KR102761143B1 - Steel for line pipe and method of manufacturing the same - Google Patents

Abstract

The present invention provides a steel for a line pipe, which comprises, in wt%, carbon (C): 0.06% to 0.09%, silicon (Si): 0.15% to 0.40%, manganese (Mn): 1.50% to 1.70%, aluminum (Al): 0.02% to 0.06%, molybdenum (Mo): 0.15% to 0.35%, niobium (Nb): 0.04% to 0.06%, vanadium (V): 0.02% to 0.04%, titanium (Ti): 0.01% to 0.03%, phosphorus (P): more than 0% to 0.02% or less, sulfur (S): more than 0% to 0.005% or less, and the remainder being iron (Fe) and other unavoidable impurities, and has a mixed structure of acicular ferrite, polygonal ferrite, and bainite ferrite. Provides.

Description

Steel for line pipe and method of manufacturing the same

The technical idea of the present invention relates to steel and a method for manufacturing the same, and more specifically, to steel for API line pipes for ensuring long-term post-heat treatment and a method for manufacturing the same.

Recently, as the problem of resource depletion has become a problem, oil drilling and transportation work in deep seabed or polar regions is increasing. As such, the demand for energy resource transportation stability is increasing as oil pipeline lines move from general areas to special areas such as permafrost and earthquake zones. Therefore, in order to develop steel for line pipes applicable to extreme environments, research on high strain-capacity steel of 65 kg class is necessary.

Conventional 65kg-class line pipe steels must secure not only high toughness and low-temperature fracture toughness but also high strength characteristics, so they are produced by adding large amounts of alloy elements, but there is a problem of decreased profitability due to increased alloy element addition. In addition, as the usage environment of line pipes has recently changed to high pressure and extreme cold, there is difficulty in responding to the trend of securing high-strength materials even after long-term post-weld heat treatment (PWHT).

Korean Patent Application No. 10-2010-0135245

The technical problem to be achieved by the technical idea of the present invention is to provide a 65 kg-class line pipe steel and a manufacturing method thereof, which can improve the decrease in profitability caused by an increase in the amount of alloying elements added, and at the same time enhance the price competitiveness of the product through optimization of manufacturing conditions in the process line, while guaranteeing the material quality after long-term post-heat treatment.

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

According to one aspect of the present invention, a steel material for a line pipe and a method for manufacturing the same are provided.

According to one embodiment of the present invention, the steel for the line pipe contains, in wt%, carbon (C): 0.06% to 0.09%, silicon (Si): 0.15% to 0.40%, manganese (Mn): 1.50% to 1.70%, aluminum (Al): 0.02% to 0.06%, molybdenum (Mo): 0.15% to 0.35%, niobium (Nb): 0.04% to 0.06%, vanadium (V): 0.02% to 0.04%, titanium (Ti): 0.01% to 0.03%, phosphorus (P): more than 0% to 0.02% or less, sulfur (S): more than 0% to 0.005% or less, and the remainder is iron (Fe) and other unavoidable impurities, and is composed of acicular ferrite, polygonal ferrite, and bainite ferrite. It has a mixed organization.

According to one embodiment of the present invention, the steel for the line pipe may have a carbon equivalent (C _eq ) of 0.40 or less. Here, C _eq = [C] + [Mn]/6 + ([Ni] + [Cu])/15 + ([Cr] + [Mo] + [V])/5.

According to one embodiment of the present invention, the steel for the line pipe may have a weld crack susceptibility index (P _cm ) of 0.19 or less. Here, 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 for line pipe can satisfy, before and after PWHT (Post Weld Heat Treatment), a tensile strength (TS): 625 MPa to 825 MPa, a yield strength (YS): 580 MPa to 705 MPa, an elongation (EL): 21% or more, a low-temperature impact toughness at a temperature of -50°C: 100 J or more, and a hardness: 248 Hv or less.

According to one embodiment of the present invention, the steel for line pipe may have, before and after PWHT (Post Weld Heat Treatment), a fraction of polygonal ferrite of 10 vol.% to 20 vol.%, a fraction of bainite ferrite of 20 vol.% to 40 vol.%, and a fraction of acicular ferrite of 50 vol.% to 70 vol.%.

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

According to one embodiment of the present invention, a method for manufacturing steel for line pipe comprises: heating a steel including, in wt%, carbon (C): 0.06% to 0.09%, silicon (Si): 0.15% to 0.40%, manganese (Mn): 1.50% to 1.70%, aluminum (Al): 0.02% to 0.06%, molybdenum (Mo): 0.15% to 0.35%, niobium (Nb): 0.04% to 0.06%, vanadium (V): 0.02% to 0.04%, titanium (Ti): 0.01% to 0.03%, phosphorus (P): more than 0% to 0.02% or less, sulfur (S): more than 0% to 0.005% or less, and the remainder being iron (Fe) and other unavoidable impurities, at a temperature of 1,000°C to 1,250°C. A method for producing a hot rolled steel sheet, the method comprising: a step of reheating at a temperature of 850°C to 950°C; a step of hot-rolling the heated steel sheet to a temperature of 850°C to 950°C; and a step of cooling the hot-rolled steel sheet.

