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, the hot press-formed member being preferably usable as a material for home appliances and 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 home appliances and automobiles.

[0002] As smart appliances come to account for the majority of household appliances, there is an increasing application of functionally corrosion-resistant steel to break away from the monotonous designs of the past and extend the actual service life for users. Examples of such steel include cold-rolled steel sheets with various surface treatments, hot-dip galvanized steel sheets, and electro-galvanized steel sheets. In particular, electro-galvanized steel sheets, which are also used for automobile bodies, require high surface quality of the plating layer, specifically high corrosion resistance as well as high fingerprint resistance to ensure a beautiful appearance. Therefore, corrosion resistance and fingerprint resistance are generally secured by applying phosphates, chromates, or organic resins to the surface layer of the plating.

[0003] Specifically, the surface of electro-galvanized steel sheets can easily become contaminated by organic substances such as oil in the workplace during production and processing, or by user fingerprints during actual use. Such contaminated areas adversely affect the corrosion resistance and paintability of the steel sheet. To prevent this, the aforementioned phosphate, chromate, or organic resin is applied to the surface layer of the plating.

[0004] However, while electro-galvanized steel sheets treated with the aforementioned surface treatment exhibit excellent properties such as corrosion resistance and fingerprint resistance, the formation of an oxidizing film on the extreme surface of the plating layer due to chemical treatment is unavoidable. In particular, organic resins form not only an oxide film but also a resin-based film; this resin-based film acts as an electrical insulator, drastically increasing the surface resistance of the material. Furthermore, it adheres to the electrode during welding, acting as a contaminant that pollutes the electrode. This issue causes current failure during resistance welding and can be considered a major factor in degrading resistance spot weldability. Such poor weldability poses significant difficulties in ensuring the weld joint robustness required for actual parts production.

[0005] Accordingly, although many measures have been devised, such as improving surface treatment materials, to enhance the resistance spot weldability of surface-treated galvanized steel sheets, there is an urgent need to develop a resistance spot welding method that can provide robust welds to the product along with improved resistance spot weldability, without additional processes that increase the production costs of the material.

[0006] One aspect of the present invention is to provide a hot-formed member and a resistance spot welding method for the hot-formed member.

[0007] 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.

[0008] 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.

[0009] One embodiment of the present invention provides a hot-formed member comprising: a galvanized steel material; and a welded portion formed by welding the galvanized steel material; wherein the galvanized steel material comprises a base steel material; a galvanized layer formed on at least one surface of the base steel material; and a surface treatment layer formed on at least one surface of the galvanized layer; wherein the welded portion comprises a nugget, and the nugget has an area of ​​55.0 to 65.0 percent relative to the total area of ​​the welded portion based on a cross-section in the thickness direction of the steel material.

[0010] The above-mentioned steel material may contain, in weight percent, carbon (C): 0.01~0.06%, manganese (Mn): 0.10~0.25%, 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.

[0011] The above zinc plating layer may be an electro-zinc plating layer.

[0012] The above zinc plating layer has a plating adhesion amount of 0.1 to 200 g / m² 2 It could be.

[0013] The above zinc plating layer may have a thickness of 0.01 to 30 μm on one side.

[0014] The above surface treatment layer may be one or more of a resin-based surface treatment layer, a phosphoric acid-based surface treatment layer, and a silane-based surface treatment layer.

[0015] The above surface treatment layer has a surface treatment adhesion amount of 0.1 to 3000 mg / m² 2 It could be.

[0016] The above surface treatment layer may have a thickness of 0.1 to 3000 μm on one side.

[0017]

[0018] The above nugget may have an area of ​​56.0 to 64.0 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 ​​57.0 to 63.0 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 ​​58.0 to 62.0 percent relative to the total area of ​​the weldment based on the cross-section in the thickness direction of the steel.

