JP2006142355A - Method for producing high strength light weight part excellent in buckling strength and produced connecting rod for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a producing method with which a buckling strength in a high strength light weight parts produced with hot-forging, such as a connecting rod, can be improved. <P>SOLUTION: In the method for producing the high strength light weight parts excellent in the buckling strength, when a part preformed material is formed, the hot-forging is applied to a portion except the strength needing portion and rolling is locally applied to the strength needing portion after hot-forging. Particularly, in the case of producing the connecting rod, a shape having thin web part and no draft part can be formed and in comparison with the case of employing only the hot-forging, the buckling strength can drastically be improved. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for manufacturing a high-strength lightweight component that can improve buckling strength by a combination of hot forging and rolling, and in particular, a high-strength lightweight component suitable for improving the buckling strength of a connecting rod for an internal combustion engine. It relates to the manufacturing method.

  Many parts that require strength, such as automobiles, construction vehicles, and construction machines, are manufactured by hot forging. This is because there are many relatively large parts, and it is judged that hot forging is the best because of the problem of required forming load.

There is a strong demand for weight reduction of these hot forged parts, and various types of high strength measures such as optimization of material chemical composition, improvement of forging conditions and heat treatment conditions have been studied, and reduction in thickness and weight has been promoted. It was.

For example, the connecting rod is composed of a large end connected to the crankshaft, a small end connected to the piston via a piston pin, and a column part connecting the large end and the small end. The part where the stress is highest during the operation of the engine is the column part, and the improvement of the strength of this part becomes a problem.

As a method for improving the strength of the column part, as a conventionally proposed method, there is a method of performing a coining process on the column part in a cold and warm state after hot forging, and thereafter performing a heat treatment such as an aging treatment, For example, methods disclosed in Patent Documents 1 to 5 have been proposed.

JP-A-5-70828 JP-A-9-196044 Japanese Patent Laid-Open No. 10-168540 JP 2003-55715 A JP 2003-147434 A

However, on the other hand, there are some parts that have reached the limit of thin and light weight due to the limitations of conventional forging methods. For example, taking the web portion of the column in the connecting rod as an example, the usual hot press forging for generating burrs has a limit of about 3 mm, and at most about 2.5 mm even if a closed press or hammer forging is used. In addition, normal hot press forging and hammer forging have a problem that a draft is required and wasteful meat is attached, and closed press forging has a problem of significant increase in cost.

In addition, the buckling strength of the connecting rod that can withstand the explosive force of the engine is affected by the shape and dimensions of the column part in addition to the material strength. The above-mentioned conventional forging method has restrictions on draft angle and limits on dimensional accuracy control, and it has been required to create a more accurate column shape at a low cost.

The present invention aims to solve the above-described problems, and proposes a new method for manufacturing a high-strength lightweight component that achieves a reduction in wall thickness and accuracy that exceeds conventional forging methods at a low cost. With the goal.

  According to the first aspect of the present invention, when forming a rough component, a portion other than the strength required portion is processed by hot forging, and the strength required portion is locally rolled after hot forging. This is a method for producing a high-strength lightweight component with excellent buckling strength.

Of course, a hot forged product cannot be manufactured to the final shape by only hot forging, and is always finished by machining. However, the machining process is very costly, and there are some reasons, such as where it comes into contact with other parts during use, high dimensional accuracy is required even without contact, and lubricating oil is circulated. For parts that are basically used without being in contact with other parts, it is often used with the forged skin to the extent that the scale is removed. Therefore, there are many portions in which the cross-sectional shape during hot forging is maintained as it is, and the product shape unique to the forged product, such as a connecting rod, may determine the buckling strength.

In normal forging, after the material is processed with a die fixed to the ram of the press, the die is easily separated from the work material forged when the ram rises from the bottom dead center and moves to the upper part. As a matter of course, a draft is necessary, and as a result, the cross-section of the forged product also has a gradient shape.

