KR101125894B1 - Method of manufacturing Graphite Steel Rod for machine structural use having lower decarburized surface property - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a graphite steel wire for mechanical structure suitable for use in machine parts such as industrial machines and automobiles. In order to provide a method for producing a graphite steel wire for mechanical structure to significantly reduce the decarburization rate on the surface of the billet to reduce the thickness of the surface decarburization layer, the purpose is to.

In the present invention, by weight%, carbon: 0.30-0.70%, silicon: 2.0-4.0%, manganese: 0.1-1.0%, phosphorus: 0.01-0.15%, sulfur: 0.01% or less, selenium (Se): 0.001- 0.05%, titanium: 0.001-0.03%, boron: 0.001-0.003%, aluminum: 0.002-0.01%, nitrogen: 0.004-0.008%, oxygen: 0.005% or less, including nickel: 0.05-1.0%, copper : 0.01-0.5%, calcium: 0.0001-0.05%, zirconium: 0.0005-0.008%, rare earth metals: one or two or more selected from the group consisting of remaining Fe and other inevitable impurities Titanium (Ti), nitrogen (N), boron (B) and aluminum satisfy the relationship of 2.0≤ (Ti + 5B + Al) /N≤5.5, while manganese, selenium (Se) and sulfur are the next relationship, 1.0≤ ( Obtaining a bloom that satisfies Mn / 5 + Se) /5S≦3.0; Stepwise air cooling the bloom to 600 ± 100 ° C .; Reheating the bloom in a range of 1260 ± 30 ° C. for 100 ± 20 minutes and rolling the steel sheet to obtain a billet; The billet is heated to a heating temperature of 1050 ± 50 ℃ at a heating rate of more than 10 ℃ / min, and maintained in this temperature section for more than 30 minutes to roll the wire rod to obtain a wire rod less mechanical structural graphite steel Wire rod manufacturing method is the main point.

Surface decarburization, graphite steel, steel strip, bloom, wire rod, ferrite

Description

Method of manufacturing Graphite Steel Rod for machine structural use having lower decarburized surface property}

The present invention relates to a method for manufacturing a mechanical structural steel wire suitable for use in machine parts such as industrial machines and automobiles, and more particularly, in a high silicon steel by forming a ferrite decarburized layer having a low carbon utilization in a steel sheet rolling process. The present invention relates to a method for producing graphite steel wire for machine structure with less surface decarburization which reduces the surface decarburization layer generated.

Mechanical parts used in industrial machines and automobiles are manufactured by cutting or cold forging steel into a predetermined shape using the same, and then securing the required characteristics as a mechanical part by hardening and annealing treatment.

Steels used as such mechanical parts should be excellent in machinability and cold forging.

As a means for improving the free machinability of mechanical structural steel, a method of adding machinable elements such as Pb, S, and Bi to the steel alone or in combination is common.

In particular, Pb has been used a lot because the action to improve the machinability is very large.

However, Pb, on the other hand, is also an element harmful to the human body and requires large exhaust facilities in the manufacturing process of steel and the processing of mechanical parts.

In addition, there is a problem in terms of recycling steel materials.

On the other hand, Bi is known as a free machinability granting element that can replace Pb, but steel with Bi is easily cracked during manufacture, which makes it difficult to manage production, and S, Te Sn, etc. Silver is widely used as a substitute element for Pb, but it is easy to crack during hot rolling and has problems of deterioration in ductility and toughness.

For this reason, in order to improve the free machinability and cold forging of mechanical structural steel at the same time, a new alternative steel is required in addition to the above-mentioned method. As an alternative, the steel structure may be considered an ideal structure of ferrite + graphite.

In Japanese Patent Laid-Open No. 3-140411, carbon has been graphitized in a steel having a general carbon content and manufactured into an abnormal structure of ferrite and graphite to significantly improve cold workability or cold forging.

Also, in view of this aspect, the present inventors have proposed a steel structure having improved cold formability of a bolt material by using a microstructure of steel as a ferrite + graphite abnormal structure. Korean Patent Application Nos. 2001-84518, 2001-84517, 2001- Proposed in 85540, Korean Patent Application Nos. 2001-85532 and 2001-85531 which propose a method for producing graphitized wire.                         

However, a large amount of silicon is inevitably added to promote graphitization, which may cause many problems in use when surface decarburization control is not appropriate in wire fabrication.

