JP5980128B2 - Manufacturing method of arc-welded structural members - Google Patents

Description

  The present invention relates to a method for producing an arc-welded structural member having excellent resistance to molten metal embrittlement cracking constituted by using a molten Zn—Al—Mg-based plated steel sheet member for one or both members to be joined.

  Since the hot dip galvanized steel sheet has good corrosion resistance, it is used in a wide range of applications including building members and automobile members. Among them, the hot-dip Zn—Al—Mg-based steel sheet maintains excellent corrosion resistance for a long period of time, and therefore, the demand is increasing as a material to replace the conventional hot-dip galvanized steel sheet.

As described in Patent Documents 1 and 2, the plated layer of the hot-dip Zn—Al—Mg-based plated steel sheet has a primary Al phase or primary Al phase in a Zn / Al / Zn ₂ Mg ternary eutectic matrix. It has a metal structure in which a Zn single phase is dispersed, and the corrosion resistance is improved by Al and Mg. Since a dense and stable corrosion product containing Mg in particular is uniformly formed on the surface of the plated layer, the corrosion resistance of the plated layer is remarkably improved as compared with the hot dip galvanized steel sheet.

  In the case of assembling building members, automobile members, and the like using a molten Zn—Al—Mg plated steel sheet, a gas shield arc welding method is often applied. When arc welding is performed on a hot-dip Zn-Al-Mg-based steel sheet, there is a problem that hot metal embrittlement cracking is likely to occur as compared with a hot-dip galvanized steel sheet. This is considered to be caused by a decrease in the liquidus temperature of the plating layer due to the Mg content (Patent Documents 3 and 4).

  When arc welding is performed on the plated steel sheet, the metal of the plating layer melts on the surface of the surrounding base material (plating original sheet) through which the arc has passed. In the case of a Zn-Al-Mg plated steel sheet, the alloy of the plating layer has a liquidus temperature lower than the melting point of Zn (about 420 ° C.) and maintains a molten state for a relatively long time. In the example of the Zn-6 mass% Al-3 mass% Mg alloy, the solidification end temperature is about 335 ° C. Molten metal derived from the Zn-Al-Mg plating layer melted on the surface of the base metal reduces the Al concentration as the Al component reacts with the underlying Fe at an early stage and is consumed as an Fe-Al alloy layer. Finally, the composition becomes close to a Zn—Mg binary system, but even with a Zn-3 mass% Mg alloy, the solidification end temperature is 360 ° C. and lower than the melting point of Zn, 420 ° C. Therefore, in the case of a Zn—Al—Mg-based plated steel sheet, the time during which the metal in the plated layer melted during arc welding stays on the surface of the base material while maintaining the liquid phase is longer than that in the galvanized steel sheet.

  When the surface of a base metal that is in a tensile stress state during cooling immediately after arc welding is exposed to molten plated metal for a long time, the molten metal penetrates into the crystal grain boundaries of the base metal and causes molten metal embrittlement cracking. It becomes a cause. When molten metal embrittlement cracking occurs, it becomes the starting point of corrosion and corrosion resistance decreases. In addition, the strength and fatigue characteristics may be reduced, causing problems.

As a method for suppressing molten metal embrittlement cracking of a molten Zn—Al—Mg-based plated steel sheet during arc welding, for example, a method of cutting and removing a plating layer before arc welding has been proposed. Patent Document 4 discloses a technique of imparting resistance to molten metal embrittlement cracking by applying a steel plate whose ferrite crystal grain boundary is reinforced by addition of B to a plating original plate. Patent Document 5 discloses a technique of suppressing molten metal embrittlement cracking by filling Zn, Al, and Mg during arc welding by filling a flux added with TiO ₂ and FeO into the outer sheath of a welding wire. .

Japanese Patent No. 3149129 Japanese Patent No. 3179401 Japanese Patent No. 4475787 Japanese Patent No. 3715220 JP 2005-230912 A

  The method of cutting and removing the above-described plating layer and the method of using a special welding wire are accompanied by a great increase in cost. The technique of using B-added steel for the plating base plate reduces the degree of freedom in selecting the steel type. Moreover, even if these methods are adopted, molten metal embrittlement cracking may not be sufficiently prevented depending on the part shape and welding conditions, and a radical arc welding structure using a Zn-Al-Mg based steel sheet is essential. It is not a measure to prevent molten metal embrittlement cracking.

