JP2006052432A - Method for manufacturing forged product for connecting rod easily separated through rupture and having high-strength - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a forged product for a connecting rod which is easily separated through rupture and has high-strengths, by efficiently strengthening only an articulated part to which high yield strength and high fatigue strength are required, without greatly spending an energy cost, a machining cost, a material cost and a die cost. <P>SOLUTION: A material for the forged product is a ferrite/pearlite type non-heat treated steel which has a composition comprising, by mass%, 0.2-0.6% C, 0.05-2% Si, 0.3-1.5% Mn, 0.01-0.2% P, 0.05-1% Cr, 0.02-0.5% V, 0.009-0.03% N and the balance Fe with inevitable impurities. The method for manufacturing the forged product comprises the steps of: hot-forging the above material into a roughly formed body for the connecting rod; and then coining the articulated part of the roughly formed body at a temperature ranging from 200 to 700°C and at a processing rate of 3 to 40% without applying distortion to a large end, in the cooling process, while simultaneously performing shape correction and strain ageing, to strengthen the articulated part. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a forging for a connecting rod, and more particularly to a method for producing a forging for a high-strength connecting rod that is later fractured and separated at a large end, and that is easily broken and separated at that time and has excellent strength at a joint.

A connecting rod (hereinafter referred to as a connecting rod) 200 of an internal combustion reciprocating engine shown as an example in FIG. 6 is conventionally formed integrally by forging, and then machined by a separating portion P after finishing machining as necessary. The first part on the main body side integrally having the small end portion 202, the connecting portion 204, and the half portion 206A of the large end portion 206, and the second part consisting of the half portion 206B of the large end portion 206. It was separated and manufactured.
Reference numeral 208 denotes a circular hole in the small end portion 202, and 210 denotes a circular hole in the large end portion 206.

However, in the case of the above-described conventional manufacturing method, an extra material is required as a cutting allowance for the cut portion, and since the separation surface is subjected to finishing processing by cutting or polishing after cutting, a great amount of time is wasted. The price was rising.
Furthermore, in such a method, no matter how much the machining is performed before assembling the crankshaft, a side slip occurs at the joint surface when disassembling after assembling and assembling to the crankshaft. There is a problem that the roundness of the circular hole 210 of the portion 206 is deteriorated.

For this reason, a side pin is prevented by inserting a knock pin or using a guide pipe. However, sufficient assembling accuracy or shape accuracy in an assembled state is not ensured.
In addition, providing a knock pin or a guide pipe is not a preferable method because it increases the price.

  As a method for solving these problems, a method of manufacturing a connecting rod using a powder sintering method has been proposed, but in this method, the powder sintering process becomes a complicated process, which hinders productivity, There was a difficulty that could not achieve the purpose of cost reduction.

  Therefore, in recent years, a method has been proposed in which a connecting rod is integrally forged into a final shape by hot forging, and then separated into the first part and the second part by breaking separation, and as a material for that, breaking separation is easy. Non-tempered steel for hot forging has been developed.

Unlike the machined surface, the connecting rod separation surface obtained by this method, that is, the assembled joint surface, is a fractured surface with random irregularities, so that no side slip occurs on the joint surface, so that it is assembled with high accuracy. be able to.
Further, when non-tempered steel is used as the material for that purpose, the quenching / tempering treatment is unnecessary, and therefore the connecting rod manufacturing process and manufacturing cost can be reduced.

Currently in Europe, XC70 (French standard) type steel is used as a material for manufacturing connecting rods by such fracture separation.
This steel is introduced in US Pat. No. 5,135,587, etc., and is almost 100% pearlite single structure steel, 0.6 to 0.75% C by mass, 0.2 to 0.5% Mn, 0.04 to 0.12% S. (Mn / S> 3), and the balance is Fe and inevitable impurities, and the impurity content is not more than 1.2%.

