JP7216902B2 - ERW steel pipe for oil well and manufacturing method thereof - Google Patents

Description

TECHNICAL FIELD The present invention relates to an electric resistance welded steel pipe suitable for oil wells and a method for manufacturing the same. More specifically, the present invention relates to an oil well electric resistance welded steel pipe having a strength equivalent to API standard 5CT R95 (yield strength YS: 655 MPa or more and 758 MPa or less, tensile strength TS: 724 MPa or more) and excellent toughness, and a method for manufacturing the same.

Oil country tubular goods (OCTG) are steel pipes used to extract gas and oil from the ground. However, with the increasing severity of natural resource drilling areas in recent years, the properties required of oil country tubular goods are changing.
As one example, deep wells are being developed, and improvements in crushing characteristics (characteristics that do not buckle against external pressure) and higher toughness have begun to be required.
The crushing property is improved by the high circumferential yield strength of the steel pipe and the high shape accuracy (especially uneven wall thickness and roundness) of the steel pipe. Due to its high shape accuracy, ERW steel pipes are known to have higher crushing characteristics than other types of pipes of the same size (outer diameter and wall thickness).
Another measure to improve the crushing strength is to increase the strength, but strength and toughness are generally contradictory properties, and it is difficult to achieve both.

Patent Documents 1 and 2 below disclose a method for manufacturing an electric resistance welded oil country tubular good having a yield strength of 655 MPa class.
Patent Document 1 discloses a hot-rolled steel sheet containing specified amounts of C, Si, Mn, Ti, B, Mo, V, and Nb and containing low amounts of P, S, and O. A hot-rolled steel sheet for electric resistance welded steel pipes in which the short diameter of the prior γ grains is 25 μm or less is disclosed.
Patent Document 2 describes an electric resistance welded steel pipe containing specified amounts of C, Si, Mn, Nb, V, Ti, Mo, Ni, and Al, and having a total value of Mo and Ni in a specified range of 10 area% Disclosed is a high-strength electric resistance welded steel pipe consisting of the following polygonal ferrite and the balance bainetic ferrite, with specific ranges of tensile strength, yield strength, and Charpy absorbed energy.

JP 2015-168864 A Japanese Patent No. 6048621

The electric resistance welded steel pipes described in Patent Documents 1 and 2 are both characterized by having a Charpy absorbed energy of 22 J or more at 0°C. Examples have been disclosed, and Patent Document 2 discloses an example with a Charpy absorbed energy of 75 to 170 J at 0°C.

However, as described above, in recent years there has been a demand for even higher toughness, specifically a Charpy absorbed energy of 100 J or more at −20° C., and the electric resistance welded steel pipes described in Patent Documents 1 and 2 satisfy this demand. I can't.

The present invention has been made in view of the actual situation as described above, and both the base material and the electric resistance welded portion have a yield strength of 655 MPa or more, a Charpy fracture surface transition temperature of −40 ° C. or less, and −20 An object of the present invention is to provide an electric resistance welded steel pipe for oil wells having a Charpy toughness value of 100 J or more at °C and a method for manufacturing the same.

The gist of the present invention, which aims to solve the above problems, is as follows.
(1) The electric resistance welded steel pipe for oil wells of this embodiment has, in mass%, C: 0.030 to 0.100%, Mn: 1.30 to 2.00%, Ti: 0.010 to 0.100%, Nb: 0.010-0.100%, N: 0.0010-0.0200%, Si: 0.01-0.50%, Al: 0.001-0.100%, Mo: 0.010- 0.500%, V: 0.010 to 0.100% , B: 0.0001 to 0.0010% , P: 0.030% or less, S: 0.010% or less , and the balance is It has a component composition consisting of Fe and unavoidable impurities, Mo% + V% (X% is mass% of element X) of 0.10% or more, a ferrite area ratio of 10% or more and 50% or less, and ferrite The average crystal grain size is 20 μm or less, the balance consists of a bainite structure, the yield strength of the base material is 655 MPa or more and 758 MPa or less, the tensile strength of the base material is 724 MPa or more, and the Charpy fracture surface transition temperature of the base material is − The Charpy absorbed energy at 40° C. or less and −20° C. is 100 J or more, and the yield strength of the electric resistance welded portion is 655 MPa or more and 758 MPa or less.

