WO2025018310A1 - 溶接継手及び自動車部材の接合構造 - Google Patents

溶接継手及び自動車部材の接合構造 Download PDF

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Publication number
WO2025018310A1
WO2025018310A1 PCT/JP2024/025325 JP2024025325W WO2025018310A1 WO 2025018310 A1 WO2025018310 A1 WO 2025018310A1 JP 2024025325 W JP2024025325 W JP 2024025325W WO 2025018310 A1 WO2025018310 A1 WO 2025018310A1
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steel sheet
steel
welded joint
content
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PCT/JP2024/025325
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English (en)
French (fr)
Japanese (ja)
Inventor
敬之 古川
卓哉 光延
隆志 大毛
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Nippon Steel Corp
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Nippon Steel Corp
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Priority to JP2025534043A priority Critical patent/JP7832577B2/ja
Priority to CN202480045981.3A priority patent/CN121488057A/zh
Priority to KR1020267000363A priority patent/KR20260021716A/ko
Publication of WO2025018310A1 publication Critical patent/WO2025018310A1/ja
Priority to MX2026000550A priority patent/MX2026000550A/es
Anticipated expiration legal-status Critical
Pending legal-status Critical Current

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K11/00—Resistance welding; Severing by resistance heating
    • B23K11/10—Spot welding; Stitch welding
    • B23K11/11—Spot welding
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K11/00—Resistance welding; Severing by resistance heating
    • B23K11/16—Resistance welding; Severing by resistance heating taking account of the properties of the material to be welded
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K35/00—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting
    • B23K35/22—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting characterised by the composition or nature of the material
    • B23K35/24—Selection of soldering or welding materials proper
    • B23K35/30—Selection of soldering or welding materials proper with the principal constituent melting at less than 1550°C
    • B23K35/3053—Fe as the principal constituent
    • B23K35/3073—Fe as the principal constituent with Mn as next major constituent
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2101/00—Articles made by soldering, welding or cutting
    • B23K2101/006—Vehicles
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2101/00—Articles made by soldering, welding or cutting
    • B23K2101/18—Sheet panels
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite

Definitions

  • the present invention relates to a welded joint and a joining structure of an automobile component. More specifically, the present invention relates to a welded joint that prevents LME cracking during manufacturing, and a joining structure of an automobile component.
  • Patent Document 2 discloses a steel sheet having an improved weldability by suppressing LME cracking, in which Si oxide particles having a particle size of 20 nm or more are present in a surface layer of the steel sheet at a number density of 3,000 to 6,000 particles/ mm2 with an appropriate particle size distribution.
  • the objective of the present invention is to provide a welded joint that suppresses LME cracking during manufacturing.
  • the inventors have thoroughly investigated means for solving the above problems. As a result, they have found that by applying distortion to a steel sheet before annealing using a projectile under appropriate conditions to create an appropriate surface condition, and then performing high dew point annealing, the surface layer of the steel sheet is decarburized and a layer with a high ferrite fraction in which the ferrite is randomly oriented is formed, making it possible to suppress LME.
  • the present invention was developed based on the above findings and through further investigation, and its gist is as follows:
  • a welded joint comprising a plurality of overlapping steel plates and a spot weld portion joining the plurality of steel plates, the spot weld portion having a nugget, an indentation portion pressed down by an electrode, and a weld shoulder portion which is the peripheral portion of the indentation portion, one or more of the plurality of steel plates being a plated steel plate having a plating layer containing Zn formed at least on a surface corresponding to the overlapping surface of the plurality of steel plates, and one or more of the plurality of steel plates constituting the overlapping surface being a high-strength steel plate having a hardness of 200 Hv or more in a first region which is a surface region located 5 mm or more away from the outer end of the nugget toward the outside of the nugget in the direction in which the plate surface of the steel plate extends, and the high-strength steel plate having a chemical composition, in mass %, of C: 0.05-0.40%, Si: 0.5-3.0%, Mn
  • the high-strength steel plate has a surface roughness Ra of more than 3.0 ⁇ m, a depth at which the C concentration is 0.02% or less in a plate thickness direction from the surface of the high-strength steel plate is 8 ⁇ m or more, and an oblique incidence angle of 1° with respect to the surface of the high-strength steel plate is 1°.
  • the thickness of a layer having an area ratio of ferrite of 90% or more in the plate thickness direction from the surface of the high-strength steel plate is 15 ⁇ m or more, provided that, in the case where the steel plate is a plated steel plate, the surface of the steel plate is the interface between the steel plate and the plated layer of the
  • a joining structure for automobile components in which any one of the welded joints (1) to (5) is used to join the automobile components when the multiple steel plates are used as automobile components.
  • the present invention makes it possible to obtain welded joints that suppress LME cracking during manufacturing.
  • FIG. 2 is a diagram illustrating an example of a welded joint of the present invention.
  • FIG. 1 is a diagram showing an example of the results of grazing incidence X-ray diffraction measurement when the ferrite phase is randomly oriented and when it is not randomly oriented. 1 is an example of a SEM micrograph of a structure near the surface layer located 0 to 100 ⁇ m away from the weld shoulder of a welded joint of the present invention.
  • FIG. 2 is a diagram for explaining the positions of cracks targeted in the LME resistance evaluation in the examples.
  • LME cracking is caused, for example, during spot welding, when the metal structure of a steel sheet is heated and transformed into austenite, and molten zinc produced by melting the plating penetrates into the grain boundaries of the austenite in the surface layer of the steel sheet.
  • the molten zinc that penetrates into the austenite grain boundaries embrittles the steel sheet, and is further caused by the tensile stress applied to the steel sheet during welding.
  • the inventors of the present invention came up with the idea of utilizing the metal structure of the surface layer of the steel sheet that constitutes the welded joint as a method for improving LME resistance.
  • the surface layer of the steel sheet has a metal structure mainly composed of a ferrite phase with a low C concentration and low LME sensitivity, and the ferrite phase is randomly oriented to suppress the occurrence of LME.
  • LME resistance means a property in which LME cracking is suppressed in a steel sheet
  • LME sensitivity means a property in which LME cracking is likely to occur in a steel sheet.
