Golf club head and method for manufacturing the same

Abstract

To improve the durability of the hitting face. [Solution] A golf club head 1 having a hollow portion i inside, including a striking face 2 made of a metal material. The striking face 2 includes a striking surface 21 for striking a golf ball and a back surface 22 opposite to the striking surface 21. At least a portion of the back surface 22 is provided with a smooth back surface portion 10 having an arithmetic mean roughness of 1 μm or less and a maximum height roughness of 10 μm or less.

Description

Technical Field

[0001] The present invention relates to a golf club head and a method for manufacturing the same.

Background Art

[0002] In recent years, various golf club heads have been proposed that aim to improve the hitting distance by enhancing the resilience performance. With the improvement of the resilience performance, such golf club heads generate high stress on the hitting face that collides with the golf ball. When high stress is repeatedly applied to the hitting face, the hitting face may be damaged.

[0003] Conventionally, in order to improve the durability of the hitting face of a golf club head, for example, Patent Document 1 below has been proposed. This technique expects to improve durability by providing a thick rib or the like on the back surface of the hitting face.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] The main object of the present invention is to provide a golf club head that can improve the durability of the hitting face by an approach different from that of Patent Document 1.

Means for Solving the Problems

[0006] The present invention relates to a golf club head having a hollow portion inside, comprising a striking face made of a metal material, wherein the striking face comprises a striking surface for striking a golf ball and a back surface opposite to the striking surface, and at least a portion of the back surface has a smooth back surface portion having an arithmetic mean roughness of 1 μm or less and a maximum height roughness of 10 μm or less. [Effects of the Invention]

[0007] By adopting the above configuration, the golf club head of the present invention can improve the durability of the striking face. [Brief explanation of the drawing]

[0008] [Figure 1] This is a front view of the golf club head of this embodiment. [Figure 2] Figure 1 is a side view of the golf club head as seen from the heel side. [Figure 3] This is a partial cross-sectional view taken along line III-III in Figure 1. [Figure 4] (A) is a view of the golf club head from the direction of the face normal, and (B) is a cross-sectional view of (A) at s1. [Figure 5] This is a cross-sectional view of the striking face of a golf club head in another embodiment. [Figure 6] (A) is a plan view of the test specimen, and (B) is a side view thereof. [Figure 7] This is a front view illustrating the layout of a bending fatigue testing machine. [Modes for carrying out the invention]

[0009] One embodiment of the present invention will be described below with reference to the drawings. The drawings may contain exaggerations or representations that differ from the actual structural dimensional ratios in order to aid in understanding the present invention. Furthermore, where there are multiple embodiments, modifications, etc., the same or common elements are denoted by the same reference numerals throughout the specification, and redundant explanations are omitted. Moreover, the specific configurations shown in the embodiments and drawings are for the purpose of understanding the content of the present invention, and the present invention is not limited to the specific configurations shown in the illustrations.

[0010] Figure 1 is a front view of the golf club head (hereinafter sometimes simply referred to as "head") 1 of this embodiment, Figure 2 is a side view of the head 1 of Figure 1 as seen from the heel side, and Figure 3 is a partial cross-sectional view taken along line III-III of Figure 1.

[0011] In Figures 1-3, head 1 is in the reference state. The reference state is when head 1 is held at the lie angle α and loft angle β specified by the manufacturing plant and placed on the horizontal plane HP. The lie angle α and loft angle β are usually listed in the manufacturer's catalog. In the reference state, the hosel axis centerline CL of head 1 is located within the reference vertical plane VP. Unless otherwise specified in this specification, head 1 is assumed to be in the reference state.

[0012] In this specification, an xyz coordinate system is shown to describe the orientation of head 1. The x-axis is parallel to both the reference vertical plane VP and the horizontal plane HP. The y-axis is perpendicular to the reference vertical plane VP and parallel to the horizontal plane HP. The z-axis is perpendicular to both the x-axis and the y-axis. With respect to head 1, the direction along the x-axis is defined as the toe-heel direction, the direction along the y-axis is defined as the front-back direction, and the direction along the z-axis is defined as the up-down direction. With respect to the front-back direction of head 1, the side with the striking face 2 is the front side, and the opposite side is the rear side.

[0013] As shown in Figure 3, the head 1 of this embodiment has a hollow section i inside. The hollow section i may be left as a cavity, or it may be filled with foamed resin or the like.

