KR102122643B1 - Steel for line pipe and manufacturing method thereof - Google Patents

Abstract

Disclosed is an invention related to a steel material for line pipes and a method for manufacturing the same. In one embodiment, the method for manufacturing the steel for the line pipe is carbon (C): 0.04 to 0.07% by weight, silicon (Si): 0.2 to 0.4% by weight, manganese (Mn): 1.4 to 1.7% by weight, phosphorus (P): More than 0 0.020% by weight or less, sulfur (S): more than 0 0.005% by weight or less, soluble aluminum (S_Al): 0.02 to 0.06% by weight, chromium (Cr): 0.2 to 0.3% by weight, molybdenum (Mo): 0.05 to 0.10 Weight percent, nickel (Ni): 0.15 to 0.25 weight percent, niobium (Nb): 0.07 to 0.10 weight percent, titanium (Ti): 0.005 to 0.015 weight percent, and the remaining amount of iron (Fe) and other unavoidable impurities Reheating; Hot rolling the reheated slab to a finish rolling temperature of 870 to 930° C. to produce a rolled material; And cooling the rolled material to a cooling end temperature: 450 to 550°C.

Description

Steel material for line pipe and its manufacturing method {STEEL FOR LINE PIPE AND MANUFACTURING METHOD THEREOF}

The present invention relates to a steel material for line pipes and a method for manufacturing the same. More specifically, the present invention relates to a steel material for line pipes having excellent strength, low temperature fracture toughness, and weldability, and a method for manufacturing the same.

Recently, as the problem of resource depletion has emerged, oil drilling and transportation operations in the deep sea or the polar regions are increasing. Accordingly, there is an increasing need for the development of high-strength, high-toughness line pipe steel having excellent low-temperature impact characteristics. In the case of API (American Petroleum Institute) steel pipes for oil pipelines used for transportation of crude oil, they are classified into API-X70, X80, and X100 according to the material strength. On the other hand, the API-X70 steel material forms a low-temperature transformation microstructure through accelerated cooling and deterioration in order to realize high strength, and in this process, there is a problem that the toughness of the material decreases.

Prior art related to the present invention is Korean Patent Application Publication No. 2008-0036476 (published on April 28, 2008. Name of the invention: steel for large-diameter line pipes having excellent resistance to hydrogen organic cracking and a method of manufacturing the same).

According to an embodiment of the present invention, it is to provide a steel material for line pipes having excellent high strength, weldability, and low temperature fracture toughness characteristics.

According to an embodiment of the present invention, it is to provide a steel material for line pipe excellent in productivity and economic efficiency.

According to an embodiment of the present invention, to provide a steel pipe manufacturing method.

One aspect of the present invention relates to a method for manufacturing a steel material for line pipes. In one embodiment, the method for manufacturing the steel for the line pipe is carbon (C): 0.04 to 0.07% by weight, silicon (Si): 0.2 to 0.4% by weight, manganese (Mn): 1.4 to 1.7% by weight, phosphorus (P): More than 0 0.020% by weight or less, sulfur (S): more than 0 0.005% by weight or less, soluble aluminum (S_Al): 0.02 to 0.06% by weight, chromium (Cr): 0.2 to 0.3% by weight, molybdenum (Mo): 0.05 to 0.10 Weight percent, nickel (Ni): 0.15 to 0.25 weight percent, niobium (Nb): 0.07 to 0.10 weight percent, titanium (Ti): 0.005 to 0.015 weight percent, and the remaining amount of iron (Fe) and other unavoidable impurities Reheating; Hot rolling the reheated slab to a finish rolling temperature of 870 to 930° C. to produce a rolled material; And cooling the rolled material to a cooling end temperature: 450 to 550° C., wherein the slab has a Ceq value of 0.39 or less according to Equation 1, and a Pcm value of Equation 2 below is 0.17 or less:

[Equation 1]

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

[Equation 2]

Pcm = [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] are included in the slab Content of carbon (C), manganese (Mn), nickel (Ni), copper (Cu), chromium (Cr), molybdenum (Mo), vanadium (V), silicon (Si) and boron (B) Weight percent)).

In one embodiment, the reheating, slab reheating temperature: 1100 ~ 1200 ℃ may be carried out under conditions.

