JP2007008212A - Part for automobile and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a tie-down having desired strength and attempted to reduce its weight and its manufacturing cost, and its manufacturing method. <P>SOLUTION: This tie-down 1 is an integral fabricated article made of a steel plate 2 and is furnished with both of a bead 5 provided in a recessed shape or in a protruded shape and a structure 6 with martensite as a main body on a peripheral part of a burring hole 3 provided on a bottom surface 1a of this integral fabricated article and having a cylindrical flange 4 in the circumference. It is manufactured by forming the bead 5 in the recessed shape or in the protruded shape on the peripheral part of the burring hole 3 on the integral fabricated article and making the high frequency quenched structure 6 with martensite as the main body by carrying out high frequency quenching. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an automotive part and a method for manufacturing the same. Specifically, the present invention relates to an automobile part that can be reduced in weight and reduced in manufacturing cost while having a desired strength, and a method for manufacturing the same.

  In light of environmental problems and economic efficiency, demands for reducing the weight of automobile bodies have increased in recent years. In connection with this, the weight reduction is calculated | required also for each components for motor vehicles (henceforth only a "component"). In order to satisfy the required strength performance, conventional automotive parts made of thin steel plates often have reinforcing parts joined to a part of the main body parts constituting the main body by spot welding or the like, and the weight inevitably increases.

  For this reason, so-called high high tension is also achieved, in which the main body part is made of a high strength material such as high tension to reduce the thickness, thereby reducing the weight. However, the high tensile strength causes molding defects such as springback deformation and cracking when the material is processed into the shape of the main body part.

  Therefore, a technology for increasing strength by forming a low-strength material having excellent tensile strength of about 440 MPa class into a desired automotive part shape and then heating and quenching using a laser quenching apparatus is disclosed. Has been.

For example, in Patent Documents 1 to 3, after a mild steel plate that is inexpensive and excellent in formability is formed into the shape of a desired automobile part, a high-density energy source (substantially) along the length direction of the formed product. An invention for manufacturing a high-strength press-molded product by irradiating a laser) to form a bead-like quench-hardened portion is disclosed.
JP-A 61-99620 JP-A-4-72010 JP-A-6-73438

  In order to obtain sufficient strength by the quenching strengthening method disclosed in these Patent Documents 1 to 3, a large number of laser irradiations are inevitably required. For this reason, in addition to the expensive laser equipment itself, the laser irradiation process becomes indispensable, resulting in a decrease in productivity and an inevitable increase in the manufacturing cost of automotive parts. As described above, in the methods disclosed in Patent Documents 1 to 3, there is a problem that the processing cost increases and the productivity decreases due to the necessity of irradiation with high density energy.

  Moreover, if the reduction of productivity is accepted and many quenching strengthening parts are formed according to the invention disclosed in Patent Documents 1 to 3, it is possible to obtain a corresponding improvement in strength, but it is obtained. There is a limit to the degree of strength improvement, and a desired strength may not be obtained depending on the required strength level.

  Furthermore, particularly when the finished vehicle is mounted on a transportation vehicle such as a truck or a railway vehicle among the parts for automobiles, the tie-down hook attached to the mounting portion of the transportation vehicle is hooked to complete the completed vehicle. For parts that have tie-down hook mounting holes (burring holes) for fixing and mounting to the transport vehicle and are joined to the bottom surface of the floor plate of the completed vehicle by spot welding (hereinafter referred to as “tie-down”) In order to increase the strength around the burring hole to which is input, this portion has to be configured as a separate part by a reinforcing member, which is disadvantageous in terms of both weight and cost.

  An object of the present invention is to provide an automotive part such as a tie-down, for example, and a manufacturing method thereof, both of which are reduced in weight and reduced in manufacturing cost while having a desired strength.

  The present invention can be obtained by combining shape strengthening that improves surface rigidity by devising the shape of an automotive part that is an integrally molded processed product that does not use reinforcing parts and partial quenching strengthening by induction hardening. It is based on the original technical idea that both the weight reduction and the manufacturing cost reduction with the strength of the above can easily provide automobile parts that are the integrally formed processed products.

