JP2001009573A - Welding joint of high tensile steel plate and welding method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a welding joint which is difficult to cut in a nugget by resistance welding a steel plate with a given value or more base metal strength and thickness, and making size of a heat-affected zone of the welding joint with a given value or more hardness to be smaller than the diameter of a corona bond part. SOLUTION: Resistance welding is performed to two or more high tensile steel plates of 440 MPa or more metal base strength and 1.2 mm or more thickness, so as to obtain a welding joint. A spot welding part is composed of a nugget to be melted/solidified and a heat-affected zone outside it, and a corona bond part to be pressure welded under high temperature exists on the interface of the plate adjacent to the nugget. The size of the heat-affected zone of the welding joint with (X+2Y)/3 or more hardness is smaller than the diameter of the corona bond part, where average hardness of the base metal is designated as X and average hardness in the nugget is as Y. The condition for performing resistance welding is set as t<=54/TS1/4. In this case, t shows a welding time (cycles/60 Hz), and TS shows the base metal strength (MPa).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a welded joint in which a welded portion is hardly broken when a high-strength steel plate is subjected to resistance welding in the fields of automobiles, industrial equipment, and the like, and a welding method therefor. .

[0002]

2. Description of the Related Art At present, resistance welding such as spot welding is a simple welding method with high productivity and is widely used in various fields such as automobile assembly lines. In the following, spot welding will be described as an example of resistance welding.

Conventionally, steel sheets used in such fields are:
Mild steel sheets and steel sheets with a base material strength up to the 350 MPa class accounted for the majority. By the way, in the case of spot welding of steel sheets,
Although base metal fracture is desirable, such a spot-welded joint made of a steel sheet does not break at the weld even when subjected to a tensile shear test as long as it is welded under practical conditions. Break.

Since the strength of the joint at this time is largely determined by the size of the nugget which is the molten and solidified portion, it has been considered that it is important to secure a predetermined diameter of the nugget in order to obtain a predetermined strength.

[0005] As shown in JIS Z 3140, the nugget diameter is usually given as a function of the plate thickness, and the quality standard is set based on the nugget diameter specified in JIS according to the required joint strength. There are many. For example, JIS defines that the minimum nugget diameter is at least 4.25√t (t: plate thickness) for Class A joints that require the most strength.

[0006] Welding conditions for this purpose are described in RWMA (Resista
nce Welder Manufacturers Association). This condition is based on the assumption that the energizing time is a function of the plate thickness, and the class A joint requiring the most joint strength is about 10 t [t:
Plate thickness (mm)], but welding is often performed with a longer conduction time.

In recent years, high-strength steel sheets having a base material strength exceeding 440 MPa have been increasingly applied to exterior materials of automobiles for the purpose of reducing vehicle body weight and improving collision safety. At present, spot welding is performed on such high-strength steel sheets under the same conditions as before, although the pressing force is increased.

[0008]

However, when spot welding is performed on a high-strength steel sheet under the conventional conditions, the strength of the welded portion does not increase as much as the strength of the base metal, and the strength of the welded portion is relatively weak. And it is easy to break at the weld.
For this reason, even if a nugget diameter of 4.25√t is obtained, breakage may occur in the nugget.

Usually, in the case of a steel sheet, the static strength of a spot weld is evaluated by a tensile shear test. At this time, the form of the fracture is important together with the fracture load. Of course, in order to cause the base material to break, the diameter of the nugget may be increased. However, obtaining a large diameter of the nugget involves difficulties in equipment or construction.

The range of welding conditions for spot welding is as follows:
It is said that the current value of the base material breaks from the current value of dust generation, but in the case of a high-strength steel sheet, because the electrical resistance of the base material is high,
Since the dust generation current is low and the nugget diameter when the base material breaks is large, the range of welding conditions is significantly narrower than that of the mild steel plate.

Accordingly, the present invention is to improve the prior art as described above and to realize a joint by resistance welding of a high-strength steel sheet without increasing the nugget diameter, even in a relatively wide range of welding conditions. It is an object of the present invention to provide a welded joint that is less likely to break in a nugget and a welding method therefor.

[0012]

Means for Solving the Problems The present inventors have studied various factors that affect the strength and fracture mode of a welded joint by spot welding using various high-strength steel sheets in order to improve the welding method.

As a result, by reducing the size of the heat-affected zone outside the nugget, ie, the width of the heat-affected zone, as shown in FIG. It has been found that the internal fracture is improved to the base metal fracture.

Heretofore, it has been demanded that a welded joint made of a high-strength steel plate be made to have a smaller nugget diameter so that the base material is broken. However, as far as the inventors know, there is no report on this solution.

