JP2010077492A - Steel pipe for line pipe and method of producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a high strength steel pipe for a line pipe, excellent in DWTT performance and HIC resistance. <P>SOLUTION: The steel pipe for a line pipe comprises 0.02-0.06% C, ≤0.5% Si, 0.8-1.6% Mn, ≤0.008% P, ≤0.0008 S, ≤0.08% Al, 0.005-0.035% Nb, 0.005-0.025% Ti, 0.0005-0.0030% Ca, ≤0.0030% O and one or more kinds of ≤0.5% Cu, ≤1% Ni, ≤0.5% Cr, ≤0.5% Mo, ≤0.1% V and the balance Fe with inevitable impurities, wherein a CP value=4.46C(%)+2.37Mn(%)/6+ä1.18Cr(%)+1.95Mo(%)+1.74V(%)}/5+ä1.74Cu(%)+1.7Ni(%)}/15+22.36P(%) is ≤0.95, and an IP value=[Ca(%)-ä0.18+130Ca(%)}*O(%)]/1.25S(%) is 1.5-2.8, and the steel pipe has, as metallurgical structure, two phase structure of ferrite-bainite having ≥50% ferritic phase in an area fraction, wherein the average grain diameter of the ferritic phase is ≤5 μm and the average aspect ratio of the bainitic phase is ≤6.0. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention is a steel pipe for high-strength line pipes used for transportation line pipes for crude oil, natural gas, etc., and is particularly suitable for line pipes with a pipe thickness of 20 mm or more that require strict DWTT performance and HIC resistance. The present invention relates to a steel pipe and its manufacturing method.

  Generally, a steel pipe for a line pipe is manufactured by forming a steel sheet manufactured by a thick plate mill or a hot rolling mill into a steel pipe by UOE forming, press bend forming, roll forming or the like and then welding. Line pipes used to transport crude oil and natural gas containing hydrogen sulfide (hereinafter sometimes referred to as “sour line pipes”) are not only strong, tough, and weldable, but also resistant to hydrogen-induced cracking (HIC resistance). ) And stress corrosion cracking resistance (SCC resistance) and so-called sour resistance is required. HIC (hydrogen-induced cracking) of steel materials is that hydrogen ions due to corrosion reactions are adsorbed on the surface of steel materials and penetrate into the steel as atomic hydrogen, resulting in non-metallic inclusions such as MnS in the steel and hard second phase structures. It is said that it diffuses and accumulates around it and causes cracks due to its internal pressure.

  Conventionally, several methods have been proposed to prevent such hydrogen-induced cracking. For example, in Patent Document 1, by reducing the S content in steel and adding an appropriate amount of Ca, REM, or the like, the formation of long extended MnS is suppressed, and a finely dispersed spherical CaS inclusion is formed. A technology to change this has been proposed. As a result, the stress concentration due to the sulfide inclusions is reduced, and the generation and propagation of cracks is suppressed, thereby improving the HIC resistance.

In Patent Documents 2 and 3, accelerated cooling is performed in the middle of transformation during cooling after rolling, reduction of elements having a high segregation tendency (C, Mn, P, etc.), reduction of segregation by soaking in the slab heating stage. Technology has been proposed. This suppresses the generation of island martensite that becomes the starting point of cracks in the center segregation part and the generation of hardened structures such as martensite that becomes the propagation path of cracks.
Japanese Patent Laid-Open No. 54-110119 JP 61-60866 A JP-A-61-165207

  However, in recent sour-resistant pipes, thick materials having a pipe thickness of 20 mm or more are increasing. DWTT performance is required for line pipes to stop brittle cracks, but DWTT performance is not only reduced for thick-walled materials, but DWTT testing is performed at a temperature lower than the design temperature when using thinned specimens. And a higher base metal toughness than the thin-walled material is required. In order to increase the base metal toughness, it is a known technique in the material design of thick steel plates that it is effective to lower the rolling end temperature in hot rolling. However, when the rolling end temperature is lowered, the metal structure of the steel sheet becomes a ferrite-bainite structure, and cracks are likely to propagate at the ferrite-bainite interface, so that there is a problem that the cracking rate in the HIC test increases. In addition, it is necessary to increase the amount of alloying element added in order to ensure the strength of the thick-walled material. In such a case, the hardness of the center segregation part increases. Is likely to occur. Furthermore, if the rolling end temperature is lowered to increase the toughness, the ferrite-bainite structure is formed up to the center of the plate thickness of the steel sheet. Will be promoted.

