JP2024500138A - Wire rod and graphite steel for graphitization heat treatment - Google Patents

Abstract

An object of the present invention is to provide a graphitizing heat-treated wire and a graphite steel in which fine graphite grains are uniformly distributed within the matrix during the graphitizing heat treatment while significantly shortening the graphitizing heat treatment time. [Solution] The wire rod for graphitization heat treatment of the present invention has C: 0.65 to 0.85%, Si: 2.00 to 3.00%, and Mn: 0.15 to 0.35% in weight percent. , Ti: 0.005 to 0.1%, N: 0.01% or less, B: 0.0005% or less, P: 0.05% or less, S: 0.05% or less, and the balance is Fe and Consisting of unavoidable impurities, the value of formula (1) below satisfies -1 or more and 1 or less, and the microstructure has an area fraction of 40% or less ferrite, 5% or less in total of bainite and martensite, and pearlite as the balance. It is characterized by including. Formula (1): 100*([Mn]-0.25)2-(100*[N])2 In the above formula (1), [Mn] and [N] mean the weight% of each alloying element. do. [Selection diagram] None

Description

The present invention relates to graphite steel that can be applied in various industrial fields such as mechanical parts, and more particularly to a wire rod for graphitization heat treatment and graphite steel that has short graphitization heat treatment time and excellent machinability.

Free-cutting steels to which machinability-imparting elements such as Pb, Bi, and S are added have been used as materials for small mechanical parts that require machinability. Pb-added free-cutting steel, which is the most typical free-cutting steel, emits harmful substances such as toxic fumes during cutting operations, which is extremely harmful to the human body and is also extremely disadvantageous for recycling steel materials. There is a problem that.

In order to replace Pb free-cutting steel which has such problems, free-cutting steels to which S, Bi, Te, Sn, etc. are added have been proposed. The machinability of S free-cutting steel to which S is added does not reach that of Pb free-cutting steel. Bi free-cutting steel to which Bi is added has a problem in that it easily cracks during manufacturing, making it extremely difficult to produce. Free-cutting steels to which Te and Sn are added also have the problem of causing cracks during hot rolling.

Graphite steel, which is a steel material replacing Pb free-cutting steel, is a steel in which graphite grains exist inside a ferrite base or a ferrite and pearlite base. Graphite steel has excellent machinability because the graphite grains inside the base structure act as a crack source during cutting, acting as a chip breaker and reducing friction with tools. .

However, graphite steel has the disadvantage that it requires a separate graphitization heat treatment for a long time to decompose the metastable cementite phase and precipitate graphite particles. This not only reduces productivity and increases costs, but also causes decarburization during the long graphitization heat treatment process, which has a negative effect on the performance of the final product.

In addition, even if graphite particles are precipitated through graphitization heat treatment, if the graphite particles are unevenly distributed in an irregular shape, the physical properties will be uneven during cutting, and the chip processability and surface roughness will be extremely poor. There is also the problem that tool life is shortened.

Korean Published Patent No. 1995-0006006

In order to solve the above-mentioned problems, the present invention provides graphitization heat-treated wire and graphite steel in which fine graphite grains are uniformly distributed within the base during graphitization heat treatment while significantly shortening the graphitization heat treatment time. This is what we are trying to provide.

The wire rod for graphitization heat treatment of the present invention has, in weight percent, C: 0.65 to 0.85%, Si: 2.00 to 3.00%, Mn: 0.15 to 0.35%, and Ti: 0. Contains .002 to 0.1%, N: 0.01% or less, B: 0.0005% or less, P: 0.05% or less, S: 0.05% or less, and the remainder is Fe and unavoidable impurities. Therefore, the value of the following formula (1) satisfies -1 or more and 1 or less, and the microstructure contains 40% or less of ferrite, 5% or less of bainite and martensite in total, and pearlite as the balance in terms of area fraction. Features.
Formula (1): 100*([Mn]-0.25) ² -(100*[N]) ²
In the above formula (1), [Mn] and [N] mean the weight percent of each alloying element.

