Line pipe steel and manufacturing method therefor

Abstract

The present invention provides line pipe steel comprising, by wt%, 0.04-0.07% of carbon (C), 0.20-0.30% of silicon (Si), 1.2-1.35% of manganese (Mn), 0.02-0.05% of aluminum (Al), 0.10% or less (excluding 0%) of copper (Cu), 0.15-0.25% of chromium (Cr), 0.02-0.08% of molybdenum (Mo), 0.15-0.25% of nickel (Ni), 0.04-0.05% of niobium (Nb), 0.01-0.018% of titanium (Ti), 0.025%-0.035% of vanadium (V), 0.01% or less (excluding 0%) of phosphorus (P), 0.001% or less (excluding 0%) of sulfur (S), 0.0005% or less (excluding 0%) of boron (B), and the balance of iron (Fe) and inevitable impurities, wherein the relative notch tensile strength (RNTS) according to ASTM G142 is 0.86 or greater.

Description

Steel for line pipes and method of manufacturing the same

[0001] The technical concept of the present invention relates to steel materials and a method for manufacturing the same, and more specifically, to steel materials for line pipes with excellent hydrogen embrittlement resistance and a method for manufacturing the same.

[0002] With the recent emergence of resource depletion issues, the demand for steel line pipes for hydrogen production, storage, transportation, and utilization is increasing in order to meet the requirements of a hydrogen society. Currently, as there are no API standards for high-pressure hydrogen transportation, the trend is to conduct SSRT (Slow Strain Rate Test) tensile tests, fracture tests, and fatigue fracture tests in a high-pressure hydrogen atmosphere and use them as indicators of hydrogen embrittlement resistance. Korean Patent Registration No. 10-2122643 serves as a prior art document.

[0003] The technical problem that the technical concept of the present invention aims to solve is to provide a steel material for line pipes with excellent hydrogen embrittlement resistance as an API steel material for high-pressure hydrogen transport, and a method for manufacturing the same.

[0004] However, these tasks are exemplary, and the technical concept of the present invention is not limited thereto.

[0005] According to one aspect of the present invention, a steel material for line pipes with excellent hydrogen embrittlement resistance and a method for manufacturing the same are provided.

[0006] According to one embodiment of the present invention, the steel material for a line pipe comprises, in weight percent, carbon (C): 0.04% or more and 0.07% or less, silicon (Si): 0.20% or more and 0.30% or less, manganese (Mn): 1.2% or more and 1.35% or less, aluminum (Al): 0.02% or more and 0.05% or less, copper (Cu): 0.10% or less (excluding 0%), chromium (Cr): 0.15% or more and 0.25% or less, molybdenum (Mo): 0.02% or more and 0.08% or less, nickel (Ni): 0.15% or more and 0.25% or less, niobium (Nb): 0.04% or more and 0.05% or less, titanium (Ti): 0.01% or more and 0.018% or less, vanadium (V): 0.025% or more and 0.035% or less, and phosphorus (P): It is characterized by having 0.01% or less (excluding 0%), sulfur (S): 0.001% or less (excluding 0%), boron (B): 0.0005% or less (excluding 0%), and the remainder being iron (Fe) and unavoidable impurities, and having a Relative Notch Tensile Strength (RNTS) of 0.86 or higher according to ASTM G142.

[0007] In the above-mentioned steel for line pipes, the final microstructure may consist of acicular ferrite, polygonal ferrite, and bainitic ferrite.

[0008] In the above steel material for line pipes, the area fraction of the acicular ferrite may be 5 to 20%, the area fraction of the polygonal ferrite may be 10 to 25%, and the area fraction of the bainitic ferrite may be 60 to 80%.

[0009] The above steel for line pipes may have a tensile strength (TS): 520 MPa to 760 MPa, a yield strength (YS): 415 MPa to 565 MPa, a yield ratio (YR): 93% or less, and an elongation (EL): 27% or more.

[0010] According to one embodiment of the present invention, the method for manufacturing the steel material for a line pipe comprises, in weight%, carbon (C): 0.04% or more and 0.07% or less, silicon (Si): 0.20% or more and 0.30% or less, manganese (Mn): 1.2% or more and 1.35% or less, aluminum (Al): 0.02% or more and 0.05% or less, copper (Cu): 0.10% or less (excluding 0%), chromium (Cr): 0.15% or more and 0.25% or less, molybdenum (Mo): 0.02% or more and 0.08% or less, nickel (Ni): 0.15% or more and 0.25% or less, niobium (Nb): 0.04% or more and 0.05% or less, titanium (Ti): 0.01% or more and 0.018% or less, and vanadium (V): 0.025% or more and 0.035% or less. The method comprises: a step of hot rolling a steel material containing phosphorus (P): 0.01% or less (excluding 0%), sulfur (S): 0.001% or less (excluding 0%), boron (B): 0.0005% or less (excluding 0%), and the remainder being iron (Fe) and unavoidable impurities; and a step of cooling the hot-rolled steel material; wherein the hot-rolling step is performed under conditions where the reheating temperature (SRT): 1050~1150℃, the rolling start temperature (FST): 850~950℃, and the rolling end temperature (FRT): 850~950℃, wherein the rolling end temperature (FRT) is lower than the rolling start temperature (FST); and the cooling step is performed under conditions where the cooling start temperature (SCT): Ar3 or less, the cooling end temperature (FCT): 400~600℃, and the cooling rate (CR): 30~60℃ / s.

[0011] According to the technical concept of the present invention, a steel material for line pipes with excellent hydrogen embrittlement resistance and a method for manufacturing the same can be realized.

[0012] However, the effects of the present invention are described by way of example, and the scope of the present invention is not limited by these effects.

[0013] FIG. 1 is a process flowchart schematically illustrating a method for manufacturing steel for line pipes according to an embodiment of the present invention.

