JP5168204B2 - Spot welding method for steel sheet - Google Patents

Description

  The present invention relates to a spot welding method for improving the fatigue strength of spot welded joints of steel plates including high-strength steel plates, and more particularly to spot welding methods for spot welding methods used in the mounting of automobile parts and the assembly of vehicle bodies. The present invention relates to a spot welding method for improving the fatigue strength of a joint.

In recent years, in order to reduce fuel consumption of automobiles, reduce CO ₂ emissions and improve collision safety, in the automobile field, thin high-strength steel sheets have been used instead of conventional thin-walled mild steel for automobile bodies and parts. There is a growing need to do so.

  Spot welding methods are mainly used for assembling automobile bodies and mounting parts. However, when high strength steel plates are welded by spot welding methods, there are the following problems.

  That is, fatigue strength is important as well as tensile strength as a quality index of spot welds (welded joints). Although the tensile strength of the welded joint increases with the tensile strength of the steel sheet, the fatigue strength of the welded joint hardly increases even if the tensile strength of the steel sheet increases.

For example, if a high strength steel plate with a tensile strength of 590 MPa is used instead of a mild steel plate with a tensile strength of 270 MPa, the tensile shear strength of the spot welded joint (the tensile strength when a tensile load is applied in the shear direction of the weld joint) Strength) is almost doubled. However, even in this case, if the fatigue strength when a repeated load is applied in the shear direction of the welded joint, for example, the highest load that does not cause fatigue fracture even when the number of stress loads is 2 × 10 ⁶ times, is defined as the fatigue strength, this fatigue The strength does not increase and shows almost the same value as that of the mild steel plate.

  As described above, the cause of the low fatigue strength may be the notch shape of the spot weld, as reported conventionally. That is, as shown in FIG. 1, since the contact portion on the overlapping surface around the nugget 2 existing between the steel plates 1 has a notch shape, a load is applied in the tensile shear direction (arrow direction) 3. When a fatigue test is performed, it is considered that the fatigue strength is not improved by this notch effect even if the tensile strength of the steel sheet is high.

  In particular, when a high-strength steel plate is used, the notch effect becomes significant because the hardness of the nugget portion increases as compared with the case where a mild steel plate is used. On the other hand, even when a fatigue test is performed with a load applied in the peeling direction {direction perpendicular to the tensile shear direction (arrow direction) 3}, the fatigue strength of the welded joint of the high-strength steel plate does not increase and is equivalent to that of mild steel. is there.

  In this case, stress concentration in the periphery of the nugget is conspicuous, and the stress load at the local area increases, so that cracks are likely to occur there. Therefore, the fatigue strength is less than that when a repeated load is applied in the tensile shear direction. Decrease by an order of magnitude.

In general, it is known that the value of the carbon equivalent Ceq represented by the following formula tends to increase as the tensile strength of the steel plate increases, and the value of Ceq of the high-strength steel plate exceeds 0.2.
Ceq = C + Si / 30 + Mn / 20 + 2P + 4S
In the formula, C, Si, Mn, P, and S indicate the respective contents (mass%) of carbon, silicon, manganese, phosphorus, and sulfur in the steel.

  As described above, the tensile strength of the high-strength steel plate increases and the carbon equivalent Ceq of the steel plate increases. Therefore, the higher strength steel plate having higher tensile strength, the hardness of the spot welded portion (nugget portion) and the heat-affected zone. As a result, the toughness is lowered and the fracture is easily caused.

  In addition, the high strength steel plate has a large amount of springback (bounce due to elastic recovery after processing) compared to mild steel, so that the residual strength of the tensile stress is generated in the spot welded part, and the fatigue strength is likely to decrease. In addition, cracking occurs due to a relative decrease in ductility, and fatigue strength and static strength tend to decrease.

  For the above reasons, it is considered that the fatigue strength of the spot welded portion of the high-strength steel plate does not increase even when the tensile strength of the high-strength steel plate increases, and is comparable to that of mild steel.

  In conventional spot welding of high-strength steel sheets, as a means of improving the fatigue strength of welded joints, temper energization is performed after a certain period of time has elapsed after spot welding energization, and the effect of heat on the spot welded part (nugget part) A method is known in which the hardness is reduced by annealing the part to change the residual stress (for example, see Non-Patent Document 1).

