KR102131523B1 - Steel wire rod having excellent spheroidizing heat treatment properties and method of manufacturing the same - Google Patents

Abstract

One aspect of the present invention is to provide a wire rod excellent in spheroidizing heat treatment and a method of manufacturing the same.
One embodiment of the present invention in weight percent, C: 0.3 to 0.5%, Si: 0.02 to 0.4%, Mn: 1.0 to 1.5%, Cr: 0.3 to 0.7%, B: 0.003% or less (excluding 0%) , Ti: 0.03% or less (excluding 0%), P: 0.03% or less (including 0%), S: 0.01% or less (including 0%), Al: 0.02 to 0.05%, N: 0.001 to 0.01% , Residual Fe and other unavoidable impurities, and the microstructure is a composite structure in which the main phase is ferrite + pearlite, and contains at least one of bainite or martensite in 5 area% or less (including 0%) Provided is a wire rod having a spheroidizing heat treatment property having an average colony size of 7 µm or less in a region from 2/5 to 3/5, and a method for manufacturing the same.

Description

Wire rod with excellent spheroidizing heat treatment and its manufacturing method{STEEL WIRE ROD HAVING EXCELLENT SPHEROIDIZING HEAT TREATMENT PROPERTIES AND METHOD OF MANUFACTURING THE SAME}

The present invention relates to a wire rod having excellent spheroidizing heat treatment properties and a method for manufacturing the same.

Boron steel is an economical material that can reduce expensive alloying elements such as Cr and Mo by improving the quenching properties of steel by adding a small amount of low-cost boron. The 800MPa-grade boron steel with low tensile strength has a low tensile strength due to the non-addition of alloying elements to improve the strength, so it is possible to omit spheroidizing heat treatment. High-strength products of 1000 MPa or more and large-diameter materials contain a large number of alloying elements such as Cr and Mn, and as a result, the tensile strength of the wire increases, and spheroidizing heat treatment is required.

Patent document 1 is a representative technology of the 800 MPa class boron steel. Patent Document 1 aims to improve the toughness of steel by minimizing the size of ferrite grains and increasing the fraction in hot-rolled bar steel wire. However, due to the limitation of the Cr content, there is a limitation in quenching, so there is a disadvantage that the use is limited in large-diameter steel bars.

In order to improve this problem, Patent Document 2 tried to improve the high-frequency hardenability by adding boron steel to which Cr, Mo, etc. were added and the microstructure contained ferrite. However, since ferrite is a structure that is difficult to austenite during austenizing heat treatment, in order to use a short heat treatment such as high-frequency heat treatment, there is a disadvantage in that the phase fraction of ferrite contained in the initial microstructure of the wire must be reduced as much as possible. In addition, since this method has to increase the finish rolling temperature in order to keep the ferrite fraction as low as possible, the wire rod strength increases, and accordingly, it is inevitable to act adversely in terms of processability.

Japanese Patent Application Publication No. 2010-053426 Japanese Patent Application Publication No. 2005-133152

One aspect of the present invention is to provide a wire rod excellent in spheroidizing heat treatment and a method of manufacturing the same.

One embodiment of the present invention in weight percent, C: 0.3 to 0.5%, Si: 0.02 to 0.4%, Mn: 1.0 to 1.5%, Cr: 0.3 to 0.7%, B: 0.003% or less (excluding 0%) , Ti: 0.03% or less (excluding 0%), P: 0.03% or less (including 0%), S: 0.01% or less (including 0%), Al: 0.02 to 0.05%, N: 0.001 to 0.01% , Residual Fe and other unavoidable impurities, and the microstructure is a composite structure in which the main phase is ferrite + pearlite, containing at least one of bainite or martensite in 5 area% or less (including 0%), and from the surface An average size of colonies of the pearlite in a region of 2/5 to 3/5 of the diameter is 7 μm or less, thereby providing a wire rod having excellent spheroidizing heat treatment properties.

