JPH11152542A - Non-heattreated steel for hot forging, having high fatigue limit ratio, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a non-heat treated steel for hot forging, capable of improvement in fatigue strength without increasing tensile strength to a higher value than is needed, and its production. SOLUTION: The non-heat treated steel for hot forging, having high fatigue limit ratio, has a composition consisting of, by mass, 0.1-0.4% C, 0.05-0.55% V, 1.0-2.5% Mn, 0.2-0.7% Cr, 0.5-3.0% Si, 0.005-0.3% S, 0.005-0.1% Al, 0.005-0.02% N, and the balance Fe with inevitable impurities and also has a structure, after hot forging, which is composed of ferrite/pearlite structure and in which ferrite fraction is regulated to 50-90% and also the ratio between ferrite hardness and pearlite hardness is regulated to >=0.60. Further, as chemical components, at least one kind among <=0.30% Pb, <=0.01% Ca, <=0.10% Te, and <=0.20% Bi can be incorporated into the above composition.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention belongs to the technical field of hot forging non-refining and a method for producing the same, which are used as parts of automobiles and industrial machines, and more particularly, without heat treatment after hot forging. The present invention belongs to the technical field of a hot forged non-heat treated steel having a high fatigue limit ratio and a method for producing the same.

[0002]

2. Description of the Related Art Hitherto, many parts of automobiles and industrial machines have been manufactured through a process of imparting necessary mechanical properties by performing a heat treatment such as quenching and tempering after hot forging, that is, a so-called tempering treatment. However, a non-heat treated steel for hot forging to which a small amount of V, Nb, or the like is added so that desired mechanical properties can be obtained even if the tempering treatment is omitted. For example, JP-A-59-9122 and JP-A-62-16785
The non-heat treated steel for hot forging disclosed in Japanese Patent Laid-Open Publication No. H10-157, has a disadvantage that the fatigue limit ratio is lower than that of the alloy tempered steel. Japanese Patent Publication No. 8-14001 discloses an example in which bainite is increased to 30% or more to increase strength. However, the machinability is inferior to that of ferrite + pearlite because of the bainite structure. I have. Furthermore, Japanese Patent Application Laid-Open No. 9-53142 discloses that fatigue characteristics can be improved by controlling the ferrite grain size and pearlite lamella spacing to be small by using Ti carbonitride without adding V. Although disclosed, the method results in unnecessarily high static tensile strength, so the machinability is not very good and the fatigue limit ratio is low. Japanese Patent Publication No. 7-59739 discloses that MnS particles and Ti
It is disclosed that the structure is refined by a synergistic effect with the compound particles to increase the strength and toughness, but no significant effect is observed in improving the fatigue limit ratio.

[0003]

In the conventional non-heat-treated steel for hot forging, in order to ensure high fatigue strength in order to increase the durability of parts, the tensile strength and hardness become higher than necessary. As a result, the workability such as cutting is reduced, and the effect of reducing the manufacturing cost by omitting the tempering treatment cannot be sufficiently obtained. This is because the fatigue limit of steel is roughly proportional to tensile strength, and in a limited manufacturing process,
It was extremely difficult to deviate from this relationship and improve the fatigue limit ratio. Here, the fatigue limit ratio is a ratio of fatigue strength / tensile strength.

The present invention has been made in order to solve the above-mentioned problems, and it is an object of the present invention to provide a hot-forged non-heat treated steel capable of improving fatigue strength without unnecessarily increasing tensile strength and a method of manufacturing the same. The purpose is to provide.

[0005]

Means for Solving the Problems The inventors of the present invention have conducted detailed studies on the relationship between the microstructure of non-heat treated steel cooled under various cooling conditions after hot forging, tensile strength, fatigue limit, and the like. The present invention has been found by finding out a structure form for obtaining a high fatigue limit ratio and a manufacturing method thereof.

That is, in the first invention, C:
0.1-0.4%, V: 0.05-0.55%, Mn: 1.0-2.5%, Cr: 0.
2 ~ 0.7%, Si: 0.5 ~ 3.0%, S: 0.005 ~ 0.3%, Al: 0.
005-0.1%, N: 0.005-0.02%, the balance consists of Fe and unavoidable impurities, the structure after hot forging is a ferrite-pearlite structure, the ferrite fraction is 50% or more,
It is a hot-forged non-heat treated steel having a high fatigue limit ratio of 90% or less and a ferrite hardness / pearlite hardness ratio of 0.60 or more.

