Spot welding method and spot welded joint

Abstract

The present invention provides a novel spot welding method and spot-welded joint that allows for tempering of the ends of the nugget and obtaining excellent joint strength, even when using high-strength steel plates with a tensile strength of 1350 MPa or more. [Solution] The spot welding method of the present invention comprises a first step (S1) of spot welding a plate assembly consisting of two or more steel plates, at least one of which is a high-strength steel plate having a tensile strength of 1350 MPa or more, to form a nugget (3) on the overlapping surface of the plate assembly, The process includes a second step (S2) in which a molten and solidified portion (5) is formed in a region inside the outer edge (3E) of the nugget (3) as viewed from above, and which penetrates the two or more steel plates in the thickness direction, The molten and solidified portion (5) is the outer diameter D of the molten and solidified portion (5) as viewed from above. L and nugget diameter D N The relationship is 0.3 ≤ D L / D N It is characterized by being formed to satisfy <1.0.

Description

[Technical Field]

[0001] This invention relates to a spot welding method and a spot welding joint. [Background technology]

[0002] In recent years, there has been a trend towards increasing the strength of steel sheets used in various fields such as automobiles, home appliances, and building materials. For example, in the automotive sector, the use of thin, high-strength steel sheets is increasing in order to lighten vehicle bodies and components and improve fuel efficiency.

[0003] Furthermore, in the case of high-strength steel sheets used in automobile bodies and parts, not only unplated high-strength steel sheets are used, but also high-strength steel sheets with a surface plated with metals such as aluminum for rust prevention.

[0004] On the other hand, the assembly of automobile bodies and the attachment of parts are mainly carried out by spot welding from the standpoint of cost and manufacturing efficiency. The quality of welds formed by such spot welding is evaluated by the tensile shear strength (TSS) and cross tensile strength (CTS), and it is known that the TSS increases as the tensile strength of the base material used increases. However, when using high-strength steel plates with a tensile strength of 780 MPa or higher as the base material, the peel strength, such as CTS, tends to decrease as the tensile strength of the base material increases. At 1350 MPa or higher, the tendency for the peel strength to decrease becomes particularly pronounced.

[0005] Generally, the reason for the decrease in CTS is thought to be that rapid cooling after nugget formation leads to the formation of a hard martensitic structure, which reduces the toughness of the nugget edges. When the toughness of the nugget edges decreases in this way, cracks propagate from the edges of the nugget, making it more susceptible to brittle fracture.

[0006] Therefore, in order to prevent such brittle fracture, various methods are being considered to improve toughness by tempering the nugget and the surrounding tissue. One such method is the post-tempering method, in which the nugget is formed by the initial current, cooled, and then tempered again by applying current.

[0007] For example, Patent Document 1 discloses a welding method for overlapping portions, comprising: a resistance spot welding step of forming a spot welded portion by resistance spot welding at the overlapping portion of a plurality of steel plate members, the spot weld having a nugget, a heat-affected zone formed around the nugget, and a softened portion within the heat-affected zone where the Vickers hardness is lowest; and a tempering step of forming a tempered region between the center of the nugget and the softened portion, which is made of tempered martensite and has a Vickers hardness of 120% or less when the Vickers hardness of the softened portion is taken as 100%.

[0008] According to the method disclosed in Patent Document 1, it is possible to improve the peel strength in spot welds.

[0009] Furthermore, Patent Document 2 describes a first energizing step in which a plate assembly made of two or more steel plates, each containing at least one steel plate with a carbon content of 0.280% or more and 0.700% by mass or less, is clamped in the thickness direction between a pair of electrodes and subjected to pressure while being energized with a current value I1 (kA), and a time t of 20 ms to 200 ms after the first energizing step. c1 A first de-energizing step in which the device is de-energized, a second energizing step after the first de-energizing step in which the device is energized with a current value I2 (kA) that satisfies equation (1): 0.60 ≤ I2 / I1 ≤ 1.10 and a time t2 (ms) that satisfies equation (2): 50 ≤ t2 ≤ 1000, and after the second energizing step, equation (3): t c2 >3.5×10 -3 × Ms 2 -3.3 × Ms + 1100 is satisfied by the time t c2 After (ms) has elapsed, the tempering temperature at the energized position is 350°C or higher, and the formula (A): H = T × (log t HTA method for manufacturing a spot welded joint is disclosed, which includes a tempering step in which tempering is performed under the condition that the tempering parameter H calculated by +(17.7-5.8×[C])) is between 8000 and 18000.

[0010] According to the method disclosed in Patent Document 2, even when using a plate assembly that includes steel plates with a relatively high carbon content, the joint strength can be significantly improved compared to when resistance spot welding is performed with single current. [Prior art documents] [Patent Documents]

[0011] [Patent Document 1] International Publication No. 2014 / 025063 [Patent Document 2] International Publication No. 2022 / 210749 [Overview of the Initiative] [Problems that the invention aims to solve]

[0012] However, conventional post-energization methods have a narrow range of conditions under which the desired effect can be obtained, and are susceptible to various disturbances that occur in actual production sites (i.e., have low robustness), making them difficult to apply to actual production sites. Examples of disturbances that occur in actual production sites include dust generation, electrode wear, electrode misalignment, and gaps between steel plates.

[0013] In particular, when using high-strength steel plates with a tensile strength of 1350 MPa or higher as the base material, such disturbances may prevent stable tempering of the weld, or more specifically, the ends of the nugget. Therefore, there is a need for a practical and novel method that can stably temper the ends of the nugget even when using high-strength steel plates.

[0014] The present invention has been made in view of such circumstances, and even when using a high-strength steel sheet with a tensile strength of 1350 MPa or more, it is possible to temper the end of the nugget and obtain excellent joint strength, and an object thereof is to provide a novel spot welding method. Further, an object of the present invention is to provide a novel spot welded joint in which, even when including the above-mentioned high-strength steel sheet, the end of the nugget is tempered and has excellent joint strength.