[0014] FIG. 1 shows a wood-type head 1 as an embodiment of the present invention. The wood-type head 1 is intended to include various fairway woods, for example, in addition to a driver (#1). As long as it has a hollow portion i, the head 1 may be configured as a hybrid type, an iron type, or even a putter type.

[0015] The head 1 of this embodiment includes, for example, a striking face 2, a crown 3, a sole 4, a toe 5, a heel 6, and a hosel 7.

[0016] The striking face 2 is formed of a metal material. In this embodiment, the entire head 1 including the striking face 2 is made of a metal material. The metal material constituting the striking face 2 is not particularly limited, and for example, pure titanium, titanium alloy, stainless steel, maraging steel, aluminum alloy, magnesium alloy, tungsten-nickel alloy, etc. can be adopted. In other embodiments, at least a part of the head 1 other than the striking face 2 may be made of a non-metal material such as CFRP.

[0017] The striking face 2 includes a striking surface 21 for striking a golf ball and a back surface 22 on the opposite side of the striking surface 21.

[0018] The striking surface 21 may be provided with markings such as face lines, punch marks, and fine grooves for enhancing the friction state with the golf ball, but are omitted in the drawings of this embodiment.

[0019] The striking surface 21 is an area surrounded by a face periphery E. In this specification, the face periphery E of the striking face 2a is specified by the edge when the edge is clearly visible in appearance. On the other hand, when the edge is not clear, it is defined as follows.

[0020] First, as shown in Figure 4(A), each cross section s1, s2, s3… containing the normal N connecting the head center of gravity CG and the sweet spot SS is identified. Here, the sweet spot SS is the point where the normal N drawn from the head center of gravity CG to the striking surface 21 intersects the striking surface 21. Next, as shown in Figure 4(B), in each of the cross sections s1, s2, s3…, the position Pe is determined at which the radius of curvature r of the contour line Lf on the outer surface of the head (which is identified by filling in the face line if there is one) first becomes 200 mm or less from the sweet spot SS side toward the face periphery side, and the connection of these positions Pe is defined as the face periphery E of the striking face 2a. Note that the "radius of curvature" of any point on a curve is determined as the radius of a single circle passing through three points: that point, a point on the curve 1 mm away on one side of that point, and a point on the curve 1 mm away on the other side of that point. These "1mm" measurements represent the distance traveled along the curve.

[0021] As shown in Figure 1, the striking surface 21 has a face center FC. The face center FC is determined as follows: First, an arbitrary point P (not shown) is selected near the approximate center of the striking surface 21 in the vertical and toe-heel directions. Next, a plane is determined that passes through this point P, extends along the normal direction of the striking surface 21 at point P, and is in the toe-heel direction. A line is drawn between this plane and the striking surface 21, and its midpoint Px is determined. Next, a plane is determined that passes through this midpoint Px, extends along the normal direction of the striking surface 21 at point Px, and is in the vertical direction. A line is drawn between this plane and the striking surface 21, and its midpoint Py is determined. Next, a plane is determined that passes through this midpoint Py, extends along the normal direction of the striking surface 21 at point Py, and is in the toe-heel direction. A line is drawn between this plane and the striking surface 21, and its midpoint Px is newly determined. Next, a plane is determined that passes through this new midpoint Px, extends along the normal direction of the striking surface 21 at point Px, and is in the vertical direction. A line is drawn where this plane intersects with the striking surface 21, and its midpoint Py is newly determined. These steps are repeated to sequentially determine Px and Py. During the repetition of this process, the new position Py (the last position Py) at which the distance between the new midpoint Py and the immediately preceding midpoint Py first becomes 0.5 mm or less is defined as the face center FC.

[0022] The crown 3 is the portion that extends from the upper face periphery E of the striking surface 21 toward the rear of the head, forming the upper surface of the head 1.

[0023] The sole 4 is the portion that extends from the lower face periphery E of the striking surface 21 toward the rear of the head, so as to form the lower surface of the head 1.

[0024] Toe 5 is the end portion of head 1 located opposite heel 6 in Figure 1.

[0025] The heel 6 is the end portion of the head 1 on the side where the hosel 7 is provided, as shown in Figure 1.

[0026] The hosel 7 includes a hosel bore 7a for mounting a golf club shaft (not shown). The hosel bore 7a is a substantially cylindrical space, and the center line of the hosel bore 7a defines the hosel axis center line CL.

[0027] As shown in Figure 3, the back surface 22 of the striking face 2 faces the hollow portion i. The periphery e of the back surface 22 is determined by connecting the intersection points of the normal 23 drawn on the periphery E of the face and the inner surface of the head.

