JP2014133259A - Manufacturing method of arc welding structural member - Google Patents

Abstract

PROBLEM TO BE SOLVED: To remarkably restrain a molten metal embrittlement crack and generation of a blow hole in an arc welding structural member of using a Zn-Al-Mg-based plated steel sheet member, without causing restriction and a large cost increase by a kind of steel of a plated original plate.SOLUTION: Joining one or both members are formed as a molten Zn-Al-Mg-based plated steel sheet member, and Ar+COgas, He+COgas or Ar+He+COgas are used as shield gas, and a gas shield is executed so that the relationship between a COuse amount F(L/cm) per unit welding length and welding input heat Q(J/cm) satisfies the following (3) formula. 0.0025≤F≤47.5Q...(3).

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

Welding Technology, Sangyo Publishing Co., Ltd., November 2006, p.112-124

  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.

  In recent years, a high-tensile steel plate having a tensile strength of 590 MPa or more has been used as a plating original 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

On the other hand, when arc welding is performed on a Zn-based plated steel sheet, the generated Zn vapor easily enters the weld metal and easily forms weld defects such as blow holes and pits (hereinafter referred to as “blowing” including “pits” in this specification). Called "Hall"). In order to construct a highly reliable welded structural member, it is also important to suppress the occurrence of blowholes. The composition of the shielding gas also affects the generation of this type of blowhole. As a typical arc welding shield gas that is widely used, there are two types of gas: CO ₂ 100% gas and Ar 80% + CO ₂ 20% gas. Ar + CO ₂ gas is known as a high-performance shield gas because it exhibits superior welding workability compared to 100% CO ₂ gas, and because it has a low penetration due to the thin penetration of thin plates, it has excellent resistance to burn-off and gap resistance. (Non-Patent Document 1). However, in order to suppress blowholes due to Zn vapor, CO ₂ 100% gas is considered to be more effective, and when importance is attached to suppression of blowholes in constant voltage arc welding of galvanized steel sheets, Ar + CO ₂ gas is not recommended (see Table 1 of Non-Patent Document 1).

  The present invention has excellent resistance to molten metal embrittlement cracking in arc-welded structural members using Zn-Al-Mg-based plated steel sheet members, without any restrictions due to the steel type of the plating base plate and significant cost increase. And it aims at providing the reliable thing in which the production | generation of the blowhole was also suppressed.

  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 this wetting spread is suppressed and cooling is completed while maintaining the state of evaporative disappearance, the penetration of the plating layer component into the base metal is avoided at a position close to the welding beat, and the molten metal embrittlement crack is It can be effectively prevented.

As a shield gas for arc welding, CO ₂ gas or a gas containing about 20% by volume of CO ₂ is often used for reasons such as suppressing oxidation around the weld bead. In particular, it is known that application of 100% CO ₂ gas is advantageous when emphasizing suppression of blowholes in arc welding applied to galvanized steel sheets (described above). However, according to investigations by the inventors, it has been found that this CO ₂ has an action of promoting the wetting and spreading of the Zn—Al—Mg-based plating layer metal. Therefore, as a result of detailed research, the above-mentioned wetting in the Zn—Al—Mg based steel sheet member is appropriately controlled by appropriately controlling the flow rate of CO ₂ supplied as a component of the shielding gas from the welding torch according to the welding heat input. It has been found that the expansion can be remarkably suppressed, and the generation of blowholes can be effectively suppressed without using 100% CO ₂ gas. The present invention has been completed based on such findings.

That is, the above object is to manufacture a welded structural member by joining steel materials by gas shielded arc welding, and one or both of the members to be joined is a molten Zn—Al—Mg based plated steel plate member, and Ar + CO ₂ as a shielding gas. Using gas, He + CO ₂ gas or Ar + He + CO ₂ gas, the amount of CO ₂ used per unit weld length F _CO2 (L / cm) expressed by the following formula (1) and welding expressed by the following formula (2) The heat input Q (J / cm) relationship is achieved by a method for manufacturing an arc welded structure member that performs gas shielding so that the following equation (3) is satisfied.
F _CO2 = D _CO2 / v (1)
Q = (I × V) / v (2)
0.0025 ≦ F _CO2 ≦ 47.5Q ^−0.74 (3)
However, D _CO2 is the CO ₂ flow rate (L / sec) in standard conditions (0 ° C, 101.3 kPa) in shield gas, I is the welding current (A), V is the arc voltage (V), and v is the welding. Speed (cm / sec).

