Patent Publication Number: US-2022219262-A1

Title: Groove processing device and groove processing method

Description:
TECHNICAL FIELD OF THE INVENTION 
     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. 2019-091044 filed on May 14, 2019, the contents of which are incorporated herein by reference. 
     RELATED ART 
     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 
     [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2002-292484 
     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     As shown in  FIGS. 1A and 1B , a laser beam LB incident on a polygon mirror  10  of the groove processing device is not a point light source and has a predetermined radius φ. 
     As shown in  FIG. 1A , when the laser beam LB is incident so as to fall within one surface of the polygon mirror  10 , the laser beam LB reflected from the polygon mirror  10  is focused on one spot on the surface of the steel sheet  20  through a condensing lens  12 , and a groove is formed at the spot on the surface of the steel sheet  20 . 
     On the other hand, as shown in  FIG. 1B , 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 LB 1  and LB 2 . The divided laser beams LB 1  and LB 2  are focused on the surface of the steel sheet  20  through the condensing lens  12 . As a result, an end portion of the groove in the scanning direction is processed by the laser beams LB 1  and LB 2  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 LB 1  and LB 2  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 on 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. 
     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 is not irradiated with the laser beams LB 1  and LB 2 . However, this configuration has a problem that the shielding plate is processed, which causes the contamination of optical components. 
     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 achieve uniform groove processing and groove depth without contaminating optical components. 
     Means for Solving the Problem 
     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 a laser beam. The groove processing device includes: a light source device that outputs the laser beam; a polygon mirror that reflects the laser beam output from the light source device; and an optical system that is provided on an optical path of the laser beam reflected from the polygon mirror and includes a condensing portion which transmits the laser beam reflected from one surface of the polygon mirror so as to be focused on the surface of the object and a non-condensing portion which is provided outside the condensing portion and transmits the laser beam reflected from a corner portion, in which two adjacent surfaces of the polygon mirror meet, so as not to be focused on the surface of the object. 
     (2) In the groove processing device according to (1), the non-condensing portion may not have a focus. 
     (3) In the groove processing device according to (1), the non-condensing portion may diverge the laser beam reflected from the corner portion of the polygon mirror. 
     (4) The groove processing device according to any one of (1) to (3) may further include: a shielding plate that is provided on the optical path of the laser beam transmitted through the non-condensing portion. 
     (5) 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 a laser beam. The groove processing method includes: an output step of outputting the laser beam from a light source device; a reflection step of reflecting the laser beam output from the light source device by a polygon mirror; a condensing portion passage step of causing the laser beam reflected from one surface of the polygon mirror to pass through a condensing portion so to be focused on the surface of the object; and a non-condensing portion passage step of causing the laser beam reflected from a corner portion, in which two adjacent surfaces of the polygon mirror meet, to pass through a non-condensing portion that is provided outside the condensing portion so as not to be focused on the surface of the object. 
     (6) In the groove processing method according to (5), the non-condensing portion may not have a focus in the non-condensing portion passage step. 
     (7) In the groove processing method according to (5), the non-condensing portion may diverge the laser beam in the non-condensing portion passage step. 
     (8) The groove processing method according to any one of (5) to (7) may further include: a shielding step of blocking the laser beam transmitted through the non-condensing portion in the non-condensing portion passage step using a shielding plate that is provided on the optical path of the laser beam. 
     Effects of the Invention 
     According to the invention, the laser beam reflected from the corner portion of the polygon mirror passes through the non-condensing portion of the optical system. Therefore, a groove is not processed in the surface of the object. As a result, it is possible to provide a groove processing device and a groove processing method that achieve uniform groove processing and groove depth without contaminating optical components. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         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. 1B  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 the size of a lens. 
         FIG. 5A  is a schematic diagram showing the rotation angle of the polygon mirror and an irradiation state of the laser beam. 
         FIG. 5B  is a schematic diagram showing the rotation angle of the polygon mirror and the irradiation state of the laser beam. 
         FIG. 5C  is a schematic diagram showing the rotation angle of the polygon mirror and the irradiation state of the laser beam. 
         FIG. 5D  is a schematic diagram showing the rotation angle of the polygon mirror and the irradiation state of the laser beam. 
         FIG. 5E  is a schematic diagram showing the rotation angle of the polygon mirror and the irradiation state of the laser beam. 
         FIG. 6  is a schematic diagram showing an aspect in which a spot of the laser beam on the surface of the steel sheet changes depending on the rotation angle of the polygon mirror. 
         FIG. 7  is a schematic diagram showing a configuration of a groove processing device according to Modification Example 1 of this embodiment as viewed from the rolling direction of the steel sheet. 
         FIG. 8  is a schematic diagram showing a configuration of a groove processing device according to Modification Example 2 of this embodiment as viewed from the rolling direction of the steel sheet. 
     
