallized grain at an active bonding surface. Furthermore, it also proposes chemical polishing of the bonding surface to form a clean and smooth bonding surface, and ensuring the wettability of the lap bonding surface by Sn plating. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] Special Publication No. 40-15644 [Patent Document 2] Special Publication No. 51-13469 [Patent Document 3] Japanese Patent Application Publication No. 05-112827 [Patent Document 4] Japanese Patent Application Laid-Open No. 2001-129356 [Patent Document 5] Special Publication No. 58-50295 [Patent Document 6] Japanese Patent Application Publication No. 61-190017 [Patent Document 7] Japanese Patent Application Publication No. 02-263926 Summary of the Invention [Problem to be solved by the invention]

[0014] However, although the techniques described in Patent Documents 1 to 4 produce grain-oriented electrical steel strips by using inhibitors or texture formation techniques to preferentially induce secondary recrystallization in the Goss orientation, which is advantageous for magnetic properties, there is a problem in that variations in the orientation of secondary recrystallized grains, which are primarily in the Goss orientation, occur due to variations in the steel tapping composition and variations in each manufacturing process.

[0015] Furthermore, the techniques described in Patent Documents 5 and 6 cannot control the Goss orientation crystals formed in the initial stage of growth of the crystal orientation grains, and the influence of the orientation variations on the magnetism cannot be ignored.

[0016] Furthermore, although Patent Document 7 describes a technology for eliminating the uncertainty of the Goss orientation crystal grains formed in the initial stage, this technology is at the laboratory level and has not yet been realized as an industrial product for continuous and large-area processing.

[0017] The present invention aims to propose a method for industrially realizing a technology that involves joining a directional electrical steel strip having a Goss orientation crystal orientation with a post-primary recrystallization steel strip (hereinafter simply referred to as the two types of steel strip) that does not have such a crystal orientation, thereby stably growing a group of crystal grains with a specific crystal orientation (Goss orientation) throughout the entire post-primary recrystallization steel strip from the very beginning, a technology that has been disclosed for a relatively long time but has not yet been industrially commercialized. [Means for solving the problem]

[0018] The inventors considered that there are two main elemental technologies that can solve the above problems. First, we investigated the joining conditions necessary to move the grain boundaries of a seed crystal with a specific orientation and cover the entire steel strip with crystal grains of a specific orientation after primary recrystallization of the polycrystalline body.

[0019] Grain-oriented electrical steel strips are generally provided in coil form with thinned sheet thickness to reduce iron loss, and are long products measuring several thousand meters in the longitudinal direction. Achieving specific grain growth in the longitudinal direction of the coil takes an enormous amount of time and is therefore unrealistic. Therefore, assuming a manufacturing method that grows grains in the coil width direction, which is about 1 meter, it is rational to perform joining on the side of the coil that is parallel to the longitudinal rolling direction.

[0020] The mother coil containing the seed crystal grains was made from a directional electrical steel strip after secondary recrystallization, which had been accumulated in the Goss orientation.The child coil in which the Goss-oriented grains were grown was made from a general steel strip after primary recrystallization, containing approximately 3% Si, which had been cold-rolled to the final thickness and then annealed for primary recrystallization.A detailed study was conducted on the joining method using electron beam butt welding, taking into account the cleanliness of the joining surface and handling after welding.

[0021] By adjusting the welding speed, acceleration voltage, cathode diameter, beam current, etc., butt welding of two types of steel strips with a thickness of 0.23 mm was performed at various energy intensities per unit time and unit length without using any weld metal material.

[0022] Because the thickness of the welding material is thin, the beam penetrates the butt joint under welding conditions that require keyhole penetration, which is a characteristic of electron beam welding, making it difficult to form a good weld.However, it was found that combining this with techniques such as under- or over-focusing the beam to expand the beam irradiation area or electrically oscillating the beam within the plate surface was effective.

[0023] Furthermore, although the lower the welding speed, the fewer defects and better weld quality can be obtained, in the case of the present invention, at low speeds the heat-affected zone at the butt joint is likely to expand and problems such as the beam penetrating the joint and not being able to form a good weld are likely to occur, so it was found that a high speed of 10 m / min or more is better.

[0024] In the present invention, "joined" refers to a state in which the steel strips do not separate during continuous welding. Therefore, it is not necessary for the entire thickness of the steel plate to be welded, and it also includes a state in which the steel plate is partially welded in the thickness direction or longitudinal direction of the plate surface in order to obtain a small area of ​​the heat-affected zone.

[0025] Secondly, the effect of grain coarsening in the heat-affected zone caused by welding of two steel strips on the growth of Goss-oriented grains was investigated.

