KR102042063B1 - Steel material for graphitization and graphite steel with improved machinability - Google Patents

Abstract

Disclosed is a steel for graphitizing heat treatment. According to the graphitizing heat treatment steel according to an embodiment of the present invention, in weight%, C: 0.60 to 0.90%, Si: 2.0 to 2.5%, Mn: 0.1 to 0.6%, Al: 0.01 to 0.05%, Ti: 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), balance Fe and inevitable impurities.

Description

Steel for Graphitization Heat Treatment and Graphite Steel with Improved Machinability {STEEL MATERIAL FOR GRAPHITIZATION AND GRAPHITE STEEL WITH IMPROVED MACHINABILITY}

The present invention relates to a graphite steel with improved machinability, and more particularly, to a graphite steel with improved machinability including a graphitizing heat treatment steel and uniform and fine graphite.

In general, free-cutting steels containing machinability-providing elements such as Pb, Bi, and S are used as materials for machinability and the like requiring machinability. Pb-added free-cutting steel, which is the most representative free-cutting steel, emits harmful substances such as toxic fume during cutting, which is very harmful to the human body and has a very disadvantageous problem in recycling steel. Therefore, in order to replace this, addition of S, Bi, Te, Sn, etc. has been proposed. However, Bi-added steel has a problem that is very difficult to produce due to easy cracking during manufacture, and S, Te, Sn, etc. are also hot rolled. There is a problem in that it causes cracking in time.

The steel proposed to solve the above problems is graphite steel. Graphite steel is a steel containing fine graphite grains in ferrite matrix or ferrite and pearlite substrate, and the fine graphite grains inside act as a crack source during cutting and serve as a chip breaker, thus providing good machinability. It is a river with nature.

However, despite the advantages of graphite steel, graphite steel is not commercialized at present. When carbon is added to steel, it is difficult to precipitate graphite without heat treatment for a long time of 10 hours or more, even though graphite is stable, and it is precipitated as metastable cementite. This is because adverse effects occur that adversely affect the performance of the final product.

In addition, even if graphite particles are precipitated through graphitization heat treatment, coarse precipitation of graphite in steel increases the possibility of cracking. Unevenness, chip segmentation and surface roughness are very poor, and tool life is also shortened, making it difficult to obtain the advantages of graphite steel.

Accordingly, there is a need for a steel for graphite steel and a graphite steel with improved machinability derived therefrom which can significantly reduce heat treatment time, and allow fine graphite grains to be uniformly distributed in a regular shape in a matrix during heat treatment.

One aspect of the present invention, while significantly shortening the heat treatment time, to provide a steel for graphite steel that can be uniformly distributed in a regular shape in the matrix during heat treatment.

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

According to the graphitizing heat treatment steel according to an embodiment of the present invention, in weight%, C: 0.60 to 0.90%, Si: 2.0 to 2.5%, Mn: 0.1 to 0.6%, Al: 0.01 to 0.05%, Ti: 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), balance Fe and inevitable impurities.

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

Equation (1): -0.01 ≤ [Ti] -3.43 k [N] ≤ 0.01

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

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

Equation (2): 400 ≤ 3.1 + 169.0 ⅹ [Si] + 127.7 ⅹ [Mn] ≤ 500

Here, [Si] and [Mn] mean each weight percent of the corresponding element.

Graphite steel with improved machinability according to an embodiment of the present invention, in weight percent, C: 0.60 to 0.90%, Si: 2.0 to 2.5%, Mn: 0.1 to 0.6%, Al: 0.01 to 0.05%, Ti: 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), balance Fe and unavoidable impurities, 2.0% by area fraction on the ferrite matrix Including the above graphite grains, the average aspect ratio of the graphite grains may be 2.0 or less.

Here, the aspect ratio of the graphite grains means the ratio of the longest axis and the shortest axis in one graphite grain.

In addition, according to an embodiment of the present invention, the machinability is improved graphite steel satisfies the following formula (1).

