JP3036396B2 - Method for producing near β type titanium alloy - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a titanium alloy used as a lightweight and high-strength material, and more particularly to a method for producing a near beta type titanium alloy having high strength and high toughness.

[0002]

2. Description of the Related Art Near β type titanium alloy is Ti-6Al-
High strength and high toughness can be obtained as compared with an α + β type titanium alloy represented by 4V. In addition, since the amount of alloying elements is smaller than that of the β-type titanium alloy and the hot deformation resistance is small, it is excellent in isothermal forgeability. Combining these excellent properties
Near β-type titanium alloys are attracting attention in the field of aircraft mainly as a substitute for Ti-6Al-4V which is a typical titanium alloy so far. Commercially available near β type titanium alloys that have been developed include Ti-10
V-2Fe-3Al or Ti-5Al-2Sn-2Zr-
4Mo-4Cr and the like.

[0003] In these Near β type titanium alloys, martensitic transformation does not occur even by rapid cooling from the β single phase region,
Becomes β single phase structure. This structure is a metastable β phase, contains a large amount of ω phase, and shows a complex phase transformation such as a martensitic transformation caused by processing. Further, precipitation hardening occurs by the subsequent aging treatment, and the strength is improved. During low-temperature aging, an aged ω phase is formed in the α phase, but during long-term aging or high-temperature aging in a medium to high temperature range, a stable structure is formed in which the α phase is precipitated in the β phase, which is the parent phase.

[0004] In such a Near β type titanium alloy, a treatment for toughening is indispensable. The mechanism of toughening by the general process is based on the solution treatment in the α + β2 phase region below the β transformation point and the subsequent aging treatment.
A fine α phase is precipitated in the β phase. Further, there are the following improved toughening methods.

[0005] Anisotropic fracture toughness is reduced by processing a Near β type titanium alloy in the α + β range during cooling from the β range and then reheating to the α + β range (Japanese Patent Laid-Open No. 63-105954). No.). The first-stage solution treatment is performed in the high temperature range of α + β, the second-stage solution treatment is performed at a lower temperature, and then the low-temperature aging is performed to improve the strength and the fracture toughness (Japanese Unexamined Patent Publication No. -217452). β transformation point-(15 to 40) ° C. for 30 minutes or more, then cooled to a high temperature by furnace cooling or air cooling,
Thereafter, after the same heating and holding, the mixture is cooled to room temperature by water cooling and aged (AMS standard 4983).

[0006]

As described above, several methods have been proposed to improve the strength and fracture toughness of the near β type titanium alloy, but all of them carry out the solution treatment in the α + β2 phase high temperature region. Including these, conventional α + β
The solution treatment in the two-phase region improves the strength-toughness balance, which is a feature of the present alloy system, and does not consider ductility. Also, no consideration is given to the homogeneity of the tissue. Therefore, when the solution treatment temperature is increased, the ductility is reduced and the structure becomes inhomogeneous. Due to these restrictions, the solution treatment temperature can be suppressed to be lower than the immediately preceding treatment temperature even though the temperature is high as shown in FIG. Is customary,
Therefore, the strength-toughness balance was also restricted.

That is, in the conventional solution treatment in the α + β2 phase high temperature region, the higher the treatment temperature, the more the subgrain grows appropriately, and the strength-toughness balance is improved. However, when the solution heat treatment temperature is just below the β transformation point,
The β grains are coarsened because the α phase is significantly reduced. Also,
When the temperature is equal to or higher than the β transformation point, a β single phase state is established. Therefore, in each case, the structure is based on very coarse β grains. As a result, although the strength-toughness balance is improved, the ductility is significantly reduced, and the reliability as an industrial material is greatly reduced.

On the other hand, even when the solution treatment temperature is lower than immediately below the β transformation point, partial β transformation due to segregation occurs as the β transformation point is approached, so-called β flecks are generated. As a result, a heterogeneous structure that can be easily recognized as a macro structure at the wall limit is generated. When such an inhomogeneous structure is generated, regions having different mechanical properties are formed, so that the reliability of the material is greatly reduced, and it is difficult to apply the material to aircraft materials in which safety is particularly important.

