JP3411686B2 - Graphite composite free-cutting steel - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel for machine structure, which is used as a material for machine parts, that is, a graphite composite free-cutting steel, and is developed especially for further improvement in machinability.

[0002]

2. Description of the Related Art Among steel materials used for machine parts, those having a predetermined shape by cutting are required to have excellent machinability. Regarding such steel materials, conventionally, the machinability has been improved by adding free-cutting elements such as Pb, Te, Bi, S and Ca to carbon steel for machine structural use alone or in combination. Above all, Pb is used as a free-cutting element because it does not cause deterioration of the mechanical properties of steel materials due to its addition and is cheaper than Te and Bi.

However, since Pb is generally harmful to the human body, large-scale exhaust equipment is required not only in the manufacturing process of steel products but also in the manufacturing process of machine parts using the same, and it is also necessary to recycle steel products. There were also many problems. Therefore, it has been desired in the past to develop a steel material that does not contain Pb or has a machinability comparable to that of Pb-added steel even with a smaller addition amount. On the other hand, research on free-cutting steel containing no Pb has recently progressed, and graphite steel has been attracting attention as an alternative material.

As free-cutting steels using such graphite, for example, the steels disclosed in JP-A-49-67816, JP-A-49-103817 and JP-A-50-96416 are known. These steel materials are intended to improve the machinability without using Pb by making C in the steel exist as graphite and utilizing the notch and the lubricating effect. However, the steel materials produced by these conventional techniques have a problem that the graphite grains are coarse and stable machinability cannot be obtained. Moreover, in order to graphitize C in the steel, quenching as a pretreatment is indispensable in all cases, which is not necessarily economical.

[0005]

SUMMARY OF THE INVENTION The present invention aims to develop a technique capable of advantageously overcoming the above-mentioned problems of the prior art. The graphite grains are made finer and a small amount of free-cutting element is added. The purpose is to propose a graphite composite free-cutting steel having significantly better machinability than the conventional free-cutting steel containing free-cutting elements. Another object of the present invention is to propose a graphite composite free-cutting steel having excellent machinability, which does not require quenching as a pretreatment during graphitization.

[0006]

[Means for Solving the Problems] As a result of intensive studies to solve the above problems, the inventors have properly controlled the amounts of Ti and N in steel, Ti / N, and Si, Mn
By adjusting the amounts of O and O, and adding a small amount of Pb, Se, Ca, and Te in combination, the machinability is significantly improved, and even without quenching as a pretreatment. We have found that it is possible to perform graphitization while hot rolling. The present invention is based on the above findings.

That is, the gist of the present invention is as follows. (1) C: 0.1 to 1.5 wt%, Si: 0.5 to 2.0 wt%, Mn: 0.1 to 2.0 wt%, Ti: 0.005 to 0.05 wt%, N: 0.0015 wt to 0.0150 wt%, O: 0.0030 wt% or less , And the above Ti and N are contained to satisfy Ti / N: 2.5 to 4.0, Pb: 0.03 to 0.3 wt%, Bi: 0.01 to 0.3 wt%, Te: 0.002 to O.5 wt%, Se: 0.003 to 0.10 wt%, P: 0.004 to 0.15 wt%, S: 0.005 to 0.25 wt% and Ca: 0.0002 to 0.30 wt% Any one or more selected from the rest, and the balance Graphite composite free-cutting steel with excellent machinability, which is composed of a composition of Fe and inevitable impurities and has a structure mainly composed of ferrite and graphite.

(2) C: 0.1 to 1.5 wt%, Si: 0.5 to 2.0 wt%, Mn: 0.1 to 2.0 wt%, Ti: 0.005 to 0.05 wt%, N: 0.0015 wt to 0.0150 wt%, O: 0.0030 wt% or less, and the above Ti and N satisfy Ti / N: 2.5 to 4.0 and further contain Pb: 0.03 to 0.3 wt%, Bi: 0.01 to 0.3 wt%, Te: 0.002 to O. 5 wt%, Se: 0.003 to 0.10 wt%, P: 0.004 to 0.15 wt%, S: 0.005 to 0.25 wt% and Ca: 0.0002 to 0.30 wt% and any one or more selected from , A graphite composite free-cutting steel excellent in machinability, further containing REM: 0.0005 to 0.2 wt%, the balance being composed of Fe and unavoidable impurities, and the structure being mainly composed of ferrite and graphite.

