JP7725440B2 - Structural joints and frameworks - Google Patents

Description

The present invention relates to structural joints, etc.

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 plate, which are connected (joined, joined) using spot welding, beam welding, etc.

The mechanical properties (particularly rigidity) of a monocoque structure are important from the perspective of driving performance as well as safety (ability to absorb external impacts). The rigidity of a structure is greatly influenced 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 related disclosures can be found in the following patent documents, for example.

JP 5-286457 JP 2004-306698 Patent Publication No. 2014-226985 JP 5-65078 JP 2002-68013

Patent Documents 1 to 3 propose connecting structures for the center pillar and side sill. While each of these documents describes providing a reinforcement to strengthen the connection between the center pillar and side sill, there is no mention of using a joint to connect them.

Patent Document 4 proposes a joint made of press-formed steel plate that connects a front pillar and a roof rail. However, because this joint is made of cast iron, which has inferior mechanical properties to steel, the entire joint must be thick, resulting in increased weight and making it unsuitable for a monocoque structure, which requires lightweight construction.

Patent Document 5 proposes connecting the ends of the sash, horizontal frame, and vertical frame by casting. This method, like Patent Document 4, also results in increased weight, etc. Furthermore, in the case of Patent Document 5, the casting process must be carried out simultaneously with the joining process, etc., which is not efficient.

The present invention was made in light of these circumstances, and aims to provide a new connection structure for the components that make up the framework.

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 put this idea into practice. By further developing this result, the present invention, described below, was completed.

Structural joints
The present invention provides a structural joint having a connecting portion at which the ends of structural members constituting a framework are joined, the connecting portion having a maximum thickness of 0.5 to 5 mm, and made of cast steel containing 0.05 to 0.5 mass % C.

The structural joint of the present invention (also simply referred to as a "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 reduced.

In addition, the use of such joints can simplify the connection structure and connection process, and increase the degree of freedom in the shape of structural components (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 by platform, space savings in manufacturing plants, and improvements in material yield and productivity when pressing structural components.

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.

"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.

FIG. 1 is a schematic diagram showing an analytical model. FIG. 2 is an enlarged schematic view showing a spot welded portion of the analysis model. FIG. 1 is a schematic diagram showing analysis conditions and an analysis target (deformation amount). 10 is a photograph showing a prototype joint made of cast steel. 1 is a photograph showing a cast steel plate subjected to spot welding. 1 is a table showing current patterns and current conditions for spot welding. 1 is a micrograph showing a cross section of a spot weld. FIG. 1 is a schematic diagram illustrating joints, structural members, and skeletal structures.

One or more components selected from this specification may be added to the components of the present invention described above. The contents of 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.

Structural joints
(1) Connecting Portion The joint has connecting portions that are connected (coupled, joined, etc.) to structural members. The connecting portions 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).

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

The connecting parts may be thin, with a maximum thickness of, for example, 0.5 to 5 mm, 1 to 3.5 mm, or 1.5 to 2 mm. This reduces the weight of the joint and therefore the skeletal structure, reduces steps at the connecting parts, and increases the degree of freedom in shape. Note that parts other than the connecting parts that connect to the ends of structural members (for example, parts where connecting parts converge (corners, bases, raised parts, etc.), reinforcing parts (ribs, etc.)) may be thicker than the connecting parts.

(2) Cast Steel The cast steel that constitutes the joint has a lower C content (C: 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 there is too little C, the effect is poor, while if there is too much C, there may be a decrease in ductility and toughness. 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 "%".

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, 0.01-2%, 0.05-0.8%, 0.1-0.6%, or 0.2-0.4%. Si contributes to improving the fluidity of the molten metal and the strength of the casting. If there is too little Si, this effect is limited, while if there is too much Si, it may result in reduced weldability and embrittlement.

Mn content should be, for example, 0.1-3%, 0.4-2.7%, or 0.8-2.4%. Mn can improve the strength and hardenability of castings. If there is too little Mn, this effect is limited, while if there is too much Mn, it can result in reduced weldability and ductility.

Note that P and S may be added with the intention of achieving a specific function, or may be treated as impurities. P may be contained, for example, at 0.03% or less, 0.025% or less, or 0.02% or less. S may be contained, for example, at 0.02% or less, 0.015% or less, or 0.01% or less.

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

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 alloying elements is, for example, 0.1-3%, 0.5-2%, or 0.8-1.5%. Specifically, for example, Cr: 0.05-2%, 0.15-1.5%, or 0.3-0.7%, Mo: 0.05-1.5%, 0.15-1%, or 0.2-0.5%, etc.

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 total is 100%.

(3) Raw Materials The raw materials of the cast steel (molten metal) and the method for preparing the molten metal are not important. Scrap may be used as at least a part 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.

At least some of the steel scraps generated during the formation of structural members 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) by reducing _CO2 emissions.

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

(4) Supplementary Information The shape of the joint is adjusted according to 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 the mold for casting steel. If the ceramic mold can be reused, the manufacturing cost of the joint can also be reduced.

Skeletal Structure
(1) Connection 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.

