JP7368716B2 - Manufacturing method of resistance spot welding joints - Google Patents

Description

The present invention relates to resistance spot weld joints. The present invention particularly relates to a method for manufacturing a resistance spot welded joint that has excellent joint strength and delayed fracture resistance.

In the field of automobiles, in order to preserve the environment, there is a need to improve fuel efficiency by reducing the weight of vehicle bodies, as well as improving collision safety. Therefore, various efforts have been made to reduce the weight of the car body and improve collision safety by using high-strength steel plates to make the car body thinner and optimizing the car body structure.
BACKGROUND ART Resistance spot welding (hereinafter sometimes referred to as "spot welding") is mainly used for welding in the manufacture of parts for automobiles and the assembly of vehicle bodies. Tensile strength is a quality indicator of welded joints formed by spot welding. The tensile strength of welded joints includes tensile shear strength (TSS), which is measured by applying a tensile load in the shear direction, and cross tensile strength (CTS), which is measured by applying a tensile load in the peeling direction.

Here, when high-strength steel plates are spot welded, there is a problem of delayed fracture (hydrogen embrittlement).
In order to achieve this strength, high-strength steel plates contain many elements with high hardenability such as Si and Mn in addition to C, and welding of welded joints formed by spot welding to high-strength steel plates. The part is hardened through the heating and cooling process of welding, becoming a martensitic structure and becoming hard. Furthermore, in the welded joint, tensile residual stress in the welded joint increases due to locally occurring transformation expansion and contraction. In particular, as the strength of the steel sheet increases, springback is more likely to occur after press forming, which increases the gap at the weld flange. Such gaps become a factor that further increases tensile residual stress when spot welding steel plates together.
For this reason, the welded parts of welded joints formed by spot welding to high-strength steel plates have high hardness and large tensile residual stress, so if hydrogen intrusion occurs, they are likely to cause delayed fracture. . When such delayed fracture occurs, sufficient tensile strength, which is a quality indicator for welded joints mentioned above, cannot be obtained, and if moisture infiltrates into that part (cracks), corrosion may occur. The problem arises that the strength is further reduced. These problems are one of the reasons why it is difficult to reduce the weight (thinner wall thickness) of vehicle bodies by using high-strength steel plates.
Under these circumstances, after a certain period of time has elapsed after the spot welding has been energized, tempering is applied or heated with high frequency to temper the weld and reduce the hardness of the weld. The technology is known. However, with this technique, the welding process takes a long time, which reduces productivity, and the welded part softens due to tempering, which tends to cause peeling and fracture within the weld metal (nugget).

Under these circumstances, various proposals have been made to suppress the occurrence of delayed fracture.
For example, Patent Document 1 discloses a resistance spot welding method that can form a welded part with excellent delayed fracture resistance even when high-strength galvanized steel plates are joined by forming small-diameter nuggets. ing.

Japanese Patent Application Publication No. 2018-171649

However, with the technique of Patent Document 1, it is difficult to achieve both excellent joint strength and delayed fracture resistance depending on the thickness of the steel plate used and the welding conditions.

The present invention has been made in view of the above-mentioned circumstances, and an object of the present invention is to provide a method for manufacturing a resistance spot welded joint that has excellent joint strength and delayed fracture resistance.

The specific method of the present invention is as follows.

[1] A first aspect of the present invention is a method for manufacturing a resistance spot welded joint by overlapping and joining a plurality of steel plates, including at least one steel plate having a tensile strength of 780 MPa or more, by resistance spot welding. a preliminary energization step of applying current while pressurizing the surface of the steel plate using a pair of DR electrodes having a tip diameter of 8 mm or more and 12 mm or less; and after the preliminary energization step, using the pair of DR electrodes. a main energization step of applying current to the surface of the steel plates under pressure, and a holding step of maintaining the pressure applied by the pair of DR electrodes after the main energization step; The thickness of each plate is 0.8 mm to 3.2 mm, and the pressure in the preliminary energization process is P1 (kN), the current is I1 (kA), and the energization time is t1 (s). When the pressure is P2 (kN) and the current is I2 (kA), the following equations (1) to (5) are satisfied.
2≦I1/h<4 ... (1) formula 1.5≦P1/h<2.5 ... (2) formula 0.1≦t1/h≦1.5 ... (3) formula 4≦I2/h ...Equation (4) 2.5≦P2/h ...Equation (5) However, h is the value of 1/2 of the total thickness (mm) of the plurality of steel plates. be.
[2] In the method for manufacturing a resistance spot welded joint according to [1] above, the holding time Ht(s) in the holding step may be 0.4×h ² or less.
[3] In the method for manufacturing a resistance spot welded joint according to [1] or [2] above, at least one of the plurality of steel plates may be a galvanized steel plate.
[4] In the method for manufacturing a resistance spot welded joint according to [3] above, the holding time Ht(s) in the holding step may be 0.08×h ² or more.

