JP7587150B2 - Resistance spot welding method and manufacturing method for joint - Google Patents

Description

This disclosure relates to a resistance spot welding method and a method for manufacturing a joint.

Spot welding is primarily used in processes such as assembling car bodies and attaching parts.
In recent years, in the automotive field, there has been an increasing demand for lighter vehicle bodies to achieve better fuel economy and reduced _CO2 emissions, as well as for higher vehicle body rigidity to improve collision safety. To meet these demands, there is an increasing need to use high-strength steel plates for vehicle bodies, parts, etc.
On the other hand, in order to achieve high strength steel plate, the carbon equivalent of the base material is large, and in spot welding, the weld is heated and then immediately cooled rapidly, so the weld becomes a martensite structure, and the hardness of the weld and the heat-affected zone increases and the toughness decreases.

As a method for improving the toughness of a spot weld and ensuring joint strength, a method of performing post-heating current application after main current application has been proposed.
For example, as a spot welding method using two-stage current application, Patent Document 1 describes a method of spot welding high-strength steel sheets containing 0.15 mass % or more of carbon and having a tensile strength of 980 MPa or more by overlapping and spot welding the sheets, in which the spot welding process is divided into three steps: a first current application process for forming a nugget, a cooling process in which no current is applied following the first current application process, and a second current application process for softening the nugget following the cooling process. In this case, when the current in the first current application process is I ₁ and the current in the second current application process is I ₂ , I ₂ /I ₁ is set to 0.5 to 0.8, and the time tc (sec) of the cooling process is set to a range of 0.8×tmin or more and 2.5×tmin or less, calculated by tmin=0.2×H ² , depending on the steel sheet thickness H (mm ₎ . This spot welding method discloses a spot welding method in which (sec) is set in the range of 0.7 × tmin or more and 2.5 × tmin or less, and welding is performed by increasing the electrode pressure after the cooling step compared to the electrode pressure up to the first current application step to obtain a spot welded joint.

Patent Document 2 describes a three-stage current spot welding method in which a sheet set of two or more overlapping steel sheets is clamped between a pair of electrodes and joined by passing current while applying pressure, in which a main current passing step is performed in which current is passed at a current value I _w (kA), and then, as a post-tempering heat treatment step, cooling is performed for a cooling time t _ct (ms) shown in formula (1), and then current is passed at a current value I _t (kA) shown in formula (2) for a current passing time t _t (ms) shown in formula (3),
800≦t _ct ...Formula (1)
0.5×I _w ≦I _t ≦I _w ...Formula (2)
500≦t _t ...Formula (3)
At least one steel plate of the plate set is
0.08≦C≦0.3 (mass%),
0.1≦Si≦0.8 (mass%),
2.5≦Mn≦10.0 (mass%),
P≦0.1 (mass%)
and the balance being Fe and unavoidable impurities.

International Publication No. 2014/060848 International Publication No. 2019/156073

Increasing the carbon content of the steel plate used for spot welding can increase the strength of the joint base material (steel plate). However, in the examples of Patent Document 1, a steel plate with a C content of 0.22% or less is used, and it is desirable to use a steel plate with a higher C content to manufacture a joint with high joint strength (toughness).
Furthermore, Patent Document 2 requires the use of steel plates having a C content of 0.08 to 0.3%, and describes, as a comparative example, that when a steel plate having a C content of more than 0.3% is used and three-stage current application is performed, the joint strength decreases.

The present disclosure aims to provide a resistance spot welding method and a method for manufacturing a joint that can significantly improve joint strength compared to resistance spot welding using a single current, even when using a sheet assembly that includes steel sheets with a carbon content of more than 0.3%.

