Resistance spot welded joint, method for manufacturing same, and method for evaluating peel strength of welded joint

Abstract

The purpose of the present invention is to provide: a resistance spot welded joint; a method for manufacturing the same; and a method for evaluating the peel strength of the welded joint. The present invention is a resistance spot welded joint having a nugget formed on a steel plate mating surface in a plate set in which a plurality of steel plates including at least one high-strength steel plate are stacked, a corona bond, and a heat-affected zone, wherein: the high-strength steel plate has a tensile strength of 980 MPa or greater; the decarburized layer thickness d (µm) of a surface layer of the high-strength steel plate and the C content X (mass%) at a position of 10 µm in the plate thickness direction from a steel plate surface of the high-strength steel plate satisfy formula (1); and the average hardness in a region from the corona bond to 30 µm in the plate thickness direction is lower than the average hardness of a base material steel plate by 10% or more. (1): 10 < d(1−X) < 300

Description

Resistance spot welded joint, method for manufacturing the same, and method for evaluating the peel strength of a welded joint.

[0001] This invention relates to resistance spot welded joints and methods for manufacturing resistance spot welded joints. Furthermore, this invention relates to a method for evaluating the delamination strength of welded joints.

[0002] In recent years, the automotive sector has seen increased use of high-strength steel plates to improve fuel efficiency through weight reduction and enhance vehicle safety during collisions. However, with the increased strength of steel plates, concerns have arisen regarding a decrease in the delayed fracture resistance of welds due to residual stress caused by welding and the presence of hydrogen that penetrates the welds during welding and in the operating environment.

[0003] As a method to prevent delayed fracture of such welded joints, the techniques described in Patent Documents 1 to 3 can be cited.

[0004] Patent Document 1 discloses a technique in which, after forming a nugget by applying current, a post-current is applied to maintain a high-temperature environment that facilitates hydrogen diffusion. This allows hydrogen that entered during welding to be expelled to the outside, preventing delayed fracture.

[0005] Patent Document 2 discloses a technique that improves delayed fracture resistance by softening the area between the steel plate pressure-welded portion and the end of the nugget through a two-stage energizing process, which involves first forming a nugget by energizing, and then energizing again for heating purposes.

[0006] Patent Document 3 discloses a technique in which, after forming a nugget by applying current, a post-current is applied, and the raising and lowering of the welding electrode is controlled to repeat alternately and continuously. This reduces tensile residual stress and improves delayed fracture resistance.

[0007] Japanese Patent Publication No. 2019-72764, International Publication No. 2014 / 171495, Japanese Patent Publication No. 2018-144098

[0008] The hydrogen ingress paths that cause delayed fracture can be mainly classified into two types: (a) oil, dirt, etc. on the steel plate surface are decomposed by the heat during welding, and (b) ingress from the use environment during the process of using the product after welding. In the case of (a) above, hydrogen diffuses within several days after welding, and this hydrogen accumulates at the stress concentration part to cause delayed fracture, so this is called "initial crack". In contrast, in the case of (b) above, it takes a relatively long period of time until cracks occur.

[0009] The technologies of Patent Documents 1 to 3 described above basically assume cracking caused by hydrogen entering from the use environment as in (b) above. Therefore, there are cases where the resistance to delayed fracture (i.e., initial crack) occurring particularly immediately after welding is insufficient. Also, since solving the initial crack is effective in improving the delayed fracture resistance characteristics, improvement of the characteristics is also required.

[0010] The present invention has been made in view of such circumstances, and an object thereof is to provide a resistance spot welding joint excellent in resistance to initial crack (specifically, initial crack resistance characteristics of the welded part) and a method for manufacturing the same. Another object of the present invention is to provide a method for evaluating the peel strength of a welded joint that can evaluate the above resistance.

[0011] The inventors of the present invention used various resistance spot welding joints obtained by resistance spot welding a plate stack in which two or more steel plates including at least one high-strength steel plate were overlapped, and studied particularly techniques for suppressing initial crack. As a result, it was found that by appropriately controlling the thickness and carbon (C) distribution of the decarburized layer existing on the surface layer of the high-strength steel plate, a welded part with improved initial crack resistance characteristics can be obtained.

[0012] Specifically, the following findings were obtained. Cracks due to initial crack progress in the order of corona bond and nugget. It was found that by controlling the thickness and carbon (C) distribution of the decarburized layer in the high-strength steel plate used for the plate stack, the hardness in the region from the corona bond to the 30 μm position in the plate thickness direction decreases. As a result, it was found that it is possible to increase the peel strength of the corona bond formed around the nugget, and as a result, the initial crack resistance characteristics of the welded part are improved.