Equation (1): -0.01 ≤ [Ti] -3.43 k [N] ≤ 0.01

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

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

Equation (2): 400 ≤ 3.1 + 169.0 ⅹ [Si] + 127.7 ⅹ [Mn] ≤ 500

Here, [Si] and [Mn] mean each weight percent of the corresponding element.

In addition, according to an embodiment of the present invention, the average grain size of the graphite grains may be 5 ㎛ or less.

In addition, according to one embodiment of the present invention, the number of graphite particles per unit area may be 1000 to 5000 / mm ² .

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

Graphite steel according to the present invention is excellent in machinability, it can be applied to the material of mechanical parts, such as industrial machinery or automobiles.

The following embodiments are presented to sufficiently convey the spirit of the present invention to those skilled in the art. The present invention is not limited to the embodiments presented herein but may be embodied in other forms. The drawings may omit illustrations of parts not related to the description in order to clarify the present invention, and may be exaggerated to some extent in order to facilitate understanding.

Throughout the specification, when a part is said to "include" a certain component, it means that it can further include other components, without excluding other components unless specifically stated otherwise.

Singular expressions include plural expressions unless the context clearly indicates an exception.

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

Hereinafter, with reference to the accompanying drawings an embodiment according to the present invention will be described in detail. First, the graphitizing heat treatment steel will be described, and then the machinability and the improved graphite steel will be described.

Steel for graphitizing heat treatment according to an aspect of the present invention, in weight%, C: 0.60 to 0.90%, Si: 2.0 to 2.5%, Mn: 0.1 to 0.6%, Al: 0.01 to 0.05%, Ti: 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), balance Fe and inevitable impurities.

Hereinafter, the reason for the numerical limitation of the content of the alloying component in the embodiment of the present invention will be described. In the following, the unit is% by weight unless otherwise specified.

The content of C is 0.60 to 0.90%.

Carbon (C) is an essential element for forming graphite grains. When the content of carbon is less than 0.60% by weight, the machinability improvement effect is insufficient and the distribution of the graphite grains is uneven even when graphitization is completed. On the other hand, when the content is excessive, graphite grains are coarsened and the aspect ratio is increased, so that machinability, in particular, surface roughness is reduced, and the upper limit thereof may be limited to 0.80% by weight.

The content of Si is 2.0 to 2.5%.

Silicon (Si) is a necessary component as a deoxidizer in the manufacture of molten steel, and it is preferable to add 2.0 wt% or more as a graphitization facilitating element that destabilizes cementite in the steel so that carbon can be precipitated into graphite. However, if the content is excessive, the effect is not only saturated, but the hardness is increased due to the solid-solution strengthening effect, thereby accelerating the tool wear during cutting, causing brittleness due to the increase of non-metallic inclusions, and excessive decarburization during hot rolling. There is a problem to cause, 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 characteristics of the steel material, and combines with sulfur in the steel to form an MnS inclusions to contribute to improved machinability, it is preferable to add 0.1% by weight or more. However, if the content is excessive, the graphitization may be inhibited and the graphitization completion time may be delayed, and the strength and hardness may be increased to decrease the depth of tool wear. Therefore, the upper limit thereof should be limited to 0.6% by weight. Can be.

The content of Al is 0.01 to 0.05%.

Aluminum (Al) is a powerful deoxidation element that not only contributes to deoxidation but also is a useful element for promoting graphitization. In the graphitization heat treatment, the decomposition of cementite is promoted and at the same time, it forms an AlN in combination with nitrogen, thereby preventing the stabilization of cementite and promoting graphitization. In addition, the aluminum oxide formed by the addition of aluminum is also effective in promoting precipitation crystallization of the graphite to promote the crystallization of the graphite, it is preferable to add 0.01% by weight or more. However, when the content is excessive, there is a problem that not only the effect is saturated, but also the hot deformation is remarkably lowered. In addition, when Al is excessive, AlN is formed at the austenite grain boundary, and graphite having nuclei therein is unevenly distributed at the grain boundary. The upper limit thereof may be limited to 0.05% by weight.

