JP2018171649A - Resistance spot-welding method and welding condition determination method for resistance spot welding - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a resistance spot-welding method capable of forming a weld zone having superior delayed fracture resistance properties even when a high-strength galvanized steel plate is joined with a small-diameter nugget formed.SOLUTION: A resistance spot-welding method is characterized in that at least one steel plate is a high-strength galvanized steel plate of 980-1,770 MPa in tensile strength which has a galvanizing layer on a surface on a weld face side, and also comprises a preliminary energization process and a main energization process energizing with a current value higher than that of the preliminary energization process after the preliminary energization process. The preliminary energization process is performed under welding conditions that a weld zone 22 has a diameter of 2√t [mm] or smaller and a region 23 where galvanizing of 3√t [mm] or larger in diameter is discharged is formed, a plate thickness of a steel plate which is less in plate thickness between two adjacent steel plates to be joined together being t [mm] at a stage where the preliminary energization process is completed. The main energization process is performed under a welding condition that a nugget of 3√t [mm] or larger is formed.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a resistance spot welding method for high-strength zinc-based plated steel sheets, and a resistance spot welding welding condition determination method for determining welding conditions excellent in delayed fracture resistance of a welded portion in resistance spot welding.

  In order to achieve both a reduction in the weight of an automobile body and an improvement in collision safety in order to improve the fuel efficiency of the automobile, efforts have been made to increase the strength of the steel sheet used and reduce its thickness. However, with the increase in tensile strength of automobile steel plates to 980 MPa class or higher, there has been concern about the deterioration of delayed fracture resistance of welds.

  In detail, the welding method mainly used in the automobile production process is resistance spot welding. However, the welded portion of this resistance spot welding is hard to cause martensitic transformation by rapidly cooling the melted portion. Become an organization. In addition, tensile residual stress is generated in the weld due to thermal contraction during the cooling process. Furthermore, hydrogen may be taken into the weld metal during welding from a plating layer, oil, moisture, or the like on the surface of the steel sheet, or hydrogen may enter the weld from the use environment (for example, in an acidic environment). Accordingly, the resistance spot welded portion may be in a very disadvantageous state from the viewpoint of delayed fracture resistance. Conventionally, since the strength (tensile strength) of the steel plate was not so high, the stress concentration on the welded portion was relatively small and delayed fracture was not a problem, but the tensile strength of the steel plate was 980 MPa class. High-strength steel sheets of higher grades contain a lot of hardenable elements such as carbon, so the nugget (the part melted and solidified by resistance spot welding) and its vicinity become very hard and prone to delayed fracture resistance. It becomes. Moreover, in the high-strength steel plate, since press forming becomes difficult, a gap is easily generated between the steel plates when they are overlapped. In resistance spot welding in which this gap is forcibly crushed by a pair of opposed electrodes and welded, tensile stress due to the gap in this steel sheet is added to the welded part, and it is considered that delayed fracture is likely to occur. .

  As a method for solving the problem of delayed fracture of the welded portion of resistance spot welding of such a high-strength steel sheet, there are the following techniques. For example, a method of softening the space between the steel plate press-contact portion and the nugget end by two-stage energization that performs non-energization after the first energization for forming the nugget and further performs the second energization, or immediately after welding energization A method of improving the toughness of the weld and reducing the tensile residual stress by increasing the applied pressure and decreasing the current (Patent Document 2) is disclosed.

International Publication No. 2014/171495 Japanese Patent Laying-Open No. 2015-93282

  In the methods of Patent Document 1 and Patent Document 2, as can be seen from the examples, the nugget diameter is as large as 4.5√t [mm] (t: plate thickness of the steel plate) or 5√t [mm] or more. Is targeted. Moreover, it mainly improves the delayed fracture resistance against the hydrogen entry from the outside by the acid immersion test, that is, the hydrogen entry from the use environment.

  However, in an actual vehicle body assembly using a steel sheet having a tensile strength of 980 MPa or more, there may be a case where it is difficult to crush the gap between the members to be joined with the electrode pressure of the resistance spot welder. In that case, in the methods of Patent Document 1 and Patent Document 2, when the energization is started, the contact state between the steel plates becomes insufficient, and when trying to obtain a nugget of a necessary diameter, spatter is generated and appropriate welding conditions are set. May not be possible. In addition, in order to guarantee the performance of automobile parts, even when the nugget diameter becomes smaller than expected due to some disturbance such as a shunt to the contact part other than the weld part between the steel plates, the delayed fracture resistance of the weld part is good. It is also important to be. Therefore, even when the nugget diameter is smaller than 4√t [mm], which is smaller than the nugget diameters described in Patent Document 1 and Patent Document 2, the delayed fracture resistance may be required to be good. . In particular, when the nugget diameter is small, the stress applied to the end of the nugget is larger than that when the nugget diameter is large, which is more disadvantageous from the viewpoint of delayed fracture resistance.