[0026] Depending on the welding conditions, the grains in the heat-affected area tend to coarsen, and the larger the heat input per unit area and unit time, the more pronounced the grain coarsening. However, if the heat input is too large, the ends of the two butted steel strips will melt and separate, making welding impossible. On the other hand, if the heat input is too small, the welding will be insufficient and the steel strips will easily separate, making it impossible to ensure weld strength, so it is necessary to maintain appropriate welding conditions.

[0027] Furthermore, it was found that the relationship between the distribution of crystal grains in the region of the steel strip after primary recrystallization, surrounded by the welded surface and a width equal to the thickness of the steel strip after primary recrystallization from the welded surface, and the average grain size of the entire steel strip after primary recrystallization excluding the weld heat-affected zone, and the area ratio of oxides in the above region, are the most important requirements for the growth of crystal grains having the Goss orientation beyond the heat-affected zone near the welded portion.

[0028] In other words, it was confirmed that when the distribution of crystal grains in the region of the primary recrystallized steel strip surrounded by the joined surface and a width equal to the thickness of the primary recrystallized steel strip from the joined surface is such that the area ratio of crystal grains having a grain size 1.8 times or more the average grain size of the entire primary recrystallized steel strip excluding the weld heat affected zone of the primary recrystallized steel strip is less than 70% and the area ratio of oxides is less than 5%, the growth of crystal grains having the Goss orientation proceeds smoothly beyond the heat affected zone near the joint and to areas not affected by the heat from welding.

[0029] Furthermore, the development history of the present invention will be explained in detail. First, we investigated the welding method. Butt welding is considered to be the most efficient method for joining thin steel strips, typically 0.23 mm thick, in the rolling direction. While welding metal wires is typically used to weld metal to metal, the present invention relies on the presence of dissimilar weld metals between butted steel strips, which inhibits grain growth across the weld joint surface. Therefore, the present invention employs butt welding, which does not use weld metal.

[0030] In butt welding of such thin steel strips, it is extremely important to accurately irradiate the beam onto the butt joint. However, when the materials to be joined are dissimilar metals, it has been reported that electron beam welding can cause the electron beam to deflect due to electromagnetic effects caused by thermoelectric power. However, in the present invention, since both the grain-oriented electrical steel strip after secondary recrystallization and the steel strip after primary recrystallization are Fe-Si alloys containing approximately 3% Si, the influence of beam deflection was not particularly apparent. We also investigated the use of laser welding for butt welding of such thin steel strips, but because laser light is not subject to electromagnetic influences, the problem of beam deflection did not arise.

[0031] Next, the effect of grain coarsening in the heat-affected zone on the growth of Goss-oriented grains was investigated. Test pieces were cut from the butt-welded steel strip at right angles to the rolling, i.e., perpendicular (vertical) to the weld surface, with a width of 10 mm and a length of 200 mm, including the weld. The cutting was performed by shearing. The test pieces were moved in a zone melting furnace near the weld while being locally heated, and the conditions for the growth of Goss grains across the weld toward the primary recrystallized grains and the crystalline structure near the weld surface were analyzed.

[0032] The zone melting furnace used was a solenoid-type induction heating system, which in principle makes it impossible to induction heat a 0.23 mm thick test piece above its Curie point of approximately 730°C. Therefore, the test piece was placed inside a ring-shaped heated object called a carbon susceptor, which was heated by induction heating, thereby indirectly heating the 0.23 mm test piece locally. It was confirmed that this method could be used to locally heat the test piece to above 1250°C, and that a temperature gradient could be created relative to the surrounding area.

[0033] The local heating temperature was changed from 950°C to 1200°C, and the moving speed of the local heating area was changed from 50mm / hour to 250mm / hour. The local heating area of ​​the crystal joint was moved, and after cooling, the success or failure of the growth of crystal grains with Goss orientation was checked visually, or if this was difficult to determine, orientation analysis was performed using EBSD, and the effect of the distribution of crystal grain size in the heat-affected area near the butt joint on the growth of crystal grains with Goss orientation was clarified.

[0034] When the thermal effects of welding are large, the grain size on the primary recrystallization side becomes larger and the range of coarsening also becomes wider, but this also increases the tendency for the erosion of Goss crystal grains to be inhibited. Therefore, by sorting out these conditions, the findings of the present invention were obtained.

[0035] Furthermore, by performing SEM observations of the area near the butt-welded joint surface, the distribution of grain size in the heat-affected area and the amount of oxide generation were analyzed under various welding conditions, and the conditions under which such welds maintain a clean joint end surface and allow grains to grow beyond the joint surface were investigated.

