JP7795102B2 - Method for manufacturing resistance spot welded joints and method for manufacturing automobile parts - Google Patents

Description

The present invention relates to a method for manufacturing resistance spot welded joints and a method for manufacturing automotive parts.

Relatively thick, high-strength steel plates are sometimes used for automotive frame components, such as center pillars. This is done to ensure the collision safety of the vehicle body and to reduce weight through part integration. On the other hand, parts such as highly decorative side members, which are formed using thin steel plates, for example, 0.6 to 0.8 mm thick, are placed on the outermost parts of the vehicle. The thin steel plates on the exterior of an automobile are often made of mild steel to ensure workability.

For the reasons mentioned above, when assembling automotive parts, sheet assemblies consisting of thick and thin steel sheets may be spot welded. Furthermore, the sheet assemblies to be spot welded may also have a three-layer structure, such as a thin steel sheet - a thick sheet - a thick sheet. The sheet thickness ratio of such sheet assemblies may be, for example, 4.5 or more. Here, the sheet thickness ratio is defined as tsum/tmin, where tmin (mm) is the thickness of the thinnest steel sheet, and tsum (mm) is the total thickness of the steel sheets included in the sheet assembly.

However, when spot welding a sheet assembly with a sheet thickness ratio of 4.5 or more and in which the thinnest steel sheet is located on the surface, poor welding is likely to occur. The nugget (which originally refers to the melted and solidified portion, but in this specification, both the molten portion and the melted and solidified portion are referred to as the nugget) formed inside the sheet assembly does not grow to the thinnest steel sheet located on the outermost surface of the sheet assembly, resulting in poor welding between the thinnest steel sheet and the adjacent steel sheet in contact with it.

One of the reasons why the nugget does not grow to the thinnest steel sheet in the sheet assembly described above is that the thinnest steel sheet is in contact with the spot welding electrode. Spot welding electrodes are designed to allow a refrigerant to flow inside, cooling the steel sheet. Therefore, the temperature of the thinnest steel sheet in contact with the electrode is less likely to rise than the temperature of the steel sheets inside the sheet assembly.

Another reason why the nugget does not grow all the way to the thinnest steel sheet is that the temperature rise starts from the center of the sheet assembly. The further away from the center of the sheet assembly, the more difficult it is for the temperature to rise.

Furthermore, if the thinnest steel sheet is mild steel, the nugget is further prevented from growing to the thinnest steel sheet. This is because thin mild steel is easily deformed. When thin mild steel is placed on the surface of a sheet assembly, the mild steel is easily deformed by applying pressure and passing current, and the contact area between the mild steel and the adjacent steel sheet tends to increase. The larger the contact area, the larger the cross-sectional area of the current path and the lower the current density. In addition, the larger the contact area between the mild steel and the electrode, the more heat is transferred from the mild steel to the electrode.

For these reasons, the temperature of the thinnest steel sheet on the surface of the sheet assembly is difficult to increase, and therefore melting and solidification are difficult to occur in the thinnest steel sheet. If the nugget does not grow to the thinnest steel sheet, joint strength cannot be ensured. This prevents the use of thick (1.6 mm or more) high-strength steel sheets in automotive frame parts and limits the range of thickness options for the steel sheets that make up the sheet assembly. Therefore, a spot welding technology that can solve these problems is eagerly awaited.

Patent Document 1 discloses a method for manufacturing a resistance spot welded joint by resistance spot welding a plate assembly made up of multiple overlapping metal plates, characterized in that the resistance spot welding is performed in two stages, a first stage and a second stage, and the second stage welding is performed using a higher welding pressure, a lower current or the same current, and a longer current flow time or the same current flow time as the first stage welding.

Patent Document 2 describes a method for resistance spot welding in which a plate assembly in which a thin steel plate is placed on at least one of two or more overlapping thick plates is sandwiched between a pair of electrodes and a pressure is applied, and the positions of the pair of electrodes are fixed when the pressure reaches a set value P, and the fixing of the electrode positions suppresses thermal expansion of the plate assembly after the start of current application, thereby increasing the pressure, and the initial set pressure P and the maximum value Pm of the pressure are:
Pm≧1.5×P
P≦4kN
The present invention discloses a resistance spot welding method characterized by:

Patent Document 3 discloses a resistance spot welding method in which a plate assembly in which a thin steel plate is placed on at least one of two or more overlapping steel plates, at least one of which is a plated steel plate, is sandwiched between a pair of electrodes and a pressing force is applied. The method divides the welding process into a first stage and a second stage, controls the pressure applied when the electrode contacts the thin steel plate at the start of the first stage so that it does not exceed 2.5 kN even if an overshoot occurs, performs resistance spot welding with a low pressing force for a short time in the first stage, and performs resistance spot welding with a high pressing force for a long time in the second stage, and further performs resistance spot welding within a range in which the radius d (mm) of the contact point between the thin steel plate and the electrode before the start of current flow in the first stage and the thickness t (mm) of the thin steel plate satisfy the relationship d < 7t.

Patent Document 4 discloses a spot welding method for joining multiple overlapping steel plates, characterized in that it includes a preliminary current application process in which current is gradually applied, a first current application process in which a constant current value I1 is applied, a second current application process in which current is then applied at a current value I2, and a third current application process in which current is then applied at a current value I3, with the relationships I1 > I2 and I2 < I3.

Japanese Patent Application Laid-Open No. 2005-262259 Japanese Patent Application Laid-Open No. 2007-203319 Japanese Patent Application Laid-Open No. 2007-268604 International Publication No. WO2015/170687

However, the techniques of Patent Documents 1 to 4 require significant limitations on the pressure and current values during the current application process and complex control, making it impossible to easily avoid poor welding when spot welding sheet assemblies with a sheet thickness ratio of 4.5 or more and in which the thinnest steel sheet is located on the surface.

In light of the above, the present invention aims to provide a method for manufacturing resistance spot welded joints and a method for manufacturing automotive parts that can easily avoid joint failures in sheet assemblies where the sheet thickness ratio is 4.5 or greater and the thinnest steel sheet is arranged on the surface.

