WO2020045678A1

WO2020045678A1 - Resistance spot welding method

Info

Publication number: WO2020045678A1
Application number: PCT/JP2019/034368
Authority: WO
Inventors: 智伸三浦; 好則大山; 朋紀柳川
Original assignee: 豊田鉄工株式会社
Priority date: 2018-08-31
Filing date: 2019-09-02
Publication date: 2020-03-05
Also published as: JPWO2020045678A1

Abstract

A method for performing resistance spot welding using an inverter DC welding machine on a sheet set obtained by overlapping two or more steel sheets, wherein: the method includes a preliminary electric-current conduction step, and a main electric-current conduction step performed by conducting a pulsed electric current; the current value of a secondary-side current in the main electric-current conduction step is set to 10 kA or less; an electric-current conduction time is set to 0.7 cycles or less with reference to 60 Hz; and a standby time is set to a time such that the secondary-side current descends to 3 kA or less.

Description

Resistance spot welding method

The present invention relates to a resistance spot welding method, and more particularly, to a resistance spot welding method using an inverter DC welding machine.

組み立て In assembling automobile bodies, press-formed steel sheets are often joined by spot welding, a type of resistance welding. In spot welding, it is required to secure a nugget diameter that can secure the strength of the welded portion according to the plate thickness.

チリ During welding, dust (spattering) may occur in which the molten base metal scatters from the steel plate. In particular, those that scatter from the overlapping surfaces of steel plates are called medium dust, and those that scatter from the contact surface between the steel plate and the electrode are called front dust. If such dust adheres to the car body of the automobile, the surface quality of the car body deteriorates. Further, if needle-shaped dust is left on the surface of the spot welded portion, the wire harness of the automobile may be damaged. Furthermore, if the scattered dust adheres to the movable part of the welding robot, the normal operation of the manufacturing equipment may be hindered.

In recent years, in the field of automobiles, the use of high-strength steel sheets for skeletal components is becoming widespread in order to reduce the weight of vehicles and improve crash safety. Among them, a hot stamped steel sheet hot-formed using a high-strength steel sheet is compatible with high forming accuracy and a small press load, and is therefore increasingly used.

However, spot welding of a high-strength steel sheet in a single-stage current-applying method, in which energization is performed only once, tends to generate dust, making it difficult to secure an appropriate current value in a wide range. In addition, if zinc plating or aluminum plating is present on the surface layer of the steel sheet before hot forming, oxidation of the plating proceeds during heating to form zinc oxide or aluminum oxide. As these oxides grow, the contact resistance of the steel sheet increases. As a result, dust tends to be generated in spot welding at the time of assembling the vehicle body, and there is also a problem that it is difficult to secure a stable nugget diameter.

To cope with such a problem, Japanese Patent Application Laid-Open No. 2010-188408 discloses a two-stage current supply system in which the contact between the contact surfaces of the steel plates is improved by preliminary current supply and then the main current is supplied. A spot welding method for suppressing generation of dust in welding is disclosed. Further, as an example of applying such a two-step energization method using the preliminary energization and the main energization to spot welding of a hot stamped steel sheet, WO 2015/005134 discloses a method in which the steel sheet is covered with a coating having high electric resistance such as zinc oxide. When spot welding hot stamped steel sheets, pre-energization is performed by pulsation energization that repeats energization and pause several times while pressing the steel sheet with the welding electrode, and then continuously for a longer time than the maximum energization time during pulsation energization Discloses a method in which main power is supplied. Further, in WO-A-2015 / 093568, when spot welding a similar steel plate, the pre-energization and the main energization are performed by pulsation energization, and the maximum current of the main energization is made higher than the maximum current of the pre-energization. A method of doing so is disclosed. In the method of these documents, vibration due to thermal expansion and contraction is applied to the electrode contact surface of the steel sheet by repeatedly conducting and stopping during pre-energization, and the high melting point oxide layer is effectively eliminated outside the welded part. In addition to this, the cooling effect of the electrode can be sufficiently exerted by suspending the current supply during the pulsation, and the rapid rise in the temperature of the welded portion can be suppressed. For this reason, adaptability between the contact surfaces of the steel plates can be improved in a short time. In addition, since an increase in current density at the contact interface can be suppressed and rapid nugget growth can be suppressed, generation of dust can be suppressed.