According to one embodiment of the present invention, the cooling step can be performed at a cooling rate of 20°C/sec to 40°C/sec.

According to one embodiment of the present invention, the cooling step can be performed at a cooling end temperature of 150°C to 350°C.

According to one embodiment of the present invention, the steel for line pipe manufactured by the method for manufacturing the steel for line pipe has a mixed structure of acicular ferrite, polygonal ferrite, and bainite ferrite, and before and after PWHT (Post Weld Heat Treatment), the fraction of the polygonal ferrite may be 10 vol.% to 20 vol.%, the fraction of the bainite ferrite may be 20 vol.% to 40 vol.%, and the fraction of the acicular ferrite may be 50 vol.% to 70 vol.%.

According to one embodiment of the present invention, the steel for line pipe manufactured by the method for manufacturing the steel for line pipe can satisfy, before and after PWHT (Post Weld Heat Treatment), a tensile strength (TS): 625 MPa to 825 MPa, a yield strength (YS): 580 MPa to 705 MPa, an elongation (EL): 21% or more, a low-temperature impact toughness at a temperature of -50°C: 100 J or more, and a hardness: 248 Hv or less.

According to one embodiment of the present invention, the steel for line pipe manufactured by the method for manufacturing the steel for line pipe may have a carbon equivalent (C _eq ) of 0.40 or less and a weld crack susceptibility index (P _cm ) of 0.19 or less. Here, C _eq = [C] + [Mn]/6 + ([Ni] + [Cu])/15 + ([Cr] + [Mo] + [V])/5, and P _cm = [C] + [Si]/30 + ([Mn] + [Cu] + [Cr])/20 + [Ni]/60 + [Mo]/15 + [V]/10 + 5[B].

According to the technical idea of the present invention, it is possible to implement a 65 kg-class line pipe steel and a manufacturing method thereof, which can improve the decrease in profitability due to an increase in the amount of alloying elements added, and at the same time enhance the price competitiveness of the product through optimization of manufacturing conditions in the process line, while guaranteeing the material quality after long-term post-heat treatment.

The effects of the present invention described above are described as examples, and the scope of the present invention is not limited by these effects.

Figure 1 is a process flow diagram schematically showing a method for manufacturing steel for line pipe according to an embodiment of the present invention.
Figure 2 is a microscope photograph showing the microstructure before post-heat treatment of steel for line pipe according to Example 2 among experimental examples of the present invention.
Figure 3 is a microscope photograph showing the microstructure of a steel material for a line pipe after post-heat treatment according to Example 2 of the experimental examples of the present invention.
Figure 4 is a microscope photograph showing the microstructure before post-heat treatment of steel for line pipe according to Comparative Example 1 among experimental examples of the present invention.
FIG. 5 is a drawing showing a microscope photograph and the results of a component analysis of precipitates appearing in a microstructure after post-heat treatment of steel for line pipes according to Example 2 among experimental examples of the present invention.

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 idea 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 idea of the present invention is not limited to the following embodiments. Rather, these embodiments are provided to more faithfully and completely convey the technical idea of the present invention to those skilled in the art. Like reference numerals throughout this specification denote like elements. Furthermore, various elements and areas in the drawings are schematically drawn. Therefore, the technical idea of the present invention is not limited by the relative sizes or intervals drawn in the attached drawings.

Typically, line pipe steel strictly limits the carbon (C) content in order to reduce the carbon equivalent (C _eq ), which is the most sensitive indicator of the relevant elements, since high toughness and weldability are important in addition to mechanical strength. This naturally leads to an increase in the use of alloy ferromagnesium. However, in order to strengthen cost competitiveness due to recent oversupply, each steel company is trying to limit the use of expensive alloy ferromagnesium. To this end, the present invention reduces the addition of alloy ferromagnesium and adds an appropriate level of carbon (C) and manganese (Mn).

In general, when the cooling end temperature is lowered during the production of line pipe steel, the yield strength tends to increase due to the grain refinement effect and the increase in the fraction of low-temperature transformation phase. However, when the temperature drops below a certain level, the yield strength decreases and the tensile strength increases. This is closely related to the fraction of the secondary phase. In addition, since the strength must be guaranteed after a long-term post-heat treatment, the steel plate of the present invention forms acicular ferrite of 50 vol.% to 70 vol.% and low-temperature transformation structure of 20 vol.% to 40 vol.%, and then tempering the low-temperature phase of the low-temperature transformation structure series through post-heat treatment to increase the toughness, and prevent the strength from decreasing by precipitating carbonitrides. The aim is to develop a steel for line pipes.

