JP2018140431A - Steel plate weld member, and spot welding method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel plate weld member of which a high strength is realized.SOLUTION: A steel plate weld member, in which plural steel plates 1A, 1B, which include at least one high strength steel plate having a tensile strength TS of 800 MPa or more are overlapped and are spot-welded at plural places, has two or more large diameter spot weld zones 4 of which a nugget diameter Dn satisfies the formula (A). Two or more large diameter spot weld zones 4 have such a probability of 70% or more that at least one of the condition that the zones have a shrinkage cavity 5 of which a width exceeds 100 μm in a nugget 3, and the condition that the zones have plural shrinkage cavities in the nugget 3 and a sum total of widths of the plural shrinkage cavities exceeds 100 μm is satisfied. Formula (A): Dn≥2.5√h (h represents 1/2 of a total thickness of the overlapped plural steel plates).SELECTED DRAWING: Figure 1

Description

  The present invention relates to a spot welding method in which a plurality of steel plates are overlapped and spot-welded in the field of manufacturing automobiles and the like, and a steel plate welded member in which a plurality of steel plates are spot-welded.

In recent years, in the automobile field, it has been required to increase the strength of vehicle body members from the viewpoint of improving collision safety while achieving weight reduction of vehicle bodies from the viewpoint of reducing fuel consumption and reducing CO ₂ emissions. . For this reason, high-strength steel plates are used for vehicle bodies and parts.
Note that spot welding, which is a type of resistance welding, is often used for assembling the vehicle body and attaching parts. However, particularly when a high-strength steel plate is welded by spot welding, it is required to improve the strength of the joint portion.

  For example, Patent Document 1 discloses a connection of a strap connected to either a positive electrode plate or a negative electrode plate of a power generation element housed in a cell on both sides of the cell interval wall through a through hole formed in the cell interval wall of the battery case. A lead-acid battery having a partition through-welding process for welding between the parts, and a welding nugget pressing process for pressing between the connection parts of the straps that have solidified after being once melted in the through holes of the cell interval wall by the partition through-welding process. The manufacturing method of this is proposed.

  Patent Document 2 discloses a one-side spot welding method in which an electrode is applied only to one side of the workpiece to a welding site of the workpiece and is welded to the workpiece while pressing the workpiece from the one side with the electrode. A nugget forming step in which main energization is performed at a predetermined current value, and after the nugget formation step, the work is energized at a current value lower than the predetermined current value while pressing the workpiece from the one side with the electrode. A one-side spot welding method including a slow cooling step has been proposed.

  Further, in Patent Document 3, when an aluminum alloy sheet is welded by a resistance spot welding gun, the aluminum alloy sheet is pressurized at a welding pressurization time ST1 with a required welding pressurizing time, and the welding pressurization time ST1 is increased. The welding current is passed through at a welding time WT1, the welding time WT1 is timed up, the required forging pressurization is applied, and the decay time WT2 is passed at the forging pressurization to cause forging pressurization. There has been proposed a resistance spot welding method for an aluminum alloy plate material in which welding is completed with a condition of holding the required holding time HT1 and finishing forging pressurization after the time WT2 is up.

  Patent Document 4 discloses a resistance spot welding method for an aluminum thin plate or an aluminum alloy thin plate having a coating layer containing an organic substance, and the current value is within a range of 5 to 60% of the welding current prior to energization of the welding current. There has been proposed a method of welding organic coated aluminum sheets and aluminum alloy sheets that are pre-energized.

  Further, Patent Document 5 discloses a resistance seam welding method in which a workpiece is pressurized through a seam electrode ring, heat / cool is repeatedly intermittently energized, and the electrode ring is rotationally driven to perform seam welding. There has been proposed a resistance seam welding method in which a continuous high and low current waveform is generated without dropping the current value of the cool time of the heat / cool timing to zero and the workpiece is seam welded.

JP 2002-313310 A JP 2013-66932 A JP 2003-145277 A Japanese Patent Laid-Open No. 8-25060 Japanese Patent Laid-Open No. 11-77326

  In the past, there was a proposal focusing on the shrinkage nest generated in the nugget during spot welding, but we tried to finely control this shrinkage in terms of improving the strength of steel welded members using high-strength steel plates. There are no examples, and there is room for further improvement in this regard.

  The present invention has been made in view of the above-described present situation, and provides a steel plate welding member that realizes high strength, and a spot welding method that realizes suppression of generation of a sink and reduction of the size of the generated sink. For the purpose.

When a joint (welded part) is formed by spot welding with a plurality of steel plates overlapped, a void called a “shink nest” may be generated near the center of the nugget formed in the spot welded part. Conventionally, when a steel sheet having a relatively small tensile strength TS is used, such a shrinkage nest has not been a problem in terms of strength.
However, when one or more high-strength steel sheets with a tensile strength TS of 800 MPa or more are used and spot welding is performed at a plurality of locations to produce a steel plate welded member, the number of spot welds having large sized nests increases too much. And it turned out that the high intensity | strength as a steel plate welding member is hard to be obtained.

