JP5210874B2 - Cold workable titanium alloy - Google Patents

Description

  The present invention relates to a cold workable α + β type titanium alloy and a heat treatment method for producing a cold workable α + β type titanium alloy. The present invention further relates to a method of using the heat-treated α + β type titanium alloy for producing a titanium alloy structural member by cold working.

  The use of titanium alloy structural members is becoming increasingly attractive in various technical fields. The reason for this attraction is that titanium alloys, in particular, have a low specific gravity and at the same time high strength and low corrosion sensitivity.

  Titanium alloys are basically classified into so-called α-type, α + β-type, and β-type titanium alloys according to the constituent phases at room temperature. Pure titanium exists in an α phase (hexagonal crystal structure) at room temperature, and transforms into a body-centered cubic β phase at about 890 ° C. (β transformation point). The transformation temperature of the titanium alloy is not only influenced by the type and amount of the alloy inclusions, but also by the mechanical, chemical or thermal pretreatment of the titanium alloy.

  By adding a specific alloy element, the α phase can be stabilized over a wider temperature range (increase in β transformation point). When other alloy elements are added, a β phase is generated and stabilized (decrease in β transformation point). Therefore, these alloy elements can be divided into so-called α-stabilizing elements and β-stabilizing elements. Technically used α-stabilizing elements are, for example, oxygen, nitrogen, carbon or aluminum. The β-stabilizing elements used technically today are, for example, hydrogen, vanadium, molybdenum, iron, chromium, copper, palladium or silicon.

  Alloys rich in α-phase usually have lower strength than alloys rich in β-phase. The specific gravity of a titanium alloy containing a large amount of β phase is usually larger than the specific gravity of a titanium alloy containing a large amount of α phase. Since the cubic lattice of β titanium has a large number of sliding surfaces, the β phase has better cold workability than the α phase. Alloys used technically usually have one compromise: adjusting the proportions of α and β phases by appropriate mixing of α and β stabilizing elements, for example, the desired production characteristics for structural members, Strength values and corrosion resistance are obtained.

  An unresolved problem in manufacturing structural members from titanium alloys is the lack of variety of available manufacturing methods compared to other metal materials or plastics. Titanium alloys are usually very expensive for welding and hot working. In addition, technically usable titanium alloys can only be cold worked in a very narrow range. “Cold workable” means that the material can be molded at room temperature without causing a significant drop in strength or cracking during the molding.

  Welding, hot working and cold working also have a significant effect on the strength of the structural member thus produced, so structural members that are subject to high stress or that are in safety-related locations are It is processed only by the method. Accordingly, when manufacturing a titanium structural member, it is necessary to rely on manufacturing by a wide and costly cutting process. Thus, today, titanium alloy structural members are limited to almost expensive products such as aviation, especially military aviation and medical fields.

  According to the Russian patent RU22111873, the following titanium alloys are known. That is, this titanium alloy contains 1.5 to 3.0% by weight of aluminum (all percentages are expressed in terms of weight%), 4.5 to 8.0% of molybdenum, and 1.0 of vanadium. -3.5% and iron containing 1.5-3.8%. This alloy is manufactured on the basis of a relatively inexpensive alloy element, reaches a specific ratio of strength and ductility after heat treatment, and can be used to manufacture several types of fixing members and springs. The main drawback of this alloy is that the heat treatment necessary to obtain these properties is expensive.

  According to Russian Patent RU1584408, the following titanium alloys are known. That is, this titanium alloy has 1.2 to 3.8% aluminum, 5.1 to 6.5% molybdenum, 4.0 to 6.5% vanadium, 0.01 to 0.05% silicon, Contains 0.005 to 0.015% hydrogen. This alloy has increased ductility during multi-stage forming and is used in the manufacture of rivets. However, this alloy has insufficient strength characteristics for a structural member subjected to high stress.

