JP2007130685A - Method for enhancing fatigue strength of spot welded joint of high-strength steel plate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method capable of enhancing the fatigue strength of a weld joint while ensuring the excellent workability in a spot welded joint of a high-strength steel plate. <P>SOLUTION: In the method for enhancing the fatigue strength of a spot welded joint of a high-strength steel plate having the tensile strength of 260-1,570 MPa and the thickness of 0.6-3.3 mm, after the temperature of the surface of a weld part is dropped to ≤ 300°C after finishing the spot welding, a center of a columnar punch is set to a center of a weld part by using the columnar punch having the fore-end radius of curvature of 50-120 mm, the weld part is pressed from both sides of upper and lower steel plates so that the pressure P of the columnar punch satisfies inequalities (1) 0.0314×t×TS≤P≤0.1571×t×TS in terms of the relationship between the tensile strength TS of the high-strength steel plate, and the thickness t, where t : the thickness (mm) of the high-strength steel plate, TS : tensile strength (MPa) of the high-strength steel plate, and P: the pressure (kN) by the columnar punch. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a spot-welding method for high-strength steel sheets mainly applied to mounting of automobile parts and assembling of a vehicle body, and more particularly to a spot-welding method for high-strength steel sheets that require high joint fatigue strength characteristics. is there.

In recent years, in the automobile field, there is a growing need for using thin high-strength steel sheets for car bodies and parts in order to reduce automobile fuel consumption, reduce CO ₂ emissions, and improve collision safety.

  A spot welding method is mainly used for assembling a vehicle body or attaching parts, but there are the following problems when spot-welding a high-strength steel plate.

  Fatigue strength is important as well as tensile strength as a quality index of spot welds (welded joints). Normally, the tensile strength in the shear direction of the welded joint increases with the tensile strength of the steel sheet, but the fatigue strength in the shear direction 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 290 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, but fatigue strength when a repeated load is applied in the shear direction of a welded joint, for example, a load at a stress load of 2 × 10 ⁶ times is defined as fatigue strength. Does not increase and shows almost the same value as in the case of mild steel plate.

  As described above, the cause of the low fatigue strength may be the notch shape of the spot welded portion, as reported conventionally. That is, as shown in FIG. 1, since the end 3 of the nugget 2 existing between the steel plates 1 has a notch shape, a fatigue test is performed by applying a load in the tensile shear direction (arrow direction) 4. In this case, it is considered that the fatigue strength is not improved by the notch effect even when a steel plate having a high tensile strength is used.

  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 was performed by applying a load in the peeling direction of the welded joint (the direction perpendicular to the tensile shear direction (arrow direction) 4), the fatigue strength of the welded joint of the high-strength steel sheet did not increase. Same as mild steel.

  In this case, the stress concentration in the nugget periphery is remarkable, and the stress load in the local area increases, and cracks are likely to occur there. Therefore, compared with the case where repeated loads are applied in the tensile shear direction, the fatigue strength is Decrease by an order of magnitude.

In general, as the tensile strength of a steel plate increases, the value of the carbon equivalent Ceq related to the toughness of the weld indicated by the following formula tends to increase, and the value of Ceq of the high strength steel plate exceeds 0.2. It is known.
Ceq = C + Si / 30 + Mn / 20 + 2P + 4S
In addition, in said formula, C, Si, Mn, P, and S show each content (mass%) of carbon in a steel, silicon, manganese, phosphorus, and sulfur, respectively.

  Thus, since the carbon equivalent Ceq of the steel plate increases as the tensile strength of the high-strength steel plate increases, the higher the strength of the steel plate, the higher the strength 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.

  Also, high strength steel plates are more susceptible to springback during welding than soft steel plates, so tensile residual stress occurs in spot welds and fatigue strength tends to decrease, and cracks occur. As a result, 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 the mild steel plate.

  Conventionally, as means for improving the fatigue strength of a spot welded joint, a method of spot welding using a steel plate having excellent joint fatigue strength characteristics is known (for example, see Patent Documents 1 to 6). However, these methods relate to spot welded joints of mild steel plates and are not methods for improving the fatigue strength of spot welded portions of high strength steel plates.

  In spot welding of high-strength steel plates, as a means to improve the fatigue strength of welded joints, after energization at the time of spot welding is completed, cool in a non-energized state for a certain period of time, and then perform temper energization, A method is known in which the nugget portion and the heat-affected zone are annealed to reduce the hardness and 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 spot welding of high-strength steel plates, as another means to improve the fatigue strength of welded joints, high-strength steel plates are spot-welded to each other under pressure by electrodes, and temporary energization to the electrodes after spot welding (post-energization) A method is known in which the spot welded portion is heated and tempered, and the tensile residual stress around the spot welded portion is reduced by increasing the pressure applied by the electrode to increase the fatigue strength of the spot welded joint. (For example, refer to Patent Document 7).

  However, this method is also similar to the above method, and there is a practical problem that the range of the appropriate condition range for energizing the temper is very narrow, and the reproducibility is poor due to changes in the operating conditions. Further, since the applied pressure after rewinding is very high, there is a problem that the electrode life is very short and the dent of the welded portion becomes large.

  As another method for improving the fatigue strength of spot welded parts of high-strength steel plates, after spot welding, leave the welded part unenergized / non-pressurized and then pressurize it with non-energized spot welded part. There is known a method for improving the fatigue strength (for example, see Patent Document 8).

  However, in this method, the optimum pressing conditions are not presented, and it is unclear what pressure should be applied to various high-strength steel plates having different tensile strengths.

  Further, as a method for improving the fatigue strength of a spot welded portion of a high-strength steel plate, a method of increasing the fatigue strength of a spot welded portion by pressurizing the spot welded portion after spot welding is known (for example, Non-Patent Document 2). ,reference).

  However, this method relates to a method for improving the fatigue strength of a spot welded joint of mild steel plate, and is not related to a method for improving the fatigue strength of a spot welded joint of high strength steel plate. In addition, in the pressurization of the spot welded portion, there has not been sufficiently studied what kind of tool is used at the tip shape and what pressure is applied to various high-strength steel plates having different strengths.

