JPH10193164A - Method for improving fatigue characteristic in lap joint - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce residual stress effectively and efficiently in a weld joint for a steel sheet by using TIG arc and the like and heating a position close to the weld zone in the lower sheet of a lap joint to the extent that the melting of the sheet is avoided. SOLUTION: An upper 1 and a lower sheet 2 are superposed and fixed with a holding jig 3. Arc welding is performed to form a fillet welded joint in the weld zone 6. The heat source generating part 4 is capable of parallel displacement for the weld zone 6. As the heat source 5, TIG arc, plasma arc and laser beam are emitted. The heat source 5 moves parallelly to the weld zone 6 at a position close to the weld zone 6, namely, a position away from the weld ending part 7 by a distance (a), and is capable of heating that part locally. The output of the heat source 5 is adjusted to the extent that the melting of the lower sheet 2 is avoided. As a result, since the heat source is excellent in heat concentration, heat efficiency is improved, reducing residual stress efficiently and enhancing fatigue characteristic.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for improving the fatigue properties of a lap welded joint, and more particularly to a method for improving the fatigue properties of a thin steel sheet welded joint used for automobiles and the like.

[0002]

2. Description of the Related Art Among steel sheets for automobiles, steel sheets used for undercarriage of vehicles are required to have high strength. Above all, fatigue strength is regarded as important. Regarding the fatigue strength, it is generally known that even if the fatigue strength of the base metal increases with the increase of the base metal strength, the fatigue strength of the welded portion hardly increases. The reason for this is that there is a location where stress is concentrated in the weld joint, and high tensile residual stress is generated. Therefore, it is desired to eliminate such causes and improve the fatigue strength of the welded portion.

As a method of reducing the stress concentration, there is a method of smoothing the shape of a weld toe where the stress is concentrated at a welded joint. As such a method, a method of reducing the toe with a grinding tool such as a grinder is known. Grinding method, JP-A-59-1
There is a remelting method by TIG welding disclosed in Japanese Patent No. 10490.

[0004] As a method of reducing the residual stress in the welded portion, there are a method disclosed in Japanese Patent Application Laid-Open No. 4-371287 and a method disclosed in Japanese Patent Application Laid-Open No. 8-112688. The former is a method in which a bead is placed near the welded portion, and then the bead is removed with a grinder to finish the surface smoothly, and the latter is a method in which the vicinity of the welded portion is linearly heated with an acetylene gas flame.

[0005]

However, in the method of grinding the toe with a grinding tool such as a grinder, the operator must hold and grind the cutting tool, and the working efficiency for that is very low. In addition, the welding length that can be processed per unit working time has to be shortened. Furthermore, it is difficult to stably and uniformly grind, and there is even a risk that the surface of the steel sheet may be damaged by the grinding tool and the fatigue strength may be reduced.

In the remelting method by TIG welding disclosed in Japanese Patent Application Laid-Open No. Sho 59-110490, the degree of improvement in fatigue strength greatly depends on welding conditions such as the remelting position and heat input at that time. Furthermore, even if these are melted under certain conditions, the shape of the toe greatly varies depending on the flow state of the molten metal, so that it is difficult to obtain a stable effect.

Further, among the methods for reducing the residual stress in the welded portion, the method disclosed in Japanese Patent Application Laid-Open No. 4-371287 may damage the steel sheet surface when grinding the bead by placing it with a grinder. There is a risk of lowering. In addition, since it takes a long time to grind, the productivity of an automobile component mass-produced is extremely reduced.

The method disclosed in Japanese Patent Application Laid-Open No. 8-112688 is intended for a thick plate used for shipbuilding and the like. It is not suitable for local heating, which heats only the intended place to a specific temperature range, and the temperature gradient around the heated part becomes gentle, so the effect of local heating to reduce the residual stress near the weld toe is reduced. It is too low to obtain a sufficient effect for improving the fatigue characteristics. Furthermore, since the gas flame does not concentrate heat so much, it takes time to heat the heating unit to a target temperature, and the efficiency is poor.

The present invention has been made in view of the above circumstances, and it is possible to effectively and efficiently reduce the residual stress of only a necessary portion in a welded joint of a thin steel plate such as a steel plate for an automobile. Accordingly, an object of the present invention is to provide a method capable of improving fatigue characteristics.

[0010]

In order to solve the above-mentioned problems, the present invention uses a TIG arc, a plasma arc, or a laser beam as a heat source and determines the position near the welded portion of the lower steel plate constituting the lap weld joint. , Parallel to the weld,
Provided is a method for improving the fatigue properties of a lap welded joint, wherein the steel sheet is heated to such an extent that the steel sheet does not melt.

