JP2004176177A - Steel superior in machinability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel superior in machinability which not only gives a tool a long life, but also has satisfactory properties of a cut surface. <P>SOLUTION: The steel superior in machinability comprises 0.005-0.2% C, 0.001-0.5% Si, 0.5-3.0% Mn, 0.001-0.2% P, 0.003-1.0% S, more than 0.005% but 0.05% or less B, 0.002-0.02% total N, 0.0005-0.035% total O and the balance Fe with unavoidable impurities; and has a pearlite area rate of 5% or less in a microstructure. <P>COPYRIGHT: (C)2004,JPO

Description

  The present invention relates to steel used for automobiles, general machines, and the like, and more particularly to steel excellent in machinability, which is excellent in tool life, cutting surface roughness, and chip disposal during cutting.

  General machines and automobiles are manufactured by combining various types of parts, and the parts are often manufactured through a cutting process from the viewpoint of required accuracy and manufacturing efficiency. At that time, cost reduction and improvement in production efficiency are required, and steel is also required to have improved machinability. In particular, conventionally, SUM23 and SUM24L have been developed with emphasis on machinability. It has been known that it is effective to add a machinability improving element such as S or Pb in order to improve machinability. However, some consumers avoid using Pb as an environmental load, and the amount of Pb used is being reduced.

  Until now, when Pb is not added, a method of forming inclusions that are soft under a cutting environment such as MnS like S and improving machinability has been used. However, so-called low-carbon lead free-cutting steel SUM24L contains the same amount of S as low-carbon sulfur free-cutting steel SUM23. Therefore, it is necessary to add a higher S amount than before. However, when a large amount of S is added, merely making MnS coarse does not only result in MnS distribution effective for improving machinability, but also causes a problem in production such as rolling flaws as a starting point of fracture in rolling and forging. Cause a lot. Further, in the case of the sulfur free-cutting steel based on SUM23, the component cutting edge easily adheres, and the cutting surface becomes uneven due to the falling of the component cutting edge and the chip separation phenomenon, and the surface roughness deteriorates. Therefore, from the viewpoint of machinability, there is a problem that accuracy is reduced due to deterioration of surface roughness. In terms of chip controllability as well, it is considered better if the chips are short and easy to separate, but the simple addition of S has a large ductility of the matrix, so that the matrix is not sufficiently separated and cannot be significantly improved.

  Further, elements other than S, such as Te, Bi, and P, are also known as machinability improving elements. However, even if machinability can be improved to some extent, cracks easily occur during rolling or hot forging. It is said that it is desirable to have as little as possible. (For example, see Patent Literature 1, Patent Literature 2, Patent Literature 3, and Patent Literature 4.)

JP-A-9-71840 Japanese Patent Application No. 2000-160284

  The present invention provides a steel which has both improved tool life and surface roughness while avoiding problems in rolling and hot forging, and has machinability equal to or higher than that of a conventional low-carbon lead free-cutting steel.

  Cutting is a breaking phenomenon that separates chips, and promoting it is one point. This effect is limited by simply increasing S. The present inventors have found that not only increasing the amount of S, but also enhancing the machinability by suppressing the unevenness of the cutting surface by extending the tool life by making the matrix brittle and facilitating fracture. did.

  The present invention has been made based on the above findings, and the gist is as follows.

  (1) By mass%, C: 0.005 to 0.2%, S: 0.5 to 1.0%, B: more than 0.005 to 0.05%, and Mn / S: 1. A steel excellent in machinability, having a pearlite area ratio of 5% or less in a microstructure of 2 to 2.8.

  (2) C: 0.005 to 0.2%, Si: 0.001 to 0.5%, Mn: 0.5 to 3.0%, P: 0.001 to 0.2%, S: 0 0.5 to 1.0%, B: more than 0.005 to 0.05%, total-N: 0.002 to 0.02%, total-O: 0.0005 to 0.035%, with the balance being the balance Is excellent in machinability, characterized by being composed of Fe and unavoidable impurities.

  (3) The steel according to (2), wherein the steel has a pearlite area ratio of 5% or less in a microstructure.

  (4) The machinability according to (2) or (3), wherein, in the steel, the ratio of Mn to S in the steel is Mn / S: 1.2 to 2.8 in mass%. Excellent steel.

