JPS62133051A - Manufacture of alpha+beta (alpha+beta)-type titanium alloy - Google Patents

Abstract

PURPOSE:To manufacture, at a low cost, an alpha+beta-type Ti alloy material having strength and ductility equal to those of parts to which sufficient alpha+beta working is applied, by subjecting an alpha+beta=type Ti alloy to a combination of beta-working and heat treatment under specific conditions. CONSTITUTION:The alpha+beta-type Ti alloy such as the one of Ti-6Al-4V, etc., is worked at >=80% reduction in area in a beta-single phase range in the range of temp. at and above beta-transus. Then the alloy is held at a temp. lower than the beta-transus by 20-90 deg.C for >=4hr and cooled slowly down to 550 deg.C or below at <=0.5-10 deg.C/sec cooling rate. Successively, the alloy is reheated to a temp. of 20-90 deg.C below the beta-transus, held at the temp. for less than 4hr, and cooled down to <=550 deg.C at 0.5-10 deg.C/sec cooling rate. The alloy is further aged at 540-650 deg.C for 4-10hr and cooled to room temp. In this way, alpha+beta-type Ti alloy parts excellent in toughness as well as in strength can be obtained.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a method for producing an α+β type titanium alloy, particularly a high strength, high ductility α→-β type titanium alloy produced by combining processing, heat treatment and aging treatment. This invention relates to a method for producing an alloy.

(Conventional technology) Titanium alloy has the highest specific gravity strength (=strength/
It is widely used as an aircraft material because of its high specific gravity) and high high-temperature strength. Furthermore, by taking advantage of its properties, it is being used to reduce the weight of automobile parts, especially engine parts, such as pulp and valve springs. However, when titanium alloys are used in general industrial products such as automobile parts, the biggest problem is that they are expensive.

By the way, titanium alloy has α-type and α-type due to its metallographic structure.
It is classified into three types of alloys: +β type and β type. Among these, the α+β type is often used as a high-strength material for automobile parts and the like. In order to obtain high strength and ductility, this α+β type titanium alloy must be sufficiently heated and processed in the α+β region to obtain a fine equiaxed structure.

However, machining in the α+β region has a large deformation resistance and the machining temperature range is limited to a narrow range, causing an increase in machining cost. On the other hand, it is known that processing in the β range is easy and a large reduction ratio can be achieved. Therefore, the degree of machining in the α+β region is significantly reduced,
If it becomes possible to perform most of the processing in the β region, it is expected that it will greatly contribute to lower processing costs.

(Problems to be Solved by the Invention) The purpose of the present invention is to produce α+β type titanium that has the same combination of strength and ductility as a wire rod that has undergone sufficient α+β processing by combining β processing and heat treatment. It is an object of the present invention to provide a method for producing alloys at low cost.

Another object of the present invention is to make it possible to produce α+β type titanium alloys, particularly bars, wire rods, and hot-rolled sheets, on a high-speed steel bar and wire rod rolling line or a hot-rolled plate rolling line. An object of the present invention is to provide a method for producing a titanium alloy.

(Means for solving the problem) By the way, in order to maintain the ductility of a titanium alloy at a high water content, it is necessary to create an equiaxed &+[11
It is necessary to set it to 1. In order to change the acicular structure to equiaxed Ml HM, in addition to the method of applying sufficient heating and processing in the α and β regions, as already disclosed in many literatures, (room temperature ~ 600) °C ~ ( A method is known in which heating and cooling are repeated 2 to 10 times in the temperature range of 850 to 1000°C.

However, as a result of intensive research by the present inventors, we found that in materials in which β grains are refined by β processing with a cross-section reduction rate of 80% or more, the β transus is less than 20% without repeated heating.
After heating once to a temperature range of ~90°C and holding for less than 4 hours,
It has been found that a sufficient equiaxed effect can be obtained by cooling to 550° C. or lower at a cooling rate of 0.1'c/sec or lower. So, we further investigated this point and found that by combining this equiaxed heat treatment method, refinement heat treatment method, and aging treatment, it is possible to obtain strength and ductility almost equivalent to α + β forged and then annealed material. The invention has been completed.

