Abstract:
An α+β-type titanium alloy which, has a high Young&#39;s modulus and strength-ductility balance is provided as a material for a face of a driver, iron, or other golf club. 
     A titanium alloy having a high strength and high Young&#39;s modulus for a golf club face comprising, by mass %, 4.7 to 5.5% of Al, 0.5 to 1.4% of Fe, 0.03% or less of N, O which satisfies an [O] eq (oxygen equivalent value) of 0.25 to 0.34% calculated by formula (1), and a balance of Ti and unavoidable impurities. By adding Al, O, and N which cause solution strengthening of the α-phase and selecting the inexpensive Fe as the β-stabilizing element and suitably limiting the amounts of addition of these elements, it is possible to achieve a high strength and a high Young&#39;s modulus which satisfies the SLE rule without relying on cold working strengthening or aging strengthening heat treatment, and simultaneously obtain a large, good total elongation and a high strength-ductility balance. 
       [O]eq=[O]+2.77[N]  formula (1)

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a titanium alloy which is used for a material for the face of a golf club, mainly a driver. 
       BACKGROUND ART 
       [0002]    In recent years, the “spring-like effect (SLE) rule” has been introduced which prohibits golf clubs from having a spring effect. Along with this, the types of titanium alloys which are used as materials for golf club faces have greatly changed. Before the SLE rule had been introduced, β-type titanium alloy comprised mainly of Ti-15% V-3% Cr-3% Sn-3% Al alloy which easily gave a high spring performance with a low Young&#39;s modulus and was high in strength and excellent in durability was the mainstream. However, along with the introduction of the SLE rule, the only way to use a low Young&#39;s modulus β-type titanium alloy and lower the spring coefficient so as to satisfy the rule was to increase the thickness of the face plate so as to raise the rigidity of the face surface. If using this method, when applying a β-type titanium alloy which contains V and Mo and other expensive alloying elements in large amounts for a face material, an increase in material costs was unavoidable. Furthermore, a β-type titanium alloy is higher in specific gravity as well compared with other titanium alloys. Making the plate thickness greater leads to a heavier face. In this way, a golf club head using a β-type titanium alloy for the face was limited in volume. The sweet spot when hitting a ball became relatively small, so there was also the problem of the user finding the club to be difficult to use. Due to such reasons, a β-type titanium alloy is no longer still the mainstream material for golf club faces. 
         [0003]    An α+β-type titanium alloy, which has a higher Young&#39;s modulus compared with a β-type titanium alloy, is now becoming the mainstream material for driver faces. By using a high Young&#39;s modulus α+β-type titanium alloy, even if the face is made thin, the spring coefficient does not easily become higher. Compared with a β-type titanium alloy, the degree of freedom of plate thickness for clearing the SLE rule increases. Further, compared with a β-type titanium alloy, the specific gravity is smaller and therefore it is possible to increase the volume of the club head even with the same mass. Furthermore, compared with a β-type alloy, the content of expensive alloying-elements is low, so there are also numerous merits such as the lower material costs. As this α+β-type titanium alloy, Ti-6% Al-4% V is representative, but in addition, for example, Ti-5% Al-1% Fe, Ti-4.5% Al-3% V-2% Fe-2% Mo, Ti-4.5% Al-2% Mo-1.6% V-0.5% Fe-0.3% Si-0.03% C, Ti-6% Al-6% V-2% Sn, Ti-6% Al-2% Sn-4% Zr-6% Mo, Ti-8% Al-1% Mo-1% V, Ti-6% Al-1% Fe, etc. are also being used. 