[0007] The second invention is further characterized in that, as a chemical component, P
b: 0.30% or less, Ca: 0.01% or less, Te: 0.10% or less, Bi:
It is a hot forged non-heat treated steel having a high fatigue limit ratio containing at least one of 0.20% or less.

[0008] A third invention is a hot forged non-heat treated steel having a high fatigue limit ratio, which further contains 0.06% or less in total of Ti and / or Nb in the chemical components of the first and second inventions. It is.

In a fourth aspect, the steel having the chemical composition of the first to third aspects is subjected to hot forging at a temperature of 1050 ° C. or more and 1300 ° C. or less, and then from 20 ° C. to 800 ° C. to 500 ° C. / min
As described above, the present invention is a method for producing a hot forged non-heat treated steel having a high fatigue limit ratio, which is cooled at a cooling rate of 100 ° C./min or less.

In a fifth aspect, in order to obtain a higher fatigue limit ratio, the steel having the chemical components of the first to third aspects is subjected to hot forging at a temperature of 1050 ° C. or more and 1300 ° C. or less,
Cooling from the forging end temperature to 700 ° C at a cooling rate of 60 ° C / min or more, and then gradually cooling from 700 ° C to 600 ° C at a cooling rate of 20 ° C / min or less. This is a method for producing high quality steel.

According to a sixth aspect of the present invention, in order to obtain a higher fatigue limit ratio, the temperature is gradually cooled from 700 ° C. to 600 ° C. at a cooling rate of 20 ° C./min or less, and then from 60 ° C./min. This is a method for producing a non-heat-treated steel for hot forging having a high fatigue limit ratio and cooling at the above cooling rate.

Hereinafter, the reasons for limiting the chemical components in the present invention will be described. C forms pearlite and has the effect of forming carbides and carbonitrides with V to strengthen ferrite. However, if it exceeds 0.4%, pearlite increases and tensile strength increases, but ferrite is strengthened. Therefore, the fatigue strength does not improve much, and the fatigue limit ratio decreases. In addition, the machinability also decreases, so the C content
0.4% or less. On the other hand, if C is less than 0.1%, sufficient tensile strength cannot be secured. Therefore, the C content is
The range is 0.1 to 0.4%. Particularly when the tensile strength is not so required, the C content is desirably in the range of 0.1 to 0.25%.

V forms carbides and carbonitrides together with C and N to increase the strength. In particular, fine carbides and carbonitrides of V precipitated in pro-eutectoid ferrite greatly contribute to ferrite strengthening and improve fatigue strength. However, the carbides and carbonitrides of V that precipitate in the high-temperature region (austenite region) during cooling after hot forging have a large grain size and do not contribute much to the strengthening of ferrite. The present inventors have investigated the precipitation behavior of carbides and carbonitrides of V in detail, and found that increasing the amount of VC (carbide of V) precipitated in ferrite as much as possible leads to strengthening of the ferrite. It was found that low C and high V are desirable. However, even if V exceeds 0.55%, the effect saturates and the price of steel increases because V is an expensive element.
The upper limit was 0.55%. On the other hand, when V is less than 0.05%, ferrite strengthening cannot be expected because the amount of precipitation of carbides and carbonitrides of V, which strengthens ferrite, decreases. Therefore, the V content is in the range of 0.05 to 0.55%. More preferably,
It is in the range of 0.2-0.5%, more preferably 0.3-0.5%.

Both Mn and Cr have the effect of strengthening the ferrite matrix and pearlite by solid solution strengthening. However, if the amount is too small, the effect does not appear. If the amount is too large, a supercooled structure such as bainite appears during cooling. As a result, the tensile strength becomes too high, and the machinability and the like are reduced. Therefore, the Mn content is in the range of 1.0 to 2.5%, and the Cr content is in the range of 0.2 to 0.7%.

[0015] Si acts not only as a deoxidizing agent, but also has an effect of increasing the fatigue strength by solid solution strengthening. However, if the addition amount is less than 0.5%, the effect is not sufficiently exhibited,
Even if it exceeds 3.0%, the effect does not increase any more, but rather brings about adverse effects such as a decrease in toughness. Therefore, the Si content is in the range of 0.5 to 3.0%.