However, this steel type was developed mainly for ease of fracture separation, and is therefore suitable for fracture separation as described above. There is a problem that the property is also bad, and it is not suitable as an automobile part.
Therefore, the development of steel types that are excellent in fatigue strength and proof stress, have good machinability, and are suitable for fracture separation is currently underway.

  On the other hand, in recent years, connecting rods for engines use non-tempered steel with V and Nb added as a raw material instead of quenching and tempering treatment for cost reduction. It is provided for use.

Furthermore, in order to achieve higher strength than component adjustment such as V addition, for example, Patent Document 1 listed below shows age hardening by imparting strain in the temperature range of Ar ₁ point or less to 200 ° C. in the cooling process after hot working. There has also been proposed a method for obtaining a high strength by causing the above to occur.
However, in the case of warm coining, if strain is introduced into the large end, which is a rupture portion, the strain cannot be recovered during cooling, so that the reassembly of the large end portion after rupture deteriorates.

  In addition, when the coining is performed warmly, if the connecting part shape of the connecting rod rough molded body after forging is non-uniform, the strain applied by warm processing is introduced non-uniformly into the connecting part. turn into.

In the patent application filed previously (Japanese Patent Application No. 2003-207393: not disclosed), the present inventors hot forged a material made of ferritic / pearlite-type non-heat treated steel to form a rough formed body for a connecting rod. When the rough molded body is formed into a shape satisfying A, B, α satisfying α / (A + B) ≦ 0.05 as defined below, only the connecting portion is formed without applying strain to the large end. In particular, it has proposed a method for producing a forged product for a high-strength connecting rod that is easily rupture-separated and that is subjected to aging treatment by cold coining and high-frequency induction heating.
A: One cross-sectional area when cut at the center line of the lower mold on the rough molded body after hot forging B: When cut at the center line of the lower mold on the rough molded body after hot forging The other cross-sectional area α: Area of the part where the A part and the B part do not overlap when folded over the center line of the lower mold on the rough formed body after hot forging

  According to this manufacturing method, it is possible to provide a high-strength connecting rod that can ensure cracking and reduce the amount of bending of the connecting rod and can be easily separated by breaking.

  However, in the case of this method, when high-frequency induction heating is performed on a joint portion that has been partially subjected to cold coining, heating tends to be non-uniform, and due to the influence, α / (A + B) must be 0.05 or less. Don't be.

  That is, the shape of the connecting portion must be made uniform with high accuracy. In this case, the die precision of the hot forging die must be kept high, so that the frequency of die replacement becomes high, which causes a problem that causes an increase in cost.

  In addition, in the method of this prior application, after the rough molded body is hot forged and cooled to room temperature, cold coining processing and subsequent aging treatment by high frequency induction heating are performed in a separate process, that is, these There is a problem that the process is forced to be performed as a separate process.

JP 2003-55714 A

  The present invention is based on the above circumstances, and it is possible to efficiently increase the strength of only the joints where high proof stress and high fatigue strength are required without adding a large amount of energy costs, processing costs, material costs, mold costs, etc. The purpose of the present invention is to provide a forged product for a high-strength connecting rod that is easy to break and separate.

  Thus, the content of claim 1 is mass%, C: 0.2-0.6%, Si: 0.05-2%, Mn: 0.3-1.5%, P: 0.01-0.2%, Cr: 0.05-1%, V : Ferrite and pearlite type non-heat treated steel with a composition of 0.02 to 0.5%, N: 0.009 to 0.03%, balance Fe and inevitable impurities, and forming a rough formed body for connecting rods by hot forging After that, in the cooling process, a coining process that combines the shape correction and the process for strain aging is performed in the warm region of 200 to 700 ° C. for the joint part of the rough molded body without adding strain to the large end part. And it is performed at a processing rate of 3 to 40% to increase the strength of the connecting portion.

