KR20060057804A - Steel for manufacturing graphite steel having good graphitizing property - Google Patents

Abstract

The present invention relates to a steel for graphite steel, and is suitable for use in mechanical parts such as industrial machines or automobiles, and when producing graphite steel, which is a mechanical structural steel having excellent cold forging and good machinability, the graphitization treatment time is remarkable. The present invention relates to a steel material for producing graphite steel which is shortened.

The steel is in weight%, C: 0.30-0.70%, Si: 2.0-4.0%, Mn: 0.1-1.0%, P: 0.01-0.15%, S: 0.01% or less, Se: 0.001-0.05%, Ti: Steel for graphite steel containing 0.001 to 0.03%, B: 0.001 to 0.003%, Al: 0.002 to 0.01%, N: 0.004 to 0.008%, O: 0.005% or less, and consisting of balance Fe and other impurities. N, B, and Al satisfy the following relational formula 1, and Mn, Se, and S satisfy the following relational formula 2, and are graphite steels excellent in graphitization processability.

(However, Ti, N, B, Al, Mn, Se and S in the following relations 1 and 2, respectively, means the weight percent of the element.)

[Relationship 1]

2.0≤ (Ti + 5B + Al) /N≤5.5

[Relationship 2]

1.0≤ (Mn / 5 + Se) /5S≤3.0

Graphite Steel, Machinability, Cold Forging, Graphitization, Ferrite

Description

Steel for graphite steel with excellent graphitization treatment {STEEL FOR MANUFACTURING GRAPHITE STEEL HAVING GOOD GRAPHITIZING PROPERTY}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to steel for graphite steel, and is suitable for use in machine parts such as industrial machines or automobiles, and when graphite steel, which is a mechanical structural steel having excellent cold forging and good machinability, is produced, the graphitization treatment time is significantly shortened. It relates to steel materials for steel production.

Mechanical parts used in industrial machines and automobiles are usually machined into complex shapes of the parts by either cutting or cold forging. However, in the case of the cutting process, in order to process the final shape of the raw material, there is a problem that the amount of processing increases, resulting in large parts loss and excessive production cost. In addition, the cold forging process has the advantage of relatively low component loss compared to the cutting process and can be carried out through a simple process. However, when the final shape of the component is complicated, the forging operation alone cannot realize the complicated shape. . Therefore, it is most suitable in terms of material saving, production cost reduction, etc., after machining into a near net shape by cold forging and final cutting into a complicated shape. In order to perform the cutting process after such cold forging, both machinability and cold forging of the material must be combined, and such steel cannot satisfy all of these properties.

That is, in order to improve the machinability of the material, a free-cutting steel containing machinable elements such as Pb, S, Bi, etc. can be used in steel. Such free-cutting steel can be used for cutting steel such as surface roughness, chip treatment, and tool life. In terms of machinability, it is very good. However, when cold forging is performed using this, cold forging is very poor due to cracks caused by micro-deformation due to cracks caused by inclusions of machinability enhancing elements.

In addition, Pb, which has been conventionally used as a machinability granting element, emits harmful substances such as toxic fume during cutting, and thus is very harmful to the human body. Although S, Bi, Te, Sn, and the like have been proposed, Bi-added steels are easily cracked during manufacturing, which is very difficult to produce, and S, Te, Sn, etc. cause cracks during hot rolling. There is a problem.

Therefore, the free-cutting steel using the conventional machinability imparting element not only has very poor cold forging, but also has a problem of causing environmental problems and low production efficiency, making it difficult to use as a steel material having both cold forging and machinability. It was.

In addition, cold forged steel is excellent in toughness and ductility, so it is advantageous for cold forging to be processed by forging to a degree similar to the final shape, but hardly cracks in the material. (chip) There is a problem that it is difficult to use because the machinability such as chipability and surface roughness is very poor.

In order to solve the above problems, the proposed steel is graphite steel. Graphite steel is a carbon steel containing fine graphite grains inside, and has excellent impact toughness and ductility, and is excellent in cold forging, and also has good machinability as the internal fine graphite grains become a crack source during cutting. It is a river.

