Patent Application
20220168839
The present disclosure relates to a welded joint manufacturing method, a welded joint, a tempering device, and a welding apparatus.
For example, when high-strength sheet steels are spot welded, insufficient joint strength may cause welds to fracture, such that members fail to perform as designed. Examples of high-strength sheet steels include sheet steel with a comparatively high carbon (C) content and tensile strength of 440 MPa or greater.
Various methods, such as post heat methods and two-stage energization methods are being investigated as joining processes for modifying weld properties. For example, Japanese Patent No. 5714537 (Patent Document 1) discloses technology for spot welding two or more overlapped high-strength steel sheets, in which welding is followed by an appropriate rest period before performing post heat for a short time. In Patent Document 1, the properties of a nugget region and a heat affected zone are modified by a tempering effect and an effect of slowing the rate of cooling (what is referred to as an auto-tempering effect), thereby enabling the joint strength to be improved. Note that a nugget or nugget region is a region where metal has melted.
However, in post heat methods or in two-stage energization methods such as that proposed in Patent Document 1, the range of conditions under which the desired effects can be obtained are narrow, and are liable to be affected by various external factors encountered at an actual manufacturing site. Application of such approaches at an actual manufacturing site therefore remains difficult. Hereafter, the quality of maintaining stable behavior under various external factors encountered at a manufacturing site is referred to as having high robustness, whereas conversely, being unable to maintain stable behavior is referred to as having low robustness.
Namely, many conventional post heat methods and two-stage energization methods are often methods in which energization is performed at least twice during a single cycle from when a sheet grouping is applied with pressure by a pair of electrodes until the electrodes are retracted after welding, as a technique for tempering the weld (nugget). However, with such methods, suitable ranges of conditions (temperature conditions and the like) in which the desired effects can be obtained are narrow. Moreover, at actual manufacturing sites, various external factors including debris, electrode wear, electrode core misalignment, and inter-steel sheet gaps may be encountered.
Since the current density during post heat varies due to the influence of such external factors, various issues exist, such as a tendency for uneven current density and difficulties in controlling the current density. Out of the various conditions that enable the desired effects to be obtained, the current passing through a weld being liable to stray from its suitable range in particular reduces robustness, making application of post heat methods and two-stage energization methods such as that in Patent Document 1 to actual manufacturing sites difficult. As a specific example, productivity of manufactured components suffers. There is accordingly a risk that the desired modified properties (tempering effect) in spot welded joints may be difficult to obtain.
An object of the present disclosure is to provide a welded joint manufacturing method, a welded joint, a tempering device, and a welding apparatus for a welded joint with excellent robustness that is not liable to be affected by external factors during a post heat process.
Specific aspects of the present disclosure are as follows.
A welded joint manufacturing method according to the present disclosure includes: preparing a welded joint including a first steel sheet, a second steel sheet overlapped with the first steel sheet, and a quenched nugget joining the first steel sheet and the second steel sheet together; abutting a first electrode against the first steel sheet at a site A, which is a location at an outer side of the nugget in a sheet-plane direction in a plane running parallel to the first steel sheet of the welded joint; abutting a second electrode against the second steel sheet at a site B, which is a location at an outer side of the nugget in a sheet-plane direction in a plane running parallel to the first steel sheet of the welded joint, and positioned on an opposite side of the nugget from the site A; and passing a current through the welded joint between the first electrode and the second electrode.
A tempering device according to the present disclosure includes a first electrode, and a second electrode. Approach and retract directions of the first electrode and approach and retract directions of the second electrode are mutually opposing directions to each other. An inter-electrode distance between the first electrode and the second electrode is at least 6 mm in a flat plane orthogonal to the approach and retract directions.
A welding apparatus according to the present disclosure includes the tempering device according to the present disclosure, a robot arm to which the tempering device is attached, a welding machine configured to form a nugget, and a position controller. The position controller is configured to control the robot arm so as to move an intermediate point between a tip of the first electrode and a tip of the second electrode to a location that has been welded as the nugget by the welding machine, and to dispose the first electrode and the second electrode at an outer side of the location that has been welded.
A welded joint according to the present disclosure includes a first steel sheet, a second steel sheet overlapped with the first steel sheet, and a quenched nugget joining the first steel sheet and the second steel sheet together. Tensile strength of the first steel sheet and the second steel sheet is at least 1180 MPa. A contact mark from a first electrode is formed on the first steel sheet at a site A, which is a location at an outer side of the nugget in a sheet-plane direction in a plane running parallel to the first steel sheet of the welded joint. A contact mark from a second electrode is formed on the second steel sheet at a site B, which is a location at an outer side of the nugget in a sheet-plane direction in a plane running parallel to the first steel sheet of the welded joint, and positioned on an opposite side of the nugget from the site A. A softened structure having Vickers hardness lower than Vickers hardness of the first steel sheet and Vickers hardness of the second steel sheet by at least 10 HV is continuously present between the contact mark from the first electrode and the contact mark from the second electrode.
The present disclosure is able to provide a welded joint manufacturing method, a welded joint, a tempering device, and a welding apparatus for a welded joint with excellent robustness that is not liable to be affected by external factors during a post heat process.
