JPH10212559A - Non-heat treated steel excellent in fatigue resistance and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a low-cost steel having excellent fatigue strength and machinability as non-heat treated and suitable as the stock for parts of machine structures and to provide a method for producing the same. SOLUTION: This steel is the one having a compsn. contg., by weight, 0.2 to 0.6% C, 0.05 to 1.5% Si, 0.1 to 2.0% Mn, 0.01 to 0.07% P, 0.01 to 0.20% S, >0.25 to 1.0% Ti, 0.002 to 0.05% Al, <=0.008% N, 0 to 2.0% Cr, 0 to 0.3% V, 0 to 0.005% Nb, 0 to 0.5% Mo, 0 to 1.0% Cu, 0 to 0.1% Nd, 0 to 0.50% Pb, 0 to 0.01% Ca, 0 to 0.5% Se, 0 to 0.05% Te, 0 to 0.4% Bi, and the balance Fe with inevitable impurities and having a ferritic-pearlitic structure composed of ferrite having >=6 JIS size number and pearlite with <=0.2μm lamellar intervals. As to the method for producing it, after heating in the temp. range of 1,050 to 1,300 deg.C, hot working is executed in the temp. range of >=900 deg.C, and next, cooling is executed at a cooling rate of 5 to 30 deg.C/min.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-heat treated steel excellent in fatigue resistance and a method for producing the same. More specifically, the present invention relates to a non-heat treated steel material having excellent fatigue strength-machinability balance without being subjected to a tempering treatment of quenching and tempering after hot working, and suitable as a material for machine structural parts and the like, and a method for producing the same. is there.

[0002]

2. Description of the Related Art Conventionally, steel mechanical structural parts that require high fatigue strength are roughly worked into a predetermined shape by hot working, and then cut into a final desired shape.
It was common to apply a quenching and tempering refining treatment. However, this refining process consumes a lot of energy and cost. Therefore, in recent years, non-heat-treated steels that can be used as they are in hot working have been actively developed in order to meet social demands for energy saving and reduce costs.

For example, Japanese Unexamined Patent Publication (Kokai) No. 4-141550 discloses a "non-heat treated steel for hot forging" having a fine bainite structure obtained by adding Cr, Mo, Nb and Ti in a complex manner. However, since this non-heat treated steel is bainite type, its machinability is inferior to that of conventional ferritic / pearlite type non-heat treated steel, and furthermore, there is a problem that bending becomes large due to large transformation strain. I was Therefore, a straightening process for bending is required, which leads to an increase in cost.

On the other hand, as an alternative to the refining treatment in which the steel material cooled after hot working is reheated to the austenite temperature range, quenched, and then tempered,
No. 2347 discloses "a hot forged product having high fatigue strength and a method for producing the same", in which steel having a specific chemical composition is quenched immediately after hot forging and then tempered to precipitate TiC. . However, the hot forged product described in this publication is quenched immediately after hot forging to have a martensitic structure, so it is necessary to control quenching cracks during quenching, and tempering to precipitate solid solution TiC. Therefore, there is also a problem that energy cost increases.

[0005] With respect to non-heat treated steel, there is an increasing demand for free-cutting steel having excellent machinability for the purpose of facilitating cutting after hot working.

[0006] Generally, the machinability of steel largely depends on the metallographic structure, and as mentioned in the section of the technology described in JP-A-4-141550, the machinability of steel having a ferrite-pearlite structure is poor. It is good, and the machinability is poor in steels having a ferrite bainite structure or a single phase structure of bainite or martensite. Also, Pb, Te, Bi, C
It is a well-known fact that the machinability is improved by adding free-cutting elements such as a and S alone or in combination. Therefore, conventionally, a method of improving the machinability after hot working by adding the above-mentioned free-cutting element to non-heat treated steel has been adopted. However, simply adding a free-cutting element to non-heat treated steel often cannot ensure a desired high fatigue strength.

Under these circumstances, for example, Japanese Patent Laid-Open No. 2-1
Japanese Patent No. 11842 and Japanese Patent Application Laid-Open No. Hei 6-279849 disclose "Machinability and sintering properties" in which C in steel is present as graphite and the machinability is improved by utilizing the notch and lubrication effect of the graphite. A hot-rolled steel excellent in machinability and a method for producing steel for machine structural use excellent in machinability have been proposed.

