JP2014151333A - Method of manufacturing weld joint and apparatus for manufacturing the weld joint - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing a weld joint capable of obtaining the weld joint having less difference in hardness between a weld metal and a base-material steel sheet, and having excellent forming workability without heat-treating a portion including the weld metal.SOLUTION: There is provided a method of manufacturing a weld joint formed by welding a plurality of base-material steel sheets 1a, 1b. The method of manufacturing the weld joint includes a welding stage of welding the base-material steel sheets at welding speed of 0.5 m/min or more using a direct diode laser 3 as a light source. The laser in which the shape of a condensed beam 5 in the half width of peak intensity is long in a welding direction, and the aspect ratio (welding direction/direction orthogonal to welding) of the length of the condensed beam 5 in the welding direction and that in the direction orthogonal to welding is in a prescribed range, is used as the direct diode laser 3. In the method of welding the weld joint, in the welding stage, the weld metal including 50% or more of ferrite having a particle size of 30-150 μm is formed.

Description

  The present invention relates to a welded joint manufacturing method and a welded joint manufacturing apparatus, and more particularly to a welded joint manufacturing method having a small difference in hardness between a weld metal and a base steel plate and having excellent formability.

  The tailored welded blank material is conventionally used for a vehicle outer plate member such as an automobile door outer or a body side panel. The tailored welded blank material is formed by welding a plurality of base material steel plates having different thicknesses and types, and has partially different material characteristics. By using a tailored welded blank material in which regions having predetermined material characteristics are appropriately arranged for the inner and outer plate members of the vehicle, the weight and strength of the vehicle body can be reduced.

  Conventionally, in a laser welding method of a high-tensile steel plate, as a technique for reducing the hardness of the weld metal and forming a welded portion having good formability, welding is performed after the welding laser beam has passed for more than 1 second. A heat treatment method for locally heating a portion containing a metal is disclosed (for example, see Patent Document 1). In the technique of Patent Document 1, since the weld metal having high hardness is tempered by locally heating the portion including the weld metal, the elongation of the welded portion is improved.

JP 2004-209497 A

In conventional tailored welded blanks, the weld metal hardness at the welded joint made by welding multiple base steel plates is higher than that of the base steel plates, so that it is easier to form than the base steel plates. Is a problem.
Specifically, for example, when a welded blank material used for a vehicle outer plate member is subjected to a forming process such as a press forming process to impart distortion, the weld metal has a high hardness, so that the weld metal In some cases, the surface of the steel plate rises from the position of the surface of the base steel plate. If the raised height of the weld metal is high and cannot be sufficiently concealed even if the surface is painted, it is difficult to obtain a predetermined appearance for use as a vehicle outer plate member.

As a method for solving this problem, it is conceivable to reduce the hardness of the weld metal by performing a heat treatment that locally heats a portion including the weld metal. However, in the case of locally heating the portion including the weld metal, there is an inconvenience that an apparatus for performing heat treatment is necessary and time-consuming.
For this reason, in a welding joint formed by welding a plurality of base steel plates, it is required to reduce the difference in hardness between the weld metal and the base steel plate without performing heat treatment.

  The present invention has been made in view of the above circumstances, and it is possible to obtain a welded joint having excellent forming workability with a small difference in hardness between the weld metal and the base steel plate without heat-treating the portion containing the weld metal. It is an object of the present invention to provide a welded joint manufacturing method and a welded joint manufacturing apparatus.

  In order to solve the above-mentioned problems, the present inventor has intensively studied paying attention to the microstructure of the weld metal of the weld joint. As a result, when the weld metal of the weld joint contains 50% or more of ferrite having a particle size of 30 to 150 μm, the difference between the hardness of the weld metal and the hardness of the base steel plate is caused by the ferrite that is soft and excellent in ductility. It was found to be a sufficiently small weld joint.

As a method for obtaining a weld metal containing 50% or more of ferrite having a particle size of 30 to 150 μm, it is conceivable to slow the cooling rate of the weld metal and promote the formation of ferrite having a particle size of 30 to 150 μm. In order to slow down the cooling rate of the weld metal, the welding speed may be slowed down. However, if the welding speed is slowed down, the productivity will decrease.
Therefore, the present inventor has studied a method for reducing the cooling rate of the weld metal while ensuring a sufficient welding speed.

  As a result, it has been found that a direct diode laser may be used as a light source provided in the welding means, and the shape of the focused beam of the direct diode laser should be sufficiently long in the welding direction. And when the aspect ratio (welding direction / welding orthogonal direction) of the welding direction length and the welding orthogonal direction length of the focused beam is 2.5 or more and satisfies the following formula (1), it is 0.5 m / min or more. Even if welding is performed at a sufficiently high welding speed, it has been confirmed that a weld metal containing a weld having a grain size of 30 to 150 μm in 50% or more and having a small difference in hardness from the base steel sheet can be obtained. I came up with. The gist of the present invention is as follows.