According to the method for manufacturing a resistance spot welded joint according to the present invention, it is possible to manufacture a resistance spot welded joint that has excellent joint strength and delayed fracture resistance.

It is a schematic diagram of an electrode and a steel plate used in the manufacturing method of the resistance spot welding joint concerning this embodiment. It is a schematic diagram of the electrode used in the same embodiment.

As a result of intensive study on measures to solve the above-mentioned problems, the present inventors found that
(A) Even for welded joints with the same nugget diameter, welded joints obtained by energizing with a high pressure force are better than welded joints obtained by energizing with a low pressure force due to the gap between steel plates. Hydrogen tends to easily enter spot welds due to water and oil as hydrogen sources adhering to the surface;
(B) If hydrogen enters the spot weld, delayed fracture will occur when the tensile stress and amount of hydrogen reach critical values at the nugget end where tensile stress is concentrated;
(C) Delayed fracture is more likely to occur as the inter-plate gap increases (as press accuracy deteriorates) and as the nugget diameter becomes smaller;
(D) Therefore, by performing preliminary energization with appropriate current, pressure, and energization time, the contact area between the steel plates is gradually increased, and water and oil, which serve as hydrogen sources, are evaporated over a wide range (area). and then by energizing, it is possible to achieve both excellent joint strength and delayed fracture resistance.
I discovered something new.
Regarding (A), the reason why the amount of hydrogen entering the weld zone tends to increase when the pressurizing force is high is that when the pressurizing force is high, the contact area between the steel plates and the contact area between the electrode and the steel plate increase. However, since the current density decreases, it is necessary to increase the current in order to obtain the same nugget diameter, which is presumably because the initial contact area and current increase.

The present invention has been made based on the above findings. Hereinafter, a method for manufacturing a resistance spot welded joint according to an embodiment of the present invention will be described in detail.

As shown in FIG. 1, in the method for manufacturing a resistance spot welded joint according to the present embodiment, a first steel plate 1 and a second steel plate 2, which are stacked on top of each other, are sandwiched between a first electrode 10 and a second electrode 20 and then pressurized. In this state, resistance spot welding is performed by performing a preliminary energization process, a main energization process, and a holding process.

(steel plate)
In this embodiment, two steel plates, the first steel plate 1 and the second steel plate 2, are overlapped and resistance spot welded, but other steel plates are also overlapped, and three or more steel plates are overlapped and resistance spot welded. Good too.
At least one of the first steel plate 1 and the second steel plate 2 may have a tensile strength of 780 MPa or more, and both may have a tensile strength of 780 MPa or more. When there are three or more steel plates, the tensile strength of at least one steel plate should be 780 MPa or more.
Furthermore, at least one of the first steel plate 1 and the second steel plate 2 may be a galvanized steel plate. When there are three or more steel plates, at least one steel plate may be a galvanized steel plate. The amount of zinc plating applied may be 30 to 100 g/m ² per side.
The thickness of each of the overlapping steel plates is not particularly limited, and may be, for example, 0.8 mm to 3.2 mm.
In this specification, the value of 1/2 of the total thickness (mm) of the steel plates to be stacked is referred to as h. For example, when three steel plates with a thickness of 1 mm are stacked, the value of h is 1.5.

(electrode)
As shown in FIG. 1, the first electrode 10 is a DR electrode, and includes a tip 11 and a main body 13 that is connected to the tip 11 via a continuous portion 12.
The tip portion 11 is a portion where the contact area with the steel plate changes depending on the pressure applied to the steel plate, and means a portion where the tip R (radius of curvature of the curved surface of the electrode tip) is a constant value as shown in FIG. do. The tip R may be 30 mm or more, for example, 40 mm. The maximum value of the tip R may be 100 mm.
The continuous portion 12 is a portion having a smaller radius of curvature than the tip portion 11. The radius of curvature of the continuous portion 12 may be, for example, 6 mm or more.
The main body part 13 is a substantially cylindrical part, and one end is connected to the continuous part 12, and the other end is connected to a structure (not shown) above the electrode. The diameter D of the main body portion 13 may be 12 mm to 20 mm.