Each wire rod for graphitization heat treatment of the present invention can satisfy the value of the following formula (2) of 6 or less.
Formula (2): 100*[Ti]+10000*[B]
In the above formula (2), [Ti] and [B] mean the weight percent of each alloying element.

Furthermore, the graphite steel of the present invention for achieving the above object has, in weight percent, C: 0.65 to 0.85%, Si: 2.0 to 3.0%, Mn: 0.15 to 0. 35%, Ti: 0.002 to 0.1%, N: 0.01% or less, B: 0.0005% or less, P: 0.05% or less, S: 0.05% or less, and the remainder is It consists of Fe and unavoidable impurities, the value of the following formula (1) satisfies -1 or more and 1 or less, and the microstructure is characterized by containing 80% or more of ferrite and graphite grains as the balance in terms of area fraction. .
Formula (1): 100*([Mn]-0.25) ² -(100*[N]) ²
In the above formula (1), [Mn] and [N] mean the weight percent of each alloying element.
In each graphite steel of the present invention, the value of the following formula (2) can satisfy 6 or less.
Formula (2): 100*[Ti]+10000*[B]
In the above formula (2), [Ti] and [B] mean the weight percent of each alloying element.

Each graphite steel of the present invention may have a tensile strength of 550 MPa or less.

The graphite steel according to the present invention can be applied to materials for precision mechanical parts such as automobiles, home appliances/electronic equipment, and industrial equipment. In particular, the present invention significantly shortens the graphitization heat treatment time by controlling the alloy composition, and dramatically reduces the manufacturing cost of graphite steel, while also ensuring uniform distribution of fine graphite grains within the matrix structure. , excellent machinability can be ensured.

As a means for achieving the above object, a wire rod for graphitization heat treatment according to an example of the present invention has, in weight percent, C: 0.65 to 0.85%, Si: 2.00 to 3.00%, Mn: 0.15 to 0.35%, Ti: 0.002 to 0.1%, N: 0.01% or less, B: 0.0005% or less, P: 0.05% or less, S: 0.05% Contains the following, with the remainder consisting of Fe and unavoidable impurities, the value of the following formula (1) satisfies -1 or more and 1 or less, and the microstructure has an area fraction of 40% or less of ferrite, the total of bainite and martensite. It may contain up to 5% and the balance pearlite.
Formula (1): 100*([Mn]-0.25) ² -(100*[N]) ²
In the above formula (1), [Mn] and [N] mean the weight percent of each alloying element.

In the following, preferred embodiments of the invention will be described. However, the embodiments of the present invention may be modified into various other forms, and the technical idea of the present invention is not limited to the embodiments described below. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

The terminology used in this application is merely used to describe specific examples. Accordingly, references to the singular include the plural unless the context clearly requires otherwise. Additionally, the words "comprising" and "comprising" as used in this application clearly refer to the presence of the features, steps, functions, components, or combinations thereof described in the specification. It should be noted that it is used for the purposes of the present invention and is not used to preliminarily exclude the existence of other features, steps, functions, components or combinations thereof.

It should be noted that, unless otherwise defined, all terms used herein are deemed to have the same meaning as commonly understood by a person of ordinary skill in the technical field to which this invention pertains. Should. Therefore, unless explicitly defined herein, certain terms should not be construed in an overly idealized or formal sense. For example, as used herein, the singular term includes the plural term unless the context clearly dictates otherwise.

Additionally, the terms "about," "substantially," and the like herein are used at or near that numerical value when manufacturing and material tolerances inherent in the recited meaning are used; Precise or absolute numbers are used to aid understanding or to prevent unconscionable infringers from taking unfair advantage of the referenced disclosures.

Graphite grains precipitated within the steel base improve machinability. Specifically, during cutting, graphite particles act as a solid lubricant to suppress the wear of the cutting tool, and act as a crack initiation point due to stress concentration to reduce cutting friction and allow cutting chips to be cut short. , improve machinability.

