JP5270928B2 - Resistance welding method for steel bars - Google Patents

Description

  The present invention relates to a resistance welding method for a steel bar, for example, a resistance welding method such as spot welding of a steel bar suitable for use in welding a basic reinforcing bar for a house. More specifically, the present invention relates to a resistance welding method for steel bars that can form a welded portion having excellent hardness and toughness without requiring a tempering step.

  Resistance welding, such as spot welding, is a method of welding metals by resistance heating, and follows a welding heat cycle in which the weld is rapidly heated and cooled by the welding current, and the weld heat affected zone becomes a structure harder than the base metal. Steel bars containing carbon, such as housing foundation rebar, can provide such a quenching effect due to rapid heating and quenching, but this also causes the adverse effect that the toughness (elongation) of the welded portion deteriorates. The reinforced concrete structure bears a tensile force by the reinforcing bars, and when the toughness deteriorates, the main bars are liable to be brittlely broken, which is dangerous.

  Therefore, conventionally, tempering is performed on the welded portion of the hardened steel bar, and instead of reducing the hardness of the welded part, a predetermined toughness is imparted to the welded part so that the steel bar does not cause brittle fracture. . In Patent Documents 1 and 2, spot welding is performed using a large welding current so that the weld strength of the main bar and stirrup welds is equal to or greater than the stirrup yield point strength, and the cooling time is approximately the same as the welding time. After that, by passing a current of 70% of the welding current through the welded portion, the welded portion is tempered, thereby ensuring the toughness of the welded portion.

  In such a spot welding method for reinforcing bars, desired toughness is ensured by performing a tempering process on a welded part that has been quenched by rapid heating and quenching in order to increase the strength of the welded part. In this quenching and tempering technology, there are many factors that restrict productivity, such as rapid cooling from quenching and gradual cooling from tempering, and such temperature control is realized under the production cycle of resistance welding. In order to achieve this, excellent technology and advanced production management are required.

Here, in Patent Document 3, the target cooling curve is stored and held in advance based on the continuous cooling transformation curve of the high-tensile steel plate, and the temperature decrease rate of the welded portion of the high-tensile steel plate is stored and held during resistance welding. Disclosed are a welding apparatus and a welding method for controlling a temperature decrease rate so as to match a target cooling curve.
Japanese Patent No. 365833 JP 2006-346745 A JP 2003-285193 A

  An object of the present invention is to propose a resistance welding method for a steel bar that can form a welded portion having excellent hardness and toughness without previously storing and holding a target cooling curve and without requiring a tempering step. There is.

In order to solve the above problems, the resistance welding method of the steel bar of the present invention is:
A welding process in which a welding current is applied to a weld portion of a steel bar to be welded having a carbon content within a range of 0.2% to 2%;
A slow cooling step of gradually cooling the weld by performing heat input control on the weld so that the rapidly heated weld is not quenched and quenched .
The steel bar to be welded is a rebar used for a reinforced concrete beam or column,
In the slow cooling step, the heat input control is performed so that a cooling time from 1070 K (800 ° C.) to 770 K (500 ° C.) in a welding heat cycle of the welded portion is at least 4.5 seconds. It is a feature.

In the present invention, the rapidly heated bar steel welded portion is gradually cooled by performing heat input control without waiting for the structural change of the rebar welded portion due to quenching for quenching after the welding process. By rapidly cooling the rapidly heated steel bar welded portion by heat input control, that is, the rapidly heated welded portion is cooled from the first temperature of 1070 K (800 ° C.) to the second temperature of 770 K (500 ° C.). It was confirmed that the desired strength and toughness can be ensured in the obtained welded part by delaying the time required for this. Therefore, according to the method of the present invention, control of rapid cooling (quenching) and tempering becomes unnecessary, and control becomes easy. Further, since tempering time is not required, the productivity of resistance welding work of the steel bar is improved. Furthermore, as compared with the case where the cooling rate is feedback-controlled along the target cooling curve, it is only necessary to manage the time from the first temperature to the second temperature, so that control management becomes easy.

