JP6369347B2 - High-strength thick-walled spiral steel pipe for conductor casing for deep well and manufacturing method thereof - Google Patents

Description

  The present invention relates to a spiral steel pipe suitable for a conductor casing used for retaining a well when excavating an oil well or a gas well, and in particular, a well used for developing a deep sea oil field or a deep sea gas field existing at a depth of 3,000 m or more. The present invention relates to a high-strength thick spiral steel pipe suitable for a conductor casing (hereinafter also referred to as a deep well) and a method for manufacturing the same.

  Conductor casings are used as earth retainings for wells that protect oil well pipes from external pressure during the early stages of oil and gas well drilling. Conventionally, a conductor casing has been manufactured by joining a UOE steel pipe and a connector (threaded forged member).

  Usually, the conductor casing is subjected to post-weld heat treatment (hereinafter also referred to as SR treatment) in a temperature range of 600 ° C. or higher in order to remove residual stress at the joint between the steel pipe and the forged member and to prevent hydrogen cracking. . Therefore, there has been a demand for a steel pipe excellent in SR treatment resistance that can suppress a decrease in strength due to SR treatment and can maintain a desired strength even after SR treatment.

  In response to such a request, Patent Document 1 describes, in weight percent, C: 0.03-0.09%, Si: 0.05-0.20%, Mn: 1.2-2.0%, Nb: 0.005-0.05%, Ti: 0.005-0.02. %, Al: less than 0.01%, or Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.5% or less, Mo: 0.30% or less, Ca: 0.0005 to 0.0025%, one or more And a base metal part containing 551 MPa or more of yield strength, and C: 0.03-0.07%, Si: 0.05-0.30%, Mn: 1.2-2.0%, Ni: 0.5-2.5%, Mo: 0.3-1.0%, O : 0.015-0.035% in addition, Cu: 0.50% or less, Cr: 1.0% or less, V: 0.10% or less, Ti: Less than 0.03%, one or more of Yield strength 551MPa or more A high strength and high toughness steel pipe having a weld metal and excellent weld zone toughness before and after SR treatment.

  Patent Document 2 describes a method for producing a high-strength steel plate of API X80 grade or higher. The technique described in Patent Document 2 is mass%, C: 0.03% or more and less than 0.07%, Si: 0.01 to 0.5%, Mn: 0.5 to 2%, Mo: 0.1 to 0.5%, Al: 0.08% or less. Containing Ti: 0.005 to 0.035%, Nb: 0.005 to 0.07%, V: 0.005 to 0.1%, or one or more of them, Ceq value is 0.32 or more, and further, Mo, Ti, in atomic% [C] / ([Mo] + [Ti] + [Nb] + [V]), which is the ratio of the total amount of Nb and V to 0.14% or more and the amount of C in atomic%, is 0.6 to 1.7. A steel is heated to a temperature of 1100 to 1250 ° C, hot-rolled at a rolling finish temperature of 750 ° C or higher, and then subjected to accelerated cooling to a temperature of 400 to 600 ° C at a cooling rate of 5 ° C / s or higher. This is a method for producing a high-strength steel sheet that is reheated to 550 to 700 ° C. at 0.5 ° C./market heating rate. As a result, fine carbides of 10 nm or less can be dispersed and precipitated, and even after stress relief annealing (SR) performed after welding, it has excellent strength and toughness without containing a large amount of alloy elements. An excellent high-strength steel sheet is obtained.

Patent Document 3 describes a method for producing a high-strength steel sheet having an API X100 grade or higher and excellent in SR resistance and deformation performance. The technology described in Patent Document 3 includes, in mass%, C: 0.03-0.10%, Si: 0.01-0.5%, Mn: 1.5-2.5%, Mo: 0.1-0.5%, Al: 0.08% or less, In addition, steel with a composition that contains one or more of Ti: 0.005-0.035%, Nb: 0.005-0.07%, V: 0.005-0.1%, with a PCM value of 0.19-0.25 and a P value of 0.20 or more. , Heated to a temperature of 1100-1300 ° C, hot-rolled at a rolling end temperature of 750 ° C or higher, accelerated to a temperature of 350-550 ° C at a cooling rate of 20 ° C / s or higher, and then immediately cooled to 0.5 ° C / This is a method for producing a high-strength steel sheet that is reheated to 550 to 700 ° C. at a temperature rising rate of s or more. As a result, the microstructure is a bainite structure containing island-like martensite with an area ratio of 2% to 10% at the center of the plate thickness, and the composite carbide mainly composed of Mo with a circle equivalent diameter of 10 nm or less per 1 μm ^{2 in the} entire cross section in the plate thickness direction. It is said that a high-strength steel sheet having a structure in which 30 or more are dispersed and the total precipitation amount is 0.03% by mass or more and excellent in SR resistance and deformation performance is obtained.

