JP2006111928A - Thick steel plate for line pipe and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thick steel plate for a line pipe whose production cost is lower than that of the conventional one and having excellent HIC (Hydrogen Induced Cracking) resistance in a small amount hydrogen sulfide environment in which the partial pressure of hydrogen sulfide is about 0.1 to 0.3atm. <P>SOLUTION: A stock comprising, by mass, 0.04 to 0.08% C, 0.05 to 0.35% Si, 1.20 to 1.80% Mn, 0.02 to 0.06% Nb, 0.005 to 0.030% Ti, 0.010 to 0.060% Al, 0 to 0.25% Mo, 0 to 0.40% Cu, 0 to 0.50% Cr, 0 to 0.50% Ni, 0 to 0.06% V, ≤0.020% P, ≤0.003% S and <0.0005% Ca, and the balance Fe with impurities, and satisfying an Mn segregation degree of ≤1.2 and a P segregation degree of ≤2.0 is heated to T<SB>1</SB>°C and is rolled at T<SB>2</SB>°C so as to be a thick steel plate satisfying a plate thickness of WTmm. After the rolling, the steel plate is water-cooled to T<SB>3</SB>°C and is air-cooled from the T<SB>3</SB>°C to room temperature. The produced thick steel plate satisfies inequality (1); 547≤0.10WT+1428C+93Si+108Mn-12Cu+113Cr+113Ni+304Mo+430Nb+238V+1315Ti+0.21T<SB>1</SB>+0.03T<SB>2</SB>-0.04T<SB>3</SB>+30≤739. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a thick steel plate and a manufacturing method thereof, and more particularly to a thick steel plate for a line pipe and a manufacturing method thereof.

  Crude oil and natural gas contain wet hydrogen sulfide. For this reason, hydrogen embrittlement caused by hydrogen sulfide becomes a problem in oil well pipes used for drilling crude oil and natural gas and in line pipes that transport drilled crude oil and natural gas.

  Hydrogen embrittlement includes hydrogen sulfide cracking that occurs in steel under static external stress and hydrogen induced cracking (hereinafter referred to as HIC) that occurs in steel without external stress. Line pipes are less subject to static external stress than oil well pipes. Therefore, the HIC resistance is particularly required for the line pipe. In particular, in line pipes (hereinafter referred to as high-strength line pipes) whose strength grades are API5L X60 to X80, HIC is likely to occur, and therefore anti-HIC measures are important.

  Generally, Ca treatment is performed on the above-described high-strength line pipe. In the Ca treatment, the form of the sulfide inclusions is changed from MnS that is easily stretched during hot rolling to CaS that is difficult to stretch. Therefore, it is possible to prevent HIC from occurring at the interface between the inclusion and the parent phase.

  The Ca treatment is effective in suppressing the generation of HIC in a high-strength line pipe used in a sour environment with a hydrogen sulfide partial pressure of 1 atm, but this process increases the production cost of the line pipe.

By the way, the sour environment where a high-strength line pipe is used is a sour environment having a relatively low hydrogen sulfide partial pressure of about 0.1 to 0.3 atm of hydrogen sulfide (hereinafter referred to as a small amount of hydrogen sulfide environment). There is. Line pipes intended for use in a sour environment with a hydrogen sulfide partial pressure of 1 atm have an excessive material design for a small amount of hydrogen sulfide environment, and high manufacturing costs are a problem.
Japanese Patent No. 347895 JP 2002-294394 A JP-A-5-93243 JP-A-5-9575 JP-A-2-240211

  The object of the present invention is a line pipe whose API X60 to X80 has a strength grade capable of suppressing the generation of HIC in a small amount of hydrogen sulfide environment where the production cost is lower than conventional and the hydrogen sulfide partial pressure is about 0.1 atm to 0.3 atm. It is to provide a thick steel plate and a manufacturing method thereof.