[0021] The above weldment may have a cross tensile strength of 750 Kgf 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 material; a zinc plating layer formed on at least one surface of the base steel material; and a surface treatment layer formed on at least one surface of the zinc plating layer; a step of contacting and pressing an electrode to a welding target area of ​​the zinc-plated member; a step of applying a preliminary current to the zinc-plated member to which the electrode is pressed; a step of cooling the zinc-plated member to which the preliminary current is applied; and a step of applying a main welding current to the cooled zinc-plated member; wherein the value of A, expressed by the following [Equation 1], satisfies 11.0 to 30.0.

[0024] [Equation 1]

[0025] (wherein in [Equation 1] above, I is the leading current (kA), t p is the leading energization time (ms), w m Silver plating thickness (g / m²) 2 ), w o is the surface treatment adhesion amount (g / m²) 2 ), T is the thickness of the galvanized steel (mm), f FZ is the ratio (area %) occupied by the molten material within the weld zone.

[0026] The above surface treatment layer can be formed by applying a surface treatment solution through roll coating or bar coating.

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

[0028] When the above-mentioned preceding current is applied, the preceding current may be 2.0 to 10.0 kA.

[0029] When the above-mentioned preceding current is applied, the preceding current application time may be 0.01 to 20 ms.

[0030] During the above cooling step, the cooling time may be 1 to 50 ms.

[0031] The above welding current application step can be performed for 10 to 30 cycles with a current range of 6.0 to 8.5 kA.

[0032] After the above-mentioned welding current is applied, a step of maintaining the electrode in a pressurized state may be further included.

[0033] 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.

[0034] 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.

[0035] FIG. 1 illustrates a resistance spot welding method according to one embodiment of the present invention.

[0036] FIG. 2 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.

[0037] FIG. 3 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.

[0038] 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.

[0039] 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.

[0040] In drawings, the shapes and sizes of elements may be exaggerated for clearer explanation.

[0041] 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.

[0042] 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.

[0043] Unless otherwise specifically defined in the specification of the present invention, % units mean weight %.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] Hereinafter, a hot-formed member according to one embodiment of the present invention will be described.

[0048] The hot-formed member of the present invention comprises a galvanized steel material; and a welded portion formed by welding the galvanized steel material; wherein the galvanized steel material may comprise a base steel material; a galvanized layer formed on at least one surface of the base steel material; and a surface treatment layer formed on at least one surface of the galvanized layer. In the present invention, the shape of the hot-formed member is not specifically limited, and as an example, it may have a plate shape or a specific part shape.

[0049] In the present invention, the type of the above-mentioned base steel material is not specifically limited, and any material commonly used in the relevant technical field may be used.

[0050] For example, the above-mentioned base steel may contain, in weight percent, carbon (C): 0.01–0.06%, manganese (Mn): 0.10–0.25%, 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. Since the above-mentioned unavoidable impurities may be unintentionally incorporated during the ordinary manufacturing process, they cannot be excluded. As such impurities are known to any person skilled in the ordinary steel manufacturing field, all details thereof are not specifically mentioned in this specification.

[0051] In the present invention, the type of zinc plating layer is not specifically limited, and any type commonly used in the relevant technical field may be used.

[0052] As an example, the above zinc plating layer may be an electro-galvanized layer.

[0053] As an example, the zinc plating layer has a plating adhesion amount of 0.1 to 200 g / m² 2 It may be. The plating adhesion amount according to one embodiment of the present invention is 1 g / m² 2 It may be more than that. The plating adhesion amount according to one embodiment of the present invention is 10 g / m² 2 It may be more than that. The plating adhesion amount according to one embodiment of the present invention is 180 g / m² 2 It may be less than or equal to the following. The plating adhesion amount according to one embodiment of the present invention is 150 g / m² 2 It may be less than or equal to the following. The plating adhesion amount according to one embodiment of the present invention is 100 g / m² 2 It may be less than or equal to the following. The plating adhesion amount according to one embodiment of the present invention is 70 g / m² 2 It may be less than or equal to the following. The plating adhesion amount according to one embodiment of the present invention is 30 g / m² 2 It may be less than.

