JPS583793A - Spot welding method of high tension steel plate - Google Patents

Abstract

PURPOSE:To elevate fatigue strength of a spot welding joint, by controlling an electrification current in case of spot welding of a high tension steel plate whose C content is <= a specific value, within a specific range exceeding a dust generating threshold current value of its steel plate. CONSTITUTION:A high tension steel plate having <=0.20wt% C and <=35kg/mm.<2> tensile strength is subjected to spot welding between a dust generating threshold value and a current value being by 3KA higher than said value. As a result, a remarkable improvement of fatigue limit is obtained by sacrificing a slight drop of tension shearing strength due to generation of dust. This phenomenon is influenced by the quantity of C of a material, and in case of C >0.20%, the tensile shearing strength drops greatly due to generation of dust, and a drop of static strength cannot be neglected. In this regard, it is also possible to execute temper electrification after the welding electrification process has accomplished.

Description

[Detailed Description of the Invention] The present invention relates to a spot welding method for high-strength steel plates, and in particular to a spot welding method that can improve the fatigue strength of welded joints. Thinner high-strength steel sheets are being used instead, but progress has been slow. The causes include a decrease in rigidity due to the thinner copper plate, and a decrease in formability due to the high strength of the steel plate. This problem has been resolved through the development of tensile steel plates. The biggest difficulty in using high-strength steel plates is that the fatigue strength of spot welds does not improve in proportion to the tensile strength of the base metal, and is at the same low level as that of mild steel plates.

In other words, Fig. 1 shows a comparison of the strength of the spot weld and the base metal, and the tensile shear strength of the spot weld increases in proportion to the tensile strength of the base metal. The fatigue fracture strength of spot welds when subjected to stress is almost independent of the tensile strength of the base material, and the tensile strength of the mild steel plate is 60] #/lEJ
It has almost the same fatigue strength as high-strength steel plates. Therefore, there are great concerns about using a thinner copper plate instead of the conventionally used mild steel plate.

The following three methods have been conventionally proposed to solve this problem.

Mark: Increased number of spot welding points.

6+1) Enlargement of nugget diameter.

←) Design changes for automobile parts.

However, these methods have the following problems.

Regarding (a), it is applicable only when there is space to increase the number of dots, and even if the number of dots is doubled, the fatigue strength does not necessarily double, and generally only a lower strength is obtained. I can't.

(For →, it is necessary to make the electrode larger and strengthen the pressing force, so there are equipment constraints and at the same time there are space constraints as mentioned in the case of octopus.

As for r+, since the conventional design is changed, the structure of the entire vehicle is subject to greater restrictions than the previous power supply method.

As mentioned above, there is no drastic method for improving the fatigue strength of spot welded joints known in the past, and there has been a demand for a revolutionary improvement method for spot welded joints of high-strength steel plates.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for spot welding high-strength steel plates, which solves the problems of the prior art described above and improves the fatigue strength of spot welded joints.

This object of the present invention is achieved by the following two inventions.

The gist of the first invention is as follows. That is, in a spot welding method for high-strength copper plates in which the C content is 120% by weight or less and the tensile strength is 35-1, the current applied during spot welding is calculated from the dust generation limit current value of the steel plate. This is a spot welding method for high-strength steel sheets, characterized by comprising a welding energization step controlled between a 3 KA high current value and a welding energization process.

The gist of the second invention is to subject the same high-strength steel plate as the first invention to a tempering energization treatment after welding using the same spot welding method as the first invention.

That is, in the present invention, high tensile strength steel plates with a C content of Q20 , a welded joint with excellent fatigue strength can be obtained.

The present inventors determined the relationship between spot welding conditions and joint fatigue strength through extensive and detailed experiments, and as a result, overturned the conventional theory that dust should not be generated in spot welding of high-strength steel plates. We found that the generation of residual dust dramatically improved the fatigue strength of spot-welded joints of high-tensile copper plates.
The present invention has been achieved.

That is, the most important constituent factor in the present invention is the dust generation limit current value, which is a fraction of the electrode pressurizing force and current application time, and is a value unique to the material. Therefore, if the welding current is increased while keeping the electrode pressure and current application time constant, dust will begin to occur at a value higher than a specific current value determined by the material. This specific current value is called a dust generation limit current value.