  • the random orientation of the ferrite phase means that the characteristics of the ferrite grain boundaries are averaged as a whole. In other words, it means that the crystal orientation of each ferrite particle in the ferrite phase is randomly oriented.
  • the random orientation of the crystal orientation of the ferrite particles prevents the grain boundaries oriented in a specific direction from being unevenly distributed and connected continuously or intermittently. It is believed that LME cracking occurs when Zn from the plating invades grain boundaries where the grain boundary energy is locally low. In other words, if there are continuous grain boundaries where the grain boundary energy is locally low, Zn from the plating will concentrate there, making LME cracking more likely to occur.
  • the random orientation of the ferrite phase is expressed by the following conditional formula.
  • a steel sheet that satisfies the following conditional formula means that the ferrite phase is randomly oriented.
  • the diffraction intensity corresponding to the (110) plane is I(110)
  • the diffraction intensity corresponding to the (200) plane is I(200)
  • the diffraction intensity corresponding to the (211) plane is I(211), 0.45 ⁇ I(110)/(I(110)+I(200)+I(211)) ⁇ 0.90
  • the cold-rolled steel plate is strained and then annealed at a high dew point. This promotes decarburization and makes it easier to form a ferrite phase on the steel plate surface. Furthermore, the present invention was made based on the discovery that the orientation of the ferrite phase can be randomized by controlling the temperature at which humidification begins.
  • the present invention will be described in detail below. First, the welded joint of the present invention will be described with reference to FIG. 1.
  • the welded joint of the present invention includes a plurality of overlapping steel sheets 1 and a spot welded portion 2 that joins the plurality of steel sheets.
  • the spot welded portion 2 has an indentation portion 3, a welded shoulder portion 4, and a nugget 5 formed on the surface of the steel sheet 1 pressed down by an electrode.
  • the welded shoulder portion 4 is the peripheral portion of the indentation portion 3, and refers to the inclined portion from the edge of the indentation portion 3 to the outer end 4a of the welded shoulder.
  • a pressure welded portion 6 is formed around the nugget 5, where two steel sheets 1 are pressure welded.
  • the first region 21 is a surface region (non-heat-affected zone) that is 5 mm or more away from the outer end of the nugget toward the outside of the nugget in the direction in which the sheet surface of the steel sheet spreads
  • the second region 22 is a surface region (heat-affected zone) that is 0 to 100 ⁇ m from the outer end of the welded shoulder.
  • the plurality of steel sheets may include both plated steel sheets and non-plated steel sheets that are not plated. It may also include both high-strength steel plates and relatively low-strength steel plates.
  • the plated steel sheet includes a base steel sheet and a plated layer.
  • the plated layer contains Zn and is formed on at least a surface corresponding to the overlapping surfaces of the plurality of steel sheets.
  • the plated layer may be formed on a surface other than the surface corresponding to the overlapping surfaces of the steel sheets.
  • the chemical composition of the plating layer can be determined by dissolving the plating layer in an acid solution containing an inhibitor that suppresses corrosion of the steel sheet, and measuring the resulting solution using ICP (inductively coupled plasma) emission spectroscopy.
  • an acid solution containing an inhibitor for dissolving the plating layer for example, a 10% by mass hydrochloric acid solution containing 0.06% by mass inhibitor (Ibit 710K, manufactured by Asahi Chemical Industry Co., Ltd.) can be used.
  • the thickness of the plating layer may be, for example, 3 to 50 ⁇ m.
  • the coating weight of the plating layer is not particularly limited, but may be, for example, 10 to 170 g/ m2 per side. In the present invention, the coating weight of the plating layer is determined from the change in weight before and after removal of the plating layer.
  • the plating layer is dissolved in an acid solution containing an inhibitor that suppresses corrosion of the base steel sheet, and the coating weight is determined from the change in weight before and after the plating layer is peeled off by pickling. After the plating layer is removed, the base steel sheet is washed with water and dried.
  • High-strength steel plate At least one of the plurality of steel plates is a high-strength steel plate.
  • "high strength” means that the hardness in the first region is 200 Hv or more.
  • the hardness of the steel plate is measured at a position that is the first region of the steel plate constituting the welded joint, at a position of 1/2 depth.
  • the hardness measurement is performed in accordance with JIS Z 2244:2009.
  • the measurement load is 200 gf.
  • the hardness in the first region may be 240 Hv or more, 270 Hv or more, or 340 Hv or more.
  • the one or more plated steel sheets and the one or more high-strength steel sheets may each be different steel sheets, or may be the same high-strength zinc-plated steel sheet that is high-strength and has been zinc-plated.
  • LME cracking occurs when molten zinc plating is present on the surface of a high-strength steel sheet during welding. For example, when considering a welded joint consisting of two steel sheets, if at least one of the two steel sheets is high strength and zinc plating is present on the overlapping surface, LME cracking may occur. In addition, even if one sheet is a relatively low-strength zinc-plated steel sheet and the other is a high-strength non-plated steel sheet, molten zinc plating is present on the overlapping surface of the steel sheets during welding, and the molten zinc plating comes into contact with the high-strength non-plated steel sheet, so LME cracking may occur.
  • the welded joint of the present invention suppresses LME cracking during manufacturing even in such cases, so it can also include a welded joint consisting of one sheet of zinc-plated steel with a relatively low strength and the other sheet of high-strength non-plated steel.
  • the welded joint of the present invention may include a welded joint in which one side is made of a relatively low-strength uncoated steel sheet and the other side is made of a high-strength zinc-coated steel sheet, or a welded joint in which one side is made of a relatively low-strength zinc-coated steel sheet and the other side is made of a high-strength zinc-coated steel sheet, or a welded joint in which both sides are made of high-strength zinc-coated steel sheets.
  • the thickness of the steel plate constituting the welded joint of the present invention is not particularly limited. For example, it can be 0.1 to 3.2 mm.
  • the thickness may be 0.2 mm or more, 0.4 mm or more, or 0.6 mm or more.
  • the thickness may be 3.0 mm or less, 2.5 mm or less, 2.0 mm or less, or 1.8 mm or less.
  • the above-mentioned high-strength steel plate has the chemical composition described below.
  • “%” regarding the chemical composition means “mass %”.
