阅读说明：本技术 电阻点焊方法、电阻点焊接头的制造方法 (Resistance spot welding method and method for manufacturing resistance spot welded joint ) 是由 远藤玲子 松下宗生 谷口公一 松田广志 池田伦正 于 2019-02-05 设计创作，主要内容包括：本发明的目的在于提供电阻点焊方法。本发明的电阻点焊方法是用一对电极夹持2张以上钢板叠合而成的板组、并一边加压一边通电进行接合的电阻点焊方法,该方法包括：进行以电流值I<Sub>w</Sub>(kA)通电的主通电工序,然后,作为回火后热处理工序,在式(3)所示的通电时间t<Sub>t</Sub>(ms)的期间以式(2)所示的电流值I<Sub>t</Sub>(kA)进行通电,板组中的至少一张钢板具有如下成分：以质量％计,含有0.08≤C≤0.3、0.1≤Si≤0.8、2.5≤Mn≤10.0、P≤0.1,且余量由Fe及不可避免的杂质构成。800≤t<Sub>ct</Sub>···式(1) 0.5×I<Sub>w</Sub>≤I<Sub>t</Sub>≤I<Sub>w</Sub>···式(2) 500≤t<Sub>t</Sub>···式(3)。(The invention aims to provide a resistance spot welding method. The resistance spot welding method of the present invention is a resistance spot welding method for sandwiching a plate group formed by stacking at least 2 steel plates between a pair of electrodes and joining the steel plates by applying a voltage and a current, the resistance spot welding method including: is carried out with a current value I w (kA) and then as a post-tempering heat treatment step, the main energization step of energization and the post-tempering heat treatment step are performed for an energization time t shown in the formula (3) t Current value I shown in formula (2) during (ms) t (kA) and at least one steel sheet in the plate package has the following composition: contains 0.08-0.3C, 0.1-0.8 Si, 2.5-10.0 Mn and 0.1P in mass%, and the balance Fe and inevitable impurities. T is more than or equal to 800 ct 0.5 × I of formula (1) w ≤I t ≤I w T is 500. ltoreq. of formula (2) t Formula (3).)

1. A resistance spot welding method for clamping a plate group formed by stacking at least 2 steel plates by a pair of electrodes and bonding the steel plates by applying a voltage and a current, the method comprising:

is carried out with a current value I_w(kA) a main energization step of energizing,

then, as a post-tempering heat treatment step,

at a cooling time t represented by the formula (1)_ctAfter the cooling has been carried out in (ms),

the energization time t shown in the formula (3)_tCurrent value I shown in formula (2) during (ms)_t(kA) the electric current is applied to the substrate,

800≤t_ctthe formula (1)

0.5×I_w≤I_t≤I_wThe type (2)

500≤t_tThe type (3)

At least one steel sheet in the set of sheets has the following composition: contains, in mass%)

0.08≤C≤0.3、

0.1≤Si≤0.8、

2.5≤Mn≤10.0、

P is less than or equal to 0.1, and the balance is Fe and inevitable impurities.

2. The resistance spot welding method according to claim 1,

a segregation reducing post-heat treatment step of further performing a segregation reducing post-heat treatment step between the main energization step and the post-tempering heat treatment step, the segregation reducing post-heat treatment step including:

at a cooling time t shown by the formula (4)_cpAfter the cooling has been carried out in (ms),

the energization time t shown in the formula (6)_pCurrent value I shown in formula (5) during (ms)_p(kA) general procedureThe power supply device can be powered on,

10≤t_cpthe type (4)

0.6×I_w≤I_p≤0.99×I_wThe type (5)

400≤t_pFormula (6).

3. The resistance spot welding method according to claim 1 or 2, wherein at least one steel sheet in the plate group has a tensile strength of 780MPa or more.

4. A method for manufacturing a resistance spot welding joint using the resistance spot welding method according to any one of claims 1 to 3.

Technical Field

The present invention relates to a resistance spot welding method and a method for manufacturing a resistance spot welded joint.

Background

In recent years, various high-strength steel sheets (high-strength steel materials) have been applied to automobile bodies from the viewpoint of weight reduction of the bodies for fuel efficiency improvement and ensuring collision safety. Resistance spot welding (hereinafter, also referred to as spot welding) is mainly used for joining members made of such high-strength steel sheets, for example, automobile structural members in an assembly line of an automobile. As described above, a welded joint joined by spot welding is required to have a strength (tensile strength) that does not break even when deformed by collision by ensuring collision safety. The strength of the welded joint (hereinafter, also referred to as joint strength) is generally evaluated by the following method. The joint strength of the spot-welded portion of the welded joint was evaluated by tss (tensile shear strength) which is the tensile strength of the joint in the shearing direction and cts (cross tensile strength) which is the tensile strength of the joint in the peeling direction.