Because the connecting portion of the joint is thin, it is suitable for spot welding to structural members made from press-formed plate material. Spot welding allows for the efficient manufacture of skeletal structures. Spot welding conditions should be adjusted depending on the material of the structural member, the thickness of the materials to be joined (joint and structural member), etc.

When spot welding a structural member made of steel plate (e.g., 0.5 to 3 mm thick) to a connecting portion of a joint, the current value may be, for example, 4 to 10 kA or 5 to 8 kA (current density: 10 to 200 A/mm ² or even 20 to 100 A/mm ² ), the pressure may be 2 to 6 kN or 3 to 5 kN, and the current application time may be 25 to 300 ms or 50 to 200 ms.

When spot welding a structural member made of an Al (alloy) plate (e.g., 0.5 to 3 mm thick) to the connecting portion of a joint, the current value may be, for example, 11 to 15 kA or 12 to 14 kA (current density: 50 to 300 A/mm ² or 100 to 250 A/mm ² ), and the current application time may be 50 to 600 ms or 150 to 400 ms.

The spot welding current application process may be divided into multiple steps. 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 the current application processes performed before and after it may be slope (upslope, downslope) current application processes in which the current value changes (rising, falling) over time.

The shape (shape, size) and material of the electrode (tip) used in spot welding are adjusted and selected depending on the shape 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.

Multiple plate materials (two or more steel plates, a steel plate and an Al alloy plate, etc.) that make up a structural member may be stacked and joined at one connecting portion of the joint.

(2) Structural Members The structural members may be of any shape, material, or manufacturing method as long as they are connected to the joint. The structural members may be formed, for example, by pressing a plate material. The plate material may be a steel plate or an aluminum alloy plate.

Steel sheets include, for example, cold-rolled steel sheets, hot-rolled steel sheets, high-strength steel sheets, and hot-stamped steel sheets. The steel sheets may be surface-treated (e.g., zinc-plated). Aluminum alloy sheets typically include 2000 to 8000 series, particularly 5000 or 6000 series. For the 5000 series, aluminum alloy sheets equivalent to, for example, A5052, A5083, or A5005 as specified in JIS are used. For the 6000 series, aluminum alloy sheets equivalent to, for example, A6022, A6016, or A6N01 as specified in JIS are used. The aluminum alloy sheets referred to in this specification also include the A1000 series.

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.

(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.

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 via joints at their multiple connection points (see Figure 4).

It is not necessary for all structural members that make up 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 between the structural members.

The mechanical properties and spot weldability of the joints were evaluated using analytical models and actual prototypes. The present invention will be explained in more detail using these specific examples.

[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 bars 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 bars was 2 mm.

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

(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.

The material of each model was structural steel (Young's modulus: 200 GPa, density: 7.85 g/cm ³ ). The spot welds (nuggets) were also made of the same structural steel, and the steel plate and spot welds of model C1 were connected integrally. 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.

(3) Evaluation As is clear from Table 1, Model 1, which was made of one 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 had approximately 12% more rigidity and approximately 17% less weight than Model C1.

[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 that of Model 1 and cooled to solidify, performing precision casting. 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 poor shape were observed in the appearance of the prototype joint.

[Third Example] (Welding)
(1) Welding Materials The weldability of a cast steel joint and a pressed steel plate molded product (structural member) was confirmed as follows. A cast steel plate and a 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.

(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 Fig. 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 a first current-flow process and a second current-flow process while controlling the DC current.

The first current application process was performed using an upslope current application, where the current value was changed linearly from 6 kA to 7 kA. The second current application process was performed at a constant current value of 7 kA. The current application time for each process is summarized in the table in Figure 3B. Note that only the first current application process was performed for samples 33 and 34.

The pressure applied to the plate assembly by the electrodes was 3 kN (constant) 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 for the electrodes.

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 the metal structure was observed under a microscope. The observed images (photographs) are summarized in Figure 3C.

As is clear from Figure 3C, no welding defects (unbalanced penetration in the nugget, large blowholes reaching the base material 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 well, even if the type of pressed steel plate (chemical composition, etc.) and welding conditions (current pattern, etc.) are different.

[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.

Tensile tests were conducted using test specimens cut from each cast steel plate, and their tensile properties (mechanical properties) were measured. The results are shown in Table 2.

The mechanical properties of the cast steel plates vary depending on their chemical composition, but their strength (tensile strength, 0.2% yield strength) is generally 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) that make up skeletal structures.

Claims (9)

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 . The structural joint described in claim 1, wherein the connecting portion is spot welded to the structural member. The structural joint described in claim 1, wherein the cast steel is formed by solidifying molten metal containing, at least in part, scrap steel plate generated during the forming of the structural member. The structural joint according to claim 1, wherein the cast steel contains, based on the total mass, 0.1 to 3 mass% Mn and/or 0.01 to 2 mass% Si. The structural joint described in claim 1, wherein the maximum thickness of the connecting portion is 1 to 3.5 mm. The structural joint according to claim 1, wherein the cast steel contains 0.06 to 0.35 mass% C. The structural joint according to claim 4, wherein the cast steel contains 0.8 to 2.4 mass% Mn. 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: The skeletal structure of claim 8, which is a monocoque structure.