(2) In the electric resistance welded steel pipe for oil wells of this embodiment, it is preferable that the plate thickness is 10 mm or more and 25 mm or less.
(3) In the electric resistance welded steel pipe for oil well of this embodiment, Cu: 0.05 to 0.50%, Ni: 0.05 to 0.50%, Cr: 0.05 to 0.50%, One or more of Ca: 0.0001 to 0.0100% and REM: 0.0001 to 0.0100% may be contained.

(4) In the method for manufacturing an electric resistance welded steel pipe for oil well according to this aspect, the method for manufacturing an electric resistance welded steel pipe for oil well according to any one of (1) to (3), wherein (1) or (3) After finish rolling the slab having the chemical composition described in 950 ° C. or less with a cumulative reduction rate of 50% or more and a finish rolling end temperature of 850 ° C. or less, the average cooling rate is 20 ° C./s to 600 to 700 ° C. After cooling to 450 to 600° C. at an average cooling rate of 2 to 10° C./s, the hot-rolled steel sheet is coiled and welded. After heating, it is cooled to 400-700°C at an average cooling rate of 10-50°C/s.

According to the present invention, both the base material and the electric resistance welded portion have a yield strength of 655 MPa or more, the Charpy fracture surface transition temperature of the base material is -40 ° C. or less, and the Charpy toughness value at -20 ° C. is 100 J or more. An electric resistance welded steel pipe for a certain oil well can be provided, and the industrial contribution is extremely effective.

An embodiment of an electric resistance welded steel pipe for oil wells according to the present invention will be described below.
First, the chemical composition of steel suitable for the electric resistance welded steel pipe for oil well use according to one embodiment of the present invention will be described. In addition, "%" in a component composition means the mass % unless there is particular notice. Further, in the numerical range in the component composition, the numerical range represented by using "~" means a range including the numerical values described before and after "~" as lower and upper limits, unless otherwise specified. Therefore, for example, 0.03 to 0.10% means a range of 0.03% or more and 0.10% or less.

As described below, the electric resistance welded steel pipe for oil wells according to the present embodiment has C: 0.030 to 0.100%, Mn: 1.30 to 2.00%, and Ti: 0.010% by mass. ~0.100%, Nb: 0.010-0.100%, N: 0.0010-0.0200%, Si: 0.01-0.50%, Al: 0.001-0.100%, Mo: 0.010 to 0.500%, V: 0.010 to 0.100%, P: 0.030% or less, S: 0.010% or less, B: 0.0010% or less , the balance being Fe and unavoidable impurities, and Mo%+V% is 0.10% or more.
The reasons for limiting the chemical composition of the electric resistance welded steel pipe for oil well use of the present invention will be described below.

"C: carbon (0.030 to 0.100%)"
C is an important element that contributes to strength development of steel, and the C content is made 0.030% or more. Below this amount of carbon, the strength of the base material decreases. On the other hand, if the C content exceeds 0.100%, the strength becomes excessive, so the upper limit of the C content is made 0.100%.

"Mn: manganese (1.30 to 2.00%)"
Mn is a solid-solution strengthening element of steel, and the Mn content is made 1.30% or more to ensure strength. If the Mn content is less than 1.3%, solid-solution strengthening is insufficient, and base material strength and electric resistance weld strength deteriorate. When Mn is contained excessively, coarse MnS is generated in the central portion of the plate thickness, which may impair the toughness of the base material. Therefore, the upper limit of the Mn content is set to 2.00%.
"Ti: titanium (0.010 to 0.100%)"
Ti is an element that forms carbonitrides in steel to improve the strength of the base material and also contributes to grain refinement. By containing 0.010% or more of Ti, it is possible to refine the structure of the steel, and the Ti content is made 0.010% or more. However, if the Ti content exceeds 0.100%, coarse carbonitrides are formed and the toughness of the base material is lowered, so the upper limit of the Ti content is made 0.100%.

"Nb: niobium (0.010 to 0.100%)"
Nb is contained in order to contribute to improvement of toughness and strength of the base metal. The Nb content is set to 0.010% or more in order to improve toughness by non-recrystallization rolling. If the Nb content exceeds 0.100%, coarse carbides deteriorate the toughness of the base material, so the upper limit of the Nb content is made 0.100%.
"N: Nitrogen (0.0010 to 0.0200%)"
N forms alloy nitrides in the steel to suppress grain coarsening and improve the toughness of the base material. In order to obtain the effect, the N content is made 0.0010% or more. On the other hand, if the N content exceeds 0.0200%, the amount of alloy nitrides produced increases and the toughness of the base material deteriorates, so the upper limit of the N content is made 0.0200%.