  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. The welding heat input Q can be, for example, in the range of 2000 to 12000 J / cm.

Further, when the molten Zn—Al—Mg-based plated steel sheet member has a plate thickness of less than 2.7 mm (for example, 0.8 mm or more and less than 2.7 mm), the following formula (4) is applied instead of the formula (3). can do.
^{_{0.0025 ≦ F CO2 ≦ 2.62Q -0.35 ...}} (4)
Thus, when plate | board thickness is thin, it is more preferable that the said welding heat input Q shall be the range of 2000-4500 J / cm, for example.

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. Those having a plating layer composed of ˜0.05%, Si: 0 to 2.0%, Fe: 0 to 2.5%, the balance Zn and unavoidable impurities are particularly suitable. Zn-Al-Mg plated coating weight per one surface can be, 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 that is inherently susceptible to molten metal embrittlement cracking is one that exhibits excellent resistance to molten metal embrittlement cracking. It became possible to realize stably without increasing the cost. The generation of blowholes due to the generation of Zn vapor is also suppressed at the same time. 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 cross section of the torch and base material in gas shielded arc welding. 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 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. For the plate thickness of the plating original plate is 6 mm, shows the CO ₂ amount F _CO2 per heat input Q and the unit weld length, the resistance to liquid metal embrittlement cracking resistance and blowholes of the relationship graph. Graph showing the relationship between welding heat input Q and CO ₂ usage F _CO2 per unit weld length, resistance to molten metal embrittlement cracking and blowhole resistance when the plate thickness is 2.6 mm .

FIG. 1 schematically shows a cross section of a torch and a base material during gas shielded arc welding. The welding torch 31 advances in the direction of the arrow while forming an arc 35 on the surface of the base material 1. A shield gas 34 is blown out from the periphery of the electrode 33 and the welding wire 32 positioned at the center of the welding torch 31 to protect the arc 35 and the surface of the base material 1 exposed to high temperatures from the atmosphere. A part of the base material 1 melted by heat input from the arc 35 is rapidly solidified after the welding torch 31 passes and forms a weld bead 2 made of weld metal. The shield gas 34 needs to be a non-oxidizing gas. Generally, CO ₂ gas or Ar + CO ₂ mixed gas is employed. A part of CO ₂ in the shielding gas 34 is considered to be separated into CO and O ₂ by the plasma arc 35, and the CO exerts a reducing action to suppress oxidation of the weld bead and its surroundings. Thereby, it is thought that the corrosion-resistant fall in a welding part is reduced.

  FIG. 2 schematically illustrates the cross-sectional structure of the welded portion of the lap fillet weld 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.

3 to 5 schematically show an enlarged cross-sectional structure of a portion corresponding to the vicinity of the bead toe 3 shown in FIG.
FIG. 3 schematically shows a cross-sectional state in the vicinity of a high-temperature weld immediately after the arc passes during gas shielded arc welding of a Zn—Al—Mg based 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. 3, the thicknesses of the Zn—Al—Mg-based molten metal 8 and the plating layer 7 are exaggerated.

