JPS634908B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a titanium alloy having a homogeneous and equiaxed α crystal structure and excellent mechanical properties. Generally, after producing a titanium alloy ingot, the ingot is broken down by hot forging or blooming in order to shape it into a slab for hot rolling and to destroy the cast structure. In order to destroy the cast structure of the titanium alloy and to reduce the deformation resistance, it is usually heated above the β transformation point, and most of the forging or blooming is performed in the region above the β 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. The titanium alloy forging temperature listed in the Metals Engineering Institute (1969) is the first.
As shown in the table. Although only the forging temperatures are shown in Table 1, the temperatures for blooming are also the same. In the cooling stage after forging or blooming,
A network-like coarse grain boundary α phase precipitates along the prior β grain boundaries, and an α+β lamellar structure becomes coarse within the prior β grains (the α+β lamellar 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 hot rolled in the α+β region and then heat treated, but this hot rolling and subsequent heat treatment produces a fine and homogeneous equiaxed α crystal structure. The purpose is to improve mechanical properties. For example, Tokukai Akira
No. 58-25423 describes that the surface temperature is controlled at 980° C. to 700° C., the degree of processing is 70% or more, and then recrystallization is performed. In general, as the degree of working in the α+β region increases, the α phase that does not form an equiaxed crystal structure tends to decrease. However, this degree of working is also limited at the manufacturing stage, and no matter how Even if it is increased, a structure that does not become an equiaxed crystal remains and has a negative effect on mechanical properties. As a result of intensive research on this point, the present inventor found that the α phase, which does not become equiaxed even after hot rolling and subsequent heat treatment, precipitates at the prior β grain boundaries that occur during the hot forging or blooming process of casting. We found that this is due to the coarsening of the network-like coarse grain boundary α phase and the coarsening of the α+β lamellar phase within the prior β grains. Therefore, after the process of hot forging or blooming rolling of an α+β type titanium alloy ingot, the coarse grain boundary α generated in this process is
In order to eliminate the phase and the coarse lamellar phase, the slab obtained by the hot forging or blooming rolling is heated to a temperature above the β transformation point and below the β transformation point + 150°C, and then heated at a rate of 50°C/min or more. We have developed a method for producing titanium alloy sheets, which is characterized by rapid cooling at a cooling rate and then hot rolling in the α+β region with a cross-section 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, from the viewpoint of quality, which involves completely destroying the forged structure in this process, and from the viewpoint of reducing manufacturing costs by increasing the degree of processing in the β region where deformation resistance is low, forging or blooming at a temperature higher than the β transformation point is preferred. It is preferable to do this. Through this process, a slab is formed and air cooled, but in the slab, coarse grain boundary α phase precipitates in a network shape at the prior β grain boundaries, and coarse α+ phase precipitates within the prior β grains.
The structure has a developed βlamellar phase. However, by heating this slab to a temperature above the β transformation point and below the β transformation point + 150°C, and then rapidly cooling it at a cooling rate of 50°C/min or more, the network-like coarse grain boundary α phase and coarse α+β lamellar phase are removed. It can be made to disappear into α' (martensite) or a fine α+β lamellar phase structure. Furthermore, after that, the cross-sectional reduction rate of the slab in the α + β region
By performing hot rolling of 50% or more, it is possible to accumulate processing strain and recrystallize it as a driving force to obtain a homogeneous and fine equiaxed α-crystal structure. As a result, we succeeded in easily manufacturing titanium alloy plates with excellent mechanical properties. The heating temperature of the slab needs to be higher than the α transformation point in order to eliminate the network-like coarse grain boundary α phase and the coarse α+β lamellar phase, but if it is too high, the surface oxidation will be severe and the β grains will be Since coarsening becomes significant, the upper limit needs to be set to β transformation point + 150°C. Hot rolling with a reduction in area of 50% or more is required in the α+β region in order to store the 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 β-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. Temperatures in the range up to are preferred. The plate that has undergone the hot rolling process in the α+β region is then subjected to annealing, solution aging treatment, etc. to obtain a homogeneous and equiaxed α crystal structure. Next, an example will be described. Example Ti-6Al-, a typical α+β type titanium alloy
Table 2 shows the comparison results between the example of the present invention and the conventional process for 4V alloy. The β transformation point of the test material is
It was 1000℃. The slab was produced by blooming using an ingot with a diameter of 550 mm. Regarding the tensile properties in Table 2, the parallel part is 8.75mm from the center of the plate thickness.
A test piece with a diameter of 35 mm and a diameter of GL of 35 mm was sampled in the final rolling direction and measured. Heat treatment after rolling (STA treatment)
The test was carried out on a board measuring 12.5 mm (thickness) x 100 mm (width) x 125 mm (length). The incidence of non-equiaxed α-crystals was expressed as the percentage of microstructure photographs taken at 70 arbitrary locations in which α-crystals that were clearly not equiaxed were observed. The microstructure observation surface is a cross section parallel to the final rolling direction (L-Z plane), and the field of view of one photograph is
The size was 180×120 μm. As is clear from Table 2, in processes No. 1 to 3 according to the method of the present invention, the generation rate of anisometric α crystals is greatly reduced compared to comparative processes No. 4 to 7, and the tensile strength and yield strength are It can be seen that the strength and ductility of elongation, drawing strength, etc. are significantly superior. Comparison process No. 6 has a processing degree of 30%, and although the other conditions satisfy the method of the present invention, it can be seen that sufficient characteristics are not obtained. 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 and equiaxed structure and excellent mechanical properties.

【table】

【table】

Claims (1)

[Scope of Claims] 1 After the process of hot forging or blooming rolling of an α+β type titanium alloy ingot, in order to eliminate the coarse grain boundary α phase and the coarse α+β lamellar phase generated in the process, the hot forging or After heating the slab obtained by blooming rolling to a range of not less than the β transformation point and not more than the β transformation point + 150°C, rapidly cooling it at a cooling rate of 50°C/min or more,
A method for producing a titanium alloy sheet, which comprises then hot rolling in the α+β region with a reduction in area of 50% or more. 2. The method for manufacturing a titanium alloy plate according to claim 1, which comprises performing heat treatment such as annealing and solution aging treatment depending on the product use.