KR101808440B1 - Steel for inverted angle and method of manufacturing the same - Google Patents

Abstract

Disclosed are steel for an inverted angle which is excellent in low temperature impact toughness and weldability; and a manufacturing method thereof. According to an embodiment of the present invention, the manufacturing method of the steel for the inverted angle comprises: a step of reheating a cast piece at a temperature of 1180-1220C; a step of hot-rolling the reheated cast piece at a rolling start temperature of 940-985C and a rolling finish temperature of 800-840C; and a step of rapidly cooling the hot-rolled steel at a cooling finish temperature of 740-780C, reheating the hot-rolled steel, and performing a quenching and self-tempering (QST) process. The cast piece is formed with 0.06-0.09 wt% of C, 0.10-0.20 wt% of Si, 1.1-1.3 wt% of Mn, 0.02-0.06 wt% of Al, greater than 0 and equal to or less than 0.012 wt% of P, greater than 0 and equal to or less than 0.003 wt% of S, 0.01-0.02 wt% of Nb, 0.007-0.017 wt% of Ti, 0.010-0.020 wt% of V, 0.15-0.25 wt% of Ni, greater than 0 and equal to or less than 0.005 wt% of N, and the remainder consisting of iron (Fe) and inevitable impurities.

Description

[0001] STEEL FOR INVERTED ANGLE AND METHOD OF MANUFACTURING THE SAME [0002]

More particularly, the present invention relates to a steel material and a method of manufacturing the steel material, and more particularly, to a steel material having high impact toughness and elongation at low temperatures and capable of controlling the yield strength to 440 MPa or less at the same time and having an inferred angle Steel material and a manufacturing method thereof.

Generally, steels for inverted angles are used as cargo tanks and secondary barriers of LPG transport vessels. In the case of such a steel for an inverted angle, a tensile strength (TS) of 440 MPa or more and an impact absorption energy value at -60 ° C with respect to the rolling direction of the base material should be an average of 41J or more.

In the case of such a steel for inverted angle, it is difficult to control the physical balance of tensile strength, yield strength and elongation due to its thin thickness, its long side and short side. In the conventional technique, a steel material whose balance of physical properties is not suitable can be a factor for increasing the defect occurrence rate.

Further, it is difficult to control the balance of physical properties of the steel for an inverted angle, and a method of separately controlling the alloy components according to the characteristics of each furnace is used in the electric furnace manufacturing process and the blast furnace manufacturing process.

A related prior art is Korean Patent Laid-Open Publication No. 10-1997-0010983 (published on Mar. 23, 1997).

An object of the present invention is to provide a steel for an inverted angle which can control the yield strength to 440 MPa or less at the same time while having low impact toughness and elongation at low temperature,

An embodiment of the present invention is a steel sheet comprising, by weight%, 0.06 to 0.09% of C, 0.10 to 0.20% of Si, 1.1 to 1.3% of Mn, 0.02 to 0.06% of Al, 0.001 to 0.03%, N: 0.01 to 0.02%, Ti: 0.007 to 0.017%, V: 0.010 to 0.020, Ni: 0.15 to 0.25% Reheating the slab composed of the impurities to 1180 to 1220 캜; Hot rolling the reheated cast slab at a rolling start temperature of 940 to 985 占 폚 and a rolling finish temperature of 800 to 840 占 폚; And performing quenching and self-tempering (QST) by accelerating and cooling the hot-rolled steel to a cooling termination temperature of 740 to 780 ° C, followed by thermal annealing.

The rolling finishing temperature and the cooling finishing temperature may be controlled so as to satisfy the following formula (1).

[Formula 1]

70 ° C <(T1 - T2) <110 ° C

In the above formula (1), T1 is the rolling finish temperature and T2 is the cooling finish temperature.

The hot rolling may be carried out at a rolling feed rate of 3.0 to 4.0 m / s.

The step of Quenching & Self-Tempering (QST) may control the cooling water to 160 to 250 m ³ / h.

The steel has a composite structure including a ferrite and a pealite, and the ferrite grains can be controlled to have a grain size of 9-11 탆.