"Si: silicon (0.01 to 0.50%)"
Si is an element used as a deoxidizing agent for steel, and has the effect of suppressing the formation of coarse oxides in the base material and improving toughness. In order to obtain this effect, the Si content is set to 0.010% or more. On the other hand, if the Si content exceeds 0.50%, inclusions may form and the toughness may decrease, so the upper limit of the Si content is made 0.50%.
"Al: aluminum (0.001 to 0.100%)"
Al, like Si, is contained in steel as a deoxidizer. The Al content is set to 0.001% or more to prevent cracks caused by free oxygen. On the other hand, if the Al content exceeds 0.100%, the toughness is lowered due to the formation of Al-based oxides, so the upper limit of the Al content is made 0.100%.

"Mo: Molybdenum (0.010-0.500%)"
The reason for containing Mo is to improve the strength by precipitation strengthening. In order to obtain the effect, the Mo content is made 0.010% or more. If a large amount of Mo is contained, Mo carbonitrides are formed, which may reduce the toughness of the base material. Therefore, the upper limit of the Mo content is made 0.500%.
"V: vanadium (0.010 to 0.100%)"
V forms carbonitrides during rolling of steel and has the effect of refining the structure due to the pinning effect. In order to obtain this effect, the V content is set to 0.010% or more. If a large amount of V is contained, V carbonitrides become coarse and the toughness of the base material is lowered, so the upper limit of the V content is made 0.100%.

"P: phosphorus (0.030% or less)"
P is an element that exists as an unavoidable impurity in steel, and if the P content exceeds 0.030%, it segregates at grain boundaries and impairs the toughness. do.
"S: sulfur (0.010% or less)"
S is an element that exists as an unavoidable impurity in steel, and an excessive S content deteriorates the toughness of the steel.
"B: boron (0.0010% or less)"
B is an element that enhances the hardenability of steel even in a very small amount. If the B content exceeds 0.0010%, the base material strength exceeds the upper limit, so the upper limit of the B content is made 0.0010%.

In the present embodiment, in addition to the above elements, the base steel plate further contains, in mass%, Cu: 0.05% to 0.50%, Ni: 0.05% to 0.50%, Cr: May contain one or more elements selected from 0.05% to 0.50%, Ca: 0.0001% to 0.0100%, REM: 0.0001% to 0.0100% .

"Cu: Copper (0.05-0.50%)"
Cu is an element effective in improving the strength of the base material, and in order to obtain this effect, the Cu content is made 0.05% or more. However, if it is contained in an excessively large amount, fine Cu particles may be generated and the toughness may be significantly deteriorated. Therefore, the upper limit of Cu content is set to 0.50%.
"Ni: Nickel (0.05-0.50%)"
Ni is an element that contributes to improving the strength and toughness of steel. In order to obtain those effects, the Ni content is set to 0.05% or more. However, if a large amount of Ni is contained, the strength becomes too high, so the upper limit of the Ni content is made 0.50%.
"Cr: Chromium (0.05-0.50%)"
Cr is a solid-solution strengthening element in steel, and the Cr content is set to 0.05% or more in order to obtain its effect. On the other hand, it is also an element that lowers weldability, and if it is contained in a large amount, welding defects will occur due to Cr-based inclusions formed in the electric resistance welded portion. Therefore, the upper limit of the Cr content is set to 0.50%.

"Ca: calcium (0.0001 to 0.0100%)"
Ca is an element that controls the morphology of sulfide inclusions and improves the low temperature toughness of steel. In order to obtain the effect, the Ca content is made 0.0001% or more. If the amount of Ca exceeds 0.0100%, Ca-based coarse inclusions and clusters are formed, which may adversely affect toughness. Therefore, the upper limit of Ca content is set to 0.0100%.

"REM: rare earth element (0.0001 to 0.0100%)"
REM is an element contained as a deoxidizing agent and a desulfurizing agent, and the REM content is made 0.0001% or more. On the other hand, if the REM content exceeds 0.0100%, coarse oxides may be formed and the toughness of the base metal may be lowered, so the upper limit of the REM content is made 0.0050%.

