Method for manufacturing spot welded joint, tempering device, and spot welded joint

Abstract

The present invention provides a method of production of a spot welded joint, a tempering apparatus, and a spot welded joint which are resistant to the effects of disturbance, are excellent in robustness, and are resistant to deformation of the steel sheet due to thermal expansion.SOLUTION: A method for manufacturing a spot welded joint according to the present invention includes a welding step of forming a nugget by energizing a plurality of overlapped steel sheets, a cooling step of cooling the nugget, and a holding step of holding the plurality of steel sheets in a thickness direction by a first electrode and a second electrode and energizing the steel sheets. And a tempering step of tempering at least a part of the nugget, wherein in the tempering step, the current is passed so that a current passing area in the plurality of steel sheets sandwiched between the first electrode and the second electrode overlaps at least a part of the nugget and the current passing area on the first electrode side is wider than the current passing area on the second electrode side.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a manufacturing method of a spot welded joint, a tempering apparatus, and a spot welded joint. [Background technology]

[0002] For example, when spot welding high-strength steel plates with a tensile strength of 440 MPa or more, the weld may fracture due to insufficient joint strength, preventing the component from achieving its designed performance. Therefore, there is a demand for a joining process that makes the weld less likely to fracture, even when using high-strength steel plates.

[0003] As one such joining process, various post-current application methods for modifying the weld zone have been studied. For example, Patent Document 1 discloses a technique for spot welding two or more overlapping high-strength steel sheets, in which an appropriate rest period and a short post-current application are performed after welding. The technique disclosed in Patent Document 1 is said to be able to modify the nugget zone and heat-affected zone and improve joint strength by using the tempering effect and the effect of slowing the cooling rate, known as the auto-tempering effect.

[0004] However, the post-current application method proposed in Patent Document 1 has a narrow conditional tolerance for achieving the desired effect and is not robust enough to withstand various disturbances that occur in actual production sites, making it difficult to apply to actual production sites.

[0005] That is, in many conventional post-current methods, current is applied two or more times during a single cycle from applying pressure to the sheet assembly with a pair of electrodes until the electrodes are released after welding to temper the welded portion (nugget). However, this method has a narrow range of appropriate conditions (e.g., temperature conditions) for achieving the desired effect. In addition, various disturbances occur in actual production sites, such as the generation of spatter, electrode wear, electrode misalignment, and gaps between steel sheets. These disturbances can cause the current density during post-current application to change outside the appropriate range, potentially making it difficult to achieve the desired modification effect (tempering effect).

[0006] In order to address these problems with the conventional post-energization method, for example, Patent Document 2 proposes a manufacturing method including: preparing a welded joint including a first steel plate, a second steel plate overlapping the first steel plate, and a quenched nugget joining the first steel plate and the second steel plate; applying a first electrode to the first steel plate at part A, which is an outer part of the welded joint in a plate surface direction of the nugget within a plane parallel to the first steel plate; applying a second electrode to the second steel plate at part B, which is an outer part of the welded joint in a plate surface direction of the nugget within a plane parallel to the first steel plate and is located on the opposite side of part A with the nugget in between; and passing a current through the welded joint between the first electrode and the second electrode.

[0007] The manufacturing method proposed in Patent Document 2 is said to provide a method for manufacturing a welded joint that is less susceptible to disturbances in the post-energization step and has excellent robustness. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent No. 5714537 [Patent Document 2] International Publication No. 2020 / 184728 Summary of the Invention [Problem to be solved by the invention]

[0009] In the manufacturing method proposed in Patent Document 2, a current is applied to stacked first and second steel sheets while they are sandwiched between an electrode and a fixing member at portions A and B, which are outer portions in the sheet surface direction of the nugget. When a current is applied to the steel sheets in this state, the steel sheets thermally expand in the portion between portions A and B sandwiched between the electrode and the fixing member due to a temperature rise caused by the current. However, because portions A and B are sandwiched between the electrode and the fixing member, there is no escape route for deformation caused by thermal expansion, and unintended deformation such as distortion may occur in the steel sheets.

[0010] Therefore, an object of the present invention is to provide a method for manufacturing a spot-welded joint, a tempering apparatus, and a spot-welded joint that are less susceptible to the influence of external disturbances, have excellent robustness, and are less susceptible to deformation of the steel plate due to thermal expansion. [Means for solving the problem]

[0011] The present invention includes the following aspects.

[0012] (Aspect 1) a welding process in which a nugget is formed by passing an electric current through a plurality of overlapping steel plates; a cooling step of cooling the nugget; a tempering process of tempering at least a portion of the nugget by clamping the plurality of steel plates between a first electrode and a second electrode in the plate thickness direction and passing a current through the electrodes, a current-carrying area of ​​the plurality of steel plates sandwiched between the first electrode and the second electrode overlapping with at least a portion of the nugget, and a current-carrying area on the first electrode side is wider than a current-carrying area on the second electrode side, in the tempering process.

[0013] (Aspect 2) The method for manufacturing a spot welded joint described in aspect 1, characterized in that the first electrode is composed of a pair of electrodes aligned in a plate surface direction perpendicular to the plate thickness direction and spaced apart so as to sandwich the nugget in a planar view.

[0014] (Aspect 3) A method for manufacturing a spot welded joint according to aspect 2, wherein the second electrode is disposed at a position overlapping the nugget in the plate thickness direction.

[0015] (Aspect 4) A method for manufacturing a spot welded joint according to aspect 2 or 3, characterized in that each of the pair of electrodes has a rod-like shape extending in a direction perpendicular to both the direction in which the pair of electrodes are arranged and the plate thickness direction.

[0016] (Aspect 5) The method for manufacturing a spot welded joint according to any one of aspects 2 to 4, wherein the tempering step is performed by clamping the plurality of steel plates between the first electrode, the second electrode, and an insulating fixing member arranged to be aligned with the second electrode in a direction parallel to the direction in which the pair of electrodes are arranged, and passing a current through the plurality of steel plates.

[0017] (Aspect 6) A method for manufacturing a spot-welded joint according to any one of aspects 1 to 5, characterized in that in the tempering step, the pressure applied when the first electrode and the second electrode clamp the plurality of steel plates in the plate thickness direction is 30 to 70% of the pressure applied by a pair of welding electrodes in the welding step.

[0018] (Aspect 7) A tempering device for a spot welded joint in which a nugget is formed on the overlapping surfaces of a plurality of overlapping steel plates, a first electrode and a second electrode arranged to sandwich the spot welded joint in the plate thickness direction, the first electrode and the second electrode are arranged such that, when current is applied, a current application range in the spot welded joint sandwiched between the first electrode and the second electrode overlaps with at least a portion of the nugget, and the current application range on the first electrode side is wider than the current application range on the second electrode side.

[0019] (Aspect 8) A tempering apparatus as described in aspect 7, characterized in that the first electrode is composed of a pair of electrodes aligned in a plate surface direction perpendicular to the plate thickness direction and spaced apart so as to sandwich the nugget in a planar view.

[0020] (Aspect 9) Aspect 9. The tempering apparatus according to aspect 8, wherein the second electrode is disposed at a position overlapping the nugget in the plate thickness direction.

[0021] (Aspect 10) A tempering apparatus according to aspect 8 or 9, wherein each of the pair of electrodes has a rod-like shape extending in a direction perpendicular to the direction in which the pair of electrodes are arranged and the plate thickness direction.

[0022] (Aspect 11) The tempering apparatus according to any one of aspects 8 to 10, further comprising an insulating fixing member arranged to be aligned with the second electrode in a direction parallel to the direction in which the pair of electrodes are arranged.

