JP2861836B2 - Laser welding method for ferritic stainless steel - 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 laser welding ferritic stainless steel, and more particularly, to a method for laser welding ferritic stainless steel capable of obtaining a welded portion having good workability.

[0002]

2. Description of the Related Art Laser welding is a method in which a laser beam is condensed into a thin beam and radiated to a welded portion to melt-join the welded portion. A low heat input and a deep penetration depth are obtained. It is widely used as a welding method for ferritic stainless steel and the like because it is a welding method capable of high-speed welding with low distortion and small deterioration of the material due to heat influence due to a narrow welding width (Japanese Patent Laid-Open No. 168988).

However, when a thick ferritic stainless steel having a thickness of 2.0 mm or more is welded, a structure in which the crystal grains of the welded portion are partially coarsened is formed. It is said to be the cause of deterioration in toughness.

Generally, the fracture toughness value of a material accompanied by cleavage fracture and the transition temperature of brittle fracture depend on the crystal grain size of the material.
It is known that the larger the crystal grain size is, the more brittle it is. Ferritic stainless steel is a typical material. Therefore, in order to prevent deterioration of the toughness of the welded part of ferritic stainless steel and to secure sufficient processing performance, it is essential to reduce the crystal grain size of the welded part.

[0005] However, it is said that when a thick ferritic stainless steel is welded, as described above, the crystal grains in the welded portion are partially coarsened and the toughness is degraded. The detailed mechanism of was unknown.

The present inventors have conducted various experimental studies to elucidate the cause of the deterioration of the toughness of the welded portion that occurs when thick ferritic stainless steel is laser-welded, and have found the following facts. .

That is, when a thick ferritic stainless steel having a thickness of 2.0 mm or more is laser-welded with the butt shape of the material to be welded being an I-shape as shown in FIG. 8, the thickness of the material to be welded is increased. The thicker the thickness is, the smaller the laser penetration force is. Therefore, it is difficult to secure a bead width on the inner surface side, and welding defects such as insufficient penetration occur, so that it is necessary to reduce the welding speed. However, when the welding speed is reduced, the amount of plasma mass generated during welding increases,
Due to the thermal energy of the plasma mass, as shown in FIG. 9, the welded material has a wine cup-shaped welded shape in which the melting width on the outer surface side is larger than that on the inner surface side.
As a result, since the solidification rate of the molten metal portion on the outer surface side of the welded portion becomes slower, the crystal grains partially coarsened in the central portion of the weld metal on the outer surface side and vertically extended toward the outer surface side. A columnar crystal structure and a fine-grained columnar crystal structure extending in the direction orthogonal to the weld center on the inner surface are mixed, and the coarse columnar crystal grains vertically extending in the outer surface direction of the former. It became clear that the crystal structure causes toughness degradation.

This is clear from the following experimental results.

FIG. 10 shows that a steel plate made of SUS430, which is a typical ferritic stainless steel, is formed into a tube, and then, as shown in FIG. A laser beam is applied to the above-mentioned I-shaped butted portion in a state where the gap between the edges is made zero, and the outer diameter of the butted portion is reduced by 4 mm.
A welded tube having a thickness of 8.6 mm and a thickness of 3 mm was prepared, and macroscopic observation of a cross section of the welded portion was performed to confirm the presence of the coarse columnar crystal structure of the crystal grains. In order to remove the coarse columnar crystal structure portion of the crystal grains extending vertically in the direction, it is 0.1 mm from the outer surface of the tube including the extra weld portion.
A test material (hereinafter referred to as a reduced thickness material) having a thickness of 2.5 mm by cutting and removing a 5 mm thick portion, and cutting and removing only an excess weld portion, while cutting a 0.5 mm thick inner surface. Eighty test pieces each having a thickness of 2.5 mm removed (hereinafter referred to as as-welded materials) were prepared, and the flattening test was performed by placing the welded portions at various temperatures in the horizontal direction. It is the result of having investigated the crack generation rate (the ratio of the number of test pieces).

As is apparent from FIG. 10, a reduced thickness material obtained by cutting and removing a 0.5 mm thick portion from the outer surface of the tube (in FIG.
), No cracking was observed even at -50 ° C. However, in the as-welded material in which only the extra welded portion and the inner surface side wall thickness were cut and removed by 0.5 mm (indicated by ● in the figure), The occurrence of cracks at 0 ° C. indicates that the columnar crystal structure in which the crystal grains are coarsened causes deterioration of the toughness of the welded portion, in other words, deterioration of the workability.