  In the case of resistance spot welding of a zinc-based plated steel sheet having a zinc-based plating layer on its surface, diffusible hydrogen incorporated into the steel sheet during the steel plate manufacturing process remains due to the presence of the zinc-based plating, causing delayed fracture The diffusible hydrogen that causes water is likely to remain in the steel sheet. That is, when resistance-welding a zinc-based plated steel sheet, diffusible hydrogen derived from a zinc-based plated layer, oil or moisture attached to the surface of the zinc-based plated layer is likely to be mixed into the weld metal during welding. , Delayed destruction is likely to occur. For this reason, it is important to use a resistance spot welding method for improving delayed fracture resistance against hydrogen derived from these zinc-based plating layers and the like.

  Moreover, the determination method of the suitability of the welding conditions excellent in the delayed fracture-proof characteristic of the weld part of the resistance spot welding which joins such a high intensity | strength zinc-plated steel plate is not established. If the determination method can be established, based on the result of the determination method, even if a nugget with a small diameter is formed on a high-strength galvanized steel sheet, a weld with excellent delayed fracture resistance is formed. Resistance spot welding that can be formed can be performed easily and appropriately. Therefore, a method for determining welding conditions excellent in delayed fracture resistance of the welded part of resistance spot welding of such a high-strength zinc-based plated steel sheet is also desired.

  The present invention has been made in view of the above problems, and even when a nugget having a small diameter is formed and joined to a high-strength zinc-based plated steel sheet, a welded portion excellent in delayed fracture resistance is provided. It is an object of the present invention to provide a resistance spot welding method and a welding condition determination method that can be formed.

The present inventors have repeated intensive studies to achieve the above object, and obtained the following knowledge.
Delayed fracture that occurs in a relatively short time after welding in a resistance spot welded joint (hereinafter also referred to as a resistance spot welded part) where a small-diameter nugget is formed on a high-strength galvanized steel sheet In order to suppress it, it is important to reduce hydrogen mixed in the weld metal during welding. Specifically, by welding the steel plate with two stages of energization, a pre-energization process and a main energization process that energizes at a higher current value than the pre-energization process after the pre-energization process, and applying an appropriate pre-energization process In addition, when the pre-energization process is completed, the diameter of the melted portion is set to a specific value or less and a region in which specific zinc-based plating is discharged is reduced, thereby reducing the hydrogen taken into the weld metal during welding, It was found that the delayed fracture resistance of the weld can be improved.

  In addition, as a method for determining the welding conditions of resistance spot welding that can improve such delayed fracture resistance, at the stage where the pre-energization process is completed, the diameter of the melted portion is set to a specific value or less and a specific zinc-based material is used. It has been found that it is effective to determine whether or not the welding condition can form the region where the plating is discharged.

  The present invention has been made on the basis of the above findings, and the gist is as follows.

[1] A resistance spot welding method in which two or more steel plates are overlapped and sandwiched between a pair of electrodes and energized while being pressed to form a nugget and join the steel plates,
At least one of the steel sheets is a high-strength zinc-based plated steel sheet having a tensile strength of 980 MPa to 1770 MPa and a zinc-based plated layer on the surface on the joining surface side,
A pre-energization step and a main energization step of energizing at a higher current value than the pre-energization step after the pre-energization step,
In the pre-energization step, when the pre-energization step is finished, the thickness of the thinner steel plate of the two adjacent steel plates to be joined is t [mm], and the diameter of the molten part is 2 √ t [mm] or less and under welding conditions in which a region where zinc-based plating with a diameter of 3 √t [mm] or more is discharged is formed,
The current energizing step is performed under a welding condition in which a nugget having a diameter of 3√t [mm] or more is formed.

  [2] The diameter of the area from which the zinc-based plating is discharged is set such that the inner diameter of the white ring-shaped area of zinc oxide observed on the steel sheet surface when the two adjacent steel sheets to be joined are peeled is −1 mm. The resistance spot welding method according to [1], wherein:

[3] When two or more steel plates are overlapped and sandwiched between a pair of electrodes and energized while being energized to form a nugget and resistance spot welding is performed to join the steel plates, at least one of the steel plates has a tensile strength. It is a high-strength galvanized steel sheet having a zinc-based plating layer on the surface at least on the joining surface side at 980 MPa to 1770 MPa, and the energization has a higher current than the pre-energization step after the pre-energization step and the pre-energization step A book that forms a nugget with a diameter of 3√t [mm] or more, where t [mm] is the thickness of the steel plate with the smaller thickness among the two adjacent steel plates to be joined. A method for determining resistance to delayed fracture of a welded portion of resistance spot welding having an energization process,
When the pre-energization process is completed, if there is a region where the diameter of the melted portion is 2√t [mm] or less and zinc-based plating with a diameter of 3√t [mm] or more is discharged, delayed fracture resistance A welding condition determination method for resistance spot welding, wherein it is determined that the characteristics are good.

  [4] At the stage where the pre-energization process is completed, a peel test is performed to peel the two adjacent steel plates to be joined together, and the diameter of the molten portion and the diameter of the region where the zinc-based plating is discharged are obtained. [3] The welding spot judgment method for resistance spot welding according to [3].

  [5] The diameter of the region from which the zinc-based plating discharged in the peel test is discharged is the inner diameter of the white ring-shaped region of zinc oxide observed on the steel plate surface—1 mm. The welding spot judgment method of resistance spot welding of description.