  On the other hand, in recent years, a high-tensile steel plate having a tensile strength of 590 MPa or more has been used as a plating base plate for reducing the weight of automobiles. In such a molten Zn-Al-Mg plated steel sheet using high-tensile steel sheet, the tensile stress in the weld heat-affected zone is increased, so that molten metal embrittlement cracking is likely to occur, and applicable part shapes and applications are limited. The

  In view of such a current situation, the present invention is excellent in resistance to molten metal brittleness in an arc welded structure member using a Zn-Al-Mg-based plated steel sheet member without being restricted by the steel type of the plating base plate and a significant increase in cost. It aims at providing what has fracturing ability.

  According to the inventors' investigation, the plating layer disappears once by evaporation in the vicinity of the weld bead during gas shielded arc welding, but after the arc passes, the plating layer metal is in a molten state at a position slightly away from the bead. It has been confirmed that the phenomenon of immediately spreading to the above disappeared portion occurs. If the molten plating layer metal can be solidified at the time until the molten plating layer metal spreads to the vicinity of the bead toe, it can be introduced into the base material at a position near the bead toe. It is considered that the penetration of the plating layer component is avoided and the molten metal embrittlement crack can be effectively prevented. As a result of detailed studies by the inventors, it is clear that the above object can be achieved by controlling the flow balance between the inner shield gas and the outer shield gas using a welding torch having a shield gas discharge nozzle having a double pipe structure. It became. The present invention has been completed based on such findings.

That is, in the present invention, when manufacturing a welded structural member by joining steel materials by gas shielded arc welding, at least one member to be joined is a molten Zn-Al-Mg-based plated steel plate member, and an inner tube around the electrode. An inner shield gas having a composition in which CO ₂ : 10 to 100%, O ₂ : 0 to 5%, and Ar: 0 to 90% in volume% from the inner pipe using a welding torch having a double pipe made of an outer pipe with ejecting, Ar volume% from the outer tube: 0~100%, He: 0~100% , CO 2: 0~100%, N 2: 0~80%, H 2: 0~25%, The outer shield gas having a composition of O ₂ : 0 to 22% is discharged, and the discharge flow rate Qout of the outer shield gas and the discharge flow rate Qin (L / min) of the inner shield gas are defined in the following formulas (1) to (3). Range Preparation of welded structural members is provided.
2 ≦ Qin ≦ 50 (1)
10 ≦ Qout ≦ 50 (2)
Qin / Qout ≧ 0.2 (3)

Here, the “molten Zn—Al—Mg-based plated steel sheet member” is a member made of a molten Zn—Al—Mg-based plated steel sheet, or a member formed by using it as a raw material.
At the outlet end portion of the outer shield gas flow path formed between the outer wall of the inner tube and the inner wall of the outer tube, when the outer diameter of the inner tube is D ₁ and the inner diameter of the outer tube is D ₂ , It is more effective to apply a welding torch that satisfies the following formula (4).
0.3 ≦ (D ₂ −D ₁ ) / D ₂ ≦ 0.5 (4)

The molten Zn—Al—Mg based steel sheet is, for example, mass%, Al: 1.0 to 22.0%, Mg: 0.05 to 10.0%, Ti: 0 to 0.10%, B: Those having a plating layer consisting of 0 to 0.05%, Si: 0 to 2.0%, Fe: 0 to 2.5%, the balance Zn and inevitable impurities are suitable targets. The plating adhesion amount per one side is, for example, 20 to 250 g / m ² .

  According to the present invention, an arc welded structure using a molten Zn-Al-Mg-based plated steel sheet member, which is inherently susceptible to molten metal embrittlement cracking, is stable when it exhibits excellent resistance to molten metal embrittlement cracking. And realized it. There is no particular restriction on the steel type of the plating base plate, and it is not necessary to adopt a steel type to which a special element is added as a measure against molten metal embrittlement cracking. Even when a high-strength steel plate is applied, excellent melt metal embrittlement cracking resistance can be obtained. Moreover, the freedom degree with respect to a component shape is also large. Therefore, the present invention is widely used in a wide variety of applications including Zn-Al-Mg-plated steel sheet arc welded structural members, including automotive arc welded structural members that use high-tensile steel sheets that are expected to increase in the future. It contributes.

