JPH10195599A - Free cutting non-heat treated steel excellent in strength and toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a free cutting non-heat treated steel having excellent strength and toughness and suitable as the stock for machine structural parts or the like at a low cost. SOLUTION: This steel is the one having a compsn. contg., by weight, 0.2 to 0.6% C, 0.1 to 1.5% Si, 0.1 to 2.0% Mn, 0.01 to 0.07% P, 0.01 to 0.2% S, 0.02 to 2.0% Cr, 0.05 to 1.0% Ti, 0.002 to 0.05% Al, <=0.008% N, 0 to 0.1% Nd, 0 to 0.3% V, 0 to 0.05% Nb, 0 to 0.5% Mo, 0 to 1.0% Cu, 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 in which >=90% of the structure is composed of ferrite-pearlite.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a free-cut non-heat treated steel having excellent strength and toughness. More specifically, the present invention relates to a free-cutting non-heat treated steel which has excellent strength and toughness without being subjected to a tempering treatment of quenching and tempering after hot working and is suitable as a material for parts for machine structural use.

[0002]

2. Description of the Related Art Conventionally, high tensile strength and fatigue strength (hereinafter, referred to as "high")
Tensile strength and fatigue strength are sometimes referred to simply as "strength".)
In addition, steel mechanical structural parts that require high toughness,
In general, after roughing into a predetermined shape by hot working, and then to a final desired shape by cutting, a tempering treatment such as quenching and tempering is generally performed. 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.

[0003] Further, for the purpose of facilitating cutting after hot working, the demand for free-cutting steel excellent in machinability has been increasing.

[0004] Generally, the machinability of steel largely depends on the metallographic structure. In the case of a steel mainly composed of ferrite / pearlite, the machinability is good, and the machinability of ferrite / bainite or bainite or martensite is simple. It is known that steel having a phase structure has poor machinability.
It is also a well-known fact that machinability is improved by adding free-cutting elements such as Pb, Te, Bi, Ca, 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, when a free-cutting element is simply added to a non-heat treated steel, a desired strength (particularly, fatigue strength) and a desired toughness cannot often be secured.

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 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.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and has been made in a non-refined condition without performing heat treatment including tempering under ordinary hot working and cooling conditions. Accordingly, an object of the present invention is to provide steel for a material such as a machine structural part having high strength and good toughness and excellent machinability at a low cost.

[0009]

SUMMARY OF THE INVENTION The gist of the present invention is a free-cutting non-heat treated steel having the following excellent strength and toughness.

That is, "in weight%, C: 0.2-0.
6%, Si: 0.1 to 1.5%, Mn: 0.1 to 2.0
%, P: 0.01 to 0.07%, S: 0.01 to 0.2
%, Cr: 0.02-2.0%, Ti: 0.04-1.
0%, Al: 0.002 to 0.05%, N: 0.008
%, Nd: 0 to 0.1%, V: 0 to 0.3%, N
b: 0 to 0.05%, Mo: 0 to 0.5%, Cu: 0 to 0%
1.0%, Pb: 0 to 0.50%, Ca: 0 to 0.01
%, Se: 0 to 0.5%, Te: 0 to 0.05% and B
i: 0 to 0.4%, the balance being Fe and inevitable impurities, and 90% or more of the structure is composed of a ferrite / pearlite structure, and free cutting is excellent in strength and toughness. Quality steel. "

[0011]

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, the present inventor has found that a steel to which Ti has been added is hot-worked, then cooled at an appropriate cooling rate, and the structure thereof is reduced. It has been found that the machinability of steel is greatly improved if it is mainly made of ferrite / pearlite.
Therefore, as a result of further research, the following items were found.

If Ti is positively added to steel in consideration of the balance with S, 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.

[0016] Ti carbosulfide produced during steelmaking does not form a solid solution with 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: 0.2 to 0.6% C is an element effective for securing strength. But,
If the content is less than 0.2%, the effect of the addition is poor, while if it exceeds 0.6%, the toughness deteriorates. Therefore,
The content of C was set to 0.2 to 0.6%. Note that the C content is preferably set to 0.25 to 0.5%.