The TSS of the spot welded portion tends to increase together with the tensile strength of the base material. However, the CTS at the spot welded portion had a tensile strength of 780N/mm in the base material²The pressure may be reduced when the pressure is (780MPa) or more. With reduced CTS, the fracture mode is from spot weldingPlug fracture (plug fracture) in which the base material or HAZ around the portion is fractured in a ductile manner is converted into interfacial fracture or partial plug fracture in which the base material or HAZ is fractured in a brittle manner within the nugget. The reasons for the reduction of CTS are generally the segregation of P, S at the end of the nugget and the solidification of the quenched nugget end, which causes brittle fracture. In order to solve the brittle fracture, various post-energization methods have been studied in which energization is performed again after main energization.

As a post-energization method in which energization is performed again after main energization, for example, patent documents 1 and 2 disclose techniques in which energization is performed for a short time as post-energization. Specifically, the method of patent document 1 describes performing post-energization that satisfies 0.70 × WC ≦ PC1 ≦ 0.90 × WC, and 40 ≦ Pt1 ≦ 80 (where WC denotes a welding current (kA), PC1 denotes a post-welding heating energization current (kA), and Pt1 denotes a post-welding heating energization time (ms)), and the method of patent document 2 describes performing post-energization that satisfies 0.70 × WC ≦ PHC1 ≦ 0.90 × WC, and 40 ≦ PHT1 ≦ 80 (where WC denotes a welding current (kA), PHC1 denotes a post-heating current (kA), and PHT1 denotes a post-heating time (ms)).

Patent document 3 describes a technique for improving the cross tensile strength by a tempering energization method in which after forming nuggets, after cooling for a long time, a post-energization is performed for a short time at a current value higher than that of an initial energization.

Disclosure of Invention

Problems to be solved by the invention

However, the techniques described in patent documents 1 and 2 are applied to a composition containing Mn: 2.5 to 10.0 mass% of a steel sheet as a steel sheet component (hereinafter, the steel sheet is referred to as a medium Mn steel sheet), the following problems arise.

In order to obtain toughness of the nugget end portion, it is necessary to first form the nugget by main energization and then perform martensite transformation by a cooling process. Then, the martensite structure is tempered by re-electrification to generate tempered martensite. Tempered martensite is a structure exhibiting higher toughness than martensite in a quenched state, and therefore CTS greatly affected by stress concentration at the end of the nugget can be increased. However, in the techniques described in patent documents 1 and 2, complete martensitic transformation cannot be performed and a tempered martensitic structure is formed by short-time cooling and post-energization. Therefore, the toughness-improving effect by tempering cannot be obtained, and stable joint strength cannot be obtained.

The technique described in patent document 3 is a method of performing post-energization at a current value higher than that of initial energization, but the tempering effect cannot be obtained similarly.

As another method for solving the brittle fracture, there is a spot welding method in which only a single current is applied. However, when this method is applied to a high-strength steel sheet having a tensile strength of 780MPa or more, the austenite structure is transformed into a hard and brittle martensite structure through martensite transformation by forming a so-called quenched structure by only energization by a single energization. As a result, there is a problem that the cross tensile strength is lowered.

In view of the above problems, an object of the present invention is to provide a resistance spot welding method and a method for manufacturing a resistance spot welded joint, which can improve joint strength by preventing embrittlement of a nugget end portion of a spot welded portion and reducing segregation of the nugget end portion, even in a high-strength steel sheet having a tensile strength of 780MPa or more, particularly a medium Mn steel sheet.

Means for solving the problems

As described above, the cross tensile strength decreases as the strength of the steel sheet increases. This is because the solidification causes segregation and rapid cooling at the time of solidification, and causes embrittlement of the end of the nugget due to formation of a hardened structure. Therefore, the present inventors have intensively studied a mechanism of lowering the cross tensile strength and a method of improving the cross tensile strength in resistance spot welding of a plate group including a high-strength steel sheet having a tensile strength of 780MPa or more, which are methods for improving the cross tensile strength of such high-strength steel sheets.

The results show that appropriate post-energization conditions exist to improve the cross tensile strength. Specifically, first, the current value I is set for the purpose of heating to a melting point or higher to form nuggets_W(kA) main electrification. It is understood that the effect of tempering the hardened portion of the nugget end portion can be obtained subsequently by performing a cooling process of quenching the molten portion to a temperature at which the molten portion is transformed from austenite to martensite via solidification and a heating process of continuing energization to heat to a temperature slightly lower than a₁Current value I for dot purpose_t(kA) heating process.

