JPWO2018020972A1 - High-strength seamless steel pipe and riser - Google Patents

Abstract

Provided is a high-strength seamless steel pipe capable of stably achieving both high strength and low hardness while ensuring weldability. The high strength seamless steel pipe has a chemical composition of mass%, C: 0.10 to 0.18%, Si: 0.03 to 1.0%, Mn: 0.5 to 2.0%, P: 0.020% or less, S: 0.0080% or less, Cr: 0.10 to 0.60%, Mo: 0.10 to 0.40%, V: 0.02 to 0.40%, Ti: 0 0.004 to 0.020%, B: 0.0005 to 0.005%, Al: 0.10% or less, N: 0.008% or less, Ca: 0.0004 to 0.0040%, Cu: 0.00. 1 to 1.0%, Ni: 0.2 to 1.0%, Nb: 0 to 0.05%, balance: Fe and impurities, which satisfy the following formula (1).
C + Si / 30 + (Mn + Cu + Cr) /20+Ni/60+Mo/15+V/10+5×B≦0.28 Formula (1)
In the element symbol of the formula (1), the content of the corresponding element is substituted by mass%.

Description

  The present invention relates to a high-strength seamless steel pipe and a riser using the same, and more particularly to a high-strength seamless steel pipe suitable for a work over riser and a riser using the same.

  In recent years, oil and natural gas resources in oil fields located on land and in shallow water have been depleted, and development of subsea oil fields has become active. In the submarine oil field, it is necessary to transport crude oil and natural gas from the wells of oil and natural gas wells installed on the sea floor to offshore platforms using steel pipes (line pipes) called flow lines and risers.

  A flow line is a steel pipe for transportation laid along the terrain on the ground or the sea bottom. A riser is a steel pipe for transportation that rises from the sea floor to an offshore platform. Flow lines and risers laid in the deep sea receive pressure from a high-pressure fluid with deep formation pressure. In addition, when operations are stopped, it is affected by seawater pressure in the deep sea. The riser is further affected by repeated distortion caused by waves. Therefore, high strength is required for the flow line and the riser, and a thick steel pipe having a thickness of 30 mm or more is used.

  Work-over risers are used for commissioning and test production of well equipment in offshore oil field development. The work over riser can come into contact with the production fluid during test production. Therefore, the work over riser may be required to have sour resistance in addition to high strength.

  Currently, steel pipes of American Petroleum Institute (API) standard X60 grade (yield strength 415 MPa or more) and X65 grade (yield strength 450 MPa or more) are used for risers and flow lines, but the X80 grade (yield strength 555 MPa or more). ) Steel pipes have also been developed.

  Japanese Patent No. 4502010 discloses a seamless steel pipe for a line pipe that can ensure high strength, stable toughness, and good corrosion resistance with a seamless steel pipe having a large thickness, and a method for manufacturing the same. As an example, this publication describes a steel pipe for a line pipe having a wall thickness of 40 mm, a yield strength of 555 MPa or more, and excellent resistance to sulfide stress corrosion cracking (SSC resistance).

  Japanese Unexamined Patent Application Publication No. 2013-32584 discloses a thick high strength seamless steel pipe excellent in sour resistance and a method for manufacturing the same. As an example, this publication describes a seamless steel pipe having a wall thickness of 30 mm, a yield strength of 600 MPa, and excellent sour resistance.

  Japanese Patent No. 5516831 has high strength and excellent hydrogen-induced cracking resistance (HIC resistance), and even in the case of circumferential welding, the HIC resistance of the heat affected zone (HAZ) is improved. An excellent seamless steel pipe suitable for a line pipe is disclosed. As an example, this publication describes a seamless steel pipe having a wall thickness of 40 mm, a yield strength of 555 MPa or more, and excellent HIC resistance.

  High-strength seamless steel pipes for line pipes are generally manufactured by heat treatment of quenching and tempering. When trying to obtain high strength with a thick material, it is conceivable to increase the carbon equivalent to increase the hardenability. However, when the carbon equivalent is increased, the weldability decreases. Since the line pipe is used by circumferential welding, it is necessary to ensure weldability. Therefore, the line pipe steel pipe is designed to have a low carbon equivalent as compared with the oil well pipe steel pipe that is constructed without being welded, and has a low hardenability.

  ISO 15156 specifies that the hardness of the surface layer of a carbon steel line pipe that requires SSC resistance is controlled to 250 Hv or less. However, since the steel pipe for line pipe has low hardenability as described above, the hardness at the center of the thickness with a small cooling rate is difficult to increase during quenching, and the hardness of the surface layer with a high cooling rate is relatively high. This hardness distribution is inherited even after tempering. As a result, it is difficult to manage the hardness of the surface layer low, particularly in thick steel pipes.