  In addition, it was found that the shrinkage nest is likely to occur particularly when spot welding is performed using one or more high-strength steel sheets having a tensile strength TS of 800 MPa or more, and a larger shrinkage nest is likely to occur.

  Some proposals for adjusting the spot welding conditions as described above have been made in the past as shown in Patent Documents 1-5. However, in Patent Documents 1 to 5, there is no focus on the fact that the degree of occurrence of the large size of the nest of the steel plate welded member obtained by spot welding contributes to strength improvement, and the applied pressure and nugget at the time of spot welding There is no focus on the point that the diameter contributes to the suppression of shrinkage nests and the reduction of its size.

  Therefore, the present inventors suppress the generation of the sink nest and reduce the size of the generated nest when spot welding is performed using one or more high strength steel plates having a tensile strength TS of 800 MPa or more. Possible spot welding methods were investigated. Further, from the viewpoint of obtaining high strength, the degree of occurrence of the shrinkage nest in the steel plate welded member and the size of the generated shrinkage nest were examined.

As a result, the probability that a large shrinkage nest exists contributes to the strength of the steel plate welded member, and the applied pressure and nugget diameter by the welding electrode during spot welding contribute to the generation of the shrinkage nest and the size of the generated shrinkage nest. It was discovered that
The present invention has been completed by repeating the above studies.
That is, the gist of the present invention is as follows.

<1> A steel plate welded member in which a plurality of steel plates including at least one high strength steel plate having a tensile strength TS of 800 MPa or more are overlapped and spot-welded at a plurality of locations,
The nugget diameter Dn has two or more large-diameter spot welds having a nugget part satisfying the following formula (A),
The two or more large-diameter spot welds have the condition that the nugget portion has a shrinkage nest having a width of more than 100 μm, and the sum of the widths of the plurality of shrinkage nests has a plurality of shrinkage nests in the nugget portion. A steel plate welded member having a probability of satisfying at least one of the conditions of exceeding 100 μm being 70% or less.
Formula (A): Dn ≧ 2.5√h
(Here, h represents 1/2 of the total thickness of the plurality of stacked steel plates.)

<2> When at least one of the large-diameter spot welds has a depression on one or both surfaces in the thickness direction, and the depression is only on one surface, the depth of the depression is The steel plate welded member according to <1>, wherein the sum of the depths of both the recesses is 5% or more of the total thickness of the plurality of the steel plates stacked when the recess is included.

<3> A superposition step of superposing a plurality of steel plates including at least one high strength steel plate having a tensile strength TS of 800 MPa or more;
The overlapped portions of the plurality of steel plates are sandwiched between welding electrodes from both sides, and the welding electrodes are energized so that the pressing force Fe and the nugget diameter Dn by the welding electrodes satisfy the following condition (B1) or condition (B2). Spot welding process for spot welding,
A spot welding method comprising:
Condition (B1): Fe ≧ 500 kgf and Dn ≧ 2.5√h
Condition (B2): Fe ≧ 100 kgf and Dn ≧ 4√h
(Here, h represents 1/2 of the total thickness of the plurality of stacked steel plates.)

  ADVANTAGE OF THE INVENTION According to this invention, the steel plate welding member which implement | achieved high intensity | strength and the spot-welding method which implement | achieved suppression of generation | occurrence | production of a dent and the reduction | decrease of the magnitude | size of the generated dent are provided.

It is a figure which shows typically an example of the nugget and sinkhole of the spot-welded part formed by spot welding. It is a figure which shows an example of arrangement | positioning of the two steel plates at the time of starting spot welding, and a welding electrode. It is a figure which shows typically an example of the steel plate welding member which concerns on this embodiment with which two steel plates were spot-welded. It is an image which shows an example of the nugget and sinkhole of the spot-welded part formed by spot welding. It is an image which shows another example of the nugget and sinkhole of the spot weld part formed by spot welding. It is a figure which shows typically the various examples of the front-end | tip shape of the welding electrode used for spot welding.

  Hereinafter, an embodiment of a method for selecting a steel plate for spot welding according to the present invention will be described.

  In the present specification, a numerical range expressed using “to” means a range including numerical values described before and after “to” as a lower limit and an upper limit unless otherwise specified. However, when there is a notice such as “exceeding” and “less than”, it means that numerical values described before and after “to” are not included as at least one of the lower limit value and the upper limit value.

[Steel plate welding member]
The steel plate welded member according to this embodiment is a member in which a plurality of steel plates including at least one high strength steel plate having a tensile strength TS of 800 MPa or more are overlapped and spot welded at a plurality of locations.

In addition, in two or more places among a plurality of spot-welded places (spot welded portions), the nugget diameter Dn has a large-diameter spot welded portion having a nugget portion satisfying the following formula (A).
Formula (A): Dn ≧ 2.5√h
(Here, h represents 1/2 of the total thickness of the plurality of stacked steel plates.)