As mentioned above, expensive alloys such as Cr, Ni, and Cu are essential elements for securing the material of API line pipe steel, which strictly limits the content of C, but they act as a major factor in increasing the manufacturing cost of semi-finished products. Therefore, in the present invention, the results of research for optimizing the addition amount of other trace elements and manufacturing conditions were reflected, excluding the alloys.

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

Steel for line pipes

The steel for line pipe, which is one aspect of the present invention, contains, in wt%, carbon (C): 0.06% to 0.09%, silicon (Si): 0.15% to 0.40%, manganese (Mn): 1.50% to 1.70%, aluminum (Al): 0.02% to 0.06%, molybdenum (Mo): 0.15% to 0.35%, niobium (Nb): 0.04% to 0.06%, vanadium (V): 0.02% to 0.04%, titanium (Ti): 0.01% to 0.03%, phosphorus (P): more than 0% to 0.02% or less, sulfur (S): more than 0% to 0.005% or less, and the remainder being 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 as follows. In this case, the content of the component elements all refers to weight%.

Carbon (C): 0.06% ~ 0.09%

Carbon is added to secure the strength of the steel. If the carbon content is less than 0.06%, it may be difficult to secure the strength. If the carbon content exceeds 0.09%, the low-temperature impact toughness and weldability may deteriorate. Therefore, it is preferable to add carbon at 0.06% to 0.09% of the total weight of the steel.

Silicon (Si): 0.15% ~ 0.40%

Silicon contributes to increasing the strength of steel. In addition, as a ferrite stabilizing element, it is effective in improving the toughness and ductility of steel by inducing ferrite formation. When the silicon content is less than 0.15%, the addition effect is insufficient. When the silicon content exceeds 0.40%, redscale is generated in the heating furnace during the hot rolling process, which deteriorates the surface quality of the steel and may deteriorate the weldability. Therefore, it is preferable that silicon be added in an amount of 0.15% to 0.40% of the total weight of the steel.

Manganese (Mn): 1.50% ~ 1.70%

Manganese is an element that contributes to the enhancement of the hardenability of steel and the strengthening of the steel. When the manganese content is less than 1.50%, the effect of addition is insufficient. When the manganese content exceeds 1.70%, the synergistic effect due to the increase in the amount of addition is minimal, and MnS inclusions and oxides may be formed, which may impair the weldability of the steel when manufacturing line pipes. Therefore, manganese is preferably added at 1.50% to 1.70% of the total weight of the steel.

Aluminum (Al): 0.02% ~ 0.06%

Aluminum acts as a deoxidizer to remove oxygen in the steel. When the aluminum content is less than 0.02%, the effect of addition is insufficient. When the aluminum content exceeds 0.06%, non-metallic inclusions, Al ₂ O _{3 ,} may be formed, which may reduce the low-temperature impact toughness. Therefore, it is preferable that aluminum be added at 0.02% to 0.06% of the total weight of the steel.

Molybdenum (Mo): 0.15% ~ 0.35%

Molybdenum effectively acts on the solution strengthening and improves strength, but if added excessively, the elongation decreases and the carbon equivalent increases, which may reduce weldability. Therefore, it is desirable to control molybdenum to 0.15% to 0.35% of the total weight of the steel sheet.

Niobium (Nb): 0.04% ~ 0.06%

Niobium precipitates carbonitride (NbC) in steel, which acts to pin grain boundaries and hinders grain boundary sliding (GBS) and dislocation movement that occur at high temperatures, thereby improving strength. When the niobium content is less than 0.04%, the addition effect is insufficient. When the niobium content exceeds 0.06%, the synergistic effect due to the increase in the addition amount is minimal, and excessive precipitation may deteriorate the playability, rollability, and elongation. Therefore, it is preferable that niobium be added in an amount of 0.04 to 0.06% of the total weight of the steel.

Vanadium (V): 0.02% ~ 0.04%

Vanadium (V) can delay recrystallization during hot rolling together with niobium (Nb) to promote grain refinement. In addition, it can contribute to strength improvement through solid solution strengthening and formation of composite precipitates. In one specific example, the vanadium is included in an amount of 0.02% to 0.04% of the total weight of the steel. If the content of vanadium is less than 0.02%, it is difficult to achieve the above-described effect, and if it is added in excess of 0.04%, it can lower weldability and cause problems during coiling due to excessive precipitation at low temperatures.

Titanium (Ti): 0.01% ~ 0.03%

Titanium improves the toughness and strength of hot-rolled products by inhibiting austenite grain growth during welding and refining the weld structure by forming Ti(C,N) precipitates with excellent high-temperature stability. When the titanium content is less than 0.01%, the addition effect is insufficient. When the titanium content exceeds 0.03%, coarse precipitates may be formed, which may reduce the toughness of the steel. Therefore, it is preferable that titanium be added in an amount of 0.01% to 0.03% of the total weight of the steel.

P: More than 0% ~ less than 0.02%

Phosphorus is a representative element that reduces impact toughness, and the lower the content, the better. If phosphorus is included in excess of 0.02%, weldability and toughness may deteriorate. It is desirable to limit phosphorus to 0% to 0.02% of the total weight of the steel.

