JPH01272750A - Production of expanded material of alpha plus beta ti alloy - Google Patents

Abstract

PURPOSE:To produce an expanded material of alpha plus beta Ti alloy excellent in strength and ductility and having homogeneous mechanical properties in the material by subjecting an ingot of alpha plus beta Ti alloy to hot forging or slabbing and then applying specific soaking to the above. CONSTITUTION:The ingot of alpha plus beta Ti alloy is subjected to hot forging or slabbing or both of the above. Subsequently, this Ti alloy stock is subjected to soaking treatment consisting of soaking at a temp. in an alpha plus beta uniformization region and slow cooling down to <=400 deg.C at <=0.2 deg.C/sec average cooling rate. When hot working such as hot rolling is applied at need, it can be carried out before or after the above soaking in an alpha plus beta region. Since metallic structure and composition are uniformized by the above soaking, the expanded material of alpha plus beta Ti alloy excellent in strength and ductility and free from the dispersion and anisotropy of the above mechanical properties in the material can be obtained.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a method for manufacturing a wrought material with improved mechanical properties by uniformizing the metal structure and component composition of an α+β type Ti alloy. It is.

[Conventional technology]

Ti alloy forged materials, hot rolled materials, and other wrought materials have high specific strength and excellent corrosion resistance, and are therefore used for various high-grade parts such as aircraft parts.

Ti alloys are α type, α+β type depending on their metallographic characteristics.
It is broadly divided into type and β type. α+β type Ti alloy has β
One with an acicular martensitic structure obtained by rapid cooling after being heated to the β region, and a lamellar two-phase structure obtained by generating a lamellar α phase during cooling after heating to the β region. Some have a granular two-phase structure obtained by generating a granular α phase during cooling after being heated to the α+β region.

Wrought α+β type Ti alloys having a granular two-phase structure have high strength and ductility, and are therefore mainly used for aircraft parts such as fuselage fasteners. Strict quality standards (for example, AMS 4967) exist for materials used in such applications, and materials are required to have a structure in which the α grains are equiaxed and the crystal grain size is uniform.

To produce α+β type Ti alloy wire rods, bars, plates, pipes, profiles, etc., the cast ingot is hot forged or bloomed, or both forged and bloomed. Accordingly, it is manufactured by performing processing such as hot rolling. In order to obtain the above-mentioned equiaxed and uniform structure, various attempts have been made in the past. For example, two-heat hot rolling (
Japanese Patent Publication No. 6, IT-4914) and beta heat treatment of hot-rolled materials in sheet manufacturing (Japanese Patent Publication No. 63-4908) are known.

[Problem to be solved by the invention]

When a wrought material of α+β type Ti alloy is manufactured by the conventional method, it is difficult to obtain one with a uniform structure of the granular α phase.
Additionally, due to the presence of component segregation, the strength and ductility were non-uniform within the material. Further, in the case of manufacturing by the method described in Japanese Patent Publication No. 63-4914 and Japanese Patent Publication No. 63-4908, although improvements have been made, they are still not sufficient.

The present invention provides a wrought material of α+β type Ti alloy with equiaxed α grains and a uniform crystal grain size, and by making the composition uniform, the material has excellent strength and ductility, and has excellent strength and ductility. The purpose is to obtain a wrought material with homogeneous mechanical properties.

[Means to solve the problem]

The present invention is manufactured by hot forging, blooming rolling, or both forging and blooming rolling on a cast α+β type Ti alloy ingot, and further processing such as hot rolling as necessary. In the manufacturing process of α+β type Ti alloy wrought material, the metal structure and component composition are controlled by soaking in the α+β homogenization area and slow cooling to 400°C or less at an average cooling rate of 0.2°C/sec or less. It equalizes.

Claim (1) of the present invention is characterized in that the soaking is performed after performing forging or blooming rolling, or forging and blooming rolling. Claim (2) is characterized in that the soaking is performed after forging or blooming. Claim (3) is characterized in that the soaking is performed after rolling or forging and blooming, and then hot working is performed in the α+β region. It is characterized in that it is applied after hot working.

