JPH02310348A - Manufacture of alpha+beta titanium alloy rolled bar and wire having good structure - Google Patents

Abstract

PURPOSE:To manufacture the alpha+beta titanium alloy rolled bar and wire having low anisotropy and good structure by heating a billet of an alpha+beta titanium alloy to a specified temp. and executing specified hot rolling for controlling the reduc tion of area in a specified temp. range. CONSTITUTION:A billet of an alpha+beta titanium alloy such as Ti-6Al-4V is heated to the temp. from >=900 deg.C to the beta transus-20 deg.C or below. The billet is hot-rolled at 60 to 95% total reduction of area. At this time, the reduction of area in the temp. range of >=900 deg.C is regulated to >=50% and that in the temp. range of <850 deg.C to <=30% to from a structure contg. equiaxed and fine alpha crystal grains. In this way, the alpha+beta titanium alloy rolled bar and wire having excellent specific strength and corrosion resistance, having low anisotropy and good structure as well as having uniformly fine crystalline structure and good forgeability can be obtd.

Description

Detailed Description of the Invention [Industrial Field of Application] The present invention is directed to α crystal grains having equiaxed and fine structures.
The present invention relates to a method for rolling +β type titanium alloy strips and wires.

[Conventional technology] Titanium alloys have a high specific strength and excellent corrosion resistance among metal materials, so they are used for space aircraft, special parts for automobiles, and large chemical industries used under severe corrosive conditions. Used in structures, etc. In these applications, materials typically manufactured in the form of rods, wires, or plates are hot or cold forged or machined into parts. Therefore, the material is required not only to have excellent workability and rigidity, but also to be able to control the quality of parts within a certain range regardless of the processing direction, that is, to have little anisotropy. Improvement in workability in forging involves improvement in ductility, and for this purpose a uniform and fine grain structure is required.

In addition, to reduce anisotropy, the crystal grains are fine and
In particular, it is necessary that the α grains have a constant shape regardless of the rolling direction, that is, be equiaxed.

The present invention proposes a manufacturing method by rolling an α+β type titanium alloy strip and wire, which has fine crystal grains and equiaxed α grains, has little anisotropy, and has excellent workability.

Since α+β type titanium alloy is one of the materials that are difficult to process, rods and wires have traditionally been manufactured mainly by forging. Regarding the manufacturing method, for example, as shown in Japanese Patent Application Laid-Open No. 53-1617, efforts have been made to optimize the combination of processing conditions in forging and heat treatment including cooling method.

However, manufacturing materials by forging is overwhelmingly inferior to rolling in terms of productivity and cost. However, in rolled rods and wires, crystal grains tend to become coarse, especially in the center, and the α grains tend to stretch significantly in the rolling direction, so it has been possible to produce only materials of significantly inferior quality as materials for processing. On the other hand, Japanese Patent Application Laid-open No. 58-25421 has 5
JP-A-58-100663 discloses a method of performing heat treatment at a high temperature after hot rolling with a working degree of 0% or more.
A method is disclosed in which the material is processed in the α+β region and then rolled in the α+β region.

However, with these methods, a heat treatment process at high temperature is essential after hot rolling, and thick rods and wires must be left on the rolling line for a long time to cool from the β range to the α+β range during hot working. Not only did this lead to a decrease in productivity, but also new anisotropy was generated due to the large temperature difference between the surface layer and the center, making it impossible to apply it in practice. Also,
In the method of achieving finer grain refinement or uniform grain refinement through heat treatment after rolling, the rolling temperature is kept as low as possible to promote recrystallization.

It is oriented to leave a lot of deformation strain within the grain being rolled down. For this reason, processing has become increasingly difficult, and the number of rolling methods has increased, resulting in a situation where subsequent finishing processing is difficult.

[Problems to be Solved by the Invention] The present invention is capable of manufacturing alpha-decade-type titanium alloy rods and wires by rolling, without requiring any new or special processes. A manufacturing method is disclosed.

[Means for Solving the Problems] As a result of various studies on the deformation behavior of α grains due to rolling and the subsequent recrystallization behavior, the present inventors have found that the amount of α grains during rolling can be controlled and the rolling reduction ratio can be controlled. The inventors have discovered that by promoting recrystallization after rolling or during cooling, it is possible to achieve refinement of α grains and uniform grain refinement without performing uniform grain refinement treatment by post-heat treatment, and have accomplished the present invention. Ta.

The deformation resistance of α grains at high temperatures is approximately 3 times higher than that of β grains.When rolling a material in which a small amount of α grains exists among the β grains, mainly the soft β grains are rolled and stretched. and hard α
The rolling and stretching of grains is small. Therefore, the present invention aims at rolling at a high temperature with few alpha grains.

Figure 1 shows 6A Q-4V titanium alloy (β transus 9
This is an optical microscopic structure of an L cross-section of a bar obtained by heating a forged billet (95°C) to 950°C and 850°C, pressing it down at an area reduction rate of 84%, and cooling it in air. The material heated at 950°C with few α grains ((a) in the same figure) is the material heated at 850°C with many α grains ((b) in the same figure).
It can be seen that the α grains after compression cooling are fine and equiaxed compared to the above.