Sulfur (S): More than 0% ~ less than 0.005%

Sulfur is an element that is inevitably included with phosphorus during the production of steel, and can deteriorate the toughness and weldability of the steel. If the sulfur content exceeds 0.005%, it may form sulfide inclusions (MnS), which may worsen the resistance to stress corrosion cracking, causing cracks to occur during processing of the steel, and consequently reducing the corrosion resistance of the steel. It is preferable to limit sulfur to more than 0% and less than 0.005% of the total weight of the steel.

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

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

[Formula 1]

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

[Formula 2]

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

In the above formulas 1 and 2, [C], [Mn], [Ni], [Cu], [Cr], [Mo], [V], [Si] and [B] represent the contents 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%.

The above steel may have a carbon equivalent (C _eq ) of 0.4 or less according to the above formula 1. For example, the carbon equivalent (C _eq ) may be 0.2 to 0.4. When the carbon equivalent (C _eq ) according to the above formula 1 exceeds 0.4, weldability may deteriorate.

The above steel may have a weld crack susceptibility index (P _cm ) of 0.19 or less according to the above formula 2. For example, the weld crack susceptibility index (P _cm ) may be 0.11 to 0.19. If the weld crack susceptibility index (P _cm ) according to the above formula 2 exceeds 0.19, the weldability may deteriorate.

The steel for line pipe manufactured by controlling the specific components of the alloy composition described above and the content range thereof and through the method for manufacturing the steel described below can satisfy the following requirements: tensile strength (TS): 625 MPa to 825 MPa, yield strength (YS): 580 MPa to 705 MPa, elongation (EL): 21% or more, low-temperature impact toughness at a temperature of -50℃: 100 J or more, and hardness: 248 Hv or less.

Furthermore, even in the steel after heat treatment, for example, PWHT (Post Weld Heat Treatment), applied during or after forming a line pipe by forming the steel of the present invention, the steel can satisfy the following requirements: tensile strength (TS): 625 MPa to 825 MPa, yield strength (YS): 580 MPa to 705 MPa, elongation (EL): 21% or more, low-temperature impact toughness at a temperature of -50°C: 100 J or more, and hardness: 248 Hv or less.

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

The fraction of the polygonal ferrite may be, for example, 10 vol.% to 20 vol.%, and the fraction of the bainite ferrite may be, for example, 20 vol.% to 40 vol.%. In addition, the fraction of the needle-shaped ferrite may include 50 vol.% to 70 vol.%, for example, 70% to 90%. The above-described fraction range may also be applied to the steel after heat treatment, for example, PWHT (Post Weld Heat Treatment), applied during or after forming the line pipe by manufacturing the steel of the present invention. Meanwhile, the fraction means a volume ratio derived from a microstructure photograph of the steel through an image analyzer.

The average particle size of the above ferrite can be in the range of 10 μm to 15 μm.

Referring to the attached drawings below, a method for manufacturing steel for line pipe according to the present invention is described.

Method of manufacturing steel

Figure 1 is a process flow diagram schematically showing a method for manufacturing steel for 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 to be subjected to the hot rolling process may be, for example, a slab. The slab in the semi-finished state can be obtained through a continuous casting process after obtaining molten steel of a predetermined composition through a steelmaking process.

The above steel contains, in wt%, carbon (C): 0.06% to 0.09%, silicon (Si): 0.15% to 0.40%, manganese (Mn): 1.50% to 1.70%, aluminum (Al): 0.02% to 0.06%, molybdenum (Mo): 0.15% to 0.35%, niobium (Nb): 0.04% to 0.06%, vanadium (V): 0.02% to 0.04%, titanium (Ti): 0.01% to 0.03%, phosphorus (P): more than 0% to 0.02% or less, sulfur (S): more than 0% to 0.005% or less, and the remainder being iron (Fe) and other unavoidable impurities.

Referring to FIG. 1, a method for manufacturing steel for 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 method for manufacturing the steel for the line pipe may include a step (S110) of reheating the steel having the above composition at a temperature of 1,000°C to 1,250°C; a step (S120) of hot-rolling the reheated steel to a temperature of 850°C to 950°C; and a step (S130) of cooling the hot-rolled steel.

The above cooling step can be performed at a cooling rate of 20°C/sec to 40°C/sec to a cooling end temperature of 150°C to 350°C.

