KR102490575B1 - Spot welding method of hot stamping steel sheet - Google Patents

Abstract

According to one aspect of the present invention, the step of arranging at least some of the hot stamping steel sheets to overlap each other and providing a pressing force, applying a first welding current to a welded portion where the hot stamping steel sheets overlap each other for a first conduction time, pre-pulse Step, a second welding current smaller than the first welding current is applied to the welding portion for a second conduction time to form a nugget, the impulse step being repeated two or more times, and a third welding current larger than the second welding current to the welding portion. Provided is a method of spot welding a hot stamping steel sheet, including a main pulse step of growing the nugget by applying a welding current for a third conduction time.

Description

Spot welding method of hot stamping steel sheet}

Embodiments of the present invention relate to a method of spot welding a hot stamped steel sheet, and more particularly, to a method of spot welding a hot stamped steel sheet capable of securing a nugget diameter of a sufficient size and preventing the occurrence of spatters.

Hot stamping steel sheets are applied to automotive parts as high-strength steel sheets with excellent formability. In addition, automobile bodies and the like can be assembled by joining hot-stamped steel sheets using a spot welding method.

The spot welding method is a method of welding by arranging materials to be welded so as to overlap each other and instantaneously flowing a large current between electrodes in a state of pressing with electrodes. When a current is applied to the electrode, resistance heat is generated between the contact surface between the electrode and the material to be welded and the contact surface between the materials to be welded. At this time, the heat between the electrode and the material to be welded is cooled by cooling water inside the electrode, and the contact surface between the materials to be welded may reach a molten state. Accordingly, the materials to be welded may be bonded to each other while being melted at the interface to form a nugget.

However, in the conventional spot welding method of hot stamping steel sheet, there is a problem in that a nugget diameter of a sufficient size cannot be secured or spatter is generated, resulting in deterioration in weldability.

Embodiments of the present invention are intended to solve various problems, including the above problems, to provide a method for spot welding a hot stamping steel sheet that can prevent spatter while securing a nugget diameter of a sufficient size. . However, these tasks are illustrative, and the scope of the present invention is not limited thereby.

According to one aspect of the present invention, the step of arranging at least some of the hot stamped steel sheets to overlap each other and providing a pressing force, applying a first welding current to a welded portion where the hot stamped steel sheets overlap each other for a first conduction time, a pre-pulse Step, a second welding current smaller than the first welding current is applied to the welding portion for a second conduction time to form a nugget, the impulse step being repeated two or more times, and a third welding current larger than the second welding current to the welding portion. There is provided a method of spot welding a hot stamping steel sheet, including a main pulse step of growing the nugget by applying a welding current for a third conduction time.

According to this embodiment, the first welding current may be greater than or equal to 6.0 kA and less than or equal to 7.3 kA, the second welding current may be greater than or equal to 5.5 kA and less than or equal to 6.5 kA, and the third welding current may be greater than or equal to 6.0 kA and less than or equal to 7.5 kA.

According to this embodiment, the second welding current may be 80 to 90% of the first welding current or 80 to 90% of the third welding current.

According to this embodiment, the first energization time may be shorter than the second energization time, and the second energization time may be shorter than the third energization time.

According to this embodiment, the first conduction time may be 30 ms or more and 50 ms or less, the second conduction time may be 100 ms or more and 160 ms or less, and the third conduction time may be 150 ms or more and 200 ms or less.

According to this embodiment, assuming that the thickness of each of the hot stamping steel sheets is t, the nugget may grow to have a diameter greater than or equal to 4√t in the main pulse step.

According to this embodiment, in the main pulse step, the nugget may grow to satisfy a diameter of 5√t or more.

According to this embodiment, the step of cooling the welded portion for a preset cooling time period, wherein the step of cooling the welded portion for a preset cooling time period, between the pre-pulse step and the impulse step, the impulse step It may be performed between and between the impulse step and the main pulse step, respectively.

According to this embodiment, the preset cooling time may be 20 ms or more and 40 ms or less.

According to the present embodiment, after the third welding current is applied to the welding region, the method may further include maintaining the hot stamping steel sheets for a predetermined holding time while maintaining the pressing force.

According to this embodiment, the preset holding time may be greater than or equal to 40 ms and less than or equal to 100 ms.

According to this embodiment, the hot stamping steel sheets may include three hot stamping steel sheets.

Other aspects, features, and advantages other than those described above will become clear from the detailed description, claims, and drawings for carrying out the invention below.

According to one embodiment of the present invention made as described above, it is possible to implement a method of spot welding a hot stamping steel sheet capable of securing a nugget diameter of a sufficient size and at the same time preventing the occurrence of spatter. Of course, the scope of the present invention is not limited by these effects.

