JP2000202563A - High strength steel plates joining method providing excellent tensile characteristic and fatigue characteristic - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a joining method of high strength plates by which excellent tensile characteristic and a fatigue characteristic are obtained. SOLUTION: A high strength plate with tensile strength 430 to 1000 MPa is selected as a material to be joined. By driving a rivet 3 into the overlaid materials to be joined, the rivet 3 is allowed to pierce through the overlaid materials, and deforming the tip of the pierced rivet 3, then the overlaid materials are mechanically joined. When using this joining method to the joining the high strength plates, a joint having high tensile strength and fatigue strength is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for joining high-strength steel sheets used for the purpose of reducing the weight of a vehicle body and improving collision safety characteristics.

[0002]

2. Description of the Related Art Recently, global warming due to an increase in CO _{2 in} the atmosphere has come to be taken up as an environmental problem.
There is a movement to reduce the amount of O ₂ emission. In response to such movements, in the field of automobiles, attempts have been made to reduce the thickness of the vehicle body by using a high-strength steel plate and reduce the weight of the vehicle body. On the other hand, separately from this, recently, the problem of the collision safety characteristics of automobiles has come to be greatly highlighted, and application of a high-strength steel plate has been studied in order to improve the collision safety characteristics. As described above, in the field of automobiles, needs for high-strength steel sheets are increasing. However, welding of high-strength steel sheets has the following problems.

[0003] Conventionally, spot welding is mainly used in the assembly process of automobiles, and spot welding is also used in welding high-strength steel sheets. When the steel plates are spot-welded, if the tensile strength of the steel plates increases, the tensile shear strength of the joint also increases. However, the fatigue strength in the tensile shear direction of the joint hardly increases even if the tensile strength of the steel sheet increases. To give an example, if a high-strength steel sheet with a tensile strength of 590 MPa is used instead of a mild steel sheet with a tensile strength of 290 MPa, the tensile shear strength of a spot welded joint will be almost 2
Although it is doubled, the fatigue strength in the tensile shear direction (for example, 2 ×
Fatigue strength at 10 ⁶ times) should not doubled, it exhibits almost the same value as that of mild steel plate. This is thought to be due to the notch shape of the spot weld as previously reported.

That is, as shown in FIG. 1, since a portion of a nugget (welded portion) 2 existing between steel plates 1 has a notch shape, a fatigue test is performed by applying a load in a tensile shear direction (arrow direction). When implemented, even if a steel plate having a high tensile strength is used, the fatigue strength is not improved by the notch effect. In particular, when a high-strength steel sheet is used, the hardness of the nugget part is increased as compared with the case where a mild steel sheet is used, so that the notch effect is remarkable. On the other hand, the tensile strength of the spot welded joint in the peeling direction (the direction perpendicular to the arrow) hardly increases even if the tensile strength of the steel sheet increases. Also,
The fatigue strength in the peeling direction hardly increases, and is at the same level as that of the mild steel sheet. This is presumably because, when a force is applied in the peeling direction, stress concentration occurs in the nugget portion, so that the notch effect becomes more remarkable than in the tensile shearing direction.

[0005] Means for improving the tensile strength in the peeling direction (cross tensile strength) and the fatigue strength in the shearing / peeling direction (hereinafter referred to as fatigue strength) of a spot welded portion of a high-strength steel sheet include, for example, iron. And Steel, Vol. 68 (1982) No. 9, P
As described in 1444 to P1451, the nugget part is annealed by performing tempering after spot welding,
Further, a method is known in which the residual stress is changed to improve the tensile strength and the fatigue strength in the peeling direction of the spot welded portion. However, in this method, there is a problem that the optimum conditions for the tempering are very narrow and the reproducibility is poor. Further, as in the case of continuously hitting a plated steel sheet, in the case where the electrode tip deteriorates due to an alloying reaction between the plating and the electrode with an increase in the number of spots, and the electrode tip diameter increases,
Since the current density decreases and the energization condition changes, there is also a problem that the tensile strength and the fatigue strength in the peeling direction of the joint are not improved due to deviation from the optimal energization condition.

On the other hand, separately from the above-described method of conducting the tempering, as shown in FIG. 2, a rivet 3 is driven into two superposed high-strength steel sheets 1 to mechanically join the two, and a joint is formed. Hahn.O. and Schulte.A .: Europaische Forsc
hungsgesellschaft fur Blechverarbeitung, Band T17,
EFB-Kolloquim, Leichtbau d. Intelligente Blechver
arbeitung, Fellbach, 1997. and Hahn.O., Kurzok.JR
and Schulte.A .: Tagungsband ,, Innovative Fugetechnik
en fur Leichbaukonstruktionen, 7-8, November, 1996
in Paderborn, Freundeskreis desLaboratoriums fur
Known from Werkstoff-und Fugetechnik eV, Paderborn, 1997.