Means for Solving the Problems and Effects of the Invention

  The present inventor has considered to reduce the manufacturing cost by omitting the Ca treatment usually performed in the manufacturing process of the high-strength line pipe. Therefore, the present inventors have examined a production method capable of preventing the generation of HIC in a small amount of hydrogen sulfide environment even if the Ca treatment is omitted, based on various experiments. As a result, the present inventor not only affects the segregation degree and chemical composition of Mn and P on the HIC resistance, but also the plate thickness of the thick steel plate, which is the manufacturing condition of the thick steel plate, and the slab before rolling. It was newly found that the heating temperature, the rolling finishing temperature during rolling, and the water cooling stop temperature when water-cooling the rolled steel plate affect the HIC resistance. The present inventor considered that the segregation degree, chemical composition, and production conditions (sheet thickness, slab heating temperature, rolling finishing temperature, and water cooling stop temperature) correlated with each other and affected HIC resistance.

  As a result of conducting various experiments based on the above idea, the chemical composition and production conditions satisfying the formula (1) with the Mn segregation degree being 1.2 or less, the P segregation degree being 2.0 or less, and It has been found that the steel plate manufactured in (1) has a strength grade corresponding to APIX60 to X80 and has HIC resistance against a small amount of hydrogen sulfide environment.

547 ≦ 0.10 WT + 1428C + 93Si + 108Mn-12Cu + 113Cr + 113Ni + 304Mo + 430Nb + 238V + 1315Ti + 0.21T ₁ + 0.03T ₂ −0.04T ₃ + 30 ≦ 739 (1)

Here, WT (mm) is the plate thickness of the thick steel plate for line pipe, T ₁ (° C.) is the slab heating temperature, T ₂ (° C.) is the rolling finishing temperature, and T ₃ (° C.) is cooling. Stop temperature. The other symbols are the content (mass%) of each element.

  In addition, the yield stress of the thick steel plate satisfying the formula (1) is 414 to 690 MPa, and the tensile strength is 517 to 827 MPa. In other words, API5L X60-X80 thick steel plate for line pipes is obtained by satisfying the formula (1).

  Based on the above findings, the present inventor has completed the following invention.

The manufacturing method of the thick steel plate for line pipes by this invention is C: 0.04-0.08%, Si: 0.05-0.35%, Mn: 1.20-1.80%, Nb by mass%. : 0.02 to 0.06%, Ti: 0.005 to 0.030%, Al: 0.010 to 0.060%, Mo: 0 to 0.25%, Cu: 0 to 0.40%, Cr: 0 to 0.50%, Ni: 0 to 0.50%, V: 0 to 0.06%, P: 0.020% or less, S: 0.003% or less, Ca: less than 0.0005% The balance consists of Fe and impurities, the Mn segregation degree, which is the ratio of the Mn concentration of the segregation part to the total Mn concentration, is 1.2 or less, and the ratio of the P concentration of the segregation part to the total P concentration is extract the material is P segregation ratio is 2.0 or less was charged into a heating furnace, and heating the T ₁ ° C., the heated material from the furnace And comprising the steps of the steel plate was rolled with a finishing temperature of T ₂ ° C., and water cooled steel plates to T ₃ ° C., and a step of cooling from T ₃ ° C. to room temperature, satisfying the equation (1).

547 ≦ 0.10 WT + 1428C + 93Si + 108Mn-12Cu + 113Cr + 113Ni + 304Mo + 430Nb + 238V + 1315Ti + 0.21T ₁ + 0.03T ₂ −0.04T ₃ + 30 ≦ 739 (1)

  Here, WT (mm) is the plate thickness of the thick steel plate for line pipe, and C, Si, Mn, Cu, Cr, Ni, Mo, Nb, V, and Ti are the contents (mass%) of each element. .

  According to the present invention, a thick steel plate for a high-strength line pipe can be obtained by satisfying the above-mentioned Mn segregation degree and P-segregation degree and satisfying the formula (1) without producing Ca. Even so, the generation of HIC can be suppressed in a small amount of hydrogen sulfide environment. Therefore, the production cost can be reduced compared with the conventional thick steel plate for high-strength line pipe because the Ca treatment is not performed, and a thick steel plate for line pipe excellent in HIC resistance in a small amount of hydrogen sulfide environment can be obtained.

