KR102503447B1 - Steel for line pipe having excellent weldability and method of manufacturing the same - Google Patents

Abstract

The present invention provides a steel material for a line pipe having excellent weldability and a manufacturing method thereof. According to one embodiment of the present invention, the steel for the line pipe, in weight%, carbon (C): 0.05% ~ 0.09%, silicon (Si): 0.15% ~ 0.40%, manganese (Mn): 1.00% ~ 1.20%, soluble aluminum (S_Al): 0.02% to 0.06%, niobium (Nb): 0.01% to 0.03%, titanium (Ti): 0.01% to 0.03%, phosphorus (P): greater than 0% to 0.02%, sulfur (S): greater than 0% to 0.005%, and the remainder including iron (Fe) and other unavoidable impurities, and may have a carbon equivalent (C _eq ) of 0.22 to 0.29.

Description

Steel for line pipe having excellent weldability and method of manufacturing the same}

The technical idea of the present invention relates to a steel material and a manufacturing method thereof, and more particularly, to a steel material for a line pipe having excellent weldability and a manufacturing method thereof.

Recently, as the problem of resource depletion has emerged, oil drilling and transportation operations in the deep sea or polar regions are increasing. In this way, as oil pipelines move from general areas to special areas such as permafrost and seismic zones, the demand for energy resource transportation stability is increasing. Therefore, in order to develop steel materials for line pipes applicable to extreme environments, research on 50kg class high deformability steel materials is required.

In the case of applying ferrite-pearlite steel as the line pipe steel, it has excellent toughness and ductility, but it is difficult to obtain a strength of 50 kg or more, so the addition of an alloy such as Mo, Cr, Ni, Cu, etc. is required. However, there is a problem in that the additive increases the cost of steel and deteriorates weldability.

Korean Patent Application No. 10-2010-0135245

A technical problem to be achieved by the technical idea of the present invention is to provide a steel material for a line pipe having excellent weldability 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, a steel material for a line pipe having excellent weldability and a manufacturing method thereof are provided.

According to one embodiment of the present invention, the steel for the line pipe, in weight%, carbon (C): 0.05% ~ 0.09%, silicon (Si): 0.15% ~ 0.40%, manganese (Mn): 1.00% ~ 1.20%, soluble aluminum (S_Al): 0.02% to 0.06%, niobium (Nb): 0.01% to 0.03%, titanium (Ti): 0.01% to 0.03%, phosphorus (P): greater than 0% to 0.02%, sulfur (S): More than 0% to 0.005%, and the balance includes iron (Fe) and other unavoidable impurities, and may have a mixed structure of acicular ferrite, polygonal ferrite, and bainitic ferrite.

According to one embodiment of the present invention, the steel material for the line pipe may have a carbon equivalent (C _eq ) of 0.22 to 0.29. (Where C _eq = [C] + [Mn]/6 + ([Ni] + [Cu])/15 + ([Cr] + [Mo] + [V])/5)

According to one embodiment of the present invention, the steel material for the line pipe may have a weld crack susceptibility index (P _cm ) of 0.11 to 0.16. (where P _cm = [C] + [Si]/30 + ([Mn] + [Cu] + [Cr])/20 + [Ni]/60 + [Mo]/15 + [V]/10 + 5[B])

According to one embodiment of the present invention, the steel material for the line pipe has tensile strength (TS): 520 MPa to 760 MPa, yield strength (YS): 415 MPa to 565 MPa, elongation (EL): 24% to 45% , It can satisfy low-temperature impact toughness: 300 J ~ 400 J, and hardness: 150 Hv ~ 248 Hv at a temperature of -50 ° C.

According to an embodiment of the present invention, the fraction of the polygonal ferrite may be greater than 0% to 10%, the fraction of the bainitic ferrite may be 10% to 20%, and the fraction of the acicular ferrite may be the remaining fraction.

According to one embodiment of the present invention, the average particle size of the ferrite may be in the range of 10 μm to 15 μm.