Si: 0.1-1.5% Si is added for deoxidizing steel and strengthening ferrite. However, if the content is less than 0.1%, the above effect is insufficient, while if it exceeds 1.5%, not only the above effect is saturated but also the toughness is reduced. Therefore,
The content of Si was set to 0.1 to 1.5%. In addition, the preferable content of Si is 0.3 to 1.3%.

Mn: 0.1 to 2.0% Mn has the effect of improving fatigue strength by solid solution strengthening. However, if the content is less than 0.1%, the desired effect cannot be obtained, and if the content exceeds 2.0%, the hardenability becomes too high to promote the formation of bainite structure and island-like martensite structure, and the durability (Fatigue limit / tensile strength) and yield ratio (proof stress / tensile strength) are reduced. Therefore, the content of Mn is set to 0.1 to 2.0%. Note that M
The n content is preferably set to 0.5 to 1.7%.

P: 0.01 to 0.07% P is a solid solution strengthening element and has an effect of improving tensile strength and fatigue strength. However, its content is 0.01%
If it is less than 0.07%, the effect is saturated, and if it exceeds 0.07%, the effect is saturated and the toughness is deteriorated and the ductility (workability) is reduced.
0.07%. The preferred content of P is 0.015
0.05%.

S: 0.01 to 0.2% S is an element effective for improving machinability. In particular, it has an effect of forming a carbosulfide of Ti by combining with Ti together with C to enhance machinability. Further, MnS or N bonded to Mn
When N is added, Nd ₂ S ₃ combined with Nd is finely dispersed and precipitated, thereby increasing the ferrite generation nucleus density.
This has the effect of increasing the amount of ferrite and miniaturizing ferrite grains. However, its content is 0.01%
If it is less than 0.2%, the desired effect cannot be obtained.
Excessive generation of nS not only reduces the machinability improving effect of Ti carbosulfide, but also deteriorates the toughness. Therefore, the content is set to 0.01 to 0.2%.
Note that the S content is preferably set to 0.02 to 0.17%.

Cr: 0.02 to 2.0% Cr has the effect of improving the fatigue strength by solid solution strengthening. However, if the content is less than 0.02%, the desired effect cannot be obtained. If the content exceeds 2.0%, the hardenability becomes too high, and the formation of bainite structure and island-like martensite structure is promoted. In addition, the yield ratio decreases. Therefore, the content of Cr is set to 0.02 to 2.0%. The preferable content of Cr is 0.05 to 1.5%.

Ti: 0.04 to 1.0% Ti is an important element in the present invention, and has an effect of forming a carbosulfide of Ti by combining with C and S to enhance machinability. However, if the content is less than 0.04%, the desired effect cannot be obtained. On the other hand, even if the content exceeds 1.0%, the effect of improving machinability due to Ti carbosulfide is saturated, which not only increases the cost, but also decreases the toughness due to excessive generation of TiC. Therefore, the content of Ti is set to 0.04
-1.0. In order to stably improve machinability and secure good toughness, the content of Ti is preferably set to 0.08 to 0.6%.

Al: 0.002 to 0.05% 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. If the content exceeds 0.05%, the effect is saturated and the machinability of the steel is rather reduced. Therefore, the content of Al is 0.002 to
0.05%. In addition, the Al content is 0.005 to
It is preferably set to 0.03%.

N: 0.008% or less In the present invention, it is extremely important to control the N content to a low level. That is, since N has a large affinity for Ti, it not only easily bonds with Ti to form TiN to greatly deteriorate toughness, but also fixes Ti, so that when N is contained in a large amount, Means that the effect of improving the machinability of the carbosulfide of Ti cannot be sufficiently exhibited.
When the N content is 0.008% or less, the effect of the Ti carbosulfide described above is secured. In order to enhance the effect of Ti carbosulfide, the upper limit of the N content is preferably set to 0.006%.

