JP2006035256A - Method for spot welding of high strength steel plate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a stable technique in the spot welding of high strength steel plates, which can manufacture a reliable joint having high fatigue strength and is suitable for a practical spot welding work. <P>SOLUTION: In the spot welding method of high strength steel plates having a yield stress of at least 270 MPa, and a plate thickness of 1.0 to 3.6 mm, a through-hole is formed at a joint portion from one side surface by plasma, and then the welding is performed by forming weld metal in the through-hole, wherein the weld metal has a yield stress of 270 MPa and a transformation starting temperature of 200 to 350°C in the case of the transformation from austenite structure to martensite or bainite structure. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a spot welding method widely applied in the field of automobiles and the like, and more particularly to a hot welding method for improving the fatigue strength of a high-strength steel spot welded joint for the purpose of reducing the weight of a vehicle body and improving collision safety. .

In recent years, the need for high-strength steel sheets as materials used in automobile bodies and parts has been increasing from the viewpoint of reducing fuel consumption, reducing CO ₂ emissions, and improving collision safety. It is considered a thing.

  On the other hand, a resistance spot welding method is widely used as a welding method used for assembling a vehicle body or attaching parts.

As shown in FIG. 1, resistance spot welding is performed, for example, by superposing high-strength steel plates 1, energizing the high-strength steel plates 1 with two water-cooled upper and lower copper electrodes 2 while pressing the high-strength steel plates 1. This is a welding method in which the contact portion (overlapping surface) is melted, and after energization, the portion is cooled with a water-cooled copper electrode to solidify the molten portion and form the nugget 3.

As a quality index of resistance spot welds (welded joints), fatigue strength is important as well as static tensile strength.
Usually, the static tensile strength of resistance spot welded joints increases with increasing tensile strength of the steel sheet. On the other hand, it is known that the fatigue strength of the resistance spot welded joint hardly increases even if the tensile strength of the steel sheet increases.
For example, when a high strength steel plate with a tensile strength of 590 MPa is used instead of a mild steel plate with a tensile strength of 290 MPa, the tensile shear strength of the resistance spot welded joint (the tensile strength when a load is applied in the shear direction) (Strength) increases to almost double the value. In contrast, the tensile shear fatigue strength of a resistance spot welded joint under the same tensile strength increase condition of the same steel sheet (fatigue strength when a repeated load is applied in the shear direction, that is, the number of stress loads is a constant value. (Load) does not increase and shows almost the same value as in the case of a mild steel plate.

Thus, as a reason why the fatigue strength of the resistance spot welded joint is not improved even if the tensile strength of the steel sheet is increased, it has been reported that the notch shape of the resistance spot welded portion causes a decrease in the fatigue strength. That is, as shown in FIG. 2, in the resistance spot welded joint, both end portions of the nugget 3 formed on the overlapping surface of the high-strength steel plate 1 are likely to have a notch shape. For this reason, when a fatigue test is performed with a load applied in the tensile shear direction (arrow direction) 4 of the resistance spot welded joint, the fatigue strength is reduced by this notch effect, and the fatigue strength is increased even if the tensile strength of the steel sheet is increased. Does not improve.

  In particular, when scattering occurs during resistance spot welding (a phenomenon in which molten metal scatters from between steel plates when the diameter of the melted part formed between the steel plates becomes larger than the tip diameter of the copper electrode during energization) Since the end portion of the nugget formed therebetween has a sharp notch shape, as a result, the fatigue strength of the joint may be further reduced as compared with the case where no scattering occurs.

Moreover, generally, when the tensile strength of a steel plate increases, the value of the carbon equivalent defined by the following formulas (1) and (2) also increases.
Ceqh = C + Si / 40 + Cr / 20 (1)
Ceqt = C + Si / 30 + Mn / 20 + 2P + 4S (2)
Here, C, Si, Cr, Mn, P and S indicate the contents (mass%) of carbon, silicon, chromium, manganese, phosphorus and sulfur in the steel sheet, respectively.
Ceqh shown by the above formula (1) is a carbon equivalent corresponding to the hardness of the nugget part, and the hardness of the nugget part increases as this value increases.
Moreover, Ceqt shown by said Formula (2) is a carbon equivalent corresponding to the crack generation sensitivity of a nugget part, and the crack generation sensitivity of a nugget part increases, so that this value increases.

In general, the carbon equivalents Ceqh and Ceqt expressed by the above formulas (1) and (2) increase as the tensile strength of the steel sheet increases in the resistance spot welded joint. For this reason, as the tensile strength of the steel sheet increases, the hardness of the nugget part in the resistance spot welded joint increases, and the cracking susceptibility increases, so that the crack is easily generated in the nugget part during the fatigue test. Become.
In a resistance spot welded joint using a high-strength steel plate, the fatigue strength is difficult to improve because of the influence of the hardness of the nugget part and cracking susceptibility in combination with the above-described notch effect.

On the other hand, when a fatigue test is performed with a load applied in the peeling direction of the resistance spot welded joint (a direction perpendicular to the tensile shear direction (arrow direction) 4), the high strength steel plate is the same as when the load is applied in the shear direction. The fatigue strength of the welded joint shows the same value as that of the mild steel plate. In this case as well, as in the case of loading in the shear direction, the notch effect causes a decrease in the fatigue strength of the resistance spot welded joint. However, in this case, compared to when a load is repeatedly applied in the shear direction of a resistance spot welded joint, the stress concentration at the periphery of the nugget becomes more prominent, and the local stress load increases and cracks occur. Therefore, the fatigue strength is reduced by about an order of magnitude compared to when the load is applied in the shear direction.

  In addition, the increase in the tensile strength of the steel plate in a resistance spot welded joint makes it difficult for deformation to occur.Therefore, the amount of deformation during the fatigue test decreases, and stress concentration increases around the nugget. descend. Furthermore, since high-strength steel sheets are more likely to spring back than mild steel, tensile residual stress is likely to occur in spot welds, which is a cause of a decrease in fatigue strength of spot welded joints.

As described above, in the conventional resistance spot welding method, even if the tensile strength of the steel sheet is increased, the fatigue strength of the welded joint is only about the same as when using a mild steel sheet.
Thus, methods for improving the fatigue strength of resistance spot welded joints have been studied.

Conventionally, as a means to improve the fatigue strength of resistance spot welded joints of high strength steel plates, after energization of resistance spot welding is completed, temper energization is performed after a certain period of time, and the nugget part and heat affected zone in the resistance spot welded part are A method is known in which the hardness is reduced by annealing and the residual stress is changed (see Non-Patent Document 1).

  However, this method has a practical problem that the appropriate condition range of the temper energization is very narrow, and the reproducibility is poor due to a change in operating conditions. In particular, when spot-welding is performed by continuously striking plated steel sheets, the electrode tip deteriorates due to an alloying reaction with plating as the number of striking points increases, and the electrode tip diameter increases and the current density decreases. Therefore, it is difficult to stably improve the fatigue strength of the joint because it deviates from the optimum temper energization condition.

In addition to this, as disclosed in Patent Documents 1 to 6 below, a method of resistance spot welding using a steel plate having excellent fatigue strength characteristics is known as a method of improving the fatigue strength of a resistance spot weld. ing.
However, all of these methods relate to resistance spot welding of mild steel plates, and there are no reported examples of methods for improving fatigue strength at resistance spot welds of high strength steel plates.