         [0004]    If using these alloys, even if reducing the face plate thickness compared to a face made by a β-type titanium alloy, it is possible to satisfy the SLE rule. In addition, by using a titanium alloy with suitable ranges of strength and ductility, the required durability can also be imparted to the golf club face. Even if making the face plate thickness thinner, in the case of a high grade golf club from which a high durability is demanded, in a round bar shaped product etc. where the face shape or structure can be changed to control the spring performance, it is preferable that the Young&#39;s modulus be 120 GPa, the tensile strength be 950 MPa or more, and the total elongation be 15% or more. In a sheet product etc. where there is little degree of changing the structure of a face when forming it, it is preferable that, in one direction in the sheet plane, the Young&#39;s modulus be 135 GPa or more, the tensile strength be 1100 MPa or more, and the total elongation be 7% or more. In such cases, for the Young&#39;s modulus to clear the SLE rule and, further, for the tensile strength and the ductility to give good durability, preferably the above values are satisfied. However, in general, hot-workability of some α+β-type alloys is not considerably good. Even if making the plate thickness thin, the material has a high durability. The α+β-type alloys in which the high strength and high Young&#39;s modulus which satisfy the SLE rule and good hot workability are simultaneously achieved are limited. 
         [0005]    For example, the most generally used α+β-type alloy, that is, a Ti-6% Al-4% V alloy, has a strength and Young&#39;s modulus which are sufficient for a face material and is already being widely used as an alloy for a golf club face. However, this alloy contains 6% of Al which exhibits a solution strengthening ability at a high temperature and increases the deformation stress during hot working, so is not good in hot workability. Further, it contains the expensive β-phase stabilizing element of V in 4% and therefore had the problem of a relatively high material cost. 
         [0006]    PLT 1 proposes an alloy which has a high specific strength similar to a Ti-6% Al-4% V alloy and is low in production cost. This is an α+β-type alloy which aims at high specific strength and low cost by replacing the β-phase stabilizing elements of V, Mo, and other expensive, heavy specific gravity elements with the inexpensive, high β-stabilizing ability Fe and by adding a large amount of the light specific gravity α-phase stabilizing element of Al. However, this alloy has the difficulty that it contains Al in 5.5 to 7% and is hard to hot work. In particular, to lower the cost of producing the face material, it is desirable to supply a sheet product which can be worked into the shape of a face by simple press forming with a light degree of working and polishing steps, but with this alloy, the high hot deformation stress makes hot-rolling the sheet product difficult. In particular, during hot rolling, the suitable hot rolling temperature range of this alloy is narrow. Due to this, there was the problem that if the temperature was lowered even a small amount, edge cracking would easily occur and the production yield would be low. 
         [0007]    PLT 2 proposes a golf club head which contains a high strength and low springiness titanium alloy face. In the titanium alloy which forms the face, the contents of Al and % Fe are prescribed. Due to this, a higher Young&#39;s modulus and tensile strength can be obtained. PLT 2 does not describe the specific method of production of this alloy, but with an alloy composition which contains Al and Fe plus unavoidable impurities as shown in the claim, to obtain the tensile strength of 1200 to 1600 MPa which is described in the claim, the method of production is considerably limited. That is, an alloy which is left as hot rolled, forged, or otherwise hot worked or an alloy which is obtained by the method of hot working or