JP5874707B2 - Titanium alloy with high strength, high Young's modulus and excellent fatigue properties and impact toughness - Google Patents

Description

  The present invention relates to a titanium alloy plate having high strength and high Young's modulus in one direction in the plate surface, excellent fatigue characteristics and / or impact toughness, and good hot workability.

  Until now, many titanium alloy products have been used, taking advantage of excellent properties such as high specific strength and high corrosion resistance, including aircraft components. Among them, it has been widely used for consumer products such as automobile / motorcycle muffler materials, eyeglass frames and various sports tools (golf club face, spike parts, metal bats, etc.).

  One of the drawbacks of titanium alloys is that the Young's modulus is lower than that of steel materials. Due to the low Young's modulus, when used as a structural material or component, it may become a problem that it is easily elastically deformed (low rigidity). Further, for example, when used as a golf club face, since the face is easily bent, the coefficient of restitution tends to increase, making it difficult to satisfy the coefficient of restitution coefficient.

  At this time, when the shape of the product is an elliptical or rectangular plate, it becomes difficult to bend when the Young's modulus increases in the short side direction, and it has been found that the product is effective as a means for increasing the rigidity of the plate. In order to obtain such a state, Patent Document 1 discloses a technique for increasing strength and Young's modulus in the plate width direction by controlling the texture by unidirectional hot rolling of an α + β type titanium alloy. This is because the α + β alloy is unidirectionally hot-rolled under specific conditions to develop a hot-rolling texture called transverse-texture in which the bottom surface of the titanium α-phase is strongly oriented in the plate width direction. The Young's modulus is increased. In that case, by setting the plate width direction of the hot-rolled plate to the short side of the oval or rectangular plate product, it becomes possible to make the plate product difficult to bend.

  Thus, for example, for golf club faces, the application of α + β type titanium alloys having a high Young's modulus as described above has become the mainstream in an environment where the coefficient of restitution coefficient has become strict. By using α + β type titanium alloy with high Young's modulus, even if the face is thinned, the coefficient of restitution is hard to increase, and compared to β type titanium alloy with low Young's modulus, the plate thickness freedom to meet the regulation of coefficient of restitution is increased. . In addition, the specific gravity is small compared to the β-type titanium alloy, the capacity of the club head can be increased even with the same mass, and the material cost is low because the content of expensive alloy elements is low compared to the β-type alloy. There are many merits. As the α + β type titanium alloy, Ti-6% Al-4% V is typical, but other examples include Ti-5% Al-1% Fe, Ti-4.5% Al-3% V. -2% Fe-2% Mo, Ti-4.5% Al-2% Mo-1.6% V-0.5Fe-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. It is used.

  Furthermore, in applications for golf club faces, for thin plate materials that have a low degree of molding during face processing and a low degree of freedom to meet the coefficient of restitution coefficient by shape control, the Young's modulus is 135 GPa or more in one direction in the plate surface. It is desirable to have a strength of 1100 MPa or more. At this time, it is desirable that the Young's modulus satisfies the rebound coefficient regulation, and the tensile strength and ductility satisfy the above values in order to obtain good fatigue properties. However, α + β type alloys generally do not have good workability, and even if the plate thickness is reduced, excellent fatigue properties and high strength and high Young's modulus satisfying repulsion coefficient regulations are compatible with good hot workability. Alloys to do are limited. Also, high values have not been obtained for fatigue properties and / or impact toughness that affect the durability of golf club faces. That is, a technique related to a titanium alloy that achieves both a high Young's modulus and high fatigue strength and / or impact toughness is not disclosed.

  For example, Ti-6% Al-4% V alloy, which is the most general-purpose α + β type alloy, has sufficient strength and Young's modulus, and is already widely used as a structural member including aircraft parts. However, this alloy contains 6% Al, which exhibits solid solution strengthening ability at high temperatures and increases deformation resistance during hot working, has poor hot workability, and is an expensive β-stabilizing element However, there is a problem that the fatigue strength is not sufficient because it is strengthened by solid solution strengthening of O.

  Patent Document 2 proposes a low-cost alloy having a high specific strength in the same manner as a Ti-6% Al-4% V alloy. This is to replace expensive and heavy specific elements such as V and Mo with low-cost and high β-stabilizing elements as β-stabilizing elements and to add a large amount of Al, which is an α-stabilizing element with low specific gravity. Therefore, it is an α + β type alloy aimed at high specific strength and low cost. However, this alloy contains 5.5 to 7% of Al and has a drawback that it is difficult to hot work. In order to reduce the processing cost of the face material, it is desirable to supply a plate product that can be processed into a face shape only by light press forming and polishing processes. However, when manufacturing hot rolled sheets of the alloy, high hot deformation resistance is required. Therefore, there is a problem that the proper hot rolling temperature range is narrow, and if the temperature is lowered even slightly, the ear cracks are likely to occur and the product yield is low.

  Patent Document 3 proposes a golf club head including a high strength and low repulsion titanium alloy face. In the titanium alloy constituting the face, the contents of Al and Fe are regulated, and it is said that high Young's modulus and tensile strength can be obtained. Although the specific manufacturing method of this alloy is not described in Patent Document 3, the tensile strength described in the claims is made of an alloy composition containing Al and Fe shown in the claims and the balance unavoidable impurities. In order to obtain 1200 to 1600 MPa, the manufacturing method is considerably limited. That is, such a strength cannot be obtained in the case of performing an annealing process after hot working such as hot rolling or forging, or after hot working or cold working. Furthermore, even when hot- or cold-worked products are subjected to aging heat treatment, products in this strength range cannot be obtained, and can only be obtained in the state of being cold-worked to a high workability. There is. However, if the material is used for a golf club face while being cold-worked, high strength can be obtained, but the fatigue characteristics are remarkably lowered. Therefore, once a fatigue crack occurs in the face, the propagation cannot be suppressed. Thus, there is a problem that the excellent fatigue characteristics required for the golf club face cannot be secured.

