KR101977499B1 - Wire rod without spheroidizing heat treatment, and method for manufacturing thereof - Google Patents

Abstract

The present invention relates to a wire rod for cool forging and, more specifically, to a wire rod for cool forging which can skip the heat treatment for spheroidizing and softening, and a manufacturing method thereof. The wire rod capable of skipping the heat treatment for spheroidizing and softening comprises: 0-1-0.7 wt% of carbon (C); 0.2-2.0 wt% of silicon (Si); 0.2-2.0 wt% of manganese (Mn); 0.01-0.05 wt% of aluminum (Al); 0.004-0.02 wt% of nitrogen (N); and a remainder consisting of Fe and other impurities. A micro structure includes ferrite and segmented pearlite, wherein 50% or more of an area fraction of the segmented pearlite is contained, and the average grain size (FGS) of the ferrite instantly after finishing hot rolling is 5 μm or less.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wire roving material,

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wire rod for cold stamping, and more particularly, to a wire rod capable of omitting a spheroidizing softening heat treatment and a manufacturing method thereof.

In order to soften the wire rod, spheroidizing heat treatment is generally performed. Spheroidal heat treatment induces spherical cementite and homogeneous particle distribution to improve cold workability during cold forming. Further, in order to improve the life of the processing die, the hardness of the material to be processed can be reduced as much as possible. In order to accomplish the above two objects, it is used as a softening concept of a material, and cutting performance can be improved more than general ferrite + pearlite steel in addition to cutting work.

These spheroidizing heat treatments are classified into two types. One is sub-critical annealing, which is mainly used for hot-rolled products, and the other is heating between the vacancy temperature and the austenitizing temperature. (Inter-critical annealing).

The spherical particles thus formed grow through a process similar to that of Ostwald ripening to form spherical tissue. These spheroidization processes are mainly observed under the austenite transformation temperature, and the base structure of the material changes from ferrite and pearlite to ferrite and spherical cementite. In other words, the portion of pearlite existing in the initial structure is changed into ferrite and spherical cementite, and the whole microstructure is composed of ferrite and spherical cementite.

The method of performing the spheroidization by heating in the two-phase region is fundamentally different from the method of spheroidizing below the vacancy temperature. When the initial structure is composed of pearlite and ferrite, when the pearlite is heated in the two-phase region, the pearlite portion and a portion of the ferrite are transformed into austenite at a high temperature, and the austenite portion existing in the pearlite- It is not completely dissolved and a part remains to maintain the form of austenite + residual cementite. After that, cementite remaining in a cold state acts as a nucleus, transformation proceeds from the austenite to the growth form of ferrite and cementite particle, not from pearlite transformation, and the spherical particles that have already formed in the cold state after transformation become Ostwald ripening ) To form a spherical tissue.

From this concept of spheroidizing heat treatment, the spheroidization softening heat treatment abbreviation techniques can be divided into two types. The first is to reheat the billet beyond the austenite single phase and then continuously rolling from Acm (cementite precipitation or dissolution temperature) to Al (vacancy transformation temperature) or below to produce a spheroidized carbide seed, It is a method to grow these seeds by applying an extreme cold pattern. This rolling pattern can also be repeatedly applied to increase the number of carbide seeds. The second is to aggressively add high temperature carbide forming elements and grow them into spheroidized carbide seeds during cooling. However, these methods have problems in that they are subjected to roll breakage or high rolling load because of the necessity of performing low-temperature reverse rolling in order to secure sufficient spheroidized carbide seeds (Patent Document 1).

Korean Patent Publication No. 2005-0000101

An aspect of the present invention is to provide a wire material capable of omitting a spheroidizing softening heat treatment by optimizing an alloy composition and manufacturing conditions and a manufacturing method thereof.

An aspect of the present invention provides a method of manufacturing a semiconductor device, comprising 0.1 to 0.7% carbon, 0.2 to 2.0% silicon, 0.2 to 2.0% manganese, 0.01 to 0.05% aluminum, , Nitrogen (N): 0.004 to 0.02%, the balance Fe and other impurities, the microstructure includes ferrite and segmented pearlite, and the segmented pearlite has an area fraction of not less than 50% .