The gist of the present disclosure to achieve the above object is as follows.
<1> A first current-passing step of sandwiching a plate set obtained by stacking two or more steel plates including at least one steel plate having a C content, in mass%, of more than 0.30% and not more than 0.70% between a pair of electrodes in a plate thickness direction and applying pressure thereto at a current value I ₁ (kA);
a first de-energization step in which, after the first energization step, no current is applied for a time _tc1 of 16 ms or more and 200 ms or less;
a second current-passing step of passing a current at a current value _I2 (kA) that satisfies the following formula (1) and for a time _t2 (ms) that satisfies the following formula (2) after the first current-passing step;
0.6≦I ₂ /I ₁ ≦1.1 (1)
50≦ _t2 ≦1000...(2)
a second de-energization step in which, after the second energization step, no current is applied for a time t _c2 (ms) that satisfies the following formula (3) and the following formula (4);
3.5×10 ⁻³ ×Ms ² −3.3×Ms+1100<t _c2 ≦9000 (3)
Ms(℃)=561-474×[C]-33×[Mn]-17×[Ni]-17×[Cr]-21×[Mo]...(4)
a third current-passing step in which, after the second current-passing step, a current is passed at a current value I ₃ (kA) that satisfies the following formula (5) and for a time t ₃ (ms) that satisfies the following formula (6);
0.4≦I ₃ /I ₁ ≦1.0 (5)
200≦t ₃ ...(6)
A resistance spot welding method in which the above steps are performed continuously.
The Ms in the formula (3) means the Ms point calculated by substituting the mass% of each element contained in the steel plates constituting the plate assembly into [name of element] in the formula (4). However, when at least two steel plates constituting the plate assembly have different compositions, the Ms point calculated for each steel plate by the formula (4) for all steel plates constituting the plate assembly is multiplied by the plate thickness ratio of each steel plate to the total thickness of the plate assembly, and the weighted average Ms point is substituted into the formula (3).
<2> In the plate assembly, the at least one steel plate comprises, in mass%,
P content is less than 0.010%;
The resistance spot welding method according to <1>,
<3> In the plate assembly, the at least one steel plate comprises, in mass%,
P content is less than 0.010%;
S content is 0.0100% or less,
The resistance spot welding method according to <1>,
<4> In the plate assembly, the at least one steel plate comprises, in mass%,
The Si content is more than 0.10%,
P content is less than 0.010%;
S content is 0.0100% or less,
The resistance spot welding method according to <1>,
<5> In the plate assembly, the at least one steel plate comprises, in mass%,
The Si content is more than 0.10%,
Mn content is 15.00% or less,
P content is less than 0.010%;
S content is 0.0100% or less,
The resistance spot welding method according to <1>,
<6> A method for manufacturing a joint using the resistance spot welding method according to any one of <1> to <5>.

According to the present disclosure, a resistance spot welding method and a method for manufacturing a joint are provided that can significantly improve joint strength compared to resistance spot welding using a single current, even when using a sheet assembly that includes steel sheets with a carbon content of more than 0.3%.

FIG. 1 is a diagram showing the relationship between the spot welding conditions applied to overlapping steel plates and the CTS (cross tensile strength) of a joint. 13A and 13B are SEM-EBSD analysis images of the vicinity of the nugget after spot welding, in which (A) is a single current flow only, and (B) is a case where a second current flow was performed after the single current flow. FIG. 4 is a diagram illustrating an example of current and time in each step of the resistance spot welding method according to the present disclosure. FIG. 1 is a diagram illustrating an example of a nugget and a heat-affected zone (HAZ) formed when resistance spot welding is performed on a plate assembly in which two steel plates are overlapped. FIG. 11 is a diagram comparing CTS when spot welding is performed by single current while changing the carbon content of the steel plate (single current CTS) with CTS when spot welding is performed by current (three-stage current) according to the present disclosure. FIG. 6 is a processed view of FIG. 5 , showing the rate of increase in CTS when spot welding is performed by current application (three-stage current application) according to the present disclosure, relative to CTS when spot welding is performed by single current application (single current CTS).

Hereinafter, an embodiment that is an example of the present disclosure will be described.
In this disclosure, the "%" designation for the content of each element means "mass %." In this disclosure, a numerical range expressed using "to" means a range that includes the numerical values written before and after "to" as the lower and upper limits, unless otherwise specified. In addition, when the numerical values written before and after "to" are followed by "greater than" or "less than," the numerical range does not include these numerical values as the lower or upper limit.
In the numerical ranges described in stages in this disclosure, the upper or lower limit of a certain numerical range may be replaced by the upper or lower limit of another numerical range described in stages, or may be replaced by a value shown in the examples.
In addition, the term "process" includes not only an independent process but also a process that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved.

The inventors have intensively studied a method for improving joint strength (cross tensile strength: CTS) when resistance spot welding is performed even when the C content of the steel sheet exceeds 0.30 mass%. Figure 1 shows the relationship between the CTS of a joint in which two types of steel sheets, each with a C content of 0.34% and a P content of 0.015% (normal P material) and 0.0007% (extremely low P material), are stacked together and resistance spot welded. The components other than the C and P contents are common to both types: S: 0.0008%, Si: 0.25%, and Mn: 1.25%. In addition, "single current" means that resistance spot welding was performed by passing a current once to form a nugget in the sheet assembly, and "temper current" means that a single current was passed through the sheet assembly to form a nugget, followed by a post-current (temper current) equivalent to an annealing process to soften the nugget (two-stage current). Three-stage current passing means that after a single current passing to form the nugget, a current value greater than the temper current is passed, and then the temper current is passed.

Both the single current joint and the temper current joint were made of normal P material and extremely low P material. The three-stage current joint used normal P material, but the CTS far exceeded that of extremely low P material subjected to temper current. Since the extremely low P material has a high CTS even without current flow after segregation relaxation, the inventors speculated that the improvement in CTS by the three-stage current cannot be explained only by the effect of segregation relaxation and tempering by temper current.
Therefore, in order to investigate the influence of the second stage of current passing in the three-stage current passing, the inventors cut the steel sheet after spot welding perpendicular to the sheet thickness, polished it, and observed the metal structure.