Means for Solving the Problems

[0015] The present invention includes the following aspects.

[0016] (Aspect 1) A spot welding method for a plate assembly composed of two or more steel sheets, at least one steel sheet constituting the plate assembly is a high-strength steel sheet having a tensile strength of 1350 MPa or more, a first step of performing spot welding on the plate assembly to form a nugget on the overlapping surface of the plate assembly; a second step of forming a molten and solidified portion penetrating the two or more steel sheets in the plate thickness direction in a region inside the outer edge of the nugget in a top view; including, the outer diameter D of the molten and solidified portion in a top view L and the nugget diameter D N and the relationship is 0.3 ≦ D L / D N < 1.0, and the spot welding method is characterized in that it is formed so as to satisfy.

[0017] (Aspect 2) The spot welding method according to the above Aspect 1, wherein the means for forming the molten and solidified portion is laser irradiation.

[0018] (Aspect 3) The spot welding method according to the above Aspect 1 or 2, wherein the region for forming the molten and solidified portion is a ring-shaped region in a top view.

[0019] (Aspect 4) The spot welding method according to embodiment 1 or 2, characterized in that the region forming the molten and solidified portion is a circular region when viewed from above.

[0020] (Appendix 5) A spot-welded joint composed of two or more steel plates, including a high-strength steel plate having a tensile strength of 1350 MPa or more, The overlapping surfaces of the two or more steel plates described above have nuggets, In a top view, a molten and solidified portion is formed in the area inside the outer edge of the nugget, penetrating the two or more steel plates in the thickness direction. The molten and solidified portion is the outer diameter D of the molten and solidified portion as viewed from above. L and nugget diameter D N The relationship is 0.3 ≤ D L / D N A spot-welded joint characterized by being formed to satisfy <1.0.

[0021] (Aspect 6) The spot welded joint according to embodiment 5, characterized in that the molten and solidified portion is formed in a ring shape when viewed from above.

[0022] (Aspect 7) The spot welded joint according to embodiment 5, characterized in that the molten and solidified portion is formed in a circular shape when viewed from above. [Effects of the Invention]

[0023] According to the spot welding method of the present invention, even when using high-strength steel plates with a tensile strength of 1350 MPa or more, the ends of the nugget can be tempered more reliably, and excellent joint strength can be obtained. Furthermore, according to the present invention, even when including such high-strength steel plates, the ends of the nugget are reliably tempered, and a spot-welded joint with excellent joint strength can be provided. [Brief explanation of the drawing]

[0024] [Figure 1]Figure 1 is a schematic cross-sectional view illustrating the first step S1 of a spot welding method, which is one embodiment of the present invention. [Figure 2] Figure 2 is a schematic cross-sectional view illustrating the second step S2 of a spot welding method, which is one embodiment of the present invention. [Figure 3] Figure 3 is a schematic diagram showing a cross-sectional view and a top view of a spot-welded joint 1 obtained by a spot welding method according to one embodiment of the present invention. [Figure 4] Figure 4 is a schematic diagram showing a cross-sectional view and a top view of a spot-welded joint 1 obtained by a spot-welding method according to another embodiment of the present invention. [Figure 5] Figure 5 is a magnified cross-sectional photograph of the spot-welded joint of Example 1 of the present invention. [Figure 6] Figure 6 is a magnified cross-sectional photograph of the spot-welded joint in Example 2 of the present invention. [Figure 7] Figure 7 is a magnified cross-sectional photograph of the spot-welded joint in Comparative Example 1. [Figure 8] Figure 8 is a magnified cross-sectional photograph of the spot-welded joint in Comparative Example 2. [Modes for carrying out the invention]

[0025] Hereinafter, preferred embodiments of the spot welding method and spot welded joint of the present invention will be described in detail with reference to the drawings.

[0026] To achieve the above objective, the inventors diligently studied post-welding processes. As a result, the inventors discovered that by forming a molten and solidified area that penetrates two or more steel plates in the thickness direction in a specific region inside the outer edge of the nugget when viewed from above, even when using high-strength steel plates, the nugget ends can be stably tempered, making them less susceptible to disturbances.

[0027] The present invention was completed based on these findings and includes the following embodiments of spot welding methods and spot welding joints.

[0028] First, preferred embodiments of the spot welding method of the present invention will be described in detail with reference to the drawings.

[0029] <Spot welding method> One embodiment of the present invention is a spot welding method for a plate assembly consisting of two or more steel plates. In the spot welding method of this embodiment, at least one of the two or more steel plates constituting the plate assembly is a high-strength steel plate having a tensile strength of 1350 MPa or more.

[0030] The spot welding method of this embodiment includes a first step of spot welding the above-mentioned plate assembly to form a nugget on the overlapping surface of the plate assembly, and a second step of forming a molten and solidified portion that penetrates two or more steel plates in the thickness direction in a region inside the outer edge of the nugget when viewed from above. Here, the molten and solidified portion has an outer diameter D of the molten and solidified portion when viewed from above. L and nugget diameter D N The relationship is 0.3 ≤ D L / D N It is formed such that it satisfies <1.0.

[0031] The spot welding method of this embodiment will be described in detail below with reference to the drawings. The following description will focus on a spot welding method for a plate assembly consisting of two steel plates.

[0032] Here, Figures 1 and 2 are schematic cross-sectional diagrams illustrating the first step S1 and the second step S2 of the spot welding method of this embodiment, respectively. Figure 3 is a schematic cross-sectional and top view of the spot-welded joint 1 obtained by the spot welding method of this embodiment.

[0033] As shown in Figure 1, the spot welding method of this embodiment first involves performing spot welding on a plate assembly consisting of two steel plates, an upper steel plate 21 and a lower steel plate 22, as the first step S1. In this embodiment, at least one of the steel plates 21 and 22 is a high-strength steel plate having a tensile strength of 1350 MPa or more.

