JP6558249B2 - Tensile test method for spot welded cross joints - Google Patents

Description

  The present invention relates to a tensile test method for evaluating peel strength characteristics of a spot welded cruciform joint.

In recent years, in the automobile field, it is required to reduce the weight of a vehicle body in order to reduce fuel consumption and reduce CO ₂ emissions, and to increase the strength of vehicle body members in order to improve collision safety. In order to satisfy these requirements, it is effective to use high-strength steel plates for vehicle body members and various parts.

  Resistance spot welding (hereinafter referred to as “spot welding”) is widely used mainly in processes such as assembling a vehicle body made of a high-strength steel plate and mounting parts. In the past, it has been considered that as the strength of a steel plate used for spot welding increases, the welded portion becomes stronger. However, when the strength of steel sheets is increased and spot welding is performed on steel sheets of a strength class called ultra high tensile strength or AHSS, there is a problem that the peel strength of the spot welded portion decreases as the base material strength increases. Therefore, in promoting the development of high-strength steel sheets, the weldability of high-strength steel sheets must be considered from the development stage.

  The decrease in peel strength of such high-strength steel plate welds results in sharp stress concentration due to increased steel plate strength, and the embrittlement factor that did not contribute to joint strength in spot welds of low-strength steel because the weld metal became harder It is thought that this is because the influence of is becoming easier to appear. In recent years, many analyzes have been made on the characteristics of welded joints using high-strength steel sheets by utilizing highly accurate analysis techniques and numerical calculations. On the other hand, the peel strength characteristic evaluation that is the basis of such an analysis is a cross tensile strength (CTS) that is measured by applying a tensile load in the peel direction to a welded joint in which welded members are assembled in a cross and spot welded. The test is known and defined in JIS Z3137.

This conventional method for testing the cross tensile strength will be described with reference to the drawings.
FIG. 1 shows a test piece for the cross tension test. In the cross tension test, the test piece 1 is a spot-welded cross in which two rectangular metal plates (for example, steel plates) 2 and 3 are superposed on each other in a cross shape and the center portion is spot welded to form a welded portion 4. It is a joint. The steel plates 2 and 3 are provided with through holes 2a, 2b, 3a, and 3b for fixing to the tensile test jig with bolts, respectively. Hereinafter, for convenience, the steel plate 2 side may be described as the upper side and the steel plate 3 side as the lower side.

  FIG. 2 shows a state in which the test piece is assembled to a cross tensile test jig. The steel plate 2 of the test piece 1 is sandwiched between the upper jig 5 and the pressers 6a and 6b, and is fixed by bolts 7a and 7b. The steel plate 3 of the test piece 1 is the lower jig 8 and the presser foot. It is sandwiched between 9a and 9b and fixed with bolts 10a and 10b. The jigs 5 and 8 are constituted by rectangular parallelepiped portions 5a and 8a that sandwich the steel plates 2 and 3, respectively, and shafts 5b and 8b that are connected to the rectangular parallelepiped shapes 5a and 8a and pull the test piece in the peeling direction. The pressers 6a, 6b, 9a, 9b have a rectangular parallelepiped shape. In the cross tension test, a tensile load is applied in the direction indicated by the arrow, the load against the tensile displacement is measured, the tensile strength and the load against the tensile displacement are measured, and a load displacement curve is obtained.

  On the other hand, in Patent Document 1, it is considered that it takes time to assemble a test piece to a tensile test jig in the conventional cross tensile test jig. It is disclosed that a test piece is firmly held by a jig for cross tension test by being pushed in with a screw.

Japanese Utility Model Publication No. 58-082649

  In the conventional cross tension test, the load displacement curve has a tendency that the load suddenly rises on the high displacement side, and the rise (inclination, etc.) of the load in this load displacement curve is the test regardless of the same specimen. Everything was different. Therefore, it has been difficult to associate the load displacement curve with the fracture phenomenon of the weld.

  In view of such a situation, an object of the present invention is to provide a method capable of improving the reproducibility of how the load rises on the load displacement curve in the cross-tension test method for spot welded joints.

  The present inventors diligently studied a means for solving the above problems. There are various theories as to why the load suddenly rises on the high displacement side of the load displacement curve in the cross tension test, and when the opinions were not in agreement, the present inventors conducted a test from the actual state of the cross tension test as follows. It was thought to be due to slippage between the piece and the tensile test jig.

