Specification
Title of invention: Method for manufacturing resistance spot welded joint
Technical field
[0001]
　The present invention relates to a method for manufacturing a resistance spot welded joint for a steel plate.
Background technology
[0002]
　The body of an automobile is assembled by joining press-formed steel sheets mainly by spot welding by resistance welding. In spot welding, it is required to secure a nugget diameter according to the plate thickness and suppress the occurrence of dust (scatter).
[0003]
　In recent years, in the field of automobiles, the use of high-strength steel sheets as skeletal components is expanding in order to reduce the weight of vehicle bodies and ensure collision safety. Above all, a hot stamped steel sheet hot-formed by using a high-strength steel sheet is capable of achieving both high forming accuracy and low press load, and therefore its adoption has been advanced.
[0004]
　However, when spot-welding a high-strength steel sheet by the one-step energization method, dust is likely to occur and it is difficult to secure an appropriate current range. Further, if the surface layer of the steel sheet for hot stamping has zinc plating or aluminum plating, the plating is oxidized during heating to form zinc oxide, aluminum oxide, or the like. When these oxides grow, the contact resistance of the steel sheet increases. As a result, there is a problem that dust is likely to occur in the spot assembly welding of the vehicle body, and it becomes difficult to secure a stable nugget diameter.
[0005]
　In order to solve such a problem, Patent Literature 1 adopts a two-step energization method in which main energization is performed after improving familiarity between contact surfaces of the steel sheets by pre-energization, thereby performing spot welding of high-tensile steel sheets. A spot welding method for suppressing the generation of dust is disclosed.
[0006]
　In Patent Document 2, after forming a nugget having a diameter of 3√t to 5√t by preliminary energization, the current value is decreased, and then the current value is increased again to perform a constant current or pulsed main energization. A spot welding method for suppressing the occurrence of dust in spot welding of a high-strength steel sheet is disclosed by adopting an energization method for performing the above.
[0007]
　In addition, as an example of applying such a two-step energization method of preliminary energization and main energization to spot welding of hot stamp steel sheets, in Patent Document 3, a hot stamp steel sheet covered with a film having high electrical resistance such as zinc oxide is used. During spot welding, preliminary energization is performed by pulsation energization in which energization and de-energization are repeated multiple times while pressing the steel plate with the electrode, and then main energization is continued for a longer time than the maximum energization time during pulsation energization. A spot welding method for doing so is disclosed.
[0008]
　Further, in Patent Document 4, when spot-welding the same steel plate as in Patent Document 3, preliminary energization and main energization are performed by pulsation energization, and the maximum current of the main energization is higher than the maximum current of the preliminary energization. Such a spot welding method is disclosed.
[0009]
　In the methods disclosed in Patent Documents 3 and 4, the energization and the de-energization are repeated during the pulsation energization of the pre-energization, thereby giving vibration due to thermal expansion and contraction to the electrode contact surface of the steel sheet, and The oxide layer can be effectively removed to the outside of the welded portion, and the cooling effect of the electrode can be sufficiently exerted by stopping the energization of the pulsation energization to suppress the rapid temperature rise of the welded portion. Therefore, while suppressing the occurrence of dust, it is possible to obtain the effect of improving the familiarity of the contact surfaces of the steel sheets in a short time, suppressing the increase in the current density at the contact interface and suppressing the rapid nugget growth. You can As a result, it is possible to suppress the occurrence of dust in spot welding of hot stamped steel sheets.
[0010]
　In Patent Document 5, the pressure of the electrode is set to an appropriate range according to the plate thickness of the steel plate, and further, the energization pattern is set to an appropriate range to secure the nugget diameter while suppressing the occurrence of indentation, and A spot welding method for preventing the generation of dust is disclosed.
