JPS5935663A - Manufacture of hot-rolled plate of alpha+beta type titanium alloy - Google Patents

Abstract

PURPOSE:To obtain a hot-rolled plate of an alpha+beta type Ti alloy which is easily workable into a thin plate and has superior cold rollability, by heating the Ti alloy to a temp. in a temp. range whose upper limit is a specified temp. below the beta transformation point and by hot rolling the heated alloy in one direction to a prescribed final plate thickness at a prescribed finishing temp. CONSTITUTION:An alpha+beta type Ti alloy such as a Ti-6Al-4V alloy having 970- 1,000 deg.Cbeta transformation point is heated to a temp. in a temp. range whose upper limit is a temp. 20 deg.C below the beta transformation point. The heated Ti alloy is hot-rolled in one direction to <=5mm. final plate thickness at >=700 deg.C finishing temp. A hot-rolled plate of the alpha+beta type Ti alloy which has extremely superior cold rollability and is easily workable into a thin plate can be efficiently manufactured.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a hot-rolled α+β titanium alloy sheet that can be easily processed into a thin sheet and has excellent cold rollability.

Titanium and titanium alloys generally have excellent corrosion resistance and high specific strength, so they are used not only in aerospace equipment but also in reaction vessels, heat exchangers, turbine materials in the chemical industry, and in environments where chlorine ions coexist. Titanium has been widely adopted in structures and equipment used in the industry, and in addition to the characteristics common to titanium materials in general, titanium has particularly excellent weldability and excellent performance against high-temperature creep. Therefore, demand for α+β type titanium alloys has shown signs of increasing in recent years.

By the way, conventionally, such titanium or titanium alloy materials have been used mainly for the following purposes: ■ Mechanical properties that fully meet standard specifications; and ■ Soundness. With two points in mind: the ability to achieve a fine microstructure, hot working was usually carried out under heating at as low a temperature as possible to obtain the sufficient low-temperature forging effect.

Of course, similar hot working methods are also used to manufacture α+β type titanium alloy sheets, and low-temperature processing, in which the heating temperature and final temperature are kept as low as possible in the final hot rolling, is an exception. <It was being implemented. Furthermore, in this case, the desired product dimensions can be easily obtained.''2.For the purpose of reducing the anisotropy of the material, so-called cross rolling, in which the rolling direction is turned by 9.0 degrees in the intermediate stage of hot rolling, is carried out. It was done normally.

However, although the α+β type titanium alloy sheet produced in this way has good performance as a hot-rolled product, it has extremely poor cold rollability.
Even if the rolling pressure was 20 to 30% υ or less, rolling cracks such as tail cracks, edge cracks, and surface cracks as shown in Figure 1 would occur, making cold rolling impossible. .

Therefore, when trying to manufacture a thin plate based on such a hot-rolled sheet, it is necessary to repeat the process of low-reduction cold rolling, intermediate annealing, and low-reduction cold rolling to reduce the thickness, which requires a large number of rolling and annealing steps. Therefore, it was practically impossible to manufacture thin plates with a thickness of about 3 mm or less on an industrial scale. Furthermore, it is unimaginable to manufacture coiled continuous rolled thin strips using a Kerc cold strip mill, since α+β type hot-rolled sheets of titanium alloy, which have poor cold rollability and cause edge cracks and surface cracks even under low pressure, require a pressure of 1 degree. There was no such thing.

FIG. 1 schematically shows the form of rolling cracks that occur when an α+β type titanium alloy hot rolled sheet is cold rolled. Tail cracks occurring at the edges 2.Ear cracks occurring at the edges perpendicular to the rolling direction 3. It can be clearly classified into three types: and surface cracks 4 that occur on the plate surface.

For this reason, attempts have been made to manufacture α+β type titanium alloy thin sheets using only hot rolling mills, but hot rolling mills have a limited ability to thin the walls and it is impossible to manufacture the desired thin sheets. there were.