    
    
     EMBODIMENTS OF THE INVENTION 
     Hereinafter, embodiments 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. 
       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 form 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 , for example, 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 and is a left-right direction of the plane of paper in  FIG. 2 . The longitudinal direction of the steel sheet  20  is, for example, the rolling direction of the steel sheet  20  and is a depth direction of the plane of paper in  FIG. 2 . 
     As shown in  FIG. 2 , the groove processing device  100  includes a polygon mirror  10 , a light source device  11 , a collimator  11 A, and a lens  13 . 
     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  11 A in one direction (horizontal direction) and is then reflected by the plane mirror (reflection step). 
     The polygon mirror  10  can be rotated on a rotation axis O 1  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 reflection direction of the laser beam LB is sequentially changed to scan the steel sheet  20  with the laser beam LB in the width direction. 
     In addition,  FIGS. 1A to 1B, 2, 3, 5A to 5E, 7, and 8  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. 
     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). 
     The collimator  11 A is connected to the light source device  11  through an optical fiber cable  15 . The collimator  11 A adjusts the radius of the laser beam output from the light source device  11  and outputs the adjusted laser beam LB. The laser beam LB has a laser diameter having a predetermined radius φ, 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  11 A and the polygon mirror  10  to change the radius of the beam along one axis (for example, a scanning direction). 
     The lens  13  is an optical system that is provided on an optical path of the laser beam reflected from the polygon mirror  10  and is manufactured by performing processing, such as grinding and polishing, on a piece of glass. The lens  13  has a condensing portion  13 A and a non-condensing portion  13 B that is integrally provided outside (in the outer circumference of) the condensing portion  13 A. In addition, in the following description, a case in which the lens  13  is composed of a single lens is described as an exemplary example. However, the lens  13  may be composed of a plurality of sets of lenses. A mirror may be adopted instead of the lens  13 . 
     The condensing portion  13 A is located on the optical path of the laser beam LB reflected from one plane mirror of the polygon mirror  10 . The condensing portion  13 A constitutes a condensing optical system that has a radius rc and a focal length f. The laser beam LB reflected from the polygon mirror  10  passes through the condensing portion  13 A and is focused on the surface of the steel sheet  20  (condensing portion passage step). In this way, a groove is formed in the surface of the steel sheet  20 . 
     The non-condensing portion  13 B is located on the optical paths of laser beams LB 1  and LB 2  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 LB 1  and LB 2  (non-condensing portion passage step). The non-condensing portion  13 B is a planar optical system of a donut-shaped flat sheet in which an inner circle has a radius rc (=the radius of the condensing portion  13 A) and an outer circle has a radius r0. The non-condensing portion  13 B does not have a focus because the focal length thereof is infinite. Therefore, the surface of the steel sheet  20  is irradiated with the laser beams LB 1  and LB 2  that have passed through the non-condensing portion  13 B. However, since the laser beams LB 1  and LB 2  are not focused, they do not have a high energy density, and no grooves are formed in the surface of the steel sheet  20 . Further, even in a case in which the surface of the steel sheet  20  is not irradiated with the laser beams LB 1  and LB 2 , the devices and the like around the steel sheet  20  are not erroneously processed by the laser beams LB 1  and LB 2  which deviate from the steel sheet  20 . 
     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) for injecting an assist gas for blowing off a molten material. 
     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 θ(°) 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 a rotation axis O 1  of the polygon mirror  10  to a plane mirror  101  is the reference position (θ=0°). The rotation angle θ 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 plane mirror and the reference position (θ=0°). In  FIG. 3 , a counterclockwise angle from the reference position (θ=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. 
     An angle θ0 formed between the reference position (θ=0°) in each plane mirror and a boundary with an adjacent plane mirror is 180°/N. The rotation angle θ of one plane mirror is defined in the range of −θ0≤θ≤+θ0. Therefore, in  FIG. 3 , the rotation angle θ=+θ0 of the plane mirror  101  and the rotation angle θ=−θ0 of a plane mirror  102  adjacent to the plane mirror  101  in the counterclockwise direction indicate the same position on the polygon mirror  10 . 
     In this embodiment, a 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 θc. 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 θc 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 C 1  of the polygon mirror  10  is R and the radius of the laser beam LB incident on the polygon mirror  10  is φ, the critical angle θc is defined by Expression (1). 
     