[0014] FIGS. 2 to 4 are photographs of the microstructure of steel according to experimental examples of the present invention.

[0015] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. The embodiments of the present invention are provided to more completely explain the technical concept of the present invention to those skilled in the art, and the following embodiments may be modified in various different forms, and the scope of the technical concept of the present invention is not limited to the following embodiments. Rather, these embodiments are provided to make the present disclosure more faithful and complete and to fully convey the technical concept of the present invention to those skilled in the art. In this specification, the same reference numerals denote the same elements throughout. Furthermore, various elements and areas in the drawings are depicted schematically. Accordingly, the technical concept of the present invention is not limited by the relative sizes or spacing depicted in the attached drawings.

[0016] Hereinafter, a steel material for line pipes with excellent hydrogen embrittlement resistance, which is one aspect of the present invention, will be described.

[0017] Steel for line pipes with excellent hydrogen embrittlement resistance

[0018] A steel material for line pipes having excellent hydrogen embrittlement resistance, which is one aspect of the present invention, comprises, in weight%, carbon (C): 0.04% or more and 0.07% or less, silicon (Si): 0.20% or more and 0.30% or less, manganese (Mn): 1.2% or more and 1.35% or less, aluminum (Al): 0.02% or more and 0.05% or less, copper (Cu): 0.10% or less (excluding 0%), chromium (Cr): 0.15% or more and 0.25% or less, molybdenum (Mo): 0.02% or more and 0.08% or less, nickel (Ni): 0.15% or more and 0.25% or less, niobium (Nb): 0.04% or more and 0.05% or less, titanium (Ti): 0.01% or more and 0.018% or less, and vanadium (V): 0.025% or more and 0.035%. Less than or equal to, phosphorus (P): 0.01% or less (excluding 0%), sulfur (S): 0.001% or less (excluding 0%), boron (B): 0.0005% or less (excluding 0%), and the remainder is iron (Fe) and unavoidable impurities.

[0019] For example, a steel material for a line pipe with excellent hydrogen embrittlement resistance according to one embodiment of the present invention may be composed of alloy elements having the above-described content range.

[0020] The role and content of each component included in the steel material for line pipes with excellent hydrogen embrittlement resistance according to the present invention are described below. In this case, the content of the component elements refers to weight percent.

[0021] Carbon (C): 0.04% or more, 0.07% or less

[0022] Carbon is added to ensure the strength of the steel. If the carbon content is less than 0.04%, it is difficult to ensure strength. If the carbon content exceeds 0.07%, low-temperature impact toughness and weldability may decrease. Therefore, it is desirable to add carbon in an amount of 0.04% or more and 0.07% or less of the total weight of the steel.

[0023] Silicon (Si): 0.20% or more, 0.30% or less

[0024] Silicon contributes to increasing the strength of steel. Additionally, as a ferrite-stabilizing element, it is effective in improving the toughness and ductility of steel by inducing ferrite formation. If the silicon content is less than 0.20%, the additive effect is insufficient. If the silicon content exceeds 0.30%, red scale is generated in the furnace during the hot rolling process, which degrades the surface quality of the steel and may lead to a deterioration in weldability. Therefore, it is desirable to add silicon in an amount of 0.20% or more and 0.30% or less of the total weight of the steel.

[0025] Manganese (Mn): 1.2% or more, 1.35% or less

[0026] Manganese is an element that contributes to solid solution strengthening and the improvement of the hardenability of steel. If the manganese content is less than 1.2%, the effect of addition is insufficient. If the manganese content exceeds 1.35%, the synergistic effect with increasing addition amount is negligible, and MnS inclusions and oxides are formed, which may impair the weldability of the steel during line pipe manufacturing. Therefore, it is desirable to add manganese in an amount of 1.2% or more and 1.35% or less of the total weight of the steel.

[0027] Aluminum (Al): 0.02% or more, 0.05% or less

[0028] Soluble aluminum acts as a deoxidizer to remove oxygen from steel. If the aluminum content is less than 0.02%, the additive effect is insufficient. If the aluminum content exceeds 0.05%, non-metallic inclusions such as Al2O3 are formed, which may reduce low-temperature impact toughness. Therefore, it is desirable to add aluminum in an amount of 0.02% or more and 0.05% or less of the total weight of the steel.

[0029] Copper (Cu): 0.10% or less (excluding 0%)

[0030] Copper is an effective element for increasing strength and improving toughness. If the copper content exceeds 0.10%, it can cause surface defects. Therefore, it is desirable to add copper at 0.10% or less (excluding 0%) of the total weight of the steel.

[0031] Chrome (Cr): 0.15% or more, 0.25% or less

[0032] Chromium (Cr), like manganese, lowers the equilibrium temperature and is added to ensure strength; as an element with high hardenability, it is added to increase strength through transformation strengthening. However, if chromium (Cr) is added in an amount less than 0.15 wt%, it is difficult to achieve the aforementioned effects, and if it exceeds 0.25 wt%, it may combine with carbon to form coarse carbides. Although the addition of chromium (Cr) increases strength, it is detrimental to toughness, so large amounts should be avoided. Furthermore, since toughness is reduced due to the formation of structures such as upper bainite, which results in an overall non-uniform final microstructure, it is desirable to control the content to 0.25 wt% or less.

[0033] Molybdenum (Mo): 0.02% or more, 0.08% or less

[0034] Molybdenum (Mo) is a substitutional element that contributes to the improvement of steel strength through solid solution strengthening. It also improves the hardenability and corrosion resistance of steel. Molybdenum (Mo) is an element with greater hardenability than chromium (Cr) and is added to increase strength through transformation strengthening. However, if molybdenum (Mo) is added in an amount of less than 0.02 wt%, it is difficult to achieve the aforementioned effects. Furthermore, if it exceeds 0.08 wt% within the carbon component range of the present invention, toughness is reduced due to the formation of a large amount of hard secondary phases such as martensite / austenite (MA) phases; therefore, it is desirable to control the content to 0.08 wt% or less.

