JP3534146B2 - Non-heat treated steel excellent in fatigue resistance and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a non-heat treated steel material excellent in fatigue resistance and a method for producing the same. More specifically, it relates to a non-heat treated steel material suitable as a material for machine structural parts and a method for producing the same, which has an excellent fatigue strength-machinability balance without being subjected to a tempering treatment after hot working. is there.

[0002]

2. Description of the Related Art Conventionally, mechanical structural parts made of steel, which require high fatigue strength, are roughly hot-worked into a predetermined shape and then cut into a final desired shape.
It was general to perform tempering treatment such as quenching and tempering. However, this conditioning process consumes a lot of energy and cost. Therefore, in recent years, non-heat treated steel that can be used as hot-worked has been actively developed in order to meet the social demand for energy saving and at the same time reduce cost.

For example, Japanese Unexamined Patent Publication (Kokai) No. 4-141550 discloses "non-heat treated steel for hot forging" having a fine bainite structure by adding Cr, Mo, Nb and Ti together. However, since this non-heat treated steel is a bainite type, it has inferior machinability compared to the conventional ferrite / pearlite type non-heat treated steel, and further has a problem that a large transformation strain causes a large bending. Was there. Therefore, a correction process for removing the bend is required, which leads to an increase in cost.

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.

Regarding non-heat treated steel, there is an increasing demand for free-cutting steel excellent in machinability for the purpose of facilitating cutting after hot working.

Generally, the machinability of steel largely depends on the metal structure, and as mentioned in the section of the technique described in Japanese Patent Laid-Open No. 4-141550, the machinability of steel has a ferrite-pearlite structure. It is good, and the machinability is poor in the steel having a ferrite / bainite structure or a single-phase structure of bainite or martensite. Also, Pb, Te, Bi, C
It is well known that the machinability is improved by adding free-cutting elements such as a and S alone or in combination. Therefore, conventionally, a method of adding the above-mentioned free-cutting element to non-heat treated steel to improve the machinability after hot working has been adopted. However, if the free-cutting element is simply added to the non-heat treated steel, the desired high fatigue strength cannot be secured in many cases.

Under such circumstances, for example, Japanese Patent Laid-Open No. 2-1
In Japanese Patent No. 11842 and Japanese Patent Laid-Open No. 6-279849, C in steel is present as graphite, and the notch and lubrication effect of this graphite are used to improve the machinability. A "hot-rolled steel having excellent workability" and a "method for manufacturing steel for machine structural use having excellent machinability" have been proposed.

However, the steel material proposed in Japanese Patent Laid-Open No. 2-111842 is one in which B is added to promote graphitization by using B nitride (BN) as a graphitization nucleus, and addition of B is essential. Therefore, there is a problem that cracking is likely to occur during solidification. On the other hand, the method described in Japanese Patent Application Laid-Open No. 6-279849 promotes graphitization as it is in hot rolling by restricting O (oxygen) in steel together with addition of Al. It is not always economical because it needs to be annealed.
Further, since both of the proposals in the above-mentioned two publications utilize graphitization, in order to impart desired mechanical properties to mechanical structural parts etc. processed into a predetermined shape, quenching and tempering must be performed. It has to undergo quality treatment,
It has not been possible to meet the demands of the industrial world to achieve both "non-heat treatment" and "improvement of machinability of high strength steel".

Iron and Steel (vol. 57 (1971) S4
84) has reported that machinability may be enhanced by adding Ti to the deoxidized controlled free-cutting steel. But T
It is also stated that the addition of a large amount of i increases tool wear due to the large amount of TiN produced, which is not preferable in terms of machinability. For example, C: 0.45%, S
i: 0.29%, Mn: 0.78%, P: 0.017
%, S: 0.041%, Al: 0.006%, N: 0.
Steel containing 0087%, Ti: 0.228%, O: 0.004% and Ca: 0.001% conversely has a shortened drill life and poor machinability. As described above, the machinability is not improved simply by adding Ti to steel.

[0010]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned current situation, and under the conditions of normal hot working and cooling,
It is also a material for machine structural parts, etc. that has excellent fatigue resistance as well as machinability without heat treatment including tempering and excellent machinability, that is, excellent fatigue strength-machinability balance. An object of the present invention is to provide a low-cost steel material suitable for use and a method for manufacturing the same.

[0011]

The gist of the present invention is to provide a non-heat treated steel material having excellent machinability and fatigue resistance characteristics as shown in (1) below and excellent machinability and fatigue resistance characteristics as shown in (2) below. There is a method for manufacturing non-heat treated steel.

