Laser welded joint and method of manufacturing the same

Abstract

The present application provides a laser-welded joint which is excellent in press formability of a welded portion even when a steel sheet having a tensile strength of 980 MPa or more is used. A laser-welded joint has a first steel sheet, a second steel sheet, and a butt-welded portion of the first steel sheet and the second steel sheet, either one or both of the first steel sheet and the second steel sheet is a high-strength steel sheet having a tensile strength of 980 MPa or more, and in a welded heat-affected portion of the high-strength steel sheet, the following formula (1) is satisfied. s－max -H s－min ≤ 200 (1)

Description

Technical Field

[0001] This invention relates to a laser welding joint and its manufacturing method. In particular, this invention relates to a laser welding joint for welding blanks (hereinafter also referred to as a laser welding joint for welding blanks) and its manufacturing method. Background Technology

[0002] A tailor welded blank (TWB) is a single blank made by welding multiple steel sheets of different thicknesses or materials together using methods such as laser welding before stamping. For example, in the automotive industry, tailor welded blanks allow for the optimal configuration of the steel sheets used as raw materials. This enables a balance between lightweight vehicle bodies and ensuring crash safety. Specifically, tailor welded blanks are used in automotive components such as door inner panels, inner side panel components, and longitudinal beams.

[0003] As a technology related to the welding of blanks, for example, it is disclosed in Patent Document 1:

[0004] "A laser butt welded steel plate for thin steel plates, characterized in that the width of the heat-affected softened portion near the weld portion of the welded steel plate is less than 25% of the plate thickness, and the tensile strength of the base material on at least one side is more than 780 MPa."

[0005] Furthermore, Patent Document 2 discloses that:

[0006] "A method for strengthening a butt-welded workpiece, characterized in that it is strengthened by plastically deforming the heat-affected softened portion near the weld of the workpiece made by butt-welding plates together."

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Application Publication No. 2006-218500

[0010] Patent Document 2: Japanese Patent Application Publication No. 2010-082685 Summary of the Invention

[0011] One of the required characteristics of laser-welded joints for welded blanks is the crack resistance of the welded portion and the heat-affected zone during stamping (hereinafter also referred to as the stamping formability of the welded portion). For example, during the stamping of welded blanks, if excessive strain is concentrated in the welded portion or the heat-affected zone, cracks will originate from the strain concentration point.

[0012] In recent years, the use of high-strength steel sheets for automobiles has been continuously promoted. When high-strength steel sheets are used in welded blanks, excellent stamping formability of the welded parts is also required.

[0013] However, in the technologies disclosed in Patent Documents 1 and 2, when high-strength steel plates are used for welding blanks, excellent stamping formability of the welded parts is not always obtained, and improvements are expected in this regard.

[0014] This invention was developed in view of the above-mentioned situation, and its object is to provide a laser-welded joint with excellent stamping formability of the welded part, even when using high-strength steel plates, especially steel plates with a tensile strength of 980 MPa or higher. Furthermore, this invention also aims to provide a method for manufacturing the aforementioned laser-welded joint. It should be noted that in this disclosure, the numerical ranges indicated by "~" refer to the ranges including the values ​​before and after "~" as the lower and upper limits, respectively.

[0015] Next, the inventors conducted repeated in-depth research in order to achieve the above objectives and obtained the following insights.

[0016] (1) In order to improve the stamping formability of the welded part, it is important to prevent fracture caused by cracks originating from the HAZ softened part by suppressing the concentration of strain in the softened area (hereinafter also referred to as the HAZ softened part) in the weld heat affected zone (hereinafter also referred to as the HAZ softened part).

[0017] (2) Especially during the stamping of welded blanks, strain tends to concentrate in the surface layer of the HAZ softened zone due to the bending deformation of the welded blanks. Here, the surface layer refers to the surface layer on the laser incident side, and more specifically, the area from the surface on the laser incident side to a depth of 300 μm. Furthermore, when high-strength steel plates, especially steel plates with a tensile strength of 980 MPa or higher, are used for the welded blanks, the concentration of strain in the surface layer of the HAZ softened zone becomes significant.

[0018] (3) In order to suppress the concentration of such strain in the softened HAZ, it is effective to minimize the hardness difference of the HAZ. More specifically, by making the difference between the maximum and minimum Vickers hardness in the surface HAZ of the high-strength steel plate less than 200, excellent stamping formability of the welded part can be obtained even when using steel plates with a tensile strength of 980 MPa or more.

[0019] (4) In addition, in order to make the difference between the maximum and minimum Vickers hardness in the surface layer of the HAZ of the high-strength steel plate less than 200, it is important to properly control the laser welding conditions, especially the relationship between the laser output, the welding speed and the average Vickers hardness at the surface layer and the center of the plate thickness of the high-strength steel plate that is being joined.

[0020] This invention was completed based on the above insights and further research.

[0021] That is, the essence of the present invention is as follows.

[0022] 1. A laser-welded joint comprising a first steel plate, a second steel plate, and a butt weld portion between the first steel plate and the second steel plate.