According to one embodiment of the present invention, in the method for manufacturing the steel for the line pipe, in weight%, carbon (C): 0.05% to 0.09%, silicon (Si): 0.15% to 0.40%, manganese (Mn): 1.00% to 1.20%, Fusible Aluminum (S_Al): 0.02% to 0.06%, Niobium (Nb): 0.01% to 0.03%, Titanium (Ti): 0.01% to 0.03%, Phosphorus (P): >0% to 0.02% %, sulfur (S): more than 0% to 0.005%, and the balance comprising iron (Fe) and other unavoidable impurities, reheating at a reheating temperature of 1,000 ° C to 1,250 ° C; Hot rolling the heated steel material to end at a temperature of 850 ° C to 950 ° C; and cooling the hot-rolled steel material.

According to an embodiment of the present invention, the cooling may be performed at a cooling rate of 20 °C/sec to 60 °C/sec.

According to one embodiment of the present invention, the cooling may be cooled to a cooling end temperature of 150 ° C to 400 ° C.

According to an embodiment of the present invention, the steel for line pipe manufactured by the method for manufacturing steel for line pipe has a mixed structure of acicular ferrite, polygonal ferrite and bainite ferrite, The fraction of the polygonal ferrite may be greater than 0% to 10%, the fraction of the bainitic ferrite may be 10% to 20%, and the fraction of the acicular ferrite may be the remaining fraction.

According to an embodiment of the present invention, the steel for line pipe manufactured by the method for manufacturing steel for line pipe has a tensile strength (TS) of 520 MPa to 760 MPa and a yield strength (YS) of 415 MPa to 565 MPa. , elongation (EL): 24% to 45%, low-temperature impact toughness at a temperature of -50 ° C: 300 J to 400 J, and hardness: 150 Hv to 248 Hv.

According to one embodiment of the present invention, the steel for line pipe manufactured by the method for manufacturing steel for line pipe has a carbon equivalent (C _eq ) of 0.22 to 0.29 and a welding crack susceptibility index (P) of 0.11 to 0.16. _cm ) can be (where C _eq = [C] + [Mn]/6 + ([Ni] + [Cu])/15 + ([Cr] + [Mo] + [V])/5) (where P _cm = [C] + [Si]/30 + ([Mn] + [Cu] + [Cr])/20 + [Ni]/60 + [Mo]/15 + [V]/10 + 5[B] ] lim)

According to the technical concept of the present invention, it does not contain expensive elements such as molybdenum, copper, nickel, chromium, etc., thereby lowering the carbon equivalent and the weld crack susceptibility index, thereby increasing weldability and improving economic feasibility, yield strength of 410 MPa or more, 520 It is possible to obtain a steel material for line pipe having high strength of tensile strength of MPa or more and excellent impact energy of 300J or more at -50 ° C and excellent low-temperature toughness. The steel material for the line pipe can secure transport stability in a special region such as an earthquake zone or permafrost.

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 illustrating a method of manufacturing a steel material for a line pipe according to an embodiment of the present invention.
2 is a photomicrograph showing microstructures of Examples and Comparative Examples manufactured using the method for manufacturing a steel material for line pipe according to an embodiment of the present invention.

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.

In the steel for line pipe according to the technical concept of the present invention, toughness and weldability are particularly important properties, and the amount of carbon needs to be strictly limited to prevent formation of hard spots. Therefore, it is necessary to keep the carbon equivalent and the weld crack susceptibility index low.

Adding molybdenum (Mo), chromium (Cr), nickel (Ni), copper (Cu), etc. helps to secure the strength of line pipe steel, which strictly limits the carbon content, but increases manufacturing cost and deteriorates weldability. so it needs to be limited. Instead of these additive elements, niobium (Nb), titanium (Ti), vanadium (V), etc. are the most important elements for microstructure control along with carbon as carbonitride forming elements of steel. Therefore, in the present invention, by adding a small amount of niobium (Nb) and titanium (Ti) to form a precipitate phase at a high temperature, recrystallization of austenite is suppressed, crystal grains are made fine, and strength is increased by forming a precipitate phase.

