WO2004078277A1 - Golf club head - Google Patents

Abstract

A golf club head, wherein the hard peening of 0.30 to 0.80 mmA in arc height value is applied to the rear surface of a face part to remain a large compressive stress in the rear surface of the face part, whereby the maximum stress produced in the rear surface of the face part when a ball is hit can be lowered and such effects as to prevent the fatigue, rupture, and deformation of the face part can be provided. Shot materials formed in a spherical shape with a diameter of 1 to 3 mm are suitably used for the peening. By this invention, the face part of the golf club head can be thinned and accordingly the repulsion and the inertia moment of the golf club head can be increased, and the gravity center thereof can be optimized. As a result, a golf club with excellent carry and directivity can be provided.

Description

 Golf club head Technical field

 The present invention relates to a metal golf club head. Background art

 Heretofore, it has been considered that, by subjecting the inner surface of the golf club head to a pinning process, it is possible to remove the scale and improve the fatigue strength by the residual stress. For example, it is described in Japanese Patent Application Laid-Open No. 2003-10366.

 However, according to the studies of the present inventors, it has been found that simply applying a peening treatment often fails to produce an effect, and depending on the conditions, it causes fatigue failure. Specifically, fatigue strength did not improve when a general short pinning process with an arc high value of less than 0.3 mmA was performed. In addition, it was found that the surface was damaged by peening, and it was predicted that this might become the start of fatigue failure.

 In addition, it was also found that peening could cause undesirable deformation of the members, and the fine flaws caused by pinning could cause fatigue failure. Ark high 0 0.80 A hard shot pealing that exceeds 8 mm A will result in significant deformation of the member by the pinning, which may impair the function and aesthetics of the golf club head, etc. It was difficult to make

 In view of such circumstances, the present invention is effective in preventing fatigue, breakage and deformation of the face portion by peening treatment, and it is thinner and has high resilience, large inertia moment, flight distance, An object is to provide a golf club head which is excellent in directionality. Disclosure of the invention

The present invention relates to a golf club head which is characterized in that a hard facing of arc height value of 0.3 mmA or more and less than 0.8 mmA is applied to the back of the face. solid In the case of shot peening using a shot material, the shot material should have a square shape, and a sphere with a diameter of 1 to 3 mm is ideal. Besides the shot peening method, the laser peening method is also suitable. The peening treatment should be performed at least in the center of the base of the substrate.

 According to the above steps, the following actions can be obtained.

 (1) A large compressive stress remains on the back of the face because a hard high-pitched surface of 0.3 mmA or more is applied to the back of the face.

 2. In the case of shot peening, since a shot material with a non-cornered shape is used, surface peening can be prevented by the peening treatment.

 3. The surface of the material has streak-like fine scratches generated during rolling and other processing, but this streak-like fine flaw disappears by the peening treatment. However, in the case of shot piling, the case is where the diameter of the shot material is ~ 3 mm.

 4. Since the arc height value is less than 0.8 mmA, deformation of the member caused by peening can be suppressed. By limiting the peening process to only the face central portion, the deformation of the member can be further minimized.

 In the golf club face portion, a large tensile stress acts on the back of the golf club as it is hit. As described in 1 above, the compressive stress remains on the back face of the golf club according to the present invention. It is offset and the stress generated upon impact is reduced. Furthermore, as described in 3 above, the micro-scratch of the surface originally possessed by the material is eliminated, and the factor that can cause fatigue failure is eliminated. As a result, the golf club head of the present invention has extremely high fatigue strength, and in addition, it becomes difficult to cause permanent deformation due to impact. However, it may be counterproductive to make a new minute flaw by the pinning. Therefore, as described in 2 above, in the case of shot peening which may cause fine scratches, shot material with no corners, ideally shot material with a spherical shape, is used to prevent this.

By using the present invention, it is possible to make the golf club head thinner, and by using this, it is possible to improve the resilience, to increase the inertia moment, and to optimize the center of gravity position. Brief description of the drawings

 FIGS. 1 and 2 are both perspective views of an example of a head head suitable for applying the present invention, wherein reference numeral 1 denotes a fuse; c--, and reference numeral 2 denotes a head body. Note that these symbols are common to all the figures below.

 FIG. 3 is a perspective view of an example of an iron head suitable for applying the present invention. FIG. 4 is a perspective view of still another example of a hood head suitable for applying the present invention, which has extensions extending rearward from the top and bottom of the face.

