JP5660285B2 - Manufacturing method of welded steel pipe for oil well with excellent pipe expandability and low temperature toughness, and welded steel pipe - Google Patents

Description

  The present invention relates to oil well welded steel pipes embedded in crude oil or natural gas oil wells and gas wells (hereinafter simply referred to as oil wells) and used particularly for oil wells excellent in pipe expandability and low temperature toughness. The present invention relates to a welded steel pipe manufacturing method and a welded steel pipe.

  In order to lay an oil well pipe from the ground surface to the underground, first, excavation from the ground surface to a predetermined depth is carried out, and a steel pipe called a casing is buried therein to prevent the collapse of the wall. Thereafter, the basement is further excavated from the tip of the casing to form a deeper well, and a new casing is buried through the previously buried casing. By repeating this operation, an oil well pipe (tubing) that finally reaches the oil field or natural gas field is laid. When excavating deep wells, many types of casings with different diameters are required. The diameter of the oil well pipe (tubing) through which crude oil and natural gas are passed is fixed, so when drilling deep wells, it is necessary to increase the drilling area in the radial direction, but this drilling area is the minimum necessary There is a strong demand to limit.

  In response to such a request, for example, Patent Literature 1 and Patent Literature 2 describe a method of expanding a casing (steel pipe) in a well by means of expansion or the like. According to the techniques described in Patent Document 1 and Patent Document 2, the diameter of each casing having a multistage structure can be kept small by expanding the casing (steel pipe) in the radial direction in the well. It is said that it becomes possible to reduce the drilling cost of the oil well by suppressing the casing size of the oil well.

  In Patent Document 3, C: 0.05 to 0.30%, Si: 0.2 to 2%, Mn: 0.7 to 4.0%, P: 0.03% or less, S: 0% by mass 0.15% or less, N: 0.007% or less, O: 0.005% or less, or one or two of Al, Cr, Ni, Cu, Nb, V, Ti, Mo, B, and Ca Contains a specified amount of seeds or more, consists of the remainder Fe and inevitable impurities, has a residual γ phase with a phase fraction of 5% by volume or more, and has a high tensile strength (TS) of 600 MPa or more, and a tube expansion rate of 30% An oil well seamless steel pipe having excellent pipe expandability and excellent pipe expandability with respect to super pipe expansion processing is described.

  In Patent Document 4, the mass% is C: 0.01 to 0.3%, Si: 0.01 to 0.7%, Mn: 0.5 to 2.0%, Nb: 0.005 to 0.00. 1%, Ti: 0.005-0.05%, Al: 0.002-0.1%, Ca: 0.0005-0.008%, P: 0.10% or less, S: 0 0.005% or less, O: 0.0040% or less, Si / Mn: 0.005 to 1.5 is satisfied, and the balance is composed of Fe and inevitable impurities. An excellent ERW steel pipe for expanded oil wells is described.

JP 7-507610 A International Publication No. W098 / 00626 Japanese Patent No. 4367259 JP 2008-202128 A

  However, since the technique described in Patent Document 3 is a seamless steel pipe manufactured by piercing and rolling a billet, it is difficult for the wall thickness accuracy to satisfy a desired value (± 10%). Moreover, in the technique described in Patent Document 4, in order to obtain the described tube expansion performance, the entire ERW steel pipe requires a heating-cooling heat treatment from the Ac1 transformation point to the Ac3 transformation point, so that the heat treatment is performed. Expenses increased, and the expansion rate that allowed tube expansion was low, at most 28% or less. In addition, the upper limit of the expansion rate range that can be expanded is hereinafter referred to as a limit expansion rate. That is, the conventional technique related to the welded steel pipe for oil wells has a problem that the wall thickness accuracy is insufficient or an excellent pipe expandability cannot be obtained.

The present invention solves the above-mentioned problems, and in the range where the tensile strength is 490 MPa or more and the yield ratio (= yield strength / tensile strength) is 0.74 to 0.92, the thickness accuracy is a desired value (± 10% It is an object of the present invention to provide means capable of achieving excellent pipe expansion and low temperature toughness while satisfying
Here, “excellent tube expandability” means that the limit tube expansion ratio obtained in the next cone tube expansion test is 46% or more.
(Cone expansion test :) Cut out a steel pipe twice as long as the outer diameter from the welded steel pipe and make both pipe ends parallel ▽ ▽ ▽ Finish (Ra: 1.6a equivalent finish, Japan Society of Mechanical Engineers, "Mechanical Engineering Handbook" new edition No. 6), Maruzen issued on July 30, 1993, see page B1-22), and after applying press oil to prevent warping, the cone with a top angle of 60 ° is pushed into the steel pipe with a press to widen the end of the pipe. When the pipe end is cracked, the indentation is stopped, the pipe end outer diameter Db on the side of the expansion after unloading is measured, and the tube expansion ratio (= (Db−Do) / Do × 100) is calculated from the outer diameter Do of the original pipe. (%)). This is repeated three times, and the average of the obtained three tube expansion rate data is defined as the limit tube expansion rate.

