JP4453422B2 - Titanium tube for hydrofoam, its manufacturing method and hydrofoam molding - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a titanium tube for hydrofoam that is mounted in a mold, applies an internal pressure in the tube, and is processed in a predetermined shape while being pushed in the axial direction as necessary, a method for producing the same, and a molded product using the hydroform. .

  Pure titanium and titanium alloys (hereinafter sometimes simply referred to as titanium) are excellent in specific strength, and thus have been used in the fields of aviation, military, space, automobiles, sports equipment and the like. Moreover, since it is excellent also in corrosion resistance, it is used also for the piping member etc. in a chemical plant. In these fields, further weight reduction and higher strength are desired.

On the other hand, component molding by hydroforming has been attracting attention as a method for reducing the weight by hollowing a solid member.
Here, `` hydroform '' means that a hollow tube (called a raw pipe) that is a material is mounted in a processing mold, and a working liquid is injected into the raw pipe to apply pressure, and at the same time as necessary. This is a processing method for manufacturing a predetermined deformed tubular product while performing bulge processing by applying axial push from the tube end.

  Fig. 1 is a diagram for explaining the hydroforming process. Fig. 1 (a) shows a partial cross-sectional view of the shape of the raw tube 2 before processing, and Fig. 1 (b) shows the result of hydroforming it. The shape of the obtained product 1 is shown in partial cross section.

  The metal tube 2 which is a raw tube for hydroform shown in FIG. 1 (a) is mounted in a processing mold (not shown), and a processing liquid is injected into the metal tube to apply pressure, and at the same time, it is necessary. In response to this, a predetermined deformed tubular product 1 shown in FIG. Normally, pipe expansion ratio [(peripheral length after hydroforming)-(perimeter of base pipe) / (perimeter of base pipe)]] is used as an evaluation index of workability in hydroform. Form is said to be difficult. In other words, it can be said that a work tube with a larger tube expansion rate has better workability.

  On the other hand, there are cases where the cross section of the product usually includes an irregular cross section such as a rectangle, and a sharper corner portion is formed without causing breakage or significant thinning during molding, that is, a smaller R portion (hereinafter referred to as a corner portion). It can be said that it is a material suitable for molding a complex and precise shape part having such an irregular cross section.

  As a method of manufacturing a titanium tube as an element tube used for such a hydroform, there are a seamless tube that is manufactured by drilling a strip material, and a welded tube that rounds a plate material into a tubular shape and welds a butt portion. In the present specification, the term “titanium (alloy) pipe” includes both seamless pipes and welded pipes.

  In general, TIG welding performed by shielding with an inert gas is used for pipe making of a pure titanium welded pipe or a titanium alloy welded pipe (hereinafter sometimes simply referred to as a titanium welded pipe). Titanium alloys are known to be hardened as the content of nitrogen, oxygen, hydrogen, etc. increases, and are shielded with an inert gas such as argon during pipe production to prevent the introduction of impure gas.

However, when applying a conventional titanium welded pipe to a hydroform, there is a problem that the pipe can only be expanded smaller than the forming limit of the original base material.
Patent Document 1 discloses a technique relating to a welding method at the time of forming a β-type titanium alloy tube, but it improves the strength (toughness) of the tube itself, and the formability at the time of processing of the tube is disclosed. It is not intended to improve.

Patent Document 2 discloses a β-type titanium alloy excellent in cold workability, but it is particularly intended for cold forging and does not assume a hydroform.
Patent Document 3 proposes a pure titanium tube having good formability in hydroform or the like by narrowing the width of the welded portion, but to satisfy the relationship of 3t ≧ W> (3 / 6.5) t, The welding width (W) is limited to a certain range defined by the plate thickness (t).

However, this method is premised on laser welding, and it must be said that the welding conditions are rather severe, and cannot be said to be practical.
Japanese Patent Laid-Open No. 11-114684 Patent 2669004 JP2003-311322

An object of the present invention is to provide a titanium tube suitable for molding with hydrofoam, a method for producing the same, and a hydrofoam molded article using the same.

  In automobiles and motorcycle parts, parts formed by hydroforming metal pipes are used. On the other hand, pure titanium material produces a unique color at high temperatures, has a high design and high-class feeling, and is used for mufflers and the like of two-wheeled vehicles. Further improvement in workability is desired.