The balance other than the above elements consists of Fe and unavoidable impurities. In addition to the above elements, a trace amount of elements that do not impair the effects of the present embodiment may be contained.
In this embodiment, Mo%+V% is set to 0.10% or more.
Mo and V are elements that contribute to the improvement of the strength of the electric resistance weld by precipitation strengthening. From the viewpoint of ensuring the strength of the electric resistance welded part, the sum of Mo% and V% is 0.10% or more.

The metal structure and ratio of the metal structure of the electric resistance welded steel pipe for oil well use according to the present embodiment are as follows.
In the present embodiment, the area ratio of ferrite is 10% or more and 50% or less, the average grain size of ferrite is 20 μm or less, and the balance is a bainite structure, so that the base material of the electric resistance welded steel pipe has strength and It was found that both toughness can be improved. In the composition range of the present embodiment, if the area ratio of ferrite is less than 10%, the toughness deteriorates, and if it exceeds 50%, the strength decreases. Further, when the average grain size of ferrite exceeds 20 μm, the toughness deteriorates. The formation of pearlite in the residual structure deteriorates the toughness. These metal structures can be produced by highly controlling the hot rolling process, which will be described later.

The area ratio of ferrite and the average crystal grain size are measured at the center of the wall thickness in a cross section (specifically, a cross section perpendicular to the pipe axis direction) at a position shifted by 90° in the pipe circumferential direction from the electric resistance welded part. In part, it can be obtained by grain average misorientation analysis (hereinafter referred to as GAM analysis) based on crystal orientation information obtained by EBSD (Electron Back Scatter Diffraction Patterns) method.

Specifically, after the cross section is mirror-polished and then finished with colloidal silica, a Field-Emission Scanning Electron Microscope (manufactured by JEOL, 7001F) is used to perform a 0.3 μm step on an area of 200 μm × 300 μm. Crystal orientation analysis is performed by the EBSD method.
In the subsequent GAM analysis, a region surrounded by 15° crystal misorientation is defined as one crystal grain, and grains with an average crystal misorientation of 1° or less are determined to be ferrite. The arithmetic mean value of the determined equivalent circle diameters of each ferrite is taken as the average grain size of the ferrite. In addition, the value obtained by performing the above measurements in 5 or more different fields and averaging the ferrite area ratios of the obtained fields is taken as the ferrite area ratio of the investigated electric resistance welded steel pipe.
The residual structure is identified by mirror-polishing the cross section on which the area ratio and grain size of ferrite are measured, etching it with nital, and observing it with an optical microscope at a magnification of 400.

Next, a method for manufacturing an electric resistance welded steel pipe for oil wells according to the present embodiment will be described.
First, a slab obtained by a continuous casting method or the like from molten steel adjusted to the above composition is charged into a heating furnace and heated. Since the steel used in the present embodiment contains a large amount of Ti and Nb, if the heating temperature of the steel slab is low, undissolved Nb carbides are formed and the toughness deteriorates. It is preferable to On the other hand, if the heating temperature is too high, the structure becomes coarse and the toughness deteriorates, so the heating temperature is preferably 1350° C. or lower.
After rough rolling of the heated slab, finish rolling is performed under the conditions that the cumulative rolling reduction at a temperature of 950° C. or less is 50% or more and the rolling end temperature is 850° C. or less. These conditions are necessary to refine the metallographic structure of steel and improve both strength and toughness.

After finish rolling, it is preferable to start cooling at a temperature of Ar3 or higher. This is because, after finish rolling, if the steel is air-cooled to less than the Ar 3 point at which ferrite transformation starts, coarse polygonal ferrite may be formed and the strength and toughness may deteriorate.
The Ar3 point can be obtained from the composition of the base steel sheet by the following (Equation 1).
Ar3 (° C.) = 910-310C%-80Mn%-55Ni%-20Cu%-15Cr%-80Mo% (Formula 1)

Here, in (Formula 1), C%, Mn%, Ni%, Cu%, Cr%, and Mo% are the contents (% by mass) of C, Mn, Ni, Cu, Cr, and Mo, respectively. .
Ni, Cu, and Cr are optional elements, and when they are not intentionally contained, they are calculated as 0 in the above (formula 1).
The cooling start temperature corresponds to the rolling end temperature. It is preferable to start cooling within 5 seconds after finishing rolling.

In the steel material constituting the electric resistance welded steel pipe for oil wells of this embodiment, it is essential to control the area ratio of the main phase bainite and the subphase ferrite and the ferrite grain size in order to balance strength and toughness. .

