JP5019781B2 - MIG arc welding method using gas shielded arc welding flux cored wire - Google Patents

Description

The present invention provides a gas shielded arc-welded flux-cored wire that can reliably obtain high joint fatigue strength when performing fillet welding of a carbon steel sheet such as an automobile, and can obtain excellent welding workability with high efficiency. The present invention relates to a MIG arc welding method to be used .

  Recently, movements to use high-strength steel sheets have become active in order to reduce vehicle weight for the purpose of improving fuel efficiency. High-strength steel sheets are arc-welded in the same way as mild steel, but weld joints have a problem that fatigue strength can only be secured to the same level as mild steel, and the original performance of high-tensile steel sheets cannot be exhibited.

  It is thought that the fatigue strength of the welded part is lower than that of the base metal, mainly due to the stress concentration at the weld toe and the tensile residual stress caused by expansion and contraction due to welding heat. Has been.

First, in order to alleviate the stress concentration at the weld toe, means for smoothing the bead shape such as a decrease in contact angle and an increase in toe radius have been devised. Patent Document 1, Patent Document 2, Patent Document 3, and Patent Document 4, only the composition of the steel plate, special component added to lower the surface tension, be achieved by limiting such welding conditions such as welding voltage shown However, the toe shape is affected by various factors such as the composition of the steel plate and wire, the surface properties of the steel plate, and welding conditions such as the welding voltage, welding position, and welding speed. Many were not very versatile. Further, in the above-described conventional technology, it is difficult to dramatically improve the shape of the toe, and a significant increase in fatigue strength cannot be achieved.

  Further, a reduction in tensile residual stress, a reduction in yield stress of weld metal, and an improvement in ductility are achieved. Patent Document 5 and Patent Document 6 propose the idea of easily deforming the weld metal plastically in order to reduce the residual stress. However, Patent Document 5 does not provide specific welding means. Moreover, in patent document 6, since a deoxidation component is reduced excessively with respect to a welding wire and a strength is reduced, pore defects are likely to occur due to insufficient deoxidation, or static joint tensile strength when applied to a high-strength steel plate. There was a problem such as shortage.

  Furthermore, stress relief annealing is performed to reduce the tensile residual stress. Conventionally, the most well-known method for extinguishing residual stress is stress removal annealing (PWHT) which is held at a high temperature in an annealing furnace. However, there are few manufacturers that have PWHT equipment for thin plates such as automobiles, and even if the equipment is introduced, the production efficiency is remarkably lowered and the cost is increased.

Furthermore, peening is performed to reduce the tensile residual stress. There is a means for applying a compressive stress after welding by means of shot peening or hammer peening and ultrasonic peening, which is also disclosed in Patent Document 7. However, this technique also requires the introduction of equipment, and the production efficiency is remarkably lowered, resulting in an increase in cost.

Furthermore, in order to reduce the tensile residual stress, a technique using a low temperature transformation welding material has been proposed. Recently, attention has been focused on a method of reducing the martensitic transformation temperature (Ms point) of the weld metal to give the compressive residual stress of the expansion transformation at room temperature or reducing the tensile residual stress. . Patent Document 8 has already proposed a method of reducing the Ms point with a high Cr + high Ni-based weld metal. Since then, numerous proposals have been made using this method. In Patent Literature 9, Patent Literature 10, Patent Literature 11, Patent Literature 12, Patent Literature 13, Patent Literature 14, Patent Literature 15, and Patent Literature 16, a high Cr, high Ni, or high Mn based weld metal or welding wire is used. Proposed. In this technique, in a general welding material, an expansion transformation occurs at a high temperature M _S ^{S as} shown by a line S in FIG. When returning to room temperature, tensile [Delta] L ^S residual stress remains. However, in the case of low transformation temperature welding material (line A) has a low M _S point (M S ^_A) for not occur shrinkage deformation at the time of room temperature, it is ideal is that the compressive residual stress of the [Delta] L ^A. However, there is a problem that the residual stress cannot always be stably improved only by lowering the Ms point. Even Ms point is low, many subsequent termination expanded immediately to become (line B) field if generated in the cooling process, the compressive residual stress does not occur only a very small [Delta] L _B. On the other hand, even if M _S point is somewhat higher (M S ^_C), then if the expansion deformation is very large (line C) is sufficiently high compressive deformation [Delta] L ^C can be secured even when the room temperature moves to shrinkage deformation. That is, in the above-described conventional technology, there is a problem that not only a decrease in Ms point but also a method for efficiently causing martensite expansion transformation has not been established.

  In addition, since the above-mentioned welding material has extremely high hardenability, in the case of a solid wire, the wire drawing property is poor and the welding material becomes a high cost. In addition, due to the high viscosity of the weld metal, the high speed required for thin plate welding and the applicability in actual welding lines such as increased spatter generation due to the difficulty of droplet transfer are not considered. there were. In some cases, a specific welding wire or a welding method is not presented by the definition of only a welded joint (metal), and in this case, the realization method is unknown. In general, the simplest means for realizing a predetermined weld metal is practically using the TIG welding method in which the penetration is extremely shallow, it is not necessary to consider dilution of the base material, and oxidation consumption does not occur. . For this reason, it cannot be said that a consumable electrode type gas shielded arc welding method such as high-efficiency MAG or MIG desired in an implementation is proposed.

  As one example of the proposed welding material, Patent Document 17 proposes a means using a flux-cored wire. However, as described above, no means for guiding efficient expansion transformation has been presented, and the improvement of fatigue strength is unstable, and it is a thick plate type that generates a large amount of slag and is not suitable for thin plates. Have problems such as poor electrodeposition and poor high speed.

