KR101778598B1 - Manufacturing method of high strength steel plate for line pipe having excellent low yield ratio property and high strength steel plate for line pipe having excellent low yield ratio property thereby - Google Patents

Abstract

The present invention relates to a method for manufacturing a thick steel plate for a low yield ratio high strength line pipe and a thick steel plate for a low yield ratio high strength line pipe manufactured thereby. The method for manufacturing a thick steel plate for a low yield ratio high strength line pipe includes: a heating step of re-heating a slab in a heating furnace; a rolling step of manufacturing a thick steel plate by rolling the slab; and a cooling step of cooling the thick steel plate manufactured in the rolling step. The slab contains 0.06 to 0.1 wt% of C, 0.1 to 0.4 wt% of Si, 1.3 to 1.7 wt% of Mn, 0.01 to 0.05 wt% of Al, 0.008 to 0.02 wt% of Ti, 0.03 to 0.06 wt% of Nb, 0.03 to 0.06 wt% of V, greater than 0 equal to or less than 180 ppm of P, greater than 0 equal to or less than 50 ppm of S, greater than 0 equal to or less than 60 ppm of N, greater than 0 equal to or less than 30 ppm of O, remaining Fe and other unavoidable impurities. The thick steel plate is satisfied with greater than or equal to 0.34 Ceq, and greater than 1 less than 8 of Ti/N, has a yield strength of greater than or equal to 450 MPa, a yield ratio of greater than or equal to 85%, and -40C shock absorbing energy of greater than or equal to 200 J. The method for manufacturing a thick steel plate for a low yield ratio high strength line pipe can minimize addition of an alloy element and allow a thick steel plate for a line pipe which has a yield strength of 450 MPa level or greater with a performance of a low temperature and a low yield ratio without an expensive alloy element, such as Ni and Mo, to be manufactured.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a post-steel plate for a high-strength and high-strength line pipe, and a post-steel plate for a low-resistance double-high-strength line pipe manufactured by the method. HAVING EXCELLENT LOW YIELD RATIO PROPERTY THEREBY}

The present invention relates to a method of manufacturing a steel sheet for a low-resistance high-strength line pipe and a steel sheet for a low-resistance high-strength line pipe manufactured by the method. More specifically, The present invention relates to a method for manufacturing a post-steel plate for a high-strength line pipe having a yield strength of 450 MPa or higher and a post-steel plate for a low-resistance, high-strength line pipe manufactured by the method.

In general, the line pipe used to transport gas or crude oil is subjected to high pressure in the pipe for efficient long-distance transportation of the production site and the consumer site.

Petroleum and natural gas resources are mainly mined in the Middle East, but recently, the amount of mined mined in extreme places such as Siberia, Arctic Ocean and Antarctica has been gradually increasing and mined in various regions.

Accordingly, there is a demand for a line pipe excellent in durability even at a low temperature of 0 占 폚 or lower, particularly an extreme condition of -20 占 폚 or below.

Therefore, there is an increasing need for the development of a steel material for a line pipe excellent in toughness at a low temperature of 0 占 폚 or lower, particularly at an extreme temperature of -20 占 폚 or lower.

In general, the steel material for a line pipe excellent in low temperature toughness can be obtained by adding an expensive alloying element such as Mo and Ni or satisfying a low resistance through a continuous yielding behavior in order to satisfy both high tensile strength of 500 MPa or more in high strength, low resistance and low temperature toughness have.

However, conventional steels for linefine require a high production cost because of the addition of expensive alloying elements such as Mo and Ni. Further, in the case of conventional steel for a line pipe, when yielding a low resistance through a continuous yielding behavior, there is a problem that the yield strength increases and the yield ratio increases.

That is, Korean Patent Laid-Open Publication No. 2001-0060759 'High Strength Steel Having Low Resistance and Method for Manufacturing the Same " has realized the resistance reduction property by using the continuous yielding behavior. However,' high strength steel having low resistance and its manufacturing method ' There is a problem that the yield ratio due to rise in yield may be increased due to work hardening after joining.

