JP7215332B2 - Manufacturing method of welded steel pipe for sour resistant line pipe - Google Patents

Description

TECHNICAL FIELD The present invention relates to a welded steel pipe for sour line pipe and a method for manufacturing the same.

Against the backdrop of rising global energy demand, the amount of crude oil and natural gas extracted is increasing year by year. We are forced to use crude oil and natural gas. Therefore, in order to ensure safety, pipelines laid in such an environment must have HIC (Hydrogen Induced Cracking) resistance and SSC (Sulfide Stress Corrosion Cracking) resistance performance. Application of line pipes with excellent sour resistance is required.

Furthermore, in order to increase the distance of pipelines and improve transportation efficiency, sour-resistant line pipes used in pipelines tend to be thicker and stronger. Therefore, with a strength grade of about API 5L X60 to X65 and a pipe thickness of about 25 to 40 mm, excellent sour resistance performance and excellent DWTT (Drop Weight Tear Test) in the A solution environment of NACE-TM0284 and NACE-TM0177 The issue is how to stably supply thick-walled, high-strength sour line pipes that ensure performance (tear test) to customers.

In addition, in API RP 5L3, which is a DWTT test standard, when evaluating the DWTT performance of welded steel pipes and thick steel plates with a thickness of 28.6 mm or more with a 19 mm reduced thickness test piece, the test temperature is shifted to the low temperature side by 17 ° C. This makes it difficult to ensure DWTT performance.

In order to stably supply sour-resistant line pipes, it is possible to use thick steel plates manufactured by so-called TMCP (Thermo-Mechanical Control Process) technology, which combines controlled rolling and controlled cooling, using continuously cast slabs as steel pipe materials. Required. Under such constraints, many studies have been conducted in the past to improve the HIC resistance performance and/or the DWTT performance.

For example, in Patent Document 1, in a thick high-strength line pipe, the heating temperature during reheating of the slab is set to a condition in which NbCN in the slab dissolves and coarsening of austenite grains is suppressed as much as possible. A method for achieving both DWTT performance and HIC resistance performance is disclosed.

Further, in Patent Document 2, by performing online rapid heating after accelerated cooling, MA (Martensite-Austenite constituent; island-shaped martensite) generated in the microstructure is decomposed, and the propagation stopping performance of HIC is improved. A method for ensuring good HIC resistance performance is disclosed. In Patent Document 3, in accelerated cooling, the cooling rate at the center of the plate thickness is lowered in the initial stage to lower the surface layer temperature to 500 ° C. or less, and then cooling to the cooling stop temperature at the center of the plate thickness at which strength can be secured at a high cooling rate. discloses a method for reducing the hardness of the surface layer and suppressing the hardening of the central segregation part, thereby ensuring excellent HIC resistance performance.

JP 2010-189722 A JP-A-2009-52137 Japanese Patent Application Laid-Open No. 2000-160245

In thick-walled high-strength sour line pipes, HIC occurs in the vicinity of the surface layer in addition to HIC occurring in the center segregation part and inclusion accumulation zone (near the 1/4t position on the slab surface side in the case of a vertical bending type continuous casting machine). HIC is often a problem. This is because (1) the thicker the wall, the greater the amount of strain received during pipe making by cold working such as UOE or press bending, and (2) when using TMCP, the strength is secured by accelerated cooling. (3) As the wall thickness increases, the difference in cooling rate between the surface layer and the center of the plate thickness increases, and the hardness of the surface layer increases. One of the reasons is that it becomes easier. However, Patent Literature 1 does not clarify a method for solving HIC occurring on the surface layer of a thick high-strength sour line pipe. Patent Documents 2 and 3 disclose a method of online rapid heating and two-stage accelerated cooling as methods for reducing surface layer hardness, respectively. It is

Moreover, the thicker the wall, the more difficult it becomes to ensure the DWTT performance, but Patent Documents 2 and 3 do not disclose a method for achieving both excellent sour resistance performance and DWTT performance in thick walls.

Furthermore, thick high-strength sour line pipes are often laid on the seabed, and in this case, high compressive strength is required in order to prevent crushing caused by external pressure during laying. However, none of Patent Documents 1 to 3 disclose a line pipe having both excellent HIC resistance and DWTT properties and high compressive strength, and a method for manufacturing the same.

Therefore, in view of the above problems, the present invention provides a welded steel pipe for sour-resistant line pipe having both excellent HIC resistance and DWTT properties even though it is thick and high compressive strength, and an advantageous manufacturing method thereof. With the goal.

In order to solve the above-mentioned problems, the present inventors individually investigated HIC occurring at each position in the pipe thickness direction of a welded steel pipe with a uniform bainite microstructure, and obtained the following findings. . First, with regard to HIC that occurs in the center segregation part, conventional knowledge can be applied to thick-walled materials, and can be suppressed by suppressing the Vickers hardness of the center segregation part to 250 or less and suppressing the generation of MnS. It turns out there is.

Regarding the central segregation part, it was found that when the microstructure is a bainite single phase, the Vickers hardness can be controlled to 250 or less by setting the index of the below-described formula (2) to 1.05 or less. Also, with respect to suppressing the generation of MnS, addition of an optimum amount of Ca is effective, and various formulas have been conventionally proposed. However, recent low O, ultra-low S steel has a high correlation with the ACRM shown in equation (3) below, and by setting the ACRM to 1.0 or more, it is possible to suppress the generation of MnS in the center segregation part. I found out.

