Tempering method, tempering device, and quality evaluation method for spot welded joints

Abstract

The present invention provides a method, a tempering device, and a quality evaluation method for tempering spot-welded joints, which enable inspection and evaluation at the production site of the quality of whether a weld has been properly tempered. The tempering method and quality evaluation method of the present invention are characterized in that, during the tempering process, an exposed indentation (P i ) and measure the temperature of the indentation (P i ) reaches a predetermined tempering temperature. The tempering device (1) of the present invention is i The first electrodes (21, 22) and the second electrode (3) are arranged to sandwich the spot welded joint (8) in the plate thickness direction with the exposed indentation portion (P) exposed, and the current path when energized overlaps with at least a part of the nugget (N). i ) and a thermometer (9) to measure the temperature of the indentation (P i A control section (C) determines whether the temperature of the M ) and is equipped with.

Description

[Technical Field]

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

[0002] When spot welding is performed using high-strength steel plates with a tensile strength of 440 MPa or more, the weld may fracture due to insufficient joint strength, and the intended design performance of the component may not be achieved. Therefore, a joining process that is less likely to fracture the weld, even when using high-strength steel plates, is required.

[0003] One such joining process is known as a post-tempering current application method, which modifies the structure of the weld by tempering the weld. For example, Patent Document 1 proposes a technique for spot welding two or more overlapping high-strength steel sheets, in which an appropriate rest period and a short post-welding current are applied after welding. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5714537 Summary of the Invention [Problem to be solved by the invention]

[0005] Even if this type of post-tempering current heating method were to be implemented in an actual production site, there is no method for inspecting and evaluating the quality of the welded joints at the production site to determine whether they have been properly tempered. Currently, the only option is to measure the hardness of the welded joints through random inspection of continuously produced welded joints, but this is not realistic to implement at an actual production site. For this reason, this type of post-tempering current heating method has not yet been put to practical use.

[0006] The present invention has been made in consideration of the above-described circumstances, and aims to provide a method for tempering a spot-welded joint, a tempering apparatus, and a quality evaluation method that enable inspection and evaluation at the production site of the quality of whether a weld has been properly tempered. [Means for solving the problem]

[0007] The present invention includes the following aspects.

[0008] (Aspect 1) A method for tempering a spot welded joint having a nugget and a pair of indentations overlapping the nugget in a plate thickness direction, comprising: a tempering process of applying current so that a current path overlaps at least a part of the nugget while at least one of the pair of indentations is exposed, The tempering method is characterized in that the tempering step includes measuring the temperature of the exposed indentation portion while applying current, and determining whether the temperature of the indentation portion has reached a predetermined tempering temperature.

[0009] (Aspect 2) The tempering method according to aspect 1, wherein the tempering step includes measuring the temperature of the exposed indented portion using a non-contact thermometer.

[0010] (Aspect 3) 3. The tempering method according to aspect 1 or 2, wherein the tempering step stops the current application when it is determined that the predetermined tempering temperature has been reached.

[0011] (Aspect 4) A tempering apparatus for tempering a spot welded joint having a nugget and a pair of indentations overlapping the nugget in a plate thickness direction, a first electrode and a second electrode arranged to sandwich the spot welded joint in the plate thickness direction with at least one of the pair of indentations exposed, and arranged so that a current path during current application overlaps with at least a portion of the nugget; a thermometer that measures the temperature of the exposed indentation portion while applying current through the first electrode and the second electrode; a management unit that determines whether the temperature of the indentation portion measured by the thermometer has reached a predetermined tempering temperature; A tempering device comprising:

[0012] (Aspect 5) A tempering apparatus according to aspect 4, wherein the thermometer is a non-contact thermometer.

[0013] (Aspect 6) 6. The tempering device according to aspect 4 or 5, wherein the management unit stops the application of electricity when it determines that the predetermined tempering temperature has been reached. (Aspect 7) A quality evaluation method for a spot welded joint having a nugget and a pair of indentations located in a plate thickness direction of the nugget, comprising: During the tempering process, current is applied so that the current path overlaps at least a portion of the nugget while at least one of the pair of indentations is exposed, and A quality evaluation method characterized by measuring the temperature of the exposed indentation while current is being applied, and determining whether or not a predetermined tempering temperature has been reached. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a method for tempering a spot-welded joint, a tempering device, and a quality evaluation method that allow the quality of a welded joint to be inspected and evaluated at the production site to determine whether it has been properly tempered. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a diagram schematically showing a tempering apparatus used in a method for tempering a spot-welded joint according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, preferred embodiments of the method for tempering a spot-welded joint, the tempering apparatus, and the quality evaluation method of the present invention will be described in detail with reference to the drawings.