 FIG. 5 is still another example of an iron head suitable for applying the present invention, having an extension extending rearward from above and below the face.

 FIG. 6 is a front view of a test head body 2-T and a test face 1-T used to verify the effect of the present invention.

 Fig. 7 is a graph showing the relationship between the arc height threshold value and the number of strokes that lead to the L-th stroke in the ball impact test.

 FIG. 8 is a graph showing the relationship between the arc high threshold value and the amount of indentation deformation of the face after 1000 hits in the ball impact test.

 FIG. 9 is a general explanatory view of shot picking, showing a collision state of the shot material 3 and the work 4. When performing shock peening in the present invention, cake 4 corresponds to face 1. BEST MODE FOR CARRYING OUT THE INVENTION

First, the general effects of peening will be described in the case of shot peening. The shot peening locally deforms near the surface of the workpiece 4 by the collision of the shot material 3 as shown in FIG. By repeating the collision with the shot material 3 many times, compressive stress remains in the thickness direction of the workpiece 4 and in the collision surface direction in the vicinity of the collision surface of the workpiece 4. As a result, the stress generated in the direction of the collision surface of the workpiece 4 has a distribution such that the surface side of the collision surface is larger and the smaller the inner side. In detail, residual stress is the largest Not the surface, but slightly inside the surface, which will be described in more detail later. Other mining methods also leave approximately the same stress.

 An outline of manufacturing the golf club of the present invention will be described. The present invention is applicable to golf clubs of all shapes and structures, but it is preferable to use a sleeve 2 having one or more parts as shown in FIG. A golf club head having a structure of joining 1 is preferable. This is because the face 1 having the face back surface to be peened is a separate part from the head main body 2 and thus peening is easy. Furthermore, it is possible to use a high strength material for the face 1 and to achieve even higher strength together with the effects of the present invention.

 The step of joining the substrate 1 and the head body 2 is preferably performed after the peening treatment. As mentioned above, it is because the processing is clean because it can be peened in the state of the base alone. Moreover, peening processing is performed on unnecessary parts, and unnecessary deformation can be prevented. However, it is also possible to perform the step of joining the face 1 and the head body 2 before the peening treatment. For example, there is a method of providing an opening in the − portion of the head body 2 and performing processing through the opening. However, the structure of the main body 2 tends to be complicated. On the other hand, the heat of welding, which is the most common joint in golf clubs, has the effect of eliminating the possibility of losing the effects of the present invention, and is selected in consideration of the balance with other elements. Should.

 The risk of loss of the effect of the present invention due to heat, and bonding of the face 1 and the head body 2 will be described. Metal materials are known to change in structure with temperature, and heat treatment uses this. If the tissue changes, the residual stress will disappear. As a temperature at which the residual stress is lost, the recrystallization temperature of the metal material is considered as a standard. Therefore, it is necessary to ensure that the part requiring the effect of peening does not rise above the recrystallization temperature. The recrystallization temperature varies depending on the type of metal, but in the case of stainless steel, maraging steel, and titanium alloy used for golf club heads, the temperature is relatively low at approximately 300 ° C. .

Therefore, when manufacturing the golf club head of the present invention, • Do you have the shape of a golf club that does not require bonding?

 • Choose a junction that does not require heating, or

 * Join before the painting process

You can think of it as ideal if you can do it.

 If the shape does not require bonding, there is no need to consider the bonding strength, and manufacturing is extremely easy. However, the shape of the golf club is extremely limited and can be applied to only a few types of golf clubs. It is impossible to manufacture a hollow golf club head generally referred to as a wood club or a hollow iron. In addition, it is also difficult to manufacture a golf club head in which a recess on the back surface, which is called a pocket-type aisle, has an sander structure.

 As an example of joining that does not require heating, mechanical joining methods such as caulking and pin 'screwing or adhesion can be considered. Although it is highly preferable in that it does not require heating, it has problems in terms of bonding strength. Another problem is that it is easy to create gaps and looseness.

 The method of bonding before the peening treatment is as described above.

 In consideration of the above, the present inventors judged that the method of joining by various welding methods after the peening treatment was more realistic than the three methods described above. We think that the method of joining before peening treatment is the second best solution. However, since the determination is based on the joining technology at the time of invention, the scope of the present invention is not limited to these.