Since the first electrode 10 has a tip diameter d of 8 mm or more, it is possible to remove oil and water, which are hydrogen sources, from the surface of the steel plate over a sufficient area in the preliminary energization process described later, thereby preventing delayed fracture. be able to.
Therefore, the tip diameter d of the first electrode 10 is 8 mm or more, preferably 9 mm or more.
On the other hand, if the tip diameter d of the first electrode 10 is more than 12 mm, it becomes difficult to obtain a desired current density due to the increased contact area. Therefore, the tip diameter d of the first electrode 10 is 12 mm or less, preferably 11 mm or less.

The second electrode 20 , like the first electrode 10 , includes a tip 21 and a main body 23 connected to the tip 21 via a continuous portion 22 . The shape and dimensions of the second electrode 20 need only be the same as those of the first electrode 10, so a description thereof will be omitted.

In the method for manufacturing a resistance spot welded joint according to the present embodiment, a preliminary energization process, a main energization process, and a holding process are performed using the first electrode 10 and the second electrode 20 described above.

(Preliminary energization process)
In the preliminary energization process, electricity is applied while the first steel plate 1 and the second steel plate 2 are pressurized by the pair of electrodes 1.
Here, when the pressurizing force in the preliminary energization step is P1 (kN), the current is I1 (kA), and the energization time is t1 (s), the following equations (1) to (3) are satisfied.
2≦I1/h<4 (kA/mm) ... (1) formula 1.5≦P1/h<2.5 (kN/mm) ... (2) formula 0.1≦t1/h≦ 1.5 (s/mm)...Equation (3)

Equation (1) defines the range of current I1 with respect to h in the preliminary energization process. When I1/h is less than 2, even if formulas (2) and (3) are satisfied, it becomes difficult to sufficiently remove oil and water, which are hydrogen sources, from the surface of the steel plate due to insufficient heat input. For this reason, hydrogen may enter the welded portion during the main energization process, causing delayed fracture. Therefore, I1/h is 2 or more.
On the other hand, if I1/h is 4 or more, it is equivalent to performing the main energization process without performing the preliminary energization process, so spot welding is performed without sufficient removal of oil and water, which are hydrogen sources, from the surface of the steel plate. will be implemented. Therefore, hydrogen may enter the welded portion and delayed fracture may occur. Therefore, I1/h is less than 4.

Equation (2) defines the range of the pressing force P1 with respect to h in the preliminary energization step. When P1/h is less than 1.5, the contact area between the steel plates (first steel plate 1, second steel plate 2) and electrodes (first electrode 10, second electrode 20) and the steel plates are reduced due to the low pressurizing force. contact area becomes smaller. Therefore, even if formulas (1) and (3) are satisfied, the range in which oil and water, which are hydrogen sources, on the surface of the steel sheet can be removed becomes narrow. For this reason, hydrogen may enter the welded portion during the main energization process, causing delayed fracture. Therefore, P1/h is 1.5 or more, more preferably 1.7 or more.
On the other hand, when P1/h is more than 2.5, the contact area between the steel plate and the electrode and the contact area between the steel plates becomes large due to the high pressing force. Therefore, the current density becomes small, and even if equations (1) and (2) are satisfied, it becomes difficult to sufficiently remove oil and water, which are hydrogen sources, from the surface of the steel sheet. For this reason, hydrogen may enter the welded portion during the main energization process, causing delayed fracture. Therefore, P1/h is 2.5 or less, more preferably 2.3 or less.

Equation (3) defines the range of energization time t1 with respect to h in the preliminary energization step. If t1/h is less than 0.1, even if formulas (1) and (2) are satisfied, the energization time is too short to sufficiently remove oil and water, which are hydrogen sources, from the surface of the steel plate. It becomes difficult. For this reason, hydrogen may enter the welded portion during the main energization process, causing delayed fracture. Therefore, t1/h is 0.1 or more, more preferably 0.5 or more.
On the other hand, even if t1/h exceeds 1.5, the effect of removing oil and water, which are hydrogen sources, from the surface of the steel sheet is saturated, and there is a concern that productivity may decrease. Therefore, t1/h is 1.5 or less.
It is preferable to immediately carry out main energization without providing a cool down period between preliminary energization and main energization in order to suppress delayed breakdown.
Preliminary energization may be upslope. In this case, the initial current of pre-energization is set to 2≦I1/h≦3 (kA/mm), the current value is gradually increased, and the average value of the current value at the end of pre-energization and the initial current value is 2≦I1/h<4 (kA/mm).

