JP6958153B2 - Gas shield arc welding method and welding joint manufacturing method - Google Patents

Description

The present disclosure relates to a gas shielded arc welding method and a welding joint manufacturing method.

In recent years, the application of wear-resistant steel, high-strength thick steel sheet, and high-strength thin steel sheet to structures brings about rationalization and weight reduction of the structure, and has many advantages, so that the range of application has been expanded. For example, in the automobile field, there is an increasing need to use high-strength thin steel sheets in order to reduce the weight of the vehicle body for the purpose of reducing _{fuel consumption and CO 2 emissions.}

On the other hand, when a steel plate having a high Pcm such as an abrasion-resistant steel, a high-strength thick steel plate, and a high-strength thin steel plate is welded, low-temperature cracks are likely to occur, and in particular, there is a problem that low-temperature cracks are likely to occur in the first layer of welding. For example, in the welding of steel sheets for automobiles, when a high-strength thin steel sheet having a tensile strength of more than 980 MPa is welded using a high-strength wire, there are cases where low-temperature cracks occur in the weld metal. Therefore, some methods for suppressing low temperature cracking have been proposed (Patent Document 1).

Japanese Unexamined Patent Publication No. 2016-137518

However, in the prior art of Patent Document 1, low temperature cracking of high Pcm steel sheets such as abrasion resistant steels, high-strength thick steel sheets, and high-strength thin steel sheets is still insufficiently suppressed, and the weld bead shape becomes poor. There was a problem that there was something.

In view of the above prior art, the present inventor uses a wire (also referred to as a molten material) having a predetermined chemical composition and a predetermined amount of diffusible hydrogen, and moves the wire forward and backward with respect to the molten pool to generate an arc. We have found that welding is performed intermittently.

The gist of the method of the present disclosure is as follows.
(1) By mass%
C: 0.05 to 0.13%,
Si: 0.35 to 1.20%,
Mn: 1.60 to 3.40%, and Cu: 0.10 to 0.50%
Contains,
P: 0.030% or less, and S: 0.020% or less,
Using a wire in which the balance is composed of Fe and impurities and the amount of diffusible hydrogen in the weld metal is 2.0 ml / 100 g or less, the advance / retreat and energization control of the wire are controlled between the molten pool and the contact tip. A gas shielded arc welding method characterized in that an arc is intermittently generated and welding is performed.
(2) The wire is in mass%.
Ni: 3.40% or less,
Cr: 1.20% or less,
Mo: 1.00% or less, and Ti: 0.25% or less,
The gas shielded arc welding method according to (1) above, which further contains one kind or two or more kinds selected from the group consisting of.
(3) The Pcm calculated from the chemical composition of the wire is lower than the Pcm calculated from the chemical composition of the steel material.
The Pcm is the following equation (1):
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1)
The gas shielded arc welding method according to (1) or (2) above, wherein the element symbol is the mass% of the additive element contained in the formula.
(4) The gas shielded arc welding method according to any one of (1) to (3) above, wherein the wire is a solid wire.
(5) The gas shielded arc welding method according to any one of (1) to (4) above, wherein the gas shielded arc welding according to any one of (1) to (4) above is performed on at least the first layer of the multi-layer welding.
(6) A method for manufacturing a welded joint, characterized in that a steel plate is gas-shielded arc-welded by using the gas-shielded arc welding method according to any one of (1) to (5).

According to the present invention, it is possible to suppress low temperature cracking in a welded joint and obtain a good weld bead shape.

FIG. 1 is a schematic view of a joint shape defined in a y-shaped weld crack test conforming to JIS Z3158. FIG. 2 is a cross-sectional photograph of the welded joint obtained in the example of the present invention. FIG. 3 is a cross-sectional photograph of the welded joint obtained in the comparative example. FIG. 4 is a schematic view of a joint shape of multi-layer welding. FIG. 5 is a schematic view of lap fillet welding used in the example of the present invention. FIG. 6 is a schematic view and a cross-sectional observation photograph for explaining the observation points of the low temperature cracks in the overlap fillet welded portion.