Figure 2 shows images of a SEM-EBSD analysis of a nugget and its vicinity after spot welding. (A) shows the result of only a single current, and (B) shows the result of a single current followed by a second current of 0.1 seconds (no tempering current). In (B), grain order can be seen near the edge of the nugget (near the fusion boundary) that is not seen much in (A). This is thought to be due to a change in grain shape caused by the grains being heated again after solidification, undergoing delta transformation, and then cooled again and undergoing gamma transformation. It is believed that this grain order improves toughness and CTS.

Generally, the higher the C content, the higher the tensile strength of the steel plate, but the toughness of the welded part decreases, resulting in a decrease in joint strength. However, the inventors of the present invention have found that for steel plates with a C content of more than 0.30%, not only segregation mitigation but also grain regulating is important. They have also found that even for plate assemblies that include steel plates with a C content of more than 0.30% and 0.70% or less, by conducting a three-stage current process that combines a current process for forming a nugget under specific conditions, a current process for grain regulating, and a current process for tempering, the toughness of the area where stress in the peeling direction is most applied in the CTS test (near the nugget boundary within the nugget) can be improved, and joint strength can be significantly improved.

That is, the resistance spot welding method according to the present disclosure includes:
a first current-passing step of sandwiching a plate set obtained by stacking two or more steel plates including at least one steel plate having a C content, in mass%, of more than 0.30% and not more than 0.70% between a pair of electrodes in a plate thickness direction and applying pressure thereto while passing a current I ₁ (kA);
a first de-energization step in which, after the first energization step, no current is applied for a time _tc1 of 16 ms or more and 200 ms or less;
a second current-passing step of passing a current at a current value _I2 (kA) that satisfies the following formula (1) and for a time _t2 (ms) that satisfies the following formula (2) after the first current-passing step;
0.6≦I ₂ /I ₁ ≦1.1 (1)
50≦ _t2 ≦1000...(2)
a second de-energization step in which, after the second energization step, no current is applied for a time t _c2 (ms) that satisfies the following formula (3) and the following formula (4);
3.5×10 ⁻³ ×Ms ² −3.3×Ms+1100<t _c2 ≦9000 (3)
Ms(℃)=561-474×[C]-33×[Mn]-17×[Ni]-17×[Cr]-21×[Mo]...(4)
a third current-passing step in which, after the second current-passing step, a current is passed at a current value I ₃ (kA) that satisfies the following formula (5) and for a time t ₃ (ms) that satisfies the following formula (6);
0.4≦I ₃ /I ₁ ≦1.0 (5)
200≦t ₃ ...(6)
This is done continuously.
The Ms in the formula (3) means the Ms point calculated by substituting the mass% of each element contained in the steel plates constituting the plate assembly into [name of element] in the formula (4). However, when at least two steel plates constituting the plate assembly have different compositions, the Ms point calculated for each steel plate by the formula (4) for all steel plates constituting the plate assembly is multiplied by the plate thickness ratio of each steel plate to the total thickness of the plate assembly, and the weighted average Ms point is substituted into the formula (3).

Figure 3 shows an example of the current and time in each step of the resistance spot welding method according to the present disclosure. The resistance spot welding method according to the present disclosure uses a sheet assembly in which two or more steel sheets are overlapped, including at least one steel sheet having a C content, in mass %, of more than 0.30% and not more than 0.70%, and as shown in Figure 3, a first current flow step, a first non-current flow step, a second current flow step, a second non-current flow step, and a third current flow step are performed consecutively, i.e., without any other steps in between. Each step will be described in detail below.

[First current application step]
First, in the first current application step, a sheet assembly consisting of two or more overlapping steel sheets, including at least one steel sheet having a C content, in mass%, of more than 0.30% and not more than 0.70%, is sandwiched between a pair of electrodes in the sheet thickness direction and pressurized while a current value _I1 (kA) is applied thereto.