The figure which showed typically the welding part cross-section of a lap fillet welded joint. The figure which showed typically the cross-sectional state of the high temperature welded part immediately after the arc passed at the time of the arc welding of a hot-dip Zn-Al-Mg type plated steel plate. The figure which showed typically the cross-section of the conventional Zn-Al-Mg type plated steel plate arc welding structural member cooled from the state of FIG. The figure which showed typically the cross-section of the welding torch applied to implementation of this invention. The figure which showed typically the cross-section of the Zn-Al-Mg system plating steel plate arc welding structural member according to this invention obtained by cooling from the state of FIG. The figure which showed the welding experiment method for investigating the molten metal embrittlement cracking resistance.

  FIG. 1 schematically illustrates a cross-sectional structure of a welded portion of a lap fillet welded joint. This type of welded joint by arc welding is frequently used for automobile chassis. A base material 1 and a base material 1 ′, which are steel plate members, are arranged so as to overlap each other, a weld bead 2 is formed on the surface of the base material 1 and an end surface of the base material 1 ′, and both members are joined. The broken lines in the figure represent the surface position of the base material 1 and the end face position of the base material 1 'before welding. The intersection of the base metal surface and the weld bead is called the “bead toe”. In the drawing, the bead toe portion of the base material 1 is indicated by reference numeral 3.

2 to 4 schematically show an enlarged sectional structure of a portion corresponding to the vicinity of the bead toe 3 shown in FIG.
FIG. 2 schematically shows a cross-sectional state in the vicinity of a high-temperature welded portion immediately after the arc passes during gas shielded arc welding of a Zn—Al—Mg-based plated steel sheet. The surface of the base material 1 was covered with the uniform plating layer 7 via the Fe—Al-based alloy layer 6 in the stage before welding, but the metal of the plating layer was near the bead toe 3 due to the passage of the arc. Evaporates and disappears (plating layer evaporation region 9). In the portion where the distance from the bead toe 3 is larger than that, the original plating layer 7 is melted to become the Zn—Al—Mg based molten metal 8, but has not disappeared due to evaporation. When the distance from the bead toe 3 is further increased, the original plating layer 7 exists without melting. In FIG. 2, the thicknesses of the Zn—Al—Mg molten metal 8 and the plating layer 7 are exaggerated.

  FIG. 3 schematically shows a cross-sectional structure of a conventional Zn—Al—Mg-based plated steel sheet arc welded structural member obtained by cooling from the state of FIG. In this case, the Zn—Al—Mg based molten metal (reference numeral 8 in FIG. 2) wets and spreads in the “plating layer evaporation region” (reference numeral 9 in FIG. 2) formed once the plating layer disappears during welding. The entire surface up to the bead toe 3 is covered with the Zn—Al—Mg alloy layer 5. A portion of the Zn—Al—Mg alloy layer 5 formed by solidification of the Zn—Al—Mg based molten metal (reference numeral 8 in FIG. 2) is called a melt solidified region 10, and the original plating layer 7 remains and is formed. The portion of the Zn—Al—Mg-based alloy layer 5 is referred to as a plating layer unmelted region 11. In a conventional Zn-Al-Mg-based plated steel sheet arc welded structural member, the bead toe 3 is usually the melt-solidified region 10 as shown in this figure. In this case, since the liquidus temperature of the Zn—Al—Mg-based molten metal 8 is low as described above, the surface portion of the base material 1 that becomes the melted and solidified region 10 after cooling becomes Zn—Al in the cooling process after welding. -The contact time with the Mg-based molten metal is relatively long. Since tensile stress is generated in the portion of the base material 1 near the bead toe due to cooling after welding, the Zn—Al—Mg based molten metal component tends to enter the crystal grain boundary. The said component which penetrate | invaded the grain boundary becomes a factor which causes a molten metal embrittlement crack.

