JP2012241241A - Titanium material and producing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a titanium material, displaying high strength without remarkably lowering high extension, even without adding expensive element and material.SOLUTION: The titanium material has a crystalline structure of a closed hexagonal lattice arranging atoms to a-axial direction and c-axial direction. The oxygen atoms having ≥4,000 ppm in the titanium, is dissolved as a solid-solution. The value of axial ratio c/a which is the ratio to the lattice constant in the a-axis and the lattice constant in the c-axis, is in the range of 1.589-1.593.

Description

  The present invention relates to a titanium material and a method for producing the same.

  Titanium is a lightweight material with a specific gravity about half that of steel, and has the characteristics of excellent corrosion resistance and strength. Therefore, parts such as aircraft, railway vehicles, motorcycles and automobiles that have strong needs for weight reduction. It is used for household appliances and building materials. It is also used as a medical material from the viewpoint of excellent corrosion resistance.

  However, since titanium has a higher material cost compared to steel materials and aluminum alloys, its application target is limited. In particular, a titanium alloy has a high tensile strength exceeding 1000 MPa, but has a problem that ductility (breaking elongation) is not sufficient and plastic workability is poor at room temperature or low temperature. On the other hand, pure titanium has a high elongation at break exceeding 25% at room temperature and is excellent in plastic workability in a low temperature range, but has a low tensile strength of about 400 to 600 MPa. .

  Prior art relating to titanium materials having high strength and high ductility will be described below. In any prior art, the basic idea is to improve the strength of the titanium material by adding an appropriate element. In many cases, it has been proposed to increase the strength of a titanium material by dissolving oxygen in the titanium substrate.

  For example, in Japanese Patent Laid-Open No. 2002-285268 (titanium alloy and manufacturing method thereof), the strength of the titanium material is increased by including 1.5 to 6 at% oxygen (O) and / or nitrogen (N). Is disclosed. Oxygen is previously contained in pure titanium powder, which is a starting material powder.

  Similarly, Japanese Patent No. 3426605 (high strength / high ductility titanium alloy and method for producing the same) discloses that oxygen, nitrogen, and iron (Fe) are incorporated into the titanium alloy by a melting method. Oxygen has a strengthening action as a solid solution element in the titanium substrate.

  In JP 2009-127083 (a method for producing a titanium alloy), the strength of a titanium alloy using a relatively inexpensive sponge titanium as a raw material is improved by increasing the content of nitrogen or oxygen based on pure titanium. A manufacturing method is proposed. Here, fine titanium oxide particles and pure titanium (sponge titanium) are mixed and molded and solidified, and then vacuum arc melting is performed to decompose titanium oxide and dissolve oxygen contained therein in pure titanium. Is disclosed. That is, according to this manufacturing method, the titanium oxide particles to be added are dissolved in the pure titanium melted in the process of arc melting, and therefore do not exist as titanium oxide particles in the solidified titanium ingot.

  In US7311873 (Process of Direct Powder Rolling of Blended Titanium Alloys, Titanium Matrix Composites, and Titanium Aluminides), titanium alloy powder containing at least one element is mixed with particles of carbide, nitride, oxide or the like. A method for producing a titanium-based composite material by sintering in a solid state after cold rolling is proposed. Here, since it does not go through the dissolving step, the above-mentioned added particles are also present in the state of particles without being dissolved or decomposed.

JP 2002-285268 A Japanese Patent No. 3426605 JP 2009-127083 A US7311873 Publication

  Since the demands for both high strength and high ductility for titanium and reduction of material costs are extremely strong, various studies have been conducted so far. In particular, from the viewpoint of cost reduction, many attempts have been made to increase the strength by using relatively inexpensive elements such as oxygen instead of expensive elements such as vanadium, scandium, and niobium. In the oxygen uptake technology by the dissolution method disclosed so far, oxygen expresses a strengthening action by solid solution in the titanium substrate. However, as the oxygen content in titanium by solid solution increases, titanium There is a problem that the ductility of the steel significantly decreases.

  The present invention has been made in order to solve the above-mentioned problems, and its object is to provide a titanium material that exhibits high strength without significantly reducing high ductility without adding expensive elements and substances. It is to be.