The present inventors have aimed to improve the cold rollability of α+β type titanium alloy hot-rolled sheets from the viewpoint of “dual series” and to efficiently produce ultrathin α+β type titanium alloy sheets at low cost, which are expected to have a wide range of applications. As a result of conducting research from the viewpoint that we should first investigate the factors that affect the cold rollability of α+β type titanium alloy hot rolled sheets, we found the following (a) to (e)
). That is, (a)-T Generally, the heating atmosphere of titanium materials is kept oxidizing in order to prevent hydrogen embrittlement, but when titanium powder is hot rolled, the material is exposed to the oxidizing atmosphere in the heating furnace. When exposed, oxygen can be scavenged and penetrated through the surface, forming an extremely hard hardened layer that can be removed by conventional descaling means such as shot blasting, mechanical polishing, or pickling. If the cracks remain over a certain depth on the surface of the cold-rolled material, they become a brittle layer that causes minute cracks during cold rolling, creating a notch effect that promotes edge cracking and surface cracking of the plate. to cause something to happen. The heating temperature has the greatest effect on the size of the hardened surface layer.

Figure 2 shows Ag: 5.96% (the first man is in charge of weight), V: 4.10%, Fe: 0.11%, O: 0.1
For the Tj-6Ag, -4V alloy with a composition consisting of 3%, Ti and other unavoidable impurities, and the remainder, the hot rolling heating temperature (held for 3 hours), hardening depth and cold rollability (here,
Although this is shown in terms of the rolling reduction rate at which surface cracks occur, similar results can be obtained with the rolling reduction rate at which edge cracks occur). It can be seen that when the temperature exceeds a predetermined value, the hardening depth increases rapidly, and in correlation with this, the cold rollability tends to deteriorate significantly.

(bl Normally, the crystal texture of metallic materials is the pole figure (P
It can be determined by measuring ate fj, gure),
For convenience, this can be replaced by the r value (plastic strain ratio) determined by a tensile test, but in α+β type titanium alloys, this r value strongly depends on the hot rolling temperature and rolling direction, and furthermore, There is an extremely good correlation between r value and cold rollability. In other words, in α+β type titanium alloys, the higher the hot rolling temperature, the better the crystal texture for cold rollability is obtained, and the smaller the r value is. Rolling in only one direction without cross rolling during rolling is effective in reducing the r value.

FIG. 3 shows the relationship between r value and cold rollability (edge cracking reduction ratio) using Ti-6AQ, which is the same as that used to obtain the results shown in FIG.
- Figure 4 is a diagram showing the results of an investigation on the 4V alloy. It is clear from FIG. 3 that good cold ductility can be obtained by reducing the r value as much as possible. From FIG. 4, it is clear that increasing the end temperature of hot rolling is effective for reducing the r value.

(C1 Generally, compressive stress and tensile stress act on the cross section of a hot-rolled sheet during roll rolling, and the thickness is reduced while being stretched in the rolling direction. At this time, the tensile stress is
The amount of deformation generated differs depending on the frictional force between the rolls and the material and the temperature difference between the surface side and the inside of the material, but in the case of α''-β type titanium alloys, this compressive deformation affects the cold rollability of the hot-rolled sheet produced. The ratio between the amount of deformation and the amount of tensile deformation that occurs has a significant effect, and when the tensile deformation is larger than the compressive deformation, that is, when excessive shear stress is generated within the material, cold rollability is significantly impaired. The hot rolling condition that most strongly influences the amount of shear deformation is the hot rolling temperature range.

FIG. 5 is a conceptual diagram for explaining a method of laboratory confirmation of the amount of shear deformation during hot rolling. It has been discovered that the amount of shear deformation can be evaluated by embedding the index line 6 inside 5 and measuring the shape of the index line 6' deformed after hot rolling at an angle θ, as shown in FIG. 5(b). It is established.

Figure 6 shows the hot rolling temperature range (temperature difference between rolling start temperature and end temperature) for Ti-6AQ and -4V alloys,
This is a diagram showing the relationship between the θ value and cold rollability measured as described above, and the closer the θ value is to 180°, the smaller the shear deformation amount is, and the closer the θ value is to 0°, the opposite is true. As is clear from the figure, the smaller the temperature difference between the start and end of rolling, the larger θ becomes and the amount of shear decreases, and in contrast, the cold rollability is improved.

(a) Furthermore, if the finished hot-rolled plate thickness reaches a certain specific value, cold rollability is good if it is smaller than that, but if it exceeds that value, it shows an extreme tendency to deteriorate.