When the forging process is in a high hydrostatic pressure state, the load increases rapidly. Therefore, when the height (thickness) of the forged product becomes thin due to compression, the load increases rapidly, and there is a limit to reducing the thickness in the same direction as the movement of the ram.

  This will be described with reference to the column portion of the connecting rod as an example as shown in FIG. That is, the outer side and inner side surfaces of the rib part have a sloped shape, and the thickness of the web part is about 3 mm in the case of a connecting rod for automobiles, and it is very difficult to process it thinner.

  Therefore, as a result of creating and investigating the test piece with regard to the presence or absence of this draft and the effect on the buckling strength due to the change in the thickness of the web part, if the cross-sectional area of the column part is the same, the draft is It has been found that the buckling strength can be increased when the web portion is small and the thickness of the web portion is small.

However, it is difficult to eliminate this gradient or reduce the thickness of the web portion by ordinary hot forging. If we try to achieve it by forging, we can consider a closed forging method. However, if closed forging is used, a very high load is required as in the case of normal forging, and the mold is complicated. There is a problem that the cost of the mold increases.

Therefore, as a result of studying other plastic working methods, parts other than the required strength parts are processed only by the conventional forging method, and the required strength parts are locally rolled after applying the conventional forging method. I came up with the idea.

Hereinafter, the content of the present invention will be described in detail for each step.
The production method of the present invention can be broadly divided into two steps, a roughing process comprising hot forging and a finishing process comprising rolling. First, the roughing process will be described.

Of course, parts such as connecting rods such as automobiles and construction vehicles are repeatedly subjected to fluctuating loads during use, but the load stress is not uniform within the parts, and there are always parts that are subject to high stress. There is a part where high stress is not applied. Of these, the latter part need not be improved in buckling strength, and the required strength can be ensured only by conventional hot forging.

In addition, there are parts that have to be processed into complicated irregularities and curved shapes, and hot forging is most suitable for efficiently mass-producing such parts. Therefore, the parts excluding the required strength parts are processed up to the final shape in a roughing process, and the required strength parts are processed into a rough shape by hot forging in order to facilitate the finishing process by subsequent rolling. That's what it meant.

In addition, since the roughing process in the roughing process needs to be performed as easily as possible in the finishing process of the subsequent process as described above, the hot forging method can be used. Therefore, it is necessary to process the shape as close to the final shape as possible.

Moreover, the strength requirement site | part said by this invention means the site | part to which the highest stress is loaded at the time of actual use, and improvement of buckling strength is required. For example, in the case of a connecting rod for an automobile engine, an intermediate column portion between the large end portion and the small end portion corresponds to the strength requirement portion.

  After the rough machining process is finished, the parts other than the strength request part are finished, and the finishing process is performed by rolling concentrated on only the strength request part. Although this rolling process can be performed after cooling without reheating, it is preferable to perform it in a hot or warm temperature range in consideration of an increase in required load. That is, it can be carried out after air forging after hot forging and reheating. However, considering energy efficiency, it is more desirable to carry out during cooling after hot forging. The rolling temperature is preferably about 1000 ° C. at which normal rolling is carried out from the viewpoint of workability. However, in order to increase the material strength, it is not re-opened at about 700 to 850 ° C. It is desirable to process in the crystallization temperature range, and thereby the crystal grains can be made finer.

  In addition, when using a steel material whose strength can be improved by aging treatment, the strength may be greatly improved by the strain introduced at the time of rolling. In that case, as the finish rolling, in the cold rolling or non-recrystallization temperature range. In some cases, it may be desirable to consider the choice of rolling.

Further, in the present invention, since the finishing process is performed by rolling, it is possible to reduce the thickness of the web portion of the connecting rod as compared to molding by forging. Specifically, it is possible to easily produce a connecting rod part having a web part of less than 3 mm, such as about 2 mm, and without changing the part weight at all (= without changing the sectional area of the column part at all). ) A connecting rod component having excellent buckling strength can be manufactured (claim 2).