By the way, when a thick ferritic stainless steel is laser-welded, a method of preventing or suppressing the formation of the coarse columnar crystal structure of the crystal grains generated on the outer surface side of the welded portion will be described below. A method is conceivable.

In the first method, the shape of the butted material to be welded is set to the above-mentioned I-shaped shape (see FIG. 8), and as shown in FIG. 12, the flow rate of the processing center gas supplied from the welding torch 10 is increased. Alternatively, a side gas nozzle 11 is disposed downstream of the welding torch 10 and a side gas is blown to suppress the amount of plasma lumps generated during laser welding to perform welding. The second method is shown in FIG. As shown in the above, a method of welding the material to be welded so that the butt shape of the material to be welded is V-shaped so as to reduce the amount of fusion of the material to be welded on the outer surface side portion due to the thermal energy of the plasma mass,
In the case of using these methods, the shape of the welded portion has a shape as shown in FIG. 14, the crystal grains on the outer surface side are suppressed from coarsening and growing vertically, and the relatively fine grains in the horizontal direction are suppressed. A columnar structure is obtained. However, in the first method, since the gas flow presses and disturbs the molten metal flow, as shown in FIG.
A welding defect such as the undercut 13 is generated. Also,
In the second method, since the groove cross-sectional area of the material to be welded is large, there is a problem that the amount of the molten metal is insufficient and welding defects such as the undercut 13 and the underbead are generated as described above.

Incidentally, Japanese Patent Application Laid-Open No. Sho 56-9088 discloses that
A laser welding method has been proposed in which two laser beams are irradiated in series on a welding line to shape a bead, but a means for preventing deterioration in the toughness of a weld portion caused when a ferritic stainless steel is welded. Is not shown at all.

[0014]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has been made in consideration of the above problems.
It is possible to effectively refine the coarsely misoriented crystal grains generated on the outer surface side of the welded part without adopting the side gas spraying and adopting the V-shaped shape for the butted material to be welded. Ferritic stainless steel having a welded part with excellent workability, in which cracks do not occur in the welded part even when, for example, a welded pipe from which a weld excess part has been removed is subjected to an adhesion flatness test at −40 ° C. An object of the present invention is to provide a method for laser welding steel.

[0015]

The gist of the present invention resides in the following laser welding method for ferritic stainless steel.

[0016] In the laser welding method of ferritic stainless steel, after the temperature of the welds penetration welding by the first laser beam becomes 400 ° C. or less, penetration by the first laser beam to the weld A laser welding method for ferritic stainless steel, comprising: irradiating a second laser beam with an output capable of obtaining a melting depth equivalent to a melting width on an outer surface of a material to be welded.

[0017]

Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings.

FIG. 1 is a schematic view showing an example of an embodiment in which the laser welding method for ferritic stainless steel according to the present invention is applied to the production of a welded pipe.

In FIG. 1, OP is an open pipe, and the open pipe OP is formed by passing a band steel made of ferritic stainless steel through a group of forming rolls (not shown) so that both edge portions E and E are relatively opposed from a U-shaped cross section. It is bent to the opposite cross section O shape. When the open pipe OP is conveyed in the direction of the outline arrow and passes through the induction heating coil 1, both edges E, E of the strip forming the V shape are preheated to a predetermined temperature.

Next, the open pipe OP is passed between the pair of left and right squeeze rolls 2 and 2 and has the above-mentioned cross-sectional shape I so that the gap in the thickness direction between both edges E and E of the steel strip becomes zero. It is butt-joined so as to have a mold shape, and is through-welded by a laser beam emitted from a first welding torch 3 arranged above the butt portion to form a welded pipe P. In the present invention, the first welding torch is used. 3 is welded through-welded by a local forced cooling device 4 including a cooling water injection nozzle 4a disposed downstream of the cooling water injection nozzle 3 and an inert gas injection nozzle 4b formed of nitrogen gas, argon gas, or the like.
After forcibly cooling B, a laser beam having a predetermined output is applied to the forcibly cooled weld WB from a second welding torch 5 disposed close to the downstream side of the local forced cooling device 4.

More specifically, the local forced cooling device 4
Melt depth equivalent to the melt width on the outer surface of the material to be welded when the welded portion WB forcibly cooled to a temperature of 00 ° C. or less is welded through by a laser beam emitted from the first welding torch 3 Is irradiated with a laser beam from the second welding torch 5 with an output that yields

After the welded portion WB welded through by the first laser beam is forcibly cooled to a temperature of 400 ° C. or less, the forcibly cooled welded portion is welded by the first laser beam. By irradiating a second laser beam with an output that can obtain a melting depth equivalent to the melting width on the outer surface of the material to be welded, the crystal grains generated on the outer surface side of the welded portion are coarsened to the outer surface side. As shown in FIG. 2, the portion of the columnar crystal structure extending vertically becomes a finely grained columnar crystal structure extending in the horizontal direction, thereby preventing a decrease in the toughness of the welded portion, in other words, processing the welded portion. Performance can be improved.