[0028] In this embodiment, at least a portion of the back surface 22 of the striking face 2 is provided with a smooth back surface portion 10 having an arithmetic mean roughness Ra of 1 μm or less and a maximum height roughness Rz of 10 μm or less.

[0029] As a result of various studies, it has been found that the fracture mechanism of the impact face 2 due to impact with a golf ball can be explained by so-called low-cycle fatigue, in which first, tensile stress acting on the back surface 22 during impact with the golf ball causes an initial crack on the back surface 22, then this crack gradually propagates toward the impact surface 21 of the impact face 2, and finally the impact face 2 fractures because it can no longer withstand the impact with the golf ball. As a countermeasure against this fracture mechanism of the impact face 2, the inventors focused on the surface properties of the back surface 22 in order to suppress cracking of the back surface 22 of the impact face 2. They found that by providing a smooth back surface portion 10 on at least a part of the back surface 22 of the impact face 2, in which the arithmetic mean roughness Ra and maximum height roughness Rz are specified within a certain range, the durability of the impact face 2 can be significantly improved.

[0030] More specifically, the smooth back surface 10 has an arithmetic mean roughness Ra of 1 μm or less and a maximum height roughness Rz of 10 μm or less. Such a smooth back surface 10 can suppress the concentration of tensile stress at specific points when a golf ball is struck, and consequently suppress (delay) the initiation of initial cracks on the smooth back surface 10. In this specification, the arithmetic mean roughness Ra and maximum height roughness Rz shall be measured in accordance with JIS B 0601-2013.

[0031] As described above, in order to suppress the concentration of tensile stress at specific points when a golf ball is struck, it is desirable for the arithmetic mean roughness Ra of the smooth back surface 10 to be as small as possible, for example, 0.9 μm or less, more preferably 0.8 μm or less, and especially preferably 0.7 μm or less. The lower limit of the arithmetic mean roughness Ra of the smooth back surface 10 is not particularly limited, but from the viewpoint of processability, it may be set to, for example, 0.3 μm or more.

[0032] Similar to the arithmetic mean roughness Ra, the maximum height roughness Rz of the smooth back surface 10 is also desirable to be as small as possible, for example, 9 μm or less, even more preferably 8 μm or less, and especially 7 μm or less. The lower limit of the maximum height roughness Rz of the smooth back surface 10 is not particularly limited, but from the viewpoint of processability, it may be set to, for example, 3 μm or more.

[0033] As described above, the head 1 of this embodiment can improve the durability of the striking face 2 by providing a smooth back surface portion 10 on the back surface 22 of the striking face 2, using a different approach than before. Furthermore, the present invention improves the durability of the striking face 2 by improving the microscopic surface properties of the back surface 22 of the striking face 2. For this reason, the head 1 of the present invention can be adopted in various forms without any particular limitations regarding the thickness distribution of the striking face 2.

[0034] The position where the smooth back surface portion 10 is formed on the back surface 22 of the striking face 2 is not particularly limited. On the other hand, empirically, cracks and other damage to the striking face 2 can occur near the face center FC, where it frequently collides with the golf ball. Therefore, it is desirable that the smooth back surface portion 10 be provided at least at a position corresponding to the face center FC on the back surface 22. Specifically, as shown in Figure 3, the smooth back surface portion 10 may be provided so as to include a position that passes through the face center FC and intersects with the normal 24 perpendicular to the striking surface 21.

[0035] In this embodiment, in the most preferred configuration, the smooth back surface portion 10 is formed over the entire area of ​​the back surface 22 of the striking face 2. In other embodiments, the smooth back surface portion 10 may be provided only on a portion of the back surface 22 of the striking face 2. In such embodiments as well, an improvement in the durability of the striking face 2 can be expected in the portion where the smooth back surface portion 10 is formed. In this case, the ratio of the area of ​​the smooth back surface portion 10 to the area of ​​the back surface 22 is not particularly limited. Preferably, the area of ​​the smooth back surface portion 10 is 18% or more, more preferably 25% or more, of the area of ​​the back surface 22 of the striking face 2.

[0036] Figure 5 shows a cross-sectional view of the striking face 2 of the head 1 according to another embodiment of the present invention. As shown in Figure 5, in this embodiment, the striking face 2 includes a first portion 201 having a first thickness t1 and a second portion 202 having a second thickness t2 which is smaller than the first thickness t1. In this embodiment, the first portion 201 is located in the central part of the striking face 2, including the face center FC, and the second portion 202 is formed on the peripheral side of the striking face 2.