S forms MnS, contributes to the refinement of the structure, improves fatigue strength, and is effective in improving machinability. However, if the addition amount is less than 0.005%, the effect is not obtained, and if it exceeds 0.3%, the effect is saturated, and the toughness and hot forgeability are reduced, and the fatigue strength is also reduced. Therefore, the S content should be in the range of 0.005 to 0.3%.

Al acts as a deoxidizing agent, but is added in an amount of 0.0
If it is less than 05%, the effect is not sufficient and the mechanical properties are reduced, and if it exceeds 0.1%, the toughness is reduced. Therefore, the Al content is in the range of 0.005 to 0.1%.

N forms V carbonitride together with V and C, and also forms nitride by combining with Al, both of which contribute to microstructural refinement and improvement in tensile strength and fatigue strength. However, if the amount added is less than 0.005%, the effect does not appear.
%, Coarse V nitrides precipitate at high temperatures,
Precipitation strengthening by fine V carbides / carbonitrides becomes insufficient. Therefore, the N content should be in the range of 0.005 to 0.02%.

The present invention relates to the above-mentioned chemical components, Pb, Ca,
At least one of Te and Bi can be contained.
Pb, Ca, and Te are added as necessary because they have the effect of improving machinability.However, even if they are added in excess, not only the effect is saturated, but also the mechanical properties are reduced by reducing hot forgeability. Cause a decrease. Therefore, an upper limit is set for each of them, so that the Pb content is 0.30% or less, the Ca content is 0.01% or less, the Te content is 0.10% or less, and the Bi content is 0.20% or less.

Ti and Nb combine with N and C to form TiN, TiC, N
Forming bN, NbC and their solid solution compounds, etc., precipitate at various temperatures to strengthen the precipitation, or pin the crystal grain boundaries to suppress the grain growth and affect the mechanical properties . In the present invention, by precipitating a fine Ti compound or Nb compound, the ferrite is strengthened, the ratio of ferrite hardness / pearlite hardness is controlled to an appropriate value range, and the fatigue limit ratio is improved. The reason why the upper limit is set to 0.06% is that if it exceeds this, the Ti and / or Nb compound is likely to be coarsened, so that the ferrite is insufficiently strengthened and the fatigue strength is reduced. The effect becomes remarkable when the addition amount of Ti and / or Nb is 0.002% or more.

Next, the reasons for limiting the structure and hardness will be described. Furthermore, the hot forged non-heat treated steel of the present invention basically comprises ferrite and pearlite, and has a ferrite fraction of 50%.
Above. It is known that when a supercooled structure such as bainite appears, the machinability is significantly reduced. Even in the steel of the present invention, the supercooled structure should be suppressed to at most about 5% by volume, and desirably does not exist at all. Better.
The reason for setting the ferrite fraction to 50% or more is that the lower the ferrite fraction, the higher the pearlite fraction, the higher the tensile strength than necessary, and the less the fatigue strength improves, resulting in a lower fatigue limit ratio. Because it will.
More preferably, the ferrite fraction is 60% or more. Also, the reason why the ferrite fraction is set to 90% or less is that if it exceeds 90%, the tensile strength decreases, and the ferrite fraction cannot be used as a structural material.

Specifying the ratio of ferrite hardness / pearlite hardness is the most important matter in the present invention. The inventors have confirmed that cracking when a repeated load is applied occurs from the ferrite portion. That is, when a load is applied, strain is concentrated on the ferrite portion, and a crack is generated on the ferrite portion. The tendency increases as the difference between the ferrite strength and the pearlite strength increases. Therefore, the inventors focused on this point and investigated the relationship between the ferrite hardness / pearlite hardness ratio and fatigue behavior. As a result, by setting this ratio to 0.60 or more, the concentration of strain on the ferrite portion was minimized. It has been found that the fatigue strength can be drastically improved. Therefore, in the present invention, the ratio of ferrite hardness / pearlite hardness is set to 0.60 or more.

Next, the reasons for limiting the manufacturing conditions will be described. The reasons for limiting the forging temperature to 1050 ° C or higher and 1300 ° C or lower are as follows. In the steel of the present invention, proeutectoid ferrite is strengthened by carbides / carbonitrides of V, and therefore it is necessary to completely re-dissolve them during forging. Within the range of the chemical composition of the steel of the present invention, it is possible to re-dissolve carbides and carbonitrides of V by setting the forging temperature to 1050 ° C. or higher. Also, the forging temperature was set to 1300 ° C or less,
If the temperature exceeds this, the grain growth in the austenite region becomes remarkable and the fatigue strength decreases.