Regarding this phenomenon, the present inventors used a high tensile strength copper plate with a thickness of 0.5 to 40 cm and a tensile strength of 351 #/j or more due to the C content to increase static strength and fatigue strength. It was measured over a period of time and examined in detail.

That is, a (α+γ) mixed structure copper plate having the chemical composition and mechanical properties shown in Table 1 was used, and a pressing force of 1000ζ was used.
Then, static tensile shear strength and fatigue strength were measured by welding by changing the welding current while keeping the current application time constant at 29 cycles.

Table 1 The results are shown in Figure 2 for the Z9■ thick copper plate and Figure 3 for the U thick steel plate. In each case, the vertical axis shows tensile shear strength (hereinafter referred to as TSS) and fatigue strength at 107 cycles (hereinafter referred to as fatigue limit), and the horizontal axis represents welding current. If we first focus on TSS in Figures @2 and 3, we can see that TSS increases as the welding current becomes higher, and shows the highest value Q at the welding current just before the critical current value for dust generation, which is a point that has not been considered in the past. This was the optimum condition. When the current is further increased, dust begins to be generated and the TSS decreases slightly, but as the current 1 increases, it tends to increase again. Such a phenomenon is already known, and therefore, it is said that the optimum condition for spot welding high-strength steel plates should be just before the critical current value for dust generation, as disclosed in, for example, Japanese Patent Laid-Open No. 55-94466. has become an established theory.

On the other hand, if we focus on the fatigue limit, in the welding current range without dust generation, which includes the conventionally considered optimal conditions, the fatigue limit is 100 to 200 ky/5pot, which is almost the same as that of an annealed copper plate.
In contrast, in the range of the dust generation limit current value with moderate dust generation and a current value 3kA higher than that, the fatigue limit increases dramatically, and is approximately twice that of the case without dust generation. The fatigue limit is improved to a strength of . When the welding current becomes higher than the dust generation limit current value +3KA, the fatigue limit decreases and approaches the value when no dust occurs.

Therefore, the TSS due to dust generation is small, that is, about lθ
A significant improvement in fatigue limit can be obtained by sacrificing a decrease of about %. This result is exactly the same in FIGS. 2 and 3. Note that although TSS variations and damage to the electrode tip slightly increase, they are negligible compared to the effect of improving the fatigue limit.

The improvement in fatigue limit as described above overturns the conventional theory,
This is possible only by controlling the welding current within the range of the dust generation limit current value and the 3KA higher current value, that is, within the range of P shown in FIG. It is a component.

As a result of further experiments, the present inventors newly discovered that the above phenomenon is not noticeable in any material, but is dependent on the C content of the material. That is, C
When powdered into materials with a content higher than α20%, the TSS decreases significantly due to dust generation (hereinafter referred to as △TSS).
Decrease in static strength cannot be ignored. The situation is shown in FIG. 4, and in the present invention, the value of △TSS/TSS is 2.
0! The range was limited to the shaded range in FIG. 4 where the C content was less than α20X, that is, the C content was less than α20X.

In addition, the tensile strength of the high tensile strength steel plate to which the present invention is applied is 351
The reason why the tensile strength is set at 9/- or more is that a high tensile strength steel plate required for automobiles etc. needs to have a tensile strength of 35 kr/- or more.

As above <w! Although the first invention has made it possible to significantly improve the fatigue limit, the object of the present invention can be achieved more effectively by adding tempering current. That is, temper energization is a well-known method for improving spot welding strength, especially static cross tensile strength, but according to the first invention, after welding energization that generates a moderate amount of dust, by adding temper energization, static cross tensile strength can be improved. It is possible to further dramatically improve the fatigue limit while improving the physical strength.

The reason for the improved fatigue limit of the joint of the present invention is not clear, but it is thought to be as follows. FIGS. 5 and 6 are schematic cross-sectional views showing the shape of the weld and the occurrence of fatigue tack when spot welding is performed. Fig. 5III shows a case where spot welding is carried out under conventional optimal conditions without dust generation, and Fig. 6 shows a case where spot welding is carried out under moderate dust generation conditions according to the present invention.