  According to US Pat. No. 4,842,652, the following method for heat treating an α + β type titanium alloy (Ti-6426) is known. That is, hot forging at a temperature higher than the β transformation point, solution treatment of the forged alloy for 1 to 4 hours in the region below the β transformation point but below the β transformation point minus 50 ° C., water cooling at room temperature, or a temperature of 100 to It is a method comprising transfer to a salt bath at 1400 ° F. (204 to 760 ° C.) and tempering (precipitation treatment) at a temperature of 1100 to 1200 ° F. (593 to 650 ° C.) for 2 to 16 hours. Such heat treatment makes it possible to improve the fracture strength and low vibration strength of the α + β type titanium alloy, but at the same time, loses the plastic workability at room temperature.

  Finally, according to US Pat. No. 5,679,183, the following method for heat treating an α + β type titanium alloy is known. That is, in this method, hot working with a working degree of 30% or more is performed in the α + β phase region, followed by processing for 60 minutes in the temperature region of the β transformation point minus 55 to 10 ° C., air cooling, and this air cooled titanium alloy. Subsequently, heat treatment is performed for 60 minutes in the temperature range of β transformation point minus 250 to 120 ° C. and air cooling. Such a method of heat treatment can increase the fracture strength without noticeably worsening the ductility. However, the ductility of the alloy is not sufficient to allow the alloy to be processed into a normal product shape at room temperature.

  However, in view of the characteristics of the particularly advantageous titanium alloy as mentioned at the beginning, the application of titanium alloy structural members to many other technical fields, especially to the field of mass-produced products that require advantageous manufacturing techniques. The application of would be intriguing.

  Therefore, the subject of this invention is providing the titanium alloy which can be processed cheaply, and the cheap processing method of this titanium alloy.

  This problem is solved by the following titanium alloy. That is, the titanium alloy has about 2 to 4.0% by weight of aluminum, about 4 to 5.5% by weight of vanadium, about 4.0 to 6.0% by weight of molybdenum, and about 0.5 to 1 of zirconium. 0.5% by weight and about 0.5 to 1.5% by weight of tin.

  This titanium alloy according to the invention is suitable, in part, for processing by cold working without any special heat treatment in advance, ie directly after semi-finished product production, for example by hot rolling. However, the α + β-type titanium alloy that can be cold-worked is not limited to this, and the following heat treatment method according to the present invention can be used to obtain a higher cold workability and higher strength of the cold-worked structural member at the same time. Suitable for use. The combination of the α + β type titanium alloy subjected to this kind of alloying and the heat treatment method according to the present invention gives particularly good results in terms of cold workability and strength of the manufactured structural member.

  By alloying the above alloy elements at the above weight ratios, an α + β type mixed phase structure at normal temperature, which is particularly advantageous for the cold workability of the semi-finished product and the strength of the product, especially this alloy is treated by the heat treatment method according to the present invention. Obtained when. Tin and zirconium are neutral substitutional alloy elements, and their addition causes effective solid solution strengthening. If the content of tin and zirconium is 0.5% or less, alloy strengthening does not occur. The optimum content of tin and zirconium in the alloy is 0.5 to 1.5% by weight. Such a concentration increases the strength of the alloy by solid solution strengthening of the α phase and the β phase, but the ductility of the alloy hardly changes. If the tin and zirconium contents are clearly higher than 1.5% by weight, the ductility of the alloy deteriorates.

  In addition, it has been found advantageous if the titanium alloy contains about 0.1 to 0.4 weight percent oxygen. This addition of alloying elements has proved advantageous to obtain the cold workability and strength of the heat treated titanium alloy. Oxygen is an element having a strong α-stabilizing action. As the oxygen content in the alloy increases, the alpha phase proportion increases and an interstitial solid solution is formed resulting in significant work hardening. The optimum oxygen content in the alloy is 0.1 to 0.4% by weight. Such oxygen content does not cause a large change in the α phase ratio (about 3 to 5%), but an improvement in the strength and overall strength level of the alloy is obtained, and the ductility is hardly reduced.

The object of the present invention is solved by the heat treatment method described at the beginning and including the following steps.
1. The titanium alloy is annealed at a low annealing temperature, that is, at a transformation point (β transformation point) minus 160 to 230 ° C.
2. Cool the titanium alloy to room temperature.