(3) C: 0.1 to 1.5 wt%, Si: 0.5 to 2.0 wt%, Mn: 0.1 to 2.0 wt%, Ti: 0.005 to 0.05 wt%, N: 0.0015 wt to 0.0150 wt%, O: 0.0030 wt% or less, and the above Ti and N satisfy Ti / N: 2.5 to 4.0 and further contain Pb: 0.03 to 0.3 wt%, Bi: 0.01 to 0.3 wt%, Te: 0.002 to O. 5 wt%, Se: 0.003 to 0.10 wt%, P: 0.004 to 0.15 wt%, S: 0.005 to 0.25 wt% and Ca: 0.0002 to 0.30 wt% and any one or more selected from , Ni: 0.10-3.0 wt%, Cu: 0.10-3.0 wt%, Cr: 0.05-1.0 wt%, Mo: 0.05-0.5 wt%, V: 0.05-0.5 wt% and Nb: 0.005-0.05 wt%, A graphite composite having excellent machinability, characterized in that it contains any one or more selected from the group consisting of ferrite and inevitable impurities in the balance, and the structure is mainly composed of ferrite and graphite. Free cutting steel.

(4) C: 0.1 to 1.5 wt%, Si: 0.5 to 2.0 wt%, Mn: 0.1 to 2.0 wt%, Ti: 0.005 to 0.05 wt%, N: 0.0015 wt to 0.0150 wt%, O: 0.0030 wt% or less, and the above Ti and N satisfy Ti / N: 2.5 to 4.0 and further contain Pb: 0.03 to 0.3 wt%, Bi: 0.01 to 0.3 wt%, Te: 0.002 to O. 5 wt%, Se: 0.003 to 0.10 wt%, P: 0.004 to 0.15 wt%, S: 0.005 to 0.25 wt% and Ca: 0.0002 to 0.30 wt% and any one or more selected from , REM: 0.0005 to 0.2 wt%, Ni: 0.10 to 3.0 wt%, Cu: 0.10 to 3.0 wt%, Cr: 0.05 to 1.0 wt%, Mo: 0.05 to 0.5 wt%, V: 0.05 ~ 0.5 wt% and Nb: 0.005 to 0.05 wt%, containing one or more selected from the group consisting of Fe and unavoidable impurities, and the structure mainly consisting of ferrite and graphite. Characterized by Graphite composite free-cutting steel with excellent machinability.

(5) However, in the above (1) to (4), the components (Pb, Bi, Te, Se, P, S, Ca) selectively added are added.
Regarding the above, it is recommended to add the following combinations within the range of the above composition. 0.03-0.3 wt% Pb- (Bi, Te, Se, P, S and Ca
0.01 to 0.3 wt% Bi- (any one or more of Te, Se, P, S and Ca) 0.002 to O.5 wt% Te- (any of Se, P, S and Ca) 0.003 to 0.10wt% Se- (any one or more of P, S and Ca) 0.004 to 0.15wt% P- (any one or more of S and Ca) 0.005 to 0.25wt% S-Ca

(6) In addition, the components (Ni, Cu, Cr, Mo, V, Nb) selectively added in the above (3) and (4) are as follows in the range of the above composition. It is recommended to add them in such a combination. 0.10-3.0 wt% Ni- (any one or more of Cu, Cr, Mo, V and Nb) 0.1-3.0 wt% Cu- (any one or more of Cr, Mo, V and Nb) 0.05-1.0 wt % Cr- (any of Mo, V and Nb 1
Type or more) 0.05 to 0.5 wt% Mo- (any one or more of V and Nb) 0.05 to 0.5 wt% V-Nb

[0013]

The basic idea of the present invention is that in low C steel,
It is a finding that fine graphite particles can be rapidly formed without quenching as a pretreatment when Ti, N and Ti / N are properly controlled. The reason for this is not necessarily clarified, but it is considered that a large number of fine TiN are formed by Ti and N, and this TiN provides a nucleus for crystallization of graphite during annealing. The reason why the fine graphite particles as described above improve the machinability is considered to be as follows. That is, the cutting of graphite steel proceeds by a mechanism in which minute cracks are generated at the interface between the graphite and the matrix phase in the steel material in the region where the work material receives shear stress from the tool, and the cracks are connected.
Therefore, the finer the graphite particles and the smaller the intervals between the graphite particles,
The number of places where cracks are generated is small, the distance between cracks is small, and it is easy to connect the cracks to each other, thus improving machinability. In addition, in the present invention, Pb, Bi, Te, Se, P,
The machinability can be further improved by adding one or more of S and Ca.

