Hot press-formed member and resistance spot welding method for hot press-formed member

Abstract

The present invention relates to a hot press-formed member and a resistance spot welding method for the hot press-formed member and, more specifically, to a hot press-formed member and a resistance spot welding method for the hot press-formed member, wherein the hot press-formed member can be suitably used as a material for automobiles.

Description

Hot-formed member and resistance spot welding method of the hot-formed member

[0001] The present invention relates to a hot-formed member and a resistance spot welding method for a hot-formed member, and more specifically, to a hot-formed member and a resistance spot welding method for a hot-formed member that can be preferably used as a material for automobiles.

[0002] Recently, as environmental pollution caused by the use of fossil fuels has emerged globally, environmentally friendly policies and regulations aimed at carbon neutrality—such as sustainable development, carbon emission control, and the development of alternative energy—are being strengthened. In line with this trend, automobile manufacturers are actively promoting the application of high-strength, lightweight materials to reduce exhaust emissions and improve fuel efficiency. Consequently, ultra-high-strength steel sheets for automobiles have been developed; however, due to the springback phenomenon—the tendency for the material to return to its original shape after firing—and low elongation, these sheets face practical difficulties in industrial applications, such as automobile bodies and parts, which require processing into complex shapes.

[0003] Meanwhile, hot-formable steel sheets, which are increasingly being used as automotive materials, are materials capable of securing high strength properties by heat-treating boron steel, which has excellent hardenability, at high temperatures and then supersaturating carbon through a cooling process. Specifically, the hot-formable steel sheets are useful for obtaining complex part shapes required for automotive parts, which is the limit of the ultra-high-strength steel sheets, by transforming them into a soft austenite phase at high temperatures and then rolling or forming them. In addition, the hot-formable steel sheets have the advantage of being advantageous for the production of automotive parts because they can suppress the springback phenomenon, which is a phenomenon in which the material attempts to return to its pre-deformation state at room temperature.

[0004] However, since decarbonation occurs during the high-temperature heat treatment process for forming hot-formed steel sheets, which is a phenomenon in which carbon within the extreme surface layer of the steel sheet is released to the outside, a process such as plating is required on the surface of the steel sheet to prevent this.

[0005] When plating steel sheets for hot forming, aluminum plating, which has a relatively higher melting point than zinc, is mainly applied. When the aluminum-plated steel sheets for hot forming undergo high-temperature heat treatment, Fe diffusion occurs from the base material (substrate steel) into the plating layer, forming an Fe-Al intermetallic compound layer. This compound layer acts as a factor that increases the resistance of the material and ultimately becomes a major factor that reduces the resistance weldability of the hot-formed member obtained by hot forming.

[0006] Furthermore, there is a growing demand for enhanced corrosion resistance and hydrogen embrittlement resistance to apply hot-formed steel sheets to actual industries, such as automotive bodies and parts. Accordingly, aluminum-based alloy plating layers with specific elements added to existing aluminum-based plating layers are being developed to keep pace with the demands and trends of the automotive industry. For example, this objective can be achieved by adding zinc above a certain level to the aluminum plating layer.

[0007] However, in the case of the steel sheet for hot forming having the aluminum-zinc plating layer, in addition to the aforementioned Fe-Al intermetallic compound layer and Fe-based oxide layer, a thick surface oxide layer with a porous structure is formed due to the high oxygen affinity of zinc within the plating layer. Furthermore, a decrease in resistance spot weldability is expected due to electrode contamination during welding caused by a drop in the melting point of the plating layer compared to existing ones. In addition, there are many difficulties in securing a certain level of weld strength required for actual application in automotive parts, such as cracking caused by liquid metal embrittlement in the weld.

[0008] Accordingly, as an effort to improve the resistance spot weldability of hot-formed components, methods such as performing pre-treatment before welding and physically ablating the plating layer before welding have been devised. However, these methods have the problem of accompanied by increased manufacturing costs due to additional processes.

[0009] Therefore, there is a need to develop a method to secure a robust weld joint by improving weldability without the need for separate additional processes during the welding of a hot-formed member obtained by hot-forming an aluminum-zinc plated steel sheet for hot forming.

[0010] One aspect of the present invention is to provide a hot-formed member and a resistance spot welding method for the hot-formed member.

[0011] A preferred aspect of the present invention is to provide a hot-formed member having a robust weld and a resistance spot welding method for the hot-formed member.

[0012] The problems of the present invention are not limited to those described above. A person skilled in the art to which the present invention pertains will have no difficulty understanding additional problems of the present invention from the overall contents of this specification.

[0013] One embodiment of the present invention comprises: an aluminum-zinc alloy plated steel material; and a welded portion formed by welding the aluminum-zinc alloy plated steel material; wherein the aluminum-zinc alloy plated steel material comprises a base steel material; and an aluminum-zinc alloy plating layer formed on at least one surface of the base steel material, and the welded portion comprises a nugget, wherein the nugget has an area of ​​50.0 to 70.0 percent relative to the total area of ​​the welded portion based on a cross-section in the thickness direction of the member.