The gist of the present invention is as follows:

[1] A manufacturing method of a resistance spot welded joint according to one aspect of the present invention is a manufacturing method of a resistance spot welded joint in which a sheet assembly formed by stacking two or more steel sheets is spot-welded, wherein, when the sheet thickness of the thinnest steel sheet among the steel sheets included in the sheet assembly is tmin (mm) and the total sheet thickness of the steel sheets included in the sheet assembly is tsum (mm), tsum/tmin is 4.5 or more, the thinnest steel sheet is arranged on at least one surface of the sheet assembly, the manufacturing method includes, in order, a first pressurizing step in which the sheet assembly is sandwiched between tips of a pair of electrodes and pressed, a pressure releasing step in which the pressing force of the pair of electrodes is reduced, and a second pressurizing step in which the sheet assembly is sandwiched between the tips of the pair of electrodes and pressed, and wherein a pressing force P1 (kN) in the first pressurizing step, a pressing force Pr (kN) in the pressure releasing step, and a pressing force P2 (kN) in the second pressurizing step are Formulas (1) to (3) are satisfied, the first pressurizing step includes a first current-carrying step of passing current between the pair of electrodes, and a current value in the pressure-releasing step is 0, the second pressurizing step includes a second current-carrying step of passing current between the pair of electrodes, a current value I1 (kA) in the first current-carrying step and a current value I2 (kA) in the second current-carrying step satisfy formulas (4) and (5), and a current-carrying time T1 (m The current flow time T2 (msec) of the second current flow process satisfies the formula (6), the current flow time T2 (msec) of the second current flow process satisfies the formula (7), and in the case where the first pressure application process has a first holding process in which the pressure that satisfies the formula (2) is maintained with the current value set to 0 after the first current flow process, the holding time Th1 (msec) of the first holding process satisfies the formula (8), and the time Tr (msec) of the pressure release process satisfies the formula (9).
P1≦P2 (1)
2<P1...(2)
Pr≦2 (3)
I1≦I2 (4)
4≦I1 (5)
50×tsum/2<T1<200×tsum/2...(6)
50×tsum/2<T2...(7)
Th1<300...(8)
20<Tr...(9)
[2] In the method for manufacturing a resistance spot welded joint described in [1] above, the pressing force P1 (kN) in the first pressing step may further satisfy formula (10).
P1<2.0×tsum/2...(10)
[3] In the method for manufacturing a resistance spot welded joint described in [1] above, the pressing force P1 (kN) in the first pressing step may further satisfy formula (11).
P1≦5 (11)
[4] In the method for manufacturing a resistance spot welded joint according to any one of [1] to [3] above, the pressing force P2 (kN) in the second pressing step may further satisfy formula (12).
1.2 × P1 ≦ P2 (12)
[5] In the method for manufacturing a resistance spot welded joint described in any one of [1] to [4] above, after the second pressurizing step, a third pressurizing step may be further provided, which includes a third current application step for the purpose of modifying the nugget.
[6] Another aspect of the present invention provides a method for manufacturing an automobile part, comprising the method for manufacturing a resistance spot welded joint according to any one of [1] to [3] above.
[7] Another aspect of the present invention provides a method for manufacturing an automobile part, comprising the method for manufacturing a resistance spot welded joint described in [4] above.
[8] Another aspect of the present invention provides a method for manufacturing an automobile part, comprising the method for manufacturing a resistance spot welded joint described in [5] above.

The present invention provides a method for manufacturing a resistance spot welded joint and a method for manufacturing an automotive part that can easily avoid joint failure in sheet assemblies where the sheet thickness ratio is 4.5 or greater and the thinnest steel sheet is arranged on the surface.

1 is a graph showing an example of changes in pressing force and current value over time in a method for manufacturing a resistance spot welded joint according to one aspect of the present invention. 1 is a schematic diagram of a method for manufacturing a resistance spot welded joint according to one aspect of the present invention. FIG. 10 is a conceptual diagram illustrating the influence of current application time in the first current application step. 10 is a cross-sectional photograph showing the experimental results of the influence of current application time in the first current application step.

A manufacturing method of a resistance spot welded joint 1 according to one aspect of the present invention is a manufacturing method of a resistance spot welded joint 1, for example, as shown in FIGS. 1 and 2 , in which a plate assembly 11 formed by stacking two or more steel plates 111 is spot-welded, and when the plate thickness of the thinnest steel plate 111 min among the steel plates 111 included in the plate assembly 11 is t min (mm) and the total plate thickness of the steel plates included in the plate assembly is t sum (mm), t sum / t min is 4.5 or more, and the thinnest steel plate 111 min is arranged on at least one surface of the plate assembly 11. Here, the manufacturing method of the resistance spot welded joint 1 includes a first pressurizing step S1 in which the sheet pair 11 is sandwiched between the tips of a pair of electrodes E and pressurized, a pressure release step S2 in which the pressure applied by the pair of electrodes E is reduced, and a second pressurizing step S3 in which the sheet pair 11 is sandwiched between the tips of the pair of electrodes E and pressurized, in that order, the pressurizing force P1 (kN) in the first pressurizing step, the pressurizing force Pr (kN) in the pressurizing release step, and the pressurizing force P2 (kN) in the second pressurizing step satisfy equations (1) to (3), the first pressurizing step S1 includes a first current application step in which current is applied between the pair of electrodes, the current value in the pressure release step S2 is 0, and the second pressurizing step S3 includes a second current application step in which current is applied between the pair of electrodes E and the pressure applied by the pair of electrodes E and the pressure applied by the pair of electrodes E is 0. The first current flow step S1 includes a second current flow step in which current is passed between the first current flow step S1 and the second current flow step S2, and the current value I1 (kA) of the first current flow step S1 and the current value I2 (kA) of the second current flow step S2 satisfy the formulas (4) and (5), the current flow time T1 (msec) of the first current flow step S1 satisfies the formula (6), and the current flow time T2 (msec) of the second current flow step S2 satisfies the formula (7). In the case where the first pressure application step S1 includes a first holding step in which a pressure that satisfies the formula (2) is maintained with the current value set to 0, the holding time Th1 (msec) of the first holding step S2 satisfies the formula (8), and the time Tr (msec) of the pressure release step S2 satisfies the formula (9).
P1≦P2 (1)
2<P1...(2)
Pr≦2 (3)
I1≦I2 (4)
4≦I1 (5)
50×tsum/2<T1<200×tsum/2...(6)
50×tsum/2<T2...(7)
Th1<300...(8)
20<Tr...(9)
Details will be explained below. Note that the "thinnest steel plate 111 min" may be referred to as the "thinnest steel plate 111 min" hereinafter.

