US20100092330A1 - Eco-friendly pb-free free cutting steel with excellent machinability and hot workability - Google Patents

Abstract

There is provided a Pb-free free cutting steel is formed of 0.03 to 0.30 wt % of carbon (C), 0.01 to 0.30 wt % of silicon (Si), 0.2 to 2.0 wt % of manganese (Mn), 0.02 to 0.10 wt % of phosphorus, 0.06 to 0.45 wt % of sulfur (S), 0.04 to 0.20 wt % of bismuth (Bi), 0.04 to 0.20 wt % of tin (Sn), 0.001 to 0.015 wt % of boron (B), 0.001 to 0.010 wt % of nitrogen (N), 0.002 to 0.025 wt % of total oxygen (T[O]), and residual Fe, and unavoidable impurities, wherein S, Bi, S, B, and N satisfy a certain relationship. The steel has excellent machinability no less than conventional free cutting steel including Pb, while being eco-friendly. Also, the steel has excellent hot ductility capable of reducing occurrence of defects on a surface, thereby improving hot rolling workability.

Description

TECHNICAL FIELD
• The present invention relates to eco-friendly Pb-free free cutting steel used as a material of precision oil pressure parts of an automobile, parts for office automation equipment, and parts for home appliances and more particularly, to eco-friendly free cutting steel with excellent machinability by using not only elements for improving the machinability, capable of replacing Pb harmful to environments or a human body, but also oxides with low melting point, formed on a steel wire rod by precision deoxidization. In addition, the present invention relates to eco-friendly free cutting steel where defects on a surface, such as corner cracks, do not occur while hot rolling, due to excellent hot ductility thereof.
BACKGROUND ART
• Free cutting steel is generally used as a material for precision components, which has excellent machinability. The excellent machinability of the free cutting steel is due to metallic or nonmetallic inclusions present in the free cutting steel. When cutting steel products by using a tool, nonmetallic inclusions such as MnS act as a stress concentration element at a portion where a tip of the tool is in contact with steel products in such a way that generation of voids and growth of cracks at an interface between the inclusions and a matrix are easily made and power required in cutting is reduced.
• Also, metallic inclusions such as Pb melt at a relatively lower temperature than cutting heat and act as a lubricant at an interface between a chip and a cutting tool, thereby restraining abrasions of the cutting tool and reducing cutting force.
• Accordingly, to improve machinability of steel products, elements capable of forming the metallic or nonmetallic inclusions are added. As conventionally used nonmetallic inclusions, there is MnS. Particularly, MnS formed in a spherical shape mixed with oxides provides most excellent machinability.
• On the other hand, metallic inclusions are generally called as machinability improving elements. Pb is a representative machinability improving element. Since Pb has low solubility with iron, it is easy to exist in free cutting steel as metallic inclusions. Also, due to an appropriately low melting point of 327.5° C., Pb is capable of being easily melted by heat generated in a cutting tip.
• Accordingly, since Pb thoroughly has properties required for improving machinability, up to now, free cutting steel containing Pb is classified as most representative free cutting steel and has been put to practical use as most suitable steel products for cutting.
• However, the free cutting steel containing Pb may generate lead vapor in a process of recycling cutting operations. Since Pb present in steel products is harmful to a human body, it has been required from long ago to replace the steel having Pb.
• As steel products developed to replace the free cutting steel containing Pb, there is free cutting steel having Bi. Since Bi is a low melting point metal and has low solubility for iron, Bi is very advantageous to improve machinability.
• However, since the melting point of Bi is 290° C., which is lower than that of Pb by 120° C., Bi is easier to be melted. Since having surface tension lower than Pb, Bi has high wettability. Such properties cause embrittlement of grain boundaries of steel products.
