KR102126971B1 - Graphite steels excellent in machinability and soft magnetism and methods for manufacturing the same - Google Patents

Abstract

This specification relates to graphite steel that can be applied to electromagnetic components in various fields, such as automobiles, robots, electronics, electricity, computers, and communications, and discloses a graphite steel having excellent machinability and softness and a method of manufacturing the same.
According to an embodiment of the disclosed graphite steel in weight%, C: 0.4 to 1.0%, Si: 2.0 to 3.0%, Mn: 0.01 to 0.4%, P: 0.03% or less, S: 0.02% or less, Al: 0.01 to 0.05%, Ti: 0.01~0.03%, Ca: 0.0005~0.003%, N: 0.003~0.008%, O: 0.005% or less, the balance contains Fe and other inevitable impurities, and the microstructure distributes graphite grains to the ferrite matrix do.

Description

Graphite steels excellent in machinability and soft magnetism and methods for manufacturing the same}

The present invention relates to graphite steel which can be applied to electromagnetic components in various fields such as automobiles, robots, electronics, electricity, computers, and communications, and a method of manufacturing the same, more specifically, it is required in a fine cutting process as well as excellent softness. Graphite steel having excellent machinability and a method for manufacturing the same.

In accordance with demands for miniaturization, precision, and productivity improvement of materials that can be applied to electromagnetic parts in various fields such as automobiles, robots, electronics, electricity, computers, and communications, demands for improving machinability are also increasing in soft magnetic materials. The typical iron-based soft magnetic material, pure iron and silicon-containing electric steel sheet, has excellent electromagnetic properties, but has low strength and difficult machining. The soft magnetic property is easily magnetized when a magnetic field is applied, but the magnetic property is easily lost when the magnetic field is removed. The electromagnetic properties of the excellent soft magnetic material include high magnetic flux density, low coercive force, and low iron loss.

For this reason, the actual industry is cutting parts using low-carbon free cutting steel in which S is added in a large amount up to about 0.3%, such as SUM22. In this case, the machinability is good, but the problem of deterioration of electromagnetic properties occurs. This is because MnS is formed due to the added S for the purpose of improving the machinability, and the formed MnS interferes with magnetic flow and deteriorates electromagnetic properties. In addition, the low-carbon free-cutting steel had a problem of deteriorating electromagnetic properties due to the presence of about 10% pearlite.

In order to solve the problem that the electromagnetic properties of low-carbon free-cutting steel are deteriorated, research has been conducted on graphite steel containing high carbon and silicon. However, since graphite is precipitated with cementite, which is a metastable phase, even when graphite is a stable phase when carbon is added, it is difficult to precipitate graphite without additional long heat treatment. In addition, there is a problem in that decarburization occurs during such a long heat treatment process and adversely affects the performance of the final product.

In addition, even if the graphite grains are precipitated through the graphitization heat treatment, the probability of cracks is increased when the graphite in the base of the steel is coarse precipitated to 10 µm or more, and if it is unevenly distributed in an irregular shape rather than a spherical shape, cutting is performed. When the distribution of physical properties is non-uniform, chip segmentation or surface roughness becomes very bad, and tool life is also shortened, making it difficult to obtain the advantages of graphite steel.

Korean Patent Registration Publication No. 1992-0007579 (announced on September 7, 1992)

In order to solve the above-mentioned problems, the present invention significantly shortens the graphitization heat treatment time, but allows the fine graphite grains to be uniformly distributed in the final microstructure after heat treatment. An object of the present invention is to provide a graphite steel excellent in soft magnetic properties and a method for manufacturing the same.

Graphite steel excellent in machinability and softness according to an embodiment of the present invention is in weight%, C: 0.4 to 1.0%, Si: 2.0 to 3.0%, Mn: 0.01 to 0.4%, P: 0.03% or less, S: 0.02% or less, Al: 0.01 to 0.05%, Ti: 0.01 to 0.03%, Ca: 0.0005 to 0.003%, N: 0.003 to 0.008%, O: 0.005% or less, including residual Fe and other unavoidable impurities, and microstructure Graphite grains are distributed in the silver ferrite matrix.

In addition, the graphitization rate may be 99% or more.

In addition, the graphite particles may be distributed in an area fraction of 2 to 5%.

In addition, the average grain size of the graphite grain may be 10㎛ or less.

In addition, the graphite particles may be distributed in more than 1000 pieces / mm ² .

