JPH10168540A - Production of non-heat treated steel for connecting rod and connecting rod - Google Patents

Abstract

PROBLEM TO BE SOLVED: To recover the proportional limit (proof stress) and fatigue strength, deteriorated by cold coining after hot forging, by means of artificial aging and natural aging under a service environment and to provide a connecting rod having proportional limit (proof stress) and fatigue strength both higher than those before coining. SOLUTION: A non-heat treated steel, having a composition consisting of 0.20-0.50% C, 0.05-1.00% Si, 0.10-1.50% Mn, 0.10-1.00% Cr, 0.10-0..50% V, 0.010-0.025% N, further one or >=2 kinds among <=0.30% Pb, <=0.20% S, <=0.30% Te, <=0.01% Ca, and <=0.30% Bi, and the balance Fe with impurities, is used as a stock and formed into a roughly formed body for connecting rod by means of hot forging. After cold coining for shape straightening is applied to the I-section part, including a minimum cross sectional part, of the roughly formed body for connecting rod, the proportional limit and fatigue strength, deteriorated after coining, are recovered by artificial aging or natural aging.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a connecting rod for connecting a piston and a crankshaft, which has a high proportionality limit without being subjected to heat treatment such as quenching and tempering after hot forging. The present invention relates to a non-heat-treated steel for connecting rods and a method for manufacturing a connecting rod, which is suitable for obtaining (high yield strength) and high fatigue strength.

[0002]

2. Description of the Related Art Conventionally, in order to omit heat treatment (tempering treatment) such as quenching and tempering after hot working such as hot forging, a medium carbon steel (C content of 0.3 to 0.3%) is required. 0.5
Wt%) plus a small amount of V +
Perlite type non-heat treated steel is widely used as a material for parts for machine structural use.

[0003] In recent years, the use of such non-heat treated steel has been further expanded due to the demand for cost reduction, and the non-heat treated steel which can be applied to parts requiring higher strength is available. The development of is desired.

In this case, the required strength is generally proof stress and fatigue strength.

[0005]

The connecting rod used to connect the piston and the crankshaft in an engine or a pump is an example in which the above-mentioned non-heat-treated steel is employed. Mn, C
By optimizing alloy elements such as r and V, proof stress and fatigue strength have been improved.

However, in this kind of connecting rod, after forming a rough connecting rod body by hot forging, a cold working called 5% to 30% called a coining is applied to the I section for correcting the shape. Is commonly done.

When such coining is performed, a large amount of movable dislocations are introduced into the I section where the strength is most required in the connecting rod.

[0008] In general non-heat treated steel applications, since hot working such as hot forging is often used as it is, mobile dislocations introduced during ferrite-pearlite transformation, etc.
Subsequent precipitation of V carbonitride and fixation by interstitial solute atoms such as O, N, and C result in passive dislocations.

Therefore, in general applications, the density of mobile dislocations is low in the product state, and high yield strength and fatigue strength can be obtained.

However, in a connecting rod in which a large amount of movable dislocations are introduced by coining after hot forging, the movable dislocations are activated by a small external force and plastic deformation occurs.

As a result, the proof stress is significantly reduced, and the initial stage of fatigue caused by the reciprocating motion of dislocations in a minute region, that is, the generation of fatigue cracks is accelerated, and the fatigue strength is also reduced.

[0012] Therefore, there is a problem that it is difficult to obtain a high proof stress and a high fatigue strength in a connecting rod in which coining for shape correction is performed cold after hot forging.

[0013]

SUMMARY OF THE INVENTION An object of the present invention is to solve such a conventional problem. In a connecting rod for performing coining for shape correction after hot forging in a cold state, conventionally, coining is avoided. Cold coining is also applied to the minimum cross-sectional area of the I section that has been considered desirable, and due to artificial aging or natural aging under the use environment in the engine, a higher proportional limit (high proof stress) than before cold coining. And high fatigue strength can be obtained.

[0014]

According to the first aspect of the present invention, there is provided a non-heat treated steel for connecting rods, wherein C: 0.20 to 0.50% and Si: 0 on a weight basis. .05
1.00%, Mn: 0.10 to 1.50%, Cr:
0.10 to 1.00%, V: 0.10 to 0.50%,
N: 0.010 to 0.025%, and the composition is characterized by having a balance of Fe and impurities.