Japanese Patent Laid-Open No. 2001-158939 JP 2004-269964 A JP 2008-274405 A

  From the viewpoint of depletion of energy resources, the development of oil and gas fields with deeper water depth has been promoted recently, and the conductor casing (conductor casing for deep wells) used there has a diameter of about 20 inches. Is now required. The steel pipe manufactured by the technique described in Patent Document 1 is a UOE steel pipe, and in order to obtain a pipe having the above-described size by this method, there is a problem that productivity is remarkably lowered. In addition, the thick steel plates manufactured by the techniques described in Patent Documents 2 and 3 are subjected to accelerated cooling after hot rolling to precipitate fine composite carbides in order to improve the SR resistance. There is also a problem that the production process becomes complicated and the productivity is lowered.

  The present invention provides a high-strength, thick-walled spiral steel pipe that is suitable for deep-well conductor casings and has high strength, high toughness, and excellent resistance to SR treatment, and a method for producing the same. For the purpose.

Here, “high strength” refers to a case where the strength is API X80 grade or higher, that is, yield strength YS: 555 MPa or more and tensile strength TS: 625 MPa or more. The term “high toughness” as used herein refers to a case where Charpy impact test absorbed energy vE ₋₄₀ at a test temperature of −40 ° C. is 27 J or more. Further, “thick” here means a case where the thickness is 15 mm or more. Especially for deep-sea burial, a thickness of 20 mm or more is often used. “Excellent SR treatment resistance” means that the strength of the spiral steel pipe in the circumferential direction maintains the strength of API X80 grade even after SR treatment at 600 ° C. or higher. Shall.

  In order to achieve the above-mentioned object, the present inventors have proposed a steel pipe material which has various effects on the strength, toughness and SR treatment resistance of a spiral steel pipe, particularly on the SR treatment resistance, which is suitable for a conductor casing for deep wells. The composition of the hot-rolled steel sheet and the influence of hot-rolling conditions were studied earnestly.

  As a result, in order to maintain the strength of spiral steel pipes of API X80 or higher even after SR treatment at 600 ° C or higher, preferably lower than 750 ° C, a hot-rolled steel plate, which is a steel pipe material, has a fine particle size of less than 20 nm It has been found that the amount of Nb precipitates (deposited Nb) needs to be 75% or less of the Nb content in terms of Nb. It has been found that when the amount of fine Nb precipitates (precipitated Nb) exceeds 75% of the amount of contained Nb, the decrease in yield strength YS during SR treatment heated to a temperature of 600 ° C. or higher cannot be suppressed.

The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.01 to 0.12%, Si: 0.05 to 0.50%, Mn: 1.0 to 2.2%, P: 0.03% or less, S: 0.005% or less, Al: 0.001 to 0.10%, N: 0.006 % Or less, Nb: 0.010 to 0.100%, Ti: 0.001 to 0.050%, the composition composed of the balance Fe and inevitable impurities, and a bainitic ferrite phase with a volume ratio of 90% or more as the main phase, the main phase And a second phase having a volume fraction of 10% or less (including 0%), the bainitic ferrite phase having an average particle size of 10 μm or less, and a fine Nb precipitate having a particle size of less than 20 nm A high-strength, thick-walled spiral steel pipe for a conductor casing for deep wells, characterized in that, in terms of Nb, the structure is a structure in which the ratio (%) to the total Nb amount is 75% or less.
(2) In (1), in addition to the above composition, in addition to mass, V: 0.1% or less, Mo: 0.5% or less, Cr: 0.5% or less, Cu: 0.5% or less, Ni: 1.0% or less, B : A high-strength thick-walled spiral steel pipe for a conductor casing for deep wells, characterized in that the composition contains one or more selected from 0.0030% or less.
(3) In (1) or (2), in addition to the above-described composition, the composition further contains one or two selected from Ca: 0.0005 to 0.0050% and REM: 0.0005 to 0.0050% by mass%. A high-strength thick-walled spiral steel pipe for conductor casings for deep wells, characterized by having a composition.
(4) A method for manufacturing a spiral steel pipe in which the end faces in the width direction of the hot-rolled steel sheet are welded together while continuously winding the hot-rolled steel sheet in a spiral shape with a spiral pipe making machine. %, C: 0.01 to 0.12%, Si: 0.05 to 0.50%, Mn: 1.0 to 2.2%, P: 0.03% or less, S: 0.005% or less, Al: 0.001 to 0.10%, N: 0.006% or less, Nb : Steel containing 0.010 to 0.100%, Ti: 0.001 to 0.050%, and the balance Fe and unavoidable impurities are heated to a temperature of 1150 to 1250 ° C. Rolling end temperature: Hot rolling at 750 ° C. or higher is performed, and after the hot rolling is finished, the average cooling rate in the temperature range of 750 ° C. to 650 ° C. is 8 to 70 ° C./s at the plate thickness center temperature. Conductor cake for deep wells, characterized in that it is a thick hot-rolled steel sheet manufactured by performing a cooling process at a coiling temperature of 400 to 580 ° C. A manufacturing method for high-strength thick-walled spiral steel pipes
(5) In (4), in addition to the above composition, in addition to mass, V: 0.1% or less, Mo: 0.5% or less, Cr: 0.5% or less, Cu: 0.5% or less, Ni: 1.0% or less, B : A method for producing a high-strength thick spiral steel pipe for a conductor casing for deep wells, characterized in that the composition contains one or more selected from 0.0030% or less.
(6) In addition to (4) or (5), in addition to the above composition, the composition further contains one or two selected from Ca: 0.0005 to 0.0050% and REM: 0.0005 to 0.0050% by mass%. A method for producing a high-strength thick-walled spiral steel pipe for a conductor casing for deep wells, characterized by having a composition.
(7) A high-strength thick conductor casing for deep wells, in which screw members are attached to both ends of the high-strength thick spiral steel pipe for conductor casings for deep wells according to any one of (1) to (3).

