KR102327928B1 - Steel material having softening resistance at high temperature and method of manufacturing the same - Google Patents

Abstract

The present invention relates to a steel material having excellent high temperature softening resistance and a method for manufacturing the same.
One embodiment of the present invention is by weight%, C: 0.15 to 0.35%, Si: 0.4 to 0.6%, Mn: 0.85 to 1.25%, Cr: 0.80 to 1.10%, Mo: 0.65 to 1.05%, P: 0.03% Below, S: 0.03% or less, the balance contains Fe and other unavoidable impurities, and the microstructure of the region from 0.05D to 0.95D from the surface is area%, ferrite: 12-18%, bainite: 80% or more, balance Provided are a steel material having excellent high temperature softening resistance containing martensite and a method for manufacturing the same.
(However, D is the diameter of the steel, and the unit is mm.)

Description

Steel material with excellent high temperature softening resistance and its manufacturing method

The present invention relates to a steel material having excellent high temperature softening resistance and a method for manufacturing the same.

In Japan, Europe, etc., high-rise buildings use fireproof steel (plate) with relatively low strength degradation at high temperatures. This is because, in the event of a fire, which is most important, the time that people can escape from the building can be greatly increased.

In Korea, KS registration for refractory steel is available, but it cannot be used because there is no performance design standard. However, since the activities to apply it are underway in Korea, there is a high possibility that a new market environment will be formed. When using a fire-resistant steel plate, the bolts that fasten it must also have fire-resistance properties. Japan (KOBE, NSSMC, JFE, etc.) has already had regulations on the use of refractory steel since the 1990s, so the development of refractory bolts has already progressed. High temperature softening resistance) is 0.30 to 0.35. Therefore, if the application of refractory steel is expanded in Korea, it is necessary to develop refractory bolts, and it is judged that KS registration for it is necessary.

Meanwhile, large-scale buildings for future cities are underway in the Middle East and Europe. One of the important items required for a super-large building is to reduce the weight of the building, and the strength of the fire-resisting structure serving as a skeleton is increased.

One of the effective methods for improving the softening resistance at high temperatures is to precipitate fine carbides. This is because high-temperature strength can be secured by pinning dislocations in ferrite grains by forming fine Mo2C and trace element carbonitride precipitates at high temperature through the addition of Cr, Mo and trace alloying elements Nb, V, etc. .

Examples of such techniques include Patent Documents 1 and 2. Patent Document 1 describes, in order to secure a yield strength of 337 to 348 MPa at 600 ° C, C: 0.15 to 0.30%, Mn: 0.30 to 0.60%, Cr: 1.0 to 2.0%, Mo: 0.30 to 0.80%, S: 0.015 % or less, P: 0.015% or less, Ti: using an alloy composition consisting of 0.10% or less, Patent Document 2, in order to secure a yield strength of 367-385 MPa level at 600 ° C, C: 0.15-0.25%, Si An alloy composition consisting of ≤ 0.2%, Mn: 0.1 to 0.7%, Cr: 0.2 to 1%, Mo: 0.3 to 0.5%, Al: 0.01 to 0.05%, and N: 0.007 to 0.03% is used. However, these technologies are not suitable for high-rise buildings because of their low yield strength at high temperature, and the development of steel materials having a novel alloy composition is required.

Japanese Laid-Open Patent Publication No. 1995-216506 Japanese Laid-Open Patent Publication No. 1998-102203

One aspect of the present invention is to provide a steel material having excellent high temperature softening resistance and a method for manufacturing the same.

One embodiment of the present invention is by weight%, C: 0.15 to 0.35%, Si: 0.4 to 0.6%, Mn: 0.85 to 1.25%, Cr: 0.80 to 1.10%, Mo: 0.65 to 1.05%, P: 0.03% Below, S: 0.03% or less, the balance contains Fe and other unavoidable impurities, and the microstructure of the region from 0.05D to 0.95D from the surface is area%, ferrite: 12-18%, bainite: 80% or more, balance To provide a steel having excellent resistance to high temperature softening containing martensite.

(However, D is the diameter of the steel, and the unit is mm.)