As described above, according to the present invention, it is possible to carry out the graphitization treatment as it is after the hot rolling, and it is possible to carry out the quenching as the pretreatment which is conventionally indispensable for promoting the graphitization. As compared with the conventional Pb free-cutting steel or Pb composite free-cutting steel, it is possible to obtain a steel for machine structural use having a better machinability. In the present invention, from the viewpoint of free-cutting property based on the lubricating action, it is desirable that the structure contains at least 0.3% by volume of the graphite phase.

Next, in the present invention, the reason why each component composition based on the above technical idea is limited will be described. C: 0.1 to 1.5 wt% C is an element that is essential not only for forming a graphite phase but also for ensuring the strength of mechanical parts. If its content is less than 0.1 wt%, its effect is small, while 1.5 wt%
If it is contained in a large amount in excess of 0.1%, the hot rolling property deteriorates, so the content was limited to the range of 0.1 to 1.5 wt%. 0.2 to 0.8
It is preferably in the range of wt%.

Si: 0.5-2.0 wt% Si not only acts as a deoxidizer during the melting of steel,
Since it also has the function of destabilizing iron carbide in steel and promoting graphitization, it is positively added. If this content is less than 0.5 wt%, its effect is poor, while if it exceeds 2.0 wt%, decarburization increases during hot rolling and the mechanical properties after quenching and tempering deteriorate. The content was set to be in the range of wt%. It is preferable to set it in the range of 0.8 to 1.7 wt%.

Mn: 0.1-2.0 wt% Mn is an element which is also useful in securing the strength of the steel material and forms MnS, MnSe and MnTe to contribute to the improvement of machinability. If the content is less than 0.1% wt, the effect is small, while if it exceeds 2.0% by weight, graphitization is hindered. Therefore, the content is set to 0.5 to 2.0 wt%. It is preferable that the content is in the range of 0.2 to 0.8 wt%.

Ti: 0.005 to 0.05 wt% Ti forms carbonitrides and suppresses the growth of γ grains in the heating process of hot rolling, thereby refining the structure after hot rolling and graphitizing. Promote. In particular, TiN formed by combining with N in steel acts as a nucleus of crystallization of graphite,
It has an important role of contributing to the miniaturization of graphite grains and the improvement of the graphitization rate. Also, due to the formation of TiN, B
It also promotes hardenability. Further, Ti forms TiC and precipitates in ferrite to contribute to the improvement of strength. Content is
If it is less than 0.005 wt%, its effect is poor, while 0.0
If added in excess of 5 wt%, the toughness will deteriorate, so 0.005
It was limited to the range of ~ 0.05wt%. 0.01-0.03 wt
It is preferably in the range of%.

N: 0.0015 wt-0.0150 wt% N forms TiN and acts as a nucleus for crystallization to promote graphitization. Further, solid solution N improves machinability by dynamic strain aging. If the content is less than 0.0015 wt%, the effect is poor. On the other hand, if the content exceeds 0.0150 wt%, the hot workability deteriorates and causes cracking and flaws in the steel.
The range is from wt to 0.0150 wt%. 0.0020 to 0.0080wt
It is preferably in the range of%.

O: 0.0030 wt% or less O forms oxide-based non-inclusions, deteriorates the fatigue strength of mechanical parts, and deteriorates workability in hot and cold. Therefore, it is desirable to reduce O as much as possible, but the content of O up to 0.0030 wt% is allowed. 0.00
It is preferably in the range of 15 wt% or less.

Ti / N: 2.5 to 4.0 Ti / N is a necessary requirement for refining graphite. That is, the refinement of graphite is achieved by the refinement of TiN, and the refinement of TiN depends on Ti / N. The stoichiometric ratio of Ti / N is ideally 3.43, but if the Ti / N ratio is in the range of 2.5 to 4.0, the graphite is sufficiently miniaturized.

Pb: 0.03 to 0.3 wt% Pb is melted by the processing heat during cutting and improves the machinability by the liquid lubrication effect. To obtain this effect, it is necessary to add 0.03 wt% or more, but if it exceeds 0.3 wt%, graphitization is hindered and machinability is rather reduced, so 0.03 to 0.30 wt%
And The range of 0.06 to 0.2 wt% is preferable.

Bi: 0.01 to 0.3 wt% Bi improves the machinability due to the liquid lubrication effect, like Pb. The effect is obtained by adding 0.01 wt% or more, but 0.3
If it exceeds wt%, graphitization is hindered and machinability is rather deteriorated, so 0.01 to 0.3 wt% is set. 0.05-0.2 wt
It is preferably in the range of%.