In addition, the magnetic flux density at a coercive force of 1000 A/m may be 1.35 T or more.

In addition, the coercive force at the magnetic flux density of 1.0T may be 200 A/m or less.

In addition, the iron loss at a frequency of 50 Hz and a magnetic flux density of 1.7 T may be 15 W/Kg or less.

Method of manufacturing a graphite steel excellent in machinability and softness according to another embodiment of the present invention is by weight, C: 0.4 to 1.0%, Si: 2.0 to 3.0%, Mn: 0.01 to 0.4%, P: 0.03% or less , S: 0.02% or less, Al: 0.01~0.05%, Ti: 0.01~0.03%, Ca: 0.0005~0.003%, N: 0.003~0.008%, O: 0.005% or less, containing residual Fe and other unavoidable impurities A step of homogenizing and heat-treating a steel material, a step of hot rolling the heat-treated steel material, and a step of graphitizing the rolled steel material, wherein the heat-treating the graphitization heats the cementite of the pearlite structure in the rolled steel material The graphite grains are distributed to the ferrite matrix.

In addition, the step of heat-treating the graphitization may be heat-treated for 2 to 6 hours in a temperature range of 730 to 770°C.

The present invention can provide a graphite steel excellent in machinability and softness suitable for electromagnetic components such as automobiles and home appliances, and a method for manufacturing the same.

Hereinafter, preferred embodiments of the present invention will be described. However, embodiments of the present invention may be modified in various other forms, and the technical idea of the present invention is not limited to the embodiments described below. In addition, embodiments of the present invention are provided to more fully describe the present invention to those skilled in the art.

The terms used in this application are only used to describe specific examples. Thus, for example, a singular expression includes a plural expression, unless the context clearly indicates it. In addition, terms such as “comprise” or “include” used in the present application are used to clearly indicate the existence of features, steps, functions, components, or combinations thereof described in the specification, and other features. It should be noted that it is not used to preliminarily exclude the presence of a field or step, function, component, or combination thereof.

On the other hand, unless otherwise defined, all terms used in this specification should be regarded as having the same meaning as generally understood by a person having ordinary skill in the art to which the present invention pertains. Accordingly, unless explicitly defined herein, certain terms should not be construed in excessively ideal or formal sense. For example, in this specification, a singular expression includes a plural expression unless the context clearly has an exception.

In addition, "about", "substantially" and the like in the present specification are used in the sense of or close to the value when manufacturing and substance tolerances unique to the stated meaning are presented, and are used to help understand the present invention. Or, an absolute value is used to prevent unscrupulous use of the disclosed content by unscrupulous intruders.

In accordance with demands for miniaturization, precision, and productivity improvement of materials that can be applied to electromagnetic parts in various fields such as automobiles, robots, electronics, electricity, computers, and communications, demands for improving machinability are also increasing in soft magnetic materials.

Graphite steel, which is a steel with a high content of carbon and silicon, is characterized by excellent machinability due to graphite grains formed in the steel, and excellent soft magnetic properties due to the large amount of silicon employed in the ferrite matrix. However, when carbon is added to the steel, despite the stability of graphite, it is precipitated with cementite, which is a metastable phase, and requires a long heat treatment process, and decarburization occurs due to the long heat treatment process, which adversely affects the performance of the final product. There is this. In addition, when graphite in the base of the steel precipitates coarsely or is irregularly distributed in an irregular shape, there is a problem of poor machinability.

Therefore, in order to solve the above-mentioned problems, the present invention significantly shortens the time of the graphitization heat treatment, and allows the fine graphite grains to be uniformly distributed in the final microstructure after the heat treatment. , In the final product, it is intended to provide graphite steel having excellent soft magnetic properties.

Graphite steel excellent in machinability and softness according to an aspect of the present invention is in weight%, C: 0.4 to 1.0%, Si: 2.0 to 3.0%, Mn: 0.01 to 0.4%, P: 0.03% or less, S: 0.02 % Or less, Al: 0.01 to 0.05%, Ti: 0.01 to 0.03%, Ca: 0.0005 to 0.003%, N: 0.003 to 0.008%, O: 0.005% or less, residual Fe and other unavoidable impurities.

Hereinafter, the reason for limiting the composition of the steel will be described in detail. The following composition of the composition means all weight percents unless otherwise specified.

