Patent Publication Number: US-11041223-B2

Title: High strength thick-walled electric-resistance-welded steel pipe for deep-well conductor casing, method for manufacturing the same, and high strength thick-walled conductor casing for deep wells

Description:
TECHNICAL FIELD 
     The present disclosure relates to an electric-resistance-welded steel pipe suitable for a conductor casing used as a retaining wall in oil or gas well drilling and more particularly to a high-strength thick-walled electric-resistance-welded steel pipe suitable for a conductor casing for wells in deep-water oil or gas field development at a depth of 3,000 m or more (hereinafter also referred to as deep wells) and to a method for manufacturing the high-strength thick-walled electric-resistance-welded steel pipe. 
     BACKGROUND ART 
     Conductor casings are used as retaining walls in wells at an early stage of oil or gas well drilling and protect oil well pipes from external pressure. Conductor casings are conventionally manufactured by joining a UOE steel pipe to a connector (threaded forged member). 
     When placed into wells, conductor casings are repeatedly subjected to bending deformation. When placed into deep wells, conductor casings are also subjected to stress loading due to their own weights. Thus, deep-well conductor casings are particularly required 
     (1) not to be broken by repeated bending deformation during placement, and 
     (2) to have strength to bear their own weights. 
     In order to prevent conductor casings from being broken by bending deformation, it is particularly necessary to reduce stress concentration, for example, caused by linear misalignment in a joint. Linear misalignment may be reduced by improving the circularity of a steel pipe to be used. 
     In general, conductor casings are sometimes subjected to post-weld heat treatment at a temperature of 600° C. or more in order to relieve the residual stress of a joint between a steel pipe and a forged member or to prevent hydrogen cracking. Thus, there is a demand for a steel pipe that suffers a smaller decrease in strength due to post-weld heat treatment, can maintain desired strength even after post-weld heat treatment, and has high resistance to post-weld heat treatment. 
     For example, Patent Literature 1 describes a high-strength riser steel pipe having good high-temperature stress relief (SR) characteristics to meet the demand. In the technique described in Patent Literature 1, a riser steel pipe having good high-temperature SR characteristics has a steel composition containing C: 0.02% to 0.18%, Si: 0.05% to 0.50%, Mn: 1.00% to 2.00%, Cr: 0.30% to 1.00%, Ti: 0.005% to 0.030%, Nb: 0.060% or less, and Al: 0.10% or less by weight. In the technique described in Patent Literature 1, in addition to these components, a riser steel pipe may further contain one or two or more of Cu: 0.50% or less, Ni: 0.50% or less, Mo: 0.50% or less, and V: 0.10% or less, and further Ca: 0.0005% to 0.0050% and/or B: 0.0020% or less by weight. In the technique described in Patent Literature 1, inclusion of a predetermined amount of Cr retards softening of the base material ferrite and increases resistance to softening, which can suppress the decrease in toughness and strength caused by post-weld heat treatment (SR treatment) and improve high-temperature SR characteristics. 
     Patent Literature 2 describes, as a technique for improving the circularity of a steel pipe, a method for expanding a UOE steel pipe by using a pipe expander in which each dice of all mounted on the pipe expander has a grooved outer surface, and changing the dies mounted on the pipe expander for each steel pipe to be expanded, each of the dies facing a piece of excess weld metal inside a steel pipe weld portion. Patent Literature 2 states that the technique can uniformize the wear loss of the dies mounted on the pipe expander and improve the circularity of a steel pipe. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Patent No. 3558198 
     PTL 2: Japanese Unexamined Patent Application Publication No. 2006-289439 
     SUMMARY 
     Technical Problem 
     In order to prevent a conductor casing from being broken by repeated bending deformation during placement, it is important to reduce stress concentration. Thus, a steel pipe to which a connector is to be joined should have a certain degree of circularity. However, Patent Literature 1 does not describe a measure to improve circularity, for example, by reducing linear misalignment. The technique described in Patent Literature 1 includes no measure to improve circularity, and a steel pipe will have insufficient circularity at its end portion, particularly when used as a deep-well conductor casing. When a steel pipe manufactured by the technique described in Patent Literature 1 is used as a deep-well conductor casing, an additional step is necessary to improve the circularity of an end portion of the steel pipe by cutting or straightening. Thus, there is a problem in the technique described in Literature 1 that the productivity of manufacturing conductor casings is decreased. 
     The technique described in Patent Literature 2 also cannot ensure sufficient circularity particularly for deep-well conductor casings, which is a problem. 
     The present disclosure solves such problems of the related art and aims to provide a high-strength high-toughness thick-walled electric-resistance-welded steel pipe having high resistance to post-weld heat treatment suitable for a deep-well conductor casing and a method for manufacturing the steel pipe. The present disclosure also aims to provide a conductor casing including the electric-resistance-welded steel pipe as a component thereof. 
     The term “high-strength thick-walled electric-resistance-welded steel pipe”, as used herein, refers to a thick-walled electric-resistance-welded steel pipe having a thickness of 15 mm or more in which both a base material portion and an electric-resistance-welded portion have high strength of at least the API X80 grade. The base material portion has a yield strength YS of 555 MPa or more and a tensile strength TS of 625 MPa or more, and the electric-resistance-welded portion has a tensile strength TS of 625 MPa or more. The term “high toughness”, as used herein, means that the absorbed energy vE −40  in a Charpy impact test at a test temperature of −40° C. is 27 J or more. For placement in deep water, the thickness is preferably 20 mm or more. 
     The phrase “high resistance to post-weld heat treatment”, as used herein, means that the base material maintains the strength of at least the API X80 grade even after post-weld heat treatment performed at 600° C. or more. 
     Solution to Problem 
     In order to achieve the objects, the present inventors have intensively studied the characteristics of a steel pipe suitable for a deep-well conductor casing. As a result, the present inventors have found that in order to prevent a conductor casing from being broken by bending deformation during placement, it is necessary to use a steel pipe having a circularity of 0.6% or less. The present inventors have found that if a steel pipe to be used has a circularity of 0.6% or less, linear misalignment between a threaded member and a joint (an end portion of the steel pipe) can be reduced to prevent the steel pipe from being broken by repeated bending deformation, without a particular additional process, such as cutting or straightening. 
     The present inventors have considered that such a steel pipe is preferably an electric-resistance-welded steel pipe rather than a UOE steel pipe. Electric-resistance-welded steel pipes have a cylindrical shape formed by continuous forming with a plurality of rolls and have higher circularity than UOE steel pipes formed by press forming and pipe expanding. The present inventors have found from their study that forming by reducing rolling with sizer rolls finally performed after electric resistance welding is effective in order to manufacture an electric-resistance-welded steel pipe having circularity suitable for a deep-well conductor casing. The present inventors have also found that in roll forming in pipe manufacturing, in addition to roll forming with a cage roll group and a fin pass forming roll group, pressing two or more portions of an inner wall of a hot-rolled steel plate being subjected to the forming process with an inner roll disposed downstream of the cage roll group is effective in further improving circularity, and further this can reduce the load of fin pass forming. 
     The present inventors have also intensively studied the effects of the composition of a hot-rolled steel plate used as a steel pipe material and the hot-rolling conditions on the steel pipe strength after post-weld heat treatment. As a result, the present inventors have found that in order that an electric-resistance-welded steel pipe maintains the strength of at least the API X80 grade even after post-weld heat treatment performed at 600° C. or more and preferably at less than 750° C., a hot-rolled steel plate used as a steel pipe material should contain fine Nb precipitates (precipitated Nb) having a particle size less than 20 nm in an amount of 75% or less of the Nb content on a Nb equivalent basis. The present inventors have found that when the amount of fine Nb precipitates (precipitated Nb) is more than 75% of the Nb content, the decrease in yield strength YS due to post-weld heat treatment performed at a temperature of 600° C. or more cannot be suppressed. 
     Embodiments of the present disclosure are described below. 
     [1] A high-strength thick-walled electric-resistance-welded steel pipe for a deep-well conductor casing, 
     the steel pipe having a composition containing, on a mass percent basis: 
     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%, and 
     Ti: 0.001% to 0.050%, 
     the remainder being Fe and incidental impurities, 
     the steel pipe having a structure composed of 90% or more by volume of a bainitic ferrite phase as a main phase and 10% or less (including 0%) by volume of a second phase, the bainitic ferrite phase having an average grain size of 10 μm or less, the structure containing fine Nb precipitates having a particle size of less than 20 nm dispersed in a base material portion, a ratio (%) of the fine Nb precipitates to the total amount of Nb being 75% or less on a Nb equivalent basis, and 
     the circularity of an end portion of the steel pipe defined by the following formula (1) being 0.6% or less.
 
Circularity (%)={(maximum outer diameter mmφ of steel pipe)−(minimum outer diameter mmφ of steel pipe)}/(nominal outer diameter mmφ)×100  (1)
 
[2] The high-strength thick-walled electric-resistance-welded steel pipe for a deep-well conductor casing according to [1], wherein the composition further contains one or two or more selected 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, and B: 0.0030% or less on a mass percent basis.
 
[3] The high-strength thick-walled electric-resistance-welded steel pipe for a deep-well conductor casing according to [1] or [2], wherein the composition further contains one or two selected from Ca: 0.0050% or less and REM: 0.0050% or less on a mass percent basis.
 
