JPS6144167A - Production of titanium alloy plate - Google Patents

Abstract

PURPOSE:To obtain an alloy made of the equiaxed crystal structure having uniform quality and excellent mechanical characteristics by subjecting a slab obtd. by hot forging or blooming of an alpha+beta type Ti alloy ingot to quick cooling at a prescribed cooling rate after heating at a specific temp. then hot rolling the slab in an alpha+beta region. CONSTITUTION:The slab obtd. by hot forging or blooming of the alpha+beta type Ti alloy ingot is quickly cooled at >=50 deg.C/min cooling rate after heating to the range of the beta transformation or above and <= the beta transformation point +150 deg.C. Such slab is then hot rolled at >=50% reduction in area in the alpha+beta region. The resultant hot rolled sheet is subjected to a heat treatment such as annealing or solutionization aging treatment according to product applications. The coarse intergranular alpha phase and coarse alpha+beta laminar phase generated in the hot forging or blooming stage can be annihilated if the slab obtd. by the hot forging or blooming is heated to the range of the beta transformation point or above and the <=beta transformation point +150 deg.C in the above-mentioned manner.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention describes a method for producing a titanium alloy having a homogeneous and equiaxed α crystal structure and excellent mechanical properties.

In general, ingot breakdown is performed by hot forging or blooming rolling in order to shape the manufacturing edge of a titanium alloy plate ingot, into a slab for hot rolling, and to destroy the casting structure. In order to destroy the cast structure of the titanium alloy and reduce its deformation resistance, it is usually heated above the β transformation point.
Most of the forging or blooming is performed in the region above the transformation point. Then, after finishing the processing or during the processing, the material is air-cooled (slowly cooled) from the β region through the β transformation point to the α+β region.

Metals Engineering Institute (196
The titanium alloy forging temperatures described in 9) are as shown in Table 1. In this Table 1, only the forging temperatures are shown, and the same applies to the temperatures in the case of blooming rolling.

At the cooling stage after forging or blooming piezoelectricity described above, a network-like coarse grain boundary α phase precipitates along the prior β grain boundaries, and α+β lamellar mtlA becomes coarse within the prior β grains (in addition, α+β1a! The IIθ1iar phase is a plate-like α
It has a structure in which phase and β phase are arranged in layers. ).

The hot rolling slab produced in this process is then α+β
The purpose of this hot rolling and subsequent heat treatment is to form a fine and homogeneous equiaxed α-crystalline structure to improve mechanical properties. For example, JP-A No. 58-25423 describes that the surface temperature is controlled from 980° C. to 700° C. while the working ratio is 70% or more, followed by recrystallization.

In general, the larger the working degree in the α+β region, the less the α phase that does not form an equiaxed heavy structure tends to decrease, but this working degree also has a limit in the manufacturing stage.
Moreover, no matter how much the degree of processing is increased, a structure that does not have equiaxed loads remains, which has a negative effect on the mechanical properties.

As a result of intensive research on this point, the present inventor found that the α phase, which does not become equiaxed during hot rolling and heat treatment of the bales, is located at the prior β grain boundaries that occur during the hot forging or blooming process of the ingot. α + β in precipitated network-like coarse grain boundary α phase and prior β grains
It was learned that this is caused by coarsening of the lamsllar phase.

Therefore, after the process of hot forging or blooming rolling of an α+β type titanium alloy ingot, the coarse grain boundary α phase and the coarse 1a phase generated in this process are
In order to eliminate the mellar phase, the slab obtained by hot molding or blooming is heated to a range from β transformation point to β transformation point + 150°C, and then heated at 50°C/min.
We have developed a method for producing a titanium alloy plate, which is characterized by rapidly cooling at the above cooling rate and then hot rolling in the α+β region with a cross-sectional reduction rate of 50% or more.

The hot-rolled plate thus obtained is subjected to heat treatments such as annealing and solution aging treatment depending on the intended use of the product.

Hot forging or blooming of the α+β type titanium alloy ingot is carried out in the β region above the β transformation point, but the temperature of the material may drop to the α+β region during this forging or rolling. However, in terms of quality and deformation resistance as the casting structure is completely destroyed in this process, forging at a temperature above the β transformation point or It is preferable to perform blooming rolling.

