JP3145228B2 - Laser welding method for thin steel plate - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser welding method for welding thin steel plates using a laser as a heat source and a filler metal.

[0002]

2. Description of the Related Art As one of processing techniques using a laser,
Laser welding is known. Since the laser beam has a very small spread angle unlike ordinary light, it can be focused on a small spot diameter, and a high power density similar to that of an electron beam can be obtained. For this reason, laser welding enables deep penetration welding at a high energy density. Compared to electron beam welding, laser welding can be performed in the atmosphere without the need for a vacuum chamber and does not emit X-rays. It is characterized by high practicality such as not receiving. Also, compared to electron beam welding, the maximum plate thickness that can be welded with a given power is small, making it more suitable for high-speed and precision welding of thin plates than for thick plates.

Conventionally, two methods have been adopted in laser welding: a method of performing without using a filler material and a method of using a filler material. However, in the case of laser welding using no filler material as in the former case, there is a problem that the allowable range for misalignment and gap of the base material is narrow, and that blow holes and pits are easily generated. On the other hand, in the case of laser welding using a filler metal, high-speed welding is performed, the cooling rate increases, the difference in strength between the weld metal and the base metal increases, and processing of thin steel sheet materials that require processing such as forming after welding is performed. Deterioration of properties has been a major problem.

Japanese Patent Application Laid-Open No. Hei 3-221280 discloses a method of using a steel wire having a low C content, a low Si content, and a low Mn content as a filler material to prevent hardening of a weld metal. Furthermore, JP-A-3-230880 discloses that
A laser welding method using a flux-cored wire as a filler material and obtaining a sound welded joint particularly in welding galvanized steel sheets has been proposed.

[0005]

DISCLOSURE OF THE INVENTION The present invention relates to a method for laser welding of a steel sheet which can be used to form a welded joint excellent in sound formability by using a filler material in welding of a steel sheet used after forming by laser welding. A welding method is provided.

[0006]

Means for Solving the Problems The present inventors have conducted various studies on the base metal components, filler metal types, welding conditions, etc. in laser welding in order to solve the above-mentioned problems. A laser welding method for obtaining a joint has been established, and the gist of the method is to use a laser as a heat source and to weld a steel sheet using a filler metal, as follows:
0.07% or less (all% below are all% by weight), Si: 0.
005 to 0.06%, Mn: 0.03 to 0.80%,
B: 0.0005% or less, balance: Fe and unavoidable impurities, C: 0.02% or less, Si: 0.00
2 to 0.05%, Mn: 0.05 to 0.30%, P:
0.03 to 0.08%, S: 0.02 to 0.06%, N
b: 0.003 to 0.03%, B: 0.001% or less,
O: 0.01 to 0.04%, the balance being a steel wire having a diameter of 0.4 to 1.2 mm consisting of Fe and unavoidable impurities as a filler material, and a filler material supply speed of 1.2 to 3.0 m / min. And a laser welding method characterized in that welding is performed by using a laser welding method. Hereinafter, the present invention will be described in detail.

[0007]

The present invention is intended to reduce the difference in strength, ie, the difference in hardness, between the weld metal of the joint and the base metal in the welding of thin steel sheets used after forming by laser welding. This has been achieved from the viewpoint that it is very important to improve the bead shape, especially the toe shape.

First, in order to reduce the difference in strength between the weld metal and the base material, it is effective to make the chemical composition of the weld metal less curable. As a result of various studies on the weld metal components of the laser welded joint, the present inventors have found that good formability can be maintained when the components of the weld metal are within the following component ranges. C: 0.05% or less Si: 0.002 to 0.04% Mn: 0.03 to 0.70% B: 0.0005% or less

In laser welding of a thin steel plate having a plate thickness of 2 mm or less, even when a filler metal is used, there is usually a base material dilution of 50% or more, sometimes as much as 90% or more. For this reason, even if only the components of the filler material used are limited, the performance of the welded joint obtained cannot be specified. The performance of the weld metal by laser welding of thin steel plates is of course affected by the components of the filler metal, but in addition, the base metal components and welding conditions, such as the type of laser beam, beam mode, output, welding speed, and filler metal supply It has been found that the chemical composition of the resulting weld metal is greatly influenced by the speed. Particularly, the components of the base metal are largely diluted in the weld metal as described above. For this reason,
As means for controlling the weld metal component of the weld joint, the chemical components of the base metal and the filler metal are both limited. In addition, the supply rate of the filler material, which greatly affects the amount of dilution of the base material, is limited as welding conditions.

