JP2023143113A - Manufacturing method for projection weld member - Google Patents

Abstract

To provide a manufacturing method for a projection weld member that can suppress low-temperature cracking.SOLUTION: In a manufacturing method for a projection weld member, which welds a non-plated steel plate or a galvanized steel plate whose tensile strength is over 1.60 GPa and whose plate thickness is 1.8 mm or more and less than 2.3 mm to a member having a protrusion part, a power distribution time is set to 120 msec or more in a main power distribution step and a holding time is set to 280 msec in a holding step. In a manufacturing method for a projection weld member according to another embodiment of the invention, which welds a non-plated steel plate or a galvanized steel plate whose tensile strength is over 1.60 GPa and whose plate thickness is 2.3 mm or more and less than 3.3 mm to a member having a protrusion part, a power distribution time is set to 120 msec or more in a main power distribution step, and a power distribution time is set to 80 msec or more in a second power distribution step, and a holding time is set to 400 msec in a holding step, where a wireless power distribution time between the main power distribution step and the second power distribution step is set to 160 msec or less.SELECTED DRAWING: Figure 2

Description

The present invention relates to a method of manufacturing a projection welding member.

In recent years, hot stamping materials have been increasingly used in the automotive technology field. For example, hot-stamped materials made by hot-pressing high-strength steel plates are often used as frame members for automobile bodies. For example, in automotive structural members such as front side members, center pillars, hinge reinforcements, etc., steel members are used in which nuts and bolts are welded to parts made of hot stamping material. Further, hot stamping materials and other high strength steel sheets (high tensile strength steel sheets, high tensile strength steel sheets) may be non-plated steel sheets or zinc-based plated steel sheets.

For example, projection welding is used to weld members such as nuts and bolts to steel plates. In projection welding of high-strength steel plates, for example, steel plates with a tensile strength of 1.60 GPa or more, there is a problem in that low-temperature cracking is likely to occur. Cold cracking is a general term for cracks that occur after the temperature of the welded part drops to around room temperature after welding. The main causes of cold cracking include hydrogen introduced into the weld, confinement stress and residual stress in the weld. High-strength steel plates are highly sensitive to hydrogen, and large residual stress is generated when they are welded. In recent years, as the strength of automotive steel materials has improved, the need to suppress low-temperature cracking in projection welding has further increased.

As an example of a projection welding method, Patent Document 1 discloses that a pierced hole is provided in a high-strength steel plate with a tensile strength of 1100 MPa or more before welding, and that the center of the pierced hole and the center of the threaded part of a welding nut or welding bolt are aligned. A structural member for an automobile obtained by joining the high-strength steel plate and the welding nut or welding bolt by projection welding in which electrical heating is performed while applying pressure in a substantially coincident state, the welding nut or welding bolt has a flange portion whose lower surface side is a joint surface with the high-strength steel plate, and a substantially hemispherical projection portion is provided on the joint surface, and further, in a longitudinal section of the flange portion, the projection portion When the center on the chord of the semicircle formed by the intersection of the substantially hemispherical arc and the joint surface is C, and the radius of the projection part is R (mm), a distance of 3R from the center C. has a recess within a range of, and the recess is locally provided on an upper surface of the flange portion opposite to the joint surface so as to approximately coincide with the projection portion at a position corresponding to the projection portion; Further, an automobile structure having excellent delayed fracture characteristics and static strength characteristics of a welded portion, wherein the total volume of the recessed portion is in a range of 0.7 to 1.3 times the total volume of the projection portion. A member is disclosed.

Patent Document 2 discloses that a pierced hole is provided in a high-strength steel plate having a tensile strength of 1100 MPa or more before welding, and the high-strength steel plate is and the welding nut are joined by projection welding in which the welding nut is electrically heated while being pressurized, the welding nut having a welding nut portion, the welding nut being connected to the high-strength steel plate. A substantially hemispherical projection part is provided on the joint surface, and the depth H1 in the plate thickness direction of the weld heat-affected zone made to appear using the metal flow corrosive liquid in the high-strength steel plate and the high-strength steel plate are A structural member for an automobile having a welded nut portion is disclosed, which is characterized in that the relationship with plate thickness H2 satisfies the following formula {H1/H2=0.05 to 0.5}.

Patent Document 3 describes a nut having a predetermined component composition, tensile strength: 750 to 1600 MPa, plate thickness: 0.8 to 3.0 mm, and carbon equivalent Ceq: in the range of 0.22 to 0.50%. When projection welding a high-strength steel plate, immediately after main energization is performed with an electrode pressing force EF and energization time Wt, post-energization is performed with a post-energization current POC1 and a post-energization time POt1, and then a post-energization is performed with an electrode holding time Ht. By holding it at A method of controlling the Vickers hardness of the joint portion and the heat affected zone so that the maximum value of the Vickers hardness of the joint portion and the heat affected zone is 550 Hv or less is disclosed.

Patent Document 4 describes a nut (or bolt) having a predetermined component composition, tensile strength: 750 to 1600 MPa, plate thickness: 0.8 to 3.0 mm, and the following formula {[C]+[Si]/30+ [Mn]/20+2[P]+4[S]} A high-strength steel plate with a carbon equivalent Ceq in the range of 0.22 to 0.50% is projection welded to the nut (or bolt). The ratio of the area SJ of the joint with the high-strength steel plate to the area SR of the nominal diameter portion of the nut (or bolt) satisfies the relationship expressed by the following formula {0.7≦SJ/SR≦1.5}. , and a projection welded joint in which the maximum value of Vickers hardness of the joint portion and the heat affected zone is 550 Hv or less is disclosed.

Patent Document 5 discloses a projection welding method that satisfies the following requirements. A first workpiece made of steel and a second workpiece made of steel having a large number of protrusions are prepared. At least one of the first workpiece and the second workpiece is pressurized so that the protrusion of the second workpiece 2 is pressed against the plate-shaped portion of the first workpiece. While performing a pressurizing operation, a first energization operation is carried out to energize under the conditions of a predetermined welding current and energization time, and then a second energization operation is carried out to energize under the conditions of a predetermined welding current and energization time. . The welding current in the first energization operation is smaller than the welding current in the second energization operation, and the energization time in the first energization operation is set smaller than the energization time in the second energization operation.