The content of Ti is 0.005 to 0.02%.

Titanium (Ti) combines with nitrogen such as boron and aluminum to form nitrides such as TiN, BN, and AlN. These nitrides act as nuclei of graphite formation during incubation. However, BN, AlN, etc., due to the low formation temperature, uneven precipitation at the grain boundary after austenite is formed, whereas TiN is crystallized before the completion of austenite formation because the formation temperature is higher than AlN or BN, uniform distribution in the austenite grain Will be Therefore, the graphite grains produced using TiN as nucleation nuclei are also finely and uniformly distributed. In order to exhibit such an effect, it is preferable to contain 0.005% by weight or more, but when the content is excessive, it becomes coarse carbonitride and consumes the carbon necessary for graphite formation, thereby preventing graphitization. It 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 mainly formed at the austenite grain boundaries. In the graphitization heat treatment, since such a nitride is formed as a nucleus, an appropriate amount of addition is necessary because it may cause uneven distribution of graphite. When the amount of nitrogen added is not combined with the nitride-forming element, and the solid nitrogen exists in the steel as a solid solution, it increases the strength, stabilizes cementite, and has a detrimental effect of delaying graphitization. Therefore, the present invention is limited to the upper limit of 0.030% by weight to the lower limit of 0.0030% by weight in order to be consumed to form the nitride acting as a graphite nucleation source, and to leave no dissolved nitrogen.

The content of P is 0.015% or less.

Phosphorus (P) is an impurity contained inevitably. Phosphorus, although the steel is weak to the grain boundary to some extent, helps to improve machinability, but due to the significant solidification effect, it increases the hardness of ferrite, reduces the toughness and delayed fracture resistance of steel, and encourages the occurrence of surface defects. Therefore, it is desirable to keep the content as low as possible. In theory, the phosphorus content is advantageously controlled to 0% by weight, but inevitably contained in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit is managed at 0.015% by weight.

The content of S is 0.030% or less.

Sulfur (S) is an impurity contained inevitably. Sulfur not only significantly inhibits the graphitization of carbon in steel, but also segregates at grain boundaries, lowers toughness, forms low melting point emulsions, and inhibits hot rolling. Therefore, it is desirable to manage the content as low as possible. When S is excessively high, the machinability may be improved by MnS formation, but mechanical anisotropy may be caused by MnS drawn by rolling. In the present invention, the addition of S induces the production of MnS within the range that can contribute to improving the machinability without causing mechanical anisotropy. Therefore, it is advantageous to control the content of sulfur to 0% by weight, but inevitably contained in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit is managed at 0.030% by weight.

The remaining component of the present invention is iron (Fe). However, in the conventional manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably mixed, and thus cannot be excluded. Since these impurities are known to those skilled in the art, all of them are not specifically mentioned in the present specification.

In addition, according to an embodiment of the present invention, the steel for graphitization heat treatment that satisfies the above-described alloy composition may satisfy the following formula (1).

Equation (1): 400 ≤ 3.1 + 169.0 ⅹ [Si] + 127.7 ⅹ [Mn] ≤ 500

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

Hardness, tensile strength, and ductility in graphitized heat-treated steels, i.e. graphite steels, are affected by the amount of Si and Mn added, so that in order to obtain satisfactory machinability in terms of chip segmentation, surface roughness, and tool abrasion, It is preferable that the value of [Si] +127.7 k [Mn] satisfies the range of 400 or more and 500 or less.

If the value of 3.1 + 169.0ⅹ [Si] + 127.7ⅹ [Mn] is less than 400, the tensile strength will be lowered, and the surface roughness may be poor or the segmentality of the chip may be reduced due to the characteristics of the soft material. In this case, the hardness value becomes high, and thus the degree of tool wear during cutting may be deepened.

According to one embodiment of the present invention, the steel for graphitization heat treatment that satisfies the above-described alloy composition may satisfy the following formula (2).