In addition, two or more large-diameter spot welds have a probability of satisfying at least one of the following conditions I and II (hereinafter also simply referred to as “large-diameter dent existence probability”) of 70% or less.
Condition I: A nest having a width exceeding 100 μm in the nugget portion Condition II: A plurality of sink nests in the nugget portion and the sum of the widths of the plurality of sink nests exceeding 100 μm

Conventionally, steel sheets have been used in various fields. For example, in the automobile field, weight reduction of the vehicle body has been demanded for the purpose of reducing CO ₂ emissions and the like, while improving the strength such as collision safety. There is a need to be compatible. Therefore, in order to achieve both weight reduction and strength improvement, it is effective to apply a high-strength steel plate to the automobile body, and a steel plate welding member in which a plurality of high-strength steel plates are assembled by welding is put into practical use. . Such steel plate welding members are used not only in the automobile field but also in various fields.
In addition, in steel plate welded members of high-strength steel sheets that are required to achieve both weight reduction and strength improvement, high strength is also required in joints (welded parts) joined by welding. It is important to establish welding conditions to achieve this.

  Here, when a joint (welded portion) is formed by spot welding in the overlapped portion of the steel plates, a void called “shink nest” may be generated near the center of the nugget formed in the spot welded portion. FIG. 1 is a view showing a steel plate welded member in which two steel plates 1A and 1B are overlapped, and spot welding is performed by energizing the electrodes while sandwiching them between welding electrodes 2A and 2B from both sides. A state in which a sink nest 5 is generated in the nugget 3 of the portion (heat affected zone) 4 is shown. Note that the cause of the occurrence of the sink nest 5 is generally due to the solidification shrinkage of the nugget after the energization is completed.

However, conventionally, in the steel plate welded member using a steel plate having a relatively small tensile strength TS, such a hollow 5 has not been considered as a problem in terms of strength. However, in a steel plate welded member that is welded by spot welding at a plurality of locations using one or more high-strength steel plates having a tensile strength TS of 800 MPa or more, the number of spot welds having a large size of the nest is excessively increased. And it turned out that the high intensity | strength as a steel plate welding member is hard to be obtained.
On the other hand, the steel plate welding member according to the present embodiment has two or more large-diameter spot welds in which the nugget diameter Dn satisfies the formula (A), and among these large-diameter spot welds, the condition I A large diameter spot where a shrinkage nest satisfying the condition II, that is, a shrinkage nest whose width exceeds 100 μm is formed, or a sink nest satisfying the above condition II, that is, a plurality of sink nests and a sum total of the widths exceeding 100 μm is formed. The probability that a weld is present (large-diameter shrinkage existence probability) is suppressed to 70% or less. Thereby, in this embodiment, the high intensity | strength as a steel plate welding member is obtained, for example, the outstanding impact resistance and fatigue fracture resistance are obtained.

-Large diameter shrinkage nest existence probability The steel plate welding member of this embodiment has a large diameter spot welded part in which the nugget diameter Dn satisfies the formula (A) in at least two or more of the plurality of spot welded parts. And among these two or more large-diameter spot welds, the ratio of the number of large-diameter spot welds satisfying at least one of the above condition I and condition II (large-diameter dent existence probability) is 70% or less. . Since the probability of existence of large-diameter shrinkage nests is suppressed to 70% or less, high strength as a steel plate welded member is realized, and, for example, excellent impact resistance and fatigue fracture resistance can be obtained.
In addition, it is preferable that this large diameter shrinkage nest existence probability is further 50% or less, and it is more preferable that it is 30% or less.

  Here, the calculation of the existence probability of the large-diameter dent nest is carried out with respect to any ten points out of the large-diameter spot welded portions (spot welded portions where the nugget diameter Dn satisfies the formula (A)) of the steel plate welded member. It is calculated by measuring the total width (the width of the sink nest when there is only one sink nest, the sum of the widths of the sink nests when there are a plurality of sink nests). In addition, when the steel plate welding member does not have the above-mentioned number of large-diameter spot welds, calculation is performed by measuring the total shrinkage width for all the large-diameter spot welds.

  In a steel plate welded member having two or more large-diameter spot welds, the method for controlling the large-diameter dent nest existence probability within the above range is not particularly limited, but a spot welding method according to this embodiment described later. The method of manufacturing a steel plate welding member by is mentioned.

-Measurement of nugget diameter Dn and measurement of width of shrinkage nest Measurement of nugget diameter Dn and measurement of the width of shrinkage nest are performed by the following methods.
First, the spot welded portion is cut in the plate thickness direction to prepare an embedded sample, and a cross-section obtained by polishing and etching is observed using an optical microscope. The nugget diameter Dn is obtained by measuring the length in the direction orthogonal to the plate thickness direction of the nugget in this cross-sectional image.