[1] A method for manufacturing a welded joint obtained by welding a plurality of base steel sheets, including a welding process of welding at a welding speed of 0.5 m / min or more using a direct diode laser as a light source, As the diode laser, the shape of the focused beam at the half-value width of the peak intensity is long in the welding direction, and the aspect ratio (welding direction / welding orthogonal direction) between the welding direction length of the focused beam and the welding orthogonal length is 2.5. A method for manufacturing a welded joint, wherein a weld metal containing 50% or more of ferrite having a particle size of 30 to 150 μm is formed in the welding step using a material satisfying the following formula (1).
Aspect ratio ≧ 5.0 × Welding speed (m / min) (1)

[2] A method of manufacturing a welded joint processed at a predetermined strain ratio, wherein the direct diode laser has an aspect ratio satisfying the following formula (2). The method for producing a welded joint according to [1], wherein a weld metal satisfying the following formula (3) is formed with a ferrite content of ˜150 μm.
Aspect ratio ≧ (5.8 + 1.6 × strain ratio) × welding speed (m / min) (2)
Ferrite content ≧ 53 + 6 × strain ratio Equation (3)

[3] Using the direct diode laser having an aspect ratio satisfying the following formula (4), and forming the weld metal in which the maximum grain size of the ferrite is 350 μm or less in the welding step. The method for manufacturing a welded joint according to [1] or [2].
Aspect ratio ≦ 16.0 × Welding speed (m / min) −3.0 Formula (4)

[4] The method for producing a welded joint according to any one of [1] to [3], wherein an average power density of the direct diode laser is 0.6 kW / mm ² or less.

[5] A welded joint manufacturing apparatus used in the welded joint manufacturing method according to any one of [1] to [4], having welding means for welding at a welding speed of 0.5 m / min or more. And
As the light source, the shape of the focused beam as a light source at the half width of the peak intensity is long in the welding direction, and the aspect ratio (welding direction / welding orthogonal direction) between the welding direction length of the focused beam and the welding orthogonal direction length is A welding joint manufacturing apparatus comprising a direct diode laser satisfying the following formula (1) that is 2.5 or more.
Aspect ratio ≧ 5.0 × Welding speed (m / min) (1)

  The method for manufacturing a welded joint according to the present invention includes a welding step of welding the steel sheet at a welding speed of 0.5 m / min or more using a direct diode laser as a light source, and the half width of peak intensity as the direct diode laser. In the welding step, a weld metal containing 50% or more of ferrite having a particle size of 30 to 150 μm is formed, and a step of heat-treating a portion containing the weld metal is used. Without doing so, a difference in hardness between the weld metal and the base steel sheet is small, and a weld joint having excellent formability can be obtained.

  Therefore, the welded joint obtained using the manufacturing method of the present invention can prevent the surface of the weld metal from rising from the position of the surface of the base steel plate even when distortion is applied by forming. Even if the weld metal rises or the bead surface becomes somewhat rough, the surface can be sufficiently concealed by slightly polishing the surface and painting the surface as necessary. Therefore, the welded joint obtained by using the manufacturing method of the present invention can be applied to the exterior of the vehicle by, for example, performing a molding process such as a press molding process and then painting the surface after slightly polishing if necessary. A molded product having a smooth and beautiful appearance that can be suitably used as a plate member is obtained.

FIG. 1 is a schematic view for explaining an example of a welding joint manufacturing apparatus according to the present invention. FIG. 2 is a plan view showing the shape of a focused beam of a direct diode laser provided in the welding joint manufacturing apparatus shown in FIG. FIG. 3 is a schematic view for explaining a state after applying strain to the welded joint obtained by using the method for manufacturing a welded joint according to the present invention, and is an enlarged cross-sectional schematic diagram viewed from a direction orthogonal to the welding direction. It is. FIG. 4 is a graph showing the relationship between the incidence of poor welding in Experimental Examples 27 and 30 and the average power density of the direct diode laser.

Hereinafter, the manufacturing method and apparatus for manufacturing a welded joint according to the present invention will be described in detail.
The method for manufacturing a welded joint according to the present embodiment is a method for manufacturing a welded joint obtained by welding a plurality of base steel plates using the welded joint manufacturing apparatus according to the present embodiment. FIG. 1 is a schematic view for explaining an example of a welding joint manufacturing apparatus according to the present invention. FIG. 2 is a plan view showing the shape of the focused beam of the direct diode laser provided in the welding joint manufacturing apparatus shown in FIG.

  In FIG. 1, the codes | symbols 1a and 1b have shown the base material steel plate. The welding joint manufacturing apparatus shown in FIG. 1 has welding means 30 for welding at a welding speed of 0.5 m / min or more. The welding means 30 includes a direct diode laser 3 as a light source and a gas supply means 4 for supplying a shielding gas for welding.

In the direct diode laser 3 shown in FIG. 1, the shape of the condensed beam 5 at the half width of the peak intensity is long in the welding direction (the direction of the arrow in FIG. 2) as shown in FIG. The aspect ratio (the welding direction / the welding orthogonal direction (A / B in FIG. 2)) of the length A and the welding orthogonal direction length B is 2.5 or more and satisfies the following formula (1).
Aspect ratio ≧ 5.0 × Welding speed (m / min) (1)

  As shown in FIG. 2, the shape of the focused beam 5 can be an elliptical shape, but it may be any shape as long as the aspect ratio has the above range, and may be, for example, an oval shape or an oval shape.