According to another aspect of the present invention, there is provided a method of manufacturing a billet, comprising: reheating a billet satisfying the above-described alloy composition at a temperature range of 900 to 1050 占 폚; Subjecting the reheated billet to a finish hot-rolling at a strain of 0.6 or more in a temperature range of 700 to 780 캜 to produce a wire rod; Cooling the rolled wire rod to a temperature of 450 to 550 ° C at a cooling rate of 10 to 20 ° C / s; And a step of second cooling the steel sheet to room temperature at a cooling rate of 5 ° C / s or less (excluding 0 ° C / s) after the primary cooling, thereby providing a method of manufacturing a wire rod capable of omitting the spheroidizing softening heat treatment.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to effectively induce pearlite segments during the wire rod manufacturing process, thereby providing a wire rod capable of omitting the spheroidizing softening heat treatment.

Further, the wire rod of the present invention can be suitably used for cold pressing.

Fig. 1 is a microstructure photograph of Comparative Example 1 (a) and Inventive Example 1 (b) measured immediately before the finish hot rolling is started according to an embodiment of the present invention.
2 is a microstructure photograph of Comparative Example 1 (a) and Inventive Example 1 (b) measured immediately after the finish hot rolling (immediately before the start of primary cooling) according to an embodiment of the present invention.
3 is a photograph of microstructure measured after completion of secondary cooling of Comparative Example 1 (a) and Inventive Example 1 (b) according to an embodiment of the present invention.

The present inventors have intensively studied a method of omitting the spheroidizing softening heat treatment in providing the wire rod for cold pressing. As a result, it has been confirmed that spheroidization can be realized during wire rod production by effectively inducing pearlite segments while suppressing crystal grain growth from fine carbides by optimizing alloy composition and manufacturing conditions, and accomplished the present invention.

Hereinafter, the present invention will be described in detail.

The cold rolled steel wire rod according to one aspect of the present invention is capable of omitting the annealing softening heat treatment. The cold rolled steel wire has 0.1 to 0.7% carbon, 0.2 to 2.0% silicon, 0.2 to 2.0% manganese, Al: 0.01 to 0.05%, and nitrogen (N): 0.004 to 0.02%.

Hereinafter, the reason why the alloy composition of the wire rod is limited as described above will be described in detail in the present invention. At this time, the content of each element means weight% unless otherwise specified.

C: 0.1 to 0.7%

Carbon (C) is an element favorable for securing strength, but if the content is less than 0.1%, the strength of the wire is insufficient, and there is a problem that the strength of the product after the final Q & T heat treatment is heated. On the other hand, when the content exceeds 0.7%, most of the structure is formed of pearlite, which makes it difficult to produce a soft wire rod.

Therefore, in the present invention, the content of C is preferably limited to 0.1 to 0.7%. And more preferably 0.2 to 0.6%.

Si: 0.2 to 2.0%

Silicon (Si) is a representative substitutional element and has a great influence on securing strength of steel. If the content of Si is less than 0.2%, it is difficult to ensure the strength and the entrapment property. On the other hand, if the Si content exceeds 2.0%, generation of decarburized structure is promoted during rolling of the wire,

Therefore, in the present invention, it is preferable to limit the Si content to 0.2 to 2.0%. More advantageously from 0.25 to 1.0%.

Mn: 0.2 to 2.0%

Manganese (Mn) forms a substitute solid solution in the matrix and lowers the Al (vacancy transformation temperature) temperature to refine the interlayer spacing of the pearlite and increase the sub-grain in the ferrite structure.

If the content of Mn exceeds 2.0%, there is a problem that physical properties are disadvantageous due to uneven structure due to manganese segregation. Segregation of manganese segregation occurs easily due to the segregation mechanism in the solidification of steel. The manganese segregation promotes segregation due to the relatively low diffusion coefficient compared to other elements, martensite). Therefore, in the present invention, it is preferable to limit the content of Mn to 2.0% or less. However, if the content is less than 0.2%, there is a possibility that sufficient incombustibility for forming a martensite structure may not be ensured after the final Q & T heat treatment.

Therefore, in the present invention, the content of Mn is preferably limited to 0.2 to 2.0%. More advantageously from 0.5 to 1.5%.

