JP3375719B2 - Manufacturing method of stainless steel welded pipe by laser welding - Google Patents

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 a stainless steel welded pipe excellent in workability and toughness by laser welding.

[0002]

2. Description of the Related Art Since laser welding is a welding method using a beam of high energy density, the width of the welding bead is narrower than that of the conventional welding method, and the thermal effect is small. Suppressed. Moreover, since the heat is concentrated on an extremely small area, the welding speed can be set high. Utilizing these characteristics, it is beginning to be adopted as a welding method suitable for speeding up the pipe making line in order to improve productivity. On the other hand, in high-purity ferritic stainless steel,
Due to the coarsening of the crystal grains of the weld metal, the toughness tends to decrease significantly. This tendency becomes more remarkable in high-purity ferritic stainless steel in which impurity elements such as C, N, O, and S are extremely reduced. In this respect, laser welding is a method suitable for welding ferritic stainless steel because it has a small heat effect and the degree of coarsening of crystal grains in the welded portion is small.

[0003]

The ferritic stainless steel contains a large amount of Cr, which easily reacts with the gas components in the atmosphere, and tends to be oxidized and nitrided by heating during welding. For example, when nitrogen is absorbed from the atmosphere, chromium nitride is deposited on the weld. Chromium nitride significantly reduces toughness, making the welds brittle. This tendency becomes more remarkable as the purity of ferritic stainless steel becomes higher. In normal welding, by making the gas shield complete, absorption of gas components from the atmosphere into the weld is suppressed. However, in laser welding with high energy density, the temperature of the molten metal is significantly higher than that in conventional TIG welding, and sometimes reaches the temperature at which the iron-based metal evaporates. The gas absorption capacity of the molten metal whose temperature has risen greatly increases depending on the temperature.

For example, the amount of nitrogen absorbed by molten steel in the atmosphere increases as shown in FIG. 1, depending on the molten steel temperature. In TIG welding, the amount of nitrogen absorbed is about 240 ppm because the molten metal reaches about 1800 K, whereas in laser welding, the amount of nitrogen absorbed rises to about 300 ppm because the molten metal reaches about 2900 K. Further, even with the molten stainless steel held at the same 2900K, the nitrogen absorption amount changes according to the Cr content as shown in FIG. In laser welding of stainless steel, it is not known how much the nitrogen concentration of the shield gas should be lowered, so the nitrogen concentration was excessively lowered.
It is inevitable to seal with an inert gas such as r.
Alternatively, a large amount of high-purity Ar gas may be circulated in order to suppress the influence of nitrogen that enters the shield box from the atmosphere. However, a large amount of high-purity Ar gas is consumed, which causes an increase in production cost. The present invention has been devised to solve such a problem, and by controlling the nitrogen concentration in the shield atmosphere in relation to the Cr content of the base metal and the allowable nitrogen content, laser welding's unique high An object of the present invention is to obtain a stainless steel welded pipe having a welded portion which is sound and excellent in toughness by suppressing the absorption of the gas component of the atmosphere by the molten metal exhibiting gas absorbing ability.

[0005]

In order to achieve the above object, the present invention allows a stainless steel sheet formed into a cylindrical shape to pass through a shield box while running, and the widthwise both ends of the stainless steel sheet in the shield box are laser-scanned. When welding and forming a pipe, the nitrogen concentration in the atmosphere near the welding point is set to the Cr concentration [% Cr] (% by weight) of the stainless steel plate, the temperature T (° C) of the molten metal and the allowable amount of nitrogen [% N] of the weld metal. ] _WM (wt%)
It is characterized by maintaining the nitrogen concentration [% N] _at (% by weight) or less determined by the following equation according to log ([% N] _at ) ≦ 2 log ([% N] _WM ) −2 × (518 / T + 1.068) −2 × (0.046 [% Cr] −0.00028 [% Cr] ² ) Allowable nitrogen amount [% N As _WM , a preset value in the range of 0.005 to 0.035% by weight is used according to the workability of the obtained welded pipe. The atmosphere control according to the present invention is
Especially suitable for laser welding of high-purity ferritic stainless steel sheets with C: 0.03 wt% or less, N: 0.025 wt% or less, O: 0.03 wt% or less, and S: 0.02 wt% or less. ing. This type of high-purity ferritic stainless steel has a strong tendency to be oxidized and nitrided due to Cr, so that the effect of controlling the nitrogen concentration in the atmosphere is remarkable.

