Structural joints and frameworks

Abstract

To provide a structural joint that is able to firmly connect structural members of a framework structure.SOLUTION: The present invention provides a structural joint made of cast steel and having a connecting portion to which an end part, etc. of a structural member constituting a framework is connected. The maximum thickness of the connecting portion is 0.5 to 5 mm. The amount of C contained in the entire cast steel is 0.05 to 0.5 mass%. The cast steel may further contain 0.1 to 3 mass% of Mn and 0.01 to 2 mass% of Si. The structural joint is also excellent in spot weldability with the structural member. When the cast steel is cast using, as at least a part of a raw material, an end material of a steel plate resulting from forming the structural member, the end material can be horizontally recycled. As a result, circular economy and carbon neutral can be promoted in the manufacture of a framework structure.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to structural joints and the like. [Background technology]

[0002] The basic framework of an automobile body is often a monocoque structure, which does not have a chassis such as a ladder frame. A monocoque structure is made up of numerous structural members (various pillars, rails, members (frames), side sills, center tunnel, roof panel, underbody, etc.) pressed from steel plates, which are connected (joined, joined) by spot welding, beam welding, etc.

[0003] The mechanical properties (especially rigidity) of a monocoque structure are important from the standpoint of driving performance as well as safety (ability to absorb external impacts). The rigidity of a structure is greatly affected not only by the shape and material of each structural member, but also by the structure of the parts where structural members overlap (multiple connection parts). For this reason, many proposals have been made regarding such connection structures, and for example, the following patent documents contain relevant disclosures: [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 5-286457 [Patent Document 2] Patent Publication No. 2004-306698 [Patent Document 3] Patent Publication No. 2014-226985 [Patent Document 4] Patent Publication No. 5-65078 [Patent Document 5] Patent Publication No. 2002-68013 Summary of the Invention [Problem to be solved by the invention]

[0005] Patent Documents 1 to 3 propose connecting structures for a center pillar and a side sill. Each of these documents describes providing a reinforcement to reinforce the connection between the center pillar and the side sill, but does not describe using a joint to connect them.

[0006] Patent Document 4 proposes a joint that connects a front pillar and a roof rail, which are made by pressing steel plates. However, this joint is made of cast iron, which has inferior mechanical properties to steel, so the entire joint has to be thick, which increases the weight and makes it unsuitable for a monocoque structure, which requires lightweight construction.

[0007] Patent Document 5 proposes to connect the ends of the sash, horizontal frame, and vertical frame by casting. This method also leads to an increase in weight, as in Patent Document 4. Furthermore, in the case of Patent Document 5, it is necessary to combine the casting process with the joining process, which is not efficient.

[0008] The present invention has been made in view of the above circumstances, and has as its object to provide a new connection structure for members that constitute a framework. [Means for solving the problem]

[0009] As a result of intensive research into solving this problem, the inventor came up with the idea of forming a frame using thin-walled joints made of cast steel, and realized this idea. By expanding on this idea, the inventor has completed the present invention, which will be described below.

[0010] Structural joints The present invention is a structural joint having a connecting portion where the ends of structural members that make up the framework are joined, the maximum thickness of the connecting portion being 0.5 to 5 mm, and made of cast steel containing 0.05 to 0.5 mass % C.

[0011] The structural joint of the present invention (also simply referred to as "joint") is made of cast steel, which has excellent rigidity and strength, and is thin-walled. By connecting structural members via this joint, the mechanical properties (rigidity, strength, etc.) of the skeletal structure composed of multiple structural members can be improved and the weight can be reduced.

[0012] Furthermore, the use of such joints can simplify the connection structure and the connection process, and increase the degree of freedom in the shape of structural members (simplification, linearization, etc.).It is also expected that this will enable the setting of processing reference points at joints, the unification of assembly processes that were previously divided for each platform, space savings in manufacturing factories, and improvements in material yield and productivity when pressing structural members.

[0013] Skeletal Structure The present invention can also be understood as a skeletal structure (also simply referred to as a "structure"). For example, the present invention may be a skeletal structure including the above-mentioned structural joint and a plurality of structural members whose ends or the like are connected to the structural joint.

