KR20190075512A - Steel material for graphitization and graphite steel with improved machinability - Google Patents

Abstract

Disclosed is a steel material for graphitizing heat treatment. According to one embodiment of the present invention, the steel material for graphitizing heat treatment comprises: 0.60-0.90 wt% of C; 2.0-2.5 wt% of Si; 0.1-0.6 wt% of Mn; 0.01-0.05 wt% of Al; 0.005-0.02 wt% of Ti; 0.0030-0.0100 wt% of N; less than or equal to 0.015 wt% of P (excluding 0 wt%); less than or equal to 0.030 wt% of S (excluding 0 wt%); and the remaining consisting of Fe and unavoidable impurities. Therefore, the present invention significantly shortens a heat treatment time and can uniformly distribute fine graphite grains in a matrix during heat treatment.

Description

TECHNICAL FIELD [0001] The present invention relates to graphite steels, graphite steels, graphite steels, graphite steels, graphite steels, graphite steels, graphite steels,

TECHNICAL FIELD The present invention relates to graphite steels improved in machinability, and more particularly to graphite steels improved in machinability including graphite-annealed steels and uniform and fine graphite.

Generally, free machining steel to which machinability-imparting elements such as Pb, Bi, and S are added is used as a material of machine parts and the like requiring machinability. In the case of Pb-added free cutting steel, which is the most representative free cutting steel, it releases harmful substances such as toxic fumes during cutting, which is very harmful to the human body and is also disadvantageous for recycling of steel. The addition of S, Bi, Te, Sn, etc. has been proposed to replace this. However, there is a problem that the steel containing Bi added is easily produced due to easy cracking during production, and S, Te, There is a problem in that cracks are generated at the cracks.

In order to solve the above problems, the proposed steel is graphite steel. Graphite steel is a ferrite base or a steel containing fine black alumina in ferrite and pearlite matrix. It acts as a source of cracks during cutting and serves as a chip breaker. It is a river with nature.

However, despite the advantages of such graphite steels, graphite steels have not been commercialized at present. This is because, when carbon is added to steel, it is difficult to precipitate graphite without a long heat treatment for a long period of time of 10 hours or more, even though the graphite is stable, and the cementite is precipitated as a metastable phase and decarburization occurs in such a long heat treatment process Which may adversely affect the performance of the final product.

In addition, even if the black coal is precipitated through the graphitization heat treatment, if the graphite in the steel base is precipitated to a large extent, the possibility of occurrence of cracks increases. If the graphite is irregularly distributed irregularly rather than spherical, The chip segmentability and surface roughness become very poor, and the tool life is shortened, making it difficult to obtain the advantages of the graphite steel.

Accordingly, there is a demand for a graphite steel material capable of uniformly distributing fine black alumina in a regular shape in a matrix while heat treatment time is greatly shortened, and graphite steel having improved machinability derived therefrom.

An aspect of the present invention is to provide a graphite steel material capable of uniformly distributing fine black allium in a regular shape in a matrix while heat treatment time is greatly shortened.

Another aspect of the present invention is to provide a graphite steel excellent in machinability.

According to an embodiment of the present invention, there is provided a graphitizing heat treatment steel comprising: 0.60 to 0.90% of C, 2.0 to 2.5% of Si, 0.1 to 0.6% of Mn, 0.01 to 0.05% of Al, 0.005 to 0.02%, N: 0.0030 to 0.0100%, P: 0.015% or less (excluding 0), S: 0.030% or less (excluding 0), the balance Fe and unavoidable impurities.

Further, according to an embodiment of the present invention, the graphitizing heat treatment steel satisfies the following formula (1).

Formula (1): -0.01? [Ti] -3.43 X [N]? 0.01

Here, [Ti] and [N] mean the weight% of the corresponding element, respectively.

Further, according to an embodiment of the present invention, the graphitizing heat treatment steel satisfies the following formula (2).

Formula (2): 400? 3.1 + 169.0? [Si] + 127.7? [Mn]? 500

Here, [Si] and [Mn] mean the weight% of the corresponding element, respectively.

