Golf club head

Abstract

To provide a material composition and implementation thereof, the material composition being suitable for various use modes expected to the golf club head.SOLUTION: A golf club head 100 includes an integrated type component. The integrated type component is formed of a stainless steel material. The integrated type component has variable hardness. A first portion of the component indicates a first hardness H1 equal to or greater than 50 HRC. A second portion of the component indicates a second hardness H2 equal to or less than 85 HRB.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] This application relates to a golf club head. This application is a non-provisional application of U.S. Provisional Application No. 63 / 614,154, filed on December 22, 2023, the disclosure of which is hereby incorporated by reference in its entirety.

Background Art

[0002] From a technical perspective, golf clubs are subject to their own stringent standards. For example, golf clubs are evaluated on their ability to meet performance criteria such as the ability to efficiently transfer energy to a golf ball at impact. Golf clubs are also evaluated in terms of their tolerance to off-center shots and mishits. Additionally, golf clubs are sometimes evaluated by their ability to impart specific spin characteristics and other attributes to a golf ball at impact in order to shape the flight trajectory and rolling characteristics of the ball. In addition to performance, club heads are required to withstand repeated use, for example, by resisting wear, rust, and material fatigue. Some club heads are also required to be capable of some adjustment by plastic deformation, such as bending the hosel to adjust loft and lie.

[0003] Golf club manufacturers want to succeed in all of these aspects of use, or as many as possible. However, these functional requirements are diverse, and due to limitations regarding mass, physical dimensions, cost, etc., design conflicts and trade-offs often occur. To increase the tolerance of a club head, for example, when increasing the moment of inertia about a desired axis, it is often necessary to sacrifice the desired feel. Similarly, to improve a club head to impart advantageous spin and wear resistance, a material selection that may negatively impact other considerations may be required. For example, conventional materials with high hardness, high yield strength, and appropriate machinability are often insufficient in other important areas such as adjustability and bendability, in terms of malleability and flexibility.

[0004] To minimize the severity of such design trade - offs, manufacturers have considered partially changing the material of the club head with respect to its structure to better match the material properties and structural functions. For example, in iron - type club heads such as wedges, face inserts made of a material different from the body that secures the face insert are employed. Thus, it is possible to select face inserts with properties optimal for impact, such as relatively high hardness, low density, sufficient wear resistance, and sufficient workability. However, the material of the body may appropriately deviate from those properties and instead may be selected to exhibit, for example, higher density and higher malleability. SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

[0005] Partially arranging different materials in the structure of the club head may seemingly be beneficial, but it is not without drawbacks. First, when the number of components constituting the club head increases, the cost rises and the manufacturing becomes complex. As a result, the range of manufacturing errors expands, and for example, the number of failure points may increase due to adhesives, mechanical fasteners, heat - affected zones by welding or brazing, etc. Also, when the club head transitions from a solid structure to a component - based structure, it may lead to a deterioration in feel and a decrease in acoustic and vibration characteristics.

[0006] Accordingly, an object of the present disclosure is to provide a material composition and its implementation that are suitable in themselves for various usage modes expected of a golf club head. Therefore, while achieving the advantages associated with material properties that selectively correspond to a specific club - head structure, it is possible to minimize or avoid the drawbacks associated with an overly component - based structure. MEANS FOR SOLVING THE PROBLEMS

[0007] In one aspect, the golf club head includes a one-piece component formed of a stainless steel material. The one-piece component has a variable hardness. A first portion of the component exhibits a first hardness H1 of 50 HRC or more. A second portion of the component exhibits a second hardness H2 of 85 HRB or less.

[0008] In another aspect of the present disclosure, the method includes forming a component of a golf club head. The component includes a stainless steel material. The method includes selectively case hardening the component such that a first portion of the component exhibits a first hardness H1 of 50 HRC or more and a second portion of the component exhibits a second hardness H2 of 85 HRB or less.

[0009] In another aspect of the present disclosure, a component of a golf club head includes a stainless steel material. The stainless steel material has a nickel content of 0.25% or less by mass ratio.

[0010] In another aspect of the present disclosure, a component of a golf club head includes a stainless steel material, and the stainless steel material has a carbon content of 0.25% or more by mass ratio.