In the present invention, for example, the apparatus shown in FIG. 3 is used to continuously laser-weld a stainless steel pipe while sealing the weld. The stainless steel plate 1 is formed into a cylindrical shape by a roll forming machine, a rollless forming machine, or the like, and is fed into the shield box 2. A pair of squeeze rolls 3 are arranged in the shield box 2, the cylindrical shape of the stainless steel plate 1 is maintained, and both ends in the plate width direction are butted against each other. After the laser beam 5 emitted from the laser oscillator 4 is reflected by the mirror 6, the beam diameter is narrowed by the condenser lens 7 and directed toward the abutting portion of the cylindrical stainless steel plate 1. A shield gas supply pipe 8 connected to an inert gas supply source such as Ar faces the shield box 2. The tip of the shield gas supply pipe 8 is preferably opened to a hood 10 surrounding the laser beam 9 that has passed through the condenser lens 7 as shown in the figure. The shield gas supply pipe 8 is provided with a flow rate controller 11, and the control mechanism 12
The flow rate of the shield gas sent to the shield box 2 is adjusted based on the control signal from the.

In the shield box 2, a nitrogen sensor 13 and a temperature sensor 14 are arranged near the welded portion. The concentration of gas such as nitrogen contained in the welding atmosphere and the temperature of the welded portion are detected by the nitrogen sensor 13 and the temperature sensor 14, respectively, and are input to the control mechanism 12 as detection signals. An oxygen sensor may be used instead of the nitrogen sensor 13 to estimate the nitrogen concentration from the detected oxygen concentration in the atmosphere. The control mechanism 12 is preliminarily input with setting data relating to appropriate nitrogen concentration and atmospheric conditions from the terminal device 15. The control mechanism 12 compares the detection signal with the setting data, and outputs the calculation result to the flow rate controller 11 as a control signal. The shield gas is sent to the vicinity of the welded portion at a flow rate according to the control signal, and the butt portion of the stainless steel plate 1 is laser-welded in a protective atmosphere in which the nitrogen concentration is adjusted. Then, the welded pipe 16 is continuously sent out from the shield box 2.

By the way, the ductility-brittleness transition temperature rises with the nitrogen concentration of the weld metal, and the toughness of the weld metal deteriorates. That is, the crack occurrence rate of the welded portion is closely related to the nitrogen concentration of the weld metal as shown in FIG. Note that FIG.
The crack occurrence rate is 0.degree. C. at a minimum temperature of 0.degree.
It is a value obtained under the condition that a ferritic stainless steel pipe having a diameter of 2 mm and an outer diameter of 30 mm is bent at a bending R of 120 mm and an angle of 90 degrees. As is clear from FIG. 4, it is necessary to reduce the nitrogen concentration of the weld metal in order to secure the toughness that can withstand the processing. Therefore, in laser welding using the apparatus shown in FIG. 3, the nitrogen concentration in the shield box 2 is kept different and the crack generation rate sharply rises, that is, the allowable nitrogen concentration [% N] _WM.
(Wt%) was investigated. As a result, the allowable nitrogen concentration [%
N] _WM is the nitrogen concentration in the atmosphere [% N] _at (% by weight),
It was found that there is a close relationship with the temperature T (° C.) of the molten metal and the Cr content [% Cr] (% by weight) of the stainless steel plate 1, and it is expressed by the formula (1). [% N] _WM = K · ([% N] _at / 100) ^1/2 / f _N ··· (1)