The forging in the present invention is performed by repeating heating and forging as conventionally generally performed, and is initially performed in the β region, and finishing is performed in the α+β region. Blossoming is also repeated in the same manner. Further, blooming rolling may be performed after forging. In addition, hot working in the present invention is a process performed after the ingot is subjected to the forging, blooming rolling, or both forging and blooming rolling, and includes plate rolling, bar rolling, shape rolling, extrusion processing,
This is a commonly used method such as piercing and rolling. Claims (
In 2), the hot working is performed in the α+β region throughout the entire region, and in claim 3, the hot working may be performed initially in the β region, but finishing is performed in the α+β region.

Next, the soaking process in the present invention will be explained. Ti-6A7-, a typical α+β type Ti alloy
FIG. 3 shows the metallographic structure of a cross section from the center of a billet obtained by forging a 4V alloy ingot.

The ingredients are as shown in Table 1. The history of the billet is that it was forged from an ingot with a diameter of 700 mm that was melted and cast twice using VAR (vacuum arc melting), and the forging was finished in the α + β region, and the C cross section was square and the length of each side was 106 mm.
It is mm. In the structure shown in Figure 3, in the area indicated by A, the space between relatively coarse α grains that appear white is filled with a transformed structure from the β phase, and the transformed structure from the β phase consists of the α phase and the remaining β phase. Consisting of The region indicated by B has the same structure as A, but has relatively fine grains, has a small amount of transformed structure from the β phase, and has crystal grain orientation.

CMA (Computer-aided Micro
Analyzer), impurities such as At, V, and Fe are lower in the BFiI region than in the A region, that is, they are negatively segregated. Such regions A and B exist adjacent to each other as shown in FIG. 3, and their size approximately corresponds to the size of β grains during solidification. The soaking treatment in the present invention is a treatment for eliminating and uniformizing such non-uniform metal structure and component segregation.

Table 1 Chemical composition (wtχ) The forged billet described above having the structure shown in Figure 3 was heated under the conditions shown in Table 2, and the average cooling rate to 400°C was 0.2°C.
It was slowly cooled at a speed of less than 1/sec. The structure of the center cross-section of the billet after heat treatment was observed under a microscope and analyzed by CMA. The results are shown in Table 2 by symbols. By selecting the soaking temperature and soaking time, a product with a uniform structure and component composition can be obtained.

Table 2 Heat treatment conditions for forged material ★: Local transformation from α+β to β has occurred ■: Both the heterogeneous structure and segregation have not been resolved ・: The structure has become homogeneous but segregation has not been resolved No ○: Both the structure and components are homogenized. As a representative example of a product in which both the structure and component composition are homogenized,
Figure 1 shows the structure of a sample that was soaked at 950°C for 6 hours and then slowly cooled to 400°C at an average cooling rate of 0.2'C7 seconds or less. The α grains are equiaxed and their size is uniform. Soaking in the present invention is performed at a temperature and time such that both the structure and component composition are homogenized, and this temperature and time is referred to as the α+β homogenization region.

Although the temperature at which two phases α+β transform into a single β phase is generally known for various Ti alloys, the value is based on the average composition of the alloy. If soaking is simply carried out at a temperature below this level, regions will locally transform into a β single phase, making it impossible to obtain a uniform structure. The upper limit of the temperature in the α+β homogenization region varies depending on the composition of the alloy. It also varies depending on material history such as ingot size, casting method, heating temperature, forging temperature, rolling temperature, degree of processing, etc.

To accurately determine this, it is sufficient to heat a preliminary sample and observe its structure. However, it is safe to set the temperature to be 30° C. lower than b obtained from the phase diagram based on the average composition of the alloy. By the way, for the above-mentioned forged billet of the alloy with the composition shown in Table 1, the
90°C, whereas the upper limit temperature of the α+β homogenization region is 960°C.