[0017] To achieve the above object, the inventors conducted extensive research, focusing on the correlation between the hardness of a welded joint after post-tempering current application and the tempering temperature. The inventors first considered that, based on the correlation between the hardness of a welded joint after post-tempering current application and the tempering temperature, it would be possible to determine whether a welded joint was properly tempered if the temperature of the welded joint surface during post-tempering current application, specifically the temperature of the indentation directly above or below the nugget, could be measured. However, in a typical post-tempering current application process in which a nugget is formed and tempered in a single pressure-release cycle, the indentation is hidden by the electrode, making it impossible to properly measure the temperature of the indentation. Therefore, the inventors discovered that, by applying current to at least one side of the welded joint in the tempering process, with the indentation exposed, and the current path overlapping at least a portion of the nugget, the temperature of the indentation can be directly measured while the welded joint is being tempered.

[0018] The present invention was completed based on these findings, and includes aspects of each embodiment described below.

[0019] A method for tempering a spot welded joint according to one embodiment of the present invention is a method for tempering a spot welded joint having a nugget and a pair of indentations overlapping the nugget in a plate thickness direction, a tempering process of applying current so that a current path overlaps at least a part of the nugget while at least one of the pair of indentations is exposed, The tempering step is a tempering method characterized in that the temperature of the exposed indentation portion is measured while current is being applied, and it is determined whether the temperature of the indentation portion has reached a predetermined tempering temperature.

[0020] Hereinafter, the method for tempering a spot welded joint according to this embodiment will be described in detail with reference to the drawings. Here, Fig. 1 is a diagram schematically showing a tempering apparatus used in the method for tempering a spot welded joint according to one embodiment of the present invention.

[0021] <Tempering method for spot welded joints> As shown in FIG. 1 , a method for tempering a spot-welded joint according to an embodiment of the present invention involves tempering a nugget N formed on the overlapping surface of two overlapping steel sheets, an upper sheet 6 and a lower sheet 7, and a pair of indentations P overlapping the nugget N in the sheet thickness direction. i The method for tempering a spot welded joint 8 has the following features.

[0022] The tempering method of this embodiment is to form a pair of indentations P i At least one of the indentations P i (In FIG. 1, the indentation P formed on the upper plate 6 i ) is exposed, the current path overlaps with at least a part of the nugget N. In FIG. 1, the current path is schematically shown as two arrows extending from the lower electrode 3 of the tempering device 1 to each of the pair of upper electrodes 21, 22.

[0023] In the tempering process, at least one indentation P of the spot welded joint 8 i By passing electricity through the exposed part, the welded part is tempered while the indentation part P i It is possible to directly measure the temperature of the indentation part P i The temperature of the indentation P i This means the surface temperature at

[0024] Then, in the tempering process, the exposed indentation part P i Measure the temperature of the indentation P i It is determined whether the temperature reaches a predetermined tempering temperature.

[0025] Here, the "predetermined tempering temperature" refers to the temperature conditions required for tempering the weld. This can be determined by conducting several prototypes within the usual range before actual production and examining the correlation between the hardness of the welded joint after current application (specifically, the hardness of the nugget edge) and the temperature of the welded joint surface (specifically, the temperature of the indentation directly above or below the nugget) obtained from the prototypes. The tempering temperature is set to the temperature of the indentation at which the welded joint after current application is sufficiently hard. If the temperature required for tempering the weld is known from past production results, the process of determining the temperature conditions through prototypes, as described above, is not necessary. As described above, the "tempering temperature" is different from the actual temperature at which the target area for tempering, i.e., the nugget edge and its surrounding heat-affected zone, is tempered (hereinafter, sometimes simply referred to as the "actual tempering temperature"), and is the controlled temperature of the indentation surface determined from the correlation.

[0026] Therefore, if the measured temperature of the indentation portion during current application reaches this tempering temperature, the welded joint after current application will have a weld that has been properly tempered and will have sufficient hardness. On the other hand, if the measured temperature of the indentation portion during current application does not reach the tempering temperature, the welded joint after current application will have a weld that has not been properly tempered and will not have sufficient hardness.

[0027] As described above, in the tempering method of the present embodiment, the exposed indentation portion P i Measure the temperature of the indentation P i By determining whether the temperature reaches a predetermined tempering temperature, the quality of the welded portion can be inspected and evaluated at the production site to determine whether the welded portion has been properly tempered.

[0028] In this specification, the term "weld" refers to a spot-welded joint including a nugget and a heat-affected zone (HAZ). The term "nugget" refers to the metal that melts and solidifies during spot welding.

[0029] The tempering step in this embodiment will be described in more detail below.

[0030] [Tempering process] In the tempering process, first, as shown in Fig. 1, the spot welded joint 8 is clamped in the plate thickness direction between a pair of upper electrodes 21, 22 and a lower electrode 3 of the tempering device 1. At this time, the pair of indentations P of the spot welded joint 8 i At least one of the indentations P i The wire is clamped with the wire exposed.