  Japanese Unexamined Patent Application Publication No. 2013-32584 describes a method of grinding a high hardness portion of a surface layer after quenching, a method of decarburizing a surface before quenching, a method of quenching in a film boiling state, and the like. However, since these methods are greatly different from the manufacturing process of a general seamless steel pipe, there is a concern about a decrease in manufacturing efficiency.

  Japanese Patent No. 4502010 and Japanese Patent No. 5516831 do not mention a specific method for hardness management.

  An object of the present invention is to provide a high-strength seamless steel pipe and a riser capable of stably achieving both high strength and low hardness while ensuring weldability.

The high-strength seamless steel pipe according to an embodiment of the present invention has a chemical composition of mass%, C: 0.10 to 0.18%, Si: 0.03 to 1.0%, Mn: 0.5 to 2.0%, P: 0.020% or less, S: 0.0080% or less, Cr: 0.10-0.60%, Mo: 0.10-0.40%, V: 0.02-0 .40%, Ti: 0.004-0.020%, B: 0.0005-0.005%, Al: 0.10% or less, N: 0.008% or less, Ca: 0.0004-0. 0040%, Cu: 0.1 to 1.0%, Ni: 0.2 to 1.0%, Nb: 0 to 0.05%, balance: Fe and impurities, satisfying the following formula (1) .
C + Si / 30 + (Mn + Cu + Cr) /20+Ni/60+Mo/15+V/10+5×B≦0.28 Formula (1)
In the element symbol of the formula (1), the content of the corresponding element is substituted by mass%.

  According to the present invention, it is possible to obtain a high-strength seamless steel pipe and a riser capable of ensuring both high strength and low hardness while ensuring weldability.

FIG. 1 is a diagram schematically showing a measurement position of HAZ hardness. FIG. 2 is a diagram showing the relationship between the tempering parameter TP, the yield strength, the tensile strength, and the hardness of the surface layer in the steel type 1. FIG. 3 is a diagram showing the relationship between the tempering parameter TP, the yield strength, the tensile strength, and the hardness of the surface layer in the steel type 2. FIG. 4 is a view showing the relationship between the tempering parameter TP, the yield strength, the tensile strength, and the hardness of the surface layer in the steel type 3. FIG. 5 is a diagram showing the relationship between the tempering parameter TP, the yield strength, the tensile strength, and the hardness of the surface layer in the steel type 4. FIG. 6 is a view showing the relationship between the tempering parameter TP, the yield strength, the tensile strength, and the hardness of the surface layer in the steel type 5. FIG. 7 is a diagram showing the relationship between the tempering parameter TP, the yield strength, the tensile strength, and the hardness of the surface layer in the steel type 6. FIG. 8 is a diagram showing the relationship between the tempering parameter TP, the yield strength, the tensile strength, and the hardness of the surface layer in the steel type 7. FIG. 9 is a diagram showing the relationship between the tempering parameter TP, the yield strength, the tensile strength, and the hardness of the surface layer in the steel type 8. FIG. 10 is a diagram showing the relationship between the tempering parameter TP, the yield strength, the tensile strength, and the hardness of the surface layer in the steel type 9. FIG. 11 is a diagram showing the relationship between the tempering parameter TP, the yield strength, the tensile strength, and the hardness of the surface layer in the steel type 10.

  The present inventors examined a high-strength seamless steel pipe that can achieve both high strength and low hardness while ensuring weldability. As a result, the following knowledge was obtained.

In order to improve hardenability, a steel material containing 0.10 to 0.18% carbon (C), which has been conventionally considered to be unsuitable for a steel material for line pipes, is used. Even if it is said C content, if PCM defined by the following formula | equation is 0.28 or less, the weldability required practically will be acquired. Specifically, if PCM is 0.28 or less, welding is possible without preheating.
PCM = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5 × B
In the element symbol of the above formula, the content of the corresponding element is substituted by mass%.

  Boron (B) is considered not to be an element that is positively contained in the steel for line pipes because it significantly deteriorates the weldability. However, the hardenability of steel can be remarkably improved by containing an appropriate amount of B within a range that satisfies PCM ≦ 0.28. In addition, it is effective to contain a predetermined amount of Cu and Ni that are elements that improve hardenability and have a relatively small effect on PCM. As a result, a hardened structure can be obtained up to the center of the thickness even with a thick steel pipe, and both high strength and low hardness can be achieved.

