[0001]The present invention relates to a groove processing device and a groove processing method that form a groove in an object using a laser. The present application claims priority based on Japanese Patent Application No. 2019-091043 filed on May 14, 2019, the contents of which are incorporated herein by reference. [Related Art] [0002] In the related art, a groove processing device is known which irradiate a surface of a steel sheet with a laser beam in a direction (scanning direction) intersecting a sheet travelling direction of the steel sheet, using a polygon mirror, to periodically form a groove in the surface of the steel sheet, thereby improving iron loss characteristics (see, for example, Patent Document 1). [Prior Art Document] [Patent Document] [0003] [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2002-292484 [Disclosure of the Invention] [Problems to be Solved by the Invention] [0004] As shown in FIGS. 1A and IB, a laser beam LB incident on a polygon mirror - 1 - 10 of the groove processing device is not a point light source and has a predetermined radius (p. [0005] As shown in FIG. 1 A, when the laser beam LB is incident so as to fall within one surface of the polygon mirror 10, the laser beam LB reflected by the polygon mirror 10 is focused on one spot on the surface of the steel sheet 20 through a condensing lens (hereinafter, simply referred to as a lens) 12, and a groove is formed at the spot on the surface of the steel sheet 20. [0006] On the other hand, as shown in FIG. IB, when the laser beam LB is incident on a corner portion in which two adjacent surfaces of the polygon mirror 10 meet, the laser beam LB is reflected from each of the two adjacent surfaces and is divided into two laser beams LBl and LB2. The divided laser beams LBl and LB2 are focused on the surface of the steel sheet 20 through the lens 12. As a result, an end portion of the groove in the scanning direction is processed by the laser beams LBl and LB2 with insufficient energy densities. Therefore, the end portion of the groove is shallow, and it is difficult to form a uniform groove. In addition, the divided laser beams LBl and LB2 are irradiated in a direction different from that of the laser beam LB. Therefore, there is a concern that a position different from the position where a groove is to be formed in the surface of the steel sheet 20 or devices and the like other than the surface of the steel sheet 20 will be erroneously processed. [0007] In order to avoid this situation, a configuration is considered in which a shielding plate, such as a mask, is provided such that a portion corresponding to the end portion of the groove in the surface of the steel sheet 20 is not irradiated with the - 2 - laser beams LB1 and LB2. However, this configuration has a problem that the shielding plate is processed and optical components are contaminated by minute pieces of the shielding plate generated by the processing. [0008] The invention has been made in view of the above-mentioned problems, and an object of the invention is to provide a groove processing device and a groove processing method that suppress the contamination of optical components and achieve uniform groove processing and groove depth. [Means for Solving the Problem] [0009] Means for solving the problems include the following aspects. (1) According to an embodiment of the invention, there is provided a groove processing device that forms a groove in a surface of an object using laser beams. The groove processing device includes: a light source device that outputs the laser beams; a polygon mirror that reflects the laser beams output from the light source device; a condensing optical system that is provided on an optical path of the laser beams reflected by the polygon mirror and focuses the laser beams; and a shielding plate that is provided between the condensing optical system and the object at a position which blocks some of the laser beams focused through the condensing optical system and blocks some of the laser beams. Among the laser beams focused through the condensing optical system, some of the laser beams that are not blocked by the shielding plate form the groove in the surface of the object at a focus of the laser beams. The shielding plate is provided closer to the condensing optical system than the focus and is rotated with respect to the surface of the object so as to block the laser beams that do not form the groove. - 3 - (2) In the groove processing device according to (1), when an angle of the shielding plate with respect to the surface of the object is \|/ and a critical angle which is a maximum angle at which the laser beam falls within one plane mirror of the polygon mirror is 0c(°), the angle \|/ of the shielding plate may be inclined in a range of 29c < \|/ < 90(°). (3) In the groove processing device according to (2), assuming that a position where a perpendicular line is drawn from a rotation axis of the polygon mirror to the plane mirror of the polygon mirror is a reference position, an angle formed between a boundary