In addition, with the addition of small amounts of niobium (Nb) and titanium (Ti), mechanical properties are improved by forming a solid solution or precipitate phase such as molybdenum (Mo), chromium (Cr), nickel (Ni), copper (Cu), etc. Since there is a limit, the rolling end temperature is increased to perform high-temperature hot rolling in the single-phase area directly above Ar3, and after hot rolling to create a low-temperature phase, at a low temperature of 150 ° C to 400 ° C at a cooling rate of 30 ° C / sec to 60 ° C / sec. Accelerated cooling can achieve the desired mechanical properties.

Hereinafter, a steel material for a line pipe having excellent weldability according to a preferred embodiment of the present invention and a manufacturing method thereof will be described in detail with reference to the accompanying drawings.

Steel for line pipe

Steel for line pipe, which is an aspect of the present invention, in weight percent, carbon (C): 0.05% to 0.09%, silicon (Si): 0.15% to 0.40%, manganese (Mn): 1.00% to 1.20%, solubility Aluminum (S_Al): 0.02% to 0.06%, Niobium (Nb): 0.01% to 0.03%, Titanium (Ti): 0.01% to 0.03%, Phosphorus (P): greater than 0% to 0.02%, Sulfur (S): More than 0% to 0.005%, and the balance includes iron (Fe) and other unavoidable impurities.

Hereinafter, the role and content of each component included in the steel for line pipe according to the present invention will be described. At this time, the content of component elements all means weight%.

Carbon (C): 0.05% to 0.09%

Carbon is added to ensure strength of steel. When the carbon content is less than 0.05%, it may be difficult to secure strength. When the carbon content exceeds 0.09%, low-temperature impact toughness and weldability may deteriorate. Therefore, carbon is preferably added in an amount of 0.05% to 0.09% of the total weight of the steel.

Silicon (Si): 0.15% to 0.40%

Silicon contributes to increase the strength of steel. Also, as a ferrite stabilizing element, it is effective in improving toughness and ductility of steel by inducing ferrite formation. When the content of silicon is less than 0.15%, the addition effect is insufficient. When the content of silicon exceeds 0.40%, red scale is generated in the heating furnace during the hot rolling process, and thus the surface quality of the steel is deteriorated and weldability may be deteriorated. Therefore, silicon is preferably added in an amount of 0.15% to 0.40% of the total weight of the steel.

Manganese (Mn): 1.00% to 1.20%

Manganese is an element that contributes to solid solution strengthening and improvement of hardenability of steel. When the content of manganese is less than 1.00%, the addition effect is insufficient. When the content of manganese exceeds 1.20%, the synergistic effect according to the increased amount of manganese is insignificant, and MnS inclusions and oxides may be formed, which may impair weldability of the steel when forming a line pipe. Therefore, manganese is preferably added in an amount of 1.00% to 1.20% of the total weight of the steel.

Soluble aluminum (S_Al): 0.02% to 0.06%

Soluble aluminum acts as a deoxidizer to remove oxygen from the steel. When the content of aluminum is less than 0.02%, the addition effect is insufficient. When the content of aluminum exceeds 0.06%, Al ₂ O ₃ , which is a non-metallic inclusion, may be formed and low-temperature impact toughness may be deteriorated. Therefore, aluminum is preferably added in an amount of 0.02% to 0.06% of the total weight of the steel.

Niobium (Nb): 0.01% to 0.03%

Niobium precipitates carbonitride (NbC) in steel, plays a role in pinning grain boundaries, and prevents grain boundary sliding (GBS) and dislocation movement occurring at high temperatures, thereby improving strength. . When the content of niobium is less than 0.01%, the addition effect is insufficient. When the content of niobium exceeds 0.03%, the synergistic effect according to the increase in the added amount is insignificant, and playability, rollability and elongation may be deteriorated due to excessive precipitation. Therefore, niobium is preferably added in an amount of 0.01% to 0.03% of the total weight of the steel.