Next, with regard to HIC, which occurs in the inclusion-accumulated zone generated by the vertical bending continuous casting machine, the generation of Ca clusters can be suppressed by setting the Ca/O ratio to 2.5 or less. It was found that by setting the height to 230 or less, the generation can be suppressed. As a result of various investigations into methods for making the surface layer hardness of welded steel pipes 230 or less, it was found that, on the premise that the surface layer is tempered to 400°C or higher after accelerated cooling, the surface layer should be tempered from 700°C to 600°C at a depth of 1 mm from the surface. was found to be achievable by setting the average cooling rate of 150° C./s or less.

Next, we investigated how to ensure DWTT performance under the above constraints. As a result, after dissolving an appropriate amount of NbC in the slab reheating stage, rolling is completed at a temperature as low as possible in a temperature range higher than the Ar3 point, thereby improving the DWTT performance and further improving the Ar3 point of the steel material. It was found that the lower the , the better the DWTT performance. Furthermore, as a result of investigating steel material properties that correlate with DWTT performance, it became clear that the degree of (211) plane integration has the best correlation.

Furthermore, the present inventors investigated a method for ensuring high compressive strength. As a result, it was found that the microstructure is a uniform bainite structure, and high compressive strength can be obtained by controlling the MA in the microstructure to 1% or less.

The gist and configuration of the present invention completed based on the above findings are as follows.
[1] % by mass, C: 0.03 to 0.06%, Si: 0.5% or less, Mn: 0.8 to 1.6%, P: 0.008% or less, S: 0.0015 % or less, Al: 0.08% or less, Mo: 0.05-0.50%, Nb: 0.005-0.050%, Ti: 0.005-0.020%, Ca: 0.0010- 0.0040%, N: 0.008% or less, and O: 0.0030% or less, the Ceq represented by formula (1) is 0.32 or more, and the PHICT represented by formula (2) is 1.05 or less, ACRM represented by formula (3) is 1.0 or more, Ca / O is 2.5 or less, and the balance is Fe and unavoidable impurities.
Ar O represented by formula (4) is 780 or less,
In the pipe thickness direction, the microstructure from the position 2 mm deep from the inner surface to the position 2 mm deep from the outer surface contains bainite with an area fraction of 95% or more and island marten with an area fraction of 1% or less. including the site
The Vickers hardness HV10 of the portion excluding the central segregation portion is 230 or less,
Vickers hardness HV0.05 of the center segregation part is 250 or less,
The degree of integration of the (211) plane of the rolled surface at the center position of the pipe thickness obtained by X-ray diffraction is 1.5 or more,
A welded steel pipe for a sour line pipe, characterized by having a tensile strength of 535 MPa or more.

[2] One or more selected from the group consisting of Cu: 0.50% or less, Ni: 1.00% or less, Cr: 0.50% or less, and V: 0.060% or less The welded steel pipe for sour resistant line pipe according to the above [1], further comprising:

[3] % by mass, C: 0.03 to 0.06%, Si: 0.5% or less, Mn: 0.8 to 1.6%, P: 0.008% or less, S: 0.0015 % or less, Al: 0.08% or less, Mo: 0.05-0.50%, Nb: 0.005-0.050%, Ti: 0.005-0.020%, Ca: 0.0010- 0.0040%, N: 0.008% or less, and O: 0.0030% or less, the Ceq represented by formula (1) is 0.32 or more, and the PHICT represented by formula (2) is 1.05 or less, ACRM represented by formula (3) is 1.0 or more, Ca/O is 2.5 or less, and the balance is Fe and unavoidable impurities. A process of manufacturing by casting,
reheating the slab to a temperature T that satisfies equation (5);
After that, the slab is hot-rolled at a final rolling temperature of (ArO + 50) ° C. or less using ArO represented by the formula (4) and a total rolling reduction of 50 to 90% in the non-recrystallization temperature range. and obtaining a thick steel plate;
Cooling start temperature: steel plate surface temperature Ar30 ° C. or higher, cooling stop temperature: steel plate surface temperature 200 to 500 ° C., from a position 1 mm deep from the surface in the plate thickness direction to a depth of 3/16 t. The average cooling rate from 700 ° C. to 600 ° C. in the region of: 150 ° C./s or less, and the average cooling rate from 700 ° C. to 600 ° C. at the center of the plate thickness: 20 ° C./s or more.
Thereafter, a step of reheating the thick steel plate under conditions of a surface layer temperature of 400 to 720° C. and a plate thickness center temperature of 350 to 550° C.;
Thereafter, the thick steel plate is cold-worked into a cylindrical shape and the butt portions are welded to obtain a welded steel pipe;
has
A method for producing a welded steel pipe for a sour resistant line pipe, wherein the Ar3O of the welded steel pipe is 780 or less.

