Welded joints and automotive parts

Abstract

The technology of the present disclosure is to suppress liquid metal embrittlement (LME) cracking in a welded joint obtained by spot welding a first steel sheet and a second steel sheet. In the welded joint of the present disclosure, a first plated layer is provided on the surface of the first steel sheet opposite the second steel sheet, no plated layer is present on the surface of the second steel sheet opposite the first steel sheet, or a second plated layer is provided, and a boundary plated layer is provided in a range of 0.5 mm from the end of a plastic metal ring region toward the outside of the spot welding portion between the first steel sheet and the second steel sheet. The higher tensile strength of the tensile strength of the first steel sheet and the tensile strength of the second steel sheet is 780 MPa or more, the area ratio of η-Zn phase in the cross section of the boundary plated layer is 5% or more, and the first plated layer and the second plated layer satisfy prescribed relationships I and II.

Description

Technical Field

[0001] This application discloses welded joints and automotive parts. Background Technology

[0002] When multiple coated steel sheets are joined by spot welding, metallic components in the coating may penetrate into the grain boundaries of the steel sheets, causing liquid metal embrittlement (LME) cracking. LME cracking is particularly problematic in high-strength steel sheets.

[0003] As a technique to suppress LME cracking during spot welding, Patent Document 1 discloses a technique that determines the post-weld holding time of the welding electrode based on a function of the total plate thickness during spot welding. Furthermore, although not directly related to LME cracking, Patent Document 2 discloses a technique that applies ultrasonic impact treatment to the spot weld area to open cracks in the weld area and inhibit the intrusion of moisture into the cracks.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2017-047045

[0007] Patent Document 2: Japanese Patent Application Publication No. 2005-103608 Summary of the Invention

[0008] The problem that the invention aims to solve

[0009] In previous techniques, LME cracking was suppressed through the processes and operations involved in spot welding. However, there has been insufficient research on suppressing LME cracking by focusing on the plating itself. In this respect, there is room for improvement in suppressing LME cracking in welded joints.

[0010] Methods for solving problems

[0011] As one means of solving the above-mentioned problems, this application discloses a welded joint comprising a first steel plate, a second steel plate, and a spot weld portion for joining the first steel plate and the second steel plate.

[0012] On the surface of the first steel plate opposite to the second steel plate, a first coating is provided.

[0013] On the surface of the second steel plate opposite to the first steel plate, there is either no coating or a second coating.

[0014] The aforementioned spot weld has a weld nugget and a corona bond.

[0015] A boundary plating layer is provided between the first steel plate and the second steel plate, and within a range of 0.5 mm from the end of the plastic metal ring region toward the outer side of the spot weld.

[0016] The higher of the tensile strengths of the first steel plate and the second steel plate is 780 MPa or higher.

[0017] The area fraction of the η-Zn phase in the cross-section of the aforementioned boundary coating is greater than 5%.

[0018] The first coating and the second coating satisfy the following relationships I and II.

[0019] Relationship I: 0.030 ≥ [(Al composition of the first coating layer (mass%)) × (adhesion amount of the first coating layer (g / m)] 2 ))+(Al composition of the second coating (mass%))×(Adhesion amount of the second coating (g / m)) 2 ))] / [(Zn composition of the first coating (mass%))×(Adhesion amount of the first coating (g / m)) 2 ))+(Zn composition of the second coating (mass%))×(Adhesion amount of the second coating (g / m) 2 ))]≥0.003

[0020] Relationship II: 0.001 ≥ [(Mg composition of the first coating above (mass%)) × (adhesion amount of the first coating above (g / m)] 2 ))+(Mg composition of the second coating (mass%))×(Adhesion amount of the second coating (g / m)) 2 ))] / [(Zn composition of the first coating (mass%))×(Adhesion amount of the first coating (g / m)) 2 ))+(Zn composition of the second coating (mass%))×(Adhesion amount of the second coating (g / m) 2 ))]

[0021] In the absence of the second coating layer, the Al composition, Zn composition, Mg composition, and adhesion amount of the second coating layer are all 0.

[0022] In the welded joint of this disclosure, the first coating and the second coating can also satisfy the following relationship I-1.

[0023] Relationship I-1: 0.010 ≥ [(Al composition of the first coating layer (mass%)) × (adhesion amount of the first coating layer (g / m)) 2 ))+(Al composition of the second coating (mass%))×(Adhesion amount of the second coating (g / m)) 2))] / [(Zn composition of the first coating (mass%))×(Adhesion amount of the first coating (g / m)) 2 ))+(Zn composition of the second coating (mass%))×(Adhesion amount of the second coating (g / m) 2 ))]≥0.003

[0024] In the welded joints disclosed herein, the aforementioned boundary coating may also have one or more oxides with a major diameter of 0.5 μm or more.

[0025] The welded joint of this disclosure may also have an internal oxide layer with a depth of 1.5 μm to 20.0 μm on one side of the first steel plate opposite to the second steel plate.

[0026] The welded joint disclosed herein can be used, for example, as an automotive component. For instance, an automotive component of this disclosure may also include the aforementioned welded joint, wherein the first steel plate is disposed on the outer side of the vehicle, the second steel plate is disposed on the inner side of the vehicle, and the second coating has a lower Al composition than the first coating.

[0027] Furthermore, the automotive component of this disclosure may also have the welded joint of this disclosure described above, with the first steel plate disposed on the outer side of the vehicle and the second steel plate disposed on the inner side of the vehicle, and the Mg composition of the second coating being lower than that of the first coating.

[0028] In the automotive components disclosed herein, the ratio of [Al composition (mass %) of the first coating layer] to [Zn composition (mass %) of the first coating layer] may also be greater than 0.030.

[0029] In the automotive parts disclosed herein, the ratio of [Mg composition (mass %) of the first coating layer] to [Zn composition (mass %) of the first coating layer] may also be greater than 0.001.

[0030] Invention Effects

[0031] In the welded joints disclosed herein, LME cracking is easily suppressed. Attached Figure Description

[0032] Figure 1 Here is a rough example of the cross-sectional structure of a welded joint.

[0033] Figure 2 This is a rough example of the cross-sectional structure of the Zn intrusion.

[0034] Figure 3 An example of the cross-sectional structure of the Zn intrusion portion.

[0035] Figure 4 An example that roughly illustrates the composition of a car component. Detailed Implementation

[0036] 1. Welded joint

[0037] like Figure 1 As shown, the welded joint 100 includes a first steel plate 10, a second steel plate 20, and a spot weld portion 30 that joins the first steel plate 10 and the second steel plate 20. A first plating layer 11 is provided on the surface of the first steel plate 10 opposite to the second steel plate 20. On the surface of the second steel plate 20 opposite to the first steel plate 10, there is no plating layer or a second plating layer 21 is provided. The spot weld portion 30 includes a weld nugget 31 and a plastic metal ring region 32. A boundary plating layer 50 is provided between the first steel plate 10 and the second steel plate 20, extending 0.5 mm outward from the end of the plastic metal ring region 32 toward the outside of the spot weld portion 30. With regard to the welded joint 100, the higher of the tensile strength of the first steel plate 10 and the tensile strength of the second steel plate 20 is 780 MPa or more, the area fraction of the η-Zn phase in the cross section of the boundary coating 50 is 5% or more, and the first coating 10 and the second coating 20 satisfy the following relationships I and II.

[0038] Relationship I: 0.030 ≥ [(Al composition of the first coating layer (mass%)) × (adhesion amount of the first coating layer (g / m)] 2 ))+(Al composition of the second coating (mass%))×(Adhesion amount of the second coating (g / m)) 2 ))] / [(Zn composition of the first coating (mass%))×(Adhesion amount of the first coating (g / m)) 2 ))+(Zn composition of the second coating (mass%))×(Adhesion amount of the second coating (g / m) 2 ))]≥0.003

[0039] Relationship II: 0.001 ≥ [(Mg composition of the first coating above (mass%)) × (adhesion amount of the first coating above (g / m)] 2 ))+(Mg composition of the second coating (mass%))×(Adhesion amount of the second coating (g / m)) 2 ))] / [(Zn composition of the first coating (mass%))×(Adhesion amount of the first coating (g / m)) 2 ))+(Zn composition of the second coating (mass%))×(Adhesion amount of the second coating (g / m) 2 ))]

[0040] 1.1 Steel Plate

[0041] In the welded joint 100, at least one of the first steel plate 10 and the second steel plate 20 has a tensile strength of 780 MPa or more. In other words, the higher of the tensile strengths of the first steel plate 10 and the second steel plate 20 is 780 MPa or more. That is, in the welded joint 100, the tensile strength of the first steel plate 10 may be 780 MPa or more and the tensile strength of the second steel plate 20 may be less than 780 MPa, or the tensile strength of the first steel plate 10 may be less than 780 MPa and the tensile strength of the second steel plate 20 may be 780 MPa or more, or both the first steel plate 10 and the second steel plate 20 may have a tensile strength of 780 MPa or more. Therefore, when the welded joint 100 includes a high-strength steel plate with a tensile strength of 780 MPa or more, the problem of LME cracking tends to become significant. The first steel plate 10 and the second steel plate 20 may have the same or different tensile strengths. Furthermore, in the welded joint 100, the higher of the tensile strengths of the first steel plate 10 and the second steel plate 20 may be 980 MPa or higher, 1180 MPa or higher, or 1470 MPa or higher. There is no particular upper limit to the tensile strength; for example, it may be below 2500 MPa, below 2200 MPa, or below 2000 MPa. It should be noted that the "tensile strength" of the steel plate referred to in this application is based on the tensile strength specified in ISO 6892-1:2009.