Japanese Unexamined Patent Publication No. 50-096416 discloses C (total): 0.45 to 0.90, Si: 1.0 to 2.5%, Mn: 0.1 to 0.7%, S: 0.015% or less, and one or two of Al and Ti 0.015 to Graphite free cutting steel containing 0.15%, containing 0.45 to 0.90% of graphite present in a distribution of 50 pieces / mm ² or more, and consisting of the balance Fe and impurities has been proposed. The invention has been proposed in terms of free machinability rather than cold forging, but can be referred to as an invention belonging to the invention of the invention for graphite-based free cutting steel. However, the composition of the present invention discloses that the silicon concentration should not exceed 2.3% and the minimum graphitization time accordingly requires about 10 hours. However, when demand is considered in the process, it is inevitable to perform graphitization heat treatment for about 10 hours in order to give free machinability. It is virtually impossible to implement.

In addition, Japanese Patent Laid-Open No. 6-212351 discloses a method for producing graphite free-cutting steel that is graphitized by heat treatment in a relatively short time in order to solve the problem that the graphitization treatment, which is a problem shown in Japanese Patent Application Laid-Open No. 50-096416, takes a long time. Doing. The technique disclosed in the above publication is C: 0.3-1.0%, Si: 0.4-1.0%, Mn to shorten the graphitization treatment time; 0.3 ~ 1.0, P: 0.02% or less, S: 0.015 ~ 0.035, B: 0.001 ~ 0.004%, N: 0.002 ~ 0.008%, and hot-rolled steel bars immediately after hot rolling composed of the balance Fe and unavoidable impurities. The water cooling device installed before and after the line cools the cooling start temperature below Ac1 point, the cooling end temperature below Ms point, and the average cooling rate from 30 ° C / s to 100 ° C / s. After that, the graphitization treatment is then performed at a heating temperature of 600 to 700 ° C. This technique uses the grating distortion, a characteristic of martensitic phase, to facilitate the precipitation of carbon into graphite, which is a technical gist of at least 9-12. It was a technique that could reach 100% graphitization rate within time.

However, even with the above technique, the time required for graphitization is at least 9 hours, so that it is difficult for the demand to be applied to the process, and there is still a need for technical advancement required to shorten the graphitization time.

As a technique for further shortening the graphitization time, Japanese Patent Application Laid-Open No. 2000-063948 has a weight%, more than C: 1.00 to 1.50%, Si: 1.00 to 2.80%, Mn: 0.01 to 2.00%, and P: 0.050% or less. Cast iron or slab containing 0.10% or less, O: 0.0050% or less, N: 0.020% or less, consisting of residual iron (Fe) and unavoidable impurities, and having a graphitization index CE of 1.30 or more, 850 to 1150 ° C. After heating to a temperature in the range of, hot-rolled, and then cooled to room temperature, the hot-rolled steel thus obtained is heated to a temperature within the range of 600 to 1000 ° C. for 3 hours or less, followed by air cooling, and then averaged in the steel. the particle size or more 1.0㎛ graphite to precipitate 100 / mm ² or more, and free cutting steel to the metal structure characterized in that the tissue and the Brinell hardness consisting of or composed of more than 20% of ferrite with the balance pearlite or ferrite only to 200 bongseon There is a description of the production method is described. If the above technique is used, the time required for graphitization can be shortened to at least 30 minutes, and thus it can be referred to as a technique of drastically improving the graphitization time compared to the prior art. However, even in the case of the above technique, it is still difficult to apply to the actual processing process because the graphitization treatment time, which is at least about 30 minutes and most of the time, is required.

In order to be applied to the actual machining process, the pre-graphitized wire rod is provided to the demand side so that the demand can be cold forged and cut without additional graphitization, or the demand is before the cold forging. Alternatively, it needs to be graphitized for a very short time so that it can be immediately used for cutting work. In order to satisfy all of the above conditions, inline graphitization that can be graphitized in a continuous process should be possible. The allowable graphitization time for graphitization is up to 7 minutes.

In view of this aspect, the present inventors have proposed that the steel microstructure of ferrite + graphite abnormal structure improves the cold forming of the bolt material. Korean Patent Application Nos. 2001-84518, 2001-84517, 2001-85540 It was proposed in the issue. According to the technique of the present inventors, the time required for graphitization could be significantly shortened compared to the prior art in about 10 minutes. However, the above technique still did not shorten the graphitization time until the maximum graphitization time necessary for complete process application.