Accordingly, the object of the present invention is to solve the above-mentioned problems of the prior art, and to provide a steel pipe for line pipes, particularly a pipe thickness, which has high strength (preferably strength of API standard X60 or higher) and excellent DWTT performance and HIC resistance. An object of the present invention is to provide a steel pipe for a line pipe that has excellent performance that can sufficiently cope with severe DWTT performance and HIC resistance required for a sour-resistant line pipe of 20 mm or more.
Moreover, the other object of this invention is to provide the manufacturing method which can manufacture such a steel pipe for line pipes stably and at low cost.

As a result of detailed investigation mainly from the viewpoint of the metal structure, the inventor has obtained the following knowledge about the HIC resistance and DWTT performance of the steel sheet in which the rolling end temperature in the hot rolling is lowered in order to improve the base material toughness. I came to get.
(A) By reducing the rolling end temperature at the time of steel plate production, the metal structure becomes a two-phase structure of ferrite and bainite, and the base metal toughness of the steel pipe is greatly improved. FIG. 1 shows the relationship between the area fraction of the ferrite phase in the steel pipe for line pipes and the fracture surface transition temperature (85% SATT) in the DWTT test (the test method is the same as in Examples described later). According to this, it turns out that the high base material toughness whose fracture surface transition temperature is -10 degrees C or less is obtained by making the area fraction of a ferrite phase into a fixed value or more.

(B) On the other hand, when the metal structure becomes a ferrite-bainite two-phase structure, the generated cracks easily propagate through the ferrite / bainite interface, so that the HIC resistance is deteriorated. However, the distance of the ferrite / bainite interface in the rolling direction is shortened by making the shape of the bainite phase not excessively elongated in the rolling direction, that is, by keeping the average aspect ratio of the bainite phase below a certain value. Crack propagation is less likely to occur. Furthermore, by reducing the crystal grain size of the ferrite phase to a certain value or less, for example, even if cracks propagate at the ferrite / bainite interface, the crack propagation in the ferrite phase stops, so the crack length ratio or crack in the HIC test An increase in area ratio can be suppressed. FIG. 2 shows the relationship between the average aspect ratio of the bainite phase of a steel pipe for a line pipe with a ferrite-bainite two-phase structure and the crack area in the HIC test (the test method is the same as in the examples described later). According to this, it can be seen that by setting the average aspect ratio of the bainite phase to a certain value or less, excellent anti-HIC performance can be obtained even in a ferrite-bainite two-phase structure.

(C) The metal structure as described above can be obtained by optimizing the slab heating temperature, rolling end temperature, accelerated cooling start temperature, and accelerated cooling stop temperature in the steel sheet manufacturing process.
(D) Furthermore, in order to improve the HIC resistance of a steel pipe having a ferrite-bainite two-phase structure, it is effective to reduce the amount of non-metallic inclusions that can cause cracks. It is important to reduce the amount of Ca-based inclusions by strictly limiting the content, detoxifying MnS inclusions by Ca treatment, and further strictly limiting the amount of Ca addition from the relationship between the S amount and the O amount. .
(E) In order to suppress cracks at the center segregation part, the amount of alloy components having a segregation tendency is strictly controlled, the increase in the hardness of the center segregation part is suppressed, and further, the NbC which becomes the starting point of cracks at the center segregation part It is effective to suppress generation.

The present invention has been made on the basis of the above-described findings and has the following gist.
[1] In mass%, C: 0.02 to 0.06%, Si: 0.5% or less, Mn: 0.8 to 1.6%, P: 0.008% or less, S: 0.00. 0008% or less, Al: 0.08% or less, Nb: 0.005-0.035%, Ti: 0.005-0.025%, Ca: 0.0005-0.0030%, O: 0.0030 In addition, Cu: 0.5% or less, Ni: 1% or less, Cr: 0.5% or less, Mo: 0.5% or less, V: 0.1% or less 1 type or 2 types or more are contained, the balance consists of Fe and inevitable impurities, the CP value represented by the following formula (1) is 0.95 or less, and the IP value represented by the following formula (2) is 1.5 to Ferrite phase with an area fraction of ferrite phase of 50% or more and a total of ferrite phase and bainite phase of 95% or more. DOO - a bainite dual phase structure, and an average particle size of the ferrite phase is 5μm or less, the steel pipe for a line pipe, wherein the average aspect ratio of bainite phase is 6.0 or less.
CP = 4.46C (%) + 2.37Mn (%) / 6+ {1.18Cr (%) + 1.95Mo (%) + 1.74V (%)} / 5+ {1.74Cu (%) + 1.7Ni (% )} / 15 + 22.36P (%) (1)
IP = [Ca (%) − {0.18 + 130Ca (%)} * O (%)] / 1.25S (%) (2)