The lower limit temperature and time of the α+β homogenization region is the temperature and time at which a part of the α phase dissolves in the β phase and the volume of the α phase becomes 50% or less, and the lower limit temperature and time of the α + β homogenization region are When soaking is performed, the α phase becomes equiaxed grains with a uniform size and the component composition becomes uniform. Specifically, the temperature in the α+β homogenization region may be set to 860° C. or higher. By the way,
The lower limits for forged billets as described above of the alloys listed in Table 1 are: 860°C for 12 hours, 900″C for 8 hours;
The temperature was 930°C for 6 hours and 960°C for 5 hours.

Ta.

Cooling in the soaking process is performed slowly so that the average cooling rate to a temperature of 400°C or less is 0.2°C/sec or less. - 9 pieces of the above-mentioned forged billet with a side length of 106 m.
As a result of observing the metal structure after soaking at 50°C for 6 hours and cooling at various cooling rates, as shown in Table 3, 40
If the average cooling rate to 0°C exceeds 0.2°C/sec, a uniform α+β granular two-phase structure cannot be obtained. Also, 0.2℃/
Even when slow cooling for less than a second was terminated at a temperature exceeding 400''C, a uniform α+β granular two-phase structure could not be obtained.

The soaking treatment in the present invention may be performed by heating the cooled material after forging, blooming rolling, or further hot working, or by heating the cooled material after forging, blooming rolling, or hot working. The heat retained in the material may be used to charge the material into a heating furnace before or during cooling.

Claim (1) of the present invention is applied to a product shape obtained by subjecting an ingot to forging, blooming rolling, or forging and blooming rolling. Claims (2) and (3) apply to those in which the product shape is obtained by further performing hot working such as rolling.

Further, the wrought material obtained by the method according to each claim of the present invention can be further processed such as rolling.

[For production]

The soaking treatment in the present invention homogenizes the non-uniform structure and components in the material that existed before the treatment, resulting in a uniform granular two-phase structure.

First, due to the soaking in the α+β homogenization region, the coarse α grains that existed before soaking are partially dissolved into the β phase, and the β
As the volume of the phase increases, each α grain becomes an equiaxed and uniformly sized grain. Furthermore, diffusion between adjacent β grains becomes active and component segregation is eliminated. Here, if the soaking temperature is too high, the α+β two phases in a local negative segregation region such as the B T1N region in FIG. 3 will be transformed into a β single phase, and new component segregation will also occur due to the transformation. - Once transformed into the β phase, it is difficult to return to the original α phase even if soaked for a long time in the α+β homogenization region, and therefore a uniform granular two-phase structure cannot be obtained. If the soaking temperature is too low or the soaking time is too short, the α-grains will not be sufficiently dissolved, and a uniform structure and components will not be obtained. What is important here is that there are local negative segregation regions of alloying components in the material before soaking, and the transformation temperature in these regions is lower than that obtained from the average composition. The first point of the present invention is to soak the material in the α+β homogenization region such that this local negative segregation region does not exceed the transformation point.

Next, by slow cooling after soaking, an α phase nucleates and grows from the β phase that existed during soaking, and an α+β granular two-phase structure having uniform α grains is obtained. During cooling from the α+β homogenization zone, a part of the β phase transforms into the α phase, but if the cooling rate is too fast, the β phase becomes an acicular α phase, that is, martensite, or transforms into the α phase. However, it becomes lamellar. Also,
Due to the difference in cooling rate between the center and surface of the material, even if the transformed α becomes granular in the center, it becomes martensite or lamellar α phase in the surface. When slow cooling is performed at an average cooling rate of 0.2°C/sec or less to a temperature of 400°C or less, α grains generated from the β phase grow to the same size as the α grains that existed during soaking. This results in a uniform α+β granular two-phase structure. If the end temperature of slow cooling is too high, a new α phase is generated from the β phase in a lamellar shape by subsequent cooling, and a uniform granular two-phase structure cannot be obtained. In the present invention, the α produced by the transformation of the β phase present during soaking is made into equiaxed and uniform grains equivalent to the α grains present during soaking, and both the surface and center of the material are uniform grains. The second point is to create a two-phase structure.

