Solid wire for welding aluminum-plated steel sheets, and method for manufacturing welded joints.

Abstract

To provide a solid wire for welding an aluminum plated steel sheet and a method for manufacturing a weld joint which enable formation of weld metal having high hardness after hot stamp even when aluminum (Al) is mixed in weld metal when high-strength aluminum plated steel sheets having tensile strength of 1,800 MPa or more are welded after hot stamp.SOLUTION: A solid wire for welding an aluminum plated steel sheet is a solid wire for welding an aluminum plated steel sheet, in which contents of a chemical composition is, by mass% with respect to the total mass of the wire, 0.25-0.35% C, less than 0.30% Si, 1.8-2.5% Mn, 0.30-1.00% Mo, 0.050% or less P, 0.030% or less S, 0.10% or less Al, 0.010% or less N, 0.50% or less Cu, and the balance Fe with inevitable impurities.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to a solid wire for welding aluminum-plated steel sheets and a method for manufacturing welded joints. [Background technology]

[0002] Hot stamping (hereinafter sometimes referred to as "hot stamping") is attracting attention as one of the technologies for forming steel sheets. In hot stamping, steel sheets are heated to a high temperature and press-formed in a temperature range above the Ar3 transformation temperature. Furthermore, in hot stamping, the press-formed steel sheet is rapidly cooled by heat dissipation using a die, and martensitic transformation occurs simultaneously with the formation while press pressure is applied. Hot stamping is a technology that can manufacture hot-press-formed products (hereinafter sometimes referred to as "hot-stamped products") that have high strength and excellent shape retention.

[0003] Tailored blanks, formed by joining the ends of at least two steel plates using laser welding, plasma welding, or other methods, are used as steel materials for hot stamping. Because multiple steel plates are joined according to the purpose, tailored blanks allow for free variation in plate thickness and strength within a single component. Therefore, by hot stamping onto a tailored blank, high-strength hot-pressed products with freely adjustable plate thickness and strength can be obtained.

[0004] When tailoring blanks are formed by hot stamping, they are heated to temperatures such as 800°C to 1000°C. Therefore, aluminum-plated steel sheets with aluminum plating such as Al-Si, which has high melting and boiling points, are often used for tailored blanks.

[0005] In tailored blanks formed by butt welding aluminum-plated steel sheets, aluminum, a component of the aluminum plating layer or intermetallic compound layer, may be mixed into the weld metal. Since aluminum is a ferrite-forming element, its inclusion prevents sufficient martensite from forming in the structure after hot stamping, resulting in a decrease in the hardness of the weld metal. Therefore, when aluminum is mixed into the weld metal, fracture can occur in the weld metal, leading to a decrease in the strength of the welded joint.

[0006] To address this challenge, Patent Document 1 proposes a tailored blank in which the average concentration of aluminum in the weld metal and the Ac3 point of the weld metal are controlled. Patent Document 2 also proposes a tailored blank in which the average concentration of aluminum in the weld metal and the product of the hardness of the weld metal after hot stamping and the thickness of the thinnest part of the weld metal are controlled to satisfy a certain relationship. Patent Document 3 discloses a method for manufacturing a tailor-made semi-finished sheet metal product using a filler wire that contains almost no aluminum and at least one alloying element that promotes the formation of austenite. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Patent No. 5316670 [Patent Document 2] Patent No. 5316664 [Patent Document 3] Patent No. 6430070 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] However, the weld metal formed by welding aluminum-plated steel sheets requires even higher strength (hardness) than the weld metal obtained by these techniques. Furthermore, in welding high-strength aluminum-plated steel sheets that achieve a tensile strength of 1800 MPa or more after hot stamping, conventional welding wires have not been able to provide sufficient strength.

[0009] The object of the present invention is to provide a solid wire for welding aluminum-plated steel sheets and a method for manufacturing a welded joint that can form a weld metal with high hardness after hot stamping, even if aluminum (Al) is mixed into the weld metal when welding high-strength aluminum-plated steel sheets that have a tensile strength of 1800 MPa or more after hot stamping. [Means for solving the problem]

[0010] Through diligent research, the inventors have discovered that by using a solid wire containing Mo, which has ferrite-forming ability but high hardenability, a reduced content of Si, which has ferrite-forming ability but low hardenability, and a C content within a predetermined range, and welding high-strength aluminum-plated steel sheets with a tensile strength of 1800 MPa or more after hot stamping, a weld metal with high hardness can be formed even if aluminum is mixed into the weld metal. The gist of the present invention is as follows. <1> A solid wire for welding aluminum-plated steel sheets according to one aspect of the present invention has a chemical composition content in mass % of the total wire mass. C: 0.25%~0.35%, Si: Less than 0.30% Mn: 1.8%~2.5% Mo: 0.30%~1.00%, P: 0.050% or less, S: 0.030% or less, Al: 0.10% or less, N: 0.010% or less, Cu: 0.50% or less, The remainder consists of Fe and impurities. The diameter is 0.6mm to 1.2mm. . <2> The solid wire for welding the aluminum-plated steel sheet according to <1> above further contains, in place of a part of the Fe, Cr: 1.5% or less, Ni: 3.0% or less, B: 0.010% or less, Ti: 0.20% or less, V: 0.50% or less, Nb: 0.20% or less, and may contain one or more of the above. <3 > The method for manufacturing a welded joint according to one aspect of the present invention welds at least one aluminum-plated steel sheet having a thickness of 0.7 mm to 3.2 mm, and uses the solid wire for welding the aluminum-plated steel sheet according to <1> above for the welding. or <2> < 4 > The above < 3 > The method for manufacturing a welded joint according to the above may perform the welding using laser welding.

