JP6584728B1 - Method of manufacturing resistance spot welded joint - Google Patents

Abstract

A method of manufacturing a resistance spot welded joint that can suppress the occurrence of dust during spot welding of a steel sheet including at least one hot stamped steel sheet, wherein two or more steel sheets are overlapped, and the overlap portion is resistance spot welded In this case, the electrode pressing force is 3.9 kN or more, the current Ia (t) (kA), and the time ta (sec), the preliminary energization process and the main energization process are performed in two stages. The pre-energization and main energization currents are all direct current, and 80% or more of the pre-energization time is continuous energization. Satisfy and make this energization pulsation energization.

Description

  The present invention relates to a method of manufacturing a resistance spot welded joint for steel plates.

  The body of an automobile is assembled by joining press-formed steel sheets mainly by spot welding using resistance welding. In spot welding, it is required to secure both the nugget diameter according to the plate thickness and the suppression of dust generation.

  In recent years, in the field of automobiles, the use of high-strength steel plates for frame parts has been increasing in order to ensure the weight reduction and collision safety of the vehicle body. Among them, a hot stamped steel sheet hot-formed using a high-strength steel sheet can adopt both a high forming accuracy and a low press load.

  However, when spot-welding a high-strength steel sheet by a one-stage energization method, dust is likely to occur, and it is difficult to ensure an appropriate current range. In addition, when zinc plating or aluminum plating is present on the surface layer of the steel sheet for hot stamping, oxidation of the plating proceeds during heating to form zinc oxide, aluminum oxide, or the like. As these oxides grow, the contact resistance of the steel sheet increases. As a result, there is a problem that dust is easily generated in spot assembly welding of the vehicle body, and it is difficult to ensure the stability of the nugget diameter.

  For such problems, Patent Document 1 discloses that in spot welding of high-strength steel sheets by adopting a two-stage energization method in which main energization is performed after improving the familiarity between the contact surfaces of the steel sheets by preliminary energization. A spot welding method that suppresses generation of dust is disclosed.

  In Patent Document 2, a nugget having a diameter of 3√t to 5√t is formed by preliminary energization, and then the current value is decreased, and then the current value is increased again to perform constant energization or pulsed energization. A spot welding method that suppresses generation of dust in spot welding of a high-tensile steel sheet by adopting an energization method that performs the above is disclosed.

  In addition, as an example in which such a two-stage energization method using preliminary energization and main energization is applied to spot welding of a hot stamped steel sheet, Patent Document 3 discloses a hot stamped steel sheet covered with a film having a high electrical resistance such as zinc oxide. During spot welding, pre-energization is performed by pulsation energization that repeats energization and de-energization several times while pressing the steel plate with electrodes, and then the main energization is continued continuously for longer than the maximum energization time during pulsation energization. A spot welding method is disclosed.

  Further, in Patent Document 4, when spot-welding the same steel plate as Patent Document 3, preliminary energization and main energization are performed by pulsation energization, and the maximum current of main energization is higher than the maximum current of pre-energization. A spot welding method as described above is disclosed.

  In the methods disclosed in Patent Documents 3 and 4, energization and energization stop are repeated during pre-energization pulsation energization, so that vibration due to thermal expansion and contraction is given to the electrode contact surface of the steel sheet, and a high melting point is obtained. The oxide layer can be effectively removed outside the welded part, and the cooling effect of the electrode can be sufficiently exerted by stopping energization of the pulsation energization to suppress a rapid temperature rise of the welded part. For this reason, while suppressing the generation of dust, it is possible to obtain the effect of improving the familiarity between the contact surfaces of the steel plates in a short time, suppressing the increase in current density at the contact interface and suppressing rapid nugget growth. Can do. As a result, generation of dust in spot welding of a hot stamped steel plate can be suppressed.

  In Patent Document 5, the nugget diameter is ensured while suppressing the occurrence of indentation by making the pressure of the electrode an appropriate range according to the plate thickness of the steel sheet, and further by making the energization pattern an appropriate range, and A spot welding method for preventing the occurrence of scattering is disclosed.