In the first current application step, it is preferable to set the current value I ₁ (kA) and the current application time t ₁ (ms) so that a nugget is formed by spot welding all the steel plates constituting the plate assembly. Fig. 4 shows an example of a nugget formed when the first current application step is performed on a plate assembly in which two steel plates are stacked. As shown in Fig. 4, current is applied between the electrodes 2A and 2B while the electrodes 2A and 2B are pressed against each other so as to sandwich the plate assembly in which the steel plates 1A and 1B are stacked in the plate thickness direction. As a result, a nugget 13 and a heat-affected zone (so-called HAZ) 14 are formed in the current application part between the steel plates 1A and 1B, and the two steel plates are welded.
In the first current application step, the current value I ₁ is a current value at which the desired nugget diameter is obtained, and when the thinnest steel plate t (mm) in the sheet assembly is used, the current application time t ₁ may be set to 10t-5 to 10t+50 cycles (50 Hz), etc. It is preferable to aim for a nugget diameter of 4√t or more from the viewpoint of joint strength and avoidance of splashing. The nugget diameter is more preferably 5√t or more. In order to form such a nugget diameter of 5√t or more without causing splashing, it is desirable to set an upslope of 1 cycle to 80 cycles (50 Hz) before the first current application step. In addition, before the first current application step, pre-current application of 2 to 80 cycles at a current value lower than that of the first current application step may be performed.
When upslope current is applied, the current value at the end of the upslope is set to the current value I ₁ (kA) in the first current application step, and the current application time t ₁ (ms) of the first current application step does not include the time required for the upslope.
The pressure of the electrodes 2A and 2B against the plate assembly is, for example, 2000 to 8000 N so as to suppress the occurrence of expulsion and to stably obtain nuggets. The pressure may be constant or may be changed during welding.

In the resistance spot welding method according to the present disclosure, at least one of the steel sheets constituting the sheet pair may have a C content of more than 0.30% and not more than 0.70% by mass. Examples of such steel sheets include cold-rolled steel sheets, hot-rolled steel sheets, and hot-stamped steel sheets. In particular, hot-stamped steel sheets with a high C content are preferable. The surface of the steel sheet may be unplated, zinc-based plated, or aluminum-based plated. The number of steel sheets constituting the sheet pair is not particularly limited as long as it is two or more, and may be selected according to the application of the joint to be manufactured.
Hereinafter, a steel plate having a C content of more than 0.30% and not more than 0.70% (hereinafter, may be simply referred to as a "steel plate") in a sheet assembly according to the present disclosure will be described.

C: more than 0.30% and not more than 0.70% C is an element that improves the hardenability of steel and contributes to improving strength. When spot welding is performed by stacking only steel plates with a C content of 0.3% or less, it is possible to ensure joint strength without applying the resistance spot welding method according to the present disclosure, so in the resistance spot welding method according to the present disclosure, the C content of at least one steel plate is made to be more than 0.30%. It is preferably 0.31% or more, more preferably 0.33% or more, and even more preferably 0.35% or more.
However, if the C content exceeds 0.70%, the toughness is too low and even if the resistance spot welding method according to the present disclosure is applied, only a low CTS is still obtained, so the C content is set to 0.70% or less, preferably 0.55% or less, and more preferably 0.48% or less.

FIG. 5 is a graph comparing the CTS when the carbon content of steel sheets is changed and spot welded by a single current (single current CTS) with the CTS when spot welding is performed under the following conditions (normal condition CTS of the present invention).
First energization step/I ₁ : 6800A, t ₁ : 500ms
1st non-current process/t _c1 : 70ms
Second energization process/I ₂ : 5800A, t ₂ : 250ms
2nd non-current process/t _c2 : 700ms
Third energization step/I ₃ : 4000A, t ₃ : 1300ms

FIG. 6 is a graph showing the rate of increase, calculated by the following formula, of the CTS when spot welding is performed by current application according to the present disclosure (invention normal condition CTS) relative to the CTS when spot welding is performed by single current application (single current CTS).
Rise rate [%] = [(normal condition CTS of the present invention - single current CTS) / single current CTS] x 100 [%]

As shown in Figures 5 and 6, when the C content is 0.3% or less, there is little improvement in CTS (Figure 6), and when it exceeds 0.70%, the CTS becomes quite low to begin with (Figure 5). Therefore, by performing three-stage current application under specific conditions on sheet assemblies that include steel sheets with a C content of more than 0.3% to 0.7%, it is possible to significantly improve the CTS.

The remainder other than C may be Fe and impurities, or may contain optional components in place of a portion of Fe. Note that impurities refer to components that are not intentionally included in the steel sheet, and are exemplified by components contained in raw materials such as ores and scraps, or components that are mixed in during the manufacturing process. Below, preferred contents of components other than C and Fe are explained. Note that the components explained below are impurities or optional components, and the lower limit may be 0% or may exceed 0%.

P: less than 0.010% P is an impurity and an element that causes embrittlement. If the P content is 0.010% or more, it is difficult to obtain sufficient joint strength, so the upper limit is preferably set to less than 0.010%, and more preferably to 0.009% or less.
It should be noted that the lower the P content, the more preferable it is, but the lower the P content, the higher the dephosphorization cost. According to the resistance spot welding method of the present disclosure, as shown in Fig. 1, even when a steel sheet with a normal P content is used, the CTS can be improved to be equal to or higher than that when a nugget is formed by current application using a steel sheet with an extremely low P content, and then current application for tempering is performed. Therefore, it is not necessary to significantly lower the P content of the steel sheet, and the lower limit of the P content may be 0.0005%.

