JP7846376B2 - Method for manufacturing spot welded joints and spot welded joints - Google Patents

Description

This invention relates to a method for manufacturing spot-welded joints and to spot-welded joints themselves.

The application of high-strength steel sheets to automotive parts is progressing with the aim of improving fuel efficiency (weight reduction) and collision safety. However, automobiles have thousands of spot welds. When zinc-based plating is applied to spot welds joining high-strength steel sheets to other steel sheets, there is a problem of increased susceptibility to liquid metal embrittlement (LME) cracking. LME cracking is particularly likely to occur when the tensile strength of the high-strength steel sheet exceeds 780 MPa. Therefore, measures to prevent LME cracking have been proposed.

Patent Document 1 discloses a spot welding method that can easily prevent liquid metal embrittlement cracking in spot welding of plated steel sheets. This method is characterized by removing the plating before spot welding, specifically in an area centered on the planned location where the nugget center will be formed, with the outer circumference including the inner circle of the outer edge of the welding-affected zone, or in an area on the overlapping surface of the steel sheets to be welded, sharing a center with the planned location that will become the nugget center formed on the overlapping surface of the steel sheets, with the outer circumference including the inner circle of the outer edge of the welding-affected zone.

Patent Document 2 discloses a resistance spot welding method in which, when joining two galvanized ultra-high-strength steel materials W1 and W2 by overlapping them, an adhesive A containing an element pre-selected to contribute to improving the toughness of the joint (at least one of Ti, Ni, V, Mo, Nb, and Al) is interposed between the two materials, and then resistance spot welding is performed.

Patent Document 3 discloses a spot-welded member in which multiple steel plates are spot-welded, wherein at least one of the multiple steel plates is a high-strength cold-rolled steel plate with a tensile strength of 780 MPa or more and has no plating layer on its surface, and at least one of the multiple steel plates is a zinc-plated steel plate having a zinc-based plating layer on its surface, and the surface Zn concentration inside the corona bond of the spot-welded portion is 1% by mass or more and less than 25% by mass.

Patent Document 4 discloses a resistance spot welding method for spot welding multiple steel plates, each containing at least one zinc-plated steel plate with a tensile strength of 980 MPa or more and containing at least 0.08% by mass or more of carbon and at least 0.50% by mass or more of silicon. The method uses a pair of electrodes comprising: a pair of electrode tips for clamping and applying pressure to the multiple steel plates; and a pressure absorption mechanism provided on at least one of the electrode tips, capable of absorbing the axial pressure applied by the electrode tip. The welding is performed by clamping the multiple steel plates between the pair of electrode tips and applying pressure to the pair of electrode tips while current is passed through, thereby absorbing the fluctuating pressure generated during current application using the pressure absorption mechanism.

International Publication No. 2016-159169 Japanese Patent Publication No. 2020-121334 Japanese Patent Publication No. 2020-179413 Japanese Patent Publication No. 2022-021770

However, the technologies disclosed in Patent Documents 1 to 4 have room for improvement in the following respects.

The technology described in Patent Document 1 requires a step to remove the plating before spot welding. This plating removal step reduces the efficiency of spot welding. Therefore, a means to suppress LME cracking without a plating removal step is desired.

In the technology described in Patent Document 2, resistance spot welding is performed after interposing an adhesive between steel plates. The adhesive application process reduces the efficiency of the spot welding. Therefore, a means of suppressing LME cracking without the adhesive application process is desired.

In the technology described in Patent Document 3, the surface Zn concentration within the corona bond is set to 1% by mass or more and less than 25% by mass. To achieve this, it is necessary to favorably adjust the composition of the zinc-based plating layer. However, from the viewpoint of increasing the flexibility in selecting the zinc-based plating layer, a means of suppressing LME cracking other than that caused by the plating layer components is desired.

In the technology described in Patent Document 4, a pressure absorption mechanism is provided in the electrode tip. However, it is sometimes difficult to provide a pressure absorption mechanism in a spot welding machine. A means of suppressing LME cracking that can be implemented in various spot welding machines is desired.

Therefore, the object of the present invention is to provide a method for manufacturing a spot-welded joint and a spot-welded joint that can more effectively suppress LME cracking.

The gist of this invention is as follows:

(1) A method for manufacturing a spot welded joint according to one aspect of the present invention is a method for manufacturing a spot welded joint comprising the steps of forming a spot welded joint that joins two or more overlapping steel plates by applying current to the steel plates, and applying current to the spot welded joint afterwards, wherein one or more of the steel plates are high-strength steel plates having a tensile strength of 780 MPa or more, one or more of the high-strength steel plates are in contact with a Zn-based plating layer, the B content of the high-strength steel plate in contact with the Zn-based plating layer is 0.0010 mass% or more, in the initial current application, the welding current value _I1 (kA), the initial current application time _t1 (sec), and the Si content per unit mass% of the high-strength steel plate in contact with the Zn-based plating layer satisfy 28 × Si + 1111 < 58 × _I12 × _t1 < ³⁰ × Si + 1250, and in the post-current application, the initial current application time _t1 (kA), the post-current application time _t2 If the Si content in unit mass% of the high-strength steel plate in contact with the Zn-based plating layer satisfies {(-14.3 × Si + 34.3) ^² / 3.7} × ^10⁻⁴ < _t² < 0.5 × _t¹ , and in the post-energization, the post-energization current value _I² (kA) satisfies 0.7 × _I¹ < _I² < 1.5 × _I¹ , and the post-energization current value _I² (kA) of the post-energization satisfies 0.7 × _I¹ < _I² < _I¹ , then the main energization and the post-energization are performed continuously, and if the post-energization current value _I² (kA) of the post-energization satisfies _I¹ ≤ _I² < 1.5 × _I¹ , then the method for manufacturing the spot welded joint further comprises a step of cooling the spot weld between the main energization and the post-energization, and in the cooling, the welding current value I The current current flow time _t1 (sec) and the cooling time _tS (sec) satisfy _the following conditions: (4.07 × _I12 × ^t1 _- 19.30)/1000 < _tS < (4.07 × _I12 × _t1 - ^17.15 )/1000.
(2) In the method for manufacturing a spot welded joint described in (1) above, preferably the method for manufacturing a spot welded joint further comprises a step of holding the pressure applied to the spot weld while the current is not supplied to the spot weld after the step of applying the current, and the holding time for the holding is 0.4 seconds or more.

(3) A spot welded joint according to another aspect of the present invention comprises two or more overlapping steel plates, a spot weld having a nugget for joining the steel plates together, a pressure-welded portion formed around the nugget where the opposing steel plates are pressed together, and a plate gap formed on the outside of the pressure-welded portion, wherein one or more of the steel plates are high-strength steel plates having a tensile strength of 780 MPa or more, and a Zn-based plating layer is disposed on the mating surface between one or more of the high-strength steel plates and the steel plate in contact therewith, The B content of the high-strength steel sheet in contact with the Zn-based plating layer is 0.0010% by mass or more. The number of BN particles (Bulbasinosulfide) with a major axis of 0.5 μm or more, located within the high-strength steel sheet in contact with the Zn-based plating layer in a cross-section passing through the center of the nugget and parallel to the thickness direction of the spot-welded joint, having a square shape with sides of 100 μm, one side of which overlaps with the pressure-welded portion, and the observation area where the center of the one side of the square coincides with the boundary between the pressure-welded portion and the gap between the plates, is 100 or less.

According to the present invention, it is possible to provide a method for manufacturing a spot-welded joint and a spot-welded joint that can more effectively suppress LME cracking.

This is a cross-sectional view showing the schematic configuration of a spot welded joint according to this embodiment. This is an enlarged cross-sectional view of the area near the boundary between the pressure-welded portion and the gap between the plates. This is a cross-sectional photograph of an example of a spot-welded joint where LME cracking occurred.

It is known that boron (B) is effective in suppressing LME cracking that occurs directly outside the pressure-welded joint of high-strength steel plates. However, the inventors have found that the amount of boron in the heat-affected zone (HAZ) of high-strength steel plates and the LME resistance of the HAZ are not necessarily proportional. The inventors have confirmed that LME cracking can occur frequently even in the HAZ of spot-welded joints obtained from high-strength steel plates containing a large amount of boron.

Further detailed investigation by the inventors revealed that when B (boron) is present as BN (boron nitride) in the HAZ (heat-affected zone) of high-strength steel sheets, the LME crack suppression effect cannot be obtained. To suppress LME cracking using B, it is necessary to keep B in a solid solution state in the HAZ. This is thought to allow B to segregate at grain boundaries, preventing molten zinc from penetrating the grain boundaries and effectively suppressing LME cracking.

Furthermore, the inventors have discovered that the optimal welding conditions for solid-solving B in the heat absorption zone (HAZ) vary depending on the composition of the high-strength steel sheet. Conventional high-strength steel sheets contain Si. Si increases the activation energy of N, thereby stabilizing BN. To achieve a solid-solution state of B in the HAZ, it is necessary to sufficiently heat the HAZ to decompose the BN, and then segregate the B in a solid-solution state at the grain boundaries. However, the Si contained in high-strength steel sheets raises the decomposition temperature of BN. Therefore, the inventors have discovered that the higher the Si content of the high-strength steel sheet, the higher the heat input to the HAZ needs to be.

Preferred embodiments of the present invention will be described in detail below with reference to the attached drawings. In this specification and the drawings, components having substantially the same functional configuration are denoted by the same reference numerals, thus omitting redundant explanations.