[0011] In another aspect of the present disclosure, a component for a golf club head includes a stainless steel material. The stainless steel material has an austenitizing temperature of 800 °C or higher.

[0012] These features and advantages of the golf club head, its composition, and its manufacturing method according to various aspects of the present disclosure, as well as other features and advantages, will become more apparent upon consideration of the following description, drawings, and appended claims. The following description and drawings are for illustrative purposes only and are not intended to limit the scope of the invention in any way.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

DETAILED DESCRIPTION OF THE INVENTION

[0014] In one aspect of the present disclosure, referring to FIGS. 1 and 2, a golf club head 100 is shown. The golf club head 100 includes a front portion 122 including a striking face 102, a top portion 106, and a sole portion 108 opposite the top portion 106. The sole portion 108 is configured to rest on a virtual ground, such as a ground contact surface 114, when in a reference position. The golf club head 100 further includes a heel portion 112 and a toe portion 110 opposite the heel portion 112. A hosel portion 104 extends from the heel portion 112. The hosel portion 104 includes a hosel bore (not shown) configured to receive a golf shaft (not shown). The golf club head 100 forms a golf club when combined with a golf shaft. The hosel portion 104 defines a virtual hosel axis 124 that is a central axis defined by the hosel bore. The hosel axis 124 in relation to the rest of the structure of the golf club head 100 defines the loft angle and lie angle of the club head.

[0015] Preferably, the golf club head 100 is an iron-type club head having a loft between, for example, 20° and 66°. More preferably, the golf club head 100 is a wedge-type club head having a loft between, for example, 40° and 66°. Additionally, or alternatively, the golf club head 100 has a lie between about 62° and about 66°, more preferably between 61° and 63°. However, the structures and material compositions described herein can be readily applied to other types of golf club heads, such as woods including drivers, fairway woods and hybrids, as well as putters, rescue clubs, and the like.

[0016] The golf club head is preferably composed of a steel material, preferably a stainless steel material. Generally, readily available grades of stainless steel have been conventionally used in the manufacture of iron-type golf club heads, particularly those including wedge types. For example, AISI 431 alloy steel is a common material for golf club heads. However, considering the specific harsh conditions and constraints associated with golf club heads, particularly wedge-type golf club heads, and from the perspective of further minimizing design trade-offs, a steel composition different from AISI 431 may be advantageous. In this application, a steel alloy having a chromium content of at least 10.5% by mass ratio is considered to be stainless steel.

[0017] Preferably, by mass, more than half (i.e., more than 50%) of the golf club head 100 is composed of such steel, more preferably at least 85% of the golf club head 100, and even more preferably substantially the whole of the golf club head 100 (taking into account auxiliary small parts such as paint, thin coatings, adhesives, ferrules, etc.). Alternatively or additionally, such steel preferably constitutes an integral part of the golf club head 100, more preferably an integral part including a first part forming at least a part of the striking face 102, even more preferably also including a second part forming at least a part of the hosel portion 104, and even more preferably also including a third part forming at least a part of the back face portion 116 of the golf club head 100. The golf club head 100 preferably has an integral shape having an upper blade portion 118 and a lower muscle portion 120 proximate to the sole portion 108. In some embodiments, substantially the whole of the golf club head 100 is integrally formed of such steel. As described above, reducing the number of parts required for manufacturing the golf club head 100 can bring advantages such as reducing manufacturing costs, reducing manufacturing tolerances, and reducing failure-prone locations while maintaining desirable tactile, acoustic, and vibration characteristics.

[0018] However, preferably, the steel composition described above is selected to provide material characteristics particularly beneficial for the use of the golf club head. Commercially available steel grades provide some suitable characteristics with respect to the club head, but may be procured to exhibit characteristics that are not particularly suitable for the use of the club head. For at least these reasons, adjusting the steel composition according to the applications associated with the golf club head may provide some benefits and have little adverse impact. Also, preferably, adjusting the steel composition according to the applications associated with the golf club head may make it possible to more flexibly vary the characteristics of the club head depending on the structure.

