JP7840752B2 - Manufacturing method for projection welded components - Google Patents

Description

This invention relates to a method for manufacturing projection welding members.

In recent years, the use of hot-stamped materials has been increasing in the automotive technology field. For example, hot-stamped materials, formed by hot-pressing high-strength steel sheets, are being used as structural components for automobile bodies. For instance, in automotive structural components such as front side members, center pillars, and hinge reinforcements, steel members made from hot-stamped materials with nuts and bolts welded to them are used. Furthermore, hot-stamped materials and other high-strength steel sheets (high-tensile steel sheets, hi-tensile steel) can be either unplated or zinc-plated.

When welding components such as nuts and bolts to steel plates, projection welding is often used. However, projection welding of high-strength steel plates, such as those with a tensile strength of 1.60 GPa or higher, presents a problem: cold cracking is likely to occur. Cold cracking is a general term for cracks that occur after welding, once the temperature of the weld has dropped to near room temperature. The main factors contributing to cold cracking include hydrogen introduced into the weld, as well as confinement stress and residual stress in the weld. High-strength steel plates are highly sensitive to hydrogen, and welding them generates significant residual stress. In recent years, with the increasing strength of automotive steel materials, the need to suppress cold cracking in projection welding has become even more critical.

As an example of a projection welding method, Patent Document 1 describes an automotive structural member obtained by projection welding, in which a piercing hole is made in a high-strength steel plate having a tensile strength of 1100 MPa or more before welding, and the high-strength steel plate and the welding nut or welding bolt are joined by applying pressure and heating with electric current while the center of the piercing hole and the center of the threaded portion of the welding nut or welding bolt are roughly aligned. The welding nut or welding bolt has a flange portion on its lower side which serves as the joining surface with the high-strength steel plate, and a substantially hemispherical projection portion is provided on the joining surface, and furthermore, the vertical cross section of the flange portion In this invention, a structural member for automobiles is disclosed, characterized in that, when the center of the semicircular chord formed by the intersection of the substantially hemispherical arc of the projection portion and the joint surface is C, and the radius of the projection portion is R (mm), a recess is provided within a distance of 3R from the center C, the recess is locally provided on the upper surface of the flange portion opposite to the joint surface, so as to roughly coincide with the position corresponding to the projection portion, and the total volume of the recess is in the range of 0.7 to 1.3 times the total volume of the projection portion, thereby exhibiting excellent delayed fracture characteristics and static strength characteristics of the welded portion.

Patent Document 2 discloses an automotive structural member having a welded nut portion, obtained by projection welding, in which a piercing hole is made in a high-strength steel plate having a tensile strength of 1100 MPa or more before welding, and the high-strength steel plate and the welded nut are joined by projection welding, in which the high-strength steel plate and the welded nut are heated under pressure. The welded nut has a substantially hemispherical projection portion on the joining surface with the high-strength steel plate, and the relationship between the depth H1 in the thickness direction of the heat-affected zone in the high-strength steel plate, revealed using a metal flow corrosion solution, and the thickness H2 of the high-strength steel plate satisfies the following equation {H1/H2 = 0.05 to 0.5}.

Patent Document 3 discloses a method for projection welding a nut having a predetermined component composition to a high-strength steel plate having a tensile strength of 750 to 1600 MPa, a plate thickness of 0.8 to 3.0 mm, and a carbon equivalent Ceq of 0.22 to 0.50%. This method involves performing main energization with an electrode pressure EF and energization time Wt, immediately followed by post-energization with a post-energization current POC1 and post-energization time POT1, and then holding the electrode for an electrode holding time Ht. This method controls the ratio of the joint area SJ between the nut and the high-strength steel plate to the area SR of the nominal diameter portion of the nut to satisfy the relationship expressed by the following formula {0.7 ≤ SJ/SR ≤ 1.5}, and ensures that the maximum Vickers hardness of the joint and heat-affected zone is 550 Hv or less.

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

Patent Document 5 discloses a projection welding method that satisfies the following requirements: A first steel workpiece and a second steel workpiece having numerous protrusions are prepared. At least one of the first and second workpieces is pressurized so that the protrusions of the second workpiece are pressed against the plate-like portion of the first workpiece. While performing the pressurizing operation, a first energizing operation is carried out, applying current at predetermined welding current and energizing time conditions. Subsequently, a second energizing operation is carried out, also applying current at predetermined welding current and energizing time conditions. The welding current in the first energizing operation is smaller than the welding current in the second energizing operation, and the energizing time in the first energizing operation is smaller than the energizing time in the second energizing operation.

However, none of these technologies have considered suppressing cold cracking in high-strength steel sheets with a tensile strength exceeding 1.60 GPa, nor have they provided any specific means for doing so.

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

In projection welding of steel members to high-strength steel plates, such as those with a tensile strength exceeding 1.60 GPa, rapid shrinkage of the weld immediately after welding causes cold cracking in the hardened portion of the steel plate at the weld. When the heat input is large, or when the steel plate thickness is large, the strain generated in the weld becomes greater, making cold cracking particularly likely.

The present invention aims to provide a manufacturing method that can suppress low-temperature cracking in the production of projection-welded components composed of unplated steel sheets or zinc-plated steel sheets with a tensile strength exceeding 1.60 GPa, and components such as bolts and nuts.