It is also known that, between the main energization step for forming nuggets and the post-tempering heat treatment step, after a short-time cooling process for completing solidification, a current value I is given for reheating to a temperature near the melting point_p(kA), thereby, solidification segregation at the end of the nugget can be alleviated.

Thus, by setting these procedures, only the power I is applied_wThe cross tensile strength is improved compared with the case of (kA). From the above results, the cross tensile strength can be improved by performing the energization pattern of the present invention.

The present invention has been completed based on the above findings, and the gist thereof is as follows.

[1] A resistance spot welding method for clamping a plate group formed by stacking at least 2 steel plates by a pair of electrodes and bonding the steel plates by applying a voltage and a current, the method comprising:

is carried out with a current value I_w(kA) a main energization step of energizing,

then, as a post-tempering heat treatment step,

at a cooling time t represented by the formula (1)_ctAfter the cooling has been carried out in (ms),

the energization time t shown in the formula (3)_tCurrent value I shown in formula (2) during (ms)_t(kA) the electric current is applied to the substrate,

800≤t_ctthe formula (1)

0.5×I_w≤I_t≤I_wThe type (2)

500≤t_tThe type (3)

At least one steel sheet in the set of sheets has the following composition: contains, in mass%)

0.08≤C≤0.3、

0.1≤Si≤0.8、

2.5≤Mn≤10.0、

P is less than or equal to 0.1, and the balance is Fe and inevitable impurities.

[2] The resistance spot welding method according to the above [1],

a segregation reducing post-heat treatment step of further performing a segregation reducing post-heat treatment step between the main energization step and the post-tempering heat treatment step, the segregation reducing post-heat treatment step including:

at a cooling time t shown by the formula (4)_cpAfter the cooling has been carried out in (ms),

the energization time t shown in the formula (6)_pCurrent value I shown in formula (5) during (ms)_p(kA) the electric current is applied to the substrate,

10≤t_cpthe type (4)

0.6×I_w≤I_p≤0.99×I_wThe type (5)

400≤t_pFormula (6).

[3] The resistance spot welding method according to the above [1] or [2], wherein at least one steel sheet in the plate group has a tensile strength of 780MPa or more.

[4] A method for manufacturing a resistance spot welding joint using the resistance spot welding method according to any one of the above [1] to [3 ].

Effects of the invention

According to the present invention, when the resistance spot welding method is performed on a plate group in which a plurality of steel plates including at least one high-strength steel plate are stacked, the joint strength of the resistance spot welded joint can be improved by reducing the segregation of the nugget end in the resistance spot welded portion of the high-strength steel plate, and an industrially advantageous effect can be achieved. In particular, when the resistance spot welding method is performed on a plate group including at least one medium Mn steel plate, the above-described effects can be further improved.

Drawings

Fig. 1 is a sectional view schematically showing resistance spot welding according to an embodiment of the present invention.

Fig. 2 is a graph showing an energization pattern of an embodiment of the present invention.

Fig. 3 is a graph showing an energization pattern in another embodiment of the present invention.

Description of the symbols

1 lower steel plate

2 go up the steel sheet

3 board group

4 lower electrode

5 Upper electrode

6 nugget

Detailed Description

Hereinafter, a resistance spot welding method and a method of manufacturing a resistance spot welded joint according to the present invention will be described with reference to the drawings. The present invention is not limited to the embodiment.

The present invention joins a total of 2 or more steel sheets including one or more high-strength steel sheets by resistance spot welding. Fig. 1 shows a case where 2 steel sheets are resistance spot welded, as an example. As shown in fig. 1, in the resistance spot welding method of the present invention, a plate group 3 is sandwiched between electrodes 4 arranged on the lower side and electrodes 5 arranged on the upper side (i.e., a pair of upper and lower electrodes) with respect to the plate group 3, and current is applied while being pressurized, the plate group 3 being formed by stacking steel plates 1 and 2. Then, the nugget 6 of the necessary size is formed, and the welded joint is obtained. In this case, a welded joint can be obtained in the same manner as the above-described welding method.

As a welding apparatus suitable for carrying out such resistance spot welding method, there may be mentioned a pressure control apparatus and a welding current control apparatus which have a pair of upper and lower electrodes, and can press and apply current to a portion to be welded while sandwiching the portion between the pair of electrodes, and further can arbitrarily control a pressure and a welding current during welding.