The term "excellent low-temperature toughness", and that _V E _-20 obtained by the following low temperature impact test is 100 J / cm ² or more.
(Low-temperature impact test :) In a 2 mm V notch Charpy test (conforming to JIS Z 2242), a test piece (conforming to JIS Z 2202) in which the notch depth direction is orthogonal to the pipe circumferential direction and wall thickness direction is used. The impact value (absorbed energy per unit area 1 cm ^{2 of the} cross section in the notch depth direction at the notch position of the test piece) is obtained by testing at −20 ° C., and this is _designated as _V E ₋₂₀ .

  In order to achieve the above-mentioned object, the present inventors systematically studied the effects of the composition of the hot-rolled material, the manufacturing conditions, and the microstructure on the limit expansion rate and low-temperature toughness of the ERW steel pipe as it is. . In addition, we have systematically researched appropriate online heat treatment conditions and microstructure conditions to obtain the desired tube expansion and low temperature toughness for welds that harden by welding. As a result, a material slab having a specific composition is hot-rolled at a specific temperature and processing conditions. It was found that a welded steel pipe for oil wells having excellent pipe expandability and low temperature toughness can be obtained by adding.

The present invention has been completed based on the above findings, and the gist of the present invention is as follows.
(1) By mass%, C: 0.05 to 0.25%, Si: 0.001 to 2.00%, Mn: 0.50 to 2.50%, Al: 0.010 to 0.100% P: 0.019% or less, Sn: 0.10% or less, S: 0.005% or less, N: 0.0049% or less, O: 0.0030% or less, and 30 * C + 100 * (P + Sn) + 1000 * (S + N + O) is less than 16.0%, and a steel slab having a composition composed of the balance Fe and inevitable impurities is heated to 1150 to 1300 ° C. and kept soaked for 30 minutes or more. Hot rolling is performed at a reduction rate of 93.0 to 98.0%, finish rolling is finished at 750 ° C. or higher, cooling time between 750 to 600 ° C. is set to 4 s or longer, and the coiling temperature is higher than 300 ° C. and lower than 600 ° C. Winding and forming a hot-rolled steel strip, slitting the hot-rolled steel strip, forming a continuous roll The arc-shaped cross-section is formed according to the shape, both ends of the arc-shaped cross-section are welded, and only the welded portion formed by welding is heated to 750 to 1000 ° C. and then cooled to 500 ° C. or less at a cooling rate of 5 ° C./s or more. Characteristic tube expandability and low temperature where the critical tube expansion ratio obtained in the conical tube expansion test is 46% or more and the impact value _V E- ₂₀ obtained in the 2 mmV notch Charpy test at −20 ° C. is 100 J / cm ² or more. A method for producing a welded steel pipe for oil wells having a tensile strength of 490 MPa or more and a yield ratio of 0.74 to 0.92 excellent in toughness.
(2) By mass%, it contains one or two selected from Cu: 0.001 to 1.00%, Ni: 0.001 to 1.00% (1) A method for producing a welded steel pipe for oil wells having a tensile strength of 490 MPa or more and a yield ratio of 0.74 to 0.92 excellent in pipe expandability and low temperature toughness.
(3) By mass%, Cr: 0.001-1.50%, Mo: 0.001-0.49%, Nb: 0.0001-0.14%, V: 0.0001-0.14% , Ti: 0.0001 to 0.14%, W: 0.0001 to 0.14%, B: 0.0001 to 0.0003%, Ca: 0.0001 to 0.0003%, REM: 0.0001 The tensile strength is 490 MPa or more excellent in tube expandability and low-temperature toughness according to (1) or (2), and the yield ratio is 0. A method of manufacturing a welded steel pipe for oil wells of .74 to 0.92.
(4) The expandability according to any one of (1) to (3), wherein only the welded portion formed by welding is replaced with the entire welded steel tube formed by welding. And a method for producing a welded steel pipe for oil wells having a tensile strength of 490 MPa or more and a yield ratio of 0.74 to 0.92 excellent in low temperature toughness.
(5) In any one of (1) to (4), after the last cooling, the entire welded steel pipe is tempered in the range of 200 to 650 ° C. and has excellent pipe expandability and low temperature toughness. A method for producing a welded steel pipe for an oil well having a tensile strength of 490 MPa or more and a yield ratio of 0.74 to 0.92.
(6) By mass%, C: 0.05 to 0.25%, Si: 0.001 to 2.00%, Mn: 0.50 to 2.50%, Al: 0.010 to 0.100% P: 0.019% or less, Sn: 0.10% or less, S: 0.005 % or less, N: 0.0049% or less, O: 0.0030% or less, and 30 * C + 100 * (P + Sn) + 1000 * (S + N + O) is less than 16.0%, or further contains the following group A and / or group B, and has a composition comprising the balance Fe and inevitable impurities, and the base material part and 46% of the limit expansion rate required in the conical tube expansion test is characterized by including 0.1 to 12 area% of the second phase in which C is concentrated to 0.4% by mass or more in the microstructure of the weld. above, and the and the impact value _V obtained by 2mmV notch Charpy test at -20 ° C. _-20 is 100 J / cm ² or more at a pipe expansion and low-temperature toughness excellent tensile strength 490MPa or more, for oil wells welded steel pipe yield ratio from 0.74 to 0.92.
Group A:% by mass, Cu: 0.001 to 1.00%, Ni: 0.001 to 1.00% selected from Group 1 or Group B:% by mass, Cr: 0 0.001 to 1.50%, Mo: 0.001 to 0.49%, Nb: 0.0001 to 0.14%, V: 0.0001 to 0.14%, Ti: 0.0001 to 0.14 %, W: 0.0001-0.14%, B: 0.0001-0.0030%, Ca: 0.0001-0.0030%, REM: 0.0001-0.10% One or more