After forging, the cooling is performed from 800 ° C. to 500 ° C. at a cooling rate of 20 ° C./min or more and 100 ° C./min or less for the following reasons. The transformation temperature is in the range of 800 ° C to 500 ° C. Therefore, when the cooling rate is less than 20 ° C / min, the ferrite fraction increases, but the C concentration of pearlite increases and the hardness of pearlite increases. This is because the ratio of ferrite hardness / pearlite hardness is less than 0.60. On the other hand, if the cooling rate exceeds 100 ° C./min, a supercooled structure such as bainite / martensite appears and the fatigue properties and machinability are reduced. In particular, those having a high content of Mn and Cr tend to cause a supercooled structure, so that the cooling rate is desirably 20 ° C / min or more and 80 ° C / min or less.
The cooling rate may be controlled by changing the cooling rate at the time of natural cooling according to the shape and size of the component, or by controlling the cooling rate by performing blast or keeping the heat as necessary.

Further, 700 ° C. from the forging end temperature of the fifth invention.
The reason for cooling at a cooling rate of 60 ° C./min or more is as follows. In order to make the carbides and carbonitrides of V precipitated in proeutectoid ferrite as fine as possible, the temperature range of austenite is cooled as quickly as possible to suppress the precipitation of coarse carbides and carbonitrides. To achieve this, 70
It is necessary to cool to 0 ° C. at a cooling rate of 60 ° C./min or more, and at a cooling rate lower than that, the above effect is not so significant. The preferred cooling rate to achieve the above effect is 1
00 ° C / min or more.

After the above cooling, 20 ° C. from 700 ° C. to 600 ° C.
The reason for slow cooling at a cooling rate of / min or less is as follows. In order to increase the lamellar spacing of pearlite and to promote the precipitation of carbides and carbonitrides of V in pro-eutectoid ferrite to strengthen ferrite, the cooling rate near and immediately below the pearlite transformation temperature is reduced. For that purpose, it is effective to set the cooling rate from 700 ° C. to 600 ° C. at 20 ° C./min or less, more preferably at 10 ° C./min or less. In some cases, it may be maintained at an appropriate temperature of 700 ° C. to 600 ° C. for about 5 to 30 minutes. The cooling rate may be controlled by changing the cooling rate at the time of natural cooling according to the shape and size of the component, or by controlling the cooling rate by performing blast or keeping the heat as necessary.

After the above-mentioned slow cooling,
The reason for cooling at a cooling rate of 60 ° C./min or more is as follows. To strengthen ferrite by supersaturation C,
It is effective to set the cooling rate after pearlite transformation to 60 ° C./min or more, whereby the ratio of ferrite hardness / pearlite hardness approaches 1 and the fatigue limit ratio is improved. If the cooling rate is less than 60 ° C / min, ferrite strengthening by supersaturated C cannot be sufficiently expected. Therefore, the forging end temperature is 700
Cool to 60 ° C at a cooling rate of 60 ° C / min or more, and then
Higher fatigue limit ratio can be obtained by gradually cooling at a cooling rate of 20 ° C / min or less from ℃ to 600 ° C and at a cooling rate of 60 ° C / min or more from 600 ° C to 300 ° C .

[0028]

Embodiments of the present invention will be described below. The test material was hot forged from a steel ingot having the chemical composition shown in Tables 1 to 4 at a temperature of 1250 ° C. to various diameters shown in Tables 5 to 9 by using a high frequency vacuum melting furnace. , At the cooling rates shown in Tables 5 to 9. Various test pieces were cut out from these test materials, and a structure observation, a hardness test, a tensile test, and a fatigue test were performed. In addition, the ferrite fraction was measured by image analysis by microstructure observation. Further, the ferrite hardness and the pearlite hardness were measured with a load of 5 g using a Vickers hardness meter. The tensile test was performed according to JIS Z 2241. The fatigue test was performed in accordance with JIS Z 2274 after using an Ono-type rotary bending fatigue test piece with a notch having a notch coefficient α = 1.9 and electrolytic polishing to remove the influence of the processed layer on the surface. Table 5 shows the results.
To Table 9 below. In the structures of Tables 5 to 9, F indicates ferrite, P indicates pearlite, and B indicates bainite.

