Tailor-welded blank and manufacturing method and manufacturing equipment of tailor-welded blank

Abstract

Provided is a tailor-welded blank which can be manufactured with high productivity and at low cost even when a thin steel sheet is used as a raw material, and which has excellent press workability. The butt joint part is provided with a stirring part and a hot working influencing part adjacent to the stirring part, the stirring part is provided with an upper stirring part and a lower stirring part adjacent to each other in the thickness direction, and on the basis, the relationship of the following formulae (1)-(3) is satisfied. Pj > = 0.9 * PbmL (1) TszL > = 0.9 * TbmL (2) TszH < = 1.1 * TbmH (3).

Description

Technical Field

[0001] The present invention relates to a tailor-welded blank and a method and equipment for manufacturing the tailor-welded blank. Background Art

[0002] In recent years, reducing CO2 emissions has become a pressing issue in the automotive and other transportation equipment sectors. Consequently, efforts are underway to further reduce the weight of automobile bodies (hereinafter referred to as body lightweighting), with various proposals being investigated from various perspectives, including materials, structures, and assembly methods.

[0003] As one of the approaches to reducing the weight of such vehicle bodies, the application of tailor-welded blanks to automotive structural components is being studied.

[0004] Typically, automotive structural components are manufactured by stamping a blank made of a single raw material (steel plate). In this case, the thickness and shape of the raw material are determined based on the areas subject to the greatest stress during stamping and the areas requiring the highest levels of properties. As a result, some automotive structural components have excessive strength and properties.

[0005] On the other hand, in a tailor-welded blank, for example, high-strength and thick steel plates can be placed only in areas requiring high strength, while low-strength and thin steel plates can be placed in other areas. A tailor-welded blank is a blank formed by joining steel plates of different thicknesses and steel grades. Furthermore, a tailor-welded blank can have rust-proof steel plates placed only in areas requiring corrosion resistance, while ordinary steel plates can be placed in other areas. This allows tailor-welded blanks to be assigned appropriate properties to each part within a single blank, which is advantageous in achieving lightweight components.

[0006] As a technology related to such tailor-welded blanks, for example, Patent Document 1 discloses a "laser welding method characterized in that, in tailor-welding in which plate materials are welded and then press-formed, a thick plate and a thin plate having different thicknesses are butted together, and a laser beam is irradiated on the butted portion to achieve fusion joining, forming a step difference (d f ), the thickness of the thick plate side (t1), the thickness of the thin plate side (t2) and the average plate thickness (tave=(t1+t2) / 2) satisfy tave·tan10≤d f Step difference (d f ) to irradiate the laser beam. ".

[0007] Patent Document 1: Japanese Patent No. 3230228

[0008] Patent Document 2: Japanese Patent Publication No. 07-505090

[0009] Patent Document 3: Japanese Patent Application Laid-Open No. 2004-50189

[0010] Patent Document 4: Japanese Patent No. 3261433

[0011] Patent Document 5: Japanese Patent No. 4838385

[0012] Patent Document 6: Japanese Patent No. 4838388

[0013] Patent Document 7: Japanese Patent No. 6825630

[0014] Patent Document 8: Japanese Patent No. 6737347

[0015] Tailor-welded blanks are typically manufactured by joining steel plates (hereinafter referred to as "joined parts") as the raw materials using laser welding, as described in Patent Document 1. Laser welding offers the advantage of low thermal strain during welding. Furthermore, laser welding offers high energy density, resulting in deep penetration and high-speed welding.

[0016] However, from the perspective of reducing vehicle body weight, thinning of steel sheets due to increased strength is being promoted, particularly the use of steel sheets with a thickness of 2.5 mm or less, particularly less than 2.0 mm (hereinafter also referred to as thin steel sheets).

[0017] In the above-mentioned laser welding method, the laser light is focused by a lens, and is usually focused to a beam diameter of 1 mm or less on the workpiece. Therefore, when performing butt welding, high butt welding accuracy is required for the steel plates used as the raw materials.

[0018] Shearing is commonly used to cut steel plates. However, the precision of the cut surface obtained by shearing is not sufficient, and this tendency becomes particularly pronounced when cutting thin steel plates. Therefore, when thin steel plates cut into a predetermined shape by shearing are butted together and laser welded together, the butting precision of the end faces of the welded parts (the cut surfaces obtained by shearing) is poor, which can easily lead to weld defects. Furthermore, the focus and aiming position of the beam can shift, resulting in poor welding and uneven joint strength.

[0019] Therefore, when manufacturing tailor-welded blanks using thin steel plates as raw materials, finishing treatment of the cut surfaces by shearing is performed to improve the butt-jointing accuracy of the end faces of the welded parts, or high-precision management of the focus and aiming position of the beam is required. This results in reduced productivity and increased manufacturing costs of the product. In addition, in laser welding, as the steel plates used as the welded parts become stronger and more alloyed, there is also the possibility of cracks caused by segregation of impurities during melting and solidification (so-called solidification cracks) and cracks caused by hydrogen intrusion (so-called hydrogen cracks). In addition, the above-mentioned welding defects, solidification cracks, and hydrogen cracks all become the starting point of damage during the stamping process of the tailor-welded blank. Summary of the Invention

[0020] The present invention has been developed to solve the above-mentioned problems, and an object thereof is to provide a tailor-welded blank that can be manufactured at low cost with high productivity and excellent in punchability even when thin steel plates are used as raw materials.

[0021] Another object of the present invention is to provide a method and equipment for manufacturing the tailor-welded blank.

[0022] To achieve the aforementioned objectives, the inventors conducted extensive research. First, they investigated whether adjusting laser welding conditions could resolve the aforementioned issues when manufacturing tailor-welded parts made of thin steel plates. However, adjusting laser welding conditions alone did not effectively resolve the aforementioned issues.

[0023] Therefore, the inventors tried to apply joining methods other than laser welding when manufacturing tailor-welded blanks made of thin steel plates. As a result, the inventors obtained the following findings.

[0024] (a) In the manufacture of tailor-welded blanks using thin steel plates as raw materials, in order to solve the above-mentioned problems, the application of friction stir welding as solid-phase joining is effective. Here, friction stir welding refers to solid-phase joining that utilizes the frictional heat between the rotating tool and the workpieces to be joined and the plastic flow of the workpieces to be joined. That is, the unjoined portion (the area to be joined) of the workpieces to be joined is frictionally stirred by the rotating tool. If the unjoined portion of the workpieces to be joined is heated by frictional heat, plastic flow begins. Moreover, the interface between the plastic flow area and the base material portion (base material steel plate) is greatly elongated. As a result, the workpieces to be joined will not melt to form a joining portion (hereinafter also referred to as a joint portion or a butt joint portion).

[0025] (b) From the perspective of suppressing defects during joining and increasing the joining speed, the application of so-called double-sided friction stir welding is effective. Furthermore, technologies related to friction stir welding are disclosed in, for example, Patent Documents 2 to 8, but their application is not limited to the production of tailor-welded blanks.

[0026] (c) The butt joint of a tailor-welded blank produced by double-sided friction stir welding comprises a stirred zone (SZ) and a thermally worked affected zone (TMAZ) adjacent to the stirred zone. Furthermore, the stirred zone comprises an upper stirred zone (U-SZ) and a lower stirred zone (L-SZ) adjacent to each other in the thickness direction of the tailor-welded blank.

[0027] (d) Here, if tensile deformation occurs in the tailor-welded blank due to stamping, strain tends to concentrate in the butt joint, particularly in the hot-working affected zone, making cracks more likely to occur starting from this zone. In this regard, satisfying the relationship of the following equation (1) can effectively prevent the occurrence of cracks starting from the hot-working affected zone when tensile deformation occurs.