In addition to this, as disclosed in Patent Document 7, as a method for improving the fatigue strength of the resistance spot welded portion, a method of applying an ultrasonic impact treatment to the resistance spot welded portion is known.
However, this method is not a preferable method in terms of workability and economy because it requires a post-treatment process after the end of welding, which increases the number of work steps and the economic load.

Conventionally, a method of increasing the number of resistance spot welding points (number of nuggets) in order to improve the fatigue strength of the welded joint is also known. However, this method has problems such as a decrease in welding work efficiency, an increase in welding construction cost, and a restriction on design freedom. Moreover, this method aims at reducing the stress concentration of the nugget periphery part per joint in a joint by increasing the number of resistance spot welding points (number of nuggets). However, when stress is applied to the joint, the stress is not necessarily applied evenly to each welding point (nugget), so that the stress dispersion effect is not sufficiently exhibited, and the stress concentrates on one of the welding points. As a result, even if the number of welding points is increased from 1 point to 2 points or 3 points, for example, the fatigue strength of the joint is not necessarily doubled or tripled.

On the other hand, as a spot welding method different from the resistance spot welding method, a spot welding method using plasma or a plasma torch is known from, for example, Non-Patent Document 2, Patent Documents 8 and 9, and the like. However, these methods are intended for improving the welding workability in a method of performing single-sided welding without using a welding material, targeting mild steel plates or steel plates having relatively low tensile strength.
In the case of welding high-strength steel plates using these spot welding methods using plasma, there are problems such as a reduction in welding work efficiency, an increase in welding construction cost, and a restriction in design freedom. In addition, by using this method, compared with the resistance spot welding method, the component composition of the weld metal is adjusted by the welding material, and the effects of increased hardness of the weld metal and cracking susceptibility associated with higher strength of the steel sheet are reduced. However, these documents do not disclose any welding materials. Furthermore, it cannot solve the problem of joint fatigue strength reduction caused by the stress concentration in the notch shape at the welded portion end formed on the steel plate overlapping surface unique to the spot welded joint and the tensile residual stress.

Further, as a method for improving the fatigue strength of fillet welded joints produced by arc welding of steel sheets, as disclosed in, for example, Patent Documents 10 and 11, the low temperature of weld metal using a low temperature transformation welding material is used. There is known a method for reducing the tensile residual stress near the welded portion of the joint, particularly in the vicinity of the bead toe, using the transformation expansion on the side.
In this method, in the cooling process of the weld metal after solidification, the temperature at which transformation starts from austenite to martensite is lowered, volume expansion associated with phase transformation is generated at low temperature, and the amount of heat shrinkage after transformation is reduced. This is a method of reducing the tensile residual stress in the vicinity of the bead toe of the fillet welded joint and improving the fatigue strength of the joint. However, this method is a fatigue improving method for fillet welded joints in which the steel plates are overlapped to arc weld the corners of the lower steel plate surface and the upper steel plate end surface, and is not a method for improving the fatigue strength of spot welded joints. In other words, this method cannot solve the problem of joint fatigue strength reduction due to stress concentration at the notch shape at the weld end and the tensile residual stress formed on the steel plate overlap surface unique to spot welded joints. Absent.

Japanese Unexamined Patent Publication No. Sho 63-317625 JP-A-2-163323 JP-A-5-263184 JP-A-9-268346 Japanese Patent Laid-Open No. 10-8187 Japanese Patent Laid-Open No. 11-279589 JP 2004-122152 A JP 60-68156 A Japanese Patent Application Laid-Open No. 07-303971 JP 2000-17380 JP2002-239722 "Iron and Steel" Vol. 68 (1982) No. 9, pp. 1444-1451 Welding Technology January 2002, pages 78-83

As mentioned above, conventionally, the fatigue strength of joints spot welded with high strength steel plates is not different from the fatigue strength of joints spot welded with mild steel plates, so even if high strength steel plates are used in the automotive field, It was not possible to fully enjoy the benefits of improving safety, reducing fuel consumption by reducing weight, and reducing CO ₂ emissions.

  In the present invention, in order to solve these problems in the prior art, a spot welding method capable of improving the fatigue strength of a welded joint while ensuring good welding workability in spot welding of a high-strength steel sheet. The purpose is to provide.

The present inventors paid attention to the fact that the fatigue strength of spot welded joints depends on the residual stress state around the nugget, and in order to improve the fatigue strength of the spot welded joint, means for improving the residual stress state around the nugget We studied diligently.

As a result, after forming a through-hole at the joining position by plasma from one side of a high-strength steel plate having a yield stress of 270 MPa or more and a plate thickness of 1.0 to 3.6 mm, the yield stress is 270 MPa in the through-hole. As described above, it has been found that the fatigue strength in a welded joint of a high-strength steel sheet can be effectively improved by forming a weld metal whose temperature at which transformation starts from austenite to martensite or bainite is 200 to 350 ° C. .

The present invention is based on the results obtained as a result of extensive studies by the inventors in order to solve the above-mentioned problems, and the gist thereof is as follows.
(1) In a spot welding method of a high strength steel plate having a yield stress of 270 MPa or more and a plate thickness of 1.0 to 3.6 mm, after a through hole is formed at a joining position by plasma from one side, The yield stress is 270 MPa or more, the temperature at which transformation starts from austenite to martensite or bainite is 200 to 350 ° C.
C: 0.01-0.2%
Si: 0.1 to 0.5%,
Mn: 0.01 to 1.5%,
P: 0.03% or less,
S: 0.02% or less,
A spot welding method for a high-strength steel sheet, characterized by welding by forming a weld metal containing Ni: 8 to 12% and the balance being iron or inevitable impurities.
(2) The weld metal is in mass%, and
Nb: 0.01 to 0.4%,
Ti: 0.01 to 0.4%,
V: 0.1 to 1.0%
Cr: 0.1 to 3.0%
Mo: 0.1 to 3.0%,
Co: 0.1-2.0% of 1 type or 2 types or more are contained, The spot welding method of the high strength steel plate of said (1) description characterized by the above-mentioned.
(3) The weld metal is in% by mass,
Cu: 0.05-0.4% is contained, The spot welding method of the high strength steel plate of the said (1) or (2) description characterized by the above-mentioned.
(4) In a spot welding method of a high strength steel plate having a yield stress of 270 MPa or more and a plate thickness of 1.0 to 3.6 mm, after a through hole is formed at a joining position by plasma from one side, The yield stress is 270 MPa or more, the temperature at which transformation starts from austenite to martensite or bainite is 200 to 350 ° C.
C: 0.001 to 0.05%,
Si: 0.1 to 0.7%,
Mn: 0.4 to 2.5%
P: 0.03% or less,
S: 0.02% or less,
Ni: 4-8%,
Cr: 10 to 15%,
N: 0.001 to 0.05% is contained, the total amount of C and N is 0.001 to 0.06%, and the remainder is welded by forming a weld metal made of iron or inevitable impurities. A spot welding method for high-strength steel sheets characterized by
(5) The weld metal is in% by mass, and
Nb: 0.005-0.3%
Ti: 0.005 to 0.3%,
V: 0.05-0.5%
Mo: 0.1 to 2.0%,
Co: 0.1-2.0% of 1 type or 2 types or more is contained, The spot welding method of the high strength steel plate of the said (4) description characterized by the above-mentioned.
(6) The weld metal is in% by mass, and
Cu: 0.05-0.4% is contained, The spot welding method of the high strength steel plate of the said (4) or (5) description characterized by the above-mentioned.
(7) In a spot welding method of a high strength steel plate having a yield stress of 270 MPa or more and a plate thickness of 1.0 to 3.6 mm, after a through hole is formed at a joining position by plasma from one side, The yield stress is 270 MPa or more, the temperature at which transformation starts from austenite to martensite or bainite is 200 to 350 ° C.
C: 0.35-0.70%,
Si: 0.1 to 0.8%,
Mn: 0.4 to 2.0%,
P: 0.03% or less,
S: A spot-welding method for high-strength steel sheets, comprising welding by forming a weld metal containing 0.02% or less and the balance being iron or inevitable impurities.
(8) The weld metal is in% by mass, and further 0.001 to 2.0% in total of one or more of Ni, Cr, Nb, Ti, V, Mo, and Co. The spot welding method for a high-strength steel sheet according to the above (7), comprising:
(9) The weld metal is in% by mass, and
Cu: 0.05-0.4% is contained, The spot welding method of the high strength steel plate of the said (7) or (8) description characterized by the above-mentioned.