  As a means for reducing the cost as a solid wire, Non-Patent Document 1 proposes a solid wire having a double structure of different components with good drawability as a single unit in order to improve the drawability. However, no means for inducing efficient expansion transformation is presented, and the cost of the wire manufacturing method is still very high because of its special method. Although this wire has been proposed to improve arc stability in MIG welding and improve weldability, in MIG welding, in order to stabilize the cathode and anode points, the oxygen content and the electron are improved. Since a means such as addition of a special element for facilitating the release is required and these considerations are not made in this solid wire, it is still insufficient.

JP-A-6-340947 JP-A-8-25080 JP 2002-361480 A JP-A-2002-361481 JP-A-7-171679 JP-A-9-227987 JP20041363631A JP 54-130451 A JP 2000-288728 A JP2001-246495 JP 2002-273599 A JP 2004-98108 A JP 2004-98109 A JP 2004-98113 A JP 2004-98114 A JP-A-2005-238305 JP 2002-307189 A Super Steel Workshop Vol.9th 58-59P, 2005/7/20]

The present invention has been made in view of such problems, and not only sufficiently lowers the Ms point, but also efficiently leads to the expansion transformation that occurs in the cooling process below the Ms point, and has excellent wire drawability, By adopting high-oxygen and high-nitrogen flux-cored wires with excellent low slag properties, fatigue strength can be improved at low cost. High-speed weldability, low spatter properties, low slag properties, and arc stability It is an object of the present invention to provide a MIG arc welding method using a gas-shielded arc welding flux-cored wire that can realize the performance and is easy to use .

In the MIG arc welding method according to the present invention, in a flux-cored wire for arc welding formed by filling a steel outer shell with a flux, the component composition of the entire wire is C: 0.02 to 0.00. 70% by mass, Si: 0.30 to 1.50% by mass, Mn: 0.50 to 5.00% by mass, Ni: 2.0 to 9.5% by mass, Cr: Ni in a total amount of 4. 0 to 18.0% by mass (including the case where Cr is not included), O: 0.020% by mass or more and N: 0.0020 to 0.0400% by mass, P: 0.030% by mass Hereinafter, S: 0.030% by mass or less, Ti: 0.15% by mass or less, Al: 0.20% by mass or less, at least one selected from the group consisting of Nb, V, Mo and Cu: per element Less than 2.00% by mass, B: 0.0100% by mass REM (rare earth element): 0.50 mass% or less, Mg: 1.00 mass% or less, at least one selected from the group consisting of F and Ca: 0.100 mass% or less for each element, K, at least one selected from the group consisting of Na and Li: regulated to 0.200 wt% or less in total, the balance being Fe and consists unavoidable impurities, and the gas shielded arc flux rate Ru 7 to 30% by mass A wire with welding flux is used, and MIG welding is performed using a mixed gas of 96 mass% or more of Ar and the balance of CO ₂ or O ₂ as a shielding gas .

  In the present invention, Cr is 4.0 to 18.0% by mass in total with Ni, but Cr may not be included if Ni alone contains this amount.

In this MIG arc welding method , the component composition of the entire wire is further at least one selected from the group consisting of Nb, V, Mo and Cu with respect to the total mass of the wire: 0.05% by mass or more for each element Less than 2.00% by mass, B: 0.0010 to 0.0100% by mass, REM (rare earth element): 0.01 to 0.50% by mass, Mg: 0.05 to 1.00% by mass, F and Ca At least one selected from the group consisting of: 0.005 to 0.100% by mass for each element, or at least one selected from the group consisting of K, Na and Li: 0.001 to 0.00 in total. It can contain 200 mass%.

In another MIG arc welding method according to the present invention, in a flux-cored wire for arc welding in which a steel outer shell is filled with a flux, the composition of the entire wire is C: 0.02 to the total mass of the wire. 0.70% by mass, Si: 0.30 to 1.50% by mass, Mn: 0.50 to 5.00% by mass, Ni: 2.0 to 9.5% by mass, and Cr: Ni 4.0 to 18.0% by mass (provided that Cr is not included), O: 0.020% by mass or more and N: 0.0020 to 0.0400% by mass, P: 0.030 % By mass or less, S: 0.030% by mass or less, Ti: 0.15% by mass or less, Al: 0.20% by mass or less, at least one selected from the group consisting of Nb, V, Mo and Cu: each Less than 2.00% by mass per element, B: 0.0100 quality % Or less, REM (rare earth element): 0.50 mass% or less, Mg: 1.00 mass% or less, at least one selected from the group consisting of F and Ca: 0.100 mass% or less for each element, K At least one selected from the group consisting of Na and Li: a gas shielded arc whose total amount is regulated to 0.200% by mass or less, the balance being Fe and inevitable impurities, and a flux rate of 7 to 30% by mass MIG welding is performed using a wire with welding flux and substantially using pure Ar gas as a shielding gas.

In this MIG arc welding method, the component composition of the entire wire is further at least one selected from the group consisting of Nb, V, Mo and Cu with respect to the total mass of the wire: 0.05% by mass or more for each element Less than 2.00% by mass, B: 0.0010 to 0.0100% by mass, REM (rare earth element): 0.01 to 0.50% by mass, Mg: 0.05 to 1.00% by mass, F and Ca At least one selected from the group consisting of: 0.005 to 0.100% by mass for each element, or at least one selected from the group consisting of K, Na and Li: 0.001 to 0.00 in total. It can contain 200 mass%.