  First, in JIS Z3137, a hole of φ20 mm is made in a test piece, the test piece is fixed to a tensile test jig with a hexagon bolt, and the hexagon bolt has a nominal diameter of M12, M14, M16, M18. It is prescribed to be either. In order to reduce the variation in the test results, it is described that it is desirable to use a bolt having a large nominal diameter and to reduce the gap between the φ20 mm hole of the test piece and the bolt.

  However, the surface condition of the test piece, the tightening torque, and the like are not defined, and the degree of slippage of the test piece changes depending on the degree of tightening of the bolts of the test operator. In addition, friction corresponding to the bolt tightening axial force during assembly occurs between the test piece and the jig and the presser foot. However, friction may decrease due to a decrease in the thickness of the test piece due to tension, and slippage may occur between the test piece and the tensile test jig.

  Therefore, the present inventors welded the test piece to the presser foot to suppress slippage between the test piece and the tensile test jig, and conducted a cross tensile test in that state. It was found that slippage was suppressed and a load displacement curve with good reproducibility was obtained.

The present invention has been made based on the above findings, and the gist thereof is as follows.
(1) In a tensile test method for joints spot welded by superimposing metal plates in a cross shape,
Welding and fixing the joint to the presser foot,
Overlay the jig on the joint, tighten the bolt between the presser foot and the jig,
A tensile test method for a spot welded cross joint, wherein a tensile load is applied in the peeling direction of the joint through the jig.

  According to the present invention, since the spot welded cruciform joint is welded and fixed to the presser foot, slippage between the spot welded cruciform joint and the tensile test jig can be suppressed, and the reproducibility of how the load rises in the load displacement curve. Can be improved.

It is a figure which shows the test piece of a cross tension test. It is a figure which shows the state which assembled | attached the test piece to the jig | tool for a cross tension test. A schematic cross section of a test piece welded to a presser foot is shown. It is a figure which shows the cross section of the state which welded the test piece to the presser foot and was assembled | attached to the jig | tool. It is a figure which shows a load displacement curve.

  The spot welded cross joint tensile test method of the present invention (hereinafter referred to as “test method of the present invention”) is a method in which a spot welded cross joint is welded and fixed to a presser foot and assembled to a jig. Thereby, the slip between the spot welded cross joint and the tensile test jig is reduced.

Next, the flow of the test method of the present invention will be described.
In the method of the present invention, first, a spot-welded cruciform joint (hereinafter also referred to as “test piece”) is manufactured according to the method defined in JIS Z3137. As shown in FIG. 1, the metal plates 2 and 3 are arranged so that the center lines of the widths are perpendicular to each other, and spot welding is performed at the center of the cross-over portion to produce the test piece 1.

Next, the test piece is fixed to the presser foot by welding. The method for welding the test piece to the presser foot is not particularly limited. Next, a method performed by laser welding will be described as an example of a method of welding a test piece to a presser foot.
FIG. 3 shows a schematic cross section of the test piece welded to the presser foot. FIG. 3 is a view corresponding to a cross section in the plate thickness direction of the test piece in FIG. 2. In addition, the through-holes of the bolts of the steel plate 2, the jig 5, and the pressers 6a and 6b are not shown.

  As shown in FIG. 3, each of the pressers 6a and 6b is provided with a wall at one end. Then, the pressers 6a and 6b are placed on the jig 5 with the ends of the jig 5 aligned with the walls of the pressers 6a and 6b. Next, the steel plate 2 is placed on the pressers 6a and 6b on the side where the jig 5 is not disposed. And the test piece 1 which consists of the steel plate 2 and the steel plate 3, and the presser foot 6a, 6b are welded. In welding the steel plate 2 and the pressers 6a and 6b, laser welding is performed by irradiating the laser beam 11 from the steel plate 2 side. Thereby, the laser welding bead 12 is formed in about two places between the steel plate 2 and the presser foot 6a and the presser foot 6b.

  Next, the steel plate 3 and the pressers 9a and 9b are welded. First, the test piece 1 is turned upside down and rotated about 90 ° about the spot welded portion 4, and the end of the jig 5 is aligned with the walls of the pressers 9a and 9b as in FIG. Then, the pressers 9 a and 9 b are placed on the jig 5. Then, laser welding is performed by irradiating the laser beam 11 from the steel plate 3 side. Thereby, the laser welding bead 12 is formed in about two places between the steel plate 3 and the presser foot 9a and the presser foot 9b.

Note that the walls of the end portions of the pressers 6a, 6b, 9a, and 9b improve the work efficiency of the alignment, and the walls need not be provided.
In addition, when the test piece 1 is assembled to the jigs 5 and 6, the raised laser welding beads 12 are ground with a belt sander or the like in order to bring the jigs 5 and 6 and the steel plates 2 and 3 into contact with each other. Also good.