First, we will explain the technical concept of the method for manufacturing a resistance spot welded joint according to this embodiment. The inventors conducted extensive research into a method for manufacturing a resistance spot welded joint that can avoid joint failures in sheet assemblies where the sheet thickness ratio is 4.5 or greater and the thinnest steel sheet is located on the surface.

The inventors first attempted to increase the heat input by lengthening the welding time. However, experimental results revealed that the effect of extending the welding time on increasing the nugget diameter saturates at a certain level.

Figure 4 shows cross-sectional photographs of resistance spot-welded joints produced using various current conditions. In all spot welds, the welding pressure was 3.5 kN, the current was 8 kA, the sheet thickness ratio of the sheet assembly was 7, and the thinnest steel sheet was positioned on the surface of the sheet assembly. The welding time alone was varied between 40 and 400 msec. As a result, within the welding time range of 40 to 200 msec, the nugget size increased with increasing welding time, and the nugget diameter at the interface between the thinnest steel sheet positioned on the surface of the sheet assembly and the adjacent steel sheet also tended to increase. However, this tendency disappeared when the welding time exceeded 200 msec. The nugget diameter at the interface between the thinnest steel sheet positioned on the surface of the sheet assembly and the adjacent steel sheet was smaller in resistance spot-welded joints with a welding time of 400 msec than in resistance spot-welded joints with a welding time of 200 msec.

The reason for this is thought to be that the surface of the sheet assembly is cooled by the spot welding electrode. The longer the current flow time, the greater the heat input, which is the product of the current value and the current flow time. However, as shown in Figure 4, the longer the current flow time, the greater the depression in the spot weld, i.e., the deeper the indentation. This increases the contact area between the steel sheet and the electrode. This is thought to promote heat transfer from the steel sheet to the electrode and reduce the current density, further hindering the melting of the thinnest steel sheet on the surface of the sheet assembly.

Another means of increasing the heat input is to increase the current value. However, the higher the current value, the more likely it is that expulsion will occur from the interface between steel sheet 111 when the number of steel sheets is three or more, making it difficult to obtain a sound weld. Increasing the welding pressure can suppress expulsion, but on the other hand, it expands the cross-sectional area of the current path, reducing the current density and making it difficult to increase the temperature of the weld. It is also estimated that the greater the welding pressure, the greater the indentation and the greater the contact area between the steel sheet and the electrode. For these reasons, it is believed that increasing the current value will not achieve an increase in the nugget diameter at the interface between the thinnest steel sheet and the adjacent steel sheet.

Taking the above circumstances into consideration, the present inventors have conducted further research and have found that poor joining can be easily avoided by spot welding that satisfies the following conditions.
(1) After the first pressure application step S1 including the first current application step, a pressure release step S2 is provided in which the pressure applied by the pair of electrodes E is reduced to 2 kN or less.
(2) After the pressure release step S2, a second pressure step S3 including a second current application step is carried out.

In the pressure release step S2, the pressure applied by the pair of electrodes E is reduced to 2 kN or less. This pressure is significantly lower than that used in conventional spot welding. This is to prevent heat transfer from the thinnest steel sheet 111 to the electrodes E. As described above, the electrodes E used in conventional spot welding have the function of cooling the steel sheet 111. However, by reducing the pressure applied by the electrodes E to 2 kN or less, the amount of heat transferred from the steel sheet 111 to the electrodes E is significantly reduced. Furthermore, since the central portion of the sheet assembly 11 is heated during the first pressure step S1, which includes the first current application step, heat transfer occurs from the central portion of the sheet assembly 11 to the thinnest steel sheet 111 in the pressure release step S2, causing the thinnest steel sheet 111 to heat up. This phenomenon is called reheating. Reheating melts the thinnest steel sheet 111, increasing the nugget diameter between the thinnest steel sheet 111 and the adjacent steel sheet 111.

Furthermore, after the pressure release step S2, a second pressure process S3, which includes a second current application step, is carried out. The width of the molten portion of the thinnest steel sheet 111 mm created by the pressure release step S2 is further expanded by the second pressure process S3. This reliably prevents poor welding.

Next, we will explain the specific configuration of the method for manufacturing a resistance spot welded joint according to this embodiment.

(Board set 11)
In the manufacturing method for a resistance spot-welded joint according to this embodiment, a sheet assembly 11 formed by stacking two or more steel sheets 111 is spot-welded. The sheet assembly 11 has a sheet thickness ratio of 4.5 or more. The sheet thickness ratio is a value defined as tsum/tmin, where tmin (mm) is the sheet thickness of the thinnest steel sheet (thinnest steel sheet 111 min) and tsum (mm) is the total sheet thickness of the steel sheets 111 included in the sheet assembly. In the manufacturing method for a resistance spot-welded joint according to this embodiment, the thinnest steel sheet 111 min is disposed on one or both surfaces of the sheet assembly 11.

When the thinnest steel plate 111min is arranged on the surface of the plate assembly 11, if the plate thickness ratio is 4.5 or greater, the nugget 12 is less likely to extend to the interface between the thinnest steel plate 111min and the adjacent steel plate 111. Therefore, a plate assembly 11 that meets the above requirements is disadvantageous in terms of ensuring joint strength. On the other hand, plate assemblies 11 that meet the above requirements are in high demand, for example, in automotive parts. For example, in automotive parts manufactured by spot welding a frame component made of thick steel plate and an exterior component made of thin steel plate, a plate thickness ratio of 4.5 or greater is preferable. From the perspective of reducing the weight of the automobile body, a larger plate thickness ratio is preferable. For example, the plate thickness ratio may be 5.0 or greater, 5.5 or greater, or 6.0 or greater.

The number of steel plates 111 constituting the plate assembly 11 is not particularly limited as long as it is two or more. For example, the number of steel plates 111 may be three or more, or four or more. In typical spot welding, when the number of steel plates 111 constituting the plate assembly 11 is three or more and tsum increases, the nugget diameter is secured by increasing the pressure and current value. However, increasing the current value makes expulsion more likely to occur between the steel plates 111. Therefore, it is difficult to prevent poor welding when the number of steel plates 111 is three or more. On the other hand, according to the manufacturing method of this embodiment, the pressure release process S2 and the second pressure process S3 are used to expand the molten region at the interface between the thinnest steel plate 111min and the adjacent steel plate 111. By employing these processes, the pressure-welded portion at the steel plate interface in the steel plate 111 expands, and the nugget grows gradually. Therefore, expulsion and poor welding can be prevented even when the number of steel plates 111 is three or more.