Titanium (Ti): 0.01% to 0.03%

Titanium creates Ti(C,N) precipitates with excellent high-temperature stability, which hinders the growth of austenite grains during welding, thereby improving the toughness and strength of hot-rolled products by refining the welded structure. When the content of titanium is less than 0.01%, the addition effect is insufficient. When the content of titanium exceeds 0.03%, coarse precipitates may be formed to lower the toughness of the steel. Therefore, titanium is preferably added in an amount of 0.01% to 0.03% of the total weight of the steel.

Phosphorus (P): greater than 0% to 0.02%

Phosphorus is a representative element that lowers the impact toughness, and the lower the content, the better. When phosphorus is included in excess of 0.02%, weldability and toughness may be deteriorated. Phosphorus is preferably limited to more than 0% to 0.02% of the total weight of the steel.

Sulfur (S): greater than 0% to 0.005%

Sulfur, along with phosphorus, is an element that is unavoidably contained in the manufacture of steel, and can impair toughness and weldability of steel. When the sulfur is included in an amount exceeding 0.005%, it is possible to generate cracks during processing of the steel by forming sulfuric inclusions (MnS) to deteriorate the resistance to stress corrosion cracking, and as a result, the corrosion resistance of the steel can be reduced. there is. Sulfur is preferably limited to more than 0% to 0.005% of the total weight of the steel.

The remaining component 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 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 carbon equivalent (C _eq ) and the weld crack susceptibility index (P _cm ) of the steel are as shown in Equations 1 and 2, respectively.

[Equation 1]

C _eq = [C] + [Mn]/6 + ([Ni] + [Cu])/15 + ([Cr] + [Mo] + [V])/5

[Equation 2]

P _cm = [C] + [Si]/30 + ([Mn] + [Cu] + [Cr])/20 + [Ni]/60 + [Mo]/15 + [V]/10 + 5[B] ]

In Equations 1 and 2, [C], [Mn], [Ni], [Cu], [Cr], [Mo], [V], [Si] and [B] are included in the steel It is the content of carbon (C), manganese (Mn), nickel (Ni), copper (Cu), chromium (Cr), molybdenum (Mo), vanadium (V), silicon (Si) and boron (B), respectively. The unit is % by weight.

The steel material may have a carbon equivalent (C _eq ) according to Equation 1, for example, 0.22 to 0.29. When the carbon equivalent (C _eq ) according to Equation 1 exceeds 0.29, weldability may be deteriorated.

The steel material may have a welding crack susceptibility index (P _cm ) according to Equation 2, for example, 0.11 to 0.16. When the weld crack susceptibility index (P _cm ) according to Equation 2 exceeds 0.16, weldability may be deteriorated.

The steel for line pipe manufactured by controlling the specific components and content ranges of the alloy composition described above and using the steel manufacturing method described below has tensile strength (TS): 520 MPa to 760 MPa, yield strength (YS): 415 MPa to 565 MPa, elongation (EL): 24% to 45%, low-temperature impact toughness at a temperature of -50 ° C: 300 J to 400 J, and hardness: 150 Hv to 248 Hv.

The steel material for the line pipe may have a mixed structure of acicular ferrite (AF), polygonal ferrite (PF), and bainite ferrite (BF).

The fraction of the polygonal ferrite may be, for example, greater than 0% to 10%, the fraction of the bainite ferrite may be, for example, 10% to 20%, and the fraction of the acicular ferrite may be the remaining fraction. It may contain, for example, it may be 70% to 90%. The fraction means an area ratio derived from a picture of the microstructure of the steel through an image analyzer.

An average grain size of the ferrite may be in the range of 10 μm to 15 μm.

Hereinafter, a method of manufacturing a steel material for a line pipe according to the present invention will be described with reference to the accompanying drawings.

Steel manufacturing method

1 is a process flow chart schematically illustrating a method of manufacturing a steel material for a line pipe according to an embodiment of the present invention.