Furthermore, aircraft parts and the like are required to have a more precise shape, and the product is required to have high strength.
In order to achieve both weight reduction and high strength of the product, it is effective to use a closed cross-sectional structure and a material with high specific strength. Therefore, the use of hydroform as a manufacturing method for closed cross-section structural members and the application of titanium material can be considered to reduce the weight of the material, but conventional hydroforms with titanium welded pipes are welded during bulging molding. In some cases, the shape that can be molded is limited.

As a result of earnest investigation, the present inventor has found that in the case of a titanium alloy, if it is a seamless pipe having no welded portion, it can be formed at the forming limit of the material.
By the way, seamless pipes are expensive to manufacture, and it is difficult to adopt them other than special applications under the circumstances where cost reduction is strongly demanded as in today, and welded pipes are usually used.

  Here, as a method for welding a titanium welded pipe, TIG welding performed by sealing with an inert gas is generally used. However, when such a titanium welded tube was hydroformed, it was found that the welded portion fractured below the expansion ratio obtained with the seamless tube.

Thus, as a result of further investigation of the fracture phenomenon by the present inventors, it has been found that the heat-affected zone extends more intensively than the base metal during pipe expansion and breaks.
That is, if the strength of the heat-affected zone is not lower than that of the base material, the formability of the material itself can be obtained even as a welded pipe. In this specification, the weld zone and the heat affected zone are collectively referred to as “welded zone”, and the average hardness of the weld zone is used as an index of the strength of the heat affected zone. Therefore, in the present invention, at least the average hardness of the welded portion is made higher than the average hardness of the base material, so that the expansion ratio of the titanium welded tube during hydroforming is improved.

  Patent Document 3 describes that the crystal grains become coarse and soften until the weld metal is solidified and cooled, but such softening can be achieved by mixing an impure gas into the inert gas used during welding. I found out that it can be prevented.

Furthermore, in order to increase the specific strength of the material, it is conceivable to apply a desirable titanium alloy.
Conventionally, titanium alloys excellent in cold workability have been proposed, but these are intended for cold and hot forging and press forming, and hydroforms were not in that category. Formability in forging is evaluated by crushing a cylindrical specimen in the axial direction and generating cracks on the surface. This crack is caused by shear stress caused by axial compression and circumferential tension. Judgment in press molding is due to tension under biaxial tensile stress that does not include the thickness direction.
On the other hand, fracture in hydroform is basically cracking in a triaxial stress state, and a material excellent in formability by conventional forging or press molding is not necessarily superior in hydroform.

Therefore, a titanium alloy excellent in forming with a hydroform has not been proposed so far.
As a method for satisfying these requirements, the present inventors have a body-centered cubic crystal structure with good cold workability, and are molded in a solution-treated state, and are subjected to an aging treatment after molding to form an α phase. It was considered that a β-type titanium alloy whose strength can be increased by precipitation is promising.

  Here, β-type titanium alloys such as Ti-3Al-8V-6Cr-4Mo-4Zr and Ti-15V-3Cr-3Al-3Zn are known as β-type titanium alloys, but these have high deformation resistance. For this reason, there is a limit to the shape that can be molded with hydroform molded by hydraulic pressure, and a material with lower deformation resistance is desired.

On the other hand, when a precise shape such as an aircraft part is required, it is necessary to form a sharper corner R portion. In general, by increasing the hydraulic pressure, the corner R can be reduced, but an infinitely small R is not selected, and it is known that R stops or breaks at the R apex during molding.
Here, as a result of intensive studies, the present inventors have obtained (1) a low yield point and (2) difficult to work harden (1) in order to obtain a sharp corner R portion with a low hydraulic pressure and without cracking. I found that the material is good (small spread and large local spread).

  Therefore, the above-mentioned method for preventing breakage at the heat affected zone during hydroforming is studied, and it is inevitable that the crystal grains become coarse due to recrystallization at the molten metal and heat affected zone during welding. It has been found that a decrease in strength due to grain coarsening may be compensated by another method.

  Here, to summarize the above, as a method for increasing the strength of the heat-affected zone that becomes the fracture site during hydroforming of a titanium welded tube, an impure gas may be mixed into the shielding inert gas during welding. Among impure gases, oxygen is particularly effective for increasing the strength.