Carbon (C): 0.4~1.0%

Carbon is an essential element for forming a graphite grain as a constituent element of the graphite grain. One of the methods to graphitize cementite produced during cooling of steel materials as quickly as possible is to increase carbon activity. When the carbon content is less than 0.4%, the carbon activity is low, the effect of improving the machinability is insufficient, and when it exceeds 1.0%, the graphite grains are coarsely generated, the aspect ratio is increased, and the machinability, in particular, the surface roughness is reduced, and the hot rolling property is reduced. Therefore, it is preferable to control the component range of carbon in the range of 0.4 to 1.0%.

Silicon (Si): 2.0~3.0%

Silicon is a necessary component as a deoxidizing agent in the production of molten steel, and is a graphitization promoting element that destabilizes cementite in the steel so that carbon can be precipitated as graphite. In addition, since it improves soft magnetic properties such as magnetic flux density, coercive force, and iron loss, it is preferably added, and it is preferable to add 2.0% or more. When added in excess of 3.0%, the effect of accelerating graphitization becomes saturated, and due to the solid solution strengthening effect, the hardness increases to accelerate tool wear during cutting, causing brittleness with an increase in non-metallic inclusions, and inducing decarburization during hot rolling. can do. Therefore, it is preferable to control the component range of silicon in the range of 2.0 to 3.0%.

Manganese (Mn): 0.01~0.4%

Manganese is an alloy element that increases the strength of steel and improves the impact properties, and it is preferable to add it in an amount of 0.01% or more since it increases rolling properties and reduces brittleness. However, if it is added in excess of 0.4%, the graphitization is inhibited to delay the completion time of graphitization, and the strength and hardness can be increased to deepen the depth of tool wear. In addition, since the MnS inclusion is formed by combining with sulfur in the steel, the coercive force and iron loss increase, resulting in poor soft magnetic properties. Therefore, it is preferable to control the component range of manganese in the range of 0.01 to 0.4%.

Phosphorus (P): 0.03% or less

Phosphorus is an element that promotes graphitization, but increases the hardness of ferrite by separating solid solution, segregates to grain boundaries, lowers toughness, reduces delay fracture resistance, and promotes surface defects. It is desirable to manage. Therefore, it is preferable to control the component range of phosphorus to 0.03% or less.

Sulfur (S): 0.02% or less

Sulfur is not only an element that greatly inhibits graphitization, but also segregates at grain boundaries to degrade toughness. In addition, it combines with manganese in steel to form low-melt emulsions, such as MnS, to improve machinability, but it is desirable to manage its content as low as possible because it interferes with magnetic flow and degrades electromagnetic properties. Therefore, it is preferable to control the component range of sulfur to 0.02% or less.

Aluminum (Al): 0.01~0.05%

Aluminum not only contributes to deoxidation as a strong deoxidizing element, but also accelerates the decomposition of cementite during the graphitization heat treatment, and at the same time forms AlN by combining with nitrogen in steel, thereby preventing stabilization of cementite and promoting graphitization.

In the present invention, for the above-described effect, it is preferable that aluminum is contained at least 0.01%. However, when the content exceeds 0.05%, the effect is not only saturated, there is a problem that the wear of the tool is accelerated when cutting by oxides of high hardness formed by bonding with oxygen in steel. In addition, when Al is excessive, AlN is generated at the austenite grain boundary, and there is a problem in that graphite using this as a nucleus is unevenly distributed at the grain boundary. Therefore, it is preferable to control the component range of aluminum to 0.01 to 0.05%.

Titanium (Ti): 0.01~0.03%

Titanium combines with nitrogen such as boron and aluminum to form TiN, thereby lowering the dissolved nitrogen that inhibits graphitization, and acting as a nucleation site for graphite to shorten the graphitization heat treatment time. In addition, BN, AlN, etc. have a low formation temperature, and austenite is formed and thus precipitates unevenly in the grain boundary, whereas TiN has a higher formation temperature than BN, AlN, and is precipitated before the austenite generation is completed. Evenly distributed in the mouth. Therefore, the graphite grain generated by using TiN as a nucleation site is finely and uniformly distributed.

When titanium is added in less than 0.01%, the number of TiN is too small, so the effect of graphite as a nucleation site is weak, so that the graphite grain is not formed finely and uniformly, and when it exceeds 0.03%, the effect is saturated and coarse carbonaceous There is a problem of inhibiting graphitization by consuming carbon necessary for forming graphite as a cargo. Therefore, it is preferable to control the component range of titanium to 0.01 to 0.03%.