<1. Method for manufacturing spot welded joints>
First, the method for manufacturing spot-welded joints will be explained. First, multiple steel plates 10 are prepared. Figure 1 shows an example of a spot-welded joint 1 including two steel plates 10, but the number of steel plates 10 may be three or more. Of these steel plates 10, one or more are high-strength steel plates 10H with a tensile strength of 780 MPa or more. Furthermore, one or more of the high-strength steel plates 10H included in the plate assembly are in contact with a Zn-based plating layer 11. The concept of "in contact with a Zn-based plating layer 11" includes the following two cases.
(1) The high-strength steel sheet 10H is a Zn-plated steel sheet. (2) A high-strength steel sheet 10H without a Zn-plated layer 11 and a Zn-plated steel sheet are stacked in contact with each other (see Figure 1).
In either case (1) or (2) above, molten zinc is formed on the surface of the heat-affected zone 123 of the high-strength steel plate 10H during spot welding. It is not necessary for all of the high-strength steel plates 10H to be in contact with the Zn-based plating layer 11. For example, if the plate assembly includes two or more high-strength steel plates 10H, it is sufficient if at least one of the high-strength steel plates 10H is in contact with the Zn-based plating layer 11.

LME cracking is prone to occur in the high-strength steel plate 10H in contact with the Zn-based plating layer 11. Therefore, in the manufacturing method of the spot-welded joint according to this embodiment, the B content of the high-strength steel plate 10H in contact with the Zn-based plating layer 11 is set to 0.0010 mass% (10 ppm) or more. Furthermore, using welding conditions described later, B is dissolved in the heat-affected zone 123 of the high-strength steel plate 10H in contact with the Zn-based plating layer 11. This suppresses LME cracking.

All of the steel plates 10 to be spot-welded may be high-strength steel plates 10H, or any of them may be steel plates with a tensile strength of less than 780 MPa, i.e., low-strength steel plates 10L. The tensile strength of the high-strength steel plates 10H is preferably 980 MPa or higher, 1300 MPa or higher, 1500 MPa or higher, 1700 MPa or higher, or 1900 MPa or higher. While there is no specific upper limit for the tensile strength of the high-strength steel plates 10H, for example, it may be 2700 MPa or lower, 2600 MPa or lower, or 2500 MPa or lower.

The high-strength steel sheet 10H in contact with the Zn-based plating layer 11 contains B. The B content is 0.0010% by mass or more, as described above. When the B content of the high-strength steel sheet 10H in contact with the Zn-based plating layer 11 is within this range, LME cracking can be effectively suppressed. If the B content of the high-strength steel sheet 10H in contact with the Zn-based plating layer 11 is less than 0.0010% by mass, there is too little solid-solution B segregated at the grain boundaries, and LME cracking cannot be effectively suppressed. The lower limit of the B content is preferably 0.0012% by mass, and more preferably 0.0015% by mass.

There is no particular upper limit specified for the B content of the high-strength steel sheet 10H in contact with the Zn-based plating layer 11. From the viewpoint of suppressing LME cracking, a higher B content in the high-strength steel sheet 10H in contact with the Zn-based plating layer 11 is preferable. On the other hand, it is even more preferable to set the B content of the high-strength steel sheet 10H in contact with the Zn-based plating layer 11 to 0.0060 mass% or less. This suppresses the amount of B precipitates and further improves the joint strength of the spot-welded joint 1. The upper limit of the B content is preferably 0.0050 mass%, more preferably 0.0040 mass%, and even more preferably 0.0020 mass%.

Other chemical components are optional. Depending on the application of the spot-welded joint 1, any chemical component can be selected and applied to the high-strength steel plate 10H in contact with the Zn-based plating layer 11.

For example, in the high-strength steel sheet 10H in contact with the Zn-based plating layer 11, the Ti content may be 0.01% by mass or less. Ti forms TiN in the steel, reducing BN and increasing the amount of B in solid solution. Therefore, from the viewpoint of suppressing LME cracking, a higher Ti content in the high-strength steel sheet 10H in contact with the Zn-based plating layer 11 is preferable. On the other hand, by reducing the Ti content of the high-strength steel sheet 10H in contact with the Zn-based plating layer 11, the amount of Ti precipitates can be reduced, further increasing the joint strength of the spot-welded joint 1. In the method for manufacturing the spot-welded joint according to this embodiment, the amount of B in solid solution can be secured in the heat-affected zone 123 of the high-strength steel sheet 10H in contact with the Zn-based plating layer 11 through welding conditions described later. Therefore, even if the Ti content of the high-strength steel sheet 10H in contact with the Zn-based plating layer 11 is 0.01% by mass or less, it is possible to suppress LME cracking using B.

Furthermore, the high-strength steel sheet 10H in contact with the Zn-based plating layer 11 may contain Si. Si increases the activation energy of N, thereby stabilizing BN. Therefore, Si reduces the amount of B in the solid solution state in the high-strength steel sheet 10H in contact with the Zn-based plating layer 11. On the other hand, Si enhances the mechanical properties, such as strength, of the high-strength steel sheet 10H. In the method for manufacturing spot-welded joints according to this embodiment, the welding conditions are adjusted according to the Si content. Therefore, the high-strength steel sheet 10H in contact with the Zn-based plating layer 11 may contain Si. In the high-strength steel sheet 10H in contact with the Zn-based plating layer 11, the Si content may be, for example, 0.01 to 2.00 mass%.