According to the present invention, the wall thickness accuracy is within ± 10%, the excellent tube expandability is 46% or more in the conical tube expansion test, and the _V E- ₂₀ is 100 J / cm ² or more. An oil well welded steel pipe having a tensile strength of 490 MPa or more and a yield ratio of 0.74 to 0.92 having low temperature toughness is obtained.

Microscopic occupied in the tissue, the graph showing the second phase area ratio of C is concentrated in the least 0.4 wt%, the limit pipe expansion _ratio, V _E -20, the relationship YR

First, the reason why the composition is limited as described above in the present invention will be described. Hereinafter, the component content is abbreviated as% in units of mass%. Also, TS means tensile strength, YS means yield strength, EL means elongation, and YR means yield ratio (= YS / TS).
C: 0.05-0.25%
C ensures the desired original pipe strength (TS, YS), and the desired second phase (second phase in which C is concentrated to 0.4% or more) in the microstructure of the base metal part and the welded part. Is an element that forms a desired area ratio (0.1 to 12 area%) and obtains good tube expansion. If it is less than 0.05%, this desired tube strength and the second phase cannot be obtained. On the other hand, if it exceeds 0.25%, the low temperature toughness of the steel pipe is lowered, so this is the upper limit. In addition, Preferably it is 0.06 to 0.13%.

Si: 0.001 to 2.00%
Si is an element that promotes ferrite transformation in the hot rolling process, and is an element for ensuring necessary tube expansion. If it is less than 0.001%, the tube expandability is insufficient. On the other hand, when it exceeds 2.00%, oxides remain and the low temperature toughness of the welded portion deteriorates. Therefore, Si was limited to 0.001 to 2.00%. In addition, Preferably it is 0.81-1.45%.

Mn: 0.50 to 2.50%
Mn ensures the desired original pipe strength (tensile strength TS, yield strength YS), and the desired second phase (C is concentrated to 0.4% or more) in the microstructure of the base metal part and the welded part. The second phase is an element that forms a desired area ratio (0.1 to 1.2 area%) and obtains good tube expandability. If it is less than 0.50%, the desired original tube strength and the second phase cannot be obtained. On the other hand, if it exceeds 2.50%, the low temperature toughness of the steel pipe is lowered, so this is the upper limit. In addition, Preferably it is 0.84-1.25%.

Al: 0.010 to 0.100%
Al is a deoxidizing element at the time of steelmaking, and is an element that suppresses the growth of austenite grains in the hot rolling process, makes the crystal grains fine, and obtains good tube expandability. If the content is less than 0.010%, these effects cannot be obtained. On the other hand, if the content exceeds 0.10%, the effect is saturated. To do. In addition, Preferably it is 0.030-0.080%.

P: 0.019% or less P lowers the low temperature toughness and lowers the pipe expandability through solidification co-segregation with Mn. If the content exceeds 0.019%, this adverse effect becomes significant, so 0.019% is made the upper limit. In addition, Preferably it is 0.009% or less.
Sn: 0.10% or less Sn is present in the steel as a low melting point solid solution element, and deteriorates the pipe expandability. If it exceeds 0.10%, the adverse effect becomes remarkable, so 0.10% is made the upper limit. In addition, Preferably it is 0.05% or less.

S: 0.005% or less S is present as an inclusion in steel as MnS or the like, and lowers the pipe expandability. If this amount exceeds 0.005%, this adverse effect becomes significant, so 0.005% is made the upper limit. In addition, Preferably it is 0.003% or less.
N: 0.0049% or less When N remains as a solid solution N, tube expandability is reduced. If this amount exceeds 0.0049%, this adverse effect becomes remarkable, so 0.0049% is made the upper limit. In addition, Preferably it is 0.0040% or less.

O: 0.0030% or less O exists as oxide inclusions, and lowers the pipe expandability and low temperature toughness. If this content exceeds 0.0030%, this adverse effect becomes significant, so 0.0030% is made the upper limit. In addition, Preferably it is 0.0020% or less.
30 * C + 100 * (P + Sn) + 1000 * (S + N + O): less than 16.0% C is an increase in the area ratio of carbide and hard second phase, P and S are segregated into the metal flow part, and Sn is a low melting solid solution As an element, N is age-hardened, and O is an oxide inclusion in the welded portion, both of which synergistically lower tube expandability and low temperature toughness. In order to ensure the desired tube expandability and low temperature toughness, it is not sufficient to individually define the component content ranges of these elements, and 30 * C + 100 * (P + Sn) + 1000 * (S + N + O) taking into account the influence of each element ) The value should be kept below a certain threshold. If the value of 30 * C + 100 * (P + Sn) + 1000 * (S + N + O) is 16.0% or more, the decrease in tube expandability and low temperature toughness increases, so the content is made less than 16.0%. In addition, Preferably it is less than 13.0%.