According to the present invention, low-temperature cracking is suppressed and the bead shape is good by welding in which a wire having a specific chemical component and a wire are moved back and forth with respect to a molten pool to generate an arc intermittently. Good welding can be performed. Low-temperature cracking can occur due to high-strength steel sheets, tensile residual stress, and invading hydrogen. However, by using a wire having a specific chemical component, low-temperature cracking and bead formation defects are less likely to occur, and the above welding Due to the intermittent generation of arcs, the divergence of hydrogen sources such as water and oil into atomic hydrogen can be suppressed, and low-temperature cracking can be suppressed.

Further, if the alloy component of the weld metal increases due to the dilution of the base metal during welding, solidification cracking and low temperature cracking are likely to occur. The amount of alloy components such as C and Ni invading the weld metal due to the above is reduced. As a result, the increase in hardness of the weld metal is suppressed, and low-temperature cracking can be suppressed. Further, the bead shape can be improved by setting the amount of Si in the wire to a specific amount. For welding that intermittently generates an arc by moving the wire forward and backward with respect to the molten pool, CMT (Cold Metal Transfer) (registered trademark) welding, Synchro Feed (registered trademark) GMA welding, and AWP (Active Wire Feed) Process) There is welding. These are the same welding methods with different names depending on the welding machine manufacturer.

Hereinafter, examples of embodiments of the method of the present disclosure will be described.

1. 1. Chemical Composition The reason why the chemical composition of the wire used in the method of the present disclosure is defined as described above will be described. In the following description, "%" representing the content of each element means mass% with respect to the total mass of the wire unless otherwise specified.

[C: 0.05% or more and 0.13% or less]
C is an element necessary for improving the strength of the weld metal. However, if C is less than 0.05%, this effect cannot be obtained. On the other hand, if it exceeds 0.13%, the welding crack sensitivity becomes high. Therefore, C is set to 0.05 to 0.13%.

The lower limit of the C content is preferably 0.07% or more, more preferably 0.08% or more. By setting the lower limit of the C content to the above range, the strength of the weld metal can be further improved. The upper limit of the C content is preferably 0.12% or less, more preferably 0.11% or less, and by setting the upper limit of the C content to the above range, the welding crack sensitivity can be further reduced. can.

[Si: 0.35% or more and 1.20% or less]
Si is indispensable as a main deoxidizer for weld metals and has an effect of stabilizing an arc in a low current range. Further, it is an element that reduces the amount of O on the surface of the molten metal and increases the surface tension of the molten metal by forming fine inclusions of Si and O in the molten metal. As a result, the molten metal can be prevented from coming off from the tip of the wire, and the arc is stabilized. However, if Si is less than 0.35%, the above effect is not sufficient, the arc in the low current region becomes unstable, and bead formation failure occurs due to arc breakage. In addition, the surface tension of the molten metal becomes low, the molten metal easily separates from the wire tip, and the amount of spatter generated increases. On the other hand, when Si exceeds 1.20%, the surface tension of the molten metal rises excessively, so that the molten metal is difficult to separate from the tip of the wire, the droplets become large, and spatter occurs. Therefore, Si is 0.35 to 1.20%.

The lower limit of the Si content is preferably 0.50% or more, more preferably 0.60% or more, and further preferably 0.70% or more. By setting the lower limit of the Si content to the above range, the deoxidizing effect and the arc stabilizing effect can be further improved. The upper limit of the Si content is preferably 1.10% or less, more preferably 1.00% or less.

[Mn: 1.60% or more and 3.40% or less]
Like Si, Mn is a major antacid and is added to ensure strength. Further, by fixing S as MnS before a low melting point compound such as FeS is produced, there is an effect of preventing high temperature cracking. If Mn is less than 1.60%, the strength and toughness of the weld metal cannot be sufficiently ensured. Also, the arc becomes unstable. On the other hand, when Mn exceeds 3.40%, the surface tension of the molten metal is excessively lowered, so that the molten metal is irregularly separated from the tip of the wire and spatter occurs. Therefore, Mn is set to 1.60 to 3.40%.

The lower limit of the Mn content is preferably 1.70% or more, more preferably 1.75% or more. By setting the lower limit of the Mn content to the above range, the deoxidizing effect, the strength and toughness improving effect, and the high temperature cracking prevention effect can be obtained more stably. The upper limit of the Mn content is preferably 3.00% or less, more preferably 2.60% or less. By setting the upper limit of the Mn content to the above range, spatter generation can be suppressed more stably.