Reheating step (S110)

In the reheating step (S110), the steel having the above composition, for example, a slab plate, is reheated at a reheating temperature (Slab Reheating Temperature, SRT) of, for example, 1,000°C to 1,250°C. Through this reheating, re-dissolution of components segregated during casting and re-dissolution of precipitates can occur. If the reheating temperature is less than 1,000°C, the rolling load may increase because the reheating temperature is low, and complete dissolution of niobium may become difficult, which may reduce the fine dispersion effect and make it difficult to secure strength. In addition, since the dissolution temperature of niobium-based precipitates such as NbC and NbN is not reached, they are not re-precipitated as fine precipitates during hot rolling, which fails to suppress the grain growth of austenite, which may cause the austenite grains to rapidly coarsen. In addition, there is a problem that the solidification of impurities and precipitate-forming elements is not sufficient, and the components segregated during casting are not evenly distributed sufficiently. If the reheating temperature exceeds 1,250℃, the austenite grains rapidly coarsen, making it difficult to secure the strength and low-temperature toughness of the steel sheet being manufactured. In addition, as the reheating temperature increases, there is a problem that the manufacturing cost increases and the productivity decreases due to the heating cost and the additional time required to match the hot rolling temperature.

Hot rolling stage (S120)

The above heated steel is first heated to adjust its shape, and then hot rolled. The above hot rolling can be performed sequentially as width rolling, rough rolling, and finish rolling. By the above hot rolling step, the steel can be formed into a steel plate.

The above hot rolling, i.e., the above-mentioned rolling, may be completed at a finish rolling temperature (FRT) of, for example, 850°C to 950°C, for example, 900°C to 950°C. These rolling conditions may be conditions under which single-phase rolling on Ar3 is performed. When the above-mentioned finish rolling temperature is less than 850°C, abnormal rolling may occur, forming a non-uniform structure, which may significantly reduce the low-temperature impact toughness. When the above-mentioned rolling finish temperature exceeds 950°C, the ductility and toughness may be excellent, but the strength may rapidly decrease.

Cooling stage (S130)

The above hot-rolled steel is cooled to a cooling end temperature of 150°C to 350°C at a cooling rate of 20°C/sec to 40°C/sec. The cooling can be performed by air cooling or water cooling. When cooling within the above cooling rate range, the phenomenon of hardness increasing and low-temperature toughness decreasing can be prevented, while sufficiently securing a low-temperature microstructure.

When the above-described rapid cooling rate and cooling end temperature are controlled within the above-described range, the material properties of the 65 kg API line pipe can be secured through the formation of needle ferrite and low-temperature transformation structure (bainite, some martensite). Furthermore, the tempering effect of the low-temperature transformation structure can be imparted through long-term post-heat treatment (e.g., PWHT; Post Weld Heat Treatment), and the prevention of strength decline after long-term post-heat treatment through carbide precipitation can be implemented.

Hereinafter, in order to help understand the present invention, preferred experimental examples are presented. However, the following experimental examples are only intended to help understand the present invention, and the present invention is not limited to the following experimental examples. Since the contents not described herein can be sufficiently technically inferred by those skilled in the art, their explanations will be omitted.

Experimental example

Tables 1 and 2 show the compositions of steels for line pipes of comparative examples and examples. The remainder in Tables 1 and 2 consists of iron (Fe) and impurities inevitably contained in the steelmaking process, etc. The unit of content of each component is weight%. Table 2 shows the carbon equivalent (C _eq ) and the weld crack susceptibility index (P _cm ).

division C Si Mn Al Mo Nb V Ti Comparative examples 1 and 2 0.06 0.25 1.7 0.04 0.20 0.04 0.02 0.015 Examples 1, 2, 3 0.07 0.25 1.6 0.04 0.25 0.05 0.03 0.020

division P S Cu Cr Ni C _eq P _cm Comparative examples 1 and 2 0.01 0.003 0.15 0.3 0.3 0.477 0.196 Examples 1, 2, 3 0.01 0.003 0 0 0 0.392 0.178

Referring to Tables 1 and 2, Examples 1, 2, and 3 satisfy the range of composition of steel for line pipe according to one embodiment of the present invention in weight %, carbon (C): 0.06% to 0.09%, silicon (Si): 0.15% to 0.40%, manganese (Mn): 1.50% to 1.70%, aluminum (Al): 0.02% to 0.06%, molybdenum (Mo): 0.15% to 0.35%, niobium (Nb): 0.04% to 0.06%, vanadium (V): 0.02% to 0.04%, titanium (Ti): 0.01% to 0.03%, phosphorus (P): more than 0% to 0.02% or less, sulfur (S): more than 0% to 0.005% or less, and the remainder is iron (Fe).

In contrast, Comparative Examples 1 and 2 have a slightly higher manganese (Mn) content and a slightly lower molybdenum (Mo), niobium (Nb), vanadium (V), and titanium (Ti) content than Examples 1, 2, and 3. In particular, Comparative Examples 1 and 2 are unique in that they contain more copper (Cu), chromium (Cr), and nickel (Ni), unlike Examples 1, 2, and 3.

Examples 1, 2, and 3 having the compositions described above have a carbon equivalent (C _eq ) of 0.392 according to Equation 1 described above, satisfying the range of 0.4 or less, and a weld crack susceptibility index (P _cm ) of 0.178 according to Equation 2 described above, satisfying the range of 0.19 or less. In contrast, Comparative Examples 1 and 2 have a carbon equivalent (C _eq ) of 0.477, which exceeds the range of 0.4 or less and thus dissatisfies, and a weld crack susceptibility index (P _cm ) of 0.196, which exceeds the range of 0.19 or less and thus dissatisfies.