Nd: 0 to 0.1% 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 increasing the strength and toughness of the steel as a fine ferrite-pearlite structure by making it finer. In order to surely obtain the above-mentioned effects, the content of Nd is preferably set to 0.005% or more. However, if the content exceeds 0.1%, Nd
₂ S ₃ itself is coarsened, resulting in a decrease in toughness. Therefore, the content of Nd is set to 0 to 0.1%. Note that N
A preferred upper limit of the d content is 0.08%.

V: 0 to 0.3% V may not be added. If added, it precipitates as fine nitrides or carbonitrides and has the effect of improving the strength of the steel, especially the 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: 0 to 0.05% 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, when the content exceeds 0.05%, not only the above-mentioned effect is saturated, but also coarse nitrides are formed and the tool is damaged,
This leads to reduced machinability. Therefore, the content of Nb is 0 to
0.05%.

Mo: 0 to 0.5% Mo may not be added. If added, ferrite
It has the effect of refining the pearlite structure to improve the strength of the steel, especially the fatigue strength. To ensure this effect,
The content of Mo 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 set to 0 to 0.5%.

Cu: 0 to 1.0% 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. However, the content is 1.0
%, The hot workability is deteriorated, and the precipitates are coarsened, so that 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%.

Pb: 0 to 0.50% Pb may not be added. If added, it has the effect of further increasing the machinability of the steel. To ensure this effect,
It is preferable that the content of Pb is 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: 0 to 0.01% Ca may not be added. If added, it has the effect of greatly improving the machinability of steel. To ensure this effect,
Preferably, the content of Ca is 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, the content of Ca is set to 0 to 0.01%.

Se: 0 to 0.5% Se need not be added. If added, it has the effect of further 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, the content of Se is set to 0.
-0.5%.

Te: 0 to 0.05% 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, if the content exceeds 0.05%, not only the above-mentioned effect is saturated, but rather coarse inclusions are formed and the fatigue strength is reduced. Further, the hot workability is remarkably deteriorated, so that the surface of the steel material is flawed. Therefore, the content of Te is set to 0 to 0.05%.

Bi: 0 to 0.4% Bi may not be added. If added, it has the effect of greatly improving the machinability of 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, Bi
Was set to 0 to 0.4%.

Incidentally, O (oxygen) as an impurity element
Forms hard oxide-based inclusions, which may damage the cutting tool during cutting and reduce machinability. In particular, if the O content exceeds 0.015%, the machinability may be significantly reduced. Therefore, in order to maintain good machinability, the content of O as an impurity element is preferably set to 0.015% or less. It is more preferable that the content of O be 0.01% or less.

(B) Structure of Steel Even if the steel has the above chemical composition, when it is cooled to room temperature (room temperature) after hot working, its structure is changed from so-called “low-temperature transformation products” such as bainite and martensite. In such cases, the machinability deteriorates. Further, in the cooling process after hot working, bending and residual stress due to transformation strain increase, which hinders the finishing process. For example, a process for correcting the bending is required, which leads to an increase in cost. Therefore, in order to obtain good machinability and reduce transformation strain, the main structure of the steel must be a ferrite-pearlite structure. If the ratio of the "low-temperature transformation product" in the structure is less than 10%, the deterioration of the machinability and the occurrence of bending and residual stress due to transformation strain do not become a serious problem. Therefore, in the present invention, it is specified that 90% or more of the structure is made of ferrite / pearlite. As a manufacturing method therefor, for example, a slab is heated to 1050 to 1300 ° C., then subjected to hot working such as hot forging, finished at a temperature of 900 ° C. or more, and then cooled at a rate of 60 ° C./min or less. Then, there is a process of air cooling or cooling to at least 500 ° C. In addition,
The chemical composition of (A) is designed so that if the steel material is cooled under the above conditions after hot working, a "low-temperature transformation product" exceeding 10% is not generated in the structure.