  The welded joint obtained by using the manufacturing method of the present embodiment using the manufacturing apparatus of the present embodiment is preferably used as a material to which distortion is imparted by molding. FIG. 3 is a schematic view for explaining a state after applying strain to the welded joint obtained by using the method for manufacturing a welded joint according to the present invention, and an enlarged cross-sectional schematic view seen from a direction orthogonal to the welding direction. It is. In FIG. 3, reference numerals 1a and 1b denote base steel plates, and reference numeral 2 denotes a weld metal.

  In the manufacturing method of the present embodiment, the base steel plates 1a and 1b are not particularly limited, but steel plates used in vehicles can be suitably used. The plurality of base steel plates to be welded may all have the same plate thickness and type, or may have different plate thicknesses and / or types. In addition, when the welded joint obtained by the manufacturing method of the present invention is a welded joint part of a tailored welded blank material, the plurality of base material steel plates have different plate thicknesses and / or types.

  Further, as the base steel plates 1a and 1b, it is preferable to use a steel plate having a B content of 0.0006% by mass or less, and more preferably 0.0003% by mass or less. B is an element that greatly affects the hardenability. When the base steel plates 1a and 1b contain B, the hardness of the weld metal of the weld joint increases. However, when the content of B in the base steel plates 1a and 1b is 0.0006% by mass or less, an improvement in the hardness of the weld metal formed after welding can be suppressed, and 0.0003% by mass In the case of the following, the hardness improvement of the weld metal can be more effectively suppressed. Therefore, when a steel plate having a B content of 0.0006% by mass or less is used as the base steel plates 1a and 1b, there is a weld joint in which the difference in hardness between the weld metal and the base steel plates 1a and 1b is much smaller. can get.

The manufacturing method of the welding joint of this embodiment has the welding process of welding at the welding speed of 0.5 m / min or more using the direct diode laser 3 as a light source.
The direct diode laser 3 used in the present embodiment has a focused beam aspect ratio (welding direction / welding orthogonal direction) of 2.5 or more and satisfies the following formula (1).
Aspect ratio ≧ 5.0 × Welding speed (m / min) (1)

  In the present embodiment, since the welding speed in the welding process is 0.5 m / min or more, a welding joint can be manufactured efficiently. The welding speed is 0.5 m / min or more from the viewpoint of improvement in productivity and stability of the molten pool. If the welding speed is less than 0.5 m / min, in addition to insufficient productivity, the formation of the molten pool becomes unstable, and welding defects are likely to occur. Further, in order to increase the welding speed while sufficiently reducing the difference in hardness between the weld metal of the weld joint and the base steel plates 1a and 1b, as shown in the equation (1), the collection of the direct diode laser 3 is performed. It is necessary to increase the aspect ratio (welding direction / welding orthogonal direction) of the light beam 5. When the welding speed is set to 10 m / min or more, undercut occurs and welding is likely to be poor. Therefore, the welding speed is preferably set to less than 10 m / min.

In the present embodiment, the welding speed in the welding process is 0.5 m / min or more, and the aspect ratio of the focused beam 5 of the direct diode laser 3 has the above range. A weld metal 2 containing 50% or more of ferrite having a diameter of 30 to 150 μm can be formed.
The ferrite particle size of the weld metal 2 is such that the cross-section of the weld metal 2 is subjected to nital corrosion, the microstructure is observed using an optical microscope or a scanning electron microscope, and a cutting method or image analysis of the microstructure photograph is performed. It can be calculated by a method or the like. The ferrite content can be the area ratio of ferrite determined by image analysis of a microstructure photograph.

  The ferrite having a particle size of 30 to 150 μm contained in the weld metal 2 is generated in the welding process. The ferrite having a particle size of less than 30 μm is insufficient in the effect of suppressing the hardness of the weld metal 2. Further, ferrite having a particle size exceeding 150 μm becomes coarse particles that cause non-uniform deformation when strain is imparted by forming, and the surface roughness (Rmax) of weld metal 2 after imparting strain by forming is increased. In addition, the appearance of the weld metal 2 may be deteriorated to an extent that cannot be corrected by light polishing.

Further, when the ferrite having a particle size of 30 to 150 μm contained in the weld metal 2 is less than 50%, an effect of reducing the difference in hardness between the weld metal 2 and the base steel plates 1a and 1b due to the ferrite is sufficiently obtained. Absent. When the ferrite having a particle size of 30 to 150 μm contained in the weld metal 2 exceeds 95%, the difference in hardness between the weld metal 2 and the base steel plates 1a and 1b becomes too small, and the weld joint is easily broken. There is a risk of becoming a thing. Therefore, it is preferable that the ferrite with a particle size of 30 to 150 μm contained in the weld metal 2 is less than 95%.
In addition, if the ferrite with a particle size of 30 to 150 μm contained in the weld metal 2 is 50% or more, the effect of reducing the difference in hardness between the weld metal 2 and the base steel plates 1a and 1b can be sufficiently obtained. In order to further reduce the difference in hardness between the weld metal 2 and the base steel plates 1a and 1b, it is preferably 70% or more.

  In the welded joint manufactured using the manufacturing method of the present embodiment, since the weld metal 2 contains 50% or more of ferrite having a particle size of 30 to 150 μm, the hardness of the weld metal 2 and the base steel plates 1a and 1b is low. The difference is small and it has excellent moldability. Therefore, when distortion is applied by forming, the surface of the weld metal 2 is prevented from rising from the position of the surfaces of the base steel plates 1a and 1b.