(e) Therefore, in order to suppress the formation of a hardened surface layer on the hot-rolled sheet material, the hot-rolling heating temperature is kept relatively low, and the hot-rolling end temperature is kept at a predetermined temperature range, resulting in a low r value. If hot-rolling is carried out within a temperature range that allows a crystal texture to be obtained and the amount of cross-sectional shear deformation to be small, and a hot-rolled sheet with a thickness below a predetermined thickness is manufactured, extremely α+β type titanium alloy hot rolled sheet with good cold rolling performance was obtained,
It is possible to obtain extremely thin cold-rolled sheets with good properties not only by sheet mills but also by continuous coil rolling using cold strip mills.

This invention was made based on the finding in Section 41, and it is possible to prepare an α+β type titanium alloy by 20°C below its β transformation point.
After heating to a temperature range with a low temperature of 1,
Finishing temperature near 0°C or lower'' 2. By hot rolling in the same direction so that the finished plate thickness is 5 mm or less, one cold rolling without intermediate annealing results in an edge cracking reduction rate of 5o%.
The present invention is characterized in that it provides an α+β type titanium alloy hot-rolled sheet that has excellent cold rollability and can be easily thinned.

The α+β type titanium alloy that is the object of the method of this invention is Ti, -6AM-4V alloy (β transformation point: 9
70~1000℃), Ti-6AQ-6V-2Sn
Alloy (β transformation point: 950-970°C) + T1-3 M
-2,5V alloy (β transformation point: 910-930C).

Ti-2AQ-2Mn alloy (β transformation A1890-92
0°C), Ti-'6Ae-2 Sn-4Zr-2Mo
Refers to all titanium alloys that have a structure in which α and β phases coexist at room temperature, such as alloys (β transformation point: 980 to 1020°C), and are limited to specific types. Of course, it is not a thing.

In addition, "21 surface crack occurrence pressure F ratio: 5o% or less" in one cold rolling without intermediate annealing, as in the case of two series.
The reason why we used the property ``70% or more'' as a guideline for cold rolling property is that if it has this level of cold rolling property, it will be a good cold rolled coil that will not crack even if it is subjected to strip method rolling with a high processing rate. This is because it is possible to obtain

The reason why the heating temperature of the material to be rolled, the rolling end temperature, and the finished plate thickness are numerically limited as described above in the method of the present invention will be explained below.

■ Heating Temperature Figure 2 above, which shows the relationship between hot rolling heating temperature, hardening depth, and cold rollability, shows that the cold rollability of a hot rolled sheet is determined when the hardening depth is 0.2 m.
It clearly shows that exceeding 71L causes rapid deterioration, and in order to maintain good cold rollability, the hardening depth should be set to 0.2 mm or more, that is, the hot rolling heating temperature should be set to 950
This shows that it is necessary to keep the temperature below ℃. this is,
T'1-6A+ with a specific composition with a β transformation point of 970°C! -4
This is the value for the V alloy, but similarly for other α-10β type alloys, the limit of the surface hardening layer (0.2 in), which determines the upper limit of the heating temperature, depends on the β transformation point of the alloy. The temperature is 20°C lower than the β transformation point, that is, (β
If heated to a temperature exceeding the transformation point (-20°C), the hardened surface layer will suddenly become deeper and the cold rollability will deteriorate.
The hot rolling heating temperature limit is set to 2 below the β transformation point of the alloy.
The temperature was set as 0°C lower.

■ Figure 3, which shows the relationship between end-of-rolling temperature r-value and cold rollability, shows that in order to exhibit good cold-rollability with an edge cracking reduction of 50% or more, the r-value must be about 0.05 or less. It is clear that in order to obtain such an r value, the final temperature must be set to 7.
It is clear that the temperature must be 00°C or higher. This value was applicable to all α+β type titanium alloys.

That is, if the rolling end temperature is less than 700°C, desired cold rollability cannot be obtained, so the end rolling temperature is set at 700°C or higher.

■ Finished Plate Thickness Figure 7, which shows the relationship between hot-rolled finished plate thickness and cold rollability, shows that when the finished plate thickness exceeds 5 mm, the cold rollability decreases rapidly, and the reduction rate at which surface cracking occurs exceeds 170%. It turns out that it will not be possible to secure it. This value also showed a constant value regardless of the type of α+β type titanium alloy.

