JPS6245486A - Production of clad steel pipe - Google Patents

Abstract

PURPOSE:To produce a joined clad steel pipe having good adhesiveness by inserting an inside pipe consisting of a two-phase stainless steel contg. specific solid soln. nitrogen into an outside pipe consisting of a carbon steel or low-alloy steel having the specific inside surface roughness, maintaining the inside surface within a specific temp. range and subjecting the pipe to pressurization and expansion from the inside. CONSTITUTION:The inside pipe consisting of the two-phase stainless steel contg. 0.05-0.25% solid soln. nitrogen and contg. Fe, Cr and Ni as the essential components is heated and is inserted into the outside pipe consisting of the carbon steel or low alloy steel having <=30mum inside surface roughness or the pipe is heated after the insertion into the outside pipe, then the inside pipe is pressurized from the inside and expanded in an inert atmosphere while the temp. of the inside pipe is at 750-1,100 deg.C. The parting and spheroidizing of the gamma phase and the recrystallization of the alpha phase are active in the case of such temp. range. The outside surface of the inside pipe and the inside surface of the outside pipe are tightly fitted and joined to each other by the superplasticity. The need for mirror finishing of the joint surfaces is eliminated and the method is effective in improving the quality and decreasing the number of working steps.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention uses duplex stainless steel exhibiting superplasticity for the inner tube and carbon steel or low alloy steel for the outer tube, which has excellent corrosion resistance and high joint strength. , relates to a method for manufacturing clad steel pipes that is also cost-effective.

[Conventional technology]

In pipe bodies that pass corrosive fluids, such as oil pipes and oil country tubular goods,
In addition to sufficient mechanical strength, a high degree of corrosion resistance is required. Stainless steel is commonly used as a metal material with excellent corrosion resistance, but this material is Cr.

Contains a large amount of Ni, and its use is restricted due to cost considerations.

Clad steel pipes are a type of double-walled pipe in which an inner pipe is joined to the inside of an outer pipe, and can be given many functions by selecting the materials for the inner and outer pipes. For example, a tube made of stainless steel for the inner tube and carbon steel or low-alloy steel for the outer tube is resistant to corrosive fluids flowing inside, has high mechanical strength, and is more durable than a tube made of stainless steel alone. It is significantly superior.

There are two methods for manufacturing such a clad steel pipe: one is to form and weld pre-manufactured clad steel plates, and the other is to separately create an outer pipe and an inner pipe and finally join them together.

The former is a common method, but in the welding step E, 2
Different types of materials must be welded separately, which not only complicates the work and increases costs, but also leaves quality problems in the welds. On the other hand, in the latter case, the welding process can be omitted by introducing a seamless pipe, and all problems associated with welding are solved.

However, joining pipes is not as easy as joining plates, and although various joining forms have been proposed, the reality is that sufficient joining strength has not been achieved.

[Problem that the invention seeks to solve]

The conventional basic idea for joining the inner and outer tubes of clad steel pipes is to determine how large a tightening force can be exerted between the outer tube and the inner tube, and how to apply force to the interface between the outer tube and the inner tube. There is no metallurgical bond. Furthermore, even if a metallurgical bond is to be obtained, mirror finishing is required on the surfaces to be joined, which is a problem when joining plates, which significantly increases the number of processing steps.

For example, JP-A-55-117515 and JP-A-57
In all of the manufacturing methods for clad steel pipes described in Publication No. 85684, the joint strength is secured by force by tightening due to expansion and contraction using the temperature difference between the inner pipe and the outer pipe, and metallurgical material is used at the interface. There are no physical connections. Therefore, if heat is applied later, there remains a risk that the tightening force may loosen.

An object of the present invention is to provide a highly efficient manufacturing method that forms a metallurgical bond at the interface between an inner tube and an outer tube and eliminates the need for mirror finishing of the surfaces to be joined.

[Means for solving problems]

In the manufacturing method of the present invention, Fe, Cr, and Ni with a solid solution nitrogen content of 0.05 to 0.25% are added to an outer tube made of carbon steel or low alloy steel with an inner surface roughness of 30 μm or less. The inner tube made of duplex stainless steel is heated and then inserted into the inner tube, or heated after the inner tube is inserted, and while the inner tube temperature is 750 to 1100℃, pressure is applied from the inside of the inner tube to the outside. It is closely bonded to the inner surface of the tube.

