JP5205167B2 - Resistance welding method and resistance welding system for steel bar - Google Patents

Description

  The present invention relates to a resistance welding method and a resistance welding system for a steel bar used for welding a basic reinforcing bar for a house. More particularly, the present invention relates to a resistance welding method and a resistance welding system for a steel bar that can form a welded portion having excellent hardness and toughness without requiring a tempering step by heating the welded portion before welding.

  Resistance welding is a method of welding metals by resistance heat generation, 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.

  Conventionally, tempering is performed on a welded portion of a hardened steel bar, and instead of reducing the hardness of the welded portion, a predetermined toughness is imparted to the welded portion 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.

  In Patent Document 3, a target cooling curve is stored and held in advance based on a continuous cooling transformation curve of a high-strength steel plate, and the temperature decrease rate of the welded portion of the high-tensile steel plate is stored and held during resistance welding. Have disclosed a welding apparatus and a welding method for controlling the temperature decrease rate so as to coincide with the above. Further, Patent Document 4 describes a technique for preventing hardening of a welded portion by suppressing rapid cooling after welding by controlling an input power density at the time of welding.

  In the patent document 5, the applicant of the present application increases the input heat amount of the welding object by preheating the welded part in a welding process using a resistance welder, and thereby the welded part is quenched due to rapid cooling. We have proposed a welding method that prevents falling into a state.

Japanese Patent No. 365833 JP 2006-346745 A JP 2003-285193 A Japanese Patent Laid-Open No. 11-197850 JP 2007-319887 A

  The present invention relates to an improvement of the welding method disclosed in Patent Document 5, and its object is to propose a spot welding method for a bar steel capable of forming a welded portion having excellent hardness and toughness.

The steel bar resistance welding method of the present invention comprises:
Ri by the preheating device, a preheating step for preheating the welding of at least one of the steel bars in the first bars and second bars of the welding object,
The resistance welding system, possess a resistance welding step of welding the first and second bars after preheating,
The first steel bar and the second steel bar are steel bars made of carbon steel, or the first steel bar and the second steel bar are rebars used in a reinforced concrete structure,
The preheating temperature of the welded portion after the preheating step is set to a temperature within a range from 175 ° C. to 475 ° C.

  In the resistance welding method of the present invention, the welded portion is heated by the preheating device prior to resistance welding by the resistance welding device so that the structural change of the reinforcing bar welded portion due to rapid cooling can be suppressed after the welding process. Therefore, since there is much calorie | heat amount of the welding part after welding, the cooling time of the rapidly heated bar steel welding part can be lengthened. That is, by increasing the time required for the rapidly heated welded portion to cool from the first temperature to the second temperature, desired strength and toughness can be secured in the obtained welded portion. According to the method of the present invention, it is not necessary to control quenching (quenching) and tempering in the resistance welding process, and the control is facilitated. 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 preheating temperature of the welded portion by the preheating device, and therefore control management is very easy.

  According to the experiments by the present inventors, if the preheating temperature of the welded part in the preheating step is set to a temperature within the range of 175 ° C. to 475 ° C., welding with excellent hardness and toughness and a low probability of occurrence of brittle fracture. It was confirmed that a part was obtained. In particular, it was confirmed that if the preheating temperature of the welded part in the preheating process is set to 200 ° C. or higher, a very good welded part can be obtained.

  Next, in the resistance welding method of the present invention, the first steel bar has a larger cross-sectional area at the weld than the second steel bar, and the pre-heating step heats the weld of the first steel bar. A welded portion of a thick steel bar having a high heat capacity is not sufficiently heated at the time of welding as a welded portion of a thin steel bar, and is likely to be rapidly cooled after welding. Since the welded portion of the thin steel bar is sufficiently heated at the time of welding, it does not fall into a rapidly cooled state after welding and tends to be gradually cooled. Therefore, it is only necessary to preheat only the welded portion of the thick steel bar, and it is economical to do so.

  Here, the resistance welding method for steel bars of the present invention is suitable for use in spot welding of welded reinforcing bars. That is, in the method of the present invention, the first steel bar and the second steel bar are rebars used for reinforced concrete members, for example, main bars and shear auxiliary bars used for reinforced concrete structures.

