Wire rod and method for manufacturing same

Abstract

The present invention relates to a wire rod and a method for manufacturing same and, more specifically, to a wire rod and a method for manufacturing same, the wire rod containing, in wt%, 0.600-1.000% of carbon (C), 1.500-1.990% of silicon (Si), 0.100-0.600% of manganese (Mn), 0.010-0.150% of sulfur (S), 0.005-0.020% of titanium (Ti), 0.003-0.015% of nitrogen (N), 0.005-0.050% of chromium (Cr), 0.005-0.050% of molybdenum (Mo), 0.005-0.150% of nickel (Ni), 0.005-0.150% of copper (Cu), 0.005-0.100% of aluminum (Al), and 0.005-0.050% of phosphorus (P), with the remainder comprising Fe and other impurities, and having a microstructure including graphite grains and ferrite.

Description

Wire rod and method of manufacturing the same

[0001] The present invention relates to a wire and a method for manufacturing the same.

[0002] Free-cutting steels with large amounts of machinability-enhancing elements such as Pb, Bi, and S are used as materials for machine parts that require machinability. By adding low-melting-point machinability-enhancing elements such as Pb, Bi, and S to the steel, liquid metal embrittlement is utilized, or a large amount of MnS is formed within the steel. Such free-cutting steels exhibit excellent machinability, including surface roughness, chip handling, and tool life, during machining.

[0003] Graphite free-cutting steel is a steel containing fine graphite particles within a ferrite matrix or a ferrite and pearlite matrix, and the fine graphite particles within it act as a crack source during cutting, thereby improving machinability by acting as a chip breaker.

[0004] Despite the advantages of graphite free-cutting steel, the bit wear characteristics are severe compared to lead free-cutting steel, resulting in a short bit replacement cycle. In addition, there was a problem that it was uneconomical because graphitization heat treatment had to be performed for a long time.

[0005] The objective of the present invention, which aims to solve the aforementioned problem, is to provide a wire rod with excellent machinability and a method for manufacturing the same, which improves bit wear characteristics by lowering material hardness while shortening the graphitization heat treatment time.

[0006] The technical problems to be solved in this document are not limited to those mentioned above, and other technical problems not mentioned will be clearly understood by those skilled in the art to which this invention belongs from the description below.

[0007] To achieve the above objective, a wire rod according to one embodiment of the present invention comprises, in weight %, carbon (C): 0.600%~1.000%, silicon (Si): 1.500%~1.990%, manganese (Mn): 0.100%~0.600%, sulfur (S): 0.010%~0.150%, titanium (Ti): 0.005%~0.020%, nitrogen (N): 0.003%~0.015%, chromium (Cr): 0.005%~0.050%, molybdenum (Mo): 0.005%~0.050%, nickel (Ni): 0.005%~0.150%, copper (Cu): 0.005%~0.150%, aluminum (Al): 0.005%~0.100%, phosphorus (P): It contains 0.005% to 0.050%, the remainder being Fe and other impurities, and the microstructure may contain graphite grains and ferrite.

[0008] In addition, according to one embodiment of the present invention, the area fraction of the graphite particles may be 1.0% to 4.0%, and the average diameter of the graphite particles may be 0.5μm to 5.0μm.

[0009] In addition, according to one embodiment of the present invention, the area fraction of the graphite particles may be 1.0% to 3.0%, and the average diameter of the graphite particles may be 1.8 μm to 4.5 μm.

[0010] In addition, according to one embodiment of the present invention, the area fraction of the graphite particles may be 3.0% to 4.0%, and the average diameter of the graphite particles may be 1.1 μm to 4.0 μm.

[0011] In addition, the HRB hardness of the wire according to one embodiment of the present invention may be 95 or less.

[0012] In addition, the wire according to one embodiment of the present invention may have a graphite fraction of 95% or more according to the following formula (1).

