Grain-oriented electrical steel sheet and its manufacturing method

Abstract

The purpose of the present invention is to provide a grain-oriented electromagnetic steel sheet for which iron loss is further improved in magnetic domain control for forming a laser groove in a cold rolled steel sheet. The present invention provides a grain-oriented electromagnetic steel sheet provided with the following requirements for achieving the abovementioned purpose. In other words, this grain-oriented electromagnetic steel, which comprises a base steel sheet having a plurality of grooves on the surface thereof, and a glass coating on the surface of the base steel sheet, is characterized in that: the absolute value of the angle θ formed by the direction perpendicular to the rolling direction of the base steel sheet and to the sheet thickness direction, and the longitudinal direction of the grooves is 0-40°; the width W of the grooves is 20-300 μm, the depth D of the grooves is 1-40 μm, and the gap P between the grooves in the rolling direction is 1.0-30 mm; and the formula (1) is satisfied, where t1 is the thickness of the glass coating in a flat section of the surface of the base steel sheet, t2 is the thickness of the glass coating at the deepest section of the grooves in the recess of the groove, and t3 is the thickness of the glass coating in the side surface sections of the grooves. (1): (t2＋t3) / 2≥t1

Description

[Technical field]

[0001] The present invention relates to a grain-oriented electrical steel sheet. [Background technology]

[0002] Grain-oriented electrical steel sheet is a steel sheet whose crystal orientation is controlled by a combination of cold rolling and annealing so that the magnetization easy axis of the crystal grains coincides with the rolling direction.

[0003] Forming an insulating coating on the surface of a base steel sheet with controlled crystal orientation is known as a technology for reducing eddy current loss, which is a type of iron loss in grain-oriented electrical steel sheets. The insulating coating not only provides electrical insulation, but also tensile strength, rust resistance, and other properties to the base steel sheet.

[0004] Another method for reducing eddy current loss is the magnetic domain control method, in which strain regions or grooves are formed in a direction intersecting the rolling direction at predetermined intervals along the rolling direction to narrow the width of the 180° magnetic domains (subdivision of the 180° magnetic domains). Magnetic domain control methods are classified into a method of applying strain to the base steel sheet of the grain-oriented electrical steel sheet and a method of forming grooves on the surface of the base steel sheet where a coating that applies tension to the base steel sheet exists.

[0005] By using grain-oriented electrical steel sheets with grooves that have been subjected to magnetic domain control, the grooves do not disappear even when the transformer core (wound core) is manufactured and stress relief annealing is performed, so the magnetic domain refinement effect can be maintained. For this reason, magnetic domain control by groove formation is sometimes adopted for wound cores as a method of reducing eddy current loss.

[0006] Fig. 1 is a schematic diagram showing an electrical steel sheet with grooves formed therein. Fig. 1 shows a state in which a plurality of grooves 2 are formed on the surface of a base steel sheet 1 at intervals in the rolling direction of the base steel sheet 1. In Fig. 1, the symbol θ indicates the angle between the longitudinal direction of the groove 2 and a direction perpendicular to the rolling direction and thickness direction of the base steel sheet 1 (the sheet width direction). The symbol W indicates the width of the groove 2, the symbol D indicates the depth of the groove 2, and the symbol P indicates the distance between adjacent grooves 2 in the rolling direction.

[0007] Various methods for forming grooves in electrical steel sheets have been proposed. For example, Patent Document 1 discloses an electrolytic etching method for forming grooves on the surface of a grain-oriented electrical steel sheet by electrolytic etching. For example, Patent Document 2 discloses a gear pressing method for forming grooves on the surface of a grain-oriented electrical steel sheet by mechanically pressing a gear onto the surface of the steel sheet.

[0008] However, the electrolytic etching method requires masking, etching, and mask removal, which makes the process more complicated than the mechanical method. In the gear press method, the teeth wear out in a short period of time due to the high hardness of the magnetic steel sheet. Furthermore, from the perspective of high-speed processing, it is difficult to achieve a line speed of 100 mpm or more, which is required in general steel manufacturing processes.

[0009] Also, for example, Patent Document 3 discloses a laser irradiation method in which a laser irradiated portion on the surface of a grain-oriented electrical steel sheet is melted and evaporated by laser irradiation. The laser irradiation method does not have the problems of tooth profile wear and complicated processes, and allows for high-speed processing.