  The present invention is an integrally molded product having a single outer surface formed of a steel plate and having a burring hole with a cylindrical flange around it, and is provided in a concave or convex shape around the burring hole. An automotive part having both a bead and an induction-hardened structure.

  The bead in the automotive component according to the present invention is (i) provided in a direction substantially along the outline of the rising portion of the cylindrical flange, or (ii) provided in an annular shape along the outline of the rising portion of the cylindrical flange. It is desirable that

  In these automobile parts according to the present invention, the molded product is a part (tie-down) for attaching a tie-down hook that restrains the automobile during transportation, and the burring hole is a tie-down hook attachment hole. desirable.

  From another point of view, the present invention provides a bead with a concave or convex shape at the periphery of a burring hole formed on the outer surface of an integrally molded product made of a steel plate and having a cylindrical flange around it. This is a method of manufacturing an automotive part characterized by quenching.

  According to the present invention, it is possible to easily provide an automotive part, for example, a tie-down, which is reduced in weight and manufacturing cost while maintaining a desired strength.

  BEST MODE FOR CARRYING OUT THE INVENTION Best modes for carrying out automotive parts and a method for manufacturing the same according to the present invention will be described below in detail with reference to the accompanying drawings. In the following description, the case where the automotive part is a tie-down having a tie-down hook mounting hole as described above is taken as an example.

FIG. 1 is an explanatory diagram illustrating a configuration example of a tie-down 1 according to the present embodiment. FIG. 2 is an explanatory view showing the AA cross section in FIG. 1 together with the tie-down hook 7.
As shown in the figure, the tie-down 1 is an integrally formed processed product formed into a substantially groove shape by press forming a steel plate 2 as a material.

  In this example, the thickness of the steel plate 2 is in the range of 1.4 mm to 1.8 mm. Moreover, the steel plate 2 should just be what can be used for induction hardening, and is not limited to a specific steel type. In this example, a material made of general ordinary steel containing carbon is used. However, the material is not limited to this. For example, a material made of martensitic stainless steel may be used.

  As shown in FIG. 2, the bottom surface 1 a forming one outer surface of the tie-down 1 has a through-hole 3 that is a tie-down hook mounting hole for hooking a tie-down hook 7 that restrains the automobile during transportation. Is formed. This through hole 3 is a burring hole with a cylindrical flange 4 formed around it. The through hole 3 is usually formed after a prepared hole is drilled at a predetermined position of the steel plate 2 as a raw material, and the steel plate 2 is subjected to press forming to form an integrally formed processed product having a predetermined shape. By burring the prepared pilot hole, it is stretch-flanged into a cylindrical shape and perforated.

  There is a bead 5 formed in a concave or convex shape within a range from the edge of the through hole 3 to a position separated by a predetermined distance (31.5 mm in this example) in the direction along the bottom surface 1a. There is an induction hardening structure 6.

  The bead 5 is generated in a direction substantially along the outline of the rising portion of the cylindrical flange 4 by a load applied to the edge of the burring hole 3 in a direction substantially parallel to the direction in which the cylindrical flange 4 is formed (burring process direction). One or more are provided in a direction substantially parallel to the direction in which the maximum principal stress is generated. In the example shown in FIG. 1, the bead 5 is provided in an annular shape along the outline of the rising portion of the cylindrical flange 4.

  FIG. 3 shows that if a load F is applied to the tip of a strip-shaped cantilever rectangular flat plate 8 (50 × 100 mm), the cantilever rectangular flat plate 8 bends in the load direction, but the length direction of the flat plate 8 ′ having the same dimensions, That is, it is an explanatory view showing material mechanics knowledge that when a concave or convex bead 9 is provided in a direction parallel to the direction in which the maximum principal stress occurs, the amount of bending when the load F is applied is reduced. The bead 5 formed in the present embodiment is based on this material mechanical knowledge.

