Manufacturing method for projection welding components

Abstract

To provide a manufacturing method for a projection weld member that can suppress low-temperature cracking.SOLUTION: In a manufacturing method for a projection weld member, which welds an AI-system plated hot stamp steel plate whose tensile strength is over 1.60 GPa and whose plate thickness is 1.8 mm or more and less than 2.3 mm to a member having a protrusion part, a power distribution time is set to 120 msec or more in a main power distribution step and a holding time is set to 280 msec or less in a holding step. In a manufacturing method for a projection weld member according to another embodiment of the invention, which welds an AI-system plated hot stamp steel plate whose tensile strength is over 1.60 GPa and whose plate thickness is 2.3 mm or more and less than 3.3 mm to a member having a protrusion part, a power distribution time is set to 120 msec or more in a main power distribution step, a power distribution time is set to 80 msec or more in a second power distribution step, and a holding time is set to 400 msec in a holding step, where a wireless power distribution time between the main power distribution step and the second power distribution step is set to 160 msec or less.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a projection-welded component. [Background technology]

[0002] In recent years, the use of hot-stamped steel has been increasing in the automotive technology field. For example, hot-stamped steel, which is hot-pressed from high-strength steel sheets, is used as the frame components of automobile bodies. For example, in automotive structural components such as front side members, center pillars, and hinge reinforcements, steel members are used in which nuts and bolts are welded to parts made of hot-stamped steel. In addition, hot-stamped steel may have an aluminum-based plating on its surface.

[0003] Projection welding, for example, is used when welding components such as nuts and bolts to steel plates. Projection welding of high-strength steel plates, such as steel plates with a tensile strength of 1.60 GPa or more, presents a problem of high cold cracking. Cold cracking is a general term for cracks that occur after welding when the temperature of the weld drops to around room temperature. Major causes of cold cracking include hydrogen introduced into the weld, as well as restraint stress and residual stress in the weld. High-strength steel plates are highly susceptible to hydrogen, and large residual stresses are generated when they are welded. In recent years, with the improvement in the strength of automotive steel materials, there has been an increasing need to suppress cold cracking in projection welding.

[0004] As an example of a projection welding method, Patent Document 1 describes a structural member for an automobile obtained by forming a pierce hole in a high-strength steel plate having a tensile strength of 1100 MPa or more before welding, and joining the high-strength steel plate and the weld nut or weld bolt by projection welding, in which pressure is applied and current heating is performed in a state where the center of the pierce hole and the center of the threaded portion of the weld nut or weld bolt are approximately aligned, and the weld nut or weld bolt has a flange portion on the underside that serves as a joining surface with the high-strength steel plate, and a substantially hemispherical projection portion is provided on the joining surface, and further, a longitudinal section of the flange portion The present disclosure discloses an automotive structural member having excellent delayed fracture properties and static strength properties at welded joints, characterized in that when C is the center of a chord of a semicircle formed by the intersection of the approximately hemispherical arc of the projection portion and the joint surface and R (mm) is the radius of the projection portion, a recess is provided within a range of a distance of 3R from the center C, the recess is locally provided on the upper surface of the flange portion opposite the joint surface so as to roughly coincide with a position corresponding to the projection portion, and the total volume of the recess is in the range of 0.7 to 1.3 times the total volume of the projection portion.

[0005] Patent Document 2 discloses an automotive structural component having a weld nut portion, which is obtained by forming a pierce hole in a high-strength steel plate having a tensile strength of 1100 MPa or more before welding, and joining the high-strength steel plate and the weld nut by projection welding in which pressure is applied to the high-strength steel plate and heating is performed while the center of the pierce hole is roughly aligned with the center of a threaded hole in a weld nut. The weld nut has a substantially hemispherical projection portion on the joint surface with the high-strength steel plate, and the relationship between the depth H1 in the plate thickness direction of a weld heat-affected zone revealed in the high-strength steel plate using a metal flow etchant and the plate thickness H2 of the high-strength steel plate satisfies the following formula: H1 / H2 = 0.05 to 0.5.

[0006] Patent Document 3 discloses a method for controlling the ratio of the area SJ of the joint between the nut and the high-strength steel plate to the area SR of the nominal diameter portion of the nut by the following formula {0.7≦SJ / SR≦1.5}, and by controlling the maximum Vickers hardness of the joint and heat-affected zone to 550 Hv or less, when projection welding a nut having a predetermined chemical composition with a high-strength steel plate having a tensile strength of 750 to 1600 MPa, a plate thickness of 0.8 to 3.0 mm, and a carbon equivalent Ceq in the range of 0.22 to 0.50%, immediately after performing a main current with an electrode pressure EF and a current time Wt, a post-current is performed with a post-current POC1 and a post-current time POt1, and then the electrode is held for a time Ht.

[0007] Patent Document 4 discloses a projection-welded joint formed by projection welding a nut (or bolt) having a predetermined chemical composition to a high-strength steel plate having a tensile strength of 750 to 1600 MPa, a plate thickness of 0.8 to 3.0 mm, and a carbon equivalent Ceq in the range of 0.22 to 0.50%, expressed by the following formula {[C] + [Si] / 30 + [Mn] / 20 + 2[P] + 4[S]}, in which the ratio of the area SJ of the joint between the nut (or bolt) and the high-strength steel plate to the area SR of the nominal diameter portion of the nut (or bolt) satisfies the relationship expressed by the following formula {0.7≦SJ / SR≦1.5}, and the maximum Vickers hardness of the joint and heat-affected zone is 550 Hv or less.

[0008] Patent Document 5 discloses a projection welding method that meets the following requirements: A first steel-based workpiece and a second steel-based workpiece having a large number of protrusions are prepared. At least one of the first workpiece and the second workpiece is pressed so that the protrusions of the second workpiece 2 are pressed against the plate-shaped portion of the first workpiece. While performing the pressure operation, a first current-carrying operation is performed in which current is applied under conditions of a predetermined welding current and current-carrying time, and then a second current-carrying operation is performed in which current is applied under conditions of a predetermined welding current and current-carrying time. The welding current of the first current-carrying operation is set smaller than the welding current of the second current-carrying operation, and the current-carrying time of the first current-carrying operation is set shorter than the current-carrying time of the second current-carrying operation.