  However, this method has a practical problem that the appropriate condition range of the temper energization is very narrow, and the reproducibility is poor due to a change in operating conditions. In particular, when spot welding is performed by continuously spotting a plated steel sheet, the electrode tip deteriorates due to an alloying reaction with the plating as the number of hit points increases, the electrode tip diameter increases, and the current density decreases, Since it deviates from the optimum temper energization condition, it is difficult to stably improve the fatigue strength of the joint.

  In addition to this, as a method for improving the fatigue strength of the spot welded portion, a method of spot welding using a steel plate having excellent fatigue strength characteristics is known (for example, see Patent Documents 1 to 6).

  However, these methods relate to spot welding of mild steel plates and are not methods for improving the fatigue strength of spot welded portions of high strength steel plates.

  Moreover, as a method for improving the fatigue strength of the spot welded portion of the high-strength steel sheet, there is a method of introducing compressive residual stress into the welded portion by setting the pressure and holding time within a certain range (for example, patents) Considering the case where the holding time becomes long and the spot welded portion of the mild steel plate and the high-strength steel plate coexist, the application in the actual welding line is not a realistic method.

Japanese Unexamined Patent Publication No. Sho 63-317625 Japanese Patent Laid-Open No. 02-163323 JP 05-263184 A JP 09-268346 A JP-A-10-008187 Japanese Patent Laid-Open No. 11-279589 JP 2004-122153 A

"Iron and Steel", Vol. 68 (1982) No. 9, pp. 1444-1451.

As described above, the fatigue strength of the welded joint when spot-welding a high-strength steel plate is the same as the fatigue strength when spot-welding a mild steel plate, so even if a high-strength steel plate is used in the automotive field, the high-strength steel plate It is not possible to fully enjoy the benefits of improving safety, reducing fuel consumption by reducing weight, and reducing CO ₂ emissions.

  In order to improve the fatigue strength of welded joints, the conventional method of increasing the number of spot welding points can be adopted, but this method has problems such as reduced work efficiency, increased costs, and restrictions on design flexibility. .

  Therefore, the present invention can improve the fatigue strength of a welded joint while ensuring good welding workability in spot welding of not only mild steel but also high-strength steel sheets in order to solve these problems in the prior art. It aims at providing the spot welding method of a steel plate.

  The present inventor believes that since the fatigue strength of the spot welded joint depends on the residual stress state around the nugget, if the residual stress state around the nugget is improved by some means, the fatigue strength of the welded joint can be increased. Based on this idea, we intensively studied a method to improve the residual stress state around the nugget.

  As a result, by increasing the welding pressure during the holding time after the end of the energizing time at the time of spot welding, it is possible to introduce compressive residual stress around the weld and effectively increase the fatigue strength of the welded joint. I found it.

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

(1) In a welding method for improving the fatigue strength of a steel plate spot welded joint,
Of the holding time Ht (ms) after spot welding, after the initial pressurizing pressure holding time Hti (ms) has elapsed with the initial pressurizing pressure Pi (kN), the pressurizing force is increased from the initial pressurizing pressure Pi to the late pressurizing pressure Pa, As a welding method in which the late pressurization holding time Hta (ms) elapses at the late pressurization pressure Pa (kN),
The latter applied pressure Pa (kN) satisfies the following (formula 1):
The initial pressure holding time Hti (ms) satisfies the following (Equation 2):
The latter pressurization holding time Hta (ms) satisfies the following (formula 3),
The initial pressure Pi (kN) is the following (Formula 4) when using a steel sheet having a tensile strength TS <430 MPa as a workpiece, and the following (Formula 5) when using a steel sheet having a tensile strength TS ≧ 430 MPa. The filling,
The above-mentioned holding time Ht (ms) is set so as to satisfy the following (Equation 6), and spot welding is performed.
1.5 × Pi ≦ Pa ≦ 2.5 × Pi (kN) (Formula 1)
0.35 × Ht ≦ Hti ≦ 0.65 × Ht (ms) (Formula 2)
Hta = Ht−Hti (ms) (Formula 3)
Pi = 2.45 × t (kN) (Formula 4)
Pi = 2.45 × t × (TS / 270) ^0.5 (kN) (Formula 5)
Ht = 200 × t−80 (ms) (Formula 6)
Where t is the steel plate thickness (mm), and TS is the tensile strength (MPa) of the steel plate.