[0013] Formula (1): (1 - Carbon content in undissolved pearlite / Carbon content in steel) x 100

[0014] In addition, a method for manufacturing a wire rod according to one embodiment of the present invention comprises, in weight%, carbon (C): 0.600%~1.000%, silicon (Si): 1.500%~1.990%, manganese (Mn): 0.100%~0.600%, sulfur (S): 0.010%~0.150%, titanium (Ti): 0.005%~0.020%, nitrogen (N): 0.003%~0.015%, chromium (Cr): 0.005%~0.050%, molybdenum (Mo): 0.005%~0.050%, nickel (Ni): 0.005%~0.150%, copper (Cu): 0.005%~0.150%, aluminum (Al): 0.005%~0.100%, phosphorus (P): The method may include the steps of: manufacturing a billet containing 0.005% to 0.050%, the remainder being Fe and other impurities; heating the billet at 950°C to 1150°C for 60 minutes or more; hot rolling the heated billet at 900°C to 1150°C; cooling the hot-rolled billet to 500°C at a cooling rate of 0.1°C / s to 10°C / s in a temperature range of 750°C to 900°C; air-cooling the cooled billet; and, after the air-cooling step, performing a graphitization heat treatment for 3 hours or more in a temperature range of A1-100°C to A1.

[0015] In addition, the microstructure of the wire rod that has undergone the graphitization heat treatment step according to one embodiment of the present invention may include graphite grains and ferrite.

[0016] In addition, according to one embodiment of the present invention, the area fraction of the graphite particles may be 1.0% to 4.0%, and the average diameter of the graphite particles may be 0.5μm to 5.0μm.

[0017] In addition, according to one embodiment of the present invention, the area fraction of the graphite particles may be 1.0% to 3.0%, and the average diameter of the graphite particles may be 1.8 μm to 4.5 μm.

[0018] In addition, according to one embodiment of the present invention, the area fraction of the graphite particles may be 3.0% to 4.0%, and the average diameter of the graphite particles may be 1.1 μm to 4.0 μm.

[0019] In addition, a method for manufacturing a wire rod having a graphitization fraction of 95% or more according to the following formula (1) of the wire rod that has undergone the graphitization heat treatment step according to one embodiment of the present invention.

[0020] Formula (1): (1 - Carbon content in undissolved pearlite / Carbon content in steel)Y 100

[0021] According to one embodiment of the present invention, by controlling the alloy composition and graphitization heat treatment conditions, the material hardness is lowered to improve bite wear characteristics, and by controlling the average diameter and area fraction of graphite grains, a wire rod with excellent machinability and a method for manufacturing the same can be provided.

[0022] Furthermore, the wire of the present invention has excellent machinability and can be applied to CNC lathes and CAM automatic lathes.

[0023] The effects obtainable from the present disclosure are not limited to those mentioned above, and other unmentioned effects will be clearly understood by those skilled in the art to which the present disclosure belongs from the description below.

[0024] Figure 1 is a photograph of the microstructure measured with an optical microscope after graphitization heat treatment according to one embodiment.

[0025] Figure 2 is a scanning electron microscope (SEM) image of the graphite grain distribution after graphitization heat treatment according to one embodiment.

[0026] Figure 3 is a photograph of the chip length distribution after a cutting evaluation according to one embodiment.

[0027] Figure 4 is a photograph of the chip length distribution after cutting evaluation according to one comparative example.

[0028] Preferred embodiments of the present invention are described below. However, embodiments of the present invention may be modified in various other forms, and the technical concept of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more completely explain the present invention to those with average knowledge in the relevant technical field.

[0029] The terms used in this application are used merely to describe specific examples. For this reason, singular expressions include plural expressions unless the context clearly requires them to be singular. Additionally, it should be noted that terms such as “comprising” or “comprising” used in this application are used to clearly indicate the presence of features, steps, functions, components, or combinations thereof described in the specification, and are not used to preliminarily exclude the existence of other features, steps, functions, components, or combinations thereof.

[0030] Meanwhile, unless otherwise defined, all terms used in this specification shall be understood to have the same meaning as generally understood by those skilled in the art to which the present invention pertains. Accordingly, unless explicitly defined in this specification, specific terms should not be interpreted in an overly ideal or formal sense.

[0031] Additionally, terms such as "about," "substantially," etc., in this specification are used to mean at or near the stated value when inherent manufacturing and material tolerances are presented in the said sense, and are used to prevent unscrupulous infringers from unfairly exploiting the disclosed content in which precise or absolute values ​​are mentioned to aid in understanding the invention.

[0032] Unless otherwise specifically stated in this specification, the % indicating the content of each element is based on weight.