  FIG. 4 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. 3) wets and spreads in the “plating layer evaporation region” (reference numeral 9 in FIG. 3) formed once the plating layer disappears during welding, and the base material 1 The entire surface up to the bead toe 3 is covered with the Zn—Al—Mg alloy layer 5. The portion of the Zn—Al—Mg alloy layer 5 formed by solidification of the Zn—Al—Mg molten metal (reference numeral 8 in FIG. 3) is referred to as a molten 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. 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. In the present invention, a gas containing CO ₂ is used as the shielding gas, but the amount of CO ₂ used per unit weld length is appropriately limited as described later. In this case, the surface of the base material 1 in the “plating layer evaporation region” (reference numeral 9 in FIG. 3) where the plating layer disappears during welding is quickly covered with a thin oxide film because the reducing action by the shielding gas is relatively weak. it is conceivable that. It is inferred that this oxide film inhibits wetting with the Zn—Al—Mg based molten metal (reference numeral 8 in FIG. 3), thereby suppressing the wetting and spreading of the Zn—Al—Mg based molten metal. As a result, the plating layer evaporation region 9 remains after cooling. That is, the surface of the base material 1 in the vicinity of the bead toe 3 is finished to cool without coming into contact with the Zn—Al—Mg-based molten metal, and the penetration of the molten metal component into the base material 1 at that portion is avoided. Is done. Therefore, excellent molten metal embrittlement cracking resistance is imparted without depending on the steel type of the base material 1. Even in a welding posture in which the height position of the Zn—Al—Mg-based molten metal (reference numeral 8 in FIG. 3) is above the bead toe 3, the above-described Zn—Al—Mg due to the above-described wetting inhibition action. Wetting and spreading of the molten metal is remarkably suppressed.

In the present invention, in order to limit the amount of CO ₂ used per unit weld length, the weld bead and its surroundings are more easily oxidized than conventional arc welding. However, by applying a molten Zn—Al—Mg-based plated steel sheet member as a member to be joined, the corrosion resistance of not only the plated layer surface but also the portion where the steel substrate is exposed in the vicinity of the welded portion is improved. That is, in addition to the anticorrosive effect by Zn, the corrosion product derived from the Zn—Al—Mg-based plated metal exhibits excellent protection, thereby improving the long-term corrosion resistance and limiting the amount of CO ₂ used. The decrease in corrosion resistance is not apparent during normal use. In addition, according to the conventional knowledge that “the formation of blow holes tends to be a problem with galvanized steel sheets when Ar + CO ₂ gas is used”, the generation of blow holes is not increased by limiting the amount of CO ₂ used. There was concern about whether there was. However, in the case of a Zn—Al—Mg-based steel sheet, unlike the zinc-plated steel sheet, by ensuring a certain amount of CO ₂ supplied from the shield gas per unit weld length, a blowhole caused by Zn vapor is prevented. It was found that generation can be sufficiently suppressed.

  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” in this specification, 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 As a result of various studies, if the plating layer evaporation region length is 0.3 mm or more, the molten metal embrittlement cracking resistance is greatly improved, and if it is 0.4 mm or more, it is more preferable. If this plating layer evaporation region length becomes too long, there will be a problem of deterioration of corrosion resistance due to the absence of the plating layer, but according to the study by the inventors, if the plating layer evaporation region length is 2.0 mm or less, It was found that the sacrificial anticorrosive action by the Zn—Al—Mg based plating layer was sufficiently obtained, and the corrosion resistance reduction at this portion was at a level that would not be a problem. By adjusting the shield gas composition as described later, the plating layer evaporation region length can be controlled within the range of 0.3 to 2.0 mm.