However, none of these techniques has considered suppressing cold cracking in high-strength steel plates having a tensile strength of more than 1.60 GPa, nor has any specific means been provided for this purpose.

Patent No. 5626025 Patent No. 5613521 Japanese Patent Application Publication No. 2013-078784 Japanese Patent Application Publication No. 2012-157900 Japanese Patent Application Publication No. 2004-050280

In projection welding of a steel member and, for example, a high-strength steel plate having a tensile strength of over 1.60 GPa, cold cracking occurs in the hard part on the steel plate side of the weld due to rapid shrinkage of the weld that occurs immediately after welding. When the amount of heat input is large or when the thickness of the steel plate is large, the strain generated in the welded part becomes large, so cold cracking is particularly likely to occur.

The present invention provides a manufacturing method that can suppress low-temperature cracking when manufacturing projection welded parts made of unplated steel sheets or zinc-plated steel sheets with a tensile strength of over 1.60 GPa and members such as bolts and nuts. The challenge is to provide the following.

The gist of the invention is as follows.

(1) A method for manufacturing a projection welded member according to one aspect of the present invention is a method for manufacturing a projection welded member having a tensile strength of more than 1.60 GPa, a plate thickness of 1.8 mm or more and less than 2.3 mm, and a non-plated steel plate or a zinc-based plated steel plate. A method for manufacturing a projection welding member, in which a steel plate, which is a steel plate, and a member having a protrusion are joined by projection welding, wherein the protrusion of the member and the steel plate are in contact with each other, and the member and the member are bonded together by projection welding. A main energization process in which the steel plate is energized while being pressurized to weld the protrusion and the steel plate, and after the main energization process, the steel plate and the member are a holding step of maintaining the pressurization, the energization time in the main energization step is 120 msec or more, and the holding time in the holding step is 280 msec or less.
(2) In the method for manufacturing a projection welded member according to (1) above, when the parameter X is the sum of the tensile strength in units of GPa and the plate thickness in units of mm of the steel plate, The energization time in the main energization step may be 26×X or more in msec, and the holding time in the holding step may be 1520/X or less in msec.
(3) The method for manufacturing a projection welded member according to (1) or (2) above includes, after the main energization step and before the holding step, while maintaining the pressurization of the steel plate and the member. , comprising a second energization step of energizing the member and the steel plate with a smaller heat input than the main energization step, and the non-energization time between the main energization step and the second energization step is 160 msec or less. The energization time in the second energization step may be 80 msec or more, and the upper limit of the holding time in the holding step may be 400 msec instead of 280 msec.
(4) In the method for manufacturing a projection welding member according to (3) above, between the second energization step and the holding step, while maintaining the pressurization of the steel plate and the member, the second a third energizing step of energizing the member and the steel plate with a smaller heat input than the energizing step; the non-energizing time between the second energizing step and the third energizing step is 160 msec or less; The energization time in the third energization step may be 80 msec or more, and the upper limit of the holding time in the holding step may be 600 msec instead of 400 msec.

(5) A method for manufacturing a projection welded member according to another aspect of the present invention is a projection welded member having a tensile strength of more than 1.60 GPa, a plate thickness of 2.3 mm or more and less than 3.3 mm, and a non-plated steel plate or a zinc-based A method for manufacturing a projection welding member, in which a steel plate, which is a plated steel plate, and a member having a protrusion are joined by projection welding, the method comprising: joining the member and the member with the protrusion of the member in contact with the steel plate; a main energization step in which the steel plate is energized while being pressurized to weld the protrusion and the steel plate; and after the main energization step, the main energization step is performed while maintaining the pressurization of the steel plate and the member. A second energization step of energizing the member and the steel plate with a smaller heat input than a holding step of holding the pressure, the energizing time in the main energizing step is 120 msec or more, the energizing time in the second energizing step is 80 msec or more, the holding time in the holding step is 400 msec or less, and the main energizing step is The non-energizing time between the step and the second energizing step is 160 msec or less.
(6) In the method for manufacturing a projection welding member according to (5) above, between the second energization step and the holding step, while maintaining the pressurization of the steel plate and the member, the second a third energizing step of energizing the member and the steel plate with a smaller heat input than the energizing step; the non-energizing time between the second energizing step and the third energizing step is 160 msec or less; The energization time in the third energization step may be 80 msec or more, and the upper limit of the holding time in the holding step may be 600 msec instead of 400 msec.
(7) In the method for manufacturing a projection welded member described in (5) or (6) above, the parameter X is the sum of the tensile strength in GPa and the plate thickness in mm of the steel plate. In this case, the energization time in the main energization step may be 26×X or more in msec.

(8) In the method for manufacturing a projection welded member according to any one of (1) to (7) above, the steel plate has C: 0.07 to 0.45%, Si: 0. 001 to 2.50%, Mn: 0.8 to 5.0%, P: 0.03% or less, S: 0.01% or less, and the remainder is Fe and impurities, and the following formula (A) The carbon equivalent Ceq of the steel sheet, expressed by , may be 0.20% by mass to 0.55% by mass.
Ceq=[C]+[Si]/30+[Mn]/20+2[P]+4[S]...(A)
Here, the element symbols included in the above formula (A) are the contents of the elements in unit mass %.
(9) In the method for manufacturing a projection welded member according to any one of (1) to (8) above, in the main energization step, the welding current value in kA and the energization time in msec are The product may be set to 3300 msec·kA or less.
(10) The projection welding member according to any one of the above (3) to (7) and any one of the above (8) and (9) that is subordinate to any one of the above (3) to (7). In the manufacturing method, the current value I1 in the main energization step and the current value I2 in the second energization step may satisfy the relationship expressed by the following formula (B).
0.2≦I2/I1≦0.8…(B)
(11) In the method for manufacturing a projection welded member according to any one of the above (4) and (6) and the above (7) to (10) that are subordinate to the above (4) or (6), the method includes: The current value I1 in the main energization step and the current value I3 in the third energization step may satisfy the relationship expressed by the following formula (C).
0.2≦I3/I1≦0.8…(C)
(12) In the method for manufacturing a projection welded member according to any one of (1) to (11) above, the steel plate has zinc plating on the surface, and the method for manufacturing the projection welded member includes: Before the main energization step, there may be a preliminary energization step in which a current smaller than that in the main energization step is applied to the member and the steel plate while maintaining the pressurization of the steel plate and the member.