[0033] A wire rod according to one embodiment of the present invention comprises, in weight%, carbon (C): 0.600%~1.000%, silicon (Si): 1.500%~1.990%, manganese (Mn): 0.100%~0.600%, sulfur (S): 0.010%~0.150%, titanium (Ti): 0.005%~0.020%, nitrogen (N): 0.003%~0.015%, chromium (Cr): 0.005%~0.050%, molybdenum (Mo): 0.005%~0.050%, nickel (Ni): 0.005%~0.150%, copper (Cu): 0.005%~0.150%, aluminum (Al): 0.005%~0.100%, phosphorus (P): It contains 0.005% to 0.050%, and the remainder is Fe and other impurities.

[0034] Hereinafter, the reason for the numerical limitation of the alloy component content in the embodiments of the present invention will be explained.

[0035] The carbon (C) content may be 0.600% to 1.000%.

[0036] Carbon is an essential element for forming graphite grains. If the carbon content is less than 0.600%, the effect of improving machinability is insufficient, and even when graphitization is complete, the distribution of graphite grains is uneven and cementite formation is difficult. On the other hand, if the carbon content is excessive at 1.000% or more, coarse graphite grains are formed, and there is a risk that surface roughness will decrease due to an increase in intergranular precipitated graphite grains. Considering this, it is desirable to control the C content to 0.600% to 1.000%.

[0037] The silicon (Si) content may be 1.500% to 1.990%.

[0038] Silicon is a necessary component as a deoxidizer during molten steel production and is actively added because it is a graphitization-promoting element that destabilizes cementite in the steel, allowing carbon to precipitate as graphite. To achieve this, the Si content must be at least 1.500% to enable commercial graphitization heat treatment. On the other hand, if the content is excessive, not only does the effect become saturated, but the solid solution strengthening effect also increases hardness, accelerating tool wear during cutting; it may induce brittleness due to an increase in non-metallic inclusions; and there is a risk of excessive decarburization during hot rolling. Considering this, it is desirable to control the Si content to between 1.500% and 1.990%.

[0039] The manganese (Mn) content may be 0.100% to 0.600%.

[0040] Manganese improves the strength and impact properties of steel and contributes to improved machinability by combining with sulfur in the steel to form MnS inclusions. In order to exhibit these effects in the present invention, it is preferable to include 0.100% or more. On the other hand, if the content is excessive, it may inhibit graphitization, potentially delaying the completion time of graphitization, and may increase strength and hardness, thereby reducing machinability. Therefore, it is preferable that the upper limit of the manganese content be 0.600%.

[0041] The content of S (sulfur) can be 0.010% to 0.150%.

[0042] Sulfur is a very important component that improves machinability by forming MnS, TiS, and Ti4C2S2 inclusions in steel. When sulfur is excessive, the formation of MnS inclusions improves machinability, but mechanical anisotropy occurs due to the MnS elongated by rolling. Therefore, in the present invention, the formation of MnS is induced by controlling the S content within a range that contributes to improving machinability without causing mechanical anisotropy, and machinability can be improved by forming Ti-based sulfides such as TiS and Ti4C2S2 through the combination of S and Ti. If the S content is less than 0.010%, it may be difficult to form sulfides such as MnS, TiS, and Ti4C2S2. Furthermore, if the S content exceeds 0.150%, surface defects may occur, causing difficulties in manufacturing parts. Considering this, it is desirable to control the S content to between 0.010% and 0.150%.

[0043] The chromium (Cr) content may be 0.005% to 0.050%.

[0044] Chromium is an element that promotes the transformation of lamellar pearlite into spherical cementite during graphitization heat treatment. This is because chromium destabilizes the cementite within the pearlite, and to exhibit this effect, it is desirable to include at least 0.005% of Cr. On the other hand, if the chromium content is excessive, the formation of graphite grains may be delayed. Considering this, it is desirable to control the Cr content to between 0.005% and 0.050%.

[0045] The molybdenum (Mo) content may be 0.005% to 0.050%.

[0046] Molybdenum is an element that promotes the transformation of lamellar pearlite into spherical cementite during graphitization heat treatment. This is because molybdenum destabilizes the cementite within the pearlite, and it is desirable to include at least 0.005% to exhibit this effect. On the other hand, if the content is excessive, the formation of graphite grains is delayed. Therefore, it is desirable that the upper limit of the molybdenum content be 0.050%.

[0047] The nickel (Ni) content may be 0.005% to 0.150%.

[0048] Nickel is an element that plays a positive role in the formation of graphite grains during graphitization heat treatment. This is because nickel destabilizes cementite within pearlite, thereby promoting graphitization. In particular, it has the effect of shortening the graphitization time associated with cold working. To achieve this effect, it is desirable to include at least 0.005%. However, if the content is excessive, it inhibits graphite grain growth, so an upper limit of 0.150% is desirable.