This is because the depth of the columnar crystal structure vertically extending to the outer surface side where the crystal grains are coarsened and formed at the time of welding by the first laser beam is based on the results of various experimental studies by the present inventors. For example, since the welding width is about the melting width of the outer surface of the welded portion, the output of the second laser beam can be obtained by the first laser beam at a melting depth equivalent to the melting width of the welded material outer surface of the welded portion. By doing so, it is possible to re-melt and refine the portion of the abnormal structure that causes toughness degradation for the first time.

Further, in order to remelt the columnar crystal structure of the coarse crystal grains of the weld portion generated at the time of penetration welding by the first laser beam into a fine grain structure, the first laser beam is used to cool the weld portion. Irradiating the second laser beam at the temperature of the crystal grain growth process in the process has no effect, and the crystal grain in the remelted portion grows and its crystal orientation and grain size are changed by the first laser beam. It will be equivalent to a beam weld. Therefore, the growth of the crystal grains of the weld by the first laser beam is completely stopped, and the crystal grains of the weld by the second laser beam grow again due to the effect of preheating the weld by the first laser beam. It is necessary to irradiate the second laser beam after the temperature falls below
This is because if the temperature of the welded portion by the first laser beam is 400 ° C. or lower, finer particles can be formed by remelting.
This is clear from the experimental results shown in FIG.

FIG. 3 shows that a second laser beam is applied to generate cracks (refinement by re-melting) in a welded portion when a welded tube from which a weld excess portion has been removed is subjected to an adhesion flatness test at -40 ° C. It is a figure which shows the result of having investigated the influence of the welding part temperature before, when the welding part temperature before irradiating a 2nd laser beam exceeds 400 degreeC, a crack will generate | occur | produce in a welding part and 400 degreeC
In the following, it is clear from the fact that there is no crack in the welded portion. In addition, the grain refinement by remelting of the abnormal structure portion by the second laser beam irradiation can be sufficiently achieved when the temperature of the welded portion by the first laser beam irradiation is 400 ° C. or lower as described above, but is more stable. In order to achieve this, it is desirable that the second laser beam be irradiated after the temperature reaches 100 ° C. or lower, more preferably, around room temperature.

In the embodiment shown in FIG. 1, both edges E and E of the steel strip are preheated by the induction heating coil 1 in order to enable high-speed welding, but this can be omitted. . Further, a local forced cooling means is provided to forcibly cool the welded portion by the first laser beam and to perform second cooling.
The laser beam was immediately irradiated, but the local forced cooling means was omitted and the second laser was placed at an appropriate position on the downstream side where the temperature of the welded portion by the first laser beam became 400 ° C. or less, or off-line. It goes without saying that a beam may be applied. Furthermore, it goes without saying that the method of the present invention can be applied not only to the production of welded pipes, but also to, for example, butt welding of flat plates, production of welded steel sections, and the like.

Further, according to the present invention, as described above, when irradiating the first laser beam and the second laser beam in series with respect to the welding line, the first laser beam and the second laser beam are used. A beam is a method of chopping a first laser beam during transmission as shown in FIG. 4 by a semi-transmissive mirror 6 to form a second laser beam. As shown in FIG. Is split into two by a split plane mirror 7 having an angle,
Twin beam method in which the laser beam is split into a first laser beam and a second laser beam by a method shown in FIG. 6, in which a first laser beam and a second laser beam are supplied from independent laser oscillators (not shown). Any of these may be used.

[0028]

EXAMPLE A 2.5 mm-thick strip steel made of ferritic stainless steel having the chemical components shown in Table 1 was used and had an outer diameter of 4 mm.
An 8.6 mm open pipe is continuously formed, and both ends of the steel strip of the open pipe are butted by a squeeze roll so that a butt shape becomes an I shape so that a gap in a thickness direction becomes zero. Under the conditions shown in Table 2, a first laser beam was applied to the abutted portion where both edges of the steel strip abuted to each other to perform penetration welding. Was applied to obtain a welded tube.