Advantages of the Invention

[0011] According to the solid wire for welding the aluminum-plated steel sheet and the method for manufacturing a welded joint of the present invention, even if aluminum (Al) is mixed into the weld metal when high-strength aluminum-plated steel sheets having a tensile strength of 1800 MPa or more after hot stamping are welded together, a weld metal having a high hardness after hot stamping can be formed.

Brief Description of the Drawings

[0012] [Figure 1] It is a schematic diagram of a cross-section of a weld metal for explaining a Vickers hardness test.

Embodiments for Carrying Out the Invention

[0013] Hereinafter, the solid wire for welding the aluminum-plated steel sheet and the method for manufacturing a welded joint according to the present embodiment will be described. ​

[0014] <Solid wire> To increase the hardness of the weld metal, it is effective to reduce the proportion of non-martensite structures such as ferrite and bainite, which have low hardness, and increase the proportion of martensite in the weld metal. In welding high-strength aluminum-plated steel sheets that have a tensile strength of 1800 MPa or more after hot stamping, the inventors diligently investigated how to obtain a weld metal with a high martensite ratio even when aluminum, a ferrite-forming element, is mixed into the weld metal, and as a result obtained the following findings.

[0015] (A) By controlling the Mo content of the solid wire to an appropriate range, the hardenability of the weld metal is improved, and the proportion of martensite in the weld metal increases. (B) By reducing the Si content of the solid wire, the formation of ferrite in the weld metal is suppressed, and the proportion of ferrite in the weld metal decreases. (C) By setting the Mo, Si, and C content of the solid wire to appropriate ranges, the appropriate hardness of the weld metal can be obtained for high-strength aluminum-plated steel sheets.

[0016] Based on the above findings, this disclosure determines the component composition of a solid wire for welding aluminum-plated steel sheets. The solid wire for welding aluminum-plated steel sheets described in this disclosure achieves the desired effect due to the synergistic effect of each component. The component composition is described below.

[0017] Solid wire is a steel wire having a predetermined composition, or a steel wire with a Cu plating on its surface. Total wire mass refers to the total mass of the solid wire, including the Cu plating. In this specification, the chemical composition of the solid wire is expressed as a mass % of the total wire mass, and the mass % is simply indicated as %. In this specification, numerical ranges expressed using "~" mean a range that includes the numbers written before and after "~" as the lower and upper limits. In this specification, the content of a component (element) may be expressed as, for example, the content of C (carbon) as "C amount". The content of other elements may also be expressed similarly.

[0018] In this specification, "weld metal" means a part of the weld that has melted and solidified during welding. Here, "melted and solidified metal" means both the molten base steel plate and the molten solid wire. Therefore, weld metal refers to the metal formed when the steel plate base material and the solid wire melt and mix together.

[0019] The solid wire for welding aluminum-plated steel sheets disclosed herein has a chemical composition in which, by mass %, the following elements are present: C: 0.25% to 0.35%, Si: less than 0.30%, Mn: 1.8% to 2.5%, Mo: 0.30% to 1.00%, P: 0.050% or less, S: 0.030% or less, Al: 0.10% or less, N: 0.010% or less, Cu: 0.50% or less, with the remainder being Fe and impurities. Each element is described below.

[0020] (C: 0.25%~0.35%) Carbon (C) is an important element for improving the hardenability of weld metal and increasing the hardness of martensite. If the C content is less than 0.25%, the hardenability will be insufficient, making it difficult to form martensite, and the hardness of the martensite will not be sufficient for high-strength aluminum-plated steel sheets that achieve a tensile strength of 1800 MPa or more after hot stamping. Therefore, the C content should be 0.25% or more. On the other hand, if the C content exceeds 0.35%, welding cracks may occur when used for welding high-strength aluminum-plated steel sheets. Therefore, the C content should be 0.35% or less. The C content may also be 0.26% or more, 0.27% or more, or 0.28% or more. Furthermore, the C content may be 0.32% or less, or 0.30% or less.

[0021] (Si: less than 0.30%) Conventional solid wires actively add Si as a deoxidizing element. However, Si is a ferrite-forming element and does not significantly improve hardenability. Therefore, a low Si content is preferable. Accordingly, the Si content should be less than 0.30%. Furthermore, the Si content is preferably less than 0.25%, more preferably 0.20% or less, even more preferably 0.15% or less, and even more preferably 0.10% or less. Note that Si does not need to be included in the solid wire; the Si content can be 0%, greater than 0%, or 0.01% or more.

[0022] (Mn: 1.8%~2.5%) Mn is a deoxidizing element and is also an effective element for improving the hardenability of weld metal and ensuring stable strength after quenching. If the Mn content is less than 1.8%, its effect may not be sufficient. Therefore, the Mn content should be 1.8% or more, preferably 2.0% or more. On the other hand, if the Mn content exceeds 2.5%, not only does its effect saturate, but it may also lead to a decrease in the toughness and ductility of the weld metal. Therefore, the Mn content should be 2.5% or less, preferably 2.3% or less.

[0023] (Mo: 0.30%~1.00%) Mo has a lower ferrite-forming ability than Al and is an effective element for increasing the proportion of martensite in the weld metal in order to improve the hardenability of the weld metal. If the amount of Mo is less than 0.30%, the effect may not be sufficient. Therefore, the amount of Mo should be 0.30% or more. Preferably, the amount of Mo is 0.40% or more. On the other hand, if the amount of Mo exceeds 1.00%, ferrite may form in the weld metal, which may reduce its hardness. Therefore, the amount of Mo should be 1.00% or less. Preferably, the amount of Mo is 0.90% or less, and more preferably 0.80% or less.

[0024] (P:0.050% or less) P is an element that is generally present as an impurity in steel, and is also typically found as an impurity in solid wire. Here, since P is one of the main elements that cause hot cracking of weld metal, it is desirable to suppress it as much as possible. If the amount of P exceeds 0.050%, hot cracking of the weld metal becomes significant, so the amount of P should be 0.050% or less. In practice, the amount of P is greater than 0%, and from the standpoint of cost and productivity of eliminating P, it may be 0.001% or more.

[0025] (S:0.030% or less) S, like P, is an element that is generally present as an impurity in steel, and is also typically found as an impurity in solid wire. Here, S is one of the main elements that cause hot cracking in weld metal, so it is desirable to suppress it as much as possible. If the amount of S exceeds 0.030%, solidification cracking occurs in the weld metal. Therefore, the amount of S should be 0.030% or less. The amount of S may also be 0.020% or less. In reality, the amount of S is greater than 0%, and from the viewpoint of the cost and productivity of S removal, it may be 0.001% or more.

[0026] (Al: 0.10% or less) Al is a ferrite-forming element. If the Al content exceeds 0.10%, ferrite and bainite may form in the weld metal even after quenching, potentially reducing the hardness of the weld metal. Therefore, the Al content should be 0.10% or less. The Al content may also be 0.070% or less, or 0.030% or less. While the Al content may be 0%, it may be 0.001% or more, as Al may be added for deoxidation.

[0027] (N:0.010% or less) Nitric oxide (N) is an element present as an impurity in solid wires used for welding aluminum-plated steel sheets. Furthermore, N forms inclusions in the weld metal, degrading the toughness after hot press forming. Therefore, the amount of N should be 0.010% or less. Preferably, the amount of N should be 0.008% or less, and more preferably 0.006% or less. While a lower amount of N is preferable, from the viewpoint of the cost of removing N, an N amount of 0.0002% is preferable.

[0028] (Cu:0.50% or less) Solid wires are sometimes coated with copper (Cu) to stabilize wire feeding and conductivity, and to provide corrosion resistance. Therefore, when copper plating is applied, solid wires contain a certain amount of copper. However, excessive copper content increases the likelihood of welding cracks; therefore, the copper content is kept below 0.50%. Since copper is not an essential element, the lower limit for copper-free solid wires is 0%.

[0029] (Cr, Ni, B, Ti, V, Nb) Cr, Ni, B, Ti, V, and Nb are not essential elements, but may be included one or more at once as needed. The effects and upper limits of each element's inclusion will be explained. Note that if these elements are not included, their amount is 0%.

[0030] (Cr:1.5% or less) Cr is a useful element for improving the hardness of weld metal by increasing its hardenability. Therefore, solid wire for welding aluminum-plated steel sheets may contain Cr. To obtain the above effect, it is preferable to contain 0.1% or more Cr. However, since Cr is a ferrite-forming element, if the solid wire contains more than 1.5% Cr, the proportion of martensite in the weld metal will decrease. Therefore, it is preferable to keep the amount of Cr at 1.5% or less. The amount of Cr may also be 1.0% or less.

[0031] (Ni:3.0% or less) Ni is an effective element for improving the tensile strength and toughness of weld metal. Therefore, Ni may be included in solid wires used for welding aluminum-plated steel sheets. To obtain the above effects, it is preferable that the amount of Ni be 0.1% or more. However, if the amount of Ni in the solid wire exceeds 3.0%, welding cracks are more likely to occur. Therefore, it is preferable to keep the amount of Ni 3.0% or less. The upper limit for the amount of Ni may be 2.0%.

[0032] (B: 0.010% or less) B is an effective element for improving the hardenability of weld metal. Therefore, solid wire for welding aluminum-plated steel sheets may contain B. If the amount of B in the solid wire exceeds 0.010%, not only will its effect saturate, but it will also lead to a decrease in the toughness and ductility of the weld metal, and welding cracks will become more likely to occur. Therefore, it is best to keep the amount of B to 0.010% or less. In order to obtain the above-mentioned effects of B, it is best to keep the amount of B to 0.0005% or more, preferably 0.001% or more.

[0033] (Ti: 0.20% or less) Ti is an effective element for refining the microstructure of weld metal. Therefore, solid wire for welding aluminum-plated steel sheets may contain Ti. If the Ti content in the solid wire exceeds 0.20%, the toughness of the weld metal decreases, making it more susceptible to welding cracks. Therefore, it is best to keep the Ti content below 0.20%. To obtain the above-mentioned effects of Ti, it is best to have a Ti content of 0.01% or more, preferably 0.05% or more.

[0034] (V:0.50% or less) V is an element effective in refining the microstructure of weld metal. Therefore, V may be contained in solid wire for welding aluminum-plated steel sheets. If the V content in the solid wire exceeds 0.50%, not only will its effect saturate, but it will also lead to a decrease in the toughness and ductility of the weld metal, and welding cracks will become more likely to occur. Therefore, it is best to keep the V content at 0.50% or less. To obtain the above-mentioned effects of V, it is best to keep the V content at 0.05% or more, preferably 0.10% or more.

[0035] (Nb:0.20% or less) Nb is an effective element for refining the microstructure of weld metal. Therefore, solid wire for welding aluminum-plated steel sheets may contain Nb. If the Nb content in the solid wire exceeds 0.20%, not only will its effect saturate, but it will also lead to a decrease in the toughness and ductility of the weld metal, and make welding cracks more likely. Therefore, it is best to keep the Nb content below 0.20%. To obtain the above-mentioned effects of Nb, it is best to have an Nb content of 0.01% or more, preferably 0.05% or more.

[0036] (Remainder) The remainder of the chemical composition of the solid wire used for welding aluminum-plated steel sheets consists of Fe and impurities. Here, impurities refer to components contained in raw materials such as ore and scrap, or components that are mixed into the solid wire for welding aluminum-plated steel sheets during the manufacturing process. Impurities refer to components that were not intentionally included in the solid wire for welding aluminum-plated steel sheets.

[0037] (diameter) The diameter of the solid wire affects weldability. If the diameter of the solid wire exceeds 1.2 mm, for example, when welding with a laser, the diameter may be too large relative to the size of the molten pool, potentially leading to insufficient or unstable melting of the wire. As a result, defects may occur, such as the solid wire being supplied to the molten area without completely melting, creating heterogeneous weld metal, or the solid wire being left embedded in the molten area. Therefore, the diameter of the solid wire for welding aluminum-plated steel sheets is preferably 1.2 mm or less. If the diameter of the solid wire is less than 0.6 mm, it tends to be difficult to supply the solid wire to the desired position due to coiling and other factors. In addition, the manufacturing cost of the wire tends to increase significantly. Therefore, the diameter of the solid wire for welding aluminum-plated steel sheets is preferably 0.6 mm or more, and preferably 0.8 mm or more.

[0038] Solid wires for welding aluminum-plated steel sheets like these can be manufactured using conventional methods. Specifically, molten steel adjusted to the desired chemical composition is solidified, a base wire is created by rolling, and then individual wires are produced by diameter reduction and annealing. Furthermore, these individual wires are drawn to produce a wire of the desired diameter.

[0039] <Method for manufacturing welded joints> Next, the manufacturing method of the welded joint according to the present disclosure will be described. The welded joint is obtained by welding an aluminum-plated steel sheet using the solid wire for welding aluminum-plated steel sheets according to the present disclosure. The aluminum-plated steel sheet used together with the solid wire for welding aluminum-plated steel sheets according to the present disclosure will be described below.

[0040] (Aluminum-plated steel sheet) Aluminum-plated steel sheets are not particularly limited as long as they are steel sheets having an aluminum plating. An aluminum-plated steel sheet comprises a base steel sheet, an intermetallic compound layer, and an aluminum plating layer. The intermetallic compound layer and the aluminum plating layer are formed in this order on the surface of the base steel sheet, starting from the base steel sheet side. The preferred ranges for the components of the base steel sheet, aluminum plating layer, and intermetallic compound layer will be described later.

[0041] (Thickness of aluminum-plated steel sheet) The thickness of the aluminum-plated steel sheet is 0.7 mm to 3.2 mm. This is because within this range, martensite is easily formed in the weld metal after the hot stamping process. Preferably, the thickness of the aluminum-plated steel sheet is 0.8 mm or more, more preferably 1.0 mm or more. Also, preferably, the thickness of the aluminum-plated steel sheet is 2.9 mm or less.

[0042] An aluminum-plated steel sheet may have exposed portions at its edges where at least a portion of the aluminum plating layer and the intermetallic compound layer has been removed. In the exposed portions, the aluminum plating layer and the intermetallic compound layer may be completely removed, a portion of the aluminum plating layer may be removed, a portion of the aluminum plating layer and the intermetallic compound layer may be removed, or the aluminum plating layer may be removed while the intermetallic compound layer remains. Such exposed portions may be welded using the solid welding wire for aluminum-plated steel sheets of this disclosure.

[0043] When punching out aluminum-plated steel sheets to obtain punched parts, burrs may occur at the edges of the aluminum-plated steel sheet due to cutting means such as shears. By removing the intermetallic compound layer and aluminum plating layer at the burred edges of the aluminum-plated steel sheet by cutting, grinding, etc., the exposed portion can be formed while leaving the intermetallic compound layer and aluminum plating layer in the burred area.

[0044] (Base material steel plate) The base steel sheet for aluminum-plated steel sheets can be any steel sheet obtained by conventional methods including hot rolling, cold rolling, and annealing processes, and is not particularly limited. The base steel sheet may be either a hot-rolled or cold-rolled steel sheet. For the base steel sheet, it is preferable to use a steel sheet that has been prepared to have high tensile strength after a hot stamping process, for example. Specifically, a steel sheet with a tensile strength of 1300 to 2500 MPa after hot stamping can be used as the base steel sheet. The solid wire for welding aluminum-plated steel sheets of this disclosure is particularly suitable for steel sheets with a tensile strength of 1800 MPa or more after hot stamping. The thickness of the base steel sheet is 0.7 mm to 3.2 mm. However, the above ranges of tensile strength and thickness of the base steel sheet are examples. Depending on the performance of the part or section to which the base steel sheet is applied, such as strength, rigidity, energy absorption, deformation behavior, and fracture behavior, a steel sheet with appropriate tensile strength and thickness should be used as the base steel sheet.

[0045] Examples of preferred chemical compositions for the base steel sheet include the following: Note that the "%" indicated below to show the content of components (elements) means "mass percent". The base steel sheet has the following composition by mass: C: 0.30%~0.50%, Si: 0%~2.00%, Mn: 0.20%~3.00%, Al: 0.005%~0.10%, P: 0.03% or less, S: 0.010% or less, N: 0.010% or less, Ti: 0%~0.20%, Nb: 0%~0.20%, V: 0%~0.50%, W: 0%~1.0% It has a chemical composition consisting of Cr: 0%~1.0%, Mo: 0%~1.0%, Cu: 0%~1.0%, Ni: 0%~1.0%, B: 0%~0.0100%, Mg: 0%~0.05%, Ca: 0%~0.05%, REM: 0%~0.05%, Sn: 0%~0.5%, Bi: 0%~0.05%, and the remainder being Fe and impurities.

[0046] (Aluminum plating layer) The aluminum plating layer is formed on the surface of the base steel sheet. The method for forming the aluminum plating layer is not particularly limited. For example, the aluminum plating layer may be formed on both sides of the base steel sheet by a hot-dip galvanizing method (a method in which the base steel sheet is immersed in a molten metal bath mainly containing aluminum to form the aluminum plating layer).

[0047] Here, the aluminum plating layer is a plating layer mainly containing aluminum, and it is sufficient if it contains 50% by mass or more of aluminum. Depending on the purpose, the aluminum plating layer may also contain elements other than aluminum (for example, Si), and may contain impurities that are introduced during the manufacturing process. Specifically, the aluminum plating layer may have a chemical composition consisting of 5% to 12% by mass of Si (silicon), with the remainder being aluminum and impurities. Alternatively, the aluminum plating layer may have a chemical composition consisting of 5% to 12% by mass of Si (silicon), 2% to 4% by mass of Fe (iron), with the remainder being aluminum and impurities. By incorporating Si into the aluminum plating layer within the above range, the deterioration of workability and corrosion resistance can be suppressed. Furthermore, the thickness of the intermetallic compound layer can be reduced.

[0048] The thickness of the aluminum plating layer is not particularly limited, but for example, it is often 8 μm (micrometers) or more in average thickness, and preferably 10 μm or more. Also, for example, the thickness of the aluminum plating layer is often 40 μm or less in average thickness, and preferably 30 μm or less.

[0049] (Intermetallic compound layer) The intermetallic compound layer is a layer formed between the base steel sheet and the aluminum plating layer when an aluminum plating layer is applied to the base steel sheet. Specifically, the intermetallic compound layer is formed by the reaction of iron (Fe) in the base steel sheet with a metal containing aluminum (Al) in a molten metal bath mainly containing aluminum. The intermetallic compound layer is mainly Fe x Al yIt is formed of multiple types of compounds represented by (x and y each represent 1 or more). If the aluminum plating layer contains Si (silicon), the intermetallic compound layer is Fe x Al y and Fe x Al y Si z It is formed from multiple types of compounds represented by (x, y, and z each representing a value of 1 or greater).

[0050] The thickness of the intermetallic compound layer is not particularly limited, but is often 1 μm or more on average, and more preferably 3 μm or more. Furthermore, the thickness of the intermetallic compound layer is often 10 μm or less on average, and more preferably 8 μm or less. Furthermore, the thickness of the intermetallic compound layer can be controlled by the temperature and immersion time of the molten metal bath, which mainly contains aluminum.

[0051] (Total thickness of aluminum plating layer and intermetallic compound layer) The combined thickness of the aluminum plating layer and the intermetallic compound layer is preferably 40 μm or less on average, and more preferably 30 μm or less. This range is preferable because it facilitates the formation of a molten metal with high hardness after hot stamping.

[0052] (Average thickness of the base steel sheet, intermetallic compound layer, and aluminum plating layer) The thickness of the intermetallic compound layer and the aluminum plating layer is obtained by performing a line analysis from the surface of the aluminum-plated steel sheet to the base steel sheet using a field emission electron probe microanalyser (FE-EPMA) on the cross-section of the aluminum-plated steel sheet, and measuring the aluminum and Fe concentrations. The measurement conditions are an acceleration voltage of 15kV, a beam diameter of approximately 100nm, an irradiation time of 1000ms per point, and a measurement pitch of 60nm. The measurement distance should be such that the thickness of the aluminum plating layer and the intermetallic compound layer can be measured; for example, the measurement distance should be approximately 30μm to 80μm in the thickness direction from the surface of the aluminum-plated steel sheet. To explain using an aluminum-plated steel sheet having the aforementioned chemical composition as an example, the measured aluminum concentration in the cross-section of the aluminum-plated steel sheet is such that the region with an aluminum (Al) concentration of 0.06 mass% or less is considered the base steel sheet, and the region with an aluminum concentration greater than 0.06 mass% is considered the intermetallic compound layer or the aluminum plating layer. Furthermore, within the intermetallic compound layer and the aluminum plating layer, the region with an iron (Fe) concentration greater than 4 mass% is considered the intermetallic compound layer, and the region with an Fe concentration of 4 mass% or less is considered the aluminum plating layer. The thickness of the intermetallic compound layer is defined as the distance from the boundary between the base steel sheet and the intermetallic compound layer to the boundary between the intermetallic compound layer and the aluminum plating layer. Furthermore, the thickness of the aluminum plating layer is defined as the distance from the boundary between the intermetallic compound layer and the aluminum plating layer to the surface of the aluminum-plated steel sheet on which the aluminum plating layer is formed.

[0053] The thickness of the aluminum plating layer is determined as follows: First, in a plan view, determine the direction in which the area of ​​the member to be measured that has the longest aluminum plating layer is formed. Divide the aluminum plating layer in that direction into six equal lengths from one end to the other. Then, mark the boundaries of these six areas as measurement points. In other words, there are five measurement points. Next, at these five points, determine the thickness from the surface of the aluminum-plated steel sheet to the intermetallic compound layer according to the aforementioned criteria. Then, take the average of the five values ​​obtained as the thickness of the aluminum plating layer. Similarly, the thickness of the intermetallic compound layer can be determined as follows: In a plan view of the aluminum-plated steel sheet, the thickness of the intermetallic compound layer is determined at the same five locations where the thickness of the aluminum plating layer was determined as described above, according to the aforementioned criteria. The average of these five values ​​is then taken as the thickness of the intermetallic compound layer. The thickness of the base steel sheet is sufficiently large compared to the thickness of the intermetallic compound layer and the aluminum plating layer. Therefore, the thickness of the aluminum-plated steel sheet may be considered as the thickness of the base steel sheet.

[0054] The aluminum-plated steel sheets described above are not limited to those described herein.

[0055] (Welding method) The welding method used in the manufacturing method of the welded joint of this disclosure is preferably one that has a small penetration depth into the aluminum-plated steel sheet and low aluminum inclusion in the weld metal. Examples of such welding methods include laser welding, plasma welding, and electron beam welding, which can heat a narrow area. Laser welding is particularly preferred when welding using the solid wire for welding aluminum-plated steel sheets of this disclosure. The conditions for laser welding will be described below.

[0056] For efficiency, a higher laser welding speed is preferable, for example, 5.0 m / min or higher. There is no particular upper limit to the laser welding speed. If the welding speed is too high, excessive spatter may occur or the weld bead shape may become unstable, depending on the laser output, oscillator type, optical system conditions, and the composition, thickness, and combination of the steel plate being welded. Therefore, the laser welding speed should be selected as high as possible, taking into account other welding conditions.

[0057] Higher laser output is preferable, for example, 5.0 kW or higher. There is no particular upper limit to the laser output. Higher output allows for a good penetration depth even with a higher welding speed to ensure efficiency. On the other hand, excessively high output can lead to excessive spatter or an unstable weld bead shape. Furthermore, high-output welding equipment has high installation costs. Therefore, the upper limit of the laser output is naturally determined by balancing the output limit of the welding equipment, welding speed, and weld quality.

[0058] During laser welding, a shielding gas may be supplied to the molten pool and its surroundings. In this case, air, argon, nitrogen, helium, carbon dioxide, oxygen, or a mixture thereof may be used as the shielding gas.

[0059] When welding an aluminum-plated steel sheet, the supply amount of the solid wire for welding the aluminum-plated steel sheet of the present disclosure can be appropriately adjusted according to the composition, plate thickness, and welding conditions of the base metal steel sheet. That is, the supply amount of the solid wire of the present disclosure is adjusted so that there is no occurrence of underfill and no excessive bulging of the molten metal to the back side.

[0060] (Hardness of the weld metal) In the welded joint manufactured using the method for manufacturing a welded joint of the present disclosure, the proportion of martensite in the weld metal becomes high. The martensite hardness H M is obtained from the following formula (1). H M = 884C(1 - 0.3C 2 ) + 294 ····· (1) Here, C represents the numerical value when the average C concentration of the weld metal is expressed in mass%. The average C concentration of the weld metal is estimated from the C amount of the base metal steel sheet, the C amount of the solid wire, and the welding conditions. The hardness of the weld metal of the welded joint of the present disclosure after hot stamping is 90% or more with respect to the martensite hardness H M estimated from the average C concentration of the weld metal.

[0061] As described above, the method for manufacturing a welded joint can be appropriately changed within a range that does not impair the object of the present invention.

Examples

[0062] Hereinafter, the effects of the present invention will be specifically described with reference to examples.

[0063] Table 1 shows the chemical composition of the base steel sheet for the aluminum-plated steel sheet, and Table 2 shows the chemical composition of the solid wire. Values ​​outside the scope of the present invention are underlined. Note that 1.8 GPa, 2.0 GPa, and 2.3 GPa in Table 1 refer to the tensile strength after hot stamping. Alloy components that were not intentionally added are left blank in the table. P, S, and N are not alloying elements that were intentionally added, but their component analysis values ​​are listed. Although Al mainly acts as a deoxidizing agent, it is a ferrite-forming element, so its analysis value is listed. Note that the amount of Cu in Table 2 includes the Cu plating. The solid wire was manufactured by melting the raw material having the chemical composition of Table 2, forging, rolling, drawing, and annealing, drawing it to the final diameter described in Tables 3-1 and 3-2, and then Cu plating the surface of some of the solid wire as shown in Table 2. Furthermore, the description "Top surface: Completely removed, Bottom surface: Completely removed" in the "Aluminum plating layer and intermetallic compound layer at the edge" column for steel plates A J1 to J15 in Tables 3-1 and 3-2 means that the following treatment was applied to the area to be welded. That is, on both the top and bottom surfaces of the edge of the aluminum-plated steel plate, the aluminum plating layer and intermetallic compound layer were removed from the area up to 1.0 mm from the edge, creating an exposed area where the base steel plate is exposed. In addition, the description "Top surface: Edge not removed, Bottom surface: Completely removed" in the "Aluminum plating layer and intermetallic compound layer at the edge" column for steel plates B J16 to J45 and steel plates A J46 to J60 in Tables 3-1 and 3-2 means that the following treatment was applied to the area to be welded. Specifically, on the upper surface of the end of the aluminum-plated steel sheet, the aluminum plating layer and intermetallic compound layer were left in an area up to 0.6 mm from the edge, while the aluminum plating layer and intermetallic compound layer were removed in an area from the remaining area up to 1.0 mm, creating an exposed area where the base steel sheet is exposed. Similarly, on the lower surface of the end of the aluminum-plated steel sheet, the aluminum plating layer and intermetallic compound layer were removed in an area up to 1.0 mm from the edge, creating an exposed area where the base steel sheet is exposed.In Tables 3-1 and 3-2, the entry "Not performed" in the "Aluminum plating layer and intermetallic compound layer at the edges" column for steel plates A (J16-J45) and steel plates B (J46-J60) means that the aluminum plating layer and intermetallic compound layer have not been removed. The aluminum plating layer and intermetallic compound layer were removed by cutting with an end mill.

[0064] [Table 1]

[0065] [Table 2]

[0066] Next, steel plates A and B shown in Tables 3-1 and 3-2 were prepared. The ends of steel plates A and B were butted together, and a welded joint was fabricated by laser welding while supplying the solid wire described in Tables 3-1 and 3-2. The plating thickness in Tables 3-1 and 3-2 represents the sum of the average thickness of the aluminum plating layer and the average thickness of the intermetallic compound layer. The plate thickness in Tables 3-1 and 3-2 represents the plate thickness of the aluminum-plated steel sheet. The laser welding was adjusted to perform through welding under conditions of laser output of 3.0 kW to 10.0 kW and welding speed of 4.0 m / min to 10.0 m / min. The supply amount of solid wire was adjusted to prevent underfill and excessive bulging of molten metal to the back side. The resulting welded joint was heated in a furnace heated to 950°C for 4 minutes, then formed in a water-cooled mold and hardened to produce a hot-stamped molded product.

[0067] (Vickers hardness test) The Vickers hardness of the obtained welded joints was measured in accordance with JIS Z 2244 (2009). The measurement method is described below. Figure 1 is a schematic diagram of a cross-section of weld metal to explain the Vickers hardness test. The obtained welded joint is cut perpendicular to the weld line at the halfway point of the weld line, as shown in Figure 1. In the obtained cross-section, the measurement line F is defined as the line that passes through the center of the width of the weld metal 200 and is parallel to the thickness direction of the aluminum-plated steel sheets 100 and 110 (measurement line F in Figure 1). Here, the center of the width of the weld metal 200 is the midpoint between the position where the weld metal 200 is narrowest on the aluminum-plated steel sheet 100 (steel sheet A) side (the position where the aluminum-plated steel sheet 100 (steel sheet A) penetrates the weld metal 200 side the most) and the position where the weld metal 200 is narrowest on the aluminum-plated steel sheet 110 (steel sheet B) side (the position where the aluminum-plated steel sheet 110 (steel sheet B) penetrates the weld metal 200 side the most), in the cross-section described above, perpendicular to the thickness direction of the aluminum-plated steel sheets 100 and 110. Vickers hardness is measured at the center D of the weld metal 200 (center D in Figure 1) on the measurement line F, under a load of 0.5 kgf. Furthermore, measurements are taken at two points above and two points below center D on the measurement line F, and the average value of these measurements is taken as the hardness of the weld metal. The distance between the centers of the indentations after each measurement shall be at least three times the diagonal length of the indentation. The measurement range is defined as a range of 1 / 4 L above and below (1 / 2 L centered on center D) when the length of the weld metal 200 on the measurement line F is L. The estimated hardness value Hv of the weld metal, which is used as the basis for evaluation, was calculated using the following formula (2), with the average carbon concentration of the weld metal 200 derived from the carbon content of aluminum-plated steel sheet 100 (steel sheet A), aluminum-plated steel sheet 110 (steel sheet B), and solid wire, as well as the welding conditions. Hv = 884C(1 - 0.3C) 2 )+294·····(2) Here, Hv represents the estimated hardness of weld metal 200 (unit: HV0.5), and C represents the average C concentration of weld metal 200 expressed as mass%. The weld metal 200 was deemed acceptable if its measured hardness (actual value) was 90% or higher than the calculated estimated hardness.

[0068] (Tensile test) Using the obtained welded joint, a JIS No. 5 test specimen was prepared in accordance with JIS Z 2241 (2011), with the weld line positioned perpendicular to the load direction. A tensile test was performed on the prepared JIS No. 5 test specimen in accordance with JIS Z 2241 (2011). The tensile test speed was kept constant at 10 mm / min. A fracture path on the steel plate A or steel plate B side was considered "Good," and a fracture path on the weld metal side was considered "Bad." "Good" was considered a pass.

[0069] (Welding defects) A welded joint was judged as "Good" if no welding cracks occurred. A joint was judged as "Bad" if welding cracks occurred. A "Good" result was considered a pass.

[0070] The results of these tests are shown in Tables 4-1 and 4-2. Wire numbers outside the scope of the present invention are underlined.

[0071] [Table 3-1]

[0072] [Table 3-2]

[0073] [Table 4-1]

[0074] [Table 4-2]

[0075] As shown in Tables 4-1 and 4-2, joints No. J1-J10, J16-J25, J31-J40, and J46-J55 according to the present invention were able to form weld metal with excellent hardness because the chemical composition of the solid wire was appropriate.

[0076] In the comparative example, joint No. J11 had a carbon content outside the appropriate range, resulting in weld metal hardness below the standard and fracture path being within the weld metal.

[0077] In the comparative example, joint No. J12 had a carbon content outside the appropriate range, resulting in weld cracking, and the fracture path was also within the weld metal.

[0078] In the comparative example, joint No. J13 had a Mo content outside the appropriate range, resulting in weld metal hardness below the standard and fracture path being within the weld metal.

[0079] In the comparative example, joint No. J14 had a Mo content outside the appropriate range, resulting in weld metal hardness below the standard and fracture path also being within the weld metal.

[0080] In the comparative example, joint No. J15 had a Si content outside the appropriate range, resulting in weld metal hardness below the standard, and the fracture path was also within the weld metal.

[0081] In the comparative example, joint No. J26 had a carbon content outside the appropriate range, resulting in weld metal hardness below the standard and fracture path being within the weld metal.

[0082] In the comparative example, joint No. J27 exhibited welding cracks due to its carbon content being outside the appropriate range, and the fracture path was also within the weld metal.

[0083] In the comparative example, joint No. J28 had a Mo content outside the appropriate range, resulting in weld metal hardness below the standard, and the fracture path was also within the weld metal.

[0084] In the comparative example, joint No. J29 had a Mo content outside the appropriate range, resulting in weld metal hardness below the standard and fracture path being within the weld metal.

[0085] In the comparative example, joint No. J30 had a Si content outside the appropriate range, resulting in weld metal hardness below the standard and fracture path being within the weld metal.

[0086] In the comparative example, joint No. J41 had a carbon content outside the appropriate range, resulting in weld metal hardness below the standard and fracture path being within the weld metal.

[0087] In the comparative example, joint No. J42 exhibited welding cracks due to its carbon content being outside the appropriate range, and the fracture path was also within the weld metal.

[0088] In the comparative example, joint No. J43 had a Mo content outside the appropriate range, resulting in weld metal hardness below the standard and fracture path being within the weld metal.

[0089] In the comparative example, joint No. J44 had a Mo content outside the appropriate range, resulting in weld metal hardness below the standard and fracture path being within the weld metal.

[0090] In the comparative example, joint No. J45 had a Si content outside the appropriate range, resulting in weld metal hardness below the standard, and the fracture path was also within the weld metal.

[0091] In the comparative example, joint No. J56 had a carbon content outside the appropriate range, resulting in weld metal hardness below the standard and fracture path being within the weld metal.

[0092] In the comparative example, joint No. J57 exhibited welding cracks due to its carbon content being outside the appropriate range, and the fracture path was also within the weld metal.

[0093] In the comparative example, joint No. J58 had a Mo content outside the appropriate range, resulting in weld metal hardness below the standard, and the fracture path was also within the weld metal.

[0094] In the comparative example, joint No. J59 had a Mo content outside the appropriate range, resulting in weld metal hardness below the standard, and the fracture path was also within the weld metal.

[0095] In the comparative example, joint No. J60 had a Si content outside the appropriate range, resulting in weld metal hardness below the standard and fracture path being within the weld metal. [Industrial applicability]

[0096] According to the present invention's method for manufacturing solid wires and welded joints for welding aluminum-plated steel sheets, even if aluminum is mixed into the weld metal during welding of high-strength aluminum-plated steel sheets with a tensile strength of 1800 MPa or more after hot stamping, a weld metal with high hardness can be formed, thus offering high potential for industrial application. [Explanation of Symbols]

[0097] 12 Base steel plate 14. Aluminum plating layer 16 Intermetallic compound layer 100 Aluminum-plated steel sheet 110 Aluminum-plated steel sheet 200 Weld metal 300 Welded Joints

Claims

1. A solid wire for welding aluminum-plated steel sheets, wherein the content of the chemical composition, in mass %, relative to the total mass of the wire, C: 0.25% to 0.35%, Si: Less than 0.30% Mn: 1.8% to 2.5%, Mo: 0.30% to 1.00%, P: 0.050% or less, S: 0.030% or less, Al: 0.10% or less, N: 0.010% or less, Cu: 0.50% or less, The remainder consists of Fe and impurities. The diameter is 0.6 mm to 1.2 mm. Solid wire for welding aluminum-plated steel sheets.

2. Furthermore, in place of a portion of the aforementioned Fe, Cr: 1.5% or less, Ni: 3.0% or less, B: 0.010% or less, Ti: 0.20% or less, V: 0.50% or less, Nb: 0.20% or less, A solid wire for welding aluminum-plated steel sheets according to claim 1, comprising one or more of the above.

3. A method for manufacturing a welded joint, comprising welding at least one aluminum-plated steel sheet having a thickness of 0.7 mm to 3.2 mm, and welding the aluminum-plated steel sheet using the solid wire for welding aluminum-plated steel sheets described in claim 1 or 2.

4. A method for manufacturing a welded joint according to claim 3, wherein the welding is performed using laser welding.

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