[4] One or more selected from the group consisting of Cu: 0.50% or less, Ni: 1.00% or less, Cr: 0.50% or less, and V: 0.060% or less The method for producing a welded steel pipe for sour-resistant line pipe according to [3] above, further comprising:

Equations (1) to (5) are as follows.
formula (1)
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5
formula (2)
PHICT = (4.46C + 0.395Mn + 0.116Cu + 0.113Ni + 0.236Cr + 0.390Mo + 0.348V + 22.36P) [{7396.2(C + 0.0023Si + 0.0344Mn - 0.2652P + 2.5275S - 0.0616Al + 0.02Cu + 0.026NiCr + 0.026Cr .02Mo-0.04Nb-0.04V+0.021Si.Mn-1.525Mn.S)-8.9423}/700] ²
Formula (3)
ACRM = {Ca-(1.23O-0.000365)}/(1.25S)
Formula (4)
Ar3O=910-310C-80Mn-20Cu-55Ni-15Cr-80Mo+0.35(t-8)
Formula (5)
6780/(2.26-log(Nb(C+12N/14)))-293≤T≤6780/(2.26-log(Nb(C+12N/14)))-223
Here, t in the above formula (4) is the pipe thickness or plate thickness (mm), and the element symbols in the above formulas (1) to (5) and Ca/O are the respective elements in the composition. It represents the content (% by mass), and is set to 0 when the element is not contained.

The welded steel pipe for sour-resistant line pipes of the present invention has excellent HIC resistance and DWTT performance even when thick, and high compressive strength.

According to the method for producing a welded steel pipe for sour-resistant line pipes of the present invention, a welded steel pipe for sour-resistant line pipes having both excellent HIC resistance and DWTT performance even with a thick wall and high compressive strength can be manufactured. can be done.

(Thick steel plates and welded steel pipes for sour line pipes)
A thick steel plate and a welded steel pipe for a sour line pipe of the present invention will be described below. Note that a welded steel pipe has a welded portion and a steel pipe base material other than the welded portion, and therefore, unless otherwise specified, the steel pipe base material, not the welded portion, is the subject of the following description.

[Component composition]
First, the chemical compositions of the steel plate and welded steel pipe of the present invention and the reasons for limiting them will be described. In addition, although the unit of the content of the element in the component composition is "mass %" in all cases, it is indicated simply as "%" unless otherwise specified.

C: 0.03-0.06%
C is an element that concentrates in the center segregation part and is an element that promotes segregation of other elements in the center segregation part, so it is better to reduce it from the viewpoint of ensuring HIC resistance performance. From this point of view, the C content should be 0.06% or less, preferably 0.05% or less. On the other hand, since C is an element that is inexpensive and greatly contributes to high strength, it is desirable to add it from the viewpoint of ensuring strength. Therefore, from the viewpoint of obtaining a predetermined strength, the amount of C is set to 0.03% or more.

Si: 0.5% or less Si is an element used for deoxidation, and a certain amount of Si cannot be avoided in order to reduce inclusions. In addition, since Si is an element that contributes to high strength and does not have a great effect on HIC resistance performance, the Si content is preferably 0.05% or more. On the other hand, if the amount of Si exceeds 0.5%, the toughness of the weld heat affected zone (HAZ) is significantly degraded, and the weldability is also degraded. Therefore, the Si content should be 0.5% or less, preferably 0.4% or less.

Mn: 0.8-1.6%
Since Mn is remarkably concentrated in the central segregation part, it is desirable to reduce it from the viewpoint of securing HIC resistance performance. If the Mn content exceeds 1.6%, the hardness of the central segregation portion increases even if other alloying elements are adjusted, and the HIC resistance performance cannot be ensured. Therefore, the Mn content is set to 1.6% or less, preferably 1.5% or less. On the other hand, Mn is an element that is inexpensive and greatly contributes to high strength, and is an element that suppresses the formation of ferrite during cooling. From the viewpoint of obtaining these effects, the Mn content is set to 0.8% or more, preferably 1.0% or more.

P: 0.008% or less P is an element that remarkably concentrates in the center segregation part, and significantly increases the hardness of the center segregation part, thereby deteriorating the HIC resistance performance. Therefore, the P content should be 0.008% or less, preferably 0.006% or less. However, from the viewpoint of steelmaking costs, the P content is preferably 0.001% or more.

S: 0.0015% or less S is an element that remarkably concentrates in the center segregation part, forms MnS in the center segregation part, and significantly deteriorates the HIC resistance performance. Therefore, the S content is set to 0.0015% or less, preferably 0.0008% or less. However, from the viewpoint of steelmaking costs, the S content is preferably 0.0001% or more.

Al: 0.08% or less Al is an essential element for reducing inclusions by deoxidation. Therefore, the Al content is preferably 0.01% or more. On the other hand, if the amount of Al exceeds 0.08%, problems such as deterioration of HAZ toughness, deterioration of weldability, and alumina clogging of the submerged nozzle during continuous casting occur. Therefore, the Al content is set to 0.08% or less, preferably 0.05% or less.

Mo: 0.05-0.50%
Mo is an element that contributes to high strength, and is an element that is less concentrated in the central segregation part. Addition is essential to obtain all strength, HIC resistance and DWTT performance in heavy wall sour wood. Therefore, the Mo content should be 0.05% or more, preferably 0.10% or more. On the other hand, when the amount of Mo exceeds 0.50%, deterioration of weldability and HAZ toughness is caused. Therefore, the Mo content is set to 0.50% or less, preferably 0.35% or less.