Prior art documents
Patent literature
[0011]
Patent Document 1: Japanese Patent Laid-Open No. 2010-188408
Patent Document 2: Japanese Patent Laid-Open No. 2010-207909
Patent Document 3: International Publication No. 2015/005134
Patent Document 4: International Publication No. 2015/093568
Patent Document 5: International Publication No. 2014/045431
Summary of the invention
Problems to be Solved by the Invention
[0012]
　Many steel sheets used for hot stamping are surface-treated with zinc-based plating, aluminum-based plating, or the like in order to prevent generation of iron scale when heated to a high temperature. When such a surface-treated steel sheet is hot-stamped, the oxidation of the plating proceeds during heating to form an oxide layer such as zinc oxide or aluminum oxide. When these oxide layers grow, the contact resistance of the steel sheet after hot stamping (hot stamped steel sheet) increases to 1 mΩ or more. In spot assembly welding of a vehicle body or the like using such a hot stamped steel plate, there is a problem that dust is easily generated and it is difficult to secure a stable nugget diameter.
[0013]
　The technology disclosed in Patent Documents 3 and 4 uses the action of pulsation energization (energization and energization repeated a plurality of times in a short time) using an inverter DC welding power source to form a high melting point oxide layer in a welded portion. By excluding it to the outside, the familiarity between the contact surfaces of the steel sheets during pre-energization is improved. However, there are cases where the effect is not sufficient, such as when the oxide layer is thick, and it is desirable to be able to further suppress the generation of dust even in such a case. In addition, since there is an advantage that the power supply is small, there is a problem that an inverter direct current, which is becoming mainstream these days, has a narrower proper current range than an alternating current as disclosed in Patent Document 4. A welding method that can obtain a wider appropriate current range is desired even when the inverter is a direct current and the pulsation energization is rarely used, and the energization is mainly continuous energization or energization with no repetition of short energization pauses. ..
[0014]
　The technique disclosed in Patent Document 5 secures the nugget diameter and suppresses the occurrence of dust by changing the applied pressure according to the plate thickness and further setting the energization pattern within an appropriate range. In some cases, the effect may not be sufficient, such as when the layer is thick, and it is desirable to be able to further suppress the generation of dust even in such cases.
[0015]
　In view of such circumstances, it is an object of the present invention to provide a spot welding technique capable of suppressing the occurrence of dust during spot welding of a steel sheet including at least one hot stamp steel sheet.
Means for solving the problem
[0016]
　Inverter DC welding power source is used, and pulsation energization is hardly used, but even if the main part is continuous energization, steel sheets with high contact resistance where substances with high electrical resistance such as zinc oxide are formed on the surface layer In the case of spot welding with a combination of the above, a method for dispersing or migrating a substance having a high electric resistance in the surface layer to suppress dust and stably securing a nugget diameter was examined.
　As a result, when an electrode having a large tip diameter is used and pre-energization is performed before the main energization under the condition that the pressure applied to the steel sheet is increased as in Patent Documents 1 to 4, the electric resistance of the surface layer is high. It has been found that the substance can be dispersed or moved effectively, and thus the current of dust generation at the time of the main energization increases, and the proper welding current range can be expanded.
　As a result of further studying the tip diameter of the electrode, the pressure applied to the steel plate, and the energization conditions for pre-energization, the substance with high electrical resistance in the surface layer is dispersed or moved to suppress dust and a stable nugget diameter can be secured. Found the conditions.
　The gist of the present invention thus made is as follows.
[0017]
(1) A method for manufacturing a resistance spot welding joint in which two or more steel plates are superposed, and the superposed portion is pressed by an electrode to conduct electricity,
　wherein a surface area having a curvature radius of a tip surface of the electrode of 40 mm or more is provided. The area of ​​a region projected onto a plane perpendicular to the pressing direction of the electrode, and the diameter of the tip of the electrode, which is the diameter of a circle having an equivalent area, is 8.0 mm or more, and a
　load of 5.5 kN or more. A pre-energizing step of energizing the current Ia(t)(kA) for the energizing time ta seconds so as to satisfy the following formulas (1) and (2) while pressurizing the electrode with pressure,
　and 5 after the pre-energizing step. A main energization step of energizing the electrode while pressurizing the electrode at 0.0 kN or more,
　all of the currents in the preliminary energization step and the main energization step are direct current, and the
　preliminary energization time ta and the energization time of the main energization step are respectively A method for producing a resistance spot welding joint, wherein 80% or more of the energization method is continuous energization for continuous energization.