ADVANTAGE OF THE INVENTION According to this invention, the fatigue strength of a welded joint can be improved, ensuring favorable workability | operativity in the spot welding of the high-strength steel plate used for the assembly of the vehicle body in the automotive field | area etc., or attachment of the components for motor vehicles. Therefore, this makes it possible to fully enjoy the benefits of improving safety by applying high-strength steel sheets, reducing fuel costs by reducing weight, and reducing CO ₂ emissions in the automotive field and the like.

The present invention will be described in detail below.
FIG. 3 shows an example of an embodiment of the present invention.
In the spot welding method for high-strength steel sheets according to the present invention, after the high-strength steel sheets 1 are overlapped with each other, the plasma 6 generated from the plasma torch 5 installed on one side of these is blown to the joining position to form the through holes 7. (See FIG. 3 (a)). Subsequently, molten metal is formed in the through-hole 7 while supplying the melt 8 into the plasma 6 (see FIG. 3 (b)), and then the weld metal is solidified. The weld metal part 9 is formed (see FIG. 3C). In normal spot welding, if there is a gap between the steel plates, the pressurizing force must be set higher to fill the gap, but if the method of the present invention is used, even if there is a gap between the steel plates. Since the melted molten material bridges and joins the steel plates (see FIG. 3D), it can be easily joined.

In addition to plasma, methods such as machining and laser are conceivable as a method for opening a through hole in a joint, but it is possible to open a sufficiently large hole efficiently in a short time, and Since plasma is considered to be the most suitable method for filling the hole by melting the molten material immediately after the drilling step, the present invention employs plasma.

In the present invention, in the above embodiment, a high-strength steel plate having a yield stress of 270 MPa or more and a plate thickness of 1.0 to 3.6 mm is used for reasons described later. After forming, a weld metal having a yield stress of 270 MPa or more and a temperature at which transformation starts from austenite to martensite or bainite is 200 to 350 ° C. is formed in the through hole 7.

,
First, the technical idea of the present invention, that is, a method for reducing the residual stress in the spot welded portion and its surroundings for improving the fatigue strength of the spot welded joint will be described.
In the welding of steel plates, the process of generating residual stress in the welded portion is roughly as follows. After welding, when the weld metal is solidified and cooled and the temperature of the weld metal reaches the transformation start temperature, the weld metal transforms from austenite to martensite or bainite and the volume expands. At this time, since the weld metal is restrained from the base material portion including the surrounding heat-affected zone, compressive residual stress is generated in the vicinity of the weld metal. However, the introduced compressive residual stress returns to the tensile stress state at room temperature if the thermal shrinkage is large in the process of further cooling the weld metal after transformation. In conventional resistance spot welding, since no welding material is used, the transformation start temperature of the weld metal is determined by the composition of the steel sheet, and when welding a normal steel sheet except for a part of stainless steel, Since the temperature at which transformation starts is high, compressive residual stress is once introduced in the vicinity of the weld toe due to transformation expansion of the weld metal. It becomes stress.
As a result, particularly in the case of resistance spot welding of high-strength steel sheets, in addition to the influence of the notch shape at the end of the nugget described above, the tensile residual stress at the end of the weld metal reduces the fatigue strength of the spot welded joint. Cause.
Residual stress conditions that occur in the vicinity of the spot weld are the compressive stress generated by the restraint of the base metal part including the surrounding heat-affected zone when the weld metal undergoes transformation expansion, and the surrounding base metal part during heat shrinkage after transformation of the weld metal. This is considered to be determined by the magnitude relationship with the tensile stress generated by the restraint.

From the above, as a method of reducing the tensile residual stress in the vicinity of the weld toe of the spot welded joint, a method of reducing the tensile stress accompanying thermal contraction after transformation expansion and a method of increasing the compressive stress during transformation expansion are considered. It is done.
It is considered that the thermal shrinkage is obtained by multiplying the temperature difference from after the transformation of the weld metal to room temperature by the thermal expansion coefficient. Therefore, if this temperature change is reduced, the tensile residual stress accompanying the thermal shrinkage can be reduced.
As a specific means, a spot welding method using plasma is applied, and a welding material that has been studied for fillet welding by arc welding and that can control the transformation start temperature of the weld metal to be low is used. It is possible to apply a method in which the transformation expansion is performed at a lower temperature and the amount of heat shrinkage from the end of transformation to room temperature is reduced.
However, the transformation start temperature of the weld metal must be lowered to 200 ° C. or lower in order to bring the weld end to a compressive residual stress state only by this means. In addition, for that purpose, the alloy content is inevitably increased, and as a result, there is a risk of increasing the cost of the welding material, lowering the workability of welding, deterioration of mechanical properties such as toughness of the weld metal, etc., which is not preferable. .
Therefore, the present invention examined specific means for generating sufficient compressive residual stress at the weld toe using the transformation expansion of the weld metal even under conditions where the transformation start temperature of the weld metal is 200 ° C. or higher. .
The compressive residual stress introduced in the vicinity of the weld end in the process of transformation expansion of the weld metal is the elastic strain (yield stress: σy, Young's modulus) of the weld metal in this transformation temperature region and the base metal part constraining it from the surroundings. : E, the elastic strain is limited to σy / E). In other words, even if the stress increases beyond the elastic strain due to transformation expansion, the weld metal and the surrounding base metal part yield and only undergo plastic deformation, increasing the compressive residual stress at the weld toe. Must not.
Therefore, in the present invention, by securing the yield stress of the steel plate and the weld metal and the plate thickness of the steel plate to a predetermined value or more, the surrounding restraint force is increased during the transformation expansion of the weld metal, and the compressive residual stress is increased. The technical idea is to generate sufficient compressive residual stress at the weld toe even when the transformation start temperature of the weld metal is 200 ° C. or higher.
In addition, the present invention is intended for automotive steel sheets having a relatively thin plate thickness. However, as the plate thickness increases, after the transformation expansion of the weld metal is completed, the surrounding restraint force in the thermal contraction to room temperature is received and tensile stress is applied. Therefore, it is also a technical idea to limit the upper limit of the steel plate thickness.