[0009] However, none of these techniques have investigated the prevention of cold cracking in high-strength steel sheets with a tensile strength of over 1.60 GPa, nor have they provided any specific means for doing so. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Patent No. 5626025 [Patent Document 2] Patent No. 5613521 [Patent Document 3] Japanese Patent Application Laid-Open No. 2013-078784 [Patent Document 4] Japanese Patent Application Laid-Open No. 2012-157900 [Patent Document 5] Japanese Patent Application Laid-Open No. 2004-050280 Summary of the Invention [Problem to be solved by the invention]

[0011] In projection welding of steel members and high-strength steel plates, for example, with a tensile strength of more than 1.60 GPa, cold cracking occurs in the hard parts on the steel plate side of the weld due to the sudden contraction of the weld immediately after welding. When the heat input is large or the steel plate is thick, the strain generated in the weld becomes large, making cold cracking particularly likely to occur. Furthermore, hydrogen penetrates more easily into aluminum-based plated steel sheets than with other types of plating, and the hydrogen concentration in the steel sheets tends to be high, which can lead to variations and reductions in joint strength and delayed fracture.

[0012] An object of the present invention is to provide a manufacturing method capable of suppressing cold cracking when manufacturing a projection-welded component composed of an Al-based plated hot stamped steel sheet having a tensile strength of more than 1.60 GPa and a component such as a bolt or nut. [Means for solving the problem]

[0013] The gist of the present invention is as follows.

[0014] (1) A method for manufacturing a projection-welded component according to one aspect of the present invention is a method for manufacturing a projection-welded component, in which a steel plate having a tensile strength of more than 1.60 GPa, a plate thickness of 1.8 mm or more and less than 2.3 mm, and which is an Al-based plated hot stamp steel plate, is joined to a component having a protrusion by projection welding, the method comprising: a main current application step in which, with the protrusion of the component and the steel plate in contact with each other, current is applied to the component and the steel plate while pressure is applied to the component and the steel plate to weld the protrusion and the steel plate; and a holding step in which, after the main current application step, current application to the component and the steel plate is stopped and the pressure on the steel plate and the component is maintained, wherein the current application time in the main current application step is 120 msec or more and the holding time in the holding step is 280 msec or less. (2) In the method for producing a projection-welded component described in (1) above, when the sum of the tensile strength in GPa and the thickness in mm of the steel plate is defined as a parameter X, the current application time in the main current application step may be set to 26×X or more in msec, and the holding time in the holding step may be set to 1520 / X or less in msec. (3) The method for manufacturing a projection-welded component described in (1) or (2) above may include a second current application process, after the main current application process and before the holding process, in which current is applied to the component and the steel plate with a smaller heat input than in the main current application process while maintaining the pressure on the steel plate and the component, wherein the non-current application time between the main current application process and the second current application process is 160 msec or less, the current application time in the second current application process is 80 msec or more, and the upper limit of the holding time in the holding process may be 400 msec instead of 280 msec. (4) The method for manufacturing a projection-welded member described in (3) above may include a third current application process between the second current application process and the holding process, in which current is applied to the member and the steel plate with a heat input smaller than that of the second current application process while maintaining the pressure on the steel plate and the member, the non-current application time between the second current application process and the third current application process may be 160 msec or less, the current application time in the third current application process may be 80 msec or more, and the upper limit of the holding time in the holding process may be 600 msec instead of 400 msec.

[0015] (5) A method for producing a projection-welded component according to another aspect of the present invention is a method for producing a projection-welded component, in which a steel plate having a tensile strength of more than 1.60 GPa, a plate thickness of 2.3 mm or more and less than 3.3 mm, and an Al-based plated hot stamp steel plate, is joined to a component having a protrusion by projection welding, the method comprising the steps of: a main current-passing step in which, with the protrusion of the component and the steel plate in contact with each other, current is passed through the component and the steel plate while applying pressure to weld the protrusion and the steel plate; and a main current-passing step in which, after the main current-passing step, current is passed through the steel plate and the component. The method comprises a second current application process in which current is applied to the member and the steel plate with a smaller heat input than in the main current application process while maintaining the above-mentioned pressurization, and a holding process in which, after the second current application process, current application to the member and the steel plate is stopped and the pressurization of the steel plate and the member is maintained, wherein the current application time in the main current application process is 120 msec or more, the current application time in the second current application process is 80 msec or more, the holding time in the holding process is 400 msec or less, and the non-current application time between the main current application process and the second current application process is 160 msec or less. (6) The method for manufacturing a projection-welded component described in (5) above may include a third current application process between the second current application process and the holding process, in which current is applied to the component and the steel plate with a heat input smaller than that of the second current application process while maintaining the pressure on the steel plate and the component, the non-current application time between the second current application process and the third current application process may be 160 msec or less, the current application time in the third current application process may be 80 msec or more, and the upper limit of the holding time in the holding process may be 600 msec instead of 400 msec. (7) In the method for producing a projection-welded component according to (5) or (6), when the sum of the tensile strength in GPa and the thickness in mm of the steel plate is defined as a parameter X, the energization time in the main energization step may be set to be 26×X or more in msec.