[0010] In the laser irradiation method, several processes for forming grooves have also been disclosed. For example, Patent Document 4 proposes forming grooves in cold-rolled steel sheets by laser irradiation, but when a cold-rolled steel sheet is irradiated with a laser, a glass coating made of forsterite is formed under the grooves in the final product. Since the glass coating is a non-magnetic oxide, the magnetic flux density of the steel sheet is reduced, which causes a problem of deterioration in iron loss. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Special Publication No. 62-54873 [Patent Document 2] Special Publication No. 62-53579 [Patent Document 3] Japanese Patent Application Laid-Open No. 2003-129135 [Patent Document 4] International Publication No. 2019 / 156127 [Patent Document 5] International Publication No. 2011 / 007771 [Summary of the Invention] [Problems to be Solved by the Invention]

[0012] The present invention has been developed in view of the above circumstances, and an object thereof is to provide a grain-oriented electromagnetic steel sheet in which iron loss is further improved in magnetic domain control for forming laser grooves (grooves formed by laser irradiation) on a cold-rolled steel sheet. [Means for Solving the Problems]

[0013] The inventors of the present invention have intensively studied to solve the above problems. In repeatedly examining the laser irradiation conditions during laser groove formation, it has been found that the glass film formed on the steel sheet surface has a magnetic domain control effect, and although the magnetic flux density decreases, it has an iron loss improvement effect. The present invention is based on this finding, and the gist thereof is as follows.

[0014] [1] A grain-oriented electromagnetic steel sheet according to an aspect of the present invention includes a base steel sheet having a plurality of grooves on the surface of the steel sheet, and a glass film formed on the surface of the base steel sheet. The angle θ formed between the direction orthogonal to the rolling direction and the thickness direction of the base steel sheet and the longitudinal direction of the groove is 0 to 40°, the width W of the groove is 20 to 300 μm, the depth D of the groove is 10 to 40 μm, the interval P between the grooves in the rolling direction is 1.0 to 30 mm, the glass film thickness of the flat portion (portion other than the groove) of the base steel sheet is t1, the glass film thickness at the deepest part of the groove in the concave portion of the groove is t2, and the glass film thickness of the side surface portion of the groove is t3. The grain-oriented electromagnetic steel sheet is characterized by satisfying the relational expression of formula (1). (t2 + t3) / 2 ≥ t1 ··· Formula (1) Furthermore, it is preferable to satisfy the relational expression of formula (2). t1 < t2 ··· Formula (2) [2] A method for producing a grain-oriented electrical steel sheet according to one aspect of the present invention is a method for producing the grain-oriented electrical steel sheet described in the above item [1], comprising the steps of: irradiating a laser onto a surface of a cold-rolled steel sheet to form grooves; Decarburization 1. A method for producing a grain-oriented electrical steel sheet, comprising: an annealing step; and, as irradiation conditions of the laser beam in the groove forming step, a focused spot diameter dL of the laser beam in the rolling direction and a focused spot diameter dC of the laser beam in the sheet width direction satisfy formula (3), and an oxygen potential (PH2O / PH2) in the annealing step is 0.20 or more and 0.85 or less. 0.010≦dL / dC≦1.000 ··· Formula (3) Effect of the Invention

[0015] According to the present invention, the magnetic domain refining effect can be fully exerted, and a grain-oriented electrical steel sheet having good core loss can be obtained. [Brief description of the drawings]

[0016] [Figure 1] FIG. 1 is a schematic diagram showing an example of an electromagnetic steel sheet having grooves formed on the surface. [Diagram 2] FIG. 2 is a diagram showing the relationship between an example of a cross section of a base steel sheet including a groove and the coating thickness. [Diagram 3] FIG. 3 is a diagram showing an example of a cross-sectional photograph of a base steel sheet including a groove. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] The inventors have conducted extensive research into how to maximize the magnetic domain refinement effect, and have found that the magnetic domain refinement effect can be maximized by controlling the thickness of the glass coating on the groove recess surface. Hereinafter, the configuration of a grain-oriented electrical steel sheet according to an embodiment of the present invention (hereinafter, abbreviated as the present electrical steel sheet) will be described.

[0018] The present electrical steel sheet comprises a base steel sheet having a plurality of grooves arranged in parallel on its surface, and a glass coating formed on the surface of the base steel sheet (see Figs. 1 to 3). In the present electrical steel sheet, a tensile coating (insulating coating) may be formed on the surface of the glass coating. As shown in Fig. 1, on the surface of the base steel sheet, a plurality of grooves are formed approximately parallel to each other and adjacent in the rolling direction of the base steel sheet. The groove direction (angle θ), groove width W, depth D, and spacing P are determined taking into account iron loss, as in the case of ordinary grain-oriented electrical steel sheets.

[0019] <Angle θ between the rolling direction of the base steel sheet and the longitudinal direction of the groove> The angle θ between the direction perpendicular to the rolling direction and thickness direction of the base steel sheet (sheet width direction) and the longitudinal direction of the groove is set to 40° or less, because if it is too large, there is no magnetic domain control effect and the iron loss improvement effect cannot be obtained. The angle θ is preferably small, and is preferably 35° or less, 30° or less, 25° or less, 20° or less, 15° or less, 10° or less, 8° or less, 6° or less, or 5° or less. The lower limit of the angle θ is 0°, that is, when the longitudinal direction of the groove is parallel to the sheet width direction. The direction of the angle θ is not important, and refers to the acute angle of the angle formed by the longitudinal direction of the groove and the sheet width direction. A plurality of grooves are arranged approximately parallel to the surface of the base steel sheet, and the angle θ of each groove may be within the range described above.