[2] In a method of hot rolling a steel slab and forming and welding the obtained steel plate to produce a steel pipe, the steel slab having the chemical composition described in [1] above is heated to 1000 to 1150 ° C, After performing hot rolling with a rolling reduction in the non-recrystallization temperature range of 60% or more and a rolling end temperature of Ar ₃ or higher represented by the following formula (3), the cooling start temperature is set to (Ar ₃ points) −50 ° C.) to Ar ₃ points, a method for producing a steel pipe for a line pipe, characterized by performing accelerated cooling at a cooling stop temperature of 300 to 550 ° C.
Ar ₃ (° C.) = 910-310C (%)-80Mn (%)-20Cu (%)-15Cr (%)-55Ni (%)-80Mo (%) (3)

  The steel pipe for line pipe of the present invention has high strength, excellent DWTT performance and HIC resistance, and can sufficiently cope with severe DWTT performance and HIC resistance required especially for sour line pipes with a pipe thickness of 20 mm or more. Excellent performance. Moreover, according to the manufacturing method of this invention, such a steel pipe for line pipes can be manufactured stably and at low cost.

Hereinafter, the detail of the steel pipe for line pipes of this invention is demonstrated.
First, the chemical components of the steel pipe for line pipes of the present invention and the reasons for limitation will be described. In addition,% of the component amount is all “mass%”.
C is the most effective element for increasing the strength of the steel sheet produced by accelerated cooling. However, if the amount of C is less than 0.02%, sufficient strength cannot be secured, while if it exceeds 0.06%, toughness and HIC resistance deteriorate. Therefore, the C content is 0.02 to 0.06%.
Si is added for deoxidation, but when the amount of Si exceeds 0.5%, toughness and weldability deteriorate. For this reason, the amount of Si shall be 0.5% or less.

Mn is added to improve the strength and toughness of the steel, but if the amount of Mn is less than 0.8%, the effect is not sufficient, while if it exceeds 1.6%, the weldability and HIC resistance deteriorate. . For this reason, the amount of Mn shall be 0.8 to 1.6%.
P is an inevitable impurity element, and the HIC resistance is deteriorated by increasing the hardness of the central segregation part, and this tendency becomes remarkable when the amount of P exceeds 0.008%. Therefore, the P content is 0.008% or less, preferably 0.006% or less.
S is generally MnS-based inclusions in steel, but the form is controlled from MnS-based to CaS-based inclusions by the addition of Ca. However, if the amount of S is large, the amount of CaS-based inclusions also increases, and a high-strength material can be a starting point for cracking. This tendency becomes remarkable when the S amount exceeds 0.0008%. For this reason, the amount of S is made 0.0008% or less.
Al is added as a deoxidizer, but if the Al content exceeds 0.08%, ductility deteriorates due to a decrease in cleanliness. For this reason, the amount of Al is made into 0.08% or less.

Nb suppresses grain growth during rolling, and improves toughness by making fine grains. However, if the Nb content is less than 0.005%, the effect is not sufficient. On the other hand, if the Nb content exceeds 0.035%, not only the toughness of the weld heat affected zone is deteriorated, but also coarse Nb carbonitride is produced. The anti-HIC performance deteriorates. Therefore, the Nb amount is set to 0.005 to 0.035%.
Ti not only suppresses grain growth during slab heating by forming TiN, but also suppresses grain growth in the weld heat affected zone and improves toughness by making the base material and the weld heat affected zone finer. However, if the Ti content is less than 0.005%, the effect is not sufficient, while if it exceeds 0.025%, the toughness deteriorates. Therefore, the Ti amount is set to 0.005 to 0.025%.