Al: 0.01 to 0.05%

Aluminum (Al) is an element to be added for the deoxidizing effect. When the content is less than 0.01%, it is difficult to secure a sufficient deoxidation power. On the other hand, if the content exceeds 0.05%, hard inclusions such as Al ₂ O ₃ may increase, and clogging of the nozzle due to inclusions may occur particularly during performance.

Therefore, in the present invention, the content of Al is preferably limited to 0.01 to 0.05%. More advantageously from 0.02 to 0.04%.

N: 0.004 to 0.02%

Nitrogen (N) is an element that forms carbon and nitride. If the content is less than 0.004%, precipitates such as Ti, Nb and V may not be sufficiently secured. On the other hand, when the content exceeds 0.02%, the amount of solid solution nitrogen is increased and the toughness and ductility of the steel may be lowered.

Therefore, in the present invention, the content of N is preferably limited to 0.004 to 0.02%.

The wire of the present invention preferably contains at least one of Nb, Ti, V, and Mo among the elements forming the carbon and nitride other than the alloy composition described above.

Nb: 0.001 to 0.03%

Niobium (Nb) forms carbon and nitride such as Nb (C, N) and has a favorable effect on the refinement of ferrite / pearlite structure during wire rolling. In addition, prior to the precipitation of carbon and nitride, the solute drag effect affects the austenitic grain refinement during rolling.

In order to sufficiently obtain the above-mentioned effect, it is preferable to add Nb in an amount of 0.001% or more. However, if the content exceeds 0.03%, the precipitation may be coarsened and the precipitation effect may be reduced.

Ti: 0.01 to 0.05%

Titanium (Ti) is the most powerful element for forming carbon and nitride, and has an effect on grain refinement in a heating furnace.

When the content of Ti is less than 0.01%, the amount of precipitation is insufficient and the above-mentioned effect can not be sufficiently obtained. When the content exceeds 0.05%, precipitates are coarsened and crack initiation sites initial site).

V: 0.2 to 0.5%

Vanadium (V) is an element that forms precipitates such as VC, VN, and V (C, N), and has a favorable effect on ferrite / pearlite texture refinement during wire rolling.

However, if the content is less than 0.2%, the distribution of the above precipitates is insufficient and the effect of fixing the ferrite grain boundaries can not be sufficiently secured, so that the effect on the improvement of the toughness is insufficient. On the other hand, when the content exceeds 0.5%, coarse vanadium carbon and nitride are formed and adversely affect the toughness.

Mo: 0.15 to 0.50%

Molybdenum (Mo) is advantageous for suppressing the strength decrease (temper softening) by forming a precipitate of Mo ₂ C upon tempering during the Q & T heat treatment for the final product.

If the Mo content is less than 0.15%, it is difficult to sufficiently secure the softening effect of temper. On the other hand, if the Mo content exceeds 0.50%, a low-temperature structure is formed during the manufacturing of the wire rod, There is a problem.

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

The wire material of the present invention satisfying the above-mentioned alloy composition includes microstructure and ferrite-fractionated pearlite, and the segmented pearlite preferably has an area fraction of 50% or more.

If the fraction of the segmented pearlite is less than 50%, a separate heat treatment step for softening the wire rods is required, which is not preferable. The higher the fraction of the segmented pearlite is, the better, but in the present invention, it can be up to 85%.

The pearlite segmented in the present invention means segmented cementite (also referred to as segmented cementite) in segmented form. In one aspect of the present invention, the segmented pearlite has an aspect ratio (short axis: major axis) of 1 to 2 (D: diameter of wire rod) observed from the surface of the wire, : Can be less than 3.

Further, it is preferable to include at least one precipitate out of Nb-based carbides, nitrides, Ti-based carbides, nitrides, V-based carbides and nitrides, and Mo-based carbides and nitrides.

The present invention can obtain the effect of suppressing the growth of austenite grains from the formation of the above-mentioned carbonitride and nitride, and securing the pro-eutectoid ferrite fraction close to the equilibrium phase.

The wire having the microstructure as described above can be obtained by controlling the above-mentioned alloy composition and by optimizing the wire rolling process and the cooling process to be described later. Particularly, by the step cooling process, the generation of the cornerstone cementite and the surface ferrite decarburization are suppressed, and the thickness of the cementite is minimized, so that the pearlite segment can be effectively induced during the wire manufacturing process.

