KR20200001182A - Steel for line pipe and manufacturing method thereof - Google Patents

Abstract

Disclosed are a steel material for line pipes and a manufacturing method thereof. More specifically, the manufacturing method thereof includes the following steps of: reheating a slab including 0.04-0.07 wt% of carbon (C), 0.2-0.4 wt% of silicon (Si), 1.4-1.7 wt% of manganese (Mn), more than 0 and no more than 0.020 wt% of phosphorus (P), more than 0 and no more than 0.005 wt% of sulfur (S), 0.02-0.06 wt% of soluble aluminum (S_Al), 0.2-0.3 wt% of chrome (Cr), 0.05-0.10 wt% of molybdenum (Mo), 0.15-0.25 wt% of nickel (Ni), 0.07-0.10 wt% of niobium (Nb), 0.005-0.015 wt% of titanium (Ti), and the remaining of iron (Fe) and inevitable impurities; manufacturing a rolled material by hot-rolling the reheated slab at a finish-rolling temperature of 870-930°C; and cooling the rolled material up to a cooling end temperature of 450-550°C. Therefore, the steel material for line pipes can have high strength as well as excellent weldability and low-temperature toughness.

Description

Steel for Line Pipe and Manufacturing Method {STEEL FOR LINE PIPE AND MANUFACTURING METHOD THEREOF}

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

Recently, with the problem of resource depletion, oil drilling and transportation operations in the deep sea or the polar regions are increasing. Accordingly, the need for development of high strength, high toughness line pipe steel with excellent low temperature impact characteristics is increasing. The API (American Petroleum Institute) steel pipe used for oil transportation is classified into API-X70, X80 and X100 according to the material strength. On the other hand, the API-X70 steel to form a low-temperature transformation microstructure through accelerated cooling harshening to achieve high strength, there was a problem that the toughness of the material is reduced in the process.

Prior art related to the present invention is Korean Patent Publication No. 2008-0036476 (published: 2008.04.28. Name of the invention: hydrogen steel for large diameter line pipes excellent in organic crack resistance and a method of manufacturing the same).

According to one embodiment of the present invention, to provide a line pipe steel excellent in high strength, weldability and low temperature fracture toughness.

According to one embodiment of the present invention, to provide a line pipe steel excellent in productivity and economy.

According to one embodiment of the present invention, there is provided a method for manufacturing a line pipe steel.

One aspect of the present invention relates to a method for producing steel for line pipes. In one embodiment, the steel pipe manufacturing method for the line pipe is 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 to 0.10 Weight%, nickel (Ni): 0.15 to 0.25 weight%, niobium (Nb): 0.07 to 0.10 weight%, titanium (Ti): 0.005 to 0.015 weight% and slab containing residual iron (Fe) and other unavoidable impurities Reheating; Manufacturing a rolled material by hot rolling the reheated slab under a finish rolling temperature of 870 ° C to 930 ° C; And cooling the rolled material to a cooling end temperature of 450 ° C. to 550 ° C .; wherein the slab has a Ceq value of 0.39 or less and a Pcm value of 0.17 or less, according to Equation 1 below:

[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 Formula 1 and Formula 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 may be performed under slab reheating temperature: 1100 to 1200 ° C.

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

Another aspect of the present invention relates to a line pipe steel produced by the method for producing a line pipe steel. In one embodiment, the line pipe steel is 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): greater than 0 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 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 amount of iron (Fe) and other unavoidable impurities, microstructure The acicular ferrite (AF) may be included at 65% by volume or more, the Ceq value according to Equation 1 below 0.39, and the Pcm value according to Equation 2 below 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 Formula 1 and Formula 2, [C], [Mn], [Ni], [Cu], [Cr], [Mo], [V], [Si], and [B] are included in the steel. 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 may be tensile strength (TS): 600 ~ 700MPa, yield strength (YS): 500 ~ 600MPa and elongation (El): 30% or more.

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

In one embodiment, the steel may have a 95% or more ductile fracture rate at 0 ° C., 95% or more at -20 ° C., and 80% or more at −30 ° C.

Steel for line pipes of the present invention is excellent in high strength, weldability and low temperature fracture toughness characteristics, it may be excellent in productivity and economic efficiency.

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

Hereinafter, the present invention will be described in detail. In this case, 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 thereof will be omitted.

The terms to be described below are terms defined in consideration of functions in the present invention, and may be changed according to intentions or customs of users or operators, and the definitions should be made based on the contents throughout the specification for describing the present invention.