[0031] Here, "a state in which the indentation is exposed" means not only a state in which the entire indentation is exposed, but also a state in which a portion of the indentation is exposed to an extent that the temperature of the indentation can be measured. Specifically, it means a state in which the indentation does not overlap the electrode at all in a plan view, and a state in which a portion of the indentation overlaps the electrode to an extent that the temperature of the indentation can be measured. The pair of indentations is a pair of recesses formed by pressure applied by the electrodes during spot welding, and is also called an indentation.

[0032] 1, the material is clamped with only the upper indentation exposed, but it may also be clamped with only the lower indentation exposed, or with both the upper and lower indentations exposed. In order to clamp the material with the lower indentation exposed, the position of the lower electrode may be changed, or the lower electrode may be configured as a pair of electrodes arranged at a predetermined interval.

[0033] When the spot-welded joint 8 is clamped in the plate thickness direction between the pair of upper electrodes 21, 22 and the lower electrode 3, the spot-welded joint 8 may be pressurized in the plate thickness direction. Applying pressure in this manner allows the first electrode and the second electrode to more reliably contact the surface of the steel plate. The pressure applied when clamping the spot-welded joint 8 in the plate thickness direction is preferably 30 to 70% of the pressure applied by the electrodes when welding the spot-welded joint 8. Setting the pressure in this range in the tempering process has the advantage of making it less likely for the steel plate to deform due to thermal expansion.

[0034] At least one of the indentations Pi Electric current is passed through the spot welded joint 8, which is clamped with the nugget N exposed, so that the current path overlaps with at least a part of the nugget N. When current is passed in this manner, the passing current and its branches can temper the martensite structure at least in the heat-affected zone (HAZ) around the nugget N.

[0035] As shown in FIG. 1 , current may be applied so that the current application area of ​​one electrode (the upper electrode in FIG. 1 ) in the spot-welded joint 8 is wider than the current application area of ​​the other electrode (the lower electrode in FIG. 1 ). Alternatively, a pair of upper and lower electrodes (i.e., one upper electrode and one lower electrode) may be arranged so that the central axes of the electrodes do not overlap, and current may be applied to the weld of the spot-welded joint in an oblique direction to temper the nugget. However, when current is applied in the former manner, the length of the current path is increased to a certain extent in areas with a wide current application area, allowing for a gradual temperature increase and tempering of not only the nugget N 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. On the other hand, in areas with a narrow current application area, the temperature increase of the steel sheet is limited to a certain range, making it possible to suppress thermal expansion, which has the advantage of reducing the deformation of the steel sheet due to thermal expansion.

[0036] In the tempering process, the current path during current application can be adjusted appropriately by adjusting the positions and numbers of the upper and lower electrodes.

[0037] The tempering process can be carried out under the same conditions as those used in the normal post-tempering current application method, as long as the current application path overlaps at least a part of the nugget. Examples of the current application conditions include the current application, the current application time, and the electrode pressure. These current application conditions are usually set according to the desired joint strength, etc. For example, if the actual temperature (actual tempering temperature) when the area to be tempered, i.e., the nugget edge and its surrounding heat-affected zone, is tempered is 500°C to 1000°C, the actual tempering temperature may be 500°C to 1000°C. c3The current application conditions include those that result in a temperature within the range of 600°C to 600°C. c1 When the actual tempering temperature is within this range, sufficient heat input makes it easy to reduce hardness, i.e., to improve toughness, and also makes it difficult for re-hardening to occur.

[0038] The tempering process is carried out by tempering at least one of the indentations P of the spot welded joint 8 as described above. i While applying current while the exposed indentation part P i Measure the temperature of this indentation P i The temperature is measured by a thermometer 9 as shown in FIG.

[0039] Indentation part P i The thermometer 9 for measuring the temperature of the indentation P may be a contact type thermometer such as a thermocouple. i It is preferable to use a non-contact thermometer because it is not necessary to attach a temperature sensor to the thermometer 9. The thermometer 9 shown in Fig. 1 is a non-contact thermometer.

[0040] In the tempering process, the exposed indentation part P is heated while applying current as described above. i The temperature of the indentation portion is measured to determine whether it has reached the predetermined tempering temperature. If the measured temperature of the indentation portion during current application reaches the predetermined tempering temperature, the welded joint after current application can be evaluated as having been properly tempered and having sufficient hardness. On the other hand, if the measured temperature of the indentation portion during current application does not reach the predetermined tempering temperature, the welded joint after current application can be evaluated as having not been properly tempered and not having sufficient hardness.

[0041] Whether or not the weld has been properly tempered can be evaluated by measuring the Vickers hardness distribution of the nugget and determining whether or not a part or all of the hardness has softened before and after energization. However, in the tempering method of this embodiment, such a measurement is not necessary, and the indentation P iThe quality of the welded portion can be evaluated as to whether it has been properly tempered simply by determining whether the measured temperature has reached the predetermined tempering temperature. Therefore, the tempering method of this embodiment makes it possible to inspect and evaluate the quality of the welded portion as to whether it has been properly tempered at the production site while the tempering process is being carried out.