[0042] The effects of the welded joint 100 of this disclosure can be achieved regardless of the chemical composition and microstructure of the first steel plate 10 and the second steel plate 20. That is, as long as at least one of the first steel plate 10 and the second steel plate 20 has a tensile strength of 780 MPa or more, the chemical composition and microstructure of each steel plate are not particularly limited. The chemical composition and microstructure of the steel plates 10 and 20 can be appropriately determined according to the application of the welded joint 100, etc. The first steel plate 10 and the second steel plate 20 may, for example, have the following chemical composition: containing, by mass %: C: 0.01–0.50%, Si: 0.01–3.50%, Mn: 0.10–5.00%, P: less than 0.100%, S: less than 0.0300%, N: less than 0.0100%, O: 0–0.020%, Al: 0–1.000%, B: 0–0.010%, Nb: 0–0.150%, Ti: 0–0.20%, Mo: 0–3.00%, Cr The composition of the chemical composition is as follows: 0–2.00% V, 0–1.00% Ni, 0–2.00% W, 0–1.00% Ta, 0–0.10% Co, 0–3.00% Sn, 0–1.00% Sb, 0–0.50% Cu, 0–2.00% As, 0–0.050% Mg, 0–0.100% Ca, 0–0.100% Zr, 0–0.100% Hf, and 0–0.100% REM, with the remainder consisting of Fe and impurities. Furthermore, in the above chemical composition, the lower limit of the content of optionally added elements can also be 0.0001% or 0.001%.

[0043] There is no particular limitation on the thickness of the first steel plate 10 and the second steel plate 20. The plate thickness can be determined appropriately according to the application. For example, the plate thickness can be 0.5 mm or more, 0.8 mm or more, 1.0 mm or more, 1.2 mm or more, or 2.0 mm or more, and can also be less than 10.0 mm, 5.0 mm or less, 4.0 mm or less, or 3.0 mm or less. The plate thickness can be the same throughout the steel plate or it can vary in different parts of the steel plate.

[0044] 1.2 Coating

[0045] In the welded joint 100, a first plating layer 11 is provided on the surface of the first steel plate 10 opposite to the second steel plate 20. Furthermore, on the surface of the second steel plate 20 opposite to the first steel plate 10, there is either no plating layer or a second plating layer 21 is provided. Figure 1The diagram shows a configuration with both a first coating 11 and a second coating 21, but the configuration of the coating in the welded joint 100 is not limited to this. In the welded joint 100, the first steel plate 10 and the second steel plate 20 can be welded together with the coating sandwiched between them. It should be noted that the surface of the first steel plate 10 that is not opposite to the second steel plate 20 may or may not have a coating. Similarly, the surface of the second steel plate 20 that is not opposite to the first steel plate 10 may or may not have a coating. The first coating 11 and the second coating 21 may be of the same type or different types. The chemical composition of the first coating 11 and the second coating 21 is not particularly limited as long as it satisfies Relations I and II described later and satisfies the specified area ratio of the η-Zn phase in the boundary coating 50. The first coating 11 and the second coating 21 may be Zn-based coatings, and may have, for example, the following chemical compositions.

[0046] (A1: 0-90.0%)

[0047] By including Al in the coating, LME cracking becomes easier to suppress. Furthermore, the corrosion resistance of the coating may be improved. The Al content in each of the first coating 11 and the second coating 21, in mass %, can be 0%, or 0.010% or more, 0.100% or more, 0.500% or more, 1.0% or more, or 3.0% or more. Furthermore, the Al content in each of the first coating 11 and the second coating 21, in mass %, can be 90.0% or less, 80.0% or less, 70.0% or less, 60.0% or less, 50.0% or less, 40.0% or less, 30.0% or less, 20.0% or less, 10.0% or less, or 5.0% or less. However, as self-evident from the above relation I, at least one of the first coating 11 and the second coating 21 contains Al. In other words, in the welded joint 100, one of the first plating layer 11 and the second plating layer 21 may be free of Al, or either or both of the first plating layer 11 and the second plating layer 21 may contain Al.

[0048] (Mg: 0-60.0%)

[0049] By including Mg in the coating, it is possible to improve the corrosion resistance of the coating. On the other hand, in order to more effectively suppress LME cracking, it is preferable to reduce the Mg content in the coating. The Mg content in each of the first coating 11 and the second coating 21, in terms of mass%, can be 0%, or more than 0.001%, 0.005% or more, or more than 0.010% by mass%. Furthermore, the Mg content in each of the first coating 11 and the second coating 21, in terms of mass%, can be less than 60.0%, less than 40.0%, less than 20.0%, less than 10.0%, less than 5.0%, less than 1.0%, less than 0.100%, less than 0.080%, or less than 0.050% by mass%. In the welded joint 100, one or both of the first coating 11 and the second coating 21 may be Mg-free, or one or both of the first coating 11 and the second coating 21 may contain Mg.

[0050] (Fe: 0–65.0%)

[0051] When heat treatment is performed after a coating is formed on the surface of a steel sheet, Fe may diffuse from the steel sheet into the coating. The Fe content in each of the first coating 11 and the second coating 21, by mass%, can be 0%, or 1.0% or more, 2.0% or more, 3.0% or more, 4.0% or more, or 5.0% or more. Furthermore, the Fe content in each of the first coating and the second coating 21, by mass%, can be 65.0% or less, 55.0% or less, 45.0% or less, 35.0% or less, 25.0% or less, 15.0% or less, 12.0% or less, 10.0% or less, 8.0% or less, or 6.0% or less.

[0052] (Si: 0~10.0%)

[0053] By including Si in the coating, it is possible to improve the corrosion resistance of the coating. The Si content in each of the first coating 11 and the second coating 21, in terms of mass%, can be 0%, 0.005% or more, or 0.010% or more. Furthermore, the Si content in each of the first coating 11 and the second coating 21, in terms of mass%, can be 10.0% or less, 5.0% or less, 3.0% or less, 2.5% or less, 2.0% or less, 1.5% or less, or 1.0% or less.

[0054] (other)

[0055] The first coating 11 and the second coating 21 may each optionally contain, by mass percent, one or more of the following: Sb: 0-0.50%, Pb: 0-0.50%, Cu: 0-1.0%, Sn: 0-1.0%, Ti: 0-1.0%, Sr: 0-0.50%, Cr: 0-1.0%, Ni: 0-1.0%, and Mn: 0-1.0%. The total content of these optional added elements may, for example, be less than 5.0% or less than 2.0%.

[0056] The remaining components in the first coating 11 and the second coating 21, other than those described above, may consist of Zn and impurities. Examples of impurities in the first coating 11 and the second coating 21 include components that may be introduced during the formation of the first coating 11 and the second coating 21 due to various reasons related to the formation process, represented by the raw materials. The first coating 11 and the second coating 21 may also contain trace amounts of elements other than those described above.

[0057] The chemical composition of the coating can be determined by dissolving the coating in an acid solution containing an inhibitor that inhibits steel corrosion, and then measuring the resulting solution using ICP (inductively coupled plasma) luminescence spectrophotometry.

[0058] The thickness of each of the first coating 11 and the second coating 21 can be, for example, 3 μm or more, or 50 μm or less. Furthermore, the adhesion amount of each of the first coating 11 and the second coating 21 is not particularly limited, but for example, it can be 10 g / m² per single side of the steel plate. 2 The above can also be 170g / m 2 The following is an explanation of the coating adhesion amount, which is determined by the weight change before and after pickling when the coating is dissolved in an acid solution containing an inhibitor that suppresses corrosion of the base metal.

[0059] (Relationship I)

[0060] In the welded joint 100, the first plating layer 11 and the second plating layer 21 satisfy the following relationship I. In other words, the average Al / Zn obtained by weighting the composition contained in the first plating layer 11 and the second plating layer 21 considering the amount of plating adhered satisfies the following relationship I. It should be noted that in this disclosure, when referring to "average" in relation to chemical composition, it refers to this weighted average.