As described above, by using a TIG arc, a plasma arc, or a laser beam as a heat source, it is possible to locally heat even a thin steel plate welded joint such as a steel plate for an automobile. For this reason, it is possible to effectively and efficiently reduce the residual stress in that portion by locally heating only a necessary portion near the weld toe of the lower steel plate constituting the lap welded joint, Characteristics can be effectively improved. In addition, since these heat sources have good heat concentration, the heating efficiency is high.

In this case, the heating position of the lower steel plate is not less than 12.5 mm and not more than 20 mm from the weld toe,
It is preferable that the heating temperature is 500 ° C. or more and 800 ° C. or less. Thereby, the residual stress can be further reduced, and the effect of improving the fatigue characteristics can be increased.

[0013]

Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a perspective view showing an embodiment of the present invention. The upper steel plate 1 and the lower steel plate 2 are fixed by a holding jig 3 in a state of being overlapped. The steel plates 1 and 2 are thin steel plates such as a steel plate for an automobile. Here, lap fillet welded joints are formed by arc welding in this state. Reference numeral 6 is a welded portion, and 7 is a weld toe.

The heat source generating section 4 can move in parallel with the welding section 6, and a TIG arc, a plasma arc, or a laser beam is emitted from the heat source generating section 4 as a heat source 5. By moving the heat source generating part 4, the heat source 5 moves parallel to the welded part 6 at a position near the welded part 6, that is, at a distance a from the weld toe part 7, and locally heats that part. can do. In this case, the output of the heat source 5 is adjusted to such an extent that the lower steel plate 2 does not melt.

In this case, T used as the heat source 5
The IG arc, the plasma arc, and the laser beam can narrow the heating portion, and even in the case of a thin steel plate welded joint as in this example, local heating can be performed without increasing the temperature rising region. Therefore, a specific portion in the vicinity of the weld toe can be locally heated in this manner, and the residual stress in that portion can be effectively and efficiently reduced. here,
In order to further reduce the residual stress, the distance a is set to 12.
It is preferable to set the heating temperature to 500 to 800 ° C. Further, the TIG arc, the plasma arc, and the laser beam used as the heat source 5 have high energy and can be efficiently heated. By reducing the residual stress in this way, the fatigue properties of the welded joint are improved.

[0016]

Embodiments of the present invention will be described below. Here, steel plates having a thickness of 2.6 mm and 3.2 mm were prepared. For each thickness, width 60mm, length 3
Two steel plates cut to 00 mm were piled up with a margin of 10 mm as shown in FIG.
After temporarily fixed at 0 mm intervals, the steel plate was fixed with the holding jig 3 and heat input along the end of the temporarily fixed upper steel plate 2.53.
MAG welding was performed at kJ / cm to produce a lap fillet welded joint. The conditions at this time were welding current 220A, voltage 23V,
And the welding speed was 120 cm / min.

After the welded joint is cooled down to about room temperature, a TIG arc is used as the heat source 5 to change the heat input and the distance a of the lower steel plate from the weld toe 7 in various ways to be parallel to the welded portion 6. The TIG arc was moved to heat the lower steel plate 2. Then, the measurement of the residual stress in the vicinity of the weld and the fatigue test were performed.

As shown in FIG. 2, the residual stress was measured by attaching a five-piece strain gauge 8 to the vicinity of the weld toe 7 of the lower steel plate 2 of the welded joint formed as shown in FIG. The portion where the strain gauge was attached was cut out to release the residual strain, and the residual stress was calculated from the amount of the strain. In addition, since the residual stress which becomes a problem in the fatigue strength of the lap fillet weld joint is a residual stress in a direction perpendicular to the welding direction, the residual stress targeted in the present example was set to be perpendicular to the welding direction. In the fatigue test, oscillating plane bending was performed on the test piece shown in FIG.

First, the measurement results of the residual stress will be described. The residual stress in the vicinity of the weld toe of the lower steel plate as welded is as shown in FIG. 4, and a welded joint using a steel plate with a thickness of 2.6 mm and a steel plate with a thickness of 3.2 mm were used. In both the welded joints, a high residual stress of 100 MPa or more was generated at the weld toe.

