JP3485805B2 - Hot forged non-heat treated steel having high fatigue limit ratio and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art Conventionally, many parts of automobiles and industrial machines are manufactured through hot forging followed by heat treatment such as quenching and tempering, that is, a so-called tempering process to impart necessary mechanical properties. However, non-heat treated steel for hot forging containing a small amount of V, Nb, etc. has been used so that desired mechanical properties can be obtained even if this heat 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 Publication has a drawback that its fatigue limit ratio is lower than that of alloy heat treated steel. In addition, Japanese Patent Publication No. 8-14001 discloses an example in which the strength of bainite is increased to 30% or more. However, since it has a bainite structure, its machinability is inferior to that of a ferrite + pearlite structure. There is. Furthermore, in Japanese Patent Laid-Open No. 9-53142, fatigue properties are improved by controlling the grain size of ferrite and the lamella spacing of pearlite to be small by using Ti carbonitride without adding V. Although disclosed, that method results in an unnecessarily high machinability and a low fatigue limit ratio because the static tensile strength becomes unnecessarily high. In addition, Japanese Patent Publication No. 7-59739 discloses that MnS particles and Ti
It is disclosed that the structure is refined by the synergistic effect with the compound particles to enhance 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, when it is attempted to secure high fatigue strength in order to improve the durability of parts, the tensile strength and hardness become unnecessarily high. Since the workability such as cutting is deteriorated, the effect of reducing the manufacturing cost due to the omission of the heat treatment cannot be sufficiently obtained. This is because the fatigue limit of steel material is roughly proportional to the tensile strength, and in the limited manufacturing process,
It was extremely difficult to deviate from this relationship and improve the fatigue limit ratio. Here, the fatigue limit ratio is the ratio of fatigue strength / tensile strength.

The present invention has been made to solve the above problems, and provides a hot forged non-heat treated steel capable of improving the fatigue strength without increasing the tensile strength more than necessary and a method for producing the same. The purpose is to provide.

[0005]

Means for Solving the Problems As a result of detailed investigations on the relationship between the microstructure and tensile strength, fatigue limit, etc. of non-heat treated steel cooled under various cooling conditions after hot forging, The present invention has been completed by finding a structure morphology for obtaining a high fatigue limit ratio and a manufacturing method thereof.

That is, the first invention is C:
0.1-0.4%, V: 0.05-0.55%, Mn: 1.0-2.5%, Cr: 0.
2 to 0.7%, Si: 0.5 to 3.0%, S: 0.005 to 0.3%, Al: 0.
005 to 0.1%, N: 0.005 to 0.02%, balance Fe and unavoidable impurities, the structure after hot forging is ferrite-pearlite structure, ferrite fraction is 50% or more,
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.

The second aspect of the invention further comprises, 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.

A third aspect of the present invention is a hot forged non-heat treated steel having a high fatigue limit ratio, which contains Ti and / or Nb in a total amount of 0.06% or less in addition to the chemical components of the first and second aspects. Is.

A fourth invention is that the steel having the chemical composition of the above-mentioned first to third inventions is hot forged at a temperature of 1050 ° C. or higher and 1300 ° C. or lower and then 20 ° C. from 800 ° C. to 500 ° C. / min
As described above, the method for producing hot forged non-heat treated steel having a high fatigue limit ratio of cooling at a cooling rate of 100 ° C / min or less.

In a fifth aspect of the invention, in order to obtain a higher fatigue limit ratio, the steel having the chemical composition of the first to third aspects is hot forged at a temperature of 1050 ° C or higher and 1300 ° C or lower,
Hot forging non-conditioning with a high fatigue limit ratio that cools from the forging end temperature to 700 ° C at a cooling rate of 60 ° C / min or more, and then gradually cools from 700 ° C to 600 ° C at a cooling rate of 20 ° C / min or less It is a method of manufacturing quality steel.

In order to obtain a higher fatigue limit ratio, the sixth aspect of the invention is to gradually cool from 700 ° C. to 600 ° C. at a cooling rate of 20 ° C./min or less and then from 600 ° C. to 300 ° C. at 60 ° C./min. A method for producing a non-heat treated steel for hot forging having a high fatigue limit ratio, which is cooled at the above cooling rate.