Detailed explanation follows regarding a favorable exemplary embodiment of a welded joint manufacturing method of the present disclosure, with reference to the drawings. A welded joint of the present disclosure is also referred to as a “spot welded joint”. In the drawings, portions that are the same or similar to one another are allocated the same or similar reference numerals. Note that the relationships between thickness and plan view dimensions, as well as thickness proportions and the like in the various devices and various members in the drawings may differ from those in reality. Accordingly, specific thicknesses and dimensions should be ascertained with reference to the forthcoming explanation. Moreover, dimensional relationships and proportions may also differ between the respective drawings.
<Spot Welded joint Manufacturing Method>
A spot welded joint manufacturing method of the present disclosure is a manufacturing method including a welding process of post-heating plural mutually overlapped steel sheets to form a nugget, a cooling process of cooling at least the nugget, and a tempering process of post-heating the plural steel sheets in an oblique direction relative to a sheet thickness direction of the steel sheets in order to temper at least the nugget.
Note that in the present specification, an “oblique direction relative to a sheet thickness direction” is also referred to simply as an “oblique direction”.
In the manufacturing method of the present disclosure, during a post heat process, this being a tempering process separate to the welding process to form the nugget, the nugget is tempered by post-heating the plural steel sheets in an oblique direction. In other words, when tempering the nugget, a pair of electrodes are disposed at an outer side of the nugget and the length of an energization path is increased such that the pair of electrodes on either side of the plural steel sheets do not overlap in a sheet thickness direction. As a result, a broad region including not only the nugget but also a heat affected zone (HAZ) peripheral to the nugget is gently heated, enabling tempering to occur. So doing enables a broad range of suitable current conditions to be secured in which a tempering effect (namely, an effect of improving toughness) can be obtained.
The manufacturing method of the present disclosure is thus capable of securing excellent robustness and is not liable to be affected by external factors during the post heat process.
Detailed explanation follows regarding each process of the manufacturing method of the present disclosure, with reference to the drawings. In the present disclosure, a welding apparatus is referred to as a “spot welding apparatus”. As illustrated in
[Welding Process]
The welding process of the present disclosure is a process of post-heating plural overlapped steel sheets to form a nugget.
The welding process may employ a similar process to a process performed during normal spot welding, as long as a nugget can be formed at mutually superimposed faces of the plural overlapped steel sheets and in a region in the vicinity of these mutually superimposed faces. An example of such a process is a process in which the plural overlapped steel sheets are interposed between a pair of electrodes, and then applied with a pressurizing force while being energized in a sheet thickness direction with a predetermined weld current for a predetermined weld time, in order to melt the mutually superimposed faces of the plural steel sheets and the region in the vicinity of these mutually superimposed faces and thereby form a nugget.
The upper electrode 2A and the lower electrode 3A each have a substantially circular cylinder shape, and have substantially the same dimensions as each other. Tips of the upper electrode 2A and the lower electrode 3A on the sheet grouping side have a reduced diameter, and outer edges of the tips as viewed along the sheet thickness direction are substantially circular in shape. In the welding process, the nugget is internally formed in the sheet grouping between the upper electrode 2A and the lower electrode 3A.
Note that predetermined weld conditions corresponding to a desired nugget diameter or the like may be employed as weld conditions during the welding process. Weld conditions may include a weld current, a weld time, and a pressurizing force to be applied by the electrodes.
A pair of electrodes such as those employed in normal spot welding may be employed as the pair of electrodes employed in the welding process, as long as they are capable of forming a nugget of a predetermined size at the mutually superimposed faces of the plural steel sheets and in a region in the vicinity of these mutually superimposed faces.
[Cooling Process]
The cooling process of the present disclosure is a process of cooling at least the nugget formed during the welding process described above.
The cooling process may employ a similar process to a cooling process performed during normal spot welding, as long as it is capable of transforming the nugget formed during the welding process into martensite. An example of such a process is a process in which the pair of electrodes are not retracted after the welding process and the pair of electrodes continue to hold the plural steel sheets in a non-energized state so as to allow heat from the steel sheets to dissipate into the electrodes. Another example of such a process is a process in which the pair of electrodes are retracted after the welding process so as to allow heat from the steel sheets to dissipate in air while the plural steel sheets are being conveyed to the spot welding apparatus employed in the tempering process.
Note that in the latter process, since the nugget cools in a state in which the pair of electrodes have been retracted, any thinning of the nugget can be suppressed since the nugget is not being applied with pressure from the electrodes. This is advantageous due to enabling a high joint strength to be stably achieved. Moreover, as a result of the process to retract the pair of electrodes, the plural steel sheets or welded joint cool while being conveyed after the welding process. This is advantageous in terms of productivity since it enables concurrent implementation of the welding process and the tempering process at separate weld locations.
Examples of cooling conditions employed during the cooling process include cooling duration or holding duration, as well as cooling temperature. Such a cooling condition may be that the nugget reaches a temperature of the Ms point or below, this being the temperature at which the nugget transforms into martensite, after the welding process. A cooling condition such that the nugget reaches a temperature of the Mf point or below is preferable.
[Tempering Process]
The tempering process of the present disclosure is a process of tempering at least the nugget by post-heating in a direction that is oblique to the sheet thickness direction of the plural steel sheets (namely, an oblique direction) after cooling.
