JPH09168878A - Manufacture of duplex stainless steel welded tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide method for manufacturing with high productivity a duplex stainless steel welded tube having an excellent toughness and corrosion resistance. SOLUTION: This method comprises the steps of (a) preparing a duplex stainless steel sheet composed of, as the chemical composition, by wt., <=0.030% C, <=1.0% Si, 0.8-2.0% Mn, <=0.03% P, <=0.01% S, 20-30% Cr, 2.5-4.0% Mo, 4.0-7.0% Ni, 0.08-0.20% N, (b) forming this steel sheet into an open pipe, (c) applying laser beam welding with both edges of the steel sheet butted after both edges are heated as they are or with an electric resistance method or an induction heating method and (d) applying an arc welding to the welded zone from one side of either the outer or inner surface of the tube or both sides.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for manufacturing a duplex stainless steel welded steel pipe.

[0002]

Duplex stainless steel is a material for chemical equipment,
A steel material with excellent corrosion resistance used in the fields of line pipes and oil country tubular goods. The metallic structure is a composite structure consisting of a ferrite phase and an austenite phase, and contains Cr, Ni, Mo, N
By controlling the amount of the component, it exhibits excellent corrosion resistance in a chloride ion environment and a carbon dioxide gas environment. Also,
It is known that strength such as proof stress and tensile strength is greater than those of austenite and ferrite.

In the case of producing a thick-walled and large-diameter welded steel pipe using the duplex stainless steel having excellent corrosion resistance and strength characteristics, a method of welding from the inner and outer surfaces of the pipe by a submerged arc welding method is generally adopted. Has been. Submerged arc welding is a highly efficient welding method capable of high heat input welding as compared with other arc welding methods, and it is also possible to join steel pipes having a thick plate by welding one pass each on the front and back sides.

In addition to this, laser welding is another welding method that has been receiving attention in recent years. The laser welding method is a method in which a laser beam is focused by a lens or a mirror, an object is locally melted, and at the same time, an inert gas is jetted to perform welding while sealing. The focused laser beam has an extremely high energy density of 10 ⁶ W / cm ² or more, and welding is performed while forming holes (keyholes) that penetrate both edges of the abutted steel plates. Is.

Since the laser welding method uses rapid heating and cooling, the solidification structure of the welded portion is fine, the strength and toughness levels are high, and the hot crack resistance is good. In addition, unlike the submerged arc welding method, the laser welding method does not use flux, so defects such as slag entrapment do not occur, and since welding is performed in an inert gas atmosphere, the oxygen concentration in the welded part is suppressed to the level of the base metal. There is no deterioration in quality. Further, the laser welding method has high welding speed and excellent productivity.

[0006]

However, since submerged arc welding uses welding flux in the atmosphere, it is inevitable that gases such as oxygen and nitrogen are mixed from the atmosphere. Gas component elements such as oxygen and nitrogen are melted into the fusion zone, and the concentration of oxygen and nitrogen in the entire weld metal is increased. Therefore, oxides and nitrides are formed over the entire weld metal, that is, over the entire thickness of the weld. In the end, there was a problem that the toughness level of the entire welded portion was lowered.

As a countermeasure against this, a flux having a high basicity is used to reduce the amount of oxygen in the weld metal.
The use of this highly basic flux is also optimal from the viewpoint of corrosion resistance, which prevents non-uniformity of the component concentration that may be the starting point of pitting corrosion.

However, since there is a limit to the reduction of the oxygen content in the weld metal by the highly basic flux, and there is no effect on the reduction of the nitrogen content, the toughness reduction is unavoidable after all. Further, the highly basic flux has a problem that defects such as slag entrapment and undercut are likely to occur.

Further, when the front and back one-pass welding is performed by submerged arc welding, there is a problem that the metal structure becomes coarse and the degree of high temperature cracking increases because of the high heat input welding method. Regarding the welding speed, since the highly basic flux has a high melting point, it is difficult to melt and is not suitable for increasing the welding speed.

On the other hand, in the laser welding method, the formation of oxides and nitrides during welding basically does not occur, so that the toughness level does not decrease. Further, since an inert gas is used instead of slag, there is no slag entrapment.

However, in the laser welding method, it is necessary to apply a compressive force (upset force) to the molten portion so that the undercut of the molten portion does not occur so that the molten metal is in the same plane as the plate thickness. There is. Such precise control is
It is not technically easy in ordinary pipe making equipment, and it is usually necessary to control the molten portion so that it travels slightly from the inner and outer surfaces, and then perform cutting, or deburring, of the molten portion.

Regarding the capability of the laser welding method, since there is a technical limit to the capacity of the laser generator, there is a limit to the welding speed corresponding to that limit, and it is difficult to achieve higher speeds.