  • the numerical range expressed by "to” means a range including the numerical values written before and after "to” as the lower and upper limits.
  • C (C: 0.05-0.40%)
  • C (carbon) is an element that ensures the strength of steel, and the C content is set to 0.05% or more.
  • the C content is 0.40% or less.
  • the C content may be 0.08% or more, 0.10% or more, or 0.15% or more. It may be 37% or less, 0.35% or less, or 0.30% or less.
  • Silicon (Si: 0.5-3.0%) is an element that promotes ferrite stabilization and decarburization. By including silicon, decarburization advances in the surface layer and the ferrite in the surface layer is stabilized by the pretreatment and heat treatment described below. By increasing the Si content, the LME resistance during the production of welded joints is improved. To obtain this effect, the Si content is set to 0.5% or more. If the Si content is too high, high dew point annealing is required.
  • the Si content is set to 3.0% or less.
  • the Si content may be 0.6% or more, 0.7% or more, or 0.8% or more.
  • Si Content may be 2.5% or less, 2.0% or less, or 1.5% or less.
  • Mn manganese
  • Mn manganese
  • the Mn content is set to 0.1% or more.
  • the Mn content is set to 5.0% or less.
  • the Mn content is set to 0.5% or more, 1.0% or more, or 1.5% or more.
  • the Mn content may be 4.5% or less, 4.0% or less, or 3.5% or less.
  • Al (aluminum) is an element that, like Si, promotes ferrite stabilization and decarburization by dissolving in steel.
  • Sol. Al is not in the form of oxides such as Al2O3 .
  • Acid-soluble Al means Al that is soluble in acid, and is calculated as the Al content measured after excluding the insoluble residue on the filter paper that is generated during the analysis of Al. However, since the role of sol. Al can be achieved by including Si, sol. Al is not essential, and the lower limit of the content of sol. Al is 0%. If the content of sol.
  • the content of sol. Al is set to 3.0% or less.
  • the content of sol. Al is set to 0.1% or more, 0.3% or more, or 0.5% or more. It's okay. sol.
  • the Al content may be 2.0% or less, 1.5% or less, or 1.0% or less.
  • Si and sol. Al are elements that reduce LME resistance when added in excess, so the total content of Si and sol. Al is preferably 1.8% or less.
  • the total content of Si and sol. Al may be 1.7% or less, 1.6% or less, or 1.5% or less.
  • P 0.0300% or less
  • P (phosphorus) is an impurity generally contained in steel. If the P content exceeds 0.0300%, there is a risk of reduced weldability. Therefore, the P content is set to 0.0300% or less.
  • the P content may be 0.0200% or less, 0.0100% or less, or 0.0050% or less. It is preferable that no P is contained, and the lower limit of the P content is 0%. From the viewpoint of cost, the P content may be more than 0%, 0.0001% or more, or 0.0005% or more.
  • S sulfur
  • S is an impurity generally contained in steel. If the S content exceeds 0.0300%, the weldability decreases, and further, the amount of MnS precipitated increases, which reduces workability such as bendability. Therefore, the S content is set to 0.0300% or less.
  • the S content may be set to 0.0100% or less, 0.0050% or less, or 0.0020% or less. It is preferable that no S is contained, and the lower limit of the content of S is 0%. From the viewpoint of desulfurization costs, the content of S may be more than 0%, 0.0001% or more, or 0.0005% or more.
  • N nitrogen
  • nitrogen is an impurity generally contained in steel. If the N content exceeds 0.0100%, there is a risk of reduced weldability. Therefore, the N content is set to 0.0100% or less.
  • the N content may be 0.0080% or less, 0.0050% or less, or 0.0030% or less. It is preferable that N is not contained, and the lower limit of the N content is 0%. Manufacturing cost From this viewpoint, the N content may be more than 0%, 0.0005% or more, or 0.0010% or more.
  • O oxygen
  • oxygen is an element that forms oxides and reduces the workability of steel sheets. If the O content is too high, excessive oxides are generated, and the workability of the steel sheet is reduced. Therefore, the O content is set to 0.0030% or less.
  • the O content may be 0.0026% or less, 0.0024% or less, 0.0020% or less, or 0.0018% or less. It is preferable that O is not contained, and the lower limit of the O content is 0%. From the viewpoint of production cost, the O content may be more than 0%, 0.0005% or more, or 0.0010% or more. good.
  • B (boron) is an element that improves hardenability, contributes to improving strength, and segregates at grain boundaries to strengthen the grain boundaries and improve toughness, so it may be contained as necessary. Since B is not an essential element, the lower limit of the B content is 0%. This effect can be obtained even with a small amount of B, but if B is contained, the B content is preferably 0.0001% or more. In order to ensure sufficient toughness, the B content is set to 0.0100% or less. The B content is set to 0.0002% or more, 0.0003% or more, or 0.0005% or more. The B content may be 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0020% or less.
  • Ti titanium
  • Ti titanium
  • Ti titanium
  • Ti titanium
  • the Ti content is 0%. This effect can be obtained even with a small amount of Ti, but when Ti is contained, the Ti content is preferably 0.0001% or more. The Ti content may be 0.0005% or more.
  • coarse TiN may be generated, which may impair toughness, so the Ti content is set to 0.1500% or less. It may be 0.1000% or less, 0.0500% or less, 0.0050% or less, or 0.0020% or less.
  • Niobium (Nb) is an element that contributes to improving strength by improving hardenability, and may be contained as necessary. Since it is not an essential element, the lower limit of the Nb content is 0%. This effect can be obtained even with a small amount of Nb, but when Nb is added, the Nb content is preferably 0.0001% or more. On the other hand, from the viewpoint of ensuring sufficient toughness, the Nb content is set to 0.1500% or less. The Nb content is set to 0.1000% or less, 0.0600% or less, 0.0200% or less. It may be the following:
  • V Vanadium
  • V is an element that contributes to improving strength by improving hardenability, and therefore may be contained as necessary. Since it is not an essential element, the lower limit of the V content is 0%. This effect can be obtained even with a small amount of V, but when V is contained, the V content is preferably 0.001% or more. On the other hand, from the viewpoint of ensuring sufficient toughness, the V content is set to 0.150% or less. % or less, and may be 0.020% or less.
