Welded joints and automotive component joining structures

Abstract

The present invention addresses the problem of providing a welded joint in which LME during production is suppressed.　A welded joint according to the present invention comprises a plurality of steel sheets and a spot-welded part, wherein at least one of the plurality of steel sheets is a plated steel sheet, and at least one of the plurality of steel sheets is a high-strength steel sheet having a prescribed chemical component. In a first region (non-heat-affected part) of the high-strength steel sheet, the surface roughness Ra is greater than 3.0 µm, and the depth from the surface having a C concentration of 0.02% or less is 8 µm or more. In oblique incidence X-ray diffraction at an incidence angle of 1° with respect to the surface of the high-strength steel sheet, when the diffraction intensity corresponding to a (110) plane is I(110), the diffraction intensity corresponding to a (200) plane is I(200), and the diffraction intensity corresponding to a (211) plane is I(211), 0.45 ≤ I(110) / (I(110) + I(200) + I(211)) ≤ 0.90 is satisfied. In a second region (heat-affected part), the thickness of a surface layer having an area ratio of ferrite of 90% or more is 15 µm or more.

Description

[Technical Field]

[0001] This invention relates to welded joints and joining structures for automotive components. More specifically, this invention relates to welded joints that suppress LME cracking during manufacturing and joining structures for automotive components. [Background technology]

[0002] In recent years, there has been a trend towards increasing the strength of steel plates used in various fields such as automobiles, home appliances, and building materials. For example, in the automotive sector, the use of welded joints made by spot-welding high-strength steel plates is increasing in order to lighten the vehicle body in order to improve fuel efficiency.

[0003] In welding zinc-plated steel plates, especially high-strength (high tensile strength and hardness) steel plates, weldability may be reduced due to liquid metal embrittlement (LME) cracking, as described in Patent Document 1, for example.

[0004] Furthermore, Patent Document 2 describes a steel sheet with improved weldability by suppressing LME cracking, wherein the surface layer of the steel sheet contains 3,000 to 6,000 Si oxide particles with a particle size of 20 nm or larger per mm². 2 with number density The invention discloses a steel sheet having an appropriate particle size distribution. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] International Publication No. 2019 / 116531 [Patent Document 2] International Publication No. 2020 / 218575 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] To suppress LME cracking during the manufacturing of welded joints, it is effective, for example, to prevent Zn and other elements contained in the plating layer from penetrating into the austenitic-transformed steel sheet during welding. There is room for improvement in this regard.

[0007] In view of these circumstances, the present invention aims to provide a welded joint that suppresses LME cracking during manufacturing. [Means for solving the problem]

[0008] The inventors diligently investigated means to solve the above-mentioned problems. As a result, they found that by applying strain to a steel sheet before annealing using a projection material under appropriate conditions to achieve 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 ferrite is randomly oriented is formed, and as a result, LME can be suppressed.

[0009] This invention was developed based on the above findings and further investigations, and its gist is as follows.

[0010] (1) A welded joint comprising a plurality of overlapping steel plates and a spot weld joint for joining the plurality of steel plates, wherein the spot weld joint has a nugget, an indentation portion pressed down by an electrode, and a weld shoulder portion which is the peripheral edge of the indentation portion, and one or more of the plurality of steel plates is a plated steel plate having a plating layer containing Zn formed on the surface corresponding to the overlapping surface of the plurality of steel plates, and of the plurality of steel plates, the overlapping surface The one or more steel plates are high-strength steel plates in which the hardness of the first region, which is the surface region, is 5 mm or more away from the outer edge of the nugget in the direction in which the surface of the steel plate expands outwards from the outer edge of the nugget, is 200 Hv or more, and the chemical composition of the high-strength steel plate is, in mass%, C: 0.05~0.40%, Si: 0.5~3.0%, Mn: 0.1~5.0%, sol.Al: 0~3.0%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0100% or less, B: 0~0.0100% It contains Ti: 0-0.1500%, Nb: 0-0.150%, V: 0-0.150%, Cr: 0-2.00%, Ni: 0-2.00%, Cu: 0-2.0000%, Mo: 0-1.00%, W: 0-1.000%, Ca: 0-0.1000%, Mg: 0-0.100%, Zr: 0-0.100%, Hf: 0-0.100%, and REM: 0-0.1000%, with the remainder being Fe and impurities, and in the first region, the surface roughness Ra of the high-strength steel sheet is greater than 3.0 μm, and the In the thickness direction from the surface of the high-strength steel plate, the depth at which the carbon concentration is 0.02% or less is 8 μm or more, and in oblique incidence X-ray diffraction at an incident angle of 1° to the surface of the high-strength steel plate, when the diffraction intensity corresponding to the (110) plane is I(110), the diffraction intensity corresponding to the (200) plane is I(200), and the diffraction intensity corresponding to the (211) plane is I(211), the condition 0.45 ≤ I(110) / (I(110) + I(200) + I(211)) ≤ 0.90 is satisfied, and the weld shoulder portion of the high-strength steel plate A welded joint characterized in that, in a second region which is a surface layer region located at a distance of 0 to 100 μm toward the outside of the spot weld in the direction in which the surface of the high-strength steel plate expands from the surface, the thickness of the layer having a ferrite area ratio of 90% or more in the thickness direction from the surface of the high-strength steel plate is 15 μm or more, provided that if the steel plate is a plated steel plate, the surface of the steel plate is the interface between the steel plate and the plating layer of the plated steel plate.

[0011] (2) The welded joint according to (1), characterized in that in the first region, 0.50 ≤ I(110) / (I(110) + I(200) + I(211)) ≤ 0.75.

[0012] (3) The welded joint according to (1) or (2), characterized in that the depth at which the C concentration is 0.02% or less in the first region is 15 μm or more.

[0013] (4) Any of the welded joints described in (1) to (3) above, characterized in that the surface roughness Ra in the first region is 3.5 μm or more.

[0014] (5) In the second region, the area ratio of the ferrite is 90% That's all. A welded joint according to any of the above (1) to (4), characterized in that the thickness of the layer is 20 μm or more.

[0015] (6) A joining structure for automobile components, wherein any of the welded joints described in (1) to (5) above joins the automobile components when the plurality of steel plates are used as automobile components. [Effects of the Invention]

[0016] According to the present invention, a welded joint can be obtained that suppresses LME cracking during manufacturing. [Brief explanation of the drawing]

[0017] [Figure 1] This figure illustrates an example of a welded joint according to the present invention. [Figure 2]It 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. [Figure 3] It is an example of a microstructure photograph by SEM near the surface layer at a distance of 0 to 100 μm from the welding shoulder of the welded joint of the present invention. [Figure 4] It is a diagram for explaining the position of the crack targeted in the LME resistance evaluation in the examples.

Embodiments for Carrying Out the Invention

[0018] Hereinafter, the present invention will be described. The present invention is not limited to the following forms. First, an outline of improving the LME resistance in the present invention will be described.

[0019] LME cracks are caused, for example, during spot welding, when the metal structure of the steel plate is heated and transformed into austenite, and molten zinc generated by melting the plating penetrates into the grain boundaries of austenite in the surface layer of the steel plate. The molten zinc that has penetrated into the austenite grain boundaries embrittles the steel plate, and it is considered to occur when tensile stress is applied to the steel plate during welding. The inventors of the present invention have come up with the idea of utilizing the metal structure of the surface layer of the steel plate constituting the welded joint as a method for improving the LME resistance. Specifically, the structure of the surface layer of the steel plate is a metal structure mainly composed of a ferrite phase with a low C concentration and low LME sensitivity, and further, by randomly orienting the ferrite phase, the occurrence of LME is suppressed. Here, in this specification, "LME resistance" means a property in which LME cracks are suppressed in the steel plate, and "LME sensitivity" means a property in which LME cracks are likely to occur in the steel plate.