[0023] (Aspect 12) A spot welded joint in which a nugget is formed on the overlapping surfaces of a plurality of overlapping steel plates, the spot welded joint has a softened tissue region that extends from one surface side to the other surface side in the plate thickness direction and overlaps with at least a portion of the nugget, A spot welded joint, characterized in that, when the plurality of steel plates include steel plates of different strengths, the steel plate with the highest strength has a softened tissue region on one surface side in the plate thickness direction that is wider than the softened tissue region on the other surface side, or when the plurality of steel plates are steel plates of the same strength, the softened tissue region on one surface side in the plate thickness direction is wider than the softened tissue region on the other surface side. [Effects of the Invention]

[0024] The present invention provides a method for manufacturing a spot-welded joint, a tempering apparatus, and a spot-welded joint that are less susceptible to the influence of external disturbances, have excellent robustness, and are less susceptible to deformation of the steel plate due to thermal expansion. [Brief explanation of the drawings]

[0025] [Figure 1] FIG. 1 is a cross-sectional view that schematically shows the periphery of an electrode of a tempering device used in the tempering step of a method for producing a spot-welded joint according to one embodiment of the present invention. [Figure 2] FIG. 2 is a plan view schematically showing a first electrode and a second electrode that can be used in the tempering step. [Figure 3] FIG. 3 is a cross-sectional view schematically showing the periphery of an electrode of a tempering device used in the tempering step of a manufacturing method according to another embodiment of the present invention. [Figure 4] FIG. 4 is a cross-sectional view schematically showing the periphery of an electrode of a tempering device used in the tempering step of a manufacturing method according to still another embodiment of the present invention. [Figure 5] FIG. 5 is a cross-sectional view schematically showing the periphery of an electrode of a tempering device used in the tempering step of a manufacturing method according to still another embodiment of the present invention. [Figure 6] FIG. 6 is a cross-sectional view that schematically shows a spot-welded joint according to still another embodiment of the present invention. [Figure 7] FIG. 7 is a cross-sectional view that schematically shows a spot-welded joint according to still another embodiment of the present invention that is different from FIG. [Figure 8] FIG. 8 is a cross-sectional view that schematically shows the form of a softened tissue region in a spot-welded joint according to the present invention that is manufactured using a plurality of steel plates having different strengths. [Figure 9] FIG. 9 is a cross-sectional view that schematically shows a tempering method and a method for measuring the deformation amount of a steel sheet according to an example of the present invention. [Figure 10] FIG. 10 is a cross-sectional view schematically showing a tempering method and a method for measuring the deformation amount of a steel sheet in a comparative example. [Figure 11] FIG. 11 is a graph showing the deformation amounts of steel sheets in the examples of the present invention and the comparative examples. DETAILED DESCRIPTION OF THE INVENTION

[0026] Hereinafter, preferred embodiments of the method for manufacturing a spot-welded joint of the present invention will be described in detail with reference to the drawings. In this specification, unless otherwise specified, viewing an object along its thickness direction is simply referred to as a "planar view." Examples of the object include a steel plate and a spot-welded joint. Examples of the thickness direction include the plate thickness direction.

[0027] In order to achieve the above object, the present inventors have conducted extensive research focusing on the current path when tempering a nugget. As a result, the present inventors have found that when tempering multiple steel sheets after welding, i.e., a nugget formed at a spot-welded joint, by limiting the current path so that the current range on one electrode side in the sheet thickness direction is wider than the current range on the other electrode side, the method is less susceptible to the influence of disturbances, is highly robust, and deformation of the steel sheets due to thermal expansion as described above can be suppressed.

[0028] The present invention was completed based on these findings, and includes aspects of each embodiment described below.

[0029] <Method for manufacturing spot welded joints> A manufacturing method for a spot-welded joint according to one embodiment of the present invention includes a welding process in which an electric current is passed through multiple overlapping steel plates to form a nugget, a cooling process in which the nugget is cooled, and a tempering process in which the multiple steel plates are clamped between a first electrode and a second electrode in the thickness direction and electric current is passed through the electrodes to temper at least a portion of the nugget.

[0030] The tempering process is a process of passing current so that the current flow range in the multiple steel plates clamped between the first electrode and the second electrode overlaps with at least a portion of the nugget, and the current flow range on the first electrode side is wider than the current flow range on the second electrode side.

[0031] As described above, in the manufacturing method of this embodiment, in the tempering process separate from the welding process for forming the nugget, current is applied so that the current application range on the first electrode side of the multiple steel plates or spot-welded joint is wider than the current application range on the second electrode side. This increases the length of the current path to a certain extent in the area with the wider current application range, allowing for a gradual increase in temperature and tempering of not only the nugget but also a relatively wide area, including the surrounding heat-affected zone (HAZ). As a result, a wide range of appropriate current conditions for achieving the tempering effect, i.e., the effect of improving toughness, can be ensured.

[0032] On the other hand, in areas where the current is applied over a narrow range, the temperature rise of the steel plate is limited to a certain range, making it possible to suppress thermal expansion, thereby making it less likely for deformation of the steel plate due to thermal expansion to occur.

[0033] As described above, the manufacturing method of this embodiment is less susceptible to external disturbances in the tempering process, ensuring excellent robustness, and making it less likely for deformation of the steel plate to occur due to thermal expansion.

[0034] Each step in the manufacturing method of this embodiment will be described in more detail below.

[0035] [Welding process] In this embodiment, the welding process is a process of passing an electric current through a plurality of overlapping steel plates to form a nugget.

[0036] The welding process can be the same as that performed in normal spot welding, as long as it can form a nugget at the overlapping surfaces of multiple overlapping steel sheets and the surrounding area. One such process is to clamp the overlapping steel sheets between a pair of electrodes, and then apply a predetermined current and current for a predetermined time in the sheet thickness direction while applying a pressure, thereby melting the overlapping surfaces of the multiple steel sheets and the surrounding area to form a nugget.

[0037] The welding conditions in the welding process may be predetermined conditions according to the desired nugget diameter, etc. Examples of the welding conditions include the current, the welding time, and the pressure applied by the electrodes.

[0038] The pair of electrodes that can be used in the welding process can be any pair of electrodes that are used in conventional spot welding, as long as they can form a nugget of a predetermined size on the overlapping surfaces of multiple steel plates and in the surrounding area.

[0039] [Cooling process] In this embodiment, the cooling step is a step of cooling at least the nugget formed in the welding step.

[0040] The cooling process can be the same as that performed in conventional spot welding, as long as the nugget formed in the welding process can undergo martensitic transformation. An example of such a cooling process is a process in which, without releasing the pair of electrodes after the welding process, the steel sheets are held by the pair of electrodes in a non-energized state, and heat from the steel sheets is dissipated to the electrodes. Another example of the cooling process is a process in which, after the welding process, the pair of electrodes is released, and the steel sheets are transported to a tempering device for a tempering process, and heat from the steel sheets is dissipated into the air.

[0041] The latter cooling process has the advantage that the nugget is cooled with the pair of electrodes open, so that the absence of pressure from the electrodes can suppress a decrease in nugget thickness and consistently achieve high joint strength. Furthermore, this cooling process is advantageous in terms of production efficiency because it cools multiple steel plates after the welding process, i.e., the welded joint, while transporting them, allowing the welding process and a tempering process at another welded location to be performed in parallel.

[0042] The cooling conditions in the cooling process are set to the predetermined cooling conditions under which the nugget after the welding process undergoes martensite transformation, i.e., M S The temperature is preferably below M fThe cooling conditions may be, for example, a cooling time or a holding time, a cooling temperature, or the like.

[0043] [Tempering process] In this embodiment, the tempering process is a process in which a plurality of welded steel sheets, i.e., spot-welded joints, are clamped between a first electrode and a second electrode in the sheet thickness direction and current is passed through the electrodes to temper at least a portion of the nugget. Note that a spot welder may be used as the tempering device.