[0036] First, cross-sectional SEM images were used to observe the structure near the weld interface, and attention was focused on the region d (mm) inward (on the primary recrystallization side) from the butted weld interface, where the plate thickness is d (mm). That is, for a region with a square cross section (hereinafter simply referred to as the square region) that is the plate thickness d (mm) of the steel strip after primary recrystallization and the distance d (mm) from the weld edge, the change in crystal grain size and the amount of oxide formation in this square region were evaluated.

[0037] After welding, the steel strip has two sides, one with Goss-oriented grains and the other with primary recrystallized grains, but it was confirmed that the amount of oxide generated is almost the same. Furthermore, because the grain size can only be evaluated on the steel strip side with primary recrystallization, the conditions for grain growth in the zone melting furnace were carefully examined based on a quantitative evaluation on the steel strip side after primary recrystallization.

[0038] As a result, it was found from the observation of the cross section that an area ratio of oxides in the cross section must be less than 5% in order to realize grain growth beyond the bonding surface. In other words, it was found that if oxides exist in the structure of the steel strip after primary recrystallization in an area ratio of 5% or more relative to the square region, encroachment of the primary recrystallized grains by Goss grains does not progress.

[0039] To achieve these conditions, it is advantageous to use a welding method that operates in a non-oxidizing atmosphere or in a vacuum, where there is no oxygen and no oxides form on the end surfaces of the steel strip. Electron beam welding requires a vacuum to emit the electron beam, and if the appropriate vacuum conditions are maintained, no oxides will form on the joint surfaces of the weld. Even with laser welding, oxidation can be suppressed by shielding the area near the weld with a non-oxidizing gas or by replacing the gas, which can reduce the amount of oxides that form on the joint surfaces of the weld. In either case, the area ratio of oxide can be reduced to less than 5% by keeping the oxygen concentration in the atmosphere near the weld to about 100 Pa or less in a vacuum, or to about several tens of ppm or less in the atmosphere.

[0040] In addition, the heat-affected zone formed by welding heat input undergoes localized grain coarsening. Therefore, the conditions under which Goss-oriented grains can encroach across the weld interface onto the partially coarsened primary recrystallized grains were investigated by statistically collating the grain sizes in the square region described above.

[0041] As a result, it was found that in the above-mentioned square region, if the area ratio of crystal grains having a grain size 1.8 times or more the average crystal grain size of the entire steel strip after primary recrystallization excluding the weld heat-affected zone (equivalent to the steel strip after primary recrystallization before welding) is less than 70%, it is possible to cause crystal grains with Goss orientation to grow by encroaching on the primary recrystallized grains by providing a driving force for the crystal grains to grow in a specific direction, such as a temperature gradient.

[0042] Here, the radius of the crystal grains that have become coarse due to the influence of heat is the radius when the crystal grain is considered to be equivalent to a circle, since the coarse grains may be elongated in the cooling direction. That is, when the area ratio of crystal grains having a grain size 1.8 times or more was 70% or more, the growth of crystal grains having the Goss orientation stopped, and they were unable to encroach on the primary recrystallized grains.

[0043] It was also found that the welding joining conditions do not need to be uniform across the entire thickness of the plate. In other words, rather than focusing the beam exactly on the steel strip surface under so-called keyhole conditions, it is effective to intentionally shift the focus, melt the irradiated surface more, and reduce the heat input to the non-irradiated surface under so-called heat conduction welding conditions. Since the beam does not penetrate to the non-irradiated surface and welding is not completed across the entire thickness of the plate, it is necessary to consider the joint strength after welding, but it is possible to suppress the area ratio of coarsening of crystal grain size due to thermal effects.

[0044] The present invention was completed based on these findings and further investigations. That is, the gist of the present invention is as follows. 1.{110} <001> At least one of the widthwise ends of the grain-oriented electrical steel strip after secondary recrystallization, which has been accumulated in the orientation, is joined to at least one of the widthwise ends of the steel strip after primary recrystallization that has been finished to the final thickness, and the distribution of crystal grains in a region on the side of the steel strip after primary recrystallization, which is perpendicular to the joined surface and extends in a width equal to the thickness of the steel strip after primary recrystallization, is adjusted to an area ratio of crystal grains having a grain size 1.8 times or more the average grain size of the entire steel strip after primary recrystallization excluding the weld heat affected zone of the steel strip after primary recrystallization: less than 70%, and an area ratio of oxides: less than 5%. Then, the grain-oriented electrical steel strip is heated in a direction perpendicular to the rolling direction and perpendicular to the joined surface, while applying a temperature gradient in which the grain-oriented electrical steel strip side is heated. <001> A method for manufacturing a grain-oriented electrical steel strip in which oriented crystal grains grow toward the steel strip after the primary recrystallization.