In addition, Korean Patent Laid-Open Publication No. 2013-0036545 ', a steel material for a line pipe excellent in resistivity characteristics and a method of manufacturing the steel pipe have a proper microstructure by controlling the composition ratio and operation of steel. However, since expensive Ni and Mo are essentially added, There was a problem. Also, in the steel material for a line pipe excellent in resistivity reduction characteristics of Korean Patent Laid-Open Publication No. 2013-0036545 and its manufacturing method, there is a problem that it is difficult to actualize the technique of cooling the primary and secondary coils during accelerated cooling in a general accelerated cooling facility.

It is an object of the present invention to provide a steel sheet for a line pipe having a yield strength of 450 MPa or more having a high strength, a low temperature toughness and a resistance to abrasion property with a minimum addition of expensive alloying elements such as Ni and Mo, To provide a method for manufacturing a steel plate for a high strength line pipe and a steel plate for a steel plate having a high resistance to a high strength line manufactured by the method.

Another object of the present invention is to provide a steel sheet for a line pipe having a yield strength of 450 MPa or more having low temperature toughness and low resistance properties without using a continuous yielding behavior, Which is capable of maintaining the yield ratio even after piping, and to provide a post-steel plate for a low-resistance, high-strength line pipe manufactured by the method.

In order to accomplish the above object, the present invention provides a method of manufacturing a slab, comprising the steps of: heating a slab in a heating furnace to reheat, rolling the slab to produce a rolled steel sheet, and cooling the rolled steel sheet Wherein the slab comprises 0.06 to 0.1% of C, 0.1 to 0.4% of Si, 1.3 to 1.7% of Mn, 0.01 to 0.05% of Al, 0.008 to 0.02% of Ti, 0.03 to 0.06% of Nb, , V: 0.03 to 0.06%, P: more than 0 and not more than 180 ppm, S: more than 0 and not more than 50 ppm, N: more than 0 and not more than 60 ppm, O: more than 0 and not more than 30 ppm and other Fe and other unavoidable impurities Ceq 0.34 or more and 1 < Ti / N < 8 is satisfied.

In the present invention, the condition that the compression ratio 3 <(slab thickness (t) x total reduction amount (%)) / (product thickness) <18 can be satisfied.

In the present invention, the heating step of reheating the slab in a heating furnace reheats the slab to a temperature of T _slab ± 50 ° C, and the T _slab is calculated by the following equation 1: to provide.

[Equation 1]

T _slab = {-6770 / (log [Nb] [C + (12/14) N] -2.26} - 273

(Nb: wt% of Nb, C: wt% of C, N: wt% of N)

In the present invention, the rolling step includes a recrystallization rolling step of rolling a plurality of rolling mills and a finishing rolling step of rolling a final finishing mill, wherein the recrystallization rolling step is carried out at a temperature of T _nr + The rolling is finished at a cumulative rolling reduction of 50% or more and Ar3 + 50 to Ar3 + 120 deg. C, and _Tnr is calculated by the following equation (2) And Ar3 is calculated by the following equation (3).

&Quot; (2) &quot;

(% By weight of Nb,% by weight of Nb,% by weight of C,% by weight of N,% by weight of V,% by weight of Ti,% by weight of Al,% by weight of Al,

&Quot; (3) &quot;

Ar 3 = 910 - (273 × C) - (74 × Mn) - (57 × Ni) - (16 × Cr) - (9 × Mo)

(C:% by weight of C, Mn:% by weight of Mn,% by weight of Ni,% by weight of Cr,% by weight of Mo,% by weight of Cu,

In the present invention, the cooling step starts cooling at a temperature of Ar 3 + 20 ° C or higher and Ar 3 + 90 ° C or lower and terminates the cooling to a temperature of Ms ~ Ms + 120 ° C at a cooling rate of 10 ° C / sec to 50 ° C / (S310) and air-cooling after the water-cooling process, and Ms is calculated by the following equation (4).

&Quot; (4) &quot;

Ms = 561 - (474 x C) - (33 x Mn) - (17 x Ni) - (17 x Cr) - (21 x Mo)

(C:% by weight of C,% by weight of Mn,% by weight of Ni,% by weight of Cr,% by weight of Mo)

In order to achieve the above object, the present invention provides a method for manufacturing a steel sheet for a line pipe excellent in toughness according to the present invention, which has a yield strength of 450 MPa or higher, a yield ratio of 85% Or more of the high strength line pipe.