  The titanium material according to the present invention has a dense hexagonal lattice crystal structure in which atoms are arranged in the a-axis direction and the c-axis direction, and has the following characteristics.

  (A) Oxygen atoms are dissolved in titanium.

  (B) The oxygen content in titanium is 4000 ppm or more.

  (C) The value of the axial ratio c / a, which is the ratio of the lattice constant in the c-axis direction to the lattice constant in the a-axis direction, is in the range of 1.589 to 1.593.

A preferable upper limit of the oxygen content in titanium is 13000 ppm. Preferably, the titanium material is obtained by sintering titanium powder and TiO ₂ particles after mixing. As another embodiment, the titanium material may be obtained by sintering titanium powder and ZrO ₂ particles after mixing.

  The manufacturing method of the titanium material according to this invention includes the following steps.

(A) A step of preparing titanium powder and TiO ₂ particles.

(B) A step of mixing the titanium powder and the TiO ₂ particles by adjusting the addition amount of the TiO ₂ particles to 0.5% to 3.0% on a mass basis with respect to the entire mixed powder.

(C) A step of sintering the mixture in a temperature range from 700 ° C. to less than the melting point of TiO _{2 in} a vacuum atmosphere to thermally decompose the TiO ₂ particles and causing dissociated oxygen atoms to dissolve in titanium. .

  The titanium material manufacturing method may further include a step of heating and extruding the sintered body after the sintering.

  The operational effects or technical significance of the above characteristic configuration will be described in the following items.

It is a figure which shows the relationship between oxygen content and tensile strength. It is a figure which shows the relationship between oxygen content and ductility (elongation). It is a figure which shows the relationship between oxygen content and axial ratio c / a value. It is a figure which shows the relationship between an axial ratio c / a value and elongation value. It is a figure which shows the compression test result of the oxygen solid solution pure titanium material produced by the powder metallurgy method. It is a figure which shows the relationship between sintering temperature and axial ratio c / a value. It is the structure | tissue photograph of a sintered compact, (a) has shown the case where sintering temperature is 600 degreeC, (b) has shown the case where sintering temperature is 1000 degreeC. It is a figure which shows the X-ray-diffraction result of a sintered compact. It is a figure which shows the measurement result of the micro Vickers hardness of each sintered compact which varied the sintering temperature.

The feature of the present invention is that oxygen is not taken into the titanium material by a melting method, but a mixture of titanium powder and TiO ₂ particles is sintered in a vacuum atmosphere at a temperature from 700 ° C. to less than the melting point of TiO _2. By doing so, the TiO ₂ particles are thermally decomposed and the dissociated oxygen atoms are dissolved in titanium. When oxygen atoms are dissolved in titanium, the strength (tensile strength, compressive strength, hardness) of the titanium material is increased.

According to X-ray diffraction, in the dense hexagonal lattice crystal structure of titanium, as the amount of TiO ₂ particles added increases, the amount of oxygen atoms dissolved in the c-axis direction increases almost proportionally, but the oxygen in the a-axis direction increases. The amount of dissolved atoms remains almost constant. The strength of the titanium material can be increased by the solid solution of oxygen atoms in the c-axis direction. On the other hand, the elongation characteristic (ductility) exceeds 20% when the value of the axial ratio c / a, which is the ratio of the lattice constant in the c-axis direction to the lattice constant in the a-axis direction, is 1.593 or less. Elongation is obtained. Therefore, in a sintered material produced using a mixed powder composed of TiO ₂ particles and titanium powder by the solid phase sintering method, in order to achieve both high strength and high ductility, the value of c / a is 1.589. It is necessary to manage in the range of ~ 1.593.

  In the dissolution manufacturing method, even when the oxygen solid solution amount in titanium is in the range of about 2000 to 3000 ppm, the oxygen atom preferentially dissolves in the c-axis direction, so the value of the axial ratio c / a exceeds 1.594. As a result, ductility (elongation characteristics) is significantly reduced. The reason why oxygen atoms are dissolved in the c-axis is that in the dissolved (liquid phase) state, the interval between titanium atoms is larger than that in the solid phase state, and a larger number of oxygen atoms are likely to enter. Therefore, the lattice constant in the c-axis direction is increased in the melting material as compared with the sintered material.