Table 3 shows the process condition values of examples and comparative examples of steel for line pipes. The steel thickness is 31.8t.

division Reheating temperature
(℃) Rolling end temperature
(℃) Cooling end temperature
(℃) Cooling rate
(℃/sec) Comparative Example 1 1172 831 482 21 Comparative Example 2 1191 840 520 25 Example 1 1198 895 350 34 Example 2 1199 903 311 35 Example 3 1196 892 286 30

Referring to Table 3, Examples 1, 2, and 3 satisfy the process conditions of the method for manufacturing steel for line pipe according to the embodiments of the present invention, specifically, they all satisfy the ranges of reheating temperature: 1,000°C to 1,250°C, rolling completion temperature: 850°C to 950°C, cooling rate: 20°C/sec to 40°C/sec, and cooling completion temperature: 150°C to 350°C.

In contrast, Comparative Examples 1 and 2 are not satisfactory because they fall below the rolling end temperature range of 850°C to 950°C, and are not satisfactory because they exceed the cooling end temperature range of 150°C to 350°C.

Tables 4 and 5 show the mechanical properties of the manufactured line pipe steel, such as tensile strength (TS, unit: MPa), yield strength (YS, unit: MPa), elongation (EL, unit: %), low-temperature impact toughness at -50℃ (unit: J), and hardness (unit: Hv), before and after PWHT (Post Weld Heat Treatment), respectively. The PWHT (Post Weld Heat Treatment) applied a heat treatment condition of maintaining the temperature at 610℃ for 240 minutes. The low-temperature impact toughness shows the results of performing the impact test three times. The hardness measurement was performed under the condition of Hv 10 max 248.

division YS
(Before PWHT) YS
(After PWHT) TS
(Before PWHT) TS
(After PWHT) EL
(Before PWHT) EL
(After PWHT) Comparative Example 1 558 571 698 705 31 39 Comparative Example 2 581 556 707 656 39 43 Example 1 662 654 776 751 29 39 Example 2 653 628 784 786 31 42 Example 3 636 641 756 771 34 46

division Low temperature impact toughness
(Before PWHT) Low temperature impact toughness
(After PWHT) hardness
(Before PWHT) hardness
(After PWHT) Comparative Example 1 308/299/34 321/313/316 217 220 Comparative Example 2 281/292/274 255/281/271 216 219 Example 1 232/206/210 361/331/231 231 221 Example 2 268/258/231 315/326/341 246 224 Example 3 223/286/261 306/301/312 223 219

Referring to Tables 4 and 5, Examples 1, 2, and 3 all satisfy the following requirements: tensile strength (TS): 625 MPa to 825 MPa, yield strength (YS): 580 MPa to 705 MPa, elongation (EL): 21% or more, low-temperature impact toughness at a temperature of -50℃: 100 J or more, and hardness: 248 Hv or less before and after PWHT (Post Weld Heat Treatment). In contrast, Comparative Example 1 fails to satisfy because it falls below the range of yield strength (YS): 580 MPa to 705 MPa before and after PWHT (Post Weld Heat Treatment), and Comparative Example 2 fails to satisfy because it falls below the range of yield strength (YS): 580 MPa to 705 MPa after PWHT (Post Weld Heat Treatment).

In Examples 1, 2, and 3, it can be confirmed that the material before and after heat treatment is superior even when the amount of alloy iron is reduced due to the effect of reducing the cooling end temperature compared to Comparative Examples 1 and 2.

Fig. 2 is a microscope photograph showing the microstructure before post-heat treatment of steel for line pipes according to Example 2 among the experimental examples of the present invention, Fig. 3 is a microscope photograph showing the microstructure after post-heat treatment of steel for line pipes according to Example 2 among the experimental examples of the present invention, and Fig. 4 is a microscope photograph showing the microstructure before post-heat treatment of steel for line pipes according to Comparative Example 1 among the experimental examples of the present invention. The post-heat treatment, PWHT (Post Weld Heat Treatment), applied heat treatment conditions of maintaining the temperature at 610°C for 240 minutes.

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

(Unit: Volume%) Example 2
(Before PWHT) Example 2
(After PWHT) Comparative Example 1
(Before PWHT) Bed type ferrite (AF) 63 68 51 Polygonal ferrite (PF) 14 12 42 Bainite ferrite (BF) 23 20 7 Average grain size (μm) 13.8 12.6 12.3

Referring to FIGS. 2 to 4 and Table 6, Example 2 satisfies all ranges in which the fraction of the polygonal ferrite is 10 vol.% to 20 vol.%, the fraction of the bainite ferrite is 20 vol.% to 40 vol.%, and the fraction of the needle-shaped ferrite is 50 vol.% to 70 vol.% before and after PWHT (Post Weld Heat Treatment).