  Patent Document 4 proposes a titanium alloy plate for a face in which a golf club head including a welded portion has high fatigue characteristics in the heat affected zone and the Young's modulus and strength can be adjusted by heat treatment. The strength is adjusted by adding appropriate amounts of Al, Fe, O, and N, the fatigue properties of the heat affected zone are improved, and the Young's modulus is controlled by controlling the heat treatment conditions such as aging strengthening heat treatment. And However, after the patent document 4 is filed, the coefficient of restitution coefficient is enforced, and only alloys having a high Young's modulus are required, and the plate product manufactured according to the alloy composition and manufacturing conditions described in the patent document 4 However, there is a problem that a high Young's modulus that satisfies the restitution coefficient regulation may not be obtained.

  In Patent Document 5, a titanium alloy containing Al, Fe, O, and N is unidirectionally hot-rolled to develop a texture called the above-mentioned transverse-texture and suppress the occurrence of plate breakage during coil winding. For example, a technique for improving cold coil handling is disclosed. This is because, along with the development of Transverse-texture, even if an ear crack that is the starting point of the plate breakage occurs, the crack propagation path is skewed and lengthened. However, the solution of technical problems concerning high Young's modulus, high fatigue properties, etc. is not considered.

  Patent Document 6 discloses an α + β-type titanium alloy containing Al, Fe, and Si, and discloses that it has fatigue strength and creep resistance equivalent to those of a conventional Al—Fe-based titanium alloy. . However, technical issues regarding high Young's modulus and the like are not considered.

  In Patent Document 7, a titanium alloy containing Al, Fe, Si, and O, and selectively containing Mo and V is heated to a β transformation point temperature or higher, and hot rolling is started below the β transformation point. And the manufacturing method of the alpha + beta type titanium alloy mainly hot-rolled at the temperature of 900 degreeC or more is disclosed. The titanium alloy produced in this way is able to reduce surface flaws generated on the surface of the hot-rolled sheet, but the disclosure of the technology relating to the titanium alloy having high Young's modulus, high strength, and excellent fatigue properties. There is no.

  Patent Document 8 discloses a near β-type α + β-type alloy having excellent fracture toughness by adding Si and a method for producing the same. However, the toughness is evaluated by the fracture toughness value, not by the characteristics related to impact toughness including deformation at a high strain rate by the Charpy test. Further, the microstructure is limited to a needle-like structure.

  Here, the fracture toughness is generally a material characteristic showing the difficulty of crack propagation at a relatively low strain rate, and is generally evaluated by performing a fracture toughness test. For example, it is evaluated using an unloading elastic compliance method as shown in Non-Patent Document 1 or the like. On the other hand, impact toughness is a characteristic indicating the difficulty of fracture under a high strain rate, and can be easily evaluated using absorbed energy of a Charpy impact test. Since golf clubs and automobile parts are exposed to high-speed deformation, high impact toughness is desirable.

  That is, there is no disclosure of a technology relating to an α + β type titanium alloy that simultaneously satisfies high grade golf club face, high Young's modulus, high strength, excellent fatigue properties and impact toughness required for some automobile parts and the like.

JP 2012-132057 A JP 2004-10963 A JP 2006-212092 A Japanese Patent Laid-Open No. 2005-220388 WO2012 / 115243A1 Japanese Patent Laid-Open No. 7-62474 JP 2012-149283 A Japanese Patent Laid-Open No. 11-343529

"Materials" Vol. 25, No. 276, September 1976, pp. 91-95

  An object of the present invention is to solve the above-mentioned problems and to provide an α + β-type titanium alloy having high Young's modulus and strength in one direction in the plate surface and high fatigue characteristics and / or impact toughness. It is.

  The inventors added Al, O, N, and Si that solidify and strengthen the α phase, and Si that shows a segregation tendency opposite to that of O, and selected Fe that is inexpensive and has high β stabilizing ability as a β stabilizing element. The amount of these elements added was appropriately limited to reduce the β phase fraction at room temperature and suppress the decrease in Young's modulus. Furthermore, it has been found that by hot rolling this alloy in one direction, both high strength and high Young's modulus can be achieved in one direction within the plate without relying on cold work strengthening or aging strengthening heat treatment. At the same time, it has been found that, while exhibiting high strength, fatigue characteristics and / or impact toughness are also increased. Since Si shows a segregation tendency opposite to that of O, by adding Si and O in combination, at the position corresponding to the top side of the original ingot accompanying the solidification segregation of O that occurs when O is added alone. It became possible to prevent excessive high strength and low ductility. Further, since Si is less prone to solidification and segregation as much as O, it has a characteristic that it becomes a starting point of fracture in a fatigue test and an impact test, or a crack is easily propagated and a hard part is hardly generated. Thus, by adding Si that does not adversely affect fatigue characteristics and / or impact toughness, it is possible to ensure strength even if the amount of O that decreases fatigue characteristics and / or impact toughness is reduced.

  In particular, this alloy is unidirectionally hot-rolled to develop a texture called transverse-texture in which the c-axis of the titanium α phase is strongly oriented in the plate width direction, thereby reducing the tensile strength and Young's modulus in the plate width direction. It is possible to increase the fatigue characteristics and / or impact toughness when bending deformation is repeated in the plate width direction. In particular, it has been found that the above-described mechanism is highly effective when Si and O are added in combination.

  In addition, this alloy has a low specific gravity and is an optimal material for a wide range of applications including golf club faces. Furthermore, compared with other α + β type alloys mainly composed of Ti-6% Al-4% V alloy, the content of Al which lowers hot workability is limited to be low, and the rolling load during hot rolling Therefore, scratches and ear cracks during hot rolling are less likely to occur, and therefore, there is an advantage that the manufacturability to products of all shapes including thin plates is good.

