JP2003145286A - Method for laser beam welding of steel members - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for laser beam welding of steel members by which such defects as a blow hole and a low temperature crack do not occur in a mutual joining of the steel members with the laser beam welding. SOLUTION: In the laser beam joining of the steel members, a gas consisting essentially of nitrogen is used as a shield gas and a filler metal is supplied whose components satisfy the relations Y>=-(1/3)X+23, Y>=12, Y>=-5X+55 (where X=Cr(%)+Mo(%)+1.5Si(%), Y=1.2Ni(%)+20C(%)+0.8Mn(%), and % is expressed in mass %), and the steel members are irradiated with the laser beam.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a laser welding method for steel materials.

[0002]

2. Description of the Related Art Joining steel materials is performed by general arc welding,
Alternatively, it is widely performed by high energy density beam welding such as electron beam and laser beam. Among them, laser welding is attracting attention as a method capable of high-speed deep-penetration welding and achieving high-efficiency welding without requiring a vacuum unlike electron beams. A large-capacity laser welder was also manufactured, making it possible to weld thick steel plates. However, laser welding has the following two problems.

The first problem is that defects such as blowholes due to pores are likely to occur. In deep penetration welding with a laser beam, the metal to be welded is vaporized by the laser beam, and a cavity called a keyhole is formed in the material by the vapor reaction force and vapor pressure. The keyhole is filled with metal vapor, and the vapor is ejected from the keyhole opening to the outside. Thus, in laser welding, a very unstable molten pool called a keyhole is formed,
There is a problem that defects such as blow holes due to pores are likely to occur.

Secondly, there is a problem that the weld metal portion is so narrow that the cooling rate is high and the weld metal portion is hardened to lower the toughness. Since steel contains C as an indispensable contained element regardless of the amount, and the cooling rate is higher than that of normal arc welding, these two factors are superimposed and the weld metal due to martensitic transformation is added. This is a problem that low temperature cracking is likely to occur.

Therefore, in order to put the laser welding into practical use, it is necessary to obtain a welded joint having both proper prevention of blowholes and prevention of cold cracking and an appropriate strength. Further, unless the welding itself can be performed stably, it cannot be put to practical use in a field where productivity is extremely important.

For the purpose of solving such problems, researches on prevention of defects and improvement of toughness of laser weld metal have been actively conducted in industry and academia, and some have been proposed.

First, as a method of preventing a welding defect called a blowhole, for example, in Japanese Patent Laid-Open No. 9-314368, a method of suppressing a blowhole generated by entrainment of a shield gas is used in which the wavelength of a laser beam is 8. A method has been proposed in which the amount of nitrogen is limited to 2 μm or less, and the shield gas composition contains nitrogen in a volume ratio of 5% or more. However, in this method, although a method for preventing welding defects is described, there is no description for a method for improving the toughness of the weld metal.

Further, as a method of improving the toughness of the weld metal, for example, a method of laser welding while supplying a filler material (Japanese Patent Laid-Open Nos. 9-122957 and 6-67).
No. 0), a method for controlling the chemical composition of the steel plate base material to be laser-welded and the quenching critical diameter Di value (JP-A-8-2762).
No. 86, JP-A-10-94890), control of chemical composition and structural parameters (crystal grain size and second phase microstructure fraction) and mechanical properties (uniform elongation and local elongation) of a steel sheet base material to be laser-welded. A method (Japanese Patent Application Laid-Open No. 11-293398) has been proposed. However, both methods focus on improving the toughness and formability of the weld metal. Among these, JP-A-8-276286 and JP-A-1
In the method proposed in Japanese Patent Application Laid-Open No. 0-94890 and Japanese Patent Application Laid-Open No. 11-293398, Mn vapor and N contained in a steel material are used.
It is described that blowholes are generated due to the above, and the content of Mn and N contained in the steel material is regulated in order to suppress the blowholes. However, as described in Japanese Patent Application Laid-Open No. 9-314368, it can be said that no measures are taken against the pore defect caused by the entrainment of the shield gas, which is essential in laser welding.

As described above, the conventional laser welding method and filler metal are insufficient in terms of preventing the occurrence of defects and improving the toughness of the weld metal portion. The current situation is that we cannot find it.

[0010]

If the above-mentioned laser welding can be applied to the joining of steel materials, the penetration depth,
It is considered to be advantageous in terms of strength, productivity and distortion. However, laser welding has a problem that a very unstable molten pool called a keyhole is formed, and defects such as blowholes due to pores are likely to occur. Further, steel contains C as an indispensable contained element regardless of its amount, and has a higher cooling rate as compared with ordinary arc welding, and these factors overlap to cause welding by martensitic transformation. There is a problem that cold cracking of metal easily occurs. Therefore, in order to put the laser welding into practical use, it is necessary to obtain both a blowhole prevention and a cold crack prevention at the same time and obtain a welded joint having an appropriate strength. Further, unless the welding itself can be performed stably, it cannot be put to practical use in a field where productivity is extremely important.