　Ia(t)≦6.0 (kA)...Equation (1)
[ Equation 1]

[0018]
(2) The method for manufacturing a resistance spot welded joint according to (1), wherein the current is increased in the preliminary energization step.
[0019]
(3) The method for producing a resistance spot welded joint according to (1) or (2) above, wherein an electric current is increased in the main energization step.
[0020]
(4) The method for producing a resistance spot welded joint according to any one of (1) to (3) above, wherein the energization method of the preliminary energization step is continuous energization.
[0021]
(5) The method for producing a resistance spot welded joint according to any one of (1) to (4), wherein the energization method of the main energization step is continuous energization.
[0022]
(6) The method for producing a resistance spot welded joint according to any one of (1) to (5) above, wherein the contact resistance of at least one of the steel plates is 1 mΩ or more.
Effect of the invention
[0023]
　According to the present invention, like a hot-stamped steel plate, a steel plate in which a substance having a high electric resistance is present in the surface layer is subjected to spot welding mainly due to continuous energization with direct current, suppressing dust, and stably providing a nugget diameter. Welding method that can secure
Brief description of the drawings
[0024]
[Fig. 1] Nugget growth behavior when spot-welding a 1800 MPa class hot stamp material with a plate thickness of 1.4 mm by continuous energization using an inverter DC welding power source while changing the energization pattern, electrode diameter, and pressure. It is a graph shown.
FIG. 2 is a diagram showing an example of an energization pattern for spot welding.
FIG. 3 is a diagram for explaining a diameter of a tip portion of an electrode.
FIG. 4 is a diagram for explaining an example of an energization pattern for spot welding.
FIG. 5 is a diagram for explaining an energization pattern when pulsation energization is used for main energization.
FIG. 6 is a diagram for explaining a method of measuring contact resistance.
MODE FOR CARRYING OUT THE INVENTION
[0025]
　Embodiments of the present invention will be described below with reference to the accompanying drawings.
[0026]
　When resistance spot welding a hot-stamped steel plate (surface-treated hot-stamped steel plate) after hot-stamping a steel plate that has been subjected to surface treatment such as hot dip coating, surface dust is likely to occur along with medium dust, and the appropriate current range is significantly narrowed. The current generated by dust is low. Therefore, when welding is performed without generating dust at a current value within the appropriate current range (excluding the current near the upper limit of the appropriate current range), the nugget diameter obtained also becomes small.
[0027]
　Here, the "appropriate current range" means increasing the current little by little, and letting the average value of the plate thicknesses of spot-welded steel plates be t, the first current (when the nugget diameter is 4√t or more) ( Hereinafter, it is referred to as “4√t current”) to a current at which dust is first generated.
[0028]
　The reason why the resistance spot welding of the surface-treated hot-stamped steel plate is likely to cause dust and the appropriate current range is narrowed is considered as follows.
[0029]
　The surface-treated hot stamped steel sheet has an intermetallic compound and an iron-based solid solution formed on its surface by an alloying reaction between the plated metal and the steel of the base material, and further has a metal derived from the plating on its 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 amount of heat generation than a steel sheet that is cold pressed.
[0030]
　On the other hand, the alloying of the plated metal and steel progresses in the hot stamping process, and the melting point near the surface has a high value close to that of iron. Is hard to soften, and expansion of the energization path is suppressed. In particular, in the (inverter) direct current system, the heat generation efficiency is higher than that in the single-phase alternating current, so that the formation of the nugget at the initial stage of the current supply becomes very rapid. Therefore, it is presumed that the growth of the pressure contact portion around the nugget does not catch up with it and the molten metal cannot be confined, resulting in the occurrence of medium dust.