In one embodiment, the cooling may be performed at a cooling rate of 40° C./sec or higher.

Another aspect of the present invention relates to a steel material for line pipes manufactured by the steel pipe manufacturing method. In one embodiment, the steel material for the line pipe is carbon (C): 0.04 to 0.07 wt%, silicon (Si): 0.2 to 0.4 wt%, manganese (Mn): 1.4 to 1.7 wt%, phosphorus (P): more than 0 0.020 wt% or less, sulfur (S): more than 0 0.005 wt% or less, soluble aluminum (S_Al): 0.02 to 0.06 wt%, chromium (Cr): 0.2 to 0.3 wt%, molybdenum (Mo): 0.05 to 0.10 wt% , Nickel (Ni): 0.15 to 0.25% by weight, Niobium (Nb): 0.07 to 0.10% by weight, Titanium (Ti): 0.005 to 0.015% by weight and residual iron (Fe) and other inevitable impurities, and microstructure This acicular ferrite (accicular ferrite, AF) contains more than 65% by volume, the Ceq value according to the following formula 1 is 0.39 or less, and the Pcm value according to the following formula 2 may be 0.17 or less:

[Equation 1]

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

[Equation 2]

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

(In the formulas 1 and 2, [C], [Mn], [Ni], [Cu], [Cr], [Mo], [V], [Si], and [B] are included in the steel material. Content of carbon (C), manganese (Mn), nickel (Ni), copper (Cu), chromium (Cr), molybdenum (Mo), vanadium (V), silicon (Si) and boron (B) Weight percent)).

In one embodiment, the steel material may have a tensile strength (TS): 600 to 700 MPa, a yield strength (YS): 500 to 600 MPa, and an elongation (El): 30% or more.

In one embodiment, the microstructure of the steel material includes 65 to 75% by volume of acicular ferrite, 10 to 18% by volume of polygonal ferrite (PF) and 12 to 20% by volume of bainitic ferrite (BF). Can be.

In one embodiment, the microstructure of the steel material may have an average grain size of 15 μm or less.

In one embodiment, the steel material may have a ductile wave front modulus of 95% or higher at 0°C, a ductile wave front modulus of 95% or higher at -20°C, and a ductile wave front modulus at -30°C of 80% or higher.

The steel material for line pipes of the present invention is excellent in high strength, weldability, and low temperature fracture toughness characteristics, and may be excellent in productivity and economy.

Figure 1 shows a steel pipe manufacturing method according to an embodiment of the present invention.
Figure 2 (a) shows a microstructure of an example steel material according to the present invention, Figure 2 (b) is an optical micrograph showing the microstructure of a comparative example steel material according to the present invention.
Figure 3 is a photograph showing the results of the DWTT test for the comparative example steel for the present invention.
Figure 4 is a photograph showing the results of the DWTT test for the steel material of the embodiment of the present invention.

Hereinafter, the present invention will be described in detail. At this time, in the description of the present invention, when it is determined that the detailed description of the related known technology or configuration may unnecessarily obscure the subject matter of the present invention, the detailed description will be omitted.

In addition, terms to be described later are terms defined in consideration of functions in the present invention, which may vary according to a user's or operator's intention or practice, and thus the definition should be made based on the contents of the present specification describing the present invention.

Line Pipe Steel manufacturing method

One aspect of the present invention relates to a method for manufacturing a steel material for line pipes. Figure 1 shows a steel pipe manufacturing method according to an embodiment of the present invention. Referring to Figure 1, the steel pipe manufacturing method (S10) slab reheating step; (S20) rolling material manufacturing step; And (S30) cooling step. More specifically, the steel pipe manufacturing method (S10) carbon (C): 0.04 ~ 0.07% by weight, silicon (Si): 0.2 ~ 0.4% by weight, manganese (Mn): 1.4 ~ 1.7% by weight, phosphorus ( P): more than 0 and 0.020 wt% or less, sulfur (S): more than 0 and 0.005 wt% or less, soluble aluminum (S_Al): 0.02 to 0.06 wt%, chromium (Cr): 0.2 to 0.3 wt%, molybdenum (Mo): 0.05~0.10% by weight, nickel(Ni): 0.15~0.25% by weight, niobium(Nb): 0.07~0.10% by weight, titanium(Ti): 0.005~0.015% by weight and residual iron(Fe) and other unavoidable impurities Reheating the containing slab; (S20) a step of hot rolling the reheated slab to a finish rolling temperature: 870 to 930°C to prepare a rolled material; And (S30) cooling the rolled material to a cooling end temperature: 450 to 550°C.

