JPH09310146A - Production of non-heat treated steel for high strength connecting rod and high strength connecting rod - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a non-heat treated steel for high strength connecting rod, having high yield ratio even after straightening while securing machinability and excellent in strength. SOLUTION: This steel has a composition consisting of 0.3-0.6% C, 0.05-2.5% Si, 0.3-2.0% Mn, 0.03-0.5% V, 0.001-0.6% Al, 0.003-0.03% N, 0.01-0.12% S, and the balance Fe with inevitable impurities and satisfying the relations in inequalities X=0.7[C]+0.1[Si]+[Mn]-1.6[S]+0.3[V]+0.4[Cr]+0.2[Mo]>=1.50 and Y=[C]+0.1[Si]+0.3[Mn]-[S]+1.7[V]+0.3[Cr]+1.2[Mo]<=1.30. At this time, each symbol of element in the inequalities means its content (%), where [Cr] and [Mo] mean the amounts contained as impurities, respectively.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-heat treated steel for connecting rods, which is an engine part of automobiles and construction machines, and a method for manufacturing connecting rods. In particular, it relates to a quenching / tempering adjustment performed after hot forging. The present invention relates to a non-heat treated steel for connecting rods that can obtain high strength even without heat treatment by omitting a quality treatment, and a method for producing a high strength connecting rod using the non-heat treated steel.

[0002]

2. Description of the Related Art Machine structural parts used in automobiles and construction machinery are usually made of carbon steel for machine structural use or alloy steel for machine structural use, and are hardened after hot forging in order to secure necessary strength and toughness. It has been manufactured by performing a tempering treatment (tempering treatment).

However, in recent years, for the purpose of saving the energy required for the above-mentioned tempering treatment and reducing the cost of work-in-process, for example, carbon steel for machine structure specified in JIS G 4051 and JIS G 4106 are specified. A non-heat treated steel in which a precipitation hardening element such as V or Nb is added to a manganese steel for machine structure has been developed, and engine parts such as the above connecting rod (hereinafter abbreviated as connecting rod), underbody parts, or other construction machinery. It is applied to parts, etc.

These non-heat treated steels are obtained by cooling after hot forging to form a ferrite / pearlite mixed structure and precipitating carbides and nitrides such as V in the ferrite part to obtain the target strength. If such non-heat treated steel is used, quenching and tempering treatment after hot forging can be omitted, and further, the heat treatment strain generated during quenching is reduced, and the subsequent straightening process is simplified. There are advantages.

On the other hand, in recent years, the demand for weight reduction of automobiles in order to reduce the fuel consumption of automobiles has become stronger and stronger, and the weight reduction of the above-mentioned mechanical structural parts is also required. In particular, since the connecting rod, which is an engine component, is a moving component, if it is possible to reduce the weight of the connecting rod, it is expected to lead to a reduction in the motion inertial force and a great effect in reducing the fuel consumption of an automobile.

In order to reduce the weight of the connecting rod, the buckling strength and bending fatigue strength of one section connecting the cylinder-side end (small end) of the engine and the crankshaft-side end (large end) are to be improved. Improvement is needed. In order to improve these strengths, it is conceivable to provide a means for increasing the tensile strength or the "hardness" having a high correlation with the tensile strength. There is a problem that the machinability of the end portion and the large end portion is extremely reduced.

Therefore, in order to provide a means capable of increasing the buckling strength and the bending fatigue strength without increasing the hardness, that is, while maintaining the machinability, the relationship between these strengths and various material characteristic values. The inventors have found that these strength characteristics have a high correlation with the yield strength rather than the tensile strength. Therefore, 1 without increasing the tensile strength (hardness)
Increasing the yield strength of the section, that is, the yield strength ratio calculated by the ratio of the yield strength to the tensile strength (hereinafter referred to as the yield strength ratio)
Is expected to be effective.