FIG. 1 is a schematic explanatory view showing the positional relationship between a nugget and a heat-affected zone. The upper half of the figure is an example of the present invention and the lower half is a comparative example. Although the nugget diameter is the same, in the case of the comparative example, the corona bond portion, that is, the heat-affected zone extends beyond the press-contact portion, whereas in the case of the present invention example, the corona-bonded portion and the heat-affected zone extend. The areas of the parts almost overlap.

In the illustrated example, two high-tensile steel sheets are subjected to resistance welding by spot welding. For example, three or more high-tensile steel sheets such as a roof part of an automobile and a welded part of a center pillar are used. Is the same as in the case of two steel plates. The same applies to the case where only one is a high-tensile steel plate. In the following, the present invention will be described by taking as an example a case where resistance welding is performed on two high-tensile steel sheets.

Here, the spot-welded portion comprises a nugget melt-solidified by an electric current and a heat-affected zone outside the nugget. And at the interface between the two plates adjacent to the nugget,
There is a corona bond pressed under high temperature. Here, the corona bond portion is, specifically, an area up to the front end of the sheet separation.

In the case of a high-strength steel plate welded joint, the heat-affected zone extends to the outside of the corona-bonded portion in the ordinary welding method, but the area of this heat-affected zone is smaller than the tip of the corona-bonded portion (sheet separation). By doing so, the fracture form is improved.

That is, as shown in FIG. 2 (a), in the case of a joint having the same heat-affected zone width even if the nugget diameter is the same, the joint is easily broken by the base metal (see the invention example). In a joint having a wide heat-affected zone, the joint easily breaks in the nugget (see the comparative example in FIG. 2 (b)).

Further, as a result of studying various methods for obtaining such a joint, it has been found that energization for a shorter time is more effective than conventional conditions. The gist of the present invention completed based on such knowledge is as follows.

(1) Base material strength of 440 MPa or more and sheet thickness of 1.2 mm
A welded joint obtained by performing resistance welding on the above high-tensile steel sheet, wherein the average hardness of the base material is X and the average hardness in the nugget is Y
Then, the size of the heat-affected zone having a hardness of (X + 2Y) / 3 or more is equal to or smaller than the diameter of the corona bond portion. (2) The welded joint according to the above (1), obtained by performing resistance welding on two or more high strength steel sheets.

(3) Base material strength of 440 MPa or more and sheet thickness of 1.2 mm
The welding method of a welded joint for performing resistance welding on one or two or more high-tensile steel sheets, wherein t ≦ 54 / TS ^1/4 t: energizing time (cycles / 60Hz) TS: base material strength ( (MPa), resistance welding is performed under conditions that satisfy the following conditions.

[0023]

Next, embodiments of the present invention will be described more specifically. According to the present invention, for example,
In a welded joint by resistance welding such as spot welding and projection welding, if the average hardness of the base material is X and the average hardness in the nugget is Y, then (X + 2Y) / 3 or more, in order to realize fracture in the base material The size of the heat-affected zone of the hardness is made equal to or smaller than the size of the corona bond.

This is based on the following reasons. Breaking form by a tensile shear test spot welded joint, the breaking load when breaking in the base metal: and F _B, determined by the magnitude relationship between the breaking load F _N in the case of break in a nugget. That is,
Is the F _B> F _N → nugget within the fracture F _B <F _N → base material rupture.

Breaking load F _B (N) when breaking by base metal
And d _f : distance (mm) from the nugget center to the breaking position,
t: sheet thickness (mm), TS: generally between tensile strength of the base material _{(MPa), F B = a} × d f × t × TS (a: constant) relation of. The breaking position when breaking with the base material is
Usually, the base material portion along the outer periphery of the cured heat affected zone.

[0026] Therefore, when the width of the heat affected zone becomes narrower, the distance d _f from the nugget center to break position decreases.
As a result, it decreases the breaking load F _B in the case of fracture in the base metal. In particular, the size of the heat affected zone equivalent to sheet separation corona bond part end surface or it if becomes smaller than, strain is concentrated on soft base metal by stress concentration portion and the hardness changing part matches, with F _B The breaking load is lower than the value shown.

On the other hand, the breaking load F _N (N) when breaking in the nugget is not affected by the size of the heat-affected zone,
d _n : almost determined by the nugget diameter (mm) and expressed as F _N = b × H _V × d _n ² (b: constant, H _V : Vickers hardness of nugget).

For this reason, even if the nugget diameter is constant,
When the width of the heat-affected zone is reduced, the strength required to break relatively on the base material side is lower than the strength required for breaking in the nugget, so that the base material side easily breaks.

In particular, when the size of the heat-affected zone is equal to or smaller than the front end of the sheet separation at the end face of the corona bond, the strain concentrates on the soft base material because the stress concentration portion coincides with the sudden change in hardness. Further, it is easy to break on the base material side. Although the above findings are based on the results of the tensile shear test, this tendency is observed regardless of the test method such as the cross tensile test and the high-speed tensile test.