Representative examples of decarburization control methods known so far include Korean Patent Nos. 92-24974, 92-24163, 92-24161, Japanese Patent Publication (Pyeong) 2-301514,

(Pyeong) 1-31960, (S) 63-216591, and the like.

Most of these are methods of controlling decarburization by lowering silicon content or adding alloying elements such as lead and tin.

However, in order to promote graphitization, silicon content cannot be lowered in steel inevitable addition of high silicon, and when alloying elements such as lead and tin are added, mechanical properties such as impact toughness are deteriorated.

The present invention has been made in the course of research to prevent the decarburization layer on the surface in the wire heating process of graphite steel for mechanical structure using the graphitization properties for the purpose of simultaneously improving the cold formability and free machinability, billet in the steel sheet rolling process By forming a ferrite decarburization layer with low carbon utilization on the surface of the wire, it is intended to provide a method of manufacturing graphite steel wire for mechanical structure, which significantly reduces the decarburization rate on the surface of the billet in the wire rolling process, thereby reducing the thickness of the surface decarburization layer. The purpose is to.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.                     

In the present invention, by weight%, carbon: 0.30-0.70%, silicon: 2.0-4.0%, manganese: 0.1-1.0%, phosphorus: 0.01-0.15%, sulfur: 0.01% or less, selenium (Se): 0.001- 0.05%, titanium: 0.001-0.03%, boron: 0.001-0.003%, aluminum: 0.002-0.01%, nitrogen: 0.004-0.008%, oxygen: 0.005% or less, including nickel: 0.05-1.0%, copper : 0.01-0.5%, calcium: 0.0001-0.05%, zirconium: 0.0005-0.008%, rare earth metals: one or two or more selected from the group consisting of remaining Fe and other inevitable impurities Titanium (Ti), nitrogen (N), boron (B) and aluminum satisfy the relationship of 2.0≤ (Ti + 5B + Al) /N≤5.5, while manganese, selenium (Se) and sulfur are the next relationship, 1.0≤ ( Obtaining a bloom that satisfies Mn / 5 + Se) /5S≦3.0;

Stepwise air cooling the bloom to 600 ± 100 ° C .;

Reheating the bloom in a range of 1260 ± 30 ° C. for 100 ± 20 minutes and rolling the steel sheet to obtain a billet;

The billet is heated to a heating temperature of 1050 ± 50 ℃ at a heating rate of more than 10 ℃ / min, and maintained in this temperature section for more than 30 minutes to roll the wire rod to obtain a wire rod less mechanical structural graphite steel It relates to a wire rod manufacturing method.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The present inventors conducted a multi-angle study to reduce the decarburization of graphite steel wire for mechanical structure, and then, after heating the bloom for the non-rolling, the bloom was properly air-cooled to a temperature of 600 ± 100 ° C. and then heated and maintained. It was found that the reheating with controlled time could lead to a uniform ferrite layer with a very low carbon solubility on the surface of the billet to 0.05-0.2mm thickness.

In addition, the temperature rising through the abnormality (the temperature between the Ac1 transformation point and Ac3 transformation point, the ferrite and austenite phases are mixed to maintain an equilibrium state) during the billet heating (when heating the wire heating) for rolling the billet. If the heating rate is properly controlled, the ferrite layer with low carbon solubility is uniformly distributed on the surface of the billet, which slows down the diffusion of carbon to the outside regardless of the heating pattern. The thickness of the surface ferrite layer can be adjusted according to the degree of oxidation, which can significantly improve the billet surface decarburization layer when extracted by wire heating.

The reason for selecting the steel composition of this invention is demonstrated as follows.

Carbon (C): 0.3 ~ 0.7%

Carbon is an important element for forming graphite phase and is an important element to secure the strength of mechanical parts, but the content of carbon is 0.3 ~ 0.7% because its effect is less than 0.3% and its effect is saturated at 0.7% or more. It is preferable to limit to the range of.

Silicon (Si): 2.0 ~ 4.0%

Silicon is a necessary component as a deoxidizer in molten steel manufacturing and is an element effective in promoting graphitization by destabilizing iron carbide (cementite) in steel. In addition, silicone is actively added because it is a component that improves strength.                     

If the addition amount is less than 2.0%, the addition effect is insufficient. If the addition amount is more than 4.0%, the effect of promoting graphitization is saturated and the temperature range where liquid phase occurs becomes low, so that the appropriate temperature range is narrowed during hot rolling. It is preferable to limit the addition amount of silicon to 2.0 to 4.0% of range.