  FIG. 4 schematically shows a cross-sectional structure of a welding torch applied to the implementation of the present invention. Around the welding wire 31 and the contact tip 32, a double pipe comprising a cylindrical inner tube 33 and an outer tube 34 with the welding wire 31 as a central axis is provided, and supply paths that are different from the tips of the inner tube 33 and the outer tube 34, respectively. More guided shield gases A and B are discharged. A mesh 35 is installed between the outer wall 36 of the inner tube 33 and the inner wall 37 of the outer tube 34 as necessary so that the shield gas B is uniformly dispersed and discharged. Although the welding wire 31 is a consumable electrode, a non-consumable electrode can be applied depending on the application. The shield gas A discharged from the inner pipe 33 is called “inner shield gas”, and the shield gas B discharged from the outer pipe 34 is called “outer shield gas”. The inner shield gas plays a role of mainly preventing surface oxidation of the material to be welded, like the shield gas applied to conventional gas shield arc welding. On the other hand, in the present invention, the outer shield gas is used for the purpose of quickly cooling the surface around the bead of the welded material. Thereby, the solidification of the Zn—Al—Mg-based molten metal (symbol 8 in FIG. 2) existing in a molten state on the surface around the bead is promoted, and wetting and spreading to the bead toe end portion is suppressed.

  As a method of increasing the cooling capacity by the shield gas, there is a method of increasing the flow rate of the shield gas in the normal gas shield arc welding method, or the nozzle shape of the welding torch so that the shield gas hits a wider area around the weld bead. A method to devise can be assumed. However, according to the study by the inventors, when the flow rate of the shielding gas is increased using a general welding torch, the arc becomes unstable, and the Zn—Al—Mg based molten metal (reference numeral 8 in FIG. 2) It has been found that it is difficult to secure sufficient cooling power to sufficiently prevent wetting and spreading. Also, when the diameter of the shield gas discharge port of the welding torch is made larger than before and the shield gas hits a wide area on the surface of the material, the arc tends to become unstable and it is difficult to obtain good results. It was.

  As a result of detailed studies, the inventors have adopted a method of independently controlling the flow rates of the inner shield gas and the outer shield gas using a double pipe type welding torch as illustrated in FIG. By applying the flow rate conditions described later, wetting and spreading of the Zn—Al—Mg based molten metal (reference numeral 8 in FIG. 2) is effectively suppressed, and it is overcome by welding using a Zn—Al—Mg based plated steel sheet. The problem of molten metal embrittlement cracking, which is considered difficult, is avoided.

In the present invention, the cooling ability of the outer shield gas is utilized to promote solidification of the molten plating layer metal, and therefore it is effective to spray the outer shield gas over as wide a range as possible around the bead. However, from the viewpoint of arc stability, it is important for the inner shield gas to prevent the flux from spreading excessively. Therefore, as a result of study, it is possible to increase the ratio of the portion due to the outer shield gas in the total shield gas discharge flux formed by the inner shield gas and the outer shield gas, so that the cooling capacity is maintained while maintaining a stable arc. It is effective in increasing Specifically, as shown in FIG. 4, the outer diameter of the inner pipe is set to D ₁ at the outlet end portion of the outer shield gas flow path formed between the outer wall 36 of the inner pipe 33 and the inner wall 37 of the outer pipe 34. , when the inner diameter of the outer tube and D _2, it is more effective to apply satisfy the following equation (4).
0.3 ≦ (D ₂ −D ₁ ) / D ₂ ≦ 0.5 (4)

The inner diameter of the inner tube 33 is more preferably in the range of 11 to 18 mm. The outer diameter D ₁ of the inner tube 33 is more preferably in the range of 15 to 22 mm, and the inner diameter D ₂ of the outer tube 34 is more preferably in the range of 22 to 44 mm.

  FIG. 5 schematically shows a cross-sectional structure of a Zn—Al—Mg based plated steel sheet arc welded structural member according to the present invention obtained by cooling from the state of FIG. A part of the outer shield gas discharged using the welding torch having the above-described structure is sprayed to the area around the bead. At this time, the surface of the Zn—Al—Mg based molten metal (reference numeral 8 in FIG. 2) is generated by blowing the outer shield gas in addition to the gas flow in the lateral direction (direction away from the bead) caused by the inner shield gas. The Zn—Al—Mg based molten metal is also exposed to a gas flow, and comes into contact with more gas per unit time. As a result, heat removal from the Zn—Al—Mg based molten metal is promoted, the solidification time is advanced, and cooling can be completed in a state where the plating layer evaporation region 9 is left to some extent in the vicinity of the toe portion 3.