Another feature of the present invention is that in welding a steel sheet to be used after forming by laser welding, it is necessary to form a smooth undercut and a weld bead having no smoothness in order to ensure formability. It is found that it is effective to increase P and S in the filler material and to specify the filler material supply rate for the bead formation.

The present invention has been made based on the above findings, and the reasons for limitation in the present invention will be described below. [C: 0.07% or less as base material, 0.0 as filler metal
[2% or less] C exhibits effects such as securing strength and stabilizing the structure. However, in laser welding of a thin steel plate, the cooling rate during welding is high, so that the hardening of the weld metal is promoted. Therefore, the C content in the weld metal must be reduced as much as possible from the viewpoint of ensuring workability after welding. If the C of the weld metal is 0.05% or less, hardening due to welding is small and workability is good. For this reason, the base material needs to be 0.07% or less. Further, if C as a filler material is 0.02% or less, C of the weld metal can be obtained in a relatively wide range of welding conditions.
Can be kept to 0.05% or less. Therefore, the C content is set to 0.07% or less as a base material and 0.02% or less as a filler material.

[Si: 0.005 to 0.06 as a base material
%, 0.002 to 0.05% as a filler metal] Si is an effective component as a deoxidizing agent, and an appropriate amount is added to a steel material depending on the purpose. The Si added to the filler metal for laser welding acts as a deoxidizing agent in the molten metal to obtain clean steel and has the effect of suppressing the occurrence of blow holes. Further, the viscosity of the molten metal is adjusted to improve the bead shape.
However, Si is also an element that promotes the hardening of the welded portion similarly to C. If the content of the molten metal exceeds 0.04%, the difference in strength between the weld metal and the base metal increases, and the formability deteriorates. Si tends to deoxidize during welding and decrease as weld metal. The base metal has a Si content of 0.06% or less and the filler metal has a Si content of 0.1% or less.
If it is at most 05%, a weld metal excellent in formability can be obtained under a wide range of welding conditions. 0.005 Si of base metal
% And less than 0.002% of the filler metal, the deoxidation at the time of welding is insufficient and blowholes are generated, and the bead shape is deteriorated due to insufficient viscosity of the molten metal. Therefore, the amount of Si is 0.005 to 0.06 as a base material.
%, And 0.002 to 0.05% as a filler material.

[Mn: 0.03-0.80 as a base material]
%, 0.05 to 0.30% as a filler material] Mn is also an effective component as a deoxidizing agent like Si, and is used for the purpose of improving strength, securing a fine microstructure, and improving toughness. Widely used for steel materials. In addition, M added to the filler metal
n has the effect of obtaining clean molten steel as a deoxidizing agent and suppressing the occurrence of blowholes. However, Mn is also an element that promotes the hardening of the welded portion, like Si and C. If the content of the weld metal exceeds 0.70%, the difference in strength between the weld metal and the base metal increases, and the formability deteriorates. If the Mn of the base material is 0.80% or less and the Mn of the filler material is 0.30% or less, a weld metal excellent in formability can be obtained. Mn of the base material is 0.0
If the content is less than 3% and the Mn content of the filler is less than 0.05%, deoxidation at the time of welding is insufficient and blowholes are generated. Therefore, the Mn content is set to 0.03 to 0.80% as a base material and 0.05 to 0.30% as a filler material.

[B: 0.0005% or less as base metal and 0.001% or less as filler metal] B is used in steel for the purpose of improving toughness by refining the structure, etc. In the case of such a high high cooling rate, even a minute amount hardens the weld metal. For this reason, it is desirable that B be reduced as much as possible in both the base material and the filler material. B in the base metal is 0.0005% or less, B in the filler metal is 0.001%
Below, the amount of B is set to 0.0005% or less as a base material and 0.001% or less as a filler metal because the weld metal hardly hardens.

[P: 0.03-0.08 as filler metal
%] P is generally contained as an impurity in steel materials, and efforts have been made to keep the mechanical properties as low as possible in steel. On the other hand, when P is added to the filler metal during laser welding, the viscosity of the molten metal is adjusted during welding to improve the wettability with the base metal, and a smooth weld bead can be obtained. If the content of P in the filler material is less than 0.03%, the effect of improving the bead shape is not sufficient, and if the content of P in the filler material exceeds 0.08%, embrittlement of the weld metal becomes severe. Therefore, the P content is set to 0.03 to 0.08% as a filler material.