When the sheet assembly 11 includes three or more steel sheets 111, the number of thinnest steel sheets 111min may be two or more. In this case, one or two of the multiple thinnest steel sheets 111min may be arranged on one or both surfaces of the sheet assembly 11. Because nuggets 12 are easily formed inside the sheet assembly 11, the thinnest steel sheets 111min included inside the sheet assembly 11 do not cause poor welding. Furthermore, the nugget diameter enlargement phenomenon using the above-mentioned pressure release process S2 and second current application process S3 occurs on both of the pair of electrodes E. Therefore, when thinnest steel sheets 111min are arranged on both surfaces of the sheet assembly 11, the nugget diameter enlargement effect is achieved on both thinnest steel sheets 111min.

When the number of steel plates 111 constituting the plate assembly 11 is two to four, the thicknesses of these steel plates 111 must naturally be different from one another. This is because when two, three, or four steel plates 111 of the same thickness are stacked, the plate thickness ratio is 2, 3, or 4, which is below the lower limit of the plate thickness ratio of "4.5" that can be welded using the manufacturing method of the resistance spot welded joint 1 according to this embodiment. On the other hand, when the number of steel plates 111 constituting the plate assembly 11 is five or more, the plate thicknesses of the steel plates 111 constituting the plate assembly 11 may all be the same. This is because the plate thickness ratio of a plate assembly 11 made up of five stacked steel plates 111 of the same thickness is 5. Even with such a plate assembly 11, the effects of enlarging the nugget diameter using the pressure release process S2 and the second pressure application process S3 described above can be enjoyed.

The configuration of the steel plate 111 other than the plate thickness ratio is not particularly limited, but suitable examples are shown below.

The total thickness tsum of the steel plates 111 included in the plate assembly 11 is preferably within the range of 2 mm to 6 mm, for example. This ensures the rigidity of the resistance spot welded joint while more effectively suppressing the occurrence of expulsion during spot welding. Furthermore, the thickness tmin of the thinnest steel plate 111min arranged on the surface of the plate assembly 11 is preferably within the range of 0.3 mm to 1.5 mm, for example.

The tensile strength of the steel plate 111 is preferably, for example, 980 MPa or more. By using a high-strength steel plate with a tensile strength of 980 MPa or more for one or more of the multiple steel plates 111, the rigidity of the resistance spot welded joint 1 can be increased. On the other hand, the steel plate 111 may also be mild steel with a tensile strength of less than 980 MPa. For example, if the resistance spot welded joint 1 is to be an automotive part, the thinnest steel plate 111 min arranged on the surface of the plate assembly 11 may be mild steel, and the other steel plates 111 may be high-strength steel plates.

One or more of the multiple steel plates 111 may be plated steel plates. Examples of plating include hot-dip galvanizing, alloyed hot-dip galvanizing, electrogalvanizing, and aluminum plating.

(First pressurization step S1, pressure release step S2, and second pressurization step S3)
As shown in Figures 1 and 2, the above-described sheet set 11 is joined through a first pressure application step S1 including a first current application step, a pressure release step S2, and a second pressure application step S3 including a second current application step to form a resistance spot welded joint 1. If necessary, a third pressure application step S4 including a third current application step may be further performed. Note that the phrase "the first pressure application step includes the first current application step" means that the period from the start to the end of the first pressure application step includes the period from the start to the end of the first current application step. This also applies to the second pressure application step and second current application step, and the third pressure application step and third current application step.

The first pressure application step S1 and the second pressure application step S3 are performed by clamping and applying pressure to the sheet assembly 11 between the tips of a pair of spot welding electrodes E, similar to regular spot welding. The first current application step included in the first pressure application step S1 and the second current application step included in the second pressure application step S3 are performed by passing current between the pair of electrodes E, similar to regular spot welding. In the pressure release step S2, which is performed between the first pressure application step S1 and the second pressure application step S3, the pressure applied by the pair of electrodes E is reduced to 2 kN or less. The conditions for performing these steps must be determined taking into account their interrelationships. The conditions for performing these steps are described below. The pressure, current, hold time, and pressure release step time described below are all settings entered into the spot welding machine.

(definition)
Generally, spot welding conditions are represented by a time-pressure graph and a time-current graph as shown in Figure 1. In this graph, the pressure release step S2 refers to a period during spot welding in which the pressure is set to 2 kN or less and the current value is set to 0 kA. In other words, the pressure release step is a period in which the following formula (3) is satisfied and no current application step is included. The condition values in this pressure release step S2 are defined as follows:
Pressure release step time Tr: The length of the period during which the pressure is kept at 2 kN or less.
Pressure Pr during pressure release process: If the pressure is set to a constant value, this refers to the pressure. If the pressure is set to fluctuate, this refers to the minimum pressure during the period in which the pressure is 2 kN or less.
In addition, there are cases where the pressure is set to more than 0 kN and 2 kN or less immediately after the start or end of spot welding, but such a low pressure period immediately after the start or immediately before the end is not included in the pressure release process S2, which is a process carried out between the first pressure process S1 and the second pressure process S3.

The first pressurizing step S1 is a pressurizing period provided before the pressurizing release step S2 for forming the nugget 12. As defined by the following formula (2), the pressurizing force in the first pressurizing step S1 is set to be more than 2 kN, and in this respect, the first pressurizing step S1 and the pressurizing release step S2 are distinguished. The first pressurizing step S1 includes a first current application step, and the condition values in the first pressurizing step S1 and the first current application step are defined as follows:
Current conduction time T1 of the first current conduction step: the length of the period during which the current value is 4 kA or more in the first current conduction step. In other words, T1 is the period during which the following formula (5) is satisfied. For example, if the first current conduction step is an upslope current conduction, the period during which the current value is less than 4 kA is not included in the current conduction time T1.
Current value I1 of the first current flow step: if the current value is set to be constant, this refers to that current value. If the current value is set to vary, this refers to the maximum current value during the period in which the following formula (5) is satisfied and the current value is 4 kA or more, in the case of direct current, or the maximum effective value during the period in which the effective value of the current is 4 kA or more, in the case of alternating current.
Pressure P1 of the first pressure step: if the pressure is set to be constant, this refers to that pressure. If the pressure is set to vary, this refers to the maximum pressure during the period in which a pressure of more than 2 kN is applied, which satisfies the following formula (2).