According to the present invention, it is possible to suppress low-temperature cracking when producing a projection welded member composed of an uncoated steel sheet or a zinc-based plated steel sheet with a tensile strength of more than 1.60 GPa and members such as bolts and nuts. A manufacturing method can be provided.

It is a schematic diagram of a projection welding member. It is an example of a current profile and a pressurizing force profile in the manufacturing method which has a main energization process and a holding process. It is an example of a current profile and a pressurizing force profile in the manufacturing method which has a main energization process, a second energization process, and a holding process. It is an example of a current profile and a pressurizing force profile in the manufacturing method which has a main energization process, a second energization process, a third energization process, and a holding process.

The present inventors have discovered that cold cracking can be suppressed by optimizing the welding current application time and holding time in projection welding. Hereinafter, an example of the method for manufacturing a projection welding member according to the present invention will be described in detail.

In the method for manufacturing a projection welded member 1 according to the first aspect of the present invention, as illustrated in FIG. , and the steel plate 11, which is a non-plated steel plate or a zinc-based plated steel plate, and the member 12 having the protrusion 121 are joined by projection welding. Projection welding is a type of resistance welding in which a protrusion formed in a base metal is brought into contact with a welding point and an electric current is passed through the weld to limit the generation of resistance heat to a relatively small, specific part. As illustrated in FIG. 2, projection welding is carried out by applying current while applying pressure to the member 12 and the steel plate 11 in a state where the protrusion 121 of the member 12 and the steel plate 11 are in contact with each other, thereby bringing the protrusion 121 and the steel plate 11 together. a main energization step S1-1 for welding, and a holding step S2 for maintaining the pressurization of the steel plate 11 and the member 12 with the current applied to the member 12 and the steel plate 11 stopped after the main energization step S1-1. have
In this specification, maintaining the pressurization in the holding step S2 is not limited to maintaining the pressurizing force P2 in the holding step S2 as the pressurizing force P1-1 at the end of the main energization step S1-1, For example, the pressing force P2 may vary during the holding step S2. Further, for example, the pressing force P2 in the holding step S2 can be set to 0.8 to 1.2 times the pressing force P1-1 at the end of the main energization step S1-1.

The tensile strength of the steel plate 11 is greater than 1.60 GPa (1600 MPa). Thereby, the method for manufacturing the projection welding member 1 according to the present embodiment can be applied to mechanical parts that require high strength. Note that, although low-temperature cracking is a problem in projection welding of the steel plate 11 with a pressure exceeding 1.60 GPa, in the manufacturing method according to the present embodiment, low-temperature cracking can be avoided by optimizing welding conditions, which will be described later.

The thickness of the steel plate 11 is 1.8 mm or more and less than 2.3 mm. By setting the plate thickness to 1.8 mm or more, the strength of the steel plate 11 is increased, and the method for manufacturing the projection welded member 1 according to the present embodiment can be applied to mechanical parts that require high strength. Further, the load on the joint during cooling shrinkage after welding is not so great if the plate thickness is thin, but it becomes significant when the plate thickness is 1.8 mm or more, which makes the present invention useful.
On the other hand, the greater the thickness of the steel plate 11, the greater the heat loss from the weld to the steel plate 11, the greater the strain produced in the weld, and the greater the residual stress in the weld. Therefore, in the method for manufacturing the projection welded member 1 according to the first embodiment, the thickness of the steel plate 11 is defined as less than 2.3 mm. A method of projection welding the steel plate 11 having a thickness of 2.3 mm or more will be described later.

The steel plate 11 is a non-plated steel plate or a zinc-based plated steel plate. Other configurations of the steel plate 11 are not particularly limited as long as the tensile strength and thickness are within the above ranges. The zinc-based plating includes, for example, hot-dip galvanizing, alloyed hot-dip galvanizing, and the like. The shape of the steel plate 11 is also not particularly specified. For example, the steel plate 11 may be a press-molded steel part, in particular a hot-pressed hot-stamped material.

The member 12 to be projection welded to the steel plate 11 has a protrusion 121 for projection welding. Thereby, the resistance heat generated by the welding current is limited to the protrusion 121 and its surroundings, and resistance welding can be performed efficiently. The members 12 are, for example, bolts and nuts.

The configuration of the member 12 is not particularly limited as long as it has a shape suitable for projection welding. According to the study results of the present inventors, the crack C occurred in the region on the steel plate 11 side of the welded portion, as schematically shown in FIG. It was determined that the strength and shape of the member 12 have little influence on cold cracking. Therefore, various configurations can be adopted for the member 12 depending on the use of the projection welding member 1.

The method for manufacturing a projection welding member according to the first embodiment includes a main energization step S1-1 and a holding step S2, as illustrated in FIG. The main energization step S1-1 is a step in which a welding current is applied while pressing the steel plate and the member against each other and applying pressure. Welding current is a current that is applied to form a weld. Post-heating current for heat-treating the welded part is not included in the concept of welding current. The holding step S2 is a step in which the pressing force on the steel plate and the member is maintained in a state where the current value flowing through the steel plate and the member is substantially zero. In addition, due to the power supply capacity of the projection welding device, even if the current value is controlled to be reduced to 0, it may take several cycles for the current value actually flowing through the steel plate and components to decrease to 0. There is. In the method for manufacturing a projection welding member according to the present embodiment, a state where the current value has decreased to a value close to 0 is considered to be a state where the current value is substantially 0.

The energization time in the main energization step S1-1 is 120 msec or more (6 cycles or more when the power frequency of the welding device is 50 Hz). The current application time is the length of time that the welding current passes through the member and the steel plate. By setting the energization time to 120 msec or more, the surface of the steel plate that is in contact with the member (hereinafter referred to as the "first surface") and the surface opposite to the first surface (hereinafter referred to as the "second surface") are separated. The temperature difference between the two surfaces can be reduced. By reducing the temperature difference between the first surface and the second surface of the steel plate, cooling shrinkage of the welded portion after welding is completed can be suppressed, and residual stress in the welded portion can be reduced. The energization time in the main energization step S1-1 is preferably 130 msec or more, 140 msec or more, or 150 msec. As long as this requirement is met, the energization conditions in the main energization step S1-1 can be appropriately selected depending on the shape, material, etc. of the steel plate and member to be welded. The energization time in the main energization step S1-1 is, for example, 140 to 280 msec.