[0023] Either or both of the aforementioned steel plate 1 and steel plate 2 are high-strength steel plates with a tensile strength of 980 MPa or higher.

[0024] The heat-affected zone of the above-mentioned high-strength steel plate meets the following relationship (1).

[0025] H s－max -H s－min ≤200 (1)

[0026] here,

[0027] H s－max The maximum Vickers hardness at the surface layer of the laser-incident side of the weld heat-affected zone of a high-strength steel plate.

[0028] H s－min The minimum Vickers hardness at the surface layer of the laser-incident side of the heat-affected zone of a high-strength steel plate welded to a weld.

[0029] 2. The laser-welded joint according to 1 above, wherein the base material portion of the high-strength steel plate satisfies the relationship of formula (2),

[0030] 0.20≤H bs / H bm ≤0.90 (2)

[0031] here,

[0032] H bm The average Vickers hardness at the center of the thickness of the base material of a high-strength steel plate.

[0033] H bs The average Vickers hardness of the surface layer on the laser-incident side of the base material of a high-strength steel plate.

[0034] 3. The laser-welded joint according to 1 or 2 above, wherein the heat-affected zone of the high-strength steel plate satisfies the relationship of the following formula (3),

[0035] 50≤H m－min -H s－min ≤300 (3)

[0036] here,

[0037] H m－min The minimum Vickers hardness at the center of the weld heat-affected zone of a high-strength steel plate.

[0038] H s－min The minimum Vickers hardness at the surface layer of the laser-incident side of the heat-affected zone of a high-strength steel plate welded to a weld.

[0039] 4. A method for manufacturing a laser-welded joint, comprising:

[0040] The process of obtaining a laser-welded joint by laser welding the materials to be joined by butt-jointing the first steel plate and the second steel plate.

[0041] Either or both of the aforementioned steel plate 1 and steel plate 2 are high-strength steel plates with a tensile strength of 980 MPa or higher.

[0042] In the above laser welding, the following relationship (4) must be satisfied.

[0043] (V / 3)+2≤P≤[(V / 3)+9]×(H bm / H bs ) (4)

[0044] here,

[0045] P: Laser output (kW)

[0046] V: Welding speed (m / min)

[0047] H bm The average Vickers hardness at the center of the thickness of a high-strength steel plate.

[0048] H bs The average Vickers hardness of the surface layer on the laser incident side of a high-strength steel plate.

[0049] 5. A method for manufacturing the laser-welded joint described in 4 above, wherein the high-strength steel plate satisfies the relationship of the following formula (2),

[0050] In the above laser welding, the following relationship (5) must be satisfied:

[0051] 0.20≤H bs / H bm ≤0.90 (2)

[0052] (V / 3)+(D / 10)+2≤P≤[(V / 3)+8]×(H bm / H bs ) + (D / 10) (5)

[0053] here,

[0054] P: Laser output (kW)

[0055] V: Welding speed (m / min)

[0056] D: The amount of defocus (mm) of the laser focal point relative to the surface of the materials being joined.

[0057] H bm The average Vickers hardness at the center of the thickness of a high-strength steel plate.

[0058] H bs The average Vickers hardness of the surface layer on the laser incident side of a high-strength steel plate.

[0059] According to the present invention, even when using steel plates with a tensile strength of 980 MPa or higher, laser-welded joints with excellent stamping formability of the welded portion can be obtained. Preferably, laser-welded joints with excellent static tensile properties, in addition to excellent stamping formability of the welded portion, can also be obtained. By applying the laser-welded joints of the present invention to automotive parts, such as welded blanks for body frame components, it is possible to achieve weight reduction of the vehicle body (thereby reducing CO2 emissions during vehicle operation) and further improve the safety performance of the vehicle. Furthermore, the laser-welded joints of the present invention offer a high degree of shape freedom and can be manufactured with consistent quality at high production efficiency, thus providing significant advantages in terms of manufacturability. Attached Figure Description

[0060] Figure 1 This is a schematic diagram showing an example of the cross-section of a laser-welded joint.

[0061] Figure 2 This is a schematic diagram illustrating an example of defocusing.

[0062] Figure 3 This is a schematic diagram illustrating an example of defocusing.

[0063] Figure 4 This is a schematic diagram illustrating an example of crack morphology in the Erikson test.

[0064] Figure 5 This is a schematic diagram illustrating an example of crack morphology in the Erikson test.

[0065] Figure 6 This is a schematic diagram illustrating an example of crack morphology in the Erikson test. Detailed Implementation

[0066] The present invention will be described based on the following embodiments. First, a laser-welded joint according to one embodiment of the present invention will be described.

[0067] [1] Laser welding joint

[0068] One embodiment of the laser-welded joint of the present invention is

[0069] A laser-welded joint having a first steel plate, a second steel plate, and a butt weld joint between the first steel plate and the second steel plate.

[0070] Either or both of the aforementioned steel plate 1 and steel plate 2 are high-strength steel plates with a tensile strength of 980 MPa or higher.

[0071] The heat-affected zone of the above-mentioned high-strength steel plate meets the following relationship (1).