  According to the present invention, it is possible to easily and inexpensively manufacture a high-strength thick-walled spiral steel pipe that is suitable for a deep well conductor casing and has high strength and high toughness of API X80 class or more and excellent SR treatment resistance. It has a remarkable industrial effect. In addition, according to the present invention, there is also an effect that a conductor casing having strength of API X80 or higher can be obtained even after SR treatment heated to 600 ° C. or higher.

It is explanatory drawing which shows the outline of the manufacturing process of a spiral steel pipe.

  The high-strength thick-walled spiral steel pipe of the present invention is a high-strength thick-walled spiral steel pipe suitable for a conductor casing for deep wells. The term “high-strength thick-walled spiral steel pipe” as used herein refers to a thick-walled spiral steel pipe having a base material strength of a high strength of API X80 grade or higher and a wall thickness of 15 mm or more. “API X80 grade or higher” refers to the case where the tensile strength in the circumferential direction is maintained at yield strength YS: 555 MPa or higher and tensile strength TS: 625 MPa or higher.

  The high-strength thick-walled spiral steel pipe of the present invention is in mass%, C: 0.01 to 0.12%, Si: 0.05 to 0.50%, Mn: 1.0 to 2.2%, P: 0.03% or less, S: 0.005% or less, Al: 0.001 0.10%, N: 0.006% or less, Nb: 0.010-0.100%, Ti: 0.001-0.050% included, or V: 0.1% or less, Mo: 0.5% or less, Cr: 0.5% or less, Cu: 0.5 %, Ni: 1.0% or less, B: One or more selected from 0.0030% or less, and / or Ca: 0.0005 to 0.0050%, REM: 0.0005 to 0.0050% It has 1 or 2 types, and has a composition which consists of remainder Fe and unavoidable impurities.

  First, the reasons for limiting the composition of the high strength thick spiral steel pipe of the present invention will be described. Hereinafter, unless otherwise specified, the mass% in the composition is simply expressed as%.

C: 0.01-0.12%
C is an important element that contributes to increasing the strength of the steel pipe, and needs to be contained in an amount of 0.01% or more in order to ensure the desired high strength. On the other hand, if the content exceeds 0.12%, weldability deteriorates. For this reason, C was limited to the range of 0.01 to 0.12%. In addition, Preferably it is 0.03-0.10%, More preferably, it is 0.03-0.08%.

Si: 0.05-0.50%
Si is an element that contributes to an increase in strength of the steel pipe by solid solution strengthening. To obtain such an effect and ensure a desired high strength, the Si content needs to be 0.05% or more. On the other hand, if it exceeds 0.50% and excessively contained, a hard phase such as MA (island martensite) is likely to be generated in the spiral welded portion, and the weld HAZ toughness is lowered. For these reasons, Si was limited to the range of 0.05 to 0.50%. In addition, Preferably it is 0.05 to 0.30%.

Mn: 1.0-2.2%
Mn is an element that contributes to increasing the strength of the steel pipe, and needs to be contained in an amount of 1.0% or more in order to ensure the desired high strength. On the other hand, if the content exceeds 2.2%, MA is likely to be produced and weldability is deteriorated similarly to Si. For this reason, Mn was limited to the range of 1.0 to 2.2%. In addition, Preferably it is 1.2 to 2.0%.

P: 0.03% or less
P is an element that is present as an impurity in steel and easily segregates at grain boundaries and has an adverse effect on steel pipe properties such as low-temperature toughness, and is preferably reduced as much as possible, but is acceptable up to 0.03%. Therefore, P is limited to 0.03% or less. In addition, Preferably it is 0.02% or less. In addition, excessive reduction leads to an increase in refining costs, so 0.001% or more is preferable.

S: 0.005% or less
S is present in the steel as coarse sulfide inclusions such as MnS and causes a decrease in ductility and toughness. Therefore, it is desirable to reduce it as much as possible, but 0.005% is acceptable. For these reasons, S is limited to 0.005% or less. In addition, Preferably it is 0.004% or less. In addition, excessive reduction leads to a rise in refining costs, so 0.0001% or more is preferable.