As described above, the B content is limited in the high-strength steel sheet 10H that is in contact with the Zn-based plating layer 11. On the other hand, the B content and chemical composition are not limited in the other steel sheets 10 included in the spot-welded joint 1. The risk of LME cracking is extremely low in the low-strength steel sheet 10L. LME cracking does not occur in the high-strength steel sheet 10H that is not in contact with the Zn-based plating layer 11. Therefore, the low-strength steel sheet 10L and the high-strength steel sheet 10H that is not in contact with the Zn-based plating layer 11 do not need to contain B. That is, in the low-strength steel sheet 10L with a tensile strength of less than 780 MPa, the B content may be less than 0.0010 mass%. Also, even in the case of the high-strength steel sheet 10H with a tensile strength of 780 MPa or more, if it is not in contact with the Zn-based plating layer 11, its B content may be less than 0.0010 mass%.

The type of high-strength steel sheet 10H is not particularly limited. Examples of high-strength steel sheets 10H include DP steel sheets, TRIP steel sheets, composite structure steel sheets, martensitic steel sheets, and hot-stamped steel sheets. Furthermore, the high-strength steel sheet 10H may be either cold-rolled or hot-rolled steel sheets.

Of the multiple steel sheets 10, at least one is a Zn-plated steel sheet having a Zn-plated layer 11. The Zn-plated layer 11 is a plating layer with Zn as the main component. The high-strength steel sheet 10H may be a Zn-plated steel sheet, or the low-strength steel sheet 10L may be a Zn-plated steel sheet. Examples of Zn-plated steel sheets include GI-plated steel sheets, GA-plated steel sheets, EG-plated steel sheets, Zn-Ni-plated steel sheets, Zn-Al-plated steel sheets, Zn-Mg-plated steel sheets, and Zn-Mg-Al-plated steel sheets.

Next, multiple steel plates 10 are overlapped. The entire area of the steel plates 10 may be overlapped, or only a portion may be overlapped. At this time, a gap (plate gap) may occur at the joint surface. However, from the viewpoint of ensuring spot welding quality, the gap should preferably be 2.5 mm or less, and more preferably 1.5 mm or less.

Next, by applying current to multiple (two or more) overlapping steel plates 10, a spot weld 12 is formed to join the steel plates 10 together. Specifically, for example, multiple overlapping steel plates 10 are placed between a pair of opposing electrodes of a spot welding device, and a welding current is applied while applying pressure. Here, the spot weld 12 is a concept that includes a nugget 121 formed between the steel plates 10, a pressure-welded portion 122 formed around the nugget 121 where the opposing steel plates 10 are pressed together, and a gap 13 formed outside the pressure-welded portion 122. Furthermore, the base material around the nugget 121 is usually a heat-affected zone (HAZ) 123, and this heat-affected zone 123 is also included in the weld. The spot welding device is not particularly limited, and any known device can be used as appropriate. A preferred example of a spot welding device is shown below.

The spot welding equipment may be either an inverter-type DC spot welding equipment or a single-phase AC spot welding equipment. The pressure mechanism of the resistance spot welding equipment may be either servo-motor-driven or air-driven. Furthermore, the gun shape may be stationary, C-type, or X-type.

There are no particular restrictions on the electrodes used for spot welding. A suitable example of an electrode is a DR-type electrode with a tip diameter of 5 to 9 mm. The upper and lower electrodes may be the same or different. The electrode material may be chromium copper, zirconium copper, or alumina-dispersed copper. From the viewpoint of suppressing welding between the electrode and the steel plate 10, surface dust generation, and LME cracking, it is preferable to use alumina-dispersed copper as the electrode material.

The welding current value _I1 (kA) during the current application, the current application time _t1 (sec), and the Si content (Si) per unit mass % of the high-strength steel plate 10H in contact with the Zn-based plating layer 11 satisfy the following equation 1.
28×Si+1111<58×I ₁ ² ×t ₁ <30×Si+1250 (Formula 1)

Equation 1 specifies the amount of heat input during the main energization. By performing the main energization in a manner that satisfies Equation 1, BN can be sufficiently decomposed in the heat-affected zone 123 of the high-strength steel plate 10H in contact with the Zn-based plating layer 11. If 58 × I ₁ ² × t ₁ is below the lower limit of Equation 1, the amount of BN becomes excessive in the heat-affected zone 123 of the high-strength steel plate 10H in contact with the Zn-based plating layer 11. As a result, B does not sufficiently segregate at the grain boundaries in the heat-affected zone 123 of the high-strength steel plate 10H in contact with the Zn-based plating layer 11, and LME cracking cannot be suppressed. On the other hand, if 58 × I ₁ ² × t ₁ is above the upper limit of Equation 1, the amount of heat input becomes excessive, and LME is more likely to occur.

The reason why the Si content of the high-strength steel plate 10H in contact with the Zn-based plating layer 11 is included in Equation 1 is that Si has the effect of stabilizing BN. The higher the Si content, the greater the energy required to decompose BN, so it is necessary to increase the heat input during this current application.

