JP7047535B2 - Resistance spot welding method - Google Patents

Description

The present invention relates to a resistance spot welding method.

In recent years, in the automobile field, it has been required to reduce the weight of the vehicle body from the viewpoint of fuel efficiency and CO ₂ emission reduction, and further, from the viewpoint of improving collision safety, the strength of the vehicle body member is increased. Is required. In order to meet these demands, high-strength steel plates are used for vehicle body members, various parts, and the like.

The vehicle body is required to have high rust resistance, and from the viewpoint of improving the rust prevention of the vehicle body, for example, a zinc-based plated steel plate having excellent corrosion resistance is applied to the members constituting the vehicle body. As the zinc-based plated steel sheet, a high-strength galvanized steel sheet subjected to zinc-based plating is used from the viewpoint of weight reduction and high strength of the vehicle body, using the high-strength steel sheet as the original plate for plating.

On the other hand, in the assembly of the vehicle body of an automobile and the attachment of parts, resistance spot welding is mainly performed (see, for example, Patent Documents 1 to 4).

For example, Patent Document 1 discloses a resistance spot welding method for welding an aluminum alloy plate with a resistance spot welding gun. In this resistance spot welding method, the pressure is applied with the required forging pressure at the time-up of the predetermined welding conditions, and the forging pressure is applied by passing the decay current in the decay time, and the required after the time-up of the decay time is required. It is disclosed that welding is completed by holding for a holding time and ending the forging pressurization.

Patent Document 2 discloses a welding method for improving the fatigue strength of a resistance spot welded joint. In this welding method, the late pressing Pa after energization satisfies 1.5Pi ≦ Pa ≦ 2.5P with respect to the initial pressing Pi, and the initial pressing holding time Hti is 0.35 × Ht ≦ Hti. It is disclosed that ≦ 0.65 × Ht is satisfied and Ht = 200 × t-80 is satisfied.

Patent Document 3 discloses that one or more steel plates and one or more aluminum alloy plates are laminated in total, and resistance spot welding is performed while pressurizing the steel plates and electrodes arranged above and below the aluminum alloy plates. Has been done. Then, in this resistance spot welding, after the pulsation energization in the pressing EF, the pressure is increased to the pressing FE satisfying 1.2 × EF ≦ FF ≦ 2.0 × EF, so that the intermetallic metal in the dissimilar metal bonding is increased. It is disclosed to suppress the formation of compounds.

In Patent Document 4, in resistance spot welding of a high-strength plated steel sheet, a pressing force satisfying the condition of 1.2WF ≤ PHF ≤ 2.0WF is applied immediately after the end of welding energization, and the condition of 300-500t + 250t ² is provided. Techniques for satisfying retention times are disclosed.

Japanese Patent Application Laid-Open No. 2003-145277 Japanese Unexamined Patent Publication No. 2010-110816 Japanese Unexamined Patent Publication No. 2012-152786 Japanese Patent Application Laid-Open No. 2003-103377

When resistance spot welding is performed using a high-strength zinc-based plated steel plate, for example, liquid metal embrittlement cracking (LME; Liquid Metal Embrittlement) may occur at the pressure-welded portion of the obtained resistance spot welded joint. When such a crack occurs, the strength of the welded portion is lowered, so that the reliability of the resistance spot welded joint is lowered. In this crack, tensile stress due to the pressing force of the electrode, thermal expansion and contraction of the steel plate is applied to the welded part, and the melt melted on the surface of the steel plate at the welded part such as zinc, zinc and the alloy of copper of the electrode is the steel plate. It is said that the cracks are caused by the so-called liquid metal brittleness, which is caused by invading the crystal grain boundaries of the above and lowering the grain boundary strength.