However, in this method, since the rivet is driven, deformation of the steel sheet on the die side becomes extremely severe, and there is a problem that cracks occur on the die side of the high-strength steel sheet. The tensile strength in the shear direction (tensile shear strength) is lower than that of spot welded joints and bolted joints, and the tensile strength in the peeling direction (cross tensile strength) is:
There is also a problem that it is higher than spot welding but lower than a bolted joint. It is considered that the reason why a sufficient value is not obtained in the tensile strength in the shearing direction and the peeling direction is due to the joining form of the rivet driving method.

[0008] That is, in this bonding mode, it is considered that when tensile stress is applied in the shearing direction and the peeling direction, the rivet comes off and breaks, so that sufficient bonding strength cannot be obtained. Furthermore, in addition to this, 2 × 1
0 Comparing the fatigue strength at ⁶ times, compared to the spot welding, the fatigue strength towards the joining by riveting system is high, at the joint of the same riveting system than the fatigue strength of high-strength steel sheet and mild steel plate There is a problem that there is almost no difference (about 1.1 times improvement).

[0009]

As described above, even when spot welding is performed using a high-strength steel plate instead of a mild steel plate, the tensile strength and fatigue strength in the peel direction of the joint are not improved. Therefore, it is necessary to increase the number of welding points in order to make the high-strength steel sheet thinner and lighter. However, this leads to an increase in production time and, consequently, an increase in cost. Also, the degree of freedom in design is considered to be considerably limited. The present invention has been made to solve such a problem, and provides a joining method for a high-strength steel sheet having excellent tensile properties and fatigue properties.

[0010]

Means for Solving the Problems There are roughly two methods for improving the tensile strength and the fatigue strength in the peeling direction of the joint of a high-strength steel sheet. In other words, there are a method in which tempering is performed after spot welding and a method in which rivets are driven and mechanically joined. In the former method, it is considered that it is difficult to improve the tensile strength and the fatigue strength in the peeling direction with good reproducibility because the tempering condition changes with the deterioration of the electrode. On the other hand, the latter method has a considerable expectation as a method for improving the tensile strength and the fatigue strength in the peeling direction since the notch effect of the nugget part is lost.
Therefore, in order to improve the tensile strength and the fatigue strength in the peeling direction with good reproducibility, it is considered preferable to use the latter method.

However, even when the latter method is used, the tensile strength in the shear direction has a lower value than that of spot welding.
Therefore, in order to improve both the tensile strength and the fatigue strength in the shear direction and the peel direction, it is considered necessary to further improve the mechanical joining method of the rivet driving method.

From such a viewpoint, the present inventors have conducted intensive studies to improve the tensile strength and the fatigue strength of the high-strength steel plate joint in the shear direction and the peeling direction, and as a result, the inventors of the present invention have found that It is possible to improve the tensile strength and fatigue strength of the joint in the shear direction and the peeling direction by mechanically joining the materials to be joined by deforming the tip of the rivet that is driven by driving a rivet from one side and penetrating the rivet. I found it.

That is, the gist of the present invention is as follows: (1) A high-strength steel plate having a tensile strength of 430 to 1000 MPa is used as a material to be joined, and a rivet is driven into the material to be joined, which is made to penetrate. A method for joining high-strength steel sheets having excellent tensile properties and fatigue properties, characterized in that the tips of the set rivets are deformed and mechanically joined. (2) A rivet having a 0.2% proof stress, a tensile strength, and a Vickers hardness value represented by the following formula is used. How to join high strength steel sheets. 1.2YP _{0.2M ≤YP 0.2R} ≤2.5YP _0.2M 1.2TS _M ≤TS _R ≤2.5TS _M 1.2Hv _M ≤Hv _R ≤2.5Hv _M where YP _0.2M : 0 of the material to be joined .2% proof stress, YP _0.2R : 0.2% proof stress of rivet TS _M : Tensile strength of material to be joined, TS _R : Tensile strength of rivet Hv _M : Vickers hardness of material to be joined, Hv _R : Rivet Vickers hardness.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The joining method according to the present invention will now be specifically described with reference to the drawings, together with the operation. FIG.
FIG. 3 is a view for explaining a mechanical joining method according to the present invention. As shown in FIG. 3A, the two high-strength steel sheets 1 to be joined are overlapped with each other, and then the rivet 3 is driven from above with a punch 4 as shown in FIG. Further, the tip of the rivet 3 penetrated as shown in (c) is deformed by the die 5 to mechanically join the two high-strength steel plates.