  The present invention is based on the following recognition. That is, the alloys known from US Pat. No. 5,679,183 recognize that heat treatment does not increase the existing α particles, but produces new particles with a fine layered morphology. The biphasic structure formed by such a method has a notable fracture strength, but at the cost of significantly reducing ductility. Next, when heating is performed in a temperature range of β transformation point minus 250 to 120 ° C., the distributed precipitation is expanded, and an increase in ductility is obtained. However, the tissue remains biphasic because it consists of a small amount of β-phase and α-phase particles of different morphology (geometrically uniform and layered). Such a structure is not a starting point for providing ductility at room temperature.

  The present invention is based on the recognition that spherical structures exhibit a good combination of strength and ductility. A spherical structure is obtained in an α + β type titanium alloy after deformation in a two-phase region near the β transformation point. However, the balance between strength and ductility depends on the size of the components of the tissue. The finer the α-phase precipitation is geometrically and uniformly, the higher the strength and the lower the ductility. If the α-phase spherical particles are very large, the increase in ductility is small, but the strength and fracture resistance are significantly reduced.

  Therefore, in order to perform forming at room temperature and achieve a sufficient workability for a typical product shape, particularly 60% or more, it is necessary to select the chemical composition of the alloy correctly and reduce the strength so much. There is a need for a heat treatment method to obtain a harmonious increase in ductility without reducing it.

  According to the heat treatment method of the present invention, it is possible to obtain an α + β-type titanium alloy that exhibits high ductility on the one hand and very little work hardening during forming on the other hand. Heating of the titanium alloy to a low annealing temperature can be performed at various heating rates. In order to prevent the formation of stress cracks, heating at a slow heating rate of 20 ° C. or less per minute is preferably selected. The titanium alloy is preferably annealed in an inert atmosphere. This is to prevent an element having an embrittlement action (for example, oxygen, nitrogen or carbon) from diffusing into the titanium alloy.

  It is also preferable to cool the titanium alloy to ambient temperature in an inert atmosphere. The α + β type titanium alloy, like many metal materials, can be hardened by quenching (quenching) from the annealing temperature. However, this effect is not desirable when it is desired to produce a titanium alloy material with good cold workability. Therefore, it is preferable that the cooling rate be a low rate that avoids quenching of the titanium alloy.

  In the first advantageous embodiment, the titanium alloy is annealed at a low annealing temperature for more than 5 hours, in particular about 7 hours. This annealing time mainly depends on the dimensions of the structural member to be annealed. A titanium alloy structural member usually does not have a wall thickness of 20 mm or more. If the annealing time is 5 hours or longer, particularly in the case of a round bar having a diameter of 10 to 20 mm, for example, a desired phase structure is obtained, and thus a desired cold workable titanium alloy semi-finished product is obtained. Seven hours of annealing proved to be advantageous for reliable results reproduction.

The method according to the invention can be developed as follows, and the following steps can be carried out before annealing at a low annealing temperature.
1. The titanium alloy is annealed at a high annealing temperature, that is, at a transformation point (β transformation point) minus 50 to 100 ° C., particularly at a transformation point (β transformation point) minus 60 to 100 ° C.
2. Cool the titanium alloy to a low annealing temperature.

  As a first stage of tempering, a temperature region of β transformation point minus 50 to 100 ° C. is selected. The alloy structure is characterized by discrete α-phase spherical particles placed in a β-matrix at this temperature. If the temperature is kept constant at this temperature, the excess (secondary) α-phase is eliminated, and not only the equilibrium state of the α-phase and the β-phase is approached, but also structural defects are reduced in the process of realizing polygonization. After completion of the isothermal holding, the alloy is cooled at a cooling rate of 0.01 to 0.02 ° C./second to a temperature of β transformation point minus 160 to 230 ° C. With such a cooling rate, new α-phase particles are not formed from the β matrix during cooling, and the existing primary α crystal can grow in the structure. The homogenization annealing process can be completed by keeping the temperature constant for 3 to 6 hours in the second stage of tempering. Subsequently, cooling to room temperature at a cooling rate of 2.5 to 3.5 ° C./second is sufficient to prevent the precipitation of the secondary α phase.