  In this specification, the length of the plating layer evaporation region 9 remaining after cooling from the bead toe 3 is referred to as “plating layer evaporation region length”, and is indicated by the symbol L in FIG. Most of the molten metal embrittlement cracks which are a problem in Zn-Al-Mg-plated steel sheet arc welded structural members are very close to the bead toe 3, specifically less than 0.3 mm from the bead toe. It has been confirmed that When a welding torch that satisfies the above formula (4) is used, it becomes easier to secure the plating layer evaporation region length of 0.3 mm or more, and it provides excellent resistance to molten metal embrittlement cracking. It is extremely effective. Even in a welding posture in which the height position of the Zn—Al—Mg molten metal (reference numeral 8 in FIG. 3) is above the bead toe 3, the cooling effect by the outer shield gas causes the Zn— Wetting and spreading of the Al—Mg based molten metal is remarkably suppressed.

  When the plating layer evaporation region length L is too long, there is a problem of deterioration in corrosion resistance due to the absence of the plating layer. According to the study by the inventors, the plating layer evaporation region length is 2.0 mm or less. In other words, it was found that the sacrificial anticorrosive action by the surrounding Zn—Al—Mg-based plating layer was sufficiently obtained, and the corrosion resistance reduction at this portion was at a level not causing a problem. By setting the flow rates of the inner shield gas and the outer shield gas in accordance with the regulations described later, the plating layer evaporation region length can be controlled in the range of 0.3 to 2.0 mm.

〔Shielding gas〕
Various gases usually used in arc welding can be applied to the inner shield gas. Specifically, Ar gas, CO ₂ gas, Ar—CO ₂ mixed gas, Ar—CO ₂ —O ₂ mixed gas, or the like may be used. Concentration range of each gas component, CO ₂ by _{volume%: 10~100%, O 2:} 0~5%, Ar: can be 0% to 90%. In addition to this, mixing of the components used in the known shield gas is allowed, but the remainder may be managed to include only inevitable impurities.

As the outer shield gas, a gas containing one or more of He gas, H ₂ gas, Ar gas, CO ₂ gas, O ₂ gas, and N ₂ gas as a component is used. Air can be regarded as a mixed gas of N ₂ , O ₂ and a small amount of Ar, and can be used. Since He gas and H ₂ gas have high thermal conductivity, a particularly excellent heat removal effect can be obtained when these gases are used. The gas composition may be the same as that of the inner shield gas. Concentration range of each gas component, Ar by volume%: 0~100%, He: 0~100 %, CO 2: 0~100%, N 2: 0~80%, H 2: 0~25%, O ₂ : 0 to 22%. In addition to this, mixing of the components used in the known shield gas is allowed, but the remainder may be managed to include only inevitable impurities.

[Gas shield arc welding conditions]
In the arc welding according to the present invention, the discharge flow rate Qout of the outer shield gas and the discharge flow rate Qin (L / min) of the inner shield gas are set within the ranges defined by the following equations (1) to (3).
2 ≦ Qin ≦ 50 (1)
10 ≦ Qout ≦ 50 (2)
Qin / Qout ≧ 0.2 (3)

  If the flow rate of the inner shield gas is too small, the arc becomes unstable. When the flow rate of the inner shield gas is too large, the cooling effect of the weld bead portion is increased, and the occurrence of blow holes increases due to solidification of the weld metal before Zn derived from the plated metal is removed as vapor. If the flow rate of the outer shield gas is too small, the cooling capacity around the weld bead will be insufficient, and it will be difficult to prevent wetting and spreading of the Zn—Al—Mg based molten metal (reference numeral 8 in FIG. 2). In that case, it is difficult to stably impart excellent molten metal embrittlement cracking resistance. If the flow rate of the outer shield gas is too large, the cooling effect is saturated and the cost increases.

If the flow ratio Qin / Qout between the inner shield gas and the outer shield gas is too small, the flow of the inner shield gas is disturbed by the discharge flow of the outer shield gas and the arc becomes unstable. When the arc becomes unstable, the weld bead may be interrupted and a continuous and healthy bead may not be formed. The upper limit of Qin / Qout is not particularly required because it is restricted by the above formulas (1) and (2). However, when the stabilization of the arc is particularly emphasized, it is more effective to apply the following expression (3) ′ instead of the above expression (3).
2.0 ≧ Qin / Qout ≧ 0.2 (3) ′

The welding heat input may be set to an optimum value according to the plate thickness or the like. If the welding heat input is too low, the welding may be insufficient and the weld bead may be discontinuous. Conversely, if the welding heat input is excessive, spatter is likely to occur. Usually, an appropriate value of welding heat input can be found in the range of 2000 to 12000 J / cm.
The welding speed is set so that the welding heat input can be obtained. For example, it can be set in the range of 0.2 to 2.5 m / min.