In order to achieve the above object, the present invention provides a ferritic stainless steel comprising 0.06 to 0.1% of C, 0.1 to 0.4% of Si, 1.3 to 1.7% of Mn, 0.01 to 0.05% of Al, 0.008 to 0.02% of Ti, S: more than 0 and not more than 50 ppm, N: more than 0 and not more than 60 ppm, O: more than 0 and not more than 30 ppm, and the balance of Fe and O, Characterized in that it has Ceq 0.34 or more and satisfies 1 < Ti / N < 8, and has a yield strength of 450 MPa or more, a yield ratio of 85% or less, Provide post-steel plate.

The present invention provides a steel sheet for a low-resistance high-strength line pipe satisfying the following conditions: the slab thickness (t) x total reduction (%)) / (product thickness)

The present invention provides a steel sheet for a low-resistance high-strength line pipe comprising 50 to 80% of needle-like ferrite and quasi-polygonal ferrite as a base structure and containing 15 to 20% of bainite.

The present invention provides a steel sheet for a low-resistance high-strength line pipe satisfying a Ceq of 0.34 or more and 1 < Ti / N < 8, a yield strength of 450 MPa or more, a yield ratio of 85% or less,

The present invention can produce a steel sheet for a line pipe having a low-temperature toughness and low-temperature resistance characteristics and a yield strength of 450 MPa or higher without adding expensive alloying elements, such as Ni and Mo, The effect is greatly reduced.

The present invention can produce a steel sheet for a line pipe having a yield strength of 450 MPa or more having a low temperature toughness and a low resistance property without using a continuous yielding behavior, It is possible to maintain the yield ratio even after piping, thereby improving the quality of the product, greatly reducing the defect rate, and ensuring the quality of the steel sheet for the line pipe stably for a long period of time.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is an overall flow chart of a method for manufacturing a steel plate for a low-resistance, high-strength line pipe according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Prior to the detailed description of the present invention, terms and words used in the present specification and claims should not be construed as limited to ordinary or dictionary terms. Therefore, the embodiments described in this specification and the configurations shown in the drawings are merely the most preferred embodiments of the present invention and do not represent all the technical ideas of the present invention. Therefore, It is to be understood that equivalents and modifications are possible.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is an overall flow chart of a method for manufacturing a steel plate for a low-resistance, high-strength line pipe according to the present invention.

Referring to FIG. 1, the method for manufacturing a steel sheet for a low-resistance double-high-strength line pipe according to the present invention includes a heating step S100 for reheating the slab in a heating furnace, a rolling step S200 for rolling the slab, And a cooling step (S300) of cooling the steel sheet produced in the rolling step.

Wherein the slab comprises 0.06 to 0.1% of C, 0.1 to 0.4% of Si, 1.3 to 1.7% of Mn, 0.01 to 0.05% of Al, 0.008 to 0.02% of Ti, 0.03 to 0.06% of Nb, More than 0 and not more than 60 ppm, more than 0 and not more than 30 ppm, and the balance of Fe and other inevitable impurities, and Ceq of 0.34 or more, 0.03 to 0.06%, P: more than 0 and not more than 180 ppm, S: more than 0 and not more than 50 ppm, 1 < Ti / N < 8.

In more detail, the slab is made of a steel slab including 0.06 to 0.1% of C, 0.1 to 0.4% of Si, 1.3 to 1.7% of Mn, 0.01 to 0.05% of Al, 0.008 to 0.02% of Ti, 0.03 to 0.06% of Nb, More than 0 and not more than 30 ppm, and the balance of Fe and other inevitable impurities, and more preferably not less than 0.03 to 0.06%, P: more than 0 and not more than 180 ppm, S: more than 0 and not more than 50 ppm, N: more than 0 and not more than 60 ppm, And alloying elements such as Mo are not added, so that the manufacturing cost can be greatly reduced.