  (2) The steel sheet according to (1), wherein the steel sheet is a microstructure-induced work-induced transformation type composite steel sheet containing a retained austenite of 5% to 25% in volume fraction. Spot welding method.

ADVANTAGE OF THE INVENTION According to this invention, the fatigue strength of a welded joint can be improved, ensuring favorable welding workability | operativity in spot welding of the high-strength steel plate etc. which are used for the attachment of the components for motor vehicles, the assembly of a vehicle body, etc. Therefore, in this field, the social contribution of the present invention is enormous, such as the safety improvement by applying high-strength steel sheets in the automobile field, the low fuel cost by weight reduction, and the benefits of CO ₂ emission reduction. is there.

It is a figure which shows typically the load condition of the fatigue test of a spot weld part with sectional drawing perpendicular | vertical to the plate | board surface containing the nugget part of a fatigue test piece. It is a figure which shows typically the present invention which piles up two steel plates between upper and lower electrodes, and is spot-welded in the sectional view which expanded between the upper and lower electrodes partially. It is a figure which shows typically the time change of the applied pressure and welding current in the spot welding of this invention. Late pressure and initial pressure ratio Pa / Pi and late pressure holding time and holding time when spot-welding (nugget diameter ND: 5.5 mm) of a high-strength steel plate (TS = 590 MPa class, plate thickness 1.2 mm) It is a figure which shows the relationship between ratio Hta / Ht and the evaluation result ((circle), x) of the fatigue strength of a welded joint. Initial pressure and late pressure ratio Pa / Pi and late pressure holding time and holding time ratio Hta when spot-welding (nugget diameter ND: 7 mm) of a high-strength steel plate (TS = 590 MPa class, plate thickness 2.0 mm) It is a figure which shows the relationship between / Ht and the evaluation result ((circle), x) of the fatigue strength of a welded joint.

The present inventor firstly, in spot welding of high-strength steel plates, as a method of improving the fatigue strength of welded joints,
(A) A method of making stress concentration difficult to occur by changing the notch shape at the end of the weld metal (nugget),
(B) a method of reducing the hardness of the weld metal (nugget) portion and the surrounding heat-affected zone (HAZ);
and,
(C) a method of generating a compressive residual stress around the weld metal (nugget) portion to relatively reduce the residual tensile stress,
Three methods were studied.

  As for the method of (a), for example, as described in Non-Patent Document 1, it is intentionally scattered during welding (during energization, the diameter of the molten part generated between the steel plates is larger than the tip diameter of the copper electrode. There is a known method of changing the shape of the end of the weld metal (nugget) by generating a phenomenon in which the molten metal scatters from the gaps between the steel plates, but this method uses a weld metal (nugget). It is known that the end shapes of the parts vary and, in fact, the fatigue strength varies considerably.

  As a method of (b), as described above, after the welding is completed, the energized portion is held (cooled) while being deenergized for a certain period of time, and then the welded portion is energized (post-energized) for a certain period of time, and the welded portion is tempered It has been known. However, as described above, this method has a problem that the optimum energization condition range for the temper treatment of the welded portion is very narrow, and the reproducibility is poor due to a change in operating conditions.

  As a method of (c), the present inventor believes that the method of improving the residual stress state by utilizing the pressure applied to the welded portion by the electrode and the volume expansion due to the martensitic transformation around the weld metal (nugget) portion is effective. I thought and conducted diligent experiments. As a result, it has been found that by increasing the welding pressure during the holding time after the end of the energization time during spot welding, compressive residual stress can be generated around the nugget and the fatigue strength of the welded joint can be improved. It was.

  The present invention has been made based on the above findings as described above. Details will be described below.

  FIG. 2 is a view for explaining spot welding according to the present invention. First, in spot welding, two steel plates 1 that are materials to be joined are overlapped, and a welding electrode 4 made of copper is energized while applying pressure (load direction) 5 to the overlapped portion. A molten metal part is formed between the two. The molten metal portion is cooled and solidified by heat removal from the water-cooled electrode or heat conduction to the steel plate after the welding energization is completed, and a weld metal (nugget) 2 is formed between the two steel plates 1. .

