JPH09194999A - Ferrite-pearlite-type non-heat treated steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a non-heat treated steel having >=500MPa tensile strength, >=250MPa fatigue strength, >=0.50 fatigue limit value, and excellent machinability and suitable for use as a stock for engine parts for compact automobile, etc., particularly crankshaft and connecting rod. SOLUTION: This steel has a composition consisting of, by weight, 0.20-0.45% C, 0.05-1.00% Si, 0.20-0.60% Mn, 0.01-0.08% S, 0.02-0.50% V, 0.004-0.03% N, 0-0.05% P, 0-0.30% Cu, 0-0.30% Ni, 0-1.00% Cr, 0-0.30% Mo, 0-0.05% Ti, 0-0.050% Al, 0-0.30% Pb, 0-0.0100% Ca, 0-0.100% Bi, 0-0.10% Te, and the balance Fe with inevitable impurities and satisfying fn1=C+(Si/10)+(Mn/5)+(5Cr/22)+1.65 V-(58/7)>=0.50 and [C+(Mn/5)-5N]/fn1<=0.80, where the symbol of the element in the inequalities means the content of the element by weight percent.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a non-heat treated steel which has excellent characteristics without being subjected to so-called “tempering treatment” of quenching and tempering after hot working, and which is suitable for applications such as machine structural members. Regarding More specifically, it has a tensile strength of 500 MPa or more and a fatigue strength of 250 MPa or more, which are suitable as materials for engine parts of automobiles, industrial machines, construction machines, etc., especially crankshafts and connecting rods, and have a fatigue limit ratio of 0.50. The above relates to a ferrite / pearlite type non-heat treated steel.

[0002]

2. Description of the Related Art Crankshafts and connecting rods, which are engine parts for automobiles, industrial machines, construction machines, etc., and steering knuckles and spindles, which are undercarriage parts, have been conventionally used as carbon for machine structures. Steel (S45C, S50C, etc.) and alloy steel (SCM4
No. 40, etc.) was used for hot forming, followed by machining and refining to ensure the desired shape and performance.

However, since a large amount of heat energy is required to carry out the above-mentioned tempering treatment, the manufacturing cost increases. Therefore, from the viewpoint of energy saving and cost reduction, non-heat treated steel has been developed which has the same characteristics as the heat treated steel in the hot working state.

As non-heat treated steels, bainite type, martensite type and ferrite / pearlite type non-heat treated steels are known. Of these, bainite-type and martensite-type non-heat treated steels provide high strength but low machinability. Therefore, there is a difficulty in finish forming by machining,
In addition, there is a problem that "bending" becomes large because a large transformation strain is generated, and a correction process for removing the bending is required, which leads to an increase in cost. For example, Japanese Patent Laid-Open No. 4-1
In the bainite type “non-heat treated steel for hot forging” proposed in Japanese Patent No. 41550 and Japanese Unexamined Patent Publication No. 4-176842, problems still remain in terms of machinability and bending. .

On the other hand, as an alternative to the refining treatment in which a steel material cooled after hot working is reheated to the austenite temperature range, quenched, and then tempered, there is disclosed in Japanese Patent Laid-Open No. 6-21.
Japanese Patent No. 2347 discloses "a hot forged product having high fatigue strength and a manufacturing method thereof" in which a steel having a specific chemical composition is quenched immediately after hot forging, and then a tempering treatment is performed to precipitate TiC. . However, since the hot forged product described in this publication is hardened immediately after hot forging to form a martensite structure, it is necessary to control quenching cracks at the time of quenching, and tempering is required to precipitate solid solution TiC. Therefore, there is also a problem that the energy cost is increased.

Japanese Unexamined Patent Publication (Kokai) No. 5-125439 discloses that steel having a specific chemical composition is hot forged and then cooled to room temperature.
A "method for producing a non-heat treated steel part having a high yield ratio" in which it is reheated to a high temperature (350 ° C to Ac ₁ point) is disclosed. However, the method proposed in this publication is also "quenching" for tempering.
Although it can be omitted, it is still insufficient in that it is reheated from room temperature to the above temperature after hot forging.