Hereinafter, a method for manufacturing a wire rod for cold-pressing capable of omitting the spheroidizing softening heat treatment, which is another aspect of the present invention, will be described in detail.

The cold-rolled steel wire rod according to the present invention can be manufactured by preparing a billet having the above-described alloy composition and then subjecting it to a reheating-wire rolling-cooling process.

[Reheating step: 900 to 1050 DEG C]

First, it is preferable to provide a billet satisfying the alloy composition described above and reheat it.

At this time, it is preferable to maintain the temperature within the range of 900 to 1050 캜 within 90 minutes at that temperature. That is, the steel is maintained in a single phase of austenite, and the temperature range is such that the austenite grains are not coarsened and is effective in removing the remaining coarse cementite.

If the reheating temperature exceeds 1050 DEG C, austenite grains are coarsened and it becomes difficult to secure super fine ferrite after the subsequent wire rolling. On the other hand, if the temperature is lower than 900 캜, the holding time of the billet becomes longer, which lowers the productivity and increases the rolling load.

[Finishing hot rolling: 700 to 780 DEG C, deformation amount of 0.6 or more]

It is preferable that the billet reheated under the above-mentioned conditions is subjected to finish hot rolling to produce a wire shape.

At this time, the finish hot rolling is performed in a temperature range of 700 to 780 캜, and the deformation amount is preferably 0.6 or more. If the temperature during the final hot rolling is less than 700 ° C, a cementite phase is formed to cause a high rolling load. On the other hand, when the temperature exceeds 780 ° C, coarse ferrite is produced, .

If the amount of deformation is less than 0.6 at the time of the final hot rolling in the above temperature range, the deformation becomes insufficient at the fine-austenite grain boundaries and the ferrite nucleation site becomes small. In the present invention, the upper limit of the amount of deformation is not particularly limited, but can be set to 2.0 or less in consideration of facility load and the like.

On the other hand, it is preferable that the austenite grain size (AGS) immediately before the finish hot rolling is 15 탆 or less.

As the austenite grain size becomes finer, a lot of deformation is induced during rolling to maximize the ferrite nucleation site, thereby making the grain finer and the ferrite having an average grain size of 5 탆 or less can be obtained.

[Step cooling]

Primary cooling: from 450 to 550 ° C at a cooling rate of 10 to 20 ° C / s

It is preferable to perform the final hot rolling under the above-mentioned conditions to form ferrite having an average grain size of 5 탆 or less and then cool it at a cooling rate of 10 to 20 캜 / s.

Since the ferrite having an average grain size of 5 μm or less is in a state of increased transformation and diffusion rate such as nucleation / growth, pearlite structure can be rapidly formed by quenching at a cooling rate of 10 ° C./s or more. In addition, the thickness of the resulting plate-shaped cementite in pearlite can be minimized, and the pearlite segment can be effectively induced in the subsequent slow cooling process.

At this time, if the cooling rate is less than 10 캜 / s, the growth of pro-eutectoid ferrite may be induced or the decarburization of ferrite may occur on the surface. However, if the cooling rate exceeds 20 ° C / s, there is a fear that low-temperature structure such as martensite may be generated.

If the cooling end temperature in the primary cooling exceeds 550 ° C, the pearlite structure may not be sufficiently transformed. On the other hand, if the cooling end temperature is lower than 450 ° C, there is a possibility that a mild phase such as low temperature structure is generated from the untransformed residual austenite phase .

Secondary cooling: Cool down to room temperature at a cooling rate of 5 ° C / s or less (excluding 0 ° C / s).

After completing the primary cooling in accordance with the above, it is preferable to perform slow cooling to room temperature at a cooling rate of 5 占 폚 / s or less (excluding 0 占 폚 / s).

At this time, if the cooling rate exceeds 5 DEG C / s, the pearlite segment can not be sufficiently induced, and there is a possibility that a subsequent spheroidizing softening heat treatment is required.

As described above, in the present invention, pearlite having a plate-like cementite thickness minimized through quenching immediately after finish rolling is formed, and then slowly cooled to form pearlite segments. Specifically, the segmented pearlite may have an area fraction of 50% or more.

