Patent Publication Number: US-2021170516-A1

Title: Method for preventing spot welding crack of galvanized steel plate

Description:
TECHNICAL FIELD 
     The present invention relates to a method for reducing cracking during spot-welding of a galvanized steel sheet. 
     BACKGROUND ART 
     Steel sheets galvanized with advanced high strength steels (hereinafter, “AHSS”) have recently been used as a material for vehicles. 
     Such galvanized AHSS has a transformed structure formed at a low temperature and thus has excellent strength and ductility, whilst having excellent corrosion resistance due to sacrificial anode corrosion of zinc. 
     In order to secure the high strength of galvanized AHSS, however, large amounts of alloy elements are added. Particularly in the case of galvanized steel sheets to which a large amount of carbon or manganese is added, cracking tends to frequently occur during spot welding. 
     The spot welding technology of AHSS steel sheets has so far been the most general method of controlling a welding current pattern to improve welding strength (Patent Documents 1 to 3). 
     However, as ultra high strength steel sheets having tensile strength of 1000 MPa or more have been developed, welding cracking due zinc has become a problem, but there is no appropriate means to resolve the problem. Further, similarly to the existing methods, pressing force and welding current patterns are only controlled during welding (Patent Documents 4 to 6), and thus, the problem cannot be fundamentally solved. 
     A main cause of a defect, such as cracking, during spot welding of a high-alloy galvanized AHSS is liquid metal embrittlement (hereinafter, “LME”), and prerequisites for the LME to occur are a load or deformation over a critical point, a molten metal and austenite appearance. 
     When spot welding is performed on a high-alloy galvanized AHSS, a temperature of the steel rises due to resistance heat as current is applied, and zinc having a low melting point first begins to melt. Thereafter, the steel is transformed into austenite. The lower the austenite formation temperature, the longer contact time between the molten zinc and the austenite structure of the steel on a surface. In this case, when a thermal stress and an external stress are applied, an austenite grain boundaries of a portion where the stress is concentrated glide and deform. When interfacial energy between the steel and the molten zinc is lower than that of the austenite grain boundaries, the grain boundary cracking occurs as the molten zinc penetrates into the austenite grain boundaries. 
       FIG. 1  shows a result of analyzing large cracks and micro cracks appearing when welding commercially available galvannealed AHSS having tensile strength of 1200 MPa. It is observed that a significant amount of zinc was detected in a cracked area. 
     Meanwhile, in order to minimize welding cracking due to LME, only one of the above-described three factors needs to be removed. 
     First, appearance of austenite is to be suppressed. As a welding temperature rises to a melting point during welding, the appearance of austenite in steel can never be prevented. Rather, the more the alloying elements, such as C and Mn, exist, the more the A3 temperature is reduced. Accordingly, high-alloy galvanized AHSS is more likely to crack due to LME as austenite appears at much lower temperatures. Next, a plating thickness is to be reduced such that the melting temperature of zinc or an amount of molten zinc is reduced. This is closely related to corrosion resistance and workability of the steel sheet. Besides, it is not easy as plating costs increase. Last, an external stress is to be reduced, which is difficult as the bonding strength and quality of the weld zone are related. 
     Most of the technologies, which have thus far been suggested, are related to controlling an austenite-appearing speed, a welding current pattern to reduce stress, controlling pressing force to reduce an external stress, and reducing an amount of a molten metal by reducing a thickness of a plating layer instead of improving corrosion resistance; however, none of the technologies can substantially and sufficiently suppress microcracking due to LME.
     Patent Document 1: Korean Patent Laid-Open Publication No. 2015-0132959   Patent Document 2: Korean Patent Laid-Open Publication No. 2006-0112701   Patent Document 3: Japanese Patent Laid-Open Publication No. 2008-093726   Patent Document 4: Korean Patent Laid-Open Publication No. 2014-0190199   Patent Document 5: Korean Patent Laid-Open Publication No. 2011-0071941   Patent Document 6: Korean Patent Laid-Open Publication No. 2013-0169656   

     DISCLOSURE 
     Technical Problem 
     An aspect of the present invention is to provide a method for preventing spot welding cracking of a galvanized steel sheet that can reduce the occurrence of cracks due to low melting point molten zinc during spot welding of a galvanized AHSS steel sheet and improve welding strength and fatigue life. 