The gist of this 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, comprising joining a steel plate having a tensile strength of more than 1.60 GPa and a plate thickness of 1.8 mm or more and less than 2.3 mm, and being an unplated steel plate or a zinc-plated steel plate, to a member having a projection by projection welding, the method comprising: a main energizing step in which the projection of the member and the steel plate are in contact, and current is applied to the member and the steel plate while applying pressure to weld the projection and the steel plate; and a holding step in which, after the main energizing step, current is stopped from being applied to the member and the steel plate, and the pressure on the steel plate and the member is maintained, wherein the energizing time in the main energizing 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 projection welded members described in (1) above, when the parameter X is the sum of the tensile strength of the steel plate in units of GPa and the plate thickness in units of mm, the energizing time in the main energizing step may be 26 × X or more in units of msec, and the holding time in the holding step may be 1520/X or less in units of msec.
(3) The method for manufacturing a projection welded member described in (1) or (2) above includes a second energizing step after the main energizing step and before the holding step, in which the member and the steel plate are energized with a smaller heat input than in the main energizing step while maintaining the pressure on the steel plate and the member, the time without energizing between the main energizing step and the second energizing step is 160 msec or less, the energizing time in the second energizing step is 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) The method for manufacturing a projection welded member described in (3) above includes a third energizing step between the second energizing step and the holding step, in which the member and the steel plate are energized with a smaller heat input than in the second energizing step while maintaining the pressure on the steel plate and the member, the time without energizing between the second energizing step and the third energizing step is 160 msec or less, the energizing time in the third energizing step is 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 method for manufacturing a projection welded member, which involves joining a steel plate having a tensile strength of more than 1.60 GPa and a plate thickness of 2.3 mm or more and less than 3.3 mm, and being an unplated steel plate or a zinc-plated steel plate, to a member having a projection by projection welding, comprising: a main energizing step in which the projection of the member and the steel plate are in contact, and the member and the steel plate are energized while pressurizing them to weld the projection and the steel plate; and after the main energizing step, the steel plate and the member The system comprises a second energizing step in which current is supplied to the member and the steel plate with a smaller heat input than in the main energizing step while maintaining the aforementioned pressure, and a holding step in which, after the second energizing step, current is supplied to the member and the steel plate and the pressure is maintained on the steel plate and the member while the current is supplied to the member and the steel plate, wherein 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 time without current supply between the main energizing step and the second energizing step is 160 msec or less.
(6) The method for manufacturing a projection welded member described in (5) above includes a third energizing step between the second energizing step and the holding step, in which the member and the steel plate are energized with a smaller heat input than in the second energizing step while maintaining the pressure on the steel plate and the member, the time without energizing between the second energizing step and the third energizing step is 160 msec or less, the energizing time in the third energizing step is 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 projection welded members described in (5) or (6) above, when the parameter X is the sum of the tensile strength of the steel plate in units of GPa and the plate thickness in units of mm, the energizing time in the energizing process may be 26 × X or more in units of msec.

(8) In the method for manufacturing a projection welded member described in any one of the above items (1) to (7), the steel sheet contains, by 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, with the remainder being Fe and impurities, and the carbon equivalent Ceq of the steel sheet, represented by the following formula (A), may be 0.20% to 0.55% by mass.
Ceq=[C]+[Si]/30+[Mn]/20+2[P]+4[S]...(A)
Here, the element symbols included in equation (A) represent the content of these elements in unit mass percent.
(9) In the method for manufacturing a projection welded member described in any one of the above items (1) to (8), the product of the welding current value in units kA and the energizing time in units msec in the energizing step may be 3300 msec·kA or less.
(10) In the method for manufacturing a projection welding member described in (3) to (7) above, and in any one of (8) and (9) above which is dependent on any one of (3) to (7) above, the current value I1 in the main energizing step and the current value I2 in the second energizing step may satisfy the relationship shown in the following equation (B).
0.2≦I2/I1≦0.8…(B)
(11) In the method for manufacturing projection welding members described in (4) and (6) above, and any one of (7) to (10) above which is dependent on (4) or (6) above, the current value I1 in the main energizing step and the current value I3 in the third energizing step may satisfy the relationship shown in the following formula (C).
0.2≦I3/I1≦0.8…(C)
(12) In the method for manufacturing a projection welded member described in any one of the above items (1) to (11), the steel plate has a zinc-based plating on its surface, and the method for manufacturing a projection welded member may include a preliminary energizing step before the main energizing step, in which a current smaller than that of the main energizing step is applied to the member and the steel plate while maintaining the pressure on the steel plate and the member.

According to the present invention, a manufacturing method is available that can suppress low-temperature cracking in the production of projection-welded members composed of unplated steel sheets or zinc-plated steel sheets with a tensile strength exceeding 1.60 GPa, and components such as bolts and nuts.

This is a schematic diagram of a projection welded component. This is an example of a current profile and pressure profile in a manufacturing method having an energizing process and a holding process. This is an example of a current profile and pressure profile in a manufacturing method having a main energizing process, a second energizing process, and a holding process. This is an example of a current profile and pressure profile in a manufacturing method having a main energizing process, a second energizing process, a third energizing process, and a holding process.