By the way, the techniques disclosed in Patent Documents 1 and 3 relate to resistance spot welding using an aluminum alloy plate, and are not techniques for high-strength galvanized steel sheets. Further, the technique disclosed in Patent Document 2 is provided with an initial pressing force holding time. When a member to be welded including a high-strength galvanized steel plate is subjected to resistance spot welding by applying this technique, liquid metal embrittlement cracks may occur in the pressure-welded portion of the resistance spot welded joint.
The technique disclosed in Patent Document 4 is out of scope when the pressing force in the holding step is high. Further, when the technique disclosed in 4 is applied to the patent document, the liquid metal embrittlement crack may occur in the pressure-welded portion of the resistance spot welded joint. In particular, when a high-strength galvanized steel sheet having a tensile strength of 980 MPa or more was used, the liquid metal embrittlement cracks generated in the pressure-welded portion of the resistance spot welded joint tended to be remarkably observed.

An object of the present invention is to provide a resistance spot welding method for suppressing the occurrence of liquid metal embrittlement cracking at a pressure welded portion of a resistance spot welded joint in a resistance spot welding method using a high-strength zinc-based plated steel plate.

The means for solving the above problems include the following aspects.

<1>
A process of energizing and welding a member to be welded, which is a stack of a plurality of steel plates including at least one high-strength galvanized steel plate, while pressurizing with a pressing force F1 by a welding electrode.
In the step of holding and cooling with the pressing force F2 from immediately after the end of the energization to the end of the holding, the relationship of the pressing force F2 with respect to the pressing force F1 is the relationship of (F2)> (F1 × 2). And the process to satisfy
Resistance spot welding method with.
<2>
When the time from the end of energization to the end of holding is t (s) and 1/2 of the total plate thickness of the member to be welded is h (mm), the relationship of t ≧ 0.065 h ² is satisfied. The resistance spot welding method according to <1>.
<3>
The resistance spot welding method according to <1> or <2>, wherein the high-strength galvanized steel sheet has a tensile strength TS of 980 MPa or more.

INDUSTRIAL APPLICABILITY According to the present invention, in a resistance spot welding method using a high-strength zinc-based plated steel plate, there is provided a resistance spot welding method in which the occurrence of embrittlement cracking of liquid metal in a pressure-welded portion of a resistance spot welded joint is suppressed.

It is a schematic diagram which shows an example of the energization pattern and the pressure pattern of the resistance spot welding method which concerns on this embodiment. It is a schematic diagram which shows an example of the energization pattern and the pressure pattern of the conventional resistance spot welding method. It is explanatory drawing which schematically represented the resistance spot welding method. It is a graph which shows the relationship between the temperature and the radial stress after the end of energization. It is a graph which shows the relationship between the pressing force and the radial stress after the end of energization. 4 is an analysis model diagram for performing FEM calculation in FIGS. 4 and 5.

Hereinafter, an example of a preferred embodiment of the method for resistance spot welding of the present invention will be described.

In the present specification, the numerical range represented by using "-" means a range including the numerical values before and after "-" as the lower limit value and the upper limit value unless otherwise specified.
Further, in the present specification, the term "process" is used not only as an independent process but also as long as the intended purpose of the process is achieved even if it cannot be clearly distinguished from other processes. , Included in this term.

In the resistance spot welding method according to the present embodiment, a member to be welded in which a plurality of steel plates including at least one high-strength zinc-based plated steel plate are laminated is energized and welded while being pressurized by a welding electrode with a pressing force F1. In the step of holding and cooling with the pressing force F2 from immediately after the end of the energization to the end of the holding, the relationship between the pressing force F1 and the pressing force F2 is (F2)> (F1 ×). It has a process that satisfies the relationship of 2).

Hereinafter, the resistance spot welding method will be described with reference to the drawings. FIG. 3 is an explanatory diagram schematically showing the resistance spot welding method. As shown in FIG. 3, in the resistance spot welding method, for example, three high-strength zinc-based plated steel plates 1A, 1B, and 1C are laminated to form a member to be welded, and the steel plates are overlapped in the overlapping direction (to be welded shown in FIG. 3). The members to be welded are sandwiched from both sides in the stacking direction of the steel plates by the welding electrodes 2A and 2B arranged on both sides in the vertical direction with respect to the member. Then, the welding electrodes 2A and 2B are used to energize while pressurizing with the pressing force F in the direction of the arrow. After the energization is completed, the state of being pressurized by the pressing force F is maintained, and after cooling by heat removal by the cooled (for example, water-cooled) electrodes 2A and 2B and heat conduction to the steel plate itself, the weld electrode is opened and the nugget is opened. A resistance spot welded joint in which 3 is formed is obtained. A welding heat-affected zone (HAZ portion) is formed around the nugget 3, and a pressure-welded portion is formed on the mating surface of the steel plate in the welding heat-affected zone. As shown in FIG. 3, the member to be welded shown in FIG. 3 has four pressure-welded portions.