[Gas shield arc welding conditions]
In arc welding according to the present invention, it is important to limit the amount of CO ₂ per unit weld length supplied by the shielding gas in accordance with the welding heat input. As described above, CO ₂ mixed in the shielding gas comes into contact with the plasma arc and partly dissociates into CO and O ₂ , and the base metal surface near the weld bead is activated by the reduction action of the CO. However, in the present invention, by suppressing the reduction action, it is possible to prevent the surface of the base material from which the plating layer in the vicinity of the welded portion has evaporated and lost from being excessively activated, and Zn—Al existing on the surface of the surrounding base material. -It suppresses that a Mg-type molten metal wets and spreads to a bead toe part. As a result of detailed examination, when CO ₂ consumption F _CO2 per unit weld length is limited so that F _CO2 ≦ 47.5Q ^-0.74 in the following formula (3) is satisfied, the wetting spread suppression effect appears, It becomes possible to control the above-mentioned plating layer evaporation region length in the range of 0.3 to 2.0 mm. It is more effective to apply the following formula (3) ′ instead of the following formula (3). Moreover, when the amount of CO ₂ used F _CO2 per unit weld length is secured so as to satisfy 0.0025 ≦ F _CO2 in the following formula (3), the generation of blow holes is sufficiently suppressed.
F _CO2 = D _CO2 / v (1)
Q = (I × V) / v (2)
0.0025 ≦ F _CO2 ≦ 47.5Q ^−0.74 (3)
0.0025 ≦ F _CO2 ≦ 47.5Q ^−0.74 −0.01 (3) ′
However, D _CO2 is the CO ₂ flow rate (L / sec) in standard conditions (0 ° C, 101.3 kPa) in shield gas, I is the welding current (A), V is the arc voltage (V), and v is the welding. Speed (cm / sec).

Only one member to be joined is a molten Zn—Al—Mg-based plated steel plate member, and the thickness of the molten Zn—Al—Mg-based plated steel plate member is less than 2.7 mm (for example, 0.8 mm or more and less than 2.7 mm). When both the members to be joined and the molten Zn—Al—Mg-based plated steel plate member having a plate thickness of less than 2.7 mm (for example, 0.8 mm or more and less than 2.7 mm) are used, the above formula (3) is substituted. Thus, the following equation (4) can be applied. In this case, (4) if limiting the CO ₂ amount F _CO2 per unit weld length to satisfy F _CO2 ≦ 2.62Q ^-0.35 in the formula, the plating layer evaporation zone above the deterrent effect of the wetting spread The length can be controlled in the range of 0.3 to 2.0 mm. It is more effective to apply the following formula (4) ′ instead of the following formula (4). Further, (4) when securing the CO ₂ amount F _CO2 of 0.0025 ≦ F per a unit weld length to meet the _CO2 in the formula, the generation of blowholes is also sufficiently suppressed.
^{_{0.0025 ≦ F CO2 ≦ 2.62Q -0.35 ...}} (4)
^{_{0.0025 ≦ F CO2 ≦ 2.62Q -0.35 -0.01}} ... (4) '
Thus, when plate | board thickness is thin, it is more preferable that the said welding heat input Q shall be the range of 2000-4500 J / cm, for example.

  If preferable welding conditions are classified according to the plate thickness, for example, the following two modes of manufacturing an arc welding structural member can be disclosed.

[Aspect 1]
When manufacturing a welded structural member by joining steel materials by gas shielded arc welding, one or both of the members to be joined is a molten Zn-Al-Mg based steel plate member, and the molten Zn-Al-Mg based plating is used. The plate thickness of the steel plate member is set to 2.7 mm to 7.0 mm, and Ar + CO ₂ gas, He + CO ₂ gas, or Ar + He + CO ₂ gas is used as a shielding gas, and CO per unit weld length represented by the following formula (1) ₂ Arc welding structural members that perform gas shielding so that the relationship between the usage F _CO2 (L / cm) and the welding heat input Q (J / cm) represented by the following equation (2) satisfies the following equation (3): Manufacturing method.
F _CO2 = D _CO2 / v (1)
Q = (I × V) / v (2)
0.0025 ≦ F _CO2 ≦ 47.5Q ^−0.74 (3)
However, D _CO2 is the CO ₂ flow rate (L / sec) in standard conditions (0 ° C, 101.3 kPa) in shield gas, I is the welding current (A), V is the arc voltage (V), and v is the welding. Speed (cm / sec).
In this case, the welding heat input Q is more preferably in the range of 2000 to 12000 J / cm. It is more effective to adopt the following expression (3) ′ instead of expression (3).
0.0025 ≦ F _CO2 ≦ 47.5Q ^−0.74 −0.01 (3) ′