[0035] Nickel (Ni): 0.15% or more and 0.25% or less

[0036] Nickel (Ni), like copper (Cu), is added to increase strength through solid solution strengthening and to improve toughness. It is an effective element for refining grain size, strengthening the matrix by solid solution in austenite and ferrite, and improving low-temperature impact toughness. However, if nickel (Ni) is added in an amount less than 0.15 wt%, it is difficult to achieve the aforementioned effects, and if it exceeds 0.25 wt%, it leads to a decrease in toughness due to the formation of precipitates; therefore, it is desirable to control the upper limit of its content to 0.25 wt%.

[0037] Meanwhile, if the sum of the elemental contents (unit: weight%) of chromium (Cr), molybdenum (Mo) and nickel (Ni) is less than 0.25%, it may be difficult to achieve the above-mentioned effects. However, if the sum of the elemental contents (unit: weight%) of chromium (Cr), molybdenum (Mo) and nickel (Ni) exceeds 0.50%, weldability and toughness of the heat-affected zone (HAZ) may be reduced, and red hot brittleness may be induced.

[0038] Niobium (Nb): 0.04% or more, 0.05% or less

[0039] Niobium precipitates carbonitrides (NbC) in steel, playing a role in pinning grain boundaries and hindering grain boundary sliding (GBS) and dislocation movement that occur at high temperatures, thereby improving strength. If the niobium content is less than 0.04%, the aforementioned additive effect is insufficient. If the niobium content exceeds 0.05%, the synergistic effect with increasing additive amount is negligible, and excessive precipitation may lead to a decrease in continuous casting, rollability, and elongation. Furthermore, from the perspective of securing an unrecrystallized reduction region, it is desirable to add niobium in an amount of 0.04% or more and 0.05% or less of the total weight of the steel.

[0040] Titanium (Ti): 0.01% or more, 0.018% or less

[0041] Titanium improves the toughness and strength of hot-rolled products by impeding austenite grain growth during welding and refining the weld microstructure through the formation of Ti(C,N) precipitates with excellent high-temperature stability. If the titanium content is less than 0.01%, the additive effect is insufficient. If the titanium content exceeds 0.018%, coarse precipitates are formed, which can reduce the toughness of the steel. Therefore, it is desirable to add titanium in an amount of 0.01% or more and 0.018% or less of the total weight of the steel.

[0042] Vanadium (V): 0.025% or more, 0.035% or less

[0043] Vanadium (V), like niobium (Nb), is a component that exhibits a precipitation strengthening effect when added in small amounts. If the vanadium content is less than 0.025 weight%, the addition effect is insufficient. In the carbon range of the present invention, if the vanadium content exceeds 0.035 weight%, it may lead to a decrease in low-temperature toughness and weldability due to a large amount of precipitates, so it is desirable to control the vanadium content to 0.035 weight% or less.

[0044] Phosphorus (P): 0.01% or less (excluding 0%)

[0045] Phosphorus is limited to 0.01% or less (excluding 0%) of the total weight of the steel. Phosphorus is a representative element that reduces impact toughness, so the lower the content, the better. If phosphorus is contained in excess of 0.01%, weldability and toughness may be reduced.

[0046] Sulfur (S): 0.001% or less (excluding 0%)

[0047] Sulfur is limited to 0.001% or less of the total weight of the steel (excluding 0%). Sulfur is an element that is inevitably contained along with phosphorus during the manufacture of steel and can impair the toughness and weldability of the steel. If the sulfur content exceeds 0.001%, it can form sulfide inclusions (MnS), which can worsen resistance to stress corrosion cracking and cause cracks during processing of the steel, thereby reducing the corrosion resistance of the steel.

[0048] Boron (B): 0.0005% or less (excluding 0%)

[0049] Boron (B) is an element that improves the hardenability of steel. However, when a large amount of boron is added with a boron content exceeding 0.0005 weight%, there is a problem of impairing the surface quality of the steel sheet due to the formation of boron oxides and a problem of rapidly increasing the brittleness of the steel.

[0050] The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the ordinary manufacturing process, they cannot be excluded. As these impurities are known to any person skilled in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0051] Carbon equivalent (C) of the above steel eq ) and weld crack susceptibility index (P cm ) is equal to Equation 1 and Equation 2, respectively.

[0052] [Equation 1]

[0053] C eq = [C] + [Mn] / 6 + ([Ni] + [Cu]) / 15 + ([Cr] + [Mo] + [V]) / 5

[0054] [Equation 2]

[0055] P cm = [C] + [Si] / 30 + ([Mn] + [Cu] + [Cr]) / 20 + [Ni] / 60 + [Mo] / 15 + [V] / 10 + 5[B]

[0056] In the above Equations 1 and 2, [C], [Mn], [Ni], [Cu], [Cr], [Mo], [V], [Si], and [B] represent the content of carbon (C), manganese (Mn), nickel (Ni), copper (Cu), chromium (Cr), molybdenum (Mo), vanadium (V), silicon (Si), and boron (B) contained in the steel, and each unit is weight%.

[0057] The above steel has a carbon equivalent (C) according to Formula 1 above. eq ) can be 0.29 to 0.39. Carbon equivalent (C according to the above Formula 1) eq If ) exceeds 0.39, the weldability of the present invention may be reduced.

[0058] The above steel has a weld crack susceptibility index (P according to Equation 2) cm ) can be 0.13 to 0.18. The weld crack susceptibility index (P according to Equation 2 above) cm If ) exceeds 0.18, weldability may be reduced.