However, in the conventional tempered steel having a tempered martensite structure, the above-mentioned yield strength ratio is improved by cold straightening of one section, which is essential in the manufacturing process of connecting rods. The non-heat treated steel having the ferrite / pearlite structure has a problem that the yield strength ratio is significantly reduced by the straightening process. For example, JISG
When straightening with a working rate of 5% is applied to a part manufactured using a steel in which V is added to medium carbon steel specified in 4051,
The proof stress ratio (0.01% proof stress to tensile strength ratio), which was about 0.7 before the straightening process, decreased to less than 0.50, and the desired buckling strength and bending fatigue strength were obtained. Absent.

Therefore, if the yield strength ratio after the straightening process can be made 0.50 or more, the strength can be improved while the machinability is ensured, so that such requirements must be satisfied. It has been earnestly desired to provide a new non-heat treated steel for a high-strength connecting rod that is capable of

[0010]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and its purpose is to provide a high yield strength ratio even after straightening while maintaining machinability. 1
An object of the present invention is to provide a non-heat treated steel for a high-strength connecting rod in which the buckling strength and fatigue strength of a section are remarkably enhanced, and a method capable of efficiently manufacturing a high-strength connecting rod using the non-heat treated steel. .

[0011]

The constitution of the non-heat treated steel for high-strength connecting rod according to the present invention, which has been able to solve the above-mentioned problems, has a composition of C: 0.3 to 0.6% (mass%, the same applies hereinafter). , S
i: 0.05 to 2.5%, Mn: 0.3 to 2.0%,
V: 0.03 to 0.5%, Al: 0.001 to 0.06
%, N: 0.003 to 0.03%, S: 0.01 to 0.
The gist is that it satisfies the requirement of 12%, the balance is Fe and unavoidable impurities, and satisfies the relationships of the following formulas (1) and (2). X = 0.7 [C] +0.1 [Si] + [Mn] -1.6 [S] +0.3 [V] +0.4 [Cr] +0.2 [Mo] ≧ 1.50 (1) Y = [ C] +0.1 [Si] +0.3 [Mn]-[S] +1.7 [V] +0.3 [Cr] +1.2 [Mo] ≦ 1.30 (2) [Symbol in the formula Means the content (%) of each.
However, [Cr] and [Mo] are included as unavoidable impurities. ]

In the present invention, as other elements,
Is a preferred embodiment, and by adopting such a constitution, the properties of the non-heat treated steel can be further enhanced as described later. Cr: 0.2-1.5% and / or Mo: 0.0
Non-heat treated steel containing 5 to 0.50%. Pb: 0.3% or less, Zr: 0.2% or less, Ca:
0.01% or less, Te: 0.1% or less, and Bi: 0.
A non-heat treated steel containing at least one selected from the group consisting of 1% or less (however, none of the elements contains 0%).

Further, the manufacturing method according to the present invention comprises the above-mentioned non-heat treated steel, which is hot-forged at a forging end temperature of 800 to 1100 ° C., and is then formed into 800-
The gist is to cool the temperature range of 500 ° C. at an average cooling rate of 1 to 300 ° C./min. By specifying such a structure, a high-strength connecting rod can be efficiently manufactured.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION As described above, according to the present invention, the machinability is determined by identifying the composition of each of the steel materials and by comprehensively considering the composition of the above in consideration of the histological viewpoint of the steel. It was possible to achieve the intended purpose of improving the proof stress ratio after straightening while maintaining the above, and as a result, to obtain a non-heat treated high-strength connecting rod excellent in buckling strength and fatigue strength. It was successful. Furthermore, by using the above-mentioned non-heat treated steel and specifying the hot forging end temperature and the average cooling rate after hot forging, the high-strength connecting rod was successfully manufactured. First of all, the reasons for defining the chemical composition of steel materials will be clarified.

C: 0.3 to 0.6% C is an element indispensable for increasing the amount of pearlite in the metal structure of the forged product after hot forging and cooling to secure the necessary strength. For that purpose, at least 0.3% or more must be contained. A preferable lower limit value is 0.35%. However, if the amount of C is too large, the toughness is reduced and the machinability is also significantly reduced, so the upper limit must be suppressed to 0.6% or less. A preferred upper limit is 0.55%.