• Accordingly, due to a decrease in hot ductility caused by the embrittlement of grain boundaries, the free cutting steel having Bi has notably deteriorated hot workability. Also, the free cutting steel has machinability not good as that of the free cutting steel containing Pb, there still exist various problems to replace the steel having Pb by the steel having Bi.
• However, there are also problems in free cutting steel containing Pb. Particularly, as CNC machine tools have been rapidly provided, high speed cutting and automation are realized. There occurs a phenomenon where a certain element of a cutting tool, such as tungsten (W) that is most important element of a tungsten carbide, diffuses to a chip at high speed by heat with a temperature of 1000° C. or more in the high speed cutting. Due to such diffusion of the element such as W, the cutting tool may be rapidly worn.
• Particularly, since the free cutting steel containing Pb is not capable of effectively preventing abrasion of tools, due to thermal diffusion, it is required to develop free cutting steel having excellent machinability in an aspect of high speed cutting.
DISCLOSURE OF INVENTION Technical Problem
• An aspect of the present invention provides eco-friendly Pb-free free cutting steel having eco-friendly properties by adding Bi and Sn capable of replacing elements harmful to environments and human bodies, such as Pb, to steel products, providing excellent machinability by forming composite oxides having a low melting point, capable of restraining abrasions of tools, and having excellent hot workability by adding elements such as Mn and B by optimal ratio.
Technical Solution
• According to an aspect of the present invention, there is provided Pb-free free cutting steel is formed of 0.03 to 0.30 wt % of carbon (C), 0.01 to 0.30wt % of silicon (Si), 0.2 to 2.0 wt % of manganese (Mn), 0.02 to 0.10 wt % of phosphorus (P), 0.06 to 0.45 wt % of sulfur (S), 0.04 to 0.20 wt % of bismuth (Bi), 0.04 to 0.20 wt % of tin (Sn), 0.001 to 0.015 wt % of boron (B), 0.001 to 0.010 wt % of nitrogen (N), 0.002 to 0.025 wt % of total oxygen (T[0]), and residual Fe, and unavoidable impurities, wherein Sn, Bi, S, B, and N satisfy one or more relations selected from a group consisting of following Relational Expressions 1 to 3,
• $\begin{array}{cc}\frac{\left(\mathrm{Bi}+\mathrm{Sn}+S\right)}{\mathrm{Mn}}\ge 0.4,& \mathrm{Relational}\mathrm{Expression}1\\ \frac{{\mathrm{Mn}}^3}{S}\ge 4.6,& \mathrm{Relational}\mathrm{Expression}2\\ \mathrm{and}& \\ \frac{B}{N}\ge 2.0.& \mathrm{Relational}\mathrm{Expression}3\end{array}$
• According to another aspect of the present invention, there is provided Pb-free free cutting steel is formed of 0.03 to 0.30 wt % of C, 0.01 to 0.30wt % of Si, 0.2 to 2.0 wt % of Mn, 0.02 to 0.10 wt % of P, 0.06 to 0.45 wt % of S, 0.04 to 0.20 wt % of Bi, 0.04 to 0.20 wt % of Sn, 0.001 to 0.015 wt % of B, 0.001 to 0.010 wt % of N, 0.002 to 0.025 wt % of T[0], and residual Fe, and unavoidable impurities, wherein the steel includes one of MnO—SiO—Al₂O₃-based oxides, CaO—SiO—Al₂O₃-based oxides, and composite oxides with a low melting point, which is a mixture of the MnO—SiO₂—Al₂O₃-based oxides and the CaO—SiO—Al₂O₃-based oxides.
Advantageous Effects
• According to an exemplary embodiment of the present invention, eco-friendly Pb-free free cutting steel having excellent machinability, no less than free cutting steel containing Pb. Also, due to excellent hot ductility, occurrence of defects on a surface while hot rolling is capable of being reduced, thereby improving hot workability.
BEST MODE FOR CARRYING OUT THE INVENTION
• As described above, the present invention provides Pb-free free cutting steel showing excellent properties not only at low speed cutting but also at high speed cutting by controlling an element system, a relationship between elements, a number of low melting point composite oxides, respectively or cooperatively.
• Hereinafter, an element system forming the Pb-free free cutting steel according to the present invention will be described in detail.
• Carbon (C): 0.03 to 0.30 wt %
• To provide surface roughness and mechanical properties, C should be added by 0.03 wt % or more, and more particularly, to 0.05 wt % or more. However, when adding C more than 0.30 wt %, machinability becomes deteriorated due to an increase of hard pearlite structures.