The present invention has been made in order to meet the above-mentioned demands, and an object thereof is a laser welding method for steel products, in which defects such as blow holes and cold cracks do not occur in joining steel products by laser welding. Is provided.

[0012]

In order to solve the above-mentioned problems, the present inventor has conducted extensive studies, and as a result, by setting the conditions unique to the composition of the filler metal and the shield gas at the time of laser welding, Found that it is possible to join
The present invention has been made here.

That is, according to the present invention, when laser welding steel materials, a gas containing nitrogen as a main component is used as a shield gas, and at least one of Cr, Mo and Si and Ni, C and Mn are selected. While supplying a component-based filler material containing at least one kind, the gist is a laser welding method of a steel material which irradiates a laser beam, and further, the filler material is a strong deoxidizer if necessary. Al, T
A predetermined amount of i or the like may be added, and the balance is substantially Fe. The structure and operation of the present invention will be described in more detail below.

[0014]

In laser welding, there is a problem that a very unstable molten pool called a keyhole is formed and defects such as blowholes due to pores are likely to occur. Further, since C is added to a general steel material for the purpose of improving the strength, when such a steel material is welded with a laser beam having an extremely high cooling rate as compared with general arc welding, it is contained in the weld metal. Due to C, the weld metal is apt to become martensite with poor toughness, and cold cracking tends to occur.

Among the above problems, the suppression of blowholes is first prevented by using nitrogen gas as a shield gas. In laser welding, generally, in order to prevent the reaction between oxygen contained in the atmosphere and molten metal, helium,
An inert gas such as argon is commonly used as a shield gas. However, once these inert gases are trapped in the keyhole and caught in the molten metal, they remain in the weld metal and cause blowhole defects unless they are discharged outside until the molten metal solidifies.

On the other hand, when nitrogen is used as the shielding gas, unlike the case where the shielding gas is shielded by an inert gas such as helium or argon, nitrogen is dissolved in the molten Fe alloy and is unlikely to cause blowholes. . Generally, in arc welding, nitrogen causes blowholes, and efforts are made to prevent nitrogen from coming into contact with molten metal as much as possible. This is a phenomenon when the cooling rate is slow as in arc welding, and when the cooling rate after welding is slow as described above, the nitrogen dissolved in the molten Fe alloy is reduced in solubility due to cooling, Even if nitrogen, which causes blowholes, is dissolved in the molten Fe alloy, bubbles are generated in the cooling process, which causes blowholes. The difference from the conventional knowledge of the present invention is that when the cooling rate after welding is extremely fast like laser welding, unlike in the case of arc welding, nitrogen that is supersaturated in the molten Fe alloy is melted after welding. That is, it can be frozen while being supersaturated even in the cooling process.

However, since cracking cannot be prevented by simply laser welding with nitrogen as the shielding gas, a filler material (filler wire) is used in the present invention. That is, in order to prevent low-temperature cracking due to martensitic transformation of the weld metal, austenite forming elements such as C, Ni and Mn are added in an appropriate ratio with respect to ferrite forming elements such as Cr, Mo and Si. By using the contained wire, the structure of the weld metal is austenite,
Alternatively, it becomes a mixed structure of austenite and martensite, which suppresses martensite transformation and prevents low temperature cracking. As described above, in the present invention, the point is to forcibly form a solid solution of nitrogen in the weld metal for the purpose of suppressing blowholes. However, N has a high austenite forming ability as with C and is accompanied by austenitization of the weld metal. Also effective in preventing cold cracking.

The effect of suppressing blowhole defects is increased by adding a predetermined amount of a strong deoxidizing agent such as Al or Ti to the filler material, if necessary. It was mentioned earlier that the cause of blowholes was mainly inert gas, but it may not be possible to completely shut off the atmosphere even if the nitrogen shield gas is blown onto the molten metal. This is to suppress blowholes generated due to oxygen mixed in from the atmosphere. Even if Mn and Si are included, if the deoxidizing agent for Mn and Si alone is insufficient for preventing the generation of blowholes, the deoxidizing agent such as Al and Ti is added in a predetermined amount (0. Addition of 2 to 3% by mass makes it possible to completely prevent the generation of blowholes.

Next, the reasons for limiting the components of the filler material of the present invention will be explained. When laser welding is performed while supplying a filler metal, it is impossible to make the entire weld metal into a filler metal composition because the amount of dilution from the base metal is large. The ratio of the filler material component to the entire weld metal is practically limited to about 50% in consideration of the wire supply stability. On such a premise, the structure of the weld metal should be austenite, or a mixed structure of austenite and martensite having a certain hardness or less (hardness without the problem of cold cracking), and in order to achieve both suppression of blowholes. , The following formula is satisfied,
A component-based filler material with the balance being essentially Fe is suitable. Y ≧ − (1/3) X + 23, Y ≧ 12, Y ≧ −5X + 5
5 However, X = Cr (%) + Mo (%) + 1.5Si (%) Y = 1.2Ni (%) + 20C (%) + 0.8Mn
(%) (%:% By mass)

In the filler metal of the present invention, the X component in the above formula is at least one of Cr, Mo and Si.
More than one kind, the Y component contains at least one kind of Ni, C and Mn. Of these elements, Cr and Mn
Has the effect of increasing the solubility of nitrogen in the molten Fe alloy. Therefore, nitrogen is easily absorbed in the molten Fe alloy, and the effect of further suppressing the blowhole is further enhanced.