1 is a flowchart schematically illustrating a part of a method for spot welding a hot stamped steel sheet according to an embodiment of the present invention.
2A to 2C are cross-sectional views schematically showing portions of hot stamping steel sheets disposed in a welding apparatus according to an embodiment of the present invention.
3 is a schematic diagram illustrating a part of a method of spot welding a hot stamping steel sheet according to an embodiment of the present invention based on time and current.
4 is an image showing a part of a nugget formed by a method of spot welding hot stamping steel sheets according to an embodiment of the present invention.
5A and 5B are graphs showing a relationship between a welding current and a nugget diameter for Examples and Comparative Examples, respectively.
6A and 6B are graphs showing the relationship between welding current and tensile shear strength for Examples and Comparative Examples, respectively.

Since the present invention can apply various transformations and have various embodiments, specific embodiments will be illustrated in the drawings and described in detail in the detailed description. Effects and features of the present invention, and methods for achieving them will become clear with reference to the embodiments described later in detail together with the drawings. However, the present invention is not limited to the embodiments disclosed below and may be implemented in various forms.

In the following embodiments, terms such as first and second are used for the purpose of distinguishing one component from another component without limiting meaning.

In the following examples, expressions in the singular number include plural expressions unless the context clearly dictates otherwise.

In the following embodiments, terms such as include or have mean that features or elements described in the specification exist, and do not preclude the possibility that one or more other features or elements may be added.

In the drawings, the size of components may be exaggerated or reduced for convenience of description. For example, since the size and thickness of each component shown in the drawings are arbitrarily shown for convenience of description, the present invention is not necessarily limited to the illustrated bar.

When an embodiment is otherwise implementable, a specific process sequence may be performed differently from the described sequence. For example, two processes described in succession may be performed substantially simultaneously, or may be performed in an order reverse to the order described.

In this specification, "A and/or B" represents the case of A, B, or A and B. And, "at least one of A and B" represents the case of A, B, or A and B.

The x-axis, y-axis, and z-axis are not limited to the three axes of the Cartesian coordinate system, and may be interpreted in a broad sense including them. For example, the x-axis, y-axis, and z-axis may be orthogonal to each other, but may refer to different directions that are not orthogonal to each other.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings, and when describing with reference to the drawings, the same or corresponding components are assigned the same reference numerals, and overlapping descriptions thereof will be omitted. .

1 is a flowchart schematically illustrating a part of a method for spot welding a hot stamped steel sheet according to an embodiment of the present invention, and FIGS. 2A to 2C are hot stamped steel sheets arranged in a welding apparatus according to an embodiment of the present invention. These are cross-sectional views schematically showing a part of the appearance.

A method of spot welding hot stamping steel sheets according to an embodiment of the present invention relates to a method of spot welding a plurality of high-strength steel sheets. As shown in FIG. 1, the method of spot welding a hot stamping steel sheet according to an embodiment of the present invention includes arranging the steel sheets and providing a pressing force (S100), a pre-pulse step (S200), an impulse step (S300), It may include a main pulse step (S400) and a sustain step (S500).

The step of arranging the steel plates and providing a pressing force (S100) is a step of arranging high-strength steel plates, which are materials to be welded, in a welding device. At least a portion of the high-strength steel sheets disposed in the welding device overlaps each other, and the overlapped portion may include a welded portion wp.

Here, there is no limitation on the type of high-strength steel sheet that is the material to be welded. For example, high-strength steel sheets include hot stamping processed steel sheets, precipitation strengthened steel sheets, DP (dual phase) steel sheets, CP (complex phase) steel sheets, HPF (hot press forming) steel sheets, and TRIP (transformation induced plasticity) steel sheets. , a twin induced plasticity (TWIP) steel sheet, and the like. In addition, the high-strength steel sheet may be a cold-rolled steel sheet or a hot-rolled steel sheet, and may be a plated steel sheet or a non-plated steel sheet. In addition, there is no limitation on the type of plating treatment.

Hereinafter, for convenience, a case in which the effect of the present invention is most greatly exhibited, a case in which the high-strength steel sheet is a hot-stamped steel sheet will be described as an example, but the same can be applied to a method of spot welding any high-strength steel sheet.

Hot-stamped steel sheets are generally surface-treated. The surface of hot-stamped steel sheets is zinc-based (pure Zn, Zn-Fe, Zn-Ni, Zn-Al, Zn-Mg, Zn-Mg-Al, etc.) or aluminum. An intermetallic compound and an iron-based solid solution may be formed by an alloying reaction between the plated film of the system (Al-Si, etc.) and the steel sheet. In addition, an oxide layer containing zinc or aluminum as a main component may be formed on the surface of the hot stamped steel sheet. Further, in some cases, a film containing zinc oxide as a main component is formed on the surface of a hot stamped steel sheet to further improve corrosion resistance on the surface of a film containing an intermetallic compound of iron and aluminum as a main component. The surface-treated hot stamping steel sheet in this way can easily generate spatter during spot welding, so the appropriate welding current range is relatively narrow and limited.