Si: 0.05 to 2.5% Si effectively acts on deoxidation during melting of steel materials, and also forms a solid solution in the ferrite material of steel materials to strengthen the forged products after hot forging and cooling. It is an effective element for improving the yield strength and fatigue strength of the forged product. In order to exert such an effect effectively, it must be contained at least 0.05% or more. A preferred lower limit is 0.1%. However, if the amount is too large, the machinability is adversely affected, decarburization during hot forging is promoted, and the fatigue strength of one section is reduced, so the upper limit is 2.5%. A preferable upper limit value is 2.0%.

Mn: 0.3 to 2.0% Mn is an element effective as a deoxidizing / desulfurizing element during the melting of steel materials, and is solid-solved in the ferrite material of the forged product to strengthen the ferrite. It enhances the hardenability of pearlite to increase the amount of pearlite, and makes the lamellar spacing in pearlite finer to contribute to the increase of the yield strength ratio and fatigue strength. In order to exert such effects effectively, at least 0.3% or more must be contained. The preferred lower limit is 0.5%
It is. However, if it is too large, bainite is generated in the metal structure after hot forging and cooling, which adversely affects the machinability, so it must be suppressed to 2.0% or less. A preferred upper limit is 1.8%.

V: 0.03 to 0.5% V is an element effective for improving the proof stress ratio and the fatigue strength by finely precipitating as carbide or nitride in the ferrite part of the forged product. In order to exert such effects effectively, at least 0.03% or more must be contained. A preferred lower limit is 0.05%. However, even if added over 0.5%, the above effect is saturated and it is economically useless, so the upper limit is made 0.5%. A preferable upper limit value is 0.4%.

Al: 0.001 to 0.06% Al effectively acts as a deoxidizing element during the melting of steel materials, and also contributes to the improvement of toughness by refining austenite crystal grains due to the formation of nitrides. Therefore, in order to effectively exhibit these effects, the content must be 0.001% or more. A preferable lower limit value is 0.005%. However, if the amount is too large, the austenite crystal grains are rather coarsened and the toughness is adversely affected. Therefore, it must be suppressed to 0.06% or less. The preferred upper limit is 0.05
%.

N: 0.003 to 0.03% N has an effect of combining with a nitride forming element such as Al to refine austenite crystal grains and improving toughness. It also has the effect of strengthening the forged product by forming VN and finely depositing it. In order to exert such effects effectively, the content must be 0.003% or more. A preferable lower limit value is 0.005%. However, if the amount is too large, the toughness of the forged steel is lowered, or the hot workability is adversely affected, and cracks are likely to occur during casting or hot working. Therefore, it must be suppressed to 0.03% or less. A preferable upper limit value is 0.02%.

S: 0.01 to 0.12% S forms MnS and has the effect of improving machinability.
It is an element that contributes to the improvement of yield strength, fatigue strength, and toughness by refining the ferrite-pearlite structure by refining austenite crystal grains and forming intragranular ferrite. Addition of 0.01% or more is necessary to effectively exhibit such an effect. However, if added excessively, conversely, the fatigue strength and the toughness of the side grain are lowered, so the upper limit must be suppressed to 0.12% or less. A preferable upper limit value is 0.10%.

X = 0.7 [C] +0.1 [Si] + [Mn] -1.6 [S] +0.3 [V] +0.4 [Cr] +0.2 [Mo] ≧ 1.50 (1) The above formula (1) is particularly set as an index for improving the yield strength ratio after the straightening process is performed by cold working. If the elements in the formula satisfy the above formula (1),
It is possible to secure a yield strength ratio of 0.50 or more even after performing the straightening process, and there is almost no effect of the work ratio if the work ratio during cold working is in the range of 2 to 20%. A preferable lower limit value is 1.60%.

Y = [C] +0.1 [Si] +0.3 [Mn]-[S] +1.7 [V] +0.3 [Cr] +1.2 [Mo] ≦ 1.30 (2) [In the formula, [Cr] and [Mo] are amounts contained as inevitable impurities. The above formula (2) is particularly set as an index for improving the drill life or the broach life during drilling or broaching. As described above, the addition of elements such as Mn and V is useful for improving the proof stress. However, if these elements are added excessively, the drill life at the large end of the connecting rod will greatly increase during drilling. And the life of the broach is greatly reduced during broaching, leading to higher cost of parts. Those in which the elements in the formula satisfy the above formula (2) are extremely useful for improving their life.