[0020] <Groove width W> The groove width W refers to the width of the groove on the surface of the base steel sheet in the cross section (groove cross section) of the groove on a plane perpendicular to the longitudinal direction of the groove. If the groove width W is too narrow, less than 20 μm, it will not become the starting point of magnetic pole generation, there will be no magnetic domain control effect, and good iron loss will not be obtained. On the other hand, if it is too wide, exceeding 300 μm, it will not become the starting point of magnetic pole generation, there will be no magnetic domain control effect, only the magnetic flux density will be significantly reduced, and good iron loss will not be obtained. Therefore, the groove width W should be 20 μm or more and 300 μm or less. The lower limit of the groove width W is preferably 25 μm, 30 μm, or 35 μm. The upper limit of the groove width W is preferably 250 μm, 200 μm, 150 μm, 100 μm, or 80 μm.

[0021] <Groove depth D> If the groove depth D is too shallow, less than 10 μm, it will not become the starting point of the magnetic pole, there will be no magnetic domain control effect, and good iron loss will not be obtained. On the other hand, if it is too deep, exceeding 40 μm, the magnetic domain control effect will reach saturation, and only the magnetic flux density will drop significantly, so good iron loss will not be obtained. Therefore, the groove depth D should be 10 μm or more and 40 μm or less. The lower limit of the groove depth D is preferably 11 μm, 12 μm, 13 μm, 14 μm, or 15 μm. The upper limit of the groove depth D is preferably 38 μm, 36 μm, 34 μm, 32 μm, 30 μm, 28 μm, or 26 μm.

[0022] <Groove Spacing P> The groove spacing P is the spacing between the longitudinal centerlines of adjacent grooves arranged approximately parallel to each other on the surface of the base steel sheet, and refers to the distance in the rolling direction of the base steel sheet. The groove centerline is a line parallel to the longitudinal direction of the groove that passes through the midpoint of the groove on the surface of the base steel sheet in the groove cross section.

[0023] If the groove spacing P is too narrow, less than 1 mm, the magnetic domain control effect is saturated, and only the magnetic flux density is significantly reduced, so good core loss cannot be obtained. On the other hand, if the groove spacing P is too wide, more than 30 mm, the magnetic domain control effect is not sufficiently obtained, and good core loss cannot be obtained. Therefore, the groove spacing P is preferably 1 mm or more and 30 mm or less. The groove spacing P does not have to be equal, but the groove spacing P between adjacent grooves should be within the above range. The lower limit of the groove spacing P is preferably 1.2 mm, 1.4 mm, 1.5 mm, 1.6 mm, 1.8 mm, or 2.0 mm. The upper limit of the groove spacing P is preferably 25 mm, 20 mm, 15 mm, 10 mm, 7 mm, or 5 mm.

[0024] <Thickness of glass coating> The glass coating on the flat portion of the base steel sheet surface (the portion where no grooves are formed, i.e. the portion other than the grooves; hereinafter sometimes simply referred to as the "flat portion") and on the surface of the laser grooves (hereinafter sometimes simply referred to as the "grooves") in this electrical steel sheet will be described. Figure 2 is a cross-sectional view of the base steel sheet containing a groove, showing the region containing the groove in a cross-section in the sheet thickness direction perpendicular to the longitudinal direction of the groove (groove cross-section). The portions on both sides of the groove become the flat portions.

[0025] This electrical steel sheet is a grain-oriented electrical steel sheet characterized in that when the glass coating thickness on the flat portion is t1, the glass coating thickness at the bottom (deepest portion) of the groove in the recessed portion of the groove is t2, and the glass coating thickness on the side portion of the groove is t3, the relationship shown in Equation (1) is satisfied. (t2+t3) / 2≧t1 ··· Formula (1)

[0026] The thicknesses of the coatings at each part, t1, t2, and t3, are explained below. The glass coating has an inset structure. The inset structure is a structure in which the end of the glass coating extends into the base steel sheet like the roots of a plant. Therefore, in the cross section of the steel sheet, the end of the inset structure may appear separated from the glass coating. For example, in FIG. 2, the glass coating can be seen as an isolated island away from the glass coating on the flat part and the groove recessed surface, but this is because the cross section of the end of the inset structure is observed. FIG. 3 is an example of a cross-sectional photograph of a steel sheet including a groove, which corresponds to FIG. 2. In FIG. 3, what appears as a black dot in the steel sheet is the end of the inset structure of the glass coating. Hereinafter, the end of the glass coating refers to the part furthest from the glass coating surface in the cross section of the steel sheet, including not only the glass coating but also the glass coating of the inset structure part that appears to exist away from these glass coatings.