If this energization is performed by continuous energization, heat is rapidly generated not only between the steel plates but also between the steel plates and the electrodes due to the effect of the surface of the steel plate having a high resistance in the initial stage of the energization. Then, in the latter half of the energization, the nugget growth in the direction perpendicular to the steel sheet increases, and surface dust tends to occur. Thus, as disclosed in WO 2015/093568, it has been found that the pulsation of the current even in the actual energization can moderately promote the heat generation of the steel sheet and suppress the generation of dust. In addition, when pulsation is performed, vibration due to thermal expansion and contraction is applied to the contact surface, so that the high melting point oxide layer can be effectively destroyed. As a result, a large number of current-carrying points can be formed between the electrode and the steel sheet or at the contact interface between the steel sheets, whereby an increase in current density at the interface can be suppressed, and rapid nugget growth can be suppressed. By this action, medium dust and surface dust can be prevented from being generated.

However, if the power supply is paused for a long time as in the above-mentioned International Publication, the nugget in the course of growth is too cold and temporarily becomes small, and the nugget is considered to be growing slowly while repeating expansion and contraction. . In other words, the duration of the main energization is long in order to obtain a required nugget diameter. Some cooling is required on the steel sheet surface where the electrodes come in contact, to suppress surface dust, but if the downtime is long, the interface between the steel sheets where the nugget should be grown will be cooled too much and hinder nugget growth There are inconveniences.

溶接 In the welding of ordinary steel plates, if an AC welding machine is used, the thermal efficiency is poor due to the downtime, so it is said that an inverter DC welding machine using an inverter DC power supply can have a shorter welding time. However, in the case of a hot stamped steel sheet having an oxide on the surface, the nugget grows rapidly when heated at a stretch with a direct current, or the nugget is formed unevenly due to the presence of the oxide on the steel sheet surface, which easily leads to surface dust. In addition, the inverter DC type has an advantage that the welding robot can be downsized because the transformer is small. However, in practice, it has been found that the AC method can suppress dust more than the inverter DC method. Therefore, the present inventors can suppress dust without increasing the welding time by adopting similar energizing conditions in pulsation energization using a DC power supply, based on knowledge about energizing conditions in an AC system. And proceeded with the study.

One aspect of the present invention is a method of performing resistance spot welding using an inverter DC type welding machine on a sheet assembly in which two or more steel sheets are overlapped. The current value of the secondary current in the main current stage is 10 kA or less, the current supply time is 0.7 cycle or less based on 60 Hz, and the pause time is such that the secondary current drops to 3 kA or less. It is a method to make time. In some embodiments, the energization time in the main energization stage is 0.3 cycles. In some embodiments, the pause time in the main energization stage is set to 0.2 cycle. In some embodiments, the number of pulses in the main energization stage is set to 40 times or more.

チリ Depending on the embodiment, the use of the above method can suppress the dust without increasing the welding time.

It is a figure which shows the current waveform of the secondary side electric current (effective value) when the energization time of the main energization performed by pulsation energization is set to 0.3 cycle, and a pause time is set to 0.2 cycle. FIG. 2 is a partially enlarged view of the current waveform of FIG. 1. It is a figure which shows the electric current waveform of the secondary side electric current (effective value) when the energization time of main energization performed by pulsation energization is set to 0.3 cycle, and a pause time is set to 0.1 cycle. FIG. 4 is a partially enlarged view of the current waveform in FIG. 3. 9 is a graph showing an example of a set waveform of a secondary current input to a power supply device in welding in which pre-energization and main energization are combined.

実施 An embodiment of the present invention will be described below with reference to the drawings.

[Steel plate used for welding]
For the welding of the present method, a steel plate formed by hot stamping (hot press) (herein referred to as a hot stamped steel plate) may be used. Hot stamping is a method in which a steel sheet material is heated to a temperature at which it can be quenched, austenitized, and then cooled and quenched simultaneously with press molding in a mold. Therefore, the hot stamped steel sheet has an oxide layer such as iron oxide on the surface. The hot stamped steel sheet has a tensile strength of, for example, 1470 MPa or more. Further, for welding, a steel sheet (surface-treated hot-stamped steel sheet) obtained by subjecting a steel sheet material subjected to surface treatment such as zinc-based plating or aluminum-based plating to hot stamping may be used. Such plating is performed to prevent an oxide layer (scale) from being generated on the surface of the steel sheet when heated to a high temperature. Such a surface-treated hot stamped steel sheet has an intermetallic compound or an iron-based solid solution formed on its surface by an alloying reaction between a zinc-based or aluminum-based plating film and a base steel, and further has an outer surface. Has an oxide layer mainly composed of a metal derived from plating (such as zinc or aluminum). Therefore, the surface-treated hot-stamped steel sheet has a higher contact resistance of 1 mΩ or more than a bare steel sheet, and generates a large amount of heat when energized. Two or more hot stamped steel plates as described above are overlapped to form a plate assembly to be welded. It is preferable that any one of the steel plates to which the electrode contacts at the time of welding in the plate assembly is a surface-treated hot stamped steel plate.

