JPH073386A - Non-refining steel for hot forging excellent in fatigue strength and production of non-refining hot forged product using the same steel - Google Patents

Abstract

PURPOSE:To produce a non-refining hot forged product excellent in fatigue strength by forging steel contg. specified small amounts of V, Ti, N or the like in a specified temp. range and thereafter specifying the cooling rate into specified value at the time of its cooling. CONSTITUTION:Steel contg., by weight, 0.10 to 0.60% C, 0.005 to 2.00% Si, 0.55 to 2.00% Mn, 0.01 to 0.10% S, 0.0005 to 0.05% Al, >0.30 to 0.70% V and 0.003 to 0.05% Ti, furthermore contg. <0.008% Ni and <0.035% P, or moreover contg. a specified small amt. of Nb or contg. Cr, Mo, Ca, Pb, Te, Se, Bi or the like is forged in the austenitization temp. range of the Ac3 transformation point to 1250 deg.C and is thereafter cooled to 300 deg.C at 0.05 to 2 deg.C/sec cooling rate. The structure is formed into ferrite-pearlite and the formation of bainite having a high dislocation density is prevented, by which the non-refining hot forged product having a high yield strength and high fatigue strength can be obtd.

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 hot forging having excellent fatigue strength and a method for producing a non-heat treated hot forged product using the steel.

[0002]

2. Description of the Related Art Generally, automobile parts, machine structural parts, etc., which require high strength and high toughness, are hot forged into a predetermined shape and then subjected to offline quenching and tempering as a refining process. Has been done. However, in recent years, the use of so-called non-heat treated steel, which does not require tempering treatments such as quenching and tempering, has become widespread in the manufacture of automobile parts from the viewpoint of reducing manufacturing costs and omitting steps.

Non-heat treated steel of ferrite / pearlite structure, which is widely used at present, lacks strength, especially low temperature toughness. Therefore, the composition and cooling rate are adjusted to form a non-heat treated structure of ferrite + pearlite + bainite. Steel has been developed (Japanese Patent Laid-Open No. 58-167751). Further, in the bainite type hot forged non-heat treated steel, the lower limit of the transformation temperature is limited by adjusting the composition, and the fatigue properties are reduced by reducing the island martensite and the retained austenite which deteriorate the fatigue properties of the bainite steel. There is also an attempt to improve the quality (Japanese Patent Laid-Open No. 4-141547).

[0004]

The strength and toughness of conventional ferrite / pearlite non-heat treated steel are not sufficient, and the strength and toughness are satisfactory in the structure in which the above bainite is mixed. On the contrary, the fatigue strength, which is one of the important properties, may decrease, and the range of use is naturally limited. An object of the present invention is to provide a non-heat treated steel for hot forging excellent in fatigue strength and a method for manufacturing a non-heat treated hot forged product using the steel.

[0005]

DISCLOSURE OF THE INVENTION The inventors of the present invention have made it possible to guarantee the fatigue characteristics of a conventional non-heat treated steel in the as-hot-forged state as it is. In order to provide a method for manufacturing a fatigue strength non-heat treated hot forged product, the inventors have made earnest studies. As a result, it was found that the two points of high dislocation density and existence of a locally low hardness portion in the matrix lead to early initiation of fatigue cracks and adversely affect fatigue properties. Therefore, by defining the cooling rate after forging to a certain value or less, the structure becomes ferrite pearlite and bainite with high dislocation density is prevented from being generated.

Further, in order to reduce the locally low hardness portion in the matrix, the ferrite solution is strengthened by utilizing the solid solution strengthening by increasing the Si content and the precipitation strengthening of V carbide by the addition of a large amount of V. Reduce the hardness difference. In addition, since the fatigue strength decrease due to surface decarburization becomes a problem in the part that is used with the forged skin as it is, the heating temperature is set low, but V used for precipitation strengthening forms a nitride and its small solubility product. Therefore, the solid solution cannot be completely dissolved at the heating temperature. Therefore, N is greatly reduced to form V carbide having a large solubility product, and Ti is added to fix N as TiN.