  In these MIG arc welding methods, it is preferable to perform MIG welding using a pulse arc welder. Moreover, it is preferable to apply to a steel plate having a plate thickness of 1 to 5 mm and a base material strength of 490 MPa or more.

  According to the present invention, a large expansion transformation occurs in the cooling process of the welded portion, thereby improving the joint fatigue strength in high-speed welding of thin plates, improving the bead shape, the amount of spatter and fume, and improving the resistance to electrodeposition coating. This effect can be reliably realized at a low running cost without requiring a large capital investment. For this reason, the wire of the present invention is extremely effective for reducing the weight of the steel sheet in the automobile industry and the like, and has great social significance that leads to environmental improvement and the like.

  Hereinafter, embodiments of the present invention will be specifically described with reference to the accompanying drawings. In order to increase the compressive residual stress as much as possible when cooling at room temperature, the inventors of the present application do not need to lower the Ms point so much, and it is more important to generate the expansion transformation after the Ms point with high efficiency. I focused on that. Therefore, the present invention aims to satisfy all of (1) reliable and highly efficient expansion transformation in the cooling process after the Ms point, (2) low cost, and (3) welding workability required for thin plate welding. As a result of repeated research, the following conclusions were obtained.

(1) The technique for obtaining a highly efficient expansion transformation is as follows.
As in the technology developed so far, a large amount of Ni and Cr is effective for lowering the Ms point, but with this alone, the transformation rate from austenite to martensite is not large below the Ms point, and the expansion ends immediately. As a retained austenite, it remains at room temperature. The present inventors have found that it is necessary to satisfy both the reduction of the oxygen content of the weld metal and the positive addition of nitrogen in order to prevent residual austenite. Furthermore, it has been found that it is preferable to add carbon at a high content. To achieve this, 1) positive addition of N and C from the flux, 2) high efficiency by reducing the amount of oxygen in the weld metal by adopting the low oxygen MAG or MIG welding method that reduces the amount of oxygen in the shield gas as much as possible. An expansion transformation could be made possible. Further, when the pulse welding method is used, the heat input is reduced, so that the cooling rate is increased, and this is advantageous in that the Ms point is lowered and a highly efficient expansion transformation is caused.

(2) The technology for cost reduction is as follows.
As a wire form, the solid wire is easy to harden and needs to be annealed many times at the time of wire drawing. By positively adding oxygen to the anode, the anode and cathode spots during MIG welding were stabilized, realizing stable arc stability and low spatter. It was also confirmed that the oxygen content of the weld metal can be suppressed to a very low level because even when the wire is a high oxygen type, the weld pool is bonded with carbon having a strong deoxidizing function and separated.

  Further, in MIG welding, it is possible to reliably suppress the generation of a large amount of fumes, which is a drawback of flux-cored wires.

  Even if it does not contain a large amount of expensive elements such as Cr and Ni, the present invention enables highly efficient expansion transformation after the Ms point, so a large amount of Cr and Ni, which are elements for lowering the Ms point, are added. The lack of necessity is also a factor in reducing the cost of material. Also for C, inexpensive graphite can be used as a part of the flux component in the flux-cored wire, and the carbon can be easily increased.

(3) Means for obtaining good workability in consideration of high-speed welding of thin plates are as follows.
When a large amount of elements such as Cr and Ni is contained, the viscosity of the molten pool becomes high and high-speed welding becomes difficult, but with this wire, these elements can be kept low, so the viscosity of the molten pool is low and high-speed welding of thin plates is possible. It is. Furthermore, since general flux-cored wires are designed for shielding gas up to about CO ₂ or Ar 80 + 20 mass% CO ₂ (MAG), in most conventional technologies, the arc stabilizer Ti, Al, and Zr are used. It is positively added in the form of TiO ₂ , Al ₂ O ₃ , ZrO ₂ . However, since Ti, Al, and Zr generate slag, in the case of electrodeposition coating such as automobile parts, there is a problem that the coating is peeled off by slag peeling. The wire of the present invention emphasizes the use of a thin plate for automobiles, and does not positively add Ti, Al and Zr, and improves workability.

  As described in (2) above, by increasing the oxygen content of the flux-cored wire, arc stability during MIG welding, low spatter, and low fume are also achieved. Further, by combining pulse welding with the wire of the present invention, it is possible to further achieve arc stabilization, reduction in spatter amount, and reduction in fume amount.

  The reasons for adding the components and limiting the composition of each component will be described below. This composition is a composition of the components of the entire wire including the hoop and the flux of the outer skin.

“C: 0.02 to 0.70 mass%”
C is a component necessary not only to ensure the strength of the weld metal, but also to cause an expansion transformation after passing through the Ms point in the cooling process with high efficiency. If C is less than 0.02% by mass, the efficiency of expansion transformation is low. In order to increase the efficiency of the expansion transformation, C is required to be 0.02% by mass or more, desirably 0.08% by mass or more, and more desirably 0.20% by mass or more. On the other hand, when C is added excessively, carbide precipitates and becomes a starting point of fatigue fracture, conversely, fatigue strength decreases and fume generation increases. When C is added in excess of 0.70% by mass, hot cracks and cold cracks are more likely to occur. Therefore, the upper limit of C is 0.70% by mass. However, from the viewpoint of reducing the amount of fume, C is desirably 0.50% by mass or less.