[Welding method]
Welding is performed by spot welding, which is a type of resistance welding. Welding is performed by a two-stage energization method that combines preliminary energization and main energization. FIG. 5 shows an example of preliminary energization and main energization. This energization is for growing a nugget to be a weld. On the other hand, the pre-energization is performed mainly for the purpose of improving the conformity of the region of the steel plate surface that contacts the electrode and the interface between the steel plates. The welding machine uses an inverter DC type. The inverter DC type has the advantage that the transformer (transformer) can be made smaller than other types such as a single-phase AC type, so that the inverter DC type can be mounted on a robot having a small payload. The power supply device of the inverter type welding machine usually includes an inverter circuit (consisting of a switching element), a transformer and a rectifier circuit, and the AC pulse current (primary current) generated by the inverter circuit has a desired current value in the transformer. The current is converted into a current (secondary current), and the secondary current is passed between the electrodes as a welding current to perform welding. The graph of FIG. 5 shows the waveform of the set secondary current. The set waveforms of the pre-energization and the main energization, that is, the set values of the durations t _a and t _b (or the number of pulses) and the currents I _a and I _b (the pulse heights) are input through a control device also called a timer. The inverter direct current welding machine further includes a rectifier circuit on the secondary side of the transformer, and uses a secondary current converted into a direct current by the rectifier circuit as a welding current. As the inverter direct current welding machine, for example, the one disclosed in JP-A-2000-158148 can be used. Any electrode can be used. For example, a DR (dome radius) type electrode can be used. As the DR electrode, for example, one having a tip diameter of 6 mm and a tip curvature radius of 40 mm can be used. However, by setting the radius of curvature to 30 mm or less, the contact pressure acting on the steel plate at the electrode tip may be increased, and the effect of dispersing oxide on the steel plate surface may be increased. During welding, the plate assembly of the steel plates is sandwiched by the electrodes at a predetermined pressure. The pressing force by this electrode can be set in a range usually used. However, by increasing the pressing force, the oxide layer on the steel sheet surface is broken and dispersed, and a part of the oxide layer is moved (excluded) in the direction outside the contact range of the electrode, thereby reducing the surface contact resistance. Can be. For example, the pressure can be 400 kgf (= 3.9 kN) or more.

[Preliminary energization]
The preliminary energization may be performed by continuous energization or pulsation energization. FIG. 5 shows an example in which preliminary energization is performed by continuous energization. When the pre-energization is performed by continuous energization, the current value _Ia is preferably set to 6 kA or less. This makes it possible to reduce the contact resistance on the steel sheet surface while suppressing the generation of dust. Further, it is preferable that the current value of the pre-energization be higher than the current value of the main energization. Further, the energization time t _a of the preliminary energization, it is preferable that the product I _a t _a between the energizing time and the current value set longer within a range equal to or less than 2 kA · ms. As a result, the effect of dispersing (removing) the oxide is increased, and the appropriate range of the current value can be expanded.

[Main energization]
When the pre-energization is completed, the main energization is started. As shown in FIG. 5, this energization is performed by pulsation energization using a DC current having a pulse waveform. At the boundary between the pre-energization, the current may be connected to the first pulse without turning off the current, or the first pulse may be entered after a minimum pause time.

[Current value]
In this energized or less 10kA settings I _b of the secondary side current. It should be noted that the current value may be changed gradually or within each pulse within a range of 10 kA or less as needed. For example, by gradually increasing the current value, it is possible to suppress the generation and rapid growth of the nugget at the time when the contact resistance is high at the beginning of energization. In general, the current in spot welding is controlled at a constant current.

[Electrification / Pause time]
In this energization, the energization time (the pulse width of the set waveform of the secondary current) is set to 0.7 cycles (11.7 ms) or less. The cycle referred to in the present application is a unit based on a frequency of 60 Hz, and one cycle is about 16.7 ms. The pause time (pulse interval of the set waveform of the secondary current) is determined based on the response characteristic of the secondary current of the power supply device of the inverter DC welding machine. Since the secondary current generally responds to the change of the rectangular pulse of the primary current with a delay, the secondary current actually flowing between the electrodes even if the current value of the set waveform suddenly drops to 0 as shown in FIG. The effective value of falls relatively slowly. Specifically, the pause time is preferably such a time that the effective value of the secondary current drops to 3 kA or less. Those skilled in the art can determine the rest time that satisfies such conditions by appropriate experiments. As an example, the energization time can be 0.3 cycles (5 ms) and the pause time can be 0.2 cycles (3.3 ms).