Another embodiment of the present invention is by weight%, C: 0.15 to 0.35%, Si: 0.4 to 0.6%, Mn: 0.85 to 1.25%, Cr: 0.80 to 1.10%, Mo: 0.65 to 1.05%, P: 0.03% Hereinafter, S: heating the billet containing 0.03% or less, the balance Fe and other unavoidable impurities at 1000 ~ 1100 ℃; obtaining a wire rod by finishing rolling the heated billet so that the exit temperature is 1000 to 1150°C; winding the wire rod at 830 to 880°C; cooling the wound wire rod to 300°C or less at a cooling rate of 2°C/s or less; spheroidizing annealing heat treatment of the cooled wire rod at 780 ~ 820 °C; drawing the heat-treated wire rod; cold forming the fresh wire rod; heating the cold-formed wire rod at 880 to 920° C. and then quenching; and tempering the quenched wire rod at 520 to 580 °C.

According to one aspect of the present invention, it is possible to provide a steel material excellent in high temperature softening resistance and a method for manufacturing the same.

1 is a microstructure photograph of Inventive Example 1 according to an embodiment of the present invention observed with an optical microscope.

Hereinafter, a steel material excellent in high temperature softening resistance according to an embodiment of the present invention will be described. First, the alloy composition of the present invention will be described. However, the unit of the alloy composition described below is considered as weight % unless otherwise specified.

C: 0.15~0.35%

C is a major element for securing strength. When the content of C is less than 0.15%, it is not easy to obtain sufficient strength, and it is not easy to secure sufficient hardenability after QT heat treatment. On the other hand, when it exceeds 0.35%, there is a high possibility that central segregation and coarse carbides are formed by the addition of high amounts of Cr and Mo. Accordingly, the content of C is preferably in the range of 0.15 to 0.35%. The lower limit of the C content is more preferably 0.18%, even more preferably 0.20%, and most preferably 0.22%. The upper limit of the C content is more preferably 0.32%, even more preferably 0.30%, and most preferably 0.28%.

Si: 0.4~0.6%

Si serves to increase the strength after QT through solid solution strengthening of ferrite. In addition, since Si exists at the interface between ferrite and carbide, it plays a role in suppressing C diffusion during heat treatment, and serves to suppress a decrease in strength during plating. When the Si content is less than 0.4%, it is difficult to sufficiently obtain the effect of increasing strength and suppressing strength decrease during plating, and when it exceeds 0.6%, it may be difficult to form a die during cold forging. Accordingly, the Si content is preferably in the range of 0.4 to 0.6%. The lower limit of the Si content is more preferably 0.43%, even more preferably 0.45%, and most preferably 0.48%. The upper limit of the Si content is more preferably 0.57%, even more preferably 0.55%, and most preferably 0.52%.

Mn: 0.85-1.25%

Mn not only serves to increase strength, but is also added for the purpose of securing hardenability by sufficiently delaying the transformation nose during QT heat treatment at the customer. When the content of Mn is less than 0.85%, it is difficult to obtain hardenability and target strength, and when it exceeds 1.25%, disconnection of processing may occur during drawing due to segregation. Accordingly, the Mn content is preferably in the range of 0.85 to 1.25%. The lower limit of the Mn content is more preferably 0.90%, even more preferably 0.95%, and most preferably 1.0%. The upper limit of the Mn content is more preferably 1.20%, even more preferably 1.15%, and most preferably 1.1%.

Cr: 0.80 to 1.10%

Cr is a major element that forms fine Cr-based carbides when maintained at a high temperature. However, according to the present invention, when only Cr among the carbide forming elements is added alone, it may be difficult to secure resistance to softening at high temperature. If the content of Cr is less than 0.80%, the yield strength at 600 ℃ may be lowered, and if it exceeds 1.10%, there is a possibility that central segregation and coarse carbides are formed, and there is a high possibility that a large amount of martensite is formed. Accordingly, the content of Cr is preferably in the range of 0.80 to 1.10%. The lower limit of the Cr content is more preferably 0.85%, more preferably 0.88%, and most preferably 0.90%. The upper limit of the Cr content is more preferably 1.05%, even more preferably 1.02%, and most preferably 1.00%.

Mo: 0.65~1.05%

Mo is expensive, but together with Cr, it is an element that greatly increases the high temperature softening resistance. As with Cr, when only Mo is added, it may be difficult to secure resistance to softening at high temperatures. If the content of Mo is less than 0.65%, the yield strength at 600 ℃ may be lowered, and if it exceeds 1.05%, there is a high possibility that a large amount of martensite is formed, and it is disadvantageous in terms of cost competitiveness. Accordingly, the Mo content is preferably in the range of 0.65 to 1.05%. The lower limit of the Mo content is more preferably 0.7%, more preferably 0.75%, and most preferably 0.80%. The upper limit of the Mo content is more preferably 0.95%, more preferably 0.90%, and most preferably 0.85%.