  In addition to this, as a method of improving the fatigue strength of the resistance spot welded portion, a method of applying an ultrasonic impact treatment to the resistance spot welded portion is known (for example, see Patent Document 9). However, this method is not a preferable method in terms of workability and economy because it requires a post-treatment process after the end of welding, which increases the number of work steps and the economic load.

  Conventionally, a method of increasing the number of resistance spot welding points (number of nuggets) in order to improve the fatigue strength of the welded joint is also known. However, this method has problems such as a decrease in welding work efficiency, an increase in welding construction cost, and a restriction on design freedom. Moreover, this method aims at reducing the stress concentration of the nugget periphery part per joint in a joint by increasing the number of resistance spot welding points (number of nuggets).

  However, when stress is applied to the joint, the stress is not necessarily applied evenly to each welding point (nugget), so that the stress dispersion effect is not sufficiently exhibited, and the stress concentrates on one of the welding points. As a result, even if the number of welding points is increased from 1 point to 2 points or 3 points, for example, the fatigue strength of the joint is not necessarily doubled or tripled.

Japanese Unexamined Patent Publication No. Sho 63-317625 JP-A-2-163323 JP-A-5-263184 JP-A-9-268346 Japanese Patent Laid-Open No. 10-8187 Japanese Patent Laid-Open No. 11-279589 JP 2001-170776 A Japanese Patent Laid-Open No. 2001-321593 JP 2004-122152 A "Iron and Steel" Vol. 68 (1982) No. 9, pp. 1444-1451 R. Spitsen, D. Kim, M. Ramulu, B. Flinn and ET Easterbrook, “THE EFFECTS OF POST-WELD COLD WORKING PROCESSES ON THE FATIGUE STRENGTH OF LOW CARBON STEEL RESISTANCE SPOT WELDS”, Proceedings of IMECE: 2004 ASME International Mechanical Engineering Congress & Exposition.

As described above, in the conventional spot-welding method for high-strength steel sheets, the joint fatigue strength is the same as the joint fatigue strength when spot-welding mild steel sheets, so the joint fatigue strength must be ensured by the steel sheet thickness. In the automotive field, it is not possible to fully enjoy the merits of improving safety by using high-strength steel sheets, reducing fuel consumption by reducing weight, and reducing CO ₂ emissions.

  The present invention provides a spot welding method capable of improving the fatigue strength of a welded joint while ensuring good welding workability in spot welding of a high-strength steel sheet in order to solve these problems in the prior art. The purpose is to do.

  The present inventors changed the residual stress state around the nugget, which is one of the factors that decrease the fatigue strength of the spot welded joint of the high-strength steel plate, from the tension side to the compression side, and the residual stress state around the nugget that becomes the stress concentration part We have intensively studied how to improve the above.

  As a result, using a punch with a tip shape that maintains good contact with the spot welded part, it is possible to efficiently apply to the nugget periphery with the optimum pressure according to the tensile strength and thickness of each high-strength steel sheet. It has been found that the fatigue strength of a high-strength steel plate welded joint can be effectively increased by introducing compressive residual stress.

  This invention is made | formed based on these knowledge, The place made into the summary is as follows.

  (1) In a method for improving the fatigue strength of a spot-welded joint of high-strength steel sheet having a tensile strength of 260 to 1570 MPa and a plate thickness of 0.6 to 3.3 mm, the temperature of the welded surface decreases to 300 ° C. or less after spot welding. After that, using a cylindrical punch having a radius of curvature at the tip of 50 to 120 mm, the center of the cylindrical punch is aligned with the center of the welded portion, and the pressure P of the cylindrical punch is applied from both the upper and lower steel plates above and below the welded portion. A method for improving the fatigue strength of a spot welded joint of high strength steel plate, wherein pressurization is performed so as to satisfy the following formula (1) in relation to tensile strength TS and thickness t of the high strength steel plate.

0.0314 × t × TS ≦ P ≦ 0.1571 × t × TS (1)
However,
t: Thickness (mm) of high-strength steel plate
TS: Tensile strength (MPa) of high-strength steel sheet
P: Pressurizing force with cylindrical punch (kN)

  (2) In a method for improving the fatigue strength of a spot-welded joint of high-strength steel sheets having a tensile strength of 260 to 1570 MPa and a plate thickness of 0.6 to 3.3 mm, the temperature of the surface of the welded portion decreases to 300 ° C. or less after spot welding. After that, the tip end has a ring-shaped projection having a radius of curvature of the projection of 2.0 to 7.0 mm and a projection height of 1.0 to 3.0 mm, and the diameter D of the ring-shaped projection is the nugget diameter ND. The projecting punch satisfying the following (2) is used, the center of the projecting punch is aligned with the center of the welded portion, and the pressure P of the projecting punch is applied to the high strength from both the upper and lower steel plates. A method for improving the fatigue strength of a high-strength steel spot welded joint, wherein pressurization is performed so as to satisfy the following expression (3) in relation to the tensile strength TS and the thickness t of the steel plate.

0.70 × ND ≦ D ≦ 1.30 × ND (2)
0.0157 × t × TS ≦ P ≦ 0.0786 × t × TS (3)
However,
ND: Nugget diameter (mm)
D: Ring-shaped protrusion diameter (mm)
t: Thickness (mm) of high-strength steel plate
TS: Tensile strength (MPa) of high-strength steel sheet
P: Pressurizing force with protrusion type punch (kN)

ADVANTAGE OF THE INVENTION According to this invention, the fatigue strength of a welded joint can be improved, ensuring favorable welding workability | operativity in the spot welding of the high-strength steel plate used for the attachment of the components for motor vehicles, the assembly of a vehicle body, etc. Therefore, by applying the present invention, in the automotive field, etc., it is possible to fully enjoy the benefits of improving safety by applying high-strength steel sheets, reducing fuel costs by reducing weight, reducing CO ₂ emissions, etc. It is.