  A titanium material obtained by the melting method and a titanium material obtained by the solid-phase sintering method show significant differences in strength and ductility when the solid solution oxygen content is about 3000 ppm or more.

  The titanium material obtained by the melting method shows the maximum tensile strength when the oxygen content is about 3100 ppm, and the tensile strength decreases as the oxygen content further increases. Regarding the elongation, when the oxygen content exceeds about 2200 ppm, it rapidly decreases.

  In the case of a titanium material obtained by the solid-phase sintering method, the tensile strength increases almost in proportion to the oxygen content in the range of the oxygen content up to about 13000 ppm, and the tensile strength increases as the oxygen content further increases. descend. The elongation gradually decreases with an increase in the oxygen content, but when it exceeds about 13,000 ppm, it rapidly decreases.

  In order to obtain a titanium material having strength characteristics that cannot be realized by the melting method by the solid phase sintering method, it is desirable that the oxygen content in titanium be 4000 ppm or more. In order to maintain good elongation characteristics, it is desirable that the oxygen content in titanium be 13000 ppm or less.

As an oxide showing the same effect as TiO ₂ , there is ZrO ₂ particles. In the case of ZrO ₂ , oxygen generated by thermal decomposition is dissolved in titanium, and at the same time, Zr (zirconium) takes a solid solution form with titanium, so that solid solution strengthening of Zr also occurs. As a result, further increase in strength can be realized. In order to obtain this effect, it is necessary to sinter at a temperature that maintains the solid phase state of the ZrO ₂ particles.

When mixing TiO ₂ particles and titanium powder, in order to achieve both the strength and ductility of the finally obtained titanium material, the amount of TiO ₂ particles added to the entire mixed powder is 0.5% on a mass basis. It is desirable to adjust to be -3.0%. When pure titanium powder is used as the titanium powder, a desirable lower limit of the amount of TiO ₂ particles added is 0.8%.

Further, it is desirable that the mixed powder is sintered in a vacuum atmosphere in a temperature range from 700 ° C. to less than the melting point of TiO ₂ . When sintering temperature exceeds 700 degreeC, the value of axial ratio c / a will increase rapidly. This means that the TiO ₂ particles were thermally decomposed and the dissociated oxygen atoms were dissolved in titanium. Therefore, in order to produce a titanium material having high strength and high ductility by oxygen solid solution from a mixture of TiO ₂ particles and titanium powder, a sintering treatment of the mixture at a temperature of 700 ° C. or higher is effective.

In order to promote the thermal decomposition of TiO ₂ , it is necessary to reduce the oxygen partial pressure in the atmosphere, so a vacuum atmosphere is desirable. However, once the atmosphere is filled with argon gas after being evacuated, the oxygen partial pressure can be maintained at a low state, so that thermal decomposition of TiO ₂ may proceed. Nitrogen and hydrogen are not preferable because they react with Ti to form a compound and reduce the ductility of the titanium powder sintered body.

  The titanium material is preferably obtained by heating and extruding a sintered body. The “titanium powder” prepared as the starting material powder may be a pure titanium powder or a titanium alloy powder.

  The inventors of the present application conducted the following tests in order to confirm the effect of the present invention.

[Preparation of mixed powder]
TiO ₂ particles (average particle size 3.1 μm) and pure titanium powder (purity 95% or more, average particle size 28.0 μm) were prepared. The added amount of TiO ₂ particles is 0.2%, 0.4%, 0.6%, 0.8%, 1.0%, 1.5%, 2.0% on the mass basis with respect to the entire mixed powder. 3.0%, 4.0%, and 5.0%, and pure titanium powder and TiO ₂ particles were mixed with a rotating ball mill for 1 hour.

[Sintering]
Using a discharge plasma sintering apparatus, each mixed powder was sintered under the following conditions.

Atmosphere: Vacuum atmosphere (4 Pa or less)
Temperature: 900 ° C
Applied pressure: 30 MPa
Holding time: 30 minutes [Production of extruded bar]
Each powder sintered body was heated for 5 minutes at 1000 ° C. in an argon gas atmosphere using an infrared image heating furnace, and then immediately extruded (extrusion ratio: 36) to produce an extruded bar having a diameter of 7 mm.