cold working, then annealing cannot give such strength. Furthermore, even when heat treating the hot or cold worked product by aging heat treatment, it is not possible to obtain this range of strength of a product. There is just a possibility of obtaining it if leaving it as cold worked up to an extremely high working rate. However, in that case, while a high strength is obtained, the ductility and toughness is remarkably deteriorated. In a golf club which uses such a state of a face, if fatigue cracks once form at the face surface, they cannot be kept from spreading. In this way, there was the problem that the high durability which is demanded from recent golf club faces cannot be secured. 
         [0008]    Further, PLT 3 proposes a titanium alloy for face use which gives a high durability of a heat affected zone at a golf club head which includes a welded zone and which can be adjusted in Young&#39;s modulus and strength by heat treatment. This is characterized by adding suitable quantities of Al, Fe, O, and N to adjust the strength and improve the fatigue properties of the heat affected zone and by controlling the aging strengthening heat treatment and other heat treatment conditions so as to control the Young&#39;s modulus. However, after PLT 3 was filed, the SLE rule was introduced and only alloys with a high Young&#39;s modulus came to be sought. With the alloy composition and heat treatment conditions which are described in the claims of PLT 3, there was the problem that sometimes a high Young&#39;s modulus which satisfies the SLE rule cannot be obtained. 
       CITATIONS LIST 
     Patent Literature 
       [0009]    PLT 1: Japanese Patent Publication No. 2004-10963A 
         [0010]    PLT 2: Japanese Patent Publication No. 2006-212092A 
         [0011]    PLT 3: Japanese Patent Publication No. 2005-220388A 
         [0012]    PLT 4: Japanese Patent Publication No. 2008-106317A 
         [0013]    PLT 5: Japanese Patent Publication No. 2008-133531A 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0014]    The present invention has as its object to solve the above problem and provide an α+β-type titanium alloy which has a high Young&#39;s modulus and strength-ductility balance. 
       Solution to Problem 
       [0015]    The inventors discovered that by adding Al, O, and N for solution strengthening the α-phase and by selecting the inexpensive Fe as a β-stabilizing element and suitably limiting the amounts of these elements so as to reduce the fraction of the β-phase at room temperature, it is possible to achieve both a high strength and a high Young&#39;s modulus which satisfies the SLE rule without cold working strengthening or aging strengthening heat treatment. At the same time, they discovered that the total elongation is high, a high strength-ductility balance is exhibited, and a high durability is obtained. Further, this α+β-type alloy contains low specific gravity as well and is optimal for a golf club face application. Furthermore, compared with other α+β-type alloys which are mainly comprised of a Ti-6% Al-4% V alloy, the content of Al, which lowers the hot workability, is kept low, so the applied load during hot rolling is low and defects or edge cracking do not easily occur during hot rolling. For this reason, there is the advantage that the manufacturability of various shapes of products—including sheet—is excellent. 
         [0016]    The present invention was made based on the above discovery and has the following content as its framework: 
         [0017]    A titanium alloy having a high strength and high Young&#39;s modulus for a golf club face comprising, by mass %, 4.7 to 5.5% of Al, 0.5 to 1.4% of Fe, 0.03% or less of N, O which satisfies an [O]eq (oxygen equivalent value) of 0.25 to 0.34% calculated by formula (1), and a balance of Ti and unavoidable impurities. 
         [0000]      [O]eq=[O]+2.77[N]  formula (1)
 