  When spot-welding a steel plate, volume expansion occurs when martensitic transformation occurs in the solidification and cooling process in the molten metal (nugget) portion and the surrounding heat-affected zone (HAZ) after welding. Thermal contraction occurs in the cooling process to room temperature, and tensile stress remains in the vicinity of the finally formed weld metal (nugget) portion. This tensile residual stress is considered to be the main cause of reducing the fatigue strength of the spot welded joint of the steel sheet together with the notch shape at the end of the nugget.

  In the present invention, during spot welding of a steel sheet, the late pressurization pressure Pa (kN) satisfying the following (formula 1), the initial pressurization holding time Hti (ms) satisfying the following (formula 2), 3) Late pressurization holding time Hta (ms) satisfying 3), when using a steel sheet with tensile strength TS <430 MPa as the welded material (Formula 4), and when using a steel sheet with tensile strength TS ≧ 430 MPa Is set to an initial pressure Pi (kN) that satisfies the following (formula 5) and a holding time Ht (ms) that satisfies the following (formula 6).

1.5 × Pi ≦ Pa ≦ 2.5 × Pi (kN) (Formula 1)
0.35 × Ht ≦ Hti ≦ 0.65 × Ht (ms) (Formula 2)
Hta = Ht−Hti (ms) (Formula 3)
Pi = 2.45 × t (kN) (Formula 4)
Pi = 2.45 × t × (TS / 270) ^0.5 (kN) (Formula 5)
Ht = 200 × t−80 (ms) (Formula 6)
Where t is the steel plate thickness (mm), and TS is the tensile strength (MPa) of the steel plate.

  The initial pressurizing force and the initial pressurizing pressure holding time, the late pressurizing force, the late pressurizing pressure holding time and the holding time are as shown in FIG.

  When a steel sheet with a tensile strength TS <430 MPa is used as the material to be welded, (Equation 4) is the most representative addition when (Equation 5) is used when a steel sheet with a tensile strength TS ≧ 430 MPa is used. This is known as a pressure calculation formula. This is because, when the tensile strength of the steel plate is increased, it is necessary to increase the electrode pressing force in order to ensure the energization. Regarding the holding time Ht, the one defined by (Expression 6) is known as the most representative one. The holding time has the purpose of preventing internal defects in the welded portion by performing water-cooling and pressurization with the electrodes until the welded portion is at a certain temperature or lower. For this reason, the holding time is not related to the tensile strength of the material to be welded, and is set by an equation that depends only on the plate thickness t. Further, the pressure setting and measurement can be performed by incorporating a torque control mechanism and a load cell inside the welding machine.

  Late pressure, initial pressure holding time, late pressure holding time, and initial pressure are set to satisfy the above (Formula 1), (Formula 2), (Formula 3), (Formula 4), and (Formula 5). As a result, the nugget portion can be deformed, and as a result, residual stress can be introduced around the nugget portion, and the fatigue strength of the welded joint is improved.

  The late pressurization pressure Pa is set according to the above (formula 1), and the late pressurization pressure holding time Hta is set according to the (formula 2), but the experimental results used as the basis for deriving the (formula 1) and (formula 2) An example is shown in FIG.

  Fig. 4 shows a nugget with a diameter of 5.5 mm by spot welding a tensile strength TS = 590 MPa grade steel plate with a thickness of 1.2 mm using a JIS-DR6φ type electrode with a tip diameter of 6 mm and a tip curvature diameter of 40 mm. Relationship between late pressurization and initial pressurization ratio Pa / Pi, late pressurization holding time and holding time ratio Hta / Ht, and fatigue strength evaluation results of welded joints (marked with ○ and × in the figure) Is shown.

  In FIG. 5, a JIS-DR8φ type electrode having a tip diameter of 6 mm and a tip curvature diameter of 40 mm was used, and a 2.0 mm thick tensile strength TS = 590 MPa grade steel plate was spot welded to form a 7 mm diameter nugget. Shows the relationship between late pressure and initial pressure ratio Pa / Pi, late pressure holding time and holding time ratio Hta / Ht, and fatigue strength evaluation results (circles and x in the figure) ing. In FIG. 4 and FIG. 5, the case where the fatigue strength of the welded joint is improved with respect to the fatigue strength when the steel plate having the tensile strength TS = 590 MPa is welded at the initial pressure Pi is constant, and the case where the fatigue strength is not improved is indicated by ×. Indicated.