[0027] The method for measuring the thicknesses t1, t2, and t3 of the glass coating at each portion will be explained below with reference to FIG. First, the glass coating thickness t1 of the flat portion will be explained. The intersections of the flat portion and the groove (i.e., the edges of the groove) are designated as points A and A'. The straight line passing through points A and A' is designated as L1U. In other words, the straight line showing the glass coating surface on the flat portion of the steel sheet surface is designated as L1U. Next, of the ends of the glass coating within a range of 25 μm from points A and A' on the flat portion, the bottom end position (deepest position in the sheet thickness direction) of the end furthest from the glass coating surface (i.e., L1U) is designated as point a. Next, the straight line passing through point a and parallel to line L1U is designated as L1L, and the distance between L1U and L1L is designated as t1.

[0028] Next, the thickness of the glass coating at the bottom (deepest part) of the groove will be described as t2. The deepest part of the groove recess surface (deepest part in the plate thickness direction) is defined as point B, and the line that passes through point B and is parallel to L1U is defined as L2U. Next, the lower end position of the end of the glass coating that is farthest from L2U in the plate thickness direction is defined as point b, and the line that passes through point b and is parallel to L2U is defined as L2L. The distance between these straight lines L2U and L2L is defined as t2.

[0029] Finally, the thickness of the glass coating on the side of the groove will be described as t3. The intersection point of the perpendicular bisector of the line segment AB, which connects point A at the end of the groove and point B at the deepest point of the groove, and the glass coating surface on the side of the groove is designated as point C, and the straight line passing through points A and C is designated as L3U. Next, the end of the glass coating on the side of the groove that is farthest from the straight line L3U is identified, and the point of that end farthest from the straight line L3U is designated as point c. The line that passes through point c and is parallel to L3U is designated as L3L, and the distance between these straight lines L3U and L3L is designated as t3. The thickness of the glass coating on the side of the groove is almost the same on both sides of the groove (the side on the side of point A and the side on the side of point A' in Figure 2), so it is sufficient to measure either one of them. Of course, it is also possible to measure the thickness of the glass coating on both sides and take the average value as the glass coating thickness t3 on the side of the groove.

[0030] The thickness of the glass coating can be measured, for example, by observing the cross section with an optical microscope or SEM after mirror polishing. Known conditions can be applied as the observation conditions for the optical microscope or SEM, and the measurement field of view and observation magnification can be appropriately adjusted according to the size of the groove and the thickness of the glass coating. In the SEM observation, a secondary electron image or a backscattered electron image can be taken.

[0031] Here, the effect of the glass coating will be described. Magnetic domain refinement in grain-oriented electrical steel sheet occurs when the magnetic poles generated on the steel sheet surface increase magnetostatic energy, and to resolve this, new 180° magnetic domain walls are generated, narrowing the magnetic domain width. Narrowing the magnetic domain width shortens the distance the magnetic domain wall moves when the steel sheet is magnetized, reducing the energy loss during magnetic domain wall movement and iron loss. By applying and baking a glass coating, which has a different expansion coefficient from the steel sheet, at high temperatures, a tensile force is applied in the rolling direction after cooling due to the difference in expansion coefficients between the steel sheet and the glass coating, which subdivides the magnetic domains and improves iron loss. Furthermore, because the glass coating is made of non-magnetic oxides and has a different magnetic permeability from the steel sheet, magnetic poles are generated at the interface and magnetic domains are subdivided. Therefore, it is thought that the glass coating acts as the starting point for magnetic pole generation and promotes magnetic domain refinement, thereby reducing iron loss. Although tension is less likely to be applied in the rolling direction in the grooves, forming grooves perpendicular to the rolling direction generates magnetic poles on the concave surfaces of the grooves, and the synergistic effect of this and the generation of magnetic poles by the glass coating promotes the subdivision of magnetic domains.

[0032] On the other hand, because the glass coating is made of non-magnetic oxides, the magnetic flux density of the steel sheet is reduced. Therefore, there is a trade-off between core loss and magnetic flux density. If the glass coating is made thick, magnetic domain refinement is promoted in the grooves, but the magnetic flux density decreases over the entire surface of the steel sheet.

[0033] The inventors have therefore discovered that it is possible to enjoy the effect of refining magnetic domains in the grooves by making the glass coating thickness on the surfaces of the recesses in the grooves relatively thick, while at the same time suppressing a reduction in magnetic flux density in the entire steel sheet by making the glass coating thickness on the flat portions relatively thin. That is, as shown in formula (1), the thickness of the glass coating in the recesses of the grooves should be made thicker than the thickness of the glass coating in the flat portions. (t2+t3) / 2≧t1 ··· Formula (1) t1: Glass coating thickness on flat surface t2: Glass coating thickness at the bottom (deepest part) of the groove in the concave part of the groove t3: Glass coating thickness on the side of the groove Preferably, the film thickness ratio [film thickness ratio = {(t2 + t3) / 2} / t1], which is the ratio of the thickness [(t2 + t3) / 2] of the glass film in the concave portion of the groove to the thickness [t1] of the glass film in the flat portion, may be 1.1 or more, 1.2 or more, 1.3 or more, 1.4 or more, 1.5 or more, 1.6 or more, 1.7 or more, 1.8 or more, 1.9 or more, or 2.0 or more. Although the upper limit of the film thickness ratio [film thickness ratio = {(t2 + t3) / 2} / t1] is not particularly limited, considering practical operations, the film thickness ratio may be 5.0 or less, 4.0 or less, or 3.0 or less.