The present invention relates to a method for finely forming an austenite mean grain size (AGS) before finishing hot rolling to obtain an ultra fine grain FGS of 5 탆 or less and maximizing a ferrite nucleation site by performing under- Can be miniaturized. Since the superfine FGS obtained by this method increases the transformation and diffusion rate such as nucleation / growth, the pearlite structure is rapidly formed through the initial quenching immediately after the cooling, and then the pearlite decomposition rapidly proceeds through the slow cooling, It is possible to provide a cold-rolled wire rod which can be omitted.

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

( Example )

Billets having the alloy compositions shown in the following Table 1 were prepared and then subjected to reheating-finishing hot-rolling-step cooling under the conditions shown in Table 2 below to prepare respective wires.

At this time, in producing each wire, the average austenite grain size (AGS) just before the final hot rolling and the ferrite average grain size (FGS) immediately after the final hot rolling are set to 1 / 2D of each wire ), And the measurement was carried out according to ASTM E112 method. The fractional pearlite fraction of each wire obtained by completing the step cooling step was measured at 1 / 2D position of the wire using an image analyzer, and the mechanical properties were measured. At this time, as the mechanical properties, hardness values were measured using Vickers hardness tester.

division Alloy composition (% by weight) C Si Mn Al N Nb Ti V Mo Comparative Example 1 0.25 0.30 1.30 0.042 0.015 0.015 0 0 0 Comparative Example 2 0.35 1.20 1.30 0.010 0.004 0.015 0.02 0 0 Comparative Example 3 0.40 0.80 1.20 0.042 0.013 0.020 0.02 0 0.2 Comparative Example 4 0.72 0.30 0.80 0.035 0.010 0 0 0 0 Inventory 1 0.35 0.20 0.70 0.035 0.010 0 0 0 0 Inventory 2 0.20 0.25 0.80 0.030 0.015 0.015 0 0.3 0.2 Inventory 3 0.25 0.30 1.20 0.040 0.020 0.015 0 0 0 Honorable 4 0.45 0.25 1.20 0.036 0.016 0 0.03 0 0.3 Inventory 5 0.50 0.25 1.50 0.032 0.012 0 0.02 0.3 0.2

(The compositions of Comparative Examples 1 to 3 satisfy the requirements of the present invention, but the following manufacturing process conditions are outside the scope of the present invention.

division
Reheating
Condition
(° C, min) Finishing hot rolling Cooling AGS (탆) before rolling Temperature
(° C) Strain Primary cooling
(° C / s) Primary cooling
End temperature (캜) Secondary cooling
(° C / s) Comparative Example 1 1000, 90 25 850 1.0 10 500 4 Comparative Example 2 1200 , 90 30 760 0.8 10 500 5 Comparative Example 3 1020, 90 9 780 0.4 10 500 5 Comparative Example 4 1000, 90 10 750 1.0 10 500 10 Inventory 1 1000, 90 10 760 1.2 10 500 3 Inventory 2 1000, 90 11 750 0.8 12 500 3 Inventory 3 1000, 90 9 730 0.6 12 500 5 Honorable 4 1000, 90 9 760 0.8 11 500 2 Inventory 5 1000, 90 10 750 1.0 10 500 One

(AGS means the mean grain size of austenite).

division Immediately after rolling, FGS
(탆) Microstructure Segmented pearlite
Area ratio (%) Hardness
(HV) Comparative Example 1 11 Ferrite + Pearlite - 165 Comparative Example 2 15 Ferrite + Pearlite - 205 Comparative Example 3 9 Ferrite + Segmented Pearlite 25 185 Comparative Example 4 11 Ferrite + Pearlite - 280 Inventory 1 3.5 Ferrite + Segmented Pearlite 60 179 Inventory 2 4.2 Ferrite + Segmented Pearlite 65 147 Inventory 3 3.8 Ferrite + Segmented Pearlite 80 155 Honorable 4 3.2 Ferrite + Segmented Pearlite 73 190 Inventory 5 3.1 Ferrite + Segmented Pearlite 75 203

(FGS means ferrite average grain size).

As shown in Tables 1 to 3, Inventive Examples 1 to 5 satisfying the alloy composition and manufacturing conditions proposed in the present invention sufficiently provided the pearlite segment, ensured that the area percentage of the pearlite pearlite was 50% or more, Can be confirmed.

1 is a photograph of microstructure measured immediately before the finish of hot rolling of Comparative Example 1 (a) and Inventive Example 1 (b).