Incidentally, the volume fraction of ferrite in the structure is 20 to 70%, and the crystal grain size of ferrite is J
When the IS particle size number is 5 or more, the strength and machinability are particularly excellent.

Further, the maximum diameter of Ti carbosulfide is 10 μm.
Below, and when the amount is 0.05% or more in cleanliness, high machinability and high strength are obtained.
When the maximum diameter of the i-carbon sulfide is about 0.5 to 7 μm and the amount thereof is about 0.08 to 2.0% in terms of cleanliness, the machinability and strength are extremely excellent. In order to set the size and cleanliness of the carbosulfide of Ti to the above-mentioned values, it is important to prevent the generation of an excessive amount of Ti oxide. It is sufficient to sufficiently deoxidize with Al and finally add Ti.

The Ti carbosulfide described above is obtained by mirror polishing a test piece taken from a steel material and using the polished surface as a test surface with a magnification of 4.
When observed with an optical microscope at a magnification of 00 or more, it can be easily distinguished from other inclusions based on the color and shape. That is, when observed with an optical microscope under the above conditions, the "color" of the Ti carbosulfide is very light gray, and the "shape" is recognized as a granular shape (spherical shape) corresponding to JIS B-based inclusions. The detailed determination of Ti carbosulfide can also be performed by observing the test surface with a microscope having an analysis function such as EDX (energy dispersive X-ray analyzer). The cleanliness of the above-mentioned Ti carbosulfide was determined by a JIS G
The value measured by the method of 0555.

[0043]

EXAMPLES Steel having the chemical composition shown in Tables 1 to 6 was melted using a 150 kg vacuum melting furnace. In order to prevent the formation of Ti oxide, the size and cleanliness of the Ti carbosulfide were adjusted by adding Ti at the end after sufficiently deoxidizing with Si and Al and adding various elements.

Steels 1 to 5 in Table 1 and Steel 1 in Table 2
5 to 21, steels 26 to 30 in Table 3, steels 40 to 40 in Table 4, steels 51 to 57 in Table 5, and steels 62 to 66 in Table 6 are steels of the present invention and steels 6-1 in Table 1.
4, steels 22 to 25 in Table 2, steels 31 to 31 in Table 3
35, steels 41 to 50 in Table 4, steel 58 in Table 5
61 to 67 and steels 67 to 71 in Table 6 are steels of comparative examples in which any of the components was out of the range of the content specified in the present invention.

[0045]

[Table 1]

[0046]

[Table 2]

[0047]

[Table 3]

[0048]

[Table 4]

[0049]

[Table 5]

[0050]

[Table 6]

Next, these steels were heated to 1250 ° C. and then subjected to hot forging to finish at 1000 ° C.
mm round bar was prepared. The cooling conditions after hot forging were air-cooled or allowed to cool to 400 ° C. so that the cooling rate was 5 to 35 ° C./min.
Adjusted to consist of perlite.

15 mm from the surface of the round bar thus obtained.
From the position (R / 2 part position, R is the radius of the round bar)
No. 14A tensile test specimen, Ono-type rotary bending test specimen (parallel part diameter 8 mm and length 18.4 mm) and JIS
A No. 3 impact test piece (2 mm U notch Charpy test piece) was sampled, and its tensile strength, fatigue strength (fatigue limit) and toughness (absorbed energy at impact test) at room temperature were examined. or,
A test specimen was collected according to FIG. 3 of JIS G 0555, and a mirror-polished test surface having a width of 15 mm and a height of 20 mm was magnified by 40%.
Observation with an optical microscope at a magnification of 0 was performed to measure the cleanliness of the Ti carbosulfide while separating it from other inclusions. In addition, the maximum diameter of Ti carbosulfide was investigated by observation with an optical microscope at a magnification of 400 times.