[0034] In the first step S1, as shown in Figure 1, spot welding is performed by clamping the plate assembly consisting of steel plates 21 and 22 between a pair of electrodes, an upper electrode 41 and a lower electrode 42, and applying current. As a result, a nugget 3 made of molten metal is formed around the overlapping surface of steel plates 21 and 22.

[0035] In the first step S1, as shown in Figures 1 and 2, an indentation IP is formed on the surface of the steel plate 21 and the steel plate 22 by pressing in a pair of electrodes.

[0036] Furthermore, in the spot welding method of this embodiment, after the first step S1, as shown in Figure 2, a second step S2 is performed to form a molten and solidified portion 5 that penetrates the steel plates 21 and 22 in the thickness direction in a region inside the outer edge 3E of the nugget 3 when viewed from above. Here, as shown in Figure 3, the outer diameter D of the molten and solidified portion 5 when viewed from above is L and nugget diameter D N The relationship is 0.3 ≤ D L / D N It is formed such that it satisfies <1.0.

[0037] (Top view) Herein, in this specification, "top view" means that when the spot welded joint is placed on a horizontal plane such that the thickness direction of each steel plate constituting the spot welded joint is the vertical direction, the object of observation in the steel plate or welded joint (e.g., the molten and solidified area) is viewed from above in the vertical direction.

[0038] (Molten and solidified part) Furthermore, in this specification, "molten and solidified area" refers to a portion of the steel plate constituting a spot-welded joint that has locally melted and solidified. The molten and solidified area can be clearly identified in the cross-section of the spot-welded joint as a portion where a characteristic solidification structure called a dendrite or dendritic crystal is formed.

[0039] (through) In this specification, "the molten and solidified portion penetrates the thickness direction of two or more steel plates" means, when a spot welded joint is placed on a horizontal plane such that the thickness direction of each steel plate constituting the spot welded joint is vertical, the molten and solidified portion extends in the thickness direction from the upper surface of the uppermost steel plate and reaches the lower surface of the lowermost steel plate (i.e., a complete penetration state), as well as the molten and solidified portion extends in the thickness direction from the upper surface of the uppermost steel plate and reaches a depth of 80% or more of the total thickness of all the steel plates constituting the spot welded joint (i.e., a substantially penetration state).

[0040] Furthermore, it is preferable that the molten and solidified portion penetrates the two or more steel plates in the thickness direction in a completely penetrating state. When the molten and solidified portion penetrates completely, the ends of the nugget can be tempered more reliably regardless of whether any of the two or more steel plates is a high-strength steel plate, and it is possible to visually confirm that the molten and solidified portion is formed from both sides of the spot weld joint (i.e., the upper surface of the upper plate and the lower surface of the lower plate).

[0041] (Outer diameter D of the molten and solidified part) L ) Furthermore, in this specification, "outer diameter D of the molten and solidified portion" L " refers to the distance between the outer edges of the molten and solidified area, centered on the nugget center. For example, if the molten and solidified area is formed in a ring shape when viewed from above, the outer diameter D of the molten and solidified area is the outer diameter D of the molten and solidified area. L This represents the outer diameter of the ring-shaped molten and solidified area.

[0042] Furthermore, in this specification, "Nugget diameter D N " is the maximum length of the nugget in the direction of the plate surface. The direction of the plate surface is the direction parallel to the plate surface of the steel plate and perpendicular to the plate thickness direction. Outer diameter D of the molten and solidified portion L and nugget diameter D N It can be measured by the following measurement method.

[0043] <Outer diameter D of the molten and solidified part> L and nugget diameter D N Measurement method > The spot-welded joint to be measured is cut along the thickness direction, passing through the center of the indentation (i.e., the nugget center), to expose the cross-section of the welded joint. This cross-section is then filled with a cold-curing resin. Next, the sample observation surface after resin filling is roughly polished using waterproof abrasive paper with grits of 80, 400, 800, and 1500, and then finely polished using a 3 μm diamond spray. Furthermore, the sample observation surface after polishing is etched to the extent that the molten boundary can be identified, making the nugget visible. At this time, the molten and solidified area is also in a state where it can be identified as a solidified structure with characteristic patterns called dendrites or dendritic crystals. The etching solution used to etch the sample observation surface is picric acid. Finally, the etched sample observation surface is photographed using a microscope at a magnification of 5 to 50 times (preferably 25 times or more) to obtain a magnified photograph of the cross-section. From the magnified cross-sectional photograph, the distance (mm) between the outer edges of the molten and solidified area centered on the nugget's center and the maximum length (mm) of the nugget in the direction of the plate surface were measured, and the outer diameter D of the molten and solidified area was determined accordingly. L and nugget diameter D N The outer diameter D of the molten and solidified portion is as follows. L Regarding this, the outer diameter D of the spot welded joint has different values ​​on one side (e.g., the side facing the laser) and the other side. L If it is measured, the minimum value of the outer diameter D L We will adopt this.

[0044] Furthermore, if the outer edge of the nugget overlaps with the molten and solidified area, the outer edge of the nugget cannot be defined. That is, the nugget diameter D N It is not possible to measure the outer diameter D of the molten and solidified part. L nugget diameter D N It is clear that it is greater than D L / D N Determined as >1.0

[0045] (Outer diameter D of the molten and solidified part) L and nugget diameter D N Relationship: 0.3≦D L / D N <1.0) As described above, in the second step S2, the molten and solidified portion 5 that penetrates the steel plates 21 and 22 in the thickness direction is defined by the outer diameter D of the molten and solidified portion 5 in a top view. L and nugget diameter D N The relationship is 0.3 ≤ D L / D N The structure is formed to satisfy <1.0. That is, the molten and solidified portion 5 is formed in a region inside the outer edge 3E of the nugget 3 when viewed from above, at a position within a specific range from the outer edge 3E of the nugget 3 that satisfies the above relation.