[Cu: 0.10% or more and 0.50% or less]
Cu has the effect of improving the strength of the weld metal. In addition, the Cu plating on the wire surface improves the electrical conductivity and wire feedability during welding and stabilizes the arc. If Cu is less than 0.10%, the arc becomes unstable and spatter occurs. On the other hand, if Cu exceeds 0.50%, segregation occurs in the weld metal and high-temperature cracking is likely to occur. Therefore, Cu is set to 0.10 to 0.50%. Cu also includes a copper-plated component on the wire surface.

The lower limit of the Cu content is preferably 0.15% or more, more preferably 0.20% or more. By setting the lower limit of the Cu content to the above range, the strength improving effect and the arc stabilizing effect can be obtained more stably. The upper limit of the Cu content is preferably 0.40% or less, more preferably 0.30% or less. By setting the upper limit of the Cu content to the above range, high temperature cracking can be suppressed more stably.

[P: 0.030% or less]
P is an impurity, and if the wire used in the method of the present disclosure contains an excessive amount of P, the solidification crack resistance of the weld metal deteriorates. Therefore, the upper limit of the P content is limited to 0.030% or less.

[S: 0.020% or less]
S is an impurity and is an element that affects the surface tension and viscosity of the molten metal. If the wire used in the method of the present disclosure contains S excessively, the surface tension of the molten metal is excessively lowered and the tip of the wire The molten metal is irregularly separated from the metal, the arc is unstable, spatter occurs, and the solidification crack resistance deteriorates. Therefore, the upper limit of the S content is limited to 0.020% or less.

[Ni: 3.40% or less]
Ni is not an essential element for the wire used in the method of the present disclosure, but Ni lowers the transformation temperature to make the structure of the weld metal finer and dissolves in the weld metal to lower the low temperature toughness. It has the effect of increasing the strength. On the other hand, if Ni exceeds 3.40%, the solidification crack resistance deteriorates. Therefore, the wire preferably contains 3.40% or less Ni, more preferably 2.70% or less Ni. When Ni is added, it is preferable that the content is 0.50% or more because more effect can be obtained.

[Cr: 1.20% or less]
Cr is not an essential element in the wire used in the methods of the present disclosure, but Cr has the effect of lowering the transformation temperature, refining the structure and improving strength and toughness. When Cr exceeds 1.20%, the hardening of the weld metal becomes remarkable and the toughness and the sensitivity to low temperature cracking decrease. Therefore, the wire preferably contains 1.20% or less of Cr, and more preferably 1.10% or less of Cr. When Cr is added, it is preferable that the content is 0.05% or more because more effects can be obtained.

[Mo: 1.00% or less]
Mo is not an essential element for the wire used in the methods of the present disclosure, but Mo has the effect of lowering the transformation temperature, refining the structure of the weld metal and improving strength and low temperature toughness. If Mo exceeds 1.00%, the strength of the weld metal becomes excessively high, and low temperature toughness cannot be stably obtained. It also increases the sensitivity to low temperature cracking. Therefore, the wire preferably contains 1.00% or less of Mo, and more preferably 0.68% or less of Mo. When Mo is added, it is preferable that the content is 0.30% or more because more effect can be obtained.

[Ti: 0.25% or less]
Ti is not an essential element for the wires used in the methods of the present disclosure, but Ti acts as a deoxidizer and produces fine oxides of Ti in the weld metal to improve the toughness of the weld metal. If Ti exceeds 0.25%, the toughness decreases. Therefore, the wire preferably contains 0.25% or less Ti, more preferably 0.21% or less Ti. When Ti is added, it is preferable that the content is 0.09% or more because more effect can be obtained.

The wire used in the present invention is preferably a solid wire. This is because if a so-called flux-cored wire in which the flux is wrapped with a steel outer skin is used, the slag generated from the flux may remain under the bead and cause welding defects. When a flux-cored wire is used, it is preferable to use a metal-based flux-cored wire having as little slag component as possible.

The combination of the wire and the steel material as the base material is not particularly limited, but it is preferable that the Pcm of the wire is lower than the Pcm of the steel material. Pcm is edited by Welding Society, Introduction to Welding and Joining Technology, 1998, Sanpo Shuppan, Tokyo, P.M. The following equation (1) described in 118:
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1)
It is represented by. Substitute the mass% of the contained element for the element symbol in the formula (1), and substitute zero for the additive-free element.