[0045] 2. The method for producing a grain-oriented electrical steel strip according to 1 above, wherein the temperature gradient is 0.1°C / mm or more.

[0046] 3. A method for manufacturing a grain-oriented electromagnetic steel strip as described in 1 or 2 above, in which the grain-oriented electromagnetic steel strip obtained by the manufacturing method described in 1 or 2 above is joined to a primarily recrystallized steel strip that has been finished to its final thickness, and the newly formed grain-oriented electromagnetic steel strip is made into a primarily recrystallized steel strip. [Effects of the Invention]

[0047] According to this invention, by applying single crystal growth technology, a completely new manufacturing process for grain-oriented electrical steel strips is proposed, making it possible to supply grain-oriented electrical steel strips with stable and excellent magnetic properties at an industrial level. DETAILED DESCRIPTION OF THE INVENTION

[0048] Preferred production conditions of the present invention will be described below. First, the preferred chemical composition of the material will be described. The chemical composition of the grain-oriented electrical steel strip after secondary recrystallization, which is mainly composed of Goss-oriented grains, can be appropriately determined from known chemical compositions. The chemical compositions specifically described below are merely examples.

[0049] In the present invention, when an inhibitor is used, for example, an AlN-based inhibitor contains appropriate amounts of Al and N, and when an MnS / MnSe-based inhibitor is used, an appropriate amount of Mn and Se and / or S is added. Of course, both inhibitors may be used in combination. In this case, the preferred contents of Al, N, S, and Se are 0.01 to 0.065 mass% Al, 0.005 to 0.012 mass% N, 0.005 to 0.03 mass% S, and 0.005 to 0.03 mass% Se, respectively.

[0050] The present invention can also be used in grain-oriented electrical steel strips that do not use inhibitors and have limited contents of Al, N, S, and Se. In this case, the amounts of Al, N, S, and Se are preferably controlled to 100 ppm by mass or less for Al, 50 ppm by mass or less for N, 50 ppm by mass or less for S, and 50 ppm by mass or less for Se, respectively.

[0051] Other basic components and optional additional components are as follows: C: 0.08% by mass or less If the C content exceeds 0.08 mass%, the burden of reducing the C content to 50 mass ppm or less, at which magnetic aging does not occur during the manufacturing process, increases, so it is preferable to set the C content to 0.08 mass% or less. Note that there is no need to set a lower limit, as secondary recrystallization is possible even in materials that do not contain C.

[0052] Si:2.0~8.0% by mass Silicon is an element effective in increasing the electrical resistance of steel and improving iron loss, and a silicon content of 2.0% by mass or more is particularly effective in reducing iron loss. On the other hand, a silicon content of 8.0% by mass or less provides particularly excellent workability and magnetic flux density. Therefore, the silicon content is preferably in the range of 2.0 to 8.0% by mass.

[0053] Mn:0.005~1.0% by mass Mn is an element advantageous in improving hot workability, but if the content is less than 0.005% by mass, the effect of adding it is poor. On the other hand, if the content is 1.0% by mass or less, the magnetic flux density of the finished sheet is particularly good. For this reason, the Mn content is preferably in the range of 0.005 to 1.0% by mass.

[0054] In addition to the above basic components, the following elements may be appropriately contained as magnetic property improving components. Specifically, it is at least one selected from Ni: 0.03 to 1.50 mass%, Sn: 0.01 to 1.50 mass%, Sb: 0.005 to 1.50 mass%, Cu: 0.03 to 3.0 mass%, P: 0.02 to 0.50 mass%, Mo: 0.005 to 0.10 mass%, and Cr: 0.03 to 1.50 mass%.

[0055] Ni is a useful element for improving the hot-rolled sheet structure and further enhancing magnetic properties. However, if the Ni content is less than 0.03% by mass, the effect of improving magnetic properties is small, while if the Ni content is 1.50% by mass or less, the stability of secondary recrystallization in particular increases, improving magnetic properties. Therefore, the Ni content is preferably in the range of 0.03 to 1.50% by mass.

[0056] Furthermore, Sn, Sb, Cu, P, Cr, and Mo are each elements useful for improving magnetic properties, but if the content of each element is less than the lower limit of the respective element, the effect of improving magnetic properties is small. On the other hand, if the content of each element is equal to or less than the upper limit of the respective element, the development of secondary recrystallized grains is optimal. Therefore, it is preferable to contain each of these elements within the above-mentioned range.

[0057] The remainder other than the above components is unavoidable impurities mixed in during the manufacturing process and Fe.