The tempering process is a tempering process in which the plural steel sheets are energized in the oblique direction after cooling so as to temper martensite structures in at least the nugget, and in particular martensite structures in the nugget and the heat affected zone. Apart from post-heating in the oblique direction, the tempering process may be performed in a similar manner to a post heat process or a tempering process employed in a normal post heat method or two-stage energization method.
Note that whether or not the nugget has been tempered by such a post heat method or two-stage energization method may be confirmed by measuring a hardness distribution of the nugget. Tempering is confirmed when partial or full softening of the hardness is present after the post heat method or two-stage energization method. Although not illustrated in the drawings, during the tempering process, some or all of the upper electrode 2, the lower electrode 3, the upper fixing member 4, or the lower fixing member 5 may be employed to apply pressure to the sheet grouping. Such pressure application enables the upper electrode 2 and the lower electrode 3 to make more reliable contact with the sheet grouping.
Specific explanation follows regarding the tempering process, with reference to an exemplary embodiment of the present disclosure.
In the manufacturing method according to an exemplary embodiment of the present disclosure, the tempering process employs the tempering device 102 included in the spot welding apparatus 1 illustrated in
The tempering device 102 of the spot welding apparatus 1 includes the upper electrode 2, the lower electrode 3, the upper fixing member 4, the lower fixing member 5, an upper holder member 6, and a lower holder member 7 as principal configuration members. The upper electrode 2 is disposed at an upper side of the sheet grouping, and the lower electrode 3 is disposed at a lower side of the sheet grouping.
The upper electrode 2 corresponds to a first electrode of the present disclosure. The lower electrode 3 corresponds to a second electrode of the present disclosure. The upper fixing member 4 corresponds to a first fixing member of the present disclosure. The lower fixing member 5 corresponds to a second fixing member of the present disclosure. Note that
The upper fixing member 4 and the lower fixing member 5 are both fixing members. The upper fixing member 4 is disposed at the upper side of the sheet grouping, and the lower fixing member 5 is disposed at the lower side of the sheet grouping. The sheet grouping can be interposed between the upper fixing member 4 and the lower fixing member 5. The upper holder member 6 holds the upper electrode 2 and the upper fixing member 4, and is capable of moving in the up-down direction. The lower holder member 7 holds the lower electrode 3 and the lower fixing member 5.
As illustrated in
Note that the upper electrode 2 and the lower electrode 3 illustrated in
Moreover, in the tempering device 102 of the spot welding apparatus 1, as illustrated in
Note that although an inter-sheet separation (gap) between the upper sheet 8 and the lower sheet 9 is formed at both ends in the left-right direction in
Moreover, in the present exemplary embodiment, as illustrated in
Note that an example of a state in which the upper electrode 2 with the constant width W along the up-down direction is separated from the upper sheet 8 is illustrated on the upper left side in
As illustrated in
In plan view, the “circular” shape of the outer edge of the indentation and the “circular” shape of the nugget N are not limited to perfectly circular shapes, and may be considered to be circular overall even when some local distortions are present, and treated as if circular. Moreover, the shape of the outer edge of the indentation and the shape of the nugget N in plan view are not limited to circular shapes, and for example elliptical shapes or other geometric shapes may be adopted therefor as appropriate.
Note that the upper fixing member 4 and the lower fixing member 5 illustrated in
In the present disclosure, the tempering device of the spot welding apparatus employed in the tempering process is not limited to a configuration such as that of the tempering device 102 of the spot welding apparatus 1 illustrated in
Note that in cases in which the placement of the electrodes or the like is such that there is little concern of movement or misalignment of the plural steel sheets during post heat, fixing members such as those described above may be omitted from the spot welding apparatus.
In the manufacturing method of the present exemplary embodiment, the tempering process is specifically formed in the following manner using the tempering device 102 of the spot welding apparatus 1 described above.
First, a welded joint 10 is prepared by the welding process and the cooling process described above. As illustrated in
Moreover, the lower electrode 3 is abutted against the lower sheet 9 at a site B, this being a location at an outer side of the nugget N in every plane running parallel to the upper sheet 8 of the welded joint 10 and positioned on the opposite side of the nugget N from the site A. In order to achieve stable post heat, this is preferably interposed between the lower electrode 3 provided on the lower sheet 9 side and the lower fixing member 5 provided on the upper sheet 8 side. To describe the site B in other words, the site B runs through the lower sheet 9 and the upper sheet 8 in a direction perpendicular to the upper sheet 8, and projections of the site B and the nugget N do not overlap when the welded joint 10 is viewed along the direction perpendicular to the upper sheet 8. The site A and the site B are positioned with axial symmetry to each other about a central axis passing through a center E of the nugget N in a direction perpendicular to the upper sheet 8.
Namely, after the cooling process, the two steel sheets are energized in an oblique direction by abutting the upper electrode 2 and abutting the lower electrode 3 of the tempering device 102 of the spot welding apparatus 1 illustrated in
Moreover, when the current is passed through the welded joint 10, as illustrated in
When this is performed, as illustrated in
Note that of
The distribution diagram
As illustrated in
On the other hand, in the welded joint corresponding to the distribution diagram
The welded joint corresponding to the distribution diagram
In cases in which such spot welding is performed during mass production, the possibility of a site of high hardness aligning with a nugget end increases due to wear of the electrode tips, variations in thermal conductivity of the electrodes due to alloys forming between the electrodes and plating of the steel sheets, and so on. Moreover, the possibility of a site of high hardness aligning with a nugget end also increases due to variation in inter-steel sheet gaps, variation in the strike angle between the electrodes and the steel sheets, and so on. In particular, the width of inter-sheet gaps is liable to increase the higher the strength of the sheet steel employed. Namely, robustness is reduced as a result of the current passing through the weld being more liable to stray from a suitable range.