  The bead 5 arranged in the vicinity of the through hole 3 is desirably formed in a region from the outline of the through hole 3 to a position where the distance from the outline is 31.5 mm. Moreover, it is desirable that the through hole 3 is provided in an annular shape along the outline of the rising portion of the cylindrical flange 4 formed around the through hole 3. When the load is applied to the through hole 3 in the vertical direction, the maximum principal stress is generated in a direction substantially parallel to the outline of the through hole 3. Therefore, the maximum principal stress is generated based on the material mechanical knowledge described above. This is because providing the beads 5 in a direction parallel to the direction increases the rigidity improvement effect most.

Induction hardening structure 6 in the present embodiment is a structure mainly composed of martensite.
Induction hardening is performed on the periphery of the through-hole 3 including the bead 5 formation range. Specifically, it is desirable to perform induction hardening in a range where the distance from the outline of the through hole 3 is at least 21.5 mm, and a range where this distance is 31.5 mm at the maximum. The reason is that if the induction hardening range is narrow, sufficient strength cannot be achieved. On the other hand, even if induction hardening is performed over a wider range than necessary, the stress reduction ratio is not proportional to the hardening range. This is because quenching deformation increases and the dimensional accuracy of the tie-down 1 is impaired.

The technical significance of the tie-down 1 of the present embodiment having both the bead 5 and the induction-hardened structure 6 in a predetermined range will be described.
FIG. 4 is an explanatory diagram showing a configuration example of a conventional tie-down 10. This tie-down 10 is constituted by superposing reinforcing parts 12 at predetermined positions of the main body part 11 and joining them together by spot welding.

  Here, in the conventional tie-down 10, if the reinforcing part 12 can be omitted and the main part 11 can be used, it is possible to reduce the weight and to form an integral part (single part) and to reinforce the reinforcing part. The step of joining 12 to the main body part 11 can be omitted, and the manufacturing cost can be greatly reduced, but the strength required for the tie-down 10 cannot be satisfied.

  Further, the material and thickness of the main body part 11 are SPFC590 and 1.4 mm, whereas the material and thickness of the reinforcing part 12 are SPFC440, 2. 0 mm. When the reinforcing component 12 is omitted, since the burring hole 12a provided in the reinforcing component 12 is directly provided in the main body part 11, the main body part 11 must be configured by the SPFC 440 having good molding processability. The strength is lower than that of the SPFC 590 constituting the main body part 11 of the conventional tie-down 10.

  As described above, if the reinforcing part 12 is omitted from the conventional tie-down 10 and only the main part 11 is configured, the reinforcing part 12 is not simply omitted, but the strength of the main part 11 is reduced. Also occurs.

  In general, as one of means commonly used to compensate for the strength reduction of parts, it is known to increase the surface rigidity by devising the shape in the vicinity of the load input point. Therefore, a numerical analysis model having various beads was created, and the strength was compared with a conventional tie-down consisting of two parts. As a result, it has been confirmed that there is a suitable bead shape for improving the strength, but it is not possible to obtain the strength level of the conventional tie-down 10 only by forming a bead of any shape. I understood it.

  On the other hand, as one of means commonly used other than the formation of beads, it is known to strengthen the vicinity of the load input point by induction hardening. Therefore, a numerical analysis model that takes into account the increase in material strength due to induction hardening was created, and the strength was compared with a conventional tie-down consisting of two parts. As a result, it was found that the strength level of the conventional tie-down 10 cannot be obtained only by strengthening by induction hardening.

  Therefore, in the present embodiment, as described above, induction hardening is further performed on the region shown in FIG. 1 of the integrally molded product having the bead 5, and the structure of the steel plate 2 in this region is set as the induction hardening structure 6. As a result, a strength level equal to or higher than that of the conventional tie-down 10 can be secured for a predetermined range of the tie-down 1 that is an integrally molded product.

  As described above, the tie-down 1 of the present embodiment forms the bead 5 in order to improve the surface rigidity of the outer portion of the rising portion of the cylindrical flange 4 that is in the vicinity of the load input point where strength is required, Induction hardening is performed in a range including the bead 5. Thus, even if the tie-down 1 is configured as an integrally molded product without configuring the rising portion of the cylindrical flange 4 and its outer shell with a high-strength reinforcing member, the desired range including the edge of the through-hole 3 The strength can be effectively increased, and the tie-down 1 having a desired strength can be provided as an integrally molded product.