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

In a method for manufacturing a projection-welded member 1 according to a first aspect of the present invention, as illustrated in Figure 1, a steel plate 11 having a tensile strength of more than 1.60 GPa and a plate thickness of 1.8 mm or more and less than 2.3 mm, and being an unplated steel plate or a zinc-plated steel plate, is joined to a member 12 having a projection 121 by projection welding. Projection welding is a type of resistance welding in which a projection formed at the welding location of the base material is brought into contact with the base material and an electric current is passed through it, limiting the generation of resistance heat to a specific area where resistance heat is relatively small. Projection welding includes, as illustrated in Figure 2, a main energizing step S1-1 in which the projection 121 of the member 12 and the steel plate 11 are brought into contact, and an electric current is passed through the member 12 and the steel plate 11 while applying pressure to them to weld the projection 121 and the steel plate 11, and a holding step S2 after the main energizing step S1-1 in which the electric current to the member 12 and the steel plate 11 is stopped and the pressure on the steel plate 11 and the member 12 is maintained.
In this specification, maintaining pressure in the holding step S2 is not limited to maintaining the applied pressure P2 in the holding step S2 at the same pressure P1-1 as at the end of the main energizing step S1-1. For example, the applied pressure P2 may fluctuate during the holding step S2. Also, for example, the applied pressure P2 in the holding step S2 can be 0.8 to 1.2 times the applied pressure P1-1 at the end of the main energizing step S1-1.

The tensile strength of the steel plate 11 is set to exceed 1.60 GPa (1600 MPa). This allows the manufacturing method of the projection-welded member 1 according to this embodiment to be applied to machine parts requiring high strength. While cold cracking is a problem in projection welding of steel plates 11 with a tensile strength exceeding 1.60 GPa, this manufacturing method according to this embodiment can avoid cold cracking by optimizing the welding conditions, as described later.

The thickness of the steel plate 11 is set to 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 manufacturing method of the projection welded member 1 according to this embodiment can be applied to machine parts that require high strength. Furthermore, the load on the joint during cooling shrinkage after welding is not so large if the plate thickness is thin, but becomes significant when the plate thickness is 1.8 mm or more, making the present invention useful.
On the other hand, the greater the thickness of the steel plate 11, the greater the heat dissipation from the weld to the steel plate 11, the greater the strain generated in the weld, and consequently the greater the residual stress in the weld. Therefore, in the manufacturing method of the projection welded member 1 according to the first embodiment, the thickness of the steel plate 11 is specified to be less than 2.3 mm. The projection welding method for steel plates 11 with a thickness of 2.3 mm or more will be described later.

The steel sheet 11 is either an unplated steel sheet or a zinc-plated steel sheet. Other components of the steel sheet 11 are not particularly limited, as long as the tensile strength and thickness are within the above-mentioned ranges. Zinc-plating refers to, for example, hot-dip galvanizing, alloyed hot-dip galvanizing, etc. The shape of the steel sheet 11 is also not particularly specified. For example, the steel sheet 11 may be a press-formed steel part, particularly a hot-stamped material.

The component 12 to be projection-welded to the steel plate 11 has projections 121 for projection welding. This limits the resistance heat generated by the welding current to the projections 121 and their surroundings, allowing for efficient resistance welding. The component 12 is, for example, a bolt and a nut.

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

The manufacturing method for projection-welded members according to the first embodiment includes a main energizing step S1-1 and a holding step S2, as illustrated in Figure 2. The main energizing step S1-1 is a step of applying welding current while pressing the steel plate and member against each other to apply pressure. The welding current is the current flowed to form the weld. The post-heat current used for heat treatment of the weld is not included in the concept of welding current. The holding step S2 is a step of maintaining the applied pressure to the steel plate and member while the current flowing through them is substantially reduced to zero. Note that, due to the power supply capacity of the projection welding apparatus, even if control is applied to reduce the current value to zero, it may take several cycles for the actual current value flowing through the steel plate and member to decrease to zero. In the manufacturing method for projection-welded members according to this embodiment, a state where the current value has decreased to a value close to zero is considered to be a state where the current value is substantially zero.

The energizing time in the energizing process S1-1 shall be 120 msec or more (6 cycles or more if the power supply frequency of the welding equipment is 50 Hz). The energizing time is the length of time that the welding current is applied to the member and the steel plate. By setting the energizing time to 120 msec or more, the temperature difference between the surface of the steel plate that is in contact with the member (hereinafter referred to as the "first surface") and the surface opposite the first surface (hereinafter referred to as the "second surface") 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 area after welding is suppressed, and residual stress in the welded area can be reduced. The energizing time in the energizing process S1-1 is preferably 130 msec or more, 140 msec or more, or 150 msec. As long as this requirement is met, the energizing conditions in the energizing process S1-1 can be appropriately selected according to the shape, material, etc., of the steel plate and member to be welded. The energizing time in this energizing process S1-1 is, for example, 140 to 280 msec.

The holding time in holding step S2 is 280 msec or less (14 cycles or less if the welding equipment's power frequency is 50 Hz). Holding time refers to the length of time from the end of the welding current flow in the main energizing step S1-1 until the electrodes are released. If the holding time exceeds 280 msec, the weld area will be overcooled, resulting in significant cooling and shrinkage of the weld area after welding is complete. On the other hand, by setting the holding time to 280 msec or less, the weld area is cooled gradually, reducing residual stress in the weld area. The holding time in holding step S2 is preferably 260 msec or less, 240 msec or less, or 200 msec or less. For example, the holding time in holding step S2 is 160 to 240 msec.

Furthermore, from the viewpoint of avoiding cold cracking, a shorter holding time is preferable, and it may even be 0 msec. That is, the pressure may be set to 0 at the same time as the termination of the welding current. A method for manufacturing a projection-welded member with a holding time of 0 msec is also considered a method for manufacturing a projection-welded member according to this embodiment. On the other hand, considering the capabilities of the projection welding apparatus, a longer holding time is preferable because it makes pressure control easier. Therefore, the holding time may be 10 msec or more, 20 msec or more, or 40 msec or more.