The wrought material obtained by the methods of claims (1) and (3) has such a uniform granular two-phase structure, has excellent strength and ductility, and has little variation or anisotropy within the material. Further, even when these materials are further processed as raw materials, the uniformity of the materials is maintained and a wrought material with excellent properties can be obtained.

The wrought material obtained by the method of claim (2) is obtained by further hot working the soaked material in the α+β region, and when compared with that obtained by the method of claim (3), Although the uniformity is slightly inferior, this case also has a uniform granular two-phase structure and is excellent in strength and ductility. Since the composition of the material is uniform, there are no regions where the material transforms locally into a single β phase during hot working in the α+β region, and
Since the structure of the material is uniform, the structure after processing will also be uniform.

The structure is deformed by hot working, but the β phase is more easily deformed than the α grains, so in normal hot working,
Aspect ratio L/H after processing (L:
A structure having α grains with a grain size in the rolling direction (H: grain size in the perpendicular direction) of less than 3 is obtained. Cooling after hot working does not require gradual cooling as in soaking. The β phase at the end of hot working may become a lamellar two-phase structure or a martensitic structure during cooling, but since the wrought material after hot working has a small cross section, it has a nearly uniform structure. Therefore, excellent mechanical properties can be obtained.

〔Example〕

(13 A Ti-6A/-4V alloy consisting of the components shown in Table 1 was forged into a 700 mm diameter ingot that had been melted and cast twice using VAR (vacuum arc melting), and the forging was finished in the α+β region to form a side length of 106 mm. billets were produced.

The hearth of the heating furnace is maintained at 950'C and the billet is charged, so that both the surface and internal temperatures of the billet reach 950'C.
After the temperature reached 50±5°C, a soaking treatment was performed for 6 hours, followed by slow cooling in a heating furnace by controlling the average cooling rate to 400°C to 0.2°C/sec or less.

A cross section passing through the center of the billet before and after soaking was observed under a microscope, and component analysis was performed using CMA. FIG. 1 shows a typical microstructure photograph of the inventive example after soaking. The billet has such a structure throughout the entire area from the center to the surface, and as mentioned above, it consists of equiaxed and uniformly sized α grains and β phase filling the spaces between them. α-grain aspe
ct ratio is 1.0, and the component segregation is AZ
, V were both recognized. As a comparative example, a typical microstructure photograph before soaking is shown in FIG. As mentioned above, the structure is heterogeneous, and if we show the segregation of components in terms of M, the average M content in the hemi region is 6.4%, and in the BSi region it is 5.4%, which is 1.
There was a difference of %.

The homogenized billet of the present invention had excellent strength and ductility, and had almost no variation within the material.

(2) The soaked billet of Example (1) was
It was hot-rolled into a bar with a diameter of 25 mm at 950"C in the β region and air-cooled. A photograph of the microstructure of the center cross section of the obtained bar is shown in Figure 2.
o was 1.2 on average, and had a nearly uniform granular two-phase structure, with lamellar fibers being seen here and there within the black β phase. No component segregation was observed. The mechanical properties of this bar were excellent in both strength and ductility, with almost no variation within the material, and passed strict quality standards such as AMS 4967.

(3) 930% billet before soaking in Example (1)
After heating to ℃ and hot rolling to make a bar with a diameter of 25 mm with a finished surface temperature of 870℃, it is charged into a heating furnace maintained at 950℃, and after the material temperature reaches 950±5℃ 6
The mixture was soaked for a period of time, cooled to 400°C at an average cooling rate of 0.2°C/second or less, and then allowed to cool. The structure of the obtained bar material was a uniform granular two-phase structure with an aspect ratio of 1.0, and no segregation of components was observed.