However, in order to graphitize, heat treatment is required to graphitize cementite within pearlite in the initial rolling structure. In order to precipitate graphite grains, a long graphitization heat treatment must be carried out, and such a long heat treatment not only increases costs, but also causes decarburization during the heat treatment, which deteriorates the quality of the final part. Problems that adversely affect performance occur.

First, in the present invention, large amounts of C and Si are added in order to shorten the long graphitization heat treatment while improving machinability. The more C there is, the more graphite grains will be formed after the graphitization heat treatment, and therefore the machinability will be more excellent. Si destabilizes cementite, promotes cementite decomposition, and as a result, graphitization heat treatment can be shortened. Among these, when Si is added excessively, there are problems such as wear of cutting tools and an increase in the difficulty of steel manufacturing.In particular, in conventional medium- and high-C based graphite steels, it is necessary to ensure cold forgeability. In addition, a small amount of Si was added. On the other hand, in the present invention, Si is added upward in an amount of 2.0% by weight or more to further promote graphitization.

Furthermore, the present invention mainly utilizes TiN nitride among AlN, BN, and TiN nitrides that act as nucleation sites for graphite grains. Since BN and AlN have low precipitation temperatures, after austenite is formed, they are concentrated at grain boundaries and precipitate non-uniformly. The unevenly precipitated BN and AlN act as nuclei for forming graphite particles during graphitization heat treatment, so there is a risk of causing uneven distribution of graphite particles. On the other hand, since TiN has a higher precipitation temperature than AlN and BN, it crystallizes before austenite formation is completed, so it is uniformly distributed at austenite grain boundaries and inside grains. In other words, TiN is more uniformly distributed within the microstructure than BN and AlN, and as a result, graphite grains formed with TiN as the nucleation target are uniformly distributed within the microstructure, and the graphite grains are Graphitization can be further promoted compared to BN and AlN, which grow in an unbalanced manner. Furthermore, uniformly distributed graphite particles can improve cutting properties such as chip processing properties.

The graphite steel according to the present invention is prepared by subjecting a wire rod for graphitization heat treatment to graphitization heat treatment. A wire rod for graphitization heat treatment according to an example of the present invention has, in weight percent, C: 0.65 to 0.85%, Si: 2.00 to 3.00%, Mn: 0.15 to 0.35%, and Ti. : 0.002 to 0.1%, N: 0.01% or less, B: 0.0005% or less, P: 0.05% or less, S: 0.05% or less, and the balance is Fe and unavoidable It can consist of impurities.

Hereinafter, the reason why the component composition of the wire rod for graphitization heat treatment is limited will be specifically explained. The reason for limiting the graphite steel alloy composition is the same as that for the wire rod for graphitization heat treatment, so it will be omitted for convenience.

The content of C is 0.65 to 0.85% by weight.
C is a component element constituting graphite grains, which is a cutting factor, and the higher the C content, the more graphite grains are formed. Further, the more C is added, the more the C activity increases, and as a result, cementite decomposition is promoted, and the graphitization heat treatment can be shortened. If the C content is less than 0.65% by weight, there is a problem that the C activity is poor and the machinability is reduced. On the other hand, when the C content exceeds 0.85% by weight, not only the effect of increasing C activity is saturated, but also the toughness of the steel material decreases due to excessively formed graphite grains, and then CD-Bar (cold drawn When drawing graphite steel to produce wire (bar), there is a risk of breakage occurring. Therefore, in the present invention, the C content is controlled to 0.65 to 0.85% by weight.

The content of Si is 2.00 to 3.00% by weight.
Si is a necessary component as a deoxidizing agent during the production of molten steel, and is a graphitization-promoting element that can destabilize cementite in steel and precipitate carbon as graphite, so it is actively added. If the Si content is less than 2.00% by weight, the graphitization rate may decrease rapidly. On the other hand, if the Si content exceeds 3.00% by weight, the graphitization promoting effect is saturated, and there is a risk of inducing brittleness due to an increase in nonmetallic inclusions and causing decarburization during hot rolling. Therefore, in the present invention, the Si content is controlled to 2.00 to 3.00% by weight.

