Method for resistance spot welding of hot press formed member

Abstract

The present invention relates to a method for resistance spot welding of a hot press formed member and, more specifically, to a method for resistance spot welding of a hot press formed member suitable as a material for an automobile.

Description

Resistance spot welding method for hot-formed parts

[0001] The present invention relates to a resistance spot welding method for a hot-formed part, and more particularly, to a resistance spot welding method for a hot-formed part suitable as an automotive material.

[0002] Among the high-strength steel plates used in automobiles, hot-formable steel plates were developed to improve vehicle fuel efficiency and crashworthiness. These steel plates undergo heat treatment for high-temperature forming, thereby maximizing the strength of the final component (hot-formed member). Specifically, unlike conventional forming methods, these steel plates achieve high strength by applying a continuous process of heating general cold-rolled steel plates to high temperatures, forming, quenching, and then rapidly cooling.

[0003] These hot-forming steel sheets are subjected to plating processes to prevent surface oxidation that occurs during heat treatment for high-temperature forming. The plating layer formed by the plating process suppresses contact with air that occurs when the material is heat-treated or moved to a mold during high-temperature heating of the hot-forming steel sheet, thereby having the effect of preventing oxide scale.

[0004] Generally, aluminum-based plating, for example, Al-Si-based plating, is applied to mass-produced steel sheets for hot forming, and the plating layer formed by the plating reacts with the base material depending on the heat treatment conditions during hot forming to form an intermetallic compound layer of Fe-Al-Si, and at the same time, an aluminum-based oxide layer is generated on the surface of the plating layer by reaction with the atmosphere or oxygen in the air.

[0005] When resistance spot welding a hot-formed part obtained by hot-forming a steel plate for hot forming having a plating layer as described above, the intermetallic compound layer and oxide layer formed during the high-temperature heat treatment process for hot forming have a somewhat detrimental effect on securing weldability. That is, the formation of a nugget during resistance spot welding varies depending on the contact resistance of the base metal, and the metallographic changes on the surface of the base metal act as a factor that excessively increases the resistance of the base metal.

[0006] Meanwhile, recently, in order to apply hot forming steel sheets with aluminum plating to actual industries such as automobile bodies and their parts, there is an increasing demand for accelerated heating speeds and reduced production costs in the heat treatment stage for high-temperature forming. Accordingly, materials with significantly reduced plating adhesion compared to existing mass-produced materials, especially those with a plating weight of 70 g / m on one side during plating, are being developed. 2 20g / m inside and outside 2 The development of hot-forming steel sheets with plating is increasing by reducing the internal and external thickness.

[0007] In this way, when a hot-forming steel plate that has been plated is subjected to high-temperature heat treatment for hot forming, not only is the influence of Fe diffusion due to a decrease in the amount of plating maximized, but the structure of the plating layer also completely changes, making it difficult to secure a certain level of weld strength required for application to actual automobile parts.

[0008] Accordingly, in an effort to improve the resistance spot weldability of materials, methods such as pre-heat treatment before welding and physically removing the plating layer before welding (ablation) have been devised. However, this additional process increases manufacturing costs.

[0009] Therefore, there is a need to develop a method for improving weldability and securing a strong weld joint without a separate additional process when welding a hot-formed member obtained by hot-forming a hot-formed steel plate with a thin plating for aluminum plating.

[0010] (Patent Document 1) Korean Patent Registration No. 10-1696121

[0011] One aspect of the present invention is to provide a resistance spot welding method that can significantly improve weldability and stably secure tensile properties of a welded portion when resistance spot welding a hot-formed member obtained by heat-treating and forming a thinly plated hot-formed steel plate at high temperature by reducing the amount of plating adhesion during aluminum plating.

[0012] The objectives of the present invention are not limited to the above-described content. The objectives of the present invention can be understood from the overall content of this specification, and those skilled in the art will have no difficulty understanding the additional objectives of the present invention.