JP 2010-188408 A JP 2010-207909 A International Publication No. 2015/005134 International Publication No. 2015/093568 International Publication No. 2014/045431

  Steel plates used for hot stamping are often subjected to surface treatments such as zinc plating and aluminum plating in order to prevent the generation of iron scale when heated to a high temperature. When such a surface-treated steel sheet is hot stamped, oxidation of the plating proceeds during heating, and an oxide layer such as zinc oxide or aluminum oxide is formed. When these oxide layers grow, the contact resistance increases to 1 mΩ or more in the steel plate after hot stamping (hot stamping steel plate). In spot assembly welding of a vehicle body using such a hot stamped steel plate, there is a problem that dust is easily generated and it is difficult to ensure a stable nugget diameter.

  The techniques disclosed in Patent Documents 3 and 4 are based on the effect of pulsation energization (energization in which energization and deactivation are repeated a plurality of times in a short time) using an inverter DC welding power source. By excluding it on the outside, the familiarity between the contact surfaces of the steel plates during preliminary energization is improved. However, the effect may not be sufficient, for example, when the oxide layer is thick. Even in such a case, it is desired that generation of dust can be further suppressed. In addition, inverter DC, which has recently become mainstream due to advantages such as a small current, has a problem that the appropriate current range is narrower than AC, as disclosed in Patent Document 4.

  The technique disclosed in Patent Document 5 is to change the applied pressure according to the plate thickness, and further to set the energization pattern within an appropriate range, thereby securing the nugget diameter and suppressing the generation of dust. In some cases, such as when the layer is thick, the effect may not be sufficient.

  In the present invention, in view of such a situation, it is an object to provide a spot welding technique capable of suppressing generation of dust during spot welding of a steel sheet including at least one hot stamped steel sheet.

  When spot welding is performed by combining steel plates with high contact resistance that have high electrical resistance such as zinc oxide formed on the surface layer, the material with high electrical resistance on the surface layer is dispersed or moved to suppress dust and stabilize Then, the means for securing the nugget diameter was examined.

  As a result, when pre-energization is performed under conditions where the applied pressure of the electrode is increased when pre-energization is performed before the main energization as in Patent Documents 1 to 5, a substance having a high electrical resistance on the surface layer is effective. In addition, by making the main energization a pulsation energization, the region where the oxide is dispersed and moved gradually expands, and the generation current of dust increases during the main energization. It has been found that the welding current range can be expanded.

  As a result of further examination of the electrode pressing force and the energization conditions of the preliminary energization, the present inventors have found a condition in which the nugget diameter can be stably ensured by dispersing or moving a substance having a high surface resistance to suppress dust.

  The gist of the present invention thus made is as follows.

  [1] A method of manufacturing a resistance spot welded joint in which two or more steel plates are overlapped and the overlapped portion is pressurized with an electrode and energized, and the overlapped portion is applied by the electrode with a pressure of 3.9 kN or more A pre-energization step of energizing the current Ia (t) (kA) during the energization time ta (sec) so as to satisfy the following formulas (1) and (2), and a main energization step after the preliminary energization step The pre-energization step and the main energization step are all direct current, and in the pre-energization step, the energization method of 80% or more of the energization time ta is continuous energization, and the book A method of manufacturing a resistance spot welded joint, wherein the energization method of the energization process is pulsation energization in which energization and energization stop are repeated a plurality of times.

Ia (t) ≦ 6.0 (kA) (1)

  [2] The method of manufacturing a resistance spot welded joint according to [1], wherein the current is increased in the preliminary energization step.

  [3] The method of manufacturing a resistance spot welded joint according to [1] or [2], wherein a current is increased in the main energization step.

  [4] The method of manufacturing a resistance spot welded joint according to any one of [1] to [3], wherein the contact resistance of at least one of the steel plates is 1 mΩ or more.

  According to the present invention, there is provided a welding method capable of suppressing dust and stably securing a nugget diameter with respect to spot welding of a steel plate having a material having a high electrical resistance on the surface layer, such as a hot stamped steel plate. .