  For example, when the first steel bar and the second steel bar are SD345 and SD295A, respectively, the preheating temperature is preferably about 200 ° C. When the first steel bar and the second steel bar are SD490 and SD295A, respectively, the preheating temperature is preferably about 300 ° C.

  In addition, the preheating temperature of the welded portion in the preheating step is set so that the cooling time between about 1070 K (800 ° C.) and 770 K (500 ° C.) in the welding heat cycle of the welded portion is 4 seconds or more. It is desirable.

  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 preheating temperature so as to be equal to or greater than the set time width. By setting, a steel bar weld having desired hardness and toughness can be obtained.

  Instead of setting the preheating temperature based on the cooling time, the preheating temperature of the welded part in the preheating step may be set so that the hardness of the welded part is Hv400 or less.

Next, according to the method of the present invention, when welding a reinforced concrete beam in a house or the like, a single reinforcing bar used for a reinforced concrete foundation in a factory or the like,
For each of the first steel bars, the second steel bars are arranged in a crossing state at a constant welding pitch along the material axis direction.
A preheating position by the preheating device is arranged at a position one welding pitch away from the welding position on the feed path passing through the welding position by the resistance welding apparatus,
Along the feed path, the first steel bar is intermittently fed in the material axis direction by the welding pitch,
What is necessary is just to repeat the operation | movement which performs the said preheating process and the said welding process with respect to the welding part of the said 1st steel bar and each 2nd steel bar positioned in the said preheating position and the said welding position, respectively.

  In addition, it is desirable to use an induction heating device that can efficiently heat the weld in the preheating step.

On the other hand, the present invention relates to a resistance welding system, the resistance welding system of the present invention,
A preheating device;
Resistance welding equipment,
A feed path for conveying the first steel bar and the second steel bar to be welded through the preheating position of the preheating device and the welding position of the resistance welding device;
The preheating temperature of the welded portion of the first steel bar and the second steel bar by the preheating device is set to a temperature within a range from 175 ° C. to 475 ° C.

  Here, it is desirable that the preheating temperature of the welded portion by the preheating device is 200 ° C. or higher.

  When welding a welded steel frame or the like, the first and second steel bars to be welded are each second with a constant welding pitch along the material axis direction with respect to the first steel bar. A steel bar is arranged in an intersecting state, a preheating position by the preheating device is arranged at a position one welding pitch away from the welding position on the feeding path passing through a welding position by the resistance welding apparatus, and the feeding path The first steel bar is intermittently sent in the direction of the material axis by the welding pitch, and the welded portion of the first steel bar and each second steel bar positioned at the preheating position and the welding position, It is desirable to repeatedly perform preheating by the preheating device and resistance welding by the resistance welding device, respectively.

  Furthermore, the preheating device is preferably an induction heating device.

  According to the resistance welding method for steel bars of the present invention, the desired hardness and toughness can be obtained by setting the preheating temperature of the welded portion in the preheating step by the preheating device performed prior to resistance welding by the resistance welding device to a predetermined temperature. It is possible to obtain a welded portion with a low probability of occurrence of brittle fracture. Therefore, it is possible to suppress deterioration of the properties of the steel bar weld due to quenching of the weld without requiring a post-process such as tempering after welding and without controlling the welding current with high accuracy. Therefore, production management becomes easy, and tempering time becomes unnecessary, so that the productivity of the welding work of the steel bar is improved.

  In the embodiment of the present invention, the resistance welding method of the present invention is applied to a spot welding method for main bars and stirrups used for housing foundations, beams and the like. A preheating process for preheating at least a main bar welded portion using a preheating device and a welding process for spot welding the main bar to the main bar using a spot welder after preheating, The preheating temperature of the weld is set to a temperature in the range from 175 ° C to 475 ° C.

  In a preferred embodiment, when the first steel bar and the second steel bar are SD345 and SD295A, respectively, the preheating temperature is about 200 ° C., and when the first steel bar and the second steel bar are SD490 and SD295A, respectively. The preheating temperature is about 300 ° C.

  By performing preheating using the preheating device in this way, the cooling time between about 1070 K (800 ° C.) and 770 K (500 ° C.) in the welding heat cycle of the welded portion can be increased to 4 seconds or more. The hardness of the part can be Hv400 or less.