[0013] According to one aspect of the present invention, a resistance spot welding method for a hot-formed part may include the steps of: preparing a hot-formed part obtained by hot-forming a base steel plate and a steel plate for hot forming including an aluminum-based plating layer on at least one surface of the base steel plate; contacting and pressing an electrode having a tip diameter of 4.0 to 10.0 mm to a welding portion of the hot-formed part; performing preliminary current application while the electrode is pressed; cooling after completing the preliminary current application; and performing the main current application after the cooling.

[0014] According to one embodiment of the present invention, the aluminum-based plating layer may have a thickness of 5.0 to 40.0 μm on one side.

[0015] According to another embodiment of the present invention, the preceding current may be performed under conditions satisfying the following [Formula 1] and [Formula 2].

[0016] [Formula 1]

[0017]

[0018] (In Equation 1, T p : Single-sided thickness of aluminum plating layer (㎛), f al : Area fraction (%) of Al-concentrated layer in aluminum-based plating layer, T: Thickness of the material to be welded (thickness of base steel plate (mm) + thickness of one side of plating layer (mm)), I: Current during prior current application (kA), d: Electrode tip diameter (mm), t w : It means the time (ms) of the preceding power-on.

[0019] The above t w satisfies Equation 2 below.

[0020] Also, in Equation 1, T p , f al And T, if there are two or more welded materials, the average value of each corresponding value is applied.)

[0021] [Formula 2]

[0022] 40 < t w ≤ 100

[0023] According to another embodiment of the present invention, the hot-formed member may include an aluminum-based plating layer on the surface with which the electrode comes into contact.

[0024] According to another embodiment of the present invention, the pressing of the electrode may be performed with a pressing force of 1.0 to 9.0 kN, the cooling may be performed for 0.01 to 1.0 seconds, and the main current may be performed for 0.2 to 0.5 seconds at a current range of 5.0 to 10.0 kA. In addition, a step of maintaining after the main current may be further included.

[0025] A weld according to another aspect of the present invention may be formed by the resistance spot welding method described above and may have a tensile strength of 18 kN or more.

[0026] According to the present invention, the range of weldable current can be drastically increased during resistance spot welding of a hot-formed part suitable as an automotive material, particularly a hot-formed part having a thinly plated aluminum-based plating layer with a reduced amount of plating adhesion, while securing more stable weld properties.

[0027] FIG. 1 is a graph illustrating an example of a resistance spot welding process according to one embodiment of the present invention.

[0028] FIG. 2 is a photograph of a cross-section of a hot-formed member having an aluminum-based plating layer as a point material for resistance spot welding according to one embodiment of the present invention, observed in the thickness direction.

[0029] Figure 3 is a photograph of a cross-section of a welded part of an invention example according to one embodiment of the present invention observed using an optical microscope.

[0030] Figure 4 is a photograph of a cross-section of a welded part of comparative examples according to one embodiment of the present invention observed using an optical microscope.

[0031] Figure 5 is a schematic diagram showing an example of an electrode tip diameter.

[0032] The inventors of the present invention studied a method for forming a strong weld joint by significantly improving the weldability during resistance spot welding of a hot-formed member obtained by heat-treating and forming a hot-formed steel sheet, for example, an aluminum-plated hot-formed steel sheet, which is increasingly being used as an automobile material, at a high temperature.

[0033] In particular, the inventors of the present invention have recently conducted in-depth research on a method for improving the weldability of the material in accordance with the trend of performing thin plating by significantly reducing the amount of plating adhesion during plating in an effort to improve productivity and reduce costs in the heat treatment stage for high-temperature forming in the actual industrial application of aluminum-plated steel sheets for hot forming.

[0034] Accordingly, according to one aspect of the present invention, there is a technical significance in providing a welding method capable of improving weldability during spot welding of a hot-formed member having an aluminum-based plating layer by plating, while simultaneously securing the tensile properties of the welded portion, without performing an additional process that is expected to increase costs other than the resistance spot welding process.

[0035] Hereinafter, the present invention will be described in detail.