[0019] As described above, the golf club head 100 is preferably an iron-type club head, and more preferably a wedge-type club head. Therefore, adapting the body material composition to a specific usage mode of such a club head may bring greater benefits. Since the relevance to a wedge-type club head is considered, particular attention is paid to hardness characteristics, wear resistance, and density. However, other characteristics may also be considered in some cases.

[0020] Hardness is an example of a property for which particularly desirable applications are expected in the case of a golf club head. On the other hand, the striking face, for example, the striking face 102, preferably has a relatively hard surface. However, other parts of the golf club head, for example, the golf club head 100, such as the hosel part 104, are preferably softer and more malleable. Such malleability enables adjustments by plastically deforming the golf club head, such as loft and / or lie corrections by bending the hosel part. This duality of hardness is a unique aspect in the function of the golf club head and is considered to justify deviation from conventional material compositions.

[0021] One concern regarding conventional steel materials is the limited ability to adjust hardness. For example, 431 stainless steel exhibits an austenitizing temperature of approximately 720°C, thereby enabling annealing at temperatures below approximately 700°C. Above this temperature, the possibility of austenite transformation and rehardening becomes significantly higher. Therefore, it can be difficult to achieve a relatively hard striking face while maintaining forgeability at the same time.

[0022] Such hardness limitations are thought to be closely correlated with the nickel content in the steel, without considering other material components. Nickel is considered a strong austenite promoter, which is thought to affect by lowering the austenitization temperature. As a result, when such steel is quenched and tempered, the achievable minimum hardness is thought to increase. For example, 431 stainless steel is thought to be only softenable to about 85 HRB. Such defects may be eliminated by improvement processes such as a softening process in which such steel is kept at a temperature near the austenitization temperature and cooled slowly. However, these improvement processes themselves are not without demerits such as a decrease in wear resistance. Such improvement processes complicate manufacturing and increase costs. Therefore, a steel composition that can achieve a desirable hardness change only by quenching and tempering is desirable.

[0023] Therefore, the steel composition of the golf club head 100 preferably has a nickel content of 0.5% or less, more preferably 0.35% or less, by mass ratio. However, it should be noted that even if the nickel composition is reduced to 0.25% or less, more preferably about 0.2% by mass ratio, the golf club head may be able to exhibit excellent properties such as hardness change and wear resistance with little impairment. With such a nickel content, the steel can exhibit an austenitization temperature of 800 °C or higher, preferably 850 °C or higher, more preferably about 870 °C. Therefore, this steel is likely to be temperable at a temperature of at least 800 °C. This makes it possible to soften the steel to 90 HRB or less after quenching and tempering, greatly improving the bendability, for example, enabling it to accommodate loft and / or lie angle adjustments by bending the hosel. Preferably, the steel exhibits material properties that allow for an angle adjustment of the hosel portion of up to about 4° in either case of loft adjustment or lie adjustment. Generally, in some alternative embodiments, especially in the case of wedge-type golf club heads, reducing the nickel content to less than 0.2% by mass has no significant adverse effect. However, the lower limit of the nickel content needs to be evaluated based on that point because it may correspond to unacceptable impact energy.

[0024] The hardness characteristics of steel are also thought to be greatly affected by the carbon content. Generally, it is considered that increasing the carbon content increases the maximum achievable hardness of the steel in the quenched state. For example, regardless of other material components, 431 stainless steel is considered to have a carbon content of about 0.1% by mass and exhibit a maximum hardness of about 40 HRC in the quenched state. 8620 stainless steel has a carbon content of about 0.2% by mass and is considered to exhibit a maximum hardness of about 45 HRC in the quenched state. The steel of the golf club head 100 preferably has a carbon content of 0.13% or more, more preferably 0.25% or more, by mass. Regardless of other components, the steel of the golf club head 100 in the quenched state can exhibit a hardness of 50 HRC or more, more preferably 55 HRC or more, even more preferably 60 HRC or more, and even more preferably between 60 HRC and 65 HRC.

[0025] In addition to its direct advantages, carbon is considered to be a somewhat effective substitute for nickel. Thus, carbon enables the advantageous reduction of the nickel content described above. In addition to such hardness advantages, such a limitation on the carbon content is thought to contribute to an improvement in wear resistance and a reduction in material density.