[Fused Zn-Al-Mg plated steel sheet member]
In the present invention, a molten Zn—Al—Mg based plated steel sheet member is applied to at least one of both members joined by arc welding.
Various steel types can be adopted as the plating original plate of the molten Zn—Al—Mg-based plated steel plate member depending on the application. High tensile steel plates can also be used. The plate thickness may be set in the range of 1.0 to 6.0 mm, for example.

  Specifically, the composition of the molten Zn—Al—Mg-based plating layer is, by mass, Al: 1.0 to 22.0%, Mg: 0.05 to 10.0%, Ti: 0 to 0.10. %, B: 0 to 0.05%, Si: 0 to 2.0%, Fe: 0 to 2.5%, the balance Zn and inevitable impurities. The plating layer composition substantially reflects the hot-dip plating bath composition. Although the method of hot dipping is not particularly limited, it is generally advantageous in terms of cost to use an in-line annealing type hot dipping equipment. Hereinafter, the component elements of the plating layer will be described. “%” Of the plating layer component element means “mass%” unless otherwise specified.

  Al is effective in improving the corrosion resistance of the plated steel sheet, and suppresses the generation of Mg oxide dross in the plating bath. In order to fully exhibit these actions, it is necessary to secure an Al content of 1.0% or more, and it is more preferable to secure an Al content of 4.0% or more. On the other hand, when the Al content increases, a brittle Fe—Al alloy layer easily grows on the base of the plating layer, and excessive growth of the Fe—Al alloy layer causes a decrease in plating adhesion. As a result of various studies, the Al content is more preferably 22.0% or less, and may be controlled to 15.0% or less, or even 10.0% or less.

  Mg exhibits the effect | action which produces | generates a uniform corrosion product on the surface of a plating layer, and raises the corrosion resistance of a plated steel plate remarkably. The Mg content is more preferably 0.05% or more, and more preferably 1.0% or more. On the other hand, if the Mg content in the plating bath increases, Mg oxide-based dross is likely to occur, which causes a reduction in the quality of the plating layer. The Mg content is desirably in the range of 10.0% or less.

  When Ti and B are contained in the hot dipping bath, there are advantages such as an increase in the degree of freedom of manufacturing conditions during hot dipping. For this reason, 1 type or 2 types of Ti and B can be added as needed. It is more effective to add 0.0005% or more in the case of Ti and 0.0001% or more in the case of B. However, when the content of Ti or B in the plating layer becomes excessive, it causes a poor appearance of the plating layer surface due to the formation of precipitates. When these elements are added, it is desirable that Ti: 0.10% or less and B: 0.05% or less.

  When Si is contained in the hot dipping bath, excessive growth of the Fe—Al alloy layer formed at the interface between the plating original plate surface and the plating layer is suppressed, and the workability of the hot-dip Zn—Al—Mg plated steel sheet is improved. This is advantageous. Therefore, Si can be contained as necessary. In that case, it is more effective to set the Si content to 0.005% or more. However, since excessive Si content causes an increase in the dross amount in the hot dipping bath, the Si content is preferably 2.0% or less.

  In the hot dipping bath, Fe is likely to be mixed because the steel sheet is immersed and passed. The Fe content in the Zn—Al—Mg plating layer is preferably 2.5% or less.

When the coating amount of the molten Zn—Al—Mg-based steel sheet member is small, it is disadvantageous for maintaining the corrosion resistance and sacrificial anticorrosive action of the plated surface for a long time. As a result of various studies, when the “plating layer evaporation region” generated in the vicinity of the bead toe according to the present invention is left, the Zn—Al—Mg based plating adhesion amount per side is preferably 20 g / m ² or more. It is effective. On the other hand, when the plating adhesion amount increases, blow holes are likely to occur during welding. When blow holes occur, the welding strength decreases. For this reason, it is desirable that the amount of plating deposited on one side be 250 g / m ² or less.

[Parts to be welded]
The mating member to be joined by arc welding to the above molten Zn—Al—Mg based plated steel sheet member may be the same molten Zn—Al—Mg based plated steel sheet member as described above, or other steel materials. It doesn't matter.