[0037] In this embodiment as well, a smooth back surface portion 10 is formed on the back surface 22 of the striking face 2, but the smooth back surface portion 10 is provided at least on the second portion 202 (the back surface of the second portion 202). Based on experience, damage such as cracks in the striking face 2 may occur in areas where the thickness of the striking face 2 is small. In this embodiment, by forming the smooth back surface portion 10 on the back side of the second portion 202, which has a small thickness, the occurrence of cracks in the second portion 202 can be suppressed, and consequently, the durability of the striking face 2 can be improved. Note that the area in which the smooth back surface portion 10 is formed may be limited to only the second portion 202. In this case, the area to be machined for the smooth back surface portion 10 is reduced, which improves the productivity of the head 1.

[0038] To further improve the durability of the striking face 2, the smooth back surface 10 may have compressive residual stress. As described above, tensile stress acts on the back surface 22 of the striking face 2 when a golf ball is struck. Therefore, by pre-applying compressive residual stress to the smooth back surface 10, the tensile stress acting on the back surface 22 when a golf ball is struck is reduced. This has the effect of suppressing (delaying) the propagation of cracks that occur on the back surface 22 to the striking surface 21, and further improving the durability of the striking face 2 against fatigue failure.

[0039] The compressible region should preferably have a compressive residual stress of, for example, 440 MPa or more, and more preferably 540 MPa or more. Generally, when manufacturing a metal striking face 2, a certain amount of compressive residual stress is applied by pressing or machining, but this value is at most 400 MPa or less. By having a compressive residual stress greater than these values ​​in the compressible region, the tensile stress acting on the back surface 22 of the striking face 2 when a golf ball is struck can be effectively relieved. There is no particular upper limit to the compressive residual stress of the compressible region, but from the viewpoint of processability, it may be, for example, 1100 MPa, more preferably 1300 MPa, and even more preferably 1500 MPa or less.

[0040] The compressive residual stress of the smooth back surface 10 can be determined by X-ray diffraction, more specifically by sin 2 It can be measured by the Ψ method.

[0041] In this embodiment, the compression region is formed over the entire area of ​​the smooth back surface 10. In other embodiments, the compression region may be formed only on a part of the smooth back surface 10. The compression region has compressive residual stress, which can be expected to further improve durability, but because it requires processing, it may be used only in areas where durability is particularly required. This makes it possible to improve the productivity and durability of the head 1 in a balanced way.

[0042] [How to manufacture the head] Next, a manufacturing method for producing the head 1 of this embodiment will be described. The manufacturing method of this embodiment includes a smoothing step to form a smooth back surface 10 on at least a portion of the back surface 22 of a metal striking face 2, which includes a striking surface 21 for striking a golf ball and a back surface 22 opposite to the striking surface 21, having an arithmetic mean roughness Ra of 1 μm or less and a maximum height roughness Rz of 10 μm or less. Other steps for manufacturing the head 1 can be carried out as appropriate in accordance with convention.

[0043] The smoothing process is not particularly limited as long as it is a processing method that can obtain the smooth back surface 10 described above, but for example, and practically, polishing is preferable. Polishing can be carried out by various methods. In a typical example, polishing is carried out by polishing the back surface 22 of the striking face 2 by hand or by processing machine using abrasive paper such as emery paper or sandpaper and polishing tools. For the abrasive paper, for example, a mesh count in the range of #240 to #2000 is preferably used. In addition, in polishing, it is preferable to replace the abrasive paper so that the abrasive particles become smaller and smaller as the polishing is carried out. Other smoothing processes include, for example, ultra-precision machining that can process the surface with nano-level accuracy.

[0044] Furthermore, the manufacturing method of the head 1 may include a residual stress application step in which compressive residual stress is applied to the back surface 22 by shot peening before the smoothing step. The projectile material, pressure, projection time, etc. for shot peening can be appropriately set according to the target value of the compressive residual stress to be applied. Note that the back surface 22 of the striking face 2 becomes uneven due to shot peening. However, the back surface 22 can be smoothed while retaining compressive residual stress by a subsequent smoothing step by polishing.

[0045] Although embodiments of the present invention have been described in detail above, the present invention is not limited to the specific disclosures described above, and can be implemented with various modifications within the scope of the technical idea described in the claims. [Examples]

[0046] In order to verify the effects of the present invention, various test materials simulating a hitting face were prepared, and bending fatigue tests were conducted on them.