Other embodiments of the present invention in weight percent, C: 0.3 to 0.5%, Si: 0.02 to 0.4%, Mn: 1.0 to 1.5%, Cr: 0.3 to 0.7%, B: 0.003% or less (excluding 0%) , Ti: 0.03% or less (excluding 0%), P: 0.03% or less (including 0%), S: 0.01% or less (including 0%), Al: 0.02 to 0.05%, N: 0.001 to 0.01% , Heating the steel material containing the remaining Fe and other inevitable impurities at a temperature of 1200° C. or higher for 60 minutes or more, followed by primary hot rolling to obtain a billet; Air-cooling the billet to 150-500°C; Cooling the air-cooled billet to room temperature at a cooling rate of 5 to 30°C/sec; After heating the cooled billet, extracting at 950 ~ 1050 ℃; Obtaining a wire rod by performing secondary hot rolling on the extracted billet; And cooling the wire rod to 2° C./sec or less, wherein the secondary hot rolling includes intermediate finishing rolling the extracted billet; And 730 ℃ ~ finishing step rolling at Ae3; provides a method for manufacturing a wire rod having excellent spheroidizing heat treatment.

According to one aspect of the present invention, it is possible to provide a wire rod capable of increasing the spheroidization rate of cementite during spheroidizing heat treatment and a method of manufacturing the same.

Hereinafter, a wire rod having excellent spheroidizing heat treatment property according to an embodiment of the present invention will be described. First, the alloy composition of the present invention will be described. The content of the alloy composition described below refers to weight percent unless otherwise specified.

C: 0.3-0.5%

C is an element added to accelerate the spheroidization of cementite. When the content of C exceeds 0.5%, residual austenite occurs excessively during quenching and tempering heat treatment after spheronization heat treatment and forging processing, and it is difficult to obtain appropriate strength, toughness, and ductility. On the other hand, boron steel having a C content of less than 0.3% is sufficiently low in tensile strength, so no separate spheroidizing heat treatment is required. In the present invention, boron steel that requires spheroidizing heat treatment is used as a target steel, and in order to accelerate the spheroidization rate of cementite, the C content is controlled to 0.3% or more. Therefore, the content of C is preferably in the range of 0.3 to 0.5%.

Si: 0.02~0.4%

Si is an element added to secure a certain level of strength. When the content of Si is less than 0.02%, the strength enhancing effect is not sufficient, and when it exceeds 0.4%, the solid solution strengthening effect is excessively high, which may be disadvantageous in securing workability of steel. Therefore, the Si content is preferably in the range of 0.02 to 0.4%. The lower limit of the Si content is more preferably 0.04%, and the upper limit of the Si content is more preferably 0.3%.

Mn: 1.0-1.5%

Mn is an element added to improve hardenability. When the content of Mn is less than 1.0%, it is difficult to obtain sufficient quenching properties due to insufficient curing ability, so it is difficult to obtain sufficient strength during quenching and tempering heat treatment after spheroidizing heat treatment and forging processing. There is a fear that the graininess is excessively increased, and thus a low-temperature structure is formed during the production of the wire rod. It is desirable to limit the content of the low-temperature structure since there is a possibility of internal cracking in the subsequent drawing process. Therefore, the content of Mn is preferably in the range of 1.0 to 1.5%. The Mn content is preferably between 1.0 and 1.3%. This is because Mn is generally an element in which fine segregation occurs well during the casting process, so if the desired level of quenching is satisfied, it is advantageous to manage the Mn content low to control the deviation of steel.

Cr: 0.3~0.7%

Cr, like Mn, is mainly used as an element that enhances the quenching properties of steel. When the content of Cr is less than 0.3%, the hardenability of the steel is insufficient, and thus the hardenability of the central portion of the large diameter material is insufficient. When the quenchability of the steel exceeds 0.7%, the presence of segregation zones inside the steel may cause low-temperature tissue bands during the wire manufacturing process and cracks may occur in the subsequent drawing process. Therefore, the Cr content is preferably in the range of 0.3 to 0.7%, and more preferably 0.3 to 0.6%.