  • Cr 0-2.00% Cr (chromium) is effective in increasing the hardenability of steel and increasing the strength of steel, so it may be contained as necessary. Since it is not an essential element, the lower limit of the Cr content is This effect can be obtained even with a small amount of Cr, but when Cr is contained, the Cr content is preferably 0.001% or more. On the other hand, if the Cr content is excessive, a large amount of Cr carbide is formed, which may adversely affect the hardenability. Therefore, the Cr content is set to 2.00% or more, and 0.10% or more. The Cr content may be 1.80% or less, 1.50% or less, 0.50% or less, or 0.20% or less.
  • Ni 0-2.00%
  • Ni nickel
  • the lower limit of the Ni content is This effect can be obtained even with a small amount of Ni, but when Ni is contained, the Ni content is preferably 0.001% or more.
  • the Ni content may be 1.80% or more, or 0.02% or more, or 0.05% or more.
  • the Ni content is set to 2.00% or less. % or less, 1.50% or less, 0.50% or less, or 0.20% or less.
  • Cu (copper) is effective in increasing the hardenability of steel and increasing the strength of steel, so it may be contained as necessary. Since it is not an essential element, the lower limit of the Cu content is This effect can be obtained even with a small amount of Cu, but when Cu is contained, the Cu content is preferably 0.0001% or more. On the other hand, from the viewpoint of suppressing deterioration of toughness and cracking of the slab after casting, the Cu content is set to 2.0000% or less. The Cu content is set to 1.8000% or less. It may be 1.5000% or less, 0.0050% or less, or 0.0020% or less.
  • Mo mobdenum
  • Mo mobdenum
  • Mo mobdenum
  • the Mo content is preferably 0.001% or more.
  • the Mo content may be 0.02% or more, or 0.03% or more.
  • the Mo content is 1.00% or less.
  • the Mo content is 0.80% or less, It may be 0.60% or less, or 0.20% or less.
  • W 0-1.000%) W (tungsten) is effective in increasing the hardenability of steel and increasing the strength of steel, so it may be contained as necessary. Since it is not an essential element, the lower limit of the W content is This effect can be obtained even with a small amount of W, but when W is contained, the W content is preferably 0.001% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness, the W content is set to 1.000% or less. . 300% or less, 0.100% or less, or 0.020% or less.
  • Ca (Ca: 0-0.1000%)
  • Ca (calcium) is an element that contributes to inclusion control, particularly to finely dispersing inclusions, and has the effect of increasing toughness, so it may be contained as necessary.
  • the lower limit of the Ca content is 0%. This effect can be obtained even with a small amount of Ca included, but when Ca is included, the Ca content is preferably 0.0001% or more. However, if the Ca content is excessive, deterioration of the surface properties may become evident, so the Ca content is set to 0.1000% or less. may be 0.0800% or less, 0.0500% or less, 0.0300% or less, 0.0100% or less, or 0.0010% or less.
  • Mg manganesium
  • Mg is an element that contributes to inclusion control, particularly to finely dispersing inclusions, and has the effect of increasing toughness, so it may be contained as necessary. Since it is not an essential element, Mg The lower limit of the Mg content is 0%. This effect can be obtained even with a small amount of Mg, but when Mg is contained, the Mg content is preferably 0.0001% or more. The Mg content may be 0.0005% or more, or 0.0008% or more. On the other hand, if the Mg content is excessive, deterioration of the surface properties may become evident, so the Mg content is set to 0.100% or less. The amount may be 0.090% or less, 0.080% or less, 0.030% or less, 0.010% or less, 0.002% or less.
  • Zr zirconium
  • Zr zirconium
  • the lower limit of the Zr content is 0%. This effect can be obtained even with a small amount of Zr, but when Zr is contained, the Zr content is preferably 0.001% or more. However, if the Zr content is excessive, deterioration of the surface properties may become evident, so the Zr content is set to 0.100% or less. may be 0.050% or less, 0.030% or less.
  • Hf (hafnium) is an element that contributes to inclusion control, particularly to finely dispersing inclusions, and has the effect of increasing toughness, so it may be contained as necessary.
  • the lower limit of the Hf content is 0%. This effect can be obtained even with a small amount of Hf content, but when Hf is contained, the Hf content is preferably 0.0001% or more.
  • the Hf content may be 0.0003% or more, or 0.0005% or more.
  • the Hf content is set to 0.100% or less.
  • the amount may be 0.050% or less, 0.030% or less, 0.010% or less, 0.005% or less, 0.002% or less.
  • REM 0-0.1000%
  • REM rare earth elements
  • the lower limit of the REM content is 0%. This effect can be obtained even with a small amount of REM content, but when REM is contained, the REM content is preferably 0.0001% or more.
  • the REM content may be 0.0003% or more, or 0.0005% or more.
  • the REM content is set to 0.1000% or less.
  • the content may be 0.0500% or less, 0.0300% or less, 0.0100% or less, 0.0050% or less, or 0.0020% or less.
  • REM is an abbreviation for Rare Earth Metal. REM refers to elements that belong to the lanthanide series. REM are usually added as misch metals.
  • the remainder other than the above chemical components consists of Fe and impurities.
  • the remainder may be Fe and impurities, that is, the remainder may consist only of Fe and impurities.
  • impurities refer to components that are mixed in due to various factors in the manufacturing process, including raw materials such as ores and scraps, when industrially manufacturing steel plate, and that do not adversely affect LME cracking during the manufacturing of the welded joint according to the present invention.
  • Analysis of the chemical components of steel plate may be performed using elemental analysis methods known to those skilled in the art, for example, inductively coupled plasma mass spectrometry (ICP-MS).
  • ICP-MS inductively coupled plasma mass spectrometry
  • C and S may be measured using the combustion-infrared absorption method
  • N may be measured using the inert gas fusion-thermal conductivity method
  • O may be measured using the inert gas fusion-infrared absorption method.
  • the high-strength steel plate constituting the welded joint according to the present invention may have a tensile strength of 780 MPa or more.
  • the present invention suppresses LME that occurs when welding high-strength steel plates, i.e., steel plates with high tensile strength and hardness, to manufacture a welded joint.
  • the welded joint according to the present invention can suppress LME even when using a steel plate with a tensile strength of 780 MPa or more.