[0036] A resistance spot welding method of a hot-formed part according to one aspect of the present invention may include the steps of preparing a hot-formed part including an aluminum-based plating layer; the step of contacting and pressing an electrode to a welding portion of the hot-formed part; the step of performing preliminary current welding while the electrode is pressed; the step of cooling after completing the preliminary current welding; and the step of performing the main current welding after the cooling.

[0037] Below, each step is explained in detail.

[0038] In one embodiment of the present invention, a high-strength hot-formed member including a high-strength material, for example, an aluminum-based plating layer, can be prepared as a weld material to be subjected to resistance spot welding.

[0039] The hot-formed member has high strength characteristics, and may have, for example, a tensile strength of 900 MPa or more or 1000 MPa or more. As an example, the hot-formed member may be obtained by heating a base steel plate and a hot-forming steel plate having an aluminum-based plating layer formed on at least one surface of the base steel plate at a high temperature and then hot-forming the steel plate.

[0040] In one embodiment of the present invention, the base steel sheet for forming the aluminum-based plating layer may be any carbon steel applicable as an automotive material. However, as an example, the carbon steel may be transformation-induced plasticity (TRIP) steel, composite structure (CP) steel, or dual structure (DP) steel containing a certain amount of carbon (C), manganese (Mn), etc., and having a tensile strength of a certain level or higher. However, it should be noted that the present invention is not limited thereto.

[0041] As a non-limiting example, it is disclosed that a steel having an alloy composition of carbon (C): 0.19 to 0.25% by weight, manganese (Mn): 1.10 to 1.40%, silicon (Si): 0.001 to 0.400%, phosphorus (P): 0.001 to 0.025%, sulfur (S): 0.001 to 0.015%, the remainder being iron and other unavoidable impurities, can be used as a base steel plate.

[0042] In one embodiment of the present invention, since the aluminum-based plating layer can be formed by a conventional aluminum plating method, there are no particular limitations on the plating conditions. For example, the aluminum plating layer can be formed on at least one surface of the base steel sheet by a process of immersing the base steel sheet in a molten aluminum plating bath, or the alloyed aluminum plating layer can be formed by subjecting the base steel sheet on which the aluminum plating layer has been formed to alloying heat treatment.

[0043] However, according to one embodiment of the present invention, in forming the aluminum-based plating layer, the plating adhesion amount is 20 g / m on one side. 2 It can be performed by plating inside and outside, and the aluminum-based plating layer can have a thickness of 5.0 to 40.0㎛ on one side.

[0044] Meanwhile, the aluminum-based plating layer may contain, in weight %, iron (Fe): 2 to 98%, silicon (Si): 0.01 to 15.0%, the remainder aluminum (Al), and unavoidable impurities, depending on the location within the plating layer.

[0045] More specifically, since the aluminum-based plating layer contains Fe diffused from the base steel sheet by undergoing hot forming in the high-temperature heat treatment and forming process described below, a region with a relatively high Fe content, i.e., a region with a high Fe content compared to the surrounding area, may exist, and such a region may be referred to as an Fe-enriched layer.

[0046] Here, the hot forming refers to a [high-temperature heat treatment process - forming and cooling process] performed to obtain a typical hot-formed part, and as an example, the high-temperature heat treatment may be performed in a temperature range where reverse transformation to austenite is possible, and the forming may be a pressing process using a mold having a specific shape.

[0047] As a non-limiting example, when utilizing a mold during the above forming process, a press mold capable of circulating water or oil within the mold for cooling purposes may be applied, in which case the forming and cooling are performed simultaneously. That is, by forming in an austenite single phase that is easy to process through high-temperature heat treatment, cooling is performed simultaneously, thereby obtaining a low-temperature transformation phase of a complex shape, and thereby ensuring high strength.

[0048] According to one embodiment of the present invention, an electrode may be brought into contact with the welding area of ​​a hot-formed member and then pressurized. The pressing force of the electrode may be applied under normal conditions and is not particularly limited. However, as an example, the pressing force may be 1.0 to 9.0 kN.