Equation (2): -0.01≤ [Ti] -3.43Ⅹ [N] ≤0.01

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

If the value of [Ti] -3.43k [N] is less than -0.01, excess nitrogen remaining after the formation of TiN is dissolved in the steel to stabilize cementite and delay the graphitization. Therefore, it is preferable that the value of [Ti] -3.43k [N] is -0.01 or more. On the contrary, when the value of [Ti] -3.43k [N] is excessive, excess Ti that is not produced by TiN is excessive in the steel. Will exist. Since the excess Ti forms carbon by which coarse carbonitride is formed, there is a possibility of reducing graphite fraction or generating coarse graphite, so that the value of [Ti] -3.43k [N] is preferably 0.01 or less.

Steel for graphitization heat treatment according to the disclosed embodiment may reach a graphitization rate of 99% or more after 300 minutes graphitization heat treatment at 730 ~ 770 ℃.

Graphitization rate means a ratio of the carbon content present in the graphite state to the carbon content added to the steel, it can be represented by the following formula (3).

Equation (3): graphitization rate (%) = (carbon content in the graphite state / carbon content in steel) × 100

Graphitization of more than 99% means that all of the added carbon has been consumed to produce graphite (the amount of dissolved carbon in the ferrite is not considered to be very small). There is no undigested pearlite, i.e. It means having a distributed microstructure.

The steel for graphitizing heat treatment of the present invention described above can be produced by various methods, the present invention does not particularly limit the method. For example, after casting an ingot having the above component range, it may be prepared by homogenizing heat treatment at 1100 to 1300 ° C. for 5 to 10 hours, followed by hot rolling at 1000 to 1100 ° C., followed by air cooling.

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

Graphite steel according to the disclosed embodiment has the same alloy composition and component range as the above-described graphitization heat treatment steel, the description of 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 may satisfy the following formula (1) or formula (2).

Equation (1): 400 ≤ 3.1 + 169.0 ⅹ [Si] + 127.7 ⅹ [Mn] ≤ 500

Equation (2): -0.01≤ [Ti] -3.43Ⅹ [N] ≤0.01

Here, [Si], [Mn], [Ti], and [N] mean each weight percent of the corresponding element.

According to one embodiment of the invention, the improved machinability of the graphite steel may include at least 2.0% graphite grains in an area fraction of the ferrite matrix. Since the machinability improves as the area fraction of graphite grains increases, the upper limit is not specifically limited.

According to an embodiment of the present invention, the average aspect ratio of the graphite grains may be 2.0 or less. The aspect ratio of the graphite grains refers to the ratio of the longest axis and the shortest axis in one graphite grain. When the graphite grains are spheroidized in this manner, the anisotropy during processing is reduced, and the machinability and cold forging property are remarkably improved.

According to one embodiment of the present invention, the average grain size of the graphite grains may be 5㎛ or less. The average grain size of graphite grains means the equivalent circular diameter of particles detected by observing one cross section of graphite steel. The smaller the average grain size, the better the surface roughness during machinability. It does not specifically limit about.

According to one embodiment of the present invention, the number of graphite particles per unit area may be 1000 to 5000 / mm ² . More specifically, the number of graphite grains having an average grain size of 3 μm or less may be 1200 to 3500 pieces / mm ² .

As such, when fine graphite grains in the graphite steel are uniformly dispersed, the formed graphite grains reduce cutting friction and serve as crack initiation sites, thereby significantly improving machinability.

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

Graphite steel of the present invention described above can be produced by various methods, the manufacturing method is not particularly limited, for example, graphitized heat treatment of graphite steel at 730 ~ 770 ℃ 600 minutes or more (air cooling after constant temperature heat treatment) It can manufacture by). The temperature range corresponds to a temperature range corresponding to the graphite generation curve nose in the isothermal transformation curve, and corresponds to a temperature range that can shorten the heat treatment time.