[0072] H s－max -H s－min ≤200 (1)

[0073] here,

[0074] H s－max The maximum Vickers hardness at the surface layer of the weld heat-affected zone of high-strength steel plates.

[0075] H s－min The minimum Vickers hardness at the surface layer of the heat-affected zone of a high-strength steel plate.

[0076] [1-1] Steel Plate 1 and Steel Plate 2

[0077] In a laser-welded joint according to one embodiment of the present invention, either or both of the first steel plate and the second steel plate constituting the laser-welded joint are high-strength steel plates with a tensile strength of 980 MPa or higher.

[0078] Tensile strength: above 980MPa

[0079] When the steel plate constituting the laser-welded joint is made high-strength, particularly with a tensile strength of 980 MPa or higher, the overall Vickers hardness of the laser-welded joint, including the butt weld portion and the heat-affected zone (HAZ) composed of weld metal, increases. Consequently, strain concentrates in the surface portion of the HAZ softening zone, leading to a significant decrease in the formability of the weld portion. That is, in laser-welded joints using steel plates with a tensile strength of 980 MPa or higher, improved formability of the weld portion is particularly required. Therefore, the tensile strength of the high-strength steel plate is 980 MPa or higher, preferably 1180 MPa or higher. There is no particular upper limit to the tensile strength of the high-strength steel plate. For example, the tensile strength of the high-strength steel plate is preferably 2500 MPa or lower.

[0080] There are no particular limitations on the composition of high-strength steel plates. For example, the following composition can be cited as an example of a high-strength steel plate:

[0081] In terms of mass %,

[0082] C: 0.04~0.40%

[0083] Si: 0.01~2.50%,

[0084] Mn: 1.00~5.00%,

[0085] P: below 0.050%

[0086] S: Below 0.010%

[0087] Ti: 0~0.20%,

[0088] Al: 0.01–0.30%, and

[0089] B: 0~0.0100%,

[0090] It contains, in total, one or more of any added elements selected from Cr, Ni, Mo, W, V, Nb, Cu, N and O (more preferably, in total, more than 0.1% when these added elements are present), with the remainder being Fe and unavoidable impurities.

[0091] It should be noted that either the first steel plate or the second steel plate may use the aforementioned high-strength steel plate, while the other may use a steel plate with a tensile strength of less than 980 MPa (hereinafter also referred to as a general-purpose steel plate). Alternatively, both the first steel plate and the second steel plate may use the aforementioned high-strength steel plate.

[0092] Furthermore, both the first and second steel plates (in other words, the aforementioned high-strength steel plates and / or general-purpose steel plates used for the first and second steel plates) can be surface-treated steel plates (hereinafter also referred to as coated steel plates) with a metallic coating on the surface of the base steel plate. The type and composition of the metallic coating are not particularly limited. Examples of metallic coatings include Zn-based coatings (coatings with a Zn content exceeding 50% by mass) or Al-based coatings (coatings with an Al content exceeding 50% by mass). When corrosion resistance is required, Zn-based coatings are preferred over Al-based coatings. This is because in Zn-based coatings, the corrosion rate of the base steel plate is reduced due to the sacrificial corrosion protection effect of Zn. Examples of Zn-based coatings include hot-dip galvanized (GI), alloyed hot-dip galvanized (GA), electroplated (EG), Zn-Ni based coatings (e.g., coatings containing 10-25% by mass of Ni in addition to Zn), Zn-Al based coatings, Zn-Mg based coatings, and Zn-Al-Mg based coatings. Furthermore, examples of Al-based coatings include Al-Si based coatings (e.g., coatings containing 10-20% by mass of Si in addition to Al). It should be noted that the amount of metal coating adhering to the coated steel sheet is not particularly limited. For example, from the viewpoint of weldability, the amount of metal coating adhering is preferably 120 g / m² per single side. 2 Furthermore, from the viewpoint of ensuring rust prevention, the amount of metal coating applied is preferably 20 g / m² or more per side.

[0093] Furthermore, the thickness of the first steel plate and the second steel plate is preferably 0.5 mm to 5.0 mm. More preferably, the thickness of the first steel plate and the second steel plate is 0.7 mm or more. More preferably, the thickness of the first steel plate and the second steel plate is 2.0 mm or less. It should be noted that the thickness of the first steel plate and the second steel plate may be the same or different.

[0094] Furthermore, the high-strength steel plates used for the first and second steel plates each have a HAZ adjacent to the butt weld and a base material portion adjacent to the HAZ, respectively. And, as described later, it is important to reduce the difference between the maximum and minimum Vickers hardness in the surface portion of the HAZ of the high-strength steel plate. It should be noted that the butt weld, HAZ, and base material portion can be defined, for example, by the methods described later.

[0095] [1-2] Butt welded parts

[0096] The butt weld portion of the laser welding joint constituting one embodiment of the present invention is the part where the first steel plate and the second steel plate are joined together, and is made of welding metal. It should be noted that the composition of the welding metal is not particularly limited, and examples may be the same composition as that of the high-strength steel plate described above, or a composition obtained by mixing the composition of the first steel plate and the composition of the second steel plate.