Here, the surface of the weld metal when distortion is applied to the weld joint manufactured by using the manufacturing method of the present embodiment will be described with reference to the drawings.
In FIG. 3, reference numerals 11 a and 11 b indicate the surfaces of the base steel plates 1 a and 1 b, and reference numeral 21 indicates the surface of the weld metal 2. Reference numeral m1a denotes an average line of the surface position of the base steel plate 1a, reference sign m1b denotes an average line of the surface position of the base steel plate 1b, reference sign m2 denotes an average line of the surface position of the weld metal 2, and reference sign Ra1a denotes the base material. The roughness (Rmax) of the steel plate 1a, the symbol Ra1b represents the roughness (Rmax) of the base steel plate 1b, the symbol Ra2 represents the roughness (Rmax) of the weld metal 2, and the symbol H represents the swell amount of the weld metal. In the present embodiment, the average lines (reference numerals m1a and m1b) of the surface positions of the base steel plate 1a and the base steel plate 1b are the same.

The roughness Ra1a and Ra1b of the base steel plates 1a and 1b can be measured using a surface roughness meter.
Further, the surface roughness Ra2 of the weld metal 2 is measured by measuring the roughness of the cross section curve of the surface of the weld metal 2 using a surface roughness meter, and only a high frequency component having a cutoff wavelength or less is measured from the cross section curve. And Rmax is calculated from the roughness curve. Note that the measurement direction of the roughness of the cross-sectional curve is a direction orthogonal to the welding direction.

  Further, the swell amount H of the weld metal 2 means a difference between the average lines m1a and m1b of the surface positions of the base steel plates 1a and 1b and the average line m2 of the surface positions of the weld metal 2. The swell amount H of the weld metal 2 can be set to half the difference between the thickness of the weld metal 2 and the thickness of the base steel plates 1a and 1b. Further, the swell amount H of the weld metal 2 is extracted from the cross-sectional curve of the surface of the weld metal 2 obtained using a surface roughness meter, and only the low frequency component having a cutoff wavelength or more is extracted as a shape deviation curve, and its maximum cross section It may be defined as height.

  In the welded joint manufactured using the manufacturing method of the present embodiment, the surface 21 of the weld metal 2 may rise from the positions of the surfaces 11a and 11b of the base steel plates 1a and 1b when strain is applied by forming. Is prevented. For this reason, as shown in FIG. 3, even if the surface 21 of the weld metal 2 (see the average line m2 of the surface position of the weld metal 2) rises, the swell amount H of the weld metal 2 sandwiches the weld metal 2 therebetween. Since the roughness Ra1a (Rmax) and Ra1b (Rmax) of the two base steel plates 1a and 1b which are arranged opposite to each other and are welded to each other, it becomes a welding joint that can be sufficiently concealed by painting the surface.

  In the manufacturing method of the present embodiment, when the aspect ratio of the focused beam 5 of the direct diode laser 3 used is less than 2.5, the cooling rate of the weld metal 2 by increasing the aspect ratio of the focused beam 5 is set. The effect of delaying is insufficient. For this reason, it becomes impossible to sufficiently promote the generation of ferrite having a particle size of 30 to 150 μm while setting the welding speed in the welding process to 0.5 m / min or more.

  Further, when the aspect ratio of the focused beam 5 does not satisfy the formula (1), the content of ferrite having a particle size of 30 to 150 μm is insufficient, and the difference in hardness from the base steel plates 1a and 1b is sufficiently small. The weld metal 2 cannot be formed. Therefore, if the aspect ratio of the focused beam 5 does not satisfy the formula (1), the manufactured weld joint has a surface 21 of the weld metal 2 of the base steel plates 1a and 1b when strain is applied by forming. From the position of the surfaces 11a and 11b, the base steel plates 1a and 1b swell with a swell amount H exceeding the roughness Ra1a and Ra1b of the base steel plates 1a and 1b.

When manufacturing a welded joint that is processed at a predetermined strain ratio determined in advance using the manufacturing method of the present invention, the aspect ratio of the focused beam 5 satisfies the following formula (2) as the direct diode laser 3 In the welding process, it is preferable to form a weld metal in which the ferrite content satisfies the following formula (3).
Aspect ratio ≧ (5.8 + 1.6 × strain ratio) × welding speed (m / min) (2)
Ferrite content ≧ 53 + 6 × strain ratio Equation (3)
Here, the strain ratio is a value obtained by dividing the minimum principal strain by the maximum principal strain out of the amount of strain generated by the action of biaxial stress.

  When press-molding a tailored welded blank material, the tailored welded blank material is usually arranged so that the maximum principal strain direction acting on the weld joint with respect to the mold is substantially parallel to the weld line direction. The reason for this is that if pressing is performed in a state where the maximum principal strain direction is at an angle with respect to the weld line direction, deformation concentrates on the base steel plate around the weld joint and the risk of breakage increases. is there.