B: 0.003% or less (excluding 0%)

B is an element added to improve quenching. When the content of B exceeds 0.003%, since the B forms Fe ₂₃ (C,B) ₆ , the amount of free boron decreases, so that the quenching property of steel decreases. Therefore, it is preferable that the content of B has a range of 0.003% or less.

Ti: 0.03% or less (excluding 0%)

Ti is an element added to fix nitrogen to maximize the effect of boron to improve the quenching properties. When the content of Ti exceeds 0.03%, a phenomenon in which TiN is precipitated in molten steel occurs, which makes it difficult to achieve the original purpose of adding titanium to fix nitrogen in the steel. Therefore, the Ti content is preferably in the range of 0.03% or less. The lower limit of the Ti content is more preferably 0.01%, and even more preferably 0.015%. The upper limit of the Ti content is more preferably 0.025%.

P: 0.03% or less (including 0%)

P is an imperatively contained impurity in the steel, and when its content exceeds 0.03%, P segregates at the austenite grain boundary to cause grain boundary brittleness, and in particular, there is a fear of lowering the low-temperature impact toughness of the steel. Therefore, the content of P is preferably in the range of 0.03% or less. The lower the P content is, the more advantageous it is to secure the health of the steel, so it is more preferably 0.02% or less, and even more preferably 0.015% or less.

S: 0.01% or less (including 0%)

S is an imperatively contained impurity in the steel, and when its content exceeds 0.01%, excessive MnS is generated, adversely affecting the impact toughness of the steel. Therefore, the content of S is preferably in the range of 0.01% or less. The content of S is more preferably 0.007% or less, and even more preferably 0.005% or less.

Al: 0.02~0.05%

Al is an element that serves to form AlN to generate austenite grains. When the content of Al is less than 0.02%, it is difficult to sufficiently obtain the effect because AlN is not sufficiently formed because there is little solid solution Al. When it exceeds 0.05%, aluminum oxide in the steel grows excessively and affects the toughness of the steel. Can give. Therefore, the content of Al is preferably in the range of 0.02 ~ 0.05%.

N: 0.001 to 0.01%

N is an element that affects the formation of austenite grains by forming TiN by reacting with Ti to improve the effect of boron to improve quenching and forming AlN by reacting with Al in steel. When N exceeds 0.01%, N combines with boron to form BN, thereby reducing the role of boron added for quenching, and also increasing the concentration of solid nitrogen to cause strength increase during processing. On the other hand, the lower the N content, the more preferable. However, in order to control the content to less than 0.001%, an excessive denitrification process is required, resulting in an increase in process cost. Therefore, it is preferable that the content of N has a range of 0.001 to 0.01%. The N content is more preferably 0.001 to 0.005%, and even more preferably 0.001 to 0.003%.

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

The microstructure of the wire rod of the present invention is preferably a ferrite + pearlite composite structure. If you look only at the spheroidizing side of the steel, there is an advantage in that bainite steel with fine cementite, but the cementite spheroidized in bainite is reported to be very fine and the growth is very slow. Unfavorable Therefore, in the present invention, by controlling the microstructure of the wire rod with a ferrite + pearlite composite structure, not only the spheroidizing heat treatment property can be improved, but also the structure can be more homogenized. At this time, the fraction of ferrite is preferably 35 area% or more, and if it is less than 35 area%, the pearlite phase fraction is relatively reduced, thereby affecting the pearlite colony size, and to be obtained in the present invention It may be difficult to effectively secure the spheroidizing heat treatment properties. In particular, even if they have the same ferrite phase fraction, if the ferrite grain size becomes fine, the colony size can be further refined. Meanwhile, in the present invention, at least one of microstructures that may be inevitably formed during manufacture, for example, bainite or martensite, may be included in an area of 5 area% or less. That is, the microstructure of the present invention is a composite structure in which the main phase is ferrite + pearlite, and may contain one or more of bainite or martensite in 5 area% or less (including 0%).