The above components form a basic composition. In the present invention, in addition to the basic composition, the group A and / or the group B can be further contained.
Group A:% by mass, Cu: 0.001 to 1.00%, Ni: 0.001 to 1.00% selected from 1 type or 2 types Cu: 0.001 to 1.00%
Cu forms a corrosion protection film, and by strengthening this, the penetration of hydrogen into the steel is suppressed, and the resistance to sulfide stress corrosion cracking is improved. Captures as sulfide and has the effect of suppressing selective corrosion of welds. These effects are manifested with a content of 0.001% or more. However, when the content exceeds 1.00%, the pipe expandability is lowered, and when the raw material is hot-rolled, Cu becomes a liquid phase, causing hot cracking and surface defects. Therefore, the upper limit is 1.00%. In addition, Preferably it is 0.001 to 0.049%.

Ni: 0.001 to 1.00%
Ni, like Cu, has the effect of suppressing the penetration of hydrogen into the steel and improving the resistance to sulfide stress corrosion cracking. Furthermore, there is an effect of improving the low temperature toughness of the base metal part and the welded part. These effects are manifested with a content of 0.001% or more. However, the content exceeding 1.00% has an upper limit of 1.00% in order to reduce tube expandability. In addition, Preferably it is 0.001-0.049%.
Group B:% by mass, Cr: 0.001 to 1.50%, Mo: 0.001 to 0.49%, Nb: 0.0001 to 0.14%, V: 0.0001 to 0.14% , Ti: 0.0001 to 0.14%, W: 0.0001 to 0.14%, B: 0.0001 to 0.0003%, Ca: 0.0001 to 0.0003%, REM: 0.0001 One or more selected from ˜0.10% Cr: 0.001 to 1.50%
Cr is an element that improves corrosion resistance such as carbon dioxide corrosion resistance and carbon dioxide stress corrosion cracking resistance. Furthermore, in the cooling process from the austenite phase during the heat treatment of the welded part after hot rolling or welding, the two-phase separation of the structure is promoted, and the desired second phase (C is contained in the microstructure of the base material part and the welded part). It is an element effective for forming a desired second phase ratio (0.1 to 12 area%) of the second phase concentrated to 0.4% or more. These effects are manifested when the content is 0.001% or more. However, when the content exceeds 1.50%, an oxide remains in the welded portion, so that the pipe expandability and the low temperature toughness of the welded portion are lowered. The upper limit is 50%. In addition, Preferably it is 0.01 to 0.49%.

Mo: 0.001 to 0.49%
Mo is an element that improves the resistance to sulfide stress corrosion cracking in an environment where hydrogen sulfide is present. Further, in the cooling process from the austenite phase during hot rolling or heat treatment of the welded portion after welding, two phases of the structure are formed. Separation is promoted, and a desired second phase (second phase in which C is concentrated to 0.4% or more) is added to a desired area ratio (0.1 to 12 area) in the microstructure of the base material part and the welded part. %) Is an effective element. These effects are manifested with a content of 0.001% or more, but if it exceeds 0.49%, the upper limit is 0.49% in order to reduce tube expandability. In addition, Preferably it is 0.01 to 0.09%.

Nb: 0.0001 to 0.14%
Nb contributes to improvement of low temperature toughness through refinement of crystal grains. If it is less than 0.0001%, this effect cannot be obtained. On the other hand, if it exceeds 0.14%, the pipe expandability is significantly reduced, so 0.14% is made the upper limit. In addition, Preferably it is 0.022 to 0.080%.
V: 0.0001 to 0.14%
V improves the hardenability, and converts the desired second phase (second phase in which C is concentrated to 0.4% or more) into a desired area ratio (0.1%) in the microstructure of the base metal part and the welded part. (About 12% by area) is an effective element. If it is less than 0.0001%, this effect cannot be obtained. On the other hand, if it exceeds 0.14%, the pipe expandability is significantly reduced, so 0.14% is made the upper limit. In addition, Preferably it is 0.011-0.080%.

Ti: 0.0001 to 0.14%
Ti is an effective element for improving the tube expandability by fixing solid solution N which adversely affects the tube expandability as TiN. If it is less than 0.0001%, this effect cannot be obtained. On the other hand, if it exceeds 0.14%, the pipe expandability decreases due to the precipitated carbide, so 0.14% is made the upper limit. In addition, Preferably it is 0.0001 to 0.0049%.

W: 0.0001 to 0.14%
W precipitates as a carbide and is an element effective for securing strength. This effect is manifested at a content of 0.0001% or more. However, if the content exceeds 0.14%, the tube expandability decreases, so 0.14% is made the upper limit. In addition, Preferably it is 0.0001 to 0.06%.
B: 0.0001 to 0.0030%
B is an element effective for securing the strength through improvement of hardenability. This effect is manifested at a content of 0.0001% or more, but a content exceeding 0.0030% lowers the tube expandability, so 0.0030% is made the upper limit. In addition, Preferably it is 0.0001 to 0.0005%.