• Silicon (Si): 0.01 to 0.30 wt %
• Si acts as a deoxidizer and generates SiO₂. To form low melting point composite oxides capable of reducing abrasions of a tool due to thermal diffusion while cutting at high speed, Si may be added to 0.01 wt % or more, and more particularly, to 0.05 wt % or more. However, when adding silicon more than 0.30 wt %, high melting point inclusions or exclusive SiO₂ inclusions area formed, thereby notably increasing speed of abrasions of tools.
• Manganese (Mn): 0.2 to 2.0 wt %
• Mn forms MnS inclusions, which prevent red shortness caused by S. Mn may be added to 0.2 wt % or more. However, when adding Mn more than 2.0 wt %, ferrites are solid-solution strengthened, which reduces machinability. Mn acts as a deoxidizer, forms MnO, and acts as a nucleus of MnS inclusions.
• Phosphorus (P) 0.02 to 0.10 wt %
• P is segregated into boundaries and improves machinability. P may be added to 0.02 wt % or more. However, to provide mechanical properties and cold workability, P may be added 0.10 or less.
• Sulfur (S): 0.06 to 0.45 wt %
• S forms MnS inclusions, which restrains generation of a built-up edge to reduce abrasions of a cutting tool and improve surface roughness of a workpiece in a cutting process. For this, S may be added to 0.06 wt % or more. However, when an amount of S becomes great, it is easy to generate FeS with a low melting point, which decreases hot ductility and make hot rolling difficult. Therefore, the amount of S should be 0.45 wt % or less.
• Bismuth (Bi): 0.04 to 0.20 wt %
• Bi independently exists as metal inclusions or attached to MnS inclusions when adding to steel products. Bi is easily melted by heat while cutting, which improves cutting properties, reduces frictional force by acting as a lubricating film between a chip and a cutting tool and restrains abrasions of the cutting tool. When a content of Bi is less than 0.04 wt %, an effect of machinability is decreased. On the other hand, when the content of Bi is more than 0.20 wt %, and more particularly, 0.16 wt %, casting and rolling properties are not good. The content of Bi may be limited within a range from 0.04 to 0.20 wt %.
• Tin (Sn): 0.04 to 0.20 wt %
• Sn may act similarly to Pb. That is, Sn may act identically to liquid metal embrittlement that is a mechanism of Pb, improving machinability of steel. In detail, this phenomenon is shown since Sn moves ferrite grain boundaries and is segregated, and decreases binding energy of grain boundaries, thereby allowing the grain boundaries to be easily weakened. Accordingly, to obtain an effect of improving machinability due to Sn, Sn may be added to 0.04 wt % or more. However, when a content of Sn is more than 0.20 wt %, and more particularly, 0.16 wt %, it may be harmful to casting and rolling properties. Therefore, the content of Sn may be limited to be in a range from 0.04 to 0.20 wt %.
• Boron (B): 0.001 to 0.015 wt %
• B segregated into austenite boundaries improves hot ductility by strengthening grain boundaries. Also, it has been known since early times that steel containing graphite has excellent machinability. When B reacts to nitrogen in steel and a B nitride BN having a grain structure and physical properties similar to the graphite is generated, there may be an effect of improving machinability identical to the steel containing graphite. When a content of B is less than 0.001 wt %, an effect of adding B is very small. Accordingly, B should be added to 0.001 wt % or more. On the other hand, when the content of boron is more than 0.015 wt %, there is no additionally increased effect and grain boundary strength is decreased due to precipitation of the boron nitride, thereby deteriorating hot workability. The content of B may be limited to be within a range from 0.001 to 0.015 wt %.
• Nitrogen (N): 0.001 to 0.010 wt %
• N should be added to 0.001 wt % or more to form BN together with boron. However, when a content of N is more than 0.010 wt %, an amount of effective boron segregated into austenite grain boundaries is reduced, thereby decreasing boundary strengthening effect.
• Total Oxygen (T[0]): 0.002 to 0.025 wt %
• It is required to add oxygen (0) of 0.002 wt % or more to prevent a decrease of machinability, due to MnS inclusion elongation while hot rolling. However, a content of T[0] should be 0.025 wt % or less to provide plastic deformability of the MnS inclusions while cutting.