[S: 0.02-0.06 as filler metal
%] S is generally contained as an impurity in steel materials except for free-cutting steel and the like, and efforts are being made to keep the content in steel as low as possible to promote deterioration of mechanical properties and high-temperature cracking during welding. On the other hand, when S is added to the filler metal of laser welding, the melting point of the molten metal can be reduced during welding, and a smooth weld bead can be obtained. Excellent weld beads can be obtained. If the S content in the filler metal is less than 0.02%, the effect of improving the bead shape is not sufficient. If the S content in the filler material exceeds 0.06%, a brittle structure due to segregation of S precipitates in the weld metal and a high temperature occurs. Cracks easily occur. Therefore, the amount of S is set to 0.02 to 0.06% as a filler material.

[Nb: 0.003 to 0.0 as filler metal]
[3%] Nb is generally used for securing strength because a small amount of Nb increases the strength. However, if a small amount is added to a filler metal for laser welding, blow holes due to N entrainment during welding and the like are generated. Has the effect of suppressing. Nb in filler metal
Is less than 0.003%, the blowhole suppressing effect cannot be obtained. On the other hand, if Nb in the filler material exceeds 0.03%, the weld metal is hardened and the formability deteriorates. For this reason, the Nb content was set to 0.003 to 0.03% as a filler material.

[O: 0.01 to 0.04 as filler metal
%] O in the filler material is effective in facilitating detachment of the droplet at the tip of the filler material during laser welding and ensuring a stable bead shape. If the amount of O in the filler material is less than 0.01%, a sufficient effect for improving the bead shape cannot be obtained, and if it exceeds 0.04%, blowholes are easily generated. Therefore, the amount of O in the filler material is set to 0.01 to 0.04%.

[Diameter of filler metal: 0.4 to 1.2 mm] The diameter of the filler metal controls the amount of molten metal in the weld joint.
It also affects the base material dilution. Filler diameter is 0.4mm
If it is less than 3, the molten metal supplied to the joint portion becomes insufficient, resulting in a concave bead and the occurrence of blowholes increases. on the other hand,
If the diameter of the filler material exceeds 1.2 mm, the amount of the molten metal becomes large and the shape becomes a convex bead, and the toe end of the bead tends to overlap and the formability deteriorates. Therefore, the diameter of the filler metal is 0.4
-1.2 mm.

[Feeding material supply speed: 1.2 to 3.0 m / m
in] The welding material supply speed among the welding conditions in laser welding greatly affects the bead shape of the welded portion, the dilution of the base metal component into the weld metal, and the like. Filler feed rate is 1.2
If it is less than m / min, the amount of the molten metal is insufficient, and the resulting bead has an irregular bead width. In addition, since the amount of dilution of the base material is increased, a welded joint having inferior formability is obtained even when a low C, Si, or Mn filler material is used. On the other hand, the filler supply speed is 3.
If it exceeds 0 m / min, the amount of molten metal becomes too large and the margin of the bead becomes large, so that it becomes necessary to grind after welding. For this reason, the filler supply speed is set to 1.2 to 3.0.
m / min.

The main conditions for laser welding include gas lasers (eg, CO ₂ laser) and solid-state lasers (eg, YAG laser) as laser beam types. Although there is a ring mode and the like, the present invention can employ any condition. However, as the type of laser light, CO
_Two lasers are desirable, and as a laser beam mode, a multi mode or a ring mode is desirable from the viewpoint of penetration characteristics and gap resistance characteristics. As a practical range of output, welding speed, etc., output is 1 to 5 kW, welding speed is 1 to 5 m / min.
It is. The steel sheet referred to in the present invention mainly has a sheet thickness of 4 m.
The present invention is suitable for butt welding and lap welding of thin steel plates.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Five types of steel sheets (thickness: 1.6 m) shown in Table 1
m), various welding materials and welding conditions were selected, and the gap between the steel sheets was set to 0 mm to perform joint welding. As the conditions for laser welding, the output was ₂ to 4 kW using a CO ₂ laser.
And the mode was multi-mode. The welding speed was kept constant at about 2 m / min, and the filler feed speed was set at 0.1 m / min.
It was changed in the range of 8 to 3.2 m / min.