Hereinafter, the steel pipe manufacturing method according to the present invention will be described in detail step by step.

(S10) Slab reheating step

The steps include: carbon (C): 0.04 to 0.07 wt%, silicon (Si): 0.2 to 0.4 wt%, manganese (Mn): 1.4 to 1.7 wt%, phosphorus (P): more than 0 and 0.020 wt% or less, sulfur ( S): more than 0 and less than 0.005% by weight, soluble aluminum (S_Al): 0.02 to 0.06% by weight, chromium (Cr): 0.2 to 0.3% by weight, molybdenum (Mo): 0.05 to 0.10% by weight, nickel (Ni): 0.15 This step is to reheat the slab containing ~0.25% by weight, niobium (Nb): 0.07 to 0.10% by weight, titanium (Ti): 0.005 to 0.015% by weight, and the remaining amount of iron (Fe) and other inevitable impurities.

Hereinafter, the components included in the slab of the present invention will be described in detail.

Carbon (C)

The carbon is added to ensure the strength of the steel. In one embodiment, the carbon is contained 0.04 to 0.07% by weight relative to the total weight of the slab. When the carbon content is less than 0.04% by weight, it is difficult to secure the strength, and when it exceeds 0.07% by weight, low-temperature impact toughness and weldability may be deteriorated.

silicon( Si )

The silicon (Si) contributes to increasing the strength of the steel. In addition, as a ferrite stabilizing element, it is effective in improving the toughness and ductility of steel by inducing ferrite formation. In one embodiment, the silicone is included in an amount of 0.2 to 0.4% by weight based on the total weight of the slab. When the silicon is contained in an amount of less than 0.2% by weight, the effect is minimal, and when it exceeds 0.4% by weight, the surface quality of the steel is deteriorated and the weldability is deteriorated by generating redscale in the heating furnace during the hot rolling process. Can be.

Manganese (Mn)

The manganese (Mn) is an element that contributes to strengthening the solid solution and improving the hardenability of the steel. In one embodiment, the manganese is included in an amount of 1.4 to 1.7% by weight based on the total weight of the slab. When the manganese content is less than 1.4% by weight, the addition effect is negligible, and when it exceeds 1.7% by weight, the synergistic effect according to the increase in the addition amount is negligible, forming MnS inclusions and oxides. Can be inhibited.

Phosphorus (P)

The phosphorus (P) is a representative element that lowers the impact toughness, the lower the content, the better. The phosphorus is contained in an amount of more than 0 and 0.020% by weight or less based on the total weight of the slab. When the phosphorus exceeds 0.020% by weight, weldability and toughness may deteriorate.

Sulfur (S)

The sulfur (S) is an element that is inevitably contained in the production of steel, and may inhibit the toughness and weldability of the steel. The sulfur is contained in an amount of more than 0 and 0.005% by weight or less based on the total weight of the slab. When the sulfur is included in an amount exceeding 0.005% by weight, an emulsion-based inclusion (MnS) is formed to deteriorate resistance to stress corrosion cracking, thereby generating cracks during steel processing, and as a result, corrosion resistance of the steel may be reduced.

Soluble aluminum (S_Al)

The soluble aluminum (S_Al) serves as a deoxidizer for removing oxygen in the steel. In one embodiment, the soluble aluminum is contained 0.02 to 0.06% by weight relative to the total weight of the slab. When the soluble aluminum is included in an amount of less than 0.02% by weight, the effect of adding it is small, and when it exceeds 0.06% by weight, there is a problem of lowering low-temperature impact toughness by forming Al ₂ O ₃ as a non-metallic inclusion.

chrome( Cr )

The chromium (Cr) is added to secure the strength. In one embodiment, the chromium is contained 0.2 to 0.3% by weight relative to the total weight of the slab. When the chromium content is less than 0.2% by weight, the effect is minimal, and when it exceeds 0.3% by weight, weldability and toughness of the heat-affected zone (HAZ) may be deteriorated.