[0020] The random orientation of the ferrite phase means that the characteristics of the ferrite grain boundaries are averaged out as a whole. In other words, it means that the crystal orientation of each ferrite particle in the ferrite phase is randomly oriented. Because the crystal orientation of the ferrite particles is randomly oriented, grain boundaries oriented in a specific direction are unevenly distributed, and the continuous or discontinuous connection of grain boundaries is suppressed. LME cracking is thought to occur when Zn from the plating concentrates and penetrates into grain boundaries where the grain boundary energy is locally low. In other words, if there are continuous grain boundaries with locally low grain boundary energy, Zn from the plating concentrates there, making LME cracking more likely. By averaging out the characteristics of the grain boundaries as a whole, grain boundaries with locally low grain boundary energy do not connect continuously, the local concentration of Zn from the plating is suppressed, and as a result, LME during the manufacturing of welded joints can be suppressed.

[0021] In this invention, the random orientation of the ferrite phase is expressed by the following conditional formula. That is, a steel sheet that satisfies the following conditional formula means that the ferrite phase is randomly oriented.

[0022] (Conditional expression) In oblique incidence X-ray diffraction at an incident angle of 1° to the surface of a steel plate, if the diffraction intensity corresponding to the (110) plane is I(110), the diffraction intensity corresponding to the (200) plane is I(200), and the diffraction intensity corresponding to the (211) plane is I(211), 0.45≦I(110) / (I(110)+I(200)+I(211))≦0.90

[0023] In order to achieve the above-described structure of the steel plates constituting the welded joint, the present invention involves applying strain to the cold-rolled steel plates during manufacturing, followed by annealing at a high dew point. This promotes decarburization and facilitates the formation of a ferrite phase on the surface of the steel plates. Furthermore, the present invention is based on the finding that the orientation of the ferrite phase can be randomized by controlling the temperature at which humidification begins.

[0024] The present invention will now be described in detail. First, the welded joint of the present invention will be described with reference to Figure 1. The welded joint of the present invention comprises a plurality of overlapping steel plates 1 and a spot weld 2 that joins the plurality of steel plates. The spot weld 2 has an indentation portion 3, a weld shoulder portion 4, and a nugget 5 formed on the surface of the steel plate 1 that has been pressed down by an electrode. The weld shoulder portion 4 is the peripheral edge 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 weld shoulder portion. A pressure-welded portion 6 is formed around the nugget 5 by pressing two steel plates 1 together. 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 in the direction in which the surface of the steel plate expands toward the outside of the nugget, and the second region 22 is a surface region (heat-affected zone) from 0 to 100 μm away from the outer end of the weld shoulder portion. The plurality of steel plates may include both plated steel plates and unplated steel plates. Furthermore, it may include both high-strength steel plates and relatively low-strength steel plates.

[0025] [Plated steel sheet] One or more of the multiple steel sheets are plated steel sheets. The plated steel sheet comprises a base steel sheet and a plating layer. The plating layer contains Zn and is formed on surfaces corresponding to the overlapping surfaces of at least the multiple steel sheets. The plating layer is not particularly limited as long as it contains Zn. Examples include Zn-0.2%Al, Zn-0.5%Al, Zn-1.5%Al-1.5%Mg, Zn-20%Al-7%Mg, and Zn-30%Al-10%Mg. The plating layer may also be formed on surfaces other than those corresponding to the overlapping surfaces of the steel sheets.

[0026] The chemical composition of the plating layer can be determined by dissolving the plating layer in an acidic solution to which an inhibitor that suppresses corrosion of the steel sheet has been added, and then measuring the resulting solution by ICP (inductively coupled plasma) emission spectroscopy. As the acidic solution to which the inhibitor has been added to dissolve the plating layer, for example, a 10% hydrochloric acid solution to which 0.06% by mass of inhibitor (manufactured by Asahi Chemical Industries, Ltd., Ibit 710K) has been added may be used.

[0027] The thickness of the plating layer may be, for example, 3 to 50 μm. The amount of plating layer is not particularly limited, but for example, 10 to 170 g / m² per side. 2 It may be so. The amount of plating layer attached is determined from the weight change before and after the 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 amount is determined from the weight change before and after the pickling and removal of the plating layer. After the plating layer is removed, the base steel sheet is washed with water and dried.

[0028] [High strength steel plate] One or more of the multiple steel plates are high-strength steel plates. Here, "high-strength" means that the hardness in the first region is 200 Hv or higher. The hardness of the steel plates is measured at a depth of 1 / 2 at the location of the first region of the steel plates constituting the welded joint. 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 higher, 270 Hv or higher, or 340 Hv or higher.

[0029] Here, the one or more plated steel sheets and the one or more high-strength steel sheets mentioned above may be different steel sheets, or they may be the same single high-strength galvanized steel sheet that is both high-strength and galvanized.

[0030] LME cracking occurs when molten zinc plating is present on the surface of a high-strength steel plate during welding. For example, considering a welded joint composed of two steel plates, if at least one of the two steel plates is high-strength and zinc plating is present on the overlapping surface, LME cracking may occur. Alternatively, even if one is a relatively low-strength galvanized steel plate and the other is a high-strength unplated steel plate, molten zinc plating will be present on the overlapping surface of the steel plates during welding, and the molten zinc plating will come into contact with the high-strength unplated steel plate, so LME cracking may occur. The welded joint of the present invention suppresses LME cracking during manufacturing even in such cases, and therefore may include welded joints composed of one relatively low-strength galvanized steel plate and the other high-strength unplated steel plate. Other welded joints of the present invention may include welded joints in which one is made of a relatively low-strength unplated steel sheet and the other is made of a high-strength galvanized steel sheet, or welded joints in which one is made of a relatively low-strength galvanized steel sheet and the other is made of a high-strength galvanized steel sheet, or welded joints in which both are made of high-strength galvanized steel sheets.

[0031] 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 plate thickness may be 0.2 mm or more, 0.4 mm or more, or 0.6 mm or more. The plate thickness may be 3.0 mm or less, 2.5 mm or less, 2.0 mm or less, or 1.8 mm or less.

[0032] [Chemical composition] The high-strength steel plate, among the steel plates constituting the welded joint of the present invention, has the chemical composition described below. Hereinafter, "%" in relation to the chemical composition means "mass%". Furthermore, in the numerical range of the chemical composition, the numerical range expressed using "~" means a range that includes the values ​​written before and after "~" as the lower and upper limits, respectively.

[0033] (C: 0.05~0.40%) Carbon (C) is an element that ensures the strength of steel, and the C content should be 0.05% or more. To prevent the C concentration in the surface layer from becoming too high, as described later, and to consider weldability, the C content should be 0.40% or less. The C content may be 0.08% or more, 0.10% or more, or 0.15% or more. The C content may be 0.37% or less, 0.35% or less, or 0.30% or less.

[0034] (Si: 0.5~3.0%) Silicon (Si) is an element that promotes ferrite stabilization and decarburization. The inclusion of Si, through the pre-treatment and heat treatment described later, promotes decarburization in the surface layer and stabilizes the ferrite in the surface layer, thereby improving LME resistance during the manufacture of welded joints. To achieve this effect, the Si content should be 0.5% or more. If the Si content is too high, even with high-dew-point annealing, external oxidation will progress, forming oxides (scale) on the surface of the steel sheet. Conversely, decarburization at the outermost surface will be suppressed, reducing the effect of improving LME resistance during the manufacture of welded joints. Considering this point, the Si content should be 3.0% or less. The Si content may be 0.6% or more, 0.7% or more, or 0.8% or more. The Si content may be 2.5% or less, 2.0% or less, or 1.5% or less.