As described above, in the conventional solution treatment in the α + β2 phase region, when the treatment temperature is increased in order to improve the strength-toughness balance, the ductility is reduced due to the coarsening of β grains and the imperfections due to the formation of β flecks are caused. Since a homogeneous structure was generated, the processing temperature had to be lowered and the strength-toughness balance was sacrificed. In addition, when the solution treatment temperature is increased at present, ductility is reduced and the structure is heterogeneous. Therefore, a member having a good strength-toughness-ductility balance and uniform characteristics could not be obtained.

An object of the present invention is to suppress the coarsening of β grains and the formation of β flecks, and to stably perform a solution treatment in the α + β2 phase region at a high temperature to obtain a high strength-toughness-ductility balance and Ne that can obtain a homogeneous member
An object of the present invention is to provide a method for producing an arβ type titanium alloy.

[0011]

Means for Solving the Problems Near β type titanium alloy is finally subjected to a solution treatment and an aging treatment in an α + β2 phase region as a general process to form a structure composed of a proeutectoid α phase and a β phase. I do. At this time, in order to improve strength and fracture toughness while securing sufficient ductility, it is necessary to raise the solution treatment temperature to just below the β transformation point and grow sub-grains to the extent that β grains do not become coarse. is necessary.
However, since β flecks are formed at a lower temperature, the solution treatment temperature cannot be increased to a point immediately below the β transformation point. Therefore, it is necessary to prevent the β fleck from growing in order for the solution treatment to function effectively.

The present inventors have found that β fleck is partially α + β
→ α + β in this part due to β transformation
→ Thinking that delaying β transformation leads to suppression of β fleck, the authors studied the effect of heat history before solution treatment. As a result, a quenching treatment from the α + β region before the solution treatment is effective as a means for delaying the partial α + β → β transformation, and the quenching before the solution treatment increases the β fleck formation temperature in the solution treatment. As a result, stable solution treatment near just below the β transformation point, which is higher than the treatment temperature immediately before the solution treatment, becomes possible, uniformly grows the subgrain to an appropriate size, and has a good strength-toughness-ductility balance and They have found that a homogeneous member can be obtained, and have completed the present invention.

In the method for producing a Near β type titanium alloy according to the present invention, in the method for producing a Near β type titanium alloy in which the final heat treatment is a solution treatment and an aging treatment in an α + β region, a temperature of 500 ° C. or more (β A pretreatment of cooling at a rate of 1.0 ° C./s or more from a temperature below the transformation point (−20 ° C.) is performed, and the subsequent solution treatment is performed at a temperature of (β transformation point−20 ° C.) or more (β transformation point−5 ° C.). ) It is characterized by performing at the following temperature.

[0014]

FIG. 1 shows typical steps in the manufacturing method of the present invention.
Shown in In the production method of the present invention, the pretreatment performed before the solution treatment and the subsequent solution treatment are important.

As a hot working material to be subjected to a solution treatment,
It is desirable to use a material processed in the α + β region. This is because it is difficult to obtain a target microstructure composed of the primary eutectoid α phase and β subgrain only by the subsequent heat treatment.

The pretreatment may be performed independently after the hot working, or may be used together with the hot working. In short, the heating-cooling process immediately before the solution treatment is the pretreatment.

The reason why the temperature in the pretreatment is set to 500 ° C. or more and lower than the β transformation point is −20 ° C. is that if the temperature is less than 500 ° C., sufficient diffusion of alloy elements does not occur, and the effect of the pretreatment is not exhibited. In the heating stage of the treatment, since the operation of suppressing the β fleck generation by rapid cooling was not performed, (β transformation point −20
This is because above (° C), β flecks occur, and just below the β transformation point, β grains become coarse.

The cooling following the heating in the pretreatment is the most important factor in the present invention, and the cooling rate is 1.0 ° C./s.
The above is because the change in composition at the interface of the α / β phase is
This is because the α + β → β transformation is delayed. FIG. 2 shows a typical Near β type titanium alloy, Ti-10V-
1 shows the structure of 2Fe-3Al.