Another embodiment of the present invention is a steel sheet comprising, by weight%, 0.06 to 0.09% of C, 0.10 to 0.20% of Si, 1.1 to 1.3% of Mn, 0.02 to 0.06% of Al, 0.001 to 0.03%, N: 0.01 to 0.02%, Ti: 0.007 to 0.017%, V: 0.010 to 0.020, Ni: 0.15 to 0.25% Characterized in that the final microstructure has a composite structure including ferrite and pealite and the grain size of the ferrite in the microstructure is 9 to 11 占 퐉. will be.

The steel may have a tensile strength (TS) of 440 to 590 MPa, a yield strength (YS) of 315 to 440 MPa, an elongation (EL) of 29 to 38% and an impact absorption energy of 300 to 345J at -60 캜.

INDUSTRIAL APPLICABILITY The present invention can provide a steel for an inverted angle which can control the yield strength to 440 MPa or less at the same time as the low temperature impact toughness and elongation rate,

FIG. 1 is a flowchart showing a method of manufacturing a steel material for an inverted angle according to an embodiment of the present invention.
Fig. 2 shows the final microstructure of the specimen according to Example 1. Fig.
Fig. 3 shows the final microstructure of the specimen according to Comparative Example 1. Fig.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention and the manner of achieving them will become apparent with reference to the embodiments described in detail below with reference to the accompanying drawings. The present invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Is provided to fully convey the scope of the invention to those skilled in the art, and the invention is only defined by the scope of the claims. Like reference numerals refer to like elements throughout the specification.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a steel material for an inverted angle according to a preferred embodiment of the present invention and a method of manufacturing the same will be described in detail with reference to the accompanying drawings.

Inverted  Steel for angle

The steel for an inverted angle according to the present invention has a tensile strength (TS) of 440 to 590 MPa, a yield strength (YS) of 315 to 440 MPa, an elongation (EL) of 29 to 38% To 345J.

For this purpose, the steel for an inverted angle according to the present invention preferably contains 0.06 to 0.09% of C, 0.10 to 0.20% of Si, 1.1 to 1.3% of Mn, 0.02 to 0.06% of Al, 0.004 to 0.017% of V, 0.010 to 0.020% of V, 0.15 to 0.25% of Ni, more than 0 to 0.005% of N, and 0.01 to 0.03% of N, Iron (Fe) and unavoidable impurities.

The steel may have a composite structure in which the final microstructure includes ferrite and pealite, and the crystal grains of the ferrite in the microstructure may be controlled to be 9 to 11 탆.

Hereinafter, the role and content of each component included in the steel for an inverted angle according to the present invention will be described.

Carbon (C)

In the present invention, carbon (C) is added to secure strength.

The carbon (C) is preferably added at a content ratio of 0.06 to 0.09% by weight based on the total weight of the steel for an inverted angle according to the present invention. When the content of carbon (C) is less than 0.06% by weight, it may be difficult to secure strength. On the other hand, when the content of carbon (C) exceeds 0.09% by weight, impact toughness of the base material is lowered and weldability is lowered.

silicon( Si )

In the present invention, silicon (Si) is added as a deoxidizer to remove oxygen in the steel in the steelmaking process. Silicon (Si) also has a solid solution strengthening effect.

It is preferable that silicon (Si) is added at a content ratio of 0.10 to 0.20% by weight based on the total weight of the steel for an inverted angle according to the present invention. When the content of silicon (Si) is less than 0.10 wt%, the effect of adding silicon can not be exhibited properly. On the contrary, when the content of silicon (Si) exceeds 0.20% by weight, surface defects may be generated or weldability may be lowered.

Manganese (Mn)

Manganese (Mn) is an element which increases the strength and toughness of steel and increases the ingotability of steel. Addition of manganese (Mn) causes less deterioration of ductility when the strength is higher than that of carbon (C).

The manganese (Mn) is preferably added at a content ratio of 1.1 to 1.3% by weight based on the total weight of the steel for an inverted angle according to the present invention. If the content of manganese (Mn) is less than 1.1% by weight, it may be difficult to secure strength even if the content of carbon (C) is high. On the other hand, when the content of manganese (Mn) exceeds 1.3% by weight, the amount of MnS-based nonmetal inclusions increases, which may lead to defects such as cracking during welding.