In the present invention, when the content of titanium is controlled in the above-described component range, TiN serving as a nucleation site for graphite has a size of 100 nm or less, and the density can be formed finely and uniformly at 10000/mm ² or more. Graphite grains generated as a nucleation site are also finely and uniformly distributed.

Calcium (Ca): 0.0005~0.003%

Calcium combines with oxygen and sulfur in steel to produce oxides or sulfides, and calcium oxides and sulfides act as nucleation sites for graphite. In addition, since dissolved oxygen or sulfur inhibits graphitization, calcium can effectively remove these dissolved elements. Calcium can be mixed with Si-Al-O-based inclusions to form Ca-Si-Al-O-based low-melting inclusions, and these low-melting inclusions melt as the temperature rises during high-speed cutting to form a tool surface protective film. Tool life is extended by suppressing wear progress.

For the above-described effect, it is preferable to add more than 0.0005% calcium. However, when the content of calcium is added in excess of 0.003%, the effect is not only saturated, but the nozzle may be blocked by an increase in the amount of CaS, a sulfide, when playing. Therefore, it is preferable to control the component range of calcium to 0.0005 to 0.003%.

Nitrogen (N): 0.003 to 0.008%

Nitrogen combines with titanium and aluminum to form a nitride, which serves as a nucleus to promote the formation and growth of graphite, but nitrogen that remains in solid state without forming nitride stabilizes cementite to significantly graphitize. Inhibit. Therefore, in order to form nitrides effective for promoting graphitization, it is preferable to add in an equivalent amount of titanium and aluminum.

For this reason, it is necessary to add more than 0.003%, but when added in excess of 0.008%, the effect is not only saturated, but can remain in solid solution in steel to inhibit graphitization. Therefore, it is preferable to control the component range of nitrogen to 0.003 to 0.008%.

Oxygen (O): 0.005% or less

Oxygen combines with aluminum to form oxides. The production of these oxides reduces the effective concentration of aluminum. As a result, since the amount of AlN useful for crystallization of graphite is reduced, graphitization is inhibited. In addition, since the alumina oxide formed by containing a large amount of oxygen damages the cutting tool during cutting, it causes a decrease in machinability, so it is preferable to keep it as low as possible. Theoretically, it is advantageous to control the content of oxygen to 0%, but inevitably, it must be contained in the manufacturing process. Therefore, it is important to manage the upper limit, and it is preferable to control the component range of oxygen to 0.005% or less because the problem caused by oxygen is not large until 0.005%.

The remaining component of the invention is iron (Fe). However, in the normal manufacturing process, unintended impurities from the raw material or the surrounding environment may be inevitably mixed, and therefore cannot be excluded. Since the impurities are known to anyone skilled in the ordinary manufacturing process, they are not specifically mentioned in this specification.

Graphite steel according to an embodiment of the present invention having the above component range has a graphitization rate of 99% or more, and the microstructure includes graphite grains based on ferrite.

Here, the graphitization rate means the ratio of the carbon content present in the graphite state to the carbon content added to the steel, and is expressed as in the following formula (1). A graphitization rate of 99% or more means that all the added carbon was consumed to produce graphite. In formula (1), the amount of solid carbon and fine carbide in ferrite are extremely small, and therefore are not considered.

(1) Graphitization rate (%) = (1-residue pearlite carbon content/carbon content in steel) Х100

The graphitization rate of 99% or more means that the unresolved pearlite is not present in the steel, and the microstructure includes graphite grains based on ferrite. On the other hand, unlike the present invention, when the residual pearlite is present in the steel, the hardness increases, thereby increasing the degree of tool wear when cutting, thereby reducing the machinability, and increasing the coercive force and iron loss, thereby reducing softness.

The graphite steel may include 2 to 5% of graphite grain in an area fraction. In order to secure excellent machinability and soft magnetic properties at the same time, a certain amount of graphite must be secured, and at least 2% of the graphite grains should be included in the area fraction to reduce the cutting friction of the graphite steel and the graphite grains act as a starting point for cracks, thereby significantly improving the machinability. Since it is possible to ensure sufficient machinability and softness. On the other hand, when the graphite grain is contained in an area fraction exceeding 5%, the machinability effect is not only saturated, but the soft magnetic property is deteriorated. In addition, in terms of manufacturing, there is a difficulty in production such as damage due to reduction in hot rolling properties.

The average grain size of the graphite grains may be 10 µm or less, and no particular lower limit is specified. This is because the finer the size of the resulting graphite grains, the shorter the graphitization time and the better the surface roughness after cutting in terms of machinability. Here, the average grain size of the graphite grain means an average diameter of particles detected by observing one cross section of the graphite steel.