  Further, in forming a welded joint, stress may be applied in a direction perpendicular to the weld line direction in addition to the weld line direction. This state is called a state in which biaxial stress acts on the weld joint. When biaxial stress is applied to the weld joint, the base steel plate around the weld joint is deformed by the stress acting in the direction perpendicular to the weld line, and the plate thickness is reduced. In FIG. 3, this is equivalent to lowering the positions of the average lines m1a and m1b of the surface positions of the base steel plates 1a and 1b. The swell amount H of the weld metal 2 is a difference between the average lines m1a and m1b of the surface positions of the base steel plates 1a and 1b and the average line m2 of the surface position of the weld metal 2. Therefore, when biaxial stress acts on the welded joint, the swell amount H of the weld metal 2 becomes larger than when uniaxial stress acts on the welded joint in the direction of the weld line.

  In this embodiment, when manufacturing a welded joint that is processed at a predetermined strain ratio that is determined in advance, a direct diode laser 3 that has an aspect ratio of the focused beam 5 that satisfies Equation (2) is used. In the process, when the weld metal having the ferrite content satisfying the formula (3) is formed, it is possible to effectively suppress the surface of the weld metal 2 from being raised by applying strain by forming. For this reason, even if the biaxial stress acts on the weld joint by the forming process and the bulge amount H of the weld metal 2 becomes larger than the case where the uniaxial stress acts in the weld line direction, the weld metal 2 The swell amount H can be set to be equal to or less than the roughness Ra1a (Rmax) and Ra1b (Rmax) of the two base steel plates 1a and 1b that are arranged to be opposed to each other with the weld metal 2 interposed therebetween.

  When the direct diode laser 3 is used in which the aspect ratio of the focused beam 5 does not satisfy the formula (2) and / or when the ferrite content of the weld metal 2 does not satisfy the formula (3), the direct diode laser 3 is formed. The phenomenon that the weld metal 2 swells due to the biaxial stress acting on the weld joint due to the processing cannot be sufficiently suppressed, and the weld metal 2 has a swell amount H exceeding the roughness Ra1a, Ra1b of the base steel plates 1a, 1b. There is a risk that it will rise and be difficult to conceal even if the surface is painted.

The aspect ratio of the focused beam 5 preferably satisfies the following formula (4). When the aspect ratio of the focused beam 5 satisfies the following formula (4), the weld metal 2 having a maximum grain size of ferrite of 350 μm or less can be formed in the welding process, and even if distortion is imparted by forming. The surface roughness (Rmax) of the weld metal 2 due to uneven deformation due to coarse grains existing in the weld metal 2 is unlikely to increase, and a weld joint having the weld metal 2 with a smooth surface is obtained.
Aspect ratio ≦ 16.0 × Welding speed (m / min) −3.0 Formula (4)

  When the aspect ratio of the focused beam 5 does not satisfy the formula (4), the aspect ratio of the focused beam 5 becomes high and the cooling rate of the weld metal 2 becomes too slow. Is a big one. For this reason, the weld metal 2 contains ferrite having a maximum particle size exceeding 350 μm, and becomes a weld joint having a large particle size difference between the weld metal 2 and the base steel plates 1a and 1b. As a result, when distortion is imparted by forming, the surface roughness Ra2 of the weld metal 2 increases due to nonuniform deformation due to the particle size difference between the weld metal 2 and the base steel plates 1a and 1b, and the surface is coated. However, it is difficult to obtain a smooth and beautiful appearance that can be suitably used as a vehicle outer plate member.

  That is, as shown in FIG. 3, the weld metal 2 of the weld joint after applying distortion by forming is welded even if the surface 21 is raised from the positions of the surfaces 11 a and 11 b of the base steel plates 1 a and 1 b. The rising amount H of the metal 2 is not more than the surface roughness Ra1a, Ra1b of the base steel plates 1a, 1b, and the surface roughness Ra2 is not more than the surface roughness Ra1a, Ra1b of the base steel plates 1a, 1b. preferable.

In the present embodiment, the average power density of the direct diode laser 3 is preferably 0.6 kW / mm ² or less. By setting the average power density to 0.6 kW / mm ² or less, it is possible to realize heat conduction mode welding in which welding defects are unlikely to occur, and to prevent poor welding. When the average power density exceeds 0.6 kW / mm ² , keyhole mode welding is performed. In the present embodiment, since the focused laser beam 5 having a predetermined aspect ratio is used as the direct diode laser 3, surface tension sufficient to close the molten pool cannot be obtained when keyhole mode welding is performed. , Welding defects are likely to occur. It is preferable that the average power density is low, so that poor welding is less likely to occur. However, if the average power density is less than 0.01 kW / mm ^2, it is difficult to make the welding speed 0.5 m / min or more. For this reason, it is preferable that an average power density is 0.01 kW / mm < ² > or more.

Further, in the present embodiment, Ar gas and / or He gas containing 0.5 to 10% by volume of oxygen O ₂ from the gas supply means 4 as a shielding gas for welding and the other containing inevitable impurities. It is desirable to supply what is.
When the above shielding gas is used, oxygen contained in the shielding gas reacts with elements contributing to hardenability such as C and Mn contained in the base steel plates 1a and 1b to form oxides. , Hardenability is reduced. Therefore, the improvement of the hardness in the weld metal 2 of the weld joint formed after welding is suppressed.