The carbon (C) is the most effective element for improving the strength of the steel, but when it is added in an excessively large amount, the weldability, formability and toughness can be lowered. If the content of carbon is less than 0.03 wt%, it is difficult to obtain the desired strength because the content of carbon is too low, so that a desired strength can be obtained by additionally containing a high-priced alloy element. However, when the content is more than 0.1% by weight, the content of carbon is too high to cause a problem of deterioration in weldability, formability and toughness as described above. Therefore, the content thereof is preferably limited to a range of 0.03 to 0.1% by weight Do.

The above-mentioned Si (silicon) is an element essential for deoxidation of steel and is an element effective for increasing the strength. However, if the content is 0.1% by weight or less, desired high strength can not be obtained. Moreover, when the content exceeds 0.4% by weight, toughness and weldability are deteriorated. Therefore, the content thereof is preferably limited to a range of 0.1 to 0.4% by weight.

The Mn (manganese) has an effect of increasing the strength at the time of heat treatment, and is also an essential element added to compensate the strength due to the limited amount of carbon added. However, if the amount of manganese is too low, the effect of improving the sintering property is hardly obtained. If it exceeds a certain range, manganese sulfide (MnS), which is a nonmetallic inclusion, is formed to lower the weldability and the structure of the phosphorus. Do.

Al (aluminum) is an element that acts as a deoxidizer for removing oxygen by reacting with oxygen present in molten steel. When the amount is too large, however, a large amount of oxide inclusions is formed to deteriorate the impact toughness of the material. Is preferably limited to 0.01 to 0.05% by weight.

Ti (titanium) plays a key role in improving the low temperature toughness through grain refinement. However, if the amount is too large, impact toughness at low temperature deteriorates. Therefore, it is preferable to limit the upper limit to 0.02% by weight, But is preferably limited to 0.008 wt% to 0.02 wt%.

The Nb (niobium) is an element which is very useful for refining the crystal grains, and is an element effective for enhancing the strength by promoting the formation of acicular ferrite or bainite which is a high strength texture. When the content of Nb (niobium) exceeds 0.07% by weight, the weldability is lowered. Therefore, the content thereof is preferably limited to 0.03 to 0.06% by weight.

The V (vanadium) reacts with carbon to form a vanadium precipitate, and precipitation strengthening and entrapping properties can be improved by vanadium precipitates. However, when it is contained in an amount of 0.08% by weight or more, the weldability and toughness may deteriorate. Therefore, the content thereof is preferably limited to 0.03 to 0.06% by weight.

The P (phosphorous) is inevitably contained in steel production, and it is preferable to control it as low as possible because it induces brittleness as an element easily segregated at the center of the slab during solidification, and theoretically the content of phosphorus can be limited to 0 wt% The manufacturing process is inevitably added. Therefore, it is important to manage the upper limit, and the upper limit of the content of phosphorus is preferably limited to 0.020 wt%, that is, 200 ppm or less.

S (sulfur) is an element which is inevitably contained in the production of steel and exists in the form of manganese sulfide (MnS) because it has good affinity with manganese (Mn), and is not squeezed during rolling and is elongated. Further, since it causes red embrittlement, it is preferable to suppress the content to the maximum. In theory, it is possible to limit the content of sulfur to 0% by weight, but it is inevitably added to the manufacturing process inevitably. Therefore, it is important to manage the upper limit, and it is preferable to limit the upper limit of the content of sulfur to 0.005 wt%, that is, 50 ppm or less.

In addition, the method for manufacturing a steel sheet for a low-resistance, high-strength line pipe in accordance with the present invention satisfies the condition that the slab thickness (t) × total reduction (%) / (product thickness) <18.

In addition, the heating step (S100) of reheating the slab in the heating furnace reheats the slab to a temperature of T _slab ± 50 ° C.

The T _slab is calculated by the following equation (1).

[Equation 1]

T _slab = {-6770 / (log [Nb] [C + (12/14) N] -2.26} - 273

(Nb: wt% of Nb, C: wt% of C, N: wt% of N)

The rolling step S200 includes a recrystallization rolling step S210 for rolling a plurality of rolling mills and a finishing rolling process S220 for rolling the final finishing mill, Rolling at an average rolling reduction rate of 10% or more per pass at a temperature of T _nr + 50 ° C or more, and rolling in the finishing rolling process (S220) at a cumulative rolling reduction of 50% or more and Ar3 + 50 to Ar3 + 120 ° C desirable.