At this time, the average colony size of the pearlite is preferably 7 μm or less. As described above, by finely controlling the average size of pearlite colonies, the segmentation effect of cementite may be improved to increase the spheroidization rate of cementite during spheroidizing heat treatment.

In addition, the average grain size of the ferrite is preferably 5㎛ or less. As described above, by controlling the average grain size of ferrite finely, the size of pearlite colonies can also be refined, thereby increasing the spheroidization rate of cementite during spheroidizing heat treatment.

On the other hand, in the present invention, the average size of the colony of the pearlite and the average size of the grains of the ferrite may be in the center of the diameter of the wire rod, for example, in the region of 2/5 to 3/5 from the surface based on the diameter. Typically, the surface layer portion of the wire rod is subjected to a strong pressing force during rolling, so the average size of the colony of pearlite and the average grain size of ferrite in the surface layer portion may be fine. However, in the present invention, it is possible to effectively increase the spheroidization rate of cementite during spheroidizing heat treatment by minimizing the average size of the colonies of pearlite and the average grain size of ferrite to the center as well as the surface layer of the wire rod.

The wire rod of the present invention provided as described above may have an average aspect ratio of cementite of 2.5 or less after a single spheroidizing heat treatment. It is generally known that the spheroidizing heat treatment is effective for spheroidizing cementite as the number of times of treatment increases. However, in the present invention, cementite can be sufficiently spheroidized by only one spheroidizing heat treatment. On the other hand, as mentioned above, since the surface layer portion of the wire is subjected to a strong pressing force during rolling, the spheroidization of cementite can also proceed smoothly. However, in the present invention, the cementite in the area of 1/4 to 1/2 from the surface based on the diameter of the center of the wire, for example, based on the diameter, can also be sufficiently spheroidized so that the average aspect ratio of cementite in the center of the wire can be 2.5 or less. have. In addition, in order to spheroidize cementite in general, a spheroidizing heat treatment process is performed to segment the microstructure, and the wire rod of the present invention can effectively increase the spheroidization rate of cementite without such a processing step.

Hereinafter, a method of manufacturing a wire rod having excellent spheroidizing heat treatment according to an embodiment of the present invention will be described.

First, a steel material having the above-described alloy composition is heated at 1200° C. or higher for 60 minutes or more, and then first hot rolled to obtain a billet. The aforementioned steel materials may be one of slabs, blooms, and billets of relatively large sizes, and the primary hot rolling is preferably a plate rolling that reduces the size or thickness of the steel materials. Ti in the boron steel serves to increase the free B content by preventing N from binding to B by fixing N, and in the present invention, the steel is heated as above to grow TiN in order to make sufficient free B. When the heating temperature of the steel is less than 1200°C or when the heating time of the steel is less than 60 minutes, TiN may be stabilized and TiN growth may not sufficiently occur.

Meanwhile, after heating the steel, the steel may have an average size of TiN of 500 μm or more. When the average TiN size is less than 500 μm, a free B increasing effect cannot be sufficiently obtained.

Thereafter, the billet is air-cooled to 150 to 500°C. When the billet air-cooled stop temperature exceeds 500°C, other precipitates other than TiN may grow to cause cracking or fracture of the wire during the rolling process, and if it is less than 150°C, productivity may decrease.

Thereafter, the air-cooled billet is cooled to room temperature at a cooling rate of 5 to 30°C/sec. The billet cooling is intended to improve productivity, and the risk of cracking is reduced even if the billet cooling rate is increased to 5°C/sec or higher at a temperature of less than 150°C. The billet cooling rate is more preferably 10°C/sec or more, even more preferably 15°C/sec or more, and most preferably 20°C/sec or more. However, when the billet cooling rate exceeds 30°C/sec, the risk of cracking may increase due to excessive cooling rate.

