JP3059318B2 - Manufacturing method of high fatigue strength hot forgings - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hot forged product having high fatigue strength.

[0002]

2. Description of the Related Art Conventionally, automotive parts and mechanical structure parts requiring high strength and high toughness have been subjected to off-line quenching and tempering as a tempering treatment after hot forging into a predetermined shape. Was. In recent years, there has been a demand for lower fuel consumption of automobiles in order to protect the global environment. One of the effective methods for achieving lower fuel consumption of automobiles is to reduce the weight of vehicles and to reduce the yield of steel materials. Miniaturization by improving strength and fatigue strength is aimed at.

However, conventionally, in order to increase the yield ratio of mechanical parts, it has been necessary to perform tempering treatment by quenching and tempering off-line. Also, "Heat treatment of steel, 5th revised edition p18
9 ”, the fatigue limit ratio of the tempered steel (fatigue strength /
The tensile strength 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 also decreases as the strength increases, it is not preferable from the viewpoint of machinability to increase the strength more than necessary to improve the fatigue strength.

[0004]

In the tempering treatment by off-line quenching and tempering after hot forging, it is impossible to improve fatigue strength without reducing machinability, and it is impossible to reduce the weight of parts It is. The purpose of the present invention is to obtain a tensile strength of 80
In ~130kgf / mm ^2, it has sufficient toughness and machinability, yield ratio 0.92 or more, is to provide a hot forged part and a manufacturing method having the above fatigue ratio 0.51.

[0005]

Means for Solving the Problems In order to provide a method for improving the yield strength and fatigue strength of steel for machine structural use which has high strength, high toughness and excellent machinability, the present inventors have conducted intensive studies and made the following studies. Was obtained. (1) In order to obtain a yield ratio of 0.92 or more and a fatigue limit ratio of 0.51 or more, the austenite grain size is sufficiently reduced, and the movement of high-density dislocations introduced by martensitic transformation is considered as a precipitate. Must be suppressed. For that purpose, the following five points are essential.

A steel containing a specific amount of a carbonitride forming element such as Nb, Ti, V, etc. Partial solid solution of the carbonitride forming element at a forging heating temperature of 1200 to 1350 ° C. Forging at a reduction rate of 10 to 90% in a recrystallization austenite temperature range of 950 to 1150 ° C. to reduce austenite grains. After that, quenching is performed immediately to transform the fine-grained austenite into martensite. Precipitation of carbonitrides by tempering in a temperature range of 400 ° C. to _one point of Ac in order to suppress the movement of dislocations with precipitates and to give the matrix a consistent strain.

(2) The effective crystal grains of martensite are refined by the above (1), and the same level of toughness as a conventional structural component can be obtained. In addition, in order to ensure sufficient hardenability for large parts and the like, a specific amount of B is added, and the N amount is suppressed to a specific value or less.

[0008] 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: 0.20 to 0.60%, Si: 0.15 to 2.00%, Mn: 0.55 to 2.00%, S: 0.01 to 0.10% , P: 0.035% or less, Al: 0.015 to 0.05%, N: 0.020% or less, and Nb: 0.02 to 0.20%, Ti: 0.01 to 0.01% A) When hot forging a steel containing one or more of 0.04%, V: 0.03 to 0.50%, and the balance being iron and unavoidable impurities, A) 1200 to 1350 ° C B) a reduction of 10 to 90 in a temperature range of 950 to 1150 ° C.
% Forging and immediately quenching at a cooling rate of 20 ° C./sec or more; and C) thereafter, tempering in a temperature range of 400 ° C. to _one point of Ac, characterized by a tensile strength of 80 to 130 kgf. / M
m ² , a method for producing a high fatigue strength hot forged product having sufficient toughness and machinability, a yield ratio of 0.92 or more, and a fatigue limit ratio of 0.51 or more.

The method for producing a high fatigue strength hot forged product according to the above (1), wherein the component (2) further contains B: 0.0003 to 0.005%.

The component (3) further comprises one or more of Cr: 0.10 to 1.50%, Mo: 0.05 to 1.00%, Ni: 0.10 to 3.60%. (1) or containing
(2) A method for producing a hot forged product having a high fatigue strength according to (2).

Hereinafter, the present invention will be described in detail. First, C
Is an element effective in increasing the strength of the final product as a mechanical part, but if it is less than 0.20%, 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 0.20 to 0.60%.

Next, Si is added as a deoxidizing element and for the purpose of increasing the strength of the final product due to solid solution hardening and tempering softening resistance. However, if it is less than 0.15%, these effects are insufficient. On the other hand, if the content exceeds 2.00%, 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 by improving the hardenability.
And MnS contribute to the improvement of machinability. However, if it is less than 0.55%, this effect is insufficient, while if it exceeds 2.00%, this effect is saturated, and the toughness of the final product is rather increased. Of 0.55 to 0.55%.
2.00%.