molybdenum( Mo )

The molybdenum (Mo) is a substitutional element, and contributes to improving the strength of steel through solid solution strengthening. In addition, it improves the hardenability and corrosion resistance of steel. In one embodiment, the molybdenum is contained 0.05 to 0.10% by weight relative to the total weight of the slab. When the molybdenum is contained in an amount of less than 0.05% by weight, the effect of the addition is negligible, and when it exceeds 0.10% by weight, the synergistic effect according to the increase in the amount of addition is small, and weldability may be deteriorated.

nickel( Ni )

The nickel (Ni) contributes to improving the strength of the steel through solid solution strengthening. In addition, it improves the hardenability and corrosion resistance of steel. In one embodiment, the nickel is contained 0.15 to 0.25% by weight based on the total weight of the slab. When the nickel is contained in an amount of less than 0.15% by weight, the additive effect is negligible, and when it is more than 0.25% by weight, the synergistic effect according to an increase in the added amount is small, and weldability may be deteriorated.

Niobium ( Nb )

The niobium (Nb) precipitates carbonitride (NbC) in the steel, and serves to pin grain boundaries, thereby preventing grain boundary sliding (GBS) and dislocation movement occurring at high temperatures in the present invention. , Can improve the strength.

In one embodiment, the niobium is contained 0.07 to 0.10% by weight relative to the total weight of the slab. When the niobium is contained in an amount of less than 0.07% by weight, the effect of the addition is negligible, and when it exceeds 0.10% by weight, the synergistic effect according to the increase in the amount of addition is negligible, and performance, rollability and elongation may be reduced due to excessive precipitation. have. For example, 0.07, 0.08, 0.09 or 0.10% by weight may be included.

titanium( Ti )

The titanium (Ti) prevents austenite grain growth during welding by generating a Ti(C,N) precipitate having excellent high-temperature stability, thereby minimizing the structure of the weld to improve the toughness and strength of the hot-rolled product. In one embodiment, the titanium is contained in an amount of 0.005 to 0.015% by weight based on the total weight of the slab. When the titanium is contained in an amount of less than 0.005% by weight, the effect of the addition is negligible, and when it exceeds 0.015% by weight, coarse precipitates are generated to deteriorate the toughness of the steel.

In one embodiment, the slab has a Ceq value according to Equation 1 below of 0.39 and a Pcm value of Equation 2 below of 0.17:

[Equation 1]

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

[Equation 2]

Pcm = [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] are included in the slab Content of carbon (C), manganese (Mn), nickel (Ni), copper (Cu), chromium (Cr), molybdenum (Mo), vanadium (V), silicon (Si) and boron (B) Weight percent)).

When the Ceq value according to Equation 1 exceeds 0.39, the weldability of the present invention decreases, and when the Pcm value according to Equation 2 exceeds 0.17, the weldability of the present invention decreases. For example, the slab may have a Ceq value according to Equation 1 of 0.33 to 0.39. For example, the slab may have a Pcm value according to Equation 2 of 0.14 to 0.17.

In one embodiment, the reheating, slab reheating temperature: 1100 ~ 1200 ℃ may be carried out under conditions. When reheating under the above conditions, austenite grains grow, and the solid solution of impurities and precipitates in the steel can easily occur.

(S20) Rolling material Manufacturing stage

The step is a step of hot rolling the reheated slab under the conditions of a finish rolling temperature: 870 to 930°C, thereby producing a rolled material. When the austenite grains are recrystallized or not recrystallized during hot rolling, the dislocation density in the grain increases, and the microstructure graininess of the final product can be refined.

When the finishing rolling temperature is performed at less than 870°C, toughness and strength are lowered by formation of a non-uniform structure, and when exceeding 930°C, the austenite grains are coarsened and ferrite grains are not sufficiently refined after transformation. It can be difficult to secure strength.

In one embodiment, the thickness of the rolled material may be less than 30 mm.

(S30) Cooling step

The step is a step of cooling the rolled material to a cooling end temperature: 450 to 550°C. In one embodiment, if winding is performed simultaneously with cooling, the cooling end temperature may be a coiling temperature (CT). When the cooling end temperature is less than 450°C, mechanical properties such as toughness of the present invention are lowered, and when it exceeds 550°C, it is difficult to secure the acicular ferrite microstructure of the present invention, and strength may be lowered.