  • the upper limit of the tensile strength is not particularly limited, but may be, for example, 2000 MPa or less from the viewpoint of ensuring toughness.
  • the tensile strength is measured by taking a JIS No. 5 tensile test piece whose longitudinal direction is perpendicular to the rolling direction and the plate thickness direction, in accordance with JIS Z 2241:2011.
  • the tensile strength may be 980 MPa or more, 1180 MPa or more.
  • the surface layer of the steel sheet means a layered region having a depth from the surface of the steel sheet (sheet surface of the steel sheet) to a specified distance in the sheet thickness direction of the steel sheet.
  • the surface of the steel sheet means the surface of the steel sheet (sheet surface) excluding the plating.
  • the specified distance in the sheet thickness direction can be the longer (deeper) length (depth) of either "the depth where the C concentration of the surface layer is 0.02% or less" or "the depth where the area ratio of the ferrite phase is 90% or more" described later.
  • the interface between the plating layer and the base steel sheet when the high-strength steel sheet has a plating layer, and the surface roughness Ra of the high-strength steel sheet when the high-strength steel sheet does not have a plating layer is more than 3.0 ⁇ m in terms of arithmetic mean height Ra defined in JIS B0601:2013.
  • the roughness may be 3.5 ⁇ m or more in terms of Ra.
  • the roughness of the interface when the high-strength steel sheet has a plating layer is the surface roughness Ra of the steel sheet measured after removing the plating.
  • the plating layer is removed by dissolving the plating layer in an acid solution to which an inhibitor that suppresses corrosion of the base steel sheet has been added, as in the measurement of the coating weight described above.
  • the surface roughness Ra when measuring the surface roughness Ra, 10 measurement points are randomly selected on the surface of the steel plate in accordance with JIS B 0601:2013 so that the distance between each measurement point is 1 mm or more.
  • the surface profile at each measurement point is measured using a laser microscope (for example, Keyence's "VK-X3000"). Specifically, an image is taken at a magnification of 20 times using a laser microscope, and the arithmetic mean roughness (Ra) at each measurement point is calculated using a reference length of 2000 ⁇ m in the image taken.
  • the arithmetic mean value of the arithmetic mean roughness (Ra) at each measurement point is defined as the "surface roughness Ra".
  • the depth at which the C concentration measured by GDS (glow discharge spectroscopy) is 0.02% or less is 3 ⁇ m or more in the sheet thickness direction from the steel sheet surface.
  • the starting point in the depth direction is the interface between the plating layer and the base steel sheet when the high-strength steel sheet has a plating layer, and the surface of the high-strength steel sheet when the high-strength steel sheet does not have a plating layer.
  • Such a surface structure (metal structure at the surface of the steel plate) can be obtained as a decarburized layer produced by setting the chemical composition of the steel plate as described above and carrying out the pretreatment and annealing described below.
  • the depth where the C concentration is 0.02% or less is 8 ⁇ m or more, this contributes to improving LME resistance, so there is no particular upper limit to the depth where the C concentration is 0.02% or less.
  • the depth where the C concentration is 0.02% or less may be, for example, 50 ⁇ m or less, 40 ⁇ m or less, or 30 ⁇ m or less.
  • the depth where the C concentration is 0.02% or less is preferably 10 ⁇ m or more, more preferably 12 ⁇ m or more, and even more preferably 15 ⁇ m or more, or 20 ⁇ m or more.
  • GDS measurements are performed at five measurement points in the plate thickness direction, and the arithmetic mean value of the depth of the area where the C concentration is 0.02% or less at each measurement point is taken as the depth where the C concentration in the surface layer is 0.02% or less.
  • the five measurement points are randomly selected so that there is an interval of 5 mm or more between each measurement point on the steel plate surface.
  • the measurement conditions are as follows. Naturally, measurement results can be obtained even if the measurement device, etc., are not exactly as follows, but if there is a difference in the measurement results, the steel plate according to the present invention is identified by the measurement results according to the following conditions.
  • the value of the middle part of the conditional formula, "I(110)/(I(110)+I(200)+I(211))" is preferably 0.85 or less, more preferably 0.80 or less, and even more preferably 0.75 or less.
  • I(110)/(I(200)+I(211)) is preferably 0.50 or more.
  • the conditional formula means that the ferrite phase is randomly oriented. When the ferrite phase is completely randomly oriented, the value of the middle part is 0.67.
  • grazing incidence X-ray diffraction also called grazing incidence XRD, low angle incidence XRD, or tilted XRD
  • grazing incidence X-ray diffraction is a measurement technique in which the angle of incidence of the incident X-rays is set small, and the detector is scanned (the detection angle is changed) while maintaining the angle of incidence.
  • the angle of incidence of the X-rays is fixed at 1° to detect the orientation of ferrite in the surface layer of the steel sheet.
  • the angle of incidence is the angle between the surface of the sample (steel sheet) and the direction of incidence of the incident X-rays.
  • the plating layer is removed before measurement.
  • the plating layer is removed by dissolving the plating layer in an acid solution to which an inhibitor that suppresses corrosion of the base steel sheet has been added, as in the above-mentioned measurement of the coating amount.
  • Figure 2 shows examples of the results of oblique incidence XRD analysis when the ferrite phase is randomized (b) and not (a).
  • (a) is the result of oblique incidence XRD analysis of a normal (conventional) steel plate, and it can be seen that it is oriented in the (110) direction. Therefore, the value of the middle part of the conditional equation "I(110)/(I(110)+I(200)+I(211))" is relatively large at 0.91.
  • (b) is the result of oblique incidence XRD analysis of a steel plate that constitutes a welded joint of the present invention, and the orientation in the (110) direction is smaller than in (a). Therefore, the value of the middle part of the conditional equation "I(110)/(I(110)+I(200)+I(211))" is relatively small at 0.58.
  • a layer having an area ratio of ferrite of 90% or more in the second region, has a thickness of 15 ⁇ m or more in the thickness direction of the base steel plate starting from the interface between the plating layer and the base steel plate.
  • the thickness of the high ferrite layer is 15 ⁇ m or more, it contributes to suppressing LME during the production of welded joints, so there is no particular upper limit on its thickness.