[0097] [1-3] Vickers hardness

[0098] In a laser-welded joint according to one embodiment of the present invention, it is extremely important to satisfy the relationship of the above formula (1).

[0099] H s－max -H s－min ≤200 (1)

[0100] As mentioned above, to improve the stamping formability of welded parts, it is important to prevent fracture caused by cracks originating from the HAZ softening zone by suppressing the concentration of strain towards the HAZ softening zone. Especially during the stamping of welded blanks, strain tends to concentrate on the surface layer of the HAZ softening zone due to the bending deformation of the blanks. Furthermore, when high-strength steel plates, particularly those with a tensile strength of 980 MPa or higher, are used for the welded blanks, the concentration of strain towards the surface layer of the HAZ softening zone becomes significant. To suppress such strain concentration towards the HAZ softening zone, it is effective to minimize the hardness difference of the HAZ as much as possible. Here, if H… s－max -H s－min If the value exceeds 200, it will promote the concentration of strain in the HAZ softened area, especially when using steel plates with a tensile strength of 980 MPa or higher, making it impossible to obtain sufficient stamping formability of the welded part. Therefore, it is necessary to satisfy the relationship of the above equation (1). H s－max -H s－min Preferably, it is 150 or less, more preferably 120 or less. s－max -H s－min The lower limit is not specifically defined and can be 0. However, since it is difficult to completely eliminate the hardness difference of HAZ when welding high-strength steel plates, H... s－max -H s－min Preferably, it is 30 or higher. It should be noted that when both the first steel plate and the second steel plate are made of high-strength steel plates, the relationship of the above formula (1) is satisfied in the heat-affected zone of each high-strength steel plate.

[0101] here,

[0102] H s－max The maximum Vickers hardness at the surface layer of the weld heat-affected zone of high-strength steel plates.

[0103] H s－min The minimum Vickers hardness at the surface layer of the heat-affected zone of a high-strength steel plate.

[0104] 0.20≤H bs / H bm ≤0.90 (2)

[0105] In a laser-welded joint according to one embodiment of the present invention, it is preferable to further satisfy the relationship of the above formula (2). For the welded blank, in addition to the stamping formability of the welded part, excellent static tensile properties are sometimes also required. Here, the ratio of the average Vickers hardness at the surface of the base material of the high-strength steel plate to the average Vickers hardness at the center of the thickness of the base material of the high-strength steel plate, i.e., H bs / H bm By controlling the value within the range of 0.20 to 0.90, in addition to improving the stamping formability of the welded part, the static tensile properties can also be improved. Therefore, it is preferable to satisfy the relationship in equation (2) above. H bs / H bm Preferably, it is 0.30 or higher. H bs / H bm Preferably, it is 0.80 or less. It should be noted that when both the first steel plate and the second steel plate are made of high-strength steel plates, it is preferable that the relationship of the above formula (2) is satisfied in the base material portion of each high-strength steel plate.

[0106] here,

[0107] H bm The average Vickers hardness at the center of the thickness of the base material of a high-strength steel plate.

[0108] H bs The average Vickers hardness of the surface layer of the base material of a high-strength steel plate.

[0109] It should be noted that the mechanical properties of the base material portion of the high-strength steel plate constituting the laser-welded joint generally remain unchanged from the mechanical properties of the high-strength steel plate used as the joined material. Therefore, as described later, the average Vickers hardness at the center of the plate thickness and the surface portion of the high-strength steel plate used as the joined material is also expressed as H, in the same way as the average Vickers hardness at the center of the plate thickness and the surface portion of the base material portion of the high-strength steel plate constituting the laser-welded joint. bm and H bs .

[0110] 50≤H m－min -H s－min ≤300 (3)

[0111] In a laser-welded joint according to one embodiment of the present invention, it is preferable to further satisfy the relationship of the above formula (3). As described above, for the welded blank, in addition to the stamping formability of the welded part, it is sometimes required to have excellent static tensile properties as well. During the stamping of the welded blank, since the welded blank will bend and deform, the strain tends to concentrate in the surface part of the HAZ softened part. On the other hand, when a static tensile load is applied to the welded blank, the strain also tends to concentrate in the HAZ softened part. However, in this case, the strain will be dispersed along the thickness direction of the laser-welded joint. Therefore, in order to improve the static tensile properties, it is important to increase the minimum Vickers hardness at the center of the thickness of the HAZ of the high-strength steel plate, which can be called the average hardness in the thickness direction, thereby increasing the difference between the minimum Vickers hardness at the center of the thickness of the HAZ of the high-strength steel plate and the minimum Vickers hardness at the surface part, i.e., H. m－min -H s－min Here, if H m－min -H s－min If the value is less than 50, the improvement in static tensile properties may become less significant. On the other hand, making H... m－min -H s－min When the value exceeds 300, it can sometimes lead to production constraints such as excessively increasing the carbon equivalent of high-strength steel plates. Therefore, it is preferable to satisfy the relationship in equation (3) above. H m－min -H s－min Preferably, it should be 60 or higher. H m－min -H s－min Preferably, it is 170 or less. It should be noted that when both the first steel plate and the second steel plate are made of high-strength steel plates, it is preferable that the relationship of the above formula (3) is satisfied in the base material portion of each high-strength steel plate.