The holding time in the holding step S2 is 280 msec or less (14 cycles or less when the power frequency of the welding device is 50 Hz). The holding time is the length of time from when the welding current in the main energization step S1-1 ends to when the electrode starts to open. If the holding time exceeds 280 msec, the welded part will be supercooled, and cooling shrinkage of the welded part after welding is completed will become significant. On the other hand, by setting the holding time to 280 msec or less, the welded part is slowly cooled and residual stress in the welded part can be reduced. The holding time in the holding step S2 is preferably 260 msec or less, 240 msec or less, or 200 msec or less. The holding time in the holding step S2 is, for example, 160 to 240 msec.

In addition, from the viewpoint of avoiding cold cracking, the shorter the holding time is, the more preferable it is, and it may be 0 msec. That is, the pressing force may be set to 0 at the same time as the welding current supply ends. A method for manufacturing a projection welded member in which the holding time is 0 msec is also considered as a method for manufacturing a projection welded member according to the present embodiment. On the other hand, considering the capability of the projection welding device, the longer the holding time, the easier the pressurization control, which is preferable. Therefore, the holding time may be set to 10 msec or more, 20 msec or more, or 40 msec or more.

The energization time in the main energization step S1-1 and the holding time in the holding step S2 may be further limited depending on the tensile strength and thickness of the steel plate. For example, when the parameter X is the sum of the tensile strength in GPa and the plate thickness in mm, the energization time is 26 x X or more in msec, and the holding time is 1520/ It may be less than or equal to X.

As described above, the greater the tensile strength of the steel plate, the greater the residual stress in the weld, and the greater the thickness of the steel plate, the greater the residual stress in the weld. Therefore, the present inventors used the parameter X, which is the sum of the tensile strength and thickness of the steel plate, as a simple indicator of the residual stress in the welded part. When following the above formula, the larger the parameter X is, the larger the lower limit value of the current application time is, and the smaller the upper limit value of the holding time is. Therefore, when the above formula is followed, the residual stress in the weld is further relaxed. The energization time is more preferably 28×X or more, or 42×X or more in msec. The retention time is more preferably 1440/X or less in msec.

As illustrated in FIG. 3, the method for manufacturing a projection welded member according to the first embodiment includes, after the main energization step S1-1 and before the holding step S2, while maintaining the pressurization of the steel plate and the member. There may be a second energization step S1-2 in which the member and the steel plate are energized with a smaller heat input than the main energization step S1-1. By providing the second energization step S1-2, the cooling rate of the welded portion can be slowed down and low-temperature cracking can be further suppressed. Further, as will be described later, by providing the second energization step S1-2, the upper limit value of the holding time can be extended.
Here, maintaining the pressure in the second energization step S1-2 means that the pressure P1-2 in the second energization step S1-2 remains the same as the pressure P1-1 at the end of the main energization step S1-1. For example, the pressing force P1-2 may be varied during the second energization step S1-2. Further, for example, the pressurizing force P1-2 in the second energizing step S1-2 can be set to 0.8 to 1.2 times the pressurizing force P1-1 at the end of the main energizing step S1-1.
Furthermore, in this specification, heat input refers to a time-integrated value of current, and under conditions where the current is constant, it refers to "heat input = current x time". In the second energization process S1-2, energization with a smaller heat input than in the main energization process S1-1 means that "I1×t1>I2×t2" under the condition that the current is constant. Here, I1, t1 are the current value and energization time of the main energization step S1-1, and I2, t2 are the current value and energization time of the second energization step S1-2.
When the current value changes during energization due to upslope energization or downslope energization, heat input is calculated as the area of a triangle (or the sum of the areas of a triangle and a square) in a current profile where the horizontal axis is time and the vertical axis is current value. ). When the current value changes stepwise, the heat input is represented by the sum of the areas of a plurality of rectangles in a current profile in which the horizontal axis represents time and the vertical axis represents current value.

When the method for manufacturing a projection welding member includes a second energizing step S1-2, the non-energizing time (so-called cool time) between the main energizing step S1-1 and the second energizing step S1-2 is 160 msec or less. It is preferable. The non-energizing time is the length of time during which the current value is substantially zero. By setting the non-energizing time to 160 msec or less, the welded portion is slowly cooled, and the residual stress in the welded portion can be further reduced. The shorter the non-energizing time is, the more preferable it is. Therefore, the lower limit of the non-energizing time is 0 msec.

Further, when the method for manufacturing a projection welding member includes a second energization step S1-2, the energization time in the second energization step S1-2 is preferably 80 msec or more. The longer the energization time in the second energization step S1-2, the more the cooling shrinkage of the weld can be suppressed and the residual stress in the weld can be further reduced.

When the method for manufacturing a projection welding member includes the second energization step S1-2 in accordance with the above-described non-energization time and energization time regulations, the holding time may be 400 msec or less. As described above, in the absence of the second energization process, a holding time exceeding 280 msec may cause cold cracking due to overcooling. However, when the second energization process is performed according to predetermined conditions, the temperature of the welded part has already decreased by the time the holding process is started, and the risk of cold cracking due to overcooling is reduced. Therefore, when performing the second energization step according to predetermined conditions, the upper limit of the holding time can be extended to 400 msec instead of the above-mentioned 280 msec.

As illustrated in FIG. 4, the method for manufacturing a projection welding member according to the first embodiment includes, between the second energization step S1-2 and the holding step S2, while maintaining the pressurization of the steel plate and the member. There may be a third energization step S1-3 in which the member and the steel plate are energized with a smaller heat input than the second energization step. By providing the third energization step S1-3 in addition to the second energization step S1-2, it is possible to further slow down the cooling rate of the welded portion and further suppress cold cracking. Further, as will be described later, by providing the third energization step S1-3, the upper limit value of the holding time can be further extended.
Here, maintaining the pressurization in the third energization process S1-3 means that the pressurization force P1-3 in the third energization process S1-3 is equal to the pressurization force P1-2 at the end of the second energization process S1-2. For example, the pressing force P1-3 may be varied during the third energization step S1-3. Further, for example, the pressurizing force P1-3 in the third energizing step S1-3 can be set to 0.8 to 1.2 times the pressurizing force P1-2 at the end of the second energizing step S1-2.
In addition, in the third energization process S1-3, energizing with a smaller heat input than in the second energization process S1-2 means that "I2×t2>I3×t3" under a constant current condition. . Here, I2, t2 are the current value and energization time of the second energization step S1-2, and I3, t3 are the current value and energization time of the third energization step S1-3. Note that the definition of the amount of heat input and the handling when the current value changes during energization are as described above.