[0035] (Mn: 0.1~5.0%) Manganese (Mn) is an effective element for improving the strength of steel by obtaining a hard structure. Considering the strength of the steel, the Mn content should be 0.1% or more. Also, considering the decrease in workability due to Mn segregation, the Mn content should be 5.0% or less. The Mn content may be 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.

[0036] (sol.Al: 0~3.0%) Aluminum (Al), when dissolved in steel, is an element that, like Si, promotes ferrite stabilization and decarburization. Sol.Al refers to acid-soluble Al, which is not in the form of oxides such as Al2O3 and is soluble in acid. It is determined by subtracting the insoluble residue on the filter paper generated during the Al analysis process. In the steel plates constituting the welded joints of the present invention, the role of sol.Al can also be achieved by including Si, so sol.Al is not essential, and the lower limit of the sol.Al content is 0%. If the sol.Al content is too high, even if high dew point annealing is performed, external oxidation will progress and oxides (scale) will form on the surface of the steel plate. Conversely, decarburization at the outermost surface will be suppressed, and the effect of improving LME resistance during the manufacture of welded joints will be reduced. Considering this point, the sol.Al content should be 3.0% or less. The sol.Al content may be 0.1% or more, 0.3% or more, or 0.5% or more. The sol.Al content may be 2.0% or less, 1.5% or less, or 1.0% or less.

[0037] As mentioned above, Si and sol.Al are elements that reduce LME resistance when added in excess, so it is preferable that the total content of Si and sol.Al be 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.

[0038] (P:0.0300% or less) Phosphorus (P) is a common impurity found in steel. A P content exceeding 0.0300% may reduce weldability. Therefore, the P content should be 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 P is not present, and the lower limit of the P content is 0%. From the viewpoint of dephosphorization costs, the P content may be greater than 0%, 0.0001% or more, or 0.0005% or more.

[0039] (S:0.0300% or less) S (sulfur) is a common impurity found in steel. If the sulfur content exceeds 0.0300%, weldability decreases, and furthermore, the amount of MnS precipitation increases, potentially reducing workability such as bendability. Therefore, the sulfur content should be 0.0300% or less. The sulfur content may be 0.0100% or less, 0.0050% or less, or 0.0020% or less. It is preferable that sulfur is not present, and the lower limit of the sulfur content is 0%. From the viewpoint of desulfurization costs, the sulfur content may be greater than 0%, 0.0001% or more, or 0.0005% or more.

[0040] (N:0.0100% or less) Nitrogen (N) is a common impurity found in steel. A N content exceeding 0.0100% may reduce weldability. Therefore, the N content should be 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 be absent, and the lower limit of the N content is 0%. From a manufacturing cost perspective, the N content may be greater than 0%, 0.0005% or more, or 0.0010% or more.

[0041] (O:0.0030% or less) Oxygen (O) is an element that forms oxides and reduces the workability of steel sheets. If the O content is too high, excess oxides are produced, which tends to reduce the workability of the steel sheet. Therefore, the O content should be 0.0030% or less. The O content may also 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 present, and the lower limit of the O content is 0%. From the perspective of manufacturing cost, the O content may be greater than 0%, 0.0005% or more, or 0.0010% or more.

[0042] (B: 0~0.0100%) Boron (B) is an element that enhances hardenability and improves strength, and also improves toughness by segregating at grain boundaries and reinforcing them. Therefore, it may be included as needed. Since it is not an essential element, the lower limit of B content is 0%. This effect can be obtained even with trace amounts, but if B is included, it is preferable that the B content be 0.0001% or more. Furthermore, from the viewpoint of ensuring sufficient toughness, the B content should be 0.0100% or less. The B content may be 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.

[0043] (Ti: 0~0.1500%) Titanium (Ti) precipitates as TiC during the cooling of steel and contributes to improving its strength, so it may be included as needed. Since it is not an essential element, the lower limit for Ti content is 0%. This effect can be obtained even with trace amounts, but if Ti is included, it is preferable that the Ti content be 0.0001% or more. The Ti content may be 0.0003% or more, or 0.0005% or more. On the other hand, if it is included in excess, coarse TiN may be formed, which may impair toughness, so the Ti content should be 0.1500% or less. The Ti content may be 0.1000% or less, 0.0500% or less, 0.0050% or less, or 0.0020% or less.

[0044] (Nb: 0~0.1500%) Niobium (Nb) is an element that contributes to increased strength through improved hardenability, and therefore may be included as needed. Since it is not an essential element, the lower limit for Nb content is 0%. This effect can be obtained even with trace amounts, but if Nb is included, it is preferable that the Nb content be 0.0001% or more. The Nb content may be 0.0005% or more, or 0.0010% or more. On the other hand, from the viewpoint of ensuring sufficient toughness, the Nb content should be 0.1500% or less. The Nb content may be 0.1000% or less, 0.0600% or less, or 0.0200% or less.

[0045] (V: 0~0.150%) Vanadium (V) is an element that contributes to increased strength through improved hardenability, and therefore may be included as needed. Since it is not an essential element, the lower limit for V content is 0%. This effect can be obtained even with trace amounts, but if V is included, it is preferable that the V content be 0.001% or more. The V content may be 0.003% or more, 0.005% or more, or 0.008% or more. On the other hand, from the viewpoint of ensuring sufficient toughness, the V content should be 0.150% or less. The V content may be 0.100% or less, 0.060% or less, or 0.020% or less.

[0046] (Cr: 0~2.00%) Chromium (Cr) is effective in increasing the hardenability and strength of steel, and may be included as needed. Since it is not an essential element, the lower limit for Cr content is 0%. This effect can be obtained even with trace amounts, but if Cr is included, it is preferable that the Cr content be 0.001% or more. The Cr content may be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, if it is included in excess, a large amount of Cr carbide may be formed, which may conversely impair the hardenability, so the Cr content should be 2.00% or less. The Cr content may be 1.80% or less, 1.50% or less, 0.50% or less, or 0.20% or less.

[0047] (Ni: 0~2.00%) Nickel (Ni) is effective in increasing the hardenability and strength of steel, and may be included as needed. Since it is not an essential element, the lower limit for Ni content is 0%. This effect can be achieved even with trace amounts, but if Ni is included, it is preferable that the Ni content be 0.001% or higher. The Ni content may be 0.01% or higher, 0.02% or higher, or 0.05% or higher. On the other hand, excessive Ni addition increases costs, so the Ni content should be 2.00% or lower. The Ni content may be 1.80% or lower, 1.50% or lower, 0.50% or lower, or 0.20% or lower.

[0048] (Cu: 0~2.0000%) Copper (Cu) is effective in increasing the hardenability and strength of steel, and may be included as needed. Since it is not an essential element, the lower limit for Cu content is 0%. This effect can be achieved even with trace amounts, but if Cu is included, it is preferable that the Cu content be 0.0001% or higher. The Cu content may be 0.0002% or higher, or 0.0005% or higher. On the other hand, from the viewpoint of suppressing a decrease in toughness and cracking of the slab after casting, the Cu content should be 2.0000% or lower. The Cu content may be 1.8000% or lower, 1.5000% or lower, 0.0050% or lower, or 0.0020% or lower.