Manganese (Mn): 0.1-1.0%

Manganese is an effective element to secure the strength of steel and is also useful as a deoxidizer in molten steel manufacturing. In addition, it combines with S to form MnS to contribute to the improvement of machinability. However, if the content is less than 0.1%, there is a problem that the contribution to the strength is small and the toughness is deteriorated at more than 1.0%, the content is preferably limited to the range of 0.1 ~ 1.0%.

Phosphorus (P): 0.01 ~ 0.15%

Phosphorus not only inhibits graphitization, but also segregates at the austenite grain boundary during the quenching treatment, thereby lowering the grain boundary strength, and thus, it is preferable to manage phosphorus low.

On the other hand, phosphorus is an element which improves machinability by hardening a ferrite phase, but is also an element which inhibits graphitization. In order to improve the machinability, it is necessary to add at least 0.01% or more. However, when adding 0.15% or more, the upper limit is preferably limited to 0.15% because it inhibits graphitization and conversely lowers machinability.                     

Sulfur (S): 0.01% or less

Sulfur forms MnS to improve chip breaking during cutting to improve machinability. Especially, it acts as a nucleus of graphitization to promote graphitization, and when the amount added is 0.01% or more, the effect is saturated and a large amount is added. If so, not only the graphitization is delayed but also the toughness is drastically lowered, so the upper limit is limited to 0.01%.

Selenium (Se): 0.001-0.05%

Selenium improves chip breaking by forming MnSe in combination with manganese. At the same time, MnSe is an element that improves machinability by acting as a graphitization nucleus to promote graphitization. This effect is limited to less than 0.001% and limited to 0.001 ~ 0.05% because the effect is saturated at 0.05% or more.

Titanium (Ti): 0.001-0.03%

Titanium defects with nitrogen in the steel to form TiN to prevent cementite stabilization and at the same time to cause the crystallization of graphite to promote graphitization. Moreover, since it functions effectively also as a deoxidizer, it adds actively. Therefore, the effect is less than 0.001% and limited to 0.001 ~ 0.03% because it hinders graphitization rather than 0.03%.

Boron (B, B): 0.001 ~ 0.003%                     

Boron is an element that is actively added because it combines with N to form BN and acts as a crystal nucleus of graphite while interfering with stabilization of iron carbide, thereby promoting graphitization and enhancing quenchability. For this reason, the addition effect is insufficient at less than 0.001%, and the effect is saturated at 0.003% or more, and the grain boundary strength is lowered due to the precipitation of boron nitride at the grain boundary, which reduces the hot workability. .

Aluminum (Al): 0.002-0.03%

Aluminum is a useful element that not only contributes to deoxidation but also promotes graphitization. At the time of annealing, it accelerates the decomposition of cementite and simultaneously binds with nitrogen to form AlN, thereby preventing the cementite from stabilizing. In addition, aluminum oxide produced in the steel by the addition of aluminum is effective in that it also becomes a precipitation nucleus of BN and promotes crystallization of graphite. In the present invention, aluminum is actively added, but if the content is less than 0.002%, the addition effect is saturated, and at 0.03% or more, the graphitization promoting action is saturated, and the hot deformation is remarkably lowered in the range of 0.01 to 0.03%. Restrict.

Nitrogen (N): 0.004-0.008%

Nitrogen is actively added because it combines with titanium, boron and aluminum to form nitrides and use these as nuclei to promote the crystallization of graphite. On the other hand, in order to form nitrides effective for promoting graphitization, it is preferable to add in stoichiometric amounts almost equivalent to those of titanium, boron, and aluminum, but in order to uniformly disperse these nitrides, it is preferable to add slightly higher than the chemical equivalents. In addition, it is advantageous to add a little excessively because nitrogen improves chip treatability by dynamic strain aging. For this reason, it is necessary to add more than 0.004%, but when added more than 0.008% saturation effect was limited to 0.004 ~ 0.008%.

Oxygen (O): 0.005% or less

In the present invention, the role of oxygen is important. Oxygen combines with aluminum to form oxides. The production of such oxides reduces the graphitization-promoting effect of aluminum described above, which inhibits nucleation for crystallization of graphite, thereby attenuating substantial graphitization.