       
         
           
             
               
                 
                   
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     The size of the lens  13  can be defined by the rotation angle θ of the polygon mirror  10 . As shown in  FIG. 4 , assuming that the distance between the reference position (θ=0°) on the plane mirror  101  and the condensing portion  13 A is L, the radius rc of the condensing portion  13 A is represented by Expression (2), and the radius of the outer circle of the non-condensing portion  13 B r0 is represented by Expression (3). 
     
       
         
           
             
               
                 
                   
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     For example, it is assumed that the groove processing device  100  is designed with the following specifications: 
     the radius φ of the laser beam LB: 6 mm; 
     the number of plane mirrors N constituting the polygon mirror 10:8; and 
     a circumscribed radius R: 140 mm. 
     Therefore, θ0=22.5° and the critical angle θc=19.9° are established. 
     When the distance L is 50 mm, rc=49.4 mm and r0=58.5 mm are established by Expressions (2) and (3). 
     Next, a relationship between the rotation angle of the polygon mirror  10  and the irradiation state of the laser beam LB will be described with reference to  FIGS. 5A to 5E  and  FIG. 6 . 
       FIGS. 5A to 5E  show the irradiation state of the laser beam LB when the polygon mirror  10  is rotated clockwise from the rotation angle θ=0° of the plane mirror  101  to the rotation angle θ=0° of the adjacent plane mirror  102 . 
     When the plane mirror  101  is at the position where the rotation angle θ is in the range of −θc≤θ≤+θc, the laser beam LB incident on the plane mirror  101  is reflected in a downward direction (a direction toward the surface of the steel sheet  20 ) from the plane mirror  101 , passes through the condensing portion  13 A, and is focused on the surface of the steel sheet  20 . In particular, when the plane mirror  101  is at the position where the rotation angle θ is 0°, as shown in  FIG. 5A , the laser beam LB incident on the plane mirror  101  is reflected in the vertical direction (a direction perpendicular to the surface of the steel sheet  20 ) from the plane mirror  101 , passes through the center of the condensing portion  13 A, and is focused on the surface of the steel sheet  20 . Therefore, when the plane mirror  101  is at the position where the rotation angle θ is in the range of −θc≤θ≤+θc, the position where the laser beam LB is reflected changes with the rotation of the polygon mirror  10  while a state in which the laser beam LB is focused on the surface of the steel sheet  20  is maintained. Therefore, a groove is formed in the surface of the steel sheet  20  in the width direction (scanning direction). 
     When the plane mirror  101  reaches the position where the rotation angle θ is +θc, as shown in  FIG. 5B , the laser beam LB reflected from the plane mirror  101  passes through a first end portion  131  of the condensing portion  13 A and is focused on the surface of the steel sheet  20 . 
     As shown in  FIG. 5C , when the plane mirror  101  exceeds the position where the rotation angle θ is +θ0 and when the plane mirror  102  is in front of the position where the rotation angle θ is −θ0 (that is, when the laser beam LB is incident on a corner portion in which the two adjacent plane mirrors  101  and  102  meet), the incident laser beam LB is reflected by each of the two plane mirrors  101  and  102  and is divided into two laser beams LB 1  and LB 2 . The two laser beams LB 1  and LB 2  pass through the non-condensing portion  13 B, and the surface of the steel sheet  20  or the devices and the like around the steel sheet  20  are irradiated with the laser beams LB 1  and LB 2 . 
     When the plane mirror  102  reaches the position where the rotation angle θ is −θc, as shown in  FIG. 5D , the laser beam LB reflected from the plane mirror  102  passes through a second end portion  132  opposite to the first end portion  131  of the condensing portion  13 A and is focused on the surface of the steel sheet  20 . 
     When the plane mirror  102  is at the position where the rotation angle is in the range of −θc≤θ≤+θc, the laser beam LB incident on the plane mirror  102  is reflected in the downward direction (the direction toward the surface of the steel sheet  20 ) from the plane mirror  102 , passes through the condensing portion  13 A, and is focused on the surface of the steel sheet  20 . In particular, when the plane mirror  102  reaches the position where the rotation angle θ is 0°, as shown in  FIG. 5E , the laser beam LB incident on the plane mirror  102  is reflected in the vertical direction from the plane mirror  102 , passes through the center of the condensing portion  13 A, and is focused on the surface of the steel sheet  20 . 
