Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B19/00—Tube-rolling by rollers arranged outside the work and having their axes not perpendicular to the axis of the work
  • B21B19/02—Tube-rolling by rollers arranged outside the work and having their axes not perpendicular to the axis of the work the axes of the rollers being arranged essentially diagonally to the axis of the work, e.g. "cross" tube-rolling ; Diescher mills, Stiefel disc piercers or Stiefel rotary piercers
  • B21B19/04—Rolling basic material of solid, i.e. non-hollow, structure; Piercing, e.g. rotary piercing mills
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B23/00—Tube-rolling not restricted to methods provided for in only one of groups B21B17/00, B21B19/00, B21B21/00, e.g. combined processes planetary tube rolling, auxiliary arrangements, e.g. lubricating, special tube blanks, continuous casting combined with tube rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies

Definitions

• the present invention relates to a heavy wall seamless steel pipe for line pipe excellent in strength, toughness and weldability, and a manufacturing method thereof.
• the heavy wall seamless steel pipe means a seamless steel pipe having a wall thickness of 25 mm or more.
• the seamless steel pipe of the present invention is a high-strength, high-toughness heavy wall seamless steel pipe for line pipe having a strength of not less than X70 regulated in API (American Petroleum Institute) Standard, that is, a strength of X70 (yield strength of 482 MPa or more), X80 (yield strength of 551 MPa or more), X90 (yield strength of 620 MPa or more), X100 (yield strength of 689 MPa or more), and X120 (yield strength of 827 MPa or more), which is particularly suitably used for submarine flow lines.
• API American Petroleum Institute
• An inner part of a pipe constituting the flow line laid in the deep sea suffers a high internal fluid pressure in addition to a deep stratum pressure, and is also subjected to repeated strains by ocean waves and influenced by the sea water pressure of the deep sea at the time of shutdown. Therefore, a heavy wall steel pipe with high strength and toughness is desired as the pipe used for this purpose.
• Such a seamless steel pipe with high strength and toughness has been manufactured by piercing a billet heated to high temperature by a piercing mill, shaping into a pipe shape of product by rolling and drawing, and then performing heat treatment.
• simplification of the manufacturing process by applying an in-line heat treatment has been examined from the viewpoint of energy and process saving.
• a process for performing quenching without once cooling beforehand to room temperature has been introduced. According to this method, substantial energy saving and increased efficiency of the manufacturing process can be attained, enabling significant reduction in manufacturing cost.
• a steel pipe manufactured in the in-line heat treatment process of performing quenching directly after finish rolling has not been subjected to transformation and reverse transformation since, unlike in the past, it is not reheated after once cooling to room temperature and rolling. Therefore, the grains are apt to be coarsened, and it is not easy to ensure the toughness and corrosion resistance.
• Patent Document 1 Japanese Patent Unexamined Publication 2001-240913 discloses a technique for making fine grains by adjusting the leading time to let it into the reheating furnace after finish rolling.
• Patent Document 2 Japanese Patent Unexamined Publication 2000-104117 discloses a technique for adjusting the chemical composition, particularly, the contents of Ti and S to provide a satisfactory performance even with a relatively large grain size.
• Patent Document 1 cannot respond to manufacture of a heavy wall steel pipe with high strength for offshore oil fields in depth, which has been increasingly demanded in recent years.
• the heavy wall steel pipe requires a high finish rolling temperature, and it takes an excessive time to ensure an intended reheating furnace temperature, and seriously reduces the production efficiency.
• the method described in Patent Document 2 is also hardly applicable to heavy wall materials. Since the cooling rate in the in-line heat treatment is reduced in the case of heavy wall materials, the toughness is deteriorated even if steel of the composition disclosed in Patent Document 2 is applied.
• Precipitation strengthening deteriorates the balance between strength and toughness in the case of in-line heat treatment materials. Although it is disadvantageous for obtaining a high strength, it is desirable to utilize the transformation strengthening and the solid-solution strengthening, without utilizing the precipitation strengthening, in order to obtain a high toughness,
• the Ti carbonitride that precipitated during casting is apt to coarsen with a reduced number of precipitated grains, since the precipitation is caused at a high temperature. Therefore, the capability of pinning the grains of the parent phase is reduced.