[0014] The above-mentioned steel material may contain, in weight percent, carbon (C): 0.19~0.25%, manganese (Mn): 1.10~1.40%, silicon (Si): 0.001~0.40%, phosphorus (P): 0.001~0.025%, sulfur (S): 0.001~0.015%, and the remainder being Fe and other unavoidable impurities.

[0015] The above aluminum-zinc alloy plating layer may contain, in weight percent, iron (Fe): 2~90%, silicon (Si): 0.1~10%, zinc (Zn): 10~98%, the remainder being aluminum (Al) and unavoidable impurities.

[0016] The above aluminum-zinc alloy plating layer may have an average thickness of 5.0 to 40.0 μm on one side.

[0017] The above aluminum-zinc alloy plating layer may have an average thickness of 0.1 to 10.0 μm.

[0018] The above nugget may have an area of ​​52.0 to 68.0 percent relative to the total area of ​​the weldment based on the cross-section in the thickness direction of the steel.

[0019] The above nugget may have an area of ​​54.0 to 66.0 percent relative to the total area of ​​the weldment based on the cross-section in the thickness direction of the steel.

[0020] The above nugget may have an area of ​​55.0 to 65.0 percent relative to the total area of ​​the weldment based on the cross-section in the thickness direction of the steel.

[0021] The above welded part may have a tensile strength of 18 kN or more.

[0022] The above hot-formed member may have a current range of 1.2kA or more when current is applied during the main welding.

[0023] Another embodiment of the present invention provides a resistance spot welding method for a hot-formed member comprising: a base steel member; and a hot-formed member comprising an aluminum-zinc alloy plating layer on at least one surface of the base steel member; a step of contacting and pressing an electrode to a welding target area of ​​the hot-formed member; a step of applying a preliminary current to the hot-formed member to which the electrode is pressed; a step of cooling the hot-formed member to which the preliminary current is applied; and a step of applying a main welding current to the cooled hot-formed member, satisfying the following [Equation 1].

[0024] [Equation 1]

[0025] [Equation 2] C eq = C + (Si / 30) + (Mn / 20) + 2P + 4S

[0026] (wherein T in [Equation 1] above p : Average thickness of one side of the aluminum-zinc alloy plating layer (㎛), T D : Average thickness (㎛) of the Fe-Al intermetallic compound layer on one side within the aluminum-zinc alloy plating layer, C eq : Carbon equivalent of the substrate steel, T: Thickness of the workpiece (Thickness of the substrate steel + Average thickness of one side of the aluminum-dissipated alloy plating layer) (mm), f FZ : Percentage of molten material within the weld (area %), I: Lead current (kA), D: Electrode tip diameter (mm), t w : Lead time (ms), t c : refers to the cooling time (ms), and the above C eq is expressed by the above [Equation 2], where C, Si, Mn, P, and S represent the weight percent of each alloying element.)

[0027] During the above contact and pressurization step, the pressurizing force may be 1.0 to 10.0 kN.

[0028] The above electrode may have a tip diameter of 4.0 to 10.0 mm.

[0029] When the above preceding current is applied, the following [Equation 3] can be satisfied.

[0030] [Equation 3] 30 ≤ t w ≤ 120

[0031] When the above preceding current is applied, the following [Equation 4] can be satisfied.

[0032] [Equation 4] 2.0 ≤ I ≤ 15

[0033] When cooling as described above, the following [Equation 5] can be satisfied.

[0034] [Equation 5] 30 ≤ t c ≤40

[0035] The above welding current application step can be performed for 12 to 30 cycles with a current range of 6.0 to 7.5 kA.

[0036] After the above-mentioned welding current is applied, a step of maintaining the electrode in a pressurized state may be further included.

[0037] According to one aspect of the present invention, a hot-formed member and a resistance spot welding method for the hot-formed member can be provided.

[0038] According to a preferred aspect of the present invention, a hot-formed member having a robust weld and a resistance spot welding method for the hot-formed member can be provided.

[0039] FIG. 1 illustrates a resistance spot welding method according to one embodiment of the present invention.

[0040] Figure 2 schematically illustrates an example of an electrode tip diameter.

[0041] FIG. 3 is a photograph of a cross-section of a hot-formed member according to one embodiment of the present invention observed with an optical microscope.

[0042] FIG. 4 is a photograph of the cross-section of the welded portion of Invention Examples 1 to 3 according to one embodiment of the present invention observed with an optical microscope, and (a) is a photograph of Invention Example 1, (b) is a photograph of Invention Example 2, and (c) is a photograph of Invention Example 3.

[0043] FIG. 5 is a photograph of the cross-section of the welded portion of Comparative Examples 1 to 3 according to one embodiment of the present invention observed with an optical microscope, and (a) is a photograph of Comparative Example 1, (b) is a photograph of Comparative Example 2, and (c) is a photograph of Comparative Example 3.

[0044] Preferred embodiments of the present invention are described below. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0045] In addition, embodiments of the present invention are provided to more fully explain the present invention to those with average knowledge in the relevant technical field.