[0049] (Mo: 0~1.00%) Molybdenum (Mo) is effective in increasing the hardenability and strength of steel, and may be included as needed. Since it is not an essential element, the lower limit for Mo content is 0%. This effect can be obtained even with trace amounts, but if Mo is included, it is preferable that the Mo content be 0.001% or more. The Mo content may be 0.01% or more, 0.02% or more, or 0.03% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness, the Mo content should be 1.00% or less. The Mo content may be 0.80% or less, 0.60% or less, or 0.20% or less.

[0050] (W: 0~1.000%) Tungsten (W) is effective in increasing the hardenability and strength of steel, and may be included as needed. Since it is not an essential element, the lower limit for W content is 0%. This effect can be obtained even with trace amounts, but if W is included, it is preferable that the W content be 0.001% or more. The W content may be 0.002% or more, or 0.003% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness, the W content should be 1.000% or less. The W content may be 0.800% or less, 0.600% or less, 0.300% or less, 0.100% or less, or 0.020% or less.

[0051] (Ca: 0~0.1000%) Calcium (Ca) is an element that contributes to inclusion control, particularly the fine dispersion of inclusions, and enhances toughness; therefore, it may be included as needed. Since it is not an essential element, the lower limit for Ca content is 0%. This effect can be obtained even with trace amounts, but if Ca is included, it is preferable that the Ca content be 0.0001% or more. The Ca content may be 0.0002% or more, or 0.0003% or more. On the other hand, excessive content may lead to deterioration of surface properties, so the Ca content should be 0.1000% or less. The Ca content may be 0.0800% or less, 0.0500% or less, 0.0300% or less, 0.0100% or less, or 0.0010% or less.

[0052] (Mg: 0~0.100%) Magnesium (Mg) is an element that contributes to inclusion control, particularly the fine dispersion of inclusions, and enhances toughness; therefore, it may be included as needed. Since it is not an essential element, the lower limit of Mg content is 0%. This effect can be obtained even with trace amounts, but if Mg is included, it is preferable that the Mg content be 0.0001% or more. The Mg content may be 0.0005% or more, or 0.0008% or more. On the other hand, excessive content may lead to deterioration of surface properties, so the Mg content should be 0.100% or less. The Mg content may be 0.090% or less, 0.080% or less, 0.030% or less, 0.010% or less, or 0.002% or less.

[0053] (Zr: 0~0.100%) Zr (zirconium) is an element that contributes to inclusion control, particularly the fine dispersion of inclusions, and enhances toughness; therefore, it may be included as needed. Since it is not an essential element, the lower limit of Zr content is 0%. This effect can be obtained even with trace amounts, but if Zr is included, it is preferable that the Zr content be 0.001% or more. The Zr content may be 0.005% or more, or 0.010% or more. On the other hand, excessive content may lead to deterioration of surface properties, so the Zr content should be 0.100% or less. The Zr content may be 0.050% or less, or 0.030% or less.

[0054] (Hf: 0~0.100%) Hf (hafnium) is an element that contributes to inclusion control, particularly the fine dispersion of inclusions, and enhances toughness, so it may be included as needed. Since it is not an essential element, the lower limit of Hf content is 0%. This effect can be obtained even with trace amounts, but if Hf is included, it is preferable that the Hf content be 0.0001% or more. The Hf content may be 0.0003% or more, or 0.0005% or more. On the other hand, excessive content may lead to deterioration of surface properties, so the Hf content should be 0.100% or less. The Hf content may be 0.050% or less, 0.030% or less, 0.010% or less, 0.005% or less, or 0.002% or less.

[0055] (REM: 0~0.1000%) Rare earth elements (REMs) contribute to inclusion control, particularly the fine dispersion of inclusions, and enhance toughness; therefore, they may be included as needed. Since they are not essential elements, the lower limit for REM content is 0%. While this effect can be achieved even with trace amounts, it is preferable that the REM content be 0.0001% or higher. The REM content may be 0.0003% or higher, or 0.0005% or higher. On the other hand, excessive REM content may lead to deterioration of surface properties; therefore, the REM content should be 0.1000% or lower. The REM content may be 0.0500% or lower, 0.0300% or lower, 0.0100% or lower, 0.0050% or lower, or 0.0020% or lower. Note that REM stands for Rare Earth. REM is an abbreviation for Earth Metal, and refers to elements belonging to the lanthanide series. REM is usually added as mischmetal.

[0056] In the high-strength steel plate constituting the welded joint according to the present invention, the remainder other than the above-mentioned chemical components consists of Fe and impurities. In the steel plate according to the present invention, the remainder may consist of Fe and impurities, that is, the remainder may consist only of Fe and impurities. Here, impurities refer to components that are mixed in during the industrial manufacture of steel plates due to various factors in the manufacturing process, including raw materials such as ore and scrap, and which do not adversely affect LME cracking during the manufacture of the welded joint according to the present invention.

[0057] The chemical composition of steel sheets can be analyzed using elemental analysis methods known to those skilled in the art, such as inductively coupled plasma mass spectrometry (ICP-MS). However, for C and S, combustion-infrared absorption spectrometry is used, and for N, inert gas fusion-thermal conductivity spectrometry is used. For O, inert gas fusion-infrared absorption spectrometry is used. These analyses can be performed on samples taken from steel sheets in accordance with JIS G0417:1999.

[0058] [Tensile strength] The high-strength steel plate constituting the welded joint according to the present invention may have a tensile strength of 780 MPa or higher. The present invention suppresses LME (Long Mass Emission) that occurs when manufacturing a welded joint by welding a high-strength steel plate, that is, a steel plate with high tensile strength and hardness. 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 higher. The upper limit of the tensile strength is not particularly limited, but from the viewpoint of ensuring toughness, it may be, for example, 2000 MPa or less. The tensile strength is measured by taking a JIS No. 5 tensile test specimen with the longitudinal direction perpendicular to the rolling direction and the thickness direction, and measuring it in accordance with JIS Z 2241:2011. The tensile strength may be 980 MPa or higher, or 1180 MPa or higher.

[0059] Next, the microstructure of a welded joint will be described. Here, the surface layer of the steel plate refers to a layered region having a depth of a predetermined distance from the surface of the steel plate (the surface of the steel plate) in the thickness direction of the steel plate. In the case of plated steel plates, the surface of the steel plate (the surface of the steel plate) refers to the surface of the steel plate (the surface of the steel plate) excluding the plating. The predetermined distance in the thickness direction can be the longer (deeper) of either "the depth at which the carbon concentration of the surface layer is 0.02% or less" or "the depth of the layer at which the area ratio of the ferrite phase is 90% or more," which will be described later.

[0060] <Characteristics of Domain 1>

[0061] [Interface between the plating layer and the base steel sheet, or surface roughness Ra of the steel sheet] The high-strength steel plate constituting the welded joint of the present invention has, in the first region, an arithmetic mean height Ra of greater than 3.0 μm at the interface between the plating layer and the base steel plate if the high-strength steel plate has a plating layer, or a surface roughness Ra of the high-strength steel plate if the high-strength steel plate does not have a plating layer, as defined in JIS B0601:2013. The roughness may be 3.5 μm or more in Ra. The roughness of the interface when the high-strength steel plate has a plating layer is the surface roughness Ra of the steel plate measured after removing the plating. The removal of the plating layer is performed by dissolving the plating layer in an acid solution to which an inhibitor that suppresses corrosion of the base steel plate has been added, similar to the measurement of the amount of adhesion described above.