[0049] The copper (Cu) content may be 0.005% to 0.150%.

[0050] Copper segregates at grain boundaries and has a positive effect on chip fragmentation during automatic lathe cutting. This is because the chip length is shortened, which reduces the negative effect of chips clumping on the cutting tool during machining. To achieve this effect, it is desirable to include at least 0.005%. However, if the content is excessive, the occurrence of surface defects increases significantly, making the product unsuitable for use. Therefore, it is desirable that the upper limit of the copper content be 0.150%.

[0051] The aluminum (Al) content may be 0.005% to 0.100%.

[0052] Aluminum combines with oxygen to form Al-O-based oxides, and these oxides serve as nuclei for the formation of graphite particles. Consequently, graphite particle nuclei are formed starting from Al oxides, accelerating the graphitization rate. To achieve this effect, it is desirable to include at least 0.005%, but since an excessive amount can cause surface defects, an upper limit of 0.100% is desirable.

[0053] The phosphorus (P) content may be 0.005% to 0.050%.

[0054] Phosphorus is segregated at grain boundaries and has a positive effect on chip fragmentation during automatic lathe cutting. This is because the chip length is shortened, which reduces the negative effect of chips clumping on the cutting tool during machining. For this effect, it is desirable to include at least 0.005%. However, since an excessive content causes inhibition of graphitization, it is desirable to have an upper limit of 0.050%.

[0055] The titanium (Ti) content may be 0.005% to 0.020%.

[0056] Titanium combines with nitrogen, similar to boron and aluminum, to form nitrides such as TiN, BN, and AlN; these nitrides act as nuclei for graphite formation during graphitization heat treatment. However, while BN and AlN precipitate non-uniformly at grain boundaries after austenite is formed due to their lower formation temperatures, TiN precipitates before austenite formation is complete because its higher formation temperature results in a uniform distribution at both the grain boundaries and within the austenite grains. Consequently, the graphite particles formed using TiN as a nucleation site are also fine and uniformly distributed. Furthermore, titanium combines with sulfur to form sulfides such as TiS and Ti4C2S2; these sulfides play a role in improving chip fragmentation during machining. Like sulfides such as MnS, they have the effect of reducing cutting forces. In order to exhibit these effects, it is desirable to include at least 0.005% by weight, but if the content exceeds 0.020%, it becomes a coarse carbonitride and consumes the carbon required for graphite formation, thereby inhibiting graphitization. Therefore, it is desirable that the upper limit of the titanium content be 0.020%.

[0057] The nitrogen (N) content can be 0.003% to 0.015%.

[0058] Nitrogen combines with titanium and aluminum to form TiN and AlN, and in particular, AlN nitrides are mainly formed at austenite grain boundaries. Since graphite is formed using these nitrides as nuclei during graphitization heat treatment, it is necessary to add an appropriate amount to prevent the uneven distribution of graphite. If the amount of added nitrogen is excessive and it fails to combine with nitride-forming elements, it remains in the steel as dissolved nitrogen, which has a harmful effect of increasing strength and stabilizing cementite to delay graphitization. Therefore, in order to ensure that the nitrogen is consumed in forming nitrides that act as graphite nucleation sites and does not remain as dissolved nitrogen, it is preferable in the present invention to control the amount to 0.003% to 0.015%.

[0059] The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the ordinary manufacturing process, they cannot be excluded. As these impurities are known to any person skilled in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0060] By controlling the graphite grain distribution through the alloy composition and manufacturing method presented in the present invention, a wire rod with improved machinability can be provided.

[0061] By lowering the material hardness through the alloy composition and manufacturing method presented in the present invention, it is possible to provide a wire rod with improved bite wear characteristics.

[0062] A wire rod according to one embodiment of the present invention may include graphite grains and ferrite in its microstructure after graphitization heat treatment.

[0063] In addition, the wire according to one embodiment of the present invention may have a graphite grain area fraction of 1.0% to 4.0% and an average graphite grain diameter of 0.5㎛ to 5.0㎛.

[0064] In addition, the wire according to one embodiment of the present invention may have a graphite grain area fraction of 1.0% to 3.0% and an average graphite grain diameter of 1.8㎛ to 4.5㎛.