For line pipe  Steel manufacturing method

One aspect of the present invention relates to a method for producing steel for line pipes. 1 shows a method for producing a line pipe steel according to one embodiment of the present invention. 1, the steel pipe manufacturing method for the line pipe (S10) slab reheating step; (S20) rolling material manufacturing step; And (S30) cooling step. More specifically, the steel pipe manufacturing method for the line pipe (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): greater than 0 and 0.020 wt% or less, sulfur (S): greater 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 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 balance of iron (Fe) and other unavoidable impurities Reheating the comprising slabs; (S20) hot rolling the reheated slab to a finish rolling temperature: 870 ~ 930 ℃ condition, to produce a rolled material; And (S30) cooling the rolled material to a cooling end temperature: 450 to 550 ° C .;

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

(S10) slab reheating stage

The step is 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): greater than 0 and 0.020% by weight or less, sulfur ( S): more than 0 and 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% 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 reheating the slab containing the remaining iron (Fe) and other unavoidable impurities.

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

Carbon (C)

The carbon is added to secure the strength of the steel. In one embodiment, the carbon is included 0.04 to 0.07% by weight based on the total weight of the slab. When the carbon is less than 0.04% by weight, it is difficult to secure the strength, and when it contains more than 0.07% by weight, low-temperature impact toughness and weldability may be reduced.

silicon( Si )

The silicon (Si) contributes to increasing the strength of the steel. Moreover, as a ferrite stabilizing element, it is effective in improving the toughness and ductility of steel by inducing ferrite formation. In one embodiment the silicon is included 0.2 to 0.4% by weight based on the total weight of the slab. When the silicon is included in less than 0.2% by weight, its addition effect is insignificant, and when it contains more than 0.4% by weight, red scale is generated in the heating furnace during the hot rolling process, resulting in deterioration of the surface quality of the steel and deterioration of weldability. Can be.

Manganese (Mn)

The manganese (Mn) is an element contributing to solid solution strengthening and improving hardenability of steel. In one embodiment the manganese is included 1.4 to 1.7% by weight based on the total weight of the slab. When the manganese is included in less than 1.4% by weight, its addition effect is insignificant, and when it is included in excess of 1.7% by weight, the synergistic effect is increased by increasing the amount added, and MnS inclusions and oxides are formed to form weldability of the steel when the line pipe is formed. Can be inhibited.

Phosphorus (P)

The phosphorus (P) is a representative element that lowers the impact toughness, the lower the content is better. The phosphorus is included 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 be reduced.

Sulfur (S)

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

Soluble Aluminum (S_Al)

The soluble aluminum (S_Al) serves as a deoxidizer to remove oxygen in the steel. In one embodiment the soluble aluminum is included 0.02 ~ 0.06% by weight based on the total weight of the slab. When the soluble aluminum is included in less than 0.02% by weight, its addition effect is insignificant, and when it exceeds 0.06% by weight, there is a problem in that low-temperature impact toughness is lowered 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 included 0.2 to 0.3% by weight based on the total weight of the slab. When the chromium is included in less than 0.2% by weight, its addition effect is insignificant, and when it contains more than 0.3% by weight, the weldability and the toughness of the heat affected zone (HAZ) may be reduced.

molybdenum( Mo )

The molybdenum (Mo) is a substituted element, contributes to the strength of the steel through solid solution strengthening. Moreover, the hardenability and corrosion resistance of steel are improved. In one embodiment the molybdenum is included 0.05 ~ 0.10% by weight relative to the total weight of the slab. When the molybdenum is included in less than 0.05% by weight, the addition effect is insignificant, and when included in excess of 0.10% by weight, the synergistic effect due to the increase in addition amount is insignificant, and the weldability may be reduced.

nickel( Ni )

The nickel (Ni) contributes to the strength improvement of the steel through solid solution strengthening. Moreover, the hardenability and corrosion resistance of steel are improved. In one embodiment, the nickel is included in an amount of 0.15 to 0.25 wt% based on the total weight of the slab. When the nickel is included in less than 0.15% by weight, its addition effect is insignificant, and when it contains more than 0.25% by weight, the synergistic effect due to the addition amount is insignificant, and weldability may be reduced.

Niobium ( Nb )

The niobium (Nb) precipitates carbonitride (NbC) in steel, pinning grain boundaries, and prevents grain boundary sliding (GBS) and dislocation movement occurring at high temperatures in the present invention. , The strength can be improved.