  The large diameter spot welded portion is a spot welded portion whose nugget diameter Dn satisfies the formula (A) (Dn ≧ 2.5√h), and “h” in the formula (A) is a plurality of overlapped portions. It represents 1/2 of the total thickness of the steel plates. If the spot weld has a dent on one or both surfaces, 1/2 of the total thickness at a location where this dent does not exist is “h”.

Further, the width of the shrinkage nest can also be obtained by measuring the length of the cross-sectional image obtained above in the direction orthogonal to the thickness direction of the shrinkage nest.
Here, examples of cross-sectional images in which the sink marks are generated are shown in FIGS. 4 and 5. As shown in FIG. 4, there is a case where only one sink nest 5 is observed near the center of the nugget 3, and as shown in FIG. 5, a plurality of sink nests 5A and 5B are dispersed in the nugget 3. May be observed. For this reason, when only one sink nest is observed in the nugget, the width of the sink nest (for example, width W in FIG. 4) is set as the total nest width. On the other hand, when a plurality of shrinkage nests are observed in the nugget, the sum total of the widths of the respective shrinkage nests (for example, the width W1 and the width W2 in FIG. 5) is defined as the total shrinkage width. That is, whether or not at least one of the condition I and the condition II is satisfied is determined by whether or not the total width of the shrinkage nest exceeds 100 μm.

-Depth of Depth In the steel plate welded member of the present embodiment, at least one of the large-diameter spot welds (spot welds where the nugget diameter Dn satisfies the above formula (A)), one side surface or both side surfaces in the plate thickness direction You may have a hollow.
Here, FIG. 3 shows an example of a cross-sectional view in which a dent has occurred. FIG. 3 shows a state in which indentations 6A and 6B are generated on both side surfaces of the large-diameter spot welded portion (heat affected zone) 4 in the plate thickness direction. The depth of the recess 6A (the difference in height in the plate thickness direction from the portion where no recess exists) is D1, while the depth of the recess 6B is D2.

And when it has a dent only on one side surface, the depth of this dent is the sum of the depths of both said dents when it has a dent on both surfaces (for example, in the case of the dents 6A and 6B shown in FIG. If present, the sum of the depth D1 and the depth D2) is preferably 5% or more of the total thickness (total thickness at a location where no dent is present) of a plurality of stacked steel plates.
The ratio of the depth of the depression (the sum of the depths of the depressions) to the total thickness of the steel sheet is in the above range, so that the above-described large diameter shrinkage nest existence probability is easily controlled to the above range, and as a result, as a steel plate welding member High strength is easily achieved, and excellent impact resistance and fatigue fracture resistance are easily achieved.

  In addition, as an upper limit of the ratio with respect to the total thickness of the steel plate of the said depth of depression (total of the depth of an indentation), 30% or less is preferable, 25% or less is more preferable, and 20% or less is further more preferable.

  In the steel plate welded member of the present embodiment, it is preferable that at least one of the two or more large-diameter spot welds satisfies the requirement of the depth of the recess (the sum of the depths of the recess). More preferably, the large-diameter spot welded portion satisfies the requirements for the depth of the recess (the sum of the depths of the recess). For example, the ratio of the number satisfying the requirement for the depth of the dent (the sum of the depths of the dent) among all the large-diameter spot welds is preferably half or more, and more preferably 80% or more. It is more preferable to satisfy all of them.

  The depth of the dent is obtained by measuring the difference in height in the plate thickness direction between the deepest part of the dent and a portion where no dent exists in the cross-sectional image obtained by the above-described measurement of the nugget diameter Dn.

[Spot welding method]
The spot welding method according to the present embodiment includes the following steps.
-Superposition | polymerization process The process of superimposing the several steel plate containing at least 1 high strength steel plate with tensile strength TS of 800 Mpa or more.
Spot welding process The welded electrode is sandwiched between the overlapped portions of the plurality of steel plates, and the welding is performed so that the pressing force Fe and the nugget diameter Dn by the welding electrode satisfy the following condition (B1) or condition (B2). The process of conducting spot welding by energizing the electrodes.
Condition (B1): Fe ≧ 500 kgf and Dn ≧ 2.5√h
Condition (B2): Fe ≧ 100 kgf and Dn ≧ 4√h
(Here, h represents 1/2 of the total thickness of the plurality of stacked steel plates.)

As described above, when a joint (welded portion) is formed by spot welding in the overlapped portion of the steel plates, a void called “shink nest” may be generated near the center of the nugget formed in the spot welded portion. The cause of the occurrence of the shrinkage nest is generally due to the coagulation contraction of the nugget after the energization is completed.
In addition, this shrinkage nest is more effective when spot welding is performed using one or more high-strength steel sheets having a tensile strength TS of 800 MPa or more, compared to a case where only a steel sheet having a lower tensile strength TS is used. It has been found that it tends to occur and a larger shrinkage nest is likely to occur.

