JP3042574B2 - Hot forged product having high fatigue strength and method of manufacturing 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 a hot forged product having high fatigue strength and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, it has been required to reduce the fuel consumption of automobiles in order to protect the global environment. One of the effective methods for achieving low fuel consumption of automobiles is to reduce the weight of vehicles. In addition, miniaturization is aimed at by improving the yield strength and fatigue strength of steel materials. In order to increase the yield ratio and the fatigue limit ratio of steel materials, precipitation strengthening is the most effective method of strengthening by precipitating microalloys such as Ti once dissolved in γ into ferrite. It is. Conventionally, microalloys such as Ti have been added in order to increase the toughness of steel. For example, as disclosed in Japanese Patent Publication No.
T by suppressing the amount of C to a predetermined amount or less for grain refinement
It is characterized by increasing the amount of N in steel by dispersing in a nitrogen atmosphere and dispersing Ti carbonitride in steel, which aims at the precipitation of iN or as disclosed in Japanese Patent Publication No. 57-25606. However, there is a method for preventing coarsening of γ grains by TiN, and there is no method for strengthening precipitation by microalloy carbonitride. In addition, there is a method in which the upper limit of the amount of N in steel is specified as in JP-A-49-123923, but this is to prevent strain aging and the added N
The purpose of the present invention is to effect the refinement by precipitation of TiN.

[0003] In addition, in the case of parts for automobiles and parts for machine structures that require high strength and high toughness, off-line quenching and tempering are performed as a tempering treatment after hot forging into a predetermined shape. However, "Heat treatment of steel, 5th revised edition p189"
As can be seen, the fatigue limit ratio (fatigue limit / tensile strength) of the tempered steel is 0.35 to 0.5, and the only way to improve the fatigue strength is to lower the tempering temperature and increase the strength. . Since the machinability will decrease with the increase in strength,
From the viewpoint of machinability, it is not preferable to increase the strength more than necessary for improving the fatigue strength. In addition, at the austenitizing temperature of normal quenching, it is not possible to form a solid solution of a microalloy effective for precipitation strengthening, and when the austenitizing temperature is increased to form a solid solution of the microalloy,
This time, coarsening of γ grains occurs, which adversely affects the material properties of the steel, such as deterioration of toughness.

[0004]

SUMMARY OF THE INVENTION By increasing the amount of N in steel,
In the refinement of γ grains by forming N in steel, Ti
High yield ratio and high fatigue limit ratio cannot be expected because precipitation strengthening cannot be achieved by heat treatment. Cannot be improved, and weight reduction of parts is impossible. An object of the present invention is to provide a tensile strength of 90 to 1
30 kgf / mm ² , yield ratio 0.93 or more, fatigue limit ratio 0.
An object of the present invention is to provide a hot forged product having a high fatigue strength of 52 or more and a method for producing the hot forged product.

[0005]

Means for Solving the Problems In order to provide a method for improving the yield ratio and the fatigue limit ratio of a hot forged product, the present inventors have made intensive studies and obtained the following knowledge. (1) In order to obtain a yield ratio of 0.93 or more and a fatigue limit ratio of 0.52 or more, it is necessary to refine γ grains and to precipitate TiC at a high density in tempered martensite. For that purpose, the following four points are essential. Steel containing a specific amount of Ti. In order to form TiC having a large solubility product in austenite in steel and to form a solid solution during hot forging, N /
The N content is such that the value of Ti is less than 0.1. To be manufactured by a hot forging method that specifies the heating temperature, forging temperature, and quenching speed. 400 ° C. to A _c1 to precipitate solid solution TiC
Tempering in the temperature range of