Example 1
The inventors made a test body shown in FIG. 1 and conducted a spot welding experiment. The test body 1 is a reinforcing bar lattice used for a basic reinforcing bar for a house, and between two main reinforcing bars (D22) 2 extending in parallel, the streaks 3 (with a constant pitch along their material axis directions). D10) is spot-welded in an orthogonal state. The spot welding position is indicated by a shaded circle. The reinforcing bars used (material of the main reinforcing bar 2 and the stirrup 3), spot welding conditions, and the testing machines used in the experiment are as follows.

Reinforcing bars used Stirrup D10 (SD295A)
Main muscle S22 (SD345)
Test equipment DC inverter 1-point welding machine (NASTOA)
Amsler Universal Testing Machine (300kN / 500kN) (Shimadzu Corporation)
Pickers hardness tester (AAV-5000) (Mitutoyo)
Fiber type radiation thermometer (FTZ6) (Japan sensor)
Spot welding conditions Current value 13.3kA
Energizing time 460ms
Applied pressure 550kgf
Electrode diameter Up and down φ30mm

  10 specimens (N1 to N10) that are the same as the specimen 1 are prepared, and for each specimen N1 to N10, the preheating temperature of those spot welds is not preheated (10 ° C), 50 ° C, 100 ° C, Spot welding was performed by switching to eight stages of 150 ° C, 175 ° C, 200 ° C, 225 ° C, and 250 ° C. Preheating was performed using an induction heating device. About each test body N1-N10, the hardness of the spot welding part, the shear strength of the main reinforcement 2, and elongation were measured.

  FIG. 2 is a graph showing measurement results of shear strength and brittle fracture occurrence rate. The shear strength is high when the preheating temperature is 170 ° C. to 225 ° C., and the brittle fracture occurrence rate is zero when the preheating temperature is 175 ° C. to 250 ° C. FIG. 3 is a graph showing measurement results of elongation at break. The elongation at break exceeds 18% in any specimen when the preheating temperature is 175 ° C. or higher. Therefore, it can be seen that the preheating temperature should be in the range of 175 ° C. to 250 ° C. in terms of shear strength and brittleness.

  4A to 4H are graphs showing measurement results of hardness (Vickers hardness) at each preheating temperature. As can be seen from these figures, when the preheating temperature is 175 ° C. to 250 ° C., the hardness is approximately 350 Hv or less. In particular, the hardness is less than 350 Hv at 200 ° C. to 225 ° C.

  From these measurement results, it is understood that when the preheating temperature is changed from 175 ° C. to 250 ° C., a spot welded portion having excellent hardness and brittleness can be obtained. In particular, it can be seen that a spot weld with excellent hardness and brittleness can be obtained at around 200 ° C.

  Next, FIGS. 5A and 5B are graphs showing temperature changes in the case of no preheating (room temperature 28 ° C.) and a preheating temperature 200 ° C. in the spot welded portion. When there is no preheating, the cooling time from about 1070 K (800 ° C.) to 770 K (500 ° C.) in the welding heat cycle of the spot weld is 2.09 seconds, and when the preheating temperature is 200 ° C. It was 4.34 seconds. The predicted hardness simulated based on these cooling times was about 494 Hv without preheating and about 298 Hv when the preheating temperature was 200 ° C. From these temperature changes, it can be seen that by appropriately setting the preheating temperature, the cooling time after spot welding can be lengthened and the toughness of the base metal is prevented from being impaired. In other words, the preheating temperature can be determined such that the cooling time from 800 ° C. to 500 ° C. is a predetermined time width or longer, for example, 4 seconds or longer. Alternatively, the preheating temperature can be determined so that the predicted hardness calculated based on the temperature change is 400 Hv or less, preferably 350 Hv or less.

(Example 2)
Using the stirrup D10 (SD295A) and the main reinforcement S22 (SD490), the same spot welding experiment as in Example 1 was performed with a spot welding current of 13 kA and an energization time of 562 ms.

  For each test specimen, the spot-welded part is preheated at a temperature of no preheating (25 ° C), 150 ° C, 200 ° C, 250 ° C, 300 ° C, 350 ° C, 400 ° C, etc. I did it. Preheating was performed using an induction heating device. About each test body, the hardness of the spot weld part, the shear strength of the main reinforcement 2, and elongation were measured.

  As a result, it was confirmed that the shear strength was high when the preheating temperature was around 300 ° C, and the brittle fracture occurrence rate was the lowest when the preheating temperature was around 300 ° C. Further, it was confirmed that when the preheating temperature is around 300 ° C., the hardness of the welded portion is 375 Hv or less.