Details of the present invention will be described below.
First, as a method of improving the fatigue strength of spot welded joints of high-strength steel plates,
(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 introducing compressive residual stress around the weld metal (nugget) portion to relatively reduce tensile residual stress,
There are three possible methods.

  As for the method of (a), for example, as described in Non-Patent Document 1, it is intentionally scattered during welding (the diameter of the molten part generated between the steel plates during energization is 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 gap between the steel plates, but this method uses a weld metal (nugget). ) Portion end shape varies, and in fact, the fatigue strength varies considerably, which is not preferable as a method for stably improving joint fatigue strength characteristics.

  As a method of (b), as described in Non-Patent Document 1 and Patent Document 7, cooling is performed in a non-energized state for a certain period of time after the end of welding, and then the welding part is energized again for a certain period of time (post-energization). A method of tempering the welded portion is 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.

  Further, as described in Patent Document 8, as a method of (c), after spot welding, after a predetermined time has passed, a method of pressurizing the welded portion with a non-energized electrode, Non-Patent Document 2 As described in, there is known a method of pressurizing a welded portion with a pressurizing jig other than an electrode after spot welding. However, this method does not present the tip shape or pressurizing condition of the pressurizing jig effective for improving the joint fatigue strength of the high-strength steel sheet, and its effect is insufficient.

  In addition, as described in Patent Document 9, a method of performing ultrasonic impact treatment on a welded portion of a spot welded joint is also known. However, since post-processing is assumed, in terms of workability and manufacturing cost. A practical method is not preferable.

  As a method of (c), the present inventors use a punch having a tip shape that maintains a good contact state with a spot welded portion, and is optimal according to the tensile strength and thickness of each high-strength steel plate. It has been found that the fatigue strength of a high-strength steel plate welded joint can be effectively increased by efficiently introducing compressive residual stress into the periphery of the nugget with a moderate pressure.

  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. In spot welding, two high-strength steel plates 1 that are materials to be joined are overlapped with each other, and the overlapped portion is vertically moved while pressing with a predetermined pressure 6 using two upper and lower copper welding electrodes 5. Energization is performed to form a molten metal portion between the two high-strength steel plates 1.

  This molten metal portion is cooled and solidified by heat removal by the electrode or heat conduction to the steel sheet in contact with the water-cooled electrode after the welding energization is completed, and a nugget between two high-strength steel sheets 1 ( Weld metal) 2 is formed.

  FIG. 3 shows the spot weld after energization / cooling. Between the two high-strength steel sheets 1, the nugget 2 is formed and the heat affected zone caused by welding heat input around the nugget 2 (hereinafter, the nugget and the surrounding heat affected zone may be collectively referred to as a weld zone). Is formed. In addition, on the upper and lower front and back surfaces of the high-strength steel plate 1, the welded portion is plastically deformed at a high temperature by pressurization by the upper and lower welding electrodes, and the recess 7 is formed at the welded portion.

  The molten metal formed between the two high-strength steel plates 1 by spot welding causes volume expansion due to martensitic transformation in the cooling process after solidification, and then heat shrinkage occurs in the cooling process to room temperature. The residual stress in the periphery 8 of the finally formed nugget 2 becomes a tensile residual stress state. The tensile residual stress introduced into the periphery 8 of the nugget 2 together with the notch shape at the end of the nugget becomes a main cause of the joint fatigue strength characteristic deterioration in spot welding of the high strength steel plate.

  In the first embodiment of the present invention, the material to be welded (high-strength steel plate 1 shown in FIG. 2) has a tensile strength TS of 260 to 1570 MPa and a thickness t in the range of 0.6 to 3.3 mm. After spot welding using a high-strength steel plate, the temperature of the weld surface decreases to 300 ° C. or less (after cooling), and then a cylindrical punch having a tip radius of curvature R1 of 50 to 120 mm shown in FIG. As shown in FIG. 5 (a), the center of the cylindrical punch is aligned with the center of the welded portion, and the cylindrical punch is added from both sides of the high strength steel plate 1 above and below the spot welded portion (nugget 2). Pressure is applied so that the pressure P (10) satisfies the condition of the following formula (1) in relation to the tensile strength TS and the thickness t of the high-strength steel plate, and compressive residual stress is introduced around the nugget. And improving the fatigue strength of the welded joint.

0.0314 × t × TS ≦ P ≦ 0.1571 × t × TS (1)
However,
t: Thickness (mm) of high-strength steel plate
TS: Tensile strength (MPa) of high-strength steel sheet
P: Pressurizing force at punch (kN)

  Hereinafter, in the embodiment of the first invention, the tensile strength TS of the high-strength steel plate, the thickness t, the surface temperature of the welded portion when pressurizing with a punch after welding, the tip shape of the punch, and the reasons for limitation of the pressurizing force will be described. To do.

(Tensile strength TS of high-strength steel sheet)
In the present invention, the tensile strength TS of the high-strength steel plate used as the material to be welded is defined in the range of 260 to 1570 MPa. The reason why the lower limit of the tensile strength TS is 260 MPa is that the lower limit of the tensile strength of the steel plate for automobiles is 260 MPa of the mild steel plate. This is because it improves.

  The upper limit of the tensile strength TS is set to 1570 MPa because, when spot welding is performed using a high-strength steel plate having a tensile strength higher than 1570 MPa, the hardness of the weld due to an increase in the carbon equivalent Ceq increases the application of the present invention. This is because a sufficient effect of improving the joint fatigue strength cannot be obtained. Moreover, since a high pressurizing force is required for pressurization with the punch, there is a demerit that the pressurization equipment becomes large.

  If the lower limit of the tensile strength TS is 440 MPa, the yield stress of the steel sheet increases and sufficient compressive residual stress is introduced. The fatigue strength can be improved. Further, if the upper limit of the tensile strength is 980 MPa, a better tensile strength improvement and fatigue strength improvement effect can be obtained with a welded joint.