However, the steel material proposed in Japanese Patent Application Laid-Open No. 2-111842 is one in which B is added to promote the graphitization using B nitride (BN) as a nucleus of graphitization, and the addition of B is essential. Therefore, there is a problem that cracks easily occur during solidification. On the other hand, the method described in Japanese Patent Application Laid-Open No. 6-279849 is to promote graphitization while hot rolling by restricting O (oxygen) in steel to a low level together with the addition of Al. Since it is necessary to perform a chemical annealing treatment, it is not necessarily economical.
Further, since the proposals in the two publications described above both utilize graphitization, in order to impart desired mechanical properties to a machine structural part or the like processed into a predetermined shape, it is necessary to adjust the quenching and tempering. Quality treatment,
It was unable to meet the demands of the industry to achieve both "non-tempering" and "improvement in machinability of high-strength steel".

Iron and steel (vol. 57 (1971) S4)
84) reports that the addition of Ti to deoxidized adjusted free-cutting steel may enhance machinability. But T
It is also described that the addition of a large amount of i increases tool wear due to generation of a large amount of TiN, and is undesirable from the viewpoint 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%, on the contrary, has a short drill life and poor machinability. Thus, the machinability is not improved simply by adding Ti to steel.

[0010]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and has been developed under ordinary hot working and cooling conditions.
It also has excellent fatigue resistance and excellent machinability without heat treatment including tempering and without heat treatment, that is, materials such as machine structural parts with excellent balance between fatigue strength and machinability. It is an object of the present invention to provide a low-cost steel material suitable for use and a method for producing the same.

[0011]

SUMMARY OF THE INVENTION The gist of the present invention is to provide a non-heat-treated steel material having excellent fatigue resistance as shown in the following (1) and (2):
The method for producing a non-heat-treated steel material having excellent fatigue resistance characteristics shown in FIG.

(1) By weight%, C: 0.2-0.6%,
Si: 0.05 to 1.5%, Mn: 0.1 to 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 the composition of Fe and unavoidable impurities, the structure being a ferrite-pearlite structure composed of ferrite having a JIS particle 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 fatigue resistance.

(2) The steel having the chemical composition described in the above (1) is heated to a temperature in a temperature range of 1050 to 1300 ° C., then hot-worked in a temperature range of 900 ° C. or higher, and then
Cool at a cooling rate of ３０30 ° C./min.
A method for producing a non-heat treated steel excellent in fatigue resistance, characterized by having a ferrite-pearlite structure composed of pearlite having an average ferrite and lamellar spacing of 0.2 μm or less.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have conducted research on the chemical composition and structure of a non-heat treated steel material in order to achieve the above-mentioned object, and have obtained the following findings.

In the case of a ferrite-pearlite structure, the fatigue strength shows a positive correlation with the yield strength (YS). Therefore, in order to increase the fatigue strength, YS may be improved. It is not preferable to increase the tensile strength (TS) from the viewpoint of machinability.
By increasing YS without increasing S, machinability and high fatigue strength (fatigue limit (σw)) can be obtained.

The refinement of ferrite grains and the pearlite lamellar spacing are effective for increasing YS without increasing TS of ferrite / pearlite structure so much.

[0017] If a proper amount of Ti is contained in a steel material whose N amount is regulated and cooling conditions after hot working are optimized, YS can be increased without increasing TS of ferrite-pearlite structure so much, and fatigue can be improved. The strength is dramatically improved. This is because (a) fine TiC precipitates during cooling and the ferrite is strengthened, and (ii) the growth of austenite grains is suppressed by TiC which is not dissolved in the heating during hot working. This is because a fine structure is obtained and the structure is strengthened (finely strengthened) by making the structure finer.

When Ti is actively 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 formed decreases.

When the S content in steel is the same, Ti carbosulfide has a greater machinability improving effect than MnS. This is based on the fact that the melting point of the carbosulfide of Ti 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 the carbosulfide of Ti, it is important to limit the N content to a low level.
This is because if the N content is large, Ti is fixed as TiN, and the generation of Ti carbosulfide is suppressed.

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

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

Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the component content means “% by weight”.

(A) Chemical composition of steel C: C is an element effective for securing strength. To ensure the effect, a 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 fraction of the ferrite phase in the ferrite-pearlite structure is reduced, and the effect of ferrite strengthening is weakened and the fatigue strength is reduced.
Therefore, the content of C is set to 0.2 to 0.6%. Note that the C content is preferably set to 0.25 to 0.5%.

