JP7151116B2 - α＋β type titanium alloy extruded shape - Google Patents

Description

The present invention relates to an α+β type titanium alloy extruded profile.

Due to its high specific strength and excellent corrosion resistance, titanium alloys have been used in various fields such as aircraft aggregates and structural members, consumer products such as golf face club heads and eyeglass frames, and medical applications such as implants. ing.

Among them, the α+β type titanium alloy has a good balance of strength and ductility and is excellent in fracture toughness, so it has been widely used mainly in the aerospace industry. In particular, α+β type titanium alloys are expected to further improve in quality as shape materials such as aggregates and seat rails in such aircraft applications.

Such α+β type titanium alloys, including Ti-6Al-4V, which is the most used, have been used for many years mainly in the aircraft field. Recently, with the increasing use of carbon fiber reinforced composite materials (CFRP) in airframes to further improve fuel efficiency, the use of titanium alloys is also on the rise, and is expected to rise further in the future. expected. This is because the aluminum alloys conventionally used in the aircraft field cause dissimilar metal galvanic corrosion when they come into contact with CFRP. ° C), there is a problem that misalignment, loosening, etc. are likely to occur. This is because it is close to

α+β type titanium alloys are sometimes used as aggregates, seat rails, and other shapes in such aircraft applications. Some shaped materials have complicated cross-sectional shapes, and conventionally, they have been manufactured by cutting large-section forged products or extra-thick materials. When cutting the α+β type titanium alloy after forging, the metal structure is made into an equiaxed structure with a high balance of strength and ductility by performing strong working at a temperature below the β transformation point temperature, and the required tensile properties, especially It had high endurance.

However, recently, as the need to reduce the manufacturing cost of aircraft parts has increased, it is expected that yield and productivity will be improved by manufacturing long shapes with a cross-sectional shape close to the final product. Manufacturing technology for shaped materials has been developed.

Extrusion includes methods such as an indirect extrusion method and a hydrostatic extrusion method, and the Eugene Sejournet method is one of them. In this method, a round billet manufactured by forging an ingot is used as a raw material. As shown in FIG. 1, a material (billet 5) is inserted into a container 1, a hydraulic load is applied to a stem 2, the billet 5 is pushed in an extrusion direction 11 through a dummy block 3, and passed through a die 4 to produce various By molding into a cross-sectional shape, it is possible to obtain a long shape member 6 .

By the way, as mentioned above, the metal structure of the α+β type titanium alloy is subjected to strong working such as forging below the β transformation point temperature (α+β temperature range) for applications that require a high balance of strength and ductility. By controlling the structure to an equiaxed structure, the required high tensile strength has been achieved. On the other hand, when the metal structure is controlled to an equiaxed structure by extrusion molding, the α + β type titanium alloy has a significantly high hot deformation resistance in a temperature range 200 ° C or more below the β transformation point temperature (T _β ), so a high extrusion load A large extrusion press that can add is required, which increases the equipment cost and may not be able to be extruded. Furthermore, even if it is possible to extrude, if the temperature of a part of the cross section of the shape exceeds the β transformation point temperature due to the heat generated during extrusion, the equiaxed structure and the β transformation point temperature will appear in the cross section of the shape. The needle-like structure obtained by the above processing is mixed, and a significant difference in mechanical properties occurs within the cross section. Therefore, in general, in the extrusion of α+β type titanium alloy, the billet is heated to a temperature higher than the β transformation point and extruded so that it can be manufactured with a low extrusion load and surface defects are unlikely to occur. controlled to

However, when the billet is heated to a temperature equal to or higher than the β transformation temperature and extruded, there is a problem that the profile after extrusion has a needle-like structure, and the balance of strength and ductility is inferior to that of the equiaxed structure. Furthermore, if the heating temperature of the billet is higher than the β-transformation temperature, the length of time that the billet is held above the β-transformation temperature after extrusion increases the β-grain growth, resulting in poor strength/ductility balance and fatigue strength. There's a problem. Insufficient strength due to an increase in inertia weight due to an increase in engine speed is a serious problem for use in parts closer to the engine.

On the other hand, if the billet heating temperature is in the vicinity of the β transformation point temperature or too lower than the β transformation point temperature, heat removal due to contact with extrusion tools such as containers and dies will also affect the processing temperature of the surface layer below the β transformation point temperature. , the equiaxed texture is mixed in the surface layer. Furthermore, the surface layer is less ductile due to the lower temperature, and defects such as cracks and flaws may occur during extrusion.

Thus, it is difficult to control the extrusion temperature of the α+β type titanium alloy extruded profile having a needle-like structure obtained by extrusion. There are problems such as surface defects and a high extrusion load that makes extrusion impossible. In order to solve these problems, the following prior arts have been disclosed.

In Patent Document 1, a Ti-6Al-4V alloy, which is an α + β type titanium alloy, is heated to an α + β temperature range and extruded to achieve high strength, high toughness, and a shape with small dimensional fluctuation in the longitudinal direction and few surface defects. A method of manufacturing a material is described.

In Patent Document 2, an α + β type titanium alloy is heated to an α + β temperature range or a β single phase temperature range and extruded, then heated to an α + β temperature range and then forcedly cooled to perform solution treatment. It describes a method of producing a shape having excellent strength and ductility by performing two-step heat treatment for aging treatment.

In Patent Document 3, an α + β type titanium alloy billet exhibiting a fine equiaxed α + β structure is extruded at a temperature equal to or higher than the β transformation point, quenched at 5 ° C./sec or higher, and then annealed to perform extrusion in the α + β region. A method is described to produce an extruded profile with comparable strength and ductility to the profile produced.

