JPH0135069B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for producing a titanium alloy material having excellent mechanical properties such as strength and ductility. [Prior art and its problems] Ti-15V-3Cr-3Sn-3Al alloy is a β-type titanium alloy with high strength and excellent cold workability, and its use has recently been expanding. This means that the titanium alloy
This is because cold workability is significantly superior to α+β type titanium alloys such as Ti-6Al-4V alloy, and it is possible to employ cold pressing, etc., thereby saving labor in the processing process. However, the above Ti−15V−3Cr−3Sn−3Al
The alloy material (hereinafter simply referred to as the present alloy material) has been considered difficult to manufacture through a hot working process. This is because the non-uniformity of mechanical properties and structure is a bottleneck in the manufacture of this alloy material, and it has been considered difficult to obtain sufficient mechanical properties and uniformity of structure through hot working alone. Therefore, this alloy material is rarely used as it is hot workable, and in the past, it was generally manufactured through three steps: hot working, solution treatment, and cold working. By adding a cold working process, improvements in mechanical properties and uniformity of mechanical properties and structure were attempted. However, this method requires a large number of steps and is not easy to manufacture, resulting in problems in productivity and production costs. [Means for Solving the Problems] The present invention was created through repeated research and experiments in order to solve the above-mentioned conventional problems, and its purpose is to solve the problems by only hot working. , conventional method (hot working process - solution treatment process -
It is an object of the present invention to provide a method for easily manufacturing this alloy material having excellent mechanical properties and structure equivalent to those obtained by cold working process. In order to achieve this objective, the present invention particularly focuses on materials for hot processing (slabs,
This method involves strictly controlling the heating temperature and hot working conditions of the billet (such as billets), and performing rigorously controlled rapid cooling after hot working. In other words, the feature of the present invention is that V:14
~16wt%, Cr: 2.5~3.5wt%, Sn: 2.5~3.5wt
%, Al: 2.5 to 3.5wt%, balance Ti and unavoidable impurities.In producing a titanium alloy material, the material for hot processing is heated to a temperature of 900°C or more and 1050°C or less, and then processing is started, and at least 900℃ above β transformation point
Processing is performed at a processing rate of 50% or more at a processing rate of 30% or more at a temperature above the β-transformation point and below 850°C, and then the same processed material is heated to 400°C from a temperature above the β-transformation point to 850°C or below.
The objective is to cool down to a temperature below 0.degree. C. at a cooling rate of 2.degree. C./sec or more. With this method, only the hot working process is required without adding a cold working process, and the same excellent mechanical properties and uniformity of structure as in the case of performing cold working after hot working in the conventional method can be achieved. A titanium alloy material with excellent properties is manufactured. Further, the titanium alloy material of the present invention may contain oxygen up to 0.3% or less, thereby further increasing the strength of the titanium alloy material. The present invention will be explained in detail below. First, the titanium alloy material to be manufactured in the present invention has V: 14 to 16 wt%, Cr: 2.5 to 3.5 wt%, and Sn:
It consists of 2.5 to 3.5 wt%, Al: 2.5 to 3.5 wt%, the remainder Ti and unavoidable impurities, and is made into a material for hot working by forging or blooming an ingot made in a vacuum arc furnace etc. obtain. The alloy material can also contain up to 0.3% oxygen. Although it is possible to increase the strength of this alloy material by adding oxygen, if it exceeds 0.3%, cold workability (cold formability) will be significantly degraded. In other words, in solution treatment (800℃ x 20min → water cooling), oxygen: 0.3
If the content is less than 0.3%, close bending is possible.
%, breakage is observed even at bending radius R=10t, making it practically unapplicable. When hot-processing this hot-processing material, processing is started after heating it to a temperature of 900°C or more and 1050°C or less in a batch furnace or continuous furnace. As mentioned above, one important point of the present invention is to control the heating temperature of the material for hot processing. This alloy material is usually used after being subjected to solution treatment and aging treatment after the hot working process, but the structure tends to become non-uniform during the aging treatment, which causes the mechanical properties of this alloy material to deteriorate. This results in a decrease in uniformity and mechanical properties. This is due to the non-uniformity of aging precipitation behavior, and is simply the main cause of segregation of added elements. Conventionally, the final product is produced through hot working, solution treatment, cold working, solution treatment, and aging treatment.Following the hot working process, a cold working process is added, and after each process, a solution treatment process (processing - By repeating the recrystallization process (twice in total), the final product is made homogeneous. In contrast, the present invention aims to homogenize the added elements and the structure by controlling the heating temperature of the material for hot processing to a constant condition, that is, in the β variable temperature range of 900°C or more and 1050°C or less. By heating the material for hot working to a temperature of , diffusion homogenization of the added elements is obtained. Here, the upper limit of the heating temperature of the material for hot processing is determined.
