JP2000205213A - Method of joining high strength steel sheet with excellent fatigue properties and high strength steel sheet for joining - Google Patents

Abstract

(57) [Problem] To provide a joining method of a high-strength steel sheet excellent in fatigue characteristics and a high-strength steel sheet used for this joining. SOLUTION: A high-strength steel sheet having a tensile strength of 490 to 1000 MPa and a composite structure including at least one of austenite, martensite, and bainite and a ferrite structure is used as a material to be joined, and the material to be joined is overlapped. This method is characterized by using rivets and mechanical joining to achieve high fatigue strength at the joints of high-strength steel sheets, compared to the spot welding method generally used conventionally. Can be

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, and a high-strength steel sheet used for the joining.

[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). In this case, 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. This tendency is the same even when the load direction is the peeling direction (the direction perpendicular to the arrow). In this case, the stress concentration occurs at the nugget part, so that the fatigue strength value is lower than the tensile shear direction. Decrease.

[0005] Means for improving the fatigue strength of the spot-welded portion of a high-strength steel plate include, for example, iron and steel, Vol. 68 (1).
982) No. 9, P1444 to P1451, a method of annealing a nugget part by applying a tempering current after spot welding and changing a residual stress to improve the fatigue strength of the spot welded part is known. Have been. 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 where a plated steel sheet is continuously hit, as the number of hit points increases, the electrode tip deteriorates due to an alloying reaction between the plating and the electrode, and when the electrode tip diameter increases, Since the current density decreases and the energization state changes, there is also a problem that the fatigue strength of the joint is 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. an
d Schulte.A .: Europaische Forschungsgesellschaft fu
r Blechverarbeitung, Band T17, EFB-Kolloquim, Leic
htbau d. intelligente Blechverarbeitung, Fellbach,
1997. and Hahn.O., Kurzok.JR and Schulte.A.:Tagu
ngsband ,, Innovative Fugetechniken fur Leichbaukons
truktionen, 7-8, November, 1996 in Paderborn, Freu
ndeskreis desLaboratoriums fur Werkstoff-und Fuget
echnik eV, Paderborn, 1997. However, in this method, since the rivet is driven, the 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. Also, 2 ×
10 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 And there is almost no difference (about 1.1 times improvement).

[0007]

As described above, even if a high-strength steel sheet is used instead of a mild steel sheet, the fatigue strength of the joint is 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,
It causes cost increase. 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 of a high-strength steel sheet having excellent fatigue characteristics and a high-strength steel sheet suitable for the method.

[0008]

Means for Solving the Problems There are roughly two methods for improving the fatigue strength at the joint of high strength steel sheets. 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 fatigue strength with good reproducibility because the tempering condition changes with the deterioration of the electrode. On the other hand, the latter method
Since the notch effect of the nugget part is lost, there is considerable expectation as a method for improving the fatigue strength. Therefore, in order to improve the fatigue strength with good reproducibility among the above two methods, it is considered that the latter method is more preferable. On the other hand, in order to improve the fatigue strength of the joint by the rivet driving mechanical joining method, it is necessary to use a high-strength steel sheet suitable for this method so that cracks do not occur on the die side when driving rivets. It is considered important.

From such a viewpoint, the present inventors have conducted intensive studies in order to improve the fatigue strength of a joint of a high-strength steel sheet. As a result, the present inventors have used a composite structure steel sheet as a material to be joined, and set rivets into it. By mechanically joining with
It has been found that the fatigue strength of the joint can be improved.

That is, the gist of the present invention is as follows: (1) A high-strength steel sheet having a tensile strength of 490 to 1000 MPa and a composite structure including at least one of austenite, martensite, and bainite and a ferrite structure. A method for joining high-strength steel sheets excellent in fatigue characteristics, wherein rivets are driven into the joined joined members and mechanically joined. (2) A high-strength steel sheet having excellent fatigue properties according to (1), wherein a rivet having a 0.2% proof stress, a tensile strength, and a Vickers hardness value represented by the following formula is used. 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 (3) A high-strength steel sheet used in the joining method according to (1) or (2), wherein the tensile strength is 490 to 1000.
A high-strength steel sheet for rivet bonding having a composite structure including at least one of austenite, martensite, and bainite and a ferrite structure at MPa. It is.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The joining method according to the present invention and the joining material used in the joining method according to the present invention will be specifically described below with reference to the drawings. FIG. 2 is a diagram for explaining the mechanical joining method according to the present invention. As shown in FIG. 2 (a), two high-strength steel sheets 1, which are materials to be joined, are overlapped, and a rivet 3 is driven from above with a punch 4,
The tip of the rivet 3 is set with the die 5 at the time of driving, as shown in FIG.
And mechanically join the two high-strength steel sheets.