[0058] In the present invention, the process for producing the grain-oriented electrical steel strip can basically follow a conventionally known production process. The steel material adjusted to the above-mentioned preferred composition may be made into a slab by a normal ingot-making method or continuous casting method, or a thin cast piece having a thickness of 100 mm or less may be produced by a direct continuous casting method. The molten steel may be produced by a blast furnace method or an electric furnace method.

[0059] The resulting slab is heated in the usual way and subjected to hot rolling, but it can also be subjected to hot rolling immediately after casting without heating. In the case of a thin cast strip, it can be hot rolled, or it can be directly proceeded to the subsequent process without hot rolling. The preferred conditions are to perform hot-rolled sheet annealing as necessary, followed by one cold rolling or two or more cold rolling steps with intermediate annealing in between to achieve the final sheet thickness. Next, after decarburization annealing, an annealing separator mainly composed of MgO is applied, followed by finish annealing for secondary recrystallization.

[0060] Since the grain-oriented electrical steel strip after secondary recrystallization annealing, which has Goss-oriented grains, is welded to a steel strip after primary recrystallization, which grows secondary recrystallized grains as described below, it is not coated with a so-called insulating coating. Furthermore, the forsterite film, which is mainly composed of Si and Mg and is formed on the surface of the grain-oriented electrical steel strip after final annealing, is removed only in the vicinity of the weld zone to facilitate stable butt welding, but it is also possible to obtain the joining conditions described in the present invention without removing it.

[0061] The chemical composition of the steel strip after primary recrystallization to be welded to the grain-oriented electrical steel strip after secondary recrystallization, which is mainly composed of Goss-oriented crystal grains, can be appropriately determined to be a composition that is advantageous for the magnetic properties required of general electrical steel strips. C: 50 mass ppm or less It is preferable to ultimately reduce the content to 50 mass ppm or less, at which point magnetic aging does not occur. However, there is no need to set a lower limit.

[0062] Si:2.0~8.0% by mass Silicon is an element effective in increasing the electrical resistance of steel and improving iron loss, and a silicon content of 2.0% by mass or more is particularly effective in reducing iron loss. On the other hand, a silicon content of 8.0% by mass or less provides particularly excellent workability and magnetic flux density. Therefore, the silicon content is preferably in the range of 2.0 to 8.0% by mass.

[0063] Mn:0.005~1.0% by mass Mn is an element advantageous in improving hot workability, but if the content is less than 0.005% by mass, the effect of adding it is poor. On the other hand, if the content is 1.0% by mass or less, the magnetic flux density of the finished sheet is particularly good. For this reason, the Mn content is preferably in the range of 0.005 to 1.0% by mass.

[0064] In addition to the above basic components, various elements may be appropriately contained as components for improving magnetic properties. The remainder other than the above components is unavoidable impurities mixed in during the manufacturing process and Fe.

[0065] In the present invention, the process for producing a steel strip after primary recrystallization, which grows crystal grains having a Goss orientation, can basically follow the conventionally known process for producing a thin steel strip that undergoes primary recrystallization annealing.

[0066] The steel material adjusted to the above-mentioned preferred composition may be made into a slab by a normal ingot-making method or continuous casting method, or a thin cast piece having a thickness of 100 mm or less may be produced by a direct continuous casting method. The molten steel may be produced by a blast furnace method or an electric furnace method.

[0067] The resulting slab is heated in a conventional manner and subjected to hot rolling, but it may also be subjected to hot rolling immediately after casting without heating. In the case of a thin cast strip, it may be hot rolled, or hot rolling may be omitted and the strip may be directly subjected to subsequent processes. The preferred conditions are to perform hot-rolled sheet annealing as necessary, followed by one cold rolling or two or more cold rolling steps with intermediate annealing between them to obtain the final sheet thickness, followed by primary recrystallization annealing.

[0068] The grain-oriented electrical steel strip after secondary recrystallization and the steel strip after primary recrystallization are butt-welded. After trimming one edge of the width direction of the two steel strips parallel to the rolling direction, the two steel strips are conveyed while being restrained from both sides to prevent gaps from forming at the butt joint, and welding is performed so that the butt joint position does not move.

[0069] The width of the grain-oriented electrical steel strip after secondary recrystallization used in the present invention is not particularly limited, but a width of about 100 mm is sufficient.

[0070] The welding used in the present invention can be performed using a general electron beam or laser, but it is preferable not to use welding metal such as welding wire, and to perform the welding in a vacuum or non-oxidizing atmosphere in order to maintain cleanliness near the weld surface.The weld cross section should be observed using an SEM or the like, and the area ratio of oxide in the square region on the steel strip side after primary recrystallization should be less than 5%.

