JPH01197005A - Manufacture of titanium seamless pipe - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a method for manufacturing seamless pipes made of industrially pure titanium or α-type or α+β-type titanium alloys, and more specifically, the present invention relates to a method for manufacturing seamless pipes made of industrially pure titanium or α-type or α+β-type titanium alloys. Relating to a combined manufacturing method.

[Conventional technology]

Titanium is classified into pure titanium and titanium alloys such as α-type and α+β-type. As an α-type titanium alloy, Tl-
0,15Pd, Ti-0,8Ni-0゜3Mo, T(-
5AI-2, 53n, Ti-8ATi-8AI-I, etc. As an α+β type titanium alloy, Ti-3Al-
2,5V, Ti-6AIt-4V, TiTi-6A1-
6V-23nSTi-6AI-23n-4Zr-6,T
i-6Aj! -23n-4Zr-2Mo, etc., and T
i-3AI-2,5V is known as an alloy that can be cold worked.

Titanium is lightweight and has high corrosion resistance, and its seamless pipes are particularly expected to be applied to chemical plants and aircraft hydraulic piping.

Incidentally, hot pipe manufacturing methods such as extrusion methods and inclined rolling methods are well known as conventional methods for manufacturing seamless metal pipes.

One type of extrusion method is the Eugene-Sejournet method, which is a method of hot extrusion molding into a tube using a glass lubricant. show).

In the inclined rolling method, a solid billet is perforated using an inclined rolling mill called a piercer, and the obtained hollow tube is rolled using a stretching mill such as a mandrel mill or a plug mill, and then a reduction rolling mill called a reducer or sizer is used. Generally, it is finished to a predetermined wall thickness and outer diameter.

Titanium inherently has poor hot workability, and the former extrusion method is mostly used to produce seamless titanium pipes.

The latter inclined rolling method has high production efficiency and is advantageous in terms of production cost, but there has been no example of it being applied to the production of seamless titanium pipes.

[Problem to be solved by the invention]

However, titanium is active and easily seizes, so even if the former extrusion method is used, the surface after extrusion will be poor and the outer surface of the extruded tube must be ground. Moreover, the extrusion method has low manufacturing efficiency and requires pre-processing such as drilling of the billet. Therefore, the yield is poor and an increase in manufacturing costs is unavoidable.

In addition, when the latter highly efficient inclined rolling method is applied to the production of titanium seamless pipes, the processing speed is high, and the unevenness of the structure occurs due to local temperature rise, and in some cases, coarse structures may be formed after hot rolling. A fatal problem arises in that the acicular structure or processed structure remains and reduces the ductility of the product.

The present invention uses a combination of inclined rolling method and cold working to achieve
The object of the present invention is to provide a method for efficiently and economically producing a titanium seamless structure having excellent mechanical properties.

[Means for solving problems]

According to the research conducted by the present inventors, when the inclined rolling method is applied to the production of seamless titanium pipes, the rolling conditions in the final rolling mill, the reducing mill, have a large effect on the properties of the pipe after hot rolling. It has been found that when cold secondary processing is performed after hot rolling, this secondary processing is also greatly affected.

Figure 2 shows the transformation temperature of pure titanium and titanium alloy, V, MO
According to diagram 0, which is a phase diagram schematically showing the relationship with the amount of β-phase stabilizing elements such as % Fe, Cr, Mn, etc., as the β-phase stabilizing element increases, the temperature changes from β phase to α + β phase,
That is, it is shown that the β transus line decreases.

The stable processing temperature range of pure titanium and titanium alloy plates or bars is below the β transus line. When titanium is at a high temperature above the β transus line, it becomes a β single phase, and when forging or plate rolling is performed in this temperature range, a coarse acicular structure is generated during cooling after processing, and hot processing Cracks occur during subsequent secondary processing (cold processing).

However, when pure titanium or titanium alloy is rolled into a tubular shape,
When the material undergoes a large deformation strain in a three-dimensional state such as in the inclined rolling process, if the material leaves the final rolling mill at a temperature below β transus + 50°C, it becomes acicular & 1l.
Although va is formed, its structure becomes fine and it is clear that no cracks occur during secondary processing (cold working) after rolling. If cold working can be performed as secondary processing on a tube after hot rolling, the mechanical properties of the tube, especially the strength, will be significantly improved.