[0044] Furthermore, in this embodiment, the tempering process is a process of passing current so that the current flow range in the multiple steel plates after welding clamped between the first electrode and the second electrode, i.e., the spot-welded joint, overlaps with at least a portion of the nugget, and the current flow range on the first electrode side is wider than the current flow range on the second electrode side.

[0045] The tempering process is a process for tempering the martensite structure in at least a portion of each of the nugget and the heat-affected zone by passing current through the cooled steel sheets so that the current passing range overlaps with at least a portion of the nugget and the current passing range on the first electrode side is wider than the current passing range on the second electrode side. The tempering process can be performed under the same conditions as the post-current passing process and tempering current passing process performed in a normal post-current passing method, except that current is passed so that the current passing range overlaps with at least a portion of the nugget and the current passing range on the first electrode side is wider than the current passing range on the second electrode side.

[0046] Whether or not the nugget has been tempered by such a tempering process can be confirmed by measuring the Vickers hardness distribution of the nugget and checking whether or not a part or all of the hardness has softened after the tempering process.

[0047] Hereinafter, the tempering process will be described in more detail with reference to the drawings using one embodiment of the present invention. In the following description, the position of the electrode is determined based on the vertical direction of the electrode (the plate thickness direction D T) The same applies to the position of the fixing member.

[0048] FIG. 1 is a cross-sectional view that schematically shows the periphery of an electrode of a tempering apparatus 1 that is used in the tempering step of a method for producing a spot-welded joint according to one embodiment of the present invention.

[0049] In a manufacturing method according to one embodiment of the present invention, a tempering apparatus 1 shown in Fig. 1 is used in the tempering step. Note that Fig. 1 shows only the periphery of the electrodes of the tempering apparatus 1. A normal spot welder may be used as the tempering apparatus 1 shown in Fig. 1. For example, the tempering apparatus 1 may be used in combination with a spot welder used in the welding step.

[0050] The tempering apparatus 1 shown in FIG. 1 is an apparatus for tempering a spot-welded joint 8 in which a nugget N is formed on the overlapping surface of two steel plates, an upper plate 6 and a lower plate 7, which are overlapped with each other.

[0051] In this specification, the phrase "a nugget is formed on the overlapping surface of the steel plates" is used to mean not only the overlapping surface of the overlapping steel plates, but also a state in which a nugget is formed in a predetermined region including the vicinity of the overlapping surface.

[0052] As shown in FIG. 1, the tempering device 1 is configured to temper a spot welded joint 8 in the thickness direction D T The spot welded joint 8 is sandwiched between a pair of upper electrodes 21, 22 arranged above the spot welded joint 8 and a lower electrode 3 arranged below the spot welded joint 8 so that the spot welded joint 8 can be sandwiched between the upper electrodes 21, 22 and the lower electrode 3. The pair of upper electrodes 21, 22 are first electrodes in this embodiment. On the other hand, the lower electrode 3 is a second electrode in this embodiment.

[0053] The tempering device 1 holds a pair of upper electrodes 21 and 22 and is arranged in the thickness direction D T and a lower holding member 5 that holds the lower electrode 3 and is movable in the vertical direction.

[0054] The tempering device 1 generates a current path C that spreads from the lower electrode 3 toward each of the pair of upper electrodes 21, 21 when energizing, i.e., when flowing current through the spot welded joint 8. P The pair of upper electrodes 21, 22 and the lower electrode 3 are arranged so that a current flows through the spot welded joint 8. That is, the pair of upper electrodes 21, 22 and the lower electrode 3 define a current flow path C of the current that flows through the spot welded joint 8 when current is applied. P The upper electrodes 21 and 22 are disposed so that a current-carrying area formed by the electrodes 21 and 22 overlaps with at least a part of the nugget N, and the current-carrying area on the pair of upper electrodes 21 and 22 side is wider than the current-carrying area on the lower electrode 3 side.

[0055] In the manufacturing method of this embodiment, the tempering step is performed as follows using a tempering apparatus 1 shown in Fig. 1. First, the two steel plates after the cooling step, i.e., the spot-welded joints 8, are tempered in the thickness direction D by a pair of upper electrodes 21, 22 and a lower electrode 3 of the tempering apparatus 1 as shown in Fig. 1. T Furthermore, the spot welded joint 8 is welded in the thickness direction D by the pair of upper electrodes 21, 22 and the lower electrode 3. T At this time, by arranging the pair of upper electrodes 21, 22 and the lower electrode 3 as described above, it is possible to pass current so that the current-carrying range overlaps with at least a part of the nugget N and the current-carrying range on the pair of upper electrodes 21, 22 side is wider than the current-carrying range on the lower electrode 3 side.

[0056] As a result, the thickness direction D of the spot welded joint 8 T In the portion on the pair of upper electrodes 21, 22 side, i.e., the portion where the current is applied over a wide range, the length of the current path is longer to a certain extent, so that not only the nugget N but also a relatively wide region including the heat-affected zone (HAZ) around it can be gradually heated and tempered. As a result, it is possible to secure a wide range of appropriate current conditions for obtaining the tempering effect, i.e., the effect of improving toughness.

[0057] On the other hand, the thickness direction D of the spot welded joint 8 TIn the portion on the side of the lower electrode 3, i.e., the portion where the current is applied over a narrow range, the temperature rise of the steel sheet is limited to a certain range, making it possible to suppress thermal expansion, and therefore making it difficult for deformation of the steel sheet due to thermal expansion to occur.

[0058] As described above, in the manufacturing method of this embodiment, the tempering process is carried out using the tempering apparatus 1 shown in FIG. 1, which makes it less susceptible to external disturbances, ensures excellent robustness, and makes it less likely for deformation of the steel plate to occur due to thermal expansion.

[0059] In the present embodiment, the tempering apparatus 1 has a pair of upper electrodes 21 and 22, which are first electrodes, arranged in the thickness direction D as shown in FIG. T The direction of the plate surface D perpendicular to W The pair of upper electrodes 21, 22 are arranged in parallel to each other and spaced apart from each other so as to sandwich the nugget N in a plan view. W Distance between electrodes d E is the nugget diameter d N are spaced apart so that they are larger than

[0060] Although the arrangement of the pair of upper electrodes 21, 22 is not particularly limited, it is preferable that the pair of upper electrodes 21, 22 are arranged at a distance from each other so as to sandwich the nugget N, as described above. When the pair of upper electrodes 21, 22 are arranged in this manner, it is possible to easily realize the current flow range during current flow overlapping with at least a part of the nugget N, and the current flow range on the pair of upper electrodes 21, 22 side being wider than the current flow range on the lower electrode 3 side.

[0061] The pair of upper electrodes 21, 22 are spaced apart so as to sandwich the nugget N. T ) are spaced apart so as to sandwich the nugget N.

[0062] In addition, in this specification, "plate thickness direction D T The direction of the plate surface D perpendicular to W" means any one direction in a virtual plane perpendicular to the thickness direction, that is, any one direction in the in-plane direction perpendicular to the thickness direction. Furthermore, in this specification, the thickness direction D T and the plate surface direction D W The direction perpendicular to each of these is called the depth direction D dep It is sometimes referred to as.

[0063] The pair of upper electrodes 21 and 22 are in the plate surface direction D W Distance between electrodes d E is not limited to the above-mentioned form, and the nugget diameter d N For example, the distance between the electrodes d E is the nugget diameter d N In the case of spot welded joints, the nugget diameter d of 6 mm may be set to 2 times or more, or may be set to 6 mm or more. N Since the nugget diameter d N This is particularly advantageous when tempering spot welded joints having

[0064] In addition, the pair of upper electrodes 21 and 22 in the plate surface direction D W Distance between electrodes d E The upper limit of the distance between electrodes d E is the nugget diameter d N It may be set to 10 times or less, 6 times or less, or 4 times or less.