The tie-down 1 of the present embodiment is configured as described above. Next, the manufacturing process of this tie-down 1 is demonstrated.
First, an integrally formed processed product including a bead 5 provided in a concave shape or a convex shape at a predetermined position is manufactured by press-forming the steel plate 2 as a raw material.

  The bead 5 is formed by sandwiching a steel plate 2 in which a burring hole which is a through hole 3 is previously punched or a steel plate 2 which is not punched between the upper die and the lower die of the press and performing press forming. The When press forming is performed on the steel plate 2 in which the burring hole 3 has not been drilled, the burring hole 3 is drilled by performing burring in a subsequent process after forming the bead 5 by press molding. May be. That is, the burring process may be performed before or after the press molding.

  Next, induction hardening is performed on the above-described range of the integrally molded product in which the burring hole 3 and the bead 5 are provided on the bottom surface which is one outer surface in this way, and this range is mainly composed of martensite. Induction hardening structure 6 is used.

  In this induction hardening, a circular spiral heating coil (see FIG. 6 to be described later) having an outer diameter corresponding to the range to be induction hardening is arranged immediately above the burring hole 3, and an alternating current is applied to the heating coil. This is exemplified by induction heating.

  The current condition for energizing the heating coil is adjusted by the heating range and the target heating temperature. The heating temperature is preferably a temperature at which a martensitic structure can be obtained after cooling, that is, the Ac3 point or higher, and the upper limit is preferably about 950 ° C. When the heating temperature exceeds 950 ° C., the austenite grains become coarse, the mechanical properties of the structure after transformation may be deteriorated, and for example, a galvanized film is formed on the surface of the integrally molded processed product that is a heated part. This is because zinc may melt and evaporate.

  After heating to the target temperature in this way, the heating coil is quickly withdrawn from directly above the burring hole 3, and cooling water is ejected from the cooling jacket to rapidly cool above the critical cooling rate for obtaining a martensite structure. That is, quenching is performed. Such a cooling rate is a sufficiently secured rate in the case of normal water quenching.

In this way, according to the present embodiment, it is possible to easily provide a tie-down 1 in which both weight reduction and manufacturing cost reduction are achieved while having a desired strength.
In other words, in the present embodiment, induction hardening is performed on the formation region of the bead 5 of the integrated press-molded product in which the bead 5 is formed and the reinforcing effect is enhanced. This press molding can be performed simply by preparing a press die having the shape of the bead 5, so that there is no particular increase in cost, and in addition to induction hardening after the bead 5 is formed, induction hardening is performed. The quenching deformation which is a concern at the time can be reduced as much as possible. For this reason, according to the present embodiment, although it has a very simple configuration, it is integrally formed without using a reinforcing member, and has a desired strength and excellent dimensional accuracy, thereby reducing weight and manufacturing. A tie-down 1 can be manufactured with reduced costs.

  Although it is considered that the weight reduction of the automobile body is extremely useful and important even when the weight reduction is about several percent at most, according to the present embodiment, it is disclosed in the examples described later depending on conditions. Thus, the weight of the tie-down 1 can be reduced by more than about 20%.

  In addition, according to the present embodiment, it is possible to omit the reinforcing parts that are indispensable in the conventional tie-down, so the number of parts is reduced, the joining process of the reinforcing parts is omitted, and further, the rust prevention treatment around the reinforcing parts is performed. It is also possible to omit (wax application to a portion where early rusting is expected, such as an end portion of the reinforcing component or a joint portion between the reinforcing component and the main body component). For this reason, it is extremely useful for rationalizing the production process of automobiles.

  Furthermore, according to the present embodiment, as a material of the tie-down 1 that is an integrally molded product, a low-strength steel sheet that has good formability and is inexpensive and is formed into a desired shape, and then induction-hardened. By doing so, the strength can be increased, so that the tie-down can be provided at a much lower cost than a conventional tie-down using, for example, 590 MPa class high tension.