The graphitized steel according to one embodiment of the present invention is characterized by containing 0.60 to 0.90% of C, 2.0 to 2.5% of Si, 0.1 to 0.6% of Mn, 0.01 to 0.05% of Al, 0.01 to 0.05% of Al, 0.005 to 0.02%, N: 0.0030 to 0.0100%, P: not more than 0.015% (excluding 0), S: not more than 0.030% (excluding 0), the balance Fe and unavoidable impurities, Or more of the black alliance, and the average aspect ratio of the black alliance may be 2.0 or less.

Here, the aspect ratio of the black alleys means the ratio of the longest axis to the shortest axis in one black alcove.

Further, according to an embodiment of the present invention, the graphitized steel having improved machinability satisfies the following formula (1).

Formula (1): -0.01? [Ti] -3.43 X [N]? 0.01

Here, [Ti] and [N] mean the weight% of the corresponding element, respectively.

Further, according to an embodiment of the present invention, the graphite steel having improved machinability satisfies the following formula (2).

Formula (2): 400? 3.1 + 169.0? [Si] + 127.7? [Mn]? 500

Here, [Si] and [Mn] mean the weight% of the corresponding element, respectively.

Also, according to an embodiment of the present invention, the average grain size of the black allium may be 5 탆 or less.

Also, according to an embodiment of the present invention, the number of black alliances per unit area may be 1000 to 5000 / mm ² .

According to an embodiment of the present invention, the hardness of the graphite steel may be 70 to 80 HRB.

The graphite steel according to the present invention is excellent in machinability and can be applied as a material for machine parts such as an industrial machine or an automobile.

The following embodiments are provided to fully convey the spirit of the present invention to a person having ordinary skill in the art to which the present invention belongs. The present invention is not limited to the embodiments shown herein but may be embodied in other forms. For the sake of clarity, the drawings are not drawn to scale, and the size of the elements may be slightly exaggerated to facilitate understanding.

Throughout the specification, when an element is referred to as "comprising ", it means that it can include other elements as well, without excluding other elements unless specifically stated otherwise.

The singular forms " a " include plural referents unless the context clearly dictates otherwise.

Hereinafter, a steel material capable of uniformly distributing fine black alumina in a regular shape in a matrix during graphitization heat treatment will be described.

Hereinafter, embodiments according to the present invention will be described in detail with reference to the accompanying drawings. First, a description will be given of a steel material for graphitization heat treatment, and then graphite steel having improved machinability will be described.

The steel for graphitizing heat treatment according to one aspect of the present invention is characterized by containing 0.60 to 0.90% of C, 2.0 to 2.5% of Si, 0.1 to 0.6% of Mn, 0.01 to 0.05% of Al, 0.005 to 0.005% of Ti, 0.0030 to 0.0100%, P: 0.015% or less (excluding 0), S: 0.030% or less (excluding 0), the balance Fe and unavoidable impurities.

Hereinafter, the reason for limiting the numerical value of the content of the component component in the embodiment of the present invention will be described. Unless otherwise stated, the unit is wt%.

The content of C is 0.60 to 0.90%.

Carbon (C) is an essential element for forming black alliance. When the content of carbon is less than 0.60% by weight, the machinability improvement effect is insufficient, and the distribution of black allied particles is uneven even when graphitization is completed. On the other hand, when the content is excessive, there is a problem that black coarse aggregation is generated to a great extent and the aspect ratio is increased, and the machinability, particularly the surface roughness, is lowered, and the upper limit can be limited to 0.80 wt%.

The content of Si is 2.0 to 2.5%.

Silicon (Si) is a component required as a deoxidizing agent in the production of molten steel, and is preferably added in an amount of 2.0% by weight or more as a graphitization accelerating element capable of destabilizing cementite in steel and precipitating carbon into graphite. However, when the content is excessive, not only the effect is saturated but also the hardness is increased due to the hardening effect, accelerating the tool wear during cutting, causing brittleness due to the increase of nonmetallic inclusions, There is a problem that the upper limit can be limited to 2.5% by weight.

The content of Mn is 0.1 to 0.6%.

Manganese (Mn) improves the strength and impact properties of the steel and forms MnS inclusions by binding with sulfur in steel, thereby contributing to improvement in machinability. However, when the content is excessive, there is a risk of retarding the graphitization and delaying the graphitization completion time, increasing the strength and hardness and decreasing the tool wear depth, and the upper limit is limited to 0.6 wt% .