The plate assembly may have two or more high-strength steel plates 10H in contact with the Zn-based plating layer 11, and these may have different Si content. In this case, Equation 1 must be satisfied for the Si content of all high-strength steel plates 10H in contact with the Zn-based plating layer 11. On the other hand, the Si content of high-strength steel plates 10H not in contact with the Zn-based plating layer 11, and the Si content of low-strength steel plates 10L, do not affect the conditions for this current application. It is sufficient that Equation 1 is satisfied only for the steel plate 10 where LME cracking is a concern.

Other conditions for energization are not particularly limited and can be appropriately selected depending on the configuration of the plate assembly. For example, the applied pressure during energization is not particularly limited and should be adjusted appropriately so that nuggets 121 are formed between the steel plates 10. As an example, the applied pressure may be set to satisfy the following equation 2. In equation 2, FE is the applied pressure (N) and h is the average plate thickness (the sum of the plate thicknesses of the multiple steel plates 10 divided by the number of steel plates 10) (mm). The applied pressure may be constant during energization or may be changed as appropriate. When the applied pressure is increased, LME cracking tends to be less likely to occur.
1960×h≦FE≦3920×h (Formula 2)

After the initial energization, the spot weld 12 is energized again. Generally, post-energization is performed to temper the hardened nugget 121 and to mitigate segregation of elements such as P. However, in the method for manufacturing spot welded joints according to this embodiment, the main purpose of post-energization is to cause B to segregate at the grain boundaries in a solid solution state in the heat-affected zone 123 of the high-strength steel plate 10H in contact with the Zn-based plating layer 11. During the initial energization, the heat-affected zone 123 of the high-strength steel plate 10H in contact with the Zn-based plating layer 11 is heated to a degree that allows BN to decompose. During post-energization, the heat-affected zone 123 of the high-strength steel plate 10H in contact with the Zn-based plating layer 11 is maintained at a temperature range that allows B to segregate at the grain boundaries in a solid solution state.

During the post-energization, the main energization time _t1 (sec), the post-energization time _t2 (sec), and the Si content (Si) per unit mass % of the high-strength steel plate 10H in contact with the Zn-based plating layer 11 satisfy the following equation 3. Also, the welding current value _I1 (kA) and the post-energization current value _I2 (kA) satisfy the following equation 4.
{(-14.3×Si+34.3) ² /3.7}×10 ⁻⁴ <t ₂ <0.5×t ₁ (Formula 3)
0.7 × I ₁ < I ₂ < 1.5 × I ₁ (Equation 4)

If the post-energizing time _t2 is less than or equal to the lower limit of Equation 3, and/or the post-energizing current value _I2 is less than or equal to the lower limit of Equation 4, it is difficult to maintain B in a solid solution state in the heat-affected zone 123 of the high-strength steel plate 10H in contact with the Zn-based plating layer 11. On the other hand, if _t2 is greater than or equal to the upper limit of Equation 3, and/or the post-energizing current value _I2 is greater than or equal to the upper limit of Equation 4, the temperature of the spot weld 12 becomes excessively high, making LME more likely to occur.

The timing for starting post-energy application is set according to the post-energy application current value (kA). Specifically, if the post-energy application current value _I2 (kA) satisfies the following equation 5, post-energy application is started immediately after the completion of main energy application. That is, main energy application and post-energy application are performed continuously. If the post-energy application current value _I2 satisfies the following equation 6, post-energy application is started after the spot welded joint 1 has been cooled for a cooling time t _s that satisfies the following equation 7. In equations 5 and 6, _I1 (kA) represents the welding current value, and _I2 (kA) represents the post-energy application current value. In equation 7, _I1 (kA) represents the welding current value _I1 (kA), _t1 (sec) represents the main energy application time, and _tS (sec) represents the cooling time.
0.7 × I ₁ < I ₂ < I ₁ (Equation 5)
_I₁ ≤ _I₂ < 1.5 × _I₁ (Equation 6)
(4.07×I ₁ ² ×t ₁ -19.30)/1000<t _s <(4.07×I ₁ ² ×t ₁ -17.15)/1000 (Formula 7)
If _I2 satisfies Equation 5, the initial and subsequent energization are performed continuously. This prevents a decrease in the temperature of the heat-affected zone and further promotes the segregation of B. On the other hand, if _I2 satisfies Equation 6, cooling is performed between the initial and subsequent energization. This cooling is performed to prevent the temperature of the spot weld 12 from becoming excessively high.

Furthermore, cooling of the spot weld 12 can be achieved by stopping the current while the spot weld 12 is still held between the pair of electrodes used for the current application. Conventional spot welding electrodes have a coolant circulating inside, keeping their tips constantly cooled. Therefore, when the current is stopped while the spot weld 12 is still held between the pair of electrodes, heat transfer occurs from the spot weld 12 to the electrodes, cooling the spot weld 12.

The pressure applied during post-energization and cooling is not particularly limited. For example, the pressure applied during post-energization and cooling may be determined in the same way as the pressure applied during initial energization, or it may be the same as the pressure applied during initial energization.