  A second aspect of the present invention is the same as in the first aspect, wherein the composition of the ferritic pearlite type non-heat treated steel is in mass%, Pb: 0.01 to 0.30%, S: 0.02 to 0.2%, Te: 0.002 to 0.3%, Ca : 0.0004 to 0.01%, Bi: 0.01 to 0.30%, or a composition further containing one or more.

A third aspect of the present invention provides the method according to any one of the first and second aspects, further comprising the following formula (1):
0.74 ≦ [C%] + 0.17 [Si%] + 0.22 [Mn%] + [P%] + 0.25 [Cr%] + 1.8 [V%] ≦ 1.59 Equation (1)
It is characterized by satisfying.

According to a fourth aspect of the present invention, in any one of the first to third aspects, when the raw material made of the ferritic / pearlite type non-heat treated steel is hot forged to form a rough formed body for a connecting rod, After the molded body is molded into a shape satisfying A / B, α defined below as α / (A + B) ≦ 0.08, no distortion is applied to the large end portion of the coarse molded body in the cooling process. By adding the coining process to the connecting part at the processing rate, the roundness at the time of reassembly after breaking and separating the large end part is set to 500 μm or less.
A: One cross-sectional area when cut at the center line of the lower mold on the rough molded body after hot forging B: When cut at the center line of the lower mold on the rough molded body after hot forging The other cross-sectional area α: Area of the part where the A part and the B part do not overlap when folded over the center line of the lower mold on the rough formed body after hot forging

Effects and effects of the invention

  As described above, the present invention uses a ferrite-pearlite non-heat treated steel containing C, Si, Mn, P, Cr, V, and N in a predetermined amount as a raw material, and performs rough forming for connecting rods by hot forging. After molding the body, the coining process 200 ~ which combines the shape correction and the process for strain aging on the joint part of the coarse molded body without applying distortion to the large end portion to be divided later in the cooling process. This is performed in a warm region of 700 ° C. and at a processing rate of 3 to 40%, and then cooled to room temperature to increase the strength of the connected part by strain aging. In the present invention, N is contained in a predetermined amount or more. Hardening by precipitating V carbonitride and precipitating V-type ferritic / pearlite non-heat treated steel, and moving dislocations introduced by coining efficiently with N and C interpenetrating solid solution elements The strength of joints is increased by hardening by strain, that is, strain age hardening. It can be improved.

  Further, in the present invention, after hot forging, coining is performed in the warm region of the cooling process, so that the joints can be coined and strain-aged at a uniform temperature, and non-uniformity such as high-frequency induction heating can be achieved. In addition to being able to solve the problem of causing bending based on heating, it also solves the problem of being forced to perform cold coining and high-frequency induction heating later as separate processes. It becomes possible.

  In addition, since no distortion is applied to the large end portion that is later broken and separated, the problem that the reassembly property of the large end portion after the breakage is deteriorated due to the fact that the strain cannot be recovered during cooling is solved. Can do.

  In the present invention, one or more of Pb, S, Te, Ca, Bi can be contained in the non-heat treated steel in the above amount as required (Claim 2). The machinability during machining can be effectively increased.

The present invention also provides the following formula (1) according to claim 3.
0.74 ≦ [C%] + 0.17 [Si%] + 0.22 [Mn%] + [P%] + 0.25 [Cr%] + 1.8 [V%] ≦ 1.59 Equation (1)
By satisfy | filling, it becomes possible to obtain appropriate hardness also in the site | part which does not give cold work and an aging treatment.

In the present invention, when forming a rough formed body for a connecting rod by hot forging, the connecting portion is partially coined by forming the rough formed body into the shape specified in claim 4. However, the connecting rod can hardly be bent, and the amount of bending can be kept within the machining allowance.
In addition, since the coining process is performed in the warm region without applying distortion to the large end, the roundness of the circular hole in the large end at the time of reassembly after breaking the large end is separated. It can be suppressed to 500 μm or less.