Patent Document 4 proposes a method of heating an α+β type titanium alloy billet to a temperature equal to or higher than the β transformation temperature, cooling the surface layer to the α+β region, and extruding the billet. In this method, since the inside of the billet is heated to the β transformation point temperature or higher during extrusion, the hot deformation resistance is small, and extrusion processing is possible with a small extrusion force. It is said to have high strength because it has an equiaxed α+β structure.

In Patent Document 5, an α + β type titanium alloy billet is heated to a temperature range in the α + β region calculated by a linear expression including an extrusion ratio and extruded, so that the subsequent heat treatment is performed by the heat generated during extrusion. An optional manufacturing method is disclosed.

In Patent Document 6, an α + β type titanium alloy billet is extruded at a temperature in the α + β region calculated by a linear expression including an extrusion ratio to control the structure, and a shape excellent in strength and elongation is manufactured. method is described.

On the other hand, as alloys whose mechanical properties are improved or adjusted by studying the component composition that does not contain V, Patent Documents 7 to 9 disclose 4.4% or more and less than 5.5% Al, 1.4% or more Contains less than 2.3% Fe, 1.5% or more and less than 5.5% Mo, and as impurities, Si is less than 0.1%, C is less than 0.01%, and the balance is titanium and unavoidable An α+β type titanium alloy comprising impurities is disclosed.

In Patent Document 7, in order to adjust the Young's modulus of an α+β type titanium alloy having a tensile strength of 1000 MPa class or higher to a predetermined value, heat treatment is performed by combining heating in a predetermined temperature range and a predetermined cooling rate corresponding thereto. It is stated that it will

JP-A-61-193719 JP-A-61-284560 JP-A-63-223155 Japanese Patent Publication No. 5-2405 Japanese Patent No. 2932918 JP 2012-52219 A JP 2007-314834 A

The α + β titanium alloy extruded profiles according to Patent Documents 1 to 6 listed in the above prior art are all subjected to forced cooling after extrusion to control the structure, or controlled to a structure other than an acicular structure to improve strength and ductility. are making improvements.

Shapes that have undergone structure control by forced cooling have a high balance of strength and ductility. This is because the growth of the side plate α-phase and the grain boundary α-phase in the acicular structure is suppressed as the cooling rate increases. However, when forced cooling is applied to a long material or a shape with a large cross-sectional area, there is a problem that the cooling rate varies over the entire length and inside and outside the shape, and there are parts where the desired structure and material properties cannot be obtained. Furthermore, during the cooling process, thermal contraction causes stress inside the profile. Therefore, if the cooling rate difference is significant and the stress is large, shape defects such as warpage may occur in the shape due to plastic deformation, or residual stress may remain even after cooling, which is not preferable.

In the method of using a profile structure other than the needle-like structure, it is necessary to control part and all of the billet in the α+β temperature range. However, the α+β type titanium alloy has a high hot deformation resistance when the working temperature drops below the β transformation temperature, and requires a large pressing force. In addition, since the amount of heat generated during processing is large in the α+β temperature range, the processing temperature may exceed the β transformation temperature due to the heat generated during extrusion. As a result, there is a problem that a profile with a uniform structure cannot be obtained and the mechanical properties are not uniform. Furthermore, in the method of heating with a temperature gradient within the cross section of the billet, there is a problem that a stable shape cannot be obtained because the degree of deformation varies due to a slight difference in temperature within the cross section.

Therefore, the present invention uses a Ti-Al-Sn-Zr-Mo system α+β type titanium alloy, which has a relatively high strength compared to Ti-Al-V, as a material and has an acicular structure, but has the same strength and strength as the prior art. It is an object to provide an α+β titanium alloy extruded profile with ductility balance.

In order to solve such problems, the gist of the present invention is as follows.
[1]
The component composition is mass%, Al: 5.5 to 6.5%, Sn: 1.8 to 2.2%, Zr: 3.6 to 4.4%, Mo: 1.8 to 2.2 %, O: 0.20% or less (including 0%), C: 0.08% or less (including 0%), N: 0.05% or less (at 0% The balance is Ti and unavoidable impurities, the metal structure consists of a needle-like structure, the average prior β grain size is 300 μm or less, and the average maximum width of the grain boundary α phase is 5 μm or less. There is an average particle size d ₁ (m) of prior β grains in a certain cross section perpendicular to the extrusion direction of the extruded shape, and a distance L (m) away from the one cross section in the extrusion direction parallel to the one cross section An α+β titanium alloy extruded profile characterized in that the value of the following formula (1) calculated from the average grain size d ₂ (m) of prior β grains in another section of the extruded profile is 25 or less. .
However, the distance L is 0.3 m or more.
|(d ₁ −d ₂ )/L|×10 ⁶ (1)
[2]
The α+β titanium alloy extruded shape according to [1], wherein the average prior β grain size is 200 μm or less .

According to the present invention, the 0.2% proof stress is 860 MPa or more because the metal structure is an acicular structure and the average prior β grain size is 300 μm or less for the α+β type titanium alloy profile having a specific composition. , an extruded profile with an elongation of 10% or more.

The α+β type titanium alloy extruded profile of the present invention has excellent strength and elongation, and can be mass-produced, so it can be used to manufacture structural members for aircraft, automobiles, motorcycles, etc. at low cost. become able to. INDUSTRIAL APPLICABILITY According to the present invention, it is possible to expand industrial applications and obtain effects such as improvement in fuel consumption of aircraft and automobiles due to its light weight and high strength characteristics.

It is a schematic diagram of an extrusion press in the Eugene Sejournet method. 1 is a micrograph showing a needle-like structure of an α+β-type titanium alloy extruded shape. 4(a) to (d) are graphs showing thermal histories illustrating the method for producing the α+β type titanium alloy extruded shape of the present invention. It is a schematic diagram of a cross-sectional shape of an extruded profile manufactured in an example. FIG. 4 is an explanatory diagram of "warpage" of extruded profiles measured in Examples.