[0059] The steel for line pipes manufactured by controlling the specific components of the alloy composition and the content range thereof as described above, and through the method of manufacturing steel described below, can satisfy tensile strength (TS): 520 MPa ~ 760 MPa, yield strength (YS): 415 MPa ~ 565 MPa, yield ratio (YR): 93% or less (excluding 0%), and elongation (EL): 27% or more.

[0060] The final microstructure of the above-mentioned steel for line pipes may consist of acicular ferrite, polygonal ferrite, and bainitic ferrite.

[0061] In the final microstructure of the steel for the line pipe, the area fraction of the acicular ferrite may be 5 to 20%, the area fraction of the polygonal ferrite may be 10 to 25%, and the area fraction of the bainitic ferrite may be 60 to 80%. The area fraction refers to the area ratio derived from an image of the microstructure of the steel using an image analyzer.

[0062] The steel for line pipes manufactured by controlling the specific components and content ranges of the alloy composition described above and the steel manufacturing method described below is characterized by having a Relative Notch Tensile Strength (RNTS) of 0.86 or higher according to ASTM G142 standards.

[0063] ASTM G142 provides a test method for evaluating the reactivity between gases and solids, so RNTS is used to specifically measure and compare the resistance of materials to structural defects under these conditions.

[0064] Specifically, RNTS (Relative Notch Tensile Strength) is an indicator of hydrogen embrittlement resistance, and the notch tensile strength (NTS) in a hydrogen atmosphere H ) and notch tensile strength in an atmospheric environment (NTS R It refers to the ratio of ). A higher RNTS value indicates that material property degradation did not occur significantly in a hydrogen environment, which can be considered excellent resistance to hydrogen embrittlement. Conversely, a lower value indicates that hydrogen embrittlement occurred more significantly in a hydrogen environment than in an atmospheric environment, which means that resistance to hydrogen embrittlement is inferior. A higher fracture toughness value of the steel indicates greater resistance to crack propagation, which can be considered excellent resistance to hydrogen embrittlement.

[0065] In this invention, the Slow Strain Rate Test (SSRT), one of the priority tests for hydrogen embrittlement resistance, was utilized to determine the excellent hydrogen embrittlement resistance performance in a high-pressure hydrogen environment. The Slow Strain Rate Test (SSRT) in a high-pressure hydrogen environment is an experiment designed to evaluate the hydrogen embrittlement of metallic materials and the resulting changes in mechanical properties. In the Slow Strain Rate Test, the tensile speed is set very slowly; for example, the strain is 10 -6 s -1 ~ 10 -5 s -1 By setting it as such, this is a test to measure the stress-strain relationship of a metal, which is necessary to understand how a material reacts in an environment such as hydrogen, particularly the changes that occur during micro-deformation processes. By analyzing the stress-strain curve obtained after the test, it is possible to determine whether a decrease in ductility and brittle fracture of the material occurred in the hydrogen environment.

[0066] Another aspect of the present invention provides a method for manufacturing a steel material for line pipes with excellent hydrogen embrittlement resistance. According to this, the method comprises the steps of: hot rolling a steel material having the aforementioned alloy composition; and cooling the hot-rolled steel material.

[0067] A method for manufacturing a steel material for line pipes with excellent hydrogen embrittlement resistance according to the present invention will be described below with reference to the attached drawings.

[0068] Method of manufacturing steel

[0069] FIG. 1 is a process flowchart schematically illustrating a method for manufacturing steel for line pipes according to an embodiment of the present invention.

[0070] In the method for manufacturing steel according to the present invention, the semi-finished product subject to the hot rolling process may be, for example, a slab. The slab in the semi-finished product state can be obtained through a continuous casting process after obtaining molten steel of a predetermined composition through a steelmaking process.

[0071] The above steel comprises, in weight%, Carbon (C): 0.04% or more and 0.07% or less, Silicon (Si): 0.20% or more and 0.30% or less, Manganese (Mn): 1.2% or more and 1.35% or less, Aluminum (Al): 0.02% or more and 0.05% or less, Copper (Cu): 0.10% or less (excluding 0%), Chromium (Cr): 0.15% or more and 0.25% or less, Molybdenum (Mo): 0.02% or more and 0.08% or less, Nickel (Ni): 0.15% or more and 0.25% or less, Niobium (Nb): 0.04% or more and 0.05% or less, Titanium (Ti): 0.01% or more and 0.018% or less, Vanadium (V): 0.025% or more and 0.035% or less, and Phosphorus (P): 0.01% or less (0%) The composition comprises (excluding), sulfur (S): 0.001% or less (excluding 0%), boron (B): 0.0005% or less (excluding 0%), and the remainder being iron (Fe) and unavoidable impurities. For example, a steel material for a line pipe with excellent hydrogen embrittlement resistance according to one embodiment of the present invention may be composed of alloying elements having the above-described content ranges.

[0072] Referring to FIG. 1, a method for manufacturing steel for a line pipe according to an embodiment of the present invention includes a hot rolling step (S100) and a cooling step (S200).

[0073] Hot rolling step (S100)

[0074] In the method for manufacturing steel for line pipes according to an embodiment of the present invention, a reheating process may be performed first before hot rolling.

[0075] In the reheating step, steel having the above composition, for example, a slab plate, is reheated at a slab reheating temperature (SRT) of 1050°C to 1150°C. Through this reheating, the re-dissolution of components segregated during casting and the re-dissolution of precipitates may occur.

[0076] When the reheating temperature is below 1050°C, there is a problem in that the solid solution of impurities and precipitate-forming elements is insufficient, and components segregated during casting are not sufficiently evenly distributed. When the reheating temperature exceeds 1150°C, very coarse austenite grains are formed, making it difficult to secure strength. Furthermore, as the reheating temperature increases, there is a problem in that it causes increased manufacturing costs and decreased productivity due to heating costs and the additional time required to meet the hot rolling temperature.