[4] A method for manufacturing a high-strength thick-walled electric-resistance-welded steel pipe for a deep-well conductor casing, including: continuously rolling a hot-rolled steel plate with a roll forming machine to form an open pipe having a generally circular cross section; butting edges of the open pipe; electric-resistance-welding a portion where the edges being butted while pressing the butted edges to control by squeeze rolls to form an electric-resistance-welded steel pipe; subjecting the electric-resistance-welded portion of the electric-resistance-welded steel pipe to in-line heat treatment; and reducing the diameter of the electric-resistance-welded steel pipe by rolling,
 
     wherein the hot-rolled steel plate is manufactured by 
     heating to soak a steel at a heating temperature in the range of 1150° C. to 1250° C. for 60 minutes or more, 
     the steel having a composition containing, on a mass percent basis, 
     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%, and 
     Ti: 0.001% to 0.050%, 
     the remainder being Fe and incidental impurities, and 
     hot-rolling the steel with a finishing delivery temperature of 750° C. or more, 
     after completion of the hot rolling, subjecting the hot-rolled steel plate to accerelated cooling such that the average cooling rate in a temperature range of 750° C. to 650° C. at the center of plate thickness ranges from 8° C./s to 70° C./s, and 
     coiling the hot-rolled steel plate at a coiling temperature in the range of 580° C. to 400° C. 
     [5] The method for manufacturing a high-strength thick-walled electric-resistance-welded steel pipe for a deep-well conductor casing according to [4], wherein the roll forming machine includes a cage roll group composed of a plurality of rolls and a fin pass forming roll group composed of a plurality of rolls.
 
[6] The method for manufacturing a high-strength thick-walled electric-resistance-welded steel pipe for a deep-well conductor casing according to [5], wherein two or more portions of an inner wall of the hot-rolled steel plate are pressed with an inner roll disposed downstream of the cage roll group during a forming process.
 
[7] The method for manufacturing a high-strength thick-walled electric-resistance-welded steel pipe for a deep-well conductor casing according to any one of [4] to [6], wherein the in-line heat treatment of the electric-resistance-welded portion includes heating the electric-resistance-welded portion to a temperature in the range of 830° C. to 1150° C. and cooling the electric-resistance-welded portion to a cooling stop temperature of 550° C. or less at the center of plate thickness such that the average cooling rate in a temperature range of 800° C. to 550° C. at the center of plate thickness ranges from 10° C./s to 70° C./s.
 
[8] The method for manufacturing a high-strength thick-walled electric-resistance-welded steel pipe for a deep-well conductor casing according to any one of [4] to [7], wherein a reduction ratio in the reducing rolling is in the range of 0.2% to 3.3%.
 
[9] The method for manufacturing a high-strength thick-walled electric-resistance-welded steel pipe for a deep-well conductor casing according to any one of [4] to [8], wherein the composition further contains one or two or more selected 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, and B: 0.0030% or less on a mass percent basis.
 
[10] The method for manufacturing a high-strength thick-walled electric-resistance-welded steel pipe for a deep-well conductor casing according to any one of [4] to [9], wherein the composition further contains one or two selected from Ca: 0.0050% or less and REM: 0.0050% or less on a mass percent basis.
 
[11] A high-strength thick-walled conductor casing for deep wells, comprising a screw member disposed on each end of the high-strength thick-walled electric-resistance-welded steel pipe for a deep-well conductor casing according to any one of [1] to [3].
 
     Advantageous Effects 
     The present disclosure has industrially great advantageous effects in that a high-strength thick-walled electric-resistance-welded steel pipe having high resistance to post-weld heat treatment can be easily manufactured at low cost without particular additional treatment. The steel pipe is suitable for a deep-well conductor casing, has high strength and toughness, and can maintain desired high strength even after post-weld heat treatment performed at 600° C. or more. The present disclosure can also reduce the occurrence of breakage of a conductor casing during placement and contributes to reduced placement costs. The present disclosure can also provide a conductor casing that can maintain the strength of at least the API X80 grade even after post-weld heat treatment performed at 600° C. or more. An electric-resistance-welded steel pipe according to the present disclosure also has an effect that it is useful as a line pipe manufactured by joining pipes together by girth welding. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic explanatory view of an example of a production line suitable for the manufacture of an electric-resistance-welded steel pipe according to the present disclosure. 
         FIG. 2  is a schematic explanatory view of an example of the shape of inner rolls. 
         FIG. 3  is a schematic explanatory view of an example of in-line heat treatment facilities. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A high-strength thick-walled electric-resistance-welded steel pipe according to the present disclosure is a high-strength thick-walled electric-resistance-welded steel pipe for a deep-well conductor casing. The term “high-strength thick-walled electric-resistance-welded steel pipe”, as used herein, refers to a thick-walled electric-resistance-welded steel pipe having a thickness of 15 mm or more in which both a base material portion and an electric-resistance-welded portion have high strength of at least the API X80 grade. The base material portion has a yield strength YS of 555 MPa or more and a tensile strength TS of 625 MPa or more, and the electric-resistance-welded portion has a tensile strength TS of 625 MPa or more. 
     A high-strength thick-walled electric-resistance-welded steel pipe according to the present disclosure has a composition containing, on a mass percent basis, 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%, and Ti: 0.001% to 0.050%, optionally further containing one or two or more selected 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, and B: 0.0030% or less, and/or one or two selected from Ca: 0.0050% or less and REM: 0.0050% or less, the remainder being Fe and incidental impurities. 
     First, the reasons for limiting the composition of a high-strength thick-walled electric-resistance-welded steel pipe according to the present disclosure will be described below. Unless otherwise specified, the mass percentage of a component is simply expressed in %. 
     C: 0.01% to 0.12% 
     C is an important element that contributes to increased strength of a steel pipe. A C content of 0.01% or more is required to achieve desired high strength. However, a high C content of more than 0.12% results in poor weldability. Furthermore, during cooling after hot rolling or during in-line heat treatment of an electric-resistance-welded portion, a high C content of more than 0.12% makes the formation of martensite easier in the case of rapid cooling or the formation of a large amount of pearlite easier in the case of slow cooling, thereby possibly reducing toughness or strength. Thus, the C content is limited to the range of 0.01% to 0.12%. The lower limit of the C content is preferably 0.03% or more. The upper limit is preferably 0.10% or less, more preferably 0.08% or less. 
     Si: 0.05% to 0.50% 
     Si is an element that contributes to increased strength of a steel pipe by solid-solution strengthening. A Si content of 0.05% or more is required to achieve desired high strength by such an effect. Si has a higher affinity for O (oxygen) than Fe and, together with Mn oxide, forms a viscous eutectic oxide during electric resistance welding. Thus, an excessive Si content of more than 0.50% results in poor quality of an electric-resistance-welded portion. Thus, the Si content is limited to the range of 0.05% to 0.50%. The Si content preferably ranges from 0.05% to 0.30%. 
     Mn: 1.0% to 2.2% 
     Mn is an element that contributes to increased strength of a steel pipe. A Mn content of 1.0% or more is required to achieve desired high strength. However, in the same manner as in C, a high Mn content of more than 2.2% makes the formation of martensite easier and results in poor weldability. Thus, the Mn content is limited to the range of 1.0% to 2.2%. The lower limit of the Mn content is preferably 1.2% or more. The upper limit is preferably 2.0% or less. 
     P: 0.03% or Less 
     P exists as an impurity in steel, tends to segregate at grain boundaries, and adversely affects the steel pipe characteristics, such as toughness. Thus, the P content is preferably minimized. In the present disclosure, the allowable P content is up to 0.03%. Thus, the P content is limited to 0.03% or less. The P content is preferably 0.02% or less. However, an excessive reduction in P content increases refining costs. Thus, the P content is preferably 0.001% or more. 
     S: 0.005% or Less 
     S exists in the form of coarse sulfide inclusions, such as MnS, in steel and reduces ductility and toughness. Thus, the S content is desirably minimized. In the present disclosure, the allowable S content is up to 0.005%. Thus, the S content is limited to 0.005% or less. The S content is preferably 0.004% or less. However, an excessive reduction in S content increases refining costs. Thus, the S content is preferably 0.0001% or more. 