The reasons for limiting the chemical components in the present invention will be described below. C has the effect of forming pearlite and strengthening ferrite by forming carbides and carbonitrides with V, but if it exceeds 0.4%, pearlite increases and tensile strength increases, but ferrite is strengthened. If not, the fatigue strength is not improved so much, and the fatigue limit ratio is reduced. Also, since the machinability is reduced, the C content is
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%. When the tensile strength is not particularly required, it is preferable that the C content be in the range of 0.1 to 0.25%.

V forms a carbide / carbonitride with C and N to increase the strength. In particular, the fine V carbides and carbonitrides precipitated in proeutectoid 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. Therefore, as a result of detailed investigation of the precipitation behavior of carbides and carbonitrides of V, the inventors found that increasing the amount of VC (carbides of V) precipitated in ferrite leads to strengthening of ferrite. , And found that low C and high V are desirable. However, even if V exceeds 0.55%, its effect is saturated, and since V is an expensive element, the price of steel increases, so the V content is
The upper limit was 0.55%. On the other hand, if V is less than 0.05%, the amount of carbide and carbonitride precipitation of V that strengthens ferrite decreases, so ferrite strengthening cannot be expected. Therefore, the V content should be in the range of 0.05 to 0.55%. More preferably,
0.2 to 0.5%, more preferably 0.3 to 0.5%.

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

Si functions as a deoxidizing agent and also has an effect of increasing fatigue strength by solid solution strengthening. However, if the addition amount is less than 0.5%, the effect will not be fully exerted,
Even if it exceeds 3.0%, the effect does not increase any more, but rather it causes adverse effects such as deterioration in toughness. Therefore, the Si content should be in the range of 0.5 to 3.0%.

S forms MnS, contributes to the refinement of the structure, improves the fatigue strength, and is effective in improving the machinability. However, if the addition amount is less than 0.005%, there is no effect, and if it exceeds 0.3%, the effect is saturated, rather the toughness and hot forgeability deteriorate, and the fatigue strength also decreases. Therefore, the S content should be in the range of 0.005 to 0.3%.

Al acts as a deoxidizing agent, but its addition amount is 0.0
If it is less than 05%, the effect is not sufficient and the mechanical properties deteriorate, and if it exceeds 0.1%, the toughness decreases. Therefore, the Al content should be in the range of 0.005 to 0.1%.

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

In addition to the above chemical components, the present invention also comprises Pb, Ca,
At least one of Te and Bi can be contained.
Pb, Ca, and Te have the effect of improving machinability, so they are added as necessary, but even if they are added in excess, not only will the effect saturate, but the hot forgeability will be reduced and mechanical properties will decrease. Cause to decrease. Therefore, the upper limit is set for each, and 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 and N
Mechanical properties are affected by forming bN, NbC and their solid-solution compounds, etc. and precipitating at various temperatures to strengthen the precipitation or pinning the grain boundaries to suppress grain growth. . In the present invention, by precipitating fine Ti compounds and Nb compounds, ferrite is strengthened, the ratio of ferrite hardness / pearlite hardness is controlled within an appropriate value range, and the fatigue limit ratio is improved. The upper limit value is set to 0.06%, because if it exceeds this value, the Ti and / or Nb compound tends to be coarsened, so that the strengthening of ferrite becomes insufficient and the fatigue strength decreases. 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 consists of ferrite and pearlite, and has a ferrite fraction of 50%.
That is all. It is known that the machinability is remarkably reduced when a supercooled structure such as bainite appears, and in the steel of the present invention, the supercooled structure should be suppressed to at most about 5% by volume, and preferably does not exist at all. Better.
The ferrite ratio of 50% or more means that if the ferrite ratio is low, the pearlite ratio will be high, the tensile strength will be unnecessarily high, and the fatigue strength will not be improved so much that the fatigue limit ratio will decrease. Because I will do it.
More preferably, the ferrite fraction is 60% or more. Further, the reason why the ferrite fraction is 90% or less is that if it exceeds 90%, the tensile strength decreases and it cannot be used as a structural material.