[0014] "others" Unless otherwise specified, "x to y" in this specification includes a lower limit value x and an upper limit value y. Any numerical value included in the various numerical values or numerical ranges described in this specification may be used as a new lower limit or upper limit value to create a new range such as "a to b." Furthermore, unless otherwise specified, "x to y mm" in this specification means x mm to y mm. The same applies to other unit systems. [Brief explanation of the drawings]

[0015] [Figure 1A] FIG. 1 is a schematic diagram showing an analytical model. [Figure 1B] FIG. 2 is an enlarged schematic view showing a spot welded portion of the analysis model. [Figure 1C] FIG. 1 is a schematic diagram showing analysis conditions and analysis target (deformation amount). [Figure 2] 10 is a photograph showing a prototype joint made of cast steel. [Figure 3A]1 is a photograph showing a cast steel plate subjected to spot welding. [Figure 3B] 1 is a table showing current patterns and current conditions for spot welding. [Figure 3C] 1 is a micrograph showing a cross section of a spot weld. [Figure 4] FIG. 1 is a schematic diagram illustrating joints, structural members, and skeletal structures. DETAILED DESCRIPTION OF THE INVENTION

[0016] One or more components selected arbitrarily from this specification may be added to the above-described components of the present invention. The contents described in this specification may also apply to joints, structural members, skeletal structures, and their manufacturing methods, as appropriate. Which embodiment is best depends on the target, required performance, etc.

[0017] Structural joints (1) Connecting part The joint has connecting parts that are connected (coupled, joined, etc.) to structural members. The connecting parts may be arranged along one direction (e.g., the X direction), two directions (e.g., the X direction and the Y direction), or three directions (e.g., the X direction, the Y direction, and the Z direction).

[0018] The connecting portion may be flat, curved, annular, etc., as long as it has a shape that corresponds to the shape of the joining portion (end portion, etc.) of the structural member. The structural member and the connecting portion may be connected by welding (spot welding, beam welding, etc.), fitting, fastening (screw connection, rivet connection, etc.), etc.

[0019] The connecting portion may be thin, for example, with a maximum thickness of about 0.5 to 5 mm, 1 to 3.5 mm, or 1.5 to 2 mm. This reduces the weight of the joint and thus the skeletal structure, reduces steps at the connecting portion, and increases the degree of freedom in shape. Note that portions other than the connecting portion that are connected to the ends of the structural members (for example, portions where connecting portions gather (corners, bases, raised portions, etc.), reinforcing portions (ribs, etc.)) may be thicker than the connecting portion.

[0020] (2) Cast steel The cast steel constituting the joint has a lower C content (less than 2.14%) than cast iron (C: 2.14 to 6.67%). The C content is, for example, 0.05 to 0.5%, 0.1 to 0.4%, or 0.2 to 0.35%. C affects the strength (hardness) and hardenability of the casting. If the C content is too low, the effect is poor, while if the C content is too high, the ductility and toughness may decrease. Unless otherwise specified, the chemical composition (composition of elements) referred to in this specification is expressed as a mass percentage relative to the entire cast steel (casting) (100 mass%) and is simply expressed as "%".

[0021] The cast steel may be ordinary cast steel (carbon steel cast steel) or alloy cast steel. Ordinary cast steel is composed of one or more of five elements (C, Si, Mn, P, S), with the balance being Fe (including impurities). The Si content may be, for example, 0.01 to 2%, 0.05 to 0.8%, 0.1 to 0.6%, or 0.2 to 0.4%. Si contributes to improving the fluidity of the molten metal and the strength of the casting. If the Si content is too low, this effect is insufficient, while if the Si content is too high, it may cause reduced weldability, embrittlement, etc.

[0022] The Mn content may be, for example, 0.1 to 3%, 0.4 to 2.7%, or 0.8 to 2.4%. Mn can improve the strength and hardenability of castings. If the Mn content is too low, this effect is poor, while if the Mn content is too high, it can cause deterioration in weldability, ductility, etc.

[0023] Note that P and S may be added for a specific purpose or may be treated as impurities. P may be contained in an amount of, for example, 0.03% or less, 0.025% or less, or 0.02% or less. S may be contained in an amount of, for example, 0.02% or less, 0.015% or less, or 0.01% or less.

[0024] An example of the chemical composition of ordinary cast steel is as follows: C: 0.05 to 0.5%, Si: 0.01 to 2%, Mn: 0.1 to 3%, and the balance: Fe and impurities, assuming the total to be 100%.

[0025] Alloy cast steel contains one or more alloying elements other than the five elements, such as Cr, Mo, Cu, Ni, V, Ni, W, and Ti. The total content of the alloying elements is, for example, 0.1 to 3%, 0.5 to 2%, or 0.8 to 1.5%. Specifically, for example, Cr: 0.05 to 2%, 0.15 to 1.5%, or 0.3 to 0.7%, and Mo: 0.05 to 1.5%, 0.15 to 1%, or 0.2 to 0.5%, etc.