When the method for manufacturing a projection welding member includes the third energization step S1-3, the non-energization time between the second energization step S1-2 and the third energization step S1-3 is preferably 160 msec or less. The non-energizing time is the length of time during which the current value is substantially zero. By setting the non-energizing time to 160 msec or less, the welded portion is slowly cooled, and the residual stress in the welded portion can be further reduced. The shorter the non-energizing time is, the more preferable it is. Therefore, the lower limit of the non-energizing time is 0 msec.

Further, when the method for manufacturing a projection welding member includes a third energization step S1-3, the energization time in the third energization step S1-3 is preferably 80 msec or more. The longer the energization time in the third energization step S1-3, the more the cooling shrinkage of the weld can be suppressed and the residual stress in the weld can be further reduced.

When the method for manufacturing a projection welding member includes the third energization step S1-3 according to the above-mentioned regulations, the holding time may be 600 msec or less. When the third energization step S1-3 is carried out according to the predetermined conditions, the temperature of the welded part is further reduced at the time the holding step is started, and the risk of cold cracking due to overcooling is further reduced. Therefore, when performing the third energization step S1-3 according to predetermined conditions, the upper limit of the holding time can be extended to 600 msec instead of the above-mentioned 400 msec.

Next, a method for manufacturing a projection welding member according to a second aspect of the present invention will be described. The method of manufacturing a projection welding member according to the second embodiment targets a steel plate that is thicker than that of the first embodiment. Specifically, the method for manufacturing a projection welded member according to the second embodiment is a method for manufacturing a projection welded member having a tensile strength of more than 1.60 GPa, a plate thickness of 2.3 mm or more and less than 3.3 mm, and a non-plated steel plate or a zinc-based steel plate. A plated steel plate and a member having a projection are joined by projection welding. As shown in Fig. 3, this projection welding includes a main energization process in which the protrusion of the member and the steel plate are in contact with each other, and electricity is applied while applying pressure to the member and the steel plate to weld the protrusion and the steel plate. , a second energization process in which the steel plate and the steel plate are energized with a smaller heat input than the main energization process while maintaining pressure on the steel plate and the member; and a state in which the energization to the member and the steel plate is stopped after the second energization process. and a holding step of holding the steel plate and the member under pressure.

The tensile strength of the steel plate is greater than 1.60 GPa, similar to the first embodiment. On the other hand, the thickness of the steel plate is 2.3 mm or more and less than 3.3 mm. By setting the plate thickness to 2.3 mm or more, the strength of the steel plate can be further increased. Furthermore, the present invention is useful because the load on the joint during cooling shrinkage after welding becomes particularly significant when the plate thickness is 2.3 mm or more.
On the other hand, in order to suppress residual stress in the welded portion, the thickness of the steel plate is specified to be less than 3.3 mm. Other configurations of the steel sheet are not particularly limited as long as the tensile strength and thickness are within the above-mentioned ranges and the steel sheet is a non-plated steel sheet or a zinc-based plated steel sheet. The various configurations illustrated in the first embodiment can also be applied to the steel plate in the second embodiment. The members to be projection welded to the steel plate can also be the same as those in the first embodiment.

As illustrated in FIG. 3, the method for manufacturing a projection welding member according to the second embodiment includes a main energization step S1-1, a second energization step S1-2, and a holding step S2. The main energization step S1-1 is a step in which the protrusion of the member and the steel plate are in contact with each other, and electricity is applied while pressurizing the member and the steel plate to weld the protrusion and the steel plate. The second energization step S1-2 is a step in which, after the main energization step S1-1, the member and the steel plate are energized with a smaller heat input than in the main energization step while maintaining pressure on the steel plate and the member. The holding step S2 is a step of holding the steel plate and the member under pressure after the second energization step S1-2 with the current applied to the member and the steel plate being stopped. In addition, due to the power supply capacity of the projection welding device, even if the current value is controlled to be reduced to 0, it may take several cycles for the current value actually flowing through the steel plate and components to decrease to 0. There is. In the method for manufacturing a projection welding member according to the present embodiment, a state where the current value has decreased to a value close to 0 is considered to be a state where the current value is substantially 0.

The main energization step S1-1 in the second embodiment is the same as that in the first embodiment. That is, the energization time in the main energization step S1-1 is set to 120 msec or more. The energization time in the main energization step S1-1 is preferably 130 msec or more, 140 msec or more, or 150 msec. The current value in the main energization step S1-1 is not particularly limited, and can be appropriately selected depending on the shape and use of the projection welding member. On the other hand, in the main energization process S1-1, the product of the welding current value in kA and the energization time in msec may be defined as 3300 msec·kA or less.

The energization time in the second energization step S1-2 is 80 msec or more. The longer the energization time in the second energization step S1-2, the more the cooling shrinkage of the weld can be suppressed and the residual stress in the weld can be further reduced.

The non-energizing time between the main energizing step S1-1 and the second energizing step S1-2 is 160 msec or less. The non-energizing time is the length of time during which the current value is substantially zero. By setting the non-energizing time to 160 msec or less, the welded portion is slowly cooled, and the residual stress in the welded portion can be further reduced.

The holding time in the holding step S2 is 400 msec or less. The holding time is the length of time during which the current value flowing through the steel plate and the member is substantially 0 and the pressing force on the steel plate and the member is greater than 0. If the holding time exceeds 400 msec, the welded part will be supercooled, and cooling shrinkage of the welded part after welding is completed will become significant. On the other hand, by setting the holding time to 400 msec or less, the welded part is slowly cooled and residual stress in the welded part can be reduced. The holding time in the holding step S2 is preferably 360 msec or less, 300 msec or less, or 200 msec or less.