The reason why we limited it to 1050℃ is that if the heating temperature exceeds this temperature, the grain growth of β grains will be significant, resulting in coarsening and unevenness of β grains in the final product.
This is because mechanical properties deteriorate. In addition, the lower limit of the heating temperature of materials for hot processing has been set to 900
The temperature was set at ℃ because if the heating temperature is lower than this, the diffusion of the additive elements will not be uniform enough, which will cause problems with the mechanical properties and non-uniformity of the structure of the final product, and will also lead to a decrease in the mechanical properties themselves. be. In addition, according to the present invention, rolling is started after heating the material for hot working under the above temperature conditions, and processing is performed in a recrystallization region of 900°C or higher to recrystallize β crystals. It is also possible to further homogenize the added elements through the process. Next, the present invention provides a hot processing material heated to a temperature range as described above, at least above the β transformation point and below 900°C, at a processing rate of 50% or more, and of which β
Hot working is applied at a processing rate of 30% or more at a temperature above the transformation point and below 850°C, and processing is completed at a temperature above the β transformation point. One important point of the present invention is to strictly control the processing steps in this way. In the conventional method, processing strain is applied to the workpiece in the cold working step that follows the hot working step, and through recrystallization behavior in the subsequent solution treatment step, uniform refinement of β crystals and homogenization of added elements are achieved. We are also working to improve mechanical properties during aging. The present invention changes this idea and provides the same effect as cold processing by controlling the processing rate in the low temperature range during hot processing itself, and the specific processing conditions are defined as above. This is because, in order to refine the β grains and improve the mechanical properties of the final product, it is necessary to control the processing rate below 900°C, which is the non-recrystallization temperature range. The reason why the finishing temperature, including the working temperature, is specified to be above the β transformation point is because if the finishing temperature is lower than this, alpha crystals will precipitate non-uniformly in the deformation zone formed by the hot working. This is because the effect of preventing α-crystal precipitation by accelerated cooling in the next step is lost. The processing rate is at least 50% at temperatures above the β-transformation point and below 900℃, and 30% at temperatures above the β-transformation point and below 850℃.
% or more, the rolling deformation structure becomes uniform, and the solution-treated structure and solution-treated structure after hot rolling also become uniform and fine. The result is an improvement in the mechanical properties and uniformity of these heat treated materials. In other words, if the processing rate is small, non-uniform recrystallization and recovery structure will occur in the solution heat treatment after hot rolling.
As a result, aging precipitation in the subsequent aging heat treatment becomes non-uniform. Next, the workpiece hot worked as described above is successively cooled from a temperature above the β transformation point to a temperature below 400°C at a cooling rate of 2°C/sec or above. One of the features of the present invention is to perform accelerated cooling after hot working. The β transformation point of this alloy is 730℃, and if the cooling start temperature and cooling temperature after hot working are inappropriate, α crystals will precipitate unevenly in the deformation zone formed by hot working. Traces of the precipitated α-crystals remain even after the next step of solution treatment and aging treatment, which is one of the causes of non-uniformity of the structure and mechanical properties and deterioration of the mechanical properties. The present invention employs accelerated cooling after hot working to prevent α crystal precipitation during air cooling after hot working. The reason why the cooling start temperature was specified as a temperature above the β-transformation point is because if the cooling start temperature is below the β-transformation point, the deformation area formed in the previous process will be uneven between the end of processing and the start of cooling. This is because α-crystal precipitation occurs, and the effect of preventing α-crystal precipitation by accelerated cooling is lost. The reason why the cooling rate was specified to be 2℃/sec or more is because α crystals will precipitate during cooling if the cooling rate is less than this. This is because if cooling is stopped at a temperature of 100°C, α crystals will be precipitated during the air cooling process to room temperature after cooling is stopped. Note that the lower limit of the cooling stop temperature does not need to be particularly limited from the viewpoint of the material, but from an industrial standpoint, room temperature is adopted from the viewpoint of cost. In addition, there is no need to limit the upper limit of the cooling temperature from the viewpoint of the material.
Due to cost constraints, the upper limit is approximately 50°C/sec. [Example] Next, an example of the present invention will be shown. Table 1 shows the chemical composition (wt%) of the sample material, and the diameter of the Ti-15V-3Cr-3Sn-3Al alloy ingot is 550 mm.