Aluminum (Al)

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

The aluminum (Al) is preferably added at a content ratio of 0.02 to 0.06% by weight based on the total weight of the steel for the inverted angle according to the present invention. When the content of aluminum (Al) is less than 0.02% by weight, the above deoxidation effect can not be exhibited properly. On the other hand, when the content of aluminum (Al) exceeds 0.06% by weight, it is difficult to perform, which lowers the productivity and lowers the impact resistance at low temperature by forming nonmetallic inclusions such as Al ₂ O ₃ have.

Phosphorus (P), sulfur (S)

Phosphorus (P) is an impurity which is inevitably contained at the time of production, and is contained in the steel to deteriorate the weldability and toughness, and segregates at the center of the bloom and at the austenite grain boundary during solidification. Therefore, in the present invention, the content of phosphorus (P) is limited to more than 0 to 0.012 wt% of the total weight of the steel for inverted angle.

Sulfur (S) is an element that is inevitably contained in the manufacture of steel together with phosphorus (P), and reacts with manganese (Mn) to form MnS, thereby lowering impact toughness at low temperatures. Therefore, in the present invention, the content of sulfur (S) is limited to more than 0 to 0.003 wt% of the total weight of the steel for inverted angle.

In particular, when the total content of phosphorus (P) and sulfur (S) exceeds 0.015% by weight, there is a problem that the low-temperature impact toughness deteriorates rapidly. The total content of phosphorus and sulfur is 0.015% Or more.

Niobium ( Nb )

Niobium (Nb) combines with carbon (C) and nitrogen (N) at high temperatures to form carbides or nitrides. Niobium-based carbides or nitrides improve the strength and low-temperature toughness of the steel by suppressing grain growth during rolling and refining the crystal grains.

The niobium (Nb) is preferably added in an amount of 0.01 to 0.02% by weight based on the total weight of the steel for the inverted angle according to the present invention. If the content of niobium (Nb) is less than 0.01% by weight, it may be difficult to exhibit the above-mentioned effects properly. On the contrary, when the content of niobium (Nb) exceeds 0.02% by weight, excessive precipitation may cause deterioration in performance, rolling property and elongation.

titanium( Ti )

Titanium (Ti) is an element which forms a nitride, and is effective for improving low-temperature toughness by fine precipitation of TiN.

The titanium (Ti) is preferably added at a content ratio of 0.007 to 0.017% by weight of the total weight of the steel for an inverted angle according to the present invention. When the content of titanium (Ti) is less than 0.007 wt%, the effect of the addition can not be exhibited properly. On the contrary, when the content of titanium (Ti) is more than 0.017 wt% and added in a large amount, the toughness is deteriorated.

Vanadium (V)

Vanadium (V) combines with carbon (C) and nitrogen (N) at high temperatures to form carbides or nitrides. In addition, vanadium (V) plays a role in improving the strength of steel through precipitation strengthening effect by precipitate formation.

The vanadium (V) is preferably added in an amount of 0.010 to 0.020 wt% of the total weight of the steel for the inverted angle according to the present invention. When the content of vanadium (V) is less than 0.010 wt%, the addition effect can not be exhibited properly. On the contrary, when the content of vanadium (V) is more than 0.020 wt% and added in a large amount, the low-temperature impact toughness may be lowered.

The weight ratio of niobium (Nb): titanium (Ti): vanadium (V) in the alloy composition may be controlled to 0.5 to 2: 1: 0.5 to 2. In this case, even when applied to both the electric furnace manufacturing process and the blast furnace manufacturing process, the uniformity of the material can be improved and the variation in physical properties can be reduced.

nickel( Ni )

Nickel (Ni) fine grains and solidify in the austenite and ferrite to strengthen the matrix. In particular, nickel (Ni) is an effective element for improving the low-temperature impact toughness.

Nickel (Ni) is preferably added at a content ratio of 0.15 to 0.25% by weight based on the total weight of the steel for an inverted angle according to the present invention. If the content of nickel (Ni) is less than 0.15% by weight, the effect of adding nickel can not be exhibited properly. On the other hand, when a large amount of nickel (Ni) is added in an amount exceeding 0.25% by weight based on the total weight, there arises a problem of inducing the red hot brittleness.