[0028] Pj ≥ 0.9 × PbmL…(1)

[0029] here,

[0030] Pj is the maximum tensile load per unit joint length of the butt joint (N / mm),

[0031] PbmL is the tensile strength of the first and second base steel plates (N / mm 2 ) and the plate thickness (mm), whichever is smaller (N / mm).

[0032] (e) Furthermore, if the thickness of the butt joint of the tailor-welded blank is too thin compared to the thickness of the base steel plate, tensile deformation caused by stamping will cause strain to concentrate in the butt joint, particularly in the stirred portion, and thus easily cause cracks. In this regard, satisfying the relationship of the following formula (2) can reduce the concentration of strain in the butt joint, effectively preventing the occurrence of cracks.

[0033] TszL≥0.9×TbmL…(2)

[0034] here,

[0035] TszL is the minimum thickness of the butt joint (mm),

[0036] TbmL is a value (mm) of the thinner thickness of the first base material steel plate and the second base material steel plate.

[0037] (f) Furthermore, if the thickness of the butt joint of the tailor-welded blank is too thick compared to the thickness of the base steel plate, and bending or stretching deformation occurs in the tailor-welded blank due to the press working, strain will concentrate on the butt joint, particularly in the hot working affected area adjacent to the stirred area, and cracks will be more likely to form. In this regard, by satisfying the relationship of the following formula (3), the concentration of strain on the butt joint can be reduced, effectively preventing the formation of cracks.

[0038] TszH≤1.1×TbmH…(3)

[0039] here,

[0040] TszH is the maximum thickness of the butt joint (mm),

[0041] TbmH is a value (mm) of the thicker one of the first base material steel plate and the second base material steel plate.

[0042] In addition, when the plate thickness of the first base material steel plate is the same as the plate thickness of the second base material steel plate, TbmL=TbmH.

[0043] (g) In order to manufacture tailor-welded blanks that simultaneously satisfy the relationships of the above-mentioned formulas (1) to (3), it is important to appropriately control the joining conditions of double-sided friction stir welding. Specifically, it is important to:

[0044] The diameter D (mm) of the shoulder of the rotary tool satisfies the following equation (4):

[0045] RS×D is expressed by the rotation speed RS (rpm), the diameter D (mm) of the shoulder of the rotation tool, and the joining speed JS (mm / min). 3 / JS satisfies the relationship of the following formula (5).

[0046] 4×TJ≤D≤10×TJ…(4)

[0047] 200×TJ≤RS×D 3 / JS≤2000×TJ…(5)

[0048] Here, TJ is the average value (mm) of the thickness of the first base steel plate and the thickness of the second base steel plate.

[0049] The present invention has been completed as a result of further research based on the above findings.

[0050] That is, the gist of the present invention is as follows.

[0051] 1. A tailor-welded blank comprising a first base steel plate, a second base steel plate, and a butt joint portion joining the first base steel plate and the second base steel plate, wherein:

[0052] The butt joint portion includes a stirred portion and a hot working affected portion adjacent to the stirred portion.

[0053] The stirring portion includes an upper stirring portion and a lower stirring portion adjacent to each other in the thickness direction of the tailor-welded blank.

[0054] The following relationships (1) to (3) are satisfied.

[0055] Pj ≥ 0.9 × PbmL…(1)

[0056] TszL≥0.9×TbmL…(2)

[0057] TszH≤1.1×TbmH…(3)

[0058] here,

[0059] Pj is the maximum tensile load per unit joint length of the butt joint (N / mm),

[0060] PbmL is the tensile strength of the first and second base steel plates (N / mm 2 ) and the plate thickness (mm), whichever is smaller (N / mm),

[0061] TszL is the minimum thickness of the butt joint (mm),

[0062] TszH is the maximum thickness of the butt joint (mm),

[0063] TbmL is the value (mm) of the thinner thickness of the first base material steel plate and the second base material steel plate.

[0064] TbmH is a value (mm) of the thicker one of the first base material steel plate and the second base material steel plate.

[0065] 2. The tailor-welded blank according to 1 above, wherein:

[0066] At least one of the first base steel plate and the second base steel plate has a thickness of 2.5 mm or less.

[0067] 3. The tailor-welded blank according to 1 or 2 above, wherein:

[0068] At least one of the first base steel plate and the second base steel plate has a tensile strength of 980 MPa or more.

[0069] 4. The tailor-welded blank according to any one of 1 to 3 above, wherein:

[0070] The plate thickness of the first base steel plate is different from the plate thickness of the second base steel plate.

[0071] 5. A method for manufacturing tailor-welded blanks, characterized by comprising the following steps:

[0072] A pair of rotating tools facing each other are used to press the butt joint portion, i.e., the unjoined portion, between the first and second base steel plates while rotating in opposite directions from both sides of the unjoined portion.

[0073] The rotating tool is moved in a joining direction to join the first base steel plate and the second base steel plate to obtain a tailor-welded blank.

[0074] The diameter D (mm) of the shoulder of the rotary tool satisfies the following equation (4), and

[0075] RS×D is expressed by the rotation speed RS (rpm) of the rotation tool, the diameter D (mm) of the shoulder of the rotation tool, and the joining speed JS (mm / min). 3 / JS satisfies the relationship of the following formula (5).

[0076] 4×TJ≤D≤10×TJ…(4)

[0077] 200×TJ≤RS×D 3 / JS≤2000×TJ…(5)

[0078] Here, TJ is the average value (mm) of the thickness of the first base steel plate and the thickness of the second base steel plate.

[0079] 6. The method for manufacturing a tailor-welded blank according to 5 above, wherein:

[0080] At least one of the first base steel plate and the second base steel plate has a thickness of 2.5 mm or less.

[0081] 7. The method for manufacturing a tailor-welded blank according to 5 or 6 above, wherein:

[0082] At least one of the first base steel plate and the second base steel plate has a tensile strength of 980 MPa or more.

[0083] 8. The method for manufacturing a tailor-welded blank according to any one of 5 to 7 above, wherein:

[0084] The plate thickness of the first base steel plate is different from the plate thickness of the second base steel plate.

[0085] 9. A manufacturing device for tailor-welded blanks, wherein:

[0086] Used for producing the tailor-welded blank described in any one of 1 to 4 above.

[0087] According to the present invention, even when thin steel sheets are used as raw materials, tailor-welded blanks with excellent stamping workability can be manufactured at low cost with high productivity. Furthermore, by using the tailor-welded blanks of the present invention as raw materials for automotive structural parts, for example, it is possible to further contribute to vehicle weight reduction and, consequently, to reductions in CO2 emissions. BRIEF DESCRIPTION OF THE DRAWINGS

[0088] Figure 1Schematic diagram showing an example of a cross section of a tailor-welded blank according to one embodiment of the present invention.

[0089] Figure 2 1 is a schematic diagram (side perspective view) illustrating an example of a method for manufacturing a tailor-welded blank according to one embodiment of the present invention.

[0090] Figure 3 yes Figure 2 AA view.

[0091] Figure 4 Schematic diagram showing an example of the shape of a rotating tool used in a method for manufacturing a tailor-welded blank according to an embodiment of the present invention.

[0092] Figure 5 Schematic diagram showing an example of the shape of a rotating tool used in a method for manufacturing a tailor-welded blank according to an embodiment of the present invention. DETAILED DESCRIPTION

[0093] The present invention will be described based on the following embodiments.

