JPH09143610A - Hot forged non-heat treated steel having high fatigue strength and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a hot forged non-heat treated steel having fatigue strength higher than that of the conventional non-heat treated steel without deteriorating its machinability and furthermore having about 70 to 100kgf/mm<2> tensile strength. SOLUTION: The componental compsn. of this steel is composed of, by mass, 0.15 to 0.60% C, 0.05 to 2.5% Si, 0.3 to 2.5% Mn, 0.01 to 0.10% S, 0.05 to 2.5% Cr, 0.05 to 0.5% V, 0.01 to 0.060% Al, 0.005 to 0.030% N and 0.001 to 0.005% O, and the balance Fe with inevitable impurities. The structure obtd. after the steel is subjected to hot forging is formed of a ferritic-pearlitic one, and moreover, as for the pearlitic structure, the average grain size is regulated to >=3.0, the average colony size in the pearlite is regulated to <=20μm, and the average lamellar distance is regulated to <=0.30μm.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot forged non-heat treated steel having high fatigue strength, which is used for parts for machine structures such as automobiles and construction machines, and efficiently manufactures forged products using the non-heat treated steel. With regard to the method, in particular, high tempering strength can be obtained without quenching and tempering that is performed after hot forging, and high tensile strength can be obtained without reducing machinability. The present invention relates to an inexpensive hot forged non-heat treated steel that can be manufactured and a method for manufacturing a forged product.

[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 carrying out a tempering process. However, in recent years, for the purpose of saving energy required for the above-mentioned heat treatment and reducing costs by cutting work-in-process, for example, carbon steel for machine structural use specified in JIS G 4051 and machines specified in JIS G 4106. Non-heat treated steels in which precipitation hardening elements such as V and Nb are added to structural manganese steels have been developed and applied to automobile engine parts, suspension parts, construction machine parts and the like.

[0003] These non-heat treated steels have a target strength by cooling after hot forging to form a ferrite-pearlite mixed structure and precipitating carbides or nitrides such as V and Nb in the ferrite part. If such a 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 can be reduced and the subsequent straightening process can be simplified. In addition to the advantage of being hardened, quench cracking is less likely to occur, the rate of defective products due to quench cracking is reduced, and it is possible to significantly reduce the component manufacturing cost.

However, along with the recent trend toward higher engine output and smaller and lighter parts in automobiles, the mechanical strength of mechanical structural parts manufactured using the above-mentioned non-heat treated steel is also increased. Is coveted. At present, fatigue strength is the bottleneck in increasing the strength of these parts, and various studies have been made to improve the fatigue strength. Among them, the simplest method for improving the fatigue strength of a non-heat treated steel having a ferrite / pearlite structure is to increase the C content of the steel material and increase the amount of pearlite, which is a hard structure, to increase the hardness of the entire member. It is to increase the sheath tensile strength. However,
On the other hand, if the hardness or tensile strength becomes too large, the machinability will be significantly reduced, and the productivity at the time of component manufacturing will be reduced, leading to an increase in manufacturing cost.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and its purpose is higher than that of conventional non-heat treated steel without reducing machinability. It has fatigue strength and tensile strength of 70-100kgf /
Providing a hot forged non-heat treated steel of about mm ² and efficiently manufacturing a forged product having the above-mentioned characteristics by using the non-heat treated steel while leaving it to be hot-forged and then left to cool or blast cooled. It is intended to provide a way to do.

[0006]

In the hot forged non-heat treated steel according to the present invention, which can solve the above problems, the composition of the steel is C: 0.15 to 0.60% (mass%, Same), Si: 0.05 to 2.5%, Mn: 0.3 to 2.
5%, S: 0.01 to 0.10%, Cr: 0.05 to
2.5%, V: 0.05 to 0.5%, Al: 0.01 to
0.060%, N: 0.005-0.030%, O:
0.001 to 0.005% and the balance: Fe and inevitable impurities, the structure after hot forging of the steel has a ferrite-pearlite structure, and the pearlite structure has an average grain size number of 3.0 or more. , The average colony size in pearlite: 20 μm or less, and the average lamellar spacing: 0.30 μm or less are essential.