(Main energization process)
In the main energization step, after the above-described preliminary energization step, the surface of the steel plate is energized under pressure using the electrode 1.
Here, when the pressing force in the main energization step is P2 (kN) and the current is I2 (kA), the following equations (4) and (5) are satisfied.
4≦I2/h...Equation (4) 2.5≦P2/h...Equation (5)

Equation (4) defines the range of current I2 with respect to h in the main energization step, and Equation (5) defines the range of pressing force P2 with respect to h in the main energization step.
In the present application, main energization is performed with a current and pressure that satisfy equations (4) and (5) after sufficiently removing oil and water, which are hydrogen sources on the surface of the steel plate, by the preliminary energization as described above. A spot welded joint with excellent delayed fracture resistance can be obtained.

Note that the upper limit value of I2/h does not need to be specified, but it is preferably 8 or less. This is because if I2/h exceeds 8, there is a high probability that expulsion will occur even if the pressurizing force is within an appropriate range. The time for main energization may be appropriately set so as to obtain a desired nugget diameter.

(holding process)
In the holding step, after the above-described main energization step, the pressure applied to the surface of the steel by the electrode is maintained for a predetermined period of time. By holding, the solidification of the molten metal can proceed, and the strength of the welded part can be increased even when the steel plate strength is high or there is a gap between the plates. The holding time Ht may be more than 0 seconds, but is preferably 0.04 seconds or more.
The upper limit of the holding time Ht is preferably set to 0.4×h ² in consideration of the productivity and the possibility that the joint strength will decrease. This is because if the holding time Ht is made longer than 0.4×h ² , the strength of the cross-tension joint, for example, decreases. The reason for this is that when the holding time Ht exceeds a predetermined range, the welded portion is rapidly cooled due to heat removal from the electrode, and the welded portion is hardened. It is thought that hardening reduces the toughness of the weld and the joint. Additionally, hardening of the weld increases susceptibility to hydrogen embrittlement. In order to further ensure the effect of suppressing delayed fracture, it is preferable to set the upper limit of the holding time Ht to 0.4×h ² . On the other hand, if the holding time Ht is 0.4× ^h2 or less, the cooling after the electrode is released will be relatively slow, so auto-tempering will proceed and the toughness of the weld will be improved. At the same time, susceptibility to hydrogen embrittlement can be improved. Therefore, the upper limit of the retention time Ht is 0.4× ^h2 .

Here, in the case where at least one of the plurality of steel plates has a galvanized layer, the holding time Ht is preferably 0.08×h ² or more.
When at least one of the plurality of steel plates has a galvanized layer, depending on the conditions of the preliminary energization process and the main energization process, LME cracking may occur due to contact of molten zinc with the solid steel plate during welding. Liquid Metal Embrittlement Crack) may occur. LME cracking occurs when molten zinc comes into contact with a solid steel plate during welding and when tensile stress (strain) is applied to that area. Therefore, by setting the holding time Ht to 0.08 x ^h2 or more, the molten zinc will solidify during the holding time (the molten zinc will decrease or disappear completely), and the holding will end and the electrode Even if tensile stress is generated when is released, LME cracking can be suppressed. Therefore, when at least one of the plurality of steel plates has a galvanized layer, the lower limit of the holding time Ht is preferably 0.08×h ² .
In addition, when at least one of the plurality of steel plates has a galvanized layer, the preliminary energization process has the effect of removing the galvanized layer on the surface of the steel plate or promoting alloying of the galvanized layer. LME cracking can be suppressed more reliably by combining the preliminary energization process and the optimization of the holding process.

As described above, according to the method for manufacturing a resistance spot welded joint according to the present embodiment, water and oil, which serve as hydrogen sources, are evaporated by performing preliminary energization with the current, pressing force, and energization time set within appropriate ranges. Then, by carrying out main energization, it is possible to achieve both excellent joint strength and delayed fracture resistance.

(Example)
Hereinafter, the effects of the present invention will be specifically explained using examples.

Steel plates a and b shown in Table 1 were prepared, each steel plate was cut into a size of W40 mm x L100 mm, and a spacer of W40 mm x L30 mm x T2.0 mm was sandwiched between both ends of the steel plate, and the center was spot welded. This central welding point was targeted for evaluation.

The electrodes were made of two types of Cr-Cu, with a main body diameter D of 16 mm, a tip radius of curvature (tip R) of 40 mm, a continuous portion radius of curvature of 8 mm, and a tip diameter d of 6 mm or 8 mm. A DR electrode was used.
Table 2 shows the steel plates, electrodes, and welding conditions used in each experimental example. Values outside the scope of the present invention are underlined. The cooling time between preliminary energization and main energization was set to zero.