[0065] In addition, the wire according to one embodiment of the present invention may have a graphite grain area fraction of 3.0% to 4.0% and an average graphite grain diameter of 1.1㎛ to 4.0㎛.

[0066] Meanwhile, in the present invention, the average refers to the average diameter measured by image processing of an optical microscope photograph using an image analysisr device.

[0067] In addition, the wire according to one embodiment of the present invention may have a graphite content of 95% or more according to the following formula (1).

[0068] Formula (1): (1 - Carbon content in undissolved pearlite / Carbon content in steel) x 100

[0069] Next, a method for manufacturing a wire rod according to one embodiment of the present invention will be described.

[0070] A method for manufacturing a wire rod according to one embodiment of the present invention comprises, in weight%, carbon (C): 0.600%~1.000%, silicon (Si): 1.500%~1.990%, manganese (Mn): 0.100%~0.600%, sulfur (S): 0.010%~0.150%, titanium (Ti): 0.005%~0.020%, nitrogen (N): 0.003%~0.015%, chromium (Cr): 0.005%~0.050%, molybdenum (Mo): 0.005%~0.050%, nickel (Ni): 0.005%~0.150%, copper (Cu): 0.005%~0.150%, aluminum (Al): 0.005%~0.100%, phosphorus (P): The method may include the steps of: manufacturing a billet containing 0.005% to 0.050%, the remainder being Fe and other impurities; heating the billet; hot-rolling the heated billet; cooling the hot-rolled billet; and naturally air-cooling the cooled billet.

[0071] The reason for the numerical limitation of the component ranges of each alloy composition above is as described above, and each manufacturing step is explained in more detail below.

[0072] After manufacturing a billet satisfying the above alloy composition, it may undergo a series of processes including heating, hot rolling, cooling, and natural air cooling.

[0073] First, the billet can be heated by maintaining it at 950°C to 1150°C for at least 60 minutes.

[0074] If the heating temperature of the billet is below 950℃, the load during rolling increases, which may reduce rolling productivity. Furthermore, if the heating temperature exceeds 1150℃, not only do costs increase, but decarburization is accelerated, resulting in a thickened decarburized layer that remains in the final product, which can adversely affect product performance. Since it is difficult to secure a uniform temperature distribution inside and outside the billet for wire rod rolling if the heating time is less than 60 minutes, it is desirable for the heating time to be 60 minutes or more.

[0075] After the above heating, hot rolling can be performed at 900°C to 1150°C.

[0076] If the above hot rolling temperature is less than 900℃, surface defects may easily occur during hot rolling or the rolling load may increase, making rolling difficult; if it exceeds 1150℃, the austenite grain size may coarsen, and the subsequent graphitization heat treatment time may be prolonged.

[0077] Next, the rolled wire rod can be cooled to 500°C at a cooling rate of 0.1 to 10°C / s in a temperature range of 750°C to 900°C.

[0078] The above cooling step is a step prior to graphitization heat treatment, and it is necessary to control the cooling rate so that graphite particles can be uniformly distributed within the matrix. If the cooling rate is 10℃ / s or higher, the hardness may increase excessively and ductility may decrease, and if it is less than 0.1℃ / s, the proeutectoid phase may be excessively formed, causing the grain size to coarsen and the graphite particles produced after graphitization heat treatment may have an uneven distribution.

[0079] In addition, the present invention may further include a natural air cooling step after the cooling step.

[0080] After the above natural air cooling step, a step of graphitizing heat treatment at A1-100℃ to A1 temperature for at least 3 hours may be further included. The eutectoid transformation of steel is called the A1 transformation, and the temperature at which this transformation occurs is called the A1 temperature. Eutectoid transformation refers to the process in which austenite changes into ferrite and cementite in the iron-carbon (Fe-C) phase diagram.

[0081] The microstructure of the wire rod prior to the above graphitization heat treatment step may include pearlite, proeutectoid ferrite, and proeutectoid cementite. These structures may undergo a phase transformation into ferrite and graphite grains through graphitization heat treatment.

[0082] The microstructure of the wire rod that has undergone the above graphitization heat treatment step may include graphite grains and ferrite.

[0083] The area fraction of the graphite particles is 1.0% to 4.0%, and the average diameter of the graphite particles may be 0.5μm to 5.0μm.

[0084] The area fraction of the graphite particles is 1.0% to 3.0%, and the average diameter of the graphite particles may be 1.8 μm to 4.5 μm.