  (5) The steel is, in mass%, V: 0.05 to 1.0%, Nb: 0.005 to 0.2%, Cr: 0.01 to 2.0%, Mo: 0. Machinability according to any one of (1) to (4), wherein one or more of 0.05 to 1.0% and W: 0.05 to 1.0% are contained. Excellent steel.

  (6) The steel is characterized in that the steel further contains one or two of Ni: 0.05 to 2.0% and Cu: 0.01 to 2.0% by mass% (1). The steel excellent in machinability according to any one of the items (1) to (5).

  (7) The steel is characterized in that the steel further contains one or two of Sn: 0.005 to 2.0% and Zn: 0.0005 to 0.5% by mass%. The steel excellent in machinability according to any one of the items (1) to (6).

  (8) The steel further contains, by mass%, Ti: 0.0005 to 0.1%, Ca: 0.0002 to 0.005%, Zr: 0.0005 to 0.1%, and Mg: 0. The steel excellent in machinability according to any one of (1) to (7), which contains one or more of 0003 to 0.005%.

  (9) The steel is, in mass%, one or more of Te: 0.0003 to 0.05%, Bi: 0.005 to 0.5%, Pb: 0.01 to 0.5%. The steel excellent in machinability according to any one of (1) to (8), which contains at least one kind.

  (10) The steel excellent in machinability according to any one of (1) to (9), wherein the steel is limited to Al: 0.015% or less.

  As described above, the present invention can be used for members for automobiles and members for general machinery because the present invention has characteristics of excellent tool life, cutting surface roughness, and chip disposability during cutting.

  The present invention embrittles the matrix to obtain sufficient machinability, especially good surface roughness, without adding lead, and to improve the lubrication of the tool / workpiece contact surface, It is characterized by adding a large amount of B. Furthermore, the amount of S is also added in a relatively large amount, and the ratio of the added amounts of Mn and S is precisely controlled in order to finely disperse them. Also, regarding the microstructure of the steel, the pearlite found in conventional carbon steel was controlled. That is, in the chemical component, the amount of C added is suppressed, and the precipitation of coarse pearlite is suppressed, or when large amounts of C are contained, the formation of coarse pearlite grains is suppressed by heat treatment. It is a steel with excellent machinability with reduced cracking.

  Next, the reasons for limiting the steel components specified in the present invention will be described.

  C has a significant effect on machinability because it relates to the basic strength of the steel material and the amount of oxygen in the steel. If the strength is increased by adding a large amount of C, the machinability decreases, so the upper limit was made 0.2%. On the other hand, it is necessary to control the amount of oxygen to an appropriate amount in order to prevent the formation of hard oxides that reduce machinability and to suppress the adverse effects of dissolved oxygen at high temperatures such as pinholes during the solidification process. If the amount of C is simply reduced too much by blowing, not only does the cost increase, but also a large amount of oxygen in the steel remains and causes problems such as pinholes. Therefore, the lower limit is set to 0.005% of C, which can easily prevent problems such as pinholes. A preferred lower limit of the amount of C is 0.05%.

  Excessive addition of Si generates a hard oxide and reduces machinability, but moderate addition softens the oxide and does not reduce machinability. The upper limit is 0.5%, above which hard oxides are produced. If the content is less than 0.001%, it becomes difficult to soften the oxide, and the cost is industrially high.

  Mn is necessary for fixing and dispersing sulfur in steel as MnS. In addition, it is necessary to soften oxides in steel and make the oxides harmless. The effect also depends on the amount of S added, but if it is 0.5% or less, the added S cannot be sufficiently fixed as MnS, and S becomes FeS and becomes brittle. When the amount of Mn increases, the hardness of the substrate increases, and machinability and cold workability decrease. Therefore, the upper limit is set to 3.0%.

  P increases the hardness of the base material in steel, and deteriorates not only cold workability but also hot workability and casting properties. Therefore, the upper limit of P must be set to 0.2%. On the other hand, the lower limit of elements that are effective in improving machinability is set to 0.001%.

  S bonds with Mn and exists as MnS inclusions. MnS improves machinability, but elongated MnS is one of the causes of anisotropy during forging. Large MnS should be avoided, but a large amount is preferable from the viewpoint of improving machinability. Therefore, it is preferable to finely disperse MnS. When Pb is not added, 0.5% or more must be added to improve the machinability over conventional free-cutting steel. On the other hand, if it exceeds 1%, not only the generation of coarse MnS is inevitable, but also cracks occur during production due to deterioration of casting properties and hot modification properties due to FeS or the like.