• Aluminum (Al) and Calcium (Ca): 10 ppm or less, respectively
• Al and Ca are required to form low melting point composite oxides formed in steel. However, it is not required to intentionally add. An amount naturally included in slag is enough. Al and Ca may be generally present by 10 ppm or less.
• Among the described elements, Bi, Sn, S, Mn, and B may provide excellent machinability and hot workability by satisfying following relational expressions. Hereinafter, the relational expressions with respect to Bi, Sn, S, Mn, and B will be described in detail.
• Relational expression with respect to Sn, Bi, S, and Mn is as follows.
• $\begin{array}{cc}\frac{\left(\mathrm{Bi}+\mathrm{Sn}+S\right)}{\mathrm{Mn}}\ge 0.4& \mathrm{Relational}\mathrm{Expression}1\end{array}$
• wherein each symbol for element indicates weight percent (wt %) of the element, the same as above.
• In addition to the restriction on contents of the elements, to provide Pb-free fee cutting steel having excellent machinability according to an exemplary embodiment of the present invention, Relational Expression 1 should be satisfied. That is, Sn and Bi causes improvement of machinability by liquid metal embrittlement in steel products, as metallic inclusions and S improves machinability by forming MnS.
• Relational expression with respect to Mn and S is as follows.
• $\begin{array}{cc}\frac{{\mathrm{Mn}}^3}{S}\ge 4.6& \mathrm{Relational}\mathrm{Expression}2\end{array}$
• In addition to the restriction on the contents of the elements, to provide the Pb-free free cutting steel having excellent hot ductility, it is required that a relation between Mn and S satisfies Relational Expression 2. Relational Expression 2 indicates that a content Mn is required to a degree to be bonded to S and restrain hot embrittlement due to S.
• Relational Expression with respect to B and N is as follows.
• $\begin{array}{cc}\frac{B}{N}\ge 2.0& \mathrm{Relational}\mathrm{Expression}3\end{array}$
• To provide the Pb-free free cutting steel having excellent hot ductility, B and N should satisfy Relational Expression 3. That is, though N is present, there is required just an amount of N capable of strengthening austenite grain boundaries by B segregated into grain boundaries.
• Though satisfying one of Relational Expressions 1 to 3, an effect thereof is shown. When satisfying two or more of Relational Expressions 1 to 3 at the same time, there is shown a notable effect. Accordingly, when satisfying one or more of Relational Expressions 1 to 3, it may be considered as being included in the scope of the present invention.
• On the other hand, the Pb-free free cutting steel of the present invention includes low melting point inclusions by Mn, Si, Ca, and Al. Hereinafter, the low melting point inclusions will be described in detail.
• In the element system of the present invention, oxidization of Mn, Si, Ca, and Al occurs, thereby various low melting point composite oxides are formed. To form the inclusions, Mn, Si, Ca, and Al may be additionally added. However, an amount of Ca and Al basically present in steel is enough to form the inclusions. In the present invention, the oxides may be present in the form of MnO—SiO₂—Al₂O₃-based or CaO—SiO₂—Al₂O₃-based.
• The MnO—SiO₂—Al₂O₃-based oxides may be formed of 20 to 65 wt % of MnO, 25 to 60 wt % of SiO , and 0 to 30 wt % of Al₂O₃. The CaO—SiO—Al₂O₃-based oxides may be formed of 10 to 55 wt % of CaO, 35 to 65 wt % of SiO₂, and 0 to 25 wt % of Al₂O₃.
• Also, one of the low melting point composite oxides such as the MnO—SiO₂—Al₂O₃-based oxides and the CaO—SiO —Al₂O₃-based oxides may be present five or more per 5 g of a steel wire rod. When there are less than five inclusions, machinability is decreased.
• Hereinafter, embodiments of the present invention will be described in detail.
Embodiment
• Turning test and high temperature tensile test were performed on inventive steels and comparative steels having compositions shown in Tables 1, 2, and 3 to investigate machinability, hot ductility thereof. Composite oxides were analyzed by extraction & separation of nonmetallic inclusion in steel by electrolysis in AA solution under ultrasonic wave (ESAA).