That is, if the finished plate thickness exceeds 5 mm, the cold rollability deteriorates rapidly, so the value was determined to be 5 mm or less.

Furthermore, from Figure 6, which shows the relationship between the hot rolling processing temperature range, shear deformation amount, and cold rollability, the reduction ratio at which surface cracking occurs is 70%.
In order to obtain the above-mentioned good cold rollability, the amount of shear deformation must be
It can be seen that the hot rolling temperature range must be such that θ shown in Figure (b) is 55° or more, that is, 250°C or less, but this value is different from the heating temperature determined in the method of the present invention. This is exactly applicable to the difference in hot rolling end temperature, and if the method of the present invention is implemented, this requirement will also be inevitably satisfied.

Next, the present invention will be explained using Examples and comparing with Comparative Examples.

Example First, an α+β type titanium alloy blank plate having the composition shown in Table 1 was manufactured by a conventional method.

The thickness of this material plate was as shown in Table 2.

Next, these blank sheets were hot-rolled under the conditions shown in Table 2 to produce hot-rolled sheets having a predetermined thickness. When the cold rolling performance of the obtained hot rolled sheet was measured, the values shown in Table 2 were also obtained.

Note that the asterisks in Table 2 indicate conditions that deviate from the conditions of the method of the present invention.

From the results shown in Table 2, the cold rollability of the α+β type titanium alloy hot-rolled sheets produced by the method of the present invention shows that the rolling reduction at which edge cracking occurs is 55% or more, and surface cracking occurs even at a rolling reduction of 70% or more. The first son is white, which is so good that it does not cause any problems.

On the other hand, it can be seen that all of the hot-rolled sheets manufactured by the comparative method in which the hot-rolling conditions or the finished sheet thickness deviate from the conditions of the method of the present invention have extremely poor cold-rollability.

According to the present invention, it is possible to efficiently produce an α+β type titanium alloy hot rolled sheet that has extremely excellent cold rollability and can be easily made into a thin sheet. Since there is no need to turn the rolling direction from hot rolling to finish hot rolling, continuous coil rolling using a strip mill can be used to further improve production efficiency, which has many useful effects in the industry. is brought about.

[Brief explanation of drawings]

Figure 1 is a schematic diagram showing the morphology of rolling cracks, Figure 2 is a diagram showing the relationship between hot rolling heating temperature, surface hardening depth, and its cold rollability, and Figure 3 is a diagram showing the relationship between r value and cold rollability. A diagram showing the relationship, FIG. 4 is a diagram showing the relationship between hot rolling heating/end temperature and r value,
Figure 5 is a schematic diagram showing the method of measuring the amount of cross-sectional shear deformation, Figure 5 (a) is a diagram showing the situation before rolling, Figure 5 (b)
1 is a diagram showing the situation after rolling, Figure 6 is a diagram showing the relationship between the shear deformation amount and cold rollability with respect to the temperature difference between the start and end of rolling, and Figure 7 is a diagram showing the relationship between the finished hot rolled plate thickness and cold rollability. It is a line diagram showing a relationship. In the drawings, l...Bhutan alloy cold-rolled plate, 2...tail crack, 3
...Ear crack, 4.Surface crack, 5.Hot rolled material, 6.Indicator line, 6'...Deformed index line. Applicant Nippon Stainless Co., Ltd. Agent Tomi 1) Shun Huo and 1 old man 1 Figure 2 Mokukenka Ayuon J('C) 3rd Prisoner 4N 300 500 700 900 Luxury Dinner' Hishiki-Yu 46/ -Note 1 (Q) (b) Year 6 million MIJ mother-in-law 1 body length n temperature and collapse difference (0C nzu 7 figure thickness (mm)

Claims (1)

[Claims] After heating an α+β type titanium alloy to a temperature range whose upper limit is 20°C lower than its β transformation point. This is α+β type titanium with excellent cold rollability, which is characterized by hot rolling in the same direction such that the final temperature is near 00℃ or less, and the finish plate thickness is 5mm or more. Method for producing hot rolled alloy sheet.