Hereinafter, welding means suitable for joining performed after the pinning process will be described. A welding method that heats only a very small area and laser welding and electron beam welding is the best. These bonding methods efficiently heat only the joints, so the amount of heat reaching the peening parts is small, so the temperature rise in the peening parts can be minimized. However, since there is still a rise in temperature in the peening part, it is more preferable to use measures to prevent this. For example, a method of welding while blowing a cooling gas to the peening part, a method of welding while contacting the heat radiation part to the peening part, etc. Selection of TIG welding etc. is also possible It comes to As a cooling gas, it is preferable to use an inert gas from the viewpoint of preventing oxidation and the like of the bonding portion and its periphery.

 Since laser welding and electron beam welding must have a small gap between the members to be joined, it is ideal to add machining to both joints.

 Even with the shape of the golf club head, it is possible to prevent heat transfer W to the peening treatment portion. As an example, there can be mentioned a face 1 having extensions extending from the top to the bottom as shown in FIGS. Since the joint is separated from the center of the face where the effect of the peening treatment is most necessary, the amount of heat conduction at the time of joining is reduced, and the temperature rise of the relevant part can be reduced.

 The reduction and disappearance of the peening effect mentioned above is not limited at the time of bonding, but relates to all the treatments after the peening treatment. It is also impossible to carry out heat treatment to reach temperature. Therefore, if heat treatment is required, it should be done before peening treatment. It goes without saying that low temperature treatment not exceeding 300 ° C. is possible even after the pinning treatment.

 As described above, the best embodiments of the present invention considered by the present inventors are as follows.

• As the head structure, use Face 1 with extensions extending from the top to the bottom as shown in Figures 4 and 5.

 'Face 1 is pre-heat treated.

 • After that, perform a pinning process on the back of face 1 with an arc high threshold value of 0.3 mmA or more and less than 0.8 mmA.

 • The peening method should be shot peening or laser peening using a 1 to 3 mm diameter shot material of spheres.

 • Before or after the peening treatment, apply mechanical erosion to the joint between the face 1 and the head body 2 so that the joint gap becomes smaller.

 After that, join by laser welding or electron beam welding.

• The above welding is performed while blowing a cooled inert gas to the peening treatment section. Hereinafter, the results of the present invention in which the present invention was practiced and tested are shown. In order to confirm the effect of peening, the golf ball is repeatedly hit with a golf club head with the test head 1-T screwed on the test head body 2 -T shown in FIG. The amount of deformation and the number of strikes that resulted in destruction was noted. The blow is a blow

-Deated at 4.0 m / sec.

 The test head body 2-T is an outer 90 圆 X 70 min, inner 70 X X 40 枠 frame, and a shaft mounting neck is provided on the side at a loft angle of 15 °. At the four corners, screw holes are provided to fix the test face 1-T. The test face 1-T is a 90 mni X 70 square board with the same outer dimensions as the test head body 2-T, and its thickness is approximately 2.2 inches. In addition, on the back side of the central part 70 mm x 40 mm not in contact with the test head body 2 -T, the ultrasonic shot peening machine performs the shock treatment almost equally, and in the holes provided at the four corners It is fixed to the test head body 2-T through the screw.

 The first test material for test face 1-T is a rolled titanium alloy having a composition of 13V-11Cr-3AI. The second test material is a rolled material of a titanium alloy having a composition of 15V-6Cr-3AI, and the third test material is an aging-treated material of a titanium alloy having a composition of 15V-3Sn-3Cr-3AI. They were shot peened with different arc height values.

The experimental results are shown in Tables 1 to 3 and FIGS. In Table 3, the number of destructive strokes is the number of strokes when the test feather 1 T was destroyed. The amount of deformation after 1000 hits represents the amount by which the center of the impact surface of the test face 1 1 T, which was initially flat, was depressed after 1000 hits. In addition, although the arc height values of Comparative Examples 1-1, 2-1 and 3 are O. OO mmA, this means that the peening process is not applied. The measured thickness is the thickness measured after shot peening, and is the average value of several measurements of the thickness of the test foil: n-s 1-T. The face height value is the value of peening applied to the test face 1-T. The diameter of the shot material used is the diameter of the shot material used for shot peening. Figures 7 and 8 show the number of destructive strokes and the amount of deformation after 1000 hits, respectively, plotted with the arc height value for each material. table 1