[0046] By forming a molten and solidified section 5 that penetrates the steel plates 21 and 22 in the thickness direction at such a specific location, the heat generated during the formation of the molten and solidified section 5 is transferred to the end 31 of the nugget, allowing for reliable tempering of the end 31. The amount of heat input during the formation of the molten and solidified section 5 is the amount of heat required to penetrate the steel plates 21 and 22 in the thickness direction. As the heat generated during the formation of this molten and solidified section 5 is transferred from the center of the nugget 3 to the end 31, it is less susceptible to disturbances caused by gaps between the steel plates, and the end 31 of the nugget can be tempered more reliably while preventing re-hardening of the end 31. Furthermore, because the molten and solidified section 5 is formed to penetrate the steel plates 21 and 22 in the thickness direction, the end 31 of the nugget can be tempered even more reliably, regardless of whether the steel plate 21 or 22 is a high-strength steel plate.

[0047] Outer diameter D of the molten and solidified part L and nugget diameter D N In the relationship between D L / D N If the lower limit is less than 0.3, there is a risk that it will be difficult to temper the ends 31 of the nugget. L / D N The lower limit may be 0.4 or higher, 0.5 or higher, or 0.6 or higher. On the other hand, D L / D N The upper limit of this value is less than 1.0, and may be 0.9 or less, or 0.8 or less, considering that the heat used to form the molten and solidified portion 5 is transferred from the center of the nugget 3 to the end portion 31.

[0048] (Effects and Benefits) As described above, the spot welding method of this embodiment forms a molten and solidified portion 5 that penetrates the steel plates 21 and 22 in the thickness direction after spot welding, satisfying the above relational equation. This makes it less susceptible to disturbances caused by gaps between the steel plates, regardless of whether the steel plates 21 or 22 are high-strength steel plates. Furthermore, it prevents re-hardening of the nugget ends 31 while more reliably tempering the nugget ends 31. Therefore, according to the spot welding method of this embodiment, even when using high-strength steel plates with a tensile strength of 1350 MPa or more, the nugget ends 31 can be stably tempered, and excellent joint strength can be obtained.

[0049] In this specification, joint strength refers to joint strength evaluated by at least cross tensile strength (CTS). CTS can be measured according to the cross tensile test based on JIS Z 3137:1999. Specific measurement conditions for CTS will be described in the examples below.

[0050] (steel plate) The two or more steel plates used in spot welding are high-strength steel plates, with at least one of them having a tensile strength of 1350 MPa or more. When using such high-strength steel plates, the risk of reduced joint strength, such as in CTS, increases significantly. However, the spot welding method of the present invention can stably temper the ends of the nugget even when using high-strength steel plates, thereby obtaining excellent joint strength. Therefore, the present invention is particularly advantageous when using such high-strength steel plates.

[0051] The tensile strength of high-strength steel sheets may be 1600 MPa or higher, 1800 MPa or higher, 2000 MPa or higher, 2300 MPa or higher, or 2500 MPa or higher. While there is no particular upper limit to the tensile strength of high-strength steel sheets, from the viewpoint of workability, for example, it is 3000 MPa or 2000 MPa.

[0052] The two or more steel plates used for spot welding may all be high-strength steel plates, or only some of the two or more steel plates may be high-strength steel plates. For example, when using three steel plates, at least one of the top or bottom plates may be a high-strength steel plate, and the intermediate plate between the top and bottom plates may be a high-strength steel plate or a steel plate other than a high-strength steel plate.

[0053] Furthermore, there are no particular limitations on steel plates other than high-strength steel plates that can be used together with high-strength steel plates, and any steel plate can be used according to the desired joint strength, etc. The tensile strength of steel plates other than high-strength steel plates is not particularly limited, but from the viewpoint of joint strength, it may be 780 MPa or higher or 980 MPa or higher.

[0054] The tensile strength of a steel plate is determined by first measuring its Vickers hardness and then converting it. The Vickers hardness of a steel plate is measured according to the method specified in JIS Z 2244:2009 "Vickers hardness test - Test method". The Vickers hardness of the steel plate is measured at a depth of 1 t / 4 of the plate thickness t, under a test load of 500 g. From the Vickers hardness (HV) of the steel plate obtained in this way, the tensile strength (MPa) of the steel plate is calculated using the formula: Tensile strength (MPa) = 3.3 × Vickers hardness (HV).

[0055] The two or more steel plates used for spot welding may be unplated steel plates, zinc-plated steel plates, or aluminum-plated steel plates. Furthermore, examples of zinc plating types for zinc-plated steel plates include hot-dip galvanizing (GI), alloyed hot-dip galvanizing (GA), hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating, and hot-dip Zn-Al-Mg-Si alloy plating. Similarly, examples of aluminum plating types for aluminum-plated steel plates include aluminum-silicon alloy plating.

[0056] The two or more steel plates used for spot welding may be of the same type or strength as all of them, some of them may be of the same type or strength as only some of them, or all of them may be of different types or strengths.

[0057] Furthermore, the number of steel plates is not particularly limited as long as it is two or more; any number of plates (for example, two, three, four, or five or more) can be used depending on the application of the welded joint. Also, the thickness of the steel plates is not particularly limited; for example, a thickness of 0.5 mm or more or a thickness of 3.5 mm or less is acceptable.

[0058] The following describes in detail each step in the spot welding method of this embodiment with reference to the drawings.

[0059] [1st step] In the spot welding method of this embodiment, the first step S1 is a step of spot welding to a plate assembly composed of an upper steel plate 21 and a lower steel plate 22, as shown in Figure 1. In this embodiment, at least one of the steel plates 21 and 22 is a high-strength steel plate having a tensile strength of 1350 MPa or more. That is, the plate assembly consisting of steel plates 21 and 22 may consist of steel plate 21 being a high-strength steel plate, steel plate 22 being a high-strength steel plate, or both steel plates 21 and 22 being high-strength steel plates.