  • the thickness of the high ferrite layer may be, for example, 100 ⁇ m or less, 80 ⁇ m or less, 60 ⁇ m or less, or 40 ⁇ m or less.
  • the thickness of the high ferrite layer is preferably 20 ⁇ m or more.
  • the structure other than ferrite in the high ferrite layer is not limited.
  • it can be one or more of martensite, bainite, and cementite.
  • the thickness of the high ferrite layer is measured by analyzing the secondary electron image by SEM observation of the observation cross section, which is mechanically polished to a mirror finish and then etched with nital.
  • a field emission scanning electron microscope e.g., JSM 7000F manufactured by JEOL Ltd., acceleration voltage: 15 kV
  • the observation field ranges from the surface of the steel sheet (sheet surface) to a depth of 500 ⁇ m in the sheet thickness direction (vertical direction of the observation cross section) and a width range of 600 ⁇ m in the direction perpendicular to the sheet thickness direction (horizontal direction of the observation cross section).
  • observation cross section Five observation fields are observed with an SEM so that the observation fields are spaced 1000 ⁇ m or more apart in the direction perpendicular to the sheet thickness direction (horizontal direction of the observation cross section), and a secondary electron image is obtained.
  • the observation resolution is 1280 ⁇ 960 pixels.
  • the surface of the steel sheet (sheet surface) is the surface of the steel sheet excluding the plating.
  • the ferrite fraction is calculated for the five secondary electron images obtained using the point counting method. More specifically, a grid with equal spacing is first drawn on the secondary electron image. Next, the number of grid points at each grid point where the structure is ferrite is found and divided by the total number of grid points to measure the ferrite fraction. The greater the total number of grid points, the more accurately the area ratio can be calculated. In the present invention, the grid spacing is 2 ⁇ m ⁇ 2 ⁇ m, and the total number of grid points is 1,500 points.
  • an area with relatively low brightness and no visible substructure can be determined as ferrite.
  • substructure means a transformed structure formed inside the old austenite phase such as laths or blocks.
  • ferrite is observed as an area with relatively low brightness and a relatively monotonous spread of brightness and color tone.
  • there is no need to distinguish metal structures other than ferrite but the discrimination criteria in the secondary electron image of tempered martensite, pearlite, ferrite, fresh martensite or retained austenite, or bainite are shown below.
  • An area that has a substructure (lath boundary, block boundary) within the grain and where carbides are precipitated with multiple variants is determined as tempered martensite.
  • an area where cementite is precipitated in a lamellar form is determined as pearlite.
  • An area with high brightness and where the substructure is not revealed by etching is determined as fresh martensite or retained austenite.
  • An area that does not fall into any of the above categories is determined as bainite.
  • the area ratio of the ferrite phase can be calculated by distinguishing between ferrite and other structures.
  • Figure 3 shows an example of a SEM micrograph of the surface layer of a steel plate that constitutes a welded joint of the present invention, located 0 to 100 ⁇ m away from the weld shoulder.
  • the white areas are ferrite. It can be seen that the surface layer is mainly composed of ferrite.
  • Ferrite has low susceptibility to LME.
  • the surface structure of the steel plate is mainly composed of ferrite.
  • Such a surface structure can be obtained by setting the chemical composition of the steel plate as described above and carrying out the pretreatment process and annealing process described below.
  • the depth at which the C concentration is 0.02% or less in the GDS measurement and the thickness at which the ferrite area ratio is 90% or more are the starting points at the interface between the base steel sheet and the plating layer.
  • the high-strength steel plate constituting the welded joint according to the present invention can be obtained by a manufacturing method including, for example, a casting process in which molten steel with adjusted chemical composition is cast to form a steel billet, a hot rolling process in which the steel billet is hot rolled to obtain a hot-rolled steel plate, a coiling process in which the hot-rolled steel plate is coiled, a cold rolling process in which the coiled hot-rolled steel plate is cold rolled to obtain a cold-rolled steel plate, a pretreatment process in which the cold-rolled steel plate is pretreated (grit blasted), and an annealing process in which the pretreated cold-rolled steel plate is annealed.
  • the hot-rolled steel plate may not be coiled after the hot rolling process, but may be pickled and then cold-rolled as is.
  • the conditions for the casting process are not particularly limited. For example, after melting in a blast furnace or an electric furnace, various secondary smelting processes may be carried out, and then casting may be carried out by a method such as ordinary continuous casting or casting by an ingot method.
  • the steel slab obtained by casting can be hot-rolled to obtain a hot-rolled steel sheet.
  • the hot rolling step is performed by reheating the cast steel slab directly or after cooling once, and then hot rolling it.
  • the heating temperature of the steel slab may be, for example, 1100 to 1250°C.
  • rough rolling and finish rolling are usually performed.
  • the temperature and reduction rate of each rolling may be appropriately changed depending on the desired metal structure and plate thickness.
  • the finishing temperature of the finish rolling may be 900 to 1050°C, and the reduction rate of the finish rolling may be 10 to 50%.
  • the hot-rolled steel sheet can be coiled at a predetermined temperature.
  • the coiling temperature may be appropriately changed depending on the desired metal structure, etc., and may be, for example, 500 to 800°C.
  • the hot-rolled steel sheet may be subjected to a predetermined heat treatment by recoiling before or after coiling. Alternatively, the hot-rolled steel sheet may be pickled after the hot rolling step without performing the coiling step, and then cold rolling, which will be described later.
  • the hot-rolled steel sheet After pickling or the like is performed on the hot-rolled steel sheet, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet.
  • the rolling reduction in the cold rolling may be appropriately changed depending on the desired metal structure and sheet thickness, and may be, for example, 20 to 80%.
  • the steel sheet After the cold rolling process, the steel sheet may be cooled to room temperature, for example, by air cooling.
  • the pretreatment includes carrying out a grit blasting treatment in which angular shot materials are shot onto the surface of the cold-rolled steel sheet.
  • the shot materials that can be used are not particularly limited, but for example, polygonal steel grits having an average particle size of 100 to 500 ⁇ m can be used.
  • An example of such grit is TGD-30 manufactured by WINOA IKK JAPAN.