That is, the present invention is based on the above knowledge, and it is said that it is possible to guarantee a high fatigue strength which has not been found in conventional non-heat treated steels, and the gist thereof is as follows. is there. (1) As a weight ratio, C: 0.10 to 0.60%, S
i: 0.005 to 2.00%, Mn: 0.55 to 2.00
%, S: 0.01 to 0.10%, Al: 0.0005 to
0.05%, V: more than 0.30% and 0.70% or less, Ti:
0.003 to 0.050%, and further N: 0.00
A non-heat treated steel material for hot forging excellent in fatigue strength, characterized in that it is limited to less than 8% and P: 0.035% or less, and the balance is composed of iron and inevitable impurities.

The component (2) further contains Nb: 0.005 to 0.005.
The non-heat treated steel material for hot forging according to claim 1, which contains 10%.

The component (3) further comprises Cr: 0.10 to 1.5.
The non-heat treated steel material for hot forging excellent in fatigue strength according to claim 1 or 2, which contains one or two of 0% and Mo: 0.05 to 1.00%.

The component (4) further contains Ca: 0.0005 to 0.00.
The non-heat treated steel material for hot forging excellent in fatigue strength according to claim 1 or 2 or 3, containing one or two of 005% and Pb: 0.04 to 0.30%.

The component (5) further contains Te: 0.01 to 0.1.
5%, Se: 0.01 to 0.15%, Bi: 0.04 to 0.
The non-heat treated steel material for hot forging excellent in fatigue strength according to claim 1, 2 or 3 or 4, containing one or more of 3%.

(6) As a weight ratio, C: 0.1 to 0.1.
60%, Si: 0.005 to 2.00%, Mn: 0.55
~ 2.00%, S: 0.01 to 0.10%, Al: 0.0
005 to 0.05%, V: more than 0.30% and 0.70% or less, Ti: 0.003 to 0.050%, N: less than 0.008%, P: 0.035% Limited to
Steel with the balance being iron and unavoidable impurities is forged at an austenitizing temperature of Ac ₃ points or more and 1250 ° C. or less, and then up to 300 ° C. 0.05 ° C./second or more 2 ° C./second
A method for producing a non-heat treated hot forged product, which has an excellent fatigue strength and is composed of a ferrite / pearlite structure, characterized by cooling at an average cooling rate of less than a second.

The component (7) further contains Nb: 0.005 to 0.5.
The method for producing a non-heat treated hot forged product excellent in fatigue strength according to claim 6, containing 10%.

The component (8) further comprises Cr: 0.10 to 1.5.
The method for producing a non-heat treated hot forged product excellent in fatigue strength according to claim 6 or 7, containing one or two of 0% and Mo: 0.05 to 1.00%.

The component (9) is further Ca: 0.0005-
The method for producing a non-heat treated hot forged product excellent in fatigue strength according to claim 6 or 7 or 8, containing one or two of 0.005% and Pb: 0.04 to 0.30%.

The component (10) further contains Te: 0.01 to 0.0.
15%, Se: 0.01 to 0.15%, Bi: 0.04 to
7. One or more of 0.3% is contained.
Alternatively, the method for producing a non-heat treated hot forged product excellent in fatigue strength according to 7 or 8 or 9.

The present invention will be described in detail below. First,
C is an element effective in increasing the strength of the final product as a mechanical part and increasing pearlite, but 0.10
If it is less than 0.1%, the effect is insufficient, and if it exceeds 0.60%, the toughness of the final product is rather deteriorated, so the content was made 0.10 to 0.60%.

Next, Si is added as a deoxidizing element and for the purpose of forming a solid solution in ferrite and strengthening the ferrite by solid solution hardening to improve fatigue characteristics.
If it is less than 5%, these effects are insufficient, while 2.
If it exceeds 0.00%, these effects are saturated and rather the toughness of the final product is deteriorated, so the content is 0.005 to 2.
It was set to 00%.

Mn is an element effective in increasing the amount of pearlite and narrowing the lamellar spacing to increase the strength of the final product, but if it is less than 0.55%, this effect is insufficient, while If it exceeds 2.00%, this effect is saturated and rather the toughness of the final product is deteriorated, so the content was made 0.55 to 2.00%.