(4) Length 2m after soaking treatment in Example (1)
The billet was cooled by various means. Table 4 shows the average cooling rate up to 400″C and the metallographic structure after cooling. It was calculated by measuring the temperature and measuring the time it took for the temperature to reach 400°C from 950°C.
Cover cooling involves taking the billet out of the furnace and immediately covering it with a stainless steel box-shaped jig lined with asbestos-based insulation material and leaving it to cool.For air cooling, the billet is taken out of the furnace and left to cool. Forced air cooling is a method in which the billet taken out of the furnace is immediately blown with air to cool it, and water cooling is a method in which the billet taken out from the furnace is immediately placed in water and cooled while stirring the water. In the example of the present invention, both the center and the surface of the billet have a uniform α+β granular two-phase structure, but in the comparative example, the surface or the entire billet does not have the desired structure.

Further, as a result of hot working each billet at an area reduction rate of 93.7%, the billets of the present invention had an aspect ratio of α grains.

It had a uniform granular two-phase structure with a grain size of 2.1, and was excellent in both strength and ductility, but the comparative example had a non-uniform structure with a lamellar α phase or a martensitic phase extending in the processing direction. The ductility was poor.

(5) Ti-6AI-2Sn 4Zr shown in Table 5
The 2Mo alloy ingot was divided into two parts at the rough forging stage, one was forged into a billet with a side length of 106 mm in the α+β region, and immediately soaked at 950°C, and the other was forged in the same way and cooled to room temperature. Thereafter, it was heated and soaked at 950°C. The soaking time for soaking was varied between 4 and 6.8 hours, and the cooling was performed at an average cooling rate of 0.2°C/sec up to 400°C. The transformation temperature T obtained from the average composition of this material is 994°C.
It is.

Table 5 Chemical composition (wtχ) No difference was observed in the structure and component segregation after the treatment between the soaking treatment performed immediately after forging and the soaking treatment performed after cooling to room temperature and heating. In the case where the soaking time was 4 hours, band-like segregation areas similar to those observed in the Ti-6A7-4V alloy were present, but in the case where the soaking time was 6 hours and 8 hours, the Ti-6Aj-4V alloy was completely soaked. Uniform granules α+β similar to those processed by the invention method
It had a two-phase structure, a uniform composition, excellent strength and ductility, and almost no variation within the material.

〔Effect of the invention〕

By the method of the present invention, an α+β type Ti alloy with equiaxed α grains, uniform grain size and composition, excellent strength and ductility, and no variation or anisotropy within the material can be obtained. We can supply materials that meet strict standards such as materials for industrial use.

[Brief explanation of the drawing]

Figures 1 and 2 are metallographic micrographs showing the metallographic structure of a wrought material obtained by the method of the present invention, and Figure 3 is a metallurgical micrograph showing the metallographic structure of a forged billet before soaking as a comparative example. be. ,・Remittance toyu, − All sheets ≧ Mi＼)ε)() Fig. 2(・It)1′:)

Claims (3)

[Claims] (1) After hot forging or blooming rolling or forging and blooming rolling on an α+β type Ti alloy ingot, α+β
Soak in the homogenization area and maintain an average cooling rate of 0.
A method for producing an α+β type Ti alloy wrought material, which comprises performing soaking to gradually cool the material at a rate of 2° C./second or less. (2) After hot forging or blooming rolling or forging and blooming rolling on an α+β type Ti alloy ingot, α+β
Soak in the homogenization area and maintain an average cooling rate of 0.
Perform soaking to slowly cool at 2℃/second or less, then α+
α+β type Ti characterized by hot working in β region
Method for manufacturing wrought alloy material. (3) After performing hot forging or blooming rolling or forging and blooming rolling on an α-10β type Ti alloy ingot, hot working is performed to finish the hot working in the α+β region, and then α+
Soak in β homogenization area and average cooling rate is 0 until below 400℃
．． A method for producing an α+β type Ti alloy wrought material, which comprises performing soaking to gradually cool the material at a rate of 2° C./second or less.