[0061] Relationship I: 0.030 ≥ [(Al composition of the first coating layer (mass%)) × (adhesion amount of the first coating layer (g / m)] 2 ))+(Al composition of the second coating (mass%))×(Adhesion amount of the second coating (g / m)) 2 ))] / [(Zn composition of the first coating (mass%))×(Adhesion amount of the first coating (g / m))2 ))+(Zn composition of the second coating (mass%))×(Adhesion amount of the second coating (g / m) 2 ))]≥0.003

[0062] The aforementioned relationship I refers to the following: when the chemical composition of the first coating 11 and the second coating 21 in the welded joint 100 is averaged, the mass ratio of Al to Zn (average Al / Zn) is 0.003 to 0.030. According to the inventor's new understanding, when the first coating 11 and the second coating 21 satisfy the aforementioned relationship I, in the boundary coating 50 described later, the rate of Zn intrusion into the grain boundaries of the steel plate slows down, making LME cracking less likely. The details of the effect of Al contained in coatings 11 and 12 on LME cracking in the boundary coating 50 are not clear, but can be considered, for example, as follows. That is, it is believed that when the mass ratio Al / Zn in the average composition of the first coating 11 and the second coating 21 of the welded joint 100 is within the range specified above, Al and Zn coexist in the boundary coating 50, and the stability of the liquid phase at the steel grain boundaries in the boundary coating 50 changes, thus suppressing the generation of Zn liquid phase that leads to LME cracking. The lower limit in the above relationship I can also be 0.003 or more, 0.005 or more, or 0.007 or more, and the upper limit can also be 0.025 or less, 0.020 or less, 0.015 or less, or 0.010 or less. In particular, when the first coating 11 and the second coating 21 in the welded joint 100 satisfy the following relationship I-1, LME cracking in the boundary coating 50 can be further effectively suppressed.

[0063] Relationship I-1: 0.010 ≥ [(Al composition of the first coating layer (mass%)) × (adhesion amount of the first coating layer (g / m)) 2 ))+(Al composition of the second coating (mass%))×(Adhesion amount of the second coating (g / m)) 2 ))] / [(Zn composition of the first coating (mass%))×(Adhesion amount of the first coating (g / m)) 2 ))+(Zn composition of the second coating (mass%))×(Adhesion amount of the second coating (g / m) 2 ))]≥0.003

[0064] (Relationship II)

[0065] In the welded joint 100, the first coating 11 and the second coating 21 satisfy the following relationship II. In other words, the average Mg / Zn obtained by weighting the composition of the first coating 11 and the second coating 21 considering the amount of coating adhesion satisfies the following relationship II.

[0066] Relationship II: 0.001 ≥ [(Mg composition of the first coating above (mass%)) × (adhesion amount of the first coating above (g / m)] 2 ))+(Mg composition of the second coating (mass%))×(Adhesion amount of the second coating (g / m)) 2 ))] / [(Zn composition of the first coating (mass%))×(Adhesion amount of the first coating (g / m)) 2 ))+(Zn composition of the second coating (mass%))×(Adhesion amount of the second coating (g / m) 2 ))]

[0067] Relationship II mentioned above refers to the following: when the average chemical composition of the first coating 11 and the second coating 21 in the welded joint 100 is combined, the mass ratio of Mg to Zn (average Mg / Zn) is 0.001 or less. According to the inventor's new understanding, when the first coating 11 and the second coating 21 in the welded joint 100 satisfy both Relationship I and II mentioned above, LME cracking in the boundary coating 50 is easily suppressed. The details of the effect of Mg contained in coatings 11 and 12 on LME cracking in the boundary coating 50 are not clear, but it can be considered, for example, as follows: It is believed that when the mass ratio of Mg / Zn in the average composition of the first coating 11 and the second coating 21 in the welded joint 100 is 0.001 or less, the Al content in the boundary coating 50 relatively increases, while the Mg content relatively decreases. This changes the stability of the liquid phase at the steel grain boundaries in the boundary coating 50, leading to the suppression of LME cracking. The upper limit in relation II above can also be below 0.0005. The lower limit in relation II above is not specifically limited and can also be 0.

[0068] Furthermore, as described above, the second plating layer 21 may also be absent in the welded joint 100. That is, when the second plating layer 21 is absent in relationships I and II described above, the Al composition, Zn composition, Mg composition, and adhesion amount of the second plating layer are 0.

[0069] Whether the above relationships I and II are satisfied in the welded joint 100 can be determined by measuring the chemical composition and adhesion amount of the first plating layer 11 and the second plating layer 21 of the welded joint 100. The chemical composition and adhesion amount of the first plating layer 11 and the second plating layer 21 can be confirmed, for example, only in the portions of plating layers 11 and 21 that are sufficiently far away from the spot weld portion 30.

[0070] 1.3 Spot weld section

[0071] In the welded joint 100, the first steel plate 10 and the second steel plate 20 are joined by a spot weld 30. For example... Figure 1As shown, if the first steel plate 10 and the second steel plate 20 are spot welded, a portion of the steel composition and / or coating composition, referred to as a melt nugget 31, is formed at the part where pressure is applied through the electrodes. Then, a plastic metal ring region 32, where the aforementioned components are joined without melting, is formed around the melt nugget 31. It should be noted that the "plastic metal ring region" refers to the area where the first and second steel plates are pressure-welded around the melt nugget. The coating at the portion normally located in the plastic metal ring region is extruded to the periphery of the plastic metal ring region during its formation. Sometimes, Zn may remain in the plastic metal ring region. In this case, Zn remains in a state of solid solution in the first and second steel plates. Even with solid solution Zn, the first and second steel plates are in contact without gaps in the plastic metal ring region, thus allowing identification of the end of the plastic metal ring region. Furthermore, the Zn intrusion portion described later is not formed by solid solution Zn in the plastic metal ring region. This is because the Zn in solid solution exists as the α-(Fe,Zn) phase even at high temperatures and does not precipitate. The melt nugget 31 and the ductile metal ring region 32, due to their different chemical compositions, can be easily distinguished, for example, by using a scanning electron microscope (SEM) with reflected electron images (BSE images). There are no particular limitations on the shape and composition of the melt nugget 31 in the welded joint 100.

[0072] 1.4 Separation Section

[0073] In the welded joint 100, a separation portion 40 exists around the spot weld portion 30 (around the ductile metal ring region 32). The separation portion 40 refers to the portion where no spot welding or pressure welding occurs. That is, the "separation portion" refers to the area around the ductile metal ring region where the first steel plate and the second steel plate are not in direct contact. For example... Figure 1 As shown, in the separation section 40 surrounding the spot weld section 30, no welding or pressure welding is performed between the first steel plate 10 and the second steel plate 20, and a gap may exist between the first steel plate 10 and the second steel plate 20. The size of the gap in the separation section 40 is not particularly limited.

[0074] 1.5 Boundary Coating

[0075] "Boundary coating" refers to the coating obtained by melting and solidifying around the plastic metal ring region through welding heat input. The boundary coating is also called the boundary portion. In the welded joint 100, the boundary coating 50 is contained within a range of 0.5 mm from the end of the plastic metal ring region 32 towards the outside of the spot weld portion 30. That is, the coating within the range of 0.5 mm from the end of the plastic metal ring region 32 towards the outside of the spot weld portion 30 of the coatings 11 and 21 present on the opposing surfaces of the first steel plate 10 and the second steel plate 20 becomes the boundary coating 50 through spot welding. Furthermore, the coating at the location of the plastic metal ring region 32 is squeezed out to the outside of the plastic metal ring region during spot welding to form part of the boundary coating 50. The higher the welding heat input of the spot weld, the further the range becoming the boundary coating 50 expands towards the outside of the spot weld. Furthermore, if there is no coating on the surface of the second steel plate 20 opposite to the first steel plate 10, it is possible that the coating of the first steel plate melts and wraps back to the side of the second steel plate 20 adjacent to the separation portion 40 of the plastic metal ring region 32, forming a boundary coating 50. The boundary coating 50 is as follows... Figure 1 As shown, it can have a fan-shaped (semi-circular) cross-sectional shape, or other shapes. The shape of the boundary plating 50 can vary depending on the spot welding conditions, etc.

[0076] In the welded joint 100, the boundary coating 50 contains components derived from the first coating 11 and the second coating 21. That is, the boundary coating 50 can be formed by solidifying the coatings 11, 21, etc., which are fused by spot welding. For example, when both the first coating 11 and the second coating 21 are present, the components derived from these two coatings 11, 21 are mixed with components derived from the steel plate in the boundary coating 50. That is, in addition to the components derived from the first coating 11 and the second coating 21, components derived from the steel plates 10 and 20 may also be present in the boundary coating 50. The chemical composition of the boundary coating 50, excluding components derived from the steel plate, can correspond to the average composition of the first coating 11 and the second coating 21 present in the welded joint 100. However, according to the inventor's understanding, the chemical composition in the boundary coating 50 fluctuates greatly, making it difficult to definitively determine the chemical composition in the boundary coating 50. In this respect, in the welded joint 100 of this disclosure, it is not necessary to determine the chemical composition of the boundary plating layer 50; it is sufficient to determine the average chemical composition of the first plating layer 11 and the second plating layer 21. That is, by satisfying the above-mentioned relationships I and II with respect to the first plating layer 11 and the second plating layer 21, the chemical composition of the boundary plating layer 50 is easily made to be a chemical composition effective in inhibiting LME cracking.