After the post-energization, the pressure applied to the spot weld 12 may be maintained while the current is stopped. During post-energization, the Zn-based plating layer 11 may be in a molten state. By solidifying the Zn-based plating layer 11 through the holding process and ending the state where LME cracking is a concern before releasing the electrodes, LME cracking can be further suppressed.

The pressure applied during holding is not particularly limited. For example, the pressure applied during holding may be determined in the same way as the pressure applied during main energization, or it may be the same as the pressure applied during main energization. The holding time is also not particularly limited, but from the viewpoint of more effectively suppressing LME cracking, a holding time of 0.04 seconds or more, or 0.4 seconds or more, is preferable.

On the other hand, in the method for manufacturing spot-welded joints according to this embodiment, the heat-affected zone 123 of the high-strength steel plate 10H in contact with the Zn-based plating layer 11 is sufficiently modified at the stage when post-energization is completed. Therefore, the holding step may be omitted. Furthermore, even if a holding step is provided, the holding time can be set to a much smaller value than usual (for example, 0.04 seconds or more as described above).

Through the above process, a spot-welded joint 1 with suppressed LME cracking can be manufactured.

<2. Configuration of Spot Welded Joint 1>
Next, the configuration of the spot-welded joint 1 according to this embodiment will be described based on Figure 1. Figure 1 is a cross-sectional view showing a cross-section that passes through the center of the nugget 121 and is parallel to the thickness direction of the spot-welded joint 1.

The spot-welded joint 1 comprises two or more overlapping steel plates 10 and a spot-welded section 12. The spot-welded section 12 includes a nugget 121 that joins multiple steel plates 10, a pressure-welded section 122 formed around the nugget 121 where opposing steel plates 10 are pressed together, and a plate gap 13 formed outside the nugget 121. In the example in Figure 1, two steel plates 10 are spot-welded, but three or more steel plates 10 may be spot-welded. Furthermore, a heat-affected zone (HAZ) 123 is typically formed around the nugget 121. The heat-affected zone 123 is also included in the spot-welded section 12.

The multiple steel plates 10 are the same steel plates 10 described in <1. Method for Manufacturing Spot Welded Joints>. That is, one or more of the multiple steel plates 10 are high-strength steel plates 10H with a tensile strength of 780 MPa or higher. All of the steel plates 10 may be high-strength steel plates 10H, or, as shown in Figure 1, one or more of the steel plates 10 may be low-strength steel plates 10L.

One or more high-strength steel plates 10H are in contact with the Zn-based plating layer 11. That is, one or more high-strength steel plates 10H are Zn-based plated steel plates, or one or more high-strength steel plates 10H are stacked in contact with a Zn-based plated steel plate. The Zn-based plating layer 11 is the Zn-based plating layer 11 described in <1. Method for Manufacturing Spot Welded Joints>. In the example in Figure 1, the Zn-based plating layer 11 is applied to the lower surface (the mating surface with the lower steel plate 10) of the upper steel plate 10, which is a low-strength steel plate 10L. The Zn-based plating layer 11 is in contact with the upper surface of the high-strength steel plate 10H.

The spot welded joint 12 comprises a nugget 121 and a pressure-welded joint 122. The nugget 121 is formed between the steel plates 10 and joins them together. The pressure-welded joint 122 is formed around the nugget 121 and is the portion where opposing steel plates 10 are pressure-welded together. The pressure-welded joint 122 is sometimes referred to as a corona bond. A gap 13 is formed on the outside of the pressure-welded joint 122.

Normally, the pressure-welded portion 122 is formed inside the heat-affected zone 123, and LME cracking is likely to occur in the pressure-welded portion 122. However, in the spot-welded joint 1 according to this embodiment, the amount of BN is reduced in the vicinity of the pressure-welded portion 122. More specifically, in the observation area A shown in the cross-section of Figure 2 (a cross-section passing through the center of the nugget 121 and parallel to the thickness direction of the spot-welded joint 1), the number of BN particles with a major axis of 0.5 μm or more is 100 or less.

As shown in Figure 2, observation area A is provided inside the high-strength steel plate 10H in contact with the Zn-based plating layer 11. It has a square shape with sides of 100 μm, one side of the square overlapping with the pressure-welded portion 122, and the center of one side of the square is set to coincide with the boundary B between the pressure-welded portion 122 and the gap portion 13. If there are two or more high-strength steel plates 10H in contact with the Zn-based plating layer 11, observation area A is provided for each high-strength steel plate 10H in contact with the Zn-based plating layer 11. Furthermore, in each of the two or more observation areas A, the number of BN particles with a major axis of 0.5 μm or more is set to 100 or less.

Observation area A is provided on the high-strength steel sheet 10H in contact with the Zn-based plating layer 11. As described above, the amount of B (biopolymer) is 0.0010% by mass or more in the high-strength steel sheet 10H in contact with the Zn-based plating layer 11. Therefore, when the number of BN particles with a major axis of 0.5 μm or more in observation area A is 100 or less, a sufficient amount of B exists in a solid solution state in observation area A. The B in a solid solution state naturally segregates at the grain boundaries, preventing the penetration of molten zinc into the grain boundaries. Observation area A is the region where LME cracking is most likely to occur, but by sufficiently reducing BN in this region, LME cracking is suppressed very effectively.