It is a graph which shows the nugget growth behavior at the time of carrying out spot welding of the 1800MPa class hot stamp material with a plate thickness of 1.4 mm while changing the energization pattern. It is a figure which shows an example of the electricity supply pattern of spot welding. It is a figure for demonstrating the diameter of an electrode front-end | tip part. It is a figure which shows the other example of the electricity supply pattern of spot welding. It is a figure for demonstrating the further another example of the electricity supply pattern of spot welding. It is a figure for demonstrating the measuring method of contact resistance.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

  When a hot stamped steel sheet (surface-treated hot stamped steel sheet) after hot stamping a surface-treated steel sheet such as hot-dip plated is resistance spot welded, surface dust is easily generated together with medium dust even at low current values, and the appropriate current range is It becomes extremely narrow and the current generated by dust becomes low. For this reason, when welding is performed without generating dust at a current value within the appropriate current range (excluding current near the upper limit of the appropriate current range), the nugget diameter obtained is also reduced.

  Here, the “appropriate current range” means that the current is increased little by little, and when the average value of the plate thickness of the spot welded steel plate is t, the first current (when the nugget diameter becomes 4√t or more ( (Hereinafter referred to as “4√t current”) to the current at which dust occurs for the first time.

  When the surface-treated hot stamped steel sheet is resistance spot welded, dust is likely to be generated and the appropriate current range is narrowed as follows.

  In the surface-treated hot stamped steel sheet, an intermetallic compound and an iron-based solid solution are formed on the surface by an alloying reaction between the plated metal and the base steel, and a metal derived from plating on the outer surface (for example, It has an oxide film mainly composed of Zn). Therefore, the surface-treated hot stamped steel sheet has a higher resistance at the contact portion between the steel sheets and a larger calorific value than a steel sheet cold-pressed.

  On the other hand, the alloying of plated metal and steel has progressed in the hot stamping process, and the melting point near the surface has a high value close to that of iron. Is difficult to soften, and the expansion of the energization path is suppressed. In particular, since the heating efficiency of the inverter direct current method is higher than that of the single-phase alternating current, the formation of the nugget at the initial stage of energization becomes very rapid. For this reason, the growth of the pressure contact portion around the nugget cannot catch up and the molten metal cannot be confined, and it is estimated that medium dust is generated.

  Further, since direct current does not have a current pause time unlike single-phase alternating current, it is difficult to obtain a cooling effect by the electrodes. For this reason, it is presumed that nuggets are likely to grow in the thickness direction, the molten portion reaches the outermost layer of the steel plate, and surface dust is generated. In the present invention, “direct current” does not include energization in which plus / minus is reversed as in alternating current. For this reason, not only energization in which current always flows like continuous energization but also pulsation energization that repeats energization and energization stop multiple times in a short time is determined to be direct current unless plus / minus is reversed.

  In spot welding by two-stage energization, the present inventors divide the oxide layer regardless of the thickness of the oxide layer, etc., and reliably remove a part of the oxide layer outside the welded portion, thereby rapidly Means to suppress the growth of molten metal were investigated.

  As a result, in the pre-energization, by applying an appropriate current with a high pressure applied to the hot stamped steel plate, the effect of dispersing and moving (excluding) oxides in the pre-energization is increased, and in the main energization, It has been found that the pulsation energization of the steel gradually expands the region in which the oxide is dispersed and moves, suppresses the rapid growth of molten metal, and makes it possible to suppress dust.

  FIG. 1 shows an example of a test result obtained from such knowledge.

  In the test, two hot-stamped galvanized steel sheets with a thickness of 1.4 mm (hot stamped steel sheets) were superposed and spot-welded with one-stage energization with only main energization, and pre-energization and 2-stage energization with main energization. In the case of spot welding, the expansion behavior of the nugget was investigated when the current value of main energization was increased until dust was generated.