Therefore, the cooling pattern is highly controlled in the ROT (run-out table) after rolling. Specifically, as the cooling pattern of the first stage, cooling is performed at an average cooling rate of 20°C/s or more from the start of cooling to a cooling stop temperature in the range of 600 to 700°C. This creates a large number of ferrite nucleation sites in the strain within the unrecrystallized structure formed by finish rolling.
If the cooling rate in the first stage is less than the average cooling rate of 20° C./s, the number of ferrite nucleation sites is small, ferrite is coarsely formed, and the toughness deteriorates. If the cooling stop temperature in the first stage is lower than 600° C., bainite transformation occurs before ferrite is sufficiently formed in the second stage cooling, and the bainite fraction increases, resulting in deterioration of toughness. If the cooling stop temperature in the first stage is higher than 700° C., the grain size of ferrite becomes coarser in the cooling in the second stage and the toughness deteriorates.

After the first stage cooling, the second stage cooling pattern is to cool from the first stage cooling end temperature to a cooling stop temperature in the range of 450 to 600° C. at an average cooling rate of 2 to 10° C./s. As a result, fine ferrite is generated from numerous nucleation sites generated during the first stage of cooling, and then the remainder transforms into bainite.
If the average cooling rate of the second stage is less than 2°C/s, pearlite is formed and the toughness deteriorates. If the cooling rate in the second stage exceeds 10° C./s as an average cooling rate, ferrite is not sufficiently formed and the bainite fraction increases, resulting in deterioration of toughness. If the cooling stop temperature of the second stage is lower than 450°C, the precipitated elements cannot be precipitated after winding, resulting in a decrease in strength. If the cooling stop temperature of the second stage exceeds 600°C, pearlite is formed and the toughness deteriorates. If the cooling time in the second step is too short, the desired metal structure may not be obtained, so the cooling time is preferably 20 seconds or longer.

Winding is preferably carried out within 10 seconds after stopping cooling in the second stage.
The steel used in this embodiment is particularly effective for steel pipes with a plate thickness of 10 to 25 mm.

After the hot-rolled steel sheet is continuously roll-formed to form an open pipe, the vicinity of the butted portion is heated to the melting point or higher, and electric resistance welding is performed by pressure welding with a squeeze roll to obtain an electric resistance welded steel pipe. Thereafter, the electric resistance welded portion is reheated and then cooled to a cooling stop temperature in the range of 400 to 700°C at an average cooling rate of 10 to 50°C/s.
The reheating temperature of the electric resistance welded portion is preferably 900 to 1050°C. If the heating temperature of the electric resistance welded portion is lower than 900° C., a coarse metal structure produced during welding remains. When the reheating temperature is higher than 1050°C, the crystal grains become coarse. If the average cooling rate after reheating the electric resistance welded portion is less than 10° C./s, the strength of the electric resistance welded portion is lowered. If the average cooling rate exceeds 50° C./s, a hard structure is generated in the electric resistance welded portion, so that the electric resistance welded portion strength exceeds the upper limit. If the cooling stop temperature is lower than 400°C, the precipitation strengthening is not sufficiently exhibited, and the strength of the electric resistance welded portion becomes low. If the cooling stop temperature exceeds 700°C, the crystal grains become coarse and the strength of the electric resistance weld zone becomes low.
After the heat treatment and cooling are completed, the material is cooled to room temperature and diameter-reducing rolling is performed using sizer rolls. The diameter reduction ratio of the diameter reduction rolling is preferably in the range of 0.3 to 5.0%.

The method for measuring the properties of the electric resistance welded steel pipe manufactured as described above is as follows.
In the tensile test of the base material portion, a full-thickness test piece in the axial direction (rolling direction) of the steel pipe was taken as a tensile test piece from the above electric resistance welded steel pipe, and a tensile test was performed to determine the yield strength (YS: 0.2% offset). and tensile strength (TS) were measured. Here, the tensile test piece of the base material is sampled from a portion corresponding to a position 90° in the circumferential direction from the seam portion of the electric resistance welded steel pipe. In the tensile test of the electric resistance welded part, a full-thickness test piece in the circumferential direction (vertical direction of rolling) of the steel pipe is used as a tensile test specimen, and the electric resistance welded part of the electric resistance welded steel pipe is placed approximately in the center between the scores of the tensile test specimen. After correcting the warp, a tensile test is performed to measure the yield strength (YS: 0.2% offset).
Furthermore, the method for measuring the toughness of the electric resistance welded steel pipe is as follows.
For toughness, a full-size V-notch Charpy test piece in the circumferential direction (perpendicular to the rolling direction) was taken from the base material of the electric resistance welded steel pipe (the portion corresponding to the position at 90° in the circumferential direction from the seam of the electric resistance welded steel pipe), A V-notch Charpy test is performed at a test temperature of 0°C to -100°C to investigate the fracture surface transition temperature, and the absorbed energy at -20°C is measured.