The present invention will be described more specifically with reference to examples.
In this embodiment, a tie-down 1 having the shape shown in FIGS. 1 and 2 is used.
In the vicinity of the through-hole 3 of the tie-down 1, a repeated low bending load in one direction due to vibration during transportation acts, so that it is required not to break when such repeated bending load acts. In addition, an excessive load due to an unexpected accident that occurs during transportation such as a rear-end collision of a transport vehicle or climbing on an obstacle may act, and the vicinity of the through-hole 3 of the tie-down 1 is such an excessive load. It is required to fix the car safely without being destroyed even if it works.

That is, the tie-down 1 is required as a necessary strength requirement not to break even with a steady repetitive low load load and an unsteady overload load.
On the other hand, a conventional tie-down 10 shown in FIG. 4 is used as a comparative example.

  As described above, the material and the plate thickness of the main body part 11 are the workable cold-rolled high-tensile steel plate SPFC590 for automobiles specified in JIS G 3135, 1.4 mm, and the material and the plate thickness of the reinforcing part 12 are It is a workable cold rolled high-tensile steel plate SPFC440, 2.0 mm, defined in JIS G 3135. The main body part 11 and the reinforcing part 12 are spot welded at predetermined positions, and the total weight of the tie-down 10 is 961 g. The through hole 12a is drilled in the reinforcing component 12 made of SPFC440 by burring.

In this example, an elasto-plastic stress analysis by a finite element method was performed on the tie-down 1 (plate thickness 1.8 t, weight 961 g) having the same weight as the conventional tie-down 10.
As a boundary condition in the stress analysis, since the tie-downs 1 and 10 are all joined to the floor panel and side frame of the automobile body by spot welding, the spot welded part is completely restrained and on the ridge line of the through hole 3. A load was applied. Since the drilling position of the through-hole 3 of the conventional tie-down 10 is offset to the vertical wall side on one side of the center in the width direction of the tie-down 10, the through-hole 3 is also located at the same position in the tie-down 1 of the present invention example. The side where the distance between the ridgeline of the through-hole 3 and the vertical wall is long, that is, the position where higher stress is generated is defined as the load input point.

The analysis was performed for both tie-downs 1 and 10 and the strength was compared.
In the tie-down 1, in order to confirm the effect of increasing the strength by introducing the bead 5, models in which beads having various shapes are arranged around the through-hole 3 were created, and the strength was examined. Fig.5 (a)-FIG.5 (c) are explanatory drawings which show the shape of three types of beads which are the representative examples in them.

  Type A shown in FIG. 5 (a) is provided with three beads 5a toward the center of the through hole 3, Type B shown in FIG. 5 (b) is provided with a bead 5b on one horizontal character, and FIG. Type C shown in (c) is provided with a bead 5 c that goes around the through hole 3 once. In addition, the detail of the cross-sectional shape of each bead 5a-5c is as having shown in FIG.

  As a result of the stress analysis, not only the tie-down not having the bead 5 but also the tie-down having only the beads 5a to 5c and not induction-hardened, there are those that exceed the strength performance of the conventional tie-down 10. There wasn't.

  Therefore, a model in which induction hardening is formed around the through-hole 3 for the tie-down having any of the beads 5a to 5c to form the induction hardening structure 6, and the strength due to the formation of the beads 5 and the induction hardening structure 6 is created. The improvement effect was confirmed by determining the material strength of the induction-hardened part by the following method and reflecting it in the numerical analysis.

  The spiral coil 13 having a substantially elliptical shape (major axis: 78 mm, minor axis: 65 mm) shown in FIG. 6 coincides with the central part of the through-hole 3 of the tie-down whose central part has any one of the beads 5a to 5c. Thus, it is arranged at a position 10 mm directly above the through hole 3, an AC current having a current of 400 A and a frequency of 10 kHz is applied to the coil 13 to perform a magnetic field analysis, and Joule heat is obtained by estimation calculation from the generated eddy current distribution. It was.