     The technical problem of the present invention is not limited to the above. Additional problems are described throughout the specification, and those skilled in the art to which the present invention pertains will understand the additional problems based on the specification without any difficulty. 
     Technical Solution 
     According to an aspect of the present invention, a method for preventing cracking during spot-welding of a galvanized steel sheet includes preparing a galvanized steel sheet on which a plating layer is formed on at least one surface of a base steel sheet; molding the galvanized steel sheet; heating a welding spot of the molded galvanized steel sheet; and spot welding the heated welding spot, wherein the heating is performed using a current in the range of 0.4 times an exclusion-occurring current to the exclusion-occurring current. 
     Advantageous Effects 
     According to the present invention, effects of significantly improving welding strength and fatigue performance of a vehicle body can be obtained by suppressing a defect, such as cracking, or the like, due to LME when welding an existing galvanized advanced high strength steel (AHSS) having GIGA level strength. 
     Further, the present invention is advantageous in that an application of the GIGA level strength-galvanized AHSS can be extended and safety and durability of the vehicle body can be improved. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a result of analyzing large cracks and micro cracks appearing during welding of existing galvannealed AHSS having tensile strength of 1200 MPa. 
         FIG. 2  is a state diagram illustrating an iron-zinc alloy. 
         FIG. 3  is a diagram illustrating a degree of occurrence of LME cracking by heat treatment/non-treatment before welding of a galvanized steel sheet, which has been molded, observed according to an exemplary embodiment of the present invention. 
         FIG. 4  is diagrams illustrating an area change and a concentration change in a plating layer depending on an amount of a current during the heat treatment before welding of the molded galvanized steel sheet, according to an exemplary embodiment of the present invention. 
         FIG. 5  is diagrams illustrating a change in an iron (Fe concentration in the plating layer depending on a current amount during the heat treatment before welding of the galvannealed steel sheet, which has been molded, according to an exemplary embodiment of the present invention. 
         FIG. 6  is diagrams illustrating a cross-section of the spot weld zone depending on the heat treatment/non-treatment before welding of the galvannealed steel sheet, which has been molded, observed according to an exemplary embodiment of the present invention. 
     
    
    
     BEST MODE 
     The present inventors endeavored to find a method for more effectively suppressing cracking due to LME in terms of spot welding of a galvanized AHSS having GIGA level strength, as compared to conventional welding methods. 
     As a result, the present inventors have confirmed that the cracking in a weld zone after welding can be greatly reduced by heat treatment of the welding spot before welding of a gigapascal (GPa) level-galvanized AHSS, thereby completing the present invention. 
     In particular, the present inventors have found that the higher the iron (Fe) content in the welding spot during welding of the galvanized steel sheet, the more it can suppress the cracking due to LME during molding. In this regard, there is technical significance in providing an optimal method to increase the iron (Fe) content in a portion to be welded before welding of the galvanized steel sheet. 
     The present invention will hereinafter be described in detail. 
     According to an aspect of the present invention, a method for preventing cracking of spot-welding of a galvanized steel sheet, including preparing a galvanized steel sheet on which a plating layer is formed on at least one surface of a base steel sheet; molding the galvanized steel sheet; heating a welding spot of the molded galvanized steel sheet; and spot welding the heated welding spot, can be provided and is described in detail below. 
     A galvanized steel sheet, on which a plating layer is formed on at least one surface of a base steel sheet, may be formed. 
     The base steel sheet is not particularly limited, but is preferably an ultrahigh steel sheet for vehicles; for example, it may be an advanced high strength steel (AHSS) mainly used as a vehicle material due to its excellent properties. As an example, the AHSS may be a transformation induced plasticity (TRIP) steel, a complex phase (CP) steel, a dual phase (DP) steel, or the like, having a high content of carbon (C) or manganese (Mn) and high tensile strength of 980 MPa or above, but is not limited thereto. 
     As an aspect, a plating layer is provided on at least one surface of such base steel sheet, and the plating layer may be a zinc-based plating layer. The zinc-based plating layer may be formed by a conventional plating method. Plating conditions are not particularly limited. The plating layer may be a zinc-based alloying plating layer. 
     Thus-prepared galvanized steel sheet may be molded. 
     The molding is to obtain a desired shape. In one aspect, a method for pressing the AHSS galvanized steel sheet to have a desired shape using a mold can be applied. 