By making the Pcm of the wire smaller than the Pcm of the steel material and softening the weld metal side, the sensitivity to low temperature cracking in the weld metal can be reduced. The Pcm of the wire is preferably 0.01 smaller, more preferably 0.03 smaller, and even more preferably 0.05 smaller than the Pcm of the steel material.

The Pcm of the wire is preferably 0.10 to 0.50, more preferably 0.15 to 0.40, and even more preferably 0.25 to 0.35. The Pcm of the steel material is preferably 0.25 to 0.50, more preferably 0.25 to 0.45, and even more preferably 0.28 to 0.39.

[Amount of diffusible hydrogen: 2.0 mL / 100 g or less]
The amount of diffusible hydrogen contained in the weld metal is 2.0 mL / 100 g or less, preferably 1.9 mL / 100 g or less, and more preferably 1.7 mL / 100 g or less. The amount of diffusible hydrogen is mainly due to the fat content of the wire. The amount of diffusible hydrogen in the weld metal is measured in accordance with JIS Z 3118 (2007). 100% CO ₂ gas will be used as the shield gas. The welding method is not the gas shielded arc welding of the present disclosure, but a normal welding method is used. When the amount of diffusible hydrogen is in the above range, the effect of suppressing low temperature cracking can be obtained. The amount of diffusible hydrogen contained in the weld metal can be controlled, for example, by adjusting the type and amount of the feed lubricating oil to be applied to the wire surface. The feed lubricating oil may be any of animal and vegetable oils, mineral oils and synthetic oils. As the animal and vegetable oil, palm oil, rapeseed oil, castor oil, pig oil, cow oil, fish oil and the like can be used, and as the mineral oil, machine oil, turbine oil, spindle oil and the like can be used. As the synthetic oil, hydrocarbon-based, ester-based, polyglycol-based, polyphenol-based, silicone-based, and fluorocarbon-based oils can be used. Further, a sulfur-containing lubricating oil which is one or more of sulfur-containing fats and oils, sulfide esters, sulfide fatty acids or sulfide olefins in one or more base oils of fats and oils or esters can also be used.

2. Manufacturing Method Next, welding used in the gas shielded arc welding method of the present disclosure will be described.

CMT welding can be mentioned as a method of intermittently generating an arc by controlling the advancing / retreating of the wire and the energization control between the molten pool and the contact tip. For CMT welding, a CMT type welding power supply manufactured by Fronius can be used. This CMT welding power supply can detect a short circuit in the arc and control the feeding of the wire 50 to 130 times per second to transfer the droplets. By pulling back the wire, it is possible to promote the separation of droplets, and to perform droplet migration with reduced spatter by controlling the current to the minimum at the time of short circuit. Then, the current value is controlled in synchronization with the feeding and pulling back of the wire, and when an arc is generated, the wire is fed toward the melting pool and the wire is pulled back immediately after a short circuit is detected. The heat can be reduced.

By interrupting the generation of arcs by CMT welding, it is possible to suppress the deviation of hydrogen sources such as water and oil into atomic hydrogen, and low-temperature cracking is less likely to occur. The shield gas and the gas flow rate are not particularly limited, but may be CO ₂ gas, Ar + CO ₂ binary mixed gas, Ar + CO ₂ + O ₂ ternary mixed gas, or the like. The welding conditions are not particularly limited, but for example, the current value is about 130A to 290A, and the welding speed is about 0.1 to 1.5 m / min.

In the synchro feed GMA welding and the AWP welding, the same welding as the above-mentioned CMT welding can be performed.

In the method of the present disclosure, the number of passes can be one or more. A pass is a single welding operation performed along the joint to be welded. When the number of passes is two or more, it is preferable to perform the gas shielded arc welding of the present disclosure on at least the first layer of the multi-layer welding. Although low-temperature cracking is likely to occur in the first layer (first layer) of welding, low-temperature cracking in the first layer can be suppressed by performing the gas shielded arc welding of the present disclosure on the first layer. When the number of passes is two or more, conventional welding that does not use the gas shielded arc welding of the present disclosure may be performed on the second and subsequent layers.