In the method for manufacturing steel 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.

The steel material, in weight percent, carbon (C): 0.05% ~ 0.09%, silicon (Si): 0.15% ~ 0.40%, manganese (Mn): 1.00% ~ 1.20%, soluble aluminum (S_Al): 0.02% ~ 0.06%, niobium (Nb): 0.01% to 0.03%, titanium (Ti): 0.01% to 0.03%, phosphorus (P): greater than 0% to 0.02%, sulfur (S): greater than 0% to 0.005%, and The balance includes iron (Fe) and other unavoidable impurities.

Referring to FIG. 1 , the method for manufacturing a steel material for a line pipe according to an embodiment of the present invention includes a reheating step (S110), a hot rolling step (S120), and a cooling step (S130).

Specifically, the manufacturing method of the steel material for the line pipe includes reheating the steel material having the above composition at a temperature of 1,000 ° C to 1,250 ° C (S110); Hot rolling the reheated steel material to end at a temperature of 850 ° C to 950 ° C (S120); and cooling the hot-rolled steel material (S130).

The cooling step may be cooled to a cooling end temperature of 150 ° C to 400 ° C at a cooling rate of 20 ° C / sec to 60 ° C / sec.

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, 1,000 ° C to 1,250 ° C. Through such reheating, re-dissolution of components segregated during casting and re-dissolution of precipitates may occur. When the reheating temperature is less than 1,000 ° C, the rolling load may increase due to the low reheating temperature, and it may be difficult to secure strength due to a decrease in fine dispersion effect due to difficulty in complete solid solution of niobium. In addition, since the solid solution temperature of niobium-based precipitates, such as NbC and NbN, is not reached, the austenite crystal grains may rapidly coarsen because they cannot be re-precipitated into fine precipitates during hot rolling, so that the growth of austenite grains cannot be suppressed. In addition, there are problems in that the solid solution of impurities and precipitate-forming elements is not sufficient, and segregated components are not sufficiently evenly distributed during casting. When the reheating temperature exceeds 1,250° C., it may be difficult to secure strength and low-temperature toughness of a steel sheet manufactured due to rapid coarsening of austenite crystal grains. In addition, as the reheating temperature increases, there is a problem in that manufacturing costs increase and productivity decreases due to heating costs and additional time required to adjust the hot rolling temperature.

Hot rolling step (S120)

The heated steel material 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, that is, the finishing rolling, may be finished at a finish rolling temperature (FRT) of, for example, 850° C. to 950° C., for example, 900° C. to 950° C. When the finish rolling end temperature is less than 850° C., abnormal reverse rolling occurs and a non-uniform structure is formed, thereby greatly reducing low temperature impact toughness. When the rolling end temperature exceeds 950° C., ductility and toughness are excellent, but strength may rapidly decrease.

Cooling step (S130)

The hot-rolled steel is cooled to a cooling end temperature of 150 ° C to 400 ° C at a cooling rate of 20 ° C / sec to 60 ° C / sec. The cooling may be performed by air cooling or water cooling. When cooling in the above cooling rate range, it is possible to sufficiently secure a low-temperature microstructure while preventing a phenomenon in which the hardness is increased and the low-temperature toughness is lowered.

The steel material for the line pipe manufactured by performing the above steps may have a thickness of, for example, 25.4 mm or less, for example, 6 mm to 25.4 mm.

Hereinafter, the present invention will be described in detail through examples, but this is only a preferred embodiment of the present invention and the scope of the present invention is not limited by the description of these examples. Contents not described herein can be technically inferred by those skilled in the art, so descriptions thereof will be omitted.