  4 and 5, when spot welding the steel sheet, the pressurizing force is increased during the holding time, and the late pressurizing pressure Pa (kN) and the late pressurizing pressurizing holding time Hta (ms) are expressed by the above (formula 1), If it sets according to (Formula 2), it turns out that it is possible to form a welded joint with favorable fatigue strength. 4 shows an example of an experimental result when the plate thickness is 1.2 mm, and FIG. 5 shows a case where the plate thickness is 2.0 mm. It has been confirmed that a sufficient effect can be similarly obtained even in a plate thickness of 1.0 to 2.3 mm.

  In the above (Equation 1) of the present invention, the tensile strength and thickness of the steel sheet are changed, and the relationship between Pa / Pi, Hta / Ht and the fatigue strength of the welded joint is tested as shown in FIGS. It was obtained after confirmation.

  When the late pressurizing force at the time of spot welding is lower than the lower limit of the above (formula 1), sufficient compressive residual stress cannot be introduced around the nugget (welded metal part) due to the expansion of the nugget diameter, The effect of improving the fatigue strength of the welded joint is hardly recognized.

  On the other hand, when the late pressurizing force at the time of spot welding is higher than the upper limit of the above (formula 1), a large indentation is generated on the surface of the pressurizing portion where the deformation resistance is reduced at the time of welding, and the appearance shape of the metal plate is deteriorated. In addition, the plate thickness of the pressurizing portion is reduced, and there arises a problem that the static strength and fatigue strength of the welded joint are lowered.

  Furthermore, when the late pressurization holding time is shorter than the lower limit of the above (Formula 2), since the welded portion is increased to a temperature close to room temperature, an increased pressure is applied, so that it is not possible to introduce compressive residual stress. .

  On the other hand, when the latter pressurizing pressure holding time is longer than the upper limit of the above (formula 2), the temperature of the welded portion is high, so that the deformation resistance is lower than the effect of introducing the compressive residual stress due to the increased pressure. There arises a problem that the effect of reducing the fatigue strength resulting from the reduction in the thickness of the surface of the pressing part becomes larger.

  In the present invention, as described above (Equation 1), by defining the late pressurization time and the late pressurization holding time during spot welding, compressive residual stress is introduced around the weld metal part, and the fatigue strength of the welded joint is reduced. It can contribute to improvement.

  Welding conditions other than the late electrode pressurization time and the late pressurization holding time, for example, the welding current at the time of welding, the welding time, and the like may be in accordance with normal welding conditions, and need not be specified.

  Since the welding electrode is water-cooled, if the electrode holding time after welding is increased, the cooling rate of the welded portion is increased, the hardness is increased, and the toughness is lowered and is easily broken. From this point of view, the electrode holding time after welding should be shorter. On the other hand, if it is set too short, the applied pressure will be lost while the molten metal does not solidify, and therefore, scattering will occur and the end of the weld metal (nugget) will end. There are harmful effects such as deterioration of the notch shape.

  Further, if the pressure applied by the electrode is unloaded while the temperature of the weld metal part and its surroundings does not drop so much, sufficient compressive residual stress is not introduced around the weld metal part, so that the fatigue strength is not improved. In addition, it is possible to set the later pressurization start time within the holding time as shown in the above (formula 2), and to apply a greater pressurization after the temperature of the weld metal part and its surroundings has decreased to some extent, the compressive residual stress Is important to introduce.

  Therefore, from the viewpoint of improving the fatigue strength of the welded joint, it is preferable to set the later pressurizing pressure holding time after welding according to the above (Equation 2).

  Further, it is not necessary to particularly define the thickness t of the material to be welded. In general, the thickness of a steel plate used for a part that requires fatigue strength, such as an automobile part or a vehicle body, is 1.0 to 2.3 mm. However, the present invention is sufficiently effective in this thickness. Can do.

  There is no particular need to define the electrode shape. As defined in JIS C 9304, there are F-type, R-type, D-type, DR-type, CF-type, CR-type, EF-type, ER-type, and P-type. The effect can be demonstrated.