In contrast, Comparative Example 1 is not satisfactory because the fraction of polygonal ferrite exceeds the range of 10 vol.% to 20 vol.%, and is not satisfactory because the fraction of bainite ferrite falls below the range of 20 vol.% to 40 vol.%.

Example 2 can be confirmed that the fraction of low-temperature transformed structure (bainite ferrite) increases from 7% to 23% due to the effect of reducing the cooling end temperature compared to Comparative Example 1. Example 2 can be confirmed that the mechanical properties before and after the heat treatment, PWHT (Post Weld Heat Treatment), are excellent due to the small change in microstructure due to the increase in the fraction of needle-shaped ferrite and low-temperature transformed structure, bainite ferrite, compared to Comparative Example 1. Meanwhile, in the example, the material can be improved after the heat treatment due to the precipitate effect.

Figure 5 is a drawing showing a microscope photograph and the results of a component analysis of precipitates appearing in the microstructure after post-heat treatment of steel for line pipes according to Example 2 among experimental examples of the present invention. The post-heat treatment, PWHT (Post Weld Heat Treatment), applied heat treatment conditions of maintaining the temperature at 610°C for 240 minutes.

Referring to Fig. 5, it can be confirmed that coarse precipitates larger than 10 nm and fine precipitates smaller than 10 nm exist after post-heat treatment. As a result of component analysis, it was confirmed that coarse precipitates larger than 10 nm are (Ti, Nb)(C, N) precipitates, and fine precipitates smaller than 10 nm are (Mo, V, Nb)(C) precipitates.

So far, the steel for line pipe and the manufacturing method thereof according to the technical idea of the present invention have been described.

At high temperatures, niobium (Nb), titanium (Ti), vanadium (V), etc. are the most important elements for controlling microstructure along with carbon as carbonitride-forming elements of steel. Therefore, in the present invention, by adding small amounts of niobium (Nb), titanium (Ti), and vanadium (V), precipitate phases are formed at high temperatures, thereby suppressing recrystallization of austenite, making grains finer, and forming precipitate phases to increase strength. In addition, molybdenum (Mo) was additionally added to form carbides such as NbC, VC, and MoC at the post-heat treatment temperature, thereby preventing a decrease in strength. Since small additions of niobium (Nb), titanium (Ti), vanadium (V), and molybdenum (Mo) elements cannot replace the role of improving mechanical properties by forming internal solid solution or precipitation phases, like chromium (Cr), nickel (Ni), and copper (Cu), a low-cost 65 kg-class API line pipe steel was developed by utilizing the following concept during the rolling process. The material properties of the 65 kg-class API line pipe can be secured through the formation of needle ferrite and low-temperature transformation structure (bainite, some martensite) by rapid internal angle. Furthermore, the tempering effect of the low-temperature transformation structure can be imparted through long-term post-heat treatment (e.g., PWHT; Post Weld Heat Treatment), and the prevention of strength decline after long-term post-heat treatment through carbide precipitation can be implemented. In order to achieve low cost and overcome material deviation due to rapid cooling, the composition range of carbon and manganese was limited, and in order to manage elements that increase the amount of pressure in the non-recrystallized zone and elements precipitated after heat treatment, the composition range of niobium, titanium, vanadium, and molybdenum was limited.

The present invention provides a 65 kg-class high-strength API line pipe steel plate that maximizes the acicular ferrite fraction and controls the low-temperature transformed structure fraction through the alloy composition and process control, thereby reducing alloy elements and having excellent properties compared to comparable steel even after long-term post-heat treatment. The steel can be applied to various customer needs by improving profitability through reducing high-cost alloy elements such as copper (Cu), nickel (Ni), and chromium (Cr), and stably securing the material before/after post-heat treatment, thereby increasing the usability of the steel.

It will be apparent to a person skilled in the art that the technical idea of the present invention described above is not limited to the above-described embodiments and the attached drawings, and that various substitutions, modifications, and changes are possible within a scope that does not depart from the technical idea of the present invention.

Claims (12)