(Thickness t of high-strength steel sheet)
In the present invention, the thickness t of the high-strength steel plate used as the material to be welded is defined in the range of 0.6 to 3.3 mm. The reason why the lower limit of the plate thickness t is 0.6 mm is that the lower limit of the plate thickness of the steel plate for automobiles is 0.6 mm. This is because the deformation of.

  The upper limit of the plate thickness t is set to 3.3 mm because the upper limit of the plate thickness of the steel plate for automobiles is 3.3 mm. This is because a high pressurizing force is required for pressurizing, and therefore a demerit that the pressurizing equipment becomes large is generated.

  If the lower limit of the plate thickness t is 1.0 mm, it is possible to improve the tensile strength and the fatigue strength of the welded joint with little deformation of the entire steel plate. Further, if the upper limit of the plate thickness is set to 2.0 mm, a good fatigue strength improvement effect can be obtained in the welded joint with a pressure applied by an appropriate punch.

(Surface temperature of weld when pressurizing with punch after welding)
In the present invention, the surface temperature of the welded portion when pressurizing with a punch is defined within a range of 300 ° C. or less. The reason why the surface temperature of the welded part is defined as 300 ° C. or lower is that the higher the temperature of the welded part, the higher the temperature around the nugget 8 in FIG. This is because stress is hardly introduced.

  Since the yield stress of a steel sheet rapidly decreases when it exceeds 400 ° C., it is important to pressurize the weld at a temperature as low as possible. If possible, it is desirable that the surface temperature of the welded part when pressurizing with a punch is 100 ° C. or less.

(Punch tip shape)
In normal spot welding, the welded part is heated while being pressurized by the electrode during welding, so that it is plastically deformed. As shown in FIG. 3, depressions 7 are formed on both surfaces of the high-strength steel plate 1 above and below the welded part. The Therefore, in order to efficiently introduce the compressive residual stress around the nugget 2 where the tensile residual stress is likely to occur, a good contact state with the ends of the recesses 7 formed on both surfaces of the high strength steel plate 1 above and below the welded portion. There is a need to pressurize using a cylindrical punch having a tip shape capable of maintaining the pressure.

  Further, when pressurizing with a cylindrical punch, it is necessary to align the center of the punch with the center of the weld. This is because it becomes impossible to maintain a good contact state between the end of the depression 7 and the cylindrical punch due to the deviation of the center. Therefore, in practice, it is necessary to pressurize the center of the depression 7 and the center of the punch. At this time, the position may be displaced, but there is no problem if the displacement is about 20% with respect to the diameter of the recess 7.

The present inventors have used a high strength steel plate (dual-phase composite steel) having a tensile strength of 780 MPa and a plate thickness of 1.2 mm, and variously changing the tip shape and the applied pressure of the cylindrical punch, so that the upper and lower steel plates The state of plastic deformation around the surface depression 9 was examined visually. Further, a load in the shear direction was applied to the spot welded joint, and the fatigue strength (the load range when the number of repetitions was 2 × 10 ⁶ times) was examined, and compared with the case of a mild steel plate.

  As a result, when a cylindrical punch having a tip radius of curvature R1 of 50 to 120 mm as shown in FIG. 4A is used to pressurize at a predetermined pressure or more, around the depressions on the upper and lower steel plate surfaces shown in FIG. It was found that the plastic deformation of 9 (around the welded portion) became prominent, and the joint fatigue strength was improved by 10% or more compared to the mild steel plate.

  That is, according to the study by the present inventors, only when the periphery 9 of the depression on the upper and lower steel plate surfaces is pressurized using a cylindrical punch having a radius of curvature R1 of the tip of 50 to 120 mm, Since sufficient pressurizing force can be applied while maintaining a good contact state with the periphery 9 of the recess, remarkable plastic deformation was observed in the periphery 9 of the recess on the upper and lower steel plate surfaces, and the fatigue strength was remarkably improved.

  This is thought to be because plastic deformation occurred by applying pressure to the periphery 9 of the depressions on the upper and lower steel plate surfaces, and the compressive residual stress was effectively introduced into the periphery 8 of the nugget 2 indirectly.

  When the curvature radius R1 of the tip is less than 50 mm, the tip of the cylindrical punch comes into contact with the depression on the upper and lower steel sheet surfaces, and the periphery 9 of the depression on the upper and lower steel sheet surface does not come into contact with the punch. The applied pressure is not sufficiently introduced into the periphery 9 of the depression on the upper and lower steel plates, and no significant plastic deformation is observed in the periphery 9 of the depression on the upper and lower steel plates.

  Further, when the radius of curvature R1 of the tip exceeds 120 mm, the tip of the cylindrical punch becomes a substantially flat shape, and the periphery 9 of the depression on the upper and lower steel plate surfaces does not come into contact with the punch. Significant plastic deformation is not recognized around the depression 9.

  Therefore, in the present invention, the pressurizing force of the cylindrical punch sufficiently acts on the periphery 9 of the depression on the upper and lower steel sheet surfaces, and significant plastic deformation occurs in the periphery 9 on the upper and lower steel sheet surface, thereby improving the joint fatigue strength. Therefore, the curvature radius R1 of the tip of the cylindrical punch is defined to satisfy the range of 50 to 120 mm.

(Punch pressure)
The present inventors use a cylindrical punch having a thickness of 1.2 to 2.0 mm and a tensile strength of 440 to 1180 MPa (double-phase composite steel) and having a tip radius of curvature R1 of 100 mm. By changing the applied pressure, the plastic deformation state around the depression 9 on the upper and lower steel plate surfaces was visually examined in the same manner as described above, and the fatigue strength of the joint was examined.

  As a result, when the pressure P of the cylindrical punch satisfies the following formula (1), significant plastic deformation is recognized around the depression 9 on the upper and lower steel sheet surfaces, and the fatigue strength of the joint is improved by 10% or more. I understood.

