KR102495181B1 - High strength hot rolled steel sheet for steel pipe having excellent impact toughness and method of manufacturing the same - Google Patents

Abstract

The present invention provides a high-strength hot-rolled steel sheet for steel pipes having excellent impact toughness and a manufacturing method thereof. According to one embodiment of the present invention, the high-strength hot-rolled steel sheet for steel pipe, in weight%, carbon (C): 0.11% ~ 0.13%, silicon (Si): 0.10% ~ 0.30%, manganese (Mn): 1.0% ~ 1.3%, Aluminum (Al): 0.01% ~ 0.06%, Niobium (Nb): 0.045% ~ 0.055%, Phosphorus (P): >0% ~ 0.03%, Sulfur (S): >0% ~ 0.003%, And the balance includes iron (Fe) and other unavoidable impurities, yield strength (YS): 379 MPa ~ 552 MPa, tensile strength (TS): 517 MPa or more, elongation (EL): 25% or more, and at 0 ℃ Charpy impact absorption energy of: 200 J or more is satisfied.

Description

High strength hot rolled steel sheet for steel pipe having excellent impact toughness and method of manufacturing the same}

The technical idea of the present invention relates to a steel sheet and a manufacturing method thereof, and more particularly, to a high-strength hot-rolled steel sheet for steel pipes having excellent impact toughness, a steel pipe manufactured using the same, and a manufacturing method thereof.

As an example of a hot-rolled steel sheet used as a material for oil wells for crude oil and gas mining, the API-J55 standard is used as a casing. Conventionally, seamless steel pipes were mainly used, but as technology is developed, the use of ERW welded steel pipes, which are superior in price competitiveness, is increasing. In API (American Petroleum Institute) standards, toughness is evaluated through CVN (charpy V-notch) and DWTT (Drop Weight Tear Test) methods based on 0℃, but in the case of oil pipelines actually used in extreme regions, 0℃ Since it is mainly used below, it is necessary to satisfy the low-temperature toughness characteristics under harsher conditions than the standard in consideration of stability.

In general, polygonal ferrite and pearlite structures are known to be vulnerable to low-temperature toughness. Therefore, in order to satisfy the low-temperature toughness of oil well tube materials, it is important to refine crystal grains by adding fine alloy elements such as niobium (Nb) and to suppress pearlite formation. do. However, the thicker the hot-rolled steel sheet is, the lower the low-temperature toughness is because it is difficult to control the microstructure in the center of the thickness when produced under the same conditions.

Korean Patent Application No. 10-2010-0061580

A technical problem to be achieved by the technical spirit of the present invention is to provide a high-strength hot-rolled steel sheet for steel pipes having excellent impact toughness, a steel pipe manufactured using the same, and a manufacturing method thereof.

However, these tasks are exemplary, and the technical spirit of the present invention is not limited thereto.

According to one aspect of the present invention, there is provided a high-strength hot-rolled steel sheet for steel pipes having excellent impact toughness and a manufacturing method thereof.

According to one embodiment of the present invention, the high-strength hot-rolled steel sheet for steel pipe, in weight%, carbon (C): 0.11% ~ 0.13%, silicon (Si): 0.10% ~ 0.30%, manganese (Mn): 1.0% ~ 1.3%, Aluminum (Al): 0.01% ~ 0.06%, Niobium (Nb): 0.045% ~ 0.055%, Phosphorus (P): >0% ~ 0.03%, Sulfur (S): >0% ~ 0.003%, and the balance includes iron (Fe) and other unavoidable impurities, and may satisfy Charpy impact absorption energy at 0°C: 200 J or more.

According to one embodiment of the present invention, the high-strength hot-rolled steel sheet for steel pipe, yield strength (YS): 379 MPa ~ 552 MPa, tensile strength (TS): 517 MPa or more, and elongation (EL): 25% or more to be satisfied can

According to one embodiment of the present invention, the high-strength hot-rolled steel sheet for steel pipe may have a mixed structure in which acicular ferrite and pearlite are mixed.

According to one embodiment of the present invention, the fraction of the acicular ferrite may be 20% to 40%, and the fraction of the pearlite may be 60% to 80%.