[0046] In drawings, the shapes and sizes of elements may be exaggerated for clearer explanation.

[0047] In describing the embodiments of the present invention, if it is determined that a detailed description of known technology related to the present invention may unnecessarily obscure the essence of the present invention, such detailed description will be omitted. Furthermore, the terms described below are defined considering their functions in the present invention, and these may vary depending on the intentions or conventions of the user or operator. Therefore, such definitions should be based on the content throughout this specification. The terms used in the detailed description are merely for describing the embodiments of the present invention and should not be limited in any way. Unless explicitly stated otherwise, expressions in the singular form include the meaning of the plural form.

[0048] In this description, expressions such as “include” or “equipped” are intended to refer to certain characteristics, numbers, steps, actions, elements, parts or combinations thereof, and should not be interpreted to exclude the existence or possibility of one or more other characteristics, numbers, steps, actions, elements, parts or combinations thereof other than those described.

[0049] Unless otherwise specifically defined in the specification of the present invention, % units mean weight %.

[0050] In this specification, terms such as 'top', 'upper', 'upper surface', 'lower', 'lower surface', 'lower surface', and 'side surface' are based on the drawings and may actually vary depending on the direction in which the elements or components are arranged.

[0051] Additionally, throughout the specification, when it is said that one part is 'connected' to another part, this includes not only cases where they are 'directly connected,' but also cases where they are 'indirectly connected' with other elements in between.

[0052] The present invention will be described in detail below through each embodiment or example of the invention. It should be noted that each embodiment or example described in this specification is not limited to a single embodiment or example, but may also be combined with other embodiments or examples. Accordingly, the citation of claims in the patent claims is merely an example of an embodiment, and the technical concept of the present invention should not be interpreted as being limited only to a combination with the cited claims; rather, combinations with various claims are also included within the scope of the technical concept of the present invention.

[0053] The inventors of the present invention have conducted in-depth research on a method to significantly improve resistance spot weldability and secure robust weld strength during resistance spot welding of a hot-formed member obtained by hot-forming a hot-formed steel sheet, which is increasingly used as a material for automobiles, specifically one in which an aluminum-zinc or alloyed aluminum-zinc plating layer is applied to at least one surface of the base steel. In particular, the present invention has technical significance in providing a welding method that can secure the required weld properties along with improved weldability without performing additional processes, such as removing the plating layer, other than the resistance spot welding process.

[0054] Hereinafter, a hot-formed member according to one embodiment of the present invention will be described.

[0055] The hot-formed member of the present invention comprises an aluminum-zinc alloy plated steel material; and a welded portion formed by welding the aluminum-zinc alloy plated steel material; wherein the aluminum-zinc alloy plated steel material may comprise a base steel material; and an aluminum-zinc alloy plating layer formed on at least one surface of the base steel material. The present invention does not specifically limit the type of the hot-formed member, and as an example, it may have a tensile strength of 1.0 GPa or more.

[0056] In the present invention, any steel suitable for use as an automotive material may be used as the base steel; however, as an example, the carbon steel may be a transformation-induced plasticity (TRIP) steel, a composite structure (CP) steel, a differential structure (DP) steel, etc., 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 invention is not limited thereto.

[0057] In the present invention, the alloy composition of the above-mentioned steel material is not specifically limited, but as an example, the above-mentioned steel material may comprise, in weight%, carbon (C): 0.19~0.25%, manganese (Mn): 1.10~1.40%, silicon (Si): 0.001~0.40%, phosphorus (P): 0.001~0.025%, sulfur (S): 0.001~0.015%, and the remainder being Fe and other unavoidable impurities.

[0058] In the present invention, the thickness of the base steel is not specifically limited, but as an example, the base steel may have a thickness of 1.0 to 2.0 mm.

[0059] In the present invention, the alloy composition of the aluminum-zinc alloy plating layer is not specifically limited, but the aluminum-zinc alloy plating layer may comprise, in weight percent, iron (Fe): 2~90%, silicon (Si): 0.1~10%, zinc (Zn): 10~98%, the remainder being aluminum (Al) and unavoidable impurities. At this time, some or all of the alloy components within the plating layer may be those already contained in the plating bath or may be alloy components contained in the substrate steel that have diffused through heat treatment. For example, the aluminum-zinc alloy plating layer may contain Fe diffused from the substrate steel by undergoing the high-temperature heat treatment and forming process described below. Among these, there may be a region in which the content of Fe is relatively high.

[0060] In the present invention, the average thickness of one side of the aluminum-zinc alloy plating layer is not specifically limited, but as an example, the average thickness of one side of the aluminum-zinc alloy plating layer may be 5.0 to 40.0 μm.

[0061] According to one embodiment of the present invention, the aluminum-zinc alloy plating layer may include an Fe-Al intermetallic compound layer having an average thickness of 1.0 to 10.0 μm. However, it should be noted that the present invention is not limited thereto.