[0065] The pair of upper electrodes 21 and 22 are in the plate surface direction D W Distance between electrodes d E As shown in Figure 1, the distance D between the central axes of the electrodes is W means the distance.

[0066] On the other hand, the second electrode, the lower electrode 3, is positioned between the nugget N and the plate thickness direction D as shown in FIG. T Here, the lower electrode 3 is positioned so that it overlaps the nugget N and the plate thickness direction D. T The overlapping of the lower electrode 3 in the vertical direction (plate thickness direction D T) overlaps with nugget N.

[0067] The arrangement of the lower electrode 3 is not particularly limited, but as described above, the lower electrode 3 is T When the lower electrode 3 is arranged in this manner, it is possible to more easily realize that the current-carrying range during current-carrying overlaps with at least a part of the nugget N and that the current-carrying range on the pair of upper electrodes 21, 22 side is wider than the current-carrying range on the lower electrode 3 side.

[0068] In addition, in the tempering apparatus 1, the pair of upper electrodes 21, 22 and the lower electrode 3 are arranged so that the current flow range when current is applied overlaps with at least a portion of the nugget N and the current flow range on the pair of upper electrodes 21, 22 side is wider than the current flow range on the lower electrode 3 side, so that their specific structure, arrangement, etc. are not limited to the above-mentioned form.

[0069] Hereinafter, embodiments in which the pair of upper electrodes 21, 22 and the lower electrode 3 have different structures and arrangements will be described with reference to the drawings.

[0070] 2 is a plan view schematically showing the first electrode and the second electrode that can be used in the tempering process of this embodiment. As shown in FIG. 2, each of the pair of upper electrodes 21 and 22 that are the first electrodes has a direction in which the pair of upper electrodes 21 and 22 are arranged and a direction in which the plate thickness D T The depth direction D is a direction perpendicular to each of the dep The slit may have a rod-like shape extending in the direction of the arrow.

[0071] Although there are no particular limitations on the structure of each of the pair of upper electrodes 21, 22, it is preferable that each of the pair of upper electrodes 21, 22 have the above-described rod-like shape. When each of the pair of upper electrodes 21, 22 has such a rod-like shape, a wider contact area with the surface of the spot-welded joint 8 can be ensured, and the above-described tempering effect can be obtained more stably.

[0072] Furthermore, in the embodiment shown in FIG. 2, the lower electrode 3 is also dep The lower electrode 3 has a rod-like shape extending in the direction of the arrow. The structure of the lower electrode 3 is not limited to this rod-like shape and may be, for example, a dome-shaped (D-type) electrode or a dome-radius (DR-type) electrode, but it is preferable that the lower electrode 3 also has a rod-like shape as described above. If the lower electrode 3 also has this rod-like shape, a wider contact area with the surface of the spot-welded joint 8 can be ensured, and the above-mentioned tempering effect can be obtained more stably.

[0073] In the embodiment shown in FIG. 2, each of the pair of upper electrodes 21 and 22 having a rod-like shape has an indentation P i , that is, they are arranged symmetrically with respect to the nugget N.

[0074] In addition, the depth direction D of each of the pair of upper electrodes 21 and 22 having a rod shape dep The length of the indentation P is not particularly limited, but it is preferable that the indentation P i Diameter or nugget diameter d N It is preferable that it is greater than .

[0075] Furthermore, the pair of rod-shaped upper electrodes 21, 22 and the lower electrode 3 may be made of conductive material, for example, in the shape of a round bar, a semi-round bar, a square bar, etc. A specific example of such a conductive material is a round bar made of a Cu-Cr alloy.

[0076] In this embodiment, the first and second electrodes that can be used in the tempering process are not limited to the rod-shaped conductive members such as the pair of upper electrodes 21, 22 and lower electrode 3. Any electrodes used in normal spot welding may be used as long as the current-carrying area overlaps at least a portion of the nugget and the current-carrying area of ​​the first electrode is wider than the current-carrying area of ​​the second electrode. Examples of such electrodes include DR-type electrodes that make point contact with the steel sheet.

[0077] However, when round rod-shaped conductive members such as the pair of upper electrodes 21, 22 and lower electrode 3 described above are used as the first electrode and the second electrode, it is easier to pass current so that the current flow range on the first electrode side is wider than the current flow range on the second electrode side, and there is an advantage that a wider area can be tempered more slowly.

[0078] Next, an embodiment in which the first electrode and the second electrode are arranged in a different manner, that is, a tempering device used in the tempering step of a manufacturing method according to another embodiment of the present invention will be described with reference to the drawings.

[0079] In the following description, the configuration is basically the same as the above-described embodiment except for the differences from the above-described embodiment, and therefore the description will be omitted.

[0080] FIG. 3 is a cross-sectional view schematically showing the periphery of an electrode of a tempering device used in the tempering step of a manufacturing method according to another embodiment of the present invention.

[0081] The tempering apparatus 1 shown in Fig. 3 includes an insulating fixing member 9 arranged to be aligned with the lower electrode 3, which is the second electrode, in a direction parallel to the direction in which the pair of upper electrodes 21 and 22 are aligned. Note that in the tempering apparatus 1 shown in Fig. 3, the lower electrode 3 and the insulating fixing member 9 are arranged in such a manner that the nugget diameter d N They are placed at a distance greater than this.

[0082] In the tempering process using the tempering apparatus 1 shown in FIG. 3, the spot welded joint 8 is clamped between a pair of upper electrodes 21, 22 which are first electrodes, a lower electrode 3, and an insulating fixing member 9, and electricity is passed through them.

[0083] Even in the embodiment shown in Figure 3, the current can be applied so that the current flow range overlaps with at least a portion of the nugget N and the current flow range on the side of the pair of upper electrodes 21, 22 is wider than the current flow range on the side of the lower electrode 3.As a result, as described above, the tempering process is less susceptible to external disturbances, excellent robustness can be ensured, and deformation of the steel plate due to thermal expansion can be made less likely to occur.

[0084] Furthermore, in the embodiment shown in FIG. 3, the provision of an insulating fixing member 9 allows the spot welded joint 8 to be fixed more stably when electricity is passed through the spot welded joint 8, making it more unlikely that disturbances such as displacement of the steel plate or clearance between the steel plate and the electrode will occur.

[0085] When the insulating fixing member 9 clamps the spot welded joint 8 together with the pair of upper electrodes 21, 22 and the lower electrode 3, the insulating fixing member 9 is extended in the depth direction D in the same manner as the pair of rod-shaped upper electrodes 21, 22 and the lower electrode 3 shown in FIG. dep The slit may have a rod-like shape extending in the direction of the arrow.

[0086] Furthermore, the rod-shaped insulating fixing member 9 may be made of an insulating material, such as a round bar, a semi-round bar, or a square bar. Specific examples of such insulating materials include ceramic round bars. When the fixing member 9 is made of such an insulating material, it can be ensured that it does not affect the electrical conduction between the pair of upper electrodes 21, 22 and the lower electrode 3.

[0087] The fixing members that can be used in the tempering process are not limited to round-bar-shaped insulating members such as the fixing member 9 described above, and any insulating member of any shape may be used depending on the desired fixing form, ease of fixing, etc., as long as it can fix the overlapping steel plates after the cooling process, i.e., the spot-welded joints, so that the multiple steel plates do not move or shift when electricity is passed through the overlapping steel plates.

[0088] FIG. 4 is a cross-sectional view schematically showing the periphery of an electrode of a tempering apparatus 1 used in the tempering step of a manufacturing method according to still another embodiment of the present invention.