The pressurizing mechanism (e.g., a cylinder, a servo motor, etc.), the current control mechanism (e.g., ac, dc, etc.), the form (e.g., a stationary type, a robot welding (robotgun), etc.), and the like are not particularly limited. The type of power source (single-phase ac, ac inverter, dc inverter) and the like are also not particularly limited. The shape of the electrode is also not particularly limited. The tip of the electrode is formed in a DR shape (dome radius) shape, R shape (rounded corner shape) shape, or D shape (dome shape) as described in JIS C9304: 1999, for example.

The present invention can be applied to a method of welding a plurality of plate groups including high-strength steel plates. For example, in the case of resistance spot welding shown in fig. 1, at least one of the steel sheets 1 and 2 in the plate group 3 is a high-strength steel sheet.

In the high-strength steel sheet, the solute is segregated at the grain boundary, and the grain boundary is easily embrittled when the impurity such as P, S is excessively enriched. In addition, the nuggets tend to be easily hardened by rapid cooling after spot welding. However, according to the present invention, the toughness of the end portion of the nugget is improved by performing segregation diffusion by reheating slightly lower than the melting point, martensitic transformation, sufficient cooling for causing tempering thereof, and reheating to an appropriate temperature. This has the effect of reducing brittle fracture by segregation of the nugget end of the resistance spot welded portion of the medium Mn high-strength steel sheet and hardening of the nugget end after rapid cooling.

Therefore, in the present invention, in the plate group to be welded, at least one steel sheet is a high-strength steel sheet having a composition containing 0.08. ltoreq. C.ltoreq.0.3 (mass%), 0.1. ltoreq. Si.ltoreq.0.8 (mass%), 2.5. ltoreq. Mn.ltoreq.10.0 (mass%), P.ltoreq.0.1 (mass%), and the balance being Fe and unavoidable impurities. The above effects can be obtained even in a high-strength steel sheet having such a composition. Hereinafter,% of each component means mass%.

C: 0.08% or more and 0.3% or less

C (carbon) is an element that contributes to the reinforcement of steel by being able to generate martensite or the like, and when the C content is less than 0.08%, the strength level becomes very low. Therefore, it is extremely difficult to produce a steel sheet having a tensile strength of 780MPa or more with a C content of less than 0.08%. On the other hand, if the C content exceeds 0.3%, the strength of the steel sheet increases, but the nugget and the heat-affected zone around the nugget become excessively hard and become brittle, and therefore it becomes difficult to improve the cross tensile strength. Therefore, the C content is 0.08% or more and 0.3% or less. The C content is more preferably 0.10% or more, and still more preferably 0.2% or less.

Si: 0.1% or more and 0.8% or less

When the Si (silicon) content is 0.1% or more, it effectively acts to reinforce the steel. On the other hand, when the Si content exceeds 0.8%, although the steel is strengthened, embrittlement sometimes causes a reduction in ductility and adversely affects toughness. Therefore, the Si content is 0.1% or more and 0.8% or less. The Si content is more preferably 0.1% or more, and still more preferably 0.5% or less.

Mn: 2.5% or more and 10.0% or less

As described above, the present invention is also applicable to medium Mn steel sheets, and the present invention is preferably directed to high strength steel sheets having a Mn (manganese) content of 2.5% or more. The reason for this is that when the Mn content is less than 2.5%, high joint strength can be obtained even without performing cooling for a long time as in the present invention. On the other hand, when the Mn content exceeds 10.0%, embrittlement of the weld portion or cracking accompanied by embrittlement occurs remarkably, so it is difficult to improve the joint strength. Therefore, the Mn content is 2.5% or more and 10.0% or less. The Mn content is more preferably 3.5% or more, and still more preferably 8.0% or less.

P: less than 0.1%

P (phosphorus) is an inevitable impurity, and when the P content exceeds 0.1%, strong segregation occurs at the nugget end of the weld portion, and therefore, it is difficult to improve the joint strength. Therefore, the P content is set to 0.1% or less. More preferably, the P content is 0.02% or less.

In the present invention, 1 or 2 or more elements selected from Cu, Ni, Mo, Cr, Nb, V, Ti, B, Al, and Ca may be contained as optional components in addition to the components of the high-strength steel sheet.

Cu (copper), Ni (nickel), and Mo (molybdenum) are elements that can contribute to the improvement in strength of steel. Cu is effective for reinforcing steel, but when added excessively, it causes cracking. Therefore, when Cu is contained, Cu is preferably 3% or less, more preferably 1% or less. In addition, from the viewpoint of improving the strength of the steel, when Cu is contained, Cu is preferably 0.005% or more.