  FIG. 4 shows a cross section in a state where the test piece is welded to the presser foot and assembled to the jig. FIG. 4 is a view corresponding to a cross section in the plate thickness direction of the test piece in FIG. In addition, the through-holes of the bolts of the steel plate 2, the jig 5, and the pressers 6a and 6b are not shown.

  As shown in FIG. 4, the steel plate 2 is sandwiched between the pressers 6a and 6b and the jig 5, and tightened with bolts (not shown). Then, the steel plates 3 are sandwiched between the pressers 9a and 9b and the jig 8 and tightened with bolts. Any of the nominal diameters M12, 14, 16 or 18 may be used as the bolt, but the nominal diameter M18 of the hexagonal bolt, which is most preferable for the variation in test results in JIS Z3137, may be used.

  In this manner, with the test piece assembled to the tensile test jig, the jigs 5 and 8 are pulled at a tensile speed of 10 mm / min or less in the direction shown by the arrow in FIG. Measure the load, measure the cross tensile strength and the load against the tensile displacement, and obtain the load displacement curve.

  As a result, slippage between the spot welded cross joint and the tensile test jig can be suppressed, and a load displacement curve having the same shape can be obtained stably regardless of the operator. Can be associated.

Next, regarding the present invention, necessary requirements and preferable requirements will be sequentially described.
The method of the present invention is a method in which a tensile test is performed using a spot welded cross joint obtained by spot welding with metal plates superimposed in a cross shape. First, the test piece will be described.

(Metal plate)
The metal plate used as the test piece has the shape and dimensions specified in JIS Z3137. As shown in FIG. 1, the metal plate is 150 mm × 50 mm, and the distance between the centers of the bolt through holes is 100 mm. The through hole of the bolt is φ20 mm.

  Further, the metal plate is not particularly limited, and various metal plates can be used, but a steel plate is preferable. The component composition of the steel plate is not particularly limited, and a high-strength steel plate can be applied. Moreover, a metal plate is good also as a metal plate which formed surface treatment films | membranes, such as a plating film, on the both surfaces or one side of a welding location at least.

(Spot welding)
The spot welding method performed by superposing metal plates in a cross shape is not particularly limited, and examples include resistance spot welding, laser welding, arc spot welding, and the like.

  Next, a jig and presser for assembling the test piece, a welding method between the test piece and the presser, and a tensile test will be described.

(Jig and presser foot)
The jig and presser foot have the shape and dimensions specified in JIS Z3137.
The jig is composed of a rectangular parallelepiped shape portion and a shaft as shown in FIG. The dimension of the rectangular parallelepiped shape portion is 150 mm × 50 mm × 12 mm, and the distance between the centers of the through holes of the bolt is 100 mm. The axis is not particularly limited.

The presser foot has a rectangular parallelepiped shape as shown in FIG. 2 and its dimensions are 50 mm × 50 mm × 12 mm. Moreover, you may provide a wall in one edge part of a presser foot for the work efficiency of alignment.
Moreover, the through hole of the bottle in the jig and the presser foot is set to φ20 mm.

(Welding method between specimen and presser foot)
The welding method of a test piece and a presser foot is not particularly limited, and laser welding, arc welding, plasma welding, and seam welding are exemplified. In laser welding, welding can be performed using a laser such as a YAG laser, a fiber laser, or a DISK laser as a laser oscillator.

The welding condition may be a condition that enables stable through welding.
The welding conditions for line welding by laser welding can be a laser output of 2 to 5 kW, a condensing area of 5 to 30 mm ² , and a welding speed of 0.5 to 5.0 m / min.
Moreover, what is necessary is just to set it as the welding conditions which can penetrate stably also in arc, plasma, and seam welding.

  A welding location will not be specifically limited if a test piece and a presser foot can be fixed. However, in order to suppress slippage of the test piece, it is preferable to form a welded portion of the test piece and the presser foot between the through hole of the bottle of the test piece and the spot welded portion. Further, the shape and number of beads are not particularly limited. Examples of the bead shape include a line shape, a wave shape, and a dot shape. The number of beads is preferably 2 or more, and more preferably 3 or more, in order to suppress the sliding of the test piece. On the other hand, if it exceeds 5, the effect of sliding the test piece is saturated and the work load increases. For example, two or more lines may be welded in the short direction of the metal plate in the vicinity of the end of the presser foot between the through hole of the bottle of the test piece and the spot welded portion.

(Tensile test)
Hold the test piece with the presser foot and jig and tighten with the bolt. Then, a tensile test is performed by a method defined in JIS Z3137.

  Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on this one example of conditions. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

  First, a cold rolled 590 MPa DP steel plate having a plate thickness of 1.0 mm was prepared as a metal plate, which was processed into a 150 mm × 50 mm having a through hole φ20 mm. Then, the center lines are arranged so as to be orthogonal to each other, and the crucifixed portions are sandwiched from both sides by a CrCu DR type tip 40R and a tip diameter electrode of 6 mm, the applied pressure is 2.5 kN, and the energization time is 12 cyc (0.24 s). ), Spot welding was performed at an energization current of 5.8 kA, and six test pieces were produced.

  Three of the test pieces were welded and fixed to the presser foot as an invention example. Moreover, the remaining 3 bodies were not welded to the presser foot as a comparative example.

  For one body (Invention Example 1), a fiber laser (a lens having a wavelength of 1.07 μm, a focal length of 300 mm, and a condensing diameter of φ0.6 mm) is used to hold the gap between the through hole of the bottle of the test piece and the spot weld. Two welds were performed near the edge of the gold (in the range of 20 mm from the edge) in the width direction of the steel sheet. The welding conditions were a laser output of 4.25 kW and a welding speed of 4.0 m / min.

Moreover, arc welding was performed on one body (Invention Example 2). Using solid wire, digital pulse power supply, Ar + 20% CO ₂ as shielding gas and pulse MAG welding, near end of presser foot between through hole of test specimen bottle and spot welded part One weld was made in the width direction of the steel sheet (in the range of 20 mm from the end). The welding conditions were 200 A, 20 V, and 100 cm / min.

  One body (Invention Example 3) was subjected to seam welding. The electrode ring is 350 mm in diameter, 8 mm in thickness, and the tip radius of curvature is R = 8 mm. Made of Cr-Cu, driven by the forging knurl method, near the end of the presser foot between the through hole of the bottle of the specimen and the spot weld Two were welded in the direction of the width of the steel sheet (in the range of 20 mm from the end). The applied pressure was 5 kN, the current value was 7.5 kA, the energization cycle was 3-2 cycle (on-off) (frequency: 50 Hz), and the welding speed was 100 cm / min.

  For each test piece, a jig is placed on the test piece, assembled with M18 bolts (tightening torque: 11 N · m), the tensile speed is 10 mm / min, the load against the tensile displacement is measured, and the load displacement curve is obtained. It was. FIG. 5 shows a load displacement curve.

  The load displacement curves of Invention Examples 1 to 3 had the same shape (e.g., how to rise the load) and had good reproducibility. In addition, the load displacement curve showed that the maximum load was reached and breakage occurred without a significant change in slope and without fluctuations in the load.

  On the other hand, the load displacement curves of Comparative Examples 1 to 3 have different shapes particularly on the high displacement side. In addition, the load displacement curve showed that the inclination decreased from a displacement of about 4 mm, and when the displacement was about 20 mm, the load rose steeply, and then the load fluctuated and reached the maximum load, leading to fracture. The inclination decreased from the displacement of about 4 mm because the load increased due to the tension due to the forced displacement, the specimen was pulled by the weld and slipped between the jigs, and the load transmitted through the steel sheet was alleviated and the load increase was small. It is thought that it becomes.

  In addition, the load after the displacement of about 20 mm steeply rises because the slope in that region is close to the slope of the load displacement curve of the invention example. It is considered that the edge of the through hole of the piece contacted the bolt, the test piece stopped sliding, and the test piece was changed to receive a load while deforming in a situation where it did not slide.

  According to the present invention, since the spot welded cruciform joint is welded and fixed to the presser foot, slippage between the spot welded cruciform joint and the tensile test jig can be suppressed, and the reproducibility of how the load rises in the load displacement curve. Can be improved. Therefore, the present invention has high industrial applicability.

1 Test piece 2 Metal plate (steel plate)
2a, 2b Through hole 3 Metal plate (steel plate)
3a, 3b Through-hole 4 Welded portion 5 Jig 5a Rectangular shape portion 5b Shaft 6a, 6b Presser foot 7a, 7b Bolt 8 Jig 8a Rectangular shape portion 8b Shaft 9a, 9b Presser foot 10a, 10b Bolt 11 Laser beam 12 Laser welding bead

Claims (1)

In the tensile test method for joints spot welded with metal plates stacked in a cross shape,
Welding and fixing the joint to the presser foot,
Overlay the jig on the joint, tighten the bolt between the presser foot and the jig,
A tensile test method for a spot welded cross joint, wherein a tensile load is applied in the peeling direction of the joint through the jig.

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