The alumina oxide formed by containing a large amount of oxygen damages the cutting tool during cutting, resulting in a decrease in machinability. For this reason it is desirable to manage as low as possible. However, because too low oxygen causes excessive damage to refining, the upper limit is limited to less than 0.005% because there is no working problem in the steelmaking industry.

In the present invention, in order to significantly shorten the graphitization and graphitization time, the ratio of the composition ratio between alloy elements is 2.0≤ (Ti + 5B + 2Al) /N≤5.5, 1.0≤ (Mn / 5 + Se) /5S≤3.0 The reason for this is as follows.

As for (Ti + 5B + Al) / N ratio, 2.0 <= (Ti + 5B + Al) / N <= 5.5 is preferable.

If the (Ti + 5B + Al) / N ratio is less than 2.0, the number of TiN and BN precipitates that contribute to the nucleation of the graphite grains is insufficient, and if the (Ti + 5B + Al) / N ratio is greater than 5.5, the graphite grain nucleation is necessary. The number of precipitates of TiN, BN, and AlN saturates, and the amount of nitrogen dissolved in the base material increases, thus adversely affecting the graphitization rate.

As for (Mn / 5 + Se) / 5S ratio, 1.0 <(Mn / 5 + Se) / 5S <= 3.0 is preferable.

If the (Mn / 5 + Se) / 5S ratio is less than 1.0, the number of MnSe precipitates contributing to nucleation of graphite grains and the effective MnS inclusions for machinability are insufficient. The number of MnSe precipitates and MnS precipitates required for nucleation is saturated, and the amount of sulfur dissolved in the base material increases, causing grain boundary segregation, which adversely affects mechanical properties.

To the above composition, one or two or more of the group consisting of 0.05-1.0% nickel, 0.01-0.05% copper, calcium: 0.0001-0.05%, and rare earth metals 0.001-0.05% are selectively added. Explain the reason.

Nickel (Ni): 0.05-1.0%                     

Nickel is effective in improving the hardenability of steel and enhancing the strength of steel by hardening, and at the same time assisting and promoting graphitization. The effect is less than 0.05%, the effect is saturated above 1.0%, and limited to 0.005 ~ 1.0% because it is not economical as expensive elements.

Copper (Cu): 0.01-0.5%

Copper not only improves machinability by making cementite unstable and promotes graphitization, but also enhances hardenability of steel to increase precipitation, and increases strength at the time of hardening of steel. If it is less than 0.01%, the effect of promoting graphitization and improvement of corrosion resistance is insufficient. If it exceeds 0.5%, the improvement effect is saturated and the melting point of grain boundary segregation is lowered. It was limited to 0.01 to 0.5% because of the high possibility of surface flaws due to grain boundary embrittlement and the impact toughness of the final product.

Calcium (ca): 0.0001-0.05%

Calcium improves machinability by forming Ca-Al-based oxides, which act as nuclei of graphitization to promote graphitization. This effect is less than 0.0001%, and the effect is limited to 0.0501% to 0.05%, since a large amount of coarse oxide-based non-metallic inclusions are generated to lower the fatigue strength of mechanical parts.

Zirconium (Zr): 0.0005-0.008%

Zirconium finely disperses oxides such as CaO and Ti2O3 and MnS sulfides. These oxides and sulfides serve as precipitation sites of the graphite to effectively disperse the graphite particles and reduce the graphitization time. However, if the addition amount of zirconium is less than 0.0005%, the effect is insufficient, and if it is over 0.008%, coarse Zr-based sulfides and carbonitrides are formed to reduce the miniaturization effect of oxides by Zr and to deteriorate fracture toughness. It was limited to the range of 0.008%.

Rare Earth Metals (REM) : 0.001-0.05%

Rare earth metals are added for the purpose of improving the hot workability of steel and further promoting graphitization. This action is useful to use La, Ce, etc., but the content is less than 0.001% because the effect is insufficient, and at 0.05% or more because the effect is saturated, it was limited to the range of 0.001 ~ 0.05%.

Hereinafter, the manufacturing method of this invention is demonstrated.

[Roll Rolling Process]

The steel formed as described above has a high Si content and a decarburized layer is generated in a heating process for manufacturing the wire rod. In order to prevent this, the steel is 600 ± 100 ° C. rather than heating from room temperature during heating for the biletro rolling. It is important to form the ferrite decarburized layer on the surface of the steel-rolled billet by controlling the heating conditions after the bloom is air-cooled to the temperature.