“Ni: 2.0 to 9.5 mass%”
Ni is a typical austenite stabilizing element and is an effective element for lowering the Ms point. In order to lower the Ms point within an appropriate range, Ni needs to be added in an amount of 2.0% by mass or more. On the other hand, since Ni is an expensive element, its addition not only increases the cost, but excessive addition increases the viscosity of the molten pool and causes humping during high-speed welding. The disadvantage is that the amount increases. These problems are permissible if Ni is 9.5% by mass or less, but in the present invention, even if the Ms point is high, the expansion transformation is extremely large in the subsequent cooling process, so the Ms point is excessively lowered. There is no need, and in that sense, 9.5% by mass or less of Ni is sufficient, and in order to reduce the above problems, Ni is more preferably 5.9% by mass or less.

“Total amount of Cr: Ni is 4.0 to 18.0% by mass (including the case where Cr is not included)”
Cr is added together with Ni to lower the Ms point. The addition of Cr alone has no effect. If the combined amount of Ni and Cr is less than 4.0% by mass, the Ms point cannot be lowered within an effective range. Therefore, the total amount of Ni and Cr needs to be 4.0% by mass or more. On the other hand, when the total amount of Ni and Cr exceeds 18.0% by mass, not only the cost is increased, but also the viscosity of the molten pool is increased and humping is performed during high-speed welding, and the amount of spatter is increased. Occurs. Therefore, the upper limit of the total amount of Ni and Cr is 18.0% by mass. Since Cr exhibits its effect when co-added with Ni, the addition of Cr alone (when Ni is not added) is not included in the scope of the present invention. Conversely, if Ni is contained in an amount of not less than 4,0% by mass, Cr can be added.

“Si: 0.30 to 1.50 mass%”
Si has an effect of improving the bead shape. If Si is less than 0.30 mass%, this function is insufficient, the conformability of the bead shape is deteriorated, the toe shape is deteriorated, and stress is easily concentrated on the welded portion. As a result, the fatigue strength of the joint is reduced. Desirably, Si is 0.60 mass% or more. On the other hand, when Si exceeds 1.50 mass%, the viscosity of the molten pool becomes excessive, and it becomes easy to hum during high-speed welding. Further, since a large amount of slag is generated, the electrodeposition paintability is also deteriorated. Therefore, Si is 1.50 mass% or less.

“Mn: 0.50 to 5.00% by mass”
Mn has the effect of improving the bead shape, increasing the hardenability by adding a large amount, and lowering the Ms point. When Mn is less than 0.50% by mass, the conformability of the bead shape is deteriorated, the toe shape is deteriorated, and stress is easily concentrated on the welded portion. As a result, the fatigue strength of the joint is reduced. Desirably, Mn is 1.00 mass% or more. On the other hand, if Mn exceeds 5.00% by mass, the viscosity of the molten pool becomes excessive, and it becomes easy to hum during high-speed welding, and the amount of generated fume becomes excessive. Moreover, when Mn is added exceeding 5.00 mass%, since a large amount of slag is generated, the electrodeposition coating property is also deteriorated. Therefore, Mn is 5.00 mass% or less, desirably 3.00 mass% or less.

“O: 0.020 mass% or more”
Oxygen is essential to stabilize the cathode spot and anode spot during MIG welding and to obtain good arc stability. Further, the addition of oxygen increases the amount of oxygen in the droplets to lower the surface tension, improves the detachability of the droplets, and enables low spattering. The effect is particularly great during pulse welding. With solid wire, it is difficult to stably add O in an amount of 0.020% by mass or more, but by using a flux-cored wire, for example, by using iron powder, surface oxygen per unit volume increases, so it is easy. In addition, a large amount of oxygen can be added. Desirably, oxygen is 0.040 mass% or more. Moreover, there is no harmful effect on adding a large amount of oxygen, and since the upper limit of oxygen is substantially determined in consideration of the flux rate, no upper limit is set as the amount of oxygen.

“N: 0.0020 to 0.0400 mass%”
In general, in the welding of carbon steel, nitrogen is reduced as much as possible because it only reduces the toughness or generates blowholes and has no particular advantage. However, in the present invention, N is positively added. Nitrogen, when added in an appropriate amount, efficiently causes martensitic transformation in the cooling process after the Ms point of the weld metal and increases the expansion. On the other hand, when N is not added, even if the Ms point is low as shown in the B line of FIG. 1, the transformation is immediately terminated, and the increase in compressive stress and the accompanying fatigue strength improvement effect remain at a very limited level.

  The effect of N that maximizes the expansion becomes effective when the wire is added in an amount of 0.0020% by mass or more. Desirably, N is 0.0035 mass% or more. On the other hand, when the amount of nitrogen increases, the stability of the arc deteriorates, and the addition of N exceeding 0.0400% by mass generates blow holes. Desirably, N is 0.0200 mass% or less. When N is added to a low-alloy solid wire, pore defects are likely to occur during melting, so melting is difficult, but by using a flux-cored wire, N should be actively added from the flux. Can do. This is one of the reasons for selecting a flux-cored wire.

“P and S: 0.030% by mass or less, respectively”
P and S are elements that lower the hot cracking resistance, and there is no particular meaning of positive addition in order to achieve the object of the present invention. Therefore, it is suppressed to 0.030% by mass or less in consideration of industrial productivity and cost as with the conventional wire.

“Ti: 0.15 mass% or less, Al: 0.20 mass% or less”
Ti and Al are added to many flux cored wires as arc stabilizers and deoxidizing elements. However, the present invention is a welding wire for a thin plate, and a slag peeling process is not planned as a use environment. The wire of the present invention is very often applied to the welding of an electrodeposited thin plate. Therefore, the addition of Ti and Al that generate slag is undesirable because it impairs the paintability. Moreover, when there are many Ti and Al, the arc stability at the time of the low current welding used with a thin plate will deteriorate, and a big spatter will be generated. Therefore, in consideration of industrial productivity and cost, Ti is suppressed to 0.15% by mass or less, and Al: 0.20% by mass or less. More preferably, both Ti and Al are 0.10% by mass or less.