A temperature analysis was performed using the Joule heat generation obtained by the magnetic field analysis as a heat input condition, and an austenite volume fraction ξA was estimated from the heating temperature T due to the Joule heat generation. At this time, if the heating temperature T is less than the Ac1 point (740 ° C.), the austenite volume fraction ξA is 0.0, and if the heating temperature T is not less than the Ac3 point (880 ° C.), the austenite volume fraction ξA is When the heating temperature T was within the range, the austenite volume fraction was obtained by applying an empirical formula defined as the following formula (1). In the equation (1), C and D depend on the material determined by the least square method so that the difference between the experimental result of investigating the relationship between the temperature and the austenite transformation rate and the approximation result by the equation (1) is minimized. It is a coefficient.
ξA = 1.0−exp (−C * ((T−Ac1) / (Ac3−Ac1)) D) (1)
Then, it is assumed that the austenite having a volume fraction ξA is transformed into martensite by cooling, and the material strength Pmixture of the mixed phase is obtained by applying the linear mixing rule of equation (2) as martensite volume fraction ξM = ξA. It was.
Pmixture = PM × ξM + PF × (1.0−ξF) (2)
In the formula (2), PM is the material strength of the martensite single phase, and PF is the material strength of the base material. Here, “material strength” refers to mechanical properties such as elastic modulus, yield stress, and work hardening coefficient. Note that (ξF + ξM), which is the sum of the volume fractions of the parent phase and the martensite phase, is 1.0.

  Depending on the cooling rate, an intermediate structure such as pearlite or bainite appears in the inside of the plate, but the phase transformation when the thin plate material is quenched may be considered that all austenite is transformed into martensite. In order to obtain the strength evaluation criteria described later, a hardened material was actually created and its mechanical material properties were investigated. In the hardness measurement and structural observation of this specimen, the transformation phase was a martensite single-phase structure. It was.

  FIG. 7 is an explanatory diagram showing an example of the distribution of the martensite hardening region estimated from the magnetic field analysis and the temperature-structure analysis. The numerical values expressed in percentage shown in FIG. 7 are martensite volume fractions. For example, the portion indicated as “80%” indicates that 80% is martensite and the remaining 20% is the parent phase. The material strength of this mixed phase region is calculated from the equation (2). Since the formation position of the through hole 3 is offset to the vertical wall side on one side from the center of the part (width direction), the eddy current density on the side closer to the vertical wall becomes higher and the distribution of the martensite phase is slightly biased. In the periphery of the through hole 3, more than 70% of the martensite is an organization mainly composed of martensite.

  Table 1 summarizes the analysis results. FIG. 8 is a plan view showing a range where induction hardening is performed.

  Analysis number 1 in Table 1 relates to the conventional tie-down 10, analysis number 2 is obtained by removing the reinforcing component 12 from the conventional tie-down 10, and the bead 5 is not provided. Analysis numbers 3 to 5 are those in which the reinforcing beads 5a, 5b, and 5c of Type A, Type B, and Type C described above are introduced. In analysis numbers 2 to 5, induction hardening is not performed, and an attempt is made to find a dominant difference due to the difference in the beads 5a to 5c. Analysis number 6 is the case where the model without the bead 5 of analysis number 2 is quenched. In addition, analysis numbers 7, 8, and 9 were obtained by changing the induction hardening range for the Type C bead model having the highest surface rigidity improvement effect among cases 3 to 5, and examining the influence of the hardening range on the strength. It is.

  Table 1 shows two strength performances required for the tie-downs 1 and 10, ie, breaking strength evaluation and fatigue strength evaluation. Each evaluation criterion was separately collected and evaluated by the following method based on the tensile test results of the base material of the SPFC 440 and the induction-hardened material shown in Table 2 and the results of the plane bending fatigue test. In addition, the quenching temperature of the induction hardened material was 950 ° C., and the fatigue test was performed with a complete swing (stress ratio R = 0.0). Moreover, the induction hardening strengthening range L in Table 1 and FIG. 8 is shown by the distance between the point where the martensite volume fraction ξM is 90% or more and the load point.