The resulting electric resistance welded steel pipe has a base material yield strength of 655 MPa or more and 758 MPa or less, a base material tensile strength of 724 MPa or more, and a Charpy fracture transition temperature of the base material of -40°C or less and a Charpy absorption of -20°C. It is preferable that the energy is 100 J or more and the yield strength of the electric resistance welded portion is 655 MPa or more and 758 MPa or less.

Examples are shown below. However, the examples described below are for explanation along specific examples, and the present invention is not limited to the conditions used in the following examples.
The slabs of invention examples No. 1 to No. 14 having the compositions shown in Table 1 and the slabs of comparative examples No. 15 to No. 49 shown in Tables 1 and 2 were produced by continuous casting at a temperature of 1200 to 1250 ° C. After heating to 17.5 mm and rough rolling, finish rolling was performed under the conditions of a cumulative rolling reduction of 950° C. or less and a finish rolling end temperature shown in Tables 3 and 4 to obtain a steel plate having a thickness of 17.5 mm.
For these hot-rolled steel sheets, in the ROT (run-out table) after finish rolling, the first-stage cooling is stopped at the first-stage average cooling rate shown in Tables 3 and 4 as the first-stage cooling pattern. Cooled to temperature. After the first stage cooling, the steel sheet was cooled to the second stage cooling stop temperature at the second stage average cooling rate shown in Tables 3 and 4, and coiled to obtain a hot-rolled steel sheet.

The obtained hot-rolled steel sheet was continuously roll-formed into an open pipe, then the vicinity of the butted portion was heated to a melting point or higher, and electric resistance welding was performed by pressure welding with squeeze rolls to obtain an electric resistance welded steel pipe.
The electric resistance welded portion of the electric resistance welded steel pipe was reheated to the temperature (ERW portion heating temperature) shown in Tables 3 and 4, and then cooled at the average cooling rate shown in Tables 3 and 4 as shown in Tables 3 and 4. Cool to stop temperature, then stop cooling and allow to cool. After cooling to room temperature, diameter reduction rolling was performed using sizer rolls to obtain an electric resistance welded steel pipe having an outer diameter of 406 mm and a wall thickness of 17.5 mm.

Regarding the electric resistance welded steel pipe manufactured as described above, the metallographic structure of the electric resistance welded steel pipe was investigated by the method described above. In addition, the tensile test of the base material and the electric resistance welded portion, and the Charpy test of the base material were performed by the methods described above.

Tables 3 and 4 show the ferrite area ratio, the ferrite average crystal grain size (μm), and the type of the residual structure, with B being bainite and P being pearlite.
In addition, Tables 3 and 4 show base material yield strength (MPa), base material tensile strength (MPa), base material Charpy absorbed energy (J), base material Charpy fracture surface transition temperature (°C), and electric resistance welded steel pipes. Weld zone yield strength (MPa) is collectively shown.

As shown in Tables 1 and 3, the samples No. 1 to No. 14 of the present invention have a base material yield strength of 655 MPa or more and 758 MPa or less and a base material tensile strength of 724 MPa or more, which are suitable for oil wells. The Charpy fracture surface transition temperature of the base material was −40° C. or less, the Charpy absorbed energy at −20° C. was 100 J or more, and the yield strength of the electric resistance weld was 655 MPa or more and 758 MPa or less.

No. shown in Table 1. Since the C content of sample No. 15 was below the lower limit of the desirable range, the yield strength of the base metal was below the lower limit of the desirable range as shown in Table 3.
No. shown in Table 1. The 16 samples exceeded the upper limit of the desired range for C content, and as shown in Table 3, exceeded the base metal yield strength.
No. shown in Table 1. In sample No. 17, the Mn content was below the desired range, so solid-solution strengthening was insufficient, and as shown in Table 3, the yield strength of the base material fell below the lower limit.
No. shown in Table 1. In sample No. 18, the Mn content exceeded the upper limit of the desirable range, so embrittlement due to MnS occurred and the toughness of the base metal deteriorated.