Nb: 0.005-0.050%
When Nb exists as solid solution Nb, it expands the non-recrystallization temperature range during controlled rolling and contributes to ensuring toughness. In order to obtain this effect, the amount of Nb should be 0.005% or more, preferably 0.010% or more. On the other hand, Nb is concentrated in the central segregation part and crystallizes coarse NbCN or NbTiCN during solidification, which becomes the origin of HIC and deteriorates the HIC resistance performance. Therefore, the Nb content is set to 0.050% or less, preferably 0.040% or less.

Ti: 0.005-0.020%
Ti, as TiN, refines the structure of the weld heat-affected zone, so addition is an essential element in order to ensure weld zone performance for high-strength line pipe applications. If the Ti content is less than 0.005%, TiN is not sufficiently generated, so the Ti content is made 0.005% or more. Further, if the Ti content exceeds 0.020%, the generated TiN is coarsened and sufficient toughness of the weld heat affected zone cannot be obtained, so the Ti content is made 0.020% or less.

Ca: 0.0010-0.0040%
Ca suppresses the formation of MnS in the central segregation portion and improves the HIC resistance performance. In order to obtain the effect, the amount of Ca should be 0.0010% or more. On the other hand, if Ca is excessively added, Ca clusters are formed in the vicinity of the surface layer and in the inclusion accumulation zone, and the HIC resistance performance is deteriorated.

N: 0.008% or less N is an impurity element, but if the amount of N is 0.008% or less, it does not deteriorate toughness and HIC resistance performance. Therefore, the amount of N is set to 0.008% or less. However, in order to ensure HAZ toughness, the N content is preferably 0.002% or more.

O: 0.0030% or less O is an impurity element, and increases the amount of Al ₂ O ₃ and CaOS produced, thereby deteriorating the HIC resistance performance in the surface layer and the inclusion accumulation zone. Therefore, the O content is set to 0.0030% or less, preferably 0.0020% or less. However, from the viewpoint of steelmaking costs, the O content is preferably 0.0001% or more.

In addition to the basic components described above, optional components are selected from the group consisting of Cu: 0.50% or less, Ni: 1.00% or less, Cr: 0.50% or less, and V: 0.060% or less. It may further contain one or more.

Cu: 0.50% or less Cu is an element that contributes to high strength, but it is also an element that concentrates in the central segregation part, so excessive addition should be avoided. Further, when the amount of Cu exceeds 0.50%, deterioration of weldability and HAZ toughness is caused, so when Cu is added, the amount of Cu is made 0.50% or less.

Ni: 1.00% or less Ni is an element that contributes to high strength, but it is also an element that concentrates in the center segregation part, so excessive addition should be avoided. Moreover, if the amount of Ni exceeds 1.00%, the weldability is deteriorated, and Ni is an expensive element.

Cr: 0.50% or less Cr is an element that contributes to high strength, but it is also an element that concentrates in the central segregation part, so excessive addition should be avoided. Moreover, if the amount of Cr exceeds 0.50%, the weldability and HAZ toughness deteriorate, so when adding Cr, the amount of Cr is made 0.50% or less.

V: 0.060% or less V is an element that contributes to high strength, but it is also an element that concentrates in the central segregation part, so excessive addition should be avoided. Moreover, if the amount of V exceeds 0.060%, the weldability and HAZ toughness deteriorate, so when adding V, the amount of V should be 0.060% or less.

The balance other than the above is Fe and unavoidable impurities.

Ceq of 0.32 or More Ceq represented by the above formula (1) is an index representing the amount of alloying elements required to ensure strength. From the viewpoint of ensuring the desired strength, Ceq is set to 0.32 or more. Although the upper limit of Ceq is not particularly limited, Ceq is preferably 0.40 or less from the viewpoint of weldability.

PHICT is 1.05 or less PHICT represented by the above formula (2) is a parameter created by the present inventors to quantify the hardness of the central segregation part. Various formulas for quantifying the hardness of the central segregation zone have been proposed in the past, but all of them are formulated based on the effect of the components on the hardness and the degree of concentration of the components in the segregation zone. On the other hand, PHICT considers the effect of the composition on the size of the formed segregation grains, which has not been considered so far, and can predict the hardness of the center segregation with higher accuracy than the conventional formula. became. The larger this value, the higher the hardness of the central segregation part, which promotes the occurrence of HIC at the center of the pipe thickness. If this PHICT is 1.05 or less, the hardness HV0.05 of the central segregation portion can be made 250 or less, and the HIC resistance performance can be ensured. Below. Although the lower limit of PHICT is not particularly limited, PHICT is preferably 0.50 or more in order to ensure strength.

ACRM of 1.0 or More The ACRM represented by the above formula (3) is an index for quantifying the control of the MnS morphology by Ca. If the ACRM is 1.0 or more, the generation of MnS at the center segregation portion is suppressed, and the HIC resistance performance at the center of the pipe thickness is improved. Therefore, ACRM shall be 1.0 or more. On the other hand, when the ACRM exceeds 4.0, CaO clusters are likely to be generated and HIC is likely to occur, so the ACRM is made 4.0 or less.

Ca/O is 2.5 or less Ca/O is an index for quantifying the limit of Ca cluster generation by Ca. When the Ca/O ratio exceeds 2.5, Ca clusters are likely to be generated, and the HIC resistance performance deteriorates in the vicinity of the surface layer and in the inclusion accumulation zone. Therefore, Ca/O is set to 2.5 or less, preferably 2.3 or less. Although the lower limit of Ca/O is not particularly limited, Ca/O is preferably 0.5 or more in order to ensure HIC resistance performance.