  Here, the roundness is a value represented by (ab) / 2, where a is the longest distance and b is the shortest distance in the large-end circular hole after fracture separation and reassembly. is there. This roundness is preferably smaller when the machining cost of the subsequent process is taken into consideration.

Next, the reasons for limiting each chemical component of the present invention will be described below.
[I] Reason for component limitation
C: 0.2-0.6%
C is an element effective for ensuring the strength of the forged product. In order to obtain such an effect, it is necessary to contain 0.2% or more.
However, if it is too much, the hardness becomes high and the machinability deteriorates, so it is necessary to make it 0.6% or less.

Si: 0.05-2%
Si has a deoxidizing action and a desulfurizing action at the time of steel melting, and improves proof stress by strengthening ferrite by solid solution in ferrite.
However, if a large amount is contained, the hardness becomes too high to deteriorate the machinability and the hot workability is deteriorated, so 2% or less is necessary.

Mn: 0.3-1.5%
Cr: 0.05-1%
Mn and Cr are effective elements for securing the strength of the forging.
However, if added in a large amount, bainite is formed after forging, the hardness is remarkably increased and the machinability is lowered, so the respective ranges are Mn: 0.3 to 1.5% and Cr: 0.05 to 1%.

P: 0.01-0.2%
P is generally kept low as an element that lowers toughness due to segregation at grain boundaries, but in the present invention where fracture separation is performed, as an element that increases the brittle fracture surface ratio and improves the adhesion of the fracture surface. Because it works very effectively, it is actively added.
However, even if added in a large amount, the effect is saturated and the hardness is increased, so the content is made 0.01 to 0.2%.

V: 0.02-0.5%
In the present invention, it is essential to add V. In general, V is an element that forms fine carbonitride with carbon or nitrogen, and increases the strength after forging. By adding V, the requirement for high strength can be satisfied.
Furthermore, in the present invention, when the aging temperature after coining applied in the warm region of 200 to 700 ° C. is set to 400 ° C. or more, in order to prevent the strength reduction due to the recovery phenomenon of strain aging by precipitation hardening of V carbonitride, The strength can be further improved, and it acts as a very useful element.
However, adding more than 0.5% saturates the effect of increasing strength and further reduces machinability, so the upper limit is made 0.5%.

N: 0.009 to 0.03%
In the present invention, N also acts as a very effective element. N fixes movable dislocations by coining performed in a warm region of 200 to 700 ° C., and effectively improves the yield strength. And in order to acquire such an effect, it is necessary to make it contain 0.009% or more.
On the other hand, since the effect is saturated even if it is added in a large amount, the addition amount is selected within the range up to the upper limit of 0.03%.

Pb: 0.01-0.30%
S: 0.02 to 0.2%
Te: 0.002 to 0.3%
Ca: 0.0004 to 0.01%
Bi: 0.01-0.30%
Pb, S, Te, Ca, Bi are all effective elements for improving machinability, so that it is required to have better machinability in the rough formed rods of forged products. It is also possible to add an appropriate amount of one or more selected from these as necessary.
However, if too much is added, the hot workability is lowered, so even if added, Pb is 0.01 to 0.30%, S is 0.02 to 0.2%, Te is 0.002 to 0.3%, Ca is 0.0004 to 0.01, Bi is 0.01 ~ 0.30%.

Formula (1): 0.74 ≦ [C%] + 0.17 [Si%] + 0.22 [Mn%] + [P%] + 0.25 [Cr%] + 1.8 [V%] ≦ 1.59
Equation (1) defines the carbon equivalent (Ceq) necessary for obtaining an appropriate strength as a connecting rod or the like.
The connecting rod hardness generally used for automobile engines is 20 to 35 HRC. Although the present invention relates to a manufacturing method for improving the strength of the connecting portion, the other portions, that is, the connecting rod large end portion and small end portion, of course, also require predetermined strength.
That is, when the hardness is 20 HRC or less, sufficient strength cannot be obtained, and the deformation at the time of break separation of the large end is large, and the break separation process cannot be applied. Furthermore, at 35 HRC or higher, machinability such as large end bolt holes and small end machining is reduced, resulting in a large cost for machining the connecting rod. For this reason, it is desirable to adjust the hardness of the connecting rod large end and small end to 20 to 35 HRC.
In order to obtain such hardness, it is necessary to define the formula (1) as 0.74 to 1.59.