[0049] The above electrode may be used with a tip diameter of 4.0 to 10.0 mm. If the tip diameter of the electrode is less than 4.0 mm, the welding current applied per unit area becomes excessively high, increasing the risk of expulsion of the molten material. On the other hand, if the tip diameter exceeds 10.0 mm, it may be difficult to secure the energy source necessary for melting the material to be welded, which may narrow the welding area.

[0050] Here, with reference to Fig. 5 as an example, the tip diameter of the electrode refers to the line length (length of the dotted line) of the part where the electrode contacts the material to be welded.

[0051] According to one embodiment of the present invention, when contacting an electrode with a welding portion of a hot-formed member, the electrode may be in contact with a surface of the hot-formed member on which an aluminum-based plating layer is formed. Here, the aluminum-based plating layer does not exclude an alloyed aluminum-based plating layer (aka, aluminum-based alloy plating layer).

[0052] As a non-limiting example, when two or more hot-formed parts having a specific shape are laminated and resistance spot welded, the uppermost laminated part may include an aluminum-based plating layer on its upper surface (the surface that the electrode contacts). In addition, the surface that the electrode does not contact (e.g., the lower surface) may also include an aluminum-based plating layer.

[0053] In addition, the steel plate laminated at the lowest portion of the two or more laminated hot-formed members may also include an aluminum-based plating layer on its upper surface (the surface in contact with the hot-formed member laminated at the top), and may also include an aluminum-based plating layer on its lower surface other than the upper surface.

[0054] At this time, if the upper surface of the hot-formed member laminated at the top includes an aluminum-based plating layer, it is to be noted that the number of laminated hot-formed members may exceed two.

[0055] Meanwhile, in performing resistance spot welding according to one embodiment of the present invention, welding of a single hot-formed member is not excluded. For example, it should be noted that both ends of a single hot-formed member in the longitudinal direction (rolling direction) may be welded to each other, or both ends in the transverse direction (perpendicular to the longitudinal direction) may be welded to each other.

[0056] Thereafter, current can be applied to the electrode that pressurizes the welding area of ​​the hot-formed member to perform energization. In the present invention, the above process is referred to as prior energization, and as described below, the present invention can perform this energization after the prior energization.

[0057] According to one embodiment of the present invention, the hot-formed member of the present invention, which is a material to be welded, is obtained by subjecting a steel plate for hot forming to a heat treatment and forming process at a high temperature for forming a component with a complex shape. In the process of performing this heat treatment process, as Fe diffuses into the aluminum-based plating layer, an Fe-Al-based intermetallic compound layer is formed along with an Fe-based oxide layer by the atmosphere within the furnace for heat treatment. When resistance spot welding is performed in a state where a surface oxide layer such as the Fe-Al-based intermetallic compound layer and the Fe-based oxide layer are formed, even when a low welding current is applied, excessive heat input occurs, causing frequent expulsion of the molten material to the outside. In addition, there is a problem that brittle fracture due to embrittlement occurs in the intermetallic compound layer having high hardness.

[0058] Moreover, the aluminum-based plating layer has a plating adhesion amount of 20 g / m on one side. 2 In cases where the inner and outer surfaces are plated, there is a problem in that the formation of the intermetallic compound layer is further accelerated.

[0059] According to one aspect of the present invention, when spot welding a hot-formed member obtained by hot-forming a steel plate for hot forming as described above, there is a technical significance in performing the welding step separately (Fig. 1) and optimizing the conditions during the prior current application.

[0060] Specifically, according to one embodiment of the present invention, through a pre-energization process, it was intended to intentionally lower the high surface resistance that inevitably occurs in the manufacturing process, and at the same time, to provide an environment in which an oxide layer and an intermetallic compound layer existing on the surface of an aluminum-based plating layer can be discharged and removed to the outside of the weld joint. In particular, in order to obtain various advantages such as productivity and cost reduction in an actual industrial environment, in the case of a hot-formed part in which an aluminum-based plating layer is formed with a lower plating adhesion amount than before, there is a higher risk of accelerated embrittlement within the weld joint, and therefore, the condition of pre-energization is important in order to secure a strong weld joint.