In addition, since the hot forging performed in the roughing process requires a draft as described above, the rib portion of the connecting rod is shown in FIG. It is impossible to obtain a uniform thickness as shown in the figure, and the closer to the web portion, the thicker the thickness. Therefore, a connecting rod component having a uniform thickness can be manufactured by processing this simultaneously with the thinning of the web portion by rolling in a finishing process. More specifically, a connecting rod for an internal combustion engine in which the angle formed by the web surface and the rib portion outer surface is 90 ± 1 ° can be manufactured. (Claims 3 and 4).

By reducing the thickness of the web portion and making the thickness of the rib portion uniform, the buckling strength can be improved even with the same cross-sectional area, and the weight of the connecting rod component can be reduced.

  Next, the features of the present invention will be clarified by showing examples. Table 1 shows the chemical components of the steel materials used in the examples. Among these, 1 steel is a steel material already put into practical use as a ferrite pearlite type non-tempered steel, and 2 steel is a bainite type high strength steel which can obtain high strength by aging treatment.

  Sample materials comprising the components shown in Table 1 were melted and prepared by manufacturing a φ30 round bar by hot rolling. Then, this was cut into an appropriate length necessary for producing a forged product of a connecting rod to obtain a forged base material for a connecting rod (FIG. 1 (a)).

  The forged base material was heated to 1200 ° C. and hot forged to produce a rough forged part as shown in FIG. In this forging, the thickness of the web portion is about 5 mm, the web surface and the outer surface of the rib portion in order to avoid excessively high load during forging and to allow the workpiece to be smoothly die-cut. The forging was performed so that the angle formed by became about 85 ° (FIG. 2 (a)). Further, in the course of cooling after forging, hot rolling is performed only on the column portion which is a strength-required portion at temperatures of 700 ° C. and 900 ° C., and the web is thinly processed (from 5 mm thickness to 2.5 mm thickness). In addition, finishing was performed so that the angle formed between the web surface and the rib portion outer surface was approximately 90 ° (FIG. 1C). The cross-sectional shape after finishing is shown in FIG.

In addition, rolling was implemented using the rolling mill which can change the space | interval of a rolling roll as shown in FIG. 3 so that only a column part can be rolled. Moreover, it evaluated simultaneously about the case where the forging components which processed the same rolling process into the rough shape were performed after cooling to room temperature.

  Further, in order to clarify that the parts manufactured according to the present invention are superior in strength compared with the forged parts manufactured by the conventional method, the length and the cross-sectional area are substantially the same as the connecting rod after the hot rolling. Thus, a connecting rod that was hot-forged by a conventional method was simultaneously manufactured. Note that the thickness and draft of the web portion aimed at this hot forging are the average level of a normal connecting rod hot forged product (web portion thickness 4.5 mm, web surface and rib portion outer surface. (An angle formed is 86.5 °).

Further, among the test steels shown in Table 1, since 2 steels are steel materials that can obtain high strength by aging treatment, after rolling, after further aging treatment under conditions of 620 ° C. × 30 minutes, Strength measurement was performed. The connecting rod manufacturing process performed in the present embodiment described above is collectively shown in FIG.

  Necessary machining was performed so that the connecting rod for strength evaluation manufactured by the above steps could be fixed at the large end and the small end in the same manner as when used as a normal engine component. Then, compressive stress is applied in the axial direction of the column part with the large end and small end fixed with a test jig, and the load when buckling deformation begins to occur (0.2% plastic strain occurs) The buckling strength was determined from the measured load. At this time, the compression test is finished as soon as the buckling phenomenon can be confirmed (finished in a state where there is no significant change in the cross-sectional shape), and the column part is cut to measure the cross-sectional dimension and shape. In addition to determining the area, the thickness of the web part and the angle formed by the web surface and the rib part outer surface were also measured to accurately confirm the relationship between the shape and the strength. The results are shown in Table 2.