This invention is made | formed based on the said knowledge, and makes the following means the main point.
(1) By mass%, 4.7 to 5.5% Al, 0.5 to 1.3% Fe, 0.03% or less N, and calculated from equation (1) [O] Rolling of hot-rolled sheet containing O and N satisfying _eq of 0.13% or more and less than 0.25%, further containing 0.15 to 0.40% Si, the balance being Ti and inevitable impurities The surface normal direction is the ND direction, the hot rolling direction is the RD direction, the plate width direction of the hot-rolled sheet is the TD direction, the normal direction of the (0001) plane of the α phase is the c-axis direction, and the c-axis direction is ND The angle formed by the direction is θ, the angle formed by the plane including the c-axis azimuth and the ND direction with the plane including the ND direction and the TD direction is φ, the angle θ is 0 ° or more and 30 ° or less, and φ is the entire circumference. Among the (0002) reflected relative intensities of X-rays by crystal grains entering (-180 degrees to 180 degrees), the strongest intensity is XND, and the angle θ is 80 degrees or more. Of the X-ray (0002) reflection relative intensities of crystal grains that are less than 100 degrees and φ is within ± 10 degrees, the strongest intensity is XTD, XTD / XND is 4.0 or more, An α + β-type titanium alloy hot-rolled sheet excellent in hot workability, having a Young's modulus of 135 GPa or more, a tensile strength in the sheet width direction of 1100 MPa or more, and excellent fatigue characteristics.
Here, the plate width direction is a direction perpendicular to the hot rolling direction within the plate surface.
[O] _eq = [O] + 2.77 [N] ... Formula (1)
Here, [O] is the oxygen concentration (mass%), and [N] is the nitrogen concentration (mass%).
(2) By mass%, 4.7 to 5.5% Al, 0.5 to 1.3% Fe, 0.03% or less N, and calculated from the formula (1) [O] Rolling of hot-rolled sheet containing O and N satisfying _eq of 0.13% or more and less than 0.25%, further containing 0.20 to 0.40% Si, the balance being Ti and unavoidable impurities The surface normal direction is the ND direction, the hot rolling direction is the RD direction, the plate width direction of the hot-rolled sheet is the TD direction, the normal direction of the (0001) plane of the α phase is the c-axis direction, and the c-axis direction is ND The angle formed by the direction is θ, the angle formed by the plane including the c-axis azimuth and the ND direction with the plane including the ND direction and the TD direction is φ, the angle θ is 0 ° or more and 30 ° or less, and φ is the entire circumference. Among the (0002) reflected relative intensities of X-rays by crystal grains entering (-180 degrees to 180 degrees), the strongest intensity is XND, and the angle θ is 80 degrees or more. Of the X-ray (0002) reflection relative intensities of crystal grains that are less than 100 degrees and φ is within ± 10 degrees, the strongest intensity is XTD, XTD / XND is 4.0 or more, An α + β-type titanium alloy hot-rolled sheet excellent in hot workability, having a Young's modulus of 135 GPa or more, a tensile strength in the sheet width direction of 1100 MPa or more, and excellent fatigue characteristics and impact toughness.
Here, the plate width direction is a direction perpendicular to the hot rolling direction within the plate surface.
[O] _eq = [O] + 2.77 [N] ... Formula (1)
Here, [O] is the oxygen concentration (mass%), and [N] is the nitrogen concentration (mass%).
(3) The titanium alloy slab having the composition described in (1) or (2) is heated to the β single phase region or the α + β2 phase region from the β transformation point to a temperature lower by 50 ° C. than the β transformation point , and unidirectionally. The method for producing an α + β-type titanium alloy hot rolled sheet according to (1) or (2) , wherein hot rolling is performed at a hot rolling rate of 90% or more .

  According to the present invention, it is possible to provide an α + β-type titanium alloy sheet having a high strength / ductility balance and Young's modulus in the sheet width direction and having excellent fatigue characteristics and / or impact toughness.

Diagram explaining crystal orientation Diagram explaining X-ray pole figure

In order to solve the above problems, the present inventors have investigated in detail the effects of the component elements and the production method on the material properties of the titanium alloy. As a result, the amount of Fe, Al, O, N, and Si is increased by controlling the amount of addition. It has been found that an α + β type titanium alloy having a balance between strength and ductility and a high Young's modulus and having good hot workability can be produced. In particular, the addition amount of O and N, which has the function of strengthening by solid solution in the α phase, is regulated within the proper range by the [O] _eq formula, and by adding the proper amount of Si, it becomes a high-end golf club face. It was found that the required high strength, Young's modulus and fatigue characteristics can be secured. Furthermore, in the present invention alloy that is strengthened by adding Al, mainly O, N, and Si, when producing plate products, the texture that causes material anisotropy is properly developed by unidirectional hot rolling or cold rolling. However, material anisotropy occurs in which the Young's modulus and strength in the sheet width direction, which is a direction perpendicular to the rolling direction, are increased as compared to the rolling direction. Furthermore, it has the characteristic that fatigue characteristics and / or impact toughness are excellent.

  In the golf club face surface, it is sufficient that the Young's modulus and the tensile strength achieve the target values in the longitudinal direction of the golf club face surface. Therefore, the Young's modulus and tensile strength required in at least one direction of the plate may be realized. Here, in the thin plate product, by performing unidirectional rolling, it is possible to achieve the targets for the Young's modulus and the tensile strength in the plate width direction. That is, if the longitudinal direction of the golf club face is taken as the plate width direction, a high Young's modulus and tensile strength in one direction required for the golf club face (longitudinal direction of the golf club face) can be obtained. . Further, it is possible to improve the bending fatigue characteristics when the bending deformation is repeatedly performed in the sheet width direction and the Charpy impact characteristics when the notches are inserted in the sheet width direction.