[0042] In this embodiment, the indentation P i The determination as to whether the measured temperature has reached the predetermined tempering temperature or not is made by the control unit C of the tempering device 1, which is connected to the thermometer 9 by wire or wirelessly, as shown in FIG. M This can be done in the management department C. M More on this later.

[0043] Furthermore, in this embodiment, the indentation portion P i The result of determining whether the measured temperature of the indentation P has reached the predetermined tempering temperature may be used not only for quality evaluation of whether the weld has been properly tempered, but also for control of the ongoing or next tempering process. i The current flow may be stopped when it is determined that the measured temperature has reached a predetermined tempering temperature. By controlling the current flow in this manner, the welded portion can be tempered more reliably and excessive current flow can be prevented. This allows the spot-welded joint to be tempered more reliably and efficiently.

[0044] In this embodiment, predetermined processing steps that are performed in the manufacturing process of a normal spot-welded joint may be included before or after the tempering step.

[0045] The tempering method of this embodiment may be carried out as one step in the manufacturing process of a spot welded joint. Next, a method of manufacturing a spot welded joint according to another embodiment of the present invention will be described.

[0046] <Method for manufacturing spot welded joints> Another embodiment of the present invention is a method for manufacturing a spot-welded joint, which includes the tempering step of the tempering method of the above-mentioned embodiment. Specifically, the method includes, prior to the tempering step of the above-mentioned embodiment, a welding step of passing an electric current through a plurality of overlapping steel plates to form a nugget, and a cooling step of cooling the nugget, as steps for manufacturing a spot-welded joint.

[0047] In the method for manufacturing a spot-welded joint of this embodiment, the steps other than the tempering step, that is, the welding step and the cooling step, can be carried out in the same manner as in the manufacture of a normal spot-welded joint.

[0048] Hereinafter, each step in the method for manufacturing a spot welded joint according to this embodiment will be described in detail.

[0049] [Welding process] In this embodiment, the welding process is a process of passing a current through a plurality of overlapping steel sheets to form a nugget on the overlapping surfaces of the plurality of steel sheets and the surrounding area. Specifically, the overlapping steel sheets are clamped between a pair of electrodes, and then a current is passed in the sheet thickness direction with a predetermined current and current time while applying a pressure. This process melts the overlapping surfaces of the plurality of steel sheets and the surrounding area to form a nugget. At this time, the plurality of steel sheets are pressed by the pair of electrodes, and a pair of indentations P overlapping the nugget N in the sheet thickness direction are formed on the surface of each steel sheet in contact with the pair of electrodes. i is formed.

[0050] The current conditions in the welding process may be set to predetermined conditions according to the desired nugget diameter, etc. Examples of the current conditions include current flow, current flow time, and electrode pressure.

[0051] 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.

[0052] After the welding process, the steel plates (spot-welded joints) are subjected to the next cooling process.

[0053] [Cooling process] In this embodiment, the cooling process is a process of cooling the nugget formed in the welding process. The cooling process can be the same as the cooling process performed in normal spot welding, as long as the nugget formed in the welding process can be martensitic. An example of such a cooling process is a process in which, without releasing the pair of electrodes after the welding process, the pair of electrodes holds the welded steel sheet (spot-welded joint) in a non-energized state, allowing heat from the steel sheet to be 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 welded steel sheet (spot-welded joint) is transported to a tempering device, allowing heat from the steel sheet to be dissipated into the air.

[0054] 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 f The cooling conditions may be, for example, a cooling time, a holding time, a cooling temperature, or the like.

[0055] The spot welded joint after the cooling step is then subjected to the subsequent tempering step. Note that the tempering step is the same as the "tempering step" in the tempering method of the above-described embodiment, and therefore a description thereof will be omitted.

[0056] In the manufacturing method of the spot welded joint of this embodiment, during the tempering process, the indentation P i The quality of the welded portion can be evaluated as to whether it has been properly tempered simply by determining whether the measured temperature has reached the predetermined tempering temperature. Therefore, in the manufacturing method of this embodiment, the quality of the welded portion can be inspected and evaluated at the production site as to whether it has been properly tempered while the tempering process is being carried out.

[0057] The manufacturing method of this embodiment may include predetermined processing steps, such as those performed in a typical manufacturing method for spot-welded joints, before and after each of the welding step, cooling step, and tempering step.

[0058] In the method for manufacturing a spot-welded joint of this embodiment, when there are multiple welding locations for one plate pair, while the cooling process is being performed on one welding location, at least one of the welding process and the tempering process may be performed on the other welding locations in parallel. The same applies when multiple plate pairs are continuously spot-welded while being transported.

[0059] (steel plate) In the manufacturing method of this embodiment, the multiple steel plates to be welded (for example, the upper plate 6 and the lower plate 7 in FIG. 1 ) can be steel plates having any strength and thickness depending on the desired joint strength, application, etc. Examples of such steel plates include high-strength 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.

[0060] The Vickers hardness of the steel sheet may be 400 HV or more, 450 HV or more, or 500 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 and the like.