In one embodiment, the cooling may be performed at a cooling rate of 40° C./sec or higher. When cooling at the cooling rate, while preventing the phenomenon that the hardness increases excessively and the low-temperature toughness decreases, a low-temperature microstructure can be sufficiently secured. For example, it can be performed at a cooling rate of 40 to 60°C/sec.

Line Pipe Manufactured by steel manufacturing method Line Pipe Steel

Another aspect of the present invention relates to a steel material for line pipes manufactured by the steel pipe manufacturing method. In one embodiment, the steel material for the line pipe is carbon (C): 0.04 to 0.07 wt%, silicon (Si): 0.2 to 0.4 wt%, manganese (Mn): 1.4 to 1.7 wt%, phosphorus (P): more than 0 0.020 wt% or less, sulfur (S): more than 0 0.005 wt% or less, soluble aluminum (S_Al): 0.02 to 0.06 wt%, chromium (Cr): 0.2 to 0.3 wt%, molybdenum (Mo): 0.05 to 0.10 wt% , Nickel (Ni): 0.15 to 0.25% by weight, Niobium (Nb): 0.07 to 0.10% by weight, Titanium (Ti): 0.005 to 0.015% by weight and residual iron (Fe) and other inevitable impurities, and microstructure This acicular ferrite (accicular ferrite, AF) contains more than 65% by volume, the Ceq value according to the following formula 1 is 0.39 or less, and the Pcm value according to the following formula 2 may be 0.17 or less:

[Equation 1]

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

[Equation 2]

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

(In the formulas 1 and 2, [C], [Mn], [Ni], [Cu], [Cr], [Mo], [V], [Si], and [B] are included in the steel material. Content of carbon (C), manganese (Mn), nickel (Ni), copper (Cu), chromium (Cr), molybdenum (Mo), vanadium (V), silicon (Si) and boron (B) Weight percent)).

Since the components and contents of the steel material for line pipes are the same as the aforementioned slabs, detailed descriptions thereof will be omitted.

When the Ceq value according to Equation 1 exceeds 0.39, the weldability of the present invention decreases, and when the Pcm value according to Equation 2 exceeds 0.17, the weldability of the present invention decreases. For example, the steel material for the line pipe may have a Ceq value according to Equation 1 of 0.33 to 0.39. For example, the steel material for the line pipe may have a Pcm value according to Equation 2 of 0.14 to 0.17.

In one embodiment, the thickness of the steel material for the line pipe may be less than 30mm.

In one embodiment, the steel material may have a tensile strength (TS): 600 to 700 MPa, a yield strength (YS): 500 to 600 MPa, and an elongation (El): 30% or more. For example, the elongation may be 30 to 45%.

If the steel microstructure of the present invention comprises acicular ferrite (accicular ferrite, AF) less than 65% by volume, strength and toughness may be reduced.

In one embodiment, the microstructure of the steel material includes 65 to 75% by volume of acicular ferrite, 10 to 18% by volume of polygonal ferrite (PF) and 12 to 20% by volume of bainitic ferrite (BF). Can be. When the microstructure of the above conditions is included, high strength and low temperature toughness may be excellent. For example, the microstructure of steel materials is 65 to 75% by volume of acicular ferrite, 10 to 18% by volume of polygonal ferrite (PF), 12 to 20% by volume of bainitic ferrite (BF), and martensite/ Austenite (M/A) may contain 1 to 5% by volume.

In one embodiment, the microstructure of the steel material may have an average grain size (or an average ferrite grain size (F.G.S)) of 15 μm or less. Under the above conditions, high strength and toughness may be excellent. For example, it may be 10 to 15㎛.

In one embodiment, the steel material may have a ductile wave front ratio of 95% or higher at -20°C and a ductile wave front ratio of 80% or higher at -30°C in the DWTT test. For example, the steel material may have a ductile wave front modulus of 95% or higher at 0°C, a ductile wave front modulus of 95% or higher at -20°C, and a ductile wave front modulus at -30°C of 80% to 95%.

Hereinafter, the configuration and operation of the present invention through a preferred embodiment of the present invention will be described in more detail. However, this is presented as a preferred example of the present invention and cannot be interpreted as limiting the present invention in any sense.