P: 0.03% or less

Since P plays a role in reducing the toughness of steel, it is dephosphorized in the secondary refining process. When the content of P exceeds 0.03%, FeP or the like is formed at the grain boundary, and the impact properties are deteriorated. It is preferable that it is 0.03% or less.

S: 0.03% or less

When S is contained in a large amount, it forms MnS inclusions at grain boundaries to reduce workability, so the S content is preferably 0.03% or less.

The remaining component of the steel material of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal steel manufacturing process, this cannot be excluded. Since these impurities are known to anyone skilled in the steel manufacturing process, all of them are not specifically mentioned in this specification.

In the steel material according to an embodiment of the present invention, the microstructure of the region from 0.05D to 0.95D from the surface is an area%, ferrite: 12 to 18%, bainite: 80% or more, it is preferable to include the remainder martensite . The D mentioned here is the diameter of the steel material, and the unit is mm. Since the ferrite has excellent ductility, it is an effective microstructure for wire drawing and forging workability. If the fraction of ferrite is less than 12%, disconnection may occur during processing, and if it exceeds 18%, it may be difficult to secure strength. The bainite is an effective microstructure for securing strength. When the fraction of the bainite is less than 80%, there is a disadvantage in that it is difficult to achieve the target strength.

In the steel according to an embodiment of the present invention, the number of carbides having a size of 20 to 100 nm is preferably 120 or more per ^{100 μm 2 .} When the number of carbides having a size of 20 to 100 nm is ^{less than 120 per 100 μm 2} , it may be difficult to secure strength and ductility. Meanwhile, the carbide may be at least one selected from the group consisting of (Fe,Cr,Mn) ₂₃ C ₆ , (Fe,Cr) ₇ C ₃ , and Mo _{6 C.}

The steel of the present invention provided as described above has tensile strength: 1000 MPa or more, yield strength: 900 MPa or more, elongation: 14% or more, and the ratio of room temperature yield strength and 600 ° C. yield strength: 0.47 or more. As such, the steel of the present invention can secure excellent strength, elongation, and resistance to high temperature softening. On the other hand, in order to impart fire-resistance properties to steel, the steel material must have fire-resistance properties or use a large amount of paint. The steel of the present invention has excellent fire-resistance properties. As it can be reduced to 3 levels, it can be expected to reduce the construction cost.

Hereinafter, a method for manufacturing a steel material having excellent high temperature softening resistance according to an embodiment of the present invention will be described.

First, the billet having the above-described alloy composition is heated at 1000 ~ 1100 ℃. When the billet heating temperature is less than 1000 ℃, there is a disadvantage that coarse carbide remains, and when it exceeds 1100 ℃, there is a disadvantage that the ductility of the wire rod decreases because the austenite grain size increases. Therefore, the billet heating temperature is preferably in the range of 1000 ~ 1100 ℃. The lower limit of the billet heating temperature is more preferably 1010°C, even more preferably 1030°C, and most preferably 1040°C. The upper limit of the billet heating temperature is more preferably 1090°C, even more preferably 1070°C, and most preferably 1060°C.

When the billet is heated, the heating time may be 90 to 120 minutes. If the heating time is less than 90 minutes, coarse carbides may remain, and if it exceeds 120 minutes, austenite grains may be coarsened. Therefore, the heating time is preferably 90 to 120 minutes. The lower limit of the billet heating time is more preferably 95 minutes, even more preferably 100 minutes, and most preferably 103 minutes. The upper limit of the billet heating time is more preferably 115 minutes, even more preferably 110 minutes, and most preferably 108 minutes.

Thereafter, the heated billet is finished-rolled so that the exit temperature is 1000 to 1150° C. to obtain a wire rod. When the exit temperature during the finishing rolling is less than 1000 ℃, there is a disadvantage that the replacement cycle of the rolling equipment increases due to the rolling load, and when it exceeds 1150 ℃, the billet heating temperature must be excessively increased or the rolling continuity must be excessively increased. There are disadvantages. Therefore, the exit temperature during the finishing rolling is preferably in the range of 1000 ~ 1150 ℃. The lower limit of the exit temperature during the finishing rolling is more preferably 1020°C, even more preferably 1040°C, and most preferably 1060°C. The upper limit of the exit temperature during the finishing rolling is more preferably 1130°C, even more preferably 1110°C, and most preferably 1090°C. On the other hand, in the present invention, in order to obtain a wire rod having a small diameter, it is necessary to increase the rolling continuity, and the rolling temperature may be similar to or higher than the billet heating temperature due to friction between the material and the rolling equipment.