[0062] Meanwhile, in a hot-formed member according to one embodiment of the present invention, the aluminum-zinc alloy plated steel material may be welded to form a welded portion. The welded portion may include a nugget. In this case, the nugget may have an area of ​​50.0 to 70.0 percent relative to the total area of ​​the welded portion based on the cross-section in the thickness direction of the member. If the area of ​​the nugget relative to the total area of ​​the welded portion is less than 50.0 percent, it may be difficult to secure a welded portion strength of a certain level or higher, and if it exceeds 70.0 percent, the softening phenomenon in the heat-affected zone around the molten portion may worsen due to excessive heat input. The area of ​​the nugget according to one embodiment of the present invention may be 52.0 percent or more. The area of ​​the nugget according to one embodiment of the present invention may be 54.0 percent or more. The area of ​​the nugget according to one embodiment of the present invention may be 55.0 percent or more. According to one embodiment of the present invention, the area of ​​the nugget may be 68.0% or less. According to one embodiment of the present invention, the area of ​​the nugget may be 66.0% or less. According to one embodiment of the present invention, the area of ​​the nugget may be 65.0% or less. The nugget may be formed as the molten material within the weldment cools in the area that melts during welding. Meanwhile, pores may be partially formed within the nugget, and the area of ​​the nugget may be the area excluding the area of ​​the pores. The weldment may be the area formed by adding the area of ​​the nugget and the area of ​​the weld heat-affected zone (HAZ).

[0063] As described above, the hot-formed member according to one embodiment of the present invention can secure stable tensile properties of the weldment. For example, the weldment according to one embodiment of the present invention may have a tensile strength of 18 kN or more. In the present invention, the upper limit of the strength of the weldment is not specifically limited, but as an example, the strength of the weldment may be 40 kN or less.

[0064] In addition, the hot-formed member according to one embodiment of the present invention may have a current range of 1.2 kA or more during the main welding process. The welding current range (kA) refers to the difference between the upper limit and the lower limit when the current at the point where spatter occurs is set as the upper limit and the current at the point where the nugget diameter is 4.0√t (t: thickness of the steel (mm)) is set as the lower limit. That is, the difference between the upper limit and the lower limit of the current during the main welding process may be 1.2 kA or more. For example, when the lower limit of the main welding current is 7.0 kA, the upper limit current may be set to 8.2 kA or more. In the present invention, the upper limit of the current range during the main welding process is not specifically limited, but as an example, the current range during the main welding process may be 15 kA or less.

[0065] Hereinafter, a resistance spot welding method for a hot-formed member according to one embodiment of the present invention will be described.

[0066] FIG. 1 illustrates a resistance spot welding method according to an embodiment of the present invention. As shown in FIG. 1, when performing resistance spot welding, the current pulse is divided into two, and the section where current is applied first can be classified as the pre-application section, and after cooling for a certain period of time, the section where current is applied second can be classified as the main welding section.

[0067] That is, a resistance spot welding method for a hot-formed member according to one embodiment of the present invention may include preparation of the hot-formed member, contact and pressure, pre-application of current, cooling, and application of current for the main welding.

[0068] First, a hot-formed member is prepared comprising a base steel material and an aluminum-zinc alloy plating layer on at least one surface of the base steel material. The hot-formed member can be obtained by forming an aluminum-zinc plating layer on at least one surface of the base steel material and then hot-forming it.

[0069] As an example, the aluminum-zinc plating layer may be formed by a process of immersing a substrate steel material in a molten aluminum-zinc plating bath. Additionally, as an example, the aluminum-zinc plating layer may be an alloyed aluminum-zinc plating layer, and the alloyed aluminum plating layer may be formed by alloying heat treating the substrate steel material on which the aluminum-zinc plating layer is formed.

[0070] The above hot forming refers to a [high-temperature heat treatment process - forming and cooling process] performed to obtain a conventional hot-formed member, 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 process of pressing using a mold having a specific shape.

[0071] For example, when using a mold during the above forming process, a press mold capable of circulating water or oil inside the mold for cooling purposes may be applied, in which case the forming and cooling proceed simultaneously. That is, forming is performed in a single austenite phase that is easy to process through high-temperature heat treatment, and cooling is performed simultaneously, thereby obtaining a low-temperature transformation phase of a complex shape, from which high strength can be secured.

[0072] Subsequently, an electrode is brought into contact with and pressed against the welding target area of ​​the hot-formed member. When bringing the electrode into contact with the welding target area of ​​the hot-formed member, the electrode may come into contact with the surface of the outermost layer where the aluminum-zinc alloy plating layer of the hot-formed member is formed. In the present invention, the pressure applied when pressing the electrode may be subject to conventional conditions used in the relevant technical field and is not specifically limited. However, as an example, the pressure applied during the contact and pressing step may be 1.0 to 10.0 kN. Furthermore, the electrode is a conventional electrode used in resistance spot welding and is not specifically limited in the present invention. However, as an example, the electrode may have 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 may become excessively high, increasing the risk of molten material expulsion. On the other hand, if the tip diameter of the electrode exceeds 10.0 mm, it becomes difficult to secure the energy source required for the workpiece to be melted, and the welding area may become narrow.

