JPH08281458A - Production of austenitic stainless steel welded tube - Google Patents

Abstract

PURPOSE: To provide a producing method using a laser beam welding method obtaining an austenitic stainless steel welded tube excellent in durability and corrosion resistance at the welded part. CONSTITUTION: At the time of welding for tube-making by using the laser beam to the austenitic stainless steel welded tube, in the case of using P (kw) for laser beam power, V (m/min) for welding velocity, (t) (mm) for the thickness of a band steel stock and T ( deg.C) for preheating temp. at bottom edge parts of the band steel, the laser beam welding is executed in the conditions satisfying the following inequalities (I), (II) and (III). 0.4<=.exp. (aT)/V.t<=1...(I). P>=15kW...(II). 800 deg.C<=T<=1200 deg.C...(III). Wherein, (a) is the constant = 0.0006 and T without preheating is made to 0 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an austenitic stainless steel welded pipe, more specifically, an austenitic stainless steel welded pipe excellent in ductility and corrosion resistance of a welded portion, which is suitable for use as a pipe for chemical machinery. Manufacturing method.

[0002]

2. Description of the Related Art Austenitic stainless steel is widely used as a pipe material for various chemical engineering machines because it is excellent in pitting corrosion resistance, mechanical properties and weldability. This austenitic stainless steel is usually 1000-11
The solution is heat-treated in a temperature range of 00 ° C. for 10 minutes to 1 hour, and then rapidly cooled to be used for solution heat treatment. This is because when austenitic stainless steel is held in the temperature range of 600 to 800 ° C. for a long time, Cr carbide precipitates at the crystal grain boundaries and a Cr-deficient layer is formed around the grain boundaries, which causes intergranular corrosion. This is to prevent the so-called sensitization phenomenon that occurs.

Conventionally, an austenitic stainless steel welded pipe is usually formed by forming a raw strip steel into an open pipe shape and subjecting the abutted portion to gas-tungsten-arc welding (hereinafter referred to as GTAW method) for pipe forming welding. Made to go. At this time, in order to obtain the performance of the welded portion (welded metal) close to the base metal, a method of adding an appropriate alloying element into the molten metal at the time of welding using a filler wire or a method of performing post heat treatment after pipe-making welding is used. Has been adopted.

Further, in recent years, a laser welding method, which has a much higher welding speed than the GTAW method, has been under development. However, in the laser welding method, due to equipment restrictions,
It is practically difficult to add an appropriate alloying element to the molten metal during welding using the above filler wire. Therefore, for example, as shown in Japanese Patent Laid-Open No. 63-278688, after the laser pipe welding under the predetermined conditions,
After heat treatment at 600 ° C, the ductility of the weld metal is improved.

[0005]

In the above GTAW method,
Since the cooling rate of the weld metal is slow, a heat affected zone (hereinafter referred to as HAZ) is formed adjacent to the weld metal. In this HAZ, since the holding time at 600 to 800 ° C. is relatively long, a sensitization phenomenon occurs and corrosion resistance is significantly deteriorated. But,
Unlike the weld metal, the HAZ is in a solid state even at the time of welding, and therefore it is impossible to improve the corrosion resistance by controlling the structure by adding an appropriate alloying element using a filler wire. For this reason, there is a problem that post-heat treatment is required after the pipe-making welding, resulting in an increase in product cost.

On the other hand, in the laser welding method, GTA
Similar to the W method, it is not possible to control the structure by adding an appropriate alloying element using a filler wire, but in the case of pipe welding with a low heat input amount, the cooling of the weld metal is fast, so that the part corresponding to HAZ is 600 to 800 ° C. Since the time of holding in the temperature range of 1 is shortened, the HAZ does not undergo a sensitization phenomenon and the corrosion resistance does not deteriorate. However, in this case, the weld metal is not held in the temperature range of 1000 to 1100 ° C. which is the solution temperature for a sufficient time, and is rapidly heated and cooled, so that strain is concentrated in the weld metal and the ductility of the weld metal is increased. Is significantly reduced. Therefore, in this case as well, the above-mentioned JP-A-63-27 is used.
As seen in Japanese Patent No. 8688, there is a problem that post-heat treatment is required after pipe-making welding, resulting in high product cost.