Ca is an element effective in improving the ductility by controlling the form of sulfide inclusions, but if Ca content is less than 0.0005%, the effect is not sufficient, while adding over 0.0030% Even so, the effect is saturated, but rather the toughness deteriorates due to a decrease in cleanliness. Therefore, the Ca content is set to 0.0005 to 0.0030%.
O is an unavoidable impurity and forms oxide inclusions in the steel and becomes the starting point of cracking in the HIC test, so the smaller the content, the better. However, if it is 0.0030% or less, the generation | occurrence | production of the crack by an oxide type inclusion can be suppressed by adding the quantity of Ca according to the amount of O. For this reason, the amount of O is made 0.0030% or less.

The steel pipe of the present invention further contains one or more selected from Cu, Ni, Cr, Mo, and V in the following ranges.
Cu is an effective element for improving toughness and increasing strength, but if added over 0.5%, weldability deteriorates. For this reason, when adding Cu, it is 0.5% or less.
Ni is an element effective for improving toughness and increasing strength, but if it exceeds 1%, weldability deteriorates. For this reason, when adding Ni, it is 1.0% or less.
Cr is an element effective for increasing the strength by enhancing the hardenability, but if added over 0.5%, the weldability deteriorates. For this reason, when adding Cr, it is 0.5% or less.
Mo is an element effective for improving toughness and increasing strength, but if added over 0.5%, weldability deteriorates. For this reason, when adding Mo, it is 0.5% or less.
V is an element that increases the strength without deteriorating the toughness, but if added over 0.1%, the weldability is significantly impaired. For this reason, when adding V, it is made into 0.1% or less.
The balance of the steel pipe of the present invention is Fe and inevitable impurities.

In the present invention, the CP value represented by the following formula (1) is further defined as 0.95 or less, and the IP value represented by the following formula (2) is defined as 1.5 to 2.8. Here, in the following formulas (1) and (2) and formula (3) described later, C (%), Mn (%), Cr (%), Mo (%), V (%), Cu (% ), Ni (%), P (%), Ca (%), O (%), and S (%) are the contents of the respective elements.
CP = 4.46C (%) + 2.37Mn (%) / 6+ {1.18Cr (%) + 1.95Mo (%) + 1.74V (%)} / 5+ {1.74Cu (%) + 1.7Ni (% )} / 15 + 22.36P (%) (1)
IP = [Ca (%) − {0.18 + 130Ca (%)} * O (%)] / 1.25S (%) (2)

  The above equation (1) relating to the CP value is an equation created to estimate the material of the central segregation part from the content of each alloy element. The higher the CP value, the higher the concentration of the central segregation part, and the central segregation part. The hardness of the part increases. By setting the CP value to 0.95 or less, the hardness of the central segregation portion can be sufficiently reduced (preferably HV250 or less), and cracks in the HIC test can be suppressed. The lower the CP value, the lower the hardness of the center segregation part. Therefore, when higher HIC resistance is required, the upper limit is desirably set to 0.92.

  The above formula (2) relating to the IP value is an index for suppressing the generation of MnS by generating CaS by adding Ca, and the generation of MnS can be suppressed by controlling the IP value within a predetermined range. . In particular, in the case of a ferrite-bainite two-phase structure, since the sensitivity to cracking is higher than that of a steel pipe having a normal bainite single-phase structure, it is necessary to strictly suppress the generation of MnS that is the starting point of cracking. When the IP value is less than 1.5, the reduction of MnS is insufficient, and cracks occur in the HIC test in the ferrite-bainite two-phase structure. On the other hand, when the IP value exceeds 2.8, although MnS generation is suppressed, a large amount of Ca-based oxide is generated, so that the cleanliness of the steel pipe is impaired and sour resistance performance is also deteriorated.