[0073] Here, regarding the tip diameter of the electrode, as an example, it refers to the diameter of the part where the electrode contacts the workpiece (length of the dotted line).

[0074] Meanwhile, when resistance spot welding is performed after stacking two or more hot-formed members, the steel material stacked at the top or bottom may have an aluminum-zinc alloy plating layer on its upper surface (the surface where the electrode contacts), and may or may not have an aluminum-zinc alloy plating layer formed on its lower surface (the surface where the electrode does not contact). In addition, the hot-formed member provided between the top and bottom may or may not have an aluminum-zinc alloy plating layer formed on it.

[0075] Afterward, the electrode is subjected to a pre-current application to the pressurized galvanized steel. The pre-current application can be performed by applying current to the electrode.

[0076] According to one embodiment of the present invention, as previously mentioned, the hot-formed member of the present invention, which is the workpiece to be welded, is obtained by undergoing a heat treatment and forming process at a high temperature to form a part into a complex shape from a steel plate for hot forming. During the process of performing this heat treatment process, an Fe-based oxide layer is formed on the aluminum-zinc alloy plating layer formed on the base steel material, along with an Fe-Al intermetallic compound layer, due to the atmosphere inside the furnace for heat treatment as Fe diffuses from the base steel material. When resistance spot welding is performed with surface oxide layers such as the Fe-Al intermetallic compound layer and the Fe-based oxide layer formed, even if a low welding current is applied, excessive heat input occurs, causing frequent expulsion of molten material to scatter outward, and there is a problem of brittle fracture due to embrittlement in the intermetallic compound layer with high hardness. Furthermore, compared to hot-forming steel sheets with an aluminum-based plating layer, hot-forming steel sheets with an aluminum-zinc plating layer have the characteristic that this problem is further accelerated because the aforementioned surface oxide layer is formed more thickly.

[0077] Accordingly, as a result of repeated research by the inventors of the present invention, there is technical significance in separating the welding step (see FIG. 1) when spot welding a hot-formed member obtained by hot-forming a steel plate for hot forming as described above, while optimizing the conditions during the preceding current application.

[0078] Specifically, according to one embodiment of the present invention, through a pre-application process, high surface resistance inevitably generated during the manufacturing process is intentionally lowered, and at the same time, an environment is provided in which oxide layers and intermetallic compound layers present on the surface of the aluminum-zinc alloy plating layer can be discharged and removed to the outside of the weld. In particular, in the case of steel sheets for hot forming having an aluminum-zinc alloy plating layer, the risk of accelerated embrittlement within the weld is higher; therefore, the conditions for pre-application are important to ensure a robust weld.

[0079] Accordingly, the aforementioned prior current application can be performed under conditions where the plating layer of the hot-formed member melts to a certain level or higher during the current application process, and the surface oxide layer and Fe-Al intermetallic compound layer of the welded area melt, while simultaneously allowing a molten material containing a specific element to be intentionally discharged through the melting.

[0080] When the above-mentioned preceding current is applied, the following [Equation 3] can be satisfied. Preceding current application time (t w If ) is less than 30ms, it may be difficult to secure sufficient thermal energy to contribute to a decrease in surface resistance, whereas if it exceeds 130ms, there is a risk of excessive molten material scattering.

[0081] [Equation 3] 30 ≤ t w ≤ 120

[0082] Meanwhile, in the present invention, the preceding current is not specifically limited during the preceding current application. However, as an example, the following [Equation 4] may be satisfied during the preceding current application.

[0083] [Equation 4] 2.0 ≤ I ≤ 15

[0084] Subsequently, the hot-formed member that has been energized in advance is cooled. The cooling can be performed by cutting off the applied current after the advance energization is completed. It is desirable to perform the cooling for a certain period of time to stably secure the molten material generated during the advance energization. Through such a cooling process, a sufficiently wide range of welding current can be applied during the subsequent main welding energization while minimizing the occurrence of defects within the weld.

[0085] According to one embodiment of the present invention, in order to further improve the strength of the weld joint, the cooling can be performed to satisfy [Equation 5] below, rather than completely cooling to room temperature, so that the latent heat remaining after the preceding current is completed can be used as a heat source.

[0086] [Equation 5] 30 ≤ t c ≤40

[0087] Subsequently, the cooled hot-formed member is subjected to main welding current. Through the main welding current, a molten material of a certain level or higher can be secured, thereby obtaining a welded part having the physical properties intended by the present invention. The current applied during the main welding is completely independent of the preceding current, and it is preferable to apply a resistance spot welding method commonly practiced in the relevant technical field and to perform it under conditions that allow for securing a nugget of sufficient size. As an example, the main welding current step can be performed for 12 to 30 cycles with a current range of 6.0 to 7.5 kA. If the welding current exceeds 7.5 kA or the main welding current time exceeds 30 cycles during the main welding current, excessive spatter occurs, the effect of the preceding current presented in the present invention is not obtained, numerous defects occur within the welded part, and consequently, the physical properties of the welded part may deteriorate. On the other hand, if the above-mentioned main welding current is less than 6.0kA or the main welding current application time is less than 12 cycles, it may be difficult to obtain a nugget of sufficient size, making it difficult to use as an actual part. Meanwhile, the above-mentioned 1 cycle may correspond to 16.67ms.