The density of graphite particles may be 1000 pieces/mm ^{2 or} more. As long as the graphite grains are formed to be fine and uniform, the cutting friction is lowered and the cutability is improved by acting as a starting point for cracking. Therefore, an upper limit is not particularly limited in density.

The graphite steel may have a magnetic flux density of 1.35T or more at a coercive force of 1000 A/m.

The graphite steel may have a coercive force at a magnetic flux density of 1.0T of 200 A/m or less.

The graphite steel may have an iron loss of 15 W/Kg or less at a frequency of 50 Hz and a magnetic flux density of 1.7T.

The graphite steel of the present invention according to the above has all of the above-described magnetic flux density, coercive force, and iron loss properties, and thus has excellent soft magnetic properties and, furthermore, excellent machinability.

Hereinafter, a method for manufacturing the graphite steel of the present invention will be described in detail.

According to an embodiment of the present invention, a method of manufacturing a graphite steel excellent in machinability and softness is first cast an ingot having the above-described component range, and then homogenized heat treatment at 1100 to 1300°C for 5 to 10 hours, and 800 to 1100°C. After hot rolling, air-cooled to produce steel. Then, the prepared steel is heat-treated (air-cooled after constant-temperature heat treatment) at a temperature of 730 to 770°C for 2 to 6 hours to produce graphite steel.

The graphitizing heat treatment process of the present invention is a process of graphitizing cementite in a steel material, and heat-treating so that graphite grains are distributed on the ferrite matrix. The graphitization heat treatment is performed in a temperature region corresponding to a graphite generation curve nose in an isothermal transformation curve.

Graphitization heat treatment temperature range may be 730 ~ 770 ℃. When the heat treatment temperature is less than 730°C, even with a heat treatment of 6 hours, the graphitization time is not sufficient, and the remaining pearlite may be present in the steel because it is not fully graphitized. On the other hand, when the heat treatment temperature exceeds 770°C, some austenite is generated during heat treatment, and pearlite may be regenerated during cooling. Therefore, the heat treatment temperature range of the present invention is preferably controlled to 730 ~ 770 ℃.

The graphitization heat treatment temperature time may be isothermal heat treatment for 2 to 6 hours in the above heat treatment temperature range. If the graphitization heat treatment time is too short, less than 2 hours, the graphitization is not completely completed, and when the graphitization heat treatment time exceeds 6 hours, there is no longer a change in structure and physical properties. Therefore, it is preferable to control the graphitizing heat treatment of the present invention to heat treatment for 2 to 6 hours in a temperature range of 730 to 770°C.

As the graphitizing heat treatment of the present invention described above, almost all cementite in the pearlite structure in the steel material can be graphitized, so that the graphite grains are distributed in the ferrite matrix.

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are only intended to illustrate the present invention in more detail and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the items described in the claims and the items reasonably inferred therefrom.

{Example}

The steel having the component system as shown in Table 1 below was cast as an ingot and then homogenized and heat-treated at 1250° C. for 8 hours. Then, after hot rolling to a thickness of 25 mm and air-cooled, a steel material for graphitization heat treatment was prepared.

Then, a graphite steel was prepared by heat treatment with graphitization of the steel. The graphitizing heat treatment was incubated for 6 hours to obtain a microstructure containing graphite grains in the ferrite matrix.

Graphitized steel, graphite grain area fraction, graphite grain average size, machinability and magnetic flux density, coercive force, and iron loss characteristics of the graphite steel prepared as described above were investigated and the results are shown in Table 2 below.

Graphitization rate, area fraction of graphite grain, and average size of graphite grain were measured using an image analyzer. The measurement method is as follows. Each specimen was cut to a certain size, and the image was photographed under a magnification of 200 times using an optical microscope in a state of polishing without etching. In the image thus obtained, it can be clearly distinguished by a distinct contrast difference between the matrix and the graphite phase, so analysis was performed using image analysis software. In addition, 15 images per specimen were used to increase the reliability of the analysis.

Machinability was evaluated by the depth of flank wear in the cutting tool and was measured under the following cutting conditions. After the graphitization heat treatment, the steel material processed with a round rod having a diameter of 25 mm was 300-turned to a length of 20 mm until the diameter became 15 mm, and then the depth of the tool was measured. At this time, the tool used was a cermet grade, and the cutting process conditions were performed under the use of cutting oil under conditions of a cutting speed of 150 mm/min, a cutting depth of 1.0 mm, and a feed rate of 0.1 mm/rev.