The present invention has been completed in a series of research process to overcome the above technical limitations of the prior art, the object of which is to add a low melting point elements such as Pb, Bi, S, Sn added to give a free machinability In order to produce graphite steel having similar high machinability and at the same time excellent cold workability (or cold forging), the graphitization heat treatment time is greatly reduced to 7 minutes or less when the graphitization heat treatment is performed. It is an object of the present invention to provide a steel wire for graphite steel capable of making the distribution fine and uniform.

By weight%, C: 0.30 ~ 0.70%, Si: 2.0 ~ 4.0%, Mn: 0.1 ~ 1.0%, P: 0.01 ~ 0.15%, S: 0.01% or less, Se: 0.001 ~ 0.05%, Ti: 0.001 ~ 0.03 As a steel for graphite steel containing%, B: 0.001 to 0.003%, Al: 0.002 to 0.01%, N: 0.004 to 0.008%, O: 0.005% or less, and remain with Fe and other impurities,

The Ti, N, B and Al satisfy the following relational formula 1, Mn, Se and S satisfy the following relational formula 2, characterized in that the graphite steel with excellent graphitization treatment.

However, Ti, N, B, Al, Mn, Se, and S of the following Formula 1 and Formula 2 each represent a weight percentage of the corresponding element.

[Relationship 1]

2.0≤ (Ti + 5B + Al) /N≤5.5

[Relationship 2]

1.0≤ (Mn / 5 + Se) /5S≤3.0

At this time, in addition to the above-mentioned emphasis Ni: 0.05 ~ 1.0%, Cu: 0.01 ~ 0.5%, Ca: 0.0001 ~ 0.05%, Zr: 0.0005 ~ 0.008%, REM (Rare Earth Metal, Rare Earth Metal): 0.001 ~ 0.05% It is preferable to contain 1 type (s) or 2 or more types selected from the group.

In addition, it is preferable that the fraction of the cornerstone ferrite in the internal structure of the steel wire of the composition is 10% or less.

Hereinafter, the present invention will be described in detail.

[Lecture composition]

The reason for limiting the alloying elements in the present invention is as follows.

Carbon (C): 0.3-0.7 wt%

Carbon is an essential element for forming graphite, and is an important element for securing the strength of mechanical parts afterwards, but 0.3 to 0.7% by weight since the effect is less than 0.3% by weight and the effect is saturated at more than 0.7% by weight. It is limited to the range of.

Silicon (Si): 2.0-4.0 wt%

Silicon is a necessary component as a deoxidizer in molten steel and is an element that makes carbon carbide precipitate by making iron carbide (cementite) in steel unstable. In addition, silicone is actively added because it is a component that improves strength. Less than 2.0% by weight, the effect is insufficient. Even if more than 4.0% by weight of silicon is added, the effect of promoting graphitization is saturated and the temperature range where liquid phase is generated becomes low, so that the appropriate temperature range is narrowed during hot rolling. It was limited to the range of weight%.

Manganese (Mn): 0.1-1.0 wt%

Manganese is an effective element to secure the strength of steel and is also useful as a deoxidizer in molten steel manufacturing. In addition, it combines with S to form MnS, contributing to the improvement of machinability. However, when the content is less than 0.1% by weight, the effect of improving strength is small, and when the amount is more than 1.0% by weight, the toughness is deteriorated.

Phosphorus (P): 0.01 wt% or less

Phosphorus not only inhibits graphitization, but also segregates at the austenite grain boundary during the quenching treatment, thereby lowering the grain boundary strength, so that quenching cracks are easily generated.

Sulfur (S): 0.01 wt% or less

Sulfur forms MnS to improve chip breaking during cutting to improve machinability, especially as nucleation of graphitization to promote graphitization, but if the amount is more than 0.01% by weight, the effect is not only saturated. On the contrary, the upper limit was limited to 0.01% by weight because it delayed the graphitization and drastically lowered the toughness of the steel, thus adversely affecting the cold forging.

Selenium (Se): 0.001 ~ 0.05 wt%

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

Titanium (Ti): 0.001-0.03 wt%

Titanium combines with nitrogen in steel to form TiN, making cementite unstable and at the same time becoming a nucleation site of graphite to promote graphitization. Moreover, since it functions effectively also as a deoxidizer, it adds actively. To attain the above useful effects, at least 0.001% by weight should be added. However, at 0.03% or more by weight, the graphitization is rather hindered, so it is limited to 0.001 to 0.03% by weight.