As illustrated in FIG. 4, the method for manufacturing a projection welding member according to the second embodiment includes, between the second energization step S1-2 and the holding step S2, while maintaining the pressurization of the steel plate and the member. There may be a third energization step S1-3 in which the member and the steel plate are energized with a smaller heat input than the second energization step S1-2. By providing the third energization step S1-3, the cooling rate of the welded portion can be made more gradual, and low-temperature cracking can be further suppressed. Further, as will be described later, by providing the third energization step S1-3, the upper limit value of the holding time can be extended.

When the method for manufacturing a projection welding member includes the third energization step S1-3, the non-energization time between the second energization step S1-2 and the third energization step S1-3 is preferably 160 msec or less. By setting the non-energizing time to 160 msec or less, the welded portion is slowly cooled, and the residual stress in the welded portion can be further reduced.

Further, when the method for manufacturing a projection welding member includes a third energization step S1-3, the energization time in the third energization step S1-3 is preferably 80 msec or more. The longer the energization time in the third energization step S1-3, the more the cooling shrinkage of the weld can be suppressed and the residual stress in the weld can be further reduced.

When the method for manufacturing a projection welding member includes the third energization step S1-3 in accordance with the above-described non-energization time and energization time regulations, the holding time may be 600 msec or less. When the third energization step S1-3 is carried out according to the predetermined conditions, the temperature of the welded part is further reduced at the time the holding step is started, and the risk of cold cracking due to overcooling is further reduced. Therefore, when performing the third energization step S1-3 according to predetermined conditions, the upper limit of the holding time can be extended to 600 msec instead of the above-mentioned 400 msec.

The energization time and holding time in the main energization step S1-1 may be further limited depending on the tensile strength and thickness of the steel plate. For example, when the parameter X is the sum of the tensile strength in units of GPa and the plate thickness in units of mm, the energization time may be set to 26×X or more in units of msec.

As described above, the greater the tensile strength of the steel plate, the greater the residual stress in the weld, and the greater the thickness of the steel plate, the greater the residual stress in the weld. Therefore, the present inventors used the parameter X, which is the sum of the tensile strength and thickness of the steel plate, as a simple indicator of the residual stress in the welded part. When following the above formula, the larger the parameter X is, the larger the lower limit value of the current application time is, and the smaller the upper limit value of the holding time is. Therefore, when the above formula is followed, the residual stress in the weld is further relaxed.

A first embodiment regarding a steel plate that is a non-plated steel plate or a zinc-based plated steel plate with a plate thickness of 1.8 mm or more and less than 2.3 mm, and a non-plated steel plate or a zinc-based plated steel plate with a plate thickness of 2.3 mm or more and less than 3.3 mm. Each of the second embodiments regarding the steel plate, which is a steel plate, has been described above. Preferred configurations applicable to both of these embodiments will be further described below.

Examples of suitable chemical components of steel sheets include, in mass %, C: 0.07 to 0.45%, Si: 0.001 to 2.50%, Mn: 0.8 to 5.0%, P: 0 .03% or less, S: 0.01% or less, the remainder is Fe and impurities, and the carbon equivalent Ceq of the steel plate expressed by the following formula (A) is 0.20% by mass to 0.55% by mass. %.
Ceq=[C]+[Si]/30+[Mn]/20+2[P]+4[S]...(A)
Here, the element symbols included in formula (A) are the content of the elements in unit mass %.
In general, increasing the content of C and alloying elements can increase the strength of the base metal, but at the same time Ceq increases, which reduces the toughness of the weld and is likely to be one of the causes of cold cracking. Steel plates having such components are used as materials for mechanical parts that require high strength, such as automobile parts, and the present invention is particularly useful. Moreover, according to the method for manufacturing a projection welded member according to the present embodiment, it is possible to suppress cold cracking in a member obtained from a steel plate having such components.

As mentioned above, the energization conditions in the main energization step S1-1, the second energization step S1-2, and the third energization step S1-3 are not particularly limited except for the energization time, and may vary depending on the shape and application of the projection welding member. A value can be selected as appropriate. On the other hand, the current value in each step may be determined as described below.

In the main energization process S1-1, the product of the welding current value in kA and the energization time in msec may be defined as 3300 msec·kA or less. The product of the welding current value and the energization time is an index of the amount of heat input in the main energization step S1-1. The smaller the amount of heat input in the main energization step S1-1, the smaller the temperature difference between the first and second surfaces of the steel plate. Thereby, cooling shrinkage of the welded portion after completion of welding can be further suppressed, and residual stress in the welded portion can be further reduced.

The current values in the second energization step S1-2 and the third energization step S1-3 may be values corresponding to the current values in the main energization step S1-1. Specifically, the current value I1 in the main energization step S1-1 and the current value I2 in the second energization step S1-2 may satisfy the relationship expressed by the following equation (B). Further, the current value I1 in the main energization step S1-1 and the current value I3 in the third energization step S1-3 may satisfy the relationship expressed by the following equation (C).
0.2≦I2/I1≦0.8…(B)
0.2≦I3/I1≦0.8…(C)

When I2/I1 is 0.2 or more and 0.8 or less, or when I3/I1 is 0.2 or more and 0.8 or less, the second energization step S1-2 or the third energization step S1-3 , it is possible to reliably exhibit the effect of slowing down the cooling rate of the welded part.

Pressure force in projection welding is not particularly limited. From the experimental results of the present inventors, no influence of pressurizing force on the incidence of cold cracking was confirmed. This is thought to be because the main cause of cold cracking is the strain introduced during cooling of the weld zone, but the pressurizing force does not affect this. Therefore, the pressing force (for example, 3 to 6 kN) in normal projection welding may be appropriately selected.

When the steel plate 11 is particularly a zinc-based plated steel plate (a steel plate plated with zinc), it is preferable to have a preliminary energization process S0 before the main energization process S1-1, and the preliminary energization process S0 and the main energization process It is more preferable that the non-energizing time between step S1-1 is 70 msec or more. The current value I0 of the preliminary energization step S0 is lower than the current value I1 of the main energization step S1-1, and can be set to, for example, 1/3 to 1/5 of the current value I1 of the main energization step S1-1. The energization time of the preliminary energization step S0 is preferably 40 msec or more.