Ar3O is 780 or less Ar3O shown in the above formula (4) is a numerical representation of the influence of the components on the Ar3 point of the steel material, and the calculated numerical value indicates the estimated Ar3 point (° C.) of the steel material. The lower the Ar3 point of the steel material, the better the toughness of the steel plate when rolled under the same rolling conditions. Ar3O is preferably 770 or less, more preferably 760 or less. Although the lower limit of Ar3O is not particularly limited, Ar3O is preferably 730 or more in order to ensure HIC resistance performance.

[Microstructure]
Area fraction of bainite: 95% or more The microstructure of the steel plate and welded steel pipe of the present invention is desirably a single-phase structure from the viewpoint of ensuring HIC resistance performance. must be phased. The area fraction of bainite is desirably 100%, but the HIC resistance performance is ensured even if the area fraction of other structures composed of one or more of ferrite, cementite, and MA is 5% or less. be. Therefore, the area fraction of bainite is set to 95% or more. Cementite contained in the bainite lath is regarded as part of the bainite.

Area fraction of martensite islands (MA): 1% or less MA increases the Bauschinger effect of the steel material and reduces the compressive strength. From the viewpoint of obtaining high compressive strength, the area fraction of MA is 0% or more and 1% or less, preferably 0.5% or less.

In the case of thick steel plates, the "microstructure" refers to the region from a position 2 mm deep from one surface to a position 2 mm deep from the other surface in the plate thickness direction, and in the case of welded steel pipes, in the pipe thickness direction. , for the area from 2 mm deep from the inner surface to 2 mm deep from the outer surface. That is, the "area fraction of bainite" in the present invention is obtained by the following method. A nital-etched sample was prepared at a total of three points, 2 mm from the inner surface of the steel pipe, 2 mm from the outer surface, and the center of the pipe thickness, and observed with an optical microscope to measure the area fraction of bainite. , take the lowest bainite area fraction of the three. Further, the "area fraction of MA" in the present invention shall be determined by the following method. Two-stage etched samples were prepared at a total of three points, 2 mm from the inner surface of the steel pipe, 2 mm from the outer surface, and the center of the pipe thickness, and SEM photographs were taken at a magnification of 2000, and the MA was analyzed by image analysis. The area fraction is determined and the area fraction of the highest MA at the three points is adopted.

[Hardness]
Vickers hardness HV10 at locations other than the central segregation: 230 or less (welded steel pipe)
In thick-walled high-strength line pipes, HIC in the vicinity of the surface layer becomes a problem, so it is desirable that the hardness in the vicinity of the surface layer, excluding the center segregation portion, be low. On the premise that the generation of Ca clusters is suppressed, the HIC resistance performance can be ensured by setting the Vickers hardness to 230 or less at locations other than the central segregation portion. Therefore, in the welded steel pipe of the present invention, the Vickers hardness HV10 of the portion excluding the central segregation portion is set to 230 or less, preferably 220 or less. Although the lower limit of the Vickers hardness HV10 of the portion excluding the central segregation portion is not particularly limited, the hardness is generally 180 or more in the present invention. In the present invention, the "hardness of the steel pipe other than the central segregation part" is measured by a Vickers hardness tester with a load of 10 kg, and a cross section perpendicular to the rolling direction is measured from a position 1 mm deep from the inner surface to a depth of 1 mm from the outer surface. Measurements are taken at 1 mm pitches in the thickness direction over 1 mm positions (excluding the center segregation part in the center of the tube thickness), and the maximum value is used.

Vickers hardness HV10 at locations other than the central segregation: 210 or less (thick steel plate)
In thick-walled high-strength line pipes, HIC in the vicinity of the surface layer becomes a problem, so it is desirable that the hardness in the vicinity of the surface layer, excluding the center segregation portion, be low. When a thick steel plate is cold-bent to form a welded steel pipe, the hardness near the surface of the steel pipe increases by about 20 with the addition of bending. Therefore, thick steel plates for line pipes need to be controlled to a hardness that anticipates this. On the premise that the generation of Ca clusters is suppressed, the HIC resistance performance after pipe making can be ensured by setting the hardness of the portion of the steel plate excluding the center segregation portion to 210 or less. Therefore, in the steel plate of the present invention, the Vickers hardness HV10 of the portion excluding the central segregation portion is set to 210 or less, preferably 200 or less. Although the lower limit of the Vickers hardness HV10 of the portion excluding the center segregation portion is not particularly limited, the hardness is generally 150 or more in the present invention. In the present invention, the "hardness of the steel plate other than the central segregation part" is measured according to the method described in the preceding paragraph.

Vickers hardness of center segregation HV0.05: 250 or less (welded steel pipe/thick steel plate)
As the hardness of the central segregation increases, the HIC resistance deteriorates. If the steel suppresses the formation of MnS with Ca and suppresses Nb and Ti to the range of the present invention, the Vickers hardness of the central segregation part can be set to 250 or less to ensure HIC resistance performance. Therefore, in the welded steel pipe and thick steel plate of the present invention, the Vickers hardness HV0.05 of the central segregation part is set to 250 or less. Although the lower limit of the Vickers hardness HV0.05 of the central segregation portion is not particularly limited, the hardness is generally 200 or more in the present invention. In the present invention, the "hardness of the central segregation part" is obtained by measuring the hardness of the central segregation part at 20 points with a micro Vickers hardness tester with a load of 50 g, and using the maximum value.