On the other hand, in the spot welding method of the present embodiment, by performing spot welding so that the applied pressure Fe and the nugget diameter Dn satisfy the condition (B1) or the condition (B2), the occurrence of shrinkage is suppressed, The size of the generated nest can also be reduced.
The reason is guessed as follows.

The nugget shrinkage that occurs after the energization ends starts to occur from outside the nugget where the cooling proceeds faster. It is considered that this shrinkage progresses sequentially from the outside to the inside in the nugget, so that the volume decrease due to the shrinkage finally remains as a void near the center of the nugget, leading to the generation of a sink.
On the other hand, when the condition (B1) is satisfied, when the nugget shrinks after energization, the nugget is pushed in the center direction by applying a high pressure force from the electrode, and the volume decreases due to the shrinkage. This is probably because the minute is suppressed from remaining as a void near the center of the nugget.
Further, when the condition (B2) is satisfied, a large nugget is formed so that the nugget is more easily pushed toward the center by the applied pressure from the electrode. As a result, the volume reduction due to the shrinkage is reduced in the nugget center. It is thought that it is suppressed that it remains as a void in the vicinity.

  Thus, according to the spot welding method of the present embodiment, the generation of a sink nest is suppressed, and the size of the generated nest nest is also reduced.

  Next, each step of the spot welding method according to the present embodiment will be described in detail.

  The spot welding method according to the present embodiment includes a superposition process and a spot welding process. The spot welding process includes an energization process for energizing from the welding electrode, and may further include a squeeze process before the energization process, or may include a holding process after the energization process. Good.

-Superposition | polymerization process In a superposition | polymerization process, the several steel plate containing at least 1 high strength steel plate with tensile strength TS of 800 Mpa or more is prepared, and the location to weld by spot welding is piled up. For example, as shown in FIG. 2, two high-strength steel plates 1A and 1B having a tensile strength of TS800 MPa or more are prepared, and spots where spot welding is performed are overlapped.
The high-strength steel plate used here will be described in detail later.

-Spot welding process In the spot welding process, first, the overlapped portions of a plurality of steel plates are sandwiched between welding electrodes from both sides. For example, as shown in FIG. 2, two superposed high strength steel plates 1A and 1B are sandwiched from both sides by welding electrodes 2A and 2B.
Next, spot welding is performed by energizing while applying pressure with the welding electrode. For example, as shown in FIG. 1, pressure is applied to the two high-strength steel plates 1A and 1B that are overlapped by welding electrodes 2A and 2B sandwiched from both sides in the directions of arrows C1 and C2, respectively. The pressure applied by the welding electrode may be applied from the other welding electrode (for example, the upper welding electrode 2A) while one welding electrode (for example, the lower welding electrode 2B) is fixed. And the spot welding part 4 used as a coupling is formed by energizing these welding electrodes 2A and 2B and performing spot welding.

At this time, control is performed so that the pressing force Fe and the nugget diameter Dn by the welding electrode satisfy the following condition (B1) or condition (B2).
-Condition (B1)-
Fe ≧ 500 kgf and Dn ≧ 2.5√h
(Here, h represents 1/2 of the total thickness of the plurality of stacked steel plates.)

Regarding condition (B1), the applied pressure Fe satisfies Fe ≧ 500 kgf and more preferably satisfies Fe ≧ 600 kgf. On the other hand, the upper limit is not particularly defined, but there is no problem as long as it is within the strength of the electrode or the welding machine.
Regarding the condition (B1), the nugget diameter Dn satisfies Dn ≧ 2.5√h. On the other hand, the upper limit is not particularly defined, but there is no problem as long as dust is large and does not cause poor welding.

-Condition (B2)-
Fe ≧ 100 kgf and Dn ≧ 4√h
(Here, h represents 1/2 of the total thickness of the plurality of stacked steel plates.)

Regarding condition (B2), it is more preferable that the applied pressure Fe satisfies Fe ≧ 100 kgf and satisfies Fe ≧ 250 kgf. On the other hand, the upper limit is not particularly defined, but there is no problem as long as it is within the strength of the electrode or the welding machine.
Regarding the condition (B2), the nugget diameter Dn satisfies Dn ≧ 4√h. On the other hand, the upper limit is not particularly defined, but there is no problem as long as dust is large and does not cause poor welding.

  Furthermore, it is preferable to satisfy both the above conditions (B1) and (B2), that is, it is preferable to satisfy the conditions of “Fe ≧ 500 kgf and Dn ≧ 4√h”.

  The applied pressure Fe refers to the pressure (kgf) applied from the welding electrode sandwiched between the stacked steel plates.

  Examples of means for controlling the nugget diameter include means for adjusting the current value during energization, adjusting the energization time, adjusting the contact area of the welding electrode, and adjusting the pressure applied from the welding electrode.

-Other conditions-
The current value at the time of energization is adjusted according to the required nugget diameter, plate thickness, etc., for example, preferably in the range of 4.0 kA to 10.0 kA, more preferably below the current value at which no dust is generated. preferable.
The energization time is adjusted according to the plate thickness or the like, and is preferably in the range of 0.1 s to 0.8 s, for example.