The content of Mn is 0.15 to 0.35% by weight.
Mn improves the strength and impact properties of the steel material, combines with S in the steel, forms MnS inclusions, and contributes to improving machinability, so it is actively added. Moreover, if the Mn content is too low, there is a problem that the graphitization rate is inhibited by S which does not form MnS, and the material becomes brittle. Considering this, in the present invention, Mn is added in an amount of 0.15% by weight or more. However, if the Mn content is too high, the strength and hardness of the steel material may increase excessively, and the wear depth of the tool may decrease. Considering this, in the present invention, the Mn content is controlled to 0.35% by weight or less.

The content of Ti is 0.002 to 0.1% by weight or less.
Ti, like B, Al, etc., forms TiN, which is a nitride, and reduces the solid solution nitrogen content that inhibits graphitization, and the formed TiN acts as a nucleation site for graphite and promotes graphitization. Reduce time. BN and AlN, which act as nucleation sites for graphite, have low precipitation temperatures, so after austenite is formed, they are concentrated at grain boundaries and precipitate non-uniformly. However, since TiN has a higher precipitation temperature than AlN and BN, it crystallizes before austenite formation is completed, so that it is uniformly distributed at austenite grain boundaries and inside grains. In other words, TiN is more uniformly distributed within the microstructure than BN and AlN, and as a result, graphite grains formed using TiN as a nucleation site are uniformly distributed within the microstructure, and the graphite grains are unbalanced. Graphitization can be further promoted compared to BN and AlN, which grow as a result, and the machinability of graphite steel can be improved. Considering this, in the present invention, Ti is added in an amount of 0.002% by weight or more. On the other hand, if the Ti content is too high, the shortening effect of graphitization heat treatment by TiN will be saturated, and coarse carbonitrides will be formed, which may actually reduce graphitization. Considering this, in the present invention, Ti is controlled to 0.1% by weight or less.

The content of N is 0.01% by weight or less.
N combines with Ti, B, and Al to produce nitrides such as TiN, BN, and AlN. Among these, BN and AlN have low precipitation temperatures, so they are concentrated mainly at austenite grain boundaries and precipitate non-uniformly. The unevenly precipitated BN and AlN act as nuclei for forming graphite particles during graphitization heat treatment, so there is a risk of causing uneven distribution of graphite particles. Therefore, in order to suppress the formation of BN and AlN to the maximum extent while mainly precipitating TiN, it is necessary to appropriately adjust the N content. Further, if the N content is too large and does not combine with nitride-forming elements and exists as solid solution nitrogen in the steel, there is a problem that it stabilizes cementite and inhibits graphitization. Considering this, in the present invention, the N content is controlled to 0.01% by weight or less.

The content of B is 0.0005% by weight or less.
B combines with N to generate BN, and the generated BN acts as a nucleus for graphite grain generation during graphitization heat treatment. However, as described above, after austenite is formed, BN is concentrated at grain boundaries and precipitates non-uniformly. As a result, the graphite grains formed using BN as a nucleation site are also distributed non-uniformly, which may reduce machinability. Therefore, in order to prevent uneven distribution of graphite particles, it is preferable to control the upper limit of B to 0.0005% by weight.

The content of P is 0.05% by weight or less.
P is an impurity that is inevitably contained. P weakens the grain boundaries within the steel material and helps improve machinability. However, P increases the hardness of ferrite through a considerable solid solution strengthening effect, reduces the toughness and delayed fracture resistance of steel materials, and promotes the occurrence of surface defects. Therefore, it is preferable that the P content is managed as low as possible. In the present invention, the upper limit of P is controlled at 0.05% by weight.