However, K is an equilibrium constant between nitrogen in the atmosphere and nitrogen in the molten metal, and log K = -518 / T-1.06.
It is represented by 8. Further, f _N is the activity coefficient of nitrogen in the molten metal, and the primary influence coefficient of Cr on nitrogen absorption is e _N
(= -0.046), the secondary influence coefficient is r _N (= 0.00
028), log f _N = e _N · [% Cr] + r
It is represented by _N · [% Cr] ² . Therefore, equation (1)
Is transformed into equation (2). [% N] _WM = K · ([% N] _at / 100) ^1/2 / e _N · [% Cr] + r _N · [% Cr] ² ··· (2) e _{N in} equation (2) = -0.046 and r _N = 0.0002
By substituting 8) and rearranging by [% N] _at which is the control target, the formula (3) is obtained. log ([% N] _at ) = 2log ([% N] _WM ) −2 × (518 / T + 1.068) −2 × (0.046 [% Cr] −0.00028 [% Cr] ² ) ... .. (3) Therefore, the nitrogen concentration [% N] of the welding atmosphere is adjusted according to the flow rate and composition of the shield gas fed into the shield box 2 so that log ([% N] _at ) becomes smaller than the value of equation (3).
By controlling _at , it is possible to obtain a welded portion in which the absorption amount of nitrogen is suppressed to the allowable nitrogen concentration [% N] _WM or less. When laser welding is performed under these conditions, the obtained welded pipe 16 has a welded portion with no deterioration in toughness and becomes a product having excellent workability.

[0010]

【Example】

Example 1: As a pipe forming material, a plate thickness of 0.5 mm and a plate width of 9
3mm low C, N-18Cr-0.5Nb-0.5Cu
A high-purity ferritic stainless steel plate was used. With respect to this stainless steel plate, a welded portion was produced by a method of bead-on-plate welding on a flat plate using an Ar atmosphere in which the mixing ratio of N ₂ was changed as a shield gas. When the test piece after welding was bent at a bending R of 1.0 mm and test temperatures of 0 ° C. and 10 ° C., the relationship shown in FIG. 5 was established between the nitrogen absorption amount and the bending limit temperature. A stainless steel plate is formed into a cylindrical shape by a rollless pipe forming method, and both ends in the plate width direction are laser-welded to have a diameter of 30 mm.
Welded pipes of Since the weld pipe is bent at a minimum temperature of 0 ° C, the allowable nitrogen concentration in the weld metal is 300pp.
set to m. Then, assuming that the molten metal reaches a maximum of 2900 ° C., the nitrogen concentration in the atmosphere in the vicinity of the welded portion was controlled according to the formula defined in the present invention.

When the effect of the nitrogen concentration in the atmosphere on the nitrogen concentration of the weld metal was investigated, as shown in Table 1, in the case of laser welding under the conditions satisfying the relational expression specified in the present invention, The nitrogen concentration was sufficiently lower than 300 ppm. Therefore, even when bending was performed at a temperature of 0 ° C., the occurrence of work cracks was not detected. On the other hand, in the comparative example in which laser welding was performed in an atmosphere with a high nitrogen concentration, the nitrogen concentration of the weld metal exceeded a permissible value of 300 ppm, and many cracks occurred after bending. As is clear from this comparison, by suppressing the nitrogen concentration in the atmosphere according to the relational expression defined in the present invention, the nitrogen concentration in the welded portion is reduced, and a welded pipe having a good welded portion in which work cracking does not occur is obtained. It was confirmed that

[0012]

[Table 1]

Example 2: As a pipe-forming material, low C, N-22Cr-0.5N having a plate thickness of 1 mm and a plate width of 131.3 mm.
A b-0.5Mo-based high-purity ferritic stainless steel plate was used. This stainless steel has a minimum temperature of 0 ° C
%, The allowable nitrogen concentration is 200ppm
Met. Then, while measuring the temperature of the molten metal during laser welding, the nitrogen concentration in the atmosphere was controlled so that the relational expression defined in the present invention was satisfied. As shown in Table 2, the obtained welded pipe was a product having excellent workability in which work cracking did not occur even when the pipe was expanded by 20% at a minimum temperature of 0 ° C. On the other hand, in the comparative example welded in an atmosphere with a high nitrogen concentration, work cracking occurred at a rate of 5% or more when the same pipe expansion process was performed.