[0060] In the first step S1, the plate assembly consisting of overlapping steel plates 21 and 22 is first clamped between a pair of electrodes, an upper electrode 41 and a lower electrode 42. Then, while applying pressure to the plate assembly in the thickness direction with this pair of electrodes, current is passed through in the thickness direction at a predetermined current value and time. As a result, the overlapping surfaces of steel plates 21 and 22 and the surrounding areas melt due to Joule heating caused by electrical resistance, and a nugget 3 is formed by the molten metal. At this time, an indentation IP is formed on the outer surface of each of the steel plates 21 and 22 by the pressing of the upper electrode 41 and the lower electrode 42. The indentation IP has a flat portion formed where it contacts the top of the electrode and a shoulder portion which is a stepped portion formed where it contacts the shoulder of the electrode.

[0061] (Welding conditions) In the first step S1, a welding machine, electrode, and welding conditions commonly used in spot welding can be employed, as long as they can form a nugget 3 on the overlapping surface of the overlapping steel plates 21 and 22 and in the vicinity thereof.

[0062] For example, spot welding machines can use various power sources such as inverter DC power supplies, inverter AC power supplies, and single-phase AC power supplies. The welding machine can be configured as a spot welding robot system combining a welding gun and an industrial robot, or as a stationary type.

[0063] For example, a pair of electrodes used in spot welding might be a DR-type electrode made of chromium copper with an electrode tip diameter of 5mm to 8mm. Furthermore, the applied pressure by this pair of electrodes could be, for example, 250kgf to 700kgf (2.451kN to 6.864kN). The current value during energization could be, for example, 4kA to 12kA. Furthermore, the energization time could be, for example, 12 cycles to 60 cycles. Note that if the power supply frequency is 50Hz, 1 cycle (1 cyc) is 1 / 50 of a second.

[0064] In the first step, there is no particular limit to the number of times the current is applied during spot welding; it may be only once or two or more times.

[0065] In addition, in the first step, pre-energization or post-energization may be performed before or after the energization that forms the nuggets, as long as it does not hinder the effects of the present invention.

[0066] [Second process] In the spot welding method of this embodiment, the second step S2 is a step in which a molten and solidified portion 5 is formed in the region inside the outer edge 3E of the nugget 3 when viewed from above, and which penetrates the steel plates 21 and 22 in the thickness direction, as shown in Figure 2. In the second step S2, the molten and solidified portion 5 is formed in the region inside the outer edge 3E of the nugget 3 when viewed from above, as shown in Figure 3, and the outer diameter D of the molten and solidified portion 5 is formed in the top view. Land nugget diameter D N The relationship is 0.3 ≤ D L / D N Form it so that it satisfies <1.0.

[0067] In the example shown in Figure 2, the molten and solidified portion 5 is formed by irradiating a predetermined position on the upper surface of the steel plate 21 with a laser LB, thereby melting the steel plate 21 and the steel plate 22 in the thickness direction. The means for melting the steel plate used to form the molten and solidified portion 5 will be described later.

[0068] In the second step S2, a molten and solidified portion 5 is formed at the specific location described above, penetrating the steel plates 21 and 22 in the thickness direction. This allows the heat generated during the formation of the molten and solidified portion 5 to be transferred from the center of the nugget 3 to the end portion 31. As a result, even when using high-strength steel plates with a tensile strength of 1350 MPa or more, the process is less susceptible to disturbances caused by gaps between the steel plates, and the end portion 31 of the nugget can be tempered more reliably while preventing re-hardening of the end portion 31. Furthermore, because the molten and solidified portion 5 is formed to penetrate the steel plates 21 and 22 in the thickness direction, the end portion 31 of the nugget can be tempered even more reliably, regardless of whether the steel plate 21 or 22 is a high-strength steel plate.

[0069] Furthermore, the depth of the molten and solidified portion 5 formed in the second step S2 (i.e., the depth of the completely penetrating or substantially penetrating state described above) can be adjusted by appropriately setting various conditions of the melting means. For example, when laser irradiation is used as the melting means, the depth of the molten and solidified portion 5 can be adjusted by appropriately setting the laser output, irradiation radius, and speed.

[0070] The melting means used to form the molten and solidified portion is a means that can melt each steel plate so as to penetrate it in the thickness direction. Examples of such melting means include laser irradiation machines, arc welding machines, and plasma welding machines. Among these, it is preferable to use laser irradiation with a laser irradiation machine. When laser irradiation is used as the means to form the molten and solidified portion, there are fewer constraints on the conditions required to temper the ends of the nugget (for example, the position and temperature of the heating target, the surface shape of the steel plate, etc.), and tempering of the ends of the nugget can be achieved with higher precision and in a simpler manner.

[0071] Furthermore, when forming the molten and solidified portion by laser irradiation, the laser irradiation conditions are not particularly limited as long as they are sufficient to melt each steel plate in the thickness direction, and any conditions can be adopted depending on the type of steel plate, productivity, etc. Examples of laser irradiation conditions include the laser output, irradiation radius, and speed.

[0072] Furthermore, the melting means, such as laser irradiation, used to form the molten and solidified portion 5 may be applied from the upper surface of the steel plate 21 (upper plate), from the lower surface of the steel plate 22 (lower plate), or from both the upper surface of the steel plate 21 and the lower surface of the steel plate 22. Since the molten and solidified portion 5 is formed by penetrating the steel plates 21 and 22 in the thickness direction, the end portion 31 of the nugget can be reliably tempered regardless of which steel plate surface the melting means is applied from.

[0073] Furthermore, the region forming the molten and solidified portion 5 has an outer diameter D of the molten and solidified portion 5 when viewed from above. L and nugget diameter D N The relationship is 0.3 ≤ D L / D N The requirements are not particularly limited, as long as they satisfy <1.0. For example, the region forming the molten and solidified portion 5 may be a ring-shaped region when viewed from above, as shown in Figure 3.