[0061] Therefore, the above-described preliminary current can be performed under conditions in which the plating layer of the hot-formed member is melted to a certain level or higher during the current passing process, the surface oxide layer and intermetallic compound layer of the area to be welded are melted, and the molten material that can act as a factor in increasing the resistance of the material is intentionally discharged.

[0062] In one embodiment of the present invention, the preceding current can be performed under conditions satisfying [Formula 1] and [Formula 2] below.

[0063] [Formula 1]

[0064]

[0065] (In Equation 1, T p : Single-sided thickness of aluminum plating layer (㎛), f al : Area fraction (%) of Al-concentrated layer in aluminum-based plating layer, T: Thickness of the material to be welded (thickness of base steel plate (mm) + thickness of one side of plating layer (mm)), I: Current during prior current application (kA), d: Electrode tip diameter (mm), tw : It means the time (ms) of the preceding power-on.

[0066] The above t w satisfies Equation 2 below.

[0067] Also, in Equation 1, T p , f al And T, if there are two or more welded materials, the average value of each corresponding value is applied.)

[0068] [Formula 2]

[0069] 40 < t w ≤ 100

[0070] According to one embodiment of the present invention, the current passing condition satisfying the above [Formula 1] and [Formula 2] is a condition that can cause a phenomenon of partially releasing the molten material in an area where an intermetallic compound layer exists, in addition to the effect of lowering the surface resistance of the hot-formed member. Accordingly, while generating a certain level or more of molten material in the subsequent current passing step, it is possible to suppress defects such as bursting and pores that may occur inside the weld, thereby stably securing a current range during the current passing, and as a result, excellent physical properties of the spot weld can be secured.

[0071] In the above [Formula 1], f al is the area fraction (%) of the Al concentration layer existing in the aluminum-based plating layer, and is the area fraction based on the thickness cross-section of the plating layer. This will be explained in more detail below using the drawings.

[0072] Fig. 2 is a photograph showing the cross-sectional shape in the thickness direction of a hot-formed member having an aluminum-based plating layer as a weld material for performing resistance spot welding according to one embodiment of the present invention. Referring to Fig. 2 (a), the area (a layer darker in color than the plating layer) that appears as a line in the intermediate layer portion (arrow mark (a) and box mark (b)) within the plating layer is an Al-concentrated layer.

[0073] According to one embodiment of the present invention, the Al-concentrated layer can be formed with an area fraction of up to 50%. However, the lower limit of the area fraction of the Al-concentrated layer is not particularly limited, but it is noted that it can be formed with a minimum of 0.1 area%, and more advantageously, 1.0 area% or more.

[0074] In addition, in the above [Formula 1], T means the thickness of the welded material, and in the present invention, the welded material corresponds to the thickness of the hot-formed part, i.e., the entire thickness of the hot-formed part including the thinly plated aluminum-based plating layer.

[0075] Specifically, the hot-formed member includes an aluminum-based plating layer on a base steel plate, and the thickness of the base steel plate may be in the range of 1.0 to 2.0 mm, and the thickness of the welded material may be a value obtained by adding the thickness (mm) of one side of the aluminum-based plating layer to the thickness (mm) of the base steel plate.

[0076] Here, one side of the aluminum plating layer means the side in contact with the electrode, and as an example, when there are two or more welded materials, the uppermost laminated welded material may be referred to as the side in contact with the electrode, and the lowermost laminated welded material may be referred to as the side in contact with another welded material. At this time, the average value of the plating layer thickness of each reference surface (one side) is measured, and then the value is added to the average thickness of the base steel plate to calculate the thickness (T) of the welded material. In addition, the average value (㎛) of the plating layer thickness is T in [Formula 1] p It applies to.