[0015] Similarly, the non-heat-treated steel for connecting rods according to the present invention is based on weight,
C: 0.20 to 0.50%, Si: 0.05 to 1.0
0%, Mn: 0.10 to 1.50%, Cr: 0.10
1.00%, V: 0.10 to 0.50%, N: 0.
010 to 0.025%, and Pb: 0.30% or less,
S: 0.20% or less, Te: 0.30% or less, Ca:
It is characterized by containing one or more selected from 0.01% or less and Bi: 0.30% or less and having a composition of the balance of Fe and impurities.

In the embodiment of the non-heat-treated steel for connecting rods according to the present invention, as described in claim 3, N: more than 0.015 to 0.025% can be contained.

Similarly, in the embodiment of the non-heat treated steel for connecting rods according to the present invention, it is necessary that the structure after hot forging be ferrite + pearlite as described in claim 4. It is desirable according to.

The method for manufacturing a connecting rod according to the present invention comprises:
As described in claim 5, the non-heat-treated steel for connecting rod according to claim 1 or 2 is used as a raw material and formed into a rough connecting rod by hot forging, and more preferably ferrite + After adding cold coining for shape correction to the I-section including the minimum cross-sectional area to the pearlite-structured rough connecting rod body, the artificial aging has a higher fatigue limit (high proof stress) and a higher fatigue strength than before the coining. It is characterized by having obtained a connecting rod.

In the above embodiment of the production method, as described in claim 6, the artificial aging is 250 days.
It is desirable, if necessary, to perform the process at a temperature of about 500 ° C.

Similarly, a method of manufacturing a connecting rod according to the present invention is as described in claim 7, wherein the non-heat-treated steel for connecting rod according to claim 1 or 2 is used as a raw material to form a rough connecting rod by hot forging. After hot-forging, a cold-coining for shape correction is added to the I-section portion including the minimum cross-section portion of the conrod roughly formed body having a ferrite + pearlite structure by hot forging. It is characterized by the fact that the connecting rod with a higher proportional limit (higher proof stress) and higher fatigue strength than before coining is obtained by natural aging underneath.

In the above embodiment of the manufacturing method, as described in claim 8, it is desirable that the natural aging under the use environment is performed by incorporating the natural aging into the combustion engine as necessary.

[0022]

Next, the reasons for limiting the component range (weight basis) of the non-heat treated steel for connecting rods according to the present invention will be described.

C: 0.20 to 0.50% C is an effective element for securing the strength of the connecting rod, and it is necessary to contain 0.20% or more to obtain such an effect. . However, if the content is too large, the hardness becomes too high and the machinability decreases, so it is necessary to make the content 0.50% or less.

Si: 0.05% to 1.00% Si has a deoxidizing effect when steel is smelted, and also forms a solid solution in ferrite to effectively improve the strength of the ferrite, thereby improving the hardness and tensile strength. It has the effect of increasing the fatigue strength without increasing the hardness. In order to obtain such an effect, it is necessary to contain 0.05% or more.
However, if the content is too large, machinability and hot workability deteriorate, so it is necessary to set the content to 1.00% or less.

Mn: 0.10 to 1.50% Mn has a deoxidizing and desulfurizing action when steel is melted, and also has an action of making the lamella spacing of the pearlite portion fine and effectively improving the fatigue strength. Therefore, in order to obtain such an effect, it is necessary to contain 0.10% or more. However, when added in a large amount, bainite is generated even in air cooling, and the machinability is significantly reduced.
% Or less.

Cr: 0.10 to 1.00% Cr has the effect of making the lamella spacing of the pearlite portion finer like Mn and effectively improving the fatigue strength. Must be contained at 0.10% or more. However, when added in a large amount, bainite is generated even in air cooling and the machinability is significantly reduced.

V: 0.10 to 0.50% V is an element effective for strengthening ferrite like Si, and greatly improves fatigue strength. In order to obtain such an effect, it is necessary to add 0.10% or more. However, the addition of a large amount is economically disadvantageous, so it is necessary to make it 0.50% or less.