Examples of the energization pattern and the pressurization pattern in resistance spot welding include the pattern shown in FIG. FIG. 2 is a schematic diagram showing an example of an energization pattern and a pressure pattern of a conventional resistance spot welding method. As shown in FIG. 2, conventionally, as a resistance spot welding method, a method of energizing while pressurizing with a pressing force F1 is performed, and after the energization is completed, the same pressing force F1 as in the energizing step is maintained. When resistance spot welding is performed on a high-strength galvanized steel sheet by such a conventional method, for example, liquid metal embrittlement cracks may occur in the pressure-welded portion shown in FIG. The occurrence of such liquid metal embrittlement cracks was particularly remarkable when a high-strength galvanized steel sheet having a tensile strength of 980 MPa or more was used.

One of the factors of the liquid metal embrittlement cracking generated by performing resistance spot welding on a zinc-based plated high-strength steel plate is, for example, tensile stress generated by cooling after energization. Therefore, we focused on reducing the stress generated in the pressure welded part of the resistance spot welded joint after welding of the high-strength galvanized steel sheet, and examined the welding conditions that suppress the occurrence of embrittlement cracking of the liquid metal.

In resistance spot welding, the steel sheet is cooled in the holding process after energization. Then, with cooling, tensile stress is generated in the plate width direction in the member to be welded after the energization is completed due to the thermal shrinkage of the nugget. It is considered that the occurrence of this tensile stress causes embrittlement cracking of the liquid metal at the pressure contact portion. From this, in the holding process of cooling after the energization process, if the pressing force on the member to be welded after the energization process is increased, the tension generated in the plate width direction due to the thermal shrinkage of the nugget is suppressed while suppressing the influence on the shape of the nugget. It is believed that the stress is reduced. This can be thought of as follows. By increasing the pressing force in the holding process, stress is generated in the direction opposite to the tensile stress generated in the pressure contact portion. Thereby, this tensile stress is offset by the stress generated in the opposite direction. Therefore, the tensile stress generated in the pressure contact portion is reduced.

Therefore, as shown in FIG. 1, energization is performed while pressurizing with the pressing force F1, and immediately after the energization is completed, the pressing force F2 is more than twice the pressing force F1 in the energizing process without opening a time. It was considered to pressurize and hold. As a result, it was found that by making the pressing force F2 in the holding process more than twice the pressing force F1 in the energizing process, the tensile stress generated in the pressure contact portion is reduced by the nugget that tries to heat shrink due to cooling. bottom. It is considered that this suppresses the occurrence of liquid metal embrittlement cracks in the pressure welded portion of the resistance spot welded joint obtained by resistance spot welding a high-strength galvanized steel sheet. For example, after the energization process is completed, if the pressing force is held unchanged and then the pressing force is doubled and held, the nugget is cooled and it is considered difficult to suppress the tensile stress due to thermal shrinkage. .. Therefore, in the resistance spot welding method according to the present embodiment, a holding step is performed immediately after the end of energization so as to satisfy the relationship (F2)> (F1 × 2). In the present specification, the pressing force F1 and the pressing force F2 represent the maximum ultimate pressing force when pressurized.

From the above, the resistance spot welding method according to the present embodiment suppresses the occurrence of liquid metal embrittlement cracks at the pressure welded portion of the resistance spot welded joint when resistance spot welding is performed using a high-strength zinc-based plated steel plate.

Hereinafter, the resistance spot welding method according to this embodiment will be described.