For electromagnetic properties such as magnetic flux density, coercive force, and iron loss characteristics, a plate-shaped specimen having a width of 60 mm, a width of 60 mm, and a thickness of 1 mm was taken from graphite steel, and then 750° C. for 2 hours in a hydrogen atmosphere furnace to remove processing stress. After heat treatment, magnetic flux density, coercive force, and iron loss were measured. The magnetic flux density is the value (B ₁₀ ) measured at a coercive force of 1000 A/m, the coercive force is the value at a magnetic flux density of 1.0T, and the iron loss is a frequency of 50 Hz. The value (W _17/50) measured at a magnetic flux density of 1.7T were compared with reference.

division C Si Mn P S Al Ti Ca N O Graphitizing heat treatment temperature/hour
(℃/hour) Inventive Example One 0.45 2.80 0.12 0.0092 0.0052 0.0390 0.0141 0.0010 0.0038 0.0032 760/2 2 0.55 2.43 0.15 0.0078 0.0063 0.0320 0.0134 0.0012 0.0067 0.0023 760/4 3 0.70 2.29 0.26 0.0080 0.0024 0.0270 0.0140 0.0008 0.0049 0.0020 760/2 4 0.82 2.33 0.32 0.0092 0.0052 0.0290 0.0123 0.0030 0.0045 0.0032 740/5 5 0.92 2.71 0.36 0.0078 0.0042 0.0320 0.0133 0.0020 0.0041 0.0043 730/6 6 0.98 2.94 0.23 0.0072 0.0030 0.0190 0.0150 0.0027 0.0062 0.0024 760/3 Comparative example One 0.25 2.52 0.27 0.0068 0.0041 0.0031 0.0120 0.0020 0.0052 0.0022 760/6 2 0.95 3.41 0.22 0.0076 0.0018 0.0027 0.0132 0.0006 0.0062 0.0031 760/6 3 0.67 1.80 0.12 0.0083 0.0034 0.0030 0.0143 0.0023 0.0045 0.0030 760/6 4 0.67 2.12 0.38 0.0082 0.0410 0.0032 0.0070 0.0021 0.0043 0.0031 760/6 5 0.78 2.29 0.26 0.0092 0.0063 0.0320 0.0404 0.0010 0.0046 0.0032 760/6 6 1.47 2.92 0.22 0.0045 0.0045 0.0032 0.0125 0.0020 0.0043 0.0031 Damage during rolling 7 0.53 2.10 0.48 0.0092 0.0024 0.0270 0.0134 0.0010 0.0041 0.0032 760/6 8 0.51 2.33 0.15 0.0078 0.0052 0.0030 0.0132 0.0002 0.0041 0.0043 760/6 9 0.68 2.43 0.22 0.7000 0.0021 0.0310 0.0140 0.0015 0.0041 0.0023 720/6 10 0.75 2.29 0.31 0.0080 0.0024 0.0270 0.0140 0.0023 0.0049 0.0020 780/6

division Constitutionally Graphitization rate (%) black smoke
area
Fraction
(%) black smoke
density
(Pcs/mm ² ) Graphite grain
Average size (㎛) tool
Wear
depth
(㎛) Magnetic flux
density
(B ₁₀ )(T) Coercive force
(A/m) _Iron loss (W _17/50 )
(W/Kg) Inventive Example One F+G 100 2.4 1078 4.12 170 1.46 176 9.2 2 F+G 100 2.5 1546 3.50 172 1.45 187 10.1 3 F+G 100 3.2 1992 3.48 120 1.41 181 11.3 4 F+G 100 2.8 2406 3.00 145 1.40 184 10.4 5 F+G 100 4.2 2193 3.80 110 1.37 189 9.9 6 F+G 100 4.4 1031 5.72 115 1.38 187 10.0 Comparative example One F+G 100 1.1 1002 2.93 182 1.46 179 11.2 2 F+G 100 4.3 1845 4.21 188 1.49 171 8.8 3 F+P+G 80 3.0 1972 3.42 215 1.21 243 28.7 4 F+P+G 95 2.1 135 10.89 210 1.31 234 19.3 5 F+G 100 3.2 1205 4.5 240 1.4 179 13.2 6 Damage during hot rolling 7 F+P+G 88 2.2 1370 3.50 192 1.21 223 12.3 8 F+G 100 2.3 1216 3.80 240 1.34 184 10.4 9 F+P+G 88 1.8 1153 3.45 234 1.23 221 20.2 10 F+P+G 92 1.9 1241 3.22 212 1.21 232 23.2

(F: ferrite, P: pearlite, G: graphite)

Hereinafter, with reference to Tables 1 and 2, the graphitization rate, the area fraction of graphite grains, the average size of graphite grains, the machinability and magnetic flux density, the coercive force, and the iron loss characteristics of each invention example and comparative example are comparatively evaluated.