Based on the above findings, the present invention was completed as a result of studying the heating temperature and pressure reduction conditions. That is, the technology that forms the basis of the present invention is that in the process of manufacturing α+β type titanium alloy rolled rods and wires, the billet is heated to a temperature of 900°C or higher and lower than β transus -20°C, and the total area reduction rate is increased to 60% or more. 95% or less, area reduction rate is 50% in the temperature range of 900℃ or more
The above is a method for producing α+β type titanium alloy rolled rods and wires with good microstructures, which is characterized by hot rolling with an area reduction rate of 30% or less in a temperature range of less than 850°C.

Next, the reason for limiting the heating temperature of the present invention will be explained.

The heating temperature is limited by the content of α grains before rolling. 9
If heated below 00°C, the area reduction rate that can be reduced becomes small due to the large α content, and it is not possible to secure the reduction rate necessary for recrystallization of β grains, which will be described later. Therefore, 900°C was set as the lower limit.

By heating and rolling at a temperature exceeding β transus -20° C. with the aim of reducing the α grains to the maximum during heating, the α grains are extremely reduced, so that the deformation of the α grains hardly affects the material. However, after cooling, a martensitic acicular structure is generated from the β grains, which not only expands the anisotropy but also significantly deteriorates the fatigue resistance. In order to avoid this, strong working in the α+β region where a large number of α grains exist during cooling (the reason for this is currently under study, but the shear strain within the β grains resulting from the presence of the hard α phase) It is thought that this method is effective in preventing the transformation into a morphological structure, but it is impossible to put it into practical use because the α grains are stretched in this case. Therefore, the upper limit of the heating temperature was set to β transus -20°C.

Next, the reason for limiting the area reduction rate will be described.

Even if β-grain-based deformation is performed, strictly speaking, it is impossible to prevent α-grain deformation. If the total area reduction rate exceeds 95%, the deformation of the α grains becomes large and the ratio of the lengths in the L direction and the T direction after cooling under pressure exceeds 5 times, so 95% was set as the upper limit.

Further, if the area reduction rate is less than 60%, an acicular structure is generated from the β grains after cooling, so 60% was set as the lower limit.

In the present invention, since the key is to deform mainly β grains, not only the heating temperature is set to 900°C or higher, but also the area reduction ratio by rolling is set to 50% or higher in the temperature range of 900°C or higher. Furthermore, it is necessary to suppress the deformation of the α grains as much as possible by setting the area reduction ratio under rolling to 30% or less in the temperature range below 850° C. where the α grains increase.

The α+β type titanium alloy to which the present invention can be applied is a typical Ti
-6A Q -4V, Ti-6A Q -6V-2S
Examples include, but are not limited to, Ti-6A Q-2Sn-4Zr-2Mo alloy.

[Function] In the present invention, by setting the heating temperature to 900°C or higher,
It does not have the effect of limiting the alpha grains within the grains when being rolled down.
Furthermore, by applying a reduction in which the total area reduction rate is 95% or less, the area reduction rate in the temperature range of 900°C or higher is 50% or more, and the area reduction rate in the temperature range below 850°C is 30% or less, α grains can be reduced. This contributes to minimizing the deformation of the structure and keeping it equiaxed. Furthermore, by limiting the total area reduction to 60% or more, the transformation of β grains into fine α grains after reduction cooling is promoted.

As a result of the above-mentioned high-temperature hot rolling, rolling is performed under conditions with low deformation resistance, which not only makes processing easier, but also significantly reduces the occurrence of surface defects and greatly reduces the finishing load in the post-process. It is being

[Example] Next, further explanation will be given with reference to examples.

A Ti-6A Q-4V alloy was vacuum arc melted by a conventional method, hot forged and surface polished to produce a billet of 120a+mφ. This billet was rolled into 87+imφ, 55a++aφ and 25m+mφ by continuous hole rolling mill.
It was rolled into a round bar. Furthermore, some materials were heat treated.

The L direction and T of α grains measured from the optical microscopic structure at the center of the L cross section of these round bars after rolling and cooling or heat treatment.
Table 1 shows the axial length ratio in the direction and the grain size in the T direction of α grains. As is clear from the table, the rolled bar according to the present invention has an equiaxed microstructure.

[Effects of the Invention] As shown above, according to the present invention, α grains can be made into fine grains without using any special rolling method to retain or accelerate cooling during rolling, and without heat treatment after rolling. It has become possible to manufacture α+β type titanium alloy rods and wires with an equiaxed structure. Further, according to the present invention, it is possible to avoid the occurrence of flaws during rolling of α+β type titanium alloy rods and wires, which are difficult-to-process materials, and manufacturing becomes easier. As a result, the manufacturing cost of α+β type titanium alloy rods and wires, which have excellent specific strength and corrosion resistance and have little anisotropy, is reduced, so it can be said that the economic effect is large.

[Brief explanation of the drawing]

Claims (1)

[Claims] (1) In the process of manufacturing α+β type titanium alloy rolled rods and wires, the billet is
Heating to a temperature below 0℃, total area reduction rate of 60% or more to 95%
Below, the area reduction rate in the temperature range of 900℃ or higher is 50
% or more and less than 850°C and has a good α+β structure by hot rolling with an area reduction rate of 30% or less
Manufacturing method of type titanium alloy rolled rod and wire.