Thereafter, the bloom is reheated in a range of 1260 ± 30 ° C. for 100 ± 20 minutes to roll the steel sheet to obtain a billet. If the heating temperature is less than 1230 ° C, the thickness of the appropriate ferrite layer on the bloom surface of the steel piece heating furnace increases more than necessary, so that it is difficult to obtain a decarburization improvement effect. In addition, when the heating temperature exceeds 1290 ℃, the oxidation rate is very increased, it is difficult to secure the thickness of the appropriate ferrite layer on the bloom surface, and the amount of oxidation is increased, there is a high possibility of surface defects during rolling. On the other hand, if the heating holding time is less than 80 minutes, the time required to secure the thickness of the appropriate ferrite layer on the bloom surface is insufficient. If it exceeds 120 minutes, the effect is saturated and the surface quality is degraded due to excessive oxidation amount.

It is preferable that the thickness of the ferrite decarburization layer be 0.05 to 0.2 mm on the surface of the thus prepared billet. If the ferrite decarburization layer is less than 0.05 mm, most ferrite layers are oxidized during billet heating, so it is difficult to expect decarburization improvement effect due to the presence of the ferrite layer. When the ferrite decarburization layer exceeds 0.20 mm, the heating time may be longer to show the effect of the present invention. Because there is.

[Wire Rolling Process]

Next, roll the billet in advance. The billet is heated to a range of 1050 ± 50 ° C at a heating rate of at least 10 ° C / min and held for at least 30 minutes in this temperature range. If the heating rate is less than 10 ° C / min is undesirable because there is a problem that the ashing time is increased by the wire heating is passed through the ideal range temperature range to promote the decarburization rate to deepen the billet surface decarburization.

If the heating temperature is less than 1000 ° C, it is difficult to control the thickness of the billet surface ferrite layer for decarburization control. In addition, it is not easy to re-use coarse precipitated vanadium-based or niobium-based precipitates during the production of billet, and the workability is poor due to the overload during rolling due to the increase in hot deformation resistance. On the other hand, when the heating temperature exceeds 1100 ℃ can not maintain a uniform ferrite layer for decarburization control. That is, in order to precipitate a surface ferrite layer having a very low carbon solubility and to sharply reduce the decarburization reaction, the surface ferrite layer must remain at the heating and holding temperature, but when the heating temperature exceeds 1100 ° C, the surface ferrite layer is transformed into austenite. As a result, the decarburization rate increases rapidly, which intensifies the surface decarburization. The heating holding time is 30 minutes or more because it is difficult to ensure a uniform temperature distribution inside the billet for wire rod rolling in less than 30 minutes.

As described above, the billet is heated to roll the wire. At this time, the decarburization area of the billet extracted from the heating furnace is the same as the decarburization area of the wire after the wire rolling. Therefore, in the present invention, in consideration of this point, it is preferable to roll the wire to a diameter of 30 mm or less.

In the present invention, a wire rod having the above-described steel component system and having a grain size of spherical austenite of 30 µm or less is used. If the spherical austenite grain size of the wire rod exceeds 30㎛, the size and thickness of cementite constituting the pearlite after the transformation of the pearlite becomes too large and thick, which causes a problem of lowering the graphitization rate. It is unpreferable because it becomes coarse and adversely affects cold forging property and machinability.

On the other hand, wire rod rolling for producing a wire rod having a grain size of spherical austenite of 30 μm or less is performed at a temperature range of 870 ± 30 ° C. This is because the control rolling effect is insufficient at 900 ° C or higher, and below 840 ° C, the effect is saturated, surface flaws are easily generated, wire rod deflection is difficult to control, and workability is reduced due to increased rolling load. .

In addition, the present invention uses a wire rod having a granite ferrite structure fraction of 10% or less after cooling the wire rod. In the case where the cornerstone ferrite structure fraction exceeds 10% after wire rod cooling, it is not preferable because the distribution of graphite grains during graphitization becomes uneven and may cause coarsening of graphite grains and adversely affect cold forging and machinability. In the ferrite region, the carbon content is very low and the carbon necessary for the formation of graphite grains during graphitization is scarce, so that the ferrite is hardly graphitized, so it must be limited to 10% or less.