During cooling in the pretreatment, the α phase increases. At this time, movement of the alloy element occurs between α / β, and the composition fluctuates near the interface between the α / β phases. More specifically, the α phase and the β phase have different chemical compositions. For example, a typical Near β type titanium alloy, Ti-10V-
In the case of 2Fe-3Al, Al exists in the α phase, V and Fe in the β phase, and V and Fe in the α phase become β in the β phase as the α phase increases during cooling. Al in the phase moves into the α phase, and a phenomenon occurs in which the rejected element is concentrated near the interface. When a region in which the element is concentrated is generated, the α phase decreases and the β phase increases again during the solution treatment following cooling, and the movement (diffusion) of the element is likely to occur in this process. That is, the α + β → β transformation is likely to occur, and β flecks are also likely to be generated.

However, the cooling rate in the pretreatment is set to 1.0 ° C. /
When s or more, the element movement during cooling is suppressed, and as a result, in the solution treatment, the β fleck generation temperature rises, and it is possible to increase the processing temperature to just below the β transformation point at which β grains coarsen. Become. Regarding the cooling rate, the higher the cooling rate, the greater the effect of suppressing element movement, so that the cooling rate is 4.0 ° C./s
The above is desirable and 10 ° C./s or more is more desirable.

In the solution treatment following the pretreatment, the quenching in the pretreatment raises the β fleck generation temperature, so that the formation of β flecks does not cause tissue inhomogeneity, and thus is higher than before, ie, higher than the immediately preceding treatment temperature. High temperature solution treatment becomes possible. The temperature in the solution treatment is (β transformation point-
(20 ° C.) or more and (β transformation point −5 ° C.) or less
If it is less than (transformation point -20 ° C.), the sub-grains cannot grow larger than before, and if it exceeds (β transformation point -5 ° C.), the β grains become coarse and the structure is heterogeneous due to the growth of β flecks. Because they cause a reduction in ductility.

As described above, in the solution treatment following the pretreatment by quenching in the pretreatment, even if the solution treatment temperature is increased to just below the β transformation point, β flecks are not generated, and as a result, β grains are coarsened. It is possible to obtain high strength and high toughness by growing sub-grains large without causing a decrease in ductility due to the formation and an inhomogeneous structure due to the generation of β fleck.

[0023]

EXAMPLES Examples of the present invention will be shown below, and the effects of the present invention will be clarified by comparison with comparative examples.

Diameter 420 obtained by vacuum arc double melting
mm of a Ti-10V-2Fe-3Al alloy ingot was heated to the β temperature range and β-forged into a 200 mm diameter round bar. This β forging material is heated to 750 ° C. in the α + β region, and α +
A slab (normally forged material) having a thickness of 100 mm and a width of 120 mm was formed by β forging. The β transformation point of this alloy is about 800
° C.

For some ordinary forged materials, a block of 150 mm in length is sampled from the material, and a block of 75 mm in the α + β region is obtained.
At 0 ° C., a constant temperature forging was performed at a strain rate of 10 ⁻³ S ⁻¹ and a reduction rate of 50%, to produce a material (constant temperature forged material) having a height of 50 mm.

The cooling rate after normal forging and constant temperature forging in the α + β range was variously changed by performing furnace cooling, standing cooling, oil cooling, and water cooling. As the cooling rate, the temperature at the center of the material was measured, and the average cooling rate based on the time required from the heating temperature to 300 ° C. was determined.

For normal and constant temperature forgings,
With the exception of some, as an independent pretreatment, it was cooled to various temperatures after heating and holding at a temperature in the α + β region for 2 hours. In the case where independent pretreatment was not performed, heating in the final forging and cooling after heating are pretreatments. Table 1 shows the conditions of forging and pretreatment that each material received.