Next, the metal structure of the steel pipe for line pipes of the present invention and the reason for limitation will be described.
The steel pipe for a line pipe of the present invention is a steel pipe having a ferrite-bainite two-phase structure, and the metal structure is composed of a ferrite phase having an area fraction of 50% or more and the remaining bainite phase (however, other metal phases are inevitable). The ferrite phase has an average particle size of 5 μm or less and the bainite phase has an average aspect ratio of 6.0 or less.
Although the DWTT performance is improved by adopting a ferrite-bainite two-phase structure, as shown in FIG. 1, if the area fraction of the ferrite phase is less than 50%, the effect cannot be sufficiently obtained. On the other hand, if the area fraction of the ferrite phase exceeds 80%, the bainite phase becomes too hard and the HIC property tends to be lowered, so the upper limit of the area fraction of the ferrite phase is preferably 80%. Basically, the balance is the bainite phase, but unavoidably other metal phases (martensite, pearlite, cementite, etc.) can be maintained at the desired DWTT performance even if the total area fraction is less than 5%. A ferrite-bainite two-phase structure in which the total of the ferrite phase and the bainite phase is 95% or more in terms of area fraction may be used.

The area fraction of the ferrite phase and the bainite phase is 200 to 400 times the metal structure of the cross section in the rolling direction at the position of the sheet thickness ¼ for the sample taken from the 90 ° portion in the pipe circumferential direction from the seam welded portion of the steel pipe. Observed and photographed with an optical microscope, and analyzed and imaged the tissue photograph.
Moreover, DWTT performance improves as the crystal grain size of the ferrite phase is smaller, and furthermore, crack propagation in the HIC test can be suppressed. However, when the average particle size of the ferrite phase exceeds 5 μm, a sufficient effect cannot be obtained.
The average particle diameter of the ferrite phase is determined by the line segment method from the structure photograph obtained by determining the area fraction of the ferrite phase.

In a steel pipe having a ferrite-bainite two-phase structure, the interface between the ferrite phase and the bainite phase becomes a crack propagation path in the HIC test. Therefore, the HIC resistance performance is significantly deteriorated. However, if the average aspect ratio of the bainite phase is 6.0 or less, the crack stops at the adjacent ferrite phase before propagating over a long distance, so that sufficient HIC resistance performance can be obtained even in a ferrite-bainite two-phase structure. can get. Moreover, since the smaller the aspect ratio of the bainite phase is, the more effective the crack propagation is suppressed in the HIC test, the more preferable average aspect ratio of the bainite phase is 5.0 or less.
As for the aspect ratio of the bainite phase, an image analysis is performed on the structure photograph obtained for the area fraction of the ferrite phase, and the average aspect ratio is obtained.

The diameter and thickness of the steel pipe for line pipe of the present invention are not particularly limited, but as described above, a steel pipe having a thickness of 20 mm or more that requires particularly severe DWTT performance and HIC resistance is particularly suitable.
The steel pipe for a line pipe of the present invention is usually manufactured by forming a steel plate manufactured by a thick plate mill or a hot rolling mill into a tubular body by UOE forming, press bend forming, roll forming, etc., and then seam welding. The

Next, the preferable manufacturing conditions of the steel plate (steel pipe raw steel plate) for obtaining the metal structure described above are as follows.
The heating temperature of the steel slab to be hot rolled is 1000-1150 ° C. If the slab heating temperature is less than 1000 ° C., the solid solution of Nb carbide is insufficient, coarse undissolved Nb carbide is formed at the center segregation part, and not only the HIC resistance is deteriorated but also sufficient strength is obtained. I can't. On the other hand, when the slab heating temperature exceeds 1150 ° C., the crystal grains become coarse and the toughness deteriorates.

In the hot rolling of the steel slab heated under the above conditions, the rolling end temperature is set to Ar ₃ points or more. Ar ₃ point is a temperature at which ferrite transformation starts in the cooling process, and can be obtained from the chemical composition of the steel material by the following equation (3).
Ar ₃ (° C.) = 910-310C (%)-80Mn (%)-20Cu (%)-15Cr (%)-55Ni (%)-80Mo (%) (3)
When the rolling end temperature is lower than the Ar ₃ point, the bainite phase becomes a structure elongated in the rolling direction, and cracks are easily propagated, so that the HIC resistance is deteriorated. However, if the rolling end temperature is too high, the ferrite grain size becomes coarse, so the rolling end temperature is preferably Ar ₃ points + 50 ° C. or less.