  [6] A resistance spot welding method, wherein the welding conditions for the pre-energization step are determined based on the welding condition determination method for resistance spot welding according to any one of [3] to [5].

  According to the present invention, in resistance spot welding of a high-strength galvanized steel sheet, the welding process is divided into a pre-energization step and a main energization step of energizing at a higher current value than the pre-energization step. Even if the nugget diameter formed in this energization process is small, it has excellent delayed fracture resistance by forming the area where the diameter of the molten part is below a specific value and the specific zinc plating is discharged. It is possible to form a welded portion (resistance spot welded portion). The present invention is suitable for resistance spot welding in which, for example, a high-strength galvanized steel sheet is formed with a small nugget diameter and used in processes such as manufacturing automobile parts and assembling a vehicle body.

FIG. 1 is a cross-sectional view schematically showing an example of a resistance spot welding method. FIG. 2A is a cross-sectional view schematically showing a molten portion and a region where zinc-based plating is discharged at the stage where the pre-energization process in the present invention is completed, and FIG. It is a top view when the steel plate of the area | region shown to (a) was peeled. FIG. 3 is a side view showing a test piece for resistance spot welding performed in an example of the present invention.

  The resistance spot welding method of the present invention is a resistance spot welding method in which two or more steel plates are overlapped and sandwiched between a pair of electrodes and energized while being pressed to form a nugget and to join the steel plates. Is a high-strength galvanized steel sheet having a tensile strength of 980 MPa to 1770 MPa and a zinc-based plating layer on the surface on the joining surface side, and a higher current value than the pre-energization step after the pre-energization step and the pre-energization step The pre-energization step is a stage where the pre-energization step is completed, and the thickness of the steel plate having the smaller thickness among the two adjacent steel plates to be joined is t [mm. ] Under the welding conditions in which the region where the diameter of the molten part is 2√t [mm] or less and the zinc-based plating with the diameter of 3√t [mm] or more is discharged is formed, Diameter 3√t [mm Those carried out in the welding conditions over nugget is formed.

  In the present invention, two or more steel plates are joined by resistance spot welding. FIG. 1 is a cross-sectional view schematically showing an example of a resistance spot welding method, and shows an example of performing resistance spot welding of two steel plates. FIG. 2 is a cross-sectional view (FIG. 2 (a)) schematically showing the molten portion and the area where zinc-based plating is discharged at the stage where the pre-energization process is completed. FIG. 2 is a plan view (FIG. 2B) when the steel sheet in the region where the water is discharged is peeled off. The resistance spot welding method of the present invention will be described below with reference to FIGS.

  First, two or more steel plates are overlapped. As shown in FIG. 1, when using two steel plates, the lower steel plate 1 arranged on the lower side and the upper steel plate 2 arranged on the upper side are overlapped.

  In the present invention, at least one steel plate to be resistance spot welded is a high-strength zinc-based plated steel plate having a tensile strength of 980 MPa to 1770 MPa and a zinc-based plating layer on at least the surface on the joining surface side. Since the problem of delayed fracture does not occur when the tensile strength of the steel sheet is less than 980 MPa, it is out of the scope of application of the present invention. Further, when the tensile strength of the steel sheet exceeds 1770 MPa, the strength is too high, so that it is difficult to obtain the effect of improving delayed fracture resistance according to the present invention. In addition, examples of the zinc-based plating that the steel sheet has include hot dip galvanizing (GI), alloyed hot dip galvanizing (GA), electric pure galvanizing, and electrolytic zinc alloy plating (Zn—Ni). The zinc-based plating layer may be provided at least on the surface of the lower steel plate 1 or the upper steel plate 2 on the side of the joining surface (the mating surface of the lower steel plate 1 and the upper steel plate 2). It may be provided on both sides. The steel sheets to be superposed may all be high-strength zinc-based plated steel sheets having a tensile strength of 980 MPa to 1770 MPa and having a zinc-based plated layer on at least the bonding surface side surface.

  In the present invention, the thickness of the steel plate to be resistance spot welded is not particularly limited, but is preferably in the range of, for example, 1.0 mm or more and 2.3 mm or less. A steel plate having a thickness within this range can be suitably used as an automobile member.

  The two or more steel plates to be resistance spot welded may be the same or different, may be the same type and the same shape, or may be different types or different shapes.

  Next, a pair of welding electrodes, that is, the electrode 4 disposed on the lower side and the electrode 5 disposed on the upper side, sandwich the stacked steel plates (the lower steel plate 1 and the upper steel plate 2), and energize while applying pressure. The structure in which the pressure is applied by the electrodes 4 and 5 and the pressing force is controlled is not particularly limited, and conventionally known devices such as an air cylinder and a servo motor can be used, and the shape (stationary type, robot gun) is also available. There is no particular limitation. Further, the present invention can be applied to both direct current and alternating current, and the type of power source (single-phase alternating current, alternating current inverter, direct current inverter) and the like are not particularly limited. In the case of alternating current, “current” means “effective current”. Further, the type of the tip of the electrode 4 or the electrode 5 is not particularly limited. For example, DR type (dome radius type), R type (radius type), D type (dome type) described in JIS C 9304: 1999, and the like. Is mentioned. The tip diameters of the electrode 4 and the electrode 5 are, for example, 4 mm to 16 mm, and the curvature radius of the tip of the electrode 4 or the electrode 5 is 6.5 mm to 200 mm. In addition, resistance spot welding is performed in a state where the electrode is always water-cooled.