A method for deforming the tip of the rivet 3 may be a cold forging method or a hot forging method. As a method of hot forging, any method may be used as long as the rivet is heated and deformed. For example, a method in which copper electrodes are used for the punch 4 and the die 5 and the rivet is energized and heated while being pressed by the electrodes, and deformed while lowering the deformation resistance of the rivet. The pressurizing and energizing conditions in this case depend on the shape of the rivet, but usually, the conditions used in spot welding of a steel plate may be used. As specific conditions, the pressing force is about 0.98 to 7.84 kN, the current value is about 3 to 10 kA, and the energizing time is 0.04 to
It is about 0.3 sec.

The electrode surface in contact with the rivet is desirably as flat as possible to prevent the occurrence of scattering.
Further, a method of heating and deforming the tip of the rivet with high frequency or the like may be used. When the hot forging method is used, the pressing force when deforming the rivet can be reduced, and cracking of the deformed portion of the rivet can be easily prevented. In addition, compared with the cold forging method, the rivet and the steel plate can be firmly contacted with each other, so that the rivet does not loosen, and it is considered that the tensile strength and the fatigue strength are increased. Other possible forms of such joining include bolt joining, but bolt joining requires drilling in advance and positioning is required when joining. It takes a long time and causes an increase in cost, which is not preferable.

The material of the rivet used for mechanical joining may be any material having a 0.2% proof stress, a tensile strength, and a hardness higher than that of a high-strength steel plate to be joined. That is, it is sufficient that each value satisfies the following equation. 1.2YP _{0.2M ≤YP 0.2R} ≤2.5YP _0.2M 1.2TS _M ≤TS _R ≤2.5TS _M 1.2Hv _M ≤Hv _R ≤2.5Hv _M where YP _0.2M : 0 of the material to be joined .2% proof stress, YP _0.2R : 0.2% proof stress of rivet TS _M : Tensile strength of material to be joined, TS _R : Tensile strength of rivet Hv _M : Vickers hardness of material to be joined, Hv _R : Rivet Vickers hardness

The values of 0.2% proof stress, tensile strength, and hardness are set to be equal to or higher than the above lower limits in order to reliably punch a high-strength steel sheet so that the rivet does not break at the time of driving. This is to prevent the rivet from breaking during the fatigue test. In addition, 0.2% proof stress, tensile strength,
The value of the hardness is set to be equal to or less than the above upper limit value because if a rivet having too high strength is used, cracks occur in the rivet at the time of driving or deformation on the die side. Specific materials include carbon steel (S45C, SK4, etc.)
Etc. may be used. The diameter of the rivet (the diameter of the part to be driven into the steel sheet) and the length may be appropriately selected according to the thickness of the high-strength steel sheet, but the diameter is 3 to 6 mm and the length is 4 to 14 mm.
What is necessary is just to use something. The shape of the die may be any shape as long as it does not crack with rivets.

FIG. 4A shows the shape of the rivet head.
May be used, or the flat or pan-shaped one shown in (b) or (c) may be used. In order to improve the tensile strength and fatigue strength of the joint as much as possible, it is desirable to use a rivet having a shape of (b) or (c) rather than (a) in which the rivet enters the material to be joined.

The high-strength steel sheet used in the present invention is not particularly limited, but a high-strength steel sheet having a tensile strength of about 430 to 1000 MPa may be used. Specifically, high-strength steel sheets such as solid-solution strengthening type, Ti precipitation strengthening type, and Nb precipitation strengthening type, and high-strength steel sheets of a composite structure type, for example, a dual-phase structure steel containing martensite in ferrite (Dual Phase Steel) Steel), a work-induced transformation type high retained austenite steel (TRIP steel) containing residual austenite in ferrite, a high burring steel containing bainite in ferrite, and the like. There are no particular restrictions on other mechanical properties, but higher elongation and higher r values are desirable to avoid cracks on the die side during rivet driving as much as possible. Further, the method for producing the steel sheet may be a hot rolling method or a cold rolling method. The plate thickness may be generally the thickness of a steel plate used in an automobile or the like, for example, about 0.4 mm to 4.0 mm.

[0021]

EXAMPLES (Example 1) As materials to be joined, six kinds of high strength steel sheets having a thickness of 1.0 mm and one kind of mild steel sheet shown in Table 1 were used. From this steel plate, 30 ×
A 100 mm test piece was used for a tensile shear fatigue test.
A test piece of × 150 mm was prepared. The test pieces were wrapped and overlapped by 30 mm and 40 mm, respectively, in a combination of similar materials, and as shown in FIG.
A rivet (material: SK4) having a length of 6.0 mm was driven in from one side and penetrated, and the tip of the rivet that was penetrated was deformed by cold forging and hot forging to be mechanically joined (joining method, respectively) A and joining method B). In addition,
The shape of the rivet head was flat as shown in FIG. Hot forging is performed by applying current and pressure to the rivet,
Pressing and energizing conditions are as follows: pressing force: 5.88 kN, current value:
7.0 kA, energization time: 0.2 sec. For comparison, a joint was manufactured by a conventionally used rivet driving type mechanical joining method as shown in FIG. 2 (joining method C). The joining rivet used had a diameter of 5.0 mm and a length of 6.0 mm. In addition, the specimen has a diameter of 5 mm.
And a joint joined with bolts was also manufactured (joining method D). In addition to these, spot-welded joints were also manufactured under the condition that the nugget diameter became 5.0 mm (joining method E).