  The present invention has been made based on the above findings. The reasons for selecting the various additive elements shown in the present invention and the reasons for limiting the addition amount range are shown below. Hereinafter, unless otherwise specified,% means mass%.

  Fe is an inexpensive additive element among the β-phase stabilizing elements and has a function of strengthening the β-phase. In addition, since the β-stabilizing ability is high, the β-phase can be stabilized even with a relatively low addition amount. Addition of 0.5% or more of Fe is necessary in order to obtain the strength required for applications such as automobile parts and consumer products, for example, golf club faces. On the other hand, Fe is prone to solidification and segregation in Ti, and if added in a large amount, the volume fraction of the β phase, which has a lower Young's modulus compared to the α phase, increases, resulting in a decrease in the bulk Young's modulus. The Young's modulus is less than 135 GPa in one of the directions, making it difficult to satisfy the rebound coefficient regulation when used as a golf club face. Furthermore, it turned out that intensity | strength raises with the raise of Fe content rate, and the fall of impact toughness is also seen as a result. Considering those effects, the upper limit of the amount of Fe added is set to 1.3%. In addition, in order to emphasize strength characteristics and to reliably satisfy the coefficient of restitution due to a decrease in Young's modulus, it is desirable that the lower limit of Fe addition is 0.7% and the upper limit is 1.2%.

  Al is a stabilizing element of the titanium α phase, has high solid solution strengthening ability, and is an inexpensive additive element. In order to obtain a tensile strength of 1100 MPa or more in the sheet width direction of a thin sheet product, which is a necessary strength level that can ensure excellent fatigue characteristics for applications such as high-grade golf club faces by combined addition with O and N described later. The lower limit of the amount added was 4.7%. On the other hand, when Al is added in excess of 5.5%, hot workability is decreased due to an increase in hot deformation resistance, and the α phase is excessively solid-solution strengthened by solidification segregation or the like to locally harden the region. In addition to reducing fatigue strength, it also reduces impact toughness. Therefore, the amount of Al needs to be 5.5% or less.

Both O and N have an interstitial solid solution in the α phase and have the effect of strengthening the α phase by solid solution strengthening at a temperature near room temperature. By combined addition with Al, it becomes possible to achieve high strength and also high Young's modulus. On the other hand, unlike Al, since the hot deformation resistance is not increased, it is possible to suppress the amount of Al added by adding O, N, and Si in combination. As described in Patent Documents 4 to 6, from the similarity of the strengthening mechanism of O and N on Ti, the action of O and N on the strength at room temperature is represented by the formula [1] It can be uniquely expressed by _eq . Even when Si is added, the addition of O and N with [O] _eq of less than 0.13% shows, for example, a thin plate product that exhibits sufficient fatigue characteristics as a high-grade golf club face. It is impossible to stably obtain a strength of 1100 MPa or more in one direction. In patent document 7, it turns out that 0.08% is made into a lower limit only by O, and it is not aiming at obtaining sufficient intensity | strength. In addition, when O and N in a range where [O] _eq is 0.25% or more are added due to the complex addition of Si, locally, along with excessive solid solution strengthening of α phase due to solidification segregation, A hard region is generated, and fatigue strength and / or impact toughness is reduced. Therefore, the lower limit of [O] _{eq represented by} the formula (1) is set to 0.13% or more and the upper limit is set to less than 0.25%.

  As for the amount of N added, when 0.030% or more of N is added by a normal addition method using sponge titanium containing a high concentration of N, undissolved inclusions called LDI (Low density Inclusion) are generated. Since it becomes easy and the yield of the product becomes low, 0.030% was made the upper limit. N may not be contained.

  Si is a stabilizing element of the titanium β phase, but is also an inexpensive additive element while being dissolved in the α phase and having a high solid solution strengthening ability. The combined addition with O and N, for example, to obtain a tensile strength of 1100 MPa or more in the sheet width direction of a thin sheet product, which is a strength level necessary to ensure fatigue characteristics as a high-grade golf club face. Was set to 0.15%. Preferably it is 0.25% or more. In addition, Si has a tendency to segregate opposite to O, and solidification segregation is less likely to occur as much as O. Therefore, by adding an appropriate amount of Si in combination with O, both high fatigue strength and high tensile strength can be achieved. Is possible. This is a feature of the Si addition effect. In Patent Documents 6 and 7, the amount of Si added is specified to be less than 0.25% from the viewpoint of reducing fatigue strength in a component system similar to the present invention. However, even if Si is added in an amount of 0.25% or more, fatigue characteristics do not deteriorate unless coarse silicide is generated. Moreover, when Si became 0.2% or more, it turned out that impact toughness also improves. That is, in the Si composition region of 0.2% or more, characteristics excellent in fatigue characteristics and impact toughness can be obtained. On the other hand, if Si is added in excess of 0.40%, coarse silicide is generated during hot rolling, hot forging, or cooling, resulting in reduced strength and a tendency to become the starting point of fatigue failure. For example, sufficient fatigue properties cannot be ensured for golf club faces, some automobile parts, etc., and impact toughness is reduced. Furthermore, Si has an action of increasing the hot deformation resistance. If Si exceeds 0.40%, the hot deformation resistance is rapidly increased and the hot workability is lowered. Therefore, the amount of Si needs to be 0.40% or less. The effect of Si on impact toughness is exacerbated when the added amount exceeds 0.40%, and when it is less than 0.2%, no effect is observed. When the Si addition amount is in the range of 0.2 to 0.40%, the impact toughness can be improved as the addition amount increases.