Boron (B): 0.001 to 0.003% by weight

Boron is an element that is actively added because it combines with N to form BN and acts as a crystal nucleus of graphite while interfering with stabilization of cementite, thereby promoting graphitization and enhancing quenchability. If it is less than 0.001% by weight, the effect of addition is insufficient, so it is necessary to add more than 0.001% by weight. On the contrary, if it is added more than 0.003% by weight, the effect can not be expected to increase any more, and at the same time, the grain boundary strength due to precipitation of boron nitride in the grain boundary The addition range was limited to 0.001 to 0.003% by weight because it lowered the hot workability.

Aluminum (Al): 0.002-0.03 wt%

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

Nitrogen (N): 0.004-0.008 wt%

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

Oxygen (O): 0.005% by weight or less

In the present invention, the role of oxygen is important. Oxygen combines with aluminum to form oxides. The production of such oxides reduces the effective concentration of aluminum. As a result, the amount of AlN produced useful for the crystallization of graphite is reduced, thus causing the effect of substantially obstructing the graphitization action. In addition, alumina oxide formed by containing a large amount of oxygen damages the cutting tool during cutting, resulting in a decrease in machinability. For this reason it is desirable to manage as low as possible. However, if the oxygen is managed too low, it causes a refining load of the steelmaking process, and the upper limit is limited to 0.005% by weight or less because the problem caused by the oxygen is not so much up to 0.005% by weight.

As can be seen from above, in order to promote graphitization, first, a carbon source necessary for graphitization is sufficiently maintained in the steel, and the cementite is destabilized so that carbon in the cementite can be easily diffused into the steel. Oxygen must be able to grow in graphite on various types of nitride or compound inclusions, which are non-homogeneous nucleation sites in the river.

However, the composition alone may not provide a sufficient nucleation site for graphitization, and the nitride- or emulsion-based inclusions generated by the composition may be effectively finely dispersed in a large amount in the steel to supply a sufficient nucleation site. Accordingly, the graphitization time can be significantly shortened and fine graphite can be produced.

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

It is preferable that (Ti + 5B + Al) / N ratio is 2.0 <(Ti + 5B + Al) / N <= 5.5.

If the (Ti + 5B + Al) / N ratio is less than 2.0, the number of TiN and BN precipitates that contribute to the nucleation of graphite grains due to the lack of Ti, B, Al is insufficient, and the amount of (Ti + 5B + Al) / N When the ratio exceeds 5.5, Ti, B, and Al are sufficient, but the amount of nitrogen is insufficient, so the number of precipitates of TiN, BN, and AlN necessary for graphite nucleation is not increased anymore, but due to excess nitrogen, The amount of nitrogen that is added increases and adversely affects the graphitization rate.

The value of (Mn / 5 + Se) / 5S is preferably 1.0 ≦ (Mn / 5 + Se) /5S≦3.0.

If the value of (Mn / 5 + Se) / 5S is less than 1.0, the number of MnSe precipitates contributing to nucleation of graphite grains and the effective MnS inclusions for machinability are insufficient, and when (Mn / 5 + Se) / 5S ratio exceeds 3.0, Not only the number of MnSe precipitates and MnS precipitates required for graphite grain nucleation is saturated, but the amount of sulfur dissolved in the base material increases, causing grain boundary segregation, which adversely affects mechanical properties.

One or more of the group consisting of 0.05 to 1.0% by weight of nickel, 0.01 to 0.05% by weight of copper, 0.0001 to 0.05% by weight of calcium, 0.0005 to 0.008% by weight of zirconium, and 0.001 to 0.05% by weight of rare earth metal. Two or more kinds are selectively added, and the reason for component limitation is explained.

Nickel (Ni): 0.05-1.0 wt%

Nickel is effective in improving the hardenability of steel and enhancing the strength of steel by hardening, and at the same time assisting and promoting graphitization. The effect was insufficient at 0.05% by weight or less, and at 1.0% by weight or more, the effect was saturated and limited to 0.005 to 1.0% by weight because it was not economical as an expensive element.