         [0000]    wherein [O] is the concentration of oxygen (mass %) and [N] is the concentration of nitrogen (mass %). 
       Advantageous Effects of Invention 
       [0018]    According to the present invention, it is possible to provide an α+β-type titanium alloy for a golf club face which has a high strength-ductility balance and Young&#39;s modulus. 
     
    
     DESCRIPTION OF EMBODIMENTS 
       [0019]    The inventors etc. worked to solve the above problem by investigating in detail the effects of the component elements and method of production on the material properties of a titanium alloy and as a result discovered that by controlling the amounts of addition of Fe, Al, O, and N, it is possible to produce an α+β-type titanium alloy which gives the high strength-ductility balance and high Young&#39;s modulus which are demanded for a material used for a high grade golf club face, where a high durability is sought even if the plate thickness is reduced, and has an excellent hot workability. In particular, they discovered that by defining the amounts of addition of O and N, which act to solid-solution-strengthen the α-phase, by the [O]eq formula to suitable ranges, it is possible to secure the high strength and Young&#39;s modulus which are demanded for a high end golf club face without causing a deterioration in the ductility. Furthermore, in the alloy of the present invention where mainly Al is used and O and N are added to strengthen the α-phase by coordination of solid-solution-strengthening effect of these alloying elements, when producing a sheet product, hot rolling or cold rolling in one direction enables the remarkable growth of a texture which causes in-plane material anisotropy. In-plane material anisotropy occurs where the Young&#39;s modulus and strength in the direction vertical to the rolling direction, that is, the sheet width direction, increase over those of the rolling direction. 
         [0020]    At the surface of a golf club face, it is enough to realize the target values of the Young&#39;s modulus and tensile strength in the vertical direction of the surface of the golf club face. Therefore, it is sufficient to realize the Young&#39;s modulus and tensile strength in as little as one direction of the sheet. Further, in a sheet product, by hot or cold rolling uni-directionally, it becomes possible to realize the targets of the Young&#39;s modulus and tensile strength in the sheet width direction. That is, if making the vertical direction of the surface of the golf club face the sheet width direction, it is possible to obtain a high Young&#39;s modulus and tensile strength in the very direction required for a golf club face (vertical direction along the surface of golf club face). 
         [0021]    The present invention was made based on the above discovery. Below, the reasons for selecting the various additive elements which are shown in the present invention and their ranges of amounts of addition will be shown. 
         [0022]    Fe is an inexpensive alloying element among β-phase stabilizing elements and has the ability of strengthening the β-phase. In addition, the β-stabilizing ability is high, so even with a relatively low amount of addition, Fe stabilizes the β-phase strongly. To obtain the strength necessary as a golf club face, 0.5% or more of Fe has to be added. On the other hand, Fe tends to segregate in Ti during melting and casting. Further, if added in a large amount, compared with the α-phase, the volume fraction of the low Young&#39;s modulus β-phase increases, so the Young&#39;s modulus of the alloy lowers, with a round bar product, the Young&#39;s modulus becomes 120 GPa, with a sheet product, the Young&#39;s modulus in one direction in the sheet plane becomes less than 135 GPa, and it becomes difficult to clear the SLE rule at the golf club face. Considering these effects, the upper limit of the amount of addition of Fe was made 1.4%. Note that, to emphasize the strength properties and reliably clear the SLE rule by lowering the Young&#39;s modulus, the lower limit of the amount of addition of Fe is preferably 0.7% and the upper limit is 1.2%. 
         [0023]    Al is an element which stabilizes the titanium α-phase. It has a high solid-solution-strengthening ability and is an inexpensive additive element. To obtain the level of strength necessary to be able to secure durability as a high grade golf club face by addition together with the later explained O and N, that is, in the case of a round bar product, a tensile strength of 950 MPa or more or, in the case of a sheet product, a tensile strength in one direction of 1100 MPa or more, the lower limit of the amount of addition was made 4.7%. On the other hand, if adding over 5.5% of Al, the deformation stress becomes too high, the ductility which is required for durability as a golf club face cannot be achieved, and the increase in deformation stress causes the hot workability to be deteriorated. Therefore, the amount of addition of Al has to be 5.5% or less. 
         [0024]    Both O and N are alloying elements which solid-solute in the α-phase and strengthen the α-phase by solution strengthening near room temperature. By addition together with Al, it becomes possible to achieve a high strength and a high Young&#39;s modulus. On the other hand, unlike Al, these do not cause the hot deformation stress to increase, so addition of O and N enables the amount of addition of Al to be lowered. As described in PLTs 3 to 5, due to the similarly of the strengthening mechanisms of O and Ni on Ti, the actions of O and N on the strength at room temperature can be unambiguously expressed by the [O]eq which is shown in the above formula (1). With addition of O and N with an [O]eq of less than 0.25%, it is not possible to stably obtain a strength where sufficient durability is expressed as a high grade golf club face, that is, for round bar products, a tensile strength of 950 MPa or more and, for sheet products, a tensile strength of 1100 MPa or more in one direction in the sheet plane. Further, if adding O and N in a range of [O]eq which exceeds 0.34%, the strength becomes too high and the ductility decreases and a total elongation of 7% can no longer be secured in one direction in the sheet plane of the sheet product. Therefore, the lower limit of the [O]eq which is shown by formula (1) was made 0.25%, while the upper limit was made 0.34%. 
         [0025]    In the case of a sheet product, the inventors discovered that the Young&#39;s modulus E of the sheet width direction of a uni-directionally hot rolled material or cold rolled material of a titanium alloy of the range of chemical composition which is prescribed in the present invention increases in proportion to [O]eq in accordance with formula (2) in the range of the [O]eq. This is because the increase in the α-phase stabilizing element causes a decrease in the β-phase which causes a drop in the Young&#39;s modulus. By adding O and N in the range of the [O]eq of the present invention, it is possible to obtain a value of the Young&#39;s modulus in the sheet width direction of around 140 GPa: 
         [0000]        E= 41.2[O]eq+130.2   formula (2)
 