  Considering golf club face applications, when manufacturing thin plate products with low room for reducing the coefficient of restitution due to the face shape as a material for the face, the width of the plate can be increased by developing a transverse-texture. The tensile strength and Young's modulus in the direction are increased, which is preferable as a face material. At this time, as shown in FIG. 1 (a), the normal direction of the hot rolled sheet is the ND direction, the hot rolled direction is the RD direction, the width direction of the hot rolled sheet is the TD direction, ) The normal direction of the surface is defined as the c-axis direction, the angle between the c-axis direction and the ND direction is defined as θ, and the angle between the surface including the c-axis direction and the ND direction as the surface including the ND direction and the TD direction is defined as φ. Next, as shown in FIG. 1B, the X-ray (0002) of the crystal grains having an angle θ of 0 degrees or more and 30 degrees or less and φ of the entire circumference (−180 degrees to 180 degrees). Among the reflected relative intensities, the strongest intensity is XND, and as shown in FIG. 1C, the angle θ is 80 degrees or more and less than 100 degrees, and the X-rays ( [0002] Of the reflected relative intensities, the strongest intensity is XTD. When XTD / XND is 4.0 or more, the tensile strength in the plate width direction satisfies 1100 MPa and the Young's modulus satisfies 135 GPa, so that the characteristics required for a high-end golf club face can be cleared. Therefore, the XTD / XND range described above was defined as 4.0 or more.

  In the manufacturing process of a thin plate product, the addition amount of Al, Fe, O, N, and Si is limited to the above component range, and the unidirectional hot rolling is performed by heating to the β single phase region or the α + β2 phase region immediately below the β transformation point. The term “below the β transformation point” means that the temperature is 50 ° C. lower than the β transformation point. Thereby, the range of XTD / XND can be 4.0 or more. This is because Transverse-texture is easy to develop. Thus, the tensile strength in the plate width direction required for high grade golf club faces of 1100 MPa or more and Young's modulus of 135 GPa or more can be obtained, and furthermore, it has high fatigue characteristics and is optimal as a face material. In addition to being able to manufacture products, it has excellent impact toughness.

  Here, the high fatigue property is defined by the fact that the fatigue strength when the three-point bending fatigue test is repeated 100,000 times is 800 MPa or more.

High impact toughness was defined as Charpy absorbed energy of 25 J / cm ² .

  The reason for rolling in one direction consistently from the start to the end of hot rolling when manufacturing this thin plate material is that the high Young's modulus in the plate width direction accompanying the material anisotropy is the purpose of the present invention. This is to obtain the transverse-texture efficiently. Thus, when the titanium alloy thin plate having a high Young's modulus and strength is used as a golf club face material, the plate width direction is arranged in the longitudinal direction of the face or a direction close thereto, thereby complying with the coefficient of restitution coefficient, and It is possible to produce a face having high fatigue characteristics, that is, high fatigue characteristics, and excellent impact toughness. The hot rolling temperature is a two-phase region in the vicinity of the β transformation point, so that the generation of nuclei that form the transverse-texture is promoted, and when the β transformation point is exceeded, the energy cost for heating is increased. Therefore, the hot rolling heating temperature is set to a temperature immediately below the β transformation point. Preferably, it is in the range of a temperature of β transformation point −65 ° C. or more and β transformation point or less. Further, the hot rolling rate is preferably 90% or more in order for Transverse-texture to grow.

  In addition, the titanium alloy slab having the above composition has no hot rolling during hot rolling and is excellent in hot workability. Here, a hot-rolled sheet without hot-rolling is used as a hot-rolled sheet with good hot workability. The hot rolled steel is a steel sheet having a depth of 0.3 mm or more, and is determined by performing a visual inspection over the front surface of the hot rolled sheet and measuring the depth of a material that may be hot rolled. .

<Example 1>
A titanium material having the chemical composition shown in Table 1 was melted by a vacuum arc melting method, and this was hot forged to obtain a slab having a thickness of 180 mm. This slab was heated to 1060 ° C., and those other than test numbers 1 and 22 were hot-rolled in one direction to produce a hot-rolled sheet having a thickness of 4 mm. In Test Nos. 1 and 22, a slab was heated to 1060 ° C., and a hot rolled sheet having a thickness of 4 mm was manufactured by cross rolling including hot rolling in the sheet width direction. This was shot blasted and then pickled to remove oxide scale.

  When the oxide scale was removed, the surface scratch depth was measured with a depth gauge to evaluate hot workability (O: maximum scratch depth ≦ 0.3 mm, x: maximum scratch depth ≧ 0.3 mm). The results and the results of examining the tensile properties are also shown in Table 1.

  Further, the texture in the plate surface direction of the hot-rolled pickled plate was measured by X-ray diffraction, and in the (0001) plane pole figure of the α phase from the ND direction from the hot rolled surface, the hatched portion (region) in FIG. As shown in B), of the α phase (0002) reflection relative intensity of the X-rays by the crystal grains (region shown in FIG. 1B) where the angle θ between the c-axis orientation and the ND direction is less than 30 degrees. The strongest strength is XND, and the angle θ between the c-axis azimuth and the ND direction is not less than 80 degrees and not more than 100 degrees as shown in the hatched area (region C) in FIG. Among the X-ray α-phase (0002) reflection relative intensities of crystal grains in the range (region shown in FIG. 1C), the strongest intensity is XTD, and the ratio XTD / XND is X-ray anisotropy. The degree of texture development was evaluated as an index.