The T _nr is calculated by the following equation (2).

&Quot; (2) &quot;

(% By weight of Nb,% by weight of Nb,% by weight of C,% by weight of N,% by weight of V,% by weight of Ti,% by weight of Al,% by weight of Al,

The above-mentioned Ar3 is calculated by the following equation (3).

&Quot; (3) &quot;

Ar 3 = 910 - (273 × C) - (74 × Mn) - (57 × Ni) - (16 × Cr) - (9 × Mo)

(C:% by weight of C, Mn:% by weight of Mn,% by weight of Ni,% by weight of Cr,% by weight of Mo,% by weight of Cu,

Meanwhile, the cooling step (S300) starts cooling at a temperature of Ar 3 + 20 ° C or higher and Ar 3 + 90 ° C or lower and cooling to a temperature of Ms ~ Ms + 120 ° C at a cooling rate of 10 ° C / sec to 50 ° C / (S310) for cooling the water-cooling process (S310), and an air-cooling process (S320) for air-cooling the water-cooling process (S310).

The Ms is calculated by the following equation (4).

&Quot; (4) &quot;

Ms = 561 - (474 x C) - (33 x Mn) - (17 x Ni) - (17 x Cr) - (21 x Mo)

(C:% by weight of C,% by weight of Mn,% by weight of Ni,% by weight of Cr,% by weight of Mo)

The steel sheet for a low-resistance, high-strength line pipe manufactured by the method for manufacturing a steel plate for a low-resistance double-high-strength line pipe according to the present invention comprises 0.06 to 0.1% of C, 0.1 to 0.4% of Si, P: more than 0 and not more than 180 ppm, S: more than 0 and not more than 50 ppm, N: more than 0:%, Al: 0.01 to 0.05%, Ti: 0.008 to 0.02%, Nb: 0.03 to 0.06% Not more than 60 ppm, not more than 0 and not more than 30 ppm and the balance of Fe and other unavoidable impurities Ceq 0.34 or more and 1 <Ti / N <8, a yield strength of 450 MPa or more, a yield ratio of 85% The shock absorption energy is more than 200J.

In addition, the steel sheet for a low-resistance, high-strength line pipe according to the present invention comprises acicular ferrite of 50 to 80% and quasi-polygonal ferrite as a base and includes 15 to 20% of bainite.

The method for manufacturing a post-steel plate for a low-resistance, high-strength line pipe according to the present invention and a post-steel plate for a low-resistance, high-strength line pipe minimize the addition of alloying elements and optimize operating conditions without adding expensive alloying elements such as Ni and Mo And the two-phase fraction are appropriately adjusted to produce a steel sheet for a line pipe satisfying all of high strength, low temperature toughness and low resistance properties.

division C Si Mn Al Nb V Ti P S O N Invention river 0.08 0.2 1.6 0.03 0.045 0.04 0.015 0.015 0.006 0.003 0.004 Comparative River 1 0.12 0.2 1.6 0.03 0.045 0.04 0.015 0.015 0.003 0.003 0.004 Comparative River 2 0.05 0.2 1.6 0.03 0.045 0.04 0.015 0.012 0.002 0.003 0.004 Comparative Steel 3 0.04 0.25 1.25 0.03 0.045 0.03 0.015 0.01 0.001 0.002 0.005

Table 1 shows composition ratios of the steels of the present invention satisfying the composition ratio of the present invention and the comparative steels 1 to 3 not meeting the composition ratios of the present invention, It is to be noted that the unit of the element is% by weight and includes Fe and other unavoidable impurities other than the composition shown in Table 1 above.

division Ti / N Ceq Ar3 temperature (占 폚) Invention river 3.75 0.35 770 Comparative River 1 3.75 0.39 759 Comparative River 2 2.0 0.32 807 Comparative Steel 3 5.0 0.27 778

Table 2 shows Ti / N, Ceq and Ar3 temperatures of the inventive steels and the comparative steels 1 to 3 in Table 1 above.