On the other hand, P causes grain boundary segregation and center segregation in steel, which causes deterioration of toughness. In particular, when P exceeds 0.035%, the deterioration of toughness becomes remarkable, so that the P content is set to 0.035% or less. S is present as MnS in steel and contributes to improvement of machinability and refining of microstructure.
If less, the effect is insufficient. On the other hand, when the content exceeds 0.10%, the effect is saturated, and rather, the toughness is deteriorated and the anisotropy is increased. For the above reasons, the content of S is set to 0.
01 to 0.10%.

Next, Al is added as a deoxidizing element and a grain refining element. If its content is less than 0.015%, its effect is insufficient, while if it exceeds 0.05%, its effect is saturated. However, since the toughness is rather deteriorated, the content is set to 0.015 to 0.05%.

Further, N contributes to refinement of the austenite structure, suppression of recrystallization, and strengthening of precipitation during tempering through the precipitation behavior of NbN, TiN, and VN. N content 0.02
If it exceeds 0%, coarse nitrides are formed and the toughness is reduced. Therefore, the upper limit of the N content is set to 0.020%. When B is added for ensuring quenching properties, the N content is preferably set to 0.1 to prevent the effect of B from disappearing due to BN formation.
006% or less is good.

Further, in the present invention, Ti, N is used for the purpose of adjusting austenite grain size, suppressing recrystallization, and strengthening precipitation.
One or more of b and V are contained as essential elements. However, if the Nb content is less than 0.02%, the Ti content is less than 0.01%, and the V content is less than 0.03%, the effect is insufficient, while the Nb content exceeds 0.2%. , T
When the i content is more than 0.04% and the V content is more than 0.50%, the effect is saturated and the toughness is rather deteriorated. Therefore, these contents are set to Nb: 0.02 to 0.20%, Ti: 0.01-0.0.
04%, V: 0.03 to 0.50%. In the case of adding B, Ti is added in the above amount as an essential element for fixing N.

[0018] Claim (2) is a forged product in which the hardenability of a large-sized machine component or the like which is hardly hardened into the inside is improved.
Therefore, B is added, but if it is less than 0.0003%, improvement in hardenability cannot be expected, and if it exceeds 0.005%, this effect is saturated and not only increases the cost but also decreases the hardenability. So that the amount added is 0.0003-
0.005%.

Claim (3) is similar to claim (2) in that the hardenability of a large machine part or the like which is hard to be hardened into the inside is improved.
It is a forged product with improved strength and toughness so that it can be used for important security parts. Therefore C
Add r, Mo, Ni. Cr is an element effective for increasing the strength of the final product by improving the hardenability. If the content is less than 0.10%, the effect is not obtained, and 1.55%
If it is excessively high, the hardness becomes too high, and the toughness is reduced. Further, the content is 0.10 to 1.0.
50%.

Mo is also an effective element for increasing the strength of the final product by improving the hardenability, but has no effect when the content is less than 0.05%, and when the content exceeds 1.00%, the hardness is low. The content is set to 0.05 to 1.00% because it causes an increase and is not preferable in terms of economy. 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%, the effect is not obtained. If the content exceeds 3.60%, the improvement of strength and toughness is saturated. In addition, its content is 0.10 to 3.6 because it is not economically preferable.
0%.

In addition, to improve machinability, the P content of up to 0.3% is increased.
b, Even if Ca is added up to 0.005%, the effect of the present invention is not impaired at all.

Next, the reason for limiting the hot forging conditions in the present invention will be described. First, set the heating temperature to 1200
When the heating temperature is lower than 1200 ° C., the carbonitrides of Nb, Ti and V do not sufficiently form a solid solution at a heating temperature lower than 1200 ° C., and thus have no effect on suppressing recrystallization or strengthening precipitation during tempering.
If the heating temperature is higher than 350 ° C., the austenite crystal grains rapidly become coarse.

Next, the forging is performed at a rolling reduction of 10 to 90% in the austenite recrystallization temperature range of 950 to 1150 ° C. in order to reduce the austenite grains to the 9th to 12th grains by recrystallization. , The rolling reduction of 10% or more
If less than this, the effect of recrystallization refinement is small.
The upper limit is set to 90% because the effect is saturated.

The lower limit of the forging temperature is set at 950 ° C., because Mo, Nb, Ti and V which are recrystallization suppressing elements are contained, and austenite is not recrystallized at lower temperatures. It is. The upper limit is set to 1150 ° C. because the recrystallized austenite becomes coarse at a temperature higher than this temperature.

If this is immediately quenched at a cooling rate of 20 ° C./sec or more, the refined austenite undergoes martensitic transformation to produce martensite having a narrow block width (effective crystal grain size) as a basic structural unit. Can be. The reason for quenching at a cooling rate of 20 ° C./sec or more after forging is to cause martensitic transformation in austenite in a recrystallized and refined state, and to mix bainite and the like at a cooling rate lower than this. . Cooling rate 20 ° C
If quenching solvent / second or more can be obtained, any quenching solvent such as water and oil may be used.