     In a case in which attention is focused on one plane mirror  101 , when the range of the rotation angle θ is −θc≤θ≤+θc, the laser beam LB reflected from the plane mirror  101  passes through the condensing portion  13 A and is focused on the surface of the steel sheet  20 . On the other hand, when the range of the rotation angle θ is −θ0□θ&lt;−θc or +θc&lt;θ□+θ0, the laser beams LB 1  and LB 2  reflected from the corner portion in which the plane mirror  101  and an adjacent plane mirror meet pass through the non-condensing portion  13 B, and the surface of the steel sheet  20  is irradiated with the laser beams LB 1  and LB 2 . However, the laser beams LB 1  and LB 2  are not focused and do not have a high energy density. 
       FIG. 6  shows a change in the spot of the laser beam on the surface of the steel sheet  20  when the polygon mirror  10  is rotated clockwise from the position where the rotation angle θ of the plane mirror  101  is 0° to the position where the rotation angle θ of the adjacent plane mirror  102  is 0° (see  FIGS. 5A to 5E ). In  FIG. 6 , a dotted line indicates the scanning direction of the laser beam. 
     As shown in  FIG. 6 , when the range of the rotation angle θ of the plane mirror  101  is 0□θ□+θc (see  FIGS. 5A and 5B ), the laser beam LB reflected from the plane mirror  101  is focused on a minute circular spot S 1  on the surface of the steel sheet  20 . As the rotation angle θ increases, the spot S 1  is moved in one direction (to the left in  FIG. 6 ). 
     When the range of the rotation angle θ of the plane mirror  101  is +θc&lt;θ□+θ0 and when the range of the rotation angle θ of the plane mirror  102  is −θ0≤θ&lt;−θc, the laser beam LB is divided into two laser beams LB 1  and LB 2  as described above. The surface of the steel sheet  20  is irradiated with the two laser beams LB 1  and LB 2  through the non-condensing portion  13 B, and two spots S 2  and S 3  corresponding to the two laser beams LB 1  and LB 2  are formed. Since the laser beams LB 1  and LB 2  are not focused on the surface of the steel sheet  20 , each of the spots S 2  and S 3  has a larger area than the spot S 1 . 
     When the range of the rotation angle θ of the plane mirror  101  is +θc&lt;θ&lt;+θ0, the amount of the laser beam LB 1  reflected from the plane mirror  101  is larger than the amount of the laser beam LB 2  reflected from the plane mirror  102 . Therefore, the spot S 2  has a larger area than the spot S 3 . 
     When the plane mirror  101  is at the position where the rotation angle θ is +θ0 and when the plane mirror  102  is at the position where the rotation angle θ is −θ0 (see  FIG. 5C ), the amount of the laser beam LB 1  and the amount of the laser beam LB 2  are equal to each other. Therefore, the area of the spot S 2  is equal to the area of the spot S 3 . 
     When the range of the rotation angle θ of the plane mirror  102  is −θ0&lt;θ&lt;−θc, the amount of the laser beam LB 2  reflected from the plane mirror  102  is larger than the amount of the laser beam LB 1  reflected from the plane mirror  101 . Therefore, the spot S 3  has a larger area than the spot S 2 . 
     When the range of the rotation angle θ of the plane mirror  102  is −θc≤θ≤0 (see  FIGS. 5D and 5E ), the laser beam LB reflected from the plane mirror  102  is focused on the minute circular spot S 1  on the surface of the steel sheet  20 . As the rotation angle θ approaches 0°, the spot S 1  is moved in one direction (to the left in  FIG. 6 ). 
     As described above, according to this embodiment, when the laser beam LB is divided and reflected from the corner portion in which two adjacent surfaces of the polygon mirror  10  meet, the divided laser beams LB 1  and LB 2  pass through the non-condensing portion  13 B and are not focused on the surface of the steel sheet  20  such that energy density is not high. Therefore, no grooves are formed in the surface of the steel sheet  20 . As a result, unlike the related art, an end portion of the groove in the scanning direction is not shallow, and it is possible to achieve uniform groove processing and groove depth and to produce a product having excellent iron loss characteristics. In addition, the devices and the like around the steel sheet  20  are not erroneously processed. 
     In the above-described embodiment shown in  FIGS. 2 to 6 , the non-condensing portion  13 B of the lens  13  is a planar optical system having no focus. However, an optical system ( FIG. 7 ) that diverges the divided laser beams LB 1  and LB 2  may be adopted. 