Copper (Cu): 0.01-0.5 wt%

Since copper is effective for promoting graphitization by making cementite unstable, not only improves machinability but also has an effect of increasing strength at the time of hardening of steel by the effect of enhancing the hardening of the steel and the strengthening of precipitation. In addition, when copper is added, the corrosion resistance of the steel can be improved. If it is less than 0.01% by weight, the effect of promoting graphitization and improvement of corrosion resistance is insufficient. If it exceeds 0.5% by weight, the improvement effect is saturated and the melting point is lowered at the grain boundary segregation. It was limited to 0.01 to 0.5% by weight because of the high probability of occurrence of surface defects due to grain embrittlement during charging and the impact toughness in the final product.

Calcium (Ca): 0.0001 ~ 0.05 wt%

Calcium improves machinability by forming Ca-Al-based oxides in the steel composition of the present invention, which acts as a nucleus of graphitization to promote graphitization. This effect is less than 0.0001% by weight, and in 0.05% by weight or more coarse oxide-based non-metallic inclusions are generated in a large amount to reduce the fatigue strength of the mechanical parts were limited to the range of 0.0001 ~ 0.05% by weight.

Zirconium (Zr): 0.0005 ~ 0.008% by weight

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

Rare Earth Metal (REM): 0.001 ~ 0.05% by weight

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

Hereinafter, the structure of the steel wire for finely controlling the size of the graphite grains will be described.

Cornerstone ferrite structure fraction: 10% or less

Since the steel wire according to the present invention has a composition of a pore-stone region, upon cooling, the steel wire passes through an austenite + ferrite abnormal region, and as a result, the cornerstone ferrite may precipitate. The cornerstone ferrite has a low carbon utilization in the steel and hardly acts as a carbon source. Therefore, the cementite constituting the residual pearlite other than the cornerstone ferrite acts as a carbon source, but when the fraction of the cornerstone ferrite is high, the cementite is difficult to be uniformly distributed, so the carbon diffusion distance becomes uneven and thus cementite Where the large amount of distribution is easy carbon diffusion, large graphite is distributed in a large amount, and the ferrite distribution is difficult to produce graphite grains, the graphitic grain distribution becomes uneven during graphitization. Therefore, a steel material having a high perlite fraction, that is, a steel wire having a structure of a cornerstone ferrite fraction, is preferable. In order to have a uniform and fine graphite grain distribution during graphitization, the fraction of the cornerstone ferrite should be controlled to 10% or less.

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

EXAMPLE

The steels having the composition as shown in Table 1 below were cast into 50 kg ingot, homogenized and heat treated at 1250 ° C. for 48 hours, and hot-rolled to 13 mm in thickness to produce air wire. At this time, the finish temperature at the time of hot rolling was set to 950 ℃ and the amount of reduction was 80% or more.

division C, wt% Si, wt% Mn, wt% S, wt% P, wt% Ti, wt% B, ppm Al, wt% Se, wt% N, ppm O, ppm Ni, wt% Cu, wt% Ca, wt% Zr, ppm REM, ppm foot                                                  persons                                                  Yes One 0.51 3.1 0.51 0.008 0.011 0.007 20 0.005 0.004 60 20 0.1 2 0.52 3.1 0.40 0.009 0.015 0.004 10 0.004 0.003 55 21 0.1 0.05 3 0.50 3.0 0.48 0.007 0.009 0.010 21 0.005 0.002 56 19 0.01 4 0.49 2.9 0.55 0.009 0.008 0.010 20 0.003 0.010 60 24 10 5 0.53 3.2 0.53 0.008 0.009 0.009 30 0.005 0.004 65 22 10 6 0.7 3.1 0.48 0.007 0.008 0.004 13 0.003 0.002 75 17 0.09 0.04 7 0.31 3.2 0.51 0.008 0.011 0.010 26 0.004 0.002 66 29 0.16 0.06 8 0.53 4.0 0.55 0.009 0.009 0.003 17 0.004 0.005 60 23 0.20 0.09 9 0.49 2.8 1.00 0.008 0.015 0.011 29 0.015 0.005 55 30 0.25 0.11 10 0.66 2.05 0.44 0.009 0.009 0.003 20 0.010 0.006 80 40 0.54 0.21 ratio                                                  School                                                  Yes One 0.51 3.0 0.51 0.008 0.011 0.001 10 0.003 0.004 65 19 0.13 2 0.52 3.1 0.48 0.009 0.008 0.010 20 0.005 0.002 40 23 0.12 0.04 3 0.50 3.0 0.22 0.007 0.130 0.004 10 0.004 0.001 55 17 0.02 4 0.49 2.9 0.71 0.009 0.009 0.003 17 0.004 0.021 60 21 20 5 0.53 3.2 0.19 0.008 0.129 0.001 10 0.003 0.001 65 20 10 6 0.8 3.1 0.70 0.007 0.009 0.010 20 0.005 0.019 40 28 0.21 0.10 7 0.31 3.2 0.69 0.008 0.009 0.001 10 0.003 0.020 65 15 0.51 0.23 8 0.53 4.0 0.20 0.009 0.131 0.010 20 0.005 0.001 40 34 0.24 9 0.49 1.5 0.52 0.008 0.015 0.011 15 0.015 30 25 10 0.66 2.5 0.44 0.009 0.009 0.044 21 0.03 100 32