As an index for increasing the strength of the heat-affected zone, the average hardness distribution of the weld zone including the molten metal zone and the heat-affected zone and the difference in Vickers hardness between the base metal and the base material may be made to exceed zero.
Furthermore, as a method for further enhancing the above effect, it is effective to reduce the width of the heat affected zone under the above-described conditions. If the width of the heat-affected zone is made sufficiently short, it will be restrained by a hard base material, so that it will be difficult for the soft heat-affected zone to be deformed.

In order to shorten the width of the heat-affected zone, it is only necessary to concentrate heat input during welding. For example, laser welding is effective as a welding method.
On the other hand, the crystal grains in the heat-affected zone become coarse during pipe-forming welding, but even in such a case, the crystal grains are subsequently refined to make the strength of the heat-affected zone equal to the strength of the base material. For example, it is possible to prevent breakage at the welded portion during hydroforming and increase the pipe expansion rate. At this time, as a technique for refining the crystal grains, a solution heat treatment may be performed after cold-working the weld and applying plastic strain.

Furthermore, in a pure titanium welded pipe, the following phenomenon occurs in addition to the above phenomenon during welding, so the strength of the heat affected zone must be higher than that of the base metal.
That is, a pure titanium plate as a base material of a pure titanium welded pipe has a texture and a large plastic anisotropy. Usually, the r value in the rolling direction is around 2 and the r value in the direction perpendicular to the rolling is particularly large 4-6. To reach. On the other hand, the molten metal part and the heat-affected zone constituting the welded part at the time of pipe-forming welding are once heated and cooled beyond the β transformation point, so that they become transformed β phases and the r value decreases to about 1. Therefore, even if the heat-affected zone is stronger than the base metal in uniaxial tension, the thermal effect is higher than that of the base metal under biaxial tensile stress close to plane strain tension such as hydroform where the deformation in the longitudinal direction is restricted. The strength of the part is significantly reduced.

  Therefore, the heat affected zone of pure titanium welded pipe has a smaller r value than the base metal, so even if it has the same strength by uniaxial tension, the axial deformation like hydroform is restrained. Under the biaxial tensile stress close to plane strain tension, the heat-affected zone shows a softer behavior.

Therefore, in the pure titanium welded pipe, the following method is effective in addition to the above-mentioned two methods as a method for preventing breakage of the heat-affected zone during hydroforming.
That is, the pure titanium sheet has a large r value in the width direction of 4 to 6, but the r value in the rolling direction is around 2. Therefore, if the rolling direction of the plate is made to be the circumferential direction of the pipe, even if the r value of the heat-affected zone decreases, the difference from the base material does not increase so much, and it is difficult to break during hydroforming. .

  Next, a preferable titanium alloy material will be described. In this case, it may be a seamless pipe or a welded pipe, and both cases are included. Since hydroform is basically a tube expansion process, the total elongation is required to be at least 10%. Further, in order to obtain a sharper corner R portion, it is necessary that the material hardly flows and is easy to flow into the corner portion. That is, it is preferable that the uniform elongation is 2% or less and the total elongation is 10% or more.

Furthermore, in order to obtain strength as a product, a means for increasing the strength after molding is necessary, and it is desirable that the tensile strength of the product after molding is 1200 MPa or more.
However, from this point of view, pure titanium and α-type titanium alloys do not have heat treatment properties, so the above conditions cannot be achieved. Since the α + β type titanium alloy has high strength and high uniformity, the above condition cannot be achieved, and only the β type titanium alloy can achieve the above condition.

Further, among the β-type titanium alloys, those having the following composition are particularly effective.
A β-type titanium alloy having V: 10 to 25%, Al: 2.5 to 5%, Sn: 0.5 to 4%, and the balance being Ti and inevitable impurities.

  According to the present invention, it is possible to provide a titanium tube which has excellent formability in hydrofoam molding and increases the strength of the product after molding, that is, a pure titanium welded tube, a titanium alloy seamless tube or a titanium alloy welded tube, and requires more severe processing than before. Processing of molded products and precision molded products is possible.

Next, a description will be given of the operational effects and reasons for limitation of welding at the time of pipe making for preventing breakage at the welded portion of the titanium welded pipe at the time of hydroforming.
FIG. 2 is a schematic explanatory view of a welded portion of a welded pipe. The welded pipe is subjected to butt welding in the molten metal part, and heat affected parts exist on both sides thereof. As already mentioned, both are collectively referred to as the “welded part”. The crystal grains in the molten metal part and the heat-affected zone are significantly coarser than the base metal part.