Further, S exists as MnS in steel and contributes to improvement of machinability and refinement of structure. Furthermore, the machinability of parts is improved by reducing cutting resistance. If it is less than 0.01%, the effect is insufficient. On the other hand, if it exceeds 0.10%, the effect is saturated and rather the toughness deteriorates and the anisotropy increases. For the above reasons, the S content is set to 0.0.
It was set to 1 to 0.10%.

Next, Al is added as a deoxidizing element and a grain refining element, but if it is less than 0.0005%, its effect is insufficient, while if it exceeds 0.05%, its effect is saturated. However, since it rather deteriorates the toughness, its content is set to 0.0005 to 0.05%.

Further, in the present invention, V is contained as an essential element for the purpose of precipitating as a carbide, strengthening the ferrite ground, and improving fatigue characteristics. However, if the V content is 0.30% or less, the effect is insufficient, while if the V content exceeds 0.70%, the effect is saturated and rather the toughness is deteriorated.
It was set to 0.30 to 0.70%.

In the method of the present invention, a specific amount of Ti is added as an essential element for the purpose of forming TiN in austenite, fixing N, and preventing coarsening of austenite grains during heating and precipitating as a carbide. But Ti
If the content is less than 0.005%, Ti for N fixation is insufficient, so that VN formation cannot be prevented, and if the Ti content exceeds 0.050%, coarse TiN is formed and the toughness is greatly reduced. The amount was 0.005-0.050%.

Further, the present invention is characterized in that N is limited to 0.008% or less. N is an element that easily forms a nitride with V. However, if VN having a low solubility in steel is formed, it cannot be dissolved in steel during heating, and precipitation strengthening can be achieved during ferrite transformation. Can not. In order to effectively utilize the precipitation strengthening of V carbide having a small solubility product in steel, N is limited to less than 0.008%.

On the other hand, P causes grain boundary segregation or center segregation in the steel and causes deterioration of toughness. In particular, when P exceeds 0.035%, the toughness is significantly deteriorated, so the content is set to 0.035% or less.

Claim (2) is intended to improve the durability ratio by further utilizing precipitation strengthening. Therefore Nb
However, if it is less than 0.005%, precipitation strengthening cannot be expected, and if it exceeds 0.10%, this effect saturates, which not only increases the cost but also lowers the toughness.
The amount of Nb added was 0.005 to 0.10%.

[0027] Claim (3) is intended to improve the strength, particularly both strength and toughness so that important safety parts and the like can be used. Therefore, Cr and Mo are added. Cr is an element effective in strengthening the ferrite base and increasing the strength of the final product. If the content is less than 0.10%, there is no effect, and if it exceeds 1.50%, the hardness becomes too high, the toughness decreases, and it is not preferable from the economical point of view. It was set to .50%. Mo is also an element effective in increasing the strength of the final product, but if the content is less than 0.05%, it does not have the effect, and if it exceeds 1.00%, the hardness increases, and the economy is low. Since it is not preferable in terms of its content, its content was set to 0.05 to 1.00%.

Claim (4) is intended to improve the machinability of the component after forging. Therefore, Ca and Pb are added.
Pb is an element that exists as a low-melting metal inclusion in steel and improves the machinability of parts. If the content is less than 0.04%, there is no effect, and if it exceeds 0.3%, the workability in hot or warm is impaired, so the content is 0.04 to 0.3.
%. Ca is also an element that forms soft inclusions and contributes to the improvement of machinability. If it is less than 0.0005%, there is no effect, and if it exceeds 0.005%, the inclusions are rather hardened, so the content is 0.0005 to 0.005.
%. When Ca is added, the Al addition amount is preferably 0.002% or less.

Claim (5) is also intended to improve the machinability of the component after forging. Therefore, Te, Se, Bi are added. Te and Se are elements that reduce cutting resistance and contribute to improvement of machinability. If it is less than 0.01%, there is almost no effect, and if it exceeds 0.15%, the mechanical properties are deteriorated.
Since the anisotropy becomes particularly remarkable, its content should be 0.01-
It was set to 0.15%. Bi is an element that exists as a low-melting metal inclusion in steel and improves the machinability of parts. If the content is less than 0.04%, there is no effect, and if it exceeds 0.3%, the workability in hot or warm is impaired, so the content was made 0.04 to 0.3%.