[0016] (8) In the method for producing a projection-welded component according to any one of (1) to (7) above, the steel plate may contain, by mass%, C: 0.05 to 0.70%, Si: 2.00% or less, Mn: 0.05 to 5.00%, P: 0.100% or less, and S: 0.0100% or less, and the carbon equivalent Ceq of the steel plate, represented by the following formula (A), may be 0.20 mass% to 0.55 mass%. Ceq=[C]+[Si] / 30+[Mn] / 20+2[P]+4[S]…(A) Here, the element symbols included in the formula (A) indicate the content of the corresponding element in unit mass %. (9) In the method for manufacturing a projection-welded member described in any one of (1) to (8) above, the product of the welding current value in kA and the welding time in msec in the main current application step may be 3300 msec·kA or less. (10) In the method for manufacturing a projection-welded member according to any one of (3) to (7) above and (8) and (9) above which are dependent on any one of (3) to (7) above, the current value I1 in the main current flow process and the current value I2 in the second current flow process may satisfy the relationship expressed by the following formula (B). 0.2≦I2 / I1≦0.8…(B) (11) In the method for manufacturing a projection-welded member according to any one of (4) and (6) above, and (7) to (10) above that are dependent on (4) or (6) above, the current value I1 in the main current flow step and the current value I3 in the third current flow step may satisfy the relationship expressed by the following formula (C): 0.2≦I3 / I1≦0.8…(C) (12) In the method for producing a projection-welded member according to any one of (1) to (11) above, the holding time in the holding step may be 100 msec or less. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide a manufacturing method capable of suppressing cold cracking in the production of projection-welded components composed of an Al-based plated hot stamped steel sheet having a tensile strength of more than 1.60 GPa and components such as bolts and nuts. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a schematic diagram of a projection welding member. [Figure 2] 10 shows an example of a current profile and a pressure profile in the manufacturing method including the current application step and the holding step. [Figure 3] 10 shows an example of a current profile and a pressure profile in a manufacturing method including a main current-carrying step, a second current-carrying step, and a holding step. [Figure 4] 10 shows an example of a current profile and a pressure profile in a manufacturing method including a main current-carrying step, a second current-carrying step, a third current-carrying step, and a holding step. DETAILED DESCRIPTION OF THE INVENTION

[0019] The present inventors have found that it is possible to suppress cold cracking by optimizing the welding current duration and holding time in projection welding. An example of a method for manufacturing a projection-welded component according to the present invention will now be described in detail.

[0020] In a manufacturing method of a projection-welded member 1 according to a first aspect of the present invention, as illustrated in FIG. 1 , a steel sheet 11, which is an Al-plated hot-stamped steel sheet having a tensile strength greater than 1.60 GPa and a thickness of 1.8 mm or more but less than 2.3 mm, is joined by projection welding to a member 12 having a protrusion 121. Projection welding is resistance welding performed by contacting a protrusion formed at a welding point of a base material and passing an electric current therethrough, thereby limiting the generation of resistance heat to a relatively small, specific area. As illustrated in FIG. 2 , projection welding includes a main current application step S1-1 in which, while the protrusion 121 of the member 12 and the steel sheet 11 are in contact with each other, an electric current is applied while applying pressure to the member 12 and the steel sheet 11 to weld the protrusion 121 to the steel sheet 11, and a holding step S2 in which, after the main current application step S1-1, the application of current to the member 12 and the steel sheet 11 is stopped and the pressure between the steel sheet 11 and the member 12 is maintained. In this specification, maintaining the pressure in the holding step S2 is not limited to maintaining the pressure P2 in the holding step S2 at the same pressure P1-1 as at the end of the main current-carrying step S1-1, but may also vary the pressure P2 during the holding step S2. For example, the pressure P2 in the holding step S2 may be 0.8 to 1.2 times the pressure P1-1 at the end of the main current-carrying step S1-1.

[0021] The tensile strength of the steel plate 11 exceeds 1.60 GPa (1600 MPa). This allows the manufacturing method of the projection-welded component 1 according to this embodiment to be applied to mechanical parts that require high strength. Note that, although cold cracking is a problem when projection welding a steel plate 11 having a tensile strength of more than 1.60 GPa, the manufacturing method according to this embodiment can avoid cold cracking by optimizing the welding conditions, which will be described later.

[0022] The thickness of the steel plate 11 is 1.8 mm or more and less than 2.3 mm. By making the plate thickness 1.8 mm or more, the strength of the steel plate 11 is increased, and the manufacturing method of the projection-welded component 1 according to this embodiment can be applied to mechanical parts that require high strength. Furthermore, the load on the joint during cooling and shrinkage after welding is not so large if the plate thickness is thin, but becomes significant when the plate thickness is 1.8 mm or more, which is why the present invention is useful. On the other hand, the thicker the steel plate 11, the greater the heat transfer from the welded portion to the steel plate 11, which increases the distortion that occurs in the welded portion and ultimately the residual stress in the welded portion. Therefore, in the method for manufacturing a projection-welded component 1 according to the first embodiment, the thickness of the steel plate 11 is specified to be less than 2.3 mm. A projection welding method for steel plates 11 with a thickness of 2.3 mm or more will be described later.

[0023] The steel sheet 11 is an Al-based plated hot stamp steel sheet. The Al-based plated steel sheet is, for example, an Al-10% Si plated steel sheet.

[0024] The member 12 to be projection-welded to the steel plate 11 has a projection 121 for projection welding. This limits the resistance heat generated by the welding current to the projection 121 and its surroundings, enabling efficient resistance welding. The member 12 is, for example, a bolt or nut.

[0025] The configuration of the member 12 is not particularly limited as long as it has a shape suitable for projection welding. According to the results of the inventors' investigations, crack C occurred in the region of the weld on the steel plate 11 side, as shown schematically in Figure 1. It was determined that the strength and shape of the member 12 have little effect on cold cracking. Therefore, various configurations can be adopted for the member 12 depending on the application of the projection-welded member 1.

[0026] As illustrated in FIG. 2 , the method for manufacturing a projection-welded component according to the first embodiment includes a main current application step S1-1 and a holding step S2. The main current application step S1-1 is a step of applying a welding current while pressing the steel plate and the member against each other and applying pressure. The welding current refers to a current that is passed to form a weld. The post-heating current used to heat-treat the weld is not included in the concept of welding current. The holding step S2 is a step of maintaining the pressure on the steel plate and the member while the current value flowing through the steel plate and the member is substantially zero. Note that, depending on the capacity of the power source of the projection welding equipment, even if control is performed to reduce the current value to zero, it may take several cycles for the current value passing through the steel plate and the member to actually decrease to zero. In the method for manufacturing a projection-welded component according to this embodiment, a state in which the current value has decreased to a value close to zero is considered to be a state in which the current value is substantially zero.