In projection welding of zinc-coated steel sheets, the uneven presence of an oxide film, which is a high-resistance substance, on the surface of the steel sheets tends to cause variations in joint strength.This variation occurs when hot-pressing zinc-coated steel sheets. This is particularly noticeable. In the preliminary energization process S0, the oxide film on the steel plate 11 in contact with each protrusion 121 is destroyed and removed or thinned, so that the thickness of the oxide film in the area where the current flows from each protrusion 121 is made uniform, and each The resistance in the current path at the contact point of the protrusion 121 becomes almost uniform. As a result, in the main current supply step S1-1 for forming a joint, the density of the current in each protrusion 121 becomes uniform, and a uniform joint can be formed at each point.

Further, by setting the non-energizing time to 70 msec or more between the preliminary energizing process S0 and the main energizing process S1-1, the temperature of each protrusion 121 is made uniform, and the current flowing through the protruding part 121 is facilitated. It is also equalized. As a result, the current flowing through each protrusion 121 in the main energization step S1-1 becomes uniform, and variations in the bonding strength between the steel plate and the member can be reduced.

In order to make the current density more uniform in the main energization step S1-1, the height of all the protrusions 121 after the end of the preliminary energization step S0 should be 0.0% higher than the height before the start of the preliminary energization step S0. It is preferable to set it to 7 to 0.9 times. By appropriately applying heat and pressure in the preliminary energization step, the protrusion 121 can be deformed so as to be crushed, and the height of the protrusion 121 can be set within the above-mentioned range.

EXAMPLES The effects of one embodiment of the present invention will be explained in more detail with reference to Examples. However, the conditions in the examples are merely examples of conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to this one example condition. The present invention may adopt various conditions as long as the objectives of the present invention are achieved without departing from the gist of the present invention.

(Example 1)
Various projection welded members were manufactured using zinc-plated hot-stamped materials under the conditions described below.
- Tensile strength, plate thickness, and Ceq of steel plate: Listed in the "TS" column, "Plate thickness" column, and "Ceq" column in Table 1 - Shape of member: Nut with three projection welding protrusions・Electrification time in the main energization process: Listed in the “Electrification time” column in Table 1 ・Current value I1 in the main energization process: Listed in the “Current value” column in Table 1 ・Press force in the main energization process: 5.0 kN
・Holding time in the holding step: Listed in the “Holding time” column in Table 1. ・Presence of second energization step and third energization step: Listed in the “Post-energization 1-stage” column and “Post-energization 2-stage” column in Table 1 ( In examples with a symbol “〇” in the “Post-energization 1-stage” column, only the second energization step is performed, and in examples with a symbol “〇” in the “Post-energization 2-stage” column, the second energization step and the (Three energization process carried out)
・No-energization time between main energization process and second energization process: 80msec
・Electrification time in second energization process: 120msec
・Current value I2 in the second energization step: 6kA
- No-energization time between second energization process and third energization process: 80msec
・Electrification time in the third energization process: 120msec
・Current value I3 in the third energization step: 6kA

The number of cracks in projection welded parts was also investigated. Three test pieces were prepared by projection welding under each of the conditions shown in Table 1. One test piece has three welds, so a total of nine welds are formed. It was confirmed whether or not cracks had occurred in these welded parts. The number of cracks is listed in the "as-welded" column of Table 1. Furthermore, the produced projection welding members were immersed in hydrochloric acid having the same pH for a predetermined time. In hydrochloric acid, hydrogen enters the joints etc., promoting delayed fracture. After immersion, the presence or absence of cracks in the projection welded member was checked again. The presence or absence of cracks can be determined by cutting each welded part along the radial direction from the center of the nut, embedding it in resin, polishing and corroding the cut surface, and observing the cut surface with an optical microscope. confirmed. The number of cracks was recorded in the "hydrochloric acid immersion" column of Table 1.

In Comparative Example 1 and Comparative Example 4, the thickness of the steel plate was 2.3 mm or more, but since the second energization step and the third energization step were not performed, cracking could not be suppressed.
In Comparative Example 2, the tensile strength of the steel plate was not over 1.60 GPa, so there was no problem of low-temperature cracking that is specific to high-strength steel plates, and it is outside the scope of the present invention. Although no cracking occurred in Comparative Example 2, the tensile strength of the steel plate was not sufficient, so it could not be used as a mechanical part requiring a tensile strength of more than 1.60 GPa.
In Comparative Example 3, the thickness of the steel plate was less than 2.3 mm, but the holding time was too long, so cracking after immersion in hydrochloric acid could not be suppressed.
In Comparative Example 5, the thickness of the steel plate was 2.3 mm or more, but the energization time was too short, so cracking could not be suppressed. In Comparative Example 5, unlike Comparative Examples 1 and 4, a second energization step was performed, but cracking could not be suppressed.

In Invention Example 1, the thickness of the steel plate was less than 2.3 mm, and the current application time and holding time were appropriate, so cracking could be suppressed.
In Invention Example 2, the thickness of the steel plate was less than 2.3 mm, the second energization step was performed, and the energization time and holding time were appropriate, so cracking could be suppressed. Inventive Example 2 had a larger plate thickness than Inventive Example 1, so although the number of cracks after immersion in hydrochloric acid was greater than Inventive Example 1, it was within the acceptable range.
In Invention Example 3, the thickness of the steel plate was 2.3 mm or more, but the second energization process and the third energization process were carried out, and the energization time and holding time were appropriate, so cracking could be suppressed. did it.
In Invention Example 4, the thickness of the steel plate was less than 2.3 mm, and the current application time and holding time were appropriate, so cracking could be suppressed. Inventive Example 4 had a larger plate thickness than Inventive Example 1, so although the number of cracks after immersion in hydrochloric acid was greater than Inventive Example 1, it was still within the acceptable range.
In Invention Example 5, the thickness of the steel plate was 2.3 mm or more, and the current application time and holding time were appropriate, so cracking could be suppressed. Inventive Example 5 had a larger plate thickness than Inventive Example 1, so although the number of cracks after immersion in hydrochloric acid was greater than Inventive Example 1, it was still within the acceptable range.
Although invention examples 6 to 8 had conditions corresponding to the upper and lower limits of claims 1, 3, and 4, respectively, the effect of suppressing cracking was confirmed.