S: 0.0100% or less S, like P, is an impurity and an element that causes embrittlement. In addition, S forms coarse MnS in steel, which reduces the workability of steel and also reduces joint strength. If the S content exceeds 0.0100%, it is difficult to obtain the required joint strength, and the workability of steel decreases, so it is preferable to keep the S content at 0.0100% or less.
In addition, the smaller the S content, the more preferable. From the same viewpoint as for P, the lower limit of the S content of the steel sheet may be 0.0003%.

Si: More than 0.10% Si is an element that increases the strength of steel by solid solution strengthening. If the Si content is 0.10% or less, the joint strength decreases, so the lower limit is preferably more than 0.10%. More preferably, it is more than 0.80%.
On the other hand, if the Si content is too high, the workability and the joint strength decrease, so the upper limit may be set to 3.5% or 3.0%.

Mn: 15.00% or less Mn is an element that increases the strength of steel. If the Mn content exceeds 15.00%, the workability and joint strength are deteriorated, so the upper limit is preferably set to 15.00%. In order to ensure a good balance between the strength, workability, and joint strength of the steel plate, 0.5 to 7.5% is more preferable. The content is further preferably 1.0 to 2.5%.

Al: 3.00% or less Al is an element that has a deoxidizing effect, stabilizes ferrite, and suppresses the precipitation of cementite. Al is contained for deoxidization and to control the steel structure, but Al is easily oxidized, and if the Al content exceeds 3.00%, inclusions increase, and the workability and joint strength decrease, so it is preferable to set the Al content at 3.00% or less. In terms of ensuring workability, a more preferable upper limit is 1.2%.

N: 0.0100% or less N is an element that increases the strength of steel plate, but also forms coarse nitrides in steel, which acts to deteriorate the formability of steel. If the N content exceeds 0.0100%, the deterioration of the formability of steel and the decrease in joint strength become significant, so it is preferable to keep the N content at 0.0100% or less.
From the viewpoint of increasing the cleanliness of the steel sheet, the N content may be 0%. From the viewpoint of reducing the production cost of N, the lower limit may be 0.0001%.

Ti: 0.30% or less Ti is an element that forms precipitates and makes the steel sheet structure fine-grained, and may be contained. In order to obtain the effect of containing, it is preferable to contain 0.001% or more. More preferably, it is 0.01% or more. On the other hand, if it is contained excessively, not only the manufacturability is reduced, cracks are generated during processing, but also the joint strength is reduced, so that the upper limit is preferably 0.30%, and more preferably 0.20% or less.

Nb: 0.30% or less Nb is an element that forms fine carbonitrides and suppresses the coarsening of crystal grains, and may be contained. In order to obtain the effect of containing, it is preferable to contain 0.001% or more. More preferably, it is 0.01% or more. If contained excessively, it not only inhibits toughness and makes manufacturing difficult, but also causes a decrease in joint strength, so the upper limit is preferably 0.30%, and more preferably 0.20% or less.

V: 0.30% or less V is an element that forms fine carbonitrides and suppresses the coarsening of crystal grains, and may be contained. In order to obtain the effect of containing V, it is preferable to contain 0.001% or more. More preferably, it is 0.03% or more. If contained excessively, it not only inhibits toughness and makes manufacturing difficult, but also causes a decrease in joint strength, so the upper limit is preferably set to 0.30%, and more preferably 0.25% or less.

Cr: 5.00% or less Mo: 2.00% or less Cr and Mo are elements that contribute to improving the strength of steel, and may be contained. In order to obtain the effect of containing them, it is preferable to contain them at 0.001% or more, respectively. More preferably, it is 0.05% or more. However, if the Cr content exceeds 5.00% or the Mo content exceeds 2.00%, not only may problems occur during pickling or hot working, but also the strength of the joint may decrease, so the upper limit of the Cr content is preferably 5.00% and the upper limit of the Mo content is preferably 2.00%.

Cu: 2.00% or less Ni: 10.00% or less Cu and Ni are elements that contribute to improving the strength of steel and may be contained. In order to obtain the effect of containing them, it is preferable to contain them at 0.001% or more, respectively. More preferably, it is 0.10% or more. However, if the Cu content exceeds 2.00% and the Ni content exceeds 10.00%, not only may problems occur during pickling and hot working, but also the joint strength may be reduced, so the upper limit of the Cu content is preferably 2.00% and the upper limit of the Ni content is preferably 10.00%.