  From these measurement results, in the case of the combination of the main reinforcement D22 (SD490) and the stirrup D10 (SD490), it is confirmed that when the preheating temperature is about 300 ° C., a spot weld having excellent hardness and brittleness can be obtained. It was.

(Spot welding system)
Next, FIG. 6 is explanatory drawing which shows an example of the spot welding system which can be used for this invention. In the illustrated spot welding system 10, the reinforcing bar frame 11 to be welded is similar to the test specimen 1, with each stirrup 3 orthogonal to the main reinforcing bar 2 at a constant welding pitch p along the material axis direction. It is arranged in the state. The crossing positions of these reinforcing bars, the positions indicated as A, B, C, and D in the figure are the welds.

  The reinforcing bar frame 11 is conveyed along a horizontal feed path 12. The feed path 12 passes through a spot welding position 13 a by the spot welding device 13. Further, a preheating position 14a by the preheating device 14 is arranged at a position one spot before the spot welding position 13a by the spot welding apparatus 13 by one welding pitch p.

  In the spot welding system 10 having this configuration, the rebar frame 11 is intermittently fed along the feed path 12 by a welding pitch p in the material axis direction. First, as shown in FIG. The leading weld A is positioned at the preheating position 14a. In this state, the welding portion A is preheated by the preheating device 14. Next, the reinforcing bar frame 11 is sent out by one welding pitch p. As a result, as shown in FIG. 6B, the welded portion A after preheating is positioned at the spot welding position 13a, and the rear welding portion B is positioned at the preheating position 14a. In this state, spot welding is performed in the welded part A, and preheating is performed in the welded part B.

  Thereafter, as shown in FIG. 6 (C), the rebar frame 11 is intermittently fed sequentially by one welding pitch, and preheating and spot welding are repeatedly performed.

  Since the welding pitch changes depending on the object to be welded, the spot welding device 13 and the preheating device 14 can be moved relatively back and forth, and the distance between the spot welding position 13a and the preheating position 14a is matched to the welding pitch. It is sufficient to make adjustments possible. As the preheating device, ones of various heating mechanisms can be adopted, but it is generally efficient to use an induction heating device.

(Other embodiments)
The above-described embodiments are for spot welding of reinforcing bars used in reinforced concrete structures, but the present invention can be similarly applied to spot welding of bar steel made of carbon steel other than this.

It is explanatory drawing which shows the test body by which the main reinforcement and the rib reinforcement were spot-welded. It is a graph which shows the measurement result of shear strength and a brittle fracture incidence. It is a graph which shows the measurement result of breaking elongation. It is a graph which shows the measurement result of the hardness in the case of 10 degreeC (no preheating). It is a graph which shows the measurement result of the hardness in the case of 50 degreeC. It is a graph which shows the measurement result of the hardness in the case of 100 degreeC. It is a graph which shows the measurement result of the hardness in the case of 150 degreeC. It is a graph which shows the measurement result of the hardness in the case of 175 degreeC. It is a graph which shows the measurement result of the hardness in the case of 200 degreeC. It is a graph which shows the measurement result of the hardness in the case of 225 degreeC. It is a graph which shows the measurement result of the hardness in the case of 250 degreeC. It is a graph which shows the temperature change of the spot welding part at the time of preheating without preheating (room temperature 28 degreeC) and 200 degreeC. It is explanatory drawing which shows the structure and operation | movement of the spot welding system to which this invention is applied.

Explanation of symbols

1 Specimen 2 Main bar (1st bar)
3 Stirrup (second bar)
DESCRIPTION OF SYMBOLS 10 Spot welding system 11 Reinforcement frame 12 Feed path 13 Spot welding apparatus 13a Spot welding position 14 Preheating apparatus 14a Preheating position A, B, C, D Welding part

Claims (15)