According to the method of spot welding hot stamped steel sheets according to an embodiment of the present invention, the nugget diameter can be controlled by adjusting the welding conditions during spot welding, and thus the mechanical properties of the hot stamped steel sheets joined can be controlled. Here, "nugget diameter" means the diameter of a nugget formed at a welded portion. A detailed description of the method for measuring the nugget diameter will be described later with reference to FIG. 4 .

In one embodiment, as a welding condition during spot welding, it is possible to control the diameter of the nugget formed at the welding portion by adjusting the conditions for applying multi-stage pulses, the range of welding current for each multi-pulse stage, and/or the range of conduction time. Through this, it is possible to secure a nugget diameter of a sufficient size to improve weldability and improve mechanical properties of parts after being joined. For example, the cross tension strength (CTS) and tensile shear strength (TSS) of the parts after being joined can be improved.

Referring to FIGS. 2A to 2C , a welding device is a device for spot welding a plurality of steel plates, and may include electrodes disposed vertically or left and right. As one embodiment, as shown in FIGS. 2A to 2C , the welding apparatus may include a first electrode e1 disposed on an upper portion and a second electrode e2 disposed on a lower portion. This welding device may be an inverter DC type spot welding device, but is not limited thereto.

The hot stamped steel plates, which are materials to be welded, may be disposed such that at least a portion overlaps each other between the first electrode e1 and the second electrode e2 of the welding device. The portion where the hot stamping steel sheets overlap each other may include a welded portion wp where a nugget is formed to form a welded portion.

Among the hot stamping steel plates disposed in the welding device, the top surface of the uppermost steel plate (eg, the third steel plate ST3) is in contact with the first electrode e1 of the welding device, and the lowermost steel plate (eg, the first steel plate ST1) The lower surface may contact the second electrode e2 of the welding device. That is, at least a part of the upper surface of the uppermost steel plate (eg, the third steel plate ST3) among the hot stamping steel plates disposed in the welding apparatus forms a contact surface with the first electrode e1 of the welding apparatus, and the lowest steel plate (eg, the first steel plate ST3) The lower surface of the first steel plate ST1 may form a contact surface with the second electrode e2 of the welding device.

The first electrode e1 and the second electrode e2 of the welding device may provide pressing force to the hot stamping steel sheets from above and below, respectively. In this case, an area where the hot stamping steel sheets overlap the first electrode e1 and the second electrode e2 of the welding device and an area adjacent thereto may be defined as a welding portion wp.

Meanwhile, the pressing force applied to the hot stamping steel sheets may be maintained even in steps after the step of disposing the steel sheets and providing the pressing force (S100). In one embodiment, the magnitude of the pressing force applied to the hot stamping steel sheets may be maintained the same throughout all steps. In another embodiment, the pressing force applied to the hot stamping steel sheets may be differently adjusted in stages.

There is no limit to the number of hot stamping steel sheets placed in the welding device. As one embodiment, as shown in FIGS. 2A to 2C , three hot stamping steel sheets may be disposed in the welding device as a material to be welded. The three hot stamping steel sheets may include a first steel sheet ST1, a second steel sheet ST2, and a third steel sheet ST3 sequentially disposed. In another embodiment, two or four or more hot stamping steel sheets may be disposed in the welding device as the material to be welded.

In addition, there is no limitation on the form in which the hot stamping steel sheets are arranged. In one embodiment, as shown in FIGS. 2A to 2C, a first steel plate (ST1), a second steel plate (ST2), and a third steel plate (ST3) are sequentially stacked as welded materials in a welding device. In this case, the width of each of the steel plates in the cross-sectional view may be variously modified. In addition, mechanical properties such as cross tensile strength (CTS) and tensile shear strength (TSS) of the joined parts and a range of welding current may vary according to the relationship between the widths of the steel plates. Here, “width” may mean a width in a first axis (eg, x-axis) direction. For reference, FIGS. 2A to 2C are diagrams illustratively illustrating a case in which the first axis (eg, the x-axis) is drawn, and the relationship between the widths of each of the steel plates in a cross-sectional view on an axis different from the first axis is shown in FIG. 2A to 2c.