[0074] By forming the molten and solidified portion 5 in such a ring shape, the entire or almost entire area of ​​the end portion 31 of the nugget can be tempered, and the energy and time required to temper the end portion 31 of the nugget can be further reduced. In other words, the entire or almost entire area of ​​the end portion 31 of the nugget can be tempered more efficiently.

[0075] The means for forming the molten and solidified portion 5 into a ring shape are not particularly limited. For example, a melting means such as laser irradiation may be moved to trace a predetermined ring shape while melting a predetermined portion of the welded joint into a ring shape, or a melting means having a ring shape may be used to melt a predetermined portion of the welded joint into a ring shape all at once.

[0076] Furthermore, the ring-shaped region forming the molten and solidified area 5 is a region in which strip-shaped regions of a predetermined width are connected in an annular shape, and the width of the strip-shaped region can be appropriately set within a range that allows the ends of the nugget to be tempered. For example, when laser irradiation is used as the melting means, the width of the ring-shaped molten and solidified area 5 can be controlled by appropriately setting the laser beam diameter, output, and speed. It is also possible to control the width of the ring-shaped molten and solidified area 5 by performing laser irradiation multiple times, and this method makes it possible to form a wider molten and solidified area 5. Note that the molten and solidified area 5, which is formed in a ring shape when viewed from above, has a cylindrical structure when viewed in three dimensions.

[0077] In this embodiment, the region where the molten and solidified area is formed is not limited to such a ring-shaped region. For example, if areas prone to strength reduction are known in advance based on experience or structure, the molten and solidified area may be formed to focus on such areas. For example, the molten and solidified area may be formed in a semi-ring shape, an intermittent ring shape, a circular shape, a semicircular shape, a fan shape, a curved shape, a straight line shape, a shape combining multiple straight lines, or any other geometric shape when viewed from above.

[0078] As described above, in this embodiment, the region forming the molten and solidified portion 5 may be a circular region when viewed from above. Here, Figure 4 is a schematic diagram of a cross-sectional view and a top view of a spot welded joint 1 obtained by a spot welding method of another embodiment of the present invention.

[0079] In the spot welded joint 1 shown in Figure 4, the circular molten and solidified portion 5 is formed to penetrate the steel plates 21 and 22 in the thickness direction, and the outer diameter D of the circular molten and solidified portion 5 is visible from above. L and nugget diameter D N The relationship is 0.3 ≤ D L / D N It satisfies <1.0.

[0080] As shown in Figure 4, the circular molten and solidified portion 5 is formed over the entire or nearly entire area inside the circular outer edge of the molten and solidified portion 5 when viewed from above. The circular molten and solidified portion 5, when viewed from above, has a cylindrical structure when viewed in three dimensions. Except for the circular shape of the molten and solidified portion 5 when viewed from above, it is the same as the spot welded joint 1 shown in Figure 3 above.

[0081] By forming the molten and solidified portion 5 in this circular shape, the entire or nearly entire end portion 31 of the nugget can be tempered more reliably.

[0082] The means for forming the molten and solidified portion 5 in a circular shape are not particularly limited. For example, a melting means such as laser irradiation may be moved in a predetermined circular pattern and across the entire area within the circle to melt a predetermined portion of the welded joint in a circular shape. Alternatively, a laser irradiator with a predetermined circular irradiation range or a melting means having a circular shape may be used to melt a predetermined portion of the welded joint in a circular shape all at once.

[0083] As described above, by forming the specific molten and solidified portion 5 in the second step S2, even when using high-strength steel plates with a tensile strength of 1350 MPa or more, the effects of disturbances caused by gaps between steel plates are less likely to occur, and the ends 31 of the nugget can be tempered more reliably while preventing re-quenching of the ends of the nugget.

[0084] (Other processes) In the spot welding method of the present invention, any steps performed before or after each of the first and second steps, as is done in a normal spot welding method, may be performed, as long as they do not hinder the effects of the present invention. Examples of such optional steps include a plate assembly step, a cooling step, and various surface treatment steps.

[0085] Next, a spot-welded joint obtained by the spot welding method of the present invention, that is, a spot-welded joint which is yet another embodiment of the present invention, will be described in detail with reference to the drawings.

[0086] <Spot welded joints> As described above, the spot-welded joint 1 shown in Figure 3, which is one embodiment of the present invention, is composed of two steel plates, where at least one of the upper steel plate 21 and the lower steel plate 22 is a high-strength steel plate having a tensile strength of 1350 MPa or more.

[0087] Furthermore, the spot welded joint 1 of this embodiment has a nugget 3 formed on the overlapping surface of steel plates 21 and 22, and a molten solidification portion 5 formed in a region inside the outer edge 3E of the nugget 3 when viewed from above, which penetrates the steel plates 21 and 22 in the thickness direction. As shown in Figure 3, the outer diameter D of the molten solidification portion 5 when viewed from above is L and nugget diameter D N The relationship is 0.3 ≤ D L / D N It is formed to satisfy <1.0.

[0088] (Effects and Benefits) As described above, in this embodiment, the spot welded joint 1 has a molten and solidified portion 5 formed in the region inside the outer edge 3E of the nugget 3 when viewed from above, which penetrates the steel plates 21 and 22 in the thickness direction, and furthermore, the molten and solidified portion 5 is 0.3 ≤ D L / D N It is formed to satisfy <1.0. That is, as described above, in the spot welded joint 1, the heat generated when forming the molten and solidified portion 5 is transferred from the center side of the nugget 3 to the end portion 31, so that even when a high-strength steel plate is included, re-hardening of the end portion 31 of the nugget does not occur, and the end portion 31 of the nugget is steadily tempered. As a result, the spot welded joint 1 of this embodiment has high toughness at the end portion of the nugget, even when a high-strength steel plate is included, and is a welded joint with excellent joint strength.