[0006] The present invention has been made based on the above-described novel findings, and the gist thereof is as follows. (1) As a weight ratio, C: 0.10 to 0.60%, S
i: 0.15 to 2.00%, Mn: 0.55 to 2.00
%, S: 0.01-0.07%, Al: 0.010
0.050%, P: 0.035% or less, further contains Ti: 0.01 to 0.20%, and N / Ti
Is limited to an N content of less than 0.1, with the balance being composed of iron and unavoidable impurities, and a tensile strength of 90 to 130.
kgf / mm ² , yield ratio 0.93 or more, fatigue limit ratio 0.52
A hot forged product having high fatigue strength characterized by the above. (2) The hot forged product having high fatigue strength according to (1), wherein the component further contains B: 0.0003 to 0.005%. (3) The component further contains Cr: 0.10 to 3.00%, M
o: 0.05-1.50%, Ni: 0.10-3.60
%, One or more of (1),
(2) A hot forged product having the high fatigue strength described in (2). (4) As a weight ratio, C: 0.10 to 0.60%, S
i: 0.15 to 2.00%, Mn: 0.55 to 2.00
%, S: 0.01-0.07%, Al: 0.010
0.050%, P: 0.035% or less, further contains Ti: 0.01 to 0.20%, and N / Ti
Is limited to an N content of less than 0.1 and the remainder is hot forged steel of a component consisting of iron and unavoidable impurities, (a) heating to a heating temperature of 1050 to 1300 ° C; B) A reduction rate of 10 to 900 ° C to a heating temperature range
90% forging and immediately quenching at a cooling rate of 20 ° C./sec or more; (c) thereafter, 400 ° C. to A _c1
A tensile strength of 90 to 90, characterized in that hot forging, quenching and tempering are performed by a method comprising a step of performing tempering in a temperature range of a point.
A method for producing a hot forged product having a high fatigue strength of 130 kgf / mm ² , a yield ratio of 0.93 or more, and a fatigue limit ratio of 0.52 or more. (5) The method for producing a hot forged product having high fatigue strength according to (4), wherein the component further contains B: 0.0003 to 0.005%. (6) The component further contains Cr: 0.10 to 3.00%, M
o: 0.05-1.50%, Ni: 0.10-3.60
%, One or more of (5),
(6) A method for producing a hot forged product having a high fatigue strength according to (6).

Hereinafter, the present invention will be described in detail. First, C increases the strength of the final product as a mechanical part,
Although it is an element effective for improving the yield ratio and the durability limit ratio through precipitation as C, if it is less than 0.10%, the strength of the final product is insufficient, and if it exceeds 0.60%, the toughness of the final product is rather deteriorated. Therefore, the content was set to 0.10 to 0.60%.

Next, Si is added as a deoxidizing element and for the purpose of increasing the strength of the final product by solid solution hardening. If the content is less than 0.15%, these effects are insufficient. If the content exceeds 00%, these effects are not saturated but rather deteriorate the toughness of the final product. Therefore, the content is set to 0.15 to 2.00%. Mn is an element effective for increasing the strength of the final product through the improvement of hardenability, and forms S and MnS in steel, and contributes to improvement of machinability and refinement of structure. If the content is less than 55%, the effect is insufficient. On the other hand, if the content is more than 2.00%, the effect is not saturated but rather deteriorates the toughness of the final product. did.

Further, S exists as MnS in steel and contributes to improvement of machinability and refining of the structure.
%, The effect is insufficient. On the other hand, 0.07%
If it exceeds, the effect saturates, but rather causes degradation of toughness and an increase in anisotropy. For the above reasons, the content of S is set to 0.
01-0.07%. Next, Al is added as a deoxidizing element and a grain refining element, but if its content is less than 0.010%, its effect is insufficient, while if it exceeds 0.050%, its effect is saturated, and the toughness is rather increased. Therefore, the content was set to 0.010 to 0.050%.

[0010] On the other hand, P causes grain boundary segregation and central segregation in steel, which causes deterioration of toughness. In particular, when P exceeds 0.035%, the toughness deteriorates remarkably. Therefore, the P content is set to 0.035% or less. Further, Ti is contained as an essential element for the purpose of strengthening the precipitation of TiC. However, when the Ti content is 0.1.
If the content is less than 01%, the effect is insufficient. On the other hand, if the Ti content is more than 0.20%, the effect is saturated and the toughness is rather deteriorated.
%.

Further, N is an element that easily forms a nitride with Ti. When the value of N / Ti exceeds 0.1, Ti is consumed due to generation of TiN having low solubility in steel,
The required strength cannot be obtained because the amount of Ti associated with C decreases. Therefore, the value of N / Ti is set to less than 0.1. Further B
In the case of addition, in order to secure free B, N / T
It is preferable that the value of i is less than 0.075.

[0012] The present invention (2) is a forged product in which the hardenability of a large-sized machine component or the like which hardly hardens into the inside is improved. Therefore, B is added. However, if less than 0.0003%, improvement in hardenability cannot be expected, and if more than 0.005%, this effect saturates, not only increases the cost but also decreases the hardenability. So the amount added is 0.000
The content was 3 to 0.005%.