between two adjacent plane mirrors of the polygon mirror and the reference position is 90(°), a position where the shielding plate, which is inclined at the angle \|/, is irradiated with the laser beam reflected by the polygon mirror at an angle of 290(°) when a rotation angle of the polygon mirror is 90(°) is a point PO, a position where the shielding plate, which is inclined at the angle \|/, is irradiated with the laser beam reflected by the polygon mirror at an angle of 29c(°) when the rotation angle of the polygon mirror is 9c(°) is a point P, a height difference between the point P and the point PO is LpO, and a distance from the condensing optical system to a height of the point P is L2, LpO < L2 may be satisfied. (4) The groove processing device according to any one of (1) to (3) may further include: a position adjustment portion that adjusts a position of the shielding plate in a scanning direction in which scanning is performed with the laser beams by the polygon mirror. (5) The groove processing device according to any one of (1) to (4) may further include: a housing that has the shielding plate disposed in a lower portion. The housing may have an upper opening portion that is located on the optical path of the laser beams focused by the condensing optical system, and a colorless and - 4 - transparent window plate that transmits the laser beams without absorbing or reflecting the laser beams may be attached to the upper opening portion. (6) According to an embodiment of the invention, there is provided a groove processing method that forms a groove in a surface of an object using laser beams. The groove processing method includes: an output step of outputting the laser beams from a light source device; a reflection step of reflecting the laser beams output from the light source device by a polygon mirror; a condensing step of focusing the laser beams on the surface of the object using a condensing optical system that is provided on an optical path of the laser beams reflected by the polygon mirror; and a shielding step of blocking some of the laser beams using a shielding plate that is provided between the condensing optical system and the object at a position which blocks some of the laser beams focused through the condensing optical system. Among the laser beams focused through the condensing optical system, some of the laser beams that are not blocked by the shielding plate form the groove in the surface of the object at a focus of the laser beams. In the shielding step, the shielding plate is provided closer to the condensing optical system than the focus and is rotated with respect to the surface of the object so as to block the laser beams that do not form the groove. (7) In the groove processing method according to (6), in the shielding step, when an angle of the shielding plate with respect to the surface of the object is \|/ and a critical angle which is a maximum angle at which the laser beam falls within one plane mirror of the polygon mirror is 6c(°), the angle \|/ of the shielding plate may be inclined in a range of 29c < \|/ < 90(°). (8) In the groove processing method according to (7), in the shielding step, assuming that a position where a perpendicular line is drawn from a rotation axis of the polygon mirror to the plane mirror of the polygon mirror is a reference position, an - 5 - angle formed between a boundary between two adjacent plane mirrors of the polygon mirror and the reference position is 90(°), a position where the shielding plate, which is inclined at the angle \|/, is irradiated with the laser beam reflected by the polygon mirror at an angle of 290(°) when a rotation angle of the polygon mirror is 90(°) is a point PO, a position where the shielding plate, which is inclined at the angle \|/, is irradiated with the laser beam reflected by the polygon mirror at an angle of 29c(°) when the rotation angle of the polygon mirror is 9c(°) is a point P, a height difference between the point P and the point PO is LpO, and a distance from the condensing optical system to a height of the point P is L2, LpO < L2 may be satisfied. (9) The groove processing method according to any one of (6) to (8) may further include: a shielding plate position adjustment step of adjusting a position of the shielding plate in a scanning direction in which scanning is performed with the laser beams by the polygon mirror. (10) The groove processing method according to any one of (6) to (9) may further include: a housing attachment step of attaching a colorless and transparent window plate that transmits the laser beams without absorbing or reflecting the laser beams to an upper opening portion of a housing that has the shielding plate disposed in a lower portion and has the upper opening portion which is located on the optical path of the laser beams focused by the condensing optical system. [Effects of the Invention] [0010] According to the invention, the shielding plate is inclined to reduce the damage of the shielding plate which occurs when the shielding plate blocks the laser