Then, the wire rod is wound at 830 ~ 880 ℃. When the coiling temperature is less than 830 ℃, there is a disadvantage that the shape of the winding coil is poor, and when it exceeds 880 ℃, there is a disadvantage in that the weight loss is large due to an increase in the scale thickness. Therefore, the coiling temperature is preferably in the range of 830 ~ 880 ℃. The lower limit of the coiling temperature is more preferably 835°C, and even more preferably 840°C. The upper limit of the coiling temperature is more preferably 870°C, even more preferably 860°C, and most preferably 850°C.

Thereafter, the wound wire rod is cooled to 300° C. or less at a cooling rate of 2° C./s or less. When the cooling stop temperature exceeds 300° C., there is a disadvantage that a low-temperature tissue may be formed. Therefore, it is preferable that the cooling stop temperature has a range of 300° C. or less. The cooling stop temperature is more preferably 290 ℃ or less, more preferably 270 ℃ or less, and most preferably 260 ℃ or less. On the other hand, in the present invention, the lower limit of the cooling stop temperature is not particularly limited, but at a cooling stop temperature of less than 200 °C, the physical properties of the wire are not significantly affected, so the lower limit of the cooling stop temperature may be 200 °C. The lower limit of the cooling stop temperature is more preferably 210°C, even more preferably 230°C, and most preferably 240°C. When the cooling rate exceeds 2° C./s, there may be a disadvantage in that a low-temperature tissue is formed. Therefore, it is preferable that the cooling rate is 2° C./s or less. The cooling rate is more preferably 1.5°C/s or less, more preferably 1°C/s or less, and most preferably 0.8°C/s or less.

Thereafter, the cooled wire rod is subjected to a spheroidizing annealing heat treatment at 780 to 820°C. When the spheroidizing annealing heat treatment temperature is less than 780 ℃, there is a disadvantage in that the strength is not sufficiently spheroidized, and when it exceeds 820 ℃, the coarse grains are partially mixed to cause material deviation. Therefore, the heat treatment temperature is preferably in the range of 780 ~ 820 ℃. The lower limit of the spheroidizing annealing heat treatment temperature is more preferably 785°C, even more preferably 790°C, and most preferably 795°C. The upper limit of the spheroidizing annealing heat treatment temperature is more preferably 815°C, even more preferably 810°C, and most preferably 805°C.

During the spheroidizing annealing heat treatment, the heat treatment time may be 10 to 22 hours. If the heat treatment time is less than 10 hours, the spheroidization may not be sufficient, and if it exceeds 22 hours, the spheroidization is almost ineffective, and there is a disadvantage in that the heat treatment cost is greatly increased. Therefore, the heat treatment time is preferably 10 to 22 hours. The lower limit of the heat treatment time is more preferably 12 hours, still more preferably 14 hours, and most preferably 15 hours. The upper limit of the heat treatment time is more preferably 20 hours, still more preferably 18 hours, and most preferably 17 hours.

Thereafter, the heat-treated wire rod is fresh. The diameter of the cooled wire rod may be reduced to be applicable to a product to be finally obtained through the wire drawing. In the present invention, the wire drawing process is not particularly limited, and a wire drawing process used in the art may be applied.

Thereafter, the drawn wire rod is cold-formed. Through the cold forming process, the present invention can be processed into a product to be applied. In the present invention, the cold forming process is not particularly limited, and a cold forming process used in the art may be applied.

Thereafter, the cold-formed wire rod is heated at 880 to 920° C. and then quenched. At the time of quenching, if the heating temperature is less than 880° C., coarse carbides may be present. On the other hand, due to the limitation of the heat treatment facility, the upper limit of the heating temperature may be 920 °C. Therefore, during the quenching, the heating temperature is preferably in the range of 880 to 920 ℃. During the quenching, the lower limit of the heating temperature is more preferably 885°C, even more preferably 890°C, and most preferably 895°C. During the quenching, the upper limit of the heating temperature is more preferably 915°C, even more preferably 910°C, and most preferably 905°C.