The present invention (3), like the present invention (2), improves the hardenability of large machine parts and the like which are hard to be hardened to the inside, and has a strength so that it can be used for important security parts. This is a forged product that improves both toughness and toughness. Therefore, Cr, Mo, and Ni are added. Cr is an element effective for increasing the strength of the final product by improving hardenability. If the content is less than 0.10%, the effect is not obtained. If the content is more than 3.00%, the hardness becomes too high, and the toughness is reduced. 0.000%. Mo is also an effective element for increasing the strength of the final product by improving the hardenability. However, if the content is less than 0.05%, there is no effect, and if the content exceeds 1.50%, the hardness is increased. , And its content is 0.05 to 1.50% because it is not preferable in terms of economy.
And Further, Ni is also an effective element for improving the strength and toughness of the final product. However, if the content is less than 0.10%, there is no effect, and if the content exceeds 3.60%, the improvement of strength and toughness is saturated, Further, the content is set to 0.10 to 3.60% because it is not preferable from the economical point.

[0014] The hot forging method is specified for the following reason. When heating to a temperature of 1050 to 1300 ° C is performed at a temperature lower than 1050 ° C, TiC in steel does not sufficiently form a solid solution, so that precipitation hardening during tempering has no effect.
This is because austenite grains are rapidly coarsened at a heating temperature exceeding ℃. The forging at a reduction rate of 10 to 90% in the recrystallization temperature range of 900 ° C. to the heating temperature is for refining the austenite grains to the 9th to 12th grains by recrystallization during heating, and the reduction rate is 10% or more. The reason is that if it is less than this, the effect of recrystallization refinement is small, and the upper limit is made 90% because the effect is saturated. The lower limit of the forging temperature was set to 900 ° C. because the recrystallization suppressing element T
This is because, since i is contained, austenite grains are not recrystallized and refined at a temperature lower than this. When this is immediately quenched at a cooling rate of 20 ° C./sec or more, the austenite which has been refined is transformed into martensite, and martensite having a narrow block width (effective crystal grain size) as a basic structure unit can be produced. Quenching at a cooling rate of 20 ° C./sec or more after forging is for causing martensitic transformation to austenite in a recrystallized and refined state, and bainite is mixed in at a cooling rate lower than this. As long as the cooling rate can be obtained at 20 ° C./sec or more, the quenching medium may be any one such as water and oil.

Next, tempering in a temperature range of 400 ° C. to A _c1 after quenching is performed not only by adjusting the balance between strength and toughness, but also by strengthening precipitation with solid solution TiC to eliminate free dislocations. It is to make it. If the tempering temperature is lower than 400 ° C., precipitation of TiC does not occur. If the tempering temperature is higher than A _c1 , the steel is transformed into austenite and the strength is rapidly lowered, so the tempering is performed in the range of 400 ° C. to A _c1 .

Hereinafter, the effects of the present invention will be described more specifically with reference to examples.

[0017]

EXAMPLES Comparative steels of the chemical compositions shown in Tables 1 and 2 and the steels of the present invention were melted in a high frequency furnace and forged into 150 kg ingots.

[0018]

[Table 1]

[0019]

[Table 2]

From this, a test piece was cut out, subjected to upsetting forging under the forging conditions of the comparative method and the method of the present invention shown in Table 3, and was quenched in water or oil.

[0021]

[Table 3]

JIS No. 14 tensile test pieces, JIS No. 3 impact test pieces, JIS No. 1 rotary bending test pieces, and drill hole test pieces were collected from the center of these materials, and the tensile strength, -50 ° C Charpy impact value, fatigue Strength and machinability were determined. For evaluation of machinability, _VL1000 was used. Drill with feed rate of 0.33 mm / s (Material: SKH51-φ
3mm) is changed variously, the total hole depth at which the drill cannot be cut at each speed is determined, a circumferential speed-drill life curve is created, and the maximum speed at which the drill life is 1000 mm is V.
_L1000 was defined as the evaluation standard for machinability. Tables 4 and 5 show the material properties of the comparative method and the method of the present invention.