[0089] Note the shape of the molten and solidified portion when viewed from above, and the outer diameter D of the molten and solidified portion. L nugget diameter D N and D in these relationships L / D N The upper and lower limits, etc., are as explained in the spot welding method described above.

[0090] The steel plates constituting the spot-welded joint 1 are not particularly limited in type or number, as long as at least one of them is a high-strength steel plate with a tensile strength of 1350 MPa or more. Various types of steel plates can be used as described in the spot welding method described above.

[0091] Furthermore, in the spot welded joint 1 of this embodiment, as described above, the molten and solidified portion 5 is formed in a ring shape when viewed from above. By forming the molten and solidified portion 5 in such a ring shape, the entire or almost entire area of ​​the end portion 31 of the nugget can be tempered, thereby more reliably obtaining superior joint strength.

[0092] The means for forming the molten and solidified portion 5 into such a ring shape is as described in the spot welding method above.

[0093] Furthermore, the molten and solidified portion 5 may be formed in a circular shape when viewed from above, as shown in the spot welded joint 1 in Figure 4 above. As described above, the circular molten and solidified portion 5 is formed over the entire or nearly entire area inside the circular outer edge of the molten and solidified portion 5 when viewed from above.

[0094] Furthermore, in the spot welded joint 1 shown in Figure 4, the circular molten and solidified portion 5 is formed to penetrate the steel plates 21 and 22 in the thickness direction, and the outer diameter D of the circular molten and solidified portion 5 in a top view L and nugget diameter D N The relationship is 0.3 ≤ D L / D N It satisfies <1.0. Except that the molten and solidified portion 5 is formed in a circular shape, it is the same as the spot welded joint 1 shown in Figure 3 above.

[0095] By forming the molten and solidified portion 5 in this circular shape, the entire or nearly entire end portion 31 of the nugget can be tempered more reliably, thus enabling the acquisition of even more reliable and superior joint strength.

[0096] The means for forming the molten and solidified portion 5 in this circular shape is as described in the spot welding method above.

[0097] (Examples of application) As described above, the spot welding method and spot welded joint of the present invention allow for stable tempering of the nugget ends even when using high-strength steel plates with a tensile strength of 1350 MPa or higher, thereby achieving excellent joint strength. Therefore, the present invention can be applied to the manufacture of various structural components such as automobiles and other transportation machinery, industrial machinery, and buildings where excellent joint strength is required. In particular, the present invention can be suitably used in the manufacture of automobile bodies and parts where high production efficiency and excellent joint strength are required.

[0098] The spot welding method and spot welding joint of the present invention are not limited to the embodiments described above or the examples described later, and can be appropriately combined, substituted, or modified without departing from the purpose and spirit of the present invention. In this specification, ordinal numbers such as "1st," "2nd," etc., are used to distinguish the items to which they are assigned, and do not indicate the order, priority, importance, etc. of each item. [Examples]

[0099] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to such examples.

[0100] (Fabrication of spot welded joints) To verify the effects of the present invention, a spot-welded joint was fabricated as follows. First, two unplated steel sheets, each 2.0 mm thick and with a tensile strength of 2500 MPa, were overlapped to form a plate assembly. The two steel sheets were overlapped in a cross shape so that the CTS (Critical Tension Spectroscopy) could be measured after fabrication. Next, spot welding was performed on the plate assembly using a stationary resistance spot welding machine with a single-phase AC power supply at a frequency of 50 Hz to form a nugget on the overlapping surface of the two steel sheets (first step).

[0101] For spot welding, a DR-type electrode made of chromium copper was used, with a tip diameter of 6 mm and a shoulder radius of 40 mm. The electrode pressure was set to 400 kgf, and current was applied after the pressure stabilized. The current was applied at a value of 7.5 kA for a duration of 20 cycles. The holding time, which is the time from the end of current application until the electrode was released, was set to 5 cycles.

[0102] Next, a laser was applied to a predetermined area on the surface of the upper steel plate of the two spot-welded steel plates, causing that area to melt in the thickness direction. In this way, a cylindrical molten and solidified portion was formed in the area inside the outer edge of the nugget when viewed from above, penetrating the two steel plates in the thickness direction (second step).

[0103] A fiber laser was used as the laser light source during laser irradiation, and the laser irradiation was performed using a remote laser system. The laser irradiation was performed on the outer diameter D of the molten and solidified area in a top view. L and nugget diameter D N The relationship is 0.3 ≤ D L / D N The irradiation was performed in a ring-shaped trajectory that satisfies <1.0, completing one full rotation.

[0104] The irradiation radius during laser irradiation was set to 2.0 mm, as shown in Table 1 below. Furthermore, the laser output was set to 5.0 kW, and the focal position was set to a position 5 mm away from the surface of the steel plate (DF5). The movement speed of the laser irradiation unit was set to 3 m / min.

[0105] As described above, the spot-welded joint of Example 1 of the present invention was fabricated.

[0106] Furthermore, the spot welded joint of the present invention example 2 was manufactured in the same manner as in the present invention example 1 described above, except that the laser irradiation pattern in the second step, i.e., the shape of the laser irradiation area in the top view, was made circular. Specifically, in the present invention example 2, the laser irradiation was performed in a circular motion twice to form a cylindrical molten and solidified portion that penetrates the two steel plates in the thickness direction. For the two rotations of laser irradiation, the irradiation radius for the first rotation was 1.5 mm and the irradiation radius for the second rotation was 2.0 mm.

[0107] As a comparative example, a spot-welded joint of Comparative Example 1 was fabricated in the same manner as the above-described example of the present invention, except that laser irradiation was not performed, i.e., the second step was not carried out.