Te: 0.002-0.5 wt% Te forms MnTe, which acts as a chip breaker during cutting and improves machinability. This effect is 0.002 wt
Although it can be obtained with a content of 0.5% or more, if it exceeds 0.5 wt%, it hinders graphitization and rather reduces machinability.
0.002 to 0.5 wt% In addition, it is preferable to set it in the range of 0.01 to 0.03 wt%.

Se: 0.003 to 0.10 wt% Se forms MnSe, which acts as a chip breaker during cutting and also acts as a core for graphitization to promote graphitization and improve machinability. To get this effect,
A content of 0.003 wt% or more is required, but even if added in excess of 0.10 wt%, the effect will saturate, so 0.003 to 0.10 wt
%. In addition, it is preferable to set it in the range of 0.010 to 0.035 wt%.

P: 0.004 to 0.15 wt% P is an element that improves the machinability by hardening the ferrite phase. To obtain this effect, it is necessary to add 0.004 wt% or more, but if it exceeds 0.15 wt%, graphitization is hindered and machinability is rather reduced, so 0.004 to 0.15
wt% In addition, it is preferable to set it in the range of 0.020 to 0.10 wt%.

S: 0.005-0.25 wt% S forms MnS and acts as a chip breaker to improve machinability, and also acts as a nucleus during graphite formation to promote graphitization. Improves machinability. It also reacts with REM to form sulfides such as (La, Ce) S, which serve as nuclei for graphitization and improve machinability. If the content is less than 0.005 wt%, the effect of addition is poor, while if it exceeds 0.25 wt%, the effect saturates.
The content should be in the range of 005 to 0.25 wt%. In addition,
It is preferably in the range of 0.020 to 0.075 wt%.

Ca: 0.0002 to 0.30 wt% Ca forms Ca-based oxides, which act as nuclei for graphitization and promote graphitization. If the added amount is less than 0.0002 wt%, the effect is poor, while if added over 0.30 wt%, a large amount of oxide-based nonmetallic inclusions are formed, and the fatigue strength of mechanical parts is reduced. Therefore, its content is 0.
0002 to 0.30 wt%. It should be noted that the range of 0.0010 to 0.0100 wt% is preferable.

Although the basic components have been described above, in the present invention, REM is further used mainly for promoting graphitization and finer graphite, and Ni is mainly used for improving strength.
One or more selected from Cu, Cr, Mo, V and Nb can be added.

REM: 0.0005 to 0.2 wt% REM such as La, Ce combines with S to form a sulfide, which acts as a nucleus for crystallization of graphite. This shortens the time required for graphitization and makes the graphite particles extremely fine. If the added amount is less than 0.0005 wt%, the effect is poor,
On the other hand, even if it exceeds 0.2 wt%, the effect will be saturated, so 0.0005 ~
0.2 wt%, preferably 0.01 to 0.05 wt%.

Ni: 0.10 to 3.0 wt% Ni improves the strength and strength of steel by strengthening the structure by quenching and tempering, and in the case of a complex addition with Cu, strengthening the precipitation of ferrite phase to improve the strength and strength. It is not only useful but also contributes effectively to the promotion of graphitization, but if it is less than 0.10 wt%, its effect is poor, and if it exceeds 3.0 wt%, its effect saturates. The range is%. In addition, it is preferable to set it in the range of 0.5 to 2.0 wt%.

Cu: 0.10 to 3.0 wt% Cu enhances the hardenability of steel and strengthens the structure by quenching and tempering treatment, and also contributes to the strength improvement by the precipitation strengthening in the combined addition with Ni, and is also graphitized. It is also an element useful for promoting However, if the content is less than 0.10 wt%, the addition effect is poor, while if it is added over 3.0 wt%, the hot deformability deteriorates.
The range is%. It is preferable that the content is in the range of 0.5 to 1.5 wt%.

Cr: 0.05 to 1.0 wt% Cr enhances the hardenability of steel and contributes to the strength improvement by quenching and tempering treatment, but if it is less than 0.05 wt%, the addition effect is poor, and if it exceeds 1.0 wt%. Stabilization of cementite hinders graphitization, so the range is 0.05 to 1.0 wt%. It should be noted that the range of 0.05 to 0.3 wt% is preferable.

Mo: 0.05 to 0.5 wt% Mo enhances the hardenability of steel and contributes to the strength improvement by quenching and tempering treatment, but if it is less than 0.05 wt%, the addition effect is poor, and if it exceeds 0.5 wt%, it is added. Stabilization of cementite hinders graphitization, so the range is limited to 0.05 to 0.5 wt%. In addition, it is preferable to set it in the range of 0.1 to 0.3 wt%.