[0077] As described above, the chemical composition of the boundary coating 50 is not particularly limited. The boundary coating 50 may also have the following chemical composition in at least a portion thereof. Alternatively, the boundary coating 50 may also have the following chemical composition as its average chemical composition.

[0078] (Al / Zn: 0.003~0.030)

[0079] In at least a portion of the boundary coating 50, or in its average chemical composition, the mass ratio of Al to Zn (Al / Zn) can be 0.003 to 0.030. According to the inventors' new understanding, by including Al in the boundary coating 50 in such a specified range, the rate of Zn intrusion into the grain boundaries of the steel sheet is slowed, making LME cracking less likely. The presumed mechanism by which Al can suppress LME cracking is as described above. In at least a portion of the boundary coating 50, or in its average chemical composition, the mass ratio of Al to Zn (Al / Zn) can be 0.003 or more, 0.005 or more, or 0.007 or more, or 0.025 or less, 0.020 or less, 0.015 or less, or 0.010 or less.

[0080] (Mg / Zn: below 0.001)

[0081] In at least a portion of the boundary coating 50, or in its average chemical composition, the mass ratio of Mg to Zn (Mg / Zn) may be 0.001 or less. According to the inventors' new understanding, by having the Mg / Zn ratio in the boundary coating 50 below a specified value, LME cracking becomes less likely to occur. That is, in the boundary coating 50, in order to effectively exhibit the LME suppression effect brought about by Al, it is preferable to reduce the amount of Mg. In at least a portion of the boundary coating 50, or in its average chemical composition, the mass ratio of Mg to Zn (Mg / Zn) may be 0.0005 or less, or may be substantially 0.

[0082] (Fe: less than 65.0% by mass)

[0083] The concentration of Fe in at least a portion of the boundary coating 50, or in its average chemical composition, may be 65.0% by mass or less, 55.0% by mass or less, 45.0% by mass or less, 35.0% by mass or less, 25.0% by mass or less, 15.0% by mass or less, 12.0% by mass or less, 10.0% by mass or less, 8.0% by mass or less, or 6.0% by mass or less. As described above, the boundary coating 50 is formed by the melting and mixing of metal components from coatings 11 and 21 and metal components from steel plates 10 and 20 during spot welding. That is, during spot welding, Fe can diffuse from steel plates 10 and 20 to the boundary coating 50. According to the inventor's new understanding, when the concentration of Fe coexisting with liquid Zn during spot welding is low (i.e., the diffusion of Fe from steel plates 10 and 20 to the boundary coating 50 is small), there is a tendency to suppress the intrusion of Zn into steel plates 10 and 20. To suppress the diffusion of Fe from steel plates 10 and 20 to the boundary coating 50, it is effective, for example, to perform an internal oxidation treatment, described later, on at least one of the steel plates 10 and 20. It should be noted that even if the coatings 11 and 21 before welding contain a large amount of Fe through hot stamping or the like, the concentration of Fe coexisting with the liquid Zn during welding may not necessarily increase. This is because Fe diffused into the coatings 11 and 21 through hot stamping or the like can form high-melting-point intermetallic compounds with other metals, making them difficult to melt during welding.

[0084] (Other ingredients)

[0085] In the boundary coating 50, there is no particular limitation on the content of components other than those described above. For example, the boundary coating 50 may contain 0.001% to 10.0% by mass of Si in at least a portion thereof or in its average chemical composition. Furthermore, as described above, the boundary coating 50 may also contain other elements or impurities derived from coatings 11, 21 or steel plates 10, 20.

[0086] (η-Zn phase: area ratio of 5% or more)

[0087] As described above, when the Fe concentration in the boundary coating 50 is low, there is a tendency to suppress the intrusion of Zn into the steel plates 10 and 20. That is, it is preferable that the boundary coating 50 contains a certain amount or more of a phase with a low Fe concentration. According to the inventors' understanding, when the area fraction of the η-Zn phase in the cross-section of the boundary coating 50 is 5% or more, LME cracking is easily suppressed. "η-Zn phase" refers to a Zn phase with an Fe concentration of 5% by mass or less. The area fraction of the η-Zn phase in the cross-section of the boundary coating 50 can also be 10% or more, 15% or more, or 20% or more, or it can be 90% or less, 80% or less, or 70% or less. The area fraction of the η-Zn phase in the cross-section of the boundary coating 50 can be determined by measuring the metal composition, etc., in the cross-section of the boundary coating 50. The method for determining the area fraction of the η-Zn phase is described in detail in the examples.

[0088] (Oxides)

[0089] The boundary coating 50 may also have one or more oxides with a major diameter of 0.5 μm or more. That is, when observing a cross-section of the boundary coating 50, oxides with a major diameter of 0.5 μm or more may be present. Furthermore, the boundary coating 50 may have two or more, three or more, five or more, ten or more, or twenty or more oxides with a major diameter of 0.5 μm or more. Moreover, the boundary coating 50 may have two or more, three or more, five or more, ten or more, or twenty or more oxides with a major diameter of 1.5 μm or more. As described below, when at least one of the steel plates 10 and 20 undergoes internal oxidation treatment, during spot welding, the internal oxides can diffuse from the steel plates 10 and 20 to the boundary coating 50. These internal oxides can be obtained by performing a prescribed annealing treatment (including pre-annealing treatment) on the steel. In addition to oxygen, the oxide contains one or more elements from the steel plates 10 and 20, typically Si, O, and Fe, and may further contain Mn depending on the circumstances. More specifically, the oxide typically contains 5-25% Si, 0-10% Mn, 40-65% O, and 10-30% Fe. The oxide may also contain the elements mentioned above in addition to these. The oxide may also be an oxide containing Si and / or Mn. Oxides containing Si and / or Mn can promote the formation of an insulating film of corrosion products in corrosive environments. Therefore, the corrosion resistance of the weld joint 100 may be improved. It should be noted that the "major diameter" of the oxide refers to the length of the longest line segment that can be transversely cut from the oxide. The shape of the oxide is not particularly limited and may be circular, approximately circular, elliptical, polygonal, etc. The major diameter of the oxide may also be 0.7 μm or more, 1.0 μm or more, or 1.5 μm or more. There is no particular upper limit to the major axis of the oxide; for example, it can be below 10.0 μm.

[0090] 1.6Zn invasion part

[0091] like Figure 2 and 3 As shown, in the welded joint 100, a Zn intrusion portion 60 may also be present in at least one of the first steel plate 10 and the second steel plate 20 adjacent to the boundary coating 50. The Zn intrusion portion 60 may advance along the steel grain boundaries from the boundary coating 50. The Zn intrusion portion 60 can be formed by the Zn contained in the coating penetrating into the steel grain boundaries of the steel plates during spot welding. It should be noted that although the Zn intrusion portion 60 is present in the welded joint 100, it does not cause LME cracking.

[0092] like Figure 2 As shown, the Zn penetration portion 60 may also include a first portion 61 formed by the diffusion of a liquid phase containing components from coatings 11 and 21 during spot welding, and a second portion 62 formed by the diffusion of a solid phase containing components from coatings 11 and 21 during spot welding. The first portion 61 may exist on the surface side of the steel plate closer than the second portion 62. Furthermore, as Figure 2 As shown, the Zn intrusion portion 60 has a front end 60a. Here, in this application, the Zn concentration is measured from the surface side of the steel plate toward the interior along the steel grain boundaries where Zn has intruded until the Zn concentration becomes 0.1% or less, and the position where the Zn concentration becomes 0.1% (within the range of 0.095 to 0.014%) is defined as "the front end 60a of the Zn intrusion portion 60".

[0093] It should be noted that if any of the above-mentioned Relationship I, Relationship II, and the area fraction of the η-Zn phase in the boundary coating 50 is not satisfied (5% or more), there is a tendency for the Zn concentration or Al concentration to be low near the tip of the Zn intrusion 60 (e.g., at a position 1.5 μm from the tip). In this case, the Zn intrusion 60 may become embrittled, potentially inducing LME cracking.

[0094] It should be noted that the Zn and Al concentrations at a position 1.5 μm from the tip of the Zn intrusion are measured as follows. Specifically, the Zn concentration is measured along the Zn-intruded grain boundaries until it falls below 0.1%, and the position where the Zn concentration reaches 0.1% is defined as the tip of the Zn intrusion. EDS line scans are then performed along the Zn intrusion at a position 1.5 μm from this tip, in a direction perpendicular to the Zn intrusion, to analyze the maximum Zn and Al concentrations.