• the Ti carbonitride is finely precipitated with an increased number of precipitated grains during the heating of the billet in the subsequent pipe making process because the precipitation is occurred at a low temperature. If the number of precipitated grains is large, the effect of pinning the crystal grains of the parent phase is increased to suppress the coarse-graining of the parent phase. Accordingly, it is extremely important to properly control the cooling rate during casting.
• the Ti carbonitride precipitates in a high temperature range during cooling. However, this precipitate in an austenite range with relatively low dislocation causes few nucleation sites, which leads to a coarsely dispersed state. Once coarsely precipitated, the Ti carbonitride cannot be finely dispersed since it is hardly dissolved in a solid phase.
• the cast ingot has no Ti carbonitride , but Ti in a dissolved state.
• the Ti carbonitride precipitates at a relatively low temperature during the subsequent heating for hot working. Since, during heating, the Ti carbonitride precipitates at a low temperature in a bainite structure with high dislocation, the Ti carbonitride precipitates as being finely dispersed with many nucleation sites. It was also found that an excessively high heating rate makes fine precipitation difficult because of the precipitation in a high temperature range.
• Ti carbonitride it is also effective for the sufficiently fine precipitation of Ti carbonitride to execute isothermal treatment in a proper temperature range during heating.
• the Ti carbonitride once finely precipitated is hardly coarsened , and even if blooming is executed, the effect of suppressing the coarse-graining can be exhibited.
• the dissolved Ti in solidification should preferably be present more than that in the case of executing without blooming.
• the C content is limited to not more than 0.08%.
• the upper limit of the Si content is set to not more than 0.25%, preferably to not more than 0.15%, and more preferably to not more than 0.10%.
• Ti content needs to be controlled in a narrow range of 0.004 to 0.010% suitable to precipitate as fine Ti carbonitride, without precipitation in the solidification, during the subsequent billet heating.
• an addition of Nb is not performed in the case of an in-line heat treatment since it causes strength dispersion in addition to deterioration of the toughness, and the upper limit as an impurity is preferably set to not more than 0.005%. Since V also deteriorates the toughness, it is not added, or should be controlled to not more than 0.08% if included.
• Mn, Cr, Ni, Mo and Cu should be selectively adjusted according to the intended strength, considering the toughness and weldability.
• A1 is added for deoxidation. It is also effective to selectively add at least one of Ca, Mg and REM to ensure the casting characteristic or improve the toughness. Further, the content of N needs to be controlled in a narrow range in order to precipitate stable Ti carbonitride.
• the Ti carbonitride is not precipitated immediately after solidification if the contents of C, Ti, and N are set to the above ranges. However, since a coarse-grained Ti carbonitride is precipitated if the subsequent cooling rate is low, the cooling after solidification needs to be performed at a specified rate or more.
• a continuous casting to a billet with a circular cross section (hereinafter, refers to "a round billet”) is ideal, but a process of continuously casting to a square mold or casting thereto as an ingot, and then blooming to the round billet can be adapted.
• the round billet is reheated to a hot workable temperature and piercing, drawing and shaping rolling are performed thereto. If the dissolved Ti is sufficiently present, the Ti carbonitride is precipitated during reheating. Since the precipitation temperature is relatively low, remarkably fine Ti carbonitride is precipitated, compared with the precipitation during cooling after solidification. Since the number of grains of the finely precipitated Ti carbonitride is large, grain migration during heating or holding the billet can be suppressed to prevent the coarse-graining. A quick heating causes no fine precipitation at low temperature so that the effect of preventing the coarse-graining cannot be obtained. Therefore, a gentle heating or a holding in a middle stage is required to promote a precipitation of fine-grained Ti carbonitride.
• the present invention according to the above-mentioned basic ideas involves the following seamless steel pipes for line pipe (1) and (2) and the following methods of manufacturing a seamless steel pipe for line pipe (3) to (6).
• a heavy wall seamless steel pipe for line pipe with high strength and increased toughness which has a chemical composition, by mass%, that consists of C: 0.03 to 0.08%, Si: not more than 0.25%, Mn:0.3 to 2.5%, Al: 0.001 to 0.10%, Cr: 0.02 to 1.0%, Ni: 0.02 to 1.0%, Mo: 0.02 to 1.2%, Ti:0.004 to 0.010%, N:0.002 to 0.008%, and 0.0002 to 0.005%, in total, of at least one selected from Ca, Mg and REM, and the balance Fe and impurities, optionally including V: 0 to 0.08%, Nb: 0 to 0.05% or Cu: 0 to 1.0%,and that P and S among impurities are not more than 0.05% and not more than 0.005% respectively.