  By improving the hardenability of the steel pipe, it is possible to reduce the hardness variation in the cross section perpendicular to the pipe axis direction. This also contributes to an improvement in the toughness of the steel pipe. Excellent toughness can be obtained by suppressing hardness variation and appropriately limiting impurities such as P and S.

  Based on the above findings, the present invention has been completed. Hereinafter, a high-strength seamless steel pipe according to an embodiment of the present invention will be described in detail.

[Chemical composition]
The high-strength seamless steel pipe according to this embodiment has a chemical composition described below. In the following description, “%” of the element content means mass%.

C: 0.10 to 0.18%
Carbon (C) increases the hardenability of the steel. If the C content is less than 0.10%, the above effects cannot be obtained sufficiently. On the other hand, if the C content exceeds 0.18%, the weldability of the steel decreases. Therefore, the C content is 0.10 to 0.18%. The lower limit of the C content is preferably 0.12%. The upper limit of the C content is preferably 0.15%.

Si: 0.03-1.0%
Silicon (Si) deoxidizes steel. If the Si content is 0.03% or more, the above effect is remarkably obtained. However, if the Si content exceeds 1.0%, the toughness of the steel decreases. Therefore, the Si content is 0.03 to 1.0%. The lower limit of the Si content is preferably 0.05%, more preferably 0.10%. The upper limit of the Si content is preferably 0.8%, and more preferably 0.5%.

Mn: 0.5 to 2.0%
Manganese (Mn) increases the hardenability of the steel. If the Mn content is less than 0.5%, the above effect cannot be obtained sufficiently. On the other hand, if the Mn content exceeds 2.0%, Mn is segregated in the steel and the toughness of the steel is lowered. Therefore, the Mn content is 0.5 to 2.0%. The lower limit of the Mn content is preferably 0.6%. The upper limit of the Mn content is preferably 1.5%, more preferably 1.0%.

P: 0.020% or less Phosphorus (P) is an impurity. P decreases the toughness of the steel. Therefore, the P content is preferably as low as possible. Therefore, the P content is 0.020% or less. The P content is preferably 0.015% or less, more preferably 0.013% or less.

S: 0.0080% or less Sulfur (S) is an impurity. S combines with Mn to form coarse MnS and lowers the toughness and HIC resistance of the steel. Accordingly, the S content is preferably as low as possible. Therefore, the S content is 0.0080% or less. The S content is preferably 0.0060% or less, and more preferably 0.0040% or less.

Cr: 0.10 to 0.60%
Chromium (Cr) increases the hardenability of the steel. Cr further increases the temper softening resistance of the steel. If the Cr content is less than 0.10%, the above effect cannot be obtained sufficiently. On the other hand, when the Cr content exceeds 0.60%, weldability and HAZ toughness are lowered. Therefore, the Cr content is 0.10 to 0.60%. The lower limit of the Cr content is preferably 0.20%, more preferably 0.25%, and further preferably 0.30%. The upper limit of the Cr content is preferably 0.55%, more preferably 0.50%.

Mo: 0.10 to 0.40%
Molybdenum (Mo) increases the hardenability of the steel. Mo further combines with C and V in the steel to increase the strength of the steel. If the Mo content is less than 0.10%, the above effects cannot be obtained sufficiently. On the other hand, if the Mo content exceeds 0.40%, the weldability and the HAZ toughness of the steel decrease. Therefore, the Mo content is 0.10 to 0.40%. The lower limit of the Mo content is preferably 0.20%, and more preferably 0.25%. The upper limit of the Mo content is preferably 0.35%.

V: 0.02 to 0.40%
Vanadium (V) combines with C in the steel to form a V carbide and increases the strength of the steel. If the V content is less than 0.02%, the above effects cannot be obtained sufficiently. On the other hand, if the V content is higher than 0.40%, the carbides become coarse and the toughness of the steel decreases. Therefore, the V content is 0.02 to 0.40%. The lower limit of the V content is preferably 0.03%. The upper limit of the V content is preferably 0.30%, more preferably 0.20%, and even more preferably 0.10%.

Ti: 0.004 to 0.020%
Titanium (Ti) combines with N in the steel to form TiN, and suppresses a decrease in the toughness of the steel due to the solid solution N. Furthermore, finely dispersed TiN increases the toughness of the steel. When the Ti content is less than 0.004%, the above effects cannot be obtained sufficiently. On the other hand, if the Ti content is higher than 0.020%, TiN coarsens or coarse TiC is generated, so that the toughness of the steel decreases. Therefore, the Ti content is 0.004 to 0.020%. The lower limit of the Ti content is preferably 0.010%.