       FIG. 7  shows a configuration of a groove processing device  200  according to Modification Example 1 of this embodiment as viewed from the rolling direction of the steel sheet  20 . The groove processing device  200  includes a lens  17  instead of the lens  13  of the groove processing device  100  shown in  FIG. 2  and  FIGS. 5A to 5E . 
     The lens  17  is an optical system that is provided on the optical path of the laser beam reflected from the polygon mirror  10  and is manufactured by performing processing, such as grinding and polishing, on a piece of glass. The lens  17  has a condensing portion  17 A and a non-condensing portion  17 B that is integrally provided outside (in the outer circumference of) the condensing portion  17 A. 
     Similar to the condensing portion  13 A of the lens  13 , the condensing portion  17 A is located on the optical path of the laser beam LB reflected from one plane mirror of the polygon mirror  10  and constitutes a condensing optical system having a focal length f. 
     The non-condensing portion  17 B is located on the optical path of the laser beams LB 1  and LB 2  that have been divided and reflected from a corner portion of the polygon mirror  10  and transmits the divided laser beams LB 1  and LB 2 . The non-condensing portion  17 B is thick toward a peripheral portion. In the non-condensing portion  17 B, a surface facing the polygon mirror  10  is a spherical surface which is concave toward the polygon mirror  10 , and a surface facing the steel sheet  20  is a flat surface. In addition, the surface of the non-condensing portion  17 B which faces the steel sheet  20  may be a spherical surface which is concave toward the steel sheet  20 . A boundary portion between the condensing portion  17 A and the non-condensing portion  17 B may have a slightly flat part. The laser beams LB 1  and LB 2  reflected from the corner portion of the polygon mirror  10  are diverged through the non-condensing portion  17 B, and the surface of the steel sheet  20  is irradiated with the laser beams LB 1  and LB 2 . 
     With this configuration, spots formed on the surface of the steel sheet  20  by the laser beams LB 1  and LB 2  passing through the non-condensing portion  17 B have a larger area than the spots S 2  and S 3  shown in  FIG. 6 . Therefore, the irradiation intensity of the laser beams LB 1  and LB 2  to the surface of the steel sheet  20  is lower, and a groove is less likely to be formed in the surface of the steel sheet  20  as compared to the embodiment shown in  FIGS. 2 to 6 . As a result, it is possible to achieve more uniform groove processing and groove depth. 
     Further,  FIG. 8  shows a configuration of a groove processing device  300  according to Modification Example 2 of this embodiment as viewed from the rolling direction of the steel sheet  20 . As shown in  FIG. 8 , the groove processing device  300  includes a shielding plate  19 , such as a mask, which is provided on the optical path of the laser beams LB 1  and LB 2  that have passed through the non-condensing portion  13 B (shielding step). Therefore, the laser beams LB 1  and LB 2  are blocked by the shielding plate  19 . The laser beams LB 1  and LB 2  that have passed through the non-condensing portion  13 B have a lower irradiation intensity than the laser beams LB 1  and LB 2  that have passed through the condensing lens  12  shown in  FIG. 1B . As a result, even when the shielding plate  19  is irradiated with the laser beams LB 1  and LB 2  that have passed through the non-condensing portion  13 B, the damage of the shielding plate  19  is small. 
     In addition, when a configuration in which the lens  17  shown in  FIG. 7  and the shielding plate  19  shown in  FIG. 8  are combined is adopted, it is possible to further suppress the damage of the shielding plate  19 . 
     Further, the content described in this embodiment can be appropriately changed without departing from the scope of the invention. 
     For example, a mirror may be used instead of the lens as the optical system constituting the groove processing device. 
     INDUSTRIAL APPLICABILITY 
     According to the invention, it is possible to provide a groove processing device and a groove processing method that can achieve uniform groove processing and groove depth without contaminating optical components. Therefore, the invention has extremely high industrial applicability. [Brief Description of the Reference Symbols]
           10  Polygon mirror     11  Light source device     11 A Collimator     13 ,  17  Lens     13 A,  17 A Condensing portion     13 B,  17 B Non-condensing portion     15  Optical fiber cable     19  Shielding plate     20  Steel sheet     100 ,  200 ,  300  Groove processing device     101 ,  102  Plane mirror   C 1  Circumscribed circle   LB Laser beam   O 1  Rotation axis   PL Perpendicular line