Next, the reason why the heating temperature and the cooling rate are limited in the present invention will be described. First, the lower limit of the heating temperature was set to Ac ₃ transformation point, the heating temperature of the Ac less than ₃ transformation point, the tissue is a mixture of ferrite and austenite, can be dissolved sufficiently in the austenite the V carbide This is because precipitation of V carbide in ferrite cannot be effectively used during cooling. The upper limit of heating temperature is 1
The reason why the temperature is 250 ° C. or lower is that at a heating temperature higher than 1250 ° C., the austenite crystal grains start to coarsen, the decarburization of the steel material surface becomes more severe, and the fatigue strength of the mechanical component after forging significantly decreases.

The upper limit of the cooling rate after forging is set to 2 ° C./sec. At a cooling rate higher than this, bainite having a high dislocation density is mixed in the structure to reduce the fatigue strength, and ferrite for improving fatigue properties is also used. This is because V carbides are not sufficiently precipitated therein. The lower limit of the cooling rate is set to 0.05 ° C./second, because at a cooling rate lower than this, coarse ferrite grains are formed and the toughness is significantly reduced, and carbides of V and Ti start to precipitate in austenite. This is because the amount of V and Ti carbides precipitated in ferrite decreases, and sufficient fatigue strength cannot be obtained. Further, the range that defines the cooling rate is set to the forging temperature to 300 ° C.
This is because even if the cooling rate is controlled at 300 ° C. or lower, the structure and mechanical properties do not substantially change. Hereinafter, the effects of the present invention will be described more specifically by way of examples.

[0032]

Example Steels having the chemical compositions shown in Tables 1 to 3 were melted in a high frequency furnace and cast into a 150 kg ingot. From this, test pieces were cut out and forged under the conditions shown in Tables 4 to 6. A JIS No. 14 tensile test piece, a JIS No. 3 impact test piece, a JIS No. 1 rotary bending test piece, and a drill drilling test piece were sampled from the central portion of those materials, and tensile strength, −
The 50 ° C. Charpy impact value, fatigue strength, and machinability were determined.

An index of VL ₁₀₀₀ was used for evaluation of machinability. Drill with feed rate of 0.33 mm / s (Material: SK
H51-φ3mm) peripheral speed is variously changed, the total hole depth at which the drill cannot cut is obtained at each speed, and the peripheral speed-drill life curve is created. The maximum speed at which the drill life becomes 1000mm is VL ₁₀₀₀ . It was defined and used as the evaluation standard of machinability. Tables 4 to 6 show the material characteristics of each steel material in comparison with the present invention and comparative examples.

As shown in Table 4, in the comparative example, steel Nos. In the cases of 1 to 7, the maximum value of the yield ratio was 0.70 and the maximum value of the durability ratio was 0.45, which were both low, and both did not reach the lower limit of the present invention. Compared with this, Steel No. 8
In the forging according to the sixth to eighth invention methods of the present invention, the yield ratio of steel No. 9 of the sixth invention method is 0.85, the durability ratio of 0.59, and the yield ratio of steel No. 17 of 0.5. 82, durability ratio 0.5
No. 9, steel No. 27 of the seventh invention method yield ratio 0.76, durability ratio 0.56, steel No. 31 of the eighth invention method yield ratio 0.8
6, durability ratio of 0.56, steel No. 36 yield ratio of 0.79,
Both the yield ratio and the durability ratio are much higher than those of the comparative method, such as the durability ratio of 0.59.

Steel Nos. 8 to 37 with further improved machinability
In the sixth to eighth invention methods of the present invention, for example, the tensile strength of steel No. 43 of the ninth invention method is 93.8 kgf / mm ² and the yield ratio is 0.8.
1, VL ₁₀₀₀ 24.8m / min while reaching a durability ratio of 0.58
And the tensile strength of Steel No. 56 of the 10th invention method are 97.7
While reaching a yield ratio of 0.75 and a durability ratio of 0.60 at kgf / mm ^2, it has a VL _{1000 of} 46.1 m / min, and is superior in yield ratio, durability ratio and machinability as compared with the comparative example.