The α + β type titanium alloy targeted by the present invention consists of an α phase having an HCP structure and a β phase having a BCC structure at a temperature below the β transformation point, and the α phase transforms to a β phase at a temperature above the β transformation point. and consists only of the β phase. The acicular structure is a morphology that occurs after processing at a temperature equal to or higher than the β transformation temperature, and the morphology of the structure is shown in FIG. Grain-boundary α-phases are generated at the boundaries of the former β-grains, which were one grain at a temperature equal to or higher than the β-transformation temperature. That is, grain boundary α phases are formed at the grain boundaries of β grains (old β grains) that exist at a temperature equal to or higher than the β transformation point temperature during cooling. In the present invention, the region surrounded by the grain boundary α-phase, which is the grain boundary of the β-grains (prior β-grains) that existed at a temperature equal to or higher than the β-transformation temperature, is referred to as "prior β-grains". Within the prior β grains, a structure in which α phases and β phases are arranged in layers, called multiple colonies, is formed. Hereinafter, the α-phase in the colony is called the side-plate α-phase, and the β-phase is called the side-plate β-phase.

Also, metal generally shrinks by heat when cooled, and its volume decreases. If the cooling rate differs depending on the part, stress is generated inside the shape because the amount of thermal contraction varies during a certain period of time during cooling. Furthermore, the extruded profile does not have a uniform temperature after extrusion. In this way, when the temperature before standing to cool differs depending on the part, the stress increases because the total amount of shrinkage during cooling differs depending on the part. When the shape member is cooled at a slow rate such as air cooling or furnace cooling, such stresses only give elastic deformation to the shape member. However, in the prior art, high strength and ductility are obtained by controlling the structure and composition by forced cooling immediately after extrusion or solution treatment (forced cooling from a high temperature range) in heat treatment after extrusion. In these forced coolings, significant stress is generated due to the large difference in cooling rate, and plastic deformation such as warpage is given to the shape. Also, even if no shape defect occurs during cooling, residual stress is generated inside the shape, causing shape defects such as warpage during processing, cutting, or the like of the shape.

Therefore, the present inventors performed hot extrusion with various heating conditions for an α + β type titanium alloy having a specific composition, and investigated the relationship between the tensile properties of the shape and the needle-like structure. As a result, the shape Even without forced cooling that causes defects, the metal structure consists of a needle-like structure and the average of the prior β grains is 300 μm or less, so that the zero is equal to or higher than that of Ti-6Al-4V having an equiaxed structure. It was found that an α+β type titanium alloy extruded profile having a 2% yield strength and practically no problem in ductility can be obtained.

The significance of determining the component composition in the present invention will be described.

The present invention is directed to titanium alloys containing Al, Sn, Zr and Mo as main elements, namely Ti--6Al--2Sn--4Zr--2Mo. The reason for limiting each component will be described below.

Al: 5.5 to 6.5% by mass
Al is an α-stabilizing element and is an element added to increase the α-phase fraction. If the content is less than 5.5% by mass, the fraction of the α phase, which has higher strength than the β phase, is too small, and sufficient strength equivalent to general Ti-6.0Al-4.0V can be obtained. 0.2% proof stress cannot be obtained. On the other hand, when the content exceeds 6.5% by mass and becomes excessive, the stacking fault energy increases and the hot and room temperature ductility deteriorates due to the suppression of twinning deformation, and Ti ₃ Al precipitates. The toughness is also deteriorated, and the workability is lowered. Furthermore, when the content exceeds 6.5% by mass, smooth local slip is induced, so cracks are likely to occur at the local slip, and hot workability deteriorates. Therefore, the Al content has a lower limit of 5.5% by mass and an upper limit of 6.5% by mass.

Sn: 1.8 to 2.2% by mass
Sn is an element effective for solid-solution strengthening, and its addition has the effect of improving the strength and 0.2% proof stress, so 1.8% by mass or more of Sn is added. On the other hand, if the content exceeds 2.2% by mass, it is not preferable because the material deteriorates and the elongation decreases when exposed to high temperatures. Therefore, the Sn content has a lower limit of 1.8% by mass and an upper limit of 2.2% by mass.

Zr: 3.6-4.4% by mass
Zr is also an element effective for solid-solution strengthening, and its addition has the effect of improving strength and 0.2% proof stress, so 3.6% by mass or more of Zr is added. On the other hand, if the content exceeds 4.4% by mass, it is not preferable because the material deteriorates and the elongation decreases when exposed to high temperature. Therefore, the lower limit of the Zr content is 3.6% by mass, and the upper limit is 4.4% by mass.

Mo: 1.8 to 2.2% by mass
Mo is a β-stabilizing element and can lower the β-transformation temperature of the titanium alloy. Further, by adding 1.8% by mass or more, 0.2% proof stress, ductility and fatigue strength are improved, and hot workability is improved. On the other hand, if the amount added exceeds 2.2% by mass, the problem of solidification segregation also arises. Therefore, the upper limit of the Mo content is set to 2.2% by mass as the amount of Mo to be added so as not to cause significant solidification segregation in large ingots.

O: 0.20 mass% or less (including 0%), C: 0.08 mass% or less (including 0%), N: 0.05 mass% or less (0% O, C, and N are inevitably contained in α + β type titanium alloys, but they are α-stabilizing elements. It has the effect of improving endurance. However, when the content of each element increases, the ductility decreases and the workability decreases. Therefore, O: more than 0% by mass and 0.20% by mass or less, C: more than 0% by mass and 0.08% by mass or less, N: more than 0% by mass and 0.05% by mass or less. Of course, O, C, and N may be 0% by mass (below the detection limit).