[Oxygen content, tensile strength, ductility (elongation)]
For each extruded material, the oxygen content was measured, and further a tensile test and a compression test (both strain rates: 5 × 10 ⁻⁴ s ⁻¹ ) were performed. As a comparative material, a pure titanium material having a different oxygen content was prepared by a melting method, and an extruded bar material having a diameter of 7 mm was similarly formed by the above-described extrusion process.

[Tensile test results]
The results of the tensile test are shown in FIGS. FIG. 1 shows the relationship between oxygen content and tensile strength (UTS), and FIG. 2 shows the relationship between oxygen content and ductility (elongation).

  Tables 1 and 2 show the numerical data on which the graphs of FIGS. 1 and 2 are based.

As shown in FIG. 1, in the powder sintered material, the tensile strength (UTS) is linearly proportional to the amount of oxygen in the range of up to 3 mass% (oxygen content: 12230 ppm) of TiO ₂ particles. The maximum value reached 1287 MPa. After reaching the maximum value, the tensile strength tended to decrease as the amount of oxygen increased.

  As shown in FIG. 2, in the powder sintered material, the elongation gradually decreases with an increase in the amount of oxygen, but rapidly decreased when the amount exceeded 12230 ppm. That is, it is recognized that the extrusion material becomes brittle due to the decrease in ductility (elongation), and as a result, the tensile strength also decreases.

  On the other hand, in the melting material, the tensile strength reached the maximum value when the oxygen content was 3090 ppm, and then decreased. The elongation gradually decreases with an increase in the amount of oxygen, but suddenly decreased when it exceeded 2240 ppm.

As can be understood from the results shown in FIG. 1 and FIG. 2, the powder sintered material can dissolve more oxygen than the dissolved material, and as a result, the tensile strength is the dissolved material. The value was significantly higher than that. In addition, the oxygen content at which the elongation of the sintered material decreases rapidly is higher than that of the melted material. Therefore, a pure titanium material produced by powder metallurgy using TiO ₂ particles can maintain high strength and high ductility as compared with a melt production material.

[Axial ratio c / a]
Regarding the oxygen solid solution, the solid solution state of oxygen atoms in titanium is determined by the change (increase) in the lattice constant in each of the a-axis and c-axis directions of titanium having a dense hexagonal lattice crystal structure by X-ray diffraction. Can be evaluated quantitatively. The relationship between the oxygen content and the value of the axial ratio c / a, which is the ratio of the lattice constant in the c-axis direction to the lattice constant in the a-axis direction, was arranged. The result is shown in FIG.

  Tables 3 and 4 show the numerical data used as the basis for the graph of FIG.

  As shown in FIG. 3, in the powder sintered material, the oxygen content and the value of the axial ratio c / a are in a proportional relationship, but in the dissolved material, the c / a value after the oxygen content exceeds 3090 ppm. Increased rapidly.

  FIG. 4 shows the relationship between the c / a value and the elongation value. Tables 5 and 6 show numerical data on which the graph of FIG.

  As shown in FIG. 4, regardless of the production method of the extruded material, the elongation value rapidly decreased in the range where the c / a value exceeded 1.594. From this result, it is recognized that it is effective to manage the c / a value to 1.593 or less in order to simultaneously develop an elongation exceeding 20% in a pure titanium material in which oxygen atoms are dissolved. In particular, in a pure titanium material, in order to achieve both high strength of 900 MPa or more and high ductility, it is desirable that the c / a value be in the range of 1.589 to 1.593.

[Results of compression test]
FIG. 5 shows a compression test result of an oxygen solid solution pure titanium material produced by a powder metallurgy method.

As the amount of TiO ₂ particles added increased, both the compressive yield strength (0.2% YS) and the stiffness increased, confirming the effect of improving the compressive strength due to oxygen solid solution.

[Sintering temperature]
Mixed powders composed of pure titanium powder (purity 95% or more, average particle diameter 28.0 μm) and 1 mass% TiO ₂ particles (average particle diameter 3.1 μm) are sintered differently in a vacuum atmosphere (oxygen partial pressure: 4 Pa). The bar was solidified at the setting temperature and extruded under the same conditions to produce an extruded bar with a diameter of 7 mm.