Furthermore, in the first pressurizing step S1, if there is a first holding step after the first current application step in which a pressure that satisfies equation (2) is maintained with the current value set to 0, the holding time of the first holding step is defined as follows.
Holding time Th1 of first holding step: This refers to the length of the period during which, in the first pressurizing step S1, a pressure of more than 2 kN is maintained, which satisfies the following formula (2), with the current value set to 0 after the first current-carrying step. Note that, although pressure is usually started before the start of current-carrying in the first current-carrying step, such a pressure-carrying period before current-carrying is not included in Th1.

The second pressurizing step S3 refers to a pressurizing period for enlarging the nugget diameter, which is provided after the pressurizing and releasing step S2, and includes the second current application step. Specifically, the second pressurizing step S3 refers to the period from when the pressurizing force is increased to P1 or more after the pressurizing and releasing step S2 until the pressurizing force becomes less than P1 through the second current application step. For definitional reasons and for technical reasons described below, the pressurizing force P2 of the second pressurizing step S3 must be P1 or more at least after the pressurizing and releasing step S2 is completed. This second pressurizing step S3 includes the second current application step, and the condition values in the second pressurizing step S3 and the second current application step are defined as follows:
Current application time T2 of the second current application step: the length of the period during which the current value is I1 or more in the second current application step. In other words, T2 is the period during which the following formula (4) is satisfied:
Current value I2 of the second current flow step: if the current value is set to be constant, this refers to that current value. If the current value is set to vary, this refers to the maximum current value during the period in which the following formula (4) is satisfied and the current value is I1 or more, in the case of direct current, or the maximum effective value during the period in which the effective value of the current is I1 or more, in the case of alternating current.
Pressure P2 in the second pressure process: if the pressure is set to be constant, this refers to that pressure. If the pressure is set to vary, this refers to the maximum pressure during the period in which a pressure of P1 or more that satisfies the following formula (1) is applied.
The second pressure application process S3 is a pressure application period for enlarging the nugget diameter, and as long as the second current application process is included, the second holding process after the second current application process may or may not be included.

The third pressurizing step S4 refers to a pressurizing period that includes a third current-applying step, which is an energizing period for modifying the nugget 12 and is provided after the second pressurizing step S3. The second pressurizing step S3 and the third pressurizing step S4 may be provided continuously or discontinuously. When the second pressurizing step S3 and the third pressurizing step S4 are provided discontinuously, the second pressurizing step S3 refers to the period from when the pressurizing force is increased to or greater than P1 after the pressure-releasing step S2 until the pressurizing force becomes less than P1, and the third pressurizing step S4 refers to the pressurized period that includes the subsequent third current-applying step. On the other hand, when the second pressurizing step S3 and the third pressurizing step S4 are provided consecutively, the second pressurizing step S3 refers to a short period of time from when the pressurizing force is increased to or greater than P1 after the pressurizing release step S2 until the pressurizing force becomes less than P1 or the current value becomes less than I1, and the third pressurizing step S4 refers to a period that follows the second pressurizing step S3 and continues until the pressurizing force becomes 0. Note that when the second pressurizing step S3 and the third pressurizing step S4 are provided consecutively, the second current-flow step and the third current-flow step may also be consecutive.
The third pressurizing process S4 is a pressurizing period for modifying the nugget, and as long as the third current application process is included, the third holding process after the third current application process may or may not be included.

(Pressure force)
The pressure P1 (kN) in the first pressurizing step S1, the pressure Pr (kN) in the pressure release step S2, and the pressure P2 (kN) in the second pressurizing step S3 satisfy the formulas (1) to (3).
P1≦P2 (1)
2<P1...(2)
Pr≦2 (3)

Equation (2) specifies the lower limit of P1. That is, the pressure P1 in the first pressure application step S1 must be greater than 2 kN. If P1 is insufficient, the contact state between the electrode E and the steel sheet 111 becomes unstable, making it difficult to perform spot welding properly. P1 may be set to 2.2 kN or more, 2.5 kN or more, 3.0 kN or more, or 3.5 kN or more.

The upper limit of P1 is not particularly defined, but P1 may also satisfy the formula (10), for example.
P1<2.0×tsum/2...(10)
As described above, tsum is the total thickness (in mm) of the steel sheets 111 included in the sheet assembly 11. Formula (10) is a formula that determines the upper limit of P1 according to the total sheet thickness of the sheet assembly 11. When P1 satisfies formula (10), the contact area between the electrode E and the steel sheet 111 and the contact area between the steel sheets 111 themselves can be reduced in the first pressurizing step S1, thereby achieving a further improvement in current density and a further suppression of heat dissipation.

Instead of determining the upper limit value of P1 based on the total plate thickness of the plate assembly 11, it may be determined as in the following formula (11).
P1≦5 (11)
Even when formula (11) is satisfied, the contact area between the electrode E and the steel sheet 111 or the contact area between the steel sheets 111 in the first pressurizing step S1 can be reduced to improve the current density and further increase the diameter of the nugget 12. P1 may be set to 4.8 kN or less, 4.5 kN or less, or 4.2 kN or less.

Equation (1) defines the lower limit of P2. That is, the pressure P2 in the second pressure step S3 must be equal to or greater than the above-mentioned P1. This is to prevent spattering. In this embodiment, the current range that does not cause spattering is referred to as the "appropriate current range." The higher the pressure, the wider the appropriate current range. As will be described later, the current value I2 must be increased in the second pressure step S3. Therefore, in order to expand the appropriate current range, P2 is set to be equal to or greater than P1.

Preferably, P2 is 1.2 times or more as large as P1. That is, P2 may satisfy the formula (11).
1.2 × P1 ≦ P2 (12)
P2 may be 1.3 times or more, 1.5 times or more, or 1.8 times or more as large as P1.

Equation (3) specifies the upper limit of Pr. That is, the pressure Pr in the pressure release step S2 must be 2 kN or less. This suppresses heat transfer from the steel sheet 111 to the electrode E and promotes nugget growth through reheating. The smaller Pr is, the better, and it may even be 0 kN. That is, the electrode E and the steel sheet 111 may be separated from each other in the pressure release step S2. Pr may also be 1.8 kN or less, 1.5 kN or less, or 1.0 kN or less.