  • the amount of grit shot is preferably 5 to 400 kg/m 2. This allows strain to be introduced into the surface layer of the steel sheet while increasing the surface roughness Ra.
  • decarburization is promoted in the annealing process described later, and a structure with stable ferrite can be efficiently formed in the surface layer of the steel sheet.
  • the projection amount per unit time and unit area at a projection amount of 400 kg/m 2 is 4.0 ⁇ 10 ⁇ 4 kg/(mm 2 ⁇ min).
  • the steel sheet to which strain is imparted by the grit blasting treatment is subjected to an annealing process including holding the steel sheet at a predetermined temperature and a high dew point.
  • the heating rate to the predetermined holding temperature is not particularly limited, and may be 1 to 10°C/sec.
  • the dew point is controlled by humidification control from 300°C or more and less than 600°C, preferably from 450 to 550°C. That is, the dew point (humidification) control start temperature is 300°C or more and less than 600°C, preferably within 450 to 550°C.
  • the dew point without dew point (humidification) control is usually less than -30°C.
  • the dew point (high dew point) during annealing is -30 to 20°C in order to promote decarburization.
  • the dew point (high dew point) during annealing is preferably -10°C or more.
  • the dew point during annealing is preferably 5°C or less.
  • the predetermined holding temperature (maximum heating temperature) in the annealing step is 750 to 900°C, preferably 770 to 870°C, in order to promote decarburization.
  • the holding time at the holding temperature (maximum heating temperature) in the annealing step is 20 to 300 seconds, preferably 50 to 200 seconds.
  • the atmosphere is preferably a non-oxidizing atmosphere, and can be, for example, N2 -1 to 10 vol% H2 or N2 -2 to 4 vol% H2 .
  • dew point, holding temperature, and holding time it is possible to promote decarburization, reduce the C concentration in the surface layer, and appropriately control the ferrite phase fraction. Furthermore, by setting the dew point (humidification) control start temperature in the above range, decarburization of the surface layer of the steel plate is promoted. At the same time, internal oxidation of Si and Mn progresses rapidly, and internal oxides are rapidly formed. The internal oxides formed function as nucleation sites, resulting in randomization of the orientation of the ferrite phase. If the dew point (humidification) control start temperature is too low, external oxidation progresses and internal oxidation of Si and Mn does not progress, making it difficult to randomize the orientation of the ferrite phase.
  • Annealing is performed under tension of, for example, 1 to 20 MPa. Applying tension during annealing makes it possible to introduce strain into the steel sheet more effectively, promoting decarburization of the surface layer.
  • the plated steel sheet constituting the welded joint according to the present invention can be obtained by performing a plating process for forming a plating layer containing Zn on the surface of the steel sheet.
  • the plating layer may be formed on the high-strength steel sheet produced as described above.
  • the plating process may be carried out according to a method known to those skilled in the art.
  • the plating process may be carried out, for example, by hot-dip plating or by electroplating.
  • the plating process is carried out by hot-dip plating.
  • the plating process conditions may be appropriately set in consideration of the chemical components, thickness, and deposition amount of the desired plating layer.
  • a known alloying process may be carried out to form alloy plating.
  • spot welding process A plurality of the above-mentioned steel plates are stacked and spot-welded to obtain a welded joint.
  • the conditions for spot welding are not particularly limited.
  • spot welding can be performed using a dome radius type welding electrode having a tip diameter of 5 to 10 mm, with a pressure of 1.0 to 5.0 kN, a current flow time of 0.2 to 1.2 seconds, and a current flow of 6 to 14 kA.
  • the welded joint according to the present invention is suitable for use in a wide range of fields, including automobiles, home appliances, and building materials, because it suppresses LME cracking during manufacturing.
  • it can be used in the automobile field as a joining structure for joining automobile components.
  • the obtained steel slab was heated to 1200°C and hot-rolled with a finish rolling end temperature of 950°C and a finish rolling reduction of 30% to obtain a hot-rolled steel sheet.
  • the obtained hot-rolled steel sheet was coiled at a coiling temperature of 650°C, pickled, and then cold-rolled with a reduction of 50% to obtain a cold-rolled steel sheet.
  • the cold-rolled steel sheet had a thickness of 1.6 mm.
  • the surface of the obtained cold-rolled steel sheet was subjected to grit blasting treatment by blasting with a blasting amount of 5 kg/m 2 using TGD-30 manufactured by WINOA IKK JAPAN Co., Ltd.
  • the surface roughness Ra of the cold-rolled steel sheet after the grit blasting treatment was 3.3 ⁇ m.
  • the cold-rolled steel sheet that had been subjected to the grit blasting was subjected to an annealing treatment in which the temperature was raised to 500°C at a rate of 6.0°C/sec in a N2-4 % H2 gas atmosphere in a furnace with an oxygen concentration of 20 ppm or less, and then the temperature was raised to 800°C at a rate of 2.0°C/sec and held for 40 seconds.
  • dew point control was started so that the dew point became 0°C from 300°C.
  • the annealing treatment was performed with the steel sheet under tension of 5.0 MPa.
  • the annealed steel sheet was immersed in a hot-dip galvanizing bath (Zn-0.14%Al) at 450°C for 3 seconds, and then pulled out at 100 mm/sec, and the coating weight was controlled to 50 g/ m2 with N2 wiping gas. Thereafter, an alloying treatment was performed at 520°C for 30 seconds to obtain a galvannealed steel sheet.
  • a hot-dip galvanizing bath Zn-0.14%Al
  • the obtained galvannealed steel sheet was stacked one by one with the same type of steel sheet as the mating material, and spot welding was performed using a dome radius type welding electrode with a tip diameter of 8 mm at an impact angle of 2°, a pressure of 4.0 kN, a current time of 0.8 seconds, and a current of 12 kA to produce a welded joint, and the LME resistance during production was evaluated.
  • the steel sheet produced in the examples is also referred to as the "example steel sheet”
  • the steel sheet combined when producing the welded joint is also referred to as the "mate steel sheet.”
  • the steel sheets or plated steel sheets were prepared under the same conditions as in Example 1, except that the chemical composition of the steel sheets was as shown in Table 1 or Table 2, the conditions of the pretreatment process, the conditions of the annealing process were as shown in Table 3, and the plating conditions were as shown in Table 4. Note that in Test No. 32, the grit blasting process was omitted, and in Test No. 35, instead of the grit blasting process, surface treatment was performed by grinding using a brush.