(1) C: 0.2 to 0.6% by weight,
Si: 0.05-1.5%, Mn: 0.1-2.0%,
P: 0.01 to 0.07%, S: 0.01 to 0.20
%, Ti: more than 0.25% to 1.0%, Al: 0.
002-0.05%, N: 0.008% or less, Cr: 0
~ 2.0%, V: 0 ~ 0.3%, Nb: 0 ~ 0.05
%, Mo: 0 to 0.5%, Cu: 0 to 1.0%, Nd:
0 to 0.1%, Pb: 0 to 0.50%, Ca: 0 to 0.
01%, Se: 0 to 0.5%, Te: 0 to 0.05%,
Bi: 0 to 0.4%, the balance being composition of Fe and unavoidable impurities, and the structure is a ferrite-pearlite structure composed of ferrite having a JIS grain size number of 6 or more and pearlite having an average lamellar spacing of 0.2 μm or less. Non-heat treated steel with excellent machinability and fatigue resistance.

(2) The steel having the chemical composition described in (1) above is heated to a temperature in the temperature range of 1050-1300 ° C., hot-worked in a temperature range of 900 ° C. or higher, and then 5
JIS particle size number 6
A method for producing a non-heat treated steel material having excellent machinability and fatigue resistance, characterized by having a ferrite-pearlite structure comprising the above ferrite and pearlite having an average lamella spacing of 0.2 μm or less.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION As a result of repeated studies on the chemical composition and structure of a non-heat treated steel in order to achieve the above-mentioned objects, the present inventors have obtained the following findings.

In the case of a ferrite / pearlite structure, the fatigue strength has a positive and good correlation with the yield strength (YS). Therefore, in order to increase the fatigue strength, YS should be improved. Since increasing the tensile strength (TS) is not preferable from the standpoint of machinability, the T of ferrite-pearlite structure is
Machinability and high fatigue strength (fatigue limit (σw)) can be achieved by increasing YS without increasing S.

Finer ferrite grains and finer pearlite lamella spacing are effective in increasing YS without significantly increasing TS in the ferrite-pearlite structure.

If proper amount of Ti is added to the steel material in which the amount of N is regulated to optimize the cooling condition after hot working, YS can be increased without increasing TS of ferrite / pearlite structure so much and fatigue. The strength is dramatically improved. This is because (a) fine TiC precipitates during cooling and strengthens the ferrite, and (b) the growth of austenite grains is suppressed by TiC that is present in an undissolved state during heating during hot working. This is because a fine structure is obtained and the structure is strengthened (finely strengthened) by miniaturization.

When Ti is positively added to steel under appropriate conditions, Ti carbosulfide is formed in the steel.

When the above-mentioned Ti carbosulfide is formed in the steel, the amount of MnS produced is reduced.

When the S content in steel is the same, Ti carbosulfide has a greater machinability improving effect than MnS. This is because the melting point of Ti carbosulfide is lower than that of MnS, so that the lubricating action on the rake face of the tool during cutting is increased.

In order to sufficiently exert the effect of Ti carbosulfide, it is important to limit the N content to a low level.
This is because when the N content is high, Ti is fixed as TiN and the generation of Ti carbosulfide is suppressed.

Ti carbosulfide produced during steelmaking does not form a solid solution in the matrix at the heating temperature for ordinary hot working.

The present invention has been completed based on the above findings.

The requirements of the present invention will be described in detail below. In addition, "%" of a component content means "weight%."

(A) Chemical composition C of steel: C is an element effective for ensuring strength. In order to secure the effect, the content of 0.2% or more is required. However, if the content exceeds 0.6%, the tool life is shortened during cutting. Further, the volume ratio of the ferrite phase in the ferrite / pearlite structure decreases, and the effect of ferrite strengthening decreases accordingly, and the fatigue strength decreases.
Therefore, the content of C is set to 0.2 to 0.6%. The C content is preferably 0.25 to 0.5%.

Si: Si acts to deoxidize the steel and strengthen the ferrite phase. Furthermore, as the Si content increases, the lubricating effect on the chip surface during cutting increases and the tool life is extended, so that it also has the effect of improving machinability. However, if the content is less than 0.05%, the effect of addition is poor, while if it exceeds 1.5%, the effect is saturated and the machinability is rather deteriorated. It was set to 0.05 to 1.5%. In addition, the preferable content of Si is 0.5 to 1.3%.