(current value)
The current value I1 (kA) in the first current-flowing step and the current value I2 (kA) in the second current-flowing step satisfy the formulas (4) and (5).
I1≦I2 (4)
4≦I1 (5)

Formula (5) defines the lower limit of I1. That is, the current value I1 in the first current application step must be 4 kA or greater. This increases the current density and increases the diameter of the nugget 12. I1 may be set to 4.2 kA or greater, 4.5 kA or greater, or 5.0 kA or greater. Furthermore, sufficient heat generation in the first pressure application step S1 allows for reheating in the subsequent pressure release step S2. Note that there is no particular upper limit for I1. If I1 is excessive, welding may become unstable due to factors such as expulsion. However, the limiting current value for expulsion can be determined, for example, by conducting a preliminary test. I1 should be set to a value equal to or less than this limiting current value for expulsion. I1 may also be set to 14.0 kA or less, 10.0 kA or less, or 6.0 kA or less.

Equation (4) defines the lower limit of I2. That is, the current value I2 in the second current application process must be equal to or greater than the current value I1 in the first current application process. By increasing I2, the diameter of the nugget 12 formed through the first pressure application process S1 and pressure release process S2 can be further enlarged. I2 may be set to 1.1 times or more, 1.2 times or more, or 1.3 times or more of I1.

In the second current application process, the nugget 12 that serves as the current path is formed in advance, so the current flow is naturally stable. Therefore, even if a current value greater than I1 is applied to the second current application process, there is little chance of problems such as expulsion occurring. On the other hand, to further stabilize the welding, I2 may be set to 14 kA or less, 12 kA or less, or 10 kA or less.

(Current application time T1 of the first current application process)
The energization time T1 (msec) in the first energization step satisfies the formula (6).
50×tsum/2<T1<200×tsum/2...(6)
Here, “tsum” in the formula (6) is the total plate thickness (mm) of the steel plates 111 included in the plate assembly 11 .

Equation (6) defines the upper and lower limits of the current flow time T1 based on tsum. The range of current flow time T1 defined by equation (6) is shorter than that of normal spot welding. Experiments conducted by the inventors have shown that if T1 is too long, the nugget 12 grows along the plate surface direction rather than along the plate thickness direction, and as a result, the effect of avoiding poor welding due to the pressure release step S2 cannot be achieved.

Figure 3 shows a conceptual diagram of this phenomenon. When the current application time T1 in the first current application process is set to less than 200 × tsum/2 and the pressure release process S2 is performed, the nugget 12 grows in the thickness direction due to reheating. As a result, the nugget diameter at the interface between the thinnest steel sheet 111 and the adjacent steel sheet 111 can be enlarged. On the other hand, when the current application time T1 in the first current application process is set to 200 × tsum/2 or more, the nugget 12 does not grow in the thickness direction sufficiently, even if reheating is achieved by the pressure release process S2. This is because, as shown in Figure 4, the longer T1 is, the greater the effect of heat transfer from the steel sheet 111 to the electrode E, which promotes solidification of the nugget 12 near the surface of the sheet assembly 11. As a result, the nugget diameter at the interface between the thinnest steel sheet 111 and the adjacent steel sheet 111 is not enlarged. For the above reasons, T1 is set to be less than 200×tsum/2. T1 may also be 180×tsum/2 or less, 150×tsum/2 or less, or 135×tsum/2 or less.

On the other hand, if T1 is too short, the amount of heat input in the first current application process will be insufficient, and a nugget 12 with a sufficient diameter will not be obtained. Therefore, T1 is set to be greater than 50 × tsum/2. T1 may also be 75 × tsum/2 or greater, 90 × tsum/2 or greater, or 120 × tsum/2 or greater.

(Current application time T2 of the second current application process)
The energization time T2 (msec) of the second energization step satisfies the formula (7).
50×tsum/2<T2...(7)
Here, "tsum" in formula (7) is the total plate thickness (mm) of the steel plates included in the plate assembly.

Equation (7) defines the lower limit of the current application time T2 based on tsum. If T2 is insufficient, the effect of enlarging the nugget diameter by the second current application process will not be sufficient. Therefore, T2 is set to greater than 50×tsum/2. T2 may also be set to 60×tsum/2 or more, 70×tsum/2 or more, or 80×tsum/2 or more. There is no particular upper limit to T2, but considering productivity, T2 may be set to 150×tsum/2 or less, 130×tsum/2 or less, or 100×tsum/2 or less, for example.

(Retention time Th1 of the first retention step)
When the first pressure application process S1 includes a first holding process after the first current application process in which a pressure that satisfies equation (2) is maintained with the current value set to 0, the holding time Th1 (msec) of the first holding process satisfies equation (8).
Th1<300...(8)
Note that "the first pressurizing step includes the first holding step" means that the period from the start to the end of the first pressurizing step includes the period from the start to the end of the first holding step.

Equation (8) defines the upper limit of Th1; specifically, Th1 is set to less than 300 msec. Generally, a holding time Th1 is set to stabilize spot welding and to harden the nugget 12. However, in the manufacturing method according to this embodiment, a shorter holding time Th1 is preferable. If Th1 is too long, heat removal may proceed before the start of the pressure release step S2, causing the nugget 12 to solidify, which may hinder the growth of the nugget 12 during the pressure release step S2. Th1 may be 200 msec or less, 150 msec or less, or 100 msec or less. The lower limit of Th1 is not particularly limited and may be 0 msec, i.e., the first holding step may not be set. However, considering the stability of spot welding, Th1 may be set to 20 msec or more, 40 msec or more, or 60 msec or more.

(Time Tr of pressure release step S2)
The time Tr (msec) of the pressure release step S2 satisfies the formula (9).
20<Tr...(9)

Equation (9) defines the lower limit of Tr; specifically, Tr is set to more than 20 msec. If Tr is too short, the effect of increasing the diameter of the nugget 12 due to reheating cannot be fully achieved. From the perspective of suppressing poor bonding, a longer Tr is preferable, so Tr may be set to, for example, 50 msec or more, 80 msec or more, or 100 msec or more. Even if solidification of the nugget 12 is completed in the pressure release step S2, the nugget diameter can be increased by applying current in the second current application step included in the subsequent second pressure application step S3. However, considering productivity, Tr may be set to, for example, 400 msec or less, 300 msec or less, or 250 msec or less.