  Further, TiN contained in the base steel plates 1a and 1b is decomposed by heating at the time of welding, and free N is generated. This N and oxygen contained in the shield gas react to form an oxide. Therefore, the amount of N which is a solid solution strengthening element is reduced, and the improvement in the hardness of the weld metal 2 formed after welding is suppressed.

[Example 1]
Two steel plates shown below are used as the base steel plate, and at the welding speed shown in Table 1, the aspect ratio of the focused beam shown in Table 1 and the laser of the light source are used, and oxygen O ₂ is used as a shielding gas in volume%. Using Ar gas containing 3%, welding was performed at an average power density shown in Table 1 to obtain weld joints of Experimental Examples 1 to 32.
The output was 900 W when a YAG laser was used as the light source, and the output was 500 W when a DDL (direct diode laser) was used.
In addition, the focused beam at the half-value width of the peak intensity of the light source was made constant in the welding orthogonal length at 0.6 mm, and the aspect ratio (welding direction / welding orthogonal direction) was changed by changing the welding direction length. .

"steel sheet"
B is contained at 0.0002% by mass, the plate thickness is 0.8 mm, the yield stress is 135.4 MPa, the tensile strength is 287 MPa, the elongation is 45%, the Vickers hardness Hv is 80.5, and the roughness Rmax is 9. A 1 μm galvannealed steel sheet was used.

  With respect to the weld metal of the weld joints of Experimental Examples 1 to 32 thus obtained, the area ratio of ferrite having a particle size of 30 to 150 μm and the maximum particle size of ferrite were examined by the following method. The results are shown in Table 1.

"Ferrite area ratio"
The cross section of the weld metal was subjected to Nital corrosion, and calculated by a point counting method (6400 points measurement) using a microstructure photograph (observation field of view 800 μm × 800 μm) taken using a scanning electron microscope.
"Maximum grain size of ferrite"
Using the method defined in JIS G0551, the grain size of ferrite was examined and the maximum grain size was determined.

Subsequently, a JIS No. 5 tensile test piece was collected from the welded joints of Experimental Examples 1 to 32, and used as Test Samples of Experimental Examples 1 to 32. In the test pieces of Experimental Examples 1 to 32, the weld metal was positioned in the center in parallel with the tensile direction.
Next, the test pieces of Experimental Examples 1 to 32 were polished by mechanically polishing both sides by 0.05 mm per side so that the surface roughness and thickness were uniform in the base steel plate and the weld metal. . The surface roughness Rmax on both surfaces of the test pieces of Experimental Examples 1 to 32 after polishing was 3.0 μm or less, and the plate thickness was 0.7 mm. In addition, the surface roughness of both surfaces of the test piece was measured using a surface roughness meter.

  Thereafter, for the test pieces of Experimental Example 1 to Experimental Example 32 after polishing, a tensile test is performed in accordance with JIS Z 2241, a strain of 15% is applied, and by the method shown below, The surface roughness of the weld metal was evaluated. The results are shown in Table 1.

“Weld metal swell amount”
The plate thickness of the weld metal and the plate thickness of the base steel plate were measured, and the difference was half the value.
"Surface roughness of weld metal"
Using a surface roughness meter, measure the roughness of the cross-section curve of the surface of the weld metal, and extract only the high-frequency component below the cutoff wavelength (0.8 mm) from the cross-section curve as the roughness curve. It was set as the value obtained by calculating Rmax from. In addition, the measurement direction of the roughness of the cross-sectional curve was orthogonal to the welding direction, the measurement distance was 10 mm, and the center of the measurement region was the center position between the base metal plates of the weld metal.

  As shown in Table 1, in Experimental Examples 7, 13, 14, 19 to 23, and 26 to 31, which are examples of the present invention, the swell amount of the weld metal is the surface roughness (9.10 μm) of the base steel plate. It turns out that it is the following.

On the other hand, Examples 1-6, 8-12, 15-18, in which the aspect ratio of the focused beam does not satisfy the above formula (1), and the ferrite having a particle size of 30 to 150 μm in the weld metal is less than 50%. In 24, 25, and 32, the swell amount of the weld metal exceeds the surface roughness of the base steel plate.
This is because in Experiments 1-6, 8-12, 15-18, 24, 25, 32, the hardness of the weld metal is higher than that of the base steel plate, so the weld metal and base material when strain was applied. It is presumed that the deformability differs greatly from the steel sheet.

  Also, as shown in Table 1, among Experimental Examples 7, 13, 14, 19-23, and 26-31, Experimental Examples 7, 13, 14, and 21 in which the aspect ratio of the condensed beam satisfies the above formula (4) In -23 and 29-31, the maximum grain size of ferrite is 350 μm or less, and the surface roughness of the weld metal is not more than the surface roughness of the base steel plate.

Further, in the welding conditions of Example 27 and Experiment Example 30 shown in Table 1, the average power density only ^{0.4kW / mm 2, 0.6kW / mm} 2, 0.8kW / mm 2, to 1.0 kW / mm ² Welding was performed, and a welding test was performed in which 1000 weld metals each having a length of 100 mm were formed for each average power density, and the presence or absence of welding failure was determined by the welding failure determination method described below. The incidence of poor welding was investigated. The result is shown in FIG.