division Steel grade Reheating temperature (℃) Abjabi Cumulative rolling reduction (%) Finishing finish temperature (° C) Water cooling start temperature (℃) Water cooling end temperature (캜) Cooling rate (° C / sec) Example 1 Invention river 1140 9.4 75 880 820 450 30 Example 2 Invention river 1140 9.4 75 860 800 580 20 Comparative Example 1 Invention river 1140 18.8 75 890 800 590 30 Comparative Example 2 Invention river 1150 12.5 75 820 750 600 15 Comparative Example 3 Invention river 1150 19.9 75 800 Air cooling Comparative Example 4 Invention river 1150 2.5 50 840 800 350 10 Comparative Example 5 Invention river 1150 13.4 75 870 800 620 25 Comparative Example 6 Invention river 1150 18.8 75 820 Air cooling Comparative Example 7 Comparative River 1 1150 9.4 75 850 790 550 20 Comparative Example 8 Comparative River 2 1140 9.4 75 860 800 560 25 Comparative Example 9 Comparative Steel 3 1150 12.5 75 850 780 580 20

Table 3 shows the results of Examples 1 and 2 of the steel sheets for low-resistance, high-strength and high-strength line pipes manufactured by satisfying the rolling conditions and cooling conditions according to the present invention, Comparative Example 1 to Comparative Example 6 of the steel sheet for low-resistance, high strength, and high-strength line manufactured by rolling conditions and cooling conditions outside the rolling conditions and cooling conditions according to the present invention using the inventive steel, And rolling conditions and cooling conditions of Comparative Examples 7 to 9 of a steel plate for a low-resistance, high-strength and high-strength line pipe manufactured by satisfying rolling conditions and cooling conditions according to the present invention using steel 3.

division Steel grade Yield strength (MPa) Tensile Strength (MPa) Yield ratio
(%) -40 C CVN (J) AF + QPF fraction Bainite fraction (%) Example 1 Invention river 526 632 83.2 220 55 15 Example 2 Invention river 506 603 83.9 260 50 15 Comparative Example 1 Invention river 535 605 88.4 310 85 5 Comparative Example 2 Invention river 519 593 87.5 280 30 0 Comparative Example 3 Invention river 545 586 93.0 172 0 0 Comparative Example 4 Invention river 488 602 81.1 183 40 20 Comparative Example 5 Invention river 520 595 87.4 315 65 10 Comparative Example 6 Invention river 476 569 8307 168 0 0 Comparative Example 7 Comparative River 1 547 661 82.8 181 70 15 Comparative Example 8 Comparative River 2 513 599 85.6 330 50 5 Comparative Example 9 Comparative Steel 3 469 537 87.3 435 40 0

Table 4 shows the yield strength, tensile strength, yield ratio, -40 캜 CVN (J), AF + QPF fraction and bainite fraction (%) of Examples 1 and 2 and Comparative Examples 1 to 9 of Table 3, ).

In Table 4, when the steel composition ratio, the rolling conditions and the cooling conditions are satisfied in the method of manufacturing the steel sheet for the low-resistance double-high strength line pipe according to the present invention, the steel sheet has a yield strength of 450 MPa or higher, a yield ratio of 85% It can be confirmed that a steel sheet for a double-high strength line pipe is manufactured.

The present invention can produce a steel sheet for a line pipe having a low-temperature toughness and low-temperature resistance property and a yield strength of 450 MPa or higher, without adding expensive alloying elements, such as Ni and Mo, The effect is saved.

The present invention can produce a steel sheet for a line pipe having a yield strength of 450 MPa or more having a low temperature toughness and a low resistance property without using a continuous yielding behavior, It is possible to maintain the yield ratio even after piping, thereby improving the quality of the product, greatly reducing the defect rate, and ensuring the quality of the steel sheet for the line pipe stably for a long period of time.

It will be understood by those skilled in the art that various changes and modifications may be made without departing from the scope of the present invention.