[0013] The present invention is based on the above findings, and its gist is as follows: [1] A resistance spot welded joint having a nugget formed on the joint surface of steel plates in a plate assembly in which a plurality of steel plates, including at least one high-strength steel plate, are stacked, and a corona bond and heat-affected zone formed around the nugget, wherein the high-strength steel plate has a tensile strength of 980 MPa or more, the thickness d (μm) of the decarburized layer on the surface of the high-strength steel plate and the C content X (mass%) at a position 10 μm from the steel plate surface in the thickness direction of the high-strength steel plate satisfy equation (1), and the average hardness in the region from the corona bond to 30 μm in the thickness direction of the plate is reduced by 10% or more compared to the average hardness of the base steel plate. 10 < d × (1 - X) < 300 …(1) [2] The resistance spot welded joint according to [1], wherein the peel strength of the corona bond is 0.05 kN or more. [3] The peel strength is the value of the maximum load before a decrease in load occurs in the load and displacement curve obtained in the load and displacement curve obtained in the tensile test, obtained in the resistance spot welded joint described in [2] above, wherein L-shaped test pieces are welded opposite each other, and then a tensile test is performed on the welded test pieces. [4] A method for manufacturing a resistance spot welded joint described in any one of [1] to [3] above, comprising a welding step of sandwiching a plate assembly made by overlapping a plurality of steel plates including at least one high-strength steel plate with a pair of welding electrodes, and applying pressure and current with the pair of welding electrodes, wherein the high-strength steel plate is a steel plate that satisfies formula (1), and in the welding step, the current value I １ (kA) and energizing time t １ A method for manufacturing a resistance spot welded joint, wherein the current is applied under the condition that (sec) satisfies equations (2) and (3). 10 < d × (1 - X) < 300 …(1) 2.0 < I １ <10.0...(2) 0.02<t １ <1.50 …(3) Here, d in equation (1) is the thickness of the decarburized layer on the surface of the high-strength steel plate (μm), and X is the C content (mass%) at a position 10 μm from the surface of the steel plate in the thickness direction. [5] The welding process is performed following the main energization, with energizing current value I ２ (kA) and energizing time t ２The method for manufacturing a resistance spot welding joint according to [4] above, wherein post-energization is performed under conditions satisfying equations (4) and (5). I １ -d × (1 - X) / 20 < I ２ < I １ -d × (1 - X) / 80...(4) 0.1 × I ２ < t ２ < 0.5 × I ２ ...(5) Here, I shown in equation (4) １ is the energization current value (kA) of the main energization, and X is the C content (mass%) at a position 10 μm in the plate thickness direction from the steel plate surface of the high-strength steel plate. [6] In the welding step, after the main energization, the post-heat treatment is performed under conditions where the furnace temperature T (°C) and the in-furnace time t ３ (min) satisfy equations (6) and (7). The method for manufacturing a resistance spot welding joint according to [4] above. 70 ≤ T ≤ 300...(6) 5 ≤ t ３ ≤ 30...(7) [7] In the welding step, after the post-energization, the post-heat treatment is performed under conditions where the furnace temperature T (°C) and the in-furnace time t ３ (min) satisfy equations (6) and (7). The method for manufacturing a resistance spot welding joint according to [5] above. 70 ≤ T ≤ 300...(6) 5 ≤ t ３ ≤ 30...(7) [8] After the welding step, there is an evaluation step of evaluating the peel strength of the corona bond of the obtained resistance spot welding joint. In the evaluation step, test pieces bent in an L shape are welded in a facing manner, and then a tensile test is performed on the welded test pieces. The value of the maximum load before load reduction occurs in the load and displacement curve obtained in the tensile test is taken as the peel strength of the corona bond. The method for manufacturing a resistance spot welding joint according to any one of [4] to [7] above. [9] A method for evaluating the peel strength of a welded joint, wherein test pieces bent in an L shape are welded in a facing manner, and then a tensile test is performed on the welded test pieces. The value of the maximum load before load reduction occurs in the load and displacement curve obtained in the tensile test is taken as the peel strength of the corona bond.

[0014] According to the present invention, the peel strength of the corona bond in the welded joint can be increased. This makes it possible to obtain a resistance spot welded joint with particularly excellent resistance to initial cracking, thus providing significant industrial benefits.

[0015] Figure 1 is a partially enlarged view of a resistance spot welded joint according to one embodiment of the present invention. Figure 2 is a diagram illustrating a method for measuring the average hardness of corona bond in the present invention. Figure 3 is a cross-sectional view illustrating a method for manufacturing a resistance spot welded joint according to one embodiment of the present invention. Figure 4 is a diagram showing an example of a tensile test specimen used for evaluating the peel strength of the present invention. Figure 5 is an example of a load-displacement graph obtained in a tensile test in an embodiment of the present invention. Figure 6 is an example of GDS (glow discharge emission spectroscopy) measurement results in an embodiment of the present invention. Figure 7 is a diagram showing an example of a gap test specimen used for evaluating the initial cracking characteristics of the present invention.

[0016] The present invention will be described below. However, the present invention is not limited to these embodiments.

[0017] [Resistance Spot Welded Joint] First, the resistance spot welded joint of the present invention will be described with reference to Figure 1. Figure 1 shows, as an example, a cross-sectional view in the thickness direction of the resistance spot weld and its surroundings in the resistance spot welded joint of the present invention.

[0018] The present invention relates to a resistance spot welded joint (hereinafter referred to as "welded joint") having a resistance spot welded section formed by resistance spot welding a plate assembly of multiple overlapping steel plates. The multiple overlapping steel plates include at least one high-strength steel plate, as described later. This "multiple steel plates" refers to two or more steel plates. There is no particular upper limit to the number of overlapping steel plates, but it is preferable to use four or fewer plates.

[0019] The example shown in Figure 1 is a welded joint 10 formed by joining two overlapping steel plates, where high-strength steel plates are used for the lower steel plate (i.e., bottom steel plate) 1 and / or the upper steel plate (i.e., top steel plate) 2. In this example, high-strength steel plates are used for both the bottom steel plate 1 and the top steel plate 2. A resistance spot weld 4, which will be described later, is formed on the steel plate joining surface 7 in this plate assembly. Although high-strength steel plates may have a plating layer, as will be described later, Figure 1 shows a steel plate without a plating layer.

[0020] [Resistance Spot Weld] The resistance spot weld (hereinafter referred to as "weld") in the welded joint of the present invention will be described in detail. As shown in Figure 1, the weld 4 has a nugget 4a formed on the steel plate mating surface 7, a corona bond 4b formed in a ring shape around the outside of the nugget 4a, and a heat-affected zone (HAZ) 4c formed around the outside of the corona bond 4b. The corona bond is the HAZ around the nugget, and is the boundary where the upper and lower steel plates are joined by pressure welding.

[0021] In this invention, the average hardness of the region from the corona bond 4b to 30 μm in the thickness direction is reduced by 10% or more compared to the average hardness of the base steel sheet. That is, the average hardness of the corona bond in that region is less than or equal to "average hardness of the base steel sheet × 0.90". The inventors have confirmed that this causes a stress relaxation effect on tensile residual stress, thereby improving the initial cracking characteristics of samples with low average hardness of the corona bond under equivalent welding and hydrogen penetration conditions. The "region from the corona bond to 30 μm in the thickness direction" refers to the region (measurement region) 11 enclosed by a rectangular frame outside the nugget 4a in the example shown in Figure 2. Preferably, the average hardness of the corona bond in that region is less than or equal to "average hardness of the base steel sheet × 0.75".

[0022] There is no specific lower limit for the average hardness of the area of ​​the corona bond. From the viewpoint of ensuring the strength of the welded joint, the average hardness of the area of ​​the corona bond is preferably 0.30 times the average hardness of the base steel plate and more preferably 0.40 times the average hardness of the base steel plate and more.