(Energization process)
First, a plurality of steel plates including at least one high-strength galvanized steel plate are superposed to prepare a member to be welded. Next, the overlapped portion of a plurality of steel plates in the member to be welded is sandwiched from both sides by welding electrodes arranged on both sides in the stacking direction of the steel plates. Then, while pressurizing with the pressing force F1 by the welding electrode, the welding electrode is energized to perform resistance spot welding.

Here, the pressing force F1 in the energization step is not particularly limited, but may be, for example, 300 kgf to 600 kgf. The pressure is applied by the welding electrode while one welding electrode (for example, the lower welding electrode 2B shown in FIG. 3) is fixed and the other welding electrode (for example, the upper welding electrode 2A shown in FIG. 3) is fixed. ) May be applied.

In the energization step, each condition such as the current value, the energization time, the contact area of the welding electrode, and the pressing force from the welding electrode is adjusted according to the required nugget diameter. For example, each condition in the energization process may be set so as to satisfy the nugget diameter shown below.

The nugget diameter (Dn) formed by resistance spot welding is preferably 3 times or more (that is, 3√h or more) the square root of the plate thickness h. For example, when the member to be welded including at least one high-strength galvanized steel sheet is composed of three steel sheets, the thickness of the first steel sheet is h1, the thickness of the second steel sheet is h2, and so on. Assuming that the plate thickness of the third steel plate is h3, the plate thickness h _D1 at the nugget diameter (D1) formed at the boundary between the steel plate arranged on one weld electrode side and the steel plate arranged in the middle. Is expressed as h _D1 = (h1 + h2) / 2. Further, in the nugget diameter (D2) formed at the boundary between the steel plate arranged on the other welding electrode side and the steel plate arranged in the middle, the plate thickness h _D2 is expressed as h _D2 = (h2 + h3) / 2. Will be done. On the other hand, when the member to be welded including at least one high-strength galvanized steel plate is composed of two steel plates, the thickness of the first steel plate is h1 and the thickness of the second steel plate is h2. Then, the plate thickness h is expressed as h = (h1 + h2) / 2. The current value when energized should be equal to or less than the current value at which dust does not occur.

In the present embodiment, post-energization may be performed in the energization step. Generally, the post-energization is performed after a cool time during which the energization is not performed after the main energization. The cool time, the current value of the post-energization, and the energization time are set according to the purpose of the post-energization and the plate thickness. In the segregation relaxation type post-energization, the cool time is generally set to the time when the nugget solidifies. Further, the energization of the post-energization is set so that the end of the nugget is within the temperature range of A ₃ points or more below the freezing point. In particular, it is desirable to set the current value of the post-energization to 85% to 95% of the current value of the main energization. Further, it is desirable to set the energization time (seconds) of the post-energization to 0.2h-0.2 when the plate thickness is h. Here, h is represented by (h1 + h2 + h3) / 2, where h1 is the thickness of the first steel plate, h2 is the thickness of the second steel plate, and h3 is the thickness of the third steel plate. .. Further, when the member to be welded is a member obtained by superimposing two steel plates, if the plate thickness of the first steel plate is h1 and the plate thickness of the second steel plate is h2, h is (h1 + h2) / 2. It is represented by. In this segregation relaxation type post-energization, it is desirable to increase the pressing force immediately after the post-energization.
On the other hand, in the temper type post-energization, the cool time is set so that the nugget and its surrounding heat-affected zone (HAZ portion) are cooled to a temperature below the Ms point. The current value and energization time of the post-energization are set so that carbides are deposited from the martensite structure generated during the cool time and the toughness is restored. In this case, it is desirable to increase the pressing force immediately after the main energization. This is because, in the temper type post-energization with a long cool time, the pressure contact portion may pass through the dangerous area and crack during the cool time.

-High-strength galvanized steel sheet-
Here, in the resistance spot welding method according to the present embodiment, a high-strength galvanized steel sheet coated with zinc-based plating is used for the high-strength steel sheet. Hereinafter, the high-strength galvanized steel sheet will be described.

The high-strength galvanized steel sheet may have a tensile strength (TS) of 980 MPa or more in order to obtain a high-strength galvanized steel sheet, for example. The tensile strength of a steel sheet is measured by performing a tensile test in accordance with JIS Z 2241 (2011). The upper limit of the tensile strength is not particularly limited, and for example, it may be 2000 MPa or less.