Then, the quenched wire rod is tempered at 520 ~ 580 ℃. When the tempering temperature is less than 520° C., there is a disadvantage in that ductility is reduced, and when it exceeds 580° C., since the width of the strength reduction is large, there is a disadvantage in that the strength is lowered. Therefore, the tempering temperature is preferably in the range of 520 ~ 580 ℃. As for the lower limit of the said tempering temperature, it is more preferable that it is 525 degreeC, and it is still more preferable that it is 530 degreeC. The upper limit of the tempering temperature is more preferably 570°C, even more preferably 560°C, and most preferably 550°C.

Hereinafter, the present invention will be described in more detail through examples. However, the following examples are only examples for explaining the present invention in more detail, and do not limit the scope of the present invention.

(Example)

After continuously casting molten steel having the alloy composition shown in Table 1, the steel slab was rolled to prepare a billet having a size of ^{160 × 160 mm 2 .} The billet is heated at 1030°C for 95 minutes, then finish-rolled so that the exit temperature is 1045°C, wound at 845°C, cooled to 280°C at a cooling rate of 1.5°C/s, and a wire having a diameter of 17 mm was prepared. After measuring the tensile strength of the wire rod, it is shown in Table 1 below. Thereafter, the wire rod was subjected to spheroidizing annealing heat treatment at 805° C., followed by wire drawing and cold forming to produce bolts, and then the bolts were heated at 900° C. and quenched, followed by tempering at 540° C. After measuring the microstructure, number of carbides, and mechanical properties of the bolts prepared in this way, the results are shown in Table 2 below.

At this time, the microstructure was measured using an average value, assuming that the crystal grains were circular after securing a total of 10 sheets by using an optical microscope at a ×200-fold field of view, and the carbide fraction having a size of 20 to 100 nm was A test piece was prepared using a replica precipitate extraction method, and an arbitrary 100 μm ² area was measured through a transmission electron microscope. The mechanical properties of yield strength and tensile strength were measured through a tensile test. At this time, the cross head speed was maintained at 12 m/m when measuring the yield strength at 600 °C.

division Alloy composition (wt%) tensile strength
(MPa) C Si Mn Cr Mo P S Comparative Example 1 0.2 0.5 0.85 0 0 0.02 0.01 320 Comparative Example 2 0.2 0.5 0.85 0.8 0 0.02 0.01 610 Comparative Example 3 0.2 0.5 0.85 0 0.8 0.02 0.01 650 Invention Example 1 0.2 0.5 0.85 0.8 0.8 0.02 0.01 820 Invention Example 2 0.2 0.5 0.85 1.1 0.8 0.02 0.01 910 Comparative Example 4 0.2 0.5 0.85 1.4 0.8 0.02 0.01 970 Comparative Example 5 0.2 0.5 0.85 0.8 0.5 0.02 0.01 760 Invention example 3 0.2 0.5 0.85 0.8 One 0.02 0.01 890 Comparative Example 6 0.2 0.5 0.85 0.8 1.4 0.02 0.01 1010 Comparative Example 7 0.2 0.5 0.85 0.6 0.8 0.02 0.01 780

division Wire rod microstructure (area%)
fresh poetry
monorail
Whether tensile strength
(MPa) yield strength
(MPa) 600℃ yield strength
/room temperature yield strength Number of carbides with a size of 20 to 100 nm
(pcs/100㎛ ² ) blow
site perla
site Bay
Night marten
site Comparative Example 1 41 59 0 0 short line 760 660 0.18 0 Comparative Example 2 32 47 20 One short line 910 810 0.19 25 Comparative Example 3 30 44 25 One short line 930 830 0.29 32 Invention Example 1 18 0 80 2 short line 1120 1040 0.48 127 Invention Example 2 12 0 86 2 short line 1195 1095 0.55 152 Comparative Example 4 0 0 38 62 monorail Unmeasured due to disconnection 89 Comparative Example 5 28 19 52 One short line 1030 930 0.32 91 Invention example 3 13 0 85 2 short line 1180 1080 0.50 165 Comparative Example 6 0 0 22 78 monorail Unmeasured due to disconnection 90 Comparative Example 7 22 0 77 One broken line 990 870 0.32 78

As can be seen from Tables 1 and 2, in the case of Inventive Examples 1 to 3 that satisfy the alloy composition and manufacturing conditions proposed by the present invention, not only the microstructure of the present invention is satisfied, but also carbides having a size of 20 to 100 nm It can be seen that the number is also 120/100㎛ ^{2 or more.} For this reason, it can be seen that not only the tensile strength and yield strength are excellent, but also the ratio of the room temperature yield strength to the 600°C yield strength is 0.47 or more, which ensures excellent high temperature softening resistance.