  In the one-stage energization, the main energization is an energization pattern with a constant current. In the two-stage energization, a pre-energization with a current value Ia: 3.5 kA and an energization time ta: 0.4 sec, followed by an energization pattern in which main energization by pulsation energization as shown in FIG. The energization pattern for performing the main energization was used. In the pulsation energization, 0.06 s energization was repeated 6 times and 0.02 s energization was alternately repeated 5 times immediately after the preliminary energization without any rest time.

  The electrode has a DR (dome radius) type having a tip curved surface (initial contact portion) and a corner curved surface as shown in FIG. 3, and an electrode tip diameter d (described later) of 6 mm. Using. The applied pressure during energization was 450 kgf (4.4 kN), and was constant from the preliminary energization to the main energization.

  FIG. 1 shows the results of spot welding in a main current only pattern with a constant current, a pre-energization + main current pattern with a constant current, and a main current pattern with pre-energization + pulsation current. The point E in FIG. 1 indicates the experimental point where dust is generated.

  As shown in FIG. 1, the upper limit current value for generating dust increases when welding is carried out by two-stage energization of preliminary energization and main energization, compared to the case of welding with the energization pattern of only main energization. When the main energization is performed by pulsation energization, the upper limit current value for generating dust increases further to 5√t or more compared to the case where the main energization is performed at a constant current, and the appropriate welding current range is expanded. Confirmed to do.

  Based on the above knowledge, the present inventors further changed the pressure applied to the electrodes and the energization conditions of the pre-energization on the premise that energization is performed in two stages of pre-energization and main energization. As a result of studying the conditions for suppressing and obtaining the necessary nugget diameter, it is possible to obtain the necessary nugget diameter without generating dust by setting at least the conditions specified in (1) and (2) above. It has been found that a wide welding current range is expanded.

  The present invention has been made based on such examination results, and the requirements and preferable requirements necessary for the present invention will be further described below.

(Steel plate subject to spot welding)
In the present invention, a raw steel plate made of high-strength steel (for example, a thin steel plate including an electroplated steel plate or a hot dip plated steel plate) is heated to a quenchable temperature and austenitized, and then cooled and baked simultaneously with press molding in a mold. Hot stamped steel sheet (hereinafter referred to as “hot stamped steel sheet”) to be placed, and the surface is subjected to surface treatment such as zinc plating or aluminum plating to prevent the generation of iron scale when heated to high temperatures. The hot stamped steel sheet hot stamped using the formed material steel sheet is the main target of spot welding. The present invention is applicable to steel plates other than hot stamped steel plates, and is not particularly limited to hot stamped steel plates.

  In many cases, the hot stamped steel sheet is not a flat plate but a molded body. However, in general, since the overlapped portion only needs to be a plate shape, the present invention may be a molded body. Including “hot stamped steel plate”. A hot stamped steel sheet obtained by hot stamping a zinc-based plated steel sheet or an aluminum-based plated steel sheet may be referred to as a “surface-treated hot stamped steel sheet” in the following description.

  The hot stamped steel sheet has an intermetallic compound and an iron-based solid solution formed on its surface by an alloying reaction between a zinc-based or aluminum-based plating film and the base steel, and is derived from plating on the outer surface. It has an oxide layer mainly composed of a metal (for example, zinc in the case of zinc-based plating). Therefore, the surface-treated hot stamped steel sheet has a high contact resistance of 1 mΩ or more and a large amount of heat generated by energization compared to a bare steel sheet. In addition, the hot stamped steel sheet has undergone alloying between plating and steel in the hot stamping process, and the melting point near the surface has a high value close to that of iron. The contact portion between the steel plates is difficult to soften. The present invention is particularly effective when applied to spot welding of a steel sheet having such a contact resistance of 1 mΩ or more. A method for measuring contact resistance will be described later.

  There is no restriction | limiting in particular about the plate | board thickness of a steel plate. In general, the thickness of a steel sheet used in automobile parts or a vehicle body is 0.6 to 3.2 mm, and the method of manufacturing a resistance spot welded joint according to the present invention has a sufficient effect in this range.