Si: Si has the effect of deoxidizing steel and strengthening the ferrite phase. Further, the increase in the Si content enhances the lubricating effect on the chip surface during cutting and extends the life of the tool, thereby improving the machinability. However, if the content is less than 0.05%, the effect of addition is poor. On the other hand, if the content exceeds 1.5%, not only the effect is saturated, but also the machinability deteriorates. 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 the fatigue strength by solid solution strengthening. However, when its content is 0.1.
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 and island martensite structure, and the durability ratio (σw / TS)
And the yield ratio (YS / TS) decreases. Therefore, the content of Mn is set to 0.1 to 2.0%. The Mn content is preferably set to 0.5 to 1.7%.

P: P is a solid solution strengthening element and has an effect of improving tensile strength and fatigue strength. However, when the content is less than 0.01%, the effect of addition is poor.
When the content exceeds 07%, the effect is saturated and the toughness is deteriorated and the ductility (workability) is reduced. Therefore, 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. It combines with C and Ti to form Ti carbosulfide and has an effect of enhancing machinability. Further, when MnS or Nd bonded to Mn is added, Nd ₂ bonded to Nd is added.
By finely dispersing and depositing S ₃ , the ferrite generation nucleus density is increased, the ferrite amount is increased, and the ferrite grains are refined. However, if the content is less than 0.01%, the desired effect cannot be obtained,
%, MnS is excessively generated, so that not only the effect of improving the machinability by Ti carbosulfide is lowered but also the toughness is deteriorated.
0.2%. In addition, S content is 0.02-0.17.
% Is preferable.

Ti: Ti is an important element in the present invention. Precipitates as fine TiC during cooling and precipitates and strengthens the steel, and the growth of austenite grains is suppressed by the pinning effect of undissolved TiC remaining without being dissolved 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 overlap to have an effect of improving the fatigue strength. Further, it has an effect of forming Ti carbosulfide by combining with C and S to enhance machinability. However, if the content is 0.25% or less, the desired effect cannot be obtained. If the content exceeds 1.0%, TiC or Ti carbosulfide is coagulated and coarsened, and the fatigue strength is rather lowered. Therefore, when the content of Ti exceeds 0.25% and the content of 1.
It was set to 0%. In order to stably improve the fatigue strength, the content of Ti is preferably set to 0.27 to 0.8%.

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

N: In the present invention, it is extremely important to control the content of N to be low. That is, since N has a large affinity for Ti, it easily binds to Ti to generate TiN and fixes Ti, so that when N is contained in a large amount, the strengthening effect by TiC and the Ti The effect of improving the machinability by the carbon sulfide cannot be sufficiently exhibited. When the N content is less than 0.008%, the above-described effects of TiC and Ti carbosulfide are ensured. Note that T
In order to enhance the effects of iC and Ti carbosulfide, the upper limit of the N content is preferably set to 0.006%.

Cr: Cr need not be added. If added, there is an effect of improving the fatigue strength by solid solution strengthening. To ensure this effect, the content of Cr is preferably set to 0.02% or more. However, if the content exceeds 2.0%, the hardenability becomes too high, and the formation of bainite structure or island martensite structure is promoted, and the durability ratio (σw / TS) and the yield ratio (YS / TS)
Will decrease. Therefore, the content of Cr is reduced to 0.
To 2.0%. When Cr is added, the content is more preferably set to 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 ensure this effect, it is preferable that the content of V is 0.05% or more. However, if the content exceeds 0.3%, the precipitates are coarsened, so that the above-mentioned effects are saturated or rather reduced. In addition, 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 or carbonitrides, has the effect of preventing austenite grains from coarsening and improving the strength of steel, especially fatigue strength. In order to surely obtain this effect, the content of Nb is preferably set to 0.005% or more. However, if the content exceeds 0.05%, not only the above-mentioned effect is saturated, but also coarse nitrides are formed, which damages the tool and lowers the machinability. Therefore, N
The content of b was set to 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. To ensure this effect, the Mo content is preferably set to 0.05% or more. However, if the content exceeds 0.5%, the structure after hot working is rather abnormally coarsened and the fatigue strength is reduced. Therefore, the content of Mo is 0 to
0.5%.

Cu: Cu need not be added. If added, it has the effect of improving the strength of the steel, especially the fatigue strength, by precipitation strengthening. In order to ensure this effect, it is preferable that the content of Cu be 0.2% or more. But,
If the content exceeds 1.0%, in addition to the deterioration of hot workability, the precipitates become coarse and the above-mentioned effects are saturated or rather deteriorated. In addition, costs are only increasing. Therefore, the content of Cu is set to 0 to 1.0%.