[Aspect 2]
When manufacturing a welded structural member by joining steel materials by gas shielded arc welding, one or both of the members to be joined is a molten Zn-Al-Mg based steel plate member, and the molten Zn-Al-Mg based plating is used. The plate thickness of the steel plate member is 0.8 mm or more and less than 2.7 mm, the welding heat input Q is in the range of 2000 to 4500 J / cm, and Ar + CO ₂ gas, He + CO ₂ gas or Ar + He + CO ₂ gas is used as a shielding gas, The relationship between the CO ₂ usage F _CO2 (L / cm) per unit weld length represented by the formula 1) and the welding heat input Q (J / cm) represented by the following formula (2) is shown in the following (4). A method of manufacturing an arc welded structural member that performs gas shielding to satisfy the equation.
F _CO2 = D _CO2 / v (1)
Q = (I × V) / v (2)
^{_{0.0025 ≦ F CO2 ≦ 2.62Q -0.35 ...}} (4)
However, D _CO2 is the CO ₂ flow rate (L / sec) in standard conditions (0 ° C, 101.3 kPa) in shield gas, I is the welding current (A), V is the arc voltage (V), and v is the welding. Speed (cm / sec).
It is more effective to adopt the following expression (4) ′ instead of expression (4).
^{_{0.0025 ≦ F CO2 ≦ 2.62Q -0.35 -0.01}} ... (4) '

In any of the above embodiments, the CO ₂ concentration in the shielding gas is more preferably in the range of 70% by volume or less. The lower limit of the CO ₂ concentration is not particularly required as long as 0.0025 ≦ F _CO2 in the above formula (3) or (4) is satisfied. However, in order to increase the flexibility of the welding heat input Q and the welding speed v, It is preferable to secure a CO ₂ concentration of 1% by volume or more, and more preferably 5% by volume or more. The flow rate of the shield gas (total flow rate of all components) can be set, for example, in the range of 0.05 to 1.0 L / sec, and more preferably in the range of 0.07 to 0.7 L / sec.

We investigated the relationship between welding heat input Q and CO ₂ consumption F _CO2 per unit weld length, and resistance to molten metal embrittlement cracking and blowholes (the magnitude of the action to suppress the occurrence of blowholes). An experimental example is introduced.

《Experimental example》
Using a 590 MPa high-tensile steel plate having a thickness of 6 mm and 2.6 mm (composition corresponding to steel B in Table 1 described later) as a plating base plate, a Zn-1.0 mass% Al-1.1 mass% Mg bath by a hot dipping method, Using a Zn-6.1 mass% Al-3.0 mass% Mg bath and a Zn-11.0 mass% Al-6.0 mass% Mg bath, 2 levels of plate thickness x 3 types of plated steel sheets were prepared. did. The plating adhesion amount was about 90 g / m ² per side (equal to both sides). In this case, the plating layer thickness per side is about 13 μm. Using the obtained plated steel sheet as a test material, the resistance to molten metal embrittlement cracking and the resistance to blowholes were examined by the following methods.

[Fused metal embrittlement cracking test method]
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.
Therefore, in this experimental example, a sample having a maximum crack depth of less than 0.5 mm was evaluated as melt metal embrittlement cracking resistance; good, and the others were evaluated as melt metal embrittlement crack resistance;

[Blow hole resistance test method]
An X-ray transmission photograph was taken for the weld bead portion after the welding test, and evaluation was performed by measuring the blowhole occupancy Bs determined by the Japan Architecture Center. The blow hole occupation ratio Bs is expressed as a ratio of the total diameter in the weld bead direction of each blow hole existing in the bead having the length L in the weld bead length L. Here, the blowhole occupancy ratio Bs was determined with L being a distance of one round at the center of the weld bead and the weld bead direction being the circumferential direction. If this Bs is 30% or less, the strength reduction of the welded part due to the blowhole is small, and it is determined as acceptable.