Here, the gist of the present invention is that after processing an α+β type titanium alloy to achieve a cross-section reduction rate of 80% or more in the β phase region in the temperature range above the β-transus, held at a low temperature for no more than 4 hours, then cooled to a temperature below 550°C at a cooling rate of no more than 0°1°C/sec, then heated again to a temperature between 20 and 90°C below the β-transus; 0.5°C below 550°C
α+β, characterized by cooling at a cooling rate of 10° C./sec or more and 10° C./sec or less, and further aging treatment at a temperature of 540 to 650° C. for 4 to 10 hours and cooling to room temperature.
This is a method for manufacturing type titanium alloy.

In addition, the α+β type alloy referred to in the present invention refers to the α type alloy and the 3 type alloy at room temperature.
In addition to having the same structure as that of a product, β
It refers to an alloy that exhibits an individual eight-component structure at temperatures above the transus, and is typified by, for example, Ti-6AQ-4V. In addition, although the alloy used in this method is mainly an ingot,
An alloy processed by some means may be provided.

(Function) Next, the method of the present invention will be further explained with reference to a heat treatment chart not shown in the accompanying drawings.

Figures 1 (8) and (B) illustrate methods for producing α+β type titanium alloys by the conventional method and the method of the present invention, respectively. In the conventional method shown in Figure 1 (8), the rolled material, e.g. First, β processing is performed to destroy the solidified structure of the ingot, and then with the aim of making the grains equiaxed and refined,
Usually sufficient alpha+beta processing is carried out so that the area reduction rate is 50% or more, and then annealing is carried out at an appropriate temperature of, for example, 700 to 800 degrees Celsius. However, processing in the α+β region is generally difficult, and processing of 50% or more is expensive and requires advanced technology.

On the other hand, in the method according to the present invention, as shown in FIG. 1(B), all processing is performed in the β region, and then a heat treatment for equiaxed crystal grains and a heat treatment for grain refinement are sequentially performed.

Here, in the present invention, an ingot may be used as a rolled material as in the past (or other appropriate α+β type titanium alloy materials may be used as a rolled material, and they are first processed by 80% or more in the β region). The reason why β processing is set at 80% or more is that β grains are not sufficiently refined by β processing of less than 80%, so the equiaxed heat treatment and refinement heat treatment of the present invention are performed in the subsequent process. This is because equiaxedization and refinement are not sufficient, and therefore, as shown as a comparative example in the Examples described later, sufficient improvement in 0.2% proof stress and reduction of area cannot be achieved.

In addition, the reason why the temperature range in the equiaxed processing was set to be lower (20 to 90°C) than the β transus is because [β
At temperatures above [transus -20°C], the ratio of α phase becomes too low, for example below 30%; on the other hand, [β transus -9
0° C.] At the ungrooved temperature, on the other hand, the ratio of the α phase is 70% or more, which is too high, making it impossible to obtain equiaxed recrystallized grains. In this case, equiaxedization first requires (20
α phase remaining in a large region at low temperature (~90℃) (referred to as primary α)
It is necessary to spheroidize (equaxed), and for this reason, it is necessary to hold it in the β transus temperature range (20 to 90°C) for an appropriate amount of time, preferably for about 1 hour. . On the other hand, holding for more than 4 hours causes the primary α phase to grow, which is not preferable.

Note that for this holding, it is necessary to once cool down to 550°C or less after β processing, and then heat and hold at a temperature 20 to 90°C lower than the β transus.

After holding for a specified time, 0.1℃/s until below 550℃
Cool down below ec. This is β transus-(20~9
During cooling, a secondary α phase precipitates from the portion that remained as a β phase when kept in the temperature range of 0°C), and in order to precipitate this secondary α phase as an equiaxed product, a temperature of 0.1°C/ This is because a slow cooling rate of sec or less is required. 55
Precipitation of the secondary α phase does not occur in a temperature range below 0°C, so the cooling rate can be controlled up to 550°C.

Next, in the present invention, a finer treatment is performed, but the heating limit in that case is also β;
) The reason for the low region is that in order to finely precipitate the secondary α phase, the primary α phase must be 30 to 70%, preferably 50%.
This is because it is necessary to suppress the growth of β-crystal grains by remaining about %, and therefore it is necessary to maintain the temperature at β-(20 to 90°C).