     Al: 0.001% to 0.10% 
     Al is an element that acts usefully as a deoxidizing agent for steel. Such an effect requires an Al content of 0.001% or more. However, a high Al content of more than 0.10% results in the formation of an Al oxide and low cleanliness of steel. Thus, the Al content is limited to the range of 0.001% to 0.10%. The lower limit of the Al content is preferably 0.005% or more. The upper limit is preferably 0.08% or less. 
     N: 0.006% or Less 
     N exists as an incidental impurity in steel and forms a solid solution or nitride, thereby reducing toughness of a base material portion or an electric-resistance-welded portion of a steel pipe. Thus, the N content is desirably minimized. In the present disclosure, the allowable N content is up to 0.006%. Thus, the N content is limited to 0.006% or less. 
     Nb: 0.010% to 0.100% 
     Nb is an important element in the present disclosure. While steel (a slab) is heated, Nb is present as Nb carbonitride in the steel, suppresses coarsening of austenite grains, and contributes to a finer structure. Nb forms fine precipitates during post-weld heat treatment performed at 600° C. or more and contributes to a smaller decrease in the strength of a base material portion of a steel pipe after the post-weld heat treatment. Such an effect requires a Nb content of 0.010% or more. However, an excessive Nb content of more than 0.100% adversely affects the toughness of a steel pipe and possibly results in an inability to achieve the desired toughness of the steel pipe for a conductor casing. Thus, the Nb content is limited to the range of 0.010% to 0.100%. The lower limit of the Nb content is preferably 0.020% or more. The upper limit is preferably 0.080% or less. 
     Ti: 0.001% to 0.050% 
     Ti forms a Ti nitride combining with N and fixes N that adversely affects the toughness of a steel pipe, and thereby has the action of improving the toughness of the steel pipe. Such an effect requires a Ti content of 0.001% or more. However, a Ti content of more than 0.050% results in a significant decrease in the toughness of a steel pipe. Thus, the Ti content is limited to the range of 0.001% to 0.050%. The lower limit of the Ti content is preferably 0.005% or more. The upper limit is preferably 0.030% or less. 
     These components are base components. In addition to the base components, a steel pipe according to the present disclosure may contain one or two or more selected 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, and B: 0.0030% or less, and/or one or two selected from Ca: 0.0050% or less and REM: 0.0050% or less. 
     One or two or more selected 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, and B: 0.0030% or less 
     V, Mo, Cr, Cu, Ni, and B are elements that improve hardenability and contribute to increased strength of a steel plate, and can be appropriately selected for use. These elements reduce the formation of pearlite and polygonal ferrite particularly in thick plates having a thickness of 15 mm or more and are effective in achieving desired strength and toughness. It is desirable to contain 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, and/or B: 0.0005% or more to produce such an effect. However, the content exceeding V: 0.1%, Mo: 0.5%, Cr: 0.5%, Cu: 0.5%, Ni: 1.0%, or B: 0.0030% may result in reduced weldability and toughness and increased material costs. Thus, the amounts of these elements are preferably 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, and B: 0.0030% or less, if any. V: 0.08% or less, Mo: 0.45% or less, Cr: 0.30% or less, Cu: 0.35% or less, Ni: 0.35% or less, and B: 0.0025% or less are more preferred. 
     One or two selected from Ca: 0.0050% or less and REM: 0.0050% or less 
     Ca and REM are elements that contribute to morphology control of inclusions in which elongated sulfide inclusions, such as MnS, are transformed into spherical sulfide inclusions, and can be appropriately selected for use. It is desirable to contain at least 0.0005% Ca or at least 0.0005% REM to produce such an effect. However, more than 0.0050% Ca or REM may result in increased oxide inclusions and reduced toughness. Thus, if present, Ca and REM are preferably limited to Ca: 0.0050% or less and REM: 0.0050% or less, respectively. 
     The remainder other than the components described above is made up of Fe and incidental impurities. 
     A high-strength thick-walled electric-resistance-welded steel pipe according to the present disclosure has the composition described above and has the structure in which a base material portion and an electric-resistance-welded portion of the high-strength thick-walled electric-resistance-welded steel pipe have a structure composed of 90% or more by volume of a bainitic ferrite phase as a main phase and 10% or less (including 0%) by volume of a second phase, the bainitic ferrite phase described above having an average grain size of 10 μm or less, fine Nb precipitates having a particle size of less than 20 nm being dispersed in the base material portion, the ratio (%) of the fine Nb precipitates to the total amount of Nb being 75% or less on a Nb equivalent basis, and the circularity of an end portion of the steel pipe is 0.6% or less. 
     Main Phase: 90% or More by Volume of a Bainitic Ferrite Phase 
     In order to achieve desired high strength and high toughness for a conductor casing, both a base material portion and an electric-resistance-welded portion of an electric-resistance-welded steel pipe according to the present disclosure have a structure composed mainly of 90% or more by volume of a bainitic ferrite phase. Less than 90% of a bainitic ferrite phase or 10% or more of a second phase other than the main phase results in an inability to achieve desired toughness. The second phase other than the main phase may be a hard phase, such as pearlite, degenerate pearlite, bainite, or martensite. Thus, the volume percentage of the bainitic ferrite phase serving as the main phase is limited to 90% or more. The volume percentage of the bainitic ferrite phase is preferably 95% or more. 
     Average Grain Size of Bainitic Ferrite Phase: 10 μm or Less 
     In order to achieve desired high strength and high toughness for a conductor casing, in the present disclosure, a bainitic ferrite phase serving as the main phase has a fine structure having an average grain size of 10 μm or less. An average grain size of more than 10 μm results in an inability to achieve desired high toughness. Thus, the average grain size of the bainitic ferrite phase serving as the main phase is limited to 10 μm or less. 
     Fine Nb precipitates having a particle size of less than 20 nm: the ratio (%) of the Nb precipitates to the total amount of Nb is 75% or less on a Nb equivalent basis 
     Fine Nb precipitates (mainly carbonitride) having a particle size of less than 20 nm effectively contribute to achieving desired high strength. Thus, the ratio (%) of the fine Nb precipitates to the total amount of Nb is preferably 20% or more on a Nb equivalent basis. However, precipitation of more than 75% of the total amount of Nb on a Nb equivalent basis results in Ostwald growth of precipitates during post-weld heat treatment performed at a temperature of 600° C. or more and reduces yield strength after post-weld heat treatment. Thus, in the present disclosure, the ratio (%) of fine Nb precipitates having a particle size of less than 20 nm in a base material portion of a steel pipe to the total amount of Nb is 75% or less on a Nb equivalent basis. Thus, fine Nb precipitates remain even after post-weld heat treatment and can suppress the decrease in yield strength. Thus, the ratio (%) of the amount of fine Nb precipitates having a particle size of less than 20 nm to the total amount of Nb on a Nb equivalent basis is limited to 75% or less. 
     The phrase “the amount of fine Nb precipitates having a particle size of less than 20 nm”, as used herein, refers to a value determined by electrolyzing an electroextraction test piece taken from a base material portion of an electric-resistance-welded steel pipe in an electrolyte solution (10% by volume acetylacetone-1% by mass tetramethylammonium chloride-methanol solution), filtering the resulting electrolytic residue through a filter having a pore size of 0.02 μm, and analyzing the amount of Nb passing through the filter. 
     A high-strength thick-walled electric-resistance-welded steel pipe according to the disclosed exemplary embodiments has the composition and structure described above, and the circularity of an end portion of the steel pipe is 0.6% or less. 
     Circularity: 0.6% or Less 
     If the circularity of an end portion of an electric-resistance-welded steel pipe is 0.6% or less, without cutting and/or straightening before the end portion of the pipe is joined to a connector by girth welding, linear misalignment in the joint is allowable, and the occurrence of breakage by repeated bending deformation can be reduced. If the circularity of an electric-resistance-welded steel pipe is more than 0.6%, the linear misalignment of a joint between the steel pipe and a connector (screw member) increases, and the joint is likely to be broken by the weight of the pipe and bending deformation during placement. Thus, the circularity of an electric-resistance-welded steel pipe is limited to 0.6% or less. The circularity of a steel pipe is defined by the following formula (1).
 