[0112] here,

[0113] H m－min The minimum Vickers hardness at the center of the weld heat-affected zone of a high-strength steel plate.

[0114] H s－min The minimum Vickers hardness at the surface layer of the heat-affected zone of a high-strength steel plate.

[0115] In addition, H s－max H s－min and H m－min The determination was performed according to JIS Z 2244-1:2020. H s－max H s－min and H m－min For example, the measurement can be performed as follows.

[0116] like Figure 1As shown, the laser-welded joint is cut with a section perpendicular to the weld line (welding direction) in the plate thickness direction as the cutting surface. Next, the cutting surface is ground, and the boundaries of the first steel plate and the butt weld (also called the first fusion portion) and the second steel plate and the butt weld (also called the second fusion portion) are defined on this cutting surface, thus delineating the butt weld portion. The method for determining the first and second fusion portions can follow conventional methods. Then, on the first steel plate, the area from the first fusion portion (towards the first steel plate side along the welding perpendicular direction) to a position of 5 mm (hereinafter also called the HAZ – base material boundary of the first steel plate) is designated as the HAZ, and the remaining area is designated as the base material. Similarly, on the second steel plate, the area from the second fusion portion (towards the second steel plate side along the welding perpendicular direction) to a position of 5 mm (hereinafter also called the HAZ – base material boundary of the second steel plate) is designated as the HAZ, and the remaining area is designated as the base material.

[0117] Figure 1 In the diagram, symbol 1 represents the butt weld section, 2 represents the first steel plate, 2-1 represents the HAZ, 2-2 represents the base material section, 3 represents the second steel plate, 3-1 represents the HAZ, 3-2 represents the base material section, 4 represents the first fusion section, 5 represents the second fusion section, 6 represents the boundary between the HAZ and base material of the first steel plate, and 7 represents the boundary between the HAZ and base material of the second steel plate. Furthermore, x represents the welding vertical direction, and y represents the plate thickness direction. It should be noted that the welding vertical direction is the direction perpendicular to both the welding direction and the plate thickness direction.

[0118] Then, when high-strength steel is used in the first steel plate, Vickers hardness tests are performed at 200 μm intervals along the welding vertical direction (x direction) at representative positions on the surface of the HAZ of the high-strength steel plate (at a depth of 20 μm from the surface of the laser incident side of the high-strength steel plate) and at the center of the plate thickness, from the first fusion portion to the boundary of the HAZ-base material portion of the first steel plate. The Vickers hardness is measured according to JIS Z 2244-1:2020. The indentation load is 20 g, and the indentation time is 15 s. Then, the maximum value of the Vickers hardness measured on the surface of the HAZ of the high-strength steel plate is set as H. s－max Set the minimum value to H s－min Furthermore, in the Vickers hardness measured at the center of the thickness of the HAZ of high-strength steel plates, the minimum value is set as H. m－min Furthermore, when high-strength steel is used in the second steel plate, a Vickers hardness test is performed according to the same procedure as described above to determine the H value of the high-strength steel used in the second steel plate. s－max H s－min and H m－min .

[0119] In addition, H bm and H bsThe determination was performed according to JIS Z 2244-1:2020. H bm and H bs For example, the measurement can be performed as follows.

[0120] Following the same procedure as described above, cut the laser-welded joint, as shown on its cut surface. Figure 1 As shown, the HAZ and base material portion, butt weld portion, and HAZ and base material portion of the first steel plate and the second steel plate were defined. Then, when high-strength steel was used in the first steel plate, Vickers hardness tests were performed at three arbitrary points on the base material portion of the high-strength steel plate, at a representative position (20 μm depth from the laser incident side of the high-strength steel plate) and at the center of the plate thickness, according to JIS Z 2244-1:2020. The indentation load was 20 g, and the indentation time was 15 s. The average Vickers hardness at the center of the plate thickness and the surface portion of the base material portion of the high-strength steel plate was then set as H. bm and H bs Furthermore, when high-strength steel is used in the second steel plate, a Vickers hardness test is performed according to the same procedure as described above to determine the H value of the high-strength steel used in the second steel plate. bm and H bs .

[0121] It should be noted that the Vickers hardness (H) at the surface layer... s－max H s－min and H bs If the depth is 20μm ± 10μm from the surface of the high-strength steel plate, deviation in the measurement location is acceptable. However, in cases where the Vickers hardness varies significantly due to depth, the average Vickers hardness at 10μm, 20μm, and 30μm from the surface of the high-strength steel plate's HAZ can be used to determine H. s－max H s－min and H bs Furthermore, regarding the Vickers hardness (H) at the center of the plate thickness... m－min and H bm If the measurement location is within ±100μm of the center of the high-strength steel plate thickness, then deviation in the measurement location is acceptable. However, if the Vickers hardness varies significantly due to the depth location, then the average value of the Vickers hardness at -100μm, the center of the plate thickness, and +100μm of the center of the plate thickness can be used to determine H. m－min and H bm .