[0054] For example, the thickness of the zinc plating layer according to one embodiment of the present invention may be 0.01 to 30 μm on a single side. The thickness of the zinc plating layer according to one embodiment of the present invention may be 0.15 μm or more on a single side. The thickness of the zinc plating layer according to one embodiment of the present invention may be 25 μm or less on a single side. The thickness of the zinc plating layer according to one embodiment of the present invention may be 20 μm or less on a single side. The thickness of the zinc plating layer according to one embodiment of the present invention may be 15 μm or less on a single side. The thickness of the zinc plating layer according to one embodiment of the present invention may be 10 μm or less on a single side. The thickness of the zinc plating layer according to one embodiment of the present invention may be 5 μm or less on a single side.

[0055] In the present invention, the type of the surface treatment layer is not specifically limited, and any type commonly used in the relevant technical field may be used.

[0056] As an example, the above surface treatment layer may be one or more of a resin-based surface treatment layer, a phosphoric acid-based surface treatment layer, and a silane-based surface treatment layer.

[0057]

[0058] As an example, the surface treatment layer has a surface treatment adhesion amount of 0.1 to 3000 mg / m² 2 It may be. The surface treatment adhesion amount according to one embodiment of the present invention is 2000 mg / m² 2 It may be less than or equal to. The surface treatment adhesion amount according to one embodiment of the present invention is 1000 mg / m² 2 It may be less than or equal to. The surface treatment adhesion amount according to one embodiment of the present invention is 500 mg / m² 2 It may be less than.

[0059] For example, the thickness of the surface treatment layer according to one embodiment of the present invention may be 0.1 to 3000 μm on one side. The thickness of the surface treatment layer according to one embodiment of the present invention may be 2000 μm or less on one side. The thickness of the surface treatment layer according to one embodiment of the present invention may be 1000 μm or less on one side. The thickness of the surface treatment layer according to one embodiment of the present invention may be 500 μm or less on one side.

[0060] The thickness of the galvanized steel according to one embodiment of the present invention may be 0.1 to 10 mm.

[0061] Meanwhile, in a hot-formed member according to one embodiment of the present invention, the galvanized steel material may be welded to form a welded portion. The welded portion includes a nugget, and the nugget may have an area of ​​55.0 to 65.0 percent relative to the total area of ​​the welded portion based on the cross-section in the thickness direction of the steel material. If the area of ​​the nugget relative to the total area of ​​the welded portion is less than 55.0 percent, it may be difficult to expect sound welded portion performance due to insufficient molten material, and if it exceeds 65.0 percent, a softening phenomenon may occur in which the physical properties of the heat-affected zone deteriorate due to excessive heat input, which may result in fracture. The area of ​​the nugget according to one embodiment of the present invention may be 56.0 percent or more. The area of ​​the nugget according to one embodiment of the present invention may be 57.0 percent or more. The area of ​​the nugget according to one embodiment of the present invention may be 58.0 percent or more. According to one embodiment of the present invention, the area of ​​the nugget may be 64.0% or less. According to one embodiment of the present invention, the area of ​​the nugget may be 63.0% or less. According to one embodiment of the present invention, the area of ​​the nugget may be 62.0% or less. Meanwhile, the nugget may be formed as the molten material within the weld zone cools down in the area that melts during welding. Meanwhile, some pores may be formed within the nugget, and the area of ​​the nugget may be the area excluding the area of ​​the pores. The weld zone may be the combined area of ​​the area that melts during welding (the nugget) and the weld heat-affected zone (HAZ).

[0062] 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 cross-tensile strength of the weldment according to one embodiment of the present invention may be 750 Kgf or more. In the present invention, the upper limit of the strength of the weldment is not specifically limited, but as an example, the cross-tensile strength of the weldment may be 1200 Kgf or less.

[0063] 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 current application. 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 current application may be 1.2 kA or more. For example, when the lower limit of the main welding current application 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 current application is not specifically limited, but as an example, the current range during the main welding current application may be 15.0 kA or less. According to one embodiment of the present invention, the current range during the main welding current application may be 10.0 kA or less.

[0064] Hereinafter, a resistance spot welding method for a hot-formed member according to an embodiment of the present invention will be described. 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, and after cooling for a certain period of time, the section where current is applied second can be classified as the main welding application.