Accordingly, with tempering under conventional tempering conditions, since rapid heating occurs from the center of the nugget, it is very difficult to achieve uniform softening over a broad region spanning both the nugget and the heat affected zone.
By contrast, as described previously, the tempering process of the present disclosure enables the broad region including not only the nugget but also the heat affected zone peripheral to the nugget to be gently heated, thereby enabling the broad region spanning both the nugget and the heat affected zone to be uniformly softened, as seen in the hardness distributions illustrated by the distribution diagram
Hardness measurement positions in the various welded joints illustrated in
First, as illustrated in
The two steel sheets employed in the respective welded joints 10 were hot stamped members with 1.8 GPa grade tensile strength in each case. The sheet thickness t of the hot stamped members was 1.6 mm.
Note that in the present disclosure, with the exception of the post heat direction, the post heat conditions employed in the tempering process (post heat current, post heat time, electrode pressurizing force, and the like) may be set to any predetermined post heat conditions according to the desired joint strength and the like, as long as such conditions are capable of tempering at least the nugget. As an example of such post heat conditions, the tempering temperature is preferably set to a temperature condition within a range from 500° C. to the Ac3 point, and is more preferably set to a temperature condition within a range from 600° C. to the Ac1 point. Setting the tempering temperature within this range facilitates lowering of the hardness (namely, facilitates improving toughness) due to ensuring sufficient heat input, while not being susceptible to re-quenching which would cause re-hardening.
Moreover, the electrodes employed in the tempering process are not limited to circular rod shaped conductive members such as the upper electrode 2 and the lower electrode 3 described above. Electrodes such as those employed in normal spot welding may be employed as long as they are capable of post-heating the plural steel sheets in an oblique direction and of tempering at least the nugget. An example of electrodes employed in normal spot welding are DR-type electrodes that make point contact with steel sheets.
Note that adopting circular rod shaped conductive members such as the upper electrode 2 and the lower electrode 3 described above as the electrodes employed in the tempering process enables a broader region to be gently tempered, and thereby enables the advantageous effects of the present disclosure to be obtained to an even greater degree.
Moreover, the upper electrode and the lower electrode employed in the tempering process are preferably set such that an inter-electrode distance de between the two electrodes in the sheet-plane direction is at least twice the diameter ϕ of the nugget. The site A and the site B in
Note that, for example, in cases in which the shape of the nugget N is a true circle, the diameter thereof may be adopted as the diameter ϕ of the nugget N. In cases in which the shape of the nugget N is not a true circle, such as an elliptical shape as illustrated in
In cases in which the nugget N has an elliptical shape as illustrated in
Note that as illustrated in
Note that the approach direction of the upper electrode 2 with respect to the sheet grouping is a direction heading from the upper side toward the lower side in
Moreover, the upper electrode and the lower electrode are preferably set such that a distance between the upper electrode and the nugget in the sheet-plane direction sheet-plane direction is equal to a distance between the lower electrode and the nugget in the sheet-plane direction. Namely, in
Note that the distances between the upper electrode and the lower electrode and the nugget refer to the sheet-plane direction distances between central axes running through the respective electrodes in the up-down direction and the nugget center.
Moreover, the fixing members employed in the tempering process are not limited to circular rod shaped electrically insulating members such as the upper fixing member 4 and the lower fixing member 5 described previously. For example, electrically insulating members of any predetermined shape may be employed according to the desired manner of holding, ease of holding, and so on, as long as they are capable of fixing the plural steel sheets so as to not move or become misaligned when the plural steel sheets are energized in an oblique direction.
However, employing circular rod shaped electrically insulating members such as the upper fixing member 4 and the lower fixing member 5 described previously as the fixing members employed in the tempering process enables the plural steel sheets to be fixed more stably. This enables the advantageous effects of the present disclosure to be obtained even more reliably.
Moreover, in the present disclosure, in cases in which plural welding target locations are present on a single sheet grouping, when performing the cooling process for one welding target location, at least one process out of the welding process or the tempering process is preferably performed concurrently at another of the welding target locations. Performing the welding process through to the tempering process concurrently at the plural welding target locations in this manner enables welded joint productivity to be further improved. Note that similar also applies in cases in which spot welding is performed in succession while conveying plural sheet groupings.
Note that in the present disclosure, processes to perform predetermined processing or the like may be provided before and/or after each process from the welding process through to the tempering process.
Next, explanation follows regarding the sheet steel employed in the manufacturing method of the present disclosure.
<Sheet Steel>
In the present disclosure, sheet steel having predetermined tensile strength and sheet thickness corresponding to the desired joint strength and the purpose of the joint (for example, for use in automobile components) may be employed for the plural steel sheets configuring a welding target. An example of such sheet steel is sheet steel with tensile strength from 270 MPa grade to 3000 MPa grade. Such sheet steel may be sheet steel plated with zinc or the like (namely, plated sheet steel).