The energizing time in the energizing process S1-1 and the holding time in the holding process S2 may be further limited according to the tensile strength and thickness of the steel plate. For example, if parameter X is the sum of the tensile strength in units of GPa and the plate thickness in units of mm, the energizing time may be set to 26 × X or more in units of msec, and the holding time may be set to 1520/X or less 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 inventors adopted a 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 weld. According to the above formula, a larger parameter X results in a higher lower limit for the energizing time and a lower upper limit for the holding time. Therefore, according to the above formula, the residual stress in the weld is further relieved. The energizing time is preferably 28 × X or more, or 42 × X or more, in msec. The holding time is preferably 1440/X or less, in msec.

The method for manufacturing a projection welded member according to the first embodiment may include a second energizing step S1-2 after the main energizing step S1-1 and before the holding step S2, in which the member and steel plate are energized with a smaller heat input than in the main energizing step S1-1 while maintaining pressure on the steel plate and member. By providing the second energizing step S1-2, the cooling rate of the welded part can be slowed down, further suppressing cold cracking. Furthermore, as will be described later, by providing the second energizing step S1-2, the upper limit of the holding time can be extended.
Here, maintaining pressure in the second energizing step S1-2 is not limited to maintaining the applied pressure P1-2 in the second energizing step S1-2 at the same pressure P1-1 as at the end of the main energizing step S1-1; for example, the applied pressure P1-2 may fluctuate during the second energizing step S1-2. Also, for example, the applied pressure P1-2 in the second energizing step S1-2 can be 0.8 to 1.2 times the applied pressure P1-1 at the end of the main energizing step S1-1.
Furthermore, in this specification, heat input refers to the time integral of the current, and under conditions where the current is constant, "heat input = current × time". In the second energizing process S1-2, energizing with a smaller heat input than in the main energizing process S1-1 means that under conditions where the current is constant, "I1 × t1 > I2 × t2". Here, I1 and t1 are the current value and energizing time of the main energizing process S1-1, and I2 and t2 are the current value and energizing time of the second energizing process S1-2.
When the current value changes during energization due to upslope or downslope energization, the heat input is represented by the area of a triangle (or the sum of the areas of a triangle and a quadrilateral) in the current profile, where the horizontal axis is time and the vertical axis is current value. When the current value changes in a stepwise manner, the heat input is represented by the sum of the areas of multiple quadrilaterals in the current profile, where the horizontal axis is time and the vertical axis is current value.

When the manufacturing method for projection welded members includes a second energizing step S1-2, it is preferable to set the time without energizing (so-called cool-down time) between the main energizing step S1-1 and the second energizing step S1-2 to 160 msec or less. The time without energizing is the length of time during which the current value is substantially zero. By setting the time without energizing to 160 msec or less, the welded area is cooled gradually, further reducing residual stress in the welded area. A shorter time without energizing is preferable. Therefore, the lower limit of the time without energizing is 0 msec.

Furthermore, if the manufacturing method for projection welded members includes a second energizing step S1-2, it is preferable to set the energizing time in the second energizing step S1-2 to 80 msec or longer. The longer the energizing time in the second energizing step S1-2, the more effectively cooling shrinkage of the welded area can be suppressed, further reducing residual stress in the welded area.

If the manufacturing method for projection welded members includes a second energizing step S1-2 in accordance with the above-mentioned provisions for non-energizing time and energizing time, the holding time may be 400 msec or less. As described above, without the second energizing step, a holding time exceeding 280 msec may cause cold cracking due to supercooling. However, when the second energizing step is performed according to predetermined conditions, the temperature of the welded area decreases at the start of the holding step, reducing the risk of cold cracking due to supercooling. Therefore, when the second energizing step is performed according to predetermined conditions, the upper limit of the holding time can be extended from the above-mentioned 280 msec to 400 msec.

The method for manufacturing a projection welded member according to the first embodiment may include, as illustrated in Figure 4, a third energizing step S1-3 between the second energizing step S1-2 and the holding step S2, in which the member and steel plate are energized with a smaller heat input than in the second energizing step while maintaining pressure on the steel plate and member. By providing the third energizing step S1-3 in addition to the second energizing step S1-2, the cooling rate of the welded part can be made even slower, further suppressing cold cracking. Furthermore, as will be described later, by providing the third energizing step S1-3, the upper limit of the holding time can be extended even further.
Here, maintaining pressure in the third energizing step S1-3 is not limited to maintaining the applied pressure P1-3 in the third energizing step S1-3 at the same pressure P1-2 as at the end of the second energizing step S1-2; for example, the applied pressure P1-3 may fluctuate during the third energizing step S1-3. Also, for example, the applied pressure P1-3 in the third energizing step S1-3 can be 0.8 to 1.2 times the applied pressure P1-2 at the end of the second energizing step S1-2.
Furthermore, in the third energizing process S1-3, energizing with a smaller heat input than in the second energizing process S1-2 means that, under the condition of a constant current, "I2 × t2 > I3 × t3". Here, I2 and t2 are the current value and energizing time of the second energizing process S1-2, and I3 and t3 are the current value and energizing time of the third energizing process S1-3. The definition of heat input and the handling of cases where the current value changes during energizing are as described above.