[0047] [Test Example 1] In Test Example 1, test specimens 1 to 4 were prepared from a rolled material (3.9 mm thick) made of α+β titanium alloy. As shown in Figure 6(A), test specimens 1 to 4 have a roughly dumbbell-shaped planar form and a constant thickness of 3.0 mm, as shown in Figure 6(B). Each test specimen is provided with eight through holes for fixing to a jig, and in the bending fatigue test, bending deformation is repeatedly applied to the constricted portion of the dumbbell shape. In Test Example 1, test specimens 1 to 4 were cut so that the roll direction (rolling direction) of the rolled material was the longitudinal direction of the test specimen. The processing steps for test specimens 1 to 4 are described below.

[0048] Test material 1: Test material 1 was prepared by milling the surface corresponding to the striking surface of the striking face of the rolled material, and then performing rough machining and two-stage fine machining with a ball end mill on the surface corresponding to the back surface of the striking face, to a thickness of 3.0 mm. After that, it was press-formed and cut out in the contour shape shown in Figure 6(A). No other processing was performed, and it does not have a smooth back surface.

[0049] Test material 2: Test specimen 2 was further modified from test specimen 1 by polishing the surface corresponding to the back of the striking face using abrasive paper to create a smooth back surface. Emery paper from #240 to #2000 was used sequentially for the polishing process.

[0050] Test material 3: Test specimen 3 was further modified from test specimen 1 by applying compressive residual stress to the surface corresponding to the back of the impact face by shot peening under the following conditions. Although test specimen 3 does not have a smooth back surface, it has a region subject to compression. Projectile material: Steel ball, 0.5 mm in diameter Pressure: Approximately 0.5 MPa Projection time: about 5 seconds

[0051] Test material 4: Test specimen 4 was subjected to abrasive polishing using abrasive paper on the surface corresponding to the impact face of test specimen 3, which had undergone shot peening. Emery paper of #240 to #2000 grit was used sequentially during the polishing process. Therefore, test specimen 4 has a smooth back surface with a compression region.

[0052] Next, bending fatigue tests were performed on test specimens 1 to 4 to simulate the bending history of the impact face when a golf ball is struck. The bending fatigue tests were conducted using a bending fatigue testing machine 100 as shown in Figure 7. This bending fatigue testing machine 100 includes a pair of sliders 101 that can reciprocate only in the horizontal direction, and a jig 103 that is rotatably connected to each slider 101 via bearings 102. Each of the pair of jigs 103 has a horizontal test specimen mounting portion 104. The test specimen is fixed so as to straddle the pair of test specimen mounting portions 104. At this time, the surface of the test specimen corresponding to the back surface of the impact face is fixed facing upward. Therefore, by moving the pair of sliders 101 closer together and further apart, the bending fatigue testing machine 100 causes the jigs 103 to tilt via the bearings 102, causing an upward convex bending deformation in the test specimen. This type of bending deformation allows tensile stress to be applied to the upper surface of the test material, similar to the impact of a golf ball on a golf club face. In this bending fatigue test, the number of bending deformations until the test material fractured was measured.

[0053] Furthermore, the compressive residual stress on the surface corresponding to the back of the impact face of each test material was measured by X-ray diffraction. The measurement was performed using an X-ray diffractometer (Rigaku Corporation, fully automated multi-purpose X-ray diffractometer SmartLab), and sin 2 The measurement was performed using the ψ method. A CuKa X-ray source was used, with a target output of 45kV-200mA, and a Rigaku HyPix-3000 detector was used. The target of measurement was the α-Ti phase, and seven points were measured at angles Ψ (angle between the diffraction plane normal (crystal plane normal) and the sample plane normal) between 0° and 45°. The test results are shown in Table 1.

[0054] [Table 1]

[0055] [Test Example 2] In Test Example 2, test specimens 5 to 8 were prepared. Test specimens 5 to 8 also use the same rolled steel as in Test Example 1, but differ in that they were cut so that the roll direction (rolling direction) of the rolled steel was aligned with the shorter side of the test specimen. Each of the test specimens 5 to 8 was manufactured according to the processing steps of test specimens 1 to 4. The test results are shown in Table 2.

