KR101808434B1 - Steel for drive shaft and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a steel material for a drive shaft. According to an embodiment of the present invention, the steel material for a drive shaft is formed of alloy comprises: 0.38-0.43 wt% of carbon (C); 0.15-0.35 wt% of silicon (Si); 0.6-0.9 wt% of manganese (Mn); 0.001-0.03 wt% of phosphorus (P); 0.0001-0.015 wt% of sulfur (S); 0.9-1.2 wt% of chrome (Cr); 0.01-0.1 wt% of molybdenum (Mo); 0.015-0.05 wt% of aluminum (Al); 0.01-0.03 wt% of titanium (Ti); 0.001-0.005 wt% of boron (B); 0.001-0.05 wt% of hafnium (Hf); and residues containing Fe and inevitable impurities. A difference value of a lattice constant of hafnium (Hf) on manganese (Mn) expressed by following equation 1 is 15-20%. [equation 1] the difference value of the lattice constant (L1)={(L_Hf_L_Mn_)-1}100. In equation 1, L_Hf_ is a lattice constant value of the hafnium (Hf), and L_Mn_ is the lattice constant value of the manganese (Mn).

Description

Technical Field [0001] The present invention relates to a steel material for a drive shaft,

The present invention relates to a steel material for a drive shaft and a manufacturing method thereof.

The drive shaft is a component that transmits the driving force generated by the engine to the transmission and the wheels. Drive shafts are manufactured through free machining due to their morphological characteristics.

The conventional steel for a drive shaft uses a method of inducing MnS formation in the molten steel to improve the free cutting property in order to prevent deterioration of workability due to high hardness. As described above, MnS generated in the steel acts as a position of micro cracks in the base to lower the processing force and shorten the chip shape, thereby improving the workability.

A background art relating to the present invention is Korean Patent Laid-Open Publication No. 2016-0055193.

It is an object of the present invention to provide a steel sheet which has excellent tensile strength and hardness and excellent free cutting ability and is excellent in the effect of spheroidizing the shape of the MnS inclusions and reducing the size thereof to prevent occurrence of segregation by sulfur (S) A steel material for a drive shaft excellent in the effect of improving fatigue strength and a method for manufacturing the same.

An embodiment of the present invention is a method of manufacturing a semiconductor device, comprising: 0.38 wt% to 0.43 wt% carbon (C), 0.15 wt% to 0.35 wt% silicon (Si), 0.6 wt% to 0.9 wt% manganese (Mn) (Al) 0.015 wt% to 0.05 wt% of aluminum (Al), 0.03 wt% to 0.03 wt% of sulfur, 0.0001 wt% to 0.015 wt% of sulfur, 0.9 wt% (Fe) and unavoidable impurities, in an amount of from 0.01% by weight to 0.03% by weight of titanium (Ti), from 0.001% by weight to 0.005% by weight of boron (B), from 0.001% (Hf) relative to 隆 manganese (Mn) represented by the following formula (1) is 15% to 20%.

[Formula 1]

Lattice constant difference value DELTA L1 = {(L _Hf / L _Mn ) -1} 100

In the above formula (1), L _Hf is the lattice constant value of? Hafnium (Hf) and L _Mn is the lattice constant value of? Manganese (Mn).

Another embodiment of the present invention is a process for the preparation of a catalyst composition comprising from 0.38 wt.% To 0.43 wt.% Carbon (C), from 0.15 wt.% To 0.35 wt.% Silicon (Si), from 0.6 wt.% To 0.9 wt. (Al) 0.015 wt% to 0.05 wt% of aluminum (Al), 0.03 wt% to 0.03 wt% of sulfur, 0.0001 wt% to 0.015 wt% of sulfur, 0.9 wt% (Fe) and unavoidable impurities, in an amount of from 0.01% by weight to 0.03% by weight of titanium (Ti), from 0.001% by weight to 0.005% by weight of boron (B), from 0.001% (Hf) relative to 隆 manganese (Mn) represented by the above-mentioned formula (1) to a value obtained by dividing the difference in lattice constant between? Hafnium To 15% to 20%. ≪ IMAGE >

The alloy composition may further include 0.001 wt% to 0.01 wt% of at least one of zirconium (Zr) and calcium (Ca).