[0061] The Vickers hardness of 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 steel plate, it is sufficient to measure it at a depth of 1t / 4 of the plate thickness t of the steel plate under a test load of 500g.

[0062] Furthermore, the type of steel sheet is not particularly limited, and any steel sheet may be used depending on the properties required for the component to which the welded joint is to be 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.

[0063] 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, four or more.

[0064] Next, a tempering apparatus 1 that can be used in the manufacturing method of the spot welded joint of this embodiment and the tempering method of the above-described embodiment will be described in detail with reference to the drawings.

[0065] <Tempering equipment> As shown in FIG. 1, a tempering apparatus 1 according to another embodiment of the present invention is configured to form a nugget N on the overlapping surface of two overlapping steel plates, an upper plate 6 and a lower plate 7, and a pair of indentations P overlapping the nugget N in the plate thickness direction. i The tempering apparatus 1 is a tempering apparatus for tempering a spot welded joint 8 having the above-mentioned. The tempering apparatus 1 can also be used as a spot welder, and may be used, for example, as a spot welder used in the welding step in the manufacturing method of the spot welded joint of the above-mentioned embodiment.

[0066] 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.

[0067] 1, the tempering apparatus 1 includes 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 as to sandwich the spot welded joint 8 in the plate thickness direction. The pair of upper electrodes 21, 22 are first electrodes in this embodiment, while the lower electrode 3 is a second electrode in this embodiment.

[0068] As shown in FIG. 1, the pair of upper electrodes 21, 22 (first electrodes) and the lower electrode 3 (second electrode) are disposed at a pair of indentations P i The indentation P formed on the upper plate 6i The pair of upper electrodes 21, 22 and the lower electrode 3 are arranged so that the current path overlaps with at least a part of the nugget N of the spot welded joint 8 when current is applied.

[0069] Then, the tempering device 1 applies current to the pair of upper electrodes 21, 22 and the lower electrode 3 while heating the exposed indentation portion P i a thermometer 9 for measuring the temperature of the indentation P i Control section C determines whether the temperature reaches the specified tempering temperature. M and further comprising:

[0070] The pair of upper electrodes 21, 22 (first electrodes) and the lower electrode 3 (second electrode), the thermometer 9, and the management unit C M The above is the basic configuration of the tempering device 1 in this embodiment. The tempering device 1 shown in Fig. 1 further includes an upper holding member 4 that holds the pair of upper electrodes 21, 22 and is movable in the vertical direction parallel to the plate thickness direction, and a lower holding member 5 that holds the lower electrode 3 and is movable in the vertical direction.

[0071] The tempering device 1 is used in the tempering step in the method for tempering or manufacturing a spot welded joint according to the above-described embodiment. That is, the tempering device 1 is used in the tempering step to temper the indentation P i The quality of the welded portion can be evaluated as to whether it has been properly tempered simply by determining whether the measured temperature has reached the predetermined tempering temperature. Therefore, the tempering apparatus 1 can inspect and evaluate the quality of the welded portion as to whether it has been properly tempered at the production site while the tempering process is being carried out.

[0072] The various components of the tempering device 1 will now be described in detail.

[0073] (electrode) In the tempering apparatus 1, as shown in FIG. 1, a pair of upper electrodes 21 and 22, which are first electrodes, are arranged in a plate surface direction perpendicular to the plate thickness direction and are aligned in a plane view at the upper indentation P of the spot welded joint 8. i That is, the pair of upper electrodes 21 and 22 are arranged at a distance from each other in the plate surface direction. i The indentations P formed on the upper plate 6 are spaced apart so that the diameter of the indentations P is larger than the diameter of the indentations P formed on the upper plate 6. i becomes exposed.

[0074] On the other hand, the lower electrode 3, which is the second electrode, is located at the lower indentation P of the spot welded joint 8 as shown in FIG. i In this way, the indentation P formed on the lower plate 7 is i is not exposed.

[0075] The positions of the pair of upper electrodes 21, 22 and the lower electrode 3 are represented by the position of the central axis extending in the up-down direction of each electrode (the plate thickness direction of the spot welded joint 8).

[0076] By arranging the pair of upper electrodes 21, 22 and the lower electrode 3 as described above, the upper indentation P i When energizing, the current can be passed so that the current path overlaps with at least a part of the nugget N of the spot welded joint 8.

[0077] Furthermore, when the pair of upper electrodes 21, 22 and the lower electrode 3 are arranged as described above, current is passed through the spot welded joint 8 so that the current passing range on the upper electrode side is wider than the current passing range on the lower electrode side, and therefore in the portion with the wider current passing range, not only the nugget N but also a relatively wide region including the surrounding heat-affected zone (HAZ) can be gradually heated and tempered. On the other hand, in the portion with the narrower current passing range, the temperature rise of the steel sheet is limited to a certain range, making it possible to suppress thermal expansion, which has the advantage of making it less likely for deformation of the steel sheet due to thermal expansion to occur.