Mn: Mn has the effect of improving fatigue strength by solid solution strengthening. However, when the content is 0.
If it is less than 1%, the desired effect cannot be obtained, and if it exceeds 2.0%, the hardenability becomes too high to promote the formation of bainite structure or island martensite structure, and the durability ratio (σw / TS).
And, the yield ratio (YS / TS) is lowered. Therefore, the Mn content is set to 0.1 to 2.0%. The Mn content is preferably 0.5 to 1.7%.

P: P is a solid solution strengthening element and has the effect of improving tensile strength and fatigue strength. However, if the content is less than 0.01%, the effect of addition is poor, while on the other hand,
If it exceeds 07%, the effect is saturated and the toughness is deteriorated and the ductility (workability) is lowered, so the content is set to 0.01 to 0.07%. In addition, the preferable content of P is 0.015 to 0.05%.

S: S is an element effective for improving machinability. Together with C, it combines with Ti to form a carbosulfide of Ti and has the effect of enhancing machinability. Furthermore, when MnS combined with Mn or Nd is added, Nd ₂ combined with Nd ₂
The finely dispersed precipitation of S ₃ has the effect of increasing the density of ferrite-producing nuclei, increasing the amount of ferrite, and refining the ferrite grains. However, if the content is less than 0.01%, the desired effect cannot be obtained and 0.2
%, MnS is excessively generated, so that not only the machinability improving effect of Ti carbosulfide decreases but also the toughness deteriorates.
It was set to 0.2%. The S content is 0.02 to 0.17.
% Is preferable.

Ti: Ti is an important element in the present invention. During cooling, it precipitates as fine TiC to strengthen the steel, and the growth of austenite grains is suppressed by the pinning effect of undissolved TiC that remains undissolved in austenite during heating for hot working. Therefore, the structure becomes fine, and the effect of fine strengthening (grain boundary strengthening) occurs. In addition, the precipitation strengthening and the fine strengthening described above are superimposed on each other to improve fatigue strength. Further, it has a function of combining with C and S to form Ti carbosulfide and enhancing machinability. However, if the content is 0.25% or less, the desired effect cannot be obtained, and if it exceeds 1.0%, TiC or Ti carbosulfide is aggregated and coarsened, and the fatigue strength is rather decreased. Therefore, if the content of Ti exceeds 0.25%, 1.
It was set to 0%. In order to stably improve the fatigue strength, the Ti content is preferably 0.27 to 0.8%.

Al: Al is an element effective for stabilizing and homogenizing deoxidation of steel. However, if the content is less than 0.002%, the desired effect cannot be obtained,
%, The effect will be saturated and, on the contrary, the machinability of the steel will be deteriorated.
It was set to 02 to 0.05%. The Al content is 0.00
It is preferably set to 5 to 0.03%.

N: In the present invention, it is extremely important to control the N content to be low. That is, since N has a large affinity with Ti, it easily binds to Ti to form TiN and fixes Ti. Therefore, when a large amount of N is contained, the strengthening effect of TiC and Ti This means that the effect of improving the machinability due to carbosulfide cannot be fully exerted. When the N content is less than 0.008%, the effects of TiC and Ti carbosulfide described above are secured. In addition, T
In order to enhance the effect of iC and Ti carbosulfide, the upper limit of the N content is preferably 0.006%.

Cr: Cr may not be added. If added, it has an effect of improving fatigue strength by solid solution strengthening. In order to surely obtain this effect, the content of Cr is preferably 0.02% or more. However, if its content exceeds 2.0%, the hardenability becomes too high, which promotes the formation of bainite structure or island martensite structure, resulting in a durability ratio (σw / TS) and a yield ratio (YS / TS).
Will be reduced. Therefore, the content of Cr is 0
˜2.0%. When Cr is added, its content is more preferably 0.05 to 1.5%.

V: V may not be added. If added, it precipitates as fine nitrides and carbonitrides,
In particular, it has the effect of improving fatigue strength. In order to reliably obtain this effect, it is preferable that the content of V be 0.05% or more. However, if the content exceeds 0.3%, the precipitates become coarse, so that the above-mentioned effect is saturated or rather decreased. Furthermore, the raw material cost only increases. Therefore, the content of V is set to 0 to 0.3%.

Nb: Nb may not be added. If added, it precipitates as fine nitrides and carbonitrides, has the effects of preventing coarsening of austenite grains and improving the strength of steel, especially fatigue strength. In order to reliably obtain this effect, the Nb content is preferably 0.005% or more. However, if the content exceeds 0.05%, not only the above effects are saturated, but also coarse nitrides are generated to damage the tool, resulting in a decrease in machinability. Therefore, N
The content of b was 0 to 0.05%.