(others)
The first pressurizing step S1 and the pressurizing release step S2 must be performed consecutively. This is because the pressurizing release step S2 is a step for growing the nugget 12 using heat generated in the first current-carrying step included in the first pressurizing step S1. Therefore, the pressurizing release step S2 must be performed before the heat diffuses into the base material of the resistance spot welded joint 1 or is dissipated to the electrode E. Specifically, the first pressurizing step S1 and the pressurizing release step S2 are performed consecutively so that the above-described holding time Th1 satisfies Equation (8). On the other hand, the pressurizing release step S2 and the second pressurizing step S3 do not need to be performed consecutively. This is because the nugget diameter can be enlarged by the second current-carrying step included in the subsequent second pressurizing step S3. Another pressurizing step including another current-carrying step may be provided between the pressurizing release step S2 and the second current-carrying step S3.

Furthermore, after the second pressurizing step S3, a third pressurizing step S4 may be provided, which includes a third current application step aimed at modifying the nugget 12. The third current application step is a so-called post-current application aimed at heat treating the nugget to modify its properties. The third current application step may be, for example, current application to reduce residual stress or current application for tempering. Various conditions can be applied to the third current application step depending on the components of the nugget 12, the configuration of the sheet assembly 11, and the application of the resistance spot welded joint 1, etc.

Other welding conditions are not particularly limited, and any conditions can be applied to the manufacturing method according to this embodiment. For example, unlike the first pressurizing step S1, the holding time of the second holding step after the second current application step in the second pressurizing step S3 is not particularly limited. The holding time of the second holding step may affect the cooling rate and degree of quench hardening of the nugget 12, but does not affect the diameter of the nugget 12 or the frequency of poor welding. Therefore, the holding time of the second holding step in the second pressurizing step S3 can be set appropriately depending on the components of the nugget 12, the configuration of the plate assembly 11, and the application of the resistance spot welded joint 1. Other welding conditions can also be appropriately selected from known conditions. Furthermore, the tip shape of the electrode E is not particularly limited. Electrodes of various shapes, such as DR, CR, CF, and R, can be used as the electrode E in the manufacturing method according to this embodiment.

Next, a method for manufacturing an automotive part according to another aspect of the present invention will be described. The method for manufacturing an automotive part according to this embodiment includes the method for manufacturing a resistance spot welded joint according to this embodiment described above. This makes it possible to easily manufacture automotive parts with a plate thickness ratio of 4.5 or more, with the thinnest steel plate located on the surface, and with reduced joint failure. When manufacturing automotive parts, thin plates used as exterior materials are often joined to thick plates used as structural materials. Therefore, the method for manufacturing an automotive part according to this embodiment is extremely suitable for manufacturing automotive parts with resistance spot welded joints with large plate thickness ratios.

However, the applications of the method for manufacturing a resistance spot welded joint according to the present embodiment described above are not particularly limited. For example, the method for manufacturing a resistance spot welded joint according to the present embodiment may be applied to the manufacture of home appliances.

The effects of one aspect of the present invention will be explained in more detail using examples. However, the conditions in the examples are merely an example of conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to this example of conditions. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and the objectives of the present invention are achieved.

Example 1
Three types of plate assemblies were prepared, each having the tensile strength (TS) and plate thickness shown in Table 1. In each plate assembly, steel plates 1, 2, 3, and 4 were stacked in this order. In each plate assembly, steel plate 1 was the thinnest steel plate arranged on the surface of the plate assembly.

These plate assemblies were spot welded using the spot welding machine and electrodes shown below.
・Welding machine: Servo pressure stationary welding machine DC (frequency 50Hz)
・Electrode: Dome radius (DR) Cr-Cu
・Electrode tip shape: φ6mm R40mm

The welding conditions were as shown in Table 2. The current and pressure were constant as shown in Table 2. Values outside the range of the invention are underlined. A supplementary explanation of the values shown in Table 2 is provided below.
Since the applied pressure in the pressure release step of Comparative Example 1 exceeds 2 kN, according to the above definition, the time Tr of the pressure release step S2 is 0. However, for convenience, the length of the period during which the applied pressure is at the value shown in Table 2 is described as Tr.
In the first current application step of Comparative Example 4, there is no period during which the current value I1 satisfies the formula (5), and therefore, by definition, the current application time T1 is 0. However, for convenience, the length of the period during which the current value is the value listed in Table 2 is listed in Table 2 as T1.
Although the pressure in the first current application step in Comparative Example 7 is 2 kN or less, for convenience, the first stage in Comparative Example 7 is treated as the first current application step. The pressure is set to the value shown in Table 2, and the time during which current application is stopped is shown as holding time Th1.
Although the pressure in the second current application step in Comparative Example 9 is less than P1, for convenience, the second stage in Comparative Example 9 is treated as the second current application step.
Although the current value in the second current application step in Comparative Example 10 is less than I1, for convenience, the second stage in Comparative Example 10 is treated as the second current application step.

Furthermore, the obtained resistance spot welded joint was cut along a plane passing through the center of the nugget and perpendicular to the surface of the steel sheet to prepare a cross section, and the nugget diameter between steel sheet 1 and steel sheet 2 was confirmed using an optical microscope. The nugget diameter is also shown in Table 3. The meanings of the symbols in Table 3 are as follows:
The nugget diameter between steel plate 1 and steel plate 2 is 4√t=3.1 (mm) or more: ◎
The nugget diameter between steel plate 1 and steel plate 2 is 3√t=2.3 (mm) or more: ◯ The nugget diameter between steel plate 1 and steel plate 2 is less than 3√t=2.3 (mm): ×
Even if the nugget diameter between steel plate 1 and steel plate 2 is 3√t=2.3 (mm) or more, expulsion occurs: △
Here, t refers to the thickness of steel plate 1, which is the thinnest steel plate arranged on the surface of the plate assembly.

In Comparative Example 1, formula (3) was not satisfied. That is, in Comparative Example 1, the applied pressure Pr during the pressure release process was excessive. As a result, the nugget solidified prematurely in Comparative Example 1, and the nugget diameter was not secured.