On the other hand, the cooled billet may include at least 80 area% TiN of all the precipitates except the oxidizing inclusions. By forming TiN in such a large amount, the effect of improving the quenching effect by B can be sufficiently obtained. The oxidative inclusions may be, for example, Al ₂ O ₃ , SiO _{2 and the} like. The TiN fraction is more preferably 90 area% or more.

Then, after heating the cooled billet, it is extracted at 950 ~ 1050 ℃. When the billet extraction temperature is less than 950°C, the rolling properties are deteriorated, and when the billet extraction temperature exceeds 1050°C, rapid cooling is required for rolling. It can be difficult to ensure quality.

Thereafter, the extracted billet is subjected to secondary hot rolling to obtain a wire rod. It is preferable that the secondary hot rolling is a ball rolling in which the billet has a wire shape. The secondary hot rolling may include the step of intermediate finishing rolling the extracted billet and finishing finishing rolling at 730°C to Ae3. The rolling speed of the wire is very fast and belongs to the dynamic recrystallization region. Studies to date have shown that under dynamic recrystallization conditions, the austenite grain size depends only on the strain rate and strain temperature. When the wire diameter is determined due to the characteristics of wire rod rolling, the amount of deformation and the rate of deformation are determined, so that the austenite grain size can be changed by adjusting the deformation temperature. In the present invention, to refine the grains by using the dynamic deformation organic transformation phenomenon during dynamic recrystallization. In order to secure the ferrite crystal grains to be obtained by the present invention by using this phenomenon, it is preferable to control the finishing finishing rolling temperature from 730°C to Ae3. When the finishing finishing rolling temperature exceeds Ae3, it may be difficult to obtain a ferrite crystal grain desired to be obtained in the present invention, and it may be difficult to obtain sufficient spheroidizing heat treatment properties. .

Meanwhile, after the intermediate finishing rolling, the average size of the austenite grains of the wire rod is preferably 5 to 20 μm. Ferrite is known to grow by nucleation at austenite grain boundaries. If the parent austenite grains are fine, the ferrites nucleating at the grain boundaries can also start to be formed finely, and thus the ferrite grain refinement effect can be obtained by controlling the average size of the austenite grains of the wire after intermediate finishing rolling as described above. When the average size of the austenite grains exceeds 20 μm, it is difficult to obtain a ferrite grain refinement effect, and in order to obtain the average size of the austenite grains of less than 5 μm, a separate facility is required to additionally apply a high strain amount such as dropping There may be a disadvantage.

Thereafter, the wire rod is cooled to 2°C/sec or less. When the cooling rate of the wire rod exceeds 2°C/sec, there is a fear that a low-temperature structure such as bainite is formed in the microsegregation portion of the wire rod. In the micro segregation portion, segregation of 2 times or more of the average of the wire rod may be formed, and as a result, a low-temperature structure may be generated even at a low cooling rate, which may adversely affect the homogenization of the steel. Meanwhile, the cooling rate of the wire rod is more preferably 0.5 to 2°C/sec in terms of ferrite grain refinement.

Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note 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)

A steel material having an alloy composition of Table 1 below was manufactured by casting using a 50 kg vacuum induction melting furnace. The steel material was heated at 1230°C for 480 minutes, air-cooled to 300°C, and then cooled to room temperature at a cooling rate of 10°C/sec to prepare a billet. The prepared billet was prepared using the conditions described in Table 2 below. For the thus prepared wire rod, after measuring the microstructure, the average grain size of ferrite, the average size of colonies of pearlite and the average aspect ratio of cementite after one spheroidizing heat treatment, the results are shown in Table 3 below.

The average size of the austenite grains after intermediate finishing rolling (AGS) was measured by cutting crops performed before finishing finishing rolling.

Ae3 displays values calculated using the commercial program JmatPro.

The average grain size (FGS) of ferrite was expressed as an average value after measuring arbitrary 3 points in the area of 2/5 points to 3/5 points from the diameter of the sample collected after removing the uncooled portion after rolling the wire.