Mo: Mo may not be added. If added, it has the effect of refining the ferrite / pearlite structure and improving the strength of the steel, especially the fatigue strength. In order to reliably obtain this effect, the Mo content is preferably 0.05% or more. However, if the content exceeds 0.5%, the structure after hot working rather becomes abnormally coarse, and the fatigue strength decreases. Therefore, the content of Mo is 0 to
It was set to 0.5%.

Cu: Cu may not be added. If added, precipitation strengthening has the effect of improving the strength of the steel, especially the fatigue strength. In order to reliably obtain this effect, the content of Cu is preferably 0.2% or more. But,
If the content exceeds 1.0%, not only the hot workability is deteriorated, but also the precipitates are coarsened and the above-mentioned effects are saturated or rather lowered. In addition, the cost is high. Therefore, the content of Cu is set to 0 to 1.0%.

Nd: Nd need not be added. If added, Nd ₂ S ₃ acts as a chip breaker and has the effect of improving machinability. In addition, Nd ₂ S ₃ is finely dispersed and formed in a relatively high temperature region of molten steel, so that MnS is finely dispersed and precipitated to increase the density of ferrite-forming nuclei, increase the amount of ferrite and form ferrite grains. It also has the effect of refining and making the steel high-strength and high-toughness as a fine ferrite-pearlite structure. In order to surely obtain the above effect, the Nd content is preferably 0.005% or more. However, its content is 0.1
If it exceeds 0.1%, Nd ₂ S ₃ itself is coarsened and rather deteriorates fatigue strength and toughness. Therefore, the content of Nd is set to 0 to 0.1%. The preferable upper limit of the Nd content is 0.08%.

Pb: Pb may not be added. If added, it has the effect of further improving the machinability of steel. In order to reliably obtain this effect, the Pb content is preferably 0.05% or more. However, if the content exceeds 0.50%, not only the above effects are saturated, but rather coarse inclusions are generated, resulting in a decrease in fatigue strength. Further, since the hot workability is deteriorated, the surface of the steel material is flawed. Therefore, the Pb content is set to 0 to 0.50%.

Ca: Ca may not be added. If added, it has the effect of increasing the machinability of steel. In order to reliably obtain this effect, the content of Ca is preferably 0.001% or more. However, if the content exceeds 0.01%, not only the above effect is saturated, but also rather coarse inclusions are generated, resulting in a decrease in fatigue strength. Therefore, Ca
Content was 0 to 0.01%.

Se: Se may not be added. If added, it has the effect of improving the machinability of steel. In order to reliably obtain this effect, it is preferable that the content of Se is 0.1% or more. However, if its content exceeds 0.5%, not only the above-mentioned effect is saturated, but rather coarse inclusions are generated to cause a decrease in fatigue strength. Therefore, Se
Content of 0 to 0.5%.

Te: Te may not be added. If added, it has the effect of further improving the machinability of steel. In order to reliably obtain this effect, the content of Te is preferably 0.005% or more. However, its content is 0.05
If it exceeds%, not only the above effect is saturated, but also rather large inclusions are formed to cause a decrease in fatigue strength. Furthermore,
Since the hot workability is remarkably deteriorated, a flaw is generated on the surface of the steel material. Therefore, the content of Te should be 0 to 0.05%.
And

Bi: Bi does not have to be added. If added, it has the effect of improving the machinability of steel. In order to reliably obtain this effect, the Bi content is preferably 0.05% or more. However, if the content exceeds 0.4%, not only the above effect is saturated, but rather, coarse inclusions are generated and fatigue strength is reduced. Further, since the hot workability is deteriorated, the surface of the steel material is flawed. Therefore, the Bi content is set to 0 to 0.4%.

(B) Structure of Steel Material Even if the steel has the above chemical composition, if the structure after hot working is composed of so-called "low temperature transformation product" such as bainite or martensite, the machinability deteriorates. To do. Further, in the cooling process after the hot working, a large transformation strain occurs in the product and the bending becomes large, so that a correction process for removing the bending is required, which leads to an increase in cost. Therefore, in order to obtain good machinability and reduce transformation strain, the steel structure must first be a ferrite-pearlite structure. The above chemical composition is designed so that a "low temperature transformation product" will not be produced if the steel material is cooled under appropriate conditions after hot working.