The object of the present invention was made in view of the above situation, and it has excellent ductility and corrosion resistance without post heat treatment after pipe-making welding using a laser welding method capable of high-speed welding. Another object of the present invention is to provide a method for producing an austenitic stainless steel welded pipe having a welded portion.

[0008]

The gist of the present invention resides in the following method for producing an austenitic stainless steel welded pipe.

A raw strip of austenitic stainless steel is continuously formed into an open pipe shape by passing through a group of forming rolls, both edges of the strip are butted against each other by a squeeze roll, and the butted portion is irradiated with a laser beam. Laser welding at P (k
W), the welding speed is V (m / min), and the thickness of the material strip steel is t
(Mm) and the preheating temperature of both edges of the steel strip is T (° C), laser welding is performed under conditions satisfying the following formulas (1), (2) and (3). Steel welded pipe manufacturing method.

0.4 ≦ P · exp (aT) / V · t ≦ 1 (1) P ≧ 15 kW (2) 800 ° C ≤ T ≤ 1200 ° C 3) However, the constant a is 0.0006, and T without preheating is 0 ° C.

As described above, the reason why the ductility of the weld metal of the austenitic stainless steel welded pipe is deteriorated regardless of the laser welding is the high temperature region due to the rapid heating and rapid cooling of the weld metal, specifically, the solution temperature. There is 1000-1
This is because the holding time in the temperature range of 100 ° C. is short. Therefore, if the amount of heat input at the time of laser welding can be increased and the weld metal can be kept in the high temperature region for a long time, the strain concentrated on the weld metal can be reduced, and the reduction in ductility can be prevented. However, in this case, since the HAZ is kept in the sensitization temperature range of 600 to 800 ° C. for a long time, HAZ is
The sensitization phenomenon occurring in Z becomes remarkable, and the corrosion resistance is further deteriorated.

[0012] Therefore, the inventors of the present invention have been able to maintain the weld metal itself in the solution temperature range for a long time, and to prevent the sensitization phenomenon from occurring in the HAZ, laser power and welding speed as laser welding conditions. As a result of earnest experimental research on the relationship between the thickness of the material strip steel and the local preheating temperature of both edges of the material strip steel, the present invention was made based on the following findings.

When the laser output P of the laser oscillator used is 15 kW or more, the apex temperature of the weld metal during welding becomes higher, so that the weld metal after welding is kept in the solution temperature range for a long time. As a result, it is possible to reduce the concentration of strain on the weld metal and reduce the reduction in ductility.

In this case, when both edges of the material strip steel are locally preheated to 800 to 1200 ° C., the cooling rate of the weld metal after welding becomes slower, and the weld metal is kept in the above solution solution temperature range. Can be held for a long time and prevent distortion from concentrating on the weld metal.

Simultaneously with the above, the welding speed V (m / m
HAZ is 600 to 800 ° C. when (in) is set to a speed that satisfies “P · exp (aT) / V · t ≦ 1” according to the material strip steel wall thickness t (mm) and the irradiation laser output P. The time for holding in the temperature range is extremely short, the sensitization phenomenon does not occur in the HAZ, and it is possible to prevent both the decrease in ductility and the decrease in corrosion resistance even after welding.

Furthermore, in order to obtain sound joint strength and shape, it is necessary to perform complete penetration welding. For this purpose, the welding speed V (m / min) is set to the material strip steel wall thickness t.
(0.4 mm) and the irradiation laser power P
It is necessary to have a speed that satisfies “exp (aT) / V · t”.

[0017]

The reason why the method of the present invention is limited as described above will be described in detail below.