         [0026]    Regarding the amount of addition of N, if adding over 0.030% of N by the ordinary method of using sponge titanium which contains a high concentration of N, unmelted inclusions called “LDI (low density inclusions)” easily form and the production yield falls, so 0.030% was made the upper limit. 
         [0027]    When, as with round bars or heavy plates etc., it is possible to keep the spring coefficient low by controlling the face shape along with the relatively large amount of working, including hot forging or hot pressing, etc, when forming them into face shapes, by having the above range of components, it is possible to obtain a golf club face which has excellent properties. The titanium alloy of the present invention which has the above range of components is provided with relatively good workability, so is preferable as a face material. 
         [0028]    On the other hand, when the amount of working to form the face shape is relatively small, mainly including a process using a sheet product as a raw material, which has little room for keeping down the spring coefficient by the face shape, if a texture called a “transverse texture” is developed, the tensile strength and the Young&#39;s modulus in the sheet width direction become higher, so the material is preferable for use for a golf club face. If limiting the Al, Fe, and O to the range of components of the present invention, heating to the single β-phase region or the α+β dual phase region temperature right below the β-transus, uni-directional hot rolling or, furthermore, uni-directional cold rolling in the same direction as the hot rolling direction, then annealing under the preferable conditions, a transverse texture easily develops and the strength and Young&#39;s modulus in the sheet width direction become higher, so it is possible to produce a material which is optimum as a material for a face. 
         [0029]    When producing this sheet material, the reason for rolling in only one direction straight from the start to the end of the hot or cold rolling is to efficiently obtain the transverse texture which enables a high Young&#39;s modulus in the sheet width direction which accompanies in-plane anisotropy in mechanical properties as targeted by the present invention. By arranging the titanium alloy sheet which has such a high Young&#39;s modulus and strength-ductility balance so that its sheet width direction becomes the vertical direction of the golf club face or a direction close to the same, it becomes possible to produce a face which complies with the SLE rule and is provided with a high durability. 
       EXAMPLES 
     Example 1  
       [0030]    A vacuum arc melting method was used to melt a titanium material of each of the compositions which are shown in Table 1. This was hot forged to obtain a diameter 100 mm billet. This billet was heated to 950° C., then hot rolled to produce a round bar with diameter of 18 mm. This round bar was annealed at 800° C. for 2 h, then an average diameter 6 mm JIS No. 14 tensile test piece was taken and investigated for tensile properties. Further, the depth of surface defects which formed during hot rolling was measured by a laser 3D roughness meter as the depth from the surface the opening of the hot rolling defect (Good: Maximum defect depth&lt;0.5 mm, Poor: Maximum defect depth≧0.5 mm). To obtain an excellent durability as a round bar material for high grade golf club face use, a tensile strength of 950 MPa or so or more and a total elongation of 15% or more are necessary. Further, a Young&#39;s modulus of 120 GPa or more is necessary. These results are shown together in Table 1. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Young&#39;s 
                 Hot rolling 
                   
               
             
          
           
               
                 Test 
                 Content of alloying elements (mass %) 
                 TS 
                 EL 
                 modulus 
                 defect 
                   
               
             
          
           
               
                 no. 
                 Al 
                 Fe 
                 V 
                 O 
                 N 
                 [O]eq 
                 Ti 
                 (MPa) 
                 (%) 
                 (GPa) 
                 evaluation 
                 Remarks 
               
               
                   
               
             
          
           
               
                 1 
                 
                   6.3 
                 
                 — 
                 
                   4.12 
                 
                 0.25 
                 0.011 
                 0.280 
                 bal 
                 1034 
                 17 
                 123 
                 Poor 
                 Comp. ex. 
               
               
                 2 
                 
                   7.2 
                 
                 1.0 
                 — 
                 0.31 
                 0.023 
                 
                   0.374 
                 
                 ″ 
                 1078 
                 16 
                 123 
                 Poor 
                 Comp. ex. 
               
               
                 3 
                 
                   3.9 
                 
                 0.9 
                 — 
                 0.25 
                 0.005 
                 0.264 
                 ″ 
                 879 
                 21 
                 123 
                 Good 
                 Comp. ex. 
               