[0077] In performing the pre-conduction according to one embodiment of the present invention, if the condition is less than the value on the right side of the above [Formula 1], sufficient thermal energy that can contribute to the reduction of surface resistance can be transferred to the material by progressing the melting of the material to be welded only a small amount at the beginning of welding, whereas if it is more than the value on the right side, there is a problem that excessive melting occurs from the pre-conduction stage, unnecessarily reducing the amount of melting in the weld zone and deteriorating the physical properties of the weld zone.

[0078] Meanwhile, if the pre-conduction is performed below the left-hand side value of the above-described [Formula 1] according to one embodiment of the present invention, sufficient thermal energy to contribute to the reduction of surface resistance is not available with only a small amount of melting at the beginning of welding, and thus, the result of partially discharging the melt containing the surface oxide layer and intermetallic compound layer cannot be obtained.

[0079] That is, since melting during spot welding of a hot-formed member, which is a material to be welded, is directly affected by the structure and thickness of the plating layer (i.e., the amount of plating adhesion) within the aluminum-based plating layer and the thickness of the material to be welded, the prior current conduction of the hot-formed member needs to be considered as a condition that satisfies [Formula 1] and [Formula 2] according to the present invention.

[0080] From this, it is possible to effectively suppress the phenomenon of weld defects that may occur in the main welding performed following the above-mentioned prior welding, and in addition to improving the stability of the worker, it is possible to secure a strong weld due to the effect of suppressing external discharge of the molten material, and there is an effect of securing the intended tensile properties of the weld.

[0081] In performing the pre-conduction according to one embodiment of the present invention, the pre-conduction time (t) represented by the above [Formula 2] w ) is less than 40 ms, it cannot exert sufficient thermal energy to contribute to the decrease in surface resistance, and on the other hand, if the time exceeds 100 ms, there is a risk that excessive melt scattering may occur.

[0082] In one embodiment of the present invention, after completing the preliminary energization, the applied current can be cut off to cool the molten plating layer for a certain period of time. This cooling process stabilizes the molten material generated during the preliminary energization process. Through this cooling process, a sufficiently wide range of welding current can be applied while minimizing the occurrence of defects within the weld during the subsequent main energization.

[0083] In order to further enhance the strength of the weld joint, the present invention may perform the cooling for approximately 1.0 seconds (sec) rather than completely cooling to room temperature so that the latent heat remaining after the completion of the preceding current application can be used as a heat source. The lower limit of the cooling time is not particularly limited, but it is noted that it may be performed for 0.01 seconds or for 1 cycle or more based on a frequency of 60 Hz of the welding power source.

[0084] In one embodiment of the present invention, after completing the cooling process, the main current corresponding to the spot welding process commonly performed in the relevant technical field can be performed, thereby manufacturing a weld having the desired properties.

[0085] The above-mentioned welding can be performed under normal spot welding conditions, but can also be performed under conditions that ensure a nugget of sufficient size.

[0086] Specifically, according to one embodiment of the present invention, the main welding can be performed at a current range of 5.0 to 10.0 kA for 0.2 to 0.5 seconds. If the welding current during the main welding exceeds 10.0 kA or the welding time exceeds 0.5 seconds, the phenomenon of flying occurs excessively, making it impossible to obtain the effect of the previous welding. In addition, there is a problem that the physical properties of the weld are deteriorated due to the occurrence of a large number of defects within the weld. On the other hand, if the welding current during the main welding is less than 5.0 kA or the welding time is less than 0.2 seconds, a nugget of sufficient size cannot be secured, which limits its application to actual components.

[0087] After the current supply according to one embodiment of the present invention is completed, a process can be performed in which the electrode is maintained in a pressurized state for a certain period of time while the supplied current is cut off.

[0088] At this time, it is advantageous to perform the above maintenance process for a period of time sufficient to allow the nugget formed during the current to sufficiently solidify, and there are no particular limitations on the conditions.