The present invention has been made on the basis of the above technical idea and knowledge. As a means for improving the fatigue strength of a spot welded joint, a spot welding method using plasma is used, and further, martensite or bainite of a weld metal is used. The transformation start temperature, the yield stress of the high strength steel plate and the weld metal, and the thickness of the high strength steel plate are optimized.
The transformation start temperature of martensite or bainite of the weld metal and the yield stress are determined by the composition of the weld metal.
In the present invention, as a component system for reducing the transformation start temperature of the weld metal depending on the mechanical properties other than the fatigue strength of the spot welded joint and the application, the Ni-based material containing Ni as the main component, Ni and Cr as the main components. The weld metal of the three main component system of Ni-Cr type | system | group and C type | system | group which made C the main component was applied.
The C-based weld metal can be expected to have an economic effect in that it can reduce expensive alloy elements as compared with Ni-based and Ni-Cr-based weld metals. In the case of a C-based weld metal, high temperature solidification cracking of the weld metal is likely to occur with an increase in C. However, in the present invention, the upper limit of the plate thickness is limited to be low, and this problem can be avoided.

Below, the transformation start temperature of the weld metal specified in the present invention, the yield strength of the weld metal and the steel plate, the plate thickness of the steel plate, and the reasons for limiting the component composition of each of the Ni-based, Ni-Cr-based, and C-based weld metals explain.

(Weld metal transformation start temperature: 200-350 ° C.)
In the present invention, after the weld metal is solidified after spot welding, compressive residual stress is introduced in the vicinity of the weld metal toe using the volume expansion of the weld metal when it is cooled and phase transformed from austenite to martensite or bainite. There is a need.
The martensite or bainite transformation start temperature of the weld metal is 500 ° C. or less, and most is 450 ° C. or less for ordinary steel plates and weld metals.
In order to introduce compressive residual stress in the vicinity of the weld metal end and maintain the compressive residual stress to room temperature, it is preferable to lower the transformation start temperature of the weld metal and to transform and expand the weld metal at a lower temperature side. However, excessively increasing the amount of alloy elements such as Ni and Cr in order to reduce the transformation start temperature of the weld metal increases the cost of the weld material, decreases the welding workability, and reduces the weld metal mechanical properties such as toughness. Since it causes deterioration, it is not preferable.
Therefore, in the present invention, any welding metal of Ni-type, Ni-Cr-type, and C-type can be realized with a welding material having industrial value, and the mechanical properties of the weld metal such as welding workability and toughness. In order to maintain a good temperature, the lower limit of the transformation start temperature was specified at 200 ° C. for martensite or bainite.

On the other hand, the upper limit of the transformation start temperature of the martensite or bainite of the weld metal is any of the Ni-based, Ni-Cr-based, and C-based weld metals. Even under the conditions of the yield stress and thickness of the weld metal and steel plate, the effect of tensile residual stress due to thermal shrinkage to room temperature after transformation expansion increases, so a sufficient compressive residual stress is introduced near the weld toe. Thus, the state cannot be maintained, and the fatigue strength of the spot welded joint cannot be sufficiently improved.
Therefore, the upper limit of the transformation start temperature of martensite or bainite is defined as 350 ° C. for all of the Ni-based, Ni-Cr-based, and C-based weld metals. In order to introduce sufficient compressive residual stress in the vicinity of the weld toe, the upper limit of the transformation start temperature is preferably lower, and desirably 300 ° C. or lower.

Even if the upper limit of the transformation start temperature is higher than 350 ° C., if the yield stress of the weld metal and steel plate is sufficiently high, the surrounding restraint force increases during transformation expansion of the weld metal and a large compressive residual stress is applied to the toe. Can be introduced. However, excessive yield stress enhancement of the weld metal and steel sheet can also reduce manufacturing costs and mechanical properties. The upper limit of the transformation start temperature of the present invention is to improve the fatigue strength of the target spot welded joint under the conditions of the yield stress of the weld metal and steel sheet of the present invention using the industrial steel sheet and welding material described later. Therefore, the temperature was set to 350 ° C.

(Yield stress of steel plate and weld metal: 270 MPa or more)
In the present invention, after the weld metal is solidified after spot welding, sufficiently compressive residual stress is introduced near the end of the weld metal by utilizing the volume expansion of the weld metal when cooled and phase transformed from austenite to martensite or bainite. In order to do this, it is necessary to ensure the yield stress of the weld metal and the steel plate at a predetermined level or more in addition to the regulation of the transformation start temperature of the weld metal.
The compressive residual stress introduced in the vicinity of the weld toe when the weld metal undergoes transformation expansion is the compression elastic strain limit (yield stress: σy) of the weld metal in the transformation temperature region and the base metal part constraining it from the surroundings. When Young's modulus is E, the compression elastic strain limit is limited to σy / E). In other words, even if the stress increases beyond the compression elastic strain limit due to transformation expansion, the weld metal and the surrounding base metal part yield and only plastically deform, increasing the compressive residual stress at the weld end. It will not be. Therefore, in order to sufficiently introduce compressive residual stress in the vicinity of the weld metal end portion and to improve the joint fatigue strength intended by the present invention more reliably, the yield stress of the weld metal and the steel sheet is set to a predetermined value or more to compressive elastic strain. It is important to increase the limits.
In any case of Ni-based, Ni-Cr-based, and C-based weld metals, when the yield stress of the weld metal and the steel sheet is less than 270 MPa, the weld metal is transformed when the weld metal undergoes phase transformation from austenite to martensite or bainite. It is difficult to sufficiently restrain the volume expansion from the surroundings and to introduce a sufficient compressive residual stress in the vicinity of the weld metal end portion. As a result, tensile residual stress is introduced by heat shrinkage after the end of transformation, the compressive residual stress state in the vicinity of the weld end can be sufficiently maintained, and the fatigue strength of the spot welded joint cannot be sufficiently improved. Therefore, the lower limit of the yield stress of the weld metal and the steel plate is defined as 270 MPa in any case of Ni-based, Ni-Cr-based and C-based weld metals.

On the other hand, the upper limit of the yield stress of the weld metal and steel plate is not particularly specified, but excessive increase of the yield stress increases the manufacturing cost of welding material and steel plate to realize this, welding workability and weld metal. This is not preferable because it causes a decrease in mechanical properties such as toughness. C weld metal is superior to Ni and Ni-Cr weld metal in terms of the cost of the welding material, but excessive increase in yield stress increases the carbon equivalent and causes hot cracks and cracks in the weld metal. This is not preferable because it increases the development sensitivity.
From these points, it is preferable that the upper limit of the yield stress of the weld metal and the steel plate is 1300 MPa in any of the Ni-based, Ni-Cr-based and C-based weld metals.

(Thickness: 1.0 to 3.6 mm)
In the present invention, in order to sufficiently introduce compressive residual stress in the vicinity of the weld metal end portion at the time of transformation expansion of the weld metal, in addition to the above specifications for the transformation start temperature of the weld metal, the yield stress of the weld metal and the steel plate, It is necessary to secure a plate thickness of a predetermined value or more.
When the thickness of the steel sheet is less than 1.0 mm, the surrounding steel sheet is plastically deformed during transformation expansion of the weld metal, and sufficient restraint force does not work, and sufficient compressive residual stress is applied in the vicinity of the weld metal toe. Since it cannot be introduced, the lower limit of the thickness of the steel sheet was set to 1.0 mm.
On the other hand, in the present invention, a relatively thin plate is used for an automobile steel plate. However, if the plate thickness becomes excessively thick, heat transfer to the surrounding base material portion of the welding heat input during spot welding by plasma of the present invention. , Which causes the weld metal to increase tensile stress during the heat shrinkage process after transformation. Further, when the plate thickness of the steel plate exceeds 3.6 mm, it is difficult to form a through hole in the steel plate during spot welding by plasma of the present invention, so the upper limit of the plate thickness is defined to be 3.6 mm or less. .