[0094] [1] Tailor-welded blanks

[0095] First, use Figure 1 A tailor-welded blank according to one embodiment of the present invention will be described. Figure 1 This is a cross-sectional view in the thickness direction of an example of a tailor-welded blank according to one embodiment of the present invention (t1=t2, where t1 is the thickness (mm) of the first parent steel plate, and t2 is the thickness (mm) of the second parent steel plate). In the figure, reference numeral 1 is the first parent steel plate (joined part), 2 is the second parent steel plate (joined part), 4 is the butt joint part, 4-1 is the stirring part, 4-2 is the upper stirring part, 4-3 is the lower stirring part, 4-4 is the hot working affected part (first parent steel plate side), and 4-5 is the hot working affected part (second parent steel plate side). In addition, in the figure, the vertical direction is the thickness direction of the tailor-welded blank (hereinafter, also referred to as the thickness direction). The horizontal direction is a direction perpendicular to the joining direction and perpendicular to the thickness direction (hereinafter, also referred to as the joining vertical direction). The direction perpendicular to the paper is the joining direction. That is, in Figure 1 The surface shown (herein, the cross section in the thickness direction) includes the joining vertical direction and the thickness direction. Figure 1 In FIG. 5 , an example of t1 = t2 is shown, but the relationship between t1 and t2 may be t1 < t2 or t1 > t2.

[0096] A tailor-welded blank according to one embodiment of the present invention comprises:

[0097] First parent steel plate;

[0098] Second base steel plate; and

[0099] The butt joint portion of the first base steel plate and the second base steel plate,

[0100] in,

[0101] The butt joint portion includes a stirred portion and a hot working affected portion adjacent to the stirred portion.

[0102] The stirring portion includes an upper stirring portion and a lower stirring portion adjacent to each other in the thickness direction.

[0103] The relationships of the above formulas (1) to (3) are satisfied.

[0104] [First base steel plate and second base steel plate]

[0105] The first base steel plate and the second base steel plate are steel plates serving as joined members.

[0106] The chemical compositions of the first and second base steel plates are not particularly limited as long as they are typical chemical compositions of steel plates. Examples of the chemical compositions of the first and second base steel plates include, by mass%, C: 0.01-0.70%, Si: 0.01-2.5%, Al: 0.01-2.5%, Mn: 0.1-3.5%, P: 0.10% or less, S: 0.05% or less, and N: 0.02% or less, with the remainder being Fe and unavoidable impurities. Furthermore, the above-described composition may optionally contain, in mass%, at least one element selected from the group consisting of Cr: 1.0% or less, Mo: 1.0% or less, Ni: 1.0% or less, Cu: 1.0% or less, Sn: 0.05% or less, Sb: 0.05% or less, Ca: 0.02% or less, REM: 0.05% or less, and Mg: 0.02% or less. The steel sheet having the above-described composition can be manufactured using conventional manufacturing methods, such as hot rolling, cold rolling, and other processes. The first and second base steel sheets may have the same or different compositions.

[0107] Furthermore, the first base steel plate and the second base steel plate may be steel plates having a coating on their surfaces, so-called coated steel plates. Examples of the coating include zinc coatings such as hot-dip galvanizing and alloyed hot-dip galvannealing with zinc as the main component (containing 50% by mass or more of zinc), and aluminum coatings with aluminum as the main component (containing 50% by mass or more of aluminum).

[0108] The thickness of the first and second base steel plates is preferably 0.6 to 3.2 mm. Furthermore, the thickness of at least one of the first and second base steel plates is preferably 2.5 mm or less, more preferably less than 2.0 mm. It is particularly preferred that the thickness of both the first and second base steel plates be 2.5 mm or less, more preferably less than 2.0 mm.

[0109] Furthermore, it is preferred that the tensile strength of at least one of the first and second parent steel plates is 980 MPa or more. As described above, in the laser welding method, along with the high strength and high alloying of the steel plates as the joined parts, there is also the possibility of cracks caused by the segregation of impurities during melting and solidification (so-called solidification cracks) and cracks caused by hydrogen intrusion (so-called hydrogen cracks). Moreover, this tendency becomes stronger when steel plates with a tensile strength of 980 MPa or more are used as the joined parts. Therefore, when the tensile strength of at least one of the first and second parent steel plates is 980 MPa or more, and in particular when the tensile strength of both the first and second parent steel plates is 980 MPa or more, it is particularly advantageous to apply the double-sided friction stir welding method as solid phase joining.

[0110] [Butt joint]

[0111] The butt joint portion where the first base material steel plate and the second base material steel plate are joined is composed of a stirred portion and a hot working affected portion adjacent to the stirred portion.

[0112] Among them, the stirred portion is a region where the microstructure becomes different from that of the base steel plate due to the frictional heat between the rotating tool and the workpiece and the plastic flow of the workpiece, and becomes a region where the structure is discontinuous with the hot working affected portion described later due to the strain caused by the plastic flow of the workpiece.

[0113] In addition, the stirring portion is composed of an upper stirring portion and a lower stirring portion adjacent to each other in the thickness direction. Figure 1 The upper stirring part can also be said to be formed by the rotating tool arranged on the upper side in the vertical direction. Similarly, the lower stirring part is located at Figure 1 The lower stirring portion can also be said to be formed by a rotating tool arranged at the lower side in the vertical direction.

[0114] The hot working affected zone is arranged adjacent to the stirred zone, particularly adjacent to the outer side of the stirred zone. Moreover, the hot working affected zone is a region where the microstructure is different from that of the base steel plate due to frictional heat between the rotating tool and the workpiece and plastic flow of the workpiece.

[0115] In addition, the hot working affected portion also includes the so-called heat affected portion. Here, the heat affected portion is an area that has a microstructure different from that of the parent steel plate due to the frictional heat between the rotating tool and the joined parts, although it is not subjected to the strain based on the plastic flow of the joined parts. That is, the hot working affected portion is usually composed of an area that is subjected to the strain based on the plastic flow of the joined parts (hereinafter also referred to as the hot working affected portion in a narrow sense) and an area that is not subjected to the strain based on the plastic flow, namely the heat affected portion. In addition, usually, in the vertical direction of the joining, with the stirring portion as the center, the stirring portion, the hot working affected portion in a narrow sense, the heat affected portion, and the parent steel plate are arranged in this order.

[0116] The stirred portion, the hot working affected portion (hot working affected portion in a narrow sense and heat affected portion), and the base material steel plate are defined as follows.

[0117] That is, the welded blank is Figure 1 The steel sheet is cut along the thickness (vertical) direction so that the indicated surface (i.e., the surface perpendicular to the joint and the thickness direction) forms the cut surface. The cut surface is then polished and etched with a saturated aqueous solution of picric acid, nital (a solution of nitric acid and ethanol), or aqua regia (a solution of concentrated hydrochloric acid and concentrated nitric acid mixed in a volume ratio of 3:1). The cut surface is then observed under an optical microscope, and the stirred portion (upper and lower stirred portion), hot work-affected portion, and base steel plate are defined based on the extent, shape, and location of the etching.

[0118] The stirred portion and the hot working affected portion can be distinguished in consideration of the following features, for example.

[0119] The stirred zone (SZ) is a region where the macroscopic (magnification: 5-10 times) rolled structure of the base steel plate is significantly strained or separated from the rolled structure due to plastic flow of the joined parts. In addition, the stirred zone has a microscopic (magnification: 100-1000 times) structure that is different from that of the base steel plate due to heating and cooling during joining and processing at high temperatures. In addition, in the stirred zone, for example, based on the direction of the strain in the macroscopic rolled structure, it can be confirmed that the upper stirred zone (U-SZ) and the lower stirred zone (L-SZ) are adjacent to each other in the thickness direction of the tailor-welded blank.

[0120] The narrowly defined hot work affected zone (TMAZ) is an area adjacent to the stirred zone where strain due to plastic flow in the joined parts is visible, while maintaining continuity with the macroscopic rolled structure of the base steel plate. Furthermore, the narrowly defined hot work affected zone develops a microstructure different from that of the base steel plate due to the heating and cooling during joining and the high-temperature processing.