  By performing this two-stage tempering, the particle size of the α-phase particles is increased from 1 to 2 μm to 5 to 7 μm, and a β-phase composition corresponding to [Mo] eq = 14 to 15 can be obtained. In addition, it is possible to reduce the defect density in the α phase generated in the process of realizing the polygonization process in the first tempering stage. The phase configuration for obtaining good cold workability can thereby be further optimized. The pre-annealing step is preferably performed in an inert atmosphere. Further, as described above, when cooling the titanium alloy, care must be taken so that the cooling rate does not cause stress cracking.

  This development can be further optimized. That is, the titanium alloy is annealed at a high annealing temperature for 1 hour or longer, particularly about 2 hours. Again, the annealing time depends on the dimensions of the titanium alloy semi-finished product. It has been demonstrated that an annealing time of 1 hour or more, especially 2 hours, can reliably reproduce the desired phase configuration.

  Furthermore, in this development of the invention, it is advantageous if the titanium alloy is annealed at a low annealing temperature for more than 3 hours, preferably 3 to 6 hours, particularly preferably about 4 hours. If annealing has been performed at a high annealing temperature in advance, the annealing time at the low annealing temperature necessary to obtain a reliable phase configuration can be reduced depending on the desired purpose. For semi-finished products of normal dimensions, for example round bar material with a diameter of 10-20 mm, it has been demonstrated that 3 hours or more, in particular 4 hours, is sufficient.

  It is particularly advantageous to air cool the titanium alloy from a high annealing temperature to a low annealing temperature at a cooling rate of 0.01 to 0.02 ° C./min. This cooling rate avoids undesirable phase composition, internal stress, and undesirable amounts of alloying element precipitation.

  It is particularly advantageous for the high annealing temperature to be about 770-830 ° C., in particular 800 ° C. This temperature range has proven effective for most α + β type titanium alloys conventionally used in the art.

  The method of the present invention can be further developed to air cool the titanium alloy from a low annealing temperature to room temperature at a cooling rate of about 2.5 to 3.5 ° C./min. This cooling rate prevents undesirable precipitation of alloying elements and undesirable phase formation, and provides optimum results in terms of cold workability and the strength of the cold worked structural member.

  The method according to the invention can also be used advantageously when the titanium alloy is processed by hot rolling before heat treatment. Hot rolling is one method for producing, for example, a semi-finished product or a semi-finished metal plate from a titanium alloy. The structure is affected by hot rolling. Tissues affected by this method are particularly suitable for the heat treatment step according to the invention.

  It is further advantageous if the low annealing temperature is about 670-730 ° C., in particular 700 ° C. This annealing temperature has proven effective for most α + β type titanium alloys conventionally used in the art.

  Furthermore, it is advantageous for the method according to the invention if at least one α-stabilizing element and at least one β-stabilizing element are added to the titanium alloy. By adding such an alloy element, a titanium alloy in which the ratio of the α phase and the β phase is optimized for a specific application can be manufactured. The proportion of stabilizing alloying elements must be adjusted in accordance with the method according to the invention so that the desired cold workability for the semi-finished product and the desired strength for the cold worked structural member are obtained.

  Furthermore, the method according to the invention removes the surface layer of the titanium alloy mechanically, in particular by cutting, after annealing with a low annealing temperature and / or after annealing with a high annealing temperature. Annealing treatment often has some effect on the surface of the titanium alloy semi-finished product, even if performed in an inert atmosphere. This effect results in embrittlement and increased cracking susceptibility of the semi-finished product, thereby reducing the cold workability and reducing the strength of the cold work product. In order to deal with these disadvantageous effects, the peripheral layer of the affected semi-finished product is removed before cold working. For this, cutting is particularly suitable.

  Another aspect of the present invention is to manufacture a titanium structural member by cold working from the α + β type titanium alloy heat-treated as described above. Thereby, a structural member can be manufactured at low cost by mass production from a titanium alloy. This is worth pursuing for a large number of structural members, for example in the automotive field, in particular for structural members used as movable structural members in the drive train.

  In this case, the method according to the invention is particularly useful for the production of titanium screws by cold heading and / or thread rolling. This application is suitable, for example, for the production of wheel bolts in the automotive field. The use of titanium alloy wheel bolts has the advantage that, on the one hand, the inertial force of the wheel can be reduced, driving performance and spring comfort can be improved, and on the other hand, the fuel consumption of the automobile can be reduced. The use of titanium bolts has the advantage that contact corrosion, which often occurs when using, for example, steel bolts, in particular in combination with light alloy wheels made of aluminum alloy or magnesium alloy, is advantageously avoided.