The content of S is 0.05% by weight or less.
S is an impurity that is inevitably contained. S forms MnS and has the effect of improving machinability. However, when S exists alone in a steel material, it not only greatly inhibits the graphitization of C, but also segregates at grain boundaries, reducing toughness, forming low melting point sulfides, and improving hot rolling properties. inhibit. Further, although MnS has the effect of improving machinability, mechanical anisotropy may appear due to MnS stretched after rolling. Therefore, it is preferable that the S content is managed as low as possible. In the present invention, the upper limit of S is controlled at 0.05% by weight.

The remaining component of the present invention is iron (Fe). However, in normal manufacturing processes, unintended impurities may inevitably be mixed in from raw materials or the surrounding environment, and this cannot be eliminated. Since these impurities are known to anyone skilled in the art of ordinary manufacturing processes, their full contents are not specifically mentioned herein.

The wire rod for graphitization heat treatment according to an example of the present invention can satisfy the value of the following formula (1) of -1 or more and 1 or less while satisfying the above-mentioned alloy composition.

Formula (1): 100*([Mn]-0.25) ² -(100*[N]) ²

In the above formula (1), [Mn] and [N] mean the weight percent of each alloying element.

If the value of the above-mentioned formula (1) is less than -1, there are problems such as decreased machinability and increased graphitization heat treatment time. On the other hand, when the value of formula (1) exceeds 1, the strength and hardness of the steel increases, machinability decreases, and the graphitization heat treatment time increases.

Moreover, the wire rod for graphitization heat treatment according to an example of the present invention can satisfy the value of the following formula (2) of 6 or less while satisfying the above-mentioned alloy composition.

Formula (2): 100*[Ti]+10000*[B]

In the above formula (2), [Ti] and [B] mean the weight percent of each alloying element.

When the content of Ti and B increases and the value of formula (2) exceeds 6, the size of TiN that crystallizes becomes large, making it difficult to function as graphite grain generation nuclei, and graphitization heat treatment becomes difficult. The problem is that it takes a long time. Moreover, BN generated along grain boundaries acts as graphite grain generation nuclei, and graphite grains are generated non-uniformly. As a result, machinability deteriorates.

The microstructure of the wire rod for graphitization heat treatment according to an example of the present invention may include, in terms of area fraction, ferrite: 40% or less, bainite and martensite total: 5% or less, and pearlite as the balance.

The graphite steel according to the present invention is preferably prepared by subjecting a wire rod for graphitization heat treatment to graphitization heat treatment, and the microstructure of the graphite steel preferably includes ferrite and graphite as the balance. If pearlite remains, the hardness of the steel increases, resulting in tool wear problems during cutting and reduced machinability. According to one example, the microstructure of graphite steel may include, in area fraction, 80% or more of ferrite and the balance graphite grains.

Moreover, according to an example of the present invention, the tensile strength of graphitized steel subjected to graphitization heat treatment may be 550 MPa or less.

According to the present invention, the graphitization heat treatment time can be dramatically reduced. The wire rod for graphitization heat treatment is subjected to graphitization heat treatment at 730 to 770° C. for 6 hours or less to prepare graphite steel, and at this time, the graphitization rate of the graphite steel may be 99% or more. Here, the graphitization rate means the ratio of the carbon content existing in a graphite state to the carbon content added to steel, and is defined by the following formula (3), and the graphitization rate is 99% or more. This means that almost all of the added carbon was used up to produce graphite. In formula (3), the amount of solid solute carbon in ferrite and fine carbides are extremely small and are therefore not taken into account. In other words, a graphitization rate of 99% or more means that undecomposed pearlite does not exist in the steel, and the steel is composed of ferrite and graphite grains as the remainder.

Formula (3): Graphitization rate (%) = (1 - pearlite carbon content as remainder/carbon content in steel) * 100

Hereinafter, the method for manufacturing graphite steel according to the present invention will be explained in detail.

The graphite steel of the present invention described above can be manufactured by various methods, and the manufacturing method is not particularly limited. and a step of graphitizing heat treatment at 730 to 770° C. for 6 hours or less.

The hot rolling step may include casting an ingot having the above alloy composition, subjecting it to homogenization heat treatment at 1100 to 1300°C for 5 to 10 hours, and then hot rolling at 1000 to 1100°C. After hot rolling, the wire rod for graphitization heat treatment can be manufactured by air cooling at 8° C. or lower.