Specifying the ferrite hardness / pearlite hardness ratio is the most important item in the present invention. The inventors have confirmed that cracking occurs from the ferrite part when a repeated load is applied. That is, when a load is applied, strain concentrates on the ferrite portion, and cracks occur in the ferrite portion. This tendency becomes stronger as the difference between the ferrite strength and the pearlite strength increases. Therefore, the present inventors have paid attention to this point and investigated the relationship between the ferrite hardness / pearlite hardness ratio and the fatigue behavior. As a result, by setting this ratio to 0.60 or more, the strain concentration on the ferrite part is minimized. It has been found that the fatigue strength can be dramatically improved by suppressing the above. 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 reason for limiting the forging temperature to 1050 ° C or higher and 1300 ° C or lower is as follows. Since the steel of the present invention is obtained by strengthening proeutectoid ferrite with V carbides and carbonitrides, it is necessary to completely re-dissolve the proeutectoid ferrite during forging. Within the chemical composition range of the steel of the present invention, it is possible to re-dissolve V carbides and carbonitrides by setting the forging temperature to 1050 ° C or higher. Also, the forging temperature was set to 1300 ° C. or lower because
This is because if the temperature exceeds this, the grain growth in the austenite region becomes remarkable and the fatigue strength decreases.

The reason for cooling from 800 ° C. to 500 ° C. at a cooling rate of 20 ° C./min or more and 100 ° C./min or less after forging is as follows. The transformation temperature is in the range of 800 ℃ to 500 ℃. Therefore, if the cooling rate is less than 20 ℃ / min, the ferrite fraction increases, but the C concentration of pearlite increases and the hardness of pearlite increases. This is because the ferrite hardness / pearlite hardness ratio is less than 0.60. Further, if the cooling rate exceeds 100 ° C./min, a supercooled structure such as bainite / martensite appears and fatigue properties and machinability deteriorate. In particular, if the content of Mn and Cr is large, a supercooled structure is likely to appear, so it is desirable to set the cooling rate to 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 depending on the shape and size of the component, or by controlling the cooling rate by applying a blast or heat retention as necessary.

From the forging end temperature of the fifth invention, 700 ° C.
The reason for cooling at a cooling rate of 60 ° C / min or more is as follows. In order to make the V carbides and carbonitrides that precipitate in proeutectoid ferrite as fine as possible, the austenite temperature range is quenched as much as possible to suppress the precipitation of coarse carbides and carbonitrides. For that, from the forging end temperature to 70
It is necessary to cool to 0 ° C at a cooling rate of 60 ° C / min or more, and if the cooling rate is lower than that, the above effect is not so great. A more preferable cooling rate for obtaining the above effect is 1
It is more than 00 ℃ / min.

After the above cooling, from 700 ° C to 600 ° C, 20 ° C
The reason for slow cooling at a cooling rate of / min or less is as follows. The lamellar spacing of pearlite is increased, and the precipitation of V carbides and carbonitrides in the pro-eutectoid ferrite is promoted to strengthen the ferrite. Therefore, the cooling rate near and immediately below the pearlite transformation temperature is slowed. For that purpose, it is effective to set the cooling rate from 700 ° C. to 600 ° C. to 20 ° C./min or less, and more preferably 10 ° C./min or less. In some cases, it may be held 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 depending on the shape and size of the component, or by controlling the cooling rate by applying a blast or heat retention as necessary.

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

[0028]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below. The test materials were steel ingots having the chemical components shown in Tables 1 to 4 melted using a high-frequency vacuum melting furnace at 1250
It was hot forged into various diameters shown in Tables 5 to 9 at a temperature of ° C and then cooled at the cooling rates shown in Tables 5 to 9. Cut out various test pieces from these test materials,
Microstructure observation, hardness test, tensile test, and fatigue test were performed. The ferrite fraction was measured by observing the structure and using image analysis. 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. Fatigue test, response
Ono type rotary bending fatigue test pieces with a notch having a force concentration coefficient α = 1.9 were used, and electrolytic polishing was performed to remove the influence of the processed layer on the surface, followed by JIS Z 2274. The results are shown in Tables 5 to 9. In the structures of Tables 5 to 9, F indicates ferrite, P indicates pearlite, and B indicates bainite.