  The thick steel plate for line pipes according to the present invention is C: 0.04 to 0.08%, Si: 0.05 to 0.35%, Mn: 1.20 to 1.80%, Nb: 0.0. 02 to 0.06%, Ti: 0.005 to 0.030%, Al: 0.010 to 0.060%, Mo: 0 to 0.25%, Cu: 0 to 0.40%, Cr: 0 -0.50%, Ni: 0-0.50%, V: 0-0.06%, P: 0.020% or less, S: 0.003% or less, Ca: less than 0.0005% The balance consists of Fe and impurities, and the Mn segregation degree, which is the ratio of the Mn concentration of the segregation part to the total Mn concentration, is 1.2 or less, and the P segregation, which is the ratio of the P concentration of the segregation part to the total P concentration. The degree is 2.0 or less and satisfies the formula (1).

  547 ≦ 0.10 WT + 1428C + 93Si + 108Mn-12Cu + 113Cr + 113Ni + 304Mo + 430Nb + 238V + 1315Ti + 0.21T1 + 0.03T2-0.04T3 ≦ 709 (1)

Here, WT (mm) is the plate thickness of the plate steel plate for line pipes, and T ₁ (° C.) is the heating temperature in the heating furnace before rolling of the material rolled to the plate steel plate for line pipes, T 2 _(° C.) is a finishing rolling temperature at which the planks steel by rolling the material, T 3 _(° C.) is a water cooling stop temperature at the time of water cooling the thick steel plates after rolling, otherwise Is the content (% by mass) of each element.

  Preferably, the yield stress of the thick steel plate for line pipe is 414 to 690 MPa, and the tensile strength is 517 to 827 MPa.

  In short, the strength grade of the thick steel plate for line pipes of the present invention corresponds to API5L X60 to X80.

  Preferably, the plate thickness WT of the thick steel plate for line pipe is 10.0 ≦ WT ≦ 45.0.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals and description thereof will not be repeated.

1. Chemical composition The thick steel plate for line pipes by embodiment of this invention has the following compositions. Henceforth,% regarding each element means the mass%.

C: 0.04 to 0.08%
C improves the strength of the steel. Excessive C content causes C segregation during pearlite transformation in the rolling process or the cooling process after rolling, and locally increases the hardness. Local hardness increase reduces HIC resistance. Therefore, the C content is 0.04 to 0.08%.

Si: 0.05 to 0.35%
Si is effective as a deoxidizer. Furthermore, Si increases the strength of the steel. When there is too little Si content, deoxidation will become inadequate. Moreover, when there is too much Si content, a martensite will be produced | generated in a welding heat affected zone (HAZ: Heat Affected Zone), and a toughness fall will be caused. Therefore, the Si content is set to 0.05 to 0.35%. A preferable Si content is 0.05 to 0.30%.

Mn: 1.20 to 1.80%
Mn contributes to the strength and toughness of the steel. If the Mn content is too small, other alloy elements must be added to obtain the strength of the steel, which raises the manufacturing cost. When the Mn content is increased, center segregation is caused in the continuously cast slab, and the HIC resistance is lowered. This is because C is concentrated in the Mn segregation part in the rolling process or the water cooling process after rolling, and the hardness is locally increased. Therefore, the Mn content is 1.20 to 1.80%.

Nb: 0.02 to 0.06%
Nb refines crystal grains and improves strength and toughness. After rolling the steel added with Nb in the austenite non-recrystallized region, the steel structure can be made into a fine lower bainite structure by quenching from a temperature not lower than the _Ar3 transformation point. When there is too much Nb content, when heating a slab, Nb carbonitride in a slab will not melt | dissolve completely, but undissolved Nb carbonitride will become the origin of HIC. Therefore, the Nb content is 0.02 to 0.06%. A preferable Nb content is 0.02 to 0.05%.