Another aspect of the present invention is a method for manufacturing a titanium bolt including the following steps.
-Manufacture round bar material by hot rolling.
-This round bar material is heat-treated by the heat treatment method described above.
-Form bolt heads by cold heading.
-Form the thread by thread rolling.

  This method makes it possible to manufacture titanium bolts as mass production structural members at a particularly low cost in terms of manufacturing technology. This titanium bolt achieves and exceeds the strength values defined in DIN classification 8.8 and is therefore suitable for use as a wheel bolt, for example.

  The titanium alloy of the present invention including an α phase portion and a β phase portion is characterized in that the size of α phase particles is about 5 to 7 μm. This titanium alloy preferably contains an alloy element that provides [Mo] eq = 14 to 15 as a molybdenum equivalent. This molybdenum equivalent is a numerical value calculated from the state and amount of the alloy composition, and is usually 0 to 2.5 for an α-type titanium alloy, 2.5 to 10 for an α + β-type titanium alloy, and β-type titanium. In the case of an alloy, it is 10 or more.

  Examples of procedures and alloy compositions are described below as a particularly preferred embodiment of the present invention.

  As the α + β type titanium alloy, a known alloy Ti-3.0Al-4.5V-5.0Mo is used. After this alloy is formed, a round bar material having a diameter of 13 mm, for example, is manufactured by hot rolling. This semi-finished product is commercially available for normal lengths.

The semi-finished product is subjected to the following heat treatment in an annealing furnace.
Heat to -800 ° C.
Annealing at −800 ° C. for 2 hours.
-Cool to 770 ° C at a rate of 0.02 ° C per second.
Annealing at -770 ° C for 30 minutes.
-Cool to 740 ° C at a rate of 0.02 ° C per second.
Anneal at −740 ° C. for 30 minutes.
-Cool to 700 ° C at a rate of 0.02 ° C per second.
Annealing at −700 ° C. for 4 hours.
-Remove the semi-finished product from the annealing furnace and cool to ambient temperature in the atmosphere.

  The semi-finished product thus treated can subsequently be further processed. For example, a screw head is generated by cold heading, and a screw thread is generated by screw rolling at room temperature. Optionally, the peripheral layer of the semi-finished product can be removed by machining before further processing.

  An alloy was produced by two vacuum melting methods using a consumable electrode. The chemical composition of the alloy is Ti-3.0% Al-5.0% Mo-4.5% V-1.0% Zr-1.0% Sn-0.25% O (β transformation point). Is 880 ° C.).

  The obtained ingot having a weight of 8 kg was isothermally forged at a temperature in the β region into a 90 × 90 mm rectangular parallelepiped, and then die forged to a height of 45 mm. Next, this ingot was cut into a filament having a square cross section of 45 × 45 mm and forged at a temperature in the α + β region to obtain a rod having a diameter of 30 mm. This bar was shaved with a lathe until the diameter reached 25 mm. The semi-finished product thus obtained was rolled in a temperature range of β transformation point minus 50 to 100 ° C. until the diameter became 16 mm. Initial heating at a predetermined temperature was for 30 minutes. Heating during the next process was for 4 minutes. The total rolling ratio was 65%.

  After rolling, a bar with a diameter of 16 mm was heat-treated in a temperature range of 860 to 780 ° C. for 2 hours, then cooled to 700 ° C. (β transformation point minus 190 ° C.) at a cooling rate of 0.02 K / second, and kept constant for 4 hours. Retained. Cooling to room temperature was performed at a cooling rate of 3 K / sec.

  This bar was shaved with a lathe to a diameter of 13 mm.

  A bar with a diameter of 16 mm was produced in the same manner as in the above two examples. After rolling, a bar with a diameter of 16 mm was heat-treated in a temperature range of 860 to 780 ° C. for 2 hours, and then air-cooled to room temperature. Next, the bar was heated to a temperature of 700 ° C. (β transformation point minus 190 ° C.) and held for 4 hours. Cooling to room temperature was performed in air.