In one embodiment, as shown in FIG. 2A , the width of the third steel plate ST3 may be relatively smaller than the widths of the first steel plate ST1 and the width of the second steel plate ST2 in a cross-sectional view. In addition, in the cross-sectional view, the first steel plate ST1 extends from the welding part wp in the first axial direction (eg, +x-axis direction), and the second steel plate ST2 extends from the welding part wp in the opposite direction of the first axis. It may extend in a direction (eg, -x-axis direction). In this case, mechanical properties such as cross tensile strength (CTS) and tensile shear strength (TSS) of the parts after being joined can be measured by applying an external force to the first steel plate (ST1), which is the lowest layer, and the second steel plate (ST2, which is the middle layer). there is.

In another embodiment, as shown in FIG. 2B , the width of the second steel plate ST2 may be smaller than the widths of the first steel plate ST1 and the third steel plate ST3 in a cross-sectional view. In addition, in cross-sectional view, the first steel plate ST1 extends from the welding region wp in the first axial direction (eg, +x-axis direction), and the third steel plate ST3 extends from the welding region wp in the opposite direction of the first axis. It may extend in a direction (eg, -x-axis direction). In this case, mechanical properties such as cross tensile strength (CTS) and tensile shear strength (TSS) of the parts after being joined can be measured by applying an external force to the first steel plate (ST1), which is the lowest layer, and the third steel plate (ST3, which is the uppermost layer). there is.

In another embodiment, as shown in FIG. 2C , in cross-sectional view, the first and third steel plates ST1 and ST3 extend in the first axial direction (eg, +x-axis direction) from the welding portion wp. And, the second steel plate ST2 may extend in a direction opposite to the first axis (eg, -x axis direction) from the welded portion wp. In addition, the ends of the first steel plate ST1 and the third steel plate ST3 far from the welding portion wp may be connected to each other by a connection portion cp. The connection part (cp) may be a part of the second steel plate (ST2), but is not limited thereto. In this case, the mechanical properties such as cross tensile strength (CTS) and tensile shear strength (TSS) of the parts after being joined are the intermediate layer. It can be measured by applying an external force to the two steel plates ST2 and the connecting portion cp.

The pre-pulse step (S200), the impulse step (S300), and the main pulse step (S400) are steps in which a welding current for spot welding is applied to the hot stamping steel sheets. In the pre-pulse step (S200), the impulse step (S300), and the main pulse step (S400), current may be applied to the hot stamping steel sheets through the first electrode e1 and the second electrode e2 of the aforementioned welding device. there is. That is, the welding current is applied while pressing force is applied to the hot stamping steel sheets disposed between the first electrode e1 and the second electrode e2 of the welding device. Meanwhile, the amount of heat input (Q) supplied to the welding portion (wp) of the hot stamped steel sheets during welding may satisfy the following equation according to Joule's law.

[Equation 1]

Q=I ² *R*t

(Q: heat input, I: welding current, R: resistance, t: conduction time)

Accordingly, the contact surface between the first electrode e1 of the welding device and the upper surface of the uppermost steel plate among the hot stamping steel plates and the contact surface between the second electrode e2 of the welding device and the lower surface of the lowermost steel plate among the hot stamping steel plates are contact resistance and Heat may be generated due to metal specific resistance, and the welded portion wp of the hot stamping steel sheets may be melted by being heated by the heat. Thereafter, the molten welded portion wp is cooled and solidified, and hot stamping steel sheets may be joined to each other.

In each of the pre-pulse step (S200), the impulse step (S300), and the main pulse step (S400), welding conditions that can maximize the nugget diameter can be applied in order to improve the welding quality and improve the mechanical properties of the joined parts. there is. For example, in each of the pre-pulse step (S200), the impulse step (S300) and the main pulse step (S400), the welding current and conduction time may be controlled to satisfy a preset range. A detailed description thereof will be described later with reference to FIG. 3 .

The maintenance step (S500) is a step of maintaining and cooling the hot stamped steel sheets after processes of applying a welding current to the hot stamping steel sheets are completed, that is, after the main pulse step (S400) is completed.

In one embodiment, in the maintenance step (S500), the pressing force provided to the hot stamping steel sheets may be maintained, but is not limited thereto. For example, it is also possible that the magnitude of the pressing force applied to the hot stamping steel sheets is reduced or increased or the pressing force is removed in the maintaining step (S500).

In the maintenance step (S500), the hot stamping steel sheets, in particular, the welded portion may be cooled. That is, in the maintenance step (S500), the welded portion and the nugget formed therein may be naturally cooled. In another embodiment, a cooling method other than a natural cooling method may be applied to the maintaining step ( S500 ). For example, various cooling methods such as air-cooled cooling, water-cooled cooling, and oil-cooled cooling may be applied to the maintenance step (S500).

3 is a schematic diagram illustrating a part of a method of spot welding a hot stamping steel sheet according to an embodiment of the present invention based on a conduction time and a welding current.

According to the method of spot welding hot stamped steel sheets according to an embodiment of the present invention, it is possible to improve weldability of hot stamped steel sheets and mechanical properties of parts after being joined by applying multi-stage pulses.

As shown in FIG. 3, in the method of spot welding hot stamped steel sheets according to an embodiment of the present invention, a first welding current I1 is applied to a welded portion where hot stamped steel sheets overlap for a first conduction time t1. A pre-pulse step (S200) to apply a second welding current (I2) smaller than the first welding current to the welding site for a second conduction time (t2), but an impulse step (S300) repeated twice or more, and a second welding current (I2) to the welding site. A main pulse step (S400) of applying a third welding current I3 greater than the welding current I2 for a third conduction time period t3 may be included.

The pre-pulse step ( S200 ) of applying the first welding current I1 to the weld area where the hot stamped steel plates overlap each other for the first conduction time t1 may be a step of deteriorating the interface of the hot stamped steel plates. Specifically, the pre-pulse step (S200) may be a step of deteriorating the interface of hot stamping steel sheets without generating nuggets.

In this way, by performing the step of deteriorating the interface before generating the nugget, it is possible to increase the contact area between the hot stamping steel sheets. At this time, it is necessary to apply a relatively high welding current for a short conduction time in order to prevent the formation of nuggets.

In one embodiment, the first welding current I1 applied to the welding region in the pre-pulse step S200 may satisfy 6.0 kA or more and 7.3 kA or less. When the first welding current I1 is less than 6.0 kA, the interface between the hot stamped steel sheets is not sufficiently deteriorated, and thus the effect of increasing the contact area between the hot stamped steel sheets is reduced, resulting in deterioration in weldability. On the other hand, when the first welding current I1 exceeds 7.3 kA, there is a problem in that spatter occurs and the welding quality deteriorates.

In addition, as an embodiment, the first conduction time t1 in which the first welding current I1 is applied to the welding portion in the pre-pulse step S200 may satisfy 30 ms or more and 50 ms or less. When the first conduction time t1 is less than 30 ms, the interface between the hot stamped steel sheets is not sufficiently deteriorated, and thus the effect of increasing the contact area between the hot stamped steel sheets is reduced, resulting in deterioration in weldability. On the other hand, if the first conduction time (t1) exceeds 50 ms, there is a problem that spatter may occur or a nugget may be generated, and furthermore, the generated nugget may be generated due to the relatively high first welding current (I1). A rapidly growing problem may arise.

Meanwhile, after the pre-pulse step (S200) is completed and before performing the impulse step (S300), a step (S600) of cooling the welded area for a preset cooling time (tc) may be performed. The step of cooling the welded portion for a preset cooling time (tc) (S600) is a step of controlling the amount of heat input by cooling the welded portion between sections to which a welding current is applied. In one embodiment, the step of cooling the welded area for a preset cooling time (S600) is between the pre-pulse step (S200) and the impulse step (S300), between the impulse step (S300) and / or the impulse step (S300) and It may be performed between the main pulse step (S400).

In one embodiment, the preset cooling time (tc) in which the step (S600) of cooling each welded area during the preset cooling time (tc) is performed may satisfy 20 ms or more and 40 ms or less. If the cooling time (tc) is less than 20 ms, the welded area is not sufficiently cooled, and spatter may occur in a subsequent process. On the other hand, when the cooling time (tC) exceeds 40 ms, the welded area is excessively cooled, which may hinder the formation of nuggets in a subsequent process.

On the other hand, in the step (S600) of cooling the welded area for a preset cooling time, a natural cooling method may be applied, but is not limited thereto. For example, in the step of cooling the welded area for a predetermined cooling time (S600), various cooling methods such as air-cooled cooling, water-cooled cooling, and oil-cooled cooling may be applied. In addition, in the step of cooling the welded area for a predetermined cooling time (S600), the pressing force provided to the hot stamping steel sheets may be maintained.

The impulse step (S300) of repeatedly applying the second welding current (I2) smaller than the first welding current (I1) to the welding area twice or more during the second conduction time (t2) may be a step of forming the base of the nugget at the welding area. there is. Specifically, in the impulse step (S300), the base of the nugget may be formed while preventing the nugget from growing in earnest.

In this way, by forming the base of the nugget before growing the nugget in the impulse step (S300) and then growing the nugget in the main pulse step (S400), it is possible to prevent expulsion and increase the nugget diameter. Here, "scattering" means a phenomenon in which the melted part of the welded material escapes to the outside of the nugget due to resistance heat during spot welding, and can act as a factor in degrading welding quality. To improve welding quality, it is necessary to minimize scattering. there is.