beam. Therefore, it is possible to provide a groove processing device and a groove processing method that suppress the contamination of optical components and achieve - 6 - uniform groove processing and groove depth. [Brief Description of the Drawings] [0011] FIG. 1A is a schematic diagram showing a state in which a laser beam reflected from a polygon mirror is focused on a surface of a steel sheet when the laser beam is incident so as to fall within one surface of the polygon mirror. FIG IB is a schematic diagram showing a state in which the laser beam reflected from each of two adjacent surfaces is focused on the surface of the steel sheet when the laser beam is incident across the two adjacent surfaces of the polygon mirror. FIG. 2 is a schematic diagram showing a configuration of a groove processing device according to an embodiment of the invention as viewed from a rolling direction of the steel sheet. FIG. 3 is a schematic diagram showing a rotation angle of the polygon mirror. FIG. 4 is a schematic diagram showing a configuration of a movable shielding plate device. FIG. 5 is a schematic diagram showing the optimum position and angle of a shielding plate. FIG. 6 is a graph showing a relationship between an angle \|/ of the shielding plate and a height difference LpO. [Embodiments of the Invention] [0012] Hereinafter, an embodiment of the invention will be described with reference to the drawings. In the specification and the drawings, the same components are designated by the same reference numerals. [0013] - 7 - FIG. 2 schematically shows a configuration of a groove processing device 100 according to the embodiment of the invention as viewed from a rolling direction of a steel sheet 20. The groove processing device 100 is a device that periodically forms a groove in a surface of the steel sheet 20, which is an object to be processed, using a laser. The steel sheet 20 is made of, for example, a well-known grain-oriented electrical steel sheet material. In the groove processing device 100, the position of the steel sheet 20 in a width direction is set on the basis of the length and position of the groove formed in the surface of the steel sheet 20, and the position of the steel sheet 20 in a longitudinal direction is set on the basis of the dimensions of the groove processing device 100. The width direction of the steel sheet 20 is a scanning direction of the laser and is a left-right direction of the plane of paper in FIG. 2. The longitudinal direction of the steel sheet 20 is the rolling direction of the steel sheet 20 and is a depth direction of the plane of paper in FIG. 2. [0014] As shown in FIG. 2, the groove processing device 100 includes a polygon mirror 10, a light source device 11, a collimator 11 A, a lens 12, and a movable shielding plate device 30. [0015] The polygon mirror 10 has, for example, a regular polygonal prism shape, and a plurality of (N) plane mirrors are provided on each of a plurality of side surfaces constituting a regular polygonal prism. A laser beam LB is incident on the polygon mirror 10 from the light source device 11 through the collimator 11A in one direction (horizontal direction) and is then reflected by the plane mirror (reflection step). [0016] The polygon mirror 10 has a configuration in which it can be rotated on a - 8 - rotation axis Ol by the driving of a motor (not shown), and the incident angle of the laser beam LB on the plane mirror changes sequentially depending on the rotation angle of the polygon mirror 10. Therefore, the polygon mirror 10 sequentially changes the reflection direction of the laser beam LB such that the steel sheet 20 is scanned with the laser beam LB in the width direction. [0017] In addition, FIGS. 1 A, IB, 2, and 3 show an example in which the polygon mirror 10 has eight plane mirrors. However, the number of plane mirrors constituting the polygon mirror 10 is not particularly limited. [0018] The light source device 11 outputs a laser beam using a predetermined irradiation method (for example, a continuous irradiation method or a pulse irradiation method) under the control of a control unit (not shown) (output step). [0019] The collimator 11A is connected to the light source device 11 through an optical fiber cable 15. The collimator 11A adjusts the radius of the laser beam output from the light source device 11 and outputs the adjusted laser beam LB to the polygon mirror 10. The laser beam LB output to the polygon mirror 10 has a laser diameter having a predetermined radius (p, and the laser diameter is that of a circle. However, the laser diameter may be that of an ellipse. In this case, an elliptical condensing shape can be formed by inserting a cylindrical lens or a cylindrical mirror between the collimator 11A and the polygon mirror 10 to change the radius of the beam along one axis (for example, a scanning direction). [0020] The lens 12 is a condensing optical system that is provided on the optical path - 9 - of the laser beam reflected by the polygon mirror 10 and is manufactured by performing processing, such