Thus, according to the method of the present invention, the tensile strength is 7
0.8 ~ 97.7kgf / mm ² , yield ratio 0.75 ~ 0.7.
89, a durability ratio of 0.51 to 0.60 can be obtained, the yield ratio and durability ratio are higher than those of the comparative method, and further it has machinability equal to or higher than that of the comparative steel.

[0037]

As described above, by using the method of the present invention, it is possible to manufacture a non-heat treated hot forged product having high yield strength and high fatigue strength, and it is possible to reduce the weight of machine structural parts. The industrial effect is extremely remarkable.

[0038]

[Table 1]

[0039]

[Table 2]

[0040]

[Table 3]

[0041]

[Table 4]

[0042]

[Table 5]

[0043]

[Table 6]

Claims (10)

[Claims] 1. A weight ratio of C: 0.10 to 0.60%, Si: 0.005 to 2.00.
%, Mn: 0.55 to 2.00%, S: 0.01 to 0.1
0%, Al: 0.0005 to 0.05%, V: 0.30
% Over 0.70%, Ti: 0.003 to 0.050%,
In addition, N: less than 0.008%, P: 0.035%
A non-heat treated steel material for hot forging excellent in fatigue strength, characterized in that the balance is limited to the following and the balance consists of iron and inevitable impurities. 2. The non-heat treated steel material for hot forging excellent in fatigue strength according to claim 1, further comprising Nb: 0.005 to 0.10%. 3. The composition further comprises Cr: 0.10 to 1.50%, Mo: 0.05 to 1.00.
%, 1 or 2 of the above are contained.
Non-heat treated steel for hot forging with excellent fatigue strength as described. 4. The composition further comprises Ca: 0.0005 to 0.005% and Pb: 0.04 to 0.04.
The non-heat treated steel material for hot forging excellent in fatigue strength according to claim 1, 2 or 3, containing one or two of 30%. 5. The composition further comprises Te: 0.01 to 0.15% and Se: 0.01 to 0.15.
%, Bi: 0.04 to 0.3%, and one or more of them are contained. The non-heat treated steel material for hot forging excellent in fatigue strength according to claim 1, 2 or 3 or 4. 6. A weight ratio of C: 0.10 to 0.60%, Si: 0.005 to 2.00.
%, Mn: 0.55 to 2.00%, S: 0.01 to 0.1
0%, Al: 0.0005 to 0.05%, V: 0.30
% Over 0.70%, Ti: 0.003 to 0.050%,
In addition, N: less than 0.008%, P: 0.035%
Steel for which the balance is iron and inevitable impurities is limited to the following, forged at an austenitizing temperature of Ac ₃ points or more and 1250 ° C or less, and then 0.05 ° C to 300 ° C.
/ Sec or more and less than 2 ° C / sec. An average cooling rate for cooling, which is characterized by comprising a ferrite / pearlite structure and excellent in fatigue strength. 7. The method for producing a non-heat treated hot forged product excellent in fatigue strength according to claim 6, wherein the component further contains Nb: 0.005 to 0.10%. 8. The composition further comprises Cr: 0.10 to 1.50%, Mo: 0.05 to 1.00.
%, 1 or 2 types are contained, 6 or 7
A method for producing a non-heat treated hot forged product having excellent fatigue strength as described above. 9. The composition further comprises Ca: 0.0005 to 0.005%, Pb: 0.04 to 0.04.
The method for producing a non-heat treated hot forged product excellent in fatigue strength according to claim 6 or 7 or 8, containing one or two of 30%. 10. The composition further comprises Te: 0.01 to 0.15%, Se: 0.01 to 0.15.
%, Bi: 0.04 to 0.3%, and a method for producing a non-heat treated hot forged product excellent in fatigue strength according to claim 6 or 7 or 8 or 9.