The average size of the colonies of pearlite was determined by selecting 10 random pearlite colonies at the same point as the FGS measurement, obtaining the (long axis + short axis)/2 value of each colony, and expressing the average value of the measured colony sizes.

On the other hand, the spheroidizing heat treatment was immediately performed without a separate processing process on the specimen of the wire rod manufactured as described above. At this time, the spheroidizing heat treatment is maintained at 4 to 6 hours after heating to 760°C at a heating rate of 100°C/Hr, and after cooling to a cooling rate of 50°C/Hr to 730°C, in a section between 730°C and 670°C After cooling at a cooling rate of 10°C/Hr, it proceeded to maintain furnace cooling at a temperature below that. The average aspect ratio of cementite is 3 times of 2000 times SEM of the 1/4~1/2 point in the radial direction of the wire after spheroidizing heat treatment, and the image measurement program is used to automatically measure the long/short axis of cementite in the field of view and perform statistical processing. It was measured through.

Steel Type No. Alloy composition (% by weight) C Si Mn Cr P S Ti B Al N Invention Steel 1 0.32 0.21 1.2 0.45 0.018 0.006 0.015 0.002 0.03 0.004 Invention Steel 2 0.36 0.2 1.15 0.51 0.015 0.006 0.018 0.003 0.02 0.005 Invention Steel 3 0.43 0.15 1.3 0.38 0.01 0.008 0.025 0.0015 0.04 0.003 Invention Steel 4 0.38 0.25 1.18 0.62 0.011 0.003 0.015 0.0015 0.03 0.005 Comparative Steel 1 0.34 0.54 1.05 0.6 0.016 0.004 0.023 0.002 0.03 0.004 Comparative Steel 2 0.43 0.56 1.22 0.43 0.013 0.005 0.014 0.0018 0.03 0.004

division Steel Type No. Billet extraction
Temperature (℃) Middle thought
AGS(㎛) after rolling Ae3
(℃) A closing thought
Rolling temperature (℃) Wire rod cooling
Speed (℃/sec) Inventive Example 1 Invention Steel 1 1032 12 792 742 0.8 Inventive Example 2 Invention Steel 2 1025 11 783 755 1.2 Inventive Example 3 Invention Steel 3 1034 14 766 764 0.9 Inventive Example 4 Invention Steel 4 1043 13 801 760 1.5 Inventive Example 5 Invention Steel 1 1021 12 778 752 1.8 Inventive Example 6 Invention Steel 2 1030 12 780 770 0.9 Comparative Example 1 Comparative Steel 1 1024 12 792 802 0.4 Comparative Example 2 Comparative Steel 2 1031 24 783 823 0.3 Comparative Example 3 Comparative Steel 1 1034 22 766 790 1.5 Comparative Example 4 Comparative Steel 2 1035 21 801 835 2 Comparative Example 5 Comparative Steel 1 1011 15 778 804 2.4 Comparative Example 6 Comparative Steel 2 1028 18 780 816 3

division Microstructure (area %) ferrite
Crystal grain
Average size (㎛) Pearlite
Colony
Average size (㎛) After spheroidizing heat treatment
Cementite
Average aspect ratio F P B+M Inventive Example 1 57 41 2 3.8 5.5 2.1 Inventive Example 2 53 47 0 4.5 5.2 2.3 Inventive Example 3 41 55 4 4.8 6.7 2.4 Inventive Example 4 53 45 2 4.3 5.4 2.2 Inventive Example 5 56 42 2 4.7 6.2 2.4 Inventive Example 6 52 46 2 4.6 5.3 2.2 Comparative Example 1 47 51 2 9.4 14 3.1 Comparative Example 2 32 66 2 11 12 2.8 Comparative Example 3 46 52 2 9.5 15 3.3 Comparative Example 4 34 64 2 12 13 2.9 Comparative Example 5 48 50 2 11 18 3.1 Comparative Example 6 35 63 2 11 17 3.3 F: ferrite, P: pearlite, B: bainite, M: martensite

As can be seen from Tables 1 to 3, in the case of Inventive Examples 1 to 6 satisfying the alloy composition and the manufacturing conditions proposed by the present invention, the visualization is performed once by securing the fine structure type and fraction as well as the fine structure type and fraction of the present invention. It can be seen that the heat treatment alone has an average aspect ratio of cementite of 2.5 or less.