[0026] However, if the carbon content is too high, it may have harmful effects. For example, manufacturing problems such as welding and solidification issues may occur. Also, the carbon content is considered to contribute to the austenitizing temperature of the steel in relation to nickel and other components. In particular, when the carbon content is high, the austenitizing temperature may decrease. This is because carbon is considered to have a strong effect of promoting austenitization. As a result, the steel may have significantly limited ability, such as not being able to increase its hardness to 90 HRB or higher by tempering. Therefore, the steel of the golf club head 100 preferably has a carbon content between 0.13% and 0.50% by mass, and more preferably between 0.25% and 0.50% by mass. However, for example, when the ability to achieve a higher hardness than the above-mentioned minimum hardness is prioritized for purposes such as the above-mentioned adjustment, a higher carbon content, for example, in the range of 0.45% to 0.50% may be particularly preferred.

[0027] Regardless of other material components that are correlated with wear resistance, an increase in hardness itself is considered to be partially correlated with an improvement in wear resistance. Therefore, if the above steel can be quenched to increase its hardness, the wear resistance and strength may be improved. This is particularly applicable to specific heat treatment processes such as laser etching and laser peening in combination with the above carbon content.

[0028] The chromium content is considered to contribute to the material properties of the steel specific to the golf club head. This is particularly applicable with respect to hardness, wear resistance, and also resistance to rust. Preferably, the chromium content included in the steel of the golf club head 100 is selected mainly based on these properties, as will be described in more detail below.

[0029] As described above, the golf club head 100 preferably exhibits a relatively high hardness at the position of the striking face and a relatively low hardness at other positions, such as the hosel portion 104 and / or the back face portion 116. In addition to the nickel and carbon described above, chromium may contribute to the realization of these desirable club head characteristics. For example, the steel composition may determine, apart from its basic hardness characteristics, which surface treatment options, such as case hardening or surface hardening, are effective and the degree of their success. For example, in some embodiments, the striking face 102 is subjected to a surface hardening process, preferably a nitriding process. In some embodiments, instead of or in addition to nitriding, other surface hardening processes such as carburizing, carbonitriding, annealing, surface hardening, induction hardening, cyaniding, flame hardening, and laser hardening may be applied. However, the nitriding treatment is preferred because it is cost-effective and is considered to give the most satisfactory results. Due to the presence of chromium, chromium nitride can be formed on the striking face 102 by the nitriding treatment. As a result, the surface hardness of the striking face 102 exhibits 1200 HV (0.05) or more. Otherwise, for example, in the case of conventional carbon steel, the maximum achievable hardness is significantly lower, for example, about 800 HV (0.05).

[0030] The nitriding treatment also has drawbacks. For example, it is known that nitriding stainless steel reduces its corrosion resistance. However, considering the overall usage characteristics of the golf club head, particularly the wedge-type golf club head 100 of FIG. 1, the advantages of the hardness obtained by the steel composition and surface hardening, such as nitriding treatment, are considered to outweigh this drawback.

[0031] Regarding rust, preferably, the steel composition is adapted to reduce or minimize the propagation of rust or natural oxidation. In some alternative embodiments, rust or oxidation of the striking face 102 of the golf club head 100 may be considered a desirable development. For example, there is a niche market for golf club heads with specially selected properties that cause or promote the generation of rust on the striking face. In such a market of golfers, the specific texture and / or surface roughness properties associated with a rusted face are preferred. However, generally, preferably, the golf club head 100 is configured to reduce the generation or propagation of rust. A club head prone to rust is thought to wear out faster than a club head resistant to rust. This is presumably because the rust on the striking face wears out faster than the non-rusted parts, resulting in a higher volume loss rate for the rusted club head. This is of particular concern regarding the structure of the score line. The inclusion of chromium in the steel can suppress the generation and progression of rust and potentially reduce the wear rate.

[0032] Based on the above considerations, the steel implemented in the golf club head 100 preferably contains chromium in a mass ratio of 13% or more, more preferably 16% or more. Additionally, or alternatively, this steel material contains chromium in an amount of 21% or less, more preferably 18% or less, by mass ratio. If the chromium content is too high, the transformation of the steel material into a martensite crystal structure may be hindered, resulting in the steel material becoming too brittle and an undesirable result of the formation of a sigma phase during tempering.

