Joined parts and joined steel plates

Abstract

This joined part includes: a first steel member; a second steel member; and a joined portion that is formed at a portion where the first steel member adjoins the second steel member and that includes a welding metal and a thermally affected portion. The first steel member includes a steel sheet substrate, and an Al-Fe coating formed on a surface of the steel sheet substrate. The tensile strength of the first steel member is more than 1500 MPa. In the case where a cross-section of the welding metal taken in a thickness direction perpendicular to an extension direction of the joined portion is a measurement plane, the average Cu content of the welding metal in the measurement plane is 0.03-3.00 mass%.

Description

[Technical Field]

[0001] The present invention relates to a joined component and a joined steel plate that is the material for the joined component. This application claims priority based on Japanese Patent Application No. 2021-131274, filed on August 11, 2021, the contents of which are incorporated herein by reference. [Background technology]

[0002] In the automotive field, the application of steel sheets with high tensile strength (high-strength steel sheets) is expanding in order to improve both fuel economy and collision safety against the backdrop of recent stricter environmental regulations and collision safety standards. However, as the strength of steel sheets increases, the press formability of the steel sheets decreases, making it difficult to manufacture products with complex shapes.

[0003] Specifically, as steel strength increases, its ductility decreases, resulting in the problem of fracture at high-pressure locations when processed into complex shapes. Furthermore, as steel strength increases, residual stress after processing causes springback and wall warping, resulting in poor dimensional accuracy. Therefore, it is not easy to process high-strength steel sheets, especially those with tensile strengths of 780 MPa or more, into products with complex shapes by press forming. While roll forming, rather than press forming, makes it easier to process high-strength steel sheets, its application is limited to parts with a uniform cross section in the longitudinal direction.

[0004] Therefore, in recent years, hot stamping technology has been adopted as a technology for press-forming difficult-to-form materials such as high-strength steel sheets, as disclosed in, for example, Patent Documents 1 to 3. Hot stamping technology is a hot forming technology in which the material to be formed is heated and then formed.

[0005] In this technology, the material is heated before being formed. Therefore, the steel is soft during forming and has good formability. This allows even high-strength steel sheets to be formed into complex shapes with high precision. In addition, with hot stamping technology, the steel is quenched at the same time as it is formed using a press die, so the steel part after forming has sufficient strength.

[0006] For example, Patent Document 1 discloses that hot stamping technology can be used to obtain steel members having a tensile strength of 1400 MPa or more after forming.

[0007] In recent years, countries around the world have set higher CO2 reduction targets, and automobile manufacturers are working to reduce fuel consumption while also considering collision safety. For gasoline-powered vehicles as well as the rapidly advancing trend toward electric vehicles, even higher strength materials are required to protect not only passengers but also batteries from collisions and to offset the resulting weight gain. For example, for steel components used in automobiles, there is a need for higher-strength hot-stamped components that exceed the strength (1.5 GPa = 1500 MPa) currently commonly used for steel components formed by hot stamping.

[0008] However, as steel components are strengthened, their deformability generally decreases. Therefore, if fracture occurs early in the collision at locations where deformation is concentrated during a vehicle collision, the load-bearing capacity and impact energy absorption commensurate with the increased strength may not be achieved. Furthermore, many metallic materials experience a deterioration in their properties as they are strengthened, resulting in a decline in deformability and hydrogen embrittlement resistance. It is known that hydrogen embrittlement susceptibility increases for steel components with tensile strengths of 1.2 GPa or higher. There have been cases of hydrogen embrittlement cracking in bolt steel, which has been the focus of increasing strength ahead of the automotive industry. Therefore, there are concerns that hot-stamped components with tensile strengths exceeding 1.5 GPa will be even more susceptible to hydrogen embrittlement.

[0009] Furthermore, when hot stamping technology is applied to steel sheets, the steel sheets are heated to high temperatures, causing the iron and other components on the surface to oxidize and form scale (oxides). Therefore, a process to remove this scale (descaling process) is required after hot pressing, which reduces productivity. Furthermore, for components that require corrosion resistance, the surface of the component must be subjected to rust prevention treatment or metal coating after processing, which requires surface cleaning and surface treatment processes, which also reduces productivity.

[0010] One example of a method for suppressing such productivity decline is to apply a coating to steel sheets. Generally, various materials, including organic and inorganic materials, are used to coat steel sheets. Among these, zinc-based coatings, which have sacrificial corrosion protection properties, are often applied to steel sheets from the perspective of their corrosion protection performance and steel sheet production technology. Meanwhile, the heating temperature in hot press working is often higher than the Ac3 transformation point of steel to achieve a hardening effect. However, this heating temperature is higher than the decomposition temperature of organic materials and the boiling point of metallic materials such as Zn. Therefore, when steel sheets coated with organic or Zn-based metallic materials are heated for hot press working, the coating layer on the surface of the steel sheet may evaporate, causing significant deterioration of the surface quality.

[0011] In order to avoid such deterioration of surface properties, it is preferable to coat steel sheets that are to be subjected to hot press working in which the steel sheets are heated to high temperatures with, for example, an Al-based metal, which has a higher boiling point than organic material coatings or Zn-based metal coatings. The use of steel sheets coated with aluminum-based metals, so-called aluminum-plated steel sheets, can prevent scale from adhering to the steel sheet surface, eliminating the need for processes such as descaling, thereby improving productivity.Al-based metal coatings also have a rust-preventing effect, improving corrosion resistance after painting.

[0012] Due to the above-mentioned background, in recent years, the use of steel components (called tailored blank materials) that are designed to have areas where deformation is concentrated during a collision and areas where deformation is not concentrated has become widespread in vehicle bodies. In particular, steel components made by butt-joining steel sheets of different strengths and thicknesses by welding are called tailored weld blank materials (TWB materials). Furthermore, aluminum-plated steel sheets are generally used for TWB materials to improve productivity and prevent deterioration of surface properties. However, when joining aluminum-plated steel sheets to produce TWB materials, there is a problem in that the aluminum on the surface dissolves into the weld metal at the joint, significantly reducing strength. Therefore, when joining aluminum-plated steel sheets to produce TWB materials, the aluminum plating on the surface of the butt joint that will become the joint is often removed before joining. In this case, there is no plating (coating) on ​​the surface of the joint.

[0013] Steel components used in automobiles are at risk of hydrogen embrittlement cracking due to hydrogen generated by corrosion of the steel during use. As mentioned above, the susceptibility of steel to hydrogen embrittlement is extremely high, especially in the strength range above 1.5 GPa. Therefore, it is thought that even small amounts of hydrogen caused by minor corrosion can cause hydrogen embrittlement. In the case of the above-mentioned TWB materials, the joints have strengths similar to those of the high-strength steel plates being joined in some locations, and since there is often no coating on the surface, it is difficult to completely prevent corrosion. Therefore, in order to apply hot-stamped components above 1.5 GPa to automobile bodies to further reduce vehicle weight, it is necessary to utilize TWB technology to reduce the risk of premature fracture during collision deformation, while also sufficiently reducing the risk of hydrogen embrittlement cracking at the joints.

[0014] There are three main reasons why weld metal at joints is susceptible to hydrogen embrittlement. Specifically, (i) there is often no coating on the surface of weld metal, making corrosion more likely, (ii) the joint is a transition zone between strength and plate thickness, making it susceptible to localized stress during a collision, and (iii) the structure of molten and solidified parts such as weld metal is coarse and prone to embrittlement, making weld metal at joints susceptible to hydrogen embrittlement. In other words, the weld metal at joints in TWBs is subject to more severe conditions than the base metal steady-state parts in terms of all of the factors that cause hydrogen embrittlement: hydrogen generation, stress application, and material limitations.

[0015] Regarding high-strength steel materials having a tensile strength exceeding 1.5 GPa, for example, Patent Document 2 discloses a hot-press-formed product having excellent toughness and a tensile strength of 1.8 GPa or more. Patent Document 3 discloses a steel material having an extremely high tensile strength of 2.0 GPa or more and also having good toughness and ductility. Patent Document 4 discloses a steel material having a high tensile strength of 1.8 GPa or more and also having good toughness. Patent Document 5 discloses a steel material having an extremely high tensile strength of 2.0 GPa or more and also having good toughness.

[0016] However, Patent Documents 2 to 5 do not adequately address the hydrogen embrittlement resistance of TWB materials, particularly in the weld lines of joints, which are susceptible to embrittlement in corrosive environments. Therefore, although the steel materials of Patent Documents 2 to 5 have a tensile strength of more than 1.5 GPa, they may not be able to fully meet the demand for higher safety when used as automotive components.

[0017] Regarding TWB materials, for example, Patent Documents 6 to 16 disclose that in order to solve the problem of reduced strength at the joint, the shape of the exposed part (the part where the Al plating has been removed) is controlled and fillers or metal powders containing gamma-genic elements are used during welding. However, none of the patent documents provide sufficient countermeasures against hydrogen embrittlement in the weld metal of the joints of TWB materials, and there are cases where the demand for higher safety cannot be fully met when applying high-strength steel materials with tensile strengths exceeding 1.5 GPa to automotive components. [Prior art documents] [Patent documents]

[0018] [Patent Document 1] Japanese Patent Publication No. 2002-102980 [Patent Document 2] Japanese Patent Application Publication No. 2012-180594 [Patent Document 3] Japanese Patent Publication No. 2012-001802 [Patent Document 4] International Publication No. 2015 / 182596 [Patent Document 5] International Publication No. 2015 / 182591 [Patent Document 6] International Publication No. 2013 / 014512 [Patent Document 7] International Publication No. 2007 / 125182 [Patent Document 8] Japan Special Publication No. 2009-534529 [Patent Document 9] International Publication No. 2015 / 121074 [Patent Document 10] International Publication No. 2017 / 050711 [Patent Document 11] Japan Special Publication No. 2018-534143 [Patent Document 12] Japan Special Publication No. 2017-512137 [Patent Document 13] International Publication No. 2015 / 086781 [Patent Document 14] Japan Special Publication No. 2016-531753 [Patent Document 15] International Publication No. 2019 / 093440 [Patent Document 16] International Publication No. 2020 / 152887 Summary of the Invention [Problem to be solved by the invention]

[0019] The present invention has been made to solve the above-mentioned problems, and aims to provide a joined component having a joint with excellent hydrogen embrittlement resistance, even if it is a steel member (TWB material) in which base steel plates of different strengths and plate thicknesses are butt-joined by welding, and a joined steel plate that is the material for such a joined component. [Means for solving the problem]

[0020] The present invention relates to the following joined parts in a steel member (TWB material) in which steel material plates with different strengths and thicknesses are butt-joined by welding, and the joined steel plate that serves as the material for the joined parts. [1] A joined component according to one aspect of the present invention includes a first steel member, a second steel member, and a joint formed at a butt joint between the first steel member and the second steel member, the joint including a weld metal and a heat-affected zone, wherein the first steel member has a steel plate substrate and an Al-Fe-based coating formed on a surface of the steel plate substrate, and has a tensile strength of more than 1500 MPa, and when a cross section of the weld metal in a plate thickness direction perpendicular to an extending direction of the joint is taken as a measurement surface, the average Cu content in the weld metal at the measurement surface is, in mass%, 0.03% or more and 3.00% or less. and Cu / Al, which is the ratio of the average Cu content to the average Al content in the weld metal on the measurement surface, is 0.15 to 3.90. . [2] In the joined component described in [1], the Vickers hardness of the weld metal on the measurement surface may be greater than the hardness of the steel plate base material of the second steel member or 350 Hv, whichever is higher. [3] The joined part according to [1] is a part of the weld metal on the measurement surface. The aforementioned The average Al content may be less than 1.00% by mass. [4] The joined part according to [2] is a part of the weld metal on the measurement surface. The aforementioned The average Al content may be less than 1.00% by mass. [ 5 ][1]~[4] The joined component according to any one of the above items (1) to (5), wherein the steel plate base material of the first steel member contains, in mass %, C: 0.25 to 0.65%, Si: 2.00% or less, Mn: 0.15 to 3.00%, P: 0.050% or less, S: 0.0100% or less, N: 0.010% or less, O: 0.010% or less, Al: 1.00% or less, B: 0.0005 to 0.0100%, Cu: 0 to 3.00%, Ti: 0. 0.100%, Nb: 0-0.10%, Mo: 0-1.00%, Cr: 0-1.00%, Ni: 0-1.00%, V: 0-1.00%, Ca: 0-0.010%, Mg: 0-0.010%, Sn: 0-1.00%, W: 0-1.00%, Sb: 0-1.00%, Zr: 0-1.00%, REM: 0-0.30%, and the balance: Fe and impurities. [ 6 ][ 5 In the joined component described in [1], the chemical composition of the steel plate base material of the first steel member may have a Cu content of 0.05 to 3.00%. [ 7 ] [1]~[4] In the joined component described in any one of 1 to 5, the total content of one or more of Mn, Cr, Mo, Ni, Sn, and W in the weld metal on the measurement surface may be 1.2% or more. [ 8 ][ 5 In the joined component described in [1], the total content of one or more of Mn, Cr, Mo, Ni, Sn, and W in the weld metal on the measurement surface may be 1.2% or more. [ 9 ][ 6 In the joined component described in [1], the total content of one or more of Mn, Cr, Mo, Ni, Sn, and W in the weld metal on the measurement surface may be 1.2% or more. [ 10 ] [1]~[4] In the joined component described in any one of the above, the second steel member may have a tensile strength of 500 MPa or more and 1500 MPa or less. [ 11 ][ 5 In the joined component described in [1], the second steel member may have a tensile strength of 500 MPa or more and 1500 MPa or less. [ 12 ][ 6 In the joined component described in [1], the second steel member may have a tensile strength of 500 MPa or more and 1500 MPa or less. [ 13 ][ 7 In the joined component described in [1], the second steel member may have a tensile strength of 500 MPa or more and 1500 MPa or less. [ 14 ][ 8 In the joined component described in [1], the second steel member may have a tensile strength of 500 MPa or more and 1500 MPa or less. [ 15 ][ 9 In the joined component described in [1], the second steel member may have a tensile strength of 500 MPa or more and 1500 MPa or less. [ 16 A welded steel plate according to another aspect of the present invention includes a first steel plate, a second steel plate, and a welded portion formed at the butt joint between the first steel plate and the second steel plate, the welded portion including a weld metal and a heat-affected zone, wherein the first steel plate has a steel plate base material and an Al-based coating formed on a surface of the steel plate base material, and when a cross section of the welded metal in a plate thickness direction perpendicular to the extending direction of the welded portion is used as a measurement surface, the average Cu content in the welded metal at the measurement surface is, in mass%, 0.03% or more and 3.00% or less. and Cu / Al, which is the ratio of the average Cu content to the average Al content in the weld metal on the measurement surface, is 0.15 to 3.90. . [ 17 ][ 16 In the welded steel plate described in the above, The aforementioned The average Al content may be less than 1.00% by mass. [ 18 ]

[16] or [ 17In the joining steel sheet described in the above, the steel sheet base material of the first steel sheet contains, in mass %, C: 0.25 to 0.65%, Si: 2.00% or less, Mn: 0.15 to 3.00%, P: 0.050% or less, S: 0.0100% or less, N: 0.010% or less, O: 0.010% or less, Al: 1.00% or less, B: 0.0005 to 0.0100%, Cu: 0 to 3.00%, Ti: 0 to 0.10 0%, Nb: 0-0.10%, Mo: 0-1.00%, Cr: 0-1.00%, Ni: 0-1.00%, V: 0-1.00%, Ca: 0-0.010%, Mg: 0-0.010%, Sn: 0-1.00%, W: 0-1.00%, Sb: 0-1.00%, Zr: 0-1.00%, REM: 0-0.30%, and the balance: Fe and impurities. [ 19 ][ 18 In the joined steel plate described in [1], the chemical composition of the steel plate base material of the first steel plate may have a Cu content of 0.05 to 3.00%. [ 20 ][ 16 ]or[ 17 In the welded steel plate described in [1], the total content of one or more of Mn, Cr, Mo, Ni, Sn, and W in the weld metal on the measurement surface may be 1.2% or more. [ 21 ][ 18 In the welded steel plate described in [1], the total content of one or more of Mn, Cr, Mo, Ni, Sn, and W in the weld metal on the measurement surface may be 1.2% or more. [ 22 ][ 19 In the welded steel plate described in [1], the total content of one or more of Mn, Cr, Mo, Ni, Sn, and W in the weld metal on the measurement surface may be 1.2% or more. [Effects of the Invention]

[0021] According to the above-described aspects of the present invention, even if the steel member (TWB material) is made by butt-joining raw steel plates with different strengths and thicknesses by welding, it is possible to provide a joined component having a joint with excellent hydrogen embrittlement resistance, and the joined steel plate that is the raw material for the joined component. The joined component according to the above aspect of the present invention has high strength and excellent resistance to hydrogen embrittlement, and therefore, when applied to automobile components, contributes to improvements in fuel economy and collision safety. [Brief explanation of the drawings]

[0022] [Figure 1] 1 is a schematic diagram illustrating an example of a joining component according to the present embodiment. FIG. [Figure 2] 1 is a schematic diagram showing an example of a joining steel plate according to the present embodiment. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0023] The present inventors investigated the influence of the structure of the weld metal at the joint and the steel sheets used as the base material on these properties in a steel member (TWB material) in which steel sheets with different strengths and thicknesses are butt-jointed by welding, in order to obtain a joint component having a joint with high tensile strength and excellent hydrogen embrittlement resistance at least in part. As a result, the following findings were obtained.

[0024] Many of the materials used in commonly manufactured hot-stamped components are coated steel sheets, with the surface coated with aluminum (Al) plating, which has excellent corrosion resistance. When this coated steel sheet is hot-stamped, an alloying reaction between the Al in the surface coating and the Fe in the steel sheet occurs during heating, resulting in a steel component (coated steel component) with a coating containing Al and Fe (hereinafter sometimes referred to as an Al-Fe-based coating). Most commonly used steel sheets that exhibit a tensile strength of about 1.5 GPa after hot stamping contain about 0.20 mass% C, which ensures strength after hot stamping. This coated steel sheet is butt-joined to another steel sheet, followed by heat treatment, to obtain a joined component with an Al-Fe-based coating.

[0025] (a) To further reduce vehicle weight, the inventors conducted detailed studies to increase the carbon content to obtain high-strength components exceeding 1.5 GPa (1500 MPa) after hot stamping. As a result, they found that by increasing the carbon content to 0.25 mass% or more in at least one of the steel sheets used for butt welding, ultrahigh tensile strength exceeding 1.5 GPa can be achieved in that area after heat treatment, including quenching, such as hot stamping. However, as the tensile strength exceeds 1.5 GPa, there are concerns that the deformability decreases and that premature fracture may occur during collision deformation. There are also concerns about increased susceptibility to hydrogen embrittlement, which could lead to hydrogen embrittlement cracking due to hydrogen generated in the corrosive environment during vehicle use. In particular, when butt-joining two Al-based coated steel sheets or an Al-based coated steel sheet to another coated steel sheet, the aluminum on the surface of the butt joint, which will become the joint, is often partially or completely removed, making it impossible to guarantee the corrosion resistance of the aluminum plating in the removed area. Therefore, although the strength of the welded joint is similar to that of the steel members being joined, there is often no coating on the surface, making corrosion more likely to progress. Also, as mentioned above, the welded joint is subject to more severe conditions in terms of both stress state and material limits than the steel members. Therefore, there is a greater risk of hydrogen embrittlement at the welded joint.

[0026] (b) Therefore, the present inventors have investigated a method for suppressing hydrogen embrittlement by improving the corrosion resistance of the weld metal of the joint, which is the starting point of embrittlement in the joined component. As a result, they have found that the corrosion resistance can be improved and hydrogen embrittlement can be suppressed by adding Cu, an element that improves corrosion resistance, to the weld metal.

[0027] (c) The inventors also investigated the hydrogen embrittlement resistance of coated steel members with a tensile strength exceeding 1.5 GPa, other than at the joints, and considered the preferable composition and structure designs to ensure hydrogen embrittlement resistance.

[0028] Based on the above findings, the inventors have developed a joint component having a joint with excellent hydrogen embrittlement resistance and a high-strength coated steel member with a tensile strength exceeding 1.5 GPa, which significantly improves hydrogen embrittlement resistance in corrosive environments by suppressing corrosion in the weld metal of the joint, reducing the amount of hydrogen penetration, and improving the hydrogen embrittlement resistance of the steel. Such a joint component has high strength yet a low risk of hydrogen embrittlement, making it safer to use in vehicle bodies.

[0029] Hereinafter, each requirement of a joint component according to one embodiment of the present invention (the joint component according to this embodiment) and a joint steel plate (the joint steel plate according to this embodiment) that is the material for the joint component will be described in detail.

[0030] (A) Joint parts 1, a joined component 1 according to this embodiment includes a first steel member 10, a second steel member 20, and a joining portion 30 formed at the butt joint between the first steel member 10 and the second steel member 20, joining the first steel member 10 and the second steel member 20. The first steel member 10 is a coated steel member having a steel plate substrate 11 having a predetermined chemical composition and a coating (Al-Fe-based coating) 12 containing Al and Fe formed on the surface of the steel plate substrate 11. The second steel member 20 has a steel plate substrate 21 and may have a coating 22 . The weld 30 includes a weld metal 31 and a heat-affected zone 32. The Al-Fe-based coating 12 of the first steel member 10 and / or the coating 22 of the second steel member 20 may be formed up to the surface of the heat-affected zone 32. Furthermore, in the joined component 1 according to this embodiment, when a cross section in the plate thickness direction perpendicular to the extension direction of the joint 30 (in Figure 1, the direction toward the depth of the paper) is used as a measurement surface, the average Cu content in the weld metal at this measurement surface is, in mass%, 0.03% or more and 3.00% or less. Each of these will be explained below.

[0031] (A1) First steel member As described above, the first steel member 10 included in the joined component 1 according to this embodiment has a steel plate substrate 11 and a coating (Al-Fe-based coating) 12 containing Al and Fe formed on the surface of the steel plate substrate 11. As will be described later, the first steel member 10 is obtained by subjecting a bonded steel plate including a coated steel plate having a steel plate substrate and an Al-based coating to heat treatment involving quenching such as hot stamping.

[0032] (A1-1) Steel plate base material [Tensile strength] In order to meet the recent demand for higher strength, the first steel member 10 included in the joined component 1 according to this embodiment has a steel plate substrate 11 with a tensile strength of more than 1.5 GPa (1500 MPa). Although there is no upper limit to the tensile strength, there are concerns that an increase in strength may result in a decrease in deformability and an increase in hydrogen embrittlement susceptibility, so the tensile strength may be set to 3000 MPa or less.

[0033] The tensile test is performed in accordance with the provisions of ASTM Standard E8. A test specimen is cut from the first steel member so that its longitudinal direction is parallel to the weld line, and both sides are uniformly ground to a thickness of 1.2 mm. A half-size plate test specimen (parallel length: 32 mm, parallel width: 6.25 mm) conforming to ASTM Standard E8 is then obtained. However, if the plate thickness is less than 1.2 mm, the coating or black scale (oxide scale) is removed before obtaining a half-size plate test specimen conforming to ASTM Standard E8. Then, a strain gauge with a gauge length of 5 mm is attached to the center of the parallel part, and a tensile test is carried out at room temperature at a strain rate of 3 mm / min to measure the tensile strength (maximum strength).

[0034] [Chemical composition] The chemical composition of the steel plate substrate 11 of the first steel member 10 provided in the joining component 1 according to this embodiment is not limited, but in order to ensure a tensile strength of more than 1.5 GPa after heat treatment and also to take into consideration the improvement of hydrogen embrittlement resistance, it is preferable to set the content of each element as follows: Specifically, the first steel member 10 contains, in mass %, C: 0.25 to 0.65%, Si: 2.00% or less, Mn: 0.15 to 3.00%, P: 0.050% or less, S: 0.0100% or less, N: 0.010% or less, O: 0.010% or less, Al: 1.00% or less, B: 0.0005 to 0.0100%, Cu: 0 to 3.00%, Ti: 0 to 0.100%, Nb: 0 to It is preferable that the alloy has a chemical composition consisting of: Mo: 0-1.00%, Cr: 0-1.00%, Ni: 0-1.00%, V: 0-1.00%, Ca: 0-0.010%, Mg: 0-0.010%, Sn: 0-1.00%, W: 0-1.00%, Sb: 0-1.00%, Zr: 0-1.00%, REM: 0-0.30%, and the balance: Fe and impurities.

[0035] The reasons for limiting the content of each element are as follows. Here, the chemical composition of the steel sheet substrate 11 refers to the chemical composition of the first steel member 10 excluding the Al-Fe-based coating 12 on the surface (for example, the position ¼ of the thickness from the surface of the steel sheet substrate). Hereinafter, % relating to the content is mass % unless otherwise specified.

[0036] C: 0.25 to 0.65% C is an element that improves the hardenability of steel and improves the strength of steel members obtained after quenching, such as hot stamping. If the C content is less than 0.25%, it becomes difficult to ensure sufficient strength (more than 1.5 GPa) in the steel member after quenching (steel member obtained by quenching). Therefore, the C content is preferably 0.25% or more. The C content is more preferably 0.28% or more. On the other hand, if the C content exceeds 0.65%, the strength of the steel member after quenching becomes too high, resulting in a significant decrease in hydrogen embrittlement resistance. Therefore, the C content is preferably 0.65% or less. The C content is more preferably 0.60% or less.

[0037] Si:2.00% or less If the Si content in steel exceeds 2.00%, the heating temperature required for austenite transformation during heat treatment (quenching) becomes significantly higher. This may increase the cost required for heat treatment and may result in residual ferrite remaining during heating, reducing the strength of the steel member. Therefore, the Si content is preferably 2.00% or less. The Si content is more preferably 1.00% or less. The Si content may be 0%, but Si is an effective deoxidizing element, and to obtain this effect, the Si content may be set to 0.01% or more, or 0.10% or more. Furthermore, Si is an element that is effective in improving the hardenability of steel and in stably ensuring the strength of the steel member after quenching. Therefore, Si may be contained. To obtain this effect, the Si content is preferably 0.25% or more.

[0038] Mn: 0.15 to 3.00% Mn is an element that is highly effective in improving the hardenability of steel and increasing the strength of steel members after quenching. Mn also lowers the Ac3 point, facilitating lowering the quenching temperature. Furthermore, by including Mn in the weld metal of a joint, it is possible to further improve corrosion resistance. If the Mn content is less than 0.15%, these effects are insufficient. Therefore, it is preferable that the Mn content be 0.15% or more. It is more preferable that the Mn content be 0.25% or more. On the other hand, if the Mn content exceeds 3.00%, there is a concern that the hydrogen embrittlement resistance of the steel member after quenching may deteriorate. Therefore, the Mn content is preferably 3.00% or less. The Mn content is more preferably 2.00% or less, and even more preferably 1.30% or less.

[0039] P:0.050% or less P is an element that reduces the hydrogen embrittlement resistance of steel members after quenching. In particular, if the P content exceeds 0.050%, the hydrogen embrittlement resistance is significantly reduced. Therefore, the P content is preferably limited to 0.050% or less. The P content is more preferably limited to 0.005% or less. The P content may be 0% since a small amount is preferable, but from the viewpoint of cost, it may be 0.001% or more.

[0040] S: 0.0100% or less S is an element that reduces the hydrogen embrittlement resistance of steel members after quenching. In particular, if the S content exceeds 0.0100%, the hydrogen embrittlement resistance is significantly reduced. Therefore, the S content is preferably limited to 0.0100% or less. The S content is more preferably limited to 0.0050% or less. The S content is preferably low, so it may be 0%, but from the viewpoint of cost, it may be 0.0001% or more.

[0041] N: 0.010% or less N is an element that reduces the hydrogen embrittlement resistance of steel members after quenching. In particular, if the N content exceeds 0.010%, coarse nitrides are formed in the steel, significantly reducing the hydrogen embrittlement resistance. Therefore, the N content is preferably 0.010% or less. It is more preferable to limit the N content to 0.006% or less. The lower limit of the N content does not need to be particularly limited and may be 0%, but a N content of less than 0.0002% increases steelmaking costs and is economically undesirable, so the N content may be 0.0002% or more, 0.0008% or more, or 0.001% or more.

[0042] O: 0.010% or less O is an element that reduces the hydrogen embrittlement resistance of steel members after quenching. In particular, if the O content exceeds 0.010%, coarse nitrides are formed in the steel, significantly reducing hydrogen embrittlement resistance. Therefore, the O content is preferably 0.010% or less. It is more preferable to limit the O content to 0.006% or less. The lower limit of the O content does not need to be particularly set and may be 0%, but an O content of less than 0.0002% increases steelmaking costs and is economically undesirable, so the O content may be set to 0.0002% or more, 0.0008% or more, or 0.001% or more.

[0043] Al: 1.00% or less Al is an element that is generally used as a deoxidizer for steel, and therefore may be contained. To obtain this effect, the Al content is preferably 0.01% or more. If the Al content exceeds 1.00%, the above effects saturate and the economic efficiency decreases. Therefore, if Al is contained, the Al content is preferably 1.00% or less. The Al content is more preferably 0.50% or less.

[0044] B: 0.0005 to 0.0100% B is an important element that has the effect of dramatically improving the hardenability of steel even in small amounts. Furthermore, B segregates at grain boundaries, strengthening the grain boundaries and improving hydrogen embrittlement resistance, and it is an element that suppresses the grain growth of austenite when the steel sheet is heated. If the B content is less than 0.0005%, the above effects may not be fully achieved. Therefore, the B content is set to 0.0005% or more. The B content is preferably 0.0010% or more. On the other hand, if the B content exceeds 0.0100%, a large amount of coarse compounds precipitates, reducing the hydrogen embrittlement resistance of the steel member. Therefore, the B content is preferably 0.0100% or less. The B content is more preferably 0.0080% or less.

[0045] Cu: 0-3.00% Cu is an element effective in improving the hardenability of steel and ensuring stable strength of steel members after quenching. Furthermore, when contained in the weld metal of a joint, it improves corrosion resistance. Therefore, its inclusion is preferable. Since these effects are insufficient when the Cu content is less than 0.05%, the Cu content is preferably 0.05% or more. The Cu content is more preferably 0.10% or more, even more preferably 0.15% or more, and even more preferably 0.20% or more. However, the improvement of hardenability can be substituted by other elements, and Cu can also be added to the weld metal using a filler wire or the like. In such cases, the Cu content of the steel member may be less than 0.05%, for example, 0%. On the other hand, if the Cu content exceeds 3.00%, the above effects saturate and the cost increases. Therefore, if Cu is contained, the Cu content is preferably 3.00% or less. The Cu content is more preferably 1.50% or less, and even more preferably 0.80% or less.

[0046] The chemical composition of the steel plate substrate 11 provided in the first steel member 10 included in the joined component 1 according to this embodiment may contain elements other than those described above, i.e., the remainder may be Fe and impurities, but in order to improve various properties (hardenability, strength, hydrogen embrittlement resistance, deoxidation, corrosion resistance, etc.) of the steel member and the joined component including this steel member, one or more elements selected from the group consisting of Ti, Nb, Mo, Cr, Ni, V, Ca, Mg, Sn, W, Sb, Zr and REM may also be contained within the ranges shown below. These elements are optional elements and do not necessarily have to be contained, so the lower limit is 0%.

[0047] Ti: 0 to 0.100% Ti is an element that suppresses recrystallization and forms fine carbides to suppress grain growth, thereby refining austenite grains, when a steel sheet is heat-treated by heating it to a temperature equal to or higher than the Ac3 point. Therefore, adding Ti improves the hydrogen embrittlement resistance of the steel member. Furthermore, Ti preferentially bonds with N in the steel, thereby suppressing the consumption of B due to the precipitation of BN and promoting the effect of B in improving hardenability, which will be described later. Therefore, Ti may be added. The Ti content is preferably 0.010% or more, and more preferably 0.015% or more. On the other hand, if the Ti content exceeds 0.100%, the amount of TiC precipitated increases and C is consumed, resulting in a decrease in the strength of the steel member after quenching. Therefore, the Ti content is preferably 0.100% or less. The Ti content is more preferably 0.080% or less.

[0048] Nb: 0 to 0.10% Nb is an element that forms fine carbides and improves the hydrogen embrittlement resistance of steel through its grain refinement effect. If the Nb content is less than 0.02%, the above effect may not be fully achieved. Therefore, in order to achieve the above effect, the Nb content is preferably 0.02% or more. The Nb content is more preferably 0.03% or more. On the other hand, if the Nb content exceeds 0.10%, the carbides become coarse and the hydrogen embrittlement resistance of the steel member decreases. Therefore, the Nb content is preferably 0.10% or less, and more preferably 0.08% or less.

[0049] Mo: 0 to 1.00% Mo is an extremely effective element for improving the hardenability of steel and for stably ensuring the strength of steel members after quenching. In particular, when Mo is added in combination with the above-mentioned B, a synergistic effect of improving hardenability can be obtained. Furthermore, when Mo is added to the weld metal of a joint, it is possible to further improve corrosion resistance. Therefore, it is preferable to add Mo. If the Mo content is less than 0.10%, these effects are insufficient, so the Mo content is preferably 0.10% or more. The Mo content is more preferably 0.20% or more. On the other hand, Mo has the effect of stabilizing iron carbides. If the Mo content exceeds 1.00%, coarse iron carbides may remain undissolved when the steel plate is heated, which may reduce the hydrogen embrittlement resistance of the steel member after quenching. Furthermore, the cost increases significantly. Therefore, if Mo is contained, the Mo content is preferably 1.00% or less. The Mo content is more preferably 0.80% or less.

[0050] Cr: 0 to 1.00% Cr is an element that is effective in improving the hardenability of steel and in stably ensuring the strength of steel members after quenching. Furthermore, by including Cr in the weld metal of a joint, it is possible to further improve corrosion resistance. Therefore, Cr may be included. To achieve the above effects, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.05% or more, and even more preferably 0.08% or more. On the other hand, if the Cr content exceeds 1.00%, the above effects saturate and costs increase. Furthermore, since Cr has the effect of stabilizing iron carbides, if the Cr content exceeds 1.00%, coarse iron carbides may remain undissolved when the steel sheet is heated, which may reduce the hydrogen embrittlement resistance of the steel member after quenching. Therefore, if Cr is contained, the Cr content is preferably 1.00% or less. A Cr content of 0.80% or less is more preferable.

[0051] Ni: 0 to 1.00% Ni is an element that is effective in improving the hardenability of steel and in stably ensuring the strength of steel members after quenching. Furthermore, Ni is an element that further improves corrosion resistance when contained in the weld metal of a joint. Therefore, it is preferable to contain Ni. Since these effects are insufficient when the Ni content is less than 0.10%, when Ni is contained, it is preferable that the Ni content be 0.10% or more. The Ni content is more preferably 0.20% or more. On the other hand, if the Ni content exceeds 1.00%, the limiting hydrogen content of the steel member decreases. Also, the cost increases significantly. Therefore, the Ni content is preferably 1.00% or less. The Ni content is more preferably 0.25% or less, and further preferably 0.20% or less.

[0052] V: 0 to 1.00% V is an element that forms fine carbides and improves the hydrogen embrittlement resistance of steel members through its grain refining effect and hydrogen trapping effect. Therefore, V may be added. To achieve the above effects, the V content is preferably 0.01% or more. The V content is more preferably 0.10% or more. On the other hand, if the V content exceeds 1.00%, the above effects saturate and economic efficiency decreases. Therefore, the V content is preferably 1.00% or less. The V content is more preferably 0.80% or less, and further preferably 0.50% or less.

[0053] Ca: 0 to 0.010% Ca is an element that has the effect of refining inclusions in steel and improving the hydrogen embrittlement resistance of steel members after quenching. Therefore, Ca may be added. To obtain the above effects, the Ca content is preferably 0.001% or more. The Ca content is more preferably 0.002% or more. On the other hand, if the Ca content exceeds 0.010%, the effect saturates and the cost increases. Therefore, when Ca is added, the Ca content is preferably 0.010% or less. The Ca content is more preferably 0.005% or less, and further preferably 0.004% or less.

[0054] Mg: 0 to 0.010% Mg is an element that has the effect of refining inclusions in steel and improving the hydrogen embrittlement resistance of the steel member after quenching. Therefore, Mg may be added. To obtain the above effects, the Mg content is preferably 0.001% or more. The Mg content is more preferably 0.002% or more. On the other hand, if the Mg content exceeds 0.010%, the effect saturates and the cost increases. Therefore, when Mg is added, the Mg content is preferably 0.010% or less. The Mg content is more preferably 0.005% or less, and further preferably 0.004% or less.

[0055] Sn: 0 to 1.00% Sn is an element that improves corrosion resistance in a corrosive environment. Furthermore, by including Sn in the weld metal of a joint, it is possible to further improve corrosion resistance. Therefore, Sn may be included. To obtain the above effects, the Sn content is preferably 0.01% or more. The Sn content is more preferably 0.03% or more. On the other hand, if the Sn content exceeds 1.00%, the grain boundary strength decreases, and the hydrogen embrittlement resistance of the steel member after quenching decreases. Therefore, the Sn content is preferably 1.00% or less. The Sn content is more preferably 0.80% or less, and even more preferably 0.50% or less.

[0056] W: 0 to 1.00% W is an element that is effective in improving the hardenability of steel and in stably ensuring the strength of steel members after quenching. Furthermore, W is an element that further improves corrosion resistance when contained in the weld metal of a joint. Therefore, W may be contained. To obtain the above effects, the W content is preferably 0.01% or more. The W content is more preferably 0.03% or more. On the other hand, if the W content exceeds 1.00%, the above effects saturate and economic efficiency decreases. Therefore, the W content is preferably 1.00% or less. The W content is more preferably 0.80% or less, and further preferably 0.50% or less.

[0057] Sb: 0 to 1.00% Sb is an element that improves corrosion resistance in a corrosive environment. Therefore, it may be contained. To obtain the above effects, the Sb content is preferably 0.01% or more. The Sb content is more preferably 0.03% or more. On the other hand, if the Sb content exceeds 1.00%, the grain boundary strength decreases, and the hydrogen embrittlement resistance of the steel member after quenching decreases. Therefore, the Sb content is preferably 1.00% or less. The Sb content is more preferably 0.80% or less, and even more preferably 0.50% or less.

[0058] Zr: 0 to 1.00% Zr is an element that improves corrosion resistance in a corrosive environment. Therefore, it may be contained. To obtain the above effects, the Zr content is preferably 0.01% or more. The Zr content is more preferably 0.03% or more. On the other hand, if the Zr content exceeds 1.00%, the grain boundary strength decreases, and the hydrogen embrittlement resistance of the steel member after quenching decreases. Therefore, the Zr content is preferably 1.00% or less. The Zr content is more preferably 0.80% or less, and even more preferably 0.50% or less.

[0059] REM: 0 to 0.30% Like Ca, REM is an element that refines inclusions in steel and improves the hydrogen embrittlement resistance of steel members after quenching. Therefore, REM may be added. To achieve the above effect, the REM content is preferably 0.01% or more. The REM content is more preferably 0.02% or more. On the other hand, if the REM content exceeds 0.30%, the effect saturates and the cost increases. Therefore, the REM content is preferably 0.30% or less. The REM content is more preferably 0.20% or less, and even more preferably 0.15% or less. Here, REM refers to a total of 17 elements, including Sc, Y, and lanthanides such as La and Nd, and the REM content refers to the total content of these elements. REM is added to molten steel using, for example, an Fe-Si-REM alloy, which contains, for example, La, Nd, Ce, and Pr.

[0060] In the chemical composition of the steel plate substrate 11 provided in the first steel member 10 included in the joined component according to this embodiment, the elements other than those described above, that is, the remainder, are Fe and impurities. Here, the term "impurities" refers to components that are mixed in from raw materials such as ores and scraps or due to various factors in the manufacturing process when industrially manufacturing steel sheets, and are acceptable within a range that does not adversely affect the characteristics of the joined component according to this embodiment.

[0061] The chemical composition of the steel sheet substrate 11 can be determined by the following method. The elemental analysis is performed at five equally spaced locations at a position 1 / 4 of the plate thickness of the steel plate substrate 11, with a distance of 1 mm or more between measurement locations, using a common method such as ICP, and the contents obtained at the five locations are averaged to obtain the value.

[0062] [Metal structure of steel plate substrate 11] The metal structure of the steel plate substrate 11 of the first steel member 10 included in the joined component 1 according to this embodiment is preferably a structure mainly composed of high-strength martensite in order to obtain a tensile strength of more than 1.5 GPa. The area fraction of martensite is preferably 70% or more, more preferably 80% or more, and may be 100%.

[0063] The metal structure of the steel sheet substrate 11 may contain one or more of retained austenite, bainite, ferrite, and pearlite as the remainder other than martensite. Martensite includes not only fresh martensite but also tempered martensite and auto-tempered martensite.

[0064] The metal structure of the steel sheet substrate 11 can be determined by the following method. The area fraction of martensite (including tempered martensite and auto-tempered martensite) is measured using a transmission electron microscope (TEM) and an electron beam diffraction device attached to the TEM. A measurement sample is cut out from 1 / 2 of the longitudinal part of the steel member (at a position 1 / 2 of the length from the longitudinal end in the longitudinal direction), 1 / 4 of the width part (at a position 1 / 4 of the member width in the width direction from the width end), and 1 / 4 of the plate thickness part of the steel plate substrate 11 (at a position 1 / 4 of the plate thickness in the plate thickness direction from the surface) to serve as a thin film sample for TEM observation. The area of ​​TEM observation is 400 μm in area. 2 To determine the extent of the defect, TEM observation is performed to identify each metal structure and measure its area. When identifying metal structures, the electron diffraction patterns of thin-film samples can be used to distinguish between martensite and bainite, which have body-centered cubic lattices, and retained austenite, which has a face-centered cubic lattice. Iron carbide (Fe3C) in martensite and bainite can then be identified from the diffraction patterns, and the precipitation morphology can be observed to distinguish between martensite and bainite. Specifically, if the precipitation morphology is three-directional precipitation, it is determined to be martensite (tempered martensite), and if it is unidirectional precipitation, it is determined to be bainite. If no iron carbide precipitation is observed, it is also determined to be martensite (fresh martensite). The structure fractions of martensite and bainite measured by TEM are measured as area percentages, but since the steel member according to this embodiment has an isotropic metal structure, the area fraction values ​​can be directly converted to volume fractions. Carbides are observed to distinguish between martensite and bainite, but in this embodiment, carbides are not included in the volume fraction of the structure.

[0065] If ferrite or pearlite is present as the remaining structure, it can be easily confirmed using an optical microscope or a scanning electron microscope. Specifically, a measurement sample is cut out from half the longitudinal part of the steel component (a position half the length from the longitudinal end in the longitudinal direction) and one-quarter of the width (a position one-quarter of the component width in the width direction from the width end), and the one-quarter part of the thickness of the cross section in the thickness direction of the steel plate substrate is observed. The observation range using the microscope is 40,000 μm in area. 2 The cut sample is mechanically polished and then mirror-finished. It is then etched with a nital etching solution to reveal ferrite and pearlite, which are then observed under a microscope to confirm the presence of ferrite or pearlite. A structure in which ferrite and cementite are arranged in alternating layers is considered pearlite, and a structure in which cementite precipitates in granular form is considered ferrite.

[0066] The plate thickness of the steel plate substrate 11 of the first steel member 10 included in the joined component 1 according to this embodiment is not limited, but is, for example, 0.8 to 3.2 mm.

[0067] (A1-2) Al-Fe coating The first steel member 10 included in the joined component 1 according to this embodiment has a coating 12 containing Al and Fe (Al-Fe-based coating) on ​​the surface of the above-described steel sheet substrate 11. In this embodiment, the Al-Fe-based coating is a coating mainly containing Al and Fe, and preferably contains 70 mass% or more of Al and Fe in total. The Al-Fe-based coating is also referred to as a coating, an alloyed plating layer, or an intermetallic compound layer. In addition to Al and Fe, the Al-Fe-based coating may further contain Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Zr, REM, and Zn, with the remainder being impurities. The thickness of the Al-Fe coating is preferably 10 μm or more. There is no particular upper limit to the thickness of the Al-Fe coating, but it may be 100 μm or less. The Al-Fe coating 12 is preferably formed on both surfaces of the steel sheet substrate 11, but may be formed on only one surface, taking into consideration the application site.

[0068] The thickness of the Al-Fe coating can be determined by observing the cross section with a scanning electron microscope. Specifically, a measurement sample is cut out from a longitudinal half portion (a position half the length from the longitudinal end in the longitudinal direction) and a width quarter portion (a position one-quarter the width from the width end in the width direction) of the steel member, and observed. The observation range under the microscope is, for example, 400 times magnification, and an area of ​​40,000 μm 2 The cut sample is mechanically polished and then mirror-finished. The thickness of the Al-Fe coating is then measured in 10 random fields of view, and the average value is taken as the thickness of the Al-Fe coating. When observing using a BSE image (or COMP image), a clear difference in contrast can be seen between the Al-Fe coating and the base steel (steel sheet substrate). Therefore, the thickness of the Al-Fe coating can be determined by measuring the thickness from the outermost surface to the point where the contrast changes. Measurements are taken at 20 equally spaced locations within the observation photograph, with the distance between measurement locations being 6.5 μm. Five fields of view are also observed in the same manner, and the average value is used to determine the coating thickness. The chemical composition of the Al-Fe coating can be determined by spot elemental analysis (beam diameter 1 μm or less) using an electron probe microanalyzer (EPMA) on the same observation area as above to determine the Al and Fe contents contained in the Al-Fe coating. A total of 10 points are analyzed on 10 arbitrary fields of view of the Al-Fe coating, and the average values ​​are used to determine the Al and Fe contents contained in the Al-Fe coating. The same method can be used even if elements other than Al and Fe are present.

[0069] (A2) Second steel member In the joined component 1 according to this embodiment, the second steel member 20 joined to the first steel member 10 via the joint 30 is not particularly limited in terms of the hydrogen embrittlement resistance of the joint 30. The second steel member 20 has a steel plate substrate containing, for example, 0.05 to 0.65% C, 2.00% or less Si, 0.15 to 3.00% Mn, 0.050% or less P, 0.0100% or less S, 0.010% or less N, 0.010% or less O, 1.00% or less Al, 0 to 0.0100% B, 0 to 3.00% Cu, 0 to 0.100% Ti, and 0 to 0.0100% Nb. 10%, Mo: 0-1.00%, Cr: 0-1.00%, Ni: 0-1.00%, V: 0-1.00%, Ca: 0-0.010%, Mg: 0-0.010%, Sn: 0-1.00%, W: 0-1.00%, Sb: 0-1.00%, Zr: 0-1.00%, REM: 0-0.30%, and the balance: Fe and impurities. The second steel member may have a coating on a portion of the surface of the steel sheet substrate. The coating may be, for example, a coating mainly made of Al-Fe or a coating mainly made of Zn-Fe. The coating is also called a coating, an alloyed plating layer, or an intermetallic compound layer.

[0070] The tensile strength of the steel plate substrate of the second steel member is not limited. When the joined component according to this embodiment is a steel member having a region where deformation is concentrated during a collision and a region where deformation is not concentrated, the tensile strength of the second steel member may be 500 MPa or more and 1500 MPa or less, or 500 MPa or more and 1000 MPa or less. On the other hand, when a joined component with high strength as a whole is to be obtained, the tensile strength of the second steel member may be more than 1500 MPa, similar to the first steel plate. The plate thickness of the steel plate substrate 21 of the second steel member 20 included in the joined component 1 according to this embodiment is not limited, but is, for example, 0.8 to 3.2 mm. The thickness of the steel plate substrate 21 of the second steel member 20 may be the same as or different from the thickness of the steel plate substrate 11 of the first steel member 10.

[0071] (A3)Joint part In the joined component 1 according to this embodiment, a first steel member 10 and a second steel member 20 are joined by (through) a joint 30. The joint 30 is formed by butting together a steel plate that will be the material for the first steel member and a steel plate that will be the material for the second steel member, and joining the butt joint by welding. The joint includes a weld metal that has melted and solidified due to the heat of welding, and a heat-affected zone. That is, the welded portion 30 is formed at the butt joint between the first steel member 10 and the second steel member 20 , and includes a weld metal 31 and a heat-affected zone 32 . Although there are no limitations on the joining method, fusion welding, in which the base material is heated above its melting point, is preferred because it allows for a high welding speed and produces a high-strength joint, rather than solid-state joining methods such as friction stir welding and friction welding, or liquid-solid-state joining brazing. Among these, methods using a high-energy density heat source are mentioned from the perspective of productivity, but laser welding is preferred because it does not use electrodes and allows for high-speed welding in the atmosphere, compared to plasma arc welding, which involves electrode wear during processing, and electron beam welding, in which the beam attenuates in the atmosphere. The width of the weld metal 31 of the joint 30 of the joint component 1 according to this embodiment, which is perpendicular to the extension direction (the direction of the so-called weld line), is not particularly limited. However, if the weld metal comes into contact with and slides against the mold during forming, excessive stress may be generated in the weld metal, potentially causing cracks in the weld. Therefore, the width is set to avoid this. The width is, for example, 0.5 to 2.2 mm in the case of laser welding, and 1.8 to 7.0 mm in the case of plasma arc welding.

[0072] (A3-1) Weld metal In the weld metal constituting the joint 30 of the joined component 1 according to this embodiment, when a cross section in the plate thickness direction perpendicular to the extension direction of the joint is used as a measurement surface, the average Cu content in the weld metal at this measurement surface is, in mass %, 0.03% or more and 3.00% or less. By including Cu in the weld metal, corrosion of the joint can be suppressed, thereby improving the hydrogen embrittlement resistance of the joint. For this reason, the average Cu content in the weld metal at the measurement surface in the weld metal is set to 0.03% or more. The average Cu content is preferably 0.05% or more, more preferably 0.10% or more, and even more preferably 0.15% or more. On the other hand, if the average Cu content in the weld metal exceeds 3.00%, the effect saturates and the cost increases, so the average Cu content is set to 3.00% or less, preferably 2.00% or less, more preferably 1.00% or less, and even more preferably 0.80% or less.

[0073] Furthermore, in the same measurement, the average Al content in the weld metal is preferably less than 1.00% by mass. If the average Al content in the weld metal is high, the weld metal may not be quenched in the heat treatment described below, resulting in a decrease in hardness. For this reason, the average Al content is preferably less than 1.00%. The average Al content is more preferably less than 0.80%. There is no particular lower limit for the average Al content, but it may be about 0.01%. Furthermore, in the same measurement surface, it is preferable that the weld metal contain one or more of Mn, Cr, Mo, Ni, Sn, and W in a total content of 1.2% or more by mass. By including one or more of Mn, Cr, Mo, Ni, Sn, and W in the weld metal, corrosion can be further suppressed and hydrogen embrittlement resistance is improved. For this reason, it is preferable that the weld metal contain one or more of Mn, Cr, Mo, Ni, Sn, and W in a total content of 1.2% or more. It is more preferable that the total content of Mn, Cr, Mo, Ni, Sn, and W be 1.4% or more.

[0074] The ratio of the average Cu content to the average Al content in the weld metal on the measurement surface, Cu / Al, is preferably 0.15 to 3.90. A Cu / Al ratio within the above range provides an isotropically homogeneous microstructure after quenching, ensuring stable weld strength. Because Cu is an austenite-forming element and Al is a ferrite-forming element, the Cu / Al ratio serves as an index of the width of the austenite region in the HS heating temperature range. With a Cu / Al ratio within the above range, the entire microstructure in the weld metal becomes a single austenite phase in the temperature range of approximately 900°C during HS heating, ensuring stable hardenability and the strength of the microstructure in the weld metal during quenching. As a result, even when an external force is applied to the weld metal after quenching, the homogeneous microstructure in the weld metal prevents localized stress concentration in the weld metal microstructure, improving the corrosion resistance of the weld, which is a potential embrittlement initiation site in a corrosive environment.

[0075] The contents of Cu, Al, Mn, Cr, Mo, Ni, Sn, and W contained in the weld metal of the joint 30 are determined by the following method. A cross section of the welded component 1 is cut in the thickness direction, perpendicular to the extension direction of the weld (weld line). A backscattered electron image is obtained for this sample using a scanning electron microscope, and spot elemental analysis (beam diameter 1 μm or less) is performed on the weld metal using an electron probe microanalyzer (EPMA) to determine the Cu, Al, Mn, Cr, Mo, Ni, Sn, and W contents in the weld metal. During measurement, the weld metal is identified based on the image shading and uneven shape (which, unlike steel plate, has a curved shape rather than a straight line) from the backscattered electron image. Ten equally spaced points are analyzed within the weld metal from the front surface to the back surface of the weld metal at the weld center line, and the average values ​​are used as the Cu, Al, Mn, Cr, Mo, Ni, Sn, and W contents in the weld metal. The weld center line is the line connecting the width-wise centers of the weld metal in the cross section (the final solidification position).

[0076] In the butt joint fabricated in this embodiment, if the Al-based coating at the planned butt welding location is not completely removed, Al will be enriched at the boundary (fusion line) between the weld metal and the base metal. However, within the scope of this embodiment, the degree of this Al enrichment does not deteriorate the strength characteristics of the welded joint, and is therefore considered to be a unique feature of the welded part, because the Al-enriched region is not continuous in the weld line direction. The measurement method for identifying this feature is as follows. First, a cross section is cut out in the sheet thickness direction perpendicular to the extension direction of the weld joint (weld line). The sample is mirror-polished and then nital-etched to identify the boundary between the weld metal and the base metal by examining the shading of the cross section or by using an electron probe microanalyzer (EPMA). Next, using the electron probe microanalyzer (EPMA), spot elemental analysis (beam diameter 1 μm or less) is performed to measure five points along the boundary between the weld metal and the base metal, and also to identify the location with the highest Al content in the width direction, evenly spaced in the depth direction from the surface of the base metal (steel sheet substrate) of the steel sheet to a depth of 0.2 mm in the sheet thickness direction, and the average value is taken as the Al content of the end portion. In this embodiment, the average Al concentration of the end portion is 0.10% or more and 1.90% or less by mass.

[0077] In order to prevent fracture at the weld metal in a butt-welded steel plate assembly, it is preferable that the weld metal have a Vickers hardness equal to or greater than that of the base material on the low-strength side. That is, the Vickers hardness is approximately 0.3 times the tensile strength (in MPa). Therefore, if the tensile strength of the first steel plate substrate on the high-strength side is greater than 1500 MPa and the tensile strength of the second steel plate substrate on the low-strength side to be combined is approximately 1000 MPa, the center of the weld metal will have a Vickers hardness exceeding 350 Hv. In this embodiment, the tensile strength of the second steel plate substrate is not particularly limited, and the Vickers hardness of the weld metal to prevent fracture at the weld metal is preferably greater than the hardness of the second steel plate substrate or 350 Hv, whichever is higher.

[0078] The hardness of the weld metal is determined by the following method. A cross section of the weld metal is cut out in the same manner as above, and Vickers hardness is measured in accordance with JIS Z 2244: 2009. The test force is 98 N, and measurements are taken at five equally spaced points within the weld metal from the front surface to the back surface of the weld metal at the weld center line, and the average value is taken as the hardness of the weld metal.

[0079] (A3-2) Heat affected zone In the joined component 1 according to this embodiment, a heat-affected zone 32 is formed around the weld metal 31 in the joint 30, but there are no particular limitations on the heat-affected zone 32. Furthermore, although there are some parts that are indistinguishable from the steel plate substrate (11 or 12) upon normal observation, it is not necessary to distinguish them.

[0080] (A4) Characteristics of joining parts In the joined component 1 according to this embodiment, the weld metal 31 of the joint 30 is controlled as described above, thereby improving the corrosion resistance of the joint. Therefore, the joined component 1 according to this embodiment has high strength, with at least a portion of the tensile strength exceeding 1.5 GPa, and the joint has excellent hydrogen embrittlement resistance.

[0081] In this embodiment, hydrogen embrittlement resistance is evaluated by an exposure test in an actual use environment of the bonded parts or an accelerated corrosion test using a combined cyclic test (CCT). For example, CCT is performed in accordance with the provisions of JASO standards M609 and M610, and the hydrogen embrittlement resistance is evaluated by the number of cycles at which the bonded parts do not fracture.

[0082] There are no particular limitations on the shape of the joining component 1. The first steel member 10 and / or the second steel member 20 may be a flat plate or a formed body. Hot-formed steel members are often formed bodies, but the term "steel member" includes cases where they are flat plates. The joined component 1 according to this embodiment is obtained by heat treating a joined steel plate as described below.

[0083] (B) Joined steel plate Next, a description will be given of a bonded steel plate (hereinafter, sometimes referred to as the bonded steel plate according to the present embodiment) that is the material for the bonded component 1 according to the present embodiment. The bonded component can be obtained by performing a heat treatment using the bonded steel plate described below as a material. 2, the joined steel plate S1 according to this embodiment includes a first steel plate S10, a second steel plate S20, and a joint S30 formed at the butt joint between the first steel plate S10 and the second steel plate S20, the joint S30 including a weld metal and a heat-affected zone. The first steel plate S10 is a coated steel plate having a steel plate substrate S11 having a predetermined chemical composition and an Al-containing coating (Al-based coating) S12 formed on the surface of the steel plate substrate S11. Furthermore, in the joining steel plate S1 of this embodiment, when the cross section in the plate thickness direction perpendicular to the extension direction (weld line) of the joint S30 is used as the measurement surface, the average Cu content in the weld metal at this measurement surface is 0.03% or more and 3.00% or less in mass%. Each of these will be explained below.

[0084] (B1) First steel plate The first steel plate S10 included in the joined steel plate S1 according to this embodiment has a steel plate substrate S11 and an Al-containing coating (Al-based coating) S12 formed on the surface of the steel plate substrate S11.

[0085] (B1-1) Steel plate base material The range of the chemical composition of the steel plate substrate S11 of the first steel plate S10 provided in the joined steel plate S1 according to this embodiment is the same as the chemical composition of the steel plate substrate 11 in the first steel member 10 described above, and the reasons for limiting it are also the same. Here, the chemical composition of the steel plate substrate S11 refers to the chemical composition of the portion of the coated steel plate excluding the Al-based coating S12 on the surface. For example, the chemical composition can be obtained by performing elemental analysis at a representative position, a position ¼ of the way through the plate thickness from the surface of the steel plate substrate S11 in the plate thickness direction, using a general method such as ICP.

[0086] [Metal structure of steel plate substrate] The metal structure of the steel plate substrate S11 of the first steel plate S10 included in the joined steel plate S1 according to this embodiment is not limited. It is often ferrite or pearlite, but may contain bainite, martensite, or retained austenite within the conditions of the manufacturing method described below. The martensite mentioned above includes tempered and auto-tempered martensite. The metallographic structure of the steel plate substrate S11 can be determined in the same manner as the metallographic structure of the steel plate substrate 11 in the joined component 1 according to the present embodiment described above.

[0087] (B1-2) Al-based coating The first steel sheet S10 included in the joined steel sheet S1 according to this embodiment has a coating S12 containing Al (hereinafter referred to as Al-based coating) on ​​the surface of the steel sheet substrate S11. The Al-based coating S12 is a coating mainly made of Al, and preferably contains 40 mass % or more of Al, more preferably 50 mass % or more. The Al-based coating is also called a coating film or a plating layer. In addition to Al, the Al-based coating may further contain one or more of Si, Mg, Ca, Sr, Ti, Zn, Sb, Sn, Ni, Cu, Co, In, Bi, and REM, with the remainder being impurities. Generally, it often contains about 10 mass % of Si. The type of Al-based coating is not limited, and may be a coating formed by hot-dip plating, electroplating, thermal spraying, or the like. The amount of Al coating applied is 25g / m 2 The upper limit of the amount of Al-based coating is not particularly limited, but it is preferable that the amount of coating be 150 g / m or more. 2 The following may also be used. Furthermore, when producing the joined steel plate S1, the coating may be partially removed from the portion to be welded (the portion where the steel plate will melt due to the heat of welding), as will be described later. Therefore, the Al-based coating S12 may also be partially removed from the surface of the first steel plate S10 of the obtained joined steel plate S1.

[0088] The chemical composition and thickness of the Al-based coating S12 can be determined by observing the cross section with a scanning electron microscope and an electron probe microanalyzer (EPMA), similar to the Al-Fe-based coating 12 of the first steel member 10.

[0089] (B2) Second steel plate In the joined steel plate according to this embodiment, the second steel plate S20 joined to the first steel plate S10 via the joint is not particularly limited in terms of the hydrogen embrittlement resistance of the joint. The steel plate substrate of the second steel plate may contain, for example, C: 0.05 to 0.65%, Si: 2.00% or less, Mn: 0.15 to 3.00%, P: 0.050% or less, S: 0.0100% or less, N: 0.010% or less, O: 0.010% or less, Al: 1.00% or less, B: 0 to 0.0100%, Cu: 0 to 3.00%, Ti: 0 to 0.100%, Nb: 0 to 0.1 The chemical composition is as follows: 0%, Mo: 0-1.00%, Cr: 0-1.00%, Ni: 0-1.00%, V: 0-1.00%, Ca: 0-0.010%, Mg: 0-0.010%, Sn: 0-1.00%, W: 0-1.00%, Sb: 0-1.00%, Zr: 0-1.00%, and REM: 0-0.30%, with the balance being Fe and impurities. The second steel sheet S20 may have a coating S22 on a portion of the surface of the steel sheet substrate S21. The coating S22 may be, for example, a coating mainly made of Al or a coating mainly made of Zn. The coating is also called a coating or a plating layer. In the case of a coating mainly containing Al, as will be described later, the coating may be partially removed from the portion to be welded (the portion where the steel plate will melt due to the heat of welding). Therefore, the coating may also be partially removed from the surface of the second steel plate S20 of the obtained joined steel plate S1.

[0090] (B3)Joint part The joined steel plate S1 according to this embodiment has a first steel plate S10 and a second steel plate S20 joined together by a joint S30. The joint includes a weld metal S31 formed by welding and a heat-affected zone (HAZ) S32 that has not melted due to the heat of welding but has undergone a structural change. The welded portion of the welded steel plate S1 according to this embodiment extends along the butt joint between the first steel plate and the second steel plate. The size in the width direction of the weld metal S31, which is perpendicular to the extending direction (weld line), is not particularly limited.

[0091] (B3-1) Weld metal In the joining steel plate S1 of this embodiment, when the cross section in the plate thickness direction perpendicular to the extension direction (weld line) of the joint S30 is used as the measurement surface, the average Cu content in the weld metal S31 at the measurement surface is 0.03% or more and 3.00% or less in mass%. By including Cu in the weld metal, corrosion can be suppressed and hydrogen embrittlement resistance can be improved. Therefore, the average Cu content in the weld metal S31 on the measurement surface is set to 0.03% or more. The average Cu content is preferably 0.05% or more. On the other hand, if the average Cu content exceeds 3.00%, the effect saturates and the cost increases. Therefore, the average Cu content is set to 3.00% or less. The average Cu content is preferably 2.00% or less.

[0092] Furthermore, if the average Al content in the weld metal S31 is high, the weld metal S31 may not be hardened during the heat treatment described below, resulting in a decrease in hardness. Therefore, in the same measurement plane, the average Al content in the weld metal S31 is preferably less than 1.00% by mass. The average Al content is more preferably less than 0.80%. However, if the amount of alloying elements in the weld metal is increased to improve hardenability, sufficient hardness (strength) can be ensured after heat treatment even if the average Al content is 1.00% or more. There is no particular lower limit for the average Al content, but it may be contained up to about 0.01%. Furthermore, in the same measurement surface, it is preferable that the weld metal S31 contains one or more of Mn, Cr, Mo, Ni, Sn, and W in a total content of 1.2% or more by mass. By including Mn, Cr, Mo, Ni, Sn, and W in the weld metal, corrosion can be further suppressed, thereby improving hydrogen embrittlement resistance. It is more preferable that the total content of Mn, Cr, Mo, Ni, Sn, and W is 1.4% or more.

[0093] The contents of Cu, Al, Mn, Cr, Mo, Ni, Sn, and W contained in the weld metal S31 of the joint are determined by the following method. A cross section of the welded steel plate is cut in the thickness direction, perpendicular to the weld line, so that the weld metal can be observed. A backscattered electron image of this sample is obtained using a scanning electron microscope, and the weld metal structure is subjected to spot elemental analysis (beam diameter 1 μm or less) using an electron probe microanalyzer (EPMA), which allows the contents of Cu, Al, Mn, Cr, Mo, Ni, Sn, and W in the weld metal to be determined. During measurement, the weld metal is identified from the backscattered electron image based on the image's shading and uneven shape (which, unlike steel plate, has a curved, not linear, shape). Ten equally spaced points are analyzed within the weld metal from the front surface of the weld metal along the weld center line to the back surface, and the average values ​​are used as the contents of Cu, Al, Mn, Cr, Mo, Ni, Sn, and W in the weld metal.

[0094] (B3-2) Heat affected zone (HAZ) In the joined steel plate S1 according to this embodiment, a heat-affected zone S32 is formed around the weld metal S31 of the joint S30, but the heat-affected zone S32 is not particularly limited. Also, in normal observation, there are some parts that are indistinguishable from the steel plate substrate (S11 or S12), but this does not have to be distinguished.

[0095] The joined steel plate according to this embodiment is obtained by joining a first steel plate and a second steel plate by welding, as will be described later.

[0096] (C) Al-based coated steel plate Next, an Al-based coated steel sheet (hereinafter, sometimes referred to as the Al-based coated steel sheet according to the present embodiment) that serves as the material for the first steel sheet S10 of the joining steel sheet S1 according to the present embodiment will be described. The Al-based coated steel sheet according to this embodiment has a steel sheet substrate made of a predetermined chemical composition, and an Al-containing coating (Al-based coating) formed on the surface of the steel sheet. The steel sheet substrate and the Al-based coating may be the same as those described above in B1-1 and B1-2, respectively. This Al-based coated steel sheet is used as the material for the first steel sheet S10 and joined to a steel sheet that is the material for the second steel sheet, thereby obtaining the joined steel sheet according to this embodiment. There are no limitations on the steel sheet that can be used as the second material, but this Al-based coated steel sheet may also be used as the material for the second steel sheet S20.

[0097] (D) Manufacturing method for joining parts Next, a method for manufacturing the joint component 1 according to this embodiment will be described. The joined component 1 according to this embodiment is obtained by subjecting the joined steel plate S1 according to this embodiment described above to a heat treatment, which will be described later. Each step will be described below.

[0098] <Heat treatment process> The heat treatment is carried out under conditions in which the joined steel sheet is heated, for example, at an average heating rate of 1.0 to 1000°C / sec to a temperature between the Ac3 point and (Ac3 point + 300)°C, and then cooled to the Ms point (°C) or below at an average cooling rate equal to or greater than the upper critical cooling rate in the steel sheet base material S11 of the first steel sheet S10. The average heating rate is the average value of the heating rate from the start of heating to "target temperature (°C) - 20"°C, and the average cooling rate is the average value of the cooling rate from the start of cooling to the Ms point (°C). An average heating rate of less than 1.0°C / sec is undesirable because it reduces the productivity of the heat treatment, while an average heating rate of more than 1000°C / sec is undesirable because it results in a duplex grain structure and reduces hydrogen embrittlement resistance. Furthermore, if the heat treatment temperature is lower than the Ac3 point (°C), ferrite will remain after cooling, resulting in insufficient strength, which is undesirable, whereas if the heat treatment temperature exceeds (Ac3 point + 300)°C, the structure will become coarse-grained, which is undesirable as it reduces hydrogen embrittlement resistance. During heating, the temperature may be held within ±10° C. of the heating temperature for 1 to 300 seconds. After cooling, the steel member may be tempered at a temperature in the range of about 100 to 600° C. to adjust its strength. The upper critical cooling rate is the minimum cooling rate at which austenite is supercooled to form martensite without precipitating ferrite or pearlite in the structure. If the material is cooled below the upper critical cooling rate, ferrite and pearlite are formed, resulting in insufficient strength.

[0099] The Ac3 point, Ms point and upper critical cooling rate are measured by the following method. A 30 mm wide, 200 mm long rectangular test piece was cut from a steel plate having the same chemical composition as the steel plate substrate S11 of the first steel plate S10 of the joining steel plate according to this embodiment. The test piece was heated to 1000°C in a nitrogen atmosphere at an average heating rate of 10°C / s, held at that temperature for 5 minutes, and then cooled to room temperature at various average cooling rates. The cooling rate was set from 1°C / s to 100°C / s in 10°C / s increments. The Ac3 point was measured by measuring the change in thermal expansion of the test piece during heating. Furthermore, among the test pieces cooled at the above cooling rates, the minimum cooling rate at which ferrite phase precipitation did not occur was defined as the upper critical cooling rate. The change in thermal expansion during cooling at the upper critical cooling rate was measured, and the transformation start point was defined as the Ms point.

[0100] Here, during the series of heat treatments described above, hot forming such as hot stamping may be performed while the steel sheet is being cooled to the Ms point after heating to a temperature range of Ac3 point to (Ac3 point + 300)°C, i.e., while cooling at or above the upper critical cooling rate. Examples of hot forming include bending, drawing, stretch forming, hole expansion, and flanging. Furthermore, the present invention may be applied to forming methods other than press forming, such as roll forming, as long as a means for cooling the steel sheet is provided simultaneously with or immediately after hot forming. Repeated hot forming may also be performed if the above-described thermal history is followed.

[0101] As described above, in this embodiment, the first steel member 10 and the second steel member 20 of the "joining part 1" include both those that have been cooled simultaneously with or immediately after hot forming to form a formed body, and those that have only been subjected to heat treatment to form a flat plate.

[0102] The series of heat treatments described above can be carried out by any method, and for example, heating may be carried out by high-frequency heating, electrical heating, infrared heating, furnace heating, etc. Cooling may also be carried out by water cooling, mold cooling, etc.

[0103] (E) Manufacturing method for bonded steel plates Next, the manufacturing method of the joined steel plate according to this embodiment will be described.

[0104] <Al-based coating removal process> In the manufacture of the joined steel plate, when butt-welding the steel plates, for at least one of the steel plates (the steel plate that becomes the first steel plate), an Al-based coated steel plate is used. In that case, if welding is performed as it is without removing the coating of the welded part, the Al content in the weld metal may be 1.00% or more. To prevent this, before manufacturing the joined steel plate, it is preferable to remove the coating of the planned welding part by mechanical grinding such as milling or brushing, or by laser ablation or the like. By removing the coating, it is preferable that the Al amount on the surface after removal makes the average Al content in the weld metal dissolved in the weld metal when welded less than 1.00%. The average Al content is more preferably 0.50% or less, and even more preferably 0.30%. At the time of removing the coating, it is sufficient to remove the coating on the surface of the planned welding part (the part where the steel plate melts due to the heat of welding), and it is not necessary to remove the coating remaining as sag during shearing. Also, when a small amount of Al melts into the weld metal, since the corrosion resistance of the welded part after HS is improved, it may be removed so as to leave a part of the Al-based coating of the planned welding part.

[0105] <Welding method> In welding, in order not to break with the weld metal during hot stamping (HS) forming, it is important to give sufficient annealing to the weld metal by cooling after welding. Therefore, it is necessary to use a welding method with a small penetration width of the steel plate and a high cooling rate after welding. As a welding method capable of such welding, a welding method using a heat source with a high energy density that can concentrate heating in a narrow area (keyhole welding can be performed), such as laser welding, electron beam welding, or plasma welding, is suitable. Among them, laser welding is most suitable. Industrially, in addition to the gas laser CO2 laser, the solid laser YAG laser and fiber laser are used, but in this embodiment, the laser type is not particularly limited. Hereinafter, the case of performing laser welding as the welding method will be described.

[0106] <Laser welding process> In the laser welding process, a steel plate that will become the first steel plate and a steel plate that will become the second steel plate are butt-welded. The first steel plate and the second steel plate may have different thicknesses and strengths or may have the same strength. When welding, consideration must be given to the thickness of the weld metal.

[0107] <Thickness of weld metal> If the thickness of the weld metal is too thin compared to the thickness of the base steel plate, the strength of the weld joint will decrease. Therefore, the thickness of the thinnest part of the weld metal should not be less than 80% of the thickness of the base steel plate (if the thicknesses of the butted steel plates are different, the thickness of the thinner plate). This rule applies whether or not filler metal such as filler wire is used. However, since the thickness tends to be thin when no filler metal is used, it is preferable to use filler metal to avoid it being less than 80%. If the thickness of the weld metal is less than 80% of the thickness of the steel plate base material, even if the weld metal has a composition that is easily quenched, there is a concern that the weld may fracture during hot forming, or even if no fracture occurs, the product strength may decrease.

[0108] On the other hand, when welding using a filler metal such as a filler wire, the strength of the weld can be ensured by raising the front and back surfaces of the weld bead relative to the surface of the steel plate to be welded, thereby increasing the thickness of the weld metal. However, if the die does not have any irregularities to prevent the weld metal from being deposited, and the weld metal deposit height becomes excessive, a gap will form between the steel plate and the die at a position slightly away from the deposit during hot forming, resulting in poor contact between the steel plate and the die and an area where quenching is insufficient. Therefore, if the mold does not have any irregularities to prevent excess weld metal buildup, the front and back surfaces of the weld metal should not protrude more than 500 μm outward from the extension line of the steel plate surface (if the steel plates have different thicknesses, the thicker steel plate surface). If the protrusion amount is 500 μm or less, the steel plate can be sufficiently hardened using a mold, especially a direct water-cooled mold (a mold that cools the steel plate by spraying cooling water from the mold).

[0109] [Selection of filler metal] In this embodiment, a filler metal may be used to adjust the chemical composition of the weld metal to be formed during laser welding. The filler metal may be in the form of either powder or wire, but from the viewpoint of yield, it is more suitable to supply the filler metal in the form of wire, i.e., as filler wire.

[0110] In the joining steel plate according to this embodiment, as described above, the average Cu content in the weld metal is 0.03% or more and 3.0% or less by mass, and preferably the average Al content in the weld metal is less than 1.00% by mass. The chemical composition of the weld metal varies depending on the steel plates to be welded. When a filler metal is used, the chemical composition of the weld metal also varies depending on the steel plates and filler metal, as well as the welding conditions. For example, the chemical composition of the weld metal varies depending on the thickness of the steel plate substrate used in welding, the composition of the steel plate substrate, the amount of Al-based coating applied, the state of removal of the Al-based coating, the gap between the butted steel plates (root gap), the chemical composition of the filler metal, the feed rate (feed rate) of the filler metal, and other factors. Therefore, by selecting the composition and feed rate of the filler metal for each steel plate used and welding conditions, weld metal with a desired chemical composition can be obtained. When specifying filler metal, it is advisable to estimate the composition and supply amount of the filler metal in advance using the following procedure, and then experimentally confirm the average Cu content and average Al content in the weld metal using the estimated composition and supply amount.

[0111] (i) First, the shape of the weld bead is estimated in advance from the plate thickness, root spacing, and welding conditions (welding heat input) of the Al-based coated steel plate to be welded. The molten width of the Al-based coating is calculated from the estimated width of the weld bead on the front and back surfaces of the steel plate. Then, the amount of Cu that will dissolve from the Al-based coated steel plate into the weld metal that forms the weld bead is estimated based on the molten width and the thickness of the Al-based coating. The amount of deposited metal is then calculated from the estimated weld bead shape, and the composition of the weld metal is estimated from the composition of the steel plate base material to be welded, the composition of the filler metal (filler wire) used, and the amount of Cu dissolved in the weld metal.

[0112] (ii) Next, the estimated composition of the weld metal is examined to determine whether the composition meets the above conditions. If it does not, it is determined whether the above conditions can be met by changing the composition of the filler metal (filler wire). If it does meet the conditions, the filler metal (filler wire) is changed.

[0113] (iii) If it does not comply, change the root spacing to increase the amount of deposited metal, and estimate the composition of the weld metal again using the procedure in (i) above to determine whether the weld metal complies with the above conditions.

[0114] [Other conditions for laser welding] Other conditions for laser welding include laser output, laser beam diameter, welding speed, and shielding gas flow rate. The welding conditions should be selected appropriately according to the judgment of the person in question so as to prevent voids or undercuts in the weld metal. There are no specific regulations regarding the laser output at the processing point (on the steel plate surface) when creating a bead by laser welding, but considering that beam welding is performed with high processing efficiency, such as forming a deep hole called a keyhole at the beam irradiation point, the power density must satisfy a certain relationship with the beam diameter. The power density is the energy density of the laser beam on the steel sheet surface (= laser output / cross-sectional area of ​​the beam πr 2 , r: beam intensity from the beam center is 1 / e 2 It is defined as the distance (radius) to the point where e is the base of the natural logarithm (Napier's constant). When adjusting the laser power and beam diameter, the lower limit of the power density was set to 5.0 × 10 required for keyhole generation. 9 W / m 2 The upper limit is 10, which is the value at which the evaporation of the steel sheet becomes excessively dominant and the base metal disappears. 13 W / m 2 When considering such a range of power density, the range of laser output is set to a lower limit of 2 kW or more and an upper limit of 15 kW or less, preferably a lower limit of 4 kW and an upper limit of 8 kW. In addition to the power density, the beam diameter must also take into consideration stable melting of the steel sheet end face even when the root spacing is large (there is a gap in the butt gap). For this reason, the beam diameter is set to be larger than the root spacing. In tailored blank welding, welding is often performed with a root spacing of 0 mm during butt welding, so the lower limit of the beam diameter is set to 0.1 mm. The upper limit is set to 1.8 mm because the required power density increases as the beam diameter increases, making it necessary to unnecessarily increase the laser output in actual production. The preferred range is 0.3 to 0.9 mm. The lower limit of the welding speed is set to 2 m / min from the viewpoint of production efficiency, and no upper limit is specifically set. However, when welding at high speeds, the laser output must be increased, which inevitably increases the equipment cost and running cost. Therefore, the upper limit is preferably set to 8 m / min or less. Shielding gases are used to cool the plasma at the processing point when the laser is irradiated and to protect the optical system such as lenses. In addition to the inert gases He and Ar, N2 and air can also be used if there is no problem with reaction with the molten metal. The gas flow rate can be selected at the discretion of the person in question, and is often in the range of 20 to 50 L / min, for example. However, since the structure of the laser optical system that forms the gas flow path varies widely, it is not limited to this range. When manufacturing tailored blank materials, the root spacing during butt welding is usually set to 0 mm, but it may be set to approximately 0.1 to 1.0 mm to adjust the composition of the weld metal, and is not limited to this range.

[0115] (F) Manufacturing method of Al-based coated steel sheet An Al-based coated steel plate suitable as a material for the first steel member included in the joining component according to this embodiment (an Al-based coated steel plate that is the material for the first steel plate included in the joining steel plate according to this embodiment) can be manufactured, for example, by using a manufacturing method including the steps shown in (i) to (vi) below. (i) a slab preparation step in which steel having the above-mentioned chemical composition is melted and cast to produce slabs; (ii) a hot rolling step in which the obtained slab is hot rolled to form a hot-rolled steel sheet; (iii) A hot-rolled sheet annealing process in which the coiled hot-rolled steel sheet is annealed as needed. (iv) a cold rolling process in which, if necessary, the hot-rolled steel sheet after coiling or after the hot-rolled steel sheet annealing process is descaled and cold-rolled to obtain a cold-rolled steel sheet. (v) An annealing step in which the hot-rolled steel sheet or the cold-rolled steel sheet is annealed as necessary to obtain an annealed steel sheet. (vi) A coating process in which an Al-based coating is applied to a hot-rolled steel sheet, a cold-rolled steel sheet, or an annealed steel sheet to produce an Al-based coated steel sheet. Each step in the manufacturing method will be described below.

[0116] <Slab preparation process> In the slab preparation step, a steel having the above-mentioned chemical composition is melted and cast to produce a slab to be subjected to hot rolling. For example, a molten steel having the above-mentioned chemical composition (the same chemical composition as the steel plate substrate of the first steel member) is melted using a converter or an electric furnace, and a slab produced by a continuous casting method can be used. Instead of the continuous casting method, an ingot casting method, a thin slab casting method, or the like may be used.

[0117] <Hot rolling> In the hot rolling process, the slab is heated and subjected to rough rolling, followed by descaling if necessary, and finally finish rolling. There are no limitations on the hot rolling conditions. In the coiling process after finish rolling, the hot-rolled steel sheet is coiled at a temperature range of 820°C or less, for example. If the coiling temperature exceeds 820°C, the steel sheet is coiled before transformation has progressed much, and transformation progresses within the coil, which may result in a defective coil shape.

[0118] <Hot-rolled sheet annealing process> In the hot-rolled sheet annealing step, the hot-rolled steel sheet is annealed for 5 hours or more at 450 to 800°C in an atmosphere of 80% by volume or more of nitrogen or in air. Although hot-rolled sheet annealing is not necessarily required, hot-rolled sheet annealing using a continuous furnace, a batch furnace, or the like is preferable because it softens the hot-rolled steel sheet and reduces the load in the subsequent cold-rolling step.

[0119] <Cold rolling process> In the cold rolling step, the hot-rolled steel sheet or the hot-rolled steel sheet after annealing the hot-rolled sheet is descaled and cold-rolled to obtain a cold-rolled steel sheet. Descaling and cold rolling are not necessarily required, but if cold rolling is performed, the cumulative reduction in cold rolling is preferably 30% or more from the viewpoint of ensuring good flatness. On the other hand, in order to avoid excessive rolling load, the cumulative reduction in cold rolling is preferably 80% or less. The descaling method is not particularly limited, but pickling is preferred. When pickling is performed, the conditions may be within known ranges, but pickling with hydrochloric acid or sulfuric acid is preferred to remove only iron scale.

[0120] <Annealing process> In the annealing step before coating formation, the hot-rolled steel sheet or cold-rolled steel sheet is annealed in a temperature range of 700 to 950°C to obtain an annealed steel sheet. Annealing before coating formation is not necessarily required, but is preferable because the annealing step softens the cold-rolled steel sheet and makes it easier to pass the sheet in the subsequent coating step.

[0121] <Coating process> In the coating process, an Al-based coating is formed on the surface of a steel sheet substrate (hot-rolled steel sheet (including hot-rolled steel sheet after annealing), cold-rolled steel sheet, or annealed steel sheet) to produce an Al-based coated steel sheet. The method for the Al-based coating is not particularly limited, and methods that can be used include hot-dip galvanization, electroplating, vacuum deposition, cladding, and thermal spraying. Hot-dip galvanization is the most widely used method industrially.

[0122] When hot-dip plating is performed, the plating bath often contains Fe as an impurity in addition to Al. In addition to the above elements, the plating bath may contain Si, Ni, Mg, Ti, Zn, Sb, Sn, Cu, Co, In, Bi, Ca, misch metal, etc., as long as it contains 70 mass% or more of Al. When hot-dip plating is performed, the annealed steel sheet after the annealing process may be cooled to room temperature and then heated again to perform plating, or may be cooled to 650 to 750°C, which is close to the plating bath temperature, after annealing and then hot-dip plating may be performed without cooling to room temperature.

[0123] There are no particular limitations on the pre-treatment and post-treatment of the Al-based coating, and pre-coating, solvent application, alloying treatment, temper rolling, etc. are possible. As an alloying treatment, for example, annealing at 450 to 800°C is possible. Furthermore, as a post-treatment, temper rolling is useful for shape adjustment, etc., and for example, a reduction of 0.1 to 0.5% is possible. [Example]

[0124] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0125] First, in order to manufacture the coated steel sheet, the bonded steel sheet, and the bonded parts, steels having the chemical compositions shown in Tables 1 and 2 were melted to obtain slabs for hot rolling.

[0126] [Table 1]

[0127] [Table 2]

[0128] Of the obtained slabs, steel Nos. A1 to A16 and a1 to a13 were hot rolled and coiled at a temperature of 800°C or less to form hot-rolled steel sheets with a thickness of 2.7 mm. The hot-rolled hot-rolled steel sheets were then cold-rolled to form cold-rolled steel sheets (steel sheet substrates) with a thickness of 1.4 to 2.3 mm. The obtained cold-rolled steel sheet (steel sheet substrate) was subjected to Al plating to obtain a coated steel sheet (first steel sheet) having an Al-based coating. In the coating process, the steel sheet was immersed in an Al plating bath containing mainly Al, 10 mass% Si, 2 mass% Fe, and the remainder being impurities (5 mass% or less in total) at 680°C, and then cooled to 200°C or less and coiled. The chemical composition of the Al-based coated steel sheet at a position from the surface to 1 / 4 of the thickness in the thickness direction was the same as that of the slab. The coating weight of the Al-based coating was 25 to 150 g / m 2 was within the range. These Al-based coated steel sheets were used as first steel sheets.

[0129] Among the slabs, steel Nos. B1 to B6 were hot-rolled and coiled at a temperature of 800°C or less to form hot-rolled steel sheets with a thickness of 2.7 mm. The hot-rolled hot-rolled steel sheets were then cold-rolled to form cold-rolled steel sheets (steel sheet substrates) with a thickness of 0.8 to 2.0 mm. A portion of the obtained cold-rolled steel sheet (steel sheet substrate) was plated with Al to obtain a coated steel sheet (second steel sheet) having an Al-based coating. In the coating process, the steel sheet was immersed in an Al-plating bath containing mainly Al, 10% Si, 2% Fe, and the remainder being impurities (5 mass% or less in total) at 680°C, then cooled to 200°C or less, and coiled. The chemical composition of the Al-coated steel sheet at a position from the surface to 1 / 4 of the thickness in the thickness direction was the same as that of the slab. The coating weight of the Al-based coating was 25 to 150 g / m 2 was within the range. In addition, some cold-rolled steel sheets were galvanized to produce coated steel sheets (second steel sheets) with a Zn-based coating. In the coating process, the steel sheets were immersed in a Zn-plating bath containing mainly Zn at 670°C with the remainder containing impurities, then cooled to 200°C or below, heated to 600°C, subjected to alloying treatment, and then coiled. The chemical composition of the Zn-coated steel sheets at a position from the surface to 1 / 4 of the thickness in the thickness direction was the same as that of the slab. The coating weight of the Zn-based coating was 25 to 150 g / m 2 was within the range. In addition, some cold-rolled steel sheets were not plated and were used as the second steel sheets as they were (steel sheet substrates).

[0130] Next, one type each of these first steel plates and second steel plates was selected, and their ends were butted together so that their respective surfaces were roughly parallel. As shown in Tables 4-1 to 4-10, the coating was removed from some of the steel plates, and then, using filler metal as necessary and setting a predetermined root gap (0.00 to 0.80 mm), the two were laser-welded to form joined steel plates. To remove the Al-based coating, the Al-based coating was ground away in the thickness direction over a 1.0 mm width from the edge of the steel plate where the joints were to be butt-jointed, and cases were created in which some of the coating remained and in which it was completely removed. When a filler metal was used for welding, a filler wire with a diameter of 0.9 mm and having the chemical composition shown in Table 3 was used.

[0131] Laser welding was performed using a focusing optical system with a focal length of 300 mm and a focused spot diameter of 0.6 mm. A shield nozzle (inner diameter 6 mm) coaxial with the laser beam was used for shielding during welding. The standoff (distance between the nozzle tip and the steel plate surface) was set to 10 mm, and the Ar gas flow rate was 30 L / min. The welding speed and processing point power were constant at 5 m / min and 5.5 kW, respectively. The filler metal feed rate was adjusted based on the plate thickness and root spacing to ensure the width of the weld metal was approximately equal to the plate thickness. The thickness of the weld metal was at least 80% of the thickness of the base steel plate (or the thinner side, if the steel plate thicknesses were different). The front and back surfaces of the weld metal did not protrude more than 200 μm beyond the extension of the front and back surfaces of the steel plate (or the thicker side, if the steel plate thicknesses were different).

[0132] [Table 3]

[0133] The contents of Cu, Al, Mn, Cr, Mo, Ni, Sn, and W in the weld metal of the obtained welded steel plates and welded components were measured by the method described above.

[0134] Next, the obtained joined steel sheets were subjected to heat treatment in which they were heated to a heating temperature at an average heating rate shown in Tables 4-1 to 4-10, held at a temperature within ±10°C of the heating temperature for 90 seconds, and cooled to below the Ms point at an average cooling rate shown in Tables 4-1 to 4-10, thereby obtaining joined parts.

[0135] The contents of Cu, Al, Mn, Cr, Mo, Ni, Sn, and W in the weld metal of the obtained welded components were measured by the method described above. In addition, the hardness of the weld metal, the tensile strength of the first steel member and the second steel member, and the limit cycle in CCT were evaluated by the following methods. In addition, when an Al--Fe coating (a coating containing 70 mass % or more of Al and Fe) was formed, its thickness was measured.

[0136] [Hardness of weld metal] A cross section of the weld metal was cut out in the same manner as above, and Vickers hardness was measured in accordance with JIS Z 2244:2009. The test force was 98 N, and measurements were taken at five equally spaced points within the weld metal from the front surface to the back surface of the weld metal along the weld center line, with the average value being taken as the hardness of the weld metal. In this example, a Vickers hardness exceeding the hardness of the steel plate substrate of the second steel member or 350 Hv, whichever is higher, was evaluated as having high hardness and being preferable.

[0137] [Tensile strength] The tensile test was conducted in accordance with the provisions of ASTM Standard E8. The first and second steel members were cut from the joined component, avoiding the edges and weld metal, so that the longitudinal direction of the test specimen was parallel to the weld line. Both sides were uniformly ground to a thickness of 1.2 mm, and then half-size plate test specimens (parallel portion length: 32 mm, parallel portion width: 6.25 mm) according to ASTM Standard E8 were obtained. When the plate thickness was less than 1.2 mm, the coating or black scale (oxide scale) was removed, and then half-size plate test specimens according to ASTM Standard E8 were obtained. A strain gauge with a gauge length of 5 mm was attached to the center of the parallel portion, and a room-temperature tensile test was conducted at a strain rate of 3 mm / min to measure the tensile strength (maximum strength). In this example, if the tensile strength of at least the first steel member exceeded 1500 MPa, the joined component was determined to have sufficient tensile strength.

[0138] [Limit cycle in CCT] Hydrogen embrittlement resistance was evaluated using accelerated corrosion testing (CCT) using a combined cyclic test (CCT). Specifically, strip-shaped test specimens measuring 8 mm wide and 68 mm long were cut from the joint components, perpendicular to the weld line and centered along the longitudinal axis of the test specimen. Strain gauges (5 mm long) similar to those used in the tensile test were attached to the surface of the test specimen at the center of the width and length of the specimen. The specimens were then bent using a four-point support jig to a strain equivalent to half the tensile strength of the first steel member. The four-point bent test specimens were subjected to CCT in accordance with JASO standards M609 and M610, and the number of cycles required for the weld to fracture was used to evaluate the resistance. The CCT was performed for up to 360 cycles, and specimens that did not fracture after 150 cycles were deemed to have excellent hydrogen embrittlement resistance. Furthermore, for test pieces that did not break after 360 cycles, the amount of corrosion loss in the welded area was measured in the thickness direction to evaluate corrosion resistance. Specifically, after removing rust from the test pieces in a solution of 10% diammonium hydrogen citrate with an inhibitor added, measurements were taken at 10 points in the thickness direction at the most corroded location in the welded area using a point micrometer with a tip SR (radius) of 0.3 mm. The average of the 10 points divided by the plate thickness before corrosion x 100 was used to calculate the amount of corrosion loss (%). Corrosion resistance was evaluated on a three-level scale: A, B, or C, depending on the degree of corrosion loss. Specifically, cases where the amount of corrosion thinning was less than 30% were rated A, 30% or more but less than 50% were rated B, and 50% or more were rated C.

[0139] [Table 4-1]

[0140] [Table 4-2]

[0141] [Table 4-3]

[0142] [Table 4-4]

[0143] [Table 4-5]

[0144] [Table 4-6]

[0145] [Table 4-7]

[0146] [Table 4-8]

[0147] [Table 4-9]

[0148] [Table 4-10]

[0149] As shown in Tables 4-1 to 4-10, in Examples D1 to D128, which satisfy the range of the present invention, at least a portion of the joined parts had a tensile strength of more than 1.5 GPa and also had excellent hydrogen embrittlement resistance. Furthermore, examples in which the Cu / Al ratio was in the preferred range also had excellent corrosion resistance. In contrast, Comparative Examples d1 to d30, which do not satisfy the range of the present invention, were inferior in at least one of hydrogen embrittlement resistance and tensile strength. [Industrial Applicability]

[0150] According to the present invention, it is possible to obtain a high-strength joined component having a welded portion with excellent hydrogen embrittlement resistance. The joined component according to the present invention is particularly suitable for use as an automobile frame component. The steel member according to the present invention has high strength and excellent hydrogen embrittlement resistance, and therefore, when applied to an automobile component, contributes to improving fuel efficiency and collision safety. [Explanation of symbols]

[0151] 1. Joint parts 10 First steel member 11 Steel plate base material 12 Al-Fe coating 20 Second steel member 21 Steel plate base material 22 Covering 30 Joint 31 Weld Metal 32 Heat-affected zone S1 jointed steel plate S10 First Steel Plate S11 Steel plate base material S12 Al-based coating S20 Second Steel Plate S21 Steel plate base material S22 Cladding S30 joint S31 weld metal S32 Heat affected zone

Claims

1. a first steel member; a second steel member; a welded portion formed at a butt joint between the first steel member and the second steel member, the welded portion including a weld metal and a heat-affected zone; Including, The first steel member comprises: A steel plate substrate; an Al-Fe-based coating formed on the surface of the steel plate substrate, The tensile strength is greater than 1500 MPa, When a cross section of the weld metal in a plate thickness direction perpendicular to an extending direction of the joint is used as a measurement surface, an average Cu content in the weld metal at the measurement surface is, in mass%, 0.03% or more and 3.00% or less, a ratio of the average Cu content to the average Al content in the weld metal on the measurement surface, Cu / Al, of 0.15 to 3.90; A joining component characterized by:

2. The Vickers hardness of the weld metal on the measurement surface exceeds the hardness of the steel plate substrate of the second steel member or 350 Hv, whichever is higher. The joining component according to claim 1 .

3. The average Al content in the weld metal at the measurement surface is less than 1.00% by mass. The joining component according to claim 1 .

4. The average Al content in the weld metal at the measurement surface is less than 1.00% by mass. The joining component according to claim 2 .

5. The steel plate substrate in the first steel member is, in mass%, C: 0.25-0.65%, Si: 2.00% or less, Mn: 0.15-3.00%, P: 0.050% or less, S: 0.0100% or less, N: 0.010% or less, O: 0.010% or less, Al: 1.00% or less, B: 0.0005-0.0100%, Cu: 0-3.00%, Ti: 0 to 0.100%, Nb: 0 to 0.10%, Mo: 0-1.00%, Cr: 0-1.00%, Ni: 0 to 1.00%, V: 0-1.00%, Ca: 0-0.010%, Mg: 0 to 0.010%, Sn: 0-1.00%, W: 0-1.00%, Sb: 0 to 1.00%, Zr: 0 to 1.00%, REM: 0 to 0.30%, and The balance is Fe and impurities. having a chemical composition consisting of The joining component according to any one of claims 1 to 4.

6. In the chemical composition of the steel plate substrate of the first steel member, The Cu content is 0.05 to 3.00%. The joining component according to claim 5 .

7. The total content of one or more of Mn, Cr, Mo, Ni, Sn, and W in the weld metal at the measurement surface is 1.2% or more. The joining component according to any one of claims 1 to 4.

8. The total content of one or more of Mn, Cr, Mo, Ni, Sn, and W in the weld metal at the measurement surface is 1.2% or more. The joining component according to claim 5 .

9. The total content of one or more of Mn, Cr, Mo, Ni, Sn, and W in the weld metal at the measurement surface is 1.2% or more. The joining component according to claim 6 .

10. The tensile strength of the second steel member is 500 MPa or more and 1500 MPa or less. The joining component according to any one of claims 1 to 4.

11. The tensile strength of the second steel member is 500 MPa or more and 1500 MPa or less. The joining component according to claim 5 .

12. The tensile strength of the second steel member is 500 MPa or more and 1500 MPa or less. The joining component according to claim 6 .

13. The tensile strength of the second steel member is 500 MPa or more and 1500 MPa or less. The joining component according to claim 7 .

14. The tensile strength of the second steel member is 500 MPa or more and 1500 MPa or less. The joining component according to claim 8 .

15. The tensile strength of the second steel member is 500 MPa or more and 1500 MPa or less. The joining component according to claim 9 .

16. a first steel plate; a second steel plate; a welded portion formed at the butt joint between the first steel plate and the second steel plate, the welded portion including a weld metal and a heat-affected zone; Including, The first steel plate is A steel plate substrate; an Al-based coating formed on the surface of the steel plate substrate, When a cross section of the weld metal in a plate thickness direction perpendicular to an extending direction of the joint is used as a measurement surface, an average Cu content in the weld metal at the measurement surface is, in mass%, 0.03% or more and 3.00% or less, a ratio of the average Cu content to the average Al content in the weld metal on the measurement surface, Cu / Al, of 0.15 to 3.90; A bonded steel plate characterized by:

17. The average Al content in the weld metal at the measurement surface is less than 1.00% by mass. The bonded steel plate according to claim 16 .

18. The steel plate base material in the first steel plate is, in mass%, C: 0.25-0.65%, Si: 2.00% or less, Mn: 0.15-3.00%, P: 0.050% or less, S: 0.0100% or less, N: 0.010% or less, O: 0.010% or less, Al: 1.00% or less, B: 0.0005-0.0100%, Cu: 0-3.00%, Ti: 0 to 0.100%, Nb: 0 to 0.10%, Mo: 0-1.00%, Cr: 0-1.00%, Ni: 0 to 1.00%, V: 0-1.00%, Ca: 0-0.010%, Mg: 0 to 0.010%, Sn: 0-1.00%, W: 0-1.00%, Sb: 0 to 1.00%, Zr: 0 to 1.00%, REM: 0 to 0.30%, and The balance is Fe and impurities. having a chemical composition consisting of The bonded steel plate according to claim 16 or 17.

19. In the chemical composition of the steel plate substrate of the first steel plate, The Cu content is 0.05 to 3.00%. The bonded steel plate according to claim 18.

20. The total content of one or more of Mn, Cr, Mo, Ni, Sn, and W in the weld metal on the measurement surface is 1.2% or more. The bonded steel plate according to claim 16 or 17.

21. The total content of one or more of Mn, Cr, Mo, Ni, Sn, and W in the weld metal on the measurement surface is 1.2% or more. The bonded steel plate according to claim 18.

22. The total content of one or more of Mn, Cr, Mo, Ni, Sn, and W in the weld metal on the measurement surface is 1.2% or more. The bonded steel plate according to claim 19.

Images