  A welded joint is obtained by energizing the steel plates stacked in this manner while being sandwiched between a pair of welding electrodes to form a nugget by resistance heating and joining the stacked steel plates. In the present invention, this energization is a pre-energization step and a main energization step of energizing at a higher current value than the pre-energization step after the pre-energization step.

  In the pre-energization process, the diameter (outer diameter) of the region where the diameter (outer diameter) of the melted portion is 2√t [mm] or less and zinc-based plating is discharged is the stage when the pre-energization process is completed. It is performed under the welding conditions formed so as to be 3√t [mm] or more. t [mm] is the thickness of the steel plate having the smaller thickness among the two adjacent steel plates to be joined.

  Here, the fusion | melting part in the stage which the pre-energization process was complete | finished, and the area | region where zinc-type plating was discharged are demonstrated using FIG. FIG. 2A shows a case where zinc-based plating layers are provided on both surfaces of the lower steel plate 1 and the upper steel plate 2. A pre-energization step in which a steel plate (lower steel plate 1 and upper steel plate 2) having a zinc-based plating layer 21 on the surface of the steel plate is overlapped with a gap and sandwiched between a pair of electrodes (not shown) and energized while being pressed. By performing, the lower steel plate 1 and the upper steel plate 2 between the electrodes are heated as shown in FIG. Since the melting point of the zinc-based plating is lower than that of the steel plate, the zinc-based plating layer 21 in the vicinity of between the steel plates immediately below the electrodes is first melted and discharged (exhausted) from the vicinity of the steel plates immediately below the electrodes by pressurization. A region where the zinc-based plating layer 21 is reduced or eliminated due to the discharge of the zinc-based plating layer 21 in the vicinity of the steel plate immediately below the electrode is a region 23 where the zinc-based plating is discharged. By forming the region 23 from which zinc-based plating is discharged in the pre-energization process, zinc-based plating is applied from the region that becomes the welded portion (welded metal (ie, nugget 3) and heat-affected zone) in the pre-energization step. Will be discharged. In the present invention, at the stage where the pre-energization process is completed, the steel sheet in this region (the region to be the welded portion described above) is not melted and the melted portion 22 is not formed or is melted and melted. The portion 22 may be formed. When the molten part 22 is formed by melting, the diameter of the molten part 22 is set to a specific size. Further, at the stage where the pre-energization process is completed, the region 23 where the zinc-based plating is discharged is set to a specific size. Specifically, the diameter of the melted portion 22 is 2√t [mm] or less (including 0 [mm]), and the diameter of the region 23 from which the zinc-based plating is discharged is 3√t [mm] or more. A pre-energization process is performed so that it may become.

  As described above, in the present invention, in the pre-energization step performed before the main energization step, the specific molten portion 22 and the specific zinc-based plating in the vicinity of the space between the steel plates immediately below the electrodes are discharged to the outside near the molten portion 22. A region 23 where the specific zinc-based plating is discharged is formed. Thereby, it is suppressed that the oil component and water | moisture content adhering to zinc type plating which can become a hydrogen source, and zinc type plating mix in the fusion | melting part 22. FIG. In the pre-energization process, the region near the welded portion to be joined is heated to a high temperature by energization in a state where compressive stress is applied due to the pressurization of the electrode, so the diffusibility trapped in the steel plate in that region Hydrogen is rapidly diffused and released, and the amount of hydrogen mixed in the melted portion 22 that becomes a nugget formed during the main energization process can be reduced. In addition, the heat-affected zone expands due to the heat input during the pre-energization process, and the periphery of the weld zone is softened widely.As a result, the tensile stress in the opening direction applied to the nugget end after the completion of the current energization process can be reduced. it can. Since the nugget is formed in the main energization process after such a pre-energization process, the nugget diameter when resistance spot welding of the high-strength galvanized steel sheet in which the deterioration of the delayed fracture resistance is a problem is 3√t [mm ] Even when the nugget diameter is as small as less than 4√t [mm], delayed fracture occurring at a relatively early stage (within 24 hours) after welding of the welded joint is suppressed, and delayed fracture resistance is improved. A good weld is formed.

  In the region 23 where zinc-based plating is discharged, it is preferable that zinc plating is completely discharged and zinc-based plating does not exist. However, even if a part of the hydrogen source adhering to the zinc-based plating surface is discharged together with the zinc-based plating, it is effective in reducing the amount of hydrogen mixed in the nugget during the main energization process, so it is completely discharged. It does not have to be. For example, a region where the thickness of the zinc-based plating layer is reduced to ½ or less of that before welding may be defined as a region 23 where zinc-based plating is discharged, and the diameter of the region may be 3√t [mm] or more. The diameter of the region 23 from which zinc-based plating formed at the stage where the pre-energization process is completed is preferably equal to or larger than the nugget diameter formed in the main energization process.