The steel for a drive shaft has a tensile strength (TS) of 76 kgf / mm < ^{2 >} To 80 kgf / mm ² , a fatigue limit of 25 kgf / mm ² to 30 kgf / mm ^2, and a fatigue ratio of 33% to 40%.

The steel material for the drive shaft includes a microstructure composed of ferrite and pearlite, the mean grain size of the austenite grains is 10 to 20 mu m, and the surface hardness may be 190 to 230 HB.

The steel material for the drive shaft may include an Fe-Hf precipitate phase, and the size of the MnS inclusions may be 10 탆 to 50 탆.

The present invention relates to a method for manufacturing a magnetic steel sheet, which has excellent tensile strength and hardness, excellent free cutting ability, and is excellent in the effect of spheroidizing the shape of the MnS inclusions and reducing the size, thereby preventing occurrence of segregation by sulfur (S) A steel material for a drive shaft having excellent effects and a method for manufacturing the same.

1 shows a method of manufacturing a steel material for a drive shaft according to the present invention.
2 is a cross-sectional view of the steel for a drive shaft manufactured in Example 1 of the present invention.
3 shows the results of the measurement of the machining force on the steel for a drive shaft manufactured in Example 1 and Comparative Example 1 of the present invention.
Fig. 4 is a photograph of a chip shape generated after machining a steel material for a drive shaft manufactured in Example 1 of the present invention. Fig.
Fig. 5 is a photograph of a shape of a chip generated after machining a steel material for a drive shaft manufactured in Comparative Example 1 of the present invention. Fig.
6 is a photograph of an austenitic crystal grain of a steel material for a drive shaft manufactured in Example 1 by an electron microscope.
7 is a photograph of an austenitic crystal grain of a steel material for a drive shaft manufactured in Comparative Example 1 of the present invention by an electron microscope.
8 is an electron microscope photograph of the microstructure of the steel for a drive shaft manufactured in Example 1 of the present invention.
9 is an electron microscope photograph of the microstructure of the steel for a drive shaft manufactured in Comparative Example 1 of the present invention.
10 is an electron microscope photograph of the MnS inclusion shape of the steel for a drive shaft manufactured in Example 1 of the present invention.
11 is an electron microscope photograph of the MnS inclusion shape of the steel for a drive shaft manufactured in Comparative Example 1 of the present invention.
12 is an enlarged view of an MnS inclusion shape of a steel material for a drive shaft manufactured in Example 1 of the present invention after it is photographed by an electron microscope.
Fig. 13 is an enlarged view of the MnS inclusion shape of the steel for a drive shaft manufactured in Comparative Example 1, taken by an electron microscope.

An embodiment of the present invention is a method of manufacturing a semiconductor device, comprising: 0.38 wt% to 0.43 wt% carbon (C), 0.15 wt% to 0.35 wt% silicon (Si), 0.6 wt% to 0.9 wt% manganese (Mn) (Al) 0.015 wt% to 0.05 wt% of aluminum (Al), 0.03 wt% to 0.03 wt% of sulfur, 0.0001 wt% to 0.015 wt% of sulfur, 0.9 wt% (Fe) and unavoidable impurities, in an amount of from 0.01% by weight to 0.03% by weight of titanium (Ti), from 0.001% by weight to 0.005% by weight of boron (B), from 0.001% (Hf) relative to 隆 manganese (Mn) represented by the following formula (1) is 15% to 20%.

[Formula 1]

Lattice constant difference value DELTA L1 = {(L _Hf / L _Mn ) -1} 100

In the above formula (1), L _Hf is the lattice constant value of? Hafnium (Hf) and L _Mn is the lattice constant value of? Manganese (Mn).