  A cold-rolled steel strip having a thickness of 3.2 mm and a width of 1000 mm having the composition shown in Table 1 is used as a plating base plate, and this is passed through a hot dipping line to obtain a molten Zn-Al-Mg system having various plating layer compositions. A plated steel sheet was produced. Tables 2 and 3 show the plating composition and the amount of plating adhered per side.

Using each molten Zn—Al—Mg-based plated steel sheet, gas shield arc welding was performed by the test method described later, and the resistance to molten metal embrittlement cracking was evaluated.
As the welding torch, a double pipe structure shown in FIG. 4 was used, and the inner shield gas and the outer shield gas were supplied from independent pipes.
In most examples, the inner tube 33 having an inner diameter of 16 mm, an outer diameter (corresponding to D ₁ ): 20 mm, and an inner diameter of the outer tube 34 (corresponding to D ₂ ): 29 mm were used. In this case, the value of (D ₂ −D ₁ ) / D _{2 in} the equation (4) is about 0.31.
In some examples, the inner tube 33 had an inner diameter of 16 mm, an outer diameter (corresponding to D ₁ ): 20 mm, and an outer tube 34 having an inner diameter (corresponding to D ₂ ): 35 mm. In this case, the value of (D ₂ −D ₁ ) / D _{2 in} the equation (4) is about 0.43.
Further, in some examples, the inner tube 33 having an inner diameter of 16 mm, an outer diameter (corresponding to D ₁ ): 20 mm, and an inner diameter of the outer tube 34 (corresponding to D ₂ ): 25 mm were used. In this case, the value of (D ₂ −D ₁ ) / D _{2 in} the equation (4) is 0.20.
In some comparative examples, welding without using an outer shield gas was performed using a welding torch generally used conventionally.
The welding conditions are as follows.
[Welding conditions]
・ Welding wire: YGW12, diameter 1.2mm
・ Shield gas composition, flow rate: listed in Tables 4 and 5 ・ Welding current: 110 to 200 A
・ Arc voltage: 12-20V
-Welding speed: 0.2 to 2.5 m / min (described in Tables 4 and 5)
・ Heat input: 2000-6000 J / cm

[Test method for molten metal embrittlement crack resistance]
As shown in FIG. 6, a boss (protrusion) 15 of a steel bar having a diameter of 20 mm and a length of 25 mm is set up vertically at the center of a 100 mm × 75 mm test piece 14 (a molten Zn—Al—Mg based steel plate member), The test piece 14 and the boss 15 were joined by performing gas shielded arc welding under the following welding conditions. Specifically, the boss 15 is rotated once around the boss 15 clockwise from the welding start point S, and after the welding start point S is passed, welding is further performed by overlapping the beads, and an overlapping portion 17 of the weld bead 16 is generated. Welding was performed up to a later welding end point E. During welding, the test piece 14 was restrained on a flat plate. This test was conducted in a situation where welding cracks are likely to occur experimentally.

  After welding, the cut surface 20 passing through the central axis of the boss 15 and passing through the bead overlap portion 17 is observed on the test piece 14 by observing the test piece 14 near the bead overlap portion 17 with a scanning electron microscope. The deepest crack depth (maximum crack depth) was measured. This crack is judged to be a “molten metal embrittlement crack”. In this test, when the maximum crack depth is less than 0.5 mm, the molten metal embrittlement cracking resistance is remarkably improved as compared with a welded structure member using a conventional Zn-Al-Mg plated steel sheet member. Can be evaluated.

Further, when the beads formed by the above-mentioned welding are visually observed, the case where the beads are continuously formed is indicated as ◯ (bead shape; good), and the case where the beads are discontinuous and there are discontinuous portions is indicated by × (beads). Shape; poor).
After observing the bead shape, an X-ray transmission photograph of the bead portion was taken, and the blowhole occupancy Bs determined by the Japan Architecture Center was measured. Bs is a ratio of the total sum of the lengths of individual blow holes in the entire length of the weld bead. Here, the circumferential length (one turn) of the center portion of the bead is defined as the total length of the weld bead. According to the evaluation standards of the Japan Architecture Center, those with Bs of 30% or less were evaluated as ○ (blowhole resistance; good) and the others were evaluated as x (blowhole resistance; poor).
The results are shown in Tables 4 and 5.