[0062] In this invention, when measuring surface roughness Ra, 10 measurement points are randomly selected on the surface of the steel plate in accordance with JIS B 0601:2013, such 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 using the laser microscope at a magnification of 20x, and the arithmetic mean roughness (Ra) at each measurement point is determined from the captured image with a reference length of 2000 μm. The arithmetic mean value of the 10 arithmetic mean roughness (Ra) values ​​obtained at each measurement point is defined as "surface roughness Ra".

[0063] [Depth where the carbon concentration in the surface layer is 0.02% or less] In the high-strength steel plate constituting the welded joint of the present invention, the depth at which the carbon concentration measured by GDS (glow discharge spectroscopy) is 0.02% or less, from the surface of the steel plate in the thickness direction, is 3 μm or more. The starting point in the depth direction is the interface between the plating layer and the base steel plate if the high-strength steel plate has a plating layer, and the surface of the high-strength steel plate if the high-strength steel plate does not have a plating layer.

[0064] Since LME susceptibility decreases as the carbon concentration decreases, lowering the carbon concentration in the surface layer improves LME resistance. Furthermore, since carbon is an austenite-stabilizing element, a lower carbon content stabilizes the less LME-sensitive layer.

[0065] Such a surface structure (metallic structure on the surface of a steel sheet) can be obtained as a decarburized layer produced by applying the pretreatment and annealing described later to the steel sheet, with the chemical composition set as described above.

[0066] Since a depth of 8 μm or more with a C concentration of 0.02% or less contributes to improved LME resistance, there is no particular upper limit to the depth with a C concentration of 0.02% or less. The depth with a C concentration of 0.02% or less may be, for example, 50 μm or less, 40 μm or less, or 30 μm or less. Preferably, the depth with a C concentration of 0.02% or less is 10 μm or more, more preferably 12 μm or more, and even more preferably 15 μm or more, or 20 μm or more.

[0067] GDS measurement is performed at five measurement points in the thickness direction of the plate, and at each measurement point, the arithmetic mean of the depth of the region where the C concentration is 0.02% or less is taken as the depth where the C concentration of the surface layer is 0.02% or less. The five measurement points are randomly determined so that there is a distance of 5 mm or more between each measurement point on the surface of the steel plate. The measurement conditions are as follows. Naturally, measurement results can be obtained even if the measurement equipment, etc., is not exactly as described below, but if there is a difference in the measurement results, the steel plate according to the present invention will be identified by the measurement results obtained under the following conditions.

[0068] Equipment: High-frequency glow discharge emission spectrometer (manufactured by LECO Japan LLC, model number "GDS850A") Ar gas pressure: 0.3 MPa Anode diameter: 4mmφ RF output: 30W Measurement time: 200-1500 seconds

[0069] [Diffraction intensity ratio of the ferrite phase by grazing incidence X-ray diffraction (XRD)] In the high-strength steel plate constituting the welded joint of the present invention, in oblique incidence X-ray diffraction at an incident angle of 1° to the steel plate surface, the diffraction intensity corresponding to the (110) plane is measured in the I(110) and (200) planes. When the corresponding diffraction intensity is I(200) and the diffraction intensity corresponding to the (211) plane is I(211), 0.45≦I(110) / (I(110)+I(200)+I(211))≦0.90 The following condition is met.

[0070] The value of the middle term of the conditional expression, "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. Also, I(110) / (I(200)+I(211)) is preferably 0.50 or more. The conditional expression is, This means the lite phase is randomly oriented. If the ferrite phase is completely randomly oriented, the value of the middle section is 0.67.

[0071] Here, grazing incidence X-ray diffraction (also called grazing incidence XRD, low-angle incidence XRD, or tilted XRD) is a measurement technique in which the incidence angle of the incident X-rays is set to a small value, and only the detector is scanned (the detection angle is changed) while maintaining that incidence angle. This makes it possible to efficiently detect information on the orientation of crystal grains up to a depth of a few micrometers in the surface layer of the sample. In this invention, the incidence angle of the X-rays is fixed at 1° to detect the orientation of ferrite in the surface layer of the steel plate. The incidence angle is the angle between the surface of the sample (steel plate) and the incidence direction of the incident X-rays. If the high-strength steel plate has a plating layer, the plating layer is removed before measurement. The removal of the plating layer is performed by dissolving the plating layer in an acid solution to which an inhibitor that suppresses corrosion of the base steel plate has been added, similar to the measurement of the amount of adhesion described above.

[0072] Figure 2 shows examples of oblique incidence XRD analysis results for cases where the ferrite phase is randomized (b) and where it is not (a). (a) is the result of oblique incidence XRD analysis on a normal (conventional) steel plate, and it can be seen that it is oriented in the (110) direction. Therefore, the value of the middle side of the condition equation "I(110) / (I(110)+I(200)+I(211))" is relatively large at 0.91. (b) is the result of oblique incidence XRD analysis on a steel plate constituting the welded joint of the present invention, and the orientation in the (110) direction is smaller compared to (a). Therefore, the value of the middle side of the condition equation "I(110) / (I(110)+I(200)+I(211))" is relatively small at 0.58.

[0073] <Characteristics of Domain 2>

[0074] [Thickness of the high-ferrite layer (a layer with a ferrite area ratio of 90% or more)] In the high-strength steel plate constituting the welded joint of the present invention, in the second region, the thickness of the layer in the thickness direction of the base steel plate, starting from the interface between the plating layer and the base steel plate, is 15 μm or more, and the area ratio of ferrite is 90% or more (hereinafter referred to as the "high-ferrite layer").

[0075] Since a high-ferrite layer thickness of 15 μm or more contributes to LME suppression during welded joint manufacturing, there is no particular upper limit to its thickness. The thickness of the high-ferrite layer may be, for example, 100 μm or less, 80 μm or less, 60 μm or less, or 40 μm or less. Preferably, the thickness of the high-ferrite layer is 20 μm or more.

[0076] The non-ferrite structures in the high-ferrite layer are not limited. For example, they can be one or more of the following: martensite, bainite, or cementite.

[0077] The thickness of the high-ferrite layer is measured by analyzing secondary electron images obtained by SEM observation of an observation cross-section that has been mirror-finished by mechanical polishing of the thickness cross-section of the steel plate and then nital-etched. A field emission scanning electron microscope (e.g., JEOL Ltd. "JSM 7000F", acceleration voltage: 15kV) is used for SEM observation. In the observation cross-section, the observation field range is defined as a depth range of 500 μm in the thickness direction (longitudinal direction of the observation cross-section) from the surface of the steel plate (plate surface) and a width range of 600 μm in the direction perpendicular to the thickness direction (lateral direction of the observation cross-section). In this observation cross-section, SEM observation is performed on five observation fields such that there is a gap of 1000 μm or more between observation fields in the direction perpendicular to the thickness direction (lateral direction of the observation cross-section), and secondary electron images are obtained. The observation resolution is 1280 × 960 pixels. In the case of plated steel plates, the surface of the steel plate (plate surface) is the surface of the steel plate excluding the plating.

[0078] The fraction of ferrite is calculated from the five obtained secondary electron images using the point counting method. More specifically, first, an equally spaced grid is drawn on the secondary electron image. Next, the number of grid points where the structure is ferrite is determined at each grid point, and the fraction of ferrite is measured by dividing this number by the total number of grid points. The larger the total number of grid points, the more accurately the area fraction can be determined. In this invention, the grid spacing is 2 μm × 2 μm, and the total number of grid points is 1500.