[0023]

[Table 1]

After welding, the bead shape, internal soundness and formability were investigated. The bead shape was visually inspected, and a bead having a constant bead width and bead height and having no defect such as undercut or overlap at the bead toe was evaluated as good. Regarding the internal soundness, an X-ray transmission test was performed over the entire length of the joint in accordance with JIS Z3104 to investigate internal defects such as blow holes. Blowhole occurrence rate is 20% of the total bead length.
% Or less as good, and more than 20% as bad. In addition, all of the specimens in which crack-like defects were observed other than the blowholes were regarded as defective.

Further, a macro test piece was taken from the center of the joint, and the hardness Hv (load 200
g) was investigated, and a difference ΔH in hardness between the base metal and the weld metal was determined as a determination of formability. Hardness difference Δ
H (hardness of weld metal−hardness of base metal) of Hv 100 or less was defined as good formability. C, Si, Mn, and B were analyzed for each of the weld metals.

Table 2 shows the combinations of the steel plates used and the filler metals and the results of various tests in this example. Symbols A-1 to A-8 are examples of the present invention, and B-1 to B-8 are comparative examples. C-1 is a comparative example in which no filler material was used.

[0027]

[Table 2]

As is clear from Table 2, A-1 to A-8 by the laser welding method of the present invention have appropriate diameters of the filler material and appropriate supply rates of the filler material. P, S, O
, All show good bead shapes. In addition, since appropriate amounts of Nb and O are contained in the filler material and Si, Mn, P, and S of the filler material are also appropriate, the internal soundness is also good. Furthermore, C, S of filler metal and weld metal
Since i, Mn, and B are also in appropriate amounts, the difference in hardness between the base metal and the weld metal is all Hv100 or less, indicating that good formability is exhibited.

On the other hand, in Comparative Example B-1, the filler metal component was within the range of the present invention, but the Si content of the base metal was large, so that the Si content of the weld metal was large and the formability was poor. Similarly, B-8 has both a filler material component and a base material component within the range of the present invention, but the bead shape and the internal soundness were poor because the diameter of the filler material was too small. B-
No. 2 has a low O in the filler, B-4 has a too low P in the filler, and B-5 has a low S in the filler and a low O in the filler.
Is too high and the filler supply speed is high, so that B-7
In each case, the bead shape was poor because the filler supply speed was too slow.

B-3 has too much P in the filler, B-4 has substantially no Nb in the filler, and B-5 has a large O in the filler. Too much, B-6
In each case, the internal soundness was poor because the Si of the filler material was too low and the S of the filler material was too large.

Furthermore, B-2 has too much C and Mn of the filler metal and the weld metal, and B-3 has too much Si and Nb of the filler metal and the weld metal, and has a large B- 6 has a large amount of B in the filler metal and the weld metal, so B-7 is C, Mn,
Since the Nb content was high and the yield was high in the weld metal, the formability was poor in all cases.

Further, C-1 which did not use a filler had a concave bead shape, many blow holes, and was inferior in internal soundness.

[0033]

As described above, according to the laser welding method of the present invention, it is possible to obtain a welded joint having excellent bead shape, internal soundness, and formability in welding a thin steel plate.

────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁷ Identification symbol FI B23K 103: 04 (56) References JP-A-63-126683 (JP, A) JP-A-3-60883 (JP, A) JP-A-62-183994 (JP, A) JP-A-3-204195 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B23K 26/00 B23K 35/30 C22C 38/00 C22C 38/04 B23K 103: 04

Claims (1)

(57) [Claims] 1. When welding a steel sheet using a laser as a heat source and a filler metal, C: 0.07% or less (all% below are all% by weight) Si: 0.005 to 0.06% Mn: 0.03 to 0.80% B: 0.0005% or less The balance is a steel material composed of Fe and inevitable impurities. C: 0.02% or less Si: 0.002 to 0.05% Mn: 0.05 P: 0.03-0.08% S: 0.02-0.06% Nb: 0.003-0.03% B: 0.001% or less O: 0.01-0. 0.4% 0.4-1.2m with the balance being Fe and unavoidable impurities
With a steel wire having a diameter of m as a filler, the filler supply speed is 1.2
A laser welding method for a steel thin plate, characterized in that welding is performed at a speed of 3.0 m / min.