Another embodiment of the present invention is a method for producing a steel material comprising the steps of hot-forging a steel material having the above-described alloy composition in a temperature range of 1150 DEG C to 1250 DEG C, And controlling the lattice constant difference value of the Hf to 15% to 20%.

It is therefore an object of the present invention to provide a steel sheet which is excellent in tensile strength and hardness and excellent in free cutting ability and is excellent in the effect of spheroidizing the shape of the MnS inclusions and reducing the size to prevent occurrence of segregation by sulfur (S) It is possible to provide a steel material for a drive shaft excellent in strength improvement effect and a manufacturing method thereof.

In particular, in the steel material for a drive shaft of the present invention and the method for producing the same, sulfur (S) is not solidified in iron (Fe) in a steel material which improves free cutting properties by using a content of sulfur (S) The effect of reducing the segregation degree is excellent.

The steel material for a drive shaft of the present invention and the method of manufacturing the same according to the present invention have a lattice constant difference value of? Hafnium (Hf) relative to? Manganese (Mn) represented by the formula 1 controlled to 15% to 20% It can be implemented to a superior degree. Further, MnS inclusions can be substituted by hafnium (Hf) included in the alloy composition by controlling the lattice constant difference value. In such a case, the solidification orientation of the steel is changed at the time of solidification of the steel, and the shape of the MnS inclusions can be spheroidized, the size can be reduced, and the dispersibility can be improved. As a result, the tensile strength and hardness of the steel for the drive shaft can be improved, and the free cutting ability can be enhanced.

When the difference in the lattice constant of? Hafnium (Hf) relative to the? Manganese (Mn) is less than 15%, it is difficult to sufficiently realize the effect of reducing the size and increasing the dispersibility of the MnS inclusions. On the contrary, when the difference in lattice constant of? Hafnium (Hf) relative to? Manganese (Mn) is more than 20%, non-uniform microstructure may be formed and the physical properties may be lowered.

The steel for the drive shaft may have a tensile strength (TS) of 76 kgf / mm ² to 80 kgf / mm ² , a fatigue limit of 25 kgf / mm ² to 30 kgf / mm ² and a fatigue ratio of 33% to 40%. Such a steel for a drive shaft has excellent tensile strength and hardness, but also has a high free cutting property and is advantageous to be applied to a drive shaft for an automobile.

The steel material for the drive shaft includes a microstructure composed of ferrite and pearlite, the mean grain size of the austenite grains is 10 to 20 mu m, and the surface hardness may be 190 to 230 HB.

The steel material for the drive shaft may include an Fe-Hf precipitate phase, and the size of the MnS inclusions may be 10 탆 to 50 탆. The tensile strength and hardness of the steel for a drive shaft and the free cutting ability can be further improved.

Hereinafter, the role and content of each component included in the steel for a drive shaft according to the present invention will be described.

Carbon (C)

In the present invention, carbon (C) is added to secure strength and hardness of the steel.

The carbon (C) is added in an amount of 0.38 wt% to 0.43 wt% of the total weight. When the content of carbon is less than 0.38 wt%, it is difficult to secure sufficient strength. On the other hand, when the content of carbon (C) exceeds 0.43% by weight, the strength of the steel increases but the core hardness decreases, a sufficient elongation is hardly secured, and the percentage of pearlite phase in the microstructure becomes high.

silicon( Si )

In the present invention, silicon (Si) is added as a deoxidizer to remove oxygen in the steel in the steelmaking process. Further, silicon (Si) is effective for improving the toughness and ductility of steel by inducing ferrite formation as a ferrite stabilizing element having a solid solution strengthening effect.

The silicon (Si) is added in an amount of 0.15 wt% to 0.35 wt% of the total weight. If the content of silicon (Si) is less than 0.15 wt%, the effect of adding silicon can not be exhibited properly. On the other hand, when the content of silicon (Si) exceeds 0.35% by weight, oxides are formed on the surface of the steel, thereby deteriorating ductility and toughness of the steel.