Ni improves hardenability, but is expensive. Therefore, when Ni is contained, Ni is preferably 3% or less, more preferably 1% or less, from the viewpoint of production cost. In addition, from the viewpoint of improving hardenability, when Ni is contained, Ni is preferably 0.005% or more.

Mo increases hardenability, but is expensive, and when the Mo content is 1.0% or more, the effect is saturated. Therefore, when Mo is contained, Mo is preferably 1.0% or less, and more preferably 0.8% or less. In addition, from the viewpoint of improving the balance between hardenability and strength ductility, when Mo is contained, Mo is preferably 0.005% or more.

Cr (chromium) is an element that can improve hardenability, but if it is contained in excess, there is a risk that toughness of the HAZ deteriorates. Therefore, when Cr is contained, Cr is preferably 1.0% or less, and more preferably 0.8% or less. In addition, from the viewpoint of improving hardenability, when Cr is contained, Cr is preferably 0.01% or more.

Nb (niobium) and V (vanadium) are elements capable of strengthening steel by controlling the structure through precipitation strengthening. However, if Nb is excessively contained, hard martensite increases, and if V is excessively contained, toughness may deteriorate. Therefore, when Nb is contained, Nb is preferably 0.2% or less, more preferably 0.1% or less. In addition, in order to obtain the HAZ strength, Nb is preferably 0.005% or more when Nb is contained. When V is contained, V is preferably 0.5% or less, more preferably 0.2% or less. When V is contained, V is preferably 0.003% or more in view of preventing the HAZ from softening.

Ti (titanium) and B (boron) are elements that improve hardenability and strengthen steel. However, if Ti is excessively contained, hard martensite may increase. Therefore, when Ti is contained, Ti is preferably 0.2% or less, and more preferably 0.1% or less. In addition, from the viewpoint of improving the precipitation strengthening, when Ti is contained, Ti is preferably 0.003% or more.

When B is contained excessively, there is a risk that the above effect is saturated. Therefore, when B is contained, B is preferably 0.005% or less, more preferably 0.004% or less. In addition, from the viewpoint of preventing the decrease in HAZ strength, when B is contained, B is preferably 0.0001% or more.

Al (aluminum) is an element capable of controlling the structure by grain refining of austenite, but if it is contained excessively, it causes deterioration of toughness. Therefore, when Al is contained, Al is preferably 2% or less, and more preferably 0.1% or less. In addition, from the viewpoint of purifying the steel by deoxidation, when Al is contained, it is preferable that Al be 0.01% or more.

Ca (calcium) is an element that can contribute to improvement of workability of steel, but if it is contained excessively, there is a risk of deterioration of toughness. Therefore, when Ca is contained, Ca is preferably 0.010% or less, and more preferably 0.005% or less. In addition, from the viewpoint of improving the influence of sulfides, when Ca is contained, Ca is preferably 0.0005% or more.

As described above, in the present invention, in order to obtain these effects, 1 or 2 or more elements selected from Cu, Ni, Mo, Cr, Nb, V, Ti, B, Al, and Ca may be added as necessary in addition to the above components.

The balance of Fe and inevitable impurities

The balance, excluding the above components, being Fe and inevitable impurities.

The tensile strength of the high-strength steel sheet having the above-described components is preferably 780MPa or more. As described above, particularly when the tensile strength of the base material is 780MPa or more, there is a concern that CTS may be reduced. According to the present invention, even in a high-strength steel sheet having a tensile strength of 780MPa or more, it is possible to prevent the brittle fracture due to the P, S segregation at the nugget end and the hardening of the nugget end after rapid cooling, and therefore, the decrease in CTS can be suppressed. It is needless to say that the above-described effects can be obtained also in a high-strength steel sheet having a tensile strength of less than 780 MPa.

It should be noted that the above-described effects can be obtained even if at least one steel sheet in the plate group to be welded is a galvanized steel sheet. In the present invention, the galvanized steel sheet is a steel sheet having a plating layer containing zinc as a main component, and the plating layer containing zinc as a main component includes all conventionally known zinc plating layers. Specifically, typical examples of the plating layer containing zinc as a main component include a molten zinc plating layer and an electrogalvanized layer, and include an Al plating layer, a Zn — Ni plating layer, and the like.

In addition, the plurality of superposed steel plates may be superposed a plurality of steel plates of the same kind, or superposed a plurality of steel plates of different kinds. There is no problem even if the thicknesses of the steel sheets are different, and a surface-treated steel sheet having a plating layer and a steel sheet having no plating layer may be laminated. Since the stress concentration at the end of the nugget is increased when the thickness of the plate is increased, it is preferably 0.4mm to 2.2 mm.