Ti: 0.005-0.030%
Ti forms Ti carbonitride and contributes to the strength of the steel. When there is too much Ti content, Ti carbonitride will coarsen and the toughness of HAZ will fall. Furthermore, an excessive Ti carbonitride is formed at the center of the continuously cast slab, causing HIC. Therefore, the Ti content is made 0.005 to 0.030%. A preferable Ti content is 0.005 to 0.025%.

Al: 0.010 to 0.060%
Al is a deoxidizer. Furthermore, Al contributes to toughness. However, if the Al content is too high, excessive inclusions are formed and the cleanliness of the steel is lowered. Therefore, the Al content is set to 0.010 to 0.060. A preferable Al content is 0.010 to 0.055%.

P: 0.020% or less P is an impurity. P segregates in the center of the slab and causes HIC by hardening the structure. Therefore, the P content is 0.020%.

S: 0.003% or less S is an impurity. S combines with Mn to form MnS, which becomes the starting point of HIC. Therefore, the one where S content is low is preferable. S content is made 0.003% or less. A preferable S content is 0.002% or less.

Ca: less than 0.0005% In the present invention, Ca is an impurity. Ca spheroidizes inclusions and suppresses the generation of HIC, but increases the manufacturing cost. Therefore, Ca is not added to the thick steel plate for line pipes according to the present invention. Therefore, the Ca content is less than 0.0005%.

Cu: 0 to 0.40%
Cu is a selective element. Cu contributes to toughness and HIC resistance. However, Cu raises manufacturing costs. Furthermore, since Cu is a ferrite forming element, if the Cu content is too large, the proportion of ferrite in the structure increases and the strength of the steel decreases. Therefore, the Cu content is set to 0 to 0.40%.

Cr: 0 to 0.50%
Cr is a selective element. Cr contributes to strength and corrosion resistance. However, Cr increases manufacturing costs. Furthermore, when there is too much Cr content, weldability will fall. Therefore, the Cr content is 0 to 0.50%.

Ni: 0 to 0.50%
Ni is a selective element. Ni contributes to strength and toughness. When adding Cu, it is preferable to also add Ni. This is because Ni prevents Cu checking. When adding Ni together with Cu, the Ni content is preferably about twice or more the Cu content. When Ni content is too high, weldability will fall. Therefore, the Ni content is 0 to 0.50%.

Mo: 0 to 0.25%
Mo is a selective element. Mo contributes to strength by improving hardenability. Specifically, Mo delays the transformation from austenite to ferrite and pearlite. Therefore, it adds as needed in order to make a thick steel plate a desired intensity | strength. The Mo content is 0 to 0.25%.

V: 0 to 0.06%
V is a selective element. V contributes to strength by precipitation hardening. When there is too much V content, manufacturing cost will become high. Therefore, the V content is 0 to 0.06%.

  The balance is composed of Fe, but other impurities may be included due to various factors in the manufacturing process. Another impurity is, for example, an oxide. In addition, the characteristic of the thick steel plate for line pipes of this invention does not change with these impurities.

  The thick steel plate for line pipes of the present invention has a low segregation degree of Mn and P. Specifically, the Mn segregation degree is 1.2 or less, and the P segregation degree is 2.0 or less. By suppressing segregation of these elements, generation of HIC can be suppressed.

  Here, the Mn segregation degree is the ratio of the Mn concentration of the Mn segregation part to the Mn concentration of the entire thick steel plate, and is measured as follows, for example.

  A plurality of samples are taken along the width direction of the thick steel plate 10 as shown in FIG. 1A. Specifically, from the end 11 of the thick steel plate 10 having a width Wmm and a plate thickness WTmm, from the positions W / 8, W / 4, W / 2, 3W / 4, 7W / 8, 50 mm × 50 mm × plate thickness WTmm, respectively. Sample 50 is taken. After polishing the cross sections of the collected five samples 50, X-ray line analysis is performed in the plate thickness WT direction as shown in FIG. 1B to measure the peak value of the Mn concentration. On the other hand, the Mn concentration of the entire thick steel plate uses a ladle value or a value (bulk value) measured by product check analysis. The peak value / bulk value is obtained for each sample, and the average of these is taken as the Mn segregation degree.