B: 0.0005 to 0.005%
Boron (B) drastically improves hardenability when contained in a small amount. This makes it possible to obtain a quenched structure up to the center of the wall thickness even for a thick steel pipe, and to achieve both high strength and low hardness. Moreover, by containing B, the range of the tempering conditions which can satisfy | fill the intensity | strength and hardness of a predetermined range simultaneously can be expanded. As a result, a seamless steel pipe that simultaneously satisfies a predetermined range of strength and hardness can be produced industrially and stably. If the B content is less than 0.0005%, the above effect cannot be obtained sufficiently. On the other hand, when B is contained excessively, weldability will fall rapidly. Therefore, the B content is 0.0005 to 0.005%. The lower limit of the B content is preferably 0.0008%, and more preferably 0.0010%. The upper limit of the B content is preferably 0.0030%, more preferably 0.0020%, and further preferably 0.0015%.

Al: 0.10% or less Aluminum (Al) combines with N to form fine nitrides and enhances the toughness of the steel. If Al is contained even a little, the above effect can be obtained. On the other hand, if the Al content is higher than 0.10%, the Al nitride becomes coarse and the toughness of the steel decreases. Therefore, the Al content is 0.10% or less. The lower limit of the Al content is preferably 0.001%, and more preferably 0.01%. The upper limit of the Al content is preferably 0.08%, more preferably 0.06%. The Al content in this specification means the content of acid-soluble Al (so-called Sol-Al).

N: 0.008% or less Nitrogen (N) combines with Al to form fine Al nitride and enhances the toughness of the steel. If N is contained even a little, the above effect can be obtained. On the other hand, if the N content is higher than 0.008%, the solid solution N reduces the toughness of the steel. If the N content is too high, the carbonitrides are further coarsened and the toughness of the steel is reduced. Therefore, the N content is 0.008% or less. The lower limit of the N content is preferably 0.001%. The upper limit of the N content is preferably 0.006%, and more preferably 0.005%.

Ca: 0.0004 to 0.0040%
Calcium (Ca) combines with S in steel to form CaS. The formation of MnS is suppressed by the formation of CaS. Therefore, Ca improves the toughness and HIC resistance of steel. It also has the function of suppressing toughening of alumina inclusions and improving toughness and HIC resistance. If the Ca content is less than 0.0004%, the above effect cannot be obtained sufficiently. On the other hand, if the Ca content is higher than 0.0040%, the cleanliness of the steel decreases, and the toughness and HIC resistance of the steel decrease. Therefore, the Ca content is 0.0004 to 0.0040%. The lower limit of the Ca content is preferably 0.0005%, more preferably 0.0008%. The upper limit of the Ca content is preferably 0.0035%, more preferably 0.0030%.

Cu: 0.1 to 1.0%
Copper (Cu) increases the hardenability of the steel and increases the strength of the steel. If the Cu content is less than 0.1%, this effect cannot be sufficiently obtained. On the other hand, if the Cu content is higher than 1.0%, the weldability of steel decreases. If the Cu content is too high, the grain boundary strength of the steel at a high temperature is further lowered, and the hot workability of the steel is lowered. Therefore, the Cu content is 0.1 to 1.0%. The lower limit of the Cu content is preferably 0.12%, more preferably 0.15%. The upper limit of the Cu content is preferably 0.5%, more preferably 0.3%, and further preferably 0.2%.

Ni: 0.2-1.0%
Nickel (Ni) increases the hardenability of the steel and increases the strength of the steel. Ni is also an element that enhances hardenability, but has little adverse effect on weldability. Ni further improves the toughness of the steel. If Ni is less than 0.2%, these effects cannot be obtained sufficiently. On the other hand, if the Ni content is higher than 1.0%, the SSC resistance decreases. Therefore, the Ni content is 0.2 to 1.0%. The lower limit of the Ni content is preferably 0.3%, more preferably 0.35%, and further preferably 0.4%. The upper limit of the Ni content is preferably 0.9%, more preferably 0.8%.

  The balance of the chemical composition of the high-strength seamless steel pipe according to this embodiment is Fe and impurities. The impurity here refers to an element mixed from ore and scrap used as a raw material of steel, or an element mixed from the environment of the manufacturing process.