In that case, miniaturization requires β transus across the entire cross section.
(20 to 90°C), and for this reason, it is necessary to hold the temperature for an appropriate time, preferably about 1 hour. If the holding time exceeds 4 hours, the primary α phase will grow, which is not preferable.

After holding for a specified time, 0.5℃/s until below 550℃
Cooling is performed at a cooling rate of ee or more and 10°C/sec or less. This is because if the cooling rate exceeds 10°C/sec, martensitic transformation will occur and the refinement of the α phase, which is the objective of the present invention, will not be achieved. On the other hand, if the cooling rate is less than 0.5°C/sec, the secondary α phase will coarsen and precipitate. Therefore, in the present invention, if the cooling rate is less than 0.5°C/sec,
A cooling rate of 0°C/secC or less is required. Furthermore, in the temperature range below 550°C, precipitation of the secondary α phase no longer occurs, and therefore the cooling rate cannot be controlled at 550°C.
That's a good thing.

The processed material of the α+β type titanium alloy obtained in this way is subjected to aging treatment, which improves the effect of the refinement treatment and
In order to obtain strength equivalent to forged and annealed materials, the temperature should be increased by 0.5℃/
It is necessary to further precipitate fine tertiary α products by aging from the β phase that remains as a parent phase after the secondary α products are precipitated when cooled for more than sec.

Aging for this is 540℃ x 4h to 650℃ x 10h
3 for conditions less than 540℃ x 4 hours.
This is because the precipitation of secondary α-crystals is not sufficient, and tertiary α-crystals rather grow at temperatures exceeding 650°C.

FIG. 2 is a schematic diagram of the metal micro-Mi weave of the α+β type titanium alloy obtained by the present invention. In the figure, the secondary α product is precipitated surrounding the primary α product, and the tertiary α product is in between. It can be seen that it is precipitated. Each of the seven items is finely dispersed, so
This results in higher strength and toughness.

Next, the present invention will be explained in more detail with reference to Examples.

Example 1 T is an α+β type titanium alloy having the composition shown in Table 1.
The i-6AQ-4V alloy was melted to obtain an ingot with a diameter of 400 mm and a length of 1800 mm.
Heated to 0℃ and β-forged at a finishing temperature of 990℃ to a diameter of 1
This was further heated to 1100° C. and β-rolled at a finishing temperature of 990° C. to obtain a round bar having a diameter corresponding to each cross-sectional reduction ratio shown in Table 2. The rods or wire rods thus obtained were then treated under the respective heat treatment conditions shown in Table 2, and test pieces were taken from the resulting α+β type titanium alloy wire rods and their mechanical properties were evaluated. The results are also summarized in Table 2.

Table 1 (weight %) In Table 2, patient 18 is β processed by the conventional method + (α + β)
This is an example of annealing by a processing worker, and the case 28 is an example of β processing + annealing. With 1m1B, a good combination of both strength and ductility was obtained, whereas with 28 mm, sufficient ductility was not obtained.

On the other hand, in the embodiments of the method of the present invention in series 1 and magnet 4, the strength
The elongation is the same compared to the 118, and the aperture is smaller than the 11 and 118, but it is much better than the 28, and it practically passes the AMS standard and the old standard (≧25%). Values without problems are obtained.

Next, the effect of the degree of β processing will be examined by comparing 1, 4, and 19. It can be seen that in 50% processing of NQI9, even if the subsequent equiaxedization and refinement heat treatments are performed, neither the strength nor the ductility is sufficiently improved, and the degree of β processing is required to be <80% or more in terms of ml and Th4.

Next, the equiaxed heat treatment conditions will be examined by comparing 1.5.6.9.10 and 20.21.22.29. Regarding the holding conditions, at 880° cxlh in No. 18, the progress of equiaxedization is not sufficient, and at 980° cxlh in No. 21, the remaining α phase is too small, and equiaxedization does not progress. Neither the ductility nor the ductility was improved, and the 900-970°C1 shown in 5.6, that is, the β transus (Ti-
It can be seen that it is necessary to keep the temperature within the range of 20 to 90°C below 6AQ-4V (990°C). In addition, when the holding time was 8 h in Kan 29, crystal grains grew and NQ9 and lO
As shown in , it is preferable to set the time to 4 hours or less.