Ca: 0.0030% or less REM: 0.050% or less Mg: 0.05% or less Zr: 0.05% or less Ca, REM (rare earth metal), Mg, and Zr are elements that refine oxides after deoxidation and sulfides present in hot-rolled steel sheets, and contribute to improving formability, and may be contained. However, if the Ca content exceeds 0.0030%, the REM content exceeds 0.050%, and each of the Mg and Zr contents exceeds 0.05%, the workability of the steel decreases. Therefore, it is preferable to set the upper limit of the Ca content to 0.0030%, the upper limit of the REM content to 0.050%, and the upper limit of each of the Mg and Zr contents to 0.05%.
In order to obtain the effects of each of these elements, it is preferable that the Ca content be 0.0005% or more, the REM content be 0.001% or more, the Mg content be 0.001% or more, and the Zr content be 0.001% or more.

Note that "REM" is a general term for Sc, Y, and lanthanides, a total of 17 elements, and the REM content refers to the total content of one or more REM elements. REM is generally contained in misch metal. For this reason, for example, REM may be contained in the form of misch metal so that the total REM content falls within the above range.

B: 0.0200% or less B is an element that segregates at grain boundaries to increase grain boundary strength, and may be contained. In order to obtain the effect of containing B, it is preferable to contain 0.0001% or more, and more preferably 0.0008% or more. On the other hand, if it is contained excessively, it not only inhibits toughness and makes manufacturing difficult, but also causes a decrease in joint strength, so that the upper limit is preferably set to 0.0200%, and more preferably 0.010% or less.

In the resistance spot welding method according to the present disclosure, of a sheet set in which two or more steel sheets are overlapped, at least one steel sheet has a C content, in mass%, of more than 0.30% and not more than 0.70%, and preferably a steel sheet having a composition within the above-mentioned range by selecting desired elements from the elements described above is used.
As steel sheet components,
C: more than 0.30% to 0.70%,
Si: more than 0.10%,
Mn: 15.00% or less,
P: less than 0.010%;
S: 0.0100% or less,
Al: 3.00% or less, and N: 0.0100% or less;
The balance of the steel plate may be iron (Fe) and impurities.
The steel sheet having the above-mentioned composition contains, in place of a part of the iron (Fe),
Ti: 0.30% or less,
Nb: 0.30% or less,
V: 0.30% or less,
Cr: 5.00% or less,
Mo: 2.00% or less,
Cu: 2.00% or less,
Ni: 10.00% or less,
Ca: 0.0030% or less,
REM: 0.050% or less,
Mg: 0.05% or less,
One or more elements selected from the group consisting of Zr: 0.05% or less and B: 0.0200% or less may be contained.
The C content of all of the steel plates constituting the sheet assembly may be more than 0.30% and 0.70% or less, or the C content of some of the steel plates in the sheet assembly may be 0.30% or less or more than 0.70%.

The thickness of the steel plates constituting the plate assembly is not particularly limited, but may be, for example, 0.5 to 3.5 mm.
The total thickness t of the plate assembly is not particularly limited, but may be, for example, 1.5 to 8.0 mm.

[First non-energized step]
After the first current application step, a time _tc1 of 16 ms or more and 200 ms or less is set as a non-current application period.
If the current-free time _tc1 is less than 16 ms, the nugget edge may not solidify before the second current-passing step. On the other hand, if the current-free time _tc1 is more than 200 ms, the nugget edge may solidify too much before the second current-passing step.
In order to avoid post-current application (second current application step) when the nugget end is insufficiently or excessively solidified and to properly advance solidification of the nugget end, the current-free time _tc1 after the first current application step is set to 16 ms or more and 200 ms or less, preferably 25 ms or more and 160 ms or less, and more preferably 30 ms or more and 150 ms or less.

[Second current application process]
The second current passing step is an important step that the inventors discovered can improve the CTS even if the C content of the steel sheet exceeds 0.3%. This step has the effect of regulating the grain size of the crystal grains near the fusion boundary in the nugget and improving the toughness of the portion where the stress applied in the peeling direction is the highest in the CTS test.
After the first de-energizing step, a current is applied at a current value I ₂ (kA) that satisfies the following formula (1) and for a time t ₂ (ms) that satisfies the following formula (2).
0.6≦I ₂ /I ₁ ≦1.1 (1)
50≦ _t2 ≦1000...(2)
In the second current flow process, in order to melt the center of the nugget without crossing the fusion boundary created in the first current flow process and to supply appropriate heat near the end of the nugget, current is passed under conditions such that the ratio ( _I2 / _I1 ) to the current value ( _I1 ) in the first current flow process and the current flow time ( _t2 ) satisfy the above equations (1) and (2), respectively.
The second current flow process corresponds to a grain control heat treatment, and by passing a current _I2 (kA) and a time _t2 (ms) that satisfy the above formulas (1) and (2), the grains of the nugget are changed, thereby improving the joint strength.
It is preferable that I ₂ /I ₁ is 0.75 to 1.05, and t ₂ is 200 to 600.