[0088] After the main welding current is applied, a process may be performed in which the electrode is maintained in a pressurized state for a certain period of time while the applied current is cut off. At this time, since it is sufficient for the maintenance process to be performed for a period sufficient for the nugget formed during the main welding to solidify, the conditions thereof are not specifically limited.

[0089] A resistance spot welding method for a hot-formed member according to one embodiment of the present invention preferably satisfies the following [Equation 1]. The following [Equation 1] is a condition that can obtain the effect of lowering the surface resistance of the hot-formed member. By doing so, a molten material of a certain level or higher is generated during the main welding current application stage, and expulsion is suppressed, thereby ensuring a stable current range during the main welding current application, and consequently, excellent physical properties of the spot weld can be secured.

[0090] According to one embodiment of the present invention, if the value of the right-hand side of [Equation 1] is less than the value of the right-hand side, only a small amount of melting of the workpiece is carried out at the beginning of welding, and sufficient thermal energy can be transferred to the material to contribute to a decrease in surface resistance. On the other hand, if the value is greater than the value, excessive scattering of molten material occurs from the preceding current application stage, causing the amount of molten material in the weld to be unnecessarily reduced and the physical properties of the weld to be degraded. Meanwhile, if the value of the left-hand side of [Equation 1] is less than the value of the left-hand side, only a small amount of melting at the beginning of welding is not enough to provide sufficient thermal energy to contribute to a decrease in surface resistance, and there is a concern that there is insufficient thermal energy to discharge the surface oxide layer and the Fe-Al intermetallic compound layer.

[0091] [Equation 1]

[0092] [Equation 2] C eq = C + (Si / 30) + (Mn / 20) + 2P + 4S

[0093] (wherein T in [Equation 1] above p : Average thickness of one side of the aluminum-zinc alloy plating layer (㎛), T D : Average thickness (㎛) of the Fe-Al intermetallic compound layer on one side within the aluminum-zinc alloy plating layer, C eq : Carbon equivalent of the substrate steel, T: Thickness of the workpiece (Thickness of the substrate steel + Average thickness of one side of the aluminum-dissipated alloy plating layer) (mm), f FZ: Percentage of molten material within the weld (area %), I: Lead current (kA), D: Electrode tip diameter (mm), t w : Lead time (ms), t c : refers to the cooling time (ms), and the above C eq is expressed by the above [Equation 2], where C, Si, Mn, P, and S represent the weight percent of each alloying element.)

[0094] In addition, T in the above [Equation 1] p , T D And T can be applied as the average value of each corresponding value when there are two or more workpieces to be welded.

[0095] The above aluminum-zinc alloy plating layer surface refers to the surface in contact with the electrode. As an example, if there are two or more materials to be welded, the materials located at the top and bottom, respectively, may be the surfaces in contact with the electrode. The thickness of the above materials to be welded can be calculated by adding the average thickness of the base steel and the average thickness of the above aluminum-zinc alloy plating layer surface.

[0096] According to the resistance spot welding method for a hot-formed member provided in one embodiment of the present invention as described above, when resistance spot welding a hot-formed member suitable for automotive materials, particularly a hot-formed member having an aluminum-zinc alloy plating layer, the weldable current range can be dramatically increased, and more stable weld properties can be secured.

[0097] The present invention will be explained in more detail below through examples. However, it should be noted that the following examples are intended only to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention.

[0098] (Example)

[0099] Two sheets of base steel (Ceq: 0.28) were prepared, containing, in weight%, carbon (C): 0.19%, manganese (Mn): 1.13%, silicon (Si): 0.25%, phosphorus (P): 0.010%, sulfur (S): 0.002%, and the remainder being Fe and other unavoidable impurities, with a tensile strength of 1500 MPa and a thickness of 1.2 mm. Each of the above base steel sheets was subjected to molten aluminum-zinc plating treatment (plating bath components: in weight%, Si: 8.0~12.0%, Fe: 0.1~2.0%, Zn: 20.0~30.0%, and the remainder being Al and unavoidable impurities) to form an aluminum-zinc plating layer on both sides, thereby producing steel sheets for hot forming. Afterwards, each hot-forming steel plate was heated to 900°C and held for 5 minutes for heat treatment, and then hot-formed to produce a hot-formed member.

[0100] FIG. 3 is a photograph of a cross-section of a hot-formed member according to an embodiment of the present invention observed with an optical microscope. As shown in FIG. 3, the average thickness (T) of one side of the aluminum-zinc alloy plating layer p ) was 25㎛, and the average thickness (T) of one side of the Fe-Al intermetallic compound layer within one side of the aluminum-zinc alloy plating layer was D ) was 6.5㎛.