0.0314 × t × TS ≦ P ≦ 0.1571 × t × TS (1)
However,
t: Thickness (mm) of high-strength steel plate
TS: Tensile strength (MPa) of high-strength steel sheet
P: Pressurizing force with cylindrical punch (kN)

  In the present invention, the above equation (1) that defines the pressure applied to the cylindrical punch is determined as follows.

  FIG. 6 and FIG. 7 show examples of the fatigue strength evaluation of joints that are the basis for deriving the above equation (1).

  FIG. 6 is an example in which the plate thickness is fixed to 1.2 mm and the strength of the steel plate is changed to examine the fatigue strength. FIG. 7 is a diagram in which the strength of the steel plate is fixed to 780 MPa and the plate thickness of the steel plate is changed. This is an example of examining the fatigue strength. In the figure, the relationship between the strength of the steel plate and the fatigue strength evaluation result and the relationship between the plate thickness of the steel plate and the fatigue strength evaluation result (◯ mark and X mark in the drawing) are shown.

  In the figure, the case where the fatigue strength of the welded joint is improved by 10% or more with respect to the fatigue strength when welding a mild steel plate having a tensile strength of 290 MPa is indicated by ○, and the case where the fatigue strength is not improved is indicated by ×.

  From FIG. 6, a relational expression between the optimum pressing force when the plate thickness of the steel plate is constant and the tensile strength of the steel plate is obtained, and from FIG. 7, the optimum pressing force when the tensile strength of the steel plate is constant and Equation (1) was derived by obtaining a relational expression with the thickness of the steel sheet.

  From FIG. 6 and FIG. 7, if the pressure P of the cylindrical punch is set so as to satisfy the relationship of the above formula (1), it is possible to form a welded joint with good fatigue strength.

  When the pressure P is lower than the lower limit (0.0314 × t × TS) of the pressure P of the cylindrical punch defined by the above formula (1), no significant plastic deformation is observed around the depression 9 on the upper and lower steel sheet surfaces. Since sufficient compressive residual stress is not introduced around the nugget 8 shown in FIG. 3, the fatigue strength is not improved.

  On the other hand, when the pressing force is higher than the upper limit (0.1571 × t × TS) of the pressing force P of the cylindrical punch specified by the above formula (1), excellent joint fatigue strength can be secured, but depending on the pressing force. Not only is the amount of compressive residual stress introduced saturated and the joint fatigue strength improved, but when the applied pressure is further increased, the depressions on the upper and lower steel sheet surfaces become larger due to the amount of plastic deformation and the plate thickness becomes thinner. Therefore, the fatigue strength of the welded joint tends to decrease.

  In addition, a large indentation is generated to deteriorate the external shape of the joint, and the static strength of the joint is also lowered, which is not desirable. In addition to this, problems such as occurrence of cracks in the welded portion are likely to occur.

  When the pressure P of the cylindrical punch satisfies the above equation (1), an appropriate pressure is applied to the periphery 9 of the depression on the upper and lower steel sheet surfaces, and proper plastic deformation occurs at the periphery 9 of the depression on the upper and lower steel sheet surfaces. As a result, a sufficient compressive residual stress is introduced around the nugget 8 so that the fatigue strength can be significantly improved.

  Therefore, in the present invention, in the spot welding method of the high-strength steel plate, pressurization is performed so that the pressure P of the cylindrical punch satisfies the above formula (1).

  Next, the reason for limiting the shape of the tip of the punch and the applied pressure in the embodiment of the second invention will be described. The reason why the tensile strength TS of the high-strength steel plate, the thickness t, and the surface temperature of the welded portion when pressurizing with a punch after welding is limited is the same as the reason described in the embodiment of the first invention.

(Punch tip shape)
As described above, in order to improve the fatigue strength of the spot welded joint, it is necessary to efficiently introduce compressive residual stress around the nugget 8 shown in FIG. 3 where tensile residual stress is likely to occur.

  Therefore, the present inventors use a projecting punch in which a ring-shaped projection is provided at the tip of the cylindrical punch used in the first embodiment of the present invention. The tip shape of the punch for efficiently introducing compressive residual stress to the periphery 8 of the nugget by applying pressure while maintaining the contact state was examined.

  The present inventors use a high-strength steel plate (dual-phase composite steel) with a tensile strength of 780 MPa and a plate thickness of 1.2 mm as in the first embodiment of the invention, and the tip shape of the protruding punch. The plastic deformation state around the depression 9 on the upper and lower steel plate surfaces was visually examined by changing the pressure force in various ways.

Further, a load in the shear direction was applied to the spot welded joint, and the fatigue strength (the load range when the number of repetitions was 2 × 10 ⁶ times) was examined, and compared with the case of a mild steel plate.

  As a result, as shown in FIG. 4B, the radius of curvature R2 of the projection provided at the tip of the cylindrical punch is 2.0 to 7.0 mm, the projection height H is 1.0 to 3.0 mm, When the projection position is a ring-shaped projection with a diameter D of the ring-shaped projection satisfying the following formula (2) with respect to the nugget diameter, and pressurizing at a predetermined pressure or more, plastic deformation around the depression 9 on the upper and lower steel sheet surfaces It became remarkable and it discovered that joint fatigue strength improved 10% or more compared with a mild steel plate.

0.70 × ND ≦ D ≦ 1.30 × ND (2)
However,
ND: Nugget diameter (mm)
D: Ring-shaped protrusion diameter (mm)

  In the present invention, the diameter D of the ring-shaped protrusion is an average diameter measured at the apex position of the ring-shaped protrusion.

  The reason why the radius of curvature R2 of the tip of the ring-shaped protrusion shown in FIG. 4B is limited to 2.0 to 7.0 mm is as follows. That is, when the radius of curvature R2 of the tip of the protrusion is smaller than 2.0 mm, the contact between the protrusion and the periphery 9 of the depression on the surface of the upper and lower steel plates becomes local, and the plastic deformation of the portion is less likely to occur. This is because it is difficult to effectively introduce compressive residual stress into 8, and as a result, fatigue strength is not improved.