However, in the case of Comparative Examples 1 to 6, which do not satisfy the alloy composition or manufacturing conditions proposed by the present invention, the cementite average at the time of spheroidizing heat treatment by not satisfying the microstructure type and fraction of the present invention or securing fine grains It can be seen that the aspect ratio is high, and in the end, it can be confirmed that additional spheroidizing heat treatment is required to apply to the final product.

Claims (8)

In weight percent, C: 0.3 to 0.5%, Si: 0.02 to 0.25%, Mn: 1.0 to 1.5%, Cr: 0.3 to 0.7%, B: 0.003% or less (excluding 0%), Ti: 0.03% or less ( 0% excluded), P: 0.03% or less (including 0%), S: 0.01% or less (including 0%), Al: 0.02 to 0.05%, N: 0.001 to 0.01%, balance Fe and other unavoidable impurities Including,
The microstructure is a composite structure in which the main phase is ferrite + pearlite, and contains at least one of bainite or martensite in 5 area% or less (including 0%),
A wire rod having excellent spheroidizing heat treatment, wherein the pearlite has an average colony size of 7 µm or less in a region from 2/5 to 3/5 of the diameter from the surface.
The method according to claim 1,
The fraction of ferrite is a wire having excellent spheroidizing heat treatment property of 35 area% or more.
The method according to claim 1,
The wire rod is a wire rod excellent in spheroidizing heat treatment, wherein the average grain size of the ferrite in the region of 2/5 to 3/5 of the diameter from the surface is 5 µm or less.
The method according to claim 1,
The wire rod is a wire rod having excellent spheroidizing heat treatment, wherein the average aspect ratio of cementite is 2.5 or less after one spheroidizing heat treatment.
In weight percent, C: 0.3 to 0.5%, Si: 0.02 to 0.25%, Mn: 1.0 to 1.5%, Cr: 0.3 to 0.7%, B: 0.003% or less (excluding 0%), Ti: 0.03% or less ( 0% excluded), P: 0.03% or less (including 0%), S: 0.01% or less (including 0%), Al: 0.02 to 0.05%, N: 0.001 to 0.01%, balance Fe and other unavoidable impurities Heating the steel material containing at least 1200° C. for at least 60 minutes, followed by primary hot rolling to obtain a billet;
Air-cooling the billet to 150-500°C;
Cooling the air-cooled billet to room temperature at a cooling rate of 5 to 30°C/sec;
After heating the cooled billet, extracting at 950 ~ 1050 ℃;
Obtaining a wire rod by performing secondary hot rolling on the extracted billet; And
Cooling the wire rod to 2 ℃ / sec or less; includes,
The secondary hot rolling may include intermediate finishing rolling the extracted billet; And
A method of manufacturing a wire rod having excellent spheroidizing heat treatment, including; finishing finishing rolling at 730°C to Ae3.
The method according to claim 5,
After the steel material is heated, the steel material is a method of manufacturing a wire rod having an spheroidizing heat treatment with an average size of TiN of 500 µm or more.
The method according to claim 5,
The cooled billet is a method of manufacturing a wire rod having excellent spheroidizing heat treatment, which contains 80% by area or more of TiN among all precipitates except for the oxidizing inclusions.
The method according to claim 5,
After the intermediate finishing rolling, the average size of the austenite grains of the wire rod is 5 ~ 20㎛ spheroidized heat treatment method of excellent wire rod manufacturing method.