The present invention is a method of overlaying steel sheets and resistance spot welding, and controls the step of energizing the plate group 3 (steel sheets 1 and 2) using the electrodes 4 and 5 shown in fig. 1 as follows.

First, a current value I is applied_w(kA) a main energization step of energization. Then, as the post-tempering heat treatment step, the cooling time t represented by the following formula (1) is set_ctAfter cooling (ms), the current is applied for a period of time t represented by the following formula (3)_tCurrent value I represented by the following formula (2) during the period (ms)_t(kA) is electrified.

800≤t_ctThe formula (1)

0.5×I_w≤I_t≤I_wThe type (2)

500≤t_tThe type (3)

[ Main electrifying process ]

The main energization step is an energization step of melting the overlapped portion of the steel sheets 1 and 2 to form the nuggets 6. In the present invention, the energization conditions and the pressurization conditions for forming the nuggets 6 in the main energization step are not particularly limited. The welding conditions currently used may be adopted.

When the high-strength steel sheet having the steel sheet component of the present invention is used, the energization condition of the main energization is preferably 120ms to 400 ms. The pressurizing condition is preferably 2.0kN to 4.0 kN.

[ Heat treatment Process after tempering ]

The post-tempering heat treatment step is a post-heat treatment step for tempering the end portions of the nuggets 6 formed in the main energization step to improve the toughness. In order to temper the end portions of the nuggets to obtain the effect of improving the toughness, it is important to control the welding conditions in the post-tempering heat treatment step as described below.

First, the cooling time t represented by the above formula (1)_ct(ms) and then cooled, and then, the current is applied for the time t shown in the above formula (3)_tThe current value I shown in the above formula (2) for the period of (ms)_t(kA) tempering energization was performed.

At cooling time t_ctIf (ms) is less than 800(ms), the nugget 6 cannot be cooled to a temperature at which the martensitic transformation occurs. As a result, the retained austenite that cannot undergo martensitic transformation is turned into a martensite structure or a retained austenite structure by energization and recooling. They are not tempered martensite and therefore, are not in a tough structure but in a hard structure. Thus, the cooling time t_ct(ms) is 800(ms) or more. For further tempering, the cooling time t_ctThe (ms) is preferably 1000(ms) or more, and more preferably 1200(ms) or more.

The cooling time t is_ctThe upper limit of (ms) is not particularly limited, and the cooling time t_ctThe (ms) is preferably 20000(ms) or less. At cooling time t_ctWhen (ms) exceeds 20000(ms), the above-mentioned effects are not observed to improve, and productivity is not impaired, which is not preferable. Cooling time t_ctPreferably 10000(ms) or less. Cooling time t_ctWhen the tempering temperature is 10000(ms) or less, a sufficient tempering effect can be obtained,therefore, most of the austenite structure at the end of the nugget can be transformed into the tempered martensite structure. In the case where productivity is more important, the cooling time t is more preferably set as the post-energization time for tempering the end portion of the nugget_ctIs set to 2000(ms) or less.

Current value I of current supply in tempering_t(kA) less than 0.5 × I_wIn the case of (kA), a temperature slightly lower than a sufficient for tempering martensite cannot be set₁The temperature of the spot. As a result, the tempered martensite structure is not formed, but the martensite structure in a hard and brittle state is formed, and the toughness of the nugget end portion cannot be improved. On the other hand, at the current value I_t(kA) value of current I exceeding main current_w(kA) is reached to exceed A₁The point temperature is such that the effect of tempering the end of the nugget cannot be obtained. Therefore, the current value I_tThe current range of (kA) was set to 0.5 × I_w(kA) or more and I_w(kA) or less, preferably 0.6 × I_w(kA) or more, and 0.99 × I_w(kA) or less.

Energization time t for energization in tempering_tWhen (ms) is less than 500(ms), the heating time is not sufficient to obtain the tempering effect targeted in the present invention. Therefore, the energization time t_t(ms) is 500(ms) or more. Time t of energization_tPreferably 1000(ms) or more. In order to further improve the joint strength by tempering the nugget end portion for a longer time, it is more preferably set to 1500(ms) or more. In order to further improve the tempering effect, it is more preferably 1800(ms) or more.

Note that the energization time t of the backfire energization_tThe upper limit of (ms) is not particularly limited, and the energization time t_tThe (ms) is preferably 20000(ms) or less. At the time of energization t_tWhen (ms) exceeds 20000(ms), productivity is impaired, and therefore, this is not preferable. Time t of energization_tMore preferably 8000(ms) or less, and still more preferably 3000(ms) or less.