[Evaluation of breaking strength]
The maximum stress [sigma] max generated by the load is evaluated by the load that reaches the tensile strength TS. Here, the maximum stress is the maximum value of the maximum principal stress, and in any stress distribution situation at the load 2000N of the analysis number 2, the cylindrical flange 4 near the load input point is illustrated in FIG. Occurs near the bottom of R. In analysis numbers 1 to 5 where induction hardening was not performed, the tensile strength TS of the base material was used as an index, and in analysis numbers 6 to 9 where induction hardening was performed, the breaking load was determined using the tensile strength TS of the induction hardening material as an index. FIG. 10 is a graph showing the relationship between the load of each analysis number and the generated stress.
[Fatigue strength evaluation]
Assuming a case where a load of 2000 N is repeatedly applied as a load condition during steady running, the number of repetitions of service life is obtained by superimposing the maximum stress generated at the load of 2000 N on the SN diagram as shown in the graph of FIG. (Lifetime) was evaluated.

  The reason why the normal load is set to 2000 N is that the maximum generated stress σmax in the conventional tie-down 10 becomes substantially equal to the fatigue limit σw of the base material at this load. That is, when the load exceeds this, even the conventional tie-down 10 is a load that causes fatigue failure. The life of analysis numbers 1 to 5 where induction hardening was not performed was evaluated from the fatigue test results of the base metal, and the life of analysis numbers 6 to 9 where induction hardening was performed was evaluated from the fatigue test results of the induction hardening material.

From Table 1, the graph of FIG. 10, and the graph of FIG. 11, the items (a) to (e) listed below can be understood.
(A) In the case of a tie-down (analysis number 2) made of an integrally formed product in which the bead 5 and the induction-hardened structure 6 are not formed and the reinforcing parts are simply removed, the tie-down breaks at 58% of the load of the conventional tie-down 10. .
(B) The breaking load of the tie-down (analysis number 6) subjected to induction hardening in the range of L = 31.5 mm in the tie-down of analysis number 2 rises to 84% of the conventional tie-down 10.
(C) In analysis numbers 3 to 5 in which the bead 5 is introduced to enhance the shape, the reinforcing effect of the type C bead 5c of analysis number 5 is the highest. Even without quenching strengthening, the breaking load increases to 98% of the conventional tie-down 10, and the generated stress at the time of steady load (2000 N) load is below the fatigue limit.
(D) A tie-down having a Type C bead 5c of analysis number 5 and a rupture load of analysis number 7 obtained by quenching strengthening with a strengthening range L = 11.5 mm is substantially the same as the conventional tie-down 10 and is a reinforced part. It becomes possible to omit and to make an integrally molded product. The ratio of the generated stress σI at a load of 2000 N to the fatigue limit σw is 1.78 times that of the conventional tie-down 10, which is advantageous for fatigue fracture.
(E) As the hardening strengthening range becomes wider as in analysis numbers 8 and 9, the breaking load increases and the ratio between the fatigue limit σw and the generated stress σI also increases. However, the breaking load accompanying the expansion of the quenching strengthening range and the rate of increase of the fatigue limit σw and σI ratio tend to converge, and there is a limit to the effect of increasing the strength even if quenching a wider range than necessary.

  From these findings (a) to (e), both the tie-down breaking strength and the fatigue strength of the conventional tie-down 10 can be obtained even if the shape strengthening by forming the bead 5 and the quenching strengthening by induction hardening are performed independently. It is impossible to make the above, and it can be seen that it is effective to use both shape strengthening by forming the beads 5 and quenching strengthening by induction hardening.

  In particular, the Type C integrated molded product with the beads 5c introduced so as to surround the through hole 3 is induction hardened at least in the range of L = 11.5 to 31.5 mm, so that both the breaking strength and the fatigue strength are obtained. This is desirable because it can be surely increased to the same level as or more than the conventional tie-down 10.

  In particular, the breaking strength under the condition of analysis number 9 (Type C bead 5c and induction hardening of L = 31.5 mm) is 1.5 times that of the conventional tie-down 10 and the fatigue strength is 2.0 times. is there. For this reason, if the goal is to secure the same performance as that of the conventional tie-down 10, the weight of the tie-down main body can be reduced by reducing the thickness.

  The graphs in Table 3 and FIG. 12 show the results of studying the fracture load and the generated stress at a load of 2000 N when the plate thickness is changed under the condition of analysis number 9.