• 	TABLE 1
  	C	Si	Mn	P	S	B	Bi	Sn	T[O]	N

  Inventive	0.079	0.067	1.155	0.053	0.304	0.0095	0.07	0.08	0.0080	0.0048
  Steel 1
  Inventive	0.073	0.060	1.151	0.067	0.328	0.0092	0.13	0.14	0.0120	0.0032
  Steel 2
  Inventive	0.102	0.080	1.235	0.058	0.350	0.0070	0.09	0.11	0.0153	0.0015
  Steel 3
  Inventive	0.044	0.030	1.570	0.059	0.380	0.0100	0.18	0.17	0.0140	0.0023
  Steel 4
  Inventive	0.038	0.010	1.250	0.061	0.310	0.0074	0.13	0.09	0.0170	0.0035
  Steel 5
  Inventive	0.104	0.086	1.380	0.058	0.360	0.0120	0.08	0.11	0.0160	0.0027
  Steel 6
  Comparative	0.080	0.138	1.449	0.050	0.376	0.0073	0.10	0.10	0.0130	0.0058
  Steel 1
  Comparative	0.070	0.004	1.162	0.076	0.344	—	0.07	0.11	0.0201	0.0043
  Steel 2
  Comparative	0.290	0.285	1.020	0.030	0.210	0.0081	—	0.05	0.0110	0.0040
  Steel 3
  Comparative	0.070	0.002	1.10	0.080	0.298	—	Pb:	—	0.0120	—
  Steel 4							0.3

• In Table 1, Inventive Steels 1 to 6 and Comparative steel 1 satisfy the element system of the present invention. On the other hand, Comparative Steels 2 and 3 are different in B and Bi and Comparative Steel 4 indicates conventional free cutting steel containing Pb.

• 	TABLE 2
  	(Bi + Sn + S)/Mn	Mn3/S	B/N

  	Inventive Steel 1	0.4	5.07	2.0
  	Inventive Steel 2	0.5	4.65	2.9
  	Inventive Steel 3	0.4	5.38	4.7
  	Inventive Steel 4	0.5	10.18	4.4
  	Inventive Steel 5	0.4	6.30	2.1
  	Inventive Steel 6	0.4	7.30	4.4
  	Comparative Steel 1	0.4	8.09	1.3
  	Comparative Steel 2	0.5	4.56	0.0
  	Comparative Steel 3	0.3	5.05	2.0

• In Table 2, it may be known that Comparative Steels 1 and 2 are out of an appropriate range of B/N and Comparative Steel 3 is out of an appropriate range of (Bi+Sn+S)/Mn. Comparative Steel 4 is not shown because Comparative Steel 4 is free cutting steel containing Pb.

• 	TABLE 3
  	MnO—SiO2—Al2O3 -	CaO—SiO2—Al2O3 -
  	based	based	Number of
  	composite oxides	composite oxides	inclusions (per

  		SiO2	Al2O3	CaO	SiO2	Al2O3	5 g of steel wire
  	MnO (%)	(%)	(%)	(%)	(%)	(%)	rod)

  Inventive	30	55	15	40	35	25	10
  Steel 1
  Inventive	45	35	20	35	45	20	7
  Steel 2
  Inventive	25	50	25	15	65	20	6
  Steel 3
  Inventive	50	25	25	45	20	15	9
  Steel 4
  Inventive	50	40	10	30	55	15	12
  Steel 5
  Inventive	35	40	25	45	50	5	8
  Steel 6
  Comparative	60	30	10	40	35	25	5
  Steel 1
  Comparative	80	10	10	25	30	45	2
  Steel 2
  Comparative	40	40	20	30	55	15	6
  Steel 3
  *ESSA: extraction & separation of nonmetallic inclusion in steel by electrolysis in AA solution under ultrasonic wave