Circularity (%)={(maximum outer diameter mmφ of steel pipe)−(minimum outer diameter mm+ of steel pipe)}/(nominal outer diameter mmφ)×100  (1)
 
     It is desirable to continuously measure the maximum outer diameter and minimum outer diameter of a steel pipe with a laser displacement meter. In the case of manual measurement from necessity, the maximum outer diameter and minimum outer diameter of a steel pipe should be determined from measurements of at least 32 points on the circumference of the steel pipe. 
     In a deep-well conductor casing including a high-strength thick-walled electric-resistance-welded steel pipe according to the present disclosure, the high-strength thick-walled electric-resistance-welded steel pipe is provided with a screw member at each end thereof. The screw member may be attached by any method, for example, by MIG welding or TIG welding. The screw member may be made of, for example, carbon steel or stainless steel. 
     A method for manufacturing a high-strength thick-walled electric-resistance-welded steel pipe according to the present disclosure will be described below. 
     An electric-resistance-welded steel pipe according to the present disclosure is manufactured using a hot-rolled steel plate as a material. 
     More specifically, an electric-resistance-welded steel pipe according to the present disclosure is manufactured by continuously cold-rolling a hot-rolled steel plate with a roll forming machine (preferably with a cage roll group composed of a plurality of rolls and a fin pass forming roll group composed of a plurality of rolls) to form an open pipe having a generally circular cross section, butting against edges of the open pipe each other, electric-resistance-welding a portion where the edges butted while pressing the butted edges to contact each other by squeeze rolls to form an electric-resistance-welded steel pipe, subjecting the electric-resistance-welded portion of the electric-resistance-welded steel pipe to in-line heat treatment, and reducing the diameter of the electric-resistance-welded steel pipe by rolling. 
     The hot-rolled steel plate used as a material is a thick-hot-rolled steel plate having a thickness of 15 mm or more and preferably 51 mm or less manufactured by subjecting a steel having the composition described above to the following process. 
     The steel may be manufactured by any method. Preferably, a molten steel having the composition described above is produced by a conventional melting method, such as with a converter, and is formed into a cast block (steel), such as a slab, by a conventional casting process, such as a continuous casting process. Instead of the continuous casting process, a steel (steel block) may be manufactured by an ingot casting and slabbing process without problems. 
     A steel having the above composition is heated to a temperature in the range of 1150° C. to 1250° C. and is subjected to hot-rolling, which includes rough rolling and finish rolling, at a finishing delivery temperature of 750° C. or more. 
     Heating Temperature: 1150° C. to 1250° C. 
     Although a low heating temperature at which finer crystal grains are expected to grow is preferred in order to improve the toughness of a hot-rolled steel plate, a heating temperature of less than 1150° C. is too low to promote solid solution of undissolved carbide, failing to achieve the desired high strength of at least the API X80 grade in some cases. On the other hand, a high heating temperature of more than 1250° C. may cause coarsening of austenite (y) grains, reduced toughness, more scales and poor surface quality, and result in economic disadvantages due to increased energy loss. Thus, the heating temperature of steel ranges from 1150° C. to 1250° C. The soaking time at the heating temperature is preferably 60 minutes or more, in order to make the temperature of steel which is heated uniform. 
     The rough rolling is not particularly limited, provided that the resulting sheet bar has a predetermined size and shape. The finishing delivery temperature of the finish rolling is adjusted to be 750° C. or more. Here, the temperature is expressed in terms of a surface temperature. 
     Finishing Delivery Temperature: 750° C. or More 
     A finishing delivery temperature of less than 750° C. causes in induction of ferrite transformation, and processing of the resulting ferrite results in reduced toughness. Thus, the finishing delivery temperature is limited to 750° C. or more. In the finish rolling, the rolling reduction in a non-recrystallization temperature range in which a temperature at the center of plate thickness is 950° C. or less is preferably adjusted to be 20% or more. A rolling reduction of less than 20% in the non-recrystallization temperature range is an insufficient rolling reduction for the non-recrystallization temperature range and may therefore result in a small number of ferrite nucleation sites, thus failing to decrease the size of ferrite grains. Thus, the rolling reduction in the non-recrystallization temperature range is preferably adjusted to be 20% or more. From the viewpoint of the load to a rolling mill, the cumulative rolling reduction in hot rolling is preferably 95% or less. 
     In the present disclosure, after the completion of the hot rolling, cooling is immediately started preferably within 5 s (s refers to second). The hot-rolled plate is subjected to accelerated cooling such that the average cooling rate in a temperature range of 750° C. to 650° C. at the center of plate thickness ranges from 8° C./s to 70° C./s, and is coiled at a coiling temperature in the range of 400° C. to 580° C. The coiled plate is left to cool. 
     Average Cooling Rate of Accelerated Cooling in the Temperature Range of 750° C. to 650° C.: 8° C./s to 70° C./s 
     An average cooling rate of less than 8° C./s in the temperature range of 750° C. to 650° C. is slow and results in a structure containing a coarse polygonal ferrite phase having an average grain size of more than 10 μm and pearlite, thus failing to achieve the toughness and strength required for casing. On the other hand, an average cooling rate of more than 70° C./s may result in the formation of a martensite phase and reduced toughness. Thus, the average cooling rate in the temperature range of 750° C. to 650° C. is limited to the range of 8° C./s to 70° C./s. The lower limit of the cooling rate is preferably 10° C./s or more. The upper limit is preferably 50° C./s or less. These temperatures are the temperatures at the center of plate thickness. The temperatures at the center of plate thickness are determined by calculating the temperature distribution in a cross section by heat transfer analysis and correcting the calculated data in accordance with the actual outer and inner surface temperatures. 
     The cooling stop temperature of the accelerated cooling preferably ranges from 400° C. to 630° C. in terms of the surface temperature. When the cooling stop temperature of the accelerated cooling is outside the temperature range of 400° C. to 630° C., the desired coiling temperature in the range of 400° C. to 580° C. may be impossible to consistently achieve. 
     Coiling Temperature: 400° C. to 580° C. 
     A high coiling temperature of more than 580° C. causes promotion of precipitation of Nb carbonitride (precipitates), a Nb precipitation ratio of more than 75% after the coiling process, and results In reduced yield strength after post-weld heat treatment performed at a heating temperature of 600° C. or more. On the other hand, a coiling temperature of less than 400° C. causes insufficient precipitation of fine Nb carbonitride (precipitates) and results in an inability to achieve desired high strength (at least the API X80 grade). Thus, the coiling temperature is limited to a temperature in the range of 400° C. to 580° C. The coiling temperature preferably ranges from 460° C. to 550° C. When the coiling temperature is adjusted to be in this temperature range, the structure can contain fine Nb precipitates having a particle size of less than 20 nm dispersed in a base material portion, and the ratio (%) of the fine Nb precipitates to the total amount of Nb is 75% or less on a Nb equivalent basis. This can suppress the decrease in yield strength due to post-weld heat treatment performed at 600° C. or more. These temperatures are expressed in terms of a plate surface temperature. 
     A hot-rolled steel plate manufactured under the conditions described above has a structure composed of 90% or more by volume of a bainitic ferrite phase as a main phase and 10% or less (including 0%) by volume of a second phase as the remainder other than the bainitic ferrite phase, the main phase having an average grain size of 10 μm or less, fine Nb precipitates having a particle size of less than 20 nm being dispersed, the ratio (%) of the fine Nb precipitates to the total amount of Nb being 75% or less on a Nb equivalent basis. The hot-rolled steel plate has high strength of at least the API X80 grade, that is, a yield strength YS of 555 MPa or more, and high toughness represented by an absorbed energy vE −40  of 27 J or more in a Charpy impact test at a test temperature of −40° C. 
     A hot-rolled steel plate (hot-rolled steel strip)  1  having the composition and structure described above is used as a steel pipe material and is continuously rolled with a roll forming machine  2  illustrated in  FIG. 1  to form an open pipe having a generally circular cross section. After that, the edges of the open pipe are butted against each other while butted edges of the open pipe are pressed to contact each other by squeeze rolls  4 , the portion where the edges being butted are heated to at least the melting point thereof and are electric-resistance-welded with a welding machine  3  by high-frequency resistance heating, high-frequency induction heating, or the like, thus forming an electric-resistance-welded steel pipe  5 . The roll forming machine  2  preferably includes a cage roll group  2   a  composed of a plurality of rolls and a fin pass forming roll group  2   b  composed of a plurality of rolls. 
     The circularity is preferably improved by pressing two or more portions of an inner wall of a hot-rolled steel plate with at least one set of inner rolls  2   a   1  disposed downstream of the cage roll group  2   a  during a forming process. Preferably, the inner rolls disposed have shape as illustrated in  FIG. 2  so as to press two or more positions from the viewpoint of improving circularity and reducing the load to facilities.  FIG. 2  illustrates two sets of inner rolls  2   a   1  (( 2   a   1 ) 1  and ( 2   a   1 ) 2 ). 
     Methods of roll forming, pressing by squeeze rolls, and electric resistance welding are not particularly limited, provided that an electric-resistance-welded steel pipe having predetermined dimensions can be manufactured, and any conventional method may be employed. 
     The electric-resistance-welded steel pipe thus formed is subjected to in-line heat treatment (seam annealing) of an electric-resistance-welded portion, as illustrated in  FIG. 1 . 
     In-line heat treatment of an electric-resistance-welded portion is preferably performed with an induction heating apparatus  9  and a cooling apparatus  10  disposed downstream of the squeeze rolls  4  such that the electric-resistance-welded portion can be heated, for example, as illustrated in  FIG. 1 . As illustrated in  FIG. 3 , the induction heating apparatus  9  preferably includes one or a plurality of coils  9   a  so as to enable one or a plurality of heating steps. By using a plurality of coils  9   a , uniform heating can be achieved. 
     In the heat treatment of an electric-resistance-welded portion, preferably, the electric-resistance-welded portion is heated so as to the minimum temperature in the thickness direction being 830° C. or more and the maximum heating temperature in the thickness direction being 1150° C. or less and is cooled with water to a cooling stop temperature (at the center of plate thickness) of 550° C. or less such that the average cooling rate in the temperature range of 800° C. to 550° C. at the center of plate thickness ranges from 10° C./s to 70° C./s. The cooling stop temperature may be lowered. When the minimum heating temperature in an electric-resistance-welded portion is less than 830° C., the heating temperature may be too low to provide the desired structure of the electric-resistance-welded portion. On the other hand, a maximum heating temperature of more than 1150° C. may result in coarsening of crystal grains and reduced toughness. Thus, the heating temperature of an electric-resistance-welded portion in heat treatment preferably ranges from 830° C. to 1150° C. 
     When the average cooling rate is less than 10° C./s, this may promote the formation of polygonal ferrite and result in an inability to provide the desired structure of an electric-resistance-welded portion. On the other hand, rapid cooling with an average cooling rate of more than 70° C./s may result in the formation of a hard phase, such as martensite, an inability to provide the desired structure of an electric-resistance-welded portion, and reduced toughness. Thus, the average cooling rate of cooling after heating preferably ranges from 10° C./s to 70° C./s. The cooling stop temperature is preferably 550° C. or less. A high cooling stop temperature of more than 550° C. may cause incomplete ferrite transformation, and formation of a coarse pearlite structure when left standing after cooling, and reduced in reduced toughness, or reduced strength. 
     The heat treatment (seam annealing) of an electric-resistance-welded portion can change the structure of the electric-resistance-welded portion into a structure similar to the structure of the base material portion, that is, a structure composed of 90% or more by volume of a bainitic ferrite phase as a main phase and 10% or less (including 0%) by volume of a second phase, the bainitic ferrite phase having an average grain size of 10 m or less. 
     Subsequently, the circularity is improved by reducing rolling. 
     The reducing rolling is preferably cold rolling with a sizer  8  composed of two or three or more pairs of rolls. In the reducing rolling, a reduction ratio in the range of 0.2% to 3.3% is preferable. A reduction ratio of less than 0.2% may result in an inability to achieve the desired circularity (0.6% or less). On the other hand, a reduction ratio of more than 3.3% may cause excessive circumferential compression and considerable thickness variations in the circumferential direction, and result in reduced efficiency of girth welding. Thus, in the reducing rolling, a reduction ratio in the range of 0.2% to 3.3% is preferable. The reduction ratio is calculated using the following formula.
 
Reduction ratio (%)={(outer perimeter of pipe before reducing rolling mm)−(outer perimeter of pipe after reducing rolling mm)}/(outer perimeter of pipe before reducing rolling mm)×100
 
     The circularity of an end portion of a high-strength thick-walled electric-resistance-welded steel pipe can be adjusted to be 0.6% or less by the reducing rolling. 
     Exemplary embodiments are described below in the following examples. 
     Examples 
     A molten steel having the composition listed in Table 1 (the remainder was made up of Fe and incidental impurities) was produced in a converter and was cast into a slab (a cast block having a thickness of 250 mm) by a continuous casting process. The slab was used as steel that is a starting material. 
     The steel obtained was reheated under the conditions (heating temperature (° C.)×holding time (min)) listed in Table 2 and was hot-rolled into a hot-rolled steel plate. The hot rolling included rough rolling and finish rolling. The hot-rolling was performed under the conditions of the rolling reduction (%) in a non-recrystallization temperature range and the finishing delivery temperature (° C.) listed in Table 2. After the finish rolling, cooling was immediately started and here, accelerated cooling, that is, cooling was performed under the conditions of temperatures at the center of plate thickness (the average cooling rate in the temperature range of 750° C. to 650° C. and the cooling stop temperature) listed in Table 2 was performed. The resultant hot-rolled steel plate was coiled at a coiling temperature listed in Table 2 to produce a steel pipe material. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Steel 
                 Chemical components (mass %) 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 No. 
                 C 
                 Si 
                 Mn 
                 P 
                 S 
                 Al 
                 N 
                 Nb 
                 Ti 
                 V, Mo, Cr, Cu, Ni, B 
                 Ca, REM 
                 Remarks 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 A 
                 0.090 
                 0.15 
                 1.90 
                 0.006 
                 0.0050 
                 0.034 
                 0.003 
                 0.037 
                 0.010 
                 — 
                 — 
                 Working example 
               