[0023] Here, the "average hardness in the region from the corona bond to 30 μm in the thickness direction" is measured by a Vickers hardness test. Specifically, as shown in Figure 2, in the lower steel plate 1 and the upper steel plate 2, a load of 10 g is applied in a direction parallel to the corona bond 4b, with a width of 50 μm, at positions 10 μm and 30 μm in the thickness direction from the corona bond 4b, and the average hardness is measured. In Figure 2 and the embodiments described later, as an example, the starting points for each measurement are the locations on the corona bond end F at the above positions of 10 μm and 30 μm in the thickness direction of each steel plate, and the hardness is measured at 10 points in the direction of the nugget end E. The number of measurement points can be adjusted as appropriate according to the corona bond width (i.e., the range from point E to point F in Figure 2). The measurement points should be adjusted so that they do not extend beyond the nugget end E, i.e., do not enter the inside of the nugget 4a.

[0024] Furthermore, the "average hardness of the base steel plate" mentioned above refers to the average hardness measured at five locations (plate thickness × 1 / 4) from the surface of the base steel plate in the thickness direction for both the lower steel plate 1 and the upper steel plate 2.

[0025] <Peel Strength of Corona Bond> In the present invention, it is preferable that the peel strength of Corona Bond 4b is 0.05 kN or higher. Increasing the peel strength of Corona Bond 4b makes it more difficult for cracks to form inside the nugget surrounded by Corona Bond, and as a result, crack propagation to the nugget is suppressed and the initial crack resistance is improved. From the viewpoint of effectively obtaining this effect, the lower limit of the peel strength is set to 0.05 kN. The peel strength is more preferably 0.07 kN or higher, and even more preferably 0.10 kN or higher. In the present invention, there is no particular upper limit specified for the peel strength, but from the viewpoint of preventing remelting of the nugget and ensuring the strength of the steel plate, the peel strength is preferably 3.0 kN or lower, and more preferably 2.5 kN or lower.

[0026] <Method for evaluating the peel strength of Corona Bond> The "peel strength of Corona Bond 4b" mentioned above is a value evaluated by the following method.

[0027] In this invention, L-shaped test pieces are welded together in an opposing manner, and then a tensile test is performed on the welded test pieces. The value of the maximum load before a decrease in load occurs in the load and displacement curve obtained from the tensile test is defined as the "peel strength of the corona bond."

[0028] Here, with reference to Figures 4 and 5, the method for evaluating the delamination strength of corona bonds in welded joints will be explained in detail. Figure 4 shows an example of a tensile test specimen used in the evaluation method for corona bond delamination strength. Figure 5 shows an example of a load-displacement graph obtained from a tensile test.

[0029] First, prepare multiple steel plates to be used in the plate assembly. For example, when using two steel plates to form a plate assembly, welded steel plates of predetermined dimensions are made from each of the two steel plates. As shown in Figure 4, a steel plate cut to 100 mm x 30 mm is bent vertically along the 30 mm portion of the longer side to create a welded steel plate with a weld surface and a gripping portion for tensile testing. The 30 mm portion of the longer side of the steel plate becomes the "welded surface," and the area excluding this weld surface and the vertically bent portion becomes the "gripping portion." In the example in Figure 4, the length of the gripping portion is 65 cm.

[0030] Next, the obtained welded steel plates are stacked on top of each other and resistance spot welded under predetermined conditions to produce tensile test specimens. For example, two welded steel plates are arranged as shown in Figure 4, and the center of the weld surface is welded. At this time, welding conditions are used to obtain a molten area with a diameter of 3 mm to 5 mm for each type of steel, and tensile test specimens are produced.

[0031] Next, a tensile test is performed immediately after welding is complete and before the hydrogen present inside the weld of the tensile test specimen escapes. Specifically, the tensile test is performed within 5 minutes after welding. The key point of the evaluation method of the present invention is that it is performed immediately after welding and before the hydrogen present inside the weld of the tensile test specimen escapes. Performing the test at this timing allows for the measurement of the corona bond peel load at the time when the amount of diffusible hydrogen inside the weld is at its highest immediately after welding. As more time passes, hydrogen diffuses from inside the weld to the outside, making it difficult to accurately measure the peel strength. The conditions for the tensile test are preferably a tensile speed of 8 to 15 mm / min.

[0032] Next, a load-displacement graph is created using the values ​​obtained from the tensile test. An example of such a graph is shown in Figure 5. As shown in Figure 5, the load increases steadily as the tensile displacement of the sample increases. However, as the corona bond delaminates, the bonding area suddenly decreases, causing the load to drop sharply. Since the load just before the corona bond delaminates is higher than the load after delamination, this represents the maximum load up to the point of corona bond delamination. Therefore, in this invention, the maximum load just before the sharp drop in load occurs is evaluated as the "corona bond delamination strength".

[0033] Here, "Condition 1" shown in Figure 5 refers to the condition where the tension is stopped midway at approximately 0.09 kN, and "Condition 2" refers to the condition where the same sample as in Condition 1 is used, but the tension is stopped midway at approximately 0.13 kN under the same tension conditions.

[0034] In the example shown in Figure 5, the maximum load of 0.13 kN, which is just before a sudden load drop occurs, is evaluated as the corona bond delamination strength. Here, "sudden load drop" refers to a load drop of 0.05 kN or more.

[0035] [Panel Assembly] In this invention, controlling the thickness of the decarburized layer (see reference numeral 1a in Figure 1) and the carbon (C) distribution in the high-strength steel sheet used in the panel assembly is important. As described above, two or more steel sheets are stacked to form a panel assembly. The steel sheet used in the panel assembly includes at least one high-strength steel sheet. The high-strength steel sheet of this invention is a steel sheet with a tensile strength of 980 MPa or more. Furthermore, when the thickness of the decarburized layer on the surface of the high-strength steel sheet is d (μm) and the C content at a position 10 μm from the surface of the high-strength steel sheet in the thickness direction is X (mass%), then d and X satisfy the following relationship (1): 10 < d × (1 - X) < 300 ... (1) Note that the "surface layer of the high-strength steel sheet" above refers to the region in the thickness direction from the surface of the base steel sheet that shows a %C lower than the average %C of the base steel sheet.

[0036] <Decarburized layer and carbon content of high-strength steel sheet surface> The above-mentioned "thickness of the decarburized layer on the surface of the high-strength steel sheet" and "carbon content at a position 10 μm from the steel sheet surface in the thickness direction (hereinafter sometimes referred to as "carbon content at a position 10 μm below the surface")" are measured using GDS (glow discharge emission spectroscopy). These measurement methods will be explained with reference to Figure 6. Figure 6 shows an example of GDS measurement results.