Example

Tables 1 and 2 show the compositions of steel materials for line pipes of Comparative Examples and Examples. In Tables 1 and 2, the remainder consists of iron (Fe) and impurities inevitably contained in the steelmaking process. The content unit of each component is % by weight. Table 2 lists the carbon equivalent (C _eq ) and weld crack susceptibility index (P _cm ).

division C Si Mn S_Al Nb Ti P S Comparative Examples 1 and 2 0.06 0.25 1.4 0.04 0.05 0.015 0.01 0.003 Examples 1 and 2 0.07 0.25 1.1 0.04 0.02 0.015 0.01 0.003

division Cu Cr Mo Ni V C _eq P _cm Comparative Examples 1 and 2 0.05 0.15 0.2 0.2 0.025 0.37 0.17 Examples 1 and 2 0 0 0 0 0 0.26 0.14

Referring to Tables 1 and 2, compared to Comparative Examples, the examples do not include copper (Cu), chromium (Cr), molybdenum (Mo), nickel (Ni), and vanadium (V), and manganese There is a difference in reducing the content of niobium and niobium. In addition, the carbon equivalent (C _eq ) of the embodiment is 0.29 or less, and the welding crack susceptibility index (P _cm ) is controlled to be less than 0.16.

Table 3 shows process condition values of Examples and Comparative Examples of steel materials for line pipes.

division reheat temperature
(℃) End of rolling temperature
(℃) Cooling end temperature
(℃) cooling rate
(℃/sec) Comparative Example 1 1144 853 570 46 Comparative Example 2 1149 860 563 48 Example 1 1160 935 343 55 Example 2 1166 931 170 57

Referring to Table 3, compared to Comparative Examples, there is a difference in that the examples are performed at a rolling end temperature of 900 ° C. or higher and a cooling end temperature of 400 ° C. or lower.

Table 4 shows tensile strength (TS), yield strength (YS), elongation (EL), low-temperature impact toughness at -50 ° C, and hardness as mechanical properties of the manufactured steel for line pipe, respectively. show the result.

division tensile strength
(MPa) yield strength
(MPa) elongation rate
(%) low temperature impact toughness
(J) Hardness
(Hv) Comparative Example 1 486 582 37 380/402/394 196 Comparative Example 2 479 570 41 399/398/386 188 Example 1 458 557 37 357/366/345 195 Example 2 498 610 35 341/355/351 203

Referring to Table 4, compared to the comparative examples, the examples did not include copper (Cu), chromium (Cr), molybdenum (Mo), nickel (Ni), and vanadium (V), but the tensile strength, yield Strength, elongation, low-temperature impact toughness at -50 ° C, and hardness were found to be almost equal.

2 is a photomicrograph showing microstructures of Examples and Comparative Examples manufactured using the method for manufacturing a steel material for line pipe according to an embodiment of the present invention.

Table 6 shows the volume fraction of the microstructures of the steel for line pipe derived from the micrograph. In Table 6, the average grain size was calculated according to ASTM E112.

(unit: volume%) Comparative Example 1 Comparative Example 2 Example 1, 2 Acicular ferrite (AF) 30 40 80 Polygonal Ferrite (PF) 47 40 5 Bainite Ferrite (BF) 30 20 15 Average grain size (μm) 10.8 11.2 12.8

Referring to FIG. 2 and Table 6, it is analyzed that, compared to Comparative Examples 1 and 2, in Examples 1 and 2, the formation of polygonal ferrite and bainitic ferrite was suppressed and acicular ferrite was promoted. Thus, Examples 1 and 2 had a microstructure composed of 80% by volume of acicular ferrite, 5% by volume of polygonal ferrite, and 15% by volume of bainitic ferrite.

Therefore, compared to general steel for line pipe, the steel for line pipe according to the technical idea of the present invention contains additives such as copper (Cu), chromium (Cr), molybdenum (Mo), nickel (Ni), and vanadium (V). It does not contain, and while the content of niobium is reduced, the rolling end temperature is raised and the cooling end temperature is lowered to control the microstructure as described above to secure desired physical properties such as strength and low-temperature impact toughness, Weldability can be increased by reducing the carbon equivalent and the weld crack susceptibility index.