For this reason, mechanical structural parts which do not require so much strength, particularly engine parts which require only a tensile strength of about 500 to 1000 MPa, such as engine parts for small automobiles, industrial machines and construction machines. For some crankshafts and connecting rods, ferrite-pearlite type non-heat treated steel with excellent machinability is required.

However, for example, Japanese Patent Laid-Open No. 62-16785.
In the conventional ferrite / pearlite type non-heat treated steel as disclosed in Japanese Patent Publication No. 5, although machinability is improved as compared with bainite type and martensite type non-heat treated steel, its structure is coarse. Since it consists of a ferrite-pearlite structure, it was not enough in terms of fatigue resistance required for engine parts. That is, in recent years, fatigue resistance has become more important in view of machinability as well as engine design, and the fatigue strength is 250 MPa or more and the fatigue limit ratio (fatigue strength / tensile strength) of 0.5 or more is stable. Is required to be secured. However, the non-heat treated steel described in the above publication cannot meet the demands of this industry.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has a tensile strength of 500 MPa or more, which is suitable as a material for engine parts of automobiles, industrial machines, construction machines, etc., especially crankshafts and connecting rods. It is an object of the present invention to provide a ferrite / pearlite type non-heat treated steel having a fatigue strength of 250 MPa or more and a fatigue limit ratio of 0.5 or more, which is excellent in machinability.

[0010]

As a result of various studies to solve the above problems, the present inventor first obtained the following findings.

It is known that C and Mn increase the static tensile strength, but excessive addition thereof lowers the fatigue limit ratio (fatigue strength / tensile strength) of the ferrite-pearlite non-heat treated steel. Therefore, in order to increase the fatigue strength and secure a high fatigue limit ratio of 0.5 or more in the ferrite-pearlite non-heat treated steel, it is necessary to strictly control the contents of C and Mn.

N has the effect of improving the fatigue limit ratio in addition to increasing the tensile strength of the ferrite-pearlite non-heat treated steel.

Then, as a result of a more detailed examination of the ferrite-pearlite structure based on the above findings and, the following findings were obtained.

The chemical composition of steel is C: 0.20 in% by weight.
%, Si: 0.05% or more, Mn: 0.20% or more, V: 0.02% or more and S: 0.01% or more, the tensile strength and fatigue strength are as follows. Can be organized with fn1.

Fn1 = C + (Si / 10) + (Mn /
5) + 1.65V- (5S / 7) + (5Cr / 22), wherein the element symbol in the above formula represents the content of the element in% by weight.

The chemical composition of steel is C: 0.45% or less, M
When n: 0.60% or less, N: 0.004% or more, and the following fn2 value is 0.8 or less, the fatigue limit ratio of 0.5 to 0.6 is stable in the ferrite / pearlite structure. Is obtained.

Fn2 = {C + (Mn / 5) -5N} / fn1, where the element symbol in fn2 also represents the content of the element in% by weight.

In the case of a ferrite / pearlite structure, if the tensile strength is controlled to about 1000 MPa or less and the content of S is optimized, sufficient machinability is achieved without adding any free-cutting element such as Pb or Ca. Is obtained.

The present invention based on the above findings is summarized as the following ferrite / pearlite type non-heat treated steel.

"% By weight, C: 0.20 to 0.45%,
Si: 0.05 to 1.00%, Mn: 0.20 to 0.6
0%, S: 0.01 to 0.08%, V: 0.02 to 0.
50%, N: 0.004 to 0.03%, P: 0 to 0.0
5%, Cu: 0 to 0.30%, Ni: 0 to 0.30%,
Cr: 0 to 1.00%, Mo: 0 to 0.30%, Ti:
0-0.05, Al: 0-0.050%, Pb: 0
0.30%, Ca: 0-0.0100%, Bi: 0-
0.100%, Te: 0 to 0.10%, the balance consisting of Fe and unavoidable impurities, and fn1 ≧
Ferrite / pearlite type non-heat treated steel characterized in that 0.50 and fn2 ≦ 0.80. "

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the chemical composition of steel in the present invention as described above will be explained below. In addition, "%" means "weight%."