     Before welding thus-molded galvanized steel sheet, heating may be performed. Specifically, a portion for welding the molded galvanized steel sheet may be heated. 
     The heating may be performed in the same manner as the welding, which is described below. In an aspect, when the welding is spot welding, the heating may be performed in the same manner as the spot welding. 
     Before welding the molded galvanized steel sheet, the portion (welding spot) to be welded is treated with high temperature in advance to increase an Fe content in a plating layer, particularly the plating layer in contact with the base steel. In other words, when the welding spot is heated with a high temperature, Fe atoms in the base steel diffuse into the plating layer, and an Fe concentration in the plating layer rapidly increases. This gives rise to an increased melting point of the plating layer, which in turn makes it possible to obtain an effect of suppressing formation of molten zinc at high temperatures at which a deformation is easily generated. 
     The heating may be performed directly after the molding or directly before the welding. Alternately, the heating may be performed in a final molding process. 
     More specifically, the heat treatment may be performed using an electrode followed by cooling. The heat treatment may be performed using a current in the range of 0.4 times an exclusion-occurring current to the exclusion-occurring current for 0.5 seconds to 2 seconds. 
     When the current amount is less than 0.4 times the exclusion-occurring current, the heating cannot be completed in 0.5 seconds to 2 seconds and the Fe diffusion cannot sufficiently diffuse in the base steel. In contrast, the current amount above the exclusion-occurring current makes subsequent welding processes difficult due to excessive exclusion. 
     When the heating is performed within the above current range, the welding spot is heated to a temperature as high as 770° C. or above, preferably 770° C. to 950° C., thereby enabling the Fe atoms in the base steel to easily diffuse. 
     Meanwhile, the cooling may be air cooling. 
     The portion for welding, that is, the welding spot may have a size corresponding to a welded zone, which is produced in the subsequent welding. In an aspect, the welding spot may be set to have an area over 1.2 times a desired nugget diameter. 
     That is, the previously described heating may be performed in an area over 1.2 times a welding nugget diameter. 
     As used herein, the expression “desired nugget diameter” refers to a nugget diameter for obtaining required welding strength for subsequent welding. 
     When the welding spot is heated over the area over the nugget diameter, a tungsten or copper electrode may be used, but is not limited thereto. Any electrode used in conventional spot welding can be used. 
     The welding spot may be heated by applying a current to the welding spot using the electrode. 
     Further, when heating the welding spot using the electrode, the pressure of the electrode can be performed with a pressing force such that the base steel sheet is not deformed. 
     As an example, when a galvanized AHSS having a tensile strength of 980 MPa or more is heated using the tungsten or copper electrode, 40% or more of the current generating spatter may be applied for a welding time of ⅙ seconds. 
       FIG. 3  illustrates an observation of a degree of occurrence of LME cracking by heat treatment/non-treatment before welding of a galvanized steel sheet, which has been molded. 
     First, a steel plate (AHSS) having tensile strength of 980 MPa or more was produced as a 10 mm rod-shaped sample followed by electro-galvanizing to a thickness of 10 μm. Subsequently, one sample was first heat-treated at 800° C. for 1 second using an electric heating-type Gleeble resistance heating device, and then cooled, followed by heat treatment at 900° C. for 5 seconds. Another sample was heat-treated at 900° C. 5 seconds. The two samples were then subject to deform per temperature to observe whether cracking occurs. The heat treatment at 900° C. can be seen as a welding simulation condition. 
     As illustrated in  FIG. 3 , in the case of sample (a), which is not heat-treated at 800° C. for 1 second, the cracking due to LME is clearly observed. In contrast, no cracking due to LME after deformation was observed at all in heat-treated sample (b). 
       FIG. 4  is diagrams illustrating an area change and a concentration change in a plating layer depending on an amount of a current during the heat treatment before welding of the galvanized steel sheet, which has been molded. 
     The samples used in  FIG. 3  are used, and the current of heat treatment varied within the range of 4 kA to 9 kA. 
     According to (A) of  FIG. 4 , it can be seen that an heat-treated area increases as the amount of current increases. According to (B) of  FIG. 4 , the temperature increases as the amount of current increases during heating, thereby increasing the Fe concentration in the plating layer. 