According to FIG. 1, the comparative material 1 has a larger austenite grain size, whereas in Inventive Example 1, the average grain size is finely formed to 10 μm.

2 is a photograph of microstructure measured immediately after the completion of the hot rolling of Comparative Example 1 (a) and Example 1 (b) (just before the start of the primary cooling).

According to FIG. 2, ferritic and segmented pearlite having crystal grains with an average grain size of 5 mu m or less were formed in Inventive Example 1, whereas in Comparative Example 1, ferrite was formed to be coarse.

3 is a photograph of microstructure measured after completion of secondary cooling of Comparative Example 1 (a) and Inventive Example 1 (b).

According to FIG. 3, in Comparative Example 1, coarse ferrite and plate-shaped cementite were formed similarly to immediately after the finish hot rolling, whereas in Inventive Example 1, the plate-shaped cementite formed immediately after the finish hot rolling was segmented, . Further, in the case of Inventive Example 1, the ferrite formed finely after the rolling was grown in a coarseness favorable to the cold rolling in the cooling process.

Therefore, the wire rod obtained according to the alloy composition and the manufacturing conditions proposed in the present invention is excellent in cold-rolling composition and can be suitably used for cold-pressing.

On the other hand, in Comparative Example 2, the temperature during reheating was excessively high, and the average austenite grain size before rolling was very coarse to 30 탆, thereby forming coarse ferrite even after rolling.

In Comparative Example 3, the amount of deformation during hot rolling was too small to secure a sufficient fraction of the segmented pearlite.

In Comparative Example 4, the content of carbon was excessive, and the pearlite fraction was not induced due to a rapid cooling rate during the secondary cooling.

Claims (9)

(Si): 0.2 to 2.0%, manganese (Mn): 0.2 to 2.0%, aluminum (Al): 0.01 to 0.05%, nitrogen (N): 0.004 To 0.02%, balance Fe and other impurities,
Wherein the microstructure comprises ferrite and segmented pearlite, wherein the segmented pearlite comprises at least 50% area fraction,
A wire rod capable of omitting a spheroidizing softening heat treatment having a ferrite grain size (FGS) of 5 탆 or less immediately after finish hot rolling.
The method according to claim 1,
The wire may further include at least one of 0.001 to 0.03% of niobium (Nb), 0.01 to 0.05% of titanium (Ti), 0.2 to 0.5% of vanadium (V), and 0.15 to 0.50% of molybdenum (Mo) Which is capable of omitting the annealing softening heat treatment.
The method according to claim 1,
Wherein the wire rod is capable of omitting a spheroidizing softening heat treatment including at least one of Nb-based carbon-nitride, Ti-based carbon-nitride, V-based carbon and nitride, and Mo-based carbon and nitride.
delete (C): 0.2 to 2.0%, Mn: 0.2 to 2.0%, Al: 0.01 to 0.05%, N (nitrogen): 0.004 To 0.02%, the remainder of Fe and other impurities in a temperature range of 900 to 1050 캜;
Subjecting the reheated billet to a finish hot-rolling at a strain of 0.6 or more in a temperature range of 700 to 780 캜 to produce a wire rod;
Cooling the rolled wire rod to a temperature of 450 to 550 ° C at a cooling rate of 10 to 20 ° C / s; And
After the primary cooling, secondary cooling to room temperature at a cooling rate of 5 DEG C / s or less (excluding 0 DEG C / s)
Wherein the method comprises the steps of:
6. The method of claim 5,
Wherein the annealing softening heat treatment can be omitted when the average grain size (AGS) of the austenite immediately before the finish hot rolling is 15 占 퐉 or less.
6. The method of claim 5,
Wherein the ferrite average grain size (FGS) immediately after the finish hot rolling is 5 占 퐉 or less can be omitted.
6. The method of claim 5,
And the fractionated pearlite is formed in an area fraction of 50% or more after the secondary cooling is completed.
6. The method of claim 5,
The billet further includes at least one of niobium (Nb): 0.001 to 0.03%, titanium: 0.01 to 0.05%, vanadium (V): 0.2 to 0.5%, and molybdenum (Mo): 0.15 to 0.50% Which is capable of omitting the annealing softening heat treatment.

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