The content of Al is 0.01 to 0.05%.

Aluminum (Al) is a powerful deoxidizing element which not only contributes to deoxidation but is also a useful element for promoting graphitization. Promotes the decomposition of cementite during the graphitization heat treatment and at the same time forms AlN by binding with nitrogen, thereby preventing stabilization of cementite and promoting graphitization. In addition, since the aluminum oxide formed by the addition of aluminum is effective in accelerating the crystallization of graphite as the precipitation nuclei of graphite, it is preferable to add it in an amount of 0.01 wt% or more. However, when the content is excessive, the effect is not only saturated but also the hot deformability is remarkably lowered. On the other hand, when Al is excessive, AlN is generated in the austenite grain boundaries, and the graphite having the nuclei is unevenly distributed in grain boundaries. The upper limit can be limited to 0.05 wt%.

The content of Ti is 0.005 to 0.02%.

Titanium (Ti) bonds with nitrogen, such as boron and aluminum, to form nitrides such as TiN, BN, and AlN. These nitrides act as nuclei of graphite during the heat treatment at constant temperature. However, since BN, AlN and the like have a low formation temperature, austenite is formed and unevenly precipitates in the grain boundaries, whereas TiN is higher than AlN and BN so that it is crystallized before the austenite formation is completed. . Therefore, the black allium generated by the nucleation site of TiN is also finely and uniformly distributed. In order to exhibit such an effect, it is preferable that the content is 0.005% by weight or more. However, when the content is excessive, coarse carbonitride is formed to consume carbon required for graphite formation, thereby inhibiting graphitization. Can be limited to 0.02% by weight.

The content of N is 0.0030 to 0.0100%.

Nitrogen (N) combines with titanium, boron, and aluminum to produce TiN, BN, AlN and the like. In particular, nitrides such as BN and AlN are formed mainly at the austenite grain boundaries. In the graphitization heat treatment, it is necessary to add an appropriate amount of graphite because it may cause non-uniform distribution of graphite because graphite is formed from nucleus as such nitride. When the amount of nitrogen added is excessive, it is not bonded to the nitride forming element. When the nitriding agent is present in the steel as solid nitrogen, it strengthens and stabilizes the cementite to deteriorate the graphitization. Therefore, 0.0030% by weight is limited to 0.0100% by weight upper limit in the present invention in order to prevent nitric acid from being consumed in forming nitride acting as a nucleation site of graphite.

The content of P is 0.015% or less.

Phosphorus (P) is an impurity inevitably contained. Although phosphorus weakens the grain boundaries, phosphorus helps the workability to some extent, but it increases the hardness of the ferrite by considerable strengthening effect, reduces the toughness and delayed fracture resistance of the steel, and promotes the occurrence of surface defects , It is desirable to manage the content as low as possible. Theoretically, it is advantageous to control the phosphorus content to 0 wt%, but it is inevitably contained in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit is controlled to 0.015 wt%.

The content of S is 0.030% or less.

Sulfur (S) is an impurity inevitably contained. Sulfur not only significantly inhibits graphitization of carbon in steel but also segregates in grain boundaries to lower toughness and form a low melting point emulsion to inhibit hot rolling property, so that its content is preferably controlled as low as possible. When S is excessively large, mechanical anisotropy is exhibited due to MnS stretched by rolling, although there is an effect of improving machinability by MnS formation. In the present invention, S is added to induce the formation of MnS within a range that can contribute to improvement of machinability without causing mechanical anisotropy. Therefore, it is advantageous to control the sulfur content to 0 wt%, but it is inevitably contained in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit is controlled to 0.030 wt%.

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

According to one embodiment of the present invention, the graphite-use heat treatment steels satisfying the above-described alloy composition can satisfy the following formula (1).

(1): 400? 3.1 + 169.0? [Si] + 127.7? [Mn]? 500

Here, [Si] and [Mn] mean the weight% of the corresponding element, respectively.

Since hardness, tensile strength and ductility are influenced by the addition amount of Si and Mn in the graphitized heat treated steel, that is, graphite steel, 3.1 + 169.0 ⅹ is required to obtain satisfactory machinability in terms of chip segmentability, surface roughness and tool wear It is desirable that the value of [Si] + 127.7 x [Mn] satisfies a range of 400 or more and 500 or less.