[0121] The heat-affected zone (HAZ) within the HAZ is located adjacent to the outer side of the narrowly defined HAZ (opposite the stirred zone). While maintaining continuity with the macroscopic rolled structure of the base steel plate, strain due to plastic flow in the joined parts is not observed. Furthermore, the HAZ develops a microstructure different from that of the base steel plate due to heating and cooling during joining and high-temperature processing.

[0122] Furthermore, in the tailor-welded blank according to one embodiment of the present invention, it is important to satisfy the relationships of the above-mentioned formulas (1) to (3).

[0123] Pj ≥ 0.9 × PbmL…(1)

[0124] If tensile deformation occurs in the tailor-welded blank due to stamping, strain is likely to concentrate on the butt joint portion, especially the hot working affected portion, and cracks starting from this portion are likely to occur. In this regard, by satisfying the relationship of the above formula (1), the generation of cracks starting from the butt joint portion can be effectively prevented. Therefore, the relationship of the above formula (1) is satisfied. Pj is preferably greater than 0.95×PbmL, and more preferably greater than 0.99×PbmL. The upper limit of Pj is not particularly limited, but for example, Pj is preferably less than 1.00×PbmL.

[0125] Here, Pj (N / mm) may be obtained as follows.

[0126] Specifically, a test piece with the same shape as the No. 1 test piece specified in JIS Z 3121 (2013) was collected from the tailor-welded blank, with the butt joint lengthwise perpendicular to the lengthwise direction of the test piece and the butt joint located at the center of the parallel section. The collected test piece was then subjected to a tensile test in accordance with JIS Z 3121 (2013) to determine the maximum test force (N). The value obtained by dividing the maximum test force (N) by the width (mm) of the parallel section of the test piece was used as Pj (N / mm).

[0127] The tensile strength of the first and second base steel plates can be measured by a tensile test in accordance with JIS Z2241 (2022). For example, JIS No. 5 test pieces are collected from each of the first and second base steel plates. Then, a tensile test is performed using the collected test pieces at a crosshead speed of 10 mm / min to measure the tensile strength.

[0128] TszL≥0.9×TbmL…(2)

[0129] In addition, when the thickness of the butt joint portion of the tailor-welded blank is too thin compared to the thickness of the base steel plate, if tensile deformation occurs in the tailor-welded blank due to stamping, the strain is concentrated in the butt joint portion, especially the stirring portion, and cracks are likely to occur. In this regard, by satisfying the relationship of the above formula (2), the concentration of strain in the butt joint portion can be reduced to effectively prevent the occurrence of cracks. Therefore, the relationship of the above formula (2) is satisfied. TszL is preferably greater than 0.95×TbmL, and more preferably greater than 0.99×TbmL. The upper limit of TszL is not particularly limited, but for example, TszL is preferably less than 1.00×TbmL.

[0130] TszH≤1.1×TbmH…(3)

[0131] Furthermore, when the thickness of the butt joint portion of the tailor-welded blank is too thick compared to the thickness of the base steel plate, if bending deformation or stretching deformation occurs in the tailor-welded blank due to stamping, strain is concentrated in the butt joint portion, especially in the hot working affected portion adjacent to the stirring portion, and cracks are likely to occur. In this regard, by satisfying the relationship of the above-mentioned formula (3), the concentration of strain in the butt joint portion can be reduced to prevent the occurrence of cracks. Therefore, the relationship of the above-mentioned formula (3) is satisfied. TszH is preferably less than 1.05×TbmH, and more preferably less than 1.03×TbmH. The lower limit of TszH is not particularly limited, but for example, TszH is preferably greater than 1.00×TbmH.

[0132] In addition, TszL, TszH, TbmL and TbmH can be measured, for example, as follows. Figure 1 The plate is cut in the plate thickness (vertical) direction so that the indicated surface (i.e., the surface including the joining perpendicular direction and the thickness direction) becomes the cut surface. Then, TszL, TszH, TbmL, and TbmH are measured on this cut surface using a vernier caliper or the like.

[0133] In addition, the tailor-welded blank according to one embodiment of the present invention may further be joined to another steel plate on at least one of the first and second base steel plates. Furthermore, the joint with the other steel plate may be a butt joint similar to that described above, or may be another type of joint (e.g., a joint formed by a joining method other than double-sided friction stir welding).

[0134] [2] Manufacturing method of tailor-welded blanks

[0135] Next, a method for manufacturing a tailor-welded blank according to one embodiment of the present invention will be described.

[0136] As described above, according to the method for manufacturing a tailor-welded blank according to one embodiment of the present invention, a first base steel plate and a second base steel plate are joined by double-sided friction stir welding.

[0137] More specifically, according to one embodiment of the present invention, a method for manufacturing a tailor-welded blank comprises: rotating a pair of opposing rotating tools in opposite directions from both sides of a first base steel plate and a second base steel plate at a butt joint, i.e., an unjoined portion, and pressing the unjoined portion.

[0138] The rotating tool is moved in a joining direction, thereby joining the first base steel plate and the second base steel plate to obtain a tailor-welded blank.

[0139] First, prepare the first and second base steel plates as the joined parts. For example, cut the steel plates (or steel strips) according to the target shape to prepare the first and second base steel plates as the joined parts. The cutting method is not particularly limited, and for example, shear cutting and laser cutting can be exemplified. Shear cutting has lower docking accuracy than laser cutting, but is very advantageous in terms of productivity and manufacturing cost. Therefore, shear cutting is preferred. The cutting conditions are not particularly limited, and conventional methods can be used.

[0140] Next, the first base steel plate and the second base steel plate prepared as described above are butted together, particularly their cut surfaces are butted together, to perform double-sided friction stir welding.

[0141] Double-sided friction stir welding uses, for example, a double-sided friction stir welding apparatus comprising a pair of opposing rotary tools, a driving device for the rotary tools, a gripping device, and a control device for controlling the movement of the rotary tools. Furthermore, the control device controls, for example, the tilt angle α of the rotary tools, the position of the tip ends of the rotary tools and the distance between the tip ends (probes) (hereinafter also referred to as the inter-probe gap), the gap between the shoulders of the rotary tools (i.e., the separation distance between the shoulder of the rotary tool on the front side and the shoulder of the rotary tool on the back side in the thickness direction), the welding speed, the press load, the rotational speed of the rotary tools, and the rotational torque.

[0142] Furthermore, if Figure 2 and Figure 3 As shown, a pair of opposing rotating tools are used to press the unjoined portion consisting of the end surfaces (butting surfaces) of the first and second base steel plates while rotating in opposite directions from both sides of the unjoined portion. In this state, the rotating tools are then moved in the joining direction to join the first and second base steel plates. Figure 2 It is a side view stereogram. Figure 3 yes Figure 2AA view. In the figure, reference numeral 1 is the first base steel plate (joined member), 2 is the second base steel plate (joined member), 3-1 is a rotary tool (surface side rotary tool), 3-2 is a rotary tool (back side rotary tool), 4 is a butt joint (joining portion), 5-1 and 5-2 are shoulders, 6-1 and 6-2 are probes (pins), 7 is a holding device, and 9-1 and 9-2 are front end portions. In addition, Figure 2 The illustration of the holding device is omitted. Figure 3 In the figure, the vertical direction is the thickness direction, the horizontal direction is the joint vertical direction, and the direction near the front side of the paper is the joint direction.