Regarding the application of the steel pipe, generally, in the construction of the welded steel pipe, it is necessary to shift the welded portion of the steel pipe to be connected at an angle in the outer peripheral direction of the steel pipe. In that case, in a welded steel pipe produced by normal arc welding, the welded portion (seam) is raised like a bead, and the welded portion can be easily detected. In addition, when used in pipelines, automatic inspection is performed to detect defective parts such as corrosion and scratches of steel pipes, but in that case there is no bead in the steel pipe by laser welding method, so the defective part and welded part are distinguished Will be difficult.

An object of the present invention is to solve these problems and to provide a method for producing a duplex stainless welded steel pipe having excellent toughness and corrosion resistance with high productivity.

[0015]

The invention of claim 1 is a method for producing a duplex stainless steel welded steel pipe, which comprises the following steps. (B) As a chemical composition, wt%, C: 0.030% or less,
Si: 1.0% or less, Mn: 0.8 to 2.0%, P:
0.03% or less, S: 0.01% or less, Cr: 20-30%,
Mo: 2.5-4.0%, Ni: 4.0-7.0%, N: 0.08
The steps of preparing a duplex stainless steel sheet containing 0.20% of steel, (b) the forming step of forming this steel sheet into an open pipe, and (c) the opposite edges of the steel sheet formed into the open pipe, as they are. Alternatively, a laser welding step of laser welding by heating by an electric resistance method or an induction heating method and then irradiating with a laser beam by butting.
(D) An arc welding process in which arc welding is performed on the laser-welded seam portion from either or both of the pipe outer surface side and the pipe inner surface side.

First, the reasons for limiting the chemical composition (expressed in wt%) of the duplex stainless steel according to the present invention will be described. In addition, it is not always necessary to add elements (C, P, S) having no lower limit.

C: C forms a Cr carbide and a Cr deficient layer is formed with it, and if it exceeds 0.030%, the corrosion resistance deteriorates. Therefore, in order to prevent this, the upper limit is made 0.030%.

Si: Si needs to be added as a deoxidizing agent in an amount of 0.1% or more, but an excessive addition exceeding 1% lowers the hot workability, so the upper limit is made 1%.

Mn: Mn must be added in an amount of 0.8% or more in order to secure the solid solubility of N, which is an element effective for pitting corrosion resistance in a chloride environment. On the other hand, excessive addition exceeding 2%
The upper limit is 2 because it reduces the pitting corrosion resistance under hydrogen sulfide environment.
%.

P: P deteriorates toughness, and if it exceeds 0.03%, stress corrosion cracking resistance is deteriorated regardless of chloride environment or hydrogen sulfide environment, so the upper limit is made 0.03%.

S: If S is a conventional steel level content,
Although it does not essentially affect the corrosion resistance, if it exceeds 0.01%, the hot workability deteriorates, so the upper limit is made 0.01%.

Cr: Cr is an element effective in corrosion resistance, but 20
If it is less than%, sufficient pitting corrosion resistance cannot be obtained, and if it exceeds 30%, the hot workability deteriorates, so the suitable range is made 20 to 30%.

Mo: Mo is an element effective in chloride corrosion resistance, but if it is less than 2.5%, sufficient pitting corrosion resistance cannot be obtained, and Mo is less than 4%.
, The hot workability will deteriorate.
~ 4%

Ni: Addition of 4% or more of Ni is effective for ensuring toughness, but addition of Ni in excess of 7% lowers pitting corrosion resistance, so the appropriate range is 4 to 7%.

N: N is effective for adjusting the ferrite fraction, and when added in an amount of 0.08% or more, it is effective for improving chloride corrosion resistance. On the other hand, if it exceeds 0.20%, the corrosion resistance decreases due to precipitation of Cr ₂ N, so the appropriate range is 0.08 to 0.2%.

Other elements may be contained or added as long as the original purpose is not impaired. A duplex stainless steel sheet containing such an element is prepared. Here, preparing a steel sheet means manufacturing or procuring a hot rolled steel sheet.

After this step, the step of forming the duplex stainless steel plate into an open pipe is performed. In the forming step, the steel plate is formed into a tubular shape by plastic working such as bending or pressing, as in the ordinary pipe making method. Here, forming the steel sheet into an open pipe means forming the edge portion of the steel sheet into a shape before welding.

After that, a part or most of the plate thickness of the edge portions of the steel plates to be joined is laser-welded, but since the welding speed is high, the productivity can be improved. Moreover, before welding, both edge portions of the steel sheet are heated by an electric resistance method or an induction heating method, and the heated edge portions are butted against each other and welded by a laser beam, so that further speedup can be achieved. Also,
By laser welding, the amount of oxygen that penetrates into the weld can be reduced, there is no slag entrapment, and the width of the heat-affected zone can be made extremely narrow.