  On the other hand, when the radius of curvature R2 of the tip of the protrusion is larger than 7.0 mm, the contact portion between the protrusion and the periphery 9 of the depression on the upper and lower steel plate surfaces becomes large, the surface pressure at that portion decreases, and plastic deformation of that portion occurs. This is because the amount is reduced and it becomes difficult to effectively introduce compressive residual stress around the nugget 8 and as a result, the fatigue strength is not improved.

  The reason why the height H of the ring-shaped protrusion shown in FIG. 4B is limited to 1.0 to 3.0 mm is as follows. That is, when the height of the protrusion is lower than 1.0 mm, the effect of intensively applying pressure to the periphery 9 of the depression on the upper and lower steel plate surfaces cannot be obtained, and the compressive residual stress is efficiently introduced into the periphery 8 of the nugget. This is because it becomes difficult to sufficiently improve the fatigue strength.

  On the other hand, if the projection height H exceeds 3.0 mm and becomes too high, a sense of stability is lost during pressurization, causing problems such as damage to the projection.

  The reason why the diameter D of the ring-shaped protrusion provided at the tip of the cylindrical punch is defined by the above equation (2) is as follows.

  That is, when the diameter D of the ring-shaped protrusion is smaller or larger than the lower limit (0.70 × ND) defined by the above formula (2), the ring-shaped protrusion and the periphery 9 of the depression on the upper and lower steel sheet surfaces are caused by the positional deviation. Contact is localized, stress is less likely to concentrate at that portion, plastic deformation is less likely to occur at that portion, and compressive residual stress is less likely to be effectively introduced around the nugget 8, resulting in fatigue. This is because it is difficult to sufficiently improve the strength.

  Therefore, in the present invention, compressive residual stress is efficiently introduced into the periphery 8 of the nugget, where tensile residual stress is likely to occur, and appropriate plastic deformation is generated in the periphery 9 of the depressions on the upper and lower steel plates, thereby efficiently increasing the fatigue strength. In order to improve this, the radius of curvature R2 of the tip of the ring-shaped protrusion provided at the tip of the cylindrical punch is 2.0 to 7.0 mm, the protrusion height H is 1.0 to 3.0 mm, and the protrusion position is A protrusion-type punch satisfying the above expression (2) in relation to the diameter D of the ring-shaped protrusion and the nugget diameter ND.

(Punch pressure)
The present inventors use a high-strength steel plate (duplex steel) having a plate thickness of 1.2 to 2.0 mm and a tensile strength of 440 to 1180 MPa, and a cylindrical punch shown in FIG. In the same manner as described above, the depressions on the surfaces of the upper and lower steel plates are changed by changing the pressing force of the protruding punch provided with a ring-shaped protrusion having a curvature radius R2 of the protrusion tip of 5.0 mm and a height H of 2.0 mm at the tip end of The state of plastic deformation at the periphery 9 was visually examined, and the fatigue strength of the joint was examined.

  As a result, when the pressure P of the projecting punch satisfies the following expression (3), remarkable plastic deformation is recognized around the depression 9 on the upper and lower steel sheet surfaces, and the fatigue strength of the joint is improved by 10% or more. I understood. Therefore, if the applied pressure is set according to the following equation (3), a welded joint with good fatigue strength can be formed.

0.0157 × t × TS ≦ P ≦ 0.0786 × t × TS (3)
However,
t: Thickness (mm) of high-strength steel plate
TS: Tensile strength (MPa) of high-strength steel sheet
P: Pressurizing force with protrusion type punch (kN)

  As can be seen by comparing the above expression (3) with the expression (1) in the embodiment of the first invention, when pressurization is performed using the protruding punch in the embodiment of the second invention, the first With about half the applied pressure in the embodiment of the invention, it is possible to introduce a compressive residual stress around the nugget 8 and sufficiently improve the fatigue strength of the joint.

  This is because when stress is applied using the projecting punch shown in FIG. 4 (b), it is possible to increase the stress concentration around the depression 9 on the upper and lower steel sheet surfaces. Since the periphery 9 of the depression on the steel plate surface can be plastically deformed, the compressive residual stress is efficiently introduced into the periphery 8 of the nugget, and the fatigue strength of the spot welded joint can be improved. I mean.

  When the pressure P is lower than the lower limit (0.0157 × t × TS) of the pressure P of the protruding punch defined by the above equation (3), the pressure is too low, and the area around the depression 9 on the upper and lower steel plates is sufficient. Since the plastic deformation does not occur and sufficient compressive residual stress is not introduced around the nugget 8, the fatigue strength is not improved.

  On the other hand, when the pressing force is higher than the upper limit (0.0786 × t × TS) of the pressing force P of the protruding punch defined by the above formula (3), excellent joint fatigue strength can be secured, but depending on the pressing force. Not only is the amount of compressive residual stress introduced saturated and the joint fatigue strength improved, but when the applied pressure is further increased, the depressions on the upper and lower steel sheet surfaces become larger due to the amount of plastic deformation and the plate thickness becomes thinner. Therefore, the fatigue strength of the welded joint tends to decrease.

  In addition, a large indentation is generated to deteriorate the external shape of the joint, and the static strength of the joint is also lowered, which is not desirable. In addition to this, problems such as occurrence of cracks in the welded portion are likely to occur.

  When the pressing force P of the protruding punch satisfies the above formula (3), an appropriate pressing force is applied to the periphery 9 of the depression on the upper and lower steel sheet surfaces, and proper plastic deformation occurs at the periphery 9 of the depression on the upper and lower steel sheet surfaces. As a result, a sufficient compressive residual stress is introduced around the nugget 8 so that the fatigue strength can be significantly improved.

  Therefore, in the present invention, in the spot welding method for high-strength steel sheets, pressurization is performed so that the pressing force P of the protruding punch satisfies the above-described expression (3).

  As described above, when pressurization is performed using a punch having this shape, it is necessary to align the center of the punch with the center of the weld. This is because, if this is not done, even if the punches defined by the above formula (2) are used as the distance between the folding projections, an appropriate contact state cannot be obtained around the welded portion.