               
                 B 
                 0.054 
                 0.15 
                 1.74 
                 0.012 
                 0.0009 
                 0.026 
                 0.0003 
                 0.060 
                 0.015 
                 V: 0.08 
                 — 
                 Working example 
               
               
                 C 
                 0.050 
                 0.20 
                 1.55 
                 0.012 
                 0.0005 
                 0.032 
                 0.004 
                 0.060 
                 0.015 
                 Mo: 0.28, Cu: 0.22, 
                 — 
                 Working example 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Ni: 0.20 
                   
                   
               
               
                 D 
                 0.066 
                 0.23 
                 1.82 
                 0.010 
                 0.0016 
                 0.037 
                 0.004 
                 0.063 
                 0.016 
                 V: 0.04, Cr: 0.13 
                 — 
                 Working example 
               
               
                 E 
                 0.022 
                 0.23 
                 1.45 
                 0.015 
                 0.0022 
                 0.026 
                 0.002 
                 0.055 
                 0.014 
                 V: 0.07, Mo: 0.15, 
                 Ca: 0.0025 
                 Working example 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Cu: 0.32 
                   
                   
               
               
                 F 
                 0.040 
                 0.18 
                 1.60 
                 0.010 
                 0.0010 
                 0.033 
                 0.002 
                 0.025 
                 0.045 
                 Mo: 0.10, Ni: 0.25 
                 Ca: 0.0020 
                 Working example 
               
               
                 G 
                 0.032 
                 0.28 
                 2.06 
                 0.010 
                 0.0019 
                 0.040 
                 0.003 
                 0.053 
                 0.012 
                 Mo: 0.37, Cr: 0.40, 
                 REM: 0.003 
                 Working example 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 B: 0.0022 
                   
                   
               
               
                 H 
                 0.004 
                 0.22 
                 1.85 
                 0.010 
                 0.0010 
                 0.030 
                 0.003 
                 0.032 
                 0.020 
                 V: 0.075, Cu: 0.22, 
                 — 
                 Comparative example 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Ni: 0.24 
                   
                   
               
               
                 I 
                 0.146 
                 0.20 
                 1.44 
                 0.012 
                 0.0025 
                 0.023 
                 0.004 
                 0.024 
                 0.008 
                 V: 0.043 
                 Ca: 0.0011 
                 Comparative example 
               
               
                 J 
                 0.042 
                 0.56 
                 1.58 
                 0.005 
                 0.0015 
                 0.038 
                 0.004 
                 0.052 
                 0.016 
                 Cr: 0.23, Ni: 0.15 
                 Ca: 0.0022 
                 Comparative example 
               
               
                 K 
                 0.037 
                 0.19 
                 0.65 
                 0.017 
                 0.0008 
                 0.021 
                 0.003 
                 0.080 
                 0.017 
                 — 
                 — 
                 Comparative example 
               
               
                 L 
                 0.036 
                 0.35 
                 2.31 
                 0.012 
                 0.0008 
                 0.048 
                 0.003 
                 0.025 
                 0.012 
                 Cu: 0.15, Ni: 0.13 
                 Ca: 0.0025 
                 Comparative example 
               
               
                 M 
                 0.050 
                 0.27 
                 1.36 
                 0.006 
                 0.0021 
                 0.045 
                 0.004 
                 0.002 
                 0.005 
                 V: 0.040 
                 — 
                 Comparative example 
               
               
                 N 
                 0.071 
                 0.21 
                 1.26 
                 0.012 
                 0.0006 
                 0.031 
                 0.003 
                 0.131 
                 0.015 
                 Mo: 0.18, Cr: 0.32 
                 — 
                 Comparative example 
               
               
                 O 
                 0.061 
                 0.23 
                 1.05 
                 0.008 
                 0.0007 
                 0.041 
                 0.001 
                 0.015 
                 0.065 
                 — 
                 — 
                 Comparative example 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                   
                 Hot rolling 
                 Cooling after 
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Heating 
                 Rolling 
                   
                 hot rolling 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Hot- 
                   
                 Heating 
                   
                 reduction in non- 
                 Finishing 
                 Average 
                   
                 Coiling 
                   
                   
               
               
                 rolled 
                   
                 temper- 
                 Holding 
                 recrystallization 
                 delivery 
                 cooling 
                 Cooling stop 
                 Coiling 
                 Plate 
                   
               
               
                 plate 
                 Steel 
                 ature 
                 time 
                 temperature range* 
                 temperature** 
                 rate** 
                 temperature*** 
                 temperature** 
                 thickness 
                   
               
               
                 No. 
                 No. 
                 (° C.) 
                 (min) 
                 (%) 
                 (° C.) 
                 (° C./s) 
                 (° C.) 
                 (° C.) 
                 (mm) 
                 Remarks 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 1 
                 A 
                 1210 
                 90 
                 40 
                 820 
                 18 
                 540 
                 520 
                 25.2 
                 Working example 
               
               
                 2 
                 B 
                 1210 
                 75 
                 40 
                 810 
                 20 
                 540 
                 530 
                 20.4 
                 Working example 
               
               
                 3 
                 C 
                 1200 
                 80 
                 50 
                 800 
                 20 
                 510 
                 500 
                 22.0 
                 Working example 
               
               
                 4 
                 D 
                 1220 
                 90 
                 20 
                 820 
                 16 
                 560 
                 540 
                 25.2 
                 Working example 
               
               
                 5 
                 E 
                 1230 
                 90 
                 85 
                 820 
                 30 
                 520 
                 500 
                 25.2 
                 Working example 
               
               
                 6 
                 F 
                 1180 
                 65 
                 55 
                 780 
                 22 
                 520 
                 500 
                 20.4 
                 Working example 
               
               
                 7 
                 G 
                 1200 
                 100 
                 60 
                 820 
                 45 
                 490 
                 470 
                 18.9 
                 Working example 
               
               
                 8 
                 H 
                 1200 
                 100 
                 20 
                 820 
                 25 
                 480 
                 460 
                 18.9 
                 Comparative example 
               
               
                 9 
                 I 
                 1200 
                 120 
                 85 
                 820 
                 18 
                 490 
                 460 
                 25.2 
                 Comparative example 
               
               
                 10 
                 J 
                 1190 
                 75 
                 40 
                 780 
                 28 
                 500 
                 480 
                 15.7 
                 Comparative example 
               
               
                 11 
                 K 
                 1170 
                 80 
                 50 
                 830 
                 16 
                 520 
                 500 
                 25.2 
                 Comparative example 
               
               
                 12 
                 L 
                 1200 
                 80 
                 20 
                 820 
                 20 
                 560 
                 540 
                 22.0 
                 Comparative example 
               
               
                 13 
                 M 
                 1210 
                 90 
                 85 
                 820 
                 35 
                 570 
                 540 
                 25.2 
                 Comparative example 
               
               
                 14 
                 N 
                 1210 
                 90 
                 40 
                 820 
                 20 
                 515 
                 500 
                 20.4 
                 Comparative example 
               
               
                 15 
                 O 
                 1230 
                 95 
                 40 
                 840 
                 25 
                 470 
                 450 
                 18.9 
                 Comparative example 
               
               
                 16 
                 A 
                 1100 
                 100 
                 50 
                 820 
                 18 
                 440 
                 420 
                 25.2 
                 Comparative example 
               
               
                 17 
                 A 
                 1300 
                 100 
                 50 
                 820 
                 60 
                 500 
                 480 
                 17.3 
                 Comparative example 
               
               
                 18 
                 A 
                 1230 
                 105 
                 20 
                 820 
                 5 
                 540 
                 520 
                 22.0 
                 Comparative example 
               
               
                 19 
                 A 
                 1200 
                 90 
                 85 
                 820 
                 100 
                 440 
                 420 
                 25.2 
                 Comparative example 
               
               
                 20 
                 A 
                 1200 
                 95 
                 40 
                 780 
                 18 
                 680 
                 650 
                 25.2 
                 Comparative example 
               
               
                 21 
                 A 
                 1200 
                 90 
                 40 
                 840 
                 45 
                 355 
                 350 
                 25.2 
                 Comparative example 
               
               
                 22 
                 C 
                 1280 
                 100 
                 50 
                 820 
                 25 
                 520 
                 500 
                 18.9 
                 Comparative example 
               
               
                 23 
                 C 
                 1220 
                 100 
                 20 
                 820 
                 120 
                 500 
                 480 
                 25.2 
                 Comparative example 
               
               
                 24 
                 C 
                 1210 
                 110 
                 85 
                 820 
                 20 
                 730 
                 700 
                 20.4 
                 Comparative example 
               
               
                 25 
                 E 
                 1110 
                 110 
                 55 
                 790 
                 20 
                 500 
                 480 
                 22.0 
                 Comparative example 
               
               
                 26 
                 E 
                 1180 
                 100 
                 60 
                 820 
                 3 
                 520 
                 500 
                 25.2 
                 Comparative example 
               
               
                 27 
                 E 
                 1180 
                 90 
                 20 
                 820 
                 15 
                 310 
                 300 
                 25.2 
                 Comparative example 
               
               
                 28 
                 F 
                 1100 
                 90 
                 20 
                 800 
                 15 
                 515 
                 500 
                 25.2 
                 Comparative example 
               
               
                 29 
                 F 
                 1170 
                 85 
                 85 
                 820 
                 5 
                 525 
                 520 
                 25.2 
                 Comparative example 
               
               
                 30 
                 F 
                 1190 
                 75 
                 40 
                 820 
                 25 
                 650 
                 630 
                 18.9 
                 Comparative example 
               