[II] Coining conditions Coining is a process that is usually performed to correct the shape of the connecting portion of the connecting rod rough molded body. However, in the coining of the present invention, it is not only performed to correct the shape, but before the coining. This is done to increase the yield strength.

Processing rate: 3-40%
In the present invention, the coining rate is 3 to 40%.
When the processing rate is lower than 3%, the yield strength after aging is not sufficiently improved, and the shape correction which is the original purpose of coining cannot be sufficiently performed.
If the coining rate exceeds 40%, the improvement in yield strength is saturated and forging cracks occur due to the increase in the rate.

The present invention is characterized in that no strain is introduced into the large end.
As described above, when warm coining is applied to the joint, if strain is applied to the large end, residual strain that cannot be recovered during cooling is released at the time of rupture, resulting in deformation, and pushing the separated fracture surface. When they are combined, there is a problem that they do not match correctly, and the break separation property is deteriorated.
Therefore, this invention solves the said problem by not giving a distortion to a large end part by contacting only a connection part and giving warm coining.

Articulated part shape: α / (A + B) ≦ 0.08
(A: One cross-sectional area when cut at the center line of the lower mold on the rough formed body after hot forging, B: Cut at the center line of the lower mold on the rough formed body after hot forging The other cross-sectional area at the time, α: the area of the portion where the A portion and the B portion do not overlap each other when folded back at the center line of the lower die on the rough formed body after hot forging.
As described above, when performing warm coining in a situation in which only the connecting portion is contacted without contacting the large end and the small end so as not to add distortion to the large end during warm coining, If the shape of the connecting part is uneven and there is a large difference in the contact area during warm coining, the connecting rod and the mold for warm coining will tend to bend easily and the connecting rod will bend easily. It will exceed the allowance for the product, and will not be established as a part.

  Therefore, according to the invention of the prior application, after forming a rough formed body for a connecting rod to α / (A + B) ≦ 0.05 by hot forging, cold coning is performed on the joint part by performing cold coining processing and aging treatment by high frequency induction heating. The non-uniform contact with the press at the time was suppressed, and the bending amount of the connecting rod was reduced, and it was possible to prevent the machining allowance at the time of machining from being exceeded and not becoming a part, but the rough molded body shape Therefore, it was necessary to increase the misalignment accuracy twice as much as the conventional method.

In the present invention, since coining is performed with a uniform temperature distribution during cooling after hot forging, there is no influence of non-uniform heating due to high-frequency induction current as in the prior application, and the range of α / (A + B) of the connecting portion shape. Can be accommodated within the machining allowance during processing even if the screw is loosened to 0.08 or less.
That is, since the mold misalignment accuracy can be relaxed, an effect of reducing the mold cost can be obtained.

Roundness of the machined surface of the large end when reassembling after breakage: 500 μm or less After roughing the connecting rod after hot forging and warm coining, a notch is added to the machined surface of the large end and reassembled after breakage separation. When the numerical value that defines the roundness of the circular hole at the large end is large (that is, the roundness is low), further roughing must be introduced before the subsequent finishing process. Cost reduction effect will be diminished. Accordingly, the roundness of the large end processed surface at the time of reassembly after fracture is set to 500 μm or less.

Next, embodiments of the present invention will be described in detail below.
<Embodiment 1>
50 mm square forging is performed by hot forging except for ingots and forging cracks and forging defects, after melting the steel types of the inventive examples having the composition shown in Table 1 below and the steel types of the comparative examples. A raw material was heated and held at 1200 ° C. for 60 minutes, and then hot forging was performed on a round bar having a diameter of 22 mm.