  As shown in Table 2, the connecting rod of the comparative example which has not been subjected to the rolling process after hot forging has a thickness of the web portion as thick as about 4.5 mm, and the angle formed between the web surface and the rib side surface is around 86.5 °. The buckling strength obtained as a result was inferior to a connecting rod manufactured by the method of the present invention described later.

On the other hand, as in the present invention, the connecting rod which has been subjected to the hot post-forging rolling process has a web portion thickness of 2 to 2.3 mm and an angle formed by the web surface and the rib portion outer surface of approximately 90 °. In addition, because it has a shape suitable for obtaining high buckling strength, the buckling load is greatly improved despite the cross-sectional area (= 140 mm ² ) that is almost the same as that of the comparative product. It was confirmed that strength was obtained.

  In particular, two steels were used, which were warm-rolled at 700 ° C. or cold-rolled, and were subjected to aging treatment in a state where the influence of strain during rolling remained, It was confirmed that the strength improvement effect of the material itself overlapped to obtain a very high strength.

  In this example, there is no difference in material components and heat treatment conditions between the example of the present invention and the comparative example, and in particular, in the case of steel 1, it is considered that there is no strength improvement effect due to strain aging as described above. Therefore, in the strength of the material itself, it is considered that there is no great difference between the product of the present invention and the connecting rod of the comparative example (that is, if a normal tensile test is performed, there is no significant difference). Nevertheless, in the present invention, the difference in the buckling load of steel 1 between the present invention and the comparative example means that the difference in the buckling load is caused by the difference in cross-sectional shape. It is. Specifically, it means that the buckling load is improved when the rib portion, particularly the tip thereof, is thicker than the web portion having a thicker thickness.

  As described above, a rough shape that is close to the final product is obtained by hot forging, and rolling is limited to the parts that require strength in the subsequent finishing process. It is possible to improve the shape of a portion that cannot be thinned due to the existence of a gradient or the problem of increased forging load. As a result, the problem that the strength is lowered due to the problem of the shape surface which has been a problem in the hot forging product can be solved, and a more suitable shape can be obtained to obtain high strength.

  In particular, when the method according to the present invention is applied to a connecting rod for an internal combustion engine, the buckling strength, which is the most problematic in terms of strength, can be greatly improved, and weight reduction can be achieved without relying on the strength improvement of the material itself. be able to.

The figure explaining the process at the time of manufacturing a connecting rod with the manufacturing method of this invention. The figure explaining the change of the cross-sectional shape of the column part at the time of manufacturing a connecting rod with the manufacturing method of this invention. The figure explaining the rolling method in the finishing process of this invention. The figure explaining the manufacturing process of the connecting rod performed as an Example.

Claims (4)

When forming a rough part, the parts other than the required strength parts are processed by hot forging, and the required strength parts are locally rolled after hot forging. A manufacturing method for excellent high-strength lightweight parts. A connecting rod for an internal combustion engine manufactured by the method according to claim 1, wherein a column portion which is a strength-requiring portion is finished by rolling and the thickness of the web portion is formed to be less than 3 mm. Connecting rod for internal combustion engines with excellent buckling strength. A connecting rod for an internal combustion engine manufactured by the method according to claim 1, wherein a column portion that is a strength-requiring portion is finished by rolling, and an angle formed between a web surface and a rib portion outer surface is 90 ° ±. A connecting rod for an internal combustion engine having excellent buckling strength, characterized by being 1 °. A connecting rod for an internal combustion engine manufactured by the method of claim 1, wherein the column portion which is a strength-requiring portion is finished by rolling, the thickness of the web portion is formed to be less than 3 mm, and the web surface A connecting rod for an internal combustion engine having excellent buckling strength, characterized in that the angle formed with the outer surface of the rib portion is 90 ° ± 1 °.

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