Comparative Example 1 is a case in which Cr and Mo are not added, and Comparative Examples 2 and 3 is a case in which Mo and Cr are not added, respectively, and not only does not secure the microstructure to be obtained by the present invention, but also 20~ It can be seen that the number of carbides having a size of 100 nm is also ^{less than 120/100 μm 2 .} For this reason, it can be seen that the mechanical properties or high temperature softening resistance to be obtained by the present invention are not secured either.

Comparative Example 4 is a case in which the Cr content proposed by the present invention is exceeded, and not only does not secure the microstructure to be obtained by the present invention, but also the number of carbides having a size of 20 to 100 nm is also 120/100 μm ² It can be seen that less than In addition, as a large amount of martensite was formed, disconnection occurred during drawing.

Comparative Example 5 is a case that is less than the Mo content proposed by the present invention, and not only does not secure the microstructure to be obtained by the present invention, but also the number of carbides having a size of 20 to 100 nm is also 120 / 100 μm ² It can be seen that less than In addition, it can be seen that the high-temperature softening resistance is low due to the low yield strength at 600°C.

Comparative Example 6 is a case exceeding the Mo content proposed by the present invention, and not only does not secure the microstructure to be obtained by the present invention, but also the number of carbides having a size of 20 to 100 nm is also 120 /100㎛ ² It can be seen that less than In addition, as a large amount of martensite was formed, disconnection occurred during drawing.

Comparative Example 7 is a case that does not meet the Cr content suggested by the present invention, and not only does not secure the microstructure to be obtained by the present invention, but also the number of carbides having a size of 20 to 100 nm is also 120 /100㎛ ² It can be seen that less than In addition, it can be seen that the high-temperature softening resistance is low due to the low yield strength at 600°C.

1 is a microstructure photograph of Inventive Example 1 observed with an optical microscope. As can be seen from FIG. 1 , in the case of Inventive Example 1, it can be seen that ferrite, bainite, and martensite of appropriate fractions proposed by the present invention are formed.

Claims (7)

By weight%, C: 0.15 to 0.35%, Si: 0.4 to 0.6%, Mn: 0.85 to 1.25%, Cr: 0.80 to 1.10%, Mo: 0.65 to 1.05%, P: 0.03% or less, S: 0.03% or less , the balance Fe and other unavoidable impurities,
The microstructure of the region from 0.05D to 0.95D from the surface is area %, ferrite: 12 to 18%, bainite: 80% or more, and the remainder contains martensite,
Tensile strength: 1000 MPa or more, Yield strength: 900 MPa or more, Ratio of room temperature yield strength to 600°C yield strength: 0.47 or more Steel with excellent resistance to high temperature softening.
(However, D is the diameter of the steel, and the unit is mm.)
The method according to claim 1,
The steel is a steel with excellent high temperature softening resistance, wherein the number of carbides having a size of 20 to 100 nm is ^{120 or more per 100 μm 2 .}
3. The method according to claim 2,
The carbide is at least one selected from the group consisting of (Fe,Cr,Mn) ₂₃ C ₆ , (Fe,Cr) ₇ C ₃ , Mo ₆ C steel having excellent resistance to high temperature softening.
delete By weight%, C: 0.15 to 0.35%, Si: 0.4 to 0.6%, Mn: 0.85 to 1.25%, Cr: 0.80 to 1.10%, Mo: 0.65 to 1.05%, P: 0.03% or less, S: 0.03% or less , heating the billet containing the balance Fe and other unavoidable impurities at 1000 ~ 1100 ℃;
obtaining a wire rod by finishing rolling the heated billet so that the exit temperature is 1000 to 1150°C;
winding the wire rod at 830 to 880°C;
cooling the wound wire rod to 300°C or less at a cooling rate of 2°C/s or less;
spheroidizing annealing heat treatment of the cooled wire rod at 780 ~ 820 °C;
drawing the heat-treated wire rod;
cold forming the fresh wire rod;
heating the cold-formed wire rod at 880 to 920° C. and then quenching; and
A method of manufacturing a steel having excellent resistance to high temperature softening, comprising the step of tempering the quenched wire rod at 520 to 580°C.
6. The method of claim 5,
When the billet is heated, the heating time is 90 to 120 minutes.
6. The method of claim 5,
In the case of the spheroidizing annealing heat treatment, the heat treatment time is 10 to 22 hours, a method of manufacturing a steel having excellent resistance to high temperature softening.

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