In the case of C-based weld metal, the risk of high-temperature solidification cracking of the weld metal increases as the plate thickness of the steel plate increases. This is because the solidification temperature of the weld metal decreases due to an increase in C, and a low melting point portion is generated in the solidification process of the weld metal, which causes hot cracking.
In the present invention, by limiting the thickness of the steel sheet to 3.6 mm or less, in the case of C-based weld metal, the solidification of the weld metal is uniformly directed in the surface direction, thereby suppressing the butt solidification. Hot solidification cracking can be avoided.

  Next, the reason for limitation of the component composition in each of the Ni-based, Ni-Cr-based, and C-based weld metals applied in the present invention will be described.

(Component composition of Ni weld metal)
The reason for limiting the component range of the Ni-based weld metal will be described below.

C acts to lower the transformation start temperature of martensite or bainite by adding it to iron. However, on the other hand, excessive addition causes problems of weld metal toughness deterioration and weld metal cracking, so the upper limit of its content was made 0.2%. However, when C is not added, it is difficult to obtain martensite or bainite, and it is not economical because residual stress must be reduced only with other expensive elements. The reason for limiting the C content to 0.01% or more was to use C, which is an inexpensive element, and set it as the minimum value that would bring about its economic merit. The upper limit of the C content is preferably set to 0.15% from the viewpoint of weld metal cracking.

Si is known as a deoxidizing element. Si has the effect of lowering the oxygen level of the weld metal. Especially during welding, there is a risk of air being mixed in, so it is extremely important to control the Si content to an appropriate value. First, regarding the lower limit of the Si content, when the Si content to be added to the weld metal is less than 0.1%, the deoxidation effect is weakened, the oxygen level in the weld metal becomes too high, the mechanical properties, In particular, there is a risk of causing toughness degradation. Therefore, the lower limit of the weld metal is set to 0.1%. On the other hand, excessive Si addition also causes toughness deterioration, so the upper limit of its content was made 0.5%.
Mn is known as an element that increases the strength. Therefore, it is an element that should be used effectively from the viewpoint of securing the yield stress at the time of transformation expansion, which is a residual stress reduction mechanism in the present invention. The lower limit of 0.01% of the Mn content was set as the minimum value at which the effect of securing the strength was obtained. On the other hand, excessive addition causes toughness deterioration of the base metal and the weld metal, so the upper limit of its content was made 1.5%.

  P and S are impurities in the present invention. However, if these elements are present in large amounts in the weld metal, the toughness deteriorates, so the upper limit of their contents was set to 0.03% and 0.02%, respectively.

  Ni is a metal having a face-centered structure as a single element, and is an element that makes the austenite state more stable when added to an iron-based weld metal. Iron itself has an austenite or face-centered structure at high temperatures and a ferrite or body-centered structure at low temperatures. Ni, when added, makes the face-centered structure in the high-temperature region of iron more stable, and therefore acts as a face-centered structure even in a lower temperature range than in the case of no addition. This means that the temperature at which it transforms into a body-centered structure is lowered. 8% of the lower limit of the Ni content was determined in the sense of the minimum addition amount at which the residual stress reduction effect appears. The upper limit of 12% of Ni content is because the effect does not change so much from the viewpoint of reducing the residual stress, and if added more than this, there is an economic disadvantage that Ni is expensive. .

The above is the basic component of the Ni-based weld metal of the present invention. However, the following Nb, Ti, V, Cr, and Mo are further added for the following purposes within the range that does not impair the characteristics of the weld metal of the present invention. , And one or more of Co, and further, an appropriate amount of Cu can be added.

  Nb combines with C in the weld metal to form a carbide. Since Nb carbide works to increase the strength of the weld metal in a small amount, the economic merit of using it effectively is great. The merit is also great from the viewpoint of increasing the yield stress at the transformation start temperature, which is the technical idea of the present invention. However, on the other hand, excessive carbide formation causes deterioration of toughness, so the upper limit of the content is naturally set. As the lower limit value of the Nb content, 0.01% was set as the lowest value at which a carbide was formed and an effect of improving the strength could be expected. The upper limit of the content was set to 0.4% as a value that does not impair the reliability of the weld due to toughness deterioration.

Ti, like Nb, forms carbides and causes precipitation hardening. However, the precipitation hardening amount in Ti is different from that in Nb. Therefore, the range of Ti content is set to a range different from Nb. The lower limit of Ti content of 0.01% was determined as the minimum amount at which the effect can be expected, and the upper limit of 0.4% of content was determined in consideration of deterioration of toughness.

  V is an element that functions similarly to Nb and Ti. However, unlike Nb and Ti, it is necessary to increase the content in order to expect the same precipitation effect. The lower limit of 0.1% for the V content is set as the lowest value at which precipitation hardening can be expected by adding, but the lower limit for the preferred content is 0.3%. The upper limit of the V content is set to 1.0% in order to cause excessive precipitation hardening and cause toughness deterioration if added more than this.

  Cr is a precipitation hardening element like Nb, V, and Ti. Cr is an element that should be effectively used because it also has the effect of reducing the transformation start temperature of martensite or bainite. However, since the Ni-based weld metal in the present invention achieves the transformation start temperature reduction of martensite or bainite mainly by adding Ni, the Cr addition amount should be less than that of Ni. Excessive Cr addition does not necessarily improve the residual stress reduction effect and is not industrially preferable because Cr is expensive. The lower limit of 0.1% of Cr content was set as a minimum value at which the residual stress reduction effect can be obtained by adding this. The upper limit of the Cr content is 3.0% because, for Ni-based weld metals, the transformation start temperature of martensite or bainite has already been reduced by addition of Ni, and the strength is secured by other precipitation elements. Even if added more than this, the residual stress reduction effect does not change so much, and the toughness deterioration becomes remarkable.

  Mo is an element having the same effect as Cr. However, Mo is an element for which precipitation hardening can be expected more than Cr. Therefore, the addition range was set narrower than Cr. The lower limit of 0.1% was set as the minimum value at which the effect of Mo addition can be expected. The upper limit of 3.0% of the content is set because when it is added more than this, the toughness deteriorates remarkably because it hardens too much.

Unlike Ti and the like, Co is not an element that causes strong precipitation hardening. However, Co is a more preferable element than Ni from the viewpoint of adding strength to increase strength and securing toughness while expecting strength increase. However, since Ni is added to the weld metal in order to secure a transformation start temperature that is low enough to expect a residual stress reduction effect, the lower limit of Co content of 0.1% can be expected to have the effect of Co addition. It was set as the minimum value. On the other hand, excessive addition causes an increase in strength and causes toughness deterioration, so the upper limit of its content was made 2.0%.

Cu is an effective element for improving welding workability because it has the effect of improving conductivity by plating on the welding wire. Moreover, since Cu is also an element which improves hardenability, the effect of promoting the transformation of martensite or bainite can be expected by adding to the weld metal. Therefore, Cu may be contained in the weld metal for these purposes. The lower limit of the Cu content is 0.05% as the minimum value necessary for improving workability and promoting the transformation of martensite or bainite. However, excessive addition of Cu not only has an effect of improving workability, but also increases the manufacturing cost of the wire, which is not preferable in the industry. Therefore, the upper limit of the content is 0.4%.

(Component composition of Ni-Cr weld metal)
The reason for limiting the component range of the Ni—Cr weld metal will be described below.