  The chemical composition of the high-strength seamless steel pipe according to the present embodiment may contain Nb instead of part of Fe. Nb is a selective element. That is, the chemical composition of the high-strength seamless steel pipe according to the present embodiment may not contain Nb.

Nb: 0 to 0.05%
Niobium (Nb) combines with C and / or N in the steel to form fine Nb carbide, and increases the strength and toughness of the steel. Nb further dissolves in Mo carbide and suppresses coarsening of Mo carbide. If Nb is contained even a little, the above effect can be obtained. On the other hand, if the Nb content is higher than 0.05%, the carbides become coarse and the toughness of the steel decreases. Therefore, the Nb content is 0 to 0.05%. The lower limit of the Nb content is preferably 0.005%. The upper limit of the Nb content is preferably 0.04%, more preferably 0.03%.

The chemical composition of the high-strength seamless steel pipe for the riser according to this embodiment satisfies the following formula (1).
C + Si / 30 + (Mn + Cu + Cr) /20+Ni/60+Mo/15+V/10+5×B≦0.28 Formula (1)
In the element symbol of the formula (1), the content of the corresponding element is substituted by mass%.

  The value on the left side of Equation (1) is called PCM. When the PCM is high, the weldability is lowered, specifically, the hardness of the weld heat affected zone (HAZ) is excessively increased or weld cracking is likely to occur. Therefore, the PCM is set to 0.28 or less. PCM is preferably 0.27 or less, more preferably 0.26 or less.

The chemical composition of the high-strength seamless steel pipe for riser according to the present embodiment preferably has a carbon equivalent Ceq defined by the following formula (2) of 0.40 or more.
Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 Formula (2)
In the element symbol of the formula (2), the content of the corresponding element is substituted by mass%.

  The carbon equivalent Ceq is used as an index of hardenability. If the carbon equivalent Ceq is too small, sufficient hardenability cannot be obtained, and particularly in a thick steel pipe, the difference between the hardness of the surface layer and the hardness in the meat becomes large. Therefore, it becomes difficult to achieve both high strength and low hardness. The lower limit of the carbon equivalent Ceq is preferably 0.42, and more preferably 0.45. On the other hand, if the carbon equivalent Ceq is too large, it becomes difficult to ensure weldability. The upper limit of the carbon equivalent Ceq is preferably 0.55, and more preferably 0.50.

[Mechanical properties]
The high-strength seamless steel pipe according to the present embodiment preferably has a yield strength of 555 MPa or more and a tensile strength of 625 MPa or more. The high-strength seamless steel pipe according to this embodiment more preferably has a yield strength of 600 MPa or more and a tensile strength of 670 MPa or more.

  The high-strength seamless steel pipe according to this embodiment preferably has a surface layer hardness of 250 Hv or less. More specifically, it is preferable that the hardness at a position of 1 mm from the inner surface of the steel pipe and the hardness at a position of 1 mm from the outer surface of the steel pipe are both 250 Hv or less. The hardness is measured according to JIS Z 2244. More preferably, the high-strength seamless steel pipe according to the present embodiment has a surface layer hardness of 240 Hv or less.

  The high-strength seamless steel pipe according to the present embodiment preferably has a hardness variation in a cross section perpendicular to the tube axis direction (hereinafter referred to as “hardness variation in the same cross section”) of 15 Hv or less. Specifically, the difference between the larger hardness value measured at a position 1 mm from the inner surface and 1 mm from the outer surface of the steel pipe and the average value of the hardness at the center of the wall thickness is preferably 15 Hv or less. In addition, let the average value of the hardness of the thickness center be an average value measured at four points at the thickness center position (1/2 thickness position). The high-strength seamless steel pipe according to the present embodiment more preferably has a hardness variation of 13 Hv or less in the same cross section.

  The high-strength seamless steel pipe according to the present embodiment preferably has a thickness of 30 mm or more. The high-strength seamless steel pipe according to the present embodiment preferably has a thickness of 35 mm or more, and more preferably has a thickness of 40 mm or more.

  The high-strength seamless steel pipe according to this embodiment is suitable for a riser. The high-strength seamless steel pipe according to this embodiment is particularly suitable for a work over riser.

[Production method]
Hereinafter, an example of the manufacturing method of the high intensity | strength seamless steel pipe by this embodiment is demonstrated. However, the manufacturing method of the high-strength seamless steel pipe by this embodiment is not limited to this.

  The steel having the above chemical composition is melted and refined. Subsequently, billets are produced from the molten steel by a continuous casting method. A slab or bloom may be produced from molten steel, and the billet may be produced by hot working the slab or bloom. The billet is hot-worked to produce a blank tube. Specifically, piercing rolling, stretching rolling, and constant diameter rolling are performed to manufacture a raw pipe.