[0095] 1.7 Supplement

[0096] As described above, in the welded joint 100, at least one of the first steel plate 10 and the second steel plate 20 may also be a steel plate that has undergone internal oxidation treatment. For example, in the welded joint 100, at least one of the first steel plate 10 and the second steel plate 20 may also have an internal oxide layer with a depth of 1.5 μm to 20.0 μm. More specifically, for example, the welded joint 100 may also have an internal oxide layer with a depth of 1.5 μm to 20.0 μm on one side of the first steel plate 10 opposite to the second steel plate 20. It should be noted that the "depth" of the internal oxide layer refers to the depth from the surface of the steel plate (base metal). When at least one of the first steel plate 10 and the second steel plate 20 has a specified internal oxide layer, LME cracking is easily suppressed, as described above. In particular, LME cracking in steel plates with internal oxide layers is easily suppressed.

[0097] As described above, the welded joint 100 may include both a first plating layer 11 and a second plating layer 21. For example, the first plating layer 11 may be present on the surface of the first steel plate 10 opposite to the second steel plate 20. Furthermore, the second plating layer 21 may be present on the surface of the second steel plate 20 opposite to the first steel plate 10. Additionally, at least one of the first plating layer 11 and the second plating layer 21 may contain both Zn and Al. Furthermore, the boundary plating layer 50 may contain components derived from the first plating layer 11 and components derived from the second plating layer 21.

[0098] In the above description, the welded joint 100 has been described as having a first steel plate 10 and a second steel plate 20. However, in addition to having a first steel plate 10 and a second steel plate 20, the welded joint 100 may also have other steel plates. That is, the welded joint 100 may also be a joint obtained by overlapping three or more steel plates and joining them by spot welding. Furthermore, the welded joint 100 may also have multiple spot weld portions. In any case, the welded joint 100 only needs to have the first steel plate 10, the second steel plate 20, the spot weld portion 30, and the portion considered as the boundary plating layer 50 in at least a portion. That is, when multiple spot weld portions are provided, there may be spot weld portions in a portion of the multiple boundary plating layers that do not meet the conditions of the boundary plating layer 50 described above.

[0099] 2. Manufacturing method of welded joints

[0100] The manufacturing method of the welded joint 100 may include: (1) manufacturing a first steel plate 10 and a second steel plate 20, wherein a first coating 11 is provided on the surface of the first steel plate 10 opposite to the second steel plate 20, and no coating or a second coating 21 is provided on the surface of the second steel plate 20 opposite to the first steel plate 10, at least one of the first coating 11 and the second coating 21 comprising Zn and Al, and the higher of the tensile strength of the first steel plate 10 and the tensile strength of the second steel plate 20 being 780 MPa or more; and (2) spot welding is performed on the basis of overlapping the first steel plate 10 and the second steel plate 20 in a manner with the coating sandwiched between them. Hereinafter, an example of a manufacturing method of the welded joint 100 will be described, but the welded joint 100 may also be manufactured by other methods.

[0101] 2.1 Manufacturing conditions of steel plates

[0102] Steel sheets can be obtained, for example, by performing the following processes: a casting process in which molten steel with adjusted composition is cast to form a billet; a hot rolling process in which the billet is hot-rolled to obtain a hot-rolled steel sheet; a coiling process in which the hot-rolled steel sheet is coiled; a cold rolling process in which the coiled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet; a pretreatment process in which the cold-rolled steel sheet is electroplated; and an annealing process in which the pretreatment cold-rolled steel sheet is annealed. Alternatively, after the hot rolling process, coiling can be omitted, and the sheet can be pickled and then directly subjected to the cold rolling process. Afterward, a coated steel sheet is manufactured by coating the surface of the steel sheet.

[0103] (Casting process)

[0104] There are no particular restrictions on the conditions of the casting process. For example, after smelting in a blast furnace or electric furnace, various secondary refining processes can be carried out, followed by casting through conventional continuous casting or casting using ingot casting methods.

[0105] (Hot rolling process)

[0106] Hot-rolled steel sheets can be obtained by hot rolling cast steel billets as described above. The hot rolling process involves directly hot rolling the cast steel billet or temporarily cooling it before reheating and hot rolling. In the case of reheating, the heating temperature of the steel billet can be, for example, 1100℃ to 1250℃. The hot rolling process typically includes roughing and finishing rolling. The temperature and reduction rate of each rolling stage can be appropriately varied according to the desired microstructure and plate thickness. For example, the finishing rolling end temperature can be set to 900–1050℃, and the finishing rolling reduction rate can be set to 10–50%.

[0107] (Winding process)

[0108] Hot-rolled steel sheets can be coiled at a specified temperature. The coiling temperature can be appropriately varied according to the desired metal structure, for example, it can be 500–800°C. Alternatively, the hot-rolled steel sheet can be uncoiled before or after coiling and subjected to a specified heat treatment. Alternatively, the coiling process can be omitted, and pickling followed by the cold rolling process described later can be performed.

[0109] (Cold rolling process)

[0110] After pickling and other processes, hot-rolled steel sheets can be cold-rolled to obtain cold-rolled steel sheets. The reduction rate during cold rolling can be appropriately adjusted according to the desired metal structure and sheet thickness, for example, it can be 20% to 80%. After the cold rolling process, the sheet can be cooled to room temperature, for example, by air cooling.

[0111] (Pre-processing step)

[0112] When a pretreatment process is performed before annealing the cold-rolled steel sheet, the external oxide film formed on the surface of the steel sheet during the rolling process is appropriately removed, making it easier for oxygen to penetrate into the interior of the steel during annealing, thus promoting the formation of oxides inside the steel sheet. Furthermore, it is also possible to promote the formation of oxides inside the steel sheet by introducing strain or the like into the surface layer. That is, with such a pretreatment process, the desired internal oxides are more easily generated during the annealing process described later. This pretreatment process may also include grinding or electrolytic treatment using brushes or the like. For example, grinding may include applying an aqueous solution containing 0.5 to 4.0% by mass NaOH to the cold-rolled steel sheet and performing brush grinding with a brush reduction of 0.5 to 4.0 mm and a rotation speed of 200 to 1200 rpm. Electrolytic treatment may, for example, involve energizing the cold-rolled steel sheet in a solution with a pH of 8.0 or higher. The current density during energizing is 1.0 to 8.0 A / dm³. 2 It is advisable to use pH, current density, and energizing time that are within these parameters. By controlling the pH, current density, and energizing time during the energizing process, internal oxides can be effectively formed during the annealing process described later.

[0113] (Annealing process)

[0114] Annealing is preferably performed under a tension of 0.1 to 20 MPa. When tension is applied during annealing, strain can be introduced into the steel sheet more effectively, and oxides are more easily formed inside the steel sheet.

[0115] To ensure proper formation of the internal oxide layer, the holding temperature during the annealing process is preferably 700–900°C, and more preferably 720–870°C. Setting the temperature within this range helps suppress the formation of the external oxide layer and allows oxides to form internally within the steel sheet. If the holding temperature is below 700°C, the desired internal oxide layer may not form sufficiently during annealing. If the holding temperature exceeds 900°C, the external oxide layer is more easily formed during annealing. The heating rate up to the holding temperature is not particularly limited, but can be 1–10°C / second. Alternatively, the heating can be performed in two stages: a first heating rate of 1–10°C / second and a second heating rate of 1–10°C / second, different from the first heating rate.

[0116] The holding time at the holding temperature in the above-mentioned annealing process can be 10 to 300 seconds, or 30 to 250 seconds. By setting it within this range, the formation of an external oxide layer can be suppressed, and oxides can be formed inside the steel sheet. If the holding time is less than 10 seconds, the desired internal oxide may not be sufficiently formed during annealing. If the holding time exceeds 300 seconds, an external oxide layer is more likely to form during annealing.

[0117] From the viewpoint of fully generating internal oxides, the dew point of the atmosphere in the annealing process should preferably be -20 to 10°C, and more preferably -10 to 5°C.

[0118] Furthermore, oxides (typically including grain boundary oxides) formed inside the steel sheet during previous processes can be removed before the annealing process. An internal oxide layer may form on the surface of the steel sheet during the aforementioned rolling processes, particularly hot rolling. This internal oxide layer formed during such rolling processes may hinder the formation of internal oxides during the annealing process; therefore, it can also be removed before annealing by pickling or similar treatments. For example, to anticipate the growth of the internal oxide layer during the annealing process, it is preferable to set the depth of the internal oxide layer in the cold-rolled steel sheet before annealing to be less than 1.5 μm, less than 1.0 μm, less than 0.5 μm, less than 0.3 μm, less than 0.2 μm, or less than 0.1 μm.