Balance: Ti and Unavoidable Impurities The balance is Ti and unavoidable impurities. Examples of unavoidable impurity elements include impurities such as Fe, Cl, Na, and Mg that are mixed in during the refining process of titanium, and Cu, Nb, and Ta that are mixed from scrap. When the content of any impurity other than Fe increases, it forms a compound with Ti, resulting in a decrease in toughness and, as a result, a decrease in workability. Also, if the total content of impurities is excessive, the workability deteriorates due to a decrease in ductility. As for Fe, if it is 0.25% by mass or less, the effect of the present invention is not impaired. Even if each of these elements is 0.1% or less, and the total amount is 0.4% by mass or less, the effect of the present invention is not impaired.

Next, the significance of limiting the prior β grain size in the present invention will be described. In the needle-like texture, some dislocations propagate easily across the α/β phase boundary, so the deposition distance for some dislocations is given by half the colony size. Colony size also decreases with decreasing prior-β particle size. Therefore, as the prior β grain size decreases, the stress field due to the accumulation of dislocations at the colony boundaries tends to decrease, and the 0.2% yield strength tends to increase due to the microstructural refinement and strengthening. Conversely, when the prior β grain size increases, the dislocation deposition distance increases and the stress concentration occurring at the colony boundary increases, resulting in a decrease in the 0.2% proof stress. Furthermore, when the prior-β grain size becomes smaller, the colony size becomes smaller, and the number of dislocations deposited at the prior-β grain boundaries and colony boundaries decreases. Rise. Therefore, in the present invention, the upper limit is set to 300 μm, which is the average prior β grain size at which the 0.2% proof stress is 860 MPa or more and the elongation is 10%. Preferably, the average prior β grain size is 250 μm or less, more preferably 200 μm or less. On the other hand, the lower limit is not necessarily limited, but is preferably 50 μm or more. In order to make the particles finer than that, it is necessary to either lower the extrusion temperature or perform strong working during extrusion, which increases the deformation resistance and places a heavy burden on the apparatus, so the above lower limit is preferable.

In the needle-like structure, the ductility decreases as the average maximum width of the grain boundary α phase increases. The grain boundary α phase is generated at the prior β grain boundaries where dislocations are likely to accumulate during working. Therefore, voids during working are likely to occur at the grain boundary α phase interface, but when the average maximum width of the grain boundary α phase increases, the voids tend to propagate along the grain boundary α. Therefore, in the present invention, it is preferable to set the upper limit of the average maximum width of the grain boundary α to 5 μm, which is the maximum width of the grain boundary α obtained by normal standing cooling. On the other hand, although the lower limit is not necessarily limited, it is preferably 1.2 μm or more in order to suppress the occurrence of warping of the extruded shape. To make it smaller, it is necessary to use forced cooling such as water cooling or fan air cooling. Because of this, the above lower limit is preferred.

A method for measuring the average maximum width of the grain boundary α phase is described. In the cross section of the extruded profile shown in FIG. 4, five prior β-grains confirmed at microscopic observation positions are arbitrarily selected, and the maximum width of each grain boundary α-phase is measured. When selecting old β-grains, avoid selecting adjacent old β-grains. The average value of the five maximum widths is defined as the average maximum width of the grain boundary α phase.

By the way, the average particle size of the prior β grains of the extruded profile is usually not uniform in the extrusion direction of the extruded profile. The reason lies in the extrusion process. The leading end of the billet 5, which first comes into contact with the extrusion die during extrusion, is likely to lose heat due to contact with the die 4 and drop in temperature. On the other hand, the rear end of the extrusion removes heat due to contact with the dummy block 3, and has a longer contact time with the container 1 than the front end, thus removing a large amount of heat. Since the degree of heat removal is different between the leading end and the trailing end, the extrusion temperature is not uniform between the leading end and the trailing end, which appears as a difference in the average particle size of prior β grains.
The difference in the average grain size of the prior β-grains appears as it is as the difference in the strength of each part. Therefore, in order to obtain an extruded profile having uniform mechanical strength in the extrusion direction, the value calculated by the following formula (1) is preferably 25 or less. Equation (1) is obtained by dividing the difference between the average grain size d ₁ (m) of prior β grains in one cross section and the average grain size d ₂ (m) of prior β grains in another cross section by L (m). is the absolute value of the value. More preferably, the value of formula (1) is 15 or less, more preferably 10 or less.
|(d ₁ −d ₂ )/L|×10 ⁶ (1)
d ₁ : Average particle diameter (m) of prior β grains in a cross section perpendicular to the extrusion direction of the extruded shape
d ₂ : Average particle size (m) of prior β grains in another cross section of the extruded shape separated by a distance L (m) in the extrusion direction from the cross section
L: Distance in extrusion direction between one cross section and another cross section (m)
In addition, the distance L is preferably 0.3 m or more, and more preferably 1 m or more. Therefore, the length of the extruded shape is preferably 2 m or longer, more preferably 3 m or longer, longer than this L.

Next, a method for producing an α+β type titanium alloy extruded profile according to the present invention will be described with reference to FIG. In FIG. 3, (a) is a manufacturing method for obtaining an acicular structure by performing hot extrusion in a temperature range of β transformation point temperature (T _β ) or higher, and (b) is a production method of β transformation point temperature (T _β ) or higher. After obtaining an _acicular structure by performing hot extrusion in a temperature range of , (c) is a manufacturing method in which strain relief annealing is performed, A production method in which a _β -single-phase region heat treatment is performed to obtain an acicular structure. This is a manufacturing method in which phase region heat treatment is performed and strain relief annealing is performed. These are merely examples, and the α+β titanium alloy extruded profile of the present invention is not limited to those obtained by these manufacturing methods.