Al: 0.001 to 0.10%
Al is an element usefully acting as a deoxidizer for steel. In order to obtain such an effect, it is necessary to contain 0.001% or more. On the other hand, when it contains more than 0.10% in a large amount, an Al oxide is generated, and the cleanliness of the steel is lowered. For this reason, Al was limited to the range of 0.001 to 0.10%. In addition, Preferably it is 0.005-0.08%.

N: 0.006% or less
N exists as an unavoidable impurity in steel and forms a solid solution or forms a nitride, leading to a reduction in the toughness of the base material portion or spiral welded portion of the steel pipe. For this reason, it is desirable to reduce it as much as possible, but it is acceptable up to 0.006%. For these reasons, N is limited to 0.006% or less.

Nb: 0.010 to 0.100%
Nb is an important element in the present invention. It is an element that exists as Nb carbonitride in steel during the heating of steel materials (slabs), suppresses coarsening of austenite grains, and contributes to refinement of the structure. Further, Nb is finely precipitated during the SR treatment that is heated to 600 ° C. or higher, and suppresses the strength reduction of the steel pipe base material portion after the SR treatment. In order to obtain such an effect, a content of 0.010% or more is required. On the other hand, an excessive content exceeding 0.100% adversely affects the toughness of the steel pipe, and there is a concern that desired toughness cannot be secured for a conductor casing. For this reason, Nb was limited to the range of 0.010 to 0.100%. In addition, Preferably, it is 0.020 to 0.080%.

Ti: 0.001 to 0.050%
Ti combines with N to form Ti nitride, fixes N which adversely affects steel pipe toughness, and has an effect of improving steel pipe toughness. In order to obtain such an effect, a content of 0.001% or more is required. On the other hand, if the content exceeds 0.050%, the steel pipe toughness is significantly reduced. For this reason, Ti was limited to the range of 0.001 to 0.050%. In addition, Preferably it is 0.005-0.030%.

  The above components are basic components. In addition to the basic composition, V: 0.1% or less, Mo: 0.5% or less, Cr: 0.5% or less, Cu: 0.5% or less, Ni: 1.0% or less, B : One or two or more selected from 0.0030% or less, and / or Ca: 0.0005 to 0.0050%, REM: One or two selected from 0.0005 to 0.0050% Also good.

V: 0.1% or less, Mo: 0.5% or less, Cr: 0.5% or less, Cu: 0.5% or less, Ni: 1.0% or less, B: 0.0030% or less
V, Mo, Cr, Cu, Ni, and B are all elements that contribute to increasing the strength of the steel pipe through the improvement of hardenability, and can be selected and contained as necessary. The inclusion of these elements is effective in preventing the formation of pearlite and polygonal ferrite and ensuring the desired strength and toughness, particularly when the thickness is 15 mm or more. In order to obtain such effects, V: 0.005% or more, Mo: 0.05% or more, Cr: 0.05% or more, Cu: 0.05% or more, Ni: 0.05% or more, B: 0.0005% or more desirable. On the other hand, V: 0.1%, Mo: 0.5%, Cr: 0.5%, Cu: 0.5%, Ni: 1.0%, B: exceeding 0.0030%, respectively, lead to deterioration of weldability and toughness and material cost Invite the soaring. For this reason, if contained, it should be limited to V: 0.1% or less, Mo: 0.5% or less, Cr: 0.5% or less, Cu: 0.5% or less, Ni: 1.0% or less, B: 0.0030% or less, respectively. Is preferred. More preferably, V is 0.08% or less, Mo is 0.45% or less, Cr is 0.3% or less, Cu is 0.35% or less, Ni is 0.35% or less, and B is 0.0025% or less.

One or two selected from Ca: 0.0005 to 0.0050%, REM: 0.0005 to 0.0050%
Both Ca and REM are elements that contribute to the form control of inclusions in which sulfide-type inclusions such as expanded MnS are spherical sulfide-type inclusions, and can be selected and contained as necessary. In order to obtain such an effect, both Ca and REM must be contained by 0.0005% or more. On the other hand, when both Ca and REM contain more than 0.0050%, oxide inclusions increase and toughness decreases. For this reason, when it contains, it is preferable to limit to Ca: 0.0005-0.0050% and REM: 0.0005-0.0050%.

  The balance other than the components described above consists of Fe and inevitable impurities.

  The high-strength thick spiral steel pipe of the present invention has the above-described composition, and has a bainitic ferrite phase of 90% or more by volume ratio as a main phase, and the main phase and 10% or less (including 0%) by volume ratio. ), The average particle size of the bainitic ferrite phase is 10 μm or less, and the fine Nb precipitate having a particle size of less than 20 nm is a ratio (%) to the total Nb amount in terms of Nb. ) And a thick-walled spiral steel pipe having a structure in which 75% or less is dispersed.