When the manufacturing method for projection welded members includes a third energizing step S1-3, it is preferable to set the time between the second energizing step S1-2 and the third energizing step S1-3 to 160 msec or less. The time without energizing is the length of time during which the current value is substantially zero. By setting the time without energizing to 160 msec or less, the welded area is cooled gradually, further reducing residual stress in the welded area. A shorter time without energizing is preferable. Therefore, the lower limit of the time without energizing is 0 msec.

Furthermore, if the manufacturing method for projection welded members includes a third energizing step S1-3, it is preferable to set the energizing time in the third energizing step S1-3 to 80 msec or longer. The longer the energizing time in the third energizing step S1-3, the more effectively cooling shrinkage of the welded area can be suppressed, further reducing residual stress in the welded area.

If the manufacturing method for projection welded members includes a third energizing step S1-3 in accordance with the above-described provisions, the holding time may be 600 msec or less. When the third energizing step S1-3 is performed according to the predetermined conditions, the temperature of the welded area decreases further at the start of the holding step, further reducing the risk of cold cracking due to overcooling. Therefore, when the third energizing step S1-3 is performed according to the predetermined conditions, the upper limit of the holding time can be extended from the above-described 400 msec to 600 msec.

Next, a method for manufacturing a projection-welded member according to a second embodiment of the present invention will be described. The method for manufacturing a projection-welded member according to the second embodiment uses a thicker steel plate as the welding target than that of the first embodiment. Specifically, the method for manufacturing a projection-welded member according to the second embodiment joins 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 being either an unplated steel plate or a zinc-plated steel plate, to a member having a projection by projection welding. This projection welding, as shown in Figure 3, includes a main energizing step in which the projection of the member and the steel plate are brought into contact, and current is applied while applying pressure to the member and the steel plate to weld the projection and the steel plate; a second energizing step in which current is applied to the member and the steel plate with a smaller heat input than in the main energizing step while maintaining the pressure on the steel plate and the member; and a holding step in which, after the second energizing step, the current to the member and the steel plate is stopped, and the pressure on the steel plate and the member is maintained.

The tensile strength of the steel plate is set to be greater than 1.60 GPa, as in the first embodiment. On the other hand, the thickness of the steel plate is set to be 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 load on the joint during cooling shrinkage after welding becomes particularly significant when the plate thickness is 2.3 mm or more, making the present invention useful.
On the other hand, in order to suppress residual stress in the welded area, the thickness of the steel plate is specified to be less than 3.3 mm. As long as the tensile strength and thickness are within the above range and the steel plate is either unplated or zinc-plated, other components of the steel plate are not particularly limited. Various components exemplified in the first embodiment can also be applied to the steel plate in the second embodiment. The components project-welded to the steel plate can also be the same as those in the first embodiment.

The manufacturing method for a projection-welded member according to the second embodiment includes a main energizing step S1-1, a second energizing step S1-2, and a holding step S2, as illustrated in Figure 3. The main energizing step S1-1 is a step in which, with the projection of the member and the steel plate in contact, current is applied while applying pressure to the member and the steel plate to weld the projection and the steel plate. The second energizing step S1-2 is a step in which, after the main energizing step S1-1, current is applied to the member and the steel plate with a smaller heat input than in the main energizing step, while maintaining the pressure on the steel plate and the member. The holding step S2 is a step in which, after the second energizing step S1-2, the pressure on the steel plate and the member is maintained while the current is stopped from being applied to the member and the steel plate. Note that, due to the capacity of the power supply of the projection welding apparatus, even if control is set to reduce the current value to 0, it may take several cycles for the current value actually supplied to the steel plate and the member to decrease to 0. In the method for manufacturing projection-welded members according to this embodiment, a state in which the current value has decreased to a value close to zero is considered to be a state in which the current value has been substantially reduced to zero.

The energizing process S1-1 in the second embodiment is the same as in the first embodiment. That is, the energizing time in the energizing process S1-1 is 120 msec or longer. Preferably, the energizing time in the energizing process S1-1 is 130 msec or longer, 140 msec or longer, or 150 msec. The current value in the energizing process S1-1 is not particularly limited, and a value can be appropriately selected according to the shape and application of the projection welding member. On the other hand, the product of the welding current value in unit kA and the energizing time in unit msec in the energizing process S1-1 may be defined as 3300 msec·kA or less.

The energizing time in the second energizing process S1-2 should be 80 msec or longer. The longer the energizing time in the second energizing process S1-2, the more effectively cooling shrinkage of the weld can be suppressed, further reducing residual stress in the weld.

The period of no current flow between the main current flow process S1-1 and the second current flow process S1-2 shall be 160 msec or less. The period of no current flow refers to the length of time during which the current value is substantially zero. By limiting the period of no current flow to 160 msec or less, the welded area is cooled gradually, further reducing residual stress in the welded area.

The holding time in holding step S2 shall be 400 msec or less. Holding time refers to the length of time during which the current flowing through the steel plate and component is substantially zero, and the pressure applied to the steel plate and component is greater than zero. If the holding time exceeds 400 msec, the weld joint will be supercooled, resulting in significant cooling shrinkage of the weld joint after welding is complete. On the other hand, by setting the holding time to 400 msec or less, the weld joint is cooled gradually, reducing residual stress in the weld joint. The holding time in holding step S2 is preferably 360 msec or less, 300 msec or less, or 200 msec or less.

The manufacturing method for projection welded members according to the second embodiment may include a third energizing step S1-3 between the second energizing step S1-2 and the holding step S2, while maintaining pressure on the steel plate and member, and energizing the member and steel plate with a smaller heat input than in the second energizing step S1-2. By providing the third energizing step S1-3, the cooling rate of the welded area can be further slowed, further suppressing cold cracking. Furthermore, as will be described later, providing the third energizing step S1-3 allows for an extension of the upper limit of the holding time.