(Board assembly)
As for the plate assembly when two or more steel plates are overlapped, it is preferable that at least one of the steel plates on the electrode contact side includes a surface-treated hot stamped steel plate. As the steel sheet combined with the surface-treated hot stamped steel sheet, a combination including a surface-treated hot stamped steel sheet or a high-tensile steel sheet of 590 MPa class or higher is preferable. In the assembly of a normal automobile body, resistance spot welding is performed on a plate assembly in which two or three steel plates are overlapped.

(electrode)
In the present invention, the surface area where the radius of curvature of the tip surface of the electrode is 40 mm or more (however, the surface area including the most advanced part of the electrode) is the same as the electrode pressing direction (usually the same as the electrode length method). The area A of the region projected onto a plane perpendicular to the surface and the diameter of a circle having an equivalent area (so-called equivalent circle equivalent diameter) is defined as the electrode tip portion diameter d. That is, the electrode tip diameter d is calculated as 2√ (A / π). According to this definition, for example, as shown in FIG. 3, a surface region having a radius of curvature of 40 mm or more is projected onto a plane perpendicular to the pressing direction of the electrode (usually the same as the electrode length method). When the formed area is circular, the diameter of the circle is the electrode tip diameter d.

  The diameter d and the curvature radius (tip R) of the tip curved surface portion of the electrode are not particularly limited, but generally the diameter is about 5 to 6 mm and the curvature radius is about 40 to 60 mm.

  As an electrode, the electrode prescribed | regulated to JISC9304: 1999 can be used, for example. For example, an electrode having a curvature radius R of 40 to 60 mm of the DR type tip curved surface portion is exemplified.

  As the electrode material, chromium copper or alumina-dispersed copper is preferable. From the viewpoint of preventing welding and surface dust, alumina-dispersed copper is preferable.

(Welding power supply)
The spot welding is energized using a DC welding power source such as an inverter DC system. The inverter direct current method has the merit of being able to reduce the transformer and mounting it on a robot with a small payload, so it is often used especially in an automated line.

  The inverter DC method has high heat generation efficiency because continuous current can be applied without current on / off unlike the conventionally used single-phase AC method. Further, in pulsation energization, the pulse current waveform can be easily controlled.

(Pressurization / energization conditions)
FIG. 2 is a time chart showing a basic example of an energization pattern in spot welding. In this energization pattern, first, preliminary energization is performed with current Ia while applying a predetermined pressure, and then pulsation energization is performed with current Ib, and main energization is performed so that the nugget has a predetermined diameter. . Then, after the energization of the main energization is completed, the electrode is separated from the steel plate when a predetermined hold time has elapsed, and the applied pressure is released.

  At that time, as described above, the electrode pressing force and the energization condition of the pre-energization are set as specific conditions.

  In the pre-energization, the applied pressure is increased, the oxide layer on the steel sheet surface is broken and dispersed, and a part of it is moved (excluded) in the direction outside the contact range of the electrode, so that the surface contact resistance is reduced. Reduce. In addition, the current value is lowered to suppress the rapid growth of the nugget at the initial stage of contact so that dust does not occur.

  For this purpose, the applied pressure is set to 3.9 kN or more so that the maximum value of the current Ia (kA) in the pre-energization satisfies Ia ≦ 6 kA. By doing in this way, the contact resistance of the steel plate surface can be reduced without generating dust.

  The applied pressure is preferably 4.4 kN or more. More preferably, it is 4.5 kN or more, 4.8 kN or more, 5.0 kN or more, or 5.4 kN or more. When the applied pressure increases beyond the appropriate range, for example, the dent of the electrode pressurizing part becomes large (a part with a thin plate thickness is locally formed), the joint strength decreases, or the current density becomes extremely high. Since the pressure may decrease and it may be difficult to form a nugget during main energization, the applied pressure is preferably 10 kN or less, 9.5 kN or less, or 9.0 kN or less.