In one embodiment the niobium is included 0.07 ~ 0.10% by weight based on the total weight of the slab. When the niobium is included in less than 0.07% by weight, its addition effect is insignificant, and when it is included in excess of 0.10% by weight, the synergistic effect is increased due to the increase in addition amount, and the playability, rolling property and elongation may be reduced due to excessive precipitation. have. For example, 0.07, 0.08, 0.09 or 0.10 wt% may be included.

titanium( Ti )

The titanium (Ti) prevents austenite grain growth during welding by producing Ti (C, N) precipitates having excellent high temperature stability, thereby minimizing the weld structure and improving the toughness and strength of the hot rolled product. In one embodiment the titanium is included 0.005 ~ 0.015% by weight relative to the total weight of the slab. When the titanium is included in less than 0.005% by weight, its addition effect is insignificant, and when included in excess of 0.015% by weight, coarse precipitates may be generated to reduce toughness of the steel.

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

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

In one embodiment, the reheating may be performed under slab reheating temperature: 1100 to 1200 ° C. When reheating under these conditions, austenite grains grow, and solid solution of impurities and precipitate forming elements in the steel may easily occur.

(S20) Rolled material  Manufacturing stage

The step is a step of hot rolling the reheated slab at the finish rolling temperature: 870 ~ 930 ℃ condition, to prepare a rolled material. If austenite grains are recrystallized or not recrystallized during hot rolling, the dislocation density in the mouth may be increased to refine the microstructure grain size of the final product.

If the finish rolling temperature is less than 870 ℃ to form a non-uniform structure, toughness and strength is lowered, and when carried out above 930 ℃ austenitic grains are coarsened and the 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 stage

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

In one embodiment, the cooling may be performed at a cooling rate of 40 ° C./sec or more. When cooling at the cooling rate, it is possible to sufficiently secure the low-temperature microstructure, while preventing the phenomenon that the hardness rises excessively to lower the low-temperature toughness. For example, it can carry out at a cooling rate of 40-60 degreeC / sec.

For line pipe  Manufactured by steel manufacturing method For line pipe  Steel

Another aspect of the present invention relates to a line pipe steel produced by the method for producing a line pipe steel. In one embodiment, the line pipe steel is 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): greater than 0 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 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 amount of iron (Fe) and other unavoidable impurities, microstructure The acicular ferrite (AF) may be included at 65% by volume or more, the Ceq value according to Equation 1 below 0.39, and the Pcm value according to Equation 2 below 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 Formula 1 and Formula 2, [C], [Mn], [Ni], [Cu], [Cr], [Mo], [V], [Si], and [B] are included in the steel. 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 content of the line pipe steel are the same as the above-described slab, detailed description thereof will be omitted.

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

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

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

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

In one embodiment, the microstructure of the steel may include 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. Including the microstructure of the above conditions may be excellent in high strength and low temperature toughness. For example, steel microstructures include 65-75% by volume of needle ferrite, 10-18% by volume of polygonal ferrite (PF), 12-20% by volume of bainitic ferrite (BF) and martensite / Austenitic (M / A) may comprise 1 to 5% by volume.

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

In one embodiment, the steel material may have a 95% or more of a ductile wavefront at -20 ° C and 80% or more at -30 ° C in the DWTT test. For example, the steel material may have a 95% or more ductile fracture rate at 0 ° C., a 95% or more ductile fracture rate at −20 ° C., and a 80% to 95% ductile fracture rate at −30 ° C.

Hereinafter, the configuration and operation of the present invention through the 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 in no sense can be construed as limiting the present invention.

Example  And Comparative example

Example  1 and 2 and Comparative example  1 ~ 2

To prepare a slab containing the components of Table 1, the remaining amount of iron (Fe) and other unavoidable impurities, Ceq according to the following formula 1 and Pcm according to the formula (2). The slab is reheated at 1150 ° C., hot rolling at a finish rolling temperature of 870 ° C. to 930 ° C. to produce a rolled material, and the rolling material is cooled at a cooling rate of 40 ° C. to 60 ° C./sec. After cooling to ℃ to produce a line pipe steel.

[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 Formula 1 and Formula 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 steels of Examples 1 and 2 and Comparative Examples 1 and 2, the microstructure and average grain size of Example 1 and Comparative Example 1 were measured and shown in Table 2 below, Examples 1 and 2 and Comparative Example 1 Tensile strength (TP), yield strength (YP) and elongation (EL) of the steel of ˜2 were evaluated and shown in Table 3 below, and the conditions of 0 ° C., −10 ° C., −20 ° C., −30 ° C., and −40 ° C. In the DWTT (Drop Weight Tear Test) test was performed, the results are shown in Table 4 below.