When the manufacturing method for projection welded members includes a third energizing step S1-3, it is preferable to set the time between the second energizing step S1-2 and the third energizing step S1-3 to 160 msec or less. By setting the time between energizing and welding to 160 msec or less, the welded area is cooled gradually, further reducing residual stress in the welded area.

Furthermore, if the manufacturing method for projection welded members includes a third energizing step S1-3, it is preferable to set the energizing time in the third energizing step S1-3 to 80 msec or longer. The longer the energizing time in the third energizing step S1-3, the more effectively cooling shrinkage of the welded area can be suppressed, further reducing residual stress in the welded area.

If the manufacturing method for projection welded members includes a third energizing step S1-3 in accordance with the above-mentioned provisions for the non-energizing time and energizing time, the holding time may be 600 msec or less. When the third energizing step S1-3 is performed according to the predetermined conditions, the temperature of the welded area decreases further at the start of the holding step, further reducing the risk of cold cracking due to supercooling. Therefore, when the third energizing step S1-3 is performed according to the predetermined conditions, the upper limit of the holding time can be extended from the above-mentioned 400 msec to 600 msec.

The energizing time and holding time in the energizing process S1-1 may be further limited according to the tensile strength and thickness of the steel plate. For example, if parameter X is the sum of the tensile strength in units of GPa and the plate thickness in units of mm, the energizing 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 inventors adopted a 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 weld. According to the above formula, a larger parameter X results in a higher lower limit for the energizing time and a lower upper limit for the holding time. Consequently, according to the above formula, the residual stress in the weld is further reduced.

The first embodiment relating to a steel sheet, which is either an unplated steel sheet or a zinc-plated steel sheet, with a thickness of 1.8 mm or more and less than 2.3 mm, and the second embodiment relating to a steel sheet, which is either an unplated steel sheet or a zinc-plated steel sheet, with a thickness of 2.3 mm or more and less than 3.3 mm, have been described above. Below, preferred configurations applicable to both of these embodiments will be further described.

A suitable steel sheet has the following chemical composition as an example: it 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: 0.01% or less, with the remainder being Fe and impurities, and the carbon equivalent Ceq of the steel sheet, represented by the following formula (A), is 0.20% to 0.55% by mass.
Ceq=[C]+[Si]/30+[Mn]/20+2[P]+4[S]...(A)
Here, the element symbols included in equation (A) represent the content of these elements in unit mass percent.
Generally, increasing the content of carbon and alloying elements can increase the strength of the base material, but at the same time, the Ceq increases, which reduces the toughness of the welded joint and is one of the causes of cold cracking. Steel sheets with such components are used as materials for machine parts that require high strength, such as automobile parts, and the present invention is particularly useful in such cases. Furthermore, the method for manufacturing projection welded members according to this embodiment can suppress cold cracking in members obtained from steel sheets with such components.

As described above, the energizing conditions in the main energizing process S1-1, the second energizing process S1-2, and the third energizing process S1-3 are not particularly limited except for the energizing time, and values can be appropriately selected according to the shape and application of the projection welding member. On the other hand, the current values in each process may be determined as described below.

In the current application process S1-1, the product of the welding current value (in kA) and the current application time (in msec) may be specified as 3300 msec·kA or less. The product of the welding current value and the current application time serves as an indicator of the heat input in the current application process S1-1. A smaller heat input in the current application process S1-1 reduces the temperature difference between the first and second surfaces of the steel plate. This further suppresses cooling shrinkage of the weld after welding is complete and further reduces residual stress in the weld.

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

When I2/I1 is set to 0.2 or more and 0.8 or less, or when I3/I1 is set to 0.2 or more and 0.8 or less, the second energizing step S1-2 or the third energizing step S1-3 can reliably exert the effect of slowing down the cooling rate of the welded area.

The applied pressure in projection welding is not particularly limited. Our experimental results showed no effect of applied pressure on the incidence of cold cracking. This is thought to be because the main cause of cold cracking is the strain introduced during the cooling of the weld, and applied pressure does not affect this. Therefore, the applied pressure used in typical projection welding (e.g., 3-6 kN) should be appropriately selected.

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

In projection welding of zinc-plated steel sheets, the uneven distribution of the oxide film, which is a high-resistance material, on the steel sheet surface easily leads to variations in joint strength. This variation is particularly pronounced when the zinc-plated steel sheet is hot-pressed. In the preliminary energizing process S0, the oxide film on the steel sheet 11 in contact with each projection 121 is destroyed, removed, or thinned, thereby uniformizing the thickness of the oxide film in the region through which current flows from each projection 121. This results in nearly uniform resistance in the current path at the contact points of each projection 121. Consequently, in the main energizing process S1-1 for forming the joint, the current density at each projection 121 becomes uniform, enabling the formation of a uniform joint at each point.

Furthermore, by making the de-energizing time between the preliminary energizing process S0 and the main energizing process S1-1 70 msec or more, the temperature of each projection 121 is made uniform, and the ease with which current flows through the projections 121 is also made uniform. As a result, the current flowing through each projection 121 in the main energizing process S1-1 becomes uniform, reducing variations in the bonding strength between the steel plate and the component.