According to the present invention, when the size of the heat-affected zone is equal to or smaller than the size of the corona-bonded portion, even if a JIS A-class minimum nugget diameter of 4.25√t is obtained under the conventional welding conditions, the nugget breaks in the nugget. The steel type that had been broken did not break in the nugget but broke on the base metal side.

The reason why the portion to be compared with the corona bond portion in the present invention is a heat-affected zone having a hardness of (X + 2Y) / 3 or more is as follows. The heat-affected zone is generally organized by heat,
This indicates the portion of the unmelted base metal that has changed in metallurgical properties, mechanical properties, and the like. In this study, we investigated the relationship between the hardness distribution of various test specimens and the fracture position.As a result, it was found that the fracture form was changed by the positional relationship between the (2X + Y) / 3 hardness region and the corona bond diameter. Therefore, it was limited to (2X + Y) / 3 in the heat affected zone.

In the example of the present invention, the joint strength at the time of fracture of the base material is lower when the nugget diameter is the same as compared with the conventional welding method, but the reduction amount is, for example, 20% or less, and there is almost no problem. The merit in quality assurance by setting the fracture position on the base material side is much greater.

In order to obtain such a narrow hardened region, at the time of welding a joint, t ≦ 54 ≦ TS ^1/4 , where t: energizing time
(cycles / 60Hz), it is desirable to weld under the condition of TS: base metal strength (MPa).

This is because the heat-affected zone becomes too large in the case of a high-tensile steel plate under the conventional conditions. That is, the higher the strength of the base material, the larger the amount of addition of elements such as C, the lower the transformation point, the lower the temperature, the wider the heat affected zone. Therefore, electricity is supplied in a short time to suppress the diffusion of heat,
It is necessary to narrow the heat-affected zone.

As a result of the study by the present inventors, it was found that the diameter of the corona bond portion and the size of the heat-affected zone were almost the same when welding was performed under the condition that t = 54 / TS ^1/4 . Therefore t
It is necessary to weld under conditions that satisfy ≦ 54 / TS ^1/4 .

FIG. 3 is a graph showing a comparison of the hardness distribution according to the energizing time. A high-strength steel sheet having a tensile strength of 590 MPa and a thickness of 1.4 mm was energized for 10 cycles / 60 Hz (Example of the present invention).
The relationship between the nugget diameter, the size of the heat-affected zone, and the size of the corona bond when spot welding is performed at a cycle of 60 Hz (comparative example) is shown. From the results shown in FIG. 3, it can be seen that the shorter the energization time, the narrower the heat-affected zone.

This condition is shorter than the manufacturing condition of the RMWA class A joint, and has the feature that there is no difference in the condition depending on the plate thickness. At this time, welding conditions other than the energizing time, such as the pressing force and the holding time, are not particularly specified because the examination did not affect the size of the heat-affected zone.

However, in the case of a high-tensile steel sheet, dust is liable to fly, so that a higher pressing condition than that of mild steel is desirable. Since the current value is completely different depending on the type of steel, surface treatment, and pressing force, welding may be performed with an optimum current value for each steel sheet. However, usually, the current value immediately before the occurrence of dust is desirable.

The specific welding conditions in the present invention are not limited by the present invention, but are as follows. Pressing force: 350 to 440 (kgf) Energizing time: 6 to 25 cycles / 60Hz Current value: 580 to 10,000 (A) Electrode holding: 0 to 60 cycles / 60Hz Thus, in the case of the welded joint according to the present invention, , 4.25
In the case of a nugget diameter of Δt, even in a normal joint, even if the inside of the nugget breaks, the inside of the nugget does not break.

[0040]

EXAMPLES Hereinafter, the working effects of the present invention will be described more specifically with reference to examples. In the spot welded joint portion of the high-tensile steel sheet according to the present invention, spot welding was performed on the test materials shown in Table 1 in order to verify whether or not the welded portion would break.

The steel type used was 440 to 780 having different plate thicknesses.
It was a high strength steel sheet of MPa class. Double R-type electrode (TC
TR-16 × 6A) was used. Table 2 shows the welding conditions. Welding of the test material was performed at a dust generation current value or less. The specimen was lightly wiped with a rag to remove dust on the surface, but was not degreased. Assuming a constant nugget diameter (4.25√t),
A comparison was made.

As shown in FIG. 4, the hardness is 0.1 mm from the plate interface.
It was determined by measuring at a position 2 mm apart. The results are shown in Table 3. In Table 3, the position of the heat-affected zone is indicated by the relative value between the position of the heat-affected zone having hardness (X + 2Y) / 3 or more and the position of the corona bond portion.
When the heat affected zone is larger, the value is +, and when it is smaller, the value is-.