[0089] In the tempering apparatus 1 shown in Fig. 4, the second electrode is composed of a pair of lower electrodes 31, 32. The pair of lower electrodes 31, 32 are arranged so as to be aligned in a direction parallel to the direction in which the pair of upper electrodes 21, 22 are aligned. Each of the pair of lower electrodes 31, 32 can be the same as the lower electrode 3 of the tempering apparatus 1 shown in Fig. 1 described above.

[0090] The pair of lower electrodes 31 and 32 are connected to the nugget N and the plate thickness direction D. T That is, the pair of lower electrodes 31 and 32 are arranged so as to overlap each other with a nugget diameter d N The electrodes are arranged with a closer distance between them than the conventional one.

[0091] In the tempering process using the tempering apparatus 1 shown in FIG. 4, the spot welded joint 8 is also clamped between a pair of upper electrodes 21, 22, which are first electrodes, and a pair of lower electrodes 31, 32, which are second electrodes, and electricity is passed through them.

[0092] Even in the embodiment shown in Figure 4, current can be applied so that the current flow range overlaps with at least a portion of the nugget N and the current flow range on the side of the pair of upper electrodes 21, 22 is wider than the current flow range on the side of the pair of lower electrodes 31, 32.As a result, as described above, the tempering process is less susceptible to external disturbances, excellent robustness can be ensured, and deformation of the steel plate due to thermal expansion can be made less likely to occur.

[0093] Furthermore, in the embodiment shown in Fig. 4, the second electrode is constituted by a pair of lower electrodes 31, 32, so that the spot welded joint 8 can be fixed more stably when current is passed through the spot welded joint 8, making it less likely that disturbances such as displacement of the steel plate or clearance between the steel plate and the electrode will occur. In addition, by ensuring a wider current passing range on the pair of lower electrodes 31, 32 side, it is possible to temper a wider area within the spot welded joint 8. Furthermore, each of the pair of lower electrodes 31, 32 has a larger area between the nugget N and the plate thickness direction D. T By arranging them so as to overlap, the area including the nugget N can be tempered more reliably.

[0094] FIG. 5 is a cross-sectional view that schematically shows the periphery of an electrode of a tempering apparatus 1 used in the tempering step of a manufacturing method according to still another embodiment of the present invention.

[0095] The tempering apparatus 1 shown in Fig. 5 includes an insulating fixing member 9 arranged to be aligned with the lower electrode 3, which is the second electrode, in a direction parallel to the direction in which the pair of upper electrodes 21 and 22 are aligned. Furthermore, the lower electrode 3 and the insulating fixing member 9 are arranged in the direction of the nugget N and the plate thickness direction D, respectively. T That is, the lower electrode 3 and the insulating fixing member 9 are arranged so as to overlap each other. N The insulating fixing member 9 is the same as the insulating fixing member 9 of the tempering apparatus 1 shown in FIG.

[0096] In the tempering process using the tempering apparatus 1 shown in Figure 5, the spot welded joint 8 is clamped and energized between a pair of upper electrodes 21, 22, which are first electrodes, a lower electrode 3, and an insulating fixing member 9.

[0097] Even in the embodiment shown in Figure 5, the current can be applied so that the current flow range overlaps with at least a portion of the nugget N and the current flow range on the side of the pair of upper electrodes 21, 22 is wider than the current flow range on the side of the lower electrode 3.As a result, as described above, the tempering process is less susceptible to external disturbances, excellent robustness can be ensured, and deformation of the steel plate due to thermal expansion can be made less likely to occur.

[0098] 5, the spot welded joint 8 can be more stably fixed by providing an insulating fixing member 9, which makes it less likely for disturbances such as displacement of the steel plate and clearance between the steel plate and the electrode to occur. T By arranging them so as to overlap, the area including the nugget N can be tempered more reliably.

[0099] As a modification of the embodiment shown in FIG. 5, the insulating fixing member 9 is dep Even if the insulating fixing members 9 are arranged in this manner, the same effects as those described above can be obtained.

[0100] The above describes various embodiments in which the first electrode and the second electrode are arranged in different ways. However, the tempering device that can be used in the present invention is not limited in configuration to the above-described embodiments, as long as it is capable of passing current through the overlapping steel plates after the cooling process, i.e., the spot-welded joint, such that the current flow range overlaps with at least a portion of the nugget and the current flow range on the first electrode side is wider than the current flow range on the second electrode side.

[0101] For example, the tempering device may have one or more any number of first electrodes and one or more any number of second electrodes. Also, the tempering device may have one or more any number of fixing members arranged so as to be aligned with at least one of the first and second electrodes in the plate surface direction.

[0102] Furthermore, in the tempering device that can be used in the present invention, the first electrode may be a lower electrode and the second electrode may be an upper electrode.

[0103] In addition, a tempering device that can be used in the present invention may be configured such that the first electrode is an upper electrode with a relatively large tip diameter and the second electrode is a lower electrode with a relatively small tip diameter, so that when current is applied, the current application range overlaps with at least a portion of the nugget and the current application range on the first electrode side is wider than the current application range on the second electrode side.

[0104] (Electrification conditions) In the present invention, the current conditions other than those related to the current path or current range in the tempering process can be any current conditions according to the desired joint strength, etc., as long as they are conditions that can at least temper the nugget. Examples of the current conditions include the current, current duration, and electrode pressure. Specific examples of the current conditions include a tempering temperature of 500°C to 500°C. c3 The temperature range is preferably 600°C to 200°C. c1 When the tempering temperature is within this range, the hardness is likely to decrease with sufficient heat input, i.e., the toughness is likely to improve, and re-hardening due to re-quenching is unlikely to occur.

[0105] Furthermore, in the tempering step, when the stacked steel sheets after the cooling step, i.e., the spot-welded joint, are sandwiched between the first electrode and the second electrode in the sheet thickness direction, the steel sheets may be pressurized in the sheet thickness direction. By applying pressure in this manner, the first electrode and the second electrode can be brought into more reliable contact with the surfaces of the steel sheets.

[0106] Furthermore, in the tempering step, the pressure applied when the plurality of steel sheets are clamped in the sheet thickness direction by the first electrode and the second electrode is preferably 30 to 70% of the pressure applied by the pair of welding electrodes in the welding step. By setting the pressure applied in the tempering step within this range, it is possible to more reliably prevent deformation of the steel sheets due to the above-mentioned thermal expansion.

[0107] Note that 30 to 70% of the pressure applied by the pair of welding electrodes means a pressure that is 30 to 70% of the pressure applied by the pair of welding electrodes when the pressure applied by the pair of welding electrodes is 100%.

[0108] Furthermore, in the present invention, when there are multiple welding locations for one plate assembly, while a cooling process is being performed on one welding location, at least one of a welding process and a tempering process may be performed on the other welding locations in parallel. By performing each process in parallel in this way, the production efficiency of spot-welded joints can be further improved. The same applies when multiple plate assemblies are spot-welded continuously while being transported.

[0109] Furthermore, in the present invention, predetermined processing steps such as those carried out in ordinary methods for manufacturing spot-welded joints may be included before or after each of the welding and tempering steps.

[0110] Next, a spot welded joint obtained by the manufacturing method of the present invention will be described.

[0111] <Spot welded joints> According to the method for manufacturing a spot-welded joint according to one embodiment of the present invention, it is possible to obtain a spot-welded joint that is less susceptible to the influence of disturbances, has excellent robustness, and is suppressed in deformation of the steel plate due to thermal expansion. That is, yet another embodiment of the present invention is a spot-welded joint manufactured by the manufacturing method of the above embodiment.

[0112] Here, Fig. 6 is a cross-sectional view schematically showing a spot-welded joint according to still another embodiment of the present invention. Furthermore, Fig. 7 is a cross-sectional view schematically showing a spot-welded joint according to still another embodiment of the present invention different from that shown in Fig. 6.