According to one embodiment of the present invention, the manufacturing method of the high-strength hot-rolled steel sheet for steel pipe, in weight%, carbon (C): 0.11% ~ 0.13%, silicon (Si): 0.10% ~ 0.30%, manganese (Mn) : 1.0% ~ 1.3%, Aluminum (Al): 0.01% ~ 0.06%, Niobium (Nb): 0.045% ~ 0.055%, Phosphorus (P): More than 0% ~ 0.03%, Sulfur (S): More than 0% ~ Reheating a steel material containing 0.003% and the balance of iron (Fe) and other unavoidable impurities at a reheating temperature of 1,180 ° C to 1,220 ° C; Hot rolling the heated steel material to end at a finish rolling end temperature of 800 ° C to 900 ° C; Cooling the hot-rolled steel material to a cooling end temperature of 580 ° C to 630 ° C; and winding the cooled steel material at a coiling temperature of 580°C to 630°C.

According to an embodiment of the present invention, the cooling may be performed at a cooling rate of 10 °C/sec to 20 °C/sec.

According to one embodiment of the present invention, the cooling may be performed at a cooling rate of 15 ° C / sec to 20 ° C / sec.

According to one embodiment of the present invention, the hot-rolled steel sheet manufactured by the method for manufacturing a high-strength hot-rolled steel sheet for steel pipe has a yield strength (YS): 379 MPa to 552 MPa, a tensile strength (TS): 517 MPa or more, an elongation ( EL): satisfies 25% or more, and Charpy impact absorption energy at 0 ° C: 200 J or more, has a mixed structure in which acicular ferrite and pearlite are mixed, the fraction of acicular ferrite is 20% to 40%, , The fraction of the pearlite may be 60% to 80%.

According to the technical concept of the present invention, high-strength hot-rolled steel sheet for steel pipe that secures low-temperature toughness of 0 ° C or less in the thick material (15t) in the target range is manufactured through control of composition content and control of manufacturing process conditions can do. To this end, by lowering the carbon content and increasing the niobium content, it is possible to secure the low-temperature impact toughness of the API standard or higher by satisfying the Charpy impact absorption energy at 0 ° C: 200 J or more.

The effects of the present invention described above have been described by way of example, and the scope of the present invention is not limited by these effects.

1 is a process flow chart schematically showing a manufacturing method of a high-strength hot-rolled steel sheet for steel pipe according to an embodiment of the present invention.
2 is scanning electron micrographs comparing the microstructure of a high-strength hot-rolled steel sheet for steel pipe according to an embodiment of the present invention with a comparative example.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The embodiments of the present invention are provided to more completely explain the technical idea of the present invention to those skilled in the art, and the following examples may be modified in many different forms, The scope of the technical idea is not limited to the following examples. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the spirit of the invention to those skilled in the art. Like reference numerals throughout this specification mean like elements. Further, various elements and areas in the drawings are schematically drawn. Therefore, the technical spirit of the present invention is not limited by the relative size or spacing drawn in the accompanying drawings.

The present invention provides a high-strength hot-rolled steel sheet for steel pipes having excellent impact toughness and a manufacturing method thereof.

In the case of conventional hot-rolled steel sheets for oil wells, there is a problem due to the high C content to have high impact toughness. In the present invention, the influence of components for impact assurance was analyzed, and the behavior in the ROT (Run Out Table, cooling table) and the resulting material were evaluated. In particular, the impact toughness is affected by the microstructure, and the manufacturing conditions that satisfy both the impact toughness and the material were secured by changing the carbon content, the niobium content, and the manufacturing conditions.

In the present invention, manufacturing conditions for producing API-J55 (standard, yield strength (YS) of 379 MPa to 552 MPa, tensile strength (TS) of 517 MPa or more), which is a conventional oil well pipe material for casing, for high impact guarantee in hot-rolled steel sheet. to secure

High-strength hot-rolled steel sheet for steel pipe with excellent impact toughness

Hereinafter, a high-strength hot-rolled steel sheet for steel pipe having excellent impact toughness, which is one aspect of the present invention, will be described.

In the high-strength hot-rolled steel sheet for steel pipe, which is an aspect of the present invention, in weight%, carbon (C): 0.11% to 0.13%, silicon (Si): 0.10% to 0.30%, manganese (Mn): 1.0% to 1.3%, Aluminum (Al): 0.01% to 0.06%, Niobium (Nb): 0.045% to 0.055%, Phosphorus (P): greater than 0% to 0.03%, Sulfur (S): greater than 0% to 0.003%, balance iron (Fe) and other unavoidable impurities.