• Also, in Table 3, it may be known that a number of inclusions included in Comparative Steel 2 are less than a reference value. Also, Comparative Steel 4 that is the free cutting steel containing Pb is excluded.
• With respect to Inventive Steels and Comparative Steels, to check whether Inventive Steels are capable of replacing the free cutting steel containing Pb by testing machinability of Inventive Steels, machinability tests were performed as follows. In the machinability test, a workpiece that was a bar with a diameter of 25 mm was turned by a CNC lathe without cutting oil. A transfer speed was 0.3 mm/rev, a cutting depth was 0.5 mm, and a cutting speed was 150 m/min. To check a degree of abrasions of a tool, after turning test for the same time, a flank wear width (VB) of the tool was measured and compared. A result of tool abrasions caused by the turning operation was shown in Table 4.

• 	TABLE 4
  	Tool flank wear width according
  	to cutting time (mm)

  	Cutting for 10	Cutting for 20	Cutting for 30
  	minutes	minutes	minutes

  Inventive Steel 1	0.12	0.21	0.30
  Inventive Steel 2	0.07	0.14	0.20
  Inventive Steel 3	0.09	0.18	0.28
  Inventive Steel 4	0.07	0.12	0.18
  Inventive Steel 5	0.08	0.16	0.24
  Inventive Steel 6	0.10	0.19	0.28
  Comparative Steel 1	0.08	0.15	0.25
  Comparative Steel 2	0.15	0.23	0.32
  Comparative Steel 3	0.20	0.34	0.40
  Comparative Steel 4	0.16	0.28	0.34

• As shown in Table 4, as a result of measuring the degree of tool abrasions via a cutting test, when comparing eco-friendly free cutting steels according to the present invention, which were Inventive Steels 1 to 6 with conventional free cutting steel containing Pb, which was Comparative Steel 4, the eco-friendly free cutting steels showed very excellent tool wear resistant properties. In the case of Comparative Steel 2, since low melting point oxides were not formed, machinability thereof was less excellent than that of Inventive Steels. Also, in the case of Comparative Steel 3, tools were most rapidly abraded.
• To perform a hot ductility test, Inventive Steels and Comparative Steels were heated at a temperature of 1250° C. that was a reheating temperature and maintained for one minute. After that, a tension test was performed. After the tests, a reduction of area (RA) of specimen was measured and shown in Table 5.

• 	TABLE 5
  		Reduction of	Reduction of	Reduction of
  	Reduction of	area at	area at	area at
  	area at 900° C.	1000° C.	1100° C.	1200° C.

  Inventive	70	72	84	89
  Steel 1
  Inventive	69	78	82	92
  Steel 2
  Inventive	76	82	85	91
  Steel 3
  Inventive	62	65	72	80
  Steel 4
  Inventive	73	78	83	93
  Steel 5
  Inventive	77	75	80	91
  Steel 6
  Comparative	50	60	62	83
  Steel 1
  Comparative	49	57	60	76
  Steel 2
  Comparative	49	57	60	76
  Steel 3
  Comparative	77	81	88	81
  Steel 4

• As shown in Tables 1 and 2, in the case of Inventive Steels 1 to 6, a value of Mn³/S was 4.6 or more, red shortness due to forming of low melting point FeS was restrained, and also, a value of B/N was 2.0 or more, an effect of strengthening austenite grain boundaries was capable being obtained. Accordingly, it was possible to obtain hot ductility with a reduction of area of 70% when performing high temperature tensile tests at 900° C. or more. Therefore, a possibility of occurrence of defects on a surface such as corner cracks was very low.
• On the other hand, as in the case of Comparative Steel 1, when a value of Mn³/S was 4.6 or more but a value of B/N was less than 2.0, since B in steel was generally precipitated and it was impossible to strengthen grain boundaries enough, a reduction of area less than 60% at a temperature of 900° C. was shown. Also, as in the case of Comparative Steel 2, when a value of Mn³/S was less than 4.6 and a value of B/N was less than 2.0, hot ductility was shown as lower than that of Comparative Steel 1.
• As described above, according to an exemplary embodiment of the present invention, there is provided eco-friendly Pb-free free cutting steel capable of providing excellent machinability by restraining tool abrasion that may be shown in cutting processes at a speed regardless of high or low speed by controlling contents of B, Sn, Mn, S, and N at appropriate relational expressions and forming low melting point composite oxides, and having excellent hot workability by adding elements such as Mn and B at optimum ratios.