C: C is an element necessary for imparting a desired static strength to steel, but it is also an element that lowers the fatigue limit ratio when it exceeds a certain amount. A content of 0.20% or more is necessary to obtain the minimum static strength (500 MPa in tensile strength). On the other hand, when the content exceeds 0.45%, the fatigue limit ratio is remarkably lowered, and it becomes difficult to obtain a desired value of 0.5 or more, and in addition, machinability is deteriorated. Therefore, the content of C is set to 0.20 to 0.45%.

Si: Si has the functions of promoting deoxidation, solid-solving in ferrite to strengthen the ferrite, and increasing static strength and fatigue strength. However, when its content is 0.1.
If less than 05%, the desired effect cannot be obtained, while on the other hand, 1.00
If it is contained in excess of%, the machinability will be deteriorated, so the content was made 0.05 to 1.00%.

Mn: Mn has a deoxidizing effect and an effect of enhancing fatigue strength. However, if the content is less than 0.20%, the effect of addition is poor. On the other hand, when the content exceeds 0.60%, the fatigue limit ratio decreases, and it becomes difficult to secure a desired value of 0.5 or more. Therefore, the content of Mn is 0.20 to 0.
It was set to 60%.

S: S has a function of improving machinability. In order to bring out the effect sufficiently, it is necessary to add 0.01% or more. On the other hand, if the content exceeds 0.08%, the fatigue limit ratio decreases. Therefore, the S content is 0.0
It was set to 1 to 0.08%.

V: V has a function of increasing static strength and fatigue strength. However, if the content is less than 0.02%, the effect of addition is poor, and if the content exceeds 0.50%, the effect is saturated, and only the cost rises and the economic efficiency is impaired. The content was 0.02 to 0.50%.

N: N is an extremely effective element for increasing the fatigue limit ratio of non-heat treated steel. In order to fully exert this effect, it is necessary to contain N in an amount of 0.004% or more.

On the other hand, if the content exceeds 0.03%, not only the effect is saturated but also the hot workability is deteriorated. Therefore, the N content is 0.004 to 0.03%.
And In addition, the preferable range of N content is 0.008-
It is 0.022%.

P: P may not be contained. If contained, it has the effect of increasing fatigue strength. In order to ensure this effect, it is preferable that the content of P be 0.005% or more.

However, if the content exceeds 0.05%, the toughness is significantly deteriorated, so the content is set to 0 to 0.05%.

Cu: Cu may not be added. If added, it has the effect of improving hardenability. In order to reliably obtain this effect, the content of Cu is preferably 0.01% or more. However, if the content exceeds 0.30%, the hot workability deteriorates. Therefore, Cu
Content of 0 to 0.30%.

Ni: Ni may not be added. If added, it has the effects of improving hardenability and enhancing toughness. To ensure this effect, Ni is 0.01%.
It is preferable to set the content as described above. However, if the content exceeds 0.30%, the machinability deteriorates, which is also disadvantageous in terms of economy. Therefore, the Ni content is set to 0 to 0.30%.

Cr: Cr may not be added. If added, it has the effects of improving hardenability and static strength and fatigue strength. In order to surely obtain these effects, it is desirable that the content of Cr be 0.02% or more. However, if the content exceeds 1.00%, bainite is likely to be formed, the tensile strength exceeds 1000 MPa, and machinability is greatly deteriorated. Therefore, the content of Cr is set to 0 to 1.00%.

Mo: Mo may not be added. If added, it has the effect of enhancing hardenability and toughness. In order to surely obtain this effect, the Mo content is preferably 0.01% or more. However, its content is 0.30%
If it exceeds, bainite is likely to be formed, resulting in deterioration of machinability, which is also disadvantageous in terms of economy. Therefore, the content of Mo is set to 0 to 0.30%.