[0026] An example of the chemical composition of alloy cast steel is as follows: C: 0.1-0.5%, Si: 0.2-1%, Mn: 1-2%, Cr: 0.3-0.7%, and the balance: Fe and impurities, assuming the whole is 100%.

[0027] (3) Raw materials The raw materials for the cast steel (molten metal) and the method for preparing the molten metal are not important. Scrap may be used as at least a portion of the raw materials. For example, if a joint is obtained by melting raw materials including scrap in an electric furnace or the like, pouring the molten metal into a mold and then cooling and solidifying it, the manufacturing cost can be reduced.

[0028] At least some of the steel scraps generated during the formation of structural components may be used for this scrap. In this case, horizontal recycling is possible rather than downgrading the scraps, and the entire manufacturing process of the framework structure will promote a circular economy (CE) and carbon neutrality (CN) through the reduction of CO2 and other emissions.

[0029] Even when molten metal is prepared using scrap containing scraps of steel plates of multiple types with different chemical compositions, the chemical composition of the molten metal does not vary much, and molten metal of the desired composition can be prepared relatively stably.

[0030] (4) Supplementary information The shape of the joint is adjusted depending on the structural members to be connected and their arrangement. The entire or part of the joint may be flat, or may be bent or curved. The joint may have a reinforcing portion to ensure rigidity, etc. The joint may be used as cast, or may be subjected to plastic processing, machining (removal processing), surface treatment, heat treatment, etc. The joint may be cast in a non-oxidizing atmosphere (vacuum atmosphere, inert atmosphere), or may be cast in an oxidizing atmosphere (air atmosphere, etc.). For example, a ceramic mold having sufficient heat resistance, a sand mold, a metal mold (made of a high-melting point metal), etc. may be used as a mold for casting steel. If the ceramic mold can be reused, the manufacturing cost of the joint can be reduced.

[0031] Skeletal Structure (1) Consolidated The skeletal structure is made up of multiple structural members connected by joints. There are various connection methods, but by joining (welding, etc.) the ends of the structural members to the connecting parts of the joints, the rigidity and strength of the skeletal structure can be increased.

[0032] Because the connecting portion of the joint is thin, it is suitable for spot welding with structural members formed by pressing plate materials. Spot welding allows for efficient manufacturing of skeletal structures. Spot welding conditions should be adjusted depending on the material of the structural member and the thickness of the joined materials (joint and structural member), etc.

[0033] When spot welding structural members made of steel plates (for example, 0.5 to 3 mm thick) at the joint connection, for example, the current value is 4 to 10 kA or 5 to 8 kA (current density: 10 to 200 A / mm 2 Furthermore, 20 to 100 A / mm 2 ), the applied pressure may be 2 to 6 kN or 3 to 5 kN, and the energization time may be 25 to 300 ms or 50 to 200 ms.

[0034] When spot welding structural members made of Al (alloy) plates (for example, 0.5 to 3 mm thick) to the joint connection, for example, the current value is 11 to 15 kA or 12 to 14 kA (current density: 50 to 300 A / mm2 or 100 to 250 A / mm 2 ), and the energization time may be 50 to 600 ms or 150 to 400 ms.

[0035] The spot welding current application process may be divided into multiple processes. The current value and pressure during the process may be approximately constant or may vary. For example, the main current application process may be divided into a first current application process and a second current application process, or a pre-current application process may be performed before the main current application process to allow the joining surfaces to blend together. Furthermore, the main current application process or current application processes performed before and after it may be slope (upslope or downslope) current application processes in which the current value changes (rises or falls) over time.

[0036] The form (shape, size) and material of the electrode (tip) used in spot welding are adjusted and selected depending on the form and material of the workpiece. For example, the outer diameter (nominal diameter) of the electrode body may be φ10-20 mm or φ12-18 mm. The basic shape of the electrode tip may be, for example, a flat (F-type), radius (R-type), dome (D-type), dome radius (DR-type), truncated cone (CF-type), truncated cone radius (CR-type), etc., as specified in JIS C9304 (1999). The electrode material may be, for example, a copper alloy (chromium copper, zirconium copper, chromium-zirconium copper, alumina-dispersed copper, beryllium copper, etc.) that has excellent thermal conductivity, electrical conductivity, strength, etc.

[0037] A plurality of plate materials (two or more steel plates, a steel plate and an Al alloy plate, etc.) constituting a structural member may be overlapped and joined at one connecting portion of the joint.

[0038] (2) Structural members The structural member may be of any shape, material, or manufacturing method as long as it can be connected to the joint. The structural member may be, for example, a molded product obtained by pressing a plate material. The plate material may be a steel plate or an aluminum alloy plate.