Nd: Nd may not be added. If added, Nd ₂ S ₃ acts as a chip breaker and has the effect of improving machinability. Further, as Nd ₂ S ₃ is finely dispersed and generated in a relatively high temperature range of molten steel, MnS is finely dispersed and precipitated to increase the ferrite generation nucleus density, increase the amount of ferrite, and increase the ferrite grain size. It also has the effect of making the steel finer and having a finer ferrite-pearlite structure to increase the strength and toughness of the steel. In order to surely obtain the above-mentioned effects, the content of Nd is preferably set to 0.005% or more. However, the content is 0.1
%, The Nd ₂ S ₃ itself becomes coarse, resulting in a decrease in fatigue strength and toughness. Therefore, the content of Nd is set to 0 to 0.1%. Note that a preferable upper limit of the Nd content is 0.08%.

Pb: Pb may not be added. If added, it has the effect of further increasing the machinability of the steel. In order to surely obtain this effect, the content of Pb is preferably set to 0.05% or more. However, if the content exceeds 0.50%, not only the above-mentioned effects are saturated, but rather coarse inclusions are formed and the fatigue strength is lowered. Further, the hot workability is deteriorated, so that the surface of the steel material is flawed. Therefore, the content of Pb was set to 0 to 0.50%.

Ca: Ca may not be added. If added, it has the effect of increasing the machinability of the steel. In order to surely obtain this effect, the content of Ca is preferably set to 0.001% or more. However, if the content exceeds 0.01%, not only the above-mentioned effects are saturated, but rather coarse inclusions are formed, and the fatigue strength is lowered. Therefore, Ca
Was set to 0 to 0.01%.

Se: Se need not be added. If added, it has the effect of improving the machinability of the steel. To ensure this effect, the content of Se is preferably set to 0.1% or more. However, if the content exceeds 0.5%, not only the above-mentioned effect is saturated, but rather coarse inclusions are formed and the fatigue strength is lowered. Therefore, Se
Was set to 0 to 0.5%.

Te: Te need not be added. If added, it has the effect of further increasing the machinability of the steel. To ensure this effect, the content of Te is preferably 0.005% or more. However, the content is 0.05
%, The above effect is not only saturated, but rather, coarse inclusions are formed to lower the fatigue strength. Furthermore,
Since the hot workability is significantly deteriorated, flaws are formed on the surface of the steel material. Therefore, the content of Te is set to 0 to 0.05%.
And

Bi: Bi may not be added. If added, it has the effect of improving the machinability of the steel. To ensure this effect, the content of Bi is preferably set to 0.05% or more. However, when the content exceeds 0.4%, not only the above-mentioned effect is saturated, but also coarse inclusions are formed and the fatigue strength is lowered. Further, the hot workability is deteriorated, so that the surface of the steel material is flawed. Therefore, the content of Bi 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 made of a so-called “low-temperature transformation product” such as bainite or martensite, the machinability deteriorates. I do. Further, in the cooling process after the hot working, a large transformation strain is generated in the product and the bending becomes large, so that a bending correction step 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 "low-temperature transformation products" are not generated if the steel is cooled under appropriate conditions after hot working.

In the ferrite / pearlite structure, when the ferrite is fine grains having a JIS particle size number of 6 or more and the average of the pearlite lamella spacing is 0.2 μm or less, YS without increasing the TS of the ferrite / pearlite structure.
Therefore, fatigue strength can be increased without lowering machinability. Therefore, the structure of the steel material was a ferrite / pearlite structure composed of ferrite having a JIS grain size number of 6 or more and pearlite having an average of lamellar spacing of 0.2 μm or less.

The average of the ferrite grain and pearlite lamella spacing is such that the smaller the average, the greater the above-mentioned effect (the effect of increasing YS without increasing the TS of the ferrite / pearlite structure). The lower limit of the average of the pearlite lamella intervals is not particularly defined.

The lamella spacing of pearlite in the present invention can be easily determined by a conventional method using, for example, a scanning electron microscope structure of a sample corroded by 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 at a temperature in a temperature range of 1,050 to 1,300 ° C. and then in a temperature range of 900 ° C. or more. There is a need.

In the case of high-temperature heating exceeding 1300 ° C.,
In addition to the quality problem that a ferrite-pearlite structure of a desired size cannot be obtained due to remarkable coarsening of austenite grains, there is an economic disadvantage that the cost is increased. Further, when heating is performed in a temperature range lower than 1050 ° C., since solid solution of Ti in austenite is not sufficient, fine TiC precipitates sufficiently even if cooled under appropriate cooling conditions after hot working. No desired texture and mechanical properties are obtained. Therefore, in the present invention, the heating temperature of the hot working is limited to 1050 to 1300 ° C.