[0027] The energization time in this energization step S1-1 is 120 msec or more (6 cycles or more when the power frequency of the welding equipment is 50 Hz). The energization time is the length of time during which the welding current is passed through the member and the steel plate. By setting the energization time to 120 msec or more, it is possible to reduce the temperature difference between the surface of the steel plate that contacts the member (hereinafter referred to as the "first surface") and the surface opposite the first surface (hereinafter referred to as the "second surface"). By reducing the temperature difference between the first surface and the second surface of the steel plate, it is possible to suppress cooling shrinkage of the weld after welding is completed and reduce residual stress in the weld. The energization time in this energization step S1-1 is preferably 130 msec or more, 140 msec or more, or 150 msec or more. As long as this requirement is met, the energization conditions in this energization step S1-1 can be appropriately selected depending on the shape, material, etc. of the steel plate and member to be welded. The energization time in the main energization step S1-1 is, for example, 140 to 280 msec.

[0028] The holding time in the holding step S2 is 280 msec or less (14 cycles or less when the power frequency of the welding equipment is 50 Hz). The holding time refers to the length of the period from the end of the welding current flow in the main current application step S1-1 to the start of opening the electrodes. If the holding time exceeds 280 msec, the welded portion will be overcooled, resulting in significant cooling shrinkage of the welded portion after welding is completed. On the other hand, by setting the holding time to 280 msec or less, the welded portion will be cooled gradually, and residual stress in the welded portion can be reduced. The holding time in the holding step S2 is preferably 260 msec or less, 240 msec or less, or 200 msec or less. The holding time in the holding step S2 is more preferably 100 msec or less, or 40 msec or less. Hydrogen tends to penetrate more easily into aluminum-based plated steel sheets than in other types of plated steel sheets, resulting in a higher hydrogen concentration in the steel sheets. This can lead to variations or a decrease in joint strength and the risk of delayed fracture, so it is preferable to further slow down the cooling rate of the weld.

[0029] From the viewpoint of avoiding cold cracking, a shorter holding time is preferable, and it may even be 0 msec. That is, the pressure may be set to 0 simultaneously with the end of the welding current flow. A method for manufacturing a projection-welded component in which the holding time is 0 msec is also considered to be a method for manufacturing a projection-welded component according to this embodiment. On the other hand, considering the capacity of the projection welding equipment, a longer holding time is preferable because it makes it easier to control the pressure. Therefore, the holding time may be 10 msec or more, 20 msec or more, or 40 msec or more.

[0030] The energization time in the main energization step S1-1 and the holding time in the holding step S2 may be further limited depending on the tensile strength and thickness of the steel sheet. For example, when the sum of the tensile strength value in GPa and the sheet thickness value in mm is defined as a parameter X, the energization time may be set to 26×X or more in msec, and the holding time may be set to 1520 / X or less in msec.

[0031] As described above, the greater the tensile strength of the steel plate, the greater the residual stress in the weld, and the greater the thickness of the steel plate, the greater the residual stress in the weld. Therefore, the inventors have adopted parameter X, which is the sum of the tensile strength and thickness of the steel plate, as a simple index of the residual stress in the weld. When the above formula is followed, the larger the parameter X, the greater the lower limit of the current flow time and the smaller the upper limit of the holding time. Therefore, when the above formula is followed, the residual stress in the weld is further alleviated. The current flow time is more preferably 28×X or more, or 42×X or more, in msec. The holding time is more preferably 1440 / X or less, in msec.

[0032] As illustrated in Fig. 3, the method for producing a projection-welded member according to the first embodiment may include a second current-flow step S1-2 after the main current-flow step S1-1 and before the holding step S2, in which current is applied to the member and the steel sheet with a lower heat input than in the main current-flow step S1-1 while maintaining pressure on the steel sheet and the member. By providing the second current-flow step S1-2, the cooling rate of the weld can be slowed, further suppressing cold cracking. Furthermore, as will be described later, by providing the second current-flow step S1-2, the upper limit of the holding time can be extended. Here, maintaining the applied pressure in the second current-flow process S1-2 does not necessarily mean maintaining the applied pressure P1-2 in the second current-flow process S1-2 at the same pressure P1-1 as when the main current-flow process S1-1 ended, but rather the applied pressure P1-2 may vary during the second current-flow process S1-2. For example, the applied pressure P1-2 in the second current-flow process S1-2 may be 0.8 to 1.2 times the applied pressure P1-1 when the main current-flow process S1-1 ended. In this specification, heat input refers to the time integral of current, and under constant current conditions, "heat input = current × time." In the second current application process S1-2, applying a smaller heat input than in the main current application process S1-1 means that under constant current conditions, "I1 × t1 > I2 × t2." Here, I1 and t1 are the current value and current application time in the main current application process S1-1, and I2 and t2 are the current value and current application time in the second current application process S1-2. When the current value changes during current flow due to upslope or downslope current, the heat input is expressed as the area of ​​a triangle (or the sum of the areas of a triangle and a rectangle) in a current profile where the horizontal axis represents time and the vertical axis represents current. When the current value changes stepwise, the heat input is expressed as the sum of the areas of multiple rectangles in a current profile where the horizontal axis represents time and the vertical axis represents current.

[0033] When the manufacturing method for projection-welded components includes the second current-carrying step S1-2, it is preferable that the no-current time (so-called cool time) between the main current-carrying step S1-1 and the second current-carrying step S1-2 is 160 msec or less. The no-current time is the length of the period during which the current value is substantially zero. By setting the no-current time to 160 msec or less, the welded portion is cooled gradually, and the residual stress in the welded portion can be further reduced. The shorter the no-current time, the better. Therefore, the lower limit of the no-current time is 0 msec.