[0119] As described above, when manufacturing steel plates 10 and 20, it is effective to form internal oxides on the surface layer of the steel plates (e.g., the region from the surface of the steel plate to 20 μm, i.e., the interior of the steel plate). For example, in order to suppress LME cracking in the first steel plate 10, it is preferable to form the aforementioned internal oxides on the surface layer of the first steel plate 10. Examples of such internal oxides include granular oxides dispersed in a granular manner within the grains or at the grain boundaries of the steel, grain boundary oxides existing along the grain boundaries of the steel, and / or dendritic oxides existing in a dendritic manner within the grains. When internal oxidation treatment is performed on the steel plates 10 and 20, for example, the Si contained in the steel plates 10 and 20 is oxidized, resulting in a state of Si deficiency at the surface layer of the steel plates 10 and 20. According to the inventor's new understanding, if Si is lacking at the surface layer of the steel plates 10 and 20, Zn becomes difficult to form a liquid phase at the surface layer of the steel plates 10 and 20. That is, in the welded joint 100, when at least one of the first steel plate 10 and the second steel plate 20 has undergone suitable internal oxidation treatment, Zn is difficult to become a liquid phase at the surface of the steel plates 10 and 20. As a result, Zn becomes difficult to penetrate into the interior of the steel plates 10 and 20, which can suppress LME cracking. When an oxide film is formed on the surface (outside) of the steel plates 10 and 20, that is, when an external oxide layer is formed, it is difficult to obtain the above-mentioned effect.

[0120] 2.2 Plating process

[0121] A coating is formed on the surface of a steel sheet through a plating process. The plating process can be performed according to methods known to those skilled in the art. For example, the plating process can be performed by hot-dip plating or electroplating. Hot-dip plating is preferred. The conditions of the plating process can be appropriately set considering the desired coating composition, thickness, and adhesion amount. Alloying can also be performed after the plating process. Typically, the conditions of the plating process are preferably set to form a coating comprising: Al: 0–90.0%, Mg: 0–60.0%, Fe: 0–15.0%, and Si: 0–10.0%, with the remainder consisting of Zn and impurities.

[0122] 2.3 Spot welding conditions

[0123] After manufacturing steel plates 10 and 20 as described above, overlap steel plates 10 and 20 and spot weld at least one location. The spot welding conditions can be any conditions known to those skilled in the art. For example, a welding electrode with a dome radius and a front diameter of 6–8 mm can be used, with a pressure of 1.5–6.0 kN, an energizing time of 0.1–1.0 seconds (5–50 cycles, power frequency of 50 Hz), and an energizing current of 4–15 kA for spot welding.

[0124] As described above, and as in the welded joint 100 of this disclosure, the first plating layer 11 and the second plating layer 21 satisfy the specified relationships I and II, and the metal structure (η-Zn phase) in the cross section of the boundary plating layer 50 satisfies the specified area ratio, thereby easily suppressing LME cracking.

[0125] 3. Applications of welded joints

[0126] The welded joint 100, as described above, readily suppresses LME cracking and is suitable for various applications. For example, it is preferably suitable for automotive parts. In a preferred embodiment, the automotive part includes the welded joint 100 described above, with a first steel plate 10 disposed on the outer side of the vehicle and a second steel plate 20 disposed on the inner side of the vehicle, and the second coating 21 having a lower Al composition than the first coating 11. In automotive parts where the second coating 21 has a lower Al composition than the first coating 11, the ratio of [Al composition (mass%) of the first coating] to [Zn composition (mass%) of the first coating] can also be greater than 0.030. Alternatively, in a preferred embodiment, the automotive part includes the welded joint 100 described above, with a first steel plate 10 disposed on the outer side of the vehicle and a second steel plate 20 disposed on the inner side of the vehicle, and the second coating 21 having a lower Mg composition than the first coating 11. In automotive parts where the Mg composition of the second coating 21 is lower than that of the first coating 11, the ratio of [Mg composition (mass%) of the first coating] to [Zn composition (mass%) of the first coating] can also be greater than 0.001.

[0127] Automotive components are composed of multiple steel sheets. When steel sheets are overlapped in an automotive component, the steel sheets located on the outer side of the vehicle are required to have higher corrosion resistance than those located on the inner side. To meet this requirement, a coating with high Al and / or high Mg content can be applied to the surface of the steel sheets located on the outer side of the vehicle. On the other hand, from a corrosion resistance point of view, it is not necessary to increase the Al and Mg content in the coating of the steel sheets located on the inner side of the vehicle compared to the steel sheets located on the outer side. If there is a concern about LME cracking at the weld joint with the steel sheets located on the inner side of the vehicle due to the high Al and / or high Mg coating on the surface of the steel sheets located on the outer side of the vehicle, the Al / Zn and Mg / Zn content ratios in the coating and the coating thickness (adhesion amount) can be set to satisfy the ranges of relationships I and II mentioned above, thus achieving a balance between the corrosion resistance of the automotive component and the integrity of the weld joint. Furthermore, if the composition and thickness of the second plating layer are adjusted to satisfy Relation I, then even if the composition of the first plating layer, which is considered a single layer, is the one most susceptible to LME cracking, LME cracking of the welded portion can be suppressed. Moreover, by paying attention to the lower limit of Relation I, LME cracking due to the second plating layer can be avoided. Furthermore, if the composition and thickness of the second plating layer are adjusted to satisfy Relation II, then even if the composition of the first plating layer, which is considered a single layer, is the one most susceptible to LME cracking, LME cracking of the welded portion can be suppressed.

[0128] The welded joint 100 disclosed herein is applicable to all automotive parts in which the first steel plate 10 and the second steel plate 20 are joined by a spot weld 30. Figure 4 The image shows one embodiment of an automotive component 1000. For example... Figure 4 As shown, the automotive component 1000 may also include a cap-shaped member 200 disposed on the outer side of the vehicle, a reinforcing member 300 disposed on the inner side of the vehicle, and a closing plate 400. Alternatively, the automotive component 1000 may not include the reinforcing member 300. In the automotive component 1000, for example, the cap-shaped member 200 may also correspond to the first steel plate 10 in the aforementioned welded joint 100. Furthermore, in the automotive component 1000, for example, the reinforcing member 300 may also correspond to the second steel plate 20 in the aforementioned welded joint 100. Furthermore, in the automotive component 1000, for example, the closing plate 400 may also correspond to the second steel plate 20 in the aforementioned welded joint 100.

[0129] Example

[0130] The effects of the welded joints of this disclosure will be further explained below while illustrating the embodiments, but the welded joints of this disclosure are not limited to these embodiments.

[0131] 1. Steel plate manufacturing

[0132] The steel molten material with adjusted composition was cast to form a steel billet, which was then hot-rolled, pickled, and cold-rolled to obtain a cold-rolled steel sheet. Next, the sheet was air-cooled to room temperature, and then pickled to remove the internal oxide layer formed during rolling. Following this, a portion of the cold-rolled steel sheets underwent brush grinding and electrolytic treatment. Brush grinding was performed twice with a brush depth of 2.0 mm and a rotation speed of 600 rpm, while the cold-rolled steel sheet was coated with an aqueous solution containing 2.0% NaOH. Electrolytic treatment involved immersing the cold-rolled steel sheet in a solution at pH 9.8 at 6.1 A / dm³. 2 The current density was applied for 7.2 seconds. Afterwards, annealing was performed according to the specified dew point, holding temperature, and holding time to produce various steel sheets. For all steel sheets, the heating rate during annealing was set to 6.0°C / second up to 500°C, and 2.0°C / second from 500°C to the holding temperature. The holding temperature was 780°C and the holding time was set to 100 seconds, with a holding atmosphere of N2-4%H2 and a dew point of 0°C. In the above annealing process, some cold-rolled steel sheets were annealed under a tension of 0.5 MPa, while other cold-rolled steel sheets were annealed without tension. Furthermore, for each steel sheet, JIS No. 5 tensile test specimens were collected with the length direction perpendicular to the rolling direction as the longitudinal direction, and tensile tests were performed according to JIS Z 2241 (2011). The thickness of all steel sheets used was 1.6 mm.

[0133] 2. Plating

[0134] After cutting each steel plate into 100mm × 200mm dimensions, each plate underwent hot-dip galvanizing, followed by alloying. In the hot-dip galvanizing process, the cut samples were immersed in a 440℃ galvanizing bath for 3 seconds. After immersion, they were drawn at 100mm / second, and the coating adhesion was controlled using N2 wiping gas. The cooling rate after coating was set to 10℃ / second, cooling from the bath temperature to below 150℃ to obtain the samples. Subsequently, a portion of the samples underwent alloying treatment at 500℃ to obtain alloyed hot-dip Zn-based coated steel sheets.

[0135] 3. Spot welding

[0136] The welded joint was obtained as follows: Two specimens were prepared by cutting each Zn-based coated steel sheet into 50mm × 100mm dimensions. For these two Zn-based coated steel sheet specimens, a dome-radius type welding electrode with a front diameter of 8mm was used. Spot welding was performed at a tilt angle of 3°, a pressure of 4.0kN, an energizing time of 0.5 seconds (20 cycles, power frequency of 50Hz), an energizing current of 7kA, and no gap (0mm). It should be noted that "tilt angle" refers to how much the angle between the electrode and the steel plate is tilted from 90°. For example, a tilt angle of 3° means welding with the electrode and steel plate in contact at an angle of 87°.