The wire used in the method of the present disclosure can be manufactured by a conventional method. That is, in the case of a solid wire, the steel having the adjusted composition is melted to make a raw wire, and the wire is reduced in diameter, annealed, and plated to make a wire, and the wire is drawn to obtain a wire having a desired diameter. Can be manufactured. Further, in the case of a flux-cored wire, the steel strip to be the outer skin is formed into an open pipe (U-shape) by a forming roll while being fed in the longitudinal direction to obtain a steel outer skin. During this molding, flux is supplied from the opening of the open tube. The facing edge surfaces of the openings are butted to form a tube shape. The tube is drawn and annealed appropriately during the wire drawing or after the wire drawing process is completed to obtain a flux-cored wire. It is preferable to seam weld the edge surface to form a so-called seamless wire.

According to the present invention, low temperature cracking can be prevented without preheating the steel material before welding. Therefore, it is possible to improve the cycle time, reduce the cost, and improve the welding workability. Further, when the outside air temperature at the time of welding is low, the plate thickness of the steel material is thick, or the Pcm of the weld metal is high, there are cases where preheating at a high temperature exceeding 100 ° C. is required in the prior art. According to the invention, the occurrence of low temperature cracking can be reliably suppressed by combining a slight preheating as necessary. Since the preheating can be performed at, for example, 15 to 35 ° C. or 20 to 30 ° C., there is an advantage that the preheating temperature can be reduced and the productivity can be improved.

(Example 1)
(Measurement of diffusible hydrogen amount)
A solid wire (molten material) having the chemical components shown in Table 2 (blank means no addition) and having a wire diameter of φ1.2 mm, the balance of which is iron and impurities, was used. The amount of diffusible hydrogen in the weld metal was adjusted by changing the amount of the feed lubricating oil (palm oil) applied to the solid wire. In accordance with JIS Z 3118, the amount of diffusible hydrogen in the weld metal was measured by performing normal welding using _{CO 2 gas as the shield gas.} Table 3-1 shows the measured diffusible hydrogen amount (Hd).

(Evaluation of mechanical properties)
Two SM400s (JIS G 3106) with a plate thickness of 25 mm are butted together to form a V-shaped groove shape as shown in FIG. 4, preheated at 100 ° C. to prevent low-temperature cracking, and have the chemical components shown in Table 2. A 12-pass multi-layer welding was performed by _{ordinary CO 2} welding for each of the solid wires (molten materials) having a wire diameter of φ1.2 mm, the balance of which was iron and impurities. The welding conditions were a distance between chip base materials: 20 mm, a current of 270 A, and a welding speed of 0.35 m / min. Tensile test pieces were collected from the weld metal welded in multiple layers according to JIS Z3111, and the mechanical properties of the weld metal were evaluated. Table 3-2 shows the yield stress (YS), tensile strength (TS), and elongation (EL) of the weld metal.

(Evaluation of low temperature cracking and bead shape)
Evaluation of low temperature cracking was performed by a y-shaped weld cracking test. Two types of wear-resistant steel sheets (plate thickness 25 mm) of HRC400 and HRC500 class shown in Table 1, and a solid wire (welded material) having the chemical components shown in Table 2 and having a wire diameter of φ1.2 mm with the balance being iron and impurities. Was used to prepare a test piece having a shape specified in the y-shaped weld crack test according to JIS Z3158 shown in FIG.

In welding, CO ₂ gas was used as the shield gas and the flow rate was set to 25 L / min. A TPS5000CMT manufactured by Fronius was used as the welding power source. The welding method was CMT welding or ordinary CO ₂ welding, and both were compared. The welding conditions were the distance between the chip base materials: 20 mm, the current: 190 A, and the welding speed: 0.20 m / min. Both CMT welding and normal welding were welded in one pass. After the above welding was performed in an environment of 5 ° C., a cross-sectional survey was conducted 48 hours or more later, and the presence or absence of low-temperature cracks and the bead shape were visually evaluated. The test conditions and test results are shown in Table 4. In the evaluation results of Table 4, the sites where cracks were observed are shown in parentheses. The bead shape was good in all samples.