[0077] The heated steel material is first subjected to hot rolling after heating to adjust its shape. The hot rolling can be performed continuously in width rolling, rough rolling, and finish rolling. Through the hot rolling step, the steel material can form a steel plate. The thickness of the hot-rolled steel plate after the hot rolling is completed can be, for example, 30 mm or less.

[0078] Among the hot rolling above, the rolling start temperature (FST) of the finishing rolling above can be set within the range of 850 to 950°C, and the rolling end temperature (FRT) of the finishing rolling above can be set within the range of 850 to 950°C. Furthermore, within the above-mentioned temperature range, the rolling end temperature (FRT) of the finishing rolling above is set lower than the rolling start temperature (FST) of the finishing rolling above.

[0079] In the present invention, the rolling end temperature (FRT) of the finishing rolling can be set within a temperature range existing in the austenite single-phase region, which is a temperature higher than Ar3. More strictly, in the present invention, the finishing rolling is performed from the recrystallization stop temperature (T) immediately above Ar3. nr It can be performed in the unrecrystallized temperature range corresponding to the temperature range below ).

[0080] When rolling in the unrecrystallization temperature range, pancake-shaped austenite is formed. The formation of deformation bands triggers nucleation within the grain bands, leading to phase transformations from austenite to ferrite. As the number of nucleation sites increases, the grain size is refined. Consequently, high-strength and high-toughness steels can be manufactured even with reduced amounts of alloying elements due to improved strengthening and toughness. This allows for the expectation of improved mechanical properties while reducing production costs.

[0081] In the present invention, if the rolling start temperature (FST) or the rolling end temperature (FRT) of the finishing rolling is lower than 850°C, the toughness and strength may be reduced as a pearlite structure is formed. If the rolling start temperature (FST) or the rolling end temperature (FRT) of the finishing rolling is higher than 950°C, it may be difficult to secure low yield ratio and low-temperature fracture resistance toughness.

[0082] Cooling step (S200)

[0083] The above hot-rolled steel can be accelerated cooling by applying cooling water.

[0084] The starting temperature (SCT) of the above accelerated cooling may be a temperature of Ar3 or lower. If the temperature difference between the finishing rolling end temperature (FRT) and the starting temperature (SCT) of accelerated cooling is significantly high, a natural air cooling step may be performed during the process of transporting the steel sheet before the hot rolling is finished and accelerated cooling is performed.

[0085] The end temperature (FCT) of the above accelerated cooling may be 400 to 600°C. The cooling step may be performed at a cooling rate of 30 to 60°C / s. When cooling within this range of cooling rates, a sufficient low-temperature microstructure can be secured while preventing the phenomenon where hardness increases and low-temperature toughness decreases. Since phase transformation must be intensively generated during cooling, cooling after hot rolling should be as fast as possible, and a cooling rate of 30 to 60°C / s may be appropriate to create a low-temperature transformation structure (e.g., acicular ferrite).

[0086] If the cooling rate of the above accelerated cooling is lower than 30℃ / s, the toughness and strength may be reduced as a pearlite structure is formed in the final microstructure. If the above hot-rolled steel is subjected to a coiling process without accelerated cooling, the cooling rate becomes relatively slow, which may result in pearlite formation in the final microstructure.

[0087] The present invention will be described in detail below through experimental examples; however, these are merely preferred embodiments of the invention, and the scope of the invention is not limited by the scope described in these experimental examples. Content not described herein can be sufficiently technically inferred by those skilled in the art, so such description is omitted.

[0088] Experimental Example

[0089] Table 1 shows the composition of the steel for line pipes in the experimental example. The content unit of each component is weight%.

[0090] CSiMnPSAlCuCr Example 1 0.05130.2491.2860.00680.00080.0350.0160.17 Example 2 0.0450.2431.2080.00640.00060.0350.0140.14 Example 3 0.07080.2331.5260.00980.00070.0450.0730.13 Comparative Example 1 0.07330.261.3940.01020.00250.0360.0510.03Comparative Example 20.19490.3521.5140.01050.00210.0370.1430.13Comparative Example 30.0770.261.060.0070.00200.0110.025Comparative Example 40.0350.451.110.0070.00200 .3130.103MoNiNbVTiBCeqPcm Example 1 0.05 0.17 0.04 40.03 20.015 0.0004 0.328 0.145 Example 2 0.04 0.17 0.04 40.03 0.015 0.0003 0.301 0.131 Example 3 0.01 0.14 0.04 30.003 0.016 0.0002 0.366 0.169 Comparative Example 100.020.0360.0420.0120.00010.3250.161 Comparative Example 200.190.0150.0030.0020.00040.4960.301 Comparative Example 30.0020.0080.02600.0060.0020.2600.141 Comparative Example 40.2110.1030.04200.01300.3110.142

[0091] Referring to Table 1, the compositions of Examples 1 to 3 are in weight%, Carbon (C): 0.04% or more and 0.07% or less, Silicon (Si): 0.20% or more and 0.30% or less, Manganese (Mn): 1.2% or more and 1.35% or less, Aluminum (Al): 0.02% or more and 0.05% or less, Copper (Cu): 0.10% or less (excluding 0%), Chromium (Cr): 0.15% or more and 0.25% or less, Molybdenum (Mo): 0.02% or more and 0.08% or less, Nickel (Ni): 0.15% or more and 0.25% or less, Niobium (Nb): 0.04% or more and 0.05% or less, Titanium (Ti): 0.01% or more and 0.018% or less, Vanadium (V): 0.025% or more and 0.035% or less, Phosphorus (P): It satisfies the compositional range in which greater than 0% and less than or equal to 0.01%, Sulfur (S): greater than 0% and less than or equal to 0.001%, Boron (B): greater than 0% and less than or equal to 0.0005%, and the remainder is Iron (Fe). In addition, it satisfies the range in which the sum of the elemental contents of Chromium (Cr), Molybdenum (Mo), and Nickel (Ni) is 0.25% or more and 0.50% or less. Furthermore, the carbon equivalent (C eq ): Satisfies the range of 0.29 ~ 0.39, and the weld crack susceptibility index (P cm ): satisfies the range of 0.13 to 0.18. The steel having the composition of Example 1 is T nr It has characteristic temperatures of : 937℃, Ar3: 776℃, Bs: 577℃, Ms: 459℃.