Further, 15% processing was performed at a temperature of 600 ° C. to cope with coining applied to the connecting portion of the connecting rod in the subsequent cooling process, and the mixture was allowed to cool to room temperature again.
Test specimens were cut out from these specimens and tested for drilling efficiency representing the elongation to break and machinability to evaluate the hardness, fatigue limit, 0.01% proof stress, fracture separability by the following methods. The results are also shown in Table 1 below.

  In addition, in the steel types of the present invention and the steel types of the comparative examples, Cu: ≦ 0.3%, Ni: ≦ 0.25%, Mo: ≦ 0.05%, S—Al: ≤0.045%, O: ≤0.005% impurities.

The hardness was measured on the C scale of the Rockwell hardness meter at the center of each test piece after warm working corresponding to the coining.
The fatigue limit was measured by an Ono rotary bending fatigue tester.
For more details, to prepare a Ono-type rotary bending test piece parallel portion φ8mm smooth shape, to create the SN curve by rotating bending test Ono-type, and the limit fatigue limit stress does not break at 10 ⁷ times of repetition rates.

  Breaking separation characteristics are as follows: JIS Z2201 14A No. 1A in the longitudinal direction of the tensile test piece was subjected to laser processing a ring cutout with a width of 1 mm and a depth of 1 mm, followed by a tensile test at room temperature and measuring the elongation at that time. evaluated.

The 0.01% yield strength was measured by a metal material tensile test method.
The reason why the yield strength is 0.01% is to clarify the influence of movable dislocations introduced during plastic working corresponding to coining.
Drilling efficiency is for evaluating the machinability of the test material. Tool: SKH51, Feed: 0.1 mm / rev, Hole depth: 10 mm, Cutting oil: None, Tool life judgment: Uncutting condition , And relative evaluation was obtained when the present invention steel No. 2 was set to 100.

  As is clear from the results shown in Table 1 above, the present invention examples No. 1 to 15 satisfying the present invention have a high strength and excellent machinability. It is recognized that

In contrast, Comparative Example A, which has a lower C content than the present invention, had a higher drilling efficiency than that of the inventive example that did not contain the machinability improving element, but the hardness, fatigue limit, and 0.01% proof stress. Both of these are significantly lower than the examples of the present invention. Further, since the hardness is not sufficient, the elongation at break increases, and the adhesion after break separation cannot be ensured.
Since Comparative Examples B and C having a higher C content or Si content than the present invention are high in hardness, the fatigue limit and 0.01% proof stress are both higher than those of the present invention examples, but the hardness is too high. In addition, the drilling efficiency is considerably lower than the example of the present invention.

  Comparative Examples D and E, which have a higher Mn content or Cr content than the present invention, have considerably higher hardness due to the occurrence of bainite, and the drilling efficiency is considerably lower than that of the present invention example.

In Comparative Example F having a P content higher than that of the present invention, forging cracks occurred and a test piece could not be prepared.
Comparative Example G, which has a higher V content than the present invention, has higher hardness, fatigue limit and 0.01% proof stress than the average values of the present invention examples. However, since the hardness is too high, drilling efficiency is higher than that of the present invention. Pretty low.

Comparative Example H, which has a lower N content than the present invention, has a small effect of strain aging, and its fatigue limit and proof stress are significantly smaller than those of the inventive examples having almost the same hardness.
In Comparative Example I having a higher N content than the present invention, a casting defect occurred and a test piece could not be produced.

  In Comparative Examples J to K having a large S content, Pb or Bi content, forging cracks occurred and a test piece could not be produced.

