JPS63230857A - Manufacture of titanium-alloy sheet for superplastic working - Google Patents

Abstract

PURPOSE:To develop an alpha+beta-type Ti alloy sheet with an isometric alpha+beta two- phase structure suitable for superplastic working, by subjecting an alpha+beta-type Ti alloy to rollings at temps. above and below the beta-transformation point at specific drafts, respectively, subjecting the rolled plate to heating up to a specific temp. and further to rapid cooling, and then applying cold rolling and recrystallization annealing to the above. CONSTITUTION:An alpha+beta-type Ti alloy represented by a Ti-6Al-4V alloy is hot-rolled at a temp. of the beta-transformation point of the alloy or above at >=75% draft, cooled down to a temp. of the beta-transformation point or below, and rolled in an alpha+beta temp. region at >=20% draft. Subsequently, the rolled plate is heated and held in a temp. region between the beta-transformation point and (beta-transformation point +70 deg.C) for 30sec-30min, and then cooled down to <=500 deg.C at >=5 deg.C/sec rolling rate. The plate is then cold-rolled at the above temp. at >=70% draft and successively subjected to recrystallization annealing at 700-800 deg.C for 30-120min. In this way, the alpha+beta-type Ti alloy sheet with superfine isometric alpha+beta two-phase structure suitable for superplastic working can be stably manufactured at a low cost.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for producing an α+β type titanium alloy plate having a fine equiaxed α+β two-phase structure suitable for superplastic working.

Background technology In general, titanium alloys have excellent corrosion resistance and exhibit high specific strength, so they have come to be used in various fields as titanium manufacturing technology has progressed in recent years. α+β type titanium alloys, represented by 6Ajl-4V alloy, have superior properties in terms of toughness and weldability, and are frequently used in aircraft, chemical equipment, heat exchangers, turbine blades, etc. ing.

By the way, α of Ti-6AI-4V alloy etc. as mentioned above
+β-type titanium alloys are known to be able to be formed into complex-shaped parts by superplastic processing, and in order to produce sufficient superplastic deformation, it is necessary to It is said that an ``α+β two-phase structure'' is required.

In order to create a material with such ultra-fine grains, it is effective to process the material at a temperature that does not cause crystal growth, especially at a high processing rate, at a cold temperature.

It is thought that.

However, in the case of α+β type titanium alloy, cold working is extremely difficult due to its unique rolling and annealing texture, and the limit cold rolling rate without cracking is at most 60
It has been reported that the amount will be only about %. and,
At such a relatively low working rate, it was extremely difficult to achieve sufficient ultra-fine refinement of the α+β two-phase structure even with cold working.

<Means for Solving the Problems> From the above-mentioned viewpoint, the present inventors have attempted to more stably produce α+β type titanium alloy plates having ultrafine equiaxed crystal grains suitable for superplastic processing on an industrial scale. As a result of intensive research into the possibility of doing so, we came to the knowledge shown in (a) to (C) below. That is, (a) In order to realize an α+β type titanium alloy plate having an ultra-fine equiaxed α+β structure, cold rolling with a high degree of deformation of 70% or more is essential; Only when α + β type titanium alloy subjected to rate cold rolling is subjected to recrystallization annealing under specific conditions can a plate material with the above-mentioned structure suitable for superplastic processing be stably and reliably obtained; The processing rate is 7 without any inconveniences such as
Normally, it is almost impossible to cold-roll an α+β-type titanium alloy with a grain size of 0% or more, but as a cold-rolled material, it is especially difficult to cold-roll a titanium alloy with a fine β grain size and (0002).
By using a material that does not form a strong α texture, cold rolling of α + β type titanium alloy with a processing rate of 70% or more is fully possible; (e) Fine β grains as described above; α-10β-type titanium alloy material with
It can be stably obtained by maintaining the temperature between the transformation point and [β transformation point + 70°C] and immediately rapidly cooling it.

This invention was made based on the above findings,
As illustrated in Figure 1, the processing rate of α+β type titanium alloy at a temperature above the β transformation point =
Processed at 75% or higher, then further processed at α + β two-phase temperature range for 2
After processing 0% or more, raise the temperature and hold it at a temperature of β transformation point ~ [β transformation point + 70°C] for 30 seconds to 30 minutes, then cool down to 500°C or less at a cooling rate of 5°C/second or more. Cooled, then cold rolled at a rolling rate of 70% or more, and further rolled at 700%
By performing recrystallization annealing for 30 to 120 minutes at a temperature range of ~800°C, it is possible to stably produce an α+β type titanium alloy plate having an ultrafine equiaxed α+β structure suitable for superplastic processing. It is characterized by the following points.

Next, in the method of the present invention, the processing rate at a temperature equal to or higher than the β transformation point, the processing rate in the α+β two-phase region, the subsequent high temperature holding conditions, the subsequent cooling conditions, the cold rolling conditions, and the recrystallization annealing conditions are set as described above. The reason for this limitation will be explained.

A) Processing rate at temperatures above the β transformation point The β grain size becomes finer as the β region processing rate increases. In order to achieve cold rolling of 70% or more, it is necessary to obtain as high a processing rate as possible for the material before the β recrystallization process.
It is important to refine the β grains in advance by performing area processing. If the processing rate of β-region processing is less than 75%, even if β recrystallization is performed, the desired cold processing is possible.
Since grain structure material cannot be obtained, the processing rate in β region processing was set at 75% or more.