In the present invention, for the purpose of obtaining higher fatigue strength, Nb: 0.05% or less (not including 0%), Ti: 0.05% or less (not including 0%), Zr. : 0.1% or less (not including 0%), Mo:
1% or less (not including 0%) and Ni: 1% or less (0
At least 1 selected from the group consisting of
Pb: 0.3% or less (not including 0%), Ca: 0.01% or less (0) as another element for the purpose of containing a seed or further improving machinability. %), Te: 0.3% or less (0% is not included), and Bi: 0.3% or less (0% is not included), at least one selected from the group consisting of It is a preferred embodiment of the present invention.

The method for manufacturing a forged product according to the present invention has a hot forging end temperature of 1000 to 1250 ° C. and 800 to 800 ° C.
In the method for producing a forged product by controlling the cooling rate at 300 ° C .: 0.1 to 5 ° C./sec, the steel having the above-described composition is used, and the steel material heating temperature (T) during hot forging is The point is that the following formula is satisfied. T (° C) ≧ {30400 / (20.5-log [V] ⁴・
[C] ³ )}-300 (In the formula, [] means mass% of each element, respectively)

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors of the present invention have found that the structure after cooling by hot forging followed by cooling by standing air cooling or blast cooling is ferrite.
In order to improve the fatigue strength of conventional hot-forged non-heat-treated steel with a pearlite structure, non-heat-treated steel with various alloy elements added in various proportions was melted and hot-forged into a round bar ( These forged materials have different hardenability and different metallographic structures due to the difference in the amount of alloying elements), and some steels have different metallographic structures by changing the hot forging conditions. The following findings were obtained by producing forged materials, processing fatigue test pieces using these forged materials, and observing fatigue tests and fatigue cracks.

(1) Fatigue cracks occur in the ferrite part, which is the weakest microstructure, but the ferrite part is formed by both actions of solid solution strengthening by increasing Si content and N content and precipitation strengthening by increasing V content. By strengthening, fatigue strength can be significantly improved without significantly increasing hardness and tensile strength.

(2) When the fatigue crack propagated, it was observed that the crack was bent due to resistance at the boundary between the ferrite / pearlite interface and the colony in the pearlite. Further, when the fatigue crack propagates in the colony, it may propagate at the interface between the cementite plate and the ferrite, but it is also observed that the fatigue crack propagates while cutting the cementite plate. As a result of considering these observation results, in order to increase the fatigue strength, the pearlite structure becomes smaller, so that not only the pearlite grain size but also the colony diameter in pearlite and the average lamellar spacing are reduced, that is, the whole pearlite. We also found that it is effective to finely control the colony and the detailed organization of the lamellar spacing.

(3) When a forged product is manufactured by hot forging and cooling the hot forged non-heat treated steel specified on the basis of the contents of knowledge of the above (1), depending on the added amount of V or C. By specifying the forging heating temperature (T), V can be completely dissolved in the steel during heating and a large amount of carbide of V can be precipitated during cooling to further strengthen the ferrite part. ,
Fatigue strength can be significantly improved.

The present invention has been made on the basis of the above findings, and specifies the chemical composition in steel capable of strengthening the ferrite part, and at the same time, the structure after hot forging of the steel (ferrite / pearlite structure). We have succeeded in obtaining a hot forged non-heat treated steel having a high level of fatigue strength by finely controlling the grain size of the pearlite structure in the above, and further, forged products using such steels. We have found a method that can efficiently produce
Hereinafter, the reason why the composition of the steel and the characteristics of the metallographic structure are defined in the present invention, and further the reason why the heating temperature of the steel material during hot forging is defined will be described in detail.

First, the reason for defining the chemical composition of the steel material will be explained. C: 0.15 to 0.60% C increases the amount of pearlite in the metal structure (ferrite pearlite) of the forged product after hot forging and cooling,
It is an element that has the effect of increasing the strength of the forged product, forms V-carbides, and exerts the effect of improving the strength of the ferrite part by the precipitation of the carbides. In order to exert these effects effectively, the content must be 0.15% or more. A preferable lower limit value is 0.20%. However, if the amount of C becomes too large, the toughness decreases and the machinability also decreases significantly, so the upper limit is set to 0.
It must be kept below 60%. A preferable C amount is 0.50% or less.

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. It is necessary to add 0.05% or more to effectively exhibit such an effect. A preferable lower limit value is 0.15%. However, if added excessively, the machinability is significantly reduced, so 2.5% is made the upper limit. The preferable upper limit value is 2.0%, and more preferably 1.50%.