FIG. 6 shows the relationship between the sintering temperature and the c / a value. As shown in FIG. 6, when the sintering temperature exceeds 700 ° C., the c / a value suddenly increases, so that the TiO ₂ particles are thermally decomposed and the dissociated oxygen atoms are dissolved in titanium. It is recognized that Therefore, in order to produce a high-strength and high-ductility pure titanium material by oxygen solid solution from a mixture of TiO ₂ particles and pure titanium powder using powder metallurgy, sintering at 700 ° C. or higher in a vacuum atmosphere is required. It is valid.

FIG. 7 is a structural photograph of the sintered body, where (a) shows the case where the sintering temperature is 600 ° C. and (b) shows the case where the sintering temperature is 1000 ° C. At a sintering temperature of 600 ° C., TiO _{2 as} a starting material exists in an unreacted state, whereas at a sintering temperature of 1000 ° C., TiO ₂ particles do not exist and heating in a vacuum atmosphere is performed. Thus, it was confirmed that the TiO ₂ particles were thermally decomposed.

FIG. 8 is a diagram showing an X-ray diffraction result of the sintered body. When sintered at a temperature of 600 ° C., a peak of TiO ₂ has been detected, TiO ₂ is unreacted is the starting material. In contrast, when sintering was performed at a temperature of 1000 ° C., no diffraction peak of TiO _2, it can be confirmed that the TiO ₂ is thermally decomposed.

  FIG. 9 shows the measurement results of the micro Vickers hardness of each sintered body with different sintering temperatures. The one having a sintering temperature of 0 ° C. is the hardness of the base of the pure titanium raw material powder. In the sintering up to a temperature of 600 ° C., the hardness is not greatly increased. However, when the sintering temperature is 800 ° C. or higher, the hardness is remarkably increased, and it is recognized that strengthening by oxygen solid solution occurs.

[Use of titanium alloy powder]
The “titanium powder” prepared as the starting material powder is not limited to pure titanium powder, and may be titanium alloy powder. The inventor of the present application has confirmed that the same effect can be obtained in a titanium material obtained by mixing and sintering a titanium alloy powder and TiO ₂ particles. Specifically, the following measurement results were obtained.

(A) In the case of extruded material of Ti-6% Al-4% V powder Tensile strength (TS): 1156 MPa
Yield strength (YS): 1107 MPa
Elongation: 26.3%
(B) In the case of extruded material of Ti-6% Al-4% V powder + 0.5 mass% TiO ₂ particles Tensile strength (TS): 1308 MPa
Yield strength (YS): 1172 MPa
Elongation: 23.3%

  INDUSTRIAL APPLICABILITY The present invention can be advantageously used as a titanium material that can be used in a wide range of fields such as aircraft, railway vehicles, automobile parts, household electrical appliance materials, architectural structural members, and medical materials, and a method for producing the same.

Claims (6)

a titanium material having a dense hexagonal lattice crystal structure in which atoms are arranged in an a-axis direction and a c-axis direction,
Oxygen atoms are dissolved in titanium,
The oxygen content in titanium is 4000 ppm or more,
A titanium material in which the value of the axial ratio c / a, which is the ratio of the lattice constant in the c-axis direction to the lattice constant in the a-axis direction, is in the range of 1.589 to 1.593.   The titanium material according to claim 1 whose oxygen content in titanium is 13000 ppm or less. The titanium material according to claim 1 or 2, wherein the titanium material is obtained by sintering and extruding after mixing titanium powder and TiO ₂ particles. The titanium material according to claim 1 or 2, wherein the titanium material is obtained by mixing and sintering titanium powder and ZrO ₂ particles and then extruding. Preparing titanium powder and TiO ₂ particles;
Adjusting the amount of TiO ₂ particles added to the entire mixed powder to be 0.5% to 3.0% on a mass basis and mixing the titanium powder and the TiO ₂ particles;
Sintering the mixture in a temperature range from 700 ° C. to less than the melting point of TiO _{2 in} a vacuum atmosphere to thermally decompose the TiO ₂ particles and dissolving dissociated oxygen atoms in titanium. , Manufacturing method of titanium material.   The method for producing a titanium material according to claim 5, further comprising a step of heating and extruding the sintered body after the sintering.