Example And Comparative example

Example 1-2 and Comparative example 1-2

A slab having the components of Table 1, residual iron (Fe), and other inevitable impurities, and having Ceq according to Equation 1 and Pcm according to Equation 2, was prepared. The slab is reheated at 1150° C., and finish rolling temperature: 870 to 930° C. is hot-rolled to produce a rolled material, and then the rolled material is cooled to a cooling rate of 40 to 60° C./sec. Cooling to ℃ to produce a steel pipe line.

[Equation 1]

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

[Equation 2]

Pcm = [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] are included in the slab Content of carbon (C), manganese (Mn), nickel (Ni), copper (Cu), chromium (Cr), molybdenum (Mo), vanadium (V), silicon (Si) and boron (B) Weight percent)).

Among the steel materials of Examples 1 and 2 and Comparative Examples 1 and 2, the microstructure and average grain size of Examples 1 and 1 were measured, and the results are shown in Table 2 below, and Examples 1 and 2 and Comparative Examples 1 The tensile strength (TP), yield strength (YP), and elongation (EL) of the steel material of ~2 were evaluated and are shown in Table 3 below, and the conditions were 0°C, -10°C, -20°C, -30°C, and -40°C. The DWTT (Drop Weight Tear Test) test was conducted in Table 4 below.

(* AF: acicular ferrite, PF: polygonal ferrite, GB: granular bainite, BF: bainite ferrite, M/A: martensite/austenite)

Figure 2 (a) shows a microstructure of an example steel material according to the present invention, Figure 2 (b) is an optical micrograph showing the microstructure of a comparative example steel material according to the present invention.

Referring to the results of Table 2 and FIG. 2, in the case of the steel material of Example 1 of the present invention, a microstructure in which acicular ferrite having a size of about 2 μm or less was homogeneously distributed was secured, but in the case of Comparative Example 1 steel, granules The bainite was formed, the fraction of martensite/austenite, and the like, and the phase fraction of the low-temperature metamorphic tissue also did not satisfy the conditions of the present invention, and through this, it can be seen that the low-temperature toughness characteristics were lower than in Example 1 above. there was.

3 is a picture showing the results of the DWTT test for the steel material of Comparative Example 2, Figure 4 is a picture showing the results of the DWTT test for the steel material of Example 2.

Referring to the results of Tables 3-4 and FIGS. 3-4, the steels of Examples 1 and 2 and Comparative Examples 1 and 2 are both tensile strength (TS): 600 to 700 MPa, yield strength (YS): 500 to 600 MPa. And elongation (El): 30% or more, but in the DWTT test, Examples 1 to 2 were found to have significantly better low temperature impact toughness than Comparative Examples 1 to 2. In addition, in Examples 1 and 2, the wavefront was formed smoothly compared to Comparative Examples 1 and 2 in the DWTT test, and it was found that the ductile wavefront properties at -20°C, -30°C, and -40°C were excellent.

Through this, the steel material for line pipes of the present invention minimizes the content of high-priced alloy elements such as copper (Cu), vanadium (V), and molybdenum (Mo), thereby reducing production costs and reducing Ceq and Pcm indices. (Nb) Through the increase in the content of NbC, the grain refining was realized compared to the conventional line pipe API-X70M steel through the strengthening effect of NbC, and the hard secondary phase, martensite/austenite (M/A), and coarse precipitate suppression It was found that the low temperature fracture toughness (DWTT) properties were improved.

Simple modifications or changes of the present invention can be easily carried out by those skilled in the art, and all such modifications or changes can be considered to be included in the scope of the present invention.