[0122] There are no particular limitations on the configuration other than those mentioned above, and the same configuration as conventional laser-welded joints may be appropriately adopted.

[0123] [2] Manufacturing method of laser welded joint

[0124] Next, a method for manufacturing a laser-welded joint according to one embodiment of the present invention will be described.

[0125] A method for manufacturing a laser-welded joint according to one embodiment of the present invention includes a step of obtaining a laser-welded joint by laser welding materials to be joined by butt-jointing a first steel plate and a second steel plate.

[0126] Either or both of the aforementioned steel plate 1 and steel plate 2 are high-strength steel plates with a tensile strength of 980 MPa or higher.

[0127] The relationship in the above laser welding is satisfied by the following equation (4).

[0128] (V / 3)+2≤P≤[(V / 3)+9]×(H bm / H bs ) (4)

[0129] here,

[0130] P: Laser output (kW)

[0131] V: Welding speed (m / min)

[0132] H bm The average Vickers hardness at the center of the thickness of a high-strength steel plate.

[0133] H bs The average Vickers hardness of the surface layer of a high-strength steel plate.

[0134] Hereinafter, a method for manufacturing a laser-welded joint according to one embodiment of the present invention will be described. It should be noted that the tensile strength, composition, plate thickness, Vickers hardness, etc. of the first steel plate and the second steel plate, which are the materials to be joined, are the same as those described in the laser-welded joint described above [1], and therefore are omitted here.

[0135] (V / 3)+2≤P≤[(V / 3)+9]×(H bm / H bs ) (4)

[0136] The inventors conducted various studies and discovered that in order to manufacture a laser-welded joint that satisfies the above formula (1), in laser welding, the average Vickers hardness at the center of the thickness of the high-strength steel plate to be joined is determined by the ratio of H to the average Vickers hardness at the surface portion at the center of the thickness of the plate. bm / H bs Properly controlling the relationship between laser output P and welding speed V, which significantly influences the welding heat input, is crucial. Here, if P exceeds [(V / 3) + 9] × (H... bm / H bs If the welding heat input is too high, the hardenability will increase in a certain region of the HAZ (the area near the heat input point). That is, in a certain region of the HAZ, where the peak temperature during welding reaches above Ac3, this region transforms into a very hard martensitic structure. Therefore, H... s－max Increase. On the other hand, in other regions of the HAZ, there are locations where the peak temperature during welding is lower than the Ac1 point, and in these regions, the tempering of martensite is promoted. Therefore, H s－min The pressure is reduced. That is, it becomes extremely difficult to satisfy the above equation (1). On the other hand, if P is less than (V / 3) + 2, the welding heat input is too small, and welding defects such as undercut are easily generated in the weld metal. Therefore, the stress concentration of the shape during stamping becomes significant, and the stamping formability is reduced. In addition, the stress concentration of the shape when a static tensile load is applied also becomes significant, and the static tensile properties are also reduced. Therefore, it is necessary to satisfy the above equation (4). P is preferably (V / 3) + 3 or more. In addition, P is preferably [(V / 3) + 7] × (H bm / H bs )the following.

[0137] here,

[0138] P: Laser output (kW)

[0139] V: Welding speed (m / min)

[0140] H bm The average Vickers hardness at the center of the thickness of a high-strength steel plate.

[0141] H bs The average Vickers hardness of the surface layer of a high-strength steel plate.

[0142] It should be noted that the inventors believe that the right side of the above equation (4), i.e., the upper limit of P, is related to H. bm / H bs The proportionality is based on the following reason: H bm / H bs A value exceeding 1 indicates that the hardness of the surface layer of the high-strength steel sheet being bonded is lower than that at the center of the sheet thickness. The lower the hardness of the surface layer of the high-strength steel sheet being bonded, the less likely tempering will occur, for example, due to a decrease in the proportion of martensite in the microstructure or a decrease in the carbon equivalent. Therefore, H... s－min The decrease is relatively suppressed. Therefore, the upper limit of P will be related to H. bm / Hbs Change proportionally.

[0143] (V / 3)+(D / 10)+2≤P≤[(V / 3)+8]×(H bm / H bs ) + (D / 10) (5)

[0144] Furthermore, in order to manufacture a laser-welded joint that satisfies the relationship in equation (3) above, it is important to use high-strength steel plates that satisfy the relationship in equation (2) above in the first and second steel plates used as the joined materials, and to perform laser welding under the condition that the relationship in equation (5) above is satisfied. Here, the defocusing amount D (hereinafter also referred to as defocusing amount) of the focal position relative to the surface of the joined materials during laser irradiation refers to, as Figure 2 and Figure 3 As shown, this is the absolute value of the offset of the laser focal position from the surface of the materials to be joined on the side closer to the laser source towards the thickness direction. Figure 2 This is a schematic diagram showing the amount of defocusing when the focal position shifts towards the laser source side. Figure 3 This is a schematic diagram showing the amount of defocusing when the focal position shifts to the side opposite to the laser source. Figure 2 and 3 In the diagram, symbol 8 represents the material being joined, 9 represents the laser source, 10 represents the laser, and 11 represents the laser focal point.