Of such sheet steel, high-strength sheet steel with tensile strength of 780 MPa or higher becomes brittle at the welds and is susceptible to fracturing after welding. Accordingly, the present disclosure is particularly advantageous when applied to a sheet grouping of which at least one steel sheet is a high-strength sheet steel with tensile strength of 780 MPa or higher. In the present exemplary embodiment, the upper sheet 8 and the lower sheet 9 are configured from high-strength sheet steel for automotive use.
Note that in the present disclosure, all of the plural steel sheets may be configured from the same type of sheet steel, or some of the plural steel sheets may be configured from the same type of sheet steel. Alternatively, all of the steel sheets may be configured from different types of sheet steel.
Moreover, the number of steel sheets is not particularly limited, and two or more thereof may be employed according to the purpose of the welded joint. Moreover, although the sheet thickness of the steel sheets is not particularly limited, the thickness of a single sheet thereof is preferably between 0.5 mm and 3.2 mm.
Moreover, in the present disclosure, the shapes of the steel sheets are not particularly limited, as long as at least a welding target location of a steel sheet has a specific sheet shaped structure to be overlapped with a welding target location of another steel sheet along the sheet thickness direction. Namely, the steel sheets employed in the present disclosure may, for example, be flat plate shaped steel sheets, or steel sheets with a flattened sheet shaped structure overall. Alternatively, for example, a steel sheet employed in the present disclosure may have a locally sheet shaped structure at a portion including a welding target location, and a bent structure or the like at portions other than the sheet shaped structure, such as an L-shaped steel sheet or a hat-shaped steel sheet.
The manufacturing method of the present disclosure is not limited to the exemplary embodiment described above nor to Examples, described below. Various combinations, substitutions, and modifications may be implemented within a range not departing from the objective and spirit of the present disclosure.
More specific explanation follows regarding the present disclosure, with reference to Examples and Comparative Examples. Note that the present disclosure is not limited to such Examples.
(Welding Process)
A sheet grouping configured by overlapping two steel sheets, namely an upper sheet and a lower sheet, was interposed between a pair of electrodes of a spot welding apparatus. Each of the two steel sheets was configured by an un-plated hot stamped steel sheet with 1.5 GPa grade tensile strength and a sheet thickness of 1.2 mm. The pair of electrodes were both DR-type 40-16 electrodes with a diameter ϕ of 6 mm.
This sheet grouping was then energized along the sheet thickness direction, employing a weld current of 5.5 kA and a weld time of 16 cyc while applying a pressurizing force of 400 kgf (approximately 3923 N) to the sheet grouping, thereby forming a nugget at the mutually superimposed faces of the two steel sheets and in a region in the vicinity of the mutually superimposed faces. The weld by the electrodes was then stopped, and the sheet grouping was maintained in a state applied with the pressurizing force from the electrodes for a holding duration of 10 cyc.
(Cooling Process)
After welding, the sheet grouping was then air-cooled until reaching room temperature (namely, the Mf point or below).
(Tempering Process)
The sheet grouping was then moved to a tempering device of a spot welding apparatus configured similarly to the tempering device 102 of the spot welding apparatus 1 illustrated in
Next, the post heat of the electrodes was stopped, and the sheet grouping was maintained in an interposed state for a holding duration of 10 cyc in order to obtain a welded joint in which the two steel sheets were joined together. Note that a total of 11 Example welded joints were produced, one for each of the tempering process post heat currents given in Table 1.
Note that the tempering device of the spot welding apparatus employed in the tempering process was provided with Cu—Cr alloy circular rod electrodes as the upper electrode and the lower electrode. The upper electrode and the lower electrode each had a length of 50 mm and a diameter ϕ of 10 mm. The inter-electrode distance de between the upper electrode and the lower electrode in the sheet-plane direction was 50 mm. The spot welding apparatus employed in the tempering process was provided with ceramic circular rods as the upper fixing member and the lower fixing member. The upper fixing member and the lower fixing member each had a length of 50 mm and a diameter ϕ of 10 mm.
A welded joint of a Comparative Example 1 was produced similarly to the Examples, with the exception that a tempering process was not performed.
Welded joints of Comparative Examples 2 were produced similarly to the Examples, with the exception that the cooling process used a cool time of 60 cyc and the tempering process was performed using the pair of electrodes of the spot welding apparatus employed in the welding process. Namely, in these Comparative Examples, post heat was performed along the sheet thickness direction instead of in an oblique direction.
The welded joints of the Examples, the Comparative Example 1, and the Comparative Examples 2 produced as described above were fractured by chisel testing in order to confirm the fracture modes thereof. Table 1 gives confirmation results for the fracture modes.
Note that the welded joints of the Examples, the Comparative Example 1, and the Comparative Examples 2 each had the same nugget diameter ϕ, this being approximately 4√t, and specifically approximately 4.4 mm.
As illustrated by the Examples appearing within a bold frame in Table 1, it can be seen that the welded joints of the Examples could be made to exhibit the fracture mode of “plug fracture” across a wide range of tempering process conditions (namely an post heat current range) spanning from 6.5 kA to 10.0 kA. The fracture mode of “plug fracture” is indicative of high joint strength. Accordingly, it can be seen that the Examples demonstrate an excellent degree of robustness.