Ti: Ti may not be added. If added, it has the effect of refining the crystal grains and increasing strength and toughness. In order to reliably obtain this effect, the Ti content is preferably 0.001% or more. However, if the content exceeds 0.05%, the Ti carbonitrides are aggregated and coarsened, making it difficult to obtain the above-mentioned effects, which is also disadvantageous in terms of economy. Therefore, the content of Ti is set to 0 to 0.05%.

Al: Al may not be added. Addition has the effect of stabilizing the deoxidation of the steel and homogenizing it. In order to reliably obtain this effect, the content of Al is preferably 0.001% or more. However, when its content is 0.1.
If it exceeds 050%, oxide-based inclusions increase and the tool life is shortened during cutting. Therefore, the Al content is 0 to
0.050%. When Pb, Ca, Bi, and Te are added to steel to improve machinability, it is desirable to control the upper limit of the Al content to 0.010%.

Pb: Pb may not be added. If added, it has the effect of enhancing machinability. In order to reliably obtain this effect, the Pb content is preferably 0.01% or more. However, if the content exceeds 0.30%, the fatigue resistance is deteriorated. Therefore, the Pb content is set to 0 to 0.30%.

Ca: Ca may not be added. If added, it has the effect of enhancing machinability. In order to reliably obtain this effect, the content of Ca is preferably 0.0003% or more. However, even if the content exceeds 0.0100%, the effect is saturated and the economical efficiency is impaired. Therefore, the content of Ca is set to 0 to 0.0100%.

Bi: Bi may not be added. If added, it has the effect of enhancing machinability. In order to reliably obtain this effect, the Bi content is preferably 0.005% or more.

However, even if the content exceeds 0.100%, the effect is saturated and the economy is impaired. Therefore, the Bi content is set to 0 to 0.100%.

Te: Te need not be added. If added, it has the effect of enhancing machinability. In order to reliably obtain this effect, the content of Te is preferably 0.01% or more. However, even if the content exceeds 0.10%, the effect is saturated and the economy is impaired. Therefore, Te
Content was 0 to 0.10%.

Fn1: The chemical composition of steel is wt%, and C:
0.20% or more, Si: 0.05% or more, Mn: 0.2
In the ferrite-pearlite structure having 0% or more, V: 0.02% or more and S: 0.01% or more, the tensile strength and the fatigue strength can be arranged by the above-mentioned fn1. When this value is 0.50 or more, the tensile strength of 500 MPa or more and the fatigue strength of 250 MPa or more can be secured. fn1
When becomes too large, bainite is easily generated,
Since the machinability will be deteriorated, fn1 is 1.20.
It is desirable to set the degree to the upper limit.

Fn2: The chemical composition of steel is C: 0.45% or less, Mn: 0.60% or less, N: 0.004% or more, and fn2 is 0.80 or less. The fatigue limit ratio of 0.5 to 0.6 can be stably obtained. It should be noted that if fn2 is made small, the cost will increase, so it is preferable to set fn2 to a lower limit of about 0.2.

The steel having the above-mentioned chemical composition is melted by a usual method, then is subjected to hot working, for example, by a usual method, and is subjected to a mechanical working or a surface hardening treatment as required to obtain a required steel. Finished on crankshafts and connecting rods.

[0045]

EXAMPLE Steels having the chemical compositions shown in Tables 1 to 3 were vacuum-melted in 200 kg in a test furnace by a usual method. Table 1,
Steels 1 to 15 in No. 2 are steels of the present invention, and steels 16 to 26 in Tables 2 and 3 are comparative steels in which any of the components is out of the range specified by the present invention.

Next, these steels were made into billets by a usual method, heated to 1250 ° C., and then 1200
Hot forging into a round bar with a diameter of 20 mm at a temperature of ~ 950 ° C,
Then, it was cooled to room temperature by air.

From the round bar thus obtained, the diameter of the parallel portion is 8 m.
An Ono-type rotary bending fatigue test piece of m was cut out and a fatigue test was performed at room temperature and in the atmosphere at 3000 rpm. or,
A JIS No. 4 tensile test piece was cut out and a tensile test was performed at room temperature.