[0034] Furthermore, when the manufacturing method for projection-welded components includes the second current-flow step S1-2, the current-flow time in the second current-flow step S1-2 is preferably 80 msec or longer. The longer the current-flow time in the second current-flow step S1-2, the more effectively the welded joint can be suppressed from cooling shrinkage and the more effectively the residual stress in the welded joint can be reduced.

[0035] If the manufacturing method for projection-welded components includes the second current-flow step S1-2 in accordance with the above-mentioned specifications for the no-current time and current-flow time, the holding time may be 400 msec or less. As mentioned above, if the second current-flow step is not included, a holding time of more than 280 msec may cause cold cracking due to overcooling. However, if the second current-flow step is performed according to specified conditions, the temperature of the weld will have dropped by the time the holding step is started, reducing the risk of cold cracking due to overcooling. Therefore, if the second current-flow step is performed according to specified conditions, the upper limit of the holding time can be extended to 400 msec instead of the above-mentioned 280 msec.

[0036] As illustrated in Fig. 4, the method for producing a projection-welded member according to the first embodiment may include a third current-flow step S1-3 between the second current-flow step S1-2 and the holding step S2, in which current is applied to the member and the steel sheet with a smaller heat input than in the second current-flow step while maintaining pressure on the steel sheet and the member. By providing the third current-flow step S1-3 in addition to the second current-flow step S1-2, the cooling rate of the weld can be further slowed, further suppressing cold cracking. Furthermore, as will be described later, by providing the third current-flow step S1-3, the upper limit of the holding time can be further extended. Here, maintaining the applied pressure in the third current-flow process S1-3 does not necessarily mean maintaining the applied pressure P1-3 in the third current-flow process S1-3 at the same pressure P1-2 as at the end of the second current-flow process S1-2, but rather may vary the applied pressure P1-3 during the third current-flow process S1-3.Furthermore, for example, the applied pressure P1-3 in the third current-flow process S1-3 may be set to 0.8 to 1.2 times the applied pressure P1-2 at the end of the second current-flow process S1-2. Furthermore, in the third current application process S1-3, energizing with a smaller heat input than in the second current application process S1-2 means that, under constant current conditions, "I2 x t2 > I3 x t3." Here, I2 and t2 are the current value and energization time of the second current application process S1-2, and I3 and t3 are the current value and energization time of the third current application process S1-3. The definition of the heat input and how to handle changes in the current value during energization are as described above.

[0037] When the manufacturing method for projection-welded components includes a third current-carrying step S1-3, it is preferable that the no-current time between the second current-carrying step S1-2 and the third current-carrying step S1-3 be 160 msec or less. The no-current time is the length of the period during which the current value is substantially zero. By setting the no-current time to 160 msec or less, the welded portion is cooled slowly, and the residual stress in the welded portion can be further reduced. The shorter the no-current time, the better. Therefore, the lower limit of the no-current time is 0 msec.

[0038] Furthermore, when the manufacturing method for projection-welded components includes a third current-flow step S1-3, the current-flow time in the third current-flow step S1-3 is preferably 80 msec or longer. The longer the current-flow time in the third current-flow step S1-3, the more effectively the welded joint can be suppressed from cooling shrinkage and the more effectively the residual stress in the welded joint can be reduced.

[0039] When the manufacturing method for projection-welded components includes the third current-flow step S1-3 according to the above-mentioned regulations, the holding time may be 600 msec or less. When the third current-flow step S1-3 according to the specified conditions is performed, the temperature of the weld is further reduced when the holding step is started, further reducing the risk of cold cracking due to overcooling. Therefore, when the third current-flow step S1-3 according to the specified conditions is performed, the upper limit of the holding time can be extended to 600 msec instead of the above-mentioned 400 msec.

[0040] Next, a method for manufacturing a projection-welded member according to a second aspect of the present invention will be described. The method for manufacturing a projection-welded member according to the second embodiment is intended to weld a thicker steel plate than that of the first embodiment. Specifically, the method for manufacturing a projection-welded member according to the second embodiment involves projection welding a steel plate that is an Al-plated hot-stamped steel plate with a tensile strength of more than 1.60 GPa and a thickness of 2.3 mm or more but less than 3.3 mm, to a member having a protrusion. As shown in FIG. 3 , this projection welding process includes a main current-passing step in which, with the protrusion of the member and the steel plate in contact with each other, current is passed through the steel plate while applying pressure to the member and the steel plate to weld the protrusion and the steel plate; a second current-passing step in which current is passed through the member and the steel plate with a smaller heat input than in the main current-passing step while maintaining the pressure on the steel plate and the member; and a holding step in which, after the second current-passing step, current is stopped and the pressure on the steel plate and the member is maintained.

[0041] The tensile strength of the steel plate is set to be more than 1.60 GPa, as in the first embodiment. Meanwhile, the thickness of the steel plate is set to be 2.3 mm or more and less than 3.3 mm. By setting the plate thickness to 2.3 mm or more, the strength of the steel plate can be further increased. Furthermore, the load on the joint during cooling contraction after welding becomes particularly significant when the plate thickness is 2.3 mm or more, and therefore the present invention is useful. On the other hand, in order to suppress residual stress in the welded portion, the thickness of the steel sheet is specified to be less than 3.3 mm. As long as the tensile strength and thickness are within the above-mentioned ranges and the steel sheet is an Al-based plated hot stamp steel sheet, other configurations of the steel sheet are not particularly limited. The various configurations exemplified in the first embodiment can also be applied to the steel sheet in the second embodiment. The member to be projection-welded to the steel sheet can also be the same as that in the first embodiment.