[0078] In the tempering device, the first electrode (upper electrode) and the second electrode (lower electrode) can clamp the spot-welded joint with at least one indentation portion of the spot-welded joint exposed, and when current is applied, the current path can be passed so as to overlap at least a portion of the nugget of the spot-welded joint. As long as these electrodes are capable of doing so, their specific arrangement and electrode structure are not limited to the form shown in Figure 1.

[0079] For example, the tempering device may be arranged with one upper electrode and a pair of lower electrodes, opposite to the arrangement shown in Fig. 1. In this case, the pair of lower electrodes are arranged spaced apart from each other in a plan view so as to sandwich the indentation formed in the lower plate of the spot-welded joint in the plate surface direction.

[0080] The tempering device may also be provided with a pair of upper electrodes and a pair of lower electrodes, which are spaced apart from each other in a plan view to sandwich at least one of a pair of indentations formed on the upper and lower plates of the spot-welded joint in the plate surface direction.

[0081] Furthermore, the tempering device may be configured such that a pair of upper and lower electrodes (i.e., one upper electrode and one lower electrode) are arranged so that the central axes of the electrodes do not overlap, so that electricity can be passed through the welded portion of the spot-welded joint in an inclined direction.

[0082] The first electrode (upper electrode) and the second electrode (lower electrode) may be of the same structure as those used in conventional spot welding, provided that the current flow area overlaps at least a portion of the nugget. Examples of such electrodes include a dome-shaped (D-type) electrode and a dome-radius (DR-type) electrode.

[0083] The first electrode and / or the second electrode may be, for example, a rod-shaped electrode extending in the depth direction, which is perpendicular to both the direction in which the pair of upper electrodes are arranged and the plate thickness direction. Specific examples of rod-shaped electrodes include electrodes made of conductive materials such as round bars, semi-round bars, and square bars. Specific examples of conductive materials include round bars made of Cu-Cr alloys. Using rod-shaped electrodes as the first electrode and / or the second electrode can ensure a wider contact area with the surface of the spot-welded joint, thereby achieving a more stable tempering effect.

[0084] (thermometer) In the tempering device 1, the thermometer 9 measures the exposed indentation P of the spot welded joint 8, which is energized by a pair of upper electrodes 21, 22 and a lower electrode 3. i The thermometer 9 is disposed at a position where it can measure the temperature of the pair of upper electrodes 21, 22. Specifically, the thermometer 9 is disposed at a position that does not overlap with the positions and movement ranges of the pair of upper electrodes 21, 22. Examples of such a position include a predetermined position in a direction that intersects with the arrangement of the pair of upper electrodes 21, 22, and a position between the pair of upper electrodes.

[0085] In addition, in the tempering device 1, the indentation part P i The thermometer 9 for measuring the temperature of the indentation P may be a contact type thermometer such as a thermocouple. i It is preferable to use a non-contact thermometer because it is not necessary to attach a temperature sensor to the thermometer.

[0086] In this embodiment, the thermometer 9 is connected to the management unit C of the tempering device 1 by wired or wireless communication means, as shown in FIG. M The temperature of the indentation P measured by the thermometer 9 during energization is i The temperature data of the control department C is transmitted via such communication means. M will be sent to.

[0087] (Management Department) In the tempering device 1, the management section CM is configured by a computer, and based on the temperature data transmitted from the thermometer 9, i It is determined whether the measured temperature of the indentation portion during current application has reached the predetermined tempering temperature. As described above, if the measured temperature of the indentation portion during current application has reached the predetermined tempering temperature, the welded joint after current application is evaluated as having been properly tempered and having sufficient hardness. On the other hand, if the measured temperature of the indentation portion during current application has not reached the predetermined tempering temperature, the welded joint after current application is evaluated as having not been properly tempered and not having sufficient hardness. Management Department C M is the indentation part P i The display may also display the result of the determination as to whether the measured temperature has reached the predetermined tempering temperature or the result of the evaluation as to whether the welded portion has been appropriately tempered.

[0088] The tempering device 1 of this embodiment is a press-fitting device for pressing the indentation P i The quality of the welded portion can be evaluated as to whether it has been properly tempered simply by determining whether the measured temperature has reached a predetermined tempering temperature. Therefore, the tempering apparatus 1 of this embodiment can inspect and evaluate the quality of the welded portion as to whether it has been properly tempered at the production site while tempering is being performed.

[0089] Furthermore, Management Department C M is the indentation part P i The determination result of whether the measured temperature has reached the specified tempering temperature can be stored as a quality evaluation result indicating whether the weld has been properly tempered, and can also be fed back to the control of the ongoing or next tempering process.

[0090] For example, Management Department C M is the temperature of the indentation P transmitted from the thermometer 9. i When it is determined that the measured temperature has reached a predetermined tempering temperature, the control unit C may stop the current supply. MHowever, controlling the current flow in this manner makes it possible to more reliably temper the welded joint and prevent excessive current flow, allowing the tempering apparatus 1 to more reliably and efficiently temper the spot-welded joint.

