JPH04193931A - Hot forged product having superior fatigue strength - Google Patents

Abstract

PURPOSE:To obtain a hot forged product having superior fatigue strength in spite of the forged surface with a black skin by hot forging steel having a specified compsn. consisting of C, Si, Mn, Cr, Al, V and Fe under specified heating conditions, and then carrying out natural air cooling. CONSTITUTION:Steel consisting of, by weight, 0.15-0.50% C, <=0.30% Si, 0.50-1.20% Mn, <=0.50% Cr, 0.010-0.060% Al, 0.15-0.60% V and the balance Fe with impurity elements or further contg. one or more among 0.04-0.12% S, 0.05-0.30% Pb and 0.0005-0.01% Ca and satisfying 100XVX(C-Si/3)>5% is hot forged under heating condition sincluding heating at >=900 deg.C for <=10min, and then natural air cooling is carried out to obtain a hot forged product having superior fatigue strength without carrying out heat treatment such as hardening and tempering after hot forging. The fatigue strength is hardly reduced in spite of the forged surface with a black skin and the forged product is suitable for use as the perform of a connecting rod, etc., of an automotive engine.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention does not undergo heat treatment such as quenching and tempering after hot forging, and has excellent fatigue strength without heat treatment, and has a smooth surface. The present invention relates to a hot forged product which is characterized by extremely little reduction in fatigue strength even when used as a black forged skin, and which is particularly useful as a rough shaped material such as connecting rods for automobile engines.

(Prior art) Conventionally, connecting rods for automobile engines are made of carbon steel for mechanical structures such as 550C and 555C, and after being formed by hot forging, they are subjected to quenching and tempering to give them high strength and high toughness. Heat treatment (hereinafter referred to as refining) had been performed.

In addition, since heat treatment after hot forging requires a large amount of energy, in order to meet the social demand for energy conservation, we have developed non-temperature steel that can obtain the necessary performance through natural air cooling after hot forging. In recent years, the development of C has been actively carried out in the range of 0.2 to 0.
V of 0.03 to 0.20χ in medium carbon steel containing about 5χ
Non-temperature steel with addition of is proposed and used.

However, the fatigue strength of parts such as condoms, which are used with black forged surfaces, is almost determined by the surface properties. The surface in question is exposed to high temperatures during manufacturing, causing decarburization and scale formation, making it extremely difficult to take full advantage of the material's inherent properties. Therefore, the strength of the actual parts is usually significantly lower than the original strength of the material, making it impossible to increase the fatigue strength of the actual parts and making it difficult to increase the strength.

Therefore, when trying to increase the strength of a part that is used with a black forged surface, it is insufficient to simply increase the strength of the material used, and it is necessary to improve the surface properties. However, the relationship between the strength of actual parts and the materials used, forging conditions, etc. is not clear, and there is a strong desire to develop a manufacturing technology for hot forged products that have excellent strength even with black forged skin. was.

(Problem B to be Solved by the Invention) The present invention has been made in consideration of the above-mentioned problems in improving the fatigue strength of parts whose surfaces are used with black forged skin, and which are formed by hot forging. An object of the present invention is to provide a hot forged product having excellent fatigue strength even when used as a black skin forging.

(Means for Solving the Problems) With the above objective in mind, the present inventor has conducted intensive research on improving the surface properties of hot forged products, especially on improving the surface decarburized layer, and has made the following findings and has developed the present invention. Obtained. In other words, it was discovered that compared to tempered carbon steel, ferrite/pearlite type non-tempered steel with ■ addition had less ferrite decarburization and the hardness of the ferrite decarburized layer was higher. As a result of further investigation, we found that the amount of ferrite decarburization is inversely proportional to the amount of C and V.
When V is contained, the ferrite decarburized layer is precipitated and strengthened by V carbonitride, suppressing the decrease in hardness, so the hardness of the ferrite decarburized layer relative to the core hardness decreases compared to the case where V is not contained. I found that it becomes smaller. Based on the above knowledge, we investigated a composition that is difficult to decarburize ferrite and has a small decrease in the hardness of the decarburized layer of ferrite relative to the hardness of the core.As a result, we determined the composition of the steel to be used as 100 X V (χ) X(C(χ)−St(χ)
/3) It was found that it is effective to limit the value to >5.

Furthermore, as a result of studying the heating conditions during forging that would further increase the fatigue strength improvement effect when using this steel, we found that 9
The present invention was completed by confirming through experiments that the heating time above 00° C. should be 10 minutes or less.

That is, the first invention of the present invention has a weight ratio of C:0.
15-0.50%, Si: 0.30% or less, Mn: 0.
50-1.20%, Cr: 0.50% or less, Al: 0.
010 to 0.060%, V: 0.15 to 0.602, and 100xV(X)
A steel that satisfies X)/3)>5 and the remainder consists of Fe and impurity elements is used, and is characterized by performing natural air cooling after hot forging at a temperature of 900°C or higher for a heating time of 10 minutes or less. The second invention is a hot forged product with excellent fatigue strength, and in order to improve machinability compared to the first invention, S:
0.04-0.12%, Pb: 0.05-0.30%,
Ca: One or more of O, 0005 to 0.01χ is contained in the steel used.