表 1 は、 上記第一の試験材料の実験結果である。 比較例 1-2はアークハイ 卜値 0. 20画 Αであり、 ゴルフクラブへッ ドへのピ一ニング処理として従来知られている方 法では、 これでもかなり強いピ一ニング処理である。 しかし、 ピーニング処理をして いない比較例 1 - 1 とほぼ同じ打数で破壊に至っており、 疲労強度の改善は見られなか つた。 一方、 アークハイ 卜値 0.40、 0. 66画 Aという非常に強いピ一ニング処理を施し た実施例 1 - 1〜3では、 破壊打数で比較例 1 -1 -2の倍以上となっており、 疲労強度が 明らかに改善された。 これは図 7により一層明らかである 1000打後の変形量を比べて も、 実施例は比較例より改善されている。

Table 1 shows the experimental results of the first test material. Comparative Example 1-2 has an arc high value of 0.20, and it is a considerably strong averaging process in the method conventionally known as the process for the golf club head. However, the failure was achieved with the same number of strokes as Comparative Example 1-1 where no peening treatment was performed, and no improvement in fatigue strength was observed. On the other hand, in Examples 1-1 to 3 in which the arc high 0.4 value 0.40 and 0.66 A were subjected to a very strong pitching process, the number of hits was at least twice the number of comparative examples 1-1 -2, The fatigue strength is clearly improved. This is more apparent than FIG. 7 in comparison with the comparative example even if the deformation after 1000 shots is more apparent.

From the above, in this test, both the improvement of the fatigue strength and the prevention of permanent deformation are more effective as the arc height is larger. Especially in the improvement of the fatigue strength, the boundary between the arc height values of 0.20 mmA and 0.40 mmA Therefore, it turned out that the effect can not be obtained without taking an arc high value more than this. Table 2

表 2は、 上記第二の試験材料の実験結果である。 1000打後の変形量を見ると、 ァ ークハイ 卜値 0. 20mmAではピーニング処理をしていない場合と同じであるが、 アーク ハイ 卜値 0. 40mmA以上では明確に変形量が小さくなつており、 この間に境界がある。 図 8からは一層これが明確である。 図 7から、 破壊打数においてはアークハイ 卜値の 臨界的境界は見いだせないが、 アークハイ 卜値が大きくなるほど破壊打数が増え、 疲 労強度が向上していることが分かる。

Table 2 shows the experimental results of the second test material. Looking at the deformation after 1000 strokes, the peak height of 0.20 mmA is the same as the case without peening treatment, but the arc height of 0.40 mmA and above shows a clearly smaller amount of deformation, There is a boundary between them. This is clearer from Figure 8. It can be seen from Fig. 7 that no critical boundary for the arc high threshold can be found in the number of destructive hits, but the number of destructive hits increases and the fatigue strength improves as the arc high threshold increases.

From the above, in this test, the larger the arc height value, the better the improvement in fatigue strength and the prevention of permanent deformation, and the more effective the prevention of permanent deformation. In particular, the boundary between arc high 卜 0.20 mmA and 0.40 mmA Therefore, it turned out that the effect can not be obtained without taking an arc high value more than this.

Table 3

表 3は、 上記第三の試験材料の実験結果である。 本例は、 ピーニング処理を施し た場合とアークハイ 卜値 0. 60mmAのピーニング処理を施した場合の比較であるが、 ピ 一二ング処理を施した場合に破壊打数、 1000打後の変形量とも向上しており、 疲労強 度の向上と永久変形の防止という本発明の効果が確認された。

Table 3 shows the experimental results of the above third test material. In this example, a comparison is made between the case of peening treatment and the case of peening treatment of arc high value of 0.60 mmA. The effects of the present invention, ie, the improvement of fatigue strength and the prevention of permanent deformation, were confirmed.

 As mentioned above, the boundary between the 0.20 mmA and 0.40 mmA high arc high threshold value can be obtained with the effect that both the improvement of fatigue strength and the prevention of permanent deformation can be obtained with general materials with golf club heads. That is, it became clear that the arc high threshold value was 0.30 mmA.

 On the other hand, such partially remaining stress is a phenomenon that occurs in the direction parallel to the peened surface.