Table 3 shows the evaluation results of delayed fracture and joint strength for each experimental example.

Delayed fracture evaluation is performed by cutting the joint perpendicular to the plate surface in the longitudinal direction of the plate with a cross section that passes through the center of the nugget, cutting out a test piece containing the nugget from this cut piece, polishing the cut surface, and cutting the polished cut. The surface was observed using an optical microscope. This test was conducted on five test pieces, and the case where no cracking occurred in any of the five test pieces was defined as "no delayed fracture."

The joint strength was evaluated by tensile shear strength (TSS) and cross tensile strength (CTS) measured by a tensile shear test (JIS Z3136) and a cross tensile test (JIS Z3137). A case where TSS was 19 kN or less or CTS was 6.2 or less was judged to be a failure.
Note that if expulsion occurs during spot welding, it is written as "Yes."

In Experimental Examples 3, 4, 5, 10, 11, and 12 according to the present invention, the preliminary energization process and the main energization process were performed under appropriate conditions, resulting in excellent joint strength and delayed fracture resistance. We were able to manufacture resistance spot welded joints.

In Experimental Example 1, which is a comparative example, the main energization process was performed without performing the preliminary energization process, and the main energization process was performed with water and oil remaining as hydrogen sources, resulting in a delay. Destruction occurred.
In Experimental Example 2, which is a comparative example, due to the small tip diameter of the electrode, the contact area between the electrode and the steel plate was small, and water and oil, which are hydrogen sources, could not be sufficiently removed, resulting in delayed fracture. There has occurred.
In Experimental Example 6, which is a comparative example, the contact area between the electrode and the steel plate was small due to the fact that P1/h in the preliminary energization process, that is, the pressing force with respect to the plate thickness was small, and water and oil, which serve as hydrogen sources, were small. It could not be removed sufficiently, resulting in delayed destruction.
In Experimental Example 7, which is a comparative example, expulsion occurred because I1/h in the preliminary energization step, that is, the current relative to the plate thickness was large.
In Experimental Example 8, which is a comparative example, due to the fact that P2/h in the main energization process, that is, the pressing force with respect to the plate thickness, was small, the contact area between the electrode and the steel plate became small, the current density increased, and scattering was reduced. Occurred.
In Experimental Example 9, which is a comparative example, heat input was insufficient due to I2/h in the main energization process, that is, the current relative to the plate thickness was small, making it difficult to form a nugget and making it impossible to obtain sufficient joint strength. There wasn't.

According to the present invention, it is possible to provide a resistance spot welded joint that has excellent joint strength and delayed fracture resistance, and has high industrial utility value.

1 First steel plate 2 Second steel plate 10 First electrode 11 Tip portion 12 Continuous portion 13 Main body portion 20 Second electrode 21 Tip portion 22 Continuous portion 23 Main body portion

Claims (4)

A method for manufacturing a resistance spot welded joint by overlapping and joining a plurality of steel plates, including at least one steel plate having a tensile strength of 780 MPa or more, by resistance spot welding, the pair having a tip diameter of 8 mm or more and 12 mm or less. a preliminary energization step of energizing the surface of the steel plate under pressure using a DR electrode;
After the preliminary energization step, a main energization step of energizing the surface of the steel plate under pressure using the pair of DR electrodes;
a holding step of maintaining pressure at the pair of DR electrodes after the main energization step;
Equipped with
The thickness of each of the plurality of steel plates is 0.8 mm to 3.2 mm,
The pressurizing force in the preliminary energization step was P1 (kN), the current was I1 (kA), the energization time was t1 (s), and the pressurizing force in the main energization step was P2 (kN), and the current was I2 (kA). A method for manufacturing a resistance spot welded joint that satisfies the following equations (1) to (5).
2≦I1/h<4 ... (1) formula 1.5≦P1/h<2.5 ... (2) formula 0.1≦t1/h≦1.5 ... (3) formula 4≦I2/h ...Equation (4) 2.5≦P2/h ...Equation (5) However, h is the value of 1/2 of the total thickness (mm) of the plurality of steel plates. be. The method for manufacturing a resistance spot welded joint according to claim 1, wherein the holding time Ht(s) in the holding step is 0.4× ^h2 or less. The method for manufacturing a resistance spot welded joint according to claim 1 or 2, wherein at least one of the plurality of steel plates is a galvanized steel plate. The method for manufacturing a resistance spot welded joint according to claim 3, wherein the holding time Ht(s) in the holding step is 0.08×h2 ^or more.

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