Hereinafter, the role and content of each component included in the high-strength hot-rolled steel sheet for steel pipe according to the present invention will be described. At this time, the contents of the component elements all mean weight% with respect to the entire steel sheet.

Carbon (C): 0.11% to 0.13%

Carbon is added to secure strength and control the microstructure. If the carbon content is less than 0.11%, it may be difficult to secure strength. When the carbon content exceeds 0.13%, weldability is deteriorated and the fraction of the pearlite phase is excessively high, making it difficult to control the desired microstructure. Therefore, carbon is preferably added in an amount of 0.11% to 0.13% of the total weight of the hot-rolled steel sheet.

Silicon (Si): 0.10% to 0.30%

Silicon, as a ferrite stabilizing element, increases the degree of supercooling during ferrite transformation to make crystal grains fine and suppresses the formation of carbides. However, when a large amount is added, the weldability of the steel is reduced, and red scale is generated during the hot rolling process and the reheating process during the hot rolling process, which can cause problems with the surface quality and impair plating properties after welding. In general, when 1.2% by weight or more of manganese is added, it is preferable that the Mn/Si ratio is in the range of 5 to 10 during ERW welding for manufacturing steel pipes. This is because Mn-Si-O oxide (Mn ₂ SiO ₄ or MnSiO ₃ ) generated during welding is formed in the region and has the lowest melting temperature, thereby facilitating the discharge of oxide during welding. When the content of silicon is less than 0.10%, the effect of adding silicon may be insufficient. When the content of silicon exceeds 0.30%, red scale is caused to deteriorate the surface quality, and cracks at the welded part may occur. Therefore, silicon is preferably added in an amount of 0.10% to 0.30% of the total weight of the hot-rolled steel sheet.

Manganese (Mn): 1.0% to 1.3%

As an austenite stabilizing element, manganese is very effective in solid solution hardening and has a great effect on increasing hardenability of steel. When manganese is added, the equilibrium temperature of the steel decreases, resulting in a decrease in ferrite, an increase in pearlite, and a decrease in the lamellar spacing of pearlite. As such, the strength, toughness, and yield ratio can be controlled according to the manganese content, but when a large amount is added, MnS inclusions are formed and central segregation occurs during casting, thereby reducing the toughness of the steel. When the content of manganese is less than 1.0%, the effect of adding manganese may be insufficient. When the content of manganese exceeds 1.3%, a large amount of MnS is formed in combination with S, and thus corrosion resistance and low-temperature toughness may be deteriorated. Therefore, manganese is preferably added in an amount of 1.0% to 1.3% of the total weight of the hot-rolled steel sheet.

Aluminum (Al): 0.01% to 0.06%

Aluminum is added for deoxidation during steelmaking. When the aluminum content is less than 0.01%, sufficient deoxidation effect cannot be obtained. When the content of aluminum exceeds 0.06%, it combines with N present in the steel to form coarse AlN-based nitride, which may impair weldability. Therefore, aluminum is preferably added in an amount of 0.01% to 0.06% of the total weight of the hot-rolled steel sheet. In addition, it is preferable to add at 0.01% to 0.03%

Niobium (Nb): 0.045% to 0.055%

Niobium delays recrystallization during hot rolling, that is, since the non-recrystallized region is enlarged, crystal grain refinement can be achieved. Niobium employed during hot rolling is known to retard the nucleation and growth of recrystallization, and since this retardation of recrystallization does not consume defect sites such as dislocations, nucleation is promoted during phase transformation to make crystal grains fine. In addition, since the carbide precipitated by strain induced serves as a nucleation site of ferrite during phase transformation, the phase transformation can be accelerated and crystal grains can be refined. When the content of niobium is less than 0.045%, the effect of adding niobium may be insufficient. When the content of niobium exceeds 0.055%, the yield strength and yield ratio increase and the elongation decreases, thereby degrading workability. Therefore, niobium is preferably added in an amount of 0.045% to 0.055% of the total weight of the hot-rolled steel sheet.

Phosphorus (P): greater than 0% to 0.03%

Phosphorus has the effect of suppressing the formation of cementite and increasing its strength. When the phosphorus content exceeds 0.03% by weight, weldability deteriorates and toughness and weldability of the steel may be deteriorated due to slab center segregation. In the present invention, phosphorus is not intentionally added and may be contained as an unavoidable impurity. Therefore, it is preferable to limit the phosphorus content to 0.03% or less of the total weight of the hot-rolled steel sheet.