Therefore, the welded portion in which the crystal grains are coarser is softer than the base material.
For example, when the pipe is expanded with a radius of curvature r ₁ by hydroforming and with an internal pressure p, if the wall thickness at that time is t ₁ , the circumferential stress σ _θ of the curved surface is uniformly σ _θ = (r ₁ · p) / t ₁
It becomes. Therefore, if there is a partially soft part in the circumferential direction, the part concentrates and eventually breaks.

  Therefore, as a method for increasing the strength of the heat-affected zone, the hardness has a correlation with the tensile strength and the yield point, and these increase as the hardness increases. As a method for increasing the hardness of the welded portion, an impure gas (oxygen, nitrogen, hydrogen, etc.) may be mixed in a shielding gas (inert gas such as argon) during welding. Since the hardness of the titanium material increases as the impure gas content increases, the titanium material is dissolved in titanium melted during welding.

  Fig. 3 shows an example of the hardness distribution of the welded portion seen in β-type titanium alloy welded pipes, which is when the hardness of the welded portion was not increased. In addition, the “difference in hardness of the weld” shown in the figure is a case where the minimum hardness of the weld is indicated. Even with such a hardness distribution, the hardness of the welded portion increases due to the mixing of impure gas during welding. As described above, it is considered that the impure gas is dissolved in titanium. Here, as an index for increasing the hardness of the welded portion, the difference between the average hardness value distributed in the welded portion including the heat-affected zone and the hardness of the base material is used. Hereinafter, this is referred to as a weld hardness difference.

In the titanium alloy welded pipe, the difference in hardness of the weld zone exceeds 0 (zero), so that the critical pipe expansion rate of the hydroform is remarkably improved.
On the other hand, in the pure titanium welded pipe, the hardness difference of the weld zone has been improved from over 0, and from 20 or more, the critical pipe expansion rate of hydroform has been significantly improved. In the present specification, hardness refers to Vickers hardness.

In the case of a pure titanium welded tube, the limit expansion rate of the hydroform is not improved unless the weld hardness difference is larger than in the case of a titanium alloy welded tube for the following reason.
The pure titanium plate, which is the material of the pure titanium welded tube, has a high r value of 4 to 5 because of the texture, whereas the welded portion is heated beyond the transformation point, so the r value decreases to about 1. End up. Therefore, even if the heat-affected zone and the base metal have the same strength in uniaxial tension, the biaxial tensile stress close to the plane strain tension in which deformation in the longitudinal direction is constrained, such as hydroform, is compared with that of the base material. The strength of the heat affected zone is significantly reduced. Therefore, in the case of a pure titanium welded pipe, the vicinity of the welded portion exhibits a softer behavior than the base material under deformation due to plane strain tension.

  On the other hand, if too much impure gas is mixed into the weld, there is a problem that the weld becomes brittle, and there is a risk of breakage during the preforming or bending process of the previous hydroform. In any of the welded pipes, the weld hardness difference is desirably 50 or less.

Next, as a second method for improving the expansion ratio of the welded tube, the crystal grains in the welded portion may be refined.
In order to refine crystal grains coarsened by recrystallization at the time of welding, it is only necessary to give a plastic strain by performing cold working once, and then rapidly cool after heating to a temperature above the transformation point. This can be applied to either a pure titanium welded tube or a titanium alloy welded tube, but the effect of the titanium alloy welded tube is large and preferable.

FIG. 4 shows the relationship between the crystal grain size and the pipe expansion rate in a β-type titanium alloy welded pipe, showing the results of Example 7 described later.
From the illustrated results, the relationship between the maximum crystal grain size Dw of the welded portion and the expansion ratio of the crystal grain size Dm of the base material is:
log (Dw) / log (Dm)> 0.65
When satisfying the above, it can be seen that the tube expansion rate is greatly improved.

Examples of the cold working method include cold drawing and cold rolling.
The crystal grain can be arbitrarily changed depending on the strain amount and heat treatment conditions given by cold working.For example, in the case of the desirable β-type titanium alloy of the present invention, 10% equivalent plastic strain is given by cold working, and the inert atmosphere. The above condition is satisfied by heating at 800 ° C. for 15 minutes and then cooling at 20 ° C./minute.