[0023]

[Table 4]

[0024]

[Table 5]

In the comparative method, steel No. 1 deviating from the composition of the present invention was subjected to off-line quenching and tempering of forging No. I, and steel No. 3 also deviating from the composition of the present invention.
In the forged No. II, neither the yield ratio nor the fatigue limit ratio reached the lower limit of the present invention. In addition, the steel No. 4 was forged No. III, the reduction ratio did not reach the lower limit of the present invention, and the tempering temperature did not reach the lower limit of the present invention.
Forging No. IV, whose forging temperature was lower than the lower limit of the present invention, also did not reach the lower limit of the present invention in both the yield ratio and the fatigue limit ratio.

On the other hand, forging N with steel Nos. 6 to 33
o. In the present invention of V to IX, for example, steel No. 13, forged No. IX
Yield ratio 0.93, fatigue limit ratio 0.60 and steel No.1
8. Yield ratio of forged No. V 0.95, fatigue limit ratio 0.5
6. Further, the yield ratio of steel No. 32 and forged No. VII was 0.93,
Both the yield ratio and the fatigue limit ratio are high, such as the fatigue limit ratio of 0.60.

As described above, according to the method of the present invention, the tensile strength of 9
Yield strength of 84.1 to ¹ at 0.4 to 127.3 kgf / mm2
19.7 kgf / mm ² , yield ratio 0.93-0.96, fatigue limit 47.9-71.3 kgf / mm ² , fatigue limit ratio 0.52
~ 0.60, higher yield ratio than the comparative method,
It turns out that it has excellent durability.

[0028]

As described above, by using the method of the present invention, it is possible to manufacture a hot forged product having high fatigue strength, to reduce the weight of mechanical structural parts, and the industrial effect is extremely remarkable. There is something.

Continuation of the front page (72) Inventor Shigeru Yasuda 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation (56) References JP-A-5-195141 (JP, A) JP-A-4-141550 (JP, A) JP-A-3-87332 (JP, A) JP-A-1-42556 (JP, A) JP-A-5-302142 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) ) C22C 38/00 301 C21D 7/13 C21D 8/00 C22C 38/38

Claims (6)

(57) [Claims] 1. A weight ratio of C: 0.10 to 0.60%, Si: 0.15 to 2.00%, Mn: 0.55 to 2.00%, S: 0.01 to 0. 07%, Al: 0.010 to 0.050%, P: 0.035% or less, further contains Ti: 0.01 to 0.20%, and the value of N / Ti is N content is limited to less than 0.1, the balance is composed of iron and unavoidable impurities, has a structure in which high-density TiC is precipitated in tempered martensite having a small block width, and has a tensile strength of 90 to 130.
A hot forged product having high fatigue strength, having a kgf / mm ² , a yield ratio of 0.93 or more, and a fatigue limit ratio of 0.52 or more. 2. The composition further comprises B: 0.0003 to 0.0.
2. The hot forged product having a high fatigue strength according to claim 1, containing 0.05%. 3. The composition further comprises Cr: 0.10 to 3.00.
%, Mo: 0.05 to 1.50%, Ni: 0.10 to 0.1%
3. The hot forged product having high fatigue strength according to claim 1 or 2, which contains one or more of 3.60%. 4. The weight ratio of C: 0.10 to 0.60
%, Si: 0.15 to 2.00%, Mn: 0.55 to
2.00%, S: 0.01 to 0.07%, Al: 0.0
N content of 10 to 0.050%, P: 0.035% or less, further containing Ti: 0.01 to 0.20%, and N / Ti value less than 0.1 When hot forging steel having a balance of iron and unavoidable impurities, (a) a step of heating to a heating temperature of 1050 to 1300 ° C, and (b) a rolling step in a range of 900 ° C to a heating temperature. Forging at a rate of 10 to 90%, and immediately quenching at a cooling rate of 20 ° C./sec or more;
A hot forging quenching and tempering method is performed by a method comprising a step of performing tempering in a temperature range of from C ° C. to A _c1 point. A tensile strength of 90 to 130 kgf / mm ² , a yield ratio of 0.93 or more, and a fatigue limit ratio of 0 A method for producing a hot forged product having a high fatigue strength of at least 52. 5. The composition further comprises B: 0.0003 to 0.0.
The method for producing a hot forged product having a high fatigue strength according to claim 4, wherein the hot forged product contains 0.05%. 6. The composition further comprises Cr: 0.10 to 3.00.
%, Mo: 0.05 to 1.50%, Ni: 0.10 to 0.1%
The method for producing a hot forged product having a high fatigue strength according to claim 4 or 5, wherein one or more of 3.60% are contained.