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

Example

Ingot was cast while changing the content of each component as shown in Table 1, followed by homogenization heat treatment at 1250 ° C. for 8 hours.

After that, the finishing temperature was set to 1000 ℃ hot rolled to a thickness of 27mm, air-cooled to produce a steel for graphitized heat treatment.

division Alloy composition (% by weight) Formula (1) Formula (2) C Si Mn P S Al Ti N Inventive Example 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 Formula (1) -0.01 <[Ti] -3.43 [N] <0.01 (where [Ti] and [N] represent the weight percentage of each corresponding element)
Formula (2) 3.1 + 169.0 * [Si] + 127.7 * [Mn], where [Si] and [Mn] represent the weight percent of each corresponding element)

Thereafter, the graphite steel was graphitized for 5 hours at 750 ° C. to obtain graphite steel. However, in Comparative Examples 17 and 18, the graphitization degree according to the heat treatment temperature was compared with the graphitization heat treatment temperature as 700 ° C and 800 ° C, respectively.

Subsequently, the graphitic grain area, the graphitic grain average size, and the graphitic grain average aspect ratio were measured for the graphitized heat treated steel using an image analyzer.

The measuring method of the area fraction, average size, and average aspect ratio of graphite grain is as follows. Each specimen was cut to a certain size, and the image was taken under a magnification of 200 times using an optical microscope with only the polishing without etching. The images thus obtained can be clearly distinguished by the apparent contrast difference between the matrix and the graphite phase, and the analysis was performed using image analysis software. In addition, 15 images were taken per specimen to increase the reliability of the analysis.

On the other hand, the area fraction of graphite is defined as the ratio of the area of graphite to the total area observed, and the average size and aspect ratio of graphite are the average circular equivalent diameter and the longest and shortest axes in one graphite grain, respectively. Means rain.

division Graphitization Heat Treatment Temperature (℃) Graphitization Heat Treatment
5 hours later Graphite area fraction (%) Graphite grains less than 3um per mm ² Graphite Grain Average Aspect Ratio Hardness (HRB) Inventive Example 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 the parts were machined for machinability evaluation, chip segmentation, tool wear depth and surface roughness, that is, roughness of the cut surface were measured. To this end, first, the plate-shaped steel was graphitized for 5 hours at the graphitization heat treatment temperature shown in Table 2, and then processed into a rod having a diameter of 25 mm, which was then cut by a CNC automatic lathe. In the chip segmentation evaluation, the chip was divided into two or less volumes, and the chip was divided into three or six volumes.

Tool wear depth was calculated by comparing the depth of the tool edge before and after machining 200 parts with a length of 200mm until the 25mm diameter rod to 15mm diameter as shown in Figure 1. At this time, cutting conditions were performed using cutting oil under the conditions of cutting speed of 100mm / min, feed rate of 0.1mm / rev, and cutting depth of 1.0mm.

division Surface Roughness (㎛) Tool Wear Depth (㎛) Chip segmentation Inventive Example 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

Referring to Tables 1 and 2, Inventive Examples 1 to 9 satisfying both the composition and the production conditions proposed in the present invention, the microstructure consists of pearlite and graphite, graphite area fraction 2% or more, graphite grain average aspect ratio 2.0 Hereinafter, the density of graphite grains is 1000 pieces / mm ² The above was shown. In addition, referring to Table 3, it can be confirmed that the graphite steel according to the disclosed embodiment has good chip segmentation, 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 Comparative Example 10, although the C content is high, the graphite area fraction satisfies the scope of the present invention, but coarse graphite grains are formed, and the aspect ratio is relatively high. As a result, as shown in Table 3, it can be seen that the surface roughness of the cutting surface is relatively inferior.

On the contrary, in the case of Comparative Example 11, the graphite content was low because the C content could not be produced in a sufficient amount, and as a result, it was confirmed that not only the tool wear depth was increased but also the chip segmentability was inferior.