[0092] In contrast, the composition of Comparative Example 1, in weight percent, exceeds the compositional range of Carbon (C): 0.04% or more and 0.07% or less, Manganese (Mn): 1.2% or more and 1.35% or less, Phosphorus (P): greater than 0% and 0.01% or less, Sulfur (S): greater than 0% and 0.001% or less, and Vanadium (V): 0.025% or more and 0.035% or less, and does not satisfy the requirements; it also falls below the compositional range of Chromium (Cr): 0.15% or more and 0.25% or less, Molybdenum (Mo): 0.02% or more and 0.08% or less, Nickel (Ni): 0.15% or more and 0.25% or less, and Niobium (Nb): 0.04% or more and 0.05% or less, and does not satisfy the requirements. The steel material having the composition of Comparative Example 1 is T nr It has characteristic temperatures of : 911℃, Ar3: 783℃, Bs: 584℃, Ms: 452℃.

[0093] The composition of Comparative Example 2, in weight percent, exceeds the compositional range of Carbon (C): 0.04% or more and 0.07% or less, Silicon (Si): 0.20% or more and 0.30% or less, Manganese (Mn): 1.2% or more and 1.35% or less, Copper (Cu): 0.10% or less (excluding 0%), Phosphorus (P): greater than 0% and 0.01% or less, and Sulfur (S): greater than 0% and 0.001% or less, and thus does not satisfy the requirements; it also falls below the compositional range of Chromium (Cr): 0.15% or more and 0.25% or less, Molybdenum (Mo): 0.02% or more and 0.08% or less, Niobium (Nb): 0.04% or more and 0.05% or less, Vanadium (V): 0.025% or more and 0.035% or less, and Titanium (Ti): 0.01% or more and 0.018% or less, and thus does not satisfy the requirements. Cannot. Steel having the composition of Comparative Example 2 is T nr It has characteristic temperatures of : 874℃, Ar3: 716℃, Bs: 556℃, Ms: 388℃.

[0094] The composition of Comparative Example 3, in weight%, exceeds the compositional range of carbon (C): 0.04% or more and 0.07% or less, sulfur (S): greater than 0% and less than 0.001%, and does not satisfy the requirements, and falls below the compositional range of manganese (Mn): 1.2% or more and less than 1.35%, aluminum (Al): 0.02% or more and less than 0.05%, chromium (Cr): 0.15% or more and less than 0.25%, molybdenum (Mo): 0.02% or more and less than 0.08%, nickel (Ni): 0.15% or more and less than 0.25%, niobium (Nb): 0.04% or more and less than 0.05%, titanium (Ti): 0.01% or more and less than 0.018%, and vanadium (V): 0.025% or more and less than 0.035%, and does not satisfy the requirements.

[0095] The composition of Comparative Example 4, in weight%, exceeds the compositional range of silicon (Si): 0.20% or more and 0.30% or less, sulfur (S): greater than 0% and less than 0.001%, copper (Cu): 0.10% or less (excluding 0%), and molybdenum (Mo): 0.02% or more and less than 0.08%, and does not satisfy the requirements, and falls below the compositional range of carbon (C): 0.04% or more and less than 0.07%, manganese (Mn): 1.2% or more and less than 1.35%, aluminum (Al): 0.02% or more and less than 0.05%, chromium (Cr): 0.15% or more and less than 0.25%, nickel (Ni): 0.15% or more and less than 0.25%, and vanadium (V): 0.025% or more and less than 0.035%, and does not satisfy the requirements.

[0096] Furthermore, Comparative Examples 1 and 3 do not satisfy the requirement because the sum of the elemental content (unit: weight%) of chromium (Cr), molybdenum (Mo), and nickel (Ni) falls below the range of 0.25% to 0.50%.

[0097] Comparative Example 2 is carbon equivalent (C eq ): Exceeds the range of 0.29 ~ 0.39 and does not satisfy, and the weld crack susceptibility index (P cm ): It exceeds the range of 0.13 to 0.18 and does not satisfy.

[0098] Comparative Example 3 is the carbon equivalent (Ceq ): It falls below the range of 0.29 to 0.39 and does not satisfy.

[0099] Table 2 shows the reheating temperature (SRT), the rolling start temperature (FST) for finishing rolling, the rolling end temperature (FRT) for finishing rolling, the cooling start temperature (SCT) for accelerated cooling, the cooling end temperature (FCT) for accelerated cooling, and the cooling rate (CR) for accelerated cooling as manufacturing methods of the experimental examples. The unit of temperature is °C, and the unit of cooling rate is °C / s.

[0100] Reheating Temperature (SRT) Rolling Start Temperature (FST) Rolling End Temperature (FRT) Cooling Start Temperature (SCT) Cooling End Temperature (FCT) Cooling Rate (CR) Example 1 1 3 2 8 9 2 8 8 4 7 5 7 5 1 7 4 7 Example 2 1 1 3 4 9 3 6 9 5 0 8 5 7 4 8 1 4 8 Example 3 1 1 3 2 9 2 3 8 6 4 7 9 7 5 5 3 4 1 Comparative Example 1 1 3 8 9 3 5 8 5 7 7 6 1 5 4 6 1 5 Comparative Example 2 1 1 4 7 9 7 9 2 4 --- Comparative Example 3 1 1 2 0 9 1 0 8 5 0 --- 1 Comparative Example 4 1 0 8 0 9 1 0 8 7 0 --- 1 0

[0101] Referring to Table 2, the process conditions of Example 1, Example 2, Example 3, and Comparative Example 1 satisfy the ranges of reheat temperature (SRT): 1050 ~ 1150℃, rolling start temperature (FST): 850~950℃, rolling end temperature (FRT): 850~950℃, cooling start temperature (SCT): Ar3 or lower (776℃ or lower), cooling end temperature (FCT): 400~600℃, and cooling rate (CR): 30~60℃ / s.