The material of the rivet used for mechanical joining may be any material as long as it has a 0.2% proof stress, tensile strength, and hardness higher than that of a high-strength steel plate as a material 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 the 0.2% proof stress, tensile strength, and hardness are set to be equal to or more than the above values in order to reliably punch a high-strength steel sheet so that the rivet does not break at the time of driving.
It is also to prevent the rivet from breaking during the fatigue test. Also, the values of 0.2% proof stress, tensile strength, and hardness were set to the above values or less because if rivets with too high strength were used, cracks occurred in the rivets during driving or deformation on the die side. Because you do. As a specific material, carbon steel (S45C, SK4, etc.), composite structure steel (two-phase structure steel, TRIP steel) or the like may be used.
The diameter of the rivet (the diameter of the portion to be driven into the steel plate) and the length may be appropriately selected according to the thickness of the high-strength steel plate, and a rivet having a diameter of about 3 to 6 mm and a length of about 4 to 14 mm may be used. The shape of the die may be any shape as long as it does not crack with rivets.

The high-strength steel sheet used in the present invention is a steel sheet having a composite structure including at least one of austenite, martensite, and bainite and a ferrite structure. Specifically, a dual phase steel containing martensite in ferrite (Dual Phase steel), a work-induced transformation type high residual austenitic steel containing retained austenite in ferrite (TRIP steel), and a high phase containing bainite in ferrite. Burring steel, etc. are raised. The mechanical properties are those having a tensile strength of 490 to 1000 MPa. Since the composite structure steel sheet is excellent in ductility, cracks hardly occur on the die side during rivet driving. Therefore, other mechanical properties are not particularly limited, but it is desirable that the elongation and the r-value are high in order to avoid cracks on the die side during rivet driving as much as possible. The manufacturing method 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.

[0015]

EXAMPLES (Example 1) Six types of high-strength steel sheets having a thickness of 1.0 mm and one type of mild steel sheet (test piece shape: 40 × 150 mm) shown in Table 1 were used as materials to be joined. This steel plate is overlapped with a combination of similar materials with a lap margin of 40 mm, and a rivet (material: SK4) having a diameter of 5.0 mm and a length of 6.0 mm is driven from one side in the manner shown in FIG. The rivet was deformed with a die and mechanically joined. For comparison, these steel sheets were spot-welded under the condition that the nugget diameter became 5.0 mm.

The joints were subjected to a fatigue test based on the JIS tensile shear fatigue test method. The fatigue test was performed by a swing test, and the stress ratio was 0.05 and the frequency was 5 Hz. Table 1 shows the fatigue strength at 2 × 10 ⁶ times. In spot welding, even if the strength of the steel sheet increases, the fatigue strength does not increase. On the other hand, solid solution type, Nb precipitation type,
In the case of using a high-strength steel sheet of a Ti precipitation type, the increase in fatigue strength was slight even when mechanical joining was used. On the other hand, when a two-phase steel sheet, a TRIP steel sheet, or a high burring steel sheet was used, the fatigue strength of the joint was significantly improved by using the mechanical joining method. 2.0mm thickness
A similar study was conducted using steel plates. The results were the same.

[0017]

[Table 1]

Example 2 The same steel plate (test piece shape: 50 × 150 mm) shown in Table 2 as in Example 1 was overlapped in a cross shape with a combination of similar materials, and had a diameter of 5.0 mm.
A rivet (material: SK4) having a length of 6.0 mm was driven in from one side, and the rivet was deformed with a die on the opposite side and mechanically joined. For comparison, these steel sheets were spot-welded under the condition that the nugget diameter became 5.0 mm. These joints were subjected to a fatigue test based on the JIS cross tensile fatigue test method. The conditions of the fatigue test are the same as in the first embodiment. Table 2 shows the fatigue strength at 2 × 10 ⁶ times. In spot welding, even if the strength of the steel sheet increases, the fatigue strength does not increase. On the other hand, when a solid solution type, Nb precipitation type, or Ti precipitation type high-strength steel sheet was used, the increase in fatigue strength was slight even when mechanical joining was used.
On the other hand, when a two-phase structure steel plate, a TRIP steel plate, or a high burring steel plate was used, the fatigue strength of the joint was significantly improved by using mechanical joining. Sheet thickness 2.0
A similar study was conducted using a steel plate of mm, but the results were the same.

[0019]

[Table 2]

[0020]

By using the joining method and the joining material of the present invention described above, a higher fatigue strength can be obtained in the joint portion of a high-strength steel sheet as compared with the conventionally generally used spot welding method.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view for explaining an example of destruction during a fatigue test of a spot weld.

FIGS. 2A and 2B are cross-sectional views for explaining an example of a joining method according to the present invention, wherein FIG.
Indicates a state after rivet driving.

[Explanation of symbols]

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

Claims (3)

[Claims] 1. A high-strength steel sheet having a tensile strength of 490 to 1000 MPa and a composite structure including at least one of austenite, martensite, and bainite and a ferrite structure is used as a material to be bonded, and the high-strength steel sheet is superposed. A method for joining high-strength steel sheets with excellent fatigue characteristics, characterized in that rivets are driven into the material and mechanically joined. 2. A high-strength steel sheet having excellent fatigue properties according to claim 1, wherein a rivet having a value of 0.2% proof stress, tensile strength and Vickers hardness represented by the following formula is used. 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 3. A high-strength steel sheet used in the joining method according to claim 1, wherein the high-strength steel sheet has a tensile strength of 490 to 1000.
A high-strength steel sheet for rivet bonding having a composite structure including at least one of austenite, martensite, and bainite and a ferrite structure at MPa.