On the other hand, since titanium easily absorbs oxygen, when the inclined rolling method is applied to the production of seamless titanium pipes, a hardened layer is formed on the inner surface of the pipe after elongation and rolling due to oxygen absorption. If a tube with a hardened layer formed on its inner surface is processed using a rolling mill that does not constrain the inner surface, such as a reducing mill, cracks will occur on the free surface of the inner surface due to the hardened layer.

According to the research conducted by the present inventors, the material is
When the material exits at temperatures above 500°C, these cracks remain within the hardened layer, but when the material exits the reducing mill at temperatures below 500°C, the cracks in the hardened layer become the starting point and form inside the tube due to the low ductility of the material itself. It became clear that a large crack with a depth of about 0.5n was generated from the surface. The hardened layer itself has a depth of about 0.1 fi, so if the crack remains within the hardened layer, it can be removed by simple internal cutting, but if the crack spreads, it becomes difficult to remove it by internal cutting. Even if removal is possible, a significant decrease in yield due to removal is unavoidable.

Cold working after hot rolling is effective for improving mechanical properties, especially strength, and when strength is mainly required, cold working is
After the next processing, stress relief annealing can be performed to produce a product. On the other hand, it has become clear that for materials requiring ductility, it is effective to perform annealing at 500° C. or higher and below the β transus after secondary processing. This annealing eliminates the processed structure that occurs during secondary processing and contributes to improving ductility.

The present invention has been made based on this knowledge, and is applicable to the production of seamless pipes made of pure titanium or α-type or α+β-type titanium alloys when hot piercing rolling, elongation rolling, and reduction rolling are performed in sequence. , compared the rolling mill outlet temperature to 500
℃ or more and β transus or less, and after hot rolling, perform cold secondary processing with a working degree of 15% or more, and then, if necessary, perform annealing at 500°C or more and β transus or less. The summary of this paper is a method for manufacturing seamless titanium pipes.

[For production]

In the method of the present invention, the reducing rolling mill exit temperature is set to 500°C or higher and β transus +50°C or lower, because if rolling is finished below 500°C, large cracks originating from the hardened layer will occur on the inner surface of the tube due to a decrease in ductility. Arose, β transus +50
This is because if rolling is completed at a temperature exceeding .degree. C., the structure will become coarse during cooling after rolling, and cracks will occur during cold secondary processing due to the coarsening.

The working degree (cross-sectional working degree) in cold secondary processing is 15
% or more because if it is less than 15%, sufficient mechanical properties cannot be obtained.

The reason why annealing after secondary processing is performed at a temperature of 500°C or higher and lower than the β transus is because the effect of annealing itself cannot be obtained at lower than 500°C, and at a temperature exceeding the β transus, coarse acicular &II weave will occur during annealing. , on the contrary, the ductility decreases.

〔Example〕

In order to clarify the effects of the present invention, two types of comparative tests were conducted. Table 1 summarizes the materials, processes, and performance tests in each comparative test.

○ Comparative test! Comparative test TEAL against Tl-3AI-2,5V.

This alloy is an α+β type titanium alloy that is easy to cold work, and its β transus is 930°C.

Incline rolling is carried out by hot rolling a solid billet using a piercing rolling mill and an elongation rolling mill to form a blank tube of 12 wt. It was assumed to be rolled.

The obtained hot-rolled tube was milled by 0.2 mm on the inner and outer surfaces for descaling, and after examining the inner surface of the tube for cracks, it was subjected to secondary processing using a cold pilger rolling mill. Alms, 4 more
Stress relief annealing was performed at 00°C for 2 hours. Then, the annealed tube was subjected to a room temperature tensile test according to ASTM8338.

The results are shown in Table 2 in correspondence with the rolling conditions in the stretch reducer, which is the final rolling mill, and the secondary processing conditions after rolling.

In mt, the reducer rolling end temperature is β transus (93
0°C) exceeds +50°C, cracks occur on the inner surface of the tube after secondary processing, and the overall evaluation is poor. This is because the hot rolling was completed at a high temperature, which caused the structure to coarsen during the cooling process after rolling, resulting in a decrease in ductility.

In Na2 to Na4, the end temperature of reducer rolling was less than 500°C, and cracks were generated on the inner surface of the tube after reducer rolling. Even though internal milling of 0.2 tsuru was carried out before the crack investigation, the cracks were detected because a hardened layer of about 0.1 fi due to oxygen absorption was formed on the inner surface of the tube before the reducer rolling. This is because this hardened layer cracked during reducer rolling, forming notches, and the cracks deepened (progressed) into the inside of the pipe wall.