Ca: 0.0001 to 0.0003%
Ca has a so-called form control effect in which expanded MnS is granular Ca (Al) S (O), and particularly suppresses cracking at the rise of the metal flow near the weld during pipe expansion, thereby improving pipe expandability. It is an effective element. This effect is manifested at a content of 0.0001% or more. However, when the content exceeds 0.0030%, the tube expandability is lowered due to an increase in nonmetallic inclusions, so 0.0030% is made the upper limit. In addition, Preferably it is 0.0001 to 0.0019%.

REM: 0.0001 to 0.10%
REM, like Ca, has a so-called form control effect that makes expanded MnS granular, and is an effective element for improving pipe expandability by suppressing cracking at the metal flow rising part in the vicinity of the weld, especially during pipe expansion forming. . This effect is manifested at a content of 0.0001% or more, but if the content exceeds 0.10%, the tube expandability is reduced, so the upper limit is made 0.10%. In addition, Preferably it is 0.01 to 0.05%.

The balance other than the above components is Fe and inevitable impurities.
Second phase in which C is concentrated to 0.4% or more in the microstructure of the base metal part and the welded part: 0.1 to 12 area%
The welded steel pipe of the present invention has a suitable YR (0.74 to 0.92) with the hot-rolled steel strip basically formed into a tubular shape except for the welded part, and includes a welded part. In order to have a desired tube expansion property, 0.1 to 12 area% of the second phase in which C is concentrated to 0.4% or more is included in the microstructure of the base material portion and the welded portion. The second phase has the effect of causing movable dislocations in the surrounding soft phase during transformation in the process of cooling from the austenite phase during hot rolling or heat treatment of the welded portion after welding, and preventing YR from becoming too high as it is formed. . Furthermore, after the cooling process, for example, transformation during tube expansion molding has an effect of relieving stress during tube expansion molding and greatly improving tube expansion. These effects are manifested when the second phase fraction in the microscopic tissue is 0.1 area% or more. On the other hand, when the area exceeds 12 area%, the tube expandability is decreased, so this is the upper limit. In addition, Preferably it is 2.0-10.0 area%.

Here, in determining the area ratio of the second phase in which C was concentrated to 0.4% or more, the following second phase fraction measurement method was used.
(Second phase fraction measurement method) EMPA surface analysis was performed with a polished circumferential cross-sectional area of 400 μm × 400 μm as the measurement area and an electron beam size of 2 μm × 2 μm, and the C concentration (content in steel) was 0.4%. The C-enriched region as described above is specified to determine the total area, and this is expressed as a percentage of the measured area as the area ratio of the second phase.

The remainder of the microstructure is the first phase that is any one or more of polygonal ferrite, acicular ferrite, and Widmanstatten ferrite (collectively referred to as ferrite): 75 area% or more, And the 3rd phase which is a mixed structure of carbide and iron such as carbide, fine pearlite, bainite, etc .: 0.0 to 15.1 area%.
Figure 1 is occupied tissue microscopic is a graph showing the second phase area ratio of C is concentrated in the 0.4% or more, the limit pipe expansion ratio, _{V E -20,} the relationship _YR, drawing as shown in, the area ratio of the second phase is in the range of 0.1 to 12 area%, the limit pipe expansion ratio: 46% or _{more, V E -20: 100J / cm} 2 or more, YR: 0.74～ 0.92 is achieved.

Next, preferable hot rolling conditions for the raw steel slab for obtaining the desired microstructure described above will be described.
Slab heating temperature and slab soaking time: 1150-1300 ° C, 30 minutes or more Slab heating conditions in the hot rolling process affect the low temperature toughness of the steel pipe through the austenite grain size, and the pipe expandability through the solid solution dispersion state of the contained elements. Effect. Slab heating temperature is 1150 to 1300 ° C., and when the slab soaking time is more than 30 _{minutes, V E -20: 100J / cm} 2 or more and critical expansion ratio: 46% or more is obtained. Austenite grain size when the slab heating temperature exceeds 1300 ° C. is extremely _coarse, V _{E -20} is below 100 J / cm ^2. On the other hand, if the slab heating temperature is below 1150 ° C. or the slab soaking time is less than 30 minutes, the solid solution dispersion state of the contained elements becomes non-uniform, deformation at the time of pipe expansion molding is concentrated locally, and the limit pipe expansion rate is Below 46%. For this reason, it is preferable that slab heating temperature shall be 1150-1300 degreeC and slab soaking time shall be 30 minutes or more.

Total rolling reduction: 93.0-98.0%
The total reduction ratio (= (H−h) / H × 100 (%)) from the slab thickness H to the hot-rolled finished sheet thickness h affects the low temperature toughness and YR of the steel pipe through the austenite grain size before transformation. . When the total rolling reduction is 93.0 to 98.0%, _V E- ₂₀ is 100 J / cm ² or more, and YR is 0.74 to 0.92. When total reduction ratio is lower than 93.0% austenite grain size before transformation _increases, V _{E -20} is below 100 J / cm ^2. Moreover, YS falls and YR after pipe making is less than 0.74. On the other hand, when the total rolling reduction exceeds 98.0%, YR exceeds 0.92. From the above, the total rolling reduction was set to 93.0 to 98.0%. In addition, Preferably it is 95.0 to 97.6%.