The non-heat treated steel according to the present invention is indispensable to satisfy the above constitutional requirements.
The following elements may be added as selectively acceptable ingredients. Cr: 0.2-1.5% and / or Mo: 0.0
5 to 0.50% These elements are effective for increasing the strength. Of these, Cr is an element effective for enhancing the pearlite hardenability and improving the proof stress ratio and the fatigue strength, like Mn,
Such effects are effectively exhibited by containing about 0.2% or more. However, if it is contained in excess of 1.5%, bainite is generated in the metal structure of the forged product, which adversely affects the machinability.

Further, Mo is an element effective in improving the strength without impairing the toughness, and such an effect is 0.0
It is effectively exhibited by containing 5% or more. However, if it exceeds 0.50% and is contained excessively, bainite is formed in the metal structure of the forged product and the machinability is deteriorated.

At least one selected from the group consisting of Pb: 0.3% or less, Zr: 0.2% or less, Ca: 0.01% or less, Te: 0.1% or less and Bi: 0.1% or less. Non-heat treated steel containing 1 type (However, each element is 0%
Not included). All of the above elements are free-cutting elements. Of these, Zr,
Ca, Te and Bi have the effect of granulating MnS to improve the anisotropy of the forged product and prevent the fatigue strength from decreasing. However, the effect of improving machinability and anisotropy is not observed even if each element is added in excess of the above-mentioned upper limit, and conversely, toughness and fatigue strength decrease, so additions exceeding the respective upper limits are avoided. There must be.

Next, a method for manufacturing the high strength connecting rod according to the present invention will be described. In manufacturing the machine structural component, after using the above-mentioned non-heat treated steel for high-strength connecting rod to perform forming by hot forging at a forging end temperature of 800 to 1100 ° C., 800 to 500 ° C.
It is necessary to cool the temperature range of 1 to 300 ° C./min at an average cooling rate.

In the manufacturing method according to the present invention, the reasons for limiting the hot forging end temperature and the average cooling rate in a specific temperature range after the hot forging, which are defined as essential constituents, will be described below.

(I) Hot forging end temperature: 800 to 110
0 ° C. When the hot forging end temperature is less than 800 ° C., the deformation resistance of the steel material increases, and the die used for forging has a significantly shortened life or lack of wall due to friction or cracking.
A preferable lower limit value is 850 ° C. On the other hand, if the temperature is 1
If it exceeds 100 ° C, the austenite crystal grains of the forged product become coarse and the yield strength ratio and the fatigue strength decrease. A preferable upper limit is 1050 ° C.

(Ii) Average cooling rate after completion of hot forging (8
(00 to 500 ° C.): 1 to 300 ° C./min If the average cooling rate exceeds 300 ° C./min, a bainite structure or a martensite structure is generated, which shortens the tool life during cutting and reduces the yield strength ratio. Therefore,
The upper limit was 300 ° C./min. A preferable upper limit value is 250 ° C./min.

On the other hand, if the average cooling rate is less than 1 ° C./min, almost no precipitation strengthening effect due to addition of V or the like can be obtained, which leads to a decrease in strength and a proof stress ratio, and extremely deteriorates productivity. The lower limit was defined as 1 ° C / min.
A preferable lower limit value is 2 ° C./min.

As described above, the manufacturing method of the present invention is characterized in that the above-mentioned constitutional requirements are specified, and other hot forging conditions, cold working conditions and the like are not particularly limited, and high strength can be obtained. General conditions under which a mechanical structural component can be obtained can be appropriately adopted.

However, the cold working rate after hot forging is 2 to 2
0% is recommended. Within such a range, there is almost no effect of the change of the working rate on the above-mentioned formulas (1) and (2), and the strength characteristics are superior to those of the case where the conventional non-heat treated steel is used. It becomes possible to obtain a part having However, if the cold working ratio exceeds 20%, the work hardening excessively causes work hardening, and the life of the die used for cold working becomes extremely short. Further, if the cold working ratio is less than 2%, the forced working is insufficient and bending causes a problem such as a decrease in fatigue strength. The cold working rate defined here means the rate of decrease with respect to the original height during upsetting.