[For production]

As is well known, duplex stainless steel has a mixed structure of α (ferrite) phase and γ (austenite) phase, and has excellent properties in terms of strength, toughness, and weldability as well as corrosion resistance. . However, in addition to the poor workability due to its two-phase structure, it is still a difficult-to-process material due to the easily precipitated σ phase. On the other hand, even materials that are difficult to process under normal deformation conditions exhibit unusually high ductility under appropriate conditions, a phenomenon known as superplasticity.

According to recent research by the present inventors, it has been found that duplex stainless steel in which a certain amount of solute N is maintained easily exhibits superplasticity under specific deformation conditions. The amount of solid solute N is 0.05-0.25%, and the deformation conditions are 750-1100.
C., and under these conditions a superplastic elongation of 1000 inches or a strain rate of 1.5.times.10-"S'-" is obtained (% Application No. 60-50542).

The present invention utilizes the superplastic deformation of this duplex stainless steel in the manufacture of clad steel pipes, and by causing the inner pipe inserted into the outer pipe to develop superplastic elongation, the inner and outer pipes can be Not only does metallurgical bonding occur at the interface, significantly increasing the bonding strength, but this metallurgical bonding is not significantly affected by the roughness of the surfaces to be bonded; on the contrary, roughness increases the bonding area.
It also gives rise to the possibility of improving the bonding strength. Therefore, according to the present invention, there is no need for mirror finishing of the joint surfaces,
At the same time, the labor required for processing is significantly reduced, and at the same time, the joint strength is increased across a wide range of areas. This will ensure stable performance (based on shear tests).

In the present invention, the mechanism by which the interface is metallurgically bonded is roughly as follows.

When observed microscopically, there are irregularities at the interface between the gold 44 planes to be joined, making it difficult for the atomic planes to approach each other. However, in the present invention, since duplex stainless steel exhibiting superplasticity is used as one of the metals, self-flow occurs easily along the uneven surface due to this superplasticity phenomenon, and as a result, the diffusion distance is extremely short. Diffusion bonding becomes possible.

[Manufacturing conditions]

a. Carbon steel or low-alloy steel is used for the outer tube, but this is mainly from the viewpoint of cost and mechanical properties, and an appropriate composition is selected from the balance between the desired mechanical properties and the cost required. may be selected from carbon steel or low alloy steel.

The surface roughness of the inner surface is allowed to be up to 30 μm (10-point average roughness defined by JISB-0601 d). 30μ
If it exceeds m, a large amount of superplastic deformation must be applied to the inner tube, which requires processing at a low strain rate, which is unfavorable in terms of manufacturing efficiency.

On the contrary, this small surface roughness means that it is close to a mirror finish, and does not directly affect the bonding condition. Therefore, there is no particular lower limit set, but the smaller the surface roughness, the more time and effort it takes.
Normally, it is desirable to set the distance north of 5 μm.

b1 The inner tube is made of two-phase stainless steel, or its main components are Fe and C.
Although it is possible to obtain a two-phase mixed structure of α phase and γ phase by combining r and Ni, considering the properties and cost of the material obtained, 1. This is because it is more advantageous to have the three elements Fe, Cr, and Ni as the main components, and preferably, the dual-phase stainless steel targeted by the present invention contains Ni: 4-18% and Cr.
:15 to 35%, and in addition to these components, if necessary, Mo≦6.0, Cu≦1%, Ti≦0.
5th section, Zr≦05 section, Me≦0.5%, ■≦0.5chi, W
≦1.0, and C10. Also included are those containing Ca and unavoidable impurities.

More preferably, Ni: 6 to 9%, Cr: 22-
27%, Mo: 1-4%, N: 0.1-0.20%
and as a deoxidizer (0,b~1.5%
It contains Sl and Mn of a certain degree.

The amount of solid solute N is set at 0.05 to 025 degrees, and the amount of solid solution N is set at 0.05 to 0.025 degrees.
This is because adding more than 0.25% of N is technically difficult, and excellent superplasticity can be obtained within this range. Note that the amount of solid solution N is preferably contained in an amount in the range of 0.1 to 0.2%. Zr, Ti, N
b. Even if a minimum amount of V is added to fix a part of N as nitride, as long as the amount of solid solute N can be effectively secured within the above range, it is of course not limited in the present invention.O More preferable is , so that the phase ratio of α phase and γ phase near 1000° C. during hot working is approximately equal, and Cr eq expressed by is approximately three times N1eq.

The reasons for this are two points: to make superplastic joining by hot deformation preferable, and to ensure the required properties of the product.