[0101] After stacking the two hot-formed members manufactured above, an electrode (ISO 5821 F1) was contacted and pressed on the weld area, and then a resistance spot welding process of [preliminary current application - cooling - main weld current application] was performed under the conditions shown in Tables 1 and 2 below.

[0102] For the hot-formed member manufactured in this way, the area of ​​the nugget relative to the total area of ​​the weld (the ratio of molten material within the weld), the welding current range, and the tensile strength of the weld were measured based on the cross-section in the thickness direction of the steel, and the results are shown in Tables 1 and 2 below.

[0103] The area of ​​the above nugget was measured using image analyzer software after photographing the above weldment with an optical microscope.

[0104] The above welding current range is set with the current at the point where spatter occurs as the upper limit and the current at the point where the nugget diameter is 4.0√t (t: thickness of the workpiece (mm)) as the lower limit, and the difference between them is calculated and presented.

[0105] The tensile strength of the weldment was measured by taking a tensile strength specimen from the part where stress is applied in the shear direction of the weldment and performing a tensile test. More specifically, specimens were taken from each of the top plate and the bottom plate with dimensions of 150 mm in width × 50 mm in length, and an overlapping area of ​​50 mm × 50 mm, and the maximum load was measured by performing a tensile test at a speed of 10 mm / min.

[0106] Classification Pre-pressurization Current-conducting Cooling Main Welding Current-conducting Electrode Tip Diameter (mm) Pressure (kN) Current (kA) Time (ms) Time (ms) Current (kA) Time (ms) Comparative Example 1 64.0---5.8320 Comparative Example 2 64.07.030106.0320 Comparative Example 3 64.08.0150308.2320 Invention Example 1 64.06.030305.8320 Invention Example 2 64.05.5120306.6320 Invention Example 3 64.08.060408.0320

[0107] Classification Holding Time (ms) Nugget (Molten) Area (Area %) [Equation 1] Satisfaction Status Welding Current Range (kA) Weld Strength (kN) Comparative Example 1 140 48.0 × 0.2 10.7 Comparative Example 2 100 42.3 × 0.0 11.3 Comparative Example 3 100 43.4 × 0.2 10.6 Invention Example 1 140 69.9 ○ 1.6 21.5 Invention Example 2 100 56.4 ○ 1.6 22.9 Invention Example 3 100 52.6 ○ 1.4 21.2 [Equation 1] (wherein T in [Equation 1] above p : Average thickness of one side of the aluminum-zinc alloy plating layer (㎛), T D : Average thickness (㎛) of the Fe-Al intermetallic compound layer on one side within the aluminum-zinc alloy plating layer, Ceq : Carbon equivalent of the substrate steel, T: Thickness of the workpiece (Thickness of the substrate steel + Average thickness of one side of the aluminum-dissipated alloy plating layer) (mm), f FZ : Percentage of molten material within the weld (area %), I: Lead current (kA), D: Electrode tip diameter (mm), t w : Lead time (ms), t c : refers to the cooling time (ms), and the above C eq is expressed by the above [Equation 2], where C, Si, Mn, P, and S represent the weight percent of each alloying element.)

[0108] As can be seen from Tables 1 and 2 above, in the case of Inventive Examples 1 to 3, which satisfy the conditions proposed by the present invention, the strength of the weld is excellent at 18 kN or more, and the weldable current range is also high to the desired level. On the other hand, in the case of Comparative Examples 1 to 3, which do not satisfy the conditions proposed by the present invention, the strength of the weld is significantly low, and the weldable current range is also very narrow.

[0109] More specifically, Comparative Example 1 is a case where the main welding current is applied without prior current application after electrode pressure. As a result, it can be confirmed that not only is the achievable appropriate welding current range narrow, but the weld strength is also inferior.

[0110] In the case of Comparative Example 2, which does not satisfy [Equation 1] and cooling time proposed by the present invention, it can be confirmed that not only is the welding current range very narrow, but the strength of the weld is significantly low due to molten material scattering.

[0111] In the case of Comparative Example 3, which does not satisfy [Equation 1] and the preceding current time proposed by the present invention, it can be confirmed that not only is the welding-capable current range very narrow, but the strength of the weld is significantly low due to molten material scattering.

[0112] FIG. 4 is a photograph of the cross-section of the welded portion of Invention Examples 1 to 3 according to an embodiment of the present invention observed with an optical microscope, where (a) is a photograph of Invention Example 1, (b) is a photograph of Invention Example 2, and (c) is a photograph of Invention Example 3. FIG. 5 is a photograph of the cross-section of the welded portion of Comparative Examples 1 to 3 according to an embodiment of the present invention observed with an optical microscope, where (a) is a photograph of Comparative Example 1, (b) is a photograph of Comparative Example 2, and (c) is a photograph of Comparative Example 3. As can be seen from FIG. 4 and 5, in the case of Invention Examples 1 to 3, it is possible to stably secure a molten amount of at least a certain level, whereas in the case of Comparative Examples 1 to 3, it can be confirmed that a molten amount of at least a certain level could not be secured.