Finish rolling end temperature: 750 ° C. or higher The finish rolling end temperature in the hot rolling process affects the low temperature toughness, YR, and tube expandability of the steel pipe. In the finish rolling end temperature is 750 ° C. or _higher, V _{E -20} is 100 J / cm ² or more, YR is 0.92, the critical expansion ratio is more than 46% is obtained. Below the finish rolling end temperature of 750 ° C., since the rolling strain is _V E _-20 for remaining below the 100 J / cm ^2, YR exceeds 0.92, also coarse grains in the surface layer portion is formed, the limit The expansion rate is below 46%. Here, the upper limit of the finish rolling finish temperature is not particularly defined, but is preferably 950 ° C. or less from the viewpoint of maintaining good surface properties. Furthermore, from the viewpoint of securing surface properties, it is preferable to perform descaling at a water pressure of 150 kgf / cm ² or more before finish rolling.

Cooling time between 750 and 600 ° C .: 4 s or more After the hot rolling is completed, the runout cooling conditions affect the presence / amount of ferrite precipitation and the state of two-phase separation. Through microstructural formation, the low temperature toughness of the steel pipe , YR, affects tube expandability. By securing a cooling time of 4 s or more between 750 and 600 ° C. corresponding to the ferrite nose temperature range of steel, ferrite is generated by 75 area% or more, two-phase separation proceeds, and C is concentrated to 0.4% or more. phased second phase is formed microstructure occupying 0.1 to 12 _area%, V _{E -20} is 100 J / cm ² or more, YR is 0.92, the critical expansion ratio is not less than 46% can get. If the cooling time between seven hundred and fifty to six hundred ° C. is below 4s, the area ratio of the second phase exceeds 12 _area%, V _{E -20} is below 100 J / cm ^2, YR exceeds 0.92, the limit pipe expansion The rate is below 46%. In addition, Preferably, the cooling time between 750-600 degreeC is 6 s or more.

Winding temperature: More than 300 ° C. and less than 600 ° C. Of the microstructure that has been separated into two phases of austenite / ferrite by cooling in a hot-rolled runout, the austenite phase is coiled and then at a temperature of more than 300 ° C. and less than 600 ° C. Partial bainite transformation, C concentration in the austenite phase proceeds, and finally a second phase in which C is concentrated to 0.4% or more is formed. When the coiling temperature is 300 ° C. or lower, the concentration of C in the austenite phase is insufficient, the area ratio of the second phase is less than 0.1 area%, the YR exceeds 0.92, and the limit tube expansion The rate is below 46%. On the other hand, when the coiling temperature is 600 ° C. or higher, C precipitates as pearlite, C concentration does not progress, and YR exceeds 0.92. From the above, the coiling temperature was set to be higher than 300 ° C and lower than 600 ° C. In addition, Preferably it is 350-550 degreeC.

Next, a preferred welded steel pipe manufacturing method will be described.
The hot-rolled steel sheet manufactured to satisfy the above hot-rolling requirements is either blackened (= with oxidized scale attached), or after removing the oxidized scale by pickling or shot blasting, slitting, and arc-shaped cross section by continuous roll forming Then, both ends of the arc-shaped cross section are heated and welded (abutting / pressure welding) by high frequency induction heating or the like. In addition, although welding may be performed in the atmosphere, the oxygen concentration is reduced (for example, 100 ppm or less) by spraying inert gas or shielding for the purpose of reducing inclusions such as oxides in the welded portion. You may weld. In addition, welding can be performed by resistance welding, laser welding, arc welding, plasma welding, or the like, or a combination thereof, instead of high-frequency induction heating.

Weld line heat treatment condition: weld only the 750 to 1000 ° C. welds rapidly cooled welding at a cooling rate of 500 ℃ 5 ℃ / s or higher to less after heating, the high _hardness, V _{E -20} is less than 100J / cm ^2. After heating this to 750-1000 degreeC on-line, by cooling with the cooling rate of 5 degrees C / s or more to 500 degrees C or less, the 2nd phase which C concentrated to 0.4% or more is 0.1 12 microstructure containing only area% is _formed, V _{E -20} can secure 100 J / cm ^2. When heating temperature exceeds 1000 degreeC, a particle size will become large and low temperature toughness will fall. On the other hand, when the heating temperature is lower than 750 ° C., the hardness is not sufficiently lowered and the low temperature toughness is lowered. After online heating, the welded portion cooled at a cooling rate of less than 5 ° C./s to 500 ° C. or less has low hardness and cannot secure TS490 MPa or more. Further cooling the way pearlite _generates, V _{E -20} is below 100 J / cm ^2. The cooling rate to 500 ° C. or less after the online heating is preferably 5 to 300 ° C./s, more preferably 5 because the area ratio of the second phase is less than 12% by area if this is too fast. ~ 200 ° C / s.