[0042] As illustrated in FIG. 3 , the method for manufacturing a projection-welded member according to the second embodiment includes a main current-flow step S1-1, a second current-flow step S1-2, and a holding step S2. The main current-flow step S1-1 is a step in which, with the protrusion of the member and the steel sheet in contact with each other, current is applied while pressure is applied to the member and the steel sheet, thereby welding the protrusion and the steel sheet. The second current-flow step S1-2 is a step in which, after the main current-flow step S1-1, current is applied to the member and the steel sheet with a lower heat input than in the main current-flow step while maintaining the pressure on the steel sheet and the member. The holding step S2 is a step in which, after the second current-flow step S1-2, current is stopped and the pressure on the steel sheet and the member is maintained. Note that, depending on the capacity of the power source of the projection welding equipment, even if control is performed to reduce the current value to zero, it may take several cycles for the current value passing through the steel sheet and the member to actually decrease to zero. In the method for producing a projection-welded member according to this embodiment, the state in which the current value has decreased to a value close to 0 is considered to be a state in which the current value is substantially 0.

[0043] The main current application step S1-1 in the second embodiment is the same as that in the first embodiment. That is, the current application time in the main current application step S1-1 is 120 msec or more. The current application time in the main current application step S1-1 is preferably 130 msec or more, 140 msec or more, or 150 msec or more. The current value in the main current application step S1-1 is not particularly limited, and can be appropriately selected depending on the shape and application of the projection-welded workpiece. On the other hand, the product of the welding current value in kA and the current application time in msec in the main current application step S1-1 may be specified to be 3300 msec·kA or less.

[0044] The energization time in the second energization step S1-2 is set to 80 msec or more. The longer the energization time in the second energization step S1-2, the more the cooling shrinkage of the welded portion can be suppressed and the more the residual stress in the welded portion can be reduced.

[0045] The no-energization time between the main current application step S1-1 and the second current application step S1-2 is 160 msec or less. The no-energization time is the length of the period during which the current value is substantially zero. By setting the no-energization time to 160 msec or less, the welded portion is cooled slowly, and the residual stress in the welded portion can be further reduced.

[0046] The holding time in the holding step S2 is 400 msec or less. The holding time refers to the length of time during which the current value flowing through the steel plate and the member is substantially zero and the pressure applied to the steel plate and the member is greater than zero. If the holding time exceeds 400 msec, the welded portion will be overcooled, resulting in significant cooling shrinkage of the welded portion after welding is completed. On the other hand, by setting the holding time to 400 msec or less, the welded portion will be cooled gradually, and the residual stress in the welded portion can be reduced. The holding time in the holding step S2 is preferably 360 msec or less, 300 msec or less, or 200 msec or less.

[0047] 4, the method for producing a projection-welded member according to the second embodiment may include a third current-flow step S1-3 between the second current-flow step S1-2 and the holding step S2, in which current is applied to the member and the steel sheet with a lower heat input than in the second current-flow step S1-2 while maintaining pressure on the steel sheet and the member. By providing the third current-flow step S1-3, the cooling rate of the weld can be further slowed, further suppressing cold cracking. Furthermore, as described below, providing the third current-flow step S1-3 can extend the upper limit of the holding time.

[0048] When the manufacturing method for projection-welded components includes the third current-flow step S1-3, it is preferable to set the no-current time between the second current-flow step S1-2 and the third current-flow step S1-3 to 160 msec or less. By setting the no-current time to 160 msec or less, the welded portion is cooled slowly, and the residual stress in the welded portion can be further reduced.

[0049] Furthermore, when the manufacturing method for projection-welded components includes a third current-flow step S1-3, the current-flow time in the third current-flow step S1-3 is preferably 80 msec or longer. The longer the current-flow time in the third current-flow step S1-3, the more effectively the welded joint can be suppressed from cooling shrinkage and the more effectively the residual stress in the welded joint can be reduced.

[0050] When the manufacturing method for projection-welded components includes the third current-flow step S1-3 in accordance with the above-mentioned specifications for the no-energization time and current-flow time, the holding time may be 600 msec or less. When the third current-flow step S1-3 is performed in accordance with the specified conditions, the temperature of the weld is further reduced when the holding step is started, further reducing the risk of cold cracking due to overcooling. Therefore, when the third current-flow step S1-3 in accordance with the specified conditions is performed, the upper limit of the holding time can be extended to 600 msec instead of the above-mentioned 400 msec.

[0051] The energization time and holding time in this energization step S1-1 may be further limited depending on the tensile strength and thickness of the steel sheet. For example, when the sum of the tensile strength in GPa and the sheet thickness in mm is defined as a parameter X, the energization time may be set to 26×X or more in msec.

[0052] As described above, the greater the tensile strength of the steel plate, the greater the residual stress in the weld, and the greater the thickness of the steel plate, the greater the residual stress in the weld. Therefore, the inventors have adopted parameter X, which is the sum of the tensile strength and thickness of the steel plate, as a simple index of the residual stress in the weld. When the above formula is followed, the larger the parameter X, the greater the lower limit of the current flow time and the smaller the upper limit of the holding time. Therefore, when the above formula is followed, the residual stress in the weld is further alleviated.

[0053] The first embodiment relating to an Al-based plated hot stamped steel sheet having a thickness of 1.8 mm or more and less than 2.3 mm, and the second embodiment relating to an Al-based plated hot stamped steel sheet having a thickness of 2.3 mm or more and less than 3.3 mm have been described above. Preferred configurations applicable to both of these embodiments will be further described below.

[0054] An example of a suitable chemical composition of the steel sheet is, in mass%, C: 0.05 to 0.70%, Si: 200% or less, Mn: 0.05 to 5.00%, P: 0.100% or less, and S: 0.0100% or less, and the carbon equivalent Ceq of the steel sheet, represented by the following formula (A), is 0.20 mass% to 0.55 mass%, in which case the remainder of the chemical composition includes Fe and impurities. Ceq=[C]+[Si] / 30+[Mn] / 20+2[P]+4[S]…(A) Here, the element symbols included in formula (A) represent the content of the corresponding element in unit mass %. Generally, increasing the content of C and alloying elements can increase the strength of the base material, but at the same time, Ceq increases, reducing the toughness of the weld and becoming one of the causes of cold cracking. Steel plates having such compositions are used as materials for mechanical parts that require high strength, such as automobile parts, and the present invention is particularly useful for these. Furthermore, the manufacturing method for projection-welded components according to this embodiment can suppress cold cracking in components obtained from steel plates having such compositions.