Also, the effect of cooling rate is k 1 , Ik 2 and 11kL2
2, it is found that when the cooling rate is 1.0°C/sec, the equiaxed formation is not sufficient and the improvement in ductility is not sufficient.
It needs to be 1°C/second or less.

Next, the refinement heat treatment conditions were changed to 7.8.11.12.1.
3.14 and 23.24.25.30.

At 850 °C improvement is not sufficient.

In addition, when holding the stage 30 at 950° C. for 8 hours, crystal grains grow and the refinement effect is not sufficient. Furthermore, looking at the effect of the cooling rate, at 0.1°C/sec for ll&125, crystal grains grow during the cooling tJI, resulting in a change in strength of 1q.
is not enough. Although not shown in Table 2, i
In o'c/sha, martensite is generated and the original purpose is not achieved.

From the above two results, the holding conditions as the refinement heat treatment conditions are 20 ~ 20 ~
It is appropriate to maintain the temperature in a temperature range of 90°C, preferably for 4 hours or less, and the cooling rate should be 0.5°C/second or more and IO°C/second or less as shown in 13.14. It can be seen that it is.

Finally, the influence of the statute of limitations is examined by comparing Ikl, 2.3.8 and Kan 26.27.31.32. 5 of 26
00'r, x4h,, th31's 600°C x 2h aging did not sufficiently increase the strength due to aging, and 750°C x 4h for No. 27 and 600°C x 24h for No. 32.
In aging, it becomes over-aged and the strength decreases. Therefore, the statute of limitations condition is as shown in Arashi 1.2.3.8 (540
~650°C)×(4-10)h, preferably (4-B)
It can be seen that h is appropriate.

Example 2 Example 1 was repeated except that in this example, plates were used instead of bars. That is, the ingot (size diameter 40
0mm x length 1800mm) was roughly rolled to a thickness of 30mm, heated to 1100°C, and the rolling end temperature was 9.
Hot rolling was performed at 90° C. with a reduction rate of 90% (unidirectional β-rolling) to obtain a plate material with a thickness of 3 mm. This was subjected to heat treatment under the conditions shown in Table 3 to obtain test materials.

The mechanical properties of each sample material are also summarized in Table 3 together with those of the comparative example.

α+β rolling (unidirectional rolling) and annealing (750'cxl
h, PC) has anisotropy in mechanical properties (particularly 0.2% proof stress), whereas the method of the present invention produces a sheet with less anisotropy. (See Table 3, Examples).

(Effects of the Invention) The present invention has been described in detail above.According to the present invention, processing in the β region, which has conventionally been considered relatively easy to process, becomes possible, and the subsequent equiaxed heat treatment It can be seen that a high-strength, high-toughness α+β type titanium alloy can be easily obtained through the refinement heat treatment, and the effect is remarkable.

In this example, as a representative α+β type titanium alloy,
Although the example of Ti 6AQ4V (β-transus = 990°C) is shown, rt-6AQ-6C2Sn, Ti-
6AQ-2Sn-4Zr-2Mo,
Ti-68Q-2Sn-4Zr-6Mo
,,Ti-6AI2-2Nh-ITa-0,8Mo
It is obvious that the method can also be applied to alloys such as the following.

[Brief explanation of drawings]

Fig. 1 (8) is a processing heat treatment chart of the conventional method; Fig. 1 ([1)] is a processing heat treatment chart 1 of the method according to the present invention.
.; and FIG. 2 are schematic diagrams of the metal structure of the α+β type titanium alloy obtained by the present invention.

Claims (1)

[Claims] After processing the α+β type titanium alloy to achieve a cross-section reduction rate of 80% or more in the β-single-phase region in the temperature range above the β-transus, hold it at a temperature 20 to 90°C lower than the β-transus for less than 4 hours. , cooled to a temperature of 550°C or less at a cooling rate of 0.1°C/second or less, then heated again to a temperature of 20 to 90°C below the β-transus and held for within 4 hours, and then heated to a temperature of 550°C or less. up to 0.5℃/sec or more, 10℃/
Cooling at a cooling rate of 540 to 65 sec or less
A method for producing an α+β type titanium alloy, which comprises aging treatment at a temperature of 0° C. for 4 to 10 hours and cooling to room temperature.