“Flux rate: 7 to 30% by mass”
When the flux rate (filling rate) is less than 7% by mass, it is impossible to include a predetermined amount of nitrogen and oxygen, segregation of the flux components occurs, the outer skin becomes thick, and the droplets increase, resulting in spattering. Will increase. On the other hand, when the flux rate exceeds 30% by mass, the outer skin becomes thin, breakage is likely to occur during wire drawing, and manufacturing becomes difficult.

“Nb, V, Mo and Cu: 0.05% by mass or more and less than 2.00% by mass for each element”
Even if Nb, V, Mo, and Cu are not added, the object of the present invention can be achieved. However, Nb, V, Mo, and Cu can reduce the Ms point by adding appropriate amounts, respectively. The effect of securing an appropriate strength can be obtained. In order to obtain these effects, it is necessary to add 0.05% by mass or more of these elements. On the other hand, if each of these elements is 2.00% by mass or more, the production cost of the wire becomes too high. Moreover, when these elements exceed 2.00 mass%, the viscosity of a molten pool will rise and it will hump at the time of high-speed welding, or a sputter | spatter generation amount will increase. In particular, defects such as hot cracking are noticeable in Cu. For this reason, when adding these elements, it is made less than 2.00 mass%. In addition, when Cu is plated on the surface of the constituent skin, the amount of Cu is restricted to less than 2.00% by mass including Cu of the plating layer.

“B: 0.0100 mass% or less, or 0.0010 to 0.0100 mass%”
Although the object of the present invention can be achieved without adding B, the addition of a small amount of B can lower the Ms point and improve the toughness of the weld metal. For this effect, it is necessary to add B in an amount of 0.0010% by mass or more. On the other hand, if B is added in an amount exceeding 0.0100% by mass, a hot crack is generated in the joint. For this reason, when adding B , it is made into 0.0100 mass% or less.

“REM (rare earth element): 0.50 mass% or less, or 0.01 to 0.50 mass%”
REM is a rare earth element such as La or Ce. The object of the present invention can be achieved without adding REM. However, when 0.01% by mass or more of REM is added, the arc stability is improved during MIG welding, and the Ms point is obtained even if the oxygen content of the weld metal is lower. Can be reduced. On the other hand, when REM is added in an amount exceeding 0.50% by mass, the arc stabilizing effect is saturated, and conversely, the droplets become large and spatter increases. Also, the cost is expensive. For this reason, when adding REM , it is 0.01 to 0.50 mass%.

“Mg: 1.00 mass% or less, or 0.05 to 1.00 mass%”
Although the object of the present invention can be achieved without adding Mg, Mg is a strong deoxidizing component, which enhances the hardenability of the weld metal and lowers the Ms point. For that effect, it is necessary to add 0.05% by mass or more of Mg. On the other hand, if Mg exceeds 1.00% by mass, the viscosity of the molten pool increases, and humping occurs during high-speed welding, and the amount of spatter generated and the amount of fume increases, so Mg is made 1.00% by mass or less. .

“F and Ca: 0.100% by mass or less, or 0.005 to 0.100% by mass, respectively”
Even if F and Ca are not added, the object of the present invention can be achieved, but F and Ca also have a strong deoxidizing action, increase the hardenability of the weld metal, and lower the Ms point. For that effect, it is necessary to add 0.005% by mass or more of F and Ca, respectively. On the other hand, when F and Ca each exceed 0.100% by mass, the viscosity of the molten pool is increased and humping is performed during high-speed welding, and the amount of spatter generated and the amount of fume increase. For this reason, when adding F and Ca, respectively, it shall be 0.100 mass% or less.

“At least one selected from the group consisting of K, Na and Li: a total amount of 0.200 mass% or less, or 0.001 to 0.200 mass%”
Although the object of the present invention can be achieved without adding K, Na and Li, K, Na and Li facilitate electron emission, smooth arc stabilization and droplet transfer, and reduce spatter generation. Has an effect. In particular, the effect is great in MIG welding. The effect of K, Na, and Li is exhibited by addition of 0.001% by mass or more in total. On the other hand, even if K, Na, and Li are added in a total amount exceeding 0.200% by mass, the effect of the addition is saturated, the arc force is weakened, the penetration depth becomes shallow, and the molten pool becomes inefficient. Problems such as stable hunting occur. Therefore, the upper limit of the total amount of K, Na and Li is 0.200% by mass. In general, K, Na, and Li are added with flux using feldspar, soda glass, potash glass, or the like mainly composed of K ₂ O, Na ₂ O, Li ₂ O.

“Ar: 96% by mass or more, balance: CO ₂ or O ₂ mixed gas, preferably pure Ar gas used as shielding gas for MIG welding”
The shield gas lowers the oxygen content of the weld metal, lowers the Ms point appropriately, and generates an expansion transformation when cooled to below the Ms point with high efficiency. Non-oxidizing is desirable. At least, if the Ar gas ratio is not 80% by mass or more, it is difficult to reduce the residual stress and suppress the amount of fume, but an Ar ratio of 96% by mass or more is recommended. Furthermore, when pure Ar gas containing substantially only impurities is used as a shielding gas, these characteristics can be dramatically improved. Although it is impossible to ensure arc stability with pure Ar with a general wire, the wire of the present invention can maintain stable arc stability with pure Ar gas. In addition, pure Ar here is allowed to contain inevitable impurities in Ar.