Nitrogen (N)

Nitrogen (N) is an element which forms a nitride, and is effective for improving low temperature toughness by fine precipitation of TiN.

However, when the content of nitrogen (N) exceeds 0.005% by weight, impact toughness and weldability are deteriorated. Therefore, in the present invention, the content of nitrogen (N) is limited to 0 to 0.005 wt% or less of the total weight of the steel for inverted angle.

Inverted  Manufacturing method of steel for angle

FIG. 1 is a flowchart showing a method of manufacturing a steel material for an inverted angle according to an embodiment of the present invention.

Referring to FIG. 1, the method for manufacturing an inverted angle steel according to an embodiment of the present invention includes a reheating step (S110), a hot rolling step (S120), and a QST cooling step (S130). At this time, the reheating step (S110) is not necessarily performed, but it is more preferable to perform the bloom reheating step (S110) in order to obtain effects such as the reuse of the precipitate.

In the method for producing steel for an inverted angle according to the present invention, the semi-finished product of the cast steel which is subject to the hot rolling process contains 0.06 to 0.09% of C, 0.10 to 0.20% of Si, 1.1 to 1.3% of Mn, P: more than 0 to 0.012%, S: more than 0 to 0.003%, Nb: 0.01 to 0.02%, Ti: 0.007 to 0.017%, V: 0.010 to 0.020, Ni: 0.15 to 0.25% N: more than 0 to 0.005% and the balance of iron (Fe) and unavoidable impurities. At this time, it is appropriate to use a bloom as the cast.

Reheating

In the reheating step (S110), the cast steel having the above composition is reheated at 1180 to 1220 ° C for 2 to 3 hours. At this time, the cast steel can be obtained through a continuous casting process after obtaining a molten steel having a desired composition through a steelmaking process. By reheating the cast steel, the segregated components can be reused.

In this step, when the reheating temperature is less than 1180 DEG C, there is a problem that the segregated components are not sufficiently reused during casting. On the contrary, when the reheating temperature is higher than 1220 ° C, the austenite grain size increases and the strength can not be ensured, and the manufacturing cost of the steel can be increased due to the excessive heating process.

Hot rolling

In the hot rolling step (S120), the reheated cast steel is hot-rolled at a rolling start temperature of 940 to 985 ° C and a rolling finish temperature of 800 to 840 ° C. By performing the hot rolling under the conditions of the rolling start temperature and the rolling finish temperature, the yield strength can be lowered and the elongation can be improved while ensuring strength and low temperature impact toughness.

When the rolling starting temperature is relatively 940 to 985 ° C during hot rolling, the yield strength can be controlled to 315 to 440 MPa while preventing the reduction of the rolling load during hot rolling.

In this step, when the rolling starting temperature is lower than 940 ° C, the rolling load may be lowered when the steel is applied to a thin steel material such as an inverted angle steel material. On the contrary, when the rolling starting temperature exceeds 985 DEG C, it may be difficult to secure the aimed elongation.

When the finish rolling temperature is relatively high at 800 to 840 캜 during hot rolling, the crystal grains of the ferrite in the final microstructure can be controlled to 9 to 11 탆.

If the finish rolling temperature (FRT) is less than 800 DEG C in this step, abnormal reverse rolling may occur and the ductility may be lowered. On the other hand, when the finishing rolling temperature (FRT) exceeds 840 DEG C, it may be difficult to obtain the desired strength.

At this time, universal rolling may be used for hot rolling, but is not limited thereto. By such universal rolling, it can be rolled into a specific shape such as 'L' or 'I'. At this time, the universal rolling is performed in such a manner that the web and the flange of the steel are pressed in the up, down, left and right directions. That is, the universal rolling can be performed in such a manner that the universal rolling is pressed at a constant speed along a universal stand having a horizontal roll for pressing the steel web and a vertical roll for pressing the steel flange.

More preferably, the hot rolling is performed at a rolling conveying speed of 3.0 to 4.0 m / s. If the rolling speed is less than 3.0 m / sec, the effect of refining the ferrite grain by the austenite recrystallization is insufficient and affects the behavior of the precipitate which has an adverse effect on the toughness of the weld heat affected zone. On the contrary, when the rolling speed exceeds 4.0 m / sec, it can not be economical because it can increase the production cost without increasing the effect.