上限 The upper limit of the pause time may be set so that a required nugget diameter is obtained. If the pause time is long, the secondary side current value drops to 0, but there is no problem if the nugget grows. However, if the downtime is lengthened, the entire welding time becomes longer. Therefore, in order to increase the production efficiency, for example, the time may be reduced to 3 kA or less.

[Number of pulses]
The number of pulses in the main energization is set according to the other conditions described above so that a desired nugget diameter can be achieved. For example, the number of pulses can be 40 or more. The number of pulses varies depending on the nugget diameter as a target value for the plate thickness t (for example, 5√t). If the pulse waveform (i.e. energizing time and pause time) is constant, the duration t _b of the current is determined by the number of pulses. Those skilled in the art can determine the number of pulses experimentally or empirically from the total energizing time.

As described above, the present invention has been described using the specific embodiments. However, the present invention is not limited to these embodiments, and those skilled in the art can make various substitutions and improvements without departing from the object of the present invention. , It is possible to make changes.

[Example of experiment]
Hot stamping was performed on a 1470 MPa alloyed hot-dip galvanized steel sheet (GA) material having a thickness of 1.4 mm, a width of 30 mm, and a length of 100 mm, thereby preparing two hot stamped steel sheet samples. Resistance spot welding was carried out by a two-stage energization method on a plate assembly in which the two samples were overlapped. The spot welding machine used was an inverter DC type equipped with a DR type electrode (made of chrome copper) having a tip curved surface portion having a diameter of 6 mm and a curvature radius of 40 mm. The preliminary energization was performed by continuous energization, the current set value was 4 kA, the energization time was 6 cycles (100 ms), and the pressing force was 4.5 kN. This energization was performed by pulsation energization. The current setting value of the main energization was variously changed, and the energization time and the pause time of the pulsation were variously changed in units of 0.1 cycle. The number of pulses was set such that the plate thickness was t (= 1.4 mm) and the nugget diameter was 5√t (= 5.9 mm). The pressurizing force at the time of this energization was 4.5 kN. Welding experiments were performed several times under each condition, and the incidence of dust on the table was calculated. In addition, the lower limit of the current dropping during the pause time was read from the graph. Table 1 shows the experimental results. As a representative, under conditions 3 and 4, the current waveform (effective value) of the secondary current is shown in FIGS. 2 and 4 are partial enlarged views of the current waveforms of FIGS.

Table 1 shows that if the energization time is 0.7 cycles or less, dust does not occur when the lower limit of the current dropped during the pause time falls below 3 kA (conditions 3, 5, 6, 7, and 9). Can be read. 2 and 4, the lower limit of 3.0 kA is indicated by a broken line for easy understanding. In particular, it was found that when the power-supply time was 0.3 cycles (5 ms) and the pause time was 0.2 cycles (3.3 ms) (condition 3), the duration of the main power-supply could be extremely short. This is considered to be because heat generation between the steel plate and the electrode is smaller than that between the steel plates having a high contact resistance due to a short energization time, the steel plate surface can be cooled in a correspondingly short time, and the surface dust is reduced. . Further, since the downtime is short, the influence on the nugget growth between the steel sheets is considered to be small, and the entire welding time can be shortened.

On the other hand, under the conditions 1 and 2 in which the energization time is extended to one cycle or more as in the related art, dust is generated even if a pause time in which the lower limit of the secondary current (effective value) falls to 0 is selected. . It is considered that, during energization, heat was generated not only between the steel plates having high contact resistance but also between the steel plates and the electrodes, which led to the generation of surface dust. To prevent the generation of dust, it is necessary to further extend the pause time.However, because it cools not only between the steel plate and the electrode but also between the steel plates, it is necessary to increase the number of pulses and secure the duration of main energization, The overall welding time is extended. Even if the energizing time is shortened to 0.3 cycles (5 ms), under the condition 4 in which the lower limit value of the current that drops due to the long rest time is 4.5 to 6.0 kA, the frequency of the table dust is high. Occurred in

Claims

A method of performing resistance spot welding using a direct current welding machine on a sheet assembly in which two or more steel sheets are overlapped,
Including a pre-energization stage and a main energization stage performed by pulsation energization, the current value of the secondary current in the main energization stage is set to 10 kA or less, the energization time is set to 0.7 cycle or less based on 60 Hz, and the pause time is set to A method in which the time is such that the secondary current drops to 3 kA or less.
(4) The method according to (1), wherein the energization time in the main energization stage is set to 0.3 cycle.
(4) The method according to the above (2), wherein the downtime in the main energization stage is 0.2 cycles.
4. The method according to claim 3, wherein the number of pulses in the main energizing step is 40 or more.