(Example 2)
Manufacturing various projection welded parts by changing the pre-energization conditions for alloyed hot-dip galvanized steel sheets (GA-plated hot-stamped materials) with TS: 2.0 GPa, Ceq: 0.45%, and plate thickness: 2.0 mm. I did it.
- Energization time in the preliminary energization process: listed in the "Preliminary energization: energization time" column in Table 2 - Current value in the preliminary energization process: listed in the "Preliminary energization: current value" column in Table 2 - Preliminary energization process and main energization Non-energizing time between processes: listed in the "Cool time" column in Table 2 - Energizing time in the main energizing process: listed in the "Electrifying time" column in Table 2 - Current value I1 in the main energizing process: " Listed in the "Current value" column - Pressure force in preliminary energization process and main energization process: 5.0kN
- Holding time in the holding step: Listed in the "holding time" column of Table 2 The number of cracks in the projection welded parts was investigated using the same method as in Example 1 for the various projection welded parts produced.

In Invention Examples 2 to 5, the number of cracks after immersion in hydrochloric acid was reduced compared to Invention Example 1 by performing preliminary energization.

1 Projection welding member 11 Steel plate 12 Member 121 Projection S1-1 Main energization process S1-2 Second energization process S1-3 Third energization process S2 Holding process C Crack

Claims (12)

A steel plate having a tensile strength of more than 1.60 GPa, a plate thickness of 1.8 mm or more and less than 2.3 mm, and which is a non-plated steel plate or a zinc-based plated steel plate and a member having a protrusion are joined by projection welding. , a method for manufacturing a projection welding member,
a main energization step of welding the protrusion and the steel plate by applying electricity while pressurizing the member and the steel plate while the protrusion of the member is in contact with the steel plate;
After the main energization step, a holding step of maintaining the pressurization of the steel plate and the member while stopping the energization to the member and the steel plate,
The energization time in the main energization step is 120 msec or more,
A method for manufacturing a projection welding member, in which the holding time in the holding step is 280 msec or less. When the parameter X is the sum of the tensile strength in GPa and the plate thickness in mm of the steel plate,
The energization time in the main energization step is 26×X or more in msec,
2. The method of manufacturing a projection welding member according to claim 1, wherein the holding time in the holding step is set to 1520/X or less in msec. After the main energization step and before the holding step, a second step of energizing the member and the steel plate with a smaller heat input than in the main energization step while maintaining the pressurization of the steel plate and the member. Has an energizing process,
The non-energizing time between the main energizing step and the second energizing step is 160 msec or less,
The energization time in the second energization step is 80 msec or more,
3. The method of manufacturing a projection welded member according to claim 1, wherein the upper limit of the holding time in the holding step is set to 400 msec instead of 280 msec. Between the second energizing step and the holding step, a second step of energizing the member and the steel plate with a smaller heat input than in the second energizing step while maintaining the pressurization of the steel plate and the member. Has three electrification processes,
The non-energizing time between the second energizing step and the third energizing step is 160 msec or less,
The energization time in the third energization step is 80 msec or more,
4. The method of manufacturing a projection welded member according to claim 3, wherein the upper limit value of the holding time in the holding step is set to 600 msec instead of 400 msec. A steel plate having a tensile strength of more than 1.60 GPa, a plate thickness of 2.3 mm or more and less than 3.3 mm, and which is a non-plated steel plate or a zinc-based plated steel plate and a member having a protrusion are joined by projection welding. , a method for manufacturing a projection welding member,
a main energization step of welding the protrusion and the steel plate by applying electricity while pressurizing the member and the steel plate while the protrusion of the member is in contact with the steel plate;
After the main energization step, a second energization step of energizing the member and the steel plate with a smaller heat input than in the main energization step while maintaining the pressurization of the steel plate and the member;
After the second energization step, a holding step of maintaining the pressurization of the steel plate and the member while stopping the energization to the member and the steel plate,
The energization time in the main energization step is 120 msec or more,
The energization time in the second energization step is 80 msec or more,
The holding time in the holding step is 400 msec or less,
A method for manufacturing a projection welding member, in which a non-energizing time between the main energizing step and the second energizing step is 160 msec or less. Between the second energizing step and the holding step, a second step of energizing the member and the steel plate with a smaller heat input than in the second energizing step while maintaining the pressurization of the steel plate and the member. Has three electrification processes,
The non-energizing time between the second energizing step and the third energizing step is 160 msec or less,
The energization time in the third energization step is 80 msec or more,
6. The method of manufacturing a projection welded member according to claim 5, wherein the upper limit of the holding time in the holding step is set to 600 msec instead of 400 msec. When the parameter X is the sum of the tensile strength in GPa and the plate thickness in mm of the steel plate,
7. The method of manufacturing a projection welding member according to claim 5, wherein the energization time in the main energization step is set to 26×X or more in msec. The steel plate contains, in mass%, C: 0.07 to 0.45%, Si: 0.001 to 2.50%, Mn: 0.8 to 5.0%, P: 0.03% or less, S : Contains 0.01% or less, the remainder is Fe and impurities,
The projection welding member according to any one of claims 1 to 7, wherein the carbon equivalent Ceq of the steel plate expressed by the following formula (A) is 0.20% by mass to 0.55% by mass. manufacturing method.
Ceq=[C]+[Si]/30+[Mn]/20+2[P]+4[S]...(A)
Here, the element symbols included in the above formula (A) are the contents of the elements in unit mass %. The projection according to any one of claims 1 to 8, characterized in that, in the main energization step, the product of the welding current value in kA and the energization time in msec is 3300 msec·kA or less. A method for manufacturing welded parts. Claims 3 to 7, wherein the current value I1 in the main energization step and the current value I2 in the second energization step satisfy the relationship expressed by the following formula (B), and claims 3 to 7. A method for manufacturing a projection welding member according to any one of claims 8 and 9 depending on any one of claims 7 to 9.
0.2≦I2/I1≦0.8…(B) Claims 4 and 6, wherein the current value I1 in the main energization step and the current value I3 in the third energization step satisfy the relationship expressed by the following formula (C), and claim 4 or The method for manufacturing a projection welding member according to any one of claims 7 to 10 depending on claim 6.
0.2≦I3/I1≦0.8…(C) The steel plate has zinc plating on the surface,
Before the main energization step, the method includes a preliminary energization step in which a current smaller than that in the main energization step is applied to the member and the steel plate while maintaining the pressurization of the steel plate and the member. A method for manufacturing a projection welding member according to any one of claims 1 to 11.

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