FIGS. 7 and 8 show the welding heat input Q, the amount of CO ₂ used per unit weld length F _CO2 , the resistance to molten metal embrittlement cracking, and the case where the plate thickness of the plating plate is 6 mm and 2.6 mm, respectively. The relationship of blow hole resistance is shown. The evaluation criteria are as follows.
X mark plot; maximum crack depth of 0.5 mm or more (molten metal embrittlement cracking resistance: poor)
Δ plot: maximum crack depth of more than 0 mm and less than 0.5 mm (melted metal embrittlement cracking resistance: good), and blowhole occupancy Bs is 30% or less (blowhole resistance: good)
○ Plot: Maximum crack depth is 0 mm, that is, no crack is observed (melted metal embrittlement cracking resistance: excellent), and the blowhole occupancy Bs is 30% or less (blowhole generation resistance: good)
● Plot: Maximum crack depth is 0 mm, that is, no cracks are observed (melted metal embrittlement cracking resistance: excellent), and the blowhole occupancy Bs exceeds 30% (blowhole resistance: poor)
Among these, ○ and Δ were accepted.
The same evaluation results were obtained for the above three types of plating compositions. Therefore, each plot in FIG. 7 and FIG. 8 shows a common evaluation result for the three types of plating compositions.

As can be seen from FIG. 7, by performing the gas shield so as to satisfy the above expression (3), it is possible to achieve both good resistance to molten metal embrittlement cracking and resistance to blowholes. If the above formula (3) ′ is adopted instead of the above formula (3), it is possible to realize a very excellent resistance to molten metal embrittlement cracking more stably.
Further, as can be seen from FIG. 8, when the plate thickness of the plating original plate is 2.6 mm (the total plate thickness including the plating layers on both sides is in the range of more than 2.6 mm and less than 2.7 mm), By performing the gas shield so as to satisfy the formula (4), it is possible to stably achieve both good resistance to molten metal embrittlement cracking and resistance to blowholes. If the plate thickness is thinner than that, the effect of suppressing the wetting and spreading of the plating layer metal will be further increased by further increasing the cooling rate during welding. Breakability is ensured. If the above formula (4) ′ is adopted instead of the above formula (4), it is possible to realize extremely excellent resistance to molten metal embrittlement cracking more stably.

[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 0.8 to 7.0 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. 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.

  For each steel type shown in Table 1, cold-rolled steel strips having a thickness of 3.2 mm, 4.5 mm, 6.0 mm, 2.6 mm, and 1.6 mm were prepared. In either case, the plate width is 1000 mm. These cold-rolled steel strips were used as plating base plates and passed through a hot dipping line to produce hot-dip Zn—Al—Mg-based plated steel plates having various plating layer compositions. The amount of plating was equal on both sides. Gas shield arc welding was performed under various welding conditions by a method according to the above-described molten metal embrittlement cracking test method (see FIG. 6) using a sample cut out from each molten Zn—Al—Mg plated steel sheet. With respect to the obtained welded portion, the maximum crack depth was measured to evaluate the resistance to molten metal embrittlement cracking. Also, the blowhole occupancy ratio Bs is obtained according to the above-mentioned blowhole resistance test method, and Bs of 30% or less is ○ (blowhole resistance; good) and the others are × (blowhole resistance) ;)).

Tables 2 and 3 show the hot dipping conditions, welding conditions, and test results for samples using a plating original plate having a thickness of 3.2 mm.
Table 4 shows the hot dipping conditions, the welding conditions, and the test results for the sample using the plating original plate having a thickness of 4.5 mm.
Table 5 shows the hot dipping conditions, welding conditions, and test results for samples using a plating original plate having a thickness of 6.0 mm.
Table 6 shows the hot dipping conditions, the welding conditions, and the test results for the sample using the plating original plate having a thickness of 2.6 mm.
Table 7 shows the hot dipping conditions, welding conditions, and test results for a sample using a plating original plate having a plate thickness of 1.6 mm.