Mn: 0.3 to 2.5% Like Si, Mn is an element effective as a deoxidizing agent during the melting of steel materials, and increases the pearlite hardenability of the forged product to increase the amount of pearlite. The particle size and colony size of pearlite are made finer, and the average lamellar spacing is made finer, which greatly contributes to the increase of fatigue strength. It is necessary to add at least 0.3% or more in order to effectively exert such an effect and to obtain high strength of 70 kgf / mm ² or more in tensile strength while considering the amounts of other main alloying elements in the present invention. . A preferred lower limit is 0.50%. However, if added excessively, bainite is generated in the metal structure after hot forging, and although the tensile strength increases, almost no improvement in fatigue strength can be obtained, and the machinability is adversely affected. The upper limit must be kept below 2.5%. A preferable upper limit value is 2.0%.

S: 0.01 to 0.10% S has an action of improving machinability, forms MnS, and promotes nucleation of ferrite during cooling after forging, so that the grain size of pearlite and the colony size are increased. Is an element useful for refining. In order to exert such an effect effectively, it is necessary to add 0.01% or more. A preferable lower limit value is 0.02%. However, if added excessively, the toughness will be adversely affected and the fatigue strength will also decrease, so the upper limit is made 0.10%. A preferable upper limit value for securing both fatigue strength and machinability is 0.07%.

Cr: 0.05 to 2.5% Cr is an element that enhances pearlite hardenability and fatigue strength, like Mn. It is necessary to add at least 0.05% or more in order to effectively exert such effects and to obtain a high strength of 70 kgf / mm ² or more in tensile strength while considering the amounts of other main alloying elements in the present invention. . A preferable lower limit value is 0.20%. However, if added in excess of 2.5%, bainite is generated in the metal structure of the forged product, and although tensile strength increases, little improvement in fatigue strength is observed and machinability is also adversely affected. The upper limit is 2.5%. A preferable upper limit value for securing both fatigue strength and machinability is 1.00%.

V: 0.05 to 0.5% V is an element effective for forming carbides or nitrides and refining austenite crystal grains, and V carbides precipitate in the ferrite part to cause fatigue fracture. It is an element that strengthens the ferrite base that becomes the starting point of and improves fatigue strength. In order to exert such effects effectively, it is necessary to add 0.05% or more. A preferred lower limit is 0.1%. However, even if added in excess of 0.5%, the above effect is saturated and useless, so the upper limit is made 0.5%. A preferable upper limit value is 0.4%.

Al: 0.01 to 0.060% 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 by the formation of nitrides. is there. In order to exert such effects effectively, it is necessary to add 0.01% or more. A preferable lower limit value is 0.015%. However, if added excessively, the austenite crystal grains will rather coarsen and adversely affect the toughness, so the upper limit is 0.0
It needs to be 60%. The preferred upper limit is 0.040
%.

N: 0.005 to 0.030% N is an element which is effective in combining with the above-mentioned nitride-forming elements such as V to refine the austenite crystal grains, and
It is an element that forms a solid solution in ferrite to strengthen the ferrite part and improve proof stress and fatigue strength. In order to effectively exhibit such effects, it is necessary to add 0.005% or more. A preferable lower limit value is 0.006%. However, even if added excessively, the effect is saturated and useless, so the upper limit was made 0.030%. A preferable upper limit value is 0.020%.

O: 0.001 to 0.005% O forms an oxide to reduce the fatigue strength, so it is desirable that the content thereof be as small as possible.
If the amount is controlled to be less than 0.1%, the manufacturing cost increases significantly, so the lower limit is made 0.001%. But 0.0
If added in excess of 05%, a large amount of oxide-based inclusions such as Al ₂ O ₃ and SiO ₂ are generated and the machinability deteriorates, so the upper limit is made 0.005%. A preferable upper limit value is 0.003%.

The steel used in the present invention contains the above-mentioned elements as essential components, but may further contain an element selected from the group (i) and / or (ii) below as a selectively acceptable component. good.

(I) Nb: 0.05%, Ti: 0.05
%, Zr: 0.1%, Mo: 1% and Ni: 1%, at least one selected from the group consisting of 0% and none of these elements. Is useful as Hereinafter, each element will be described.

Nb: 0.05% or less Nb is an element effective in forming carbides or nitrides to make austenite crystal grains finer and reduce the grain size and colony size of pearlite, and in hot forging. When such a high-temperature heat treatment is performed, it is an element that improves the fatigue strength by partly dissolving the added Nb as a solid solution to strengthen the precipitation during cooling. Addition of 0.005% or more is preferable in order to effectively exhibit such effects. But 0.0
Even if added in excess of 5%, the effect is saturated and useless, so the upper limit is preferably made 0.05%.