  The energization time in the preliminary energization is set to be longer than the time during which the oxide layer in contact with the electrode on the surface of the steel sheet is destroyed and a part can be excluded from the contact range. Specifically, energization is performed for ta (sec) so as to satisfy the following expressions (1) and (2).

Ia (t) ≦ 6.0 (kA) (1)

  However, Ia (t) (kA) in the equations (1) and (2) is the current value of the pre-energization when t (sec) has elapsed since the start of the pre-energization.

  In order to develop the effect of the preliminary energization, the current integrated value S in the preliminary energization defined by the following formula (3) is set to 0.5 kA · s or more as shown in the formula (2). If necessary, the lower limit of the current integral value S may be 0.6 kA · s, 0.8 kA · s, 1.0 kA · s, or 1.2 kA · s. The energization time in the preliminary energization process is not particularly required, but is often 0.05 to 1 s. If necessary, the lower limit of the energization time may be set to 0.1 s, 0.15 s, or 0.2 s. The upper limit may be 0.9 s, 0.8 s, 0.7 s, or 0.8 s.

  As described above, in the embodiment of the present invention, the current in the pre-energization (when the current fluctuates during the pre-energization, the maximum value of the current during the pre-energization) is 6 kA or less. Although it is not necessary to specifically define the lower limit of the pre-energization current, the lower limit is 0 kA in consideration of pulsation energization. It may be 1 kA or 2 kA as necessary.

  In the preliminary energization, the main purpose is to destroy and separate the oxide layer on the surface of the steel sheet and the part where the steel plates are in contact with each other, and move the part outside the contact range. Nuggets need not be formed.

  The energization time ta in the pre-energization is longer than the time during which the oxide layer on the steel sheet surface can be separated, and more preferably, the energization is performed so as to satisfy the above relationship in relation to the current value I (t).

  In the energization by the preliminary energization, 80% or more of the pre-energization time is continuous energization. Continuous energization refers to energization so that the magnitude of the DC current does not become 0 amperes. In addition to continuing to flow a constant current, the magnitude of the DC current is increased over time. Alternatively, the magnitude of the direct current may be increased or decreased over time so that the magnitude of the direct current does not become 0 amperes. However, continuous energization does not include energization that is not normal pulsation energization but has a long energization stop (for example, an energization stop of 1 s or longer). Moreover, it is preferable that 85% or more of the pre-energization time is continuous energization, and 100% continuous energization may be performed. It should be noted that a short time (for example, about 0.01 to 0.1 s) energization pause time such as pallet energization is included in the energization time, but an energization pause time of 1 s or more is excluded from the energization time.

  In the main energization subsequent to the pre-energization, as shown in FIG. 2, pulsation energization is performed by repeating a cycle consisting of a period t1 in which a pulse current is energized and an energization suspension period t0 twice or more.

  In pulsation energization, vibration due to thermal expansion and contraction due to repetition of energization and energization interruption can be given to the electrode contact surface, which can further promote oxide dispersion / migration (exclusion) during pre-energization. By stopping energization, it is possible to suppress a rapid temperature rise of the weld during main energization, and to suppress generation of surface dust compared to continuous energization.

  As the energization condition in the pulsation energization, a condition in which a nugget having a predetermined diameter can be obtained without generating dust. In general, a nugget diameter of 4√t or more is often used as a standard for production management. In the present invention, as shown in FIG. 1, a welded joint having a larger nugget diameter (for example, 4√t or more) can be obtained without generation of dust.

  As the energization condition, the current value Ib is preferably energized at a current value higher than the pre-energization current value Ia, that is, Ib> Ia. Further, for each time of the energization period t1 and the rest period t0, for example, an energization method in which 0.02s is energized and then 0.02s is deactivated. Further, the shape of the pulse current is not limited to a rectangle as shown in FIG. 2, and the rising and falling portions of the pulse may be inclined with respect to time.

  In FIG. 2, the first period of pulsation energization is energized continuously after the preliminary energization. However, as shown in FIG. 4, the energization of the first period can be started after a rest period. .