As shown in Table 2, Table 4, and Table 5, in gas shielded arc welding of hot-dip Zn-Al-Mg plated steel plate members, the relationship between CO ₂ usage F _CO2 per unit weld length and welding heat input Q However, by performing the gas shield so as to satisfy the above-described expression (3), a welded structure member having excellent resistance to molten metal embrittlement cracking and occurrence of blowholes can be obtained.
Further, as shown in Tables 6 and 7, when the thickness of the molten Zn—Al—Mg-based plated steel sheet member (including the plating layer thickness) is less than 2.7 mm, the above-described formula (4) By performing the gas shield so as to satisfy the condition, a welded structure member having excellent resistance to molten metal embrittlement cracking and well-suppressed blowhole generation can be obtained.

Table 3 exemplifies a case where a gas shield is performed on a sample having a plate thickness of 3.2 mm that does not satisfy the expression (3). Nos. 23, 24, 27 to 30, and 33 to 35 indicate that the amount of CO ₂ used per unit weld length F _CO2 was excessive with respect to the welding heat input Q, so that “plating layer evaporation region” near the bead. , The reduction action by CO ₂ was not sufficiently reduced, and the phenomenon that the Zn—Al—Mg based molten metal spreads to the bead toe could not be prevented. As a result, molten metal embrittlement cracking occurred. Nos. 25, 26, 31, and 32 have a sufficient effect of suppressing the generation of blow holes due to Zn vapor derived from the plating layer due to insufficient CO ₂ usage F _CO2 per unit weld length. It was not demonstrated.

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 torch 32 Welding wire 33 Electrode 34 Shielding gas 35 Arc

Claims (6)

When manufacturing a welded structural member by joining steel materials by gas shielded arc welding, one or both members to be joined are made of a molten Zn—Al—Mg-based plated steel plate member, and Ar + CO ₂ gas and He + CO ₂ gas are used as shielding gases. Alternatively, using Ar + He + CO ₂ gas, the amount of CO ₂ used per unit weld length F _CO2 (L / cm) expressed by the following formula (1) and the welding heat input Q (J expressed by the following formula (2): / Cm) is a method of manufacturing an arc-welded structural member that performs gas shielding so that the following equation (3) is satisfied.
F _CO2 = D _CO2 / v (1)
Q = (I × V) / v (2)
0.0025 ≦ F _CO2 ≦ 47.5Q ^−0.74 (3)
However, D _CO2 is the CO ₂ flow rate (L / sec) in standard conditions (0 ° C, 101.3 kPa) in shield gas, I is the welding current (A), V is the arc voltage (V), and v is the welding. Speed (cm / sec).   The manufacturing method of the arc welding structural member of Claim 1 which makes the said welding heat input Q into the range of 2000-12000 J / cm. When manufacturing a welded structural member by joining steel materials by gas shielded arc welding, one or both of the members to be joined is a molten Zn-Al-Mg based steel plate member, and the molten Zn-Al-Mg based plating is used. The amount of CO ₂ used per unit weld length represented by the following formula (1) using Ar + CO ₂ gas, He + CO ₂ gas or Ar + He + CO ₂ gas as shielding gas, with the plate thickness of the steel plate member being less than 2.7 mm F _A method of manufacturing an arc welded structure member that performs gas shielding so that the relationship between _CO2 (L / cm) and welding heat input Q (J / cm) represented by the following formula (2) satisfies the following formula (4).
F _CO2 = D _CO2 / v (1)
Q = (I × V) / v (2)
^{_{0.0025 ≦ F CO2 ≦ 2.62Q -0.35 ...}} (4)
However, D _CO2 is the CO ₂ flow rate (L / sec) in standard conditions (0 ° C, 101.3 kPa) in shield gas, I is the welding current (A), V is the arc voltage (V), and v is the welding. Speed (cm / sec).   The manufacturing method of the arc welding structure member of Claim 3 which makes the said welding heat input Q the range of 2000-4500J / cm.   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. It has a plating layer consisting of ˜0.05%, Si: 0 to 2.0%, Fe: 0 to 2.5%, the balance Zn and unavoidable impurities. Manufacturing method of arc welded structural members. The hot-dip Zn-Al-Mg plated steel sheet, method of producing arc welding structure according to any of claims 1 to 5 coating weight per one side is 20 to 250 g / m ^2.

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