Ti: 0.05% or less Ti is also effective for improving fatigue strength by forming carbides or nitrides to make austenite crystal grains finer, reduce pearlite grain size and colony size, like Nb. Is an element. In order to effectively exhibit such an effect, 0.005% or more is preferably added, but if added in excess of 0.05%, the above effect is saturated and useless.
It is recommended that the upper limit be 0.05%.

Zr: 0.1% or less Zr, like Nb and Ti, is an element that suppresses the growth of austenite crystal grains during heating, and exerts an effect of improving fatigue strength. In order to effectively exert such effects, it is preferable to add 0.001% or more, but even if added over 0.1%, the effect is saturated and useless, so the upper limit is 0.1%. Is recommended.

Mo: 1% or less, Ni: 1.00% or less These elements are effective elements for increasing fatigue strength without impairing toughness. In order to effectively exert such an action, addition of 0.05% or more is preferable in all cases, but if added in excess of 1%, bainite structure is formed and fatigue strength is significantly reduced. 1
% Is recommended.

(Ii) Pb: 0.3% or less, Ca: 0.0
1% or less, Te: 0.3% or less and Bi: 0.3% or less (at least one element is not included in any element). And then Z
r, Ca, Te and Bi have an effect of improving the anisotropy of the forged product by granulating MnS, and by containing these in the above lower limit values or more, their effects are effectively exhibited. You can However, even if the content of each element exceeds the upper limit, the effect is saturated and it is useless.

The chemical composition of the steel in the non-heat treated steel of the present invention is as described above, but the structure after hot forging the steel naturally has a ferrite-pearlite structure as the non-heat treated steel. In addition, the following requirements (1) to (4) are satisfied.

Average particle size number of pearlite (Gf particle size): 3.0 or more, average colony size of pearlite: 20 μm or less, and average lamellar spacing: 0.30
μm or less, that is, when the fatigue crack propagates as described above, resistance is received at the interface between the ferrite and pearlite and the boundary portion of the colony in the pearlite, the crack bends, and when the fatigue crack propagates in the colony, In some cases, the interface between the cementite plate and ferrite progresses, but in some cases, it progresses while cutting the cementite plate.
As a result of comprehensively considering such observation results, the above requirements aim to achieve the intended purpose by finely controlling not only the entire pearlite tissue but also the colony level and the lamellar interval level.

The range recommended for the purpose of achieving further fatigue strength without reducing machinability is the average particle size number of pearlite: 4.0 or more (more preferably 5.0 or more), average The colony size is 15 μm or less (more preferably 10 μm or less), and the average lamellar spacing is 0.25 μm or less (more preferably 0.20 μm or less).

Next, the reasons for limiting the requirements in the forged product manufacturing method of the present invention will be described. First, in hot forging using the above steel, the steel material heating temperature (T) during hot forging needs to satisfy the above formula, and the manufacturing method of the present invention is It has the greatest feature in that the temperature is specified according to the V concentration and the C concentration.

That is, by setting the heating temperature of the steel material based on the V concentration and the C concentration, it is possible to sufficiently exert the precipitation strengthening action of the ferrite portion by the V carbide described above. Outside the range of the above formula, V does not completely form a solid solution in austenite, so that the precipitation strengthening effect described above cannot be effectively exhibited during controlled cooling such as cooling after cooling or blast cooling after forging, and an effect of improving fatigue strength. Will not be obtained sufficiently. In the present invention, the upper limit is not particularly specified, but if it exceeds 1350 ° C, grain boundary oxidation occurs,
Since the crystal grains become coarser than necessary, the upper limit is 135
It is preferably 0 ° C. or lower (more preferably 1300 ° C. or lower).

Next, the forging end temperature is 1000 to 1250.
Control within ° C. If the upper limit exceeds 1250 ° C, the austenite crystal grains become coarse, the grain size of pearlite and the colony size become coarse, and the fatigue strength decreases.
A preferable upper limit value is 1200 ° C. On the other hand, 1000 ° C
If it is less than the above range, the solid solution ability of Nb or V during heating forging decreases and the precipitation strengthening effect during cooling decreases, or the deformation resistance increases and the life of the die decreases significantly. A preferable lower limit value is 1050 ° C.