  C acts to lower the transformation start temperature of martensite or bainite by adding it to iron. However, on the other hand, excessive addition causes problems of weld cracking and toughness deterioration, so the upper limit of the content was set to 0.05%. However, when C is not added, it is difficult to obtain martensite or bainite, and it is not economical because residual stress must be reduced only with other expensive elements. The reason for limiting the C content to 0.001% or more was to use C, which is an inexpensive element, and set it as the minimum value that would bring about its economic merit.

  Si is known as a deoxidizing element. Si has the effect of lowering the oxygen level of the weld metal. In particular, in welding work, there is a risk of air being mixed during welding, so it is extremely important to control the Si content to an appropriate value. First, regarding the lower limit of Si, when the Si content to be added to the weld metal is less than 0.1%, the deoxidation effect is weakened, the oxygen level in the weld metal becomes too high, and mechanical properties, particularly toughness. There is a risk of causing deterioration. Therefore, the lower limit of the content of the weld metal is set to 0.1%. On the other hand, excessive Si addition also causes toughness deterioration, so the upper limit of its content was made 0.7%.

  Mn is known as an element that increases the strength. Therefore, it is an element that should be used effectively from the viewpoint of securing the yield stress at the time of transformation expansion, which is the technical idea of the present invention. The lower limit of 0.4% of the Mn content was set as the minimum value at which the effect of securing the strength was obtained. On the other hand, excessive addition causes toughness deterioration of the weld metal, so the upper limit of its content was set to 2.5%.

  P and S are impurities in the present invention. However, if these elements are present in large amounts in the weld metal, the toughness deteriorates, so the upper limit of their contents was set to 0.03% and 0.02%, respectively.

  Ni is a metal having a face-centered structure by itself. Iron itself has an austenite or face-centered structure at high temperatures and a ferrite or body-centered structure at low temperatures. Ni, when added, makes the face-centered structure in the high-temperature region of iron more stable, and therefore acts to form a face-centered structure even in a lower temperature range than in the case of no addition. This means that the temperature at which it transforms into a body-centered structure is lowered. Moreover, Ni has the effect of improving the toughness of the weld metal by adding it. The lower limit of 4% of the Ni content in the Ni and Cr-based weld metals was determined from the viewpoint of ensuring the minimum addition amount and toughness in which the residual stress reduction effect appears. The upper limit of the Ni content is 8% in the case of Ni and Cr-based weld metals. From the viewpoint of reducing the martensite or bainite transformation temperature to some extent by the Cr content described below and reducing the residual stress. This value was set because the effect did not change much even when added, and the economic disadvantage of Ni being expensive would occur if added more than this.

Unlike Ni, Cr is a ferrite forming element. However, when Cr is added to iron, it is ferrite in the high temperature range, but forms austenite in the medium temperature range, and forms ferrite again when the temperature is further lowered. In the welded part, ferrite on the low temperature side is not formed by the heat history due to the welding heat input, but martensite or bainite is formed. This is because hardenability is increased by adding Cr. That is, the transformation of martensite or bainite by addition of Cr means that the ferrite transformation does not occur due to the increase in hardenability and that the transformation start temperature itself of martensite or bainite is lowered. . The lower limit of 10% of the content was set as the Cr-containing range that effectively utilizes transformation expansion for reducing the residual stress while satisfying both of these effects. The upper limit of 15% is set to this value because the effect is not increased even if an amount exceeding this is added, and the disadvantage is increased economically.

  N is an element known as an austenite forming element. Since it becomes easy to obtain martensite or bainite by adding N, the minimum addition is necessary. The lower limit of 0.001% of the N content was set as the lowest value for obtaining a low transformation start temperature, as in the case of C. However, excessive addition forms nitrides and causes problems of toughness deterioration and ductility deterioration, so the upper limit of the content was made 0.05%.

C and N form carbides and nitrides, respectively, and are similar in function, such as being an austenite forming element, and the total amount of C and N needs to set an upper limit and a lower limit. The lower limit of 0.001% of the total amount of C and N is the minimum value for easily obtaining martensite or bainite and lowering the transformation start temperature of martensite or bainite, and the total amount of C and N. The upper limit of 0.06% was determined as a limit value at which problems of toughness deterioration and ductility deterioration due to carbides and nitrides did not occur.

The above are the basic components of the Ni—Cr weld metal of the present invention, but within the range that does not impair the characteristics of the weld metal of the present invention, the following Nb, Ti, V, Cr are further included for the following purposes: One or more of Mo, Co, and Co, and further, Cu can be added in appropriate amounts.

Nb combines with C in the weld metal to form a carbide. Nb carbide works to increase the strength of the weld metal in a small amount, and therefore, the economic merit of effective use is great. The merit is also great from the viewpoint of increasing the yield stress at the transformation start temperature of martensite or bainite, which is a residual stress reduction technique in the present invention. However, excessive carbide formation, on the other hand, naturally sets an upper limit because toughness degradation occurs. The lower limit of the Nb content is set to 0.005% as the lowest value at which a carbide is formed and an effect of increasing the strength can be expected. The upper limit of the content was set to 0.3% as a value that does not impair the reliability of the weld due to toughness deterioration.

Ti, like Nb, forms carbides and causes precipitation hardening. However, the precipitation hardening amount of Ti is different from Nb. Therefore, the range of Ti content is set to a range different from Nb. The lower limit of the Ti content is 0.005%, and the upper limit of 0.3% is determined in consideration of deterioration of toughness.

  V is an element that functions similarly to Nb and Ti. However, unlike Nb and Ti, in order to expect the same precipitation effect, it is necessary to increase the amount of addition than Nb and Ti. The lower limit of 0.05% of V content was set as the lowest value at which precipitation hardening can be expected by addition. The upper limit of the V content is set to 0.5% in order to cause excessive precipitation hardening and cause deterioration in toughness when added more than this.

  Mo, like Nb, Ti and V, is an element that can be expected to be precipitation hardened. However, Mo needs to be added more than Nb, V, Ti in order to obtain the same effect as Nb, Ti, V. The lower limit of 0.1% of Mo content was set as the lowest value at which an increase in yield strength due to precipitation hardening can be expected. Moreover, 2.0% of the upper limit of the content was determined in consideration of toughness deterioration as in the case of Nb, V, and Ti.

Unlike Ti and the like, Co is not an element that causes strong precipitation hardening. However, Co is a more preferable element than Ni from the viewpoint of adding strength to increase strength and securing toughness while expecting strength increase. However, since Ni is added to the weld metal in order to secure a transformation start temperature that is low enough to expect a residual stress reduction effect, the lower limit of Co content of 0.1% can be expected to have the effect of Co addition. It was set as the minimum value. On the other hand, excessive addition causes an increase in strength and causes toughness deterioration, so the upper limit of its content was made 2.0%.

Cu is an effective element for improving welding workability because it has the effect of improving conductivity by plating on the welding wire. Moreover, since Cu is also an element which improves hardenability, the effect of promoting the transformation of martensite or bainite can be expected by adding to the weld metal. Therefore, Cu may be contained in the weld metal for these purposes. The lower limit of the Cu content is 0.05% as the minimum value necessary for improving workability and promoting the transformation of martensite or bainite. However, excessive addition not only has no effect of improving workability, but also increases the manufacturing cost of the wire, which is not preferable in the industry. Therefore, the upper limit of the content is set to 0.4%.

(Component composition of C-based weld metal)
Below, the technical idea and the reason for limiting the component range of the C-based weld metal will be described.