               
                 31 
                 G 
                 1300 
                 75 
                 40 
                 790 
                 20 
                 600 
                 580 
                 25.2 
                 Comparative example 
               
               
                 32 
                 G 
                 1200 
                 80 
                 50 
                 820 
                 110 
                 565 
                 550 
                 15.7 
                 Comparative example 
               
               
                   
               
               
                 *Temperature range of 930° C. or less 
               
               
                 **Surface temperature 
               
               
                 ***Temperature at the center of plate thickness 
               
            
           
         
       
     
     The hot-rolled steel plate serving as a steel pipe material was continuously cold-rolled with a roll forming machine including a cage roll group composed of a plurality of rolls and a fin pass forming roll group composed of a plurality of rolls, thereby forming an open pipe having a generally circular cross section. Then, the edges of the open pipe, which were opposite each other, were butted together. While butted edges of the open pipe were pressed to contact each other by squeeze rolls, the portion where the edges were butted was electric-resistance-welded to form an electric-resistance-welded steel pipe. In some electric-resistance-welded steel pipes, at least two portions, which were separate each other in the width direction, of the inner wall of the semi-formed product were pressed with inner rolls disposed downstream of the cage roll group. 
     The electric-resistance-welded portion of the electric-resistance-welded steel pipe was then subjected to in-line heat treatment under the conditions listed in Table 3. The in-line heat treatment was performed with an in-line heat treatment apparatus disposed downstream of the squeeze rolls. The in-line heat treatment apparatus included an induction heating apparatus and a water cooling apparatus. The average cooling rate and the cooling stop temperature were expressed in terms of a temperature at the center of plate thickness. The average cooling rate listed was an average cooling rate in the temperature range of 800° C. to 550° C. 
     The electric-resistance-welded steel pipe subjected to the in-line heat treatment was subjected to reducing-cold-rolling with a reducing rolling mill (sizer roll) at the reduction ratio listed in Table 3, thereby forming an electric-resistance-welded steel pipe having the dimensions listed in Table 3. The reducing rolling mill included 2 to 8 sets of rolls, as listed in Table 3. Some electric-resistance-welded steel pipes were not subjected to reducing rolling. The circularity of an end portion of a pipe was calculated using the formula (1). The outer diameters listed in Table 3 were nominal outer diameters. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                   
                 Heat treatment of electric- 
                   
                   
                   
               
               
                   
                 resistance-welded portion 
                   
                 Dimensions of steel pipe 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Hot- 
                   
                 Maximum 
                 Average 
                   
                 Reducing rolling 
                   
                 Circularity 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Steel 
                 rolled 
                   
                 heating 
                 cooling 
                 Cooling stop 
                 Number of 
                   
                   
                 Outer 
                 of end 
                   
               
               
                 pipe 
                 plate 
                 Steel 
                 temperature 
                 rate 
                 temperature 
                 rolls in 
                 Reduction 
                 Thickness 
                 diameter 
                 portion of 
                   
               
               
                 No. 
                 No. 
                 No. 
                 (° C.) 
                 (° C./s) 
                 (° C.) 
                 sizer mill 
                 ratio (%) 
                 (mm) 
                 (mmϕ) 
                 pipe (%) 
                 Remarks 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                  1 
                 1 
                 A 
                 1120 
                 15 
                 450 
                 2 
                 0.4 
                 25.4 
                 558.8 
                 0.45 
                 Working example 
               
               
                  2 
                 2 
                 B 
                 1080 
                 25 
                 500 
                 2 
                 0.4 
                 20.6 
                 558.8 
                 0.43 
                 Working example 
               
               
                  3* 
                 3 
                 C 
                 1100 
                 20 
                 500 
                 3 
                 0.5 
                 22.2 
                 558.8 
                 0.32 
                 Working example 
               
               
                  4* 
                 4 
                 D 
                 1100 
                 15 
                 500 
                 3 
                 0.5 
                 25.4 
                 609.6 
                 0.35 
                 Working example 
               
               
                  5 
                 5 
                 E 
                 1090 
                 15 
                 480 
                 4 
                 0.4 
                 25.4 
                 558.8 
                 0.27 
                 Working example 
               
               
                  6* 
                 6 
                 F 
                 1060 
                 20 
                 400 
                 4 
                 0.4 
                 20.6 
                 558.8 
                 0.26 
                 Working example 
               
               
                  7* 
                 7 
                 G 
                 1050 
                 25 
                 450 
                 8 
                 0.3 
                 19.1 
                 660.4 
                 0.15 
                 Working example 
               
               
                  8 
                 8 
                 H 
                 1050 
                 25 
                 350 
                 2 
                 0.3 
                 19.1 
                 558.8 
                 0.42 
                 Comparative example 
               
               
                  9 
                 9 
                 I 
                 1080 
                 15 
                 350 
                 2 
                 0.5 
                 25.4 
                 558.8 
                 0.45 
                 Comparative example 
               
               
                 10 
                 10 
                 J 
                 1100 
                 33 
                 300 
                 2 
                 0.5 
                 15.9 
                 558.8 
                 0.44 
                 Comparative example 
               
               
                 11 
                 11 
                 K 
                 1120 
                 15 
                 480 
                 4 
                 0.5 
                 25.4 
                 558.8 
                 0.33 
                 Comparative example 
               
               
                 12 
                 12 
                 L 
                 1100 
                 15 
                 450 
                 4 
                 0.5 
                 22.2 
                 558.8 
                 0.34 
                 Comparative example 
               
               
                 13 
                 13 
                 M 
                 1020 
                 15 
                 500 
                 4 
                 0.5 
                 25.4 
                 558.8 
                 0.29 
                 Comparative example 
               
               
                  14* 
                 14 
                 N 
                 1000 
                 20 
                 300 
                 4 
                 0.5 
                 20.6 
                 558.8 
                 0.28 
                 Comparative example 
               
               
                 15 
                 15 
                 O 
                 1040 
                 30 
                 300 
                 4 
                 0.5 
                 19.1 
                 457.2 
                 0.28 
                 Comparative example 
               
               
                  16* 
                 16 
                 A 
                 1070 
                 15 
                 350 
                 3 
                 0.4 
                 25.4 
                 558.8 
                 0.32 
                 Comparative example 
               
               
                 17 
                 17 
                 A 
                 1075 
                 30 
                 400 
                 2 
                 0.4 
                 17.5 
                 609.6 
                 0.42 
                 Comparative example 
               
               
                 18 
                 18 
                 A 
                 1060 
                 15 
                 350 
                 2 
                 0.4 
                 22.2 
                 508.0 
                 0.45 
                 Comparative example 
               
               
                 19 
                 19 
                 A 
                 1050 
                 15 
                 350 
                 2 
                 0.4 
                 25.4 
                 609.6 
                 0.42 
                 Comparative example 
               
               
                 20 
                 20 
                 A 
                 1100 
                 15 
                 400 
                 2 
                 0.6 
                 25.4 
                 457.2 
                 0.45 
                 Comparative example 
               
               
                 21 
                 21 
                 A 
                 1100 
                 15 
                 300 
                 2 
                 0.6 
                 25.4 
                 558.8 
                 0.44 
                 Comparative example 
               
               
                 22 
                 22 
                 C 
                 1100 
                 25 
                 300 
                 2 
                 0.6 
                 19.1 
                 558.8 
                 0.42 
                 Comparative example 
               
               
                 23 
                 23 
                 C 
                 1120 
                 15 
                 350 
                 2 
                 0.6 
                 25.4 
                 558.8 
                 0.40 
                 Comparative example 
               
               
                 24 
                 24 
                 C 
                 1080 
                 20 
                 350 
                 2 
                 0.6 
                 20.6 
                 558.8 
                 0.40 
                 Comparative example 
               
               
                 25 
                 25 
                 E 
                 1070 
                 20 
                 400 
                 2 
                 0.6 
                 22.2 
                 508.0 
                 0.44 
                 Comparative example 
               
               
                 26 
                 26 
                 E 
                 1080 
                 15 
                 400 
                 2 
                 0.6 
                 25.4 
                 558.8 
                 0.44 
                 Comparative example 
               
               
                 27 
                 27 
                 E 
                 1060 
                 15 
                 380 
                 2 
                 0.5 
                 25.4 
                 558.8 
                 0.44 
                 Comparative example 
               
               
                 28 
                 28 
                 F 
                 1100 
                 15 
                 450 
                 2 
                 0.5 
                 25.4 
                 508.0 
                 0.48 
                 Comparative example 
               
               
                 29 
                 29 
                 F 
                 1100 
                 20 
                 440 
                 2 
                 0.5 
                 25.4 
                 558.8 
                 0.38 
                 Comparative example 
               
               
                 30 
                 30 
                 F 
                 1030 
                 25 
                 430 
                 2 
                 0.5 
                 19.1 
                 558.8 
                 0.40 
                 Comparative example 
               
               
                 31 
                 31 
                 G 
                 1100 
                 20 
                 470 
                 2 
                 0.5 
                 25.4 
                 558.8 
                 0.41 
                 Comparative example 
               
               
                 32 
                 32 
                 G 
                 990 
                 55 
                 450 
                 2 
                 0.4 
                 15.9 
                 558.8 
                 0.40 
                 Comparative example 
               
               
                 33 
                 17 
                 A 
                 1080 
                 25 
                 300 
                 — 
                 — 
                 17.5 
                 406.4 
                 0.86 
                 Comparative example 
               
               
                   
               
               
                 *With use of inner rolls 
               
            
           
         
       