[0143] In double-sided friction stir welding, the rotating tools of the friction stir welding device are respectively arranged on both sides of the first and second parent steel plates, which are the workpieces to be joined. In addition, the rotating tools arranged on the surface side (upper side in the vertical direction) of the first and second parent steel plates are sometimes referred to as surface-side rotating tools, and the rotating tools arranged on the back side (lower side in the vertical direction) of the first and second parent steel plates are referred to as back-side rotating tools. The first and second parent steel plates are arranged parallel to the joining center line shown in the figure and are respectively held by holding devices. Then, the rotating tools are rotated and pressed on both sides of the unjoined portion (predetermined joining area) located on the joining center line, that is, the butt joint portion between the end of the first parent steel plate and the end of the second parent steel plate. Then, in this state, the rotating tool is moved along the joining direction. As a result, the workpieces to be joined are softened by the frictional heat between the rotating tool and the first and second parent steel plates. Then, the softened portion is stirred with a rotating tool, thereby generating plastic flow to join the first base steel plate and the second base steel plate as the joined parts. In addition, a butt joint portion (joining portion) is formed in the portion where the joining is completed. In addition, in the butt joint portion, a hot working affected portion is arranged adjacent to the stirring portion. In addition, in the butt joint portion formed by double-sided stirring joining, the stirring portion is composed of an upper stirring portion formed by a surface side rotating tool and a lower stirring portion formed by a back side rotating tool. The upper stirring portion and the lower stirring portion are adjacent to each other in the thickness direction.

[0144] Furthermore, in the method for manufacturing tailor-welded blanks according to one embodiment of the present invention, the above-mentioned double-sided friction stir welding is applied as a joining method. On this basis, it is important that

[0145] The diameter D (mm) of the shoulder of the rotary tool satisfies the relationship of the above formula (4),

[0146] RS×D is expressed by the rotation speed RS (rpm), the diameter D (mm) of the shoulder of the rotation tool, and the joining speed JS (mm / min). 3 / JS satisfies the relationship of the above formula (5). Thus, a tailor-welded blank that satisfies the relationships of the above formulas (1) to (3) is manufactured.

[0147] 4×TJ≤D≤10×TJ…(4)

[0148] By appropriately controlling the diameter D of the shoulder of the rotating tool (hereinafter also referred to as the shoulder diameter D) according to the thickness of the unjoined portion, it is possible to effectively impart a temperature rise caused by frictional heat and a shear stress caused by frictional force to the joined parts. Here, if the shoulder diameter D is less than 4×TJ (mm), sufficient material flow may not be obtained. On the other hand, if the shoulder diameter D exceeds 10×TJ (mm), the area where material flow occurs is unnecessarily expanded, and excessive heat is input to the joint (butt joint). As a result, the strength of the joint is reduced. As a result, it is impossible to obtain a tailor-welded blank that satisfies the relationships of the above-mentioned formulas (1) to (3) at the same time. Therefore, with respect to the shoulder diameter D, the relationship of the above-mentioned formula (4) is satisfied.

[0149] 200×TJ≤RS×D 3 / JS≤2000×TJ…(5)

[0150] RS×D 3 / JS is a parameter related to the heat generation per unit joint length. 3 The range of / JS is 200×TJ to 2000×TJ, which can effectively apply the temperature rise caused by frictional heat and the shear stress caused by frictional force to the welded parts. 3 If / JS is less than 200×TJ, the heat generation may be insufficient. Therefore, it is not possible to form a metallurgically bonded joint interface between the first base steel plate and the second base steel plate, and it may be difficult to meet the specified relationship. On the other hand, if RS×D 3 If / JS exceeds 2000×TJ, the heat generated by friction stirring becomes too large, and too much heat is input into the joint. As a result, the peak temperature (maximum temperature reached) of the joint rises, or the cooling rate decreases, resulting in a decrease in the strength of the joint. As a result, it is impossible to obtain a tailor-welded blank that satisfies the relationships of the above-mentioned formulas (1) to (3) at the same time. Therefore, from the perspective of satisfying the prescribed relationship, RS×D 3 / JS preferably satisfies the relationship of the above formula (5). RS×D 3 / JS is more preferably 280×TJ or more. 3 / JS is more preferably 1600×TJ or less.

[0151] Furthermore, when the rotation speed RS and the shoulder diameter D of the front side rotation tool and the back side rotation tool are different, the front side rotation tool and the back side rotation tool respectively satisfy the relationship of the above equations (4) and (5).

[0152] Furthermore, in the method for manufacturing a tailor-welded blank according to one embodiment of the present invention, it is preferable that the tilt angle α of the rotating tool satisfies the relationship of the following formula (6).

[0153] 0°<α≤2°…(6)

[0154] Here, α is the inclination angle of the plane including the joining direction and the thickness direction (the direction perpendicular to the surface of the joined parts) relative to the rotation axis of the rotary tool (hereinafter also referred to as the rotation axis of the tool) in the thickness direction (the direction perpendicular to the surface of the joined parts). In addition, the orientation (angle) of the front end of the rotary tool with respect to the joining direction is set to +.

[0155] That is, the rotating tool is made of a material that is harder than the workpiece being joined. However, in a rotating tool made of a material that lacks toughness, such as ceramic, if a force in the bending direction is applied to the probe, stress will be locally concentrated, and there is a concern that it may cause damage. In this regard, if the tool's rotation axis is tilted by α (°) from the thickness direction and the front end of the probe is advanced relative to the joining direction, the load on the rotating tool can be borne by the rotating tool as a component force compressed in the direction of the rotation axis. This can reduce the force in the bending direction and avoid damage to the rotating tool.

[0156] Here, if the inclination angle α of the rotating tool exceeds 0°, the above-mentioned effect can be obtained. However, if the inclination angle α of the rotating tool exceeds 2°, the front and back surfaces of the butt joint portion tend to become concave. As a result, the minimum thickness of the butt joint portion tends to be lower than the thickness of the parent steel plate. As a result, this has an adverse effect on the joint strength and sometimes leads to a reduction in stamping workability. Therefore, the inclination angle α of the rotating tool is preferably set to the range of 0°<α≤2° for both the front side rotating tool and the back side rotating tool.

[0157] Furthermore, in the method for manufacturing a tailor-welded blank according to one embodiment of the present invention, it is preferable that the gap G (mm) between the shoulders of the rotating tool satisfies the relationship of the following formula (7).

[0158] 0.5×TJ-0.1×D×sinα≤G≤0.9×TJ-0.1×D×sinα…(7)

[0159] In double-sided friction stir welding, from the perspective of suppressing the occurrence of defects during welding while achieving a high welding speed, it is advantageous to appropriately control the gap G between the shoulders of the rotating tool (hereinafter referred to as the shoulder gap G). In addition, the shoulder gap G can also be said to be the separation distance between the shoulder of the surface side rotating tool and the shoulder of the back side rotating tool in the thickness direction. In particular, if the shoulder gap G is within the range of 0.5×TJ-0.1×D×sinα to 0.9×TJ-0.1×D×sinα, the shoulders of the rotating tools facing each other become in close contact with or pressed into the surface and back sides of the welded parts. As a result, the welded parts are pressed by the shoulders of the rotating tool from the surface and back sides with sufficient load, which is advantageous in suppressing the occurrence of defects during welding while achieving a high welding speed. Therefore, the shoulder gap G is preferably set to the range of 0.5×TJ-0.1×D×sinα to 0.9×TJ-0.1×D×sinα.

[0160] Furthermore, conditions other than the above are not particularly limited as long as they can obtain a tailor-welded blank that satisfies the relationships of the above formulae (1) to (3) simultaneously, and conventional methods may be used.

[0161] For example, the rotational tool speed is preferably 300 to 9000 r / min. Setting the rotational tool speed within this range is advantageous because it can effectively maintain the surface shape while suppressing the degradation of mechanical properties caused by excessive heat input. The rotational tool speed is more preferably 400 r / min or higher. Furthermore, the rotational tool speed is more preferably 8000 r / min or lower.