"Pulse welding machine"
Even if the welding machine to be used is a constant voltage characteristic power source used for general consumable electrode arc welding, there is no particular problem in reducing residual stress. However, in order to achieve high-speed weldability, arc stability, and low fume in thin plate welding, a combination with a pulse welder is recommended. In particular, when pure Ar is used as the shielding gas, a pulse welder is effective for ensuring arc stability. Although there is no particular limitation on the pulse setting, a peak current of 350 to 600 A, a base current of 30 to 100 A, and a peak of 0.8 to 5.0 msec are generally used in a period of one peak (starting start to peak steady period to falling end). Is done.

  In pulse welding, compared to welding with a constant voltage characteristic waveform, the current value is reduced by about 10 to 20% and the heat input is reduced in the case of the same welding amount, so that the cooling rate of the welded portion is increased. As a result, hardenability is enhanced, which leads to a decrease in the Ms point, which is also preferable for reducing residual stress. It is more effective to apply the present invention to the following steel plates and plate thicknesses.

“Base material strength: Steel plate of 490 MPa or more”
The reason why the residual stress generated in the steel heat-affected zone due to the transformation expansion of the weld metal can be reduced is that the stress generated on the steel material side when the weld metal expands also becomes a compressive stress due to the reaction force to the weld metal. For this reason, it can be expected that a higher strength steel sheet that can be expected to have a higher reaction force has a greater improvement in fatigue properties. This is because when the steel material strength is low, the reaction force also has to be low, and there is a risk of returning to the tensile stress state again due to thermal contraction after the end of transformation. If tensile stress remains, improvement in fatigue strength cannot be expected. Therefore, in the present invention, the strength of the applied base material is 490 MPa or more as a lower limit value that can be expected to improve the fatigue strength. The upper limit of the base material strength is not particularly limited. The strength of a thin steel plate that is currently in practical use is about 1500 MPa, and if it is a steel plate up to this level, the wire of the present invention can improve the fatigue strength, and also in terms of joint tensile strength. Overmatching of weld metal can be achieved.

“Thickness: 1 to 5 mm”
If the plate thickness is excessively thin, the front and back surfaces of the steel plate are heated almost uniformly due to heat input during welding, and the molten metal reaches the back side, resulting in a state called back wave. In such a state, the weld metal expands almost freely during martensitic transformation. Therefore, no reaction force is generated on the steel material heat-affected zone side, and the effect of improving fatigue strength is limited. The lower limit plate thickness for effectively improving the fatigue strength is 1 mm.

  On the other hand, if the plate thickness exceeds 5 mm, the binding force becomes excessive and cold cracking may occur in the wire of the present invention having the property of becoming a high-strength weld metal. In addition, as the fillet leg length increases, the throat thickness inevitably increases, and hot cracking tends to occur. The upper limit of the plate thickness at which such a problem does not occur is 5 mm.

  Next, examples demonstrating the effects of the present invention will be described in comparison with comparative examples that are out of the scope of the present invention. Using two types of high-tensile steel plates shown in Table 1 below, overlapped fillet welding was performed according to the welding procedure shown in FIG. The steel plate 1 was stacked and welded. The protruding length of the wire 2 is 15 mm, the width of the overlapping portion is 7 mm, and the root gap is 0 to 0.5 mm. Also, welding speed is 1.2m / min, wire diameter is 1.2mm, shield gas flow rate is 15l / min, no torch advance angle (perpendicular to welding line direction), welding current is plate thickness is 3.2mm Was 300 A and 230 A in the case of 2.3 mm.

The test items and judgment methods are as follows.
(1) Fatigue test A fatigue test piece shown in Fig. 3 was taken from the welded workpiece and subjected to a double vibration plane bending fatigue test. The maximum value of the width is 30 mm, the minimum value is 20 mm, the length is 90 mm, and the curvature of the minimum width portion has a curvature radius of 40 mm. Reference numeral 3 denotes a welded portion. In the fatigue test, a sine wave stress having a frequency of 25 Hz was applied 2 million times, and the time strength was measured as the fatigue strength. In the case of SP1 of a 780 MPa grade steel plate, 200 MPa or more is ◎, 170 MPa or more and less than 200 MPa is ◯, less than 170 MPa is x, in the case of 490 MPa grade steel plate SP2, 170 MPa or more is ◎, 140 MPa or more and less than 170 MPa is ○, and less than 140 MPa is x In each case, the case of x was rejected as no fatigue improvement effect. Even when a humping bead was generated, the test piece was searched for a stable part, and the test piece was collected from that place.
(2) Stability of arc The sensory evaluation of arc stability during welding was performed in three stages of △△ ×. The case where it was good was evaluated as ◯, the case where some spatter was generated was evaluated as Δ, and the case where the arc was staggered or large spatter was generated was evaluated as x. ○ and △ were accepted, and × was rejected as unusable for practical use.
(3) Fume generation amount The value of fume generation amount measured under the same welding conditions based on JIS Z3930 was evaluated in four stages, ◎ ○ Δ ×. The generated amount is 300 mg / min or less, ◎, 300 to 500 mg / min or less is ◯, 500 to 700 mg / min or less is Δ, and 700 mg / min or more is ×. ◎ ○ △ was accepted, and × was rejected because it could not withstand practical use.
(4) Bead shape The fillet bead shape was subjected to sensory evaluation in three stages of ○ Δ × with a sensory sense. “Good” indicates that the conformability is slightly inferior, and “△” indicates that the shape is an overlapped toe shape or the bead width in the weld line direction is not uniform. ○ and △ were accepted, and × was rejected as unusable for practical use.
(5) Paintability In order to evaluate the risk of the paint being peeled off due to the slag peeling in the electrodeposition coating process after welding, the area of the slag generated on the bead was sensory-evaluated in three stages: . When the slag area was less than 10% by mass with respect to the bead surface area, ◯, 10% by mass or more but less than 20% by mass were evaluated as Δ, and 20% by mass or more were evaluated as x.
(6) Presence / absence of defects All occurrences of cracks in the welded portion, pore defects such as blowholes and pits, or humping phenomenon that the bead breaks occurred were all rejected.
(7) Price The price of the wire incorporating the material cost and the manufacturing cost is 3 times or less in comparison with the cost of the general-purpose wire JIS Z3312 YGW12 that is most commonly applied for thin plates. Δ: A value exceeding 4 times was marked as x, and x was rejected as impractical.
(8) Others A case where it was difficult to stably manufacture a wire as an industrial product was marked as rejected.