In the ferrite / pearlite structure, if the ferrite is fine particles having a JIS grain size number of 6 or more and the average pearlite lamella spacing is 0.2 μm or less, the YS can be obtained without increasing the TS of the ferrite / pearlite structure.
Therefore, the fatigue strength can be increased without decreasing the machinability. Therefore, the structure of the steel material is a ferrite-pearlite structure composed of ferrite having a JIS grain size number of 6 or more and pearlite having an average lamella spacing of 0.2 μm or less.

Since the above-mentioned effect (the effect of increasing YS without increasing TS of the ferrite-pearlite structure) is greater as the average of the ferrite grain and pearlite lamella spacing is finer, the upper limit of the JIS grain size number of ferrite and The lower limit of the average pearlite lamella spacing is not particularly specified.

The lamellar spacing of pearlite referred to in the present invention can be easily determined by a usual method using, for example, a scanning electron microscope structure of a sample corroded with nital or a transmission electron microscope structure of a two-step replica taken from the sample. You can ask.

(C) Hot working In order to obtain the desired structure of the steel material, hot working is performed in a temperature range of 900 ° C or higher after heating to a temperature range of 1050-1300 ° C. There is a need.

In the case of high temperature heating exceeding 1300 ° C.,
In addition to the problem of quality that a desired size of ferrite-pearlite structure cannot be obtained because the austenite grains are significantly coarsened, there is also an economical disadvantage of increased cost. Further, when heated in a temperature range lower than 1050 ° C., Ti does not form a solid solution in austenite sufficiently, and therefore fine TiC precipitation is sufficient even if cooled under proper cooling conditions after hot working. It does not occur and the desired texture and mechanical properties are not obtained. Therefore, in the present invention, the heating temperature for hot working is limited to 1050-1300 ° C.

Performing the hot working in the temperature range of 900 ° C. or higher means that if the hot working is carried out in the temperature range of lower than 900 ° C., TiC is induced by work-induced precipitation during the working, so that recrystallization is suppressed and a desired temperature is desired. This is because the organization is difficult to obtain. Further, the deformation resistance of the steel material becomes high, which leads to the occurrence of flaws and cracks. Therefore, the hot working is performed in the temperature range of 900 ° C. or higher. The upper limit of this processing temperature is preferably about 1050 ° C. The workability during hot working is 1 in terms of cross-section reduction rate.
It is preferably about 0 to 90%. In addition, in order to obtain desired characteristics more stably, it is more preferable that the working ratio during the hot working is about 30 to 90% in terms of cross-sectional reduction rate.

(D) Cooling after hot working After the hot working is finished, the steel material needs to be air-cooled or allowed to cool to at least 500 ° C at a cooling rate of 5 to 30 ° C / min. When cooled at a cooling rate of over 30 ° C / min,
The desired structure and mechanical properties cannot be obtained because a sufficient amount of fine TiC is not precipitated. On the other hand, if the cooling rate is less than 5 ° C./minute, TiC becomes coarse and a desired fine structure cannot be obtained, and desired mechanical properties cannot be obtained. The cooling rate after cooling to 500 ° C at a cooling rate of 5 to 30 ° C / min does not have to be particularly limited.

The steel material having the composition shown in (A) above is subjected to hot working / cooling under the conditions shown in (C) and (D) above to obtain the result shown in (B) above. It is possible to manufacture a non-heat treated steel having a structure.

[0053]

EXAMPLES Steels having the chemical compositions shown in Tables 1 to 5 were melted by a usual method using a 150 kg vacuum melting furnace. Steels 1 to 5 in Table 1, steels 14 to 21 in Table 2, steels 27 to 31 in Table 3, steels 37 to 46 in Table 4 and steels 47 to 56 in Table 5 are steels of the examples of the present invention, steels in Table 1 6
.About.13, Steels 22 to 26 in Table 2 and Steels 32 to 36 in Table 3 are steels of Comparative Examples in which any of the components is out of the range of the content specified in the present invention.

[0054]

[Table 1]

[0055]

[Table 2]

[0056]

[Table 3]

[0057]

[Table 4]

[0058]

[Table 5]

Then, these steels were heated to a temperature of 1250 ° C. for 1 hour and then hot forged for finishing at 900 ° C. or higher was performed once or 2-3 times to produce a round bar having a diameter of 60 mm. In the final hot forging step for forming a round bar having a diameter of 60 mm, the cooling condition after hot forging is 15 ° C /
Minutes and cooled to room temperature.