Sulfur (S): greater than 0% to 0.003%

Sulfur inhibits toughness and weldability of steel, and may generate cracks during processing of steel by forming MnS in combination with manganese. In the present invention, sulfur is not intentionally added and may be contained as an unavoidable impurity. Therefore, it is preferable to limit the sulfur content to 0.003% or less of the total weight of the hot-rolled steel sheet.

Nitrogen (N): >0% to 0.006%

Nitrogen combines with niobium, etc. to form carbonitride to refine crystal grains, but when added in large amounts, dissolved nitrogen increases, reducing the impact properties and elongation of steel and greatly impairing the toughness of welded joints. In the present invention, nitrogen is not intentionally added and may be contained as an unavoidable impurity. Therefore, it is preferable to limit the nitrogen content to 0.006% or less of the total weight of the hot-rolled steel sheet.

The remaining components of the high-strength hot-rolled steel sheet for steel pipes are iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal manufacturing process, this cannot be excluded. Since these impurities are known to anyone skilled in the ordinary manufacturing process, not all of them are specifically mentioned in this specification.

The high-strength hot-rolled steel sheet for steel pipe manufactured by controlling the specific components and content ranges of the above-described alloy composition and manufacturing the hot-rolled steel sheet described later has yield strength (YS): 379 MPa to 552 MPa, tensile strength (TS ): 517 MPa or more, elongation (EL): 25% or more, and Charpy impact absorption energy at 0 ° C: 200 J or more. The high-strength hot-rolled steel sheet for steel pipe, yield strength (YS): 379 MPa ~ 552 MPa, tensile strength (TS): 517 MPa ~ 650 MPa, elongation (EL): 25% ~ 40%, and Charpy impact at 0 ℃ Absorbed energy: 200 J ~ 250 J can be satisfied.

The high-strength hot-rolled steel sheet for the steel pipe may have a mixed structure in which acicular ferrite and pearlite are mixed. The fraction of the acicular ferrite may be, for example, 20% to 40%, and the fraction of the pearlite may be, for example, 60% to 80%. The fraction means an area ratio derived from a microstructure photograph through an image analyzer.

Hereinafter, a method for manufacturing a high-strength hot-rolled steel sheet for steel pipe according to the present invention will be described with reference to the accompanying drawings.

Manufacturing method of high-strength hot-rolled steel sheet for steel pipe

1 is a process flow chart schematically showing a manufacturing method of a high-strength hot-rolled steel sheet for steel pipe according to an embodiment of the present invention.

In the manufacturing method according to the present invention, the semi-finished product subject to the hot rolling process may be illustratively a slab. The semi-finished slab can be secured through a continuous casting process after obtaining molten steel of a predetermined composition through a steelmaking process.

Steel for forming the hot-rolled steel sheet, in weight%, carbon (C): 0.11% ~ 0.13%, silicon (Si): 0.10% ~ 0.30%, manganese (Mn): 1.0% ~ 1.3%, aluminum (Al ): 0.01% to 0.06%, niobium (Nb): 0.045% to 0.055%, phosphorus (P): greater than 0% to 0.03%, sulfur (S): greater than 0% to 0.003%, and balance iron (Fe) and other unavoidable impurities.

Referring to FIG. 1, the method for manufacturing a high-strength hot-rolled steel sheet for steel pipe according to an embodiment of the present invention includes a reheating step (S110), a hot rolling step (S120), a cooling step (S130), and a winding step (S140). do.

Reheating step (S110)

In the reheating step (S110), a steel material having the above composition, for example, a slab plate material, is reheated at a reheating temperature (Slab Reheating Temperature, SRT) of, for example, 1,180 ° C to 1,220 ° C. Through this reheating, re-dissolution of components segregated during casting may occur. When the reheating temperature is less than 1,180° C., a rolling load may be caused during hot rolling. When the reheating temperature exceeds 1,220° C., the strength of the final steel sheet may decrease due to coarsening of crystal grains.

Hot rolling step (S120)

In the hot rolling step (S120), the heated steel is first subjected to hot rolling after heating to adjust its shape. The hot rolling may be continuously performed by width rolling, rough rolling, and finishing rolling. By the hot rolling step, the steel material may form a steel sheet.