[0031]
　Moreover, since the direct current does not have a current down time unlike the single-phase alternating current, it is difficult to obtain the cooling effect by the electrode. Therefore, it is presumed that the nugget easily grows in the plate thickness direction, the molten portion reaches the outermost surface layer of the steel plate, and surface dust occurs. In the present invention, “DC” refers to a current whose flowing direction (plus/minus) does not change even if the magnitude changes with time, and includes the case where the magnitude becomes 0 ampere with time. For this reason, not only the energization in which the current is always flowing like the continuous energization, but also the pulsation energization in which the energization and the energization suspension are repeated a plurality of times in a short time is determined as the direct current as long as the plus/minus is not reversed.
[0032]
　The inventors of the present invention firstly cut the oxide layer to secure it outside the welded portion during the pre-energization step of spot welding by two-stage energization by the continuous DC energization method, regardless of the thickness of the oxide layer or the like. I examined the means to eliminate.
[0033]
　As a result, when a high pressure is applied to the hot-stamped steel sheet by the electrode with a large tip diameter, the contact area between the electrode tip and the steel sheet increases, the range in which oxides can be dispersed and moved is expanded, and the applied force is increased. The increase in pressure brings about the increase in surface pressure, and the effect of dispersing/moving (excluding) oxide is increased. Furthermore, it has been found that the generation of surface dust is particularly suppressed because the cooling effect of the electrode has a high cooling effect on the surface layer of the steel sheet.
[0034]
　FIG. 1 shows an example of the test results obtained from such knowledge.
[0035]
　In the test, two hot-stamped 1.4 mm thick galvanized steel sheets (hot-stamped steel sheets) were superposed and spot-welded by a single-stage energization of only the main energization, and a pre-energization step and a main energization step. In the case of spot welding by stepwise energization, the tip diameter of the electrode and the pressure applied to the overlapping portion of the steel plate of the electrode are changed, respectively, and the current value of the main energization is increased until dust occurs. The expansion behavior of the nugget was investigated.
[0036]
　In the two-stage energization, as shown in FIG. 2, the pre-energization is performed at the current value Ia: 3.5 kA for the energization time ta (=0.4 s), and then the main energization for the energization time tb is performed at various current values ​​Ib. An energization pattern for performing (the energization time of the main energization is 0.28 s) was used.
[0037]
　As the electrode, use a DR (dome radius) type as shown in FIG. 3, which has an electrode tip diameter d (initial contact portion) of 6.0 mm (normal electrode) and 8.0 mm (thick electrode) described later. I was there. The applied pressure during energization was 5.5 kN (low pressurization) when an electrode with a tip diameter of 6.0 mm was used, and 6.9 kN when an electrode with a tip diameter of 8.0 mm was used. (High pressure).
[0038]
　Fig. 1 shows 4 patterns: low pressure + normal electrode + main current only, low pressure + normal electrode + preliminary current, high pressure + thick electrode + main current only, high pressure + thick electrode + preliminary current The results of spot welding at are shown below. Point E in FIG. 1 indicates an experimental point where dust was generated.
[0039]
　As shown in FIG. 1, as compared with the case where spot welding is performed in the energization pattern in which only the main energization is performed and the pre-energization is not performed, the upper limit current value in which dust occurs is increased by welding by the two-step energization. Especially, in addition to pre-energization, if high pressure and thick electrode are combined, it is compared with normal condition (low pressure + normal electrode + main energization only) or even when pre-energization is low pressure + normal electrode. It was confirmed that the upper limit current value of dust generated was greatly increased and the appropriate welding current range was expanded.
[0040]
　Based on the above findings, the present inventor further changed the diameter of the tip portion of the electrode, the pressure applied to the electrode, and the energization condition of the pre-energization on the assumption that the energization is performed by the two-stage energization of the pre-energization and the main energization. Then, as a result of studying the conditions for suppressing the dust and obtaining the necessary nugget diameter, by setting the conditions defined by the formulas (1) and (2), the required nugget can be generated without causing the dust. It was found that the proper welding current range for obtaining the diameter is expanded.
[0041]
　The present invention has been made based on the results of such studies, and the requirements and preferable requirements for the present invention will be further described below.