[0034] More preferably, the glass film thickness t1 of the flat portion is preferably thinner than the glass film thickness t2 of the bottom surface portion (deepest portion) of the groove (Equation (2)). By making the glass film on the bottom surface portion of the groove thicker than the flat portion, a decrease in the magnetic flux density of the entire steel sheet is suppressed, and the generation of magnetic poles due to the groove and the glass film is promoted, enabling further improvement in magnetism. t1 < t2 ··· Equation (2)

[0035] <Manufacturing method> First, a cold-rolled steel sheet for this electromagnetic steel sheet is manufactured by a known method. The steel sheet components and the manufacturing method of the cold-rolled steel sheet are not particularly limited, and known methods, for example, the steel sheet components and steel sheet manufacturing methods described in Patent Document 5, can be adopted. However, the groove forming step of forming grooves by laser irradiation is preferably performed after cold rolling and before the decarburization annealing step. Since the shape of the grooves formed by laser irradiation is decarburization annealed and finish annealed without being cold rolled, the controllability of the groove shape is improved.

[0036] <Laser irradiation conditions> The groove forming step will be described. By irradiating a cold-rolled steel sheet with a laser, a plurality of grooves are formed on the steel sheet surface in a direction intersecting the rolling direction so that the groove width W, groove depth D, and a predetermined interval (groove interval P) are within a specified range. Among the laser irradiation conditions, the type of laser light source, laser output, laser scanning speed, and steel sheet moving speed during laser irradiation are not particularly limited, but conditions under which the groove width W, groove depth D, and the predetermined interval (groove interval P) of the grooves are within a specified range may be appropriately selected.

[0037] [Laser light source] As the laser light source, for example, a high-power laser generally used for industrial purposes such as a fiber laser, a YAG laser, a semiconductor laser, or a CO2 laser can be used. As long as the grooves can be stably formed, a pulsed laser or a continuous wave laser can be used.

[0038] [Laser output] If the laser output is less than 200 W, the laser scanning speed to form the desired groove is significantly reduced, resulting in a reduction in industrial productivity, so it is preferable to set it to 200 W or more. It is preferably 1000 W or more, and more preferably 1500 W or more. If the laser output is more than 3000 W, the power supply capacity becomes large and the equipment cost becomes enormous, which is not industrially realistic, so it is preferable to set it to 3000 W or less. It is preferably 2800 W or less, and more preferably 2500 W or less.

[0039] [Laser scanning speed] When the laser scanning speed is less than 5 m / s, the steel sheet passing speed must be slowed down, resulting in reduced productivity, and therefore the speed is set to 5 m / s or more. It is preferably set to 20 m / s or more, and more preferably set to 40 m / s or more. When the laser scanning speed is faster than 100 m / s, a higher output is required, which increases the equipment cost, and therefore the speed is set to 100 m / s or less. It is preferably set to 80 m / s or less, and more preferably set to 60 m / s or less.

[0040] [Laser light focusing spot shape] The shape of the focused spot of the laser light on the surface of the base steel sheet may be circular or elliptical, slightly spreading in the sheet width direction. To prevent the spot from spreading too much, for example, the focused spot diameter dL of the laser light in the rolling direction and the focused spot diameter dC of the laser light in the sheet width direction may satisfy formula (3). For example, the focused spot diameter dL of the laser light in the sheet width direction may be set to 5 to 100 μm, the focused spot diameter dC of the laser light in the sheet width direction may be set to 5 to 100 μm, the laser output may be set to 200 to 3000 W, and the laser scanning speed V may be set to 5 to 100 m / s.

[0041] 0.010≦dL / dC≦1.000 ··· Formula (3) If dL / dC is greater than 1, the laser spot diameter becomes elongated in the rolling direction, making it difficult to control the laser groove shape. If dL / dC is smaller than 0.010, the laser spot diameter becomes an extremely elongated ellipse in the plate width direction, and in this case too, it becomes difficult to control the laser groove shape.

[0042] The inventors conducted repeated experiments and found that by applying the above-mentioned laser irradiation conditions, the thickness of the molten layer in the laser irradiated portion can be controlled, and by forming an internal oxide layer by subsequent recrystallization annealing, it is possible to obtain an electrical steel sheet having a controlled thickness of the glass coating as described above.

[0043] The mechanism by which the laser irradiation conditions contribute to controlling the thickness of the glass coating in the groove recesses is believed to be as follows.

[0044] First, the glass coating is an oxide made of Mg2SiO4, and is formed when the internal oxide layer SiO2 formed during decarburization annealing reacts with the annealing separator MgO applied to the steel sheet surface before finish annealing during the high-temperature annealing at 1200°C in the finish annealing process. At this time, a glass film is also formed depending on the thickness of the internal oxide layer. For example, it is possible to adjust the thickness of the internal oxide layer by increasing the dew point of the decarburization annealing.