  Therefore, in practice, it is necessary to pressurize the center of the depression 7 and the center of the punch. At this time, the position may be displaced, but it is desirable to set the displacement by 10% or less with respect to the diameter of the recess 7.

  The diameter of the punch used for pressurization is not particularly specified, but in order to efficiently introduce the residual stress, a diameter equivalent to the electrode diameter is preferable. The material of the punch is not particularly defined, but tool steel or the like may be used so that it can withstand the applied pressure.

  The electrode shape may be any of F type, R type, D type, DR type, CF type, CR type, EF type, ER type, and P type defined in JIS C 9304. In order to improve the contact state of the welded portion, DR type, CF type, CR type and the like are desirable if possible.

  The tip diameter of the electrode is not particularly specified, but normally, a thickness of 5√t to 6√t (mm) is used when the plate thickness is t (mm), and the tip diameter within this range is also used in the present invention. It is desirable to use an electrode having

  The spot welding conditions, that is, the welding current at the time of welding, the welding time, the electrode holding time after welding, and the like may be in accordance with normal welding conditions and need not be specified. The nugget diameter is not particularly specified, but in normal spot welding, if the plate thickness is t (mm), the nugget diameter is set to 3√t to 6√t (mm). It is desirable to apply the present invention to a spot welded joint.

  There is no particular limitation on the type of the 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 sheet may be a hot rolling method or a cold rolling method.

Moreover, also when spot-welding the high-strength plated steel plate with the steel plate surface layer plated by the method of the present invention, a joint having excellent fatigue strength can be realized without impairing the properties of the high-strength 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.). And 100/100 g / m ² or less is desirable.

  Further, the method of the present invention is not limited to the combination of the same type and the same thickness steel plate, and may be the same type of different thickness, different types of same thickness, and different types of different thickness combinations as long as the specification is satisfied.

  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 Z3138) was prepared from steel plates having thicknesses of 1.2 and 1.6 mm and tensile strengths of 295 to 1475 MPa shown in Table 1.

  The types of steel plates are mild steel (symbol: 270S), solid solution strengthened high strength steel (symbol: 440S), precipitation strengthened high strength steel (symbol: 590P), and dual-phase composite structure type high strength steel (symbol: 590D, 780D, 980D, 1180D), work-induced transformation type composite structure high strength steel (symbol: 590T, 780T), and quenching strengthened high strength steel (symbol: 1470H). All the steel plates were bare steel plates.

  These test pieces are overlapped with a combination of the same steel type and the same plate thickness, and spot welding is performed under welding conditions such that the diameter of the nugget 2 shown in FIG. 2 is the diameter shown in Table 1 and Table 2. A welded joint was produced.

  Next, pressure was applied to both of the test pieces from both sides with a punch using a hydraulic press.

About the obtained welded joint, based on the fatigue test method (JIS Z3138) of a spot welded joint, it applied to the shear direction and the fatigue test was implemented. Tables 1 and 2 show the results. The fatigue strengths shown in Tables 1 and 2 are fatigue strengths at 2 × 10 ⁶ times when a one-way swing fatigue test is performed under the conditions of stress ratio: 0.05 and frequency: 30 Hz.

  First, the effect of the present invention was verified using various high-strength steel plate joints having a plate thickness of 1.2 mm. As shown in Table 1, the radius of curvature of the punch tip is within the range defined in claim 1. When the weld is pressurized using the punch within the range defined in claim 1, Even if the radius of curvature is changed (No. 1 to No. 3), or even if the pressing force is changed by a joint of various steel types (No. 4 to No. 12), the mild steel plate joint that does not perform pressurization (No. 26) and the high strength steel plate joints (No. 29, No. 31, No. 33), the fatigue strength of the joints was remarkably improved.

  Next, the fatigue strength was investigated using a high-strength steel plate joint having a plate thickness of 1.6 mm (No. 13 to No. 18), and in all cases, a significant improvement in fatigue strength was observed. Furthermore, the fatigue strength was investigated using a mild steel plate joint (No. 19) having a thickness of 1.2 mm and various high-strength steel plate joints (No. 20 to No. 25). Compared with mild steel plate joints (No. 26) and high-strength steel plate joints (No. 27, No. 28, No. 30, No. 32, Nos. 34 to No. 35), the fatigue strength of the joints is It was remarkably improved.

  On the other hand, when pressurization was performed with the pressurizing force defined in claim 1, but the radius of curvature at the punch tip was outside the range defined in claim 1 (No. 36 to No. 37), the fatigue strength The improvement of was not recognized.

  Further, the radius of curvature at the punch tip is within the range defined in claim 1, but when the applied pressure is lower than the value defined in claim 1 (No. 38, No. 40, No. 42, No. 44). ), No improvement in fatigue strength was observed in the case of plate thicknesses of 1.2 mm and 1.6 mm.

  Further, the radius of curvature at the tip of the punch is within the range defined in claim 1, but the pressure is higher than the value defined in claim 1 (No. 39, No. 41, No. 43, No. 45). In the case of plate thickness: 1.2 mm and 1.6 mm, the fatigue strength is improved, but the dent on the surface of the welded portion is large and the applied pressure is lower than that (No. 6, No.12, No.15, No.18) The improvement rate of fatigue strength also showed a low value. Moreover, generation | occurrence | production of the partial crack was observed on the surface.

  Next, in the same manner as described above, the effects of the present invention were verified using various high-strength steel plate joints having a plate thickness of 1.2 mm.

  As shown in Table 2, the radius of curvature of the tip of the protrusion, the height of the protrusion, and the distance between the protrusions are within the range defined in claim 2, and within the range defined in claim 2, using the punch, When pressure is applied, even if the radius of curvature of the protrusion tip is changed (No. 1 to No. 3) or the height of the protrusion is changed (No. 4 to No. 5), the distance between the protrusions Even if the pressure is changed (No. 6 to No. 7), or even if the pressing force is changed with the joints of various steel types (No. 8 to No. 13), the mild steel plate joint (No. .27) and high strength steel plate joints (No. 30, No. 32, No. 34), the fatigue strength of the joints was significantly improved.