[0033] In addition to the factors affecting the generation of rust and wear described above, the above chromium content, in relation to the above carbon and nickel contents, preferably reduces the density of the entire steel. The density of a metal material is mainly affected by the following two attributes. (1) The composition of the alloy, and (2) the structure in which its atoms are arranged. A simple method for estimating the density of an alloy is to determine the mass percentage of each constituent element, divide it by the density of that element to calculate the total volume of each element. Then, assuming a 100-gram sample, divide by the total volume as shown in Equation 1 below.

[0034]

Number

[0035] Another factor that greatly affects the density of steel is the atomic arrangement structure, or its crystal structure. The steel of the golf club head 100 based on the above-mentioned component composition preferably has a mixture of ferrite and martensite having a body-centered cubic structure (BCC), or is substantially entirely or entirely martensite after being quenched and / or tempered. Martensite shows a body-centered tetragonal (BCT) structure in the quenched state and a body-centered cubic (BCC) structure in the tempered state. Therefore, the relative proportion of the crystal structure of martensite depends on the amount of tempering after quenching. The atomic packing factor of the BCC structure and the BCT structure, that is, the amount of atomic volume per unit cell, is 0.68. Since the packing factors of the BCC structure and the BCT structure are similar, the overall material density is considered to be mainly determined by the composition of the alloy.

[0036] Another common metal crystal structure is the face-centered cubic (FCC) structure, and the atomic packing factor is 0.74, which is higher than that of BCC and BCT, indicating a dense structure. Since austenite has an FCC structure, although the calculated density according to the above formula 1 is low, generally, it has a higher density than the mixed composition of the steel of the golf club head 100. From the above, the density of the steel is preferably 7.85 g / cm 3 Hereinafter, more preferably 7.65 g / cm 3 Hereinafter, even more preferably 7.60 g / cm 3 or less. When the density is reduced, any mass of the golf club head 100, that is, the mass that is not mainly required for the structural integrity of the golf club head increases, so that it can be intentionally arranged for the purpose of improving various mass characteristics of the golf club head 100, such as the position of the center of gravity and the moment of inertia around the relevant axis passing through the center of gravity.

[0037] The nitrogen component composition in the steel of the golf club head 100 is also important. Nitrogen, like carbon, can affect steel. This is because the sizes of nitrogen and carbon are similar, and both nitrogen and carbon can be regarded as interstitial elements. For example, nitrogen is a strong austenite promoter. Therefore, when the nitrogen composition increases beyond a certain amount, the minimum hardness after quenching and tempering may increase adversely. Furthermore, if the nitrogen content is too high, since nitrogen has a small size and a relatively high diffusion rate, steel splitting problems may occur during welding and solidification. Additionally, if the nitrogen content is too high, the ductility, toughness, and corrosion resistance of the steel will decrease due to the formation of CrN. Although nitrogen is considered to have less influence than carbon, it may affect the maximum strength of stainless steel.

[0038] Based on the above considerations, the steel of the golf club head 100 preferably has a nitrogen content of 0.035% or less by mass ratio, more preferably 0.15% or less, and even more preferably a nitrogen content in the range of about 0% to 0.06%. Since nitrogen has common characteristics with carbon, the total content of carbon and nitrogen is also important. Preferably, this total content in the steel of the golf club head 100 is in the range of 0.13% to 0.75% by mass ratio, and more preferably in the range of 0.20% to 0.35% by mass ratio. In this case, if such a content is too low, the steel cannot be hardened to the desired degree, the wear resistance decreases, and it may exhibit an undesirably high density.

[0039] The above description is the details of an embodiment of a preferred steel composition for use in the golf club head 100. Table 1 below summarizes some exemplary steel compositions corresponding to the above description. Steel A of the example corresponds to a first general example of the steel used for the golf club head 100. 410 SS, 440 SS, 8620 SS, and 431 are stainless steel alloys considered to be known in the golf club industry.