A tensile shear test piece was prepared in accordance with JIS Z 3136, and the influence of welding conditions was evaluated by a tensile shear test.
Table 3 also shows the results of the investigation on the fracture mode. As can be seen from these results, it was confirmed that the inside of the nugget was broken in the comparative example, whereas the base material was broken in the example of the present invention. Table 4 shows the results of a cross tension test using the same steel type in accordance with JIS Z 3137.

The fracture mode in this case is as shown in FIGS. 5 (a) and 5 (b). Even in the comparative example of FIG. 5 (b), the fracture does not occur on the entire surface of the nugget as in the tensile shear test, but the crack is formed in the nugget. The morphology of the rupture evolving to the surface is seen. On the other hand, all the test pieces welded according to the present invention were broken on the base material side as shown in FIG. 5 (a). This is also because the heat-affected zone becomes narrower, so that stress is concentrated on the base material side rather than inside the nugget, and the base material is easily broken with a relatively small load.

FIG. 6 shows the shape of the test piece used in the high-speed peeling test in this example. As shown in the drawing, portions where the test pieces were bent at 90 degrees were overlapped and spot-welded. A hole having a diameter of 10 mm was formed in the grip portion of the test piece, and the hole was attached to a high-speed tensile tester, and a tensile load was applied in the direction indicated by a white arrow in the figure. At this time, a stress in the peeling direction is applied to the nugget. The strain rate was adjusted to be 4.0 × 10 ³ / sec.

As shown in FIGS. 7 (a) and 7 (b), the fracture mode at this time is a fracture of the base material and a fracture in the nugget as in the case of the tensile shear test. The results of the experiment are summarized in Table 5. As shown in Table 5, in the comparative example, the crack propagated in the nugget and broke, whereas in the present invention example, the crack did not propagate in the nugget, and all fractured in the base material.

As described above, the high-strength steel sheet resistance welding joint according to the present invention is characterized in that the size of the heat-affected zone is equal to or smaller than the diameter of the corona bond portion. Instead, the base material can be broken by concentrating the stress on the base material side.

[0048]

[Table 1]

[0049]

[Table 3]

[0050]

[Table 4]

[0051]

[Table 5]

[0052]

As described above in detail, according to the present invention, it is possible to enhance the reliability of a welded joint of a high-tensile steel plate having a base material strength of 440 MPa or more, and particularly to a welding method effective for reducing the weight of an automobile. Since the joint is realized, the practical significance of the present invention is great.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory view showing a fracture position in a tensile shear test in a welded joint according to an example of the present invention and a comparative example.

FIGS. 2 (a) and 2 (b) are schematic explanatory views showing fracture positions in a tensile shear test of welded joints according to the present invention and comparative examples, respectively.

FIG. 3 is a graph showing a comparison of a hardness distribution according to an energization time.

FIG. 4 is an explanatory diagram of a hardness measurement position in the embodiment.

5A and 5B are schematic explanatory views of a fracture mode by a cross tension test, in which FIG. 5A shows a case of a base material fracture and FIG. 5B shows a case of a nugget internal fracture.

FIG. 6 is a schematic explanatory view of a test piece shape in a high-speed peel test.

FIGS. 7A and 7B are schematic explanatory views of a fracture mode by a high-speed peeling test, in which FIG. 7A shows a case of a base material fracture and FIG. 7B shows a case of a fracture in a nugget.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kiyoyuki Fukui 4-33, Kitahama, Chuo-ku, Osaka City Within Sumitomo Metal Industries Co., Ltd. (72) Inventor Masato Katsukura 2 Takaracho, Kanagawa-ku, Yokohama-shi, Kanagawa Nissan Automobile Co., Ltd. F-term (reference) 4E065 EA04 EA06

Claims (3)

[Claims] 1. A welded joint obtained by performing resistance welding on a high-strength steel plate having a base material strength of 440 MPa or more and a plate thickness of 1.2 mm or more, wherein the average hardness of the base material is X, the average hardness in the nugget. Let Y be the size of the heat-affected zone with a hardness of (X + 2Y) / 3 or more,
A welded joint characterized by being equal to or smaller than the diameter of the corona bond. 2. The welded joint according to claim 1, wherein two or more high-strength steel sheets are subjected to resistance welding. 3. A welding method for a welded joint for performing resistance welding on one or two or more high-tensile steel sheets having a base material strength of 440 MPa or more and a sheet thickness of 1.2 mm or more, wherein t ≦ 54 / TS ^{1 / 4} t: Energizing time (cycles / 60Hz) TS: Welding method for welded joints characterized by performing resistance welding under conditions that satisfy base metal strength (MPa).