  The energization pattern has been described with reference to an example of a pattern in which preliminary energization is performed with a constant current value and main energization is performed with a pulse current having a constant height, as shown in FIGS. Instead, the current value can be gradually increased. Increasing the current value gradually may be continuous or stepwise.

  FIG. 5A shows an example in which energization for gradually increasing the current, that is, up-slope energization is performed at the beginning of preliminary energization. A solid line indicates an example in which up-slope energization is performed from the beginning and a broken line indicates an intermediate current value. By starting the pre-energization by up-slope energization, it is possible to suppress the generation and rapid growth of the nugget when the contact resistance at the initial energization is high.

  FIG. 5B shows an example in which up-slope energization is performed to gradually increase the peak current at the beginning of the main energization. FIG. 5C shows an example in which the peak current is increased stepwise during the main energization. Each example is shown.

  By starting the main energization with upslope energization, rapid growth of the nugget can be suppressed. Further, the energization time can be shortened by increasing the current halfway.

  The total energization time in the main energization by the pulsation energization can be within a normal condition range. For example, as a guide for energization time, about 10 times the plate thickness per sheet (0.2 s for welding of steel plates having a thickness of 1 mm) is recommended. Also in the present invention, the total energization time in the main energization is exemplified by about 10 times the plate thickness. However, the temperature of the steel plate rises due to the preliminary energization, and the interface of the steel plate melts depending on the conditions. That is, depending on the pre-energization conditions, the total energization time during main energization may be shortened to about 4 to 8 times the plate thickness.

  In the present invention, the definitions of preliminary energization and main energization are as follows.

  First, in the case of single-stage energization with constant current energization (regardless of continuous energization or pulsation energization, whether or not there is an energization stop time and the length of the energization stop time), there is no preliminary energization and only main energization And In the case of energization at the stage of energizing different constant current after energization of constant current (regardless of continuous energization or pulsation energization, whether or not energization suspension time is present and the length of energization suspension time) Pre-energization and the second stage are main energization. Although current is different in the preceding and following stages, each stage is energized with a constant current, and in the case of energization with three or more stages (whether continuous energization or pulsation energization, the presence or absence of energization suspension time and the length of energization suspension time) Regardless of the current, all energization after the stage exceeding 6 kA for the first time is assumed to be main energization, and all energization before the main energization is assumed to be pre-energization (however, if the current at each stage is less than 6 kA, The energization is the main energization, and the energization before the main energization is the pre-energization.)

  When there is an increase / decrease in current during energization, such as up-slope energization (whether continuous energization or pulsation energization, regardless of energization stop time and length of energization stop time), it exceeded 6 kA for the first time All energization after the time is assumed to be main energization, and all energization before the main energization is assumed to be preliminary energization. Therefore, when there is an increase / decrease in current during energization as in such up-slope energization, and when all the current is less than 6 kA, it is not determined as an embodiment of the present invention.

(Contact resistance)
A method for measuring the contact resistance is shown in FIG. A steel plate (which may or may not be plated) is sandwiched between one spot welding electrode. A current I of 1 A is applied to the electrode. The voltage V1 between the upper electrode 1a and the steel plate 2 and the voltage V2 between the lower electrode 1b and the steel plate 2 are measured.

  The electric resistance between the upper electrode and the steel plate is R1, the electric resistance between the lower electrode and the steel plate is R3, and the resistance resulting from the specific resistance of the steel plate bulk (base material) itself is R2. R2 can be approximated to zero. Also, the resistance of the upper and lower electrodes can be approximated to zero. Therefore, the relationship between the measured voltages V1 and V2 and the electric resistances R1 and R3 can be approximated as follows.

V1 = (R1 + R2) × I ≒ R1 × I = R1 × 1 (A) = R1
V2 = (R2 + R3) x I ≒ R3 x I = R3 x 1 (A) = R3

  The larger one of R1 and R3 is the contact resistance in the present invention.