The number of BN molecules is measured, for example, by SIMS (Secondary Ion Mass Spectrometry). The measurement conditions are as follows:
• Irradiation ions: ^Bi ₁₊
Acceleration voltage: 300 keV
The number of BNs with a major axis of 0.5 μm or larger included in the image of observation region A obtained in this way is counted. The major axis of a BN is the diameter of the smallest circle that can enclose the BN. As shown in Figure 1, two observation regions A can be defined for one high-strength steel plate 10H, but the above evaluation can be performed in either observation region A.

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.

Steel plate 1 (tensile strength approximately 2 GPa) shown in Table 1 and steel plate 2 (GA980DP steel) shown in Table 2 were used as the base material for spot-welded joints. Spot welding was performed on the plate assembly composed of steel plate 1 and steel plate 2 under the conditions shown in Tables 3 to 5 to obtain the spot-welded joints shown in Table 6. In these spot-welded joints, both steel plate 1 and steel plate 2 fall under the category of "high-strength steel plates in contact with a Zn-based plating layer."

The following conditions are not shown in the table.
Electrodes: Both upper and lower electrodes are made of CrCu, DR type, φ6-R40.
• Pressure applied: 400 gf from the start of the main energization to the end of the holding process.
・Holding time: 0.1sec
- Board thickness: 1.6 mm for both the top and bottom boards
• Basis weight of the Zn-based plating layer on steel plate 2: 45 g/ ^m² per side
- Plating of steel plate 1: None. Also, the values underlined in the table are outside the scope of the present invention.

Table 3 shows the suitability of the current value _I1 and energizing time _t1 for steel plate 1 in the " _I1 - _t1 -Steel Plate 1" column, and the suitability of the current value _I1 and energizing time _t1 for steel plate 2 in the " _I1 - _t1 -Steel Plate 2" column. Specifically, regarding the Si content of steel plate 1, if the relationship "28 × Si + 1111 < 58 × _I12 ^× _t1 < 30 × Si + 1250" is satisfied, "Good" is written in the " _I1 - _t1 -Steel Plate 1" column, and "Bad" is written otherwise. Similarly, regarding the Si content of steel plate 2, if the relationship "28 × Si + 1111 < 58 × _I12 ^× _t1 < 30 × Si + 1250" is satisfied, "Good" is written in the " _I1 - _t1 -Steel Plate 2" column, and "Bad" is written otherwise.

Table 5 shows the suitability of the post-energization time _t2 for steel plate 1 in the "Steel Plate 1 - Post-Energization Time _t2" column, and the suitability of the post-energization time _t2 for steel plate 2 in the "Steel Plate 1 - Post-Energization Time _t2" column. Specifically, with respect to the Si content of steel plate 1, if the relationship "{(-14.3 × Si + 34.3) ^² / 3.7} × ^10⁻⁴ < _t2 " is satisfied, "Good" is written in the "Steel Plate 1 - Post-Energization Time _t2 " column, and "Bad" is written otherwise. Similarly, with respect to the Si content of steel plate 2, if the relationship "{(-14.3 × Si + 34.3) ^² / 3.7} × ^10⁻⁴ < _t2 " is satisfied, "Good" is written in the "Steel Plate 2 - Post-Energization Time _t2 " column, and "Bad" is written otherwise.

The "number of BNs" listed in Table 6 refers to the number of BNs with a major axis of 0.5 μm or larger contained in the observation area A described above. The number of BNs with a major axis of 0.5 μm or larger was measured according to the measurement method described above.

The "Crack Length" column in Table 6 records the observation results of LME cracks confirmed in a cross-section passing through the center of the nugget and parallel to the thickness direction of the spot weld joint. Observation of the cross-section of a spot weld joint where LME cracking occurred clearly shows a crack near the boundary between the welded area and the gap between the plates, as shown in the photograph in Figure 3. The length of this crack was measured, and if the length was 0.25 mm or less, it was judged that LME cracking was suppressed, and "Pass" was recorded in the "Crack Length" column. Otherwise, "Fail" was recorded in the "Crack Length" column.

In the spot-welded joints of Comparative Examples 2, 5, and 6, the crack length was unacceptable. In the spot-welded joints of Comparative Examples 2, 5, and 6, a large amount of BN was present in the HAZ of the high-strength steel plate in contact with the Zn-based plating layer. Therefore, it is presumed that the LME crack suppression effect using solid-solution B did not manifest in the spot-welded joints of Comparative Examples 2, 5, and 6.