[0079] In secondary electron images, regions with relatively low brightness and no visible underlying structure can be identified as ferrite. Here, underlying structure refers to older materials such as lath or block. This refers to the transformation structure formed within the austenite phase. In secondary electron images, ferrite is observed as a region with relatively low brightness and a relatively monotonous extent of brightness and color. In this invention, there is no need to distinguish between metallic structures other than ferrite, but the criteria for distinguishing between tempered martensite, pearlite, ferrite, fresh martensite, retained austenite, or bainite in secondary electron images are shown below. A region with a substructure (lath boundary, block boundary) within the grains and in which carbides precipitate with multiple variants is judged to be tempered martensite. A region in which cementite precipitates in a lamellar pattern is judged to be pearlite. A region with high brightness and in which the substructure is not revealed by etching is judged to be fresh martensite or retained austenite. A region that does not fall under any of the above is judged to be bainite. Simply put, the area ratio of the ferrite phase can be determined by distinguishing between ferrite and other structures.

[0080] Figure 3 shows an example of a microstructure image taken by SEM of the surface layer of a steel plate constituting the welded joint of the present invention, at a distance of 0 to 100 μm from the weld shoulder. In the microstructure image of Figure 3, the white areas are ferrite. It can be confirmed that the surface layer has a microstructure mainly composed of ferrite.

[0081] Ferrite has low LME susceptibility. A surface microstructure of steel sheet that is mainly composed of ferrite is preferable from the viewpoint of suppressing LME during the manufacture of welded joints. Such a surface microstructure can be obtained by setting the chemical composition of the steel sheet as described above and applying the pretreatment and annealing processes described later.

[0082] When a steel sheet has a plating layer, the starting point for the depth at which the carbon concentration is 0.02% or less and the thickness at which the ferrite area ratio is 90% or more, as measured by GDS, is the interface between the base steel sheet and the plating layer.

[0083] 《Manufacturing method》 Next, a method for manufacturing a welded joint according to the present invention will be described. First, a method for manufacturing the high-strength steel plate constituting the welded joint will be described.

[0084] The high-strength steel sheet constituting the welded joint according to the present invention can be obtained, for example, by a manufacturing method comprising a casting step of casting molten steel with adjusted chemical composition to form a steel billet, a hot rolling step of hot rolling the steel billet to obtain a hot-rolled steel sheet, a winding step of winding the hot-rolled steel sheet, a cold rolling step of cold rolling the winded hot-rolled steel sheet to obtain a cold-rolled steel sheet, a pre-treatment step of pre-treating (grit blasting) the cold-rolled steel sheet, and an annealing step of annealing the pre-treated cold-rolled steel sheet. Alternatively, the hot-rolled steel sheet may be pickled and then cold-rolled without winding after the hot-rolling step.

[0085] <Casting Process> The conditions for the casting process are not particularly limited. For example, after melting in a blast furnace or electric furnace, various secondary smelting processes may be carried out, followed by casting using methods such as conventional continuous casting or ingot casting.

[0086] <Hot rolling process> Hot-rolled steel sheets can be obtained by hot-rolling steel billets obtained by casting. The hot-rolling process is carried out by hot-rolling the cast steel billet either directly or after it has been cooled and then reheated. When reheating is performed, the heating temperature of the steel billet can be, for example, 1100 to 1250°C. In the hot-rolling process, rough rolling and finish rolling are usually performed. The temperature and reduction ratio of each rolling step can be appropriately changed according to the desired metal structure and sheet thickness. For example, the finishing temperature of finish rolling can be 900 to 1050°C, and the reduction ratio of finish rolling can be 10 to 50%.

[0087] <Winding process> Hot-rolled steel sheets can be wound at a predetermined temperature. The winding temperature can be appropriately changed according to the desired metal structure, etc., for example, 500 to 800°C is acceptable. Before or after winding, the hot-rolled steel sheet may be unwound to provide a predetermined heat treatment. Alternatively, the winding process can be omitted, and the hot-rolled steel sheet can be pickled after the hot-rolling process and then cold-rolled as described later.

[0088] <Cold rolling process> After pickling or performing other treatments on hot-rolled steel sheets, cold-rolled steel sheets can be obtained by cold-rolling them. The reduction ratio during cold rolling can be appropriately adjusted according to the desired metal structure and sheet thickness; for example, 20-80% is sufficient. After the cold-rolling process, the sheet can be cooled to room temperature, for example, by air cooling.

[0089] <Pre-treatment process> To obtain the surface structure of the steel sheet as described above, it is necessary to perform a prescribed pretreatment followed by annealing.

[0090] The pretreatment includes grit blasting the surface of the cold-rolled steel sheet using a square-shaped grit material. While there are no particular limitations on the grit material that can be used, for example, polygonal steel grit with an average particle size of 100-500 μm can be used. An example of such grit is TGD-30 manufactured by WINOA IKK JAPAN. The grit projection rate is 5-400 kg / m². 2 It would be good if this were the case. This would increase the surface roughness Ra. This process allows for the introduction of strain into the surface layer of the steel sheet. By performing this grit blasting treatment, decarburization is promoted during the annealing process described later, and a stable ferrite structure can be efficiently formed on the surface of the steel sheet. While a higher grit blasting rate increases the effect on improving LME resistance, a certain grit blasting rate, such as 400 kg / m², is sufficient. 2 Beyond a certain point, the effect saturates. The projection rate is 400 kg / m². 2 The value per unit time / unit area at this level is 4.0 × 10⁻⁶ -4 k g / (mm 2 It is min.

[0091] <Annealing process> The cold-rolled steel sheet, which has undergone pretreatment, is then annealed.

[0092] In this invention, a steel sheet that has been strained by grit blasting is subjected to an annealing process that includes holding it at a predetermined temperature and high dew point. The heating rate to the predetermined holding temperature is not particularly limited and may be 1 to 10°C / second. Dew point control is performed by humidification control from 300°C or higher and less than 600°C, preferably from 450 to 550°C. That is, the dew point (humidification) control start temperature is 300°C or higher and less than 600°C, preferably within 450 to 550°C. The dew point when dew point (humidification) control is not performed is usually below -30°C. The dew point (high dew point) during annealing is set to -30 to 20°C in order to promote decarburization. The dew point (high dew point) during annealing is preferably -10°C or higher. Also, the dew point during annealing is preferably 5°C or lower. The predetermined holding temperature (maximum heating temperature) in the annealing process is set to 750-900°C, preferably 770-870°C, in order to promote decarburization. The holding time at the holding temperature (maximum heating temperature) in the annealing process is set to 20-300 seconds, preferably 50-200 seconds. Furthermore, a non-oxidizing atmosphere is preferred, and can be, for example, N2-1~10 vol%H2 or N2-2~4 vol%H2.

[0093] By setting the dew point, holding temperature, and holding time within the above ranges, decarburization is promoted, the carbon concentration in the surface layer is reduced, and the ferrite phase fraction can be appropriately controlled. Furthermore, by setting the dew point (humidification) control start temperature within the above range, decarburization of the surface layer of the steel sheet is promoted. At the same time, internal oxidation of Si and Mn proceeds rapidly, internal oxides are rapidly formed, and the formed internal oxides function as nucleation sites, resulting in the randomization of the ferrite phase orientation. If the dew point (humidification) control start temperature is too low, external oxidation proceeds, and internal oxidation of Si and Mn does not proceed, making it difficult for the ferrite phase orientation to become randomized.

[0094] Annealing is performed under a tension of, for example, 1 to 20 MPa. Applying tension during annealing allows for more effective introduction of strain into the steel plate, promoting decarburization of the surface layer.