Quench the manufactured tube. Quenching is a heat treatment in which the base tube is rapidly cooled from the austenite region. Quenching may be so-called direct quenching in which a hot raw tube after hot working is rapidly cooled as it is from a temperature of ₃ or more points of Ar. So-called in-line quenching in which the temperature is soaked at _three or more points and then rapidly cooled may be used. Alternatively, so-called reheating and quenching may be used, in which the elementary tube once cooled is reheated to a temperature of Ac ₃ point or higher and then rapidly cooled.

Temper the quenched pipe. Tempering is usually carried out at a temperature of Ac ₁ point or less. The tempering conditions are adjusted according to the yield strength and hardness. Tempering conditions can be managed using the following tempering parameter TP.
TP = (T + 273) × (20 + log (t))
In the formula, T is a tempering temperature expressed in ° C, t is a tempering time expressed in time, and log (t) is a common logarithm of t.

  The higher the tempering parameter TP, the lower the yield strength and surface hardness. As described above, the high-strength seamless steel pipe preferably has a high yield strength and a low surface hardness. The tempering parameter TP is adjusted so that necessary characteristics can be obtained.

  Heretofore, an example of a method for producing a high-strength seamless steel pipe has been described. The high-strength seamless steel pipe and the riser using the same according to the present embodiment can achieve both high strength and low hardness while ensuring weldability.

  Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited to these examples.

  Steels having chemical compositions of steel types 1 to 10 shown in Table 1 were melted in a converter and round billets were produced by continuous casting. In Table 1, “-” indicates that the content of the corresponding element was at the impurity level.

  Steel type 1, steel type 2, and steel type 10 are steels that satisfy the preferred conditions of this embodiment. Steel type 3 is a comparative example in which a carbon equivalent Ceq is about the same as steel type 1 from the chemical composition of a general steel product for line pipes with a low C content. Steel type 4 is a comparative example in which a carbon equivalent Ceq is about the same as that of steel type 1 from the chemical composition of a general steel product for line pipes containing Nb. Steel type 5 is a comparative example in which the C content and carbon equivalent Ceq are set to the same level as steel type 1 and the B content is lowered. Steel type 6 is a comparative example simulating a high carbon equivalent steel material used for oil well steel pipes and the like. Steel type 7 is a comparative example in which the Ca content is low. Steel type 8 is a comparative example in which the Cu and Ni contents are low. Steel type 9 is a comparative example in which the Mo content is low.

  The manufactured round billet was heated to 1100 to 1300 ° C. in a heating furnace, and pierced and rolled by a piercing machine. Further, the steel pipe was drawn and rolled by a mandrel mill and fixed diameter rolled by a sizer to produce a seamless steel pipe having an outer diameter (OD) and a wall thickness (WT) shown in Table 2. Each seamless steel pipe was quenched and tempered under the conditions shown in Table 2 to produce Item A to L seamless steel pipes.

[Hardness measurement]
A test piece for hardness measurement was taken from each seamless steel pipe after tempering, and the hardness was measured in accordance with JIS Z 2244. The higher hardness at the position of 1 mm from the inner surface of the steel pipe and the higher hardness at the position of 1 mm from the outer surface of the steel pipe are shown in the “surface layer” column of Table 2. In addition, the average hardness at four points measured at the center of the thickness is shown in the column “in the meat” of Table 2. The difference between the “surface” value and the “meat” value is shown in the “Difference” column of Table 2.

[Tensile test]
From each seamless steel pipe after tempering, arc test pieces (width 38.1 mm, gauge distance 50.8 mm) specified in ASTM E8 / E8M, with the long sides of the test pieces in the longitudinal direction (L direction) of the steel pipe Collected in parallel. Using the collected specimens, a tensile test was carried out in the air at room temperature (25 ° C.) to determine yield stress and tensile strength. Yield stress was determined by the 0.5% total elongation method. The yield stress is shown in the “YS” column of Table 2, and the tensile strength is shown in the “TS” column of Table 2.

[HIC test]
From each seamless steel pipe after tempering, a test piece including the inner surface, a test piece including the wall thickness center, and a test piece including the outer surface were collected. Each test piece had a thickness of 30 mm, a width (circumferential direction) of 20 mm, and a length of 100 mm. According to NACE TM0284-2003, the HIC resistance of each test piece was evaluated. The test bath was a room temperature 5% salt + 0.5% acetic acid aqueous solution saturated with 1 atm hydrogen sulfide gas. After 96 hours from the immersion, each test piece was cut into three equal parts in the longitudinal direction, and the presence or absence of cracks was visually confirmed. Furthermore, the presence or absence of a crack was confirmed with respect to the test piece including the inner surface of the steel pipe by ultrasonic flaw detection.