3.1 + 169.0 ⅹ [Si] + 127.7 × [Mn] value less than 400 results in a lower tensile strength. Due to the nature of the soft material, the surface roughness may be poor during cutting and the chip segmentation may be deteriorated. , The hardness value becomes high, and the degree of tool wear can be increased at the time of cutting.

According to one embodiment of the present invention, the graphite-use heat treatment steels satisfying the above-described alloy composition can satisfy the following formula (2).

(2): -0.01? [Ti] -3.43 X [N]? 0.01

Here, [Ti] and [N] mean the weight% of the corresponding element, respectively.

When the value of [Ti] -3.43 X [N] is less than -0.01, excessive nitrogen remaining after the formation of TiN is dissolved in the steel to stabilize the cementite and delay the graphitization. Therefore, it is preferable that the value of [Ti] -3.43X [N] is at least -0.01. On the contrary, when the value of [Ti] -3.43X [N] is excessively large, excess Ti, . [Ti] -3.43X [N] is preferably 0.01 or less, since excessive Ti may reduce the graphite fraction or generate coarse graphite by consuming carbon which is required to form graphite by forming coarse carbonitride.

The graphite-annealed steel according to the disclosed embodiment can reach a graphitization rate of 99% or more after graphitization annealing at 730 to 770 ° C for 300 minutes.

The graphitization ratio means a ratio of the carbon content present in the graphite state to the carbon content added to the steel, and can be expressed by the following equation (3).

(3): Graphitization rate (%) = (carbon content present in graphite state / carbon content in steel) x 100

Graphitization of more than 99% means that all of the added carbon has been consumed to generate graphite. (The amount of dissolved carbon in ferrite is not considered to be a trace amount.) There is no non-decomposed pearlite, And have distributed microstructure.

The graphite for heat treatment of graphite according to the present invention described above can be produced by various methods, and the method is not particularly limited in the present invention. For example, the ingot having the above-mentioned composition range may be cast and then homogenized at 1100 to 1300 ° C for 5 to 10 hours, hot-rolled at 1000 to 1100 ° C and then air-cooled.

Hereinafter, another graphite steel having improved machinability, which is another aspect of the present invention, will be described in detail.

The graphite steel according to the disclosed embodiment has the same alloy composition and composition range as the above-mentioned graphitizing heat treatment steels, and the reason for limiting the numerical value of the alloy element content is as described above.

That is, the graphite steel according to the disclosed embodiment can satisfy the following formula (1) or (2).

(1): 400? 3.1 + 169.0? [Si] + 127.7? [Mn]? 500

(2): -0.01? [Ti] -3.43 X [N]? 0.01

Here, [Si], [Mn], [Ti], and [N] refer to the weight% of the corresponding element, respectively.

According to an embodiment of the present invention, the graphite steel having improved machinability may include black alumina in a ferrite base at an area fraction of 2.0% or more. The higher the area fraction of black alumina is, the better the machinability is, and the upper limit is not particularly limited.

According to an embodiment of the present invention, the average aspect ratio of the black alliance may be 2.0 or less. The aspect ratio of black alliance means the ratio of the longest axis to the shortest axis in a black alliance. When the black alumina is spheroidized in this way, the anisotropy is reduced during machining, and the machinability and cold-rolled composition are remarkably improved.

According to an embodiment of the present invention, the average grain size of the black alliance may be 5 탆 or less. The average grain size of the black alleys means the equivalent circular diameter of particles detected by observing one end face of the graphite steel. The smaller the average grain size is, the better the surface roughness during machining. Therefore, Is not particularly limited.

According to an embodiment of the present invention, the number of black alliances per unit area may be 1000 to 5000 / mm ² . More specifically, the number per unit area of less black 3㎛ coalition size of the average grain may be 1200 to 3500 / mm ^2.

When the fine black alumina grains in the graphite steel are uniformly dispersed in this manner, the formed black alumina reduces the cutting friction and acts as a crack initiation site, thereby remarkably improving the machinability.

According to one embodiment of the present invention, the hardness of the graphite steel satisfies the range of 70 to 80 HRB.