[0102] In contrast, Comparative Example 2 does not satisfy the range of rolling start temperature (FST): 850~950℃ and exceeds it. In addition, Comparative Examples 3 and 4 do not satisfy the range of cooling rate (CR): 30~60℃ / s and fall below it.

[0103] Table 3 shows the strength, elongation, yield ratio, and grain size of the steel for line pipes in the experimental example. In Table 3, YS represents yield strength, TS represents tensile strength, EL represents elongation, and YR represents yield ratio.

[0104] YS(MPa)TS(MPa)YR(%)EL(%) Particle Size (FGS) Example 1 52 3.360 8.88640 12.9 Example 2 50 9.859 1.88642 12.8 Example 3 48 3.956 9.18545 13.9 Comparative Example 1 39 7.449 9.68049 11.5 Comparative Example 2 35 8.756 1.86443 - Comparative Example 3 36 1.045 0.080 -8~8.5 Comparative Example 4 58 3.466 5.288 -10~10.5

[0105] Referring to Table 3, it can be confirmed that the steel of Examples 1, 2, and 3 satisfies the physical properties of tensile strength (TS): 520 MPa ~ 760 MPa, yield strength (YS): 415 MPa ~ 565 MPa, elongation (EL): 27% or more, and yield ratio (YR): 93% or less (excluding 0%).

[0106] In contrast, the steel of Comparative Example 1 does not satisfy the requirements as it falls below the range of tensile strength (TS): 520 MPa ~ 760 MPa and yield strength (YS): 415 MPa ~ 565 MPa, the steel of Comparative Example 2 does not satisfy the requirements as it falls below the range of yield strength (YS): 415 MPa ~ 565 MPa, the steel of Comparative Example 3 does not satisfy the requirements as it falls below the range of tensile strength (TS): 520 MPa ~ 760 MPa, and the steel of Comparative Example 4 does not satisfy the requirements as it exceeds the range of yield strength (YS): 415 MPa ~ 565 MPa.

[0107] Table 4 shows the results of the hydrogen embrittlement evaluation of the steel for line pipes in the experimental example.

[0108] In this experimental example, the Slow Strain Tensile Test (SSRT), one of the priority tests for hydrogen embrittlement resistance, was used to determine the excellent hydrogen embrittlement resistance performance in a high-pressure hydrogen environment. The evaluation criteria for this are as follows.

[0109] Test specimens were prepared as notched specimens according to ASTM G142, with three specimens prepared for each test group and control group (except for Comparative Example 2, which was prepared as only one specimen). The chamber atmosphere for the test group was a hydrogen atmosphere pressurized to a design pressure (e.g., 80 Bar or higher) at room temperature, and the chamber atmosphere for the control group was an atmospheric atmosphere at room temperature, consisting of an inert gas (e.g., nitrogen) atmosphere. The test speed for the notched specimens was applied under conditions of 0.02 mm / s ± 10% according to ASTM G142.

[0110] In Table 4, the TS(@H2) item is the hydrogen notch tensile strength (NTS), which is the notch tensile strength in a hydrogen atmosphere according to ASTM G142. H ) and the TS(@N2) item is the general notched tensile strength (NTS), which is the notched tensile strength in an atmospheric environment according to ASTM G142 standards. R ) and the RNTS item is hydrogen notch tensile strength (NTS H ) and general notch tensile strength (NTS R It represents the ratio of ), and the AVG_RNTS item represents the average RNTS.

[0111] Sample No.TS(@H2)TS(@N2)RNTSAVG_RNTS Example 1 1 1 5 6 1 1 7 8 0.9 8 0.9 2 1 0 5 2 1 1 7 4 0.9 3 1 0 2 8 1 1 7 2 0.8 8 Example 2 1 0.9 1 3 Example 3 1 0.9 5 8 Comparative Example 1 1 8 9 3 9 1 3 0.9 8 0.9 3 2 8 4 4 9 1 5 0.9 2 3 8 0 7 9 1 4 0.8 8 Comparative Example 2 1 8 9 0 1 1 3 0 0.7 9 0.7 9 Comparative Example 3 1 0.8 5 Comparative Example 4 1 0.8 5 6

[0112] Referring to Table 4, it can be confirmed that the steel of Example 1 satisfies the Relative Notch Tensile Strength (RNTS) of 0.86 or higher according to ASTM G142 standards, and strictly speaking, is at the level of 0.88 to 0.98.

[0113] In addition, it can be confirmed that the steel of Examples 2 and 3 has an average Relative Notch Tensile Strength (RNTS) of 0.86 or higher according to ASTM G142 standards, and strictly speaking, is at the level of 0.88 to 0.98.

[0114] In contrast, it can be seen that the steel of Comparative Example 2 does not satisfy the Relative Notch Tensile Strength (RNTS) of 0.86 or higher according to ASTM G142 standards and falls below that level.

[0115] It can be confirmed that the steel materials of Comparative Examples 3 and 4 do not satisfy the average Relative Notch Tensile Strength (RNTS) of 0.86 or higher according to ASTM G142 standards and fall below that level.

[0116] FIGS. 2 to 4 are photographs of the microstructure of steel according to experimental examples of the present invention.