  The results are shown in the “HIC” column of Table 2. “No HIC” in the “HIC” column indicates that no crack was confirmed. “HIC” indicates that a crack was confirmed.

[SSC test]
From each seamless steel pipe after tempering, a test piece including the inner surface, a test piece including the wall thickness center, and a test piece including the outer surface were collected. Each test piece had a thickness of 2 mm, a width (circumferential direction) of 20 mm, and a length of 100 mm. These test pieces were immersed in the same NACE test bath as the HIC test described above for 720 hours in a state where a stress of 90% of the yield strength was applied. The presence or absence of the crack of the test piece after immersion was investigated.

  The results are shown in the “SSC” column of Table 2. “No SSC” in the “SSC” column indicates that no crack was observed in any of the test pieces. “SSC” indicates that a crack was confirmed in any of the test pieces.

[Weldability]
A circumferential welded joint was prepared using each seamless steel pipe after tempering, and a HAZ hardness test was performed. The groove shape is 5 ° narrow groove, the welding process is GMAW (gas metal arc welding), and the welding conditions are heat input at welding of 1.0 kJ / mm, preheating / interlayer temperature of 200-250 ° C, shielding gas It was dialed Gon (80 vol% Ar + 20 vol% CO _2).

  FIG. 1 is a diagram schematically showing a measurement position of HAZ hardness. The circumferential weld joint was cut in parallel with the pipe axis direction. At a position 0.3 mm away from the melting line (FL), 7 points of hardness were measured at intervals of 1.0 mm from the position of 1.5 mm from the inner surface of the steel pipe toward the outer surface in the thickness direction. The highest hardness in the measurement point was the highest HAZ hardness. The results are shown in the column of “maximum HAZ hardness” in Table 2.

[Toughness]
The toughness of the manufactured steel pipe was confirmed by a Charpy impact test. A 2 mm V notch test piece having a cross section of 10 × 10 mm (10 × 8 mm in the notch portion) is taken from the vicinity of the thickness center of the steel pipe so that the long side of the test piece is parallel to the longitudinal direction (L direction) of the steel pipe, The test was performed at -40 ° C. The results are shown in the “absorbed energy” column of Table 2. In this Charpy impact test, a material that showed an absorbed energy exceeding 100 J was determined to have high toughness.

[Test results]
As shown in Table 2, Item A, Item B, and Item L manufactured from Steel Type 1, Steel Type 2, and Steel Type 10 that satisfy the preferred conditions of this embodiment have a yield strength of 555 MPa or more and a tensile strength of 625 MPa or more. And the hardness of the surface layer was 250 Hv or less. In Item A, Item B, and Item L, the variation in hardness in the same cross section (difference between the hardness of the surface layer and the hardness at the center of the inner thickness) was 15 Hv or less. Further, in Item A, Item B, and Item L, the maximum HAZ hardness was 250 Hv or less. Also in the Charpy impact test, the absorbed energy exceeded 100 J and the toughness was high.

  Item C manufactured from steel type 3 did not satisfy the requirements of the X80 grade in yield strength and tensile strength. In addition, Item D manufactured by changing the tempering conditions from the same steel type 3 satisfied the specification of the X80 grade, but the hardness of the surface layer exceeded 250 Hv. Therefore, Item D confirmed cracks in the SSC test.

  Item E produced from steel type 4 had a yield strength and a tensile strength satisfying the specifications of the X80 grade, and the hardness of the surface layer was 250 Hv or less. However, the variation in hardness of the same cross section was greater than 15 Hv. The yield strength and tensile strength of Item E are close to the lower limit of the X80 grade, and the hardness of the surface layer is close to the upper limit specified in ISO 15156. Therefore, these regulations may not be met due to fluctuations in operating conditions. Item F manufactured by changing the tempering conditions from the same steel type 4 has a surface hardness exceeding 250 Hv. Therefore, in Item F, cracks were confirmed in the SSC test.

  Item G manufactured from steel type 5 did not satisfy the definition of the X80 grade in yield strength.

  Item H produced from steel type 6 had a yield strength and a tensile strength satisfying the rules of the X80 grade, and the hardness of the surface layer was 250 Hv or less. However, the maximum HAZ hardness showed a high value exceeding 260 Hv. Further, the absorbed energy was less than 100 J in the Charpy impact test.