In one embodiment, the impulse step (S300) may be repeated two or more times to improve the base formation effect of the nugget. In this regard, FIG. 3 illustrates a case in which the impulse step (S300) includes the first impulse step (S310) and the second impulse step (S320), but is not limited thereto. For example, it is also possible that the impulse step (S300) is repeated three or more times.

In one embodiment, the second welding current I2 applied to the welding area in each impulse step (S300) may be smaller than the first welding current I1 applied to the welding area in the pre-pulse step (S200). For example, the second welding current I2 may satisfy 80 to 90% of the first welding current I1.

In each of these impulse steps (S300), the second welding current I2 applied to the welding portion may satisfy 5.5 kA or more and 6.5 kA or less. When the second welding current I2 is less than 5.5 kA, the nugget base formation effect is reduced, and thus a sufficient nugget diameter may not be secured. On the other hand, when the second welding current I2 exceeds 6.5 kA, there is a problem in that spatter occurs and the welding quality deteriorates.

In addition, as an embodiment, the second conduction time (t2) in which the second welding current (I2) is applied to the welding part in each impulse step (S300) is the first welding current (I1) to the welding part in the pre-pulse step (S200). may be longer than the first energization time t1 applied. That is, the first conduction time t1 may be shorter than the second conduction time t2.

For example, the second conduction time t2 in which the second welding current I2 is applied to the welding portion in each impulse step (S300) may satisfy 100 ms or more and 160 ms or less. If the second conduction time (t2) is less than 100 ms, the effect of forming the base of the nugget is reduced, so it may not be possible to secure a sufficient nugget diameter. On the other hand, when the second conduction time t2 exceeds 160 ms, there is a problem that spatter may occur or nuggets may grow.

Meanwhile, between the impulse steps (S300), a step (S600) of cooling the above-described welded area for a preset cooling time (tc) may be performed. For example, between the first impulse step (S310) and the second impulse step (S320), a step (S600) of cooling the welded area for a preset cooling time (tc) may be performed. At this time, the cooling time tc may satisfy 20 ms or more and 40 ms or less. If the cooling time (tc) is less than 20 ms, the welded area is not sufficiently cooled, and spatter may occur in a subsequent process. On the other hand, when the cooling time (tC) exceeds 40 ms, the welded portion is excessively cooled and the base of the nugget is not sufficiently formed, resulting in a reduced nugget diameter.

The main pulse step (S400) of applying a third welding current (I3) greater than the second welding current (I2) to the welding region for a third conduction time (t3) is a step of growing the base of the nugget formed in the impulse step (S300). can be Specifically, in the main pulse step (S400), the nugget may be grown so that the nugget diameter (nugget diameter) satisfies a preset range.

In one embodiment, assuming that the thickness of each of the hot stamping steel sheets is t, the nugget may grow to have a diameter of 4√t or more, more preferably 5√t or more, in the main pulse step (S400). When the diameter of the nugget (nugget diameter) satisfies the above range, the strength of the welded portion can be improved, and furthermore, the mechanical properties of the parts after being joined, such as cross tensile strength (CTS) and tensile shear strength (TSS) ) can be improved. In this way, in order to sufficiently grow the diameter of the nugget (nugget diameter) to satisfy the above-described range, it is necessary to control the welding conditions of the main pulse step (S400).

In one embodiment, the third welding current I3 applied to the welding area in the main pulse step S400 may be greater than the second welding current I2 applied to the welding area in each impulse step S300. For example, the second welding current I2 may satisfy 80 to 90% of the third welding current I3. Meanwhile, FIG. 3 exemplarily shows a case where the first welding current I1 applied to the welding area in the pre-pulse step (S200) and the third welding current I3 applied to the welding area in the main pulse step (S400) are the same. but is not limited thereto. For example, the first welding current I1 may be greater than the third welding current I3 or the third welding current I3 may be greater than the first welding current I1. However, in any case, the first welding current I1 and the third welding current I3 may be greater than the second welding current I2.

In this main pulse step (S400), the third welding current I3 applied to the welding area may satisfy 6.0 kA or more and 7.5 kA or less. When the third welding current I3 is less than 6.0 kA, the nugget does not grow sufficiently and the nugget diameter decreases. On the other hand, when the third welding current I3 exceeds 7.5 kA, there is a problem in that spatter occurs and the welding quality deteriorates.

In addition, as an embodiment, the third conduction time (t3) in which the third welding current (I3) is applied to the welding area in the main pulse step (S400) is the first welding current (I1) to the welding area in the pre-pulse step (S200). It may be longer than the first conduction time (t1) when is applied and/or the second conduction time (t2) when the second welding current (I2) is applied to the welding part in the impulse step (S300). As a specific example, the first energization time t1 may be shorter than the second energization time t2, and the second energization time t2 may be shorter than the third energization time t3.