The table shows the 100,000 times fatigue strength when a three-point bending fatigue test is performed at room temperature. For the test piece for fatigue property evaluation, the surface is processed by processing t2.0 (mm) × w15 (mm) × L60 (mm) with the plate width direction as the longitudinal direction from near the center of the thickness of the hot-rolled plate. A smooth finish was used. By pushing a jig (punch) with a curvature of R = 2mm at the tip into the longitudinal center of the test piece in the manner of three-point bending, a repeated load of 6Hz is given at a stress ratio of 0.1, and a fatigue test is performed. went. That is, a repeated three-point bending fatigue test. The distance between the load point and each fulcrum on both sides was 20 mm. That is, the distance between the fulcrums on both sides is 40 mm, and a punch for applying a bending stress load is located at the center thereof. Here, the stress ratio is defined as the ratio of the minimum load stress to the maximum load stress applied to the test piece. The stress applied to the test piece is obtained by measuring the indentation load of the punch and substituting each dimension of the test piece into the bending equation of material mechanics. The strain generated by bending may be obtained from the above formula of material mechanics, or may be obtained by measuring a strain generated in the longitudinal direction by attaching a strain gauge to the sample. The pushing amount of the punch corresponding to the maximum stress and the minimum stress determines the upper limit and the lower limit of the punch stroke. The load is repeatedly applied by the punch repeatedly reciprocating between the upper limit and the lower limit. Performing a fatigue test at a stress ratio of 0.1 means that the ratio of maximum stress to minimum stress is 0.1. For example, when the maximum stress is 800 MPa, the indentation load is adjusted so that the minimum stress is 80 MPa, and the stress is repeatedly applied. In the present invention, the 100,000 times fatigue strength (10 ⁵ times fatigue strength) is defined as the maximum load stress that does not break by applying 10 ⁵ times repeated load, and it is characterized by maintaining 800 MPa or more. This indicates that it has very high fatigue properties and that it has the high durability required for high grade golf club faces. On the other hand, when a repeated load is applied at a maximum load stress of 800 MPa or less, if the fracture occurs at a repeat count of 10 ⁵ times or less, the intended fatigue characteristics of the present invention are not satisfied. For samples that did not break even after repeated application of 10 ⁵ times or more, the maximum load stress was increased for different specimens of the same material, repeated load application was performed, and when the sample did not break again even 10 ⁵ times, Under the condition that the maximum load stress was increased, a repeated load test was performed on a new specimen, and this procedure was repeated until a fracture was observed, and a fatigue test was performed.

Further, when comparing test number 18 in Table 1 which is a comparative example not containing Si and test number 20 in Table 1 which is an example containing Si, the comparative example is inferior in 10 ⁵ times fatigue strength, It can be seen that the combined effect of Si, oxygen, and nitrogen, which is one of the features of the present invention, is shown.

  Furthermore, the Charpy impact test piece (subsize, t2.5 (mm) x w10 (mm) x L55 (mm)) defined in JIS Z2242 is processed from the longitudinal direction of the hot-rolled sheet, and the Charpy impact test is performed. And impact toughness was evaluated. The impact test piece was processed with a V notch having a depth of 2 mm in a direction corresponding to the width direction of the original hot-rolled sheet. The Charpy impact test was performed at 22 ° C., and the value obtained by dividing the absorbed energy obtained from the height of the hammer by the cross-sectional area of the test piece was evaluated as the Charpy impact absorbed energy.

In Table 1, test number 1 is the result of cross-rolling a Ti-6% Al-4% V alloy including hot rolling in the sheet width direction. Test number 2 is Ti-7% Al-1% Fe. This is the result when unidirectional hot rolling is performed. XTD / XND of test number 1 is less than 3.0, and the tensile strength in the plate width direction does not reach 1100 MPa. In Test No. 2, XTD / XND exceeds 3.0, the tensile strength (TS) in the plate width direction is 1100 MPa or more, and the Young's modulus is 135 GPa or more, but the depth is 0.5 mm or more. In addition to poor hot workability, the Charpy impact absorption energy is below 25 J / cm ² , and the impact toughness is low. This reduction in impact toughness is due to the high Al content. Test numbers 18 and 19 are addition amounts lower than the Si amount defined in the present invention, satisfy Young's modulus of 135 GPa, tensile strength of 1100 MPa, and good hot rollability, but 10 ⁵ times fatigue strength is 800 MPa. The fatigue characteristics are not sufficient. Also, impact toughness is low.

On the other hand, test numbers 4, 5, 8, 9, 12, 13, 15, 16, 20, 23, and 24, which are examples of the present invention, have a high tensile strength (EL) of 1100 MPa or more in the plate width direction. And a high 10 ⁵ times fatigue strength exceeding 800 MPa. From these characteristics, for example, it has excellent characteristics when used as a golf club face. Furthermore, test numbers 4, 5, 12, 13, 15, 16, 20, 23, and 24 having a Si content of 0.2% or more also have high Charpy impact absorption energy exceeding 25 J / cm ² . In particular, in Test Nos. 4, 5, 12, 13, 20, 23, and 24 with a high Si addition amount, Charpy impact absorption energy exceeds 30 J / mm ² and has extremely good impact toughness.

On the other hand, in test numbers 3, 7, 7A, and 11, the tensile strength in the plate width direction is 1100 MPa or less, and the strength is not sufficient for use in the face. Since the amounts of Al, Fe, and [O] _eq were below the lower limit of the present invention in the order of test numbers 3, 7, 7A, and 11, respectively, the solid solution strengthening ability was not sufficient, and the tensile strength was low. It is.

In Test No. 14, the fatigue strength is 10 ⁵ times lower than that of the example of the present invention, and sufficient fatigue characteristics cannot be imparted. Charpy impact absorption energy is also low. In Test No. 14, because [O] _eq exceeded the upper limit, a region of high hardness was locally generated due to solidification segregation of O, and fatigue strength and impact toughness were reduced. In test number 17, N was added in excess of the upper limit of the present invention, and the occurrence of LDI was confirmed, so the test was stopped.