Main phase: Bainitic ferrite phase with a volume ratio of 90% or more In order to combine the desired high strength and toughness for conductor casings for deep wells, the spiral steel pipe of the present invention has a bainitic with a volume ratio of 90% or more. It has a structure whose main phase is a ferrite phase. Note that the bainitic ferrite phase here also includes an acicular ferrite phase. If the bainitic ferrite phase is less than 90%, that is, the second phase other than the main phase is 10% or more, the desired toughness cannot be secured. Examples of the second phase other than the main phase include hard phases such as pearlite, degenerate pearlite, bainite phase, and martensite phase. For this reason, the volume fraction of the bainitic ferrite phase that is the main phase is limited to 90% or more. In addition, Preferably it is 95% or more.

Average particle size of bainitic ferrite phase: 10 μm or less In order to combine desired high strength and toughness for conductor casings for deep wells, the average particle size of the bainitic ferrite phase is the main phase in the present invention. The microstructure is 10 μm or less. When the average particle size exceeds 10 μm, the desired high toughness cannot be maintained. For this reason, the average particle size of the bainitic ferrite phase that is the main phase is limited to 10 μm or less. The “grain size” here refers to the size of a region where the orientation difference is within 15 ° by obtaining the orientation difference between adjacent crystal grains by the SEM / EBSD method.

Particle size: Fine Nb precipitates of less than 20 nm: Nb conversion, ratio (%) to the total Nb amount, 75% or less Nb that was not precipitated before SR treatment was applied in a temperature range of 600 ° C or higher During the SR treatment, it precipitates as fine Nb precipitates (mainly carbonitrides) of less than 20 nm and suppresses the decrease in yield strength due to recovery and recrystallization during SR treatment, that is, excellent SR resistance treatment Has an effect of imparting sex. For this reason, in the present invention, fine Nb precipitates having a particle size of less than 20 nm deposited on the steel pipe base material portion are made to have a ratio (%) to the total Nb amount of 75% or less in terms of Nb. . When the amount of fine Nb precipitates exceeds 75% in terms of Nb, Ostwald growth occurs during SR treatment, Nb carbonitrides disappear and coarsen, and the yield strength decreases due to recovery and recrystallization. It becomes difficult to suppress this, and the desired SR processing resistance cannot be ensured. For this reason, the amount of fine Nb precipitates having a particle diameter of less than 20 nm is limited to 75% or less in terms of Nb, as a ratio (%) to the total Nb amount. In order to increase the strength of the base material by precipitation strengthening, the amount of fine Nb precipitates having a particle size of less than 20 nm is 20% or more in terms of Nb conversion (%) with respect to the total Nb amount. It is preferable to do.

  The “particle size: the amount of fine Nb precipitates of less than 20 nm” referred to here is obtained by using an electrolytic solution (10% acetylacetone-1% tetramethylammonium chloride— The electrolytic residue obtained by electrolysis in a methanol solution) is filtered through a filter having a pore size of 0.02 μm, and the value obtained by analyzing the amount of Nb that has passed through the filter is used.

  The high-strength thick-walled spiral steel pipe of the present invention is made of a thick-walled hot-rolled steel sheet (hot-rolled steel strip) 1 having the above-described composition and structure as described above. Through the pipe process, the spiral steel pipe 6 having a predetermined size is obtained, and then cut into a predetermined length to be manufactured.

  The hot-rolled steel sheet (hot-rolled steel strip) used as the raw material is a thick-walled hot-rolled steel sheet (hot-rolled steel) having a thickness of 15 mm or more, preferably 26 mm or less, manufactured through the following steps on the steel material having the above composition. Steel strip).

  In addition, although it is not necessary to specifically limit the manufacturing method of the steel material in the present invention, the molten steel having the above composition is melted by a conventional melting method such as a converter, and a normal casting method such as a continuous casting method is used. It is preferable to use a slab or other slab (steel material). In addition, it replaces with a continuous casting method and there is no problem even if it uses as a steel raw material (steel piece) using an ingot-making-slabbing method.

  The steel material having the above composition is heated to a temperature in the temperature range of 1150 to 1250 ° C., and then comprises hot rolling and rough rolling and finish rolling, and finish rolling finish temperature: 750 ° C. or higher. Apply.

Heating temperature: 1150-1250 ° C
In order to improve the toughness of the hot-rolled steel sheet, it is preferable to set the heating temperature to such a low level that crystal grains can be expected to be fine.However, if the heating temperature is less than 1150 ° C, the heating temperature is too low and the undissolved carbide In some cases, solid solution does not progress and the desired high strength of API X80 grade or higher cannot be secured. On the other hand, when the heating temperature is higher than 1250 ° C., the austenite (γ) grains are coarsened, the toughness is reduced, the amount of scale is increased, and the surface properties may be deteriorated. In addition, heating at an excessively high temperature causes an increase in energy loss, which is economically disadvantageous. For this reason, the heating temperature of the steel material was set to a temperature range of 1150 to 1250 ° C. In addition, it is preferable from the viewpoint of uniformizing the heating temperature of the slab that the soaking at the heating temperature (heating time) is 60 minutes or more.