To make the current density more uniform in the main energizing process S1-1, it is preferable that the height of all protrusions 121 after the completion of the preliminary energizing process S0 be 0.7 to 0.9 times the height before the start of the preliminary energizing process S0. By appropriately heating and pressurizing during the preliminary energizing process, the protrusions 121 can be deformed to flatten them, thereby bringing their heights within the above-mentioned range.

The effects of one aspect of the present invention will be further explained in detail by the 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 these examples of conditions. The present invention can adopt various conditions as long as it does not depart from the spirit of the invention and achieves the objectives of the present invention.

(Example 1)
Various projection-welded components were manufactured using hot-stamped material with zinc plating under the conditions described below.
- Tensile strength, thickness, and Ceq of the steel plate: Listed in the "TS", "Thickness", and "Ceq" columns of Table 1. - Shape of the component: A nut with three projections for projection welding. - Energizing time in this energizing process: Listed in the "Energizing Time" column of Table 1. - Current value I1 in this energizing process: Listed in the "Current Value" column of Table 1. - Pressing force in this energizing process: 5.0 kN
- Holding time in the holding process: As indicated in the "Holding Time" column of Table 1. - Presence or absence of the second and third energizing processes: As indicated in the "One Stage of Post-Energy Entrance" and "Two Stages of Post-Energy Entrance" columns of Table 1 (In cases where the symbol "〇" is placed in the "One Stage of Post-Energy Entrance" column, only the second energizing process is performed; in cases where the symbol "〇" is placed in the "Two Stages of Post-Energy Entrance" column, both the second and third energizing processes are performed).
- Time without power between the main power-on process and the second power-on process: 80 msec
• Power-on time in the second power-on process: 120 msec
- Current value I2 in the second energization process: 6kA
- Time without power between the second and third power-on processes: 80 msec
• Power-on time in the third power-on process: 120 msec
- Current value I3 in the third energization process: 6kA

Furthermore, the number of cracks in the projection-welded components was also investigated. For each of the conditions shown in Table 1, projection welding was performed to create three test specimens. Each test specimen had three welds, resulting in a total of nine welds. The presence or absence of cracks in these welds was checked. The number of cracks was recorded in the "As-Welded" column of Table 1. Next, the manufactured projection-welded components were immersed in hydrochloric acid at the same pH for a predetermined time. Hydrochloric acid allows hydrogen to penetrate the joints, accelerating delayed fracture. After immersion, the presence or absence of cracks in the projection-welded components was checked again. The presence or absence of cracks was confirmed by cutting each weld radially from the center of the nut, embedding it in resin, polishing and etching the cut surface, and observing the cut surface under an optical microscope. The number of cracks was recorded in the "Hydrochloric Acid Immersion" column of Table 1.

In Comparative Examples 1 and 4, although the steel plate thickness was 2.3 mm or more, cracking could not be suppressed because the second and third energizing processes were not performed.
Comparative Example 2 did not have a tensile strength of over 1.60 GPa in the steel sheet, so it did not have the problem of low-temperature cracking specific to high-strength steel sheets, and was therefore outside the scope of the present invention. Although no cracking occurred in Comparative Example 2, the tensile strength of the steel sheet was not sufficient, so it could not be used as a machine part requiring a tensile strength of over 1.60 GPa.
In Comparative Example 3, the steel plate thickness was less than 2.3 mm, but the holding time was too long, so cracking after hydrochloric acid immersion could not be suppressed.
In Comparative Example 5, the steel plate thickness was 2.3 mm or more, but the current application time was too short, so cracking could not be suppressed. In Comparative Example 5, unlike Comparative Examples 1 and 4, a second current application process was performed, but cracking could not be suppressed.

In Invention Example 1, the steel plate thickness was less than 2.3 mm, and the energizing time and holding time were appropriate, so cracking was suppressed.
In Invention Example 2, the steel plate thickness was less than 2.3 mm, and since the second energizing process was performed and the energizing time and holding time were appropriate, cracking was suppressed. Although Invention Example 2 had a greater plate thickness than Invention Example 1, resulting in more cracks after hydrochloric acid immersion than Invention Example 1, it was still within the acceptable range.
In Invention Example 3, although the steel plate thickness was 2.3 mm or more, cracking was suppressed because the second and third energizing processes were carried out, and the energizing time and holding time were appropriate.
In Invention Example 4, the steel plate thickness was less than 2.3 mm, and the energizing time and holding time were appropriate, so cracking was suppressed. Although Invention Example 4 had a greater plate thickness than Invention Example 1, resulting in more cracks after hydrochloric acid immersion than Invention Example 1, it was still within the acceptable range.
In Invention Example 5, the steel plate thickness was 2.3 mm or more, and the energizing time and holding time were appropriate, so cracking was suppressed. Although Invention Example 5 had a greater plate thickness than Invention Example 1, resulting in more cracks after hydrochloric acid immersion than Invention Example 1, it was still within the acceptable range.
Examples 6 to 8 correspond to the upper and lower limits of claims 1, 3, and 4, respectively, and the effect of suppressing cracking was confirmed.

(Example 2)
Various projection-welded components were manufactured using alloyed hot-dip galvanized steel sheets (GA-plated hot-stamped material) with a TS of 2.0 GPa, Ceq of 0.45%, and a plate thickness of 2.0 mm, by changing the pre-current conditions.
- Powering time during the pre-powering process: Listed in the "Pre-powering: Powering Time" column of Table 2 - Current value during the pre-powering process: Listed in the "Pre-powering: Current Value" column of Table 2 - Non-powering time between the pre-powering process and the main powering process: Listed in the "Cooling Time" column of Table 2 - Powering time during the main powering process: Listed in the "Powering Time" column of Table 2 - Current value I1 during the main powering process: Listed in the "Current Value" column of Table 2 - Pressing force during the pre-powering and main powering processes: 5.0 kN
- Holding time during the holding process: As shown in the "Holding Time" column of Table 2. The number of cracks in the various projection welded members manufactured was investigated using the same method as in Example 1.