"Judgment method of poor welding"
Whether or not there is a hole (defect) penetrating in the thickness direction in the weld metal was determined visually, and a case where a hole was present was judged as “existent”.

FIG. 4 is a graph showing the relationship between the incidence of poor welding in Experimental Examples 27 and 30 and the average power density of the direct diode laser.
As shown in FIG. 4, in Experimental Example 27 and Experimental Example 30, no welding failure occurred when the average power density of the direct diode laser was 0.6 kW / mm ² or less.

[Example 2]
As the base material steel plate, the same two steel plates as in Example 1 were used, and at the welding speed shown in Table 2, the condensed beam aspect ratio shown in Table 2 and the light source laser were used, and oxygen O ₂ was used as the shielding gas. Using Ar gas containing 3% by volume, welding was performed at an average power density shown in Table 2 to obtain weld joints of Experimental Examples 33 to 73.
In addition, DDL (direct diode laser) was used as a light source, and the output was 500W.
In addition, the focused beam at the half-value width of the peak intensity of the light source was made constant in the welding orthogonal length at 0.6 mm, and the aspect ratio (welding direction / welding orthogonal direction) was changed by changing the welding direction length. .

  For the weld metal of the weld joints of Experimental Examples 33 to 73 obtained in this manner, the area ratio of ferrite having a particle size of 30 to 150 μm and the maximum particle size of ferrite were examined in the same manner as in Example 1. It was. The results are shown in Table 3.

Subsequently, a Marciniac test piece was collected from the weld joints of Experimental Example 33 to Experimental Example 73, and used as a test piece of Experimental Example 33 to Experimental Example 73. In addition, the test pieces of Experimental Example 33 to Experimental Example 73 were arranged such that the welding direction of the weld metal was located in the center in parallel with the maximum principal strain direction.
Next, the test pieces of Experimental Example 33 to Experimental Example 73 were polished by mechanically polishing both sides by 0.05 mm per side so that the surface roughness and thickness were uniform in the base steel plate and the weld metal. . The surface roughness Rmax on both surfaces of the test pieces of Experimental Examples 33 to 77 after polishing was 3.0 μm or less, and the plate thickness was 0.7 mm. In addition, the surface roughness of both surfaces of the test piece was measured using a surface roughness meter.

Then, the marcinic test was done based on ISO12004 about the test piece of Experimental example 33-Experimental example 73 after grinding | polishing. In the Marcinic test, a strain of 15% was applied as the maximum principal strain, and the strain ratio was -0.5 (uniaxial deformation), 0 (plane strain deformation), 0.5, 1.0 (equal biaxial deformation). The test was conducted so that
After the test, the amount of swell of the weld metal and the surface roughness of the weld metal were evaluated in the same manner as in Example 1. The results are shown in Table 3.

"When strain ratio is -0.5"
As shown in Table 3, in Experimental Examples 33 to 35, 40 to 51, and 54 to 72, which are examples of the present invention, the swell amount of the weld metal is equal to or less than the surface roughness (9.10 μm) of the base steel plate. You can see that

On the other hand, the ferrite content 50 in which the aspect ratio of the focused beam does not satisfy the above formulas (1) and (2) and the ferrite having a particle size of 30 to 150 μm in the weld metal is calculated from the above formula (3). In Experimental Examples 36 to 39, 52 to 53, and 73 that are less than%, the swell amount of the weld metal exceeds the surface roughness of the base steel plate.
In Experimental Examples 36 to 39, 52 to 53, and 73, since the hardness of the weld metal is higher than that of the base steel plate, the deformability of the weld metal and the base steel plate when applying strain is greatly different. It is estimated that

  Moreover, as shown in Table 2 and Table 3, among the experimental examples 33 to 35, 40 to 51, and 54 to 72, the experimental examples 33 to 35 and 43 to 43 in which the aspect ratio of the condensed beam satisfies the above formula (4). In 51 and 59 to 72, the maximum grain size of ferrite is 350 μm or less, and the surface roughness of the weld metal is less than or equal to the surface roughness of the base steel plate.

“When the strain ratio is 0”
As shown in Tables 2 and 3, in Experimental Examples 33 to 34, 40 to 48, and 54 to 68 in which the aspect ratio of the focused beam satisfies the above formula (2), the amount of swell of the weld metal is that of the base steel plate. It can be seen that the surface roughness is not more than 9.10 μm.

On the other hand, the experimental example in which the aspect ratio of the focused beam does not satisfy the above formula (2), and the ferrite content of the weld metal with a particle size of 30 to 150 μm is less than 53% calculated from the above formula (3). In 35-39, 49-53, 69-73, the amount of swell of the weld metal exceeds the surface roughness of the base steel plate.
In Experimental Examples 35 to 39, 49 to 53, and 69 to 73, since the hardness of the weld metal is higher than that of the base steel plate, the deformability between the weld metal and the base steel plate when strain is applied. This is presumably due to a large difference.

  Moreover, as shown in Table 2 and Table 3, among Experimental Examples 33-34, 40-48, 54-68, Experimental Examples 33-34, 43- In 48 and 59 to 68, the maximum grain size of ferrite is 350 μm or less, and the surface roughness of the weld metal is less than or equal to the surface roughness of the base steel plate.