Manganese (Mn)

Manganese (Mn) is a substitutional element having an atomic diameter similar to iron. It increases the strength and toughness of the steel and increases the incombustibility of the steel, while lowering the ductility when the strength is increased as compared with the addition of carbon (C). Manganese (Mn) is an element highly effective in solid solution strengthening and contributes to improvement of hardenability of steel, and improves hardenability of steel.

The manganese (Mn) is added in an amount of 0.6 wt% to 0.9 wt% of the total weight. If the content of manganese (Mn) is less than 0.6 wt%, it may be difficult to secure the strength even if the content of carbon (C) is high. On the contrary, when the content of manganese (Mn) exceeds 0.9% by weight, the amount of MnS-based nonmetal inclusions increases, which can lead to defects such as cracking during cauterization and the curing ability is excessively improved, There is a high possibility that cold microstructure is expressed.

In (P)

Phosphorus (P) is an element that contributes to strength improvement in some degree, but it causes deterioration of steel ductility when it is included excessively, and causes the final material deviation by slab center segregation.

The phosphorus (P) is limited to an amount of 0.001 wt% to 0.03 wt% of the total weight. If the content of phosphorus (P) is less than 0.001% by weight, the effect of improving the strength due to the addition of phosphorus can not be exhibited properly because the amount of phosphorus (P) added is insignificant. On the contrary, when the content of phosphorus (P) exceeds 0.03% by weight, not only center segregation but also micro segregation is formed, which adversely affects the material and may deteriorate ductility and toughness.

Sulfur (S)

Sulfur (S) contributes partly to the improvement of workability and improves fatigue strength. However, when it is excessively contained, it inhibits the toughness and ductility of steel and forms MnS nonmetallic inclusion by binding with manganese.

The sulfur (S) is limited to an amount of 0.0001 wt.% To 0.015 wt.% Of the total weight. When the content of sulfur (S) is less than 0.0001% by weight, it is difficult to improve the workability due to sulfur, and the content of sulfur must be controlled to a minimum so that the cost of steel production increases. On the other hand, when the content of sulfur (S) exceeds 0.015% by weight, there is a problem that ductility is greatly deteriorated and MnS nonmetallic inclusions are excessively generated.

chrome( Cr )

Chromium (Cr) is an effective element added to secure strength, which can improve the hardenability of steel and improve the strength by bonding with carbon to form carbides. It is possible to form a homogeneous oxide film on the surface of the plate material to reduce the decarburization part of the surface layer of the plate material.

Cr (Cr) is added in an amount of 0.9 wt% to 1.2 wt% of the total weight. If the content of chromium (Cr) is less than 0.9 wt%, it may be difficult to secure strength even if the content of carbon (C) is high. On the contrary, when the content of chromium (Cr) exceeds 1.2% by weight, there is a problem that the ductility and the heat affected zone (HAZ) toughness are lowered.

molybdenum( Mo )

Molybdenum (Mo) contributes to improving the strength and toughness of steel and securing stable strength at room temperature and high temperature.

The molybdenum (Mo) is added in an amount of 0.01 to 0.1% by weight based on the total weight of the steel material according to the present invention. If the content of molybdenum (Mo) is less than 0.01% by weight, it may be difficult to exhibit the effect of adding molybdenum (Mo) properly. On the contrary, if the content of molybdenum (Mo) exceeds 0.1 wt%, the ductility may be lowered.

Aluminum (Al)

Aluminum (Al) acts as a deoxidizer to remove oxygen in the steel.

The aluminum (Al) is added in an amount of 0.015 wt% to 0.05 wt% of the total weight. When the content of aluminum (Al) is less than 0.015 wt%, the effect of deoxidation is insufficient. On the other hand, when the content of aluminum (Al) exceeds 0.05% by weight, Al ₂ O ₃ is formed to deteriorate toughness.

titanium( Ti )

In the present invention, titanium (Ti) forms a carbide upon reheating to inhibit the growth of austenite grains, to refine the steel texture and increase the strength.