The graphite steel of the present invention can be produced by various methods, and the production method thereof is not particularly limited. For example, the graphite steel may be subjected to graphitization heat treatment at 730 to 770 DEG C for 600 minutes or more ). The temperature region corresponds to a temperature region near the nose of the graphite production curve in the isothermal transformation curve, and corresponds to a temperature region where the heat treatment time can be shortened.

Hereinafter, preferred embodiments of the present invention will be described in more detail.

Example

Ingots were cast while changing the content of each component as shown in Table 1 below, and then subjected to homogenization heat treatment at 1250 ° C for 8 hours.

Thereafter, the steel sheet was hot-rolled at a finishing temperature of 1000 캜 to a thickness of 27 mm, and air-cooled to produce a graphite-annealed steel.

division Alloy composition (% by weight) Equation (1) Equation (2) C Si Mn P S Al Ti N Honor One 0.82 2.35 0.32 0.0138 0.0052 0.043 0.0122 0.0030 0.0019 441.1 2 0.65 2.58 0.30 0.0101 0.0030 0.030 0.0130 0.0055 -0.0059 477.4 3 0.73 2.35 0.41 0.0085 0.0042 0.023 0.0182 0.0030 0.0079 452.6 4 0.76 2.48 0.42 0.0080 0.0048 0.026 0.0190 0.0050 0.0019 475.9 5 0.75 2.45 0.40 0.0085 0.0051 0.028 0.0124 0.0033 0.0011 468.2 6 0.80 2.20 0.37 0.0077 0.0050 0.038 0.0143 0.0033 0.0030 422.1 7 0.87 2.12 0.41 0.0120 0.0282 0.028 0.0124 0.0033 0.0011 413.7 8 0.65 2.20 0.33 0.0032 0.0251 0.030 0.0154 0.0040 0.0017 417.0 9 0.65 2.12 0.33 0.0070 0.0030 0.033 0.0110 0.0040 -0.0027 403.5 Comparative Example 10 1.02 2.30 0.30 0.0121 0.0050 0.030 0.0120 0.0050 -0.0052 430.1 11 0.45 2.43 0.55 0.0125 0.0050 0.032 0.0123 0.0050 -0.0049 484.0 12 0.73 1.90 0.30 0.0102 0.0050 0.030 0.0122 0.0050 -0.0050 362.5 13 0.83 2.91 0.80 0.0082 0.0064 0.026 0.0128 0.0022 0.0053 597.1 14 0.87 2.32 0.90 0.0082 0.0064 0.023 0.0125 0.0022 0.0050 510.1 15 0.72 2.22 0.08 0.0082 0.0064 0.026 0.0120 0.0022 0.0045 388.5 16 0.81 2.27 0.57 0.0074 0.0064 0.023 0.0022 0.0062 -0.0191 459.5 17 0.82 2.41 0.39 0.0080 0.0280 0.045 0.0231 0.0030 0.0128 460.2 18 0.65 2.45 0.39 0.0080 0.0254 0.039 0.0140 0.0050 -0.0032 467.0 19 0.78 2.27 0.23 0.0082 0.0080 0.022 0.0012 0.0100 -0.0331 492.0 20 0.82 2.32 0.33 0.0078 0.0102 0.032 0.0383 0.0019 0.0318 452.3 (Ti), [N] means the weight% of each of the elements, and is expressed by the following formula (1) -0.01 <[Ti] -3.43 [N] <0.01
3.1 + 169.0 * [Si] + 127.7 * [Mn] (where [Si], [Mn] means the weight% of each element)

The graphite steel was then subjected to graphitization heat treatment at 750 ° C for 5 hours to obtain graphite steel. In the case of Comparative Examples 17 and 18, the degree of graphitization with the heat treatment temperature was compared with the graphitization heat treatment temperatures of 700 ° C and 800 ° C, respectively.

Then, the black coarse area ratio, the black coarse average size and the black coarse average aspect ratio were measured for the graphitized heat treated steel using an image analyzer.

The method of measuring the area fraction, average size and average aspect ratio of black alliance is as follows. Each specimen was cut to a certain size and images were taken under a magnification of 200 times using an optical microscope in a state of only polishing without etching. In the image thus obtained, the image can be clearly distinguished by the difference in contrast between the base and the graphite, so the image analysis software was used for the analysis. To increase the reliability of the analysis, 15 images were taken per sample.