[0014]

[Table 2]

Example 3: As a pipe forming material, a low C, N-30Cr-0.5Nb having a plate thickness of 1 mm and a plate width of 87.3 mm.
A -2.0Mo high purity ferritic stainless steel plate was used. This stainless steel had an allowable nitrogen concentration of 150 ppm when subjected to pipe end flare processing at a minimum temperature of 10 ° C. Then, while measuring the temperature of the molten metal during laser welding, the nitrogen concentration in the atmosphere was controlled so that the relational expression defined in the present invention was satisfied. The obtained welded pipe is shown in Table 2.
As shown in (1), the product was excellent in workability and did not generate work cracks even by pipe end flare processing at the minimum temperature of 10 ° C. On the other hand, in the comparative example welded in an atmosphere with a high nitrogen concentration, when the same pipe end flare processing was performed, work cracking occurred at an occurrence rate of 40% or more.

[0016]

[Table 3]

[0017]

As described above, in the present invention, the permissible nitrogen concentration determined in advance in accordance with the processing conditions when subjecting the welded pipe to bulging, flaring, etc. It is taken in together with the temperature of the molten metal as a control factor to control the nitrogen concentration in the atmosphere to which the molten metal generated during laser welding is exposed. This allows
The nitrogen concentration of the formed welded portion is maintained below the allowable nitrogen concentration, and a welded pipe excellent in workability with no defects such as work cracks can be obtained. Moreover, since it is not necessary to use an inert gas in which the nitrogen concentration in the atmosphere is extremely reduced or to continuously feed a large amount of high-purity inert gas to dilute the nitrogen concentration in the atmosphere, a large amount of inert gas is required. The high-speed pipe forming property inherent to laser welding is fully utilized without increasing the production cost due to consumption.

[Brief description of drawings]

Fig. 1 Effect of molten steel temperature on nitrogen absorption

Fig. 2 Effect of Cr content on nitrogen absorption

[Fig. 3] Part of a laser welding pipe-making line equipped with a shield box

[Fig. 4] Effect of nitrogen concentration of weld metal on crack initiation

FIG. 5: Relation between nitrogen absorption amount and bending limit temperature of stainless steel used in Example 1

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI B23K 103: 04 B23K 103: 04 (56) References JP-A-6-670 (JP, A) JP-A-4-258390 (JP , A) JP 56-168988 (JP, A) JP 50-5066 (JP, A) JP 60-87919 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB) Name) B23K 26/00-26/26 B21C 37/08

Claims (3)

(57) [Claims] 1. A stainless steel plate formed into a cylindrical shape is run.
While passing the shield box,
When both ends of the stainless steel plate in the width direction are laser-welded to form a pipe in the box, the nitrogen concentration in the atmosphere near the welding point is determined by the Cr concentration [% Cr] (% by weight) of the stainless steel plate, the temperature T of the molten metal ( ℃) and the allowable nitrogen amount [% N] _WM (wt%) of the weld metal, the nitrogen concentration determined by the following formula is maintained _at [% N] _at (wt%) or less. Welded pipe manufacturing method. log ([% N] _at ) ≦ 2 log ([% N] _WM ) −2 × (518 / T + 1.068) −2 × (0.046 [% Cr] −0.00028 [% Cr] ² ) 2. The allowable nitrogen amount [% N] _WM according to claim 1 is 0.005 to 5 depending on the workability of the obtained welded pipe.
A method for manufacturing a stainless steel welded pipe using a preset value in the range of 0.035% by weight. 3. C: 0.03% by weight or less, N: 0.02
5% by weight or less, O: 0.03% by weight or less and S: 0.0
The method for producing a stainless steel welded pipe according to claim 1 or 2, wherein a ferritic stainless steel plate regulated to 2% by weight or less is used.