[0039] Examples of the steel sheet include cold-rolled steel sheets, hot-rolled steel sheets, high-strength steel sheets, and hot-stamped steel sheets. The steel sheet may be surface-treated (e.g., galvanized). The Al alloy sheet typically includes 2000 to 8000 series, particularly 5000 or 6000 series. For the 5000 series, Al alloy sheets corresponding to, for example, A5052, A5083, A5005, etc., specified in JIS are used. For the 6000 series, Al alloy sheets corresponding to, for example, A6022, A6016, A6N01, etc., specified in JIS are used. The Al alloy sheet referred to in this specification also includes the A1000 series.

[0040] The thickness of each plate material may be the same or different. The thickness of the steel plate is, for example, 0.4 to 2.5 mm, 0.6 to 1.8 mm, or 0.8 to 1.4 mm. The thickness of the Al alloy plate is, for example, 0.8 to 3 mm, or 1 to 2 mm.

[0041] (3) Specific examples The skeletal structure is, for example, a monocoque structure, which is used in moving bodies such as automobiles (including motorcycles), aircraft, railroad cars, and ships.

[0042] In the case of an automobile's monocoque structure, two or more structural components that make up various pillars, rails, members (frames), side sills, center tunnels, panels, etc. are joined together via joints at their multiple connection points (see Figure 4).

[0043] It is not necessary for all structural members constituting the skeletal structure to be connected via joints. Furthermore, resin materials such as adhesives and sealants may be interposed, filled, or supplemented at the joints of the structural members. [Example]

[0044] The mechanical properties and spot weldability of the joints were evaluated using analytical models or actual prototypes. The present invention will be described in more detail with reference to these specific examples.

[0045] [First Example] (Numerical Analysis) (1) Model As shown in Figure 1A, a three-dimensional simplified model of a multi-joint in three orthogonal directions was created using SpaceClaim (manufactured by ANSYS, Inc.). Specifically, Model 1, which simulates an integral joint made of steel casting, and Model C1, in which the ends of angle iron made of steel plate are overlapped and spot welded together, were created. The detailed dimensions of each model are as shown in Figure 1A. The plate thickness of both the joint and the angle iron was 2 mm.

[0046] As shown in Figure 1B, model C1 is assumed to have two overlapping steel plates joined at four spot welds (each nugget: φ5 mm × thickness 0.1 mm) in each of three orthogonal planar areas (connection sections).

[0047] (2) Analysis As shown in Figure 1C, the applied load, boundary conditions (fixed end), analysis items (deformation amount), etc. were set for each model, and FEM analysis was performed using ANSYS Mechanical (manufactured by ANSYS, Inc.). The results are summarized in Table 1.

[0048] The material of each model is structural steel (Young's modulus: 200 GPa, density: 7.85 g / cm 3 The spot weld (nugget) was also made of the same structural steel, and the steel plate and spot weld of Model C1 were connected as a single unit. The axial stiffness (kN / cm) shown in Table 1 was calculated by dividing the load (kN) by the deformation (cm) along the X-axis of the apex of the model.

[0049] (3) Evaluation As is clear from Table 1, Model 1, which was made of a single piece of cast steel, was more rigid and lighter than Model C1, which was made by simply spot-welding the ends of structural members made of steel plates. Specifically, Model 1 achieved approximately 12% higher rigidity and approximately 17% lighter weight than Model C1.

[0050] [Second Example] (Casting) A cast steel joint (actual product) with the same shape as Model 1 was fabricated as follows. A raw material consisting of structural high-tensile carbon steel (SCMnCr-2A: Fe-1.36% Mn-0.41% Si-0.43% Cr-0.29% C) specified in JIS (G 5111-1991) was melted in a vacuum. The resulting molten metal (1580°C) was poured into a mold with a cavity identical to Model 1 and cooled to solidify, a precision casting process. This casting was performed in an air atmosphere. A ceramic mold fired at 900°C was used as the mold. In this way, the cast steel joint (Sample 2) shown in Figure 2 was actually obtained. As is clear from Figure 2, no defects (shrinkage cavities, cracks, etc.) or shape defects were observed in the external appearance of the prototype joint.