In other words, if the amount ratio of the α phase and the γ phase is almost the same, the higher the amount of light elements C and N among the γ phase forming elements such as Ni, Mn, C, and N, the higher the amount of the γ phase. This promotes dispersion and spheroidization during deformation, which is advantageous for superplastic deformation. However, since C easily forms carbides and impairs the properties of the product, it is best to reduce it as much as possible. Generally, C is 10.05%.

In this way, most of the superplastic deformation of duplex stainless steel occurs in the two-phase state of α phase and γ phase, and the relatively hard γ phase is divided and spheroidized, and the relatively soft α phase is deformed. This is achieved through the process of dynamic recrystallization of
It is important that 4. This is the H condition.

To manufacture the inner tube, a steel ingot of duplex stainless steel having a composition as shown in E, obtained by the ingot method or the CC method, or in some cases a billet by forging, is processed by the Eugene-Séjournet method or the Mannesmann method. It is possible to make a seamless pipe by rolling, or by welding a steel plate obtained by rolling. The same applies to the outer tube. However, if the inner tube is in trouble, it is better to use a seamless tube because it has a uniform structure and can be deformed evenly since it will be superplastically deformed later and will be brought into close contact with the outer tube.

Although no special pretreatment is required for the obtained tube, for the inner tube to be superplastically deformed, a greater deformation effect may be obtained by adding cold working.

The outer surface of the inner tube may be more rough than the inner surface of the outer tube. That is, no finishing process is particularly required, and the surface may be rough-cut or pickled to remove scale.

The reason for this is that the duplex stainless steel used for the inner tube undergoes superplastic deformation, resulting in good counterflow.

An important aspect of tube dimensions is the difference between the outer diameter of the inner tube and the inner diameter of the outer tube, that is, the clearance when inserting the inner tube into the outer tube, and it is desirable that this be as small as possible as long as it does not interfere with the insertion process. . This is because if the clearance is large, the amount of deformation imparted to the inner pipe becomes large, reducing work efficiency.

Other pipe dimensions may be determined based on the dimensions of the clad steel pipe to be obtained.

c1 Clad process: Insert the inner tube into the outer tube, and while heating the inner tube to 750 to 1100° C., expand the inner tube from the inside.

That is, the inner tube may be heated and then inserted into the outer tube, the inner tube may be inserted into the outer tube and both the inner and outer tubes may be heated, or the inner tube may be inserted into the outer tube and then heated by induction heating, etc. The inner tube may be heated intensively. The point is that when the inner tube is expanded, the temperature of the inner tube is 7.
This means that it is sufficient if the temperature is maintained within the range of 50 to 1100°C.

The heating temperature was 750-1100°C.
Below C, γ-phase fragmentation, spheroidization, α-phase recrystallization, etc.
The action of the thermal activation process becomes insufficient, making it difficult to obtain superplasticity, and at temperatures above 1100°C, the amount of γ phase is extremely reduced, the γ phase as a second phase becomes dispersed and spheroidized, and the α phase is regenerated. The same applies because the effect of promoting crystallization cannot be obtained.

To push (expand) the inner tube, gas may be injected into the inner tube, or mechanical tube expansion means may be used.

What is important from the heating process to the tube expansion process is not to oxidize the surfaces to be joined of the inner and outer tubes. That is, if scale is generated on the surfaces to be joined due to oxidation, it prevents contact between the metal surfaces and metal atoms do not diffuse.

Therefore, it is recommended that heating be performed under an inert gas atmosphere and that the subsequent tube expansion be performed in an inert gas atmosphere or at least with an inert gas maintained or flowing between the inner tube and the outer tube. When expanding the pipe by injecting gas, it is best to use an inert gas as the injected gas.

Any type of inert gas may be used, but N2 gas, which is contained in a large amount in duplex stainless steel, is preferably used.

The pressure required for pipe expansion is sufficient as long as it can secure the contact pressure necessary to create a metallurgical bond at the interface between the inner pipe and the outer pipe, and is usually 1.5 mm per wall thickness of the inner pipe. 5 atm (I
. 5 seconds -) or more, and if work efficiency is taken into account, the distance is 1 m.
It is preferably around 5 atm/art (5.2 to 9/art).

However, if it exceeds 20 atm (20 KVam'), workability will be impaired, so it is desirable that the pressure be less than this.

No particular post-treatment is required for clad steel pipes after the inner pipe has been expanded, but if the outer pipe is heated along with the heating of the inner pipe,
In order to ensure the strength and toughness of the outer tube made of carbon steel or low alloy steel, it is desirable to cool the outer tube to at least 450°C at a cooling rate of 5 to 50'C/sec, depending on the required performance.