[0037] In this invention, the "thickness of the decarburized layer on the surface of the high-strength steel sheet" (see Figure 1) refers to the distance from the surface of the steel sheet until the C concentration matches the bulk C concentration (i.e., the C concentration becomes steady). Specifically, as shown in Figure 6, it is the thickness in the plate direction (unit: μm) from the surface of the steel sheet to the position where the C concentration becomes steady.

[0038] This "C concentration" is measured using the following method. First, the C concentration is measured from the surface of the steel plate in the thickness direction using GDS. In this case, the surface of the steel plate is set to 0 μm. Next, the graph shown in Figure 6 is created using the values ​​obtained in this way.

[0039] Furthermore, in this invention, "C content at a position 10 μm below the surface" refers to the C concentration at a depth of 10 μm from the surface of the steel plate in the thickness direction. Since the C concentration on the surface of the steel plate fluctuates greatly, the C concentration at a depth of 10 μm is used for evaluation. This C concentration is measured using the same method as described above for the decarburized layer thickness.

[0040] As described above, when the relationship between the decarburized layer thickness (d) and the carbon content (X), expressed as "d × (1 - X)", satisfies the relationship in equation (1), the strength of the corona bond formed around the nugget is improved, and the resistance to initial cracking is improved. If the value of "d × (1 - X)" is 10 or less, the thickness of the decarburized layer on the surface of the steel sheet is insufficient, and the carbon concentration on the surface of the steel sheet is too high, causing the hardness around the corona bond to increase, and as a result, the effect of improving the peel strength of the corona bond is not sufficiently obtained. Therefore, the value of "d × (1 - X)" should be greater than 10. Preferably, the value of "d × (1 - X)" should be greater than 15, and more preferably greater than 18.

[0041] However, if the value of "d × (1 - X)" is 300 or more, the region where the C concentration is lower than that of the base steel plate becomes wider, and the fatigue strength of the steel plate cannot be guaranteed. For this reason, the value of "d × (1 - X)" should be less than 300. Preferably, the value of "d × (1 - X)" should be less than 280.

[0042] <Preferred Components of High-Strength Steel Sheets> High-strength steel sheets having the above-mentioned tensile strength (TS), etc., preferably contain the following components. Unless otherwise specified, the "%" in the component composition described below refers to "mass%".

[0043] C: 0.10-0.30% C is used for microstructure control to achieve a TS of 980 MPa or higher in steel plates and for controlling the hardness of nuggets. If the C content is less than 0.10%, it becomes difficult to achieve a TS of 980 MPa or higher. On the other hand, if the C content exceeds 0.30%, the hardness inside the nugget increases, making it difficult to suppress nugget fracture. Therefore, the C content should be 0.10-0.30%. Preferably, the C content should be 0.13% or higher. Also, preferably, the C content should be 0.25% or lower.

[0044] Si: 0.1–5.0% Si is a useful element as a deoxidizing agent and contributes to increased fatigue strength through solid solution strengthening. However, adding more than 5.0% Si deteriorates hot workability, making manufacturing difficult, and also reduces toughness. Furthermore, it increases the likelihood of void formation due to brittle cracking during spot welding. Therefore, the Si content should be 5.0% or less. Since a Si content of 0.1% or more is necessary for deoxidation, the lower limit for Si is set at 0.1%. Thus, the Si content should be 0.1% or more. In addition, considering refining costs, oxidation resistance, and corrosion resistance, a Si content of 0.5% to 3.0% is desirable.

[0045] Mn: 0.8–5.0% Mn is an element added as a deoxidizing agent and also serves as an element that stably forms the austenite phase in place of Ni. If Mn is added in excess, the austenite phase will soften and will not provide resistance to fatigue crack propagation, so the upper limit is set at 5.0%. Therefore, the Mn content should be 5.0% or less. If the Mn content is too low, the hardenability of the steel sheet cannot be guaranteed, so the Mn content should be 0.8% or more. Furthermore, considering oxidation resistance and pickling properties during manufacturing, a Mn content of 1.0–4.5% is desirable.

[0046] P: 0.001 to 0.010% P is contained as an impurity, which degrades hot workability and toughness during manufacturing, and is prone to void formation due to brittle cracking during spot welding. Therefore, the upper limit of P is set at 0.010%. However, excessive reduction leads to increased refining costs, and considering crack initiation due to phosphide formation, the P content should be 0.001 to 0.010%. The P content is preferably 0.002% or more, and preferably 0.009% or less.

[0047] S: 0.0005 to 0.0010% S is contained as an impurity, which degrades the hot workability and toughness during manufacturing, and is prone to void formation due to brittle cracking during spot welding. Therefore, the upper limit of S is set to 0.0010% or less. However, excessive reduction leads to an increase in refining costs, so the S content is set to 0.0005% or more. The S content is preferably 0.0006% or more, and preferably 0.0009% or less.

[0048] Al: 0.010-0.030% Al can be used as a deoxidizing agent and improves oxidation resistance and corrosion resistance. Furthermore, it has been found that the addition of an appropriate amount of Al acts as a solidification nucleus during weld solidification by finely dispersing inclusions, contributing to refinement of the weld structure, improvement of toughness and fatigue strength, and suppression of void formation during spot welding. This effect is observed when the Al content is 0.010% or more, so the lower limit is set at 0.010%. On the other hand, when the Al content exceeds 0.030%, the improvement in oxidation resistance and corrosion resistance saturates, and AlN and Al-based oxides aggregate and coarse, becoming the initiation point for impact and fatigue cracks, so the upper limit is set at 0.030%. However, considering toughness, an Al content of 0.010-0.030% is desirable. The Al content is preferably 0.012% or more, and preferably 0.028% or less.

[0049] N: 0.001-0.004% N exists as nitride in steel. If the N content exceeds 0.004%, the deformability of the steel sheet decreases, making it difficult to suppress brittle fracture in the nugget during or after welding. Therefore, the N content should be 0.004% or less. However, due to production technology constraints, the N content should be 0.001% or more. Furthermore, the N content should preferably be 0.003% or less.

[0050] The high-strength steel sheet of the present invention contains each of the above elements, with the remainder being Fe and unavoidable impurities.