Claims (6)

1. A Pb-free free cutting steel comprising 0.03 to 0.30 wt % of carbon (C), 0.01 to 0.30 wt % of silicon (Si), 0.2 to 2.0 wt % of manganese (Mn), 0.02 to 0.10 wt % of phosphorus (P), 0.06 to 0.45 wt % of sulfur (S), 0.04 to 0.20 wt % of bismuth (Bi), 0.04 to 0.20 wt % of tin (Sn), 0.001 to 0.015 wt % of boron (B), 0.001 to 0.010 wt % of nitrogen (N), 0.002 to 0.025 wt % of total oxygen (T[0]), and residual Fe, and unavoidable impurities,

wherein S, Bi, S, B, and N satisfy one or more relationships selected from a group consisting of following Relational Expressions 1 to 3,

$\begin{array}{cc}\frac{\left(\mathrm{Bi}+\mathrm{Sn}+S\right)}{\mathrm{Mn}}\ge 0.4,& \mathrm{Relational}\mathrm{Expression}1\\ \frac{{\mathrm{Mn}}^3}{S}\ge 4.6,& \mathrm{Relational}\mathrm{Expression}2\\ \mathrm{and}& \\ \frac{B}{N}\ge 2.0.& \mathrm{Relational}\mathrm{Expression}3\end{array}$

2. A Pb-free free cutting steel comprising 0.03 to 0.30 wt % of C, 0.01 to 0.30 wt % of Si, 0.2 to 2.0 wt % of Mn, 0.02 to 0.10 wt % of P, 0.06 to 0.45 wt % of S, 0.04 to 0.20 wt % of Bi, 0.04 to 0.20 wt % of Sn, 0.001 to 0.015 wt % of B, 0.001 to 0.010 wt % of N, 0.002 to 0.025 wt % of T[0], and residual Fe, and unavoidable impurities,

wherein the steel comprises one of MnO—SiO₂—Al₂O₃-based oxide, CaO—SiO₂—Al₂O₃-based oxide, and composite oxides with a low melting point, which is a mixture of the MnO—SiO₂—Al₂O₃-based oxide and the CaO—SiO₂—Al₂O₃-based oxide.

3. The steel of claim 2, wherein the MnO—SiO₂—Al₂O₃-based oxide is formed of 20 to 65 wt % of MnO, 25 to 60 wt % of Si02, and 0 to 30 wt % of Al₂O₃.

4. The steel of claim 2, wherein the CaO—SiO₂—Al₂O₃-based oxide is formed of 10 to 55 wt % of CaO, 35 to 65 wt % of SiO₂, and 0 to 25 wt % of Al₂O₃.

5. The steel of claim 2, wherein there are five or more of the composite oxides with a low melting point in 5 g of a steel wire rod.

6. The steel of claim 2, wherein the S, Bi, S, B, and N satisfy one or more relationships selected from a group consisting of following Relational Expressions 1 to 3,

$\begin{array}{cc}\frac{\left(\mathrm{Bi}+\mathrm{Sn}+S\right)}{\mathrm{Mn}}\ge 0.4,& \mathrm{Relational}\mathrm{Expression}1\\ \frac{{\mathrm{Mn}}^3}{S}\ge 4.6,& \mathrm{Relational}\mathrm{Expression}2\\ \mathrm{and}& \\ \frac{B}{N}\ge 2.0.& \mathrm{Relational}\mathrm{Expression}3\end{array}$