[0071] Although there are no particular restrictions on the welding speed, unit time of electron beam or laser irradiation for welding, or heat input per unit length, it is necessary that the distribution of grain size in the above-mentioned square region is such that the area ratio of grains having a grain size 1.8 times or more the average grain size before welding is less than 70%. In other words, if it is 70% or more, grains with Goss orientation cannot grow beyond the grain group affected by heat due to welding.

[0072] The welding of grain-oriented electrical steel strips that have undergone secondary recrystallization and have a Goss orientation can be carried out on either one edge of the steel strip after primary recrystallization, or on both edges. In the latter case, the growth distance of the Goss-oriented crystal grains can be halved, which is industrially more advantageous.

[0073] A well-known technique is a method for effectively growing crystal grains having the Goss orientation toward the steel strip side after primary recrystallization following butt welding. This can be achieved by increasing the temperature of the directional electrical steel strip side having the Goss orientation desired for grain growth relative to the vicinity of the joint, and creating a temperature gradient perpendicular to the joint surface, thereby growing crystal grains having the Goss orientation toward the steel strip side after primary recrystallization.

[0074] If the joined coil is to be subjected to continuous grain growth treatment, for example, by welding secondarily recrystallized directional electromagnetic steel strips to both ends of a steel strip after primary recrystallization, and then sandwiching the coil above and below with a transverse IH coil and running the coil continuously, it is possible to take advantage of so-called edge overheating, which is a problem with transverse IH, in which the edge parts have higher temperatures than the center, and create a temperature gradient from both ends of the coil toward the center, allowing Goss crystal grains to grow simultaneously from both edges of the coil, thereby achieving the effects of the present invention.

[0075] Furthermore, when gradient annealing is performed by batch processing, two types of steel strips are joined under the above-mentioned conditions, and then an adhesion inhibitor mainly composed of MgO is applied thereto, and the resultant is wound into a coil. The coil is then placed so that the end faces of the coil face upward, and annealing is performed in a batch furnace having a heater built into the hearth that supports the coil so that a temperature gradient is created in the vertical direction, thereby enabling grain growth of Goss crystal grains.

[0076] When grain-oriented electrical steel strips in which Goss-oriented crystal grains have grown on the side containing primary recrystallized grains are used as laminated core materials for transformers, etc., an insulating layer is required for interlayer insulation, and commonly used inorganic coatings can be used as the additional insulating coating. In particular, coatings with a tensioning effect are extremely effective in achieving low iron loss and low noise.

[0077] As a type of tension-imparting coating, a coating containing silica, which reduces the thermal expansion coefficient, is effective, and a phosphate-colloidal silica-chromic acid-based coating, which has traditionally been used for grain-oriented electrical steel strips with a forsterite coating, is preferred in terms of its effectiveness, cost, and uniform treatment ability.

[0078] Furthermore, as a magnetic domain refining process, it is possible to further reduce iron loss by carrying out a thermal distortion-inducing magnetic domain refining process such as laser or electron beam irradiation after insulating coating. It is also effective to reduce iron loss by forming physical grooves mechanically or electrochemically to achieve magnetic domain refining. [Example]

[0079] Example 1 The steel slab, containing, by mass% and mass ppm, 600 ppm C, 3.4% Si, 0.05% Mn, 150 ppm Al, 50 ppm S, 100 ppm Se, 50 ppm N, 0.06% P, 0.07% Sb, and 0.015% Mo, with the remainder being Fe and unavoidable impurities, was heated to 1350°C and hot-rolled to a thickness of 2.2 mm. It was then annealed at 1080°C for 30 seconds and cold-rolled in a tandem mill to a final thickness of 0.23 mm. It was then heated to 840°C and decarburized for 90 seconds in a wet hydrogen atmosphere to produce a steel strip with primary recrystallization. Next, a magnesia-based annealing separator was applied to the steel strip having the primary recrystallization, and a finish annealing for secondary recrystallization and purification was carried out at 1200°C to obtain a grain-oriented electrical steel strip having a secondary recrystallization with a Goss orientation (hereinafter referred to as a secondary recrystallized sheet). Magnetic measurements of the secondary recrystallized sheet showed that the magnetic flux density B8 was 1.93 T at a magnetic field strength of 800 A / m.