  This bar was shaved with a lathe to a diameter of 13 mm.

  Further advantageous processes of the heat treatment according to the invention are shown in Tables 1 to 3.

  The table below compares the advantageous properties of the present invention with the commonly used alloys Ti-3.8% -6.5% V-5.1% Mo-0.01% H-0.05% Si. Indicates.

^*第２段階への冷却は、冷却速度０.０２℃／秒で行った。
^**熱処理終了後の冷却は、冷却速度３℃／秒で行った。

^* Cooling to the second stage was performed at a cooling rate of 0.02 ° C./sec.
^** Cooling after the heat treatment was performed at a cooling rate of 3 ° C./second.

^*第１段階後、常温への冷却は大気中で行った。
^**熱処理終了後の冷却は空気中で行った。

^{* After} the first stage, cooling to room temperature was performed in air.
^** Cooling after heat treatment was performed in air.

  The addition of tin and zirconium can increase the strength, but the plasticity remains at a high level (Table 1). Along with the hot working and heat treatment conditions described in the claims when compared with the prior art (Table 2), it is possible to realize deformation with a working degree of 60% or more by press working at room temperature (Table 1). ).

Claims (15)

A step of annealing the varying state point (beta transus) minus 160 to 230 ° C. 3 hours or more titanium alloys at a lower annealing temperature is a temperature of,
A heat treatment method for producing a cold can alpha + beta type titanium alloy comprising the steps of cooling the titanium alloy to room temperature,
Before annealing at the above low annealing temperature,
Annealing the titanium alloy at a high annealing temperature of a transformation point (β transformation point) minus 60-100 ° C. for 1 hour or more;
A heat treatment method comprising cooling the titanium alloy to the low annealing temperature . The heat treatment method according to claim 1 , wherein the titanium alloy is annealed at the high annealing temperature for 2 hours. In the heat treatment method according to claim 1 or 2, a heat treatment method characterized by the titanium alloy, for moderation-out sintered between time 3-6 above lower annealing temperatures. 4. The heat treatment method according to claim 2, wherein the titanium alloy is air-cooled from the high annealing temperature to the low annealing temperature at a cooling rate of 0.01 to 0.02 [deg.] C./min. In the heat treatment method according to any one of claims 2 to 4, a heat treatment method characterized by the high annealing temperature is 7 seventy to eight hundred and thirty ° C.. Claim 1 in the heat treatment method according to any one of 5, the cooling speed 2 the titanium alloy from the lower annealing temperature. A heat treatment method comprising air-cooling to room temperature at 5 to 3.5 ° C./min. The heat treatment method according to any one of claims 1 to 6 , wherein the titanium alloy is processed by hot rolling before the heat treatment. The heat treatment method according to any one of claims 1 to 7 , wherein the low annealing temperature is 670 to 730 ° C. The heat treatment method according to any one of claims 1 to 8 , wherein the titanium alloy is alloyed by adding at least one α-stabilizing element and at least one β-stabilizing element. Heat treatment method. In the heat treatment method according to claim 9, said titanium alloy, aluminum from 2 to 4.0 wt%, vanadium from 4 to 5.5 wt%, 4 molybdenum. A heat treatment method comprising 5 to 6.0% by weight , 0.5 to 1.5% by weight of zirconium and 0.5 to 1.5% by weight of tin, with the balance being titanium and impurities . Wherein in the heat treatment method according to any one of claims 1-10, after it was annealed at least one of the lower annealing temperature and the high annealing temperature, that mechanically dividing removed by the surface layer of the titanium alloy And how to. A method comprising using the α + β-type titanium alloy heat-treated by the heat treatment method according to any one of claims 1 to 11 for manufacturing a titanium structural material by cold working. 13. The method according to claim 12 , wherein the method is used for producing a titanium bolt by at least one of cold forging and thread rolling. Manufacturing round bar material by hot rolling,
The round bar material is heat-treated by the method according to any one of claims 1 to 13 ,
A method of manufacturing a titanium bolt, wherein a head of a bolt is formed on the round bar material by cold heading. The manufacturing method according to claim 14 , wherein a thread is formed on the round bar material by thread rolling.