N: 0.010% to 0.025% N is an element that combines with V to form a nitride and strengthens ferrite by fine precipitation of the nitride. Therefore, like N, V, and Si, they are elements that effectively improve fatigue strength. N is an element effective for fixing mobile dislocations by artificial aging after coining or natural aging under the use environment and improving the proportional limit (proof stress) and fatigue strength. In order to obtain such an effect, 0.010
% Or more, and 0.015% to more effectively obtain the effect of strain aging.
It is more desirable to exceed. However, even if it is added in a large amount, the effect is saturated, so that the content is set to 0.025% or less.

Pb: 0.30% or less, S: 0.20% or less, Te: 0.30% or less, Ca: 0.01% or less, B
i: One or two or more selected from 0.30% or less P, S, Te, Ca, and Bi are all effective elements for improving machinability. When it is required that the machinability of the rough molded body is further improved, one or more selected from these may be added in an appropriate amount as necessary.

However, if the addition amount is too large, the hot workability and the fatigue limit are reduced.
Pb is 0.30% or less, S is 0.20% or less, Te is 0.30% or less, Ca is 0.01% or less, and Bi is 0.3% or less.
It is necessary to make it 0% or less.

The non-heat treated steel for connecting rods according to the present invention comprises:
It has the above-mentioned chemical component composition, so that a structure of ferrite + pearlite can be obtained after hot forging.

The method for manufacturing a connecting rod according to the present invention is as follows.
After forming the above-mentioned non-heat treated steel for connecting rod as a raw material into a rough connecting rod by hot forging, and after adding cold coining for shape correction to the I section including the minimum cross section, the rough connecting rod is formed. In addition, a higher proportional limit (high proof stress) and a higher fatigue strength than before coining are obtained by artificial aging or natural aging under the use environment.

In this case, the artificial aging is performed at a temperature of 250 to 50.
It is preferable that the heating is performed at 0 ° C. for 30 minutes or more.
If the temperature is lower than 0 ° C., the aging time becomes too long, which is economically disadvantageous. If the temperature is higher than 500 ° C., the strength decreases due to the coarsening of V carbonitride.
It is desirable to carry out at a temperature of 0 ° C. for 30 minutes or more.

It is also desirable, if necessary, to incorporate natural aging in a use environment into a combustion engine such as a gasoline engine or a diesel engine. ° C
For a longer time, for example, 10
It is desirable to perform aging for 0 minutes or more.

[0035]

In the non-heat treated steel for connecting rods according to the present invention, C: 0.20 to 0.50%, Si:
0.05 to 1.00%, Mn: 0.10 to 1.50%,
Cr: 0.10 to 1.00%, V: 0.10 to 0.5
0%, N: 0.010-0.025%, and the balance is made up of Fe and impurities, so that a connecting rod having high proof stress and fatigue strength can be obtained without refining. The effect is brought about.

In the above composition, Pb: 0.30% or less, S: 0.20% or less, Te: 0.30% or less, C:
By including one or more selected from a: 0.01% or less and Bi: 0.30% or less, the machinability at the time of performing cutting can be further improved. There is a remarkably excellent effect.

Further, in the above N content, N: 0.0
By setting the content to be more than 15 to 0.025%, an excellent effect that the action due to strain aging can be obtained more stably is provided.

Still further, by making the structure after hot forging a ferrite + pearlite, it is possible to obtain a connecting rod having a high strength and a high proof stress and a high fatigue strength without heat treatment such as quenching and tempering. Is significantly improved.

In the method for manufacturing a connecting rod according to the present invention, a non-heat treated steel for a connecting rod having the above-mentioned composition is used as a raw material to be formed into a rough connecting rod by hot forging. After adding inter-coining to the I section including the minimum cross-sectional area, artificial aging was used to obtain a higher proportional limit (high proof stress) and higher fatigue strength than before coining, so that cold forging was performed after hot forging. In the case of manufacturing a connecting rod by performing cold working,
The proportional limit (proof stress) and fatigue limit reduced by cold coining can be recovered by artificial aging to further increase the strength. A remarkably excellent effect is obtained that the production can be performed without performing the refining treatment.

By performing the artificial aging at a temperature of 250 to 500 ° C., a connecting rod having high proof stress and high fatigue strength can be produced in a relatively short time and without enlarging V carbonitride. A significant advantage of being able to do so.