[0051] The above component composition is the basic component composition of high-strength steel sheet. In this invention, in addition to this basic component composition, one or more elements selected from the following Mo, Cr, Ca, Sb, B, Cu, and Ni may be included as needed for the purpose of further improving properties. Since each of the following components Mo, Cr, Ca, Sb, B, Cu, and Ni can be included as needed, these components may be present at 0%.

[0052] Mo: 0.500% or less. Mo is an element that improves hardenability and is effective in generating a hard phase. If the Mo content exceeds 0.500%, the area ratio of the hard phase increases, and the dimensional accuracy and ductility during molding decrease. Therefore, when Mo is included, the Mo content should be 0.500% or less. The lower limit of the Mo content may be 0.000%, but from the viewpoint of improving hardenability and bringing the TS within a more suitable range, it is more preferable for the Mo content to be 0.010% or more. The Mo content is more preferably 0.300% or less, and even more preferably 0.100% or less. Furthermore, the Mo content is even more preferably 0.030% or more.

[0053] Cr: 0.300% or less. Cr is an element that improves hardenability and is effective in generating a hard phase. If the Cr content exceeds 0.300%, the area ratio of the hard phase increases, and the dimensional accuracy and ductility during molding decrease. Therefore, when Cr is included, the Cr content should be 0.300% or less. The lower limit of the Cr content may be 0.000%, but from the viewpoint of improving hardenability and bringing the TS within a more suitable range, it is more preferable for the Cr content to be 0.010% or more. Furthermore, the Cr content should more preferably be 0.250% or less, and even more preferably 0.100% or less.

[0054] Ca: 0.0200% or less. Ca exists as an inclusion in the steel. If the Ca content exceeds 0.0200%, and the steel sheet contains diffusible hydrogen, the inclusion will become a crack initiation point during bending tests, thus reducing the bendability. Therefore, when Ca is included, the Ca content should be 0.0200% or less. The Ca content is more preferably 0.0180% or less. The lower limit of the Ca content may be 0.0000%, but due to production technology constraints, it is more preferable for the Ca content to be 0.0001% or more. Furthermore, the Ca content is more preferably 0.0020% or more.

[0055] Sb: 0.200% or less. Sb is an effective element for suppressing oxidation of the steel sheet surface during annealing and controlling the thickness of the surface softening. Sb is also an element that can reduce the frequency of corresponding grain boundaries on the surface of the steel sheet by suppressing nitriding of the surface during annealing. If the Sb content exceeds 0.200%, a softened surface layer cannot be formed, resulting in a decrease in bendability and LME resistance. Therefore, when Sb is included, the Sb content should be 0.200% or less. The lower limit of the Sb content may be 0.000%, but to reduce the frequency of corresponding grain boundaries and obtain better LME resistance, it is more preferable to have an Sb content of 0.001% or more. The Sb content is even more preferably 0.002% or more, and even more preferably 0.005% or more. Furthermore, the Sb content is more preferably 0.050% or less, and even more preferably 0.020% or less.

[0056] B: 0.0100% or less. B is used to control the structure of the base steel sheet and the nugget. B is an element that can improve hardenability by segregating at austenite grain boundaries, and by adding B to the steel, it is possible to suppress the formation of ferrite and grain growth during annealing cooling. To obtain these effects, if B is included, it is preferable that the B content be 0.0001% or more. On the other hand, if the B content exceeds 0.0100%, cracks will occur inside the steel sheet during hot rolling, reducing the ultimate deformability of the steel sheet, which increases the total void number density after punching and reduces the stretch flangeability. The bendability also decreases. The B content should be 0.0100% or less. Therefore, if B is included, it is more preferable that the content be 0.0001 to 0.0100%. The B content is even more preferably 0.0002% or more, and even more preferably 0.0050% or less.

[0057] Cu: 1.00% or less Cu is an element that greatly increases hardenability, and is an effective element for bringing the area ratio of the hard phase and TS within a more suitable range, and for further improving dimensional accuracy during forming. To obtain these effects, when Cu is included, it is preferable to have a Cu content of 0.01% or more. On the other hand, if the Cu content exceeds 1.00%, the area ratio of the hard phase increases, and the dimensional accuracy and ductility during forming decrease. In addition, coarse precipitates and inclusions increase, and if diffusible hydrogen is contained in the steel sheet, it becomes a crack initiation point during bending tests, thus reducing bendability. The Cu content should be 1.00% or less. Therefore, when Cu is included, it is more preferable that the content be between 0.01% and 1.00%. The Cu content is even more preferably 0.02% or more, and even more preferably 0.20% or less.

[0058] Ni: 0.50% or less. Ni is an element that greatly increases hardenability, and is an effective element for bringing the area ratio of the hard phase and TS within a more suitable range, and for further improving dimensional accuracy during forming. To obtain these effects, when Ni is included, it is preferable to have a Ni content of 0.01% or more. On the other hand, if the Ni content exceeds 0.50%, the area ratio of the hard phase increases, and the dimensional accuracy and ductility during forming decrease. In addition, coarse precipitates and inclusions increase, and if diffusible hydrogen is contained in the steel sheet, it becomes a crack initiation point during bending tests, thus reducing bendability. The Ni content should be 0.50% or less. Therefore, when Ni is included, it is more preferable that its content be between 0.01% and 0.50%. The Ni content is even more preferably 0.02% or more, and even more preferably 0.20% or less.

[0059] [Plating layer] As mentioned above, high-strength steel sheets may have a plating layer. The type of plating is not specifically defined. Examples include Zn plating, Cr plating, and Ni plating.

[0060] [Method for Manufacturing a Resistance Spot Welded Joint] Next, with reference to Figure 3, an embodiment of the method for manufacturing the welded joint of the present invention having the welded portion described above will be explained. Figure 3 shows a cross-sectional view in the thickness direction illustrating the welding process in the manufacturing method of the present invention as an example.

[0061] The welded joint of the present invention can be manufactured by performing a resistance spot welding process (hereinafter referred to as the "welding process") in which a plate assembly made of two or more overlapping steel plates, including at least one of the high-strength steel plates, is sandwiched between a pair of welding electrodes, and the steel plates are joined by applying current while applying pressure with the pair of welding electrodes.

[0062] Furthermore, the manufacturing method of the present invention may include an evaluation step, which will be described later, after the welding step.