Claims (8)

Carbon (C): 0.04 to 0.07 wt%, silicon (Si): 0.2 to 0.4 wt%, manganese (Mn): 1.4 to 1.7 wt%, phosphorus (P): more than 0 and 0.020 wt% or less, sulfur (S): More than 0 and less than 0.005% by weight, soluble aluminum (S_Al): 0.02 to 0.06% by weight, chromium (Cr): 0.2 to 0.3% by weight, molybdenum (Mo): 0.05 to 0.10% by weight, nickel (Ni): 0.15 to 0.25% by weight %, niobium (Nb): 0.07 to 0.10% by weight, titanium (Ti): 0.005 to 0.015% by weight, and the remaining amount of iron (Fe) and other unavoidable impurities to reheat the slab;
Hot rolling the reheated slab under a finish rolling temperature of 870 to 930°C to produce a rolled material; And
Cooling the rolling material to the end temperature: 450 ~ 550 ℃; includes,
The slab has a Ceq value according to the following Equation 1 is 0.39 or less,
It is a method for manufacturing a steel pipe line characterized in that the Pcm value according to the following formula 2 is 0.17 or less,
The prepared steel material for line pipe contains 65 to 75% by volume of acicular ferrite, 10 to 18% by volume of polygonal ferrite (PF) and 12 to 20% by volume of bainitic ferrite (BF). Steel pipe manufacturing method characterized in that:
[Equation 1]
Ceq = [C] + [Mn]/6 + ([Ni] + [Cu])/15 + ([Cr] + [Mo] + [V])/5
[Equation 2]
Pcm = [C] + [Si]/30) + ([Mn] + [Cu] + [Cr])/20 + [Ni]/60 + [Mo]/15 + [V]/10 + 5[B ]
(In the formulas 1 and 2, [C], [Mn], [Ni], [Cu], [Cr], [Mo], [V], [Si] and [B] are included in the slab Content of carbon (C), manganese (Mn), nickel (Ni), copper (Cu), chromium (Cr), molybdenum (Mo), vanadium (V), silicon (Si) and boron (B) Weight percent)).
According to claim 1,
The reheating, slab reheating temperature: 1100 ~ 1200 ℃ steel pipe manufacturing method characterized in that carried out under the conditions.
According to claim 1,
The cooling, the method for manufacturing a steel material for a line pipe, characterized in that carried out under a cooling rate of 40 ℃ / sec or more conditions.
Carbon (C): 0.04 to 0.07 wt%, silicon (Si): 0.2 to 0.4 wt%, manganese (Mn): 1.4 to 1.7 wt%, phosphorus (P): more than 0 and 0.020 wt% or less, sulfur (S): More than 0 and less than 0.005% by weight, soluble aluminum (S_Al): 0.02 to 0.06% by weight, chromium (Cr): 0.2 to 0.3% by weight, molybdenum (Mo): 0.05 to 0.10% by weight, nickel (Ni): 0.15 to 0.25% by weight %, Niobium (Nb): 0.07~0.10% by weight, Titanium (Ti): 0.005~0.015% by weight and residual iron (Fe) and other inevitable impurities.
The microstructure includes 65 to 75% by volume of acicular ferrite (AF), 10 to 18% by volume of ferrite (polygonal ferrite, PF), and 12 to 20% by volume of bainitic ferrite (BF),
Ceq value according to the following formula 1 is 0.39 or less,
Steel material for a line pipe, characterized in that the Pcm value according to the following formula 2 is 0.17 or less:
[Equation 1]
Ceq = [C] + [Mn]/6 + ([Ni] + [Cu])/15 + ([Cr] + [Mo] + [V])/5
[Equation 2]
Pcm = [C] + [Si]/30) + ([Mn] + [Cu] + [Cr])/20 + [Ni]/60 + [Mo]/15 + [V]/10 + 5[B ]
(In the formulas 1 and 2, [C], [Mn], [Ni], [Cu], [Cr], [Mo], [V], [Si], and [B] are included in the steel material. Content of carbon (C), manganese (Mn), nickel (Ni), copper (Cu), chromium (Cr), molybdenum (Mo), vanadium (V), silicon (Si) and boron (B) Weight percent)).
The method of claim 4,
The line pipe steel material is a tensile strength (TS): 600 ~ 700MPa, yield strength (YS): 500 ~ 600MPa and elongation (El): 30% or more steel material for the line pipe.
delete The method of claim 4,
The microstructure of the steel material for line pipe is a steel material for line pipe, characterized in that the average grain size is 15㎛ or less.
The method of claim 4,
The steel material for line pipe is a steel material for line pipe, characterized in that the ductile wave front modulus at 0°C is 95% or higher, the ductile wave front modulus at -20°C is 95% or higher, and the ductile wave front modulus at -30°C is 80% or higher.

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