[0137] 4. Analysis and calculation of the metallic composition of the coating

[0138] For each welded joint obtained, the chemical composition of the first coating formed on the surface of the first steel plate opposite to the second steel plate and the chemical composition of the second coating formed on the surface of the second steel plate opposite to the first steel plate were analyzed, and the average Al / Zn and average Mg / Zn were calculated as expressed by the following formulas. The composition analysis of the first and second coatings focused on the portions that were more than 10 mm away from the boundary coating or the portions that were not welded. The composition of the coatings was determined by immersing samples cut into 30 mm × 30 mm pieces in 10% hydrochloric acid with added inhibitors to dissolve the coatings, and then performing ICP analysis on the coating components dissolved in the solution.

[0139] Average Al / Zn = [(Al composition of the first coating (mass%)) × (Adhesion amount of the first coating (g / m)) 2 ))+(Al composition of the second coating (mass%))×(Adhesion amount of the second coating (g / m)) 2 ))] / [(Zn composition of the first coating (mass%))×(Adhesion amount of the first coating (g / m)) 2 ))+(Zn composition of the second coating (mass%))×(Adhesion amount of the second coating (g / m) 2 ))]

[0140] Average Mg / Zn = [(Mg composition of the first coating (mass%)) × (Adhesion amount of the first coating (g / m)) 2 ))+(Mg composition of the second coating (mass%))×(Adhesion amount of the second coating (g / m)) 2 ))] / [(Zn composition of the first coating (mass%))×(Adhesion amount of the first coating (g / m)) 2 ))+(Zn composition of the second coating (mass%))×(Adhesion amount of the second coating (g / m) 2 ))]

[0141] 5. Confirmation of the metallic structure in the cross-section of the boundary coating

[0142] The area fraction of the η-Zn phase in the boundary coating was determined by cross-sectional SEM observation. The η-Zn phase was defined as having Zn content of 95% or more by mass, Fe content of 5% or less by mass, and other components totaling 2% or less by mass. The observation section was the weld joint section passing through the center of the weld nugget and along the thickness direction of the first steel plate. The area fraction of the η-Zn phase was determined using the following steps: First, SEM images of the boundary coating were obtained. Then, in the SEM images, the coating within a 0.5 mm radius from the end of the ductile metal ring region towards the outside of the weld was defined as the boundary coating. An elemental map of the defined boundary coating was obtained using SEM-EDS. In the obtained elemental map, the phases conforming to the η-Zn phase and those not conforming to it were binarized using image analysis software to derive the area fraction.

[0143] 6. Evaluation of the presence or absence of LME cracking

[0144] The spot welds of each welded joint were observed to evaluate the presence or absence of LME cracking. The evaluation criteria are as follows.

[0145] Rating AA: No LME cracking

[0146] Evaluation A: LME crack length exceeds 0 μm but is less than 100 μm

[0147] Rating B: LME crack length exceeding 100 μm but less than 300 μm

[0148] Rating C: LME crack length exceeds 300μm

[0149] 7. Evaluation Results

[0150] The following table shows the strength, plating composition and other properties of the first and second steel plates used in each weld joint, the area ratio of the η-Zn phase in the boundary plating of the weld joint, and the evaluation results of LME cracking for the weld joint.

[0151]

[0152]

[0153] (Table 3)

[0154]

[0155] As the results shown in Tables 1-3 indicate, LME cracking is significantly suppressed even when the welded joint contains high-strength steel plates with a tensile strength of 780 MPa or more, provided that the following necessary conditions are met (Nos. 2-5, 8-10, 15-18, 20, 21, 25, 26, 28, 29).

[0156] (1) The first steel plate has a first coating on the surface opposite to the second steel plate, and the second steel plate has no coating or has a second coating on the surface opposite to the first steel plate.

[0157] (2) The first coating and the second coating satisfy the following relationships I and II.

[0158] (3) The area occupied by the η-Zn phase in the cross section of the boundary coating is more than 5%.

[0159] Relationship I: 0.030 ≥ [(Al composition of the first coating layer (mass%)) × (adhesion amount of the first coating layer (g / m)] 2 ))+(Al composition of the second coating (mass%))×(Adhesion amount of the second coating (g / m)) 2 ))] / [(Zn composition of the first coating (mass%))×(Adhesion amount of the first coating (g / m)) 2 ))+(Zn composition of the second coating (mass%))×(Adhesion amount of the second coating (g / m) 2 ))]≥0.003

[0160] Relationship II: 0.001 ≥ [(Mg composition of the first coating above (mass%)) × (adhesion amount of the first coating above (g / m)] 2 ))+(Mg composition of the second coating (mass%))×(Adhesion amount of the second coating (g / m)) 2 ))] / [(Zn composition of the first coating (mass%))×(Adhesion amount of the first coating (g / m)) 2 ))+(Zn composition of the second coating (mass%))×(Adhesion amount of the second coating (g / m) 2 ))]

[0161] It should be noted that, regarding Nos. 1, 6, and 11, it is believed that: the high average Mg / Zn ratio of the coating results in a relatively lower Al content at the boundary, which helps suppress LME cracking. This alters the stability of the liquid phase at the steel grain boundaries, making it easier for Zn to invade the grain boundaries and thus increasing the likelihood of LME cracking. Furthermore, regarding No. 1, large LME cracks occurred, leading to rapid alloying starting from these cracks, and a decrease in the area fraction of the η-Zn phase.

[0162] Regarding No. 7, 12, and 14, it is believed that: the lower average Al / Zn ratio of the coating reduces the amount of Al that helps suppress LME cracking in the boundary coating, altering the stability of the liquid phase at the steel grain boundaries and making it more susceptible to LME cracking. Furthermore, for any of No. 7, 12, and 14, alloying begins with LME cracking, resulting in a decrease in the area fraction of the η-Zn phase.

[0163] Regarding No. 19, it is believed that due to the excessively large average Al / Zn ratio of the coating, the stability of the liquid phase at the steel grain boundaries in the boundary coating changes, resulting in increased susceptibility to LME cracking.

[0164] Regarding Nos. 13, 14 and 19, it is believed that the internal oxidation treatment conditions of cold-rolled steel sheets are unsuitable. As a result, Fe becomes more likely to diffuse to the boundary during welding, the area ratio of the η-Zn phase in the cross section of the boundary coating decreases, and the liquid phase of Zn becomes more likely to be generated on the surface of the steel sheet, which in turn makes it more prone to LME cracking.

[0165] Furthermore, even without a second coating, LME cracking can be suppressed by ensuring that the proportions of the η-Zn phase in the cross-sections of Relation I, Relation II, and the boundary coating meet the prescribed conditions. In examples No. 21-25 and No. 28, there is no second coating. Examples No. 21, No. 25, and No. 28 are all embodiments where the proportions of the η-Zn phase in the cross-sections of Relation I, Relation II, and the boundary coating meet the prescribed conditions. No. 22-24 are comparative examples. In No. 22, Relation I is below the lower limit, and the proportion of the η-Zn phase is also too low. In No. 23, Relation I exceeds the upper limit, and the proportion of the η-Zn phase is also too low. In No. 24, Relation II exceeds the upper limit. Therefore, these comparative examples cannot suppress LME cracking.

[0166] Examples No. 26, No. 27, and No. 29 all contain Mg in both the first and second coating layers. When Mg is present in both the first and second coating layers, insufficient LME crack suppression is not achieved if the range of Relation II is deviated from. No. 26 and No. 29 are examples where the proportion of the η-Zn phase in the cross-section of Relation I, Relation II, and the boundary coating meets the specified conditions. No. 27 is a comparative example where Relation II exceeds the upper limit. Sufficient LME crack suppression was not achieved in No. 27.

[0167] Furthermore, the details regarding the internal oxide layer are as follows. If the internal oxide layer has a sufficient thickness in the steel sheet beneath the coating, the alloying behavior between the coating and the base metal during welding can be suppressed, and the amount of Fe fused into the boundary coating can be suppressed. If the amount of Fe fused into the boundary coating is low, the proportion of the η-Zn phase in the boundary coating becomes high. That is, if the internal oxide layer has a sufficient thickness in the steel sheet beneath the coating, it becomes easier to set the proportion of the η-Zn phase in the boundary coating to a predetermined range. To suppress the alloying behavior between the coating and the base metal, it is preferable to have an internal oxide layer with a thickness of 1.5 μm or more in the steel sheet beneath the coating. Furthermore, when both the first and second steel sheets have internal oxide layers of sufficient thickness, it becomes easier to set the proportion of the η-Zn phase in the boundary coating to a predetermined range.