[0091] In addition, Management Department C M may be connected to the lower electrode 3 and the pair of upper electrodes 21, 22, etc. via any communication means so as to be able to perform the above-mentioned control.

[0092] Furthermore, the tempering device is not limited to the form shown in FIG. 1 described above. For example, instead of the measured temperature of the indentation portion transmitted from the thermometer being determined by the management unit, the measured temperature of the indentation portion may be displayed by the thermometer and determined on the spot by another means.

[0093] As described above, the tempering apparatus 1 of this embodiment can realize the spot-welded joint tempering method and manufacturing method of the above-mentioned embodiments. The tempering apparatus 1 of this embodiment may also be applied to a spot-welded joint quality evaluation method. Next, a spot-welded joint quality evaluation method, which is yet another embodiment of the present invention, will be described.

[0094] <Quality evaluation method for spot welded joints> The quality evaluation method for a spot-welded joint according to this embodiment is a method for evaluating the quality of a spot-welded joint having a nugget and a pair of indentations located in the plate thickness direction of the nugget. In this quality evaluation method, during the tempering process, current is passed through the nugget so that the current path overlaps at least a portion of the nugget, with at least one of the pair of indentations exposed. Then, while passing current, the temperature of the exposed indentation is measured to determine whether the predetermined tempering temperature has been reached.

[0095] As described above, the quality evaluation method of this embodiment is an application of the spot-welded joint tempering method of the above-described embodiment to a spot-welded joint quality evaluation method. Therefore, similar to the tempering method of the above-described embodiment, the quality evaluation method of this embodiment measures the temperature of the exposed indentation while current is applied during the tempering process, and determines whether the temperature of the indentation has reached a predetermined tempering temperature, thereby making it possible to inspect and evaluate the quality of the weld at the production site to determine whether it has been properly tempered.

[0096] The quality evaluation method of this embodiment is the same as the above-described embodiment except that the tempering method for spot-welded joints of the above-described embodiment is applied to the quality evaluation method, and therefore description thereof will be omitted.

[0097] (Application example) As described above, the spot-welded joint tempering method, tempering apparatus, and quality evaluation method of the present invention enable inspection and evaluation of the quality of a welded joint at the production site to determine whether it has been properly tempered, and are therefore applicable to various structural parts that require quality assurance, such as automobiles and other transportation machinery, industrial machinery, and buildings. In particular, the present invention is particularly suitable for use in the manufacture of automobile bodies and parts.

[0098] The manufacturing method of the present invention is not limited to the above-described embodiments, and suitable combinations, substitutions, modifications, etc. are possible within the scope that does not deviate from the object and intent of the present invention. [Example]

[0099] The present invention will be explained in more detail below by way of examples, but the present invention is not limited to these examples.

[0100] (Fabrication of spot welded joints) First, a sheet assembly consisting of two overlapping steel sheets (top and bottom sheets) was clamped between a pair of electrodes in a spot welding machine. Both steel sheets were unplated steel sheets for hot stamping with a tensile strength of 1.5 GPa (Vickers hardness approximately 480 HV) and a thickness of 2.0 mm. The pair of electrodes used were DR-type chromium copper electrodes with a tip diameter of 6 mm and a tip radius of 40 mm.

[0101] Next, a pair of electrodes was used to apply a pressure of 400 kgf to the sheet assembly, while a current of 6.3 kA was passed through the electrodes for 24 cycles in the sheet thickness direction to form a nugget with a diameter of 4√t at the overlapping surface of the two steel sheets and in the adjacent area (welding process). Then, the current to the electrodes was stopped, and this state, i.e., the state in which the electrodes applied pressure to the sheet assembly, was maintained for a holding time of 10 cycles. Note that 1 cycle is 1 / 60 seconds.

[0102] After that, the plate assembly after welding, i.e., the welded joint, is f The mixture was air-cooled to room temperature below the temperature limit (cooling step).

[0103] After cooling, the welded joint was transferred to the tempering apparatus shown in Figure 1. The welded joint was then clamped in the plate thickness direction between a pair of upper electrodes positioned to sandwich the upper indentation of the welded joint in the plate surface direction, and a single lower electrode positioned to contact the lower indentation of the welded joint. The pair of upper electrodes were each composed of round chromium copper electrodes with a diameter of 10 mm and a length of 50 mm, and were positioned so that the length of each electrode coincided with the depth direction of the tempering apparatus and the electrode spacing was 40 mm. Meanwhile, the single lower electrode was composed of a DR-type chromium copper electrode with a tip diameter of 6 mm and a tip radius of 40 mm, and was positioned so that it contacted the lower indentation of the welded joint as described above.

[0104] Next, a pressure of 200 kgf was applied to the welded joint using a pair of upper electrodes and one lower electrode, and current was applied at various values ​​of 3.2 to 5.0 kA for a current application time of 99 cycles as shown in Table 1 below, thereby tempering at least the region including the nugget edge (tempering process). During this tempering process, the temperature of the indentation on the upper side of the welded joint was measured using a non-contact thermometer in the tempering device during current application.