Looking at Tables 1 and 2, as in Comparative Examples 1,2, 3, and 6, when the amount of added carbon and silicon elements for graphite formation was outside the scope of the present invention, machinability or electromagnetic properties were deteriorated. .

In the amount of carbon addition, as in Comparative Example 1, when the amount of added carbon was 0.25% and less than 0.4%, graphitization was performed 100% to have a microstructure of a ferrite substrate containing graphite grains without pearlite. However, since the graphite area fraction is very small, less than 2% at 1.1%, it does not significantly contribute to the improvement in machinability and the depth of tool wear is deepened to 182 µm.

On the other hand, as in Comparative Example 6, when the amount of added carbon exceeds 1.47% and 1.0%, the hot rolling property deteriorates and is broken during rolling.

From these results, it can be seen that comparative evaluation of Inventive Examples 1 to 6 and Comparative Examples 1 and 6 controls the amount of added carbon to 0.4 to 1.0% as in the present invention.

In the amount of silicon added, as in Comparative Example 3, when the amount of added silicon was less than 2.0% at 1.80%, the graphitization rate was delayed to complete the graphitization within 6 hours (graphitization rate 80%), and as a result, the remainder pearlite was It can be confirmed that it exists (see Table 2). Due to this, the wear of the cutting tool was accelerated, and the depth of the tool wear was increased to 215 μm. In addition, referring to Table 2, as in Comparative Example 3, when the residual pearlite is present, the magnetic flux density value decreases to 1.21T, while the values of coercive force and iron loss rise to 243A/m and 28.7W/kg, respectively, resulting in soft magnetic properties. It can be confirmed that the deterioration occurred.

On the other hand, as in Comparative Example 2, when the amount of added silicon exceeded 3.0% with 3.41%, the graphitization was completed, but the tool wear was accelerated due to the solid solution strengthening effect due to excessive silicon, resulting in a deeper tool wear depth of 188 μm. .

From these results, it can be seen that when the comparative examples of Inventive Examples 1 to 6 and Comparative Examples 2 and 3 are controlled, the amount of added silicone is controlled to 2.0 to 3.0% as in the present invention.

In the amount of manganese added, as in Comparative Example 7, when the amount of manganese added exceeds 0.4% to 0.48%, the graphitization rate is delayed and graphitization cannot be completed within 6 hours (graphitization rate 88%), As a result, the residual pearlite was present, and the tool wear depth increased to 192µm, the magnetic flux density value decreased to 1.21T, and the coercive force and iron loss values increased to 223A/m and 12.3W/kg, respectively, indicating that the softness deteriorated. Able to know.

From these results, it can be seen that the comparative evaluation of Inventive Examples 1 to 6 and Comparative Example 7 controls the amount of manganese added to 0.01 to 0.4% as in the present invention.

In the addition amount of titanium and sulfur, as in Comparative Example 4, when the titanium content is less than 0.01% at 0.0070% and the sulfur content exceeds 0.02% at 0.0410%, it cannot be fully graphitized even after a 6-hour graphitization heat treatment (graphitization rate of 95%). , It can be confirmed that the residual pearlite is present (see Table 2). Due to this, the wear of the cutting tool was accelerated, and the depth of tool wear was deepened to 210 µm. The magnetic flux density value decreased to 1.31T, while the values of coercive force and iron loss increased to 234A/m and 19.3W/kg, respectively. This deterioration can be confirmed.

In addition, as in Comparative Example 5, when the Ti content exceeds 0.03% to 0.0404%, the wear of the cutting tool is accelerated due to the increase of the Ti-based oxide having a high hardness, and the depth of tool wear is deepened to 240 μm.

From these results, when comparative evaluation of Inventive Examples 1 to 6 and Comparative Examples 4 and 5, the amount of added titanium was controlled to 0.01 to 0.03% as in the present invention. It can be seen that it is preferable to control the amount of sulfur to 0.02% or less.