2 and 3 show cross-sectional photographs of the welded joints of the y-shaped weld crack test obtained in Examples 1 and 6 cut perpendicular to the longitudinal direction. No cracks were found in the welded joint obtained in the example of the present invention. On the other hand, in the welded joint obtained in the comparative example, low temperature cracking was observed in at least one of the HAZ (heat-affected zone) and the weld metal.

(Example 2)
(Measurement of diffusible hydrogen amount)
A solid wire (molten material) having the chemical components shown in Table 6 (blank means no addition) and having a wire diameter of φ1.2 mm, the balance of which is iron and impurities, was used. The amount of diffusible hydrogen in the weld metal was adjusted by changing the amount of the feed lubricating oil (palm oil) applied to the solid wire. In accordance with JIS Z 3118, the amount of diffusible hydrogen in the weld metal was measured by performing normal welding using _{CO 2 gas as the shield gas.} Table 7-1 shows the measured diffusible hydrogen amount (Hd).

(Evaluation of mechanical properties)
Two SM400s (JIS G 3106) having a plate thickness of 25 mm are butted together to form a V-shaped groove shape as shown in FIG. 4, preheated at 100 ° C. to prevent low-temperature cracking, and the chemical components shown in Table 6 are obtained. A 12-pass multi-layer welding was performed by _{ordinary CO 2} welding for each of the solid wires (molten materials) having a wire diameter of φ1.2 mm, the balance of which is iron and impurities. The welding conditions were the distance between the chip base materials: 20 mm, the current: 260 A, and the welding speed: 0.30 m / min. Tensile test pieces were collected from the weld metal welded in multiple layers according to JIS Z3111, and the mechanical properties of the weld metal were evaluated. Table 7-2 shows the yield stress (YS), tensile strength (TS), and elongation (EL) of the weld metal.

(Evaluation of low temperature cracking and bead shape)
Evaluation of low temperature cracking was performed by a y-shaped weld cracking test. Using the steel plate (thickness 25 mm) shown in Table 5 and the solid wire (welded material) having the chemical components shown in Table 6 and the balance consisting of iron and impurities and having a wire diameter of φ1.2 mm, JIS Z3158 shown in FIG. A test piece having a shape specified in the y-shaped weld crack test according to the above was prepared.

In welding, CO ₂ gas was used as the shield gas and the flow rate was set to 25 L / min. A TPS5000CMT manufactured by Fronius was used as the welding power source. The welding method was CMT welding or ordinary CO ₂ welding, and both were compared. The welding conditions were the distance between the chip base materials: 20 mm, the current: 190 A, and the welding speed: 0.20 m / min. Both CMT welding and normal welding were welded in one pass. After the above welding was performed in an environment of room temperature, a cross-sectional survey was conducted after 48 hours or more had passed, and the presence or absence of low-temperature cracks and the bead shape were visually evaluated. The test conditions and test results are shown in Table 8. In the evaluation results of Table 8, the sites where cracks were observed are shown in parentheses. The bead shape was good in all samples.

No cracks were found in the welded joint obtained in the example of the present invention. On the other hand, in the welded joint obtained in the comparative example, lateral cracks were observed in the weld metal.

(Example 3)
(Measurement of diffusible hydrogen amount)
Three types of high-strength solid wires (3-1-3-) having the chemical components shown in Table 10 (blanks mean no additives) and the balance consisting of iron and impurities, with tensile strength classes of 800 MPa class and 950 MPa class. 3) (Wire diameter: φ1.2 mm) and flux-cored wire (3-4) (wire diameter: φ1.2 mm) were used. In addition to the alloy components shown in the table, the flux-cored wire (3-4) in Table 10 contains TiO ₂ : 5.5%, SiO ₂ : 0.3%, Al ₂ O ₃ : 0.1%, and F conversion value: Contains 0.06% of the slag component of the fluoride. In the case of a flux-cored wire,% is expressed as a mass% with respect to the total mass of the wire including the outer skin.

The amount of diffusible hydrogen in the weld metal was adjusted by changing the amount of the lubricating oil (palm oil) supplied to the wire. In accordance with JIS Z 3118, the amount of diffusible hydrogen in the weld metal was measured by performing normal welding using _{CO 2 gas as the shield gas.} Table 11-1 shows the measured diffusible hydrogen amount (Hd).