In the manufacturing method in which the extrusion temperature is set to the β transformation point temperature or higher and the β single-phase region heat treatment is not performed after extrusion, as shown in FIGS. When hot extrusion is carried out, it is necessary that both the surface and the center of the billet are uniformly heated to a predetermined temperature equal to or higher than the β transformation temperature.

First, the manufacturing method shown in FIG. 3(a) in which annealing is not performed after extrusion will be described.
Since titanium has a low thermal conductivity, in order to uniformize the temperature of the titanium alloy billet to a predetermined temperature, the temperature rise rate during heating is slowed down, or the time in the heating furnace is lengthened so that the center of the billet is included. to reach the target temperature. When trying to reach the target temperature to the center of the billet in this way, the billet surface reaches the β transformation point temperature or higher earlier than the center, so the residence time after reaching the β transformation point temperature or higher is long. Become. As a result, the growth of β grains is promoted on the billet surface, and the β grain size before extrusion increases. When the β grains before extrusion are coarsened, the β grains after extrusion are also coarsened due to fewer recrystallization nucleation sites for the β grains after extrusion, and the average prior β grain size exceeds 300 μm, which is 0.2. % proof stress decreases.

Therefore, preheating is performed to soak the billet at a predetermined temperature equal to or lower than the β transformation point temperature, and then rapid heating is performed to bring the entire billet to a predetermined temperature equal to or higher than the β transformation point temperature. We conceived of a method of performing hot extrusion by shortening the temperature holding time. In the preheating, the billet is soaked at a temperature equal to or lower than the β transformation temperature, so the β grains do not become coarse. Since preheating is performed, it is possible to perform rapid heating after that, and when the billet center reaches a predetermined temperature equal to or higher than the β transformation point temperature, the billet surface is held at the β transformation point temperature or higher for a short period of time. It becomes possible to As a result, it is possible to prevent the β grains from becoming coarse before extrusion, including the surface of the billet, and also prevent the β grains from being coarsened after extrusion. did.

Preheating is performed so that the temperature of the billet surface and center is (T _β −500) to (T _β −80)° C. and the temperature difference between the surface and center is 50° C. or less.

It is preferable to measure the temperature of the billet surface with a radiation thermometer. On the other hand, to measure the temperature at the center of the billet, prior to heating, a hole is drilled at the center position of the circle that is the bottom of the billet, a drill hole is drilled to the center of the billet, and a thermocouple protected by an insulating tube is inserted. should be done by

If the billet temperature after preheating is too low, it becomes necessary to increase the holding time after rapid heating in order to raise the billet center to a predetermined β transformation point temperature or higher when performing rapid heating thereafter, and as a result, As a result, the holding time at the β transformation point temperature or higher of the billet surface increases, and the β grains become coarse. In the present invention, by setting the lower limit of the preheating temperature to (T _β −500)° C., the holding time after rapid heating can be shortened, and the average prior β particle size after extrusion can be made 300 μm or less. became.

Titanium is easily oxidized when heated in the air, and when heated above a certain temperature, it forms a hardened layer called α-case on its surface, the thickness of which increases as the heating temperature rises. Since the α case is hard and has poor ductility, it becomes the starting point of cracks during extrusion, resulting in cracks in the extruded product. In addition, since the die is remarkably worn by the abrasive action of the hardened surface layer, the cross-sectional dimension varies greatly in the longitudinal direction of the extruded material. Therefore, the upper limit of the preheating temperature of the billet was set at (T _β −80)° C. where the formation of α case is not significant.

Titanium has poor thermal conductivity, and if rapid heating is performed from the billet surface in a state in which the temperature of the billet is not sufficiently uniform after preheating, the entire billet will not be uniformly heated. Therefore, during rapid heating, the time from when a part of the billet reaches the β transformation point temperature to when the entire billet reaches the β transformation point temperature is short, and the old β grain size after extrusion is within the cross section of the old β grain size. The upper limit of the temperature difference between the billet surface and the center during preheating was set to 50° C. so as not to exceed the average upper limit of 300 μm. In actual operation, the temperature difference is preferably 20°C or less.

After preheating the billet, it is heated to T _β to (T _β +200)° C. at a temperature rising rate of 1.0° C./s or more by electric heating or induction heating, and then extruded.

The higher the billet temperature after rapid heating, the higher the prior-β grain size. This is because the β grains processed during extrusion recrystallize while being held at the β transformation point temperature or higher after extrusion, but as the billet temperature before extrusion increases, the β grains after extrusion This is because the time for which the grains are held at the high temperature increases, and the grain growth time after recrystallization becomes longer. It has been found that when the billet temperature after rapid heating exceeds (T _β +200)° C., the average prior β grain size of the shape exceeds 300 μm and the 0.2% yield strength falls below 860 MPa.

In addition, titanium is easily oxidized when heated in the air, and when heated above a certain temperature, it forms a hardened layer called α-case on its surface, the thickness of which increases as the heating temperature increases. Since the α case is hard and has poor ductility, it becomes the starting point of cracks during extrusion, resulting in cracks in the extruded product. In addition, since the die is remarkably worn by the abrasive action of the hardened surface layer, the cross-sectional dimension varies greatly in the longitudinal direction of the extruded material. Therefore, the upper limit of the billet rapid heating temperature is (T _β +200)° C. where the average prior β grain size is 300 μm or less and the formation of α case is not significant.

On the other hand, when the temperature after rapid heating is close to the β transformation point temperature (T _β ), the processing temperature of the surface layer of the shaped material decreases to T _β or less due to heat dissipation when contacting the die during extrusion. Has axial tissue. As the extrusion progresses, the temperature of the die rises, so the processing temperature of the surface layer of the shaped material also rises. The temperature after rapid heating is preferably (T _β +50)° C. or higher.