15 mm from the surface of the round bar thus obtained
From the position (R / 2 position, R is the radius of the round bar),
The No. 14A tensile test piece and the Ono-type rotary bending test piece (the diameter of the parallel part is 8 mm and the length thereof is 18.4 mm) are sampled,
The tensile strength (TS) and fatigue strength (σw) at room temperature were investigated. Further, the structure (phase) was examined by an optical microscope, and the pearlite lamella spacing (average value) was obtained from the scanning electron micrograph.

Machinability was also evaluated by a drilling test. That is, a round bar with a diameter of 60 mm is cut into a slice with a length of 45 mm, and a depth of 40 mm in the length direction is used.
The number of holes was determined when the holes were made and the holes could not be drilled due to the abrasion of the cutting edge. Drilling conditions are JIS high speed tool steel SKH
A φ6 mm drill No. 51 was used and a water-soluble lubricant was used, and the feed was 0.15 mm / rev and the rotation speed was 980 rpm.

The results are shown in Tables 6 and 7. Moreover, FIG.
The relationships between fatigue strength and machinability are summarized and shown in ~ 4. In addition,
FIG. 1 shows steels 1 to 13, FIG. 2 shows steels 14 to 26, FIG. 3 shows steels 27 to 36, and FIG. 4 shows steels 37 to 56.
Steels of the present invention and steels of all comparative examples (steels 6 to 13, steel 2).
2 to 26, steels 32 to 36), the relationship between fatigue strength and machinability is arranged.

[0063]

[Table 6]

[0064]

[Table 7]

From Tables 6 and 7 and FIGS. 1 to 4, the steels of the examples of the present invention have high fatigue strength and good machinability at the fatigue strength level. It is clear that the machinability balance is excellent.

On the other hand, the steels of Comparative Examples are clearly inferior in the balance of fatigue strength and machinability.

[0067]

INDUSTRIAL APPLICABILITY The non-heat treated steel material of the present invention has high fatigue strength and good machinability at the fatigue strength level, and thus can be used as a material for machine structural parts and the like. This non-heat treated steel material having excellent fatigue resistance can be manufactured relatively easily and at low cost by the method of the present invention.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between fatigue strength and machinability of steels 1 to 13 used in Examples.

FIG. 2 is a diagram showing a relationship between fatigue strength and machinability of steels 14 to 26 used in the examples.

FIG. 3 is a diagram showing the relationship between the fatigue strength and the machinability of the steels 27 to 36 used in the examples.

FIG. 4 is a diagram showing a relationship between fatigue strength and machinability of steels 6 to 13, steels 22 to 26, and steels 32 to 56 used in the examples.

Continued front page       (56) Reference JP-A-7-157824 (JP, A)                 JP-A-61-264162 (JP, A)                 Japanese Patent Laid-Open No. 50-57914 (JP, A)                 JP-A-6-228713 (JP, A)                 JP-A-50-20917 (JP, A)                 Japanese Patent Publication Sho 34-2405 (JP, B1)                 "Materials and Processes" Vol. 7               (1994), No. 3, P.I. 819

Claims (2)

(57) [Claims] 1. C: 0.2 to 0.6% by weight, Si:
0.05-1.5%, Mn: 0.1-2.0%, P:
0.01 to 0.07%, S: 0.01 to 0.20%, T
i: more than 0.25% to 1.0%, Al: 0.002
~ 0.05%, N: 0.008% or less, Cr: 0-2.
0%, V: 0 to 0.3%, Nb: 0 to 0.05%, M
o: 0 to 0.5%, Cu: 0 to 1.0%, Nd: 0
0.1%, Pb: 0 to 0.50%, Ca: 0 to 0.01
%, Se: 0 to 0.5%, Te: 0 to 0.05%, B
i: 0 to 0.4%, the balance being Fe and unavoidable impurities, and the structure is a ferrite-perlite structure composed of ferrite having a JIS grain size number of 6 or more and pearlite having an average lamella spacing of 0.2 μm or less. Non-heat treated steel with excellent machinability and fatigue resistance. 2. A steel having the chemical composition according to claim 1,
90 after heating to a temperature in the temperature range of 1050 to 1300 ° C
Hot working in the temperature range of 0 ℃ or more, then 5-30 ℃
Machinability and fatigue resistance characterized by cooling at a cooling rate of 1 / min to form a ferrite / pearlite structure consisting of ferrite with a JIS grain size number 6 or more and pearlite with an average lamella spacing of 0.2 μm or less Excellent non-heat treated steel manufacturing method.