Similarly, in the method for manufacturing a connecting rod according to the present invention, a non-heat-treated steel for connecting rods having the above-described composition is used as a raw material to be formed into a rough-formed connecting rod by hot forging, and the rough-formed connecting rod is subjected to shape correction. After adding cold coining to the I section including the minimum cross-sectional area, natural aging in the operating environment is used to obtain a higher proportional limit (high proof stress) and higher fatigue strength than before coining, so hot forging In the case of manufacturing a connecting rod by performing cold working for shape correction later, it becomes possible to recover the proportional limit (proof stress) and fatigue limit reduced by coining by natural aging and further increase the strength, High proportional limit (proof stress) and high fatigue due to natural aging without tempering, without tempering, without tempering and also without artificial aging Significantly excellent effect that it is possible to produce a connecting rod having a degree is provided.

By incorporating natural aging under the use environment into the combustion engine, a connecting rod having high proof stress and fatigue strength can be obtained without being subjected to separate aging treatment. A remarkably excellent effect is achieved.

[0043]

EXAMPLES Example 1 of the present invention shown in Table 1 was prepared. Each of the steels of Comparative Examples A to I shown in Tables 1 to 12 and Table 2 was smelted and then ingot and hot forged to form a 50 mm square forged material. The forged material was heated at 1200 ° C. for 60 minutes. After heating and holding, a 25 mm square rod was hot forged to correspond to the I section of the connecting rod, and allowed to cool to room temperature at appropriate intervals.

Next, a test piece of 20 mm square was cut out from the square bar and subjected to cold working of 10% as a material corresponding to coining to be added to the I section of the connecting rod. The aging took place. And cut out the test piece from this artificial aging product,
The test piece was tested for hardness, fatigue limit, and 0.01% proof stress.

The above-mentioned forged material of 50 mm square was heated to a temperature of 1%.
The material subjected to normalization at 100 ° C. to simulate hot working was used as a test material, and the drilling efficiency of this test material was measured to evaluate the machinability.

In this evaluation, regarding the hardness, the hardness at the center of each forged product after cold working corresponding to the above-mentioned coining was measured by a Rockwell hardness tester (C scale). It was measured by a testing machine.

Further, in order to clarify the influence of movable dislocation due to cold working, the proof stress was measured as 0.01% proof stress.

Further, the drilling efficiency was measured by performing a drill test under the conditions shown in Table 3. The relative value when 2 was set to 100 was evaluated as the drilling efficiency.

The results are also shown in Tables 1 and 2.

[0050]

[Table 1]

[0051]

[Table 2]

[0052]

[Table 3]

As is clear from the results shown in Tables 1 and 2, Example N in which the component composition satisfies the present invention.
o. Nos. 1 to 12 all have high proof stress and fatigue strength, and are also excellent in machinability.

On the other hand, Comparative Example A having a low C content
In the case of, it can be seen that sufficient strength was not obtained.
In addition, in the case of Comparative Example B, since the Si content was too large, the hardness was high and the machinability was poor. Furthermore, in the case of Comparative Examples C and D, the Mn and Cr contents were too large, so that a bainite structure was generated after hot forging and the hardness was significantly increased, so that the machinability was considerably reduced. I understand.

Further, in the case of Comparative Example E, it was found that the V content was too small, so that sufficient hardness was not obtained. Furthermore, in the case of Comparative Example F, since V was added excessively, the hardness was too high, and the drilling efficiency was significantly reduced. Furthermore, in the case of Comparative Example G, since the N content was small, the proof stress and fatigue limit were not recovered by aging after cold working, and the N content was within the range of the present invention in Example 2. It can be seen that both the proof stress and the fatigue limit are low.

Further, in Examples 10, 11, and 12, a free-cutting element in an appropriate range was added in order to further improve the machinability. It can be seen that it has better machinability than No. 2.

On the other hand, in Comparative Examples H and I in which the free-cutting elements were excessively added, the machinability was improved, but the proof stress and fatigue limit were lowered.