  C is the most important component in the present invention in the sense that it increases the strength of the weld metal and lowers the transformation start temperature of martensite or bainite. The lower limit of the C content is 0.35% because if the amount is less than this, the transformation temperature is not sufficiently reduced, the residual stress is not reduced, and as a result, the fatigue strength is not improved. Further, the upper limit of 0.70% of C is the same effect even if an amount exceeding this is added, and the load of the manufacturing process when producing a welding wire increases, so the upper limit of the content is 0. 70%.

  Si is an element to be added mainly as a deoxidizing element. The lower limit of Si content of 0.1% has a risk that the deoxidation effect is insufficient and the oxygen in the weld metal cannot be sufficiently reduced if the addition amount is less than this. The increase in oxygen causes deterioration of mechanical properties, particularly toughness, so the lower limit of the content was made 0.1%. The upper limit of the content is set to 0.8% because adding more than 0.8% causes toughness deterioration.

  Mn is added because it has the effect of increasing the strength and lowering the transformation temperature. The lower limit of 0.4% of the Mn content was set as the minimum value at which the effect of securing the strength was obtained. From the viewpoint of lowering the transformation temperature, there is no problem even if the content of Mn exceeds the upper limit of 2.0% in the present invention, but in the present invention, the transformation temperature is sufficiently reduced by C already described, And excessive Mn addition makes the welding material expensive and detracts from the spirit of the present invention, so the upper limit of the content was set to 2.0%.

P and S are impurities in the present invention. However, if these elements are present in large amounts in the weld metal, the toughness deteriorates, so the upper limit of their contents was set to 0.03% and 0.02%, respectively.

The above are the basic components of the C-based weld metal of the present invention. However, the following Ni, Cr, Nb, Ti, V, One or two or more of Mo and Co, and further, an appropriate amount of Cu can be added.

Ni, Cr, Nb, Ti, V, Mo, and Co are component elements that have the effect of improving the strength and further toughness of the weld metal, and one or more of these elements depending on the required characteristics of the weld metal Can be added. However, if a large amount of these alloy elements are added by paying attention only to the strength and toughness of the weld metal, the result may be contrary to the purpose of improving the fatigue strength of the welded joint with an inexpensive material.
In the present invention, one or two kinds of Ni, Cr, Nb, Ti, V, Mo and Co are added as additional elements for the purpose other than improving the fatigue strength of the joint, for example, for improving the strength and toughness of the weld metal. The above can be added by 0.001% or more in total. The lower limit of the total amount of these contents: 0.001% was defined as the minimum content at which these alloying elements can be added and a toughness improving effect can be expected. On the other hand, for the purpose of improving the fatigue strength with an inexpensive material, it is necessary to keep the total of these to 2.0% or less, and preferably the total of these alloy elements is 1.0% or less. .

Cu is an effective element for improving welding workability because it has the effect of improving conductivity by plating on the welding wire. Moreover, since Cu is also an element which improves hardenability, the effect of promoting the transformation of martensite or bainite can be expected by adding to the weld metal. Therefore, Cu may be contained in the weld metal for these purposes. The lower limit of the Cu content is preferably about 0.05% as the minimum value necessary for improving workability and promoting the transformation of martensite or bainite. However, excessive addition not only has no effect of improving workability, but also increases the manufacturing cost of the wire, which is not preferable in the industry. Therefore, the upper limit of the content is set to 0.4%.

The reason for limiting the range of the components of the weld metal according to the present invention has been described above. As a method for controlling the component composition of each of the weld metals of Ni-based, Ni-Cr-based, and C-based, welding wire, welding wire, and This can be realized by adjusting each component of the flux, or the welding core wire and the coating flux.

In the spot welding of the present invention, it is not necessary to specifically limit the type of the steel plate, and a solid solution type, a precipitation type (for example, Ti precipitation type, Nb precipitation type), a two-phase structure type (for example, martensite in ferrite). Even if any type of steel sheet is used, such as a structure containing sites, a structure containing bainite in ferrite), a processing-induced transformation type (structure containing residual austenite in ferrite), or a fine crystal type (ferrite-based structure). By applying the spot welding method of the invention, a joint having excellent fatigue strength can be realized without impairing the properties of the steel sheet.

Moreover, even when spot-welding a high-strength plated steel sheet with the steel sheet surface layer plated by the method of the present invention, a joint having excellent fatigue strength can be realized without impairing the properties of the high-strength plated steel sheet. .
In particular, when spot-welding a high-strength plated steel sheet according to the method of the present invention, low melting point plating components such as Zn in the steel sheet plating layer evaporate and disperse at the time of formation of the through-holes by plasma, and then a solution material is introduced into the through-holes. Suppresses the occurrence of blown-out defects such as blown-down defects due to the internal pressure of Zn vapor confined in the molten metal between the steel plates during welding, which is a problem in conventional resistance spot welding because it forms a nugget by supplying it can do.

In addition, the kind of the plating layer applied to the surface layer of the high-strength plated steel sheet is not particularly limited. For example, Zn, Zn—Fe, Zn—Ni, Zn—Al, Sn—Zn can be used as long as it is Zn-based. The weight per unit area of these plating layers is preferably 100/100 g / m ² or less on both sides.
Further, the method of the present invention is not limited to the same type and the same thickness steel plate combination, and may be the same type of different thickness, different type of different thickness, or different types of different thickness combination as long as the specification is satisfied.

The effects of the present invention will be described below with reference to examples, but the present invention is not limited to the conditions used in the examples.

Example 1
As welding materials for plasma spot welding, Ni-based melts (WA1 to WA5), Ni-Cr melts (WB1 to WB5), and C-based melts (WC1 to WC4) with a diameter of 1.2 mm shown in Table 1 are used. Prototype. Further, as a welding material for comparison, a molten material for 490, 590, 780, and 980 MPa grade steel sheets having a diameter of 1.2 mm (commercial product for MAG welding, WD1: for 490 MPa grade steel sheets, WD2: for 590 MPa grade steel sheets, WD3: 780 MPa) Grade steel plate, WD4: 980 MPa grade steel plate).
As test materials, a soft bare steel plate (270E) having a plate thickness of 0.8, 1.6 mm and a tensile strength of 270 MPa shown in Table 2, a plate thickness of 1.6 mm and a tensile strength of 370 MPa is shown. High-strength bare steel plate (370R: 370MPa class solid solution strengthened steel plate), high-strength bare steel plate (590T: 590MPa class TRIP type composite with plate thickness of 0.8, 1.6, 4.2mm and tensile strength of 590MPa) (Structure steel plate) was used. Further, as shown in Table 3, a soft bare steel plate (270E) having a thickness of 1.2 mm and a tensile strength of 270 MPa, and a high strength bare steel plate having a tensile strength of 440, 590, 780, 980, 1180, and 1470 MPa. (440W: 440 MPa class solid solution strengthened steel sheet, 590R: 590 MPa class precipitation strengthened steel sheet, 590Y: 590 MPa class DP composite steel sheet, 590T: 590 MPa class TRIP composite steel sheet, 780Y: 780 MPa class DP composite steel sheet, 780T: 780 MPa class TRIP type composite structure steel sheet, 980Y: 980 MPa class DP type composite structure steel sheet, 1180Y: 1180 MPa class DP type composite structure steel sheet, 1470Y: 1470 MPa class DP type composite structure steel sheet), and tensile at 1.2 mm High-strength alloyed galvanized steel sheets with strengths of 590 and 780 MPa (59 Y: 590 MPa grade DP type composite structure steel sheet, 780Y: 780 MPa grade DP type composite structure steel sheet, basis weight: one side 45/45 g / m ²⁾ was used.