In order to examine the effect of each wire structure on the size of graphite grain, an analysis of the cornerstone ferrite area fraction was carried out for the area of 300mm ² per specimen using an image analyzer for the pre-graphitized wire structure. Table 2 summarizes the ferrite area fraction analysis results and the calculation results of the relational expressions 1 and 2.

division Relationship 1 Relation 2 Cornerstone ferrite fraction (%) (Ti + 5B + Al) / N (Mn / 5 + Se) / 5S Inventive Example 1 4.5 1.9 5 Inventive Example 2 3.1 1.1 4 Inventive Example 3 5.5 2.2 5 Inventive Example 4 4.3 3.0 4 Inventive Example 5 5.4 2.4 4 Inventive Example 6 2.1 2.5 0 Inventive Example 7 4.1 1.9 10 Inventive Example 8 5.2 2.6 9 Inventive Example 9 3.3 2.7 3 Inventive Example 10 4.1 2.1 0 Comparative Example 1 1.4 1.9 35 Comparative Example 2 6.5 2.5 25 Comparative Example 3 3.1 0.6 32 Comparative Example 4 5.2 3.6 21 Comparative Example 5 1.4 0.6 26 Comparative Example 6 6.5 3.6 23 Comparative Example 7 1.4 3.6 30 Comparative Example 8 6.5 0.6 24 Comparative Example 9 11.2 - 20 Comparative Example 10 8.4 - 22

Thereafter, graphitization heat treatment was performed to examine the graphitization behavior during the graphitization treatment of the steel. The graphitization heat treatment temperature was fixed at 750 ° C. and then air cooled. After varying the graphitization heat treatment time for the samples of the same composition, the graphitization rate was confirmed for an area of 300 mm ² per specimen using an image analyzer, and the graphitization time was defined as the graphitization time for the graphitization rate. It was.

In order to confirm the performance of the graphite steel manufactured through the graphitization heat treatment from the viewpoint of machinability and cold forging, the following experiment was performed.

Cold forging was performed by using a test piece manufactured with a diameter of 19 mm x 25 mm in height, and the critical volume change rate until cracking occurred at room temperature compression was used as an evaluation criterion for the critical cold forging rate. At this time, the critical cold forging rate was performed 10 times, and the rest was evaluated as the average value except the maximum minimum value.

An automatic lathe was used for the machinability test, and the machinability of the graphite steel was determined by the chip treatment and the tool life. Chip chipping was used as the chip processing method. Chips were divided into two or less books, and chips were divided into three or six books. In the test of tool life, the tool life was measured by measuring the time from the tip of the die tip to the point where it became impossible to cut when cutting in the environment using cutting oil at cutting speed 150m / min and 0.20mm / rev.