The numbers 1 to 5 show the influence of the amount of C.
When the amount of C 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 high, 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%, but a high fatigue limit ratio is obtained particularly in the range of 0.10 to 0.25%. In the examples, the tensile strength is 800 MPa or more and the fatigue limit ratio is 0.260 or more.

The numbers 6 to 10 show the influence of the amount of Si.
When the amount of Si 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, the hot forgeability is reduced, and defects during forging lead to a reduction in fatigue strength, and the fatigue limit ratio is not improved.

The numbers 11 to 15 and the numbers 16 to 20 are Mn and
The effect of Cr amount is shown. When the addition amount is small, the ferrite hardness / pearlite hardness ratio becomes lower than the range of the present invention and the effect of improving fatigue strength does not appear, and when it is too large, a bainite structure appears. The fatigue limit ratio is decreasing.

Nos. 21 to 27 show the influence of the V content. When the V content is too small, the ferrite hardness is low, and therefore the fatigue strength is low and the fatigue limit ratio is also low. On the other hand, even if the amount of V is too large, the fatigue limit ratio does not improve at the expense of cost.

The numbers 28 to 32 show the influence of the S amount, and the numbers 33 to 37.
Shows the influence of the Al amount, and numbers 38 to 42 show the influence of the N amount, respectively, and the fatigue limit ratio is low when the addition amount is too small or too large. In addition, Nos. 43 to 46 are examples in which Pb, Ca, and Te were added to improve machinability, and within the scope of the present invention, mechanical properties are not particularly deteriorated.

Nos. 47 to 51 are those in which the cooling rate from 800 ° C. to 500 ° C. was changed by changing the diameter after forging in the range of 10 to 70 mm. If the cooling rate is too fast, the ferrite fraction becomes low. , Bainite has also appeared, and the fatigue limit ratio is decreasing. On the other hand, if the cooling rate is too slow, the ferrite hardness is low and the ferrite hardness / perlite hardness ratio is also lower than the range of the present invention, so the fatigue limit ratio is lowered.

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

The numbers 67 to 73 are obtained by changing the amounts of addition of Ti and Nb, while the numbers 67 to 72 are proper in the amounts of addition and a high fatigue limit ratio is obtained. No. 73 has an excessively large amount of addition, so that the ferrite hardness is rather reduced and the fatigue limit ratio is also reduced. Nos. 74 to 81 are examples in which various machinability improving elements were added, and since they are all within the scope of the present invention, they 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]

To facilitate the understanding of the results in Tables 5-9, some of these results are shown in FIGS. Fig. 1 shows the effect of C, Si, Mn, and Cr contents on the fatigue limit ratio, and Fig. 2 shows the effect of V, S, Al, and N contents 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 ratio of ferrite hardness / pearlite hardness on the fatigue limit ratio. FIG. 5 shows the effect of the cooling rate on the fatigue limit ratio in the fourth aspect of the invention. FIG. 6 shows the influence of the cooling rate on the fatigue limit ratio in the fifth and sixth inventions. Further, FIG. 7 shows the effect of Ti and Nb contents on the fatigue limit ratio. The subscripts in the figure 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-conditioning having a high fatigue limit ratio is performed. Quality steel can be provided.

[Brief description of drawings]

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

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

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

FIG. 4 is a diagram showing an influence of a ratio of ferrite hardness / pearlite hardness on a fatigue limit ratio.

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

FIG. 6 is a diagram showing an influence of a cooling rate on a 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.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masato Rokuhi, 2 Nadahamahigashicho, Nada-ku, Kobe-shi, Hyogo Prefecture Kobe Steel Works, Kobe Steel Works (72) Inventor Jun Inada 2 Nadahama-higashi, Nada-ku, Kobe, Hyogo Prefecture Stock Company Kobe Steel Works Kobe Steel Works (56) References JP-A-9-143610 (JP, A) JP-A-7-3386 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) ) C22C 38/00-38/60 C21D 8/00

Claims (6)

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