The titanium (Ti) is added in an amount of 0.01 wt% to 0.03 wt% of the total weight. When the content of titanium (Ti) is less than 0.01% by weight, the effect of adding titanium can not be exhibited properly. On the contrary, when the content of titanium (Ti) exceeds 0.03% by weight, carbonized precipitates become coarse, the effect of suppressing crystal grain growth is deteriorated, and it is difficult to secure ductility.

Boron (B)

Boron (B) contributes to the improvement of the strength and toughness of the steel and to securing a stable strength at room temperature or high temperature.

The boron (B) is added in an amount of 0.0010 wt% to 0.0050 wt% of the total weight of the steel material according to the present invention. If the content of boron (B) is less than 0.0010% by weight, it may be difficult to exhibit the effect of adding boron (B) properly. On the other hand, when the content of boron (B) exceeds 0.0050 wt%, grain boundary knitting can be caused and the brittleness can be increased.

hafnium( Hf )

Hafnium (Hf) is added to improve fatigue strength, free cutting, machinability, and grain size of steel. Further, by adding hafnium (Hf), the shape of the MnS inclusions can be made spherical, and the size of the MnS inclusions can be controlled to 10 to 50 mu m, and occurrence of segregation by sulfur (S) can be prevented.

The hafnium (Hf) is added in an amount of 0.001 wt% to 0.05 wt% of the total weight of the steel material according to the present invention. When the content of hafnium (Hf) is less than 0.001% by weight, it may be difficult to exhibit the effect of adding hafnium (Hf) properly. On the contrary, when the content of hafnium (Hf) exceeds 0.05% by weight, it is difficult to control the sizes of Fe and Mn inclusions, and the fatigue quality is deteriorated.

Hafnium: manganese (Hf: Mn)

The weight ratio of hafnium (Hf) to manganese (Mn) in the alloy composition may be 1:12 to 1:18, specifically 1:12. In such a case, the shape of the MnS inclusions may be spheroidized through addition of hafnium (Hf), the size of the MnS inclusions may be controlled to 10 탆 to 50 탆, and the efficiency of preventing segregation by sulfur (S) have. Accordingly, the present invention can realize the effect of improving the free cutting ability of steel, improving workability, and reducing grain size, and minimizing the content of MnS inclusions.

The alloy composition may further include 0.001 wt% to 0.01 wt% of at least one of zirconium (Zr) and calcium (Ca).

zirconium( Zr )

Zirconium (Zr) acts as a nucleation site in the formation of MnS, contributes to ensuring processability through spheroidization of sulfide inclusions, and has an effect of increasing strength and fatigue strength.

Zirconium (Zr) may be contained in an amount of 0.001 wt% to 0.01 wt%. In this case, the surface properties and process efficiency of the steel can be increased.

Calcium (Ca)

Calcium (Ca) forms CaS inclusions, thereby inhibiting the formation of MnS inclusions and improving electrical resistance weldability. Calcium (Ca) has a higher affinity with sulfur (S) than manganese (Mn), so CaS inclusions are formed when calcium is added and the formation of MnS inclusions can be further reduced.

Calcium (Ca) may be contained in an amount of 0.001 wt% to 0.01 wt%. In this case, the generation of MnS inclusions can be lowered, the free cutting property can be improved, the workability can be improved, and the grain size can be further miniaturized.

In addition to the components of the alloy composition described above, the remainder is composed of iron (Fe) and impurities inevitably included in the steelmaking process and the like.

Manufacturing method of steel

The method for manufacturing a steel material for a drive shaft according to the present invention comprises hot-forging a steel material having the alloy composition described above in a temperature range of 1150 ° C to 1250 ° C and cooling the molten steel, The lattice constant difference value of? hafnium (Hf) is controlled to 15% to 20%.