The austenitic stainless steel welded pipe produced by pipe welding according to the method of the present invention must naturally have excellent ductility and corrosion resistance of the base metal portion, so that it is related to the production history. However, it is necessary to use a material strip steel (hot rolled steel sheet) having excellent ductility and corrosion resistance, but the component composition is not particularly limited. But,
As a material having excellent ductility and corrosion resistance, for example, JI
SUS304, SUS304L, S specified in S standard
It is preferable to use a standard strip material such as US316, SUS316L and the like, which is a raw material steel strip that is solution-treated and adjusted so that the austenite ratio is 90% or more.

The above material strip steel is continuously formed into an open pipe shape by passing it through a group of forming rolls by an ordinary method, both edges of the strip steel are butted against each other by a squeeze roll, and the butted portion is irradiated with a laser beam from above. And pipe welding. At this time, in the present invention, a laser oscillator having a laser output P of 15 kW or more is used after locally preheating both edges of the steel strip to 800 to 1200 ° C.,
Also, the material strip steel wall thickness is t (mm) and the welding speed is V (m / m).
in) and the preheating temperature of both edges of the material strip steel is T
(0.4 ° C), the value is “0.4 ≦ P · exp (aT) /
Laser welding is performed at a welding speed V (m / min) satisfying V · t ≦ 1 ”, and the product is obtained as it is without post heat treatment.

In the above, if the local preheating temperature T of both edges of the material strip steel is less than 800 ° C., the weld metal after welding cannot be kept in the solution heat treatment temperature region for a sufficient period of time to prevent the deterioration of ductility. I can't. On the other hand, when the temperature exceeds 1200 ° C., the precipitation amount of δ ferrite increases, and the distribution of the components becomes remarkable between ferrite and austenite, so that the corrosion resistance deteriorates in the preheated portion.

[0021] Further, the entire material strip steel is the above 800-120.
Preheating to 0 ℃ slows the cooling of the steel strip,
Since the material is kept in the temperature range of 0 to 800 ° C for a long time, the material strip steel itself becomes sensitive and the corrosion resistance deteriorates.

If the laser output P is less than 15 kW or the welding speed V is more than 1 in "Pexp (aT) / Vt", can the weld metal after welding be kept in the solution temperature range for a sufficient time? , Or HAZ causes a sensitization phenomenon, and the ductility or corrosion resistance of the welded portion is significantly reduced. On the other hand, if the welding speed V is “P · exp (aT) / V · t” of less than 0.4, the amount of heat input to the predetermined thickness of the material strip steel is insufficient, and complete penetration welding becomes impossible. .

The upper limit of the laser output P does not have to be specified. However, the above “0.4 ≦ P · exp (a
T) / V · t ≦ 1 ”, the larger the laser output P, the faster the welding speed V. Therefore, from the viewpoint of improving productivity, the laser output P
Is preferably as large as possible.

For the preheating of both edges of the strip steel, a high frequency heating means using an annular induction heating coil or a contact tip capable of local heating used in the well-known ERW method is arranged in the preceding stage of the squeeze roll. The input power may be controlled to preheat to a predetermined temperature.

[0025]

[Example] A material strip steel (hot rolled steel sheet) made of four kinds of austenitic stainless steels having the composition shown in Table 1
Prepared.

[0026]

[Table 1]

These material strip steels were formed into an open pipe shape having outer diameters shown in Tables 2 and 3 by a conventional method, and a laser beam was applied from above to the abutting portions of both edges of the strip steels. Laser welding was performed under the conditions shown in Table 2 and Table 3 to obtain a product as it was. In addition, for comparison, a welded pipe as-made by the GTAW method was also prepared.