  Next, the fatigue strength was investigated using a high-strength steel plate joint having a thickness of 1.6 mm (No. 14 to No. 19), and in all cases, a significant improvement in fatigue strength was observed.

  Further, the fatigue strength was investigated using a mild steel plate joint (No. 20) having a thickness of 1.2 mm and various high-strength steel plate joints (No. 21 to No. 26). Compared with mild steel plate joints (No. 27) and high-strength steel plate joints (No. 28, No. 29, No. 31, No. 33, No. 35 to No. 36), the fatigue strength of the joints is It was remarkably improved.

  On the other hand, when pressurization was performed with the pressurizing force defined in claim 2, but the radius of curvature of the protrusion tip is outside the range defined in claim 2 (No. 37 to No. 38), or the height of the protrusion Is outside the range specified in claim 2 (No. 39 to No. 40), and when the distance between the protrusions is outside the range specified in claim 2 (No. 41 to No. 42), No improvement in fatigue strength was observed.

  Further, the radius of curvature at the tip of the protrusion, the height of the protrusion, and the distance between the protrusions are within the range defined in claim 2, but the pressure is lower than the value defined in claim 2 (No. 43, No. 4). 45, No. 47, No. 49), no improvement in fatigue strength was observed even when the plate thickness was 1.2 mm or 1.6 mm.

  Further, the radius of curvature at the tip of the punch is within the range defined in claim 2, but when the applied pressure is higher than the value defined in claim 2 (No. 44, No. 46, No. 48, No. 50). In the case of plate thickness: 1.2 mm and 1.6 mm, an improvement in fatigue strength is observed, but the dent on the surface of the welded portion is large, and the applied pressure is lower (No. 10, No. 13, No. 16, No. 19), the fatigue strength improvement rate was also low. Moreover, generation | occurrence | production of the partial crack was observed on the surface.

  Furthermore, even if various steel types were used or the thickness of the steel sheet was changed, the results were the same. Furthermore, the results were the same even when the plated steel sheet was used, and even when the experiment was carried out by changing the plating type and the basis weight.

ADVANTAGE OF THE INVENTION According to this invention, the fatigue strength of a welded joint can be improved, ensuring favorable welding workability | operativity in the spot welding of the high-strength steel plate used for the attachment of the components for motor vehicles, the assembly of a vehicle body, etc. Therefore, this makes it possible to fully enjoy the benefits of improving safety by applying high-strength steel sheets, reducing fuel costs by reducing weight, and reducing CO ₂ emissions in the automotive field and the like.

It is sectional drawing for demonstrating the fatigue test of a spot welded joint. It is sectional drawing for demonstrating spot welding. It is sectional drawing for demonstrating a spot weld part. It is sectional drawing for demonstrating the punch shape of this invention. It is sectional drawing for demonstrating the pressurization method by the punch of this invention. Punch pressure P and tensile strength TS of the steel plate when a spot welded joint (nugget diameter ND: 5.5 mm) of a high-strength steel plate (thickness t: 1.2 mm) is pressed, and fatigue strength of the welded joint It is a figure which shows the relationship with evaluation result ((circle), *) of this. Evaluation of punch pressure P and steel plate thickness t, and fatigue strength of welded joints when a spot welded joint (nugget diameter ND: 5.5 mm) of a high strength steel sheet (tensile strength TS: 780 MPa) is pressed It is a figure which shows the relationship with a result ((circle), x).

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 High-strength steel plate 2 Nugget 3 End part of nugget 4 Load direction in fatigue test 5 Welding electrode 6 Electrode pressure 7 Depression on weld surface 8 Nugget circumference 9 Weld area 10 Punch pressure

Claims (2)

In the fatigue strength improving method of a high strength steel plate spot welded joint with a tensile strength of 260 to 1570 MPa and a plate thickness of 0.6 to 3.3 mm, after spot welding, the surface temperature of the welded portion is reduced to 300 ° C. or lower, Using a cylindrical punch having a radius of curvature of 50 to 120 mm at the tip, the center of the cylindrical punch is aligned with the center of the welded portion, and the pressure P of the cylindrical punch is applied to the high strength steel plate from both sides of the steel plate above and below the welded portion. A method for improving the fatigue strength of a spot-welded joint of high-strength steel sheets, wherein pressurization is performed so as to satisfy the following formula (1) in relation to the tensile strength TS and the sheet thickness t.
0.0314 × t × TS ≦ P ≦ 0.1571 × t × TS (1)
However,
t: Thickness (mm) of high-strength steel plate
TS: Tensile strength (MPa) of high-strength steel sheet
P: Pressurizing force with cylindrical punch (kN) In the fatigue strength improving method of a high strength steel plate spot welded joint with a tensile strength of 260 to 1570 MPa and a plate thickness of 0.6 to 3.3 mm, after spot welding, the surface temperature of the welded portion is reduced to 300 ° C. or lower, The tip has a ring-shaped protrusion with a curvature radius of 2.0-7.0 mm and a protrusion height of 1.0-3.0 mm, and the diameter D of the ring-shaped protrusion is related to the nugget diameter ND. The projecting punch satisfying the following (2) is used, the center of the projecting punch is aligned with the center of the welded portion, and the pressure P of the projecting punch is applied to the tensile strength of the high-strength steel plate from both the upper and lower steel plates. A method for improving the fatigue strength of a high-strength steel plate spot welded joint, wherein pressure is applied so as to satisfy the following expression (3) in relation to the strength TS and the plate thickness t.
0.70 × ND ≦ D ≦ 1.30 × ND (2)
0.0157 × t × TS ≦ P ≦ 0.0786 × t × TS (3)
However,
ND: Nugget diameter (mm)
D: Ring-shaped protrusion diameter (mm)
t: Thickness (mm) of high-strength steel plate
TS: Tensile strength (MPa) of high-strength steel sheet
P: Pressurizing force with protrusion type punch (kN)

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