<Embodiment 2>
In the above embodiment, an example of the case where the temperature is 600 ° C. and the processing rate is 15% corresponding to coining applied to the connecting rod connecting portion is shown, but the steel type of Invention Example No. 2 in Table 1 corresponds to coining. The effect on 0.01% proof stress and fatigue limit when the temperature during processing was changed was investigated. The results are shown in Table 2 and FIG.
Table 2 also shows the measurement results of 0.01% proof stress and fatigue limit in the hot forging state.

As is apparent from Table 2 and FIG. 1, it can be seen that when the temperature at the time of processing is 100 ° C., the yield strength after processing is not sufficient as compared with the one with hot forging.
In addition, it can be seen that the proof stress and fatigue limit are improved at a temperature of 200 ° C. or higher than that of the hot forging.
However, the strength decreases at 800 ° C. For this reason, the temperature at the time of coining needs to be 200-700 degreeC.

  Table 2 shows the effect on the 0.01% proof stress and fatigue limit when the machining rate corresponding to the coining applied to the connecting rod connecting portion is 15% and the machining temperature conditions are changed. Table 3 and FIG. Investigate the effect on the 0.01% proof stress and fatigue limit when changing the processing rate at a processing temperature of 400 ° C, which showed the best values for both 0.01% proof stress and fatigue limit. Results are shown.

As shown in Table 3 and FIG. 2, it can be seen that when the processing rate is less than 3%, the effect of improving the strength is not sufficient as compared with the hot forging.
Further, although the strength is improved up to a processing rate of 40%, the yield strength is lowered when the processing rate is increased up to 50%.
When considering coining to the connecting parts of the actual connecting rods, the corrective action, which is the purpose of coining, becomes very unstable at less than 3%, and sufficient effects are not exhibited. Further, even if the processing rate is excessively increased, not only the effect on the strength is saturated, but also the occurrence of cracking during coining processing, so the processing rate was set to 3 to 40%.

Next, forging trial manufacture of a solid connecting rod using the steel type of invention example No. 2 in Table 1, and the relationship between the shape of the joint after warm coining and the amount of bending when warm coining is performed after hot forging. This is shown in FIG.
At this time, 30% warm coining is performed at 600 ° C. for each condition. In addition, as a comparative example, the relationship between the shape of the joint and the amount of bending is also shown in the case of holding for 20 seconds after heating to 600 ° C. with high frequency induction heating (induction current) after 30% coining after hot forging. It was.

Here, the shape of the connecting portion in each invention example and comparative example in FIG. 3 is as follows.
Comparative Example (1): Warm α / (A + B) = 0.10
Comparative Example (2): Warm α / (A + B) = 0.19
Invention Example (1): Warm α / (A + B) = 0.08
Invention Example (2): Warm α / (A + B) = 0.05
Comparative Example (3): Cold α / (A + B) = 0.08
Comparative Example (4): Cold α / (A + B) = 0.05

In FIG. 4, 10 represents a connecting portion of the rough molded body when the substantial connecting rod is hot forged, and A is a cross-sectional area on the upper mold side when cut along the center line X of the upper and lower molds. B represents the cross-sectional area on the lower mold side.
Further, α represents the area of the portion where the A portion and the B portion do not overlap when the connecting portion 10 of the roughly molded body is folded back along the center line X.

When the above-mentioned α / (A + B) is larger than 0.08 when only the connecting part is touched and warm coining is applied (Comparative examples (1) and (2)), the bending amount is larger than the machining allowance. On the other hand, when α / (A + B) is within 0.08 or less (invention example (1), invention example (2)), the bending amount is within the machining allowance. . α / (A + B) is desirably as small as possible.
On the other hand, when heating with high frequency induction current after cold coining, uneven heating will inevitably occur. Therefore, if α / (A + B) is not smaller than 0.05, the bending amount cannot be suppressed within the machining allowance. Thus, it can be seen that warm coining is more advantageous in preventing bending.
The connecting rod of the inventive example was manufactured by increasing the die misalignment accuracy at the time of hot forging 1.3 times that of the prior art (0.65 times that of Comparative Example (4)).