Here, fig. 2 shows an example of the energization pattern of the resistance spot welding method of the present invention described above. The main energization step and the post-tempering heat treatment step are controlled to the energization pattern shown in fig. 2.

Specifically, the current value in the main energization step as the main energization is set to I_w(kA) setting the conduction time of the main conduction as t_w(ms). In addition, the cooling time in the post-tempering heat treatment step as post-energization is set to t_ct(ms) the current value is I_t(kA), the current value I is adjusted_t(kA) is set to a current value I lower than the main current_w(kA) and setting the energization time to t_t(ms). Then, as shown in fig. 1, the plate group 3 (steel plates 1 and 2) is sandwiched between the pair of electrodes 4 and 5, and energization is performed in the energization pattern shown in fig. 2, thereby forming nuggets 6 at the boundaries between the steel plates 1 and 2.

According to the present invention, the nugget 6 formed by main energization is made slightly lower than A in the post-tempering heat treatment step₁Since tempering energization is performed at a point temperature and for a long time of 500(ms) or more, even when a high-strength steel sheet having the above steel sheet component is included in the plate group 3, the end portions of the nuggets 6 can be tempered to improve toughness.

In the present invention, from the viewpoint of more effectively improving the joint strength, a segregation reducing post-heat treatment step may be further performed between the main energization step and the tempering post-heat treatment step, the segregation reducing post-heat treatment step being: at a cooling time t represented by the following formula (4)_cpAfter cooling (ms), the current is applied for a period of time t represented by the following formula (6)_pCurrent value I represented by the following formula (5) during the period (ms)_p(kA) is electrified.

10≤t_cpThe type (4)

0.6×I_w≤I_p≤0.99×I_wThe type (5)

400≤t_pFormula (6).

[ Segregation reducing post-heat treatment Process ] (preferred conditions)

The segregation reducing post-heat treatment step is a post-heat treatment step for reducing segregation of the nugget end portion of the nugget 6 formed in the main energization step. In order to obtain the effect of reducing segregation at the end of the nugget, it is preferable to control the welding conditions in the segregation reducing post-heat treatment step performed between the main energization step and the post-tempering heat treatment step as follows.

Preferably, the cooling time t represented by the above formula (4) is first set_cp(ms) and then cooled, and then, at the energization time t shown in the above-mentioned formula (6)_pThe current value I shown in the above formula (5) for the period of (ms)_p(kA) the current was again applied.

Cooling time t in the post-segregation-relaxation heat treatment step_cpWhen (ms) is less than 10(ms), there may be a case where the time is insufficient for completing solidification of the molten nugget 6. As a result, the nugget is kept in a molten state, and the effect of the segregation reducing post-heat treatment step, which is the segregation reduction of impurities due to diffusion of solidified solute atoms, cannot be achieved. Thus, the cooling time t_cpThe (ms) is preferably 10(ms) or more, more preferably 100(ms) or more, and further preferably 200(ms) or more.

The cooling time t is_cpThe upper limit of (ms) is not particularly limited, but the cooling time t is preferably set_cp(ms) is 750(ms) or less. When the cooling time t_cpWhen (ms) exceeds 750(ms), it takes time to excessively cool the steel sheet and reheat the steel sheet to a temperature slightly lower than the melting point in a subsequent heating process (segregation mitigation energization in the post-segregation mitigation heat treatment step), which is not preferable. More preferably 700(ms) or less, and still more preferably 250(ms) or less.

Current value I of segregation reducing electrification in post-segregation reducing heat treatment step_p(kA) less than 0.6 × I_w(kA) has a problem that reheating to around the melting point is impossible. On the other hand, at the current value I_p(kA) exceeding 0.99 × I_wIn the case of (kA), the nugget 6 is melted again, and therefore, there is a possibility that the effect of alleviating solidification segregation cannot be obtained by diffusion in a solid phase state slightly lower than the melting point. Therefore, the current value I_pThe current range of (kA) is preferably set to 0.6 × I_w(kA) or more and 0.99 × I_w(kA) or less, more preferably 0.8 × I_w(kA) or more, and further preferably 0.87 × I so as to approach a temperature slightly lower than the melting point_w(kA) or more, more preferably 0.90 × I_w(kA) or more, and (kA) or less,more preferably 0.98 × I_w(kA) or less.

Energization time t of segregation reducing energization in the post-segregation-reducing heat treatment step_pWhen (ms) is less than 400(ms), there is a possibility that the effect of alleviating solidification segregation by diffusion cannot be sufficiently obtained. Therefore, the energization time t_pThe (ms) is preferably 400(ms) or more, and more preferably 600(ms) or more.