     The welding spot heated as the above has an increased Fe concentration in the plating layer. It is preferable that the Fe concentration in the plating layer at an interface of the plating layer and the base steel be 65 wt % or more. 
     When the Fe concentration in the plating layer at an interface of the plating layer and the base steel is secured at 65 wt % or more by the heating process, the melting temperature of the plating layer increases. As such, a liquid metal does not exist in a temperature range in which the austenite grain boundaries are most vulnerable. That is, when the subsequent welding is performed while the melting temperature of the plating layer is increased, the liquid metal, such as molten zinc, is not generated at the temperature at which welding is performed. Accordingly, the molten metal may be prevented from penetrating into the austenite grain boundaries, thereby effectively blocking the occurrence of cracking, and the like, by LME. 
     In addition, the heated welding spot may have an average Fe concentration of 50 wt % or more in the entire plating layer. That is, when measuring glow discharge spectrometry (GDS) of the plating layer of the heated welding spot, an average Fe concentration, represented by Equation (1) below, may be 0.5 or more. 
     As previously described, the melting point of the plating layer can be increased to a temperature or above, for example, 900° C. or above, at which the liquid iron-zinc metal appears, by securing the Fe concentration in the plating layer at 50 wt % or above. This enables to prevent the penetration of the molten metal into the austenite grain boundaries during the subsequent welding. 
       Average Fe concentration in the plating layer=Fe (%)/(Fe (%)+Zn (%)).  Equation (1)
 
     Thus heat-treated galvanized steel sheet is spot welded. In this case, the pre-heated welding spot may be spot welded. 
     The spot welding may be performed under spot welding conditions for a conventional vehicle steel sheet, and the conditions are not particularly limited. 
     As previously described, as an aspect, a state diagram of the iron-zinc alloy of  FIG. 2  can describe that cracking due to LME during welding can be prevented by heat-treating the welding spot before welding. 
     A conventional galvannealed steel sheet (GA) or galvanized steel sheet (GI) contains 15 wt % or less of Fe in the galvanized layer. When such plated steel sheet is rapidly re-heated, molten zinc is produced at a temperature of 650° C. or less. In the case of the galvanized steel sheet having tensile strength of 980 MPa or more, however, deformation occurs due to a slip of the grain boundary when a deformation force acts in the temperature range of 750° C. to 850° C., which is a lowest austenite single-phase temperature range, and cracking easily occurs in the grain boundary when the grain boundary is brittle. In this case, in the presence of a molten metal, the molten metal easily penetrates into an austenite crystal grain boundary, and brittle fracture by molten zinc occurs due to a significant decrease in the grain boundary bonding force. Accordingly, in order to suppress defects, such as brittle fracture by LME, a liquid metal such as molten zinc needs to be removed. Further, a means for increasing the melting point of the liquid metal is provided in the present invention. 
       FIG. 5  is diagrams illustrating a change in an iron (Fe concentration in the plating layer depending on a current amount during the heat treatment before welding of the galvannealed steel sheet, which has been molded. The current amount refers to a heating means of the heat treatment. 
     As for the sample, to which a current of 4 kA to 9 kA is applied for ⅙ seconds, glow discharge-optical emission spectrometer (GDS) was used to completely remove the plating layer and measured until the base steel appears. A concentration curve of zinc was integrated to obtain the average Fe concentration (Fe (%)/(Fe (%)+Zn (%)) in the plating layer. 
     As illustrated in  FIG. 5 , the average Fe concentration in the plating layer of the sample (current of 0 kA), which is a GA material of 12% alloying degree without heat treatment, is 0.46, while in the heat-treated sample, the average Fe concentration in the plating layer increases as the current amount (4 kA to 9 kA) increases. 
       FIG. 6  is diagrams illustrating a cross-section of the spot-welded zone of the sample heat-treated with 4 kA current and that of the sample which is not heat-treated. 
     As illustrated in  FIG. 6 , in the case of sample (a), which is not heat-treated, the cracking remarkably occurs in the spot-welded zone. In contrast, in the case of the heat-treated sample (b), cracking is effectively suppressed in the spot-welded zone as compared to sample (a). 
     This is due to the fact that the molten zinc is sufficiently removed by the heat treatment by the means presented in the present invention before welding, and non-uniformity of contact resistance during spot welding is also reduced, thereby forming a larger nugget. This makes it advantageous in securing welding strength.