[0089] A hot-formed member according to one embodiment of the present invention, particularly a hot-formed member obtained by hot-forming a steel sheet for hot forming having an aluminum plating layer to which plating has been applied in order to obtain effects such as improved productivity and reduced manufacturing costs, is a material suitable as a material for automobiles.

[0090] Automotive parts, etc., require not only heat treatment for hot forming but also welding. When welding the aforementioned hot-formed parts, if the resistance spot welding method according to the present invention is applied, the surface resistance due to the surface oxide layer and intermetallic compound layer generated during the heat treatment process for hot forming can be effectively reduced, thereby providing an advantage in that a wider current range can be secured in the current-conducting step. In addition, by effectively suppressing pores within the molten zone, there is an effect of providing a weld having a certain level or higher of physical properties while securing a strong weld.

[0091] Specifically, by using the resistance spot welding method provided as an embodiment of the present invention, a weld having a tensile strength of 18 kN or more can be obtained.

[0092] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are intended only to illustrate and further illustrate the present invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters set forth in the patent claims and matters reasonably inferred therefrom.

[0093] (Example)

[0094] Two sheets of base steel plates (Ceq: 0.28) having a tensile strength of 1500 MPa and a thickness of 1.5 mm, containing, by weight%, carbon (C): 0.19%, manganese (Mn): 1.13%, silicon (Si): 0.250%, phosphorus (P): 0.010%, sulfur (S): 0.002%, the remainder iron and other unavoidable impurities, were prepared. Each of the base steel plates was subjected to molten aluminum plating (plating bath components: by weight%, Si 8 to 11%, Al 88 to 91%, the remainder iron and unavoidable impurities) to form aluminum-based plating layers on both sides, thereby manufacturing steel plates for hot forming. Thereafter, each steel plate for hot forming was subjected to heat treatment by heating to 900°C and maintaining it for 5 minutes, and then hot forming to manufacture a hot-formed part. At this time, the average single-sided thickness (T) of the aluminum plating layer of two hot-formed parts p ) was 18.77㎛. In addition, the average area fraction of the Al-concentrated layer of each hot-formed member was 23.5% (Fig. 2 (a)) and 21.1% (Fig. 2 (b)), and the average value (22.3%) was calculated using f in [Equation 1]. al was applied.

[0095] After laminating the two hot-formed parts above, an electrode (ISO 5821 F1) was brought into contact with and applied pressure to the welding area, and then a resistance spot welding process of [pre-energization - cooling - main energization] was performed under the conditions shown in Table 1 below.

[0096] After completing the resistance spot welding according to the above, in order to confirm the robustness of the weld, test pieces were manufactured under the same conditions as each resistance spot welding condition, and then the tensile strength (kN) of the weld was measured. At this time, the upper limit of the welding current range (kA) was set to the current at the point where fly-off occurred, and the lower limit was set to the current at the point where the nugget diameter was less than 4.0√t (where t is the thickness of the weld material (mm), i.e., the sum of the base steel thickness (mm) and the single-sided thickness of the plating layer (mm), and the difference between them was calculated and expressed. The respective results are shown in Table 2.

[0097] Electrode condition Pre-current cooling Current maintenance [Formula 1] Satisfaction Tip diameter (mm) Applied pressure (kN) Current (kA) Time (ms) Time (s) Current (kA) Time (s) Time (s) Comparative example 185.00005.80.300.15×Comparative example 285.04.0400.306.60.300.20×Comparative example 365.09.51000.305.20.280.10×Invention example 185.06.5450.306.80.280.15○Invention example 285.09.0500.305.40.300.20○Invention example 365.04.5800.306.00.300.10○

[0098] Welding current range (kA)Welding strength (kN)Comparative example 10.011.7Comparative example 20.412.5Comparative example 30.210.9Inventive example 11.019.2Inventive example 21.421.1Inventive example 31.424.2The welding current range is the range of the minimum and maximum values ​​in the current range where welding is possible during the current application, and as an example, it means the range value between the minimum current value that can secure the minimum nugget diameter and the maximum current value that generates spatter.