If the temperature rise rate during rapid heating of the billet before extrusion is slow, the billet surface will be kept at a temperature above the β transformation temperature for a long time, and the prior β grain size before extrusion will become coarser. β grain size increases. Therefore, the lower limit of the heating rate was set at 1.0° C./s at which the growth of β grains on the billet surface before extrusion is suppressed and the average of the old β grains after extrusion is 300 μm or less.

Since titanium has poor thermal conductivity, it is preferable to provide a predetermined holding time after rapid heating in order to uniformly heat the entire billet when performing rapid heating by electrical heating or induction heating. In order to heat the entire billet to a temperature equal to or higher than the β transformation temperature, it is desirable to hold the billet for 20 seconds or more after the rapid heating. On the other hand, if the holding time after rapid heating is too long, the β grains become coarse during the holding time, and the β grains after extrusion also become coarse, which is not preferable. In the present invention, by setting the holding time after rapid heating to 20 minutes or less, the average prior β grain size of the shape can be made 300 μm or less.

After extrusion, it is preferable to cool to room temperature at a cooling rate of less than 5°C/sec. The cooling rate referred to here refers to the cooling rate up to 500°C. If forced cooling is performed at 5°C/sec or more after extrusion, the cooling rate becomes uneven, stress is generated inside the shape due to the temperature difference inside the shape, and plastic deformation such as warping and bending occurs. Moreover, even if plastic deformation does not occur, residual stress is generated inside the shape after cooling to room temperature, which causes shape defects such as warpage during processing and cutting of the shape. Therefore, after the extrusion process, it is preferable to stand to cool at a cooling rate of less than 5°C/second. In addition, if the cooling rate is slow, the β grain size increases during cooling, and the grain boundary α phase grows to reduce the strength and ductility. Therefore, it is preferable that the cooling rate after extrusion is allowed to cool at 0.5° C./second or more. In practical operation, standing cooling (approximately 1° C./sec) is preferred.

The above is the description of the manufacturing _method shown in FIG. 3(a). In addition, as in the manufacturing _method shown in FIG. )°C, strain relief annealing may be performed.

Even if the extrusion is allowed to cool, internal stress is generated due to the temperature difference inside the shape during cooling. Therefore, in the present invention, by annealing the shape after standing to cool for a time sufficient to remove the internal strain and reduce the internal stress, it is possible to reduce the bending that occurs during cutting and the like.

On the other hand, as the annealing temperature rises, the width of the α-phase (side-plate α-phase) in the colony increases, and the deposition distance of some dislocations increases, so the 0.2% proof stress and strength decrease. Therefore, the upper limit of the annealing temperature is defined as (T _β −200)° C. at which the width of the side plate α phase begins to increase.

Unlike the production method shown in FIGS. 3(a) and 3(b), the present invention can be produced by extrusion below the β transformation temperature as shown in FIGS. 3(c) and 3(d).
In the manufacturing method shown in FIGS. 3(c) and 3(d), the extrusion is performed in a temperature range below the β transformation point temperature (T _β ), so the structure after extrusion becomes an equiaxed structure. Therefore, in order to make the equiaxed structure portion into an acicular structure, a β single-phase region heat treatment is performed next. The conditions for the β single-phase region heat treatment are described in detail below.

A manufacturing method shown in FIG. 3(c) in which annealing is not performed after the β single-phase region heat treatment will be described.
For α+β titanium alloys, when the cross-sectional shape of the hot extruded shape is complicated, or when the processing rate of extrusion is high, the heat generated during processing increases, and the heat generated can be used, so the structure tends to become a needle-like structure. , When the cross-sectional shape is simple or when the processing rate of extrusion is low, the heat taken away by contact with the die or container exceeds the heat generated during processing, so it is necessary to make the entire needle-like structure by extrusion. is difficult.

Therefore, by heating to the _β -transformation point temperature Tβ or higher after extrusion, the whole is transformed into the β-phase, and an acicular structure can be obtained after cooling. Therefore, after extrusion, a β single-phase region heat treatment is performed with _Tβ as the lower limit temperature.

However, as the β single-phase region heat treatment temperature rises, the diffusion rate of atoms increases, the growth rate of β grains increases, and the time required for cooling to the β transformation point temperature or less increases, and β Grain growth is promoted. As a result, if the β single-phase region heat treatment is too high, the β grain size (old β grain size) grows to exceed 300 μm, the dislocation deposition distance increases, and the stress concentration occurring at the colony boundary increases. The 0.2% yield strength is greatly reduced. Therefore, the upper limit temperature of the β single-phase region heat treatment is set to (T _β +200)° C. where the growth rate of β grains is not significant and the cooling time to the β transformation temperature or lower is short.

In addition, the heating time of the β single-phase region heat treatment is also important in order to prevent the β grain size (old β grain size) of the acicular structure portion from being changed coarsely. The β grain size (old β grain size) increases as the β single-phase heat treatment time increases. This is because if the holding temperature above the β transformation point temperature is long, the interfacial energy of the β grains is lowered, so that the β grains start to coalesce. In addition, titanium is easily oxidized when heated in the air, and when heated above a certain temperature, it forms a hardened layer called α-case on its surface, the thickness of which increases as the heating temperature increases. Since the α case is hard and has poor ductility, it becomes the starting point of cracks, resulting in cracks in the product. Therefore, the average β grain size (old β grain size) is 300 μm or less, and the shape is held at T _β ° C. or higher for 1000 seconds or less (β single-phase region heat treatment temperature) at which the α case is not significantly formed. , all equiaxed texture portions can be made into needle-like textures without coarsely changing the prior β grain size of the needle-like texture portions. On the other hand, the lower limit of the β single-phase region heat treatment time depends on the thickness of the shape, but considering the heat transfer time to the center part of the shape, 10 seconds for heating the whole to the β transformation point temperature or higher degree is preferred.