[0145] here,

[0146] P: Laser output (kW)

[0147] V: Welding speed (m / min)

[0148] D: The amount of defocus (mm) of the laser focal point relative to the surface of the materials being joined.

[0149] H bm The average Vickers hardness at the center of the thickness of a high-strength steel plate.

[0150] H bs The average Vickers hardness of the surface layer of a high-strength steel plate.

[0151] The energy density of the laser is highest at the laser focal point, and decreases with increasing distance from the focal point. That is, the greater the defocusing, the lower the energy density, resulting in a decrease in welding heat input. Therefore, by considering the defocusing and optimizing welding conditions, specifically by shifting the range of P towards the high-output side in accordance with the increase in defocusing, the aforementioned effects, particularly the improvement in static tensile properties, can be more effectively obtained. Furthermore, laser-welded joints satisfying the relationship in equation (3) can be manufactured. P is preferably (V / 3) + (D / 10) + 2.5 or higher. Furthermore, P is preferably [(V / 3) + 6] × (H... bm / H bs ) + (D / 10) and below.

[0152] There are no special restrictions on welding conditions other than those mentioned above, and conventional methods can be followed.

[0153] For example, V is preferably 1.0 m / min to 10.0 m / min. If V is less than 1.0 m / min, it may sometimes lead to an increase in construction time. On the other hand, if V exceeds 10.0 m / min, the welding stability will decrease, and sometimes there will be an increase in spatter, etc. V is more preferably 2.0 m / min or more. Furthermore, V is more preferably 7.0 m / min or less.

[0154] Shielding gases can be used in laser welding, or not. There are no particular restrictions on the type of shielding gas used; examples include Ar, He, CO2, O2, N2, and mixtures thereof.

[0155] Furthermore, filler wire can be used or not in laser welding. Types of filler wire include solid wire, flux-cored wire, and metal-cored wire. From a cost perspective, solid wire is preferred. Examples of filler wire compositions include:

[0156] In terms of mass %,

[0157] C: 0.03~0.2%

[0158] Si: 0.005~2.00%

[0159] Mn: 0.05~5.00%,

[0160] P: below 0.050%

[0161] S: Below 0.010%

[0162] Ti: 0~0.20%,

[0163] Al: 0–0.30%, and

[0164] O: 0~0.01%,

[0165] Furthermore, the weld contains one or more of any additive elements selected from Cr, Ni, Mo, W, V, B, Nb, Cu, and N, with a total content of less than 10% (more preferably 0.01% or more when these additive elements are present), with the remainder being Fe and unavoidable impurities. When using filler wire, it is desirable to adjust the wire diameter and feed rate to prevent the weld from becoming too thick.

[0166] The distance (gap) between the first and second steel plates is preferably 1.0 mm or less, more preferably 0.5 mm or less. Furthermore, from the viewpoint of welding stability, it is desirable to be close to 0 mm.

[0167] Example

[0168] The first and second steel plates shown in Table 1 were butt-jointed under the conditions shown in Table 2 to obtain a laser-welded joint. In all the above laser welding, a fiber laser with a maximum output of 13kW and a beam diameter of 0.5mm was used. Furthermore, the entire width of the first and second steel plates was welded. It should be noted that in some examples, filler wire equivalent to YGW11 as specified in JIS Z 3312:2009 was used for welding.

[0169] For the obtained laser-welded joint, H was measured according to the above-mentioned procedures. s－max H s－min H m－min H bm and H bs The measurement results are shown in Table 3.

[0170] Furthermore, the stamping formability and static tensile properties of the obtained laser-welded joints were evaluated according to the following criteria. The evaluation results are recorded in Table 3.

[0171] Evaluation of the stamping formability of welded parts

[0172] From the center of the welded part of the laser-welded joint, samples were collected along the welding direction. Figures 4-6 The test specimens of the shape shown were subjected to the Erikson test (cupping test) according to JIS Z 2247:2006. Then, the location of crack initiation in the test specimens was identified, and the stamping formability of the weld was evaluated according to the following criteria.

[0173] A (Qualified, Excellent): Resulting in... Figure 4 The crack shown is located at ±10° to the vertical direction (x-direction) of the weld.

[0174] F (Non-conforming): Cracks appearing in the direction other than ±10° of the weld perpendicular to the weld (especially such as...) Figure 5 and Figure 6 The crack shown is located in the welding direction (z-direction) of the butt weld and the HAZ.

[0175] It should be explained that Figures 4-6 In the diagram, symbol 1 represents the butt weld, 2 represents the first steel plate, 3 represents the second steel plate, 12 represents a crack, and 13 represents the outer edge of the drawn part. Furthermore, x represents the vertical direction of welding, and z represents the welding direction.