Next, a forging test of the solid connecting rod is conducted using the invention example No. 7 steel type in Table 1, and after the hot forging, the warm coining process is performed in two ways: a method in which the large end and the small end are clamped, and a method in which it is not performed. I went there.
At this time, 30% warm coining is performed at 600 ° C. for each condition.
Further, after machining the large end portion and the small end portion of these connecting rods, notches were introduced into the large end processed surface to perform break separation.
FIG. 5 shows the roundness measurement result of the large end processed surface of the connecting rod which has been reassembled after the fracture separation.

When a large end clamp is applied and strain is introduced, the residual strain that cannot be recovered during cooling is released at the time of breakage, resulting in deformation, and when the separated fracture surfaces are abutted, there is a problem that they do not fit correctly. I got up and my roundness deteriorated.
Warm-coined so that no distortion is introduced at the large end (without clamping), the roundness after reassembly is within the range of 500μm or less, and is completed only by finishing. I was able to make it.
In addition, in order to obtain the roundness within a specified range, the cooling rate after hot forging of the fractured portion is increased twice as much as the conventional method.

It is a figure showing the relation between processing temperature, 0.01% proof stress, and fatigue limit. It is a figure showing the relation between a processing rate, 0.01% proof stress, and a fatigue limit. It is the figure showing the relationship between (alpha) / (A + B) and the amount of bending after coining process. It is a figure showing α, A, B in α / (A + B). It is a figure showing the roundness measurement result of the big end part processed surface of a connecting rod in a solid connecting rod forging test. It is a figure which shows an example of the conventional connecting rod.

Claims (4)

In mass%
C: 0.2-0.6%
Si: 0.05-2%
Mn: 0.3-1.5%
P: 0.01-0.2%
Cr: 0.05-1%
V: 0.02-0.5%
N: 0.009 to 0.03%
After forming a rough shaped body for a connecting rod by hot forging using a ferritic pearlite non-heat treated steel with a composition comprising the balance Fe and inevitable impurities, the large end is strained during the cooling process. Coining that combines shape correction and processing for strain aging is performed on the connecting portion of the rough molded body without adding a material in a warm region of 200 to 700 ° C. and a processing rate of 3 to 40%. A method for producing a forged product for a high-strength connecting rod, which is easy to break and separate, characterized by increasing the strength of the part. 2. The composition of the ferritic pearlite non-heat treated steel according to claim 1 in mass%.
Pb: 0.01-0.30%
S: 0.02 to 0.2%
Te: 0.002 to 0.3%
Ca: 0.0004 to 0.01%
Bi: 0.01-0.30%
A method for producing a forged product for a high-strength connecting rod that is easy to break and separate, wherein the composition further comprises one or more of the above. The following formula (1) in any one of claims 1 and 2
0.74 ≦ [C%] + 0.17 [Si%] + 0.22 [Mn%] + [P%] + 0.25 [Cr%] + 1.8 [V%] ≦ 1.59 Equation (1)
A method for producing a forged product for a high-strength connecting rod, which is easy to break and separate. 4. The method according to claim 1, wherein when the material comprising the ferrite-pearlite-type non-heat treated steel is hot-forged to form a rough formed body for a connecting rod, the rough formed body is defined as follows. A, B, α are formed into a shape satisfying α / (A + B) ≦ 0.08, and then the coining process is performed at the processing rate without adding strain to the large end portion of the rough formed body in the cooling process. Forgings for high strength connecting rods with easy breaking separation, wherein roundness in reassembly after breaking the large end portion is 500 μm or less by adding to the connecting portion Manufacturing method.
A: One cross-sectional area when cut at the center line of the lower mold on the rough molded body after hot forging B: When cut at the center line of the lower mold on the rough molded body after hot forging The other cross-sectional area α: Area of the part where the A part and the B part do not overlap when folded over the center line of the lower mold on the rough formed body after hot forging

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