[0113] The spot welded joint 8 shown in Fig. 6 is a spot welded joint in which a nugget N is formed on the overlapping surface of two overlapping steel plates, an upper plate 6 and a lower plate 7. In the tempering process during manufacturing, the spot welded joint 8 is energized so that the energized range overlaps with at least a part of the nugget N and the energized range on the pair of upper electrodes 21, 22 side is wider than the energized range on the lower electrode 3 side. Therefore, the spot welded joint 8 has a thickness in the plate thickness direction D T In the softened tissue region A, the softened tissue region A extends from one surface side (specifically, the upper surface side of the upper plate 6) to the other surface side (specifically, the lower surface side of the lower plate 7). s It has the following characteristics.

[0114] Soft tissue area A s As shown in FIG. 6, overlaps at least a part of the nugget N and is in the thickness direction D T In this case, the softened tissue region on one surface side is formed as a wider region than the softened tissue region on the other surface side. When the nugget edge and the HAZ hardened portion are tempered in this manner, the effects of improving joint strength and suppressing hydrogen embrittlement can be obtained.

[0115] In addition, softened tissue area A s The shape of the re-hardened portion A on the lower electrode side is not limited to that shown in FIG. 6. For example, as shown in FIG. R Even in such cases, the nugget edge and HAZ hardened portion are tempered, which can improve joint strength and suppress hydrogen embrittlement.

[0116] Furthermore, in the case where the plurality of steel plates constituting the spot-welded joint include steel plates of different strengths, the steel plate with the highest strength may have a softened tissue region on one surface side in the plate thickness direction that is wider than the softened tissue region on the other surface side, and in the case where the plurality of steel plates have the same strength, any of the steel plates may have a softened tissue region on one surface side in the plate thickness direction that is wider than the softened tissue region on the other surface side. Here, Fig. 8 is a cross-sectional view that schematically shows the morphology of the softened tissue region in a spot-welded joint of the present invention produced using plurality of steel plates with different strengths.

[0117] The example shown in Fig. 8(a) is a spot welded joint 8 formed by two steel plates with different strengths. In the example shown in Fig. 8(a), between a steel plate P1 with the highest strength and a steel plate P2 with a relatively low strength, the steel plate P1 with the highest strength has a softened structure region A on one surface side in the plate thickness direction. s On the other hand, the softened tissue area A on the surface s This soft tissue area A is wider than the s The width of the thickness can be determined by comparing the hardness distribution (Vickers hardness distribution) in the sheet surface direction on one surface side in the sheet thickness direction with the hardness distribution in the sheet surface direction on the other surface side, as shown in Figure 8(a).

[0118] The example shown in Fig. 8(b) is a spot welded joint 8 formed by three steel plates with different strengths. In the example shown in Fig. 8(b), a steel plate P1 with the highest strength and two steel plates P2 and P3 with relatively low strength are stacked in this order. In the example shown in Fig. 8(b), in the steel plate P1 with the highest strength, the softened structure region A on one surface side in the plate thickness direction is s On the other hand, the softened tissue area A on the surface s It is wider than.

[0119] Furthermore, the example shown in Fig. 8(c) is also a spot welded joint 8 formed by three steel plates with different strengths. In the example shown in Fig. 8(c), the steel plate P1 with the highest strength is sandwiched between two steel plates P2 and P3 with relatively low strengths. In the example shown in Fig. 8(c), the steel plate P1 with the highest strength also has a softened structure region A on one surface side in the plate thickness direction.s On the other hand, the softened tissue area A on the surface s It is wider than.

[0120] Even in the examples shown in FIGS. 8(a) to 8(c), the nugget edge and the HAZ hardened portion are tempered, which can improve the joint strength and suppress hydrogen embrittlement.

[0121] (Soft tissue area) Soft tissue area A s is a region tempered in the tempering process described above, and is a region softened by tempering. For example, if the metal structure after the cooling process is martensite, this is a region softened to tempered martensite by tempering.

[0122] Soft tissue area A s are defined as follows for the inside and outside of the nugget, respectively: s is a region where the hardness is 5% or more lower than the predicted value of the nugget hardness. On the other hand, the softened tissue region A outside the nugget s is a region where the hardness is 5% or more lower than the predicted hardness of the HAZ hardened part.

[0123] The predicted values ​​of the nugget hardness and the hardness of the hardened HAZ can be calculated using the following formula (1). Predicted hardness (HV) = 1063 × Ceq + 191 (1) Here, Ceq can be calculated from the following equation (2). Ceq=C+Si / 77+Mn / 21+Cr / 28+Mo / 30 (2) However, when calculating the predicted value of nugget hardness, Ceq is a value obtained by averaging the chemical compositions of each steel plate in which the nugget is formed according to the melting ratio and substituting the average into formula (2). On the other hand, when calculating the predicted value of hardness of the HAZ hardened portion, Ceq is a value obtained by substituting the chemical composition of the corresponding steel plate (if multiple steel plates have different strengths, the steel plate with the highest strength) into formula (2).

[0124] Therefore, soft tissue area A s By measuring the Vickers hardness of the spot welded joint 8, it is possible to determine that the hardness of the spot welded joint 8 is 5% or more lower than the predicted value of the nugget hardness or the predicted value of the hardness of the HAZ hardened part. The Vickers hardness of the spot welded joint can be measured by a method conforming to JIS Z 2244:2009 "Vickers hardness test - Test method". When measuring the Vickers hardness, T Measurements can be made at multiple locations at a predetermined depth under a test load of 500 g.

[0125] Next, a steel sheet that can be used in this embodiment will be described.

[0126] (steel plate) In this embodiment, the multiple steel plates to be welded can be steel plates having any strength and thickness depending on the desired joint strength, application, etc. Examples of such steel plates include steel plates having a Vickers hardness of 350 HV or more. When such high-strength steel plates are used, the risk of the welded joint becoming embrittled and breaking after welding increases significantly, so the present invention is particularly advantageous when such high-strength steel plates are used.

[0127] In this embodiment, the Vickers hardness of the steel sheet may be 360 ​​HV or more, 400 HV or more, 450 HV or more, 500 HV or more, 550 HV or more, or 600 HV or more. The upper limit of the Vickers hardness of the steel sheet is not particularly limited, but is, for example, 900 HV from the viewpoint of workability, etc.

[0128] The Vickers hardness of a steel plate can be measured using a method that conforms to JIS Z 2244:2009 "Vickers hardness test - Test method." When measuring the Vickers hardness of a steel plate, it is sufficient to measure it at a depth of 1 / 4 of the plate thickness of the steel plate under a test load of 500 g.

[0129] The multiple steel plates may all be steel plates of the same strength, all be steel plates of different strengths, or only some of the multiple steel plates may be steel plates of different strengths.

[0130] The thickness of the steel plate is not particularly limited, and any thickness can be adopted depending on the strength required for the part to which the welded joint is applied, etc. For example, the thickness of the steel plate may be 0.5 mm or more, 0.8 mm or more, or 1.0 mm or more. The thickness of the steel plate may be 3.5 mm or less, 3.2 mm or less, or 3.0 mm or less, for example.

[0131] The multiple steel plates may all have the same thickness, or may all have different thicknesses, or only some of the multiple steel plates may have different thicknesses.

[0132] Furthermore, the type of steel sheet is not particularly limited, and any steel sheet can be used depending on the properties required for the component to which the welded joint is applied. For example, the steel sheet may be an unplated steel sheet, or may be a steel sheet plated with zinc, aluminum, or the like.

[0133] The multiple steel plates may all be the same type of steel plate, all be different types of steel plates, or only some of the multiple steel plates may be different types of steel plates.

[0134] Furthermore, the number of steel plates is not particularly limited, and may be any number of two or more depending on the strength required of the part to which the welded joint is applied, etc. For example, the number of steel plates may be two, three, or four.