  The microstructure of a general steel weld is largely determined by the cooling time of about 1070 K (800 ° C.) to 770 K (500 ° C.) of the welding heat cycle. This physical property is known as a welding CCT diagram (continuous cooling transformation diagram). By analyzing the hardness, structure and the like of the welded portion of the steel bar from the CCT diagram for welding, it is possible to estimate the cooling rate of the welding process that can obtain a structural state having the most suitable hardness and toughness as the welded part of the steel bar used. Therefore, even if the target cooling curve is not stored, the time width required for the temperature decrease in the above range is appropriately set according to the composition of the steel bar to be welded and the time width is equal to or greater than the set time width. By performing heat input control, a steel bar weld with desired hardness and toughness can be obtained.

  The steel bar resistance welding method of the present invention is characterized in that, in the heat input control of the slow cooling step, a slow cooling current is applied to the welded portion in a predetermined form. By repeatedly applying a constant current to the welded portion in a pulsed manner, or by applying a current having a gradually increasing or gradually decreasing value to the welded portion, it is possible to control the steel bar welded portion from being rapidly cooled.

The method of resistance welding steel bar of the present invention is Ru used reinforcing bars used in reinforced concrete beams or columns. For example, it is suitable for use in welding a main bar and a stirrup used as a basic reinforcing bar for housing.

Experiments such as welding for CCT diagram and the present inventors, by performing the heat input controlled so that the cooling rate needed at least about 4.5 seconds to cool down to 770K from 1070K, provided the desired hardness and toughness It was confirmed that a rebar weld was obtained.

  Moreover, it confirmed that the reinforcing bar weld part with the desired toughness was obtained by performing the said heat input control so that the hardness of the said weld part after slow cooling might be less than Hv400.

  According to the resistance welding method of a steel bar of the present invention, it is not necessary to wait for a change in the weld structure due to rapid cooling (quenching) after applying a welding current, and a tempering step after quenching is not necessary. Therefore, it is possible to suppress the deterioration of the characteristics of the reinforcing bar weld due to the quenching, the production management is facilitated, and the tempering time is not required, so that the productivity of the bar steel welding operation is improved.

  DESCRIPTION OF EMBODIMENTS Embodiments of a resistance welding method for steel bars to which the present invention is applied will be described below with reference to the drawings.

  As shown in FIG. 1, the present inventors conducted a spot welding experiment of a main reinforcement 1 (D22: SD345) and a stirrup reinforcement 2 (D10: SD205A) that are generally used as basic reinforcing bars. In the welding process, a 11000 A pulsed welding current was used. In the slow cooling step, the cooling time until the weld surface temperature in the reinforcing bar weld 3 after the welding current application is changed from 800 ° C. to 500 ° C. is 4.5 seconds (sample 1), 2.8 seconds (sample 2) and Heat input control was performed so as to be 1.9 seconds (sample 3). About each sample 1, 2, and 3, the hardness measurement in the mesh-shaped area | region in the reinforcement welding part 3 and the tension test of the main reinforcement were performed.

  FIG. 2 (a) is a graph showing changes in the weld surface temperature for Sample 1 employing the method of the present invention, FIG. 2 (b) is a photograph showing the results of a tensile test, and FIG. 3 is a hardness measurement. It is a hardness distribution figure which shows the result. As a result of the tensile test, the reinforcing bar weld 3 was sound and a ductile fracture surface was observed. As a result of hardness measurement, it was confirmed that the maximum value of hardness was less than Hv350, and the average value of hardness was about Hv290.

  4A is a graph showing changes in the weld surface temperature for Sample 2, FIG. 4B is a photograph showing the results of a tensile test, and FIG. 5 is a hardness distribution showing the results of hardness measurement. FIG. As a result of the tensile test, a fracture surface having properties intermediate between brittle fracture and ductile fracture was observed. As a result of hardness measurement, it was confirmed that the maximum value of hardness was in the range of Hv400 to 425, and the average value of hardness was about Hv400.