  The diameter of the melted portion 22 at the stage where the pre-energization process is completed is the maximum diameter of the melted portion 22 at the joint surface between two adjacent steel plates to be joined. Moreover, the diameter of the area | region 23 where zinc-type plating was discharged is the minimum diameter of the area | region 23 where zinc-type plating was discharged in the joining surface of two adjacent steel plates joined.

  The diameter of the melted portion 22 and the diameter of the region 23 where the zinc-based plating is discharged are, for example, a sample in which welding is stopped at the stage where the pre-energization process is completed, and two adjacent sheets joined to this sample. A peel test for peeling the steel plates (the lower steel plate 1 and the upper steel plate 2) can be performed and the peeled surface can be observed. For example, the molten zinc-based plating discharged to the outside from the vicinity of the melted portion 22 and the discharged region 23 accumulates in a ring shape outside the region 223 where the zinc-based plating is discharged. A region where the molten zinc-based plating is accumulated in a ring shape outside the region 23 where the zinc-based plating is discharged is indicated by a region 25 in FIG. Further, vaporized zinc is oxidized and adhered as zinc oxide on the outer side. When a peel test is performed, a white ring-shaped region 24 is observed on the steel plate surface as shown in FIG. . Accordingly, the inner diameter of the region 25 where the molten zinc-based plating is accumulated in a ring shape becomes the diameter (outer diameter) of the region 23 where the zinc-based plating is discharged. Here, although the diameter (outer diameter) of the region 23 from which the zinc-based plating is discharged may be difficult to discriminate, the width of the region 25 is about 0.5 mm or less depending on the plating thickness and shape. . The diameter obtained by subtracting 1 mm from the inner diameter of the white ring-shaped region 24 is the diameter (outer diameter) of the region 23 where zinc-based plating is discharged, and the region where zinc-based plating is easily discharged by observing the inner diameter of this zinc oxide. The diameter of 23 may be obtained. That is, the conditions of the pre-energization process are welding conditions in which the diameter of the melted portion 22 is 2√t [mm] or less and the inner diameter of the white ring-shaped region 24 of zinc oxide −1 mm is 3√t [mm] or more. You may make it do. In addition, by observing the cross section without peeling off the lower steel plate 1 and the upper steel plate 2, the diameter of the melted portion 22 and the region 23 where the zinc-based plating is discharged can be obtained.

  In addition, in FIG. 2, although the form which overlapped the steel plate (the lower steel plate 1 and the upper steel plate 2) so that it might have a clearance, and carried out resistance spot welding was shown, you may overlap so that a steel plate may not have a clearance gap. . However, in the case of resistance spot welding with the steel plates overlapped so as to have a gap, the applied pressure applied to crush the gap increases, and as a result, the residual stress increases, so that delayed fracture is likely to occur. Therefore, the effect of the present invention is remarkable.

  In the main energization process, energization is performed at a higher current value than in the pre-energization process. And in this energization process, it carries out on the welding conditions in which the nugget 3 whose diameter is 3√t [mm] or more is formed. Similar to the pre-energization step, t [mm] is the thickness of the steel plate having the smaller thickness among the two adjacent steel plates to be joined. Here, the nugget diameter of 3√t [mm] or more is set to be less than 3√t [mm], and the mechanical properties such as tensile shear strength and cross tensile strength of the welded joint are low, and delayed fracture resistance is exhibited. This is because the mechanical properties cannot be obtained even if the improvement is made. Further, the nugget diameter may be 4√t [mm] or more, but when the nugget diameter is large, the delayed fracture resistance per se, which is a problem in the present invention, often does not become a problem.

  In the present invention, in the pre-energization process, the region 23 in which the diameter of the melted portion 22 is set to a specific value or less and specific zinc-based plating is discharged is formed, and thereafter, 3√ at a higher current value than in the pre-energization process. The main energization process is performed under welding conditions in which a nugget 3 of t [mm] or more is formed. For this reason, the resistance spot welding excellent in the delayed fracture resistance of the welded portion in which the amount of hydrogen mixed in the nugget 3 to be formed is small and the tensile stress in the opening direction applied to the end of the nugget 3 is reduced. It becomes possible. The “nugget” is a melt-solidified portion generated in a welded portion in lap resistance welding, and the nugget diameter here is the maximum diameter at the joint surface of adjacent steel plates.

  Although FIG. 2 shows an example in which two steel plates are overlapped and welded, the present invention can be similarly applied to a case where three or more steel plates are welded. When welding three or more steel plates, the fusion zone 22 formed at the stage of the pre-energization process on the joint surface where the zinc-based plating layer of the high-strength zinc-based plated steel plate having a tensile strength of 980 MPa to 1770 MPa is present and What is necessary is just to make it the diameter of the area | region 23 where zinc-type plating was discharged | emitted, and the nugget diameter formed by this energization process become the said range.

  The resistance spot welding method of the present invention having such a specific pre-energization step and a main energization step performs, for example, the following pre-energization step, cooling step, main energization step, and holding step in this order.

(Pre-energization process)
The pre-energization step is an energization condition that satisfies the following formulas (1) and (2), preferably an energization condition that satisfies the following formulas (1) and (3).
2 <Ip ≦ 1.3F (1)
Tp ≧ 7tt (2)
Tp ≧ 10tt (3)
However, in the above formulas (1) to (3), Ip: current value [kA] in the pre-energization process, F: electrode pressure [kN], Tp: energization time in the pre-energization process [cycle (50 Hz)], tt: The total thickness [mm] of two adjacent steel plates. If the applied pressure F is too low, it is difficult to stably perform resistance spot welding of a high strength steel plate. The total plate thickness tt is 1.5 mm or more.

  In the present specification, each of the above formulas defines a relationship of only numerical values.

When the current value (welding current) Ip in the pre-energization process is 2 kA or less, the welding current is too low, so that the region 23 in which the zinc-based plating layer is discharged becomes 3√t [mm] or more, which is a very long energization time. Necessary. On the other hand, the higher the electrode pressing force F, the larger the contact diameter between the steel plates, and it becomes possible to prevent the excessive melted portion 22 from being formed even when a large current is applied, so that a high current can be applied. When the current value Ip exceeds 1.3 F [kA], the large melted portion 22 grows rapidly during the pre-energization process, and the zinc-based plating is sufficiently discharged from the vicinity of the melted portion 22 into the molten metal. In some cases, zinc-based plating and deposits that become hydrogen sources, such as water and oil, are taken in. Therefore, the current value Ip in the pre-energization process preferably satisfies the above formula (1).
In addition, when the energization time Tp in the pre-energization process is less than 7 tt [cycle (cyc)], the energization time is short, and the size of the region 23 from which the zinc-based plating is discharged is insufficient. In addition, the effect of releasing hydrogen in the steel sheet by applying pressure and heat is small. Therefore, it is preferable that the energization time Tp in the pre-energization process satisfies the above formula (2). More preferably, the energization time Tp of the pre-energization step is 10 tt [cycles] or longer as expressed by the above formula (3). Here, in particular, the upper limit of the energization time Tp in the pre-energization process is not provided, but it is preferably 20 tt [cycle] or less from the viewpoint of productivity.
In the energization pattern of the pre-energization process, if the region 23 where the diameter of the melted portion 22 is 2√t [mm] or less and zinc-based plating with a diameter of 3√t [mm] or more is discharged is formed. In addition to the above-described one-stage energization, multistage energization conditions of two stages or three or more stages may be used. In the present invention, 1 cycle = 20 ms.

(Cooling process)
The cooling step is a step of cooling by holding the work in a state where the work is pressurized with an electrode (non-energized) after the pre-energization step, and may be performed as necessary in the present invention.

(Main energization process)
In this energization process, after the cooling process, welding is performed with an energization time and a welding current which are larger than the current value of the pre-energization process and are necessary to obtain a target nugget diameter.
The diameter of the target nugget 3 is desirably 4√t [mm] or more. As described above, according to the present invention, even if the nugget diameter formed in this main energization process is 3√t [mm] or more and less than 4√t [mm] due to some influence, delayed fracture of the resistance spot weld occurs. It becomes difficult to do.

(Holding process)
The holding step is a step of cooling and solidifying the welded portion by holding the workpiece in a state where current is stopped (non-energized) while pressing the workpiece with the electrode after the main energizing step. The holding time is preferably 1 cycle or more. Here, the upper limit of the holding time is not particularly set, but it is preferably 10 cycles or less from the viewpoint of productivity.

  Note that the pressure applied to the electrodes is constant throughout the pre-energization process, the cooling process, the main energization process, and the holding process.

  As described above, in the present invention, delayed fracture occurring at a relatively early stage (within 24 hours) after welding of the welded joint can be suppressed by using the specific pre-energization step and the main energization step.

  In addition, a method for determining welding conditions excellent in delayed fracture resistance of a welded part in resistance spot welding according to the present invention based on the above knowledge will be described below. A part of the description overlapping with the resistance spot welding method of the present invention is partially omitted.

The method of determining the welding conditions for resistance spot welding according to the present invention includes at least 1 when performing resistance spot welding in which two or more steel plates are overlapped and energized while being pressed between a pair of electrodes to form a nugget and join the steel plates. The steel sheet is a high-strength galvanized steel sheet having a tensile strength of 980 MPa to 1770 MPa and having a zinc-based plating layer on at least the surface of the joint surface. A nugget with a diameter of 3√t [mm] or more, where t [mm] is the thickness of the thinner steel plate of two adjacent steel plates to be joined. This is a method for determining the resistance conditions of resistance spot welding in which a welded portion having excellent delayed fracture resistance is obtained by a resistance spot welding method having a main energization step of forming a metal.
In the determination method, at the stage where the pre-energization process is completed, there is a region where zinc-based plating having a diameter of 3√t [mm] or more is discharged, and at that time, a molten part is not formed or a molten part When the diameter of the melted portion is 2√t [mm] or less, the delayed fracture resistance is determined to be good. On the other hand, at the stage where the pre-energization process is completed, the diameter of the area where zinc-based plating is discharged is less than 3√t [mm], and when the melted part is formed, the diameter of the melted part is 2√t. When satisfying at least one of exceeding [mm], it is determined that the delayed fracture resistance is poor under the welding conditions. The diameter of the melted part and the diameter of the area where the zinc-based plating is discharged are preferably obtained by observing a cross section of the welded part at the stage where the pre-energization process is completed. It can also obtain | require by performing the peeling test which peels a steel plate. It is preferable to use the value calculated | required by the internal diameter -1mm of the white ring-shaped area | region of the zinc oxide observed by a peeling test as the diameter of the area | region where zinc-type plating was discharged.