[0065] That is, a resistance spot welding method for galvanized steel according to one embodiment of the present invention may include preparing galvanized steel, contact and pressurization, pre-application of current, cooling, and application of current for main welding.

[0066] First, another embodiment of the present invention prepares a galvanized steel material comprising: a base steel material; a galvanizing layer formed on at least one surface of the base steel material; and a surface treatment layer formed on at least one surface of the galvanizing layer. The present invention does not specifically limit the preparation process of the galvanized steel material, and methods commonly used in the relevant technical field may be used. However, for example, the preparation may be made to satisfy the aforementioned type of base steel material, type and amount of galvanizing layer, type and amount of surface treatment layer, and thickness of the galvanized steel material.

[0067] Meanwhile, the above surface treatment layer can be formed by applying a surface treatment solution through roll coating or bar coating. As previously mentioned, the above surface treatment layer may be one or more of a resin-based surface treatment layer, a phosphoric acid-based surface treatment layer, and a silane-based surface treatment layer, and can be formed by applying a surface treatment solution capable of forming the surface treatment layer. As an example, the above resin-based surface treatment layer can be formed using a conductivity-enhancing resin solution (AC) or a weldability-enhancing resin solution (AW), etc.

[0068] Subsequently, an electrode is brought into contact with and pressed against the welding target area of ​​the galvanized steel. When bringing the electrode into contact with the welding target area of ​​the galvanized steel, the electrode may come into contact with the surface of the outermost layer where the surface treatment layer of the galvanized steel is formed. In the present invention, regarding the pressure applied when pressing the electrode, conventional conditions used in the relevant technical field may be applied and are not specifically limited. However, as an example, the pressure may be 1.0 to 10.0 kN during the step of contact and pressing. Furthermore, the electrode is a conventional electrode used in resistance spot welding and is not specifically limited in the present invention.

[0069] Meanwhile, when resistance spot welding is performed after stacking two or more galvanized steel materials, the steel material stacked at the top or bottom may have a galvanized layer and a surface treatment layer on its upper surface (the surface where the electrode contacts), and a galvanized layer or a surface treatment layer may or may not be formed on its lower surface (the surface where the electrode does not contact). In addition, the galvanized steel material provided between the top and bottom may also have a galvanized layer or a surface treatment layer formed on it, or may not be formed.

[0070] 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.

[0071] In one embodiment of the present invention, resistance spot welding is performed on a galvanized steel material in a state where an oxide film layer (e.g., a chromate film, etc.) formed on the surface and / or a surface treatment layer such as a resin-based film having electrical insulating properties is present. In this case, there is a problem in that frequent expulsion occurs, where molten material is scattered outward due to excessive heat input even when a low welding current is applied. Furthermore, if the oxide film layer and / or surface treatment layer are formed thickly, there is a very high possibility that a problem of inability to conduct current will occur even when a current above a certain level is applied. Accordingly, according to one embodiment of the present invention, the current conduction step can be separated when resistance spot welding is performed on a galvanized steel material.

[0072] Specifically, the pre-current application in the present invention is a process that forcibly lowers the high surface resistance inevitably generated during the manufacturing process. At the same time, the pre-current application can serve to create an environment where the oxide film layer and / or surface treatment layer present on the surface of the zinc-based plating layer can be discharged and removed to the outside of the weldment. Furthermore, the pre-current application is also intended to suppress the scattering of molten material that may occur due to excessive heat input and to ensure robust mechanical properties of the weldment.

[0073] Therefore, it is desirable to perform the aforementioned prior current application under conditions that allow the galvanized steel to melt to a certain level or higher, remove the oxide film layer and / or surface treatment layer of the welded area, and at the same time allow the molten material formed through the melting to be intentionally discharged.

[0074] In the present invention, the leading current during the leading current application is not specifically limited. However, as an example, the leading current during the leading current application may be 2.0 to 10.0 kA. According to one embodiment of the present invention, the leading current during the leading current application may be 2.0 to 8.0 kA.