On the other hand, the controlled cooling for producing a wire rod having a saltpeter ferrite structure fraction of 10% or less is followed by winding the wire rod after rolling, and the winding temperature at this time is preferably 750-800 ° C. The coiling temperature is secured by cooling water spraying immediately after the wire is rolled. If the coiling temperature is less than 750 ℃ it is likely to cause a poor winding due to cold winding. If the coiling temperature is higher than 800 ° C, it is difficult to control the cornerstone ferrite fraction.

The wound wire is cooled down to 3.0 ± 0.3 ° C / sec to 670 ± 20 ° C. The conditions of cooling temperature and cooling rate are taken into account the difference in the degree of cooling of wire rod integrated state, ie overlapping part and non-overlapping part. If the cooling temperature is higher than 690 ℃, the formation rate of the cornerstone ferrite increases, and if the cooling temperature is lower than 650 ℃, the perlite transformation rate decreases. In addition, when the cooling rate is faster than 2.7 ° C / sec is because the low-temperature structure (martensite or bainite) is produced, and when it is slower than 2.7 ° C / sec is not preferable because the increase in the cornerstone ferrite phase fraction.

Hereinafter, the present invention will be described in more detail with reference to Examples.

[Example]

The graphite steel bloom for mechanical structure having the component system of the invention example and the comparative example as shown in Table 1 was rolled into a steel sheet (billet) under the conditions of Table 3, then rolled at high speed, rapidly cooled by water spraying, and then wound up at 950 ° C. The mixture was slowly cooled to 1.0 ° C./sec to ° C., and then air cooled to obtain a wire rod having a diameter of 20 mm.

The surface decarburized layer depth of the billet and the wire rod was measured according to the KS standard (KD D 0216).

According to this standard, an optical microscope observation method and a microhardness measuring method are proposed, but the microhardness measuring method was used in the present invention.

The measured decarburized bed depths are shown in Table 3 below. Austenitic grain size was measured by KS standard (KS D 0205). The results are shown in Table 2 below.

As shown in Table 3 below, the surface ferrite decarburized layer of the billet after rolling the steel sheet shows a range of 0.06 to 0.09 mm in the case of the invention examples, while the comparative examples show a range of 0.20 to 0.33 mm, and also of the present invention after wire rolling. While the surface decarburization depth of the wire shows a range of 0.04-0.07 mm, in the comparative example, the range of 0.21-0.30 mm shows that the present invention is very effective in improving the wire decarburization.

As described above, the present invention provides a steel workpiece for mechanical structures and the like that can simultaneously improve the graphite grain cold forging and free machinability, and is appropriate for heating by reheating in a steel piece heating furnace by appropriately air-cooling the playing bloom to medium temperature. The ferrite decarburization layer can be distributed, and then, by suppressing the decarburization reaction during billet heating by wire heating, it is possible to provide a graphite steel wire for mechanical structure with less surface decarburization.

Claims (3)

By weight, carbon: 0.30-0.70%, silicon: 2.0-4.0%, manganese: 0.1-1.0%, phosphorus: 0.01-0.15%, sulfur: 0.01% or less, selenium (Se): 0.001-0.05%, titanium : 0.001-0.03%, boron: 0.001-0.003%, aluminum: 0.002-0.01%, nitrogen: 0.004-0.008%, oxygen: 0.005% or less, nickel: 0.05-1.0%, copper: 0.01-0.5 %, Calcium: 0.0001 to 0.05%, zirconium: 0.0005-0.008%, and rare earth metals: one or two or more selected from the group consisting of the remaining Fe and other impurities, and the titanium (Ti) , Nitrogen (N), boron (B), aluminum satisfy the following relationship, 2.0≤ (Ti + 5B + Al) /N≤5.5, while manganese, selenium (Se), sulfur are the next relationship, 1.0≤ (Mn / Obtaining a playing bloom satisfying 5 + Se) /5S≦3.0; Stepwise air cooling the bloom to 600 ± 100 ° C .; Reheating the bloom in a range of 1260 ± 30 ° C. for 100 ± 20 minutes and rolling the steel sheet to obtain a billet; And The billet is heated to a heating temperature of 1050 ± 50 ℃ at a heating rate of 10 ℃ / min or more and maintained for 30 minutes in this temperature section to obtain a wire rod by rolling the wire rod Manufacturing method of graphite steel wire for machine structure delete The method of producing a graphite steel wire for mechanical structure with low surface decarburization according to claim 1, wherein the granite ferrite structure fraction after cooling the wire is 10% or less.