  B is effective in improving machinability when precipitated as BN. These effects are not remarkable at 0.005% or less, and even if added over 0.05%, the effects are saturated, and if too much BN is precipitated, the casting properties and the hot denaturing properties are rather deteriorated. Cracks occur inside. Therefore, the range is more than 0.005 to 0.05%.

  N (total-N) hardens steel in the case of solid solution N. In particular, in cutting, it hardens near the cutting edge due to dynamic strain aging and shortens the life of the tool, but also has the effect of improving the cutting surface roughness. In addition, BN is generated in combination with B to improve machinability. If the content is 0.002% or less, the effect of improving the surface roughness due to solid solution nitrogen and the effect of improving the machinability due to BN are not recognized. On the other hand, if the content exceeds 0.02%, a large amount of solute nitrogen is present, so that the tool life is rather shortened. In addition, bubbles are generated during casting, which causes flaws and the like. Therefore, in the present invention, the upper limit is set to 0.02% at which these adverse effects become remarkable.

  When O (total-O) is present in a free state, it becomes bubbles at the time of cooling and causes pinholes. Control is also required to soften oxides and suppress hard oxides harmful to machinability. Furthermore, when finely dispersing MnS, an oxide is used as a precipitation nucleus. If it is less than 0.0005%, MnS cannot be sufficiently finely dispersed, coarse MnS is generated, and mechanical properties are adversely affected. Therefore, the lower limit is made 0.0005%. Further, if the oxygen content exceeds 0.035%, bubbles are formed during casting and pinholes are formed. Therefore, the upper limit is set to 0.035% or less.

Next, the reason why the pearlite area ratio is set to 5% or less will be described. Generally, when a steel containing carbon is cooled from a temperature higher than the transformation point, a ferrite-pearlite structure is formed. For relatively small steel subject to C content of the present invention, after air cooling transformation point (A ₃ points) or higher, when etched with nital its internal and mirror-polished cut, micro like Figure 1 The tissue can be observed. The black grains are a composite structure of ferrite and cementite called pearlite. Usually, grains that appear black due to nital are harder than ferrite grains that appear white, and the ferrite grains are locally localized in the deformation / fracture behavior of steel. Shows different behavior. This hinders uniform deformation / rupture in the chip breaking behavior in cutting, and thus greatly contributes to the generation of a constituent cutting edge, and further deteriorates the surface roughness of the cut surface. Therefore, it is important to eliminate systematic nonuniformity caused by C as much as possible. Therefore, black particles etched with nital were regarded as pearlite particles, and if the number of pearlite particles was too large, the structure became nonuniform and the surface roughness deteriorated. Therefore, the area ratio was limited to 5% or less. FIG. 4 shows the relationship between the pearlite area ratio and the surface roughness.

  Here, the details of the measurement method will be described. The rolled or forged steel was cut into a longitudinal section (L section), and the resin-embedded sample was mirror-polished and nital etched. Of the materials that were black-etched with Nital, particles having a particle diameter (equivalent circle diameter) of 1 μm or more, excluding gray MnS, were analyzed with an image processing apparatus, and the area ratio was determined. At the time of image processing of area ratio measurement, adjust the image density by setting the "threshold" according to the pearlite that looks black, and eliminate grayish inclusions (MnS, etc.) from the screen, so that only pearlite can be measured. did. The minimum perlite recognized at this time is about 1 μm, but perlite smaller than 1 μm does not affect the machinability, so there is no effect even if it is not recognized.

In the present invention, one visual field of 0.2 mm ² (0.4 mm × 0.5 mm) was measured at a magnification of 400 times or more in 20 visual fields, and the pearlite area ratio was calculated for a total area of 4 mm ² .

  It is known that Mn / S has a great influence on hot ductility, and usually, unless Mn / S> 3, the productivity is greatly reduced. The cause is the formation of FeS. In the present invention, it has been found that the ratio can be further reduced to Mn / S: 1.2 to 2.8 in a low C and high S region. If Mn / S is 1.2 or less, a large amount of FeS is generated, and the hot ductility is extremely reduced, and the productivity is greatly reduced.