In the spot welding process, a squeeze process may be provided before the energization process. The squeeze process is a process of sandwiching the welding electrode while holding the applied pressure Fe before energizing the welding electrode. The squeeze time in the squeeze step is not particularly limited, but it is preferable to make the squeeze time longer than the time for the applied pressure to reach a certain value.
In the spot welding process, a holding process may be provided after the energization process. The holding step is a step of continuing the sandwiching by the welding electrode while holding the applied pressure Fe after the energization from the welding electrode is finished. The holding time in the holding step is preferably carried out until, for example, the coagulation shrinkage of the nugget is completed, and is not particularly limited, but is specifically preferably 0.1 s or more, and 0.3 s or more. Is more preferable, and it is more preferable to set it as 1.3 s or more. The upper limit of the holding time may be set mainly from the viewpoint of productivity.

  After the spot welding process, spot welding is completed by releasing the pressure applied by the welding electrode and separating the welding electrode from the steel plate.

-Welding electrode-
For example, a pair of cylindrical electrodes are used as the welding electrodes. For example, as shown in FIG. 1, spot welding can be performed by sandwiching stacked steel plates by pressing the tips of cylindrical electrodes 2 </ b> A and 2 </ b> B from both sides.

As the tip shape of the cylindrical electrode, for example, as shown in FIG. 6, (A) flat shape, (B) radius shape (arc shape), (C) dome radius shape (contact surface is flat or loose) (D) cone flat shape (contact surface is flat or curved with a gentle curvature, and its periphery is tapered) A certain shape) and (E) point shape (a shape where the contact surface is a flat surface or a curved surface having a gentle curvature and the periphery thereof is a tapered shape) are generally used.
Among these, (A) flat shape, (C) dome radius shape, (D) cone flat shape, and (E) point shape, where the contact surface is a flat surface or a curved surface having a gentle curvature, are more preferable. ) A dome radius shape is more preferable, and a contact surface is a curved surface having a gentle curvature (for example, R40), and a (C) dome radius shape is particularly preferable.

  When the tip shape of the cylindrical electrode is a flat surface or a curved surface having a gentle curvature as the contact surface (that is, in the case of the shapes (A), (C), (D), and (E)), The diameter φ of the tip portion (that is, a flat portion or a curved portion having a gentle curvature) is adjusted according to the required nugget diameter, plate thickness, etc., and is preferably in the range of 4 mm to 10 mm, for example.

  Although FIGS. 1 to 3 show an example in which two steel plates are overlapped and spot-welded, even when spot welding three or more steel plates including at least one high-strength steel plate, Spot welding can be performed by the same method as shown in FIG.

(High strength steel plate)
Next, a high-strength steel plate (a steel plate having a tensile strength TS of 800 MPa or more) used in the steel plate welding member and the spot welding method according to the present embodiment will be described.

・ Tensile strength TS
In this embodiment, a high strength steel plate having a tensile strength TS of 800 MPa or more is used for at least one of the plurality of steel plates to be overlapped.

  The tensile strength TS in a steel plate is measured by conducting a tensile test according to JIS Z 2241 (2011).

  In addition, it is more preferable that there are two or more high-strength steel plates among a plurality of steel plates to be overlapped, and it is more preferable that all are high-strength steel plates.

-Composition Here, although the composition of the steel plate (especially high strength steel plate whose tensile strength TS is 800 MPa or more) used in this embodiment is not particularly limited, for example, the following composition is preferable.

% By mass
C: 0.100% to 0.400%,
Si: 0.30% to 3.00%,
Mn: 1.00% to 10.00%,
P: 0% to 0.010%,
S: 0% to 0.0050%,
Steel, the balance of which is Fe and impurities, or a part of the Fe is replaced with other additive elements.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples. It is obvious for those skilled in the art that various changes or modifications can be conceived within the scope of the idea described in the claims, and these naturally belong to the technical scope of the present invention. It is understood.

-Steel plate In Table 1, the component composition of C, Si, Mn, P, and S contained in the used "steel plates 1-4" and its tensile strength TS are shown. In addition, each steel plate is a cold rolled material without plating, and is a steel plate with a plate thickness of 1.6 mm.

<Example A: Influence of nugget diameter>
First, the effect of the nugget diameter on the generation and size of the sink nest is shown in the following examples.
As described below, the shrinkage nests were observed when the nugget diameter (target value) was changed to 3√h, 4√h, and 5√h for each steel plate.