  Moreover, it can be set as the resistance spot welding method which determines the welding conditions of a pre-energization process based on the welding condition determination method of resistance spot welding of this invention. In the resistance spot welding method for determining the welding conditions in the pre-energization process based on the welding condition determination method excellent in delayed fracture resistance of the welded portion of the resistance spot welding of the present invention, first, pre-energization is performed under certain welding conditions. When the pre-energization process is completed, the diameter of the molten portion 22 and the diameter of the region 23 where the zinc-based plating is discharged are obtained by a peeling test or the like, and these are 2√t [mm] or less and 3 respectively. It is determined whether or not √t [mm] or more is satisfied. And when it determines with it being favorable, the welding conditions determined to be favorable are determined as the welding conditions of the pre-energization process of resistance spot welding. In the subsequent resistance spot welding, the pre-energization process and the main energization process under the determined welding conditions are performed. On the other hand, when it is not determined to be good, the welding conditions that are not determined to be good are changed based on the determination result (the diameter of the molten portion, the diameter of the region where zinc-based plating is discharged). This determination and change are repeated until it is determined that the changed welding conditions are good. And the welding conditions determined to be good are determined as the welding conditions in the pre-energization process of resistance spot welding. As described above, when a high-strength galvanized steel sheet is resistance spot welded, even if a nugget 3 having a small diameter is formed, a resistance spot that can form a welded portion having excellent delayed fracture resistance. The welding conditions of the welding method can be easily determined by a test up to the pre-energization process, and a welded portion having excellent delayed fracture resistance can be easily formed.

  Hereinafter, the present invention will be described by way of examples for further understanding of the present invention. However, the examples do not limit the present invention.

(Invention Example and Comparative Example)
As shown in FIG. 1, the lower steel plate 1 and the upper steel plate 2 were overlapped, resistance spot welding was performed, and the delayed fracture resistance was evaluated.

  The steel plates used are 980 MPa class galvannealed steel sheet (GA980), 1180 MPa class galvanized steel sheet (GI1180), 1180 MPa class galvannealed steel sheet (GA1180), and Zn-Ni plating hot stamp material (HS). . Table 1 shows the tensile strength and thickness of each steel plate. The tensile strength is a tensile strength obtained by conducting a tensile test on a JIS No. 5 tensile test piece in accordance with the provisions of JIS Z 2241: 2011. The Zn-Ni plating hot stamp material is obtained by heating a steel material to 900 ° C. for 3 minutes in an atmospheric furnace and then cooling the mold, and has a tensile strength of 1470 MPa class. The welding machine was an inverter DC resistance spot welding machine. The electrodes 4 and 5 were both DR type chromium copper electrodes having a tip diameter (tip diameter) of 6 mm and a curvature radius of 40 mm.

  As shown in FIG. 3, resistance spot welding is performed by using a spacer 33 having a thickness of 1.8 mm and a 40 mm square between the test pieces (lower steel plate 31 and upper steel plate 32) of the steel plate (long side 125 mm, short side 40 mm). Was temporarily welded with the spacer 33 in a state in which the gap 34 was present between the steel plates. The central part of the specimen was welded under the welding conditions described above and in Table 1. FIG. 3 is a side view showing a test piece of resistance spot welding performed in [Example] (invention example and comparative example).

As shown in Table 1, a pre-energization process, a cooling process, a main energization process, and a holding process are performed in this order to produce a welded joint, and welding is stopped when the pre-energization process described in Table 1 is completed separately. A sample was prepared. And the peeling test was done as follows with respect to the obtained sample.
(Peel test)
The spacer part provided with the spacer was cut off from the sample which was stopped from welding at the stage where the pre-energization process was completed, and then peeled off at the joining surface of the two steel plates with a scissors. The bonded surface of the peeled lower steel plate 31 is observed, the diameter of the melted portion 22 is measured, and the case where the diameter is 2√t [mm] or less exceeds the symbol “◯” and 2√t [mm] Was evaluated as a symbol “x”. In addition, by observing the bonded surface of the peeled lower steel plate 31, the inner diameter of the white ring of zinc oxide formed around the melted portion 22 is measured, and the zinc plating exhales the value obtained by subtracting 1 mm from the inner diameter. When the diameter was 3√t [mm] or more, the case where the diameter was less than 3√t [mm] was evaluated as the symbol “X”. The obtained evaluation results are shown in Table 1. The measured value is written in parentheses next to each symbol. In addition, the diameter of the fusion | melting part 22 formed in the lower steel plate 31 and the upper steel plate 32, respectively, and the diameter of the area | region 23 where the zinc-type plating was discharged were the same with the lower steel plate 31 and the upper steel plate 32. FIG.