The connecting rod obtained by the above-mentioned method of the present invention has a yield strength ratio of 0.50 or more even after being subjected to the straightening process, so that the buckling strength and the fatigue strength of the connecting rod can be improved while ensuring good machinability. It can be improved and can greatly contribute to the weight reduction of automobiles.

[0035]

EXAMPLES Next, the structure and operation and effect of the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples, and can be adapted to the spirit of the preceding and following examples. Of course, the present invention can be implemented with modifications within the scope, and all of them are included in the technical scope of the present invention.

Example 1 After steel materials having the chemical composition shown in Table 1 were melted on a laboratory scale,
Using the steel material, the tensile properties of one section of the connecting rod were evaluated as follows. First, the cross-sectional shape is φ25
Hot forging was performed so as to obtain a mm square rod. Then 12
After heating to 50 ° C, the plate thickness is 8mm and the plate width is 50mm.
Forged into the plate material. The forging end temperature at this time is 95
The average cooling rate of 0 ° C. and 800 to 500 ° C. after hot forging was 150 ° C./min. A compression test piece having a plate thickness of 8 mm and a plate width of 14 mm was prepared from the plate material thus obtained, and imitation of the straightening process was performed, and the compression process was performed at a predetermined compression rate without restraint. Further, after the above compression processing, a tensile test piece specified in JIS No. 14B was prepared, a tensile test was performed, and the strain of the tensile test piece was measured with a strain gauge.
Tensile stress at 01% strain was measured as proof stress. The yield strength ratio to the tensile strength thus measured was calculated as the yield strength ratio. The results are shown in Tables 2 and 3.

[0037]

[Table 1]

[0038]

[Table 2]

[0039]

[Table 3]

Next, the drill machinability of the large end of the connecting rod was evaluated in the following manner. First, the cross-sectional shape is φ40 mm
It was hot forged so that it became a round bar. Then 1250
After being heated to ℃, forged into a round cap of φ25 mm, air-cooled, and adjusted to obtain hardness equivalent to that of the large end. In evaluating the drill life, the material is SKH51, φ
Using 10 drills, the cutting speed was 20 m / min, the feed rate was 0.15 mm / rev, and no lubrication was performed. The total cutting length before the drill was broken was taken as the drill life. The results are shown in Table 4.

[0041]

[Table 4]

From the results of Tables 2 to 4, the following can be considered. Steel type symbols 1 to 19 are examples satisfying all the requirements of the present invention, and the yield strength ratio tends to decrease due to cold working, but any steel has a yield strength of 500.
N / mm ² or more and the proof stress ratio is 0.50 or more, and it can be seen that it has excellent characteristics. Further, it was found that the hardness was 350 HV or less as it was forged, and when machining the part which is not subjected to the straightening process, the conventional problem that the tool life becomes extremely short does not occur.

On the other hand, the steel grade symbols 20 to 29 are comparative examples lacking any of the prescribed requirements defined in the present invention, and each of them has the following problems. Since the value of 20: X was 1.50 or less, the yield strength ratio was much lower than the target value of 0.50. 21: Hardness is 350 because Y value is larger than 1.30
The drill life was significantly shortened beyond HV. 22: Since the amount of C is small, the value of X becomes less than 1.50,
Both yield strength and yield ratio decreased.

23: Due to the large amount of C, the value of Y became larger than 1.30, the hardness exceeded 350 HV, and the drill life was significantly shortened. 24: Due to the large amount of Si added, the value of Y became larger than 1.30, the hardness exceeded 350 HV, and the drill life was significantly shortened. 25: Bainite was generated due to the large amount of Mn added, and the Y value became larger than 1.30, the hardness exceeded 350 HV, and the drill life was significantly shortened. 26: Bainite was generated due to the large amount of added Cr, the value of Y became larger than 1.30, the hardness exceeded 350 HV, and the drill life was significantly shortened.