Comparative Examples 12 to 15 are steels to which Mn and Si amounts are added in a range outside of Equation (1), and it can be seen that the hardness measurement results are also out of the range of hardness values presented in the present invention. Specifically, in the case of Comparative Examples 13 and 14, the hardness was 89.2 and 82.3, it can be seen that the degree of tool wear was deepened.

On the contrary, in Comparative Examples 12 and 15, the hardness is 61.3 and 66.3, it can be seen that the surface roughness characteristics are inferior.

In the case of Comparative Examples 16 and 19, the added N was too high to satisfy the formula (2) compared to the Ti addition amount, which resulted in an excess of solid solution nitrogen remaining in the steel without forming TiN, thus completely during a given heat treatment time. It could be confirmed that there was no graphitization and some pearlite remained, which resulted in an increase in tool wear due to hardness of 82.6 and above 80.

In Comparative Example 17, the graphitization heat treatment temperature was lowered to 700 ° C., so that the pearlite was not completely graphitized during the graphitization heat treatment, and pearlite was observed in the microstructure. This can be seen that the hardness increased to 83.1 above 80, resulting in a deeper tool wear depth.

In the case of Comparative Example 18, the graphitization heat treatment temperature is high at 800 ° C., and the phase transformation into austenite causes pearlite to be produced again upon cooling. As a result, the hardness of the tool is increased to 94.3.

In the case of Comparative Example 20, the added Ti is excessive compared to the amount of N added, did not satisfy the formula (2), thereby forming coarse graphite grains can be confirmed that the surface roughness is relatively inferior.

In the graphite steel according to the embodiment of the present invention, the graphite grains are sufficiently formed in the matrix, and the fine graphite grains are uniformly distributed in a regular shape to improve machinability.

As described above, the exemplary embodiments of the present invention have been described, but the present invention is not limited thereto, and a person skilled in the art does not depart from the spirit and scope of the following claims. It will be understood that various changes and modifications are possible in the following.

Claims (9)

By weight, C: 0.60 to 0.90%, Si: 2.0 to 2.5%, Mn: 0.1 to 0.6%, Al: 0.01 to 0.05%, Ti: 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), including residual Fe and inevitable impurities,
Steel for graphitizing heat treatment that satisfies the following formulas (1) and (2).
Equation (1): -0.01 ≤ [Ti] -3.43 k [N] ≤ 0.01
(Where [Ti] and [N] are the weight% of the corresponding element, respectively.)
Equation (2): 400 ≤ 3.1 + 169.0 ⅹ [Si] + 127.7 ⅹ [Mn] ≤ 500
(Here, [Si] and [Mn] mean each weight% of the corresponding element.) delete delete By weight, C: 0.60 to 0.90%, Si: 2.0 to 2.5%, Mn: 0.1 to 0.6%, Al: 0.01 to 0.05%, Ti: 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), including residual Fe and inevitable impurities,
Satisfy the following formula (1) and formula (2),
Contains at least 2.0% graphite grains in an area fraction of the ferrite matrix,
Graphite steel with improved machinability, wherein the average aspect ratio of the graphite grains is 2.0 or less.
(The aspect ratio of the graphite grains here means the ratio of the longest axis and the shortest axis in one graphite grain.)
Equation (1): -0.01 ≤ [Ti] -3.43 k [N] ≤ 0.01
(Where [Ti] and [N] are the weight% of the corresponding element, respectively.)
Equation (2): 400 ≤ 3.1 + 169.0 ⅹ [Si] + 127.7 ⅹ [Mn] ≤ 500
(Here, [Si] and [Mn] mean each weight% of the corresponding element.) delete delete The method of claim 4, wherein
Graphite steel with improved machinability, wherein the average grain size of the graphite grains is 5 µm or less. The method of claim 4, wherein
The number of graphite grains per unit area of the graphite steel is improved machinability of 1000 to 5000 / mm ² . The method of claim 4, wherein
Hardness of the graphite steel is graphite steel with improved machinability of 70 to 80 HRB.