Based on the spot welded joint fatigue test method (JIS Z3138), a tensile shear fatigue test piece was cut out from the above specimen, and as shown in FIG. A joint was made. As the plasma gas, those Ar + 7% _{H 2} composition, as the shielding gas, the naked steel ones Ar + 7% _{H 2} composition, the galvannealed steel sheet used was the Ar + 50% _{O 2} composition. For comparison, a joint for fatigue test was prepared by resistance spot welding.
Next, based on the spot welded joint fatigue test method (JIS Z3138), a tensile shear fatigue test was conducted under the conditions of a repetition frequency of 20 Hz and a stress ratio of 0.05, and the load at a repetition rate of 2 × 10 ⁶ times. Was defined as fatigue strength.

Condition No. in Table 2 11-No. In the joint manufactured by resistance spot welding, which is a comparative example shown in FIG. 14, the fatigue strength of the 590T joint shows the same level as that of the 270E joint, and the fatigue strength of the joint increases even if the tensile strength of the steel sheet increases. The result was not.
On the other hand, condition No. 2 in Table 2 was welded by the plasma spot welding method using the welding material having the composition defined in the present invention. 1-No. In each case, the 590T joint, which is an example of the present invention shown in FIG. 10, showed a higher fatigue strength than the resistance spot welded joint of the comparative example (conditions No. 13 to No. 14).
Moreover, although the plasma spot welding method was used, condition No. in Table 2 using a welding material having a component composition outside the range specified in the present invention. No. 15 to No. 19 and conditions No. 2 in Table 2 using steel sheets having yield stress that is outside the range defined by the present invention. 20-No. In the comparative example shown in 21, the fatigue strength was as low as or less than that of a welded joint produced by resistance spot welding.
Further, although the plasma spot welding method was used, the condition No. 2 in Table 2 in the case where it is lower than the plate thickness range of the steel plate specified in the present invention. The comparative example shown in 22 also had a low fatigue strength equivalent to or less than a welded joint produced by resistance spot welding.
Moreover, although the plasma spot welding method was used, condition No. in Table 2 in the case where it is higher than the plate thickness range of the steel plate specified in the present invention. In the comparative example shown in FIG. 23, it was impossible to form a healthy spot weld (nugget).

(Example 2)
In the same manner as in Example 1, the welding materials shown in Table 1 were used, and as shown in Table 3, various bare steel plates and alloys having a plate thickness of 1.2 mm and a tensile strength of 270 to 1470 MPa class. Using galvanized steel sheets (see Table 3), joints were similarly produced, and the fatigue strength of the joints was examined.
Joints produced by plasma spot welding using the welding material of the present invention (see conditions No. 1 to No. 33 in Table 3) and joints produced by resistance spot welding (conditions No. 34 to No. 3 in Table 3). 45) and a joint fatigue strength that is higher than that of a joint (see conditions No. 46 to No. 57 in Table 3) produced using a welding material deviating from the specified range of the present invention. Could get.

  In the above, the experimental results were the same regardless of whether steel plates with different thicknesses were used, other steel types, or steel plates with different plating types.

The present invention can be used not only for body parts, underbody parts, and collision safety countermeasure reinforcement parts in the automotive field, but also for parts that require high fatigue strength and require weight reduction. There is sex.

It is sectional drawing for demonstrating the resistance spot welding of a high strength steel plate. It is sectional drawing for demonstrating the tensile shear fatigue test of a resistance spot welded joint. It is a conceptual diagram for demonstrating an example of embodiment of the plasma spot welding of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... High-strength steel plate 2 ... Copper electrode 3 ... Nugget 4 ... Load application direction 5 ... Plasma torch 6 ... Plasma 7 ... Through-hole 8 ... Low temperature transformation molten material 9 ... Weld metal part

Claims (9)

In a spot welding method of a high strength steel plate having a yield stress of 270 MPa or more and a plate thickness of 1.0 to 3.6 mm, a through hole is formed at a joining position by plasma from one side, and then a yield stress is formed in the through hole. Is at least 270 MPa, the temperature at which transformation starts from austenite to martensite or bainite is 200 to 350 ° C.
C: 0.01-0.2%
Si: 0.1 to 0.5%,
Mn: 0.01 to 1.5%,
P: 0.03% or less,
S: 0.02% or less,
A spot welding method for a high-strength steel sheet, characterized by welding by forming a weld metal containing Ni: 8 to 12% and the balance being iron or inevitable impurities. The weld metal is in% by mass;
Nb: 0.01 to 0.4%,
Ti: 0.01 to 0.4%,
V: 0.1 to 1.0%
Cr: 0.1 to 3.0%
Mo: 0.1 to 3.0%,
Co: 0.1-2.0% of 1 type or 2 types or more are contained, The spot welding method of the high strength steel plate of Claim 1 characterized by the above-mentioned. The weld metal is in% by mass;
Cu: 0.05-0.4% is contained, The spot welding method of the high strength steel plate of Claim 1 or 2 characterized by the above-mentioned. In a spot welding method of a high strength steel plate having a yield stress of 270 MPa or more and a plate thickness of 1.0 to 3.6 mm, a through hole is formed at a joining position by plasma from one side, and then a yield stress is formed in the through hole. Is at least 270 MPa, the temperature at which transformation starts from austenite to martensite or bainite is 200 to 350 ° C.
C: 0.001 to 0.05%,
Si: 0.1 to 0.7%,
Mn: 0.4 to 2.5%
P: 0.03% or less,
S: 0.02% or less,
Ni: 4-8%,
Cr: 10 to 15%,
N: 0.001 to 0.05% is contained, the total amount of C and N is 0.001 to 0.06%, and the remainder is welded by forming a weld metal made of iron or inevitable impurities. A spot welding method for high-strength steel sheets characterized by The weld metal is in% by mass;
Nb: 0.005-0.3%
Ti: 0.005 to 0.3%,
V: 0.05-0.5%
Mo: 0.1 to 2.0%,
Co: 0.1-2.0% of 1 type or 2 types or more are contained, The spot welding method of the high strength steel plate of Claim 4 characterized by the above-mentioned. The weld metal is in% by mass;
Cu: 0.05-0.4% is contained, The spot welding method of the high strength steel plate of Claim 4 or 5 characterized by the above-mentioned. In a spot welding method of a high strength steel plate having a yield stress of 270 MPa or more and a plate thickness of 1.0 to 3.6 mm, a through hole is formed at a joining position by plasma from one side, and then a yield stress is formed in the through hole. Is at least 270 MPa, the temperature at which transformation starts from austenite to martensite or bainite is 200 to 350 ° C.
C: 0.35-0.70%,
Si: 0.1 to 0.8%,
Mn: 0.4 to 2.0%,
P: 0.03% or less,
S: A spot-welding method for high-strength steel sheets, characterized by welding by forming a weld metal containing 0.02% or less, the balance being iron or inevitable impurities.   The weld metal contains 0.001 to 2.0% in total by mass% of one or more of Ni, Cr, Nb, Ti, V, Mo, and Co. The high-strength steel sheet spot welding method according to claim 7. The weld metal is in% by mass;
Cu: 0.05-0.4% is contained, The spot welding method of the high strength steel plate of Claim 7 or 8 characterized by the above-mentioned.

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