Numbers 1 to 5 show the effect of the amount of C.
If the C content is less than the range of the present invention, the fatigue limit ratio is high, but the ferrite fraction is high and the tensile strength is low. On the other hand, if the C content is too large, the ferrite fraction falls below 50% and the fatigue limit ratio decreases. The C content is preferably in the range of 0.1 to 0.4%, and particularly, a high fatigue limit ratio can be obtained in the range of 0.10 to 0.25%. In the examples, the criteria for the determination are a tensile strength of 800 MPa or more and a fatigue limit ratio of 0.260 or more.

Numbers 6 to 10 show the effect of the amount of Si.
If the Si content is too small, the ferrite is not sufficiently strengthened, so that the ratio of ferrite hardness / pearlite hardness is lower than the range of the present invention, and the fatigue strength and the fatigue limit ratio are low. If the Si content is too large, defects during forging lead to a decrease in fatigue strength due to a decrease in hot forgeability, and the fatigue limit ratio does not improve.

The numbers 11 to 15 and 16 to 20 are Mn and
It shows the effect of Cr content. If the amount is small, the ratio of ferrite hardness / pearlite hardness is lower than the range of the present invention, and the effect of improving fatigue strength does not appear. If too large, bainite structure appears. The fatigue limit ratio has decreased.

Numbers 21 to 27 show the effect of the amount of V. If the amount of V is too small, the ferrite hardness is low, so that the fatigue strength is low and the fatigue limit ratio is low. On the other hand, even if the amount of V is too large, the fatigue limit ratio does not improve despite the high cost.

Nos. 28 to 32 show the effect of the amount of S, and Nos. 33 to 37
Indicates the effect of the amount of Al, and Nos. 38 to 42 indicate the effect of the amount of N. The fatigue limit ratio is low even if the amount of addition is too small or too large. In addition, Nos. 43 to 46 are examples in which Pb, Ca, and Te are added in order to improve machinability, and there is no particular decrease in mechanical properties within the range of the present invention.

Nos. 47 to 51 change the cooling rate from 800 ° C. to 500 ° C. by changing the diameter after forging in the range of 10 to 70 mm. If the cooling rate is too high, the ferrite fraction is low. However, bainite has also appeared, lowering the fatigue limit ratio. On the other hand, if the cooling rate is too slow, the ferrite hardness is low, and the ratio of ferrite hardness / pearlite hardness is lower than the range of the present invention, so that the fatigue limit ratio is low.

Numbers 52 to 61 relate to the cooling method of the fifth invention, and a high fatigue limit ratio can be obtained by cooling at a cooling rate within the limited range of the present invention. Further, reference numerals 62 to 66 relate to the cooling method of the sixth invention. By cooling at a cooling rate within the limited range of the present invention, a higher fatigue limit ratio can be obtained.

Nos. 67 to 73 are obtained by changing the addition amounts of Ti and Nb, while Nos. 67 to 72 show proper addition amounts and a high fatigue limit ratio. In the case of No. 73, since the addition amount is too large, the ferrite hardness is rather lowered and the fatigue limit ratio is also lowered. Nos. 74 to 81 are examples to which various machinability improving elements are added, all of which are within the scope of the present invention, and thus have a high fatigue limit ratio.

[0037]

[Table 1]

[0038]

[Table 2]

[0039]

[Table 3]

[0040]

[Table 4]

[0041]

[Table 5]

[0042]

[Table 6]

[0043]

[Table 7]

[0044]

[Table 8]

[0045]

[Table 9]

Some of these results are shown in FIGS. 1 to 7 to make the results in Tables 5 to 9 easier to understand. FIG. 1 shows the effect of the amounts of C, Si, Mn and Cr on the fatigue limit ratio, and FIG. 2 shows the effect of the amounts of V, S, Al and N on the fatigue limit ratio. FIG. 3 shows the effect of the ferrite fraction on the tensile strength and the fatigue limit ratio. FIG. 4 shows the effect of the ferrite hardness / pearlite hardness ratio on the fatigue limit ratio. FIG. 5 shows the effect of the cooling rate on the fatigue limit ratio in the fourth invention. FIG. 6 shows the effect of the cooling rate on the fatigue limit ratio in the fifth and sixth inventions. FIG. 7 shows the effect of Ti and Nb amounts on the fatigue limit ratio. The subscripts in the figures correspond to the numbers in each table.