[0105] Thereafter, the current to the electrode was stopped, and this state was maintained for a holding time of 10 cycles to obtain a spot-welded joint in which at least the nugget edge was tempered. As shown in Table 1, a total of seven types of spot-welded joints (Joints No. 2 to 8 in Table 1) were produced using different current values ​​during the tempering process in the range of 3.2 to 5.0 kA. The specific current values ​​during the tempering process were 3.2 kA, 3.5 kA, 3.8 kA, 4.1 kA, 4.4 kA, 4.7 kA, and 5.0 kA.

[0106] In addition, a welded joint (Joint No. 1 in Table 1) that had not undergone the tempering process was also produced, which served as a criterion for determining whether the nugget edge had been tempered or not.

[0107] The Vickers hardness of the nugget edge was measured for seven types of welded joints prepared as described above, which had different current values ​​in the tempering process, and a welded joint that had not been subjected to the tempering process.

[0108] The Vickers hardness of the nugget edge was measured in accordance with JIS Z 2244:2009 "Vickers hardness test - Test method" at a position 1t / 4 (i.e., 0.5 mm) away from the overlapping surface (sheet interface) of each steel sheet toward the upper sheet, under a test load of 500 gf, where t is the sheet thickness.

[0109] The results of measuring the Vickers hardness of the nugget edge of each welded joint are shown in Table 1 below, along with the current value in the tempering process and the temperature of the indentation.

[0110] [Table 1]

[0111] As shown in Table 1, it can be confirmed that there is a correlation between the temperature of the indentation during the tempering process and the hardness of the nugget edge after the tempering process. Specifically, it can be confirmed that as the temperature of the indentation during the tempering process increases, the hardness of the nugget edge decreases (i.e., toughness improves), and that the hardness of the nugget edge increases when the temperature of the indentation reaches 717°C.

[0112] From this correlation, in this embodiment, the "predetermined tempering temperature", i.e., the control temperature of the indentation surface during the tempering process, is set to a temperature in the range of 407°C to 921°C, preferably a temperature in the range of 500°C to 750°C, and particularly preferably a temperature in the range of 600°C to 720°C, and by determining whether the temperature of the indentation measured during the current application during the tempering process has reached the above-mentioned predetermined tempering temperature, it is possible to inspect and evaluate the quality of the weld at the production site to determine whether it has been properly tempered.

[0113] Note that welded joint No. 8 in Table 1 is an example in which the tempering temperature was too high, causing re-hardening due to re-quenching. This example shows that the quality of the weld can be evaluated more accurately by setting a control temperature, for example a re-quenching temperature, at around 1000°C in addition to the above-mentioned specified tempering temperature, and further determining whether the temperature of the indentation measured during current application in the tempering process has reached the re-quenching temperature. [Explanation of symbols]

[0114] 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 Thermometer C M Management department N Nugget P i Indentation part

Claims

1. A method for tempering a spot welded joint having a nugget and a pair of indentations overlapping the nugget in a plate thickness direction, comprising: a tempering process of applying current so that a current path overlaps at least a portion of the nugget while at least one of the pair of indentations is exposed, The tempering method is characterized in that the tempering step includes measuring the temperature of the exposed indentation portion while applying electricity, and determining whether the temperature of the indentation portion has reached a predetermined tempering temperature.

2. 2. The tempering method according to claim 1, wherein the tempering step comprises measuring the temperature of the exposed indented portion using a non-contact thermometer.

3. 3. The tempering method according to claim 1, wherein the tempering step stops energizing when it is determined that the predetermined tempering temperature has been reached.

4. A tempering apparatus for tempering a spot welded joint having a nugget and a pair of indentations overlapping the nugget in a plate thickness direction, a first electrode and a second electrode arranged to sandwich the spot welded joint in the plate thickness direction with at least one of the pair of indentations exposed, and arranged so that a current path during current application overlaps with at least a portion of the nugget; a thermometer that measures the temperature of the exposed indentation portion while current is being applied by the first electrode and the second electrode; and a management unit that determines whether the temperature of the indentation portion measured by the thermometer has reached a predetermined tempering temperature; A tempering device comprising:

5. 5. The tempering apparatus according to claim 4, wherein the thermometer is a non-contact thermometer.

6. 6. The tempering device according to claim 4, wherein the management unit stops the current supply when it determines that the predetermined tempering temperature has been reached.

7. A quality evaluation method for a spot welded joint having a nugget and a pair of indentations located in a plate thickness direction of the nugget, comprising: During the tempering process, current is applied so that a current path overlaps at least a portion of the nugget while at least one of the pair of indentations is exposed, and A quality evaluation method characterized by measuring the temperature of the exposed indentation portion while current is being applied, and determining whether or not a predetermined tempering temperature has been reached.

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