[0055] As described above, the current conditions in the main current application step S1-1, the second current application step S1-2, and the third current application step S1-3 are not particularly limited except for the current application time, and can be appropriately selected depending on the shape and application of the projection-welded workpiece. Meanwhile, the current value in each step may be determined as described below.

[0056] The product of the welding current value in kA and the welding time in msec in the main welding step S1-1 may be specified as 3300 msec·kA or less. The product of the welding current value and the welding time is an indicator of the heat input in the main welding step S1-1. The smaller the heat input in the main welding step S1-1, the smaller the temperature difference between the first and second sides of the steel plate can be. This further suppresses cooling shrinkage of the weld after welding is completed, and further reduces residual stress in the weld.

[0057] The current values ​​in the second current-flow process S1-2 and the third current-flow process S1-3 may be values ​​corresponding to the current value in the main current-flow process S1-1. Specifically, the current value I1 in the main current-flow process S1-1 and the current value I2 in the second current-flow process S1-2 may satisfy the relationship expressed by the following formula (B). Furthermore, the current value I1 in the main current-flow process S1-1 and the current value I3 in the third current-flow process S1-3 may satisfy the relationship expressed by the following formula (C). 0.2≦I2 / I1≦0.8…(B) 0.2≦I3 / I1≦0.8…(C)

[0058] When I2 / I1 is set to 0.2 or more and 0.8 or less, or when I3 / I1 is set to 0.2 or more and 0.8 or less, the second current application process S1-2 or the third current application process S1-3 can reliably exert the effect of slowing down the cooling rate of the weld.

[0059] The pressure applied in projection welding is not particularly limited. Experimental results by the inventors did not confirm the effect of pressure applied on the incidence of cold cracking. This is thought to be because the main cause of cold cracking is strain introduced during cooling of the weld, and pressure applied does not affect this. Therefore, the pressure applied in ordinary projection welding (e.g., 3 to 6 kN) may be selected appropriately.

[0060] The holding time in the holding step S2 is preferably 100 msec or less, and more preferably 40 msec or less. Hydrogen tends to penetrate more easily into aluminum-based plated steel sheets than other types of plated steel sheets, resulting in a higher hydrogen concentration in the steel sheets. This can lead to variations or reductions in joining strength and the risk of delayed fracture, so it is preferable to further slow down the cooling rate of the weld. [Example]

[0061] The effects of one embodiment of the present invention will be explained in more detail using examples. However, the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to these examples. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

[0062] Example 1 Various projection-welded components were manufactured under the conditions described below. Tensile strength, thickness, and Ceq of steel plate: See the "TS" column, "Thickness" column, and "Ceq" column in Table 1 Shape of component: Nut with three projections for projection welding - Energization time in this energization process: See the "Energization time" column in Table 1 Current value I1 in this current application process: See the "Current value" column in Table 1 Pressure applied during this current application process: 5.0 kN Holding time in the holding step: See the "Holding time" column in Table 1 Presence or absence of the second energization process and the third energization process: as stated in the "Post-energization one-stage" and "Post-energization two-stage" columns of Table 1 (in cases where the symbol "0" is added to the "Post-energization one-stage" column, only the second energization process is performed, and in cases where the symbol "0" is added to the "Post-energization two-stage" column, both the second energization process and the third energization process are performed). - De-energization time between the main energization process and the second energization process: 80 msec - Second energization process time: 120 msec Current value I2 in the second current application process: 6kA - De-energization time between the second and third energization steps: 80 msec - Current application time in the third current application process: 120 msec Current value I3 in the third energization process: 6kA

[0063] The number of cracks in the projection-welded components was also investigated. Three test pieces were produced by projection welding under each of the conditions shown in Table 1. Each test piece had three welds, resulting in a total of nine welds. The presence or absence of cracks in these welds was confirmed. The number of cracks was recorded in the "As-welded" column of Table 1. Immediately after welding, the projection-welded joints were immersed in hydrochloric acid of the same pH for a specified period of time. Hydrochloric acid allows hydrogen to penetrate the joints, promoting delayed fracture. After immersion, the three joints of the projection-welded joints were cut, polished, and examined at 200x magnification to determine whether cracks were present. The number of cracks was recorded in the "Hydrochloric Acid Immersion" column of Table 1. [Table 1]

[0064] In Comparative Examples 1 and 4, the steel plate had a thickness of 2.3 mm or more, and the second current-carrying step and the third current-carrying step were not performed, so cracking could not be suppressed. In Comparative Example 2, the tensile strength of the steel sheet was not more than 1.60 GPa, so there was no problem of cold cracking specific to high-strength steel sheets, and the steel sheet is outside the scope of the present invention. Although no cracks occurred in Comparative Example 2, the tensile strength of the steel sheet was insufficient, so it cannot be used for mechanical parts that require a tensile strength of more than 1.60 GPa. In Comparative Example 3, the steel plate had a thickness of less than 2.3 mm and the holding time was too long, so that cracking after immersion in hydrochloric acid could not be suppressed.