  The rough rolling is not particularly limited as long as it can be a sheet bar having a predetermined size and shape. In finish rolling, the finish rolling finish temperature is adjusted to 750 ° C or higher. This temperature is the surface temperature.

Finish rolling end temperature: 750 ° C. or more If the finish rolling end temperature is less than 750 ° C., ferrite transformation starts and the generated coarse ferrite is processed, resulting in a decrease in strength. For this reason, the finish rolling finish temperature was limited to 750 ° C. or higher. In the finish rolling, it is preferable to adjust the rolling reduction in the non-recrystallization temperature range of 930 ° C. or less at the plate thickness center temperature to 20% or more. If the rolling reduction in the non-recrystallization temperature region is less than 20%, the rolling reduction in the non-recrystallization temperature region is small, and the ferrite nucleation sites are small, and the ferrite grains may not be refined. Therefore, it is preferable to adjust the rolling reduction in the non-recrystallization temperature range to 20% or more. From the viewpoint of the load on the rolling mill, the cumulative rolling reduction in hot rolling is preferably 95% or less.

  In the present invention, immediately after the above-described hot rolling is finished, cooling is started preferably within 5 s, and the average cooling rate in the temperature range of 750 ° C. to 650 ° C. at the plate thickness center temperature is 8 to 70. Accelerated cooling is performed at a temperature of ℃ / s, and the coil is wound in a coil shape at a temperature in the range of 400 to 580 ° C. In addition, after winding up in a coil shape, it cools.

Average cooling rate in the temperature range of 750 ° C to 650 ° C for accelerated cooling: 8 to 70 ° C / s
When the average cooling rate in the temperature range of 750 ° C to 650 ° C is less than 8 ° C / s, the cooling rate is slow, and the resulting structure becomes a coarse polygonal ferrite phase with an average particle size exceeding 10 μm and pearlite, deep well The toughness and strength required for conductor casings cannot be secured. On the other hand, when the average cooling rate exceeds 70 ° C./s, a martensite phase is generated and the toughness may be lowered. Therefore, the average cooling rate in the temperature range of 750 ° C. to 650 ° C. is limited to the range of 8 to 70 ° C./s. In addition, Preferably it is 10-50 degreeC / s. All the above-mentioned temperatures are plate thickness center temperature.

  The cooling stop temperature for accelerated cooling is preferably a temperature in the temperature range of 400 to 620 ° C., which is the plate surface temperature. If the cooling stop temperature of accelerated cooling is out of the temperature range of 400 to 620 ° C, the desired coiling temperature: 400 to 580 ° C cannot be secured stably.

Winding temperature: 400 ~ 580 ℃
When the coiling temperature exceeds 580 ° C., precipitation of fine Nb precipitates having a particle size of less than 20 nm is promoted. Therefore, the amount of fine Nb precipitates having a particle size of less than 20 nm in the hot-rolled steel sheet that has undergone the winding process exceeds 75% in terms of Nb, and the ratio (%) to the total Nb amount exceeds 75%. On the other hand, when the coiling temperature is lower than 400 ° C., the precipitation of Nb carbonitride becomes insufficient, and it may be impossible to secure a high strength of X80 grade or higher. For this reason, winding temperature was limited to the range of 400-580 degreeC. As a result, a fine Nb precipitate with a particle size of less than 20 nm can be obtained in a Nb equivalent, with a ratio (%) to the total Nb content of 75% or less. It is possible to prevent a decrease in yield strength in the SR process. In addition, winding temperature becomes like this. Preferably it is 460-550 degreeC. All of the above temperatures are plate surface temperatures.

The hot-rolled steel sheet obtained under the above-mentioned production conditions has a bainitic ferrite phase with a volume ratio of 90% or more as the main phase and the balance bainitic ferrite phase with a volume ratio of 10% or less (including 0%). A fine Nb precipitate with an average particle size of the main phase of 10 μm or less and a particle size of less than 20 nm is 75% or less in terms of Nb conversion (%) with respect to the total Nb content. It has a distributed structure, and the strength in the direction of 30 ° relative to the rolling direction of the hot-rolled steel sheet corresponding to the circumferential direction of the spiral steel pipe is high strength of API X80 grade or higher, that is, yield strength YS: 555 MPa or higher. Has strength. In addition, the hot-rolled steel sheet obtained under the above-described manufacturing conditions is the absorbed energy of the Charpy impact test at a test temperature of 30 ° relative to the rolling direction of the hot-rolled steel plate corresponding to the circumferential direction of the spiral steel pipe: −40 ° C. It is a hot-rolled steel sheet having low temperature toughness with vE- ₄₀ of 27 J or more.