  FIG. 2 shows an example in which fine MnS in the case of Mn / S ≦ 2.8 and Mn / S> 2.8 is observed by a transmission electron microscope using a replica method. When Mn / S> 2.8, only coarse MnS as shown in FIG. 2B is obtained, and the surface roughness cannot be reduced. On the other hand, when Mn / S is regulated to 1.2 to 2.8, fine MnS is generated as shown in FIG.

  The number of the fine MnS can be increased by repeating heating at 900 ° C. or more after continuous casting or casting by ingot.

  In addition, MnS means not only pure MnS but also an inclusion mainly containing MnS, and sulfides such as Fe, Ca, Ti, Zr, Mg and REM coexist with MnS by solid solution or bonding. And inclusions such as MnTe in which an element other than S forms a compound with Mn to form a compound with MnS to form a solid solution / bond and coexist with the MnS, or includes the above-mentioned inclusions precipitated with an oxide as a nucleus. In the chemical formula, Mn sulfide-based inclusions that can be expressed as (Mn, X) (S, Y) (here, X: an element other than Mn and a sulfide-forming element other than Y: S that binds to Mn) are collectively referred to. That's what they say.

  Next, in the present invention, one or more of V, Nb, Cr, Mo, W, Ni, Sn, Zn, Ti, Ca, Zr, Mg, Te, Bi, and Pb, in addition to the components described above. Can be added as needed.

  V forms carbonitrides and can strengthen the steel by secondary precipitation hardening. If it is less than 0.05%, there is no effect on increasing the strength, and if it exceeds 1.0%, a large amount of carbonitride precipitates and mechanical properties are rather impaired.

  Nb also forms carbonitrides and can strengthen the steel by secondary precipitation hardening. If it is less than 0.005%, there is no effect on increasing the strength, and if it exceeds 0.2%, a large amount of carbonitride precipitates and mechanical properties are rather impaired.

  Cr is an element that imparts hardenability and temper softening resistance. Therefore, it is added to steels requiring high strength. In that case, 0.01% or more must be added. However, if added in a large amount, Cr carbides are formed and embrittled, so the upper limit is 2.0%.

  Mo is an element that imparts temper softening resistance and improves hardenability. If the content is less than 0.05%, the effect is not recognized, and even if added over 1.0%, the effect is saturated. Therefore, the addition range is set to 0.05% to 1.0%.

  W forms carbides and can strengthen the steel by secondary precipitation hardening. If it is less than 0.05%, there is no effect on increasing the strength, and if it exceeds 1.0%, a large amount of carbide will precipitate and the mechanical properties will be impaired.

  Ni is effective in strengthening ferrite, improving ductility, improving hardenability, and improving corrosion resistance. If the content is less than 0.05%, the effect is not recognized. If the content exceeds 2.0%, the effect is saturated in terms of mechanical properties. Therefore, the upper limit is set.

  Cu strengthens ferrite and is also effective for improving hardenability and corrosion resistance. If it is less than 0.01%, the effect is not recognized. Even if it is added more than 2.0%, the effect is saturated in terms of mechanical properties. In particular, it is preferable to add it at the same time as Ni because it reduces the hot ductility and easily causes flaws during rolling.

  Sn embrittles ferrite, prolongs tool life, and is effective in improving surface roughness. If the content is less than 0.005%, the effect is not recognized. If the content exceeds 2.0%, the effect is saturated in terms of mechanical properties, so the upper limit is set.

  Zn has the effect of making the ferrite brittle, extending the tool life, and improving the surface roughness. If the content is less than 0.0005%, the effect is not recognized. Even if the content exceeds 0.5%, the effect is saturated in terms of mechanical properties. Therefore, the upper limit is set.

  Ti also forms carbonitrides and strengthens the steel. It is also a deoxidizing element, and can improve machinability by forming a soft oxide. If the content is less than 0.0005%, the effect is not recognized, and even if added over 0.1%, the effect is saturated. Further, Ti becomes a nitride even at a high temperature and suppresses the growth of austenite grains. Therefore, the upper limit is set to 0.1%. Note that Ti combines with N to form TiN, but TiN is a hard substance and reduces machinability. Further, the amount of N required to produce BN effective for improving machinability is reduced. Therefore, the addition amount of Ti is preferably 0.010% or less.