-Generate a membrane, and cause deformation and warping where the peened surface side is convex. By this warpage, the compressive stress in the vicinity of the collision surface side is reduced, and a tensile stress is generated in the vicinity of the opposite surface, so that the moment becomes zero and becomes stable. In other words, warping is inevitable as long as compressive stress in the surface direction remains in the vicinity of the surface. Here, the term "Panning treatment part" means all portions where stress remains due to the peening treatment, and the term "Peening treatment surface" means the surface of the peening treatment part on the side subjected to the treatment.

The inventors also tested shot peening with an arc high value of 0.80 mmA under the materials and conditions described in Tables 1 to 3, but the shot peening caused a large warpage in the test face 1-T. , I could not do the impact test. The golf club head face must be convex out for the wooden head and flat for the iron head. Although it does not, if the back side of the face 1 is subjected to a pinning process, a sled will be generated in the direction to dent the face, and if the sled is large it will not function as a golf club head. That is, the larger the stress to be retained by the peening treatment, the smaller the stress generated upon impact, but the greater the warpage of the face surface, the more difficult it is to manufacture the golf club. As described above, the test face 1-T with an arc height of 0.66 mmA was sufficiently flat, and could be attached to the test head body 2-T and hit. From the above, it was considered that the arc high は value should be less than 0.80 mmA, because it is difficult to use it in an actual head when the arc high 卜 value is 0.80 mmA or more.

 However, there is also a method in which a test is performed in advance, the amount of deformation due to peening is measured, and the face 1 is formed into a shape in consideration of the deformation and peened to obtain the face 1 having a desired shape. . It can be applied to cases requiring a harder peening treatment with an arc high threshold value of 0.80 mmA or more, or an arc height threshold value of less than this, if higher accuracy noise is required. I will. If the arc high threshold value is less than 0.80 mmA, no such preliminary test or molding is required.

 The details of the peening process will be described below. First, as mentioned above, since the peening treatment leaves compressive stress, maximum stress can be reduced if it is applied to the part where tensile stress occurs at impact. On the other hand, if it is applied to a part where compressive stress is generated, compressive stress due to the pinning process and compressive stress due to the impact are applied, the maximum stress becomes large, it may be fatigued, and fracture may be caused. A golf club head peened on the surface side of the face 1 of the golf club head is known, but it should be noted that this is exactly the case of the opposite effect. If peening treatment is applied to the surface side of face 1, for the purpose of removing scale at best, it should be limited to treatment with extremely small residual stress.

In addition, since the peening treatment causes deformation, it is preferable to keep the processing range as small as necessary, and it is optimal to apply it only near the center of the back of the face where a large tensile stress is generated at the time of striking. Since the range in which large tensile stress is generated changes depending on the shape of the golf club head or face 1, the simulation using FEM etc. is performed. Although it is desirable to determine the processing range separately. As a rough guide, it may be preferable to peen around the center of the face in the range of 3 cm in width and 2 cm in height around the center of the face. As mentioned above, the method of peening is other than shot peening Laser peening is also suitable. In laser peening, the surface of a material is irradiated with a laser, and stress is retained by utilizing high impulses (intensities) generated by high pressure plasma generated by ablation of a material. Laser peening is generally considered to be able to retain stress to a deeper layer than shot peening, and can further enhance fatigue strength. In addition, since no chopsticks are used, there is no risk that dents caused by chopsticks will have an adverse effect.

 In the case of shocking, ultrasonic shocking is advantageous. This is because the number of grains of the shot material is small, replacement is easy, and a smooth surface can be easily obtained. Air shot peening is disadvantageous in that it uses a large amount of expensive grains, and the grain newness is broken in long-term use, making it difficult to obtain a stable and smooth surface. It is also conceivable to inject liquid simultaneously with the shot material. It is called wet peening, water-only peening, or wet-weather peening.

 In the case of shot peening, the shot material 3 to be used needs to use a material having hardness higher than that of the face 1 material. In the above-mentioned test conducted by the present inventors, high carbon chromium bearing steel SUJ2 was used. The hardness is about Hv 790, which is larger than the hardness (about Hv 350) of various titanium alloys used in Test Base 1-T. If the hardness of the shock material is low, even if it collides with the face 1, + partial deformation can not be generated, and compressive stress can not be retained.