The hot rolling may be finished at a finish delivery temperature (FDT) of, for example, 800° C. to 900° C. The finishing rolling needs to be finished at an appropriate temperature to form pearlite, which is the main structure, and a small amount of ferrite. When the finish rolling end temperature is less than 800° C., since rolling proceeds in the two-phase region of austenite and ferrite, non-uniformity of crystal grains increases and the fraction of ferrite increases, thereby degrading the material. In addition, it is difficult to control rolling in the center of the thickness according to the thickening, and it may cause the electric field material deviation of the hot-rolled coil. When the finish rolling end temperature exceeds 900 ° C., the austenite grains become coarse and the number of nucleation sites decreases. Due to the phase transformation delay in the cooling zone (ROT), coarse pearlite is generated after winding, resulting in tensile strength may be lowered.

Cooling step (S130)

In the cooling step (S130), the hot-rolled steel material is cooled to a cooling end temperature of, for example, 580°C to 630°C. The cooling may be performed at a cooling rate of, for example, 10°C/sec to 20°C/sec.

In order to secure low-temperature toughness, fine ferrite must be formed and pearlite formation must be suppressed, so cooling control is important. The cooling rate at which polygonal ferrite and pearlite can be generated through the continuous cooling curve is less than 10° C./sec. Since the generation of polygonal ferrite and pearlite must be suppressed and phase transformation must be intensively generated during winding, cooling after rolling is better as soon as possible. When the cooling rate is less than 10° C./sec, polygonal ferrite and pearlite may be excessively generated. When the cooling rate exceeds 20° C./sec, a large amount of bainite may be generated. Accordingly, the cooling rate may be, for example, 10°C/sec to 20°C/sec, and may be, for example, 15°C/sec to 20°C/sec.

When the cooling end temperature is less than 580° C., bainite may be generated and hard spots may occur. When the cooling end temperature exceeds 630 ° C., even if the target temperature is reached at the coiling temperature, the transformation is not completed before coiling, and coarse pearlite may be generated, which may result in insufficient material.

The cooling end temperature may be the same as the winding temperature. The cooling step (S130) may be referred to as a CT (Coiling Temperature) step. In addition, both air cooling and water cooling are possible for the cooling.

Winding step (S140)

In the winding step (S140), the cooled steel material is wound at a coiling temperature (CT) of 580 ° C to 630 ° C. When the coiling temperature is less than 580° C., strength increases due to crystal grain refinement, making it difficult to manufacture pipes and reducing weldability. When the coiling temperature exceeds 630° C., it may be difficult to secure sufficient strength.

Experimental example

Hereinafter, a preferred experimental example is presented to aid understanding of the present invention. However, the following experimental examples are only for helping understanding of the present invention, and the present invention is not limited by the following experimental examples.

Table 1 shows the composition of the hot-rolled steel sheets of Examples and Comparative Examples. In Table 1, the remainder consists of iron (Fe) and impurities inevitably contained in the steelmaking process. The unit is % by weight.

division C Si Mn Al Nb P S Example 0.113 0.158 1.013 0.021 0.047 0.011 0.0010 comparative example 0.1662 0.225 1.26 0.028 0.021 0.0097 0.0015

Referring to Table 1, compared to the comparative example, the example has a low carbon content and a high niobium content.

Table 2 shows process condition values for forming hot-rolled steel sheets of Comparative Examples and Examples.

division reheat temperature
(℃) finishing rolling
end temperature
(℃) winding temperature
(℃) thickness
(mm) Example 1,200 872 583 15 comparative example 1,200 821 619 9 to 12

Referring to Table 2, compared to the comparative example, the example has a high finish rolling end temperature and a low coiling temperature. However, the finish rolling end temperature and coiling temperature of Comparative Example are included in the range proposed by the present invention. In addition, the thickness of the hot-rolled steel sheets of Examples was thicker than that of Comparative Examples.

Table 3 shows the results obtained by measuring yield strength (YS), tensile strength (TS), elongation (EL), and Charpy impact absorption energy at 0° C. as mechanical properties of the prepared hot-rolled steel sheet.

division yield strength
(MPa) tensile strength
(MPa) elongation rate
(%) Charpy shock absorption energy at 0°C (J) average 1 time Episode 2 3rd time Example 497.5 614.5 36.05 235 226 252 226 comparative example 458 581 34 173 - - -

Referring to Table 3, the examples have yield strength (YS): 379 MPa to 552 MPa, tensile strength (TS): 517 MPa or more, elongation (EL): 25% or more, and Charpy impact absorption energy at 0 ° C: The physical properties of 200 J or more were satisfied.