as grinding and polishing, on a piece of glass. In addition, a mirror may be adopted as the condensing optical system constituting the groove processing device 100 instead of the condensing lens 12. [0021] The lens 12 may have a non-condensing portion (not shown) that is integrally provided outside (in the outer circumference of) the lens 12. The non-condensing portion is located on the optical paths of laser beams LBl and LB2 that have been divided and reflected from a corner portion in which two adjacent plane mirrors of the polygon mirror 10 meet and transmits the divided laser beams LBl and LB2. The non-condensing portion is a planar optical system of a donut-shaped plate. The non-condensing portion does not have a focus because the focal length thereof is infinite. Since the laser beams LB 1 and LB2 that have passed through the non-condensing portion are not focused, they do not have a high energy density. Therefore, even when a shielding plate 35 is irradiated with the laser beams LBl and LB2 that have passed through the non-condensing portion, the damage of the shielding plate 35 is small. In addition, the non-condensing portion may not be the planar optical system and may be, for example, an optical system that diverges the divided laser beams LBl and LB2. [0022] The movable shielding plate device 30 which will be described below is provided between the lens 12 and the steel sheet 20. The movable shielding plate device 30 is disposed on the optical path of the laser beam LB that is reflected by the polygon mirror 10 and passes through the lens 12. The laser beam LB reflected by the polygon mirror 10 passes through the lens 12 and the movable shielding plate - 10 - device 30 and is focused on the surface of the steel sheet 20 (condensing step). Therefore, a groove is formed in the surface of the steel sheet 20. [0023] Further, in a groove processing method which irradiates the surface of the steel sheet 20 with the laser beam LB to form a groove, base steel sheet is melted and removed to form a groove. Therefore, as the groove becomes deeper, the probability that a molten protrusion will occur on the surface becomes higher. Therefore, the groove processing device 100 may be configured to include a supply nozzle (not shown) which injects an assist gas for blowing off a molten material and is provided at a predetermined position. Further, the collimator 11 A, the polygon mirror 10, the lens 12, and the movable shielding plate device 30 of the groove processing device 100 may be covered with a housing (not shown), and the inside of the housing may be filled with a clean gas such that the internal pressure of the housing is positive. In this case, it is possible to prevent a molten material and the like from entering the housing and to prevent the optical system of the groove processing device 100 from being contaminated by the molten material and the like. [0024] Next, the rotation angle of the polygon mirror 10 will be described with reference to FIG. 3. In this embodiment, it is assumed that the rotation angle 9(°) of the polygon mirror 10 is defined by a central angle with respect to a reference position for each of the plane mirrors constituting the polygon mirror 10. As shown in FIG. 3, it is assumed that a position where a perpendicular line PL is drawn from the rotation axis Ol of the polygon mirror 10 to a plane mirror 101 is the reference position (9 = 0(°)). The rotation angle 9 of the polygon mirror 10 is an angle (central angle) formed between the position of a center LBc of the laser beam LB incident on each - 11 - plane mirror and the reference position (9 = 0(°)). In FIG. 3, a counterclockwise angle from the reference position (9 = 0(°); the perpendicular line PL) is defined as a positive angle, and a clockwise angle from the reference position is defined as a negative angle. [0025] An angle 90 formed between the reference position (9 = 0(°)) in each plane mirror and a boundary with an adjacent plane mirror is 180(°)/N. The rotation angle 9 of one plane mirror is defined in the range of -90 ^ 9 ^ +90. Therefore, in FIG. 3, the rotation angle 9 = +90 of the plane mirror 101 and the rotation angle 9 = -90 of a plane mirror 102 adjacent to the plane mirror 101 in the counterclockwise direction indicate the same position on the polygon mirror 10. [0026] In this embodiment, the maximum angle at which the incident laser beam LB falls within one surface (one plane mirror) of the polygon mirror 10 is defined as a critical angle 9c. That is, when the laser beam LB is totally reflected by one plane mirror without being divided by a corner portion in which two adjacent plane mirrors of the polygon mirror 10 meet, the critical angle 9c is the maximum angle at which the center LBc of the laser beam LB is located. Assuming that the radius (circumscribed radius) of a circumscribed circle CI of the polygon mirror 10 is R and the radius of the laser beam LB incident on the polygon mirror 10 is (p, the critical angle 9c is defined by the following Expression (1). [0027] 9c = sin1[(R x sin90-