On the other hand, from the weld toe of the lower steel plate,
The heating temperature is 650 ° C at a position 5mm away in parallel with the weld.
As shown in FIG. 5, in the welded joint heated by TIG arc, the residual stress state greatly changes from the residual stress state of the as-welded joint shown in FIG.
The residual stress at the weld toe was removed or even up to the compressive residual stress.

Similarly, the residual stress at the weld toe when the TIG arc was heated at various heating temperatures and heating positions was measured. As a result, as shown in FIG.
Under the condition of 00 to 900 ° C. and the heating position being 12.5 to 35 mm from the weld toe of the lower steel plate of the welded joint, the residual stress σRS ^* after TIG arc heating and the residual stress σRS ^{0 as} welded are the ratio σRS ^* / σRS ⁰ is, σRS ^* / σRS ⁰
<1 and a residual stress reduction effect was observed. Further, under the condition that the heating temperature is 500 to 800 ° C. and the heating position is from the weld toe of the lower steel sheet to 12.5 to 20 mm, σ
RS ^* / σRS ⁰ ≦ 0, and the tensile residual stress is removed or
Furthermore, the effect of reducing the compressive residual stress was found.

In the fatigue test, the as-welded welded joint and the above-mentioned TIG arc heating conditions (TIG arc heating was performed at a position 15 mm away from the weld toe of the lower steel sheet so that the heating temperature was 650 ° C. in parallel with the welded portion. (Heating)), a test piece was collected from the welded joint that was subjected to the heat treatment, and the test was performed using the test piece. As a result, as shown in FIG.
It was confirmed that the fatigue characteristics were improved by IG arc heating. Specifically, the fatigue strength of the TIG arc-heated specimen having a rupture life of 2 × 10 ⁶ times was about 1.6 times that of the as-welded state, indicating that the fatigue strength was greatly improved. .

Similarly, when the TIG arc was heated at various heating temperatures and heating positions, the fatigue strength of the welded joint when the fatigue life was 2 × 10 ⁶ times was compared with the same fatigue strength as welded. FIG. 8 shows the result. As shown in this figure, the heating temperature was set to 500 to 800 ° C., and the heating position was set to 12.5 to 12.5 from the weld toe of the lower steel plate of the lap joint.
Under the condition of 20 mm, (Fatigue strength after TIG heating) / (As-welded fatigue strength) ≧ 1.5, and it was confirmed that the fatigue strength was greatly improved.

[0024]

As described above, according to the present invention,
By using a TIG arc, a plasma arc, or a laser beam as a heat source, it is possible to locally heat even a welded joint of a thin steel plate such as a steel plate for an automobile.
For this reason, it is possible to effectively and efficiently reduce the residual stress in that portion by locally heating only a necessary portion near the weld toe of the lower steel plate constituting the lap welded joint,
Fatigue characteristics can be effectively improved. In addition, since these heat sources have good heat concentration, the heating efficiency is high, and the residual stress can be efficiently reduced to improve the fatigue characteristics.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an embodiment of an embodiment of the present invention.

FIG. 2 is a view for explaining a residual stress measurement method.

FIG. 3 is a view showing a test piece for a fatigue test.

FIG. 4 is a view showing a residual stress state near a weld toe of a lower steel plate as welded.

FIG. 5 is a diagram showing a residual stress state near a weld toe of a lower steel plate in a TIG arc-heated welded joint.

FIG. 6 is a diagram showing the results of measuring residual stress at the weld toe when TIG arc heating is performed at various heating temperatures and heating positions.

FIG. 7 is a diagram showing a comparison between fatigue characteristics of a welded joint subjected to TIG arc heating and a welded joint as-welded.

FIG. 8 is a graph showing the results of comparing the fatigue strength of a welded joint when the fatigue life is 2 × 10 ⁶ times with TIG arc heating at various heating temperatures and heating positions, and the same fatigue strength as it is as welded. .

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Upper steel plate 2 ... Lower steel plate 3 ... Holding jig 4 ... Heat source generating part 5 ... Heat source 6 ... Welded part 7 ... Toe welded part 8 ... Strain gauge

Claims (2)

[Claims] 1. Using a TIG arc, a plasma arc, or a laser beam as a heat source, heating a position near a welded portion of a lower steel plate constituting a lap weld joint in parallel with the welded portion to such an extent that the steel plate does not melt. A method for improving the fatigue properties of a lap welded joint characterized by the following. 2. The heating position of the lower steel plate is 12.5 mm or more and 20 mm or less from the weld toe, and the heating temperature is 500 ° C. or more and 800 ° C. or less. Method of improving fatigue characteristics of