After hot forging is performed in this manner and then formed into a predetermined shape, it is air-cooled or wind-blast cooled in a temperature range of 800 to 300 ° C. at a cooling rate of 0.1 to 5 ° C./sec. The cooling rate in this temperature range is 0.1 ° C / se
If it is less than c, the volume fraction particles of proeutectoid ferrite increase and the fatigue strength decreases, and V carbides that precipitate in ferrite aggregate, and the amount of fine V carbides that can increase the strength of ferrite. Is reduced, and the effect of improving fatigue strength cannot be fully exerted. The lower limit is preferably 0.3 ° C./sec. On the other hand, if the cooling rate exceeds 5 ° C./sec, bainite is generated in the structure to reduce the fatigue strength, and cracks and strains easily occur after cooling. The upper limit is preferably 3 ° C / se
c.

Next, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples, and changes may be made within a range compatible with the spirit of the preceding and following statements. Of course, it is possible to carry out, and all of them are included in the technical scope of the present invention.

[0038]

【Example】

Example 1 Steels having the chemical compositions shown in Tables 1 and 2 were melted in a vacuum furnace or a production furnace, then forged into a round bar of φ70 mm by hot forging, and then cut into a length of 200 mm. Then 1
After heating to 300 ° C, φ5 at the forging end temperature of 1150 ° C
It was hot forged to 0 mm and then air cooled. No. 29 is steel equivalent to JIS SCR440 and has a diameter of 50 mm.
After forging into a round bar, quenching and tempering (tempering)
The heat treated steel was used as an example.

After cutting the D / 4 portion in the L direction from each round bar thus obtained, JIS No. 4 tensile test piece, J
An IS3 impact test piece and a JIS1 rotary bending fatigue test piece were sampled and tested at room temperature. The obtained results are shown in Table 3 and Table 4, respectively.

[0040]

[Table 1]

[0041]

[Table 2]

[0042]

[Table 3]

[0043]

[Table 4]

From these results, it can be considered as follows. Nos. 1 to 16 are examples satisfying all the specified requirements of the present invention, and both the tensile strength and the fatigue strength (σw) show high values, and these values are the same as those of the conventional steel (No. It has a high value equal to or higher than that of the quenched and tempered material of .31) and is significantly improved compared to the conventional non-heat treated steel (No. 32). . Further, in the steel of the present invention, the fatigue limit ratios (σw / TS) are all higher than those of the conventional steel and the comparative steel. This means
This means that it is possible to improve only the fatigue strength without significantly increasing the tensile strength.

On the other hand, No. 17 has a low fatigue strength because the C content is lower than the lower limit value specified in the present invention. Also, N
O.18 has an excessively high tensile strength because the C content exceeds the upper limit of the present invention, and the obtained structure is a mixture of a bainite structure in addition to the ferrite / pearlite structure. Since No. 19 has a large amount of Si, the tensile strength becomes too high, and the obtained structure is also a ferrite bainite martensite structure.

No. 20 had a low Mn content, and both the pearlite colony size and the lamellar spacing exceeded the upper limits specified in the present invention, resulting in low fatigue strength.
On the contrary, since the Mn content was large, the tensile strength was high and the impact value was low, and in addition to the ferrite structure, a bainite structure and a martensite structure were also mixed.

No. 22 is an example in which the pearlite grain size becomes coarse due to the small amount of S and the pearlite colony size and the lamellar interval exceed the upper limit values, and the fatigue strength is high, while No. 23 is due to the large amount of S. Both impact value and fatigue strength are reduced. Since No. 24 has a large amount of Cr, the tensile strength becomes too high and the impact value is also low.
The obtained structure is also a mixture of bainite and martensite structures.

No. 25 has a small amount of V and has a low fatigue strength because the pearlite colony size and the lamellar spacing exceed the upper limit values, while No. 26 has a large amount of V and the fatigue strength improving effect is saturated. Is. No. 27 has a small amount of Cr and a large amount of Al, and the pearlite colony size and the lamellar spacing exceed the upper limit values, so that the fatigue strength is low.

No. 28 has a small amount of N and coarse pearlite grain size and the like, so that the precipitation effect is not sufficient and therefore the fatigue strength is low. On the other hand, No. 30 has an excessive addition of N, and the effect is saturated. There is. No. 29 has a large amount of O and is Al ₂ O ₃
Both the impact value and the fatigue strength are low because a large amount of is generated.