  Ca is a deoxidizing element, generates a soft oxide and not only improves the machinability, but also dissolves in MnS to reduce its deformability, and the MnS shape is formed even by rolling or hot forging. It works to control distraction. Therefore, it is an element effective for reducing anisotropy. If the content is less than 0.0002%, the effect is not remarkable. Even if 0.005% or more is added, not only the yield is extremely deteriorated, but also a large amount of hard CaO is generated, and the machinability is rather reduced. Therefore, the addition range was defined as 0.0002 to 0.005%.

Zr is a deoxidizing element and generates an oxide. The oxide serves as a precipitation nucleus of MnS, and is effective in fine and uniform dispersion of MnS. Further, it has the function of dissolving in MnS to reduce its deformability and suppressing the elongation of the MnS shape even in rolling or hot forging. Therefore, it is an element effective for reducing anisotropy. If it is less than 0.0005%, the effect is not remarkable. Even if it is added at 0.1% or more, not only the yield is extremely deteriorated, but also hard ZrO ₂ , ZrS and the like are generated in large amounts, and the machinability is rather reduced. Lower. Therefore, the addition range was defined as 0.0005 to 0.1%. In order to achieve fine dispersion of MnS, it is preferable to add Zr and Ca in combination.

  Mg is a deoxidizing element and generates an oxide. The oxide becomes an nucleus for precipitation of MnS, has an effect on fine and uniform dispersion of MnS, and is an element effective in reducing anisotropy. If it is less than 0.0003%, the effect is not remarkable. Even if 0.005% or more is added, the yield is extremely deteriorated and the effect is saturated. Therefore, the addition range was defined as 0.0003 to 0.005%.

  Te is a machinability improving element. In addition, by producing MnTe or coexisting with MnS, it has a function of reducing the deformability of MnS and suppressing the elongation of the MnS shape. Therefore, it is an element effective for reducing anisotropy. This effect is not recognized at less than 0.0003%, and the effect is saturated at more than 0.05%.

  Bi and Pb are elements that are effective in improving machinability. The effect is not recognized at 0.005% or less, and even if added over 0.5%, not only the machinability improving effect is saturated, but also the hot forging property is reduced, which is likely to cause flaws.

Al is a deoxidizing element and forms Al ₂ O ₃ and AlN in steel. However, since Al ₂ O ₃ is hard, it causes tool damage during cutting and promotes wear. Therefore, the content is limited to 0.015% or less which does not generate a large amount of Al ₂ O ₃ . In particular, when giving priority to the tool life, 0.005% or less is preferable.

  The effects of the present invention will be described with reference to examples. Table 1, Table 2 (continuation 1 of Table 1), Table 3 (continuation 2 of Table 1), Table 4 (continuation 3 of Table 1), Table 5 (continuation 4 of Table 1), Table 6 (continuation of Table 1) No. 5 among the test materials shown in 5). 13 is a 270-t converter, and the others are melted in a 2-t vacuum melting furnace, then disassembled and rolled into billets, and further rolled to φ60 mm.

  In the heat treatment section of the table, the examples described as normalization were held at 920 ° C. for 10 minutes or more and air-cooled. The invention example marked as QT was put into a water tank at the rear end of the rolling line from 920 ° C., rapidly cooled, and then kept at 700 ° C. for 1 hour or more by annealing. Thereby, the pearlite area ratio was adjusted. Even in the invention examples, those having a low C content can reduce the pearlite area ratio even in normalizing.

  In the evaluation of the machinability of the materials shown in Examples 1 to 86 in Tables 1 to 6, cutting conditions are shown in Table 7 in a drilling test. The machinability was evaluated at the highest cutting speed (so-called VL1000, unit: m / min) capable of cutting to a cumulative hole depth of 1000 mm.

  Furthermore, the cutting surface roughness indicating the surface quality in cutting was evaluated. The cutting conditions are shown in Table 8, and the outline of the evaluation method (hereinafter referred to as plunge cutting test) is shown in FIG. In the plunge cutting test, the tool repeats cutting for a short time. In a single cut, the tool does not move in the longitudinal direction of the work material but moves toward the center of the rotating work material, so the tool is pulled out after a short cut, but the shape is basically the tool Is transferred to the surface of the work material. The surface roughness of the transferred cutting surface is affected by the attachment of the structural cutting edge and wear damage of the tool. The surface roughness was measured with a surface roughness meter. Ten-point surface roughness Rz (μm) was used as an index indicating the surface roughness.