[0042]
(
　Steel plate to be spot-welded) The present invention is to heat a material 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) to a quenchable temperature to austenite, A hot-stamped steel plate (hereinafter referred to as a hot-stamped steel plate) that is cooled and hardened simultaneously with press forming with a die, and zinc-based plating for preventing iron scale from being generated on the surface when heated to high temperature. The main target of spot welding is a hot-stamped steel sheet hot-stamped with a material steel sheet that has been subjected to a surface treatment such as aluminum plating. The present invention can be applied to steel plates other than hot stamped steel plates, and is not particularly limited to hot stamped steel plates.
[0043]
　Note that the hot-stamped steel plate is not a flat plate in many cases, but a molded product that has been molded, but the point is that the overlapping portions need only be plate-shaped, so in the present invention, it may also be a molded product. Including "hot stamped steel plate". Further, 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.
[0044]
　Hot-stamped steel sheets have an intermetallic compound and an iron-based solid solution formed on their surface by an alloying reaction between a zinc-based or aluminum-based plating film and the steel of the base material, and the outer surface is derived from the plating. It has an oxide layer containing a metal (for example, zinc in zinc-based plating) as a main component. Therefore, the surface-treated hot-stamped steel sheet has a contact resistance as high as 1 mΩ or more and a large amount of heat generated by energization, as compared with a bare steel sheet. In addition, since the hot stamping steel plate undergoes plating and alloying with steel in the hot stamping process, and the melting point near the surface has a high value close to that of iron, it is higher than that of the steel plate having the plating film before heating. , It is difficult for the contact part between steel plates to soften. The present invention is particularly effective when applied to spot welding of steel sheets having a contact resistance of 1 mΩ or more. The method for measuring the contact resistance will be described later.
[0045]
　There is no particular limitation on the plate thickness of the steel plate. Generally, the plate thickness of a steel plate used for automobile parts or car bodies is 0.6 to 3.2 mm, and the method for producing a spot welded joint of the present invention has a sufficient effect in this range.
[0046]
(Plate Assembly) In the plate assembly
　for stacking two or more steel plates, it is preferable that at least one of the steel plates on the side where the electrodes hit includes a surface-treated hot-stamped steel plate. As a steel plate to be combined with the surface-treated hot-stamped steel plate, a combination including a surface-treated hot-stamped steel plate and a high-tensile steel plate of 590 MPa class or higher is preferable. In a typical automobile body assembly, resistance spot welding is performed on a plate assembly in which two or three steel plates are superposed.
[0047]
(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 tip of the electrode) is the pressing direction of the electrode (usually the electrode length method). The area A of the region projected onto a plane perpendicular to the surface of the electrode and the diameter of a circle having an equivalent area (so-called equivalent circle diameter) are defined as the tip end diameter d of the electrode. That is, the tip end diameter d of the electrode is calculated as 2√(A/π). According to this definition, for example, as shown in FIG. 3, the surface region having a radius of curvature of 40 mm or more is in the pressing direction (usually the same as the electrode length method) to the overlapping portion of the steel plates of the electrode. On the other hand, when the area projected on the plane perpendicular to the surface is circular, the diameter of the circle is the diameter d of the tip of the electrode.
[0048]
　In the present invention, the tip end diameter d of the electrode is set to 8.0 mm or more. It is preferably more than 8.0 mm. It may be 8.5 mm or more, 9.0 mm or more, 9.5 mm or more, or 10.0 mm or more. The upper limit is not particularly limited, but is limited by the shape of the welded portion and the structure of the electrode attachment portion of the welding machine, and is generally about 12.0 mm. If necessary, it may be 11.0 mm or less or 10.5 mm or less.　
[0049]
　By using such an electrode having a large tip portion diameter, that is, an electrode having a thick tip portion diameter, the contact area with the steel plate is increased, and the range in which oxides can be excluded is expanded. Further, by using an electrode having a thick tip portion diameter, the cooling effect of the electrode on the surface layer of the steel sheet is enhanced, so that the occurrence of surface dust is particularly suppressed.