In Test Nos. 6, 17, and 21, surface defects having a depth exceeding 0.5 mm occurred frequently after hot rolling. In Test Nos. 6 and 21, Al and Si that reduce hot workability were added in excess of the upper limit of the present invention, and hot-rolled scratches occurred. This is because in Test No. 17, LDI was generated due to excessive N content, and an object near the surface was recognized as a defect. In Test No. 21, coarse silicide was precipitated due to excessive Si content, and voids were generated and connected between the silicide and the matrix during hot working, resulting in surface defects. In Test No. 6, the Charpy impact absorption energy is also lower than 25 J / cm ² and the impact toughness is low. This is because the amount of Al added is high and the strength is too high. Furthermore, in test number 21, the 10 ⁵ times fatigue strength was less than 800 MPa. Charpy impact absorption energy is also lower than 25 J / cm ² and impact toughness is low. This is because coarse silicide is used as a starting point, and these characteristics are deteriorated.

  In test numbers 10 and 10A, the amount of Fe was too high and the Young's modulus was below 135 GPa. Moreover, since the strength was high, the impact toughness was also reduced.

  In test number 22, as a result of cross rolling including hot rolling in the sheet width direction, XTD / XND was less than 3.0, tensile strength of 1100 MPa, Young's modulus of 135 GPa was not obtained, and fatigue strength was also high. It is low. This is because Trans-texture did not develop by cross rolling.

In addition, Si is added at 0.15% or more and less than 0.20%, other alloy elements are added within the content range of the present invention, and the test has XTD / XND defined in the present invention. Numbers 8, 9, 8A, 9A and 24A showed high 10 ⁵ fatigue strength, but Charpy impact absorption energy was slightly lower than 25 J / cm ² . This is because the amount of Si added is sufficient to increase the fatigue strength but is insufficient to increase the impact toughness.

  From the above results, the titanium alloy hot rolled sheet having the element content and XTD / XND defined in the present invention has high tensile strength and Young's modulus in the sheet width direction, and is an excellent material as a material for high-end golf club faces. It has properties and good hot workability. On the other hand, if the amount of the alloying element specified in the present invention is deviated, the hot workability is deteriorated, and the materials necessary for the golf club face such as tensile strength in the sheet width direction, Young's modulus, fatigue strength and / or impact toughness. The property cannot be satisfied.

Moreover, the present invention material was compared with a commonly used Ti—Al—V conventional material. What changed the amount of added oxygen with Ti-6% Al-4% V as the base composition is a titanium alloy used for general purposes, and its strength (tensile strength) is adjusted by the amount of added oxygen. be able to. Therefore, an alloy having the same strength as that of the present invention alloy is manufactured by adding oxygen to Ti-6% Al-4% V having a strength of about 1000 MPa and adjusting the strength to about 1100 to 1200 MPa. The fatigue properties of the present invention were compared with those of the present invention. The Ti-6% Al-4% V conventional material often generates cracks during hot rolling, and all the samples were lower and inferior to the alloy according to the present invention in 10 ⁵ times fatigue strength.

<Example 2>
Titanium materials having chemical compositions shown in Test Nos. 5 and 9 in Table 1 were melted by a vacuum arc melting method and hot forged to obtain a slab having a thickness of 180 mm. This slab was hot-rolled in one direction under the conditions shown in Tables 2 and 3 to produce a hot-rolled sheet having a thickness of 4 mm. This was shot blasted and then pickled to remove oxide scale.

  When the oxide scale was removed, the surface scratch depth was measured with a depth gauge to evaluate the hot workability (◯: maximum scratch depth ≦ 0.3 mm, ×: maximum scratch depth> 0.3 mm). The results and the results of examining the tensile properties are also shown in Tables 2 and 3.

  Further, the texture in the plate surface direction of the hot-rolled pickled plate was measured by X-ray diffraction, and in the (0001) plane pole figure of the α phase from the ND direction from the hot rolled surface, the hatched portion (region) in FIG. B) As shown in FIG. 2, the strongest intensity is XND among the α-phase (0002) reflection relative intensities of X-rays by crystal grains whose angle θ between the c-axis orientation and the ND direction is less than 30 degrees. As shown in the hatched portion (region C), the angle θ formed between the c-axis azimuth and the ND direction is 80 degrees or more and 100 degrees or less, and the φ is in the range of ± 10 degrees. Of the α-phase (0002) reflection relative intensities, the strongest intensity was XTD, and the ratio XTD / XND was taken as the X-ray anisotropy index to evaluate the degree of texture development.

The table shows the 10 ⁵ times fatigue strength when a three-point bending fatigue test is performed at room temperature. The surface of the test piece was smoothed by processing t2.0 (mm) × w15 (mm) × L60 (mm) with the plate width direction as the longitudinal direction from the vicinity of the center of the thickness of the hot-rolled plate. I used something. A fatigue test was performed by applying a 6 Hz repetitive load at a stress ratio of 0.1 by pushing a jig having a curvature of R = 2 mm at the tip into the center in the longitudinal direction of the test piece. The distance between the load point and the fulcrum on both sides was 20 mm. The 10 ⁵ times fatigue strength is 800 MPa or more, the fatigue strength is sufficiently high, and it can be said that the fatigue strength is excellent.

  Tables 2 and 3 show the results when the plate products having the chemical compositions shown in Test Nos. 5 and 9 in Table 1 are hot-rolled in one direction, respectively. Among these, the plates manufactured under the conditions of test numbers 26, 27, 28, 29, 31, 32, 33, 34 all have a heating temperature before hot rolling in the β single phase region (above the β transformation temperature), or Because of the α + β2 phase temperature range immediately below the β transformation point (up to a temperature lower by 50 ° C. than the β transformation point), a transverse-texture develops, the tensile strength in the plate width direction (1100 MPa or more) and the Young's modulus (135 GPa or more) ) Is sufficiently satisfied and has high fatigue strength. When these plate materials are used as golf club faces, they have characteristics that conform to the coefficient of restitution coefficient and excellent fatigue characteristics. Moreover, the surface defect of the depth exceeding 0.3 mm does not generate | occur | produce in these hot-rolled pickling plates, and favorable hot ductility is shown. Accordingly, these thin plate materials are suitable as golf club face materials.