For example, in the main pulse step (S400), the third conduction time (t3) in which the third welding current (I3) is applied to the welding area may satisfy 150 ms or more and 200 ms or less. When the third conduction time (t3) is less than 150 ms, there is a problem in that the nugget does not grow sufficiently and the nugget diameter decreases. On the other hand, when the third conduction time t3 exceeds 200 ms, there is a problem in that spatter occurs and the welding quality deteriorates.

The maintenance step (S500) is a step of maintaining the hot stamping steel sheets for a predetermined holding time (ts) after the application of the third welding current (I3) is completed in the main pulse step (S400).

In one embodiment, the sustaining time ts during which the sustaining step S500 is performed may be 40 ms or more and 100 ms or less. In another embodiment, it is also possible that the maintenance step (S500) is omitted.

Hereinafter, the present invention will be described in more detail through Examples and Comparative Examples. However, the following Examples and Comparative Examples are intended to explain the present invention in more detail, and the scope of the present invention is not limited by the following Examples and Comparative Examples. The following Examples and Comparative Examples may be appropriately modified or changed by those skilled in the art within the scope of the present invention.

4 is an image showing a part of a nugget formed by the spot welding method of hot stamping steel sheets according to an embodiment of the present invention, and FIGS. 5A and 5B show the relationship between welding current and nugget diameter for each of the examples and comparative examples. 6A and 6B are graphs showing the relationship between welding current and tensile shear strength for Examples and Comparative Examples, respectively.

Specifically, FIGS. 5A and 6A are graphs showing the relationship between welding current and nugget diameter and welding current and tensile shear strength (TSS) when the welding conditions of Table 1 are applied as an embodiment of the present invention, 5B and 6B are graphs showing the relationship between welding current and nugget diameter and welding current and tensile shear strength (TSS) when welding conditions at least partially different from the above-described welding conditions are applied as comparative examples.

In addition, Type A shown in FIGS. 6A and 6B corresponds to the case where steel plates are arranged as shown in FIG. 2A, Type B corresponds to the case where steel plates are arranged as shown in FIG. 2C, and Type C corresponds to the case where steel plates are arranged as shown in FIG. 2C. Corresponds to the case where are placed.

According to the spot welding method of hot stamping steel sheets according to an embodiment of the present invention, a nugget ng may be formed at a welded portion where the hot stamping steel sheets overlap. As shown in FIG. 4, when the sequentially disposed first steel plate ST1, second steel plate ST2, and third steel plate ST3 are in contact with each other, the nugget ng is connected to the first steel plate ST1, It may be formed to overlap at least a portion of each of the second steel plate ST2 and the third steel plate ST3.

In one embodiment, the diameter of the nugget ng is the first width w1 of the portion where the nugget ng overlaps the first contact surface, which is the contact surface between the first steel plate ST1 and the second steel plate ST2, and/or Alternatively, the nugget ng may be measured as a second width w2 of a portion overlapping a second contact surface, which is a contact surface between the second steel plate ST2 and the third steel plate ST3. That is, that the diameter of the nugget ng satisfies the preset range may be understood as satisfying the first width w1 and/or the second width w2 within the preset range. Also, the first width w1 and the second width w2 may be the same as or different from each other.

The diameter of the nugget ng may satisfy 4√t or more, more preferably 5√t or more, when t is the thickness of each of the hot stamping steel sheets.

Referring to FIGS. 5A to 6B , a welding current range is shown. Here, the "welding current range" means a range of welding current that secures the required nugget diameter and tensile shear strength (TSS) after bonding and at the same time does not generate spatter. That is, when a welding current smaller than the lower limit of the welding current range is applied, a nugget diameter of a sufficient size cannot be secured, and thus welding quality may be deteriorated or tensile shear strength (TSS) of parts after being joined may be deteriorated. On the other hand, if a welding current greater than the upper limit of the welding current range is applied, spatter may occur. According to the embodiments of the present invention, there is an effect of securing a relatively wide welding current range. In an optional embodiment, the diameter of the nugget ng may satisfy 6.4 mm or more, and the tensile shear strength (TSS) may satisfy 2,032 kgf/spot or more, but is not limited thereto. In addition, the welding current range may satisfy Δ1.2kA or more based on nugget diameter or Δ1.7kA or more based on tensile shear strength (TSS), but is not limited thereto.

As an embodiment, referring to FIGS. 5A and 5B, within the welding current range, the nugget ng is the contact surface between the first steel plate ST1 and the second steel plate ST2, and the overlapping portion of the first contact surface. The first width w1 and the second width w2 of the portion where the nugget ng overlaps the second contact surface, which is the contact surface between the second steel plate ST2 and the third steel plate ST3, may all satisfy 6.4 mm or more. there is.