               
                 4 
                 4.9 
                 0.8 
                 — 
                 0.25 
                 0.005 
                 0.264 
                 ″ 
                 954 
                 19 
                 125 
                 Good 
                 Inv. ex. 
               
               
                 5 
                 5.3 
                 0.8 
                 — 
                 0.25 
                 0.005 
                 0.264 
                 ″ 
                 1003 
                 19 
                 125 
                 Good 
                 Inv. ex. 
               
               
                 6 
                 
                   6.7 
                 
                 0.8 
                 — 
                 0.25 
                 0.005 
                 0.264 
                 ″ 
                 1089 
                 14 
                 124 
                 Poor 
                 Comp. ex. 
               
               
                 7 
                 5.1 
                 
                   0.2 
                 
                 — 
                 0.29 
                 0.011 
                 0.320 
                 ″ 
                 924 
                 22 
                 122 
                 Good 
                 Comp. ex. 
               
               
                 8 
                 5.1 
                 0.7 
                 — 
                 0.29 
                 0.011 
                 0.320 
                 ″ 
                 1027 
                 19 
                 125 
                 Good 
                 Inv. ex. 
               
               
                 9 
                 5.1 
                 1.1 
                 — 
                 0.29 
                 0.011 
                 0.320 
                 ″ 
                 1061 
                 18 
                 126 
                 Good 
                 Inv. ex. 
               
               
                 10 
                 5.1 
                 
                   1.9 
                 
                 — 
                 0.29 
                 0.011 
                 0.320 
                 ″ 
                 1104 
                 19 
                 118 
                 Good 
                 Comp. ex. 
               
               
                 11 
                 5.2 
                 1.0 
                 — 
                 0.15 
                 0.020 
                 
                   0.205 
                 
                 ″ 
                 924 
                 22 
                 121 
                 Good 
                 Comp. ex. 
               
               
                 12 
                 5.2 
                 1.0 
                 — 
                 0.23 
                 0.020 
                 0.285 
                 ″ 
                 1012 
                 19 
                 125 
                 Good 
                 Inv. ex. 
               
               
                 13 
                 5.2 
                 1.0 
                 — 
                 0.27 
                 0.020 
                 0.325 
                 ″ 
                 1068 
                 18 
                 126 
                 Good 
                 Inv. ex. 
               
               
                 14 
                 5.2 
                 1.0 
                 — 
                 0.39 
                 0.020 
                 
                   0.445 
                 
                 ″ 
                 1182 
                 13 
                 127 
                 Good 
                 Comp. ex. 
               
               
                 15 
                 5.0 
                 0.9 
                 — 
                 0.25 
                 0.002 
                 0.256 
                 ″ 
                 993 
                 20 
                 124 
                 Good 
                 Inv. ex. 
               
               
                 16 
                 5.0 
                 0.9 
                 — 
                 0.25 
                 0.007 
                 0.269 
                 ″ 
                 1001 
                 20 
                 124 
                 Good 
                 Inv. ex. 
               
               
                 17 
                 5.0 
                 0.9 
                 — 
                 0.25 
                 
                   0.055 
                 
                 
                   0.402 
                 
                 ″ 
                 — 
                 — 
                 — 
                 Poor 
                 Comp. ex. 
               
               
                 18 
                 4.9 
                 1.1 
                 — 
                 0.23 
                 0.021 
                 0.288 
                 ″ 
                 1041 
                 19 
                 125 
                 Good 
                 Inv. ex. 
               
               
                 19 
                 4.9 
                 1.1 
                 — 
                 0.28 
                 0.011 
                 0.310 
                 ″ 
                 1068 
                 18 
                 126 
                 Good 
                 Inv. ex. 
               
               
                 20 
                 4.9 
                 1.2 
                 — 
                 0.32 
                 0.004 
                 0.331 
                 ″ 
                 1081 
                 18 
                 126 
                 Good 
                 Inv. ex. 
               