[0050]
　As the electrode, for example, an electrode defined in JIS C9304:1999 can be used. Among these, in order to set the electrode tip diameter d to be 8.0 mm or more, it is possible to use a DR type electrode having a tip radius of curvature of 40 mm or more, or a CR type electrode having a large cone tip diameter of the electrode tip. it can. For example, an electrode having a DR type tip curved surface with a curvature R of 40 to 60 mm is exemplified.
[0051]
　Chromium copper or alumina-dispersed copper is preferable as the material of the electrode, but alumina-dispersed copper is more preferable from the viewpoint of preventing welding and surface dust.
[0052]
(Welding power supply) For
　spot welding, power is supplied using a DC welding power supply such as an inverter DC system. Inverter DC method has a merit that the transformer can be made small and can be mounted on a robot with a small load capacity, so it is often used especially in automation lines.
[0053]
　The inverter direct current method does not have the on/off of the current unlike the conventionally used single phase alternating current method, and continuously applies the current, so that the heat generation efficiency is high.
[0054]
(Pressure/Electrification Conditions)
　FIG. 2 shows a basic example of an electricity conduction pattern in spot welding in a time chart. In this energization pattern, first, preliminary energization is performed in which a predetermined pressing force is applied to the superposed portion of the steel sheets at a current value Ia, and then a current is applied at a current value Ib so that the nugget has a predetermined diameter. Perform main energization. Here, Ib is preferably higher than Ia. Then, after the end of the energization of the main energization, the electrode is separated from the steel plate at the time when a predetermined hold time has elapsed, and the pressing force is released.
[0055]
　At that time, as described above, the electrode having the electrode tip diameter of 8.0 mm or more is used, and the electrode pressing force and the energization condition of the pre-energization are set as the specific conditions.
[0056]
　In pre-energization, with the electrode and the steel plate surface in contact with each other over a wide area, the applied pressure is increased to disperse the oxide layer on the steel plate surface and move part of the oxide out of the contact range of the electrode. (Elimination) to lower the contact resistance of the surface. Further, the current value is lowered to suppress the rapid growth of the nugget at the initial stage of contact and prevent the occurrence of dust.
[0057]
　Therefore, the applied pressure is set to 5.5 kN or more. The applied pressure is preferably 5.9 kN or more. More preferably, it is 6.0 kN or more, 6.3 kN or more, 6.5 kN or more, or 6.9 kN or more. If the applied pressure becomes larger than the appropriate range, for example, the depression of the electrode pressurizing part becomes large (locally thin part is formed) and the joint strength decreases, or the current density becomes extremely high. In some cases, it may decrease and it may be difficult to form a nugget during the main energization. Therefore, the applied pressure is preferably 10.0 kN or less, 9.5 kN or less, or 9.0 kN or less.
[0058]
　Further, in the pre-energization, the electrode is energized by the pressing force and energized for ta seconds so as to satisfy the following formulas (1) and (2).
　Ia(t)≦6.0(kA) ・・・Equation (1)
[ Equation 2]

[0059]
　However, Ia(t)(kA) in the equations (1) and (2) is the current value of the preliminary energization when the time t has elapsed from the start of the preliminary energization, and the current Ia(t) is 80% of the ta. In the above, continuous energization is performed.
[0060]
　In order to bring out the effect of the pre-energization, the current integral value S in the pre-energization defined by the following equation (3) is set to 0.5 kA·s or more as shown in the equation (2). If necessary, the lower limit of the integrated current value S may be set to 0.6 kA·s, 0.8 kA·s, 1.0 kA·s, or 1.2 kA·s. It is not necessary to specifically set the energization time of the preliminary energization, but it is often 0.05 to 1 s. If necessary, the lower limit of the energization time may be 0.1 s, 0.15 s, or 0.2 s. The upper limit may be 0.9s, 0.8s, 0.7s or 0.8s.
[Number 3]

[0061]
　As described above, in the embodiment of the present invention, the current in the preliminary energization (the maximum value of the current in the preliminary energization when the current changes during the preliminary energization) is 6.0 kA or less. Although it is not necessary to set the lower limit of the pre-energization current, the lower limit is 0 kA in consideration of the pulsation energization. If necessary, it may be 1.0 kA or 2.0 kA.