[0117] Specifically, FIG. 2 is a photograph of the microstructure of a steel material according to Example 1 of the present invention, FIG. 3 is a photograph of the microstructure of a steel material according to Comparative Example 1 of the present invention, and FIG. 4 is a photograph of the microstructure of a steel material according to Comparative Example 2 of the present invention.

[0118] Referring to FIG. 2, it can be seen that the final microstructure of Example 1 consists of acicular ferrite, polygonal ferrite, and bainitic ferrite, wherein the area fraction of the acicular ferrite is about 13%, the area fraction of the polygonal ferrite is about 17%, and the area fraction of the bainitic ferrite is about 70%. That is, in the final microstructure of Example 1, the area fraction of the acicular ferrite is 5 to 20%, the area fraction of the polygonal ferrite is 10 to 25%, and the area fraction of the bainitic ferrite satisfies the range of 60 to 80%.

[0119] Referring to FIG. 3, it can be seen that the final microstructure of Comparative Example 1 consists of polygonal ferrite and pearlite, wherein the area fraction of the polygonal ferrite is about 90% and the area fraction of the pearlite is about 10%.

[0120] Referring to Fig. 4, it can be seen that the final microstructure of Comparative Example 2 consists of polygonal ferrite and pearlite, wherein the area fraction of the polygonal ferrite is about 50% and the area fraction of the pearlite is about 50%.

[0121] Up to now, steel for line pipes and a method for manufacturing the same have been described according to the technical concept of the present invention.

[0122] In this invention, to secure hydrogen embrittlement resistance for line pipe steel grades (e.g., API-X60 grade), high-purity steel with low content of phosphorus (P) and sulfur (S) was used to minimize central segregation and inclusion formation, thereby minimizing factors that could adversely affect hydrogen embrittlement. Regarding heating and rolling conditions, the pancaking effect was maximized through rolling in the unrecrystallized zone to achieve a fine microstructure. Additionally, through a rapid cooling process, the remaining austenite was cooled to below the formation temperature of fine bainitic ferrite, thereby realizing a composite microstructure consisting of fine and uniform acicular ferrite, polygonal ferrite, and bainitic ferrite. Accordingly, a steel material with excellent hydrogen embrittlement resistance performance was developed while simultaneously maintaining the strength of the existing API-X60 grade.

[0123] It will be obvious to those skilled in the art that the technical concept of the present invention described above is not limited to the aforementioned embodiments and attached drawings, and that various substitutions, modifications, and changes are possible within the scope of the technical concept of the present invention.

Claims

1. In wt%, Carbon (C): 0.04% or more and 0.07% or less, Silicon (Si): 0.20% or more and 0.30% or less, Manganese (Mn): 1.2% or more and 1.35% or less, Aluminum (Al): 0.02% or more and 0.05% or less, Copper (Cu): 0.10% or less (excluding 0%), Chromium (Cr): 0.15% or more and 0.25% or less, Molybdenum (Mo): 0.02% or more and 0.08% or less, Nickel (Ni): 0.15% or more and 0.25% or less, Niobium (Nb): 0.04% or more and 0.05% or less, Titanium (Ti): 0.01% or more and 0.018% or less, Vanadium (V): 0.025% or more and 0.035% or less, Phosphorus (P): 0.01% or less (excluding 0%), Sulfur (S): 0.001% or less (excluding 0%), Boron (B): 0.0005% or less (excluding 0%), and the remainder consists of iron (Fe) and unavoidable impurities, Characterized by having a Relative Notch Tensile Strength (RNTS) of 0.86 or higher according to ASTM G142 standards, Steel for line pipes.

2. In Paragraph 1, The final microstructure is characterized by being composed of acicular ferrite, polygonal ferrite, and bainitic ferrite. Steel for line pipes.

3. In Paragraph 2, The area fraction of the above needle-shaped ferrite is 5 to 20%, and The area fraction of the above polygonal ferrite is 10 to 25%, and The area fraction of the above bainitic ferrite is 60 to 80%, Steel for line pipes 4. In Paragraph 1 Characterized by tensile strength (TS): 520 MPa ~ 760 MPa, yield strength (YS): 415 MPa ~ 565 MPa, yield ratio (YR): 93% or less, and elongation (EL): 27% or more. Steel for line pipes.

5. In wt%, Carbon (C): 0.04% or more and 0.07% or less, Silicon (Si): 0.20% or more and 0.30% or less, Manganese (Mn): 1.2% or more and 1.35% or less, Aluminum (Al): 0.02% or more and 0.05% or less, Copper (Cu): 0.10% or less (excluding 0%), Chromium (Cr): 0.15% or more and 0.25% or less, Molybdenum (Mo): 0.02% or more and 0.08% or less, Nickel (Ni): 0.15% or more and 0.25% or less, Niobium (Nb): 0.04% or more and 0.05% or less, Titanium (Ti): 0.01% or more and 0.018% or less, Vanadium (V): 0.025% or more and 0.035% or less, Phosphorus (P): 0.01% or less (excluding 0%), A step of hot-rolling steel containing sulfur (S): 0.001% or less (excluding 0%), boron (B): 0.0005% or less (excluding 0%), and the remainder being iron (Fe) and unavoidable impurities; and The method includes the step of cooling the hot-rolled steel material; The above hot rolling step is performed under conditions where the reheat temperature (SRT): 1050~1150℃, the rolling start temperature (FST): 850~950℃, and the rolling end temperature (FRT): 850~950℃, wherein the rolling end temperature (FRT) is lower than the rolling start temperature (FST). The above cooling step is characterized by being performed under conditions where the cooling start temperature (SCT) is Ar3 or lower, the cooling end temperature (FCT) is 400~600℃, and the cooling rate (CR) is 30~60℃ / s. Method for manufacturing steel for line pipes.

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