Further, as shown in Table 1, the pre-energization process, the cooling process, the main energization process, and the holding process were performed in this order. The welding conditions for main energization were adjusted so that the nugget diameter obtained by main energization would be 3√t to 4√t [mm]. The obtained welded joint was subjected to a delayed fracture resistance test as follows.
(Delayed fracture resistance test)
The obtained welded joint was left in the atmosphere at room temperature, and when 24 hours had elapsed after the end of welding, the spacer portion provided with the spacer was cut off and cut in the thickness direction at the center of the weld. The cross section of the nugget was observed with an optical microscope by polishing the cross section with a saturated aqueous solution of picric acid to reveal a nugget shape. Delayed fracture resistance was evaluated by assuming that no cracks extending into the nugget were observed as “no cracking” and cracks extending into the nugget as “cracked”. Further, on the cut surface, the joint surface between the lower steel plate 31 and the upper steel plate 32 was observed, and the diameter of the nugget 3 was measured. The evaluation results and the measured values of the nugget diameter are shown in Table 1.

As is clear from Table 1, the diameter of the region where the zinc-based plating was discharged at the stage where the pre-energization process was completed is 3√t [mm] or more, and the molten part at the stage where the pre-energization process was completed In the example of the present invention having a diameter of 2√t [mm] or less, the nugget diameter of the obtained welded joint is 3√t [mm] or more, and no delayed fracture occurred in the nugget cross section (in the nugget) No cracks to reach).
On the other hand, the comparative example in which the pre-energization process was not performed and the diameter of the region where the zinc-based plating was discharged at the stage where the pre-energization process was completed were 3√t [mm] or more and the diameter of the melted part was 2√t [mm] In the comparative example not satisfying the following, a crack reaching the nugget occurred.

DESCRIPTION OF SYMBOLS 1, 31 Lower steel plate 2, 32 Upper steel plate 3 Nugget 4, 5 Electrode 21 Zinc-based plating layer 22 Molten part 23 Area where zinc-based plating is discharged 24 White ring-shaped area 25 Area 33 Spacer 34 Gap

Claims (6)

It is a resistance spot welding method in which two or more steel plates are stacked and energized while being pressed between a pair of electrodes to form a nugget and join the steel plates,
At least one of the steel sheets is a high-strength zinc-based plated steel sheet having a tensile strength of 980 MPa to 1770 MPa and a zinc-based plated layer on the surface on the joining surface side,
A pre-energization step and a main energization step of energizing at a higher current value than the pre-energization step after the pre-energization step,
In the pre-energization step, when the pre-energization step is finished, the thickness of the thinner steel plate of the two adjacent steel plates to be joined is t [mm], and the diameter of the molten part is 2 √ t [mm] or less and under welding conditions in which a region where zinc-based plating with a diameter of 3 √t [mm] or more is discharged is formed,
The current energizing step is performed under a welding condition in which a nugget having a diameter of 3√t [mm] or more is formed.   The diameter of the area from which the zinc-based plating is discharged is set to be the inner diameter of the white ring-shaped area of zinc oxide observed on the steel sheet surface when the two adjacent steel sheets to be joined are peeled to −1 mm. The resistance spot welding method according to claim 1, wherein: At least one of the steel plates has a tensile strength of 980 MPa or more and 1770 MPa when two or more steel plates are stacked and sandwiched between a pair of electrodes to form a nugget by energizing and to perform resistance spot welding for joining the steel plates. A high-strength galvanized steel sheet having a zinc-based plating layer on the surface at least on the joining surface side, wherein the energization is conducted at a higher current value than the pre-energization step after the pre-energization step and the pre-energization step. A main energization step of forming a nugget having a diameter of 3√t [mm] or more, where t [mm] is the thickness of the steel plate having the smaller thickness among the two adjacent steel plates to be joined; A method for determining delayed fracture resistance of a welded portion of resistance spot welding that has
When the pre-energization process is completed, if there is a region where the diameter of the melted portion is 2√t [mm] or less and zinc-based plating with a diameter of 3√t [mm] or more is discharged, delayed fracture resistance A welding condition determination method for resistance spot welding, wherein it is determined that the characteristics are good.   At the stage where the pre-energization process is completed, a peeling test is performed to peel the two adjacent steel plates to be joined, and the diameter of the molten part and the diameter of the region where the zinc-based plating is discharged are obtained. A welding condition determination method for resistance spot welding according to claim 3.   5. The resistance according to claim 4, wherein the diameter of the region from which the zinc-based plating is discharged obtained in the peeling test is an inner diameter of a white ring-shaped region of zinc oxide observed on the steel plate surface—1 mm. A welding condition determination method for spot welding.   A resistance spot welding method, wherein welding conditions for a pre-energization step are determined based on the welding condition determination method for a welded portion of resistance spot welding according to any one of claims 3 to 5.

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