After hot rolling, accelerated cooling is performed to obtain a predetermined strength. In order to obtain the ferrite-bainite two-phase structure defined by the present invention, it is necessary to set the accelerated cooling start temperature to Ar ₃ or less. On the other hand, when the accelerated cooling start temperature is lower than (Ar ₃ points-50 ° C.), the bainite phase becomes a stretched structure and the HIC resistance is deteriorated. Thus the accelerated cooling start temperature is set to (Ar ₃ point -50 ° C.) to Ar ₃ point. When higher toughness is required, it is effective to increase the area fraction of the ferrite phase, and the accelerated cooling start temperature is (Ar ₃ points-50 ° C.) to (Ar ₃ points-10 ° C.). preferable.
The average cooling rate of accelerated cooling is preferably 10 ° C./s or more in order to obtain sufficient strength.

The accelerated cooling stop temperature is 300 to 550 ° C. The present invention mainly aims to improve the performance of steel pipes for line pipes having a pipe thickness of 20 mm or more, and in such steel pipes, in order to obtain high strength, the cooling stop temperature in the accelerated cooling process. However, if the accelerated cooling stop temperature is less than 300 ° C., a hard structure such as martensite and lower bainite is formed, and the HIC resistance is deteriorated. On the other hand, when the accelerated cooling stop temperature exceeds 550 ° C., sufficient strength cannot be obtained.
The rolling reduction at the time of hot rolling may be a condition generally applied to the production of a steel plate for high-strength line pipe, but in order to obtain sufficient toughness, it is in an unrecrystallized temperature range (about 950 ° C. or less). The rolling reduction ratio is set to 60% or more.
After accelerated cooling, the steel plate may be cooled as it is by air cooling, but may be reheated by a gas combustion furnace, an induction heating furnace or the like for the purpose of uniformizing the material inside the steel plate.
A steel pipe for a line pipe is manufactured by forming the steel plate obtained as described above into a tubular body by UOE forming, press bend forming, roll forming or the like and then welding.

  In addition, the above-described steel plate temperature (rolling end temperature, accelerated cooling start temperature, accelerated cooling stop temperature) is an average temperature in the plate thickness direction when there is a temperature distribution in the plate thickness direction of the steel plate. When the temperature distribution in the direction is relatively small, the temperature of the steel plate surface may be the steel plate temperature. In addition, there is a temperature difference between the steel plate surface and the interior immediately after accelerated cooling, but the temperature difference is eliminated by heat conduction after a while, and a uniform temperature distribution is obtained in the plate thickness direction. You may obtain | require the steel plate temperature at the time of an acceleration cooling stop based on a steel plate surface temperature.

  Steels having the chemical components shown in Table 1 (steel types A to N) were continuously cast into slabs. The slab was heated and hot-rolled, and then accelerated cooling was performed to produce a thick steel plate having a thickness of 22 to 38 mm. Table 2 shows the slab heating temperature, unrecrystallized zone reduction ratio, rolling end temperature, accelerated cooling start temperature, and accelerated cooling stop temperature at this time. The average cooling rate of accelerated cooling was set to 10 ° C./s or more. The obtained thick steel plate was cold formed by the UOE process and then seam welded to produce a steel pipe having an outer diameter of 914.4 mm. Seam welding was performed by submerged arc welding of one inner layer and one outer layer.

  The metal structure of the obtained steel pipe was observed, and the area fraction of the ferrite phase, the average grain size of the ferrite phase, and the average aspect ratio of the bainite phase were determined. For samples taken from 90 ° in the pipe circumferential direction from the seam welds of each steel pipe, the metallographic structure of the cross section in the rolling direction at the position of the plate thickness (= the cross section etched with nital after cutting the cut surface) Was observed and photographed with a 400 × optical microscope, and the structure photograph was subjected to image analysis to measure the area fraction of the ferrite phase and the average aspect ratio of the bainite phase. Moreover, the average particle diameter of the ferrite phase was measured by a line segment method using the same structural photograph.

The performance test of the steel pipe was performed as follows. These results are shown in Table 2 together with the measurement results of the metal structure of the steel pipe.
(1) Tensile strength The tensile strength in the pipe circumferential direction of the steel pipe was determined by an API standard full thickness tensile test.
(2) DWTT performance The fracture surface transition temperature (85% SATT) at which the ductile fracture surface ratio was 85% was determined by the DWTT test. In this example, a fracture surface transition temperature of −10 ° C. or lower was regarded as acceptable.
(3) HIC resistance performance 6-9 HIC test pieces were collected from a plurality of locations, and a 5% NaCl + 0.5% CH ₃ COOH aqueous solution (usually saturated with hydrogen sulfide having a pH of about 3 by the HIC test) The test piece was immersed in the NACE solution for 96 hours, and then the presence or absence of cracks on the entire surface of the test piece was investigated by ultrasonic flaw detection, and the crack area ratio (CAR) was evaluated. Here, among 6 to 9 test pieces of each steel pipe, the one with the largest crack area ratio was taken as the crack area ratio representing the steel pipe. In this example, a crack area ratio of 5% or less was accepted.