Thereafter, the wire rod for graphitization heat treatment is subjected to graphitization heat treatment at 730 to 770° C. for 6 hours or less, and can be prepared as graphite steel. According to the present invention, it is necessary to subject the wire rod for graphitization heat treatment to graphitization heat treatment to graphitize the cementite within the steel. In order to speed up graphitization, it is preferable to perform the heat treatment in a temperature range corresponding to the nose of the isothermal transformation curve. A preferable range of graphitization heat treatment temperature is 730 to 770° C. In this temperature range, all the cementite in pearlite in the steel material can be completely graphitized through isothermal heat treatment for 6 hours or less.

The graphite steel of the present invention as described above can be applied to materials for precision mechanical parts such as automobiles, home appliances/electronic equipment, and industrial equipment. In particular, the present invention significantly shortens the graphitization heat treatment time by controlling the alloy composition, and dramatically reduces the manufacturing cost of graphite steel, while also ensuring uniform distribution of fine graphite grains within the matrix structure. , excellent machinability can be ensured.

The graphite steel according to the present invention can be manufactured into precision mechanical parts for automobiles, home appliances/electronic devices, industrial equipment, etc. by wire drawing, cold forging, cutting, etc. In some cases, Q/T (quenching and tempering) heat treatment may be performed to ensure surface hardness after cutting.

Hereinafter, the present invention will be explained more specifically based on Examples. However, it should be noted that the following examples are intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention. This is because the scope of rights in the present invention is determined by matters stated in the claims and matters reasonably inferred from these matters.

{Example}
An ingot having the composition shown in Table 1 below was cast, subjected to homogenization heat treatment at 1250° C. for 6 hours, and then hot rolled and air cooled to produce a wire rod for graphitization heat treatment. During hot rolling, the finishing temperature was 1000°C.

"Equation (1)" and "Equation (2)" in Table 1 are values derived by substituting the alloy composition content into Equation (1) and Equation (2) described above.

Thereafter, the wire rods for graphitization heat treatment shown in Table 1 were subjected to graphitization heat treatment at 750° C. for 4 hours to produce graphite steel. However, in Comparative Examples 8 and 9, the graphitization heat treatment was performed at a graphitization heat treatment temperature of 711° C. and 803° C., respectively. The wire rod structure for graphitization heat treatment in Table 2 means the fine structure of the wire rod for graphitization heat treatment before the graphitization heat treatment. The graphite steel structure in Table 2 means the microstructure of graphite steel after graphitization heat treatment.

The wear depth of the tools in Table 2 was calculated by cutting 200 pieces of graphite steel of each invention example and comparative example with a diameter of 25 mm to a diameter of 15 mm, and then comparing the blade depths of the tools before and after cutting. I asked for the degree. At this time, cutting was performed using cutting oil at a cutting speed of 100 mm/min, a transfer speed of 0.1 mm/rev, and a cutting depth of 1.0 mm.

（Ｆ：ｆｅｒｒｉｔｅ、Ｐ：Ｐｅａｒｌｉｔｅ、Ｇ：Ｇｒａｐｈｉｔｅ）

(F: ferrite, P: Pearlite, G: Graphite)

Referring to the results in Tables 1 and 2, in Inventive Examples 1 to 9, there is no pearlite after the graphitization heat treatment, so the graphitization rate is 99% or more even with a short graphitization heat treatment of 4 hours, and the tensile strength The strength was 550 MPa or less, the tool wear depth was 200 mm or less, and the machinability was excellent.

On the other hand, in Comparative Examples 1 to 7, graphitization heat treatment was performed under the same graphitization heat treatment conditions as in the present invention, but the pearlite structure remained, graphitization was not completed, the tensile strength exceeded 550 MPa, and the tool was worn out. The depth exceeded 200 mm and machinability was poor.