  Inventive Examples 1 to 75 all had better drill tool life than Comparative Examples 76 to 86, and had good surface roughness in plunge cutting. This is considered to be because the ferrite was locally embrittled by B and the surface was created smoothly, so that good surface roughness was obtained.

  Although the effect of improving the surface roughness is remarkable when S is more than 0.5%, even when the amount of S is smaller than that, the effect on the chip disposition was observed.

  Further, even when the ratio of Mn to S is about 3, which is often found in conventional steels, the effect is recognized. However, when Mn / S is reduced, the tool life is further improved and the surface roughness is also improved. This is considered to be because fine MnS is finely dispersed in ferrite in an environment where a large amount of B is added, and effectively functions on both the lubricating effect and the embrittlement effect. However, when Mn / S is too small as in Example 85, FeS is generated, so that rolling cracks occur. In the evaluation of the present invention, Example 85 was not able to be evaluated at all, such as machinability, due to rolling cracks. Therefore, the evaluation results are not shown in the table.

  Even when the amount of C is slightly changed (Tables 1 to 6, Examples 37 to 75), a good tool life and a cut surface roughness can be obtained by adding a large amount of B and controlling the pearlite area ratio. I was able to.

  Regarding the chip handling property, it is preferable that the chip has a small curvature at the time of curling or the chip is cut. Accordingly, chips that were continuously curled and extended longer than three turns with a radius of curvature exceeding 20 mm were regarded as defective. If the number of windings was large and the radius of curvature was small, or if the chip length did not reach 100 mm even if the radius of curvature was large, it was regarded as good.

4 is a micrograph showing a ferrite-pearlite structure of a steel according to the present invention. (A) is a micrograph showing the state of fine dispersion of MnS according to the present invention, and (b) is a micrograph showing the state of existence of coarse MnS in conventional steel. (A), (b) is a figure which shows a plunge cutting test method. It is a figure which shows the relationship between a pearlite area ratio and surface roughness.

Claims (10)

  By mass%, C: 0.005 to 0.2%, S: 0.5 to 1.0%, B: more than 0.005 to 0.05%, and Mn / S: 1.2 to 2 0.8, the steel having excellent machinability, characterized in that the pearlite area ratio in the microstructure is 5% or less. C: 0.005 to 0.2%,
Si: 0.001 to 0.5%,
Mn: 0.5-3.0%,
P: 0.001-0.2%,
S: 0.5-1.0%,
B: more than 0.005 to 0.05%,
total-N: 0.002 to 0.02%,
total-O: 0.0005 to 0.035%
A steel excellent in machinability, characterized by containing Fe and the balance being Fe and inevitable impurities.   The steel with excellent machinability according to claim 2, wherein the steel has a pearlite area ratio of 5% or less in a microstructure.   The steel with excellent machinability according to claim 2 or 3, wherein the steel has a ratio of Mn / S in the steel of Mn / S: 1.2 to 2.8 in mass%.   The steel further contains, by mass%, V: 0.05 to 1.0%, Nb: 0.005 to 0.2%, Cr: 0.01 to 2.0%, Mo: 0.05 to 1 The steel with excellent machinability according to any one of claims 1 to 4, wherein the steel contains one or more of 0.0% and W: 0.05 to 1.0%.   6. The steel according to claim 1, wherein the steel further contains one or two of Ni: 0.05 to 2.0% and Cu: 0.01 to 2.0% by mass%. The steel excellent in machinability according to any one of the above items.   7. The steel according to claim 1, wherein the steel further contains one or two of Sn: 0.005 to 2.0% and Zn: 0.0005 to 0.5% by mass%. The steel excellent in machinability according to any one of the above items.   The steel further contains, by mass%, Ti: 0.0005 to 0.1%, Ca: 0.0002 to 0.005%, Zr: 0.0005 to 0.1%, Mg: 0.0003 to 0%. The steel having excellent machinability according to any one of claims 1 to 7, comprising one or more of 0.005%.   The steel further contains, by mass%, one or more of Te: 0.0003 to 0.05%, Bi: 0.005 to 0.5%, and Pb: 0.01 to 0.5%. The steel excellent in machinability according to any one of claims 1 to 8, wherein the steel is contained.   The steel according to any one of claims 1 to 9, wherein the steel is limited to Al: 0.015% or less.

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