[Formula 1]

Lattice constant difference value DELTA L1 = {(L _Hf / L _Mn ) -1} 100

In the above formula (1), L _Hf is the lattice constant value of? Hafnium (Hf) and L _Mn is the lattice constant value of? Manganese (Mn). It is therefore an object of the present invention to provide a steel sheet which is excellent in tensile strength and hardness and excellent in free cutting ability and is excellent in the effect of spheroidizing the shape of the MnS inclusions and reducing the size to prevent occurrence of segregation by sulfur (S) A steel material for a drive shaft excellent in strength improvement effect and a method of manufacturing the same.

Example

Hereinafter, the configuration and operation of the present invention will be described in more detail with reference to preferred embodiments of the present invention. It is to be understood, however, that the same is by way of illustration and example only and is not to be construed in a limiting sense.

The contents not described here are sufficiently technically inferior to those skilled in the art, and a description thereof will be omitted.

1. Preparation of specimens

Table 1 shows the composition of the test piece according to Example 1 and Comparative Example 1. At this time, specimens having compositions according to Example 1 and Comparative Example 1 were heated to 1150 ° C to 1250 ° C, subjected to a hot forging process, and then cooled to an optimal controlled temperature. The temperature at the end of hot forging was 1030 ± 50 ° C, the cooling start temperature was 860 ± 50 ° C, and the cooling end temperature was 490 ± 50 ° C.

C Si Mn P S Cu Ni Cr Mo Al Ti Hf B Example 1 0.43 0.35 0.9 0.03 0.015 0.3 0.25 1.2 0.1 0.05 0.03 0.05 0.005 Comparative Example 1 0.43 0.35 0.9 0.03 0.03 0.3 0.25 1.2 - - - - -

2. Property evaluation

Table 2 shows the results of evaluation of mechanical properties of the specimens prepared according to Example 1 and Comparative Example 1. [

Property evaluation items Example 1 Comparative Example 1 Occurrence of segregation radish U Macro image 1 2 Processability Great Good Processability evaluation test 3 3 Chip shape after machining Particle type Helical and Particle Type Mixing Chip shape after machining 4 5 Average grain size 13.0 탆 20.5 탆 Grain image 6 7 Microstructure Ferrite + Pearlite Ferrite + Pearlite Microstructure image 8 9 MnS inclusion shape conception Shipboard MnS inclusion shape image 10 11 MnS inclusion average size 10 μm or more and 50 μm or less 55 μm to 90 μm MnS inclusion size image 12 13 Tensile strength (kgf / mm ² ) 77 75 Fatigue limit (kgf / mm ² ) 27 24 Tiredness ratio (%) 35 32 Surface hardness 192 HB 188 HB

≪ Evaluation of physical properties &

(1) Presence or absence of segregation: The cross sections of the specimens prepared in Examples and Comparative Examples were cut out and observed visually to see whether or not segregation bands were generated. In addition, the macro image of each specimen is shown in Figs. 1 and 2. Fig.

(2) Evaluation of workability: The machining force consumed under the conditions shown in the following Table 3 was evaluated, and the results were shown in Fig. A low processing force means that the processability is excellent. In addition, the size and shape of chips generated after machining were observed. In the observation result, it is estimated that the machining force consumed is small when the shape of chip generated after machining is finely crushed. On the contrary, when machining is spiral or linear, the machining force consumed is high.

Infeed depth Processing speed Feed rate Cooling oil Processing conditions 1 mm 1,600 rpm 0.3 rev / mm Wet

(3) Grain size: The austenite grains generated in the specimens prepared in Examples and Comparative Examples were photographed by an electron microscope, and their shapes were confirmed and their sizes were measured.