  On the other hand, in the hot rolled sheets shown in test numbers 25 and 30, XTD / XND is 3.0 or less, and shows a tensile strength of 1100 MPa or less and a Young's modulus of 135 GPa or less in the sheet width direction. It is not suitable as a golf club face material. In Test Nos. 25 and 30, since the heating temperature before hot rolling was a relatively low temperature in the α + β2 phase region, the β single phase region (above the β transformation point temperature) or just below the β transformation point (β transformation) This is because the development of the transverse-texture is small and the material anisotropy is not increased as compared with the case of heating to the α + β2 phase temperature (up to 50 ° C. below the point).

  From the above results, in order to have high Young's modulus, tensile strength, and excellent fatigue properties and / or impact toughness in the sheet width direction, a titanium alloy having an additive element in the component range shown in the present invention is subjected to β transformation. It can be manufactured by heating in the temperature range above or below the point and unidirectionally hot rolling. This titanium alloy can be used for a wide range of applications requiring high specific strength and fatigue characteristics, but has excellent characteristics particularly for golf club faces and automobile parts.

  In addition, by using the slab used for the hot rolled sheet of the test number 12, several hot rolled sheets having a hot rolling rate of less than 90% were manufactured, all of which are intended strength, Young's modulus, It was not possible to obtain a transversal-texture that has developed enough to obtain fatigue properties and impact toughness. However, here, the rolling rate (%) was defined as “100 × (plate thickness before rolling−plate thickness after rolling) / plate thickness before rolling”.

  The titanium alloy of the present invention has a Young's modulus of 135 GPa or more and a tensile strength of 1100 MPa or more in one direction within the plate surface of the thin plate product, and also has excellent fatigue properties and / or impact toughness. It also has good hot workability. This alloy has excellent fatigue characteristics and satisfies the coefficient of restitution coefficient, and can provide a material suitable for applications such as high grade golf club faces and automobile parts.

Claims (3)

In mass%, 4.7 to 5.5% Al, 0.5 to 1.3% Fe, 0.03% or less N, and [O] _eq calculated from Equation (1) is 0 O, N satisfying 13% or more and less than 0.25%, further containing 0.15 to 0.40% Si, the balance being Ti and unavoidable impurities, and the direction of the normal to the rolling surface of the hot-rolled sheet is ND Direction, hot rolling direction is RD direction, plate width direction of hot-rolled sheet is TD direction, normal direction of α phase (0001) plane is c-axis direction, and angle between c-axis direction and ND direction is θ , The angle formed by the plane including the c-axis azimuth and the ND direction and the plane including the ND direction and the TD direction is φ, the angle θ is 0 ° or more and 30 ° or less, and φ is the entire circumference (−180 ° to 180 °). Of the X-ray (0002) reflection relative intensities of crystal grains entering (degrees), the strongest intensity is XND, and the angle θ is 80 degrees or more and less than 100 degrees Yes, among X-ray (0002) reflection relative intensities of crystal grains with φ of ± 10 degrees, when XTD is the strongest intensity, XTD / XND is 4.0 or more, and Young's modulus in the plate width direction Is an α + β-type titanium alloy hot-rolled sheet excellent in hot workability, characterized by having a tensile strength in the sheet width direction of 1100 MPa or more and excellent fatigue properties.
Here, the plate width direction is a direction perpendicular to the hot rolling direction within the plate surface.
[O] _eq = [O] + 2.77 [N] ... Formula (1)
Here, [O] is the oxygen concentration (mass%), and [N] is the nitrogen concentration (mass%). In mass%, 4.7 to 5.5% Al, 0.5 to 1.3% Fe, 0.03% or less N, and [O] _eq calculated from Equation (1) is 0 O, N satisfying 13% or more and less than 0.25%, further containing 0.2 to 0.40% of Si, the balance being Ti and unavoidable impurities. Direction, hot rolling direction is RD direction, plate width direction of hot-rolled sheet is TD direction, normal direction of α phase (0001) plane is c-axis direction, and angle between c-axis direction and ND direction is θ , The angle formed by the plane including the c-axis azimuth and the ND direction and the plane including the ND direction and the TD direction is φ, the angle θ is 0 ° or more and 30 ° or less, and φ is the entire circumference (−180 ° to 180 °). Among the X-ray (0002) reflected relative intensities of X-rays by crystal grains entering the degree), the strongest intensity is XND, and the angle θ is 80 degrees or more and less than 100 degrees Among the X-ray (0002) reflection relative intensities of crystal grains with φ of ± 10 degrees, when the strongest intensity is XTD, XTD / XND is 4.0 or more, and the Young's modulus in the plate width direction An α + β-type titanium alloy hot-rolled sheet excellent in hot workability, characterized by having a tensile strength in the sheet width direction of 1100 MPa or more and excellent fatigue properties and impact toughness.
Here, the plate width direction is a direction perpendicular to the hot rolling direction within the plate surface.
[O] _eq = [O] + 2.77 [N] ... Formula (1)
Here, [O] is the oxygen concentration (mass%), and [N] is the nitrogen concentration (mass%). The titanium alloy slab having the composition according to claim 1 or 2 is heated in the β single phase region or the α + β2 phase region from the β transformation point to a temperature lower by 50 ° C. than the β transformation point , and hot-rolled in one direction. The method for producing an α + β type titanium alloy hot-rolled sheet according to claim 1 or 2 , wherein hot rolling is performed at a rate of 90% or more .

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