[0045] When a cold-rolled steel sheet is irradiated with a laser, the laser-irradiated portion of the cold-rolled steel sheet (base steel sheet) melts and forms a groove. Crystals grow oriented on the surface of the groove, resulting in the formation of columnar crystals.

[0046] It was confirmed that, when the focused spot shape is an ellipse appropriately in the steel sheet width direction as in the present invention, the thickness of the molten layer on the groove recess surface becomes appropriately thick when dL / dC is small. It is considered that when the laser spot shape is an ellipse appropriately long in the sheet width direction, when the laser beam is continuously irradiated at an angle of 40° or less with respect to the sheet width direction, the heat by the laser irradiation is easily transmitted in the groove formation direction, and the controllability of the columnar crystals formed on the groove recess surface is improved. Therefore, when dL and dC are within the above-mentioned range, the heat input to the steel sheet is large, the molten layer is formed appropriately thick, and the columnar crystals are generated appropriately thick. It is considered that in the subsequent decarburization annealing, the grown columnar crystals become a diffusion path for oxygen, the internal oxide layer develops, and the glass coating also becomes thick in the finish annealing.

[0047] Therefore, it is desirable to satisfy the relationship of formula (3). The lower limit of dL / dC is 0.010, but preferably 0.015, 0.020, 0.025, 0.030, 0.035, 0.040, 0.045, or 0.050. The upper limit of dL / dC is 1.000 (circular spot), but the laser spot shape is preferably an ellipse elongated in the sheet width direction, and is preferably 0.980, 0.960, 0.940, 0.920, 0.900, 0.880, 0.860, 0.840, 0.820, 0.800, 0.780, 0.760, 0.740, 0.720, or 0.700.

[0048] On the other hand, in the flat portion, since there is no laser irradiation and therefore no molten layer is formed, the internal oxide layer does not develop as much as in the groove portion, and as a result, the glass coating does not grow as thick as in the groove portion. The focusing spot diameter is not particularly limited as long as the relationship defined by dL and dC is satisfied, but from the viewpoint of making the remaining recrystallized grains fine, it is preferable that the focusing spot diameter is also small. The focusing spot diameter may be selected in relation to other properties of the electrical steel sheet, and practically, the upper limit of dC is set to 300 μm, preferably 250 μm, 200 μm, 150 μm, or 100 μm.

[0049] <Assist gas> Simultaneously with the irradiation of the laser light, an assist gas may be sprayed onto the portion of the steel sheet to be irradiated with the laser light. The assist gas plays a role in removing components melted or evaporated from the steel sheet by the laser irradiation. By spraying the assist gas, the laser light reaches the steel sheet stably, so that grooves are formed stably. The flow rate of the assist gas is preferably, for example, 10 to 1000 liters per minute. The assist gas is preferably air or an inert gas.

[0050] <Process after groove formation> After forming grooves in the cold-rolled steel sheet in the groove forming process, the cold-rolled steel sheet is annealed (decarburization annealing), nitrided by a known method, and then an annealing separator mainly composed of MgO is applied, heated, held, and then cooled to form a glass coating.

[0051] The decarburization conditions can be any known conditions, but to further reduce iron loss, it is effective to control the glass film formation behavior more precisely. For example, the decarburization annealing conditions are as follows: the steel sheet with grooves is heated to 800 to 880°C, the annealing atmosphere is a hydrogen-inert gas atmosphere with an oxygen potential (PH2O / PH2) of 0.20 to 0.85, and the temperature is maintained for 60 seconds or more, and then the steel sheet is cooled. If the oxygen potential (PH2O / PH2) is less than 0.20, the steel sheet cannot be decarburized sufficiently, and the magnetic properties of the final product are deteriorated. In addition, the supply of oxygen is insufficient, and the internal oxide layer is hardly generated, and as a result, the glass film is hardly obtained, and the magnetic properties are significantly deteriorated. On the other hand, if the oxygen potential (PH2O / PH2) is greater than 0.85, the supply of oxygen becomes excessive, and a thick internal oxide layer is formed not only in the groove recesses having the molten layer that serves as a diffusion path, but also in the flat parts, and as a result, a thick glass film is formed in places other than the groove recesses, causing iron loss deterioration due to a decrease in magnetic flux density. The glass coating is a non-magnetic layer, and if it is formed too thick it will cause a decrease in magnetic flux density.

[0052] Furthermore, by controlling the temperature rise process conditions of the decarburization annealing, it is possible to promote internal oxidation of the groove recesses in the subsequent decarburization annealing process, and to form a thick glass coating in the subsequent finish annealing process. For example, the residence time at 200 to 700°C in the temperature rise process of the decarburization annealing is preferably 50 seconds or less. If the residence time is 50 seconds or less, internal oxidation is promoted in the subsequent decarburization annealing soaking process, which is preferable. In addition, the oxygen potential (PH2O / PH2) in the temperature rise process of the decarburization annealing is also preferably 0.85 or less, more preferably 0.60 or less. If it is 0.85 or less, internal oxidation is easily promoted in the subsequent decarburization annealing soaking process, which is preferable.