  A hot-rolled steel sheet (hot-rolled steel strip) 1 having the above characteristics wound in a coil shape is cold, for example, as shown in FIG. The end is subjected to groove processing (trimming) with an end face processing machine 2 such as an edge mirror, and then processed into a spiral shape with a spiral pipe making machine 3, and then the ends of adjacent hot rolled steel sheets 1 are spiral welded to each other. After being made into a spiral steel pipe 6 through a so-called spiral pipe making process that is commonly used for welding from the inner surface and the outer surface by the machine 4 (inner surface welding 41, outer surface welding 42), it is cut to a predetermined dimension. For welding in spiral pipe making processes (spiral welding), the submerged arc welding method is usually used, but the welding conditions are adjusted so that the spiral welded part has the same strength or toughness as the base metal part. Needless to say, since the spiral welded portion does not delete the surplus, it is not necessary to limit the strength and toughness of the welded portion. Further, in the present invention, any welding method that can be welded with the same efficiency as the submerged arc welding method can be used in place of the submerged arc welding method, and is not limited to the submerged arc welding method. Nor.

  Hereinafter, the present invention will be described more specifically based on examples.

  Molten steel shown in Table 1 was melted in a converter, and slabs (cast slab: thickness 250 mm) were formed by a continuous casting method to obtain steel materials.

  The obtained steel material was reheated under the conditions shown in Table 2 (heating temperature (° C.) × heating time), and then subjected to hot rolling consisting of rough rolling and finish rolling to obtain a hot rolled steel sheet. The hot rolling was performed under the conditions shown in Table 2 with the rolling reduction (%) in the non-recrystallization temperature range and the finish rolling finishing temperature (° C.). Immediately after finishing rolling, cooling is started, and accelerated cooling is performed at the sheet thickness center temperature under the conditions shown in Table 2 (average cooling rate in the temperature range of 750 to 650 ° C., cooling stop temperature). The coil was wound into a coil shape at the winding temperature shown in FIG.

  The obtained hot-rolled steel sheet was used as a steel pipe material, and the end faces in the width direction of the hot-rolled steel sheet were welded from the inner and outer surfaces by using a submerged arc welding method while being spirally wound to form a spiral steel pipe. In the submerged arc welding, the heat input was 35 kJ / cm, and a wire and a flux (manufactured by Lincoln) were combined so that the welded portion was adjusted to have a strength equal to or higher than that of the base material portion.

Test pieces were sampled from the obtained spiral steel pipe and subjected to structure observation, tensile test, impact test, and SR treatment test. The test method is as follows.
(1) Microstructure observation A specimen for microstructural observation is taken from the base material part of the obtained spiral steel pipe and polished so that the central position of the thickness of the cross section in the tube axis direction (L cross section) becomes the observation surface. Liquid: Nital), and the tissue was observed using a scanning electron microscope SEM (magnification: 1000 times) and imaged in at least two fields of view. Using the obtained tissue photograph, image analysis was performed to determine the tissue identification and the fraction of each phase.

The crystal grain size was obtained by calculating the orientation difference between adjacent crystal grains using the SEM / EBSD (Electron Back Scattering Diffraction) method, and determining the size of the region where the orientation difference was within 15 °. Was the grain size of the crystal grains.
In addition, a test piece for electrolytic extraction was taken from the base material of the obtained spiral steel pipe, and the current density was 20 mA / cm ² in an electrolytic solution (10% acetylacetone-1% tetramethylammonium chloride-methanol solution). Electrolyzed. The obtained electrolytic residue was dissolved in the liquid, collected with an aluminum filter (pore size: 0.02 μm), and the liquid that passed through the aluminum filter was analyzed for Nb content by ICP emission spectroscopy. The amount of precipitated Nb was calculated as a ratio (%) to the total amount of Nb.

(2) Tensile test In accordance with ASTM A 370, plate-like tensile test specimens were collected from the base material of the obtained spiral steel pipe so that the tensile direction would be the circumferential direction, and tensile properties (yield) Strength YS and tensile strength TS) were determined.
(3) Impact test V-notch specimens were taken from the base material of the spiral steel pipe obtained so that the longitudinal direction of the specimen was the circumferential direction (C direction) in accordance with ASTM A370. , Test temperature: Three Charpy impact tests were conducted at −40 ° C., the absorbed energy vE ₋₄₀ (J) was determined, and the average value of the three was taken as vE _{−40 of the} steel pipe.
(4) SR treatment test Test material was collected from the base material of the obtained spiral steel pipe, and the collected test material was placed in a heat treatment furnace maintained at a heating temperature assuming the SR treatment shown in Table 5, and tested. From the time when the temperature of the material reached (heating temperature −10 ° C.), after a predetermined holding time shown in Table 4, it was taken out from the heat treatment furnace and allowed to cool. In accordance with ASTM A 370, a tensile test (yield strength YS, tensile strength TS) is taken from the heat-treated test material in accordance with ASTM A 370 so that the tensile direction is the circumferential direction. Asked. Note that the yield strength difference ΔYS before and after the SR treatment was calculated. If the strength after SR processing is low, ΔYS is negative. For reference, a specimen for electrolytic extraction was collected from the test material after SR treatment, and the amount ratio of precipitated Nb was determined in the same manner as (1).