In other words, such cracks occur at a rate of 0.2 after reducer rolling.
It is impossible to remove it by internal milling of about m. In addition, 1. This cracking cannot be prevented even if the degree of outer diameter working after reducer rolling is lowered.

In floors 5 to 7, the degree of secondary processing is less than 15%, so the yield point and tensile strength do not meet the ASTM B337 standard.

On the other hand, in Nos. 8 to 13, both the end temperature of reducer rolling and the working degree of secondary processing are within the range of the present invention, so no cracks were observed after reducer rolling or secondary processing, and the room temperature tensile properties were also within the standard. I am satisfied with everything. The solid billet-to-product yield is 82 to 75%, which is greatly improved compared to 65% in the conventional Nigin extrusion method.

O Comparative Test■ In a comparative test against industrially pure titanium, ASTM Grad
Em equivalent material was used. Its β transus is 915℃
It is.

Hot tilt rolling and cutting after tilt rolling were conducted using comparative test NaI.
is the same as After cutting the inner and outer surfaces, secondary processing was performed using a cold pilger rolling mill or cold drawing, and further annealing was performed using some materials to ensure ductility.

Table 3 shows the results when the obtained product was subjected to a test similar to Comparative Test I.

1) h1. In No. 2, the reducer rolling end temperature exceeds β transus (915° C.) + 50° C., cracks occur on the inner surface of the tube after secondary processing, and the overall evaluation is poor. This cracking does not depend on the type of secondary processing.

This is because coarse acicular crystals are formed during the cooling process after reducer rolling. Since cracks occurred during secondary processing, annealing after secondary processing was omitted.

Na3-5 are cases where the reducer rolling end temperature is less than 500°C. Cracks were observed even when the inner surface was cut after reducer rolling. The cause is that, as in the case of Comparative Test (3), cracks that occurred in the hardened layer up to the stretch rolling were spread by the reducer rolling and were not removed by the inner surface cutting after the reducer rolling. Since cracks occurred during reducer rolling, secondary processing and annealing after rolling were omitted.

Il&lL6 to 10 are cases where the processing degree of secondary processing is less than 15%. The tensile strength of the fired 84 material did not reach the standard value (45.8 pupil f/m" or higher).

1) hll~13 is β when the annealing temperature is less than 500°C
This is the case beyond the transus. The elongation does not reach the standard value (18% or more) in any case, because recrystallization does not proceed during annealing at temperatures below 500°C, and coarse acicular structures are generated by annealing at temperatures above the β transus.

On the other hand, for Na14-25, the end temperature of reducer rolling, the degree of secondary working, and the annealing temperature are all within the range of the present invention, so no cracks are observed after reducing rolling or secondary working, and the tensile performance of the annealed material is is ASTM B337
It satisfies all the standards. The yield between the solid billet and the product is 78 to 82%, which is greatly improved compared to the 65% produced by the Nijin extrusion method.

〔Effect of the invention〕

As described above, the present invention efficiently and economically manufactures titanium seamless pipes with excellent mechanical properties by combining the inclined rolling method and cold working, and reduces the manufacturing cost of titanium seamless pipes. It has great effects.

[Brief explanation of the drawing]

FIGS. 1(A) and 1(B) are conceptual diagrams showing the thermal history in the method of the present invention, and FIG. 2 is a phase diagram schematically showing the relationship between the transformation temperature and the β-phase stabilizing element.

Claims (2)

[Claims] (1) In the production of seamless pipes made of pure titanium or α-type or α+β-type titanium alloys, when performing hot piercing rolling, elongation rolling, and reduction rolling in sequence, the exit temperature of the reduction rolling mill should be set to 500°C or higher. , β transus or less, and after hot rolling, cold processing is performed with a degree of working of 15% or more. (2) In the production of seamless pipes made of pure titanium or α-type or α+β-type titanium alloys, when performing hot piercing rolling, elongation rolling, and reduction rolling in sequence, the exit temperature of the reduction rolling mill should be set to 500°C or higher. , β transus or less, and after hot rolling, cold processing is performed with a working degree of 15% or more, and then further annealing is performed at 500°C or more and β transus or less. Production method.