The chemical composition and metallographic structure of the high-strength steel sheet, which is the base material of the high-strength galvanized steel sheet, are not particularly limited. For example, the chemical composition of the high-strength steel sheet is, in terms of mass%, C: 0.100% to 0.400%, Si: 0.30% to 3.00%, Mn: 1.00% to 10.00%. , P: 0% to 0.010%, and S: 0% to 0.0050%, and the balance may be a steel plate made of Fe and impurities. Further, a steel sheet in which a part of the Fe is replaced with other additive elements may be used. Further, it may be a steel plate having a carbon equivalent Ceq of 0.15% or more. Ceq is shown in the following formula.
Formula Ceq = [C] + [Si] / 30 + [Mn] / 20 + 2 [P] + 4 [S]
However, [C], "Si", [Mn], [P], and [S] are the contents (mass%) of C, Si, Mn, P, and S. It is 0% by mass.

In the resistance spot welding method according to the present embodiment, as a member to be welded, a plurality of steel sheets including at least one high-strength galvanized steel sheet are laminated, and a steel sheet coated with zinc-based plating is provided on the laminated surface. If so, the number of high-strength galvanized steel sheets is not particularly limited. For example, as shown in FIG. 3, three high-strength galvanized steel sheets 1A to 1C may be superposed. Further, two high-strength galvanized steel sheets may be superposed. Further, a plurality of steel sheets may be used in which the laminated surfaces of all the steel sheets are coated with zinc-based plating. A surface coated with zinc-based plating and a surface not coated with zinc-based plating may be superimposed on the overlapped surface. A high-strength galvanized steel sheet and another steel sheet may be superposed. However, considering the corrosion resistance of the resistance spot welded joint, it is preferable that the contact surface with the welded electrode is also coated with zinc-based plating. In this respect, it is more preferable that the member to be welded is a member to be welded by laminating a plurality of high-strength galvanized steel sheets.

The plurality of steel plates in the member to be welded by resistance spot welding may be two steel plates or three or more steel plates. The plate thickness of each steel plate to be spot-welded by resistance is not particularly limited, and may be, for example, 0.5 mm to 3.0 mm. Further, the total plate thickness H of the plurality of steel plates may be 1.0 mm or more. The upper limit of the total plate thickness H of the plurality of steel plates is not particularly limited, and for example, the total plate thickness H may be 7.0 mm or less.

The range of the total plate thickness H is preferably 2.4 mm to 4.8 mm in terms of achieving both weight reduction and high strength of the resistance spot welded joint.

The steel plate may have at least a part having a plate-shaped portion and may have a portion in which the plate-shaped portions are stacked on each other, and the entire steel plate does not have to be a plate. Further, the plurality of steel plates are not limited to those composed of separate steel plates, and may be obtained by superimposing one steel plate formed into a predetermined shape such as a tubular shape.

The zinc-based plating coated on the high-strength steel sheet is not particularly limited as long as it is a plating containing zinc. For example, examples of the plating type include alloyed hot-dip galvanizing, hot-dip galvanizing, and electrozinc plating. The plating containing zinc (zinc plating) includes, for example, plating containing only zinc; plating containing zinc and iron; plating containing zinc and nickel; plating containing zinc and aluminum; plating containing zinc and magnesium; zinc. , Aluminum, magnesium, silicon and the like, and any of these may be used.

Zinc-based plating may be applied to only one side or both sides. The basis weight of these plating layers is not particularly limited, and for example, the basis weight on one side may be 100 g / m ² or less.