  Furthermore, it is not necessary to specifically limit the type of steel plate. Solid solution type, precipitation type (for example, Ti precipitation type, Nb precipitation type), two-phase structure type (for example, structure containing martensite in ferrite, structure containing bainite in ferrite), work-induced transformation type (in ferrite) The present invention can be applied to any type of steel sheet such as a structure containing residual austenite.

The manufacturing method of the steel plate may be a hot rolling method or a cold rolling method, and may be a bare steel plate or a plated steel plate. The type of plating to be coated may be any type as long as it is conductive (for example, Zn, Zn—Fe, Zn—Ni, Zn—Al, Sn—Zn, etc.). Both are preferably 100 g / m ² or less.

  In particular, when the steel sheet is a work-induced transformation type composite structure steel sheet containing 5% or more and 25% or less residual austenite in the ferrite, the fatigue strength of the welded joint is remarkably improved. It is known that this work-induced transformation type composite structure steel sheet contains retained austenite in the structure, and high elongation characteristics are obtained by transforming the retained austenite into martensite during the processing of the steel sheet.

  Based on the results of various experiments using a work-induced transformation type composite steel sheet as a material to be welded, the present inventors have found that when the welding conditions at the time of spot welding are set within a range satisfying each of the above expressions, residual austenite in the structure It has been found that the fatigue strength of the welded joint is improved as compared with other steel plates not containing selenium.

  The mechanism of fatigue strength improvement has not been fully clarified. However, when a work-induced transformation type composite steel sheet is used, the retained austenite in the heat affected zone (HAZ) of the steel sheet around the nugget (welded metal part) is It is presumed that the martensitic transformation is caused by the applied pressure at the electrode tip, the elastic expansion is accumulated around the nugget due to the volume expansion due to this transformation, and finally high compressive residual stress is introduced.

  In addition, among the processing-induced transformation type composite structure steel plates, those containing a retained austenite having a microstructure of less than 5% of the volume fraction cannot sufficiently exhibit the effects of the present invention. Moreover, the thing containing the retained austenite exceeding 25% of a volume fraction cannot be produced from the manufacturing restrictions of a steel plate.

  Normally, tensile residual stress is introduced around the nugget formation area due to shrinkage after welding, so in the case of a fatigue test in which a repeated load is applied in the shear direction, fatigue failure was likely to occur in this area. In the invention, this is mitigated by the introduction of compressive residual stress around the nugget forming portion, and it is considered that the fatigue strength of the welded joint is improved as compared with the conventional case.

The effects of the present invention will be described below with reference to examples, but the present invention is not limited to the conditions used in the examples.
Example 1
A tensile shear fatigue test piece based on a fatigue test method for spot welded joints (JIS Z 3138) was prepared from a steel sheet having a thickness of 1.0 to 2.3 mm and a tensile strength of 270 to 1180 MPa shown in Table 1.

  The types of steel plates are mild steel (symbol: 270E), two-phase composite structure type high strength steel (symbol: 590Y, 780Y, 980Y, 1180Y), precipitation strengthened high strength steel (symbol: 340P, 370P), solid solution strengthened type High strength steel (symbol: 440 W). In addition, these types of steel sheets refer to Japan Iron and Steel Federation standard products.

  Thereafter, these test pieces were superposed with a combination of the same steel types, and spot welding was performed under the welding conditions shown in Table 1 using a JIS-DR type electrode to produce a welded joint.

About the obtained welded joint, based on the fatigue test method (JIS Z 3138) of a spot welded joint, the fatigue test was implemented by loading in the shear direction of the welded joint. Table 1 shows the results. The fatigue strength shown in Table 1 is the fatigue strength at 2 × 10 ⁶ times when the fatigue test is performed in a single swing test under the conditions of stress ratio: 0.05 and frequency: 20 Hz.