In the laser welding of the present invention,
It is not necessary to control the protrusion (burr) and the undercut of the fusion zone. Burrs and undercuts in the welded portions of these laser welds are melted in the next arc welding process and the traces do not stop.

After that, since arc welding is performed on the laser-welded welded portion, the heat input amount of arc welding is much smaller than that in the case of welding in one pass on the front and back. Therefore, according to the present invention, coarsening of the metal structure of the welded portion is prevented, and the occurrence of hot cracking is suppressed. In this way, at high welding speeds that are impossible with conventional arc welding alone,
Surface weld metal can be obtained with high efficiency.

The invention of claim 2 is the method for producing a duplex stainless welded steel pipe according to claim 1, wherein in the forming step, the steel sheet is continuously formed into a cylindrical open pipe by a bending roll method.

Here, the bending roll method is a well-known forming method in the ordinary manufacturing method of electric resistance welded steel pipes. Also in this invention, similarly to the electric resistance welded steel pipe, the hot-rolled steel plate is continuously rolled into a cylindrical shape by using a multi-stage forming roll group with the direction parallel to the longitudinal direction as the axis of the steel pipe. This method has a high manufacturing efficiency because the steel sheet is continuously formed. The cylindrical open pipe means a state in which both edges of the hot rolled steel sheet in the width direction are butted. After that, the laser welding process and the arc welding process are performed as described in claim 1.

A third aspect of the present invention is the method for producing a duplex stainless steel welded steel pipe according to the first aspect, wherein the steel sheet is formed into a cylindrical open pipe by U pressing and O pressing in the forming step.

In the forming process of the present invention, steel plates such as thick plates are pressed one by one to form a cylindrical open pipe. Therefore, in the step of preparing a steel sheet, as compared with the invention of claim 2, it is possible to perform processing heat treatment (TMCP) during rolling and various heat treatments for each steel sheet. Can be prepared. Further, since heat treatment with a cut plate is possible, there are less restrictions on heat treatment conditions than with a hot rolled steel sheet, and a steel sheet of higher quality can be prepared.

A fourth aspect of the present invention is the method for producing a two-phase stainless welded steel pipe according to the first aspect, wherein in the forming step, the steel sheet is continuously wound in a spiral shape and formed into a spiral open pipe.

In the present invention, a hot rolled steel sheet is prepared and continuously formed. Forming into a spiral shape is a well-known pipe-making method that is generally well known as a method for manufacturing a spiral steel pipe. Here, the direction oblique to the longitudinal direction of the hot-rolled steel sheet is spirally wound in the axial direction of the steel pipe,
The edge of the part that is being rolled is butted against the edge of the part of the hot-rolled steel sheet that is one roll before to form an open pipe. Since the dimensions (inner and outer diameters) of the steel pipe are determined by the length per spiral, it can be arbitrarily selected regardless of the width of the hot rolled steel sheet.

[0037]

BEST MODE FOR CARRYING OUT THE INVENTION In carrying out the present invention, as the arc welding, submerged arc welding (SAW), plasma welding (PAW), TIG welding, MIG welding or the like can be appropriately selected. Part or most of the thickness of the seam of the steel pipe is
Since they are joined by laser welding, the ability of arc welding does not need to be so large, and welding methods other than submerged arc welding with a slightly lower welding speed can also be applied.

After manufacturing or procuring a duplex stainless steel sheet, it is desirable from the viewpoint of operation in both laser welding and arc welding that the edge of the steel sheet is grooved for welding. Various dimensions and shapes of the groove are possible depending on the plate thickness and distribution of laser welding and arc welding. When the plate thickness is large, the X groove is preferable because arc welding is performed on both the inner and outer surfaces of the steel pipe. When arc welding is applied only to the outer surface of the steel pipe, the Y groove is used. When the plate thickness of the steel plate is particularly thin, arc welding may be performed only on the outer surface of the steel pipe as an I groove having no bevel portion.

In any case, the portion of the plate thickness to be laser-welded needs to be processed with high precision so that a large gap is not formed in the butted portion. Here, the large gap is
A gap that allows the laser beam to pass through,
This is because such a gap reduces the efficiency of laser welding.

[0040]

Example Duplex stainless steel having the chemical composition shown in Table 1 was cast in a laboratory and hot-rolled to a plate thickness of 12 to 25 mm.

[0041]

[Table 1]

The duplex stainless steel sheets were groove-processed on the assumption of the joints of the bending roll method, the UOE method and the spiral method, and welded by the following method to obtain samples of the joints. When the steel plate was thin (12 mm or less), laser welding was performed on the Y groove to form a weld metal, and then submerged arc welding was performed from the outer surface of the pipe. Steel plate is thick (12
(mm or more), laser welding was first performed from one side to the X groove to form a weld metal, and then submerged arc welding was performed from both sides of the pipe to form a weld metal.