The machinability was also evaluated by a drilling test. That is, a round bar having a diameter of 60 mm was cut into a 25 mm-length round bar, and a through-hole was made in the length direction. The number of through-holes when drilling was impossible due to abrasion of the cutting edge was counted. Was evaluated. The drilling conditions were as follows: using a JIS high-speed tool steel SKH51 φ5 mm straight shank drill, using a water-soluble lubricant, and feeding 0.15 m.
m / rev and the number of rotations were 980 rpm.

Tables 7 and 8 show the results of the various tests described above. 1 to 6 show the relationship between the fatigue strength and machinability of each steel.
7 and 8 show the relationship between the tensile strength and the toughness.
1 is for steels 1 to 14, and FIG.
3 is for steels 26 to 35, and FIG.
FIG. 5 summarizes the relationship between fatigue strength and machinability for steels 51 to 61 and FIG. 6 for steels 62 to 71. 7 shows the relationship between tensile strength and toughness for steels 1 to 35, and FIG. 8 shows the relationship between steels 36 to 71.

[0055]

[Table 7]

[0056]

[Table 8]

First, from Table 7 and FIGS. 1 to 3 and FIG. 7, it is clear that the steel of the present invention has good machinability and excellent strength (tensile strength and fatigue strength) and toughness. . Among the steels of the present invention, the durability ratio (fatigue strength / tensile strength) of steels 15 to 21 is as large as more than 0.45, and the balance between fatigue strength and tensile strength is particularly excellent.

On the other hand, the steel of the comparative example is inferior in at least one of tensile strength, fatigue strength, toughness (absorbed energy), and machinability (number of through holes).

Next, from Table 8 and FIGS. 4 to 6 and FIG.
It is evident that the steel of the present invention to which the steel of the present invention is added has good machinability, excellent strength (tensile strength and fatigue strength), and extremely excellent toughness. Among the steels of the present invention, the durability ratio of steels 51 to 57 exceeds 0.45, and the balance between fatigue strength and tensile strength is particularly excellent.

On the other hand, the steel of the comparative example is inferior in at least one of tensile strength, fatigue strength, toughness (absorbed energy), and machinability (number of through holes).

[0061]

The free-cutting non-heat treated steel of the present invention has excellent strength and toughness and excellent machinability, so that it can be used as a material for machine structural parts. This free-cutting non-heat treated steel excellent in strength and toughness can be manufactured relatively easily at low cost.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between fatigue strength and machinability of steels 1 to 14 used in Examples.

FIG. 2 is a diagram showing a relationship between fatigue strength and machinability of steels 15 to 25 used in Examples.

FIG. 3 is a diagram showing a relationship between fatigue strength and machinability of steels 26 to 35 used in Examples.

FIG. 4 is a diagram showing a relationship between fatigue strength and machinability of steels 36 to 50 used in Examples.

FIG. 5 is a diagram showing the relationship between the fatigue strength and machinability of steels 51 to 61 used in Examples.

FIG. 6 is a diagram showing the relationship between the fatigue strength and machinability of steels 62 to 71 used in Examples.

FIG. 7 is a diagram showing a relationship between tensile strength and toughness of steels 1 to 35 used in Examples.

FIG. 8 is a diagram showing a relationship between tensile strength and toughness of steels 36 to 71 used in Examples.

Claims (1)

[Claims] C .: 0.2 to 0.6% by weight, Si:
0.1-1.5%, Mn: 0.1-2.0%, P: 0.
01-0.07%, S: 0.01-0.2%, Cr:
0.02-2.0%, Ti: 0.04-1.0%, A
l: 0.002 to 0.05%, N: 0.008% or less,
Nd: 0 to 0.1%, V: 0 to 0.3%, Nb: 0 to 0%
0.05%, Mo: 0 to 0.5%, Cu: 0 to 1.0
%, Pb: 0 to 0.50%, Ca: 0 to 0.01%, S
e: 0 to 0.5%, Te: 0 to 0.05%, and Bi: 0
A free-cutting non-heat-treated steel having excellent strength and toughness, characterized in that the steel contains up to 0.4%, the balance being Fe and unavoidable impurities, and more than 90% of the structure being a ferrite-pearlite structure.