For comparison, the first laser beam was irradiated and pierced and welded under the conditions shown in Table 2, and the pierced weld was welded to the weld at the temperature shown in Table 2 without forced cooling. Table 2
1 was irradiated with the first laser beam under the conditions shown in FIG. Each of the laser oscillators is a carbon dioxide gas laser oscillator. The second laser beam has a rated output of 2 kW and its output is adjusted. The first laser beam has its focal point on the outer surface of the tube and the second laser beam has the second laser beam. The laser beam was irradiated with its focus set on the outer surface of the weld bead. Further, the irradiation position of the second laser beam in the case where the welded portion performed for comparison is not forcibly cooled is the first laser beam.
At a position 250 mm downstream from the irradiation position of the laser beam.

[0030]

[Table 1]

[0031]

[Table 2]

After grinding the excess weld portion of the obtained weld tube along the outer diameter of the tube, 20 tubular test pieces each having a length of 50 mm were sampled, and the welded portion was positioned horizontally. The flattening test was carried out at various temperatures at each position to determine the crack occurrence rate of the welded portion, and the workability of the welded portion was evaluated. The result is shown in FIG. In FIG. 7, the mark ○ indicates the case according to the method of the present invention, the mark 場合 indicates the case according to the conventional method using a single laser beam, and the mark × indicates the case according to the comparative method of the present invention in which the weld is not forcibly cooled. It is.

As is apparent from FIG. 7, in the conventional method of single laser beam welding, the transition temperature of crack initiation is 0 ° C., and the workability of the weld is inferior. In the comparative method in which the laser beam was irradiated, the transition temperature for crack initiation was −20 ° C., and the workability of the weld was inferior. On the other hand, in the method of the present invention, the workability of the welded portion is remarkably improved when the transition temperature for crack initiation is −50 ° C.

[0034]

According to the method of the present invention, the gas is generated on the outer surface side of the welded portion without increasing the flow rate of the processing center gas, adopting the side gas spraying, and adopting the V-shape in the butt shape of the material to be welded. Since the coarse crystal grains can be effectively refined, a steel product made of ferritic stainless steel having excellent weldability can be manufactured with high efficiency.

[Brief description of the drawings]

FIG. 1 is a view schematically showing one embodiment in which the laser welding method of the present invention is applied to the production of a welded pipe.

FIG. 2 is a diagram schematically showing a shape and a structure of a welded portion obtained by a laser welding method of the present invention.

FIG. 3 is a diagram showing the effect of the temperature of a weld before irradiation with a second laser beam on grain refinement by remelting according to the present invention.

FIG. 4 is a diagram schematically showing an example of a method for supplying a first laser beam and a second laser beam in the method of the present invention.

FIG. 5 is a diagram schematically showing another example of a method for supplying a first laser beam and a second laser beam in the method of the present invention.

FIG. 6 is a diagram schematically showing still another example of a method of supplying a first laser beam and a second laser beam in the method of the present invention.

FIG. 7 is a diagram showing the results of an example.

FIG. 8 is a diagram schematically illustrating a conventional laser welding method using a single laser beam by abutting an I-shaped shape.

FIG. 9 is a diagram schematically showing a shape and a structure of a welded portion obtained when a conventional single laser beam is pierced and welded by butt of an I-shaped shape.

FIG. 10 is a view showing the influence of the shape and structure of a welded portion obtained when a conventional single laser beam is pierced by butting of an I-shaped shape on the workability of the welded portion.

FIG. 11 is a diagram showing a simulated welding test method in a case where a conventional laser welding method using a single laser beam having an I-shaped butt is applied to the production of a welded pipe.

FIG. 12 is a view schematically showing a method for reducing the amount of generated plasma lumps in a conventional laser welding method using a single laser beam having an I-shaped shape butting.

FIG. 13 is a diagram schematically showing a conventional laser welding method using a single laser beam by abutting V-shaped shapes.

FIG. 14 shows welding obtained by a conventional laser welding method using a single laser beam with a V-shaped butt or a conventional laser welding method employing a means for reducing the amount of plasma mass generated by a single laser beam with an I-shaped butt. It is a figure which shows the shape and structure | tissue of a part typically.

[Explanation of symbols]

OP: Open pipe 1: Induction heating coil, 2
: Squeeze roll 3: Welding torch for irradiating first laser beam 4: Local forced cooling device 5: Welding torch for irradiating second laser beam WB: Welded part P: Welded pipe

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁶ , DB name) B23K 26/00 310 B21C 37/08

Claims (1)

(57) [Claims] 1. A laser welding method of ferritic stainless steel, after the temperature of the first laser beam welds penetration welding by became 400 ° C. or less, the first laser beam to the weld A method of laser welding ferritic stainless steel, comprising irradiating a second laser beam with an output capable of obtaining a melting depth equivalent to a melting width on an outer surface of a material to be welded at the time of penetration welding by the method.