Further, in order to suppress blowholes caused by air entrainment due to nitrogen shielding failure, it is possible to add a deoxidizing agent typified by Al or Ti. In order to exert the effect of deoxidation, at least one kind or a total of 0.2 mass% or more is necessary. However, even if added in excess of 3% by mass, the effect is saturated. Therefore, an appropriate range of one kind or a total amount of Al and Ti in the metal powder is 0.2 to 3 mass% with respect to the total weight of the wire.

By the way, when laser welding is performed while supplying the filler material having the above composition, it cannot be said to be a technique that can be practically used unless a stable and sound weld portion is obtained under various welding conditions. Therefore, it is preferable that the diameter of the filler material is 1.6 mmφ or less, and the wire supply speed is properly regulated in accordance with the laser output condition.

The filler material according to the present invention may be either solid wire or cored wire, and the solid wire is excellent in supply stability, and can be supplied more stably if it has a strand structure. Next, examples of the present invention will be described.

[0024]

[Example 1] SM490 steel plates (100w x 300L)
A butt laser welding test of × 12t) was performed under the conditions shown in Table 1. The filler material used was a φ1.2 mm cored wire, and the chemical composition is as shown in Tables 2 and 3. Figure 1 shows the welding situation.

[0025]

[Table 1]

After welding, the presence or absence of defects and low temperature cracks was examined by X-ray and cross-section of the welded portion and by penetrant flaw inspection. The results are also shown in Tables 2 and 3. Regarding the defects, the number of defects per 30 mm length of the bead was measured by X-ray inspection, and those having 5 (or less) defects were accepted.

[0027]

[Table 2]

[0028]

[Table 3]

As shown in Tables 2 and 3, when Ar, which is an inert gas, was used as the shield gas, blowhole defects could not be suppressed. On the other hand, FIG. 2 shows the relationship between the chemical composition of the filler material and the presence / absence of low temperature cracking and blowhole defects when the shielding gas is nitrogen. From the figure, it is possible to achieve both low temperature cracking and suppression of blowholes by using a filler material having a chemical composition in which Y is − (1/3) X + 23 or more, 12 or more, and Y is −5X + 55 or more. ing. In other words, it is impossible to suppress blowhole defects only by using nitrogen as the shield gas, and it is necessary to appropriately combine the shield gas and the filler material component.

From the above results, it is preferable to use nitrogen as the shield gas and a filler material having a chemical component of Y of − (1/3) X + 23 or more, 12 or more, and Y of −5X + 55 or more. As a result, it can be understood that all defects such as blowholes and cold cracks can be solved.

[0031]

Example 2 SS400 steel plate (100w × 500L × 2
A 0 t) bead-on-plate welding test was conducted to investigate the supply stability of the filler material. No. in Table 3 above. 36
The effect of the wire diameter on the feeding stability of the filler metal was investigated. The laser welding conditions are as shown in Table 4, and the presence or absence of supply failure such as wire deviation was examined. The results are shown in Table 5.

[0032]

[Table 4]

[0033]

[Table 5]

As shown in Table 5, the wire diameter is 1.6 mm.
When φ is exceeded, the wire cannot be normally fed, and it is understood that the wire diameter is preferably 1.6 mmφ or less in order to secure stable feeding stability.

[0035]

As described in detail above, according to the laser welding method for steel products of the present invention, in joining steel products,
It has become possible to prevent blowholes and weld metal cold cracks, which have been problems with conventional laser welding methods.
It is possible to provide a joining technique having high reliability as well as productivity.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a procedure of laser welding and a welding condition.

FIG. 2 is a diagram showing a relationship between a chemical composition of a filler material and a cold crack defect.

[Explanation of symbols]

1: Base material 2: Filler material 3: Laser ejection nozzle 4: Laser beam 5: Weld beads

Claims (2)

[Claims] 1. When laser welding steel materials, a gas containing nitrogen as a main component is used as a shield gas, and
The composition of the filler material is Y ≧ − (1/3) X + 23, Y ≧ 1
2, Y ≧ −5X + 55 (where X = Cr (%) + Mo
(%) + 1.5Si (%), Y = 1.2Ni (%) + 2
A laser welding method for a steel material, which comprises irradiating a laser beam while supplying a filler material containing so as to satisfy 0 C (%) + 0.8 Mn (%), where% is mass%. 2. The laser welding method for steel according to claim 1, wherein the wire diameter of the filler metal is 1.6 mm or less.