In Comparative Example 2, formula (6) was not satisfied. Specifically, in Comparative Example 2, the current application time T1 of the first current application step exceeded the upper limit value of formula (6). In Comparative Example 2, the heat input increased, but the amount of heat removed from the steel sheet 1 to the electrode also increased, and the current density decreased. As a result, the nugget solidified prematurely on the surface of the sheet assembly, and the nugget diameter was not secured.

In Comparative Example 3, formula (6) was not satisfied. Specifically, in Comparative Example 3, the current application time T1 of the first pressurizing step did not meet the lower limit of formula (6). As a result, in Comparative Example 3, the steel sheet did not melt sufficiently during the first current application step, and the nugget diameter was not secured.

In Comparative Example 4, equation (5) was not satisfied. That is, in Comparative Example 4, the current value I1 in the first pressurizing step was insufficient. As a result, in Comparative Example 3, the nugget did not grow to the steel plate 1 during the first current application step, and the nugget diameter was not secured.

In Comparative Example 5, equation (9) was not satisfied. That is, the time Tr during the pressure release process was insufficient in Comparative Example 5. As a result, heat recovery during the pressure release process was insufficient in Comparative Example 5, and the nugget diameter was not secured.

In Comparative Example 6, formula (8) was not satisfied. That is, in Comparative Example 6, the holding time Th1 in the first pressurizing step was excessive. As a result, the nugget solidified prematurely in Comparative Example 6, and the nugget diameter was not secured.

In Comparative Example 7, formula (2) was not satisfied. That is, in Comparative Example 7, the pressure P1 in the first pressurizing step was insufficient. As a result, in Comparative Example 7, expulsion occurred during the first pressurizing step, and the nugget diameter was not secured.

In Comparative Example 8, formula (7) was not satisfied. That is, in Comparative Example 8, the current application time T2 in the second pressurizing step was insufficient. As a result, in Comparative Example 8, the nugget did not grow sufficiently along the plate surface direction, and the nugget diameter was not secured.

In Comparative Example 9, formula (1) was not satisfied. That is, in Comparative Example 9, the pressure P2 in the second pressurizing step was lower than P1. As a result, in Comparative Example 9, although the nugget diameter was secured, expulsion occurred in the second pressurizing step.

In Comparative Example 10, formula (4) was not satisfied. That is, in Comparative Example 10, the current I2 in the second current application step was insufficient. As a result, in Comparative Example 10, the nugget did not grow sufficiently along the plate surface direction, and the nugget diameter was not secured.

On the other hand, in the resistance spot-welded joints of Examples 1 to 9, which were obtained by spot welding in which all of equations (1) to (9) were satisfied, a sufficiently large nugget diameter was ensured, and poor welding was suppressed.

1 Resistance spot welded joint 11 Sheet combination 111 Steel sheet 111 min Thinnest steel sheet 12 Nugget tmin Sheet thickness of thinnest steel sheet tsum Total sheet thickness E of steel sheets included in sheet combination Electrode S1 First pressure application process S2 Pressure release process S3 Second pressure application process P1 Pressure Pr of first pressure application process Pressure P2 of pressure release process Pressure I1 of second pressure application process Current value I2 of first current application process Current value T1 of second current application process Current application time Th1 of first current application process Holding time T2 of first holding process Current application time Tr of second current application process Time of pressure release process

Claims (8)

A method for manufacturing a resistance spot welded joint by spot welding a plate assembly formed by stacking two or more steel plates,
When the plate thickness of the thinnest steel plate among the steel plates included in the plate combination is tmin (mm) and the total plate thickness of the steel plates included in the plate combination is tsum (mm), tsum/tmin is 4.5 or more,
The thinnest steel plate is disposed on at least one surface of the plate assembly,
The manufacturing method comprises:
a first pressing step of sandwiching the plate assembly between tips of a pair of electrodes and applying pressure;
a pressure release step of reducing the pressure applied to the pair of electrodes;
a second pressing step of sandwiching the plate assembly between the tips of the pair of electrodes and applying pressure;
in order,
The pressure P1 (kN) in the first pressurizing step, the pressure Pr (kN) in the pressurizing and releasing step, and the pressure P2 (kN) in the second pressurizing step satisfy the following formulas (1) to (3):
the first pressurizing step includes a first current-carrying step of applying current between the pair of electrodes,
The current value in the pressure release step is 0,
the second pressurizing step includes a second current-carrying step of applying current between the pair of electrodes,
a current value I1 (kA) in the first current-flowing step and a current value I2 (kA) in the second current-flowing step satisfy equations (4) and (5),
The energization time T1 (msec) of the first energization step satisfies the formula (6),
The energization time T2 (msec) of the second energization step satisfies the formula (7),
In the first pressurizing step, when a first holding step is included in which a pressing force that satisfies formula (2) is maintained with a current value of 0 after the first current application step, a holding time Th1 (msec) of the first holding step satisfies formula (8),
A method for manufacturing a resistance spot welded joint, wherein the time Tr (msec) of the pressure release step satisfies formula (9).
P1≦P2 (1)
2<P1...(2)
Pr≦2 (3)
I1≦I2 (4)
4≦I1 (5)
50×tsum/2<T1<200×tsum/2...(6)
50×tsum/2<T2...(7)
Th1<300...(8)
20<Tr...(9) The method for manufacturing a resistance spot welded joint according to claim 1, wherein the applied pressure P1 (kN) in the first pressurizing step further satisfies formula (10).
P1<2.0×tsum/2...(10) The method for manufacturing a resistance spot welded joint according to claim 1, wherein the applied pressure P1 (kN) in the first pressurizing step further satisfies formula (11).
P1≦5 (11) The method for manufacturing a resistance spot welded joint according to any one of claims 1 to 3, wherein the pressing force P2 (kN) in the second pressing step further satisfies formula (12).
1.2 × P1 ≦ P2 (12) A method for manufacturing a resistance spot welded joint according to any one of claims 1 to 3, characterized in that after the second pressurizing step, a third pressurizing step is further provided, which includes a third current application step for the purpose of modifying the nugget. A method for manufacturing an automotive part, comprising the method for manufacturing a resistance spot welded joint described in any one of claims 1 to 3. A method for manufacturing an automotive part, comprising the method for manufacturing a resistance spot welded joint described in claim 4. A method for manufacturing an automotive part, comprising the method for manufacturing a resistance spot welded joint described in claim 5.