[0113] [Explanation of the symbol]

[0114] 1: Base steel

[0115] 2: Fe-Al intermetallic compound layer

[0116] 3: Aluminum-zinc alloy plating layer

Claims

1. Aluminum-zinc alloy plated steel; and A welded portion formed by welding the above aluminum-zinc alloy plated steel material; comprising The above aluminum-zinc alloy plated steel comprises a base steel; and an aluminum-zinc alloy plating layer formed on at least one surface of the base steel. The above welded portion includes a nugget, The above nugget is a hot-formed member having an area of ​​50.0 to 70.0 percent relative to the total area of ​​the weldment based on the cross-section in the thickness direction of the member.

2. In Paragraph 1, The above-mentioned base steel is a hot-formed member comprising, in weight percent, carbon (C): 0.19~0.25%, manganese (Mn): 1.10~1.40%, silicon (Si): 0.001~0.40%, phosphorus (P): 0.001~0.025%, sulfur (S): 0.001~0.015%, and the remainder being Fe and other unavoidable impurities.

3. In Paragraph 1, The above aluminum-zinc alloy plating layer comprises, in weight percent, iron (Fe): 2~90%, silicon (Si): 0.1~10%, zinc (Zn): 10~98%, the remainder being aluminum (Al) and unavoidable impurities, a hot-formed member.

4. In Paragraph 1, The above aluminum-zinc alloy plating layer is a hot-formed member having a single-sided average thickness of 5.0 to 40.0 μm.

5. In Paragraph 1, The above aluminum-zinc alloy plating layer comprises a Fe-Al intermetallic compound layer having an average thickness of 0.1 to 10.0 μm, and is a hot-formed member.

6. In Paragraph 1, The above nugget is a hot-formed member having an area of ​​52.0 to 68.0 percent relative to the total area of ​​the weldment based on the cross-section in the thickness direction of the steel.

7. In Paragraph 1, The above nugget is a hot-formed member having an area of ​​54.0 to 66.0 percent relative to the total area of ​​the weldment based on the cross-section in the thickness direction of the steel.

8. In Paragraph 1, The above nugget is a hot-formed member having an area of ​​55.0 to 65.0 percent relative to the total area of ​​the weldment based on the cross-section in the thickness direction of the steel.

9. In Paragraph 1, The above welded portion is a hot-formed member having a tensile strength of 18 kN or more.

10. In Paragraph 1, The above hot-formed member is a hot-formed member having a current range of 1.2kA or more when current is applied during the main welding.

11. A step of preparing a hot-formed member comprising a base steel material; and an aluminum-zinc alloy plating layer on at least one surface of the base steel material; A step of contacting and pressing an electrode onto the welding target area of ​​the above hot-formed member; A step of applying a preliminary current to a hot-formed member to which the above electrode is applied; A step of cooling the previously energized hot-formed member; and The above-mentioned cooled hot-formed member includes a step of applying current to the main welding, and Resistance spot welding method for a hot-formed member satisfying the following [Equation 1]. [Equation 1] [식 2] C eq = C + (Si / 30) + (Mn / 20) + 2P + 4S (wherein T in [Equation 1] above p : Average thickness of one side of the aluminum-zinc alloy plating layer (㎛), T D : Average thickness (㎛) of the Fe-Al intermetallic compound layer on one side within the aluminum-zinc alloy plating layer, C eq : Carbon equivalent of the substrate steel, T: Thickness of the workpiece (Thickness of the substrate steel + Average thickness of one side of the aluminum-dissipated alloy plating layer) (mm), f FZ : Percentage of molten material within the weld (area %), I: Lead current (kA), D: Electrode tip diameter (mm), t w : Lead time (ms), t c : refers to the cooling time (ms), and the above C eq is expressed by the above [Equation 2], where C, Si, Mn, P, and S represent the weight percent of each alloying element.) 12. In Paragraph 11, A resistance spot welding method for a hot-formed member in which, during the above contact and pressurizing step, the pressurizing force is 1.0 to 10.0 kN.

13. In Paragraph 11, The above electrode is a resistance spot welding method for a hot-formed member having a tip diameter of 4.0 to 10.0 mm.

14. In Paragraph 11, A resistance spot welding method for a hot-formed member satisfying the following [Equation 3] when the above-mentioned preceding current is applied. [Equation 3] 30 ≤ t w ≤ 120 15. In Paragraph 11, A resistance spot welding method for a hot-formed member satisfying the following [Equation 4] when the above-mentioned preceding current is applied. [Equation 4] 2.0 ≤ I ≤ 15 16. In Paragraph 11, A resistance spot welding method for a hot-formed member satisfying the following [Equation 5] during the above cooling. [Equation 5] 30 ≤ t c ≤40 17. In Paragraph 11, The above-mentioned welding current application step is a resistance spot welding method for a hot-formed member, performed for 12 to 30 cycles with a current range of 6.0 to 7.5 kA.

18. In Paragraph 11, A resistance spot welding method for a hot-formed member, further comprising the step of maintaining the electrode in a pressurized state after the welding is performed.

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