The joints were subjected to a tensile shear test based on the JIS tensile shear test method. Also, J
A fatigue test was performed based on the IS tensile shear fatigue test method. Fatigue test is performed by oscillating test, stress ratio: 0.05,
Frequency: 5 Hz. Table 1 shows the values of the tensile shear strength, and Table 2 shows the values of the fatigue strength at 2 × 10 ⁶ times. As can be seen from Table 1, in the joining method (joining method A, joining method B) according to the present invention, the same level of tensile shear strength as in spot welding (joining method E) was obtained. The value was higher than the mechanical joining method (joining method C) or the joining method using bolts (joining method D).

On the other hand, as can be seen from Table 2, in spot welding (joining method E), even if the tensile strength of the steel sheet increases, the fatigue strength does not increase. On the other hand, when the joining method (joining method A, joining method B) of the present invention is used, the fatigue strength is lower than that of the joining method using bolts (joining method D), but the conventional mechanical method is used. It showed a higher value than the mechanical joining method (joining method C). A similar study was conducted using a steel plate having a thickness of 2.0 mm. The results were the same.

[0024]

[Table 1]

[0025]

[Table 2]

(Example 2) The same steel plate as in Example 1 shown in Table 3 was used as a material to be joined. From this steel plate, a test piece of 50 × 150 mm for a cross tensile test and a test piece of 50 × 150 mm for a cross tensile fatigue test were prepared. The test pieces were wrapped in a cross shape and overlapped with each other in a combination of the same kind of materials, and joined by five kinds of joining methods (joining methods A to E) in the same manner as in the first embodiment to form a joint. Produced. These joints were subjected to a cross tension test based on the JIS cross tension test method. Also, J
A fatigue test was performed based on the IS cross tension fatigue test method. The fatigue test conditions are the same as in the first embodiment. Table 3 shows the values of the cross tensile strength, and Table 4 shows the values of the fatigue strength at 2 × 10 ⁶ times.

As can be seen from Table 3, in the joining method (joining method A, joining method B) according to the present invention, the value of the cross tensile strength is
The values were higher than spot welding (joining method E), conventional mechanical joining method (joining method C), and bolt joining method (joining method D). On the other hand, as can be seen from Table 4, in spot welding (joining method E), even if the tensile strength of the steel sheet increases, the fatigue strength does not increase. On the other hand, when the joining method (joining method A, joining method B) of the present invention was used, the fatigue strength was lower than that of the joining method using a bolt (joining method D). The value was higher than that of the mechanical joining method (joining method C). A similar study was conducted using a steel plate having a thickness of 2.0 mm. The results were the same.

[0028]

[Table 3]

[0029]

[Table 4]

[0030]

When the above joining method of the present invention is used for joining high strength steel sheets, a joint having high tensile strength and fatigue strength can be obtained.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view for explaining the fatigue strength of a spot weld.

FIGS. 2A and 2B are cross-sectional views illustrating an example of a conventional joining method, in which FIG. 2A shows a state at the start of rivet driving, and FIG. 2B shows a state after rivet driving.

FIGS. 3A and 3B are cross-sectional views illustrating an example of a joining method according to the present invention, wherein FIG.
(C) shows a step of deforming the rivet tip after the rivet is driven, and (c) shows each step.

FIGS. 4A to 4C are cross-sectional views illustrating examples of rivet shapes.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 High-strength steel plate 2 Nugget 3 Rivet 4 Punch 5 Dice

Claims (2)

[Claims] 1. A high-strength steel sheet having a tensile strength of 430 to 1000 MPa is used as a material to be joined, a rivet is driven into the material to be joined, and the rivet is penetrated. A method for joining high-strength steel sheets having excellent tensile properties and fatigue properties, characterized by being joined to steel sheets. 2. A rivet having a 0.2% proof stress, a tensile strength, and a Vickers hardness represented by the following formulas is used, and excellent in tensile properties and fatigue properties according to claim 1. High strength steel sheet joining method. 1.2YP _{0.2M ≤YP 0.2R} ≤2.5YP _0.2M 1.2TS _M ≤TS _R ≤2.5TS _M 1.2Hv _M ≤Hv _R ≤2.5Hv _M where YP _0.2M : 0 of the material to be joined .2% proof stress, YP _0.2R : 0.2% proof stress of rivet TS _M : Tensile strength of material to be joined, TS _R : Tensile strength of rivet Hv _M : Vickers hardness of material to be joined, Hv _R : Rivet Vickers hardness