[0095] By performing the processes described above, decarburization is promoted in the surface layer of the steel sheet, and a high-strength steel sheet can be obtained in which the surface layer of the steel sheet has a structure mainly composed of randomly oriented ferrite phases.

[0096] <Method for manufacturing plated steel sheets> The plated steel sheet constituting the welded joint according to the present invention can be obtained by performing a plating treatment to form a plating layer containing Zn on the surface of the steel sheet. Alternatively, the plating layer may be formed on a high-strength steel sheet manufactured as described above.

[0097] <Plating process> The plating process may be carried out according to methods known to those skilled in the art. The plating process may be carried out, for example, by hot-dip plating or by electroplating. Preferably, the plating process is carried out by hot-dip plating. The conditions for the plating process may be set appropriately considering the desired chemical composition, thickness, and amount of deposition of the plating layer. After the plating process, a known alloying process may be carried out to perform alloying plating.

[0098] <Spot welding process> Multiple steel plates as described above are stacked and spot-welded to obtain a welded joint. The conditions for spot welding are not particularly limited. For example, spot welding can be performed using a dome-radius type welding electrode with a tip diameter of 5 to 10 mm, with a pressing force of 1.0 to 5.0 kN, an energizing time of 0.2 to 1.2 seconds, and an energizing current of 6 to 14 kA.

[0099] The welded joint according to the present invention suppresses LME cracking during manufacturing and can therefore be suitably used in a wide range of fields, such as automobiles, home appliances, and building materials. In particular, it can be suitably used as a joining structure for joining automobile components in the automotive field. [Examples]

[0100] The present invention will be described in more detail below with reference to examples. The present invention is not limited to these examples.

[0101] <Exam No. 1> The molten steel adjusted to the chemical composition described in Test No. 1 in Table 1 was melted in a blast furnace and cast by continuous casting to obtain a steel slab. The obtained steel slab was heated to 1200 °C and hot-rolled with the finishing rolling end temperature of 950 °C and the reduction ratio of finishing rolling 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 at a reduction ratio of 50% to obtain a cold-rolled steel sheet. The thickness of the cold-rolled steel sheet was 1.6 mm.

[0102] Next, on the surface of the obtained cold-rolled steel sheet, using TGD-30 manufactured by WINOA IKK JAPAN as a projectile, grit blasting was performed with a projection amount of 5 kg / m 2 by projection. The surface roughness Ra of the cold-rolled steel sheet after grit blasting was set to 3.3 μm.

[0103] Subsequently, the cold-rolled steel sheet subjected to grit blasting was heated in a furnace with an oxygen concentration of 20 ppm or less in an N2-4%H2 gas atmosphere at a heating rate of 6.0 °C / second up to 500 °C, and further heated at a heating rate of 2.0 °C / second up to 800 °C and held for 40 seconds for annealing treatment. At this time, the control of the dew point was started so that the dew point became 0 °C from 300 °C. The annealing treatment was performed with a tension of 5.0 MPa applied to the steel sheet.

[0104] Furthermore, the annealed steel sheet was immersed in a molten zinc plating bath (Zn-0.14%Al) at 450 °C for 3 seconds, then pulled out at 100 mm / second, and the plating adhesion amount was controlled to 5 0 g / m 2 by N2 wiping gas. Then, an alloying treatment was performed at 520 °C for 30 seconds to obtain an alloyed molten zinc plated steel sheet.

[0105] The obtained alloyed hot-dip galvanized steel sheet and one sheet of the same type of steel sheet were stacked together, and spot welding was performed using a dome radius welding electrode with a tip diameter of 8 mm, at a striking angle of 2°, a pressing force of 4.0 kN, an energizing time of 0.8 seconds, and an energizing current of 12 kA to manufacture a welded joint, and the LME resistance during manufacturing was evaluated. In the following, the steel sheet manufactured in the example will be referred to as the "example steel sheet," and the steel sheet combined during the manufacturing of the welded joint will be referred to as the "counterfeit steel sheet."

[0106] <Exam No. 2-55> Steel sheets or plated steel sheets were manufactured under the same conditions as in Example 1, except that the chemical composition of the steel sheet was as described in Table 1 or Table 2, the conditions for the pretreatment process and annealing process were as described in Table 3, and the plating conditions were as described in Table 4. In Test No. 32, the grit blasting treatment was omitted, and in Test No. 35, surface treatment by grinding with a brush was performed instead of grit blasting. In Table 4, "a" means alloyed hot-dip galvanizing, "b" means hot-dip galvanizing with the alloying treatment omitted as in Example 1, "c" means the plating bath is Zn-1.5%Al-1.5%Mg with the alloying treatment omitted, and "unplated" means cold-rolled steel sheet that has not undergone plating treatment. Welded joints were manufactured using the same welding conditions as in Test No. 1, in combination with the steel sheet described in Table 4. "Same type" for the mating material indicates that the same type of steel sheet as in the example was used as the mating material. Furthermore, "Unplated Same Type" indicates that the mating steel sheet used was the same type of steel sheet as in the example but without plating, while "GA Same Type" indicates that the same type of steel sheet as the steel sheet of the test number (test no.) was alloyed zinc-plated. Additionally, "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, and "GA590" indicates that a commercially available alloyed hot-dip galvanized steel sheet with a tensile strength of 590 MPa was used as the mating steel sheet.

[0107] [Table 1]

[0108] [Table 2]

[0109] [Table 3]

[0110] (Surface roughness after pretreatment) The "surface roughness after pretreatment" listed in Table 3 refers to the surface roughness Ra of the steel sheet, measured after the pretreatment process and before the annealing process, in accordance with JIS B 0601:2013.

[0111] Rating AA: 3.5 μm or larger Rating A: Greater than 3.0 μm, less than 3.5 μm Rating B: 3.0 μm or less

[0112] The following evaluations were performed on welded joints manufactured using spot welding.

[0113] (1st area) The hardness of the example steel plate was measured in the first region of the welded joint using the method described above. The hardness of the steel plate was measured at a depth of 1 / 2 of the example steel plate, in accordance with JIS Z 2244:2009. The measurement load was 200 gf. The hardness was evaluated as follows.

[0114] Rating AAA: 340Hv or higher Rating AA: 270Hv or higher, less than 340Hv Rating A: 200 Hv or higher, less than 270 Hv

[0115] Furthermore, for unplated steel sheets, the surface roughness Ra of the steel sheet was measured, and for plated steel sheets, the plating was removed and the surface roughness Ra of the exposed base steel sheet was measured using the same method as before annealing. Plating removal was performed by dissolving the plating layer in a 10% hydrochloric acid solution to which a 0.06% by mass inhibitor (Asahi Chemical Industries, Ltd., Ibit 710K) was added to suppress corrosion of the base steel sheet.

[0116] Furthermore, a 30mm x 30mm sample was taken from the first region of the welded joint, and five GDS measurements were performed in the thickness direction using the method described above to determine the depth at which the carbon concentration was 0.02% or less. In Table 4, "C ≤ 0.02% depth" is the average value of the depth at which the carbon concentration was 0.02% or less, as determined by the five GDS measurements.

[0117] Furthermore, samples cut into 20mm x 20mm sections were taken, and the incident angle of X-rays was set to 1°. The diffraction intensity of obliquely incident X-rays I(110) from the surface of the (110) side of the steel plate at an incident angle of 1° was measured, and the diffraction intensity of the (200) side of the steel plate was measured. Obliquely incident X-ray diffraction intensity I(200) at an incident angle of 1° from the surface, (211) from the surface of the steel plate The grazing incidence X-ray diffraction intensity I(211) was measured at an incidence angle of 1°. In Table 4, "grazing incidence XRD" refers to the value of I(110) / (I(110)+I(200)+I(211)). For plated steel sheets, see above. Similarly, the plating layer was dissolved in a 10% hydrochloric acid solution to which a 0.06% by mass inhibitor (Asahi Chemical Industries, Ltd., Ibit 710K) was added to suppress corrosion of the base steel plate. After removing the plating layer, the grazing incidence X-ray diffraction intensity was measured.