In the amount of calcium added, as in Comparative Example 8, when the amount of added calcium is less than 0.0005% as 0.0002%, a low melting point inclusion that melts according to the temperature rise during high-speed cutting to form a tool surface protective film is not formed, compared to each invention example. The depth of tool wear was increased to 240 µm.

From these results, it can be seen that comparative evaluation of Inventive Examples 1 to 6 and Comparative Example 8 controls the amount of added calcium to 0.0005 to 0.003% as in the present invention.

In the graphitization heat treatment condition, when the graphitization heat treatment temperature is 720°C or less than 730°C as in Comparative Example 9, the graphitization time is not sufficient even with a 6 hour heat treatment, so that it is not fully graphitized (graphitization rate 88%). , It can be confirmed that the residual pearlite is present (see Table 2). The graphite area fraction was 1.8%, which was measured as a small value of 2% or less. In addition, referring to Table 2, the machinability and softness (magnetic flux density, coercive force, iron loss) of Comparative Example 9 are 234 µm (tool wear depth), 1.23T, 221A/m, and 20.2W/kg, respectively. Compared to graphite steel of 6, machinability and softness were deteriorated.

On the other hand, when the graphitization heat treatment temperature exceeds 770 °C to 780 °C, as in Comparative Example 10, it can be confirmed that some austenite is generated during the heat treatment and pearlite is regenerated (92% graphitization) during cooling (Table 2). Reference). The graphite area fraction was 1.9%, which was measured as a small value of 2% or less. In addition, referring to Table 2, the cutting and soft magnetic properties (magnetic flux density, coercive force, iron loss) of Comparative Example 10 are 212 µm (tool wear depth), 1.21T, 232A/m, and 23.2W/kg, respectively. Compared to graphite steel of 6, machinability and softness were deteriorated.

From these results, it can be seen that, when comparative evaluation of Inventive Examples 1 to 6 and Comparative Examples 9 and 10, it is preferable to control the graphitization heat treatment temperature to 730 to 770°C as in the present invention.

As described above, although exemplary embodiments of the present invention have been described, the present invention is not limited thereto, and a person having ordinary skill in the art does not depart from the concept and scope of the following claims. It will be understood that various modifications and variations are possible.

Claims (10)

In weight percent, C: 0.4 to 1.0%, Si: 2.0 to 3.0%, Mn: 0.01 to 0.4%, P: 0.03% or less, S: 0.02% or less, Al: 0.01 to 0.05%, Ti: 0.01 to 0.03% , Ca: 0.0005 to 0.003%, N: 0.003 to 0.008%, O: 0.005% or less, residual Fe and other inevitable impurities,
The microstructure is a ferrite matrix with an area fraction of 2 to 5%, and graphite grains with an average grain size of 10 µm or less are distributed over 1000 particles/mm ² ,
Graphite steel with a graphitization rate of 99% or more and a magnetic flux density of 1.35T or more at a coercive force of 1000 A/m. delete delete delete delete delete According to claim 1,
Graphite steel with excellent machinability and softness of 200A/m or less at a magnetic flux density of 1.0T. According to claim 1,
Graphite steel with excellent machinability and softness with iron loss of 15 W/Kg or less at a frequency of 50 Hz and a magnetic flux density of 1.7T. In weight percent, C: 0.4 to 1.0%, Si: 2.0 to 3.0%, Mn: 0.01 to 0.4%, P: 0.03% or less, S: 0.02% or less, Al: 0.01 to 0.05%, Ti: 0.01 to 0.03% , Ca: 0.0005 to 0.003%, N: 0.003 to 0.008%, O: 0.005% or less, homogenizing heat treatment of the steel containing residual Fe and other unavoidable impurities;
Hot rolling the heat-treated steel; And
Graphite heat-treating the rolled steel material,
The step of heat-treating the graphitization heats for 2 to 6 hours in a temperature range of 730 to 770°C to graphitize the cementite of the pearlite structure in the rolled steel material, so that the graphite grains are distributed on the ferrite base,
The microstructure of the graphitized heat-treated graphite steel has an area fraction of 2 to 5% on the ferrite base, and graphite grains having an average grain size of 10 µm or less are distributed in 1000 pieces/mm ² or more, and the graphitization rate is 99% or more, A method of manufacturing graphite steel having a high magnetic flux density at a coercive force of 1000 A/m and a cut and softness of 1.35 T or more.


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