[0162] The joining speed is preferably 800 to 5000 mm / min, more preferably 1000 mm / min or higher, and more preferably 4000 mm / min or lower.

[0163] The position of the tip of the rotation tool, the press-fit load, the rotation torque, the gap between the probes, and the like may be appropriately set according to conventional methods.

[0164] In addition, it is preferred that the rotation direction of the surface side rotating tool and the rotation direction of the back side rotating tool are opposite to each other when viewed from the surface side (or back side) of the joined parts, and that the rotational speeds are the same. Thus, the rotational torque applied to the joined parts by the surface side rotating tool and the back side rotating tool can be offset. As a result, compared to the single-sided friction stir welding method in which the unjoined part is pressed from one surface to perform the joining, the structure of the fixture constraining the joined parts can be simplified.

[0165] In addition, if the rotation direction of the surface side rotating tool and the rotation direction of the back side rotating tool are observed as the same direction from the surface side (or back side) of the joined parts, the relative speed of the rotating tool of one party relative to the rotating tool of the other party is close to zero. As a result, the plastic flow of the joined parts approaches a homogeneous state and the plastic deformation also becomes smaller. Therefore, the heat generated by the plastic deformation of the material cannot be obtained, and it is therefore difficult to achieve a good joining state. Thus, from the viewpoint of obtaining a temperature rise and shear stress that are sufficient to achieve a good joining state in a homogeneous manner relative to the thickness direction of the joined parts, the rotation direction of the surface side rotating tool and the rotation direction of the back side rotating tool are observed as opposite directions from the surface side (or back side) of the joined parts.

[0166] Furthermore, the rotating tool used in the method for manufacturing a tailor-welded blank according to one embodiment of the present invention is not particularly limited as long as a tailor-welded blank that satisfies the relationships of the above-mentioned formulas (1) to (3) can be obtained.

[0167] For example, the front end portion of the rotating tool contacts the first and second parent steel plates as the joined parts during joining. Therefore, the front end portion of the rotating tool is formed of a material that is harder than the first and second parent steel plates under the high temperature state exposed during joining. As a result, during joining, the rotating tool can deform the first and second parent steel plates while maintaining the shape of the front end portion. As a result, high stirring capacity can be continuously achieved, and appropriate joining can be performed. In addition, the hardness of the front end portion of the rotating tool, the first and second parent steel plates can be measured and compared using a high-temperature Vickers hardness test method. In addition, only the front end portion of the rotating tool can be formed of a material that is harder than the first and second parent steel plates. Alternatively, the entire rotating tool can be formed of a material that is harder than the first and second parent steel plates.

[0168] Figure 4 and Figure 5 1 and 2 show examples of rotating tools used in the method for manufacturing tailor-welded blanks according to one embodiment of the present invention. In the figure, reference numeral 5 is a shoulder, 6 is a probe, and 8 is a peripheral portion. Figure 4 and 5 As shown, in the rotary tool, the front end portion of the rotary tool includes a shoulder (the range indicated by the shoulder diameter in the figure) and a probe (the range indicated by the probe diameter in the figure) arranged on the shoulder and sharing a rotation axis with the shoulder.

[0169] exist Figure 4 In the example of the rotary tool shown, the rotary tool has a tool diameter of 25 mm, a probe diameter of 4 mm, a shoulder diameter of 12 mm, an outer peripheral width of 6.5 mm, a probe length of 0.2 mm, and an outer peripheral taper angle θ of 15°.

[0170] exist Figure 5 In the example of the rotary tool shown, the rotary tool has a tool diameter of 25 mm, a probe diameter of 6.7 mm, a shoulder diameter of 20 mm, an outer peripheral width of 2.5 mm, a probe length of 0.7 mm, and an outer peripheral taper angle θ of 15°.

[0171] The front end portion of a conventional rotary tool is composed of a shoulder and a probe. The shoulder is a flat shape formed by a roughly flat surface or a gently curved surface. The shoulder has the function of generating frictional heat by contacting the first base steel plate and the second base steel plate while rotating during joining. In addition, the shoulder has the function of preventing the dispersion of the material by pressing the portion softened by heat and promoting plastic flow in the direction of rotation. The probe has a shape that is discontinuous with the shoulder and is in a shape that protrudes roughly perpendicularly toward the workpiece (not shown). The probe has the function of improving the stirring capacity near the center of the thickness by penetrating into the softened portion of the first base steel plate and the second base steel plate in the thickness direction during joining. In addition, the probe is usually located at the center of the shoulder.

[0172] The shoulder diameter D (mm) can be any value as long as it satisfies the relationship of the above-mentioned equations (4) and (5). Furthermore, the pin diameter and pin length of the rotary tool are not particularly limited and can be appropriately set according to conventional methods. For example, when butt-joining is performed when the thickness of the first and second base steel plates differs, the pin diameter and pin length of the rotary tool can be set according to conventional methods by taking into account the average thickness of the first and second base steel plates.

[0173] In addition, when the first base steel plate and the second base steel plate as the joined members have different plate thicknesses, as in Figure 4 and Figure 5 As shown, the tip portion further includes an outer peripheral portion adjacent to the periphery of the shoulder portion, and a rotary tool having a tapered outer peripheral portion is preferably used.

[0174] In friction stir welding, the rotating tool penetrates into the portion of the workpiece that is softened during the joining process, near the center in the thickness direction. Here, when the workpieces with different plate thicknesses are butt-joined, a step difference is generated on the surface of at least one side of the workpieces. In order to smooth the step difference when the rotating tool passes through, it is necessary to take measures such as tilting the rotating axis of the rotating tool toward the direction perpendicular to the joining (especially the side of the workpiece with smaller plate thickness). However, in the case where the workpieces are high-strength steel plates, especially steel plates with a tensile strength of 980 MPa or more, a large load is generated due to the tilting of the rotating tool toward the direction perpendicular to the joining process, so it is sometimes difficult to form a proper weld. In addition, there are also problems such as damage to the rotating tool or the need to greatly increase the rigidity of the device for setting the rotating tool.

[0175] In this regard, by forming the outer peripheral portion adjacent to the peripheral edge of the shoulder into a tapered shape, even without tilting the rotating axis of the rotating tool toward the direction perpendicular to the joining process, the step difference in the workpiece is smoothed during the passage of the rotating tool, resulting in a suitable weld. As a result, even when the first and second base steel plates, the workpieces, differ in thickness, a suitable weld can be obtained, resulting in a tailor-welded blank that satisfies the above-mentioned equations (1) to (3). Furthermore, the joining speed can be increased.

[0176] Here, the outer peripheral portion is defined as the area (annular area) from the periphery of the shoulder of the rotary tool to the circumferential end of the rotary tool at the front end of the rotary tool. Furthermore, the width of the outer peripheral portion is preferably within the range of the rotary tool diameter (mm) x 0.05 to the rotary tool diameter (mm) x 0.35.

[0177] In addition, from the viewpoint of obtaining the above-mentioned effects, the taper angle θ of the peripheral portion is preferably set to 2 to 45°. Here, when the taper angle θ of the peripheral portion is less than 2°, there is a concern that the ability to smooth the step difference of the workpiece to be joined will be reduced when the rotating tool passes. In addition, since a large stress is applied to the peripheral portion of the rotating tool, the risk of damage to the rotating tool increases. On the other hand, when the taper angle θ of the peripheral portion is greater than 45°, the contact area between the peripheral portion and the workpiece to be joined is reduced. Therefore, there is a concern that the joining ability will be reduced. Therefore, the taper angle θ of the peripheral portion is preferably set to 2 to 45°. The taper angle θ of the peripheral portion is more preferably 8° or more. In addition, the taper angle θ of the peripheral portion is more preferably 20° or less.