  • the plating types in Table 4 are "a” for alloyed hot-dip galvanizing, "b” for hot-dip galvanizing with the alloying process omitted in Example 1, "c” for a plating bath of Zn-1.5%Al-1.5%Mg with the alloying process omitted, and "non-plated” for a cold-rolled steel sheet that was not plated.
  • the welded joints were produced under the same welding conditions as in Test No. 1, in combination with the steel sheets listed in the mating material in Table 4.
  • the "same type" of mating material indicates that the same type of steel sheet as in the examples was used as the mating material steel sheet.
  • non-plated same type indicates that the same type of steel sheet as in the example was used as the mating steel sheet, but was not plated;
  • GA same type indicates that the same type of steel sheet as the steel sheet with the corresponding test number (Test No.) was used as the mating steel sheet, but was plated with alloyed zinc;
  • GI270IF indicates that a commercially available hot-dip galvanized steel sheet with a tensile strength of 270 MPa was used as the mating steel sheet;
  • GA590 indicates that a commercially available hot-dip galvanized steel sheet with a tensile strength of 590 MPa was used as the mating steel sheet.
  • Grade AA 3.5 ⁇ m or more Grade A: More than 3.0 ⁇ m, less than 3.5 ⁇ m Grade B: 3.0 ⁇ m or less
  • the hardness of the example steel plate was measured in the first region of the welded joint by the above-mentioned method.
  • the hardness of the steel plate was measured at a position of 1/2 depth of the example steel plate in accordance with JIS Z 2244:2009.
  • the measurement load was 200 gf.
  • the hardness was evaluated as follows.
  • Rating AAA 340Hv or more Rating AA: 270Hv or more, less than 340Hv Rating A: 200Hv or more, less than 270Hv
  • the surface roughness Ra of the unplated steel sheets and the exposed base steel sheets after removing the plating were measured in the same manner as before annealing.
  • the plating was removed by dissolving the plating layer in a 10 mass% hydrochloric acid solution containing 0.06 mass% inhibitor (Ibit 710K, manufactured by Asahi Chemical Industry Co., Ltd.) that suppresses corrosion of the base steel sheets.
  • a sample cut to 30 mm x 30 mm was taken from the first region of the welded joint, and five GDS measurements were performed in the plate thickness direction using the method described above to determine the depth at which the C concentration was 0.02% or less.
  • the "C ⁇ 0.02% depth” in Table 4 is the average depth at which the C concentration was 0.02% or less, determined by five GDS measurements.
  • the plating layer was dissolved in a 10 mass% hydrochloric acid solution containing 0.06 mass% inhibitor (Ibit 710K, manufactured by Asahi Chemical Industry Co., Ltd.) that inhibits corrosion of the base steel sheet, as described above, and the plating layer was removed, and then the oblique incidence X-ray diffraction intensity was measured.
  • Ibit 710K 0.06 mass% inhibitor
  • Samples cut to 25 mm x 15 mm were taken from the second region of the welded joint, and were subjected to nital etching.
  • the T-section of each sample was observed with an SEM to measure the thickness of the layer with an area ratio of ferrite phase of 90% or more (high ferrite layer).
  • the thickness was measured at five equally spaced points within a range of 500 ⁇ m in the T-direction, and the average value was calculated.
  • the starting point of the "thickness" is the surface of the steel sheet for unplated steel sheets, and the starting point is the interface between the plating layer and the base steel sheet for plated steel sheets.
  • LME resistance With reference to FIG. 4, the evaluation method of LME resistance will be described.
  • the LME resistance was evaluated by the length of the LME crack (crack 11 immediately outside the pressure welded portion) that occurred immediately outside the pressure welded portion 2 formed by overlapping two steel plates 1 and spot welding.
  • the two steel plates 1 are the steel plates with each test number (Test No.) and their mating steel plates.
  • the immediately outside the pressure welded portion of the welded portion refers to the outer part of the pressure welded portion 6, which is the part pressure welded by spot welding on the overlapping surface of the two steel plates, and refers to the position in the vicinity of the pressure welded portion 6 (within a range of about 1 mm from the end of the pressure welded portion 6 to the outside).
  • the crack length of the crack 11 immediately outside the pressure welded portion was evaluated.
  • the spot welding test was performed three times, and the crack 11 immediately outside the pressure welded portion with the longest crack length was evaluated.
  • the evaluation criteria were as follows. In this example, if the evaluation was A or higher (i.e., evaluation A, AA, AAA), it was determined that the LME resistance was excellent.
  • Grade AA More than 0 ⁇ m, less than 60 ⁇ m Grade A: 60 ⁇ m or more, less than 120 ⁇ m Grade B: 120 ⁇ m or more
  • Test Nos. 1 to 25 and 37 to 55 are examples of the present invention and had high LME resistance.
  • Test No. 35 instead of grit blasting, pretreatment was performed by grinding with a brush, so no strain was introduced into the surface layer. This is thought to have reduced the thickness where the ferrite phase was 90% or more. It is also thought that internal oxidation of Si and Mn did not progress, and the orientation of the ferrite phase did not become random. As a result, the LME suppression during the production of the welded joint was inferior.
  • the dew point control start temperature in the annealing process was low, which is thought to have caused external oxidation to progress, preventing decarburization and resulting in a shallow depth at which the C concentration was 0.02% or less in the GDS measurement. It is also thought that internal oxidation of Si and Mn did not progress, preventing randomization of the orientation of the ferrite phase. As a result, the welded joint had poor LME resistance when manufactured.
  • the present invention makes it possible to provide a welded joint that suppresses LME during manufacturing, and the welded joint can be used in applications such as automobiles, home appliances, and building materials, particularly automobiles. Therefore, the present invention has extremely high industrial applicability.

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PCT/JP2024/025325 2023-07-18 2024-07-12 溶接継手及び自動車部材の接合構造 Pending WO2025018310A1 (ja)

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JP2025534043A JP7832577B2 (ja) 2023-07-18 2024-07-12 溶接継手及び自動車部材の接合構造
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