[Second non-energized step]
After the second current application step, a time t _c2 (ms) that satisfies the following formula (3) and the following formula (4) is set as a no-current application time.
3.5×10 ⁻³ ×Ms ² −3.3×Ms+1100<t _c2 ≦9000 (3)
Ms(℃)=561-474×[C]-33×[Mn]-17×[Ni]-17×[Cr]-21×[Mo]...(4)
Ms in formula (3) means the Ms point calculated by substituting the mass% of each element contained in the steel plate constituting the plate assembly in [element name] in formula (4). However, when at least two steel plates constituting the plate assembly have different compositions, the plate thickness is taken into consideration, and the weighted average Ms point of the value obtained by multiplying the Ms point calculated for each steel plate by formula (4) for all steel plates constituting the plate assembly by the plate thickness ratio of each steel plate to the total thickness (total thickness) of the plate assembly is substituted into formula (3). Note that, for elements in formula (4) that are not contained in the steel plate, zero is substituted.

For example, when spot welding a sheet assembly consisting of three overlapping steel plates α, β, and γ with different compositions, if the Ms point (°C) calculated from the composition of each steel plate using formula (4) is Ms _α , Ms _β , Ms _γ , the sheet thicknesses (mm) of each steel plate are t _α , t _β , t _γ , and the total thickness of the sheet assembly is t, the weighted average Ms point (Ms _ave ) taking into account the sheet thicknesses of each steel plate in this sheet assembly is calculated as follows:
Ms _ave = Ms _α × (t _α /t) + Ms _β × (t _β /t) + Ms _γ × (t _γ /t)

In the second non-energizing step, the temperature of the entire weld needs to be equal to or lower than the Ms point in order to temper the nugget and HAZ in the following third energizing step. Therefore, the required time varies depending on the steel composition. When the Ms point was found using the above formula (4) and the time required for the temperature of the entire weld to be equal to or lower than the Ms point was calculated, it was found that 3.5×10 ⁻³ ×Ms ² −3.3×Ms + 1100<t _c2 was required.
However, if the second non-current-passing time _tc2 exceeds 9000 ms, the fatigue strength in cross tension may decrease. The reason for this is thought to be that strong residual stress remains in the welded portion, even though there is a third current-passing step following the second non-current-passing time _tc2 . Therefore, the second non-current-passing time _tc2 needs to be 9000 ms or less.

[Third current application process]
After the second de-energizing step, a current is applied at a current value I ₃ (kA) that satisfies the following formula (5) and for a time t ₃ (ms) that satisfies the following formula (6).
0.4≦I ₃ /I ₁ ≦1.0 (5)
200≦t ₃ ...(6)
The third current passing step corresponds to a tempering heat treatment, and the current value _I3 and current passing time _t3 may be set so as to reheat the nugget cooled to the Ms point or below to an appropriate tempering temperature. Experimental results have shown that the toughness can be improved by passing the current ( _I3 ) in the third current passing step under conditions where the ratio ( _I3 / _I1 ) to the current value ( _I1 ) in the first current passing step and the current passing time ( _t3 ) satisfy the formulas (5) and (6), respectively.
If the current flow time in the third current flow step is too long, productivity will decrease, so it is preferable to set the current flow time to 5000 ms or less. A downslope may be provided after the third current flow step. The downslope further improves the characteristics of the welded portion by reducing cracking due to liquid metal embrittlement, reducing blowholes, and suppressing delayed fracture. When downslope current flow is performed, the current value at the start of the downslope is set to the current value _I3 (kA) in the third current flow step, and the current flow time _t3 (ms) of the third current flow step does not include the time required for the downslope.
After the third current application step, it is preferable to provide a so-called holding time during which pressure is applied but no current is applied in order to prevent cracking due to liquid metal embrittlement. The holding time is preferably 5 cycles (50 Hz) or more.

By performing resistance spot welding consisting of the above-mentioned steps on a sheet assembly in which two or more steel sheets are overlapped, including at least one steel sheet having a C content of more than 0.30% and 0.70% or less, the CTS can be significantly improved compared to the case of performing resistance spot welding with a single current. In addition, in the sheet assembly, a TSS (tensile shear strength) that matches the nugget hardness may not be obtained with a single current. According to the present invention, the TSS can also be improved by improving the toughness of the welded portion. Furthermore, the delayed fracture resistance of the spot welded portion can also be improved by improving the toughness and softening the welded portion.
The field of application of the resistance spot welding method according to the present disclosure is not particularly limited, but it is believed to be particularly effective in processes such as vehicle body assembly and part installation.

The resistance spot welding method and joint manufacturing method according to the present disclosure will be described below using examples. Note that the resistance spot welding method and joint manufacturing method according to the present disclosure are not limited to these examples.