Through this process, a slab is formed and cooled in air, but in the slab, a coarse grain boundary α phase precipitates in a network shape at the prior β grain boundaries, and a coarse α-decade β lam@1llar exists within the prior β grains.
The structure becomes a well-developed phase. However, after heating this slab to a range from β transformation point to l transformation point + 150°C,
By rapid cooling at a cooling rate of 0°C/min or more, the network-like coarse grain boundary α phase and coarse α+βlamel
The lar phase can be eliminated to form α' (martensite) or a fine α+β1 runellar phase structure.

Furthermore, the slab is then hot-rolled in the α+β region with a cross-section reduction rate of 50% or more, thereby accumulating processing strain and recrystallizing this as a driving force to obtain a homogeneous and fine equiaxed α-crystal structure. Can be done. As a result, we successfully succeeded in producing a titanium alloy plate with excellent mechanical properties.

The heating temperature of the slab is set to β in order to eliminate the network-like coarse grain boundary α phase and the coarse α+β lamellar phase.
It is necessary to heat above the transformation point, but if the temperature is too high, the oxidation of the surface will be severe and the β grains will become coarse, so the upper limit should be set to the β transformation point + 150°C. Hot rolling with a cross-section reduction rate of 50% or more is required in the α+β region in order to accumulate processing strain that becomes the driving force for recrystallization.

There are no particular regulations regarding the temperature at this time as long as it is in the α+β range, but
Just below the transformation point, the material temperature may rise above the β-transformation point due to processing heat, and if the temperature is too low, cracks will occur due to processing, so the temperature should be 50°C below the β-transformation point to 200°C below the β-transformation point. Temperatures in the range up to are preferred.

The plate subjected to the hot rolling process in the α+β region is then subjected to annealing, solution aging, etc. to obtain a homogeneous and equiaxed α crystal structure.

Next, an example will be described.

Example Typical α+β pear prickly tough alloy Ti-6Al-4v
Table 2 shows the comparison results between the examples of the present invention and conventional processes for alloys. The β transformation point of the test material was 1000°C. The slab was produced by blooming using an ingot with a diameter of 5,501,111 mm. The tensile properties shown in Table 2 were measured by sampling test specimens with a parallel portion of 6mm φ and GL 35'm from the center of the plate thickness in the final rolling direction. Heat treatment after rolling 9i (8T manual treatment) is 12.5F + 1 (thickness) xlO
The test was carried out using a board with an O length (width) of 25 mm (length). The incidence of non-equiaxed α-crystals was expressed as the percentage of microstructure photographs taken at 70 locations in which clearly non-equiaxed α-crystals were observed. The microstructure observation plane was a cross section parallel to the final rolling direction (LZ plane), and the field of view of one photograph was 180 x 120 am.

As is clear from Table 2, Step A according to the method of the present invention
It was found that for Nos. 1 to 3, the incidence of anisometric α crystals was greatly reduced compared to Comparative Steps A4 to A7, and the tensile strength, yield strength, elongation, drawing strength, etc., and ductility were significantly superior. Ru.

The comparative process unevenness is 30% in processing degree and the others are Ki-nii
+,) The method satisfies L-(I, I also obtained sufficient characteristics.)

I know he's not there. Note that although cross rolling was performed in the α+β region rolling in Table 2, similar results were obtained with unidirectional rolling.

As described above, the method of the present invention is an excellent method for obtaining a titanium alloy plate having a homogeneous, equiaxed m weave and excellent mechanical properties.

Table 1

Claims (2)

[Claims] (1) After the process of hot forging or blooming rolling of α+β type titanium alloy ingot, coarse grain boundary α phase and coarse α+ generated in this process
In order to eliminate the βlamellar phase, the slab obtained by hot forging or blooming is heated to a temperature above the β transformation point and below the β transformation point + 150°C, and then heated at 50°C/
1. A method for producing a titanium alloy sheet, which comprises rapidly cooling at a cooling rate of min or more, and then hot rolling in the α+β region with a cross-section reduction rate of 50% or more. (2) The method for manufacturing a titanium alloy plate according to claim 1, wherein heat treatment such as annealing and solution aging treatment is performed depending on the product use.