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

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

Claims (12)

A method for manufacturing a projection-welded member, comprising joining a steel sheet having a tensile strength of more than 1.60 GPa and a thickness of 1.8 mm or more and less than 2.3 mm, and being either an unplated steel sheet or a zinc-plated steel sheet, to a member having a protrusion by projection welding,
The current application process involves applying pressure to the member and the steel plate while bringing the projection of the member and the steel plate into contact, thereby welding the projection and the steel plate.
The process includes, after the above energizing step, a holding step in which the pressure on the steel plate and the member is maintained while the energizing to the member and the steel plate is stopped,
The energizing time in the above energizing process shall be 120 msec or more.
A method for manufacturing a projection welded member, wherein the holding time in the holding step is 280 msec or less. When the parameter X is the sum of the tensile strength of the steel plate in units of GPa and the plate thickness in units of mm,
The energizing time in the above energizing process shall be 26 × X or more in units of msec.
The method for manufacturing a projection welded member according to claim 1, characterized in that the holding time in the holding step is 1520/X or less in units of msec. After the main energizing step and before the holding step, there is a second energizing step in which current is applied to the member and the steel plate with a smaller heat input than in the main energizing step, while maintaining the pressure on the steel plate and the member.
The period of time without power between the aforementioned main power-on process and the aforementioned second power-on process shall be 160 msec or less.
The energizing time in the second energizing process is set to 80 msec or more.
The method for manufacturing a projection welded member according to claim 1 or 2, characterized in that 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, there is a third energizing step in which current is applied to the member and the steel plate with a smaller heat input than in the second energizing step, while maintaining the pressure on the steel plate and the member.
The time between the second energizing process and the third energizing process is set to 160 msec or less.
The energizing time in the third energizing process is set to 80 msec or more.
The method for manufacturing a projection welded member according to claim 3, characterized in that the upper limit of the holding time in the holding step is set to 600 msec instead of 400 msec. A method for manufacturing a projection-welded member, comprising joining a steel sheet having a tensile strength of more than 1.60 GPa, a thickness of 2.3 mm or more and less than 3.3 mm, and being either an unplated steel sheet or a zinc-plated steel sheet, to a member having a protrusion by projection welding,
The current application process involves applying pressure to the member and the steel plate while bringing the projection of the member and the steel plate into contact, thereby welding the projection and the steel plate.
After the aforementioned energizing step, a second energizing step is performed in which, while maintaining the pressure on the steel plate and the member, current is applied to the member and the steel plate with a smaller heat input than in the aforementioned energizing step.
The process includes a holding step in which, after the second energizing step, the energizing of the member and the steel plate is stopped, and the pressure on the steel plate and the member is maintained,
The energizing time in the above energizing process shall be 120 msec or more.
The energizing time in the second energizing process is set to 80 msec or more.
The holding time in the holding process is set to 400 msec or less.
A method for manufacturing a projection welded member, wherein the period of no power supply between the main power supply step and the second power supply step is 160 msec or less. Between the second energizing step and the holding step, there is a third energizing step in which current is applied to the member and the steel plate with a smaller heat input than in the second energizing step, while maintaining the pressure on the steel plate and the member.
The time between the second energizing process and the third energizing process is set to 160 msec or less.
The energizing time in the third energizing process is set to 80 msec or more.
The method for manufacturing a projection welded member according to claim 5, characterized in that 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 units of GPa and the plate thickness in units of mm of the aforementioned steel plate,
The method for manufacturing a projection welding member according to claim 5 or 6, characterized in that the energizing time in the energizing step is 26 × X or more in units of msec. The steel sheet contains, by mass%, C: 0.07-0.45%, Si: 0.001-2.50%, Mn: 0.8-5.0%, P: 0.03% or less, S: 0.01% or less, with the remainder being Fe and impurities.
A method for manufacturing a projection welded member according to any one of claims 1 to 7, characterized in that the carbon equivalent Ceq of the steel plate, represented 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 equation (A) represent the content of these elements in unit mass percent. A method for manufacturing a projection welding member according to any one of claims 1 to 8, characterized in that the product of the welding current value in units of kA and the energizing time in units of msec in the energizing process is 3300 msec·kA or less. A method for manufacturing a projection welding member according to claims 3 to 7, characterized in that the current value I1 in the main energizing step and the current value I2 in the second energizing step satisfy the relationship expressed by the following formula (B), and any one of claims 8 and 9 that is dependent on any one of claims 3 to 7.
0.2≦I2/I1≦0.8…(B) A method for manufacturing a projection welding member according to claims 4 and 6, characterized in that the current value I1 in the main energizing step and the current value I3 in the third energizing step satisfy the relationship expressed by the following formula (C), and any one of claims 7 to 10 that is dependent on claim 4 or 6.
0.2≦I3/I1≦0.8…(C) The steel plate has a zinc-based plating on its surface.
A method for manufacturing a projection welding member according to any one of claims 1 to 11, characterized in that, prior to the main energizing step, a preliminary energizing step is performed in which a current smaller than that of the main energizing step is applied to the member and the steel plate while maintaining the pressure on the steel plate and the member.