[0040]

Table 1

[0041] A more detailed evaluation of the chemical composition of the exemplary steel A is shown in Table 2 below. Some of the material properties exhibited by the exemplary steel A are summarized in the chart of FIG. 4 as compared to known steel alloys.

[0042]

Table 2

[0043] Based on the exemplary embodiments of the above steel, after quenching, tempering and surface hardening treatments, the steel of the golf club head 100 preferably exhibits a maximum hardness of 50 HRC or more, more preferably 55 HRC or more, and still more preferably in the range of 60 HRC to 65 HRC. The same steel components of the golf club head 100 preferably include a minimum hardness of 90 HRB or less, more preferably 85 HRB or less. Preferably, the components of the golf club head 100 made of this steel include a first position on the striking face 102 of the golf club head 100, preferably a second position on the hosel portion 104, and preferably a third position on the back portion 116 of the golf club head. In such an embodiment, preferably, the first position has a hardness of 50 HRC or more, more preferably 55 HRC or more, and still more preferably in the range of 60 HRC to 65 HRC. Alternatively or additionally, the maximum hardness of the steel component preferably coincides with the first position and is located, for example, on the striking face 102. Preferably, the second position (optionally the third position) has a hardness of 90 HRB or less, more preferably 85 HRB or less. Alternatively or additionally, the minimum thickness of the steel component is preferably located in a portion other than the striking face 102 and preferably on the hosel portion 104. However, in some embodiments, the minimum hardness position of the steel component is the back portion 116 or another portion of the golf club head 100.

[0044] Additionally or alternatively, the ratio of the maximum hardness to the minimum hardness of the steel of the golf club head 100 (when both the maximum and minimum values are expressed as quantities related to the Rockwell C hardness (HRC) scale) is preferably 4.5 or more, more preferably 6 or more, still more preferably 9 or more, and yet more preferably 12 or more. In some specific embodiments, preferably, such a ratio is in the range of 12 to 16.25.

[0045] FIG. 3 shows a flowchart 200 of a process for explaining the preferred steps performed when forming the golf club head 100 using an exemplary steel composition embodiment described herein. The steps of process 200 are preferably arranged in order and are intended to occur in chronological order in the order shown, although it is conceivable that one or more steps may occur in a different order or be omitted. Further, in some embodiments, additional steps or processes may occur in chronological order before, after, or between the process steps shown and described.

[0046] In step 202, for example, an intermediate club head is formed by investment casting or lost wax casting. Next, optionally, in step 204, welding is applied to add and / or repair material to any area of the intermediate cast club head body due to unintended defects or flaws in the casting process. For example, welding material can be applied as a filler material to a portion with a porosity problem. The welding material is preferably a stainless steel material. However, it is also possible to alternatively use other materials, in which case it is preferably used in combination with additional post-treatment.

[0047] Next, optionally, in step 206, it is preferable to polish the intermediate club head body to remove the remains of the gate and other defects resulting from the casting process 202.

[0048] Next, in step 208, the intermediate club head body undergoes a heat treatment process 208. Preferably, the heat treatment process 208 includes at least a quenching step 208A and a tempering step 208B. First, in step 208A, preferably, the intermediate club head body is held at a temperature of about 1040 °C for about 90 minutes to about 120 minutes, preferably about 90 minutes. Thereafter, the intermediate club head body is preferably quenched by immersion in a nitrogen solution. As a result, a harder martensite structure transitions over most of the intermediate club head body (i.e., more than 50% of the mass), more preferably over 60% of the mass, and even more preferably over substantially the entire intermediate club head body.

[0049] Next, in step 208B, the intermediate club head body undergoes tempering. In this step, the intermediate club head body is held at a temperature of 800 °C or higher, more preferably 850 °C or higher, still more preferably in the range of 865 °C to 870 °C, and even more preferably about 870 °C for about 2 hours. Preferably, thereafter, the intermediate club head body is cooled in an N2 solution. This step temper the martensite and softens the intermediate club head body.