[0065] In Example 1, the steel plate had a thickness of less than 2.3 mm and the current application time and holding time were appropriate, so cracking could be suppressed. In Example 2, the steel plate had a thickness of less than 2.3 mm and the holding time was more appropriate than in Example 1, so the number of cracks was reduced compared to Example 1. In Example 3, the steel plate had a thickness of less than 2.3 mm and the holding time was more appropriate than in Example 2, so the number of cracks was reduced compared to Example 2. Inventive Example 4, the plate thickness was smaller than that of Inventive Example 1, and the current application time and holding time were appropriate, so cracking could be suppressed. In Example 5, the steel plate had a thickness of 2.3 mm or more, and the second current application step was applied, so cracking could be suppressed. In Example 6, the steel plate had a thickness of 2.3 mm or more and the third current application step was applied, so the number of cracks was reduced compared to Example 5. In Example 7, the steel plate had a thickness of 2.3 mm or more, and the second current application step was applied, so cracking could be suppressed. In Example 8, the steel plate had a thickness of 2.3 mm or more, and the third current application step was applied, so cracking could be suppressed. In Example 9, the steel plate had a thickness of less than 2.3 mm and the second current application step was applied, so cracking could be suppressed. In Example 10, the steel plate had a thickness of less than 2.3 mm and the third current application step was applied, so cracking could be suppressed. [Explanation of symbols]

[0066] 1 Projection welding components 11 Steel Plate 12 parts 121 Protrusion S1-1 This power supply project S1-2 Second Power Supply Project S1-3 Third Power Supply Project S2 Retention Engineering C cut

Claims

1. A method for manufacturing a projection-welded member, comprising joining a steel sheet having a tensile strength of more than 1.60 GPa, a sheet thickness of 1.8 mm or more and less than 2.3 mm, and being an Al-based plated hot stamp steel sheet, to a member having a projection by projection welding, a main current application process in which, with the protrusion of the member and the steel plate in contact with each other, a current is applied while applying pressure to the member and the steel plate to weld the protrusion and the steel plate; a holding step of holding the pressurization of the steel plate and the member in a state where the current application step is stopped after the main current application step, The energization time in the main energization step is 120 msec or more, The holding time in the holding step is 280 msec or less. A method for manufacturing projection welded components.

2. When the value obtained by adding the tensile strength value of the steel plate in GPa and the plate thickness value in mm is defined as a parameter X, The energization time in the main energization step is set to 26×X or more in msec, The holding time in the holding step is 1520 / X or less in msec.

2. The method for manufacturing projection-welded components according to claim 1.

3. After the main current application process and before the holding process, a second current application process is further included in which, while maintaining the pressure on the steel plate and the member, a current is applied to the member and the steel plate with a heat input smaller than that of the main current application process, The non-energizing time between the main energizing step and the second energizing step is set to 160 msec or less, The energization time in the second energization step is 80 msec or more, The upper limit of the holding time in the holding step is changed from 280 msec to 400 msec.

3. The method for manufacturing projection-welded components according to claim 1 or 2.

4. a third current application process between the second current application process and the holding process, in which current is applied to the member and the steel plate with a heat input smaller than that of the second current application process while maintaining the pressurization of the steel plate and the member, The non-energizing time between the second energizing step and the third energizing step is set to 160 msec or less, The energization time in the third energization step is 80 msec or more, The upper limit of the holding time in the holding step is set to 600 msec instead of 400 msec.

4. The method for manufacturing projection-welded components according to claim 3.

5. A method for manufacturing a projection-welded member, comprising joining a steel sheet having a tensile strength of more than 1.60 GPa, a sheet thickness of 2.3 mm or more and less than 3.3 mm, and being an Al-based plated hot stamp steel sheet, to a member having a projection by projection welding, a main current application process in which, with the protrusion of the member and the steel plate in contact with each other, a current is applied while applying pressure to the member and the steel plate to weld the protrusion and the steel plate; After the main current application process, a second current application process is performed in which current is applied to the member and the steel plate with a heat input smaller than that of the main current application process while maintaining the pressure on the steel plate and the member. a holding step of holding the pressurization of the steel plate and the member in a state where the current supply to the member and the steel plate is stopped after the second current supply step, The energization time in the main energization step is 120 msec or more, The energization time in the second energization step is 80 msec or more, The holding time in the holding step is 400 msec or less, The non-energizing time between the main energizing step and the second energizing step is set to 160 msec or less. A method for manufacturing projection welded components.

6. a third current application process between the second current application process and the holding process, in which current is applied to the member and the steel plate with a heat input smaller than that of the second current application process while maintaining the pressurization of the steel plate and the member, The non-energizing time between the second energizing step and the third energizing step is set to 160 msec or less, The energization time in the third energization step is 80 msec or more, The upper limit of the holding time in the holding step is set to 600 msec instead of 400 msec.

6. The method for manufacturing projection-welded components according to claim 5.

7. When the value obtained by adding the tensile strength value in GPa and the thickness value in mm of the steel plate is defined as a parameter X, The energization time in the main energization step is set to 26×X or more in msec.

7. The method for manufacturing projection-welded components according to claim 5 or 6.

8. The steel plate contains, in mass%, C: 0.05 to 0.70%, Si: 2.00% or less, Mn: 0.05 to 5.00%, P: 0.100% or less, and S: 0.0100% or less, The carbon equivalent Ceq of the steel plate, represented by the following formula (A), is 0.20 mass % to 0.55 mass %.

8. The method for manufacturing a projection-welded member according to claim 1. Ceq=[C]+[Si] / 30+[Mn] / 20+2[P]+4[S]...(A) Here, the element symbols included in the formula (A) indicate the content of the corresponding element in unit mass %.

9. A method for manufacturing a projection-welded member according to any one of claims 1 to 8, characterized in that the product of the welding current value in kA and the welding time in msec in the main current application process is 3300 msec·kA or less.

10. The method for manufacturing a projection-welded member according to any one of claims 3 to 7, and claims 8 and 9 dependent on any one of claims 3 to 7, wherein the current value I1 in the main current flow step and the current value I2 in the second current flow step satisfy the relationship represented by the following formula (B): 0.2≦I2 / I1≦0.8…(B)

11. The method for manufacturing a projection-welded member according to any one of claims 4 and 6, and claims 7 to 10 dependent on claim 4 or 6, wherein the current value I1 in the main current flow step and the current value I3 in the third current flow step satisfy the relationship represented by the following formula (C): 0.2≦I3 / I1≦0.8…(C)

12. The method for producing a projection-welded member according to any one of claims 1 to 11, wherein the holding time in the holding step is 100 msec or less.

Images