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

Claims (12)

In weight percent, carbon (C): 0.05% to 0.09%, silicon (Si): 0.15% to 0.40%, manganese (Mn): 1.00% to 1.20%, soluble aluminum (S_Al): 0.02% to 0.06%, niobium (Nb): 0.01% to 0.03%, titanium (Ti): 0.01% to 0.03%, phosphorus (P): greater than 0% to 0.02%, sulfur (S): greater than 0% to 0.005%, and balance iron ( Fe) and other unavoidable impurities,
It has a mixed structure of acicular ferrite, polygonal ferrite and bainite ferrite,
The fraction of the polygonal ferrite is greater than 0% to 10%, the fraction of the bainite ferrite is 10% to 20%, the fraction of the acicular ferrite is the remaining fraction,
The average grain size of the ferrite is in the range of 10 μm to 15 μm,
Tensile strength (TS): 520 MPa ~ 760 MPa, yield strength (YS): 415 MPa ~ 565 MPa, elongation (EL): 24% ~ 45%, low temperature impact toughness at -50℃: 300 J ~ 400 J , and hardness: 150 Hv ~ 248 Hv,
Steel materials for line pipes. According to claim 1,
The steel material for the line pipe,
With a carbon equivalent (C _eq ) of 0.22 to 0.29,
(Where C _eq = [C] + [Mn]/6 + ([Ni] + [Cu])/15 + ([Cr] + [Mo] + [V])/5)
Steel materials for line pipes. According to claim 1,
The steel material for the line pipe,
Having a welding crack susceptibility index (P _cm ) of 0.11 to 0.16,
(where P _cm = [C] + [Si]/30 + ([Mn] + [Cu] + [Cr])/20 + [Ni]/60 + [Mo]/15 + [V]/10 + 5[B])
Steel materials for line pipes. delete delete delete In weight percent, carbon (C): 0.05% to 0.09%, silicon (Si): 0.15% to 0.40%, manganese (Mn): 1.00% to 1.20%, soluble aluminum (S_Al): 0.02% to 0.06%, niobium (Nb): 0.01% to 0.03%, titanium (Ti): 0.01% to 0.03%, phosphorus (P): greater than 0% to 0.02%, sulfur (S): greater than 0% to 0.005%, and balance iron ( Reheating the steel material containing Fe) and other unavoidable impurities at a temperature of 1,000 ° C to 1,250 ° C;
Hot rolling the heated steel material to end at a temperature of 850 ° C to 950 ° C; and
Cooling the hot-rolled steel material to a cooling end temperature of 150 ° C to 343 ° C or less at a cooling rate of 30 ° C / sec to 60 ° C / sec;
Steel materials for line pipes manufactured by the method for manufacturing steel materials for line pipes,
It has a mixed structure of acicular ferrite, polygonal ferrite and bainite ferrite,
The fraction of the polygonal ferrite is greater than 0% to 10%, the fraction of the bainite ferrite is 10% to 20%, the fraction of the acicular ferrite is the remaining fraction,
The average grain size of the ferrite is in the range of 10 μm to 15 μm,
Tensile strength (TS): 520 MPa ~ 760 MPa, yield strength (YS): 415 MPa ~ 565 MPa, elongation (EL): 24% ~ 45%, low temperature impact toughness at -50℃: 300 J ~ 400 J , and hardness: 150 Hv ~ 248 Hv,
A method for manufacturing steel materials for line pipes. delete delete delete delete According to claim 7,
Steel materials for line pipes manufactured by the method for manufacturing steel materials for line pipes,
With a carbon equivalent (C _eq ) of 0.22 to 0.29,
Having a welding crack susceptibility index (P _cm ) of 0.11 to 0.16,
(Where C _eq = [C] + [Mn]/6 + ([Ni] + [Cu])/15 + ([Cr] + [Mo] + [V])/5)
(where P _cm = [C] + [Si]/30 + ([Mn] + [Cu] + [Cr])/20 + [Ni]/60 + [Mo]/15 + [V]/10 + 5[B])
A method for manufacturing steel materials for line pipes.

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