[0062]
　Since the main purpose of the pre-energization is to destroy the oxide layer on the surface of the steel sheet that contacts the electrode and to remove a part of the oxide layer outside the contact area, it is not necessary to form the nugget during pre-energization.
　The energization time during the pre-energization is longer than the time for which the oxide layer on the surface of the steel sheet can be separated/excluded, and the energization is performed so as to satisfy the above relationship with the current value Ia(t).
[0063]
　In the pre-energization, as described above, 80% or more of the pre-energization time is continuous energization. Here, the continuous energization means energization so that the magnitude of the direct current does not become 0 amperes, and not only the current of a constant magnitude is continuously flown, but also the magnitude of the direct current is changed over time. Or 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 having a long energization suspension (for example, energization suspension of 1 s or more) that is not normal pulsation energization. In the pre-energization, preferably 85% or more of the pre-energization time is continuous energization, and may be 100% continuous energization. It should be noted that short-time (for example, about 0.01 to 0.1 s) energization suspension time such as pulsation energization is included in energization time, but energization suspension time of 1 s or more is excluded from energization time.
[0064]
　In the main energization following the pre-energization, energization is performed while pressing the electrode at 5.0 kN or more. In the embodiment of the present invention, the proper current range is also sufficiently wide. For this reason, spot welding can be performed under the same conditions as the non-hot-stamped steel plate except that the pressure is increased as described above. Therefore, it is not necessary to determine the details of the conditions regarding the main energization, except that the electrodes are energized while being pressurized at 5.0 kN or more. If necessary, a preliminary test within the range of conventional knowledge may be performed to determine the welding conditions for the main energization. The energization time of the main energization does not have to be particularly specified, but it is often 0.05 to 1 s (second). If necessary, the lower limit of the energization time may be 0.1 s, 0.15 s, or 0.2 s. The upper limit may be 0.9s, 0.8s, 0.7s or 0.8s.
[0065]
　The time integration range of the current value during main energization (corresponding to the left side of equation (2) during pre-energization) does not need to be specified in particular, but is often 1.0 to 20.0 kA·s. If necessary, the lower limit may be 2.0 kA·s, 3.0 kA·s or 5.0 kA·s. The upper limit may be 15.0 kA·s, 12.0 kA·s, 10.0 kA·s, or 9.0 kA·s. The time integration of the current value of the main energization is usually larger than the time integration of the current value of the preliminary energization.
[0066]
　The current range for the main energization need not be specified, but may be 1.0 to 10.0 kA except for the case of pulsation energization. The lower limit may be 2.0 kA, 3.0 kA, 5.5 kA, 6.0 kA, 6.5 kA. The upper limit may be 12.0 kA, 11.5 kA, 11.0 kA, 10.5 kA or 10.0 kA. Considering pulsation energization, the lower limit of the current is 0 kA. The maximum current value of the main energization is usually larger than the maximum value of the preliminary energization.
[0067]
　In general, a nugget diameter of 4√t or more is often used as a standard for production control. According to 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 causing dust.
[0068]
　In the above description, as the energization pattern, the pattern in which the preliminary energization and the main energization are continuously energized at a constant current value as shown in FIG. 2 has been described as an example. It can be increased or increased in stages.
[0069]
　FIG. 4A shows an example in which the current is gradually increased, that is, the up-slope current is applied, at the initial stage of starting the pre-energization. The solid line shows an example in which the upslope energization is performed from the beginning and the broken line shows the current value in the middle. By starting the pre-energization by up-slope energization, 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.
[0070]
　Further, FIG. 4B shows an example of performing up-slope energization in which the current is gradually increased at the initial stage of main energization, and FIG. 4C shows an example of increasing the current stepwise during the main energization. Shown respectively. However, as described above, when the current Ia(t) exceeds 6.0 kA from the start of preliminary energization, it is determined that the main energization is started.
　By starting the main energization by the up-slope energization, rapid growth of the nugget can be suppressed. In addition, the energization time can be shortened by increasing the current on the way.