[0099] As shown in Tables 1 and 2 above, the invention examples 1 to 3, which performed [preliminary current-flow - cooling - main current-flow] by the spot welding method proposed in the present invention, all secured a weld strength of 19.0 kN or more, confirming that the mechanical properties of the weld were excellent. In addition, there is an advantage in that very stable conditions can be secured when performing welding by expanding the weldable current range.

[0100] On the other hand, it can be confirmed that the weld strength of Comparative Example 1, in which no prior energization was performed, is considerably inferior.

[0101] Meanwhile, Comparative Examples 2 and 3, which performed pre-conduction but did not satisfy the conditions proposed in the present invention, showed a narrow weldable range and low weld strength. This is confirmed to be due to the surface resistance of the welded area not being sufficiently reduced during the pre-conduction process.

[0102] Fig. 3 shows a photograph of a cross-section of a welded part of invention examples 1 to 3 observed with an optical microscope, and Fig. 4 shows a photograph of a cross-section of a welded part of comparative examples 1 to 3 observed with an optical microscope.

[0103] As can be seen in Figure 3, in the case of the invention examples, it is possible to stably secure a melting amount above a certain level. However, as shown in Figure 4, in the case of the comparative examples, it is impossible to secure a melting amount above a certain level due to the expulsion phenomenon caused by excessive heat input into the weld, and it can be confirmed that the properties of the weld are deteriorated due to the numerous pore phenomena that occur in the unstable melt state.

Claims

1. A step for preparing a hot-formed member obtained by hot-forming a steel plate for hot forming including a base steel plate and an aluminum-based plating layer on at least one surface of the base steel plate; A step of contacting and applying pressure to the welding area of ​​the hot-formed member with an electrode having a tip diameter of 4.0 to 10.0 mm; A step of performing preliminary energization while the above electrode is pressurized; A step of cooling after completing the above-mentioned preliminary energization; and Including the step of performing the main current after the above cooling, A resistance spot welding method for a hot-formed member, wherein the aluminum-based plating layer has a thickness of 5.0 to 40.0 ㎛ on one side, and the preceding current passing is performed under conditions satisfying the following [Formula 1] and [Formula 2]. [Formula 1] (In Equation 1, T p : Single-sided thickness of aluminum plating layer (㎛), f al : Area fraction of Al-concentrated layer in aluminum-based plating layer (%), T: Thickness of the welded material (thickness of base steel plate (mm) + thickness of one side of plating layer (mm)), I: Current during prior current application (kA), d: Electrode tip diameter (mm), t w : It refers to the time (ms) of the preceding transmission. Above t w satisfies Equation 2 below. Also, in Equation 1, T p , f al And T, if there are two or more welding materials, the average value of each corresponding value is applied.) [Formula 2] 40 < t w ≤ 100 2. In paragraph 1, A resistance spot welding method wherein the hot-formed member includes an aluminum-based plating layer on the surface with which the electrode comes into contact.

3. In paragraph 1, A resistance spot welding method in which the above electrode is pressed with a pressing force of 1.0 to 9.0 kN.

4. In paragraph 1, A resistance spot welding method in which the above cooling is performed for 0.01 to 1.0 seconds.

5. In paragraph 1, The above resistance spot welding method is performed with a current range of 5.0 to 10.0 kN for 0.2 to 0.5 seconds.

6. In paragraph 1, A resistance spot welding method further comprising a step of maintaining the above-mentioned main current.

7. In paragraph 1, A resistance spot welding method wherein the above steel plate contains, in weight %, carbon (C): 0.19 to 0.25%, manganese (Mn): 1.10 to 1.40%, silicon (Si): 0.001 to 0.400%, phosphorus (P): 0.001 to 0.025%, sulfur (S): 0.001 to 0.015%, the remainder iron and other unavoidable impurities.

8. A weld formed by any one of the resistance spot welding methods of clauses 1 to 7 and having a tensile strength of 18 kN or more.

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