In the above embodiment, the working ratio 1 corresponding to the cold coining applied to the I section of the connecting rod is set.
0% cold working, then at a temperature of 300 ° C for 30 hours
Although the case where the artificial aging was performed for 5 minutes is shown, the example No. of Table 1 was used. With respect to No. 2 steel, the influence on the 0.01% proof stress and the fatigue limit when the temperature and time during artificial aging were changed was examined. Table 4 shows the results.

In order to cope with the case where a connecting rod is incorporated in a combustion engine and natural aging is performed in a use environment, aging treatment is performed at a temperature of 180 ° C., and 0.01% proof stress based on the aging time in this case. The effect on fatigue limit was also investigated. The results are also shown in Table 4.

Table 4 also shows the measurement results of the 0.01% proof stress and the fatigue limit under the condition of hot forging and cold working.

[0061]

[Table 4]

As is clear from Table 4, when the artificial aging temperature is 100 ° C., the aging time of 30 minutes does not sufficiently recover the proof stress and fatigue limit after cold working. Further, it can be seen that at a temperature of 300 ° C. or more, the proof stress and the fatigue limit recover more than those of hot forging. However, it can be seen that at 700 ° C., the strength is reduced due to the coarsening of the V carbonitride. Therefore, the artificial aging temperature is 250
~ 500 ° C was considered appropriate.

In the aging at 180 ° C. simulating the use environment in the engine, the aging up to 30 minutes does not sufficiently recover the reduction of the yield strength and the fatigue limit after cold working, but the aging up to 100 minutes or more causes the hot working. It turns out that it recovers to the level after processing. Further, even if aging is performed for a longer time, the recovery of the proof stress and fatigue limit is saturated. For this reason, it was recognized that in the case of natural aging under the use environment, holding for 100 minutes or more was necessary.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Takuro Yamaguchi 2 Takaracho, Kanagawa-ku, Yokohama, Kanagawa Prefecture Inside Nissan Motor Co., Ltd. (72) Inventor Koji Itakura 2 Takaracho, Kanagawa-ku, Yokohama, Kanagawa Nissan Motor (72) Inventor Hideki Usuki Nissan Motor Co., Ltd. 2 Takaracho, Kanagawa-ku, Yokohama, Kanagawa

Claims (8)

[Claims] 1. C: 0.20-0.50%, Si: 0.05-1.00%, Mn: 0.10-1.50%, Cr: 0.10-1. A non-heat-treated steel for connecting rods, comprising: 00%, V: 0.10 to 0.50%, and N: 0.010 to 0.025%, the balance being Fe and impurities. 2. C: 0.20 to 0.50%, Si: 0.05 to 1.00%, Mn: 0.10 to 1.50%, Cr: 0.10 to 1. 00%, V: 0.10 to 0.50%, N: 0.010 to 0.025%, and Pb: 0.3
0% or less, S: 0.20% or less, Te: 0.30% or less, Ca: 0.01% or less, Bi: 0.30% or less. A non-heat treated steel for connecting rods, comprising Fe and impurities. 3. The non-heat treated steel for connecting rods according to claim 1, wherein the steel contains N: more than 0.015 to 0.025%. 4. The non-heat treated steel for connecting rods according to claim 1, wherein the structure after hot forging is ferrite + pearlite. 5. A non-heat treated steel for connecting rods according to claim 1 or 2, which is formed by hot forging into a rough formed connecting rod, and the rough formed connecting rod minimizes cold coining for shape correction. A method of manufacturing a connecting rod, wherein a higher proportion limit (high proof stress) and a higher fatigue strength are obtained by artificial aging after adding to an I section including a cross-sectional portion than before coining. 6. The method for producing a connecting rod according to claim 5, wherein the artificial aging is performed at a temperature of 250 to 500 ° C. 7. A non-heat treated steel for connecting rods according to claim 1 or 2, which is formed into a rough connecting rod body by hot forging, and a cold coining for correcting the shape of the rough connecting rod body is minimized. A method of manufacturing a connecting rod, wherein after adding to an I section including a cross-sectional portion, a higher proportional limit (high proof stress) and a higher fatigue strength are obtained by natural aging in a use environment than before coining. 8. The method for manufacturing a connecting rod according to claim 7, wherein the natural aging under the use environment is performed by incorporating the natural aging into a combustion engine.