Next, after quenching, perform tempering in a temperature range of 400～Ac ₁ point, other adjustments strength and toughness balance, Nb by tempering at this temperature range, Ti, carbonitride V Is deposited to fix the high-density dislocations inherited from unrecrystallized austenite or to inhibit the movement. If the tempering temperature is lower than 400 ° C., Nb, Ti,
Can not be fixed dislocations since precipitation of carbonitrides of V does not occur, also, since the tempered martensite is ₁ point than Ac decreases sharply intensity will be austenitic transformation, tempering 400 ° C. to Ac ₁ Perform in the temperature range of the point. Hereinafter, the effects of the present invention will be more specifically described with reference to examples.

[0027]

EXAMPLES Steels having the chemical components shown in Tables 1 and 2 were melted in a high-frequency furnace and cast into 150 kg ingots. This is φ
After rolling to a 60 mm steel bar, a test piece for forging was cut out and subjected to forging and heat treatment under the conditions shown in Table 5. In the comparative example, forging and heat treatment were performed under the conditions shown in 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, the -50 ° C Charpy impact value, the fatigue strength, and the The sharpness was sought.

For evaluation of machinability, VL ₁₀₀₀ was used. Drill with a feed rate of 0.33 mm / s (Material: SKH51-φ
The peripheral speed of 10 mm) was changed variously drill obtains a total hole depth becomes impossible cutting at each speed, the peripheral speed - to create a drill life curve, the maximum speed at which the drill life is 1000mm defined as VL ₁₀₀₀ , And evaluation criteria for machinability.

[0029]

[Table 1]

[0030]

[Table 2]

[0031]

[Table 3]

[0032]

[Table 4]

[0033]

[Table 5]

[0034]

[Table 6]

[0035]

[Table 7]

Tables 4, 6 and 7 show the material properties of each steel material in comparison with the present invention and comparative examples. As shown in Table 4, in Comparative Examples, steel Nos.
IVIV did not reach the lower limit of the present invention in both the yield ratio and the fatigue limit ratio. The steel No. 6 of the present invention was used for forging No. II under the conditions outside the scope of the present invention. .III, No. 1 steel of the present invention
Forging No. IV with cooling rate less than the lower limit of the present invention in No. 2
In the present invention steel No. 17 forging No. III in the tempering temperature of the upper limit or more of the present invention also yield ratio,
Both the fatigue limit ratios did not reach the lower limit of the present invention.

In comparison with the steel Nos. Shown in Tables 6 and 7,
In the present invention of forging Nos. V to XIII according to Nos. 6 to 50, for example, the steel No. 11 of the first invention, the yield ratio of forging No.
Fatigue limit ratio 0.55, yield ratio of steel No.13, forged No.IX 0.92, fatigue limit ratio 0.59, steel No.22, forged No.IX
Yield ratio of 0.95, fatigue limit ratio of 0.56 and the steel of the second invention
No.27, Forging No.X yield ratio 0.96, fatigue limit ratio 0.5
5, the yield ratio of steel No. 42 and forged No. VII of the third invention is 0.9.
2. Both the yield ratio and the fatigue limit ratio are high, such as the fatigue limit ratio of 0.58.

As described above, according to the method of the present invention, the tensile strength of 8
Yield strength of 73.7 to ¹ at 0.4 to 126.5 kgf / mm ²
17.6 kgf / mm ² , yield ratio 0.92 to 0.96, fatigue strength 42.6 to 69.1 kgf / mm ² , fatigue limit ratio 0.5
2 to 0.60 can be obtained, which indicates that the sample has a higher yield ratio and better durability than the comparative example.

[0039]

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.

──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Shigeru Yasuda 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation (56) References JP-A-58-31064 (JP, A) JP-A-54-101709 (JP, A) JP-A-3-211227 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 8/00

Claims (3)

(57) [Claims] 1. The weight ratio of C: 0.20 to 0.60%, Si: 0.15 to 2.00%, Mn: 0.55 to 2.00%, S: 0.01 to 0.1%. 10%, P: 0.035% or less, Al: 0.015 to 0.05%, N: 0.020% or less, Nb: 0.02 to 0.20%, Ti: 0.2% When hot forging a steel containing one or more of V. 0.01 to 0.04% and V: 0.03 to 0.50%, with the balance being iron and unavoidable impurities. Heating to 1350 ° C., and B) a rolling reduction of 10 to 90 in a temperature range of 950 to 1150 ° C.
% Forging and immediately quenching at a cooling rate of 20 ° C./sec or more; and C) thereafter, tempering in a temperature range of 400 ° C. to _one point of Ac, characterized by a tensile strength of 80 to 130 kgf. / M
m ² , a method for producing a high fatigue strength hot forged product having sufficient toughness and machinability, a yield ratio of 0.92 or more, and a fatigue limit ratio of 0.51 or more. 2. The method for producing a high fatigue strength hot forged product according to claim 1, wherein the component further contains B: 0.0003 to 0.005%. 3. The composition further comprises one or more of Cr: 0.10 to 1.50%, Mo: 0.05 to 1.00%, Ni: 0.10 to 3.60%. Claim 1 or 2 containing
A method for producing a hot forged product having a high fatigue strength according to the description.