V: 0.05 to 0.5 wt% V enhances the hardenability of steel and contributes to the strength improvement by quenching and tempering. It also contributes to the acceleration of graphitization by miniaturizing the structure after hot rolling. If the content is less than 0.05 wt%, the effect of addition is poor, while if added over 0.5 wt%, the effect is saturated, so the range is 0.05 to 0.5 wt%. In addition, it is preferable to set it in the range of 0.1 to 0.3 wt%.

Nb: 0.005 to 0.05 wt% Nb contributes to improving the hardenability of steel and improving the strength by quenching and tempering, and the precipitation of fine Nb carbonitrides. It also contributes to the acceleration of graphitization by miniaturizing the structure after hot rolling. If the content is less than 0.005 wt%, the effect of addition is poor, while if added over 0.05 wt%, the effect is saturated, so the range is from 0.005 to 0.05 wt%.
It should be noted that the range of 0.01 to 0.04 wt% is preferable.

By adjusting the composition range of the above components, it is possible to obtain a steel material having a metal structure mainly composed of ferrite and graphite and having excellent machinability, without the need for quenching as a pretreatment. Further, in the present invention, not only the component composition,
The metal structure is important, and as described above, it is necessary to mainly have the structure of ferrite and graphite. This is because the lubricating effect of graphite suppresses the temperature rise of the cutting tool during cutting,
The purpose of this is to improve the life of the cutting tool.
This is because it is an essential requirement for this invention. Here, the graphitization degree of C in steel is preferably 50% or more.

Next, the manufacturing method of the present invention will be described. First, regarding the production of the raw material, it is melted in a conventionally known converter, electric furnace, etc., and then made into a slab by a continuous casting method or an ingot-casting method. Then, it can be manufactured by hot rolling into a predetermined shape and then graphitization annealing to precipitate a predetermined amount of graphite phase in the metal structure. The steel of the present invention is machined into a predetermined part shape and then used as a machine part. The product may be subjected to a nitriding treatment.

Here, as hot rolling conditions, since hot workability is deteriorated by free cutting elements, heating at 1000 ° C. or higher and 85
It is desirable to carry out rolling at 0 ° C or higher. As the heat treatment for graphitization, it is sufficient to maintain the temperature at 600 ° C. to Ac ₁ for 5 to 30 hours. However, as free-cutting elements Te, P, B
When elements such as i and Pb are added alone, the treatment time should be prolonged within the above range.

[0040]

EXAMPLES Steels having the chemical compositions shown in Tables 1 and 2 were converted into converters,
It was melted by vacuum degassing, made into bloom by continuous casting, then hot-rolled into a 150 mm square billet, and further billet-rolled to obtain a 52 mmφ steel bar. Then 700
After heating at 0 ° C for 10 hours, a graphitization treatment by air cooling was performed, and then the graphitization ratio, hardness and machinability were investigated. Machinability test, using high speed tool steel SKH4, cut: 2
mm, feed rate: 0.25 mm / rev. and cutting rate: 70 m / min
It was carried out under the conditions described above, and the time until the cutting became impossible was evaluated as the tool life. Table 3 shows the obtained investigation results.

[0041]

[Table 1]

[0042]

[Table 2]

[0043]

[Table 3]

Nos. 1 to 29 in Tables 1 and 2 are invention steels, No. 30 to
No. 44 is a comparative steel, No. 45 is a Pb-SP composite free-cutting steel (SAE standard 12L14 equivalent steel), and NO. 46 is a JIS S45C free-cutting steel with Pb added.

As is clear from Table 3, in all the invention steels, C in the steel is graphitized by 50% or more after the graphitization treatment. Therefore, the invention steel has excellent machinability and has a tool life of 57.4 m.
It shows a very good life above in. In contrast, conventional Pb
The tool life of free-cutting steel is remarkably inferior, and that of comparative steels that deviate from the proper range of the present invention has a tool life of min.
It is below.

[0046]

As described above, according to the present invention,
Machinability that is far superior to conventional Pb free-cutting steel can be obtained without using a large amount of Pb, enabling industrial manufacturing of mechanical parts without significantly adversely affecting the environment. Becomes Further, according to the present invention, quenching as a pretreatment is not required before graphitization annealing, and graphitization is achieved as it is rolled, so that it greatly contributes to improvement in productivity. Furthermore, according to the present invention, since fine graphite particles can be effectively formed, the machinability can be easily improved.