  In addition to the above, if the width of the heat-affected zone is sufficiently short, the shear stress at the interface acts as a tension in the thickness direction of the heat-affected zone due to the difference in strength between the base material and the heat-affected zone, causing deformation in the circumferential direction. Will be restrained.

  According to various experiments by the inventor, it has been found that the critical tube expansion ratio increases as the width of the heat affected zone decreases. In particular, the critical tube expansion ratio increases remarkably when the width of the heat affected zone is less than 1 times the plate thickness. did.

  Although it is a method of shortening the width of the heat affected zone, in TIG welding conventionally performed by titanium welding, it is difficult to narrow the width of the heat affected zone because the heat input is large. Therefore, the titanium welded pipe of the present invention can be manufactured by the following manufacturing method.

  Titanium coils are supplied to a group of forming rolls, the laser beam is melted by irradiating the groove part continuously formed into an open pipe shape, and the heat-affected zone is formed in a pipe making method by joining up by setting up. The width should be less than 1 times the plate thickness.

Since hydroform is basically a pipe expansion process by hydraulic pressure, if the strength of the material is too high for molding, it is difficult to form, so it is desirable that it is relatively soft during processing. In view of the equipment used, it is desirable that the yield point is 800MP or less as a desirable titanium alloy.
Next, deformation characteristics will be described.

  In general, the extension until reaching the maximum load is uniformly extended, the extension from the maximum load to the break is locally extended, and the total of these is called the total extension. If the stretch is the same, it can be said that a material having a larger uniform elongation is easier to work and harden.

  Generally, steel pipes that are easy to work harden are desirable for hydroform steel pipes used for automobile parts, etc., but for automobile parts, steel pipes with a tensile strength of about 400 to 600 MPa are used, and corner R is also used. This is because it may be relatively large, such as 8 mm or more, and a sufficient pressure with respect to the strength level of the material can be obtained relatively easily, and the need for sharpening the corner R is low. Materials that are easy to work harden are desirable.

  In a member aiming at high strength and light weight, it is desired that the strength is low because of molding at the time of hydroforming, but it is desirable that the strength after molding is high. Therefore, paying attention to β single-layer titanium alloy (β-type titanium alloy) that has a body-centered cubic crystal structure with good cold workability, hydrolyze a material that has been improved in workability by lowering the strength by solution treatment. After forming with a foam, the above-mentioned purpose can be achieved by applying an aging heat treatment to precipitate the α phase to increase the strength.

  The α-type titanium alloy increases with increasing strain and breaks immediately after reaching a high maximum load. The α + β titanium alloy is not as strong as the α alloy, but has high strength, small local spread, and is unsuitable for hydroforming.

On the other hand, the β-type titanium alloy used in the present invention reaches the maximum load at the initial stage of strain increase after yielding, and then a large elongation is obtained while the stress gradually decreases.
On the other hand, in the aircraft field where titanium alloys are suitable, products with higher precision and higher strength are required. Therefore, the strength of the work-hardening material increases during the molding, and therefore enormous hydraulic pressure is required to allow the material to flow into the corner R portion at the end of molding. Therefore, for materials that require precise shapes, materials that are difficult to work harden, that is, materials with small uniform spread and large local spread are desirable. For example, uniform stretch is 2% or less, and total stretch is 10% or more. A material that is

  In the β-type titanium alloy tube of the present invention, the effects of each component and the reasons for limitation will be described with respect to the components desirable for the hydroform. The “%” indicating the alloy composition is “mass%” unless otherwise specified.

V: 15-25%
V dissolves in the titanium alloy substrate to stabilize the β phase, and is placed at room temperature to form a single β phase structure, thereby improving cold workability. However, when the V content is less than 15%, the β phase alone cannot be obtained even if the solution treatment is performed, and a martensite structure is formed. When it is more than 25%, it becomes a β phase alone, but the age hardening is poor and the time required for the aging treatment becomes long. Moreover, when V is added excessively, raw material costs increase and it is not economical.

  In addition to V, there are other elements such as Mo, Ta, Nb, Cr, Fe, and Mn as β-phase stabilizing elements. Among these, β-phase simple alloys are inexpensive and have low strength in a solution state. Elements are limited to Mo and V. Mo has a high melting point and is difficult to dissolve, so segregation is likely to occur. Eventually, V is the most preferable β-phase stabilizing element, and its content is 15 to 25% for the above-mentioned reason.