From the thus obtained ordinary forged material and constant temperature forged material, a block having a thickness of 20 mm is sampled, each block is maintained at various temperatures for 2 hours, and then subjected to a solution treatment of cooling with water to generate β fleck. The temperature was determined. The temperature at which β flecks occur is determined by cutting a material that has been subjected to solution treatment at various temperatures along a plane passing through its center, and cutting each cut surface with hydrofluoric acid 5 vo.
After etching with a solution consisting of l%, nitric acid 20vol% and the balance water, observe the macrostructure by the thickness limit and select the material that has undergone the solution treatment at the lowest temperature among the β-flaked materials. Investigated by Table 1 shows the results
Shown in

The cooling rate received immediately before the solution treatment is
Those having a flicker generation temperature of less than 1.0 ° C / s have a β flicker generation temperature of 780 ° C or lower in the solution treatment, while those having been cooled at a speed of 1.0 ° C / s or higher have a β fleck generation temperature of less than 1.0 ° C / s. The temperature rose by 10 ° C. or more. However, if the temperature in the pretreatment is too high, β flecks are generated in the pretreatment stage, and β flecks remain after the solution treatment even if they are rapidly cooled (No.
7).

Next, a part of the obtained normal forging material and the constant temperature forging material before solution treatment (Nos. 1, 4, 6, 9, 11, 11).
13, 14, 16, 17, 18) are subjected to a solution treatment in which water is cooled after holding for 2 hours at a temperature 5 ° C. lower than the obtained β-fleck generation temperature and an aging treatment in which air is cooled after holding for 8 hours at 535 ° C. did. 6.25 diameter from the material after aging treatment
mm, a round bar tensile test piece having a flat diameter of 32 mm in length was taken from the direction perpendicular to the forging direction, and a half-size CT test piece was used for fracture toughness measurement. In addition, the crack was collected so as to grow in a direction perpendicular to the longitudinal direction of the forging. Tensile test shows 0.2% strain rate at room temperature
0.5% / min up to proof stress, 15% / mi after 0.2% proof stress
n, and the fracture toughness test was performed according to ASTM-E3.
99. Table 2 shows the results.

In each case, since the solution treatment temperature was lowered by 5 ° C. from the β-flicker generation temperature, no heterogeneity of the structure due to the generation of β-fleck occurred. At the solution treatment temperature, coarsening of β grains was also prevented, so that the ductility was sufficient.

However, the comparative example (No. 1, 9, 13, 1)
6, 18) is that the β flicker generation temperature in the solution treatment is 7
75 to 780 ° C, and 770 to 770 ° C
Since the solution treatment temperature was set to 775 ° C., the fracture toughness was not sufficient, and for some of them, the 0.2% proof stress was also insufficient.

On the other hand, the present invention example (No. 4, 6, 1)
1, 14, 17) have a β flicker generation temperature of 790 to 795 ° C. in the solution treatment, and as a result, 785 to 79
Since it was able to undergo a solution treatment at a high temperature of 0 ° C., the fracture toughness as well as the 0.2% proof stress became sufficient.

It should be noted that, even though the material was quenched immediately before the solution treatment, if the solution treatment temperature exceeded 795 ° C., the ductility became insufficient due to the coarsening of β grains, and the β
Tissue heterogeneity due to fleck formation is also a problem. On the other hand, when the solution treatment temperature is lower than 780 ° C., the 0.2% proof stress and the fracture toughness decrease to the same level as the comparative example.

[0035]

[Table 1]

[0036]

[Table 2]

[0037]

As described above, the method for producing the near β type titanium alloy of the present invention suppresses the generation of β fleck in the solution treatment by performing rapid cooling in the pretreatment immediately before the solution treatment. As a result, the solution heat treatment temperature could be increased to just below the β transformation point without generating β flecks, so that a member having a good strength-toughness-ductility balance and uniform properties can be obtained.

[Brief description of the drawings]

FIG. 1 is a heat pattern diagram showing typical steps in the method of the present invention.

FIG. 2 is a schematic diagram showing the structure of Ti-10V-2Fe-3Al.

FIG. 3 is a heat pattern diagram showing typical steps in a conventional method.

Claims (1)

(57) [Claims] 1. A method for producing a Near β type titanium alloy in which the final heat treatment is a solution treatment and an aging treatment in an α + β region, wherein 500 ° C. or more (β transformation point −
(20 ° C.), a pretreatment of cooling at a rate of 1.0 ° C./s or more is performed, and the subsequent solution treatment is performed at the (β transformation point−
20. A method for producing a Near β type titanium alloy, which is performed at a temperature of 20 ° C. or more and (β transformation point −5 ° C.) or less.