“When the strain ratio is 0.5”
As shown in Tables 2 and 3, in Experimental Examples 33, 40 to 47, and 54 to 65 in which the aspect ratio of the focused beam satisfies the above formula (2), the swell amount of the weld metal is the surface roughness of the base steel plate. It can be seen that the degree is not higher than 9.10 μm.

On the other hand, an experiment example in which the aspect ratio of the focused beam does not satisfy the above formula (2), and the ferrite content of the weld metal with a particle size of 30 to 150 μm is less than 56% calculated from the above formula (3). In 34 to 39, 48 to 53, and 66 to 73, the swell amount of the weld metal exceeds the surface roughness of the base steel plate.
This is because, in Experimental Examples 34 to 39, 48 to 53, and 66 to 73, the hardness of the weld metal is higher than that of the base material steel plate, so that the deformability of the weld metal and the base material steel plate when strain is applied. This is presumably due to a large difference.

  Moreover, as shown in Table 2 and Table 3, among Experimental Examples 33, 40 to 47, and 54 to 65, Experimental Examples 33, 43 to 47, and 59 to 65 in which the aspect ratio of the condensed beam satisfies the above formula (4). In 65, the maximum grain size of ferrite is 350 μm or less, and the surface roughness of the weld metal is less than or equal to the surface roughness of the base steel plate.

“When the strain ratio is 1.0”
As shown in Table 2 and Table 3, in Experimental Examples 33, 40 to 46, and 54 to 63 in which the aspect ratio of the focused beam satisfies the above formula (2), the amount of swell of the weld metal is the surface roughness of the base steel plate. It can be seen that the degree is not higher than 9.10 μm.

On the other hand, the experimental example in which the aspect ratio of the focused beam does not satisfy the above formula (2), and the ferrite content of the weld metal with a particle size of 30 to 150 μm is less than 59% calculated from the above formula (3). In 34 to 39, 47 to 53, and 64 to 73, the swell amount of the weld metal exceeds the surface roughness of the base steel plate.
In Experimental Examples 34 to 39, 47 to 53, and 64 to 73, since the hardness of the weld metal is higher than that of the base steel plate, the deformability between the weld metal and the base steel plate when strain is applied. This is presumably due to a large difference.

  Moreover, as shown in Table 2 and Table 3, among Experimental Examples 33, 40 to 46, and 54 to 63, Experimental Examples 33, 43 to 46, and 59 to 63 in which the aspect ratio of the condensed beam satisfies the above formula (4). In No. 63, the maximum grain size of ferrite is 350 μm or less, and the surface roughness of the weld metal is not more than the surface roughness of the base steel plate.

  DESCRIPTION OF SYMBOLS 1a, 1b ... Base material steel plate, 2 ... Weld metal, 3 ... Direct diode laser, 4 ... Gas supply means, 5 ... Condensed beam, 11a, 11b ... Surface of base material steel plate, 21 ... Surface of weld metal, 30 ... Welding means, Ra1 ... roughness of base steel plate, Ra2 ... roughness of weld metal, H ... swell amount of weld metal.

Claims (5)

A method for producing a welded joint by welding a plurality of base steel plates,
Having a welding process of welding at a welding speed of 0.5 m / min or more using a direct diode laser as a light source;
As the direct diode laser, the shape of the focused beam at the half-value width of the peak intensity is long in the welding direction, and the aspect ratio (welding direction / welding orthogonal direction) between the welding direction length of the focused beam and the welding orthogonal direction length is 2. More than 5 and satisfying the following formula (1)
In the welding step, a weld metal including 50% or more of ferrite having a particle size of 30 to 150 μm is formed.
Aspect ratio ≧ 5.0 × Welding speed (m / min) (1) A method for manufacturing a welded joint processed at a predetermined strain ratio,
As the direct diode laser, a laser whose aspect ratio satisfies the following formula (2) is used:
2. The method for manufacturing a welded joint according to claim 1, wherein in the welding step, a weld metal is formed in which the content of the ferrite having a particle size of 30 to 150 μm satisfies the following formula (3).
Aspect ratio ≧ (5.8 + 1.6 × strain ratio) × welding speed (m / min) (2)
Ferrite content ≧ 53 + 6 × strain ratio Equation (3) As the direct diode laser, the aspect ratio satisfies the following formula (4),
The method for manufacturing a welded joint according to claim 1 or 2, wherein, in the welding step, the weld metal having a maximum grain size of the ferrite of 350 µm or less is formed.
Aspect ratio ≦ 16.0 × Welding speed (m / min) −3.0 Formula (4) The method for manufacturing a welded joint according to any one of claims 1 to 3, wherein an average power density of the direct diode laser is 0.6 kW / mm ² or less. A welded joint manufacturing apparatus used in the welded joint manufacturing method according to any one of claims 1 to 4,
Having welding means for welding at a welding speed of 0.5 m / min or more;
As the light source, the shape of the focused beam as a light source at the half width of the peak intensity is long in the welding direction, and the aspect ratio (welding direction / welding orthogonal direction) between the welding direction length of the focused beam and the welding orthogonal direction length is A welding joint manufacturing apparatus comprising a direct diode laser satisfying the following formula (1) that is 2.5 or more.
Aspect ratio ≧ 5.0 × Welding speed (m / min) (1)

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