-Spot welding-
First, two steel sheets of steel type 1 (plate thickness 1.6 mm, tensile strength TS590 MPa) are used, the plates are overlapped (superposition process), sandwiched between the welding electrodes from both sides, and this welding electrode is energized for spot welding. Performed (spot welding process).
The welding machine used is a stationary type, DC inverter, air pressurizing type welding machine, and the welding electrode used is DR-φ6-R40 (material Cr—Cu, electrode tip diameter φ6 mm, curvature with a gentle contact surface (R40). Curved surface dome radius shape), the applied pressure between the electrodes is 400 kgf, the squeeze time is 0.6 s, the energizing time is 0.36 s, and the holding time (after the energization from the electrodes is finished, the electrodes are further kept holding the applied pressure) The time during which the pinching is continued is 0.40 s.
Further, the nugget diameter (target value) is “3√h” (where h is 1/2 of the total thickness of the two stacked steel plates, that is, “1.6 mm”). The current value was controlled.

  In addition, spot welding with a nugget diameter (target value) of 3√h using this steel type 1 was performed three times (three locations).

  Table 2 below shows the results (actual measurement values) of measuring the nugget diameter for each spot welded portion that was performed three times.

・ Observation of sink nest (measurement of total width of sink nest)
As described above, each spot welded portion that was performed three times was cut to prepare an embedded sample, and a polished and etched cross section was observed using an optical microscope. When a sink nest was observed, the width of the sink nest (total width of the sink nest) was measured.
In some cases, the sink nest was observed only at one location in the center of the nugget, or a plurality of the nests were observed dispersedly. Therefore, when there is only one sink nest observed in the nugget, the width of the sink nest is the total width of the sink nest, and when multiple nests are observed in the nugget, the width of each sink nest is The sum was taken as the total width of the sink.
The results are shown in Table 2 below. If no sink marks are observed, “no sink marks” is indicated, and if observed, the “total width of sink marks” is displayed.

  Furthermore, the condition that the number of the sink nests observed in the nugget is one and the width of the sink nest exceeds 100 μm, or there are a plurality of the observed nests and the total width of the sink nests (the width of each sink nest Calculate the probability of satisfying the condition that the total sum exceeds 100 μm, that is, the ratio of the number of spot welds satisfying the above condition (probability of large-diameter shrinkage nests) among the spot welds performed at three locations as described above. did. The results are shown in Table 2 below.

-Example of changing nugget diameter and steel type-
Next, spot welding was performed three times each using two steel types 1 and changing the nugget diameter (target value) to “4√h” and “5√h”. Specifically, the nugget diameter was controlled to be 3√h, 4√h, and 5√h by adjusting the current value in the range of 5.1 kA to 7.5 kA.

Furthermore, the type of steel plate used is steel type 2 (plate thickness 1.6 mm, tensile strength TS980 MPa), steel type 3 (plate thickness 1.6 mm, tensile strength TS1180 MPa), steel type 4 (plate thickness 1.6 mm, tensile strength TS1310 MPa). In the same manner as in the case of steel type 1, spot welding was performed three times each so that the nugget diameter was 3√h, 4√h, and 5√h.
The nugget diameter is controlled by adjusting the current value in the range of 4.6 kA to 6.8 kA in the example using the steel type 2 and from 4.5 kA to 6.7 kA in the example using the steel type 3. In the example using the steel type 4, the current value was adjusted in the range of 4.6 kA to 7.0 kA.

  Also for these, as a result of the actual measurement of the nugget diameter and the observation of the shrinkage nest (measurement of the total width of the shrinkage nest), the probability that the total width of the nests exceeds 100 μm at the three spot welds (probability of existence of the large diameter nest nest), Is shown in Table 2 below.

In each “reference example” using steel type 1 having a tensile strength TS of 590 MPa, no sink marks were observed.
On the other hand, in each “Comparative Example” using steel types 2, 3, and 4 with a tensile strength TS of 800 MPa or more and a nugget diameter (target value) of 3√h, a shrinkage nest was observed.
In each “Example” using steel types 2, 3, and 4 and having a nugget diameter (target value) of 4√h or 5√h, the tendency of shrinkage of the shrinkage nests was recognized as compared with the above comparative example. It is done.

<Example B: Effect of applied pressure>
Next, the influence of the pressure applied between the electrodes on the generation and size of the shrinkage nest is shown in the following examples.
As described below, the shrinkage nest when the pressing force was changed to 400 kgf and 600 kgf for the steel plate 4 was observed.

-Spot welding-
First, two steel sheets of steel type 4 (thickness 1.6 mm, tensile strength TS1310 MPa) were overlapped (superposition process), then sandwiched between welding electrodes from both sides, and the welding electrodes were energized to perform spot welding (spot welding). Welding process).
The welding machine and the welding electrode used are the same as in Example A.
The current value from the electrode was controlled so that the nugget diameter (target value) was “3√h” (h is 1.6 mm).
Moreover, it implemented about the case where the applied pressure between electrodes is 400 kgf (comparative example), and 600 kgf (example).
The squeeze time was 0.6 s, the energization time was 0.36 s, and the holding time was changed between 0.16 s and 1.98 s.
About each, spot welding was implemented 3 times (3 places).