Cooling

In the cooling step (S130), the hot-rolled steel is accelerated and cooled, and then quenching and self-tempering (QST) is performed.

When cooling QST, it is desirable to control the cooling water to 160 to 250 m ³ / h in the quenching section. If the cooling water content is less than 160 m ³ / h, the cooling may be insufficient and it may be difficult to secure the strength and impact toughness of the target steel. On the other hand, when the cooling water exceeds 250 m ³ / h, there is a problem that the elongation and the low-temperature impact toughness are lowered by supercooling.

After QST cooling is finished, the temperature of the surface of the steel rises due to the double thermal effect due to the temperature difference between the inside and the surface. At this time, the finishing cooling temperature (FCT) corresponding to the double heat temperature may be 740 to 780 캜 at the cooling end temperature.

The rolling finishing temperature and the cooling finishing temperature may be controlled so as to satisfy the following formula (1).

[Formula 1]

70 ° C <(T1 - T2) <110 ° C

In the above formula (1), T1 is the rolling finish temperature and T2 is the cooling finish temperature.

When the rolling start temperature and the cooling end temperature are controlled so as to satisfy the above-mentioned formula 1, the effect of lowering the yield strength and improving the elongation at the same time, while securing the strength and impact resistance at low temperature, is more excellent.

The steel composition for the inverted angle manufactured by the above-described alloy composition and the above-described processes (S110 to S130) has an effect of reducing the incidence of defects even when applied to both the electric furnace manufacturing process and the blast furnace manufacturing process. In this case, irrespective of the type of furnace used, it is possible to provide a steel material for an inverted angle which is excellent in strength and low temperature impact toughness while lowering the yield strength and improving the elongation at the same time. The incidence can be lowered.

As a result, the steel for an inverted angle manufactured by the method according to the present invention has a composite structure in which the final microstructure includes ferrite and pealite, wherein the ferrite has a grain size of 9-11 탆 (YS) of 315 to 440 MPa, an elongation (EL) of 29 to 38%, and an impact absorption energy of -30 to 345 J at -60 캜 by controlling the tensile strength (TS) of 440 to 590 MPa, the yield strength

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. Specimen Manufacturing

The specimens according to Examples 1 to 6 and Comparative Examples 1 to 6 were prepared by preparing casts having the alloy compositions shown in Table 1, respectively, and controlling them under the process conditions shown in Table 2, respectively. Thereafter, tensile tests and impact toughness tests were carried out on the specimens according to Examples 1 to 6 and Comparative Examples 1 to 6.

C Si Mn P S Al Ni Nb V Ti N 0.09 0.20 1.3 0.012 0.003 0.06 0.25 0.20 0.02 0.17 0.005

(Unit: wt%)

Reheating temperature (℃) Rolling start temperature (캜) Rolling end temperature (캜) Cooling end temperature (캜) (T1-T2) Feeding speed
(m / s) Cooling water
(m ³ / h) Example 1 1180 ~ 1220 945-985 840 760 80 3.0 to 4.0 160 to 250 Example 2 1180 ~ 1220 945-985 845 762 83 3.0 to 4.0 160 to 250 Example 3 1180 ~ 1220 945-985 835 755 80 3.0 to 4.0 160 to 250 Example 4 1180 ~ 1220 945-985 832 748 84 3.0 to 4.0 160 to 250 Example 5 1180 ~ 1220 945-985 850 750 105 3.0 to 4.0 160 to 250 Example 6 1180 ~ 1220 945-985 845 747 98 3.0 to 4.0 160 to 250 Comparative Example 1 1180 ~ 1220 945-985 847 712 135 3.0 to 4.0 250-300 Comparative Example 2 1180 ~ 1220 945-985 860 708 152 3.0 to 4.0 250-300 Comparative Example 3 1180 ~ 1220 945-985 827 707 120 3.0 to 4.0 250-300 Comparative Example 4 1180 ~ 1220 945-985 830 703 127 3.0 to 4.0 250-300 Comparative Example 5 1180 ~ 1220 945-985 880 695 185 3.0 to 4.0 250-300 Comparative Example 6 1180 ~ 1220 945-985 876 700 176 3.0 to 4.0 250-300

In Table 2, (T1-T2) is a value derived by the following equation (1).