  In this example, the change in the tensile strength TS before and after the SR treatment was obtained in the tube axis direction (L direction). If the hot-rolled steel sheet of the present invention example was used, the precipitation form of Nb carbonitride was anisotropic. Therefore, it is clear that the strength of API X80 or higher can be obtained after SR treatment regardless of the specimen collection direction.

  The obtained results are shown in Tables 3 and 4.

  All examples of the present invention are API X80 grade suitable for conductor casings for deep wells, and have high strength with yield strength YS: 555 MPa or more, tensile strength TS: 625 MPa or more, and excellent low temperature toughness. In addition, spiral steel pipes with little reduction in strength after SR treatment heated to 600 ° C or higher, high strength of X80 grade or higher, little reduction in low-temperature toughness, and excellent SR treatment resistance It has become. On the other hand, in the comparative examples that are outside the scope of the present invention, the strength is insufficient, the low-temperature toughness is lowered, or the SR process resistance is lowered.

1 Hot-rolled steel sheet (hot-rolled steel strip)
2 End face processing machine (edge mirror)
3 Spiral pipe making machine (forming roll)
4 Spiral welder 5 Roll holding jig 6 Spiral steel pipe

Claims (7)

% By mass
C: 0.01 to 0.12%, Si: 0.05 to 0.50%,
Mn: 1.0-2.2%, P: 0.03% or less,
S: 0.005% or less, Al: 0.001 to 0.10%,
N: 0.006% or less, Nb: 0.010 to 0.100%,
Ti: 0.001 to 0.050%
Including the balance Fe and inevitable impurities,
The main phase is a bainitic ferrite phase having a volume ratio of 90% or more, and is composed of the main phase and a second phase having a volume ratio of 10% or less (including 0%). A structure in which fine Nb precipitates having a particle size of 10 μm or less and a particle size of less than 20 nm are dispersed by 75% or less in a ratio (%) to the total Nb amount in terms of Nb,
A high-strength, thick-walled spiral steel pipe for conductor casings for deep wells with excellent toughness and SR resistance, characterized by having   In addition to the above composition, it is further selected by mass% from V: 0.1% or less, Mo: 0.5% or less, Cr: 0.5% or less, Cu: 0.5% or less, Ni: 1.0% or less, B: 0.0030% or less. The high-strength thick-walled spiral steel pipe for a conductor casing for deep wells according to claim 1, wherein the composition contains one kind or two or more kinds.   2. In addition to the above composition, the composition further comprises one or two kinds selected from Ca: 0.0005 to 0.0050% and REM: 0.0005 to 0.0050% by mass%. The high-strength thick spiral steel pipe for conductor casings for deep wells according to claim 2. A method for producing a spiral steel pipe, in which hot-rolled steel sheets are continuously wound in a spiral shape by a spiral pipe making machine while welding the end faces in the width direction of the hot-rolled steel sheets,
The hot-rolled steel sheet in mass%,
C: 0.01 to 0.12%, Si: 0.05 to 0.50%,
Mn: 1.0-2.2%, P: 0.03% or less,
S: 0.005% or less, Al: 0.001 to 0.10%,
N: 0.006% or less, Nb: 0.010 to 0.100%,
Ti: 0.001 to 0.050%
In the steel material of the composition consisting of the balance Fe and unavoidable impurities,
Heating temperature: After heating soaking in a temperature range of 1150 to 1250 ° C, finish rolling finish temperature: Hot rolling to 750 ° C or higher is performed. Thick-walled hot-rolled steel manufactured by applying accelerated cooling so that the average cooling rate in the temperature range from ℃ to 650 ℃ is 8 to 70 ℃ / s, and performing a winding process at a coiling temperature of 400 to 580 ℃ A steel plate , the spiral steel pipe, the composition and a bainitic ferrite phase having a volume ratio of 90% or more as a main phase, the second phase having a volume ratio of 10% or less (including 0%). A fine Nb precipitate having an average particle size of the bainitic ferrite phase of 10 μm or less and a particle size of less than 20 nm is 75% in terms of Nb in terms of the ratio (%) to the total Nb content. % and a less dispersed becomes the tissue, characterized by excellent spiral pipe with high toughness, yet resistant SR processibility high strength High-strength thickness method of manufacturing a meat spiral steel pipe for a well for conductor casing.   In addition to the above composition, it is further selected by mass% from V: 0.1% or less, Mo: 0.5% or less, Cr: 0.5% or less, Cu: 0.5% or less, Ni: 1.0% or less, B: 0.0030% or less. The method for producing a high-strength thick-walled spiral steel pipe for a conductor casing for deep wells according to claim 4, wherein the composition contains one or more kinds.   5. In addition to the composition, the composition further comprises one or two selected from Ca: 0.0005 to 0.0050% and REM: 0.0005 to 0.0050% by mass%. 5. A method for producing a high-strength thick spiral steel pipe for a conductor casing for deep wells according to 5.   A high-strength thick conductor casing for deep wells, wherein screw members are attached to both ends of the high-strength thick-walled spiral steel pipe for conductor casings for deep wells according to any one of claims 1 to 3.

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