In the segregation reducing post-heat treatment step, the energization time t of the segregation reducing energization_pThe upper limit of (ms) is not particularly limited, and the energization time t_pThe (ms) is preferably 8000(ms) or less. When the power is on for a time t_pIf (ms) exceeds 8000(ms), it is difficult to improve the above-mentioned effects, and productivity is impaired, so that this is not preferable. Time t of energization_pPreferably 7000(ms) or less, and more preferably 2000(ms) or less.

Here, fig. 3 shows an example of the current application pattern in the resistance spot welding method of the present invention further including the segregation reducing post-heat treatment step between the main current application step and the post-tempering post-heat treatment step. The main energization step, the segregation reducing post-heat treatment step, and the post-tempering heat treatment step are controlled to the energization pattern shown in fig. 3.

Specifically, the current value in the main energization step as the main energization is set to I_w(kA) setting the conduction time of the main conduction as t_w(ms). In addition, the cooling time in the post-segregation-relaxation heat treatment step is set to t_cp(ms) the current value for segregation relaxation energization is I_p(kA), the current value I is adjusted_p(kA) is set to a current value I of main electrification_w(kA) × 0.99.99 or less, and setting the current-carrying time of segregation-reducing current-carrying as t_p(ms). In addition, the cooling time in the post-tempering heat treatment step as post-energization is set to t_ct(ms) current value of post-energization is set to I_t(kA), the current value I is adjusted_t(kA) is set to a current value I of main electrification_w(kA) or less, and the conduction time of the post-conduction is set to t_t(ms). Then, as shown in FIG. 1, the plate group 3 (steel plates 1, 2) is sandwiched between a pair of electrodes 4, 5, and energization is performed in the energization pattern shown in FIG. 3, whereby the steel plates 1, 2,The boundary of 2 forms a nugget 6.

The current value I is_p(kA) and a current value I_tThe relationship (kA) is preferably I_p≥I_t. Segregation relaxation energization I_pSegregation is moderated at a temperature slightly lower than the melting point of the end of the nugget. On the other hand, the current value I of the energization for the heat treatment after the tempering_tBy reaction with A₁Tempering is performed while maintaining the end of the nugget at a temperature below the point. A. the₁The temperature of the spot is considerably lower than the melting point, even at a cooling time t_ctCooling is performed, and the relation of the current value is also I_p≥I_t。

According to the present invention, since the nugget 6 formed by the main energization is cooled for a short period of time and then reheated to the vicinity of the melting point in the segregation reducing post-heat treatment step, the solidification segregation can be reduced by diffusion in a solid phase state slightly lower than the melting point, and the segregation at the end portion of the nugget 6 can be reduced. Thereby, the CTS of the resulting welded joint can be improved.

Next, a method of manufacturing the resistance spot welded joint will be described.

The present invention is a method for manufacturing a resistance spot welded joint using the above resistance spot welding method. In the method of manufacturing a resistance spot welded joint according to the present invention, for example, resistance spot welding is performed to form nuggets of a necessary size to obtain a resistance spot welded joint, the resistance spot welding being: a plate group formed by stacking at least 2 steel plates is sandwiched between a pair of electrodes, and current is applied under a predetermined welding condition while applying pressure. The steel sheet, welding conditions, and the like are the same as those described above.

As described above, the resistance spot welding method and the method of manufacturing a resistance spot welded joint according to the present invention temper the nugget end portion and improve toughness by appropriately controlling the welding conditions in the post-tempering step, i.e., the post-tempering heat treatment step. This can improve the joint strength of the obtained welded joint. Further, by performing a reheating step (segregation reducing post-heat treatment step) between the main energization step and the post-tempering heat treatment step, solidification segregation at the end of the nugget is reduced, and CTS is increased. Therefore, even when the medium Mn steel sheet having the above steel sheet component is included in the plate group as the high-strength steel sheet, the joint strength can be further improved.

The composition of the nuggets obtained by the present invention is preferably in the range of 0.05. ltoreq. C.ltoreq.0.35 (mass%), 0.1. ltoreq. Si.ltoreq.0.8 (mass%), and 2.0. ltoreq. Mn.ltoreq.10 (mass%). The calculation method of the components in the nugget can be determined by cutting out the nugget from the sample produced by the above method and performing chemical analysis. Alternatively, the ratio may be calculated from photographs of the cross-section of the welded portion and converted from the ratio of the cross-sectional area of the molten portion of each of the upper and lower steel sheets and the content of each steel sheet component.