[0050] In some embodiments, in step 208, the intermediate club head body undergoes a plurality of heat treatment cycles. Applying a plurality of heat treatment cycles is preferred in some cases to maintain the relatively high potential for face hardness of the steel in the striking face 102 and to further reduce the final club head hardness at locations including the hosel portion 104 and / or the back portion 116. Such a plurality of cycles may include a plurality of tempering cycles. As shown in FIG. 6, applying a double tempering process results in a lower hardness compared to a single tempering at the same temperature. For example, when double tempering is performed at a temperature of about 863 °C for about 2 hours, the hardness is 80 HRB, but when a single tempering is performed at the same temperature and duration, the hardness is about 96 HRB. The heat treatment process of a plurality of cycles includes, for example, the sub-steps shown in Table 3 below.

[0051]

Table 3

[0052] Next, in step 210, it is preferable to mill the surface of the striking face 102. Next, in step 212, the hosel / neck region of the intermediate club head body is polished to harmonize the appearance of the striking face 102 and the hosel portion 104. Next, in step 214, preferably, a score line is machined into the striking face 102 by milling.

[0053] Next, in step 216, surface hardening of the striking face 102 is performed. Preferably, the surface hardening step 216 includes a nitriding treatment 216A and additional hardening treatments such as laser hardening or laser peening in step 216B.

[0054] The nitriding treatment 216A is preferably performed at a temperature of about 550 °C or higher, more preferably about 575 °C or higher, and even more preferably about 580 °C for about 50 minutes. However, alternative or additional surface hardening or other protective or aesthetic surface finishing processes (such as carburizing, carbonitriding, quenching, surface hardening, induction hardening, cyaniding, flame hardening, PVD, ceramic, and / or coating with diamond, etc.) are also conceivable. Due to the various component compositions described above, the steel material applied to the golf club head 100 is considered to exhibit sufficient wear resistance and corrosion resistance by itself, and there is no need to perform plating treatment for that purpose. On the other hand, for example, in the case of 8620 steel, nickel-chromium plating treatment is generally considered necessary for the purpose of enhancing wear resistance and corrosion resistance to an acceptable level.

[0055] Preferably, in step 216B, laser hardening is applied to the nitrided striking face. In this step, concentrated thermal energy is directed towards the striking face 102 for a short period of time (e.g., several seconds). Cooling of the striking face 102 after the laser peening process is preferably performed using, for example, gas or water. This laser hardening process preferably affects the material to a depth of 0.5 mm or more, more preferably 0.7 mm or more, and even more preferably about 0.8 mm from the striking face 102. Such a laser hardening process further hardens the striking face 102 to a hardness of, for example, 50 HRC or more, more preferably 52 HRC or more. Based on the above steps, the striking face 102 of the golf club head 100 preferably achieves hardness and wear resistance values according to the embodiment of the golf club head 100 described above.

[0056] As a result of the face hardening treatment, the hardness of the golf club head 100, more specifically the hardness of any part made of the inventive steel described herein, exhibits a hardness gradient throughout its thickness. Preferably, the golf club head 100 exhibits a martensitic structure from the striking face (i.e., measured rearward in a direction perpendicular to the virtual striking face plane generally coplanar with the striking face 102) over 50% or more of the overall depth of the golf club head 100, more preferably over 90% or more of the overall depth of the golf club head 100, and even more preferably over substantially the entire depth of the golf club head 100. In a particular embodiment where the striking face 102 is nitrided, preferably, a CrN layer with a thickness of 0.02 mm or more is formed on the striking face 102 by the nitriding treatment, more preferably a CrN layer with a thickness of 0.03 mm or more is formed, and even more preferably a CrN layer with a thickness of about 0.04 mm is formed.

[0057] In the foregoing description, the present invention has been described with reference to its specific exemplary embodiments. However, it is apparent that various modifications and changes can be made to these exemplary aspects without departing from the broader spirit and scope of the present invention. The exemplary steel compositions described herein are assumed to be applicable to similar scenarios, for example, to uses known to be functionally similar to the functions of the golf club heads described herein. For example, such steel compositions can be applied to figure skating or ice skating blades that require strength and ductility, considering typical cyclic loading and the desirability of forging or cold working. However, even in such cases, surface hardening may be beneficial. Similarly, for the same reasons, such steel compositions can be applied to the edges used in skis, snowboards, and related snow sliding equipment. In particular, in the case of snow sliding equipment, such edges must exhibit sufficient malleability to be formed into irregular or complex outer peripheral shapes, but must also exhibit sufficient surface hardness and wear / corrosion resistance in view of the expected repeated contact with snow and ice. Accordingly, the foregoing discussion and the accompanying drawings are not to be construed as limiting the scope in any way, but are to be regarded as merely illustrative of the present invention.