  Item I produced from steel type 7 had a yield strength and a tensile strength satisfying the specifications of the X80 grade, and the height of the surface layer was 250 Hv or less. However, cracks occurred in the HIC test, and the absorbed energy was less than 100 J in the Charpy impact test.

  ItemJ manufactured from steel type 8 has a surface layer hardness exceeding 250 Hv. Therefore, ItemJ confirmed cracks in the SSC test.

  ItemK manufactured from steel type 9 did not satisfy the definition of the X80 grade in yield strength.

[Strength-hardness balance test]
For each of steel types 1 to 10, seamless steel pipes were produced while changing the tempering conditions, and the yield strength, tensile strength, and surface hardness were measured. From the measured values, the relationship between the tempering parameter TP and these characteristics was determined by regression analysis. Then, a TP range (allowable TP width) satisfying all the three conditions of yield strength of 555 MPa or more, tensile strength of 625 MPa or more, and surface layer hardness of 250 Hv or less was determined.

More specifically, the allowable TP width is defined as the minimum TP at which the hardness of the surface layer is 250 Hv or less as TP _MIN , the maximum TP at which the yield strength is 555 MPa or more, and the maximum TP at which the tensile strength is 625 MPa or more. The smaller one is defined as TP _MAX and is defined by the following equation.
Allowable TP width = TP _MAX -TP _MIN

  If the allowable TP width is wide, the strength of the X80 grade and the hardness of 250 Hv or less can be achieved even if the operating conditions vary somewhat. Therefore, a high-strength seamless steel pipe having the above performance can be produced industrially stably. The allowable TP width is preferably 200 or more, more preferably 300 or more, and still more preferably 400 or more.

  The results are shown in FIG. 2 to FIG. 11 and the column “Allowable TP width” in Table 2. FIG. 2 is a diagram showing the relationship between the tempering parameter TP, the yield strength, the tensile strength, and the hardness of the surface layer in the steel type 1. Similarly, FIGS. 3 to 11 are diagrams showing the relationship between the tempering parameter TP, the yield strength, the tensile strength, and the hardness of the surface layer in the steel types 2 to 10, respectively.

  As shown in FIGS. 2 to 11 and Table 2, Steel Type 1, Steel Type 2, and Steel Type 10 that satisfy the preferable conditions defined in this embodiment have a wider allowable TP width than Steel Types 3-9. It was. Therefore, even if the operating conditions fluctuate somewhat, it is possible to achieve both the strength of the X80 grade and the hardness of 250 Hv or less. Therefore, steel type 1, steel type 2, and steel type 10 can industrially stably produce a high-strength seamless steel pipe having the above performance.

  The embodiment of the present invention has been described above. The above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit thereof.

Claims (5)

Chemical composition is mass%,
C: 0.10 to 0.18%,
Si: 0.03-1.0%,
Mn: 0.5 to 2.0%
P: 0.020% or less,
S: 0.0080% or less,
Cr: 0.10 to 0.60%,
Mo: 0.10 to 0.40%,
V: 0.02 to 0.40%,
Ti: 0.004 to 0.020%,
B: 0.0005 to 0.005%,
Al: 0.10% or less,
N: 0.008% or less,
Ca: 0.0004 to 0.0040%,
Cu: 0.1 to 1.0%
Ni: 0.2-1.0%
Nb: 0 to 0.05%,
Balance: Fe and impurities,
A high-strength seamless steel pipe that satisfies the following formula (1).
C + Si / 30 + (Mn + Cu + Cr) /20+Ni/60+Mo/15+V/10+5×B≦0.28 Formula (1)
In the element symbol of the formula (1), the content of the corresponding element is substituted by mass%. The high-strength seamless steel pipe according to claim 1,
The chemical composition is mass%,
Nb: 0.005 to 0.05%,
High strength seamless steel pipe containing The high-strength seamless steel pipe according to claim 1 or 2,
Having a wall thickness of 30 mm or more,
Having a yield strength of 555 MPa or more and a tensile strength of 625 MPa or more;
A high-strength seamless steel pipe having a surface layer hardness of 250 Hv or less. The high-strength seamless steel pipe according to any one of claims 1 to 3,
A high-strength seamless steel pipe having a hardness variation of 15 Hv or less in a cross section perpendicular to the pipe axis direction.   The riser which consists of a high intensity | strength seamless steel pipe as described in any one of Claims 1-4.

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