By contrast, as illustrated in Table 1, the welded joint of the Comparative Example 1 exhibited a fracture mode of “interfacial fracture”. The fracture mode of “interfacial fracture” is indicative of low joint strength. Moreover, as illustrated by the Comparative Examples 2 appearing within a bold frame in Table 1, although the welded joints of the Comparative Examples 2 did exhibit a fracture mode of “plug fracture”, the range of tempering process conditions across which “plug fracture” was obtained was very narrow, namely from 4.5 kA to 5.5 kA. Namely, it can be seen that Comparative Example 1 and Comparative Examples 2 demonstrate poor robustness.
It is thought that the wide range of tempering process conditions for the Examples is due to the gentle change in temperature as the temperature rises in response to the current. Specifically, as illustrated in
This is thought to be since the energization path in the tempering process of the Examples is broader, in other words the current density is lower, than in conventional tempering such as that employed for the Comparative Examples 2. This may also be inferred from the fact that the Examples are capable of obtaining the fracture mode of plug fracture at higher currents than the Comparative Examples 2.
In the manufacturing method of the welded joint 10 according to the present exemplary embodiment, the pair of electrodes are separated from each other such that the pair of electrodes on either side of the steel sheets do not overlap each other in the thickness direction. Accordingly, when passing a tempering current between the pair of electrodes, the length of the energization path CP is longer than it would be were the electrodes to overlap each other. In other words, the nugget is tempered by being energized in the oblique direction.
Note that “the length of the energization path CP is longer” encompasses a state in which the area of the energization path CP is larger in plan view. Namely, the present disclosure encompasses cases in which the shapes of the respective regions of the steel sheets contacted by the pair of electrodes are planar shapes instead of points in plan view.
Increasing the length of the energization path CP enables the current density per unit area of the energization path CP to be lowered. Since this enables the passage of current to be suppressed from becoming excessive, the current can be easily controlled when performing tempering in the post heat process, for example without the need to introduce a new current control mechanism when employing welding equipment provided with an existing current control mechanism. This makes the post heat process less liable to be affected by external factors, enabling a manufacturing method of a welded joint with excellent robustness to be provided.
Moreover, in the manufacturing method of the welded joint 10 according to the present exemplary embodiment, the site A and the site B are each separated from the center E of the nugget N by at least the maximum diameter of the nugget N. This enables tempering to be performed gently over an even broader region not limited to the nugget N but including both the nugget N and the heat affected zone.
Moreover, in the manufacturing method of the welded joint 10 according to the present exemplary embodiment, the distance between the site A and the center E of the nugget N is equal to the distance between the site B and the center E of the nugget N. This enables more uniform heating and tempering of a broad region including the nugget N.
Moreover, in the manufacturing method of the welded joint 10 according to the present exemplary embodiment, the site A is interposed between the upper electrode 2 and the upper fixing member 4, thereby eliminating a gap between the upper sheet 8 and the lower sheet 9 at the position of the site A. Moreover, the site B is interposed between the lower electrode 3 and the lower fixing member 5, thereby eliminating a gap between the upper sheet 8 and the lower sheet 9 at the position of the site B. Gaps in regions around the nugget N are thus eliminated, placing the respective steel sheets in close contact with each other. As a result, a broad region including not only the nugget N but also the heat affected zone peripheral to the nugget N can be gently heated. This enables uniform melting to be achieved over the broad region spanning both the nugget N and the heat affected zone.
In the manufacturing method of the welded joint 10 according to the present exemplary embodiment, the widths W of the upper electrode 2 and the lower electrode 3 are set to at least the maximum diameter ϕ of the nugget N, enabling the nugget N to be more reliably energized. Note that although the widths W of the upper electrode 2 and the lower electrode 3 may be the same as the maximum diameter ϕ of the nugget N, setting the widths W to at least the maximum diameter ϕ further increases the length of the energization path CP. This enables burn-through and the like to be prevented, and is thus advantageous from the perspective of increasing robustness.
Moreover, in the manufacturing method of the welded joint 10 according to the present exemplary embodiment, when current is passed through the welded joint 10 between the upper electrode 2 and the lower electrode 3, the current is passed through the inside of the welded joint 10 in a region outside of the nugget N, thus increasing the length of the energization path CP. This enables the current density per unit area to be further lowered on the energization path CP.
Moreover, the manufacturing method of the welded joint 10 according to the present exemplary embodiment is not liable to be affected by external factors during the post heat process, enabling a welded joint 10 with excellent robustness to be realized. Note that in the welded joint 10 illustrated in the example of
In the tempering device 102 according to the present exemplary embodiment, the inter-electrode distance de between the upper electrode 2 and the lower electrode 3 is set to at least 6 mm. This is advantageous when forming a nugget N with a width of 6 mm, this being a commonly employed width in such welded joints 10.
In the tempering device 102 according to the present exemplary embodiment, the upper fixing member 4 is provided on an approach/retract direction axis of the upper electrode 2. Moreover, the lower fixing member 5 is provided on an approach/retract direction axis of the lower electrode 3. This enables the post heat process to be implemented more stably.