Explanation of Signs

[0058] 100 Golf club head 102 Impact face 104 Hosel portion 116 Back face

Claims

1. A golf club head, comprising an integral part formed of a stainless steel material, wherein the integral part has a variable hardness such that a first part of the part exhibits a first hardness H1 of 50 HRC or more and a second part of the part exhibits a second hardness H2 of 85 HRB or less, golf club head.

2. The golf club head according to claim 1, wherein the stainless steel material has a carbon content of 0.25% or more by mass ratio.

3. The golf club head according to claim 1, wherein the stainless steel material has a nickel content of 0.35% or less by mass ratio.

4. The golf club head according to claim 3, wherein the nickel content is 0.25% or less by mass ratio.

5. The golf club head according to claim 1, wherein the stainless steel material has a chromium content of 16% or more by mass ratio.

6. The density of the stainless steel material is 7.55 g / cm ３ The golf club head according to claim 1, wherein the density is as follows.

7. Further comprising a striking face, a back portion, and a hosel portion, The golf club head according to claim 1, wherein the first part of the integral part is located on the striking face.

8. The golf club head according to claim 7, wherein the second part of the integral part is located on the hosel portion or the back portion.

9. The golf club head according to claim 1, wherein the first hardness H1 is 60 HRC or more.

10. The golf club head according to claim 1, wherein the hardness ratio H1 / H2 based on a value corresponding to the Rockwell C hardness (HRC) scale is 4.5 or more.

11. Further comprising a head mass Mh, The golf club head according to claim 1, wherein the integral part includes a part mass Mc such that the ratio Mc / Mh is greater than 0.

5.

12. The golf club head according to claim 1, wherein substantially the entire head is formed of the integral part.

13. Further comprising a club head depth Dc, The golf club head according to claim 1, wherein the integral part has a martensite structure over a depth Dm from the striking face, and the ratio Dm / Dc is greater than 0.

5.

14. The golf club head according to claim 1, wherein the striking face is composed of a nitrided surface.

15. A step of forming a part of a golf club head, the part including a stainless steel material, and, A method comprising a step of selectively case-hardening a component such that a first portion of the component exhibits a first hardness H1 of 50 HRC or more and a second portion of the component exhibits a second hardness H2 of 85 HRB or less.

16. The method according to claim 15, wherein the stainless steel material has a carbon content of 0.25% or more by mass ratio.

17. The method according to claim 15, wherein the stainless steel material has a nickel content of 0.35% or less by mass ratio.

18. The method according to claim 17, wherein the nickel content is 0.25% or less by mass ratio.

19. The method according to claim 15, wherein the stainless steel material has a chromium content of 16% or more by mass ratio.

20. The density of the stainless steel material is 7.55 g / cm ３ The method according to claim 15, wherein the density is as follows.

21. The method according to claim 15, wherein the first hardness H1 is 60 HRC or more.

22. The method according to claim 15, wherein the hardness ratio H1 / H2 based on a value corresponding to the Rockwell C hardness (HRC) scale is 4.5 or more.

23. A component for a golf club head, comprising a stainless steel material having a nickel content of 0.25% or less by mass ratio.

24. Furthermore, 7.6 g / cm ３ The component according to claim 22, having the following density.

25. The density is 7.55 g / cm ３ The component according to claim 24, wherein the density is 7.55 g / cm or less.

26. The component according to claim 22, wherein the nickel content is 0.20% or less by mass ratio.

27. A golf club head comprising the component according to claim 10, wherein the golf club head further comprises a striking face, a back portion, and a hosel portion, and the component includes at least a first portion located on the striking face and a second portion located on the back portion or the hosel portion.

28. A component for a golf club head, comprising a stainless steel material having a carbon content of 0.25% or more by mass ratio.

29. A component for a golf club head, comprising a stainless steel material having an austenitizing temperature of 800 °C or more.

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