[0053] The nitriding can be carried out by a known method, and the amount of nitriding can be set within the range of, for example, 50 to 400 ppm, with particularly good characteristics being obtained within the range of 180 to 250 ppm.

[0054] The glass coating is formed by winding up the steel sheet coated with the annealing separator into a coil, holding it at a temperature of 1150 to 1250° C. for 10 to 30 hours, and then cooling it. The composition of the annealing separator may be a known one, for example, MgO: 100 parts by mass, TiO2: 5 parts by mass, and the additive may be, for example, FeCl2 added to give 200 ppm of chlorine.

[0055] To further reduce iron loss, the moisture release rate from the annealing separator during final annealing (room temperature to 700°C) is preferably 0.5% to 6.0%. If the moisture release rate is 0.5% to 6.0%, the aggregation of the internal oxide layer on the steel sheet surface side is suppressed during the temperature rise process of final annealing, and the formation of fine grains in the glass coating is promoted.

[0056] Although the glass coating alone can provide tension to the steel sheet, a tension coating (insulating coating) may be formed on the glass coating to enhance the magnetic domain control effect. The tension coating may be, for example, one whose main component is aluminum phosphate, and may have a thickness of about 1 μm. EXAMPLES

[0057] Next, an example of the present invention will be described. The example is one embodiment of the present invention, and the present invention is not limited to this embodiment.

[0058] <Example 1> A slab containing 3.3 mass% Si, 0.10 mass% Mn, 0.006 mass% S, 0.060 mass% C, 0.027 mass% acid-soluble Al, 0.008 mass% N, with the balance being Fe and impurities, was used as the raw material and hot-rolled by a known method, followed by hot-rolled sheet annealing, and then cold-rolled to obtain a steel sheet with a final sheet thickness of 0.22 mm.

[0059] Next, the surface of the steel sheet was irradiated with a laser to form a plurality of grooves extending in a direction intersecting the rolling direction along the rolling direction. The groove formation direction was inclined at an angle of 0° to 50° to the rolling direction with respect to the width direction of the steel sheet, with the groove width set to 3 to 320 μm, the groove depth set to 5 to 50 μm, and the groove interval set to 0.9 to 32 mm.

[0060] The laser beam irradiation conditions were a laser output of 2000 W, a laser scanning speed of 45 m / s, and a focal spot diameter ratio of 0.25 and 1.50 in the rolling direction of the laser beam.

[0061] During laser irradiation, air was blown at 100 liters / min as an assist gas to efficiently remove the metal from the steel plate that had been melted and evaporated by the laser.

[0062] The cold-rolled steel sheet with the grooves was decarburized and then subjected to nitriding. The decarburization annealing conditions were to heat up to 850°C, hold for 60 seconds, and then cool. The decarburization annealing atmosphere was a hydrogen-nitrogen atmosphere with an oxygen potential (PH2O / PH2) of 0.30 to 0.50. The amount of nitriding was 200 ppm.

[0063] After that, an annealing separator mainly composed of MgO was applied at a rate of 4 g / m on one side. 2 The composition of the annealing separator was 100 parts by mass of MgO, 5 parts by mass of TiO2, and FeCl2 was added to the annealing separator so that the amount of FeCl2 was 200 ppm in terms of chlorine.

[0064] Next, the steel sheet coated with the annealing separator was wound into a coil, held at 1200°C for 20 hours, and then cooled to form a glass coating on the surface. A tension coating containing aluminum phosphate as the main component was then formed to a thickness of 1 μm to obtain a grain-oriented electrical steel sheet. The tension in this case, including the glass coating, was 12 MPa in the rolling direction.

[0065] After applying the tension insulation coating, the core loss W17 / 50 (energy loss measured under excitation conditions of 1.7 T and 50 Hz) and magnetic flux density B8 (magnetic flux density at a magnetizing force of 800 A / m) were measured. The results are shown in Table 1.

[0066] The magnetic flux densities of Nos. A1 to A11 and a1 to a16 are all about the same. However, while the iron loss of Nos. A1 to A11 is good at less than 0.750 W / kg, the iron loss of Nos. a1 to a16, whose groove shape or glass coating thickness falls outside the range of the above-mentioned specified values, is inferior at 0.750 W / kg or more.

[0067] <Example 2> A slab containing 3.3 mass% Si, 0.10 mass% Mn, 0.006 mass% S, 0.060 mass% C, 0.027 mass% acid-soluble Al, 0.008 mass% N, with the balance being Fe and impurities, was used as the raw material and hot-rolled by a known method, followed by hot-rolled sheet annealing, and then cold-rolled to obtain a steel sheet with a final sheet thickness of 0.22 mm.