In the spot-welded joint of Comparative Example 2, the following conditions were not met with respect to steel plate 1: 28 × Si + 1111 < 58 × I ₁ ² × t ₁ < 30 × Si + 1250, and (4.07 × I ₁ ² × t ₁ - 19.30) / 1000 < t _s < (4.07 × I ₁ ² × t ₁ - 17.15) / 1000. Furthermore, in the spot-welded joint of Comparative Example 2, the post-energizing current value I ₂ was inappropriate. In addition, in the spot-welded joint of Comparative Example 2, the post-energizing time was inappropriate with respect to steel plate 2. The reason for the high amount of BN in the spot-welded joint of Comparative Example 2 is thought to be that the temperature of the weld during the initial energizing did not reach a temperature at which BN could be sufficiently decomposed, and furthermore, the cooling time was long, causing the temperature to drop too low during post-energizing, preventing solid solution of B.

In the spot-welded joint of Comparative Example 5, the condition 28 × Si + 1111 < 58 × I ₁ ² × t ₁ < 30 × Si + 1250 was not satisfied for both steel plate 1 and steel plate 2. Furthermore, the post-energizing time t ₂ was also inappropriate in the spot-welded joint of Comparative Example 5. The reason for the high amount of BN in the spot-welded joint of Comparative Example 5 is thought to be that the temperature of the weld during the main energizing did not reach a temperature at which BN could be sufficiently decomposed.

In the spot-welded joint of Comparative Example 6, the condition 28 × Si + 1111 < 58 × I ₁ ² × t ₁ < 30 × Si + 1250 was not satisfied for both steel plate 1 and steel plate 2. Furthermore, the post-energizing current value I ₂ was also inappropriate in the spot-welded joint of Comparative Example 6. The reason for the high amount of BN in the spot-welded joint of Comparative Example 6 is thought to be that the temperature of the welded area did not reach a temperature at which BN could be sufficiently decomposed during the main energization.

1 Spot welded joint 10 Steel plate 10L Low-strength steel plate 10H High-strength steel plate 11 Zn-based plating layer 12 Spot welded section 121 Nugget 122 Pressure welded section 123 Heat-affected zone (HAZ)
13. Gap between plates A. Observation area B: Boundary between the contact area and the gap between plates.

Claims (3)

A step of forming spot welds that join two or more overlapping steel plates by applying current to them,
The process of applying current to the spot welded area afterwards,
A method for manufacturing a spot welded joint comprising,
One or more of the aforementioned steel plates are high-strength steel plates with a tensile strength of 780 MPa or more.
One or more of the aforementioned high-strength steel plates are in contact with a Zn-based plating layer.
The B content of the high-strength steel sheet in contact with the Zn-based plating layer is 0.0010% by mass or more.
In the current application described above, the welding current value _I1 (kA), the current application time _t1 (sec), and the Si content per unit mass % of the high-strength steel plate in contact with the Zn-based plating layer satisfy 28 × Si + 1111 < ⁵⁸ × _I12 × _t1 < 30 × Si + 1250.
In the aforementioned post-energization, the main energization time _t1 (kA), the post-energization time _t2 (sec), and the Si content in unit mass % of the high-strength steel sheet in contact with the Zn-based plating layer satisfy {(-14.3 × Si + 34.3) ^² / 3.7} ^{× 10⁻⁴} < _t2 < 0.5 × _t1 .
In the aforementioned post-energization, the post-energization current value _I² (kA) satisfies 0.7 × _I¹ < _I² < 1.5 × _I¹ .
If the post-energy current value _I2 (kA) of the post-energy application satisfies 0.7 × _I1 < _I2 < _I1 , then the main energy application and the post-energy application are performed continuously.
If the post-energization current value _I2 (kA) satisfies _I1 ≤ _I2 < 1.5 × _I1 , the method for manufacturing the spot welded joint further includes a step of cooling the spot welded portion between the main energization and the post-energization,
A method for manufacturing a spot welded joint, wherein, during the cooling process, the welding current value _I1 (kA), the energizing time _t1 (sec), and the cooling time _tS (sec) satisfy the following condition: ( ^4.07 × _I12 ^{× t1 - 19.30)/1000 < tS < (4.07 × I12} _{× t1 - 17.15} )/1000. The method for manufacturing the spot welded joint further comprises a step of maintaining pressure applied to the spot weld while suspending the supply of current to the spot weld after the step of applying current,
The method for manufacturing a spot welded joint according to claim 1, characterized in that the holding time for the holding is 0.4 seconds or more. Two or more overlapping steel plates,
A spot weld having a nugget for joining the steel plates together, a pressure-welded portion formed around the nugget where the opposing steel plates are pressed together, and a gap portion formed on the outside of the pressure-welded portion,
A spot welded joint comprising,
One or more of the aforementioned steel plates are high-strength steel plates with a tensile strength of 780 MPa or more.
A Zn-based plating layer is provided on the joint surface between one or more of the aforementioned high-strength steel plates and the steel plates in contact with them.
The B content of the high-strength steel sheet in contact with the Zn-based plating layer is 0.0010% by mass or more.
A spot welded joint having a square shape with sides of 100 μm, where one side of the square overlaps with the pressure-welded portion, and the center of the one side of the square is included in an observation area where the boundary between the pressure-welded portion and the gap between the plates coincides, and the number of BN particles with a major axis of 0.5 μm or more is 100 or less.