As shown in Table 1, the late pressurization pressure Pa (kN) after welding and the late pressurization holding time Hta (ms) after welding are defined by the invention of claim 1 (Formula 1) and (Formula 2) The fatigue strength of the 270E joint (condition No. 3) and 980Y joint (condition No. 4) within the range of 270E joint (condition No. 1), 980Y joint (No. 2), after welding 270E joint (condition No.) in which the later applied pressure Pi (kN) and the later applied pressure holding time Hta (ms) after welding are not within the range of (Expression 1) and (Expression 2) defined in the invention of claim 1 .11) and 980Y joints (condition No. 12).
1.5 × Pi ≦ Pa ≦ 2.5 × Pi (kN) (Formula 1)
0.35 × Ht ≦ Hti ≦ 0.65 × Ht (ms) (Formula 2)

  In addition, the 270E joint (conditions No. 5, 7, and 7) within the range of (Formula 1) and (Formula 2) defined in the invention of claim 1 is the late pressurization after welding and the late pressurization holding time after welding. 9), 980Y joints (No. 6, 8, 10) were also investigated, but 270E joints (conditions) that are not within the range of (Formula 1) and (Formula 2) defined in the invention of claim 1 No. 1) and 980Y joints (condition No. 2) showed higher values.

  Further, for each steel type, the late pressurization pressure Pa (kN) after welding and the late pressurization holding time Hta (ms) after welding are defined by the invention of claim 1 (formula 1) and (formula 2) The other cases (Condition Nos. 13 to 18) that were not included were investigated, but both were the same as the conventional examples of the 270E joint (No. 1) and the 980D joint (No. 2). It was found that high fatigue strength can be obtained when set within the condition range.

  The investigation was also conducted when the plate thickness was changed, but the late pressurizing force after welding and the late pressurizing holding time after welding were within the ranges of (Formula 1) and (Formula 2) defined in the invention of claim 1 Each of the welded joints having a lower plate thickness of 1.0 mm (No. 21) and an upper limit of 2.3 mm (No. 22) is 1.0 mm (No. 19), 2.3 mm (which is a conventional example). An improvement in fatigue strength was observed with respect to No. 20).

  For the 590Y joint (No. 23, 25, 27, 29) and the 780Y joint (No. 24, 26, 28, 30), the same results as those of the 270E joint and the 980 joint were obtained.

  Just in case, 340P joints (No. 31, 35), 370P joints (No. 32, 36), 440W joints (No. 33, 37) and 1180Y joints (No. 34, 38) with different steel plate strengths are also used. The effect of the present invention was confirmed.

  590T joints (No. 42), 780T joints (No. 43), and 980T joints (No. 44), which are work-induced transformation type composite structure steel plates (equivalent products of the invention described in JP-A-2002-129286), are In addition to confirming the effects of the invention, a further improvement in strength was recognized as compared with other steel types 590Y, 780Y, and 980Y of the same strength class.

1 High-strength steel plate 2 Weld metal (nugget)
3 Load direction of fatigue test load 4 Welding electrode 5 Applied pressure (load direction)

Claims (2)

In the welding method that improves the fatigue strength of the steel plate spot welded joint,
Of the holding time Ht (ms) after spot welding, after the initial pressurizing pressure holding time Hti (ms) has elapsed with the initial pressurizing pressure Pi (kN), the pressurizing force is increased from the initial pressurizing pressure Pi to the late pressurizing pressure Pa, As a welding method in which the late pressurization holding time Hta (ms) elapses at the late pressurization pressure Pa (kN),
The latter applied pressure Pa (kN) satisfies the following (formula 1):
The initial pressure holding time Hti (ms) satisfies the following (Equation 2):
The latter pressurization holding time Hta (ms) satisfies the following (formula 3),
The initial pressure Pi (kN) is the following (Formula 4) when using a steel sheet having a tensile strength TS <430 MPa as a workpiece, and the following (Formula 5) when using a steel sheet having a tensile strength TS ≧ 430 MPa. The filling,
The above-mentioned holding time Ht (ms) is set so as to satisfy the following (Equation 6), and spot welding is performed.
1.5 × Pi ≦ Pa ≦ 2.5 × Pi (kN) (Formula 1)
0.35 × Ht ≦ Hti ≦ 0.65 × Ht (ms) (Formula 2)
Hta = Ht−Hti (ms) (Formula 3)
Pi = 2.45 × t (kN) (Formula 4)
Pi = 2.45 × t × (TS / 270) ^0.5 (kN) (Formula 5)
Ht = 200 × t−80 (ms) (Formula 6)
Where t is the steel plate thickness (mm), and TS is the tensile strength (MPa) of the steel plate.   The steel sheet spot welding method according to claim 1, wherein the steel sheet is a microstructure-induced work-induced transformation type composite steel sheet containing a retained austenite having a volume fraction of 5% to 25%.

Images