Front Page Continuation (72) Inventor Shinichi Amano 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Mfg. Co., Ltd. Iron and Steel Research Laboratory (56) References ) JP-A-50-96416 (JP, A) JP-A-6-220578 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (4)

(57) [Claims] 1. C: 0.1-1.5 wt%, Si: 0.5-2.0 wt%, Mn: 0.1-2.0 wt%, Ti: 0.005-0.05 wt%, N: 0.0015 wt-0.0150 wt%, O: 0.0030 wt. % Or less, and the above Ti and N are contained to satisfy Ti / N: 2.5 to 4.0, and further Pb: 0.03 to 0.3 wt%, Bi: 0.01 to 0.3 wt%, Te: 0.002 to O.5 wt. %, Se: 0.003 to 0.10 wt%, P: 0.004 to 0.15 wt%, S: 0.005 to 0.25 wt% and Ca: 0.0002 to 0.30 wt%, and any one or more selected from Graphite composite free-cutting steel with excellent machinability, characterized in that the balance is composed of Fe and inevitable impurities, and the structure is mainly composed of ferrite and graphite. 2. C: 0.1 to 1.5 wt%, Si: 0.5 to 2.0 wt%, Mn: 0.1 to 2.0 wt%, Ti: 0.005 to 0.05 wt%, N: 0.0015 wt to 0.0150 wt%, O: 0.0030 wt. % Or less, and the above Ti and N are contained to satisfy Ti / N: 2.5 to 4.0, and further Pb: 0.03 to 0.3 wt%, Bi: 0.01 to 0.3 wt%, Te: 0.002 to O.5 wt. %, Se: 0.003 to 0.10 wt%, P: 0.004 to 0.15 wt%, S: 0.005 to 0.25 wt% and Ca: 0.0002 to 0.30 wt%, and any one or more selected from Further, a graphite composite free-cutting steel excellent in machinability characterized by containing REM: 0.0005 to 0.2 wt%, the balance being composed of Fe and unavoidable impurities, and having a structure mainly composed of ferrite and graphite. 3. C: 0.1-1.5 wt%, Si: 0.5-2.0 wt%, Mn: 0.1-2.0 wt%, Ti: 0.005-0.05 wt%, N: 0.0015 wt-0.0150 wt%, O: 0.0030 wt. % Or less, and the above Ti and N are contained to satisfy Ti / N: 2.5 to 4.0, and further Pb: 0.03 to 0.3 wt%, Bi: 0.01 to 0.3 wt%, Te: 0.002 to O.5 wt. %, Se: 0.003 to 0.10 wt%, P: 0.004 to 0.15 wt%, S: 0.005 to 0.25 wt% and Ca: 0.0002 to 0.30 wt%, and any one or more selected from Further, Ni: 0.10 to 3.0 wt%, Cu: 0.10 to 3.0 wt%, Cr: 0.05 to 1.0 wt%, Mo: 0.05 to 0.5 wt%, V: 0.05 to 0.5 wt% and Nb: 0.005 to 0.05 wt%. Graphite composite excellent in machinability, characterized by containing any one or more selected from the rest, the balance being composed of Fe and inevitable impurities, and the structure being mainly composed of ferrite and graphite. Cutting steel. 4. C: 0.1-1.5 wt%, Si: 0.5-2.0 wt%, Mn: 0.1-2.0 wt%, Ti: 0.005-0.05 wt%, N: 0.0015 wt-0.0150 wt%, O: 0.0030 wt. % Or less, and the above Ti and N are contained to satisfy Ti / N: 2.5 to 4.0, and further Pb: 0.03 to 0.3 wt%, Bi: 0.01 to 0.3 wt%, Te: 0.002 to O.5 wt. %, Se: 0.003 to 0.10 wt%, P: 0.004 to 0.15 wt%, S: 0.005 to 0.25 wt% and Ca: 0.0002 to 0.30 wt%, and any one or more selected from Further, it contains REM: 0.0005 to 0.2 wt%, Ni: 0.10 to 3.0 wt%, Cu: 0.10 to 3.0 wt%, Cr: 0.05 to 1.0 wt%, Mo: 0.05 to 0.5 wt%, V: 0.05 to 0.5 wt% and Nb: 0.005 to 0.05 wt%, containing one or more selected from the group consisting of Fe and unavoidable impurities, the balance being mainly ferrite and graphite. Characterized by Graphite composite free cutting steel with excellent machinability.