  The P segregation degree is the ratio of the P concentration of the P segregation part to the P concentration of the entire thick steel plate, and is obtained, for example, by the same measurement method as the Mn segregation degree.

  Moreover, the yield stress of the steel plate for line pipes of this invention is 414-690 MPa, and the tensile strength is 517-827 MPa. In other words, the strength grade of the thick steel plate for line pipes of the present invention is API5L X60 to X80.

3. Manufacturing method Steel having the above chemical composition is melted and refined by a well-known refining process. Next, molten steel is used as a raw material. Specifically, molten steel is made into a slab by continuous casting. At this time, by adjusting the casting speed, the casting temperature, and the cooling rate, the Mn segregation degree in the slab is made 1.2 or less, and the P segregation degree is made 2.0 or less. In order to obtain the Mn segregation degree and the P segregation degree, electromagnetic induction stirring may be performed. Subsequently, the slab is hot rolled into a thick steel plate. Specifically, the slab is charged into a heating furnace and heated at a heating temperature T ₁ ° C. Preferred heating temperatures _{T 1} is 1000 to 1200 ° C.. A preferable heating time is 180 to 300 minutes.

After heating for a predetermined time, the slab is extracted from the heating furnace. The extracted slab is rolled by a rough rolling mill and a finish rolling mill to form a thick steel plate. Preferred finishing rolling temperature _{T 2} at this time is 700 to 950 ° C.. Here, the rolling finishing temperature refers to the temperature of the material before the final pass rolling when the slab is rolled with a rolling mill to form a thick steel plate. A preferable plate thickness WT after rolling is 10.0 to 45.0 mm.

The rolled thick steel plate is water cooled to a water cooling stop temperature T ₃ ° C and then air cooled to room temperature. Water cooling stop temperature _{T 3} ° C. is preferably 400 to 550 ° C.. The temperature of the thick steel plate after rolling is not less than the _Ar3 transformation point. C is prevented from concentrating in the center segregation area by quenching from A _r3 transformation point or above the temperature, improve the HIC resistance.

Furthermore, in the manufacturing method of the thick steel plate for line pipes by this invention, the following formula | equation (1) is satisfy | filled.
547 ≦ 0.10 WT + 1428C + 93Si + 108Mn-12Cu + 113Cr + 113Ni + 304Mo + 430Nb + 238V + 1315Ti + 0.21T ₁ + 0.03T ₂ −0.04T ₃ + 30 ≦ 739 (1)

That is, not only the chemical composition but also the plate thickness WT, the heating temperature T ₁ , the rolling finishing temperature T ₂ and the water cooling stop temperature T ₃ affect the HIC resistance of the thick steel plate for line pipe. By making the Mn segregation degree 1.2 or less, the P segregation degree 2.0 or less, and manufacturing with the chemical composition and production conditions satisfying the formula (1), the resistance in a small amount of hydrogen sulfide environment without Ca treatment. A thick steel plate for line pipes having excellent HIC properties can be obtained.

In addition, by satisfy | filling Formula (1), the yield stress (0.2% yield strength) of the thick steel plate for line pipes will be 414-690 MPa, and tensile strength will be 517-758 MPa. In other words, a thick steel plate for line pipes corresponding to API5L X60 to X80 can be obtained.
Furthermore, when α shown in Formula (2) satisfies the value in the α value column of Table 1, a thick steel plate for line pipes having a strength grade corresponding to α in Table 1 can be obtained.

α = 0.10WT + 1428C + 93Si + 108Mn-12Cu + 113Cr + 113Ni + 304Mo + 430Nb + 238V + 1315Ti + 0.21T ₁ + 0.03T ₂ −0.04T ₃ +30 (2)

  For example, if 547 ≦ α ≦ 613, a thick steel plate corresponding to API5L X60 can be obtained.