[0135] The shape of the steel plate is not particularly limited as long as it has a predetermined plate-like structure in which at least the portions to be welded can be overlapped with the portions to be welded of other steel plates in the plate thickness direction. For example, the shape of the steel plate may be a flat plate-like structure as a whole, or a plate-like structure in a portion including the portions to be welded and a curved structure in other portions. Specific examples of the latter include an L-shaped steel plate and a hat-shaped steel plate.

[0136] (Application example) As described above, the method for manufacturing a spot-welded joint of the present invention is less susceptible to disturbances, ensures excellent robustness, and reduces deformation of the steel plate due to thermal expansion. Therefore, the method can be applied to various structural parts, such as automobiles and other transportation machinery, industrial machinery, and buildings, which require excellent joint strength and shape accuracy. In particular, the present invention can be particularly suitably used in the manufacture of automobile bodies and parts.

[0137] The manufacturing method of the present invention is not limited to the above-described embodiments or the examples described below, and appropriate combinations, substitutions, modifications, etc. are possible within the scope that does not deviate from the object and intent of the present invention. [Example]

[0138] The present invention will be described in more detail below by way of examples and comparative examples, but the present invention is not limited to these examples. Fig. 9 is a cross-sectional view schematically showing the tempering method and the method for measuring the deformation amount of a steel sheet in an example of the present invention, and Fig. 10 is a cross-sectional view schematically showing the tempering method and the method for measuring the deformation amount of a steel sheet in a comparative example.

[0139] <Example 1 of the present invention> First, a pair of two overlapping steel sheets, the upper and lower sheets, was clamped between a pair of electrodes (both DR type 40-16, Φ6 mm) in a spot welding machine. Both steel sheets were hot-stamped, uncoated steel sheets (Vickers hardness approximately 480 HV) with a tensile strength of 1.5 GPa and a sheet thickness of 2.0 mm. The steel sheets were 50 mm wide and 150 mm long, and the sheet pair was formed by overlapping the two steel sheets in a cross shape so that their longitudinal directions were perpendicular to each other. Next, while applying a pressure of 400 kgf to the sheet pair, a current of 6.3 kA was applied in the thickness direction for 24 cycles to form a nugget with a diameter of 4√t at the overlapping surface of the two steel sheets and in the adjacent region (welding process). Then, the current to the electrodes was stopped, and the state, i.e., the state in which the electrodes applied pressure to the sheet pair, was maintained for a holding time of 10 cycles. Note that 1 cycle is 1 / 60 seconds.

[0140] After that, the plate assembly after welding is f The sheet assembly was air-cooled to room temperature below the temperature limit (cooling step). The cooled sheet assembly was then transferred to a spot welding machine having a configuration similar to that of the tempering apparatus 1 shown in Fig. 1, and the sheet assembly was clamped in the sheet thickness direction between a pair of upper and lower electrodes. Next, while applying a pressure of 200 kgf to the sheet assembly, current was applied at various values ​​of 2.2 to 5.0 kA for a current application time of 99 cycles as shown in Table 1 below, thereby tempering a region including at least a portion of the nugget (tempering step).

[0141] Thereafter, the current to the electrodes was stopped, and this state was maintained for a holding time of 10 cycles, thereby obtaining a spot-welded joint of the present invention in which the two steel sheets were joined. Note that, as shown in Table 1, a total of 10 types of spot-welded joints of the present invention were produced with different post-energization currents in the range of 2.2 to 5.0 kA. Furthermore, two joints of each type were produced for measuring the cross tensile strength, which will be described later.

[0142] The spot welding machine used in the tempering process was equipped with a Cu-Cr alloy round rod-shaped electrode with a length of 50 mm and a diameter of 10 mm as the upper electrode. EThe lower electrode was a DR type 40-16, Φ6 mm, and was placed so that it was in contact with the indentation of the previous point.

[0143] <Comparative Example 1> The spot-welded joint of Comparative Example 1 was produced in the same manner as the Inventive Examples, except that the tempering process was carried out using conventional post-tempering energization. Specifically, the spot-welded joint of Comparative Example 1 was produced in the same manner as the Inventive Examples, except that the tempering process was carried out with a rest time of 99 cycles, post-energization at currents of 3.2 to 5.0 kA, and a post-energization time of 99 cycles. As shown in Table 1, a total of seven types of spot-welded joints were produced for the Comparative Examples, with different post-energization currents ranging from 3.2 to 5.0 kA. Two joints of each type were also produced for measuring the cross tensile strength, as described below.

[0144] The cross tensile strength (CTS) of the spot-welded joints of the invention examples and comparative examples produced as described above was measured according to the following measurement method.

[0145] (Measurement of cross tensile strength (CTS) of spot welded joints) The CTS of spot-welded joints was measured in accordance with the cross tension test based on JIS Z 3137:1999. The CTS measurement was carried out at a speed of 10 mm / min. The CTS measurement was carried out on two samples, and the average value of the measurement results was used.

[0146] The measurement results of the CTS of the spot-welded joints of the invention examples and comparative examples are shown in Table 1 below. Table 1 also shows the CTS of joints produced by single-current welding, i.e., joints that were not tempered, as reference examples. Joints whose CTS improved by 150% or more compared to the single-current joints of the reference examples were judged as good (◯), and joints whose improvement rate was less than 150% were judged as poor (×).

[0147] [Table 1]

[0148] As shown in Table 1, in the comparative example of post-tempering current, good joints were obtained in a narrow range of post-heating currents from 4.1 to 4.7 kA. On the other hand, in the examples of the present invention, good joints were obtained in a wide range of post-heating currents from 2.5 to 4.7 kA. In other words, it was found that the manufacturing method of the examples of the present invention is less susceptible to the influence of external disturbances and has excellent robustness.

[0149] <Example 2 of the present invention> First, a pair of two overlapping steel sheets, the upper and lower sheets, was clamped between a pair of electrodes (both DR type 40-16, Φ6 mm) in a spot welding machine. Both steel sheets were hot-stamped, uncoated steel sheets (Vickers hardness approximately 480 HV) with a tensile strength of 1.5 GPa and a sheet thickness of 1.4 mm. The steel sheets measured 30 mm wide and 75 mm long, and the sheet pair was formed by overlapping the two steel sheets so that their longitudinal directions coincided. Next, while applying a pressure of 350 kgf to the sheet pair, a current of 5.5 kA was applied in the sheet thickness direction for 18 cycles. This formed a nugget with a diameter of 4√t at the overlapping surface of the two steel sheets and its adjacent region at the center of the test specimen (welding process). Then, the current to the electrodes was stopped, and the state, i.e., the state in which the electrodes applied pressure to the sheet pair, was maintained for a holding time of 10 cycles.

[0150] After that, the plate assembly after welding is f The sheet assembly was air-cooled to room temperature below the temperature point (cooling step). The cooled sheet assembly was then transferred to a tempering device (spot welding machine) shown in Fig. 9(a), where it was clamped in the sheet thickness direction between a pair of upper electrodes, a lower electrode, and a pair of insulating fixing members. Next, while applying a pressure of 200 kgf to the sheet assembly, currents of 2.6 to 3.5 kA were applied for a current application time of 99 cycles, thereby tempering a region including at least a portion of the nugget (tempering step).

[0151] Thereafter, the current to the electrodes was stopped, and this state was maintained for a holding time of 10 cycles, thereby obtaining a spot-welded joint of the present invention in which the two steel sheets were joined. A total of nine types of spot-welded joints of the present invention were produced, each with a different post-welding current in the range of 2.6 to 3.5 kA. The specific post-welding current values ​​for the nine types of joints were 2.6 kA, 2.7 kA, 2.8 kA, 2.9 kA, 3.0 kA, 3.1 kA, 3.2 kA, 3.3 kA, and 3.5 kA.