On the other hand, the area fraction of graphite is defined as the ratio of the area occupied by graphite to the total area observed, and the average size and aspect ratio of graphite are respectively the equivalent circular diameter and the longest axis and the shortest axis &Lt; / RTI &gt;

division Graphitization heat treatment temperature (캜) Graphitization heat treatment
Air-cooled after 5 hours Graphite area fraction (%) 3mm or less per mm ² Number of black alliances Black Alignment Mean Aspect Ratio Hardness (HRB) Honor One 730 F + G 2.86 3253 1.36 72.3 2 730 F + G 2.14 2621 1.45 78.1 3 730 F + G 2.55 2401 1.47 74.6 4 750 F + G 2.68 3402 1.61 78.8 5 750 F + G 2.25 2203 1.33 75.6 6 750 F + G 2.60 3048 1.41 72.3 7 750 F + G 2.71 1890 1.43 71.0 8 770 F + G 2.14 1820 1.62 72.8 9 770 F + G 2.13 1720 1.62 70.0 Comparative Example
10 750 F + G 3.37 987 2.82 72.8 11 750 F + G 1.63 876 1.62 78.8 12 750 F + G 2.70 1593 1.36 61.3 13 750 F + G 2.62 1231 1.45 89.2 14 750 F + G 3.10 1340 1.47 82.3 15 750 F + G 2.36 1120 1.45 66.3 16 750 F + P + G 1.92 243 1.43 82.6 17 700 F + P + G 1.61 332 1.62 83.1 18 800 F + P + G 1.79 642 1.31 94.3 19 750 F + P + G 1.76 893 1.45 82.3 20 750 F + G 2.36 972 1.76 72.4

Then, after machining the parts for machinability evaluation, the chip segmentability, the depth of tool wear and the surface roughness, that is, the roughness of the machined surface were measured. For this purpose, first, plate-like steels were subjected to graphitization heat treatment for 5 hours at the graphitizing heat treatment temperature in Table 2, and then processed into bars having a diameter of 25 mm, and then machined with a CNC automatic lathe. Chip segmentation was evaluated as excellent when chips were divided under 2 volumes, bad when divided into 3 to 6 volumes, and bad when 7 or more chips were divided.

As shown in Fig. 1, the depth of the tool wear was 200 mm in length until the diameter of the rod was 25 mm, and then the depth of the tool was measured before and after the machining. At this time, cutting conditions were performed using cutting oil at a cutting speed of 100 mm / min, a feed rate of 0.1 mm / rev, and a cutting depth of 1.0 mm.

division Surface roughness (탆) Tool wear depth (㎛) Chip segmentation Honor One 5.9 119 Great 2 6.2 107 Great 3 6.8 121 Great 4 6.1 128 Great 5 5.4 112 Great 6 6.3 139 Great 7 6.7 122 Great 8 6.6 88 Great 9 6.3 101 Great Comparative Example
10 9.2 119 usually 11 7.8 201 Bad 12 10.8 118 Bad 13 6.6 222 usually 14 6.8 282 Great 15 9.8 134 Bad 16 6.8 211 Bad 17 10.2 208 Bad 18 7.2 298 Great 19 9.8 278 Great 20 10.5 134 Bad

Examples 1 to 9, which satisfy both the composition and the manufacturing conditions proposed in the present invention, have microstructure consisting of pearlite and graphite, and have a graphite area fraction of 2% or more, a black-line average average aspect ratio of 2.0 Hereinafter, the density of the black allied bond is 1000 / mm &lt; ^{2 &gt;} Respectively. Also, referring to Table 3, it can be confirmed that the graphite steel according to the disclosed embodiment has good chip segmentality, surface roughness, and tool life characteristics.

Referring to Table 2, it can be seen that the graphitized area fraction is generally proportional to the amount of carbon added. Therefore, in the case of Comparative Example 10, although the C content was high and the graphite area fraction satisfied the range of the present invention, a coarse black allium was formed and the aspect ratio was relatively high. As a result, it can be seen that the surface roughness of the cutting surface is relatively poor as shown in Table 3.