  In the present invention, a steel sheet having a contact resistance of 1 mΩ or more is mainly applied, but the present invention can also be applied to a steel sheet having a contact resistance of less than 1 mΩ, and need not be limited to a steel sheet having a contact resistance of 1 mΩ or more. If necessary, the lower limit of the contact resistance may be limited to 2 mΩ, 5 mΩ, 8 mΩ, or 10 mΩ. Although it is not necessary to set the upper limit of contact resistance in particular, the upper limit may be 100 mΩ, 50 mΩ, 30 mΩ, or 20 mΩ.

  Although the present invention is configured as described above, the feasibility and effects of the present invention will be further described below using examples.

A 1500MPa class GA plating with a plate thickness of 2.0mm using a servo-pressurized inverter DC spot welder equipped with a DR-type electrode (chromium copper) with a diameter of 6mm at the tip curved surface and a radius of curvature (tip R) of 40mm at the curved surface. ^Two test pieces prepared from hot stamped steel sheets (plating adhesion amount before hot stamping: 55 g / m ² per side, heating condition: 900 ° C. for 4 minutes furnace heating) were superposed and resistance spot welding was performed. The shape of the test piece was a strip having a width of 30 mm and a length of 50 mm. When the contact resistance of the steel sheet was measured by the above method, it was all 12 mΩ.

  Table 1 shows the welding conditions. The current value for this energization was varied from 4 kA to the current value at which dust was generated. The applied pressure in the two-stage energization was constant from the preliminary energization to the main energization. Under any condition, the total energization time in the main energization was set to be about 0.4 s. All power supplies were inverter DC power supplies.

  In welding, only the main energization is performed with the current values shown in Table 1, or after the preliminary energization process is performed with the current values shown in Table 1, the pulse current value of the pulsation energization in the main energization process is changed, The nugget diameter and the occurrence of dust were investigated. Table 1 also shows the plate thickness, strength (tensile strength), and test results (appropriate current range of the main energization process) of the test steel plates in each test number.

  As can be seen from Table 1, the present invention example can increase the upper limit current in the main energization process, so the comparative example in which the first stage energization is performed and the pulsation energization is not performed in the main energization in the second stage energization. Compared with the comparative example, a wider appropriate current range of about 1.5 kA can be obtained at the test piece level.

  Accordingly, in the present invention, the current value in the main energization process is set to a value not less than 4√t current and not more than the generation current of dust, so that no dust is generated even in welding of actual parts, and the current is shunted and the electrode is worn. Even if there is a disturbance due to, a spot weld with a nugget diameter of 4√t or more can be secured stably. On the other hand, in the comparative example, the appropriate current range did not satisfy the target of 1.5 kA or more.

  The embodiment of the present invention has been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately changing the above-described embodiment without departing from the spirit thereof.

DESCRIPTION OF SYMBOLS 1 Spot welding electrode 1a Upper electrode 1b Lower electrode 2 Steel plate 3 Plating layer

Claims (4)

A method of manufacturing a resistance spot welded joint in which two or more steel plates are overlapped, and the overlapped portion is pressurized with an electrode and energized,
During the energization time ta (sec) so that the current Ia (t) (kA) satisfies the following formulas (1) and (2) while pressurizing the overlapping portion with the electrode with a pressure of 3.9 kN or more. A pre-energization step for energization;
A main energization step is provided after the preliminary energization step,
The currents in the preliminary energization process and the main energization process are all direct current,
In the preliminary energization step, the energization method of 80% or more of the energization time ta is continuous energization in which energization is continuously performed,
The method of manufacturing a resistance spot welded joint, wherein the energization method of the main energization step is pulsation energization in which energization and energization stop are repeated a plurality of times.
Ia (t) ≦ 6.0 (kA) (1)

  2. The method of manufacturing a resistance spot welded joint according to claim 1, wherein the current is increased in the preliminary energization step.   3. The method of manufacturing a resistance spot welded joint according to claim 1, wherein the current is increased in the main energization step.   The method of manufacturing a resistance spot welded joint according to any one of claims 1 to 3, wherein the contact resistance of at least one of the steel plates is 1 mΩ or more.

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