[0063] [Welding Process] In the welding process, as shown in Figure 3 for example, two steel plates (in this example, a lower steel plate and an upper steel plate) 1 and 2 are stacked to form a plate assembly 3. Next, the plate assembly 3 is clamped between a pair of welding electrodes 5 and 6 positioned on the lower and upper sides of the plate assembly 3, and current is applied while applying pressure and controlling the welding conditions to predetermined levels. This joins the plates that form the steel plate joining surface 7, thereby forming the welded portion 4 described above (see Figure 1).

[0064] When high-strength cold-rolled steel sheets and high-strength galvanized steel sheets are layered to form a sheet assembly, multiple sheets should be layered so that the side of the high-strength galvanized steel sheet with the galvanized layer faces the high-strength cold-rolled steel sheet. As mentioned above, the high-strength steel sheet used is a decarburized steel sheet with a tensile strength of 980 MPa or more (i.e., a steel sheet that satisfies equation (1) above).

[0065] In this invention, the welding process includes a main energizing step. If necessary, a post-energizing step and / or a post-heat treatment step may be included after the main energizing step. Each step of the welding process will be described in detail below.

[0066] <Main energizing process> In the main energizing process, the energizing current value I is applied to the above-mentioned board assembly. １ (Unit: kA) and energizing time t １ (Unit: sec) The welding conditions satisfy equations (2) and (3) and then energize (main energization) the weld. 2.0 < I １ <10.0...(2) 0.02<t １ <1.50 …(3)

[0067] Current value I １If the current is 2.0 (kA) or less, the heat input will be insufficient, resulting in a smaller nugget diameter, and thus the joint strength cannot be guaranteed. Therefore, the energizing current value I １ The current value I will be set to exceed 2.0 (kA). １ Preferably, the current value I １ If the current is 10.0 (kA) or higher, scattering is likely to occur. Therefore, the current value I １ The current value I shall be less than 10.0 (kA). １ Preferably, it is less than 8.0 (kA).

[0068] Energization time t １ If the current is 0.02 (sec) or less, the heat input will be insufficient, resulting in a smaller nugget diameter and thus insufficient joint strength. Therefore, the energizing time t １ This is defined as exceeding 0.02 (sec). The energizing time t １ Preferably, this should exceed 0.05 (sec). Also, in order to prevent the welding process from becoming prolonged, the energizing time t １ The energizing time t shall be less than 1.50 (sec). １ Preferably, it shall be less than 1.30 (sec).

[0069] The applied pressure during this energization process shall be between 2.0 kN and 7.0 kN. If the applied pressure is less than 2.0 kN, there will be insufficient force to press the steel plates together, and dust will be generated due to the force of the nuggets expanding before they become sufficiently large. On the other hand, if the pressure exceeds 7.0 kN, the force applied by the electrodes will be too great, resulting in dust generation.

[0070] By performing the main energizing process under these welding conditions, a welded joint having the aforementioned weld can be manufactured. As described above, a post-energizing process and / or a post-heat treatment process may be performed after the main energizing process in order to further improve the properties.

[0071] <Post-energization process> When a post-energization process is performed following the main energization process described above, the following welding conditions are preferable. In the post-energization process, the energizing current value I ２ (kA) and energizing time t ２(sec) applies current (i.e., post-current) under welding conditions that satisfy equations (4) and (5). １ -d×(1-X) / 20<I ２ <I １ -d×(1-X) / 80…(4) 0.1×I ２ <t ２ <0.5 × I ２ …(5) Here, I shown in equation (4) １ is the current value (kA) for this energizing process, and X is the C content (%) at a position 10 μm from the surface of the high-strength steel plate in the thickness direction.

[0072] The post-energization process aims to improve the toughness of the nugget by reheating the formed weld and tempering it, and to improve the initial crack resistance by increasing the peel strength of the corona bond through pressurization and heat input.

[0073] Equation (4) is a newly defined equation by the present inventors, and is a relational equation that takes into account the application of additional heat input by applying an electric current after the nugget and heat-affected zone have been formed, thereby strengthening the toughness of the nugget and corona bond.

[0074] Current value I ２ (kA) is "I １ If the current value exceeds the value expressed as -d × (1 - X) / 80, the weld area may overheat, causing remelting within the nugget and potentially generating dust. Therefore, the current value I ２ is "I １ It is preferable that the current value I is less than -d × (1 - X) / 80. ２ More preferably, "I １ -d × (1 - X) / 82 shall be less than this. Also, in order to prevent the welding process from becoming prolonged, the energizing current value I ２ is "I １ It is preferable that the value exceeds the value expressed as -d × (1 - X) / 20. Current value I ２ More preferably, "I １ The value should be greater than -d × (1 - X) / 18.

[0075] Energization time t ２ is "0.1 × I ２If the value is less than or equal to the value expressed as (sec), the heat input is insufficient, and as a result, sufficient tempering cannot be achieved, and the toughness of the nugget and corona bond cannot be improved by subsequent energizing. Therefore, the energizing time t ２ is "0.1 × I ２ (sec) is considered to be the limit. Power supply time t ２ Preferably, "0.15 × I ２ The limit is set to exceed (sec). In addition, to prevent the welding process from becoming prolonged, the energizing time t ２ , is "0.5 × I ２ The energizing time t shall be less than (sec). ２ Preferably, "0.45 × I ２ Less than (sec)

[0076] The applied pressure during the post-energization process should be basically the same as the applied pressure during the main energization.

[0077] <Post-heat treatment process> When a post-heat treatment process is performed following the main energizing process or the post-energizing process described above, the following conditions are preferable.

[0078] In the post-heat treatment process, the furnace temperature is set to T (°C) and the time spent in the furnace is set to t ３ When (min), the T and the t ３ Heat treatment (post-heat treatment) is performed under conditions that satisfy equations (6) and (7). 70 ≤ T ≤ 300 …(6) 5 ≤ t ３ ≤30 …(7)

[0079] If the furnace temperature T is below 70°C, the temperature will not be high enough to temper the weld, and the tempering effect will not be obtained. On the other hand, if the furnace temperature T exceeds 300°C, the weld will become brittle, resulting in intergranular fracture in the weld and a decrease in resistance to initial cracking. Therefore, the furnace temperature T should be between 70 and 300°C. More preferably, the furnace temperature T should be 100°C or higher, and more preferably 280°C or lower.