  Tables 2 to 5 below show the components of the flux welding wire (some solid wires are included), the flux rate, the shielding gas composition, the type of welding power source, and the applicable steel plate. The evaluation results of each test under these conditions are shown in Tables 6 and 7 below.

  As a result of the above test, Example No. 1 to 25 are invention examples satisfying the claims of the present invention, exhibiting good joint fatigue strength, and welding workability such as high-speed welding stability, spatter and fume generation, and slag generation And the cost is also practical enough.

  On the other hand, Comparative Example No. 26 to 60 are outside the scope of the present invention. Comparative Example No. Since 26 and 27 had little C, Ms point did not fall and fatigue strength was low. Comparative Example No. In No. 28, since C was excessive, the amount of generated fumes was excessive, and further cracks were generated. Comparative Example No. Since 29 had less Si, the bead shape deteriorated, and the fatigue strength decreased due to stress concentration. Comparative Example No. In No. 30, since Si was excessive, the viscosity of the molten pool became excessive, it could not withstand high speed, and humping occurred. Also, a lot of slag was generated, and the electrodeposition paintability deteriorated. Comparative Example No. Since 31 had less Mn, the bead shape deteriorated and the fatigue strength was poor due to stress concentration.

  Comparative Example No. In No. 32, since Mn was excessive, the viscosity of the molten pool became excessive, and it was not able to withstand high speed, and humping occurred. Also, a lot of slag was generated, and the electrodeposition paintability deteriorated. The amount of fume was also excessive. Comparative Example No. Since P and S were excessive in Nos. 33 and 34, high-temperature cracking occurred, and the fatigue strength was low due to cracking as a starting point. Comparative Example No. No. 35 was low in oxygen, so the anode spot and cathode spot were unstable and the arc fluctuated. The bead shape also meandered in the direction of the weld line. Comparative Example No. Since 36 and 37 were low in nitrogen, the Ms point was not lowered and the fatigue strength was poor. Comparative Example No. In No. 38, nitrogen was excessive, blowholes were generated, and the fatigue strength was low starting from that.

  Comparative Example No. In Nos. 39 and 40, Ti and Al were excessive, so that the droplets became large and the arc stability was deteriorated. Furthermore, the amount of slag was large and the electrodeposition coating property was also poor. Comparative Example No. No. 41 is a high Cr-high Ni-low C-low N system that has been conventionally proposed as a means for realizing a low Ms point. Since Ni and Ni + Cr are excessive, the cost is extremely high and humping is not possible in high-speed welding. Occurred. The droplets increased and spatter was generated. For the welding of a thin plate with a high base metal dilution rate, the decrease in the Ms point was insufficient, and since the N amount was low, the expansion after the Ms point was insufficient, and tensile residual stress was generated. As a result, fatigue strength was also poor. Comparative Example No. No. 42 has excessive Ni and excessive cost, and humping occurred in high-speed welding. The droplets increased and spatter was generated. Comparative Example No. For Ni 43, there is no problem with Ni alone, but Ni + Cr is excessive, the cost is excessive, and humping occurred during high-speed welding. The droplets became larger and more spatter was generated.

  Comparative Example No. 44, 45, and 46 are excessive in Nb, V, and Mo, respectively, and are also extremely expensive, and humping occurred in high-speed welding. The droplets became larger and more spatter was generated. Comparative Example No. In No. 47, Cu was excessive, and hot cracking occurred, which was the starting point and the fatigue strength was poor. The droplets became larger and more spatter was generated. Comparative Example No. In 48, B was excessive, and hot cracking occurred, which was the starting point, and the fatigue strength was poor. Comparative Example No. No. 49 has excessive REM, is very expensive, and many large spatters are generated. Comparative Example No. No. 50 had a low flux rate and could not contain the necessary amount of O or N, so the Ms point was not lowered and the fatigue strength was poor. Furthermore, since the flux component did not enter stably as a wire, the arc stability was also poor.

  Comparative Example No. No. 51 had a high flux rate, and breakage and flux spillage occurred frequently during the wire drawing process, making it difficult to manufacture as a wire. Comparative Example No. 52 is a very general flux-cored wire conforming to JIS Z3313 YFW-C50DM. Since there is no method for lowering the Ms point and it is not designed for high-speed welding of thin plates, the fatigue strength is poor, the amount of slag is large, the electrodeposition paintability is poor, and the arc in MIG welding There were a number of disadvantages, such as poor stability and humping with high speed welding. Comparative Example No. Reference numeral 53 denotes a real component of the specified component of the present invention. However, with a solid wire, it was difficult to increase O and N, the arc stability was poor, the Ms point was not lowered, and the fatigue strength was also poor. The bead shape was also slightly inferior to the toe shape because the arc did not spread compared to the flux-cored wire. This is also considered to help the fatigue strength not increase. Furthermore, when a component system with high hardenability that realizes a low Ms point with a solid wire is manufactured, it is necessary to reduce the wire strength by annealing several times during the wire drawing in order to ensure the wire drawing property. For this reason, productivity was very bad and the cost became very high.