S100: heating step S200: rolling step
S210: Recrystallization rolling process S220: Finishing rolling process
S300: Cooling step S310: Water cooling process
S320: Air cooling process

Claims (10)

A heating step of reheating the slab in a heating furnace, a rolling step of rolling the slab to produce a steel sheet, and a cooling step of cooling the steel sheet produced in the rolling step,
Wherein the slab comprises 0.06 to 0.1% of C, 0.1 to 0.4% of Si, 1.3 to 1.7% of Mn, 0.01 to 0.05% of Al, 0.008 to 0.02% of Ti, 0.03 to 0.06% of Nb, More than 0 and not more than 60 ppm, more than 0 and not more than 30 ppm, and the balance of Fe and other unavoidable impurities, and Ceq of 0.34 or more, 0.03 to 0.06%, P: more than 0 to 180 ppm, 1 < Ti / N < 8 and the slab thickness (t) x total reduction (%)) / (product thickness) <
The heating step of reheating the slab in a heating furnace reheats the slab to a temperature of T _slab ± 50 ° C, and the T _slab is calculated by the following equation (1)
Wherein the rolling step includes a recrystallization rolling process in which a plurality of rolling mills are rolled and rolled, and a finishing rolling process in which a final finishing mill is rolled,
The recrystallization rolling process is performed at an average rolling reduction rate of 10% or more per pass at a temperature of T _nr + 50 ° C or higher, and the rolling process is finished at a cumulative rolling reduction of 50% or more and Ar 3 +50 to Ar 3 + 120 ° C In addition,
T _nr is calculated by the following equation (2)
The Ar3 is calculated by the following equation (3)
The cooling step starts cooling at a temperature of Ar 3 + 20 ° C or higher and Ar 3 + 90 ° C or lower and terminating the cooling to a temperature of Ms ~ Ms + 120 ° C at a cooling rate of 10 ° C / sec to 50 ° C / S310) and an air cooling process for cooling the water after the water cooling process,
The Ms is calculated by the following formula (4) to prepare a steel sheet for a low-resistance high-strength line pipe having a base structure of 50 to 80% of acicular type ferrite and quasi-polygonal ferrite and containing 15 to 20% of bainite Wherein the step of forming the steel plate comprises the steps of:
[Equation 1]
T _slab = {-6770 / (log [Nb] [C + (12/14) N] -2.26} - 273
(Nb: wt% of Nb, C: wt% of C, N: wt% of N)
&Quot; (2) &quot;

(% By weight of Nb,% by weight of Nb,% by weight of C,% by weight of N,% by weight of V,% by weight of Ti,% by weight of Al,% by weight of Al,
&Quot; (3) &quot;
Ar 3 = 910 - (273 × C) - (74 × Mn) - (57 × Ni) - (16 × Cr) - (9 × Mo)
(C:% by weight of C, Mn:% by weight of Mn,% by weight of Ni,% by weight of Cr,% by weight of Mo,% by weight of Cu,
&Quot; (4) &quot;
Ms = 561 - (474 x C) - (33 x Mn) - (17 x Ni) - (17 x Cr) - (21 x Mo)
(C:% by weight of C,% by weight of Mn,% by weight of Ni,% by weight of Cr,% by weight of Mo)
delete delete delete delete A manufacturing method of a post-steel plate for a low-resistance high-strength line pipe according to claim 1,
The steel sheet according to any one of claims 1 to 3, wherein the steel contains 0.06 to 0.1% of C, 0.1 to 0.4% of Si, 1.3 to 1.7% of Mn, 0.01 to 0.05% of Al, 0.008 to 0.02% of Ti, 0.03 to 0.06% of Nb, %, P: more than 0 and not more than 180 ppm, S: more than 0 and not more than 50 ppm, N: more than 0 and not more than 60 ppm, O: more than 0 and not more than 30 ppm, and other Fe and other unavoidable impurities, / N <8, and has a yield strength of 450 MPa or higher, a yield ratio of 85% or lower, a shock absorption energy of 200 J or higher at -40 ° C, a compressive strength of 3 <(slab thickness t) Thickness) < 18,
Characterized in that the base structure is composed of 50 to 80% of acicular type ferrite and quasi-polygonal ferrite and contains 15 to 20% of bainite. delete delete delete delete

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