[Table] After heating the ingot with the above ingredients to 1050℃, it was forged to a thickness of 100 mm to obtain a material for hot processing.This material for hot processing was subjected to various hot processing conditions and cooling conditions. The mechanical properties of each were investigated. The results are shown in Table 2. For hot working, a test piece is precipitated from the above hot working material, and the test piece is heated from 1100°C to 875°C.
After heating to a temperature range of 800℃ to 740℃
The tests were carried out over a temperature range of Finished plate thickness is 25mm and 15mm
There are two types. The solution treatment conditions after hot working are 800℃×
20min→Water cooling, aging treatment conditions are 480℃
Two conditions were set: x 14 hr → air cooling (STA1) and 510°C x 14 hr → air cooling (STA2). Further, for comparison, some materials (No. 24 in Table 2) were subjected to hot working and then solution treatment and cold working (5% in the L direction). The mechanical properties of each processed material in Table 2 are data obtained by collecting 10 plate-shaped tensile test pieces in the L direction with a thickness of 7 mm from the center of the plate, a parallel part of 12.5 mm, and a GL of 50 mm under each processing condition. It is. Further, the β grain size in the same table was determined by measuring the β grain size in the LZ plane (thickness section parallel to the rolling direction) using the line segment method for each processing condition.

【table】

[Table] As is clear from Table 2, processing is started after heating the material for hot processing to a temperature of 900°C or higher and 1050°C or lower, and the processing rate is at least 50% at at least the β transformation point or higher and 900°C or lower. And eventually, β metamorphosis point or above 850
℃ or less, the processing rate is 30% or more, and the same processed material is then heated from the β transformation point or higher to 400℃ or less.
When cooling at a cooling rate of ℃/sec or higher (No.1
~14) only, STA1 has a strength of 135Kgf/mm2 ^or more,
Elongation 5% or more, STA2 strength 130Kgf/mm ² or more,
Excellent strength and ductility values with an elongation of 5% or more have been obtained. In addition, the standard deviation of the intensity is less than 0.3, and the variation is small. These mechanical properties are equivalent to the mechanical properties of the material obtained by the conventional method in which a cold working step was added after the hot working step of No. 24 in Table 2. On the other hand, if the processing conditions such as heating conditions, processing rate, finishing temperature, etc. and cooling conditions in the hot working process are outside the scope of the present invention, it will not be possible to obtain both strength and ductility in a well-balanced manner. , the variation in strength is also large. [Effect of the invention] According to the present invention explained above, Ti-15V
In the production of -3Cr-3Sn-3Al alloy materials, in particular, hot-processing materials of the alloy are processed by strictly controlling the heating temperature and processing conditions, and then rapidly cooled under predetermined conditions. An excellent effect can be obtained in that this type of titanium alloy material having excellent mechanical properties can be easily manufactured through only one process.

Claims (1)

[Claims] 1 V: 14 to 16 wt%, Cr: 2.5 to 3.5 wt%, Sn:
In producing a titanium alloy material consisting of 2.5 to 3.5 wt%, Al: 2.5 to 3.5 wt%, balance Ti and unavoidable impurities, processing begins after heating the material for hot processing to a temperature of 900°C or higher and 1050°C or lower. However, the processed material is processed at least at a processing rate of 50% or more at temperatures above the β-transformation point and below 900°C, and at a processing rate of at least 30% at temperatures above the β-transformation point and below 850°C. A method for producing a titanium alloy material with excellent strength and ductility, characterized by cooling from a temperature of 400°C or less at a cooling rate of 2°C/sec or more. 2. The method for producing a titanium alloy material according to claim 1, wherein the titanium alloy material contains 0.3% or less of oxygen.