[0028]

[Table 2]

[0029]

[Table 3]

Welded portion of the obtained welded pipe (including HAZ)
Therefore, the length in the pipe axis direction is 10 mm, and the length in the pipe circumferential direction is 3
The test piece was sampled so that the weld seam was located at the center of the arc cross section at 0 mm. These test pieces were polished to JIS
-According to the Strauss test specified in G0575,
It was immersed in a boiling water solution of 6.0 wt% CuSO4 + 16.0 wt% H2 SO4 for 16 hours. At this time, JIS-H3
A large number of small copper pieces having the same quality as specified C1100 were immersed in 100 and brought into contact with the test piece. After that, the test piece was taken out, and a bending test was performed centering on the weld metal part or the HAZ part, and then observed with an optical microscope of 200 times to confirm the occurrence of cracks due to intergranular corrosion and evaluate the corrosion resistance.

Similarly, the welded portion of the obtained welded pipe (HAZ
From 10 to 20 mm, the tube circumferential dimension is 40 to 100 mm, and the test piece is sampled so that the weld seam portion is located at the center of the arc cross section. A 180 ° bending test with a bending radius of 3 times the thickness of the test piece (the thickness of the raw material steel strip) was performed to examine the occurrence of cracks in the weld, and the ductility of the weld was evaluated. The results of these tests are shown in Tables 2 and 3.

As is clear from Tables 2 and 3, the corrosion resistance and the ductility of the welded portion of the welded pipe produced by the method of the present invention are good, but the comparative examples deviating from the conditions specified by the method of the present invention. In either case, either or both of the corrosion resistance and the ductility of the welded portion are inferior.

That is, in Comparative Examples (Nos. 1 to 5) in which the laser output P is less than 15 kW, the holding time of the weld metal in the high temperature region is short, or the holding time of the HAZ in the sensitization temperature region is long. Therefore, either or both of corrosion resistance and ductility are inferior. The laser output P is 15 kW
As above, the value of “P · exp (aT) / V · t” is 1
In Comparative Example (No. 6), the HAZ holding time in the sensitization temperature range was long, and the corrosion resistance was poor. Similarly, in the comparative example (No. 7 to 9) in which the laser output P is 15 kW or more, but the preheating temperature of the edge portion of the material strip steel is out of the range of the present invention, the holding time of the weld metal in the high temperature region is high. Δ due to heat effect above the solution temperature
The precipitation amount of ferrite increases, and either corrosion resistance or ductility is poor. Furthermore, the laser output P is 15
In the comparative example (No. 10) in which the value of “P · exp (aT) / V · t” is less than 0.4, though it is kW or more, penetration heat welding was impossible because the heat input was too small. Furthermore,
The laser output P, the “P · exp (aT) / V · t” value, and the preheating temperature T of the material strip steel are within the scope of the present invention.
In the comparative example (No. 11) in which the entire material strip steel was preheated, HA was used.
Since the holding time of Z in the sensitization temperature range becomes long, the corrosion resistance of not only the welded portion but the entire material strip steel is poor.

[0034]

According to the method of the present invention, an austenitic stainless steel welded pipe excellent in ductility and corrosion resistance of the welded portion can be manufactured inexpensively and highly efficiently without performing post heat treatment after pipe forming welding. Therefore, its industrial value is extremely large.

[0035]

Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI Technical display location C22C 38/44 C22C 38/44

Claims (1)

[Claims] 1. A material strip made of austenitic stainless steel is continuously formed into an open pipe shape by passing through a group of forming rolls, both edges of the strip are abutted against each other by a squeeze roll, and a laser beam is applied to the abutting portion. When irradiating and performing pipe welding, the laser output is P (k
W), the welding speed is V (m / min), and the thickness of the material strip steel is t
(Mm) and the preheating temperature of both edges of the steel strip is T (° C), laser welding is performed under conditions satisfying the following formulas (1), (2) and (3). Steel welded pipe manufacturing method. 0.4 ≦ P · exp (aT) / V · t ≦ 1 (1) P ≧ 15kW (2) 800 ° C ≤ T ≤ 1200 ° C (3) , Constant a is 0.0006, and T without preheating is 0 ° C.