(Evaluation of mechanical properties)
Two SM400s (JIS G 3106) with a plate thickness of 25 mm are butted together to form a V-shaped groove shape as shown in FIG. 4, preheated at 100 ° C. to prevent low-temperature cracking, and have the chemical components shown in Table 10. A 12-pass multi-layer welding was carried out by _{ordinary CO 2} welding for each of the wires (molten materials) having a wire diameter of φ1.2 mm, the balance of which was iron and impurities. The welding conditions were a distance between chip base materials: 20 mm, a current of 270 A, and a welding speed of 0.35 m / min. Tensile test pieces were collected from the weld metal welded in multiple layers according to JIS Z3111, and the mechanical properties of the weld metal were evaluated. Table 11-2 shows the yield stress (YS), tensile strength (TS), and elongation (EL) of the weld metal.

(Evaluation of low temperature cracking and bead shape)
Three types of thin steel sheets for automobiles with tensile strengths of 1180 to 1800 MPa class shown in Table 9 and wires (molten materials) with a wire diameter of φ1.2 mm having the chemical components shown in Table 10 and the balance being iron and impurities. Using. The steel sheet was left with rust preventive oil attached. As shown in FIG. 5, two thin steel plates for automobiles having a length of 150 mm and a width of 60 mm and 40 mm are overlapped with each other by 10 mm on one side and placed on a thick plate having a thickness of 12 mm. After restraint welding of the end portion over a length of 150 mm, overlay fillet welding was performed over 110 mm on the overlapped side in one pass.

In welding, Ar + 20% CO ₂ gas was used as the shield gas and the flow rate was set to 20 L / min. A TPS5000CMT manufactured by Fronius was used as the welding power source. The welding method was CMT welding or pulse MAG welding without CMT, and both were compared. The welding conditions were a distance between chip base materials: 20 mm, a current of 170 to 230 A, and a welding speed of 0.80 m / min.

After 48 hours or more have passed since the above welding was performed in a room temperature environment, a cross-sectional survey was conducted in the direction orthogonal to the welding line as shown in FIG. 6, and the presence or absence of low-temperature cracks and the bead shape were visually evaluated. bottom. The test conditions and test results are shown in Table 12. In the evaluation results of Table 12, the sites where cracks were observed are shown in parentheses.

No cracks were observed in the welded joint obtained in the example of the present invention. On the other hand, in the welded joints obtained in Comparative Examples 22, 23, 26, 27, 30, and 31, low temperature cracks were observed in the weld metal. In Comparative Example 32, low-temperature cracking was not observed, but the weld bead shape was poor because the amount of Si in the wire was less than the range of the present invention. The bead shape was good except for Comparative Example 32. As a result of visually observing the cross section of the joint after welding only in Example 33 of the present invention, slag entrainment was slightly observed.

Claims (5)

By mass%
C: 0.05 to 0.13%,
Si: 0.35 to 1.20%,
Mn: 1.60 to 3.40%, and Cu: 0.10 to 0.50%
Contains,
P: 0.030% or less, and S: 0.020% or less,
Limited to
Ni: 3.40% or less,
Cr: 1.20% or less,
Mo: 1.00% or less, and
Ti: 0.25% or less,
Further containing one or more selected from the group consisting of
Using a wire in which the balance is composed of Fe and impurities and the amount of diffusible hydrogen in the weld metal is 2.0 ml / 100 g or less, the wire is moved back and forth and energization is controlled between the molten pool and the contact tip to generate an arc. A gas shielded arc welding method characterized in that welding is performed by generating it intermittently. The Pcm calculated from the chemical composition of the wire is lower than the Pcm calculated from the chemical composition of the steel material as the base material.
The Pcm is the formula (1):
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1)
In the formula, the element symbol is the mass% of the additive element contained,
The gas shielded arc welding method according to claim 1. The gas shielded arc welding method according to claim 1 or 2 , wherein the wire is a solid wire. The welding method according to any one of claims 1 to 3 , wherein the gas shielded arc welding according to any one of claims 1 to 3 is performed on at least the first layer of the multi-layer welding. A method for manufacturing a welded joint, which comprises performing gas shielded arc welding of a steel sheet by using the gas shielded arc welding method according to any one of claims 1 to 4.

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