[Increase of (211) plane of rolled surface at pipe thickness center position or plate thickness center position]
Base metal toughness such as DWTT performance required for line pipes is affected by the microstructure and texture of the steel material. The inventors of the present invention have found that there is a good correlation between the (211) plane concentration of the rolled surface at the center position of the pipe thickness or the center position of the sheet thickness, which develops when transforming from austenite to bainite, and the toughness of the base metal. I found something. When the degree of accumulation is 1.5 or more, the toughness of the base material is improved. Therefore, the degree of integration is set to 1.5 or more, preferably 1.7 or more. Although the upper limit of the degree of integration is not particularly limited, it is approximately 3.0 or less in the present invention.

The degree of accumulation of the (211) plane of the rolled surface at the pipe thickness and plate thickness center position is measured by the inverse method using an X-ray diffractometer by collecting a 5 mm thick thin film so that the rolled surface is the measurement surface. values are used. Here, the degree of accumulation of the (211) plane is a numerical value representing the degree of accumulation of the (211) crystal plane of the target material, and is taken from the center position of the pipe thickness of the target material parallel to the steel plate rolling surface ( The ratio of the X-ray diffraction intensity of the 211) reflection (I ₍ 211) ) to the X-ray diffraction intensity of the (211) reflection (I ₀₍₂₁₁₎ ) of a random standard sample without texture (I ₍₂₁₁ )/ I ₀₍₂₁₁₎ ).

[Tensile strength]
The steel plates and welded steel pipes of the present invention have a tensile strength of 535 MPa to 760 MPa, which is in the range of X65MS of API 5L.

[thickness]
It is in the case of a thick material that compatibility between HIC resistance performance and DWTT performance under the surface layer becomes a problem. The plate thickness and pipe thickness are not particularly specified in the present invention, but are preferably 28.6 mm or more, more preferably 30 mm or more.

(Manufacturing method of thick steel plate and welded steel pipe for sour resistant line pipe)
The method for producing a thick steel plate according to the present invention includes the steps of producing a slab having the chemical composition described above by continuous casting, the steps of reheating the slab to a predetermined temperature, and then heating the slab under predetermined conditions. a step of rolling to obtain a thick steel plate; a step of controlled cooling of the thick steel plate under predetermined conditions; and a step of reheating the thick steel plate under predetermined conditions thereafter. After the controlled cooling, the method for manufacturing a welded steel pipe according to the present invention comprises the step of cold working the thick steel plate into a cylindrical shape and welding the butt portions to obtain a welded steel pipe. Each step will be described below.

[Slab reheating]
Slab reheating temperature T: Shall satisfy formula (5).
Formula (5) X-293≤T≤X-223
where X=6780/[2.26-log{Nb(C+12N/14)}].

The lower the slab reheating temperature T, the finer the crystal grains. However, in the case of Nb-added steel, if it is too low, the amount of dissolved Nb during hot rolling decreases, and the toughness deteriorates. If T is (X-293)° C. or more, the amount of dissolved Nb can be ensured. On the other hand, if the slab reheating temperature is raised, the strength increases, but the crystal grains coarsen and the toughness deteriorates. If T is (X-223)° C. or less, excellent DWTT performance can be ensured, and is preferably (X-243)° C. or less. This temperature is the thickness average temperature of the slab when it is removed from the heating furnace, and is generally calculated from the actual temperature of the atmosphere inside the furnace by heat conduction calculation such as the finite difference method.

[Hot rolling]
Total rolling reduction in non-recrystallization temperature range: 50 to 90%
Reduction in the non-recrystallization temperature range has the effect of flattening the microstructure and improving toughness. To obtain this effect, the total rolling reduction is 50% or more, preferably 60% or more. On the other hand, if the total rolling reduction exceeds 90%, the HIC resistance will be degraded. Therefore, the total rolling reduction should be 90% or less, preferably 85% or less.

Final rolling temperature: (Ar3O+50)°C or less The lower the final rolling temperature, the better the DWTT performance. In order to obtain the desired DWTT performance, it is important that the final rolling temperature be (Ar3O+50)°C or lower, preferably (Ar3O+40)°C or lower. Although the lower limit of the final rolling temperature is not particularly limited, the final rolling temperature is preferably Ar30° C. or higher in order to ensure HIC resistance performance.

[Controlled cooling]
Cooling start temperature: Steel plate surface temperature Ar 30° C. or higher In order to ensure HIC resistance performance, it is necessary to have a uniform bainite structure. For this purpose, the cooling start temperature must be Ar30°C or higher, preferably (Ar3O+10)°C or higher. Although the upper limit of the cooling start temperature is not particularly limited, the cooling start temperature is preferably 850° C. or lower in order to ensure DWTT performance.

Cooling stop temperature: Steel plate surface temperature 200 to 500°C
The lower the cooling stop temperature, the higher the strength. On the other hand, if the cooling stop temperature is less than 200° C., the bainite lath spaces transform into MA, and furthermore, the center segregation part transforms into martensite, thereby deteriorating the HIC resistance performance. Therefore, the cooling stop temperature should be 200° C. or higher, preferably 300° C. or higher. However, if the cooling stop temperature exceeds 500° C., part of the untransformed austenite transforms into MA, degrading the HIC resistance performance. Therefore, the cooling stop temperature should be 500° C. or lower, preferably 450° C. or lower.