[0176] Evaluation of static tensile properties

[0177] Tensile test specimens (No. 5) as specified in JIS Z 2241:2011 were collected from the laser-welded joint with the weld perpendicular to the long side as the direction of the weld, and tensile tests were performed. The test speed was 10 mm / s. Next, the fracture location in the test specimens was confirmed and the joint efficiency was calculated. The static tensile properties were evaluated according to the following criteria.

[0178] A (Qualified, Excellent): The fracture location is in the base material.

[0179] B (Qualified, Excellent): The fracture location is in the HAZ or at the butt weld joint and the joint efficiency is above 90%.

[0180] F (Unacceptable): Fracture location is in the HAZ or at the butt weld joint and the joint efficiency is less than 90%.

[0181] Here, the joint efficiency is calculated as follows.

[0182] When the fracture location is in the HAZ

[0183] Joint efficiency (%) = Maximum load during tensile test (N) × 100 / (Thickness of steel plate with HAZ on the fracture side (mm) × Width of parallel part of test piece (mm) × Tensile strength of steel plate on the fracture side (MPa))

[0184] When the fracture occurs at the butt weld joint

[0185] The smaller of the joint efficiency for the first steel plate and the joint efficiency for the second steel plate.

[0186] The joint efficiency (%) of the first steel plate = maximum load (N) during the tensile test × 100 / (thickness of the first steel plate (mm) × width of the parallel part of the test piece (mm) × tensile strength of the first steel plate (MPa))

[0187] The joint efficiency (%) of the second steel plate = maximum load (N) during the tensile test × 100 / (thickness of the second steel plate (mm) × width of the parallel part of the test piece (mm) × tensile strength of the second steel plate (MPa))

[0188]

[0189]

[0190]

[0191] As shown in Table 3, excellent stamping formability of the welded parts was achieved in all the inventive examples. Furthermore, the static tensile properties were also excellent in the inventive examples. On the other hand, sufficient stamping formability of the welded parts was not achieved in the comparative examples.

[0192] Symbol Explanation

[0193] 1: Butt welding section

[0194] 2: First steel plate

[0195] 2-1: HAZ

[0196] 2-2: Base Material Section

[0197] 3: Second steel plate

[0198] 3-1: HAZ

[0199] 3-2: Base Material Section

[0200] 4: First fusion section

[0201] 5: Second fusion section

[0202] 6: HAZ boundary of the first steel plate – base material section

[0203] 7: HAZ boundary of the second steel plate – base material section

[0204] 8: Materials to be joined

[0205] 9: Laser source

[0206] 10: Laser

[0207] 11: Laser focus position

[0208] 12: Cracks

[0209] 13: Outer edge line of the stretch forming section

Claims

1. A laser-welded joint comprising a first steel plate, a second steel plate, and a butt weld portion between the first steel plate and the second steel plate. Either or both of the first steel plate and the second steel plate are high-strength steel plates with a tensile strength of 980 MPa or higher. The weld heat-affected zone of the high-strength steel plate satisfies the relationship of equation (1). in, H s－max The maximum Vickers hardness at the surface layer of the laser-incident side of the weld heat-affected zone of a high-strength steel plate. H s－min The minimum Vickers hardness at the surface layer of the laser-incident side of the heat-affected zone of a high-strength steel plate welded to a weld.

2. The laser-welded joint according to claim 1, wherein, The base material of the high-strength steel plate satisfies the relationship of the following formula (2). in, H bm The average Vickers hardness at the center of the thickness of the base material of a high-strength steel plate. H bs The average Vickers hardness of the surface layer on the laser-incident side of the base material of a high-strength steel plate.

3. The laser-welded joint according to claim 1 or 2, wherein, The weld heat-affected zone of the high-strength steel plate satisfies the relationship of the following equation (3). in, H m－min The minimum Vickers hardness at the center of the weld heat-affected zone of a high-strength steel plate. H s－min The minimum Vickers hardness at the surface layer of the laser-incident side of the heat-affected zone of a high-strength steel plate welded to a weld.

4. A method for manufacturing a laser-welded joint, comprising: The process of obtaining a laser-welded joint by laser welding the materials to be joined by butt-jointing the first steel plate and the second steel plate. in, Either or both of the first steel plate and the second steel plate are high-strength steel plates with a tensile strength of 980 MPa or higher. The following relationship (4) is satisfied in the laser welding process. in, P: Laser output, measured in kW V: Welding speed, in m / min H bm The average Vickers hardness at the center of the thickness of a high-strength steel plate. H bs The average Vickers hardness of the surface layer on the laser incident side of a high-strength steel plate.

5. The method for manufacturing a laser-welded joint according to claim 4, wherein, The high-strength steel plate satisfies the following relationship (2). The following relationship (5) is satisfied in the laser welding process. (V / 3)+(D / 10)+2≤P≤[(V / 3)+8]×(H bm / H bs )+(D / 10) (5) in, P: Laser output, measured in kW V: Welding speed, in m / min D: The amount of defocusing of the laser focal point relative to the surface of the materials being joined, measured in mm. H bm The average Vickers hardness at the center of the thickness of a high-strength steel plate. H bs The average Vickers hardness of the surface layer on the laser incident side of a high-strength steel plate.

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