[0085] The area fraction of the graphite particles is 3.0% to 4.0%, and the average diameter of the graphite particles may be 1.1 μm to 4.0 μm.

[0086] The graphitization fraction of the wire rod that has undergone the above graphitization heat treatment step according to the following equation (1) can be 95% or more.

[0087] Formula (1): (1 - Carbon content in undissolved pearlite / Carbon content in steel) x 100.

[0088] A graphitization fraction of 95% or higher means that most of the added carbon has been consumed in the formation of graphite. Therefore, if the graphitization fraction satisfies 95% or higher, it can be determined that a microstructure has been realized in which almost no undecomposed pearlite exists and graphite grains are uniformly distributed within the ferrite matrix. Meanwhile, since the amount of carbon dissolved in the ferrite and carbon dissolved in the fine carbides is extremely small, it was not considered in this invention.

[0089] If the graphitization heat treatment temperature is below A1-100℃, the graphitization heat treatment time becomes prolonged, making commercial application difficult; conversely, if the temperature is above A1, austenite transformation occurs, preventing graphitization from proceeding and causing uneven graphitization during cooling, which is undesirable. Considering this, it is preferable to control the temperature to between A1-100℃ and A1.

[0090] If the above graphitization heat treatment time is less than 3 hours, the hardness increases due to insufficient graphitization and the machinability decreases.

[0091] In addition, the present invention may further include a step of producing a steel wire after drawing the wire that has undergone the graphitization heat treatment step, and then cutting it using a CNC lathe or a CAM automatic lathe.

[0092] Examples

[0093] Billets were manufactured to satisfy the various alloy compositions shown in Table 1 below. The units in Table 1 below are weight%.

[0094] The above billet was maintained at 950°C to 1150°C for 90 minutes, and then wire rod was rolled at 900°C to 1150°C to produce a wire rod with a diameter of 16 mm.

[0095] CSiMnSTiNCrMoNiCuAlP Example 1 0.65 1.94 0.43 0.073 0.007 0.013 0.019 0.04 0.011 0.109 0.007 0.049 Example 2 0.61 1.72 0.3 0.072 0.009 0.005 0.03 0.037 0.109 0.013 0.038 0.022 Example 3 0.84 1.62 0.13 0.088 0.008 0.014 0.018 0.029 0.057 0.032 0.099 0.034 Example 4 0.83 1.65 0.46 0.01 0.013 0.01 0.015 0.05 0.035 0.0130.0590.012 Example 50.771.620.410.0170.0060.0090.0140.0260.0550.1350.0260.006 Example 60.711.790.150.1320.0130.0040.0210.0310.0780.0 410.0840.026 Example 70.841.570.270.1040.0190.0060.0160.0140.1230.0770.0820.031 Example 80.971.760.120.0580.0170.0040.0180.0170.1240.0660 .0780.016 Example 90.681.570.430.0660.0150.0060.0440.0280.0090.0200.0860.033 Example 100.911.960.600.0540.0060.0090.0370.0220.0130.1340.0 880.035 Example 1 10.9 41.75 0.3 70.01 70.02 00.01 30.02 00.03 70.00 90.08 60.09 80.041 Example 1 20.7 01.65 0.2 00.01 10.01 00.01 40.04 60.04 40.09 90.10 50.09 70.039 Example 1 30.62 1.61 0.43 0.12 20.013 0.005 0.008 0.016 0.071 0.028 0.089 0.022 Example 1 40.961 84 0.55 0.063 0.011 0.015 0.034 0.030 0.144 0.139 0.033 0.039 Example 1 50.78 1.6 10.2 00.09 50.02 00.01 50.03 00.03 10.01 30.07 10.09 90.008 Comparative Example 1 0.5 31.4 20.07 0.00 60.00 40.00 10.00 30.00 30.00 10.00 40.00 10.00 10.00 10.00 40.00 10.004Comparative Example 20.46 1.39 0.04 0.008 0.003 0.001 0.004 0.004 0.003 0.002 0.003 0.002 Comparative Example 30.41 1.01 0.09 0.006 0.001 0.001 0.002 0.003 0.001 0.002 0.002 0.004Comparative Example 40.35 1.360 .050.0050.0040.0010.0030.0030.0010.0010.0040.001Comparative Example 50.331.040.050.0020.0020.0010.0030.0030.0040.0020.0040.003Comparative Example 61.182.160.840.1870.0500.03 10.09 40.07 40.18 60.19 40.17 50.09 4 Comparative Example 7 1.05 2.4 70.6 70.23 00.04 70.03 40.07 10.10 90.16 10.17 20.19 60.10 2 Comparative Example 8 1.1 42.2 70.9 70.20 70.03 10.05 40.09 70.08 70.176 0.1920.1850.112 Comparative Example 91.192.350.860.1570.0410.0550.0610.0670.1810.1740.1270.069 Comparative Example 101.132.160.920.1820.0550.0470.0970.0730.2000.1620.1300.074.