Thick steel plates having the chemical composition shown in Table 2 were produced, and the strength and HIC resistance of the produced thick steel plates were evaluated.

Molten steel having the chemical composition shown in Table 2 was made into a slab by a continuous casting method. The cast slab was charged into a heating furnace and heated at a heating temperature T ₁ ° C shown in Table 2 for 180 to 330 minutes. The heated slab was extracted from the heating furnace and formed into a thick steel plate having a thickness of WT mm shown in Table 2 by hot rolling. At this time, the rolling finishing temperature T ₂ ° C was as shown in Table 2. The rolled steel plate was water cooled to a water cooling stop temperature T ₃ ° C shown in Table 2 and then air cooled to room temperature.

  After air cooling, the Mn segregation degree and the P segregation degree of each thick steel plate were investigated. Specifically, samples of 50 mm × 50 mm × plate thickness WT mm were taken at positions W / 8, W / 4, W / 2, 3W / 4, and 7W / 8 from one end of the thick steel plate. After polishing the cross sections of the five samples collected, X-ray line analysis was performed in the plate thickness WT direction to measure the peak value of the Mn concentration. On the other hand, the ladle value was used for the Mn concentration (bulk value) of the entire thick steel plate. The peak value / bulk value was obtained for each sample, and the average of these values was taken as the Mn segregation degree. The degree of P segregation was also determined by the same method. Table 2 shows the degree of Mn segregation and the degree of P segregation.

  Furthermore, the α value was calculated based on the chemical composition and production conditions of each thick steel plate. Table 2 shows the α value of each thick steel plate.

[Survey on yield stress and tensile strength]
A tensile test was performed, and the yield stress YS and tensile strength TS of each thick steel plate were investigated. The test piece was a flat plate test piece and was prepared based on the API standard.

[HIC resistance evaluation test]
An HIC resistance evaluation test (HIC test) was performed on each thick steel plate. At this time, 0.5% acetic acid + 5% brine saturated with 0.3 atm hydrogen sulfide was used as the NACE solution. For other conditions, the HIC test was performed based on NACE TM-0284. The test piece was immersed in the NACE solution for 96 hours. The presence or absence of cracking of each test piece after the test was investigated by ultrasonic flaw detection. A specimen having no cracks was accepted (indicated by “◯” in the HIC column of Table 2), and a specimen having cracks was rejected (indicated by “x” in the HIC column of Table 2).

[Investigation result]
Since the test materials 1 to 8 had the chemical composition, the Mn segregation degree, the P segregation degree, and α within the specified ranges of the present invention, no cracks occurred in the HIC test. Further, the yield stress YS and the tensile strength TS of the test materials 1 to 8 were within the specified range of the present invention, and the strength grade was within the range of API5L X60 to X80. Specifically, the α value of the specimen 1 was 591 and the strength grade was X60. The α values of the test materials 2 to 4 were 583 to 618, and the strength grade was X65. The α values of the test materials 5 and 6 were 635 and 627, respectively, and the strength grade was X70. The α values of the test materials 7 and 8 were 727 and 726, respectively, and the strength grade was X80.

  On the other hand, HIC occurred in the test materials 9 to 18. It is thought that the Mn segregation degree or the P segregation degree exceeded the specified range of the present invention. Further, although the test materials 19 and 20 had a chemical composition within the range of the present invention, HIC occurred because α exceeded the specified range of the present invention.

  Since the chemical composition of the test materials 21 to 42 exceeded the specified range of the present invention, HIC occurred. Moreover, as for the test materials 43-45, the chemical composition exceeded the regulation range of this invention, and (alpha) was less than the lower limit. Therefore, the tensile strength was less than the lower limit of the present invention, and the strength corresponding to APIL5 X60 could not be obtained.