Average cooling rate from 700° C. to 600° C. in the surface layer: 150° C./s or less If the cooling rate in the surface layer is fast, the hardness of the surface layer increases and the HIC resistance deteriorates. Assuming that the surface layer is tempered to 400° C. or more after accelerated cooling, the average cooling rate of the surface layer must be 150° C./s or less in order to make the surface layer hardness of 230 or less after pipe making. Although the lower limit of the average cooling rate of the surface layer portion is not particularly limited, the average cooling rate of the surface layer portion is preferably 10° C./s or more in order to ensure the HIC resistance performance. The term "surface layer" as used herein refers to a region (a pair of regions) from a depth of 1 mm to a depth of 3/16t from the surface in the plate thickness direction.

Average cooling rate from 700°C to 600°C at the thickness center: 20°C/s or more The faster the cooling rate at the thickness center, the higher the strength. In order to obtain the desired strength in a thick-walled material, the average cooling rate at the thickness center is set to 20° C./s or more. Although the upper limit of the average cooling rate at the thickness center is not particularly limited, the average cooling rate at the thickness center is preferably 60° C./s or less in order to ensure HIC resistance performance.

Although the temperature inside the steel plate cannot be directly measured physically, based on the surface temperature at the start of cooling measured by a radiation thermometer and the surface temperature at the target cooling stop, for example, a process computer can be used to obtain the temperature distribution in the thickness cross section in real time by difference calculation. Based on the temporal change of the temperature distribution, the average cooling rate of the "surface layer" and the average cooling rate of the "thickness center" can be obtained.

[Reheat]
Surface layer temperature: 400-720°C, plate thickness center temperature: 350-550°C
Reheating is performed immediately after accelerated cooling in order to reduce the surface layer hardness and the area fraction of martensite islands. The temperature of the surface layer is desirably higher from the viewpoint of reducing the hardness. However, if the surface layer temperature exceeds 720° C., the surface layer undergoes reverse transformation and the HIC resistance deteriorates. The term "surface layer" as used herein refers to a region (a pair of regions) from a depth of 1 mm to a depth of 3/16t from the surface in the plate thickness direction. The thickness center temperature is set to 350° C. or higher, preferably 400° C. or higher, in order to decompose MA generated by accelerated cooling. On the other hand, from the viewpoint of ensuring strength and DWTT performance, the thickness center temperature is set to 550° C. or less.

In order to fully exhibit the effect of such reheating, it is preferable to reheat to a temperature higher by 50° C. or more than the surface layer temperature and plate thickness center temperature at the end of accelerated cooling. Note that reheating immediately after stopping accelerated cooling means reheating within 120 seconds after stopping accelerated cooling. In addition, the temperature increase rate in reheating is not particularly limited, but if the temperature increase rate is small, the cementite in the bainite will aggregate and coarsen, the Charpy impact absorption energy of the base material will decrease, and the DWTT performance will deteriorate. Therefore, it is preferable that the heating rate is 3° C./s or more. Although the upper limit of the temperature increase rate for reheating is not particularly limited, it is inevitably limited by the capacity of the heating means.

[Cold working/welding]
A welded steel pipe can be obtained by cold working the thick steel plate after controlled cooling into a cylindrical shape by press bending, roll forming, UOE forming, or the like, and then welding the butt portions. It is also possible to expand the welded steel pipe to improve the roundness of the steel pipe.

A steel having the chemical composition shown in Table 1 (the balance being Fe and unavoidable impurities) is made into a slab by a continuous casting method, the slab is reheated under the conditions shown in Table 2, and hot-rolled under the conditions shown in Table 2 to thick steel plate. was obtained, and further subjected to accelerated cooling and reheating under the conditions shown in Table 2, followed by air cooling. Further, the thick steel plate was made by UOE molding (O press compression rate = 0.25%, expansion rate = 1.00%) to obtain a welded steel pipe.

[Specification of microstructure]
The area fractions of bainite and island martensite were obtained by the method described above. Table 3 shows the results.

[Measurement of hardness]
"Vickers hardness HV 10 other than the center segregation part" and "Vickers hardness HV 0.05 at the center segregation part" were measured by the method described above. Table 3 shows the results.

[Measurement of (211) surface integration]
The degree of accumulation of the (211) plane of the rolled surface at the center position of the tube thickness was determined by the method described above. Table 3 shows the results.

[Measurement of tensile strength]
In the tensile test, a full-thickness test piece specified in API 5L was sampled in the circumferential direction of the welded steel pipe, and 535-760 MPa, which is the tensile strength range of API 5L X65MS, was accepted. Table 3 shows the results.

[Evaluation of Compressive Strength]
The compressive strength in the circumferential direction of the steel pipe is measured by a round bar test with a diameter of 20 mm and a length of 60 mm in accordance with ASTM E9 so that the inner surface depth is 1 mm to 21 mm at 90° or 270° in the circumferential direction from the seam weld. A strip was taken and measured. Compressive yield stress was evaluated by the average value of 0.5% proof stress measured in duplicate under each condition. A passing value was defined as 0.85 times or more the yield stress in a tensile test conducted at the same sampling position and specimen diameter. Table 3 shows the results.