Also, when the β-single-phase region heat treatment is performed in this way, it is necessary that the billet including the surface and the center be uniformed to a predetermined temperature equal to or higher than the β transformation temperature. Therefore, in the manufacturing method shown in FIGS. 3A and 3B, β single-phase region heat treatment is performed to heat to a temperature equal to or higher than the β transformation point, similarly to the case of heating to a temperature equal to or higher than the β transformation temperature before hot extrusion. In this case, the billet is soaked at a predetermined temperature below the β transformation point temperature (surface and center temperature (T _β -500) to (T _β -80) ° C, temperature difference between surface and center is 50 ° C or less) After that, rapid heating (heating rate of 1.0° C./s or more) is performed to bring the entire billet to a predetermined temperature equal to or higher than the β transformation point temperature, thereby shortening the temperature holding time equal to or higher than the β transformation point temperature. β single-phase region heat treatment is performed. As a result, the average prior β grain size can be made 300 μm or less.

After the β single-phase region heat treatment, it is preferable to cool to room temperature at a cooling rate of less than 5° C./second. As a result, plastic deformation such as warping and bending is prevented, and residual stress does not occur inside the shaped member.

The above is the description of the manufacturing method shown in FIG. 3( _c ). In addition, as in the manufacturing _method shown in FIG. ) °C for strain relief annealing. As a result, the strain generated inside can be removed, and the bending that occurs during secondary processing such as cutting can be suppressed.

As described above, the α+β type titanium alloy extruded profile of the present invention is not obtained only by these manufacturing methods. For example, in the manufacturing method shown in FIGS. 3(c) and 3(d), extrusion may be performed at a temperature equal to or higher than the β transformation point. Diffusion annealing may be performed continuously during cooling after extrusion or during cooling after β single-phase region heat treatment.

In the extruded shape, in order to make the value of the formula (1) 25 or less, the container 1, the stem 2, the dummy block 3, the die The amount of heat removal is calculated from the temperature, contact time, and heat capacity of 4, and heating is performed at the front end and the rear end to compensate for the temperature drop due to the heat removal amount, and a temperature gradient is applied to both.

(Example 1)
In order to confirm the influence of the metal structure of the extruded shape, extruded shapes were manufactured by changing the metal structure by changing the manufacturing conditions using billets of the composition shown in Table 1, and for each extruded shape , 0.2% proof stress, tensile strength, elongation, warpage, and metallographic structure were measured.

A Ti-6Al-2Sn-4Zr-2Mo ingot with a diameter of 700 mm and a weight of 5 tons obtained by melting twice with a vacuum arc and having the composition shown in Table 1 is hot forged in the α + β temperature range to an area reduction rate of 60%. Then, the surface oxidized layer of the obtained billet was cut to obtain a billet for extrusion. In addition, alloy No. in Table 1. 1 satisfied the ranges of O: 0.20% or less, C: 0.08% or less, and N: 0.05% or less.

Table 2 shows the manufacturing method and the measurement results of the metal structure and properties of the extruded profile for each test number. Here, for the manufacturing methods (a) and (b), the billet was preheated to 700° C. (the temperature difference between the surface and the center was 5° C.) by induction heating in an Ar gas atmosphere, and then the temperature was raised at a rate of 1. The temperature was raised at a rate of 3° C./s, and the extruded material was extruded into a convex cross-sectional shape under the manufacturing conditions shown in Table 2, and then allowed to cool to room temperature. For the production method (b), the hot extruded material was then subjected to strain relief annealing under the conditions shown in Table 2. For manufacturing methods (c) and (d), an extruded shape extruded into a convex cross-sectional shape at the extrusion temperature shown in Table 2 was induction-heated to 700°C in an Ar gas atmosphere (the temperature difference between the surface and the center was 5° C.), the temperature was raised at a temperature elevation rate of 1.3° C./s, and the β single-phase region heat treatment shown in Table 2 was performed. For the production method (d), the hot extruded material was then subjected to strain relief annealing under the conditions shown in Table 2. Induction heating "with" in Table 2 means that induction heating was performed in preheating before extrusion in the case of (a) and (b), and in the case of (c) and (d) , means that induction heating was performed in the preheating before the β single-phase region heat treatment.

<Tensile test>
An ASTM E8 half size tensile test piece (parallel part φ6.35 mm, gauge length 25 mm) was obtained from the position shown in FIG. 0.2% proof stress, tensile strength, and elongation at break were measured by a tensile test.

<Tissue observation test>
A tissue observation test piece was sampled from the same position as that of the tensile test piece, and the L section was subjected to tissue observation using an optical microscope observation photograph.
For the prior β grain size, the equivalent circle diameter was measured by a cutting method, and the average of 3 mm×6 mm (minimum number of grains: about 200) was obtained.
Regarding the average maximum width of the grain boundary α phase, as described above, in the cross section of the extruded profile shown in FIG. Measure the maximum width of the grain boundary α phase. When selecting old β-grains, avoid selecting adjacent old β-grains. Then, the average value of the five maximum widths was obtained as the average maximum width of the grain boundary α-phase.

<Tissue distribution>
Equiaxed texture and needle-like texture can be determined by macrostructural observation. The macrostructure is divided into two regions, a high metallic luster region and a low luster region that looks white. In any region, light is reflected by the unevenness of the surface caused by macroetching, resulting in metallic luster. However, in the region containing fine equiaxed α-grains, unevenness generated on the surface is finer than in the region of the acicular structure, and light is diffusely reflected. Therefore, the equiaxed texture area appears whiter than the acicular texture area. For the tissue distribution, cross-sections (including the end face of the most distal portion) obtained by dividing a shape with an overall length of 4000 mm into 200-mm intervals were investigated.