[0096] Table 2 below shows the results of the graphite grain distribution (average diameter and area fraction), hardness, and machinability evaluation according to the heat treatment conditions for graphitization. The average diameter and area fraction of graphite particles were measured based on images taken with a scanning electron microscope (SEM). The area fraction was calculated and presented as the area fraction of graphite particles within a 100㎛ x 100㎛ area range. Hardness was evaluated as having good tool wear characteristics if it was lower than or equal to the hardness of commonly used lead free-cutting steel. The HRB hardness of lead free-cutting steel was measured to be 98 or lower. In addition, the machinability evaluation results were measured using a CAM-type automatic lathe; results were evaluated as good if similar to lead free-cutting steel and poor if they fell short.

[0097] Graphitization Temperature (°C) Graphitization Time (hr) Average Graphite Grain Diameter (μm) Graphite Grain Area Fraction (%) Hardness (HRB) Machinability Example 1 A1-90 8.2 3.3 2.789 Good Example 2 A1-59 4.1 1.8 1.289 Good Example 3 A1-47 4.3 3.994 Good Example 4 A1-99 4.5 3.1992 Good Example 5 A1-99 7.5 3.3 3.190 Good Example 6 A1-96.2 2.1 3.790 Good Example 7A1-473.94.5192 Good Example 8A1-569.32.33.985 Good Example 9A1-917.93.93.992 Good Example 10A1-859.43.43.594 Good Example 11A1-9810.73.72.195 Good Example 12A1-1004.42.33.991 Good Example 13A1 -783.81.13.291 Good Example 14A1-229.72.63.789 Good Example 15A1-807.643.490 Good Comparative Example 1A1-1971.70.10.5105 Poor Comparative Example 2A1-1750.70.30.8103 Poor Comparative Example 3A1-1881.30.40.2103 Poor Comparative Example 4A1-1380.80.10 .8100 Defective Comparison Example 5A1-1642.50.20.4101 Defective Comparison Example 6A1-1120.95.94.2100 Defective Comparison Example 7A1-1292.16.54.4102 Defective Comparison Example 8A1-1492.25.7599 Defective Comparison Example 9A1-1372.85.54.2101 Defective Comparison Example 10A1-1981.05.94.8100 Defective

[0098] As shown in Table 2, it was confirmed that the area fraction of graphite grains in the microstructure measured after graphitization heat treatment according to an example satisfying the alloy composition and graphitization heat treatment conditions presented in the present invention was 1.0% to 4.0%, and the average diameter was 0.5 μm to 5.0 μm. Specifically, it was confirmed that when the area fraction of graphite grains was 1.0% to 3.0%, the average diameter of the graphite grains was 1.8 μm to 4.5 μm. In addition, it was confirmed that when the area fraction of graphite grains was 3.0% to 4.0%, the average diameter of the graphite grains was 1.1 μm to 4.0 μm. It was found that machinability was also good when the distribution of graphite grains was as described above. On the other hand, in the case of Comparative Examples 1 to 5, which do not satisfy the alloy composition and graphitization heat treatment conditions presented in the present invention, the average diameter of the graphite grains was less than 0.5 μm and the graphite grain area fraction was less than 1%, which did not satisfy the requirements presented in the present invention, and accordingly, the hardness was high and the machinability was poor.

[0099] Meanwhile, in the case of Comparative Examples 6 to 10, which do not satisfy the alloy composition and graphitization heat treatment conditions presented in the present invention, the average diameter of the graphite grains exceeded 5 μm and the graphite grain area fraction exceeded 5%, thus failing to satisfy the requirements presented in the present invention, and accordingly, the hardness was high and the machinability was poor.