  For the spot welds that were carried out three times each, the actual value of the nugget diameter and the observation of the shrinkage nests (measurement of the total width of the shrinkage nests) resulted in a probability that the total width of the nests exceeds 100 μm at the three spot welds (large Table 3 and Table 4 below show the probability of existence of radial shrinkage nests.

  In each “Comparative Example” in which the applied pressure between the electrodes was 400 kgf and the holding time was changed between 0.16 s and 1.98 s, the total width of the shrinkage nest was about 240 μm on the average, whereas the applied pressure was 600 kgf In each “Example” in which the holding time was changed between 0.16 s and 1.98 s, the total width of the sink nest was about 15 μm on average. That is, in each “Example” with a pressurizing force of 600 kgf, there is a tendency that the shrinkage nest shrinks compared to each “Comparative Example” with a pressurizing force of 400 kgf.

<Example C: Effect of depression>
Next, the effect of the size of the indentation on the occurrence and size of the sink nest is shown in the following examples.
As described below, the size of the dent and the shrinkage nest when the pressing force of the steel plate 4 was changed to 400 kgf and 600 kgf were observed.

-Spot welding-
First, two steel sheets of steel type 4 (thickness 1.6 mm, tensile strength TS1310 MPa) were overlapped (superposition process), then sandwiched between welding electrodes from both sides, and the welding electrodes were energized to perform spot welding (spot welding). Welding process).
The welding machine and the welding electrode used are the same as in Example A.
The nugget diameter (target value) was changed to “3√h”, “4√h”, “5√h” (h is 1.6 mm), and the applied pressure between the electrodes was changed to 400 kgf and 600 kgf. And carried out.
The squeeze time was 0.6 s, the energization time was 0.36 s, and the holding time was 0.4 s.
About each, spot welding was implemented 3 times (3 places).

As for the spot welded portions that were carried out three times, as a result of observing the measured value of the nugget diameter, the depth of the indentation, and the shrinkage nest (measurement of the total width of the shrinkage nest), the total width of the sunken nest at the three spot welds was 100 μm. Table 5 below shows the probability (existence probability of large diameter nest of nest) exceeding.
The depth of the depression was measured from a cross-sectional photograph at the time of observing the sinkhole.

As the nugget diameter increases, that is, as the nugget diameter (target value) increases to 3√h, 4√h, and 5√h, the tendency of the dent to become deeper is recognized. Further, it is recognized that as the pressurizing force between the electrodes is increased, that is, as the pressurizing force is increased to 400 kgf and 600 kgf, the depression becomes deeper.
And the tendency for shrinkage | contraction to shrink | contract is recognized, so that a hollow becomes deep.

  By utilizing the present invention, a high-strength steel plate welded member joined by spot welding with a plurality of steel plates overlapped, and suppression of the occurrence of shrinkage when spot welding with a plurality of steel plates overlapped And the spot welding method which implement | achieved reduction of the magnitude | size of the generated shrinkage nest can be provided. For this reason, the present invention has high applicability in industries that manufacture steel products by spot welding.

1A, 1B High-strength steel plate 2A, 2B Welding electrode 3 Nugget 4 Spot weld (heat affected zone)
5, 5A, 5B Shrink webs 6A, 6B

Claims (3)

A steel plate welding member in which a plurality of steel plates including at least one high strength steel plate having a tensile strength TS of 800 MPa or more are overlapped and spot-welded at a plurality of locations,
The nugget diameter Dn has two or more large-diameter spot welds having a nugget part satisfying the following formula (A),
The two or more large-diameter spot welds have the condition that the nugget portion has a shrinkage nest having a width of more than 100 μm, and the sum of the widths of the plurality of shrinkage nests has a plurality of shrinkage nests in the nugget portion. A steel plate welded member having a probability of satisfying at least one of the conditions of exceeding 100 μm being 70% or less.
Formula (A): Dn ≧ 2.5√h
(Here, h represents 1/2 of the total thickness of the plurality of stacked steel plates.)   When at least one of the large-diameter spot welds has a recess on one surface or both surfaces in the plate thickness direction, and the recess is only on one surface, the depth of the recess is such that the recess on both surfaces. 2. The steel plate welded member according to claim 1, wherein the sum of the depths of both of the recesses is 5% or more of the total thickness of the superposed steel plates. A superposition step of superposing a plurality of steel plates including at least one high strength steel plate having a tensile strength TS of 800 MPa or more;
The overlapped portions of the plurality of steel plates are sandwiched between welding electrodes from both sides, and the welding electrodes are energized so that the pressing force Fe and the nugget diameter Dn by the welding electrodes satisfy the following condition (B1) or condition (B2). Spot welding process for spot welding,
A spot welding method comprising:
Condition (B1): Fe ≧ 500 kgf and Dn ≧ 2.5√h
Condition (B2): Fe ≧ 100 kgf and Dn ≧ 4√h
(Here, h represents 1/2 of the total thickness of the plurality of stacked steel plates.)