For reference, the above-mentioned 20 mm of steel 25 is used.
Heat the round bar at 845 ° C for 1 hour and oil quench it to 600 ° C.
After tempering for 1 hour, the Ono-type rotating bending fatigue test piece and the JIS No. 4 tensile test piece with a parallel part diameter of 8 mm were cut out,
A fatigue test and a tensile test were performed under the above conditions.

The test results are shown in Table 4.

For each of the steels 1 to 15 of the present invention, the desired tensile strength of 500 MPa or more, fatigue strength of 250 MPa or more, and fatigue limit ratio of 0.5 or more were obtained at 1000 MPa or less.

On the other hand, among the comparative steels in which any of the components deviates from the content range specified in the present invention, the C content,
Steels 16 and 18 with low Si content and fn1
Nos. 22 and 22 do not reach the tensile strength of 500 MPa.

Further, the fatigue limit ratios of the steels 17, 19 and 20 in which the amounts of C, Mn, and S deviated to a high degree and the steel 26 in which the amount of N deviated relatively low did not reach 0.50.

Further, steels 23 and 24 with fn2 deviated to a higher degree
Nos. 25 and 25 have a low fatigue limit ratio of 0.37 to 0.48, and the fatigue strength of Steel 24 is 250MP.
a has not been reached.

The steel 21 in which C, Mn and fn2 are deviated a little and V is deviated a little, the fatigue strength does not reach 250 MPa and the fatigue limit ratio is as low as 0.36.

In the case of the conventional type in which the tempering treatment of quenching and tempering is applied to the steel 25, the tensile strength, fatigue strength and fatigue limit ratio all reach the target values. However, in this case, it has a tempered martensite structure, and the machinability is inferior to that of the ferrite-pearlite type non-heat treated steel,
Further, after the quenching, some bending occurred.

[0056]

[Table 1]

[0057]

[Table 2]

[0058]

[Table 3]

[0059]

[Table 4]

[0060]

When the ferrite / pearlite type non-heat treated steel of the present invention is used, the tensile strength of 500 MPa or more, 250
Fatigue strength of MPa or more, fatigue limit ratio of 0.50 or more can be easily obtained, and tensile strength can be controlled to 1000 MPa or less, so that machinability is good, engine parts of automobiles, industrial machines, construction machines, etc. Above all, it can be used as a material for crankshafts and connecting rods, and has a great industrial effect.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Harinori Kakimi 5-1-1109 Shimaya, Konohana-ku, Osaka City, Osaka Prefecture Sumitomo Metal Industries, Ltd. Kansai Works Steel Works (72) Inventor Masayuki Mino Osaka, Osaka Prefecture Sumitomo Metal Industries, Ltd., 4-53-3 Kitahama, Chuo-ku, Yokohama (72) Inventor Masato Kurita, 4-53-3 Kitahama, Chuo-ku, Osaka-shi, Osaka Prefecture Sumitomo Metal Industries, Ltd.

Claims (1)

[Claims] 1. C: 0.20 to 0.45% by weight, S
i: 0.05 to 1.00%, Mn: 0.20 to 0.60
%, S: 0.01 to 0.08%, V: 0.02 to 0.5
0%, N: 0.004 to 0.03%, P: 0 to 0.05
%, Cu: 0 to 0.30%, Ni: 0 to 0.30%, C
r: 0 to 1.00%, Mo: 0 to 0.30%, Ti: 0
.About.0.05, Al: 0 to 0.050%, Pb: 0 to 0.
30%, Ca: 0 to 0.0100%, Bi: 0 to 0.1
00%, Te: 0 to 0.10%, the balance consisting of Fe and unavoidable impurities, and fn1 ≧ 0.50 and f
Ferrite / pearlite type non-heat treated steel characterized by n2 ≦ 0.80. However, fn1 = C + (Si / 10) + (Mn / 5) +1.65
V- (5S / 7) + (5Cr / 22) fn2 = {C + (Mn / 5) -5N} / fn1, the element symbol in the formula represents the content of the element in% by weight.