From the results shown in Table 3 and the graph of FIG. 12, the following can be understood.
(F) Even if the plate thickness is reduced to 1.4 mm, the breaking load of the conventional tie-down 10 is slightly higher.
(G) Even if the plate thickness is reduced to 1.4 mm, the generated stress at a load of 2000 N is below the fatigue limit σw of the quenched material.
(H) The minimum plate thickness that achieves the same performance as the conventional tie-down 10 is 1.4 mm. In this case, the conventional tie-down 10 and an integral molding process with a plate thickness of 1.8 mm without reinforcing parts. Compared to a tie-down made of products, the weight can be reduced by about 22%.

  Incidentally, regarding the fact that the stress reduction effect was particularly high in the shape of the Type C bead 5c, the maximum principal stress of the analysis number 2 in which the bead is not introduced is a direction surrounding the through hole 3 as indicated by a black arrow in FIG. That is, it is generated substantially parallel to the through-hole 3 and it can be seen that forming the bead 5 in a direction parallel to the maximum principal stress direction is effective in reducing the stress. This is because in the bending deformation of the cantilevered rectangular flat plate 8 described with reference to FIG. 3, which is the origin of forming the bead in the present invention, the bead 9 is placed in a direction parallel to the direction in which the maximum principal stress is generated. This is the same as introducing low deformation.

It is explanatory drawing which shows the structural example of the tie-down of embodiment. It is explanatory drawing which shows the AA cross section in FIG. 1 with a tie-down hook. If a load F is applied to the tip of the cantilevered rectangular flat plate, the cantilevered rectangular flat plate will bend in the load direction, but if a concave or convex bead is provided in a direction parallel to the direction in which the maximum principal stress is generated, the amount of bending will be It is explanatory drawing which shows the material-mechanical knowledge that is decreased. It is explanatory drawing which shows the structure of the conventional tie-down comprised by superposing | stacking a reinforcement component in the predetermined position of a main body component, and carrying out spot welding. FIG. 5A to FIG. 5C are explanatory views showing tie-downs of Type A to C formed with three types of beads subjected to strength examination, respectively. It is explanatory drawing which shows the spiral coil which has a substantially elliptical shape. It is explanatory drawing which shows an example of distribution of the martensitic hardening area | region estimated from the magnetic field analysis and the temperature-structure analysis. It is a top view which shows the range which performed induction hardening. It is explanatory drawing which shows the stress distribution condition of the load input point vicinity. It is a graph which shows the relationship between a load and generated stress. It is a graph which piles up and shows the maximum stress which generate | occur | produces at the time of 2000N load on a SN diagram. It is a graph which shows the generation | occurrence | production stress at the time of a rupture load and 2000N load when plate | board thickness is changed on the conditions of the analysis number 9. FIG. It is explanatory drawing which shows the generation | occurrence | production direction of the largest principal stress.

Explanation of symbols

1 Tie-down 1a Bottom 2 Steel plate 3 Through hole (burring hole)
4 Cylindrical flange 5 Bead 6 Induction hardening structure 7 Tie-down hook 8 Cantilevered rectangular plate 9 Bead 10 Tie-down 11 Body part 12 Reinforcement part 13 Heating coil

Claims (5)

An integrally molded product made of a steel plate and having one outer surface formed with a burring hole with a cylindrical flange around it, and a bead and induction hardening provided in a concave or convex shape around the burring hole An automotive part characterized by having an organization. The automotive part according to claim 1, wherein the bead is provided in a direction substantially along an outline of a rising portion of the cylindrical flange. The automotive part according to claim 1, wherein the bead is provided in an annular shape along an outline of a rising portion of the cylindrical flange. The said molded product is a part for attaching the tie-down hook which restrains the motor vehicle at the time of transportation, and the said burring hole is a tie-down hook attachment hole. Automotive parts. For automobiles characterized in that a bead is provided in a concave or convex shape at the periphery of a burring hole with a cylindrical flange on the outer periphery formed on one outer surface of an integrally molded processed product made of a steel plate and induction hardening is performed. Manufacturing method of parts.

Images