[0075] In the present invention, the leading current time is not specifically limited during the leading current application. However, as an example, the leading current time during the leading current application may be 0.01 to 20 ms. If the leading current time is less than 0.01 ms, it may be difficult to secure a molten material of a certain level or higher, and if it exceeds 20 ms, the physical properties of the weld may deteriorate due to the generation of excessive heat input. According to one embodiment of the present invention, the leading current time may be 1 ms or more. According to one embodiment of the present invention, the leading current time may be 5 ms or more. According to one embodiment of the present invention, the leading current time may be 18 ms or less.

[0076] Subsequently, the aforementioned galvanized steel material that has been energized in advance is cooled. The cooling can be performed by cutting off the applied current after the preceding 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 preceding 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.

[0077] In the above cooling step, the cooling time may be 1 to 50 ms. In the present invention, in order to further improve the strength of the weld joint, the cooling may be performed for a maximum of 50 ms rather than completely cooling to room temperature so that the latent heat remaining after the preceding current application is completed can be used as a heat source. In the present invention, the lower limit of the cooling time is not specifically limited, but as an example, it may be performed for 1 ms or more.

[0078] Subsequently, the main welding current is applied to the cooled galvanized steel. Through the main welding current, a molten material of a certain level or higher can be secured, thereby obtaining a weld with the physical properties intended by the present invention. The current applied during the main welding is completely independent of the preceding current, and a resistance spot welding method commonly practiced in the relevant technical field can be applied; it is preferable to perform the welding under conditions that allow for securing a nugget of sufficient size. As an example, the main welding current can be applied for 10 to 30 cycles within a current range of 6.0 to 8.5 kA. If the welding current exceeds 8.5 kA or the main welding current application time exceeds 30 cycles during the main welding application, excessive spatter occurs, the effect of the preceding current proposed in the present invention is not obtained, numerous defects occur within the weld, and consequently, the physical properties of the weld 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 10 cycles, it may be difficult to secure 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.

[0079] After the main welding is performed, a process may be carried out 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 carried out for a period sufficient for the nugget formed during the main welding to solidify, the conditions thereof are not specifically limited.

[0080] In a resistance spot welding method for a hot-formed member according to one embodiment of the present invention, it is preferable that the value of A, expressed by the following [Equation 1], satisfies 11.0 to 30.0.

[0081] According to one embodiment of the present invention, by controlling the value of A, the effect of lowering the surface resistance of galvanized steel during pre-application can be obtained. In addition, by ensuring that molten material is generated above a certain level through subsequent main welding current application and suppressing bursting, a stable main welding current range can be secured and excellent spot weld properties can be secured. If the value of A below is 30.0 or less, only a small amount of melting can be carried out at the beginning of welding, and sufficient thermal energy can be transferred to the material to contribute to a reduction in the surface resistance of the workpiece. On the other hand, if the value of A exceeds 30.0, excessive scattering of molten material occurs from the pre-application stage, which may lead to an unnecessary reduction in the amount of molten material in the weld and a deterioration in the physical properties of the weld. Meanwhile, if the value of A is less than 11.0, only a small amount of melting of the workpiece can be carried out at the beginning of welding, and there is insufficient thermal energy to contribute to a reduction in surface resistance, and there may not be enough thermal energy to expel the surface oxide layer or surface treatment layer to the outside. In other words, during spot welding of galvanized steel, the melting process can be directly influenced by the metallurgical elements of the galvanized steel, such as the plating thickness, surface treatment thickness, and the thickness of the galvanized steel.

[0082] [Equation 1]

[0083] (wherein in [Equation 1] above, I is the leading current (kA), tp is the leading energization time (ms), w m Silver plating thickness (g / m²) 2 ), w o is the surface treatment adhesion amount (g / m²) 2 ), T is the thickness of the galvanized steel (mm), f FZ is the ratio (area %) occupied by the molten material within the weld zone.

[0084] According to the resistance spot welding method according to one embodiment of the present invention provided as described above, when resistance spot welding is performed on steel having a zinc plating layer that has been surface-treated with a solution having high resistance to surface contamination during use, the surface resistance caused by the oxide film layer and / or surface treatment layer generated during the surface treatment process can be lowered, thereby dramatically increasing the weldable current range. Furthermore, when current is applied during this welding process, the weldable current range can be increased, and more stable tensile properties of the weldment can be secured. In addition, the required weldment properties along with improved weldability can be secured without performing additional processes, such as removing the surface treatment layer or zinc plating layer, other than the resistance spot welding process.