Moreover, in the present exemplary embodiment, the tensile strength of the upper sheet 8 and the tensile strength of the lower sheet 9 are both at least 440 MPa. Accordingly, in cases in which high-strength sheet steel for automotive use is employed for the upper sheet 8 and the lower sheet 9, a desirable welded joint 10 for automotive use can be obtained. Note that in the present disclosure, either the upper sheet 8 or the lower sheet 9 may be configured from sheet steel with tensile strength of at least 440 MPa. Moreover, in cases in which a welded joint is configured from three or more steel sheets, the tensile strength of at least one of the steel sheets should be at least 440 MPa.
Moreover, in the present exemplary embodiment, when performing tempering, the upper electrode 2 and the lower electrode 3 are separated from indentations of the nugget N. Namely, even if a step has been formed on the upper sheet 8 by such an indentation, the upper electrode 2 does not contact this step. Similarly, the lower electrode 3 does not contact such a step on the lower sheet 9. This enables the upper electrode 2 to contact the upper sheet 8 smoothly, and enables the lower electrode 3 to contact the lower sheet 9 smoothly.
In a tempering device 102A according to a first modified example illustrated in
The tempering device 102A according to the first modified example includes an energization controller 20 configured to alternately execute energization between the upper electrode 2 and the lower electrode 3 and energization between the upper fixing member 4 serving as a third electrode and the lower fixing member 5 serving as a fourth electrode. Accordingly, in the post heat process, the four electrodes that are the upper electrode 2, the lower electrode 3, the upper fixing member 4, and the lower fixing member 5 are able to execute post heat with energization paths that form an X shape.
The tempering device 102A according to the first modified example alternately executes energization between the upper electrode 2 and the lower electrode 3 and energization between the third electrode and the fourth electrode, thus enabling the post heat process to be implemented with greater efficiency.
A welding apparatus according to a second modified example illustrated in
The robot arm 12 illustrated on the upper side in
The robot arms 12, 13, or the gripper fingers 12, 13 attached to the leading end of a robot arm, move an intermediate point between the tip of the upper electrode 2 and the tip of the lower electrode 3 to a location that has been welded as the nugget by the welding machine 101, and dispose the upper electrode 2 and the lower electrode 3 at an outer side of this welded location. Note that the welding machine that forms the nugget in the second modified example is similar to the welding machine 101 illustrated in
Similarly to the case described in the present exemplary embodiment, the welding apparatus according to the second modified example is not liable to be affected by external factors during the post heat process, and is able to provide a welding apparatus capable of manufacturing the welded joint 10 with excellent robustness.
A welded joint of the present disclosure may be configured by three or more overlapped steel sheets.
For example, in welded joints for automotive use, an outer side steel sheet may be thinner than an inner side steel sheet. Accordingly, a welded joint according to the present disclosure that includes steel sheets having different thicknesses to each other may be advantageous as a welded joint for automotive use. Moreover, high-strength sheet steel may be employed in welded joints for automotive use. In the present disclosure, at least one of the steel sheets out of the plural steel sheets included in the welded joint should be configured by high-strength sheet steel.
The present disclosure may be configured using a combination of elements of the respective configurations illustrated in
The manufacturing method of the present disclosure is not liable to be affected by external factors and is able to secure excellent robustness during a post heat process, and is thus favorably applied to spot welding employing high-strength sheet steel such as sheet steel for automotive use.
The present disclosure accordingly has a high level of industrial applicability.
The present specification conceptualizes the following aspects.
Specifically, a first aspect is a welded joint manufacturing method including:
preparing a welded joint including a first steel sheet, a second steel sheet overlapped with the first steel sheet, and a quenched nugget joining the first steel sheet and the second steel sheet together;
abutting a first electrode against the first steel sheet at a site A, which is a location at an outer side of the nugget in a sheet-plane direction in a plane running parallel to the first steel sheet of the welded joint;
abutting a second electrode against the second steel sheet at a site B, which is a location at an outer side of the nugget in a sheet-plane direction in a plane running parallel to the first steel sheet of the welded joint, and positioned on an opposite side of the nugget from the site A; and
passing a current through the welded joint between the first electrode and the second electrode.
A second aspect is the welded joint manufacturing method of the first aspect, wherein the site A and the site B are each separated from a center of the nugget in the plane running parallel to the first steel sheet by at least a maximum diameter of the nugget.
A third aspect is the welded joint manufacturing method of the second aspect, wherein a distance between the site A and the center of the nugget and a distance between the site B and the center of the nugget are equal to each other.
A fourth aspect is the welded joint manufacturing method of any one of the first aspect to the third aspect, further including:
abutting the first electrode against the first steel sheet such that the site A is interposed between the first electrode and a first fixing member provided on a second steel sheet side; and
abutting the second electrode against the second steel sheet such that the site B is interposed between the second electrode and a second fixing member provided on a first steel sheet side.
A fifth aspect is the welded joint manufacturing method of any one of claim 1 to claim 4, wherein:
any gap between the first steel sheet and the second steel sheet at a position of the site A is eliminated by interposing the site A between the first electrode and the first fixing member; and
any gap between the first steel sheet and the second steel sheet at a position of the site B is eliminated by interposing the site B between the second electrode and the second fixing member.
A sixth aspect is the welded joint manufacturing method of any one of the first aspect to the fifth aspect, wherein:
each of the first electrode and the second electrode has a constant width in the plane running parallel to the first steel sheet when the current is passed through the welded joint; and
the width of the first electrode and the width of the second electrode are at least a maximum diameter of the nugget in the plane running parallel to the first steel sheet.