On the contrary, in the case of Comparative Example 11, since the C content was low, a sufficient amount of graphite could not be produced, and the area fraction of the graphite was low. As a result, not only the tool wear depth but also the chip segmentability was confirmed.

In the case of Comparative Examples 12 to 15, the steel materials to which the amounts of Mn and Si were added in the range outside the formula (1), and the results of hardness measurement were also out of the range of the hardness values shown in the present invention. Specifically, in the case of Comparative Examples 13 and 14, it can be confirmed that the degree of the tool wear is intensified due to the hardness exceeding 80 by 89.2 and 82.3.

On the contrary, in the case of Comparative Examples 12 and 15, it is confirmed that the hardness is 61.3 and 66.3, which is less than 70, and the surface roughness characteristic is in the open position.

In the case of Comparative Examples 16 and 19, since N added was excessive as compared with the amount of Ti added, it did not satisfy the formula (2), so that the solid nitrogen remaining in the steel without forming TiN was excessive, It was not graphitized and a part of the pearlite remained. As a result, it was confirmed that the degree of hardness of the tool was worsened due to the hardness exceeding 80 to 82.6.

In the case of Comparative Example 17, the graphitization heat treatment temperature was as low as 700 ° C, and pearlite was observed in the microstructure because the graphite was not completely graphitized during the graphitization heat treatment. As a result, the hardness increased to 83.1, which was more than 80, indicating that the depth of tool wear was intensified.

In the case of Comparative Example 18, the graphitization heat treatment temperature was as high as 800 占 폚 and was transformed into austenite and pearlite was again formed upon cooling. As a result, the hardness was as high as 94.3, which indicates that the degree of tool wear was intensified.

In the case of Comparative Example 20, Ti added to the amount of N added was too large to satisfy the formula (2). As a result, coarse black allium was formed and the surface roughness was comparatively poor.

The graphite steel according to one embodiment of the present invention can sufficiently form black allium in the matrix, and fine black allium can be uniformly distributed in a regular shape to improve machinability.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited thereto. Those skilled in the art will readily obviate modifications and variations within the spirit and scope of the appended claims. It will be understood that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

Claims (9)

0.001 to 0.02% of N, 0.0030 to 0.0100% of N, 0.015% of P, 0.01 to 0.5% of Al, 0.01 to 0.05% of Al, 0.005 to 0.02% of Ti, (Excluding 0), S: not more than 0.030% (excluding 0), the balance Fe and unavoidable impurities. The method according to claim 1,
A graphite for heat treatment which satisfies the following formula (1).
Formula (1): -0.01? [Ti] -3.43 X [N]? 0.01
(Where, [Ti] and [N] each represent the weight% of the corresponding element). The method according to claim 1,
A graphite for heat treatment which satisfies the following formula (2).
Formula (2): 400? 3.1 + 169.0? [Si] + 127.7? [Mn]? 500
(Where, [Si] and [Mn] mean the weight% of the corresponding element, respectively). 0.001 to 0.02% of N, 0.0030 to 0.0100% of N, 0.015% of P, 0.01 to 0.5% of Al, 0.01 to 0.05% of Al, 0.005 to 0.02% of Ti, (Excluding 0), S: not more than 0.030% (excluding 0), the balance Fe and unavoidable impurities,
The ferrite base contains black alumina at an area fraction of 2.0% or more,
Wherein an average aspect ratio of the black alcoves is 2.0 or less.
(Here, the aspect ratio of the black alleys means the ratio of the longest and shortest axes in one black alleys.) 5. The method of claim 4,
A graphite steel having improved machinability satisfying the following formula (1).
Formula (1): -0.01? [Ti] -3.43 X [N]? 0.01 5. The method of claim 4,
A graphite steel having improved machinability satisfying the following formula (2).
Formula (2): 400? 3.1 + 169.0? [Si] + 127.7? [Mn]? 500 5. The method of claim 4,
Wherein an average grain size of the black alumina is 5 mu m or less. 5. The method of claim 4,
Wherein the number of the black alleys per unit area is 1000 to 5000 pieces / mm &lt; ² &gt;. 5. The method of claim 4,
Wherein the hardness of the graphite steel is 70 to 80 HRB.