Al: 2.5-5%
The titanium alloy of the present invention is a metastable β-phase single substance in a solution state, and when this is subjected to an aging treatment, an α-phase is precipitated and an increase in strength is obtained. In order to increase the strength by aging precipitation of the α phase, not only the dispersion strengthening of the α layer but also the strengthening of the precipitated α phase itself is effective. Al is the most effective alloy element for solid solution strengthening of α-titanium. The addition of Al also has the effect of suppressing the precipitation of the ω phase that embrittles the alloy and promoting the precipitation of the α phase.

  Such an effect of Al does not appear remarkably when the content is less than 2.5%. On the other hand, if the Al content exceeds 5%, the strength in the solution treatment state increases and the workability decreases. That is, the proper content of Al is 2.5 to 5%.

Sn: 0.5% to 4.0%
Sn promotes and stabilizes the aging precipitation of the α phase and suppresses the formation of the ω phase, so that it has the effect of widening the appropriate temperature range for the aging treatment and increases the aging strength. Furthermore, while Al strengthens the alloy substrate in solid solution, Sn does not harden the substrate so much, so reducing Al and replacing it with Sn is effective for reducing deformation resistance. The effect of Sn is poor if it is less than 0.5%, and if it exceeds 4%, an increase in the hardness of the substrate is inevitable. Therefore, the Sn content is 0.5-4%.

  In addition to the above alloy components, the balance is substantially Ti. The fact that it is substantially Ti means that it is accompanied by impurities inevitably contained in the case of industrial production. However, in the alloy of the present invention, oxygen and hydrogen in impurities are particularly suppressed as follows.

Oxygen: 0.12% or less Oxygen is an α-phase stabilizing element. If it is present in a large amount, it inhibits the β-phase element, hardens the substrate, and degrades the hydroform processability. That is, the deformation resistance is increased, the deformability is lowered, and it becomes a cause of generating cracks during hydroforming. Since the effect is small if it is 0.12% or less, it was made 0.12% or less.

Hydrogen: 0.05% or less Hydrogen is a β-phase stabilizing element, and if its content is excessive, aging precipitation is delayed. However, if it is 0.05% or less, the effect is small, so 0.05% or less was set.

By satisfying these conditions, it is obvious that a material suitable for a welded pipe for hydroform can be provided. If a seamless pipe using these materials is used, a titanium pipe for hydroform having the characteristics of the material can be provided.
Next, the present invention will be described more specifically with reference to examples.

Examples 1-2
In FIG. 5, the metal mold | die used for the evaluation test of hydroform moldability in this invention is shown.
The mold dimensions are D1 = φ50.9 mm, D2 = φ120 mm, L1 = 140 mm, L2 = 30 mm.

The mold is composed of a lower shape and an upper shape, and the raw tube is set in the hole shape.
The workability was evaluated based on the limit tube expansion rate. In other words, the hydrofoam pressurization pattern applies axial push to the pipe end under a constant internal pressure, bulges in the mold cavities, and stops processing when cracks occur in the bulges. The perimeter of the part was measured to calculate the maximum tube expansion rate when the fracture occurred. The internal pressure was tested at various pressures, and the one with the highest maximum tube expansion rate at the time of occurrence of fracture was used for evaluation as the limit tube expansion rate.

 For this test, JIS type 1 and JIS type 2 pure titanium welded tubes having an outer diameter of 50.8 mm, a total length of 480 mm, and wall thicknesses of 1.5 mm and 2.0 mm were prepared.

TIG welding was performed with 0-5% nitrogen gas mixed in the shielding gas (argon gas), and a titanium welded tube with varying hardness of the heat affected zone was produced.
FIG. 6 is a graph showing the Vickers hardness distribution of the weld. It can be seen that the hardness of the weld is higher than that of the base metal at any location.

  Fig. 7 is a graph showing the relationship between the weld hardness difference and the tube expansion ratio. As can be seen from the graph, a large tube expansion ratio is obtained when the weld hardness difference is increased, and the tube expansion ratio is improved from over 0 Hv. However, the tube expansion rate was further improved from 20Hv or higher.