[0178] Here, the taper angle θ of the outer periphery refers to the angle formed by a line connecting the peripheral edge of the shoulder of the rotary tool and the circumferential end of the rotary tool, in a cross-section of the rotary tool viewed from the side (a cross-section that includes and is parallel to the rotation axis), and a line perpendicular to the rotation axis. Furthermore, the shape of the outer periphery of the cross-section of the front end face (a cross-section that includes and is parallel to the rotation axis) is not particularly limited. Examples of the shape of the outer periphery of the cross-section of the front end face include a straight line (line segment) and a continuous curve, with a straight line (line segment) being preferred.

[0179] The conditions other than those described above are not particularly limited, and conventional methods may be followed.

[0180] [3] Manufacturing equipment for tailor-welded blanks

[0181] Next, a manufacturing facility for tailor-welded blanks according to one embodiment of the present invention will be described.

[0182] The manufacturing apparatus for tailor-welded blanks according to one embodiment of the present invention is used to manufacture the tailor-welded blanks described in [1]. Examples of the manufacturing apparatus for tailor-welded blanks according to one embodiment of the present invention include a manufacturing apparatus having a double-sided friction stir welding apparatus, and a manufacturing apparatus having a steel plate cutting apparatus and a double-sided friction stir welding apparatus.

[0183] The form of the steel plate cutting device is not particularly limited as long as it can cut the steel plate into the desired shape to obtain the joined parts (the first base steel plate and the second base steel plate). As such a steel plate cutting device, a shearing machine can be exemplified. Compared with laser cutting machines and the like, a shearing machine is very advantageous in terms of productivity and manufacturing cost. The shearing machine, for example, has a punch (upper blade), a punch (lower blade) and a driving device for the punch. The driving method of the driving device is not particularly limited, and for example, an electric driving method can be adopted.

[0184] In addition, the form of the double-sided friction stir welding apparatus is not particularly limited.

[0185] The double-sided friction stir welding apparatus includes, for example:

[0186] A holding device for holding the joined parts;

[0187] a pair of rotating tools disposed opposite each other; and

[0188] The driving device is capable of rotating the rotating tool and moving it in the joining direction.

[0189] As the form of the holding device, for example, there are:

[0190] A gripping device having a movable gripping member and a sliding device for the movable gripping member; and

[0191] A gripping device comprising a fixed gripping member, a movable gripping member, and a sliding device for the movable gripping member.

[0192] The shape of the rotating tool is as shown in the example above [2].

[0193] The driving device of the rotary tool may include a rotary driving unit for the rotary tool and a device for moving the rotary tool in the joining direction. The driving method of the rotary driving unit and the moving device is not particularly limited, and for example, an electric driving method may be adopted.

[0194] The double-sided friction stir welding apparatus may also include a control device that controls the operation of at least one of the gripping device and the driving device of the rotary tool. Examples of the control device include an input unit that inputs data such as various set values, a calculation unit that performs calculations on the input data, a storage device that stores data, and an output unit that outputs an action signal to the gripping device and the driving device of the rotary tool based on the calculation results of the calculation unit. The control device controls, for example, the tilt angle α of the rotary tool, the position of the front end of the rotary tool and the gap between the probes, the gap between the shoulders of the rotary tool, the joining speed, the press load, the rotation speed of the rotary tool, and the rotational torque.

[0195] In addition, there is no particular limitation on the device configuration other than the above-mentioned ones, and any conventionally known device configuration may be appropriately adopted.

[0196] Example

[0197] Hereinafter, the functions and effects of the present invention will be described using examples. However, the present invention is not limited to the following examples.

[0198] A steel plate having the thickness and component composition shown in Table 1 (the remainder is Fe and unavoidable impurities) was cut into a size of 0.2m×0.5m using a shearing machine to prepare a first base steel plate and a second base steel plate as the joined parts. Then, the long sides of the cut surfaces of the first base steel plate and the second base steel plate were butted against each other according to the combination described in Table 2, and double-sided friction stir welding was performed on them as the joined parts under the conditions described in Table 2 to produce tailor-welded blanks. The joining length was set to 0.5m. Here, the groove is a so-called I-type groove in which a groove angle is not formed on the end faces of the two base steel plates as the joined parts. In addition, the tensile strength, Vickers hardness and Erichsen cupping value (Erichsen value) of the steel plates used as the first base steel plate and the second base steel plate are also noted in Table 1. Here, the tensile strength is the value measured by the above-mentioned method. The Erichsen cupping value is a value measured according to the Erichsen cupping test method specified in JIS Z 2247 (2022). Conditions not explicitly described were set according to conventional methods.

[0199] In the above-mentioned double-sided friction stir welding, Figure 2 In this way, the rotation direction of the surface side rotating tool arranged on the upper side in the vertical direction is clockwise when viewed from the upper side in the vertical direction, and the back side rotating tool arranged on the lower side in the vertical direction is counterclockwise when viewed from the upper side in the vertical direction. That is, when the front end of each rotating tool is viewed from the front, they all rotate counterclockwise. Figure 4 and Figure 5Any of the rotating tools with the cross-sectional dimensions and shapes shown in the figure was used. Furthermore, the front-side rotating tool and the back-side rotating tool used rotating tools with the same cross-sectional dimensions and shapes. Furthermore, these rotating tools were all made of tungsten carbide (WC) with a Vickers hardness of HV1090, which is harder than the workpiece.

[0200] Next, the butt joint of the manufactured tailor-welded blank was observed according to the above method, and the stirring area and the hot working affected area were demarcated. Figure 1 As shown, the butt joints of the manufactured tailor-welded blanks all have a stirred portion and a hot working affected portion. In addition, the stirred portion includes an upper stirred portion and a lower stirred portion adjacent to each other in the thickness direction. The hot working affected portion includes a hot working affected portion in a narrow sense and a heat affected portion. Then, for the manufactured tailor-welded blanks, Pj (N / mm), TszL (mm) and TszH (mm) were calculated according to the above-mentioned method. In addition, these results are recorded together with PbmL (N / mm), TbmL (mm) and TbmH (mm) in Table 3. In addition, Table 3 also records the fracture position when the maximum test force (N) was applied in the tensile test according to JIS Z 3121 (2013) when calculating Pj. In addition, with respect to Comparative Example 4, significant surface defects were confirmed in the following (I) Confirmation of the Presence of Surface Defects, so the measurement of TszL (mm) and TszH (mm) was omitted.

[0201] In addition, the manufactured tailor-welded blanks were subjected to (I) confirmation of the presence or absence of surface defects, (II) confirmation of the presence or absence of internal defects, and (III) Erichsen cupping test according to the following procedures. Moreover, when all (I) to (III) are qualified and the fracture position when the maximum test force (N) is applied in the tensile test according to the above-mentioned JIS Z 3121 (2013) is the first base steel plate or the second base steel plate (that is, the fracture position is not the butt joint portion (joint portion), that is, the stirring portion (SZ), the narrow sense of the hot working affected zone (TMAZ) and the heat affected zone (HAZ)), the stamping workability is evaluated as excellent. On the other hand, in cases other than the above, the stamping workability is evaluated as insufficient.

[0202] The results are shown in Table 3. In addition, when a significant defect was observed in at least one of (I) and (II), the subsequent evaluation was omitted.