[0168] Examples No. 3, 4, 5, 8, and 13 are instances where the values ​​of Relation I and Relation II are equivalent. Among these, examples No. 3, 4, 5, and 8 show that the proportion of the η-Zn phase in the boundary coating meets the specified conditions. On the other hand, comparative example No. 13 shows that the proportion of the η-Zn phase in the boundary coating does not meet the specified conditions. For No. 13, there is no sufficiently thick internal oxide layer, which differs from the examples. Therefore, it can be seen that if there is a sufficiently thick internal oxide layer in the steel sheet beneath the coating, it becomes easier to set the proportion of the η-Zn phase in the boundary coating to a specified range.

[0169] Examples No. 26 and No. 29 are instances where the values ​​of Relation I and Relation II are equivalent. Both No. 26 and No. 29 are embodiments where the proportion of the η-Zn phase in the boundary coating meets the specified conditions. No. 26 has a higher proportion of the η-Zn phase in the boundary coating compared to No. 29. For No. 26, there is an internal oxide layer of sufficient thickness in both the first and second steel plates. For No. 29, there is no internal oxide layer of sufficient thickness. From this, it can be seen that if there is an internal oxide layer of sufficient thickness, the proportion of the η-Zn phase in the boundary coating becomes higher. While there is no significant difference in the proportion of the η-Zn phase in the boundary coating between No. 26 and No. 29, when measuring the Zn concentration at a position 1.5 μm from the tip of the Zn intrusion, No. 26 has 73.1% by mass, and No. 29 has 30.8% by mass. Furthermore, when measuring the Al concentration at a position 1.5 μm from the tip of the Zn intrusion, No. 26 has 0.34% by mass, and No. 29 has 0.10% by mass. The influence of the internal oxide layer is significant in the Zn-intruded region. Differences in Zn and Al concentrations within the Zn-intruded region affect the evaluation results of LME cracking. Furthermore, for No. 9, where Relationship I and Relationship II are equivalent, the Zn concentration at a location 1.5 μm from the tip of the Zn-intruded region is 60.5% by mass, and the Al concentration is 0.36% by mass, which are high values. Therefore, No. 9 exhibits a high LME cracking suppression effect.

[0170] Examples No. 7 and No. 14 are cases where Relation I and Relation II are equal. Both No. 7 and No. 14 are comparative examples where Relation I is below the specified range. No. 7 has a higher proportion of the η-Zn phase in the boundary coating compared to No. 14. No. 7 has a sufficiently thick internal oxide layer. No. 14 does not have a sufficiently thick internal oxide layer. From this, it can be concluded that if the internal oxide layer has a sufficiently thick layer, the proportion of the η-Zn phase in the boundary coating becomes higher.

[0171] Examples No. 25 and No. 28 are embodiments where the proportion of the η-Zn phase in Relationship I, Relationship II, and the boundary coating meets the specified conditions. For No. 25, there is no sufficiently thick internal oxide layer in the first and second steel plates. For No. 28, there is a sufficiently thick internal oxide layer only in the first steel plate. Regarding the proportion of the η-Zn phase in the boundary coating, No. 28 is higher than No. 25. Therefore, it can be concluded that when there is a sufficiently thick internal oxide layer in both the first and second steel plates, the proportion of the η-Zn phase in the boundary coating becomes higher.

[0172] Explanation of symbols

[0173] 10. Steel Plate No. 1

[0174] 11 First coating

[0175] 20 Steel Plate No. 2

[0176] 21 Second coating

[0177] 30 Spot welds

[0178] 31 Melting Core

[0179] 32 Plastic Metal Ring Region

[0180] 40 Separation Section

[0181] 50 Boundary Coating

[0182] 60 Zn Intrusion Section

[0183] 100 Welded Joint

[0184] 200 Hat-shaped component

[0185] 300 Reinforcing Components

[0186] 400 Closed panel.

Claims

1. An automotive component having a welded joint, The welded joint comprises a first steel plate, a second steel plate, and a spot weld portion that joins the first steel plate and the second steel plate. A first coating is provided on the surface of the first steel plate opposite to the second steel plate. A second coating is provided on the surface of the second steel plate opposite to the first steel plate. The spot weld section has a weld nugget and a plastic metal ring region. A boundary plating layer is provided between the first steel plate and the second steel plate, and within a range of 0.5 mm from the end of the plastic metal ring region toward the outer side of the spot weld. The higher of the tensile strength of the first steel plate and the tensile strength of the second steel plate is 780 MPa or higher. The area fraction of the η-Zn phase in the cross-section of the boundary coating is more than 5%, wherein, η-Zn phase refers to the Zn phase with an Fe concentration of less than 5% by mass. The first coating and the second coating satisfy the following relationships I and II. The first steel plate is positioned on the outer side of the vehicle. The second steel plate is located on the inside of the vehicle. The Al composition of the second coating is lower than that of the first coating. Relationship I: 0.030 ≥ [(Al composition of the first coating in mass %) × (Al composition of the first coating in g / m 2 (Adhesion amount) + (Al composition of the second coating in mass %) × (Al composition of the second coating in g / m 2 [(adhesion amount)] / [(Zn composition of the first coating in mass %) × (the amount of the first coating in g / m 2 (Adhesion amount) + (Zn composition of the second coating in mass %) × (Zn composition of the second coating in g / m 2 [Calculated amount of adhesion] ≥ 0.003 Relationship II: 0.001 ≥ [(Mg composition of the first coating in mass %) × (Mg composition of the first coating in g / m 2 (Adhesion amount) + (Mg composition of the second coating in mass %) × (Mg composition of the second coating in g / m 2 [(adhesion amount)] / [(Zn composition of the first coating in mass %) × (the amount of the first coating in g / m 2 (Adhesion amount) + (Zn composition of the second coating in mass %) × (Zn composition of the second coating in g / m 2 (The amount of adhesion calculated).

2. The automotive component according to claim 1, wherein, The ratio of the Al composition of the first coating (in mass%) to the Zn composition of the first coating (in mass%) is greater than 0.

030.

3. An automotive component, the automotive component having a welded joint, The welded joint comprises a first steel plate, a second steel plate, and a spot weld portion that joins the first steel plate and the second steel plate. A first coating is provided on the surface of the first steel plate opposite to the second steel plate. A second coating is provided on the surface of the second steel plate opposite to the first steel plate. The spot weld section has a weld nugget and a plastic metal ring region. A boundary plating layer is provided between the first steel plate and the second steel plate, and within a range of 0.5 mm from the end of the plastic metal ring region toward the outer side of the spot weld. The higher of the tensile strength of the first steel plate and the tensile strength of the second steel plate is 780 MPa or higher. The area fraction of the η-Zn phase in the cross-section of the boundary coating is more than 5%, wherein, η-Zn phase refers to the Zn phase with an Fe concentration of less than 5% by mass. The first coating and the second coating satisfy the following relationships I and II. The first steel plate is positioned on the outer side of the vehicle. The second steel plate is located on the inside of the vehicle. The Mg composition of the second coating is lower than that of the first coating. Relationship I: 0.030 ≥ [(Al composition of the first coating in mass %) × (Al composition of the first coating in g / m 2 (Adhesion amount) + (Al composition of the second coating in mass %) × (Al composition of the second coating in g / m 2 [(adhesion amount)] / [(Zn composition of the first coating in mass %) × (the amount of the first coating in g / m 2 (Adhesion amount) + (Zn composition of the second coating in mass %) × (Zn composition of the second coating in g / m 2 [Calculated amount of adhesion] ≥ 0.003 Relationship II: 0.001 ≥ [(Mg composition of the first coating in mass %) × (Mg composition of the first coating in g / m 2 (Adhesion amount) + (Mg composition of the second coating in mass %) × (Mg composition of the second coating in g / m 2 [(adhesion amount)] / [(Zn composition of the first coating in mass %) × (the amount of the first coating in g / m 2 (Adhesion amount) + (Zn composition of the second coating in mass %) × (Zn composition of the second coating in g / m 2 (The amount of adhesion calculated).

4. The automotive component according to claim 1 or 3, wherein, The first coating and the second coating satisfy the following relationship I-1, Relationship I-1: 0.010 ≥ [(Al composition of the first coating in mass %) × (Al composition of the first coating in g / m 2 (Adhesion amount) + (Al composition of the second coating in mass %) × (Al composition of the second coating in g / m 2 [(adhesion amount)] / [(Zn composition of the first coating in mass %) × (the amount of the first coating in g / m 2 (Adhesion amount) + (Zn composition of the second coating in mass %) × (Zn composition of the second coating in g / m 2 [The amount of adhesion (calculated)] ≥ 0.

003.

5. The automotive component according to claim 1 or 3, wherein, The boundary coating has one or more oxides with a major diameter of 0.5 μm or more.

6. The automotive component according to claim 1 or 3, wherein the first steel plate has an internal oxide layer with a depth of 1.5 μm to 20.0 μm on one side of the surface of the first steel plate opposite to the second steel plate.

7. The automotive component according to claim 3, wherein, The ratio of the mass percentage of Mg in the first coating to the mass percentage of Zn in the first coating is greater than 0.001.

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