Next, the reasons for limiting the composition and manufacturing conditions of the steel for the hot forged product with excellent fatigue strength of the present invention will be explained below.

C: 0.15 to 0.50χ C is an element necessary for ensuring strength and reducing the amount of ferrite decarburization, and must be contained in an amount of 0.15% or more. However, even if the content exceeds 0.50χ, the effect will be saturated and the toughness will decrease, so the upper limit was set at 0.50χ.

Si: 0.30% or less It is desirable to keep the Si content as low as possible in order to reduce the amount of ferrite decarburization, but since it is essential as a deoxidizing agent during steel manufacturing, the upper limit was set to 0.30χ.

Mn: 0.50 to 1.20χ Mn is an element added for deoxidation during steel manufacturing and for adjusting the strength/toughness balance of steel, and must be contained in an amount of 0.501 or more. However, if it is added in excess, the hardenability will improve too much and a haynite structure will be formed, and the precipitation strengthening of the ferrite decarburized layer by carbonitride will become insufficient, which will hinder the strengthening of the ferrite decarburized layer and the improvement of fatigue strength. Become. Therefore, in the present invention, the upper limit is set to 1.20χ in order to prevent the formation of a haynite structure during forging and cooling.

Cr: 0.50% or less Cr is also an effective element for adjusting the strength/toughness balance of steel, and is added as necessary, but the upper limit is set as excessive addition produces bainite like Mn. 0.50
It was set as χ.

AI: O, OIO ~ 0.060χ AI is an element with a strong deoxidizing effect, but 0.010
If the content is less than χ, the deoxidizing effect will not be observed, so the lower limit was set to 0.010χ. However, if the content exceeds 0.060χ, the above effect will be saturated and the machinability will decrease, so the upper limit was set at 0.060χ.

V: 0.15 to 0.60χ ν combines with C and N in the steel to form carbonitrides, which precipitate finely during cooling after forging, thereby strengthening the ferrite by precipitation. is an essential element, and is usually added at around 0.1χ. However, in the case of the present invention, other effects include that V combines with C and tramps, reducing the amount of ferrite decarburized, and that the generated ferrite decarburized layer is strongly precipitation-strengthened by carbonitrides of V. This is also the aim, and the minimum amount to obtain these effects is 0.15! was set as the lower limit. However, even if the content exceeds 0.60χ, the above effect is saturated and the cost increases, so the upper limit is set to 0.60χ.
And so.

S: 0.04-0.12%, Pb: 0.05-0.30
Z, Ca: 0.0005 to 0.01χ S, Pb, and Ca are elements effective in improving machinability, and are added in necessary amounts depending on the degree of cutting after hot forging. In order to obtain the above effect, each 0.04
%, 0.05%, and 0.0005%.

However, even if a large amount is contained, the effect will be saturated and the toughness will decrease, so the upper limits have been set at 0.12% and 0.12%, respectively.
．． 30% and 0.01χ.

100 X V (χ)X(C(χ)-Si(χ)/3
)>5100×V(X) X (C(Z)-3i(χ)
/3)>5 is an essential condition to minimize the formation of ferrite decarburization and the resulting decrease in hardness, and to obtain excellent fatigue strength. If the content of C1ν is low or Si is excessively contained, the 100 × V (χ) 100xv(z)x(C($)-Si($)/
3) It is necessary to set >5.

Heating time at 900°C or higher is 10 minutes or less The limited manufacturing condition that heating time at 900°C or higher is 10 minutes or less is an essential condition for maximizing the performance of hot forged products manufactured using the above-mentioned steel. It is.

If the heating time at 900° C. or higher exceeds 10 minutes, the surface quality deteriorates and the fatigue strength decreases significantly, so the heating time at 900° C. or higher was set to 10 minutes or less.

(Example) The features of the present invention will be clarified by examples below.

Table 1 shows the chemical components of the test materials used in the examples.

(Margin below) Table 1 (gl)-, -1(XlxV(X)x(C(X)-3i
1m/3)>5.0-Satisfied, HE is steel that falls under the second invention. In addition, I to ME are comparison steels that do not partially satisfy the conditions for steels used in the present invention,
The N steel is 555C, which is a conventionally used mesh.

The test material used as an example was a round bar with a diameter of 50 mm produced by hot rolling, heated to 1200 °C in a high frequency heating furnace under conditions such that the heating time at 900 °C or higher was about 5 minutes, and then heated to 1150 °C. It is forged at 1100℃ into the shape shown in Figure 1 and naturally air-cooled to room temperature.
Only the forged products manufactured using M were air-cooled, heated at 880°C, quenched in an oil bath, and tempered at 580°C.