[0080] Furnace time t ３ If the time in the furnace is too short, the tempering effect on the weld will not be sufficient. On the other hand, the furnace time t ３ If the time is too long, the weld may become brittle. Also, the time spent in the furnace t ３If the time is too long, the workability will be poor. From these perspectives, the furnace time t ３ Let the time be 5 to 30 minutes. In-furnace time t ３ The duration is more preferably 10 min or more, and more preferably 25 min or less.

[0081] [Evaluation Step] As described above, the present invention may include an evaluation step after the welding step. This evaluation step is a step of evaluating the peel strength of the corona bond in the welded joint obtained after the welding step.

[0082] In this evaluation process, L-shaped test specimens are welded together, and then a tensile test is performed on the welded specimens. The value of the maximum load before a decrease in load occurs in the load and displacement curve obtained from the tensile test is used to measure the peel strength of the corona bond. The peel strength evaluation method for corona bond described above can be used to evaluate this peel strength.

[0083] If the peel strength of Corona Bond 4b obtained in the evaluation process is 0.05 kN or higher, it is evaluated as having excellent resistance to initial cracking.

[0084] In the evaluation process, the peel strength may be evaluated for all welded joints after the welding process, or it may be evaluated for a selection of welded joints.

[0085] The present invention will be described below with reference to examples. However, the present invention is not limited to these examples.

[0086] First, we prepared steel plates of the steel types shown in Table 1.

[0087] Table 1 shows the following measurements for each steel sheet: (1) tensile strength (TS), (2) decarburization layer thickness, surface carbon concentration (i.e., carbon content at 10 μm below the surface), and (3) average hardness of the base steel sheet. The obtained values ​​are shown in Table 1. The presence or absence of a metal plating layer on the surface of the steel sheet is also shown in Table 1.

[0088] (1) Tensile Strength (TS) The above "tensile strength" was determined by preparing a JIS No. 5 tensile test specimen from each steel plate in a direction parallel to the rolling direction, and conducting a tensile test using this specimen in accordance with the provisions of JIS Z 2241:2011, and the value obtained was defined as the tensile strength (unit: MPa).

[0089] (2) Decarburized layer thickness and surface C concentration of high-strength steel sheets The above-mentioned "decarburized layer thickness" and "surface C concentration" were determined by measuring the range from the surface of the steel sheet to a depth of 300 μm using GDS (glow discharge emission spectroscopy). As stated above, "decarburized layer thickness" was defined as the distance from the surface of the steel sheet to a depth that matches the bulk C content of the base steel sheet. Furthermore, since the C concentration on the surface of the steel sheet fluctuates greatly, the "surface C concentration" was evaluated using the C concentration at a depth of 10 μm. Note that the measurement methods for the decarburized layer thickness and surface C concentration have been described above, so a detailed explanation is omitted.

[0090] (3) Average hardness of the base steel sheet The "average hardness of the base steel sheet" mentioned above is the average value of the hardness (unit: HV) measured at five locations (1 / 4 of the sheet thickness) in the thickness direction from the surface of each steel sheet. In Table 1, "Base steel sheet hardness" shows the average value of the hardness measured at five locations for each steel sheet.

[0091]

[0092] Next, using steel plates of the steel types shown in Table 1, a lower steel plate 1 and an upper steel plate 2 were prepared, and these steel plates were overlapped with the specified gaps to form the plate assembly shown in Table 2. The steel plates used in the plate assembly may be of the same type or different types. Resistance spot welding was performed on the plate assembly under the welding conditions shown in Table 2 to produce a welded joint to be used as a sample.

[0093] Resistance spot welding was performed at room temperature, with the welding electrodes (i.e., lower and upper electrodes) constantly water-cooled. Both the lower and upper electrodes were DR-type electrodes made of chromium copper, with a tip diameter of 6 mm and a radius of curvature of 40 mm. The pressure applied was controlled by driving the lower and upper electrodes with servo motors, and a single-phase AC current of 50 Hz was supplied when energizing.

[0094]

[0095] Tables 1 and 2 show the results of whether each relation is satisfied or not in the columns labeled "(1)" to "(5)". The symbol "○" indicates that the relation is satisfied, and the symbol "×" indicates that the relation is not satisfied.

[0096] Next, the obtained welded joints were left to stand in the air at room temperature (20°C), and then the following procedures were performed: (4) average hardness of the corona bond, (5) evaluation of initial cracking characteristics of the gap test specimen, (6) cross-sectional observation of the weld using an optical microscope, and (7) evaluation of the peel strength of the corona bond.

[0097] (4) Average hardness of corona bond The average hardness of the corona bond was measured in the measurement area (see Figure 2) of the welded joint after the completion of welding as described above. This average hardness was determined using the measurement method described above.

[0098] (5) Evaluation of initial cracking characteristics of gap test specimens The initial cracking characteristics were evaluated by the following method.

[0099] First, steel plates with a long side of 100 mm and a short side of 30 mm were cut from each of the steel plates used in the plate assembly. Next, spacers (size: 30 mm x 30 mm x 2.0 mm) were inserted at both ends of the steel plates so that a 2.0 mm gap was formed between the overlapping steel plates that make up the plate assembly. Tack welding was performed in the area where the spacers were located, and then the center of the steel plates was welded. As shown in Figure 7, in the case of a plate assembly using two steel plates 1 and 2, spacers 8 were inserted at both ends of the lower steel plate 1 and the upper steel plate 2 and tack welded 9, and then the center of steel plates 1 and 2 was welded 10. After welding, the welded joints were left to stand in the air at room temperature (20°C) for more than 24 hours and were used as samples to check for delamination. The check was performed visually.

[0100] In Table 3, samples in which the nugget did not detach are marked with the symbol "〇" or "◎", and samples in which the nugget did detach are marked with the symbol "×". "Nugget detachment" refers to the phenomenon in which the nugget separates into two at the bonding interface.

[0101] Here, the symbol "〇" indicates that the material possesses "excellent initial cracking characteristics."

[0102] Furthermore, samples that were judged to have the symbol "○" in this initial cracking characteristic evaluation were subjected to cross-sectional observation to check for the presence or absence of cracks within the nugget. Cross-sectional observation was performed using an optical microscope by cutting a cross section parallel to the longitudinal direction of the test piece, passing through the center of the weld, from the welded sample. The presence or absence of "cracks within the nugget" was observed at both ends of the nugget, which are stress concentration points, at a magnification of 40x. As a result of the observation, samples that did not have cracks within the nugget and whose corona bond had not delaminate were marked with the symbol "◎". Samples marked with the symbol "◎" were evaluated as having "superior initial cracking characteristics".