[0100] Figure 2 is a scanning electron microscope (SEM) image of the graphite grain distribution after graphitization heat treatment according to an embodiment of the present invention. Referring to Figure 2, it can be seen that the microstructure has a uniform distribution of graphite grains, and the graphitization fraction is 95% or more.

[0101] Through these results, it was found that only by satisfying both the alloy composition and graphitization heat treatment conditions presented in the present invention can the heat treatment time be significantly shortened while controlling the uniform distribution of graphite particles within the matrix, thereby improving machinability and suppressing wear of the cutting tool, and that such a wire rod can be manufactured, and that such a wire rod can be applied to CNC lathes and CAM automatic lathes.

[0102] Although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and those skilled in the art will understand that various changes and modifications are possible within the scope and concept of the claims set forth below.

Claims

1. In wt%, Carbon (C): 0.600%–1.000%, Silicon (Si): 1.500%–1.990%, Manganese (Mn): 0.100%–0.600%, Sulfur (S): 0.010%–0.150%, Titanium (Ti): 0.005%–0.020%, Nitrogen (N): 0.003%–0.015%, Chromium (Cr): 0.005%–0.050%, Molybdenum (Mo): 0.005%–0.050%, Nickel (Ni): 0.005%–0.150%, Copper (Cu): 0.005%–0.150%, Aluminum (Al): 0.005%–0.100%, Phosphorus (P): 0.005%–0.050%, remainder Fe A wire rod containing and other impurities, and having a microstructure containing graphite grains and ferrite.

2. In Claim 1, A wire rod having an area fraction of the graphite grains of 1.0% to 4.0% and an average diameter of the graphite grains of 0.5μm to 5.0μm.

3. In Claim 1, A wire rod having an area fraction of the graphite grains of 1.0% to 3.0% and an average diameter of the graphite grains of 1.8 μm to 4.5 μm.

4. In Claim 1, A wire rod having an area fraction of the graphite grains of 3.0% to 4.0% and an average diameter of the graphite grains of 1.1 μm to 4.0 μm.

5. In Claim 1, The above wire has an HRB hardness of 95 or less.

6. In Claim 1, A wire with a graphite content of 95% or more according to the following formula (1). Formula (1): (1 - Carbon content in undecomposed pearlite / Carbon content in steel) x 100 7. In wt%, Carbon (C): 0.600%–1.000%, Silicon (Si): 1.500%–1.990%, Manganese (Mn): 0.100%–0.600%, Sulfur (S): 0.010%–0.150%, Titanium (Ti): 0.005%–0.020%, Nitrogen (N): 0.003%–0.015%, Chromium (Cr): 0.005%–0.050%, Molybdenum (Mo): 0.005%–0.050%, Nickel (Ni): 0.005%–0.150%, Copper (Cu): 0.005%–0.150%, Aluminum (Al): 0.005%–0.100%, Phosphorus (P): 0.005%–0.050%, remainder Fe A step of manufacturing a billet containing and other impurities; A step of heating the above billet at 950 to 1150℃ for at least 60 minutes; A step of hot rolling the heated billet at 900 to 1150℃; A step of cooling the hot-rolled billet to 500℃ at a cooling rate of 0.1 to 10℃ / s in a temperature range of 750 to 900℃; A step of naturally air-cooling the above-mentioned cooled billet; and A method for manufacturing a wire rod comprising: a step of graphitizing heat treatment for at least 3 hours in a temperature range of A1-100℃ to A1 after the above natural air cooling step.

8. In Claim 7, A method for manufacturing a wire rod in which the microstructure of the wire rod that has undergone the above graphitization heat treatment step includes graphite grains and ferrite.

9. In Claim 8, A method for manufacturing a wire rod in which the area fraction of the graphite grains is 1.0% to 4.0% and the average diameter of the graphite grains is 0.5μm to 5.0μm.

10. In Claim 8, A method for manufacturing a wire rod in which the area fraction of the graphite grains is 1.0% to 3.0% and the average diameter of the graphite grains is 1.8 μm to 4.5 μm.

11. In Claim 8, A method for manufacturing a wire rod in which the area fraction of the graphite grains is 3.0% to 4.0% and the average diameter of the graphite grains is 1.1 μm to 4.0 μm.

12. In Claim 7, A method for manufacturing a wire rod having a graphitization fraction of 95% or more according to the following formula (1) of the wire rod that has undergone the above graphitization heat treatment step. Formula (1): (1 - Carbon content in undissolved pearlite / Carbon content in steel) x 100

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