  FIG. 6A is a graph showing the change in the weld surface temperature for Sample 3, FIG. 6B is a photograph showing the result of the tensile test, and FIG. 7 is a hardness distribution showing the result of the hardness measurement. FIG. As a result of the tensile test, a brittle fracture surface was observed in the reinforcing bar weld 3. That is, a chevron pattern peculiar to a brittle fracture surface was confirmed on the fracture surface. As a result of the hardness measurement, it was confirmed that the maximum value of hardness was a limit hardness close to Hv500, and the average value of hardness was also close to Hv500.

[Experimental Example 1]
FIG. 8 is an explanatory diagram showing experimental data when spot welding is performed on the same main bars and stirrups as in sample 1 described above. The welding current was 11000 A, and in Experiment 1-1, the cooling time Δt8 / 5 required from 800 ° C. to 500 ° C. in the slow cooling process was 3.6 seconds, and 10 samples were tested. In Experimental Example 1-2, the same experiment was performed on 10 samples with a cooling time Δt8 / 5 of 6.8 seconds. Experimental Example 1-2 is based on the method of the present invention.

[Experiment 2]
FIG. 9 is an explanatory diagram showing experimental data when spot welding is performed on the same main bar and stirrup as in sample 1 described above. The welding current was 13300 A, and in Experiment 2-1, the cooling time Δt8 / 5 required from 800 ° C. to 500 ° C. in the slow cooling process was set to 3.5 seconds, and experiments were performed on 10 samples. In Experimental Example 2-2, ten samples were similarly tested with a cooling time Δt8 / 5 of 4.0 seconds. In Experimental Example 2-3, a cooling time Δt8 / 5 was set to 5.2 seconds, and ten samples were similarly tested. Experimental Example 2-3 is based on the method of the present invention.

(Other embodiments)
Each of the above examples relates to spot welding of reinforcing bars used in reinforced concrete structures, but the present invention is a resistance welding method for steel bars made of low alloy steel having a carbon content in the range of 0.2% to 2%. Can be used as

It is explanatory drawing which shows the spot welding part of a main reinforcement and a stirrup. (A) is a graph which shows the change of the weld surface temperature about the sample 1 which employ | adopted the method of this invention, (b) is a photograph which shows the result of the tension test about the sample 1. FIG. 6 is a hardness distribution diagram showing the results of hardness measurement of Sample 1. FIG. (A) is a graph which shows the change of the weld surface temperature about the sample 2, (b) is a photograph which shows the result of the tension test about the sample 2. 6 is a hardness distribution diagram showing the results of hardness measurement of sample 2. FIG. (A) is a graph which shows the change of the welding part surface temperature about the sample 3, (b) is a photograph which shows the result of the tension test about the sample 3. 6 is a hardness distribution diagram showing the results of hardness measurement of sample 3. FIG. It is explanatory drawing which shows the experimental data in Experimental example 1 which carried out the spot welding of the main reinforcement and the stirrup. It is explanatory drawing which shows the experimental data in Experimental example 2 which carried out the spot welding of the main reinforcement and the stirrup.

Explanation of symbols

1 Main bar 2 Staggered bar 3 Reinforcing bar weld

Claims (3)

A welding process in which a welding current is applied to a weld portion of a steel bar to be welded having a carbon content within a range of 0.2% to 2%;
A slow cooling step of gradually cooling the weld by performing heat input control on the weld so that the rapidly heated weld is not quenched and quenched .
The steel bar to be welded is a rebar used for a reinforced concrete beam or column,
In the slow cooling step, the heat input control is performed so that a cooling time from 1070 K (800 ° C.) to 770 K (500 ° C.) in a welding heat cycle of the welded portion is at least 4.5 seconds. A resistance welding method for steel bars. In the resistance welding method of the steel bar according to claim 1,
In the slow cooling step, the heat input control is performed so that the hardness of the welded portion after slow cooling is less than Hv400 . In the resistance welding method of the steel bar according to claim 1 or 2,
In the heat input control of the slow cooling step, a current for slow cooling is applied to the welded portion, and the resistance welding method for the steel bar is characterized in that: