Welded joint and automobile component

Abstract

A welded joint 100 includes a first steel sheet member 11 that is a hot stamp–molded body of a galvanized steel sheet, a second steel sheet member 12, and resistance spot weld parts 13 that join the first steel sheet member 11 and the second steel sheet member 12. When a first position 11a is a position on the first steel sheet member 11 that is separated by at least 12 mm but less than 30 mm from a resistance spot weld part 13a and a second position 11b is a position that is separated by at least 30 mm from every resistance spot weld part 13 and is not in contact with the other member, the absolute value of the difference between the Vickers hardness at the first position 11a and the Vickers hardness at the second position 11b is no more than 10.0% of the Vickers hardness at the second position 11b, the thickness of a zinc oxide layer at the first position 11a is no more than 85% of the thickness of the zinc oxide layer at the second position 11b, and the thickness of a solid solution layer as measured at the first position 11a is no more than 90% of the thickness of the solid solution layer as measured at the second position 11b.

Description

Welded joints and automotive parts 【0001】 This invention relates to welded joints and automotive parts. 【0002】 To achieve both weight reduction and collision safety in automobiles, there is a demand for higher strength materials used in vehicles. For this reason, in recent years, hot-stamped molded bodies with strengths up to 2.0 GPa have been put into practical use. Furthermore, welded joints, which consist of multiple steel plate members including hot-stamped molded bodies welded together by resistance spot welding, are being used as automotive parts. 【0003】 When performing resistance spot welding, it is necessary to ensure a sufficient nugget diameter and a suitable current range that does not generate spatter. In other words, the hot-stamped molded body that constitutes part of the welded joint is required to have excellent resistance spot weldability (see, for example, Patent Document 1). 【0004】 Japanese Patent Publication No. 2022-071515 【0005】 Incidentally, the inventors have so far investigated ways to improve the properties of hot-stamped molded articles, such as increasing the heating temperature before hot stamping, increasing the heating time, and performing hot stamping multiple times. 【0006】 In some cases, zinc-plated steel sheets are used as the material for hot-stamped molded bodies. That is, the resulting molded bodies have a zinc-plated layer. As a result of our research, we have found that when hot stamping is performed on zinc-plated steel sheets by increasing the heating temperature and / or the heating time, the appropriate current range for the resulting hot-stamped molded bodies becomes significantly narrower. In other words, the resistance spot weldability of the hot-stamped molded bodies decreases, making it difficult to stably obtain welded joints of the required quality (nugget diameter, joint strength). 【0007】 The present invention aims to solve the above problems and provide a welded joint including a hot-stamped molded body that exhibits excellent resistance spot weldability even when the heating temperature before hot stamping is increased and / or the heating time is increased. 【0008】 This invention was made to solve the above problems and is characterized by the following welded joints and automotive parts. 【0009】 (1) A welded joint comprising a first steel sheet member which is a hot-stamped molded body of a zinc-plated steel sheet, a second steel sheet member, and a plurality of resistance spot welds that join the first steel sheet member and the second steel sheet member, wherein, in the first steel sheet member, a predetermined position 12 mm or more but less than 30 mm away from the center of one of the plurality of resistance spot welds is designated as the first position, and a predetermined position 30 mm or more away from the center of any of the plurality of resistance spot welds and not in contact with any other member is designated as the second position, the absolute value of the difference between the Vickers hardness measured at the first position and the Vickers hardness measured at the second position is 10.0% or less of the Vickers hardness measured at the second position, the thickness of the zinc oxide layer measured at the first position is 85% or less of the thickness of the zinc oxide layer measured at the second position, and the thickness of the solid solution layer measured at the first position is 90% or less of the thickness of the solid solution layer measured at the second position. 【0010】 (2) The welded joint according to (1) above, wherein the thickness of the solid solution layer measured at the first position is 80% or less of the thickness of the solid solution layer measured at the second position. 【0011】 (3) Automotive parts including the welded joints described in (1) or (2) above. 【0012】 According to the present invention, even when hot stamping is performed after increasing the heating temperature and / or the heating time, the resulting hot-stamped molded body exhibits excellent resistance spot weldability. Therefore, welded joints can be manufactured using the hot-stamped molded body as a material in a stable manner without generating joint defects or dust. 【0013】 Figure 1 shows a welded joint according to one embodiment of the present invention. 【0014】The present inventors investigated a method to suppress the decrease in resistance spot weldability of hot-stamped molded products obtained when hot stamping is performed on zinc-plated steel sheets by increasing the heating temperature and / or heating time, and as a result obtained the following findings. 【0015】 (a) The resistance spot weldability of hot-stamped molded articles having a zinc-based plating layer decreases as the surface resistance increases. In other words, if the increase in surface resistance can be suppressed, it is possible to improve the resistance spot weldability of hot-stamped molded articles. 【0016】 (b) The thickness of the zinc oxide layer has the greatest influence on the increase in surface resistance. However, as mentioned above, when hot stamping is performed by increasing the heating temperature and / or the heating time, the zinc oxide layer becomes thicker. 【0017】 (c) As a method to reduce the thickness of the zinc oxide layer, a method of removing the zinc oxide layer by shot blasting or the like can be considered. However, as a result of the inventors' investigation, simply removing the zinc oxide layer was not sufficient to improve the resistance spot weldability of the hot-stamped molded product. 【0018】 (d) Further research by the inventors revealed that, in addition to the thickness of the zinc oxide layer, the thickness of the solid solution layer also affects resistance spot weldability, and that the thicker the solid solution layer, the more likely dust is to be generated. 【0019】 (e) It is difficult to remove the solid solution layer by shot blasting or the like. Therefore, it is necessary to suppress the formation of the solid solution layer in the heating process before hot stamping. 【0020】 (f) In the heating process before hot stamping, by stacking dummy material for covering at least the areas where welding is planned, the supply of oxygen is restricted, and the thickness of the zinc oxide layer and the solid solution layer can be partially reduced. As a result, resistance spot weldability can be ensured in the areas where welding is planned. 【0021】(g) On the other hand, in areas where welding is not planned, heating is performed without overlapping dummy material in order to suppress an excessive decrease in manufacturability and economic efficiency. As a result, in the welded joint obtained in the manner described above, there will be a difference in the thickness of the zinc oxide layer and the solid solution layer between the area where resistance spot welding was performed and at least a portion of the other areas. 【0022】 This invention is based on the above findings. Hereinafter, a welded joint according to an embodiment of the present invention will be described with reference to the drawings. 【0023】 1. Schematic diagram 1 of the welded joint shows a welded joint according to one embodiment of the present invention. Specifically, Figure 1(a) is a schematic perspective view showing a part of the welded joint, and Figure 1(b) is a schematic cross-sectional view showing the A-A portion of Figure 1(a). 【0024】 As shown in Figure 1, the welded joint 100 according to this embodiment has a first steel plate member 11, a second steel plate member 12, and a plurality of resistance spot welds 13 that join the first steel plate member 11 and the second steel plate member 12. The first steel plate member 11 is a hot-stamped molded body of zinc-plated steel sheet. 【0025】 The types of zinc-plated steel sheets are not particularly limited, but examples include hot-dip galvanizing, alloyed hot-dip galvanizing, electroplated zinc, Zn-Ni plating (electroalloyed zinc plating), alloyed electroplated zinc, hot-dip zinc-aluminum alloy plating, hot-dip zinc-aluminum-magnesium alloy plating, hot-dip zinc-aluminum-magnesium-Si alloy plating, and zinc vapor deposition Al plating. 【0026】Furthermore, the first steel plate member 11 has a first position 11a located 12 mm or more but less than 30 mm from the center of one of the multiple resistance spot welds 13a, and a second position 11b located 30 mm or more from the center of any of the multiple resistance spot welds 13 and not in contact with any other member. In this embodiment, it is sufficient that the first steel plate member 11 has a first position 11a and a second position 11b that satisfy the conditions described later. Also, the "other members" refer to members other than the first steel plate member 11, and include the second steel plate member 12. Note that the center of the resistance spot weld refers to the center of the approximate circle of the indentation of the resistance spot weld observed when viewed from the thickness direction. 【0027】 In the configuration shown in Figure 1, the first steel plate member 11 is a hat-shaped member, and the second steel plate member 12 is a flat plate closure. However, it is not limited to this, and the second steel plate member 12 may also be a hot-stamped molded body such as a hat-shaped member. The first steel plate member 11 has a pair of flange portions 11c (only one flange portion 11c is shown in Figure 1(a)), a pair of wall portions 11d rising from the pair of flange portions 11c (only one wall portion 11d is shown in Figure 1(a)), and a top plate portion 11e connecting the pair of wall portions 11d. 【0028】 The flange portion 11c is provided to extend in the first direction D1. The wall portion 11d is provided to rise from the edge of the flange portion 11c in the second direction D2, which is perpendicular to the first direction D1, when viewed from the thickness direction of the flange portion 11c. The first direction D1 is the direction along the boundary (ridge) between the flange portion 11c and the wall portion 11d, when viewed from the thickness direction of the flange portion 11c. 【0029】 A pair of flange portions 11c of the first steel plate member 11 are joined to the second steel plate member 12 by a plurality of resistance spot welds 13. In Figure 1, a plurality of resistance spot welds 13a to 13d that join one flange portion 11c to the second steel plate member 12 are shown. In this embodiment, the resistance spot welds 13a to 13d are arranged with spacing between them so as to be aligned in the first direction D1. 【0030】As shown in Figure 1(b), the resistance spot welds 13a to 13d each include the weld metal 14 and a first heat-affected zone 15a formed on the first steel plate member 11 and a second heat-affected zone 15b formed on the second steel plate member 12, respectively. The first heat-affected zone 15a and the second heat-affected zone 15b are heat-affected zones formed when the first steel plate member 11 and the second steel plate member 12 are resistance spot welded. 【0031】 In the configuration shown in Figure 1, the first position 11a is located in the flange portion 11c, and the second position 11b is located in the top plate portion 11e. In this embodiment, the second position 11b is located in the top plate portion 11e, but it may also be located in the wall portion 11d, or in both. 【0032】 In the welded joint 100 according to this embodiment, the absolute difference between the Vickers hardness measured at the first position 11a and the Vickers hardness measured at the second position 11b is 10.0% or less of the Vickers hardness measured at the second position 11b. Preferably, the absolute difference between the Vickers hardness measured at the first position 11a and the Vickers hardness measured at the second position 11b is 5.0% or less, and more preferably 3.0% or less. A large difference in hardness between the first position 11a and the second position 11b means that some part of the welded joint is softened overall. In that case, when the welded joint is used as a structural member, there is a risk that the deformation resistance during impact will be insufficient. 【0033】 There are no particular restrictions on the Vickers hardness value measured at the second position 11b. However, when the welded joint 100 according to this embodiment is used as a part of an automobile, from the viewpoint of achieving both weight reduction and improved collision safety of the automobile, the Vickers hardness measured at the second position 11b is preferably 400 to 750 HV1, and more preferably 430 to 630 HV1. 【0034】Similarly, there are no particular restrictions on the hardness of the second steel plate member 12, but the Vickers hardness measured at a third position 12a in the second steel plate member 12, which is 30 mm or more away from the center of any of the multiple resistance spot welds 13 and not in contact with other members, is preferably 90 to 620 HV1, and more preferably 200 to 400 HV1. 【0035】 The Vickers hardness should be measured with a test force of 9.807 N (1 kgf), and other conditions should be in accordance with JIS Z 2244-1:2024. The measurement should be taken at five locations at 0.5 mm intervals, centered on the first position 11a, the second position 11b, and the third position 12a, at 1 / 4 of the plate thickness in a cross section perpendicular to the surface, and the average of the five measured values ​​should be taken as the Vickers hardness. "HV1" refers to the "hardness symbol" when the Vickers hardness test is performed with a test force of 9.807 N (1 kgf) (see JIS Z 2244-1:2024). "Plate thickness" here refers to the thickness of the base material. 【0036】 Furthermore, in the welded joint 100 according to this embodiment, the thickness of the zinc oxide layer measured at the first position 11a is 85% or less of the thickness of the zinc oxide layer measured at the second position 11b. As described above, when manufacturing the hot-stamped molded body that will be the material for the welded joint 100, the resistance spot weldability of the hot-stamped molded body is improved by controlling the zinc oxide layer to be relatively thinner in the area where welding is planned. As a result, in the welded joint 100, there is a difference in the thickness of the zinc oxide layer between the first position 11a, which is close to the location where resistance spot welding was performed, and the second position 11b, which is far from the location where resistance spot welding was performed. 【0037】The thickness of the zinc oxide layer measured at the first position 11a is preferably 70% or less of the thickness of the zinc oxide layer measured at the second position 11b, more preferably 50% or less, and even more preferably 30% or less. Since a thinner zinc oxide layer is preferable, there is no need to set a lower limit on its thickness, and the thickness of the zinc oxide layer measured at the first position 11a may be 0% of the thickness of the zinc oxide layer measured at the second position 11b. However, considering manufacturing efficiency, it may be greater than 0%, 5% or more, or 10% or more. 【0038】 Furthermore, the thickness of the zinc oxide layer measured at the first position 11a is preferably 2.0 μm or less, more preferably 1.5 μm or less, even more preferably 1.0 μm or less, and even more preferably 0.5 μm or less. For the same reasons as above, the thickness of the zinc oxide layer measured at the first position 11a may be 0 μm, greater than 0 μm, 0.1 μm or more, or 0.2 μm or more. 【0039】 In addition, in the welded joint 100 according to this embodiment, the thickness of the solid solution layer measured at the first position 11a is 90% or less of the thickness of the solid solution layer measured at the second position 11b. As described above, in addition to the thickness of the zinc oxide layer, the thickness of the solid solution layer also affects the resistance spot weldability. By controlling the zinc oxide layer to be relatively thin in the area where welding is planned, it is possible to simultaneously control the thickness of the solid solution layer to be relatively thin. Here, in the present invention, the solid solution layer refers to a layer formed by the reaction of zinc in the plating layer and iron in the base material, and mainly consists of an Fe-rich phase in which Zn is solid-dissolved in α-Fe, and may also contain Zn-rich metallic zinc and / or intermetallic compounds. That is, the solid solution layer in the present invention refers to the layer between the zinc oxide layer formed on the outermost part of the zinc-based plating layer and the base material. 【0040】The thickness of the solid solution layer measured at the first position 11a is preferably 85% or less, more preferably 80% or less, still more preferably 78% or less, and still more preferably 75% or less of the thickness of the solid solution layer measured at the second position 11b. There is no need to set a lower limit for the thickness of the solid solution layer, but having a solid solution layer can suppress the occurrence of liquid metal embrittlement (LME) during hot stamping. Therefore, the thickness of the solid solution layer measured at the first position 11a is preferably 50% or more, more preferably 60% or more, and still more preferably 70% or more of the thickness of the solid solution layer measured at the second position 11b. 【0041】 Also, the thickness of the solid solution layer measured at the first position 11a is preferably 25.0 μm or less, more preferably 23.0 μm or less, still more preferably 20.0 μm or less, and still more preferably 18.0 μm or less. For the same reason as above, the thickness of the solid solution layer measured at the first position 11a is preferably 5.0 μm or more, more preferably 10.0 μm or more, and still more preferably 15.0 μm or more. 【0042】 In the present invention, the thicknesses of the zinc oxide layer and the solid solution layer are measured by a high-frequency glow discharge optical emission spectrometry (GDS). The specific measurement method will be described below. 【0043】 At each of the first position 11a and the second position 11b, while sputtering in the depth direction from the surface, the concentration of each element of Fe, Mn, Zn, Si, Al, O, Cr, Ni, Mg, Cu, and Sn at each depth is measured. 【0044】In this zinc-based plating layer, a zinc oxide layer is formed on the outermost surface, and a solid solution layer is formed inside it. In addition, in the welded joint 100, a coating may be applied to the outside of the zinc-based plating layer. Therefore, the concentrations of each element described above are measured in the depth direction from the surface, and the depth at which the total content of Fe, Zn, and O reaches 80% by mass or more is defined as the boundary between the coating and the zinc-based plating layer. In the zinc-based plating layer, the layer with an O content of 5% by mass or more is defined as the zinc oxide layer, and the layer with an O content of less than 5% by mass and a Zn content of 5% by mass or more is defined as the solid solution layer. That is, the depth at which the Zn content becomes less than 5% by mass is defined as the boundary between the zinc-based plating layer and the base material. 【0045】 For GDS measurement, for example, a Marcus-type high-frequency glow discharge emission spectrometer GD-Profiler2 (manufactured by HORIBA) is used. In this case, for example, the discharge conditions are 35W, the Ar pressure during measurement is 600Pa, the discharge area is 4mm in diameter, the electrode distance is 0.15 to 0.25mm, and the measurement pitch in the plate thickness direction is 0.01 to 0.05μm. 【0046】 There are no particular restrictions on the thickness of the first steel plate member 11 and the second steel plate member 12. However, when the welded joint according to this embodiment is used as an automobile part, the thickness of the first steel plate member 11 at the second position 11b is preferably 0.6 to 5.0 mm, and more preferably 1.4 to 2.3 mm. Similarly, the thickness of the second steel plate member 12 at the third position 12a is preferably 0.6 to 5.0 mm, and more preferably 0.6 to 2.3 mm. 【0047】 2. Chemical Composition The chemical composition of the welded joint according to this embodiment is not particularly limited. In this embodiment, the following steel plate A can be used as the material for the first steel plate member 11, and the following steel plate B can be used as the material for the second steel plate member 12. 【0048】(Steel plate A) Chemical composition in mass percent is: C: 0.10-0.70%, Si: 0.01-2.00%, Mn: 0.10-3.00%, P: 0.050% or less, S: 0.0200% or less, N: 0.0200% or less, O: 0.100% or less, Al: 0.010-0.100%, Cr: 0.01-1.00%, Nb: 0-0.200%, Ti: 0-0.200%, Mo: 0-1.00%. A steel sheet containing B: 0-0.0100%, Co: 0-4.00%, Ni: 0-2.00%, Cu: 0-1.00%, V: 0-1.00%, W: 0-1.00%, Ca: 0-0.100%, Mg: 0-0.100%, REM: 0-0.100%, Sb: 0-0.100%, Zr: 0-0.100%, Sn: 0-1.00%, As: 0-0.100%, with the remainder being Fe and impurities. 【0049】 (Steel plate B) Chemical composition in mass% is: C: 0.05-0.35%, Si: 0.01-2.00%, Mn: 0.10-3.00%, P: 0.050% or less, S: 0.0200% or less, N: 0.0200% or less, O: 0.100% or less, Al: 0.010-0.100%, Cr: 0.01-1.00%, Nb: 0-0.200%, Ti: 0-0.200%, Mo: 0-1.00%. A steel sheet containing B: 0-0.0100%, Co: 0-4.00%, Ni: 0-2.00%, Cu: 0-1.00%, V: 0-1.00%, W: 0-1.00%, Ca: 0-0.100%, Mg: 0-0.100%, REM: 0-0.100%, Sb: 0-0.100%, Zr: 0-0.100%, Sn: 0-1.00%, As: 0-0.100%, with the remainder being Fe and impurities. 【0050】 Furthermore, impurities refer to components that are mixed in during the industrial manufacturing of steel plates due to raw materials such as ore and scrap, or other factors, and are not components that were intentionally added to the steel plates according to this embodiment. 【0051】The chemical composition of the first and second steel plate members described above can be measured using general analytical methods. For example, it can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S can be measured using the combustion-infrared absorption method, N using the inert gas fusion-thermal conductivity method, and O using the inert gas fusion-non-dispersive infrared absorption method. Furthermore, the zinc-based plating layer on the surface should be removed by mechanical grinding before analysis. 【0052】 3. Applications The applications of the welded joint according to this embodiment are not particularly limited, but it can be suitably used as an automotive part. Examples of automotive parts include center pillars, front side members, rear side members, side sills, bumpers, etc. 【0053】 4. Manufacturing Method An example of a manufacturing method for the welded joint according to this embodiment will be described using the configuration shown in Figure 1 as an example. 【0054】 First, a steel plate (blank) to be used as the material for the first steel plate member 11, and a steel plate to be used as the second steel plate member 12 or a steel plate (blank) to be used as the material for the second steel plate member 12 are manufactured. Molten steel having the above-mentioned chemical composition is manufactured, and a slab is manufactured using this molten steel. As the slab to be subjected to hot rolling, a continuously cast slab or one manufactured by a thin slab caster can be used. The above method for manufacturing steel plates is suitable for processes such as continuous casting-direct rolling (CC-DR), in which hot rolling is performed immediately after casting. 【0055】 It is preferable to heat the slab at 1100°C or higher. Heating the slab at temperatures below 1100°C leads to a decrease in the finish rolling temperature, which tends to result in higher strength during finish rolling. As a result, rolling may become difficult or lead to defects in the shape of the steel sheet after rolling, so it is preferable to heat the slab at 1100°C or higher. There is no need to set an upper limit on the slab heating temperature, but from the viewpoint of suppressing excessive scale formation, it is preferable to keep it at 1300°C or lower. 【0056】The finish rolling completion temperature is preferably 800°C or higher. If the finish rolling completion temperature falls below 800°C, the rolling load will increase, which may make rolling difficult or lead to defects in the shape of the steel sheet after rolling. Therefore, the lower limit of the finish rolling completion temperature is preferably 800°C. There is no particular need to set an upper limit for the finish rolling completion temperature, but if the finish rolling completion temperature is set too high, the slab heating temperature must be set too high in order to maintain that temperature. Therefore, the upper limit of the finish rolling completion temperature is preferably 1100°C. After the finish rolling is completed, the average cooling rate up to the winding temperature, which will be described later, is preferably 10 to 100°C / s. 【0057】 The winding temperature is preferably 700°C or lower. If the winding temperature exceeds 700°C, the thickness of the oxide formed on the surface of the steel sheet may increase excessively, potentially reducing its pickling properties. If cold rolling is to be performed afterward, the lower limit of the winding temperature is preferably 400°C. If the winding temperature is below 400°C, the strength of the hot-rolled steel sheet increases drastically, making it prone to sheet fracture and shape defects during cold rolling; therefore, the lower limit of the winding temperature is preferably 400°C. However, if the wound hot-rolled steel sheet is to be softened by heating it in a box-type annealing furnace or continuous annealing equipment, it is acceptable to wind it at a low temperature of less than 400°C. In addition, rough-rolled sheets may be joined together during hot rolling and continuous finish rolling may be performed. Furthermore, the rough-rolled sheets may be wound up once. 【0058】 The above hot-rolled steel sheet may be subjected to pickling, and the hot-rolled steel sheet after pickling may be cold-rolled to produce a cold-rolled steel sheet. When cold-rolling is performed, the reduction ratio can be, for example, 30 to 80%. The pickling treatment can be carried out by immersing the sheet in an aqueous solution containing an inhibitor and with an acid concentration of 3 to 20% by mass at a temperature of 80°C or higher but less than 100°C for 30 seconds or more. Furthermore, the above hot-rolled or cold-rolled steel sheet may be annealed to produce a hot-rolled annealed sheet or a cold-rolled annealed sheet. When annealing is performed, the annealing temperature can be, for example, in the range of 700 to 950°C. 【0059】A steel sheet of a predetermined size can be cut from the various steel sheets mentioned above to form the second steel sheet member 12. On the other hand, the steel sheet that will be used as the material for the first steel sheet member 11 is further plated to form a zinc-plated steel sheet. From the viewpoint of manufacturing cost, the plating process is preferably carried out on a continuous hot-dip galvanizing line. For example, after annealing, the steel sheet is cooled to a temperature range of 500°C or less at an average cooling rate of 1 to 60°C / s, and then, by conventional methods, it is immersed in a plating bath containing molten zinc or a zinc alloy (for example, a zinc alloy containing 5% or less Al) and pulled out to obtain a hot-dip galvanized steel sheet. The temperature of the plating bath at this time is preferably 400 to 600°C, and more preferably 440 to 520°C. The amount of plating deposited is controlled by adjusting the pulling speed or the flow rate of the wiping gas blown from the nozzle. 【0060】 The hot-dip galvanized steel sheet may be further heated in a gas furnace, induction heating furnace, etc., to produce an alloyed hot-dip galvanized steel sheet. In this case, the temperature should be 500°C or higher. ３ It is preferable to heat to a temperature range below 5°C, and more preferably to a temperature range of 550 to 650°C. In this specification, hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets are collectively referred to as zinc-plated steel sheets. 【0061】 Next, hot stamping is performed on the zinc-plated steel sheet. In this process, the zinc-plated steel sheet is heated, followed by press forming and quenching. 【0062】 Here, in the area that will become the flange portion 11c where resistance spot welding is planned after hot stamping, a dummy material for covering is superimposed to partially reduce the thickness of the zinc oxide layer and restrict the supply of oxygen. There are no particular restrictions on the hot stamping conditions, but as described above, the welded joint 100 according to this embodiment can be manufactured even when hot stamping is performed under conditions that normally reduce the resistance spot weldability of the hot stamped body, such as when the heating temperature before hot stamping is increased, when the heating time is extended, or when hot stamping is performed multiple times. 【0063】Furthermore, during heating before hot stamping, the blank is set to Ac ３ Ac above points ３ After heating to a temperature of +65°C or lower, it is common to hold the temperature in that range for 10 to 240 seconds. On the other hand, when increasing the heating temperature before hot stamping, for example, the heating temperature of the blank is set to Ac ３ The temperature can exceed 65°C above the point. While there is no need to set an upper limit on the heating temperature, it is preferable to keep it below 1100°C to avoid grain coarsening and damage to the plating layer. Furthermore, if the heating time is extended, for example, it can exceed 240 seconds. There is no particular upper limit on the heating time either, but it is preferable to keep it below 900 seconds per hot stamp. 【0064】 After removing the blank from the furnace, molding can be started at a temperature of 700°C or higher, and then quenching can be performed by cooling to a temperature below the Mf point without demolding. In this case, it is preferable that the average cooling rate from the molding start temperature to the Ms point be 50°C / s or higher, and the average cooling rate from the Ms point to the Mf point be 10°C / s or higher. Considering the performance of the manufacturing equipment, it is preferable that the average cooling rate from the molding start temperature to the Ms point and from the Ms point to the Mf point be 500°C / s or lower. After demolding in the temperature range below the Mf point, it is sufficient to allow it to cool to, for example, 25°C. 【0065】 Furthermore, if hot stamping is to be performed multiple times, it is sufficient to perform hot stamping under the various conditions described above two or more times. Alternatively, one or more non-forming quenching treatments may be performed before and / or after hot stamping. In this case, the quenching conditions are the same as the hot stamping conditions described above, with the forming start temperature being replaced with the cooling start temperature. When quenching is performed before hot stamping, for example, cooling can be done using a flat die. On the other hand, when quenching is performed after hot stamping, for example, cooling can be done using the die used for hot stamping. Tempering treatment may be performed after hot stamping, or even after quenching. In this case, the temperature should be 300°C or higher. １ It is sufficient to hold the device in a temperature range below 1.5°C for 600 to 7200 seconds. 【0066】 Here, in the present invention, Ac １ point, Ac ３ point, Ms point, and Mf point are each determined based on the following formulas (I) to (IV). Ac １ (°C) = 723 - 10×Mn - 16×Ni + 29×Si + 6×Cr + 290×N - 25×Mo... (I) Ac ３ (°C) = 850 + 10×(C + N)×Mn + 350×Nb + 250×Ti + 40×B + 10×Cr + 100×Mo... (II) Ms (°C) = 550 - 361×C - 39×Mn - 35×V - 20×Cr - 17×Ni - 10×Cu - 5×(Mo + W) + 15×Co + 30×Al... (III) Mf (°C) = 410.5 - 407.3×C - 7.3×Si - 37.8×Mn - 20.5×Cu - 19.5×Ni - 19.8×Cr - 4.5×Mo... (IV) However, the element symbols in the above formulas represent the content (% by mass) of each element. 【0067】 Hereinafter, the present invention will be described more specifically by way of examples, but the present invention is not limited to these examples. 【0068】 Molten steel (steel grades A to E) having the chemical compositions shown in Table 1 was cast by the continuous casting method to produce slabs. These slabs were hot-rolled and cold-rolled under the conditions shown in Table 2 to obtain cold-rolled steel sheets having the plate thicknesses shown in Table 2. Thereafter, for steel grades A to D, annealing was performed in a continuous melting plating line and then alloying treatment was performed under the conditions shown in Table 2. For steel grade E, annealing was performed in a continuous annealing line under the conditions shown in Table 2. 【0069】 【0070】 【0071】The obtained alloyed hot-dip galvanized steel sheets of steel grades A to D were subjected to two heat treatments, heat treatment 1 followed by heat treatment 2, under the conditions shown in Table 3. In this example, heat treatment 1 simulated the heat treatment in hot stamping, and quenching was performed by cooling without forming by sandwiching the sheet between flat molds. Then, in heat treatment 2, as described in the "Treatment Details" column of Table 3, quenching similar to heat treatment 1 was performed, or tempering was performed by heating under the conditions shown in Table 3 and then allowing it to cool outside the furnace. "None" in the "Treatment Details" column of Table 3 means that heat treatment 2 was not performed. 【0072】 In Table 3, "cooling rate" refers to the average cooling rate from the cooling start temperature to the release temperature. In all examples, the average cooling rate from the cooling start temperature to the Ms point was 50°C / s or higher, and the average cooling rate from the Ms point to the Mf point was 10°C / s or higher. 【0073】 In this process, for each test number, two steel plates were prepared: one corresponding to the area where resistance spot welding was planned (hereinafter referred to as "steel plate 1"), and another corresponding to the area where resistance spot welding was not planned (hereinafter referred to as "steel plate 2"). Subsequently, each steel plate was subjected to heat treatment separately to obtain heat-treated materials. In the following explanation, the heat-treated material obtained by heat-treating steel plate 1 will be referred to as heat-treated material 1, and the heat-treated material obtained by heat-treating steel plate 2 will be referred to as heat-treated material 2. 【0074】 In Table 3, for the cases where "Yes" is indicated in the "Presence or Absence of Dummy Material" column, heat treatment 1 and / or heat treatment 2 described above were performed only on steel plate 1 with a dummy material for covering placed on top. On the other hand, heat treatment 1 and heat treatment 2 described above were performed on steel plate 2 without using any dummy material. Mild steel with a thickness of 0.6 mm was used for the dummy material. 【0075】 After the heat treatment described above, for test numbers 15 to 21, shot blasting was performed on heat-treated material 1 under the conditions shown in the "Shot Blasting" column of Table 3 to remove a portion of the zinc oxide layer. 【0076】 【0077】Subsequently, multiple 30 mm x 50 mm test pieces were cut from heat-treated material 1 of each test number. Each piece was then joined with a test piece of the same size cut from cold-rolled annealed steel sheet of steel type E. Resistance spot welding was performed at a single point with different current values ​​for each test piece to create multiple welded joints. A stationary AC power spot welding machine was used for resistance spot welding, with a pressing force of 400 kgf. A DR (dome radius) type electrode with a tip diameter of φ6 and tip radius of R40 was used, and welding was performed by varying the current in the following cycles: squeeze 60 cycle - welding current 16 cycle - holding 10 cycle. The power supply frequency was 60 Hz. Using this method, the lowest current value at which the nugget diameter was 4√t or greater, and the lowest current value at which spatter occurred, were determined, given the thickness of heat-treated material 1 as t. The results are shown in Table 4. 【0078】 In Table 4, "hardness" refers to Vickers hardness, "4√t" refers to the lowest current value at which the nugget diameter is 4√t or greater, and "dust generation" refers to the lowest current value at which dust is generated. "Appropriate range" refers to the appropriate current range, which is the value obtained by subtracting the lowest current value at which the nugget diameter is 4√t or greater from the lowest current value at which dust is generated. 【0079】 In cases where dust is generated at a current lower than 4√t, the current is increased until the nugget diameter reaches 4√t, while dust is being generated. Therefore, these current values ​​are reference values ​​with low reproducibility. Furthermore, in cases where dust is generated at a current lower than 4√t, there is no appropriate current range, and therefore, "None" is indicated in the "Appropriate Range" column of Table 4. 【0080】 Subsequently, for the welded joint produced with the lowest current value at which the nugget diameter was 4√t or greater, the first position was defined as a location 15 mm away from the center of the resistance spot weld in the heat-treated material 1 portion. The third position was defined as a location 15 mm away from the center of the resistance spot weld in the mating material portion of the same welded joint. Here, the center of the resistance spot weld refers to the center of the approximate circle of the resistance spot weld indentation observed from both the heat-treated material 1 and the mating material in the thickness direction. 【0081】 Furthermore, one 30 mm x 50 mm test specimen was cut from heat-treated material 2, and the center position, which is the intersection of the two diagonals when the specimen is viewed from the thickness direction, was designated as the second position. Then, at the first and second positions, Vickers hardness measurements, observation of the solid solution layer structure, and measurement of the thickness of the zinc oxide layer and the solid solution layer were performed using the method described above. In addition, Vickers hardness measurements were also performed at the third position. 【0082】 In the "Solid Solution Layer Composition" section, "α-Fe" refers to an Fe-rich phase in which Zn is solid-dissolved in α-Fe, and "Γ" refers to an intermetallic compound. Furthermore, since the Vickers hardness at the third position is expected to be similar for all test numbers, measurement was performed only for test number 1, and that value was applied to all test numbers. 【0083】 These results are summarized in Table 4. 【0084】 【0085】 As shown in Table 4, in test numbers 1-3, 9, 10, 12, 14, 23, and 24, which satisfy the provisions of the present invention, it was possible to secure an appropriate current range of 1.0 kA or more. In contrast, in comparative example test numbers 4-8, 11, and 15-21, since dummy material was not used when preparing the heat-treated material 1, it was not possible to control the thickness of the zinc oxide layer and / or solid solution layer, and therefore an appropriate current range could not be secured. In test numbers 15-21, the thickness of the zinc oxide layer was reduced by shot blasting, but due to the influence of the solid solution layer, the resistance spot weldability could not be sufficiently improved. 【0086】 Furthermore, in comparative example test number 13, a dummy material was used to prepare the heat-treated material 1, resulting in a thinner zinc oxide layer, which in turn allowed for securing a sufficient appropriate current range. However, in both heat treatments, the heating temperature was relatively low and the heating time was relatively short, so sufficient hardness could not be obtained at the first position of the heat-treated material 1 prepared by stacking dummy materials. Therefore, it is expected that the deformation resistance will be insufficient when used as a structural member. 【0087】 In reference example test number 22, dummy material was not used when preparing heat-treated material 1, but because the heating temperature before hot stamping was not increased, the heating time was not prolonged, and hot stamping was not performed multiple times, a sufficient appropriate current range was secured. This also shows that when hot stamping is performed under conditions different from conventional methods, such as increasing the heating temperature before hot stamping, the appropriate current range becomes significantly narrower, and the effects of the present invention are remarkably obtained under such conditions. 【0088】 According to the present invention, even when hot stamping is performed after increasing the heating temperature and / or the heating time, the resulting hot-stamped molded body exhibits excellent resistance spot weldability. Therefore, welded joints can be manufactured using the hot-stamped molded body as a material in a stable manner without generating joint defects or spatter. For this reason, the welded joint according to this embodiment can be suitably used as an automotive part.

Claims

1. A welded joint comprising a first steel plate member which is a hot-stamped molded body of a zinc-plated steel sheet, a second steel plate member, and a plurality of resistance spot welds that join the first steel plate member and the second steel plate member, wherein, in the first steel plate member, a predetermined position 12 mm or more but less than 30 mm away from the center of one of the plurality of resistance spot welds is designated as the first position, and a predetermined position 30 mm or more away from the center of any of the plurality of resistance spot welds and not in contact with any other member is designated as the second position, the absolute value of the difference between the Vickers hardness measured at the first position and the Vickers hardness measured at the second position is 10.0% or less of the Vickers hardness measured at the second position, the thickness of the zinc oxide layer measured at the first position is 85% or less of the thickness of the zinc oxide layer measured at the second position, and the thickness of the solid solution layer measured at the first position is 90% or less of the thickness of the solid solution layer measured at the second position.

2. The welded joint according to claim 1, wherein the thickness of the solid solution layer measured at the first position is 80% or less of the thickness of the solid solution layer measured at the second position.

3. An automotive part comprising a welded joint according to claim 1 or claim 2.

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