[0047]

As is apparent from the above description,
According to the present invention, by adjusting the chemical composition and controlling the cooling rate after hot forging, without performing heat treatment such as quenching and tempering after hot forging, hot forging non-forming with a high fatigue limit ratio is performed. Quality steel can be provided.

[Brief description of the drawings]

FIG. 1 is a view showing the influence of the amounts of C, Si, Mn, and Cr on a fatigue limit ratio.

FIG. 2 is a diagram showing the effect of the amounts of V, S, Al, and N on the fatigue limit ratio.

FIG. 3 is a diagram showing the effect of the ferrite fraction on the tensile strength and the fatigue limit ratio.

FIG. 4 is a graph showing the effect of the ferrite hardness / pearlite hardness ratio on the fatigue limit ratio.

FIG. 5 is a diagram showing the effect of the cooling rate on the fatigue limit ratio in the fourth invention.

FIG. 6 is a diagram showing the effect of the cooling rate on the fatigue limit ratio in the fifth and sixth inventions.

FIG. 7 is a diagram showing the influence of Ti and Nb amounts on the fatigue limit ratio.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Atsushi Atsushi 2 Nadahama-Higashi-cho, Nada-ku, Kobe City, Hyogo Prefecture Inside Kobe Steel Works Kobe Works

Claims (6)

[Claims] C: 0.1 to 0.4%, V: 0.05 to 0% by mass%.
55%, Mn: 1.0-2.5%, Cr: 0.2-0.7%, Si: 0.5-3.0
%, S: 0.005 to 0.3%, Al: 0.005 to 0.1%, N: 0.005 to
It contains 0.02%, the balance being Fe and unavoidable impurities. The structure after hot forging is a ferrite-pearlite structure, with a ferrite fraction of 50% or more and 90% or less, and of ferrite hardness / pearlite hardness. A hot forged non-heat treated steel having a high fatigue limit ratio, wherein the ratio is 0.60 or more. 2. C: 0.1 to 0.4%, V: 0.05 to 0% by mass%.
55%, Mn: 1.0-2.5%, Cr: 0.2-0.7%, Si: 0.5-3.0
%, S: 0.005 to 0.3%, Al: 0.005 to 0.1%, N: 0.005 to
0.02%, Pb: 0.30% or less, Ca: 0.01%
The following contains at least one of Te: 0.10% or less and Bi: 0.20% or less, the balance being Fe and unavoidable impurities. The structure after hot forging is a ferrite / pearlite structure, and the ferrite fraction is 50%. A hot forged non-heat treated steel having a high fatigue limit ratio, characterized in that the ratio is not more than 90% and the ferrite hardness / pearlite hardness ratio is not less than 0.60. 3. The method according to claim 2, further comprising:
The hot forged non-heat-treated steel having a high fatigue limit ratio according to claim 1 or 2, which contains 0.06% or less in total. 4. A steel having the chemical composition according to claim 1, which is subjected to hot forging at a temperature of 1050 ° C. or more and 1300 ° C. or less, and then 20 ° C./800° C. to 500 ° C. The method for producing a hot-forged non-heat treated steel having a high fatigue limit ratio according to any one of claims 1 to 3, wherein the steel is cooled at a cooling rate of not less than min and not more than 100 ° C / min. 5. The steel having the chemical composition according to claim 1 is subjected to hot forging at a temperature of 1050 ° C. or more and 1300 ° C. or less, and 60 ° C./min from a forging end temperature to 700 ° C. Cool at the above cooling rate and then from 700 ° C to 600 ° C
The method for producing a hot-forged non-heat-treated steel having a high fatigue limit ratio according to any one of claims 1 to 3, wherein the steel is gradually cooled at a cooling rate of not more than ° C / min. 6. After gradually cooling from 700 ° C. to 600 ° C. at a cooling rate of 20 ° C./min or less, 60 ° C. from 600 ° C. to 300 ° C.
6. The method for producing a non-heat treated steel for hot forging having a high fatigue limit ratio according to claim 5, wherein the cooling is performed at a cooling rate of not less than / min.