[0152] The spot welding machine used in the tempering process was equipped with a Cu-Cr alloy round rod-shaped electrode with a length of 50 mm and a diameter of 10 mm as the upper electrode. E The lower electrode was a DR type 40-16 with a diameter of 6 mm, and was positioned so that it was in contact with the indentation of the previous point. A pair of insulating fixing members was also positioned opposite the upper electrode.

[0153] <Comparative Example 2> The welding process and cooling process were carried out in the same manner as in Example 2 of the present invention. The spot welder used was the one shown in Figure 10(a), and the sheet assembly was clamped in the thickness direction between an upper electrode and an insulating fixing member, and an insulating fixing member and a lower electrode arranged to face each other. The electrodes used were round rods made of Cu-Cr alloy, 50 mm long and 10 mm in diameter. The electrode distance d E The distance between the welds was 40 mm, and the weld was positioned at the center of the weld. Next, while applying a pressure of 200 kgf to the sheet assembly, current was applied at 4.5 to 5.8 kA for a current application time of 99 cycles, thereby tempering a region including at least a part of the nugget (tempering process).

[0154] In this manner, comparative spot-welded joints were produced. Six types of comparative spot-welded joints were produced using different post-welding currents in the range of 4.5 to 5.8 kA. The specific post-welding current values ​​for the six types of joints were 4.5 kA, 4.8 kA, 5.0 kA, 5.3 kA, 5.5 kA, and 5.8 kA.

[0155] Then, for the spot-welded joints of the invention examples and comparative examples produced as described above, the steel plate deformation amounts were measured according to the following measuring method.

[0156] (Measurement of steel plate deformation in spot-welded joints) In the example of the present invention, as shown in Figure 9(b), the steel sheets warp and deform before and after tempering. In such cases, the deformation amount of the steel sheets in the joint is measured as follows. First, as shown in Figure 9(b), one end of the joint in the longitudinal direction is fixed on a horizontal reference plane with the warped concave surface of the joint after tempering facing up. At this time, the joint is fixed so that one end of the joint in the longitudinal direction and its neighboring part are aligned with the horizontal reference plane. Next, the height of the other end of the joint in the longitudinal direction, where the height from the horizontal reference plane is greatest, is measured, and this is taken as the deformation amount of the steel sheets.

[0157] On the other hand, in the comparative example, as shown in Figure 10(b), the steel sheet deforms in a wave-like manner before and after tempering. In such a case, the deformation amount of the steel sheet of the joint is measured as follows. First, as shown in Figure 10(b), one end of the joint in the longitudinal direction is fixed on a horizontal reference plane with the curved concave surface extending from one end of the joint after tempering facing upward. At this time, the joint is fixed so that one end of the joint in the longitudinal direction and its neighboring portion are aligned with the horizontal reference plane. Next, the height of the part with the greatest height from the horizontal reference plane (for example, the other end of the joint in the longitudinal direction) is measured, and this is taken as the deformation amount of the steel sheet.

[0158] The deformation amounts of the steel sheets of the invention example and the comparative example measured as described above are shown in Fig. 11. The temperature of the indentation was measured during the tempering process of each of the invention example and the comparative example, and in the graph shown in Fig. 11, the maximum value was plotted on the horizontal axis.

[0159] As shown in Fig. 11, in the example of the present invention, the temperature dependency was small and the steel sheet deformation amount was generally constant. In the comparative example, the steel sheet deformation amount increased as the temperature increased. In all temperature ranges, the steel sheet deformation amount of the example of the present invention was smaller than that of the comparative example.

[0160] As shown in Table 1 and FIG. 11, it was found that the spot-welded joints of the examples of the present invention were less susceptible to the effects of external disturbances, had excellent robustness, and also suppressed deformation of the steel plate due to thermal expansion. [Explanation of symbols]

[0161] 1 Tempering equipment 21 upper electrode (first electrode) 22 upper electrode (first electrode) 3 Lower electrode (second electrode) 4 Upper holding member 5 Lower holding member 6 Upper Plate 7 Lower plate 8 Spot welded joints 9 Fixing member 31 Lower electrode (second electrode) 32 Lower electrode (second electrode) N Nugget C P Current path P i Indentation A S Soft tissue area

Claims

1. a welding process in which a nugget is formed by passing an electric current through a plurality of overlapping steel plates; a cooling step of cooling the nugget; a tempering process of tempering at least a portion of the nugget by clamping the plurality of steel plates between a first electrode and a second electrode in the plate thickness direction and passing a current through the electrodes, a current-carrying section for carrying out the current-carrying process on the first electrode and the second electrode, the current-carrying section being located between the first electrode and the second electrode, the current-carrying section being located between the first electrode and the second electrode, the current-carrying section being located between the second electrode and the first ...

2. 2. The method for manufacturing a spot welded joint according to claim 1, wherein the first electrode is composed of a pair of electrodes aligned in a plate surface direction perpendicular to the plate thickness direction and spaced apart so as to sandwich the nugget in a plan view.

3. The method for manufacturing a spot welded joint according to claim 2, wherein the second electrode is disposed at a position overlapping the nugget in the plate thickness direction.

4. 4. The method for manufacturing a spot welded joint according to claim 2, wherein each of the pair of electrodes has a rod-like shape extending in a direction perpendicular to both the direction in which the pair of electrodes are arranged and the plate thickness direction.

5. 4. The method for manufacturing a spot welded joint according to claim 2, wherein the tempering step comprises passing a current through the plurality of steel sheets while sandwiching the plurality of steel sheets between the first electrode, the second electrode, and an insulating fixing member arranged to be aligned with the second electrode in a direction parallel to a direction in which the pair of electrodes are arranged.

6. 3. The method for manufacturing a spot welded joint according to claim 1, wherein in the tempering step, a pressing force when the first electrode and the second electrode clamp the plurality of steel plates in the plate thickness direction is 30 to 70% of a pressing force applied by a pair of welding electrodes in the welding step.

7. A tempering device for a spot welded joint in which a nugget is formed on the overlapping surfaces of a plurality of overlapping steel plates, a first electrode and a second electrode arranged to sandwich the spot welded joint in a plate thickness direction; the first electrode and the second electrode are arranged such that, when current is applied, a current application range in the spot welded joint sandwiched between the first electrode and the second electrode overlaps with at least a portion of the nugget, and the current application range on the first electrode side is wider than the current application range on the second electrode side.

8. The tempering apparatus according to claim 7, characterized in that the first electrode is composed of a pair of electrodes aligned in a plate surface direction perpendicular to the plate thickness direction and spaced apart so as to sandwich the nugget in a planar view.

9. The tempering device according to claim 8, wherein the second electrode is disposed at a position overlapping the nugget in the plate thickness direction.

10. 10. The tempering device according to claim 8, wherein each of the pair of electrodes has a rod-like shape extending in a direction perpendicular to both the direction in which the pair of electrodes are arranged and the plate thickness direction.

11. The tempering apparatus according to claim 8 or 9, further comprising an insulating fixing member arranged to align with the second electrode in a direction parallel to the direction in which the pair of electrodes are arranged.

12. A spot welded joint in which a nugget is formed on the overlapping surfaces of a plurality of overlapping steel plates, the spot welded joint has a softened tissue region that extends from one surface side to the other surface side in the plate thickness direction and overlaps with at least a portion of the nugget, A spot welded joint, characterized in that, when the plurality of steel plates include steel plates of different strengths, a softened tissue region on one surface side in the plate thickness direction is wider than a softened tissue region on the other surface side in the steel plate with the highest strength, or in any of the steel plates when the plurality of steel plates are steel plates with the same strength.

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