At this time, in FIG. 5A, when the welding current is 6.5 kA, the first width w1 and the second width w2 satisfy 6.4 mm or more. Accordingly, since the welding current range satisfies about Δ1.2kA, it can be confirmed that a relatively wide welding current range is secured. On the other hand, in FIG. 5B , the first width w1 or the second width w2 does not satisfy 6.4 mm or more even when the welding current exceeds 6.5 kA and is less than 7.0 kA at which spatter occurs. Accordingly, it can be confirmed that the welding current range is Δ0kA, that is, the welding current range is not secured. It can be understood that the spatter start section is increased and a wider welding current range is secured by applying the welding conditions of the present invention described above.

As an example, referring to FIGS. 6A and 6B , all types A to C may satisfy 2,032 kgf/spot or more within a welding current range. That is, the welding current range may be a range of welding current that satisfies the condition required for the tensile shear strength (TSS) of the parts after being joined regardless of the type and at the same time does not generate spatter. It can be seen that this welding current range is only about Δ0.7kA in the comparative example of FIG. 6b, whereas about Δ1.7kA is secured in the embodiment of FIG. 6a. That is, the welding current range of the embodiment of FIG. 6A may be relatively wide compared to the welding current range of the comparative example of FIG. 6B. It can be understood that the spatter start section is increased and a wider welding current range is secured by applying the welding conditions of the present invention described above.

Although the present invention has been described with reference to the embodiments shown in the drawings, this is merely exemplary, and those skilled in the art will understand that various modifications and equivalent other embodiments are possible therefrom. Therefore, the true technical protection scope of the present invention should be determined by the technical spirit of the appended claims.

wp: welded area
ng: nugget
ST1: 1st steel plate
ST2: 2nd steel plate
ST3: 3rd steel plate
e1: first electrode
e2: second electrode
I1: 1st welding current
I2: 2nd welding current
I3: 3rd welding current
t1: first energization time
t2: Second energization time
t3: 3rd energization time
tc: cooling time
ts: retention time

Claims (12)

arranging at least a portion of the hot stamping steel sheets to overlap each other and providing a pressing force;
a pre-pulse step of applying a first welding current to a welding portion where the hot stamped steel sheets overlap for a first conduction time;
forming a nugget by applying a second welding current smaller than the first welding current to the welding region for a second conduction time, the impulse step being repeated two or more times; and
a main pulse step of growing the nugget by applying a third welding current greater than the second welding current to the welding region for a third conduction time;
Including, spot welding method of hot stamping steel sheet. According to claim 1,
The first welding current is 6.0 kA or more and 7.3 kA or less,
The second welding current is 5.5 kA or more and 6.5 kA or less,
The third welding current is 6.0 kA or more and 7.5 kA or less, a method of spot welding a hot stamping steel sheet. According to claim 1,
The second welding current is 80 to 90% of the first welding current or 80 to 90% of the third welding current, a method of spot welding a hot stamping steel sheet. According to claim 1,
The first energization time is shorter than the second energization time,
The second conduction time is shorter than the third conduction time, a method of spot welding a hot stamping steel sheet. According to claim 4,
The first conduction time is 30 ms or more and 50 ms or less,
The second conduction time is 100 ms or more and 160 ms or less,
The third conduction time is 150 ms or more and 200 ms or less, a method of spot welding a hot stamping steel sheet. According to claim 1,
When the thickness of each of the hot stamping steel sheets is t,
In the main pulse step, the nugget grows to satisfy a diameter of 4√t or more, a method of spot welding a hot stamping steel sheet. According to claim 6,
In the main pulse step, the nugget grows to satisfy a diameter of 5√t or more, a method of spot welding a hot stamping steel sheet. According to claim 1,
cooling the welded area for a preset cooling time;
Including more,
The step of cooling the welded area for a preset cooling time is performed between the pre-pulse step and the impulse step, between the impulse steps, and between the impulse step and the main pulse step, respectively, for spot welding of hot stamping steel sheets. Way. According to claim 8,
The preset cooling time is 20 ms or more and 40 ms or less, a method of spot welding a hot stamping steel sheet. According to claim 1,
holding the hot stamped steel sheets for a predetermined holding time while maintaining the pressing force after the third welding current is applied to the welding portion;
Further comprising, spot welding method of hot stamping steel sheet. According to claim 10,
The preset holding time is 40 ms or more and 100 ms or less, a method of spot welding a hot stamping steel sheet. According to claim 1,
The method of spot welding of hot stamping steel plates, wherein the hot stamping steel plates include three hot stamping steel plates.

Images