27: Since V was not added and the value of X was less than 1.50, both the yield strength and the yield strength ratio were low. 28: Bainite is generated because a large amount of Mo is added, and Y
Value was larger than 1.30, the hardness exceeded 350 HV, and the drill life was significantly shortened. 29: Since a large amount of Al was added, cracking occurred during compression processing.

Example 2 Using the steel types 1 (inventive steels) and 20 (comparative steels) shown in Table 1 above, as shown in Table 5, the hot forging end temperature was 8
The yield strength, tensile strength, yield strength ratio, fatigue limit and fatigue limit ratio were measured for various average cooling rates in the temperature range of 00 to 500 ° C. Of these, the proof stress and the tensile strength were measured in the same manner as in Example 1. The fatigue test was carried out on an Ono-type rotary bending fatigue test piece having a parallel portion with a diameter of 8 mm. The results are also shown in Table 5.

[0047]

[Table 5]

As is clear from Table 5, those produced using Steel 1 of the present invention with the production requirements specified in the present invention have a high yield strength ratio and can obtain excellent fatigue limit and fatigue limit ratio. On the other hand, it was found that when the manufacturing requirements were out of the requirements of the present invention, the yield strength ratio, the fatigue limit and the fatigue limit ratio were all lowered. It was also found that when Comparative Steel 20 was used, the proof stress ratio decreased, and the fatigue limit and the fatigue limit ratio also decreased under any of the conditions.

[0049]

EFFECTS OF THE INVENTION Since the present invention is configured as described above, as a result of having a high yield strength ratio even after being subjected to straightening processing, for connecting rods excellent in buckling strength and fatigue strength while maintaining machinability. Non-heat treated steel can be obtained. Further, by using the manufacturing method of the present invention, it is possible to efficiently manufacture a connecting rod that can effectively exhibit the above-mentioned characteristics.

Front page continued (72) Inventor Koji Itakura 2 Takaracho, Kanagawa-ku, Yokohama, Kanagawa Nissan Motor Co., Ltd. (72) Inventor Takuro Yamaguchi 2 Takaracho, Kanagawa-ku, Yokohama, Kanagawa Nissan Motor Co., Ltd. (72) Invention Person Masahiro Yoshikawa 2 Takara-cho, Kanagawa-ku, Yokohama-shi, Kanagawa Nissan Motor Co., Ltd.

Claims (4)

[Claims] 1. C: 0.3 to 0.6% (mass%, the same hereinafter), Si: 0.05 to 2.5%, Mn: 0.3 to 2.
0%, V: 0.03-0.5%, Al: 0.001-
0.06%, N: 0.003 to 0.03%, S: 0.0
A high-strength non-heat treated steel for connecting rod, which satisfies the requirement of 1 to 0.12%, the balance is Fe and inevitable impurities, and satisfies the relations of the following formulas (1) and (2). X = 0.7 [C] +0.1 [Si] + [Mn] -1.6 [S] +0.3 [V] +0.4 [Cr] +0.2 [Mo] ≧ 1.50 (1) Y = [ C] +0.1 [Si] +0.3 [Mn]-[S] +1.7 [V] +0.3 [Cr] +1.2 [Mo] ≦ 1.30 (2) [Symbol in the formula Means the content (%) of each.
However, [Cr] and [Mo] are included as unavoidable impurities. ] 2. As another element, Cr: 0.2 to 1.5%
And / or Mo: 0.05 to 0.50%, The non-heat treated steel for high-strength connecting rod according to claim 1. 3. As other elements, Pb: 0.3% or less (not including 0%), Zr: 0.2
% Or less (not including 0%), Ca: 0.01% or less (0
%), Te: not more than 0.1% (not including 0%) and Bi: not more than 0.1% (not including 0%), at least one selected from the group consisting of The non-heat treated steel for a high strength connecting rod according to claim 1 or 2. 4. The steel according to any one of claims 1 to 3 is formed by hot forging at a forging end temperature of 800 to 1100 ° C., and then a temperature range of 800 to 500 ° C. A method for producing a high-strength connecting rod, which comprises cooling at an average cooling rate of 300 ° C./min.