  The steel plate for line pipe according to the present invention can be used for a line pipe for conveying crude oil or natural gas, and in particular, a line pipe used in a small amount of hydrogen sulfide environment having a hydrogen sulfide partial pressure of 0.1 to 0.3 atm. Is available.

It is a schematic diagram for demonstrating the measuring method of Mn segregation degree and P segregation degree of the thick steel plate for line pipes by embodiment of this invention. It is a schematic diagram for demonstrating the method of the line analysis using the test piece in FIG.

Claims (4)

C: 0.04 to 0.08% by mass, Si: 0.05 to 0.35%, Mn: 1.20 to 1.80%, Nb: 0.02 to 0.06%, Ti: 0 0.005 to 0.030%, Al: 0.010 to 0.060%, Mo: 0 to 0.25%, Cu: 0 to 0.40%, Cr: 0 to 0.50%, Ni: 0 to 0.50%, V: 0 to 0.06%, P: 0.020% or less, S: 0.003% or less, Ca: less than 0.0005%, the balance consisting of Fe and impurities, the whole A material whose Mn segregation degree, which is the ratio of the Mn concentration of the segregation part to the Mn concentration, is 1.2 or less and whose P segregation degree, which is the ratio of the P concentration of the segregation part to the total P concentration, is 2.0 or less. Charging the furnace and heating to T ₁ ° C;
Extracting the heated material from the heating furnace and rolling it into a thick steel plate at a rolling finishing temperature of T ₂ ° C;
Water-cooling the steel plate to T ₃ ° C and air-cooling from T ₃ ° C to room temperature,
The manufacturing method of the thick steel plate for line pipes characterized by satisfy | filling Formula (1).
547 ≦ 0.10 WT + 1428C + 93Si + 108Mn-12Cu + 113Cr + 113Ni + 304Mo + 430Nb + 238V + 1315Ti + 0.21T ₁ + 0.03T ₂ −0.04T ₃ + 30 ≦ 739 (1)
Here, WT (mm) is the plate thickness of the thick steel plate for line pipe, and C, Si, Mn, Cu, Cr, Ni, Mo, Nb, V, and Ti are the contents (mass%) of each element. . C: 0.04 to 0.08% by mass, Si: 0.05 to 0.35%, Mn: 1.20 to 1.80%, Nb: 0.02 to 0.06%, Ti: 0 0.005 to 0.030%, Al: 0.010 to 0.060%, Mo: 0 to 0.25%, Cu: 0 to 0.40%, Cr: 0 to 0.50%, Ni: 0 to 0.50%, V: 0 to 0.06%, P: 0.020% or less, S: 0.003% or less, Ca: less than 0.0005%, the balance consisting of Fe and impurities,
The Mn segregation degree, which is the ratio of the Mn concentration of the segregation part to the total Mn concentration, is 1.2 or less, and the P segregation degree, which is the ratio of the P concentration of the segregation part to the entire P concentration, is 2.0 or less, A thick steel plate for line pipes characterized by satisfying formula (1).
547 ≦ 0.10 WT + 1428C + 93Si + 108Mn-12Cu + 113Cr + 113Ni + 304Mo + 430Nb + 238V + 1315Ti + 0.21T ₁ + 0.03T ₂ −0.04T ₃ + 30 ≦ 739 (1)
Here, WT (mm) is the plate thickness of the thick steel plate for line pipe, T ₁ (° C.) is the heating temperature in the heating furnace before rolling of the material rolled into the thick steel plate for line pipe, and T ₂ (° C.) is a rolling finishing temperature when the material is rolled into a thick steel plate, T ₃ (° C.) is a water cooling stop temperature when water cooling the thick steel plate after rolling, and other than that of each element Content (mass%). It is a thick steel plate for line pipes according to claim 2,
A thick steel plate for line pipes having a yield stress of 414 to 690 MPa and a tensile strength of 517 to 827 MPa. A thick steel plate for a line pipe according to claim 2 or claim 3,
Thick steel plate for line pipes, wherein 10.0 ≦ WT ≦ 45.0.

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