B) Processing rate in the α+β two-phase region If the processing rate in the α+β two-phase region processing is less than 20%, the recrystallized β grains will not be refined, and therefore the desired cold working will not be possible. The processing rate in the area is 2
It was set as 0% or more.

C) High temperature holding conditions after two-phase region processing After α + β two-phase region processing of 20% or more, the β transformation point ~
The treatment of holding in the temperature range [β transformation point +70° C.] for 30 seconds to 30 minutes is performed to obtain fine β grains by recrystallization. If the holding temperature is lower than the β-transform point or the holding time is less than 30 seconds, sufficient recrystallized β grains cannot be obtained, whereas if the holding temperature is higher than [β-transform point + 70°C] or the holding time is 30 seconds, If the temperature exceeds 100%, recrystallized β grains will grow, making cold working of 70% or more impossible in any case, so the high temperature holding conditions after working in the two-phase region were limited as described above.

0) Cooling conditions after holding at high temperature If the cooling rate from the temperature range from β transformation point to (β transformation point + 70°C) is slower than 5°C/sec, pro-eutectoid α will occur at the grain boundaries of β grains. precipitates and significantly impedes cold workability, and if this quenching treatment is terminated before the temperature drops to 500°C or less, recrystallized β grains grow and deteriorate cold workability. 50 at a cooling rate of 5°C/sec or more
It was decided that the test would be carried out to temperatures below 0℃.

E) Cold rolling rolling ratio If the rolling ratio in cold rolling is less than 70%, the α+β structure will not be sufficiently refined, making it impossible to obtain a titanium alloy sheet suitable for the desired superplastic working. The rolling ratio of cold rolling was set at 70% or more.

D) Recrystallization annealing conditions The micro [%] of the titanium plate after cold rolling is in a state where the α crystal grains are elongated in the rolling direction, and the equiaxed fine α + β structure that has been passed through superplastic processing is In order to obtain this, recrystallization annealing of α grains is required.

In this case, if the annealing temperature is lower than 700°C or the treatment time is less than 30 minutes, recrystallization of α grains after cold working will not be sufficient;
If the processing time is exceeded or the treatment time is longer than 120 minutes, the α grains after recrystallization will grow and the desired fine equiaxed α+β structure will not be obtained.
It was determined that the test should be carried out in a temperature range of 00°C for 30 to 120 minutes.

Next, the present invention will be specifically explained using Examples and comparing with Comparative Examples.

<Example> A Ti-6Ai-4■ alloy ingot with a diameter of 400 mm and a composition as shown in Table 1 obtained by vacuum arc melting was subjected to β-range processing under the conditions shown in Table 2. , α+β area processing, β recrystallization heat treatment, cooling, cold rolling, and recrystallization annealing to produce a cold rolled sheet.

Ti-6Aj thus obtained! The alpha grain size and superplastic elongation (test temperature = 850°C, strain rate: 5 x 10-3 sec-') of the -4V alloy plate were measured, and the results are also shown in Table 2.

From the results shown in Table 2, it is clear that if the treatment is carried out according to the conditions specified in the present invention, extremely fine equiaxed α+β of 2 μm or less will be produced.
Ti that has a structure and exhibits superplastic elongation exceeding 1000%
While a -6AI-4V alloy plate can be obtained, it is clear that a plate material exhibiting sufficient superplastic elongation cannot be obtained if even one of the processing conditions deviates from the conditions specified in the present invention.

By the way, in the comparative example, when the treatment of "Test No. 3" was applied, a plate material with a relatively fine α grain size was obtained, but the recrystallization annealing conditions were inappropriate, so the recrystallization of α was not sufficient. Therefore, large superplastic elongation has not been achieved.

In this example, only the results for Ti-6All-4V alloy, which is a typical α+β type titanium alloy, were shown, but other α+β type titanium alloys, such as Ti-6A
β-6V-25n alloy, Ti-6AA'-2Sn
-4Zr-2Mo alloy or Ti-6812-25n
It goes without saying that similar results were obtained for -4Zr -6Mo alloys and the like.

<Summary of Effects> As explained above (according to this invention, α+β having an “ultrafine equiaxed α+β two-phase structure” suitable for superplastic processing)
This brings about extremely useful effects industrially, such as making it possible to stably manufacture type titanium alloy plates on an industrial scale at low cost.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing an example of the manufacturing process of an α+β type titanium plate according to the present invention.

Claims (1)

[Claims] 75% or more processing is performed at a temperature equal to or higher than the β-transformation point, and then 20% or more processing is performed in the α+β two-phase temperature range, and then the temperature is raised to the β-transformation point to [β-transformation point] 30 to +70℃] temperature
After holding for seconds to 30 minutes, cooling rate: 5℃/second or more
Cooled to 00℃ or less, then cold rolled at a rolling rate of 70% or more, and then further rolled in a temperature range of 700 to 800℃ for 3
characterized by performing recrystallization annealing for 0 to 120 minutes,
A method for manufacturing α+β type titanium alloy plates for superplastic processing.