Each of these test materials was machined into a finished stove and used as a test material, and subjected to a physical fatigue test using a tensile/compressive load method using an electro-hydraulic fatigue tester to determine the durability limit. In addition, the hardness at 0.1 mm from the surface and the center, the depth of the ferrite decarburized layer, and the microstructure were also observed. Based on the above measurement results, we investigated the effects of differences in the composition of the steel used on the durability limits of actual parts, as well as changes in hardness and decarburization level. Table 2 shows these results. The hardness was measured using a Pinkers hardness meter at a measurement load of 500 gf, and the depth of the ferrite decarburized layer and observation of the microstructure were performed using an optical microscope at a magnification of 400 times.

(Left below) Table 2 (G) ν. , 1-Hardness at 7 ko r of 0.1 m) As is clear from Table 2, Example No. 9 is a hot forged product manufactured using comparative steel and conventional steel. Comparing No. 1 to No. 14 with the hot forged products of the present invention, No. 9 has a low C content, so the depth of the ferrite decarburized layer is large, and the depth is 0.1 from the surface.
The hardness of mm is low (and the durability limit is inferior, and 1
No. 0 has a high Si content, so the durability limit is poor for the same reason as No. 9, and No. 11 has a high Cr content, so the hardenability is too improved and the ferrite pearlite structure is mixed with the heinite structure. The durability limit was poor because the ferrite decarburized layer was not sufficiently precipitation strengthened by V carbonitride, No. 12 had a low V content, and No. 13 had a chemical composition within the scope of the present invention, but 1) (see Table 1), the depth of the ferrite decarburized layer is large and the hardness near the surface is low, resulting in poor durability. In addition, 1 is a conventional hot forged product using 555C.
In No. 4, the surface quality was not improved even after thermal refining, but rather the depth of the ferrite decarburized layer increased due to the heating during thermal refining, resulting in poor durability.

On the other hand, Example No. 0, which is a hot forged product of the present invention.
1 to 8 added appropriate amounts of C and ■, and suppressed the amount of Si added.
00 X V (χ)X(C(%)-Si(χ)/3)
By using mackerel that satisfies >5 and performing forging under appropriate heating conditions, the amount of ferrite decarburization was reduced to 0.2 mm or less, and as a result, the decrease in hardness near the surface was minimized.
It shows excellent fatigue strength with a durability limit of 34 kgf/mm2 or more in a physical fatigue test.

Next, an example in which the influence of changes in heating conditions during forging was investigated will be described below. Among the forgings shown in Table 1, hot-rolled steel bars with a diameter of 50 mm of J steel, which is the target of the present invention, HI3 and comparative steel, and N steel, which is the conventional needle, were heated to 1200''C under various heating conditions. , Figure 1 at 1150-1100℃
A test material was produced by forging into the parts shown in , air cooling to room temperature, heating only NwA at 880°C, quenching in an oil bath, and subsequently tempering at 580°C.

Then, using the finished test material, a physical fatigue test, a hardness test, and observation of the decarburized layer and microstructure were conducted in the same manner as in the above example to evaluate its performance. Table 3 shows these results. The heating conditions are as follows: 5 to 30 hours of heating at 900°C or higher using a high frequency furnace and an electric furnace.
The effect of changing the heating time was evaluated.

(margins below C) As is clear from Table 3, steels subject to the present invention, H
Even if heat is used, if the heating time at 900° C. or higher exceeds a certain time, the depth of ferrite decarburization becomes large, the hardness decreases near the surface becomes large, and the durability limit becomes low. From this data, it can be seen that in order to fully bring out the effects of the present invention, it is necessary to use a high frequency furnace or the like and keep the heating time at 900''C or higher to 10 minutes or less.

(Effects of the Invention) The present invention has made it possible to clarify the relationship between the fatigue strength of actual parts and manufacturing conditions, which was previously unclear in the manufacturing of hot forged products. As a result, even when the surface is used with black forged skin, the strength of the actual part does not decrease significantly compared to the fatigue strength originally possessed by the material used, and the hot forged product has excellent fatigue strength. This makes it possible to provide the following information. It is particularly useful as a rough-shaped material for automobile engine burners, etc., and has extremely significant industrial contributions such as weight reduction and fuel efficiency.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the shape of a connecting rod produced as an example.

Claims (2)

[Claims] (1) C: 0.15-0.50%, Si: in weight ratio
0.30% or less, Mn: 0.50-1.20%, Cr:
0.50% or less, Al: 0.010-0.060%, V
: Contains 0.15 to 0.60%, and 100×V(%
) × (C (%) - Si (%) / 3) > 5, using steel with the balance consisting of Fe and impurity elements, and heated at 900 ° C.
A hot forged product with excellent fatigue strength characterized by performing natural air cooling after hot forging with heating time of 10 minutes or less. (2) C: 0.15-0.50%, Si: in terms of weight ratio
0.30% or less, Mn: 0.50-1.20%, Cr:
0.50% or less, Al: 0.010-0.060%, V
: Contains 0.15 to 0.60%, and further S: 0.04
~0.12%, Pb: 0.05~0.30%, Ca: 0
．． 0005 to 0.01%, and 100 x V (%) x (C (%) - Si (%)
/3) Fatigue strength characterized by using a steel that satisfies >5, the remainder consisting of Fe and impurity elements, and performing natural air cooling after hot forging at 900°C or higher for 10 minutes or less. Excellent hot forged products.