• a heavy wall seamless steel pipe for line pipe with high strength and increased toughness which has 0.0003 to 0.01% of boron in addition to the chemical composition above.
• a method of manufacturing a heavy wall seamless steel pipe for line pipe with high strength and increased toughness characterized by comprising the following steps (a) to (e):
• a method of manufacturing a heavy wall seamless steel pipe for line pipe with a high strength and increased toughness characterized by comprising the following steps (a) to (f):
• a method of manufacturing a heavy wall seamless steel pipe for line pipe with a high strength and increased toughness characterized by comprising the following steps (a) to (e):
• a method of manufacturing a heavy wall seamless steel pipe for line pipe with a high strength and increased toughness characterized by comprising the following steps (a) to (f):
• C is an important element for ensuring the strength of steel.
• a content of not less than 0.03% is needed in order to improve the hardenability and strength in a heavy wall material. Since a content exceeding 0.08% causes deterioration of toughness, the C content is set to 0.03 to 0.08%.
• Si has an effect as a deoxidizer in steel production, but it is better to add as little as possible. Because it seriously deteriorates the toughness, particularly, of a heavy wall material. If the content of Si exceeds 0.25%, the toughness of the heavy wall material is remarkably deteriorated. Therefore, the content is set to 0.25% or less if it is added as the deoxidizer. A content of 0.15% or less enables further improvement in toughness. The content is most desirably controlled to less than 0.10%. Although it is difficult to extremely reduce Si as impurity from the point of the steel making process, extremely satisfactory toughness can be obtained if the content is limited to less than 0.05%.
• Mn needs to be included in a relatively large quantity since it enhances the hardenability and therefore strengthens the center even in a heavy wall material and also enhances the toughness.
• a content of less than 0.3% cannot provide these effects, and a content exceeding 2.5% causes deterioration of the HIC resisting characteristic, therefore, the Mn content is set to 0.3 to 2.5%.
• Al is added as a deoxidizer in steel making. In order to obtain this effect, it needs to be added so as to have a content of not less than 0.001%. On the other hand, if the content of Al exceeds 0.10%, the inclusions are clustered, thereby deteriorating the toughness, and surface defects are frequently generated during the bevel face working of pipe ends. Therefore, the content of Al is set to 0.001 to 0.10 %. From the point of preventing the surface defects, it is desirable to provide an upper limit, and the upper limit is preferably set to 0.03% and more preferably to 0.02%. Since a high deoxidation effect by the addition of Si cannot be expected in the steel pipe of the present invention, the lower limit of Al content is preferably set to 0.010% for sufficient deoxidation..
• Cr is an element that improves the hardenability and strength of steel in a heavy wall material. The effect becomes remarkable when 0.02% or more of Cr is included. However, since an excessive content thereof causes some deterioration of toughness, the content is limited to 1.0% or less.
• Ni is an element that improves the hardenability and strength of steel in a heavy wall material. The effect becomes remarkable when 0.02% or more of Ni is included. However, since Ni is an expensive element, and the effect is saturated if it is excessively included, the upper limit thereof is set to 1.0%.
• Mo is an element that improves the strength of steel by transformation strengthening and solid-solution strengthening. The effect becomes remarkable when 0.02% or more of Mo is included. However, since an excessive addition thereof causes deterioration of the toughness, the upper limit is set to 1.2%.
• the content of Ti needs to be controlled in a narrow range of 0.004% to 0.010% suitable to precipitate as fine Ti carbonitride, without precipitation in the solidification, during the subsequent billet heating. If the content is less than 0.004%, a sufficient number of precipitated grains of the Ti carbonitride cannot be ensured, and if it exceeds 0.010%, the Ti carbonitride is coarsely precipitated in the cooling after solidification. Therefore, a proper content of Ti is 0.004 to 0.010%.
• N needs to be included in a content of 0.002% or more to ensure finely dispersed Ti carbonitride. Since a content exceeding 0.008% results in precipitation of coarse-grained Ti carbonitride in the solidification, the content needs to be controlled in a narrow range of 0.002 to 0.008%.
• V is an element whose content is to be determined depending on the balance between strength and toughness. If sufficient strength can be ensured by other alloy elements, increased toughness can be obtained without the addition thereof.
• the content is preferably set to 0.02% or more. Since the toughness is seriously deteriorated if the content exceeds 0.08%, the upper limit of the content is set to 0.08% if added.