  All of the examples of the present invention shown in Table 4 exhibited excellent resistance to molten metal embrittlement cracking, and had no problem in bead shape and blowhole resistance. In particular, in the case of using a welding torch satisfying the formula (4), the plated layer metal melted during welding solidifies before spreading to the bead toe, and as a result, the occurrence of molten metal embrittlement cracks Not recognized (maximum crack depth = 0 mm). Even in No. 29 using a welding torch that does not satisfy the provisions of equation (4), the amount of hot-dip plated metal that wets and spreads to the bead toes by adjusting the flow rates of the inner shield gas and the outer shield gas according to the present invention is as follows. The occurrence of cracks was extremely small even in the welding test under the severe conditions described above.

  Among the comparative examples shown in Table 5, No. 42 in which the flow rate of the inner shield gas does not meet the provisions of the present invention, and the No. in which the flow ratio Qin / Qout of the inner shield gas and the outer shield gas does not meet the provisions of the present invention In .43, 45, 46 and 49, the arc became unstable and a healthy bead could not be formed. No. 41 using a conventional general welding torch whose shield gas discharge part has a single cylinder structure, and No. 44, 47, 48, 50, 51 where the flow rate of the outer shield gas does not meet the provisions of the present invention. Since the surroundings were insufficiently cooled, the plated layer metal melted during welding spread to the toe end of the bead, resulting in large molten metal embrittlement cracks. In Nos. 52 and 53 where the flow rate of the inner shield gas exceeds the provisions of the present invention, the weld metal solidified before Zn derived from the plating layer contained in the molten weld metal escaped as a vapor, so The hall has increased.

DESCRIPTION OF SYMBOLS 1, 1 'base material 2 Weld bead 3 Bead toe part 5 Zn-Al-Mg type alloy layer 6 Fe-Al type alloy layer 7 Plating layer 8 Zn-Al-Mg type molten metal 9 Plating layer evaporation area 10 Melt solidification Area 11 Plating layer unmelted area 14 Test piece 15 Boss 16 Weld bead 17 Bead overlap part 31 Welding wire 32 Contact tip 33 Inner tube 34 Outer tube 35 Mesh 36 Outer wall of inner tube 37 Inner wall of outer tube

Claims (4)

When manufacturing a welded structural member by joining steel materials by gas shielded arc welding, at least one member to be joined is a molten Zn—Al—Mg-based plated steel plate member, and an inner tube and an outer tube are formed around the electrode 2. Using a welding torch equipped with a heavy pipe, and discharging the inner shield gas having a composition of CO ₂ : 10 to 100%, O ₂ : 0 to 5%, Ar: 0 to 90% in volume% from the inner pipe, 0~100%, He:: Ar% by volume from the outer tube _{0~100%, CO 2: 0~100%} , N 2: 0~80%, H 2: 0~25%, O 2: 0~ The outer shield gas having a composition of 22% is discharged, and the discharge flow rate Qout of the outer shield gas and the discharge flow rate Qin (L / min) of the inner shield gas are set in the ranges defined by the following equations (1) to (3). Manufacture of arc welded structural members .
2 ≦ Qin ≦ 50 (1)
10 ≦ Qout ≦ 50 (2)
Qin / Qout ≧ 0.2 (3) At the outlet end portion of the outer shield gas flow path formed between the outer wall of the inner tube and the inner wall of the outer tube, when the outer diameter of the inner tube is D ₁ and the inner diameter of the outer tube is D ₂ , The method for manufacturing an arc welded structural member according to claim 1, wherein the welding torch satisfies the following expression (4).
0.3 ≦ (D ₂ −D ₁ ) / D ₂ ≦ 0.5 (4)   The molten Zn—Al—Mg-based plated steel sheet is, by mass, Al: 1.0 to 22.0%, Mg: 0.05 to 10.0%, Ti: 0 to 0.10%, and B: 0. The arc welded structure according to claim 1 or 2, which has a plating layer comprising -0.05%, Si: 0-2.0%, Fe: 0-2.5%, the balance Zn and unavoidable impurities. Manufacturing method of the member. The hot-dip Zn-Al-Mg plated steel sheet, method of producing arc welding structure according to any of claims 1 to 3 coating weight per one side is 20 to 250 g / m ^2.

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