A seventh aspect is the welded joint manufacturing method of any one of the first aspect to the sixth aspect, wherein when passing the current through the welded joint between the first electrode and the second electrode, an energization path is increased in length by passing the current through a region inside the welded joint other than the nugget.
An eighth aspect is a tempering device including:
a first electrode; and
a second electrode, wherein:
approach and retract directions of the first electrode and approach and retract directions of the second electrode are mutually opposing directions to each other; and
an inter-electrode distance between the first electrode and the second electrode is at least 6 mm in a flat plane orthogonal to the approach and retract directions.
A ninth aspect is the tempering device of the eighth aspect, further including:
a first fixing member provided coaxially with the approach and retract directions of the first electrode; and
a second fixing member provided coaxially with the approach and retract directions of the second electrode.
A tenth aspect is the tempering device of the ninth aspect, wherein:
the first fixing member is a third electrode;
the second fixing member is a fourth electrode; and
the tempering device further includes an energization controller configured to alternately execute energization between the first electrode and the second electrode and energization between the third electrode and the fourth electrode.
An eleventh aspect is a welding apparatus including:
the tempering device of any one of the eighth aspect to the tenth aspect;
a robot arm to which the tempering device is attached;
a welding machine configured to form a nugget; and
a position controller configured to control the robot arm so as to move an intermediate point between a tip of the first electrode and a tip of the second electrode to a location that has been welded as the nugget by the welding machine, and to dispose the first electrode and the second electrode at an outer side of the location that has been welded.
A twelfth aspect is a welded joint including:
a first steel sheet;
a second steel sheet overlapped with the first steel sheet; and
a quenched nugget joining the first steel sheet and the second steel sheet together; wherein:
tensile strength of the first steel sheet and the second steel sheet is at least 1180 MPa;
a contact mark from a first electrode is formed on the first steel sheet at a site A, which is a location at an outer side of the nugget in a sheet-plane direction in a plane running parallel to the first steel sheet of the welded joint;
a contact mark from a second electrode is formed on the second steel sheet at a site B, which is a location at an outer side of the nugget in a sheet-plane direction in a plane running parallel to the first steel sheet of the welded joint, and positioned on an opposite side of the nugget from the site A; and
a softened structure having Vickers hardness lower than Vickers hardness of the first steel sheet and Vickers hardness of the second steel sheet by at least 10 HV is continuously present between the contact mark from the first electrode and the contact mark from the second electrode.
The present specification further conceptualizes the following alternative aspects.
Specifically, a first alternative aspect is a manufacturing method of a spot welded joint configured by plural steel sheets joined together by spot welding, the manufacturing method including:
a welding process of post-heating the plural mutually overlapped steel sheets to form a nugget;
a cooling process of cooling at least the nugget; and
a tempering process of post-heating the plural steel sheets in an oblique direction relative to a sheet thickness direction in order to temper at least the nugget.
A second alternative aspect is the spot welded joint manufacturing method of the first alternative aspect, wherein in the tempering process, an upper electrode and a lower electrode are disposed such that the plural steel sheets are interposed therebetween and also disposed at positions on mutually opposing sides of the nugget in a horizontal direction that is orthogonal to the sheet thickness direction, and the upper electrode and the lower electrode are employed to energize the plural steel sheets in the oblique direction.
A third alternative aspect is the spot welded joint manufacturing method of the second alternative aspect, wherein an inter-electrode distance between the upper electrode and the lower electrode in the horizontal direction is at least twice a diameter of the nugget.
A fourth alternative aspect is the spot welded joint manufacturing method of the second alternative aspect or the third alternative aspect, wherein a distance between the upper electrode and the nugget in the horizontal direction is equal to a distance between the lower electrode and the nugget in the horizontal direction.
A fifth alternative aspect is the spot welded joint manufacturing method of any of the second alternative aspect to the fourth alternative aspect, wherein in the tempering process, a fixing member is employed to fix the plural steel sheets during post heat.
A sixth alternative aspect is the spot welded joint manufacturing method of the fifth alternative aspect, wherein:
the fixing member includes an upper fixing member and a lower fixing member disposed such that the plural steel sheets are interposed therebetween; and
the upper fixing member is positioned on the opposite side of the nugget from the upper electrode in the horizontal direction; and
the lower fixing member is positioned on the opposite side of the nugget from the lower electrode in the horizontal direction.
The alternative aspects described above exhibit the following operation and advantageous effects.
In the spot welded joint manufacturing method according to the alternative aspects, the nugget is tempered by post-heating the plural steel sheets in the oblique direction with respect to the sheet thickness direction during the tempering process, this being a separate process to the welding process in which the nugget is formed. This approach is less liable to be affected by external factors, enabling excellent robustness to be secured.
The disclosure of Japanese Patent Application No. 2019-047020, filed on Mar. 14, 2019, is incorporated in its entirety by reference herein.
All cited documents, patent applications, and technical standards mentioned in the present specification are incorporated by reference in the present specification to the same extent as if each individual cited document, patent application, or technical standard was specifically and individually indicated to be incorporated by reference.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2019-047020 | Mar 2019 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2020/011563 | 3/16/2020 | WO | 00 |