               
                   
               
               
                 Good: Maximum defect depth &lt; 0.5 mm 
               
               
                 Poor: Maximum defect depth ≧ 0.5 mm 
               
             
          
         
       
     
         [0031]    In Table 1, Test Nos. 1 and 2 respectively are the results of a Ti-6% Al-4% V alloy and Ti-7% Al-1% Fe. In both Test Nos. 1 and 2, the tensile strength (TS) exceeds the target value of 950 MPa, but 0.5 mm or more depth hot rolling defects occur and the hot workability of these alloys is poor. 
         [0032]    As opposed to this, the examples of the present invention, that is, Test Nos. 4, 5, 8, 9, 12, 13, 15, 16, 18, 19, and 20, have a 950 MPa or more high tensile strength (TS) and an over 15% high total elongation (EL), so production of a face which has excellent durability is possible. 
         [0033]    On the other hand, in Test Nos. 3, 7, and 11, the tensile strength is 950 MPa or less or not a sufficient strength as a face material. In the order of Test Nos. 3, 7, and 11, respectively the amounts of Al, Fe, and [O]eq fell below the lower limit values of the present invention, so the solution strengthening ability was not sufficient and the tensile strength became low. 
         [0034]    In Test Nos. 6 and 14, the total elongation is below 15%, so ductility and toughness are insufficient and a high durability cannot be imparted. Test No. 6 had Al added in an amount over the upper limit value of the present invention, so in Test No. 14, the [O]eq exceeded the upper limit, so the strength rose excessively and the ductility decreased. Further, in Test No. 17, N was added over the upper limit of the present invention. The formation of LDI was confirmed, so the test was suspended. 
         [0035]    Among the above, in Test Nos. 6 and 17, after hot rolling, numerous surface defects with depths over 0.5 mm formed. In Test Nos. 6, Al, which lowers the hot workability, was added over the upper limit of the present invention and hot rolling defects formed. Further, in Test No. 17, due to excessive inclusion of N, LDI formed. The ones near the surface were recognized as being defects. 
         [0036]    In Test No. 10, the amount of Fe was beyond the upper limit of this invention and the Young&#39;s modulus fell below 120 GPa. 
         [0037]    From the above results, a titanium alloy which has the contents of elements which are prescribed by the present invention is high in tensile strength and total elongation, has excellent material properties as a material for a golf club face, and has good hot workability. On the other hand, if outside the amounts of alloy elements which are prescribed in the present invention, the hot workability decreases and the required material properties of the tensile strength and ductility cannot be satisfied. 
       Example 2  
       [0038]    The vacuum arc melting method was used to melt the titanium material of each of the chemical compositions which are shown in Test Nos. 5, 9, and 12 of Table 1 and ingots were cast. Each ingot was hot forged to obtain a thickness 180 mm slab. The slab was uni-directionally hot rolled to 4 mm thick sheet under the conditions which are shown in Table 2. The hot rolled sheet was shot blasted, then pickled to remove the oxide scale. At that time, the depth of surface defects was measured by a depth gauge to evaluate the hot workability (Good: maximum defect depth&lt;0.3 mm, Poor: maximum defect depth≧0.3 mm). The results on hot workability and the tensile properties are shown together in Table 2. 
         [0000]    
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                 Reheating 
                 tensile 
                 Young&#39;s 
                 total 
                   
               
               
                   
                   
                   
                 temperature 
                 strength in 
                 modulus in 
                 elongation in 
               
               
                   
                 Table 1 
                 β transus 
                 prior to 
                 sheet width 
                 sheet width 
                 sheet width 
                 Hot rolling 
               
               
                 Test 
                 test 
                 temperature 
                 hot rolling 
                 direction 
                 direction 
                 direction 
                 defect 
               
               
                 no. 
                 no. 
                 (° C.) 
                 (° C.) 
                 (MPa) 
                 (GPa) 
                 (%) 
                 evaluation 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 21 
                 5 
                 1020 
                 1010 
                 1135 
                 142 
                 10 
                 Good 
               
               
                 22 
                   
                   
                 940 
                 1110 
                 139 
                 11 
                 Good 
               
               
                 23 
                   
                   
                 1040 
                 1154 
                 142 
                 10 
                 Good 
               
               
                 24 
                   
                   
                 1100 
                 1167 
                 142 
                 9 
                 Good 
               