No. shown in Table 1. In sample No. 19, the Ti content was below the lower limit of the desirable range, so the crystal grain size increased and the toughness of the base material deteriorated.
No. shown in Table 1. In sample No. 20, the Ti content exceeded the upper limit of the desirable range, so a large amount of Ti-based carbonitrides was generated, and as shown in Table 3, the toughness of the base material deteriorated.
No. shown in Table 1. In sample No. 21, the Nb content was below the lower limit of the desirable range, so the crystal grain size of ferrite increased and, as shown in Table 3, the toughness of the base material deteriorated.
No. shown in Table 1. In sample No. 22, the Nb content exceeded the upper limit of the desirable range, so a large amount of Nb-based carbonitrides was formed, and as shown in Table 3, the toughness of the base material deteriorated.

No. shown in Table 1. In sample No. 23, since the N content was below the lower limit of the desirable range, carbonitrides were not formed, the crystal grain size became coarse, and the toughness of the base material deteriorated.
No. shown in Table 1. In sample No. 24, since the N content exceeded the upper limit of the desirable range, the formation of alloy carbide increased and the toughness of the base material deteriorated.

No. shown in Table 2. In sample No. 25, the Si content was below the lower limit of the desired range, so deoxidation was insufficient, and as shown in Table 4, the toughness of the base material deteriorated.
No. shown in Table 2. In sample No. 26, the Si content exceeded the upper limit of the desirable range.
No. shown in Table 2. In sample No. 27, the Al content was below the lower limit of the desired range.
No. shown in Table 2. In sample No. 28, the Al content exceeded the upper limit, so a large amount of Al oxide was generated, and as shown in Table 4, the toughness of the base material deteriorated.

No. shown in Table 2. In sample No. 29, the Mo content was below the desired lower limit, so the precipitation strengthening was insufficient and the base metal strength decreased as shown in Table 4.
No. shown in Table 2. In sample No. 30, the Mo content exceeded the upper limit of the desirable range.
No. shown in Table 2. In sample No. 31, since the V content was below the lower limit of the desirable range, the crystal grain size increased and, as shown in Table 4, the toughness of the base material deteriorated.
No. shown in Table 2. In sample No. 32, the V content exceeded the upper limit of the desirable range, so a large amount of V carbonitride was formed, and as shown in Table 4, the toughness of the base material deteriorated.

No. shown in Table 2. In sample No. 33, since the P content exceeded the upper limit of the desirable range, intergranular embrittlement occurred and the toughness of the base material deteriorated as shown in Table 4.
No. shown in Table 2. In sample No. 34, since the S content exceeded the upper limit of the desirable range, coarse inclusions were formed and, as shown in Table 4, the toughness of the base material deteriorated.
No. shown in Table 2. In sample No. 35, the B content exceeded the upper limit of the desirable range, so the hardenability was high, and as shown in Table 4, the base material strength exceeded the upper limit.
No. shown in Table 2. Sample No. 36 had a lower electric resistance weld strength as shown in Table 4 because the value of (Mo%+V%) was below the desired lower limit.

No. shown in Table 2. In sample No. 37, the finish rolling end temperature exceeded the upper limit of the desired range, so the ferrite average crystal grain size increased and, as shown in Table 4, the base metal toughness deteriorated.
No. shown in Table 2. In sample No. 38, the cumulative rolling reduction fell below the lower limit of the desirable range, so the average ferrite crystal grain size increased and, as shown in Table 4, the toughness of the base material deteriorated.
No. shown in Table 2. In sample No. 39, the cooling rate in the first stage after hot rolling was below the lower limit of the desirable range, so the average grain size of ferrite increased and, as shown in Table 4, the toughness of the base material deteriorated.

No. shown in Table 2. In sample No. 40, the cooling stop temperature of the first stage fell below the lower limit of the desired range, so the metal structure fraction did not satisfy the regulation, and as shown in Table 4, the base metal toughness deteriorated.
No. shown in Table 2. In sample No. 41, the cooling stop temperature in the first stage exceeded the upper limit of the desirable range, so the ferrite average crystal grain size increased and, as shown in Table 4, the base metal toughness deteriorated.
No. shown in Table 2. In sample No. 42, the second-stage cooling rate fell below the lower limit of the desired range, so a pearlite structure was formed, the metal structure fraction did not satisfy the regulation, and the base metal toughness deteriorated as shown in Table 4.
No. shown in Table 2. In sample No. 43, the second-stage cooling rate exceeded the upper limit of the desirable range, so the metal structure fraction did not satisfy the regulation, and as shown in Table 4, the base metal toughness deteriorated.