(* AF: Needle Ferrite, PF: Polygonal Ferrite, GB: Granular Bainite, BF: Bainite Ferrite, M / A: Martensite / Austenite)

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

Referring to the results of Table 2 and FIG. 2, in the case of the steel of Example 1 of the present invention, a microstructure in which needle-like ferrites of about 2 μm or less were uniformly distributed was secured, but in the case of Comparative Example 1 steel, granular It was found that bainite was formed, and the martensite / austenite fraction was increased, such that the phase fraction of the low temperature metamorphic tissue did not satisfy the conditions of the present invention. there was.

3 is a photograph showing the result of the DWTT test for the Comparative Example 2 steel, Figure 4 is a photograph showing the result of the DWTT test for the Example 2 steel.

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

Through this, the line pipe steel of the present invention minimizes the content of high-priced alloying elements such as copper (Cu), vanadium (V) and molybdenum (Mo), thereby reducing the production cost and reducing the Ceq and Pcm index, niobium (Nb) Increased NbC precipitation strengthening effect to realize grain refinement compared to conventional linepipe API-X70M steel grades, through the hard second phase martensite / austenite (M / A), and coarse precipitate suppression It was found that the low temperature fracture toughness (DWTT) characteristics 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 seen to be included in the scope of the present invention.

Claims (8)

Carbon (C): 0.04-0.07 wt%, Silicon (Si): 0.2-0.4 wt%, Manganese (Mn): 1.4-1.7 wt%, Phosphorus (P): greater 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 to 0.10 wt%, nickel (Ni): 0.15 to 0.25 weight %, Reheating the slab comprising niobium (Nb): 0.07-0.10 weight percent, titanium (Ti): 0.005-0.015 weight percent and remaining iron (Fe) and other unavoidable impurities;
Manufacturing a rolled material by hot rolling the reheated slab under a finish rolling temperature of 870 ° C to 930 ° C; And
It comprises a; cooling the rolled material to the cooling end temperature: 450 ~ 550 ℃;
The slab has a Ceq value of 0.39 or less according to Equation 1 below,
Steel pipe manufacturing method for a line pipe, characterized in that the Pcm value according to the following formula 2: 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 Formula 1 and Formula 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)).
The method of claim 1,
The reheating is a slab reheating temperature: 1100 ~ 1200 ℃ condition characterized in that the line pipe steel manufacturing method.
The method of claim 1,
The cooling is a line pipe steel manufacturing method, characterized in that carried out under the cooling rate conditions of 40 ℃ / sec or more.
Carbon (C): 0.04-0.07 wt%, Silicon (Si): 0.2-0.4 wt%, Manganese (Mn): 1.4-1.7 wt%, Phosphorus (P): greater 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 to 0.10 wt%, nickel (Ni): 0.15 to 0.25 weight %, Niobium (Nb): 0.07 to 0.10 weight percent, titanium (Ti): 0.005 to 0.015 weight percent and residual iron (Fe) and other unavoidable impurities,
The microstructure comprises 65% or more by volume of acicular ferrite (AF),
Ceq value according to the following formula 1 is 0.39 or less,
Steel pipe for line pipes, characterized in that the Pcm value according to Equation 2 below 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 Formula 1 and Formula 2, [C], [Mn], [Ni], [Cu], [Cr], [Mo], [V], [Si], and [B] are included in the steel. 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, wherein
The line pipe steel is a line pipe steel, characterized in that the tensile strength (TS): 600 ~ 700MPa, yield strength (YS): 500 ~ 600MPa and elongation (El): 30% or more.
The method of claim 4, wherein
The microstructure of the line pipe steel is 65 to 75% by volume of the needle-like ferrite, 10-18% by volume of polygonal ferrite (PF) and 12-20% by volume of bainitic ferrite (BF) Steel for line pipes.
The method of claim 4, wherein
The microstructure of the linepipe steels is a linepipe steel, characterized in that the average grain size is 15㎛ or less.
The method of claim 4, wherein
The line pipe steel is a line pipe steel, characterized in that the ductile wave ratio of 95% or more at 0 ℃, the ductile wave ratio of 95% or more at -20 ° C and 80% or more at the -30 ° C.

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