[0051] [Third Example] (Welding) (1) Welding material The weldability of a cast steel joint and a pressed steel plate molded product (structural member) was confirmed as follows. The cast steel plate and pressed steel plate shown in FIG. 3A were prepared. The cast steel plates were cast in the same manner as in Example 2, using ceramic molds with different cavity shapes. Four types of cold-rolled steel plates (SPC270, SPC590, SPC780, and SPC1180) specified in JIS (G 3141-2005) were used for the pressed steel plates. The thickness of the cast steel plate was 2 mm, and the thickness of the pressed steel plate was 1 mm. Both had a length of 100 mm and a width of 30 mm.

[0052] (2) Spot welding The cast steel plate and each pressed steel plate (plate thickness: t) were spot-welded according to the current pattern shown in Figure 3B so that the two plates were joined with a nugget of φ5 mm (5√t). Specifically, a servo pressure spot welder (manufactured by Aichi Sangyo Co., Ltd.) was used to perform the first and second current-flow processes while controlling the DC current.

[0053] The first current application step was performed by up-slope current application, where the current value was changed linearly from 6 kA to 7 kA. The second current application step was performed at a constant current value of 7 kA. The current application time for each step is summarized in the table in Figure 3B. Note that only the first current application step was performed for Samples 33 and 34.

[0054] The electrode pressure applied to the plate assembly was kept constant at 3 kN from the start to the end of the spot welding. A pair of DR-type (JIS C9304) commercially available tips (tip diameter φ6 mm / WWT-CT-155 manufactured by Yamaichi Co., Ltd.) were used.

[0055] In this way, spot-welded test pieces (samples 31 to 34) were obtained. The center of the weld (nugget) of each test piece was cut and processed, and its metal structure was observed under a microscope. The observed images (photographs) are summarized in Figure 3C.

[0056] As is clear from Figure 3C, no welding defects (unbalanced penetration in the nugget, large blowholes reaching the base metal surface, cracks at the weld edge, etc.) were observed in any of the samples. In other words, it was confirmed that spot welding of cast steel plate and pressed steel plate can be performed satisfactorily, even if the type of pressed steel plate (chemical composition, etc.) and welding conditions (current pattern, etc.) are different.

[0057] [Fourth Example] (Mechanical Properties) As shown in Table 2, several cast steel plates (100 mm x 30 mm x 2 mm thick) with different chemical compositions were produced by precision casting in the same manner as in Example 3 (samples 41 to 45). The molten metal was prepared using raw materials with known chemical compositions, such as iron ingots, carbon sources, ferrosilicon, and ferromanganese, without using scrap such as mill ends. The chemical compositions (compositions) shown in Table 2 are the analytical values of the obtained cast steel plates.

[0058] Tensile tests were conducted using test pieces cut out from each cast steel plate to measure their tensile properties (mechanical properties). The results are also shown in Table 2.

[0059] The mechanical properties of the cast steel plates vary depending on their chemical composition, but their strength (tensile strength, 0.2% yield strength) is roughly equivalent to that of pressed steel plates in the 270-780 MPa range. Furthermore, all samples had elongation of 7% or more. These results confirmed that cast steel plates are also sufficiently reliable for use as structural members (joints) in skeletal structures.

[0060]

Table 1

[0061]

Table 2

Claims

1. a plate-shaped connecting portion to which the ends of the structural members constituting the framework are joined; the connecting portion has a maximum thickness of 0.5 to 5 mm and is made of cast steel; The cast steel is ordinary cast steel or alloy cast steel containing 0.05 to 0.5 mass% of C based on the total amount of the cast steel, The ordinary cast steel is composed of one or more of five elements (C, Si, Mn, P, S) and the balance being Fe and impurities, The alloy cast steel for structural joints contains alloy elements other than the five elements in a total amount of 0.1 to 3 mass % relative to the entire steel.

2. 10. The structural joint of claim 1, wherein the connecting portion is spot welded to the structural member.

3. 2. A structural joint according to claim 1, wherein the cast steel is formed by solidifying a molten metal containing, as a raw material, at least a portion of the steel plate scraps generated during the forming of the structural member.

4. 2. The structural joint according to claim 1, wherein the cast steel contains, based on the total mass of the cast steel, 0.1 to 3 mass % of Mn and / or 0.01 to 2 mass % of Si.

5. 2. The structural joint according to claim 1, wherein the maximum thickness of the connecting portion is 1 to 3.5 mm.

6. 2. The structural joint according to claim 1, wherein the cast steel contains 0.06 to 0.35 mass % of C.

7. 5. The structural joint according to claim 4, wherein the cast steel contains 0.8 to 2.4 mass % of Mn.

8. A structural joint according to any one of claims 1 to 7; a plurality of structural members connected to the structural joint; A skeletal structure comprising:

9. 9. The skeletal structure according to claim 8, which is a monocoque structure.

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