[0108] Furthermore, the laser irradiation in the second step is directed to a position that overlaps with the edge of the nugget, i.e., D L / D N A spot-welded joint for Comparative Example 2 was fabricated in the same manner as in Example 1 of the present invention described above, except that the position was set to >1.0, the laser output during laser irradiation was set to 5.5kW, and the irradiation radius was set to 3.5mm.

[0109] Regarding the spot weld joints of the above Invention Example 1, Invention Example 2, Comparative Example 1, and Comparative Example 2, for the <outer diameter D of the fusion-solidified part L and nugget diameter D N measurement method>, after obtaining a cross-sectional enlarged photograph according to the method, the outer diameter D of the fusion-solidified part L and nugget diameter D N were measured respectively from the cross-sectional enlarged photograph. These measurement results are shown in Table 1 below. For Comparative Example 2, as shown in FIG. 8, the outer edge of the nugget overlaps with the fusion-solidified part, and the outer edge of the nugget cannot be defined. That is, the nugget diameter D N cannot be measured. However, since it is clear that the outer diameter D of the fusion-solidified part L is larger than the nugget diameter D N , it was determined that D L / D N > 1.0.

[0110] Also, the cross-sectional enlarged photographs of the spot weld joints of Invention Example 1, Invention Example 2, Comparative Example 1, and Comparative Example 2 are shown in FIGS. 5 to 8 respectively.

[0111] Furthermore, for the spot weld joints of Invention Example 1, Invention Example 2, Comparative Example 1, and Comparative Example 2, the cross-tensile strength (CTS) was measured according to the following measurement method.

[0112] (Measurement of cross-tensile strength (CTS) of spot weld joint) The CTS of the spot weld joint was measured according to the cross-tension test based on JIS Z 3137:1999. The measurement of CTS was carried out at a speed of 10 mm / min. Also, the measurement of CTS was carried out with 2 samples, and the average value of each measurement result was adopted. The measurement result of CTS is judged that 5.00 kN or more is an excellent joint strength, and less than 5.00 kN is a poor joint strength.

[0113] The measurement results of CTS of the spot weld joints of Invention Example 1, Invention Example 2, Comparative Example 1, and Comparative Example 2 are shown in Table 1 below.

[0114]

Table 1

[0115] As shown in Table 1, Figure 5, and Figure 6, the spot welded joints of the present invention, Example 1 and Example 2, have a molten and solidified portion that penetrates the two steel plates in the thickness direction, and 0.3 ≤ D L / D N The joints are formed to satisfy <1.0. As shown in Table 1, the spot welded joints of Examples 1 and 2 of the present invention have high CTS and excellent joint strength because the ends of the nuggets are steadily tempered by the formation of such molten and solidified areas. In other words, even when high-strength steel plates are used as the base material, the toughness of the ends of the nuggets is improved and the spot welded joints of Examples 1 and 2 of the present invention have excellent joint strength. Notably, as shown in Table 1, the spot welded joint of Example 1 of the present invention has a CTS improvement of approximately 48% compared to Comparative Example 1, in which the second step was not performed. Similarly, the spot welded joint of Example 2 of the present invention also has a CTS improvement of approximately 53% compared to Comparative Example 1.

[0116] On the other hand, in Comparative Example 1, where the second step of laser irradiation was not performed, the spot-welded joint did not have a molten and solidified area formed, as shown in Figure 7, and the weld was not tempered. As a result, as shown in Table 1, the CTS was low and sufficient joint strength was not obtained.

[0117] Furthermore, the laser irradiation in the second step is directed to a position that overlaps with the edge of the nugget, i.e., D L / D N In Comparative Example 2, the spot welded joint, which was performed at a position where the value was >1.0, formed a molten and solidified zone as shown in Figure 8. However, because the ends of the nugget were not tempered, the CTS was low as shown in Table 1, and sufficient joint strength was not obtained. [Explanation of Symbols]

[0118] 1. Spot welded joint 21 (Top plate) steel plate 22 (Bottom plate) steel plate 3 nuggets 31 The end of the nugget 41 Upper electrode 42 Lower electrode 5. Molten and solidified portion IP impression area LB laser irradiation

Claims

1. A spot welding method for a plate assembly consisting of two or more steel plates, At least one of the steel plates constituting the aforementioned plate assembly is a high-strength steel plate having a tensile strength of 1350 MPa or more. The first step involves spot welding the plate assembly to form a nugget on the overlapping surface of the plate assembly, A second step involves forming a molten and solidified portion in a region inside the outer edge of the nugget, as viewed from above, which penetrates the two or more steel plates in the thickness direction. Includes, The molten and solidified portion has an outer diameter D of the molten and solidified portion when viewed from above. Ｌ and nugget diameter D Ｎ The relationship is 0.3 ≤ D Ｌ / D Ｎ A spot welding method characterized by being formed to satisfy <1.

0.

2. The spot welding method according to claim 1, characterized in that the means for forming the molten and solidified portion is laser irradiation.

3. The spot welding method according to claim 1 or 2, characterized in that the region forming the molten and solidified portion is a ring-shaped region when viewed from above.

4. The spot welding method according to claim 1 or 2, characterized in that the region forming the molten and solidified portion is a circular region when viewed from above.

5. A spot-welded joint composed of two or more steel plates, including a high-strength steel plate having a tensile strength of 1350 MPa or more, The overlapping surfaces of the two or more steel plates have nuggets, In a top view, a molten and solidified portion is formed in the region inside the outer edge of the nugget, penetrating the two or more steel plates in the thickness direction. The molten and solidified portion has an outer diameter D of the molten and solidified portion when viewed from above. Ｌ and nugget diameter D Ｎ The relationship is 0.3 ≤ D Ｌ / D Ｎ A spot-welded joint characterized by being formed to satisfy <1.

0.

6. The spot welded joint according to claim 5, characterized in that the molten and solidified portion is formed in a ring shape when viewed from above.

7. The spot welded joint according to claim 5, characterized in that the molten and solidified portion is formed in a circular shape when viewed from above.

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