The reason why the hot working is performed in a temperature range of 900 ° C. or more is that if the hot working is performed in a temperature range lower than 900 ° C., TiC precipitates during the working during the working, so that recrystallization is suppressed and the desired recrystallization is suppressed. This is because it is difficult to obtain an organization. Further, the deformation resistance of the steel material is increased, which leads to generation of flaws and cracks. Therefore, the hot working is performed in a temperature range of 900 ° C. or more. The upper limit of the processing temperature is preferably about 1050 ° C. The degree of working during hot working is 1 in area reduction rate.
It is preferable to set it to about 0 to 90%. In order to obtain the desired characteristics more stably, it is more preferable that the working ratio at the time of the above-mentioned hot working is about 30 to 90% in terms of a cross-sectional reduction rate.

(D) Cooling after hot working After the hot working is completed, the steel material needs to be air-cooled or cooled to at least 500 ° C at a cooling rate of 5 to 30 ° C / min. When cooling at a cooling rate exceeding 30 ° C./min,
Since a sufficient amount of fine TiC is not deposited, the desired structure and mechanical properties cannot be obtained. On the other hand, if the cooling rate is less than 5 ° C./min, 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 may not be particularly limited.

The steel having the component composition shown in the above (A) was subjected to hot working and cooling under the conditions shown in the above (C) and (D), whereby the steel shown in the above (B) was obtained. A non-heat treated steel material having a structure can be manufactured.

[0053]

EXAMPLES Steel having the chemical composition shown in Tables 1 to 5 was melted by a usual method using a 150 kg vacuum melting furnace. The steels 1 to 5 in Table 1, the steels 14 to 21 in Table 2, the steels 27 to 31 in Table 3, the steels 37 to 46 in Table 4, and the steels 47 to 56 in Table 5 are steels of the present invention and steels in Table 1. 6
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]

Next, these steels were heated to a temperature of 1250 ° C. for 1 hour and then hot forged to finish at 900 ° C. or more once or 2-3 times to produce round bars having a diameter of 60 mm. In the final hot forging step for obtaining a round bar having a diameter of 60 mm, the cooling condition after hot forging was 15 ° C. /
And cooled to room temperature.

15 mm from the surface of the thus obtained round bar
From the position (R / 2 part position, R is the radius of the round bar)
A 14A tensile test specimen and an Ono-type rotary bending test specimen (parallel portion having a diameter of 8 mm and a length of 18.4 mm) were collected.
The tensile strength (TS) and the fatigue strength (σw) at room temperature were investigated. In addition, the structure (phase) was examined with an optical microscope, and the pearlite lamella spacing (average value) was determined from a scanning electron micrograph.

The machinability was evaluated by a drilling test. That is, a round bar having a diameter of 60 mm was cut into 45 mm-length round bars, and a depth of 40 mm was used in the length direction.
Were drilled, and the number of holes when drilling was impossible due to abrasion of the cutting edge was determined. Drilling conditions are JIS high speed tool steel SKH
Using a 51 φ6 mm drill, a water-soluble lubricant was used at a feed rate of 0.15 mm / rev and a rotation speed of 980 rpm.

Tables 6 and 7 show the results. Also, FIG.
4 to 4 show the relationship between fatigue strength and machinability. In addition,
1 is for steels 1 to 13, FIG. 2 is for steels 14 to 26, FIG. 3 is for steels 27 to 36, and FIG.
Steel of the present invention and steels of all comparative examples (steel 6 to 13, steel 2)
2 to 26 and steels 32 to 36) in which 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 steel of the present invention has high fatigue strength, and has good machinability at that fatigue strength level, that is, fatigue strength-workability. It is clear that the balance is excellent.

On the other hand, the steel of the comparative example is clearly inferior in terms of the balance between fatigue strength and machinability.

[0067]

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

[Brief description of the drawings]

FIG. 1 is a view 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 Examples.

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

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

Claims (2)

[Claims] 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 to 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 the composition of Fe and unavoidable impurities, the structure being a ferrite-pearlite structure composed of ferrite having a JIS particle 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 fatigue resistance. 2. A steel having the chemical composition according to claim 1,
After heating to a temperature in the temperature range of 1,050 to 1,300 ° C, 90
Perform hot working in a temperature range of 0 ° C or higher, then 5 to 30 ° C
At a cooling rate of / min to obtain a ferrite-pearlite structure comprising ferrite having a JIS particle size number of 6 or more and pearlite having an average lamellar spacing of 0.2 μm or less, characterized by excellent fatigue resistance. Method of manufacturing high quality steel.