Steel sheets having the compositions shown in Table 1 were prepared, and resistance spot welding was performed under the conditions shown in Table 2 (sheet assembly, pressure, current conditions, etc.).
In Table 1, even if P, S, and N were not intentionally added, the composition analysis was performed. Moreover, notations using inequality signs such as "<0.0002%" mean that the content is less than the analytical lower limit. If other elements are not intentionally added, "-" is written and the composition analysis was not performed. The balance is Fe and impurities. Moreover, in Table 1, values in which the C content of the steel plate is 0.30% or less or exceeds 0.70% are underlined. Note that steel plate r and steel plate s have a C content of 0.30% or less, but are prepared for use in the sheet assembly of the invention example in which the steel plate is combined with a steel plate having a C content of more than 0.30% and 0.70% or less, and are not underlined.

The underlines in Table 2 indicate that the requirements of the present disclosure are not met. Additionally, the "left side of formula (3)" refers to the value calculated by "3.5×10 ⁻³ ×Ms ² −3.3×Ms+1100," and the "single-current CTS" refers to the CTS when a sample is produced under only the first current flow (I ₁ , t ₁ ) among the current flow conditions.
The rate of increase was compared with the single current CTS, and any increase exceeding 10% was determined to have an effect of improving joint strength.
Rise rate [%]=[(CTS under the current-carrying conditions of the present disclosure−CTS with single current-carrying)/CTS with single current-carrying]×100
In addition to the conditions shown in Table 2, in No. 43, upslope current was applied for 500 ms before the first current application step, and in No. 44, pre-current was applied with a current of 6.7 kA for a current application time of 500 ms before the first current application step.

In Nos. 1, 3 to 6, 8 to 19, and 32 to 44, resistance spot welding satisfying the conditions of the present disclosure was performed on sheet assemblies in which the C content of at least one steel sheet was more than 0.30% and not more than 0.70% by mass, and in all cases, the increase rate of CTS exceeded 10% compared to the case of resistance spot welding using a single current.
On the other hand, in Nos. 2, 7, and 20 to 31, which do not satisfy the conditions of the present disclosure, the increase rate of CTS was less than 10% compared to the case where resistance spot welding was performed by single current, and in some cases the CTS actually decreased.

1A, 1B Steel plate 2A, 2B Electrode 13 Nugget 14 Heat affected zone (HAZ)

Claims (6)

a first current application step of forming a nugget by sandwiching a sheet set of two or more steel sheets, including at least one steel sheet having a C content, in mass%, of more than 0.30% and not more than 0.70%, between a pair of electrodes in a sheet thickness direction and applying pressure thereto while passing a current _I1 (kA);
a first de-energization step in which, after the first energization step, no current is applied for a time _tc1 of 16 ms or more and 200 ms or less;
a second current-passing step of passing a current at a current value _I2 (kA) that satisfies the following formula (1) and for a time _t2 (ms) that satisfies the following formula (2) after the first current-passing step;
0.6≦I ₂ /I ₁ ≦1.1 (1)
50≦ _t2 ≦1000...(2)
a second de-energization step in which, after the second energization step, no current is applied for a time t _c2 (ms) that satisfies the following formula (3) and the following formula (4);
3.5×10 ⁻³ ×Ms ² −3.3×Ms+1100<t _c2 ≦9000 (3)
Ms(℃)=561-474×[C]-33×[Mn]-17×[Ni]-17×[Cr]-21×[Mo]...(4)
a third current-passing step in which, after the second current-passing step, a current is passed at a current value I ₃ (kA) that satisfies the following formula (5) and for a time t ₃ (ms) that satisfies the following formula (6);
0.4≦I ₃ /I ₁ ≦1.0 (5)
200≦t ₃ ...(6)
A resistance spot welding method in which the above steps are performed continuously.
The Ms in the formula (3) means the Ms point calculated by substituting the mass% of each element contained in the steel plates constituting the plate assembly into [name of element] in the formula (4). However, when at least two steel plates constituting the plate assembly have different compositions, the Ms point calculated for each steel plate by the formula (4) for all steel plates constituting the plate assembly is multiplied by the plate thickness ratio of each steel plate to the total thickness of the plate assembly, and the weighted average Ms point is substituted into the formula (3). Of the plate set, the at least one steel plate comprises, in mass%,
The P content is less than 0.010%.
2. The resistance spot welding method of claim 1. Of the plate set, the at least one steel plate comprises, in mass%,
P content is less than 0.010%;
S content is 0.0100% or less,
The resistance spot welding method according to claim 1 . Of the plate set, the at least one steel plate comprises, in mass%,
The Si content is more than 0.10%,
P content is less than 0.010%;
S content is 0.0100% or less,
The resistance spot welding method according to claim 1 . Of the plate set, the at least one steel plate comprises, in mass%,
The Si content is more than 0.10%,
Mn content is 15.00% or less,
P content is less than 0.010%;
S content is 0.0100% or less,
The resistance spot welding method according to claim 1 . A method for manufacturing a joint using the resistance spot welding method described in any one of claims 1 to 5.