The pipes were manufactured using TIG welding or laser welding to control the welding speed to produce test materials with various heat affected zone widths. The relationship between the heat affected zone / thickness ratio and the limit tube expansion ratio was determined.
The test results are shown in FIG. 8, and a larger tube expansion ratio was obtained as the ratio Wh / t of the heat affected zone width to the plate thickness was smaller.

Examples 3-7
Using the VAR method, a titanium alloy having the composition shown in Table 1 is melted, and then hot forging and hot rolling are performed on these ingots, followed by solution treatment and scale removal (hydrofluoric acid pickling). Finally, cold rolling was performed to a thickness of 1.75 mm to obtain a cold rolled sheet.
Meanwhile, seamless pipes with an outer diameter of 25.4 mm and a wall thickness of 1.75 mm were manufactured by piercer drilling and mandrel rolling.
The mechanical properties of each material were investigated using a JIS No. 5 test piece cut out from the plate material.

(Processability evaluation method 1)
FIGS. 9A and 9B are schematic explanatory views showing a mold used in an evaluation test of the tube expansion characteristics of the hydroform in this example.

The mold dimensions are D1 = 25.5mm, D2 = 33mm, L1 = 100mm. When the pipe is expanded into a mold shape, the expansion ratio is 30%. The mold is composed of a lower shape and an upper shape, and the base tube is set in the hole shape.
For the evaluation of workability, the hydrofoam pressurization pattern was applied to the end of the tube with a constant internal pressure, and the tube was swollen in the mold cavity, and the processing was stopped when a crack occurred in the swollen part. And the perimeter of the same diameter part was measured and the maximum tube expansion rate at the time of fracture occurrence was computed. The internal pressure was tested at various pressures, and the one with the highest maximum tube expansion rate at the time of fracture occurrence was used for evaluation as the limit tube expansion rate.

(Processability evaluation method 2)
FIGS. 10 (a) and 10 (b) are schematic explanatory views showing a mold used in the evaluation test of the corner R formability of the hydroform in this example.

The mold dimensions are D1 = 25.5, L3 = 25.5mm, L2 = 100mm.
The mold is composed of a lower shape and an upper shape, and the raw tube is set in the hole shape.
Evaluation of workability was performed by measuring the corner radius of the center section of the molded product when the hydrofoam pressure pattern was axially pressed with a constant inner pressure of 200 MPa and 20 mm.

A test of workability evaluation method 1 was performed with a seamless pipe using the materials shown in Table 1.
Table 1 also shows the measured values of tensile strength after aging treatment.
The aging treatment was 475 ° C. × 20 hours for the β-type titanium alloy, and the No. 14 α + β-type titanium alloy was heated at 750 ° C. followed by furnace cooling heat treatment. No. 16 α-type titanium alloy has no heat treatment property, so heat treatment is not performed and the value of the material after hydroforming is shown.

As shown in Table 1, all of the titanium alloy tubes for hydrofoams of the present invention achieved a tube expansion rate of 30%.
It can be seen that No. 14 which is an α + β type titanium alloy as a comparative alloy has a tube expansion rate of 10%, and No. 16 which is an α type titanium alloy is 5%, which is not suitable for hydroforming.

  Of the titanium alloy tubes for hydrofoams of the present invention, No. 1 to 7 and 15 are low strength titanium alloys that have a tensile strength of 800 MPa or less and are easy to form. Has a tensile strength of 1200 MPa or more.

  No. 8 with low V content does not become a β simple structure, so the strength is too high, and No. 9, 10 and 11 with too high contents of Al, Sn, and oxygen also have a tensile strength of 800 MPa or more, and hydroform The strength is slightly higher for molding with.

  In No. 12, since the V content is too much, the strength in the solution state is low and there is no problem, but the tensile strength after aging treatment is 110 MPa, and it does not reach 120 MPa, which is desirable as a product after hydroforming.

  No. 15 is Ti-15V-3Cr-3Sn, which is well known as a β-type titanium alloy, but has a slightly high tensile strength.

Using the seamless pipes of Nos. 5, 8, and 15 shown in Table 1, the workability evaluation method 2 was tested to investigate the moldability.
The obtained minimum corner radius No. 15 was limited to a radius of 10 mm, but Nos. 5 and 8 could be molded to a radius of 5 mm.

  This is because No. 5 and 8 have a small uniform elongation and hardly work harden, so that the material flowed in to fill the mold corner.