(4) Formation of MnS inclusions: The MnS inclusions generated in the specimens prepared in Examples and Comparative Examples were photographed by an electron microscope, and their shapes were observed and their sizes were measured.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. Such changes and modifications are intended to fall within the scope of the present invention unless they depart from the scope of the present invention. Accordingly, the scope of the present invention should be determined by the following claims.

Claims (8)

(P), 0.001 wt% to 0.03 wt% phosphorous (P), 0.1 wt% to 0.35 wt% silicon (Si), 0.6 wt% to 0.9 wt% manganese (Mn) (Al), 0.015 wt% to 0.05 wt% of molybdenum (Mo), 0.1 wt% to 0.015 wt% of chromium (Cr), 0.1 wt% (Fe) and unavoidable impurities, wherein the alloy composition contains 0.01 to 0.03 wt% of boron, 0.001 to 0.005 wt% of boron (B), 0.001 to 0.05 wt% of hafnium (Hf)
The weight ratio of hafnium (Hf) to manganese (Mn) in the alloy composition is 1:12 to 1:18,
The difference value of the lattice constant of? Hafnium (Hf) relative to? Manganese (Mn) represented by the following formula 1 is 15% to 20%
A tensile strength (TS) of 76 kgf / mm ² to 80 kgf / mm ² , a fatigue limit of 25 kgf / mm ² to 30 kgf / mm ² and a fatigue ratio of 33% to 40%
Ferrite, and pearlite, wherein the austenite grains have an average grain size of 10 mu m to 20 mu m, a surface hardness of 190 HB to 230 HB,
A steel for a drive shaft having an Fe-Hf precipitate phase and a size of MnS inclusions of 10 mu m to 50 mu m;
[Formula 1]
Lattice constant difference value DELTA L1 = {(L _Hf / L _Mn ) -1} 100
In the above formula (1), L _Hf is the lattice constant value of? Hafnium (Hf) and L _Mn is the lattice constant value of? Manganese (Mn).
The method according to claim 1,
Wherein the alloy composition further comprises 0.001 wt% to 0.01 wt% of at least one of zirconium (Zr) and calcium (Ca), respectively.
delete delete delete (P), 0.001 wt% to 0.03 wt% phosphorous (P), 0.1 wt% to 0.35 wt% silicon (Si), 0.6 wt% to 0.9 wt% manganese (Mn) (Al), 0.015 wt% to 0.05 wt% of molybdenum (Mo), 0.1 wt% to 0.015 wt% of chromium (Cr), 0.1 wt% A steel material having an alloy composition containing 0.01 to 0.03 wt% boron (B), 0.001 wt% to 0.005 wt% boron (B), 0.001 wt% to 0.05 wt% hafnium (Hf), and balance iron (Fe) To 1250 < 0 > C, followed by cooling,
And controlling the lattice constant difference value of? Hafnium (Hf) to? Manganese (Mn) represented by the following formula 1 to 15% to 20%
The weight ratio of hafnium (Hf) to manganese (Mn) in the alloy composition is 1:12 to 1:18,
The steel material is comprising a microstructure, and the austenitic grain to the average particle diameter 10㎛ 20㎛ consisting of ferrite and pearlite, the surface hardness of 190 HB to 230 HB is, tensile strength (TS): 76 kgf / mm 2 to 80 kgf / mm ² , fatigue limit of 25 kgf / mm ² to 30 kgf / mm ² and fatigue fat percentage of 33% to 40%
Forming a Fe-Hf precipitate phase in the steel material and controlling the size of the MnS inclusions to 10 占 퐉 to 50 占 퐉;
[Formula 1]
Lattice constant difference value DELTA L1 = {(L _Hf / L _Mn ) -1} 100
In the above formula (1), L _Hf is the lattice constant value of? Hafnium (Hf) and L _Mn is the lattice constant value of? Manganese (Mn).
The method according to claim 6,
Wherein the alloy composition further comprises 0.001 wt% to 0.01 wt% of at least one of zirconium (Zr) and calcium (Ca), respectively.
delete

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