-Welding electrode-
Here, the welding electrode will be described. As the welding electrode, for example, a pair of cylindrical electrodes are used. The tip shape of the cylindrical electrode is, for example, (A) flat shape, (B) radius shape (arc shape), (C) dome radius shape (contact surface is flat or has a gentle curvature (for example, R40)). (D) Cone flat shape (shape in which the contact surface is flat or curved with a gentle curvature and the circumference is tapered), (E) ) A point shape (a shape in which the contact surface is flat or curved with a gentle curvature and the periphery thereof is tapered at a steep angle) or the like is generally used. Among these, (A) flat shape, (C) dome radius shape, (D) cone flat shape, and (E) point shape in which the contact surface is flat or curved with a gentle curvature are more preferable. The C) dome radius shape is more preferable, and the (C) dome radius shape in which the contact surface is a curved surface having a gentle curvature (for example, R40) is particularly preferable.

When the tip shape of the cylindrical electrode is a flat surface or a curved surface having a gentle curvature as a contact surface (that is, in the case of the shapes (A), (C), (D), and (E)). The diameter φ of this tip portion (that is, a flat surface or a curved surface having a gentle curvature) is adjusted according to the required nugget diameter, plate thickness, and the like. The diameter φ is preferably in the range of, for example, 4 mm to 10 mm. The material of the weld electrode is not particularly limited, and examples thereof include alloys containing copper and chromium.

(Holding process)
The resistance spot welding method according to the present embodiment has a holding step after the energizing step. The holding step is a step of holding and cooling the member to be welded while being sandwiched between the weld electrodes by the pressing force F2 from immediately after the end of energization of the above-mentioned energization step to the end of holding. Then, the relationship between the pressing force F1 in the energizing step and the pressing force F2 in the holding step is held so as to satisfy the relationship (F2)> (F1 × 2). The member to be welded that has undergone the energization step is cooled by a cooled (for example, water-cooled) weld electrode in a non-energized state. Here, from immediately after the end of energization to the end of holding, at the same time as the energization in the energization process is completed, the holding is started, the pressurization by the welding electrode is continued, and then the pressurization by the welding electrode is released. The period from the opening of the weld electrode from the member to be welded to the time when the pressurization is completed is shown (see, for example, FIG. 1).

For example, when the post-energization is performed by the pressing force F1 after a short cool time such that the nugget solidifies after the main energization, as typified by the segregation relaxation type post-energization, the post-energization is performed. The pressing force F2 in the holding step may satisfy the above relationship with respect to the pressing force F1. For example, when the pressing force F1 in the post-energization is higher than the pressing force in the main energization, the pressing force F2 in the holding step is more than twice the pressing force F1 in the post-energizing process. It is sufficient to pressurize. When performing post-energization, immediately after the end of energization means immediately after the end of post-energization.
On the other hand, for example, when the nugget and its surrounding heat-affected zone (HAZ part) are cooled to the Ms point or less after a long cooling time, as typified by the temper type post-energization, the post-energization is performed. It is sufficient that the pressing force F2 in the holding step satisfies the above relationship with respect to the pressing force F1 in the main energization.
Here, in the resistance spot welding method according to the present embodiment, the pressing force F1 in the energizing step indicates the pressing force F1 as a reference in the energizing step.

The relationship between the pressing force F1 in the energizing step and the pressing force F2 in the holding step satisfies (F2)> (F1 × 2). It is preferable to satisfy (F2) ≧ (F1 × 2.1) in terms of further suppressing the occurrence of liquid metal embrittlement cracking. The upper limit of the pressing force F2 is not particularly limited as long as the pressing force F2 satisfies the above relationship. For example, as the upper limit of the pressing force F2, the relationship of (F1 × 5) ≧ (F2) may be satisfied.

In the holding step, the holding time is not particularly limited as long as the time until the nugget solidifies can be secured. When the time from the end of energization to the end of holding is t (s) and 1/2 of the total plate thickness of the member to be welded is h (mm), the relationship of t ≧ 0.065 h ² is satisfied. Is preferable. If the holding time satisfies this condition, the occurrence of liquid metal embrittlement cracks at the pressure welded portion of the resistance spot welded joint when performing resistance spot welding using this high-strength galvanized steel sheet is effectively suppressed. .. Here, as described above, the plate thickness h of the member to be welded including at least one high-strength galvanized steel sheet is represented by 1/2 of the total plate thickness of the member to be welded. Specifically, when the member is a member to be welded by stacking three steel plates, the thickness of the first steel plate is h1, the thickness of the second steel plate is h2, and the plate thickness of the third steel plate is h2. Assuming that the thickness is h3, h is represented by (h1 + h2 + h3) / 2. Further, when the member to be welded is a member obtained by superimposing two steel plates, if the plate thickness of the first steel plate is h1 and the plate thickness of the second steel plate is h2, h is (h1 + h2) / 2. It is represented by. The upper limit of t is not particularly limited, and examples thereof include time for cooling to room temperature (for example, 25 ° C.).