  Comparative Example No. No. 54 was insufficient in Ni + Cr, the Ms point was lowered and the subsequent expansion transformation was insufficient, and the fatigue strength was low. Comparative Example No. 55 is within the range as Ni + Cr, but since Ni was not added, the Ms point was lowered and the subsequent expansion transformation was insufficient, and the fatigue strength was low. Comparative Example No. Similarly, Ni + Cr 56 was within the specified range, but Ni was added, but it was insufficient. Therefore, the Ms point was lowered and the subsequent expansion transformation was insufficient, and the fatigue strength was low. Comparative Example No. Nos. 57 and 58 had excessive amounts of Mg and F, respectively, had poor arc stability, and generated a large amount of spatter and fumes. Moreover, the viscosity of the molten pool was excessive and humping occurred. Comparative Example No. In 59, K + Na + Li was excessive, the arc force became too small, the molten pool became unstable, and humping occurred. Comparative Example No. No. 60 had excessive Ca, poor arc stability, and a large amount of spatter and fumes. The viscosity of the molten pool was excessive and humping occurred.

It is the relationship between the temperature and elongation of the weld metal. This is the groove condition of the welding test. The shape of the fatigue test piece.

Explanation of symbols

1: Steel plate 2: Wire 3: Welded part

Claims (5)

In a flux-cored wire for arc welding formed by filling a steel outer shell with a flux, the composition of the entire wire is C: 0.02 to 0.70 mass%, Si: 0.30 to 1.50% by mass, Mn: 0.50 to 5.00% by mass, Ni: 2.0 to 9.5% by mass, and Cr: Ni in a total amount of 4.0 to 18.0% by mass (however, (Including the case of not containing Cr), O: 0.020% by mass or more and N: 0.0020 to 0.0400% by mass, P: 0.030% by mass or less, S: 0.030% by mass or less Ti: 0.15 mass% or less, Al: 0.20 mass% or less, at least one selected from the group consisting of Nb, V, Mo and Cu: less than 2.00 mass% for each element, B: 0 0.0100% by mass or less, REM (rare earth element): 0.50 % At most, Mg: 1.00% by mass or less, at least one selected from the group consisting of F and Ca: at least 0.100% by mass for each element, at least selected from the group consisting of K, Na and Li one: regulated to 0.200 wt% or less in total, the balance being Fe and unavoidable impurities, and the flux rate using a gas shielded arc welding flux cored wire Ru 7 to 30% by mass, Ar 96 A MIG arc welding method comprising performing MIG welding using a mixed gas of at least mass% and the balance of CO ₂ or O ₂ as a shielding gas . The component composition of the entire wire is at least one selected from the group consisting of Nb, V, Mo and Cu with respect to the total mass of the wire: 0.05% by mass or more and less than 2.00% by mass for each element, B: 0.0010 to 0.0100 mass%, REM (rare earth element): 0.01 to 0.50 mass%, Mg: 0.05 to 1.00 mass%, selected from the group consisting of F and Ca At least one type: 0.005 to 0.100% by mass for each element, or at least one type selected from the group consisting of K, Na and Li: 0.001 to 0.200% by mass in total The MIG arc welding method according to claim 1. In a flux-cored wire for arc welding formed by filling a steel outer shell with a flux, the composition of the entire wire is C: 0.02 to 0.70 mass%, Si: 0.30 to 1.50% by mass, Mn: 0.50 to 5.00% by mass, Ni: 2.0 to 9.5% by mass, and Cr: Ni in a total amount of 4.0 to 18.0% by mass (however, (Including the case of not containing Cr), O: 0.020% by mass or more and N: 0.0020 to 0.0400% by mass, P: 0.030% by mass or less, S: 0.030% by mass or less Ti: 0.15 mass% or less, Al: 0.20 mass% or less, at least one selected from the group consisting of Nb, V, Mo and Cu: less than 2.00 mass% for each element, B: 0 0.0100% by mass or less, REM (rare earth element): 0.50 % At most, Mg: 1.00% by mass or less, at least one selected from the group consisting of F and Ca: at least 0.100% by mass for each element, at least selected from the group consisting of K, Na and Li Type 1: The total amount is regulated to 0.200% by mass or less, the balance is made of Fe and inevitable impurities, and the flux-cored wire with a flux rate of 7 to 30% by mass is used. A MIG arc welding method, comprising performing MIG welding using Ar gas as a shielding gas . The component composition of the entire wire is at least one selected from the group consisting of Nb, V, Mo and Cu with respect to the total mass of the wire: 0.05% by mass or more and less than 2.00% by mass for each element, B: 0.0010 to 0.0100 mass%, REM (rare earth element): 0.01 to 0.50 mass%, Mg: 0.05 to 1.00 mass%, selected from the group consisting of F and Ca At least one type: 0.005 to 0.100% by mass for each element, or at least one type selected from the group consisting of K, Na and Li: 0.001 to 0.200% by mass in total The MIG arc welding method according to claim 3 . MIG arc welding method according to claim 1 to 4, characterized in that the MIG welding using a pulsed arc welder.

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