For comparison, a test piece at the seam portion of the X groove was processed and a weld metal was prepared only by submerged arc welding which is a conventional method. An evaluation test was conducted on these joint samples to investigate the properties of the deposited metal obtained by each welding method.

First, all the weld metals are subjected to solution heat treatment (10
After holding at 50 ° C for 30 minutes, water cooling was performed, and a weld hardness test, an impact test, a corrosion test and a stress corrosion cracking test were performed. The maximum hardness of the weld metal was determined by micro Vickers measurement with a load of 1 kgf. In addition, a JIS No. 4 impact test piece (full size) was sampled from the base metal and the weld, and an impact test was performed.

Further, a test piece was sampled from the seam sample and a corrosion test in a wet carbon dioxide environment was conducted. For a corrosion test in a wet carbon dioxide environment, a test temperature of 150
In an autoclave at ℃ under the condition of carbon dioxide partial pressure of 10 atm
After being immersed in a 3% NaCl aqueous solution for 30 days, the corrosion rate was calculated from the weight change before and after the test. Generally, a material is considered to be corrosion resistant and usable if the corrosion rate in an environment is less than 0.1 g / m ² hr.

The stress corrosion cracking test was carried out in an autoclave at a test temperature of 150 ° C. under the conditions of carbon dioxide gas partial pressure of 10 atm and 3% Na.
The material was immersed in a cl aqueous solution for 30 days, and at the same time, a bending stress of 80% of the yield stress of the base material was applied to the material. After the test, the presence or absence of cracks generated in the test piece was evaluated.

The results of these tests are shown in Table 2. Regarding the maximum hardness of weld metal and heat-affected zone, ○ has a maximum hardness of 30.
Less than 0, x means that the maximum hardness was 300 or more. In the impact test results, ◯ indicates that the impact absorption energy at -50 ° C of the weld was 70 J or more. × indicates the impact absorption energy at -50 ° C of the weld is 70J
It means that it was below.

In the corrosion test, ○ indicates a corrosion rate of 0.1 g /
Below m ² hr, x indicates that the corrosion rate exceeded 0.1 g / m ² hr. In the stress corrosion cracking test, ◯ means that no cracking occurred, and x means that cracking occurred.

It can be seen from Table 2 that the strength, pitting corrosion resistance and stress corrosion cracking resistance of the welded portion are good when laser welding is performed to produce a pipe according to the present invention.

[0050]

[Table 2]

[0051]

According to the present invention, when a duplex stainless steel pipe is manufactured by the bending roll method, the UOE method and the spiral method, the welded portion is compared with the case where the seam is welded only by the conventional submerged arc welding (SAW). Toughness and other mechanical properties of

Further, by combining the laser welding method and the arc welding method, the welding speed can be increased and a thick-walled pipe can be manufactured, so that not only the quality but also the productivity can be improved.

Continuation of front page (51) Int.Cl. ⁶ Identification number Office reference number FI Technical display location B21D 5/12 B21D 5/12 J C22C 38/00 302 C22C 38/00 302H 38/44 38/44 38/58 38/58 // B23K 13/00 B23K 13/00 A

Claims (4)

[Claims] 1. A method for producing a duplex stainless steel welded steel pipe comprising the following steps. (A) As a chemical composition, wt%, C: 0.030% or less, Si: 1.0% or less, Mn: 0.8 to 2.0%, P: 0.03% or less, S: 0.01% or less, Cr: 20 to 30%, Mo : A step of preparing a duplex stainless steel sheet containing 2.5 to 4.0%, Ni: 4.0 to 7.0%, N: 0.08 to 0.20%, (b) a forming step of forming this steel sheet into an open pipe, and (c) A laser welding process in which both opposite edges of a steel sheet formed into an open pipe are heated as they are or by an electric resistance method or an induction heating method, and then laser welding is performed by irradiating a laser beam with butting (d). An arc welding process in which the laser-welded seam is arc-welded from either or both of the pipe outer surface side and the pipe inner surface side. 2. The method for producing a two-phase stainless welded steel pipe according to claim 1, wherein in the forming step, the steel sheet is continuously formed into a cylindrical open pipe by a bending roll method. 3. The method for producing a duplex stainless welded steel pipe according to claim 1, wherein in the forming step, the steel sheet is formed into a cylindrical open pipe by U press and O press. 4. The method for producing a duplex stainless welded steel pipe according to claim 1, wherein, in the forming step, the steel plate is formed into a spiral open pipe while being continuously wound in a spiral form.