The preheating device, a preheating step for preheating the welding of at least one of the steel bars in the first bars and second bars of the welding object,
The resistance welding system, possess a resistance welding step of welding the first and second bars after preheating,
The first steel bar and the second steel bar are steel bars made of carbon steel, or the first steel bar and the second steel bar are rebars used in a reinforced concrete structure,
A steel bar resistance welding method , wherein a preheating temperature of the welded portion after the preheating step is set to a temperature within a range from 175 ° C to 475 ° C. In the resistance welding method of the steel bar according to claim 1,
A resistance welding method for steel bars, wherein a preheating temperature of the welded portion after the preheating step is set to 200 ° C or higher . A preheating step of preheating a welded portion of at least one steel bar in the first steel bar and the second steel bar to be welded by a preheating device;
A resistance welding process for welding the first and second steel bars after preheating by a resistance welding device;
The first steel bar and the second steel bar are steel bars made of carbon steel, or the first steel bar and the second steel bar are rebars used in a reinforced concrete structure,
The first steel bar has a larger cross-sectional area at the weld than the second steel bar,
In the preheating step, the welded portion of the first steel bar is heated . In the resistance welding method of the steel bar according to claim 3 ,
A steel bar resistance welding method , wherein a preheating temperature of the welded portion after the preheating step is set to a temperature within a range from 175 ° C to 475 ° C. In the resistance welding method of the steel bar according to claim 3 ,
A resistance welding method for steel bars, wherein a preheating temperature of the welded portion after the preheating step is set to 200 ° C or higher . In the resistance welding method of the steel bar according to claim 1 or 3 ,
A resistance welding method for a steel bar, wherein the preheating temperature is 200 ° C. when the first steel bar and the second steel bar are SD345 and SD295A, which are rebars used in a reinforced concrete structure, respectively. In the resistance welding method of the steel bar according to claim 1 or 3 ,
Wherein the first bars and the second bars respectively, in the case of SD490 and SD295A is a rebar for use in reinforced concrete structures, a method of resistance welding bars, characterized in that the preheating temperature of 3 00 ° C.. In the resistance welding method of the steel bar according to any one of claims 1 to 5 ,
The first steel bar and the second steel bar are each a reinforcing bar used for a reinforced concrete structure,
Wherein as cooling time between the weld of the welding heat 1 that put the cycle 070K (800 ℃) to 770K (500 ℃) is more than 4 seconds, to set the preheating temperature of the welded portion in the preheating step A method for resistance welding of steel bars. In the resistance welding method of the steel bar according to any one of claims 1 to 5 ,
The first steel bar and the second steel bar are reinforcing bars used in a reinforced concrete structure,
A resistance welding method for a bar steel, wherein a preheating temperature of the welded portion in the preheating step is set so that a hardness of the welded portion is Hv400 or less. In the resistance welding method of the steel bar according to any one of claims 1 to 9,
For each of the first steel bars, the second steel bars are arranged in a crossing state at a constant welding pitch along the material axis direction.
A preheating position by the preheating device is arranged at a position one welding pitch before the welding position on the feed path passing through the welding position by the resistance welding device;
Along the feed path, the first steel bar is intermittently fed in the material axis direction by the welding pitch,
Resistance welding of a bar steel, wherein the preheating process and the welding process are repeated for the bond portions of the first steel bar and each second steel bar positioned at the preheating position and the welding position, respectively. Method. In the resistance welding method of the steel bar according to any one of claims 1 to 10,
An induction heating apparatus is used as the preheating apparatus. A preheating device;
Resistance welding equipment,
A feed path for conveying the first steel bar and the second steel bar to be welded through the preheating position of the preheating device and the welding position of the resistance welding device;
A resistance welding system, wherein the preheating temperature of the welded portion of the first steel bar and the second steel bar by the preheating device is set to a temperature within a range from 175 ° C to 475 ° C. The resistance welding system according to claim 12,
A resistance welding system, wherein a preheating temperature of the welded portion by the preheating device is set to 200 ° C or higher. The resistance welding system according to claim 12 or 13,
The first and second steel bars to be welded are arranged in an intersecting state with each second steel bar at a constant welding pitch along the material axis direction with respect to the first steel bar.
A preheating position by the preheating device is disposed at a position one welding pitch away from the welding position on the feed path via the welding position by the resistance welding device;
Along the feed path, the first steel bar is intermittently fed in the direction of the material axis by the welding pitch, and the welded portion of the first steel bar and each second steel bar positioned at the preheating position and the welding position. In contrast, the resistance welding system is characterized in that preheating by the preheating device and resistance welding by the resistance welding device are repeatedly performed. The resistance welding system according to any one of claims 12 to 14,
The preheating device is an induction heating device, and is a resistance welding system.