[0068] Next, the surface of the steel sheet was irradiated with a laser to form multiple grooves extending in a direction intersecting the rolling direction at intervals of 3 mm along the rolling direction. The groove formation direction was inclined at 10° to the rolling direction with respect to the width direction of the steel sheet, with the groove width being 30 μm and the groove depth being 25 μm.

[0069] The laser beam irradiation conditions were a laser output of 1800-2000 W, a laser scanning speed of 40-60 m / s, and the focal spot diameter ratio of the laser beam in the rolling direction was controlled in the range of 0.03-27.

[0070] During laser irradiation, air was blown at 100 liters / min as an assist gas to efficiently remove the metal from the steel plate that had been melted and evaporated by the laser.

[0071] The cold-rolled steel sheet with the grooves was decarburized and then subjected to nitriding. The decarburization annealing conditions were to heat up to 850°C, hold for 60 seconds, and then cool. The decarburization annealing atmosphere was a hydrogen-nitrogen atmosphere with an oxygen potential (PH2O / PH2) of 0.12 to 1.10. The amount of nitriding was 200 ppm.

[0072] After that, an annealing separator mainly composed of MgO was applied at a rate of 4 g / m on one side. 2 The composition of the annealing separator was 100 parts by mass of MgO, 5 parts by mass of TiO2, and FeCl2 was added to the annealing separator so that the amount of FeCl2 was 200 ppm in terms of chlorine.

[0073] Next, the steel sheet coated with the annealing separator was wound into a coil, held at 1200°C for 20 hours, and then cooled to form a glass coating on the surface. A tension coating containing aluminum phosphate as the main component was then formed to a thickness of 1 μm to obtain a grain-oriented electrical steel sheet. The tension in this case, including the glass coating, was 12 MPa in the rolling direction.

[0074] After applying the tensile insulation coating, the core loss W17 / 50 (energy loss measured under excitation conditions of 1.7 T and 50 Hz) and magnetic flux density B8 were measured. The results are shown in Table 2.

[0075] The magnetic flux density is almost the same for Nos. B1 to B12 and b1 to b4. However, the iron loss of Nos. B1 to B12, whose glass coating thickness relationship satisfies the above-mentioned specification, is less than 0.750 W / kg, which is favorable, whereas the iron loss of Nos. b1 to b4, whose glass coating thickness relationship falls outside the above-mentioned range, is inferior, being 0.750 W / kg or more. In addition, the glass coating thickness relationship of b5 and b6 falls outside the above-mentioned range, and at the same time, the iron loss is significantly inferior due to poor decarburization. Furthermore, the glass coating thickness relationship of b7 and b8 falls outside the above-mentioned range, and the magnetic flux density is low, resulting in significantly inferior iron loss.

[0076] [Table 1]

[0077] [Table 2] [Industrial Applicability]

[0078] The present invention can be used in industrial equipment that uses grain-oriented electrical steel sheets, such as winding cores for transformers. [Explanation of symbols]

[0079] 1 Base steel plate 2 grooves 3 Base steel plate 4 Glass Coating 5 Base steel plate 6 Glass Coating θ is the angle between the direction perpendicular to the rolling direction of the base steel sheet (sheet width direction) and the longitudinal direction of the groove W Groove width D Groove Depth P Groove Spacing t1 Glass coating thickness on the flat part of the base steel plate t2 Glass coating thickness at the deepest part of the groove t3 Glass coating thickness on the side of the groove

Claims

1. A grain-oriented electrical steel sheet comprising a base steel sheet having a plurality of grooves on a surface of the steel sheet, and a glass coating on the surface of the base steel sheet, The angle θ between the direction perpendicular to the rolling direction and the plate thickness direction of the base steel plate and the longitudinal direction of the groove is 0 to 40°, the width W of the groove is 20 to 300 μm, the depth D of the groove is 10 to 40 μm, and the interval P of the groove in the rolling direction is 1.0 to 30.0 mm, a glass coating thickness at the flat portion of the surface of the base steel sheet is t1, a glass coating thickness at the deepest portion of the groove in the recess of the groove is t2, and a glass coating thickness at the side portion of the groove is t3, the glass coating satisfying the relational expression (1). (t2+t3) / 2≧t1... Formula (1)

2. A method for producing the grain-oriented electrical steel sheet according to claim 1, comprising the steps of:

1. A method for producing a grain-oriented electrical steel sheet, comprising: a groove formation step of irradiating a surface of a steel sheet with a laser to form grooves; and a decarburization annealing step subsequent to the groove formation step, wherein the irradiation conditions of the laser beam in the groove formation step are such that a focused spot diameter dL of the laser beam in the rolling direction and a focused spot diameter dC of the laser beam in the sheet width direction satisfy formula (3), and the oxygen potential (PH2O / PH2) in the annealing step is 0.20 or more and 0.85 or less. 0.010≦dL / dC≦1.000... Formula (3)

Images