Compared with the comparative examples, the examples have high yield strength and tensile strength, and the elongation is slightly reduced. However, the yield strength and tensile strength of the comparative example are included in the range proposed by the present invention. On the other hand, in the case of the Charpy impact absorption energy at 0 ° C., the Example showed an increase of about 36% compared to the Comparative Example, and thus satisfied 200 J or more.

2 is scanning electron micrographs comparing microstructures of high-strength hot-rolled steel sheets for steel pipes according to an embodiment of the present invention with comparative examples.

Referring to FIG. 2 , in the case of the embodiment, the acicular ferrite structure is well developed and the fraction is formed high. In the case of Comparative Example, the polygonal ferrite fraction in the elongated form is high. It is analyzed that the acicular ferrite structure of this embodiment provides high low-temperature impact toughness characteristics of 200 J or more, together with the content control of reducing the carbon content and increasing the niobium content.

The technical idea of the present invention described above is not limited to the above-described embodiment and the accompanying drawings, and various substitutions, modifications, and changes are possible within the scope of the technical idea of the present invention. It will be clear to those skilled in the art to which it pertains.

Claims (8)

In % by weight, carbon (C): 0.11% to 0.13%, silicon (Si): 0.10% to 0.30%, manganese (Mn): 1.0% to 1.3%, aluminum (Al): 0.01% to 0.06%, niobium ( Nb): 0.045% to 0.055%, phosphorus (P): greater than 0% to 0.03%, sulfur (S): greater than 0% to 0.003%, and the balance including iron (Fe) and other unavoidable impurities,
Charpy impact absorption energy at 0 ° C: satisfies 200 J or more,
It has a mixed structure in which acicular ferrite and pearlite are mixed,
The fraction of the acicular ferrite is 20% to 40%,
The fraction of the pearlite is 60% to 80%,
High-strength hot-rolled steel sheet for steel pipe. According to claim 1,
The high-strength hot-rolled steel sheet for the steel pipe,
Yield strength (YS): 379 MPa ~ 552 MPa, tensile strength (TS): 517 MPa or more, and elongation (EL): 25% or more,
High-strength hot-rolled steel sheet for steel pipe. delete delete In % by weight, carbon (C): 0.11% to 0.13%, silicon (Si): 0.10% to 0.30%, manganese (Mn): 1.0% to 1.3%, aluminum (Al): 0.01% to 0.06%, niobium ( Nb): 0.045% to 0.055%, phosphorus (P): greater than 0% to 0.03%, sulfur (S): greater than 0% to 0.003%, and the remainder containing iron (Fe) and other unavoidable impurities 1,180 Reheating at a reheating temperature of ° C to 1,220 ° C;
Hot rolling the reheated steel material to end at a finish rolling end temperature of 800 ° C to 900 ° C;
Cooling the hot-rolled steel material to a cooling end temperature of 580 ° C to 630 ° C; and
Including; winding the cooled steel at a winding temperature of 580 ° C to 630 ° C;
As a method for manufacturing a high-strength hot-rolled steel sheet for steel pipes,
The high-strength hot-rolled steel sheet for steel pipe manufactured by the manufacturing method of the high-strength hot-rolled steel sheet for steel pipe,
Charpy impact absorption energy at 0 ° C: satisfies 200 J or more,
It has a mixed structure in which acicular ferrite and pearlite are mixed,
The fraction of the acicular ferrite is 20% to 40%,
The fraction of the pearlite is 60% to 80%,
Manufacturing method of high-strength hot-rolled steel sheet for steel pipe. According to claim 5,
The cooling step is performed at a cooling rate of 10 ° C / sec to 20 ° C / sec,
Manufacturing method of high-strength hot-rolled steel sheet for steel pipe. According to claim 5,
The cooling step is performed at a cooling rate of 15 ° C / sec to 20 ° C / sec,
Manufacturing method of high-strength hot-rolled steel sheet for steel pipe. According to claim 5,
The high-strength hot-rolled steel sheet for the steel pipe,
Yield strength (YS): 379 MPa ~ 552 MPa, tensile strength (TS): 517 MPa or more, and elongation (EL): 25% or more,
Manufacturing method of high-strength hot-rolled steel sheet for steel pipe.

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