• Nb is remarkably effective for suppressing the coarse-graining during heating for quenching in the case of an off-line heat treatment.
• Nb is desirably included in a content of 0.005% or more.
• the upper limit is set to 0.05%.
• the allowable upper limit should be set to 0.005%.
• Cu does not need to be added. However, it can be added, if improvement in the HIC resisting characteristic (hydrogen-induced cracking resisting characteristic) is intended, since it has an effect of improving the HIC resisting characteristic.
• the minimum content that improves the HIC characteristic is 0.02%. Since the effect is saturated even with a content that exceeds 1.0%, the content may be set to 0.02 to 1.0% if added.
• Ca, Mg, and REM 00002 to 0.005%, in total of at least one selected therefrom.
• These elements are added for the purpose of improving the toughness and the corrosion resistance by shape controlling of inclusions and for the purpose of suppressing nozzle clogging in casting to improve the casting characteristic.
• a content of 0.0002% or more, in total of at least one selected therefrom is needed. If the content exceeds 0.005%, in total of at least one selected therefrom, not only the effects are saturated, but also the inclusions are easily clustered, thereby somewhat deteriorating the toughness and the HIC resisting characteristic. Therefore, when one of these elements are added, each content is set to 0.0002 to 0.005%, and when two or more selected therefrom are added, the total content is set to 0.0002 to 0.005%.
• REM means 17 elements that include lanthanoide elements, Y and Sc.
• B does not need to be added. However, since addition thereof leads to improvement in hardenability even if it is a trace, the addition is effective when further high strength is needed. In order to obtain this effect, a content of 0.0003% or more is desirable. However, since an excessive addition thereof causes deterioration of the toughness, the content of B is set to 0.01% or less if added.
• the steel pipe for line pipe of the present invention contains the above chemical composition and the balance Fe and impurities.
• the upper limits of content of P and C among the impurities should be controlled as follows.
• P is an impurity element that deteriorates the toughness, and the content is preferably as little as possible. Since a content exceeding 0.05% causes remarkable deterioration of the toughness, the allowable upper limit is set to 0.05%.
• the content of P is preferably 0.02% or less and, further preferably, 0.01% or less.
• S is also an impurity element that deteriorates the toughness, and the content is preferably as little as possible. Since a content exceeding 0.005% causes remarkable deterioration of the toughness, the allowable upper limit is set to 0.005%.
• the content of S is preferably 0.003% or less and, further preferably, 0.001% or less.
• a continuous casting to a round billet shape is ideal for the manufacturing process, however, a process for continuously casting to a square mold or casting thereto as an ingot and then blooming to the round billet can be performed. In this case, it is important to further strictly control the cooling rate after solidification to suppress the precipitation of a coarse-grained TiN.
• An average cooling rate in a temperature range of 1400 to 1000°C, where the Ti carbonitride is apt to be generated after solidification, is required to be not less than 6°C /min in the case of casting to the round billet and to be not less than 8°C /min in the case of executing the blooming.
• the average cooling rate is more preferably set to be not less than 8°C /min in the case of casting to the round billet and to be not less than 10°C /min in the case of executing the blooming. In each case, no upper limit is provided since the larger average cooling rate is more desirable.
• the cooling rate of the bloom varies depending on portions of the bloom.
• the cooling rate is controlled at a place distant from a center by the distance of 1/2 of the radius.
• the cooling rate is controlled at a middle position between the center of gravity and the surface on a line passing the center of gravity of the square in parallel to the long sides thereof.
• the temperature can be measured by attaching a thermocouple, or instead, by numerical simulation corrected with the temperature history of the surface.
• the round billet is reheated to a hot workable temperature, and piercing, drawing and shaping rolling are performed thereto.
• the bloom or slab cast into a square cross section is reheated and then made into a round billet by forging or/and rolling, and piercing, drawing and shaping rolling are then performed thereto.
• a reheating temperature is required to be 1150°C or higher since hot deformation resistance is increased at a lower temperature than 1150°C, which increases flaws.
• the upper limit thereof is set to 1280°C, since a temperature exceeding 1280°C leads to an excessive increase in the heating fuel raw unit, and a reduction in yield by increased scale loss, which uneconomically shortened the life of the heating furnace, and the like. Since a lower heating temperature makes finer grains and increases the toughness, a preferable heating temperature is 1200°C or lower.