-Other processes-
Further, in the resistance spot welding method according to the present embodiment, a squeeze step may be provided before the energization step. The squeeze step is a step of sandwiching by the welding electrode while holding the pressing force before energizing from the welding electrode. For example, referring to FIG. 1, the squeeze process is a process in which a member to be welded is sandwiched between weld electrodes, pressurization is started by a pressing force F1, and electricity is applied from the weld electrode. The squeeze time in the squeeze process is not particularly limited. The squeeze time is preferably longer than, for example, the time it takes for the pressing force to reach a certain value.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples. It is clear that a person skilled in the art can come up with various modifications or modifications within the scope of the ideas described in the claims, and these also naturally belong to the technical scope of the present invention. It is understood that it is a thing.

Assuming that three high-strength galvanized steel sheets are superposed to form a member to be welded and resistance spot welding is performed on this member to be welded, the temperature inside the pressure-welded part and the stress generated in the pressure-welded part in the holding process. The relationship with and was analyzed by the finite element method (FEM). For the high-strength zinc-based plated steel sheet, a high-strength steel sheet was used as the original plating plate, and zinc-based plated steel sheets were applied on both sides thereof. The stress generated in the pressure contact portion was analyzed as the radial stress of the nugget. Table 1 shows the analysis conditions by the finite element method. In this analysis, it is assumed that the post-energization is not performed in the energization process, but only the main energization is performed. That is, the analysis conditions are resistance spot welding in the energization pattern and pressurization pattern as shown in FIG. 1 (when the pressurization F1 and the pressurization F2 are the same, the energization pattern and the pressurization pattern as shown in FIG. 2). Is assumed.

Further, the results of analysis by the finite element method under the conditions shown in Table 1 are shown in FIGS. 4 and 5. Further, FIG. 6 shows an analysis model diagram for performing the FEM calculation of FIGS. 4 and 5. P shown in FIG. 6 indicates a portion where the stress in the pressure contact portion is calculated. The pressure contact portion P outputs the temperature and radial stress shown in FIGS. 4 and 5. In addition, 1A shown in FIG. 6 represents an upper plate, 1B represents a middle plate, and 1C represents a lower plate.

FIG. 4 is a graph showing the relationship between the temperature after the end of energization and the radial stress. In the graph shown in FIG. 4, the horizontal axis represents the temperature of the member to be welded immediately after the end of energization. The notation of the end of energization indicates the time when the energization is completed in the energization process. After the energization is completed, the member to be welded is not energized and is cooled by holding the pressure applied by the weld electrode. Therefore, the horizontal axis represents that the temperature decreases from the right to the left of the horizontal axis after the energization is completed. That is, the time when the energization ends is the start point of the holding process. In addition, in FIG. 4, it is cooled while holding the pressing force without energization in the range from immediately after the end of energization to the lowest temperature. After that, the weld electrode is opened.

On the other hand, the vertical axis represents the stress generated in the pressure contact portion as the radial stress of the nugget. The negative stress on the vertical axis indicates that the stress in the compressed state is generated in the pressure-welded portion because the steel sheet is expanded as compared with that before the resistance spot welding. Further, the positive stress on the vertical axis indicates that the nugget shrinks as the nugget is cooled, so that tensile stress is generated in the pressure contact portion in the radial direction of the nugget. That is, at the 0 point on the vertical axis, the radial stress generated in the pressure contact portion changes from compression to tension.