     
     Test pieces were taken from the electric-resistance-welded steel pipe and were subjected to structure observation, a tensile test, an impact test, and a post-weld heat treatment test. These test methods are described below. 
     (1) Structure Observation 
     A test piece for structure observation was taken from a base material portion (a position at an angle of 90 degrees with respect to the electric-resistance-welded portion in the circumferential direction) and the electric-resistance-welded portion of the electric-resistance-welded steel pipe. The base material portion was polished and etched (etchant: nital) such that the observation surface was at a the central position of the plate thickness, that is, at a center of the thickness, in a cross section in the longitudinal direction of the pipe (L cross section). The electric-resistance-welded portion was polished and etched (etchant: nital) such that the observation surface was a cross section in the circumferential direction of the pipe (C cross section). The structure was observed with a scanning electron microscope (SEM) (magnification: 1000), and images were taken in at least 2 fields. The structure images were analyzed to identify the structure and to determine the fraction of each phase. The average of the area fractions thus determined was treated as the volume fraction. 
     Grain boundaries having an orientation difference of 15 degrees or more were determined by a SEM/electron back scattering diffraction (EBSD) method. The arithmetic mean of the equivalent circular diameters of the grains determine was defined to be the average grain size of the main phase. “Orientation Imaging Microscopy Data Analysis”, which is a software available from AMETEK Co., Ltd., was used for the calculation of the grain size. 
     Specimen for an electroextraction test piece was taken from the base material portion of the electric-resistance-welded steel pipe (a position at an angle of 90 degrees with respect to the electric-resistance-welded portion in the circumferential direction) and was electrolyzed at a current density of 20 mA/cm 2  in an electrolyte solution (10% by volume acetylacetone-1% by mass tetramethylammonium chloride-methanol solution). The resulting electrolytic residue was dissolved in a liquid and was collected with an aluminum filter (pore size: 0.02 μm). The amount of Nb in the filtrate was measured by ICP spectroscopy and was considered to be the amount of precipitated Nb having a grain size of 20 nm or less. The ratio (%) of the amount of precipitated Nb to the total amount of Nb was calculated. 
     (2) Tensile Test 
     A plate-like tensile test piece was taken from the base material portion (a position at an angle of 180 degrees with respect to the electric-resistance-welded portion in the circumferential direction) and the electric-resistance-welded portion of the electric-resistance-welded steel pipe according to ASTM A 370 such that the tensile direction was a direction perpendicular to the longitudinal direction of the pipe (C direction). The tensile properties (yield strength YS and tensile strength TS) of the tensile test piece were measured. 
     (3) Impact Test 
     A V-notched test piece was taken from the base material portion (a position at an angle of 90 degrees with respect to the electric-resistance-welded portion in the circumferential direction) and the electric-resistance-welded portion of the electric-resistance-welded steel pipe according to ASTM A 370 such that the longitudinal direction of the test piece was the circumferential direction (C direction). The absorbed energy vE −40  (J) each of three test pieces for a steel pipe was measured in a Charpy impact test at a test temperature of −40° C. The average value of the three measurements was considered to be the vE −40  of the steel pipe. 
     (4) Post-Weld Heat Treatment Test 
     A test material was taken from the base material portion of the electric-resistance-welded steel pipe. The test material was placed in a heat treatment furnace maintained at a heating temperature simulating post-weld heat treatment listed in Table 5. When a predetermined holding time listed in Table 5 elapsed since the temperature of the test material reached (heating temperature—10° C.), the test material was removed from the heat treatment furnace and was left to cool. A plate-like tensile test piece was taken from the heat-treated test material according to ASTM A 370 such that the tensile direction was a direction perpendicular to the longitudinal direction of the pipe (C direction). The tensile properties (yield strength YS and tensile strength TS) of the tensile test piece were measured. A difference ΔYS in yield strength between before and after the post-weld heat treatment was calculated. If the strength is decreased after the post-weld heat treatment, the ΔYS is negative. For reference, an electroextraction test piece was taken from the test material after the post-weld heat treatment, and the ratio of the amount of precipitated Nb was determined in the same manner as in (1). 
     Tables 4 and 5 show the results. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                   
                 Base material portion 
                 Electric-resistance-welded portion 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Hot- 
                   
                 Structure 
                 Strength 
                 Toughness 
                 Structure 
                 Strength 
                 Toughness 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Steel 
                 rolled 
                   
                   
                 Fraction of main 
                 Average grain 
                 Precipitated 
                 Yield 
                 Tensile 
                 Absorbed 
                   
                 Fraction of main 
                 Average grain 
                 Tensile 
                 Absorbed 
                   
               
               
                 pipe 
                 plate 
                 Steel 
                   
                 phase structure 
                 size of main 
                 Nb ratio** 
                 strength 
                 strength 
                 energy 
                   
                 phase structure 
                 size of main 
                 strength 
                 energy 
                   
               
               
                 No. 
                 No. 
                 No. 
                 Type* 
                 (vol %) 
                 phase (μm) 
                 (%) 
                 YS (MPa) 
                 TS (MPa) 
                 vE-40(J) 
                 Type* 
                 (vol %) 
                 phase (μm) 
                 TS (MPa) 
                 vE-40(J) 
                 Remarks 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 1 
                 1 
                 A 
                 BF + B 
                 BF: 98 
                 4.5 
                 62 
                 582 
                 664 
                 234 
                 BF 
                 100 
                 5.6 
                 650 
                 196 
                 Working example 
               
               
                 2 
                 2 
                 B 
                 BF 
                 BF: 100 
                 5.1 
                 57 
                 624 
                 701 
                 311 
                 BF 
                 100 
                 5.3 
                 660 
                 225 
                 Working example 
               
               
                 3 
                 3 
                 C 
                 BF 
                 BF: 100 
                 6.6 
                 48 
                 574 
                 650 
                 341 
                 BF 
                 100 
                 6.2 
                 654 
                 189 
                 Working example 
               
               
                 4 
                 4 
                 D 
                 BF + B 
                 BF: 96 
                 4.3 
                 67 
                 610 
                 692 
                 300 
                 BF 
                 100 
                 6.3 
                 680 
                 199 
                 Working example 
               
               
                 5 
                 5 
                 E 
                 BF 
                 BF: 100 
                 4.9 
                 45 
                 596 
                 676 
                 340 
                 BF 
                 100 
                 6.6 
                 672 
                 194 
                 Working example 
               
               
                 6 
                 6 
                 F 
                 BF 
                 BF: 100 
                 4.1 
                 48 
                 580 
                 674 
                 336 
                 BF 
                 100 
                 6.8 
                 666 
                 223 
                 Working example 
               
               
                 7 
                 7 
                 G 
                 BF 
                 BF: 100 
                 4.2 
                 45 
                 722 
                 849 
                 215 
                 BF 
                 100 
                 7.1 
                 801 
                 237 
                 Working example 
               
               
                 8 
                 8 
                 H 
                 BF 
                 BF: 100 
                 4.0 
                 38 
                 412 
                 460 
                 452 
                 BF 
                 100 
                 7.0 
                 650 
                 169 
                 Comparative example 
               
               
                 9 
                 9 
                 I 
                 F + BF + P 
                 F: 92 
                 5.5 
                 41 
                 486 
                 609 
                 20 
                 B 
                 100 
                 7.5 
                 630 
                 88 
                 Comparative example 
               
               
                 10 
                 10 
                 J 
                 BF + F 
                 BF: 97 
                 5.9 
                 49 
                 563 
                 634 
                 282 
                 BF 
                 100 
                 5.4 
                 651 
                 16 
                 Comparative example 
               
               
                 11 
                 11 
                 K 
                 BF + F 
                 BF: 85 
                 8.3 
                 54 
                 529 
                 608 
                 360 
                 BF 
                 100 
                 5.1 
                 580 
                 255 
                 Comparative example 
               
               
                 12 
                 12 
                 L 
                 B + M 
                 B: 90 
                 3.7 
                 71 
                 576 
                 677 
                 10 
                 B 
                 100 
                 6.0 
                 640 
                 25 
                 Comparative example 
               
               
                 13 
                 13 
                 M 
                 BF 
                 BF: 100 
                 7.2 
                 — 
                 492 
                 562 
                 386 
                 BF 
                 100 
                 6.1 
                 627 
                 221 
                 Comparative example 
               
               
                 14 
                 14 
                 N 
                 BF 
                 BF: 100 
                 4.3 
                 53 
                 605 
                 685 
                 11 
                 BF 
                 100 
                 6.4 
                 675 
                 173 
                 Comparative example 
               
               
                 15 
                 15 
                 O 
                 BF + F 
                 BF: 95 
                 5.5 
                 32 
                 612 
                 699 
                 8 
                 BF 
                 100 
                 6.6 
                 633 
                 162 
                 Comparative example 
               
               
                 16 
                 16 
                 A 
                 BF + B 
                 BF: 96 
                 4.4 
                 15 
                 541 
                 637 
                 356 
                 BF 
                 100 
                 6.9 
                 644 
                 190 
                 Comparative example 
               
               
                 17 
                 17 
                 A 
                 BF + B 
                 BF: 86 
                 11.5 
                 68 
                 585 
                 678 
                 20 
                 BF 
                 100 
                 6.8 
                 643 
                 189 
                 Comparative example 
               
               
                 18 
                 18 
                 A 
                 F + P 
                 F: 92 
                 12.8 
                 66 
                 499 
                 640 
                 14 
                 BF 
                 100 
                 5.7 
                 667 
                 217 
                 Comparative example 
               
               
                 19 
                 19 
                 A 
                 M + B 
                 M: 55 
                 2.7 
                 38 
                 524 
                 760 
                 17 
                 BF 
                 100 
                 5.4 
                 651 
                 215 
                 Comparative example 
               
               
                 20 
                 20 
                 A 
                 BF + F + P 
                 BF: 80 
                 8.6 
                 85 
                 624 
                 711 
                 22 
                 BF 
                 100 
                 6.3 
                 646 
                 231 
                 Comparative example 
               
               
                 21 
                 21 
                 A 
                 BF + B 
                 BF: 89 
                 4.4 
                 18 
                 533 
                 605 
                 410 
                 BF 
                 100 
                 6.4 
                 659 
                 166 
                 Comparative example 
               