No. shown in Table 2. In sample No. 44, the second-stage cooling stop temperature was below the lower limit of the desirable range, so precipitates were not formed and the base material strength decreased as shown in Table 4.
No. shown in Table 2. In sample No. 45, the cooling stop temperature in the second stage exceeded the desired range, so a pearlite structure was formed, the metal structure fraction did not satisfy the regulation, and the base metal toughness deteriorated as shown in Table 4.
No. shown in Table 2. In the sample No. 46, the cooling rate during the heat treatment of the electric resistance welded portion was below the lower limit of the desirable range, so the strength of the electric resistance welded portion decreased as shown in Table 4.
No. shown in Table 2. Sample No. 47 exceeded the upper limit of the desirable range for the cooling rate during the heat treatment of the electric resistance welded portion, so that the electric resistance welded portion strength exceeded the upper limit as shown in Table 4.

No. shown in Table 2. In sample No. 48, the cooling stop temperature during the heat treatment of the electric resistance welded portion was below the lower limit of the desirable range, so precipitates were not formed and the strength of the electric resistance welded portion decreased as shown in Table 4.
No. shown in Table 2. In sample No. 49, the cooling stop temperature during the heat treatment of the electric resistance welded portion exceeded the upper limit of the desirable range, so the electric resistance welded portion strength decreased as shown in Table 4.

From the descriptions in Tables 1 to 4, if it is a steel sheet for electric resistance welded steel pipe and an electric resistance welded steel pipe having the composition range described above and the ferrite area ratio and ferrite average grain size described above, the above It has been found that it has desirable yield strength and tensile strength ranges, a Charpy fracture surface transition temperature of -40°C or less, and a Charpy absorbed energy of -20°C of 100 J or more, which are excellent properties.

Claims (4)

in % by mass,
C: 0.030 to 0.100%,
Mn: 1.30-2.00%,
Ti: 0.010 to 0.100%,
Nb: 0.010 to 0.100%,
N: 0.0010 to 0.0200%,
Si: 0.01 to 0.50%,
Al: 0.001 to 0.100%,
Mo: 0.010 to 0.500%,
V: 0.010 to 0.100% ,
B: 0.0001 to 0.0010%
including
P: 0.030% or less,
S: 0.010% or less
with the balance being Fe and unavoidable impurities, Mo% + V% (X% is the mass% of element X) of 0.10% or more, and a ferrite area ratio of 10% or more and 50% and the average grain size of ferrite is 20 μm or less, the balance is composed of a bainite structure, the yield strength of the base material is 655 MPa or more and 758 MPa or less, the tensile strength of the base material is 724 MPa or more, and the Charpy An electric resistance welded steel pipe for an oil well, characterized by having a fracture surface transition temperature of −40° C. or less, a Charpy absorbed energy at −20° C. of 100 J or more, and an electric resistance welded portion having a yield strength of 655 MPa or more and 758 MPa or less. 2. The electric resistance welded steel pipe for oil well use according to claim 1, wherein the plate thickness is 10 mm or more and 25 mm or less. in % by mass,
Cu: 0.05% to 0.50%,
Ni: 0.05% to 0.50%,
Cr: 0.05% to 0.50%,
Ca: 0.0001% to 0.0100%,
REM: 0.0001% to 0.0100%
3. The electric resistance welded steel pipe for oil well use according to claim 1 or 2, characterized by containing one or more of The method for manufacturing the electric resistance welded steel pipe for oil wells according to any one of claims 1 to 3, wherein the slab having the chemical composition according to claim 1 or claim 3 is heated to a cumulative temperature of 950 ° C. or less. After finish rolling under the conditions of a rolling reduction of 50% or more and a finish rolling end temperature of 850 ° C. or less, it is cooled to 600 to 700 ° C. at an average cooling rate of 20 ° C./s or more, and then to 450 to 600 ° C. with an average cooling rate of 2. The hot-rolled steel sheet that is cooled at ~10°C/s and coiled is pipe-formed and welded, then the electric resistance welded portion is heated to 900-1050°C, and after heating, the average cooling rate is 10-50°C/s to 400-700°C. A method for manufacturing an electric resistance welded steel pipe for oil wells, characterized by cooling to °C.