[0085] 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.

[0086] (Example)

[0087] Two sheets of galvanized steel with a thickness of 1.0 mm or 1.2 mm were prepared by forming an electro-galvanized layer on both sides of a base steel material containing, in weight percent, carbon (C): 0.04%, manganese (Mn): 0.19%, silicon (Si): 0.001%, phosphorus (P): 0.01%, sulfur (S): 0.005%, and the remainder being Fe and other unavoidable impurities, and then forming a surface treatment layer by applying a silane-based resin solution (AC) onto the electro-galvanized layer via roll coating. At this time, the plating adhesion amount of the galvanized layer was 12 g / m² per side. 2 , 15g / m 2 , 20g / m 2 The two sheets of galvanized steel prepared in this manner were laminated, and then electrodes were brought into contact and pressed under the conditions listed in Tables 1 and 2 below. Subsequently, a hot-formed member was manufactured by applying current in advance, cooling, and then applying current to the main welding.

[0088] 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 cross-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.

[0089] The area of ​​the above nugget was measured using image analyzer software after photographing the above weldment with an optical microscope.

[0090] The above welding current range is set with the current at the point where spattering occurs as the upper limit and the current at the point where the nugget diameter is 4.0√t (t: thickness of steel (mm)) as the lower limit, and the difference between them is calculated and presented.

[0091] The tensile strength of the weldment was measured by taking cross-tensile strength specimens from the part where stress is applied in the vertical direction of the weldment and performing a tensile test. More specifically, specimens were taken with dimensions of 150 mm in width × 50 mm in length for the top plate and 50 mm in width × 150 mm in length for the bottom plate, and the maximum load was measured by performing a tensile test at a speed of 10 mm / min.

[0092] Classification Pre-pressurization Current-conducting Cooling Main Welding Current-conducting Electrode Pressure (kN) Current (kA) Time (ms) Time (ms) Current (kA) Time (ms) Comparative Example 1 ISO 5821 Standard EO Tip Diameter 4.5mm 2.0---6167 Comparative Example 2 2.05.5 25108.5167 Comparative Example 3 2.07.5 3107167 Invention Example 1 2.05.5 10107167 Invention Example 2 2.05.5 15106167 Invention Example 3 2.04.0 20108167

[0093] Classification plating amount (g / m²) 2 ) Thickness of galvanized layer (㎛) based on one side Surface treatment adhesion amount (mg / m²) 2 ) Surface treatment layer thickness (㎛) Steel thickness (mm) Nugget (molten) area (area %) [ Formula 1] Welding Current Range (kA) Weldment Cross Tensile Strength (kgf) Comparative Example 1 121.69 1.0 1.0 1.25 2.4 -0.44 60.8 Comparative Example 2 15 2.1 10.8 0.8 1.06 7.13 2.1 0.24 50.0 Comparative Example 3 20 2.8 20.7 0.7 1.25 1.5 5.0 0.44 40.9 Invention Example 1 121.69 1.0 1.0 1.26 2.2 17.4 1.47 57.0 Invention Example 2 15 2.1 10.8 0.8 1.05 5.2 15.9 1.47 86.5 Invention Example 3 20 2.8 20.7 0.7 1.26 3.4 11.8 1.27 82.8 [Equation 1] (wherein in [Equation 1] above, I is the leading current (kA), t p is the leading energization time (ms), w m Silver plating thickness (g / m²) 2 ), w o is the surface treatment adhesion amount (g / m²) 2 ), T is the thickness of the galvanized steel (mm), f FZis the ratio (area %) occupied by the molten material within the weld zone.

[0094] 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 750 Kgf or higher, 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.

[0095] 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.

[0096] In the case of Comparative Examples 2 and 3, which do not satisfy Equation 1 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.

[0097] FIG. 2 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. 3 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. 2 and 3, 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.