Example 2 Steels having the chemical compositions shown in Table 5 were melted in a vacuum furnace, and then hot-forged into a 70 mm-dia.
Cut to length. Then, after heating to 1300 ° C,
Hot forging to φ50 mm at a forging end temperature of 1000 to 1260 ° C., and then an average example speed of 800 to 300 ° C. of 0.
Cooling was performed while changing the temperature in the range of 05 to 10 ° C / sec. After cutting D / 4 part in the L direction from each round bar thus obtained, various characteristics were evaluated in the same manner as in Example 1. Table 6 shows the results. The heating temperature (T) during hot forging obtained by substituting the steel composition used in this example into the above formula is 1175 ° C, and the steel material heating temperature of 1300 ° C in this example is within the range of the present invention. Is.

[0051]

[Table 5]

[0052]

[Table 6]

As is clear from Table 6, No. 1, No. 4 and No. 5 satisfying the requirements of the present invention have high tensile strength and fatigue strength, and a high fatigue limit ratio. On the other hand, with No. 2 having a high cooling rate, a bainite-martensite structure is obtained, the tensile strength becomes too high, and the impact value is low. Further, No. 3 is an example in which the cooling rate is slow, and No. 6 is an example in which the forging end temperature is high, and in both cases, the impact value, the fatigue limit and the fatigue limit ratio are low.

Example 3 Steel having the chemical composition shown in Table 5 was melted in a vacuum furnace, and then hot forged into a round bar of φ70 mm, and then 200
It was cut to a length of mm. Then 1100 to 1300 ° C
After that, hot forging was performed to a diameter of 50 mm at a forging end temperature of 1000 ° C., and then the average cooling rate of 800 to 300 ° C. was cooled at 0.38 ° C./sec. After cutting D / 4 part in the L direction from each round bar thus obtained, various characteristics were evaluated in the same manner as in Example 1. Table 7 shows the results. The heating temperature (T) during hot forging obtained by substituting the steel composition used in this example into the above formula was 117.
5 ° C.

[0055]

[Table 7]

As is clear from Table 7, in No. 1 and No. 2 satisfying the requirements of the present invention, the tensile strength and fatigue limit are high, and the fatigue limit ratio is also high. On the other hand, No. 3 and No. 4 are examples in which the steel material heating temperature is low, and both have low impact values, fatigue limits and fatigue limit ratios.

[0057]

The present invention is constructed as described above, and
Higher than conventional non-heat treated steel without reducing machinability
Has fatigue strength and tensile strength of 70-100kgf
/ Mm ^Two Can provide the degree of hot forged non-heat treated steel
Was. And, if the manufacturing method of the present invention is used, after hot forging
Forging with the above-mentioned characteristics as it is left to cool or wind blown
The product could be manufactured efficiently.

Claims (4)

[Claims] 1. The composition of the steel is C: 0.15 to 0.60.
% (Mass%, the same applies hereinafter), Si: 0.05 to 2.5%,
Mn: 0.3 to 2.5%, S: 0.01 to 0.10%,
Cr: 0.05 to 2.5%, V: 0.05 to 0.5%,
Al: 0.01 to 0.060%, N: 0.005 to 0.
030%, O: 0.001 to 0.005%, and the balance: Fe and inevitable impurities, the structure after hot forging of the steel has a ferrite-pearlite structure, and the pearlite structure has an average grain size number. : 3.0 or more, average colony size in perlite:
20 μm or less, and average lamellar spacing: 0.30 μm
A hot forged non-heat treated steel having high fatigue strength, characterized by satisfying the following requirements. 2. Further, Nb: 0.05% or less (not including 0%), Ti: 0.
05% or less (not including 0%), Zr: 0.1% or less (not including 0%), Mo: 1% or less (not including 0%) and Ni: 1% or less (not including 0%) The hot forged non-heat treated steel according to claim 1, which contains at least one selected from the group consisting of: 3. As other elements, Pb: 0.3% or less, Ca: 0.01% or less, Te:
The hot forged non-heat treated steel according to claim 1 or 2, containing at least one selected from the group consisting of 0.3% or less and Bi: 0.3% or less. 4. Hot forging end temperature: 1000 to 1250
C., and cooling rate at 800 to 300.degree. C .: 0.1
In the method for producing a forged product by controlling to ~ 5 ° C / sec, the steel having the component composition according to any one of claims 1 to 3 is used, and the steel material heating temperature (T) during hot forging is A method for manufacturing a forged product, characterized by satisfying the following formula. T (° C) ≧ {30400 / (20.5-log [V] ⁴・
[C] ³ )}-300 (In the formula, [] means mass% of each element, respectively)