               
                 22 
                 22 
                 C 
                 BF + B 
                 BF: 88 
                 7.8 
                 55 
                 642 
                 682 
                 9 
                 BF 
                 100 
                 5.7 
                 640 
                 190 
                 Comparative example 
               
               
                 23 
                 23 
                 C 
                 M + B 
                 M: 60 
                 3.3 
                 53 
                 559 
                 780 
                 19 
                 BF 
                 100 
                 5.4 
                 642 
                 192 
                 Comparative example 
               
               
                 24 
                 24 
                 C 
                 BF + F + P 
                 BF: 95 
                 8.5 
                 95 
                 571 
                 680 
                 30 
                 BF 
                 100 
                 5.7 
                 639 
                 225 
                 Comparative example 
               
               
                 25 
                 25 
                 E 
                 BF 
                 BF: 100 
                 3.5 
                 13 
                 489 
                 555 
                 415 
                 BF 
                 100 
                 5.4 
                 671 
                 202 
                 Comparative example 
               
               
                 26 
                 26 
                 E 
                 F + B 
                 F: 94 
                 10.5 
                 65 
                 470 
                 553 
                 287 
                 BF 
                 100 
                 6.3 
                 675 
                 145 
                 Comparative example 
               
               
                 27 
                 27 
                 E 
                 BF + B 
                 BF: 94 
                 3.8 
                 18 
                 522 
                 639 
                 311 
                 BF 
                 100 
                 6.4 
                 664 
                 166 
                 Comparative example 
               
               
                 28 
                 28 
                 F 
                 BF 
                 BF: 100 
                 4.5 
                 12 
                 538 
                 674 
                 382 
                 BF 
                 100 
                 6.7 
                 653 
                 178 
                 Comparative example 
               
               
                 29 
                 29 
                 F 
                 F + P 
                 F: 93 
                 11.2 
                 73 
                 460 
                 541 
                 366 
                 BF 
                 100 
                 6.9 
                 658 
                 227 
                 Comparative example 
               
               
                 30 
                 30 
                 F 
                 BF + P 
                 BF: 96 
                 7.7 
                 88 
                 593 
                 706 
                 333 
                 BF 
                 100 
                 7.2 
                 668 
                 210 
                 Comparative example 
               
               
                 31 
                 31 
                 G 
                 BF 
                 BF: 100 
                 10.2 
                 70 
                 660 
                 880 
                 16 
                 B 
                 100 
                 7.1 
                 810 
                 194 
                 Comparative example 
               
               
                 32 
                 32 
                 G 
                 B + M 
                 B: 70 
                 4.5 
                 68 
                 734 
                 895 
                 22 
                 B 
                 100 
                 7.6 
                 812 
                 197 
                 Comparative example 
               
               
                 33 
                 17 
                 A 
                 BF + B 
                 BF: 95 
                 11.1 
                 65 
                 580 
                 675 
                 19 
                 BF 
                 100 
                 6.7 
                 650 
                 176 
                 Comparative example 
               
               
                   
               
               
                 *BF: bainitic ferrite, B: bainite, P: pearlite, M: martensite, F: ferrite 
               
               
                 **Amount of precipitated Nb: Amount of precipitated Nb having a particle size less than 20 nm (Ratio (%) relative to the total amount of Nb on a Nb equivalent basis) 
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                   
                 Difference in 
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 Post-weld heat 
                 Strength after post- 
                 strength between 
                   
                   
               
               
                   
                 Hot- 
                   
                 treatment conditions 
                 weld heat treatment 
                 before and after post- 
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Steel 
                 rolled 
                   
                 Heating 
                   
                 Yield 
                 Tensile 
                 weld heat treatment 
                 Precipitated 
                   
               
               
                 pipe 
                 plate 
                 Steel 
                 temperature 
                 Holding 
                 strength 
                 strength 
                 ΔYS 
                 Nb ratio* 
                   
               
               
                 No. 
                 No. 
                 No. 
                 (° C.) 
                 time (h) 
                 YS (MPa) 
                 TS (MPa) 
                 (MPa) 
                 (%) 
                 Remarks 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 1 
                 1 
                 A 
                 620 
                 2 
                 622 
                 666 
                 40 
                 95 
                 Working example 
               
               
                 2 
                 2 
                 B 
                 620 
                 2 
                 670 
                 695 
                 46 
                 90 
                 Working example 
               
               
                 3 
                 3 
                 C 
                 670 
                 1 
                 622 
                 643 
                 48 
                 88 
                 Working example 
               
               
                 4 
                 4 
                 D 
                 670 
                 1 
                 650 
                 684 
                 40 
                 89 
                 Working example 
               
               
                 5 
                 5 
                 E 
                 650 
                 2 
                 634 
                 662 
                 38 
                 85 
                 Working example 
               
               
                 6 
                 6 
                 F 
                 650 
                 2 
                 640 
                 660 
                 60 
                 92 
                 Working example 
               
               
                 7 
                 7 
                 G 
                 650 
                 4 
                 766 
                 839 
                 44 
                 92 
                 Working example 
               
               
                 8 
                 8 
                 H 
                 620 
                 2 
                 435 
                 455 
                 23 
                 91 
                 Comparative example 
               
               
                 9 
                 9 
                 I 
                 620 
                 2 
                 530 
                 579 
                 44 
                 95 
                 Comparative example 
               
               
                 10 
                 10 
                 J 
                 650 
                 1 
                 606 
                 627 
                 43 
                 96 
                 Comparative example 
               
               
                 11 
                 11 
                 K 
                 675 
                 2 
                 570 
                 603 
                 41 
                 96 
                 Comparative example 
               
               
                 12 
                 12 
                 L 
                 620 
                 2 
                 618 
                 662 
                 42 
                 94 
                 Comparative example 
               
               
                 13 
                 13 
                 M 
                 650 
                 2 
                 493 
                 521 
                 1 
                 — 
                 Comparative example 
               
               
                 14 
                 14 
                 N 
                 675 
                 2 
                 627 
                 681 
                 22 
                 94 
                 Comparative example 
               
               
                 15 
                 15 
                 O 
                 620 
                 2 
                 633 
                 690 
                 21 
                 90 
                 Comparative example 
               
               
                 16 
                 16 
                 A 
                 620 
                 2 
                 511 
                 588 
                 −30 
                 10 
                 Comparative example 
               
               
                 17 
                 17 
                 A 
                 620 
                 2 
                 623 
                 663 
                 38 
                 92 
                 Comparative example 
               
               
                 18 
                 18 
                 A 
                 650 
                 2 
                 538 
                 625 
                 39 
                 90 
                 Comparative example 
               
               
                 19 
                 19 
                 A 
                 620 
                 2 
                 568 
                 745 
                 44 
                 92 
                 Comparative example 
               
               
                 20 
                 20 
                 A 
                 675 
                 2 
                 604 
                 696 
                 −20 
                 50 
                 Comparative example 
               
               
                 21 
                 21 
                 A 
                 650 
                 2 
                 575 
                 653 
                 42 
                 56 
                 Comparative example 
               
               
                 22 
                 22 
                 C 
                 620 
                 2 
                 672 
                 685 
                 30 
                 90 
                 Comparative example 
               
               
                 23 
                 23 
                 C 
                 675 
                 2 
                 593 
                 765 
                 34 
                 90 
                 Comparative example 
               
               
                 24 
                 24 
                 C 
                 620 
                 2 
                 554 
                 622 
                 −17 
                 63 
                 Comparative example 
               
               
                 25 
                 25 
                 E 
                 620 
                 2 
                 495 
                 540 
                 6 
                 17 
                 Comparative example 
               
               
                 26 
                 26 
                 E 
                 675 
                 2 
                 503 
                 538 
                 33 
                 90 
                 Comparative example 
               
               
                 27 
                 27 
                 E 
                 650 
                 2 
                 560 
                 624 
                 38 
                 68 
                 Comparative example 
               
               
                 28 
                 28 
                 F 
                 620 
                 2 
                 540 
                 659 
                 2 
                 20 
                 Comparative example 
               
               
                 29 
                 29 
                 F 
                 650 
                 2 
                 500 
                 526 
                 40 
                 89 
                 Comparative example 
               
               
                 30 
                 30 
                 F 
                 675 
                 2 
                 550 
                 691 
                 −43 
                 60 
                 Comparative example 
               
               
                 31 
                 31 
                 G 
                 620 
                 2 
                 694 
                 865 
                 34 
                 92 
                 Comparative example 
               
               
                 32 
                 32 
                 G 
                 650 
                 2 
                 769 
                 880 
                 35 
                 90 
                 Comparative example 
               
               
                 33 
                 17 
                 A 
                 650 
                 2 
                 615 
                 658 
                 35 
                 90 
                 Comparative example 
               
               
                   
               
               
                 *Amount of precipitated Nb after post-weld heat treatment (Ratio (%) relative to the total amount of Nb on a Nb equivalent basis) 
               
            
           
         
       
     
     All the working examples of the present disclosure are electric-resistance-welded steel pipes that are suitable for a deep-well conductor casing, have high strength of the API X80 grade, that is, a yield strength YS of 555 MPa or more and a tensile strength TS of 625 MPa or more, have good low-temperature toughness, suffer a smaller decrease in strength even after post-weld heat treatment, and have high resistance to post-weld heat treatment. The comparative examples outside the scope of the present disclosure are insufficient in strength, low-temperature toughness, or resistance to post-weld heat treatment. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  Hot-rolled steel plate (hot-rolled steel strip) 
               2  Roll forming machine 
               3  Welding machine 
               4  Squeeze roll 
               5  Electric-resistance-welded steel pipe 
               6  Bead cutter 
               7  Leveler 
               8  Sizer 
               9  Inline heat treatment apparatus (induction heating apparatus) 
               10  Cooling apparatus 
               11  Thermometer