〔Example〕

Table 1 shows the component composition, and the dimensions are 16'ODX19mt
The outer tube is made of low-alloy steel, and the component composition is shown in the $2 table.
After fitting the inner tube with dimensions of 14' ODX 4 mm t and variously adjusted inner surface roughness, it was heated to various temperatures in a batch furnace filled with N2 gas, and the inner and outer tubes were welded by gas purging. While maintaining the surface in an inert gas atmosphere,
The diameter of the inner tube was expanded by injecting two gases and using a mechanical expander. For those in which N2 gas was to be injected, both ends of the inner tube were closed in advance, leaving an injection hole. The obtained clad steel pipe was subjected to a shear test specified in JISGO601, and the joint strength between the inner pipe and the outer pipe was investigated. Survey results and
Table 3 shows the specifications of the steel pipes investigated. Note that the outer surfaces of the inner tubes were all left pickled.

Table 1 (wt section) Table 2 (wt section) Table 3 In Example 1, the surface roughness of the inner surface of the outer tube, the composition of the inner tube (particularly the solid solution nitrogen content), and the heating temperature of the inner tube are all the same. This is within the scope of the invention, and the shear strength of the inner tube and outer tube far exceeds 20 mm, which is the objective of the present invention.

In Example 2, the surface roughness of the inner surface of the outer tube was extremely large, exceeding the range of the present invention. In the case of this surface roughness, the conventional amorphous insert diffusion bonding method either does not bond at all, or even if it does bond, only a strength of about 51 can be obtained. However, in the present invention, since the surface roughness was outside the range, the target shear strength was not achieved, but the strength of 16-4 was still secured.

In Example 3, the composition of the inner tube is B, and the solid solution nitrogen content is below the range of the present invention. Therefore, even though all other conditions were within the scope of the present invention, no bonding was performed at all. This means that superplasticity did not occur in the inner tube.

In Example 4, the temperature during expansion of the inner tube was below the range of the present invention, and the shear strength was zero. All other conditions are within the scope of the present invention, so if the heating temperature of the inner tube is insufficient,
It is clear that superplasticity does not occur.

In Example 5, contrary to Example 4, the temperature of the inner tube exceeds the range of the present invention, and it can be seen that superplasticity does not occur in this case as well.

In Example 6, since the pressure applied from the inside of the inner tube was intentionally lowered, it was not possible to achieve metallurgical bonding due to superplasticity. No matter how superplastic, the inner tube must be pressed against the outer tube with a certain amount of force in order to achieve a metallurgical bond. Incidentally, the pressurizing force in this example is 0.5 atm per thickness of the inner tube.

In Examples 1 to 6, the diameter of the inner tube was expanded by injecting N2 gas, whereas in Example 7, the diameter of the inner tube was expanded using a mechanical expander.

Even in the case of mechanical pipe expansion, it is possible to obtain the target shear strength if the various conditions fall within the scope of the present invention.

〔Effect of the invention〕

As is clear from the above description, the present invention enables the inner tube to develop superplasticity in the production of clad steel pipes, thereby achieving metallurgical bonding at the interface, which was difficult with conventional clad pipe manufacturing methods that do not use welding. Not only is it possible to manufacture high-quality rad steel pipes with no welds and high bonding strength, but metallurgical bonding at interfaces using superplasticity is a major processing hurdle for conventional cladding methods in general, whether for pipes or plates. This eliminates the need for mirror-finishing of the surfaces to be joined, which had previously been required, and has a significant effect not only in terms of quality but also in terms of processing man-hours.

Claims (2)

[Claims] (1) Inside the outer tube made of carbon steel or low alloy steel with an inner surface roughness of 30 μm or less, the solid solution nitrogen content is 0.05 to 0.
．． An inner tube made of duplex stainless steel whose main components are 25% Fe, Cr, and Ni is heated and then inserted into the inner tube, or heated after the inner tube is inserted while the inner tube temperature is 750 to 1100°C. A method for producing a clad steel pipe, which comprises: expanding an inner pipe by applying pressure from the inside, and closely joining the outer surface of the inner pipe to the inner surface of the outer pipe. (2) Manufacturing the clad steel pipe according to claim 1, wherein the heating expansion of the inner pipe is performed while at least the surfaces to be joined of the inner pipe and the outer pipe are maintained in an inert gas atmosphere. Method.