The graphitization time, graphite steel internal structure, cold forging property and free machinability test results are shown in Table 3. The internal structure of the graphite steel was analyzed for an area of 300 mm ² per specimen using an image analyzer in the same manner as the cornerstone ferrite fraction analysis.

division Graphitization properties Cold Forging Machinability Graphitization Time (min) Graphite fraction (%) Graphite Grain Average Size (㎛) Critical cold forging rate (%) Chip Processing Tool life (min) Inventive Example 1 5 2.1 7 130 Great 120 Inventive Example 2 6 2.2 8 140 Great 110 Inventive Example 3 5 2.0 9 130 Great 130 Inventive Example 4 7 1.9 7 135 Great 115 Inventive Example 5 7 2.1 6 140 Great 140 Inventive Example 6 6 2.5 8 130 Great 120 Inventive Example 7 7 1.2 7 140 Great 130 Inventive Example 8 5 2.1 7 135 Great 125 Inventive Example 9 6 2.0 6 135 Great 140 Inventive Example 10 5 2.7 9 140 Great 135 Comparative Example 1 350 2.0 25 70 usually 40 Comparative Example 2 400 2.0 30 75 usually 35 Comparative Example 3 10 2.1 20 80 usually 50 Comparative Example 4 9 2.2 17 90 usually 60 Comparative Example 5 600 2.1 21 85 usually 40 Comparative Example 6 440 2.9 33 70 usually 30 Comparative Example 7 530 1.3 20 80 usually 15 Comparative Example 8 380 2.0 19 80 usually 45 Comparative Example 9 3600 2.2 24 75 usually 20 Comparative Example 10 3900 2.6 35 65 usually 25

As shown in Table 3, when the emphasis of the present invention is used, the graphitization time is within 7 minutes, and 100% graphitization treatment is possible within the graphitization time target of the present invention, while the comparative examples are at least 9 minutes to the maximum. The deviation was shown up to 3900 minutes. Thus, it was possible to confirm the superiority of the graphitization performance of the steel according to the present invention.

In addition, it can be seen that the size of the graphite grains of the graphite steel graphitized by using the steel of the present invention has a graphite finer than that of the comparative example of 17 to 35 μm at about 7 to 9 μm. The cold critical forging of these shows a 65 to 90% range while the present invention can be seen that it is quite excellent in the 130 ~ 140% range. In addition, as a result of determining the machinability, in the case of graphite steel according to the present invention, the chip treatment property is all divided into two or less books, whereas in the comparative example, graphite chips manufactured from the steel for graphite steel according to the present invention are divided into three or more books. It was confirmed that this has a much better chip treatment, the tool life was also found that the tool life of the graphite steel made of the steel for graphite steel according to the present invention improved at least 55 minutes over the comparative example.

As can be seen from the above, according to the present invention, it is possible to provide a graphite steel which is significantly shortened in graphitization time within 7 minutes as compared with the graphite steel provided by the prior art, and the graphite steel is an inline graphite immediately after rolling. It is possible to graphitize without performing a process load before or after cutting or cutting, thereby completely eliminating the problems of the prior art, which is difficult to apply graphite steel to a process due to a long graphitization treatment time. In addition, the graphite steel produced from the steel for graphite steel according to the present invention is significantly improved machinability and cold forging compared to other steel materials, which is very advantageous for machining parts.

Claims (3)

By weight%, C: 0.30 ~ 0.70%, Si: 2.0 ~ 4.0%, Mn: 0.1 ~ 1.0%, P: 0.01 ~ 0.15%, S: 0.01% or less, Se: 0.001 ~ 0.05%, Ti: 0.001 ~ 0.03 As a steel for graphite steel containing%, B: 0.001 to 0.003%, Al: 0.002 to 0.01%, N: 0.004 to 0.008%, O: 0.005% or less, and remain with Fe and other impurities, The Ti, N, B and Al satisfy the following relational formula 1, Mn, Se and S satisfy the following relational formula 2, characterized in that the graphite steel with excellent graphitization treatment. However, Ti, N, B, Al, Mn, Se, and S of the following Formula 1 and Formula 2 each represent a weight percentage of the corresponding element. [Relationship 1] 2.0≤ (Ti + 5B + Al) /N≤5.5 [Relationship 2] 1.0≤ (Mn / 5 + Se) /5S≤3.0 According to claim 1, Ni: 0.05 ~ 1.0%, Cu: 0.01 ~ 0.5%, Ca: 0.0001 ~ 0.05%, Zr: 0.0005 ~ 0.008%, REM (Rare Earth Metal, Rare Earth Metal): 0.001 ~ 0.05% Graphite steel excellent in graphitization treatment, characterized in that it comprises one or two or more selected from the group. 2. The graphite steel having excellent graphitization treatment property according to claim 1, wherein the fraction of the cornerstone ferrite in the internal structure of the steel is 10% or less.