• the Ti carbonitride When the dissolved Ti is sufficiently present, the Ti carbonitride is precipitated during reheating, however, the precipitation occurs at a relatively low temperature, unlike the precipitation during the cooling after solidification. Therefore, the precipitated Ti carbonitride is much more fine-grained than the one during the cooling after solidification. An increased number of fine-grained Ti carbonitride are formed, and this suppresses the grain migration during heating the billet to prevent the coarse-graining. However, since rapid heating disables minute precipitation at a low temperature, the effect of preventing coarse-graining cannot be obtained.
• the average heating rate should be 15°C /min or less at a temperature between 550 and 900°C or that the isothermal treatment should be executed for 15 minutes or more at a temperature between 550 and 1000°C.
• the piercing, drawing and shaping rolling can be executed in the manufacturing conditions for a general seamless steel pipe.
• the quenching treatment is based on the in-line heat treatment of executing quenching, without once cooling to room temperature, in succession with hot rolling. However, if the reheating and the quenching are performed after cooling, finer grains are made, improving the toughness.
• the quenching is executed in succession with isothermal treatment in an isothermal furnace after the end of hot work, a steel pipe with minimized strength dispersion can be obtained.
• the necessary average cooling rate is 8°C /sec or more at a temperature between 800 and 500°C, more preferably, 10°C /sec or more, and most preferably not less than 15°C /sec.
• the cooling end temperature is also important for ensuring excellent toughness, in addition to the cooling rate. It is important to use a steel with an adjusted chemical composition and forcedly cool it to an end temperature of not higher than 100°C.
• the forced cooling is continuously performed, preferably to 80°C or lower, more preferably to 50°C or lower, and most preferably to 30°C or lower. According to this, the generation of transformation strengthened structure in which C is partially concentrated or retained austenite can be prevented, therefore, the toughness is significantly improved.
• tempering is performed at a temperature between 500 and 700°C.
• the tempering is performed in order to adjust the strength and improving the toughness.
• the holding time at the tempering temperature may be properly determined according the wall thickness or the like of the steel pipe, and is generally set to about 10 to 120 minutes.
• a JIS (Japan Industrial Standard) No. 12 tensile test piece for tensile test was prepared from each of the resulting steel pipes to measure tensile strength (TS) and yield strength (YS).
• the tensile test was carried out according to JIS Z 2241.
• As an impact test piece a V notch test piece of 10mm ⁇ 10mm, 2mm was prepared from the longitudinal direction of the heavy wall center according to No. 4 test piece of JIS Z 2202, and subjected to the test.
• Test No. 1 of Table 2 two examples of branch numbers of 1 and 2 are described.
• Steel A the invention, is used in 1-1 and 1-2, and the manufacturing condition of 1-1 is within the range restricted by the present invention, where increased toughness is obtained.
• the manufacturing condition of 1-2 is deviated from the manufacturing process defined by the present invention with an excessively high heating rate for pipe making, where increased toughness cannot be obtained.
• Each of Test Nos. 2 to 24 also has branch numbers 1 and 2, and the same steel grade is used in the same test number.
• the manufacturing condition of each branch number 1 is within the range restricted by the present invention, where increased toughness can be obtained.
• the manufacturing condition of each branch number 2 is deviated from the manufacturing process defined by the present invention, where increased toughness cannot be obtained.
• each branch number 1 corresponds to the manufacturing process within the range restricted by the present invention, where increased toughness is obtained.
• each branch number 2 is deviated from the manufacturing process defined by the present invention, where increased toughness is not obtained.
• Test Nos. 25 to 30 are examples of comparative steels that are deviated from the alloy composition range restricted by the present invention. Each of the steels is insufficient in toughness, and has insufficient performances as a line pipe requiring increased thickness and high toughness.
• a seamless steel pipe, even a heavy wall steel pipe, for line pipe excellent in toughness while having high strength such as yield strength of X70 class (yield strength of not less than 482 MPa), X80 class (yield strength of not less than 551 MPa), X90 class (yield strength of not less than 620 MPa), X100 class (yield strength of not less than 689 MPa), and X120 class (yield strength of not less than 827 MPa) can be manufactured.
• the seamless steel pipe of the present invention is a steel pipe that can be laid in a severer circumstance of the deep sea, particularly, for the use of a submarine flow line. The present invention is thus greatly contributable to stable supply of energies.

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• 2005
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