[0056] [Table 2]

[0057] The test results confirmed that test specimens 2 and 6, corresponding to embodiments of the impact face of the present invention, showed a significantly increased number of bending deformations before fracture compared to test specimens 1 and 5, respectively. Furthermore, test specimens 4 and 8, to which compressive residual stress was applied to test specimens 2 and 6, showed an even greater increase in the number of bending deformations before fracture. Therefore, those skilled in the art will easily understand that the durability against impact from golf balls will also be improved in golf club heads using these test specimens.

[0058] [Note] The present invention includes the following embodiments.

[0059] [Invention 1] A golf club head having a hollow section inside, Includes a striking face made of metal material, The striking face includes a striking surface for striking a golf ball and a back surface opposite to the striking surface. At least a portion of the aforementioned back surface is provided with a smooth back surface portion having an arithmetic mean roughness of 1 μm or less and a maximum height roughness of 10 μm or less. Golf club head. [Invention 2] The golf club head according to the present invention 1, wherein the arithmetic mean roughness of the smooth back surface is 0.9 μm or less. [Invention 3] The golf club head according to invention 1 or 2, wherein the maximum height roughness of the smooth back surface is 9 μm or less. [4th Invention] The smooth back surface has a compressive residual stress of 440 MPa or more, as described in any one of inventions 1 to 3. [5th ​​Invention] The striking surface includes the face center, The golf club head according to any one of inventions 1 to 4, wherein the smooth back surface is provided such that it passes through the face center and includes a position that intersects with a normal perpendicular to the striking surface. [Invention 6] The golf club head according to any one of inventions 1 to 5, wherein the smooth back surface has an area of ​​18% or more of the area of ​​the back surface of the striking face. [7th Invention] The striking face includes a first portion having a first thickness and a second portion having a second thickness smaller than the first thickness. The smooth back surface portion is provided in the second portion, as described in any one of inventions 1 to 6. [8th Invention] A method for manufacturing a golf club head, The invention includes a smoothing step of forming a smooth back surface portion on at least a portion of the back surface of a metal striking face, which includes a striking surface for striking a golf ball and a back surface opposite to the striking surface, having an arithmetic mean roughness of 1 μm or less and a maximum height roughness of 10 μm or less. A method for manufacturing golf club heads. [Invention 9] The method for manufacturing a golf club head according to the present invention, wherein the smoothing step includes polishing. [Invention 10] A method for manufacturing a golf club head according to the present invention, comprising a residual stress application step of applying compressive residual stress to the back surface by shot peening before the smoothing step. [Explanation of Symbols]

[0060] 1 head 2. Hitting Face 3 Crown 4 soles 5 Tou 6 heels 7. Hosel 10 Smooth back surface 21. The hitting surface of the hitting face 22 Back of the hitting face 201 Part 1 202 Part 2 FC Face Center i Hollow part

Claims

1. A golf club head having a hollow section inside, Includes a striking face made of metal material, The striking face includes a striking surface for striking a golf ball and a back surface opposite to the striking surface. At least a portion of the aforementioned back surface is provided with a smooth back surface portion having an arithmetic mean roughness of 1 μm or less and a maximum height roughness of 10 μm or less. Golf club head.

2. The golf club head according to claim 1, wherein the arithmetic mean roughness of the smooth back surface is 0.9 μm or less.

3. The golf club head according to claim 1, wherein the maximum height roughness of the smooth back surface is 9 μm or less.

4. The golf club head according to claim 1, wherein the smooth back surface has a compressive residual stress of 440 MPa or more.

5. The striking surface includes the face center, The golf club head according to any one of claims 1 to 4, wherein the smooth back surface is provided such that it passes through the face center and includes a position that intersects with a normal perpendicular to the striking surface.

6. The golf club head according to claim 5, wherein the smooth back surface has an area of ​​18% or more of the area of ​​the back surface of the striking face.

7. The striking face includes a first portion having a first thickness and a second portion having a second thickness smaller than the first thickness. The smooth back surface portion is provided in the second portion, as described in any one of claims 1 to 4.

8. A method for manufacturing a golf club head, The invention includes a smoothing step of forming a smooth back surface portion on at least a portion of the back surface of a metal striking face, which includes a striking surface for striking a golf ball and a back surface opposite to the striking surface, having an arithmetic mean roughness of 1 μm or less and a maximum height roughness of 10 μm or less. A method for manufacturing golf club heads.

9. The method for manufacturing a golf club head according to claim 8, wherein the smoothing step includes polishing.

10. A method for manufacturing a golf club head according to claim 8 or 9, further comprising a residual stress application step of applying compressive residual stress to the back surface by shot peening before the smoothing step.

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