  In addition, it replaces with online heat processing of a welding part, and after heating the whole welded steel pipe to 750-1000 degreeC, it is 5 degreeC / s or more to 500 degreeC or less, Preferably it is 5-300 degreeC / s, More preferably, it is 5-200 degreeC / s. Cooling at a cooling rate of s (referred to as all-tube heat treatment) can also provide a welded steel pipe for oil wells having a tensile strength of 490 MPa or more and a yield ratio of 0.74 to 0.92 excellent in pipe expandability and low temperature toughness.

  Further, for uniform stabilization of the characteristics, the entire welded steel pipe can be tempered in the range of 200 to 650 ° C. after the online heat treatment of the weld or after the whole pipe heat treatment.

Example 1
A steel slab having the composition shown in Table 1 was heated to about 1240 ° C., soaked for about 60 minutes, extracted, subjected to hot rolling with a total rolling reduction of about 96.8%, finished at about 840 ° C., and finished with rolling. Air cooling was performed in a temperature range between about 700 ° C. with a rolled runout, a cooling time of 750 to 600 ° C. was secured for 8 s, and the steel was wound at about 450 ° C. to obtain a hot-rolled steel strip (plate thickness of about 8 mm). Next, slitting these hot-rolled steel strips to a predetermined width dimension, continuous roll forming to make an open pipe, both ends of the arc-shaped cross section of the open pipe are electro-welded by high frequency resistance welding to make a pipe, Continuously, on-line seam heat treatment is performed under the conditions of a heating temperature of about 900 ° C. and a cooling rate of 30 ° C./s (by mist cooling) to 200 ° C. to obtain a welded steel pipe having an outer diameter of φ203.2 mm and a wall thickness of about 8 mm. It was.

Test pieces were collected from these welded steel pipes and subjected to a structure observation test, a pipe expansion test, a tensile test, and a low temperature toughness test.
(1) Microstructure observation test Microstructure observation specimens with the circumferential cross-section of the welded steel pipe serving as the observation surface were sampled from the base metal part and the welded part, polished, and subjected to nital corrosion, and then subjected to a microstructure with a scanning electron microscope (3000 times). Were observed and imaged, and the area ratio of ferrite was measured using an image analyzer. Furthermore, the area ratio of the second phase in which C was concentrated to 0.4% or more was determined by the above-described second phase fraction measurement method.
(2) Tensile test An arc-shaped test piece is cut out from the base metal part in accordance with ASTM A-370 so that the L direction (pipe length direction) of the welded steel pipe is the tensile direction. A tensile test was performed to determine tensile properties (TS, YS, EL, YR).
(3) Tube expansion test The conical tube expansion test was carried out to determine the limit tube expansion rate.
(4) In the low-temperature impact test of the low-temperature toughness test described above, the test used was a 1/2 size taken from the base metal and the weld of the welded steel pipe as the test piece was determined _V E _-20.

The obtained results are shown in Table 2. From the table, all the inventive examples satisfy the steel composition requirements and manufacturing method requirements stipulated by the present invention, and the obtained welded steel pipes satisfy the second phase area ratio requirements stipulated by the present invention. It has excellent tensile strength of 490 MPa or more and a yield ratio of 0.74 to 0.92.
On the other hand, none of the comparative examples satisfy at least one of the steel composition requirement and the manufacturing method requirement defined in the present invention, and the obtained welded steel pipe does not satisfy the second phase area ratio requirement defined in the present invention. The tensile strength is not less than 490 MPa and the yield ratio is 0.74 to 0.92, which is excellent in tube expandability and low temperature toughness.
(Example 2)
The steel slab having the composition of steels B and C in Table 1 was hot-rolled under the conditions shown in Table 3 (hot rolling production conditions for the steel strip) to form a hot-rolled steel strip. Next, slitting these hot-rolled steel strips to a predetermined width dimension, continuous roll forming to make an open pipe, both ends of the arc-shaped cross section of the open pipe are electro-welded by high frequency resistance welding to make a pipe, Continuously, on-line seam heat treatment was continuously performed under the conditions shown in Table 3 (on-line heat treatment conditions for the welded portion. Cooling here: air cooling or mist cooling) to obtain a welded steel pipe having an outer diameter and a wall thickness shown in Table 3. . In addition, as shown in Table 3, some of the pipes after ERW welding were subjected to whole pipe heat treatment instead of online seam heat treatment, or subjected to tempering heat treatment after online seam heat treatment or whole pipe heat treatment. .

Test specimens were collected from these welded steel pipes in the same manner as in Example 1, and subjected to a structure observation test, a pipe expansion test, a tensile test, and a low temperature toughness test in the same manner as in Example 1.
Table 4 shows the obtained results. From the table, all the inventive examples satisfy the steel composition requirements and manufacturing method requirements stipulated by the present invention, and the obtained welded steel pipes satisfy the second phase area ratio requirements stipulated by the present invention. It has excellent tensile strength of 490 MPa or more and a yield ratio of 0.74 to 0.92.