               
                 25 
                   
                   
                 960 
                 1117 
                 140 
                 11 
                 Good 
               
               
                 26 
                 9 
                 1017 
                 1010 
                 1231 
                 143 
                 9 
                 Good 
               
               
                 27 
                   
                   
                 950 
                 1191 
                 141 
                 11 
                 Good 
               
               
                 28 
                   
                   
                 1050 
                 1244 
                 145 
                 9 
                 Good 
               
               
                 29 
                   
                   
                 1070 
                 1247 
                 144 
                 8 
                 Good 
               
               
                 30 
                   
                   
                 970 
                 1201 
                 141 
                 11 
                 Good 
               
               
                 31 
                 12 
                 1018 
                 1010 
                 1164 
                 143 
                 11 
                 Good 
               
               
                 32 
                   
                   
                 950 
                 1122 
                 141 
                 12 
                 Good 
               
               
                 33 
                   
                   
                 1030 
                 1178 
                 142 
                 10 
                 Good 
               
               
                 34 
                   
                   
                 1050 
                 1197 
                 143 
                 10 
                 Good 
               
               
                 35 
                   
                   
                 980 
                 1135 
                 141 
                 12 
                 Good 
               
               
                   
               
               
                 Hot rolling defect evaluation 
               
               
                 Good: Maximum defect depth &lt; 0.3 mm 
               
               
                 Poor: Maximum defect depth ≧ 0.3 mm 
               
             
          
         
       
     
         [0039]    Table 2 shows the results in sheet products of chemical compositions which are shown in Test Nos. 5, 9, and 12 of Table 1. Among these, all of the sheets which were produced under the conditions of Table 2 were sufficiently satisfactory in the properties which are required for sheet products which are used for golf club faces, that is, the tensile strength in the sheet width direction (1100 MPa or more) and the Young&#39;s modulus (135 GPa or more), and secured a total elongation in the sheet width direction of 7% or more as well. Golf club faces which were produced by using these sheet materials are provided with both properties complying with the SLE rule and good durability. Further, hot rolled and pickled sheets did not have surface defects of a depth over 0.3 mm and exhibited good hot ductility. Therefore, these sheet members are suitable as a material for golf club faces. 
         [0040]    In particular, Test Nos. 21, 23, 24, 26, 28, 29, 31, 33, and 34 have a 142 GPa or more high Young&#39;s modulus in the sheet width direction and, when comparing alloys of the same chemical compositions, have a higher tensile strength compared with Test Nos. 22, 25, 27, 30, 32, and 35, have excellent performance with respect to the SLE rule, and have good durability. This is because, in Test Nos. 22, 25, 27, 30, 32, and 35, the reheating temperature prior to hot rolling was a relatively low temperature of the α+β dual phase region, so compared with the case of a single β-phase region or heating up to the α+β dual phase temperature right under the β-transus, transverse texture does not sufficiently develop and the in-plane anisotropy in mechanical properties is not strengthened, while in Test Nos. 23, 24, 28, 29, 33, and 34, the material is heated to the single β-phase region and hot rolled, so in particular the transverse texture develops, the in-plane anisotropy in mechanical properties is strengthened, and a high Young&#39;s modulus and tensile strength can both be obtained high in the sheet width direction. 
         [0041]    From the above results, a sheet material for a golf club face which is provided with excellent properties of a high Young&#39;s modulus, tensile strength, and ductility in the sheet width direction can be produced by hot rolling a titanium alloy which has additive elements in the range of components which is shown in the present invention. 
       INDUSTRIAL APPLICABILITY 
       [0042]    In a round bar shaped product of the titanium alloy of the present invention, a Young&#39;s modulus of 120 GPa, a tensile strength of 950 MPa, and a total elongation or 15% or more are obtained. In a sheet product, in one direction in the sheet plane, a Young&#39;s modulus of 135 GPa, a tensile strength of 1100 MPa, and a total elongation of 7% or more are obtained. As a result, it is possible to provide a material which satisfies the SLE rule when working it into a golf club face and which has excellent and is suitable for high grade golf club face applications.