[0098] Although the present invention has been described in detail through embodiments above, other forms of embodiments are also possible. Therefore, the technical concept and scope of the claims described below are not limited to the embodiments.

Claims

1. Galvanized steel; and A welded portion formed by welding the above-mentioned galvanized steel material; comprising The above galvanized steel comprises: a base steel; a galvanized layer formed on at least one surface of the base steel; and a surface treatment layer formed on at least one surface of the galvanized layer. The above welded portion includes a nugget, 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.

2. In Paragraph 1, The above-mentioned base steel is a hot-formed member comprising, in weight percent, carbon (C): 0.01~0.06%, manganese (Mn): 0.10~0.25%, 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 zinc plating layer is an electro-galvanized layer of a hot-formed member.

4. In Paragraph 1, The above zinc plating layer has a plating adhesion amount of 0.1 to 200 g / m² 2 Hot-formed member.

5. In Paragraph 1, The above zinc plating layer is a hot-formed member having a thickness of 0.01 to 30 μm on one side.

6. In Paragraph 1, The above surface treatment layer is one or more of a resin-based surface treatment layer, a phosphoric acid-based surface treatment layer, and a silane-based surface treatment layer, and is a hot-formed member.

7. In Paragraph 1, The above surface treatment layer has a surface treatment adhesion amount of 0.1 to 3000 mg / m² 2 Hot-formed member.

8. In Paragraph 1, The above surface treatment layer is a hot-formed member having a thickness of 0.1 to 3000 μm on one side.

9. In Paragraph 1, The above nugget is a hot-formed member having an area of ​​56.0 to 64.0 percent relative to the total area of ​​the weldment based on the cross-section in the thickness direction of the steel.

10. In Paragraph 1, The above nugget is a hot-formed member having an area of ​​57.0 to 63.0 percent relative to the total area of ​​the weldment based on the cross-section in the thickness direction of the steel.

11. In Paragraph 1, The above nugget is a hot-formed member having an area of ​​58.0 to 62.0 percent relative to the total area of ​​the weldment based on the cross-section in the thickness direction of the steel.

12. In Paragraph 1, The above welded part is a hot-formed member having a cross-tensile strength of 750 Kgf or more.

13. 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.

14. A step of preparing a galvanized steel material comprising: a base steel material; a galvanized layer formed on at least one surface of the base steel material; and a surface treatment layer formed on at least one surface of the galvanized layer; A step of contacting and pressing an electrode onto the welding target area of ​​the above-mentioned galvanized steel; A step of performing a preliminary current flow to the zinc-plated steel material to which the above electrode is pressurized; The step of cooling the aforementioned previously energized galvanized steel material; and The method includes the step of applying current to the cooled galvanized steel material for the main welding process; A resistance spot welding method for a hot-formed member satisfying a value of A expressed by [Equation 1] below of 11.0 to 30.

0. [Equation 1] (wherein in [Equation 1] above, I is the leading current (kA), t p is the leading energization time (ms), w m Silver plating thickness (g / m²) 2 ), w o is the surface treatment adhesion amount (g / m²) 2 ), T is the thickness of the galvanized steel (mm), f FZ is the ratio (area %) occupied by the molten material within the weld zone.

15. In Paragraph 14, The above surface treatment layer is formed by applying a surface treatment solution through roll coating or bar coating. A resistance spot welding method for a hot-formed member.

16. In Paragraph 14, 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.

17. In Paragraph 14, Resistance spot welding method for a hot-formed member, wherein the preceding current is 2.0 to 10.0 kA when the preceding current is applied.

18. In Paragraph 14, Resistance spot welding method for a hot-formed member, wherein the leading current application time is 0.01 to 20 ms when the leading current application is described above.

19. In Paragraph 14, Resistance spot welding method for a hot-formed member, wherein the cooling time during the above cooling step is 1 to 50 ms.

20. In Paragraph 14, The above-mentioned welding current application step is a resistance spot welding method for a hot-formed member performed for 10 to 30 cycles with a current range of 6.0 to 8.5 kA.

21. In Paragraph 14, 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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