On the other hand, none of the comparative examples satisfy at least one of the steel composition requirement and the manufacturing method requirement defined in the present invention, and the obtained welded steel pipe does not satisfy the second phase area ratio requirement defined in the present invention. The tensile strength is not less than 490 MPa and the yield ratio is 0.74 to 0.92, which is excellent in tube expandability and low temperature toughness.
Example 3
In Example 3, welding from an open pipe to a pipe in No. 34 of Example 2 was carried out in two ways: laser welding and TIG welding instead of electric resistance welding by high-frequency resistance welding. Steel pipes were manufactured, and these were designated as Invention Examples Nos. 59 and 60 shown in Table 3, respectively, and these were tested in the same manner as in Example 2. The results are shown in Table 4. From the same table, also in Example 3, the obtained welded steel pipe, like Example 2, satisfies the second phase area ratio requirement of the present invention, and has a tensile strength of 490 MPa or more, excellent in pipe expandability and low temperature toughness, yield. It has the characteristic of ratio 0.74-0.92.

Claims (6)

In mass%, C: 0.05 to 0.25%, Si: 0.001 to 2.00%, Mn: 0.50 to 2.50%, Al: 0.010 to 0.100% , P: 0.019% or less, Sn: 0.10% or less, S: 0.005% or less, N: 0.0049% or less, O: 0.0030% or less, and 30 * C + 100 * (P + Sn ) + 1000 * (S + N + O) is less than 16.0%, and the steel slab having the composition of the balance Fe and inevitable impurities is heated to 1150 to 1300 ° C. and kept soaked for 30 minutes or more, and the total reduction ratio is 93 Hot rolling is performed at 0.0 to 98.0%, finish rolling is finished at 750 ° C. or higher, cooling time between 750 to 600 ° C. is set to 4 s or longer, and winding is performed at a winding temperature of more than 300 ° C. and lower than 600 ° C. A hot-rolled steel strip, slitting the hot-rolled steel strip, and continuous roll forming Therefore, the arc-shaped cross section is formed, both ends of the arc-shaped cross section are welded, and only the welded portion formed by the welding is heated to 750 to 1000 ° C. and then cooled to 500 ° C. or less at a cooling rate of 5 ° C./s or more. Expandability and low temperature toughness with a critical tube expansion ratio determined by a conical tube expansion test of 46% or more and an impact value _V E _-20 calculated by a 2 mmV notch Charpy test at −20 ° C. of 100 J / cm ² or more . A method for producing a welded steel pipe for an oil well having a tensile strength of 490 MPa or more and a yield ratio of 0.74 to 0.92.   It contains 1 type or 2 types chosen from Cu: 0.001-1.00% and Ni: 0.001-1.00% by mass%. A method for producing a welded steel pipe for oil wells having a tensile strength of 490 MPa or more and a yield ratio of 0.74 to 0.92 excellent in pipe expandability and low temperature toughness.   In mass%, Cr: 0.001-1.50%, Mo: 0.001-0.49%, Nb: 0.0001-0.14%, V: 0.0001-0.14%, Ti: 0.0001-0.14%, W: 0.0001-0.14%, B: 0.0001-0.0030%, Ca: 0.0001-0.0030%, REM: 0.0001-0. The tensile strength of 490 MPa or more excellent in tube-expandability and low-temperature toughness according to claim 1 or 2, characterized by containing at least one selected from 10%. 92. Manufacturing method of welded steel pipe for oil well of 92.   The excellent weldability and low temperature toughness according to any one of claims 1 to 3, wherein only the welded portion formed by welding is replaced with the entire welded steel pipe formed by welding. A method for producing a welded steel pipe for an oil well having a tensile strength of 490 MPa or more and a yield ratio of 0.74 to 0.92.   In any one of Claims 1-4, after the last cooling, the tensile strength 490 Mpa or more excellent in the pipe expandability and low-temperature toughness characterized by carrying out the tempering heat processing in the range of 200-650 degreeC of the whole welded steel pipe, A method for producing a welded steel pipe for an oil well having a yield ratio of 0.74 to 0.92. In mass%, C: 0.05 to 0.25%, Si: 0.001 to 2.00%, Mn: 0.50 to 2.50%, Al: 0.010 to 0.100% , P: 0.019% or less, Sn: 0.10% or less, S: 0.005 % or less, N: 0.0049% or less, O: 0.0030% or less, and 30 * C + 100 * (P + Sn ) + 1000 * (S + N + O) is less than 16.0%, or further contains the following group A and / or group B, and has a composition comprising the balance Fe and inevitable impurities, and the base material part and the welded part The critical tube expansion ratio obtained by the cone tube expansion test is 46% or more, characterized by containing 0.1 to 12 area% of the second phase in which C is concentrated to 0.4% by mass or more in the microscopic tissue . and impact value _V E sought 2mmV notch Charpy test at -20 ° C. ₂₀ 100 J / cm ² or more at a pipe expansion and low-temperature toughness excellent tensile strength 490MPa or more, for oil wells welded steel pipe yield ratio from 0.74 to 0.92.
Group A:% by mass, Cu: 0.001 to 1.00%, Ni: 0.001 to 1.00% selected from Group 1 or Group B:% by mass, Cr: 0 0.001 to 1.50%, Mo: 0.001 to 0.49%, Nb: 0.0001 to 0.14%, V: 0.0001 to 0.14%, Ti: 0.0001 to 0.14 %, W: 0.0001-0.14%, B: 0.0001-0.0030%, Ca: 0.0001-0.0030%, REM: 0.0001-0.10% One or more

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