[0103] (6) Cross-sectional observation of the welded joint In the cross-sectional observation, the nugget diameter of the welded joint after the completion of welding as described above was measured. The nugget diameter was measured by observing the entire welded joint at 10x magnification using an optical microscope, and the intersection point of the steel plate mating surface and the nugget was defined as the nugget end (see point E in Figure 2), and the line segment connecting the two points passing through the center of the nugget and the nugget end was measured.

[0104] (7) Evaluation of peel strength of corona bond First, steel plates cut to 100 mm x 30 mm from each steel plate used in the plate assembly were bent vertically along the 30 mm long side so that the curvature was 5 mm, and welded steel plates with a weld surface and a "gripping section" for tensile testing were prepared.

[0105] Next, as shown in Figure 4, the steel plates to be welded were stacked and arranged, and the center of the weld surface was welded to create a tensile test specimen. After welding was completed, a tensile test was performed at a tensile speed of 10 mm / min, specifically within 5 minutes after welding, before the hydrogen present inside the weld of the specimen could escape.

[0106] Next, using the values ​​obtained from the tensile test, a load-displacement graph like the one shown in Figure 5 was created. As shown in Figure 5, the maximum load immediately before a sharp load drop (here, a load drop of 0.05 kN or more) occurred was evaluated as the corona bond peel strength. Details of the evaluation method are described above and will be omitted here. Note that the case where the value of "Corona Bond Peel Strength" in Table 3 was 0.00 (kN) is as follows: That is, if the corona bond had already peeled before the test, there was no load drop due to corona bond peeling, and therefore the corona bond peel strength was 0. Note that this evaluation can be performed as an evaluation step in the manufacturing method described above.

[0107] The results obtained are shown in Table 3.

[0108]

[0109] As is clear from Table 3, the welded joint in the example of the present invention was a good welded joint with excellent resistance to initial cracking. In contrast, a good welded joint could not be obtained in the comparative example.

[0110] 1. Lower steel plate 2. Upper steel plate 3. Plate assembly 4. Welded joint 4a. Nugget 4b. Corona bond 4c. HAZ 5, 6. Welding electrodes 7. Steel plate joint surface 8. Spacer 9. Tack weld 10. Weld

Claims

1. A resistance spot welded joint having a nugget formed on the joint surface of multiple steel plates, each containing at least one high-strength steel plate, and a corona bond and heat-affected zone formed around the nugget, wherein the high-strength steel plate has a tensile strength of 980 MPa or more, the thickness d (μm) of the decarburized layer on the surface of the high-strength steel plate and the carbon content X (mass%) at a position 10 μm from the steel plate surface in the thickness direction satisfy equation (1), and the average hardness in the region from the corona bond to 30 μm in the thickness direction is 10% or more lower than the average hardness of the base steel plate. 10 < d × (1 - X) < 300 …(1) 2. The resistance spot welded joint according to claim 1, wherein the peel strength of the corona bond is 0.05 kN or more.

3. The peel strength is defined as the value of the maximum load before a decrease in load occurs in the load and displacement curve obtained in the load and displacement curve obtained in the tensile test, obtained by welding L-shaped test pieces opposite each other, and then performing a tensile test on the welded test pieces, according to claim 2.

4. A method for manufacturing a resistance spot welded joint according to any one of claims 1 to 3, comprising a welding step of sandwiching a plate assembly made by overlapping a plurality of steel plates including at least one high-strength steel plate between a pair of welding electrodes, and applying pressure and current with the pair of welding electrodes, wherein the high-strength steel plate is a steel plate that satisfies formula (1), and in the welding step, the current value I １ (kA) and energizing time t １ A method for manufacturing a resistance spot welded joint, wherein the current is applied under the condition that (sec) satisfies equations (2) and (3). 10 < d × (1 - X) < 300 …(1) 2.0 < I １ <10.0...(2) 0.02<t １ <1.50 …(3) Here, in equation (1), d is the thickness of the decarburized layer on the surface of the high-strength steel sheet (μm), and X is the C content (mass%) at a position 10 μm from the surface of the steel sheet in the thickness direction.

5. The welding process is, following the main energization, to perform post-energization under conditions where the energization current value I ２ (kA) and the energization time t ２ (sec) satisfy equations (4) and (5). The method for manufacturing a resistance spot welding joint according to claim 4. I １ -d × (1 - X) / 20 < I ２ < I １ -d × (1 - X) / 80...(4) 0.1 × I ２ < t ２ < 0.5 × I ２ ...(5) Here, I shown in equation (4) １ is the energization current value (kA) of the main energization, and X is the C content (mass%) at a position 10 μm in the plate thickness direction from the steel plate surface of the high-strength steel plate.

6. The welding process, following the initial energization, involves setting the furnace temperature T (°C) and the time spent in the furnace t. ３ A method for manufacturing a resistance spot welded joint according to claim 4, wherein (min) is subjected to post-heat treatment under conditions that satisfy equations (6) and (7). 70 ≤ T ≤ 300 …(6) 5 ≤ t ３ ≤30 …(7) 7. The welding process, following the post-energization, involves the furnace temperature T (°C) and the time spent in the furnace t. ３ A method for manufacturing a resistance spot welded joint according to claim 5, wherein (min) is subjected to post-heat treatment under conditions that satisfy equations (6) and (7). 70 ≤ T ≤ 300 …(6) 5 ≤ t ３ ≤30 …(7) 8. A method for manufacturing a resistance spot welded joint according to claim 4, comprising an evaluation step after the welding step for evaluating the peel strength of the corona bond of the obtained resistance spot welded joint, wherein in the evaluation step, L-shaped test pieces are welded opposite each other, and then a tensile test is performed on the welded test pieces, and the value of the maximum load before a load reduction occurs in the load and displacement curve obtained in the tensile test is defined as the peel strength of the corona bond.

9. A method for evaluating the peel strength of a welded joint, comprising welding two L-shaped test pieces opposite each other, then conducting a tensile test on the welded test pieces, and defining the value of the maximum load before a load reduction occurs in the load and displacement curve obtained from the tensile test as the peel strength of the corona bond.

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