According to Table 2, no. Each of Nos. 1 to 10 has a strength equivalent to APIX65, and the fracture surface transition temperature in the DWTT test is −16 ° C. or lower, and has excellent toughness. Furthermore, the crack area ratio by the HIC test is small, and the HIC resistance is very good. In all of the examples of the present invention, the structure other than the ferrite phase is substantially a bainite phase, and the total area fraction of the structure other than the ferrite phase such as island martensite and cementite and the bainite phase is 3% or less. Met.
On the other hand, No. which is a comparative example. In Nos. 11 to 17, the chemical composition satisfies the scope of the present invention, but the manufacturing conditions of the steel sheet do not satisfy the scope of the present invention, so an appropriate metal structure cannot be obtained, and therefore either DWTT performance or HIC resistance is inferior. ing. Moreover, No. which is a comparative example. In Nos. 18 to 24, although the manufacturing conditions and the metallographic structure of the steel sheet satisfy the scope of the present invention, the chemical components do not satisfy the scope of the present invention, so the DWTT performance is good, but the HIC resistance is poor. Moreover, No. which is a comparative example. Nos. 25 and 26 are inferior in both DWTT performance and HIC resistance because the chemical composition, steel plate production conditions, and metal structure do not satisfy the scope of the present invention.

The graph which shows the relationship between the area fraction of the ferrite phase of the steel pipe for line pipes, and the fracture surface transition temperature in a DWTT test The graph which shows the relationship between the average aspect-ratio of the bainite phase of the steel pipe for line pipes which has a ferrite-bainite two phase structure, and the crack area rate in a HIC test

Claims (2)

In mass%, C: 0.02 to 0.06%, Si: 0.5% or less, Mn: 0.8 to 1.6%, P: 0.008% or less, S: 0.0008% or less Al: 0.08% or less, Nb: 0.005-0.035%, Ti: 0.005-0.025%, Ca: 0.0005-0.0030%, O: 0.0030% or less Further, Cu: 0.5% or less, Ni: 1% or less, Cr: 0.5% or less, Mo: 0.5% or less, V: 0.1% or less It contains two or more, the balance is Fe and inevitable impurities, the CP value represented by the following formula (1) is 0.95 or less, and the IP value represented by the following formula (2) is 1.5 to 2.8. Ferrite having a metal structure with an area fraction of ferrite phase of 50% or more and a total of ferrite phase and bainite phase of 95% or more A bainite dual phase structure, and an average particle size of the ferrite phase is 5μm or less, the steel pipe for a line pipe, wherein the average aspect ratio of bainite phase is 6.0 or less.
CP = 4.46C (%) + 2.37Mn (%) / 6+ {1.18Cr (%) + 1.95Mo (%) + 1.74V (%)} / 5+ {1.74Cu (%) + 1.7Ni (% )} / 15 + 22.36P (%) (1)
IP = [Ca (%) − {0.18 + 130Ca (%)} * O (%)] / 1.25S (%) (2) In a method of hot rolling a steel slab and forming and welding the obtained steel plate to produce a steel pipe,
The steel slab having the chemical composition according to claim 1 is heated to 1000 to 1150 ° C., the rolling reduction in the non-recrystallization temperature range is set to 60% or more, and the rolling end temperature is represented by the following formula (3): A line characterized in that after performing hot rolling at ₃ points or more, accelerated cooling is performed at a cooling start temperature of (Ar ₃ points-50 ° C.) to Ar ₃ points and a cooling stop temperature of 300 to 550 ° C. Manufacturing method of steel pipe for pipes.
Ar ₃ (° C.) = 910-310C (%)-80Mn (%)-20Cu (%)-15Cr (%)-55Ni (%)-80Mo (%) (3)

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