[0080] On the other hand, a steel slab containing, in mass% and mass ppm, 25 ppm C, 3.3% Si, 0.05% Mn, 50 ppm S, 0.03% Sn, with the remainder being Fe and unavoidable impurities, was heated to 1100°C, hot rolled to a thickness of 2.2 mm, annealed at 1000°C for 30 seconds, and cold rolled once in a tandem mill to a final thickness of 0.23 mm. This was then annealed at 800°C to obtain a steel strip with primary recrystallization (hereinafter referred to as a primary recrystallized sheet). The secondary recrystallized plate was sheared to a width of 100 mm in the rolling direction, and the primary recrystallized plate was sheared to a width of 500 mm in the rolling direction, and then the two crystallized plates were aligned in the rolling direction and butt-welded using an electron beam. The vacuum during welding was approximately 6.5 Pa, the welding speed was 5 to 40 m / min, the acceleration voltage was 40 kV, and the beam current was varied between 6 and 40 mA. To change the welding depth, we performed two conditions: JUST, in which the beam focus was set exactly at the center, and UPPER, in which the focus was shifted. Welds were cut out from various samples, and cross-sectional SEM observations were used to quantitatively evaluate the grain size distribution and oxides near the joint. The welding condition on the opposite side of the welded surface was visually inspected to determine whether the electron beam had penetrated.

[0081] Test pieces 600 mm long in the direction perpendicular to the rolling direction and 100 mm wide in the rolling direction were cut out and placed in an annealing furnace with a 100°C / 300 mm temperature gradient. The temperature difference between the two ends of the test piece was maintained at 200°C, so that the secondary recrystallized sheet was on the high side and the primary recrystallized sheet was on the low side. The test pieces were heated at a constant rate of 10°C / h from room temperature to 1100°C on the high side. After reaching this temperature, the power was turned off and the furnace was cooled. After gradient annealing, the central 100 mm of the original 500 mm length of the primary recrystallized sheet was cut out, and the magnetic flux density B8 in the rolling direction was measured using a single sheet testing machine (SST) with a test piece size of 100 mm x 100 mm. The results are listed in Table 1, along with the results of a cross-sectional examination after welding.

[0082] [Table 1]

[0083] As shown in Table 1, electron beam welding requires a certain degree of vacuum for beam irradiation, so no oxide formation was observed near the joint, and there was no effect. As long as the requirement of the present invention is met—in the SEM observation range of a 0.23 mm × 0.23 mm square cross section from the butt joint to the primary recrystallized grain side—that is, the area ratio of coarse grains, which are 1.8 times or more the average primary recrystallized grain size before welding, is less than 70%—the steel strips can be joined by welding even under conditions where humping (wavy weld beads) is observed or where the weld does not completely penetrate the back surface—the Goss orientation crystals grow, and an extremely high B8 electrical steel strip can be obtained (No. 2, No. 5, No. 8).

[0084] On the other hand, even if the bead shape after welding was good, the heat-affected zone was large and the area ratio of the coarse grains in the evaluation region was 70% or more, and Goss-oriented grain growth due to gradient annealing was not observed (No. 3, No. 7). In addition, there were some cases where the butt welding of thin steel strips did not work well or the beam strength was too strong, causing it to melt (No. 1, No. 4, No. 6).

[0085] Example 2 The steel slab, containing, in mass% and mass ppm, 350 ppm C, 3.4% Si, 0.07% Mn, 60 ppm Al, 50 ppm S, 30 ppm N, 0.07% P, 0.05% Sb, and 0.010% Mo, with the remainder being Fe and unavoidable impurities, was heated to 1220°C, hot-rolled to a thickness of 2.2 mm, annealed at 1050°C for 30 seconds, and cold-rolled to a final thickness of 0.23 mm in a single pass on a tandem mill. It was then heated to 860°C and decarburized for 70 seconds in a wet hydrogen atmosphere to produce a primary recrystallized sheet. Next, a magnesia-based annealing separator was applied to the primary recrystallized plate, and a finish annealing for secondary recrystallization and purification was performed at 1200°C to obtain a secondary recrystallized plate with Goss orientation. Magnetic measurements of the secondary recrystallized plate showed that the magnetic flux density B8 was 1.92 T at a magnetic field strength of 800 A / m.

[0086] On the other hand, a 30 mm thin slab containing, in mass% and mass ppm, 25 ppm C, 3.2% Si, 0.15% Mn, 15 ppm S, 0.01% Sn, with the remainder being Fe and unavoidable impurities, was heated to 1000°C in a tunnel furnace, hot rolled to a thickness of 2.2 mm, annealed at 950°C for 30 seconds, and cold rolled once in a tandem mill to a final thickness of 0.23 mm. This was then recrystallized at 750°C to obtain a primary recrystallized sheet. The secondary recrystallized plate was sheared into two 100 mm wide pieces in the rolling direction, and the primary recrystallized plate was sheared into one 800 mm wide piece in the rolling direction. The secondary recrystallized plates were then lined up on both ends of the primary recrystallized plate in the rolling direction, and butt-welded at two locations using a fiber laser under the same conditions. This welding was carried out in a vacuum chamber with a vacuum level of approximately 50 Pa to 1 kPa, a welding speed of 1 to 10 m / min, and an output of 20 kW or less. Because the beam diameter was too small, the focusing conditions were changed as needed to find conditions under which welding was possible. The welded parts of various samples were cut out, and cross-sectional SEM observation was used to quantitatively evaluate the grain size distribution and oxides near the welded parts. The welding condition on the opposite side of the welded surface was visually inspected to determine whether the laser penetrated. The evaluation was also performed by averaging the evaluations of both edges.