[Evaluation of DWTT performance]
DWTT performance was performed with reduced-thickness DWTT specimens processed to a specimen thickness of 19 mm. A press-notch type DWTT test piece conforming to API-5L with the longitudinal direction in the C direction was taken, and welded steel pipes with a pipe thickness of 28.6 to 35.0 mm At −27° C., two samples were tested, and the ductile fracture surface ratio (SA) of the fracture surface was determined. Samples with an average ductile fracture surface rate of 85% were judged as acceptable. Table 3 shows the results.

[Evaluation of HIC resistance performance]
The HIC test was carried out using solution A of NACE TM0284-2003 for each three pieces (for pipes with a thickness exceeding 32 mm, a test piece with a thickness of 30 mm was taken from the plate thickness direction in accordance with the standard provisions), and the steel pipe cracked. Those having a maximum value of 10% or less in length ratio (CLR) evaluation were regarded as acceptable. Table 3 shows the results.

All of the welded steel pipes of the present invention satisfy the tensile strength, compressive strength, and DWTT performance required for line pipes, and also satisfy excellent HIC resistance performance. On the other hand, the welded steel pipe of the comparative example does not satisfy any of these properties.

The welded steel pipe of the present invention and the welded steel pipe produced by the production method of the present invention are suitable for sour line pipe applications that require excellent HIC resistance, DWTT resistance, and high compressive strength.

Claims (2)

in % by mass,
C: 0.03 to 0.06%,
Si: 0.5% or less,
Mn: 0.8-1.6%,
P: 0.008% or less,
S: 0.0015% or less,
Al: 0.08% or less,
Mo: 0.05-0.50%,
Nb: 0.005 to 0.050%,
Ti: 0.005 to 0.020%,
Ca: 0.0010 to 0.0040%,
N: 0.008% or less, and O: 0.0030% or less,
Ceq represented by formula (1) is 0.32 or more,
PHICT represented by formula (2) is 1.05 or less,
ACRM represented by formula (3) is 1.0 or more,
Ca/O is 2.5 or less,
A step of continuously casting a slab having a composition in which the balance is Fe and unavoidable impurities;
reheating the slab to a temperature T that satisfies equation (5);
Then the slab is
Using ArO represented by formula (4) with a total rolling reduction in the non-recrystallization temperature range of 50 to 90%, the final rolling temperature is (ArO + 50) ° C. or less. a process of obtaining
The thick steel plate,
Cooling start temperature: steel plate surface temperature Ar 30 ° C. or higher,
Cooling stop temperature: steel plate surface temperature 200 to 500 ° C.,
Average cooling rate from 700°C to 600°C in the area from 1 mm deep from the surface to 3/16t deep from the surface in the thickness direction: 150°C/s or less, and 700°C to 600°C at the center of the thickness Average cooling rate to: Controlled cooling under conditions of 20 ° C./s or more;
After that, the thick steel plate,
Surface layer temperature: 400 to 720°C, and plate thickness center temperature: 350 to 550°C
A step of reheating under the conditions of
Thereafter, the thick steel plate is cold-worked into a cylindrical shape and the butt portions are welded to obtain a welded steel pipe;
has
The ArO of the welded steel pipe is 780 or less, and the welded steel pipe has an area fraction of 95% of the microstructure from a position 2 mm deep from the inner surface to a position 2 mm deep from the outer surface in the pipe thickness direction. The above bainite and island-shaped martensite with an area fraction of 1% or less, the Vickers hardness HV10 of the portion excluding the center segregation part is 230 or less, and the Vickers hardness HV0.05 of the center segregation part is 250 or less A sour resistant line characterized by having a degree of accumulation of (211) planes of the rolled surface at the center position of the pipe thickness obtained by X-ray diffraction of 1.5 or more and a tensile strength of 535 MPa or more. A method for manufacturing welded steel pipes for pipes.
formula (1)
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5
formula (2)
PHICT = (4.46C + 0.395Mn + 0.116Cu + 0.113Ni + 0.236Cr + 0.390Mo + 0.348V + 22.36P) [{7396.2(C + 0.0023Si + 0.0344Mn - 0.2652P + 2.5275S - 0.0616Al + 0.02Cu + 0.026NiCr + 0.026Cr .02Mo-0.04Nb-0.04V+0.021Si.Mn-1.525Mn.S)-8.9423}/700] ²
Formula (3)
ACRM = {Ca-(1.23O-0.000365)}/(1.25S)
Formula (4)
Ar3O=910-310C-80Mn-20Cu-55Ni-15Cr-80Mo+0.35(t-8)
Formula (5)
6780/(2.26-log(Nb(C+12N/14)))-293≤T≤6780/(2.26-log(Nb(C+12N/14)))-223
Here, t in the above formula (4) is the pipe thickness (mm), and the element symbols in the above formulas (1) to (5) and Ca/O are the contents of each element in the above component composition ( % by mass), and is set to 0 when the element is not contained. The component composition further contains one or more selected from the group consisting of Cu: 0.50% or less, Ni: 1.00% or less, Cr: 0.50% or less, and V: 0.060% or less. The method for manufacturing a welded steel pipe for sour line pipe according to claim 1 .