[0203] (I) Presence of surface defects

[0204] The presence or absence of surface defects was checked at the stable portion of the butt joint of the tailor-welded blank. The presence or absence of surface defects was checked by visually checking for groove-shaped unjoined portions (hereinafter referred to as "unjoined portions") caused by insufficient plastic flow, and for recessed portions (hereinafter referred to as "recessed portions") at the butt joint. When the unjoined portions and recessed portions were confirmed, their depth D was measured using a laser displacement meter. d (mm). The presence or absence of surface defects was then determined according to the following criteria. The evaluation results are summarized in Table 3. The term "steady-state section" refers to the area where joining is performed while the joining speed reaches the set value. Typically, the steady-state section extends from 25 mm from the end of the tailor-welded blank at the start of joining to 25 mm from the end of the tailor-welded blank at the end of joining in the joining direction. The term "end" here refers to the end in the joining direction, and the same applies hereinafter.

[0205] <Judgment Criteria>

[0206] No defects (acceptable, excellent): No unbonded state or recessed portion was observed.

[0207] Slight defect (acceptable): Although either the unbonded state or the concave portion is confirmed, the D d / TbmL are all below 0.1.

[0208] Significant defects (unqualified): confirmed to D d At least one of an unbonded state where / TbmL exceeds 0.1, a recessed portion, and an unbonded state penetrating from the front surface to the back surface.

[0209] (II) Presence of internal defects

[0210] The presence or absence of internal defects was confirmed in the steady-state portion of the butt joint of the tailor-welded blank. Figure 1 The tailor-welded blank was cut along the thickness (vertical) direction, with the indicated surface (i.e., the surface including the surface perpendicular to the joint and the thickness direction) serving as the observation surface. Three test pieces were collected. Furthermore, the cut positions (observation surfaces) in the joining direction were set at 20 mm from the end of the stable portion at the start of the joint, 20 mm from the end of the stable portion at the end of the joint, and in the middle of the stable portion. The test pieces were collected with the cut surfaces at these cut positions serving as the observation surfaces. The observation surfaces of the resulting test pieces were then observed using an optical microscope (magnification: 10x). The presence of internal defects was then determined based on the following criteria.

[0211] <Judgment Criteria>

[0212] No defect (acceptable, excellent): No tunnel-shaped unbonded state was observed in any of the three test pieces.

[0213] Slightly defective (acceptable): In one of the three test pieces, a tunnel-shaped unbonded state was observed.

[0214] Significant defect (failure): Tunnel-shaped unbonded state was observed in two or more of the three test pieces.

[0215] (III) Erickson cupping test

[0216] The Erichsen cupping value of the tailor-welded blank was measured according to the Erichsen cupping test method specified in JIS Z 2247 (2022). Specifically, a test piece was collected from the approximate center position of the tailor-welded blank in a manner that includes the butt joint portion. Next, an Erichsen cupping tester was used to measure the height of the plastic deformation until the cracks occurred in the butt joint portion of the test piece, that is, the Erichsen cupping value. Then, the measured Erichsen cupping value was divided by the Erichsen cupping value of the parent steel plate on the side close to the crack occurrence position and multiplied by 100 to calculate the ratio of the Erichsen cupping value. Then, based on the calculated ratio of the Erichsen cupping value, the qualification was determined according to the following criteria. Table 3 also shows the Erichsen cupping value, the crack occurrence position and the ratio of the Erichsen cupping value.

[0217] Pass: Eriksson cupping value ratio is 85% or more

[0218] Disqualified: Eriksson cupping value ratio is less than 85%

[0219] [Table 1]

[0220]

[0221] [Table 2]

[0222]

[0223] [Table 3]

[0224]

[0225]

[0226] As shown in Table 3, in the inventive examples, excellent stamping properties were achieved in all tailor-welded blanks made from directly cut thin steel plates. In other words, even when made from thin steel plates, tailor-welded blanks with excellent stamping properties were obtained, which could be manufactured at low cost with high productivity.

[0227] On the other hand, in the comparative examples, sufficient stamping workability was not obtained in the tailor-welded blanks made of thin steel plates that were directly cut.

[0228] Description of Reference Numerals

[0229] 1…first base steel plate (joint); 2…second base steel plate (joint); 3-1…rotating tool (surface-side rotating tool); 3-2…rotating tool (back-side rotating tool); 4…butt joint (joining portion); 4-1…stirring portion; 4-2…upper stirring portion; 4-3…lower stirring portion; 4-4, 4-5…heat-working-affected portion; 5, 5-1, 5-2…shoulder; 6, 6-1, 6-2…probe (pin); 7…holding device; 8…peripheral portion; 9-1, 9-2…front end portion.

Claims

1. A tailor-welded blank comprising a first base steel plate, a second base steel plate, and a butt joint portion for joining the first base steel plate and the second base steel plate, wherein: The butt joint portion includes a stirred portion and a hot working affected portion adjacent to the stirred portion. The stirring portion includes an upper stirring portion and a lower stirring portion adjacent to each other in the thickness direction of the tailor-welded blank. Satisfying the following relationships (1) to (3): Pj ≥ 0.9 × PbmL…(1) TszL≥0.9×TbmL…(2) TszH≤1.1×TbmH…(3) here, Pj is the maximum tensile load per unit joint length of the butt joint (N / mm), PbmL is the tensile strength of the first and second base steel plates (N / mm 2 ) and the plate thickness (mm), whichever is smaller (N / mm), TszL is the minimum thickness of the butt joint (mm), TszH is the maximum thickness of the butt joint (mm), TbmL is the value (mm) of the thinner thickness of the first base material steel plate and the second base material steel plate. TbmH is a value (mm) of the thicker one of the first base material steel plate and the second base material steel plate.

2. The tailor-welded blank according to claim 1, characterized in that: At least one of the first base steel plate and the second base steel plate has a thickness of 2.5 mm or less.

3. The tailor-welded blank according to claim 1, characterized in that: At least one of the first base steel plate and the second base steel plate has a tensile strength of 980 MPa or more.

4. The tailor-welded blank according to claim 2, characterized in that: At least one of the first base steel plate and the second base steel plate has a tensile strength of 980 MPa or more.

5. The tailor-welded blank according to any one of claims 1 to 4, characterized in that: The plate thickness of the first base steel plate is different from the plate thickness of the second base steel plate.

6. A method for manufacturing tailor-welded blanks, characterized in that: The process is as follows: A pair of rotating tools facing each other are used to press the butt joint portion, i.e., the unjoined portion, between the first and second base steel plates while rotating in opposite directions from both sides of the unjoined portion. The rotating tool is moved in a joining direction to join the first base steel plate and the second base steel plate to obtain a tailor-welded blank. The diameter D (mm) of the shoulder of the rotary tool satisfies the following equation (4): RS×D is expressed by the rotation speed RS (rpm), the diameter D (mm) of the shoulder of the rotation tool, and the joining speed JS (mm / min). 3 / JS satisfies the following relationship (5): 4×TJ≤D≤10×TJ…(4) 200×TJ≤RS×D 3 / JS≤2000×TJ…(5) Here, TJ is the average value (mm) of the thickness of the first base steel plate and the thickness of the second base steel plate.

7. The method for manufacturing tailor-welded blanks according to claim 6, characterized in that: At least one of the first base steel plate and the second base steel plate has a thickness of 2.5 mm or less.

8. The method for manufacturing tailor-welded blanks according to claim 6, characterized in that: At least one of the first base steel plate and the second base steel plate has a tensile strength of 980 MPa or more.

9. The method for manufacturing tailor-welded blanks according to claim 7, characterized in that: At least one of the first base steel plate and the second base steel plate has a tensile strength of 980 MPa or more.

10. The method for manufacturing a tailor-welded blank according to any one of claims 6 to 9, characterized in that: The plate thickness of the first base steel plate is different from the plate thickness of the second base steel plate.

11. A manufacturing device for tailor-welded blanks, characterized in that: Used for producing the tailor-welded blank according to any one of claims 1 to 4.

12. A manufacturing device for tailor-welded blanks, characterized in that: Used for the production of the tailor-welded blank according to claim 5.

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