[0071]
　The main energization is performed by continuous energization for 80% or more of the energization time. Therefore, the present invention does not include an embodiment in which the main energization as shown in FIG. 5 is all performed by an energization method such as pulsation energization. Preferably, 85% or more of the energization time of the main energization is performed by the energization method by continuous energization, and 100% continuous energization may be performed. In the case of power interruption for a short period of time, such as for the duration of the pallation current (for example, the normal pulsation current is usually 0.01 to 0.1 s), the current interruption time should be included in the current duration. And However, when the energization suspension time is 1 s or more, the energization suspension time is excluded from the energization time, and 80% or more of the energization time of the preliminary energization may be continuous energization.
[0072]
　In the present invention, pre-energization and main energization are defined as follows.
　First, in the case of one-step energization with constant current energization (whether continuous energization or pulsation energization, regardless of the presence or absence of energization suspension time and length of energization suspension time), there is no preliminary energization and only main energization. And In the case of energizing at the stage of energizing different constant current after energizing constant current (whether continuous energization or pulsation energization, regardless of presence or absence of energization suspension time and length of energization suspension time), Pre-energization and second-stage main energization.
[0073]
　Although the current is different in the front and back stages, it is a constant current in each stage, and in the case of three or more stages of energization (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 energizations after the stage that exceeds 6.0kA for the first time are main energizations, and all energizations before the main energization are pre-energizations (provided that the currents at each stage are all less than 6.0kA, The final stage of energization is the main energization, and the energization before the main energization is the preliminary energization.)
[0074]
　If there is an increase or decrease in the current during energization such as up-slope energization (whether continuous energization or pulsation energization, regardless of the presence or absence of energization suspension time and the length of energization suspension time), 6.0 kA for the first time All energizations after the point of time when the current is exceeded are main energization, and all energizations before the main energization are pre-energization. Therefore, in the case where there is an increase or decrease in the current during energization such as the up-slope energization and all the currents are less than 6.0 kA, it is not judged as the embodiment of the present invention.
[0075]
(Contact Resistance)
　FIG. 6 shows a method for measuring the contact resistance. A steel plate 2 (the plating layer 3 may be omitted) is sandwiched by one spot welding electrode 1a, 1b. A current I of 1 A is applied to the welding electrodes 1a and 1b. 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 electrical resistance between the upper electrode 1a and the steel sheet is R1, the electrical resistance between the lower electrode 1b and the steel sheet is R3, and the resistance due to the specific resistance of the steel sheet bulk (base material) itself is R2. R2 can be approximated to zero. Further, the resistances of the upper and lower electrodes 1a and 1b can be approximated to zero. Therefore, the relationship between the measured voltages V1 and V2 and the electrical resistances R1 and R3 can be approximated as follows.
　V1= (R1+R2)×I ≈ R1×I = R1×1(A)= R1
　V2= (R2+R3)×I ≈ R3×I = R3×1(A)= R3
　Either R1 or R3 The larger resistance value 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 is also applicable to a steel sheet having a contact resistance of less than 1 mΩ, and is not limited to a steel sheet having a contact resistance of 1 mΩ or more. If necessary, the lower limit of contact resistance may be limited to 2 mΩ, 5 mΩ, 8 mΩ or 10 mΩ. The upper limit of the contact resistance does not have to be specified, but the upper limit may be 100 mΩ, 50 mΩ, 30 mΩ or 20 mΩ.
[0076]
　Although the present invention is configured as described above, the practicability and effects of the present invention will be further described below using examples.
Example 1
[0077]
　Using a servo pressure type inverter DC spot welder equipped with DR type electrodes (chromium copper) having a plurality of kinds of electrode tip diameters, except for the processing number 24 described later, strength (tensile strength) of a plate thickness of 2.0 mm. ) Is a 1500 MPa class GA-plated hot-stamped steel sheet (amount of coating before hot-stamping: 55 g/m 2 per side , heating condition: furnace heating at 900° C. for 4 minutes), and two resistance spot welding tests are performed. Then, the proper current range was measured. However, a part of the same test was performed by stacking two non-hot stamp steel plates. All energization was performed under the condition of Ia(t)