[0118] (Second area) Samples were taken from the second region of the welded joint, cut to 25 mm x 15 mm, and subjected to Nital etching. The T-section of each sample was observed using SEM, and the thickness of the layer with a ferrite phase area ratio of 90% or more (high-ferrite layer) was measured. The thickness was measured at five equally spaced points in the T-direction within a range of 500 μm, and the average value was used. Here, the starting point for "thickness" is the surface of the steel sheet in the case of unplated steel sheets, and the interface between the plating layer and the base steel sheet in the case of plated steel sheets.

[0119] (LME resistance) Refer to Figure 4 to explain the evaluation method for LME resistance. LME resistance is evaluated using two steel plates. Two steel plates 1 were overlapped and spot-welded, and the length of the LME crack (crack 11 directly outside the pressure-welded area) that occurred directly outside the pressure-welded area of ​​the formed spot-welded area 2 was evaluated. The two steel plates 1 refer to the steel plate of each test number (test No.) and the steel plate of the mating material. The area directly outside the pressure-welded area of ​​the weld refers to the area outside the pressure-welded area 6, which is the part that is pressure-welded by spot welding on the overlapping surface of the two steel plates, and is located in the vicinity of the pressure-welded area 6 (a range of about 1 mm outward from the end of the pressure-welded area 6). The length of the crack 11 directly outside the pressure-welded area was evaluated. The spot-welding test was performed three times, and the one with the longest crack length directly outside the pressure-welded area 11 was evaluated. The evaluation criteria were as follows. In this example, an evaluation of A or higher (i.e., evaluation A, AA, AAA) was judged to be excellent in LME resistance.

[0120] Rating AAA: 0μm Rating AA: Greater than 0 μm, less than 60 μm Rating A: 60 μm or larger, less than 120 μm Rating B: 120 μm or larger

[0121] The results of each evaluation are shown in Table 4.

[0122] [Table 4]

[0123] Tests No. 1-25 and 37-55 are embodiments of the present invention and demonstrated high resistance to LME.

[0124] In Test No. 32, grit blasting was not performed, so no strain was introduced to the surface. Therefore, decarburization was not promoted during the annealing of the steel sheet, resulting in a shallower depth at which the carbon concentration was 0.02% or less in GDS measurements, and a smaller thickness at which the ferrite phase accounted for 90% or more. Furthermore, it is thought that internal oxidation of Si and Mn did not progress, and the orientation of the ferrite phase did not become randomized. As a result, the LME suppression during the manufacturing of welded joints was inferior.

[0125] In Test No. 35, pretreatment using brush grinding was performed instead of grit blasting, so strain was not introduced into the surface layer. Therefore, it is thought that the thickness at which the ferrite phase accounts for more than 90% was reduced. In addition, it is thought that internal oxidation of Si and Mn did not progress, and the orientation of the ferrite phase did not become randomized. As a result, the LME suppression during welded joint manufacturing was inferior.

[0126] In Test No. 36, the dew point control start temperature during the annealing process was low, which likely led to excessive external oxidation and insufficient decarburization. This resulted in a shallower depth at which the carbon concentration was 0.02% or less, as measured by GDS. Furthermore, internal oxidation of Si and Mn did not progress, and the orientation of the ferrite phase was not randomized. As a result, the LME resistance during the manufacturing of welded joints was poor. [Industrial applicability]

[0127] According to the present invention, it is possible to provide a welded joint that suppresses LME during manufacturing, and this welded joint can be suitably used in applications such as automobiles, home appliances, and building materials, particularly for automobiles. Therefore, the present invention has extremely high industrial applicability. [Explanation of Symbols]

[0128] 1 steel plate 2 Spot welds 3. Indentation Section 4. Weld shoulder 4a Outer end of weld shoulder 5 nuggets 6. Press-fit portion 11. Cracks directly outside the pressure-welded area 21 First area 22 Second area

Claims

1. Multiple overlapping steel plates, The spot welds that join the plurality of steel plates, A welded joint comprising, The spot welded portion has a nugget, an indentation portion that is compressed by the electrode, and a weld shoulder portion which is the peripheral edge of the indentation portion. One or more of the plurality of steel plates is a plated steel plate having a Zn-containing plating layer formed on at least the surface corresponding to the overlapping surface of the plurality of steel plates, Of the plurality of steel plates, one or more steel plates constituting the overlapping surface are high-strength steel plates in which the hardness of the first region, which is a surface region located 5 mm or more away from the outer end of the nugget in the direction in which the surface of the steel plate expands toward the outside of the nugget, is 200 Hv or more. The aforementioned high-strength steel plate has a chemical composition in mass%, C: 0.05-0.40%, Si: 0.5-3.0%, Mn: 0.1 to 5.0%, Sol. Al: 0-3.0%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0100% or less, B: 0 to 0.0100%, Ti: 0 to 0.1500%, Nb: 0 to 0.150%, V: 0 to 0.150%, Cr: 0-2.00%, Ni: 0-2.00%, Cu: 0-2.0000%, Mo: 0-1.00%, W: 0-1.000%, Ca: 0-0.1000%, Mg: 0-0.100%, Zr: 0 to 0.100%, Hf: 0-0.100%, REM: 0~0.1000% It contains, with the remainder being Fe and impurities. In the first region, The surface roughness Ra of the aforementioned high-strength steel plate is greater than 3.0 μm. In the thickness direction from the surface of the aforementioned high-strength steel plate, the depth at which the C concentration is 0.02% or less is 8 μm or more. In oblique incidence X-ray diffraction at an incidence angle of 1° onto the surface of the high-strength steel plate, when the diffraction intensity corresponding to the (110) plane is I(110), the diffraction intensity corresponding to the (200) plane is I(200), and the diffraction intensity corresponding to the (211) plane is I(211), Satisfying 0.45 ≤ I(110) / (I(110) + I(200) + I(211)) ≤ 0.90, and, In the second region, which is a surface region of the high-strength steel plate, located at a distance of 0 to 100 μm toward the outside of the spot weld in the direction in which the plate surface of the high-strength steel plate expands from the weld shoulder, In the thickness direction from the surface of the high-strength steel plate, the thickness of the layer in which the area ratio of ferrite is 90% or more is 15 μm or more. However, if the steel sheet is a plated steel sheet, the surface of the steel sheet shall be the interface between the steel sheet and the plating layer of the plated steel sheet. A welded joint characterized by the following features.

2. The welded joint according to claim 1, characterized in that in the first region, 0.50 ≤ I(110) / (I(110) + I(200) + I(211)) ≤ 0.

75.

3. The welded joint according to claim 1, characterized in that the depth at which the C concentration in the first region is 0.02% or less is 15 μm or more.

4. The welded joint according to claim 1, characterized in that the surface roughness Ra in the first region is 3.5 μm or more.

5. The welded joint according to claim 1, characterized in that, in the second region, the thickness of the layer having an area ratio of 90% or more of ferrite is 20 μm or more.

6. A joining structure for automobile components, wherein the welded joint according to any one of claims 1 to 5 is used to join automobile components when the plurality of steel plates are used as automobile components.

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