More specifically, in Comparative Example 1, since the C content was low, the graphitization driving force was low and graphitization was not completed. In Comparative Example 2, since the Si content was high, brittleness was induced due to an increase in nonmetallic inclusions and decarburization occurred during hot rolling. In Comparative Example 3, MnS inclusions were formed, the remaining Mn inhibited graphitization, and graphitization was not completed. In Comparative Example 4, since there was little Mn to form MnS, MnS was not formed, and the remaining S inhibited graphitization, so graphitization was not completed. In Comparative Example 5, since the N content was high, nitrides such as TiN, AlN, and BN were formed, and the remaining N inhibited graphitization, and graphitization was not completed. In Comparative Example 7, the B content was too high, and most of the BN precipitated at the grain boundaries, inhibiting graphitization.

In particular, in the case of Comparative Examples 3 to 5, the graphitization was not completed and machinability deteriorated because the value range of formula (1) defined by the present invention was not satisfied.

In addition, Comparative Examples 6 and 7 did not satisfy the value range of formula (2) defined by the present invention, so the crystallized TiN size became very large and graphitization was not completed. Further, BN generated along grain boundaries acted as graphite grain generation nuclei, and graphite grains were generated non-uniformly. As a result, machinability deteriorated.

Comparative Examples 8 and 9 do not meet the graphitization heat treatment temperature defined by the present invention. As a result, in Comparative Example 8 in which the graphitization heat treatment temperature was very low, pearlite was not completely graphitized during the graphitization heat treatment. On the other hand, in Comparative Example 9, in which the graphitization heat treatment temperature was very high, the phase transformed to austenite, and pearlite was formed again upon cooling.

Although the exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and a person having ordinary knowledge in the technical field will understand the concept and scope of the claims described in this specification. It can be understood that various changes and modifications can be made without departing from the scope.

The wire rod and graphite steel for graphitization heat treatment according to the present invention can be used industrially because fine graphite grains are uniformly distributed within the base during graphitization heat treatment while the graphitization heat treatment time is significantly shortened. .

Claims (5)

In weight%, C: 0.65 to 0.85%, Si: 2.00 to 3.00%, Mn: 0.15 to 0.35%, Ti: 0.002 to 0.1%, N: 0.01% or less, B: 0.0005% or less, P: 0.05% or less, S: 0.05% or less, and the remainder consists of Fe and unavoidable impurities, and the value of the following formula (1) is -1 or more and 1 or less,
A wire rod for graphitization heat treatment, characterized in that the microstructure contains, in terms of area fraction, 40% or less of ferrite, 5% or less of a total of bainite and martensite, and pearlite as the balance.
Formula (1): 100*([Mn]-0.25) ² -(100*[N]) ²
(In the above formula (1), [Mn] and [N] mean the weight percent of each alloying element.) The wire rod for graphitization heat treatment according to claim 1, wherein the value of the following formula (2) satisfies 6 or less.
Formula (2): 100*[Ti]+10000*[B]
(In the above formula (2), [Ti] and [B] mean the weight percent of each alloying element.) In weight%, C: 0.65 to 0.85%, Si: 2.0 to 3.0%, Mn: 0.15 to 0.35%, Ti: 0.002 to 0.1%, N: 0.01% or less, B: 0.0005% or less, P: 0.05% or less, S: 0.05% or less, and the remainder consists of Fe and unavoidable impurities, and the value of the following formula (1) is -1 or more and 1 or less,
A graphite steel characterized in that the microstructure contains, in terms of area fraction, 80% or more of ferrite and the balance of graphite grains.
Formula (1): 100*([Mn]-0.25) ² -(100*[N]) ²
(In the above formula (1), [Mn] and [N] mean the weight percent of each alloying element.) The graphite steel according to claim 3, wherein the value of the following formula (2) satisfies 6 or less.
Formula (2): 100*[Ti]+10000*[B]
(In the above formula (2), [Ti] and [B] mean the weight percent of each alloying element.) The graphite steel according to claim 3, having a tensile strength of 550 MPa or less.