In addition, the shot material 3 should be of a certain size or more. As mentioned above, the present invention also has the effect of eliminating fine scratches on the surface to be treated. The fine scratches are streaks that are produced by rolling of metal materials. This is because if the shot material 3 is small, this fine scratch can not be erased sufficiently. In addition, since a shot pitting makes a recess by a shot tip 3, if the shot depth 3 is small, the dent will also be small, and the dent itself may become the start of fatigue failure. In the tests of the present inventors, the diameter of the chopsticks 3 is 1 or more. With the upper ball, the micro-scratch was sufficiently erased, and the depression formed by the shot material 3 did not become the beginning of fatigue failure. From the above, it is considered that the shock material 3 should have a shape without corners and a certain size, and a sphere with a diameter of 1 mm or more is ideal.

 However, considering the current peening equipment that can be used to process golf club heads, the upper limit of the shot material diameter should be 3 mni. If it is larger than this, the shot material will be heavy, and the peening apparatus can not accelerate the shot material to + minutes, and the required arc height value can not be obtained.

 A further preferred example of peening will be described. First, there is a method to perform peening twice. A second round of the peening process, which is milder than the first peening process, has a smaller arc height value. The peening treatment maximizes the hardness and the stress remains most at the inner side, not at the surface. However, since it is the surface that generates the largest tensile stress at the time of impact, we would like to increase the surface hardness and residual stress if possible. Therefore, there is a method of coating the resin on the surface to be treated as a measure to prevent the above-mentioned depression by the shock material from becoming the starting point of fatigue failure, which is to realize this by adding a somewhat soft peening. . As the resin, epoxy resin is preferable, and it is also preferable to mix metal powder with it. This is because the notch effect can be prevented by the resin getting into the small scratched recess.

It is also preferable to use nitriding treatment in combination with the present invention. As mentioned above, since residual stress due to shock peening is limited to a relatively shallow layer, the stress generated by impact may be large or insufficient. Nitriding forces nitrogen atoms or molecules into the metallographic structure, resulting in residual stress. Moreover, since nitrogen penetrates deeper than the layer to which the effect of the stressing is applied, residual stress can be generated even to the deep layer _c. However, since the amount of nitrogen atoms to be penetrated is limited, There is also a limit to the residual stress that can be generated. On the other hand, the tensile stress generated on the face 1 by impact is the largest at the rear surface and smaller as it gets closer to the inside, and becomes zero near the center of the thickness. The residual stress may be generated so as to cancel this, so the stress is left to the deep layer by nitriding, and the residual stress in the vicinity of the back surface is further increased by the pinning process. As described above, since residual stress disappears when the recrystallization temperature is exceeded, it is preferable to perform nitridation first and then to perform pinning treatment later. However, it is possible to carry out after the peening treatment, if the low temperature nitriding method which is being carried out recently is being carried out. For the application of such low temperature nitriding to a golf club, refer to the specification and drawings of Japanese Patent Application No. 2003-121259 filed by the applicant.

 It is also preferable to perform shot peening by mixing a shot material 3 with a fullerene such as C60 or C70, or a nanoscale substance such as a carbon nanotube or a nanodiamond. This is because the nanoscale material penetrates into the metal material during shot peening, and the strength can be improved by this. In addition, about manufacture of a golf club head from the metal material which mixed the nano scale substance, the specification of the Japan patent application 2000-190192 (Japanese Unexamined-Japanese-Patent No. 2002-000778) which the applicant applied was performed. Please refer to the drawings. The nano-diamonds include single crystal diamonds having a diameter of about 5 nm or diamonds having a polycrystalline structure having a diameter of about 15 nm or cluster-uniformized.

 The golf club head of the present invention is not limited to the above-described embodiment, and it goes without saying that various modifications can be made without departing from the scope of the present invention. Industrial applicability

 As described above, in the golf club head according to the present invention, it is possible to reduce the thickness of the face, to improve the resilience, to increase the moment of inertia, and to optimize the position of the center of gravity. This makes it possible to provide a golf club with excellent operability for the golfer.

Claims

The scope of the claims  1. A golf club head characterized by hard peening of arc high 0.33 A or more on the back of the face.  2. The golf club according to 1 above, which is peened using a shot material having a particle size of 1 to 3 miti.  3. The golf club head according to 1 above, characterized in that only the center portion of the back of the face is hard-beaned.  4. The golf club head according to 1 above, characterized in that the arc height value is less than 0.8 mmA.