There is a nugget 4 in the center of the upper and lower base materials 2, and around it there is a nugget 6 indicated by diagonal lines, and there is a dust residue 8 in the 6th wI. In FIG. 5, the fatigue crack IOA propagates within the nugget 4, while in FIG. 6, the fatigue crack IOB propagates through the base material 2. Therefore, in the method of the present invention shown in FIG. This is thought to be because the notch shape of the pronabond part is improved and the stress concentration in the corona bond part is alleviated.

Examples The hot rolled and cold rolled bulk tensile steel plates shown in Table 2 were spot welded under the welding conditions shown in Table 3, and the static strength and fatigue strength were measured and the results are shown in Table 3. In Table 3, the welding current of the conventional method is less than the limited range of the present invention, and the welding current of the comparative method is greater than the limited range of zero firing. As is clear from Table 3, the fatigue limit of the specimens welded by the method of the present invention is greatly improved, and the static strength decreases only slightly. Furthermore, tempering current has a great effect on improving the fatigue limit. On the other hand, both the conventional method and the comparative method have much lower fatigue limits than the method of the present invention.

As is clear from the above examples, the method of the present invention is a joint that can effectively utilize the strength of high-strength steel plates by controlling the welding current to within 3 KA and performing spot welding under conditions with moderate dust generation. The present invention can be applied not only to automobiles but also to fields and parts where the fatigue strength of spot welded joints is a problem.

[Brief explanation of drawings]

Figure 81 is a correlation diagram showing the relationship between tensile shear strength and fatigue strength and the tensile strength of the base metal, and Figures 2 and 3 are spot welded using 2.9- and 1.2-meter thick steel plates as the base metal, respectively. Figure 4 is a diagram showing the relationship between the tensile shear strength at 10 cycles and the fatigue strength at 10 cycles, and the welding current.
A diagram showing the relationship between /rss and C content, Figure 5 is a schematic cross-sectional view showing the weld shape and fatigue crack occurrence status of spot welding by the conventional method, and Figure 6 is a weld part by spot welding according to the present invention. FIG. 3 is a schematic cross-sectional view showing the shape and the state of occurrence of fatigue cracks. 2...Base metal 4...Nugget 6...Heat affected zone 8...Remaining agent of dust (KA) 3rd t! 1 N 1! Connected Seed (Ni A) Figure 4 C*fliiC9Q Procedural Amendment July 1982 228 Japan Patent Office Chotomi Haruki Shimada Ki 1 ° Display of 11 items 5 B sum! $ 6 Patent Application No. 100791, 1999 2 Title 3 of invention 3 □lI□,,o,□ Relation to the high tensile strength steel plate 0-point joint fiber incident - No. 11128 (Section 9' Note name) aS8) Kawasaki Steel Corporation 8 Contents of the amendment As shown in the attached sheet 8, Contents of the amendment (1) "Additional additions" from page 5, line 20 to page 6, line 1 of the specification. ``The pressure is 1000 kg and the energization time is 29 cycles''.
The pressing force is 1000# for 19m thick steel plate, 400# for 1.2I thick steel plate, and the energization time is 2 for 2.91 thick steel plate.
9 cycles, and 16 cycles for 1.2 ffl x 1 thick steel plate. (2) rlo(IOA, 10B) on page 13, line 8 of the specification
)... Add "fatigue crack". 1/Sat (2)

Claims (1)

[Claims] (υ C content is 0% by weight or less and tensile strength is 35% by weight)
- A method for spot welding static high-tensile steel plates, including a welding energization step in which the current applied during spot welding is controlled between the -plate dust generation limit current value and a current value 3 KA higher than the -plate dust generation limit current value. A method for spot welding high-strength steel plates, characterized by comprising: (2) In a spot welding method for high-strength steel plates with a C content of 0.20% by weight or less and a tensile strength of 35 kf/lJ or more, the current applied during the point clearing is applied to the four areas where dust occurs on the steel plate. Between the current value and the current value l113KA higher than that