[Formula 1]

70 ° C <(T1 - T2) <110 ° C

In the above formula (1), T1 is the rolling finish temperature and T2 is the cooling finish temperature.

2. Evaluation of mechanical properties

Table 3 shows the results of evaluation of mechanical properties of the specimens prepared according to Examples 1 to 6 and Comparative Examples 1 to 6. The impact toughness (impact absorption energy) is an average value of six samples (three rolling directions and three vertical directions) for each of the specimens according to Examples 1 to 6 and Comparative Examples 1 to 6.

TS (MPa) YS (MPa) EL (%) -60 ° C Impact Toughness (J) Example 1 474 408 32 312 Example 2 478 407 34 320 Example 3 485 411 35 322 Example 4 491 416 29 328 Example 5 480 414 31 310 Example 6 484 416 30 303 Comparative Example 1 497 440 25 310 Comparative Example 2 502 443 25 321 Comparative Example 3 495 444 34 324 Comparative Example 4 491 447 33 330 Comparative Example 5 509 452 25 302 Comparative Example 6 511 449 26 313

Tensile Strength (TS): 440 to 590 MPa, Yielding Strength (YS): 315 to 440 MPa, and Elongation (EL), which correspond to the target values, according to Tables 1 to 3, 29 to 38%, and the impact absorption energy at -60 DEG C: 300 to 345J.

On the other hand, in the case of the specimens according to Comparative Examples 1 to 6, it can be seen that the yield strength (YS) exceeds 440 MPa or the elongation (EL) is less than 34% even when the cast steel having the same alloy composition is used. The specimens of Comparative Examples 1 to 6 have a yield strength (YS) exceeding 440 MPa, which can increase the defect occurrence rate in the process, and it is difficult to lower the yield strength and increase the elongation (EL) to 34% or more.

Fig. 2 shows the final microstructure of the specimen according to Example 1, and Fig. 3 shows the final microstructure of the specimen according to Comparative Example 1. Fig.

Referring to FIG. 2 and FIG. 3, it can be confirmed that the final microstructure of the test piece according to Example 1 includes ferrite and pearlite, and the ferrite grain size is controlled to 10.59 μm. On the other hand, the specimen of Comparative Example 1 shows that the final microstructure is composed of bainite and needle-like ferrite.

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.

S110: Reheating step
S120: Hot rolling step
S130: QST cooling step

Claims (8)

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.06 to 0.09% of C, 0.10 to 0.20% of Si, 1.1 to 1.3% of Mn, 0.02 to 0.06% of Al, (Fe) and unavoidable impurities are contained in an amount of 0.01 to 0.02% of Nb, 0.007 to 0.017% of Ti, 0.010 to 0.020 of V, 0.15 to 0.25% of Ni, Reheating to ~ 1220 ° C;
Hot rolling the reheated cast slab at a rolling start temperature of 940 to 985 占 폚 and a rolling finish temperature of 800 to 840 占 폚; And
Cooling the hot-rolled steel to a cooling end temperature of 740 to 780 DEG C, and performing quenching and self-tempering (QST)
The hot rolling was carried out at a rolling conveying speed of 3.0 to 4.0 m / s,
The quenching and self-tempering (QST) step controls the cooling water to 160 to 250 m ³ / h,
Wherein the rolling finish temperature and the cooling finish temperature are controlled so as to satisfy the following formula (1): &quot; (1) &quot;
[Formula 1]
70 ° C <(T1 - T2) <110 ° C
In the above formula (1), T1 is the rolling finish temperature and T2 is the cooling finish temperature. delete delete delete The method according to claim 1,
Wherein the steel has a composite structure in which a final microstructure includes ferrite and pealite and the ferrite grain size of the microstructure is controlled to be 9 to 11 占 퐉. Way. The method according to claim 1,
Characterized in that the steel has a tensile strength (TS) of 440 to 590 MPa, a yield strength (YS) of 315 to 440 MPa, an elongation (EL) of 29 to 38% and an impact absorption energy of 300 to 345 J at -60 캜 (EN) METHOD FOR PRODUCING STEEL MATERIAL FOR INVERTED ANGLE.

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