[0087] As in Example 1, test pieces were cut out to a length of 1000 mm transverse to the rolling direction and a width of 100 mm in the rolling direction. The test pieces were placed in the center of a 1200 mm temperature gradient furnace with a 6-zone control system, with the ends of the furnace being hotter and the center being colder. A temperature gradient (150 °C / 300 mm) was applied from both ends of the primary recrystallized sheet to the center, and the secondary recrystallized sheet was heated from room temperature to 1100 °C at a constant heating rate of 10 °C / h. After reaching 1100 °C, the power was turned off and the furnace was cooled. Of the test pieces after gradient annealing, a central 100 mm was cut out from the 800 mm length that was originally the primary recrystallized sheet. The magnetic flux density B8 in the rolling direction was measured using a single sheet testing machine (SST) with a test piece size of 100 mm × 100 mm. The results are listed in Table 2, along with the results of a cross-sectional survey after welding.

[0088] [Table 2]

[0089] As shown in Table 2, laser welding itself can be performed even at a high vacuum level (1 kPa). However, depending on the vacuum level, oxides are formed near the weld. SEM observation of the area from the butt joint toward the square region (0.23 mm × 0.23 mm) revealed that when the area ratio of such oxides was 5% or more, growth of Goss-oriented crystals did not occur, and even if it did occur partially, it was not possible for the Goss-oriented crystals to cover the entire surface of the test piece (No. 1, No. 5, No. 6). Furthermore, even if the joining itself was strong and good, if the heat input was too large and the crystal grains near the weld became coarse, and the area ratio of coarse grains in the square region mentioned above, which was 1.8 times or more the average primary recrystallized grain size before welding, reached 70% or more, the grain growth of crystals with Goss orientation did not occur normally, and it was not possible to obtain a steel strip with a good magnetic flux density by covering the entire surface with good secondary recrystallized grains (No. 1, No. 2, No. 6).

[0090] On the other hand, if the conditions of the present invention are met, even if the welding does not completely reach the back surface, as long as the steel strips are joined to an extent that they do not separate, the Goss orientation crystal grains will grow, and an electromagnetic steel strip with extremely high magnetic flux density can be obtained (No. 3, No. 7).

[0091] Furthermore, because the thin steel strips were butt-welded, if the joining was not successful or if the laser output was too strong and caused melting, it was not possible to evaluate grain growth (No. 4, No. 8).

Claims

1. 1. A method for manufacturing a grain-oriented electrical steel strip, comprising: joining at least one widthwise end portion of a grain-oriented electrical steel strip after secondary recrystallization, which has been accumulated in the {110}<001> orientation, to at least one widthwise end portion of a steel strip after primary recrystallization that has been finished to a final thickness, by butt welding without using weld metal; and setting the distribution of crystal grains in a region on the side of the steel strip after primary recrystallization, which extends perpendicular to the joined surface and has a width equal to the thickness of the steel strip after primary recrystallization, to an area ratio of less than 70% of crystal grains having a grain size 1.8 times or more the average grain size of the entire steel strip after primary recrystallization excluding the heat-affected zone of the weld of the steel strip after primary recrystallization, and an area ratio of oxides less than 5%, and then applying a temperature gradient in which the grain-oriented electrical steel strip side is elevated in a direction perpendicular to the rolling direction and perpendicular to the joined surface, thereby causing grains having the {110}<001> orientation to grow on the side of the steel strip after primary recrystallization.

2. 2. The method for producing a grain-oriented electrical steel strip according to claim 1, wherein the temperature gradient is 0.1°C / mm or more.

3. 3. A method for producing a grain-oriented electrical steel strip according to claim 1 or 2, wherein the grain-oriented electrical steel strip obtained by the method for producing a grain-oriented electrical steel strip according to claim 1 or 2 is further joined to a steel strip after primary recrystallization that has been finished to a final thickness, and the steel strip after primary recrystallization is used as a new grain-oriented electrical steel strip.

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