In the graph shown in FIG. 4, the region represented by the crack danger region is a region where liquid metal embrittlement cracks are likely to occur, the zinc melting point (420 ° C.) or higher, and the radial stress is 0 kgf / mm ² or higher. Represents the area surrounded by the range of. In this region, it is considered that the more the zinc-based plating is melted and the tensile stress is generated in the pressure-welded portion, the more likely the liquid metal embrittlement crack is to occur. That is, the higher the temperature at which the compressive stress changes to the tensile stress and the higher the radial tensile stress generated in the pressure contact portion at the melting point of zinc, the more likely the liquid metal embrittlement cracking is to occur.

As shown in FIG. 4, when the pressing force F1 in the energization process and the pressing force F2 in the holding process are the same (F1 = F2), the radial stress generated in the pressure contact portion changes from the compressive stress to the tensile stress. Is high, and the radial tensile stress generated at the pressure contact portion at the melting point of zinc is high. Therefore, in this case, liquid metal embrittlement cracking is likely to occur at the pressure-welded portion of the resistance spot welded joint.

Further, in the comparative example (F2 = F1 × 1.5) in which the pressing force F2 in the holding step is 1.5 times the pressing force F1 in the energizing step, the radial stress generated in the pressure contact portion is the compressive stress. The temperature at which the stress changes from to tensile stress becomes low. However, the radial tensile stress generated at the pressure contact portion at the melting point of zinc is about the same as when the pressing force F1 and the pressing force F2 are the same. In this case, since the effect of increasing the pressing force F2 in the holding step is low, embrittlement cracking of the liquid metal at the pressure welding portion of the resistance spot welded joint is likely to occur.

On the other hand, in each invention example, the pressing force F2 in the holding step exceeds twice the pressing force F1 in the energizing step ((F2)> (F1 × 2)), the radial stress generated in the pressure contact portion is compressed. The temperature at which the stress changes to tensile stress is lowered, and the radial tensile stress generated at the pressure contact portion at the melting point of zinc is also lowered. Therefore, in each of the invention examples, the occurrence of liquid metal embrittlement cracks at the pressure-welded portion of the resistance spot welded joint is suppressed.

Further, FIG. 5 is a graph showing the relationship between the pressing force in the holding step of pressurizing immediately after the end of energization and the radial stress. The horizontal axis represents the pressing force in the holding process. The vertical axis is the radial stress generated in the pressure contact portion at the melting point of Zn, and represents the tensile stress. In FIG. 5, the pressing force F1 in the energization step is 400 kgf. As shown in FIG. 5, when the relationship between the pressing force F1 in the energizing step and the pressing force F2 in the holding step is (F2) = (F1), and (F2) = (F1 × 1.5). ), The radial tensile stress generated in the pressure contact portion is high. On the other hand, when the relationship is (F2)> (F1 × 2), the radial tensile stress generated in the pressure contact portion is low. Therefore, when the relationship (F2)> (F1 × 2) is satisfied, the occurrence of liquid metal embrittlement cracks in the pressure-welded portion of the resistance spot welded joint is suppressed.

1A, 1B, 1C High-strength galvanized steel sheet 2A, 2B Welding electrode 3 Nugget P Pressure welding part

Claims (4)

A plurality of steel plates including at least one high- strength steel plate having a tensile strength of 980 MPa or more are present so that the high-strength steel plate is present on the laminated surface and zinc-based plating is present on the laminated surface. The process of energizing and welding the overlapped members to be welded while applying pressure with F1 by the welding electrode.
In the step of holding and cooling with the pressing force F2 from immediately after the end of the energization to the end of the holding, the relationship of the pressing force F2 with respect to the pressing force F1 is the relationship of (F2)> (F1 × 2). And the process to satisfy
Resistance spot welding method with. When the time from the end of energization to the end of holding is t (s) and 1/2 of the total plate thickness of the member to be welded is h (mm), the relationship of t ≧ 0.065 h ² is satisfied. The resistance spot welding method according to claim 1. The resistance spot welding method according to claim 1 or 2, wherein the high-strength steel sheet is coated with zinc-based plating. The resistance spot welding method according to claim 1 or 2, wherein the other steel sheet to be superposed with the high-strength steel sheet is coated with zinc-based plating.

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