  Test materials with various heat-affected widths are manufactured by controlling the welding speed using TIG welding or laser welding while making pipes from the plate material No. 5 shown in Table 1 and sealing with Ar gas. Then, an evaluation test was conducted in the workability evaluation method 1 described above.

The test results are shown in FIG. 11. As the ratio of the heat affected zone width to the plate thickness is smaller, a larger tube expansion rate was obtained, and the limit tube expansion rate was significantly improved from the heat affected zone width / thickness of 1.0 or less.
Tensile test pieces were cut out from aging at 475 ° C for 20 hours, and the tensile strength reached 1300MPa.

Titanium alloy welded pipes made from plate material No. 5 shown in Table 1 and mixed with various proportions of impure gas in shield gas (argon gas) by TIG welding to change the hardness of the weld. Produced.
An evaluation test was performed in the processability evaluation method 1 described above. The width of the molten metal part and heat-affected zone is large due to TIG welding, and in a completely gas-sealed state, the altitude is lowered due to the coarsening of the crystal grains, but the hardness is reduced while the crystal grains are coarsened due to the inclusion of impure gas. The test results are shown in FIG. 12. As the difference between the hardness of the base metal and the average hardness of the welded portion increases, a large expansion ratio is obtained, and the expansion ratio is significantly improved from above 0 Hv.

  Shielded with argon gas with the material of the present invention, manufactured welded tubes of each outer diameter by TIG welding, then cold drawn to plastically deform the entire tube, outer diameter φ25.4, wall thickness A 1.75 mm tube was manufactured, and solution treatment was performed to refine the crystal grains in the weld. The results are shown in a graph in FIG. 4, where the average crystal grain size Dm at a position sufficiently distant from the weld and the maximum crystal grain size Dw of the weld are as Dw approaches Dm, that is, log (Dw) The larger the / log'Dw), the larger the tube expansion rate, and the tube expansion rate improved remarkably from 0.65 or more.

It is explanatory drawing of the processing point of hydroforming, FIG. 1 (a) shows the shape of the raw pipe | tube before a process, FIG.1 (b) shows the example of a shape obtained by hydroforming. It is the figure which showed the welding part of the welded pipe typically. It is a figure explaining the hardness distribution of the welding part of a welded pipe. It is a figure which shows the relationship between the ratio of the maximum crystal grain size of a weld part with respect to the crystal grain size of the base material concerning embodiment of this invention, and a limit pipe expansion rate. It is a figure of the metal mold | die used for the Example of this invention. It is a figure explaining the hardness distribution of the welding part of a welded pipe. It is a figure which shows the relationship between the average hardness of the welding part concerning embodiment of this invention in a pure titanium welded pipe, and a limit pipe expansion rate. It is a figure which shows the relationship between the ratio of the heat affected zone width | variety with respect to the thickness concerning embodiment of this invention, and a limit pipe expansion rate. FIG. 10 is a view of a mold used in an example of the present invention, FIG. 10 (a) is a side view, and FIG. 10 (b) is a cross-sectional view. FIG. 11A is a side view of the mold used in the example of the present invention, and FIG. 11B is a cross-sectional view. It is a figure which shows the relationship between the ratio of the heat affected zone width | variety with respect to the thickness concerning embodiment of this invention, and a limit pipe expansion rate. It is a figure which shows the relationship between the average hardness of the welding part concerning embodiment of this invention, and a limit pipe expansion rate in a titanium alloy welded pipe.

Claims (4)

Β-type titanium or pure titanium welded pipe for hydrofoam, characterized in that the average value of Vickers hardness of the welded portion satisfies the following formula.
Hv ^W -Hv ^M > 0 (1)
Hv ^W : Average value of Vickers hardness of welded portion Hv ^M : Average value of Vickers hardness of base material The method for producing β-type titanium or pure titanium welded pipe for hydroforming according to claim 1, wherein cold working is performed after pipe forming welding and solution heat treatment is performed. The said titanium welded pipe is a beta-type titanium welded pipe , Comprising: The manufacturing method of hydroform molded article characterized by performing an aging heat treatment after hydroforming shaping | molding following the manufacturing method of Claim 2. Method. The pure titanium welded pipe for hydroform molding according to claim 1, wherein the rolled direction of the plate is formed in the circumferential direction of the pipe.

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