In weight %, carbon (C): 0.06% to 0.09%, silicon (Si): 0.15% to 0.40%, manganese (Mn): 1.50% to 1.70%, aluminum (Al): 0.02% to 0.06%, molybdenum (Mo): 0.15% to 0.35%, niobium (Nb): 0.04% to 0.06%, vanadium (V): 0.02% to 0.04%, titanium (Ti): 0.01% to 0.03%, phosphorus (P): more than 0% to 0.02% or less, sulfur (S): more than 0% to 0.005% or less, and the remainder includes iron (Fe) and other unavoidable impurities.
It has a mixed structure of needle-shaped ferrite, polygonal ferrite and bainite ferrite.
Having a carbon equivalent (Ceq) of 0.40 or less,
Steel for line pipes.
[Equation 1] Ceq = [C] + [Mn]/6 + ([Ni] + [Cu])/15 + ([Cr] + [Mo] + [V])/5
(In Formula 1, [C], [Mn], [Ni], [Cu], [Cr], [Mo] and [V] represent the contents of carbon (C), manganese (Mn), nickel (Ni), copper (Cu), chromium (Cr), molybdenum (Mo) and vanadium (V) contained in the steel, and each unit is wt%. However, if the steel for line pipes does not contain Ni, Ni in [Formula 1] is set to 0, if the steel for line pipes does not contain Cu, Cu in [Formula 1] is set to 0, and if the steel for line pipes does not contain Cr, Cr in [Formula 1] is set to 0.) delete In paragraph 1,
The steel for the above line pipe is,
Having a welding crack susceptibility index (P _cm ) of 0.19 or less,
Steel for line pipes.
[Equation 2] Pcm = [C] + [Si]/30 + ([Mn] + [Cu] + [Cr])/20 + [Ni]/60 + [Mo]/15 + [V]/10 + 5[B]
(In Formula 2, [C], [Si], [Mn], [Cu], [Cr], [Ni], [Mo], [V], and [B] represent the contents 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 wt%. However, if the steel for line pipes does not contain Ni, Ni in [Formula 2] is set to 0, if the steel for line pipes does not contain Cu, Cu in [Formula 2] is set to 0, if the steel for line pipes does not contain Cr, Cr in [Formula 2] is set to 0, and if the steel for line pipes does not contain B, B in [Formula 2] is set to 0.) In paragraph 1,
The steel for the above line pipe satisfies the following requirements before and after post-weld heat treatment (PWHT): tensile strength (TS): 625 MPa to 825 MPa, yield strength (YS): 580 MPa to 705 MPa, elongation (EL): 21% or more, low-temperature impact toughness at a temperature of -50℃: 100 J or more, and hardness: 248 Hv or less.
Steel for line pipes. In paragraph 1,
The steel for the above line pipe has, before and after post-weld heat treatment (PWHT), the fraction of the polygonal ferrite is 10 vol.% to 20 vol.%, the fraction of the bainite ferrite is 20 vol.% to 40 vol.%, and the fraction of the needle-shaped ferrite is 50 vol.% to 70 vol.%.
Steel for line pipes. In paragraph 1,
The average particle size of the above ferrite is in the range of 10 μm to 15 μm.
Steel for line pipes. A step of reheating a steel material containing, in wt%, carbon (C): 0.06% to 0.09%, silicon (Si): 0.15% to 0.40%, manganese (Mn): 1.50% to 1.70%, aluminum (Al): 0.02% to 0.06%, molybdenum (Mo): 0.15% to 0.35%, niobium (Nb): 0.04% to 0.06%, vanadium (V): 0.02% to 0.04%, titanium (Ti): 0.01% to 0.03%, phosphorus (P): more than 0% to 0.02% or less, sulfur (S): more than 0% to 0.005% or less, and the remainder being iron (Fe) and other unavoidable impurities, at a temperature of 1,000°C to 1,250°C;
A step of hot rolling the above heated steel to a temperature of 850℃ to 950℃; and
A step of cooling the hot-rolled steel material is included;
The above cooling step is performed at a cooling rate of 20°C/sec to 40°C/sec,
Cooled to a cooling end temperature of 150℃ ~ 350℃,
Method for manufacturing steel for line pipe. delete delete In paragraph 7,
The steel for line pipe manufactured by the above method for manufacturing steel for line pipe is,
It has a mixed structure of needle-shaped ferrite, polygonal ferrite and bainite ferrite.
Before and after post-weld heat treatment (PWHT), the fraction of the polygonal ferrite is 10 vol.% to 20 vol.%, the fraction of the bainite ferrite is 20 vol.% to 40 vol.%, and the fraction of the needle-shaped ferrite is 50 vol.% to 70 vol.%.
Method for manufacturing steel for line pipe. In paragraph 7,
The steel for line pipe manufactured by the above method for manufacturing steel for line pipe is,
Before and after post-weld heat treatment (PWHT, Post Weld Heat Treatment), tensile strength (TS): 625 MPa to 825 MPa, yield strength (YS): 580 MPa to 705 MPa, elongation (EL): 21% or more, low-temperature impact toughness at -50℃: 100 J or more, and hardness: 248 Hv or less.
Method for manufacturing steel for line pipe. In paragraph 7,
The steel for line pipe manufactured by the above method for manufacturing steel for line pipe is,
Having a carbon equivalent (C _eq ) of less than 0.40,
Having a welding crack susceptibility index (P _cm ) of 0.19 or less,
Method for manufacturing steel for line pipe.
[Formula 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] represent the contents 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 wt%. However, when the steel for line pipes does not contain Ni, Ni in [Equations 1 and 2] is set to 0, when the steel for line pipes does not contain Cu, Cu in [Equations 1 and 2] is set to 0, when the steel for line pipes does not contain Cr, Cr in [Equations 1 and 2] is set to 0, and when the steel for line pipes does not contain B, B in [Equation 2] is set to 0.) )

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