<Warp>
As shown in Fig. 5, warp is defined as the distance at the center of the extruded shape with a length of 4 m (4000 mm) in the longitudinal direction of the shape with respect to a straight line connecting both ends of the shape in the longitudinal direction. did. The actual measurement was carried out by attaching strings to points A (Fig. 4) at both ends of the shape.

The underlined items in Table 2 are outside the scope of the present invention. Measured at position A shown in 4. The preferred range was 860 MPa or more for 0.2% yield strength, 10% or more for elongation, and 9 mm (2.25 mm per 1000 mm) for warpage.
The results are also shown in Table 2. All of the extruded profiles of test numbers 1 to 12 had needle-like structures with a homogeneous metal structure.

In test number 9 of the comparative example, the billet temperature after induction heating exceeded (T _β +200) ° C., so in test number 10 of the comparative example, the temperature of the β single-phase region heat treatment exceeded (T _β +200) ° C. In both cases, β grains grew while the temperature was kept at _Tβ or higher after extrusion. As a result, the number of recrystallization nucleation sites after extrusion decreased, the average prior β grain size exceeded 300 μm, the 0.2% proof stress was below 860 MPa, and the elongation was below 10%.

In test number 11 of the comparative example, the billet was heated without rapid heating (preheating), so the β grains became coarse before extrusion, and the number of recrystallization nucleation sites after extrusion was small. The average β grain size also exceeded 300 μm. Therefore, the 0.2% proof stress was below 860 MPa and the elongation was below 10%.

In Test No. 12, the cooling rate after extrusion was slow, the average prior β grain size exceeded 300 μm, and the average maximum width of the grain boundary α phase exceeded 5 μm. also fell below 10%.

On the other hand, Test Nos. 1 to 8, which are examples of the present invention, had a 0.2% proof stress of 860 MPa or more and an elongation of more than 10% for any alloy composition, and had good properties. In addition, in Test Nos. 1 to 7, since the average width of the grain boundary α was suitable, warpage was also small.
On the other hand, Test No. 8, which is the present invention, was produced by forcibly cooling with water after extrusion at a temperature higher than the β transformation point. As a result, the cooling rate after the β-single-phase region heat treatment is excessively high, and the average maximum width of the grain boundary α-phase is small outside the preferred range, resulting in large warpage of the shaped material. Action is required.

(Example 2)
Next, in the extruded shape, we tried to reduce the difference in the size of the prior β grains in the direction of extrusion to make the mechanical properties uniform in the direction of extrusion.

Table 3 shows the production conditions and the results. In Test Nos. 13 to 18, while observing the production methods a to d, a temperature gradient was given to the front end and rear end of the billet when heating the billet before extrusion, and heat removal by a die or the like was compensated. These were described as "presence" of the temperature gradient at the front and rear ends of the billet. On the other hand, Test Nos. 19 and 20 complied with the production method of a, but did not compensate for the amount of heat removed. These were described as "no" temperature gradients at the leading and trailing ends of the billet.
The measurement of the average of the prior β grain size at the leading end and the trailing end and the sampling of test pieces for measuring the proof stress and elongation at the leading end and the trailing end were performed at positions 300 mm from the leading and trailing ends respectively. All of the extruded profiles of test numbers 13 to 20 had needle-like structures with a homogeneous metal structure.

In test numbers 13 to 18, as shown in Table 3, in order to compensate for the temperature drop due to heat removal, a gradient was applied to the extrusion heating temperature at the leading end and the trailing end, so that the leading end of the extruded shape It is possible to reduce the difference between the average of the old β grain size at the rear end and the rear end. As a result, it was possible to homogenize the mechanical properties of the extruded profile in the direction of extrusion. On the other hand, in Test Nos. 19 and 20, both yield strength and elongation at each part exceeded the preferable values, but the difference in the average of the prior β grain sizes in the extrusion direction was large, and a difference appeared in the mechanical properties.
In Test Nos. 1 to 12, no gradient was applied to the extrusion heating temperature at the leading end and the trailing end.

According to the present invention, by controlling the metal structure of the shaped material to a needle-like structure with a prior β grain size of 300 μm or less, it has practically no problem tensile properties and is compared to the case of forced cooling. It is possible to obtain a profile with a good shape. Therefore, the cost of cooling equipment and shape correction can be reduced, which is particularly useful industrially. In addition, the α+β type titanium alloy extruded shape of the present invention has both high strength and good elongation, and can be warped as needed. Since the bending is small, it is useful for applications such as aircraft.

1 container 2 stem 3 dummy block 4 die 5 billet 6 profile 11 extrusion direction

Claims (2)

The component composition is mass%, Al: 5.5 to 6.5%, Sn: 1.8 to 2.2%, Zr: 3.6 to 4.4%, Mo: 1.8 to 2.2 %, O: 0.20% or less (including 0%), C: 0.08% or less (including 0%), N: 0.05% or less (at 0% The balance is Ti and unavoidable impurities, the metal structure consists of a needle-like structure, the average prior β grain size is 300 μm or less, and the average maximum width of the grain boundary α phase is 5 μm or less. There is an average particle size d ₁ (m) of prior β grains in a certain cross section perpendicular to the extrusion direction of the extruded shape, and a distance L (m) away from the one cross section in the extrusion direction parallel to the one cross section An α+β titanium alloy extruded profile characterized in that the value of the following formula (1) calculated from the average grain size d ₂ (m) of prior β grains in another section of the extruded profile is 25 or less. .
However, the distance L is 0.3 m or more.
|(d ₁ −d ₂ )/L|×10 ⁶ (1) 2. The α+β type titanium alloy extruded profile according to claim 1, wherein the average prior β grain size is 200 μm or less.

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